Reversing CdS Preparation Order and Its Effects on Photocatalytic

ARTICLE pubs.acs.org/JPCC

Reversing CdS Preparation Order and Its Effects on Photocatalytic Hydrogen Production of CdS/Pt-TiO2 Hybrids Under Visible Light Hyunwoong Park,*,†,‡ Young Kwang Kim,† and Wonyong Choi§ †

Department of Physics and ‡School of Energy Engineering, Kyungpook National University, Daegu 702-701, Korea § School of Environmental Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang 790-784, Korea

bS Supporting Information ABSTRACT: A facile synthesis of high efficiency semiconductor photocatalyst hybrids is of great importance in making the photocatalytic systems more viable and applicable. This study presents that simply reversing chemical precipitation order of CdS results in significantly different photocatalytic activity in terms of hydrogen production from water under visible light when hybridized with platinized TiO2 particles (Pt-TiO2). It has been found that CdS obtained via dropping an aqueous cadmium cation in aqueous sulfide solution (i.e., Pt-TiO2 suspension with S2
) with equal molar ratios (hereafter CdSR) has a maximum >10-fold greater amount of hydrogen than that obtained by simply reversing the dropping order (i.e., dropping S2
to Pt-TiO2 suspension with Cd2þ; hereafter CdRS). Such a high activity of CdSR, however, is very sensitive to photocatalytic running conditions, in particular, kind and concentration of electron donor (Na2S and/or Na2SO3) which largely changes the hydrogen production ratio (RH) of CdSR to CdRS. Detailed surface analyses indicate that physicochemical properties of CdSR are very different from those of CdRS including larger and red-shifted onset light absorption and altered photoluminescence, S/Cd atomic ratios >1, and hexagonal crystallinity (vs cubic-CdRS), the differences of which were attributed to the primary reasons for higher activity of CdSR. Finally, the photocatalytic hydrogen production mechanism was proposed based on the experimental results.

’ INTRODUCTION Despite its limits such as photocorrosion, CdS photocatalyst has received growing attention from diverse scientific fields, particularly the areas of photocatalytic hydrogen production1
6 and photoelectrochemical cells7,8 due to its suitable energetics for harvesting solar light, e.g., proper bandgap energy of around 2.5 eV and ideal levels of conduction band (CB:
0.75 V vs NHE) and valence band (VB: 1.75 V vs NHE). In such applications, the photocatalytic efficiency of CdS has been reported to be highly dependent on the crystallinity (e.g., cubic vs hexagonal)9
11 related to annealing temperatures and surface treatments,4,12 particle sizes6,12,13 affecting bandgap energy (quantum size effect), 14,15 surface-loaded noble metals, 16
18 electron donors or electrolytes, 1,2,5 and secondary materials3,15,18
20 when hybridized. Recently, we have evaluated the photocatalytic activities of ternary hybrids of CdS, TiO2, and Pt cocatalyst in terms of hydrogen production in the presence of an electron donor (Na2SO3 þ Na2S) under visible light (λ > 420 nm).3 The most noteworthy result is that the configuration of CdS/Pt-TiO2 (CdS-attached Pt-TiO2 particles) has significantly higher photocatalytic activity than TiO2/Pt-CdS configuration (TiO2-attached Pt-CdS particles). Such a high efficiency of the former r 2011 American Chemical Society

was speculated to result from a thermodynamically allowed vectorial electron transfer via CdS f TiO2 f Pt. Instead of the TiO2 semiconductor, multiwalled carbon nanotubes (CNT) were also employed as a support for CdS and metal cocatalyst.21 In this case, the surface treatment of CNT has been found to be critically important in determining overall photocatalytic hydrogen production in visible-light irradiated CdS/Pt-CNT suspension. Typical synthesis of CdS particles is simply dropping an aqueous sulfide ion (S2
) solution to an aqueous cadmium ion (Cd2þ) solution with equal molar ratio. This study reports, however, that the simple reverse process (dropping an aqueous cadmium ion solution in an aqueous sulfide ion solution) has resulted in far more efficient CdS which produced hydrogen from water by a factor of 4
10 when hybridized with the M-TiO2 particle (where M refers to metal cocatalyst loaded on the TiO2 particle). The chemical precipitation of the former appears to provide a cadmium-rich microenvironment locally and transiently (hereafter CdRS), whereas the latter provides a reversed Received: November 19, 2010 Revised: February 27, 2011 Published: March 15, 2011 6141

