| dc.description.abstract | Amorphous silica (a-SiO2) is used in a wide range of applications that include separations, sensing, optical processing, drug delivery, and catalysis. Its advantages include low cost, optical transparency, ready synthesis, and structural and chemical tunability. At the same time, the properties of a-SiO2 in these and future applications are not completely understood. Amorphous silica is difficult to characterize due to its intrinsic disorder, the wide variety of synthetic approaches, and the critical role of buried interfaces in its mesoporous forms. This gives an impetus to modeling studies that can provide insight into the relationships between the structural features and functional properties of a-SiO2.In this work, we address this issue through the development and characterization of realistic, atomistic a-SiO2 slab models. Syntheses of amorphous silica involve steps that occur on timescales of minutes to hours and are therefore not feasible for direct computational simulations. Thus, the present approach is an artificial one designed to reproduce the range of experimental a-SiO2 structures with methods accessible by molecular dynamics methods. Specifically, a melt-quench- functionalize procedure is described that permits variation of a number of the key a-SiO2 properties including surface roughness, defect density, silanol density, and silanol spatial distribution. The developed melt-quench-functionalize algorithm generates a disordered silica structure via melting, by heating to a high temperature within molecular dynamics simulations, a crystalline silica sample and cutting it to make a slab. The molten slab is equilibrated before cooling back to room temperature at a rate that is too rapid to allow recrystallization, producing an amorphous silica solid. Finally, the resulting solid slab is functionalized using an ad hoc procedure that mimics a hydroxylation reaction by adding a water molecule across a silicon-oxygen bond. The melt-quench-functionalize approach is applied to produce more than two thousand slab models using multiple force fields that have previously been developed to describe crystalline and amorphous silica. The properties of the resulting slabs are characterized in detail as a function of the cooling rate and the details of the functionalization algorithm. In nearly all cases the bond and angle distributions of the silica structure are in excellent agreement with prior experimental measurements, but we further analyze the density and location of coordination defects and ring structures as well as the atomic-scale surface roughness. We show that the cooling rate has important effects on the quenched disorder silica structure. In particular, it modulates the surface roughness of the slabs, which is a key feature of silica surfaces that has been generally overlooked in modeling efforts. We find that faster cooling rates result in rougher slab surfaces while slower cooling rates generate smoother interfaces. The cooling rate also influences the number of coordination defects present in the solid, with more rapid cooling preventing rearrangement of five-coordinate silicon and three-coordinate oxygen atoms. The functionalization method has been developed to be flexible and is automated within a computer code. Specifically, the total silanol density can be controlled by continuing to select silicon-oxygen bonds for hydroxylation until the desired number of surface hydroxyl groups is achieved. We also show that the spatial distribution of silanols can also be varied by identifying silicon-oxygen bonds for functionalization in different ways, i.e., by finding strained (elongated) bonds or by a random selection of bonds. The former leads to more clustered silanols due to the correlation position of strained bonds in the silica. Finally, the effect of the roughness of the a-SiO2 surfaces on the dynamics of a liquid at its interface is examined. Understanding water confined within silica frameworks is important both because of its occurrence in natural geological structures and its relevance to applications that involve aqueous solutions. Thus, we use the developed slab models to create three nanometer wide amorphous silica slit pores, using surfaces of varying roughness and investigate the confined water dynamics, specifically OH reorientation, using molecular dynamics simulations. We find that the reorientational dynamics are slowed in the pores with larger surface roughness. The effect is limited to the first one to two water layers at the silica interface, which, however, is where much of the important chemistry in mesoporous materials occurs. | |
