Microporous and Mesoporous Materials 117 (2009) 356–361

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Microporous and Mesoporous Materials

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Preparation of highly dispersed TiO2 in hydrophobic mesoporesby simultaneous grafting and fluorinating

Shuai Yuan a,*, Liyi Shi a, Kohsuke Mori b,1, Hiromi Yamashita b,1

a Research Center of Nanoscience and Nanotechnology, Shanghai University, 99 Shangda Road, Shanghai 200444, PR Chinab Division of Materials and Manufacturing Science, Graduate School of Engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan

a r t i c l e i n f o

Article history:Received 27 May 2008Received in revised form 20 June 2008Accepted 9 July 2008Available online 17 July 2008

Keywords:MesoporousTiO2

TiF4

Hydrophobic photocatalystX-ray absorption fine structure

1387-1811/$ - see front matter � 2008 Elsevier Inc. Adoi:10.1016/j.micromeso.2008.07.012

* Corresponding author. Tel./fax: +86 21 66134852E-mail address: s.yuan@shu.edu.cn (S. Yuan).

1 Tel./fax: +81 6 6879 7457.

a b s t r a c t

TiO2 photocatalysts highly dispersed in the MCM-41 mesoporous silica material were synthesized usingtwo different TiO2 precursors of TiF4 and (NH4)2TiO(C2O4)2. The catalysts were characterized by X-ray dif-fraction (XRD), N2 adsorption–desorption, transmission electron microscopy (TEM), UV–vis absorption,X-ray photoelectron spectroscopy (XPS), and Ti K-edge X-ray absorption fine structure (XAFS). The hydro-phobic properties and catalytic activities of photocatalysts with different TiO2 precursors and loadingamount were examined by adsorption and degradation of iso-butanol diluted in water. The resultsshowed that the TiO2 loaded MCM-41 prepared from TiF4 exhibited higher photocatalytic activity thanfrom (NH4)2TiO(C2O4)2. The fluorine-modified hydrophobic pore walls formed in the impregnation pro-cess using TiF4 as well as highly dispersed TiO2 nanoparticles play a crucial role for the high photocata-lytic efficiency.

� 2008 Elsevier Inc. All rights reserved.

1. Introduction

In recent years, the large-scale emission of toxic agents, such asdyes, chlorobenzene and dioxins, into the atmosphere and water,causing pollution and destruction on a global scale [1]. Most ofthe organic toxins are too stable to be decomposed into harmlesssubstances in natural environment [2]. So the design of completelynew, clean and safe chemical processes is the most urgent andchallenging issue today.

Photocatalytic reactions are ideal answers to fulfill these needssince they actually work as environmentally harmonious catalystsat room temperature in a clean manner using only light as the en-ergy source in contrast to the conventional resource-burning en-ergy sources [3]. The application of such photocatalytic systemswill not only convert inexhaustible solar energy into chemical en-ergy but also recover our environment. Among them, TiO2 of ana-tase phase is the most suitable photocatalyst reported so far [4,5].However, hydrophilic surfaces and poor adsorption properties ofTiO2 crystals lead to great limitation in exploiting the photocatalystto the best of its photoefficiency [6,7].

It is effective to use the hybrid materials, photocatalysts com-bined with adsorbents, for the purification of water. Zeolite andmesoporous molecular sieve materials, which have high surface

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area and porous structure, have been employed as conventionaladsorbents [8]. The concentration properties of these adsorbentsshould depend on their surface hydrophilic–hydrophobic nature.The highly hydrophobic surface is more suitable than hydrophilicsurface for the adsorption of organic compounds diluted in water.Furthermore, less UV-light scattering is observed in the above sil-ica composites. Therefore, the combination of TiO2 photocatalystand hydrophobic porous materials is a significantly promisingand attracting study.

The synthesis of MCM-41 mesoporous silica was first reported byMobil in 1992, which has large surface area as well as ordered hex-agonal mesopore channels ranging from 15 to 100 Å [9]. Previouslyinvestigation showed that the mesopores of MCM-41 have hydro-phobic property [10], and removing „SiAOH from hexagonal mes-oporous silica (HMS) can enhance the hydrophobic property [11].Although TiO2 supported on MCM-41 has been prepared [12–16],the TiO2 photocatalysts loaded in hydrophobic zeolite or mesopor-ous molecular sieve have opened new possibilities for the photocat-alytic degradation of various organic compounds diluted in water.In this paper, fluorine-modified MCM-41 supported TiO2 were pre-pared by impregnation method from TiF4. The pore walls werehydrophobilized by fluorination simultaneously with the formationof TiO2 nanoparticles in the mesopores. With the aim of investigat-ing the effects of surface properties, (NH4)2TiO(C2O4)2 was used asanother kind of TiO2 precursor to avoid the fluorination of porewalls. We also mentioned the relationship between surface adsorp-tion capacities and photocatalytic activities of the TiO2 supported onMCM-41.