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The Journal of Physical Chemistry C condition, i.e., local and transient sulfur-rich microenvironment (hereafter CdSR), despite equal molar amounts of both ions. In addition, the photocatalytic activity ratios of CdSR/CdRS are sensitively varied with the concentration of electron donor and kind of metal cocatalyst. Detailed surface analyses of the samples (UV
vis, XRD, XPS, photoluminescence, EDX, ICP-MS, SEM, TEM, Raman) have been attempted to investigate the reasons for the different photocatalytic activities between CdRS and CdSR.

’ EXPERIMENTAL SECTION Materials. TiO2 (Degussa P25) was employed as a supporting material of CdS. It is a mixture of anatase and rutile (8:2) and has a BET surface area of ca. 50 m2/g. Its primary particle size is ca. 27 nm for anatase and ca. 16 nm for rutile.22 For preparation of metal-loaded TiO2 (M-TiO2), 0.5 g/L of TiO2 suspension with 0.1 mM H2PtCl6, PdCl2, or AuCl3 and 1 M methanol was irradiated and collected. Such photodeposition has been known to be very effective for loading metal nanoparticles on the TiO2 surface.3,23,24 In the case of RuO2
TiO2, RuCl3 was dissolved in TiO2 suspension at pH 5 and stirred overnight, and the precipitate was collected and annealed at around 450 °C. The amount of metals was fixed to be around 0.3 wt % with respect to TiO2. In Situ Synthesis of CdS/M-TiO2 and Photocatalytic Hydrogen Production. For CdRS/M-TiO2, a cadmium salt (cadmium-acetate, -nitrate, or -chloride; cadmium-acetate unless indicated) was dissolved in M-TiO2 (or TiO2) suspensions and agitated over 1 h at ambient temperature and pressure. Then a solution of sodium sulfide (Na2S) with equimolar amount to cadmium cation was added dropwise to the M-TiO2 (or TiO2) suspension with cadmium ion, immediately forming CdRS/MTiO2. In the case of CdSR/M-TiO2, M-TiO2 was suspended in aqueous sulfide solution, where the equimolar amount of cadmium was dropped. For surface analysis of CdS and CdS/MTiO2, they were filtered with 0.45 μm PTFE (Millipore), washed with distilled water, and dried overnight at 60 °C after collection (ex situ). The hydrogen production was achieved with visible light irradiation to an aqueous suspension (25 mL) of CdS/MTiO2 (or TiO2) at 0.5 g/L without the filtration step (in situ), where Na2S (4
400 mM) and Na2SO3 (4
400 mM) were added as an electron donor. A 450 W Xe arc lamp was used as a light source. Light passed through a 10 cm IR water filter and a cutoff filter (λ > 420 nm), and then the filtered light was focused onto the reactor. Prior to light illumination, nitrogen was purged for 30 min. During irradiation, the headspace gas (ca. 10 mL) of the reactor was intermittently sampled and analyzed for H2 using a gas chromatograph (Agilent) equipped with a thermal conductivity detector and a 5 Å molecular sieve column. Light intensity was measured by chemical actinometry using (E)a-(2,5-dimethyl-3-furylethylidene) (isopropylidene)succinic anhydride (Aberchrome 540).3 The typical incident light intensity was measured to be about 2  10
3 einstein L
1 min
1 in the wavelength range of 420
550 nm. Surface Analysis and Characterization. The X-ray diffraction (XRD) patterns of samples were obtained with an X-ray diffractometer (Rigaku D, Max-2500) employing Cu KR radiation (λ = 0.15406 nm) with 40 kV and 100 mA at 0.02 degree scan rate. Scanning electron microscopy (SEM) measurements were performed by a field emission scanning electron microscope (Hitachi, S-4800) at an operating voltage of 3 kV. High-resolution transmission emission microscopy (HRTEM, Hitachi, HD-2300)

