TiO2(B) Core−Shell

Dec 2, 2008 - The core-shell anatase/TiO2(B) nanofiber shows enhanced photocatalytic activity in iodine oxidation reaction with a 20-50% increase in e...
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J. Phys. Chem. C 2008, 112, 20539–20545

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Enhanced Photocatalytic Activity in Anatase/TiO2(B) Core-Shell Nanofiber Wei Li,† Chang Liu,† Yaxin Zhou,† Yang Bai,† Xin Feng,† Zhuhong Yang,† Linghong Lu,† Xiaohua Lu,*,† and Kwong-Yu Chan‡ State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing UniVersity of Technology, Nanjing 210009, China, and Department of Chemistry, The UniVersity of Hong Kong, Pokfulam Road, Hong Kong, China ReceiVed: September 15, 2008; ReVised Manuscript ReceiVed: NoVember 3, 2008

A bicrystalline titanium dioxide nanofiber with enhanced photocatalytic activity was synthesized from potassium titanate K2Ti2O5 via ion exchange and calcination. The nanofiber has a core-shell crystalline structure with a thin TiO2(B) phase sheathing the anatase core, as characterized by X-ray diffraction, Raman spectroscopy, and high-resolution transmission microscopy (HRTEM). From HRTEM and local electron diffraction patterns, the two crystalline phases form a coherent interface with the 0.34-nm spacing between the (102) planes of TiO2(B) matching that between the anatase (101) lattice planes. The core-shell anatase/TiO2(B) nanofiber shows enhanced photocatalytic activity in iodine oxidation reaction with a 20-50% increase in extent of reaction compared to either single-crystal anatase or single-crystal TiO2(B) nanofibers. Anatase and TiO2(B) have the same band gap value of 3.2 eV, while theoretical calculations show the conduction band (CB) and valence band (VB) energies in anatase are both lower than the corresponding CB and VB energies in TiO2(B). The enhanced photocatalytic property may be due to enhanced and concerted charge mobility toward or away from the anatase/TiO2(B) interface. The special structure-property relationship can provide a new strategy to design and fabricate efficient photocatalytic and photovoltaic materials. 1. Introduction 1,2

Semiconducting materials for photocatalysis and photovoltaics3-5 are of increasing importance to applications related to environment and energy. TiO2 is presently believed to be the most promising material because of its superior photoreactivity, nontoxicity, stability, and low cost.6 The commercial titania P-25 (Degussa) composed of two crystalline phases anatase and rutile was shown to have better photocatalytic activity compared to most other TiO2 materials.7-9 The high photocatalytic activity of P-25 titania was explained by the increase in charge separation efficiency resulting from interfacial electron transfer between anatase and rutile.8-10 P-25 has been frequently used not only as a benchmark but also as a model for the design of a more effective composite TiO2. Investigations have been reported with titania composites containing a anatase/rutile junction.11-15 In addition to anatase and rutile, TiO2 can form two other crystal phases, brookite and TiO2(B). Very recently, increasing research efforts have been devoted to the new composites of TiO2 including anatase/brookite16-18 and anatase/ TiO2(B).19-24 Efficient charge separation and migration is of primary importance to the overall properties of the semiconductor material. Composite semiconductors with a heterojunction of matching band potentials can enhance photogenerated charge separation and migration.25-33 Charge transport can be adversely affected when charge recombination are enhanced at bulk defects,2,4,34 especially in phase boundaries of semiconducting composites.35-37 Defect-free interfaces provide more effective charge transport in the composite.16,18,28,38 However, most * To whom correspondence should be addressed. E-mail: [email protected]. Phone: +86-25-83588063. Fax: +86-25-83588063. † Nanjing University of Technology. ‡ The University of Hong Kong.