S. Yuan et al. / Microporous and Mesoporous Materials 117 (2009) 356–361 357

2. Experimental section

2.1. Chemicals and synthesis

Si(OEt)4 (TEOS), C16H33(CH3)3NBr (CTAB), NH3 � H2O, TiF4 and(NH4)2TiO(C2O4)2 are commercial products and used as received.Because TiF4 is air sensitive, the manipulations were carried outin glovebox with N2 protection.

For the synthesis of MCM-41, CTAB was dissolved in the aque-ous NH3 solution, and then TEOS was added with stirring at roomtemperature. The mole ratio of the mixture was Si: CTAB:NH3:H2O = 1:0.12:8.6:82. After 3 h, the gel was transferred into an auto-clave and kept at 393 K for 48 h. After filtering, washing, dryingand calcination at 823 K for 24 h, MCM-41 was obtained.

The TiO2/MCM-41 was prepared by impregnating MCM-41powder in the solution of TiF4 and stirring for 3 h. Then the waterwas removed by evaporation. After drying at 383 K for 5 h, thepowder was calcined at 873 K for 3 h. The loading amount ofTiO2 was 1 at%, 2 at% and 4 at%. The products were named asTMF-1, TMF-2 and TMF-4, respectively.

According to the same procedure, TiO2/MCM-41 was also pre-pared with (NH4)2TiO(C2O4)2 as a precursor. The loading amountof TiO2 was 1 at%, 2 at% and 4 at%. The products were designatedas TM-1, TM-2 and TM-4, respectively.

2.2. Characterization

X-ray diffraction patterns of all samples were measured by Rig-aku RINT2500 diffractometer with Cu Ka radiation (k = 1.5406 Å).The porous textures of the powders were analysed from nitrogenadsorption–desorption isotherms at 77 K. By using an ASAP 2000system (Shimadzu), the BET and BJH methods were applied forthe determination of the specific surface area, and the mean mes-opore equivalent diameter, respectively. The diffuse reflectanceabsorption spectra were recorded with Shimadzu UV-2200Aphotospectrometer. The XAFS spectra (XANES and EXAFS) weremeasured at the BL-7C facility of the Photon Factory at the NationalLaboratory for High-Energy Physics, Tsukuba [17]. A Si(111) dou-ble crystal was used to monochromatize the X-rays from the2.5 GeV electron storage ring. The Ti K-edge absorption spectrawere recorded in the fluorescence mode at 295 K. The Fouriertransformation was performed on k3-weighted EXAFS oscillationsin the range of 3–10 Å�1. In a typical experiment, the sample wasloaded into the in situ cell having the plastic windows. The preedgepeaks in the XANES regions were normalized for atomic absorp-tion, based on the average absorption coefficient of the spectral re-gion. The instrument employed for XPS studies was JEOL jps-9200

Fig. 1. Low angle XRD patterns (a) TMF-1, (b) TMF-

with Mg Ka radiation (hm = 1253.6 eV) operated at 30 mA and 10kV. The binding energy of F 1s was referenced to the C 1s peak at284.8 eV.

2.3. Photocatalytic activity

The photocatalytic activity of catalyst was evaluated by thephotodegradation of iso-butanol (i-BuOH). 0.05 g of catalyst wasdispersed in 25 ml of aqueous i-BuOH solution (2.61 mmol l�1).After stirring under dark conditions for 30 min, the solution wasbubbled by oxygen for another 30 min. Then the solution was irra-diated using UV light (k > 280 nm) from a 100 W high-pressure Hglamp. The progress of the reactions were monitored by gas chro-matography analysis (GC-14B, Shimadzu). The adsorption propertywas measured by stirring the catalyst (0.01 g) dissolved in theaqueous i-BuOH solution (5 ml) without the light irradiation at303 K for 1 h.

3. Results and discussion

3.1. Mesostructure of TiO2/MCM-41

The well-defined mesostructures of TiO2/MCM-41 and the crys-tallization of TiO2 were investigated by X-ray powder diffraction(XRD). The low angle XRD patterns are shown in Fig. 1. Althoughthe intensities of low angle XRD diffraction peaks decrease withincreasing TiO2 loading amount, the diffraction peaks assigned to(100), (110) and (200) reflections from hexagonal mesostructureare almost at the same locations, which indicates that the long-range order of MCM-41 was still kept after impregnation and cal-cination [18]. From the wide angle XRD patterns, no characteristicdiffraction peak assigned to TiO2 crystal, anatase or rutile, can beobserved, suggesting TiO2 clusters are very small in size and highlydispersed in MCM-41.

