Water Diffusion on TiO2 Anatase Surface

Compatibility between biological molecules and inorganic materials, such as crystalline metal oxides, is strongly dependent on the selectivity properties and the adhesion processes at the interface between the two systems. Among the many different aspects that affect the adsorption processes of peptides or proteins onto inorganic surfaces, such as the charge state of the amino acids, the peptide 3D structure, the surface roughness, the presence of vacancies or defects on and below the surface, a key role is certainly played by the water solvent whose molecules mediate the interaction. Then the surface hydration pattern may strongly affect the adsorption behavior of biological molecules. For the particular case of (101) anatase TiO2 surface that has a fundamental importance in the interaction of biocompatible nano-devices with biological environment, it was shown, both theoretically and experimentally, that various hydration patterns are close in energy and that the water molecules are mobile at as low temperature values as 190 K. Then it is important to understand the dynamical behavior of first hydration layer of the (101) anatase surface. As a first approach to this problem, density functional calculations are used to investigate water diffusion on the (101) anatase TiO2 surface by sampling the potential energy surface of water molecules of the first hydration layer thus calculating the water molecule migration energy along some relevant diffusion paths on the (101) surface. The measured activation energy of water migration seems in contrast with the observed surface mobility of the water molecules that, as a consequence could be explained invoking a strong role of the entropic term in the context of the transition state theory.