reported composites are synthesized by physical mixing of two phases or chemical modification without compatible interfaces.39-41 In the synthesis of TiO2, phase transformation between different crystal phases of TiO2 occurred during heat treatment. Coherent interfaces could be obtained in intermediate bicrystalline products during the phase transformation.42-44 In this work, anatase/TiO2(B) bicrystalline core-shell nanofibers were fabricated via ion exchange and calcination of K2Ti2O5. A smooth and coherent interface between anatase and TiO2(B) was synthesized with gradual phase transformation and crystallization. Increased photocatalytic oxidation of iodide was observed due to efficient charge separation in the composite. 2. Experimental Section The bicrystalline titania is prepared as follows: (I) Preparation of K2Ti2O5 fibers. The starting materials, TiOSO4 and K2CO3, were commercially available with purities of 99.5%. TiO2 · nH2O (hydrous titanium dioxide) was prepared by hydrolyzing TiOSO4 in hot water with vigorous stirring. Deionized water was added to the mixture of TiO2 · nH2O and K2CO3. The chemical compositions (TiO2/K2O molar ratio) were controlled at 1.9. The mixture processes were preformed by ball milling with water. Final mixtures were dried in an oven at 90 °C for 10 h. Calcinations were performed in a muffle oven at a heating rate of 5 °C/min, at 810 °C for 2 h. (II) Ion exchange of K2Ti2O5 fibers. K2Ti2O5 (10 g) was suspended in 100 mL of vigorously stirred 0.1 M HCl solution until K+ ion was completely exchanged. The product was filtered and washed with distilled water and dried in a desiccator and at 60 °C under vacuum. (Residual of K+ < 0.2 wt % in the final product, detected by inductively coupled plasma-mass spectrometry (ICP-MS).) (III) Thermal treatment of hydrated titanate fibers. Calcination of the dried sample was performed in a muffle oven at 600 °C in air for 2 h. After natural cooling, bicrystalline titania fibers were

10.1021/jp808183q CCC: $40.75  2008 American Chemical Society Published on Web 12/02/2008

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Figure 1. SEM image of the TiO2(A+B) fibers.

synthesized. The single-phase anatase was obtained by calcination of the TiO2 in temperature up to 800 °C. Single-phase TiO2(B) was prepared via an additional water vapor hydration process between steps I and II. The as-prepared samples are denoted as TiO2(A+B), TiO2(A), and TiO2(B), in which TiO2(A+B) indicates the bicrystalline TiO2 contains anatase and TiO2(B), TiO2(A) indicates the single-phase anatase, and TiO2(B) indicates the single-phase TiO2(B). The crystal phase of raw material and product was determined by powder X-ray diffraction (XRD, Bruker D8, Cu KR radiation) and Raman spectra (Super LabRam). The sample morphology was evaluated by scanning electron microscopy (SEM, JEOL JSM-5900). The microstructure was observed by transmission electron microscopy (TEM) using a JEOL JEM2010 transmission electron microscope at 200 kV. The lattice parameters were measured from high-resolution TEM images using DigitalMicrograph. The fast Fourier transformation (FFT) and selected area electron diffration (SAED) patterns were checked using CaRIne. The indexes with a subscript “A” correspond to the anatase, whereas the TiO2(B) are denoted with a subscript “B”. The photocatalytic reactor is a quartz cell with a circulating water jack and a 300-W high-pressure mercury lamp with a maximum emission at 365 nm placed inside the quartz cell. Before photocatalytic reaction, 0.25 g of catalyst powder was added into 500 mL of 25 mmol/L KI solution. UV illumination was applied after the suspension was magnetically stirred in the dark for 30 min. During irradiation, either air, oxygen, or nitrogen was bubbled through the solution. About 5 mL of suspension was taken from the reaction cell at given time intervals for subsequent I2- concentration analysis. Concentration of I2- was determined by 288-nm absorbance (UV-vis spectroscopy, Hitachi U-2001). 3. Results and Discussion 3.1. Coherent Bicrystalline Structure of the TiO2 Fiber. The TiO2 calcined at 600 °C was examined by scanning electron microscopy (SEM). The titania material is fiberlike with fairly uniform width of 150 nm but variable length of 1-10 µm as shown in Figure 1. The K2Ti2O5 precursor material for preparation of titania was also fiberlike. The XRD spectra of these fibers are shown in Figure 2. All major diffraction peaks match the standard peaks of anatase phase well, while weak peaks correspond to a TiO2(B) phase can be observed. The coexistence of the two titania solid phases can also be confirmed by their corresponding characteristic peaks in Raman spectra, as shown in Figure 3.

