Photodeposition of CdS Quantum Dots on TiO2: Preparation

Sep 1, 2009 - A paste containing anatase TiO2 particles with a mean size of 20 nm (PST-18NR, ... The resulting sample (0.1 g) was dispersed to conc. ...
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J. Phys. Chem. C 2009, 113, 16711–16716

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Photodeposition of CdS Quantum Dots on TiO2: Preparation, Characterization, and Reaction Mechanism Masashi Fujii,† Kazuki Nagasuna,† Musashi Fujishima,† Tomoki Akita,‡ and Hiroaki Tada*,† Department of Applied Chemistry, School of Science and Engineering, Kinki UniVersity, 3-4-1, Kowakae, Higashi-Osaka, Osaka 577-8502, Japan and National Institute of AdVanced Industrial Science and Technology, Midorigaoka 1-8-31, Ikeda, Osaka 563-8577, Japan ReceiVed: June 16, 2009; ReVised Manuscript ReceiVed: August 4, 2009

Ultraviolet light irradiation (λex > 300 nm) to a mixed ethanol solution of cadmium perchlorate and elemental sulfur (S8) under deaerated conditions has led to deposition of CdS quantum dots on the surfaces of TiO2 particles and films (CdS/TiO2). High-resolution transmission electron microscopy (HRTEM) confirmed that the hexagonal CdS crystals are in good contact with TiO2. The loading amount and band gap of CdS can be controlled by irradiation time. Photoelectrochemical measurements using cyclic voltammetry and photochronopotentiometry indicated the photodeposition of CdS on TiO2 preferentially proceeds via an atomic route (Cd0 + S f CdS), whereas that on Au nanoparticle-loaded TiO2 (Au/TiO2) follows an ionic route (Cd2+ + S2- f CdS). This difference was revealed to result from the predominant adsorption of Cd2+ over S8 on TiO2 (adsorbent-selective adsorption) in the former system and the selective adsorption of S8 and Cd2+ ions on Au and TiO2, respectively, (site-selective adsorption) in the latter system. I. Introduction The establishment of environmental purification methods and sustainable energy sources replacing fossil fuels are the urgent subjects imposed on scientists. A great deal of attention has been focused for the last few decades on semiconductor photocatalysts represented by TiO2 with a strong power for decomposing environmental pollutants.1 The present major challenge in photocatalysis is to develop highly active visible light photocatalysts enabling effective utilization of sunlight as an energy source. A promising strategy to achieve this is appropriate coupling of narrow gap semiconductors (NGS) and wide gap semiconductors (WGS).2 On the other hand, dyesensitized solar cell (DSSC), in which dye-adsorbed mesoporous TiO2 films are used as a photoanode, has attracted much interest because of its power conversion efficiency reaching 10% and low cost.3 However, the efficiency and lifetime of DSSC need to be improved before its practical use, for which the replacement of the organic photosensitizers by robust inorganic NGS quantum dots (QDs) has a great potential due to the tunable band gap by the size4 and multiple exciton generation.5 In these manners, the NGS-WGS coupling systems are of importance in both photocatalysis and photovoltaics, where the common key process is the visible light-induced electron transfer from NGS to WGS.6 In order to build the representative CdS-TiO2 coupling system, the successive ionic layer adsorption and reaction technique has frequently been used.7 Although this method is convenient for preparing CdS QD-loaded TiO2 films, it is unsuitable for the application to the particulate system because several 10 times repetition of adsorption and rinsing is necessary. On the other hand, photocatalytic synthesis, which has been used mainly for loading metals on semiconductors since the discovery by Kraeutler and Bard,8 is currently being * To whom correspondence should be addressed. Phone: +81-6-67212332. Fax: +81-6-6727-2024. E-mail: [email protected]. † Kinki University. ‡ National Institute of Advanced Industrial Science and Technology.

