TiO2 Photocatalysts for

Oct 11, 2008 - The results indicate that the location as well as the electronic state of Pt is important for the high performance of platinized CdS/Ti...
3 downloads 6 Views 423KB Size
Download PDF
17200

J. Phys. Chem. C 2008, 112, 17200–17205

Location and State of Pt in Platinized CdS/TiO2 Photocatalysts for Hydrogen Production from Water under Visible Light Jum Suk Jang,† Sun Hee Choi,‡ Hyun Gyu Kim,§ and Jae Sung Lee*,† Eco-friendly Catalysis and Energy Laboratory (NRL), Department of Chemical Engineering, Pohang UniVersity of Science and Technology (POSTECH), San 31 Hyoja-dong, Pohang 790-784, Korea, Beamline Research DiVision, Pohang Accelerator Laboratory, POSTECH, San 31, Hyojadong, Pohang 790-784, Korea, and Busan Center, Korea Basic Science Institute (KBSI), Busan 609-735, Korea ReceiVed: May 28, 2008; ReVised Manuscript ReceiVed: September 2, 2008

CdS/TiO2 composite photocatalysts were platinized by different methods such as photodeposition (PD), wet impregnation (WI), and chemical reduction (CR), and studied for hydrogen production from water under visible light. All Pt species were in the metallic state, yet PD and WI photocatalysts contained electrondeficient Pt. In particular, Pt-Ti formation was identified in the WI catalyst, which contributed to electron deficiency of Pt. These two photocatalysts of electron-deficient Pt exhibited higher rates of hydrogen evolution due to favorable diffusion of photoelectrons from excited CdS toward the Pt. Between PD and WI photocatalysts, the PD catalyst showed a lower rate because part of the Pt in the catalyst resided on CdS, whereas all Pt species were located on TiO2 nanoparticles for WI and CR catalysts. The results indicate that the location as well as the electronic state of Pt is important for the high performance of platinized CdS/TiO2 photocatalysts in hydrogen production from water. 1. Introduction Visible light-driven photocatalysts that could produce hydrogen from hydrogen-containing compounds under sun light have been extensively studied.1-6 Cadmium sulfide (CdS) is an n-type semiconductor with an ideal band gap energy (2.4 eV) and band positions that can drive both oxidation and reduction of water under visible light irradiation.7 In an attempt to improve its photoactivity, CdS has been combined with another material such as ZnO, TiO2, or LaMnO38-11 to form a composite photocatalyst. The second component in the composite photocatalyst accepts electrons photogenerated in the conduction band of CdS to its own conduction band and thereby improves the charge separation of photogenerated electrons and holes. Recently, we reported CdS/TiO2 nanoparticle-bulk composite photocatalysts where bulk CdS particles are decorated with nanosized TiO2 particles. This configuration formed a heterojunction composite and exhibited a high rate of hydrogen production from aqueous solution containing sulfide and sulfite as hole scavengers under visible light (λ e 420nm).12,13 In general, a transition metal is loaded on the photocatalyst as a cocatalyst to enhance the rate of the photoreduction of the proton.14-16 The barrier height at the photocatalyst/noble metal interface of the photocatalyst is lowered relative to that at the photocatalyst/solution interface, causing efficient charge separation of photogenerated electrons and holes.17-21 Nozik designated this system as a Schottky type photochemical diode.22 Sathish et al. have reported that there is a direct correlation between the rates of hydrogen evolution and properties of the transition metal cocatalysts.23 Among Pt, Pd, and Rh, Pt metal with higher redox potential and work function, and lower metal * Corresponding author. E-mail: [email protected]. Fax: 82-54-2795799. Tel: 82-54-279-2266. † Department of Chemical Engineering, Pohang University of Science and Technology (POSTECH). ‡ Pohang Accelerator Laboratory, POSTECH. § Korea Basic Science Institute (KBSI).