Li et al.

Figure 2. XRD pattern of the TiO2(A+B) fibers. The insets show the local details. 0, Anatase; (, TiO2(B).

Figure 3. Raman spectra of the TiO2(A+B) fibers. 0, Anatase; (, TiO2(B).

TEM image of the TiO2 in Figure 4 shows the same fiber width as observed in SEM. The overall electron diffraction (ED) pattern of the whole fiber (Figure 4b) indicates the view axis as the [001]A vector of anatase (a ) 0.379 and c ) 0.951 nm).43 On the other hand, a local HRTEM image near the edge of the fiber in the marked region revealed the presence of TiO2(B) crystal lattice with [100]B vector in line with the view axis (a ) 1.216, b ) 0.374, c ) 0.651 nm, and β ) 107.29°),43 as shown in Figure 4c. FFTs of the local lattice pattern (Figure 4d) also confirm the structure of TiO2(B). From repeated ED and HRTEM characterizations of various local surface regions of the TiO2 fiber and the ED, XRD, and Raman characterizations of the overall structure, it can be concluded that the bulk of the titania is anatase while most of the surface is covered with a crystalline layer of TiO2(B) phase. From the TEM images, the thickness of the TiO2(B) phase varies from 2 to 20 nm. Figure 5 is a schematic of the bicrystalline structure inferred from the HRTEM, ED, XRD, and Raman characterizations. The orientation relationship between the anatase and TiO2(B) is such that the [010]A vector in anatase is parallel to the [010]B vector in TiO2(B) and the [001]A vector in anatase is parallel to the [100]B vector in TiO2(B). The cross sectional view of the fiber in Figure 5 is along the common [010] direction and shows the orthogonal lateral orientation relationship between the two crystal phases. At the end of the fiber (i.e., along the [010] direction), where the surface layers are broken (Figure 6), both of the two phase can be observed clearly in HRTEM. The bulk crystal phase of

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Figure 4. (a) TEM image of the TiO2(A+B) fiber. (b) The corresponding ED image of part a. (c) HRTEM image of the fiber marked in part a. (d) The corresponding FFT image of part c.

Figure 5. The schematic illustration of crytallographic orientation for the TiO2(A+B) fiber.

Figure 7. Polyhedral representation of the structure of anatase and TiO2(B). Octahedrons highlighted in yellow showed the same type of sheets of octahedral structure.

Figure 6. HRTEM image of the interface between anatase and TiO2(B) in the TiO2(A+B) fiber.

anatase was identified by the lattice domain in the lower right region of Figure 6. The upper left region in Figure 6 indicates the lattice pattern of TiO2(B). The HRTEM images of Figure 6 are in excellent agreement with the schematic in Figure 5. The interface between the anatase and TiO2(B) phases shows little defect with a smooth transition from one crystal structure to another. Despite the differences in lattice parameters, the coherent interface is possible in a special relative orientation between the lattices with matching lattice spacing. The spacing between the (101) planes in anatase is 0.34 nm and matches

Figure 8. XRD patterns of the (a) raw K2Ti2O5 and (b) as-synthesized H2Ti2O5. 1, K2Ti2O5; b, H2Ti2O5; 0: H2Ti5O11).

the spacing between the (102) planes in TiO2(B) phase. A schematic diagram of the interface is shown in Figure 7 with polyhedral structures of anatase and TiO2(B). It illustrates a highly structured interface with octahedrons of both phases sharing a common boundary (highlighted in yellow). This kind of interface has been previously discussed in theory and observed in naturally occurred minerals.42,43,45 And, anataseTiO2(B) bicrystalline structure with this kind of interfaces can

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Figure 9. XRD patterns of (a) anatase fibers and (b) TiO2(B) fibers. (0, Anatase; (, TiO2(B).)

