www.elsevier.com/locate/apcatb

Applied Catalysis B: Environmental 55 (2005) 81–91

Photocatalytic degradation of azo dyes by organic-capped anatase

TiO2 nanocrystals immobilized onto substrates

R. Comparellia, E. Fanizzaa, M.L. Currib,*, P.D. Cozzolia, G. Mascoloc,R. Passinoc, A. Agostianoa,b

aDipartimento di Chimica, Universita di Bari, via Orabona 4, I-70126 Bari, ItalybCNR IPCF, Sez Bari c/o Dip. di Chimica, Universita di Bari,

via Orabona 4, I-70126 Bari, ItalycCNR-IRSA, Sez. Bari, Via F. De Blasio 5, I-70123 Bari, Italy

Received 18 June 2004; received in revised form 28 July 2004; accepted 30 July 2004

Available online 12 September 2004

Abstract

The UV-induced photocatalytic degradation of two azo dyes, Methyl Red and Methyl Orange, has been carried out in aqueous media in the

presence of oleic acid (OLEA)- and tri-n-octylphosphine oxide (TOPO)-capped anatase TiO2 nanocrystal powders (mean particle size: 6 nm)

deposited onto a quartz substrate. The progress of photodegradation was followed by combining UV–vis absorption measurements with

HPLC–MS analysis. The abatement efficiency for the two organic compounds was compared with that obtained with commercial TiO2 P25

Degussa by evaluating a few significant variables, such as the dye chemical structure, pH of the solution, and catalyst surface status.

Identification of several by-products by HPLC–MS analysis has allowed to propose a reasonable degradation pathway for both target

molecules. Significantly, although all titania catalysts were effective in removing both parent dyes and their related derivatives, the

degradation rate by the OLEA-capped TiO2 nanocrystals was double as that obtained with both its TOPO-capped analogous and TiO2 P25

Degussa. It is suggested that efficient catalysis strictly depends on microscopic mechanisms that occur at the catalyst surface, basically

involving specific dye adsorption and local density of terminal –OH moieties.

# 2004 Elsevier B.V. All rights reserved.

Keywords: Organic-capped nanocrystals; TiO2; UV-induced photocatalysis; Azo dyes

1. Introduction

In recent years, a new class of techniques devoted to

pollutant remediation, broadly referred to as advanced

oxidation processes (AOPs) [1,2] has emerged. AOP

methods are characterized by a common chemical action,

which basically relies on the primary reactivity of OH

radicals in driving oxidation processes, ultimately resulting

in the mineralization of a variety of pollutants.

Within AOPs, photocatalyst-based degradation methods

represent a very interesting branch of research in continuous

development. Band-gap photo-excitation of semiconductors

* Corresponding author. Tel.: +39 080 5442027; fax: +39 080 5442129.

E-mail address: m.curri@area.ba.cnr.it (M.L. Curri).

0926-3373/$ – see front matter # 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.apcatb.2004.07.011

generates [3–5] electron–hole pairs capable to attack the

organic matter either directly or indirectly by means of

highly reactive species deriving from their reaction with the

solvent and/or additives. In the case of TiO2, generally

recognized as one of the most efficient, non-toxic, and

inexpensive photocatalysts, most extensive studies have

been carried out by using the oxide powder suspended in

aqueous solution containing the target molecules [6]. More

recently, TiO2-based catalysts deposited onto suitable

substrates have become an attractive alternative to circum-

vent the technological difficulty as well as the high costs

related to the catalyst recovery. Unfortunately, the potential

of this approach has been limited by the inevitable reduction

of the overall surface active area associated to catalyst

immobilization, leading to significant loss of performances

R. Comparelli et al. / Applied Catalysis B: Environmental 55 (2005) 81–9182

[7,8]. In contrast to TiO2 Degussa P25, the most widely used

form of commercial titania photocatalyst, nanostructured

oxide crystals can be expected to compensate for such a

drawback due to the significant benefits associated to the

reduction of semiconductor particle size down to a few

nanometers [3–5]. First, the extremely high surface-to-

volume ratio in small particles scales with the surface

density of active sites available for substrate adsorption,

thus increasing the overall photoreaction rate. Further-

more, when the semiconductor nanocrystal size is com-

parable or smaller than the bulk exciton diameter, the band-

gap become size-dependent due to quantization effects [3],

making in principle possible to tune the electron–hole pairs

redox potentials and consequently achieve a certain

selectivity in the related photoreactions. Obviously, a

narrow size distribution is amenable in order to precisely

correlate the redox properties to the crystal size. Finally,

crystal defect concentration associated to trap states for

the photogenerated carriers may be low in high quality

nanocrystals, so that electron–hole separation can be

maximized [4,5].

