Adsorption and surface energy (II): Thermal faceting from minimization of surface energy

Abstract

The thermodynamic criterion governing the occurrence of surface energγ-induced thermal faceting is developed and related to the alterations in shape of the γ-plot that can result from gaseous adsorption. When single and double ledges are preferred sites of adsorption, it is shown that faceting will occur if the curvature of a parabolically-shaped cusp exceeds a critical value. This defines a critical pressure that must be exceeded before faceting can occur, and it is predicted under certain conditions that this critical pressure will vary exponentially with temperature. An exponential temperature dependence of the critical pressure for thermal faceting has been previously observed, allowing a determination of the heat of adsorption for double ledge sites from this relationship. When surface sites are preferred for adsorption, a decrease in curvature of the γ-plot occurs. It is shown, however, that it is still possible to have a critical pressure for faceting, due to the simultaneous, general shrinking of the γ-plot towards the origin. In this case, the temperature dependence of the critical faceting pressure yields the heat of adsorption for surface sites.