Electrochemistry at Back-Gated, Ultrathin ZnO Electrodes: Field-Effect Modulation of Heterogeneous Electron Transfer Rate Constants by 30× with Enhanced Gate Capacitance

We report steady-state voltammetry of outer-sphere redox species at back-gated ultrathin ZnO working electrodes in order to determine electron transfer rate constants k_ET as a function of independently controlled gate bias, V_G. We demonstrate that k_ET can be modulated as much as 30-fold by application of V_G ≤ 8 V. The key to this demonstration was integrating the ultrathin (5 nm) ZnO on a high dielectric constant (k) insulator, HfO₂ (30 nm), which was grown on a Pd metal gate. The high-k HfO₂ dramatically decreased the required V_G values and increased the gate-induced charge in ZnO compared to previous studies. Importantly, the enhanced gating power of the Pd/HfO₂/ZnO stack meant it was possible to observe a nonmonotonic dependence of k_ET on V_G, which reflects the inherent density of redox acceptor states in solution. This work adds to the growing body of literature demonstrating that electrochemical kinetics (i.e., rate constants and overpotentials) at ultrathin working electrodes can be tuned by V_G, independent of the conventional electrochemical working electrode potential.