5TH INTERNATIONAL CONFERENCE ON SOL-GEL MATERIALS- TRZEBIESZOWICE, POLAND

Sol–gel doped TiO2 nanomaterials: a comparative study

Malina Raileanu Æ Maria Crisan Æ Nicolae Dragan Æ Dorel Crisan ÆAnouk Galtayries Æ Ana Braileanu Æ Adelina Ianculescu ÆValentin S. Teodorescu Æ Ines Nitoi Æ Mihai Anastasescu

Received: 2 September 2008 / Accepted: 6 June 2009 / Published online: 6 July 2009

� Springer Science+Business Media, LLC 2009

Abstract Among the great number of sol–gel prepared

nanomaterials, TiO2 has attracted significant interest due to

its high photocatalytic activity, excellent functionality,

thermal stability and non-toxicity. The photocatalytic

degradation of pollutants using un-doped and doped TiO2

nanopowders or thin films is very attractive for applications

in environmental protection, as a possible solution for

water purification. The present work describes a compar-

ative structural and chemical study of un-doped TiO2 and

the corresponding S- and Ag-doped materials. The photo-

catalytic activity was established by testing the degradation

of organic chloride compounds from aqueous solutions.

Sol–gel Ag-doped TiO2 coatings, prepared by co-gelation

and sol–gel Ag-doped TiO2 coatings obtained from

nanopowders were also compared. Their structural evolu-

tion and crystallization behaviour (lattice parameters,

crystallite sizes, internal strains) with thermal treatment

were followed by thermal analysis, X-ray diffraction,

transmission electron microscopy, atomic force micros-

copy and specific surface areas measurements. X-ray

photoelectron spectroscopy analyses were performed to

characterize the surface composition and S or Ag specia-

tion, which was used to interpret the catalytic data.

Keywords Sol–gel � Nanopowders � Coatings �S-doped TiO2 � Ag-doped TiO2 � Water purification

1 Introduction

Titania represents one of the most studied inorganic com-

pounds in chemistry. Its preparation, physical and chemical

properties together with its applications for all crystallo-

graphic forms (anatase, rutile and brookite) have been the

object of numerous studies. The literature supplies thou-

sands of articles dedicated to TiO2. Let us mention at least

some reviews [1–8] and books [9–13]. The explanation of

such a huge interest is simple. It is well known that among

the various photocatalysts, titania occupies a very impor-

tant place, due to its high photocatalytic activity, excellent

functionality, high chemical stability, thermal stability and

non-toxicity. These properties make it useful in different

fields, ranging from optics to gas sensors via solar energy.

All the mentioned properties are improved in the case of

nanostructured TiO2, which can be easily obtained by the

sol–gel method. An advantage of nanometer sized materi-

als stems from the changes in electrochemical potentials of

the photogenerated charge carriers that accompany

decreasing particle size. The sol–gel method facilitates the

M. Raileanu � M. Crisan (&) � N. Dragan � D. Crisan �A. Braileanu � M. Anastasescu

Romanian Academy, Institute of Physical Chemistry Ilie

Murgulescu, 202 Splaiul Independentei, 060021 Bucharest,

Romania

e-mail: mcrisan@icf.ro; msidcrisan@yahoo.com

A. Galtayries

Laboratoire de Physico-Chimie des Surfaces (UMR CNRS

7045), Ecole Nationale Superieure de Chimie de Paris,

11 rue P. et M. Curie, 75005 Paris, France

A. Ianculescu

Department of Oxide Materials Science and Engineering,

‘‘Politehnica’’, University of Bucharest, 1-7 Gh. Polizu,

P.O. Box 12-134, 011061 Bucharest, Romania

V. S. Teodorescu

National Institute for Physics of Materials, Atomistilor 105 bis,

P.O. Box MG 07, Bucharest, Magurele, Romania

I. Nitoi

INCD-ECOIND, 90-92 Sos. Panduri, 050663 Bucharest,

Romania

123

J Sol-Gel Sci Technol (2009) 51:315–329

DOI 10.1007/s10971-009-2017-z

preparation of the photocatalyst in both forms (powders

and coatings). Moreover, it is very convenient for pro-

ducing nanocomposites materials in which different phases

could be highly dispersed in an inorganic matrix, that is for

the doping of titania with different metal or non-metal ions.

Recent studies [6, 7, 14] point out the doping procedure as

a possibility to enhance the efficiency of the photocatalytic

process. The dopant ions can behave as both hole and

electron traps or they can mediate interfacial charge

transfer [14]. Also known as ‘‘impurity doping’’, the pro-

cedure ensures the extension of the spectral response of a

wide band gap semiconductor to visible light.

The sol–gel method was extensively used in order to

improve the photocatalytic activity of TiO2 by doping with:

(a) different metals: Pt [15–17], Pd [18–21], Cu and

Co [22–25], Cr [26, 27], Ni [28], Fe [29–36], Mn [37], V

[38, 39], Nb [40–42], W and Al [43, 44], Au [45–48], Ag

[7, 19, 49–53]; (b) lanthanides: Nd [54], La and Eu [55], Ce

[56–58]; (c) non-metals: C [59–68], N [68–74], S [64, 66,

68, 72, 74–86], N–S co-doping [87]; (d) metal non-metal

co-doping: Zr–S [88].

The photocatalytic activity of TiO2 strongly depends on

its physicochemical properties, such as crystal structure,

lattice defects, surface area, particle size, content of sur-

face hydroxyls etc. The photocatalytic activity of TiO2 is

known to be a function of its surface properties and

increases with the specific surface area (SSA) of the

samples used [72]. Conventional TiO2 powder catalysts

present the disadvantages of agglomeration and of difficult

separation of the final particle-fluid for the catalyst recy-

cling. Thus, the application of TiO2 thin films has attracted

much attention in recent years. In addition to this, an

increasing interest toward sol–gel synthesis of nanostruc-

tured oxide thin films is due to the method advantages:

simple and cheap technological equipment, low processing

temperature for film densification and wide scope for

varying film properties by changing the composition of the

precursor solution.