hydrogen bond strength was found to be the most favorable for hydrogen evolution activity. Without cocatalysts, photocatalysts show very poor performance and many of them do not function at all. Despite the critical role of the cocatalysts, however, there have been only a limited number of studies regarding fundamental aspects of the metal/semiconductor systems. In this work, we platinized CdS/TiO2 composite photocatalysts by using three different methods: chemical reduction, wet impregnation, and photodeposition. The location and the state of loaded Pt were investigated by using transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), fluorescence extended X-ray absorption fine structure (EXAFS), and theoretical calculation of X-ray absorption near-edge structure (XANES). The results were then correlated with the activity of photocatalytic hydrogen production from water under visible light. 2. Experimental Procedures 2.1. Preparation of Platinized CdS/TiO2 Composite Photocatalysts. The CdS/TiO2 composite photocatalysts were prepared according to the procedure that we reported previously.12 Briefly, bulky CdS, calcined at 1073 K for 1 h under He flow, was stirred in isopropyl alcohol and tetratitanium isopropoxide in a mole ratio of Ti to CdS of 4 as H2O was added drop-by-drop. The prepared composite powders were filtered, dried, and then calcined at 673 K for 1 h under air flow to increase the crystallinity of TiO2 in CdS/TiO2 composite photocatalysts. Platinum was loaded as a cocatalyst on the calcined CdS/ TiO2 photocatalysts by three different methods. For in situ photodeposition (PD), the composite photocatalyst was dispersed in an aqueous solution containing a required amount of H2PtCl6 to obtain 2.0 wt % Pt in the final platinized CdS/TiO2 composite photocatalysts. The solution was illuminated for 2 h under visible light (λ g 420 nm), filtered, and then dried in a static oven at 343 K. For platinization by a wet impregnation (WI)

10.1021/jp804699c CCC: $40.75  2008 American Chemical Society Published on Web 10/11/2008

Platinized CdS/TiO2 Photocatalysts for Hydrogen Production method, CdS/TiO2 was impregnated with an aqueous solution containing H2PtCl6 · nH2O precursor (to give 2 wt % Pt) followed by drying in an atmospheric environment. The resulting products were calcined in a furnace at 573 K for 1 h and then reduced in H2 flow (18.6 µmol/s) at 673 K for 2 h. For platinization by chemical reduction (CR) method, an aqueous solution of H2PtCl6 (corresponding to 2 wt % Pt) was added drop-by-drop to the aqueous CdS/TiO2 slurry. To this mixture, an aqueous NaBH4 solution of pH 12 (adjusted using NaOH solution) was slowly added. The resulting solution was continually stirred for 30 min, filtered, and then dried in a static oven at 343 K. The platinized photocatalysts prepared by the above procedures are denoted as Pt(PD), Pt(WI), and Pt(CR). 2.2. Characterization. The morphology of photocatalysts was investigated by TEM (JEOL JEM 2010F, field emission electron microscope) operated at 200 kV. The chemical states of platinum in the samples were determined from XPS measurements (VG Scientific, ESCALAB 220iXL) using Mg KR radiation (1253.6 eV). The binding energy calibration was performed using C 1s peak in the background as the reference energy (284.6 eV). The structure of loaded platinum was investigated with fluorescence EXAFS. The X-ray absorption measurements were performed at the 5A wiggler beamline of Pohang Accelerator Laboratory (2.5 GeV, stored current 150-200 mA) located in Pohang, Korea. The radiation was monochromatized with a Si(111) double crystal monochromator, and the incident beam was detuned by 30% in order to minimize the higher order reflections from the silicon crystals. The Pt LIII-edge spectra were taken in a fluorescence mode with two detectors; a Hefilled IC Spec ionization chamber for incident beam and a passivated, implanted, planar silicon (PIPS) detector for fluorescence signals from the sample. The fluorescence mode was chosen because the strong background absorption of Cd constituent in CdS/TiO2 made the transmission mode measurements difficult. The energy was calibrated with respect to the LIII-edge energy of Pt foil, 11564 eV, before examining the photocatalyst samples. The obtained data were analyzed with the IFEFFIT suite of software programs24,25 and FEFF 8 code26 for identification. The pre-edge background was removed by using a simple linear fit in the region of 11414-11504 eV. The spectra were then normalized by using the background function µ0(E) at the edge, fitted to quadratic polynomials in the region of 11714-12364 eV. The postedge background function was approximated with a piecewise spline that could be adjusted so that the low-R components of pre-Fourrier transformed data were minimized. The normalized and background-removed spectra were converted to the EXAFS function χ(k) in k-space. The k3-weighted EXAFS function k3χ(k) was Fourier-transformed to obtain the radial structural function (RSF) in R-space. A shell of interest in the RSF was back-Fourier transformed into momentum space (q-space). The reference materials used as standards for fitting the experimentally derived RSFs were generated with FEFF 8 code, and the Fourier-filtered data were fitted in q-space. The detailed procedure for data analysis is described elsewhere.27-29 2.3. Photocatalytic Reaction. The photocatalytic reactions were carried out at room temperature under atmospheric pressure in a closed photoreactor using a Hg-arc lamp (500 W) equipped with IR liquid filter and UV cut off filter (λ g 420 nm). The rate of H2 evolution was determined in an aqueous solution (100 mL) containing 0.1 g of catalyst and 0.1 M Na2S + 0.02 M Na2SO3. The evolved H2 gas was collected and analyzed by

