Continuous and Reversible Modulation of Interfacial Electrochemical Phenomena on Back-Gated Two Dimensional Semiconductors

Abstract

The continuous increases in global population and energy consumption have raised major concerns about the security of our energy future. Limited amount of fossil fuels and increasing concerns over their combustion product, carbon dioxide, on climate change make renewable energy sources, such as sun light and wind, attractive options. Accordingly, energy conversion and storage devices based on (photo)electrochemical processes (e.g., batteries, fuel cells, water splitting, solar-to-fuel conversion, etc.) have received great attention as promising solutions to overcome the challenges in the intermittent renewable energy sources. To develop highly efficient and economically viable energy devices, fundamental understanding of electrochemical phenomena occurring at the electrode/electrolyte interfaces is essential. In this dissertation, we introduce a new approach, inspired by the operating mechanism of field-effect transistors (FETs), to modify and study such electrochemical interfaces. The devices studied in this dissertation project have a “gate-insulator-electrode” stack structure, which is essentially similar to that of a FET but the (typically) thick semiconductor layer in regular FETs is replaced with an ultrathin or two-dimensional (2D) active electrode for electrochemical processes in our devices. In such a device, due to the extreme thinness of the active electrode, electronic properties at the electrode surface can be dramatically altered by extra charge carriers induced with a voltage bias to the gate. This, in turn, makes thermodynamics and kinetics of electrochemical processes at the electrode/electrolyte interface be determined by the gate bias as well as by the electrode potential (with respect to the solution or reference electrode potential). In this project, three interfacial electrochemical phenomena are of our main interest: (1) electric double layer charging, (2) electron transfer across interface, and (3) surface binding of reaction species on electrode surface. First, responses of electric double layer structure to the gate bias is investigated using graphene devices, and a method to experimentally separate the band filling potential and the double layer charging potential has been developed. Second, continuous and reversible modulation of outer-sphere electron transfer kinetics by a gate bias was demonstrated on ZnO devices for the first time using cyclic voltammetry. Third, quantitative analysis of the electron transfer kinetics has been conducted using microchannel flow cells, in which continuous supply of fresh electrolyte through the microfluidic channel generates time-invariant diffusion layers near the active electrode surface, allowing electrochemical measurements in steady states. To collectively explain our observations, a simple but very useful physical model is proposed; the model indicates that the observed changes in the interfacial electrochemical phenomena essentially result from the gate-induced band alignment shift at the electrode/electrolyte interfaces. Lastly, based upon the results and the insight gained from the previous experiments, possibilities and challenges in field-effect control of surface binding energies in electrocatalytic systems are explored, and rational strategies to overcome the difficulties are proposed.