Enhanced photocatalytic activity of TiO2 nano-structured

thin film with a silver hierarchical configuration

Jinyu Zheng a,b, Hua Yu c, Xinjun Li a,*, Shanqing Zhang c,**a Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou 510640, PR China

b Graduate School of the Chinese Academy of Sciences, Beijing 100039, PR Chinac Australian Rivers Institute and Griffith School of Environment, Gold Coast Campus, Griffith University, QLD 4222, Australia

Received 14 June 2007; received in revised form 10 July 2007; accepted 10 July 2007

Available online 28 July 2007

Abstract

TiO2 sol–gels with various Ag/TiO2 molar ratios from 0 to 0.9% were used to fabricate silver-modified nano-structured TiO2 thin films using a

layer-by-layer dip-coating (LLDC) technique. This technique allows obtaining TiO2 nano-structured thin films with a silver hierarchical

configuration. The coating of pure TiO2 sol–gel and Ag-modified sol–gel was marked as T and A, respectively. According to the coating order and

the nature of the TiO2 sol–gel, four types of the TiO2 thin films were constructed, and marked as AT (bottom layer was Ag modified, surface layer

was pure TiO2), TA (bottom layer was pure TiO2, surface layer was Ag modified), TT (pure TiO2 thin film) and AA (TiO2 thin film was uniformly

Ag modified). These thin films were characterized by means of linear sweep voltammetry (LSV), X-ray diffraction (XRD), scanning electron

microscopy (SEM), electrochemical impedance spectroscopy and transient photocurrent (Iph). LSV confirmed the existence of Ag0 state in the

TiO2 thin film. SEM and XRD experiments indicated that the sizes of the TiO2 nanoparticles of the resulting films were in the order of

TT >AT > TA > AA, suggesting the gradient Ag distribution in the films. The SEM and XRD results also confirmed that Ag had an inhibition

effect on the size growth of anatase nanoparticles. Photocatalytic activities of the resulting thin films were also evaluated in the photocatalytic

degradation process of methyl orange. The preliminary results demonstrated the sequence of the photocatalytic activity of the resulting films was

AT > TA > AA >TT. This suggested that the silver hierarchical configuration can be used to improve the photocatalytic activity of TiO2 thin film.

# 2007 Elsevier B.V. All rights reserved.

www.elsevier.com/locate/apsusc

Applied Surface Science 254 (2008) 1630–1635

Keywords: Photocatalysis; TiO2 thin film; Ag loading

1. Introduction

Substantial effort has been devoted to study the photo-

catalytic degradation of organic pollutants using nano-

structured TiO2 catalyst due to its high oxidation efficiency,

outstanding chemical stability and environmentally friendly

nature [1]. However, the efficiency of the photocatalytic

degradation reaction is limited by the high recombination rate

of photoinduced electrons and holes. Much effort has been

contributed to improve the photocatalytic efficiency of TiO2 by

ion dispersion and metal loading [2–9].

* Corresponding author. Tel.: +86 20 87057781; fax: +86 20 87057677.

** Corresponding author. Tel.: +61 7 5552 8155; fax: +61 7 5552 8067.

E-mail addresses: lixj@ms.giec.ac.cn (X. Li), s.zhang@griffith.edu.au

(S. Zhang).

0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.apsusc.2007.07.165

The sol–gel method is one of the popular techniques used for

the fabrication of TiO2 thin film modified by metal ions, which

were uniformly modified in the thin film in most of studies. In

recent years, we have developed a novel method using layer-by-

layer dip-coating (LLDC) processes for the fabrication of TiO2

thin film. In our previous works, metal ions, such as Mo6+ [10]

and Mn4+ [4] that can replace Ti4+ in the lattice of TiO2 due to

their similar ion radius, have been selected to modify TiO2 thin

film. The preliminary result suggested that the layer-by-layer

process (previously referred to as ‘‘uneven doping’’) was able to

improve the photocatalytic activity significantly compared with

the traditional process (previously referred to as ‘‘even doping’’).

Ag is one of the most promising transition metals for the

improvement of photocatalytic activity of TiO2 thin film [11]

and has been investigated extensively in the literature [12–16].

