Nobel metal-TiO2 nanocomposites - Investigo

NOBEL METAL-TiO2

NANOCOMPOSITES:

SYNTHESIS,

CHARACTERIZATION

AND CATALYTIC

ACTIVITY

Memoria que presenta

Ana Cláudia Lobão do Nascimento

para optar al grado de Doctor

Vigo, Septiembre de 2015

DEPARTAMENTO DE QUÍMICA FÍSICA

NOBEL METAL-TiO2 NANOCOMPOSITES:

SYNTHESIS, CHARACTERIZATION AND

CATALYTIC ACTIVITY

Memoria que presenta Ana Cláudia Lobão do Nascimento

Para optar al grado de Doctor

Vigo, 15 de Septiembre de 2015

Contents

Thesis Scope 1

1.General introduction 5

1.1. Metal nanoparticles 5

1.1.1. Localized surface plasmon resonance 7

1.1.1.1. Spherical particles. Mie theory 8

1.1.1.2. Core-shell particles. Correct Mie Theory 10

1.1.2. Catalysis by metal nanoparticles 11

1.1.2.1. Reduction of hexacyanoferrate (III) by borohydride 12

1.2. Titanium dioxide 15

1.2.1. Gold nanoparticles doped titania 18

1.2.2. Catalysis by gold nanoparticles supported on TiO2 20

1.2.3. Elementary steps in photocatalytic TiO2 systems 21

1.2.3.1. Photocatalytic TiO2@Au systems 24

2. Titania-coated gold nanoparticles and growth of titania shell 27

2.1. Introduction 28

2.2. Experimental section 29

2.3. Results and discussion 32

2.4. Conclusions 39

3. Synthesis and characterization of Au-doped TiO2 nanoparticles 41



3.3. Results and discussion

3.3.1. Anatase titanium dioxide

3.3.2. Gold-doped titanium dioxide nanoparticles

47

47

53

3.2.3.1. Electrostatic adsorption of AuCl4− on TiO2 surface 54

3.2.3.2. Adsorption of Au(Cl)4−n(OH)n− ions/Au(OH)3on TiO2 surface 55

3.2.3.3. Deposition-precipitation with urea 57

3.4. Conclusions 63

4. TiO2@Au catalyzed reduction of ferrycianate (III) by borohydride ions 65




4.3.1. Influence of borohydride concentration 69

4.3.2. Influence of the amount of catalyst 70

4.3.3. Influence of temperature 72

4.3.4. Recyclability of TiO2@Au catalyst 74

4.3.5. Effect of titania on the reaction rate 75

4.3.6. Model proposed for the catalytic process 77

4.4. Conclusions 78

5. Photochemical activity of TiO2 / TiO2@Au systems 81




5.3.1. Photoreduction of gold species on TiO2@Au composites 84

5.3.2. Photocatalytic degradation of rhodamine B 86

5.4. Conclusions 92

Conclusions 93

Appendix I 97

Appendix II 105

Appendix III 111

Resumen 113

Acknowledgements 135

References 137

1

Thesis Scope

The work presented in this thesis is focused on the synthesis, characterization and catalytic

activity of gold-TiO2 composites. We wanted to take advantage of the experience of the

Colloid Chemistry Group, whose activity is strongly focused on the synthesis,

characterization and evaluation of the formation mechanism of metal nanocrystals (mainly

gold and silver) with size and shape control, which allows a fine-tuning of the optical

response of these colloids in the UV-vis-NIR spectral range. Moreover, their experience also

allowed the possibility to design core-shell nanoparticles of metal nucleus (e.g. silver, gold)

surrounds by a layer of a material such as silica or titania. These core-shell systems allow

control the properties of the colloid by means of careful modification of the dimensions of the

core-shell geometry and of the nature of both the core and the shell. For instance, coating of

gold nanoparticles with a semiconductor material like titanium dioxide make them interesting

materials to be used for energy conversion of solar to electrical energy inside an

electrochemical cell, for the photocatalytic degradation of organic pollutants and for chemical

sensors, among others. In this context, we wanted, at first, coated gold nanoparticles with a

uniform shell of titanium dioxide, Au@TiO2. However, nanoparticles coated with titanium

dioxide are generally difficult to synthesize, because common titania precursors are highly

reactive, and thus control over their hydrolysis and condensation is not straightforward.

Different strategies were followed to achieve a homogeneous titania shell with controlled

thickness. Only, an approach based on the combination of the layer-by-layer self-assembly

technique and the hydrolysis and condensation of titanium (IV) butoxide allowed obtain

core-shell gold-titania nanocomposites, as describe in Chapter 2. Characterization of these

Au@TiO2 core-shell nanocomposites was carried out by UV-vis spectroscopy and electron

microscopy techniques.

In a second stage, composites of titanium dioxide doped by gold nanoparticles, TiO2@Au,

were synthesized in a two-step protocol. The strategy is based on the preparation of

spindle-shaped TiO2 nanoparticles and after the Au deposition following different strategies,

as explored in Chapter 3. Is generally known, that nanostructured catalysts with porosity are

one effective way to improve the surface area and the elementary processes in

THESIS SCOPE

2

(photo)catalysis, namely they offer a shorter path for the reagents reached the surfaces actives.

Therefore, we decided to synthesize nanoporous anatase titania nanoparticles with a high

surface area. Also, it is well known that the catalytic behavior of TiO2@Au (photo)catalysts is

strongly influenced by the size of the gold nanoparticles and by their reciprocal interaction.

Therefore, TiO2 nanoparticles homogeneously doped with gold NPs with about 2 and 4 nm in

diameter were obtained only by the method of deposition-precipitation with urea followed by

reduction with sodium borohydride. The structural, physico-chemical and morphological

properties of TiO2 and Au-doped TiO2 were examined by transmission electron microscopy,

high resolution transmission electron microscopy, X-ray photoelectron spectroscopy,

UV-visible absorption spectroscopy, X- ray diffraction, dynamic light scattering, selected area

electronic diffraction, and Brunauer-Emmett-Teller surface area studies.

In Chapter 4, knowing that citrate stabilized gold nanoparticles are very efficiency catalyst for

the reduction of ferricyanide ion to ferrocyanide by sodium borohydride, we studied the

catalytic activity of TiO2@Au composites in the this reduction reaccion, in order to

understand the importance of TiO2 support on kinetic results and provide a clear and relevant

characterization of this catalytic system. Therefore, the catalytic activity of TiO2@Au catalyst

was compared with the catalytic activity of anatase titania mesocrystals and citrate-stabilized

Au nanoparticles. Furthermore, it is well know that the catalytic activity of TiO2@Au

composites is affected by the gold nanoparticles size and the gold loading. So, TiO2@Au

samples with different Au loading (Au particle size remained unchanged) and with gold

nanoparticles of about 2 and 4 nm in diameter were used to study effect of the Au loading and

the gold NP size effect, respectively on the reaction rate of reduction of ferricyanide by

borohydride ions. Moreover, the colloidal stability, proven by reusing the TiO2@Au catalyst

without loss of catalytic activity, during the reaction allows us to propose a mechanism of the

catalyzed reaction after the kinetic analysis of the results.

In Chapter 5, knowing that heterogeneous photocatalysis based on TiO2 has been the focal

point of numerous investigations in recent years because of the chemical stability of this

material, its lack of toxicity, and its potential utility for total destruction of organic

compounds in polluted air and wastewater. Furthermore, TiO2 samples synthesized exhibits

stronger absorption in the UV-visible range with a red shift in the band gap transitions. So, the

photocatalytic activity for degradation of rhodamine B was investigated in water under visible

light using the prepared TiO2/TiO2@Au samples. In addition to this, the electrons transfer

from anatase titanium dioxide nanoparticles to the gold ions (Au1+

/Au3+

) in the TiO2@Au

THESIS SCOPE

3

composites, prepared by deposition-precipitation with urea and before NaBH4 reduction,

under sunlight irradiation was checked.

In the last chapter, a general conclusions of this thesis are presented. Finally, a synopsis, in

Spanish, of this dissertation is included at the end of the thesis as required by the regulation of

University of Vigo.

THESIS SCOPE

4

5

CHAPTER 1

General Introduction

1.1. Metal nanoparticles

Nanoparticles (NPs) has become an area of intense scientific activity, which are able to

establish a bridge between bulk materials and molecular structures and have a wide variety of

potential applications in biomedical, environmental, optical, and electronic fields. At the

nanoscale range the percentage of atoms at the surface of a material becomes significant, high

surface to volume ratio. It gives rise to interesting and sometimes unexpected properties of

NPs which are not observed in bulk material. Importantly, at the nanoscale the properties are

size and even shape dependent.

Nowadays, metal NPs of different composition, shape and size can be easily obtained through

chemical, photochemical, physical, and biologicalI strategies. For instance, the most

commonly used method to synthesize quasi-spherical gold NPs in the range of 10-40 nm is

the Turkevich method1. This method consists on the reduction of tetrachloroauric acid,

HAuCl4, in a boiling sodium citrate aqueous solution, leading to the formation of citrate

stabilized gold colloids with low polydispersity. This strategy also allows the production of

Au NPs larger than 40 nm, however the obtained NPs are then rather polydisperse in size and

shape. Nevertheless, uniform Au NPs larger than 40 nm have been synthesized using seeded

growth process. For example, Rodríguez-Fernández et al.2 have synthesized

cetyltrimethylammonium bromide, CTAB, stabilized gold NPs in a wide range, from 12 up to

180 nm, with low polydispersity by a multistep seeded growth process, using in the first

growth step a seed solution of 12 nm citrate-stabilized Au NPs, synthesized according to the

Turkevich method and diluted in a CTAB solution, ascorbic acid as a reducing agent and

CTAB as the stabilizer.

I Biological systems such as bacteria, fungi, actinomycetes, yeasts, viruses, and plants have been

reported to synthesize various metal and metal oxide NPs.

GENERAL INTRODUCTION

6

Bimetallic nanoparticles, in which two different metal elements are assembled, are attractive

because their properties often differ from those of its individual constituents3. Besides, the

properties depend on the arrangement of the individual metallic atoms within the particle, that

is, alloys, core-shell, heterodimers or core-satellites. In the alloy structure the metals A and B

are located completely at random (see Figure 1.1 (a)). In the core-shell structure a shell of A

atoms surround a core of B atoms (see Figure 1.1 (b)). The heterodimers structures often

exhibit small metal clusters which are located on/within a larger cluster (see Figure 1.1 (c)). It

should be noted there are other possible structures of bimetallic NPs, including intermetallic

structures and sub-surface shells4 (see Figure 1.1 (d) and (e), respectively).

Figure 1.1: Schematic illustration of various structures of bimetallic nanoparticles. Random alloy (a);

core-shell (b); heterodimers (c); intermetallic (d); and sub-surface shells (e).

Bimetallic NPs can be prepared by chemical methods, via simultaneous or successive

reduction of two metal ions (see Figure 1.2) in the presence of a suitable capping or template

stabilization strategy. To prepare core-shell bimetallic nanoparticle the seeded growth method

is generally used. In this method, the second metal is deposited on the surface of pre-formed

metallic NPs. The simultaneous reduction of the precursors often yields a heterodimer

structure, or a mixture of two kinds of monometallic NPs.

Figure 1.2: Schematic view of the synthesis of bimetallic nanoparticles by co-reduction method (a)

and successive reduction method (b).


7

1.1.1. Localized surface plasmon resonance

The localized surface plasmon resonance (LSPR) is an optical phenomena found in metallic

nanoparticles responsible for their remarkable optical properties. The LSPR arises from the

resonant interaction of the free electrons in the metal nanoparticles with an incident

electromagnetic radiation. When a small spherical nanoparticle is irradiated by light, the free

electrons are displaced with respect to fixed metal “positive core”, creating a dipole in the

spherical nanoparticle. The metal “positive core” acting as a restoring force due to the

Coulomb attraction, allows the free electrons cloud oscillates forth and back of the core (see

Figure 1.3).

Figure 1.3: Schematic representation of the LSPR induced in a metallic sphere upon excitation with

an electromagnetic radiation. E = Electric field, B = Magnetic field and k = Propagation direction.

Reproduced from reference [5].

This collective oscillation of conduction electrons is known as plasmon. It is observed

experimentally as a plasmon bandII in the UV region for Pb, Sn, Hg nanoparticles or in the

visible region for gold, silver or copper. Therefore, Au, Ag or Cu nanoparticles tend to exhibit

very bright colorsIII

(see Figure 1.4) and show applications in a different fields (sensing6,

photonics7

, bioelectronic devices8 and in medical imaging/diagnostics/treatment

9, etc.).

The resonance frequency (𝜔) is given by the following expression:

𝜔 = (𝑒2

𝜖0𝑚𝑒𝑓𝑓4𝜋𝑅𝑠3)

1

2 (1.1)

II As long as the metallic sphere is small compared to the radiation wavelength, the collective

oscillation of the electrons is dipolar in nature. III

The Roman cup illustrating the myth of King Lycurgus (4th century, exposed in the British

Museum) or the glass windows of Sainte Chappelle in Paris are examples of old applications of

metallic colloids. The glass Lycurgus cup contains Au and Ag NPs and it is wine-red in transmitted

light and appears opaque green in reflected light.


8

where 𝑅𝑠 is the radius of a sphere, 𝑚𝑒𝑓𝑓 is the effective mass and 𝜖0 is the vacuum

permittivity and 𝑒 is the electron charge. The resonance frequency depends on other

additional parameters, such as particle shape, composition and surrounding medium.

Figure 1.4: Dispersions of gold nanodecahedra, prepared by seeded growth in DMF using different

amounts of Au seed solution, with different particle sizes (increasing from left to right). The LSPR

determines the color. Samples prepared by the Colloid Group of the University of Vigo.

1.1.1.1. Spherical particles. Mie theory

The LSPR occurs when light interacts with NPs that are much smaller than the incident

wavelength. Besides it is strongly dependent on nanoparticle size, shape, composition and the

dielectric constant of the environment3,10

.

The optical response of a metal nanoparticle with a dielectric function 𝜖(𝜔) can be modelled

by solving Maxwell equations taking into account particles geometry, illumination conditions

and assuming a local (𝜖(𝜔)) description of the materials involved. In 1908, Gustav Mie11

analytically solved Maxwell’s equations for an electromagnetic light wave interacting with a

metal sphere, much smaller than the wavelength of light, with a complex and frequency

dependent dielectric constant 𝜖(𝜆) = 𝜖′(𝜆) + 𝑖𝜖"(𝜆), embedded in a medium with dielectric

constant 𝜖𝑚. SPR can be quantitatively explained according to the following equation:

𝐶𝑒𝑥𝑡 =24𝜋2𝑅3𝜖𝑚

3/2

𝜆

𝜖′′

(𝜖′+2𝜖𝑚)2+ 𝜖′′2 (1.2)

where 𝐶𝑒𝑥𝑡 is the extinction cross-section, 𝑅 is the radius of the particle, is the wavelength

of the incident electromagnetic radiation and 𝜖′and 𝜖′′are the real and imaginary parts of the

dielectric constant, respectively.

From Equation 1.2, it can be concluded that there is no absorption if 𝜖′′ = 0 (i.e.,

non-absorbing particles) or 𝜖′′ = ∞ (i.e., particles reflecting all the radiation). The origin of

the strong color changes displayed by small particles lies in the denominator of Equation 1.2,

which predicts the existence of an absorption peak when 𝜖′ = −2𝜖𝑚, if 𝜖′′ is small.

In the case of many metals, the electromagnetic region up to bulk plasma frequency is

dominated by the free electron behavior, and the dielectric response is well described by the


9

Drude model12

. According to this theory, the real and imaginary parts of the dielectric

function are given by3:

𝜖′ = 𝜖∞ −𝜔𝑝

2

𝜔2+ 𝜔𝑑2 (1.3)

𝜖′′ =𝜔𝑝

2 𝜔𝑑

𝜔(𝜔2+𝜔𝑑2)

(1.4)

where 𝜖∞ is the high frequency dielectric constant due to interband and core transitions, 𝑝

the bulk plasma frequency, and 𝑑 is the damping frequency. The bulk plasmon frequency is

given by:

𝜔𝑝2 =

𝑁𝑒2

𝑚𝜖0 (1.5)

were 𝑁 is the density of conduction electrons, 𝑚 is their effective optical mass, and 𝑒 the

electron charge. Additionally, the damping frequency (𝑑) is related to the mean free path of

the conduction electrons (𝑅bulk) and the velocity of the electrons at the Fermi energy (𝑣𝑓) by3:

𝜔𝑑 =𝑣𝑓

𝑅bulk (1.6)

When the particle radius, R, is smaller than the mean free path in the bulk metal, conduction

electrons are additionally scattered off the surface, and the effective mean free path, 𝑅eff,

becomes size dependent with:

1

𝑅eff=

1

𝑅+

1

𝑅bulk (1.7)

This equation was experimentally verified by Kreibig13

for silver and gold NPs down to 2 nm.

The advantage of using the Drude model is that it allows changes in the absorption spectrum

to be directly interpreted in terms of the material properties of the metal.

From equation 1.3 it can be observed that over the whole frequency regime below the bulk

plasma frequency, 𝜖′, is negative. It is due to the fact that the electrons oscillate out of phase

with the electric field vector of the light wave. This is why metal particles display extinction

spectra strong dependent of the particle size.

In a small metal particle, when the condition 𝜖𝑟() = −2𝜖𝑚 is fulfilled, the long wavelength

absorption by the bulk metal is condensed into a single localized surface plasmon resonance

band. But LSPR can be damped by interband transitions (transitions of bound electrons from

occupied to empty bulk bands of different index), by surface dispersion of the free electrons

(when their mean free path is similar to the particle size) or by scattering (retardation) effects

(more and more important when the particle size increases). While surface dispersion effects

produces the broadening and intensity decrease of the LSPR band, the scattering effects

(important for nanoparticles larger than 40 nm) also give rise a remarkable red-shift of the


10

band14

. Besides, while for small particles only dipolar modes are observed, new high

multipolar plasmon modes can be accommodated within the particle surface for sizes above

100 nm (the size limit decreases with the anisotropy)15

. The multipolar modes are always

located at higher energies with respect to the dipolar ones, which are always shifted to lower

energies by the presence of the electric field from the multipolar charge distributions.

Another important parameter affecting the LSPR is the surrounding environment. The

electron oscillations confined at the surface of NPs are very sensitive to any change in the

metal-dielectric interface. According to equation 1.2, it is apparent that the dielectric constant

of the surrounding medium determines the surface plasmon resonance of the nanoparticle. An

increase in the dielectric constant, particularly the refractive index 𝑛, (𝑛 = 𝜖𝑚

12⁄), of the

surrounding medium results in a decrease in the restoring force for the electron oscillation,

thus decreasing the plasmon oscillation frequency16

. More specifically, the LSPR wavelength

(λmax) of the NPs has been found to increase linearly with the local refractive index3,17

.

Finally the change in the electron density causes shifts in LSPR band of metallic nanoparticles

(see Equation 1.5), such as predicted by the Drude Model3. Kamat et al.

18,19 demonstrated that

the blue-shift band is associated with the electron density around metallic nanoparticles when

irradiated by UV light. The same behavior can be observed by the addition of an electron

donor such as sodium borohydride ions3. Extensive optical studies on bimetallic NPs colloids

performed during eighties and early nineties are summarized in the review by Mulvaney3.

1.1.1.2. Core-shell particles. Corrected Mie theory

For the particular case of a homogeneous sphere uniformly coated with a shell of a different

material (core-shell structure, Figure 1.5), the Mie theory has been modified obtaining an

analytical expression for the extinction cross-section of a small, concentric sphere is given

by3:

C𝑒xt = 4𝜋𝑅2k × Im{Q} (1.8)

where Q is determined by the following equation:

Q =(𝜖𝑠− 𝜖𝑚)(𝜖𝑐−2𝜖𝑠)+(1−𝑔) (𝜖𝑐− 𝜖𝑠)(𝜖𝑚+2𝜖𝑠)

(𝜖𝑠+2𝜖𝑚) (𝜖𝑐+2𝜖𝑠)+(1−𝑔) (2𝜖𝑠−2𝜖𝑚) (𝜖𝑐− 𝜖𝑠) (1.9)

where ϵc is the complex dielectric function of the core material, ϵs is that of the shell, ϵm is the

real dielectric function of the surrounding medium, 𝑔 is the volume fraction of the shell layer,

and 𝑅 the radius of the core-shell particle. When 𝑔 = 0 the equation 1.8 is reduced to


11

Equation 1.2 (for an uncoated sphere), whereas for 𝑔 = 1 the equation 1.8 yields the

extinction cross section for a sphere of the shell material7.

Figure 1.5: Sketch of a core-shell particle. A core, with a dielectric function ϵc, surrounded by a shell,

with a dielectric function of ϵs. The particle is embedded in a medium with a dielectric function ϵm. R

is the radius of the coated particle.

1.1.2. Catalysis by metal nanoparticles

Catalyst is a substance that increases the rate of a reaction by lowering the activation energy

required to convert reactants into products and without being consumed or produced by the

reaction. Metallic nanoparticles are very attractive to use as catalysts20

due to their high

surface-to-volume ratio and their high surface energy. A wide variety of reactions can be

catalyzed by metallic nanoparticles, for example, electron-transfer reactions21

, oxidations of

alcohols and alkenes22

, carbon cross-coupling reactions23

, and hydrogenation reactions of

unsaturated substrates24

.

Generally, the size, shape and the dispersion state of metal nanoparticles are important factors

in explaining its catalytic properties. Besides, in the particular case of bimetallic NPs the

catalytic performance depends on their composition, as well as structure. The relationship

between the Au NPs size and catalytic activity was reported by Carregal et al.25

using as

model reaction the reduction of hexacyanoferrate (III) by borohydride ions. They attributed

the enhancement of the catalytic activity to an increase in the Au surface area as the particle

size decreased.

However, the problem associated with the use of nanoparticles as catalysts is that they have

limited stability (tend to aggregate) at higher temperatures and harsh reaction conditions. In

most of the cases, NPs aggregation leads to loss of properties associated with their nanometer

size. Hence, the stabilization of the metallic nanoparticles in colloidal solution is an important

aspect to be considered during their preparation. However, the capping material can diminish

the catalytic performance of the nanoparticles by blocking access to the steps and kinks of the


12

surface. One way to solve this problem is to immobilize the metallic nanoparticles in a

support like titania, silica, etc..

1.1.2.1. Reduction of hexacyanoferrate (III) by borohydride

In this thesis, we have set up as model reaction the reduction of hexacyanoferrate (III) by

borohydride ions to evaluate the catalytic activity of our catalytic platform. A model reaction

is defined in the following way:

1. The chemical reaction takes place just in presence of nanoparticles. Moreover the

chemical reaction should be well-controlled from Fe(CN)63− to Fe(CN)6

4− in the

presence of nanoparticles without side reactions or by-products.

2. Kinetic measurement of the reaction rate should be highly accurate. Therefore the

analysis of the temperature dependence of the reaction rate should be possible, leading

to a detailed understanding of the reaction mechanism.

3. The reaction should proceed under mild conditions, preferably at around room

temperature and in mild solvents like water. This condition implies that no degradation

or transformation of the nanoparticles must occur during the reaction. It excludes all

reactions in which the nanoparticles serve merely as a source for metal ions in

solution.

Several redox reactions are used to investigate the catalytic activity of transition metal

nanoparticles such as the reduction of p-nitrophenol, hexacyanoferrate (III), and fluorescent

dyes26

all by borohydride ions; the reduction of hexacyanoferrate (III) by thiosulfate ions; and

the reduction of organic compounds (e.g. nitro-aryls and alcohols). These model reactions can

be easily followed spectroscopically in order to obtain a complete kinetic analysis. The

reduction of hexacyanoferrate (III) ions by borohydride ions in alkaline aqueous solution is an

electron transfer reaction used as a model reaction to investigated catalysis by metallic NPs.

This redox reaction is particularly interesting because (a) the iron ions oxidation states +3 and

+2 are stable with respect to dissociation, have the same geometry, and chemical

composition27,28

; (b) the process of reduction of hexacyanoferrate involves one electron

transfer; and (c) the course of the reaction can be monitored with high precision by UV-vis

spectroscopy, by the reduction in absorbance of the hexacyanoferrate (III) complex at 420 nm.

The redox reaction can be written as27

:

BH4− + 8Fe(CN)6

3− + 3H2O → H2BO3− + 8Fe(CN)6

4− + 8H+ (R. 1.1)


13

BH4−+ 2H2O → 4H2 + BO2

− (R. 1.2)

As shown above the reaction results in the formation of hexacyanoferrate (II) ions

and H2BO3−. Besides, the hydrolysis of the borohydride ion can also occur during the

reduction of hexacyanoferrate (III) (see Reaction 1.2). This parallel process can be inhibited

by working at basic pH29

. This reaction was employed by Carregal et al.25

to investigate the

catalytic activity of Au NPs. They found that the presence of gold NPs in the reduction

medium increased the reaction rate, as well as changed the order of the reaction with respect

to [Fe(CN)63−] from zero-order to pseudo-first-order kinetics (see Figure 1.6), and decreased

to half the activation energy. They explained the catalytic process as follows: reduction of

hexacyanoferrate (III) ions by borohydride ions in the presence of Au NPs took place in two

steps. In the first step, BH4− ions inject rapidly electrons onto the Au NPs, which act as a

reservoir. Then in a second, slow step, Fe(CN)63− ions diffuse toward the Au NP surface and

are reduced by the excess surface electrons (see scheme in Figure 1.6). No chemical reaction

occurs between the reactants and Au NPs since the optical properties of the Au NPs remain

unchanged during the redox process (see Figure 1.6).

Under pseudo-order conditions, where NaBH4 is always in excess over [Fe(CN)63− ], the

reduction of ferrocyanide ion obeys:

−d[Fe(CN6

3−)]

dt= kobs[Fe(CN6

3−)]n (1.10)

where kobs is the pseudo-nth-order rate constant for the reaction. Since this catalytic reaction

occurs at the Au surface and the relationship between the kobs and the Au surface area is

linear, Equation (1.10) can be written as follows:

− d[Fe(CN)6

3−]

dt = ks [Au]NP S [Fe(CN)6

3−]t (1.11)

where ks is the apparent rate constant normalized to the total surface area of gold per unit

volume of solution, S is the surface area per particle and [Fe(CN)63−] refers to the

concentration at time 𝑡.


14

Figure 1.6: Spectral evolution of a mixture of hexacyanoferrate (III) and Au nanoparticles upon

borohydride addition. Inset: kinetic trace of the absorbance at 420 nm during [Fe(CN)63−] reduction

and linearized data for first order analysis. The model proposed for the catalytic mechanism is

presented on the top. Reproduced from reference [28].

According to Carregal and coworkers25

the catalytic activity of the Au NPs in the

ferrocyanide reduction depends on the size of Au NPs, noting that 3.5 nm Au NPs exhibited

larger catalytic activity than the 6 nm particles. It was explained in terms of larger shifts on

Fermi energy for smaller nanoparticles. Other studies, developed by Carregal et al.30,31

, reveal

that using Au NPs encapsulated in a thermoresponsive microgel; or gold nanorods in

metallodielectric composites like SiO2 or TiO2; or Pt NPs, spherical and dendritic, supported

on carbon nanotubes; also acts as catalysts in the reduction of hexacyanoferrate (III) ions by

sodium borohydride ions.


15

1.2. Titanium dioxide

Titanium dioxide or titania (TiO2) is a multifunctional material with applications in

photocatalysis, namely degradation of environmental toxic dyes and organic pollutants, solar

cells, sensor devices, energy storage, spintronic devices, electrodes in lithium batteries,

photoelectrochemical splitting of water into hydrogen and oxygen, and so on. TiO2

applicability strongly depends on parameters such as crystalline structure, crystallite size,

specific surface area, porosity and morphology32

. For instance, high TiO2 surface area

provides a large interface, small primary crystals offer short diffusion paths and the particle

size has a tremendous effect on the mechanical, electronic and optical properties of the

material.

Titania crystallizes naturally in three major different structures: brookite (rhombohedral),

rutile (tetragonal), and anatase (tetragonal) where the two last are frequently used as

photocatalysts. Table 1.1 presents the most common synthetic routes to obtain the different

titania polymorphs. In the three structures each Ti4+

ions is octahedrally coordinated to six O2-

ions, but the distortion of each octahedron and the connection pattern of such octahedra chains

differs for the different structures (see Figure 1.7). In anatase the octahedra are connected by

their vertices, while in rutile are connected by their edges. The rutile structure is the densest

(smallest unit cell) and the most thermodynamically stable phase, and therefore the most

extensively studied among all TiO2 forms. While at low temperatures the anatase and brookite

phases are more stable, both will irreversibly revert to the rutile phase when subjected to high

temperatures (> 600 ºC). A summary of most relevant physical and chemical properties for

the three polymorphic forms of TiO2 are listed in the Table 1.2. Anatase is generally

considered the most active polymorph of TiO2. It is attributed to its higher degree of

hydroxylation, when compared with rutile phase, and its higher surface area33

. Brookite is the

least studied phase, due to the difficulties in synthesizing of pure brookite samples.


16

Figure 1.7: Geometrical illustration of the crystal structures of (a) rutile, (b) anatase, and (c)

brookite. Representation of: planar Ti3O building-block (left) and TiO6 polyhedra (right) for

each structure. The white spheres represent titanium, Ti, atoms and the red ones represent

oxygen, O, atoms. Reproduced from reference [34].

Titanium dioxide is considered as an n-type semiconductor due to the presence of oxygen

vacancies (VO) on its surface. The oxygen vacancy is responsible for the color centers in

TiO2. The VO are formed upon the release of two electrons and molecular oxygen leaving a

positive oxide ion vacancy. The redistribution of the two electrons creates Ti3+

sites in the

lattice which are highly reactive due to their unsaturated coordination. It explains the

electrical conductivity of TiO235

.


17

Table 1.1: Selected synthetic methods for obtaining TiO2 polymorphs.

Synthetic method Phases formed dBET

m2 g

-1

Reference

aA

bR

A+R

cB

A+B

Oxidation of

Ti(III)precursors

X X X X 200

Fröschl et al.

36

*Sol-gel X X X ~110-150 Fröschl et al.

36

X

X

X

X

X

200

115-345

NA

Schneider et al. 37

Niederberger et al. 38

Pottier et al.

39

Emulsions (mini/micro) X X X ~100-300 Fröschl et al.

36

Reverse micelle

X

X X NA Li et al.

40

Moran et al.

41

Hydrothermal X

X

X

X

X

X

X

X

X

75, 124, 190

70, 151, 253

43.1, 70,

151, 253

Chae et al. 42

Aruna et al.

43

Wang et al. 44

Solvothermal X X X ~215 Fröschl et al.

36

X 114 Ye et al.

45

Direct oxidation X X NA Wu et al. 46

Metalorganic/Δchemical

vapour deposition

X

X

X

X

X

X

3-300

NA

Fröschl et al.

36

Wu et al. 47

Physical/chemical vapour

deposition

X X X NA Fröschl et al.

36

Microwave

Microwave hydrothermal

X X ~250 Fröschl et al.

36

X X NA Ma et al. 48

Sonochemical X X X 230-400 Fröschl et al.

36

X X 26.3, 91.4,

97.6

Yu et al. 49

•Sonoelectrochemical

anodization

X X NA Mohapatra et al. 50

*Electrodeposition X NA Liu et al. 51

aA = Anatase

bR = Rutile

cB = Brookite

dBET = Brunauer, Emmett and Teller model, specific surface

area NA = Not Available

*The TiO2 samples prepared, in most case, have amorphous structure and calcination or hydrothermal

treatment is necessary to induce crystallinity.

ΔThe temperature and the pressure influence the crystalline

phase. •After annealing.


