Phase stability and transformation in titania nanoparticles in aqueous solutions dominated by surface energy

The surface free energy of small particles in an aqueous solution consists of the electrostatic energy of charged surfaces and the interfacial energy. For nanoparticles in an aqueous solution, the two terms can be modified by solution chemistry and be manipulated to control phase stability and transformation kinetics. Here we show that the phase stability of titania (TiO2) nanoparticles strongly depends on the solution pH. At small sizes, rutile is stabilized relative to anatase in very acidic solutions, whereas in very basic solutions anatase is stabilized relative to rutile and brookite. Rutile is the stable phase at large particle sizes regardless of pH. These results indicate that the activity of potential determining ions (protons or hydroxyl groups) is a factor that can determine the phase stability of nanoparticulate titania in aqueous solutions at pH values far from the point of zero charge of titania. The phase transformation proceeds via a dissolution-precipitation mechanism under hydrothermal conditions.