The effect of near-interface network strain on proton trapping in SiO/sub 2/

Vanheusden, K.; Korambath, Prakashan; Kurtz, Henry A.; Karna, Shashi P.; Fleetwood, D.M.; Shedd, W.; Pugh, R.D.

doi:10.1109/23.819121

Cited by 23 publications

(25 citation statements)

References 14 publications

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“…50 These quantum mechanical studies performed on molecules and small clusters provide a foundation for the existence of protonated bridging oxygens ͑BOs͒ at the interface between silica and water. Thus, certain bridges at the silica-water interface should serve as adsorption sites for excess protons, and these sites may contribute to the anomalously high proton conductivity observed in wet nanoporous silica glasses.…”

Section: Introductionmentioning

confidence: 99%

Bridging oxygen as a site for proton adsorption on the vitreous silica surface

Lockwood

Garofalini

2009

The Journal of Chemical Physics

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Molecular dynamics computer simulations were used to study the protonation of bridging oxygen (Si-O-Si) sites present on the vitreous silica surface in contact with water using a dissociative water potential. In contrast to first-principles calculations based on unconstrained molecular analogs, such as H(7)Si(2)O(7)(+) molecules, the very limited flexibility of neighboring SiO(4) tetrahedra when embedded in a solid surface means that there is a relatively minor geometric response to proton adsorption, requiring sites predisposed to adsorption. Simulation results indicate that protonation of bridging oxygen occurs at predisposed sites with bridging angles in the 125 degrees-135 degrees range, well below the bulk silica mean of approximately 150 degrees, consistent with various ab initio calculations, and that a small fraction of such sites are present in all ring sizes. The energy differences between dry and protonated bridges at various angles observed in the simulations coincide completely with quantum calculations over the entire range of bridging angles encountered in the vitreous silica surface. Those sites with bridging angles near 130 degrees support adsorbed protons more stably, resulting in the proton remaining adsorbed for longer periods of time. Vitreous silica has the necessary distribution of angular strain over all ring sizes to allow protons to adsorb onto bridging oxygen at the surface, forming acidic surface groups that serve as ideal intermediate steps in proton transfer near the surface. In addition to hydronium formation and water-assisted proton transfer in the liquid, protons can rapidly move across the water-silica interface via strained bridges that are predisposed to transient proton adsorption. Thus, an excess proton at any given location on a silica surface can move by either water-assisted or strained bridge-assisted diffusion depending on the local environment. The result of this would be net migration that is faster than it would be if only one mechanism is possible. These simulation results indicate the importance of performing large size and time scale simulations of the structurally heterogeneous vitreous silica exposed to water to describe proton transport at the interface between water and the silica surface.

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Section: Introductionmentioning

confidence: 99%

Bridging oxygen as a site for proton adsorption on the vitreous silica surface

Lockwood

Garofalini

2009

The Journal of Chemical Physics

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“…Consequentially, protons rapidly diffuse along these defect tracks into the subsurface by hopping between adjacent defects [21,24,25], and these excess subsurface protons will readily passivate neighboring NBOs formed by subsequent collision cascades. The net result is the inhibition of structural healing in the subsurface caused by H + ; despite the fact that H 2 O molecules were not able to diffuse down these defect tracks, their presence at the surface still caused increased damage accumulation away from the interface.…”

Section: Discussionmentioning

confidence: 99%

“…4; rather, they are the result of excess protons diffusing through the silica network from the near-surface reaction region (minor formation of H 2 O from O in the silica occurs and will be discussed below). The excess H + on SiOH þ 2 and BOH sites are acidic and highly mobile [22][23][24][25]37], and this mobility facilitates the increase in both subsurface H + concentration and penetration depth.…”

Section: Effect Of Radiation Damage On Subsurface H + Concentrationsmentioning

confidence: 98%

“…For example, abnormally high proton conduction has been observed in mesoporous silica [17][18][19][20], and evidence suggests that defects play a significant role in H + mobility in water-silica systems [21][22][23][24][25]. Given the fact that ballistic radiation creates large concentrations of defects in silica, it is not unreasonable to consider how radiation damage affects the mobility of H + at the silica surface and into the bulk glass.…”

Section: Introductionmentioning

confidence: 95%

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Reactions between water and vitreous silica during irradiation

Lockwood

Garofalini

2012

Journal of Nuclear Materials

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“…These were likely formed during the reaction of water with the surface Si atoms forming a silanol and an H + , which penetrated several layers within the crystal and attached to a bridging oxygen. Protonated bridging oxygens in a vitreous silica and water system have been observed in MD 18 and molecular orbital 19,20 calculations and have been previously discussed. 18 Furthermore, a small layer of water molecules is attached to the Si on the quartz surface (SiÀO distance <1.9 Å).…”

Section: ' Introductionmentioning

confidence: 90%

Molecular Dynamics Investigation of Solution Structure between NaCl and Quartz Crystals

2011

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INTRODUCTIONForces between solids immersed in a liquid are important to many research areas including the stability of colloids, 1 flotation and separation of minerals, 2 pushing of particles by growing crystals, 3À5 and crystallization pressure. 6,7 When such forces are repulsive and occur between a crystal growing in confinement and the confining body (such as can occur within a porous material), damaging stresses can arise. Such is the case with destructive crystallization pressure by salt or ice, which is responsible for sculpting mountains and damaging monuments. 8 However, in some systems, the net interaction is attractive. It has been reported that quartz and sodium chloride are such a system, where the attraction has been attributed to the modification of bulk solution structure through the orientation of the water dipoles at the surface. 2 This is important for mineral flotation and also for understanding the potential for damage to quartzitic sandstone monuments exposed to marine salts.In this study, we use molecular dynamics to confirm the nature of the interaction between halite and quartz in the presence of pure water or a slightly supersaturated solution. We also examine the relative influence of crystal separation on the structuring of the solution near the two crystal surfaces. We first describe the potential model, the development of the systems of study, and the computational simulation method. The following section shows the effects of concentration and crystal separation on the structuring of the confined solutions and pressure felt by the confined sodium chloride crystal.' COMPUTATIONAL PROCEDURE Potential Model. A fully atomistic potential developed by Mahadevan and Garofalini for dissociative water 9 and water Àsilica studies, 10 which allows for bonds to be broken or formed between all atoms, was used in this work. This potential contains both two-body and three-body terms. The form of the pair potential used for all interactions iswhereABSTRACT: Molecular dynamics simulations with a dissociative interatomic potential were used to examine the interaction forces between quartz and sodium chloride crystals in the presence of pure water and a supersaturated (7 m NaCl) solution at crystal separations between 7 and 24.5 Å. The force between the crystals was found to be attractive for all separations at both ion concentrations. The structuring of interfacial water molecules, leading to ordering of the dipole moments at both interfaces, is primarily responsible for the attraction. Modifications of the interfacial water structure by the decreasing crystal separation are more prevalent at the quartz/liquid interface than at the sodium chloride/water interface.

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The effect of near-interface network strain on proton trapping in SiO/sub 2/

Cited by 23 publications

References 14 publications

Bridging oxygen as a site for proton adsorption on the vitreous silica surface

Bridging oxygen as a site for proton adsorption on the vitreous silica surface

Reactions between water and vitreous silica during irradiation

Molecular Dynamics Investigation of Solution Structure between NaCl and Quartz Crystals

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