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Synthesis of CdS Nanorods by an Ethylenediamine Assisted Hydrothermal Method for Photocatalytic Hydrogen Evolution Yuexiang Li,*,† Yuanfang Hu,† Shaoqin Peng,† Gongxuan Lu,‡ and Shuben Li‡ Department of Chemistry, Nanchang UniVersity, Nanchang 330031, People’s Republic of China, and State Key Laboratory for Oxo Synthesis and SelectiVe Oxidation, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, People’s Republic of China ReceiVed: February 18, 2009; ReVised Manuscript ReceiVed: April 6, 2009
CdS nanocrystals were synthesized by a hydrothermal method using ethylenediamine (en) as the template agent and coordination agent, and characterized by transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), BET, and UV-vis absorption spectroscopic techniques. Their photoactivity was evaluated by hydrogen evolution from aqueous solution containing formic acid as a hole scavenger under visible light (λ g 420 nm) irradiation. The nanocrystals display rodlike and granular shapes, and their amount and morphology depend on the hydrothermal temperature and the added content of en. Photoactivity of the nanorods is higher than that of the nanograins. The effect of deposited Pt as cocatalyst on photoactivity for hydrogen evolution has been investigated. Pt can be dispersed highly on CdS so that the Pt content for effective hydrogen evolution is very low. 0.050 wt % Pt-loaded CdS shows the highest activity for hydrogen evolution under visible light irradiation, and the apparent quantum yield amounts to 13.9%. A possible mechanism was discussed. Introduction Photocatalytic hydrogen evolution from water using semiconductors is of great interest due to its possible application for converting sunlight energy into chemical energy.1,2 Many semiconducting photocatalytic materials have been developed for hydrogen generation from water under light irradiation.3-5 Among them, CdS has extensively been studied6-9 because of its excellent properties: the band gap (2.4 eV) corresponds well with the spectrum of sunlight, and the conduction band edge is more negative than the H+/H2 redox potential. It is well-known that many properties of CdS such as crystalline phase, size, morphology, specific surface area, and defects can affect its photocatalytic activity.10 Thus, controlling the size and shape of CdS is a key factor in a highly active photocatalyst. A number of methods have been explored to fabricate CdS nanocrystals, such as thermal evaporation, chemical vapor deposition, solvothermal process, and hydrothermal process.11-14 Among them, hydrothermal synthesis is an important technology for synthesizing nanostructures at low temperature. CdS nanowires or nanorods were synthesized in the solvent ethylenediamine (en) or other amines through the solvothermal route.12,15-17 Jang et al. prepared CdS nanowires using en as the solvent and template agent and investigated the photoactivity for hydrogen evolution.7 However, to the best of our knowledge, there is no report on the photoactivity of CdS nanorods prepared by the hydrothermal method using en as the coordination agent and template agent. For photocatalytic hydrogen evolution, CdS particles are usually platinized by either physical mixing with Pt particles (e.g., Pt black, Pt sol) or photoplatinization. The Pt loading content in general is larger than 1.0 wt % in the literature for * Corresponding author. Telephone: +86-791-3969983. Fax: +86-7913969983. E-mail:
[email protected]. † Nanchang University. ‡ Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences.
