Photoelectrochemical CO2 reduction on CuO-Cu2O nanocomposites with noble metal co-catalysts enhances the production of C1 oxygenates and acetate

Multi-junction copper oxide nanocomposite photocathodes formed by CuO nanowire arrays coated with Cu₂O nanopyramids or stacked nanoprisms, i.e. CuO(NWA)|Cu₂O(NPy), exhibit remarkable electrochemical stability toward reductive corrosion. Whilst bare CuO nanowires are extremely prone to decomposition even at the mildest potentials applied, the core@shell morphology of CuO(NWA)|Cu₂O(NPy) resists cathodic potentials up to −0.7 V (vs. AgCl/Ag). Precisely controlled electrodeposition of silver or gold nanoparticles resulted in Ag nanotrails arranged on flatter surface regions in CuO(NWA)|Cu₂O(NPy)|Ag, whereas Au nanoaggregates were deposited on both flat areas and prominent apexes in CuO(NWA)|Cu₂O(NPy)|Au. Photocurrent measurements demonstrated redox process activation, namely above three-fold current density increases, by light for CuO(NWA)|Cu₂O(NPy). The participation of CO₂ in them was also confirmed by around 70 % photocurrent increases (higher than 0.2 mA cm⁻²) and by sensibly milder cathodic potential onsets (≤ −0.4 V), as recorded by linear sweep voltammetry. Photoelectrochemical CO₂ reduction experiments under simulated sunlight yielded oxygenated products in the liquid phase, chiefly formate, although acetate was also consistently produced, especially in the presence of noble metal co-catalysts. Specifically, acetate yields increased by 30–40 % for CuO(NWA)|Cu₂O(NPy)|Ag and CuO(NWA)|Cu₂O(NPy)|Au up to 56 and 54 μM, respectively. Methanol could be also formed under specific conditions. A mechanistic proposal is postulated to account for all stability and selectivity phenomena observed.