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Title: Water Dynamics in Nanoporous Silica: Ultrafast Vibrational Spectroscopy and Molecular Dynamics Simulations

Abstract

Nanoporous silica materials are important in catalysis, energy, and materials applications in which water is an essential component. System performance is intimately connected to the water dynamics occurring in the confined environment. However, the dynamics and associated structures of water in mesoporous silica are challenging to measure or predict. Here, water dynamics subject to nanoscale confinement are examined via the ultrafast infrared spectroscopy of selenocyanate (SeCN -) dissolved in the hydrated ~2.4 nm silica nanopores of MCM41. Polarization selective pump-probe and two-dimensional infrared measurements on the CN stretching mode of SeCN - are used to probe the effect of confinement on orientational relaxation and spectral diffusion dynamics. The dynamics of SeCN - provide information on water hydrogen bond dynamics. The long CN stretch lifetime (~36 ps), relative to the water hydroxyl stretch (< 2 ps), significantly extends the timescales that can be accessed. Complete orientational relaxation (C 2(t), orientational correlation function) and spectral diffusion (C ω(t), frequency-frequency correlation function) dynamics are presented and compared to the simulated time correlation functions in a model silica pore of the same size. A slow decay component not present in the bulk liquid is observed in both experiments, indicating that the hydrogen bond dynamicsmore » are significantly altered by confinement. The simulations reveal a qualitative difference in the functional dependence of C 2(t; d) and C ω(t;d) on d, the distance from the interface. Finally, the former becomes exponentially faster with distance while the latter makes an abrupt transition from slower to faster dynamics midway between the surface and pore center, d ≅ 6 Å.« less

Authors:
 [1];  [1]; ORCiD logo [2]; ORCiD logo [1]
  1. Stanford Univ., CA (United States)
  2. Univ. of Kansas, Lawrence, KS (United States)
Publication Date:
Research Org.:
Stanford Univ., CA (United States)
Sponsoring Org.:
USDOE Office of Science (SC), Basic Energy Sciences (BES); National Science Foundation (NSF); US Air Force Office of Scientific Research (AFOSR)
OSTI Identifier:
1594153
Grant/Contract Number:  
FG03-84ER13251; CHE-1339661; FA9550-16-1-0104
Resource Type:
Journal Article: Accepted Manuscript
Journal Name:
Journal of Physical Chemistry. C
Additional Journal Information:
Journal Volume: 123; Journal Issue: 9; Journal ID: ISSN 1932-7447
Publisher:
American Chemical Society
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; water dynamics; mesoporous silica; confined environments; orientational relaxation; 2D IR; ultrafast IR experiments

Citation Formats

Yamada, Steven A., Shin, Jae Yoon, Thompson, Ward H., and Fayer, Michael D. Water Dynamics in Nanoporous Silica: Ultrafast Vibrational Spectroscopy and Molecular Dynamics Simulations. United States: N. p., 2019. Web. doi:10.1021/acs.jpcc.9b00593.
Yamada, Steven A., Shin, Jae Yoon, Thompson, Ward H., & Fayer, Michael D. Water Dynamics in Nanoporous Silica: Ultrafast Vibrational Spectroscopy and Molecular Dynamics Simulations. United States. doi:10.1021/acs.jpcc.9b00593.
Yamada, Steven A., Shin, Jae Yoon, Thompson, Ward H., and Fayer, Michael D. Wed . "Water Dynamics in Nanoporous Silica: Ultrafast Vibrational Spectroscopy and Molecular Dynamics Simulations". United States. doi:10.1021/acs.jpcc.9b00593. https://www.osti.gov/servlets/purl/1594153.
@article{osti_1594153,
title = {Water Dynamics in Nanoporous Silica: Ultrafast Vibrational Spectroscopy and Molecular Dynamics Simulations},
author = {Yamada, Steven A. and Shin, Jae Yoon and Thompson, Ward H. and Fayer, Michael D.},
abstractNote = {Nanoporous silica materials are important in catalysis, energy, and materials applications in which water is an essential component. System performance is intimately connected to the water dynamics occurring in the confined environment. However, the dynamics and associated structures of water in mesoporous silica are challenging to measure or predict. Here, water dynamics subject to nanoscale confinement are examined via the ultrafast infrared spectroscopy of selenocyanate (SeCN-) dissolved in the hydrated ~2.4 nm silica nanopores of MCM41. Polarization selective pump-probe and two-dimensional infrared measurements on the CN stretching mode of SeCN- are used to probe the effect of confinement on orientational relaxation and spectral diffusion dynamics. The dynamics of SeCN- provide information on water hydrogen bond dynamics. The long CN stretch lifetime (~36 ps), relative to the water hydroxyl stretch (< 2 ps), significantly extends the timescales that can be accessed. Complete orientational relaxation (C2(t), orientational correlation function) and spectral diffusion (Cω(t), frequency-frequency correlation function) dynamics are presented and compared to the simulated time correlation functions in a model silica pore of the same size. A slow decay component not present in the bulk liquid is observed in both experiments, indicating that the hydrogen bond dynamics are significantly altered by confinement. The simulations reveal a qualitative difference in the functional dependence of C2(t; d) and Cω(t;d) on d, the distance from the interface. Finally, the former becomes exponentially faster with distance while the latter makes an abrupt transition from slower to faster dynamics midway between the surface and pore center, d ≅ 6 Å.},
doi = {10.1021/acs.jpcc.9b00593},
journal = {Journal of Physical Chemistry. C},
issn = {1932-7447},
number = 9,
volume = 123,
place = {United States},
year = {2019},
month = {1}
}

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