Continuous and Reversible Tuning of Electrochemical Reaction Kinetics on Back-Gated 2D Semiconductor Electrodes:...

Here we report the steady state kinetic analysis of field-effect-controlled outer-sphere electrochemistry on ultrathin back-gated ZnO working electrodes (i.e., 5 nm ZnO electrodes prepared on SiO₂/degenerate Si back gates). To achieve steady state conditions in the electrolyte phase, gate-tunable electrochemical flow cells were prepared by integrating a silicone microfluidic channel on the back-gated ZnO electrode. In these flow cells, continuous supply of fresh electrolyte generates time-invariant diffusion layers near the ZnO surface, allowing steady-state kinetic analysis as in other hydrodynamic methods. From the steady-state analysis, it was found that the electron density on the ZnO surface increases with the voltage bias, V_BG, applied to the back gate, while the rate constant for electron transfer decreases with V_BG. The observed trend can be explained as a result of the field-effect-induced band alignment shift at the ZnO/electrolyte interface which is predicted by our conceptual model; a positive back gate bias shifts the conduction band edge down at a given working electrode potential, leading to an increased surface electron density on ZnO, but simultaneously less overlap of the band edge with the electron acceptor states in solution, which means a lower electron transfer rate constant. Overall, the results quantitatively demonstrate that back gates and the ensuing field effect can be used to control kinetics of interfacial electron transfer at two-dimensional (2D) semiconductor electrodes.