highly active hydrogen evolution. Reber and Rusek reported that platinized CdS (Pt-CdS) obtained by photodeposition showed an enhanced hydrogen production rate of 300 mL · h-1 at 1.5 wt % Pt.18 Bao et al. reported that the hydrogen yield rapidly increased with increasing Pt loading content and reached a maximum rate of 4.1 mmol · h-1 at 13 wt % Pt.19 However, we found that the Pt loading content for the prepared CdS was very low to achieve a maximum activity, i.e., 0.050 wt % Pt. It is of great interest to lower the cost of photocatalysts for practical application. In this paper, we investigated the formation of CdS nanorods via a hydrothermal route using en as a template agent and coordination agent at different reaction temperatures with different molar ratios of en to Cd2+. Their photoactivity was evaluated by hydrogen evolution from aqueous solution containing formic acid as sacrificial reagent under visible light irradiation (λ g 420 nm). The photoactivity of the nanorods is higher than that of the nanograins. The effect of Pt loading was studied. A possible mechanism was discussed. Experimental Section Photocatalyst Preparation. All reagents were of analytical grade and were used without any further purification. In a typical preparation process, CdSO4 · 8/3H2O (24 mmol), thioacetamide (48 mmol), and a given amount of en were added into a 100 mL Teflon-lined stainless steel autoclave which had been filled with distilled water to 80% of its capacity. The solution was continuously stirred for ca. 20 min, and then it was closed. The autoclave was maintained at a given temperature for 8 h and then cooled to room temperature naturally. The product was washed with distilled water and absolute ethanol several times to remove the excess reactants and byproduct. Finally, the sample was dried in a vacuum oven at 60 °C for 8 h. Characterization. The crystalline phases of the products were determined by powder X-ray diffraction (XRD) on a Bede D1 System multifunction X-ray diffractometer, employ-
10.1021/jp901505j CCC: $40.75 2009 American Chemical Society Published on Web 04/30/2009
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ing Cu KR radiation (λ ) 0.154 06 nm). The optical properties were analyzed on a UV-vis diffuse reflectance spectrometer (HITACHI U-3310). The specific surface areas of the samples were determined on a ST-08 analyzer by the volumetric BET method using nitrogen as adsorbent. The morphologies of the prepared samples were observed with a JEM-2010 transmission electron microscope (TEM) and a JEOL JEM-2010F highresolution transmission electron microscope (HRTEM), using an accelerating voltage of 200 kV. The HRTEM was equipped with an energy-dispersive spectrometer (EDS). Photocatalytic Reaction. The photocatalytic reaction was carried out in a Pyrex flask of 187 cm3 with a flat window. In all experiments, aqueous solution (100 mL) containing 5.0 mL of 88 wt % reagent formic acid, and 50 mg of CdS photocatalyst were added to the cell. Cocatalyst Pt was photodeposited in situ on the photocatalyst using a given amount of H2PtCl6 · H2O.20 A 400 W high-pressure Hg lamp was used as the light source, and an optical glass cutoff filter was used to cut the light with wavelength smaller than 420 nm. The IR fraction of the beam was removed by a cool water filter. Before irradiation, the catalyst was dispersed in an ultrasonic bath for 1 min, and nitrogen was bubbled through the reaction mixture for 30 min to remove oxygen. The catalyst was maintained in suspension by means of magnetic stirring. The top of the cell was sealed with a silicone rubber septum. Sampling was made intermittently through the septum during experiments. Hydrogen was analyzed on a gas chromatograph (TCD, 13X molecular sieve column, N2 as gas carrier). In the stability test of the CdS photocatalyst, the reaction system was flushed periodically with nitrogen. In all cases, the reproducibility of the measurements was within (10%. The apparent quantum yield was estimated by the following equation.
Φ(H2) ) 2
moles of hydrogen evolved moles of incident photons
(1)
The average photon flux of the incident light determined on a FGH-1 Ray virtual radiation actinometer (light spectrum 400-700 nm) was 830 µmol · m-2 · s-1. Results and Discussion Effect of Molar Ratio of en to Cd2+. Figure 1 shows TEM images of the CdS samples prepared at 150 °C with different ratios of en to Cd2+. The sample synthesized without en displays granular and agglomerated lamellar shapes (Figure 1a). When the ratio of en to Cd2+ is 1.0, nanograins (major) and nanorods (minor) are obtained (Figure 1b). When the ratio is 2.0, nanorods are major, whereas nanograins are minor (Figure 1c). When the ratio is 3.0, both nanorods and nanograins are also obtained (Figure 1d). However, the rod length of sample d is shorter than that of sample c; namely the latter has a higher aspect ratio. Figure 2 displays the XRD patterns of the samples prepared at 150 °C with different molar ratios of en to Cd2+. In cases without en (sample a), the prepared CdS particles consisted of a mixture of hexagonal (major) and cubic (minor) forms (JCPDS 41-1049 and JCPDS 80-0019), because the characteristic peak of cubic (200) plane is observed clearly. This is because the energy difference of the two phases of CdS is very small,21 which makes it difficult to controllably isolate the growth of one phase at a time. In cases with en (samples b-d in Figure 2), the peak becomes weak, indicating that en could prevent formation of cubic CdS phase. The samples b, c, and d mainly consisted of hexagonal phase. The diffraction peaks of the hexagonal (002) plane of the three samples were stronger and
Figure 1. TEM images of CdS synthesized at 150 °C with different molar ratios of en to Cd2+. (a) 0, (b) 1.0, (c) 2.0, and (d) 3.0.
