Mechanism of nitrogen-concentration dependence on pH value: Experimental and theoretical studies on nitrogen-doped TiO₂

Experiments and density functional theory (DFT) calculations were combined in an attempt to investigate the origin of the differences in nitrogen concentration incorporated by N-doped TiO₂. X-ray photoelectron spectroscopy (XPS), UV-vis diffusive reflectance spectra (UV-vis), and photodegradation experiments were conducted to set some premises for DFT calculations. Fourier transform infrared (FT-IR) spectroscopy was applied to investigate the NH-containing species in the dried precipitate from the hydrolysis. [Ti(H₂O)_al(NH₃)_d(OH) _cCl_d]^(4-c-d) (a + b + c + d = 6) is defined as the Ti complex produced by the hydrolysis process, and its pH-dependent character was further determined by Gibbs energy calculations for the reaction based on those of related equations. Adsorption energies of NO on TiO ₂ surfaces were calculated to study the ability of NO_x to be incorporated into the TiO₂ matrix under different pH conditions. Finally, we propose three processes as being responsible for the dependence of the nitrogen concentration on the pH value.