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was employed for studying the morphology of samples. The surface atomic composition of samples was determined by X-ray photoelectron spectroscopy (XPS, Kratos, XSAM 800 pci) using the Mg KR line (1253.6 eV) as the excitation source. The binding energies of all peaks were referenced against the Au 4f line originating from the gold powder mixed with the samples. Infrared spectra were obtained by a Fourier transform-infrared spectrometer (FTIR, Thermo, 5700&continuum). The optical absorption spectra of samples were obtained with a UV
vis spectrophotometer (Shimadzu UVPC-2401) equipped with a diffuse-reflectance attachment (Shimadzu IRS-2200). All sample powders were mixed with BaSO4 (1:1 by weight), and their absorption spectra were measured against BaSO4. Photoluminescence spectra were obtained with a PL spectrometer (Acton Research Co., Spectrograph 500i, USA) equipped with an intensified photo diode array detector (Princeton Instrument Co., IRY1024, USA). The PL light source was a He
Cd laser (Kimon, 1K, Japan) with a wavelength of 325 nm and a power of 50 mW.

’ RESULTS AND DISCUSSION Photocatalytic Hydrogen Production: CdRS vs CdSR. Figure 1a shows the effect of CdS hybrid (i.e., CdS/Pt-TiO2) amount on hydrogen production in the presence of Na2S and Na2SO3 as an electron donor under visible light (λ > 420 nm). It is obvious that the CdRS hybrid produces hydrogen rather linearly, and its H2 amount increases with increasing amount of CdS; yet the maximum H2 amounts are always lower than 30 μmol at 2 h photocatalysis. On the other hand, CdSR has a much larger amount of H2 production, which reaches ca. 50 μmol in 90 min. This H2 production pattern is quite interesting in that irrespective of CdS amount H2 production is linearly increased until 30 min and then gradually leveled off thereafter. Such a kinetic fashion was similarly reported and attempted to explain elsewhere.25,26 In addition, it is of note that irrespective of CdS amount H2 amounts always reach the same level of 50 μmol at final stages likely due to complete depletion of electron donors. Elementary steps for CdS-based photocatalytic hydrogen production might be listed as follows (R1
R7)

CdS þ hν ðλ > 420 nmÞ f ecb
þ hvb þ

ðR1Þ

2ecb
þ 2H2 O f H2 þ 2OH


ðR2Þ

2hvb þ þ 2S2
f S2 2


ðR3Þ

S2 2
þ SO3 2
f S2 O3 2
þ S2


ðR4Þ

S2
þ SO3 2
þ 2hvb þ f S2 O3 2


ðR5 = R3 + R4Þ

SO3 2
þ 2OH
þ 2hvb þ f SO4 2
þ H2 O 2SO3 2
þ 2hvb þ f S2 O6 2


ðR6Þ ðR7Þ

Here, SO32
was also considered to participate in the photoreaction since it has been known to regenerate disulfide (or polysulfide) and/or consume valence band (VB) holes (R4
R7). If SO32
works suitably, therefore, the stoichiometric amount of H2 production at plateau ([H2]plateau) should be larger than the initially added amount of S2
([S2
]0). In this system, [S2
]0 and [H2] plateau were 100 μmol (= 4 mmol/L  0.025 L) 6142

