Enhanced visible light-induced charge separation and charge transport in Cu₂O-based photocathodes by urea treatment

Carrier density, photocharge transfer kinetics, and charge transfer resistance of the anodized Cu-Cu₂O-CuO photocathode were greatly improved using thermal treatment with urea. Time-correlated single-photon counting (TCSPC) results revealed the faster electron transfer kinetics from Cu₂O to CuO in the urea-treated Cu-Cu₂O-CuO composite photoelectrodes. Preservation of the metallic copper component via the intermediate Cu₃N during the treatment facilitated higher bulk conductance of the Cu-Cu₂O-CuO photocathode for improved charge transport. Higher carrier density was also observed in the urea-treated photoelectrode, which was possibly attributed to the presence of nitrogen as a dopant. Furthermore, the compact outer layer of CuO protected the underlayer Cu₂O from being in direct contact with the aqueous solution. This suppressed the photocorrosion of Cu₂O and resulted in the higher photostability of the Cu-Cu₂O-CuO film. When these advantages were combined, the urea-treated Cu-Cu₂O-CuO film showed a higher photocurrent of 2.2 mA/cm² and improved stability versus that of the conventional Cu-Cu₂O-CuO film (1.2 mA/cm²). To improve the charge transfer kinetics and carrier density, this paper provides a new strategy for synthesizing effective and stable Cu₂O-based photoelectrodes by using urea treatment.