In this paper, we prepared the TiO2 thin films loaded with Ag

using the LLDC sol–gel process. The effects of various loading

modes and contents on the separation efficiency of photo-

J. Zheng et al. / Applied Surface Science 254 (2008) 1630–1635 1631

generated carriers as well as the mechanism of photocatalytic

activity enhancement were investigated.

2. Experimental

2.1. Preparation of TiO2 sol and Ag–TiO2 sol

Precursor solutions for TiO2 thin films were prepared at

room temperature by the method illustrated in our previous

work [10]. Briefly, 68 mL of tetrabutyl titanate and 16.5 mL of

diethanolamine were dissolved in 210 mL absolute ethanol,

and then the mixture was stirred vigorously for 1 h at room

temperature (solution A). A mixture of 3.6 mL H2O and

100 mL ethanol (solution B) was added dropwise into the

solution A under stirring. The resulting solution, an alkoxide

solution, aged for 24 h at room temperature will result in a

TiO2 sol.

The preparation of Ag–TiO2 sol was similar to that of TiO2

sol. The only difference was that various content of AgNO3

(AR grade) were added into solution B before it was added into

solution A. The Ag–TiO2 sols containing Ag loading content of

0.1, 0.3, 0.5 and 0.9% were prepared. The percentage of the Ag

concentration indicated, in this paper, refers to the molar

percentage to TiO2.

2.2. Preparation of thin films

Firstly, soda lime glass slices (200 mm � 35 mm � 2 mm)

were pretreated using acid solution and distilled water in an

ultrasonic bath before being coated with a SiO2 layer. Secondly,

the glass slices were dipped into TiO2 sol or Ag–TiO2 sol and

withdrawn at the speed of 2 mm s�1. They were allowed to dry

and then baked at 100 8C for 10 min. The second process was

repeated according to Table 1 in order to obtain a hierarchical

TiO2 structure. In this paper, this technique is defined as layer-

by-layer dip-coating (LLDC). Finally, the glass slices with

various TiO2 hierarchical structures were annealed at 500 8Cfor 2 h in an electric muffle furnace to assure the mechanical

strength of the TiO2 film. The coatings of pure TiO2 sol–gel and

Ag-modified sol–gel were marked as T and A, respectively.

According to the coating order and the nature of the TiO2 sol–

gel, four types of the TiO2 thin films were constructed and

marked as AT (bottom layer was Ag modified, surface layer was

pure TiO2), TA (bottom layer was pure TiO2, surface layer was

Ag modified), TT (pure TiO2 thin film) and AA (uniformly Ag

modified TiO2 thin film).

Table 1

Preparation of TiO2 thin films using the layer-by-layer dip-coating technique

Coating mode Thin film composition

Bottom Surface

TT Four coats of TiO2 sol Four coats of TiO2 sol

TA Four coats of TiO2 sol Four coats of Ag–TiO2 sol

AA Four coats of Ag–TiO2 sol Four coats of Ag–TiO2 sol

AT Four coats of Ag–TiO2 sol Four coats of TiO2 sol

2.3. Characterization

The thickness of TiO2 thin films was measured by scanning

electron microscopy (SEM) at 240 nm which was consistent

with our previous work [10]. The crystalline phase of the TiO2

thin films was determined by XRD using a diffractometer (D/

MAX-IIIA) with Cu Ka radiation. The transmittance spectra

of thin films were obtained using a UV–vis spectrophotometer

(U-3010).

2.4. Electrochemical experiments

2.4.1. Preparation of thin film electrode

The four types of the TiO2 thin films (i.e., AT, TA, AA and TT)

were coated on conducting substrates (ITO glasses) using the

same protocol described in Section 2.2, except without the

coating of SiO2 layer. ITO glasses were used in LSV, photo-

electrochemical current (Iph) and ESI Nyquist measurements.

