Continuous and Reversible Tuning of Electrochemical Reaction Kinetics on Back-Gated 2D Semiconductor Electrodes: Steady-State Analysis Using a Hydrodynamic Method.

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 SiO/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, applied to the back gate, while the rate constant for electron transfer decreases with V. 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.