The specific surface area, pore volume and pore diameter werecharacterized by N2 adsorption–desorption, and the results arepresented in Fig. 2 and Table 1. N2 adsorption–desorption for allsamples shows the typical type IV isotherm without significanthysteresis loop. At the relative pressure range from 0.3 to 0.4, theisotherms exhibit a sharp inflection indicative of capillary conden-sation inside the mesopores. There is no obvious differences in theshapes of isotherms between the pure MCM-41 and all TiO2 loadedMCM-41 samples. Compared with the pure MCM-41, the decreasesof specific surface areas are observed. The average pore diametersof TM series ((NH4)2TiO(C2O4)2 as titanium precursor) slightly de-crease to 2.8 nm. In the cases of TMF series (TiF4 as titanium pre-cursor), similar results were obtained. From the analyses of XRD

2, (c) TMF-4, (d) TM-1, (e) TM-2, and (f) TM-4.

Fig. 2. The N2 adsorption–desorption isotherms and the BJH pore size distribution (inset) of (a) TMF-1, (b) TMF-2, (c) TMF-4, (d) TM-1, (e) TM-2, and (f) TM-4.

Table 1XRD and N2 adsorption–desorption results of TiO2 loaded MCM-41

Sample d100a (nm) SBET

b (m2 g�1) DBJHc (nm) Wall thicknessd (nm)

MCM-41 4.2 780 3.1 1.7TM-1 4.2 766 2.8 2.0TM-2 4.2 755 2.8 2.0TM-4 4.2 749 2.8 2.0TMF-1 4.2 689 2.8 2.0TMF-2 4.2 638 2.8 2.0TMF-4 4.2 608 2.7 2.1

a d-Value of (100) reflection.b BET surface areac BJH adsorption average pore diameter.d The wall thickness of hexagonal mesostructure was calculated by subtracting

the pore diameter from 2d100/p

3.

Fig. 3. TEM image of the TMF-1.

358 S. Yuan et al. / Microporous and Mesoporous Materials 117 (2009) 356–361

and N2 adsorption–desorption, it can be concluded that Ti-oxidesspecies are highly dispersed on the pore walls of MCM-41.

Fig. 3 shows the transmission electron microscopy (TEM) imageof the TMF-1, which demonstrates that the (100) direction still re-tains a regular hexagonal array of uniform channels characteristicsof the parent MCM-41. Hexagonal mesostructures were not af-fected after the loading of the TiO2 and no crystalline TiO2 wasdetected.

3.2. Analysis of Ti-oxide species

The diffuse reflectance UV–vis (DRUV–vis) absorption spectraare shown in Fig. 4. The pure MCM-41 exhibited almost no absorp-

Fig. 4. UV–vis absorption spectra of (a) TMF-1, (b) TMF-2, (c) TMF-4, (d) TM-1, (e) TM-2, and (f) TM-4.

S. Yuan et al. / Microporous and Mesoporous Materials 117 (2009) 356–361 359

tion in the UV–vis range. With the increase of loading amount ofTiO2, the intensities of absorption bands in UV range increased,and the band edges shifted to the longer wavelength region. Theabsorption band in the 210–220 nm range is originated from li-gand-to-metal charge-transfer band (LMCT) from O2� to Ti4+ ionsin isolated tetrahedral site [19]. For sample TM-1, TM-2, TM-4,TMF-1 and TMF-2, their absorption located in the range of 220–330 nm due to the formation of oligomerized Ti–O–Ti linkage[20]. For TMF-4 sample, the wide absorption band with an edgeat around 350 nm indicates the formation of TiO2 nanoparticles[21]. The binding energy (Eg) calculated from a plot of (F(R) � hm)2

versus hm for this sample is found to be 3.68 eV, which correspondsto the nanoparticles of less than 2 nm in diameter according to theprevious report [22]. On the other hand, the Eg of the commercialbulk anatase crystals is 3.44 eV. This difference can be explainedby the quantum size effect responsible for the degradation of Eg va-lue in small TiO2 nanoparticles.