narrower than the diffraction peaks of the (100) and (101) planes. This indicates a relatively high crystalline order along the c-axis,15 whereas the order in the x-y plane is relatively poor, as verified from all other broad peaks.7 The diffraction peaks of sample c are stronger and narrower than those of samples b and d, indicating that sample c has the highest crystallinity and the best orientation growth along the c-axis of nanorods among the three samples. As shown in Table 1, specific surface areas of the samples prepared with en are much larger than that of the sample prepared without en. This can be attributed to that en was adsorbed on CdS to prevent its agglomeration. However, specific surface areas of the samples prepared with various ratios of en to Cd2+ are almost identical. Effect of Hydrothermal Temperature. It is well-known that the hydrothermal temperature plays an important role in the formation of crystal structure, shape, and size.22 Figure 3 shows
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Li et al. TABLE 2: Specific Surface Area of CdS Prepared at Different Temperatures with a Molar Ratio of en to Cd2+ of 2.0 temperature(°C) 90 120 150 180
Figure 2. X-ray diffraction patterns of CdS synthesized at 150 °C with different molar ratios of en to Cd2+. (a) 0, (b) 1.0, (c) 2.0, and (d) 3.0.
TABLE 1: Specific Surface Area of CdS Prepared at 150 °C with Different Molar Ratios of en to Cd2+ molar ratio of en to Cd2+
specific surface area (m2 g-1)
0 1.0 2.0 3.0
15.99 46.91 48.54 44.41
specific surface area (m2 g-1) 77.60 66.64 48.54 29.73
agonal CdS phase. The crystallinity of the sample obtained at 120 °C is very poor (Figure 4A-c), whereas that of the samples obtained at 150 and 180 °C is relatively high (Figure 4A-a,b). The crystallinity of sample a is close to that of sample b, indicating that when the temperature was high enough, it did not enhance the crystallinity markedly. Figure 4B shows the UV-vis absorption spectra of the samples obtained at different temperatures. The shapes of absorption edges are almost identical. However, the edges shift markedly toward longer wavelengths with increase of the temperature. This is due to the size quantization effect,22,23 indicating that the crystal size increases with the increase of the temperature. This is consistent with the results from Table 2. Photocatalytic Performance of the Prepared CdS. Figure 5A shows the hydrogen evolution rate of the samples prepared at 150 °C with different molar ratios of en to Cd2+. The rate
TEM images of the samples prepared at 120 and 180 °C for 8 h. When the temperature is 120 °C, nanograins (major) and short nanorods (minor) are observed (Figure 3a). When the temperature is 180 °C, longer and wider nanorods are major, whereas nanograins are minor (Figure 3b). This suggests that higher temperature favors the growth of nanorods. The nanorods prepared at 180 °C are much longer than those prepared at 150 °C (Figure 1c), demonstrating that the length of nanorods increases with the increase of the temperature. As shown in Table 2, specific surface areas of the prepared samples decrease quickly with the increase of the temperature. Figure 4A displays XRD patterns of the samples obtained at different temperatures. All samples mainly consisted of hex-
Figure 3. TEM images of CdS synthesized at (a) 120 and (b) 180 °C with a molar ratio of en to Cd2+ of 2.0.
Figure 4. (A) X-ray diffraction patterns of CdS synthesized at (a) 180, (b) 150, and (c) 120 °C with a molar ratio of en to Cd2+ of 2.0. (B) UVsvis absorption spectra of CdS synthesized at (a) 90, (b) 120, (c) 150, and (d) 180 °C with a molar ratio of en to Cd2+ of 2.0.
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Figure 6. Time course of photocatalytic hydrogen evolution over 0.050 wt % Pt-CdS synthesized at 150 °C with a molar ratio of en to Cd2+ of 2.0. Reaction conditions as in Figure 5 except irradiation time.