J. Phys. Chem. C, Vol. 112, No. 44, 2008 17201 gas chromatography equipped with a thermal conductivity detector and molecular sieve 5-Å column with Ar carrier gas. 3. Results and Discussion 3.1. Characterization of Platinized CdS/TiO2 Composite Photocatalysts. CdS/TiO2 composite photocatalyst is made of highly crystalline bulk CdS particles and TiO2 nanoparticles as shown in Figure 1a. Large CdS particles of ca. 1-2 µm size are decorated with TiO2 nanoparticles of ca. 10-20 nm forming a multiparticle layer on the CdS surface (see the inset in Figure 1). As we reported in the previous work,12,13 X-ray diffraction showed that CdS was in the hexagonal wurtzite phase and TiO2 in anatase phase. The morphology after loading Pt onto CdS/ TiO2 by the PD method was examined by TEM as shown in Figure 1b. It is clearly seen that Pt particles of 4-5 nm are deposited on CdS/TiO2. The size distribution of platinum particles is fairly uniform. The EDX (energy-dispersive X-ray spectroscopy) analyses for several spots of a sample gave similar spectra to the one in Figure 1c showing the presence of Pt, Cd, S, and O. Since the sampling spot size of the EDX technique lies in a submicrometer range, the result indicates that Pt and TiO2 are homogeneously distributed on CdS without forming big agglomerates. Figure 2 shows the TEM images of photocatalysts platinized by WI and CR methods. Regardless of Pt loading method, the morphologies of the photocatalysts are similar, and Pt particle sizes are also similar. Thus, platinum has been evenly loaded on CdS/TiO2 composite photocatalysts without serious aggregation. The electronic state of Pt on platinized CdS/TiO2 photocatalyst was probed by XPS measurements. Figure 3 shows Pt 4f XPS spectra of the platinized composite photocatalysts. The Pt(CR) sample exhibits a large 4f7/2 peak at approximately 71.2 eV, which is almost the same as the standard binding energy of Pt metal (71.0 eV). However, Pt(PD) and Pt(WI) exhibit the binding energy of 71.8 eV, slightly higher than that of the Pt metal. Since the value is greatly removed from the binding energies of PtO (74.2 eV) or PtO2 (75.0 eV), the platinum in both Pt(PD) and Pt(WI) photocatalysts should be considered zero-valent Pt metal but in an electron-deficient state. 3.2. Structural and Electronic Characterization by Fluorescence EXAFS and Computed XANES. The local structure of Pt in the platinized CdS/TiO2 was characterized with EXAFS. Figure 4 shows the Fourier transformed spectra of Pt LIII-edge EXAFS for the platinized photocatalysts. The Pt(PD) catalyst has a strong peak at 1.0-2.3 Å and a weak one at 2.3-3.1 Å. But Pt(WI) and Pt(CR) catalysts show two peaks of comparable peak intensity at 1.0-2.0 and 2.0-3.1 Å. While the first peak at 1.0-2.0 Å in Pt(WI) and Pt(CR) is believed to be the contribution from the nearest oxygen atoms, the peak for Pt(PD) has shifted significantly to a longer distance. This indicates the presence of another backscatterer whose distance is slightly longer than that of Pt-O. In order to identify the nature of the other backscatterer, theoretical EXAFS spectra of PtS and PtO were synthesized with FEFF by using their respective structural information.30,31 In Figure 5, these theoretical spectra are compared with that of Pt(PD) catalyst. The first peak of Pt(PD) is asymmetric and ranges between 1.0 and 2.3 Å covering the PtS peak at 1.2-2.3 Å, as well as the PtO peak at 1.0-2.0 Å. The peak maximum of the Pt(PD) is closer to that of PtS rather than that of PtO. Since the first peak in PtO and PtS structure represents Pt-O and Pt-S scattering, respectively, it is possible that both Pt-O and Pt-S contribute to construction of the first shell of Pt in the Pt(PD) catalyst. To substantiate this qualitative analysis, we