2.4.2. Electrochemical measurement

Electrochemical characterization of LSV plots, and Iph and

ESI Nyquist plots were performed in a three-electrode system

made of quartz cells linked with CHI660 electrochemical

station. A TiO2 thin film electrode, a platinum sheet

(20 mm � 30 mm) and an Ag/AgCl electrode served as the

working electrode, counter electrode and reference electrode,

respectively. A 0.5 mol L�1 Na2SO4 aqueous solution was

employed as a supporting electrolyte. All the photoelectro-

chemical tests were carried out under the illumination of a

mercury lamp (5 W, lp = 365 nm) at room temperature.

2.5. Evaluation of photocatalytic activity

The photocatalytic degradation of methyl orange was carried

out in a home-built reactor. A high-pressure mercury lamp

(125 W, lp = 365 nm) was used as the light source. In each run

five glass slices of TiO2 thin film were placed into a 10 mg L�1

methyl orange solution in the reactor. After bubbling for 30 min

in order to obtain absorption equilibrium on the surface of the

thin films, the illumination was applied to initiate the reaction.

The UV–vis spectrophotometer was used to determine the

concentration of methyl orange before and after photocatalytic

degradation. The degradation percentage of methyl orange in the

reaction process could be calculated by the following formula:

Degradation percentage ¼ C0 � Ct

C0

� 100%

where Ct is the concentration of methyl orange after 80 min

reaction and C0 is the initial concentration.

3. Results and discussion

3.1. Characterizations of the thin film structure

Metallic Ag nanoparticles, formed on the surface of the Ag-

modified TiO2 films from Ag+-containing TiO2 sol–gel, can be

observed under SEM [17,18]. Fig. 1 shows the SEM images of

Fig. 1. SEM images of the TiO2 thin films modified with 0.5% Ag content (a, b

and c correspond to AA, TA and AT, respectively) and TT (d) thin films.

J. Zheng et al. / Applied Surface Science 254 (2008) 1630–16351632

the various TiO2 thin films annealed at 500 8C for 2 h. The Ag/

TiO2 ratio of the Ag-loaded TiO2 sol was 0.5%. The SEM

indicated that the average particle size of the thin films is

approximately 20–30 nm. More importantly, it can be observed

that the average particle size decreased according to the

sequence of AA <TA < AT < TT. The XRD patterns of these

TiO2 films (as shown in Fig. 2) demonstrated that the films

consist of pure anatase with the average crystallite size of the

TiO2 films being ca. 17, 18, 21 and 28 nm for AA, TA, AT and

TT, respectively, calculated using the Scherrer equation [19]

which was consistent with the SEM results. On the basis of the

SEM observation and XRD calculation results, we can

conclude that Ag loading in TiO2 thin film is deemed to

inhibit the growth of the average particle size [18] and the

extend of the inhibition is dependent on the amount of Ag

present in the TiO2 thin films. This is in line with the reported

Fig. 2. The XRD patterns of the Ag-modified TiO2 thin films with 0.5% Ag

content and TT thin film under UV illumination.

XRD investigation [17]. Ag+ ions introduced by the sol–gel

process cannot get into the lattice of the TiO2 because the radius

of Ag+ ions (ca. 126 pm) is much larger than that of Ti4+ (ca.

68 pm) [11]. The Ag+ ions will, therefore, spread evenly on

TiO2 nanoparticles during the dip-coating process. In other

words, the TiO2 nanoparticles will be ‘‘wrapped’’ and separated

by the Ag+ ions. During the sintering process, the Ag element

here became a physical barrier to confine the TiO2

nanoparticles. Hence the energy necessary for the movement

of the grain boundary for the merging between TiO2

nanoparticles increased and this consequently led to the

inhibition of the TiO2 particle size growth. Apparently, the

extent of the inhibition is dependent on how well the TiO2

particles are separated, which relies on the amount of Ag+

present in the films. This size inhibition effect can be used to

explain the sequence of the average TiO2 particle size shown in

the SEM. The reason the AA film had the smallest size, is that it

had eight coats of Ag+ loaded on the TiO2 sol. As the TT film

does not have any Ag+ ions at all, it’s free from the size

inhibition effect and subsequently has the largest TiO2

particles. The AT film and TA film contain the same amount

of Ag and TiO2, which should lead to the same size of TiO2.

