Titanium dioxide (M-TiO_2), which was doped with 13 different metal ions (i.e., silver (Ag^+), rubidium (Rb^+), nickel (Ni^(2+)), cobalt (Co^(2+)), copper (Cu^(2+)), vanadium (V^(3+)), ruthenium (Ru^(3+)), iron (Fe^(3+)), osmium (Os^(3+)), yttrium (Y^(3+)), lanthanum (La^(3+)), platinum (Pt^(4+), Pt^(2+)), 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 TiO_2 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 TiO_2 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 TiO_2. 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 TiO_2 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 TiO_2, which had relatively high percentages of rutile in the particle phase, showed significantly enhanced visible-light photocatalytic activity for all three reaction classes
