Effects of N Precursor on the Agglomeration and Visible Light Photocatalytic Activity of N-Doped TiO₂ Nanocrystalline Powder

N-doped TiO₂ nanocrystalline powders were prepared by the sol–gel method using various N precursors, including triethylamine, hydrazine hydrate, ethylenediamine, ammonium hydroxide, and urea. The samples were characterized by X-ray diffraction, N₂ adsorption isotherms, transmission electron microscopy, ultraviolet-visible diffuse reflectance spectroscopy, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy. The photocatalytic activities of as-prepared samples under irradiation of visible light ( > 405 nm) were evaluated by photodecomposition of methyl orange. The alkalinity of N precursor was found to play a key role in the gel process. The N precursor with moderate alkalinity causes TiO₂ nanoparticles to be sol-transformed into a loosely agglomerated gel. This transformation facilitates the preparation of an N-doped TiO₂ powder with small nanocrystal size, large specific surface area, and high N doping level and results in high visible light photocatalytic activity. The N in TiO₂ with N 1s binding energy at 399–400 eV may be assigned to the N-H species located in interstitial sites of TiO₂ lattice which is the active N species responsible for the visible light photocatalytic activity. The N species of N 1s peak at ∼402 and ∼405 eV are ineffective to the visible light photocatalytic activity and may inhibit the photocatalytic activity. Moreover, a TiO₂ nanoparticle powder with large specific area can be achieved by using urea as a template and then by using ammonium hydroxide to transform the sol into gel.