Enhancing the Photocatalytic Activity of LaTiO₂N Inverse Opal Photonic Crystals by La/Ti Stoichiometry Optimization to Suppress Ti³⁺ Defects

Polycrystalline LaTiO 2 N inverse opal (IO) photonic crystal photocatalysts with different cation ratios (La/Ti) were synthesized using polymer colloidal crystals (CC) as sacrificial templates. By adjusting the La/Ti ratio in LaTiO 2 N IO powders, the number of defects (i.e., Ti 3+ centers) generated during the thermal nitridation of La 2 Ti 2 O 7 IO precursors to LaTiO 2 N IO with ammonia could be controlled. As the La/Ti ratio increases, the number of Ti 3+ centers and oxygen vacancies generated in LaTiO 2 N decreases. Excessively precipitated La 2 O 3 acts as a protective agent that suppresses the formation of defects occurring during the thermal nitriding process. LaTiO 2 N IO prepared with a La/Ti ratio of 1.05 exhibited superior photocatalytic activity for tetracycline degradation under visible light compared to LaTiO 2 N IO photocatalysts with higher or lower La/Ti ratios. The enhanced activity was due to the minimization of Ti 3+ defects (bulk electron traps). These results demonstrate that the combination of photonic crystal engineering and the reduction of defect density in oxynitride is a promising approach for enhancing the photocatalytic activity of oxynitride photocatalysts.