revealed to have a wide possibility of constructing the NGSWGS coupling particle and film systems.9-15 The important feature of this method is that the efficient interfacial charge transfer between the semiconductors is inherently guaranteed. As chalcogenide-oxide coupling systems, CdSe-TiO2 and PbSe-TiO2 two-component systems,13-15 and a CdS-Au-TiO2 three-component system10 were prepared by the photocatalytic method. Very recently, the direct CdS-TiO2 coupling taking the advantage of the TiO2 photocatalysis (CdS/TiO2) has been reported;16,17 however, the reaction mechanism is not fully understood yet. Here we report the preparation and characterization of CdS/ TiO2 and the essential mechanism of the photodeposition on TiO2 in comparison with that on the photodeposition on Au/ TiO2. II. Experimental Section A. Photodeposition of CdS QDs on TiO2. A paste containing anatase TiO2 particles with a mean size of 20 nm (PST18NR, Nikki Syokubai Kasei) was coated on SnO2-filmcoated glass substrates (12 Ω/0) by a squeegee method, and the sample was heated in air at 773 K to form mesoporousTiO2 films (mp-TiO2/SnO2). Au particles (0.38 mass %) with a mean size of 3.2 nm were loaded on TiO2 particles with a crystal form of anatase and a specific surface area of 8.1 m2 g-1 (A-100, Ishihara Sangyo) by the deposition-precipitation method using HAuCl4 as a raw material (Au/TiO2).18 TiO2 particles, Au/TiO2 particles, and mp-TiO2/SnO2 were used as supports of CdS QDs. TiO2 particles (1 g) ethanol suspension (250 mL) containing S8 (1.72 × 10-4 mol dm-3) and Cd(ClO4)2 (1.38 × 10-2 mol dm-3) had been bubbled with argon for 0.5 h in the dark; irradiation was carried out for a given period with a high-pressure mercury lamp at 298 K; the light intensity integrated from 320 to 400 nm (I320-400 nm) was 3.7 mW cm-2. After irradiation, the particles were recovered by centrifugation, and the resulting particles were washed with ethanol 3 times to be dried under vacuum.

10.1021/jp9056626 CCC: $40.75  2009 American Chemical Society Published on Web 09/01/2009

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Figure 1. TEM (A), HRTEM (B), and energy-dispersive X-ray (ED) spectrum (C) of a particulate sample obtained after 3 h irradiation: [Cd2+]0 ) 1.38 × 10-2 mol dm-3, [S8]0 ) 1.72 × 10-4 mol dm-3.

The gas-phase products were identified by gas chromatography (Shimadzu, GC-2014) with an fid column SHINCARBON A (3 mmφ × 3 m). The resulting sample (0.1 g) was dispersed to conc. HCl (10 mL), and the deposits were thoroughly dissolved into the solution by stirring for 1 h. The solution was diluted 5 times in volume with water, and then the Cd concentration was determined by inductively coupled plasma spectroscopy (ICPS-7500, Shimadzu). B. Photoelectrochemical Measurements. Cyclic voltammograms of TiO2/SnO2 were measured in ethanol solutions containing Cd2+ ions (1 × 10-3 mol dm-3) or/and S8 (1.25 × 10-4 mol dm-3) and 0.1 mol dm-3 NaClO4 supporting electrolyte under deaerated conditions using glassy carbon and Ag/AgCl as a counter electrode and a reference electrode, respectively. The photoelectrochemical cell (PEC) was designed using TiO2/ SnO2 photoelectrode, a glassy carbon counter electrode, and a Ag/AgCl reference electrode. All photochronopotentiometry (PCP) measurements were carried out in a 0.1 mol dm-3 NaClO4 electrolyte solution. After a constant potential had been reached by argon bubbling for 0.5 h in the dark, irradiation (λ > 300 nm, I320-400 nm ) 7 mW cm-2) was started by using a 300 W Xe lamp as a light source (Wacom HX-500). Electrochemical response with irradiation was followed for the PEC connected with a potentio/galvanostat (HZ-5000, Hokuto Denko). HRTEM observation and ED spectroscopic measurements were performed using a JEOL JEM-3000F and Gatan Imaging Filter at an applied voltage of 300 or 297 kV. C. Adsorption Measurements. Adsorption isotherms of Cd2+ ions were obtained by exposing TiO2 and Au/TiO2 particles (0.2 g) to ethanol solutions with different concentrations of Cd(ClO4)2 (20 mL) at 298 K for 18 h in the dark. After the suspension was filtered by a membrane filter, the solvent of the filtrate was removed by evaporation to be replaced by water. The concentration of Cd2+ in the solution was determined by inductively coupled plasma spectroscopy. Adsorption isotherms of sulfur were obtained by equilibrating TiO2 and Au/TiO2 (0.25 g) to ethanol solutions of S8 with varying concentrations (50 mL) in the dark followed by filtration using a membrane filter and spectrophotometric analysis (ε263 nm ) 8.11 × 102 mol-1 dm3 cm-1). III. Results and Discussion UV-light irradiation (λ > 300 nm) to ethanol solutions containing Cd2+ ions and S8 in the presence of TiO2 turned the color of the particles from white to yellow, whereas no change was observed without TiO2. Figure 1A and 1B shows a typical TEM image and a HRTEM image of the sample obtained after 3 h irradiation, respectively. As is apparent from Figure 1A, a number of nanometer-sized particles are deposited on the TiO2 surfaces in a highly dispersed state. Figure 1B shows two parallel