18

Table 1.2: Properties of anatase, rutile and brookite.

Property Anatase Rutile Brookite Reference

Crystal structure Tetragonal Tetragonal Rhombohedral Mo et al. 52

Molecules per unit cell 4 2 8 Kang et al 53

Mo et al.52

Lattice parameters (nm) a= b= 0.37842

c = 0.95146

a= b= 0.45937

c = 0.29587

a= 0.5447

b = 0.9184

c = 0.5145

Smyth et al. 54

Kang et al 53

Mo et al.52

Unite cell volume (nm3) 0.1363 0.0624 0.2575 Smyth et al.

54

Space group I41/amd P42/mnm Pbca Mo et al.52

Density (g cm-3

) 3.894 4.25 3.99 Kang et al53

Mo et al.52

Index of refraction 2.54, 2.49 2.79, 2.903 2.61-2.63 Hanaor et al. 55

Band gap (eV) 3.20

(~ 384 nm)

3.00

(~ 413 nm)

3.10 – 3.40

(~365–400 nm)

Fisher et al. 56

Brayner et al.57

Ti-O bond length (nm) 0.1937 (4)

0.1965 (2)

0.1949 (4)

0.1980 (2)

0.1993 (2)

0.1865 (1)

0.1919 (1)

0.1945 (1)

0.2040 (1)

Zallen et al.

58

Brayner et al.57

Mo et al.52

O-Ti-O bond angle 77.7 º

92.6º

81.2 º

90.0 º

77.0º ~ 105º Hanaor et al.55

Mo et al.52

Ti-Ti bond length (nm) 0.379

0.304

0.357

0.296

Linsebigler, et al.59

∆Gf° (kcal mol

-1) -211.4 -212.6 Linsebigler et al.

59

Solubility in HF Soluble Soluble Ohno et al. 60

Anatase TiO2 dissolves into an HF solution more easily than rutile TiO2.

1.2.1. Gold nanoparticles doped titania

Small metal nanoparticles (< 10 nm) can be thermodynamically unstable because of their high

surface energies and large surface areas. It is difficult to stabilize these nanoparticles by

maintaining a small size range while retaining their catalytic activity. Besides, the catalyst

separation from the products and reactants at the end of the reaction is difficult. To overcome

this problem, metal nanoparticles can be deposited in wide variety of support materials such

as titania, silica, carbon, zeolites, and alumina.

Different methods have been reported in the literature for doping TiO2 particles with small Au

NPs, TiO2@Au (see Table 1.3). The most widely used method for preparing TiO2@Au

catalysts with small Au sizes (<5nm), intimate interaction with TiO2, and good Au particle

distribution is the deposition-precipitation (DP) method61,62

. This technique involves the

deposition of gold hydroxide on the TiO2 surface by raising the pH of the gold chloride

precursor. The fact that the active component, the gold chloride precursor, remains on the

surface of the support and not buried in it and the easy removal of chloride ions, which poison

the NPs are advantages of this method over others.


19

Table 1.3 Selected synthetic methods for doping TiO2 with Au nanoparticles.

Synthetic

method

Interaction

with TiO2

Diameter

Au NPs

(nm)

Advantages /

Disadvantages

Reference

aAdsorption of

preformed Au NPs

Weak 5

Coalescence and

agglomeration of Au NPs

Buso et al. 63

Incipient wetness

impregnation

Weak

NA

10

Haruta et al.

64

Lin et al. 65

Zanella et al.

66

DP(NaOH) Strong 3.3

5

Part of the Au is not

deposited on TiO2 Zanella

et al.

66,79

Tsubota et al. 67

Tsubota et al.67

DP (NH4OH) Strong NA Sangeetha et al.

68

DP (CO(NH2)2) Strong 2-6

Depending

DP time

All of the Au is

deposited on the TiO2

Wen et al.

69

Hermans et al.

70

Zanella et al.

79

bAnion adsorption AuCl4

−

+ chemical reduction

4-6 Low Au loading Zanella et al.

66

Hidalgo et al.

71

Cation adsorption with

Au(en)23+ complex (en =

ethanediamine)

2-5

Contact

time ~ 1 h

Increasing the adsorption

time increases the

loading and the size of

Au and vice versa

Zanella et al.

66,79

Guillemot et al. 72

Guillemot et al.

73

Electrodeposition Quite

satisfactory

30-40 Uniform dispersion of

Au NPs on TiO2

Hosseini et al.

74

Metal sol method 4.5-5.0 Nguyen et al.

75

UV photoreduction 5 Good dispersion of Au

NPs in the TiO2. Au NPs

are deposited on TiO2

without requiring the

introduction of any toxic

agents

Chen et al.

76

aAu NPs synthesized by Turkevich

1 method or Brust method.

bTiO2 colloids prepared in acid medium were

positively charged. NA = Not Available. DP = Deposition-precipitation.

The control of the pH using NaOH77

allows to tune the size of the gold. Besides, Haruta et

al.78

found that uniform Au NPs distribution were achieved when the pH of the gold precursor

solution was adjusted to the isoelectric point of titania and the working temperature was

maintained between 47 and 87 ºC, so that Au(OH)3Cl− would be deposited on the support

without precipitating in the liquid. Nevertheless DP with NaOH present several disadvantages:

low metal loading (≤ 3 wt.%), deposition yield < 100% at pH range of 7-10, and

inapplicability of some supports with a point of zero charge below 566,

. For this reason,

Hermans et al.70

and Zanella et al.79

improved the method employing urea CO(NH2)2 as pH

adjusting agent.


20

1.2.2. Catalysis by gold nanoparticles supported on TiO2

The direct use of metal nanoparticles (homogeneous nanocatalysts) as catalysts is often

difficult because of their high tendency to aggregate, difficult recovering, among others

reasons. Hence research has been developed to create new catalysts where nanoparticles are

supported on a solid matrix (heterogeneous nanocatalysts).

Gold NPs on metal oxide support exhibit exceptionally high catalytic activity for different

reactions, such as, oxidation of carbon monoxide80

, hydrochlorination of acetylene81

, partial

oxidation of hydrocarbons82

at low temperature, hydrogenation of carbon oxides83

, and

reduction of nitrogen oxide84

. Generally, Au NPs (< 5 nm) supported on TiO2 particles show

higher catalytic activity than TiO2 and Au catalysts separately.

In the '80s Haruta and co-workers85

demonstrated that gold nanoparticles supported on

titanium, iron or cobalt oxides (Ti/Fe/CoO2@Au) exhibited highly catalytic activity for CO

oxidation at temperatures below 0 ºC. Since then, many research groups have investigated the

origin of the exceptional catalytic properties of supported Au NPs. Most of the published

work in this area are focused on elucidating; (i) if the catalytic efficiency depends on the NPs

size; (ii) the role of the supports; (iii) the nature of the active sites; and (iv) the reaction

mechanism for the low temperature CO oxidation. Several studies have found that the

reaction rate per unit area of gold surface is Au NPs size dependent, showing the Au NPs of 2

to 3 nm the highest activity86

. The remarkable catalytic performance of these Au NPs can be

attributed to, e.g., quantum size effects, a high activity of undercoordinated Au atoms at edges

and corners, or the increase perimeter length at the interface between support material and Au

NPs with decreasing Au NPs size. Some groups associate the reduction of gold NPs size with

the increase of steps, edge and kink sites, giving to the material different properties for the

chemisorption of gases reaction87

. Cleveland and coworkers88

found that as gold metal NPs

become smaller their face centered cubic structure transforms into decahedral (<2.5 nm) or

icosahedral (<1.6 nm).

The type of supporting material and the strategy for the immobilization of gold NPs are

factors which can determine the interaction gold NPs-support and therefore their physical

properties and catalytic efficiency89

. Frequently the main role of the support is to provide

stability to the nanoparticles90

. In the particular case of using TiO2 as support: (i) its nature

(e.g. its crystal phase, specific surface area, porosity, density of defects and morphology), (ii)

its chemical surface state (e.g. the presence of hydroxyl groups on the TiO2 surface), and (iii)


21

its intimate interaction with the Au NPs, are parameters involved in the catalytic activity.

Haruta105

reported, for first time, the direct role of TiO2 in the CO oxidation reaction.

Schubert91

and Liu92

proposed that the TiO2 support provided the activated oxygen species for

the CO oxidation reaction. Nevertheless, other reports claimed that the catalytically active

species are either oxidized species (Au+1

/ Au3+

)93

, or Au0, or both (Au

0 and Au

3+)94

.

Additionally, Haruta81

also reported the excellent selectivity ( 99 %) of TiO2@Au in the

partial oxidation of propene to propene oxide at 323 K.

1.2.3. Elementary steps in photocatalytic TiO2 systems

Semiconductors are characterized by their electronic structure, which can be described by the

band theory95

. Thus, the electronic structure is discussed in terms of energy bands made up of

large numbers of interacting atomic orbitals forming a continuum of energy levels. The

energy levels of interest are the highest occupied, the valence band (VB) and the highest

unoccupied, the conduction band, CB. Between these bands there is a forbidden energy gap

called band gap where no electrons can be accommodated. The band bap determines the

properties of the material. The electrons have, on average, a potential energy known as the

Fermi level, which is just below that of the CB in n-type semiconductors and just above that

of the VB in p-type semiconductors96

. For an intrinsic semiconductor, the Fermi level is

defined as the energy level in the middle of the bandgap (see Figure 1.8).

Figure 1.8: Schematic representation of the electronic band structure of semiconductors. E = Energy

and 𝐸𝑓 = Fermi energy.

For an n-type semiconductor, like TiO2, the Fermi level can be calculated using the following

expression:


22

Ef = Ei + kBT ln (ND

ni) (1.12)

where Ef is the Fermi energy, Ei is the initial energy, kB is the Boltzmann constant, T is the

temperature (K), ND is the concentration of donors, and ni is the intrinsic carrier density.

When an n-type semiconductor is in contact with a metal, the Fermi level equilibration

thermodynamically occurs by redistribution of the charge carriers from the lower work

function side to the higher work function side. Assuming that the metal work function is

higher than the semiconductor work function, the electrons migrations from the

semiconductor to the metal occur until the two Fermi level are aligns. As a consequence, a

double-layer is built-up at the interface where the surface of the metal presents an excess of

negative charge, while the semiconductor side exhibits an excess of “positive” charge. Due to

this electron accumulation the Fermi level of the metal increases to more negative potentials.

The Fermi level alignments, closer to the conduction band of the semiconductor, cause a

potential barrier across the interface known as Schottky barrier, which inhibits the

recombination of electron-hole pairs. It has been reported that smaller metallic NPs induced

more negative Fermi level shifts, than bigger ones. The height of the Schottky barrier is

defined as the difference between the semiconductor conduction band and Fermi level of the

metal97

.

Excitation of a semiconductor is initiated by the absorption of photon with energy equal to or

greater than the semiconductor band gap energy, Eg,, promoting an electron from the VB to

the CB and leaving a hole (h+) behind in the valence band (see Figure 1.9). In semiconductors

the lifetime of the charge carriers in the excited states is on the order of nanoseconds59

. Hence

the excited-state electrons and holes can recombine and dissipate the input energy as heat, or

get trapped in metastable surface states98

, or participate in redox process at the

semiconductor/water interface99

.

Figure 1.9: Schematic diagram of promotion of an electron from the valence band to the conduction

band on semiconductor after irradiation.


23

In the case of a photocatalytic redox reaction, it proceeds effectively when the top level of CB

is more negative than the reduction potential of the adsorbed acceptor species (A + e− →

A∙−). Nevertheless, for a hole transfer the position of the top level of VB should be lower

(more positive) than the oxidation potential of the adsorbed donor species D (D + h+ →

D∙+). The electron and hole transfer processes should occur simultaneously in order to

regenerate the photocatalyst. There are several ways to minimize charge carrier

recombination, e. g., incorporation of metal particles, manipulation of interfacial junctions,

adding dopants, and trapping sites.

Titania is the most widely investigated photocatalyst due to its strong oxidative properties at

ambient temperature and pressure, low cost, nontoxicity, and chemical stability again

photocorrosion100

. TiO2 presents a large intrinsic band gap energy of ~3.0 eV for rutile and

~3.2 eV for anatase (see Figure 1.10). This means, that anatase TiO2 can only be excited

under irradiation of UV light at wavelengths < 380 nm, which covers only ~ 3% of the solar

radiation. On the other hand, the rutile phase can be excited by wavelength that extent into the

visible range (410 nm).

Upon light absorption with energy equal to or greater than the TiO2 semiconductor band gap,

electrons (e-) are excited from the valence band to the conduction, leaving behind holes (h

+) in

the valence band (Reaction 1.3).

TiO2 + ℎ → eCB− + hVB

+ (R. 1.3)

The eCB− and hVB

+ migrate to the surface of TiO2 and participate in interfacial redox reaction.

The valence holes in the TiO2 surface react with adsorbed hydroxide ions or water molecules

generating adsorbed OH∙ radicalsIV

. The specific area plays an important role in the activity of

the TiO2 photocatalyst by providing a platform for the reactants. As a general rule, more

surface area, more number of surface active sites, more reactants adsorb on the active sites of

the surface and consequently more surface reactivity. Increased surface area of TiO2 may be

obtained by using highly porous materials, and/or reducing their size, and/or using specific

shapes such as nanotubes or whiskers. The chemical reaction that takes place at the surface

active site is either reduction (gain of electrons) or oxidation (loss of electrons) or both

(redox).

IV

This photodecomposition generally involves one or more radicals or intermediate species such as

O2∙− or H2O2.


24

A lot of studies reported that the photocatalytic activity of titania is also affected by particle

size, crystalline phase, surface properties (e.g. surface OH and oxygen vacancy) and defects

(active sites for the adsorption and dissociation of molecules on the surface).

As already stated, anatase TiO2 have no visible light response due to their large band gap. The

shift in the optical response of TiO2 from the UV to the visible range will have a positive

effect on the practical applications of the material. Various methods have been developed to

reduce the band gap of TiO2, for example, dope TiO2 with nonmetal such as carbon, nitrogen,

fluoride, iodine101

, or with transition metal NPs (e.g. copper, silver, gold102

); utilize

mixed-phase TiO2, particularly anatase-rutile mixtures103

, sensitize TiO2 with organic dyes or

coupled semiconductors104

. The incorporation of dyes molecules or metallic NPs on the TiO2

surface is known as photosensitization. This is the operating principle of dye-sensitized based

solar cell systems.

Figure 1.10: Band gaps and band edge positions of VB and CB versus standard hydrogen electrode

(SHE) at pH 7 of TiO2 anatase and TiO2 rutile.

The anatase phase has a higher photocatalytic activity over rutile due to the difference in

position of the top of valence band and the bottom of CB, i.e., its CB lies 0.2 eV above rutile

CB that gives more reducing power than rutile (see Figure 1.10). It is important refer that the

band levels of the oxide materials, like TiO2, usually are shifted with pH, surface impurities

and adsorbed compounds.

1.2.3.1. Photocatalytic TiO2@Au systems

Several studies reported that the presence of Au NPs enhanced the photocatalytic activity of

the titania support. However, there are numerous approaches to elucidate the process that

promotes the high photocatalytic performance of TiO2@Au or TiO2@noble metals, such as,

Ag, Cu, etc.


25

Several reports explaining the enhanced visible activity of TiO2@Au due to local electric

near-field enhancement in the TiO2 surface by the Au surface plasmon resonance105

.

Excitation of the surfaces plasmon resonances creates polarization at the metal nanoparticle

resulting in the generation of near-fields localized at the metal surface. Interaction of a

semiconductor nanoparticle with this kind of localized electric field could allow the formation

of electron/hole pairs in the near surface region (see Figure 1.11(a)). To transfer plasmon

energy to the TiO2 through near field effect the plasmon energy must be equal to or greater

than the band gap of the TiO2. Only NPs with size of 5-20 nm in diameter, with small

scattering cross-section, are able to achieve enhanced field, since in the larger particles there

is the creation of multipole resonance and dynamic depolarization of plasmon as indicated

above.

Christopher and coworkers106

reported that the enhanced photochemical reactivity of

TiO2@Ag materials is attributed to radiative transfer of energy, mediated by surface plasmon,

from Ag NPs to the semiconductor. This process produces high concentrations of charge

carriers (e−/h

+ pairs) in the TiO2 (see Figure 1.11(b)). Again, this plasmon relaxation process

is strongly dependent on the shape and size of metallic nanoparticles.

Furube and coworkers107

reported that spherical Au NPs are excited due to plasmon

resonance, inducing electron transfer from Au NPs to TiO2 conduction bandV. Specifically,

the electrons in the filled d-band of gold are excited to sp conduction band; electronic states

above the Fermi level (see Figure 1.11(c)). These energetic electrons, in metal conduction

levels above the Fermi energy, designated of hot electrons, have sufficient energy and

momentum to cross the Schottky barrier108

between metal and semiconductor (the Schottky

barrier between Au and TiO2 is about 1 eV). The semiconductor must be in intimate contact

with the plasmonic metal, allowing the LSPR-excited hot electrons to overcome the Schottky

barrier.

Noble metal nanoparticles with absorption in the visible region, such as Ag, Au, Cu,

deposited on the TiO2 surface can enhance, through the excitation of surface plasmon

resonance, the TiO2 photocatalytic efficiency. The excitation can induce the injection of

photogenerated electrons in the TiO2 CB causing photocurrent or reduction. Sellappan109

suggested the plasmonic heating (see Figure 1.11(d)) where the plasmon relaxation through

absorption in the nanoparticles leads to heating the nanoparticles in composites of

TiO2@metal NPs. Within this SPR-mediated heating process, the light absorbed by the NP

V TiO2 presents electron-accepting properties, due the high density of states in the conduction band.


26

generates a non-equilibrium electron distribution that decays via electron-electron scattering.

The hot electron gas equilibrates with lattice phonons which transfer this energy into the

surrounding medium and induces temperature increase in the vicinity of the Au NP surface.

The plasmonic heating depends on the size and shape of NPs, heat conductivity of the

surrounding environment and the incident light intensity.

Figure 1.11: LSPR energy transfer mechanism from the metal NP to TiO2: (a) Near-field nonradiative

enhancement; (b) Far-field radiative scattering; (c) Hot-electron transfer from the metal NP to TiO2;

and (d) Plasmonic heating. Reproduced from reference [111].

27

CHAPTER 2

Titania-coated gold nanoparticles and growth of titania shell

ABSTRACT

The aim of work described in this chapter is the synthesis and characterization of gold-titania

core-shell nanoparticles, Au@TiO2. In order to achieve homogeneous TiO2 coatings different

approaches based on the sol-gel reaction of titania precursors on the gold nanoparticle surface

are analysed. The surface chemistry of the Au nanoparticles is a critical parameter in the

titania deposition. Therefore Au nanoparticles with different capping ligands

cetyltrimethylammonium bromide (CTAB)-stabilized, poly(vinylpyrrolidone) (PVP) and

polyelectrolytes-PVP were employed.

2.1. INTRODUCTION

28

2.1. Introduction

Metal nanoparticles are of great interest because of their unique electronic and optical

properties110,111,112,113

. Furthermore, coating of those nanoparticles with a semiconductor like

titanium dioxide make them interesting materials for applications in photovoltaics114,115,116

,

photocatalytic degradation of pollutants117,118,119,120,121

, and for chemical sensors122,123,124

,

among others. However, titanium dioxide-coated metal nanoparticles are generally difficult to

synthesize, because titania precursors are highly reactive, and thus control over their

hydrolysis and condensation is not straightforward. The reaction mechanism describing the

sol-gel conversion of titanium alkoxides to titania involves two main reaction types:

hydrolysis and condensation of titanium alkoxides (see Scheme 2.1)125,126

. During hydrolysis,

the alkoxide groups (-OR) are replaced via the nucleophilic attack of the oxygen atom of a

water molecule under release of alcohol and the formation of titanium hydroxide (Equation

2.1). In a second step, condensation reactions between two hydroxylated metal species occur.

The partial negative charge of the oxygen atom in the hydroxide group (≡ Ti − OH) is

responsible for the nucleophilic attack to the partial positive charge of titanium (≡ Ti − OX ,

being X a H atom or alkyl group), with the transfer of a proton from −OH to the−OX group

and liberation of alcohol molecule (Equation 2.2).

≡ Ti − OR + H2O →≡ Ti − OH + ROH (2.1)

≡ Ti − OH + ≡ Ti − OX →≡ Ti − O − Ti ≡ + XOH X = H/alkyl group (2.2)

Main reactions in the sol-gel process using titanium alkoxides. Hydrolysis (Equation 2.1) and

condensation (Equation 2.2), involving oxolation and alkoxolation.

Since in our study the titanium alkoxide is titanium (IV) butoxide (TBT) or titanium

isopropoxide (TIP), the overall reactions are, respectively the following:

Ti(OC4H9)4 + 2H2O → TiO2(aq) + 4C4H9OH (2.3)

Ti{OCH(CH3)2}4 + 2H2O → TiO2(aq) + 4C3H7OH (2.4)

During the last decade, few works have been published on the coating of metal nanoparticles

with a titania shell. For instance, Pastoriza-Santos and co-workers127

synthesized silver-titania

core-shell nanoparticles (Ag@TiO2) through the high-temperature reduction of Ag+ by a

dimethylformamide/ethanol mixture; in the presence of titanium butoxide and acetylacetone.

The same procedure was adopted by Renjis et al.128

for the synthesis of Au@TiO2, Au@ZrO2,

Ag@TiO2 and Ag@ZrO2 nanoparticles. Later, Kamat and coworkers19

modified the

procedure using as titania precursor titanium(triethanolaminato)isopropoxide129

. Zhang et

2.1. INTRODUCTION

29

al.130

reported a route to synthesize polystyrene-silver-titania multishell spheres based on the

use of acetone and polyvinylpyrrolidone (PVP) as sweller and coupler, respectively, and using

tetra-n-butyl titanate as titania precursor. Following a different approach, Mayya and

co-authors131

presented the first titania coatings on gold nanoparticles based on complexation

of titanium (IV) bis(ammonium lactato) dihydroxide (TALH) with

poly(dimethydiallylammonium chloride) (PDDA, a positive polyelectrolyte), and then the

hydrolysis of TALH132,133

.

Herein, we report three different strategies to prepare Au@TiO2 core-shell nanoparticles

where titania precursors and surface chemistry of Au nanoparticles are varied. In the first

route, Au NPs stabilized in CTAB are coated with a titania shell using the sol-gel reaction of

TIP with acetylacetone. In the second, the Au NPs wrapped with a polymer such as PVP were

employed for the titania coating. This water soluble polymer is widely used as capping ligand

in the synthesis of colloidal particles, besides it easily adsorbs onto oxide surfaces (e. g.

titania, iron oxide, alumina, kaolinite)134

, metal (e.g. gold, silver, iron), polystyrene135

,

silica136

, and graphite137

. The third approach, is based on the method developed by

Pastoriza-Santos and co-workers138

to obtain Au@SiO2 nanorods which combines the

layer-by-layer, LBL, technique139,140

and the hydrolysis and condensation of TBT. The titania

layer thickness can be tuned through successive steps of hydrolysis and condensation of TBT

on the Au@TiO2 NPs.

2.2. Experimental section

Chemicals

Ascorbic acid, cetyltrimethylammonium bromide, acetylacetone (AA), polyallyamine

hydrochloride (Mw 15 000 g mol-1

), titanium isopropoxide, and titanium (IV) butoxide were

supplied by Aldrich. Tetrachloroauric (III) acid, trisodium citrate dihydrate and sodium

chloride were supplied by Sigma. Poly(sodium 4-styrenesulfonate) (Mw 14 900 g mol-1

) was

purchased from Polymer Standards Service. Poly(vinylpyrrolidone) (Mw 24 000 g mol-1

and

Mw 40 000 g mol-1

) was supplied by Fluka. All reactants were used without further

purification.

Pure grade propanol, ethanol and Milli-Q water, with a resistivity higher than 18.2 M . cm-1

,

were used in all preparations.

2.2. EXPERIMENTAL METHODS

30

Synthesis of Au@CTAB@TiO2

Au@CTAB nanoparticles. A gold seed solution of about 15 nm diameter was synthesized

according to the standard sodium citrate reduction method141

. Typically, 25 mL of a warm

sodium citrate (1 wt %) solution was added to 500 mL of a boiling 0.5 mM HAuCl4 aqueous

solution, under vigorous stirring. After boiling for 15 min, the gold sol was cooled at room

temperature. Subsequently, citrate molecules were exchanged by CTAB upon dilution of an

aliquot of the Au@citrate seeds with the same volume of CTAB 0.03 M. At this stage the

concentration of gold and CTAB are about 3.58 × 10-4

M and 0.015 M, respectively.

The preformed 15 nm Au NPs were grown up to 66 nm followed the procedure described by

Rodríguez-Fernández and co-authors2. Briefly, 32.47 μL of gold seed solution was added onto

a mixture of aqueous CTAB solution (1.79 mL, 0.03 M), Milli-Q water (3.18 mL), aqueous

ascorbic acid solution (17.9 μL, 0.1 M) and aqueous HAuCl4 solution (8.4 μL, 0.1 M) under

stirring. The temperature of the growth solution was kept constant at 35 ºC. After 1 hour, Au

NPs dispersion (~ 0.227 mM) was centrifuged at 3500 rpm for 30 min. the supernatant

discarded, and the nanoparticles redispersed in 5 mL of Milli-Q water. Thus, the final gold

dispersion presented a concentration of about 0.6 mM in CTAB and 0.454 mM in gold.

Au@CTAB@TiO2. The coating reaction was carried out by hydrolysis of TIP in ethanol, at

room temperature. To 5 mL of the gold solution, 71.8 μL of AA (9.74 M) and 500 μL of

ethanol were added. Then, 40 μL of ethanolic TIP solution (0.1 M), were added, under

vigorous stirring, to the gold NPs suspension. When the addition was over, the suspension

was gently stirred for twelve hours. The resulting titania-coated gold NPs were centrifuged at

3000 rpm for 30 min, the supernatant was removed and the nanoparticles redispersed in 5 mL

of ethanol.

The amount of TIP was calculated to overgrow a titania shell of 10 nm on the Au NPs.

Synthesis of Au@PVP-K30@TiO2

Au@PVP-K30. Gold NPs ([Au] = 2.74 × 10-4

M, [seeds Au] = 2.33 × 10-5

M, [PVP] = 0.8

mM) with about 26.6 nm of diameter were synthesized in the microwave by the procedure

describe by Pastoriza-Santos and co-workers142

. One milliliter of this gold NPs suspension

was transferred to an Eppendorf tube and centrifuged at 4500 rpm for 30 min and the

nanoparticles were dispersed in the same volume of ethanol. This washing cycle was repeated

two more times. The resulting gold dispersion presented a concentration of ≈ 8.64 × 10-5

mM

in PVP and 2.37 × 10-4

M in gold.


31

Au@PVP-K30@TiO2 preparation. To Au@PVP NPs in ethanol (1 mL), 40 μL of Milli-Q

water were added and stirred for five minutes. Subsequently, 12 μL of 0.086 M TBT in

ethanol (freshly prepared) were added to the mixture under vigorous stirring. After 5 min. the

stirring was stopped and it was allowed to react for 30 min. Then, the mixture was centrifuged

at 4500 rpm for 30 minutes, the supernatant was discarded and the nanoparticles redispersed

in 1 mL of ethanol.

Synthesis of Au@CTAB@PSS@PAH@PVP@TiO2

Au@CTAB@PSS@PAH@PVP preparation. The gold colloids are synthesized according

the procedure described above (see Au@CTAB preparation). The polyelectrolyte coating of

the gold nanoparticles was carried out following the method described by Pastoriza-Santos et

al.31

. Briefly, the gold colloids were added dropwise to 5 mL of 2 mg mL-1

PSS aqueous

solution (6 mM NaCl, previously sonicated during 30 minutes) under vigorous stirring. The

solution was allowed to react for 3 h, after which the excess polyelectrolyte was removed via

centrifugation at 3500 rpm for 30 minutes and redispersed in 5 mL of Milli-Q water.

Thereafter, it was added dropwise to 5 mL of 2 mg mL-1

PAH aqueous solution (6 mM NaCl,

previously sonicated during 30 min) under vigorous stirring. After 30 min, it was centrifuged

at 3500 rpm for 30 minutes and redispersed in 5 mL of Milli-Q water. The polyelectrolyte

wrapping was followed by the surface adsorption of PVP. The amount of PVP was calculated

to provide the colloids with about 80 PVP molecules per nm2 gold surface. The PVP was

dissolved in water by ultrasonication for 15 min. Then, 5 mL of Au@CTAB@PSS@PAH

colloids were mixed with 5 mL of PVP (Mw 24 000 g mol-1

) aqueous solution and stirred for

15 h at room temperature. Finally, the mixture was centrifuged at 2500 rpm for 30 min, the

supernatant removed and the nanoparticles dispersed in 0.2 mL of Milli-Q water.

Subsequently, the mixture was added dropwise to 5 mL of ethanol under vigorous stirring.

The concentration of gold at this stage was around 0.376 mM.

Au@CTAB@PSS@PAH@PVP@TiO2 preparation. Briefly, 60 μL of a freshly prepared

85.5 mM TBT solution in ethanol was added dropwise onto the Au dispersion

(Au@CTAB@PSS@PAH@PVP in ethanol) under nitrogen atmosphere and vigorous stirring.

After the addition was completed, the mixture was gently stirred for 1 hour. Then, the mixture

was centrifuged at 2500 rpm for 20 min, the supernatant removed and the particles dispersed

in 5 mL of ethanol.


32

Instrumentation

A JEOL JEM 1010 transmission electron microscope operating at an acceleration voltage of

100 KV was used to analyze the nanoparticles morphology. The UV-vis absorption spectra

were recorded on a VARIAN CARY 50 spectrophotometer. The zeta potential measurements

of aqueous dispersions of Au NPs coated with CTAB, PSS, PAH and PVP were performed

with a Malvern Zetasizer Nano ZS90 instrument at 25 °C.


CTAB or PVP are both commonly used capping ligand employed to provide stability to Au

nanoparticles. Besides there are some reports that evidence that both ligands could induce the

deposition of titania on metal surfaces. For instance, Sakai and co-authors143

demonstrated

that silver nanoparticles could be successfully coated with titania by the sol-gel reaction of

titanium tetraisopropoxide in the presence of CTAB. In the case of PVP coated nanoparticles

the TiO2 deposition is justified by the ability of PVP to form hydrogen bonds with some

oxides27

. Therefore, the first approach was to apply the sol-gel reaction of titania precursor on

Au nanoparticles stabilized by CTAB or PVP. Thus, CTAB- or PVP-stabilized Au

nanoparticles were coated with titania by the hydrolysis and polycondensation process using

TIP and TOB in ethanol solution, respectively. The direct mixing of the CTAB-stabilized

aqueous gold suspension with the sol-gel solution is complicated because of rapid

hydrolysis/condensation reactions of the TIP promoted by the water in the gold solution.

Therefore, this issue was obviated by addition of acetylacetone to the solution, which retards

the hydrolysis rate of titania precursor.

The characterization of gold-titania core-shell nanoparticles was carried out using

transmission electron microscopy and Visible-NIR absorption spectroscopy. Figure 2.1 shows

the Visible-NIR absorption spectra of Au NPs stabilized by CTAB (A) or (B) PVP before and

after titania coating. As shown, LSPR band of both samples red-shifted after the titania

coating. While shifts of 10 nm were observed for Au@CTAB NPs, the coating of Au@PVP

NPs gave rise to 3 nm shifts (from 531 to 534 nm). It suggested that the Au NPs were coated

by titania. The high refractive index of the titania shell could cause the red-shifted (LSPR

strongly depends on the dielectric constant of the medium144

and the refractive index of

amorphous titanium dioxide is 2.45145

, much higher than that of the water (1.330). Also an

2.3. RESULTS AND DISCUSSION

33

increase of LSPR band width was observed when the gold nanoparticles are coverage with a

titania shell.

