Investigation of p-CuNb₂O₆ for use as photocathodes for photoelectrochemical water splitting

P-type copper niobate (CuNb2O6) was prepared for the first time by a two-step spray pyrolysis method on FTO glass substrates. Its feasibility in photoelectrochemical (PEC) decomposition water was systematically evaluated. The physical and photophysical properties of the thin film, including band edge position, bandgap energy, carrier lifetime, and incident photo-to-current efficiency (IPCE), were characterized and analyzed. The optical band gap of the prepared gray CuNb2O6 film is similar to 1.41 eV, which indicates that it can absorb visible light and achieve sunlight-driven water splitting. The photocurrent density under simulated sunlight at AM1.5 was 0.25 mA/cm(2), and the IPCE value could reach up to 3% at 400 nm in an aqueous solution (pH = 6.8) when applying a bias of 0.4 V vs. RHE. Despite exhibiting a good optical band gap and photocurrent density, the fabricated p-CuNb2O6 thin films remained unstable under illumination in aqueous solutions due to the reduction reaction of copper. Based on these findings, this study provides a new opinion on the applicability of CuNb2O6 as a photocathode material for PEC water splitting.