N-Doped titanium dioxide nanosheets: Preparation, characterization and UV/visible-light activity

A series of nitrogen-doped 2D-titanium dioxide nanosheets was synthesized via green and facile procedure from the lyophilized aqueous colloids of peroxo titanic acid by urea addition and annealing in the temperature range of 350–500 °C. Detailed structural characterization (SEM, TEM with EDX, XRD) of N-doped TiO2 confirmed their 2D-foil morphology composed of packed anatase nanocrystals. CHNS analysis showed that the total nitrogen content in the N-doped TiO2 nanosheets is comparable (0.3 wt. %), and as shown in the EPR measurements in solid state, the annealing temperature determines the character of the nitrogen species incorporated in the anatase lattice. Paramagnetic nitrogen bulk centers (Nb•) dominate the X- and Q-band EPR spectra of the synthesized N-doped TiO2 annealed up to 400 °C, while NO species were detected in samples annealed at higher temperatures. The photoexcitation of the N-doped anatase nanosheets resulted in an intense increase of the Nb• signal intensity, especially upon VIS-light exposure, reflecting the selective photoexcitation of the material via diamagnetic Nb– centers. Nevertheless, the situation upon irradiation of dispersed systems is rather different and to link the information on the structure of the nanosheets with their photoinduced performance in suspensions, the indirect techniques of EPR spectroscopy were applied. Effective generation of paramagnetic reactive oxygen species (ROS) upon UV photoexcitation of the N-doped TiO2 nanopowders dispersed in water or dimethylsulfoxide was confirmed by EPR spin trapping technique. The VIS-light-induced ROS formation was significantly lower and correlates well with the results obtained by the photocatalytic decomposition of 4-chlorophenol upon VIS light. Even though the OH-induced capacity of N-doped TiO2 prevails upon UV exposure, the VIS irradiation evokes the formation of photoelectrons capable of the selective reduction processes as demonstrated by the one-electron reduction of 2,2′-azinobis(3-ethylbenzothiazoline-6-sulfonate) radical cation, where the form of nitrogen dopant (e. g. the presence of Nb–/Nb•) in the anatase structure showed a clear effect on the reaction rate. The prepared visible-light active N-doped TiO2 nanosheets exhibiting also adequate UV photoactivity represent promising materials for further development of solar-light active photocatalysts.