lattice fringes, of which the nearest distances are in agreement with the values for the (101) and (011) planes of hexagonal CdS (International Center for Diffraction Data, No. 41-1049). In the energy-dispersive X-ray (ED) spectrum (Figure 1C), the signals of Cd and S are present besides those of Ti and O. Closer inspection of the interface in Figure 1B shows good contact between TiO2 and CdS. Evidently, UV-light irradiation to TiO2 in an ethanol solution of Cd2+ ions and S8 causes the deposition of CdS to yield the nanoscale CdS-TiO2 heterojunction system (CdS/TiO2). Time courses for the CdS photodeposition were examined by quantifying the amount of Cd in the deposits per unit mass (X/mmol g-1). The amounts of Cd in the deposits on TiO2 were quantified by ICP for the samples washed 3 times by ethanol to remove the Cd2+ adsorbed. Figure 2A shows plots of X vs irradiation time (tp) under various Cd2+ concentrations with [S8]0 maintained at 1.72 × 10-4 mol dm-3. In each case, the X value increases with increasing tp, and the deposition rate significantly increases with an increase in [Cd2+]0. Figure 2B shows X values as a function of tp under various S8 concentrations with [Cd2+]0 kept at 1.38 × 10-3 mol dm-3. Even without S8 (a), Cd2+ ions were removed from the solution with irradiation as a result of the reduction to Cd0 (vide infra), while the deposition apparently ceases at tp > 1 h. The addition of S8 accelerates the deposition, although the dependence of the rate on [S8]0 is much weaker as compared to that on [Cd2+]0. Figure 3A shows electronic absorption spectra of CdS/TiO2 prepared by varying tp: [Cd2+]0 ) 1.38 × 10-2 mol dm-3, [S8]0 ) 1.72 × 10-4 mol dm-3. For comparison, the spectrum of authentic bulk CdS is also shown. As tp increases, new absorption due to the CdS interband transition grows in the visible region at λ < 520 nm, and the absorption edge significantly blue shifts as compared to that for the bulk. The band gap (Eg) was determined as a function of tp by the Tauc plot,19 and further the mean CdS particle size (d) was calculated from the Brus equation (eq 1) using the Eg value20 2 ∆Eg ) (π2p2 /2R2)(m*-1 + m*-1 e h ) - 1.8e /4πε0εR

(1) where ∆Eg is a shift with respect to the bulk Eg, R is the radius of CdS particle, me* () 0.19me, me is electron mass) and mh* () 0.80me) are the effective masses of electron and hole in CdS, respectively, ε0 is vacuum permittivity, and ε is the relative permittivity of CdS (5.7).21 Figure 3B shows plots of the Eg and d () 2R) as a function of tp. At tp j 4 h (i.e., at d j 7 nm), the Eg of CdS increases with respect to the bulk value (2.41 eV) because of its size quantization.22 Thus, not only the loading amount but also the