For the purposes of the present work, surfactant-capped

anatase TiO2 nanocrystals with high crystallinity and with

a narrow size distribution have been used as photocatalysts.

The TiO2 nanocrystal powders were obtained by two

synthetic routes, being basically characterized by the type

of reaction, i.e. either hydrolytic or non-hydrolytic,

respectively, involved in titania nucleation and growth.

Therefore, TiO2 nanocrystals were prepared, which

essentially differed with respect to: (1) the sterical

hindrance offered by the passivating ligand, i.e. either

oleic acid or tri-n-phosphine oxide; (2) the nature of the

surfactant functional group binding to the crystal surface,

i.e. either –P=O or –CO�2 ; (3) the available surface density

of Ti–OH groups.

The obtained nanocrystals of a selected size have been

deposited onto transparent support in order to exploit them

in a photocatalytic process devoted to the photodegradation

of two azo dyes (Methyl Red and Methyl Orange). The

efficiency of the different types of nanocatalysts and that of

their equivalent commercial oxide have been compared

under the same conditions. The dye chemical structure, the

nature of catalyst surface, and pH were kept into account

in order to elucidate the catalytic process. HPLC–MS

analysis was used to identify the degradation by-product of

both target molecules and to gain an insight into the

possible decay pathways. Although the complete disap-

pearance of both parent dyes and of their related derivatives

occurred upon photocatalysis with all titania catalysts, the

oleic acid-capped TiO2 nanocrystals were found superior

catalysts than both their tri-n-phosphine oxide-capped

counterparts and TiO2 P25 Degussa. It is proposed that

efficient catalysis strictly depend on microscopic mechan-

isms that occur at the catalyst surface, basically involving

specific dye adsorption and local density of terminal –OH

moieties.

2. Experimental

2.1. Materials

Commercial TiO2 was TiO2 ‘‘Degussa P25’’ (non-porous

anatase; surface area, 50 m2 g�1; mean diameter, approxi-

mately 30 nm). Methyl Red (2-(4-dimethylamino-pheny-

lazo)-benzoic acid—C.I. 13020, MeRed), Methyl Orange

(Acid Orange 52—13025, MeOr), n-heptadecane (C7H16 or

HD, 99%), titanium tetrachloride (TiCl4, 99%), titanium

tetraisopropoxide (Ti(OPri)4 or TTIP, 98.9%), tri-n-octyl-

phosphine oxide ((C8H17)3PO or TOPO, 90%), trimethyla-

mino-N-oxide dihydrate ((CH3)3NO�2H2O or TMAO, 98%),

oleic acid (C18H33CO2H or OLEA, 90%), ethylene glycol

(C2H4(OH)2 or EG, 98%) were purchased from Aldrich.

Acetone, chloroform, methanol were of the highest purity

available and purchased from Aldrich. All aqueous solutions

were made by using twice-distilled water.

2.2. Catalyst preparation

Two different methods were applied to synthesis TiO2

nanocrystals.

2.2.1. Non-hydrolytic synthesis of TiO2 nanocrystals

All manipulation were performed using standard air-free

techniques. By this method, TOPO-capped TiO2 nanocrys-

tals were synthesised by reacting TiCl4 with TTIP at 300 8Cin a mixture of heptadecane and TOPO as surfactant, as

previously reported [9]. Briefly, two precursor stock

solutions, 1 M TiCl4 and 1 M TTIP in HD, respectively,

were prepared in a glove box. In a three-neck flask, a mixture

of TOPO and HD (6 g) was degassed at 100 8C for 1 h under

vacuum. The flask was then put under nitrogen flow and

1 mL of the TiCl4 stock solution was introduced into the

reactor. The temperature was raised to 300 8C at which 1 mL

of the TTIP solution was rapidly injected into the vessel to

nucleate titanium dioxide. The temperature was then

lowered to 250 8C and the nanocrystals growth was

completed in 10 min. The final nanocrystal size could be

tailored by varying the TOPO:Ti molar ratio in the starting

reaction mixture. TiO2 were collected by centrifugation after

adding an excess of MeOH and washed three times with

acetone to remove surfactant residuals. The obtained

nanocrystals were soluble in apolar solvents, such CHCl3or hexane, due to their non-polar surface coating (TOPO).

