Nitrogen and sulfur co-doped TiO2nanosheets with exposed {001} facets: synthesis, characterization and visible-light photocatalytic activity

Nitrogen and sulfur co-doped TiO2 nanosheets with exposed {001} facets (N–S–TiO2) were prepared by a simple mixing-calcination method using the hydrothermally prepared TiO2 nanosheets powder as a precursor and thiourea as a dopant. The resulting samples were characterized by transmission electron microscope, X-ray diffraction, N2 adsorption–desorption isotherms, X-ray photoelectron spectroscopy, and UV-Vis absorption spectroscopy. The electronic properties of N,S co-doped TiO2 were studied using the first-principle density functional theory (DFT). The photocatalytic activity of N–S–TiO2 was evaluated by degradation of 4-chlorophenol (4-CP) aqueous solution under visible light irradiation. The production of hydroxyl radicals (˙OH) on the surface of visible-light-irradiated samples was detected by photoluminescence technique using terephthalic acid as a probe molecule. The results show that nitrogen and sulfur atoms were successfully incorporated into the lattice of TiO2, which resulted in N–S–TiO2 samples exhibiting stronger absorption in the UV-visible range with a red shift in the band gap transition. The first-principle DFT calculations further confirm that N and S co-dopants can induce the formation of new energy levels in the band gap, which is associated with the response of N–S–TiO2 nanosheets to visible light irradiation. Surprisingly, pure TiO2 nanosheets show the visible-light photocatalytic activity for the degradation of 4-CP mainly due to the substrate-surface complexation of TiO2 and 4-CP, which results in extending absorption of titania to visible light region through ligand-to-titanium charge transfer. The N–S–TiO2 samples studied exhibited an enhanced visible-light photocatalytic activity than pure TiO2. Especially, the doped TiO2 sample at the nominal weight ratio of thiourea to TiO2 powder of 2 showed the highest photocatalytic activity, which was about twice greater than that of Degussa P25. The enhanced activitiy of N–S–TiO2 can be primarily attributed to the synergetic effects of two factors including the intense absorption in the visible-light region and the exposure of highly reactive {001} facets of TiO2 nanosheets. The former is beneficial for the photogeneration of electrons and holes participating in the photocatalytic reactions, and the latter facilitates adsorption of 4-CP molecules on the surface of TiO2 nanosheets.