Single-step rapid aerosol synthesis of N-doped TiO2 for enhanced visible light photocatalytic activity

Abstract

For the first time, nitrogen-doped TiO2 was successfully synthesized by a novel single-step flame aerosol method. Our XPS and EDX results illustrate that the nitrogen was effectively doped into the crystal lattice 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 (101) 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 lattice 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. This occurrence narrows the band gap of TiO2 (from 3.12 to ~2.51 eV) in our N-doped TiO2. The lowering of band-gap energy for our flame-made N-doped TiO2 materials indicates that the nitrogen doping in TiO2 by aerosol method is highly effective in achieving a red-shift on adsorption to potentially make use of the energy of visible light. The presence of high amount interstitial nitrogen would modify the band structure and suppress the recombination efficiency of the photogenerated electron–hole pairs, resulting in enriched photocatalytic ability of TiO2 under solar environment or visible irradiation. The incorporation of nitrogen atoms into the lattice structure of TiO2 modifies the electronic band structure of titania, which leads to a new mid-gap energy state N 2p band formed above O 2p valance band. The photocatalytic degradation of phenol as a probe reaction was used to evaluate the photocatalytic properties of the as-synthesized N-doped materials under visible light.