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(ttt) 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(iit) concentrations of 0.51.0 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 2 923). 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, a-Fe,O, and Fe,Ti O,.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.

Enhanced photocatalytic activity in Al-substituted Bi2Fe4O9 submicrocrystals

“…On the other hand, it is well established that only good Fe 3? dispersion in a singlephase system are found to be photoactive [24]. However, It was reported that for x B0.5, Fe 3?…”

New Fe2TiO5-based nanoheterostructured mesoporous photoanodes with improved visible light photoresponses

“…[7][8][9] The most popular dopants for modication of both the optical and photo-electrochemical properties of TiO 2 are transition metals such as Cr, Fe, Ni, V, Mn and Cu. 18 The functionality of these materials results both from the bulk properties of their various components as well as from the properties that are directly related to their interface properties. 3,10 However, an optimum concentration of doping metal ions exists above which the photocatalytic activity decreases.…”

Visible Light-Responsive Titanium Oxide Photocatalysts: Preparations Based on Chemical Methods

“…A mechanism in which shallow trapping of the photoelectron by Fe 3+ followed by a rapid, iron-facilitated transfer of the electron to externally incorporated molecular oxygen has also been suggested as a tentative explanation for the enhancement of photocatalytic activity by this doped system (Wang et al 2003). However, it has also been reported that the presence of iron can be detrimental to the photoactivity as a consequence of the Fe 3+ centers acting as electron/hole recombination centers (Bickley et al 1994;Navío et al 1999;Beydoun et al 2000). Certainly, the structural details in each case, which basically depend on the preparation parameters employed, and the iron concentration of the systems are most relevant to explain such discrepancies (Litter et al 1996).…”