Figure 2.1: Visible-NIR spectra of Au NPs stabilized in CTAB (A) and PVP (B) before and after

titania coating. The spectra have been normalized at the plasmon absorption maximum.

In order to further characterize the titania deposition on the nanoparticles surface TEM

analysis was performed. The TEM images of Au@CTAB@TiO2 (see Figure 2.2 (A) and (B))

indicated that the gold NPs have an inhomogeneous coating thickness of titania layer. Also,

an excess of titanium dioxide was observed in the medium. It could occur, because of the

presence of CTAB in excess in the medium during the sol-gel reaction of the TIP. The CTAB

excess could not be perfectly removed and therefore it interfered in the subsequent titania

coating.

In the case of Au@PVP@TiO2 core-shell nanoparticles (see Figure 2.2 (C)-(F)) the coating

was not uniform in term of shell thickness, though the coverage was complete. The typical

shell thickness was in the range 1-10 nm.


34

Figure 2.2: TEM images of (A-B) Au@CTAB@TiO2 and (C-F) Au@PVP@TiO2. Average diameter

of Au NPs: 66 nm (A-B) and 26.6 nm (C-F).

In the last method, the CTAB-stabilized Au nanoparticles (66 nm in diameter)35

were coated

following a combination of the polyelectrolyte layer-by-layer technique and the hydrolysis

and condensation of the titania precursor in ethanol (see Scheme 2.1). Although PVP

functionalization of the CTAB-coated nanoparticles allows the transfer of the particles into

ethanol/2-propanol (requirement for titania coating) and although PVP presents a good

affinity for titania, the PVP coating was not sufficient to screen the effect of CTAB during the

hydrolysis of the titania precursor, as previously observed for silica coating31

.

Since CTAB confers the Au nanoparticles with a high positive surface charge, LBL wrapping

of PSS (a negative polyelectrolyte) around the metal nanoparticles is strongly favored. The


35

PSS deposition reversed the Au surface charge from +21 to –39 mV (see Figure 2.3). After a

washing step to remove excess PSS, the particles were coated with PAH (a positive

polyelectrolyte) reversing the ξ-potential from –39 mV to +40 mV. Two polyelectrolyte

layers have been found to completely screen the effect of CTAB on the gold nanoparticle

surface. In this step it is very important to choose polymers with an appropriate molecular

weight and to control the ionic strength of the polymer solutions and avoid aggregation32

.

Finally, the nanoparticles were functionalized with PVP to allow redispersion in ethanol. The

ξ-potential at this point was –6 mV (PVP is slightly negatively charged under experimental

conditions). Once the particles were in ethanol, TBT was added to the solution at room

temperature and under inert atmosphere (the precursors used are highly reactive in the

presence of water traces). The hydrolysis and condensation of the TBT produced a shell of

TiO2 onto the Au nanoparticles. Figures 2.4 (C) and 2.4 (D) show transmission electron

micrographs (TEM) of some titania coated gold spheres. It can be observed that the gold

nanoparticles are coated with a homogeneous titania shell of 10±4 nm.

Scheme 2.1: Schematic representation of titania coating of Au nanoparticles following the LBL

approach.


36

Figure 2.3: ξ-potential for gold nanoparticles coated with CTAB, PSS, PAH and PVP, as indicated.

Figure. 2.4: TEM micrographs of: 66.4 ± 5.0 nm gold nanoparticles before (A-B), and after (C-D) the

titania coating. In image (D), the TiO2 shell thickness is about 6 nm.


37

Focusing on the optical properties, Figure 2.5 (A) shows the Visible-NIR spectra of the gold

colloid at different coating stages. The aqueous dispersion of 66 nm gold nanoparticles

presented a LSPR band at 546 nm. We can observe that the polyelectrolyte coating does not

affect to the optical properties since the absorption spectra of the gold nanoparticles before

and after the coating are the same. However, some changes are observed when the particles

are coated with PVP and transferred into ethanol, the plasmon band red-shifted by 5 nm. It is

well known that the LSPR band position depends on the refractive index of the medium in

which the particles are dispersed146

, therefore these changes can be attributed to the solvent

exchange. Finally, the LSPR band further red-shifted after the titania coating, due to an

increase in the local refractive index (see discussion above).

Titania thickness of Au@TiO2 core-shell nanoparticles could be tuned through successive

steps of hydrolysis and condensation of TBT on the Au@TiO2 NPs (see Scheme 2.1). Thus,

Au@TiO2 core-shell nanoparticles with 5 nm Au@TiO2, 20 nm and 30 nm TiO2 thicknesses

were prepared by performing 1, 2 and 3 additions of TBT in 2-propanol, respectively. Figure

2.6 shows TEM images of gold-titania core-shells NPs with different shell thicknesses.The

increase of the titania thickness shell was produced the red-shift of LSPR band and an

increase of the band width (see Figure 2.5 (B)), as explained above.

Figure 2.5: Visible-NIR absorption spectra of gold particles at different coating stages (A) and gold

nanoparticles at different stages of titania coating (B). The spectra have been normalized at the surface

plasmon maximum.


38

Figure 2.6: TEM images of titania-coated gold NPs with an average diameter of 60 nm, with

increasing titania shell thickness: Titania shells of: 5 nm in Au@TiO2 (A) and (B); 20 nm in

Au@1TiO2 (C) and (D); 30 nm in Au@2TiO2 (E) and (F).

2.4. CONCLUSIONS

39

2.4. Conclusions

Different strategies have been analysed to coat Au nanoparticles with a titania shell. In the

case of CTAB and PVP stabilized Au nanoparticles the hydrolysis and condensation of titania

precursor led to inhomogeneous titania coatings. The cationic surfactant CTAB and the

amphiphilic polymer PVP makes the affinity of the gold to TiO2 sufficiently high, so the

titania deposited directly on the surface of Au@CTAB or Au@PVP colloids. Nevertheless, it

was very difficult to have the ideal amount of CTAB/PVP in the medium (gold colloids are

frequently prepared in an excess of CTAB/PVP) in order to get homogeneous coatings.

In the third method, core-shell gold-titania nanoparticles with a homogeneous shell of

titanium dioxide were obtained via the combination of the LBL technique and the hydrolysis

and condensation of TBT. Besides, the titania thickness could be tuned by additions steps of

titania precursor. The control of the titania shell thickness can be used to control the dipolar

interactions between particles and improves the versatility of the particles.

2.4. CONCLUSIONS

41

CHAPTER 3

Synthesis and characterization of Au-doped TiO2 nanoparticles

ABSTRACT

Anatase titanium dioxide nanoparticles doped by gold nanoparticles were synthesized in a

two-step protocol. The strategy is based on the preparation of spindle-shaped TiO2

nanoparticles and after the further Au deposition. Thus, anatase TiO2 nanoparticles were

fabricated on a large scale through mesoscale assembly in the titanium (IV) butoxide-acetic

acid system without any additives under solvothermal conditions. For the deposition of Au

nanoparticles on the titania surface three different strategies were analyzed: two

adsorption-based processes and the deposition-precipitation method with urea. The structural,

physico-chemical and morphological properties of TiO2 and Au-doped TiO2 were examined

by TEM, DLS, BET, SAED, XPS, and UV-visible absorption spectroscopy.

3.1. INTRODUCTION

42

3.1. Introduction

Several methods are reported in the literature for the synthesis of TiO2 particles (see

Introduction, Table 1.1). Among them, hydrothermal/solvothermal syntheses present

advantages since they allow to obtain crystalline TiO2 structures in the nanometer scale.

Hydrothermal synthesis involves the reaction of aqueous solutions under controlled

temperature and/or pressure (often performed in steel autoclaves). The solvothermal method

is almost identical but the reaction takes place in organic solvent, allowing higher

temperatures and therefore a better control over the structure, morphology and phase

composition of the TiO2.

In particular mesoporous TiO2 particles are very attractive due to their high crystallinity,

porosity and oriented subunit alignment. These properties make them promising substitutes

for single-crystalline or porous polycrystalline materials in many applications such as

catalysis, sensors, optoelectronics and biomedicals materials, and in the energy storage and

conversion147,148,149,150

. For example, the mesoporous channels offer a larger surface area and

their connected porous structure allows an efficient transport of reactants and products. It is

known that chemical reactions are more effective when the transport paths through which

molecules move into or out of the nanostructured materials are included as an integral part of

the architectural design151

.

Ye and coauthors45

developed the first additive-free strategy to synthesize anatase TiO2

mesocrystals. This solvothermal method is based on the reaction of titanium (IV) butoxide,

TBT, with acetic acid, HAc, which forms an unstable titanium acetate complex,

(CH3COO)𝑥Ti(OC4H9)4−𝑥, as well as butanol. Then, both hydrolysis-condensation and

nonhydrolytic condensation processes take place leading to the formation of a metastable a

transient precursor. It gradually release soluble titanium-containing species which give rise to

anatase nanocrystals through nucleation and growth processes. Finally, the anatase

nanocrystals undergo oriented aggregation along the [001] direction, with entrapped butyl

acetate, resulting in the formation of the spindle-shaped anatase mesocrystals elongated along

the [001] direction. Acetic acid played multiple roles: (i) decreasing the TBT reactivity by

forming a complex, (ii) stabilizing the anatase nanocrystals, and (iii) reacting with TBT to

form butyl acetate which acted as porogen during the mesocrystals formation.

TiO2 is often used as either catalyst or catalyst support. The primary catalytic use of TiO2 is in

photocatalysis. When TiO2 absorbs light of greater energy than his band gap, electron-hole

3.1. INTRODUCTION

43

pairs are generated and redox reactions with surface species are initiated, before

recombination. TiO2 photocatalysts are used in many ways including water purification,

hydrogen production, air purification, and selective organic synthesis.

Noble metals such as Au, Pt, and Pd supported on anatase TiO2 are particularly useful in a

number of oxidation reactions, e.g. oxidation of CO at low temperatures64

, direct synthesis of

hydrogen peroxide152

, selective oxidation of primary C-H bonds153

and oxidation of alcohols

to aldehydes154

. The advantages of supporting noble metals NPs in a platform such as TiO2, is

the enhancement of the nanoparticles stability towards ionic strength or solvent exchange, the

easy removal from reaction medium or improvement of reusability. Furthermore in some

cases, the synergistic effects could also improve the catalytic efficiency of the nanostructure.

Several methods have been reported in the literature for the deposition of Au NPs on TiO2

particles (see Chapter 1, Table 1.3). The deposition-precipitation, DP, method is the mostly

recommended method for obtaining TiO2@Au catalysts. It is explained in terms of: (i) small

and uniform gold NPs, (ii) reproducible properties, and (iii) a strong interaction between the

gold NPs and the TiO2 support61,62,155

. The DP technique involves the deposition of gold

hydroxide on the TiO2 support by raising the pH of AuCl4− solution. Although the pH adjust

could be carried out using NaOH66,80,

. The use of urea is recommended70,80

since urea

facilities surface complex formation and metal loadings of around 100 wt.% are achieved.

Zanella et al.80

proposed a mechanism based on the adsorption of AuCl4− and/or

AuCl3(OH)− which act as nucleation sites for the precipitation and growth of Au(C0(NH2)2).

Au(C0(NH2)2) resulted from the reaction between gold anions and urea or products of its

decomposition. The pH increase from 3 to ∼ 7 leads to the formation of gold nuclei and lead

to a decrease in the size of the gold nuclei on the TiO2 support156

.

TiO2@Au composites can be also obtained by chemical reduction of gold complexes

deposited on the titania surface. The nature of the gold complexes is strongly dependent of

pH157

. While at pH 3-4 the major species are neutral (AuCl3. H2O) at 7 and 10 the

predominant species are [AuCl(OH)3]− and [Au(OH)4]−, respectively. As is well known, at

higher pH anatase TiO2 particles are negatively charged (see Appendix I, Scheme I.1) and the

predominant gold precursor species in suspension were probably [AuCl(OH)3]− and/or

[Au(OH)4]− resulting in an electrostatic repulsing, whereby some other mechanism was

required. Haruta et al.86

proposed that Au(OH)3 was formed between pH 6 and pH 11 (see

Reaction (3.1)) and deposited on the TiO2 support (see Reaction (3.2)). Moreau and

3.1. INTRODUCTION

44

coworkers158

suggested also an adsorption of gold neutral species, Au(OH)3, on negatively

charged TiO2 surface.

Au(OH)4− ↔ Au(OH)3 + OH− (3.1)

Au(OH)3 + 2TiO− → [Au (OH)2(OTi)2]− + OH− (3.2)

In this chapter three different approaches for the synthesis of anatase TiO2 nanoparticles

doped with gold nanoparticles (TiO2@Au) were explored. The strategies are based on the

preparation of spindle-shaped TiO2 nanoparticles and the further Au deposition. The

deposition routes are based on either adsorption of AuCl4− or (Au(Cl)4−n(OH)n

−) ions /

Au(OH)3 on TiO2 surface and their further reduction with NaBH4 or the

deposition-precipitation with urea (DPU) followed by chemical reduction using NaBH4. This

last method, allowed modulate the size and density of the gold nanoparticles on titanium

dioxide surface by varying the amount of titania used in the process of DPU. The structural,

physico-chemical and morphological properties of TiO2 and Au-doped TiO2 were examined

by TEM, DLS, BET, SAED, XPS, and UV-visible absorption spectroscopy.


Chemicals

Titanium (IV) butoxide (Ti(OC4H9)4), reagent grade 97%, acetic acid (CH3CO2H),

tetrachloroauric acid trihydrate (HAuCl4. 3H2O), sodium borohydride (NaBH4), ammonia

25% (NH3), sodium ascorbate (C6H7NaO6) and urea (CO(NH2)2) were purchased from

Aldrich. All reactants were used without further purification. Milli-Q grade water with a

resistivity higher than 18.2 MΩ . cm-1

was used in all the preparations.

Anatase titanium dioxide synthesis

Nanoporous anatase TiO2 mesocrystals with two different sizes were prepared via the

solvothermal method described by Ye and co-authors6 with some modifications. In a typical

synthesis, 0.7 or 0.4 mL of titanium (IV) butoxide was added dropwise to 20 mL of acetic

acid with continuous stirring, yielding 1TiO2 and 2TiO2, respectively. Then the TiO2

suspension was transferred to a 40 mL Teflon-lined autoclave to perform the solvothermal

treatment at 200 ºC for 24 h. After that, the suspension was cooled at room temperature and

the product was collected and washed with absolute ethanol several times. Finally, it was

redispersed in Milli–Q water.


45

Synthesis of gold doped anatase titanium dioxide nanoparticles

Adsorption of 𝐀𝐮𝐂𝐥𝟒− on TiO2 surface. In a typical experiment, 200 µL of HAuCl4 (13.2

mM) was added to 10 mL of an aqueous solution of as-synthesized anatase TiO2 (0.05 mg

mL-1

). It was stirred for 30 min in the dark at 25 ºC to allow complete adsorption of AuCl4−

ions over the TiO2 surface and then centrifuged. The supernatant was discarded, the pellet was

redispersed in 10 mL of Milli-Q water and 25 µL of NaBH4 (100 mM) were added under

stirring. Finally the suspension was centrifuged (3500 rpm for 15 min), the supernatant was

removed and the residue was redispersed in Milli-Q water. This washing cycle was repeated

once more.

Adsorption of Au(Cl)4-n(OH)-n / Au(OH)3 on TiO2 surface. In the standard preparation

conditions, 10 mL of aqueous suspension of TiO2 (0.05 mg mL-1

) was placed, for 30 minutes,

in a water bath at 40 °C, until achieve thermal equilibrium. Then, 10 µL of 1.0 M ammonia

(25 %) and 61.3 µL of 38.8 10-3

M HAuCl4 solution were added. After 15 min at 40 °C, the

suspension was centrifuged, the supernatant was discarded and the residue redispersed in 10

mL of Milli-Q water. Then 25 µL of NaBH4 (100 mM) were added under stirring and after 10

min it was centrifuged (3500 rpm for 15 min), the supernatant discarded and the residue

(TiO2@Au) redispersed in Milli-Q water. This washing cycle was repeated once more.

The Au NPs deposited in the TiO2 surface (TiO2@Au) were subjected to a process of seeded

growth. Briefly, 10 mL of the suspension of TiO2@Au was placed in an ultrasound bath.

Then, 10 µL of 0.2 M sodium ascorbate was added and 1.0 mL of 0.35 10-3

M HAuCl4 was

injected, in 100 µL portions at an interval of 1 min each, using an automatic injector, under

magnetic stirrer. The sample was then centrifuged, the supernatant was discarded, and the

residue was redispersed in Milli-Q water.

Deposition-precipitation with urea. TiO2@Au composites were prepared by the method of

deposition-precipitation with urea developed by Zanella and co-workers80

with some

modifications. In a typical procedure, the certain amount of as-synthesized titanium dioxide

was added to 1 mL of aqueous solution containing HAuCl4. 3H2O (4.2 mM) and urea (42

mM). The suspension was heated at 90 ºC through heater-stirrer and reflux during 4 h under

magnetic stirrer. After that, the suspension was cooled naturally at room temperature. Then,

the suspension was centrifuged (3500 rpm for 15 min), the supernatant discarded and the


46

pellet redispersed in water. This washing procedure was repeated several times to remove the

residual urea as well as gold species non-adsorbed on titania. Finally, 25 µL of NaBH4 (100

mM) were added to the suspension under stirring and after 10 min it was centrifuged (3500

rpm for 15 min), the supernatant discarded and the pellet, TiO2@Au, redispersed in Milli-Q

water. This washing cycle was repeated once more.

In order to tune the size of the gold NPs, different amounts of titania nanoparticles were added

in the process of deposition precipitation with urea. The detailed experimental conditions for

the preparation of the aforementioned samples are listed in Table I.1 (see Appendix I). For

each preparation the initial pH (pHi) of the HAuCl4, urea and TiO2 suspension and the final

pH (pHf) at the end of the preparation were recorded.

Instrumentation


100 kV and a HITACHI H-8100 microscope operating at an accelerating voltage of 200 kV

were used to perform general examination of samples at low magnification, including bright

field and dark field imaging and Selected Area Diffraction (SAD). A JEOL JEM 2010 FEG

(field emission gun) high resolution transmission electron microscope (HRTEM) operating at

an acceleration voltage of 200 kV was used to obtain SAED and HRTEM images. High Angle

Annular Dark-Field images were obtained in the same system working in STEM mode,

elemental maps were also obtained in STEM mode coupled with the X-ray spectrometer

(INCA Energy 200 from Oxford). The UV-Vis absorption spectra were recorded on a

VARIAN CARY 50 spectrophotometer. The zeta potential measurements were performed

with a Malvern Zetasizer Nano ZS90 instrument. The textural properties were determined by

nitrogen physisorption at 77 K, employing the Brunauer–Emmett–Teller (BET) method in a

Micromeritics ASAP 2020 apparatus. Before analysis the samples were out-gassed at 200 ºC.

Nitrogen adsorption/desorption isotherms at 77 K were obtained by plotting the amount of

nitrogen adsorbed vs. the corresponding relative pressure of nitrogen. The total surface area of

the TiO2@Au samples was determined using the BET method. The Barrett, Joyner, and

Halenda (BJH) method was used to calculate pore size distribution and total pore volume.

X-ray diffraction (XRD) measurements were carried out at room temperature by using

Siemens X-ray diffraction D5000 with Cu Kα radiation as incident beam (k = 1.5406 Å, 40

kV and 30 mA) with automatic data acquisition. Data were collected over the 2 range of 20–

70 º. XPS experiments were performed in a SPECS Sage HR 100 spectrometer with a


47

non-monochromatic X-ray source (Magnesium Kα line of 1253.6 eV energy and 250 W and

calibrated using the 3d5/2 line of Ag with a full width at half maximum (FWHM) of 1.1 eV.

An electron flood gun was used to compensate for charging during XPS data acquisition. The

selected resolution for the spectra was 15 eV of Pass Energy and 0.15 eV/step for the detailed

spectra. All measurements were made in an ultra-high vacuum (UHV) chamber at a pressure

around 5 10-8

mbar. In the fittings Gaussian-Lorentzian functions were used (after a Shirley

background correction) where the FWHM of all the peaks were constrained while the peak

positions and areas were set free. The etching of the samples was done with an Ar+ beam with

energy of 3 kV at several times up to 1100 s. The gold content in the TiO2@Au sample

prepared by DPU was determined using an Inductively Coupled Plasma-Atomic Emission

Spectrometer (ICP-AES) system (Horiba Jobin-Yvon, France, Model: Ultima) equipped with

RF generator of 40.68 MHz, Czerny-Turner monochromator with 1.00 m (sequential), AS500

autosampler, and CMA device (Concomitant Metals Analyzer). Concentrated hydrochloric

acid and nitric acid were added to TiO2@Au suspension until completely dissolution of the

Au NPs. The resulting suspension was sonicated for 1 hour, then syringe-filtered and diluted

with Milli-Q water prior to analysis.


3.3.1. Anatase titanium dioxide

Anatase titania nanoparticles, of two different sizes, were were prepared via the solvothermal

reaction of TBT in HAc at 200 ºC. The particle size was varied by adding different amounts

of TBT in the reaction medium. The resulting mesocrystals were characterized by various

techniques, including transmission electron microscopy, selected area electron diffraction,

BET specific surface area measurement, dynamic light scattering and X-ray photoelectron

spectroscopy.

TEM images (Figure 3.1 (A-D)) indicate that the resulting particles presented spindle-like

morphology, independently of the amount of TBT added. Nevertheless, the TiO2 dimensions

depended on the TBT concentration. While the addition of 0.7 mL of TBT resulted in

particles (named 1TiO2) of 175.7 ± 38.6 nm in length and 116.8 ± 28.8 nm in diameter (see

Appendix I, Figure I.1 (A) and (B)), the addition of 0.4 mL of TBT into 20 mL of HAc gave

rise to larger particles (named 2TiO2, 303.8 ± 72.8 nm in length and 226.4 ± 48.0 nm in

diameter) (see Appendix I, Figure I.1 (C) and (D)).

4.4. CONCLUSIONS

Figure 3.1: TEM images of anatase 1TiO2 (A-B), 2TiO2 (C-D) and High Angle Annular Dark-Field

STEM images of 2TiO2 (E-F) mesocrystals.

The characterization by high angle annular dark-field STEM images (Figure 3.1 (E-F))

showed that 2TiO2 particles presented areas with different contrasts indicating the presence of

defects in the crystal structure. In order to determine the phase of the TiO2 particles the

selected area electronic diffraction (SAED) patterns were analyzed. Figure 3.2.A shows that

the SAED pattern is constituted by polycrystalline diffraction rings that can be identified as

anatase structure (Figure 3.2A). Since there were no other diffraction rings corresponding to

rutile or brookite in the SAED pattern, it suggested that 2TiO2 particles were entirely anatase.

Besides, the analysis of a single nanoparticle reveal that the anatase TiO2 mesocrystals were

consisted of anatase subunits highly oriented along the [001] direction exhibiting diffraction

spots corresponding to the [100] zone axis of the anatase phase (Figures 3.2 (B)). Similar

results were found for 1TiO2 particles.


49

Figure 3.2: (A) Electron diffraction ring pattern from the 2TiO2 particles showed in the inset, the

pattern was indexed using the anatase structure. (B) Electron diffraction pattern obtained from a single

particle showed in the inset. In plane rotation of SAED pattern was compensated.

The titania stability was measured on the basis of Zeta potential measurements. The TiO2

particles exhibited a high zeta potential values (1TiO2 = +38.5 7.58 mV and 2TiO2 = +32.1

11.4 mV) indicating that the suspension was highly stable in water. Figure 3.3(A) shows the

UV-visible absorption spectra for the TiO2 samples. A strong absorption in the ultraviolet

region, associated with the electron promotion of TiO2 from the valence to the conduction

band, was clearly observed with the absorption edge at wavelength around 335 nm. Besides,

both anatase TiO2 particles showed absorption spectra extended into the visible-NIR region

over the range of 400 to 800 nm. In semiconductors a shift in the optical edge corresponds to

a change in the band gap.159

Therefore, the observed absorption edge shift toward longer

wavelengths indicated a decrease in the TiO2 band gap energy7. To confirm this hypothesis

the energy band gap, Eg, of TiO26 samples was estimated from the relationship of the

absorption coefficient and the photon energy160

, which is given by (αℎʋ)1

2⁄ = ℎʋ, where

ℎʋ is the photon energy and α is absorption coefficient. α can be determined from Lambert

Beer Law, α =2.303A

d, where A is the absorbance and d is the optical path length. Thus

band-gap energy could be obtained from the extrapolation of the linear part of the curve

6 The band structure of the anatase single crystals has two maxima, M and Γ, near the top of the VB,

which are separated by a very small energy difference (~ 2 meV). The transitions from these states to

the bottom of the CB can be either direct (Γ (VB) → Γ (CB)) or indirect (M (VB) → Γ (CB)). This

depends upon the crystalline structure, the lattice parameters, and the material dispersion.


50

(αℎʋ)1

2⁄ versus ℎʋ (photon energy). Figure 3.3 (B) shows a band gap value of 1.99 and 1.92

eV for 1TiO2 and 2TiO2, respectively.

Figure 3.3: (A) UV-vis absorption spectra of 1TiO2 and 2TiO2 samples. The spectra were normalized

at the maximum absorbance to facilitate comparison. (B) Plot of (αℎʋ)1

2⁄ versus the energy of light

for TiO2 samples.

The reduction in the band gap of anatase TiO2 can be analyzed by using the model of the band

structure. In the band structure of stoichiometric TiO2, the conduction band is Ti 3d and 4s in

character and the valence band is oxygen 2p in character. Scheme 3.1 shows that all the four

electrons from Ti are found in the half-filled O 2p states and it leaves the Ti 3d and 4s orbitals

empty forming the conduction band. The filled O 2p orbitals form the valence band of TiO2.

Theoretical analysis161,162

confirmed that when an oxygen atom is removed, three 2p orbitals

are removed with four electrons and the remaining two electrons from Ti localized on two

adjacent Ti atoms reduces them to Ti3+

. The introduction of dopants, such as carbon, reduce

the band gap by replacing oxygen 2p states in the valence band with 2p states that float up

into the band gap. In addition, carbon doped TiO2 samples exhibited an enhanced absorption

in the whole visible light region and a red shift in the absorption edges163,164

. Hence, XPS

measurements have been performed to analyze the presence of C and Ti3+

in the TiO2

structure. Both TiO2 samples contain Ti, O, and C elements (see Table 3.1). Additionally, the

stoichiometry of TiO2 samples was confirmed by the O/Ti ratio, 2.26 for 1TiO2 and 2.00 for

2TiO2, which indicated the absence of oxygen vacancies on the titania surface.


51

Scheme 3.1: Representation of the electronic structure of TiO2. Adapted from reference [165].

Table 3.1: XPS elemental analysis (in at. %) for TiO2 samples

Sample

Atomic (%)

Ti O C O/Ti

1TiO2 28.1 63.6 8.3 2.26

2TiO2 31.0 62.0 7.0 2.00

Figure 3.4 shows the fitted Ti 2p, O 1s and C 1s spectra of the 2TiO2 sample. The C 1s XPS

spectrum (Figure 3.4 (C)) revealed an intense peak at 285 eV assigned to C-H or C-C bonds166

and other two peaks at 286.5 and 288.6 eV corresponding to C-O and C=O groups,

respectively, probably from carbonate species167

. No signals from Ti-C bond were observed at

281.8 eV. It indicated that C had been incorporated in the TiO2 mesocrystals essentially as

elemental carbon, probably within the tetrahedral and octahedral interstices existing in the

anatase lattice, and a small amount as carbonate species. Carbon from Ti(OC4H9)4 or HAc or

C4H9OCOCH3 was retained during the solvothermal process, which resulted in carbon

self-doping into the lattice of anatase TiO2 mesocrystals. These hypotheses were consistent

with the results obtained by Wu and coworkers168

who found that C-doped TiO2 could be

prepared by solvothermal reaction (180 ºC, 2 h) of tetra n-butoxide in ethanol/water without

the addition of extra carbon sources. The presence of C-O bonds in TiO2 was further observed

in the O 1s XPS spectrum from 2TiO2. Figure 3.4 (B) shows the expected positions of the

C-O, C=O and TiO2 bonds at 533.0, 531.8 and 530.0 eV43

, respectively. The XPS analysis

clearly showed that the oxygen atoms are bonded essentially to Ti, with some oxygen atoms

attached to the carbon. Figure 3.4 (A) shows Ti 2p XPS region169

where the major peak

located at 458.6 eV could be attributed to Ti-O bonds. The peak characteristic for Ti-C at


52

455.3 eV was not present, indicating the absence of Ti-C bonds in the TiO2 lattice. But, the

fittings revealed the presence of a few amount (3.4%) of Ti (III). This Ti3+

species could be

also responsible of the visible absorption contribution observed in these samples170,171

. The

XPS analysis was also performed for 1TiO2 sample obtaining similar results (see Appendix I,

Figure I.2 (A)).

In summary, the absorption of TiO2 samples in the visible range were probably due to a

combination of different effects, namely defects in the lattice, incorporation of carbon atoms

into the anatase lattice, and the presence of a small amount of Ti3+

species.

Figure 3.4: XPS spectra of Ti 2p (A), O 1s (B) and C 1s (C) for 2TiO2 sample.

The specific surface area as well as the pore size/shape of the TiO2 is an important factor in

catalysis. Hence, nitrogen adsorption-desorption isotherms were measured to determine such

properties (Figure 3.5 and Figure I.3 in Appendix I). According to the IUPAC172

classifications, the isotherms obtained in both TiO2 samples were identified as type IV with a

type H3 hysteresis loop, which is characteristic of a mesoporous non-uniform in pore

size/shape. The hysteresis loops shifted to higher relative pressures is usually characteristic of

slit-like mesopores with a pore size decreasing gradually. As shown in Figure 3.5 (B) and

Figure I.3 (B) Appendix I, the size of slit-shaped pores varied from 10.5 nm (1TiO2) and 6.05


53

nm (2TiO2) to pore sizes smaller than 2 nm. Besides, slight differences were observed in the

specific surface area, 124.77 and 151.22 m2g

-1 for 1TiO2 and 2TiO2, respectively, and in the

pore volumes, 0.32 and 0.22 cm3g

-1 for 1TiO2 and 2TiO2, respectively (see Table 3.2).

Figure 3.5: (A) Nitrogen adsorption-desorption isotherms for 2TiO2. (B) Pore size distribution

obtained from the adsorption branch of the sample 2TiO2.

Table 3.2 shows the physical properties of the synthesized anatase 1TiO2 and 2TiO2 particles.

It should be noted that the titanium dioxide mesocrystals, the 1TiO2 or 2TiO2, obtained at

each new solvothermal synthesis present slight differences in the lengths and widths.

However, both samples are rather similar in terms of crystalline structure, pore size, surface

charge, or chemical composition.

Table 3.2: Physical properties of anatase TiO2 particles.

Phase aSA

m2g

-1

bPore V

cm3g

-1

cPore D

nm

ξ

mV

Length

nm

Diameter

nm

dBG

eV

1TiO2 Anatase 124.8 0.32 10.1 +38.57.6 175.7±38.6 116.8±28.8 1.99

2TiO2 Anatase 151.2 0.22 6.1 +32.111.4 303.8±72.8 226.4±48.0 1.92

Method SAED BET BET BET DLS TEM a

BET surface area calculated from the linear part of the BET. b

Pore volume and caverage pore size

determined by nitrogen adsorption volume. dBand gap.

