Phase Engineering of Titanium Oxynitride System and Its Solar Light-Driven Photocatalytic Dye Degradation, H₂ Generation, and N₂ Fixation Properties

In the present work, a phase engineering strategy is explored toward forming a titanium oxynitride (TiOxNy) phase by nitriding a sol–gel-derived TiO2-based precursor at different nitridation temperatures ranging from 450 °C to 950 °C in an ammonia gas environment. The evolved Ti-oxynitride phase is confirmed using XRD, Rietveld refinement, micro-Raman, and HRTEM lattice fringes analysis. Various physicochemical properties of the Ti-oxynitride phase are investigated in comparison with the Ti-oxide and nitride phases obtained in this study. The XPS analysis of oxynitride phase shows dual +3/+4 oxidation states of Ti, which can be attributed to Ti–N and Ti–O network in the oxynitride system. The optical absorption and band gap energy of Ti-oxynitride are found to be favorably altered, compared to the typical Ti-oxides, which are attributed to the plasmonic material-like feature of Ti-nitride phase in the system. From the time-resolved photoluminescence spectra, lifetime of the excited carriers in oxide, nitride, and oxynitride systems is estimated to be ∼3.89, 3.83, and 4.59 ns, respectively, which ascribed to the Ohmic-interface-driven improved electron delocalization in oxynitride phase and corroborated with various photoelectrochemical analysis using voltammetry (cyclic and linear sweep), impedance, and photocurrent measurements. The photocatalytic dye degradation (expressed as a percentage), H2 evolution (in units of μmol g–1 h–1) and NH3 formation (in units of μmol g–1 h–1) over the developed Ti-oxynitride system (∼91–96/1278.2/215) is found to be improved compared to the sole oxide (∼85–88/458.6/102) and nitride (∼79–77/619.32/174) systems. The UV–visible light to H2 conversion efficiency of the developed oxynitride system is estimated to be ∼2.55%. The manifested improved photocatalytic efficiency of oxynitride could be attributed to the synergy of oxide-nitride phases facilitating the effective light harvesting properties via plasmonic features and inter/intratransfer of charge carriers in the system via Ohmic contacts, which eventually promote the multifacet redox reactions toward various photocatalytic applications, as demonstrated in this study