| In recent years, great attention has been focused on the nanostructured materials due to their special structures and excellent properties. Vanadium oxide nano-materials have a large number of selective effects with gas sites that can be used in the production of high-sensitivity gas sensor. Among various kinds of them, vanadium oxide nanotubes has great potential for improving gas materials sensitivity, stability selectivity, and other properties because of the larger surface area and more suitable for gas absorption. Thus, we selected vanadium oxide nanotubes (VONTs) as our research objects, and modified the surface morphology, systemically studied the synthesis, the relations of materials structure, morphology and gas sensing properties. Some progresses were gained. The details of our research works are presented as follows:1. The gas sensing properties of VONTs were measured by mixing detected gas and air in static state. The test results showed that VONTs have good gas sensitivity to ethanol and shorter response and recovery time. The best working temperature for VONTs is 270℃. The sensitivity can reach 2.4 for 1000 ppm ethanol at 300℃and the threshold value is 50 ppm. It can be known that one dimensional nanomaterials can formed a large number of gas channel by crossing each other, more big surface volume ratio is the reason caused a higher gas sensitivity of VONTs.2. Fe2O3 nanoparticles surface modified VONTs was synthesized by means of hydrothermal methods. The effects of the factors such as solvent, reactive temperature, reactive time on the structure and gas sensing properties of the samples were studied. The results of gas sensing properties showed that Fe2O3 modified VONTs gas sensor has higher sensitivity than VONTs gas sensor. The threshold value is as low as 10 ppm, and the sensitivity can reach 7.4 for 1000 ppm ethanol at 300℃. It can be guessed that Fe2O3 modified VONTs belongs to surface-controlled sensing mechanism according to the curves of resistance vs temperature and sensitivity vs temperature as well as Fe2O3 size effect of VONTs gas sensitivity. The appearance of Fe2O3 supply more active sites, and VONTs can provide a channel for charge transfer, lead to an improvement of sensitivity. 3. Ag or Pd surface modified VONTs was synthesized by microwave heating method. The gas sensing properties showed that have the best stability and lower working temperature among the tested VONTs. Modified VONTs with Ag can improve the selectivity to ethanol, and modified with Pd can improve the sensitivity to NH3. This is to take advantage of the spillover effects afforded by the metal nanoparticles as a result of the "electronic sensitization" mechanism.4. VONTs/polyaniline (VONTs/PANI) and VONTs/polypyrrole (VONTs/PPY) core-shell structure with have been synthesized through an in situ polymerization of poly monomers in the presence of prepared VONTs. The thickness of PANI and PPY coatings are 20 and 40 nm, respectivly. The hybrids were characterized by TEMS SEM,XRD,IR,TG. Experimental data showed certain synergetic interaction existed in the hybrids, probably resulting in the enhanced thermal stability of polymer coatings. Gas sensing tests showed that the core-shell structure possessed very fast response and high sensitivity at room temperature, implying its potential application for gas sensor. Especially, the sensitivity of VONTs/PANI to 1000 ppm NH3 is as high as 6.4, and the sensitivity of VONTs/PPY to 1000 ppm ethanol is 2.4. The very fact that the synergic interaction between intimately contacted p-type Polymer and n-type VONTs form p-n junctions. The p-n junctions formed in the donor-acceptor system could increase the depletion barrier height, thus leading to an improved response of the sensor. |
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