824
J. Phys. Chem. 1986, 90, 824-834
Photochemical Hydrogen Production with Platinized Suspensions of Cadmium Sulfide and Cadmium Zinc Sulfide Modified by Silver Sulfide Jean-Franqois Reber*+ and Mil6s Rusek Ciba-Geigy, Central Research Laboratories, CH-4002 Basel, Switzerland (Received: May 14, 1985)
An efficient hydrogen production can be achieved by irradiating suspensions of platinized CdS in solutions of S2- and/or S032-ions. However, the photocatalytic activity of CdS powders strongly depends on their specific surface area. Only CdS photocatalysts with very low (C6.7 m2/g) specific surface areas produce hydrogen with a significant rate. CdS powders precipitated by classical methods have specific surface areas larger than 10 m2/g and are consequently almost inactive. However, coprecipitation of CdS with about 0.5-3 wt % silver sulfide or surface modification of CdS with a large specific surface area by silver ions permitted preparation of very active platinized photocatalysts for the photochemical production of hydrogen from solutions containing S2-ions as hole scavenger. The enhancement of activity is not limited to the own absorption range of CdS, but also results from a significant extension of the spectral response up to about 620 nm. Further improvement of the photoactivity can be achieved by doping the Ag2S activated CdS powders with zinc sulfide. The rate of hydrogen formation in solutions containing S2-ions decreases with irradiation time due to the formation of disulfide ions which compete with the proton reduction. Addition of sulfite ions, which efficiently suppress the disulfide formation, allows hydrogen to evolve at a higher rate. In solutions containing both S2-and SO,2- ions, hydrogen is generated concomitantly with thiosulfate ions with a quantum yield of 0.37 with the most active photocatalyst of cadmium zinc sulfide containing Ag2S. However, these photocatalysts are less stable than platinized powders of pure CdS of low specific surface area.
Introduction During the past few years, photoelectrochemical processes at semiconductor-electrolyte interfaces found new interest because of their possible application in the conversion of solar energy to electrical or chemical energy. Since only the light absorbed in a thin layer close to the semiconductor-electrolyte interface can initiate chemical reactions, large surface areas, like those provided by powders, are desirable. Various endothermic reactions with semiconductor powders were first achieved with T i 0 2 suspension~.'-~These early results have stimulated many studies of the photochemical production of hydrogen with suspensions of various CdS,'2-1s ZnS,I9 semiconductors, such as Ti02,4-9SrTi03,S.10~11 etc. For an efficient production of hydrogen by sunlight, a sufficiently negative flat-band potential and good absorption properties in the visible (smaller band gap than Ti02) are essential. In both respects cadmium sulfide is a promising material. Unfortunately, this semiconductor is not stable in aqueous solutions under irradiation and undergoes anodic dissolution leading to the formation of sulfur and/or sulfate ions.2w22 However, the pbssibility of stabilizing CdS by aqueous solutions of reducing agents acting as hole scavengers, such as S2-,S032-,or S2032- ions, has been rep~rted.~~-~~ Exploiting the stabilizing properties of these ions, we have achieved an efficient hydrogen production by irradiating suspensions of metallized CdS powders in solutions containing S2or S032-ions, as well as mixtures of S2- ions with S032-or hypophosphite ions.17s27.ZsHowever, these results have been obtained with CdS powders sf a definite quality and cannot be transposed quantitatively to other cadmium sulfide samples, because the photocatalytic behavior of CdS powders strongly depends on their physicochemical properties. Although this fact has been recognized by other author^,^^,^^ no result pertaining to the influence of the quality of CdS powders on their photoactivity has been published heretofore. In this paper, results concerning the influence of the physicochemical properties of CdS powders on their photocatalytic behavior are reported. Moreover, it is shown that inactive CdS powders with unsuitable properties can be activated by deposition of silver sulfide and that active photocatalysts can also be obtained by coprecipitation of cadmium sulfide with silver sulfide. The photocatalytic behavior of even more active photocatalysts consisting of platinized powders of cadmium sulfide simultaneously 'Present address: Ilford AG, CH-1701 Fribourg, Switzerland.
