Representation of a H on a BO. Large blue atoms represent silicon, smaller gray atoms represent oxygen, and small red atom is hydrogen. Si–O bonds are drawn for Si–O separations less than 2.0 Å. Not all atoms and bonds are drawn in the image, so that the apparent NBO are actually BO to Si not drawn.
Ring size distribution of the BO atoms that were observed to be adsorption sites for protons, called bridging OH (BOH), and the distribution of all dry surface siloxane rings.
Average bridging angle as a function of ring size for BOH sites and bridges that do not become protonated. For reference, the mean bridging angle in vitreous silica is in the vicinity of 150°, consistent with the average angle of dry bridge sites. Note that the average angles for BOH sites weigh all sites equally. As a result, the angle of a site which sustained a proton for a long period of time contributed to the average angle with the same weight as a site which sustained a proton very briefly.
First peak in the normalized Si–O pair correlation function. Dashed line corresponds to the pair correlation function around oxygen coordinated to two silicons and one hydrogen (BOH); solid line corresponds to the pair correlation function around oxygen coordinated to only two silicons (siloxane bridges).
Average O–H separation distance as a function of bridging angle for BOH groups. O–H distances were averaged over intervals of 2°. Data for bridges with an angle greater than 160° are not shown; these high-angle data comprised only 0.6% of the total measurements and were prone to very high scatter as a result.
Energy of BOs as a function of bridging angle. Energies are referenced to the energy of widest dry siloxane bridge encountered for ease of comparison. The data were averaged in 0.5° intervals, and scatter at the extremes is due to small available sample sizes.
Concentration of BOH (%) as a function of Si–O–Si bond angle, showing the significant distribution of BOH in the range, consistent with ab initio calculations and Figs. 4 and 5.
Adsorbed proton lifetime as a function of the bridging angle on the adsorbing site. Note that because BOH sites tend to exhibit several long adsorption lifetimes amidst a number of very quick adsorption/desorption reactions, the mean lifetimes in this diagram include only the lifetimes measured for BOH which remained protonated for more than 0.1 ps.
Two-body potential parameters.
Point and diffuse charges for each atom.
Three-body potential parameters.
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