Nanoscale surficial films and a surface transition in V2O5–TiO2-based ternary oxide systems

Nanoscale, vanadia-based, quasi-liquid films of self-selecting (equilibrium) thickness were observed on TiO2 surfaces in six ternary oxide systems (Ti–V–X–O; X = P, Na, K, Nb, Mo or W). It is demonstrated that the film appearance and thickness could be tailored via co-doping or changing the equilibration temperature. Furthermore, the observed discontinuous changes in film thickness, hysteresis and bimodal thickness distributions indicate a first-order monolayer-to-multilayer adsorption transition, which is interpreted as a coupled prewetting and premelting transition. The film thickness and stability are measured as functions of equilibration temperature, anneal time, thermal treatment history, co-doping, overall composition and TiO2 phase and orientation. The characterization of more than 850 independent films represents the most systematic measurement of similar interfacial films to date, providing insights into the formation mechanisms and the thermodynamic stability of equilibrium-thickness surficial films and analogous intergranular films.