Concerning the S-dopant, the literature data report that it

can act both as anion, replacing the lattice oxygen in TiO2

[78] and cation, replacing Ti ions [77, 79, 80]. Very few

data could be found regarding the S-doping of TiO2 that

refer to the preparation of thin coatings [86].

Taking into account that prevention and control of the

effects due to different environmental pollutants represent

one of the most acute current problems of humanity and

that pollution of the water can be considered as one of the

most important aspects, research in the domain of materials

with catalytic properties for water pollutant degradation

represents a very important topic. Knowing that TiO2 is an

excellent photocatalyst that allows the degradation and

finally the mineralization of stable organic pollutants

(detergents, dyes, pesticides) in water, the present work has

extended the field of investigation from S to Ag-doped

TiO2, aiming at accomplishing a comparison of their effi-

ciency for the removal of chloride organic compounds

from water. The selection of silver as metal ion dopant for

TiO2 was based on two well known properties: it enhances

the photoactivity in the visible domain [19] and it produces

the highest Schotty barrier among the metals, which

facilitates the electron capture [53].

Nanostructured coatings are preferred for environmental

protection due to their high surface/volume ratio, taking

into account that photocatalytic process is based on the

chemical reactions at the surface of the material. In addi-

tion, sol–gel processing allows control of the microstruc-

ture of the film. In order to increase the thickness of the

coatings many depositions are necessary, but these induce

tensile stress by shrinkage of the layers in drying and

thermal processing. This problem can be resolved either by

increasing the viscosity of the starting solution, or by

embedding a secondary phase (e.g. precalcinated powders,

fibers, etc) as a filler, to reinforce the resulting composite

structure [89]. Other papers also pointed out the processing

of sol–gel derived titanium dioxide composites coatings

[21, 90–92].

That is why one of the purposes of the present work was

to obtain not only sol–gel S-doped TiO2 powders but also

sol–gel S-doped TiO2 coatings with photocatalytic prop-

erties which make them useful in the environmental pro-

tection. Another objective was to realize a comparative

structural study between sol–gel Ag-doped TiO2 coatings

directly prepared by co-gelation and sol–gel Ag-doped

TiO2 coatings obtained from nanopowders.

2 Experimental

In order to undertake comparative studies, sol–gel doped

TiO2 materials have been prepared in both forms (powders

and coatings) and two kinds of dopants (S and Ag) have

been used. To a better assess of the effect of dopant on the

final properties of the products obtained, all doped mate-

rials have also been compared with the corresponding

materials in the absence of dopant.

2.1 Un-doped and S-doped TiO2 powders

Pure TiO2 sol–gel nanopowders have been prepared by

controlled hydrolysis-condensation of three different titanium

alkoxides: tetraethylorthotitanate (Merck), Ti(OC2H5)4 for

sample 1, tetraisopropylorthotitanate (Merck), Ti(O–iC3H7)4

for sample 2 and tetrabutylorthothitanate (Fluka),

Ti(O–C4H9)4 for sample 3. The corresponding parent

alcohols (absolute ethanol, provided by Riedel de Haen,

for sample 1; 2-propanol, provided by Fluka, for sample 2;

316 J Sol-Gel Sci Technol (2009) 51:315–329

123

and normal butanol, provided by J. T. Baker, for sample 3)

have been used as solvents. The hydrolysis of the Ti

alkoxides proceeded with a water excess and in non-

catalyzed conditions. In order to investigate the role of the

alkoxide type on the final properties of the TiO2 powders

obtained all other parameters reaction have been kept

unchanged. Thus, the molar ratios were: water/alkoxide = 5

and alcohol/alkoxide = 85, the pH value was 4 and the

reaction temperature was 298 K. The reaction times were

between 1 and 6 h, depending on the Ti alkoxide used as

precursor.

The corresponding sol–gel S-doped TiO2 powders have

been obtained under similar conditions as samples 1–3.

They were doped with 5% wt. S related to the TiO2 content

and were identified as 4, 5 and 6. The sulfur source for the

doped sol–gel powders was thiourea (H2NCSNH2) from

Alfa Aesar, which was introduced in the reaction mixtures

dissolved in the corresponding alcohol. The resulting sols

have been concentrated and dried at 353 K.

The un-doped and S-doped TiO2 powders obtained have

been subjected to thermal analysis, in order to establish the

thermal schedule for the subsequent investigations. The

thermal analysis was performed up to 1,000 �C, using a

Mettler Toledo Star System TGA/SDTA851/LF1600 �C

apparatus with a heating rate of 10 K min-1, in flowing air

atmosphere of 50 mL min-1. Thus, thermal treatments at

573, 673 and 773 K have been applied with heating rates of

1 K min-1 and a 1 h plateau in all cases.

X-ray diffraction (XRD) analysis was performed with a

Shimadzu XRD 6000 diffractometer, using Ni-filtered

CuKa radiation (k = 1.5418 A) with a scan step of 0.02�and a counting time of 1 s/step, for 2h diffraction angles

ranging between 20 and 80�. The calculated values of

microstructural factors have been obtained from comput-

erized analysis of XRD spectra with an XRAY5.0 program

[93, 94].

Morphological characterizations were conducted using a

transmission electron microscope (TEM) from Jeol

(200CX equipment), and X-ray photoemission spectros-

copy (XPS) analyses were performed using an ESCALAB

250 spectrometer from Thermo Electron Corporation

(TEC), with a monochromatic, focussed A1 Ka source. The

data processing was performed with the commercial

AVANTAGE program, provided by TEC. The measure-

ments of the BET SSA were performed using a Quanta-

chrome Analyzer—Model Nova 1000e with N2 adsorption

at 77 K. Before analysis the samples were heated at 573 K,

under vacuum for 4 h, for sample outgasing.

In order to establish the role of S in the photocatalytic

activity, the prepared powders using titanium ethoxide as

TiO2 source have been selected in order to be tested in the

degradation of different organic chloride compounds from

aqueous solutions.

