Photocatalytic Comparison of TiO2 Nanoparticles and Electrospun

Sep 9, 2010 - and DiVision of Nano-Bio Technology, Daegu Gyeongbuk Institute of Science and Technology (DGIST),. Daegu 704-230, Republic of Korea...
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J. Phys. Chem. C 2010, 114, 16475–16480

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Photocatalytic Comparison of TiO2 Nanoparticles and Electrospun TiO2 Nanofibers: Effects of Mesoporosity and Interparticle Charge Transfer Sung Kyu Choi,† Soonhyun Kim,*,‡ Sang Kyoo Lim,‡ and Hyunwoong Park*,† School of Physics and Energy Science, Kyungpook National UniVersity, Daegu 702-701, Republic of Korea, and DiVision of Nano-Bio Technology, Daegu Gyeongbuk Institute of Science and Technology (DGIST), Daegu 704-230, Republic of Korea ReceiVed: May 12, 2010; ReVised Manuscript ReceiVed: August 18, 2010

The development of a high-efficiency TiO2 photocatalyst is of great importance to a variety of solar light conversion and application fields; the desired high efficiency can be achieved by employing well-controlled TiO2 nanoarchitectures. In this study, we have successfully synthesized well-ordered and aligned high surface area mesoporous TiO2 nanofibers (TiO2-NF) by electrospinning of TiO2 powder dispersed in viscous polymer solution and subsequent calcination. For comparison, TiO2 nanoparticles (TiO2-NP) are also prepared from calcination of the same TiO2 powder. The TiO2-NF of ca. 500 nm in diameter and a few micrometers in length consist of compactly and densely packed spherical nanoparticles of ca. 20 nm in size and have mesopores of 3-4 nm in radius. Photocatalytic comparison between TiO2-NF and TiO2-NP indicated that the former had far higher photocatalytic activities in photocurrent generation by a factor of 3 and higher hydrogen production by a factor of 7. The photocatalytic superiority of TiO2-NF is attributed to effects of mesoporosity and nanoparticle alignment, which could cause efficient charge separation through interparticle charge transfer along the nanofiber framework. Finally, various surface characterization experiments were conducted and included to understand the photocatalytic behaviors of TiO2-NF and TiO2-NP. Introduction In recent decades, TiO2 photocatalysts have been extensively studied with regard to synthesis, properties, modification, and applications.1 In principle, the electron-hole pairs created by band-gap excitation of TiO2 drive redox chemical reactions, such as water splitting, CO2 conversion, and pollutant degradation.2 Despite its merits, some intrinsic drawbacks of TiO2, such as limited light absorption due to wide band gaps and charge carrier recombination, hinder its efficiency and thus its use in more widespread applications. Various attempts have been made to overcome these problems, such as impurity doping,3,4 noble metal deposition,5-7 and surface fluorination.8-10 Very recently, mesoporous TiO2 has received great attention throughout the field. It is known that TiO2 mesoporosity improves light absorption due to increased surface area and multiple interparticle scattering and that it increases the surface adsorption capacity of reactants, leading to the enhancement of photocatalytic reactions.11,12 Generally, highly ordered mesoporous TiO2 is prepared via either template methods12-16 or template-free approaches.17,18 However, most studies have focused mainly on synthesis, arguing its superiority without systematic or detailed investigations of the photocatalytic mechanism behind the mesoporosity. In TiO2 photocatalysis, the porosity alteration significantly affects the primary particle size and crystal structure of TiO2, making a quantitative study of the effect of mesoporosity on photocatalytic activity very difficult. Tang et al. synthesized a highly crystalline and ordered mesoporous TiO2 thin film and reported that the effect of * To whom correspondence should be addressed. Tel: +82-53-430-8434 (S.K.), +82-53-950-7371 (H.P.). Fax: +82-53-430-8443 (S.K.), +82-53950-1739 (H.P.). E-mail: [email protected] (S.K.), [email protected] (H.P.). † Kyungpook National University. ‡ Daegu Gyeongbuk Institute of Science and Technology (DGIST).

