J. Chem. SOC., Faraday Trans. I , 1987, 83, 1395-1404 Photogeneration of Hydrogen from Water over an Alumina-supported ZnS-CdS Catalyst Junya Kobayashi, Kenichi Kitaguchi, Hiroshi Tanaka, Hideyasu Tsuiki and Akifumi Ueno* Department of Materials Science, Toyohashi University of Technology, Tempaku, Toyohashi, Aichi 440, Japan The rate of hydrogen generation from water with an alumina-supported mixed semiconductor catalyst (ZnS-CdS/Al,O,) has been found to be much higher than those from water with singly supported catalysts (ZnS/Al,O, and CdS/Al,O,) under irradiation by both U.V. and visible light. The activity is not improved by the physical mixture of these singly supported catalysts even when the amounts of ZnS and CdS are the same as those present in the ZnS-CdS/Al,O,. The significant improvement in the rate of hydrogen production over ZnS-CdS/Al,O, catalyst is attributed to the intimate contact between ZnS and CdS particles that results from the deposition of fine ZnS particles over the CdS surface. Sites for hydrogen production seem to be newly generated on these contacting regions.Light absorption is an important step in the photogeneration of hydrogen, and it takes place over the surface of a catalyst particle, suggesting that the efficiency of light absorption of the catalyst increases with increasing surface area. Thus attention has been paid to fine semiconductor particles so as to improve the activity for hydrogen production from water. Chalcogenides have been employed as photocatalysts, since their photocor- rosion may be overcome by the presence of a sacrificial reagent such as Na,S.l Among the chalcogenides, ZnS2 and CdS3 have been studied extensively, since much knowledge is available about their electronic and electrochemical properties.Henglein et al.* observed a blue shift of the onset in the absorption spectrum for colloidal ZnS when the colloidal ZnS and CdS were co-precipitated in Na,S solution. These authors also studied the relationship between the electronic properties of ZnS and their activity for hydrogen prod~ction.~ Reber and Meier6 observed the enhancement in the rate of hydrogen generation from water when colloidal ZnS was employed as a photocatalyst in the presence of Na,S. The electronic properties of semiconductor catalysts may be varied not only by changing their particle size but also by the addition of an appropriate material that is in contact with the catalysts.Ueno et al.' observed a significant acceleration in the rate of hydrogen production under illumination by both U.V. and visible light over ZnS- CdS/SiO, catalyst, where the fine ZnS particles coat the surface of the CdS particles deposited over the SiO, support. Electron transfer between ZnS and CdS particles probably occurs at their contacting region and may result in the acceleration of hydrogen generation. Serpone et al.* are the first who proposed an interparticle electron transfer to explain the enhanced rate of hydrogen production with the CdS/TiO,/RuO, mixed semiconductor system. Finlayson et aZ.9 believe that the active site for the photogener- ation of hydrogen over ZnS-CdS/SiO, catalyst might be Cd metal produced during the light irradiation. They also observed a flat-band shift of CdS by 0.7 V to more negative potentials when CdS powder was passivated with Zn2+ ions, resulting in the formation of Cd metal and in the reduction of the number of recombination sites. Recently, Reber and RuseklO reported that a solid solution of zinc and cadmium sulphides exhibited significant activity for hydrogen generation from water even without any noble xnztcils.13951396 Photogeneration of Hydrogen on a Semiconductor Catalyst The purpose of this work was to confirm that the particular structure of the ZnS-CdS system, i.e. a model comprising a CdS core and a ZnS shell, is a key to the enhanced rate of hydrogen generation from water. Alumina powder, instead of silica, was used here as a catalyst support, and similar experiments to previous ones7 were carried out.Experimental In the present work alumina powder was used as a catalyst carrier. The alumina powder was prepared by the hydrolysis of aluminium isopropoxide dissolved in n-butanol, which is one of the best methods to obtain pure alumina.ll The specific surface area of the alumina was varied by controlling the pH value of the solution and the amount of water added for hydrolysis,12 with powders of 260 and 400 m2 g-l being employed in this work. In most of the experiments the alumina powder with 260 m2 g-l surface area was used. The catalysts employed were as follows. (1) ZnS/Al,O, and CdS/Al,O,. These singly supported catalysts were prepared by a conventional impregnation method and the amounts of ZnS and CdS over alumina were 24 and 26 wt % , respectively.