Chang, Chia‐WeiChia‐WeiChangLai, Bo‐YingBo‐YingLaiCHANG-MING JIANG2025-08-202025-08-202025-07-26https://scholars.lib.ntu.edu.tw/handle/123456789/731527Tantalum 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 VRHE for a 100 nm Ta3N5 film on n+-Si(111), outperforming a 500 nm film derived from TaOx 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.enenergy conversionheterojunctionmetastabilityphotoelectrochemistrytantalum nitride[SDGs]SDG7journal article10.1002/smll.202505487