crystallinity was greater than that of the mesoporous structure.19 Yu et al also synthesized a nanocrystalline mesoporous TiO2 powder and argued that its high photocatalytic activity could be attributed to the synergistic effects of hydroxyl content, surface area, mesoporous structure, and crystallization.20 This study systematically compares the photocatalytic activities of mesoporous TiO2 nanofibers and nanoparticles. A polymer-assisted electrospinning approach was employed to control TiO2 mesoporosity, according to a method reported elsewhere.21 The electrospinning technique is an excellent method for producing synthetic ultrathin fibers, including carbon nanofibers and organic/inorganic hybrid nanofibers.22 The synthesis of TiO2 nanofibers can be achieved via electrospinning either from a TiO2 precursor (e.g., titanium tetraisopropoxide)23 or from TiO2 nanoparticles.24,25 In the latter case, TiO2 particles are added directly to a polymeric solution and electrospun at various conditions to control the TiO2 fiber geometry. In the present study, we prepared highly crystalline mesoporous TiO2 nanoparticles (TiO2-NP) and TiO2 nanofibers (TiO2-NF) and compared their photocatalytic activities in terms of photocurrent generation and hydrogen production. In addition, a detailed study was conducted on TiO2-NP and TiO2-NF physicochemical properties to thoroughly understand the photocatalytic phenomena and mechanisms. Experimental Section Synthesis of TiO2 Nanoparticles and TiO2 Nanofibers. TiO2 nanoparticles (TiO2-NP) were synthesized by a sol-gel method. A mixture of titanium tetraisopropoxide (30 mL, Junsei) and absolute ethanol (5 mL, J.T. Baker) was added to an aqueous solution (180 mL) containing 2 mL of concentrated HNO3 (Duksan) under vigorous stirring. Transparent colloidal particles were formed immediately; the

10.1021/jp104317x  2010 American Chemical Society Published on Web 09/09/2010

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suspension was kept stirred at 80 °C for over 6 h and then evaporated to obtain a yellowish TiO2 powder. Heat treatment of the as-prepared TiO2 powder at 500 °C for 3 h changed the amorphous, low crystalline TiO2 into highly crystalline mesoporous TiO2 nanoparticles. TiO2 nanofibers (TiO2-NF) were prepared by an electrospinning method.24,25 As-obtained yellowish TiO2 powder was dispersed in a 10 wt % viscous solution of polyacrylonitrile (PAN, Aldrich) in N,N-dimethylformamide (DMF, Samchun). This TiO2 suspension was placed in a hypodermic syringe with a 25 gauge (0.2 mm) stainless steel nozzle, positioned at a fixed distance (∼15 cm) from a metal cathode (collector). An electrical potential of 20 kV was applied between the nozzle and the collector, and then dense nanofiber webs were obtained at the collector. Finally, these webs were calcined at 500 °C for 3 h to remove the remaining PAN or DMF and to obtain highly crystalline mesoporous TiO2 nanofibers. To investigate the effects of the PAN polymer (not electrospinning), TiO2 nanoparticles are additionally prepared from the calcinations of as-obtained yellowish TiO2 powder mixed with PAN polymer solution (TiO2-NPP) and its photocatalytic activities are compared with TiO2-NF and TiO2-NP. Surface Characterization. The surface morphological images and transmission electron micrographs were obtained using a field emission scanning electron microscope (FE-SEM, Hitachi S-4200) and high-resolution transmission electron microscope (HR-TEM, JEM-2200FS), respectively. X-ray diffraction (XRD) patterns were obtained with an X-ray diffractometer (Rigaku D/MAX-2500, 18 kV) using Cu KR1 radiation. The BrunauerEmmett-Teller (BET) surface area and Barret-Joyner-Halenda (BJH) pore size distribution measurements were carried out using N2 as the adsorptive gas. The oxidation states of the Ti atoms were determined by X-ray photoelectron spectroscopy (XPS) (Kratos XSAM 800 pci) using the Mg KR line (1253.6 eV) as the excitation source. Photoluminescence spectra were recorded at room temperature using a spectrometer (f ) 0.5 m, Acton Research Co., Spectrograph 500i, U.S.A.) equipped with an intensified photodiode array detector (Princeton Instrument Co., IRY1024, U.S.A.). A He-Cd laser (Kimmon, 1K, Japan) with a wavelength of 325 nm and power of 50 mW was utilized as the excitation light source. Photocatalytic Activity Measurements. Photocurrent generation was measured using a three-electrode system as reported elsewhere.26 TiO2-NP (or TiO2-NF) was suspended at 0.5 g/L in a 90 mL quartz reactor containing an electrolyte (KNO3, 0.1 M), an electron donor (CH3OH, 10 vol %), and an electron shuttle (methyl viologen, MV2+, 1 mM). A Pt wire (1.5 mm in diameter, 250 mm in length), a Ag/AgCl electrode, and a graphite rod were immersed in the reactor as working, reference, and counter electrodes, respectively. N2 gas was continuously purged through the suspension before and during UV irradiation (λ > 295 nm). The photocurrents were collected by applying a potential (0.1 V vs Ag/AgCl) to the working Pt electrode using a potentiostat (VSP, Princeton Applied Research) connected to a computer. Photocatalytic activities were evaluated for the hydrogen evolution. TiO2 photocatalysts were suspended at 0.2 g/L in water with methanol (10 vol %), and N2 gas was purged through the suspension for 30 min to remove oxygen. A 300 W Xe arc lamp (Oriel) was used as a light source, and the light beam was passed through a 10 cm IR water filter and a UV cutoff filter (λ > 295 nm) and then focused onto a cylindrical quartz reactor with a 40 mm diameter window. The reactor was sealed with ambient air during irradiation,

