Formation of flower-like Cu₂O thin films induced by nitrate through electro-deposition for PEC water reduction

Cuprous oxide (Cu₂O) is a highly promising photocatalyst that facilitates efficient water splitting and hydrogen production under light conditions. In this study, Cu₂O thin film photocathodes were prepared through electro-deposition, with the inclusion of  \(\mathrm{NO}^{-}_{3}\) ${\mathrm{NO}}_3^{-}$  ions resulting in the formation of a flower-like microstructure. The size, distribution and roughness of these clusters were found to be greatly influenced by the concentration of the \(\mathrm{NO}^{-}_{3}\) ${\mathrm{NO}}_3^{-}$   ions as confirmed by SEM and AFM characterizations. When 0.4 M \(\mathrm{NO}^{-}_{3}\) ${\mathrm{NO}}_3^{-}$  ions were used, a flat and compact structure with the smallest ‘flower bud’ was obtained. This structure achieved a maximum photocurrent density of − 2.90 mA/cm² @0 V vs. RHE, which is 2.2 times greater than that of bare Cu₂O. UV–Vis absorption, steady-state fluorescence spectroscopy and EIS measurements suggest that the compact microstructure facilitates enhanced ultraviolet absorption and separation of photogenerated holes and electrons. This results in a lower charge transfer resistance and a significant increase in photocurrent density. Additionally, a growth mechanism for the flower-like Cu₂O was proposed. The XPS and EDS analyses indicate that the addition of \(\mathrm{NO}^{-}_{3}\) ${\mathrm{NO}}_3^{-}$  during Cu₂O formation results in the adsorption of  \(\mathrm{NO}^{-}_{3}\) ${\mathrm{NO}}_3^{-}$ onto the surface of the initial Cu₂O grain. This, in turn, catalyses the electrocatalytic reduction of  \(\mathrm{NO}^{-}_{3}\) ${\mathrm{NO}}_3^{-}$ on the surface of Cu₂O, leading to the formation of NH + 4 ions as evidenced by XPS.