17202 J. Phys. Chem. C, Vol. 112, No. 44, 2008

Jang et al.

Figure 1. TEM images of CdS/TiO2 (a) before and (b) after platinization by photodeposition (Pt(PD)) and (c) TEM-EDX spectrum of the platinized Pt(PD) photocatalyst. The insets are expanded images of pointed areas.

Figure 2. TEM images of CdS/TiO2 composite photocatalysts platinized by (a) wet impregnation, Pt(WI), and (b) chemical reduction, Pt(CR).

fitted the shell at 1.0-2.3 Å in the Pt(PD) catalyst with those two scatterings as standard. The fitting results are shown in Table 1. An excellent fitting was obtained with Pt-O at 2.075 Å and Pt-S at 2.323 Å. Relative contributions of the two scatterings are reflected in the coordination numbers (N), that is 2.57 for Pt-O and 0.59 for Pt-S. Thus, indeed, the Pt-S bond makes a significant contribution, although Pt-O still makes a greater contribution to the first shell of the Pt(PD) catalysts. The only possible source of sulfur in the photocatalyst system is CdS. The result therefore indicates that Pt in Pt(PD) is located on CdS as well as TiO2. In contrast, the first shell of Pt(WI) and Pt(CR) was made solely of Pt-O, indicating that Pt in these photocatalysts is present solely on TiO2. For Pt(WI) and Pt(CR) photocatalysts, two peaks at 1.0-3.2 Å were simultaneously considered in the fitting procedure. The first peak at 1.0-2.0 is clearly due to Pt-O scattering as discussed above, and the second peak at 2.0-3.2 Å is associated with a metal-metal bonding. The peak at 2.0-3.2 Å for Pt(CR) is clearly a single peak, denoting that it is due to scattering

from a neighboring Pt. In contrast, Pt(WI) shows two overlapped peaks with the new peak at a shorter distance. There seems to be two kinds of metal backscatter in Pt(WI). The most probable backscatterer at this distance is titanium. Therefore, we tried two types of fitting procedures for Pt(WI) and Pt(CR) photocatalysts, namely, the two-shell fit considering two scatterings of Pt-O and Pt-Pt and the three-shell fit considering three scatterings of Pt-O, Pt-Pt, and Pt-Ti. Table 1 shows the results of both fits for Pt(WI) and Pt(CR) photocatalysts. The goodness of fit, R-factor, indicates that the experimental data of Pt(WI) are better fitted with the theoretical model consisting of three scatterings, whereas the two-shell fit is more appropriate for Pt(CR). Thus, intermetallic interaction between Pt and Ti is observed only for the Pt(WI) photocatalyst. The Pt-O distances for all samples are longer by about 0.02-0.05 Å than usual Pt-O distances in PtO or PtO2 structures. The Pt-S distance in Pt(PD) is also longer than that in PtS structure. The elongated distances are induced from the Pt species residing on the surface of CdS/TiO2, not inside, which

Platinized CdS/TiO2 Photocatalysts for Hydrogen Production

J. Phys. Chem. C, Vol. 112, No. 44, 2008 17203

Figure 5. Comparison of the Fourier-transformed spectra of Pt(PD) (solid line), PtO (dashed line), and PtS (dotted line). Those of PtO and PtS were processed from theoretically calculated χ(k) with the FEFF code.

