Characterisation of iron/titanium oxide photocatalysts. Part 2.—Surface studies

Abstract

A study of the surfaces of particulate solids, formed by the dispersion of iron(III) ions in various titanium dioxide preparations, has revealed that significant changes occur during the calcination of these materials in air at progressively higher temperatures (T/K ⩽ 1273). In the more dilute systems, containing nominal iron(III) concentrations of 0.5–1.0 atom%, solid solutions in the anatase phase exhibit good degrees of dispersion which are maintained as the phase transformation into rutile progresses at more elevated temperatures (T/K 923). With the larger nominal iron concentrations (2 and 5 atom%) X-ray photoelectron (XPS) and diffuse-reflectance (DRS) spectroscopic measurements show that the formation of iron-rich surfaces has commenced already at 773 K through there being a lower limit of solubility of iron in the rutile phase and through the anatase to rutile phase transition becoming detectable at lower temperatures in these solids. The Fe : Ti atomic ratios at the surface with the larger nominal concentrations of iron are consistent with the formation of iron-rich surface phases, α-Fe₂O₃ and Fe₂TiO₅. Inhomogeneities in the distribution of the surface ion species have also been demonstrated by EDX measurements.

The structural character of these solids has been discussed in relation to their photocatalytic activities for the reductive fixation of dinitrogen by water, in which only single-phase systems were found to be active. The inactivity of the multi-phasic solids has been considered in terms of the masking of the active phase by surface layers which create electronic heterojunctions that encourage hole–electron recombination.