Photocatalytic decomposition of 4-chlorophenol over an efficient N-doped TiO₂ under sunlight irradiation

Using a new nitrogen precursor of a mixture of ammonia and hydrazine hydrate, N-doped TiO₂ photocatalyst with a high efficiency under visible light was synthesized by a precipitation method. The analysis of X-ray photoelectron spectroscopy (XPS) suggested that the doping concentration of nitrogen was 0.45 at%, while it was 0.21 at% or 0.24 at% using single ammonia or hydrazine hydrate as nitrogen precursor. The patterns of the electron paramagnetic resonance spectroscopy (EPR) indicated that the paramagnetic species of NO₂^2-, NO and Ti³⁺ existed as the proposed active species. The ultraviolet-visible (UV-vis) spectra revealed that the band-gap of the N-doped TiO₂ was 3.12 eV, which was slightly lower than 3.15 eV of pure TiO₂. The N-doped TiO₂ showed higher efficiency under both ultraviolet (UV) and visible light irradiations. Moreover, the degradation grade of 4-chlorophenol (4-CP) using the as-synthesized N-doped TiO₂ under sunlight irradiation for 6 h was 82.0%, which was higher than 66.2% of the pure TiO₂, 60.1% or 65.2% of the N-doped TiO₂ using single ammonia or hydrazine hydrate as precursor. Density functional theory (DFT) calculations were performed to investigate the visible light response of the N-doped TiO₂. Our study demonstrated that the visible activities vary well with the concentrations of NO₂^2- species incorporated by N-TiO₂ series photocatalysts and the higher activity of the as-prepared N-doped TiO₂ was attributed to the enhancement of the concentration of NO₂^2- species.