2.2.2. Hydrolytic synthesis of TiO2 nanocrystals

By this approach, OLEA-capped TiO2 nanocrystals were

obtained by slow hydrolysis of TTIP in technical grade

OLEA as both solvent and surfactant at low temperatures

(80–100 8C), as reported elsewhere [10]. In a typical

synthesis, 2–10 mmol of TTIP was added to 35 g of

previously degassed OLEA under nitrogen flow at 100 8Cand allowed to stir for 5 min. A 4–10 mmol of anhydrous

TMAO in 3.2–6.4 g (50–100 mmol) of EG was subsequently

R. Comparelli et al. / Applied Catalysis B: Environmental 55 (2005) 81–91 83

added. The solution was maintained in a close system and

stirred at 100 8C over 72 h. The reaction mixture appeared

clear during the whole period of the particle growth.

Hydrolysis of the titanium precursor occur upon reaction

with water released slowly upon esterification of OLEA and

EG. The final nanocrystal size could be tailored by varying

the TTIP content in the initial reaction mixture. The TiO2

nanocrystals were readily precipitated upon addition of an

excess of ethanol to the reaction mixture at room

temperature. The resulting precipitate was isolated by

centrifugation and washed three times with ethanol to

remove surfactant residuals. At this stage, the OLEA-coated

TiO2 nanoparticles were easily redispersed in non-polar

solvents, due to their non-polar surface coating (OLEA).

The TiO2 powders used for routine XRD and FT-IR

analysis were prepared by washing the precipitate repeat-

edly and evaporating the residual solvent under vacuum at

room temperature. Finally, there were subjected to the same

thermal treatments as used for the powder employed in the

photocatalytic experiments.

2.3. Catalyst characterization

FT-IR spectra of TiO2 powders were collected with a

Perkin Elmer Spectrum GX FT-IR Spectrometer with a

resolution of 4 cm�1. Measurements were performed with

pressed pellets which were made using KBr powder as

diluent.

XRD patterns were collected with a Philips PW1729

diffractometer in a conventional u–2u reflection geometry

using filtered Cu Ka radiation (l = 1.54056 A). For XRD

measurements the nanocrystal powder was placed on an Al

sample holder.

Transmission electron microscopy (TEM) images were

obtained using Philips EM 430 microscope operating at

300 kV. The samples for the analysis were prepared by

dropping dilute solutions of TiO2 nanocrystals onto 400-

mesh carbon-coated copper grids and leaving the solvent to

dry. The samples were stable under the electron beam and

did not degrade within the typical observation times.

2.4. Photocatalytic degradations

All photocatalytic experiments were assisted by catalysts

deposited onto a quartz slide by casting from either an

Table 1

Chemical structures of the dyes employed as target substrates in the TiO2-assist

Organic dye Chemical structure

Methyl Red (MeRed)

Methyl Orange (MeOr)

optically clear CHCl3 solution of organic-capped TiO2

nanocrystals, or a CHCl3 suspension of TiO2 P25 Degussa.

Typically, 0.1 mmol of TiO2 was spread over a support area

of 3.6 cm2. After deposition, the resulting films were

thermally treated at 150 8C for 20 min to improve catalyst

adhesion to the substrate. The transparent support was of

suitable shape in order to fit a 1 cm � 1 cm quartz cell and

positioned against the cuvette wall, which was further with

respect to light beam. The radiating source was a medium

pressure 200 W mercury lamp (l > 250 nm). The system

was arranged in a suitable geometry in order to monitor in

situ the reaction course by UV–vis spectroscopy. All

experiments were performed under ambient atmosphere

keeping the system under vigorous stirring.

MeRed and MeOr, two azo dyes, were chosen as target

compounds (Table 1). The initial concentration of the dyes

(whose respective pKa values are reported in Table 1) was

typically 3 � 10�5 M. The desired pH was obtained by

adding the proper amount of HCl or NaOH 0.1 M.

2.5. UV–vis and HPLC–MS analysis

Dye decoloration was monitored by using an Ocean

Optics UV–vis diode array spectrophotometer equipped

with an optical fibre and a deuterium lamp. The

determination of the concentration of the dyes and the

identification of their respective by-products were per-

formed by HPLC-UV-MS using a Varian 9012 chromato-

graphic system equipped with a Ultimate UV detector (LC-

Packing Dionex), set at 425 and 220 nm, interfaced to a

QSTAR hybrid Qq-TOF mass spectrometer (Applied

Biosystem/MSD Sciex, Canada) equipped with a turbo

ion spray interface. Samples, injected by a Gilson 234

autosampler equipped with a 9010 Rheodyne valve and a

40 ml loop, were eluted at 0.4 ml/min through a Luna

phenyl-hexyl column (3 mm, 150 mm � 3 mm) and pre-

column (Phenomenex) with the following gradient: from 5/

25/70 (ammonium acetate 50 mM in methanol/methanol/

water) to 5/75/20 in 10 min, which was then maintained for

5 min. The interface conditions were, for positive and

negative ion mode, respectively, as follows: nebulizer gas

(air) = 1.2 L/min, curtain gas (nitrogen) =1 L/min,

turboionspray gas (nitrogen at 350 8C) = 6 L/min, needle

voltage = 5000 and �4400 V, orifice declustering potential

= 40 and �40 V and focusing potential = 150 and �120 V.

