Sol gel synthesis and characterization of tin oxide and doped-tin oxide nanosized materials used for gas-sensing
SnO2 -based gas sensors have been shown to be convenient tools for detecting inflammable or toxic gases diluted in air. Grain size reduction and mixed tin oxidation states are two of the main factors enhancing sensor properties of undoped and doped tin oxides. Two tin(II) halide precursors were utilized along with variations in solvent, aging times, drying atmosphere and annealing temperatures to synthesize SnO, SnO2 or the SnO/SnO2 mixed powders by a modified sol-gel method. X-ray diffraction and UV-Visible Diffuse Reflectance Spectroscopy were the primary techniques used to characterize powder structure and properties. It has been found that SnCl2 is a suitable precursor for synthesis of nanocrystalline mixed Sn(II)/Sn(IV) oxide powders. The ratio of SnO to SnO2 can be adjusted by sintering/annealing of the SnO/SnO 2 mixture in air. A rarely observed form of Sn3 O4 and another intermediate phase, orthorhombic SnO2 , can be generated at the range of 500-600°C. SnBr2 and SnBr4 can be used as precursors for an efficient low-temperature, atmospheric pressure vapor deposition of nanocrystalline SnO2 , most likely via a partially hydrolyzed Sn(IV) bromide intermediate. As vapor-deposited, the average SnO 2 crystallite size is approximately 3 nm, and can be increased systematically from 3 nm to 16 nm by annealing in air. The presence of amorphous material and the remaining hydroxyl groups in the SnO2 crystals are possible causes of the crystalline deformation of SnO/SnO2 materials precipitated from SnCl2 . SnO 2 obtained from the SnO/SnO2 mixture are more disordered than that from tin(H) oxyhydroxide and from SnO2 gel heated at the same sintering temperature. The more disordered material has lower band gaps and bigger Urbach energies. Coprecipitation of a second metal, such as Cr, Fe, Zn, Co, K and Cu, results in variations in SnO2 crystallite growth patterns, depending on identity of the added dopant. Metal oxide materials mixed with polypyrrole were tested as composite thick film gas sensors at room temperature. The materials exhibited sensitivity and selectivity patterns depend on phase composition and morphology. The materials exhibit promise for construction of an integrated array of multiple SnO 2 -based sensors to analyze organic vapor mixtures at ambient temperature.
"Sol gel synthesis and characterization of tin oxide and doped-tin oxide nanosized materials used for gas-sensing"
(January 1, 2004).
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