Photochemistry of semiconductor colloids. 22. Electron ejection from illuminated cadmium sulfide into attached titanium and zinc oxide particles

Abstract

Colloidal solutions of CdS containing colloidal TiO, or ZnO were illuminated with visible light. The fluorescence of CdS (decay time -50 ns) was quenched by Ti02, several Ti02 particles being required per CdS particle. The rate of photoanodic corrosion in aerated solution was drastically increased in the presence of Ti02 In deaerated CdS solutions containing methanol and Cd2+ ions, cadmium metal was formed when Ti02 was present. Methyl viologen was reduced with a quantum yield of close to one, while it reacted about ten times more slowly in the absence of Ti02 These effects are explained in terms of improved charge separation by rapid electron injection from illuminated CdS into the conduction band of attached Ti02 particles. Electron injection into ZnO was less efficient and occurred only in the case of Q-CdS particles (very small particles having a greater band gap). The injected electrons caused a blue shift of the absorption threshold of ZnO. The photocatalytic action of colloidal or suspended semicon- ductor particles is based on the generation of electrons and positive holes which rapidly move to the surface of the particles and initiate redox processes. The efficiency of charge separation is often increased by contacting the semiconductor particle with a metal or another semiconductor. Typical examples are platinized ti- tanium dioxide' and cadmium sulfide2 as well as Ru02-covered Ti02.3 Serpone et al. reported a few years ago that H2 was formed from H2S on CdS powder illuminated with visible light in aqueous solution and that the yield was slightly increased in the presence of Ti02 powder! The effect was explained by an improved charge separation due to electron transfer from the illuminated CdS particles into the conduction band of the Ti02 particles. The increase in yield was only 20%, Le., little above the increase which could be explained by more efficient light absorption of CdS due to the increased internal light scattering by the TiO, additive. In the present paper, experiments with transparent colloidal solutions of CdS containing colloidal Ti02 or ZnO as additives are described. Efficient electron injection from the excited CdS part of the sandwich colloids to the Ti02 or ZnO part was observed with three methods of observation: (1) With use of a CdS colloid that fluoresces with a high quantum yield, the quenching of the fluorescence by added TiO, or ZnO was studied. (2) Redox processes, such as the reduction of excess Cd2+ ions and of methyl viologen and the photoanodic dissolution of CdS, were initiated by visible light illumination and the influence of added Ti02 investigated. (3) In the case of ZnO as additive, the electron injection was accompanied by the typical changes in the absorption spectrum of ZnO which have recently been observed in other experiments on the deposition of excess electrons on small semiconductor particles.s-6