ed photocatalysis

Catalyst-interacting functionality pK

–COOH 5.3

�SO�3 3.8

a

R. Comparelli et al. / Applied Catalysis B: Environmental 55 (2005) 81–9184

The flow from the HPLC-UV was split to allow 200 ml/min

to enter the turbo ion spray interface.

Fig. 2. FT-IR spectra in the 3500–400 cm�1 region of (A) OLEA-capped

and (B) TOPO-capped TiO2 nanocrystals.

3. Results and discussion

3.1. Structural and surface characterization of the

catalysts

The TiO2 nanocrystal powders employed in the present

photocatalytic experiments were obtained by two different

synthetic routes. The first approach, based on the high

temperature (300 8C) reaction of molecular organometallic

precursors (TiCl4 and Ti(OPri)4) in anhydrous heptadecane

(HD), a non-coordinating solvent, in the presence of tri-n-

phosphine oxide (TOPO), a strong surfactant and size-

regulating agent, yields TOPO-capped TiO2 nanoparticles.

Due to the non-hydrolytic reaction pathway involved in

titania nucleation, the as-prepared oxide nanocrystals

possess a negligible coverage of surface hydroxyl groups

[9]. This material will be referred to as nh-TiO2. The second

approach [10] consists of the low-temperature (100 8C)

base-catalyzed hydrolysis reaction of Ti(OPri)4 in oleic acid

(OLEA) as both solvent and surfactant. The as-obtained

OLEA-capped TiO2 nanoparticles are obviously character-

ized by a high surface density of surface hydroxyl groups,

and thus referred to as h-TiO2.

In Fig. 1 a representative X-ray diffraction (XRD) profile

of the as-synthesized nanocrystals is reported. Irrespective

of the synthetic method, the peak positions and their relative

intensities could be exclusively indexed by the known

standard TiO2 anatase pattern. Notably, the characteristic

line broadening of the diffraction peaks pointed to nanosized

crystalline domains. The average grain size, estimated to be

about 6.0 nm by applying the Debye–Sherrer formula, was

in good agreement with the mean particle size measured by

Fig. 1. XRD pattern of organic-capped 6 nm TiO2 nanocrystal powders

prepared by hydrolytic method (OLEA 35 g, TTIP 5 mmol, 2 M TMAO

5 mL). In the column diagram, the standard diffraction peaks of TiO2

anatase are reported.

transmission electron microscopy. The moderate thermal

treatment applied to improve the stability of catalyst film

induced no significant grain growth or phase transition.

TEM images (data not shown) confirmed the approximately

spherical morphology of the nanoparticles and their almost

perfect crystallinity, in agreement with literature results

[9,10].

The nature of the organic coating on the surface of the

TiO2 nanocrystals was investigated by FT-IR spectroscopy

and interpreted on the basis of pertinent studies on the

surface functionalization of titania [11,12]. In Fig. 2 the

typical IR spectra in the region 3250–400 cm�1 of the

thermally treated organic-capped TiO2 nanocrystals are

reported (Fig. 2A: OLEA-capped TiO2; Fig. 2B: TOPO-

capped TiO2). Above 2000 cm�1, both samples exhibit the

intense antisymmetric and symmetric C–H stretching

vibrations (at about 2920 and 2850 cm�1, respectively) of

the –CH2 groups in the hydrocarbon moiety. The shoulder at

2950–60 cm�1 can be associated with the asymmetric

stretching of the terminal –CH3 group of the alkyl chains.