3.3.2. Gold-doped titanium dioxide nanoparticles

For the doping of anatase TiO2 with Au nanoparticles three different strategies were explored:

adsorption of AuCl4− or (Au(Cl)4−n(OH)n

−) ions / Au(OH)3 followed by reduction with

sodium borohydride and deposition–precipitation with urea.


54

3.3.2.1. Electrostatic adsorption of 𝐀𝐮𝐂𝐥𝟒− on TiO2 surface

The first approach to dope TiO2 with Au nanoparticles is based on electrostatics adsorption.

Titania colloids exhibits a positive zeta potential (+ 38.5 7.58 mV) therefore the Au doping

was performed via the electrostatic adsorption of negatively charged AuCl4− ions on the

positively charged titania surface and their further reduction with sodium borohydride (see

Scheme 3.2). As shown in Figures 3.6 (A-B), 1TiO2 particles are homogeneously doped with

Au NPs. The average size of the Au NPs in the 1TiO2@Au was 1.3 ± 0.5 nm (see Figure I.4

(A) in Appendix I). The process was monitored by UV-visible absorption spectroscopy. As

shown in Figure 3.6 (C) the optical properties of TiO2 mesocrystals remained unperturbed

after the AuCl4− adsorption and upon their reduction with NaBH4 an increase in the visible

absorption was observed. However, 1TiO2@Au did not show a defined plasmon band due to

the small Au particle size.

Scheme 3.2: Synthesis of Au-doped TiO2 nanoparticles via electrostatic adsorption of AuCl4− ions

followed by reduction with sodium borohydride.

Figure 3.6: (A-B) TEM images of 1TiO2@Au nanoparticles. (C) UV-visible absorption spectra of

1TiO2, AuCl4−-adsorbed 1TiO2 and 1TiO2@Au colloids.

In order to increase the size of Au nanoparticles deposited on the TiO2 surface, the amount of

AuCl4− ions adsorbed on the titania surface was increased by successive additions of HAuCl4.


55

The HAuCl4 concentrations tested were 0.26, 0.53, 0.79 and 1.06 mM. As shown in Figure

3.7 (C) at higher HAuCl4 concentration the absorption band attributed to AuIII

(221 nm) could

be observed, confirming the Au ions adsorption. Then, the Au NPs were obtained by adding a

strong reducing agent such as NaBH4. As shown in Figure 3.7 (A-B) higher HAuCl4

concentration led to larger Au nanoparticles (9.4 ± 2.6 nm, see Figure I.4 (B) in Appendix I),

but not to larger Au loadings.

Figure 3.7: (A-B) TEM images of TiO2@Au nanoparticles. (C) UV-visible absorption spectra of

1TiO2, AuCl4−-adsorbed 1TiO2 and 1TiO2@Au colloids. Au NPs with average diameters of 9.4 2.6

nm (A and B).

3.3.2.2. Adsorption of (𝐀𝐮(𝐂𝐥)𝟒−𝐧(𝐎𝐇)𝐧−) ions / 𝐀𝐮(𝐎𝐇)𝟑 on TiO2 surface

The second approach carried out to dope TiO2 mesocrystals with Au NPs was also based on

the adsorption of Au species and their further reduction. But in this case AuCl4− ions were

added to TiO2 colloids in the presence of ammonia. In basic medium Au(Cl)4−n(OH)n−

complexes were formed77

. The addition of Au salt decreased the pH to around 4. Taking into

account that titania is positively charge at this pH the adsorption took place via electrostatic

interactions.

The TEM characterization of TiO2@Au nanoparticles obtained after the addition of NaBH4

(see Figure 3.8 (A-B)) showed a nonuniform distribution of 6.9 ± 2.4 nm of Au NPs on the

TiO2 surface (see Figure I.5 (A) in Appendix I). Besides, the overgrowth of the Au NPs via

further addition of HAuCl4 and reduction with ascorbic acid was also studied. But it did not

improve the Au size distributions as observed in Figure 3.8 (C-D) and Figure I.5 (B) in

Appendix I.


56

Figure 3.8: TEM images of 2TiO2@Au, before (A-B) and after (C-D) the growth of the Au NPs

deposited on the 2TiO2 surface. Au NPs with average diameters of 6.9 2.4 nm (A and B), and 12.2

5.5 nm (C and D).

Regarding the optical properties, the LSPR band of Au NPs could be distinguished just in the

case of the overgrown Au nanoparticles, TiO2@Au2, (see Figure 3.9). In the case of smaller

Au NPs, TiO2@Au1, LSPR band could not distinguished probably due to the overlapping with

the optical properties of titania as well as the low optical efficiency of the Au NPs.

Figure 3.9: UV-vis absorption spectra of TiO2, TiO2@Au1 (Au NPs with average diameters of 6.9

2.4 nm) and TiO2@Au2 (Au NPs with average diameters of 12.2 5.5 nm) colloids.


57

3.3.2.3. Deposition-precipitation with urea

The third approach to dope anatase TiO2 particles with Au was based on the

deposition-precipitation with urea. Both 1TiO2 and 2TiO2 samples were used to perform this

studied. Yet similar results of gold nanoparticles deposition over titania surface by DPU

method were obtained with both TiO2 samples. So it was decided to present only the results

obtained with 2TiO2 nanoparticles and the 1TiO2@Au results were recorded in Appendix I. In

this method, the formation of gold nanoparticles on titanium support can be briefly described

as follows: initial, the surface of TiO2 is positively charged (pH ~ 2) and the gold species in

suspension are probably AuCl4− and/or AuCl3(OH)−, thereby these gold species can interact

electrostatically with the TiO2 support. Then the growth of the particles of gold precipitate,

probably Au(CO(NH2)2 species, on these sites occurs which act as nucleation sites, due to the

progressive increase of pH by the urea decomposition. The obtained gold species in TiO2@Au

are cationic gold species, as it will be seen later in the XPS studies, and so the addition of

NaBH4 was required to reduce them to metallic Au. To modulate the size of the gold

nanoparticles, i.e. 17.5 5.3; 4.0 1.4 and 1.4 0.5 nm (see Figure I.8 in Appendix I), on

titanium dioxide surface the amount of titania used in DPU was 0.7, 7.0 and 10.5 mg/mL,

respectively (see Appendix I, Table I.1) yielding the nanostructures named TiO2@Au0,

TiO2@Au1 and 2TiO2@Au2, respectively. The resulting Au-doped TiO2 particles were

characterized by various techniques, including transmission electron microscopy, HTREM,

XRD, and X-ray photoelectron spectroscopy. Figure 3.11 shows TEM images of 2TiO2@Au

nanostructures where the amount of titanium dioxide added in the DPU was increased from

0.7 to 10.5 mg/mL. As shown in the TEM images of 2TiO2@Au the Au NPs were uniformly

distributed over the TiO2 surface7. Moreover it can be observed that the Au nanoparticle

loading and size decreased with the amount of titania used in the DPU method. Finally, Au

loading and size also varied with the amount of urea. The 2TiO2@Au sample, obtained using

42 mM of urea, (Figure 3.10 (E-F)) presented lower Au density compared with particles

obtained with 42 × 10-2

M urea, 2TiO2@Au4, (see Figure I.6 (A-B) in Appendix I). It also

produced remarkable differences in the optical properties, the 2TiO2@Au4 sample exhibited a

broad absorption in the visible light region (see Figure I.6 (C) in Appendix I).

7 See Figure I.7 in Appendix I for TEM images of 1TiO2@Au1-2 composites obtained using different

amounts of 1TiO2.


58

Figure 3.10: TEM images of 2TiO2@Au composites obtained using different amounts of 2TiO2

nanoparticles. (A-B) 0.7 mg/mL of 2TiO2, 2TiO2@Au0 sample; (C-D) 3.5 mg/mL of 2TiO2,

2TiO2@Au1 sample; and (E-F) 10.5 mg/mL of 2TiO2, 2TiO2@Au2 sample. The Au NPs dimensions,

as determined from TEM, are summarized in Table I.1, Appendix I.

The optical properties of titania composites were analyzed by UV-vis absorption

spectroscopy. Figure 3.11 shows that LSPR band were not distinguished independently of the

composite. The absorption spectra of 1TiO2@Au1/2 and 2TiO2@Au1/2 in comparison to the

1TiO2 and 2TiO2 absorption spectra, respectively, show two prominent features: (i) the

overall visible-light absorbance intensities were enhanced, probably, due to the presence of

small gold nanoparticles on titania support and (ii) the maximum absorption edge shows an

obvious red shift, more accentuate for 1/2TiO2@Au1 sample that present the higher amount of

small gold NPs in the TiO2 support. Thus, these TiO2@Au nanostructures are potentially very

actives in visible region becoming suitable for many applications, such as, solar energy

conversion devices or photocatalysts of organic dye pollutants under sunlight irradiation as

discussed in Chapter 5.


59

Figure 3.11: UV-visible spectra of (A) 1TiO2, 1TiO2@Au1, and 1TiO2@Au2; (B) 2TiO2 and

2TiO2@Au1 and 2TiO2@Au2. All spectra are normalized at the absorbance maximum.

The composites were further analyzed by high resolution transmission electron microscopy

(HRTEM). Figure 3.12 shows HRTEM images of 1TiO2@Au1 composites. The gold

nanoparticles in the titania surface showed interplanar distances of 2.3 Å, assigned to the

spacing of the (111) planes of gold. The lattice fringes on the crystal face had spacings of 3.5

Å which corresponded to the (011) planes of anatase. It was consistent with the results

observed in the SAED pattern (see Figure 3.2 (A)) for bare titania nanoparticles. Moreover,

HAADF STEM characterization of 1TiO2@Au1 nanoparticles (Figures 3.12 (C-D))

demonstrated the uniform distribution of the Au nanoparticles on the titania surface and the

presence of Au nanoparticles as small as 0.8 nm inside of the anatase titania mesocrystals.


60

Figure 3.12: (A-B) HRTEM images of 1TiO2@Au1 composite. Gold nanoparticles are indicated with

arrows. (C-D) High Angle Annular Dark-Field STEM images of 1TiO2@Au1. The Au NPs, as small as

0.8 nm, were deposited inside the porous channel system of the TiO2 nanoparticles (D).

Additionally, the phase structure of TiO2@Au samples was investigated by XRD (see Figure

3.13). Apart from the characteristic peaks at 2θ = 25.34 º, 37.8 º, 48.11 º, 55.04 º and 62.75 º

attributed to different diffraction planes of anatase TiO2. Weak diffraction peaks at 2θ =

44.14º and 64.81º corresponding to [200], [220] reflections of the face centered cubic, fcc,

structure of metallic Au could be distinguished.

Since, the Au loading is high in the 1TiO2@Au1 (as confirmed by ICP, see Appendix I, Table

I.1) the lack of intense XRD peaks could be due to the small size of Au nanoparticles and

their distribution inside the mesocrystalline structure.


61

Figure 3.13: XRD pattern of 1TiO2@Au1.

In order to obtain information about chemical composition of the composites synthesized by

DPU, the 1/2TiO2@Au samples were subjected to XPS analysis. As the XPS results obtained

for 1/2TiO2@Au samples are identical, we present only those obtained with 2TiO2@Au

samples, and the XPS information of 1TiO2@Au samples were included in Appendix I.

The survey spectra showed the presence of Au, Ti, O, and C species in all TiO2@Au

composites (see Table I.1 in Appendix I). To characterize the valence state of the gold on the

2TiO2@Au samples, obtained with different amount of titania, the XPS spectra of Au 4f were

made and the results are represented in Figure 3.14. The deconvoluted spectra (see Figure

3.14 (A) and (B)) showed that there are mainly Au (III), Au (I) but not Au (0) in 2TiO2@Au1,

while Au0 species (main component) and Au (I) were the main Au species in 2TiO2@Au2

composite. Similar results were obtained for 1TiO2@Au1-2 and they have been included in the

Appendix I, Figure I.9. Therefore it was found that samples obtained with larger ratios TiO2

to Au salt gave rise to TiO2@Au composites with larger concentrations of Au0 species respect

to Au (III), Au (I) species. As mentioned, the XPS analysis of Au 4f for the 2TiO2@Au3

sample (sample obtained by DPU with 8.5 mg/mL of 2TiO2) confirms that the amount of gold

ions decreases and consequently increases the amount of metallic gold with the increase of

titania (see Figure 3.14 (A), (B) and (C)). Probably, the presence of Ti3+

in the 2TiO2 sample,

supported by XPS (see Figure 3.4 (A)) reduce the Au cations to neutral metals atoms, and so

in the spectra of Ti 2p of 2TiO2@Au2 has not observed the peak at 457.3 eV corresponding to

Ti3+

(see Figure 3.14 (E)). This observation indicates the occurrence of electron transfer from

TiO2 to Au nuclei´s. In this context, where all TiO2@Au samples exhibit gold ion over the


62

titania support in a greater or lesser amount depending of the initial ratios of TiO2 to Au salt,

NaBH4 was added to all TiO2@Au samples, obtained by DPU method, to ensure complete

reduction of gold cations to metallic gold. After chemical reduction by NaBH4, XPS results

reveal that Au is only present in the metallic state (see Figure 3.14 (D)).

Figure 3.14: XPS spectra: (A, B, and C) at the Au 4f energies for 2TiO2@Au1*, 2TiO2@Au2*,

2TiO2@Au3* samples prepared using different amount of TiO2, namely 3.5, 10.5 and 8.5 mg/mL and

before NaBH4 addition; (D) at the Au 4f energies for 2TiO2@Au1 after NaBH4 addition; and (E) at the

Ti 2p energies for 1TiO2@Au2 and 2TiO2@Au2 composites.

4.4. CONCLUSIONS

Also XPS depth profiling was performed for 1TiO2@Au1 and 2TiO2@Au2 samples in 30 nm

of depth, to corroborate if gold were just on the titania surface or also inside of titania

mesocrystals. In both samples the results (see Figure I.10 in Appendix I,) indicated that gold

was also distributed inside of the anatase TiO2 mesocrystals. However, as expected, the

1TiO2@Au1 sample presented a larger amount of gold inside (mean of Au/Ti is 0.60 ± 0.1 and

0.046 ± 0.02 for 1TiO2@Au1 and 2TiO2@Au2, respectively).

3.4. Conclusions

TiO2 mesocrystals with high specific surface area, anatase structure, slit-shaped pores and

strong absorption in the UV visible range were obtained through solvothermal method of

titanium (IV) butoxide in acetic acid. To homogeneously dope these mesocrystals with gold

nanoparticles three distinct approaches are used, namely two adsorption-based processes and

the deposition-precipitation method with urea followed by reduction with NaBH4. In the case

of electrostatic adsorption of AuCl4− ions on the titania surface, the method allowed not

increase the size of gold nanoparticles or control the density of gold NPs over the titanium

dioxide surface. The method of adsorption of Au(Cl)4-n(OH)-n / Au(OH)3 on TiO2 surface led

to a heterogeneous sizes distribution and inhomogeneous density of Au NPs on the titania

support. Only the deposition-precipitation method with urea allowed control the size/density

of gold NPs on the titania surface by varying the amount of TiO2 used in the preparation of

TiO2@Au composites.

4.4. CONCLUSIONS

65

CHAPTER 4

TiO2@Au catalyzed reduction of ferrycianate (III) by

borohydride ions

ABSTRACT

The reduction of hexacyanoferrate (III) ions to hexacyanoferrate (II) ions by sodium

borohydride in aqueous solution was found to be strongly catalyzed by TiO2@Au composites.

The kinetics of this electron transfer reaction was followed by UV-vis absorption

spectroscopy monitoring the hexacyanoferrate (III) ions consumption at 420 nm with time. To

carry out the kinetic study different parameters (concentration of TiO2@Au and NaBH4, and

temperature) were analyzed. The fact that gold NPs are immobilized on the TiO2 particles not

just ensured their stability against aggregation but also improved their catalytic activity, which

indicate a synergic effect between Au and TiO2 NPs and facilitated their reusability. The

reduction of the specific surface area of the TiO2 NPs by the amount of loaded Au NPs has

strong effects in the TiO2@Au catalytic efficiency. A mechanism relating the electron transfer

from de surface of Au NPs to the conduction band of TiO2 and the Fe(CN)63− ions diffusion

on the Au and TiO2 surface followed by reduction by an excess of electrons on these surfaces

is proposed.

4.1. INTRODUCTION

66

4.1. Introduction

The gold, the most stable among all metals, was long considered to be a catalytically inactive

metal until 1987 when Haruta and coworkers85

reported the high activity of supported gold

catalysts, when its size is in the nanometer range, in the low-temperature oxidation of carbon

monoxide. Since then, high activity of gold nanoparticles was demonstrated in several

oxygen–transfer reactions such as propene oxidation, water gas shift reaction, synthesis of

H2O2, selective oxidation of alcohols and aldehydes173,174

. Although, one major drawback of

these frees gold nanoparticles is their tendency to agglomerate toward factors such was ionic

strength or solvent exchange. Furthermore, the catalyst should be easily removed from

product upon completion of the catalytic process. A simple way to avoid these problems is to

deposit the gold nanoparticles on a support such as TiO2, Fe2O3, Al2O3, SiO2, etc.. Also, the

cooperation between gold nanoparticles and the support, in the catalytic cycle, has been

claimed frequently to rationalize the better catalytic performance of support gold

nanoparticles. Wherefore, the chemical nature of the support plays important role in

determining the catalytic performance. Therefore, the semiconductor nature of titania can be

an important factor to study the catalysis by gold support in titania in the electron transfer

reactions, such as the reduction of hexacyanoferrate (III) by borohydride ions, once the titania

can receive electrons from Au and so can also act as a catalyst for the reaction175

. It should be

noted also that titania mesoporous materials, are potentially better (photo)catalysts176,177,178

because the channels of the material offer a larger surface area and a connect pore system

which can help to concentrate molecules for the electron-transfer reactions. It is common

knowledge that chemical reactions are most effective when the transport paths through which

molecules move into or out of the nanostructured materials are included as an integral part of

the architectural design179

. In this chapter we analyzed the catalytic performance of the

TiO2@Au obtained employing the precipitation-deposition method with urea (see Chapter 3).

In order to perform the analysis, the reduction of hexacyanoferrate (III) by borohydride ions

in alkaline aqueous solution was selected as model electron-transfer reaction. The influence of

the TiO2@Au catalysts concentration, the borohydride concentration and the temperature (in

the range of 283 - 303 K) in the model reaction rate constant of reduction of Fe (III) by BH4−

were investigated. The catalytic performance of the TiO2@Au composites is also compared

with the catalytic performance of pure Au and TiO2.


67


Chemicals

Potassium hexacyanoferrate (III), sodium borohydride and sodium hydroxide were purchased

from Aldrich. All reactants were used without further purification. Milli-Q grade water with a

resistivity higher than 18.2 MΩ . cm-1

was used in all the preparations.

Electron transfer reaction preparation

A 2 10-3

M stock solution of hexacyanoferrate (III) ions was prepared using the potassium

hexacyanoferrate (III) salt. A 10 10-2

M stock solution of borohydride ions was prepared

using the sodium borohydride salt. The pH of this stock solution was adjusted to 11.5, by the

addition of NaOH, in order to inhibit the chemical decomposition of the reducing agent29

.

The electron transfer reaction between hexacyanoferrate (III) ions and borohydride ions was

carried out as follows: 1.25 mL of an aqueous solution of Fe(CN)63− (2 10

-3 M) and 1.5 mL

of Milli-Q water containing a certain amount of Au-doped anatase TiO2 particles were placed

in a quartz cuvette with an optical path length of 1 cm and maintained at 25 ºC for 5 minutes.

Then, 250 µL of freshly prepared NaBH4 aqueous solution (10 10-2

M) were subsequently

added into the above suspension. The suspension was shaken and immediately transferred into

UV-vis spectrometer. The temperature of the suspension, in the quartz cuvette, was controlled

by water regulated Peltier assembly connect to a water bath. All solutions were placed in a

thermostatic bath at desired temperature. The catalytic electron transfer reaction was repeated

three times, for each catalyst, to ensure the reproducibility.

Recycling of TiO2@Au

In order to study the recyclability of the 1TiO2@Au1 and 2TiO2@Au2 catalysts, 1.250 mL of

an aqueous solution of Fe(CN)63− (2 10

-3 M) and 1.5 mL of Milli-Q water containing a

certain amount of TiO2@Au were placed in a quartz cuvette with an optical path length of 1

cm. Then, 250 µL of freshly prepared NaBH4 aqueous solution (10 10-2

M) was added into

the above suspension. The reaction is followed by UV-vis spectroscopy. After the first cycle

of the electron transfer reaction is completed, the next cycle of electron transfer reaction is

initiated by adding 5 µL of 0.5 M Fe(CN)63−to the suspension. A total of six cycles were

made. Since the overall volume increase after 6 cycles is of 30 µL in a total volume of 3 mL,

the catalyst concentration in each cycle could be considered as constant.


68

Instrumentation

The UV-Vis absorption spectra were recorded on a VARIAN CARY 50 spectrophotometer

with temperature control CARY single cell peltier accessory. The pH was measured using

Crison pH-Meter Basic 20+.


The catalytic performance of TiO2@Au composites, obtained in Chapter 3, was evaluated

using a model redox reaction between hexacyanoferrate (III) and borohydride ions (see

Introduction for further details). Herein we just showed the catalytic analysis performed for

2TiO2@Au2 catalyst, however similar results were obtained for 1TiO2@Au1 and they have

been included in the Appendix II.

The redox reaction can be written as27

:

BH4− + 8Fe(CN)6

3− + 3H2O → H2BO3− + 8Fe(CN)6

4− + 8H+ (4.1)

Nevertheless, during this reaction the hydrolysis of borohydride ions also takes place. To

inhibit the decomposition of the reducing agent during the reduction of hexacyanoferrate (III)

the pH was maintained at 11.529

. The reaction can be monitored with high precision by

UV-vis spectroscopy, following the decrease of the characteristic absorption peak of

hexacyanoferrate (III) at 420 nm with time (see Figure 4.1 (A)). The kinetic measurements

were performed under pseudo-nth-order conditions, where [BH4−] was always maintained in

large excess over [Fe(CN)63−]. Under these conditions, the reaction of the ferrocyanide ions

obeys:

− d[Fe(CN)6

3−]

dt= kobs[Fe(CN)6

3−]n (4.2)

where kobs is the pseudo-nth-order rate constant for the reaction. It has been reported that this

reaction in the absence of a catalyst follows zero-order kinetics with respect to [Fe(CN)63−]

lasting hours27

. In our case, we observed that the presence of the TiO2@Au in small amounts

catalyzed the process as well as change the reaction order to first order, as occurred with Au

colloids25

.

Values of pseudo-first-order rate constant, kobs, were obtained by fitting the exponential decay

of the absorbance at 420 nm using the following equation:

At = (A0 − A∞) exp(−kobst) + A∞ (4.3)

where At represents the absorbance at 420 nm. In all cases, we found that experimental results

showed an excellent agreement with first order kinetics.


69

Figure 4.1: (A) UV-vis absorption spectral evolution of a mixture of hexacyanoferrate (III) and

2TiO2@Au2 catalyst upon the addition of sodium borohydride. (B) Kinetic trace of the absorbance at

420 nm during the Fe(CN)63− reduction and corresponding fitting for first order analysis according to

equation (4.3). Conditions: [Fe(CN)63−] = 8.33 × 10

-4 M, [BH4

−] = 8.33 × 10-3

M, [2TiO2@Au2] = 10.0

× 10-6

g mL-1

, T = 25 ºC, pH = 11.5. kobs= 9.7 × 10-2

s-1

.

Figure 4.1 (B) shows the kinetic trace of the absorbance at 420 nm during the Fe(CN)63−

reduction and corresponding fitting for first order kinetics according to equation 4.3 when

2TiO2@Au2 where used as catalyst (see Chapter 3 for details)8. A rate constant of 9.7 × 10

-2

s-1

was obtained for the catalyzed reaction ([Fe(CN)63−] = 8.33 × 10

-4 M, [BH4

−] = 8.33 × 10-3

M, [2TiO2@Au2] = 10.0 × 10-6

g mL-1

, T = 25 ºC, pH = 11.5).

4.3.1. Influence of borohydride concentration

The effect of the borohydride concentration on the reaction rate was studied keeping all

experimental conditions constant and by varying the initial concentration of BH4− from 4.16

mM to 50 mM (see Appendix II, Table II.1). It was found that the rate of reaction increases

with the concentration of BH4− (see Figure 4.2) observing a linear relationship. It suggested

that the reaction was first order with respect to the concentration of BH4− , which also

indicated that the reduction is a surface controlled process. Similar results were found for

1TiO2@Au1 catalyst (see Figure II.2 in Appendix II).

8 See Appendix II, Figure II.1 for 1TiO2@Au1 nanocomposite.


70

Figure 4.2: Influence of NaBH4 concentration on the observed rate constant. Conditions: [Fe(CN)63−]

= 8.33 × 10-4

M, [2TiO2@Au2] = 1.33 × 10-5

g mL-1

, T = 25 ºC, pH = 11.5.

4.3.2. Influence of the amount of catalyst

The effect of the amount of catalyst on the electron transfer reaction rate was studied at six

different concentrations of 2TiO2@Au2, keeping other parameters constant. Table 4.1 shows

the kobs obtained from the fittings absorbance-time data for the different concentrations (see

Appendix II, Table II.2 for 1TiO2@Au1). The results are summarized in Figure 4.3 showing

that there is linear dependence of kobs on the catalyst concentration fixed reaction conditions,

which indicates that 2TiO2@Au2 acted as a catalysts involved in the rate-determining step of

this electron transfer reaction. Similar results were found for 1TiO2@Au1 catalyst (see

Appendix II, Figure II.3).

Table 4.1: Rate constant (kobs, s-1

) with their standard deviations () of catalysed reactions

between Fe(CN)63− and BH4

− ions at different concentrations of 2TiO2@Au2 catalyst.

10-6

[2TiO2@Au2] (g mL-1

) 10-2

kobs 10-3

(s-1

)

3.33 4.07 2.33

6.66 7.70 3.20

10.0 9.81 6.60

13.3 14.9 6.66

20.0 22.5 12.0

23.3 26.4 9.82


71

Figure 4.3: Influence of the amount of 2TiO2@Au2 catalyst on the observed pseudo-first-order rate

constant. Conditions: [Fe(CN)63−] = 8.33 × 10

-4 M, [BH4

−] = 8.33 × 10-3

M, T = 25 ºC, pH = 11.5.

Besides, in order to analysis the surface effect we have plotted the observed rate constant

versus the total gold surface area (see Figure 4.4(A)). The total gold surface area was been

calculated taking into account the Au NP diameter and the amount of Au in the TiO2@Au

catalyst, which were determined by TEM and ICP, respectively (see Appendix II, Table II.3

and II.4). As shown in Figure 4.4 (A) there is a linear relationship between the observed rate

constant and the surface area of Au demonstrating that the Au particles participate in the

reduction of hexacyanoferrate (III) by borohydride ions. It is predicted by the following

equation:

− d[Fe(CN)6

3−]

dt= kobs [Fe(CN)6

3−] = kSA[Au][Fe(CN)63−] (4.4)

where kS is the rate constant normalized to the total surface area of gold per unit volume of

solution. As it can be observed, the 2TiO2@Au2 catalyst doped with smaller Au NPs (1.4 nm

in diameter) is more catalytic active than the 1TiO2@Au1 with bigger gold nanoparticles

(average size of 4.5 nm). Probably, the small size of gold nanoparticles facilitates the contact

with the titania support and the rapid electron transfer from gold nanoparticles, charged with

electrons by the borohydride, to the TiO2.


72

Figure 4.4: (A) Influence of the amount of TiO2@Au catalysts in terms of gold surface area on the

observed pseudo-first-order rate constant. (B) Values of kobs normalized to the total surface area of

gold, averaged for different TiO2@Au concentrations. The dashed lines represent the mean value in

each case. Conditions: [Fe(CN)63−] = 8.33 × 10

-4 M, [BH4

−] = 8.33 × 10-3

M, T = 25 ºC, pH = 11.5.

Also, the observed rate constant normalized to the unit gold surface area is constant (see

Figure 4.4 (B)) indicating that the catalytic activity of these compounds depends on the total

surface area of gold. The 2TiO2@Au2 catalyst revealed better catalytic efficiency for exhibit

small gold NPs (~ 1.4 nm) and low density of Au NPs in the titania support and a higher

available specific surface area of titania (see Chapter 3, Figure 3.10 (E-F)). Probably, the

greater number of Au NPs at the surface and inside of 1TiO2 in 1TiO2@Au1 catalyst (see

Appendix I, Figure I. 7) decreases the catalytic efficiency by the screening effect of surface

area of 1TiO2, making less accessible the Fe(CN)63−, which results in less reaction rate of

reduction. This can be clearly seen from the high ratio of Au/Ti (15.6 at.%) in the 1TiO2@Au1

as reported in Table I.1 (see Appendix I).

4.3.3. Influence of temperature

In order to analyse the temperature effect the catalysed reaction was studied between 283 and

303 K. The values of kobs obtained for 2TiO2@Au2 at different temperatures are given in

Table 4.3 (see Appendix II, Table II.5 for 1TiO2@Au1). As expected the catalyzed reaction

rate increased as temperature increases for both catalysts. Besides, Figure 4.5 (A) displays

that ln kobs presented a linearly dependence with 1/T which indicates that the reduction of

ferrycianate (III) by borohydride ions obeys the Arrhenius temperature dependence. The

activation energy was calculated following the Arrhenius equation:

lnkobs = lnA −E𝐴

RT (4.5)


73

where A is a constant, Ea is the activation energy and R is the gas constant. An Ea of 98.64 ±

0.96 kJ mol-1

was obtained for the catalyzed model. Additionally, the activation parameters

ΔH≠ and ΔS

≠ were obtained from the slope and intercept of the Eyring’s plot

9 of ln kobs/T

versus 1/T (see Figure 4.5 (B)). The thermodynamic activation parameters are given in Table

4.4. The positive sign of the ΔH≠ reflects the endothermic nature of the process of reduction

of Fe (III) in the presence of TiO2@Au catalysts. Similar observations were found for

1TiO2@Au1 (see Appendix II, Figure II.4).

Table 4.3: Rate constants (kobs, s-1


between Fe(CN)63−and BH4

− ions at different temperatures (283 - 303 K).

2TiO2@Au2

10-2

kobs 10-3

(s-1

)

283 1.09 1.55

286 1.64 0.51

288 1.87 0.90

290 2.90 1.83

293 4.80 1.45

298 8.07 5.18

303 19.4 15.19

Ea (kJ/mol)

98.64 ± 0.96

Table 4.4: The values of activation parameters for reduction of Fe(CN)63− by BH4

− ions in the

presence of 2TiO2@Au2 catalyst.

Catalyst Ea

kJ mol-1

ΔH≠

kJ mol-1

ΔS≠

J K-1

mol-1

2TiO2@Au2 98.64 99.93 70.45

9 The enthalpy and entropy of activation were obtained by Eyring equation. The linear form of the

Eyring equation is:

lnk

T=

−∆H≠

R ×

1

T+ ln

kB

h+

∆S≠

R (4.6)

where k = reaction rate constant; T = absolute temperature; −∆H≠= enthalpy of activation; R = gas

constant; kB= Boltzmann constant; h = Planck’s constant and ∆S≠= entropy of activation. The plot of

lnk

T vs

1

T gives a straight line with slope (

−∆H≠

R) from which the enthalpy of activation can be derived

and with intercept (lnkB

h+

∆S≠

R) from which the entropy of activation is derived.