To investigate the photocatalytic properties of the

sol–gel TiO2-based prepared powders, un-doped samples

of chlorobenzene synthetic solution (0.096 9 10-3 M)

were subjected to photolysis. A laboratory installation was

used consisting of an UV reactor system comprising: (a) an

UV immersion Heraeus lamp TQ 150 Z3 (medium pressure

mercury lamp) with spectral distribution of 200–360 and

460–510 nm, input rating of 150 W and quartz cooling

jacket (U = 39 mm); (b) the reaction vessel (1,500 cm3)

with magnetic stirrer and power pack; and (c) the photo-

catalyst (50 mg L-1). The reaction conditions were:

pH = 7; O2 = 7 mg L-1; irradiation time: 3,600 s; irradi-

ation pathlength l = 2 cm; k = 200–280 and 400–450 nm.

The chlorobenzene concentration was determined by the

Gas Chromatography (GC) technique.

2.2 Un-doped, S-doped and Ag-doped TiO2 coatings

Pure TiO2 sol–gel coatings (C–TiO2) have been prepared

by controlled hydrolysis-condensation of tetraethylorthoti-

tanate (Merck), Ti(OC2H5)4. Absolute ethanol, provided by

Riedel de Haen, has been used as solvent. The hydrolysis

of the Ti alkoxide was initiated by water addition in sub-

stoichiometric quantity. Unlike the powders, the prepara-

tion of coatings requires nitric acid as catalyst. The molar

ratios used in order to obtain coatings were: water/alkox-

ide = 1.5, alcohol/alkoxide = 55 and catalyst/alkox-

ide = 0.258, at pH 3.5 and a reaction temperature of

323 K. The reaction time was 1 h.

The corresponding sol–gel S-doped TiO2 coatings

(C–S–TiO2) have been obtained under similar conditions to

C–TiO2 sample. They were doped with 2% wt. S related to

the TiO2 content. The sulfur source for the S-doped sol–gel

coatings was thiourea (H2NCSNH2) from Alfa Aesar. It

was introduced in the reaction mixtures dissolved in the

corresponding alcohol, as in the case of doped powders

(see Sect. 2.1). In a recent paper [86] it has been estab-

lished that this concentration of sulfur dopant was optimum

for achieving the best photocatalytic activity of the coat-

ings in the water purification process.

Concerning the sol–gel Ag-doped TiO2 coatings, two

kinds of films have been prepared: in situ by cogelation of

precursors (C–Ag–TiO2) and from nanopowders (CN–Ag–

TiO2). For C–Ag–TiO2, the concentration of the dopant

was 2 wt.% Ag related to the TiO2 content (same for C–S–

TiO2). The silver source was AgNO3. CN–Ag–TiO2 films

were obtained from 2 wt.% Ag-doped TiO2 nanopowder

which was prepared by ultrasonically assisted hydrother-

mal synthesis and embedded in sol–gel TiO2 matrix. This

matrix was directly obtained from tetraethylorthotitanate

(Merck), Ti(OC2H5)4 as TiO2 precursor, using absolute

ethanol provided by Riedel de Haen, as solvent, water for

hydrolysis and nitric acid as catalyst. The scheme of

J Sol-Gel Sci Technol (2009) 51:315–329 317

123

preparation for the Ag-doped TiO2 coatings obtained from

nanopowders is the same as that presented in our previous

paper, for similar Pd-doped TiO2 coatings [21].

In all cases (C–TiO2, C–S–TiO2, C–Ag–TiO2 and

CN–Ag–TiO2) single and double-layers coatings have been

deposited on glass substrates using the dip-coating proce-

dure, with a withdrawal rate of 5 cm/min.

Regarding the characterization of the prepared coatings,

thermal analysis, XRD and XPS were performed under the

same conditions as described in Sect. 2.1. Concerning the

photocatalytic tests the same laboratory UV-reactor

Heraeus system was used. In the reaction vessel

(1,500 cm3) containing the CB solution, a rod was intro-

duced along which were fixed at 2 cm distances, ten glass

plates (7 9 2.5 cm) with the catalyst film. The irradiation

surface area was 175 cm2. In addition, atomic force

microscopy (AFM) characterization with an EasyScan 2

model from Nanosurf� AG Switzerland as well as Spec-

troellipsometric measurements (SE) have been performed.

SE was accomplished with a null-type Ellipsometer in the

wavelength range of 350–700 nm, at an angle of incidence

of 70�. The experimental ellipsometric spectra were fitted

with multilayer and multicomponent Bruggemann’s effec-

tive medium approximation (BEMA) model [95].

3 Results and discussion

3.1 Un-doped and S-doped TiO2 powders

Because the XRD patterns for un-doped and S-doped TiO2

prepared powders obtained from different Ti alkoxides

(samples 1–6), thermally treated at 673 K for 1 h are very

similar, only one representative XRD pattern is presented

in Fig. 1.

In all cases, anatase as single crystalline phase has been

detected.

The size distributions and the mean size of particles

have been obtained from the statistical processing of the

TEM images. Regarding the size distributions for samples

(1–6) only minor differences are evident. Figure 2 presents

these data for sample 1.

The mean size obtained for a measured average number

of particles is presented in Table 1.

The mean size decreases in the presence of sulfur.

Conventional TEM images revealed the presence of

spheroidal TiO2 nanocrystallites, joined in aggregates with

sizes between 300 and 800 nm, regardless of the Ti-alk-

oxide type. For the samples obtained from Ti(OEt)4 (1 and 4),

these aggregates have spherical shape (Fig. 3).

Taking into account that the S-doped sample obtained

from Ti(OEt)4 (sample 4), exhibits particles with a large size

distribution and spherical shape, the pair of samples (1, 4)

have been chosen for detailed experiments.

A detailed analysis of the thermal behaviour of the

selected powders is available in Ref. [84]. It shows sharp

crystallization of anatase at 677 K for pure TiO2 (sample 1)

and at 671 K for S-doped TiO2 (sample 4).

Fig. 1 XRD pattern of sample 1

Fig. 2 Size distributions from the statistical processing of the TEM

images for sample 1

Table 1 The mean size of samples 1–6

Sample Mean size (D) nm

1 11.7

2 11.4

3 12.7

4 10.7

5 11.3

6 8.9

318 J Sol-Gel Sci Technol (2009) 51:315–329

123

The structural evolution (XRD) of samples 1 and 4, as a

function of temperature, is presented in Table 2. The XRD

analysis supplied valuable information regarding the

influence of S on TiO2 crystallization by means of lattice

constants and microstructural factors. The phase compo-

sition points out the presence of anatase as the single

crystalline phase for both samples thermally treated in the

range of temperatures between 573 and 773 K.

