State analysis of fluorine-doped SnO2 (FTO) by 57Fe Mossbauer spectroscopy

Fe doped FTO powders were prepared by a chemical precipitation method. Mossbauer spectra of FTO with several concentrations of Fe dopants were decomposed into two doublets (D1 and D2). A doublet with a small electric field gradient (D1) is assigned to Fe3+ in a small distorted site of the rutile structure, and another doublet (D2) to Fe3+ in a large distorted site due to F- ion and oxygen defect. Furthermore, with increase of the annealing temperature the isomer shifts tended to decrease. It suggests that Fe3+ bonds are more covalently bonded than ionic at the higher annealing temperatures. The surface layer of grains were also characterized by Fe-57 CEMS. The defect structures of Fe doped FTO were estimated using the oxygen defect models of tin oxide