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and 50 μmol, respectively, and production of one H2 molecule needs two conduction band (CB) electrons (i.e., releasing two VB holes), indicating that all the H2 molecules are sulfide ionfootprinted and that SO32
was not likely to work. This speculation is further supported by the same [H2]plateau in the absence of SO32
(i.e., only in the presence of S2
) (Figure S1 in

Supporting Information) despite slower kinetics. Therefore, the H2 production rates at initial states (0
30 min) are compared between CdRS and CdSR hybrids as shown in Figure 1b. Both H2 production rates have similar dependence on the amount of CdS and reach plateaus with 0.9 and 0.1 μmol/min for CdSR and CdRS hybrids, respectively, at ca. 12.5 μmol CdS (0.5 mM). Furlong et al. also similarly reported in their Pt-CdS system that H2 production rates increased with increasing CdS concentration up to ca. 0.1 mM and was unchanged by further increases in concentration.26 In addition, the H2 production ratio of CdSR and CdRS (RH) is highest with around 10 at 2.5 μmol CdS (0.01 mM), which linearly decreases with increasing amount of CdS. Such a decrease indicates that the photocatalytic activities of CdS are highly dependent on its amount. No hydrogen was produced in the absence of Pt (i.e., CdS/TiO2), suggesting that Pt plays a very important role in reducing proton/water. Table 1 presents the effects of electron donor (ED), and the results were summarized with the following. First, irrespective of CdRS and CdSR hybrids, mixed ED (sulfide plus sulfite) is usually more effective than sulfide alone in obtaining higher H2 rates (1a vs 3a; 1c vs 3c); however, as discussed above, [H2]plateau is not affected. Also no H2 production occurs only when sulfite is present (2a), indicating that the direct quenching of VB holes by sulfite R6 and R7 may not work. This further suggests that although sulfite contributes somehow to enhancing H2 production its role is likely to just indirectly aid the reaction between VB holes and sulfide without direct involvement of the reaction. Second, despite the larger amount of cumulative H2, higher ED concentration is not always desirable for obtaining higher H2 production rates at initial stages (1a,b,c and 3a,b,c; also see Figure S1 in Supporting Information). In particular, the CdSR hybrid has reduced H2 rates at higher ED concentrations, which causes RH to decrease with increasing ED concentration. Third, pH condition might play a role in a complicated way. Actually reversing CdS preparation order changes pH condition of precipitation: circum-neutral condition with CdRS due to cadmium salt solution vs alkaline condition with CdSR due to sulfide solution (∼pH 11
12). An increase in pH up to ca. 11
12, however, did not change the H2 rates in the CdRS hybrid, whereas it reduced the H2 rates in the CdSR hybrid. This implies, therefore, that there should be a certain and delicately narrow, local pH region for forming highly active CdSR, which is easily perturbed by external pH change. Sulfide concentration in CdS also seems to affect the photocatalytic activity. In CdSR synthesis,

Figure 1. Effects of CdS amount on (a) time-profiled hydrogen production and (b) ratios (RH) of hydrogen production rates between CdRS and CdSR hybrid suspensions (CdS/TiO2 and CdS/Pt-TiO2) under visible light (λ > 420 nm). For notation of subscript “R”, see text. [TiO2] = [Pt/TiO2] = 0.5 g/L (total reaction volume, 25 mL); [Na2S] = [Na2SO3] = 4 mM. In (b), 1 mM CdS (ca. 0.144 g/L) corresponds to 25 μmol.

