The discovery of new materials, and their organization into efficient nanoscale architectures, has the potential to transform and modernize energy conversion technologies. This is especially true for new devices for solar fuel and electricity generation based on photoelectrochemistry, which at present lack the required efficiency and product selectivity for commercialization.
I study the design of materials for solar energy conversion devices using two general methodologies. The first involves the discovery and characterization of new materials and nanoscale architectures that enable the water oxidation reaction, a key half reaction for solar fuel production, as well as the electrochemical reduction of CO2 to fuels. The second approach presented emphasizes the use of simplified model systems to assess the physical origins of efficiency. Here primarily spectroscopic techniques are used to (1) study the electronic structures of active interfaces, and (2) characterize the interaction of molecular catalysts and reactants with relevant crystal surfaces.
When a new promising material is developed and applied in an optoelectronic device, open questions inevitably present themselves regarding the atomic- or nanoscale nature of the associated new interfaces. Much of my work in model systems is aimed at elucidating the role of active interfaces in promoting efficiency in photoelectrochemical and photovoltaic applications. Understanding the electronic structure and chemical nature of interfaces is critical because these are the source of electrochemical potential gradients for charge separation (which influences efficiency) and of active sites for electrocatalytic reactions (which influences reaction selectivity and efficiency).
Oxides are emphasized for the design of water oxidation photoanodes because they are among the only materials that remain stable during light irradiation and anodic polarization in aqueous electrolytes. In designing photoanodes for this application, I leverage the diversity of metal oxides to combine materials into unique heterostructures to create materials specialized to perform the optical, electronic, and chemical functions required of photoelectrochemical cells. I will present research on photoelectrochemical water oxidation using anodes comprised of alpha-Fe2O3, ZnO, and WO3.
For the study of electrochemical CO2 reduction, my experiments are designed to explain the role of the semiconductor electrode surface in driving the high selectivity observed in pyridine and other N-heterocycle-catalyzed CO2 reduction systems. These systems have been shown to selectively reduce CO2 to methanol using only solar energy and water. I use a surface science approach to examine the preferential adsorption sites, bonding interactions, and reactivities of these molecular catalysts and other relevant adsorbates on the surfaces of photoactive III-V semiconductors. Surface-sensitive core-level and vibrational spectroscopy measurements on single crystals are used to supplement electrochemistry investigations, and generate unique information describing the role of the electrode surface in CO2 reduction. Conclusions from experimental results on these well-defined systems are supported by calculations using density functional theory.
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