PHOTOELECTROCHEMISTRY IN SEMICONDUCTOR PARTICULATE SYSTEMS .17. PHOTOSENSITIZATION OF LARGE-BANDGAP SEMICONDUCTORS - CHARGE INJECTION FROM TRIPLET EXCITED THIONINE INTO ZNO COLLOIDS

The kinetics and mechanism of a diffusion-controlled interfacial charge transfer between excited triplet thionine and ZnO colloids in ethanol have been elucidated with laser flash photolysis. The singlet excited state of thionine, which has a lifetime of 450 ps in ethanol, did not participate in the charge injection process. However, the triplet excited state of thionine was quenched by ZnO colloids. The triplet lifetime (tau(T)) decreased from 88 to 15-mu-s with increasing ZnO colloid concentration. The maximum charge injection efficiency (phi(TH.2+)) which was measured from the yield of dye cation radical was 0.1. The increase in the rate constant for reverse electron transfer (k(r)) at high ZnO concentration resulted in a small decrease in charge-transfer efficiency. The dependence of tau(T), phi(TH.2+), and k(r) on the concentration of ZnO is described.