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 degrees C to 950 degrees 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 similar to 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), H-2 evolution (in units of mu mol g(-1) h(-1)) and NH3 formation (in units of mu mol g(-1) h(-1)) over the developed Ti-oxynitride system (similar to 91-96/1278.2/215) is found to be improved compared to the sole oxide (similar to 85-88/458.6/102) and nitride (similar to 79-77/619.32/174) systems. The UV-visible light to H-2 conversion efficiency of the developed oxynitride system is estimated to be similar to 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.