Evidence and origin of anomalous diffusion of Ag+ ion in amorphous silica: A molecular dynamics study with neural network interatomic potentials.

The release of Ag+ ions into the environment through silica layers is a promising strategy for the development of anti-microbial surface coating devices. The aim of the present study is to provide some insight into the elementary mechanisms of diffusion of Ag+ ions through silica with the objective of proposing control strategies. Thanks to the development of interaction potentials based on neural networks, the diffusion processes were studied via molecular dynamics simulations. Silver diffusion was found to be anomalous and sub-diffusive, the origin of which could be attributed to deceleration and temporal anti-correlations. This sub-diffusion has been attributed primarily to the disordered nature of the silica matrix. Furthermore, it is magnified by the presence of coordination defects within the silica matrix. These defects, in particular the under-coordinated oxygen atoms, act as traps for Ag+ by forming O-Ag bonds, thereby limiting the jump length and retaining the ion for long duration. By comparison with existing diffusion models, the diffusion mechanism in the absence of defects appears to be of the fractional Brownian motion type, substantially modified by the presence of defects. Two possible approaches have emerged to tune the release of Ag+ ions through the silica layer: the monitoring of the number of defects and the opening/closing of diffusion paths via, e.g., a modification of the silica density.