Catal Lett DOI 10.1007/s10562-015-1612-6

Photocatalytic Hydrogen Production Under Visible Light by Using a CdS/WO3 Composite Katherine Villa1 • Xavier Dome`nech1 • Ulises M. Garcı´a-Pe´rez2 • Jose´ Peral1

Received: 27 June 2015 / Accepted: 24 August 2015 Ó Springer Science+Business Media New York 2015

Abstract Generation of H2 by visible irradiation of CdS/ WO3 composite aqueous slurries in presence of sacrificial organic molecules has been studied. The composite was chosen by taking into account the advantageous position of conduction and valence band of both semiconductor and the fact that they can absorb visible light. A potential improvement of the photogenerated charge carrier separation has been tested by depositing onto the catalyst surface Pt and RuO2, the metal acting as electron sink and the oxide capturing the photogenerated holes. A tentative scheme of charge carrier transfer between the different composite phases is initially proposed but experimental data seems to invalidate such a hypothesis. A composite configuration based in the co-precipitation of CdS over a commercial WO3 followed by Pt deposition gave the best hydrogen productions, a production that is in the same order of magnitude of the efficiency of other photocatalysts previously tested in our laboratory. The characterization of the main properties of the catalysts seems to indicate that the improved generation of hydrogen found with the CdS/WO3 composite is directly related to the available surface area rather than being a consequence of the design of a favorable charge carrier circulation. Keywords CdS  WO3  Photocatalyst composite  Hydrogen generation  Visible light & Jose´ Peral [email protected] 1

Departament de Quı´mica, Edifici Cn, Universitat Auto`noma de Barcelona, 08193 Bellaterra, Cerdanyola Del Valle`s, Spain

2

Facultad de Ingenierı´a Meca´nica y Ele´ctrica, Centro de Investigacio´n e Innovacio´n en Ingenierı´a Aerona´utica, Universidad Auto´noma de Nuevo Leo´n, Carretera a Salinas Victoria Km 2.3, C.P. 66600, Apodaca, NL, Mexico

1 Introduction Photocatalytic generation of H2 from water is an issue of unquestionable practical importance. The possibility of generating such an energy vector by using a cheap and innocuous solid catalyst (traditionally TiO2) and solar energy has attracted a large interest and many research efforts are being made in this field [1–3]. Nevertheless, the efficiencies of the energy conversion obtained so far are rather modest and thus, further effort is still needed to find more efficient photocatalytic systems. In this sense, those configurations based in the use of TiO2 are affected by the inherent limitations of this catalyst: high electron–hole recombination, no solar visible photon absorption, and a conduction band edge potential too close to the redox potential of the H?/H2 couple. Thus alternative catalysts, or combinations of catalyst, are being tested in order to find more suitable photocatalytic systems for hydrogen production. The combination of two catalysts brings the advantage of efficient charge separation (photogenerated charges can be transferred from one catalyst to the other hampering charge recombination), the possibility of tailoring conduction and valence band energy levels in order to enhance both reduction and oxidation processes, and the opportunity to choose visible light absorbing catalysts. Many examples can be found in the scientific literature describing the use of semiconductor composites for photocatalytic hydrogen evolution. So far the most studied combination involves the mixing of TiO2 and CdS particles [4, 5]. This combination has two appealing features: CdS absorbs visible light, and the photogenerated conduction band electrons can be easily transferred to TiO2 thus increasing charge separation. However, with such an electron transfer (from conduction band to conduction band) the high energy level of electrons at the CdS, an