Figure 10. Photocatalytic oxidation of iodide in KI solution saturated with air. The insets show the final I3- ion absorbs light at 288 and 350 nm.

be generally obtained from layered titanates followed by ion exchange, thermal dehydratation and postheat treatment.44 However, in these approaches, bicrystalline composites are intermediate products from the phase transformation of single phase TiO2(B). The location where the crystal structures start to transform is uncertain. In this work, the distribution and location of the interfaces were presented clearly. The coherent interface in a synthesized TiO2 material was observed by explicit HRTEM image for the first time. In addition, TiO2(A+B) from K2Ti2O5 have higher surface area (39 m2 g-1) than generally obtained TiO2 fibers from other titanates (with typical surface area of 20 m2g-1) and the anatase-TiO2(B) interfaces are very close to surface. These two factors are a benefit to photocatalytic properties discussed later. 3.2. Formation Mechanism of the Coherent Bicrystalline Interfacial Structure. It is of interest to know how the synthesis led to the bicrystalline TiO2(B)/anatase structure with a coherent interface. Only a preliminary understanding can be deduced from the synthetic parameters affecting the final structure. Precursor K2Ti2O5 was suspended in HCl solution to exchange K+ for H+, as illustrated in the reactions in eqs 1-3). During this process, a water gradient developed between solution/solid interface and interior of the fiber. Depending on synthesis conditions and available water, different titanates can be formed as in eqs 2 and 3 and via different phase transformation routes.46,47 The XRD spectrum of the acid treated intermediate product dried at 120 °C for 12 h was compared with that of the precursor in Figure 8. Diffraction patterns correspond to both H2Ti2O5 · H2O and H2Ti5O11 · H2O were identified clearly in the acid treated titanate. At the beginning of the ion-exchange

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Figure 11. Photocatalytic oxidation of iodide in KI solution saturated with O2.

Figure 12. Photocatalytic oxidation of iodide in KI solution saturated with N2.

TABLE 1: ∆EFa between TiO2(B) and anatase

photovoltage measurements Mott-Schottky measurements CASTEP DMol3

EFa of anatase (V vs NHE)

EFa of P25 (V vs NHE)

-0.51a

-0.58a -0.60b

EFa of TiO2(B) (V vs NHE)

∆EFa between TiO2(B) and anatase from EF of single phase (V vs NHE) 0.60c

-1.11b 0.63 0.54

a Data from ref 56. b Data from ref 57. c Data concluded from these two references. The absolute values of EF* from calculations have no physical meaning and are not shown here.

process with limited water, it is likely that the wetted surface K2Ti2O5 transformed into K2Ti5O11. With a H2O gradient developed between the wetted solid/solution interface and the core of the fiber, it is likely that H2Ti5O11 is more likely formed at the surface and H2Ti2O5 is likely formed in the core.

5K2Ti2O5 + 3H2O f 2K2Ti5O11 + 6KOH

(1)

K2Ti2O5 + 2H+ f H2Ti2O5 + 2K+

(2)

K2Ti5O11 + 2H+ f H2Ti5O11 + 2K+

(3)

It was suggested that H2Ti5O11 is favorable to the formation of a TiO2(B) structure in the subsequent calcination process,48 whereas H2Ti2O5 will transform into anatase.49,50 The five edgesharing octahedral structures of H2Ti5O11 form on the surface of fibers and the trigonal bipyramidal structures of H2Ti2O5, which are the same as the original K2Ti2O5 are retained in the bulk. This mechanism of different hydrogen titanates leading

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Figure 13. The schematic illustration of energy band and photogenerated charge carriers for the TiO2(A+B) fiber.