74

Figure 4.5: (A) Arrhenius plot of the temperature dependence of the electron transfer reaction in the

presence of 2TiO2@Au2. (B) Plot of ln (kobs/T) vs. 1/T for the electron transfer reaction in the presence

of 2TiO2@Au2 .Conditions: [Fe(CN)63−] = 8.33×10

-4 M, [BH4

−] = 8.33×10-3

M, [2TiO2@Au2] = 1.34 ×

10-5

g mL-1

, pH=11.5.

4.3.4. Recyclability of TiO2@Au catalyst

The recyclability of our catalysts was checked by the sequential reduction of hexacyanoferrate

(III) ions by borohydride ions in aqueous alkaline solution using the same catalyst and fresh

hexacyanoferrate (III) ions in each cycle. As shown in Figure 4.6, no reduction in the catalytic

efficiency of the 2TiO2@Au2 (data for 1TiO2@Au1 in Appendix II, Figure II.5) catalyst was

observed even after six cycles.

Figure 4.6: Absorbance kinetic traces at 420 nm, registered during the sequential reduction of

Fe(CN)63− in the presence of 2TiO2@Au2. Initial concentrations [Fe(CN)6

3−] = 8.33 × 10-4

M, [BH4−] =

8.33 × 10-3

M, [2TiO2@Au2] = 3.33 × 10-6

g mL-1

, T = 25 ºC, pH = 11.5.


75

4.3.5. Effect of titania on the reaction rate

Finally we need to take into account the Au nanoparticles were supported on anatase titania

mesocrystals and the anatase titania is among the most widely investigated material for

photocatalysis and sensing. Therefore the catalytic activity of TiO2@Au catalyst was

compared with the activity of anatase titania mesocrystals and citrate-stabilized Au

nanoparticles of around 4 nm.

As shown in Figure 4.7, in the absence of Au nanoparticles the reduction of Fe(CN)63− does

not takes place since no change in the absorbance at 420 nm was observed with time.

Therefore, the Au nanoparticles as expected played the main role in the Au doped titania

catalyst.

But in order to check if the TiO2 mesocrystals could have any influence in the catalytic

performance of Au nanoparticles, the catalytic performance of citrate stabilized Au

nanoparticles, Au@cit, was studied for reduction of ferrocyanate (III) by borohydride ions.

Since the citrate-stabilized Au nanoparticles were around 4 nm in diameter (see Appendix II,

Figure II.6), 1TiO2@Au1 was chosen to carried out the comparison since Au particles

presented similar dimensions.

Figure 4.7: Kinetic trace of the absorbance at 420 nm during the reduction of Fe(CN)63−in the

presence of TiO2 (solid line) and Au-doped TiO2 (1TiO2@Au1, dashed line)

As previously reported, citrate stabilized gold nanoparticles are very efficiency catalyst for the

reduction of ferricyanide ion to ferrocyanide by sodium borohydride25

. As observed for Au


76

doped TiO2 mesocrystals, Au nanoparticles alone change the order of the reaction from

zero-order kinetics to first order with respect to the amount of Fe(CN)63−

(Figure 4.8 (A)).

Besides, there is a linear relationship between the observed rate constant and the

concentration of gold, which demonstrated that reaction took place on the gold surface and the

catalytic activity depending on the total surface area.

Figure 4.8: Kinetic trace of the absorbance at 420 nm during the reduction of Fe(CN)63− in the

presence of Au@cit and corresponding fitting for first order analysis according to eq. 4.3. Conditions:

[Au@cit] = 4.6 10-6

mol L-1

, [BH4−] = 8.33×10

-3 M, pH = 11.5 e T = 25 ºC.

The effect of the titania support on the observed pseudo-first order rate constant was analyzed

comparing the kobs obtained for 1TiO2@Au1 and the Au NPs (both catalyst presenting the

same Au nanoparticle size). Figure 4.9 shows kobs plotted as a function of the gold

concentration for 1TiO2@Au1 and the Au NPs. The slope for 1TiO2@Au1 is much larger than

for the Au NPs under similar experimental conditions. Therefore Au nanoparticles supported

on titania mesocrystals exhibited higher catalytic performance than citrate-stabilized Au

colloids which indicated that titania played a role in the reaction mechanism. The catalytic

cooperation of the titania support and gold in TiO2@Au arise probably by a higher catalytic

surface due to the semiconductor nature of titania and, so that after charging the gold NPs by

borohydride, part of electrons can be transferred to the titania support, which can also act as a

catalyst for the model reaction.


77

Figure 4.9: Comparative study of the influence of the amount of Au on the observed rate constant for

the reduction of Fe (III) in the presence of two different catalysts (1TiO2@Au1 and citrate-stabilized

Au nanoparticles). Conditions: [Fe(CN)63−] = 8.33×10

-4 M, [BH4

−] = 8.33×10-3

M, pH=11.5.

4.3.6. Model proposed for the catalytic process

Based on the experimental results, the possible mechanism for the TiO2@Au catalyzed

reduction of hexacyanoferrate (III) ions by BH4− ions was proposed (see Figure 4.10). In a first

step the oxidation of BH4− ions generated electrons that were injected onto the Au

nanoparticles:

BH4− + 8OH− + Aun → H2BO3

− + 5H2O + 8e−(Aun) (4.7)

As the electrons accumulate within the Au core, the Fermi level shifts to more negative

potential (close to the value of bulk Au, EF = +0.45 V vs NHE) closer to the conduction band

edge of titania180

. It should be noted the fact that the energy levels in the gold are discrete

expecting a greater shift in the energy level for each accumulated electron in smaller size Au

NPs than the larger ones21

. Then, the electrons migration from the Au NPs to the conduction

band of TiO2 occurs until the two systems attain equilibrium resulting in a more potential of

the Fermi level of TiO2@Au composites (see Figure 4.11), and hence greater reductive

power181

. The electrons in TiO2 were captured by Ti4+

reducing it to Ti3+

:

eCB− + > TiIVOH → {> TiIIIOH} (4.8)

Finally, the Fe(CN)63− ions diffuse to the Au and TiO2 surface and are reduced by excess

surface electrons in the Au NPs and by the electrons from the Ti3+

, which acts as an electron

donor:

4.4. CONCLUSIONS

78

8e−(Aun) + Fe(CN)63− → 8Aun + 8Fe(CN)6

4− (4.9)

{> TiIIIOH} + Fe(CN)63− → > TiIVOH + 8Fe(CN)6

4− (4.10)

According to Subramanian182

, the TiO2 can function as a reservoir of electrons and can be

used as a strong reductive source for any species.

Figure 4.10: Schematic diagram for the possible mechanism for the reduction of Fe(CN)63− ions by

BH4− ions catalyzed by TiO2@Au.

Figure 4.11: Equilibration of TiO2@Au (a) before and (b) after electrons injection. Ef – Apparent

Fermi Level before electrons injection and Ef∗ – Apparent Fermi Level after electrons injection.

4.4. Conclusions

In summary, we demonstrated that TiO2@Au act as a very efficient catalyst for the reduction

of ferricyanide to ferrocyanide ions by borohydride ions. Also, we observed that TiO2@Au

composites exhibited higher catalytic performance than Au@cit (with similar size to the Au

NPs on the surface of the TiO2@Au nanocomposite) or TiO2 colloids suggesting a synergistic

effect between TiO2 and Au NPs in the studied redox reaction, probably a higher catalytic

surface in the process of reduction.

Furthermore, the density of Au NPs is an important factor for the catalytic efficiency of

TiO2@Au composites. It has been found that the catalytic efficient for the reduction of

Fe(CN)63− by BH4

− ions decreases at higher loadings which proves that Au NPs take up much

surface area of titania. Additionally, TiO2@Au can be used as a reusable catalyst applicable to

4.4. CONCLUSIONS

79

chemical reactions. The catalytic properties of the catalysts TiO2@Au are not significantly

degraded after its repeated use.

4.4. CONCLUSIONS

81

CHAPTER 5

Photochemical activity of TiO2 / TiO2@Au nanostructures

ABSTRACT

The nanoporous anatase TiO2 nanoparticles showed obvious absorption in the 400-600 nm

range with a red shift in the band gap transition. This visible-light activity of TiO2 was

investigated in the photoreduction of cationic gold species of the TiO2@Au composites,

prepared by deposition precipitation method with urea and before NaBH4 reduction, and in

the photodegradation of rhodamine B dye in aqueous solution, utilizing sunlight as the

irradiation source. It was found that TiO2 is a visible–light active photocatalytic material, able

to reduce gold cations to gold metal nanoparticles and degrade the rhodamine B dye when

excited with solar light irradiation. The photocatalytic activity of TiO2@Au systems in the

degradation of rhodamine B under solar light irradiation was also evaluated concluding that

these composites are active photocatalytic species in this process.

5.1. INTRODUCTION

82

5.1. Introduction

Since the discovery of the photocatalytic water splitting on titania electrodes in 1972 by

Fujishima and Honda183

, a significant effort has been made to understand fundamental

processes involved in the photocatalytic efficiency of titania. TiO2 in any of its common

phases present wide band gaps (3.2 eV for anatase and 3.0 eV for rutile phase), which can

only be effectively activated under UV irradiation (about 2–3 % of the total solar energy), but

not by visible light. Several approaches have been reported to develop effective TiO2

photocatalysts under visible light, including TiO2 doping with noble metal NPs or other

elements such as sulfur, iodine, nitrogen or carbon which act as electron donor or acceptor in

the forbidden band of titania. For example, carbon doping anatase titania particles exhibited

an obvious red shift at the maximum absorption with an enhanced absorption in the whole

visible region184,185,186

. Besides, the change of lattice parameters, and the presence of trap

states within the conduction and valence bands from electronic perturbations, gives rise to

band gap narrowing187

(band gap of C-TiO2 < 3.2 eV). Furthermore, it was also reported that

carbon doping can enhance the conductivity of TiO2 nanostructures, i.e., facilitate the charge

transfer from bulk region to the surface region where the oxidation reactions take place.

Several studies confirm the capacity of C-TiO2 to degrade various organic and inorganic

compounds under visible light. For example, Huang et al.188

demonstrated that mesoporous

TiO2 doped with carbon were effective photocatalysts for visible-light degradation of nitric

oxide. Irie and co-workers189

reported that C-doped anatase TiO2 exhibited visible-light

photocatalytic activity for the decomposition of 2-propanol. Wu et al.190

synthesized C-doped

TiO2 nanotubes, nanowires and nanorods and reported their visible-light photocatalytic

activity for the degradation of toluene. Also, noble metal NPs exhibits considerable visible

light absorption due to their plasmon resonance (SPR) effect on the surface. In this context,

the doping of TiO2 with noble metals NPs such as gold or silver has demonstrated to be an

interesting approach to extend the optical response of titania. TiO2@Au composites exhibit

exceptionally high photocatalytic activity for different reactions under visible light. Silva et

al.191

reported that Au NP-doped TiO2 exhibit photocatalytic activity in water splitting when

excited under visible light. The excitation at wavelengths corresponding to the localized

surface plasmon resonance of gold produced the absorption of photons by Au NPs and the

electrons injection into the conduction band of TiO2. Finally, several authors studied the


83

electron transfer from the Au NPs to the TiO2 in the TiO2@Au composites under visible-light

irradiation192,193

.

As mentioned in Chapter 3, the anatase titania particles synthesized in this work exhibit an

enhanced absorption in the whole visible-light region. Therefore in this chapter the

photocatalytic properties of titania and Au NP-doped titania mesocrystals are analyzed. Thus,

first the photoreduction of gold ions deposited on anatase TiO2 mesocrystals by precipitation

deposition by urea is studied. Besides, the photocatalytic activity of anatase TiO2 and Au

NPs-doped TiO2 mesocrystals is tested using as model reaction the degradation of a dye,

Rhodamine B, using sun light as irradiation source.


Chemicals

Rhodamine B dye were purchased from Sigma-Aldrich. All reactants were used without

further purification. Milli-Q grade water with a resistivity higher than 18.2 M cm-1

was used

in all the preparation. Anatase TiO2 mesocrystals and TiO2@Au composites were used as

photocatalysts (see Chapter 3 for the synthetic procedure).

Photoreduction of gold species in TiO2@Au composites

In a typical experiment, 500 µL of 1.2 mg mL-1

2TiO2@Au1 (see Chapter 3 for the synthetic

procedure) were dispersed in 15 mL of Milli-Q water in a conventional glass vial. Then, the

glass vial was closed and the suspension was exposed directly to natural sunshine during 3.5

hours under stirred. Time-sequenced aliquots were collected for UV-Visible analysis.

Photocatalytic degradation of rhodamine B with TiO2 and TiO2@Au over solar

irradiation

In a typical experiment, 15 mL of an aqueous 1.0 × 10-5

M RhB solution containing TiO2 or

TiO2@Au particles (5 × 10-5

g/mL) were stirred in a glass vial in the dark for 30 min to ensure

adsorption-desorption equilibrium between the TiO2 or TiO2@Au and the RhB dye. After

that, the reaction glass vial was exposed directly to natural sunshineduring 2 hours at room

temperature. Time-sequenced aliquots of 500 L were collected from the glass vial during the

time-course of illumination for UV-visible analysis. The TiO2/TiO2@Au particles were


84

removed by centrifuging at 4000 rpm. After that, the degraded RhB solutions were analyzed

with the UV-Vis spectrophotometer. The kinetics of the photodegradation was achieved by

monitoring the main absorption peak at 554 nm.

Characterization and measurements


100 kV was used to perform general examination of samples at low magnification.

Absorption spectra were recorded in a VARIAN CARY 50 UV-Vis-NIR spectrophotometer.


5.3.1. Photoreduction of gold species on TiO2@Au composites

As mentioned in Chapter 3, the visible range absorption of TiO2 samples are probably due to a

combination of different effects, namely defects in the lattice, incorporation of carbon atoms

into the anatase lattice, and the presence of a small amount of Ti3+

species. In this context and

with the aim of investigating the electron transfer from TiO2 to gold species (Au3+

and Au+)

under visible light irradiation, anatase titania mesocrystals doped with Au species deposited

by deposition precipitation method with urea was subjected to sunlight irradiation for 210 min

(see Scheme 5.1). Thus 2TiO2@Au1 before the NaBH4 treatment was selected to perform the

experiments (see Chapter 3 for further details), as XPS analysis revealed that this sample

presented only Au3+

and Au+

ions doping the anatase TiO2 nanoparticles (see Figure 5.1(A)).

Scheme 5.1: Photoreduction of Au

3+ and Au

1+ on anatase TiO2 mesocrystals (e

–: photoexcited

electrons; h+: photoexcited holes).

The spectral evolution of the 2TiO2@Au1 suspension under sunlight irradiation was

monitored by UV-vis absorption spectroscopy. As shown in Figure 5.1 the irradiation with

sunlight gave rise to the appearance after 30 minutes of a broad band centered at 600 nm. This

band, which increased in intensity with the sun exposure, corresponded to the localized

surface plasmon resonance of the Au nanoparticles resulting of the reduction of gold ions to


85

Au0. As a result the suspension turned from yellow to pink (see Scheme 5.1). The process

finished after 120 min since no further evolution were observed in the spectra. The

photoreduction of the Au ions was also analyzed by XPS (see Figure 5.2 (B)). The Au 4f XPS

spectrum obtained for the 2TiO2@Au1 after the sun exposure showed two peaks at binding

energies of 87.6 and 83.6 eV which were assigned to Au 4f5/2 (87.7 eV) and Au 4f7/2 (84.0

eV) from Au0. The absence of any peak assigned to Au

3+ and Au

1+, indicated the complete

photoreduction of the Au ions.

Figure 5.1: Time evolution of the UV-visible absorption spectra of 2TiO2@Au1 colloids upon sunlight

irradiation.

Figure 5.2: Au 4f X-ray photoelectron spectrum of the 2TiO2@Au1 nanocomposite (A) before and (B)

after solar irradiation for 210 minutes.


86

Finally, TEM analysis of the 2TiO2@Au1 composites showed the presence of nanoparticles

before and after the photoreduction of 4.44 ± 1.62 and 8.92 ± 3.08 nm, respectively (see

Figure 5.3 and Appendix III, Figure III.1). The absence of Au0 (see Figure 5.2 (A)) before the

irradiation indicated that the nanoparticles should be constituted by Au(CO(NH2)2).

Nevertheless the NPs after the irradiation were Au0. All the results indicated that the sun

exposure of 2TiO2@Au1 composites produced the absorption of photons with energy equal to

or greater than the TiO2 band gap. It created electron-hole pairs which moved to the surface.

The photogenerated electrons reduced the gold ions (Au3+

and Au1+

) to Au0, while the holes

were scavenged by adsorbed water or hydroxyl molecules194,195,196

.

Figure 5.3: TEM images of 2TiO2@Au1 composites before (A-B) and after (C-D) sun irradiation.

In conclusion, the anatase TiO2 nanoparticles synthesized in the present work have

visible-light absorption, which was proved by the electrons transfer from TiO2 to gold ions,

that are in the titania support, reducing them to metallic gold, when the 2TiO2@Au sample

was sunlight irradiated.

5.3.2. Photocatalytic degradation of rhodamine B

The photocatalytic performance of anatase TiO2 and TiO2@Au nanostructures were also

evaluated by analyzing the photodegradation of rhodamine B, RhB, in water with sun.

Although several studies have showed that rhodamine B is stable in water under UV and

visible light irradiation, the photodegradation of RhB might occur in the presence of


87

nanoparticles, such as, TiO2, TiO2@Au, TiO2@Ag. The RhB is a xanthene dye whose

degradation could be monitored by UV-vis spectroscopy since it presents a characteristic

absorption band centered at 554 nm. First, a control experiment was performed in absence of

TiO2 or TiO2@Au particles showing that the sun light irradiation just produced a slightly

decreased of the absorbance intensity of the 554 nm band (6.3% in 120 min of irradiation, see

Appendix III, Figure III.2). It indicated almost no degradation of RhB in the absence of

particles. Nevertheless, photodegradation of the dye was observed in the presence of TiO2 or

TiO2@Au particles. Figure 5.4 shows the time evolution of the absorption band from RhB in

water during irradiation with solar light presence of the TiO2 (Figure 5.4 (A) and (B)) or

TiO2@AuX particles (5.4 (C) and (D)). Thus, the main absorption band from RhB centered at

554 nm gradually decreased indicated the degradation of this aromatic chromophore, but after

certain irradiation time a blue-shift of the band was observed. The hypsochromic shifts might

attribute to the formation of a series of stepwise N-de-ethylated intermediates of RhB197

such

as N,N-diethyl-N′-ethylrhodamine, N-ethyl-N′-ethylrhodamine, N,N-diethylrhodamine,

N-ethylrhodamine, and rhodamine. Nevertheless, the degradation seemed to be the main

process. It should be pointed out that after mixing the RhB with the nanoparticles and under

dark conditions, a decrease in the absorption band was observed. It was explained in terms of

RhB adsorption on the TiO2 surface. Thus, from absorption spectra it was estimated that

~30.9% of the dye was adsorbed on 1TiO2 and ~26% for 2TiO2, and on TiO2@Au surface,

about 19.3% and 29.4% for the catalysts 1TiO2@Au1 and 2TiO2@Au2, respectively.

The degradation of RhB dye could be described by a pseudo-first order reaction with a

simplified Langmuir–Hinshelwood model:

lnA

A0= − kobst (5.1)

where A

A0 corresponds to the normalized RhB concentration (A0 is the initial absorbance at 554

nm after equilibrium adsorption, and A is the absorbance at 554 nm at time t, during the

reaction under solar irradiation) and kobs is the pseudo-first-order rate constant. The rate

constants were obtained by the linear fittings of lnA

A0 versus time t, and is equal to the

corresponding slope (see Figure 5.5 (F)). Table 5.2 presents the detailed information of rate

constants.

X To evaluate the photocatalytic performance, we selected the 1TiO2@Au1 and 2TiO2@Au2 samples,

synthesized with different sizes of anatase titania particles and doped with Au NPs of 4.5 ± 1.5 and 1.4

± 0.5 nm in diameter (see Chapter 3 for further details).


88

Figure 5.4: Spectral evolution of a mixture of RhB and (A) 1TiO2, (B) 2TiO2, (C) 1TiO2@Au1 and

(D) 2TiO2@Au2 upon different visible light irradiation times. Comparison of photodegradation of RhB

aqueous solution with 1TiO2, 2TiO2, 1TiO2@Au1 and 2TiO2@Au2 under sunlight as the irradiation

source (E), and first-order kinetics in RhB photodegradation under solar irradiation in the presence of

1TiO2, 2TiO2, 1TiO2@Au1 and 2TiO2@Au2 (F). Negative time (-30 min) indicates the period of RhB

adsorption on the TiO2 or TiO2@Au surface without solar irradiation. Conditions: [RhB] = 1.0 ×

10−5 mol dm−3, [1TiO2] = [2TiO2] = [1TiO2@Au1] = [2TiO2@Au2] = 5.0 × 10−5 g mL−1,

sunlight.


89

Table 5.2: Rate constants of the pseudo first-order fit of RhB photodegradation under solar

irradiation.

Photocatalyst *kobs (10

–2 min

-1)

•R

1TiO2 2.2 ± 0.28 0.924

2TiO2 3.1 ± 0.25 0.968

1TiO2@Au1 1.4 ± 0.16 0.939

2TiO2@Au2 2.3 ± 0.37 0.886 *Pseudo-first-order rate constant,

• Adj. R-Square.

The high photocatalytic activity of TiO2 particles could be attributed to synergic effects like

of its large surface area, visible-light absorption induced from C-doping of titania (see

Appendix III, Table III.1), and existence of porous structure that allows multiple reflections of

visible light within the interior of titania particles, allowing more efficient use of the visible

light. Deposition of Au NPs on anatase titania mesocrystals caused a decrease in the

photoactivity with respect to anatase TiO2 nanoparticles and the photocatalytic performance

of TiO2@Au decreases with increasing Au content. Concretely, the catalyst 2TiO2@Au2 that

exhibits a ratio of Au/Ti of 0.2 presents better photocatalytic efficiency, compared to the

catalyst 1TiO2@Au1 that present a ratio of Au/Ti of 15.6. This can be explained from that

excess Au NPs may: act as a (i) charge recombination center, or (ii) cover the active sites of

TiO2198

, or (iii) reduce of light penetration depth. In this regard, more studies are necessary to

confirm/exclude the influence of individual parameters.

Based on the experimental results, a mechanism for the degradation of RhB photocatalyzed

by TiO2, was proposed. The process started with dye adsorption on TiO2 surface, followed by

photochemical steps of the dye and TiO2, and finally a series of redox process involving the

active species produced in the photochemical steps, concretely, superoxide radical anions,

O2∙−, and/or hydroxyl radical, OH∙. Briefly, RhB was adsorbed at the TiO2 surface,

TiO2{RhB}ads (Reaction 5.2). Under solar irradiation the adsorbed RhB dye was excited to

produce singlet and triplet states, TiO2{RhB}ads∗ (Reaction 5.3). Subsequently, the

{RhB}ads∗ injected an electron into the conduction band of TiO2 with TiO2{RhB}ads

∗ being

converted to cationic dye radicals, TiO2{RhB}ads∙+ (Reaction 5.4). The TiO2 provided sufficient

unoccupied electron-acceptor states with a wide and continuous energy distribution, which

ensured rapid and efficient electron injection, and avoided useless loss of the dye excited

states. During this process, no hole on the valence band of TiO2 was involved and

consequently no formation of OH∙ by H2O oxidation or by photogenerated holes. The

electrons from the TiO2 conduction band were transferred to oxygen molecules


90

surface-adsorbed (in aerated systems) to form the superoxide radical anions, O2∙− (Reaction

5.5). Once the dispersion discolored completely, the O2∙− formation terminated automatically.

In this context, the oxygen played an important role in the RhB photodegradation, in the

formation of reactive oxygen species for the mineralization of RhB, and in the scavenging the

electron, and consequently suppressed recombination between RhB∙+ and e-. The superoxide

radical anion can transform to H2O2 (Reaction 5.6 and 5.7). H2O2 was transformed by reacting

with the electron in the conduction band of TiO2199

or by the visible induced decomposition,

via formation of a TiO2-H2O2 surface complex200

(Reaction 5.8). Once RhB was discolored

completely, no visible light could be absorbed and consequently the photoreaction via dye

excitation ceased. However, despite the important role in the electron-transfer mediation by

anatase TiO2 nanoparticles they also are excited by solar light irradiation, with generation of

electrons and holes on TiO2 (Reaction 5.9) followed by the reactions 5.5 to 5.7 to yield the

same active oxygen species which lead to RhB mineralization (Reaction 5.13). The

photoinduced holes, generated in this process are apt to react with surface-bound OH- or H2O

to produce the hydroxyl radical species, OH∙, (Reaction 5.10 and Reaction 5.11) which is an

extremely strong oxidant for the mineralization of organic chemicals. OH∙ formation was

crucial for the N-de-ethylation of RhB and therefore required for the complete degradation of

RhB dye. The direct excitation of TiO2 by solar light also led to the cationic dye

radical RhB∙+ (Reaction 5.12). The radical cation TiO2{RhB}ads∙+ reacted with the reactive

oxygen radicals and/or hydroxyl radical until mineralization (Reaction 5.13).

RhB + TiO2 → TiO2{RhB}ads (5.2)

TiO2{RhB}ads →ℎ TiO2{RhB}ads∗ (5.3)

TiO2{RhB}ads∗ + TiO2 → TiO2{RhB}ads

∙+ + TiO2(eCB− ) (5.4)

TiO2(e𝐶𝐵− ) + O2 → TiO2 + O2

∙− (5.5)

O2∙− + 2H+ + TiO2(e𝐶𝐵

− ) → H2O2 + TiO2 (5.6)

O2∙− + 2H+ → O2 + H2O2 (5.7)

H2O2 + Ti(IV)complex →ℎ decomposition (5.8)

TiO2 →ℎ 𝑒𝐶𝐵− + ℎ𝑉𝐵

+ (5.9)

TiO2(hVB+ ) + OH− → OH∙ + TiO2 (5.10)

TiO2(hVB+ ) + H2O → OH∙ + H+ + TiO2 (5.11)

TiO2{RhB}ads + TiO2(hVB+ ) → TiO2{RhB}ads

∙+ + TiO2 (5.12)

TiO2{RhB}ads∙+ + (OH∙, O2

∙− ) → Degradated produts + TiO2 (5.13)


91

In conclusion, the proposed mechanism for the photodegradation of RhB consists in a

synergetic pathway to produce reactive O2∙− and OH∙ species through the two pathways,

namely, dye sensitization pathway and TiO2-photocatalyzed pathway.

To confirm the possibility of recycling of the TiO2 NPs, the 2TiO2 photocatalyst was

repeatedly used for the RhB photodegradation reaction 2 times (see Figure 5.5) under visible

light as the irradiation source. For this purpose, 22.2 L of RhB ([RhB] = 1.173 mM) was

again added to the 10 mL of RhB and 2TiO2 suspension ([2TiO2] = 1.5 10-4

g mL-1

, [RhB] =

1.0 10-5

mol dm-3

), after complete degradation of the dye was attained. Two successive

catalytic cycles were performed and the photocatalytic performance of the second cycle of

RhB degradation was similar to the first run (see Figure 5.5 (C)). No filtration or centrifuging

was effectuated between each cycle of use of the TiO2 photocatalyst, or for the TiO2

suspension after the photocatalysis procedure and before UV-VIS spectrophotometric

analysis.

Figure 5.5: Spectral evolution of a mixture of RhB and (A) 2TiO2 – first reuse (B) 2TiO2 – second

reuse, upon different irradiation times. (C) Photodegradation of RhB aqueous solution with 2TiO2

under sunlight, as the irradiation source, in the first and second reuse. Condition: [RhB] = 1.0 ×

10−5 mol dm−3, [2TiO2] = 1.5 × 10−4 g mL−1, sunlight.

5.4. CONCLUSIONS

92

The results indicated that TiO2 photocatalyst are reusable, however more studies are needed to

evaluate the number of possible reuses without loss of catalytic performance, and the stability

of the mesoporous structure of titania nanoparticles during the RhB photodegradation process.

5.4. Conclusions

In conclusion, anatase TiO2 mesocrystals, with absorption extended to visible light region,

presented ability to reduce gold ions adsorbed on their surface under solar light irradiation.

Besides, anatase TiO2 and gold-doped TiO2 nanoparticles, TiO2@Au, showed high

photocatalytic activity for the degradation of RhB in aqueous solution under solar light

irradiation. The results further concluded that the synthesized TiO2 possess superior

photocatalytic activity than TiO2@Au composites in the degradation of RhB dye, using

present conditions, under direct sunlight irradiation. The RhB degradation rate decrease with

the increasing of Au nanoparticles on the titania support, probably due to the fact that more

Au nanoparticles may act as the centers of electron-hole recombination, or cover the active

sites of TiO2, or reduce the light penetration depth. The photodegradation of RhB by TiO2

nanoparticles consists in a cooperative effect to produce reactive radical species such as

superoxide radical anion, O2∙−, and hydroxyl radical, OH∙, from the sunlight absorption by the

RhB dye and TiO2 nanoparticles. Subsequently, the RhB dye reacts with highly potential

radicals, OH∙ and O2∙−, which results in generation of range of intermediates followed by

complete mineralization of dye. The narrow band gap of TiO2 favors the transfer of

photogenerated electrons and results in more electrons, thereby allowing holes to participate

in the photocatalytic redox reaction. A possible mechanism for this synergetic degradation

was proposed.

The recycling test revealed that the TiO2 photocatalyst are stable and possess recyclability,

which may have excellent application potentials in water treatment.

In conclusion, the nanoporous anatase TiO2 nanoparticles could be useful for environmental

applications such as water purification and hazardous waste remediation due to their high

visible-light photocatalytic activity, large specific surface area, and easy and cheap

preparation process on large scale.

The degradation of RhB dye can be driven by sunlight during daylight hours which represents

an economic and environmentally friendly perspective.

93

Conclusions

Even though specific conclusions have been address at the end of each chapter, the most

relevant, global, conclusions of the research described in this thesis are presented herein.

1. Gold nanoparticles, 66 nm in diameter, were coated with a homogeneous layer of TiO2

through an approach based on the combination of the the layer-by-layer self-assembly

technique and the hydrolysis and condensation of titanium precursor in alcoholic

medium. Besides, the sequential addition of the titania precursor allows grow the

semiconductor layer in a controlled manner.

2. Spindle-shaped mesoporous anatase titania nanoparticles were obtained by a

solvothermal method using acetic acid and titanium (IV) butoxide. The TiO2

nanoparticles exhibits strong absorption in the visible light region probably, as has been

shown, due to the combination of different effects, such as defects in the lattice,

incorporation of carbon atoms into the anatase lattice, and the presence of a small

amount of Ti3+

species. In addition, from the UV-vis absorption spectra of TiO2

nanoparticles, the band gap of the semiconductor nanoparticles was estimated using the

Kubelka-Munk equation, obtaining quite lower values (1.99 and 1.92 eV for 1TiO2 and

2TiO2, respectively).

Different strategies were explored to Au-doped TiO2 nanoparticles. Only the deposition-

precipitation method with urea followed by reduction with NaBH4 allowed control the

size and density of gold nanoparticles on the titania surface by varying the amount of

titania used in the preparation of TiO2@Au nanocomposites. In addition, we found by

XPS studies that the gold species in TiO2@Au after the step of deposition-precipitation

with urea are cationic and metallic. This fact, justifies the further treatment with

borohydride. Also the amount of cationic gold species respect to metallic gold in the

TiO2@Au nanocomposites decreases with the increase of the amount of titania in the

process of deposition-precipitation with urea. Probably, as confirmed by XPS analysis,

the presence of Ti3+

surface defect on TiO2 nanocrystals reduced the cationic gold

species, Au3+

and Au1+

, to metallic gold, Au0.

