Electron Transfer From a Semiconductor to a Metal and Its Implication on Photocatalysis for Hydrogen Production

Charge transfer from or to a metal deposited on an oxide semiconductor is central to photocatalysis. To probe into this phenomenon, the effect of gold coverage on the chemical state of Ti cations, upon photo-excitation of rutile TiO2 (110) single crystal, was investigated. Photo-catalytic reaction of gas phase ethanol (a hole scavenger) on TiO2 (110) and Au-x/TiO2(110) resulted in the formation of Ti3+ cations. Increasing the Au coverage led to a gradual decrease of these Ti3+ cations. Under the investigated reaction condition, the "quasi" total consumption of these reduced states was found at a ratio of Au atoms to reacted Ti3+ cations close to one: [Au]/[Ti+3](h nu)-> 1; this corresponded to about 0.50 at. % of Au/TiO2. The relationship, which is similar to that of hydrogen production rates, obtained on model and practical photocatalytic systems, suggests that the slow reaction rates, generally observed in photocatalysis, are intrinsic to the metal-semiconductor properties.