TABLE 1: The EXAFS Least-Square Fitting Results of Platinized CdS/TiO2 Photocatalystsa Figure 3. XPS spectra of Pt 4f of CdS/TiO2 composite photocatalysts platinized by different methods: (a) Pt(PD); (b) Pt(WI); (c) Pt(CR). Pt(PD) Pt(WI)

Pt(CR)

shell

N

R (Å)

σ2 (Å2)

∆E (eV)

R-factor

Pt-O Pt-S Pt-O Pt-Pt Pt-O Pt-Pt Pt-Ti Pt-O Pt-Pt Pt-O Pt-Pt Pt-Ti

2.57 0.59 1.30 2.06 1.39 3.16 0.70 2.18 5.13 1.74 1.70 0.87

2.08 2.32 2.06 2.68 2.06 2.77 2.87 2.05 2.75 2.05 2.80 2.76

0.001 0.0018 0.001 0.0018 0.001 0.0029 0.0029 0.001 0.0054 0.001 0.0011 0.0011

13.62 13.62 11.39 -3.57 13.54 -6.33 -6.33 13.17 2.81 13.17 13.04 13.04

0.0072 0.0692 0.0284 0.0261 0.0656

a Energy variation (∆E), bond length (R), coordination number (N), and variation in Debye-Waller factor (σ2) obtained from FEFF code. The goodness of the fit is represented as R-factor which gives N a sum-of-squares of the fractional misfit defined as ∑i)1 { [Re(fi)]2 N 2 2 2 + [Im(fi)] }/∑i)1{ [Re(χ˜ data i)] + [Im(χ˜ data i)] }.

Figure 4. Fourier-transformed spectra of k3χ(k) in Pt LIII-edge EXAFS for CdS/TiO2 photocatalysts platinized by difference methods: ( · · · ) Pt(PD); (s) Pt(WI); (---) Pt(CR).

is in accordance with the TEM results in Figure 1. Although the Pt species exist on the TiO2 nanoparticles as well as on the bulky CdS for the Pt(PD) photocatalyst, the species on TiO2 are dominant because the Pt-O coordination number is higher than that of Pt-S by a factor of ca. 4. The intensity of the metal-metal peak for Pt(PD) is very weak as shown in Figure 4. This catalyst exhibits an electrondeficient state of Pt as identified by XPS results. The catalyst has the largest coordination number of the first shell made of oxygen and sulfur in Table 1 and the strongest first peak in Figure 4. Thus, a lower degree of reduction appears to be responsible for the electron-deficient state of Pt in the Pt(PD) photocatalyst. The platinum in both Pt(WI) and Pt(CR) is wellreduced as evidenced by the strong Pt-Pt scattering in Table 1 and in Figure 4. Nevertheless, the binding energy of Pt 4f in Pt(WI) also shows an electron deficiency similar to that in Pt(PD). To account for this apparent contradiction, we studied the effect of Pt-Ti bond formation on the electron deficiency because it is the unique feature of the Pt(WI) catalyst. Figure 6 shows the theoretical Pt LIII-edge XANES spectra of Pt metal and Pt0.5Ti0.5. The theoretical absorption spectra were calculated with the FEFF code for a cluster of 55 atoms by

using the Hedin-Lundqvist self-energy. A radius of 4.0 Å was chosen for the self-consistent potential, ensuring that the second nearest neighboring atoms were all considered in the calculations. For the calculation of Pt0.5Ti0.5, Ti atoms were stoichiometrically situated in the lattice of Pt metal. The theoretical spectrum of pure Pt metal in Figure 6b shows an excellent match with the experimentally obtained spectrum for a Pt foil in Figure 6c, demonstrating the validity of the computation. This measurement for Pt foil was taken in the transmission mode in order to prevent the reduction of white line (WL) intensity by selfabsorption during the measurements in the fluorescence mode. When Ti atoms are introduced, the WL intensity increases from 1.17 to 1.42. The increased strength of the WL feature with alloy formation has been reported for Pt1-xMx (M ) Al, Si, Ge; x ) 0-0.7).32 The increased WL intensity represents the increased Pt 5d hole count due to the formation of the Pt-Ti bond. Hence, the Pt species in Pt(WI) catalyst shows an electron deficiency originating from the Pt-Ti bond, although Pt-Pt bond formation is still prevalent. As far as the coordination number is concerned, Pt(WI) has a smaller metal-metal coordination number of 3.86 (Pt-Pt + Pt-Ti) than Pt(CR) (N ) 5.13). The Pt-Pt coordination number should be even smaller for Pt(PD) catalyst. It appears that this difference in N values is not directly related with the size of Pt species because TEM images in Figures 1 and 2 show all

17204 J. Phys. Chem. C, Vol. 112, No. 44, 2008

Jang et al.

