(668g) Molecular Additives Steer Selectivity of CO2 Photoelectrochemical Reduction over Gold Nanoparticles on Gallium Nitride

Rising levels of greenhouse gases necessitate reduction in the amounts of these harmful compounds in the atmosphere and transition to sustainable production of fuels and chemicals. Photoelectrochemical (PEC) CO₂ reduction is an appealing solution to convert the harmful gas into these higher-value products. However, PEC suffers from poor selectivity due to the competitive hydrogen evolution reaction dominant in aqueous electrolytes. Our approach to overcome this challenge consists of (1) synthesis of metal/semiconductor structures with controlled properties and (2) surface functionalization of the metal/semiconductor interface with molecular additives. Here we demonstrate this two-pronged approach with gold 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 metals, such as gold nanoparticles, the semiconductor-metal interface forms a Schottky barrier. Because of the downward bending of the conduction and valence bands, electrons excited in p-GaN are pushed towards the metal-electrolyte interface, while the holes are transferred into the bulk of the semiconductor. First, by synthesizing gold nanoparticles with controlled properties (size, optical absorption, Schottky barrier height), we study the effects of these parameters on the performance of the Au/p-GaN devices to identify catalytically active sites. Second, by functionalizing the surface of these devices with molecular additives, we steer selectivity of the CO₂ reduction process toward carbon products. Overall, our work establishes a rigorous platform to elucidate structure-property relationships in photoelectrocatalysts and engineer active, stable, and selective materials for sustainable energy applications.