Platforms for Analyzing and Controlling Charge Transfer Processes at Semiconductor/Liquid Interfaces

Abstract

This thesis describes new methods for the analysis and control of charge transfer processes at semiconductor/liquid interfaces. The main aim of this work is to utilize electrochemical methods to further understand, and ultimately, optimize semiconductor/electrolyte interfaces for solar energy conversion technologies. These strategies rely mainly on electrochemical techniques in which redox/precursor molecule flux can be precisely controlled for analysis or deposition by the aid of simple electronics. As such, the work presented herein is broadly applicable and easily adaptable for a myriad of applications.

The first portion of this thesis develops a new semiconductor ultramicroelectrode (SUME) platform for the analysis of charge transfer kinetics and thermodynamics at semiconductor/liquid contacts. Chapter 2 examines the geometrical dependence of the error in rate constant and transfer coefficient for electron transfer at a recessed metal UMEs to aid in design of the SUME platform. Simulated and experimental voltammetry demonstrate that recessed UMEs with thin insulating layers exhibit small errors in the rate constant and transfer coefficient for outer-sphere charge transfer reactions relative to their inlaid counterparts, especially when near-reversible kinetics are operative. Chapter 3 details the fabrication process and electrochemical behavior of n-Si SUMEs in aqueous media. The platform demonstrated behavior characteristic of metallic UMEs (e.g. high current densities) while maintaining inherent semiconductor properties. The SUMEs were shown to be highly sensitive to dynamic surface conditions, such as oxidation, and were used to broadly fit several outer-sphere redox couple to kinetic parameters in line with predictions from classical charge-transfer theory. Chapter 4 extends the utility of the SUME voltammetric response by considering how the applied potential is distributed across the interface. In doing so, nearly all energetic and kinetic parameters relevant to charge transfer at the semiconductor/liquid interface can be extracted directly from the voltammetric response.

The second portion of this thesis describes an electrochemical process for protective layer deposition directly on photoelectrodes for solar hydrogen production. Chapter 5 describes the photoelectrodeposition of MoSx on p-GaInP2 photocathodes. By controlling the deposition parameters, 8-10 nm films were deposited that exhibited minimal parasitic absorption of incident radiation and high catalytic activity for the hydrogen evolution reaction. The thin layers displayed excellent stability for over 50 hours of photoelectrolysis, highlighting this method as a simple strategy for protective layer formation with comparable photoelectrochemical properties to catalyst thin films formed by more energy-intensive and complex methods.