Bias-dependent diffusion of a <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi mathvariant="normal">H</mml:mi><mml:mn>2</mml:mn></mml:msub><mml:mi mathvariant="normal">O</mml:mi></mml:mrow></mml:math> molecule on metal surfaces by the first-principles method under the grand-canonical ensemble

Abstract

We investigate the process by which a water molecule diffuses on the surface of metal electrodes under constant bias voltage by first-principles density functional theory. In this study, we present the constant electron chemical potential (constant-${\ensuremath{\mu}}_{\mathrm{e}}$) methods combined with the nudged elastic band method. The water diffusion on the Al(111) was calculated using the minimum energy paths (MEPs) for understanding the difference between the constant-${\ensuremath{\mu}}_{\mathrm{e}}$ and conventional methods. The simulation shows that the MEP of the water molecule, its adsorption site, and the activation barrier strongly depend on the applied bias voltage. Comparing to the constant total number of electrons (constant-${N}_{\mathrm{e}}$), we found the larger change in the tilted angle of the water dipole in the MEP by the constant-${\ensuremath{\mu}}_{\mathrm{e}}$ method. For the comparison between the theoretical results and the previous experiment, we simulate the MEP for a single water diffusion on the Pt(111) surface using constant-${\ensuremath{\mu}}_{\mathrm{e}}$ method. When we applied positive bias voltage to the Pt electrode, the result of the activation barrier for a water molecule decreases with increasing the bias voltage, which is consistent with the previous scanning tunneling microscopy (STM) experiment. The proposed constant-${\ensuremath{\mu}}_{\mathrm{e}}$ method plays a significant role in understanding the interaction between the electric field and the surface of the material, and is a reliable tool for the simulation of reactions under bias voltage not only using STM but also at the electrochemical interface.

Topics & Concepts