Choi et al.

Figure 1. SEM images of (a, b) TiO2-NF and (c, d) TiO2-NP.

and the headspace gases were monitored with gas chromatography (GC, HP6890) equipped with a Porapak-Q column and a thermal conductivity detector (TCD). For comparison, TiO2 photocatalysts were photodeposited with Pt nanoparticles by following a method presented in the literature.6 In brief, TiO2 photocatalyst suspensions (0.5 g/L) containing CH3OH (20 vol %) (as an electron donor) and H2PtCl6 (typically 1.5 wt %) were irradiated with a 250 W Hg lamp for 30 min. After irradiation, Pt-deposited TiO2 photocatalyst (Pt/TiO2-NF, Pt/TiO2-NP, Pt/TiO2-NPP, or Pt/P25) particles were filtered, washed, and dried at 100 °C. Results and Discussion Surface Characterization. Figure 1 shows the FE-SEM images of TiO2-NF and TiO2-NP; it is obvious that the morphology of TiO2-NF is different from that of TiO2-NP. TiO2NF prepared through the electrospinning process consists of fibers (or rods) with diameters of around 500 nm. A magnified image (Figure 1b) clearly shows that a single nanofiber was compactly packed with nanoparticles that had a primary particle size of ca. 20 nm. The nanoparticle components are very similar to TiO2-NP in both shape and size (Figure 1c,d), indicating that TiO2-NP agglomerated to form TiO2-NF during the electrospinning process. This similarity is likely due to the use of the same precursor and the same synthetic route. Thus, one can expect the crystalline phases also would be very similar between TiO2-NF and TiO2-NP. To confirm this conjecture, XRD analyses were performed for both. As compared in Figure 2, both samples have identical anatase phases with minor rutile fractions. The XPS spectra for Ti 2p and O 1s indicated that their chemical compositions and binding energies are also similar (Figure S1, Supporting Information). The Ti 2p1/2 and Ti 2p3/2 signals are seen to be located at binding energies of 464.5 and 458.8 eV, respectively, originating from the lattice Ti atom of TiO2.27,28 HR-TEM images are presented in Figure 3; consistent with Figure 1, TiO2-NF consists of highly crystalline and compact nanoparticles, whereas TiO2-NP is made up of loosely and randomly aggregated nanoparticles. The SAED pattern (insets of Figure 3a,c) displayed obvious rings, indicative of a polycrystalline structure. In addition, the calculation of the d-spacing from the radius rings and the lattice space (Figure S2, Supporting Information) confirms once again that both TiO2NF and TiO2-NP are of the anatase phase. Figure 4 shows the N2 adsorption-desorption isotherms and pore size distributions of TiO2-NF and TiO2-NP. Both samples exhibited type IV isotherms with type H3 hysteresis (Figure

Photocatalytic Comparison of TiO2 NPs and TiO2 NFs

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Figure 2. XRD patterns of (a) TiO2-NF and (b) TiO2-NP.

