Effect of intervalence charge transfer on photocatalytic performance of cobalt- and vanadium-codoped TiO2 thin films

X-ray photoelectron spectroscopy data revealed the presence of Ti4+, Co2+, possibly Co3+, V3+, and V4+, where the three clearly detected dopant valences were present as thermodynamically non-equilibrium valences under the annealing conditions used. The present work introduces the concept of intervalence charge transfer to explain the presence of these non-equilibrium species. The present work also considers briefly the potential roles of crystal field stabilization and defect equilibria relative to intervalence charge transfer.