Improving Charge Separation in Cu₂O/g-C₃N₄/CoS Photocathodes by a Z-Scheme Heterojunction to Achieve Enhanced Performance and Photostability

Fabricating efficient heterojunction photocathodes to accelerate charge transport and long-term stability is important to promote visible light-driven hydrogen evolution. With the strategic combination of type II band edge heterojunctions and passivation layers of graphitic carbon nitride (g-C₃N₄), Cu₂O/g-C₃N₄/CoS photocathodes that achieve high photostability have been fabricated. We used spin-coating and electrospinning techniques to synthesize g-C₃N₄nanosheets and nanowires, respectively, which were deposited on Cu₂O on fluorine-doped tin oxide (FTO) substrates. In case of Cu₂O/g-C₃N₄nanosheets, the loading of g-C₃N₄was varied from 1 to 7 wt% and the highest photocatalytic activity (5.8 mA/cm²at 0 V RHE) was obtained for the heterojunction prepared from the 5 wt% solution. For g-C₃N₄nanowires, varying the electrospinning time (from 0.5 to 12 min) controlled the loading of these nanowires, and the heterojunction with 8 min spinning time displayed the highest photocurrent density (6.6 mA/ cm²at 0 V vs RHE). The higher photocatalytic activity and better stability of the Cu₂O/g-C₃N₄nanowires (8 min)/CoS heterojunction arises from the effective separation and transport of photo-generated charge carriers, which was confirmed by photoluminescence and photocurrent measurements, and from proper protection of the underlying Cu₂O photocathode. Importantly, the Cu₂O/g-C₃N₄heterojunction decorated by CoS proved to be effective for enhancing the stabilization of the Cu₂O photocathode. About 90-95% of the photocurrent density was retained after 5 h of illumination and the faradaic efficiency for hydrogen evolution reached 80%. These results and protocols contribute to the progress of Cu₂O-based photocathodes to be applied in high-efficiency solar hydrogen devices with prolonged photostability.