Figure 4. (a) N2 adsorption-desorption isotherms and (b) pore size distribution of TiO2-NF and TiO2-NP.

Figure 3. HR-TEM images of (a, b) TiO2-NF and (c, d) TiO2-NP. The insets in (a) show the corresponding SAED pattern and TEM image of TiO2-NF, and the inset in (c) shows the corresponding SAED pattern of TiO2-NP.

4a), which is indicative of mesoporosity.29 The initially abrupt increase at P/P0 ∼ 0.05 further indicates completion of monolayer coverage and initiation of multilayer adsorption.29 As a reference material, P25 was also tested, showing the type II isotherm pattern that indicates nonporosity or macroporosity (Figure S3, Supporting Information).30 It is of note that the mesoporosity was created not only in TiO2-NF but also in TiO2NP; thus, this similar mesoporosity might be ascribed to growth of the same crystalline phase (i.e., anatase) during the heat treatment. As reported elsewhere,21 the pore structures could be correlated with anatase growth and rutile formation as the sintering temperature is raised. Despite the similar porosity, the BJH pore size analysis showed that the average pore radius and pore volume of TiO2-NF (ca. 3.5 nm and 1.2 cm3/g, respectively) were larger than those of TiO2-NP (ca. 2 nm and 0.5 cm3/g, respectively). This result suggests that the mesoporosity of TiO2NF was improved during the electrospinning process, in particular, by the polymer-preoccupied space and/or the nanofiber structure. Although the polymer-preoccupied space did not directly contribute to the formation of pores as Chen et al.

TABLE 1: Physicochemical Properties and Photocatalytic Activitiesa

TiO2-NF TiO2-NP TiO2-NPP P25

surface area (m2 · g-1)

pore radius (nm)

96.3 40.2

3.5 2.2

51.5

H2 (mmol · g-1)b without Pt

with Pt

1.42 0.19 0.08 0.16

19.5 5.05 10.85 15.6

a

b

See the Experimental Section for more detailed information. After 120 min of irradiation.

reported,21 it should affect the porosity of the TiO2-NF. The improved porosity might increase the specific surface area of TiO2-NF from 45 m2/g (TiO2-NP) to 96 m2/g, whereas the surface area of P25 was 51 m2/g. The data for the surface areas and pore sizes of TiO2-NF, TiO2-NP, and P25 are summarized in Table 1. Photocatalytic Activities. To investigate the photocatalytic activities of TiO2-NF and compare them with those of TiO2NP, photocurrent generation measurements were conducted in a UV-irradiated TiO2-NF (or TiO2-NP) suspension, including a MV2+/MV+ couple as an electron shuttle and methanol as an electron donor.26 Figure 5 shows that, as soon as UV light was irradiated, the photocurrent in the TiO2-NF suspension was generated and increased quickly initially, finally reaching a plateau of 0.085 mA at around 10 min. In the case of TiO2-NP, the photocurrent reached a plateau of as low as ca. 0.025 mA

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Figure 5. Photocurrent generation in UV-irradiated suspensions of TiO2-NF and TiO2-NP: [TiO2] ) 0.5 g/L; CH3OH in water (10 vol %); [MV2+] ) 1 mM; [KNO3] ) 0.1 mM; applied potential ) +0.1 V (vs Ag/AgCl); nitrogen continuously purged; λ > 295 nm.