However, a very minor difference between the AT film and TA

film can be observed if we compare the two images carefully. In

particular, the particle size of AT film was slightly bigger than

that of the TA film. This occurred because the SEM image

shows the surface structure rather than the size of the whole

film, and hence SEM image does not reflect the composition of

the whole film. In this case, although the same amount of Ag

was distributed unevenly in AT and TA films, Ag was

distributed with a concentration gradient in these two films.

Compared with TA film, AT film has a lower Ag concentration

at the surface because of the last four coatings of pure TiO2 sol.

Therefore, AT has a larger particle size than TA film. Compared

with TT film, AT film has a higher Ag concentration at the

surface because of the Ag diffusion to the surface from the

bottom layer. Consequently, AT has a smaller size than TT film.

In general, the average particle size sequence of the TiO2 films

(AA <TA < AT < TT) is opposite to the order of Ag amount

on the surface of the film (AA >TA > AT > TT). This

suggests that the Ag distribution in the thin film was in a

fashion of vertically gradient rather than in a way of perfect

layer form.

3.2. Electrochemical characterization

Electrochemical tools are powerful and effective tools for

the characterization of TiO2 thin film [20,21]. Immobilization

of TiO2 onto conducting substrates, such as Ti sheet and ITO

glass, makes the application of the electrochemical techniques

(such as LSVand chronoamperometry) possible to the resulting

thin films.

LSV was firstly conducted to study the redox properties of

the TiO2 films. Fig. 3a shows the LSV voltammograms of the

various TiO2 thin films described in Fig. 1. The experiments

were carried out at a scanning rate of 2 mV s�1 without UV

illumination. Distinctive oxidation peaks in the range of 0.175–

Fig. 3. LSV voltammograms of the TiO2 thin films in (a) 0.5 M Na2SO4

aqueous solution in the dark; (b) 0.5 M Na2SO4 aqueous solution under UV

illumination and (c) 0.5 M Na2SO4 aqueous solution under illumination with

0.03 mM ethanol.

Fig. 4. The photocurrent of AT thin films with varied Ag content and the TT

thin film under zero bias.

J. Zheng et al. / Applied Surface Science 254 (2008) 1630–1635 1633

0.25 V of the Ag-loaded thin films (AT, TA, AA) suggests the

existence of metal Ag with its valence state being Ag0.

Electrochemical behavior of Ag/TiO2 composite surfaces

suggested that Ag can be oxidized electrochemically [22].

Fig. 3b shows LSV voltammogram of the TiO2 thin films under

UV illumination. Under UV illumination, the current remark-

ably increased compared to the LSV voltammograms without

UV illumination (see Fig. 3a). Distinctive oxidation peaks were

observed in the range of 0.175–0.25 V, within the same

potential range as in Fig. 3a. This again indicated that the

oxidation state of the Ag element in the films was 0, i.e., Ag0.

This conclusion is in line with the result that Ag+ in the TiO2

thin film was converted into Ag0 atom or cluster after annealing

at 500 8C in the literature [18], which was confirmed by XPS.

In Fig. 3b, the current for all the TiO2 films was enhanced by

the UV illumination. The photocurrents can be used to represent

the photocatalytic activity of the TiO2 films over the oxidation

of water. The profiles of TA and AT thin films are similar, and

their photocurrents were much higher than that of AA and TT

thin films, under the same experimental conditions. The

photocatalytic activity sequence for the TiO2 films over the

oxidation of water were AT > TA > AA >TT.

The current was further enhanced with the addition of

0.03 mM ethanol into the supporting electrolyte (see Fig. 3c).

Organic compounds are generally more effective photohole

scavengers than water, i.e., easier to be oxidized than water at

the TiO2 electrodes [20]. The enhanced current was due to more

electrons being captured from ethanol besides water. The

photocatalytic activity sequence for the TiO2 films over the

oxidation of ethanol were AT > TA > AA >TT, which is

consistent with the results in Fig. 3b.