Notably, for the OLEA-capped TiO2, a weak but definite

band at 3008 cm�1 is also present, attributable to the olefinic

C–H stretching. In addition, the C–H stretching signals in

the TiO2 sample are superimposed on an underlying surface

O–H broad stretching band centred at 3300 cm�1, being due

to titanol groups in the OLEA-capped materials, and

adsorbed H2O from the non-anhydrous solvent used in the

post-synthesis extraction procedures. Below 2000 cm�1, the

COO� antisymmetric and symmetric stretching vibrations

(two characteristic bands centred at 1520 and 1436 cm�1,

respectively) of carboxylate anions complexed with surface

Ti centers dominate in the spectra of the OLEA-capped TiO2

nanocrystals (Fig. 2A). From the value of their frequency

difference (Dn � 84 cm�1), the mode of binding of

carboxylate adsorbates onto the TiO2 surface might be

interpreted as being chelating bidentate. The lack of clear

R. Comparelli et al. / Applied Catalysis B: Environmental 55 (2005) 81–91 85

evidence for the free C=O stretching band at around 1650–

1720 cm�1 (cf. 1775 cm�1 for OLEA in the vapor phase)

seems to exclude the presence of both unionized OLEA

monomers and dimers possibly [13] having the C=O

involved in H-bonding with a Ti–OH2+ surface group.

Moreover, the expected weak contributions of the –CH2–

bending (�1450 cm�1), of the C–O–H bending

(�1410 cm�1) and of the C–OH stretching (�1280 cm�1)

bands cannot be unambiguously discerned, owing to the

possible coincidence of many signals. In the fingerprint

region (below 1300 cm�1), the broadness and the complex-

ity of the peaks make their unique interpretation quite

difficult. However, a weak P=O stretching centred at about

1240 cm�1 visible in pure TOPO is still present in the

spectrum of our TOPO-modified TiO2 sample (Fig. 2B).

Below 950 cm�1, the characteristic vibrations of the

inorganic Ti–O–Ti network in titanium dioxide can be seen.

To summarize, the titania catalysts employed in the

present photocatalytic experiments essentially differed with

respect to: (1) the chemical nature of the surface passivating

ligand, basically related to the different binding mode of the

–P=O and –CO�2 functionalities in TOPO and OLEA,

respectively; (2) the different sterical hindrance offered by

the outermost exposed alkyl chains of the surfactants on the

nanocrystal surfaces (TOPO has a rather bulky hindrance

due to the three C8-alkylchains, whereas OLEA possesses a

unique C18-chain with cis-conformation with respect to the

double bound in C8 position); (3) the initial density of

surface –OH groups, approximately corresponding to the

fraction of surface Ti centres which do not accommodate

capping surfactant molecules. As opposed to the surfactant-

capped TiO2 nanocrystal-based catalysts, the commercial

TiO2 P25 Degussa is composed of a mixture of the anatase

and the rutile phases with a considerably larger mean

particle size, a rather broad size-distribution, and organic-

free surface. The features of the employed catalysts are

summarized in Table 2.

3.2. Photocatalytic experiments

Photocatalytic degradation of dyes has been extensively

studied for several decades [3,4,15–29]. It is broadly

accepted that hydroxyl radicals (OH�) are produced in water

matrices from the direct oxidation of H2O, OH� ions or

terminal –OH groups present on the catalyst surface by the

photogenerated holes (h+). Under ambient atmosphere,

hydroxyl radicals can also arise from a series of reactions

Table 2

Characteristics of the different type of TiO2 photocatalysts

Catalyst Acronym Size Surface c

TiO2 P25 Degussa P25-TiO2 25–30 nma (polydisperse) None

Hydrolytic TiO2 h-TiO2 6 nmb (monodisperse) OLEA

Non-hydrolytic TiO2 nh-TiO2 6 nmb (monodisperse) TOPO

a [11].b From TEM and XRD.

initiated by the scavenging of the photogenerated electrons

(e�) by molecular oxygen [26,27].

3.2.1. Effect of pH

Despite of the large number of accurate studies, the effect

of solution pH during photocatalysis is still debated. The

variations of pH induce modification in the ionization state

of titania surface due to the establishment of acid–base

equilibria:

Ti�OH þ Hþ$Ti�OHþ2 pKa1 (1)

Ti�OH þ OH�$Ti�O� þ H2O pKa2 (2)

For nanosized TiO2, the pHZPC (zero point charge) is known

to be 5.1 [30], so that the surface is positively or negatively

charged at low or high pH, respectively. This behaviour can

be expected to primarily influence the adsorption of the dye

on the catalyst, thus affecting the overall photocatalytic

process.

