Effect of Intervalence Charge Transfer on Photocatalytic Performance of Cobalt- and Vanadium-Codoped TiO₂ Thin Films

Co- and V-codoped TiO₂ thin films were prepared by sol–gel, spin coated on glass substrates, and annealed at 450 °C for 2 h. The data suggest substitutional solid solution formation in the anatase films. Macroscopic roughening at the highest dopant levels was attributed to oversaturation, precipitation, and grain boundary pinning. Methylene blue decomposition correlated directly with the grain size and this was attributed to the associated changes in surface area and number of active sites as well as partial amorphisation.

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.