Fig. 4 shows the photocurrent profiles of the AT with

different Ag content under the same potential bias (0 mV versus

Ag/AgCl). The photocurrent increased gradually with the

increase of Ag loading from 0.1 to 0.5%, but then descended

sharply at 0.9% content. The optimal content of AT thin films

was 0.5%. The photocurrent response reflects the amount of

free carriers in the catalyst. The results indicate that the Ag

loading content has an effect on the amount of free carriers in

the catalyst, which in turn affect the photocatalytic activities of

TiO2 thin films.

3.3. Evaluation of photocatalytic activity

Fig. 5 shows the effect of the LLDC modes and Ag loading

percentage of the thin films on the photocatalytic degradation of

methyl orange aqueous solutions under UV illumination. For

the AT and TA thin films, the photocatalytic activities were

Fig. 5. The dependence of photocatalytic degradation for aqueous methyl

orange on loading modes and loading content of the thin films.

J. Zheng et al. / Applied Surface Science 254 (2008) 1630–16351634

improved with the initial increase in Ag loading percentage.

The maximum degradation percentage was acquired at 0.5%.

Afterward, the photocatalytic activities decreased with the

increasing Ag loading percentage. For the AA thin film, the

photocatalytic activity was marginally better than that for TT,

with the optimal loading content being 0.3%. Overall, at the

optimal Ag loading percentage, the photocatalytic activities for

the investigated films are in the order of AT > TA > AA >TT.

This can be explained by the experimental results from the

characterization of the investigated TiO2 films.

In TiO2 photocatalysis, the surface area plays an important

role on the photocatalytic activity. A larger crystalline size will

lead to a smaller specific photoactive area, and therefore, lower

photocatalytic activity, vice versa. The fact that TT had the

lowest photocatalytic activity among all the thin films is

consistent with the observation from SEM, where TT had the

largest TiO2 particle size. If the particle size effect is the

dominant factor for the photocatalytic activity, the size

sequence of AT > TA > AA resulted form the Ag size

inhibition effect should have resulted in the photocatalytic

activity order of AT < TA < AA. This is opposite to the

experimental observation as the photocatalytic activity order

was reversed being AT > TA > AA. This conflict suggested

that the size variable was not the determining factor.

The dominant factor responsible for the photocatalytic

activity order of AT > TA > AA >TT may be the formation

of Schottky barriers between TiO2 and Ag0. As aforementioned,

the radius of the Ag+ and Ag atom is much larger than the Ti4+, so

very unlikely for Ag+ and Ag atom to enter the TiO2 lattice. In

fact, the Ag atoms were in direct contact with the TiO2

nanoparticles only. Because the work function of the metal Ag is

higher than that of TiO2, electrons are removed from the TiO2

particles to the vicinity of the Ag particle. This results in the

formation of Schottky barriers at the Ag–TiO2 contact region and

results in charge separation [23]. More specifically, Schottky

barriers facilitate the electron transfer from TiO2 nanoparticles

(with high Fermi level) to Ag (with low Fermi level), resulting in

higher transferring efficiency of electrons [24]. The effect of

Schottky barriers in our particular case was evidenced by the fact

that all the Ag-loaded films (AT, TA and AA) had higher

photocatalytic efficiencies than the pure TiO2 film (TT).

The photocatalytic activity order of AT > TA > AA needs

to be explained from the features brought by the proposed

LLDC technique combined with the Schottky barriers theory.

The use of the proposed LLDC technique will result in the

vertically gradient distribution of Ag in the TiO2 thin film with

the Ag element concentration at the surface being in the order

of AT < TA < AA. This will have double effect on the

photocatalytic activity. Firstly, the Schottky barriers facilitate

the photoelectron movement to a certain direction, from the

semiconductor TiO2 nanoparticles to adjacent Ag atom or

cluster. AT has the highest photocatalytic activity and can be

attributed to the fact that Ag was distributed at the back of the

TiO2 surface layer for the AT film. For the AT thin films, the

Schottky barriers attract the electrons from the surface layer to

the bottom layer, and the holes will be left on the surface layer

of the thin film. This attraction greatly inhibits the recombina-

tion of electron–hole pairs. The enriched holes in the surface

layer can directly degrade methyl orange, at the same time, it

can oxidize H2O adsorbed on the surface and produce hydroxyl

radicals (OH�) that can degrade organic matters effectively.

