(389b) Elucidating Structure-Property Relationships in Photoelectrochemical Energy Conversion Systems

The rising levels of greenhouse gases necessitate reduction in the amounts of these harmful species in the atmosphere and transition to sustainable energy conversion and chemical manufacturing. To address this issue, the photoelectrochemical (PEC) approach presents an appealing solution due to the decreasing price of renewable electricity and advancements in solar technology. Employing this approach on a large scale requires us to identify the active sites of photoelectrocatalysts to engineer active, stable, and selective materials. Prior efforts to design such structures have resulted in an exceedingly complex nature of photoelectrocatalysts, in which it becomes difficult – if not impossible – to identify these active sites. Our approach to elucidate these structure-property relationships consists of 1) synthesis of well-defined metal/semiconductor structures with controlled properties; 2) surface functionalization of these metal/semiconductor interfaces; and 3) semiconductor heterostructure engineering.

Here we demonstrate the application of this three-pronged approach for CO₂ PEC conversion using plasmonic nanoparticles on p-type gallium nitride (p-GaN). p-GaN is a wide bandgap semiconductor with good stability under CO₂ PEC conditions due to the nitrogen rich surface. Additionally, its conduction band minimum is more negative than the CO₂ reduction potential. When combined with metal nanoparticles, such as copper and gold, the semiconductor-metal interface forms a Schottky barrier for efficient transfer of excited electrons and holes. Specifically, because of the downward bending of the conduction and valence bands, the electrons are channeled to the metal-electrolyte interface, while the holes are pushed into the bulk of p-GaN. The extent of the band bending and, thus, the effectiveness of the electron charge transfer to the semiconductor-electrolyte interface can be further increased by applying external bias. First, to study how the structure of such metal/semiconductor interfaces affects their CO₂ PEC activity, we create metal nanoparticles on the surface of p-GaN via electron beam deposition. Careful control of deposition parameters and thermal annealing treatments allows us to tune the size and composition of the nanoparticles. We then study these systems under the full solar spectrum illumination to explore the role of the direct interband transitions in p-GaN on the CO₂ PEC performance of these materials. Second, to suppress the formation of hydrogen via the parasitic hydrogen evolution reaction and to promote C-C coupling towards forming higher value products, we functionalize the semiconductor surfaces with molecular additives. Lastly, to overcome the limits on the maximum photocurrent of the wide bandgap semiconductor, we develop GaN-based heterostructures by modeling their optical, electrical and thermal effects at the physical level. Overall, these efforts establish a rigorous platform to elucidate structure-property relationships in photoelectrocatalysts and provide the knowledge to engineer active, stable, and selective materials for sustainable energy applications.