Visible light induced photoreduction of heteropolytungstates on colloidal cadmium sulphide semiconductor. I. Reduction of K4SiW12O40 under steady state irradiation

The reduction of the heteropolyanion SiW12O404-was studied under steady state illumination on CdS colloidal dispersions using visible light. SiW12O404- can be reduced both by one electron to SiW12O405- and by two electrons to SiW12O406-. The yield in reduced products is a function of the hole scavenger employed in the following order: COO-≫-propanol >ethylene glycol ≅ ethanol n-propanol>methanol>;n-butanol >t-butanol ≅ no hole scavenger. The two electron reduction product is formed only when COO- and 2-propanol are used as hole scavengers. A mechanism is proposed whereby the reduction of SiW12O404- to SiW12O405- is due to both the semiconductor conduction band electrons and to reducing radicals formed in the oxidation of the hole acceptor, whereas reduction to SiW12O406- is mainly due to radicals. The influence of the adsorptive interaction of the hole scavenger with the CdS surface on the overall redox processes at the interface is discussed. © 1987.