The Effects of Sintering on the Photocatalytic Activity of N-Doped TiO2 Nanoparticles

N-Doped titanium oxide nanoparticles (NPs) were synthesized through the hydrolysis of N-substituted titanium isopropoxide precursors and postsynthesis treated by sintering at different temperatures in a nitrogen atmosphere and in air. X-ray diffraction (XRD) results revealed that the presence of oxygen during the sintering also affects the crystallization of the N-doped TiO 2 NPs. The air and N 2 sintering produced different nitrogen concentrations and visible-light absorbances. The N-doped TiO 2 NPs sintered at 200–250 °C in air for 1/2 h produced the best photocatalytic nanoparticles for visible-light decomposition of methylene blue in water. On the other hand, sintering in N 2 atmosphere at any temperature provided photocatalysts with about half-the activity for visible-light decomposition of methylene blue compared to the air-sintered NPs. The zeta potential of the N-doped TiO 2 NPs solution used in the photocatalytic tests shows that higher sintering temperatures create more negative charges on the NP surface and that the sintering in air creates more negative charges compared to the treatment in N 2, due to the carboxylation of organic surface residues. Computational and ATR-FTIR results indicate that the doping of the nitrogen into the matrix occurs as a complexation between the Ti central metal ion and the N atom and not from a covalent Ti−N bond.