The Nanocrystalline SnO2–TiO2 System‒Part II: Surface Energies and Thermodynamic Stability

Abstract

The thermodynamic stability of nanocrystalline SnO2–TiO2 solid solutions was studied experimentally. Microcalorimetry of water adsorption revealed a systematic decrease in the surface energy with increasing Ti4+ content in the SnO2-rich compositions, consistent with previous reports of Ti4+ segregation on the surface. The surface energy change was accompanied by an increase in the magnitude of the heat of water adsorption, also indicating a modification of the SnO2 surface by Ti4+. Supporting the water adsorption data, calculations using high-temperature oxide melt solution calorimetry data also suggest a decrease in the interface energies. A thermodynamic analysis showed that the observed surface energy decrease is responsible for an increase in the stability of solid solutions in the nanophase regime. Although a miscibility gap is expected in this system from bulk phase diagrams, the surface energy contribution modifies the bulk trend and promotes extensive solid solutions when the surface area is above a critical value dependent on the surface energy and the bulk enthalpy of mixing.