From these data one can establish that:

a) In the case of pure TiO2 (sample 1) as expected with

increasing temperature, the lattice becomes more

structured, so the lattice strains\S[quickly decrease.

At the same time and in agreement with\D[and\S[evolution, the geometry of the elementary cell (EC)

tends to the most stable state, resulting in a slight

contraction of the EC with temperature;

b) For the S-doped TiO2 (sample 4), the effect of sulfur

counters that of increasing temperature; indeed the rate

of crystallite growth is larger for sample 1 compared to

sample 4. Concerning u.c.v. (unit cell volume), the

decrease with increasing temperature is less significant

for sample 1 than for sample 4.

XPS measurements (Fig. 4) of the S 2p core level

indicate the presence of SO42- species on the sample

surface (S 2p core level binding energy of about 169 eV),

and the absence of sulfide species S2- (S 2p core level

binding energy of about 162 eV), at 573 and 773 K. This

indicates that the sulfur is present as S6? ion. These data

may be correlated with u.c.v. variations which suggest that

the substitution of some Ti4? ions (ionic radius = 0.68 A)

by S6? ions (ionic radius = 0.29 A) is the most probably

Fig. 3 TEM images of pure TiO2, sample 1 (a) and S-doped TiO2,

sample 4 (b)

250

350

450

550

650

750

160162164166168170172174

Binding energy (eV)

Inte

nsi

ty (

arb

. un

its)

sample 4 annealed at573 Ksample 4 annealed at773 K

169.0 eV

S 2p

Fig. 4 XPS S 2p core level spectra of sample 4, thermally treated at

573 and 773 K

Table 2 Calculated values of microstructural factors obtained from computerized analysis of XRD spectra

Sample/T (K) Identified phases Lattice constants Microstructural factors

a [A] c [A] u.c.v. [A3] \D[ [A] 10?3 9 \S[

1–573 Anatase 3.7752 (45) 9.4973 (164) 135.36 (56) 74 (7) 10.0 (8)

1–673 Anatase 3.7815 (13) 9.4709 (47) 135.43 (16) 178 (34) 1.4 (6)

1–773 Anatase 3.7750 (19) 9.4828 (68) 135.14 (23) 346 (51) 0.3 (3)

4–573 Anatase 3.7812 (18) 9.4778 (64) 135.51 (22) 172 (25) 1.6 (5)

4–673 Anatase 3.7783 (16) 9.4735 (58) 135.24 (20) 247 (37) 1.2 (4)

4–773 Anatase 3.7676 (55) 9.4561 (193) 134.23 (66) 302 (60) 1.5 (4)

a, c Lattice parameters, u.c.v. unit cell volume, D crystallite size, S internal strain

J Sol-Gel Sci Technol (2009) 51:315–329 319

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occurring. However, the accommodation of S6? ions in the

anatase lattice results in an increase in the lattice strains.

The difference between the ionic radii and change of Ti4?

and S6?, leads to a more distorted crystalline lattice. This

supplementary positive charge must be compensated to

maintain the electro-neutrality of the lattice, presumably by

O2- ions from the lattice or from the atmosphere.

Both XPS and XRD data led to the conclusion that the

initial distribution of the local electrostatic forces is dis-

torted, which leads to more structural defects. This fact

certainly suggests the possibility of highly chemically

reactive sites. This could explain the higher catalytic per-

formances of S-doped TiO2 materials (see later discussion).

The XPS S/Ti atomic ratio is 0.15 at 573 K and 0.08 at

773 K, characteristic of S diffusion in the material after

annealing at higher temperature.

Table 3 presents the influence of the temperature of the

thermal treatment of samples 1 and 4, on the chlorobenzene

(CB) photodegradation and on the mineralization of

organic chlorine during water purification.

The presence of S dopant in the TiO2 powders slightly

improved their photocatalytic activities and led to better

chlorobenzene removal yields ([CB] and gCB) than for the

un-doped samples. The sintering temperature also slightly

enhances the photocatalytic activity of the sol–gel powders.

The mineralization yield of the organic chlorine (gCl�) also

increases with increasing temperature.

The S-doped powder was also tested for the degradation

of different chloride organic compounds from aqueous

solutions. The results are presented in Table 4, together with

the influence of the irradiation time. The experiments were

performed with photocatalyst thermally treated at 673 K.

Table 3 The influence of the temperature of the thermal treatment of the pure (sample 1) and S-doped TiO2 photocatalyst (sample 4) on the

chlorobenzene (CB) photodegradation and on the mineralization of organic chloride during water purification

Photo-catalyst Temperature (K) CB and Cl content in contaminated water gCB (%) gCl� (%)

[CB] [Cl-]

mg/L M 9 103 mg/L M 9 103

1 573 0.34 0.0030 2.56 0.072 97 70

4 573 0.23 0.0020 2.27 0.064 98 70

4 673 0.18 0.0016 2.49 0.070 98 77

4 773 0.11 0.0010 2.66 0.075 99 82

Table 4 The influence of the irradiation time of S-doped photocatalyst (sample 4) in the degradation of different chloride organic compounds

during water purification

Pollutant-chloride

organic compound

Time (min) CB and Cl content in contaminated water gpollutant (%) gCl� (%)

Pollutant Cl-

mg/L M 9 10-3 mg/L M 9 10-3

MCB 0 10.23 0.091 – – – –

60 0.18 0.0016 2.49 0.070 98 77

90 0.04 0.0004 2.53 0.071 100 78

1,2 DCB 0 18.59 0.126 – – – –

60 1.65 0.011 7.39 0.210 91 82

90 0.80 0.0064 7.80 0.220 96 87

120 0.17 0.001 8.05 0.227 99 90

1,3 DCB 0 14.40 0.098 – – – –

60 0.77 0.005 5.94 0.171 97 85

90 0.22 0.002 6.15 0.173 99 88

120 0.04 0.0003 6.29 0.177 100 90

1,2,4 TCB 0 17.24 0.095 – – – –

60 0.87 0.005 8.17 0.230 94 81

90 0.33 0.002 8.49 0.239 98 84

120 0.10 0.0006 8.62 0.243 99 85

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123

The performance of the catalyst for the different types of

chloride organic pollutant is noted. Regardless of the

composition of the organic chloride pollutant, for irradia-

tion times exceeding 60 min, removal yields (gpollutant)

between 96 and 100% and mineralization yields of the

organic chlorine (gCl� ) between 78 and 90% have been

obtained.