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Figure 2. (A) Plots of the amount of Cd in the deposits per unit mass (X/mmol g-1) vs irradiation time (tp) under various Cd2+ concentrations with [S8]0 maintained at 1.72 × 10-4 mol dm-3: (a) [Cd2+]0 ) 1.38 × 10-4 mol dm-3; (b) [Cd2+]0 ) 1.38 × 10-3 mol dm-3; (c) [Cd2+]0 ) 1.38 × 10-2 mol dm-3. (B) Plots of X vs tp under various S8 concentration conditions with [Cd2+]0 maintained at 1.38 × 10-3 mol dm-3: (a) [S8]0 ) 0; (b) [S8]0 ) 1.72 × 10-5 mol dm-3; (c) [S8]0 ) 1.72 × 10-4 mol dm-3; (d) [S8]0 ) 1.72 × 10-3 mol dm-3.

Figure 3. (A) Electronic absorption spectra of CdS/TiO2 prepared by varying tp: [Cd2+]0 ) 1.38 × 10-2 mol dm-3, [S8]0 ) 1.72 × 10-4 mol dm-3. (B) Plots of the Eg and d values as a function of tp.

Eg of CdS can be controlled through tp in this photodeposition process. This is of great consequence for the applications of CdS/TiO2 to the visible light photocatalysts and photoanodes of solar cells.23 There could be two possible mechanisms of the CdS photodeposition as suggested in the two-step photodeposition of CdSe on TiO2 by the research group of Rajeshwar.24 One is the reduction of S8 to S2- ions followed by the reaction with Cd2+ ions (ionic route), and another is vice versa, i.e., the reduction of Cd2+ ions to Cd0 and the successive reaction with S8 (atomic route). To gain information about the mechanism of the CdS photodeposition on TiO2, cyclic voltammetric (CV) measurements were performed for mesoporous TiO2-film-coated SnO2 electrodes (mp-TiO2/SnO2) in ethanol solutions containing each and both of the starting components (Cd2+ and S8). Figure 4A shows CV curves of mp-TiO2/SnO2 in the presence of S8 (a), Cd2+ ions (b), and S8 and Cd2+ ions (c): reference electrode ) Ag/AgCl. In curve a, a small current of the S8 reduction flows in the region of the electrode potential U < -0.3 V. On the other hand, in curve b, a much larger current due to the reduction of Cd2+ ions is observed at U < -0.1 V accompanied by the corresponding oxidation current peak at -0.37 V. In curve c, the reduction current begins to flow at U ≈ -0.1 V in the same manner as curve b, whereas the oxidation peak is absent. PCP is also a powerful tool to clarify the photodeposition mechanism.14 Figure 4B shows PCP profiles of mp-TiO2/SnO2: (a) Cd2+ ions addition at tp ) 20 min/S8 addition at tp ) 40 min, (b) S8 addition at tp ) 20 min/Cd2+ ion addition at tp ) 40 min, (c) simultaneous addition of Cd2+ ions and S8 at tp ) 20 min. Upon UV-light irradiation, the U abruptly shifts to the cathodic direction as much as -0.80 ( 0.05 V, which can be attributed to the Fermi energy upward shift due to the current doubling effect of ethanol.25 In curve a, the addition of Cd2+ ions (first