The hydrolysis of TiF4 in water is easier than that of (NH4)2-TiO(C2O4), in which the intermediate of TiF4 hydrolysis, Ti(OH)x-F4�x, and HF are formed. The intermediate of Ti(OH)xF4�x can belocated on the MCM-41 silica wall by the condensation betweenTi–OH and surface „SiAOH groups, which result in the increaseof the wall thickness. The process is expressed by the followingEqs. (1) and (2) [23].

When TiF4 is used as TiO2 precursor, the formed HF can reactwith the walls of MCM-41 to produce „SiAF group (Eq. (3)). Theaggregation of TiO2 nanoparticles should be accelerated by the de-crease of the number of surface „SiAOH groups [12]. Possiblereactions for the formation of Ti–O–Ti bond are expressed in Eqs.(4) and (5). These are clear explanation for the higher degree ofaggregation of Ti-oxide species by using TiF4 compared to that with(NH4)2TiO(C2O4)2 as confirmed by UV–vis analysis. This processcan be illustrated by Scheme 1

SiO

OSi

OSi

OSi

OSi

O

OH

OH

OH

OH

OH

TiF4

SiO

OSi

OSi

OSi

OSi

O

F

Ti

TiO

O

O

Δ

Scheme 1. The process of grafting TiO2 and fluorination of mesoporous silica.

TiF4 þ xH2O! TiðOHÞxF4�x þ xHF ð1ÞBSi—OHþ TiðOHÞxF4�x ! BSi—O—TiðOHÞx�1F4�x ð2ÞBSi—OHþHF! BSi—FþH2O ð3ÞTi—OHþHO—Ti! Ti—O—TiþH2O ð4ÞTi—FþHO—Ti! Ti—O—TiþHF ð5Þ

From the F 1s XPS spectrum of TMF-4 shown in Fig. 5, two con-tributions to the original curve can be observed. The F 1s X-rayphotoelectron bands at around 688.7 eV and 687.7 eV are probablyassigned to F doped in TiO2 crystal lattice (Ti(OH)xF4�x) and Fbelonging to „SiAF bonds, respectively [24]. The area ratios ofpeaks at 688.7 eV and 687.7 eV are 60% and 40%, respectively.

X-ray absorption fine structure (XAFS), including X-ray absorp-tion near edge structure (XANES) and extended X-ray absorptionfine structure (EXAFS), is sensitive in probing short-range structurearound specific metal atoms[25–27]. For TiO2 nanocrystals, the lo-cal environment around Ti in bulk is different from that on surface,which will be reflected on XAFS spectra. With a decrease in particlesize, more and more Ti atoms are exposed on surface. The fractionof surface TiO2 can be estimated by 1.25/d, where d is the particlediameter in nanometer [25]. Thus, about 63% TiO2 are exposed onsurface for 2 nm. From the analysis by XRD, N2 adsorption and UV–vis, we concluded that the diameter of TiO2 highly dispersed in

Fig. 5. F 1s XPS spectrum of TMF-4. The thin and thick solid lines are the measuredand fitted data, respectively. The dot and dash lines are the deconvoluted spectra.

Fig. 6. Ti K-edge XANES spectra of (a) TMF-1, (b) TMF-2, (c) TMF-4, (d) TM-1, (e)TM-2, and (f) TM-4.

Fig. 7. Fourier transform of Ti K-edge EXAFS for (a) TMF-1, (b) TMF-2, (c) TMF-4, (d)TM-1, (e) TM-2, and (f) TM-4.

Fig. 8. Investigation of adsorption properties (A)

360 S. Yuan et al. / Microporous and Mesoporous Materials 117 (2009) 356–361

MCM-41 was smaller than 2 nm. These results indicate that mostof Ti species in TiO2 exist in 4- and 5-fold coordination rather than6-fold coordination.

The Ti K-edge XANES spectra are shown in Fig. 6. For TiO2 load-ing MCM-41 with TiF4 as TiO2 precursor (TMF series), two or threepreedge peaks (A1, A2, and A3) are observed. The preedge peak atdifferent position reveals the different coordination of Ti. The pre-vious studies show that preedge peaks at 4967.9 (A2) and4971.6 eV (A3) should be assigned to Ti with 4- and 6-fold coordi-nation, respectively [26]. The origin for A1 at 4966. 4 eV is assignedto an excitation band or a transition from 1s ? 1t1g. With an in-crease in loading amount, the intensity of A2 relative to the totalpreedge intensities (IA2=IAtotal

) apparently decrease. For TiO2 loadedMCM-41 with (NH4)2TiO(C2O4)2 as TiO2 precursor (TM series), thesame tendency is observed. However, the 4-fold coordinate Ti isobviously predominant, which means the degree of aggregationof TiO2 in TMF is higher than that in TM. This result agrees wellwith the analysis of UV–vis spectra.