BET (Brunauer, Emmett, Teller) SSA values for un-

doped (sample 1) and S-doped TiO2 (sample 4), as a

function of temperature are presented in Table 5.

As expected, the surface areas decrease with increasing

temperature, for both types of samples. Except for the

temperature of 773 K, the presence of sulfur led to a

decrease of the surface area. The significant variations in

surface areas do not have a consistent influence on the

photocatalytic activity, as could be observed from Tables 3

and 4. It may be concluded that the photocatalytic activity

is strongly influenced by the lattice defects induced by the

dopant as evidenced by XPS and XRD measurements.

3.2 Un-doped, S-doped and Ag-doped TiO2 coatings

Using the experimental conditions described in Sect. 2,

continuous and homogeneous coatings with good adher-

ence to the support were obtained. The conditions used for

thermal processing of the films were determined taking into

account the thermal behaviour of bulk gels obtained from

the sol used for coatings preparations. Thermal analysis

results of C–S–TiO2 (a) and C–Ag–TiO2 gels (b) are pre-

sented in Fig. 5.

In Fig. 5a, the differential thermal analysis curve (DTA)

shows an endothermic effect at 390 K, due to adsorbed

water and alcohol removal associated with mass loss on the

thermogravimetric curve (TG). Two exothermic peaks at

470 and 568 K with corresponding mass losses can be

attributed to the thermo-oxidative degradation of organics.

A broad exothermic peak with a maximum at 815 K could

be due to the transformation of the inorganic matrix after

the removal of the organic components with the amorphous

to anatase phase transformation.

In Fig. 5b, the DTA curve shows an intense endothermic

effect at 378 K, due to adsorbed water and alcohol removal

and a slight effect at 601 K due to the decomposition of the

organic species, both associated with a corresponding mass

loss (23% total mass loss). The exothermic effect at 813 K

on the DTA curve can be assigned to the crystallization of

rutile.

Because the coatings are very thin (see SE results

below) there is an overlap between their diffraction pat-

terns and the diffraction pattern of the support, which is

amorphous, irrespective of the thermal treatment. For this

reason, the phase composition was established on

un-supported gels obtained from the gelation of the sols

used for coatings preparation. Structural evolution (XRD

results) with temperature of the S- and Ag-doped TiO2

gels compared with the pure TiO2 gel is presented in

Table 6.

The calculated values of the microstructural factors

obtained from analysis of the XRD patterns have been

obtained with the same program mentioned in Sect. 3.1.

Table 5 BET specific surface area values for un-doped (sample 1)

and S-doped TiO2 (sample 4), as a function of temperature

Sample/T (K) BET specific

surface area (m2 g-1)

1 573 154

673 81

773 8

4 573 132

673 66

773 35

Fig. 5 Thermal behaviour of C–S–TiO2 (a) and C–Ag–TiO2 (b) gels

J Sol-Gel Sci Technol (2009) 51:315–329 321

123

The Ag-doping favors TiO2 crystallization in anatase

simultaneously with the transformation of anatase to rutile,

even at 573 K. This fact explains the decrease of the cat-

alytic activity in comparison with the S-doped sample (see

Table 9). In the Ag-doped TiO2 samples, annealed at 673

and 773 K, the anatase phase exhibits a larger mean

crystallite size and smaller internal strains than in the case

of similar S-doped samples.

XPS spectra of C–S–TiO2 and C–Ag–TiO2 coatings,

annealed at 573, 673 and 773 K for 1 h, are presented in

Fig. 6.

The XPS normalized spectra of the Ti 2p core level

(Fig. 6a) for the C–S–TiO2 sample treated at 573 K, show

the presence of a peak at a binding energy of 457.9 eV (Ti

2p3/2), which may be attributed to Ti2O3 (Ti3?). The second

peak at about 463 eV corresponds to the Ti 2p1/2 of the Ti

2p doublet (spin-orbit coupling). From the examination of

the S 2p core level (Fig. 6b), the presence of only one

contribution, at a binding energy of about 167 eV, attrib-

uted to SO32- like species can be noticed, as well as the

absence of sulfide like species. For this sample, the XPS

S/Ti atomic ratio is 0.06 (573 K), and 0.08 (773 K), compa-

rable to the case of the S-doped powders annealed at 773 K.

Regarding the C–Ag–TiO2 coatings, treated at 573 K,

Fig. 6c presents the spectrum of the Ti 2p core level, in

which a peak at 458.3 eV (Ti 2p3/2) can be observed that

may also be attributed to Ti2O3 (Ti3?). The second peak at

about 464 eV corresponds to Ti 2p1/2 of the Ti 2p doublet

(spin-orbit coupling). No changes of the intensity as a

function of temperature have been observed. The normal-

ized spectra of the Ag 3d core level are presented in

Fig. 6d. No significant change of the binding energy of Ag

Table 6 Calculated values of microstructural factors obtained from computerized analysis of XRD spectra

Sample Temperature [K] Identified phases Lattice constants Microstructural factors

a [A] c [A] u.c.v. [A3] \D[ [A] 10?3 9 \S[

C–S–TiO2 573 A 3.7849 (34) 9.4671 (133) 135.62 (44) 100 (36) 3.0 (3.1)

C–Ag–TiO2 A [97% 3.7838 (52) 9.5215 (184) 136.32 (64) 93 (11) 4.0 (7)

R \3% – – – –

C–S–TiO2 673 A 3.7775 (26) 9.4850 (91) 135.35 (31) 107 (20) 3.0 (1.2)

