CuS-Decorated GaN Nanowires on Silicon Photocathodes for Converting CO₂ Mixture Gas to HCOOH

Hybrid materials consisting of semiconductors and cocatalysts have been widely used for photoelectrochemical (PEC) conversion of CO₂ gas to value-added chemicals such as formic acid (HCOOH). To date, however, the rational design of catalytic architecture enabling the reduction of real CO₂ gas to chemical has remained a grand challenge. Here, we report a unique photocathode consisting of CuS-decorated GaN nanowires (NWs) integrated on planar silicon (Si) for the conversion of H₂S-containing CO₂ mixture gas to HCOOH. It was discovered that H₂S impurity in the modeled industrial CO₂ gas could lead to the spontaneous transformation of Cu to CuS NPs, which resulted in significantly increased faradaic efficiency of HCOOH generation. The CuS/GaN/Si photocathode exhibited superior faradaic efficiency of HCOOH = 70.2% and partial current density = 7.07 mA/cm² at -1.0 V_RHE under AM1.5G 1 sun illumination. To our knowledge, this is the first demonstration that impurity mixed in the CO₂ gas can enhance, rather than degrade, the performance of the PEC CO₂ reduction reaction.