Urea-glass preparation of titanium niobium nitrides and subsequent oxidation to photoactive titanium niobium oxynitrides

Abstract

Titanium niobium oxynitrides (TiNbON) are an attractive category of potential photocatalysts, but strategies for preparing them remain limited. We adapt the wet chemical “urea glass” method for pure transition metal nitrides to single-phase mixed-metal titanium niobium nitrides for a range of niobium mole fractions. We then oxidize the nitrides by heating in air to prepare titanium niobium oxynitride that absorbs visible light of λ ≤ 550 nm. The materials are characterized by powder X-ray diffraction, scanning electron microscopy, and diffuse reflectance UV-vis spectroscopy. Their photochemical activity as a function of Nb fraction is benchmarked with methylene blue photomineralization promoted by full-spectrum AM 1.5G solar irradiation with and without a λ ≥ 400 nm cut-on filter. First-order Langmuir–Hinshelwood rate constants for photomineralization reveal a composition with ∼8% Nb to have superior reactivity. Full compositional analysis by Kjeldahl chemical nitrogen determination and energy-dispersive X-ray spectroscopy yields a chemical formula of Ti_0.92Nb_0.08O_1.97N_0.03. Finally, electron paramagnetic resonance spectroscopy correlates a localized Nb⁴⁺ defect with increased photochemical reaction rate.