Saila Karvinen

The effects of trace elements on the crystal properties of TiO2

Abstract The effects of trace element doping of TiO2 on the crystal growth and on the anatase-to-rutile phase transformation of TiO2 were investigated. The co-precipitation process, from sulfate solution, of doped (Cr, Fe, V, Nb, Si, P) TiO2 was also studied. The heating temperatures were 473, 673, 873, 993, 1133 K and a higher temperature needed to achieve a rutile content of 98–99%. Traces of reduced titanium were found in freshly calcined anatase by X-ray diffraction. Pure anatase structure was found in 85% of the samples heated below 1000 K. Anatase-to-rutile transformation was accelerated by the mmol% content of Nb, Cr, Si, and Fe in TiO2. Interaction of co-precipitated or impregnated cations was found critical in the phase transformation process. Nb retarded the crystal growth during calcination. Sulfate ions minimized the specific surface area of TiO2 heated at low temperatures. These results of doped TiO2 serve to promote the development of new high-technology TiO2 products for photocatalytic purposes.

Ab initio quantum chemical studies of cluster models for doped anatase and rutile TiO2

Abstract Cluster models for the anatase and rutile structures of bulk TiO2 were calculated by an ab initio Hartree–Fock method with the purpose of establishing qualitative trends for the size of the band gap upon transition metal doping. Different sizes and stabilization methods of the models (Onsager reaction field method, surrounded by water) were tested. The effect of doping cations on the size of the band gap was studied with the anatase model Ti20M(n+)O42 and the rutile model Ti14M(n+)O30. Doping the anatase model with Ti3+, V3+, Cr3+, Mn3+ and Fe3+ narrowed the anatase band gap and the results were principally in agreement with the photocatalytic activity results of doped anatase. Doping of the rutile model with V3+, Mn3+, and Fe3+ did not have a noticeable effect on the size of the rutile band gap. Chromium(3+), on the contrary, broadened the band gap of the rutile model.