In the case of sulfonated dyes [31–33], a model has been

proposed to account for the dramatic role played by pH in

determining the critical step of the decomposition. The

description considers the charge status of both the target

substrate and the catalyst surface by applying a simple

electrostatic reasoning. Accordingly, the increase of

bleaching recorded at acid pH was attributed to strong

dye adsorption, through the deprotonated �SO�3 moiety, on

the catalyst. This favourable interaction would enhance the

encounter probability of nascent OH radicals with the

organic dye. As opposed, at alkaline pH, the Coulombic

repulsion arising between �SO�3 and the negative oxide

surface would make the dye access to the catalyst a

diffusion-controlled process. In this case, the OH radicals

generated at the catalyst surface would more hardly attack

the target molecule. Consequently, a comparatively slower

decomposition rate was measured. The results of the present

work for MeOr are in good agreement with this model. For

all catalysts, the highest percentage of dye discoloration was

actually recorded at pH 2 following a constant illumination

period (Fig. 3B).

However, in the case of MeRed, a remarkably different

trend was recorded (Fig. 3A). For this dye, the carboxylic

group should be responsible for the absorption of the dye on

the catalyst. As pKa for MeRed is around 5.3, a Coulombic

repulsion can be expected to occur at extreme pH values.

Nevertheless, we found that the optimal condition was again

pH 2 for all catalysts. This evidence could be accounted for

apping Density of surface –OH groups Crystal phase

Higha 70% anatase, 30% rutile

High 100% anatase

Low 100% anatase

R. Comparelli et al. / Applied Catalysis B: Environmental 55 (2005) 81–9186

Fig. 3. Percentages of decoloration for: (A) MeRed and (B) MeOr obtained with different titania photocatalysts (½dyet¼0 = 3 � 10�5 M; irradiation time:

120 min) at different pH values. The values were estimated from the relative decay of the visible absorbance maximum.

by invoking other competitive factors in the mechanism of

photocatalysis. It can be suggested that the presence of

positive charge on titania surface could make the photo-

generated electrons reach more readily the catalyst surface,

thus preventing detrimental electron–hole recombinations to

a higher extent. Surprisingly, although nanosized TiO2

colloids are known to be subjected to a facile acid-catalyzed

dissolution [34], h-TiO2 encountered inherent chemical

instability problems that prevented photocatalysis at high

pH. Such drawback has been exceptionally found in

previous studies on TiO2 P25 films, however, no explanation

was provided [35,36]. As this phenomenon was exclusively

detected upon catalyst illumination, a self-oxidation process

induced by the photogenerated holes may be tentatively

proposed, in analogy with what found for other photo-

catalysts [37]. Such an UV-driven corrosion would be

possibly favoured by removal of the ligands from the surface

of titania. Previous studies on the interaction of OLEA

molecules with titania actually proved that the binding of

this surfactant is made labile at alkaline pH due to increased

solubility of deprotonated OLEA in water [11]. On the other

hand, the TOPO-capped nh-TiO2 films were inherently

stable due to the high resistance of the Ti–O–P bond toward

hydrolysis [38,39] and to the negligible solubility of TOPO

in water, being expected also to be pH-independent.

3.2.2. Effect of catalyst

To study the effect of the nature of the catalyst,

experiments were carried out under optimal pH conditions.

The efficiency of the various photocatalysts was compared

by evaluating the percentage of decoloration from the decay

of both the visible absorbance and the quasi-molecular ion

peak of the dye detected by HPLC–MS analysis at the same

irradiation time.

Controlled photocatalytic experiments carried out on a

blank sample in a scaled-up system were performed in order

to investigate the fate of the organic coating on the TiO2

surface. Significantly, the global increase of TOC value

represented only a negligible percentage of the TOC

measured in the presence of the target dyes within the

typical timescale of a photodegradation experiment (about

150 min). Such evidence allowed to exclude that the

degradation rate of each dye over the different photo-

catalysts could be significantly affected to a varying extent

by partial consumption of active oxidant species by the

residual capping ligand molecules. Moreover, for both dyes

only a negligible bleaching was observed under illumination

when the catalyst was absent or in the presence of

unirradiated catalyst. In the case of MeRed, nanostructured

h-TiO2 was found to be the best catalyst (Fig. 4A). As a

matter of fact, h-TiO2 catalyzed a 100% of dye bleaching

within 30 min of reaction, whereas discoloration remained

at 70 and 40% with P25-TiO2 and nh-TiO2, respectively. The

same trend was observed by monitoring the dye removal

decay by HPLC-MS (Fig. 4B). The slightly higher

percentage in the abatement process found can be ascribed

to the formation of by-products having absorption features

similar to parent dye (see also Section 3.3), due to minor

changes in their chemical structure. In the case of MeOr

(Fig. 5), the photocatalytic performance of the three

photocatalysts resembled the same order of efficiency:

again h-TiO2 demonstrated to be more effective than P25-

TiO2, while nh-TiO2 remained the least effective.

