Stable Nitrogen-Doped TiO2 Sols with Enhanced Visible-Light Response and Surface Wettability for Air Purification

Photocatalytic coatings that can be immobilized on outdoor surfaces provide a promising route for distributed and passive purification of atmospheric pollutants. The practical application requires not only visible-light photocatalytic activity but also stable dispersion, facile coating and favorable interfacial properties. In this work, N-doped TiO2 (N-TiO2) sols with different nominal doping contents were prepared through hydrothermal treatment of peroxotitanic acid precursors containing different amounts of urea at 130 °C for 4 h. All samples exhibited anatase phase, while the shifts in XRD and Raman peaks indicated nitrogen-doping-induced lattice distortion. The sols exhibited monomodal particle size distributions with average hydrodynamic diameters in the range of 20.13 nm to 27.07 nm, and the high absolute zeta potentials (>30 mV) demonstrated good dispersion stability. The XPS results confirmed that nitrogen atoms were introduced at interstitial sites with Ti-O-N bonds and increased the relative proportion of surface hydroxyl oxygen from 13.9% to 34.2%. Nitrogen doping improved the visible-light absorption by decreasing the bandgap from 3.06 eV to 2.81 eV and reduced the water contact angle of the sol-derived films from 24.0° to as low as 8.6°. Among all samples, 1.0N-TiO2 exhibited the highest NO removal efficiency of 72.5% under visible-light irradiation and retained good stability over five photocatalytic cycles. These results demonstrate that moderate nitrogen doping can simultaneously regulate the colloidal, optical and interfacial properties of TiO2 sols, making them promising coating materials for functionalization of building envelopes or other outdoor surfaces for air purification.