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Title: Hydration Water Dynamics and Structure and Rutile and Cassiterite Nano-powders Studied by Quasielastic Neutron Scattering and Molecular Dynamics Simulations

Authors:
 [1];  [2];  [1];  [1];  [1];  [3];  [1];  [4];  [5]
  1. ORNL
  2. Vanderbilt University
  3. {Larry} M [ORNL
  4. National Institute of Standards and Technology (NIST)
  5. University of Maryland
Publication Date:
Research Org.:
Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States)
Sponsoring Org.:
USDOE Office of Science (SC)
OSTI Identifier:
1081586
DOE Contract Number:
DE-AC05-00OR22725
Resource Type:
Journal Article
Resource Relation:
Journal Name: Journal of Physical Chemistry C; Journal Volume: 111; Journal Issue: 11
Country of Publication:
United States
Language:
English

Citation Formats

Mamantov, Gleb M, Vlcek, L., Wesolowski, David J, Cummings, Peter T, Wang, Wei, Anovitz, Lawrence, Rosenqvist, Jorgen K, Brown, C H, and Sakai, V. Garcia. Hydration Water Dynamics and Structure and Rutile and Cassiterite Nano-powders Studied by Quasielastic Neutron Scattering and Molecular Dynamics Simulations. United States: N. p., 2007. Web. doi:10.1021/jp067242r.
Mamantov, Gleb M, Vlcek, L., Wesolowski, David J, Cummings, Peter T, Wang, Wei, Anovitz, Lawrence, Rosenqvist, Jorgen K, Brown, C H, & Sakai, V. Garcia. Hydration Water Dynamics and Structure and Rutile and Cassiterite Nano-powders Studied by Quasielastic Neutron Scattering and Molecular Dynamics Simulations. United States. doi:10.1021/jp067242r.
Mamantov, Gleb M, Vlcek, L., Wesolowski, David J, Cummings, Peter T, Wang, Wei, Anovitz, Lawrence, Rosenqvist, Jorgen K, Brown, C H, and Sakai, V. Garcia. Mon . "Hydration Water Dynamics and Structure and Rutile and Cassiterite Nano-powders Studied by Quasielastic Neutron Scattering and Molecular Dynamics Simulations". United States. doi:10.1021/jp067242r.
@article{osti_1081586,
title = {Hydration Water Dynamics and Structure and Rutile and Cassiterite Nano-powders Studied by Quasielastic Neutron Scattering and Molecular Dynamics Simulations},
author = {Mamantov, Gleb M and Vlcek, L. and Wesolowski, David J and Cummings, Peter T and Wang, Wei and Anovitz, Lawrence and Rosenqvist, Jorgen K and Brown, C H and Sakai, V. Garcia},
abstractNote = {},
doi = {10.1021/jp067242r},
journal = {Journal of Physical Chemistry C},
number = 11,
volume = 111,
place = {United States},
year = {Mon Jan 01 00:00:00 EST 2007},
month = {Mon Jan 01 00:00:00 EST 2007}
}
  • 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 nano-powder 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 nano-powders had similar levels of hydration: about 3.5 (OH/H2O) molecules per Ti2O4 surface structural unitmore » 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 L1, L2, and L3, where the L1 species are either associated water molecules or dissociated hydroxyl groups in direct contact with the surface, L2 water molecules are hydrogen bonded to L1 and structural oxygen atoms at the surface, and L3 water molecules are more weakly bound. At the hydration levels studied, L3 is incomplete compared with axial oxygen density profiles of bulk SPC/E water in contact with these surfaces, but the structure and dynamics of L1 "L3 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 L2 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 L3 water molecules in TiO2 exhibited faster dynamics with Arrhenius temperature dependence. On the other hand, the slow diffusion component in L2 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 L2 and L3 hydration layers.« less
  • 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{sub 2}) and cassiterite (SnO{sub 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 3.5 (OH/H{sub 2}O) molecules per Ti{submore » 2}O{sub 4} surface structural unit of TiO{sub 2} and about 4.0 (OH/H{sub 2}O) molecules per Sn{sub 2}O{sub 4} surface unit of SnO{sub 2}. 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{sub 1}, L{sub 2}, and L{sub 3}, where the L{sub 1} species are either associated water molecules or dissociated hydroxyl groups in direct contact with the surface, L{sub 2} water molecules are hydrogen bonded to L{sub 1} and structural oxygen atoms at the surface, and L{sub 3} water molecules are more weakly bound. At the hydration levels studied, L{sub 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{sub 1}-L{sub 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{sub 2} compared to TiO{sub 2}. In TiO{sub 2} hydration water, the more strongly bound L{sub 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{sub 3} water molecules in TiO{sub 2} exhibited faster dynamics with Arrhenius temperature dependence. On the other hand, the slow diffusion component in L{sub 2} hydration water on SnO{sub 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{sub 2} and L{sub 3} hydration layers.« less
  • Quasielastic neutron scattering (QENS) was used to investigate the diffusion dynamics of hydration water on the surface of rutile (TiO{sub 2}) nanopowder. The dynamics measurements utilizing two inelastic instruments, a backscattering spectrometer and a disk chopper spectrometer, probed the fast, intermediate, and slow motions of the water molecules on the time scale of picoseconds to more than a nanosecond. We employed a model-independent analysis of the data collected at each value of the scattering momentum transfer to investigate the temperature dependence of several diffusion components. All of the probed components were present in the studied temperature range of 230-320 K,more » providing, at a first sight, no evidence of discontinuity in the hydration water dynamics. However, a qualitative change in the elastic scattering between 240 and 250 K suggested a surface freezing-melting transition, when the motions that were localized at lower temperatures became delocalized at higher temperatures. On the basis of our previous molecular dynamics simulations of this system, we argue that interpretation of QENS data from such a complex interfacial system requires at least qualitative input from simulations, particularly when comparing results from spectrometers with very different energy resolutions and dynamic ranges.« less
  • The high energy resolution, coupled with the wide dynamic range, of the new backscattering spectrometer (BASIS) at the Spallation Neutron Source, Oak Ridge National Laboratory, has made it possible to investigate the diffusion dynamics of hydration water on the surface of rutile (TiO2) nano-powder down to a temperature of 195 K. We attribute the dynamics measured on the BASIS on the time scale of tens to hundreds of picoseconds to the mobility of the outer hydration water layers. The data obtained on the BASIS and in a previous study using the backscattering and disk-chopper spectrometers at the NIST Center formore » Neutron Research are coupled with molecular dynamics simulations extending to 50 nanoseconds. The results suggest that the scattering experiments probe two types of molecular motion in the surface layers, namely (1) a fast component that involves dynamics of the outermost hydration layer with unsaturated hydrogen bonds and localized motions of all water molecules and (2) a slower component, representing translational jumps of the fully hydrogen-bonded water molecules. The temperature dependence of the relaxation times associated with the fast dynamic component remains Arrhenius down to at least 195 K, whereas the slow translational component shows a dynamic transition to non-Arrhenius behavior above about 210K. Thus, an Arrhenius-type behavior of the fast dynamic component extends below the temperature of the dynamic transition in the slow translational component. We suggest that the qualitative difference in the character of the temperature dependence between the slow and fast component may be due to the fact that the latter involves motions that require breaking fewer hydrogen bonds.« less