This behaviour could be accounted for by considering the

intrinsically different amounts of hydroxyl groups available

at the surface of the catalyst particles. As discussed above,

the presence of –OH groups is directly related to the local

production of OH radicals, while also providing acidic–

basic sites for adsorption of the substrates. As both h-TiO2

and P25-TiO2 [14] are derived from hydrolysis of Ti

molecular precursors, they should presumably have a higher

density of surface –OH moieties, as opposed to the case of

nh-TiO2, being, in fact, synthesized by using a water-free

method. The higher efficiency of h-TiO2 with respect

commercial P25-TiO2 can be justified by the increase in

surface-to-volume ratio in the nanocrystal-based oxide due

R. Comparelli et al. / Applied Catalysis B: Environmental 55 (2005) 81–91 87

Fig. 5. (A) Spectrophotometric evaluation of MeOr decoloration.

½MeOrt¼0 = 3 � 10�5 M; pH = 2. The values were estimated from the

relative decay of the absorbance maximum. (B) Percentage of target

molecule removal. The values were estimated from the relative decay of

the parent ion peak by MS.

Table 3

Percentages of dye removal measured after 30 min of illumination

Catalyst MeRed (dye removal) MeOr (dye removal)

h-TiO2 98% 55%

nh-TiO2 53% 15%

P25-TiO2 50% 20%

½Dyet¼0 = 3 � 10�5 M; pH = 2. The values were estimated from the relative

decay of the parent ion peak by MS.

Fig. 4. (A) Spectrophotometric evaluation of MeRed decoloration.

½MeRedt¼0 = 3 � 10�5 M; pH = 2. The values were estimated from the

relative decay of the absorbance maximum. (B) Percentage of target

molecule removal. The values were estimated from the relative decay of

the parent ion peak by MS.

to its considerably smaller mean particle size. This feature

would compensate for the OLEA-passivated sites (i.e. for

the surface fraction not available for catalysis), which would

not actually occupy a high fraction of surface area in h-TiO2.

Furthermore, in h-TiO2 100% of anatase phase, the most

photoactive phase for TiO2 [2,40], is present, being expected

to enhance photocatalysis with respect to P25-TiO2 which,

in fact, contains only 70%.

In the case of nh-TiO2 a lower activity than h-TiO2 was

observed, although the constituent nanocrystals possessed a

mean particle size quite close to h-TiO2. Due to the non-

hydrolytic reaction pathways involved on the synthesis of

this material, the resulting oxide can be expected to be

characterized by an almost fully TOPO-passivated surface

and, in turn, by a comparatively scarce density of free –OH

groups with respect to h-TiO2. Accordingly, as the

coordination of –P=O to titania is known to be very strong,

the ligand is even more hardly expected to be displaced in

aqueous environment (see Section 3.2.1).

3.2.3. Influence of the molecular structure of dye

Experiments taken under the best reaction conditions (pH

= 2; irradiation time: 30 min) point out that each investigated

catalyst degraded MeRed more quickly than MeOr (Table

3). This evidence suggests that a precise role must be played

by the specific functional group, which differentiates the

molecular structure of each dye. Probably, the adsorption of

the target molecule on the catalyst surface should be

regarded as a critical step toward efficient catalysis. MeRed

adsorption through the carboxylic moiety can be reasonably

expected to be weak especially at low pH, being mainly

electrostatic in nature. As opposed, due to its favourable

dimension and spatial geometry, the �SO�3 attaches to

surface TiIV centres by assuming a bidentate coordination

through the two sulfonilic oxygens. This process would be

accompanied by substitution of a surface coordinated –OH

moieties. Because of the strong overlap between the 3d

orbitals of the TiIV atoms and the 2p orbitals of oxygens, the

formation of Ti–O bonds would have a strong covalent

character [41]. As a result of MeOr complexation, a number

of surface sites would be temporarily passivated, thus

R. Comparelli et al. / Applied Catalysis B: Environmental 55 (2005) 81–9188

Table 4

Proposed chemical structures of identified by-products during MeOr degradation

By-product Chemical structure Molecular weight P25-TiO2 nh-TiO2 h-TiO2

1 291 X X X

2 277 X

3 227 X X X

4 321 X X X

5 242 X X X

6 151 X X

leading to catalyst deactivation [41] and leaving the surface

depleted in terminal –OH groups. This detrimental

mechanism can remain operative over large periods of

photocatalysis, as primary by-products of MeOr actually

keep on bearing the sulfonic groups (see Section 3.3).