to different titaniate crystal structures can explain the abovediscussed lattice characterizations of TiO2(A+B) fibers. Under slow hydration and ion-exchange steps, a smooth and uniform H2Ti5O11/H2Ti2O5 boundary can be formed and leads to the subsequent lattice-matched crystal structure of the interface. Choosing different synthesis parameters, single crystal anatase fiber and single crystal TiO2(B) fiber were prepared separately, and their corresponding XRD spectra are shown in Figure 9. The single-phase anatase was obtained by calcination of the TiO2 at 800 °C, during which the surface TiO2(B) transformed to anatase completely. On the other hand, single-phase TiO2(B) was prepared with an additional vapor hydration step before the ion change. With the additional hydration step, K2Ti2O5 had sufficient time to transform into K2Ti5O11 completely. Then single-phase H2Ti5O11 and TiO2(B) formed after subsequent ion exchange and calcination. 3.3. Photocatalytic Oxidation of Iodide. Iodide ion is an excellent scavenger which easily captures valence band hole.51 It is widely used in investigations of efficiency of photoinduced holes, initial mechanism in photocatalysis, and delivery of electrons in dye-sensitized solar cells.4,52,53 We selected photocatalytic oxidation of KI to compare the different behavior of photoinduced charge carriers in bicrystalline structure of TiO2(A+B), TiO2(A), TiO2(B). In addition, a commercially available Degussa P-25 composed of anatase/rutile interfaces was also compared. In a detailed mechanism as shown in eqs 4-567), a hole reacts with the adsorbed iodide ion to form an iodine atom which further reacts with iodide ion to produce I2-. The I2- disproportionates to I3- and I-. The final I3- ion absorbs light at 288 and 350 nm and can be analyzed by spectrometer, as shown in insets of Figure 10. Thus, the increase of absorbance is proportional to the amount of iodide ions that reacted with the holes. TiO2 + hV f h+ (vb) + e(cb)

(4)

+ I(ads) + h(vb) f I

(5)

I + I- f I2

(6)

2I2 f I + I3

(7) I3-

Figure 10 shows the absorbance due to formation of ions in KI solution saturated with air upon irradiation in separate experiments using TiO2(A+B), TiO2(A), TiO2(B), or P25 catalysts. The highest yield of I3- ions is found in a core-shell

anatse/TiO2(B) nanofiber catalyst. Further kinetics experiments were performed with oxygen and nitrogen aerations to investigate the effect of gas molecules present, as shown in Figures 11 and 12, respectively. For all types of catalysts used, more I3- was formed in the presence of pure oxygen, and less was formed in the presence of pure nitrogen. The possibility of chemical oxidation by oxygen is ruled out since the final I3absorbance are the same whether it was pure nitrogen, pure oxygen, or air when no catalyst was added in the irradiation experiment. In the presence of oxygen, it is likely that photogenerated electrons are effectively removed by contact with adsorbed oxygen, leaving the holes to proceed in reaction 5. In the absence of oxygen, an efficient electron scavenger, electrons will recombine with the holes or reduced I2- and I3- to give back I- ions as shown in eqs 8 and 9. All of the curves show reaction equilibrium in the formation of I3- ions because of the reverse reaction. However, TiO2(A+B) presented remarkable higher rate and yield of I3- ions formation before the reaction equilibrium. I2 + e(cb) f 2I

(8)

I3 + e(cb) f I2 + I

(9)

The kinetics experiments demonstrate significantly higher efficiency of photoinduced holes in the core-shell TiO2(A+B) for substrates oxidation compared to either single-phase TiO2(A), single-phase TiO2(B), or P25 with composite anatase/rutile structure. The enhanced photocatalytic activity is due to the structural and electronic properties of the interface (heterojunction) in the composite. To facilitate some meaningful discussion, we performed theoretical calculations of the two titania structures. 3.4. Energy Calculations and Band Structure of the Interface. The energy band structure of anatase-TiO2(B) bicrystalline composites can be determined by three values, respective individual band gap of TiO2(B) and anatase and difference of potentials between TiO2(B) and anatase. The band gaps of anatase and TiO2(B) have been repeatedly reported in previous literatures; TiO2(B) and anatase have the same band gap of 3.2 eV.20,54,55 Potential difference between different semiconductors is generally identified by difference of quasiFermi levels of electrons (∆EF*, or difference of flatband potentials (∆Efb), equally). Since the value of ∆EF* between TiO2(B) and anatase can only be concluded from two separate sources56,57 as shown in Table 1, some calculation will be essential for the verification. Software modules CASTEP and DMol3 in Materials Studio from Accelerys have been employed to calculate the band structure (BS) and density of states (DoS) for two polymorphs of TiO2, anatase, and TiO2(B). For the CASTEP calculation of BS and DoS based on density function theory (DFT), ultrasoft pseudopotentials (PPs) and a plane-wave (PW) basis set with a 10 eV cutoff were used. The generalized gradient approximation (GGA) with the revised Perdew-Burke-Ernzerhof (RPBE) functional was used to describe the exchange-correlation effects. Each superlattice is composed of one unit cell and with periodical conditions in three dimensions. The BS and DoS were also investigated with the periodic DFT code Dmol3. The local density approximation (Perdew-Wang) was used with a double numerical basis set with polarization functions (DNP). A comparison of the results from CASTEP and DMol3 showed the same band gap with the value of 3.2 eV in both crystalline phases. The individual values of ECB, EVB, and EF* in CASTEP and DMol3 are off-scale and have no physical meaning. The corresponding band gap values and ∆EF* are

