Molecular-Like Equilibrium Behavior of Anatase Nanoparticles in Compressed Hydrothermal Conditions

Nanocrystals are intermediate in size between molecules and micron-scale crystallites. They usually exhibit size-dependent properties and behaviors that are closer to the latter than the former. However, in this work, we found that small titania nanoparticles present molecular-like phase equilibrium behavior under compressed hydrothermal conditions, as evidenced by the fact that both the phase contents and particle sizes of involved titania nanoparticles can increase or decrease in response to altered temperature and/or pressure applied to shift a phase equilibrium. Thermodynamic analysis showed that the phase content of a solid nanophase contributes to the free energy change of the phase equilibrium, in contrast to the textbook knowledge that the thermodynamic activity of a pure solid phase equals one and as such the phase content of a pure solid phase makes no contribution to the equilibrium. This result suggests that much more frequent and stronger collisions among nanoparticles at high-T&P conditions can enhance the energy transfer among individual nanoparticles in a fluid, making them behave collectively more like solute molecules in a solution. This finding renews our understanding of the physical-chemical properties of nanoparticles with strong interparticle interactions.