3.3. By-products formation

By-products formation was also investigated by per-

forming HPLC–MS analysis of irradiated reaction mixtures.

In Tables 4 and 5 the chemical structures of several

identified by-products are reported for MeOr and MeRed

Table 5

Proposed chemical structures of identified by-products during MeRed degradatio

By-product Chemical structure Molecul

1 255

2 241

3 271

4 287

5 285

6 317

degradation, respectively. Experimental data for both dyes

are consistent with two different mechanisms involved in

the degradation of parent compound. The first one involves

the homolytic rupture of the nitrogen–carbon bond of the

amine group leading to the substitution of methyl group

with a hydrogen atom. Such a photolytic route occurs

consecutively giving rise to by-products 1 and 2 of both

dyes (Tables 4 and 5). As for the second mechanism,

experimental results showed the formation of by-products

whose molecular weights are consistent with hydroxyl

group substitution on benzene rings. These latter are by-

products 3–5 for both MeOr and MeRed. The formation of

n

ar weight P25-TiO2 nh-TiO2 h-TiO2

X X X

X X X

X X X

X

X X X

X X X

R. Comparelli et al. / Applied Catalysis B: Environmental 55 (2005) 81–91 89

Fig. 6. Time evolution of by-product (1) during MeRed degradation, by

monitoring the corresponding quasi-molecular ion by MS.

such by-products can be explained by the attack of hydroxyl

radicals to benzene rings by substitution reactions being

well-known in the literature. It is reasonable to assume that,

for both dyes, OH substitution takes first place on the

benzene ring carrying the dimethylamino group due to its

capability to stabilize the intermediate hydroxy-benzene

radical, as opposed to the other benzene ring that carries a –

COOH and –SO3H moiety in MeRed and MeOr, res-

pectively. The formation of by-products 3 and 5 from MeOr

Fig. 7. Proposed degradation pathway for MeOr during photocatalysis. The schem

demonstrates that the ipso-substitution by a hydroxy radical

can also take place at the carbon which carries the sulfonic

moiety. Furthermore, for both dyes the presence of by-

product 3, derived from both OH substitution and

photolysis, suggests that the two mechanisms are indepen-

dently active. In Fig. 6 the temporal formation-decay of by-

product 1 for MeOr is reported as a representative example

of by-product evolution, being the evolution of other by-

products similar. The trends reported in the figure show that

the production of the intermediate compound was max-

imized at reaction times between 15 and 90 min and slowly

disappeared in the late stages of irradiation. After 120 min

no by-product could be detected by HPLC–MS. Due to the

lack of standards for the identified by-products, the reported

trends must be regarded as semiquantitative only. However,

as the structures of identified by-products are similar to that

of the parent dye, it is reasonable to assume that their MS

response are similar too. By this argument, it is actually

found that the identified by-products represent just a minor

fraction (<5% as compared to initial concentration of

parent dye) as compared to other polar degradation by-

products that could not be detected by HPLC–MS. As such

polar compounds should not have the azo-bond in their

structure, the reaction mixture results in being colorless.

This behaviour suggests the occurrence of the opening of

the aromatic ring due to consecutive oxidation reactions

which ultimately yield low molecular weight compounds,

as already reported during the extensive oxidation of

organic compounds [42].

es illustrate that two mechanisms of degradation are independently active.

R. Comparelli et al. / Applied Catalysis B: Environmental 55 (2005) 81–9190

Fig. 8. Proposed degradation pathway for MeRed during photocatalysis. The schemes illustrate that two mechanisms of degradation are independently active.

On the basis of the above results, possible degradation

pathways have been proposed for MeOr and MeRed in Figs.

7 and 8, respectively. The schemes illustrate the two

mechanisms of degradation, previously suggested, based on

photolytic attack to the dimethyl-amino group and on the

attack of hydroxyl radicals to the aromatic rings.

4. Conclusions

This study demonstrates that efficient photocatalysis can

be realized with nanosized oxide catalysts supported onto

substrates. The decrease in the exposed catalyst surface area

upon immobilization can be effectively counterbalanced by

employing colloidal TiO2 nanocrystal powders of a few

nanometers in diameter, provided that the organic capping at

their surface does not prevent dye access to the catalyst and/

or limit the local density of –OH groups. In this regard,

organic-capped TiO2 nanocrystals offer the perspective of

developing a new generation of efficient photocatalysts by

tailoring the oxide particle size and its surface character-

istics.

Acknowledgment

The authors wish to acknowledge Vito Locaputo for the

assistance in carrying out the HPLC–MS analyses.

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