SnO <sub> 2 </ Sub> One-dimensional Materials Preparation, Characterization, And Characteristics

Nanoscale one-dimensional (1-D) materials of SnO₂ nanorods, SnO₂ nanorods doped Cd²⁺ and SnO₂ nanorods doped Zn²⁺ were produced by a new method and technology of preparation. It was researched that the method and the technology of preparation, the structures, the growth mechanism of SnO₂ nanorods were studied. Comparing SnO₂ nanorods materials with SnO₂ nano-powders and SnO₂ nano-particles coating on SiO₂ ball prepared, the properties in infrared absorption spectra (IR), redox, gas sensitive of SnO₂ nanorods materials were studied. High sensitivity and high selectivity new gas sensors of SnO₂ nanorods materials were investigated too. Some new results were obtained in this paper. The results are as follows:1. Preparation and characterizations of SnO₂ nanorodsA new synthesis method of SnO₂ nanorods developed based on the preparing way of SnO₂ nanorods via annealing the precursor powders, which were prepared by microemulsion system consisting of oil phase, water phase and surfactant. SnO₂ nanorods were produced via the growth in molten salt medium of precursors, which prepared applying by solid-state reaction coated surfactants at room temperature. It was obtained that the preparing techniques of SnO₂ precursors and SnO₂ nanorods.It is found that SnO₂ nanorods prepared via the new synthesis method were single crystal materials. The diameter and length of the SnO₂ nanorods are in the range of 5nm to 100nm and several micrometers depended on the growth temperature and time. The atomic composition of Sn and O was calculated by using peak area sensitivity factors. The results showed that the atomic ratio of Sn/O was changed from 1:1 to 1:2. The deviation of composition from stoichiometry caused by oxygen vacancies was strongly affected by the growth temperature. Lower growth temperature, higher atomic ratio of Sn and O. Contradictorily, higher growth temperature, closer to the stoichiometry of atomic ratio of Sn and O.2. Synthesis and characterizition of SnO₂ nanorods doped Cd²⁺ or Zn²⁺In molten salt medium, two kinds of SnO₂ nanorods doped Cd²⁺ or Zn²⁺ were obtained via annealing the precursors of SnO₂ powders doped Cd²⁺ or Zn²⁺, which were prepared by redox reaction in microemulsion system. The structure and chemical composition of SnO₂ nanorods doped Cd(2+) or Zn(2+) were characterized by means of XPS and XFS(X-ray fluorescence spectrometry). It is found that the content of CdO in SnO₂ nanorods materials doped Cd²⁺ is up to 3%, and the content of ZnO in SnO₂ nanorods materials doped Zn²⁺ is up to 1.5 %. The crystalline structure of SnO₂nanorods doped Cd²⁺ or Zn²⁺ is a substitutional solid solution crystalline, which was formed by Sn⁴⁺ replaced with Cd²⁺ or Zn²⁺. The work was not reported.3. Growth processes and mechanism of SnO₂ nanorods in molten salt mediumThe growth processes and mechanism of SnO₂ nanorods in molten salt medium were investigated. The growth of SnO₂ nanorods is a process of homogeneously dispersed particles of SnO₂ precursors growing into SnO₂ whiskers via self assembly, forming thin SnO₂ nanorods, and gradually growing up in molten salt medium. The main feature of this procedure is a solid transformation forming process of SnO₂ crystalline from nano-particles of 0-D to nanorods of 1-D in molten salt medium.In molten salt, the growth mechanism of SnO₂ nanorods is that molten salt provide the liquid environment, in which dispersed particles of SnO_2-δ nano-crystal can diffuse along the dislocations in tubes. SnO₂ was formed by divergent reaction of SnO contained in SnO_2-δ, and take the tube dislocations as "template" oriented growth via self assembly derived by decreasing the surface energy of homogeneously dispersed SnO_2-δ particles, and controlled by VLS growth mechanism. Eventually, SnO_2-δ particles grow into SnO₂ nanorods in the tube dislocations of molten salt. The growth mechanism of SnO₂ nanorods in molten salt medium is a new