(2) ZnS/Al,O,- CdS/Al,O,. This comprised the physical mixture of the singly supported catalysts in (1) with the atomic ratio of Zn/Cd being unity. (3) ZnS-CdS/Al,O,. This catalyst was prepared by a coimpregnation method using an aqueous solution of Zn(NO,), - 6H,O and Cd(NO,), - 4H20 and water saturated with H,S. Then 12 and 13 wt % of ZnS and CdS, respectively, were loaded over alumina, as determined by atomic absorption spectroscopy. This corresponds to a Zn/Cd atomic ratio of unity. (4) ZnS(CdS/Al,O, ) and CdS(ZnS/Al,O, ) : the ZnS(CdS/Al,O, ) catalyst was prepared by sequential depo- sition of CdS over alumina followed by deposition of ZnS over the CdS particles, and the CdS(ZnS/Al,O,) catalyst was made by sequential deposition in the opposite order.In both catalysts the loadings of ZnS and CdS were 12 and 13 wt% , respectively, with a Zn/Cd atomic ratio equal to unity. (5) ZnS-CdS. The unsupported ZnS-CdS catalyst was prepared by coprecipitation of ZnS and CdS using Zn(N03);6H,0 and Cd(NO3),-4H,O and H,S-saturated water as a precipitant. The Zn/Cd atomic ratio was also kept at unity. The catalyst thus prepared was placed (not suspended) on the bottom of a 20 cm3 Pyrex glass vial in which 10 cm3 of water containing 0.1 mol of Na,S was poured, the pH value of this solution being 12.5. Usually, the amount of catalyst employed was 50 mg. The solution was de-aerated by ultrasonication and purged with N,.A 450-W Xe lamp, equipped with a water filter for the removal of i.r. light, was used to irradiate the catalyst from the bottom of the vial. A 440 nm cutoff filter was used for the experiments with visible light. The top of the vial was sealed with a rubber septum, through which the hydrogen produced was transferred to a gas chromatograph by a gas syringe. For analysis a column packed with molecular sieve 13X and N, as a carrier gas were used. The catalysts were examined by X-ray diffraction spectroscopy (XRD), transmission electron microscopy (TEM) and Auger electron spectroscopy (AES). XRD (Rigakud- enki, Geigerflex) was operated at 30 kV and with a filament current of 15 mA using an Ni filter for Cu Ka irradiation. AES (JEOL, Jump 10) was operated at an accelerating voltage of 10 kV with a sample current of ca.1 A. The surface composition of the catalyst was measured by the AES sputtering technique using Ar+ ions at an accelerating voltage of 3 kV and a beam current of 25 mA. The particle sizes of ZnS and CdS in ZnS/Al,O, and CdS/Al,O,, respectively, were monitored by TEM (Hitachi, H-800) operated at an accelerating voltage of 200 kV with a magnification of x lo5. The photographs obtained were not clear enough for the ZnS and CdS particles to be hardly distinguished from the shadows of alumina powder. The concentrations of Zn and Cd ions in the catalyst were measured by atomic absorption spectroscopy. The catalyst powder was first ground in an agate mortar and then treated with nitric acid at an elevated temperature to extract the Zn and Cd ionsJ .Chem. SOC., Faraday Trans. I , Vol. 83, part 5 Plate 1 Plate 1. TEM photographs of the unsupported ZnS-CdS (a) and the singly supported ZnS (6) and CdS ( c ) catalysts. J. Kobayashi, K. Kitaguchi, H. Tanaka, H. Tsuiki and A. Ueno (Facing p . 1397)J . Kobayashi, K. Kitaguchi, H. Tanaka, H. Tsuiki and A . Ueno 1397 100 0 . 