Effects of Single Metal-Ion Doping on the Visible-Light Photoreactivity of TiO2

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 (Cr 3+, Cr 6+ )) 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.