Novel one-step synthesis of nitrogen-doped TiO2 by flame aerosol technique for visible-light photocatalysis: Effect of synthesis parameters and secondary nitrogen (N) source

Abstract

For the first time, nitrogen-doped TiO2 was successfully synthesized by a novel single-step flame spray pyrolysis (FSP) method. Our X-ray photoelectron spectroscopy results illustrate that the nitrogen was effectively doped into the crystal structure of TiO2 in our as-synthesized N-TiO2 catalysts predominantly in the form of interstitial nitrogen (Ti–O−N) rather than substitutional nitrogen (Ti–N). The shift of the (1 0 1) plane anatase diffraction peaks to lower angles in our N-doped TiO2 catalysts compared to pristine TiO2 revealed the distortion and strain in the crystal structure instigated by the incorporation of the nitrogen atoms. The growth or expansion of crystal lattice can be attributed to the larger atomic radius of respective nitrogen atoms (r = 1.71 Å) compared to oxygen (r = 1.40 Å). Our single-step rapid aerosol synthesis method directs the nitrogen atoms mainly occupy interstitial positions in TiO2 lattice. The increase in the primary nitrogen content does not impact the bandgap energies (from 2.54 eV to 2.53 eV), whereas increase in the secondary source monotonically decreased (from 2.95 eV to 2.47 eV) the bandgap energies. This observed lowering of the band-gap energy for the flame made N-doped TiO2 materials implies that the nitrogen doping in TiO2 by aerosol method is highly effective in extending the optical response of TiO2 in the visible region. The nitrogen atoms incorporation into the crystal structure of titania alters the electronic band structure of TiO2, resulting to a new mid-gap energy state N 2p band formed above O 2p valance band. This occurrence narrows the band gap of TiO2 (from 3.07 to ∼2.47 eV) in our N-doped TiO2 and shifts the optical absorption to the visible region.