Multivalence Charge Transfer in Doped and Codoped Photocatalytic TiO2

The present work reports data for the mineralogical and chemical properties of anatase thin films individually doped or codoped with chromium and vanadium, fabricated by sol gel spin coating on glass substrates and annealing at 450 degrees C for 2 h. X-ray photoelectron spectroscopy data indicated the presence of Ti4+, Ti3+, Cr3+, and possibly Cr4+ in the Cr-doped thin films; Ti4+, Ti3+, V3+, V4+ and possibly V6+ in the V-doped thin films; and Ti4+ Cr3+, Cr4+, V3+, V4+ and possibly V6+ in the codoped thin films. While the thermodynamically stable valences Ti4+, Cr3+ and V5+ would be expected to have formed, the presence of the nonequilibrium valences Ti4+, Cr3+, V3+, and V4+ is considered to have resulted from intervalence charge transfer for the Cr-doped and V-doped systems but from multivalence charge transfer (MVCT) for the codoped system. The latter phenomenon, which is introduced as a new conceptual term, describes the nature of the mutual exchange of electrons during valence changes of both dopant (Cr, V) and matrix (Ti) ions during annealing. In the present case, MVCT appears to be a transient metastable condition that acts during annealing, but subsequent UV irradiation can alter its effects.