Efficient Ta ₃ N ₅ Photoanodes via Interface Engineering of Bixbyite‐Type Ta ₂ N ₃ Precursors

Abstract Tantalum nitride (Ta ₃ N ₅ ) shows promising prospects for photoelectrochemical (PEC) water splitting due to its suitable band gap and band edge positions. However, high‐performing Ta ₃ N ₅ photoanodes typically require Ta foil substrates and film thicknesses of several hundred nanometers, leading to increased tantalum usage and fabrication costs. This study introduces bixbyite‐type Ta ₂ N ₃ as a superior precursor for synthesizing thin (≈100 nm) Ta ₃ N ₅ films on silicon substrates, substantially reducing tantalum consumption while maintaining excellent PEC performance. The metastable Ta ₂ N ₃ phase undergoes disproportionation into Ta ₃ N ₅ and conductive Ta‐subnitrides during ammonolysis, enabling efficient charge separation without relying on Ta foils. Optimization of ammonolysis conditions reveals a critical balance between Ta ₃ N ₅ crystallinity, subnitride content, and surface Ta ³⁺ defect concentrations. Furthermore, degenerately doped silicon substrates enhance hole extraction by suppressing charge accumulation and leakage at the Ta ₃ N ₅ /Si junction, as revealed by photoelectrochemical impedance spectroscopy. This synthesis strategy yields an anodic photocurrent density of 3.86 mA cm ⁻² at 1.23 V _RHE for a 100 nm Ta ₃ N ₅ film on n ⁺ ‐Si(111), outperforming a 500 nm film derived from TaO _x precursor and achieving a 0.15 V cathodic shift in onset potential. These findings establish a scalable, low‐Ta usage platform for advancing Ta ₃ N ₅ ‐based PEC systems in solar fuel technologies.