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energy that makes them especially suitable for H? reduction and H2 generation, is partially lost. The electrons that end up at the TiO2 conduction band have only an small extra energy for H? reduction and this is probably the most important handicap that those semiconductor systems based on TiO2 face for H2 generation. We propose here the study of a photocatalytic system for the simultaneous H2 generation and sacrificial organic oxidation based on the combination of two catalysts, CdS and WO3, a system that could be conveniently loaded with suitable redox co-catalyst like Pt and RuO2 deposits. Such a combination may present several advantages: (a) both catalyst absorb visible light and can efficiently carry out solar-driven reactions; (b) photogenerated conduction band electrons in CdS have an advantageous energy position for H? reduction; (c) photogenerated valence band holes in WO3 have an advantageous energy position for sacrificial organic molecule oxidation; (d) Pt deposits onto the CdS surface can act as electron sink thus acting as H? reduction centers and avoiding electron transfer to the lower energy WO3 conduction band; (e) RuO2 deposits onto the WO3 surface can act as hole sinks thus acting as organic molecule oxidation center and avoiding hole transfer to the higher energy CdS valence band; (f) the only possibility for inter-particle charge transfer would then be the combination of photogenerated electrons of WO3 conduction band and photogenerated holes of CdS valence band, a transfer that would prevent CdS photocorrosion, i.e., reaction of CdS with its own photogenerated holes. A diagram showing the possible charge transfers that would take place in such a photoreactive system is presented in Fig. 1. The combination CdS–WO3 (in this work the dash will indicate any possible combination morphology) for H2 evolution has been previously tested by Ashokkumar et al. [6] and they found rather modest H2 generation efficiencies. No use of Pt or RuO2 co-catalysts was reported. On the other hand, the possibility of the existence of charge transfer between conduction band and valence band of Ox 1

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Fig. 1 Charge transfer scheme proposed for visible light irradiated Pt/CdS–WO3/RuO2 composites

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2 Experimental 2.1 Reagents All reagents used in this work were of analytical grade. High purity N2 (99.999 %) was used to provide an O2 free atmosphere. A commercial gas mixture of 1000 ppmv of H2 in N2 was used for GC calibration. Water used to prepare aqueous solutions was of Milli-Q grade. 2.2 Catalyst Preparation 2.2.1 CdS Two different methods (A, B) were employed for the preparation of CdS. (A) Commercial CdS (Scharlau) was directly used as precursor material to the CdS cristallization by a thermal treatment at 650 °C for 2 h with a heating rate of 10 °Cmin-1. The resulting sample was denoted as CdS-A. (B) CdS powders were prepared by co-precipitation method as follows: 1.5 g of CdSO4 were dissolved in 4.9 mL of (NH4)2S (10 % w/w aqueous solution) under overnight stirring. The resulting solid was collected by filtration, washed with distilled water, dried and finally heated at 650 °C for 2 h with heating rate of 10 °C min-1. The resulting sample was denoted as CdS-B. 2.2.2 WO3 Two different powders of WO3 (A, B) were used as catalysts. (A) Commercial WO3 (Aldrich) was directly used as catalyst material. The resulting sample was denoted as WO3-A. (B) WO3 powders were prepared by co-precipitation method as follows: 6.21 g of W powder were dissolved in 4.9 mL 100 mL of H2O2 (30 % aqueous solution), a white precipitate was formed after 10 min of reaction. The suspension was heated at 100 °C to promote the evaporation of the solvent. The resulting solid was washed with distilled water at 70 °C, and finally heated at 450 °C for 0.5 h (temperature ramp of 10 °C min-1). The resulting sample was denoted as WO3-B. 2.2.3 Pt–CdS, Pt–WO3

WO3 Red 2

particles of two different semiconductors has been previously considered by other authors [7].

The platinization of CdS or WO3 powders was carried out by a chemical reduction method. In a typical procedure, 30 mg of H2PtCl6 were dissolved in 120 mL of water, followed by addition of 30 mL of a 1 % ammonium citrate solution, and taking the mixture to reflux during 4 h. The

Photocatalytic Hydrogen Production Under Visible Light by Using a CdS/WO3 Composite