The Fourier transform of Ti K-edge EXAFS of sample preparedwith different TiO2 precursor and different loading amount areshown in Fig. 7. The strong peak at around 1.4 Å exhibited in allspectra can be assigned to the neighboring oxygen atoms (Ti–O)[27,28] In the case of TMF series prepared with TiF4 as TiO2 precur-sor, the peaks at around 2.4 Å due to the neighboring titaniumatoms (Ti–O–Ti) can be observed [27,28]. The relative intensity ofpeaks of Ti–O–Ti compared with that of Ti–O increase with the in-crease of loading amount, which indicates the aggregation of octa-hedral TiO2 species in these samples, as confirmed by the UV–visand XANES spectra. On the contrary, the samples of TM series pre-pared with (NH4)2TiO(C2O4) as TiO2 precursor scarcely exhibit thecontiguous Ti–O–Ti bond. It can be said that the Ti species mainlyexist in a tetrahedral coordination geometry. These results are ingood agreement with the results of the UV–vis and XANES spectra.

3.3. Investigation of photocatalytic activity of TiO2 /MCM-41

The hydrophobic properties of samples were measured by theiso-butanol adsorption. With an increase of TiO2 loading amount,the adsorption of iso-butanol on per m2 catalysts increases, whichindicates that the hydrophobic properties are enhanced by the con-densation of surface „SiAOH and Ti–OH (Fig. 8A). Furthermore,the replace of OH group by F can also increase the hydrophobicproperties of MCM-41 surface. As expected, the TMF samples showhigher hydrophobic properties than TM samples with same TiO2

loading amounts.

and photocatalytic activities (B) of samples.

S. Yuan et al. / Microporous and Mesoporous Materials 117 (2009) 356–361 361

Fig. 8B shows that the photocatalysts with the lower TiO2 load-ing amount exhibits higher photocatalytic ability in the degrada-tion of iso-butanol. Higher TiO2 loading amount can result in theaggregation of TiO2 species and larger cluster size, while Ti-oxidespecies can highly dispersed in MCM-41 at lower TiO2 loadingamount. The results indicate that the aggregation of Ti-oxide spe-cies has a great effect on photocatalytic activity. The increasingnumber of surface Ti sites as well as possible corner defects insmall nanoparticles may be the main reason for the high efficiency.Compared with TM samples, TMF samples prepared from TiF4 withthe same TiO2 loading amount has higher photocatalytic activity inspite of higher degree of aggregation of TiO2. It should be caused bythe higher hydrophobic property which facilitates the concentra-tion and degradation of hydrophobic organic compounds in mes-opores. Our experimental results revealed that a proper balancebetween the dispersity of the TiO2 nanoparticle and affinity towardorganic compounds diluted in water may be the keys for achievingthe efficient photocatalytic degradation.

4. Conclusions

TiO2 loaded MCM-41 with different TiO2 loadings were pre-pared from TiF4 and (NH4)2TiO(C2O4)2, respectively. The loadingamount of TiO2 significantly influence on the photocatalytic activ-ity. Ti-oxide species can be highly dispersed in MCM-41 at lowerTiO2 loading amount, which exhibited high photo degradationactivity. Using TiF4 as TiO2 precursor, HF produced in the processof hydrolysis can generate fluorine-modified pore walls of MCM-41. The replacement of „SiAOH by Si–F on the surface of porewalls can increase the hydrophobic property of MCM-41, whichbenefits the adsorption of organic compounds diluted in waterand facilitates their photocatalytic degradation. With the sameloading amount of TiO2, samples prepared from TiF4 have higherphotocatalytic activity than that from (NH4)2TiO(C2O4)2.

Acknowledgments

The Project Sponsored by Academic Leader Program of ShanghaiScience and Technology Committee (07XD14014); Technical Inno-vation Team Project of Shanghai Science and Technology Commit-tee (06DZ05902); Key Subject of Shanghai Municipal Education

Commission (J50102); The Scientific Research Foundation for theReturned Overseas Chinese Scholars, State Education Ministry;Innovation Program of Shanghai Municipal Education Commission(08YZ09); Shanghai-Unilever Research and Development Fund(06SU07001). This work is also partly performed under the projectof collaborative research at the Joining and Welding Research Insti-tute (JWRI) of Osaka University. The X-ray adsorption experimentswere performed at the Photon Factory of KEK (2004G295,2005G039) with helpful advice from Prof. M. Nomura and Prof. Y.Inada.

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