Dynamics and Structure of Hydration Water on Rutile and Cassiterite Nanopowders Studied by Quasielastic Neutron Scattering and Molecular Dynamics Simulations

Abstract

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 (TiO 2 ) and cassiterite (SnO 2 ) 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. (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 SnO 2 compared to TiO 2 . In TiO 2 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 TiO 2 exhibited faster dynamics with Arrhenius temperature dependence. On the other hand, the slow diffusion component in L 2 hydration water on SnO 2 , 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.

Diffusion and reaction pathways of water near fully hydrated TiO2 surfaces from ab initio molecular dynamics

“…These simulations have enabled the determination of the full hydration layer on TiO 2 from first principles. Our calculations corroborate earlier theoretical results on TiO 2 surface reactivity 43,68 and also explain NMR and neutron scattering data from hydrated TiO 2 nanoparticles 63 and surfaces 64,65 in terms of near-surface waters with varying mobilities. We present a general surface model for TiO 2 , where hydration layers are classified as "hard" or "soft" depending on the orientational freedom of the water molecules.…”

“…The microscopic model of water structure and dynamics shown here is in agreement with the experimental interpretation of increasingly mobile waters from an inner core of irreversibly adsorbed molecules on TiO 2 nanoparticles 63 and surfaces. 64,65 E. Water reaction pathways on fully hydrated TiO 2 surfaces Theoretical efforts to calculate water dissociation on TiO 2 surfaces have been driven by potential applications in renewable energy and focalized around anatase surfaces because of their higher photocatalytic activity compared to rutile surfaces. 120 The high reactivity of anatase (001) is undisputed, 41,68 while both dissociation and molecular adsorption have been reported on anatase (101).…”

“…[24][25][26][27][28]62 Although the exact structure of the fully hydrated TiO 2 -water interface is still not fully resolved, 32 the surface interactions are strong enough to induce three ordered solvent layers (∼10 Å boundary layer). 10,[63][64][65] Here, we use ab initio molecular dynamics (AIMD) simulations to determine the structure of the hydration layer for six different TiO 2 surfaces at full water coverage (Fig. 1).…”

Insight on Tricalcium Silicate Hydration and Dissolution Mechanism from Molecular Simulations

“…These three water regimens were also observed for Rutile and Cassiterite surfaces both from MD simulations and quasi-elastic neutron-scattering experiments. 58 …”

Understanding the interface between silicon-based materials and water: Molecular-dynamics exploration of infrared spectra

“…The dynamical behaviour of interfacial water with TiO 2 and SiO 2 has been probed by various spectroscopic methods [11][12][13][14][15] including neutron scattering (inelastic, quasielastic and backscattering) and infrared spectroscopy. Attenuated Total Reflection-Infrared (ATR-IR) spectroscopy has been used to monitor the growth of different water layers on a SiO 2 surface.…”