A Ta3N5 photoanode with few deep-level defects derived from topologic transition of ammonium tantalum oxyfluoride for ultralow-bias photoelectrochemical water splitting

An open challenge for developing solar-driven Ta₃N₅-based photoanodes with the ability to induce low-bias photoelectrochemical (PEC) water splitting is that their deep-level defects originated from low-valent tantalum cations (Ta³⁺) and nitrogen vacancies (V_N) seriously reduce the photovoltage and thus increase the bias for water splitting. Herein, we developed an effective topotactic transition synthesis route of producing few deep-level defects porous Ta₃N₅ film from the precursor film of ammonium tantalum oxyfluoride compound ((NH₄)₂Ta₂O₃F₆) pyramids on the Ta foil. The highly electronegative fluoride ions in (NH₄)₂Ta₂O₃F₆ could weaken the Ta–O bonds and the accompanied porous structure facilitates reactant diffusion, which favors the complete nitridation. Consequently, the resulting porous Ta₃N₅ film has very few deep-level defects, enabling an ultralow photocurrent onset potential at 0.2 V (vs. RHE) and a short-circuit photocurrent density (J_sc) of 3.28 mA cm⁻² after decorating oxygen evolution reaction (OER) cocatalysts under AM 1.5 G irradiation. Moreover, the J_sc can retain 85% of the initial value for a 5 h continuous stability test. By reducing the particle size of (NH₄)₂Ta₂O₃F₆ pyramid precursor, the deep-level defects could be further lowered in the Ta₃N₅ film, achieving the photoactivity for water oxidation at 0 V (vs. RHE) after modifying the OER co-catalyst.