Electric field tuning of oxygen stoichiometry at oxide surfaces: molecular dynamics simulations studies of zirconia

Abstract

Ultra-thin metal-oxides such as zirconia have tremendous technological applications such as electrolyte membranes for advanced solid oxide fuel cells, fuel cladding material for light water nuclear reactors, pressure tube materials for heavy water nuclear reactors and corrosion resistant coatings. Oxide non-stoichiometry is an important factor which significantly affects their functional properties and applicability. Here, we report on the ability to athermally control oxygen non-stoichiometry in ultra-thin zirconia films through local electric field perturbations from simulations. Variable charge molecular dynamics simulations indicate significantly enhanced oxidation kinetics on Zr (0001) substrate in the presence of an electric field. Natural oxidation with no field resulted in an amorphous oxide scale with a self limiting thickness of ∼10 A which increased to ∼17–26 A for applied electric fields of 1–10 MV/cm. Electric field (∼107 V/cm) lowers the activation energy barrier for ionic migration through the oxide film and leads to significantly increased oxygen incorporation into the oxide film. Activation energy barrier for oxidation decreased from 1.13 eV with no field to 0.08 eV for an applied field of 10 MV/cm. This manifests itself in the form of dramatic density and stoichiometry improvements of the grown ultra-thin oxide film, as indicated by the calculated structural and dynamical correlation functions. Oxide stoichiometry (O/Zr ratio) for natural oxidation was 1.42 indicative of a sub-stoichiometric and oxygen deficient oxide which increased to near stoichiometric value of 1.86 for 10 MV/cm field assisted oxidation. The simulation findings agree well with previously reported experimental observations. Our results demonstrate a pathway to athermally control oxygen concentration in near-surface regions that is of great importance to technologies utilizing ultra-thin oxides ranging from catalysis, energy and electronic device technologies.