step) shifts the U to -0.57 V, close to the standard electrode potential of Cd2+/Cd0 [E0(Cd2+/Cd0) ) -0.60 V], and the subsequent S8 addition (second step) causes a slight cathodic shift to -0.55 V. In curve b, the addition of S8 (first step) hardly changes the U, while the subsequent Cd2+ addition (second step) shifts the U to -0.55 V. In curve c, for the simultaneous addition of Cd2+ ions and S8, U sharply shifts to -0.57 V. After measurements a-c, all the mp-TiO2/SnO2 electrodes changed from white to yellow, which is indicative of the CdS formation on the TiO2 surface. The agreement of U for the CdS formation (-0.56 ( 0.01 V) with E0(Cd2+/Cd0) leads to the conclusion that the CdS photodeposition on TiO2 proceeds by way of the reduction of Cd2+ ion to Cd0 (atomic route). Similar photoelectrochemical measurements were carried out for the Au nanoparticle (NP)-loaded mp-TiO2/SnO2 electrodes (Au/mp-TiO2/SnO2) to shed light on the mechanism of the CdS photodeposition on Au/TiO2.10 Figure 5A shows CV curves of Au/mp-TiO2/SnO2 in the presence of S8 (a), Cd2+ ions (b), and S8 and Cd2+ ions (c): reference electrode ) Ag/AgCl. In contrast to the mp-TiO2/SnO2 system, the relative magnitude of the reduction currents of S8 (curve a) and Cd2+ ions (curve b) is reversed, i.e., the reduction current of S8 is much larger than that of Cd2+ ions. Also, the U value, where the current rises (Ur) for the sulfur reduction, shifts from -0.34 V (mp-TiO2/ SnO2) to -0.19 V and concomitantly the current increases. A similar excellent electrocatalytic effect of Au NPs was also confirmed in the TiO2-photocatalyzed reduction of S8 to S2ions.26 In curve c, the shift in Ur to -0.05 V further increases the current. Figure 5B shows PCP profiles of Au/mp-TiO2/SnO2: (a) Cd2+ ion addition at tp ) 20 min/S8 addition at tp ) 40 min, (b) S8 addition at tp ) 20 min/ Cd2+ ion addition at tp ) 40 min, and (c) simultaneous addition of Cd2+ ions and S8 at tp ) 20 min. When UV light is irradiated, U shifts to -0.45 V from

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Figure 4. (A) Cyclic voltammograms of mp-TiO2/SnO2 obtained for the first scan in the presence of S8 (a), Cd2+ ions (b), and S8 and Cd2+ ions (c): reference electrode ) Ag/AgCl, [Cd2+] ) 1 × 10-3 mol dm-3, [S8 ] ) 1.25 × 10-4 mol dm-3, [NaClO4] ) 0.1 mol dm-3. (B) Photochronopotentiometry profiles of TiO2/SnO2 in a 0.1 mol dm-3 NaClO4 (aq): (a) Cd2+ ion addition at (1.38 × 10-2 mol dm-3) at tp ) 20 min/S8 (1.72 × 10-3 mol dm-3) addition at tp ) 40 min, (b) S8 addition (1.72 × 10-3 mol dm-3) at tp ) 20 min/ Cd2+ ion addition (1.38 × 10-2 mol dm-3) at tp ) 40 min, and (c) simultaneous addition of Cd2+ ions and S8 ([Cd2+] ) 1.38 × 10-2 mol dm-3, [S8 ] ) 1.72 × 10-3 mol dm-3) at tp ) 20 min.

Figure 5. (A) Cyclic voltammograms of Au/mp-TiO2/SnO2 obtained for the first scan in the presence of S8 (a), Cd2+ ions (b), and S8 and Cd2+ ions (c): reference electrode ) Ag/AgCl, [Cd2+] ) 1 × 10-3 mol dm-3, [S8 ] ) 1.25 × 10-4 mol dm-3, [NaClO4] ) 0.1 mol dm-3. (B) PCP profiles of Au/mp-TiO2/SnO2 in a 0.1 mol dm-3 NaClO4 (aq): (a) Cd2+ ion addition (1.38 × 10-2 mol dm-3) at tp ) 20 min/S8 (1.72 × 10-3 mol dm-3) addition at tp ) 40 min, (b) S8 addition (1.72 × 10-3 mol dm-3) at tp ) 20 min/ Cd2+ ion addition (1.38 × 10-2 mol dm-3) at tp ) 40 min, and (c) simultaneous addition of Cd2+ ions and S8 ([Cd2+] ) 1.38 × 10-2 mol dm-3, [S8 ] ) 1.72 × 10-3 mol dm-3) at tp ) 20 min.