0 0 I I 20 30 40 50 2ei0 Fig. 1. X-Ray diffraction patterns of (a) unsupported and (b) alumina-supported ZnS-CdS catalysts; Q-ZnS and Q-CdS are denoted and 0, respectively. for analysis. The concentrations of Zn and Cd ions in the impregnating solution were also measured by atomic absorption spectroscopy during the coimpregnation procedure. Thus the rates of precipitation of Zn and'Cd ions over the alumina surface as ZnS and CdS were measured.The power dependence on the rate of hydrogen generation was studied by adjusting the power of the incident light using a few kinds of gauzes made of stainless steel. The power of the incident light was monitored by a photomultiplier. Results The X-ray diffraction patterns of the unsupported ZnS-CdS and alumina-supported ZnS-CdS catalysts are shown in fig. 1. All the peaks observed for the unsupported catalyst were assigned to either P-ZnS (cubic) or P-CdS (cubic), suggesting that the crystallographic structures of ZnS and CdS in the supported catalysts are probably P-ZnS and P-CdS, respectively, although only a small and broad peak appeared at 28 = 28" in the case of the supported catalyst.The activities of the catalysts for the photogeneration of hydrogen under irradiation by u.v.-visible light are shown in fig. 2, where the amount of hydrogen evolved is expressed in units of cm3 per 50mg of each catalyst. TEM photographs of the unsupported ZnS-CdS and the singly supported ZnS/A1,03 and CdS/Al,O, catalysts are shown in plate 1. The size of the particles in the unsupported catalyst was as large as 4 x lo3 A, while CdS particles in CdS/A1,0, were ca. 200 A in size. It was hard to identify the ZnS particles in the ZnS/Al,O, catalyst partly because of their small size. Since the lower limit of TEM under the conditions employed here is ca. 20 A, the size of the ZnS particles was estimated as < 20 A. The change in the concentrations of the Zn and Cd ions in the solution during impregnation is depicted in fig.3. The concentration of Cd ions decreased rapidly when H,S-saturated water was poured into the impregnating solution, while the concentration of Zn ions decreased less rapidly than that of Cd ions. The depth profiles of the ZnS-CdS/AI,O,, ZnS(CdS/Al,O,) and CdS(ZnS/Al,O,) catalysts measured by AES are given in fig. 4. Since the sputtered depth could not be precisely estimated, the sputtering time was employed in fig. 4. The composition was expressed by the peak height ratio of Zn/Cd in the corresponding AES spectrum, the1398 Photogeneration of Hydrogen on a Semiconductor Catalyst 0 1 2 3 4 5 6 7 8 irradiation time/h Fig. 2. Activities of the catalysts for photogeneration of hydrogen under the irradiation of u.v.-visible light.0, ZnS-CdS/Al,O,; (>, ZnS/Al,O,; 0, CdS/Al,O,; 0, ZnS/Al,O,- CdS/Al,O,; 0, CdS-ZnS. The amounts of the catalysts employed were 50 mg. - * - Y v I5 1 2 24 coprecipitat ing time/ h Fig. 3. Changes in the concentrations of Zn (0) and Cd (0) ions in the solution during coprecipitation of ZnS and CdS over alumina powder. peaks due to Zn (LMM) and Cd (MNN) being observed at 997 and 376 eV, respectively. The activities of the designed catalysts, ZnS(CdS/Al,O,) and CdS(ZnS/Al,O,), were compared with that of the ZnS-CdS/Al,O, catalyst under illumination by u.v.-visible light (see fig. 5). The ZnS-CdS/Al,O, catalyst showed a high activity for hydrogen generation even under illumination by visible light (A > 440 nm) and the activity was compared with those of the designed catalysts (see fig.6).J. Kobayashi, K. Kitaguchi, H. Tanaka, H. Tsuiki and A . Ueno 1399 2.0 I- - 0 5 10 15 Fig. 4. Depth profiles of Zn and Cd ions in (a) ZnS(CdS/Al,O,), (b) ZnS-CdS/Al,O, and (c) CdS(ZnS/Al,O,). sputtering t ime/m in P 0 1 2 3 4 5 6 7 8 Fig. 5. Activities of ZnS-CdS/Al,O, (O), ZnS(CdS/Al,O,) (0) and CdS(ZnS/Al,O,) (a) catalysts for photogeneration of hydrogen under irradiation by u.v.-visible light. The amounts of catalysts employed were 50 mg. irradiation time/ h1400 Photogeneration of Hydrogen on a Semiconductor Catalyst 0.10 0.08 \ x - 0 0.06 z (4- 0 c u 2 0.04 m irradiation time/h P 0 2 4 6 a Fig. 6. The activities of ZnS-CdS/A120, (O), ZnS(CdS/Al,O,) (0) and CdS(ZnS/Al,O,) (0) catalysts under irradiation by visible light.The amounts of catalysts employed were 50 mg. Fig. 7. Power dependence of the activity of the ZnS-CdS/Al,O, catalyst for photogeneration of hydrogen. The amount of catalyst employed was 50 mg. Fig. 7 shows the power dependence of the activity of ZnS-CdS/Al,O, catalyst under irradiation by u.v.