One-step flame synthesis of N-doped titanium (sub)oxides in rich NH₃-H₂-O₂-N₂ stagnation flames: A parametric study

In this work, we report an experimental parametric study of a fuel-rich ammonia-fueled flame, which serves as a versatile tool for the synthesis of nitrogen-doped titanium sub-stoichiometric oxides (N-TiO₂₋ₓ) with tunable stoichiometry and N-doping levels. By leveraging the unique chemical environment of NH₃/H₂/O₂/N₂ flames, characterized by high temperatures (up to 2000 K) and abundant reactive nitrogen species (NH₂, NH, N), we demonstrate precise control over phase transitions, defect engineering, and the type and level of N-doping in TiO₂₋ₓ nanoparticles (NPs). Parametric variation of the equivalence ratio (ϕ = 1.88–2.61), particle residence time (10–75 ms), and deposition time (10–40 min) enables in situ regulation of NP stoichiometry (e.g., TiO₂, Ti₃O₅, TiO) and N-doping levels (up to 17.8 at.%) without post-synthetic treatment. In situ thermophoretic sampling, followed by ex situ analyses, reveals ultrafast coalescence-etching dynamics (on a ∼ 10 ms timescale) that govern morphology evolution, accompanied by oxidation state reduction and N-doping as TiO_2-x particles evolve in the flame. By integrating simulations of flame chemistry and kinetics with experimental findings, this work establishes a platform for the synthesis of tailored metastable sub-oxides, offering mechanistic insights into NH₃-driven N-doping and composition control.