Annealing gas driven preparation of mesoporous bicrystalline N-doped TiO₂ nanomaterials for sustainable sunlight degradation activity

In our study, N-doped titanium dioxide (N-TiO₂) nanomaterials were successfully prepared using titanium butoxide and guanidinium chloride by simple sol-gel method. The significance of annealing gas environment (air, argon, and nitrogen) on their physicochemical properties and photocatalytic degradation against rhodamine B (RhB) under direct sunlight irradiation was investigated. The XRD and Raman data revealed that the crystal structure of N-TiO₂ nanoparticles was changed from monophase anatase with less crystallinity in argon, and nitrogen to dual-phase anatase/rutile with higher crystallinity in the air. Moreover, DRS and PL results revealed that the introduction of N in the TiO₂ matrix not only led to a red shift towards visible light but also narrowed the band gap (2.35 eV) and suppressed charge carries recombination unlike unmodified material. The BET showed a typical IV isotherm of mesoporous N-TiO₂ materials with high specific surface area in the rage of 80-103 m² g^-1. Furthermore, their RhB photodegradation performance was dictated by the annealing gas type; nobly, the N-TiO₂ prepared in air demonstrated the highest degradation performance (99%) with the highest apparent rate constant (0.0158 min^-1) which is twice faster than the undoped TiO₂. Such improved performance is mainly attributed to its higher crystallinity, mixed phase, aqueous-dispersion character, and lower recombination rate.