suspension of metallic Pt particles obtained produces a clearly visible dark grey color (even in the walls of the glassware). 50 mL of the resulting solution were mixed with 1 g of the catalysts (CdS-A, CdS-B, WO3-A or WO3B) and 5.8 g of NaCl during 30 min. Finally, the platinized slurry was filtered and rinsed several times with deionized water. The dark metallic color of the Pt deposits was clearly observed on the catalyst surfaces. The resulting samples were denoted as Pt–CdS-A, Pt–CdS-B, Pt–WO3-A and Pt–WO3-B. 2.2.4 RuO2–WO3 The as-prepared WO3-B sample was impregnated with RuO2 (5 %) by adding 0.025 g of commercial RuO2 and 0.5 g of WO3 to hot water under 30 min of sonication. Afterwards, water was removed by soft heating and sintering was obtained by placing the catalyst in an oven at 400 °C during 3 h. 2.2.5 CdS/WO3 Composites Two different procedures (A, B) were used to prepare platinized CdS/WO3 composites. (A) Pt–CdS ? WO3 composite was obtained by direct mixing of pure solid catalysts. 1 g of the as-prepared WO3-B and Pt–CdS-B samples were mechanically mixed (sonicated in an aqueous slurry) and heated at 700 °C in presence of N2. To prepare the Pt–CdS ? Pt–WO3 and Pt–CdS ? RuO2–WO3 composites the above procedure was followed by mixing of Pt– CdS-B and Pt–WO3-B; Pt–CdS-B and RuO2–WO3 samples, respectively. The sign ‘‘?’’ is used to refer to those composites obtained by physical mixing. (B) Pt–(CdS/ WO3) composites were obtained by deposition of one phase onto another. 1.5 g of CdSO4 was dissolved in 10 mL of H2O. In the same solution 1 g of WO3-B catalyst was added to form a slurry. Under continuous stirring 4.9 mL of (NH4)2S are then added to form a yellow precipitate of CdS. The slurry is stirred during 24 h, and afterwards filtered and rinsed with distilled water. Finally, the resulting solid mixture is dried and sintered at 700 °C in presence of N2. After that, the powders were platinized following the above-mentioned procedure. To prepare the Pt–(CdS/RuO2–WO3) composites the above procedure was carried out in presence of RuO2–WO3 sample.

ASAP 2020 V3H apparatus. Microstructure of the composite was observed by transmission electronic microscopy (TEM, JEOL JEM-1400). The absorbance of the different catalyst samples was obtained with UVA–vis diffuse reflectance spectrometry (UV-3600 SHIMADZU spectrophotometer). 2.4 Photocatalytic Setup The experiments were carried out in a double wall waterrefrigerated cylindrical Pyrex reactor of 200 mL volume equipped with gas inlet and outlet and a liquid sample port. The aqueous slurries of the catalysts were magnetically stirred. The reactor gas phase was recirculated by using a gas-tight membrane pump (Enamoto CM-15-6). A six port valve with a 2 mL loop was placed in the recirculation circuit to allow gas sample injection into a gas chromatograph. Four 15 W visible compact fluorescent lamps placed around the reactor were used to provide a steady photon flux. A 0.1 M NaNO2 solution was recirculated through the external chamber of the double wall photoreactor both to provide refrigeration, and to filter any photon below 385 nm. In a typical experiment 50 mL of the formic acid aqueous solution (10-3 M) containing 0.1 g of catalyst were placed in the reactor. The remaining gas phase volume (&170 mL taking into account piping and pump death volume) was then pumped out and refilled with pure N2 (15 min of N2 bubbling). After that, the lamps were turned on and gas samples were periodically taken for analysis. 2.5 Analytical Procedures Detection and quantification of H2 was carried out by using a Shimadzu GC-2014 chromatograph equipped with a packed column (Carboxen 1000 stationary phase) and a TCD detector. Pure N2 was used as carrier gas. An isothermal chromatographic separation (50 °C oven temperature) was chosen. Under those conditions other gases like CO2 or O2 could also be detected. Reinecke’s salt actinometry [8] was performed to quantify the amount of visible photons entering the reactor (5.3 9 10-7 Einstein s-1). Cd2? detection was carried out by mass spectroscopy with inductively coupled plasma (ICP-MS Agilent, model 7500ce). Organic degradation was quantified by using a Shimadzu VCSH TOC analyzer.