C–Ag–TiO2 A 89% 3.7788 (32) 9.4936 (112) 135.56 (39) 111 (24) 2.7 (1.3)

R 11% 4.6020 (106) 2.9528 (69) 62.54 (43) 389 (128) 8.2 (6.8)

C–S–TiO2 773 A 3.7856 (62) 9.4771 (242) 135.81 (79) 161 (78) 2.7 (2.6)

C–Ag–TiO2 A 54% 3,7859 (22) 9.4875 (78) 135.99 (27) 267 (112) 0.6 (5)

R 46% 4.5978 (47) 2.9602 (38) 62.58 (21) 619 (223) 1.4 (6)

A anatase, R rutile, a, c lattice parameters, u.c.v. unit cell volume, D crystallite size, S internal strain

0

4000

8000

12000

16000

20000

24000

452457462467

Binding energy (eV)

Inte

nsi

ty (

arb

. un

its) C-S-TiO2 annealed at 773 K

C-S-TiO2 annealed at 573 K457.9 eV

Ti 2p3/2

Ti 2p1/2

1050

1150

1250

1350

1450

1550

160162164166168170172174

Binding energy (eV)In

ten

sity

(ar

b. u

nit

s) C-S-TiO2 annealed at 573 K

C-S-TiO2 annealed at 773 K

167.7 eV

S 2p

0

2000

4000

6000

8000

10000

12000

452457462467

Binding energy (eV)

Inte

nsi

ty (

arb

. un

its)

C-Ag-TiO2 annealed at 573 K

Ti 2p1/2

458.3 eV

Ti 2p3/2

400

500

600

700

800

900

1000

362364366368370372374376378

Binding energy (eV)

Inte

nsi

ty (

arb

. un

its)

C-Ag-TiO2 annealed at 573 KC-Ag-TiO2 annealed at 673 KC-Ag-TiO2 annealed at 773K

367.7 eV

Ag 3d5/2

Ag 3d3/2

a

dc

bFig. 6 XPS Ti 2p, S 2p and Ag

3d core level spectra of

C–S–TiO2 (a, b) and

C–Ag–TiO2 (c, d) coatings,

thermally treated at 573, 673

and 773 K for 1 h

322 J Sol-Gel Sci Technol (2009) 51:315–329

123

3d5/2 (367.7 eV), probably corresponding to Ag?, has been

observed. The second peak at about 374 eV corresponds to

the Ag 3d3/2 of the Ag 3d doublet (spin-orbit coupling). A

drastic loss of the intensity increasing temperature was

observed: the XPS Ag/Ti atomic ratio varies from 0.02

(573 K) to 0.00 (773 K), via 0.01 (673 K). The surface

quantities of the Ag dopant are significantly lower than the

S surface concentrations.

The thickness of the C–S–TiO2 and C–Ag–TiO2 coatings

and volume fraction of the components versus temperature,

determined from spectroellipsometry measurements are

presented in Tables 7 and 8. The dispersion of the refractive

index (n) versus wavelength for mono- (a) and bi-layers (b)

for C–S–TiO2 and C–Ag–TiO2 coatings with thermal

treatment is presented in Fig. 7.

The fitting parameters were the thickness of the layers

and volume fraction of the components (amorphous TiO2,

anatase-TiO2, sol-containing thiourea, air-filled voids). The

dielectric constants of these compounds were taken from

different optical databases or handbooks [96–98] while for

sol-containing thiourea a Pulfrich-type refractometer has

been used. The optical constants of the films were obtained

from the best fit. The fitting error was calculated according

to equation:

Error ¼XN

i¼1

Reci � Ree

i

� �2þ Imci � Ime

i

� �2h i

=N

where Re, the real part of ellipsometric function (tanW*

cosD); Reci , the calculated value; Ree

i , the measured value;

Im, the imaginary part of ellipsometric function (tanW*

sinD); Imci , the calculated value; Ime

i , the measured value;

and N, the number of experimental points.

From SE measurements one can observe the increase of

the refractive index and consequently the decrease of the

film thickness with temperature as a result of the film

densification. The S-doped TiO2 coatings exhibit thickness

Table 7 The thickness (d) of

the C–S–TiO2 coatings, initial

and thermally treated, and the

volume fractions of the

components

Sample No. of layers d (A) TiO2 (%) Thiourea (%) Voids (%) Error 9 10-4

C1–S–TiO2-i 1 702 51.58 49.38 0.04 1.4

C1–S–TiO2-573 K 433 77.47 22.51 0.02 2.1

C1–S–TiO2-673 K 422 76.56 23.42 0.02 1.4

C1–S–TiO2-773 K 361 83.47 16.52 0.01 1.5

C2–S–TiO2-i 2 884 77.65 22.33 0.02 2.1

C2–S–TiO2-573 K 761 82.89 17.10 0.01 2.1

C2–S–TiO2-673 K 670 85.80 14.18 0.01 2.5

C2–S–TiO2-773 K 644 85.12 14.87 0.01 14.1

Table 8 The thickness (d) of the C–Ag–TiO2 coatings, initial and

thermally treated, and the volume fractions of the components

Sample No. of

layers

d(A)

TiO2

(%)

Voids

(%)

Error 9

10-4

C1–Ag–TiO2-i 1 500 65.75 34.25 2.2

C1–Ag–TiO2-573 K 370 76.30 23.70 2.0

C1–Ag–TiO2-673 K 430 72.46 27.54 2.0

C1–Ag–TiO2-773 K 350 79.03 20.97 1.7

C2–Ag–TiO2-i 2 880 77.03 22.97 6.0

C2–Ag–TiO2-573 K 620 85.58 14.42 2.5

C2–Ag–TiO2-673 K 600 85.06 14.94 1.9

C2–Ag–TiO2-773 K 520 92.94 7.06 1.0

0.40 0.45 0.50 0.55 0.60 0.65 0.70 0.751.70

1.75

1.80

1.85

1.90

1.95

2.00

2.05

2.10

2.15

2.201-deposition coatings

n

λ (µm)

