Bias-dependent diffusion of H$_2$O molecules on an Al(111) surface

TL;DR

This study uses first-principles calculations to show that water molecule diffusion on Al(111) surfaces is highly dependent on bias voltage, aligning well with experimental STM results and emphasizing the importance of accurate bias treatment.

Contribution

It introduces a constant-$ m extbf{ extmu}_e$ scheme for realistic simulation of surface reactions under bias voltage, improving understanding of electric field effects on surface diffusion.

Findings

Diffusion paths and activation barriers depend strongly on bias voltage.

The constant-$ m extmu_e$ scheme aligns well with experimental STM results.

Different schemes (constant-$ m N_e$ vs. constant-$ m extmu_e$) lead to different insights into water diffusion.

Abstract

We investigate the process by which a water molecule diffuses on the surface of an Al(111) electrode under constant bias voltage by first-principles density functional theory. To understand the diffusion path of the water on the Al(111), we calculated the minimum energy path (MEP) determined by the nudged elastic band method in combination with constant electron chemical potential (constant-$\mu_{\rm e}$) 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. This strong dependence of the water diffusion process on the bias voltage is in good agreement with the result of a previous scanning tunneling microscopy (STM) experiment. The agreement between the theoretical and experimental results implies that accurate treatment of bias voltage plays a significant role in understanding the interaction between the electric field and the surface of the material. Comparative studies of the diffusion process with the constant total number of electrons (constant-$N_\mathrm{e}$) scheme show that the absence of strong interaction between the molecular dipole and the electric field leads to a different understanding of how water diffuses on a metal surface. The proposed constant-$\mu_{\rm e}$ scheme is a realistic tool for the simulation of reactions under bias voltage not only using STM but also at the electrochemical interface.