2.3 Sample Characterization

3 Results and Discussion X-ray diffraction was carried out by means of a Philips X-Pert diffractometer, using a Cu Ka radiation, and a 2h range between 20° and 70°. A JEOL JEM-6300 microscope was used to perform scanning microscopy of the samples. BET surface areas were obtained with a Micromeritics,

In order to have a valid reference of the photocatalytic activity of the CdS/WO3 composites prepared in this work, a preliminary study of the activity and the characterization of the CdS catalysts used as starting materials was carried

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out. The surface area of commercial CdS (after improving its crystallinity with a thermal treatment at 650 °C) was ca 1.1 m2 g-1, while the CdS prepared by co-precipitation turned out to have 0.3 m2 g-1. In both cases the surface area is rather small, and could probably be improved, at least for the sample prepared at the laboratory, if other synthetic routes were chosen. Nevertheless, since the work was aimed at comparison of the activity of the CdS/WO3 composites versus the CdS starting material, any CdS with photocatalytic activity could be used for this purpose. XRD patterns of both starting materials showed clear crystalline phases. The diffraction peaks can be indexed to the hexagonal phase of CdS according to the JCPDS card No. 41-1049. The SEM micrographs of the commercial CdS showed agglomerates of CdS plates of polyhedral shape and an average diameter of 1 lm, while the CdS prepared by co-precipitation contained an agglomeration of rather spherical particles of 0.2–0.5 lm of diameter. Figure 2 displays the time course of H2 generation when using either the commercial or the laboratory made CdS as catalysts. The reactions were carried out under a N2 atmosphere and using 10-3 M formic acid as hole scavenger. The activities of the corresponding platinized samples are also included in the figure. As can be seen all samples were able to generate hydrogen under visible light irradiation. Also, there is a clear difference between the platinized and no platinized samples. Since the experimental error calculated for H2 generation was no superior to 7 % it can be said that a clear difference also exists between the performance of the two different platinized CdS samples, with a clear crossing of the curves taking place. Platinization clearly favors hydrogen generation for both the commercial and the laboratory made catalysts. This fact has been previously observed for other photocatalysts like 1,2 CdS-A Pt-CdS-A CdS-B Pt-CdS-B

H2 generated / mol

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Fig. 2 H2 generation versus irradiation time of several samples of pure and platinized CdS. 0.1 g of catalyst in 50 mL of formic acid aqueous solution (10-3 M) under N2 atmosphere

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TiO2 [9, 10]. The electron transfer in the different assynthesized photocatalyst systems in this study is illustratively explained in Fig. 3. The positive role played by Pt has been explained taking into account the capacity of this metal to act as electron ‘‘sink’’, see Fig. 3a. Thus, when electron–hole pairs are photogenerated, the electrons are captured by the Pt phase where they are stabilized, and this avoids electron–hole recombination and loss of photocatalytic efficiency. The curves of the non-platinized catalysts indicate that the commercial sample is moderately more active than the co-precipitation sample. Since both samples are crystalline this activity difference might be simply due to the different surface area (the commercial sample has almost four times more surface area than the co-precipitation sample). This seems also to be the case for platinized samples at moderate reaction times. However, above 250 min the efficiency order is inverted. No clear reason has been found for that behavior. Having the activity of pure and platinized CdS samples as reference the following step was to check the activity of different CdS/WO3 composites. Since the straightforward way to prepare such composites was to mix the pure materials, this was the first choice (see details in the experimental section). The mixtures were always submitted to high temperature treatments in order to increase interparticle contact, a key aspect if charge transfer between particles is to be expected. In fact, the simple mixing of CdS and WO3 without subsequent thermal treatment rendered a composite that, under visible irradiation, was unable to produce H2. Also, the CdS prepared by co-precipitation was chosen to prepare such mixed composites because, as shown later, this composite configuration will be compared to another configuration based on the coprecipitation of CdS onto commercial WO3 samples, and that could not be achieved by using the CdS commercial material. Thus it seemed more convenient to compare the results of two configurations that were obtained by using the same CdS source. Figure 4 shows the performance of three possible configurations based on the proposed mixing. As can be seen, one of them is the result of mixing Pt– CdS with WO3. The generation of H2 was clearly improved when using this composite. Indeed, while 1 lmol of H2 were obtained with the Pt–CdS catalyst after 300 min of irradiation (see Fig. 2), around 1.6 lmol of the gas were obtained with the Pt–CdS ? WO3 composite after the same irradiation time. As expected, the CdS ? WO3 mixture (no previous CdS platinization) gave a much lower H2 production (data not shown). Thus, there is a clear positive effect derived from the mixing of the two catalysts, especially when taking into account that irradiated slurries of WO3 produced no H2. In order to check if our initial hypothesis of interparticle charge transfer shown in Fig. 1 is valid, a Pt–

