Abstract
The energy required to form and remove vacancies on metal surfaces mediates the rate of mass transport during a wide range of processes. These energies are known to be sensitive to environmental conditions. Here, we use electronic structure density functional theory calculations to show that the surface vacancy formation energy of silver changes markedly in the presence of adsorbed and dissolved oxygen. We found that adsorbed atomic oxygen can reduce the surface vacancy formation energy of the Ag(111) surface by more than 30%, whereas surface vacancy formation becomes exothermic in the presence of pure subsurface oxygen. We went on to show that the total directionality of the topologically defined bond paths can be used to understand these changes. The resulting structure-property relationship was used to predict the behavior of silver in different atmospheres. We show that the surface vacancy formation energy decreases when electronegative elements are adsorbed on the surface, but that it can increase when electropositive elements are adsorbed.
Highlights
The in-plane dimensions of the (1 Â 1) surface unit cell were fixed at 2.93 Å using the bulk silver lattice constant we calculated within Quantum Espresso (QE), which is within 2% of the experimentally measured value.[73]
The surface vacancy formation energy is substantially lower than the bulk vacancy formation energy, which we calculated to be 0.84 eV without relaxation and 0.81 eV with relaxation, in agreement with previous theoretical work, 0.80 eV74 and 0.86 eV,[75] but approximately 0.2 eV below the vacancy formation energies derived from positron annihilation, 0.99–1.10 eV,[76,77] quenching, 1.10 eV,[1,78] and length change experiments, 1.09 eV.[79]
The ionic relaxation is small as suggested by the small reduction in surface vacancy formation energy that accompanies relaxation which principally serves to reduce the interlayer spacing in the [111] direction upon formation of the surface vacancy
Summary
Silver is a classic example of a metal whose surface selfdiffusion is dependent on the atmosphere. It has long been known that silver surfaces are etched to show simple crystallographic planes at high temperature in air.[15,16] And because silver can be thermally etched in air at standard atmospheric pressure, unlike other metals that require a more restrictive atmosphere, it has been the focus of numerous experimental investigations.[15,16,17,18,19]
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