CONCLUSIONS

94

The TiO2@Au nanocomposites were used as catalyst for the reduction of ferricyanide to

ferrocyanide ions by sodium borohydride ions. It was found that:

(a) All TiO2@Au nanocomposites (synthesized by deposition-precipitation with urea,

using different amounts of TiO2 nanoparticles for the same amount of gold salt and

followed by reduction with NaBH4), in small amounts, act as a very efficient

catalyst for the reduction of ferricyanide to ferrocyanide by borohydride ions

increasing the reaction rate compared with the non-catalyzed reaction, under our

experimental conditions. Furthermore, the presence of TiO2@Au nanocomposite

changes de order of the reaction of reduction of hexacyanoferrate (III) to

hexacyanoferrate (II) with borohydride ions from zero-order kinetics to first-order

with respect to the amount of Fe(CN)63−, indicating a change in the reaction

mechanism regarding to the non-catalyzed reaction.

(b) Under pseudo-nth-order conditions, such that the concentration of NaBH4

exceeded the concentration of hexacyanoferrate (III), and maintained de pH of at

NaBH4 solution at 11.5, to avoid competitive reactions, the reaction is first order

with respect to the concentration of borohydride ions, as confirmed by the typical

kinetic trace of the absorbance at 420 nm during the reduction of Fe(CN)63−

highlighting the high-quality, first-order nature of the reaction, for the 2TiO2@Au2

and 1TiO2@Au1 catalysts.

(c) There is a linear relationship between the observed rate constant and the

concentration of TiO2@Au, which indicates that the TiO2@Au nanocomposite acts

as a catalyst involved in the rate–determining step of the model reaction in study.

(d) The dependence of observed rate constant of the reaction, kobs on temperature, for

the TiO2@Au nanocomposites in study, increases linearly with a typical Arrhenius

behavior.

(e) Au nanoparticles supported on titania mesocrystals, TiO2@Au, exhibit higher

catalytic performance than citrate-stabilized gold nanoparticles, Au@Cit, of

similar size, as evidenced by the greater slope in the representation of the observed

rate constant, kobs, as a function of the gold concentration for TiO2@Au and

Au@Cit, under similar experimental conditions, which indicated that titania

played a role in the reaction mechanism. Probably, the catalytic cooperation of the

titania support and gold in TiO2@Au nanocomposites arise by a higher catalytic

surface due to the semiconductor nature of titania and, so that after charging the

gold NPs by borohydride, part of electrons can be transferred to the titania support,

CONCLUSIONS

95

which can also act as a catalyst for the model reaction. Furthermore, in the absence

of gold nanoparticles on titania support the reduction of ferrocyanate (III) by

borohydride ions does not take place.

(f) The nanocomposite of TiO2@Au with small gold nanoparticles (1.4 0.5 nm in

diameter) show larger catalytic activity than the nanocomposite with larger gold

nanoparticles (4.5 1.5 nm) in the reduction of hexacyanoferrate (III) by

borohydride ions, as confirmed by its higher value of the observed pseudo first

order rate normalized per square nanometer of gold surface area. Probably, the

small size of gold nanoparticles facilitates the contact with the titania support and

the rapid electron transfer from gold nanoparticles, charged with electrons by the

borohydride, to the TiO2.

(g) The high density of Au NPs in the titania surface can making this surface less

active for catalysis, such as verified for the 1TiO2@Au1 catalyst, with a high

density of gold nanoparticles (4.5 1.5 nm) over and inside of titania

nanopartículas, as evidenced by TEM, HRTEM images and XPS depth profiles

analysis up to 30 nm, that shows the slowest kinetic in the model redox reaction,

under the same experimental conditions of other TiO2@Au catalysts in study.

(h) The heterostructured TiO2@Au catalysts show reusability, which can be

repeatedly applied for complete reduction of ferrocyanate (III) by borohydride ions

for at least 6 successive cycles (with identical values obtained for constant reaction

observed, kobs, in all cycles of catalysis, under the same reaction conditions). This

demonstrates the stability of the TiO2@Au nanocomposites in the reaction media

as well the reproducibility of its catalytic activity.

(a) From the kinetic results obtained with TiO2@Au nanocomposite and Au@Cit

nanoparticles (greater slope of kobs as a function of gold concentration for the

TiO2@Au than Au@Cit, indicating that titania play a role in the reaction

mechanism), and TiO2 nanoparticles (the characteristic absorption peak of

hexacyanoferrate (III) at 420 nm is constant with time in the presence of TiO2

nanoparticles upon borohydride addition, indicating an inefficient electron transfer

from borohydride ions to the titania nanoparticles) in the redox model reaction in

study, a reaction mechanism was proposed involving:

(a) the cathodic polarization of the metallic gold nanoparticles by sodium

borohydride;

CONCLUSIONS

96

(b) the electrons transfer from gold nanoparticles to the titania semiconductor until

the two system attain equilibrium;

(c) the reduction of ferricyanide ions when reaching the gold nanoparticles surface

or/and the titania surface.

(3) The TiO2 nanoparticles present visible light activity, as proven by the photoreduction

of cationic gold species, Au3+

and Au1+

, on titania support of the TiO2@Au

nanocomposites, obtained after deposition-precipitation with urea and gold salt, under

solar irradiation. The appearance of a plasmon band at 594 nm, in the absorption spectra

of UV-vis-NIR of the TiO2@Au suspension during irradiation with sunlight and the

XPS analysis before and after the photoreduction (a single peak of Au 4f 7/2, centered

at 83.6 eV, corresponding to metallic gold), confirms that electrons photoexcited in the

conduction band of TiO2 by sunlight, reduce the cationic species to metallic species on

the surface of titania.

(4) The TiO2 nanoparticles and the TiO2@Au nanocomposites act as efficient

photocatalysts in degradation of rhodamine B dye in water under direct sunlight

irradiation, as it has been observed by gradual decrease of the characteristic absorption

band at 554 nm. It was found that deposition of gold nanoparticles on TiO2 support

caused a decrease in the photoactivity with respect to anatase titania nanoparticles, as

confirmed by the lower obtained rate constant values, kobs, for photodegradation of

Rhodamine B under solar irradiation in the TiO2@Au nanocomposites, compared with

those obtained for the nanoparticles of titanium (to the same amount of TiO2@Au/TiO2

and the same reaction conditions). In addition, the photocatalytic performance of

TiO2@Au decreases with increase Au content. Such occurs, probably, because the

excess of gold nanoparticles may act as a charge recombination center, or cover the

active sites of titania, or reduces the depth of penetration of sunlight. The high

photocatalytic performance of the anatase TiO2 nanoparticles in the degradation of

rhodamine B dye, under visible-light irradiation, is ascribed to the synergistic effects,

like: (i) larger surface area, providing more surface active sites for the adsorption of

rhodamine B; (ii) porous structure, favoring the harvesting of exciting light due to

enlarged surface area and multiple scattering within the porous framework; and (iii)

visible-light absorption.

97

APPENDIX I

Synthesis and characterization of Au-doped TiO2 nanoparticles

Figure I.1: Length and width of the titania particles: (A) and (B) 1TiO2, and (C) and (D) Experimental

conditions: 20 mL of CH3CO2H and 0.7 or 0.4 mL of Ti(OC4H9)4 for 1TiO2 or 2TiO2, respectively,

and T = 200 ºC for 24 h.

APPENDIX I

98

Figure I.2: XPS spectra of Ti 2p (A), O 1s (B) and C 1s (C) for 1TiO2 sample.

Figure I.3: (A) Nitrogen adsorption-desorption isotherms for 1TiO2 and (B) pore size distribution

obtained from the adsorption branch of the sample 1TiO2.

APPENDIX I

99

Figure I.4: Size histogram of gold NPs. (A) and (B) 1TiO2@Au composites synthesized by the

approach of electrostatic adsorption of AuCl4− on TiO2 surface. In (B) the amount of AuCl4

− ions on the

TiO2 surface was increased by successive additions of HAuCl4.

Figure I.5: Diameter of Au NPs in the TiO2@Au sample synthesized by the approach of adsorption of

(Au(Cl)4−n(OH)n−) ions / Au(OH)3 on TiO2 surface, before (A) and after the growth (B) of the Au

NPs deposited on the 2TiO2 surface.

APPENDIX I

100

Figure I.6: (A) and (B) TEM images of 2TiO2@Au4 composite obtained using 42 10

-2 M of urea.

(C) UV-Visible spectra of 2TiO2 and 2TiO2@Au4 obtained using 42 10-2

M of urea. Experimental

conditions: [HAuCl4. 3H2O] = 4.2 10-3

M, [CO(NH2)2] = 42 10-2

M, [TiO2] = 10.5 mg mL-1

and T

= 90 ºC.

Figure I.7: TEM images of 1TiO2@Au samples obtained using different amounts of 1TiO2

nanoparticles: (A and B) 0.7 mg/mL of 1TiO2 (1TiO2@Au1 sample with Au NPs of 4.5 1.5 nm in

diameter), (C and D) 7.0 mg/mL of 1TiO2 (1TiO2@Au2 sample with Au NPs of 2.0 0.8 nm in

diameter). Experimental conditions: [HAuCl4. 3H2O] = 4.2 10-3

M, [CO(NH2)2] = 42 10-3

M, and

T = 90 ºC.

APPENDIX I

101

Figure I.8: Size distribution of Au NPs in the TiO2@Au samples obtained by DPU method using

different amounts of TiO2 nanoparticles: (A) 1TiO2@Au1 (0.7 mg/mL of 1TiO2); (B) 2TiO2@Au1 (3.5

mg/mL of 2TiO2); (C) 1TiO2@Au2 (7.0 mg/mL of 1TiO2); and (D) 2TiO2@Au2 (10.5 mg/mL of

2TiO2). Experimental conditions: [HAuCl4. 3H2O] = 4.2 10-3

M, [CO(NH2)2] = 42 10-3

M, and T =

90 ºC.

Figure I.9: XPS spectra at the Au 4f energies for 1TiO2@Au1 and 1TiO2@Au2 composites obtained

by DPU method using different amounts of TiO2 nanoparticles: 0.7 mg/mL of 1TiO2 for 1TiO2@Au1,

and 7.0 mg/mL for 1TiO2@Au2. Experimental conditions: [HAuCl4. 3H2O] = 4.2 10-3

M,

[CO(NH2)2] = 42 10-3

M, and T = 90 ºC.

APPENDIX I

102

Figure I.10: XPS depth profile composition of the Au and Ti elements in the 1TiO2@Au1 and

2TiO2@Au2 samples, synthesized via DPU method using 0.7 mg/mL and 10.5 mg/mL of titania

respectively. Experimental conditions: [HAuCl4. 3H2O] = 4.2 10-3

M, [CO(NH2)2] = 42 10-3

M,

and T = 90 ºC.

Scheme I.1: Relation between TiO2 supporting, pH and isoelectronic point of the titania.

APPENDIX I

103

Table I.1: Experimental conditions for the preparations of TiO2@Au composites by DPU and their physicochemical properties.

Composite

[HAuCl4]

mM

[CO(NH2)2]

mM

[TiO2]

mg/mL nm

Au3+

Au1+

Au0

Au Ti O C O/Ti Au/Ti Amount Au

ICP Wt. % Morphology

Area % (in at.%)

1T

iO2

1TiO2@Au1 4.2

42 0.7 4.51.5 59 41 0 15.6 1.0 55.3 28.2 55.3 15.6 20.48

1TiO2@Au2 7.0 2.00.8 0 16 84 0.6 20.8 59.9 18.7 2.9 0.029 8.1

2TiO2@Au0 4.2 42 0.7 17.55.3

2T

iO2

2TiO2@Au1 3.5 4.01.4 54 46 0 12.7 6.2 54.4 26.8 8.8 2.05 15.3

2TiO2@Au2 10.5 1.40.5 0 31 69 0.4 2.0 72.3 25.3 36.1 0.2 4.7

2TiO2@Au3 8.5 12 34 54

2TiO2@Au4 420 10.5 core-shell

APPENDIX I

104

105

APPENDIX II

TiO2@Au catalyzed reduction of ferrycianate (III) by

borohydride ions

Figure II.1: (A) UV-vis absorption spectral evolution of a mixture of hexacyanoferrate (III) and

1Au@TiO1 catalyst upon the addition of sodium borohydride. (B) Kinetic trace of the absorbance at

420 nm during the Fe(CN)63−reduction and corresponding fitting for first order analysis according to

equation (3). Conditions: [Fe(CN)63−] = 8.33 × 10

-4 M, [BH4

− ] = 8.33 × 10-3

M, [1TiO2@Au1] = 2.27 ×

10-5

g mL-1

, T = 25 ºC, pH = 11.5. kobs= 6.9 × 10-2

s-1

.

APPENDIX II

106

Figure II.2: Influence of NaBH4 concentration on the observed rate constant. Conditions: [Fe(CN)63−]

= 8.33 × 10-4

M, [1TiO2@Au1] = 1.33 × 10-5

g mL-1

, T = 25 ºC, pH = 11.5.

Figure II.3: (A) Influence of the amount of 2TiO2@Au2 catalyst (on the observed pseudo-first-order

rate constant. Conditions: [Fe(CN)63−] = 8.33 × 10

-4 M, [BH4

− ] = 8.33 × 10-3

M, T = 25 ºC, pH = 11.5.

Figure II.4: (A) Arrhenius plot of the temperature dependence of the electron transfer reaction in the

presence of 1TiO2@Au1. (B) Plot of ln (kobs/T) vs. 1/T for the electron transfer reaction in the presence

of 1TiO2@Au1 .Conditions: [Fe(CN)63−] = 8.33×10

-4 M, [BH4

− ] = 8.33×10-3

M, [1TiO2@Au1] = 1.34 ×

10-5

g mL-1

, pH = 11.5. From the intercept of Figure II.4 (B) we derived a value for activation entropy,

∆S≠, of 149.14 Jmol-1

K-1

.

APPENDIX II

107

Figure II.5: Absorbance kinetic traces at 420 nm, registered during the sequential reduction of

Fe(CN)63− in the presence of 1TiO2@Au1. Initial concentrations [Fe(CN)6

3−] = 8.33 × 10-4

M, [BH4− ] =

8.33 × 10-3

M, [1TiO2@Au1] = 6.66 × 10-6

g mL-1

, T = 25 ºC, pH = 11.5.

APPENDIX II

108

Figure II.6: (A) UV-visible spectra of citrate-stabilized Au colloids. (B) Representative TEM images

and (C) size distribution of gold nanoparticles.

APPENDIX II

109

Table II.1. Volume of NaBH4 (10 10-2

M), of K3Fe(CN)6 (2 10-3

M), of Milli-Q water and

of 1TiO2@Au1 (2 10-3

g mL-1

) used in the catalysed reactions between Fe(CN)63− and NaBH4

to different borohydride concentrations.

V (mL)

10-3

[NaBH4]f

(M) NaBH4

(10 10-2

M)

H2O

K3Fe(CN)6

(2.0 10-3

M)

1TiO2@Au1

(2.0 10-3

g mL-1

)

0.125 1.625 1.250 0.020 4.16

0.250 1.500 8.33

0.375 1.375 12.5

0.500 1.250 16.7

0.750 1.000 25.0

1.500 0.250 50.0

Table II.2: Rate constant (kobs, s-1



− ions at different concentrations of 1TiO2@Au1 catalyst.

10-6

[1TiO2@Au1] (g mL-1

) 10-3

kobs 10-3

(s-1

)

3.33 8.79 1.82

6.66 17.48 0.596

10.0 29.70 0.848

13.3 34.54 2.90

20.0 55.33 3.11

23.3 69.93 1.84

Table II.3: Specific surface area of Au in the 2TiO2@Au2 catalyst.

[2TiO2@Au2]

g mL-1

aVTotal Au

cm3

DAu

nm

bVAu NPs

cm3

Amount of Au

NPs

cSAu NP

nm2

STotal

nm2

kobs

s-1

3.33 10-6 2.3810-8

1.4

1.44 10-21

1.661013

6.2

1.031014 4.0710-2

6.67 10-6 4.7710-8 3.321013 2.061014 8.0010-2

1.00 10-5 7.1610-8 4.971013 3.081014 9.8110-2

1.33 10-5 9.5210-8 6.611013 4.101014 1.4910-1

2.00 10-5 1.4310-7 9.941013 6.161014 2.2510-1

2.3310-5 1.6710-7 1.161014 7.181014 2.64 10-1

a The total volume of gold was calculated as: gold mass ([2TiO2@Au2] 3 mL amount of Au by

ICP) was divided by the gold density (19.7 g cm-3

). b

The Au nuclei were assumed as sphere and the

volume of a single Au nuclei is 4/3 R3 and the

c surface area is 4 R

2.

APPENDIX II

110

Table II.4: Specific surface area of Au in the 1TiO2@Au1 catalyst.

[1TiO2@Au1]

g mL-1

aVTotal Au

cm3

DAu

nm

bVAu NPs

cm3

Amount of Au

NPs

cSAu NP

nm2

STotal Au

nm2

kobs

s-1

3.33 10-6 1.0410-7

4.5

4.77 10-20

2.181012

63.6

1.381014 8.7910-3

6.67 10-6 2.0810-7 4.361012 2.771014 1.7510-2

1.00 10-5 3.1210-7 6.541012 4.161014 2.9710-2

1.33 10-5 4.1510-7 8.701012 5.531014 3.4510-2

2.00 10-5 6.2410-7 1.311013 8.321014 5.5310-2

2.3310-5 7.2710-7 1.521014 9.691014 6.9910-2

a The total volume of gold was calculated as: gold mass ([1TiO2@Au1] 3 mL amount of Au by

ICP) was divided by the gold density (19.7 g cm-3

). b

The Au nuclei were assumed as sphere and the

volume of a single Au nuclei is 4/3 R3 and the

c surface area is 4 R

2

Table II.5: Rate constants (kobs, s-1



− ions at different temperatures (283 - 303 K).

T (K)

1TiO2@Au1

10-3

kobs 10-4

(s-1

)

283 3.35 1.63

286 4.08 0.71

288 6.34 7.0

290 7.25 2.81

293 16.9 11.4

298 42.5 4.35

303 98.6 123.27

Ea (kJ/mol)

135.95 ± 0.59

111

APPENDIX III

Photochemical activity of TiO2 / TiO2@Au nanostructures

Figure III.1: Size distribution of Au NPs before and after solar irradiation of the 2TiO2@Au1 sample

Figure III.2: UV–vis spectral evolution of RhB solution under sunlight irradiation.

Conditions: [RhB] = 1.0 × 10−5 mol dm−3, sunlight.

APPENDIX III

112

Table III.1: XPs elemental chemical compositions of the 1TiO2 and 2TiO2 samples (in at.%).

Sample Ti (at.%) C (at.%) O (at.%)

1TiO2 28.1 8.3 63.6

2TiO2 31.0 7.0 62.0

113

RESUMEN

Nanocompuestos de TiO2 y metales nobles: síntesis, caracterización y

actividad catalítica

En este resumen se pretende ofrecer una visión global del trabajo presentado en los distintos

capítulos de la presente tesis. En primer lugar se han diseñado las rutas para la obtención de

nanopartículas tipo núcleo-corteza de Au@TiO2 y de partículas de TiO2 dopadas con

nanopartículas de oro, TiO2@Au. Seguidamente, se investigó el efecto catalítico de la

presencia de nanopartículas de TiO2@Au en la reacción de reducción de iones

hexacianoferrato (III) por iones borohidruro en agua. Se realizaron estudios sobre la

influencia de la variación de ciertas propiedades del catalizador, concretamente, el tamaño y

densidad de las nanopartículas de oro en el suporte de TiO2. También se analizaran los efectos

de parámetros como la temperatura o cantidad de catalizador y de iones de borohidruro.

Además, se ha comparado la actividad catalítica de las nanopartículas de TiO2@Au con la de

nanopartículas de TiO2 y de oro estabilizadas en citrato de tamaño (4 nm de diámetro) similar

al de las nanopartículas de oro depositado en el nanocompuesto de TiO2@Au. En base en los

datos cinéticos obtenidos se propuso un mecanismo para el proceso catalítico.

Finalmente se han llevado a cabo estudios de fotocatálisis heterogénea con las partículas de

TiO2 y/o TiO2@Au empleando como reacción modelo la degradación del colorante textil

rodamina B usando luz solar.

RESUMEN

114

6.1. Objetivos

Basándose en la experiencia del grupo de investigación de Química Coloidal en la síntesis de

coloides metálicos con control de tamaño y forma, y en la propiedades catalíticas de

nanopartículas de oro con tamaños nanométricos y TiO2, se ha querido, en primer lugar,

sintetizar nanopartículas de oro y recubrir con una capa uniforme de dióxido de titanio,

Au@TiO2, de espesor controlado. Seguidamente, se han sintetizado nanopartículas de TiO2

con gran área superficial y dopadas con oro, TiO2@Au, y se ha estudiado su actividad

catalítica empleando la reducción de iones hexacianoferrato (III) por iones borohidruro como

reacción modelo, con el fin de entender la importancia del suporte de TiO2 en el proceso

catalizado por el Au. Finalmente, se ha analizado la actividad fotocatalítica de las

nanopartículas de TiO2 y los nanocompuestos de TiO2@Au empleando la fotodegradación del

colorante textil rodamina B como reacción modelo.

En este contexto, en el Capítulo 1, se presenta una breve introducción general para abordar la

temática relacionada con las nanopartículas metálicas, desde sus propiedades ópticas, pasando

por la teoría de Mie, hasta la catálisis. También se aborda, brevemente, las principales

propiedades del dióxido de titanio y el uso de nanopartículas de oro suportadas en el TiO2 en

catálisis y fotocatálisis.

En el Capítulo 2, se describe la síntesis de nanopartículas tipo núcleo-corteza (comúnmente

denominadas core-shell) de Au@TiO2 empleando diferentes aproximaciones y se presenta su

caracterización estructural y óptica. Como se sabe, una de las principales limitaciones de las

nanopartículas es su estabilidad coloidal. Es por ello, que para su aplicación tecnológica a

menudo son depositadas sobre un sustrato; o son dispersadas en soluciones con agentes

anti-aglomerantes; y/o son recubiertas con una corteza de naturaleza polimériza o inorgánica.

Además, el recubrimiento de las nanopartículas puede dar lugar a nuevos sistemas

hetero-estructurados con nuevas propiedades o propiedades mejoradas resultantes de la

combinación de más de un material. En el presente estudio, se hicieron tres rutas diferentes

para el recubrimiento de las nanopartículas de oro con dióxido de titanio. Sólo fue posible

obtener nanopartículas de oro con una capa homogénea de dióxido de titanio mediante un

enfoque basado en la combinación de la técnica de “capa por capa” y la hidrólisis y

condensación del precursor de dióxido de titanio en medio alcohólico. La capa de óxido de

titanio se puede ajustar por la adición sucesiva de precursor de titanio a los nanocompuestos

RESUMEN

115

de Au@TiO2. El control del espesor de la capa de titania se podría utilizar para controlar las

interacciones dipolares entre las partículas y mejorar la versatilidad de las mismas.

En el Capítulo 3, se presenta, la síntesis de nanopartículas porosas de TiO2 con dos tamaños

diferentes y su caracterización estructural y óptica. A continuación, se explora la deposición

homogénea de nanopartículas de oro, con diámetros comprendidos entre 2 y 5 nm, sobre la

superficie del dióxido de titanio, así como su caracterización estructural y óptica. Para tal fin,

se analizaron tres métodos distintos, siendo el método que combinaba la técnica de

deposición-precipitación con urea, DPU, y la reducción química con borohidruro el que

permitió obtener una distribución más homogénea de nanopartículas de oro sobre la superficie

del TiO2 y controlar su tamaño variando la cantidad de partículas de TiO2 usadas durante la

DPU.

En el Capítulo 4, se estudia la actividad catalítica de los nanocompuestos de TiO2@Au

empleando como reacción modelo la reducción de iones hexacianoferrato (III) por iones

borohidruro en medio acuoso. Se estudió la influencia del tamaño y densidad de las

nanopartículas de oro en el suporte de TiO2 durante el proceso catalítico. También se

analizaron los efectos en la actividad catalítica de parámetros como la: temperatura o la

cantidad de catalizador y borohidruro. Además, se comparó la actividad catalítica de las

nanopartículas de TiO2@Au con la de nanopartículas de TiO2 y de oro estabilizadas en citrato

de tamaño (4 nm de diámetro) similar al de las nanopartículas de oro depositado en el

nanocompuesto de TiO2@Au. En base en los datos cinéticos obtenidos se propuso un

mecanismo para el proceso catalítico.

En el Capítulo 5, se emplean las nanopartículas de TiO2 y los nanocompuestos de TiO2@Au,

sintetizadas anteriormente, para estudiar la degradación fotocatalítica del colorante rodamina

B, RB, bajo la luz solar directa. Se quiso demonstrar la calidad de las nanopartículas de TiO2

y TiO2@Au como fotocatalizadores empleando luz solar, dada su gran capacidad de

absorción en la zona del visible. También se comprobó la reducción fotoquímica de los

cationes de oro, Au3+

y Au1+

, presentes en las hetero-estructuras de TiO2@Au obtenidas por

DPU y antes de su reducción con borohidruro.

Finalmente, se resumieron las conclusiones generales más relevantes que se desprenden del

trabajo realizado.

RESUMEN

116

6.2. Introducción

En general, cuando se reduce a la escala nanométrica cualquier material, surgen nuevas

propiedades ópticas, catalíticas y magnéticas, entre otras, distintas a las del material masivo.

En el caso particular de los metales y más concretamente del oro, plata, o cobre, el

confinamiento de los mismos en la nanoescala da lugar a una modificación drástica de sus

propiedades ópticas, fundamentalmente en la región visible del espectro electromagnético. El

origen de dicha respuesta óptica se encuentra en la oscilación coherente de los electrones de la

banda de conducción del metal, por interacción con la radiación electromagnética incidente,

que se conoce como resonancia de plasmón superficial localizada. Dichas resonancias

plasmónicas son altamente sensibles al tamaño y morfología de las nanopartículas, así como

también a la distancia entre partículas o las propiedades dieléctricas del medio que las rodea.

Esto hace que estos materiales presenten infinidad de posibilidades tecnológicas en

diagnóstico y terapia médica, tecnología de almacenamiento de dados, biosensores,

optoelectrónica, catálisis, fotocatálisis, entre muchas otras.

El dióxido de titanio es uno de los materiales semiconductores más estudiado debido a sus

numerosas aplicaciones, que van desde celdas solares a (foto)catálisis, pasando por la

industria de pinturas y cosméticos, implantes de huesos, y desinfección de aguas, entre

muchas otras. De todas sus aplicaciones, nos interesa conocer su comportamiento y

características como soporte de metales para el estudio de reacciones catalíticas y

fotocatalíticas en su superficie.

En un proceso fotocatalítico la absorción de fotones, con una energía mayor que la banda

prohibida, band gap, del semiconductor da lugar a la excitación de los electrones desde la

banda de valencia hasta la banda de conducción generando el par electrón-hueco. Una vez

creado el par, este puede ser capturado por especies adsorbidas en la superficie, por impurezas

contenidas en el sistema, o por especies que están en fase gaseosa en contacto con la

superficie. Por ejemplo, el electrón en la banda de conducción puede ser capturado por

moléculas de oxígeno, mientras el hueco en la banda de valencia puede ser capturado por las

especies OH− o H2O, adsorbidas en la superficie del catalizador, generando radicales

hidroxilos. La recombinación electrón-hueco es un proceso que suele competir con la

transferencia de carga a las especies adsorbidas. La introducción de metales, como por

ejemplo el oro, o no metales, como por ejemplo el carbono, entre otros, es capaz de disminuir

o eliminar la recombinación electrón-hueco, visto que estas impurezas crean niveles

RESUMEN

117

intermedios, entre la banda de valencia y la banda de conducción, que actúan como trampa, y

posibilitan, además, la absorción de fotones de menores energías. Otros factores que

determinan la eficiencia del catalizador son la selectividad, la actividad y la estabilidad, así

como poseer una cuantidad suficiente de sitios activos en un área superficial de dimensiones

adecuadas y expuestos a las especies reactivas.

El trabajo de investigación presentado en esta memoria está centrado en la fabricación y

caracterización de nanopartículas de oro soportados en dióxido de titanio, TiO2@Au, y en su

aplicación en la catálisis de la reacción de transferencia electrónica entre los iones de

hexacianoferrato (III) y los iones de borohidruro en agua. Este estudio visa ampliar el

conocimiento existente de este tipo de sistemas en catálisis. En general, los materiales de

TiO2@Au son catalizadores heterogéneos porque el catalizador y los reactivos se encuentran

en fases distintas, en este caso, un catalizador sólido que acelera una reacción en fase acuosa.

Esto implica, que las reacciones catalíticas ocurren en las superficies de los catalizadores. Por

eso, la naturaleza de los átomos de estas superficies determinan cuan rápidas y de qué manera

ocurren estas reacciones. La facilidad de los reactivos en alcanzar la superficie catalizadora

para ser quimisorbidos con el mínimo de energía, o transferir electrones, favorece os procesos

catalíticos en los nanomateriales, llevando dichas reacciones por un camino de reacción de

menor energía. Así que conocer las propiedades estructurales y electrónicas de estos sistemas

es fundamental para entender los complejos procesos que tienen lugar durante la catálisis.

6.3. Fabricación de Nanopartículas núcleo-corteza de oro y dióxido de

titanio.

En el Capítulo 2, se estudian diferentes rutas para el recubrimiento de nanopartículas de oro,

de aproximadamente 66 nm de diámetro, con una capa homogénea de dióxido de titanio y

controlar su espesor. El proceso experimental consta de dos partes bien diferenciadas. Una

primera, donde se lleva a cabo la formación de semillas de oro de aproximadamente 15 nm de

tamaño y su crecimiento hasta unos 66 nm, siguiendo el proceso descrito por

Rodríguez-Fernández et al, y una segunda, donde se recubren las NPs de Au, estabilizadas en

bromuro de hexadeciltrimetilamonio, CTAB, con TiO2. De las tres rutas analizadas, sólo la

metodología que combina la técnica de “capa por capa” (deposición de polielectrolitos y PVP

mediante fuerzas electrostáticas sobre la superficie de las nanopartículas de oro) y la hidrólisis

y condensación de un precursor de dióxido de titanio ha permitido el recubrimiento

RESUMEN

118

homogéneo de las NPs de Au con titania. La presencia del surfactante catiónico, CTAB, en

exceso dificulta el recubrimiento homogéneo con TiO2 cuando se intenta recubrir las

nanopartículas simplemente empleando la hidrólisis y condensación de un precursor de

dióxido de titanio. Por eso primero se enmascara el surfactante por medio de la adsorción de

polielectrolitos sobre la superficie de las partículas de oro. El CTAB confiere à las

nanopartículas de oro una carga superficial positiva, por lo cual la deposición de sulfonato

sódico de poliestireno, PSS, un polielectrolito cargado negativamente, está fuertemente

favorecida. La capa de PSS invierte la carga de la superficie del oro y por lo tanto las

nanopartículas se pueden recubrir nuevamente con un polielectrolito con carga positiva como

el poli(hidrocloruro de alilamina), PAH. Seguidamente, las nanopartículas son

funcionalizadas con polivinilpirrolidona, PVP, para la redispersión en medio alcohólico. De

esta forma, es posible depositar una capa uniforme de TiO2 a través de la adición de butóxido

de titanio, TBT, a la solución alcohólica de Au@CTAB@PSS@PAH@PVP a temperatura

ambiente y bajo atmósfera inerte (el precursor de titanio es altamente reactivo en presencia de

trazas de agua). Para aumentar el espesor de la capa de titania, las hetero-estructuras de

Au@CTAB@PSS@PAH@PVP@TiO2 fueron sujetas a dos nuevos procesos de hidrólisis y

condensación del TBT. Las imágenes de TEM de las nanopartículas núcleo-corteza de

Au@TiO2, Figuras 6.1 (B) y 6.1 (D), confirmaron el aumento de espesor de la capa de

dióxido de titanio durante sus etapas de crecimiento. También, en la Figura 6.2 (B), el

desplazamiento continuo hacia el rojo de la banda de absorbancia debido a la resonancia de

plasmon superficial (SPR) y los aumentos del ancho de la banda SPR confirman el aumento

de espesor de la capa de TiO2 en las nanopartículas de Au@TiO2. Durante la adsorción de los

polielectrolitos no se observó cambio significativos en las propiedades ópticas de las

partículas (ver Figura 6.2 (A)).

