<i>In Situ</i> Vibrational Spectroscopic Study for Photoelectrochemical CO₂ Reduction over the Au/p-GaN Catalyst: The Role of HCO₃^– for Selective Reaction

The photoelectrochemical (PEC) reduction of CO2 has been paid much attention to produce fuels and valuable chemicals (e.g., CO, CH4, CH3OH). PEC CO2 reduction is generally performed in solutions with a high concentration of bicarbonate (HCO3–) solutions. Recently, gold nanoparticles supported on p-type gallium nitride (Au/p-GaN) exhibited superior selectivity for CO production under visible-light conditions. A determining factor for selectivity could be electrolyte effects. So far, some controversial models have been proposed for the role of HCO3– involved in the PEC CO2 reduction (e.g., proton donor, carbon source). Understanding the role of electrolytes is crucial for controlling the product selectivity. However, only a few studies have addressed how HCO3– behaves in PEC CO2 reduction. In this study, we applied in situ surface-enhanced Raman spectroscopy (in situ SERS) to investigate the behavior of HCO3– near the surface during the PEC CO2 reduction on Au/p-GaN catalysts. The amount of bidentate carbonate (CO32–), which originates from HCO3–, takes a maximum at a certain potential (−1.3 V vs Ag/AgCl). Both CO and CH4 are produced depending on the applied potential as the PEC CO2 reduction products. The potential dependence of bidentate CO32– can be correlated to the selectivity of CH4 production. Our results suggest that the HCO3– plays a role in improving the selectivity of CH4 over CO via the adsorption of the CO32– and donation of protons.