Table 1. Effects of Electron Donors on Photocatalytic Hydrogen Production of CdRS and CdSR Hybrid Suspensions under Visible Lighta CdRS entry #

Na2SO3 (mM)

others

H2 (μmol/min)

H2 (μmol/min)

RH

1a

4

-

0.077

0.66

8.5

1b

40

-

0.223

0.53

2.4

1c

400

-

0.011

0.027

2.4

2a

-

4

0

0

-

2b

-

4

0.14

0.16

1.2

3a

4

4

0.15

1.01

6.9

3a1

4

4

0.14

0.43

3.1

40 400

40 400

0.19 0.059

0.54 0.078

2.8 1.3

3b 3c a

Na2S (mM)

CdSR

Cd:S = 1:5 pH 11
12

[CdS] = 1 mM = 0.144 g/L; [Pt-TiO2] = 0.5g/L; pH not adjusted unless indicated; RH = H2(CdSR)/H2(CdRS). 6143

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Figure 2. Effects of (a) counteranions for Cd salt and (b) metal cocatalysts (loaded on TiO2) on initial hydrogen production rates and their RH values in CdRS and CdSR hybrid suspensions under visible light. [M-TiO2] = 0.5 g/L; [Na2S] = [Na2SO3] = 4 mM; λ > 420 nm. In (a), M-TiO2 means Pt-TiO2.

Figure 3. UV
vis absorption spectra of (a) in situ CdRS and CdSR and (b) ex situ CdRS and CdSR samples (nonhybridized ones; i.e., w/o PtTiO2). In situ and ex situ mean without filtration and with filtration, respectively (see Experimental Section). In (b), absorbance was expressed as a Kubelka
Munk (K
M) unit, and the inset shows the relative K
M absorbance (%) with respect to K
M absorbance at 400 nm.

instead of equimolar ratio, dropping 1 mM Cd2þ solution to 5 mM sulfide solution would result in 1 mM CdSR with unreacted 4 mM sulfide (2b), which is identical to 1 mM CdSR plus postadded 4 mM sulfide (3a). Nevertheless, the H2 rate of the former was only a one-sixth that of the latter. Conversely, no significant change of H2 rate was observed in CdRS hybrids, which indicates once again that the local microenvironment is very critical in forming active CdS particles. The higher photocatalytic activity of CdSR could be associated with various factors such as types of cadmium salt and specific interaction of CdS precursors with metal loaded at the TiO2 surface, etc. Therefore, we have checked every possible factor. As shown in Figure 2a, three kinds of cadmium salt (cadmium acetate, nitrate, and chloride) are compared for their hydrogen production rates and corresponding RH values. Despite slight variation among the salts, however, CdSR always has higher activities than CdRS by a factor of 4
7. This suggests that the counteranions for cadmium cation have a minor effect on the photocatalytic activity (Figure S2 in Supporting Information). In addition, the effect of metal cocatalysts that are loaded on the TiO2 surface has been compared among Pt, Pd, Au, and RuO2 (Figure 2b). It is obvious that the metal effect in CdRS hybrid suspensions is highest with Pd and Pt and lowest with Au with the following order: Pd > Pt > RuO2 > Au. In the case of CdSR hybrid suspensions, the metal dependence is similar, yet the sensitiveness to metal is much greater (e.g., AuH2/PtH2 = 18.5 with CdSR vs 2 with CdRS). Such sensitiveness causes RH values to vary very largely with metals (RH,Pt = 6.9; RH,RuO2 = 1.3).

Surface Analysis and Characterization of CdRS and CdSR. All the aforementioned results indicate that the CdSR hybrid is highly active in generating hydrogen as compared to the CdRS hybrid, and such a high activity of the CdSR hybrid is very sensitive to experimental conditions like amount of CdS, concentration of ED, and kind of metal cocatalyst. To investigate the possible reasons for higher activity of the CdSR hybrid, therefore, surface properties of CdS samples without and with Pt-TiO2 were studied. Figure 3a shows the UV
vis absorption spectra of in situ prepared CdS nanoparticles (without Pt-TiO2) suspended in water as a function of CdS amount. It is obvious that the absorbance increases with increasing amount of CdS, and CdSR has larger absorbance than CdRS at the same amount. Such enhanced optical property was further confirmed with ex situ samples (Figure 3b and inset), and CdSR was also found to be able to absorb a larger fraction of light (λ < ∼600 nm). The different optical property of CdS simply by reversing the preparation order has not been reported according to our best knowledge and was speculated to result from different form and/ or crystallinity of CdS. According to SEM analysis, however, the surface morphologies of CdRS and CdSR (ex situ) were found to be very similar with particle sizes of 10
20 nm and not changed much even for CdS hybrids (Figure 4; see Figure S3 for TiO2 and Pt-TiO2 images in Supporting Information). The similarity of surface morphological images between CdRS and CdSR hybrids was also found in the TEM analysis of the samples. As shown in Figure 5, a number of spherical Pt nanoparticles of ca. 2
3 nm in size were uniformly deposited on TiO2 particles of ca. 30 nm 6144

