Tin-Doped Hematite Nanoparticles for Gas-Sensing Applications
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Tin-Doped Hematite Nanoparticles for Gas-Sensing Applications Monica Sorescu1, Lucian Diamandescu1,2 and Doina Tarabasanu-Mihaila2 Duquesne University, Department of Physics, Pittsburgh, PA, U.S.A 2 National Institute for Materials Physics, Bucharest-Magurele, Romania 1
ABSTRACT
Structural and morphological characteristics of (1-x)α-Fe2O3-xSnO2 (x=0.0-1.0) nanoparticles obtained under hydrothermal conditions have been investigated by X-ray diffraction (XRD) and Mössbauer spectroscopy. On the basis of the Rietveld structure refinements of the XRD spectra at low tin content, it was found that Sn4+ partially substitutes for Fe3+ at the octahedral sites. The mean particle dimension decreases from 70 to 6 nm as the molar fraction x increases. The estimated solubility limits in the system of tin-doped hematite nanoparticles synthesized under hydrothermal conditions are x0.7 for Fe3+ in SnO2.
INTRODUCTION
The study of semiconducting oxides has become increasingly important due to their sensing properties in the detection of toxic and dangerous gases, such as CO, NO2, Cl2 and CH4 [1-3]. Enhanced gas properties are expected for nanostructured semiconducting oxides due to the great surface activity provided by their high surface areas [4-8]. It was suggested that the content of Sn4+ may have an important role in the gas sensing activity of hematite [9, 10]. However, the mechanism of sensing in (1-x)α-Fe2O3-xSnO2 is not well understood, due to incomplete understanding of its microstructure characteristics. In this paper we report the hydrothermal synthesis of (1-x)α-Fe2O3-xSnO2 nanoparticles over the entire concentration range of x=0.0-1.0. X-ray diffraction (XRD) and Mössbauer spectroscopy have been used to correlate the structure, morphology and phase dynamics in this system in connection with the tin concentration. Experimental evidence of the solubility limits of Sn4+ in hematite and Fe3+ in SnO2 are discussed.
EXPERIMENTAL
Hydrothermal synthesis of (1-x)α-Fe2O3-xSnO2 system was carried out in a 50 mL Teflon lined autoclave. The reaction precursors were aqueous mixtures of Fe (III) chloride hexahydrate and tin (IV) chloride pentahydrate. Precipitation agent was an ammonium hydroxide solution and a pH equal to 8 was achieved. The mixture was heated at 200oC for 4 hours at 15 atm. The resulted precipitate was filtered, washed and dried in a furnace at 105oC.
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The structure of the powders was examined using Rigaku D-2013 X-ray diffractometer with CuKα radiation (λ=1.540598 Å). The 57Fe Mössbauer spectra were recorded at room temperature using a 57Co in Rh source and an MS-1200 constant acceleration spectrometer.
RESULTS AND DISCUSSION
The hematite structure is based on the hexagonal close packing of oxygen with iron in 2/3 of the octahedral vacancies. The lattice parameters are a=5.038 Å, c =13.772 Å and the space group is R32/c. The spin reorientation transition at the Morin point of 260 K results in a weak ferromagnetic moment along the electric field gradient axis. SnO2 is tetragonal,
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