Figure 8. Structural (a) and operational (b) models of platinized CdS/ TiO2 composite photocatalysts for photocatalytic hydrogen production from water in the presence of hole scavengers (0.1 M Na2S + 0.02 M Na2SO3).

Figure 6. Theoretically calculated Pt LIII-edge XANES spectra of (a) Pt0.5Ti0.5 and (b) Pt metal. The experimental spectrum for Pt foil (c) was compared. Spectra a and b are shifted by 0.5 and 0.25 in Y-axis, respectively.

Figure 7. Photocatalytic hydrogen production over CdS/TiO2 composite photocatalysts platinized by different methods: (a) Pt(PD); (b) Pt(WI); (c) Pt(CR). Catalyst ) 0.1 g loaded with 2 wt % Pt; electrolyte ) 0.1 M Na2S + 0.02 M Na2SO3; light source ) Hg-arc lamp (500 W) equipped with UV cutoff filter (λ g 420 nm). The inset shows a correlation between absorption and hydrogen generation activity (action spectrum) for Pt(PD) sample.

comparable Pt sizes for three photocatalysts. Instead, it may originate from the different degrees of static disorder of the coordination sphere due to the distributed distances. In addition, there is also the possibility of a different extent of exposure to the surface as seen by XPS depending on the oxidation state of Pt in the samples. 3.3. Photocatalytic Activity of Platinized CdS/TiO2 Composite Photocatalysts. Figure 7 shows the time course of H2 evolution from the aqueous electrolyte solution containing 0.1 M Na2S and 0.02 M Na2SO3 as sacrificial reagents under visible light irradiation (using a cutoff filter of λ g 420 nm for all catalysts). Hydrogen is produced from reduction of protons in water by the photoelectrons, while holes are consumed to oxidize sulfide ions in the electrolyte solution. Following a short induction period initially, all photocatalysts showed stable rates of hydrogen evolution. The behavior is similar to what was observed previously.12,13 The photocatalytic activity increased in a sequence of Pt(CR) < Pt(PD) < Pt(WI). The inset shows a correlation between absorption and hydrogen generation

activity (action spectrum) for Pt(PD) sample. The activity pattern resembles the absorption spectrum of CdS. In order to account for this difference in photoactivity, we propose in Figure 8 structural (a) and operational (b) models of platinized CdS/TiO2 composite photocatalysts. As evident from TEM images of Figures 1 and 2, a large and crystalline CdS particle of ca. 1-2 µm is decorated with a multiparticle layer of TiO2 nanoparticles of ca. 10-20 nm on the CdS surface. This TiO2 layer is not continuous nor compact but is loose and porous. When Pt is loaded, it is naturally deposited preferentially on TiO2 forming 4-5 nm Pt particles. But in the case of Pt(PD) photocatalyst, some Pt species penetrate through the TiO2 layer and are deposited on CdS, as indicated by Pt-S interaction in EXAFS observed for the catalyst. An operational model of the current platinized CdS/TiO2 composite photocatalyst is depicted in Figure 8b. Under visible light irradiation (g420 nm), only CdS with proper band gap energy can absorb the photons to generate photoelectrons and holes in its conduction and valance bands, respectively. The holes are rapidly scavenged on the CdS surface to oxidize the sacrificial reagents, Na2S and Na2SO3, in the electrolyte. The electrons in the conduction band of CdS are transferred to the conduction band of surrounding TiO2 particles driven by the different conduction band positions of the two semiconductors. The charge transfer from CdS to TiO2 is supported by the resemblence between the hydrogen evolution activity pattern and the absorption spectrum of CdS as shown in the action spectrum in the inset of Figure 7. The charge injection from excited CdS to TiO2 was found to be very fast, on the order of a picosecond.33-35 As we reported earlier,12,13 the superior activity of CdS/TiO2 relative to CdS alone is due to this efficient charge separation with holes remaining in CdS while electrons migrate to TiO2. When platinum is loaded on TiO2 of CdS/TiO2, the charge separation may be further enhanced when electrons are now collected by Pt, which catalyzes the reduction of proton to hydrogen with these photoelectrons. According to this model, we expect that the efficiency of the photocatalytic hydrogen production would depend on the location and electronic state of Pt in this platinized CdS/TiO2 composite photocatalyst. As an electron collector, Pt in an electron-deficient state would be more effective to receive electrons. Thus, Pt(PD) and Pt(WI) photocatalysts, in which platinum is electron-deficient, perform better than Pt(CR) catalyst with Pt in its usual metallic state. However, the electron deficiency of the two photocatalysts seems to originate from different sources. The Pt(PD) catalyst achieves the electron deficiency by prevalent interaction with electron-withdrawing anioic species (oxygen and sulfur), whereas the Pt(WI) photocatalyst does it by forming Pt-Ti alloy bonds. The Pt(PD) and