Figure 5. (A) Photocatalytic H2 evolution of CdS samples synthesized at 150 °C with different molar ratios of en to Cd2+. (B) Photocatalytic H2 evolution of CdS samples synthesized at different temperatures with a molar ratio of en to Cd2+ of 2.0. Conditions: 50 mg of CdS with 0.050 wt % Pt loaded by in situ photoreduction, 5.0 mL of formic acid in 100 mL of distilled water, and 3 h irradiation.
increases with the increase of the ratio to a maximum at the ratio 2.0, beyond which the rate decreases. The rate of CdS prepared at 150 °C without en was 4.11 µmol · h-1 under the same conditions, much lower than those of CdS samples prepared with en. Based on results from Figures 1 and 2, we could conclude that the activity of CdS nanorods for hydrogen evolution should be higher than that of CdS nanograins, and the activity increased with the increase in the length of the nanorods. Also, high crystallinity of the samples can enhance the photocatalytic activity. Figure 5B shows the photocatalytic activity of the samples prepared at different temperatures. The CdS samples prepared at lower temperatures (90 and 120 °C) have lower activity, which is due to lower crystallinity and a smaller amount of nanorods. The CdS sample prepared at 150 °C has the highest activity. The activity of the sample prepared at 180 °C is lower than that of the sample prepared at 150 °C, which would be attributed to that the CdS nanorods prepared at 180 °C were much longer than those prepared at 150 °C and the specific surface area prepared at 180 °C was smaller than that prepared at 150 °C, which lowered the activity. In general, an efficient photocatalyst requires a large surface area and a high crystallinity because many reaction steps of photocatalysis take place on the surface, and a defect in the material structure could provide a site for energy-wasteful
electron-hole recombination that reduces the efficiency of photocatalysis by absorbed photons.24,25 The two factors should be important to the photocatalysis. However, the amount of nanorods of the prepared nanocrystals and their length should also be key factors to the activity. Figure 6 shows H2 evolution time course of 0.050 wt % Pt-CdS prepared at 150 °C with a molar ratio of en to Cd2+ of 2.0 under visible light (λ g 420 nm) irradiation from an aqueous solution containing formic acid as sacrificial reagent. The average rate of hydrogen evolution is nearly maintained at 223 µmol · h-1 in the first five cycles. The total amount of hydrogen evolved under visible light irradiation for 50 h is 6958 µmol. The turnover number, which is defined as the ratio of the total amount of hydrogen evolved to the amount of CdS, reaches 20 in the first five cycles. These indicate that the prepared CdS was very stable and hydrogen was produced photocatalytically. According to eq 1, the average apparent quantum yield amounts to 13.9% during 50 h of irradiation. Pt Loading Effect. Figure 7 shows a TEM image with a selective EDS analysis area and high-resolution HRTEM images with EDS analysis areas of CdS with 0.050 wt % Pt loading content. No deposited Pt particles on CdS can be observed from the TEM image and HRTEM images. This indicates that Pt should be dispersed highly on CdS. The Pt contents in the selective areas 1eds (Figure 7a) and 2eds and 3eds (Figure 7c) were 0.13, 0.10, and 0.00 wt %, respectively. Area 2eds is located at the nanorods, whereas area 3eds at the nanograins. This demonstrates that Pt should be deposited selectively on nanorods. Pt loading is important to photocatalytic hydrogen evolution due to its acting as a photoinduced electron trap and to lowering overpotential for hydrogen evolution.1,8 Figure 8 shows the effect of Pt loading content on the activity of photocatalytic H2 evolution over CdS prepared at 150 °C with a ratio of en to Cd2+ of 2.0. Without Pt loading, CdS alone shows low activity for photocatalytic H2 evolution (ca. 10.96 µmol · h-1). With Pt loading, the activity increases remarkably, and when the Pt loading content is 0.050 wt %, the activity achieves a maximum. The activity is increased by up to 21 times compared to that of pure CdS. However, the activity decreased when the Pt loading content was excessive.1 The Pt loading content in general is larger than 1.0 wt % in the literature for highly active hydrogen evolution. Low Pt loading content can be due to its high dispersion. It is interesting that the maximal activity can be
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Figure 7. TEM image and HRTEM images with selective EDS analysis areas of CdS (150 °C and a molar ratio of en to Cd2+ of 2.0) with 0.050 wt % loading Pt content.