Figura 6.1: Imágenes de TEM de nanopartículas de oro con aproximadamente 66 nm de diámetro (A)

y de nanopartículas tipo núcleo-corteza de Au@TiO2 con capas homogéneas de titania de espesor

controlado: 5 nm (B); 20 nm (C); y 30 nm (D).

RESUMEN

119

Figura 6.2: Espectros de absorción Vis-NIR de dispersiones acuosas de nanopartículas de oro

recubiertas con CTAB, PSSS, PAH y PVP (A), y nanopartículas de oro en diferentes etapas del

recubrimiento de dióxido de titanio (B). Los espectros se normalizaron en el máximo de absorción

para facilitar comparación.

En este capítulo además se analizan dos métodos más para la síntesis de nanopartículas

núcleo-corteza de Au@TiO2 basados en la hidrólisis de precursores de TiO2 directamente

sobre coloides de oro estabilizados con CTAB o PVP. Los resultados obtenidos permitieran

concluir que el tensioactivo catiónico CTAB y el polímero anfifílico PVP hacen que la

afinidad del oro para el TiO2 sea suficientemente alta, por lo que, las partículas de titania

hidrolizados se depositan directamente sobre la superficie de coloides de Au@CTAB o

Au@PVP durante el procedimiento de revestimiento con dióxido de titanio. Sin embargo, es

muy difícil tener la cantidad ideal de CTAB o de PVP en el medio de modo a obtener un

recubrimiento homogéneo.

7.4. Síntesis y caracterización de nanopartículas de TiO2 dopadas con oro

En el Capítulo 3, nanopartículas de TiO2 anatasa en forma de huso, con dos tamaños

diferentes (175.7 ± 38.6 de largo y 116.8 ± 28.8 nm de ancho para el 1TiO2, y 303.8 ± 72.8

nm de largo y 226.4 ± 48.0 nm de ancho para el 2TiO2) fueron sintetizados en gran escala

mediante un método solvotermal a 200 ºC. Se empleó como precursor el butóxido de titanio y

como disolvente el ácido acético y no se adicionaron aditivos. Se ha observado que el tamaño

y área específica de las partículas de dióxido de titanio dependían de la cantidad de precursor

de titanio. De este modo la muestra 2TiO2, sintetizada con menor cantidad de precursor de

titanio para igual volumen de ácido acético, presentaba mayor tamaño y menor área

específica. Las propiedades estructurales, fisicoquímicas y morfológicas de las nanopartículas

de TiO2 obtenidas fueron examinadas por microscopía electrónica de transmisión (TEM),

RESUMEN

120

dispersión dinámica de luz (DLS), el método de Brunauer-Emmett-Teller (BET),

espectroscopia fotoelectrónica de rayos X (XPS), y espectroscopia de absorción UV-vis.

Como bien se sabe, el área de superficie específica, el tamaño y forma del poro de las

nanopartículas de TiO2 son factores importantes en la catálisis. Por lo tanto, se llevaron a cabo

estudios de adsorción-desorción de nitrógeno para determinar tales parámetros. Las isotermas

de adsorción-desorción obtenidas para el 1TiO2 y el 2TiO2 fueron del tipo IV, con un ciclo de

histéresis tipo H3, características de materiales mesoporosos con una distribución no uniforme

de tamaño y forma de poro.

En lo referente a las propiedades ópticas, se observó que ambas muestras, 1TiO2 y 2TiO2,

absorbían radiación UV en longitudes de onda inferiores a 400 nm, este efecto está asociado

con la promoción de los electrones de la banda de valencia a la banda de conducción del TiO2,

con un máximo de absorción a ~ 335 nm. Además la absorción se extendía en la zona del

visible y el infrarrojo cercano (ver figura 6.3 (A)). Se evaluó la energía de banda prohibida o

band gap, Eg, del semiconductor a través de la representación espectral de la función

Kubelka-Munk (Figura 6.3 (B)), suponiendo transiciones indirectas. De este modo se obtuvo

un valor de Eg de 1.99 eV y de 1.92 eV para las partículas de 1TiO2 y 2TiO2, respectivamente.

Esta absorción de las muestras de TiO2 en el rango visible es probablemente debido a una

combinación de diferentes efectos, tales como: defectos en la red, la incorporación de carbono

y la presencia de una pequeña cantidad de Ti3+

en los mesocristales de TiO2. Mientras que los

defectos en la red se caracterizaron por microscopia electrónica de transmisión por barrido de

campo oscuro anular de alto ángulo (STEM-HAADF), la presencia de carbono y Ti3+

fueron

confirmadas por XPS.

Figure 6.3: (A) Espectros de absorción UV-vis-NIR de los coloides de 1TiO2 y 2TiO2. Los espectros

se normalizaron a la absorbancia máxima para facilitar comparación. (B) Relación de (αℎʋ)1

2⁄ vs

energía de la luz para las muestras de TiO2.

RESUMEN

121

Posteriormente, ambas nanopartículas de TiO2 fueren dopadas con oro, TiO2@Au, a través

del método de deposición-precipitación con urea (DPU). Así se prepararon diferentes

muestras manteniendo constante la cantidad de sal de Au y variando la cantidad de partículas

de TiO2. El análisis de las diferentes muestras por XPS mostraron que aumentando la

cuantidad de titania usada en el proceso de DPU aumenta la cuantidad de oro metálico, frente

a cationes Au+ y Au

3+, presente en la superficie de las partículas semiconductoras (ver Figuras

6.4 (A) –(C)). Esto ocurre probablemente porque los centros de Ti3+

presentes en TiO2, datos

suportados por XPS, son capaces de reducir los cationes de oro a Au0, y por eso el pico a

457.3 eV correspondiente a Ti3+

deja de observarse en los espectros de XPS. La completa

reducción de los cationes de Au tras la deposición-precipitación con urea se realizó mediante

la adición de borohidruro a los nanocompuestos de TiO2@Au tal y como se demostró por

XPS (ver Figura 6.4 (D)).

Los nanocompuestos de TiO2@Au obtenidos con el 1TiO2 y el 2TiO2 presentan características

similares en lo que respecta al tamaño y densidad de nanopartículas de oro, y por eso se hay

decidido presentar apenas los datos de una amuestra, 2TiO2, colocando los restantes

resultados en el anexo. Los nanocompuestos preparados se denominaron de 2TiO2@Aux,

donde X representa el contenido de oro, como una función de la cantidad de TiO2 utilizado la

etapa de DPU. De esto modo: X = 0[2TiO2]= 0.7 mg/mL, X = 1[2TiO2]=3.5 mg/mL,

X=2 [2TiO2]=10.5 mg/mL (ver Figura 6.4). Se ha observado que el porcentaje de

nanopartículas de oro y su diámetro, sobre el soporte de TiO2, disminuye al aumentar la

cuantidad de TiO2 empleado en el proceso de DPU (ver Figura 6.5).

RESUMEN

122

Figura 6.4: Espectros de XPS del nivel Au 4f de los nanocompuestos de 2TiO2@Au preparados por

deposición-precipitación con urea y antes de su reducción con borohidruro (A-C). (A) [2TiO2] = 3.5

mg/mL; (B) [2TiO2] = 8.5 mg/mL; (C) [2TiO2] = 10.5 mg/mL. (D) Espectro de XPS del nivel Au 4f

de la muestra mostrada en A después de la reducción con NaBH4.

En este capítulo, se analizan además otros dos métodos de preparación de los nanocompuestos

de TiO2@Au, los cuales están basados en la adsorción de especies de oro, AuCl4− o

Au(Cl)4-n(OH)n− sobre la superficie del TiO2 y su posterior reducción con borohidruro sódico.

Estos métodos se basan en el hecho de que los coloides de TiO2 sintetizados exhiben un

potencial zeta positivo, + 38.5 7.58 mV, por lo que el dopaje con NPs de oro a través de la

adsorción electrostática de especies de oro con carga negativa, AuCl4− o Au(Cl)4-n(OH)n

−, en

la superficie de TiO2, es favorecida. Estés métodos no han permitido aumentar y controlar el

tamaño de las nanopartículas de oro ni su densidad sobre la superficie de TiO2.

RESUMEN

123

Figura 6.5: Imágenes de TEM de (A) 2TiO2 y 2TiO2@Au obtenidos usando diferentes cantidades de

NPs de 2TiO2: (B) 0.7 mg/mL (muestra 2TiO2@Au0); (C) 3.5 mg/mL (muestra 2TiO2@Au1), y (D)

10.5 mg/mL (muestra 2TiO2@Au2). El tamaño promedio de las NPs de oro en las muestras de

2TiO2@Au0, 2TiO2@Au1, 1TiO2@Au2 es 17.5 ± 5.3 nm, 4.0 ± 1.4 nm y 1.4 ±0.4 nm, respectivamente.

6.5. TiO2@Au como catalizador en la reducción de ferrocianato (III) por

iones borohidruro

En el Capítulo 4, se estudia la actividad catalítica de los nanocompuestos de TiO2@Au

preparados en el Capítulo 3 empleando la reducción de hexacianoferrato (III) por iones

borohidruro como reacción modelo. Esta reacción fue seguida, por espectroscopia de

absorción UV-vis, vía diminución de la banda de absorción del hexacianoferrato (III) a 420

nm. Se ha verificado que todos los nanocompuestos de TiO2@Au actúan como eficientes

catalizadores en la reducción de ferrocianato por iones borohidruro aumentando altamente la

velocidad de reacción respecto de la reacción sin catalizar. El estudio de la reacción fue

llevado a cabo usando una concentración de iones borohidruro en exceso respecto a

hexacianoferrato (III), de modo que la concentración de iones de borohidruro se pueda

considerar constante. También, se trabajó a pH 11.5 para inhibir la hidrólisis del borohidruro

en agua. En estas condiciones, los resultados muestran que el orden de reacción con respecto a

la concentración de borohidruro, [NaBH4] y de catalizador, [TiO2@Au], es de primer orden.

Además se calculó la energía de activación para la reacción catalizada por 1TiO2@Au1 y por

RESUMEN

124

2TiO2@Au2 obteniendo unos valores de 135.95 ± 0.59 kJ mol-1

y 98.64 ± 0.96 kJ mol-1

,

respectivamente.

El efecto del soporte de TiO2 en la constante de velocidad observada de pseudo primer orden

se analizó comparando las constantes de velocidad observadas, kobs, obtenidas para el

nanocompuesto de 1TiO2@Au1 y para NPs de Au estabilizadas con citrato, Au@Cit, con

diámetro similar al de las NPs de oro presentes en 1TiO2@Au1, 4.0 0.7 nm. Para una mejor

comparativa se representó las kobs un función de la concentración de oro (ver Figura 6.6 (B).

Los resultados obtenidos mostraron que las nanopartículas de Au soportadas sobre TiO2

exhiben un mayor rendimiento catalítico que los coloides de oro estabilizados con citrato,

indicando que el soporte de TiO2 tiene un papel en el mecanismo de reacción. Probablemente,

la cooperación catalítica del TiO2 y oro, en el nanocompuesto TiO2@Au, surge por el

aumento de la superficie catalítica debido a la naturaleza semiconductora del TiO2. Es decir,

después de cargar con electrones las NPs de oro mediante la adición de borohidruro, parte de

los electrones se pueden transferir al soporte de óxido de titanio, qué puede así también actuar

como catalizador para el modelo de reacción en estudio. Además se verificó que bajo nuestras

condiciones experimentales, el borohidruro no consigue inyectar electrones en las

nanopartículas de TiO2 sin dopar ya que los iones de ferrocianuro no se reducen (ver Figura

6.6(A)). Así, las nanopartículas de oro, presentes en los nanocompuestos de TiO2@Au son la

única especie receptora de los electrones del borohidruro.

Figure 6.6: (A) Traza cinética de la absorbancia a 420 nm durante la reducción de Fe(CN)63− en

presencia de TiO2 ((línea continua) y TiO2 dopado con Au (1TiO2@Au1, línea discontinua). (B)

Estudio comparativo de la influencia de la cantidad de Au en la constante de velocidad observada para

la reducción de Fe (III) en presencia de dos catalizadores diferentes 1TiO2@Au1 y Au@Citrato.

Condiciones: [Fe(CN)63−] = 8.33 × 10

-4 M, [BH4

− ]= 8.33 × 10-3

M, T = 25 ºC, pH = 11.5.

El mecanismo de reacción propuesto se define del siguiente modo. Inicialmente el

borohidruro carga negativamente la superficie metálica de las nanopartículas de oro

RESUMEN

125

soportadas en el TiO2. En esta situación, su nivel de Fermi se desplaza a valores más

negativos, acercándose al borde de la banda de conducción del TiO2, permitiendo así la

transferencia de electrones desde el metal a la banda de conducción del TiO2 hasta que haya

equilibrio de cargas en el sistema de TiO2@Au (ambos los niveles de Fermi se igualan).

Posteriormente, los electrones presentes en la superficie del TiO2 y del oro son capaces de

reducir el hexacianoferrato (III). Se ha observado que la disminución de densidad de NPs de

oro en los nanocompuestos de TiO2@Au aumenta ligeramente su capacidad catalítica. Esto

puede ser debido a la diminución de la cantidad de nanopartículas de oro sobre la superficie

del TiO2, es decir, al aumento de área superficial de TiO2 libre, lo que aumenta la

probabilidad de contacto entre los iones de hexacianoferrato (III) y la superficie del TiO2.

Consecuentemente, aumenta a superficie catalítica, que ahora puede ser la del oro y a del

TiO2. Cuando el TiO2 se encuentra completamente dopado con NPs de oro, como en el caso,

por ejemplo, del catalizador 2TiO2@Au0, se observa un apantallamiento de la superficie del

TiO2 (ver Figura 6.5 (A)), disminuyendo así la probabilidad de reducción del Fe3+

por la

superficie del TiO2, por lo que la reducción se hace esencialmente vía superficie de las

nanopartículas de oro.

En lo que se refiere al efecto del tamaño de las nanopartículas de oro en los nanocompuestos

de TiO2@Au, se ha visto que la mayor capacidad catalítica se obtuvo para los

nanocompuestos de TiO2@Au con nanopartículas de oro de 1.4 0.5 nm de diámetro. Estas

pequeñas dimensiones facilitan el contacto entre el Au y el TiO2, y por otro, la rápida

transferencia de electrones del Au al TiO2 al ser cargadas con electrones por el borohidruro.

Finalmente también se ha estudiado la capacidad de reusabilidad de los catalizadores de

TiO2@Au empleando la misma reacción modelo. Los resultados no mostraron una reducción

importante en la eficacia catalítica de los catalizadores de TiO2@Au incluso después de seis

ciclos (ver Figura 6.7) lo que indica la excelente estabilidad de estos nanocompuestos.

RESUMEN

126

Figure 6.7: Trazas cinéticas de la absorbancia a 420 nm, registradas durante la reducción secuencial

de ferrocianato (III) por iones borohidruro en presencia de 2TiO2@Au2. Concentraciones iniciales

[Fe(CN)63−] = 8.33 × 10

-4 M, [BH4

− ] = 8.33 × 10-3

M, [2TiO2@Au2] = 3.33 × 10-6

g mL-1

, T = 25 ºC,

pH = 11.5.

6.6. ESTUDIOS FOTOQUÍMICOS CON NANOPARTÍCULAS DE TiO2 Y

TiO2@Au

En el Capítulo 6, se investigó la capacidad catalítica del TiO2, utilizando la luz solar como

fuente de irradiación, en la fotorreducción de las especies catiónicas de Au presentes en la

superficie de TiO2 tras la deposición-precipitación con urea de una sal de Au. También se

estudió la eficiencia de las nanopartículas de TiO2 y de TiO2@Au en la fotodegradación de la

rodamina B en solución acuosa.

De este modo se observó que cuando la suspensión acuosa de TiO2 dopado con cationes Au+

y Au3+

, contenida en un vial de vidrio cerrado, era expuesta a la luz solar, los electrones

fotoexcitados, en la banda de conducción de TiO2 eran capaces de reducir las especies

catiónicas a Au0. El proceso fue seguido por espectroscopia de absorción UV-Vis

observándose la aparición de una banda plasmónica a 594 nm que indicaba la presencia de

NPs de Au (ver Figura 7.7A). Estos resultados coinciden con los obtenidos en la

caracterización del nanocompuesto de 2TiO2@Au1, mediante XPS (ver Figura 6.7 (C) y (D)),

antes y después de la fotoreducción. Tras la fotoreducción aparece un único pico de Au 4f7/2,

centrado a 83.6 eV, correspondiente a oro metálico. También, la mudanza de color de la

RESUMEN

127

suspensión acuosa de TiO2@Au de amarillo para rosa (ver Figura 6.8 (B)) comprueba la

formación de NPs de oro.

Figura 6.8: (A) Evolución de los espectros de absorción UV-vis-NIR de una dispersión de

nanopartículas de 2TiO2@Au1 durante la irradiación con luz solar. (B) Esquema de la fotorreducción

de Au3+

y Au1+

a Au0

catalizada por las nanopartículas de TiO2 en los nanocompuestos de TiO2@Au

obtenidos por deposición-precipitación con urea (e- = electrones fotoexcitados, h

+ = huecos

fotogenerados). (C-D) Espectros de XPS del nivel Au 4f de 2TiO2@Au1 antes (C) y después (B) de

irradiación con luz solar durante 210 minutos.

Se investigó también la fotodegradación del colorante textil rodamina B con luz solar en

presencia de nanopartículas de TiO2 y TiO2@Au. El proceso se estudió por espectroscopia de

absorción UV-vis-NIR siguiendo la evolución de la banda de absorción característica de la

rodamina B a 554 nm. Así esta banda disminuía gradualmente en presencia de TiO2 o de

TiO2@Au (ver Figura 6.9) indicando que ambos actúan como eficientes fotocatalizadores en

la degradación de la RB por irradiación con luz solar. También, se notó un desplazamiento

hace al azul, indicando la N-des-metilación de la RB. Además la disminución de la

absorbancia a 554 nm en condiciones de oscuridad indicaba que las moléculas de RB era

pre-adsorbidas en la superficie de TiO2 y de TiO2@Au (ver Figuras 6.9 (A)-(D)).

RESUMEN

128

Así los estudios de la actividad catalítica efectuados mostraron que las nanopartículas de TiO2

cuando están dopadas con oro presentan un comportamiento cinético más lento en la

fotodegradación de la RB (ver Figura 6.9 (F)). También se observó que aumentando la

densidad de nanopartículas de oro sobre la superficie del TiO2, como en el caso del

fotocatalizador 1TiO2@Au1, la cinética de degradación del colorante RB es más lenta. Tal

ocurre, probablemente, porque el exceso de nanopartículas de oro funciona como centros de

recombinación de carga, o cubre los sitios activos del TiO2, o reduce la profundidad de

penetración de la luz solar.

El mecanismo sugerido para la descomposición fotocatalítica de la RB en presencia de TiO2

consiste, simplificadamente, en la acumulación de electrones en la banda de conducción de

TiO2, inyectados por la fotoexcitación del colorante textil de RB, TiO2{RB}ads∗ y del TiO2,

que entonces reaccionan con el oxígeno reduciéndolos a aniones radicales superóxidos, O2∙−,

responsables por la generación del radical hidroxilo, OH∙. También los huecos, fotogenerados

en el proceso de fotoexcitación del TiO2, son responsables por la formación de OH∙. A

continuación, el colorante rodamina B reacciona con los radicales OH∙ y O2∙− generando una

gama de productos intermedios seguido por su completa mineralización.

La importancia de este estudio reside en las excepcionales propiedades de las nanopartículas

de TiO2 en la región visible, que permiten su uso en fotocatálisis solar para resolución de

problemas de contaminación de aguas con materiales no biodegradables.

Para concluir, probablemente, a alta actividad fotoreductora y fotocatalítica de las

nanopartículas de TiO2 con luz solar podría atribuirse a efectos sinérgicos como: (i) la

presencia de carbono en los mesocristales de dióxido de titanio, que induce la absorción de

luz visible; (ii) su gran área de superficie, que proporciona más posiciones de adsorción para

las especies reactivas; y (iii) su estructura porosa, que permite múltiples reflexiones de la luz

visible en el interior de las partículas de dióxido de titanio, lo que hace un uso más eficiente

de la luz visible.

RESUMEN

129

Figure 6.9: Evolución espectral de una mezcla de rodamina B y (A) 1TiO2, (B) 2TiO2, (C)

1TiO2@Au1 y (D) 2TiO2@Au2 en diferentes tiempos de irradiación de luz visible.

RESUMEN

130

6.7. Conclusiones

A continuación se recogen las conclusiones más relevantes que se han obtenido en la presente

tesis.

1. Se han sintetizado nanopartículas núcleo-corteza de Au@TiO2 mediante el

recubrimiento de nanopartículas de oro presintetizadas con una capa homogénea de

TiO2 a través de una metodología que combina la técnica de “capa por capa” con la

hidrolisis y condensación del precursor de titanio en medio alcohólico. Además la

adición secuencial del precursor de TiO2 permite crecer la capa del semiconductor de

una manera controlada.

2. Se han sintetizado y caracterizado mesocristales de dióxido de titanio (anatasa) en

forma de huso mediante síntesis solvotermal con ácido acético y butóxido de titanio.

Estas nanopartículas presentan fuerte absorción en la región espectral del visible

probablemente debido, tal y como se ha comprobado, a la combinación de diferentes

efectos, tales como la incorporación de carbono en los mesocristales de dióxido de

titanio, y/o defectos en su red cristalina originados por la presencia de Ti3+

. Además, a

partir del espectro de absorción UV-visible se ha calculado el band gap de las partículas

semiconductoras empleando la relación de Kubelka-Munk, obteniéndose valores

bastante bajos (1.99 y 1.92 eV para el 1TiO2 y 2TiO2, respectivamente).

Se han desarrollado diferentes estrategias para dopar las nanopartículas de TiO2 con Au

y se ha encontrado que la ruta basada en la deposición-precipitación del precursor de

oro con urea y un posterior tratamiento con borohidruro es la que obtiene los mejores

resultados en cuanto a homogeneidad y control de tamaño. Además, hemos comprobado

por XPS que tras la etapa de deposición-precipitación con urea los mesocristales de

TiO2 estaban dopados no solo con Au metálico pero también con especies catiónicas de

oro, Au3+

y Au+. Hecho que justifica el posterior tratamiento con borohidruro. También

hemos visto que la cantidad de especies catiónicas de oro respecto a Au0 disminuía al

aumentar la cantidad de nanopartículas TiO2, relativamente a la sal de Au durante el

método de deposición-precipitación con urea. Los análisis de XPS llevados a cabo

indicaron que el Ti3+

, presente en la red cristalina de los cristales de TiO2, se

comportaba como agente reductor de iones Au3+

/ Au1+

a Au0.

RESUMEN

131

Se ha analizado la eficiencia catalítica de los nanocompuestos de TiO2@Au en

reacciones de transferencia de carga empleando como reacción modelo la reducción de

ferrocianato por iones borohidruro. Se ha verificado que:

(a) Todos los nanocompuestos de TiO2@Au (sintetizados por

deposición-precipitación con urea, usando diferentes cantidades de nanopartículas

de TiO2 para la misma cantidad de sal de oro, seguido de reducción química con

borohidruro), en pequeñas cantidades, actúan como eficientes catalizadores en la

reducción de ferrocianato por iones borohidruro aumentando altamente la

velocidad de reacción respecto de la reacción sin catalizar, sobe nuestras

condiciones experimentales. Además, la presencia de los nanocompuestos de

TiO2@Au cambia la orden de reacción de reducción del hexacianoferrato (III) a

hexacianoferrato (II) con borohidruro de pseudo-orden zero para pseudo-orden uno

respecto de la concentración de ferrocianato (III), indicando un cambio en el

mecanismo de reacción respecto de la reacción sin catalizar.

(b) Utilizando una concentración de iones borohidruro en exceso respecto de

hexacianoferrato (III), de manera que la concentración de iones borohidruro se

pueda considerar constante, y trabajando a pH 11.5 para evitar reacciones

competitivas, se obtuvo un pseudo-orden reacción uno respecto de la

concentración de borohidruro para los nanocompuestos 2TiO2@Au2 y

1TiO2@Au1.

(c) Hay una relación lineal entre la constante de velocidad observada y la

concentración de TiO2@Au, lo que indica que el nanocompuestos de TiO2@Au

actúa como un catalizador implicado en la etapa determinante de la velocidad de la

reacción modelo en el estudio.

(d) La dependencia de la constante observada de reacción kobs con la temperatura para

los nanocompuestos de TiO2@Au estudiados aumenta linealmente con un

comportamiento de Arrhenius típico.

(e) Las nanopartículas de Au soportadas sobre el TiO2, TiO2@Au, exhiben un mayor

rendimiento catalítico que los coloides de oro estabilizados con citrato, Au@Cit,

(de tamaño similar), tal y como se ha comprobado pela representación de las kobs

en función de la concentración de oro para TiO2@Au y Au@Cit. Lo que indica

que el soporte de TiO2 tiene un papel en el mecanismo de reacción redox elegido.

Probablemente, ocurre una cooperación catalítica del TiO2 y oro por el aumento de

la superficie catalítica, debido a la naturaleza semiconductora del TiO2. Además, el

RESUMEN

132

borohidruro no consigue inyectar electrones en las nanopartículas de TiO2 sin

dopar, ya que, tal como se ha comprobado, los iones de ferrocianuro na presencia

de nanopartículas de TiO2 no se reducen, bajo nuestras condiciones

experimentales. Por eso, son las nanopartículas de oro, en los nanocompuestos de

TiO2@Au, la única especie receptora de los electrones del borohidruro.

Posteriormente, las nanopartículas de titania, debido a su naturaleza

semiconductora, poden aceptar electrones procedentes de las nanopartículas de

oro, cargadas pelo borohidruro, y así actuar también como superficie catalizadora

aumentando, y por lo tanto, la capacidad catalítica de las nanopartículas de oro

suportado en titania, TiO2@Au, se ve incrementado.

(f) El nanocompuesto 2TiO2@Au2 con nanopartículas de oro más pequeñas (1.4 0.5

nm) presenta mayor capacidad catalítica en la reducción de iones hexacianoferrato

(III) por iones borohidruro que el nanocompuesto 1TiO2@Au1 con nanopartículas

de oro mayores (4.5 1.5), tal como se ha comprobado pelos mayores valores de la

constante de velocidad observada para la reacción, normalizada por nanómetro

cuadrado de área superficial de oro, kobsA-1

, (a una misma cantidad de TiO2@Au y

las mismas condiciones de reacción). Probablemente, la pequeña dimensión da las

nanopartículas de oro facilita el contacto con el suporte de titania y la rápida

transferencia de electrones de las nanopartículas de oro, cargadas con electrones

por el borohidruro, para al TiO2.

(g) La elevada densidad de nanopartículas de oro en la superficie del TiO2 en los

nanocompuesto de TiO2@Au disminuí su capacidad catalítica en la reacción redox

modelo en estudio, tal como se hay observado para el nanocompuesto 1TiO2@Au1

con elevada densidad de nanopartículas de oro en la superficie e dentro del

mesocristal de TiO2 comprobada por TEM, HRTEM y por análisis de XPS en

perfiles de profundidad de 30 nm. Se hay propuesto que el aumento de área

superficial libre de las nanopartículas semiconductoras de TiO2, aumenta la

probabilidad de contacto entre los iones de hexacianoferrato (III) y la superficie

del TiO2 y consecuentemente, aumenta a superficie catalítica, que puede ser la del

oro y a del TiO2.

(h) La capacidad catalítica de los nanocompuestos de TiO2@Au no se ve reducida

hasta pelo menos seis ciclos de catálisis, tal y como se comprobado empleando el

mismo catalizador de TiO2@Au en la reacción modelo durante seis ciclos de

catálisis y obteniéndose valores idénticos para la constante observado de reacción,

RESUMEN

133

kobs, en todos los ciclos de catálisis (bajo las mismas condiciones de reacción).

Este hecho demuestra la estabilidad de los nanocompuestos de TiO2@Au durante

la reacción catalizada.

(i) A partir dos resultados cinéticos obtenidos con los nanocompuestos de TiO2@Au,

las nanopartículas de Au estabilizadas en citrato de tamaño similar (mayor valor de

pendiente en la representación del kobs en función de la concentración de oro para

el TiO2@Au que para el Au@Cit, indicando que el titania desempeña un papel en

el mecanismo de reacción), y las nanopartículas de TiO2 (la banda de absorción

característica para el hexacianoferrato (III) a 420 nm é constante con el tiempo na

presencia de nanopartículas de TiO2 tras la adición de borohidruro, indicando una

ineficiente transferencia de electrones del borohidruro para el titanio) en la

reacción redox elegida, se propuso un mecanismo de reacción para el proceso

catalítico que consiste en la:

(a) polarización catódica de las nanopartículas de oro por los iones borohidruro;

(b) transferencia de los electrones de las nanopartículas de oro, cargadas

negativamente, para las nanopartículas semiconductoras de TiO2;

(c) reducción de los iones de hexacianoferrato (III) en la superficie metálica del

oro y/o na superficie semiconductora del TiO2.

3. Se ha analizado la capacidad catalítica del TiO2, utilizando la luz solar como fuente de

irradiación, en la fotoreducción de las especies catiónicas de oro, Au3+

y Au1+

, presentes

en la superficie de TiO2 tras la deposición-precipitación con urea de una sal de Au. Se

ha comprobado, pela aparición de una banda plasmónica a 594 nm en los espectros de

absorción UV-vis-NIR de la dispersión de nanopartículas de TiO2@Au durante la

irradiación con luz solar y pela análisis de XPS antes y después de la fotoreducción (un

único pico de Au 4f7/2, centrado a 83.6 eV, correspondiente a oro metálico), que los

electrones fotoexcitados en la banda de conducción de TiO2, por la luz solar, reducen las

especies catiónicas de oro a especies metálicas, Au0, sobre la superficie del titania.

4. Se ha analizado la eficiencia fotocatalítica de las nanopartículas de TiO2 y de los

nanocompuestos de TiO2@Au en la degradación del colorante textil rodamina B cuando

iluminados con luz solar directa. Estos sistemas, TiO2 y TiO2@Au, actúan como

eficientes fotocatalizadores en la degradación de la rodamina B por irradiación con luz

solar, tal y como se y comprobado, pela disminución gradual de la banda característica

de la rodamina B a 554 nm con el tiempo. Se ha verificado que las nanopartículas de

TiO2 cuando están dopadas con oro presentan un comportamiento cinético más lento en

RESUMEN

134

la fotodegradación de la rodamina B, tal y como se ha comprobado pelos menores

valores de la constante observado de reacción, kobs, de degradación de la rodamina B

bajo irradiación de luz solar, en los nanocompuestos de TiO2@Au (a una misma

cantidad de TiO2@Au/TiO2 y las mismas condiciones de reacción). Tal ocurre,

probablemente, porque el exceso de nanopartículas de oro funciona como centros de

recombinación de carga, o cubre los sitios activos del TiO2, o reduce la profundidad de

penetración de la luz solar. La mayor capacidad fotocatalítica de las nanopartículas de

TiO2 en la degradación de la rodamina B con luz solar puede atribuirse a efectos

sinérgicos como: (i) la presencia de carbono en los nanopartículas de dióxido de titanio

y/o defectos en su red cristalina originados por la presencia de Ti+3

que induce la

absorción de luz visible; (ii) su gran área de superficie, que proporciona más posiciones

de adsorción para la rodamina B; y (iii) su estructura porosa, que permite múltiples

reflexiones de la luz visible en el interior de las nanopartículas de dióxido de titanio, lo

que hace un uso más eficiente de la luz visible.

