Synergistic Metal-Support Interactions in Au/GaN Catalysts for Photoelectrochemical Nitrate Reduction to Ammonia

Metal-support interactions are crucial in the electrochemical synthesis of ammonia (NH₃) from nitrate (NO₃ ^-) reduction reaction, enabling efficient NH₃ production under mild conditions. However, the complexity of the reaction pathways often limits efficiency. Here, a photoelectrochemical system composed of gold (Au) nanoclusters supported on gallium nitride (GaN) nanowires is introduced, grown on a n⁺-p Si wafer, for selective reduction of NO₃ ^- to NH₃ under solar illumination. NO₃ ^- ions are preferentially adsorbed and reduced to nitrite (NO₂ ^-) on the GaN nanowires, which then transfer to adjacent Au nanoclusters to complete the NH₃ synthesis. This mechanism is confirmed by both experimental data and theoretical calculations. Optimizing the surface coverage and size of Au nanoclusters on the GaN nanowires significantly enhanced catalytic activity compared to that on planar n⁺-p Si photoelectrodes, achieving a faradaic efficiency of 91.8% at -0.4 V_RHE and a high NH₃ production rate of 131.1 µmol cm^-2 h^-1 at -0.8 V_RHE. These findings highlight the synergetic effect between metal co-catalysts and semiconductor supports in designing photoelectrodes for multi-step NO₃ ^- reduction.