Photoinduced structural modulation of CuO/C photocathode governs product selectivity in photoelectrochemical nitrate reduction.

Although potential-induced surface restructuring of catalysts is well documented, light irradiation can further enhance this phenomenon, leading to significant changes in catalytic activity and product selectivity. Despite its significance, light-assisted surface reconstruction has not been adequately investigated in previous studies. In this work, we compare the yield and selectivity of ammonia production at a CuO/C photocathode under applied potential and combined light-potential conditions, demonstrating how dynamic surface modulation influences catalytic performance. During the photoelectrochemical nitrate reduction reaction (PEC-NTRR), the CuO/C photocathode achieves a maximum ammonia yield of ∼36.9 µmol h-1 cm-2 at -1.4 V (vs. Ag/AgCl) under illumination, representing a 2.8-fold enhancement over dark conditions at the same potential. Meanwhile, the maximum nitrite yield reaches ∼43 µmol h-1 cm-2 at -1.1 V, nearly 90 times higher than in the dark. This significant increase in both ammonia and nitrite formation under illumination clearly indicates that light plays a crucial role in altering product yield and selectivity. In situ X-ray absorption spectroscopy revealed light-dependent evolution of the CuO local structure under applied potential. While no significant structural transformation occurs at open-circuit potential, applying a negative potential (-1.3 V vs. Ag/AgCl) induces partial reduction of Cu2+ to Cu(0) (Cuδ+), which becomes more pronounced when combined with light irradiation, leading to a more dynamically modulated Cu(0) state (Cu(δ*)+) that affects product selectivity. In situ FTIR spectroscopy tracks the formation of surface-bound intermediates, showing that the reaction proceeds through successive deoxygenation and hydrogenation steps consistent with the conventional mechanism. Overall, this study establishes a clear correlation between light-driven local structural modulation and the resulting enhancement in product yield and selectivity.