Effects of single metal-ion doping on the visible-light photoreactivity of TiO₂

Titanium dioxide (M-TiO₂), which was doped with 13 different metal ions (i.e., silver (Ag⁺), rubidium (Rb⁺), nickel (Ni²⁺), cobalt (Co²⁺), copper (Cu²⁺), vanadium (V³⁺), ruthenium (Ru³⁺), iron (Fe³⁺), osmium (Os³⁺), yttrium (Y³⁺), lanthanum (La³⁺), platinum (Pt⁴⁺, Pt²⁺), and chromium (Cr³⁺, Cr⁶⁺)) at doping levels ranging from 0.1 to 1.0 at. %, was synthesized by standard sol-gel methods and characterized by X-ray diffraction, BET surface area measurement, SEM, and Uv-vis diffuse reflectance spectroscopy. Doping with Pt(Iv/II), Cr(III), v(III), and Fe(III) resulted in a lower anatase to rutile phase transformation (A-R phase transformation) temperature for the resultant TiO₂ particles, while doping with Ru(III) inhibited the A-R phase transformation. Metal-ion doping also resulted in a red shift of the photophysical response of TiO₂ that was reflected in an extended absorption in the visible region between 400 and 700 nm. In contrast, doping with Ag(I), Rb(I), Y(III), and La(III) did not result in a red shift of the absorption spectrum of TiO₂. As confirmed by elemental composition analysis by energy dispersive X-ray spectroscopy, the latter group of ions was unable to be substituted for Ti(Iv) in the crystalline matrix due to their incompatible ionic radii. The photocatalytic activities of doped TiO₂ samples were quantified in terms of the photobleaching of methylene blue, the oxidation of iodide (I-), and the oxidative degradation of phenol in aqueous solution both under visible-light irradiation (λ > 400 nm) and under broader-band Uv-vis irradiation (λ > 320 nm). Pt- and Cr-doped TiO₂, which had relatively high percentages of rutile in the particle phase, showed significantly enhanced visible-light photocatalytic activity for all three reaction classes.