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Abstract
In models of Pt 111 and Pt 100 surfaces in water, motions of molecules in the first hydration layer are spatially and temporally correlated. To interpret these collective motions, we apply quantitative measures of dynamic heterogeneity that are standard tools for considering glassy systems. Specifically, we carry out an analysis in terms of mobility fields and distributions of persistence times and exchange times. In so doing, we show that dynamics in these systems is facilitated by transient disorder in frustrated twodimensional hydrogen bonding networks. The frustration is the result of unfavorable geometry imposed by strong metalwater bonding. The geometry depends upon the structure of the underlying metal surface. Dynamic heterogeneity of water on the Pt 111 surface is therefore qualitatively different than that for water on the Pt 100 surface. In both cases, statistics of this adlayer dynamic heterogeneity responds asymmetrically to applied voltage.
We are grateful to Aaron Keys for comments on an earlier version of the manuscript. Work on this project in its early stages was supported by the Helios Solar Energy Research Center of the U.S. Department of Energy under Contract No. DEAC0205CH11231. In its final stages, it was supported by the Director, Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering Division, and Chemical Sciences, Geosciences, and Biosciences Division under the same DOE contract number.
INTRODUCTION
MODEL
ORIENTATIONAL MOBILITY AND FIELDS
CORRELATION AND DISTRIBUTION FUNCTIONS
Key Topics
 Electrodes
 23.0
 Surface dynamics
 22.0
 Hydrogen bonding
 20.0
 Surface patterning
 8.0
 Metal surfaces
 7.0
Figures
An instantaneous configuration and dynamic heterogeneity of the water adlayer on the Pt 100 surface. The adlayer is in equilibrium with adjacent bulk water (not shown). (a) Hydrogen bonding patterns showing heterogeneous distribution of line defects. For the 100 surface there are four (as dictated by lattice symmetry) particularly stable relative arrangements of water molecules which can be generated by tiling the surface with unit cells containing four water molecules (in this panel starting at the bottom lefthand corner). In the image above each of the four unit cells has been assigned a different color code (see lefthand side of panel) which is projected onto the underlying electrode atoms to highlight distinct domains of adlayer waters with specific hydrogen bond arrangements. (b) and (c) Instantaneous and time averaged mobility fields, q(a; t) and , respectively, with t obs = τp/3. Horizontal and vertical axes represent position in the plane parallel to the electrode surface. The snap shots in (a) and (b) are taken at the midpoint of the trajectory that is averaged to produce (c). Color code for the mobility field is given by q(a, t)Δx 3 where Δx = 0.1 Å.
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An instantaneous configuration and dynamic heterogeneity of the water adlayer on the Pt 100 surface. The adlayer is in equilibrium with adjacent bulk water (not shown). (a) Hydrogen bonding patterns showing heterogeneous distribution of line defects. For the 100 surface there are four (as dictated by lattice symmetry) particularly stable relative arrangements of water molecules which can be generated by tiling the surface with unit cells containing four water molecules (in this panel starting at the bottom lefthand corner). In the image above each of the four unit cells has been assigned a different color code (see lefthand side of panel) which is projected onto the underlying electrode atoms to highlight distinct domains of adlayer waters with specific hydrogen bond arrangements. (b) and (c) Instantaneous and time averaged mobility fields, q(a; t) and , respectively, with t obs = τp/3. Horizontal and vertical axes represent position in the plane parallel to the electrode surface. The snap shots in (a) and (b) are taken at the midpoint of the trajectory that is averaged to produce (c). Color code for the mobility field is given by q(a, t)Δx 3 where Δx = 0.1 Å.
An instantaneous configuration and dynamic heterogeneity of the water adlayer on the Pt 111 surface. The adlayer is in equilibrium with adjacent bulk water (not shown). (a) Hydrogen bonding patterns: water molecules engaging in the preferred hydrogen bond pattern (see lefthand side of panel) have their underlying electrode atom colored blue. (b) and (c) Instantaneous and time averaged mobility fields, q(a; t) and , respectively, with t obs = τp/3. Horizontal and vertical axes represent position in the plane parallel to the electrode surface. The snap shots in (a) and (b) are taken at the midpoint of the trajectory that is averaged to produce (c). Color code for the mobility field is given by q(a, t)Δx 3 where Δx = 0.1 Å.
Click to view
An instantaneous configuration and dynamic heterogeneity of the water adlayer on the Pt 111 surface. The adlayer is in equilibrium with adjacent bulk water (not shown). (a) Hydrogen bonding patterns: water molecules engaging in the preferred hydrogen bond pattern (see lefthand side of panel) have their underlying electrode atom colored blue. (b) and (c) Instantaneous and time averaged mobility fields, q(a; t) and , respectively, with t obs = τp/3. Horizontal and vertical axes represent position in the plane parallel to the electrode surface. The snap shots in (a) and (b) are taken at the midpoint of the trajectory that is averaged to produce (c). Color code for the mobility field is given by q(a, t)Δx 3 where Δx = 0.1 Å.
(a) The dipole autocorrelation function for water molecules adsorbed to the 100 (dotted red line) and 111 (solid blue line) electrode. The corresponding quantity for molecules in the bulk liquid is plotted as a dashed black line. (b) The probability distributions for the persistence, t p, and exchange times, t x, for the 111 surface, shown on a log scale where P(log t) = tP(t). (c) The probability distributions for the persistence and exchange times for the 100 surface.
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(a) The dipole autocorrelation function for water molecules adsorbed to the 100 (dotted red line) and 111 (solid blue line) electrode. The corresponding quantity for molecules in the bulk liquid is plotted as a dashed black line. (b) The probability distributions for the persistence, t p, and exchange times, t x, for the 111 surface, shown on a log scale where P(log t) = tP(t). (c) The probability distributions for the persistence and exchange times for the 100 surface.
(a) The probability distribution for the total orientational mobility, Q, plotted for the 100 electrode surface (dashed red line) and the 111 electrode surface (solid blue line). (b)–(c) The probability distribution p(Q) at different values of the applied electrode potential. Dotted black line corresponds to the results at zero applied potential. Red and blue lines correspond to the results at the negative electrode (V 0 = −1.36V) and positive electrode (V 0 = 1.36V), respectively.
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(a) The probability distribution for the total orientational mobility, Q, plotted for the 100 electrode surface (dashed red line) and the 111 electrode surface (solid blue line). (b)–(c) The probability distribution p(Q) at different values of the applied electrode potential. Dotted black line corresponds to the results at zero applied potential. Red and blue lines correspond to the results at the negative electrode (V 0 = −1.36V) and positive electrode (V 0 = 1.36V), respectively.
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Abstract
In models of Pt 111 and Pt 100 surfaces in water, motions of molecules in the first hydration layer are spatially and temporally correlated. To interpret these collective motions, we apply quantitative measures of dynamic heterogeneity that are standard tools for considering glassy systems. Specifically, we carry out an analysis in terms of mobility fields and distributions of persistence times and exchange times. In so doing, we show that dynamics in these systems is facilitated by transient disorder in frustrated twodimensional hydrogen bonding networks. The frustration is the result of unfavorable geometry imposed by strong metalwater bonding. The geometry depends upon the structure of the underlying metal surface. Dynamic heterogeneity of water on the Pt 111 surface is therefore qualitatively different than that for water on the Pt 100 surface. In both cases, statistics of this adlayer dynamic heterogeneity responds asymmetrically to applied voltage.
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