0022-3654/86/2090-0824$01.50/0
doped with zinc and silver sulfides have been investigated. The preparation of CdS photocatalysts containing silver sulfide and
(1) Freund, T.; Gomes, W. P. In "Catalysis Reviews"; Heinemann, H., Ed.; Marcel Dekker: New York, 1969; Vol. 3, pp 22-26. (2) Krasnovsky, A. A,; Brin, G. P.; Nikandrov, V. V. Dokl. Akad. Nauk SSSR 1975, 220, 1214; 1976, 229, 990. (3) Frank, S.N.; Bard, A. J. J . Phys. Chem. 1977, 81, 1484-1488. (4) Bard, A. J. J . Photochem. 1979, 10, 59-75. ( 5 ) Lehn, J.-M.; Sauvage, J.-P.; Ziessel, R. N o w . J . Chim. 1981, 5 , 29 1-295. (6) Pichat, P.; Herrmann, J.-M.; Disdier, J.; Courbon, H.; Mozzanega, M.-N. N o w . J . Chim. 1981, 5, 627-636. (7) Sakata, T.; Kawai, T. Chem. Phys. Lett. 1981, 80, 341-344. (8) Oosawa, Y . J . Chem. SOC.,Chem. Commun. 1982, 221-222. (9) Mills, A. J . Chem. SOC.,Chem. Commun. 1982, 367-368. (10) Lehn, J.-M.; Sauvage, J.-P.; Ziessel, R. Nouu. J . Chim. 1980, 4, 623-627. (1 1) Domen, K.; Naito, S.;Onishi, T.; Tamaru, K. Chem. Phys. Lett. 1982, 92, 433-434. (12) Darwent, J. R.; Porter, G. J . Chem. Soc., Chem. Commun. 1981, 145-146. (13) Darwent, J. R. J . Chem. Soc., Faraday Trans. 2 1981, 77, 1703-1709. (14) Harbour, J. R.; Wolkow, R.; Hair, M. L. J . Phys. Chem. 1981, 85, 4026-4029. (15) Kalyanasundaram, K.; Borgarello, E.; Duonghong, D.; Gratzel, M. Angew. Chem. 1981, 93, 1012-1013. (16) Borgarello, E.; Kalyanasundaram, K.; Gratzel, M.; Pelizetti, E. Hela Chim. Acta 1982,65, 243-248. (17) Biihler, N.; Reber, J.-F.; Meier, K.; Rusek, M. European Patent Application, Publication No. 58136, 1982. (18) Reber, J.-F.; Meier, K.; Buhler, N. "Book of Abstracts", 4th International Conference on Photochemical Conversion and Storage of Solar Energy, Jerusalem, Israel, Aug. 8-13, 1982; Weizmann Science Press: Jerusalem, Israel, 1982-1983; 252-254. (19) Reber, J.-F.; Meier, K. J . Phys. Chem. 1984, 88, 5903-5913. (20) Lal, P.; Ganguly, P. B. J . Indian Chem. Soc. 1929, 6, 547-556. (21) Williams, R. J . Chem. Phys. 1960, 32, 1505-1514. (22) Gerischer, M.; Meyer, E. 2.Phys. Chem. (N.F.) 1971, 74, 302-318. (23) Inoue, T.; Watanabe, T.; Fujishima, A,; Honda, K.; Kohayakawa, K. J . Electrochem. SOC.1971, 124, 719-722. (24) Ellis, A. B.; Kaiser, S. W.; Wrighton, M. S . J . Am. Chem. SOC.1976, 98, 6855-6866. (25) Minoura, H.; Oki, T.; Tsuiki, M. Chem. Lett. 1976, 1279-1282. (26) Minoura, H.; Tsuiki, M. Electrochim. Acta 1978, 23, 1377-1382. (27) Reber, J.-F.; Buhler, N.; Meier, K.; Rusek, M. European Patent Application, Publication No. 100 299, 1984. (28) Biihler, N.; Meier, K.; Reber, J.-F. J . Phys. Chem. 1984, 88, 3261-3268.
0 1986 American Chemical Society
The Journal of Physical Chemistry, Vol. 90, No. 5, 1986 825
Photochemical Hydrogen Production TABLE I: Specific Surface Area, Specific Pore Volume, Silver Concentration, and Impurity Content of the Samples of Cadmium Sulfide Modified with Silver Sulfide Used in Figure 9
sample
specific surface area, m2/g
I
65.7
I1
* 1.3
specific pore volume, mL/g
Ag concn, ppm
0.368
lo0 m2/g) or very low (