Photocatalytic Hydrogen Production Under Visible Light by Using a CdS/WO3 Composite

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Fig. 3 Illustrative diagrams of the electron transfer in the different as-synthesized photocatalyst systems: a Pt–CdS; b Pt–(CdS/RuO2–WO3); c Pt–(CdS/WO3) and d Pt–CdS/WO3 3,5

H2 generated / mol

3,0 2,5 2,0 1,5 1,0 Pt-CdS+WO3 (sintered at 700 ºC)

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Fig. 4 H2 generation versus irradiation time of several samples of mixed CdS and WO3 composites (CdS ? WO3). 0.1 g of catalyst in 50 mL of formic acid aqueous solution (10-3 M) under N2 atmosphere

CdS ? RuO2–WO3 was prepared and tested. Its H2 production capability is also shown in Fig. 4. According to the proposed charge transfer mechanism the presence of RuO2, being a material that can easily trap holes, should had reinforced electron–hole separation and, thus, H2 generation. In fact, that is the key point of this study: to hamper the apparently more favorable charge transfer of electrons

between particle conduction bands, or holes between particle valence bands by driving CdS conduction band electrons towards Pt, and WO3 valence band holes towards RuO2, and thus forcing an electron exchange between WO3 conduction band and CdS valence band. However, no significant increase in the H2 production was observed after 400 min of irradiation pointing toward the absence of any beneficial role that could be assigned to the use of RuO2. Moreover, when a new configuration based on the mixing of previously platinized CdS and WO3 was checked, a noticeable increase of the H2 production was observed. In this sense, the amount of H2 generated after 400 min of irradiation is nearly doubled (Fig. 4). This observation works against the proposed scheme of charge transfer in CdS/WO3 composites. The platinization of WO3 should force a charge transfer of the WO3 conduction band electrons toward the Pt dots deposited in that material, and would probably favor the transfer of some electrons from the CdS to the WO3 particles. Having WO3 a band energy distribution that is not suitable for H2 production, the accumulation of electrons in the Pt deposited on the WO3 would involve a decrease of the composite efficiency. The observed increase of H2 can only be explained if mechanisms of CdS photocatalytic efficiency improvement others than the one proposed in our initial hypothesis are involved. In this sense, the existence of surface plasmon

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resonance (SPR) in the Pt particles deposited on the WO3 and irradiated with visible light could be a sound explanation of the increased H2 generation (Fig. 3c). SPR of Pt and other metal nanoparticles has been previously described [11, 12]. This phenomenon produces high energy electrons that could be used to reduce H? and generate H2. Thus, although the photoexcited electrons of the WO3 conduction band have not enough energy to reduce H?, when those electrons are captured by the Pt particle the surface plasmonic resonance effect could provide them with an extra energy that would be enough to favour the electron transfer and H? reduction.

5 Pt-(CdS/WO3) Pt-(CdS/RuO2-WO3)

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Fig. 5 H2 generation versus irradiation time of several samples of CdS co-precipitated onto WO3 composites (CdS/WO3). 0.1 g of catalyst in 50 mL of formic acid aqueous solution (10-3 M) under N2 atmosphere

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Fig. 6 XRD diffractograms of pure commercial WO3, pure coprecipitated CdS, and CdS/WO3 composite

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Fig. 7 a, b TEM micrographs of the Pt–(CdS/WO3) composite. c c SEM-EDS elemental analysis of the composite

