One-Step Flame Synthesis of White-to-Yellow-to-Black Titanium (Sub-) Oxides with Tunable Oxygen Vacancies and N-Doping: Demonstration of Efficient Photocatalytic Hydrogen Evolution

The synthesis of nitrogen-doped titanium (sub-)oxide (N-doped TiO_2-x) nanoparticles is reported via a novel one-step flame method that enables a significant enhancement of photocatalytic properties. The NH₃-fueled quasi-1D stagnation flame provides a unique high-temperature environment (up to 2000 K), enabling rapid, scalable production of tailored N-doped metal oxides through reactive nitrogen radicals. The as-synthesized nanoparticles exhibit nitrogen doping levels varying from 2.5% to 17.1%, which lead to considerable bandgap narrowing and an impressive visible light absorption range of 280–2000 nm. Characterization reveals mean particle sizes ranging from 15 to 25 nm, tunable crystal phases from anatase to rutile, and specific surface areas ranging from 60.1 to 101.9 m² g⁻¹. Notably, the optimized 1.0-TiO_2-x demonstrates a remarkable hydrogen evolution rate of 12.5 mmol·g⁻¹·h⁻¹ under solar irradiation. This exceptional performance is attributed to the synergistic effects of suitable nitrogen doping and oxygen vacancies, along with specific structural configurations, all of which facilitate charge separation and enhance light absorption. This study highlights the potential of NH₃-fueled flame synthesis for developing multifunctional photocatalysts, thereby paving the way for advancements in sustainable hydrogen production and diverse applications in photocatalysis and materials science.