SCHEME 1: Schematic Illustration of the Growth Mechanism of Wurtzite CdS Nanorods with Selectively Adsorbed en
Figure 8. Effect of Pt loading content on photoactivity of CdS synthesized at 150 °C with a molar ratio of en to Cd2+ of 2.0. Reaction conditions as in Figure 5.
achieved with very low Pt loading content, which can lower the cost of the photocatalyst for practical application. Mechanism. The reaction process for the formation of CdS from CdSO4 aqueous solution containing en can be expressed as follows. First, Cd2+ ion coordinated with en molecules to form [Cd (en)2]2+ coordinating ion:
Cd2+ + 2en f [Cd(en)2]2+
(2)
Thioacetamide decomposed to produce S2- in the base environment:
CH3CSNH2 + 2OH- f CH3COO- + NH4+ + S2-
(3) Finally, the CdS nanocrystals nucleated and grew via the following reaction:
[Cd(en)2]2+ + S2- f CdS + 2en 2+
(4)
Without en, the concentration of Cd in aqueous solution was high, which could react with S2- released from reaction 3 at higher temperature to form a large amount of CdS crystal nuclei, so the monomer concentration of CdS for crystal growth was lower. The growth of CdS crystals should mainly be controlled by thermodynamic equilibrium to obtain granular and lamellar nanocrystals as shown in Figure 1a, because the CdS nanocrystal with granular shape has a minimum surface energy. However, with en, due to forming a stable coordinating ion [Cd(en)2]2+ (log β2 ) 10.09, where β2 represents the stability constant of coordinating ion [Cd(en)2]2+), the growth rate of crystal nuclei via reaction 4 was relatively slow so that the
monomer concentration of CdS for crystal growth was higher. The crystal growth should be controlled by the kinetic rate. The rodlike structures would be obtained by increasing the rate of growth in one direction with respect to other growth orientations.26 Scheme 1 shows the formation mechanism of the nanorods with wurtzite structure. The orientation growth can be observed when some en molecules adsorb to the side facets (100) of CdS nanocrystals to terminate in the lateral direction growth. Thus, in the hydrothermal process CdS nucleated in the solution and grew along the (001) facet according to closedpacking effect, which caused the formation of CdS nanorods.27 The (100) facet can be terminated by sulfide atoms or cadmium atoms. If sulfide atoms are terminal atoms, the uncoated facet grows quickly along that direction. For any cadmium atoms exposed on the (100) facet, there are always three dangling bonds for each cadmium atom, and en can be adsorbed on the facet in the chelate structure to terminate the growth. However, due to an excess amount of dangling bonds, crystal growth on this facet is still active.28 With increase of the ratio of en to Cd2+, the concentration of CdS crystal nuclei decreased, whereas the selective adsorption amount of en on the (100) facet increased, which led to the development of nanorods very well. Thus, at the ratio 2.0 the nanorods had the highest aspect ratio. However, when the ratio was high enough, such as 3.0, higher en concentration would result in its nonselective adsorption on all facets. This impeded formation of the nanorods. Thus, when the ratio was 3.0, nanorods were not developed very well. Reaction 4 happened at high temperature. With increasing temperature, high concentration of the monomers was expected.
CdS Nanorods for Photocatalytic H2 Evolution SCHEME 2: Schematic Formation Mechanism of Highly Dispersed Pt on Wurtzite CdS Nanorods
Thus, the longer and wider nanorods were obtained at 180 °C, as shown in Figure 3b. In Qian’s work,29 they proved by IR that the en molecules coordinated with Cd2+ in trans conformation with one coordination atom (monodentate coordination) on the surface of CdS. This can be attributed to the sulfur bonded with Cd2+ that impeded the formation of the chelate structure between en molecule and Cd2+ on the surface of CdS. Because nanorods have a high aspect ratio, the adsorption amount of en on the (001) facet should be major, whereas that on (100) should be minor. Cumulative stability constants for methylamine (monodentate) coordinated with Cd2+ are 2.75 (log β1) and 4.85 (log β2),30 much smaller than those for ethylenediamine (bidentate) coordinated with Cd2+ (log β1 ) 5.45, log β2 ) 10.09). The bonding ability of en with Cd2+ on the surface of CdS in trans conformation should be close to that of methylamine with Cd2+. This indicates that the interaction between the en molecules and Cd2+ ions on the (001) facet is weaker. Thus, the en molecules adsorbed