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 (VN) 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 (Jsc) of 3.28 mA cm−2 after decorating oxygen evolution reaction (OER) cocatalysts under AM 1.5 G irradiation. Moreover, the Jsc 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.