CuO/Cu₂O nanoflake/nanowire heterostructure photocathode with enhanced surface area for photoelectrochemical solar energy conversion

Branched heterostructures play an important role in solar energy harvesting through an efficient separation of photogenerated charges as well as enhancement in catalytic reaction sites. Here we report the CuO/Cu₂O nanoflake/nanowire heterostructure with the enhanced surface area directly grown on Cu foil through scalable process steps ensuring easy large-scale fabrication. A hydrothermal treatment on Cu₂O nanowires, obtained through anodization of Cu foil and argon annealing, leads to a partial conversion of Cu₂O into CuO nanoflakes resulting in a heterostructure with a highly enhanced surface area facilitating a higher semiconductor-electrolyte interface. Brief annealing at 250 °C for 5 min on the sample results in a photocurrent density of −1.9 mA/cm² (at −0.3 V vs. Ag/AgCl under AM1.5G illumination), which is about an order of magnitude higher than the Cu₂O nanowires. Further, CuO/Cu₂O shell/core heterostructure is fabricated through direct annealing of Cu₂O nanowires to understand the effect of highly enhanced surface area over the charge separation due to heterostructure. A comparative study on both heterostructures by photocurrent, electrochemical impedance, and Mott Schottky measurements reveals that although CuO/Cu₂O heterojunction helps in charge separation, however, the enhanced surface area obtained through nanoflake morphology has a dominating effect in deciding high photocurrent density.