C1-Ag-TiO2-i

C1-Ag-TiO2-573K

C1-Ag-TiO2-673K

C1-Ag-TiO2-773K

C1-S-TiO2-i

C1-S-TiO2-573K

C1-S-TiO2-673K

C1-S-TiO2-773K

0.40 0.45 0.50 0.55 0.60 0.65 0.70 0.75

1.85

1.90

1.95

2.00

2.05

2.10

2.15

2.20

2.25

C2-Ag-TiO2-i

C2-Ag-TiO2-573K

C2-Ag-TiO2-673K

C2-Ag-TiO2-773K

C2-S-TiO2-i

C2-S-TiO2-573K

C2-S-TiO2-673K

C2-S-TiO2-773K

2-deposition coatings

n

λ (µm)

a

b

Fig. 7 The dispersion of the refractive index (n) versus wavelength

for mono- (a) and bi-layers (b) for C–S–TiO2 and C–Ag–TiO2

coatings, thermally treated

J Sol-Gel Sci Technol (2009) 51:315–329 323

123

and refractive indices greater than the Ag-doped TiO2

samples. More details concerning the ellipsometric data of

the C–S–TiO2 are presented in our previous paper [86].

AFM images of C–S–TiO2 and C–Ag–TiO2 thermally

treated at 573 K, measured over a 9.15 9 9.15 lm area,

are presented in Fig. 8. The roughness values, Root–Mean–

Square (RMS) deviation, for Ag-doped TiO2 coatings were

greater (14 nm) than for S-doped samples (2.8 nm).

The results of the photocatalytic tests are presented in

Table 9.

The photocatalytic activity tests of the coatings obtained

with S- and Ag-doped TiO2 in the photodegradation of

chlorobenzene at neutral pH have shown that a prolonged

irradiation time and two layers lead to better chlorobenzene

removal yields (gCB). The chlorobenzene removal yield is

slightly better for C–S–TiO2 coating. This could be

explained either by the presence of anatase as a single

crystalline phase in the S-doped sample, or by the lesser

amount of Ag (compared to S) in the surface of the

coatings.

3.3 Ag-doped TiO2 sol–gel coatings obtained

from nanopowders

Using the experimental conditions described in Sect. 2.2, con-

tinuous and homogeneous coatings, labeled CN–Ag–TiO2,

with good adherence to the support were obtained. The

thermal processing of the films was determined from the

thermal behaviour of the gels obtained from the gelation of

the sol used for coatings preparations.

Thermal analysis results obtained for the CN–Ag–TiO2

gel are presented in Fig. 9b and are compared with the

corresponding data for the TiO2 matrix (TiO2-M) gel

(Fig. 9a).Fig. 8 AFM images of C–S–TiO2 (a) and C–Ag–TiO2 (b) coatings,

thermally treated at 573 K for 1 h

Table 9 The influence of the

number of layers and of the

irradiation time of C–S–TiO2

and C–Ag–TiO2 photocatalysts,

thermally treated at 573 K for

1 h in the photodegradation of

chlorobenzene during water

purification

Photocatalyst Number

of layers

Irradiation

time (h)

Remanent

chlorobenzene

(mg/L)

Chlorobenzene

removal

yield, gCB (%)

C1–TiO2 1 0.5 2.70 75

C1–Ag–TiO2 2.75 76

C1–S–TiO2 2.37 78

C1–TiO2 1 1.19 89

C1–Ag–TiO2 1.08 91

C1–S–TiO2 0.62 94

C2–TiO2 2 0.5 2.59 76

C2–Ag–TiO2 2.46 78

C2–S–TiO2 1.95 82

C2–TiO2 1 0.76 93

C2–Ag–TiO2 0.58 95

C2–S–TiO2 0.32 97

324 J Sol-Gel Sci Technol (2009) 51:315–329

123

In Fig. 9a, the DTA curve shows an intense endothermic

effect at 378 K, due to adsorbed water and alcohol removal

and a slight feature at 598 K due to the decomposition of

the organic matter, both associated with a mass loss (21%

total mass loss). The presence of two maxima, at 546 and

593 K, on the DTG curve illustrates that the decomposition

takes place in two steps. The exothermic effect at 797 K on

the DTA curve can be assigned to the crystallization of

rutile. Figure 9b, presents the thermal behaviour of the

CN–Ag–TiO2 gel. The DTA curve shows an endothermic

effect at 353 K due to adsorbed water and alcohol removal

and a broad exothermic effect between 423 and 693 K with

three maxima at 439, 581 and 642 K, probably due to the

thermo-oxidative degradation of the organic species. The

DTA effects are associated with mass losses, with the total

mass loss being 11%.

The structural evolution (XRD results) of the CN–Ag–

TiO2 gel with temperature, is presented in Table 10 and

compared to that of pure TiO2 (TiO2-M) and C–Ag–TiO2

gel, obtained by co-gelation.

The Ag-doping favors the transformation of TiO2 from

anatase to rutile, the presence of rutile being detected even

at 673 K in both samples (C–Ag–TiO2, CN–Ag–TiO2),

which does not occur for the S-doped samples. This pro-

cess is more evident for the sample prepared by co-gela-

tion. In this case, the rate of growth of the anatase

crystallites dimension is much greater, and is accompanied

by a decrease in the tensile strains of the anatase phase with

increasing temperature.

The morphological characterization by TEM of the

CN–Ag–TiO2 gel, CN–Ag–TiO2 coating and TiO2-M

coating are presented in Figs. 10, 11 and 12. In order to

illustrate the transition zone between the quasi-amorphousFig. 9 Thermal behaviour of: TiO2 matrix (TiO2–M) (a) and

CN–Ag–TiO2 gel (b)

Table 10 Microstructural

factors calculated from the

computerized profiles analysis

of the XRD spectra

A anatase, R rutile, a, c lattice

parameters, u.c.v. unit cell

volume, D crystallite size, Sinternal strain

Sample Identified

phases

Microstructural factors

a [A] c [A] u.c.v. [A3] \D[ [A] 103 9 \S[

TiO2–M-573 K A 3.7770 (12) 9.4651 (42) 134.90 (14) 127 (25) 3.7 (1.1)

TiO2–M-673 K A 3.7835 (19) 9.4883 (67) 135.83 (23) 161 (26) 1.3 (7)

