The Origin of Nano-Scale Stability Reversals in Titanium Oxide Minerals

Fine-grained titanium oxide minerals are important in soils, where they affect a variety of geochemical processes. They are also industrially important as catalysts, pigments, food additives, and dielectrics. Recent research has indicated an apparent reversal of thermodynamic stability between TiO2 phases at the nanoscale thought to be caused by an increased contribution of surface energy to the total free energy. Time-resolved X-ray diffraction experiments in which titanium oxides crystallize from aqueous TiCl4 solutions confirm that anatase, a metastable phase, is always the first phase to nucleate. Rutile peaks are observed only minutes after the first appearance of anatase, after which anatase abundance slowly decreases while rutile continues to form. Whole pattern refinement of diffraction data reveals that lattice constants of both phases increase throughout the crystallization process.

Density functional theory (DFT) calculations were employed to model the energetics of nano-sized anatase and rutile using our refined structures. The calculated change in free energy between the incipient, surface-strained nanoparticles and the final microcrystals was only ~ 0.5 kJ/mol for each phase, far less than the 6 kJ/mol difference in free energy of formation between the two minerals needed to account for stability reversal. Surface energy calculations of model 1 and 2 nm nanoparticles did yield lower surface energies for anatase than for rutile by ~ 16 kJ/mol for both particle sizes, but these whole-particle surface energies were much higher than the sum of energies of each particle's constituent crystallographic surfaces. We attribute the excess energy to defects associated with the edges and corners of nanoparticles which are not present on a 2-D periodic surface. This previously unreported edge and corner energy may play a dominant role in the stability reversal of nanocrystalline titanium oxides, as well as other mineral systems susceptible to reversals in phase stability at the nano-scale.