Dynamics and structure of hydration water on rutile and cassiterite nanopowders studied by quasielastic neutron scattering and molecular dynamics simulations

Quasielastic neutron scattering (QENS) experiments carried out using time-of-flight and backscattering neutron spectrometers with widely different energy resolution and dynamic range revealed the diffusion dynamics of hydration water in nanopowder rutile (TiO2) and cassiterite (SnO2) that possess the rutile crystal structure with the (110) crystal face predominant on the surface. These isostructural oxides differ in their bulk dielectric constants, metal atom electronegativities, and lattice spacings, which may all contribute to differences in the structure and dynamics of sorbed water. When hydrated under ambient conditions, the nanopowders had similar levels of hydration: about 3.5 (OH/H2O) molecules per Ti2O4 surface structural unit of TiO2 and about 4.0 (OH/H2O) molecules per Sn2O4 surface unit of SnO2. Ab initio optimized classical molecular dynamics (MD) simulations of the (110) surfaces in contact with SPC/E water at these levels of hydration indicate three structurally distinct sorbed water layers L-1, L-2, and L-3, where the L-1 species are either associated water molecules or dissociated hydroxyl groups in direct contact with the surface, L-2 water molecules are hydrogen bonded to L-1 and structural oxygen atoms at the surface, and L-3 water molecules are more weakly bound. At the hydration levels studied, L-3 is incomplete compared with axial oxygen density profiles of bulk SPC/E water in contact with these surfaces, but the structure and dynamics of L-1-L-3 species are remarkably similar at full and reduced water coverage. Three hydration water diffusion components, on the time scale of a picosecond, tens of picoseconds, and a nanosecond could be extracted from the QENS spectra of both oxides. However, the spectral weight of the faster components was significantly lower for SnO2 compared to TiO2. In TiO2 hydration water, the more strongly bound L-2 water molecules exhibited slow (on the time scale of a nanosecond) dynamics characterized by super-Arrhenius, "fragile" behavior above 220 K and the dynamic transition to Arrhenius, "strong" behavior at lower temperatures. The more loosely bound L-3 water molecules in TiO2 exhibited faster dynamics with Arrhenius temperature dependence. On the other hand, the slow diffusion component in L-2 hydration water on SnO2, also on the time scale of a nanosecond, showed little evidence of super-Arrhenius behavior or the "fragile"-to-"strong" transition. This observation demonstrates that the occurrence of super-Arrhenius dynamic behavior in surface water is sensitive to the strength of interaction of the water molecules with the surface and the distribution of surface water molecules among the different hydration layers. Analysis of energy transfer spectra generated from the molecular dynamics simulations shows fast and intermediate dynamics in good agreement with the QENS time-of-flight results. Also demonstrated by the simulation is the fast (compared to 1 ns) exchange between the water molecules of the L-2 and L-3 hydration layers.