An electron spin resonance study of rutile and anatase titanium dioxide polycrystalline powders treated with transition-metal ions

Abstract

Rutile and anatase forms of titanium dioxide polycrystalline powders have been treated with low levels, ca. 200 ppm wt/wt, of Cr³⁺ and Fe³⁺ ions. The effects of calcination temperature upon these powders are described. At temperatures below 500 °C, both rutile and anatase forms of doped powder exhibited e.s.r. resonances at g= 1.97 and g= 4.3 ascribed to surface chromium and iron species, respectively. At higher temperature migration of these surface bound ions into the bulk of the rutile powder is observed. In the case of both chromium and iron substitutionally incorporated within the rutile lattice, photochromic activity is observed. Diffusion of ions into the bulk upon calcination is also demonstrated (using e.s.r.) by the corresponding anatase powder, although the results appear to indicate that the process is more difficult for the anatase case. Trapping of transition-metal ions at substitutional sites within the anatase structure is also possible by impregnation of a hydrous precursor pulp of TiO₂ prior to calcination.