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Figure 14. The schematic illustration of iodide photocatalytic oxidation by photogenerated charge carriers in TiO2.

meaningful since the same ground energies are used in the individual single-phase calculations. The calculated ∆EF* values of 0.63 and 0.54 V, respectively, both agree well with the data from literature, 0.60 V, as shown in Table 1. Accordingly, a schematic energy band structure of anatase-TiO2(B) bicrystalline composites was shown in Figure 13. In Table 1, the values of ∆EF* between TiO2(B) and anatase are calculated from the values of pure phases. A first principle calculation of the actual composite interface would be more desirable but not executed. The ∆EF* value from refs 56 and 57 was also calculated from difference of experimentally measured values of pure anatase and pure TiO2(B) phases. According to the energy diagram of the interface in Figure 13, the holes stimulated in anatase would migrate toward the TiO2(B) phase because of the higher VB edge potentials. These holes will subsequently oxidize a mobile species efficiently. At the same time, the photoinduced electrons generated in the conduction band of TiO2(B) favor migration to the anatase phase which has a lower CB potential. The electrons will be consumed in a reduction process. This disjoint in the band structure provides alternative mechanisms for movement of holes and electrons which may reduce recombination. Efficient transport of photoinduced charge carriers will be affected by defects and crystallinity near the interface. Smooth transition of the interface with fewer defects can improve the charges mobility. The coherent interface between anatase and TiO2(B) of the TiO2 shown in Figure 6 with a defect-free transition may be conducive to the enhanced photocatalytic activity observed in the anatase/TiO2(B) core-shell nanofiber. This feature is distinct from other approaches for combining the different semiconductors. The process of iodine photooxidation in the presence of oxygen is shown in Figure 14 with comparison of the energy structures in pure anatase, pure TiO2(B), anatase/rutile as in P-25, and the core-shell anatase/ TiO2(B) composite. Photoinduced charge carriers can be separated easily in TiO2(A+B), owing to the different VB and CB edge potentials. The energy structure of the anatase/rutile interface promotes electrons photoinduced in anatase to move toward the rutile phase. But there is no promotion of hole movement and distribution since the VB energy is the same in anatase and rutile. This difference in energy structures may explain the better photocatalytic activity of the core-shell TiO2(A+B) compared to P-25. 4. Conclusions We have shown a novel anatase/TiO2(B) heterostructure synthesized via ion exchange and calcination of K2Ti2O5. The fibers are composed of anatase in the core and thin layers of TiO2(B) on thesurface. The orientation relationship of the bicrystalline composites is determined to be [010]A | [010]B and [001]A | [100]B. The interface between the two phases shows little defect with coherent integration of two crystal structures with matching spacings.