En general, durante el proceso de investigación que ha llevado al desarrollo de esta

tesis, se ha acumulado experiencia en la optimización de varios métodos de fabricación

de muestras, caracterización y análisis de las propiedades estructurales y ópticas de

diversos materiales, y se ha avanzado un poco en la área de catálisis heterogénea de

nanopartículas de oro suportadas en titania, aumentando el conocimiento de la química

subyacente y permitiendo el diseño de futuros materiales y dispositivos.

135

Acknowledgements

I want to thank to my supervisor, Professor Isabel Pastoriza Santos for her whole support,

guidance, patience, and the background information that provided me during the whole thesis.

I am also thankful to Professor Jorge Pérez-Juste for their support and for sharing their

scientific knowledge and Professor Eulália Pereira for their support and for giving me the

opportunity to work in the REQUIMTE laboratory.

I want to thank all the people that have contributed to the development of my PhD Thesis.

I want to thank to my work colleagues and my students, for their interest in learning concepts

of Nanoscience.

And finally, I want to thank to my family, especially Jorge and Margarida, for her patience,

understanding and support during my PhD study.

REFERENCES

137

References

1 Turkevich, J.; Stevenson, P. C.; Hillier, J. J. Phys. Chem. 1953, 57, 670.

2 Rodríguez-Fernández, J.; Pérez-Juste, J.; García de Abajo, F. J.; Liz-Marzán, L. M. Langmuir: the

ACS Journal of Surfaces and Colloids 2006, 22, 7007

3 Mulvaney, P. Langmuir. 1996, 12, 788.

4 Cable, R. E.; Schaak, R. E. Chem. Mater. 2005, 17, 6835.

5 Trügler, A. Optical Properties of Metallic Nanoparticles, PhD thesis, Institut für Physik, Fachbereich

Theoretische Physik, Karl–Franzens: Universität Graz, 2011.

6 Pastoriza-Santos, I.; Liz-Marzán, L. M. J. Mater. Chem. 2008, 18,1724.

7 Liz-Marzán, L. M.; Mulvaney, P. J. Phys. Chem. B 2003, 107, 7312.

8 Hepel, M.; Zhong, C.-J. Functional Nanoparticles for Bioanalysis, Nanomedicine, and Bioelectronic

Devices, Volume 1112, November 26, American Chemical Society, 2012.

9 Rengan, A. K.; Bukhari, A. B.; Pradhan, A.; Malhotra, R.; Banerjee, R.; Srivastava, R.; De, A. Nano

Lett. 2015, 15 (2), 842.

10 Wang, H.; Brandl, D. W.; Le, F.; Nordlander, P.; Halas, N. J. Nano Lett. 2006, 6, 827.

11 Mie, G. Annalen der Physik 1908, 330 (3), 377.

12 Liz-Marzán, L. M.; Kamat, P. V. (Ed.), Nanoscale Materials; Kluwer Academic Publishers: Boston,

2003.

13 Kreibig, U. Phys. B: Condens. Matter Quanta 1978, 31, 39.

14 Meier, M.; Wokaun, A. Opt. Lett. 1983, 8 (11), 581.

15 Liz-Marzán, L. M. Langmuir 2006, 22, (1), 32.

16 Tam, F.; Moran, C.; Halas, N. Journal of Physical Chemistry B 2004, 108 (45), 17290.

17 McFarland, A. D.; Van Duyne, R. P. Nano Letters 2003, 3 (8), 1057.

18 Kamat, P. V. Journal of Physical Chemistry B 2002, 106 (32), 7729.

19 Hirakawa, T.; Kamat, P. V. J. Am. Chem. Soc. 2005, 127 (11), 3928.

20 Henglein, A. Chem. Rev. 1989, 89, 1861.

21 Kamat, P. V. J. Phys. Chem. C 2007, 111, 2834.

22 Hou, W.; Dehm, N. A.; Scott, R. W. J. J. Catal. 2008, 253, 22.

23 Littke, A. F.; Dai, C.; Fu, G. C. J. Am. Chem. Soc. 2000, 122 (17), 4020.

24 Wilson, O. M.; Knecht, M. R.; Garcia-Martinez J. C. J. Am. Chem. Soc. 2006, 128 (14), 4510.

http://pubs.acs.org/action/doSearch?ContribStored=Rengan%2C+A+K

http://pubs.acs.org/action/doSearch?ContribStored=Bukhari%2C+A+B

http://pubs.acs.org/action/doSearch?ContribStored=Pradhan%2C+A

http://pubs.acs.org/action/doSearch?ContribStored=Malhotra%2C+R

http://pubs.acs.org/action/doSearch?ContribStored=Banerjee%2C+R

http://pubs.acs.org/action/doSearch?ContribStored=Srivastava%2C+R

http://pubs.acs.org/action/doSearch?ContribStored=de%2C+A

REFERENCES

138

25

Carregal-Romero, S.; Pérez-Juste, J.; Hervés, P.; Liz-Marzán, L. M.; Mulvaney, P. Langmuir 2010,

26 (2), 1271.

26 Mallik, M.; Witcomb, M. J.; Scurrell, M. S. Appl. Phys. A: Mater. Sci. Process. 2005, 80, 797.

27 Freund, T. J. Inorg. Nucl. Chem. 1959, 9, 246.

28 Hervés, P.; Pérez-Lorenzo, M.; Liz-Marzán, L. M.; Dzubiella, J.; Lu, Y.; Ballauff, M. Chem. Soc.

Rev. 2012, 41, 5577.

29 Chatenet, M.; Micoud, F.; Roche, I.; Chainet, E. Electrochim. Acta 2006, 51, 5459.

30 Carregal-Romero, S.; Buurma, N. J.; Perez-Juste, J.; Liz-Marzán, L. M.; Herves, P. Chem. Mater.

2010, 22, 3051.

31 Sanles-Sobrido, M.; Correa-Duarte, M. A.; Carregal-Romero, S.; Rodriguez-Gonzalez, B.;

Alvarez-Puebla, R. A.; Herves P.; Liz- Marzán, L. M. Chem. Mater. 2009, 21, 1531.

32 Zhang, H.; Finnegan, M.; Banfield, J. F. Nano Lett. 2001, 1, 81.

33 Stafford, U.; Gray, K. A.; Kamat, P. V.; Varma, A. Chem. Phys. Lett. 1993, 205, 55.

34 Landmann, M.; Rauls, E.; Schmidt, W. G. J. Phys.: Condens. Matter 2012, 24, 195503.

35 Fujishima, A.; Zhang, X. T.; Tryk, D. A. Surf. Sci. Rep. 2008, 63, 515.

36 Fröschl, T.; Hörmann, U.; Kubiak, P.; Kucerová, G.; Pfanzelt, M.; Weiss, C. K.; Behm, R. J.;

Hüsing, N.; Kaiser, U.; Landfesterd, K.; Wohlfahrt-Mehrens, M. Chem. Soc. Rev. 2012, 41, 5313.

37 Schneider, M.; Baiker, A. J. Mater. Chem. 1992, 2 (6), 587.

38 Niederberger, M.; Bartl, M. H.; Stucky, G. D. Chem. Mater. 2002, 14, 4364.

39 Pottier, A.; Cassaignon, S.; Chanéac, C.; Villain, F.; Tronc, E.; Jolivet, J.-P. J. Mater. Chem. 2003,

13, 877.

40 Li, G. L.; Wang, G. H. Nanostruct. Mater. 1999, 11, 663.

41 Moran, P. D.; Bartlett, J. R.; Bowmaker, G. A.; Woolfrey, J. L.; Cooney, R. P. J. Sol-Gel Sci.

Technol. 1999, 15, 251.

42 Chae, S. Y.; Park, M. K.; Lee, S. K.; Kim, T. Y.; Kim; S. K.; Lee, W. I. Chem. Mater. 2003, 15,

3326.

43 Aruna, S. T.; Tirosh, S.; Zaban, A. J. Mater. Chem. 2000, 10 (10), 2388.

44 Wang, C.-C.; Ying, J. Y. Chem. Mater. 1999, 11, 3113.

45 Ye, J.; Liu, W.; Cai, J.; Chen, S.; Zhao, X.; Zhou, H.; Qi, L. J. Am. Chem. Soc. 2011, 133, 933.

46 Wu, J. M.; Hayakawa, S.; Tsuru, K.; Osaka, A. Scripta Mater. 2002, 46, 101.

47Wu, J. J.; Yu, C. C. J. Phys. Chem. B 2004, 108, 3377.

48 Ma, G.; Zhao, X.; Zhu, J. Int. J. of Mod. Phys. B 2005, 19, 2763.

REFERENCES

139

49

Yu, J. C.; Yu, J.; Ho, W.; Zhang, L. Chem. Commun. 2001, 1942.

50 Mohapatra, S. K.; Kondamudi, N.; Banerjee, S.; Misra, M. Langmuir 2008, 24, 11276.

51 Liu, S.; Huang, K. Sol. Energy Mater.& Sol. Cells 2004, 85, 125.

52 Mo, S.-D.; Ching, W. Y. Phys. Rev. B 1995, 51 (19), 13023.

53 Kang, S. H.; Lim, J.-W.; Kim, H. S.; Kim, J.-Y.; Chung, Y.-H.; Sung, Y.-E. Chem. Mater. 2009, 21,

2777.

54 Smyth, J. R. and Mccormick, T. C. Crystallographic Data for Minerals, in Mineral Physics &

Crystallography: A Handbook of Physical Constants; Ahrens, T. J. (Ed), American Geophysical

Union: Washington, D. C., 2013.

55 Hanaor, D. A. H.; Sorrell, C. C. J. Mater. Sci. 2011, 46, 855.

56 Fisher J.; Egerton T. A. Titanium Compounds, Inorganic. In Kirk-Othmer Encyclopedia of

Chemical Technology. New York: John Wiley & Sons, 2001.

57 Brayner, R.; Fiévet, F.; Coradin, T. (Eds.) Nanomaterials: A Danger or a Promise? A Chemical and

Biological Perspective; Springer-Verlag: London, 156, 2013.

58 Zallen, R.; Moret, M. P. Solid State Communications 2006, 137 (3), 154.

59 Linsebigler, A. L.; Lu, G. Q.; Yates, J. T. Chem Rev, 1995, 95, 735.

60 Ohno, T.; Sarukawa, K.; Matsumura, M. J. Phys. Chem. B 2001, 105, 2417.

61 Hidalgo, M. C.; Murcia, J. J.; Navio, J. A.; Colon, G. Applied catalysis A: General 2011, 397, 112.

62 Hutchings, G. J.; Haruta, M. Appl. Catal. A 2005, 291, 2.

63 Buso, D.; Pacifico, J.; Martucci, A.; Mulvaney, P. Adv. Funct. Mater. 2007, 17 (3), 347.

64 Haruta, M.; Tsubota, S.; Kobayashi, T.; Kageyama, H.; Genet, M. J.; Delmon, B. J. of Catal. 1993,

144 (1), 175.

65 Lin, S.; Vannice, M. A. Catal. Lett. 1991, 10, 47.

66 Zanella, R.; Giorgio, S.; Henry, C. R.; Louis, C. J. Phys. Chem. B 2002, 106, 7634.

67 Tsubota, S.; Haruta, M.; Kobayashi, T.; Ueda, A.; Nakahara, Y. Stud. Surf. Sci. Catal. 1991, 72,

695.

68 Sangeetha, P.; Chang, L.-H.; Chen Y.-W. Materials Chemistry and Physics 2009, 118, 181.

69 Wen, Y.; Liu, B.; Zeng, W.; Wang, Y. Nanoscale 2013, 5, 9739.

70 Hermans, L. A. M.; Geus, J. W. Stud. Surf. Sci. Catal. 1979, 4, 113.

71 Hidalgo, M. C.; Maicu, M.; Navió, J. A.; Colón, G. J. Phys. Chem. C 2009, 113, 12840.

72 Guillemot, D.; Polisset-Thfoin, M.; Fraissard, J. Catal. Lett. 1996, 41, 143.

REFERENCES

140

73

Guillemot, D.; Borovskov, V. Y.; Kazansky, V. B.; Polisset-Thfoin, M.; Fraissard, J. J. Chem. Soc.,

Faraday Trans. 1997, 93, 3587.

74 Hosseini, M.; Momeni, M. M.; Faraji, M. J. Mol. Catal. A: Chem. 2011, 335, 199.

75 Nguyen, L. Q.; Salim, C.; Hinode, H. Applied Catalysis A: General 2008, 347, 94.

76 Chen, S. F.; Li, J. P.; Qian, K.; Xu, W. P.; Lu, Y.; Huang, W. X.; Yu, S. H. Nano Res 2010, 3, 244.

77 Tsubota, S.; Cunningham, D. A. H.; Bando, Y.; Haruta, M. Stud. Surf. Sci. Catal. 1995, 91, 227.

78 Haruta, M. J. New Mater. Electrochem. Syst. 2004, 7, 163.

79 Zanella, R.; Delannoy, L.; Louis, C. Appl. Catal. A 2005, 291, 62.

80 Haruta, M. Catal. Today 1997, 36, 153.

81 Thompson, D. T. Gold Bull. 1998, 31, 111.

82 Hayashi, T.; Tanaka, K.; Haruta, M. J. Catal. 1998, 178, 566.

83 Sakurai, H.; Haruta, M. Catal. Today 1996, 29, 361.

84 Ueda, M.; Haruta, M. Appl. Catal. B 1998, 18, 115.

85 Haruta, M.; Kobayashi, T.; Sano, H.; Yamada, N. Chem. Lett. 1987, 16, 405.

86 Valden, M.; Pak, S.; Lai, X.; Goodman, D. W. Catal. Letters 1998, 56, 7.

87 Mills, G.; Gordon, M. S.; Metiu, H. J. Chem. Phys. 2003, 118, 4198.

88 Cleveland, C. L.; Landman, U.; Schaaff, T. G.; Shafigullin, M. N.; Stephens, P. W.; Whetten, R. L.

Phys. Rev. Lett. 1997, 79, 1873.

89 Horsley, J. A. J Am. Chem. Soc. 1979, 101 (11), 2870.

90 Gluhoi, A. C.; Bogdanchikova, N.; Nieuwenhuys, B. E. J. Catal. 2005, 232 (1) 96.

91 Schubert, M. M.; Hackenberg, S.; Veen, A. C. v.; Muhler, M.; Plzak, V.; Behm, R. J. J. of Catal.

2001, 197, 113.

92 Liu, H., Kozlov, A. I.; Kozlova, A. P.; Shido, T.; Asakura, K., Iwasawa, Y. J. of Catal. 1999, 185

(2), 252.

93 Park, E. D.; Lee, I. S. J. of Catal. 1999, 186, 1.

94 Hodge, N. A.; Kiely, C. J.; Whyman, R.; Siddiqui, M. R. H.; Hutchings, G. J.; Pankhurst, Q. A.;

Wagner, F. E.; Rajaram, R. R.; Golunski, S. E. Catalysis Today 2002, 72, 133.

95 Kolasinski, K. Surface Science: Foundations of Catalysis and Nanoscience; John Wiley and Sons

Ltd.:London, 2002.

96 Wardle, B. Principles and Applications of Photochemistry, John Wiley & Sons, Ltd: United

Kingdom, 2009.

http://gen.lib.rus.ec/get?md5=27359331B225EA82D869F6B684C87D8A&open=0

REFERENCES

141

97

Mandal, S. S.; Bhattacharyya, A. J. J. Chem. Sci. 2012, 124 (5), 969.

98 Hurum, D. C.; Gray, K. A.; Rajh, T.; Thurnauer, M. C. J. Phys. Chem. B, 2005, 109 (2), 977.

99 Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W. Chem Rev. 1995, 95, 69.

100 Fujishima, A.; Rao, T. N.; Tryk, D. A. J. Photochem. Photobiol. C 2000, 1, 1.

101 Tojo, S.; Tachikawa, T.; Fujitsuka; M.; Majima, T. J. Phys. Chem. C 2008, 112 (38), 14948.

102 Wang, X.; Caruso, R. A. J. Mater. Chem. 2011, 21, 20.

103 Henderson, M. A. Surface Science Reports 2011, 66, 185.

104 Serpone, N.; Borgarello, E.; Graetzel, M. J. Chem. Soc., Chem. Commun. 1984, 342.

105 Seh, Z. W.; Liu, S.; Low, M.; Zhang, S.-Y.; Liu, Z.; Mlayah, A.; Han, M.-Y. Advanced Materials

2012, 24 (17), 2310.

106 Christopher, P.; Ingram, D. B.; Linic, S. J. Phys. Chem. C 2010, 114 (19), 9173.

107 Furube, A.; Du, L.; Hara, K.; Katoh, R.; Tachiya, M .J. Am. Chem. Soc. 2007, 129, 14852.

108 McFarland, E. W.; Tang, J. Nature 2003, 421, 616.

109 Sellappan, R. Mechanisms of Enhanced Activity of Model TiO2/Carbon and TiO2/Metal

Nanocomposite Photocatalysts; Phd Thesis, Department of Applied Physics, Chalmers University of

Technology, Göteborg, Sweden, 2013.

110 Liz-Marzán, L. M. Materials Today February 2004; 26.

111 Pérez-Juste, J.; Pastoriza-Santos, I.; Liz-Marzán, L. M.; Mulvaney. P. Coord. Chem. Rev. 2005,

249,1870.

112 Sánchez-Iglesias, A.; Pastoriza-Santos, I.; Pérez-Juste, J.; Rodríguez-González, B.; García de

Abajo, F. J.; Liz-Marzán. L. M. Adv. Mater. 2006, 18, 2529.

113 McConnell, W. P.; Novak, J. P.; Brousseau, L. C.; Fuierer, R. R.; Tenent, R. C.; Feldheim, D. L. J.

Phys. Chem. B 2000, 104, 8925.

114 Kondapaneni, S. C.; Singh, D.; Srivastava O. N. J. Phys. Chem. 1992, 96, 8094.

115 Stergiopoulos, T.; Arabatzis, I. M.; Katsaros, G.; Falaras, P. Nano Letters 2002, 2, 1259.

116 Lin, Cheng-Lan; Yeh, Mei-Yu; Chen, Chih-Hsien; Sudhakar, S.; Luo, Shr-Jie; Hsu, Ying-Chan;

Huang, Chung-Yi; Ho, Kuo-Chuan; Luh, Tien-Yau Chem. Mater. 2006, 18, 415.

117 Dagan, G.; Tomkiewicz, M. J. Phys. Chem. 1997, 49, 12652.

118 Rideh, L.; Wehrer, A.; Ronze, D.; Zoulalian, A. Ind. Eng. Chem. Res. 1997, 36, 4712.

119 Ranjit, K. T.; Willner, I.; Bossmann, S.; Braun, A. J. Phys. Chem. B 1998, 102, 9397.

120 Goutailler, G.; Guillard, C.; Faure, R.; Païssé, O. J. Agric. Food Chem. 2002, 50, 5115.

http://pubs.acs.org/action/doSearch?ContribStored=Tojo%2C+S

http://pubs.acs.org/action/doSearch?ContribStored=Tachikawa%2C+T

http://pubs.acs.org/action/doSearch?ContribStored=Fujitsuka%2C+M

http://pubs.acs.org/action/doSearch?ContribStored=Majima%2C+T

http://pubs.rsc.org/en/results?searchtext=Author%3AXingdong%20Wang

http://pubs.rsc.org/en/results?searchtext=Author%3ARachel%20A.%20Caruso

http://pubs.acs.org/action/doSearch?action=search&author=Christopher%2C+P&qsSearchArea=author

http://pubs.acs.org/action/doSearch?action=search&author=Ingram%2C+D+B&qsSearchArea=author

http://pubs.acs.org/action/doSearch?action=search&author=Linic%2C+S&qsSearchArea=author

REFERENCES

142

121

Mills, A.; Crow, M.; Wang, J.; Parkin, I. P.; Boscher, N. J. Phys. Chem. C 2007, 111, 5520.

122 Zhu, Y.; Shi, J.; Zhang, Z.; Zhang, C.; Zhang, X. Anal. Chem. 2002, 74, 120.

123Du, X.; Wang, Y.; Mu, Y.; Gui, L; Wang, P.; Tang, Y. Chem. Mater. 2002, 14, 3953.

124 Lobato, K.; Peter, L. M. J. Phys. Chem. B 2006, 110, 21920.

125 Stallings, W. E.; Lamb, H. H. Langmuir 2003, 19, 2989.

126 Murakami, Y.; Matsumoto, T.; Takasu, Y. J. Phys. Chem. B 1999, 103, 1836.

127 Pastoriza-Santos, I.; Koktysh, D. S.; Mamedov, A. A.; Giersig, M.; Kotov, N. A.; Liz-Marzán, L.

M. Langmuir 2000, 16, 2731.

128 Tom, R. T.; Nair, A. S.; Singh, N.; Aslam, M.; Nagendra, C. L.; Philip, R.; Vijayamohanan, K.;

Pradeep, T. Langmuir 2003, 19, 3439.

129 Kreibig, U.; Vollmer, M. Optical Properties of Metal Clusters. Springer: Berlin, 1995.

130 Zhang, J. H.; Wang, S. Z.; Liu, J. B.; Wang, Z. L.; Ming, N. B. J. Mater. Res. 2005, 20, 965.

131 Mayya, K. S.; Gittins, D. I.; Caruso, F. Chem. Mater 2001, 13, 3833.

132 Hanprasopwattana, A.; Rieker, T.; Sault, A. G.; Datye, A. K. Catal. Lett. 1997, 45, 165.

133 Möckel, H.; Giersig, M.; Willig, F. J. Mater. Chem. 1999, 9, 3051.

134 Pattanaik, M.; Bhaumik, S. k. Mater. Lett. 2000, 44, 352.

135 Smith, J. N.; Meadows, J.; Williams, P. A. Langmuir 1996, 12, 3773.

136 Esumi, K.; Matsui, H. Colloid Surf., A: Physicochem. Eng. Asp. 1993, 80, 273.

137 Otsuka, H.; Esumii, H. J. Colloid Interface Sci. 1995, 170, 113.

138 Pastoriza-Santos, I.; Pérez-Juste, J.; Liz-Marzán, L. M. Chem. Mater. 2006, 18, 2465.

139 Decher, G. Science 1997, 277, 1232.

140 Gittins, D. I.; Caruso, F. J. Phys. Chem. B 2001, 105, 6846.

141 Enüstün, B. V.; Turkevich, J. J. Am. Chem Soc. 1963, 85, 3317.

142 Pastoriza-Santos, I.; Liz-Marzán, L Langmuir 2002, 18, 2888.

143 Sakai, H.; Kanda, T.; Shibata, H.; Ohkubo, T.; Abe, M. J. Am. Chem Soc. 2006, 128, 4944.

144 Kelly, K. L.; Coronado, E.; Zhao L. L.; Schatz, G. C. J. Phys. Chem. B 2003, 107, 668.

145 Lee, S.; Rhee, Sung-Gyu.; Oh, Soo-Ghee J. Korean Phys. Soc. 1999, 34, 319.

146 Underwood, S.; Mulvaney, P. Langmuir 1994, 10, 3427.

147 Song, R.-Q.; Cölfen, H. Adv. Mater. 2010, 22, 1301.

REFERENCES

143

148

Zhou, L.; O'Brien, P. Small 2008, 4, 1566.

149 Fang, J.; Ding, B.; Gleiter, H. Chem. Soc. Rev. 2011, 40, 5347.

150 Zhou, L.; O’Brien, P. J. Phys. Chem. Lett. 2012, 3, 620.

151 Rolison, D. R. Science 2003, 299, 1698.

152 Edwards, J. K.; Carley, A. F.; Herzing, A. A.; Kiely, C. J.; Hutchings, G. J., Faraday Discuss.

2008, 138, 225.

153 Kesavan, L.; Tiruvalam, R.; Ab, R. M. H.; bin, S. M. I.; Enache, D. I.; Jenkins, R. L.; Dimitratos,

N.; Lopez-Sanchez, J. A.; Taylor, S. H.; Knight, D. W.; Kiely, C. J.; Hutchings, G. J. Science 2011,

331 (6014), 195.

154 Enache, D. I.; Edwards, J. K.; Landon, P.; Solsona-Espriu, B.; Carley, A. F.; Herzing, A. A.;

Watanabe, M.; Kiely, C. J.; Knight, D. W.; Hutchings, G. J. Science 2006, 311 (5759), 362.

155 Bamwenda, G. R.; Tsubota, S.; Nakamura, T.; Haruta, M. Catal. Lett. 1997, 44, 83.

156 Jong, K. P. Synthesis of Solid Catalysts; WILEY-VCH Verlag GmbH & Co. KGaA: Weinheim,

2009.

157 Nechayev, Y. A.; Zvonareva, G. V. Geokhimiya 1983, 6, 919.

158 Moreau, F.; Bond, G. C.; Taylor, A. O. Journal of Catalysis 2005, 231, 105.

159 Yu, J. C.; Yu, J..; Ho, W.; Jiang, Z.; Zhang, L. Chem. Mater. 2002, 14, 3808.

160 Santara, B.; Giri, P. K. Materials Chemistry and Physics 2013, 137, 928.

161 Morgan, B. J.; Watson, G. W. Journal of Physical Chemistry C 2009,113 (17), 7322.

162 Henrich, V. E.; Dresselhaus, G.; Zeiger, H. J. Physical Review Letters 1976, 36, 1335.

163 Yu, J.; Dai, G.; Xiang, Q., Jaroniec, M. J. Mater. Chem. 2011, 21, 1049.

164 Chen, X.; Mao, S. S. Chem. Rev. 2007, 107, 2891.

165 Nie, X.; Zhuo, S.; Maeng, G.; Sohlberg, K. International Journal of Photoenergy 2009, 1.

166 Nagaveni, K.; Sivalingam, G.; Hegde, M. S.; Madras, G. Applied Catalysis B: Environmental 2004,

48, 83.

167 Gu, D.-e.; Lu, Y.; Yang, B.-c.; Hu, Y.-d. Chem. Commun. 2008, 2453.

168Wu, X.; Yin, S.; Dong, Q.; Guo, C.; Li, H.; Kimura, T.; Sato, T. Applied Catalysis B:

Environmental 2013, 142–143, 450.

169 Moulder, J. F.; Stickle, W. F.; Sobol, P. E.; Bomben, K. D. Handbook of X-Ray Photoelectron

Spectroscopy, Physical Electronics, Inc., Eden Prairie, MN, USA, 1995.

170 Kormann, C.; Bahnemann, D. W.; Hoffmann, M. R. The Journal of Physical Chemistry 1988, 92,

5196.

REFERENCES

144

171

Zuo, F.; Wang, L.; Wu, T.; Zhang, Z.; Borchardt, D.; Feng, P. Journal of the American Chemical

Society 2010, 132, 11856.

172 IUPAC J. Colloid Interface Chem.; Pure Appl. Chem. 1972, 31, 578.

173 Corma, A., Garcia, H. Chem. Soc. Rev. 2008, 37, 2096.

174 Hashmi, A.S.K., Hutchings, G.J., Angew. Chem. Int. Ed. 2006, 45, 7896.

175 Carregal-Romero, S.; Catalytic Activity of Gold Nanoparticles and Other Colloidal

Nanocomposites on a Model Redox Reaction; PhD Thesis, Universidade de Vigo: Vigo, 2009.

176 Ismail, A. A.; Kandiel, T. A.; Bahnemann, D. W. J. Photochem. and Photobiol. A: Chemistry 2010,

216, 183.

177 Ismail, A. A.; Bahnemann, D. W.; Robben, L.; Yarovyi, V.; Wark, M. Chem. Mater. 2010, 22, 108.

178 Li, H.; Bian, Z.; Zhu, J.; Huo, Y.; Li, H.; Lu, Y. J. Am. Chem. Soc. 2007, 129, 4538.

179 Zhang, L.; Yu, J. C. Chem. Commun. 2003, 2078.

180 Haroa, M.; Abargues, R.; Herraiz-Cardona, I.; Martínez-Pastor, J.; Giménez, S. Electrochimica

Acta 2014, 144, 64.

181 Kiyonaga, T.; Fujii, M.; Akita, T.; Kobayashic, H.; Tada, H. Phys. Chem. Chem. Phys. 2008, 10,

6553.

182 Subramanian, V., Photoelectrochemical and Photocatalytic Aspects of Semiconductor–Metal

Nanocomposites; PhD Thesis, Notre Dame: Indiana, 2004.

183 Fujishima, A.; Honda, K. Nature, 1972, 238, 37.

184 Asahi, R.; Morikawa, T.; Ohwaki, T.; Aoki, K.; Taga, Y. Science 2001, 293, 269.

185 Kamisaka, H.; Adachi, T.; Yamashita, K. J. Chem. Phys. 2005, 123, 84704.

186 Valentin, C. D.; Pacchioni, G.; Selloni, A. Chem. Mater. 2005, 17, 6656.

187 Hamal, D. B.; Klabunde, K. J. J. Colloid Interface Sci. 2007, 311, 514.

188 Huang, Y.; Ho, W. K.; Lee, S. C.; Zhang, L. Z.; Li, G. S.; Yu, J. C. Langmuir 2008, 24, 3510.

189 Irie, H.; Watanabe, Y.; Hashimoto, K. Chem. Lett. 2003, 32, 772.

190 Wu, Z. B.; Dong, F.; Zhao, W. R.; Wang, H. Q.; Liu, Y.; Guan, B. H. Nanotechnology 2009, 20,

235701.

191 Silva, C. G.; Juárez, R.; Marino, T.; Molinari, R. Garcia, H. J. Am. Chem. Soc. 2012, 134, 6309.

192 Zheng, Z.; Huang, B.; Qin, X.; Zhang, X.; Daib, Y.; Whangbo, M.-H. J. Mater. Chem. 2011, 21,

9079.

193 Primo, A.; Marino, T.; Corma, A.; Molinari, R.; Garcia, H. J. Am. Chem. Soc. 2011, 133, 6930.

REFERENCES

145

194

Deepa, J.; Christopher, M. S.; Sadhana, S. R.; Khadga, M. S.; Kenneth, J. K. International Journal

of Photoenergy 2013, 2013.

195 Yang, S.-y.; Chen, Y.-x.; Lou, L.-p.; Wi, X.-n. Journal of Environmental Sciences 2005, 17, 761.

196 Kuvarega, A. T.; Krause, R. W. M.; Mamba, B. B. J. Phys. Chem. C 2011, 115, 22110.

197 Yang, J.; Chen, C.; Ji, H.; Ma, W.; Zhao, J. J. Phys. Chem. B 2005, 109, 21900.

198 Ge, L.; Han, C. C.; Liu, J.; Li, Y. F. Appl. Catal. A: Gen. 2011, 409-410, 215.

199 Yu, J.; Dai, G.; Huang, B. J. Phys. Chem. C. 2009, 113, 16394.

200 Chen, C.; Ma, W.; Zhao, J. Chem. Soc. Rev. 2010, 39, 4206.

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Nobel metal-TiO2 nanocomposites - Investigo

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