Tuning Optoelectronic Properties and Photoelectrochemical Performance of β-TaON via Vanadium Doping

The application of β-TaON for solar-driven water splitting is hindered by limitations in phase purity, stoichiometry, crystallinity, visible-light absorption, carrier mobility, and high recombination rates. This study investigates the impact of vanadium doping (0-25 at.% V) on the structural, optoelectronic, and photoelectrochemical properties of β-TaON using both experimental and density functional theory (DFT) approaches. Phase-pure β-TaON is retained up to 10 at.% V, beyond which secondary phases (Ta₂O₅ and VN) form, indicating a threshold of ∼10 at.% under the applied synthesis conditions. All samples exhibit a porous microstructure. Increasing vanadium content induces a redshift in the absorption edge, reducing the bandgap from 2.72 eV (undoped) to 2.38 eV at 25 at.% V for the main β-TaON phase, in agreement with DFT results. X-ray photoelectron spectroscopy confirms substitutional incorporation of V⁵⁺ for Ta⁵⁺ in the β-TaON lattice. DFT calculations reveal reduced electron effective mass, enhanced n-type conductivity, and favorable band edge shifts enabling spontaneous overall water splitting at ≤10 at.% V. Photoelectrochemical measurements show improved photocurrent and more negative onset potentials for 5-10 at.% V, while higher V doping degrades performance due to phase segregation, which likely increases recombination and hinders interfacial charge transport. Vanadium doping (≤10 at.% V) is an effective strategy for tuning the electronic structure and enhancing the optical properties and photoelectrochemical performance of β-TaON.