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 photoexcitation
of rutile TiO2(110) single crystal, was investigated. Photocatalytic
reaction of gas phase ethanol (a hole scavenger) on TiO2(110) and Aux/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]hν→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
