Nitrogen and sulfur co-doped TiO2 nanosheets 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, N-2 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)-O-center dot) 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.