Energetics at the nanoscale: Impacts for geochemistry, the environment, and materials

Nanoparticles are ubiquitous in both natural and synthetic environments, providing much of the chemical reactivity for geochemical, planetary, environmental, and technological processes. However, this reactivity and differences between the bulk and nanoscale are thermodynamically, as well as kinetically, controlled. Energetic effects arising from differences in surface energies of different nanomaterials lead to changes in which phases are thermodynamically stable under given conditions. This results in crossovers in polymorphic stability as a function of particle size and substantial shifts in the positions of dehydration and redox equilibria. Examples of these phenomena in aluminum, cobalt, iron, and manganese oxides are presented, and implications for catalysts, battery materials, and other functional oxides are discussed. A hypothesis is presented that low surface energy and the resulting relatively weak water binding on the surface leads to better function when electrons or ions are transferred at the solid-solution interface.