Secondly, the most efficient UV illumination (in terms of

effective illumination geometric area) was achieved on the AT

film because there was the lowest amount of Ag on the AT film

surface and the Ag particles on the top of TiO2 surface block the

UV light. This was certainly beneficial to higher photocatalytic

activity.

For the TA and AA thin films, the Schottky barriers force the

photoelectrons being transferred from the TiO2 nanoparticles to

the Ag atom or cluster on the surface, which will physically

extend the distance between photoholes and photoelectrons.

Therefore, they have stronger photocatalytic activities than the

TT thin film. Among the AT, TA and AA film, the Ag loaded

amount on the surface was in the order of AT < TA < AA, and

the Ag atom or Ag cluster will block light illumination, with the

efficiency of UV illumination being in the order of

AT > TA > AA. In other words, under the same UV

illumination intensity, the amounts of the resulting photoholes

and photoelectrons were in the order of AT > TA > AA.

The Ag loading content has an important effect on the

photocatalytic activity. Too much Ag loading content may

result in a photohole trapping effect. Due to the fact that the Ag

particles and clusters on the TiO2 nanoparticles are relatively

negative charged due to the formation of the Schottky barriers,

the photoholes in the interfacial region of the TiO2 film may be

trapped by the negatively charged Ag particles and clusters

before they react with water and organics. This trapping effect

was neglectable when the Ag concentration is below 0.5%, and

the Schottky barrier predominantly promoted the TiO2 charge

separation efficiency. The TiO2 photocatalytic activities,

therefore, increased with the increasing Ag concentration.

With the Ag concentration increasing beyond 0.5%, however,

the trapping effect became prevailing and the photocatalytic

activity declined. The decline in the photocatalytic activity can

also be associated with the change of the reaction site density.

Fig. 6. EIS Nyquist plots of the Ag-modified TiO2 thin films with 0.5% Ag

content and TT thin film under UV illumination.

J. Zheng et al. / Applied Surface Science 254 (2008) 1630–1635 1635

As the Ag+ concentration increased beyond 0.5%, the coverage

of Ag on TiO2 surface will increase, in other words, the density

of reaction sites (for adsorption of UV light, produce photohole

and photoelectrons) at the surface of the resulting film would

decrease. Consequently, the TiO2 photocatalysis is depressed

when the Ag+ concentration was greater than 0.5%.

It is well established that of EIS Nyquist plots are associated

with the charge transfer resistance and the separation efficiency

of the photohole–electron pairs [25]. Fig. 6 shows the results of

EIS Nyquist plots of the TiO2 thin film investigated. A larger

circular radius usually has a larger charge transfer resistance,

and therefore, a lower separation efficiency of the photohole–

electron pairs [26,27]. The frequencies for EIS measurement

are scanned from 105 to 0.1 Hz. As shown in Fig. 6, the

curvature radiuses varied with the loading mode following the

below sequence: AT < TA < AA �TT. In other words, the

electron separation efficiency of the TiO2 films was in the order

of AT > TA > AA �TT. These experimental results supported

the conclusions from the photoelectrochemical characterization

and photocatalytic degradation of methyl orange.

4. Conclusions

Four types of the TiO2 thin films (i.e., AT, TA, AA and TT)

were prepared using the LLDC sol–gel technique, and were

characterized using SEM, XRD and electrochemical methods.

The experimental results demonstrated that the AT film has the

higher photocatalytic activity to water and organic compound

oxidation than conventional Ag modified TiO2 thin films (AA)

and pure TiO2 thin film (TT). The sequence of the photocatalytic

activity was AT > TA > AA >TT, which was explained

systematically using the Schottky barrier, the photohole trapping

effect and the reactive reaction site density. This investigation

demonstrated the LLDC coating technique can be used to

construct a tailored hierarchical configuration of TiO2 nano-

structured thin film, which has a significantly higher photo-

catalytic activity over traditional evenly-modified method.

Acknowledgements

The authors are grateful for Griffith University encourage-

ment grant and financial support from Guangzhou Science and

Technology Bureau (No. 2005J1-C0261), China, and Austra-

lian Research Council discovery grant.

