Molecular Additives Steer Selectivity of CO₂ Photoelectrochemical Reduction over Gold Nanoparticles on Gallium Nitride

We demonstrate that functionalizing Au/p-GaN cathodic photoelectrochemical devices with molecular additives steers the selectivity for CO₂ reduction process toward reduced products of carbon dioxide and suppresses hydrogen generation via water splitting. The working hypothesis for this selectivity change is the suppressed proton transfer through hydrophobic molecular additive films, which results in the diminished hydrogen evolution reaction rates (Faradaic Efficiency to H₂decreases from 90% to 18 %). Our work establishes a rigorous platform to elucidate structure-property relationships in photoelectrocatalysts and engineer active, stable, and selective materials for sustainable energy applications.<br/><br/>The wide bandgap semiconductor p-GaN exhibits stability under CO₂ photoelectrochemical conditions due to its 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. The downward bending of the conduction and valence bands drives electrons excited in p-GaN towards the metal-electrolyte interface, while the holes are transferred into the p-GaN bulk semiconductor.<br/><br/>Rising levels of greenhouse gases necessitate a reduction in the amounts of these harmful compounds in the atmosphere and a transition to sustainable production of fuels and chemicals. Photoelectrochemical CO₂ reduction (CO2R) is an appealing solution to convert carbon dioxide into higher-value products. However, CO2R in aqueous electrolytes suffers from poor selectivity due to the competitive hydrogen evolution reaction dominant in aqueous electrolytes. As noted above, our approach to overcome this challenge consists of (1) synthesis of metal/semiconductor structures with controlled properties and (2) functionalizing the metal/semiconductor surface with molecular additives.