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Figure 4. SEM images of (a) CdRS, (b) CdSR, (c) CdRS hybrid (CdRS/Pt-TiO2), and (d) CdSR hybrid (CdSR/Pt-TiO2) samples.

Figure 5. TEM images of (a) Pt-TiO2, (b) CdRS, (c) CdSR, (d) CdRS/Pt-TiO2, and (e) CdSR/Pt-TiO2 hybrid samples.

(Figure 5a) indicating the employed photodeposition method is a very effective way to load Pt nanoparticles. In the case of CdS, CdRS and CdSR have no distinctly different morphology along with similar primary particle size of ca. 10 nm (Figure 5b and c). Such similar configuration was still observed when CdS was

hybridized with Pt-TiO2 as shown in Figure 5d and e, where agglomerated CdS nanoparticles were obviously located at the surface of Pt-TiO2. This indicates that the surface of Pt-TiO2 did not significantly influence the creation and growth mechanism of CdS particles. Nevertheless, EDX analysis reveals that atomic 6145

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Table 2. EDX Analysis for Atomic Ratios of S and Cd (S/Cd) for Differently Prepared CdS Samples ratio

CdRS

CdSR

CdRS/Pt-TiO2

CdSR/Pt-TiO2

S/Cd

0.98 ( 0.01

1.05 ( 0.02

0.52 ( 0.08

1.31 ( 0.15

ratios of S and Cd (S/Cd) are different with 0.98 for CdRS and 1.05 for CdSR (Table 2 and Figure S4 in Supporting Information). Such difference becomes more pronounced with CdS/Pt-TiO2 (CdRS/Pt-TiO2: 0.52 vs CdSR/Pt-TiO2: 1.31) suggesting the presence of some interaction between CdS precursors and Pt-TiO2. An adsorption test via ICP-MS analysis of Cd2þ and S2
, however, showed that both ions are hardly adsorbed on the Pt-TiO2 surface implying that although the interaction between the Cd2þ (or S2
) precursor and Pt-TiO2 is very weak and disturbed easily it might become more intensified upon hybridization. To check their interactions, XPS analysis for the samples was made since different local environments in CdS formation might affect the binding states among component elements. As shown in Figure 6, Cd and S of CdSR have different binding energies from those of CdRS in the absence and presence of Pt-TiO2. In the absence of Pt-TiO2, the binding energies of Cd3d (404.6 eV) and S2p (161.0 eV) of CdSR are 0.1
0.2 eV less than those of the CdRS sample suggesting that CdSR has a slightly reduced environment. When hybridized with Pt-TiO2, however, the binding energies of Cd3d (405.1 eV) and S2p (161.3 eV) of CdSR become higher with 0.4 eV ∼ 0.8 eV than those of CdRS indicating CdSR has a more oxidized environment or is more susceptible to oxidation than CdRS. Furthermore, comparison between CdS and CdS/Pt-TiO2 indicates that the Pt-TiO2 shifts CdRS to lower binding energy region and CdSR to higher binding energy region. Such different shifts might result from different interaction of Cd and S with Pt-TiO2. In the case of CdRS, both the Cd cation (Cd2þ) and Lewis-acidic Pt-TiO2 compete for the electron of sulfide, and thus the chance of sulfide for interaction with Pt-TiO2 should be lower due to more favored reaction with the Cd cation, likely resulting in rather loosely connected hybrid configuration. XRD analyses of CdRS and CdSR were performed as well to check the effect of the preparation order on the crystalline structure of CdS (Figure 7). Both samples have similar peaks at 26.5° (002), 44° (110), 52° (112), and 71° (211), all of which are characteristic peaks of Greenockite CdS (hexagonal or wurtzite). On the other hand, CdRS has additional peaks at 81° (300) and 87° (511) which are associated with metastable Hawleyite CdS (cubic or zincblende). The peak at 26.5° can be also assigned to be Hawleyite (111) and was not always found in CdSR. Hence, CdRS is speculated to have a mixed phase with hexagonal and cubic structure, whereas CdSR seems to possess only a single hexagonal structure. Such structure difference is quite surprising since simply reversing the addition order results in structural change. Generally, different synthetic routes have been employed to control or obtain CdS crystalline structures. For example, Singh and Chauhan obtained hexagonal CdS (hCdS) via chemical precipitation (Cd:S = 1:1) at ambient temperature and pressure,27 whereas Lozada-Morales prepared cubic CdS (c-CdS) film via chemical bath deposition at room temperature.28 In some cases, an excess amount of Cd with respect to S resulted in formation of h-CdS,29 and amorphous CdS nanoparticles were transformed either to cubic or hexagonal

