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.