mechanism of 1-D nano-materials.4. Study on infrared absorption characteristics of SnO₂ nanorodsThe infrared absorption characteristics of SnO₂ nanorods with different diameters were investigated by infrared absorption spectra (IR). It is found that there existdouble peaks at 475.2cm^-1 and 680.5cm^-1 of the vibrational modes of Sn-O band in the low frequency range 400-1000 cm^-1. The IR modes of Sn-O band depend on the diameter of SnO₂ nanorods closely. The smaller the diameter is, the wider the IR peaks become. The active IR mode at 475.2cm^-1 shifts to high frequency(blue shift) in accordance with decrease of diameter of SnO₂ nanorods. On the other hand, the active IR mode at 680.5cm^-1 shifts to low frequency(red shift) in accordance with decrease of diameter of SnO₂ nanorods. This novel phenomenon about the infrared absorption spectra of SnO₂ nanorods with different diameters was investigated for the first time.Being similar to the infrared absorption spectra of SnO₂ nanorods, the infrared absorption spectra of SnO₂ nanorods doped Cd²⁺ or Zn²⁺ have double peaks of the vibrational modes of Sn-O band in the low frequency range 400-1000 cm^-1, too. But the two active IR modes shift to low frequency(red shift) in accordance with decreasing of nanorods diameter at the same time. It is different from the infrared absorption spectra of SnO₂ nanorods due to the substitutional solid solution crystalline structure of SnO₂ nanorods doped Cd²⁺ or Zn²⁺.5. Redox properties and new reaction thermodynamics mechanism of SnO₂ nanorodsThe redox properties of SnO₂ nanorods materials were investigated by means of H₂-TPR. The results showed that the better redox properties of SnO₂ nanorods ascribed to higher oxidizing activity induced adsorbed oxygen on the surface of SnO₂ nanorods. The slow-reaction temperature of H₂ consumption was started at 320℃. The reaction mechanism of SnO₂ nanorods is in the employ of H₂ consumption reaction of the desorbed oxygen and the divergent reaction of SnO₂.The redox properties of SnO₂ nanorods materials doped Cd²⁺ or Zn²⁺ were similar to redox properties of SnO₂ nanorods materials. But the started slow-reaction temperature of H₂ consumption were reduced in a large degree due to the catalytic effect of CdO or ZnO. The reaction was started at 270℃and the H₂ consumption peak was at 510℃in H₂-TPR profile of SnO₂ nanorods materials doped Cd²⁺. In the meanwhile, the reaction was started at 180℃and the H₂ consumption peak was at 420℃in H₂-TPR profile of SnO₂ nanorods materials doped Zn²⁺. The reaction mechanism submits to catalytic reaction of CdO or ZnO cooperated with H₂ consumption reaction of the desorbed oxygen, the divergent reaction of SnO₂ and reduction reaction of Cd²⁺ or Zn²⁺. This work of reaction mechanism was reported for the first time.6. Research on gas sensing properties and new gas sensitive mechanism of SnO₂ 1-D nano-materials.Two kinds of gas sensor have been desired with new gas-sensing materials based on SnO₂ nanorods and mixture materials of SnO₂ nanorods with SnO₂ nano-powders for the first time. It was discovered that sensors exhibited higher sensitivity, better selectivity, better stability, and better response and reversion to ethanol (C₂H₅OH). Sensors based on SnO₂ nanorods materials doped Zn²⁺ illustrated the best sensitivity to ethanol and sensors based on SnO₂ nanorods materials was inferior to it. Sensitivity to ethanol of the sensors based on SnO₂ nanorods materials doped Cd²⁺ followed them. The work was not still reported.The gas sensitive mechanism of SnO₂ nanorods was put forward according to structure characteristics of SnO₂ nanorods gas sensitive materials with deviated from stoichiometry, to adsorption properties and interface principle, to grain size and surface-to-volume ratios associated with 1-D nanostructures, and to the tunnel of electrical conductivity theory. Relative experiment results of the gas sensing properties of SnO₂ nanorods materials can be explain very well with using the mechanism pattern.