Cu-Ion-Implanted and Polymeric Carbon Nitride-Decorated TiO₂ Nanotube Array for Unassisted Photoelectrochemical Water Splitting

Photoelectrochemical (PEC) water splitting over TiO₂ photoanodes is a promising strategy for hydrogen production due to its eco-friendly, energy-saving, and low-cost nature. However, the intrinsic drawbacks of TiO₂, i.e., the too wide band gap and rapid exciton recombination, significantly limit further enhancement of its performance. Herein, we report a TiO₂ nanotube array (TNA), which is implanted by Cu ions and decorated by polymeric carbon nitride (PCN) nanosheets, as a photoanode for the high-efficiency PEC water splitting. In such designed material, Cu-ion implantation can effectively tailor the electronic structure of TiO₂, thus narrowing the band gap and enhancing the electronic conductivity. Meanwhile, the PCN decoration induces TiO₂/PCN heterojunctions, enhancing the visible light absorption and accelerating the exciton separation. Upon this synergistic effect, the modified TNA photoanode shows significantly improved PEC capability. Its photocurrent density, solar-to-hydrogen efficiency, and applied bias photon-to-current efficiency achieve 1.89 mA cm^-2 at 1.23 V_RHE (V vs reversible hydrogen electrode), 2.31%, and 1.20% at 0.46 V_RHE, respectively. Importantly, this modified TNA supported on a meshlike Ti substrate can be readily integrated with a perovskite solar cell to realize unassisted PEC water splitting.