Figure 6. XPS spectra of (a) Cd3d and (b) S2p for CdRS, CdSR, CdRS hybrid, and CdSR hybrid samples.

Figure 7. XRD patterns of CdRS and CdSR samples. G and H refer to Greenockite (hexagonal or wurtzite) and Hawleyite (cubic or zincblende), respectively.

phases depending on the applied heat temperature.30 In general, hexagonal structure is more stable thermodynamically at normal conditions,9 and cubic structure is formed only under nonequilibrium conditions.31 In addition, phase transformation of CdS from the cubic to hexagonal structure needs high temperature (∼800 °C)4,32 despite very small differences of cohesive energies with the order of a few tens of 10
3 eV per atom.33 As for photocatalytic activities, h-CdS has been reported to be more active than the cubic one. As examples, Matsumura et al.34 obtained max 10-fold enhanced hydrogen production in h-CdS suspension as compared to c-CdS, and Jang et al.32 and Silva et al.4 reported similar results. Since CdSR is a hexagonal phase while CdRS possesses mixed phases, the higher activity of CdSR 6146

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Scheme 1. Schematic Illustration for Photocatalytic Hydrogen Production Mechanisms of CdRS and CdSR Hybrids

Figure 8. Photoluminescence (PL) spectra of (a) CdRS, CdSR, CdRS hybrid, and CdSR hybrid samples. PL spectra of (b) CdRS and (c) CdSR were deconvoluted into green, yellow, and red emission bands.

for hydrogen production in this study is consistent with literature results. CdS samples were also analyzed with photoluminescence (PL) with excitation at 325 nm (Figure 8). As compared to CdRS, CdSR has a PL spectrum of stronger intensity yet similar pattern with apparent two bands centered at ca. 476 nm (2.6 eV) and 682 nm (1.8 eV) along with a hidden band at ca. 600 nm (2.0 eV) (Figure 8a). The highest-energy band (2.6 eV) of the PL spectrum corresponds to Green emission (GE) resulting from electronic transition from the CdS CB to an acceptor level (i.e., band-edge emission) (Figure 8b,c).35 On the hand, the other two bands (ca. 2.0 eV and ca. 1.8 eV) correspond to Yellow emission (YE) and Red emission (RE), respectively, which are ascribed most likely to surface-states-induced recombination and vacancies of Cd28 or S,36 respectively. The PL spectrum therefore might indicate the followings: (1) The band-edge emission (i.e., GE) band of CdSR has a longer wavelength than that of CdRS by 8 nm (∼0.04 eV) which is qualitatively consistent with

