Isokinetic molecular dynamics simulations of argon adsorbed on several model amorphous surfaces of titanium dioxide were carried out. The solid was represented by the Bernal Model, in which the surface was taken to be the exposed face of a dense random packing of oxide ions. This surface was roughened by deleting varying numbers of oxides at random from the outer layer of the solid. Surface diffusion in the limit of very low coverage (tracer diffusion) was evaluated by following the dynamics of non-interacting single adatoms. The diffusion coefficient in the zero-density limit was calculated from velocity autocorrelation functions for adatoms at 85 to 300K. The temperature dependence of the diffusion constants obtained in this way revealed an almost Arrhenius behavior over this temperature range. It was shown that surface roughening at the atomic scale gave rise to a higher apparent activation energy for diffusion. A statistical characterization of the adsorptive field through distributions of local maxima and minima in the adsorption energy surface along the diffusion paths was performed in order to relate the surface diffusion of monatomic species on amorphous surfaces to the dual site-bond description of heterogeneous surfaces.

Molecular Dynamics Study of Tracer Diffusion of Argon Adsorbed on Amorphous Surfaces. Riccardo, J.L., Steele, W.A.: Journal of Chemical Physics, 1996, 105[21], 9674-85