Calorimetric studies of nanomaterials, using oxide melt solution calorimetry and water vapor adsorption calorimetry, have produced a wealth of data on the energetics of phase transformation and the surface energies of hydrated and dehydrated surfaces of nanoparticulate oxides. Competition between surface energy and phase transformation energy lead to crossovers in phase stability at the nanoscale. The surface energies of polymorphs metastable in the bulk are smaller than those of the stable polymorph and decrease with increasing metastability, regardless of the sign of the density difference. Wet surfaces have smaller surface energies than dry ones. Oxyhydroxides have smaller surface energies than oxides. In ZnO, for which nanoparticles of different morphologies can be prepared, surface energy increases in the order nanoparticles, nanorods, nanoneedles, reflecting the predominance of different surface planes. Very recently, the interfacial energy of a dense nanoceramic sample of yttria stabilized zirconia (YSZ) has been measured. These results and systematic trends provide benchmarks for theoretical calculations.
