Selective synthesis of β-TaON: The critical influence of oxygen partial pressure in ammonolysis

Tantalum oxynitride (TaON), a member of the transition metal oxynitride family, shows strong potential for solar water splitting due to its tunable optoelectronic properties and visible-light absorption. However, the selective synthesis of thermodynamically stable β-TaON with high phase purity remains challenging because of competing phase formation and sensitivity to processing conditions. In this study, we demonstrate a systematic approach to synthesizing β-TaON by precisely controlling the oxygen partial pressure during the ammonolysis of Ta₂O₅. The oxygen partial pressure is found to critically govern the sequential phase transformation from Ta₂O₅ to Ta₃N₅, with 0.25 L h^-1 delineating the narrow synthesis window required for obtaining phase-pure β-TaON. Comprehensive structural, morphological, optical, and chemical characterizations, complemented by density functional theory (DFT) calculations, reveal strong correlations between nitrogen incorporation, band structure modulation, light absorption, and charge transport properties. Photoelectrochemical (PEC) measurements under simulated solar illumination present that mixed-phase β-TaON/Ta₃N₅ heterostructures exhibit significantly enhanced photocurrent densities, the possibility of operation at higher power densities, and a favorable band alignment conducive to efficient water oxidation due to enhanced light absorption, effective spatial separation of charge carriers, and improved interfacial charge transfer. The introduction of a sacrificial electron donor further enhances PEC performance by promoting interfacial charge transfer and suppressing charge carrier recombination. These findings establish oxygen partial pressure as a critical synthesis parameter for controlling phase composition and tuning the optoelectronic properties of tantalum (oxy)nitrides, offering a promising strategy for the development of high-performance photoanodes for solar fuel production.