-visible light. The rate of hydrogen production increased as the first order of the incident light power, indicating that the hydrogen was produced through a one-photon absorption mechanism on the present catalyst. By varying the surface area of the alumina support of the ZnS-CdS/Al,O, catalysts, the activity for photogenerationJ . Kobayashi, K. Kitaguchi, H, Tanaka, H. Tsuiki and A. Ueno 1401 0 1 2 3 4 5 6 7 8 Fig. 8. Change in the activity of the ZnS-CdS/Al,O, catalyst for photogeneration of hydrogen with the change in the surface area of the alumina support.0, 400 m2 g-l; 0, 260 m2 g-l. The amounts of catalysts employed were 50 mg. irradiation time/h of hydrogen was significantly modified (shown in fig. 8). The schematic models for ZnS(CdS/Al,O,) and CdS(ZnS/Al,O,) catalysts are shown in fig. 9, where the area of contact between ZnS and CdS particles in the former is apparently higher than that in the latter. Discussion There have been many works on the photocatalytic properties of ZnS,13 and Kakuta et aZ.14 might be the first who reported the enhanced rate of hydrogen generation under the irradiation of u.v.-visible light over ZnS-CdS/Nafion system. Silica-supported ZnS-CdS catalysts also exhibited a rate much higher than the singly supported ZnS and CdS catalysts.' Similar results were obtained for the alumina-supported ZnS-CdS catalyst in the present work.The rate decreased in the following sequence: ZnS- CdS/Al,O, > unsupported ZnS-CdS > ZnS/Al,O, physical mixture with CdS/Al,O, (see fig. 2). Although the amounts of ZnS and CdS in the unsupported catalyst were four times greater than those in the ZnS-CdS/Al,O, catalyst, the rate with the unsupported .catalyst was four times lower than that with the supported one. This may be due, in part, to the particle sizes of ZnS and CdS in the catalysts. As is shown in plate 1, the particles in the unsupported catalyst are > 4 x lo4 A, while the particles of ZnS and CdS in the singly supported catalysts are ca.20 and 200 A, respectively. Since the preparation procedures and the conditions of the ZnS-CdS/Al,O, catalyst were the same as those of the singly supported catalysts, the particle sizes of ZnS and CdS in the ZnS-CdS/A120,1402 Photogeneration of Hydrogen on a Semiconductor Catalyst sample would also be ca. 20 and 200 A, respectively. The formation of a solid solution between ZnS and CdS in the ZnS-CdS/Al,O, catalyst might be one way of interpreting the enhanced rate of hydrogen generation over ZnS-CdS/Al,O,. Since the X-ray diffraction spectrum of the catalyst showed only a small and broad peak at 28 = 28", the structures of the ZnS and CdS particles could not be precisely determined (see fig. 1). Nevertheless, the formation of a solid solution is unlikely in the ZnS-CdS/Al,O, catalyst, since it was shown by U.V.reflection spectroscopy7 that the solid solution was not observed in the ZnS-CdS/SiO, catalyst. However, the question arises as to why the rates of hydrogen generation with the singly supported catalysts and with their physical mixture were so much lower than that with ZnS-CdS/Al,O,, although the sizes of ZnS and CdS particles are the same in these catalysts. This might be explained by the idea that intimate contact between ZnS and CdS particles is necessary for an acceleration of the rate of hydrogen generation under the illumination of u.v.-visible light and that such intimate contact between the particles is much more likely in the ZnS-CdS/AI,O, catalyst than in the physically mixed sample.As may be seen in fig. 3, Cd ions were deposited more rapidly over the alumina surface than Zn ions during the coprecipitation, suggesting that the ZnS-CdS/Al,O, is composed of fine ZnS particles deposited over CdS particles coating the alumina surface. In order to see if this particular structure causes an enhanced rate of hydrogen production, the activities of the designed catalysts, ZnS(CdS/Al,O,) and CdS(ZnS/Al,O,), were compared with that of ZnS-CdS/Al,O,. The absolute amounts of Zn and Cd ions in the vicinity of the surface of the catalyst powders were hard to determine, but the sequential deposition of Zn and Cd ions in the designed catalysts is clearly shown in fig. 4. The particulate structure of the ZnS-CdS/Al,O, catalyst, i.e. comprising a ZnS shell and a CdS core, was also shown, since the depth profile of the ZnS-CdS/Al,O, catalyst was similar to that of ZnS(CdS/Al,O,).The activity of the ZnS(CdS/Al,O,) catalyst was, as expected, the same as that of ZnS-CdS/Al,O, at the