Robust mesoporous N-doped TiO₂ nanoparticles for wastewater treatment under sunlight

Photocatalysts are vital for tackling environmental crises; however, their poor solar-energy utilization is a bottleneck. Herein, N-doped titania (N/TiO₂) nanomaterials were successfully synthesized using a facile sol–gel technique. The importance of the annealing gases' environment on physicochemical properties and photocatalytic efficiency under sunlight was examined. Spectroscopy data revealed that the spheroidal N/TiO₂ crystals were transformed from monophase anatase with less crystallinity to dual-phase anatase/rutile (A/R) with higher crystallinity in argon/nitrogen and air, respectively. Moreover, XPS confirmed the incorporation of interstitial nitrogen in the bare titania structure; this introduction not only led to a red shift towards visible light but also lowered the bandgap energy (2.35 eV) and suppressed charge-carrier recombination according to DRS and PL results. Furthermore, they showed a typical IV isotherm of mesoporous nanomaterials with a high surface area, up to 103 m²/g. Particularly, their rhodamine B photodegradation and thermal stability were dictated by the annealing gas type. Notably, the N/TiO₂ prepared in air demonstrated the highest degradation performance of 99% with the fastest rate of 0.0158 min^-1, which is twice faster than the control TiO₂. This improved performance is mostly attributed to its higher crystallinity, A/R mixed phase, aqueous-dispersion character, and lower charge recombination. Such a gas-driven synthesis of catalysts has practical applications in designing other solar-energy conversion systems.