by a few seconds after UV irradiation. The fact that photocurrent generation of TiO2-NF was more than 3 times higher than that of TiO2-NP is very significant and further implies that more conduction band electrons are generated and/or that a larger fraction of the conduction band electrons are available in the TiO2-NF suspension. To confirm this conjecture, we carried out a series of experiments on the photocatalytic hydrogen production from water in UV-irradiated TiO2 suspensions. The suspensions are compared in Figure 6a, which shows that the time-profiled hydrogen production in the TiO2-NF was far higher than those in TiO2-NP and even than TiO2-NPP and P25 by a factor of 7. TiO2-NP and TiO2-NPP had a hydrogen production rate very similar to that of P25 (see Table 1). P25, a representative TiO2 mixture of anatase and rutile, has been known to be very active for a variety of photocatalytic reactions due to its anatase/rutile hybrid structure. Because the anatase has a slightly elevated conduction position compared with rutile TiO2, while both valence bands maintain a similar position, the conduction band electrons of anatase TiO2 can be effectively transferred to the conduction band of rutile TiO2 by following the formed potential gradient, leading to a facilitated charge separation/transfer. Despite this well-known excellent mechanism, P25 has a disappointingly low hydrogen production compared with TiO2NF. Thus, in addition to plain TiO2, platinized TiO2 particles were also tested for their photocatalytic hydrogen productions to study the effect of Pt. Platinum nanoparticles on TiO2 have been known to act as a kind of electron reservoir, pulling the electron from the TiO2 conduction band via the Mott-Schottky interface. Therefore, if the charge separation/transfer worked as a limiting factor in the plain TiO2, then removing this factor via platinization should increase the hydrogen production rate. In this study, the amount of Pt loading on TiO2 was fixed to be 1.5 wt % because the photocatalytic hydrogen production was highest with this amount (Figure S4 in the Supporting Information). The smaller amount of Pt loading (e.g., 0.2 wt %) did not significantly affect the photocatalytic hydrogen product, whereas the larger amount of Pt loading (e.g., 5 wt %) reduced the hydrogen production likely due to shielding effect of Pt nanoparticles. However, the hydrogen production ratios between TiO2-NF and TiO2-NP are almost invariable irrespective of the amount of Pt loading.

Figure 6. Time-profiled H2 evolution in UV-irradiated suspensions of (a) TiO2 and (b) Pt deposited TiO2 (Pt/TiO2): [TiO2] ) [Pt/TiO2] ) 0.2 g/L; Pt loading at 1.5 wt %; CH3OH in water (10 vol %); nitrogenpurged; λ > 295 nm.

At 1.5 wt % of Pt loading, TiO2-NF (Pt/TiO2-NF) displayed the highest hydrogen production rate at ca. 20 mmol/g, around 4 times and 2 times those of Pt/TiO2-NP (ca. 5 mmol/g) and Pt/TiO2-NPP (ca. 11 mmol/g), respectively (Figure 6b). In the case of P25, the hydrogen production rate was significantly enhanced (to 16 mmol/g) and even comparable to Pt/TiO2-NF. This implies that, in the absence of Pt, the charge separation process occurring at the anatase/rutile interface of P25 did not operate so effectively. Because the plain TiO2-NF has a much higher hydrogen production and photocurrent generations, we can speculate that TiO2-NF has a far more efficient charge separation/transfer process and/or recombination inhibition mechanism than the other photocatalysts. The comparison of ratios for the hydrogen production rates (RH’s) between the plain TiO2 and the platinized TiO2 supports this speculation. The RH for TiO2-NF was around 14, whereas the RH’s for TiO2-NP, TiO2-NPP, and P25 were 27, 136, and 98, respectively, indicating that the effect of Pt was lowest in TiO2-NF and highest in TiO2-NPP (NF < NP < P25 < NPP). This order of platinization effects strongly supports that TiO2-NF has the intrinsically superior charge separation/transfer mechanism. To further support the superiority of TiO2-NF, photoluminescence (PL) spectra were obtained for TiO2-NF and TiO2NP (Figure 7); it is clear that TiO2-NP exhibited a higher emission intensity than TiO2-NF. The PL emission intensity is

Photocatalytic Comparison of TiO2 NPs and TiO2 NFs

Figure 7. PL emission spectra (excited at 325 nm) of TiO2-NF and TiO2-NP.

related to the recombination of excited electrons and holes, and thus, the lower emission intensity is indicative of a decrease in recombination rate.31 Lakshminarasimhan et al. argued similarly that the dense packing of TiO2 nanoparticles in mesoporous TiO2 plays an important role in efficient charge separation through interfacial charge transfer.17 Origin of High Photocatalytic Activity of Mesoporous TiO2 Nanofibers. In principle, TiO2 photocatalytic activity is governed by several elementary steps, such as charge separation, charge transport to the surface, and charge recombination; these steps compete with one another to determine the overall activity. To obtain higher photocatalytic activities, therefore, one should facilitate the charge separation and transfer while inhibiting charge recombination. The well-ordered and aligned TiO2 particle assembly is an ideal structure to enhance the separation of electron-hole charge pairs and facilitate interparticle charge transfer. In addition, the mesoporous structure created via the TiO2 particle-particle interconnection is very advantageous in enhancing the adsorption of reactants and desorption of products.17 In this regard, a well-ordered and well-aligned mesoporous TiO2 particle assembly should serve as a photocatalytic platform that satisfies those requirements.

