Monte Carlo simulations of amorphous hydroxylated silica (SiO2) nanoparticles

Abstract

Monte Carlo simulations were carried out on bulk amorphous silica (SiO2) and amorphous hydroxylated silica nanoparticles with different particle sizes (diameter ≈1–10nm). The potential developed by D. M. Tether, and modified and tested by Cormack, Du et al. was used to model the interatomic interactions of SiO2 in both cases (bulk and nanoparticles). The bulk system was realized using periodic boundary conditions. The SiO2 nanoparticles were generated by cutting out a sphere from the bulk silica melt to the required particle size. Free valences on the nanoparticle surfaces were saturated via additional hydroxyl groups and then quenched to T=300K under free boundary conditions.The effect of particle size on bulk and surface properties of the nanoparticles were calculated at T=300K and P=0GPa and studied via radial distribution functions, bond angle distributions, bond distances, coordination numbers, different types of OH group concentrations (isolated, geminol and vicinal groups) and radial density profiles. In addition, a comparison study was done between amorphous hydroxylated SiO2 nanoparticles and bulk amorphous silica properties in order to study differences between them.The study shows that the bulk properties of amorphous hydroxylated SiO2 nanoparticles are size-dependent and different from those of the bulk silica. However, increasing the particle size leads to an approach of the particle’s bulk properties to the bulk properties of the (quasi) infinite system. In addition, the study also shows that decreasing the particle size (i.e., increase in surface area-to-volume and surface-to-core atom ratios) results in increasing the surface effects and surface OH group concentration. Accordingly, small-sized SiO2 nanoparticles have higher surface OH group concentration and larger surface effects than large-sized SiO2 nanoparticles. The simulation results also show that decreasing the particle size not only results in increasing the surface effects but also results in shifting the bond distances and angles towards higher values. Details of the modeling, simulations results and the study are presented in this paper.