N-doped TiO2 for photocatalytic degradation of colorless and colored organic pollutants under visible light irradiation

Titanium dioxide (TiO₂) can only be stimulated by UV light, making its real application for photocatalytic water treatments ineffective, particularly under sunlight and visible light irradiation. As a result, significant efforts have been conducted over the last decades to fabricate visible light-active TiO₂ photocatalysts through band-gap engineering. Herein, nitrogen-doped titanium dioxide (N-TiO₂) photocatalysts were effectively prepared by utilizing a simple sol–gel process with ethanol as a single solvent and urea as the nitrogen source under ambient temperature and pressure. The effects of urea concentration (0, 2, 4, 6 urea/TTIP mol ratio) on the optical, structural, morphological, and photocatalytic properties of the photocatalysts were investigated. SEM morphology revealed an aggregated nano-spherical shape in all samples. HR-TEM and SAED patterns showed an anatase phase of 2-N-TiO₂. The X-ray diffraction analysis also showed a pure anatase phase for pure TiO₂, 2-N-TiO₂, and 4-N-TiO₂. However, the crystalline phase transformed to amorphous for 6-N-TiO₂. The crystallite size reduced from 14.16 to 9.76 nm upon increasing urea concentration. The band-gap energy of N-TiO₂ also decreased from 3.25 to 2.95 eV. Furthermore, the photocatalytic experiment was examined for the degradation of colorless and colored pollutants, such as salicylic acid (SA), methyl blue (MB), and rhodamine B (RhB). The results showed the photocatalytic activity of 2-N-TiO₂ exhibited an optimum efficiency compared to the 4-N-TiO₂ and 6-N-TiO₂, for photocatalytic degradation of SA (k = 0.0265 min⁻¹), MB (k = 0.0180 min⁻¹) and RhB (k = 0.1071 min⁻¹), under visible light irradiation. Therefore, the results suggest that crystallite size, urea (as an N dopant) concentration, and organic model pollutants were critical parameters for the photocatalytic activity of N-TiO₂ under visible irradiation.