Platinized CdS/TiO2 Photocatalysts for Hydrogen Production Pt(WI) photocatalysts have a similar degree of electron deficiency evidenced by similar XPS binding energy, yet Pt(WI) displays superior photoactvity. This must be due to the location of Pt particles. Platinum performs its expected roles of an electron collector as well as a hydrogen-generating catalyst when it is deposited on TiO2 particles. Since the electrons flow from excited CdS to TiO2, Pt should perform better when it is located on TiO2. In the case of Pt(WI), all Pt is located on TiO2 particles, whereas in Pt(PD), part of the Pt is also photodeposited on the bulk CdS. The Pt species on CdS would trap photoelectrons and serve as the recombination site for holes and the trapped electrons. Thus the Pt(PD) photocatalyst containing Pt deposited on CdS is less effective. The results indicate that the location as well as the electronic state of Pt is important for the high performance of platinized CdS/TiO2 photocatalysts in hydrogen production from water. The Pt(WI) catalyst shows the highest rate of hydrogen evoloution because the Pt species in the catalyst is in an electron-deficient metallic state, and the Pt species is deposited only on TiO2 particles. 4. Conclusions The platinized CdS/TiO2 photocatalysts, fabricated by different methods of photodeposition, wet impregnation, and chemical reduction, were characterized for hydrogen production from water under visible light irradiation. The Pt atoms were evenly located on TiO2 of CdS/TiO2, except that a part of the Pt was deposited on CdS for photodeposited catalyst. The photocatalysts with Pt loaded by wet impregnation and photodeposition possessed electron-deficient Pt species and thus could promote efficient charge separation by fast diffusion of photoelectrons from excited CdS to Pt through TiO2. The catalyst is also more effective when the Pt species is deposited only on TiO2 nanoparticles. Thus, the location as well as the electronic state of Pt is important for the high performance of platinized CdS/TiO2 photocatalysts in hydrogen production from water. Acknowledgment. This work was supported by the Hydrogen Energy R&D Center, one of the 21st Century Frontier R&D Program, General Motors R& D Center, and the Brain Korea 21 Project. References and Notes (1) Ishikawa, A.; Takata, T.; Kondo, J. N.; Hara, M.; Kobayashi, H.; Domen, K. J. Am. Chem. Soc. 2002, 124, 13547.

