Characterisation of iron/titanium oxide photocatalysts. Part 1.—Structural and magnetic studies

Abstract

An extensive study of the structural and morphological properties of the dispersion of iron(III) ions in titanium dioxide has revealed that well dispersed biphasic solids (Fe³⁺ ions dispersed into the lattice of both modifications of TiO₂, anatase and rutile) are formed with co-precipitated solids which nominally contain 0.5 and 1.0 atom% of iron. Heavily iron-loaded ( >5%) solids become multi-phasic at 923 K and above, with the production of α-Fe₂O₃ at the lower segregation temperature (923 K) and Fe₂TiO₅ at the highest temperatures (1073 and 1273 K) from a reaction between the segregated α-Fe₂O₃ and the saturated iron-doped titanium dioxide. Magnetic susceptibility studies of the solids confirm that a good degree of dispersion of iron exists in the co-precipitated solids at 0.5 and 1.0% iron loadings (Curie law behaviour), and that stronger magnetic interactions (Curie–Weiss law behaviour) arise in heavily loaded specimens that have been fired at the highest temperatures (1273 K). At lower temperatures, two unusual phenomena have been observed with specimens that have been loaded heavily (5%) by impregnation. At 773 K the temperature dependence of χ_g produces a ‘double slope’ Curie–Weiss plot owing to the formation of an incipient haematite layer on the surface of the TiO₂ particles, while at 923 K this solid produces antiferromagnetic behaviour owing to the appearance of α-Fe₂O₃. The transition from Curie behaviour to Curie–Weiss behaviour occurs in the region of nominal iron concentration between 1 and 2 atom%, which is below the estimated solubility limit of ca. 3 atom% of Fe³⁺ in the rutile phase at 923 K.