stationary state of the reaction and was higher than that of the CdS(ZnS/Al,O,) catalyst (see fig. 6). It is concluded that in order to enhance the rate of hydrogen production, (1) the ZnS and CdS particles must be fine, (2) the ZnS and CdS particles should be in intimate contact with each other and (3) the ZnS particles should be superimposed on the surface of CdS particles first deposited over the Al,O, surface. Similar conclusions have been obtained for a silica-supported ZnS-CdS ~atalyst,~ indicating that with any kind of support this particular structure exhibits an accelerated rate of hydrogen production from water under the irradiation of u.v.-visible light.Why has this particular structure an advantage for hydrogen generation under illumination? Experiments with visible light (A > 440 nm) were carried out in order to answer the question. Since the band gap between the conduction and valence bands of ZnS is 3.7 eV (320 nm) and that of CdS is 2.4 eV (520 nm),15 the visible light employed in this work can only excite the electrons from the valence band of CdS to the conduction band. As clearly shown in fig. 6 , the ZnS-CdS/Al,O, and ZnS(CdS/Al,O,) catalysts again showed high activities for hydrogen production even under the irradiation of visible light. The activity of ZnS-CdS/Al,O, was much higher than that of the singly supported CdS/Al,O, catalyst, suggesting that the interaction between ZnS and CdS particles at their boundary plays an important role in the hydrogen generation.Part of the interaction may be interpreted by electron transfer from CdS to ZnS particles through impurity levels in the band of the ZnS semiconductor, the levels originating from structural deficiencies such as anion vacancies and interstitial cations.16 The electrons excited to the conduction band of CdS by the irradiation of visible light might be transferred to an impurity level in ZnS if the potential of the impurity level is nearly equal to that of the conduction band of CdS. The electrons thus trapped in the impurity level of ZnS might further absorb another photon to be excited to the conduction band of ZnS to react with H+ in the liquid phase for hydrogen production.According to this electron-transfer model, hydrogen will beJ . Kobayashi, K. Kitaguchi, H . Tanaka, H. Tsuiki and A . Ueno 1403 \ ZnS (CdS/Al2O3) A1203’ Fig. 9. Schematic models for the ZnS(CdS/Al,O,) and CdS(ZnS/Al,O,) catalysts. produced through a two-photon absorption mechanism. This is in contrast to the results given in fig. 7, where a one-photon mechanism is suggested for hydrogen production with the alumina-supported ZnS-CdS catalyst. Consequently, the electron-transfer model seems unable to explain the enhanced rate of hydrogen generation with the ZnS- CdS/Al,O, catalyst. Structural models of ZnS(CdS/Al,O, ) and CdS(ZnS/Al,O,) are presented schemat- ically in fig.8. Since the particle size of ZnS is much smaller than that of CdS, the contacting area between ZnS and CdS particles in the ZnS(CdS/Al,O,) catalyst will be larger than that in the CdS(ZnS/Al,O,) catalyst, resulting in the higher rate of hydrogen production over the ZnS(CdS/Al,O,) catalyst. The area of contact between the particles increases with increasing surface area of the alumina support; hence the activity of the ZnS-CdS/Al,O, catalyst increases with increase in the surface area of the support (see Thus we believe that the key to the enhanced rate of hydrogen generation with the supported ZnS-CdS catalyst systems lies in how large is the area of contact between the ZnS and CdS particles, since at the contacting surface the photocatalytic activity of ZnS(CdS/Al,O,) must be the same as that of the CdS(ZnS/Al,O,) catalyst.Only the particular structure with a ZnS shell and a CdS core supplies the large contact area required between ZnS and CdS particles. The problem to be solved is what kind of interactions are in the contact area and how they contribute to the enhanced rate of hydrogen generation under the illumination of light. fig. 9). References 1 2 3 4 5 6 7 8 9 10 11 12 A. B. Ellis, S. W. Kaiser and M. S. Wrighton, J. Am. Chem. SOC., 1976,98, 6855; T. Inoue, T. 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Riehl, Trans. Faraday SOC., 1939, 35, 135. Paper 61873; Received 6th May, 1986