J. Phys. Chem. C, Vol. 112, No. 44, 2008 17205 (2) Kim, H. G.; Hwang, D. W.; Lee, J. S. J. Am. Chem. Soc. 2004, 126, 8912. (3) Tsuji, I.; Kato, H.; Kudo, A. Angew. Chem., Int. Ed. 2005, 44, 3565. (4) Kim, H. G.; Pramod, H. B.; Choi, W.; Lee, J. S. Angew. Chem., Int. Ed. 2005, 44, 4585. (5) Reber, J. F.; Meier, K. J. Phys. Chem. 1986, 90, 824. (6) Jang, J. S.; Kim Vangala, H. G.; Reddy, R.; Bae, S. W.; Ji, S. M.; Lee, J. S. J. Catal. 2005, 231, 213. (7) Buhler, N.; Meier, K.; Reber, J. J. Phys. Chem. 1984, 88, 3261. (8) Sobczynski, A.; Bard, A. J.; Campion, A.; Fox, M. A.; Mallouk, T.; Webber, S. E.; White, J. M. J. Phys. Chem. 1987, 91, 3316. (9) Spanhel, L.; Weller, H.; Henglein, A. J. Am. Chem. Soc. 1987, 109, 6632. (10) Kida, T.; Guan, G.; Yoshida, A. Chem. Phys. Lett. 2003, 371, 563. (11) Shangguan, W.; Yoshida, A. J. Phys. Chem. 2002, 106, 12227. (12) Jang, J. S.; Lee, J. S.; Li, W.; Oh, S. H. Chem. Phys. Lett. 2006, 425, 278. (13) Jang, J. S.; Ji, S. M.; Bae, S. W.; Son, H. C.; Lee, J. S. J. Photochem. Photobiol. A: Chem. 2007, 188, 112. (14) Matsumura, M.; Saho, Y.; Tsubomura, H. J. Phys. Chem. 1983, 87, 3807. (15) Harbour, J. R.; Wolkow, R.; Hair, M. L. J. Phys. Chem. 1981, 85, 4026. (16) Uchihara, T.; Matsumura, M.; Yamamoto, A.; Tsubomura, H. J. Phys. Chem. 1989, 93, 5870. (17) Tsubomura, H.; Nakato, Y. NouV. J. Chim. 1987, 11, 167. (18) Harada, H.; Sakata, T.; Ueda, T. J. Am. Chem. Soc. 1985, 107, 1773. (19) Nakato, Y.; Ueda, K.; Yano, H.; Tsubomura, H. J. Phys. Chem. 1988, 92, 2316. (20) Sakata, T.; Kawai, T.; Hashimoto, K. Chem. Phys. Lett. 1982, 88, 50. (21) Aspnes, D. E.; Heller, A. J. Phys. Chem. 1983, 87, 4919. (22) Nozik, A. J. Appl. Phys. Lett. 1977, 30, 567. (23) Sathish, M.; Viswanathan, B.; Viswanath, R. P. Int. J. Hydrogen Energy 2006, 31, 891. (24) Newville, M. J. Synchrotron Radiat. 2001, 8, 322. (25) Ravel, B.; Newville, M. J. Synchrotron Radiat. 2005, 12, 537– 541. (26) Ankudinov, A. L.; Bouldin, C.; Rehr, J. J.; Sims, J.; Hung, H. Phys. ReV. B 2002, 65, 104107. (27) Choi, S. H.; Lee, J. S. J. Catal. 1997, 167, 364. (28) Park, E. D.; Choi, S. H.; Lee, J. S. J. Phys. Chem. B 2000, 104, 5586. (29) Sayer, D. E.; Bunker B. A. In X-ray Absorption: Principles, Applications, Techniques of EXAFS, SEXAFS and XANES; Koningsberger, D. C., Prins, R., Eds.; John Wiley& Sons: New York, 1988; p 211. (30) Moore, W. J., Jr.; Pauling, L. J. Am. Chem. Soc. 1941, 63, 1392. (31) Grønvold, F.; Haraldsen, H.; Kjekshus, A. Acta Chem. Scand. 1960, 14, 1879. (32) Jeon, Y.; Qi, B.; Lu, F.; Croft, M. Phys. ReV. B 1989, 40, 1538. (33) Gopidas, K. R.; Bohorquez, M.; Kamat, P. V. J. Phys. Chem. 1990, 94, 6436. (34) Hotchandani, S.; Kamat, P. V. J. Phys. Chem. 1992, 96, 6834. (35) Piris, J.; Ferguson, A. J.; Blackburn, J. L.; Norman, A. G.; Rumbles, G.; Selmarten, D. C.; Kopidakis, N. J. Phys. Chem. C 2008, 112, 7742.

JP804699C