Photo-Chemical Energy Conversion by Pillared Hydrotalcite

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CdS and CdS-ZnS mixed semiconductor particles were incorporated into the interlayer of hydrotalcite by the chemical reactions between Cd(edta)^<2-> and S^<2-> in the interlayer. The incorporated particles seemed to be small as less than 0.4 nm in thickness. The band gap energies of CdS and sequentially precipitated CdS followed by ZnS in the interlayer were slightly larger than that of normal-crystalline CdS. On the other hand, the band gap energy of CdS/ZnS mixture coprecipitated in the interlayer was almost equal to the average value of those of normal-crystalline CdS and ZnS. The CdS and CdS/ZnS mixture incorporated into hydrotalcite were capable of efficient hydrogen evolution by irradiation with visible light in the presence of Na_2S and/or Na_2SO_3 as a sacrificial donor. The hydrogen production activities of the catalyst incorporated in hydrotalcite were in the order of sequentially precipitated CdS followed by ZnS > simultaneously precipitated CdS-ZnS mixture >> CdS. Mass transfer problems of the products through the interlayer restricted the efficiency of the semiconductor incorporated into hydrotalcite for the visible light induced hydrogen evolution. Almost the same amounts of S_2O_6^<2-> and SO_4^<2-> were formed by the photochemical oxidation of SO_3^<2-> aqueous solution with unsupported CdS/ZnS, but that of S_2O_6^<2-> produced in the same reaction with CdS/ZnS incorporated into hydrotalcite significantly decreased.