References

[1] T.T.Y. Tan, D. Beydoun, R. Amal, J. Phys. Chem. B 107 (2003) 4296.

[2] Y.H. Wang, T.F. Tian, X.Q. Liu, G.Y. Meng, J. Membr. Sci. 280 (2006)

261.

[3] W. Kubo, T. Tatsuma, J. Mater. Chem. 15 (2005) 3104.

[4] W. Xu, X.-J. Li, S.-J. Zheng, J.-G. Wang, Chem. J. Chinese Univ. 26

(2005) 2297.

[5] K. Nagashima, H. Kokusen, N. Ueno, A. Matsuyoshi, T. Kosaka, M.

Hasegawa, T. Hoshi, K. Ebitani, K. Kaneda, H. Aritani, S. Hasegawa,

Chem. Lett. 3 (2000) 264.

[6] V. Vamathevan, H. Tse, R. Amal, G. Low, S. McEvoy, Catal. Today 68

(2001) 201.

[7] E.P. Reddy, B. Sun, P.G. Smirniotis, J. Phys. Chem. B 108 (2004) 17198.

[8] N. Sasirekha, S.J.S. Basha, K. Shanthi, Appl. Catal. B 62 (2006) 169.

[9] H. Yamashita, M. Honda, M. Harada, Y. Ichihashi, M. Anpo, T. Hirao, N.

Itoh, N. Iwamoto, J. Phys. Chem. B 102 (1998) 10707.

[10] Y. Yang, X.-J. Li, J.-T. Chen, L.-Y. Wang, J. Photochem. Photobiol. A 163

(2004) 517.

[11] C. He, Y. Xiong, J. Chen, C. Zha, X. Zhu, J. Photochem. Photobiol. A 157

(2003) 71.

[12] J.C. Colmenares, M.A. Aramendia, A. Marinas, J.M. Marinas, F.J.

Urbano, Appl. Catal. A 306 (2006) 120.

[13] V. Vamathevan, R. Amal, D. Beydoun, G. Low, S. McEvoy, J. Photochem.

Photobiol. A 148 (2002) 233.

[14] W. Dai, X. Wang, P. Liu, Y. Xu, G. Li, X. Fu, J. Phys. Chem. B 110 (2006)

13470.

[15] A. Kumar, A.K. Jain, J. Mol. Catal. A, Chem. 165 (2001) 265.

[16] B. Xin, L. Jing, Z. Ren, B. Wang, H. Fu, J. Phys. Chem. B 109 (2005)

2805.

[17] C. He, Y. Yu, X. Hu, A. Larbot, Appl. Surf. Sci. 200 (2002) 239.

[18] J. Yu, J. Xiong, B. Cheng, S. Liu, Appl. Catal. B 60 (2005) 211.

[19] G.J. Wilson, G.D. Will, R.L. Frost, S.A. Montgomery, J. Mater. Chem. 12

(2002) 1787.

[20] S. Zhang, D. Jiang, H. Zhao, Environ. Sci. Technol. 40 (2006) 2363.

[21] S. Zhang, W. Wen, D. Jiang, H. Zhao, R. John, G.J. Wilson, G.D. Will, J.

Photochem. Photobiol. A 179 (2006) 305.

[22] I. Boskovic, S.V. Mentus, M. Pjescic, Electrochim. Acta 51 (2006) 2793.

[23] K.V.S. Rao, B. Lavedrine, P. Boule, J. Photochem. Photobiol. A 154

(2003) 189.

[24] S. Sen, S. Mahanty, S. Roy, O. Heintz, S. Bourgeois, D. Chaumont, Thin

Solid Films 474 (2005) 245.

[25] S. Ningshen, U. Kamachi Mudali, G. Amarendra, P. Gopalan, R.K. Dayal,

H.S. Khatak, Corros. Sci. 48 (2006) 1106.

[26] B. Xin, Z. Ren, P. Wang, J. Liu, L. Jing, H. Fu, Appl. Surf. Sci. 253 (2007)

4390.

[27] J. Li, L. Liu, Y. Yu, Y. Tang, H. Li, F. Du, Electrochem. Commun. 6 (2004)

940.