Novel continuous single-step synthesis of nitrogen-modified TiO2 by flame spray pyrolysis for photocatalytic degradation of phenol in visible light

A novel rapid and continuous process has developed for the synthesis of nitrogen-doped TiO₂ (N-TiO₂) with flame spray pyrolysis (FSP) method. The nitrogen incorporation into TiO₂ was achieved by a facile modification (addition of dilute nitric acid) in the precursor for the synthesis. The catalysts were characterized by X-ray diffraction, Raman spectroscopy, transmission electron microscopy, diffuse reflectance spectroscopy, and X-ray photoelectron spectroscopy. The doping of nitrogen into the TiO₂ was confirmed by X-ray photoelectron spectroscopy (XPS) and energy dispersive X-ray (EDX) spectroscopy. The UV–vis spectra of the modified catalysts (with primary N source) exhibited band-gap narrowing for 4N-TiO₂ with band gap energy of 2.89 eV, which may be due to the presence of nitrogen in TiO₂ structure. The introduction of secondary N-source (urea) into TiO₂ crystal lattice results in additional reduction of the band gap energy to 2.68 eV and shows a significant improvement of visible light absorption. The N-TiO₂ nanoparticles modified by using secondary N-source showed significant photocatalytic activity under visible light much higher than TiO₂. The higher activity is attributed to the synergetic interaction of nitrogen with the TiO₂ lattice. The 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 atomic percentage has increased monotonically (0.09%–0.15%) with the increase in primary nitrogen source (nitric acid), and significantly boosted to 0.97% when secondary nitrogen source (urea) was introduced. The highest rate of phenol degradation was obtained for catalysts with secondary N source due to increase in N content in the catalyst.