Mechanochemical Synthesis of Red-Light-Active Green TiO2 Photocatalysts with Disorder: Defect-Rich, with Polymorphs, and No Metal Loading

Disorder in crystal structures, chemical and physical doping and polymorphs (mixed phase), can extend the absorption of TiO2 from the ultraviolet (UV) to visible and near-infrared (NIR) regions. The photocatalytic activity of colored TiO2 can be controlled by increasing the absorption but with the trade-off of reaction deactivation due to disorders. Herein, we report the facile syntheses of four colored TiO2 photocatalysts, green, gray, orange, and yellow, via a mechanochemical method. The photocatalysts were synthesized by milling TiO2 with or without melamine for 2 h at room temperature. Dry milling under air or argon significantly affected the electronic structure of TiO2 due to the resultant defect and dopant densities. All four colored TiO2 photocatalysts, which had absorptions in the UV-to-NIR regions, were characterized using spectroscopy, microscopy, and diffraction methods. The densities of the defects, oxygen vacancies, and Ti3+ in TiO2, were quantified and were used to evaluate the photocatalytic activity in conjunction with the contents of the four polymorphic phases, i.e., anatase, rutile, high-pressure (TiO2-II; α-PbO2 or columbite), and amorphous phases. Green and orange TiO2, which contained all four phases and had a high density of dopants, oxygen vacancies, and heterojunctions, exhibited a 5-fold enhancement of photocatalytic activity with respect to that of P25. According to the action spectrum of the reaction rate constant, red light from around the NIR region was vital in enhancing the photocatalytic reaction. In addition, the reduced band gap (Eg = 2.3 eV) was an essential factor for high photocatalytic activity in the visible region. Furthermore, we determined the thresholds of absorbance and defect density for the enhancement of the photocatalytic activity under visible light. All of the prepared TiO2 photocatalysts did not need promoters or metal loadings in the reaction.