the rest potential in the dark, approaching -0.36 V. In curve a, after a transient small shift in U with the addition of Cd2+ ions (first step), U slowly changes to -0.43 V by the addition of S8 (second step). Curve b shows clear and sharp shifts to -0.43 and -0.31 V by the addition of S8 (first step) and the subsequent addition of Cd2+ ions (second step), respectively. In curve c, for the simultaneous addition of Cd2+ ions and S8, U approaches -0.31 V after a transient cathodic shift due to the sulfur reduction. The samples subjected to measurements b and c were yellow, whereas the sample after measurement a remains deep purple, which is the color of Au/mp-TiO2/SnO2. The underpotential deposition of Cd on Au is reported to form alloy thin films through the diffusion of Cd into Au,27 which may be responsible for the fact that CdS is not produced after measurement a. Clearly, the CdS photodeposition on Au/TiO2 follows the reduction of S8 to S2- (ionic route). The U value for the CdS photodeposition on Au/TiO2 (-0.31 V) is shifted toward the anodic direction by ca. 0.24 V from that on TiO2, which should arise from the difference of the mechanisms of the CdS photodeposition.

To clarify the origin of the striking difference in the mechanisms, adsorption properties of TiO2 and Au/TiO2 for Cd2+ ions and S8 were examined. Figure 6A shows the adsorption isotherms of Cd2+ ions and S8 on TiO2 at 298 K: Y denotes the adsorption amounts of Cd2+ and S8 determined from the decreases in their concentrations after the adsorption equilibrium is achieved. TiO2 acts as a good adsorbent for Cd2+ ions, whereas the adsorption amount of S8 is negligibly small (adsorbate-selective adsorption). As is obvious from the inset, the Langmuir plots for the adsorption of Cd2+ on TiO2 show a straight line, whose analysis provides the saturated adsorption amount and the adsorption equilibrium constant (K) of 3.20 × 10-5 mol g-1 and 6.15 × 103 mol-1 dm3, respectively. Figure 6B shows the adsorption isotherms of Cd2+ ions and S8 on Au/TiO2 at 298 K. A drastic increase in the adsorption amount of S8 is induced by loading such a small amount of Au (0.38 mass %) due to the specific strong S-Au interaction,28 whereas the adsorption amount of Cd2+ ions decreases to ca. 65%. This means that S8 and Cd2+ are

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Figure 6. Adsorption isotherms of Cd2+ ions and S8 on TiO2 (A) and Au/TiO2 (B). The inset in Figure 6A shows Langmuir plots for the Cd2+ adsorption on TiO2: W expresses the mass of TiO2.

SCHEME 1: Proposed Mechanism on the CdS Photodeposition on TiO2 (Atomic Route)

Figure 7. Plots of (d[Cd2+]dep/dt)t ) 0-1 vs [Cd2+]0-1.

almost selectively adsorbed on the Au and TiO2 surfaces of Au/TiO2, respectively (site-selective adsorption). On the basis of the results, we proposed a mechanism of the initial CdS photodeposition on TiO2, where the effect of the CdS photoexcitation can be neglected (Scheme 1). In the dark, Cd2+ ions are in adsorption equilibrium, S1. UV-light irradiation (λ > 300 nm) of TiO2 initiates the interband transition to generate electron-hole pairs, S2. The valence band holes (h+vb(TiO2)) escaping the recombination, S3, oxidizes ethanol to •C2H4OH radical, S4. This radical, having a strong reduction power, directly injects another electron to the conduction band of TiO2 [e-cb(TiO2)] to produce CH3CHO and H+, S5.29 The formation of CH3CHO was confirmed by gas chromatography. The electrons accumulated in the cb(TiO2) reduce the Cd2+ ions adsorbed preferentially on TiO2 to Cd0, S6. The reaction of Cd0 and S8 yields CdS QDs via the atomic route, S7. Somasundaram et al. reported that TiO2-photocatalyzed reduction of Cd2+ ions is very slow in water, and the two-step photodeposition of CdSe on TiO2 obeys an ionic route.14 However, this reaction could proceed under the present conditions, since the photopotential of TiO2 in ethanol (-0.85 V) is much higher than that in water (ca. -0.2 V). In the absence of S8, the oxidation of Cd0 to Cd2+ by h+vb(TiO2) taking place simultaneously with the reduction of Cd2+ to Cd0 may inhibit Cd growth (Figure 2B(a)). In this scheme, the concentration of highly active •C2H4OH radicals can be assumed to be negligibly small under the