J. Phys. Chem. C, Vol. 114, No. 39, 2010 16479 The single TiO2-NF consists of compactly packed spherical particles of around 20 nm in diameter that form a cylindertype fiber of around 500 nm in diameter and a few micrometers in length, as illustrated in Figure 8. A simple numerical calculation suggests that more than 600 discrete TiO2 nanoparticles compose each cross-section layer, and around 230 000 discrete TiO2 particles are necessary to form a single 5 µm long TiO2 fiber. However, considering the presence of mesopores of around 3.5 nm in radius, the actual number of TiO2 particles necessary for the nanofiber is reduced. Because TiO2 nanoparticles are three-dimensionally interconnected, a very rapid interparticle, vectorial transport of photogenerated charge carriers (electrons and holes) is likely to occur through the grain boundaries. This suggests that the redox reaction sites are away from the photoexcitation sites (Figure 8), which seems to be responsible for the high activities for photocurrent generation and hydrogen production. In addition, TiO2-NF is very rigid and robust and thus can maintain its high activity under various conditions (e.g., pH changes) for the period of prolonged UV irradiation. On the other hand, TiO2-NP have mesoporosity as well; yet they are easily aggregated in their loose and random states, lowering the efficiency. Conclusions The present study shows that the electrospun, mesoporous TiO2 nanofiber photocatalyst is far more efficient than its nanoparticle analogue in photocurrent generation and hydrogen production. The highly enhanced photocatalytic activity of the mesoporous TiO2 nanofiber is ascribed to a synergistic effect of (1) the mesoporosity enhancing adsorption/desorption of substrates and (2) the well-aligned TiO2 nanoparticle framework effectively separating charge pairs and facilitating charge transfer. In addition, the polymer-assisted electrospinning of the TiO2 nanoparticle is a very easy and reproducible way to synthesize rigid and robust mesoporous TiO2 nanofibers. Therefore, more detailed and ongoing studies on the alteration of fiber diameters, in particular, would be interesting and highly applicable in a variety of research areas, such as dye-sensitized solar cells, sensors, and environmental remediation.

Figure 8. Schematic illustration for a mesoporous TiO2 nanofiber and its photocatalytic reaction mechanisms.

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Acknowledgment. This work was also financially supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) grant, also funded by the Korean Government (MEST) (No. 2009-0075415). Supporting Information Available: Figures S1-S4 as noted in the text. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Chen, X.; Mao, S. S. Chem. ReV. 2007, 107, 2891. (2) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W. Chem. ReV. 1995, 95, 69. (3) Choi, W. Y.; Termin, A.; Hoffmann, M. R. J. Phys. Chem. 1994, 98, 13669. (4) Ikeda, S.; Sugiyama, N.; Pal, B.; Marci, G.; Palmisano, L.; Noguchi, H.; Uosaki, K.; Ohtani, B. Phys. Chem. Chem. Phys. 2001, 3, 267. (5) Izumi, I.; Dunn, W. W.; Wllbourn, K. O.; Fan, F.-R. F.; Bard, A. J. J. Phys. Chem. 1980, 84, 3207. (6) Kim, S.; Choi, W. J. Phys. Chem. B 2002, 106, 13311. (7) Lee, J. S.; Choi, W. Y. EnViron. Sci. Technol. 2004, 38, 4026. (8) Minero, C.; Mariella, G.; Maurino, V.; Pelizzetti, E. Langmuir 2000, 16, 2632. (9) Vohra, M. S.; Kim, S.; Choi, W. J. Photochem. Photobiol., A 2003, 160, 55. (10) Park, H.; Choi, W. J. Phys. Chem. B 2004, 108, 4086. (11) Antonelli, D. M.; Ying, J. Y. Angew. Chem., Int. Ed. 1995, 34, 2014. (12) Sreethawong, T.; Laehsalee, S.; Chavadej, S. Int. J. Hydrogen Energy 2008, 33, 5947. (13) Ostomel, T. A.; Stucky, G. D. Chem. Commun. 2004, 1016.

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