Vanadia-based equilibrium-thickness amorphous films on anatase (101) surfaces

Nanometer-thick, surficial amorphous films are found to form in a model “monolayer” catalyst system: vanadia on TiO2 anatase (101) surfaces. These films exhibit a self-selecting or “equilibrium” thickness; once a thermodynamic equilibrium is reached, the film thickness, which corresponds to the Gibbsian surface excess of vanadia adsorbates, is independent of synthesis methods, the fraction of secondary vanadia phase, and the heat treatment history. These (multilayer) adsorbate films are largely amorphous (quasiliquid) at subeutectic temperatures, where analogies to premelting and prewetting phenomena are made. Reversible film thickness versus temperature (with a hysteresis loop) is observed and explained from a force-balance model.