Probing Nanoscale Electron Transfer and Hydrogen Evolution Reaction at Two-Dimensional Electrodes by Scanning Electrochemical Microscopy

Electron transfer plays a significant role in many biological and chemical processes, especially in the catalytic reactions involved in photoelectrochemical energy conversion and storage. Revealing the electron transfer behaviour at the interface of electrode-electrolyte is thus of great importance. Here, we fabricate atomically thin tungsten diselenide (WSe₂) by chemical vapor deposition method and directly map the outer-sphere and inner-sphere electron transfer using Atomic Force Microscopy combined with Scanning Electrochemical Microscopy (AFM-SECM). Using AFM-SECM, the topography of the sample as well as the mechanical, electrical, and electrochemical properties can be simultaneously obtained. AFM-SECM feedback mapping shows layer-dependent electrocatalytic ability of WSe₂ to oxidize the redox species Ru²⁺ back to Ru³⁺. Moreover, AFM-SECM substrate generation and tip collection mode show layer-dependent hydrogen evolution reaction of WSe₂. Compared with monolayer and bilayer, few-layer WSe₂ shows better stability in electrochemical environment, faster electron transfer, and higher hydrogen production. First principal calculations show that the layer-dependent electron transfer and hydrogen production is highly correlated with the higher electronic density of states and more suitable Fermi level position in few-layer WSe₂ for specific redox reactions. Furthermore, finite element method-based numerical simulations using MATLAB® and COMSOL Multiphysics® are performed to calculate the electron transfer rate constants k⁰ and simulate the steady-state concentration gradient. Finally, the electrochemistry at WSe₂ electrode-electrolyte interface is spatially resolved at the nanoscale, and understanding this behaviour will be useful for future electrochemical devices.