photostatinary state, and the relation of [e-cb] ) 2[h+vb] would be valid because of the current doubling effect of C2H5OH. The application of the steady-state approximation for [e-cb] yields eq 2 at the initial stage of the reaction -1 (d[Cd2+]dep /dt)t)0 ) (1/Iφ){1 + (krec /2Kkr)[Cd2+]-1 0 }

(2) where [Cd2+]dep expresses the decrease in the Cd2+ concentration ([Cd2+]0 s [Cd2+]) with irradiation. Figure 7 shows plots of (d[Cd2+]dep/dt)t ) 0-1 vs [Cd2+]0-1: the ordinate is the reciprocal of the initial rate of the decrease in the Cd2+ ion concentration calculated from the X value after 1 h irradiation. As expected from eq 2, the plots give a straight line, where the slope and intercept yield the krec/Kkr value of 2.49 × 10-4 mol dm-3. From the values of krec/Kkr and K, the reaction efficiency defined as kr/(kr + krec) was estimated to be ca. 0.4. On the other hand, the mechanism of the CdS photodeposition on Au/TiO210 at the initial stage can be explained within the framework of the concept of “Reasonable Delivery Photocatalytic Reaction Systems (RDPRS)” (Scheme 2).30 S8 is strongly and selectively adsorbed on the Au surface of Au/TiO2, S1′. Electron-hole pairs are produced in TiO2 by UV-light irradiation of Au/TiO2, S2′, and the e-cb(TiO2) is effectively transferred to Au with large work function, S3′. The holes left in the valence band escaping the recombination, S4′, oxidizes ethanol to

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SCHEME 2: Proposed Mechanism on the CdS Photodeposition on Au/TiO2 (ionic route)

Acknowledgment. This work was supported by a Grant-inAid for Scientific Research (B) No. 20350097 from the Ministry of Education, Science, Sport, and Culture, Japan. The authors acknowledge Dr. S. Naya for helpful discussion. H.T. acknowledges Ishihara Techno Co. Ltd. for the gift of TiO2 (A-100) sample. References and Notes

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C2H4OH radical, S5′. This radical injects another electron to the cb(TiO2) to produce CH3CHO and H+, S6′, and the e-cb(TiO2) is further transferred to Au, S7′. The electrons accumulated in Au reduce S selectively adsorbed to S2- ions, S8′. The reaction of S2- ions and Cd2+ ions yields CdS on the Au surface via an ionic route to form Au(core)-CdS(shell) (Au@CdS) type NPs, S9′.10 Consequently, in the CdS photodeposition on TiO2, Cd2+ ions selectively adsorbed on TiO2 are reduced to Cd0 reacting with S8 to deposit CdS QDs on TiO2 (atomic route), whereas in the CdS photodeposition on Au/TiO2, S8 (oxidant) selectively adsorbed on the Au NP surface (reduction sites) of Au/TiO2 is reduced to S2- ions reacting Cd2+ to produce Au@CdS NPs on TiO2 (ionic route). IV. Conclusions CdS QDs have been directly coupled with TiO2 particles and films by a one-pot photochemical method at ambient temperature and pressure. The loading amount and band gap of CdS can be controlled by irradiation time. Photoelectrochemical and adsorption experiments indicated the photodeposition of CdS on TiO2 predominantly proceeds via an atomic route (Cd0 + S f CdS), whereas the CdS photodeposition of Au/TiO2 follows an ionic route (Cd2+ + S2- f CdS). This study has presented the basis of the photocatalytic synthesis of chalcogenide-oxide nanocoupling systems, which is highly expected for applications to visible light photocatalysts and photoelectrodes of solar cells.

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