Preparation and nonstoichiometric property of wide compositional Fe(III)-doped TiO 2 (anatase)
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Fe(III)-doped TiO2 (anatase) was prepared by the oxidation of FexTiS2. Two calcination methods were used to oxidize FexTiS2. In the first, sulfide was calcined in air at a given temperature for 2 h. In the second method, the sulfide was heated in air at a finite heating rate (2.5 K/min) and then held at a constant temperature for 2 h. Fe(III) ions completely dissolved into TiO2 (anatase), forming Fe(III)-doped TiO2 (anatase), in the composition range of 0 艋 Fe/Ti 艋 0.3 (mole ratio). The properties of the obtained oxide depended on the oxidation method of FexTiS2. The electronic property and the valence stage of the Fe(III)-doped TiO2 (anatase) were examined. The activation energy of electronic conduction decreased with an increase of the doped amount of Fe(III) ions. The x-ray photoelectron spectroscopy result showed that the electron density on the Ti ion in the Fe(III)-doped TiO2 (anatase) was decreased by the Fe(III) doping.
I. INTRODUCTION
Transition metal oxides exhibit various kinds of important electrical and magnetic properties due to their electrons in the d or f orbital.1 The semiconductivity of transition metal oxides can be controlled by doping with foreign cations of different valences.2 In previous papers, we reported that valence-controlled transition metal oxide thin films show unique electronic and optical characteristics, compared with powder samples.3–5 However, the preparation of valence-controlled oxides in a wide compositional range is quite difficult.6 When the solidphase reaction method is employed to obtain valencecontrolled oxides, the oxide mixture must be calcined at high temperature as much as possible because the rapid diffusion of ions is required. In general, calcination at high temperature produces a thermally stable compound with a crystal structure different from the original oxide. In recent years, many solution processes such as the sol-gel method for preparing homogeneous multicomponent oxides have been developed.7 This method provides homogeneously mixed oxides without serious segregation.8,9 Alternatively, if mixed precursors can be oxidized at low temperatures, it is expected that the oxide solid solutions will reflect the homogeneity of the precursors. Since multicomponent sulfides can be obtained
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Address all correspondence to this author. e-mail: [email protected] J. Mater. Res., Vol. 15, No. 4, Apr 2000
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in a wide compositional range, compared with that of the oxides, the sulfides should be appropriate as the precursor of the valence-controlled oxides.10 Anatase type TiO2 has attracted much attention as a photocatalyst.11,12 Valence control can regulate the photocatalytic activity of anatase; however, anatase is unstable above 800 K, so regulation of the solubility of cations in anatase is difficult, even with the aid of the solid-phase reaction method. For example, the solubility of the iron ions in rutile ranges from 3% at 1623 K to 1% at 1073 K.13 In this study, the Fe(III)-doped TiO2 (anatase)
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