Taking into account the above results and considering that other configurations of the CdS/WO3 composite could possibly render improved H2 production, an alternative combination of the two catalysts was tested. As mentioned in the Experimental Section a catalyst consisting on the coprecipitation of CdS onto WO3 followed by Pt deposition was prepared Pt–(CdS/WO3). Proceeding in this way Pt could end up deposited both on the CdS and the WO3, a situation that taking into account our initial hypothesis about the charge transfer mechanism in the CdS/WO3 composites should not be the best one. Surprisingly, this catalyst configuration gave the largest hydrogen generation (see Fig. 5). Indeed, after 350 min of irradiation the total H2 production was close to 5 lmol, which is 2 lmol larger than the best situation shown in Fig. 4. In fact, this has been the best hydrogen production obtained in this work. When a similar composite based on the co-precipitation of CdS onto a RuO2–WO3 support followed by Pt deposition Pt–(CdS/RuO2–WO3) was tested the hydrogen generation remained below 1 lmol after 300 min of irradiation (Fig. 3b). Again, the inclusion of RuO2 on the composite had not the expected beneficial effect, even hampering the hydrogen production, this experimental observation working against the validity of our initial hypothesis. Also, and according to the hypothesis of the existence of SPR on the Pt deposited on to the WO3 particles, any catalyst modification that could lead to a lower contact between the Pt and the WO3 phases would be detrimental for an effective electron transfer between them, and would decrease H2 generation. This could be a sound explanation for the detected loss o activity of the Pt–(CdS/RuO2–WO3) where the presence of the RuO2 phase attached to WO3 leaves less surface area available for Pt–WO3 contact. TOC measurements were performed to verify that mineralization of formic acid occurred simultaneously to the photocatalytic hydrogen production. The initial TOC value of the 10-3 M formic acid aqueous solution (pH: 3.50) was 136 mg L-1; this value decreased to 124 mg L-1 after 6 h of irradiation in the Pt–(CdS/WO3) experiment, while the amount of produced hydrogen increased from 0 to 4.7 lmol, which is in stoichiometric agreement with the value of formic acid degraded. This fact confirms that the photocatalytic hydrogen production and the decomposition of formic acid occurred simultaneously. The average photonic efficiency of the Pt–(CdS/WO3) composite, calculated by using the following mathematical expression: u¼

2  Number of H2 molecules produced  100 Number of incident photons

Photocatalytic Hydrogen Production Under Visible Light by Using a CdS/WO3 Composite

and taking into account the number of photons entering the reactor (5.3 9 10-7 Einstein s-1) and the data of Fig. 5 (4.74 lmol of H2 were produced after 360 min of irradiation) was 0.083 %. In order to have a closer idea of the significance of such a hydrogen production a comparison can be established with the hydrogen generated when using other catalysts and catalysts composites. For example, a catalyst based on a platinized mixture of CdS and ZnS, a composite that has repeatedly shown in the literature an acceptable capability for hydrogen production [13–15], was prepared in our laboratory and tested in under exactly the same experimental conditions (reactor configuration, photon flux, solution volume, mass of catalyst, hole scavenger nature and concentration, etc.) reported here, and the amount of hydrogen produced in that case was around 10 lmol after 360 min of irradiation [16], i.e., the Pt– (CdS/WO3) produced around half the hydrogen of a similar Pt–(CdS/ZnS) composite. On the other hand, the photonic efficiency of the hydrogen generation with a N-doped and platinized TiO2 (experiments also carried out in our laboratory), visible light irradiation, and using also formic acid as hole scavenger was 0.24 % [16], a larger efficiency, although in the same order of magnitude than the performance of the present Pt–(CdS/WO3) catalyst studied here. Since the Pt–(CdS/WO3) configuration turned out to be the best among the studied CdS/WO3 composites, special attention was paid to its characterization. As seen in Fig. 6 (XRD of the composite) the material had a clear crystalline structure, and the diffraction lines obtained in the diffractogram are the sum of the reflections corresponding to pure CdS and WO3 materials. On the other hand the surface area found by BET analysis was 3.6 m2 g-1, i.e., an area that is one order of magnitude larger than the surface area of the co-precipitated CdS alone. This could be a plausible explanation of the improved activity observed for that composite, since surface area is a key parameter that affects the rate of many heterogeneous photocatalytic processes. A possible cause of the appearance of such a larger surface area can be found by paying attention to the TEM micrographs. As seen in Fig. 7a, the composite material contained an agglomeration of particles of approximately 0.1–0.2 lm of diameter, while TEM of pure CdS obtained by co-precipitation showed particles of 0.2–0.5 lm of diameter. Thus, it seems that WO3 was acting as nucleation center for CdS particles of lower size and larger area. Figure 7b shows a closer view of one of those particles. As can be seen the CdS phase is built around several WO3 particles and supports small Pt deposits. Figure 7c shows the SEM–EDS elemental analysis of the composite. The peaks that appear in the figure indicate the presence of both CdS and WO3 phases. Pt was not

