Mechanistic studies of solar fuel generation at electrode surfaces

Solar fuel generation addresses the inherent intermittency of solar energy and can be part of the global solution to tackling climate change. The idea involves harnessing solar energy to drive chemical reactions to produce high energy density fuels. Thus, when the Sun is not shining, the sustainably produced fuels can be converted to energy to meet demands. Key to the future wide-scale adoption of this process is the development of efficient solar fuel generation devices from cheaper, more stable and abundant materials. Mechanistic studies on both state-of-the-art (and expensive) materials as well as the cheaper, less efficient alternatives can provide vital information on both efficient catalytic process and causes of inefficiencies in cheaper materials. This mechanistic understanding can be used to inform the design of improved materials in the future. In situ vibrational spectroscopy can provide direct chemical information on the identity of surface intermediates, making it a powerful tool in mechanistic studies. Unfortunately, almost all solar fuel reactions occur at heterogeneous interfaces and the spectroscopic response is often dominated by the inactive bulk of these materials. Surface-sensitive vibrational spectroscopies are inherently blind to the majority of the bulk response, thus selectively providing mechanistic information from the catalytically active interface. This thesis explores the development of several surface-sensitive spectroscopic experiments for studying photo- and electrocatalytic water splitting materials to produce hydrogen and oxygen as the solar fuels. Electrochemical shell-isolated nanoparticle-enhanced Raman spectroscopy is used to detect surface intermediates at state-of-the-art hydrous iridium oxide electrodes. In situ vibrational sum frequency generation is applied to studying the interfacial water structure at hematite and TiO2 electrode surfaces under water oxidation conditions. Finally, photoinduced absorption and electric field-induced second harmonic generation spectroscopy are used to follow slow surface hole kinetics and understand the impact of surface trap states on interfacial electric fields