Processing of iron-doped titania powders in flame aerosol reactors

Abstract

A flame aerosol reactor was used to synthesize Fe(III)-doped titania powders. The processing conditions were controlled to obtain varying ratios of Fe:Ti in the as processed powders. The iron was incorporated into the titania lattice and promoted the conversion of the anatase to the rutile phase. With an increase in the iron dopant concentration, a decrease in the crystal size of the resultant titania particles was observed, along with a conversion to the amorphous state. The defect structure was further explored by Raman spectroscopy, revealing an increased shift and broadening of the anatase peaks with an increasing iron dopant concentration, and was attributed to shrinkage in the grain size. Absorption spectra revealed a shift of the absorption band toward the visible frequencies. Powders with Fe:Ti ratio exceeding 0.8 resulted in a binary mixture that had superparamagnetic characteristics.

Introduction

Combustion processes are used in industry for the large scale production of powders such as silica, titania and carbon black [1]. These processes typically operate at atmospheric pressure, are low cost and are readily scalable for production of large volumes [2]. Flame aerosol reactors have been extensively used in laboratory scale systems to produce ceramic powders such as magnetic oxides, high temperature superconductors and photocatalysts [3], [4], [5], [6], [7]. The gas phase route used in a flame reactor provides for a well-mixed system of precursors at the atomic level, thus making it feasible to process chemically homogenous powders. A mechanistic understanding of flame aerosol processes [8] allows the choice of processing conditions to also produce non-homogeneous composite powders and particulate coatings [9].

Titania is a biologically and chemically inert compound that finds applicability as a pigment and as a photocatalyst. Titania has been widely studied for use in air and water purification [10], photosplitting of water to produce hydrogen, for odor control, and as a disinfectant for destroying microorganisms [11]. Photocatalytic or photoactivated reactions are applicable to a wide range of valuable industrial processes such as organic synthesis [12], photodestruction of toxic compounds, and purification of drinking water [13], [14], [15], [16]. The anatase form of TiO₂ has been the most extensively employed in photocatalytic reactions because of its high activity and chemical stability [10], [13], [14], [15], [16]. For example, the anatase phase of titania has applicability as a photocatalyst for several problems of environmental interest [10], [17], as a catalyst for sulfur removal [18], for toxic metals capture [19], [20], [21], and as an additive in cosmetics due to its efficient sun screen properties [22]. Recently, Yang et al. [7], [23] have used it for coating of steel substrates to provide its stainless and corrosion resistant characteristics. The electronic structure of titania is characterized by a filled valence band and an empty conduction band. When a photon with energy equal to or exceeding the optical band gap energy is incident, an electron is readily excited, creating an electron-hole pair which may then participate in oxidation–reduction reactions [24]. Two factors appear to be important in establishing the overall efficiency of the photoreaction: grain size and role of dopants [25]. It has been reported that as the grain size approaches the quantum domain, the photoreactivity increases due to higher efficiencies of interfacial charge transfer (before possible recombination of electron and hole). An increase in photo reactivity has been observed for doped titania powders, possibly due to the trapping of the charge carriers in defect centers [26], [27]. In addition, the use of dopants has resulted in the shifting of the absorption spectra towards the visible spectrum [28] providing a potential for using solar light.

Fe³⁺ has been used by many researchers as a dopant for titania. Bockelmann et al. [28] used a wet method of solution–evaporation–drying to obtain quantum-sized titania doped with 5% to 50% iron. The absorption band-gap edge was observed to shift toward the visible solar spectrum. Choi et al. [25] used a similar wet process to dope several transition metals (including iron) in titania, and the effect of dopants on its quantum yield and bandgap shift was studied. Borgalleo et al. [29] used a solid state surface doping technique (Cr) to extend the absorption band-gap of anatase titania in the visible region. Ohshima et al. [30] used a spray pyrolysis method to prepare a mixture of titania and ZnO composites, and a wider range of UV shielding was observed from 200 to 370 nm. The reported quantum efficiency and the band gap shift varied depending on the defect structures, grain sizes and attendant surface morphology that resulted from the different preparation techniques. Optical characterization of titania by absorption spectroscopy and Raman scattering has been carried out and reported as a function of stoichiometry [31] and grain size in the submicrometer and nanometer ranges [7], [38]. It was found that the 141- (E_g), 394- and 516.2-cm⁻¹ modes blue shifted whereas the 638-cm⁻¹ mode red shifted as the grain size of anatase titania decreased. This was attributed to the dispersion of optical phonos with k near the zone center (k=0) [7], [32].

There is a need to develop processes to produce powders with well-tailored properties for a variety of applications. For example, iron-doped titanium dioxide has applicability as a photocatalyst that is activated using solar light. At higher ratios of Fe:Ti, superparamagnetic materials with applications in magnetic refrigeration can be produced. In this paper, a flame aerosol process for synthesizing nanostructured, iron-doped titania powders is described. By controlling the processing conditions, it is demonstrated that powders with varying ratios of Fe:Ti can be readily produced. A detailed solid state characterization (XRD, Raman spectroscopy, UV–Vis absorption spectroscopy, Mössbauer spectroscopy) is carried out to establish the properties of these powders.