Photocatalytic oxidation of iodide shows that anatase/TiO2(B) composite outperforms either the single crystal anatase or single crystal TiO2(B). This may be attributed to the smooth interfacial structure and the band energy structure of the two phases, facilitating efficient charge separation and migration. Acknowledgment. The collaborative research between Nanjing University of Technology and University of Hong Kong was assisted by a NSFC-RGC Joint Research Award (Nos. 20731160614 and HKU 735/07). The authors also thank the support by Changjiang Scholars and Innovative Research Team in University (No. IRT0732), Joint Research Fund of NSFC for Young Scholars Abroad (No. 20428606), Fund of NSFC (Nos. 20236010, 20676062, 20736002, 20706029, and 20706028), National High Technology Research and Development Program of China (No.2006AA03Z455), and the Key Science Foundation of Jiangsu Province, China (BK2007051). References and Notes (1) Hoffmann, M. R.; Martin, S. T.; Choi, W. Y.; Bahnemann, D. W. Chem. ReV. 1995, 95, 69. (2) Thompson, T. L.; Yates, J. T. Chem. ReV. 2006, 106, 4428. (3) Gratzel, M. Nature 2001, 414, 338. (4) Hagfeldt, A.; Gratzel, M. Chem. ReV. 1995, 95, 49. (5) McFarland, E. W.; Tang, J. Nature 2003, 421, 616. (6) Chen, X.; Mao, S. S. Chem. ReV. 2007, 107, 2891. (7) Ding, Z.; Lu, G. Q.; Greenfield, P. F. J. Phys. Chem. B 2000, 104, 4815. (8) Hurum, D. C.; Gray, K. A.; Rajh, T.; Thurnauer, M. C. J. Phys. Chem. B 2005, 109, 977. (9) Hurum, D. C.; Agrios, A. G.; Gray, K. A.; Rajh, T.; Thurnauer, M. C. J. Phys. Chem. B 2003, 107, 4545. (10) Kawahara, T.; Konishi, Y.; Tada, H.; Tohge, N.; Nishii, J.; Ito, S. Angew. Chem., Int. Ed. 2002, 41, 2811. (11) Li, G. H.; Ciston, S.; Saponjic, Z. V.; Chen, L.; Dimitrijevic, N. M.; Rajh, T.; Gray, K. A. J. Catal. 2008, 253, 105. (12) Zachariah, A.; Baiju, K. V.; Shukla, S.; Deepa, K. S.; James, J.; Warrier, K. G. K. J. Phys. Chem. C 2008, 112, 11345. (13) Zhang, J.; Xu, Q.; Feng, Z.; Li, M.; Li, C. Angew. Chem., Int. Ed. 2008, 47, 1766. (14) Li, G. H.; Gray, K. A. Chem. Mater. 2007, 19, 1143. (15) Li, G. H.; Chen, L.; Graham, M. E.; Gray, K. A. J. Mol. Catal. A: Chem. 2007, 275, 30. (16) Ardizzone, S.; Bianchi, C. L.; Cappelletti, G.; Gialanella, S.; Pirola, C.; Ragaini, V. J. Phys. Chem. C 2007, 111, 13222. (17) Ozawa, T.; Iwasaki, M.; Tada, H.; Akita, T.; Tanaka, K.; Ito, S. J. Colloid Interface Sci. 2005, 281, 510. (18) Tian, G. H.; Fu, H. G.; Jing, L. Q.; Xin, B. F.; Pan, K. J. Phys. Chem. C 2008, 112, 3083. (19) Daoud, W. A.; Pang, G. K. H. J. Phys. Chem. B 2006, 110, 25746. (20) Kuo, H. L.; Kuo, C. Y.; Liu, C. H.; Chao, J. H.; Lin, C. H. Catal. Lett. 2007, 113, 7. (21) Yin, S.; Sato, T. Ind. Eng. Chem. Res. 2000, 39, 4526. (22) Zhu, J. F.; Zhang, J. L.; Chen, F.; Anpo, M. Mater. Lett. 2005, 59, 3378. (23) Jitputti, J.; Pavasupree, S.; Suzuki, Y.; Yoshikawa, S. Jpn. J.Appl. Phys 2008, 47, 751. (24) Jitputti, J.; Suzuki, Y.; Yoshikawa, S. Catal. Commun. 2008, 9, 1265. (25) Beranek, R.; Kisch, H. Angew. Chem., Int. Ed. 2008, 47, 1320. (26) Gross, D.; Susha, A. S.; Klar, T. A.; Da Como, E.; Rogach, A. L.; Feldmann, J. Nano Lett. 2008, 8, 1482.

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