Effects of precipitation, liquid formation, and intervalence charge transfer on the properties and photocatalytic performance of cobalt- or vanadium-doped TiO2 thin films

Abstract

Anatase thin films were prepared by sol-gel, spin coated on glass, and annealed (450 °C/2 h). The effects of cobalt and vanadium doping (0.05–1.00 mol%) on the mineralogical (GAXRD, Raman), nanostructural (AFM), optical (UV-Vis), and chemical (XPS) properties were considered. A new perspective of the role of ionic size effects on substitutional (Co2+) and interstitial (V3+) solid solubility in the elongated TiO6 octahedron and the consequent roles of lattice deformation and charge compensation is presented. The role of intervalence charge transfer (transient, permanent) and the effects of electronegativity and electron affinity on the presence and absence of ions of different valence states also are considered, explaining the detection of nonequilibrium V4+ and the nondetection of Ti3+. The data are interpreted according to a model for nanostructural evolution during annealing, which allows interpretation of the film thickness and roughness, grain and crystallite sizes, and band gap as a function of dopant concentration. The detection of high levels of contamination from the glass substrates, which decreased during aqueous methylene blue (MB) testing (≤24 h), indicates the likelihood of mass transport by grain boundary diffusion of these ions (Na+, Mg2+, Ca2+, Si4+). This leaching impacted on the loss of dopant ions precipitated on the grain surfaces following the achievement of saturation solubility, as suggested by the calculation of the relevant speciation and Pourbaix diagrams. The photocatalytic performance by MB testing revealed that Co (precipitated) had a deleterious effect at all levels while V (dissolved) showed slight improvements over undoped TiO2 at most levels.