Efficient Ta₃N₅ Photoanodes via Interface Engineering of Bixbyite-Type Ta₂N₃ Precursors

Tantalum nitride (Ta₃N₅) shows promising prospects for photoelectrochemical (PEC) water splitting due to its suitable band gap and band edge positions. However, high-performing Ta₃N₅ 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 Ta₂N₃ as a superior precursor for synthesizing thin (≈100 nm) Ta₃N₅ films on silicon substrates, substantially reducing tantalum consumption while maintaining excellent PEC performance. The metastable Ta₂N₃ phase undergoes disproportionation into Ta₃N₅ and conductive Ta-subnitrides during ammonolysis, enabling efficient charge separation without relying on Ta foils. Optimization of ammonolysis conditions reveals a critical balance between Ta₃N₅ crystallinity, subnitride content, and surface Ta³⁺ defect concentrations. Furthermore, degenerately doped silicon substrates enhance hole extraction by suppressing charge accumulation and leakage at the Ta₃N₅/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 Ta₃N₅ 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 Ta₃N₅-based PEC systems in solar fuel technologies.