Type-II WO ₃ /NiOOH nanoflakes enable efficient and selective low-bias photoelectrochemical saline water-splitting reaction

Tungsten trioxide (WO₃) is an n-type semiconductor composed of earth-abundant and low-cost elements, known for its excellent stability in both acidic and neutral environments. Despite these advantages, its photoelectrochemical (PEC) efficiency is significantly hindered by high charge-carrier recombination and restricted visible light absorption. To address these limitations, a highly active WO₃/NiOOH heterostructure is developed by electrodepositing γ-NiOOH nanoparticles (NPs) on WO₃ nanoflakes. The heterostructure exhibits a notable enhancement in photocurrent density, achieving 2.23 mA cm^-2, in contrast to bare WO₃, which can generate 1.53 mA cm^-2 at an applied bias of +1.2 V vs. Ag/AgCl. It is attributed to the suppression of surface charge-carrier recombination, along with enhanced charge-carrier separation and injection in the heterostructure. Furthermore, the WO₃/NiOOH heterostructure demonstrates a cathodic shift of ∼400 mV in onset potential, significantly reducing energy losses during the PEC process. The superior PEC performance arises from the excellent hole transport property of γ-NiOOH, which facilitates oxygen evolution reaction (OER) kinetics at the interface. Additionally, the heterostructure exhibits enhanced light absorption capabilities, which contribute to its overall efficiency. The photostability of the heterostructure is examined over a five-hour test period. NiOOH undergoes initial delamination within one hour, followed by surface reconstruction of NiOOH and its partial conversion into Ni(OH)₂ within the next two hours, maintaining its performance under highly corrosive saline water conditions. Post-photostability X-ray photoelectron spectroscopy (XPS) analysis reveals the upshifting of the Fermi level due to the presence of the mid-gap states composed of oxygen vacancies. The heterostructure exhibits excellent efficiency and selectivity towards water oxidation in corrosive saline water conditions, which enhances the long-term durability of the photoelectrode.