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observed due to its low content (less than 1 %) that is below the detection limit of the instrument. Nevertheless, the presence of Pt is justified by the fact that the small particles that are signed as Pt in Fig. 7b only appeared after platinization was carried out. It is important to see that WO3 is not completely covered by CdS and, consequently, it keeps the contact with the solution and could exchange charges with the hole scavenger. Even with this advantageous configuration the inclusion of RuO2, a cocatalyst that should potentiate such a charge transfer, rendered a worse hydrogen production (see Fig. 5), thus invalidating our initial hypothesis (Fig. 1). Finally, and taking into account the possibility of CdS photocorrosion, i.e., the reaction of the semiconductor with its own valence band holes that takes to solid disintegration with the release of Cd2? to the solution and the oxidation of S2- to other forms of S of higher oxidation state, detection of Cd2? in solution was carried out at the end of each experiment. Appearance of Cd2? was detected in all cases, although the data showed a non systematic behavior with Cd2? concentrations ranging from 1.5 to 7.0 mg L-1. The presence of photocorrosion is another fact that works against the existence of a charge transfer mechanism like the Z-scheme shown in Fig. 1 because the transfer of WO3 conduction band electrons to the CdS valence band should efficiently eliminate the reaction of valence band holes with CdS and the appearance of photocorrosion. From the data presented in this work it can be concluded that the charge transfer scheme proposed in Fig. 1 is not taking place in the CdS/WO3 composite systems, or is not the main cause of the improved hydrogen generation observed with those composites. Also, other causes like surface area increase or SPR can be invoked to explain the improved H2 generation observed when CdS and WO3 are properly mixed and platinized. In order to gain more knowledge about charge distribution and charge transfer during irradiation of this mixed semiconductor system future work on hydrogen generation by using photoelectrochemical systems based on separated CdS and WO3 electrodes (both materials deposited over conducting glass) is advisable.

4 Conclusions The generation of hydrogen by visible light irradiation of aqueous slurries of CdS/WO3 composites in presence of a hole scavenger like formic acid has been studied. Several composite configurations have been tested. In particular, the use of Pt and RuO2 deposits acting as reduction and oxidation catalyst, respectively, has been studied. A sound hypothesis of the charge transfer mechanism that could

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take place in that mixed semiconductor system, and the advantages that it would involve in terms of hydrogen generation, have been proposed. Nevertheless, the expected high performance of the proposed Pt–CdS/RuO2–WO3 systems has not been detected, being the addition of RuO2 always detrimental towards the hydrogen production. Other catalysts configurations based on the co-precipitation of CdS onto commercial WO3 followed by platinization (Pt–(CdS/WO3) catalyst) turned out to be more effective. Thus, the charge transfer scheme initially proposed in this work is not endorsed by the experimental data. Characterization of the Pt–(CdS/WO3) catalyst showed an agglomerated of particles of larger surface area than the pure CdS particles, probably being this one reason of the hydrogen production improvement detected for that composite. The possible existence of SPR of the illuminated Pt nanoparticles deposited onto the WO3 surface could also be used as a sound explanation. In any case, the amount of hydrogen produced was lower but in the same order of magnitude than the one reported by other catalysts like the CdS/ZnS composite or a N-doped Pt–TiO2 catalyst. Photoelectrochemical studies of systems based on separated CdS and WO3 electrodes are advised to clarify the charge transfer scheme taking place in the composite systems studied here. Acknowledgments The authors want to thank to the Spanish Government for financial support trough Project CTQ2013-47103-R.

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