Visible-light-responsive bicrystalline (anatase/brookite) nanoporous nitrogen-doped TiO₂ photocatalysts by plasma treatment

The plasma treatment method was employed to characterize nitrogen (N)-doped nanoporous TiO₂ photocatalysts (N-nTiO₂) of ~18nm diameter. With respect to the organic dye degradation efficiencies and antibacterial properties after exposure to visible-light irradiation, N-nTiO₂ showed better performance than the alternative commercial 5nm anatase TiO₂ (TiO₂), as-grown nanoporous TiO₂ (as-grown nTiO₂), and Ar-plasma-treated nanoporous TiO₂ (nTiO₂) photocatalysts. This was attributed mainly to N-nTiO₂'s higher anatase/brookite phase crystallinity and large surface area (375.9m²g^-1). The N doping of N-nTiO₂ was confirmed by a shift of ~0.25° toward the higher angles in the XRD patterns and high-resolution X-ray photoelectron spectroscopy (HR-XPS) surface analysis results. Additionally, N-nTiO₂'s total surface energy was significantly increased. Its photoluminescence (PL) response and apparent quantum yield inhibited the recombination of holes and electrons and decreased the band-gap energy (Eg<3.2eV), both of which contribute to activation in an expanded visible-light range. Ultraviolet (UV) and visible-light irradiation of the organic dye model Rhodamine B (Rho B), both of 70min duration, resulted in rate constants of 3.381 and 4.269h^-1 for N-nTiO₂, respectively. In contrast, under exposure to visible light, the value for nTiO₂ was 0.395h^-1. Moreover, the antibacterial activity indicated that ~100% of both Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus) were destroyed after 30min visible-light irradiation while photostability was maintained. Considering all of these results, it was concluded that N-nTiO ₂ is an excellent solar-light-activated photocatalyst candidate for water/waste treatment and medical applications.