red-shifted UV-absorption spectrum of CdSR. (2) YE band positions of CdRS and CdSR are exactly the same with 613.2 nm indicative of similar surface states and recombination. On the other hand, the RE band of CdSR (683.6 nm) is red-shifted by 23 nm as compared to CdRS (660.4 nm), which suggests that the former might be attributed to Cd vacancies and the latter to S vacancies. (3) YE and RE bands completely disappear with PtTiO2 hybrids (Figure 8a) indicating that the recombination by surface states is inhibited due to efficient charge transfers from CdS to Pt-TiO2. (4) The stronger PL intensity of CdSR suggests that it has a greater absorption of excitation light, that more charge carriers are produced, and thus that the recombination might happen with higher probability. This is likely to be reasonable since the PL experiment was carried out in the absence of an electron donor, and thus all the photoproduced CB electrons were destined to undergo recombination with VB holes. The actual photocatalytic experiments, however, were performed with the electron donor; once produced, therefore, the CB electrons are more likely to be transferred to proton/water. Photocatalytic Hydrogen Production of CdRS and CdSR. As discussed in the aforementioned results, the remarkable differences of CdSR from CdRS can be listed with (1) larger and redshifted onset absorption, (2) S/Cd atomic ratios greater than one, and (3) hexagonal crystallinity. These differences, however, appear to vanish with running conditions such as electron donor and metal cocatalyst indicating the sensitiveness of CdSR. On the basis of such differences, the photocatalytic hydrogen production mechanism of CdRS and CdSR hybrids was illustrated in Scheme 1. In the case of the CdRS hybrid, additional cadmium cations are likely to be present on the surface of CdS as forms of >CdOH and/or >CdOH2þ (The notation “>” represents CdS surface) and thus can play a role of CB electron trapping site,4,6 reducing CB electron transfer to TiO2 and Pt. On the contrary, the CdSR hybrid is likely to have rich extra sulfide ions on the surface of CdS as forms of >CdSH2þ and/or >CdSH, and such a sulfide-rich microenvironment becomes more pronounced in the presence of Pt-TiO2. Thus, photogenerated CdS VB holes are 6147

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The Journal of Physical Chemistry C effectively quenched by the extra sulfide ions, enhancing CB electron transfers to Pt-TiO2. In addition, the larger absorption property of CdSR contributes to additional generation of CdS CB electrons, which are more efficiently transferred to TiO2 via densely interconnected CdS framework.

’ CONCLUSIONS This study presents that the photocatalytic activity of CdS is significantly changed only by reversing the addition order of CdS precursors under identical preparation conditions, and such change is primarily attributed to altered physicochemical properties of CdS particles. Although CdS needs stoichiometrically equal molar ratios between Cd and S, the sulfur-rich microenvironment results in CdS with higher photocatalytic activity. This suggests that the surface states of CdS do no need to have stoichiometry in the subnano time scale or transiently. In the aspect of broader implication, this finding might stress the importance of cautious selection for CdS preparation conditions and further suggest that synthesis of CdS and its photocatalytic activity have to be still revisited despite long and extensive investigation on this issue. ’ ASSOCIATED CONTENT

bS

Supporting Information. Figures S1
S4 as described in the text. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Phone: þ82-53-950-7371.

’ ACKNOWLEDGMENT This research was supported by Basic Science Research Programs (No. 2009-0089904, No. 2009-0071350, No. 20100002674), by the Korea Center for Artificial Photosynthesis (NRF-2009-C1AAA001-2009-0093879) through the National Research Foundation of Korea (NRF), and by KOSEF NRL program (No. R0A-2008-000-20068-0) funded by the Ministry of Education, Science and Technology, Korea.

ARTICLE

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