TiO2–M-773 K A 70% 3.7756 (16) 9.4740 (57) 135.05 (19) 443 (92) 0.5 (4)

R 30% 4.5886 (38) 2.9561 (30) 62.24 (17) 685 (193) 1.1 (4)

CN–Ag–TiO2-573 K A 3.7836 (17) 9.5069 (61) 136.10 (21) 178 (31) 3.4 (1.0)

CN–Ag–TiO2-673 K A 3.7717 (30) 9.4896 (106) 135.00 (36) 165 (27) 1.9 (6)

R \3% – – – – –

CN–Ag–TiO2-773 K A 90% 3.7731 (18) 9.4449 (61) 134.46 (21) 198 (42) 1.8 (7)

R 10% 4.6048 (134) 2.9487 (64) 62.52 (50) 610 (31) 1.5 (3)

C–Ag–TiO2 573 K A 3.7838 (52) 9.5215 (184) 136.32 (64) 93 (11) 4.0 (7)

C–Ag–TiO2-673 K A 89% 3.7788 (32) 9.4936 (112) 135.56 (39) 111 (24) 2.7 (1.3)

R 11% 4.6020 (106) 2.9528 (69) 62.54 (43) 389 (128) 8.2 (6.8)

C–Ag–TiO2-773 K A 54% 3.7859 (22) 9.4875 (78) 135.99 (27) 267 (112) 0.6 (5)

R 46% 4.5978 (47) 2.9602 (38) 62.58 (21) 619 (223) 1.4 (6)

J Sol-Gel Sci Technol (2009) 51:315–329 325

123

and crystalline structure for CN–Ag–TiO2 coating treated

at 773 K for 1 h, the SAED (selected area electron dif-

fraction) image is also included between the TEM ones

(Fig. 11c).

The un-supported gels CN–Ag–TiO2 exhibits aggregates

of nanocrystallites with the nanocrystallite size in the range

10–30 nm at 573 K (Fig. 10a) and 20–50 nm at 773 K

(Fig. 10b). The CN–Ag–TiO2 coatings have a mixed struc-

ture. There are quasi-amorphous zones with granular struc-

ture of a few nanometers, together with crystalline zones

with the crystallite size of 5–10 nm at 573 K (Fig. 11a) or

10–20 nm at 773 K (Fig. 11b). The SAED pattern (Fig. 11c)

is taken from the area of the transition zone between quasi-

amorphous to crystalline structure shown in Fig. 11b. For

comparison, Fig. 12 presents the TEM image of the pure

TiO2 film at 573 K, obtained from the sol matrix (M). The

TiO2 films are amorphous and their morphology exhibits

nanometric porosity [21]. The increase of temperature at

773 K determined the beginning of the crystallization and

led to almost a doubling in the sizes of pores.

AFM images of Ag-doped TiO2 coatings obtained by

co-gelation and from nanopowders, measured over a

9.15 9 9.15 lm area, are presented in Fig. 13. The

roughness values Root–Mean–Square (RMS) deviation for

CN–Ag–TiO2 coatings were greater (20 nm) than for

C–Ag–TiO2 ones (14 nm). Regardless of the method used

to prepare the coating (from nanopowder or by co-gelation)

the roughness values of the Ag-doped films are greater than

those for the S-doped ones.Fig. 10 TEM images of CN–Ag–TiO2 gel thermally treated at 573 K

(a) and 773 K (b) for 1 h

Fig. 11 TEM images of

CN–Ag–TiO2 coating thermally

treated at 573 K (a) and 773 K

(b) for 1 h; SAED image of

CN–Ag–TiO2 coating thermally

treated at 773 K for 1 h (c)

326 J Sol-Gel Sci Technol (2009) 51:315–329

123

4 Conclusions

Sol–gel doped TiO2 powders and coatings have been

obtained by the alkoxide route. The role of S and Ag as

dopants of TiO2 has been studied. The structure and pho-

tocatalytic activity of the nanomaterials have been com-

pared, as follows:

A. Pure and S-doped TiO2 sol–gel nanopowders

• The influence of the type of Ti-alkoxide, of the

dopant and of the thermal treatment on the

nanopowders structure was established.

• The materials obtained from titanium ethoxide as

TiO2 source were tested for the degradation of

different chloride organic compounds from aque-

ous solutions.

• The presence of S as dopant in the TiO2 powders

only slightly improved their photocatalytic activ-

ities and led to slightly better chlorobenzene

removal yields than for un-doped samples.

B. Un-doped, S-doped and Ag-doped TiO2 nanometer

coatings

• The influence of the number of layers and of the

temperature of the thermal treatment on the

coatings structure and on their catalytic properties

has been established for each dopant.

• The catalytic activity of the prepared coatings has

been tested in the chlorobenzene degradation from

aqueous solutions.

• Both dopants (S and Ag) have improved the

photocatalytic activity of TiO2. The doped-sam-

ples exhibit higher removal yields than the

un-doped ones, regardless of the number of layers

and the irradiation times.

• Based on characterizations and photocatalytic tests

comparisons, the S-doped samples were more

efficient than the Ag-doped materials. The higher

photocatalytic activities of the S-doped TiO2

samples are attributed both to the presence of

anatase as the single crystalline phase of TiO2 as

well as to a more significant surface concentration

of dopant. In the Ag-doped case, the presence of

rutile together with anatase led to the slight

decrease of the photocatalyst efficiency.

C. Ag-doped TiO2 coatings from nanopowders

• Both effects of Ag as dopant and of temperature on

the structural behaviour were studied.

• The Ag-doping favors the transformation of TiO2

from anatase to rutile even at 673 K. This process

is more evident for the sample prepared by

cogelation.

Fig. 12 TEM image of TiO2–M coating thermally treated at 573 K

for 1 h

Fig. 13 AFM images of C–Ag–TiO2 (a) and CN–Ag–TiO2

(b) coatings, thermally treated at 573 K for 1 h

J Sol-Gel Sci Technol (2009) 51:315–329 327

123

Acknowledgments This work was supported by the National

Research Program CEEX, grants No. 642/2005-2008, 6113/2005-

2008 and 59/2007-2008.

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