on the (001) facet of CdS nanorods could dissociate easily and be replaced by water molecules during the washing process, whereas en molecules on the (100) facet in chelate form could not be removed. When prepared CdS was added to aqueous PtCl62- solution, PtCl62- ions would be adsorbed on the exposed cadmium atoms (ions) by coordination bonds between Cl atoms of PtCl62- and the cadmium atoms (ions) or by electrostatic attraction, which makes PtCl62- ions disperse highly on the (001) facet. The surface cadmium atoms with dangling atom on the (001) facet should be effective trap centers of photoinduced electrons when irradiated. Thus, PtCl62- ions were reduced into metal Pt effectively and dispersed highly on the facet so that Pt particles could not be observed by HRTEM. This mechanism can be described by Scheme 2. Due to high dispersion of Pt, the optimal loading amount is very low. For a photocatalyst to be highly active, the separation of photoinduced electrons and holes should be effective. As shown in Scheme 3, there is a permanent dipole along the c-axis which could separate photoinduced electron-hole pairs effectively. Terminal sulfide atoms exposed on the (100) facet with dangling bonds could act as the hole traps, whereas Pt deposited on the (001) facet could act as the electron traps. The holes can be scavenged by electron donor formic acid, whereas protons are reduced on Pt for hydrogen evolution. Therefore, the separation efficiency of the electron-hole pairs, and the photocatalytic activity of nanorods with wurtzite structure should be very high. It should be noted that terminal cadmium atoms exposed on the (100) facet could act as the electron traps, in which recombination of the electron-hole pairs would take place to lower the activity. With increase of the aspect ratio or the length of nanorods, the effect should decrease. Thus, photocatalytic
J. Phys. Chem. C, Vol. 113, No. 21, 2009 9357 SCHEME 3: Schematic Illustration of the Mechanism of Effective Separation of Photoinduced ElectronsHole Pairs for Photocatalysis on Wurtzite CdS Nanorods
activity should increase with the aspect ratio. However, when the aspect ratio was too high, the transferring distance of the holes along the c-axis would enhance and the reaction surface area of the holes on the (100) facet would decrease. These led to lower the activity. Thus, there was an optimal aspect ratio to achieve high activity. Conclusion CdS nanocrystals have been synthesized by a hydrothermal method using en as the template agent and coordination agent. The nanocrystals display rodlike and granular shapes, and their amount and morphology depend on the hydrothermal temperature and added content of en. The photoactivity of the nanorods is higher than that of the nanograins. For the nanorods, there is an optimal aspect ratio to achieve the highest activity when the molar ratio of en to Cd2+ is 2.0. Pt can be dispersed highly on CdS nanorods, so the Pt content for effective hydrogen evolution is very low. Under the optimal conditions (molar ratio of en to Cd2+ 2.0, hydrothermal temperature 150 °C, loading Pt content 0.050 wt %), the highest apparent quantum yield amounts to 13.9% for hydrogen evolution under visible light irradiation. Acknowledgment. The financial support of National Basic Research Program of China (2009CB220003), the National Nature Science Foundation of China (No. 20763006), Program for Changjiang Scholars and Innovative Research Team in University (IRT0730), and the Nature Science Foundation of the Jiangxi Province (No. 2007GZH1754) is gratefully acknowledged. References and Notes (1) Li, Y. X.; Lu, G. X.; Li, S. B. Appl. Catal., A 2001, 214, 179. (2) Li, Q. Y.; Jin, Z. L.; Peng, Z. G.; Li, Y. X.; Li, S. B.; Lu, G. X. J. Phys. Chem. C 2007, 111, 8237. (3) Zou, Z. G.; Ye, J. H.; Sayama, K.; Arakawa, H. Nature 2001, 414, 625. (4) Khan, S. U. M.; Al-Shahry, M.; Ingler, W. B., Jr. Science 2002, 297, 2243. (5) Peng, S. Q.; Li, Y. X.; Jiang, F. Y.; Lu, G. X.; Li, S. B. Chem. Phys. Lett. 2004, 398, 235. (6) Jing, D. W.; Guo, L. J. J. Phys. Chem. B 2006, 110, 11139. (7) Jang, J. S.; Joshi, U. A.; Lee, J. S. J. Phys. Chem. C 2007, 111, 13280. (8) Li, Y. X.; Du, J.; Peng, S. Q.; Xie, D.; Lu, G. X.; Li, S. B. Int. J. Hydrogen Energy 2008, 33, 2007. (9) Sathish, M.; Viswanathan, B.; Viswanath, R. P. Int. J. Hydrogen Energy 2006, 31, 891. (10) Ashokkumar, M. Int. J. Hydrogen Energy 1998, 23, 427. (11) Kar, S.; Chaudhuri, S. J. Phys. Chem. B 2006, 110, 4542.
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