| Tungsten trioxide (WO3) has been under an extensive research because of its importance for its electro-optical, electrochromic, ferroelectric and catalytic etc. properties. Moreover, it has received much attention because of its good sensitivity. Volatile organic compound (VOC) gases, for example, toluene, are the primary resource of indoor environmental pollutants and are considered seriously harmful to human body. Most notable are the strong correlations between VOC gases emissions and cancers. So there has been much demand to develop gas sensor to detect VOC gases. In the thesis, it has been investigated the gas-sensing characteristics to VOC gases, microstructure and electrical properties of WO3 nanoparticles influenced by the preparation and the doping elements,and the mechanisms for such behaviours were discussed. The results include as follows:Firstly, nano-scaled tungsten-trioxide powder has been synthesized by sol-gel process using tungsten powders. The effect of sintering temperature and time on the gas-sensing properties of WO3 nanoparticles was investigated. The good gas-sensing properties of the WO3 nanoparticles sintered at 600oC for 1 hour to VOC gases were found. The WO3 nanoparticles are nearly spherical in shape and have a small grain size, large surface area and the proper aperture size. Complex impedance analysis indicates that depressed angles of semicircular arc will reduce with the extension of sintering time. This results show the distribution of defects and pores, and the grain connected shapes at the grain-boundary tend to be more uniform and stable with proper extension of sintering time.Secondly, the effect of Cerium oxide (CeO2) on the gas-sensing properties of WO3 to VOC gases was investigated. The sensitivity of the samples exposed to 50 vol ppm of xylene, toluene, acetone, benzene and ethanol under different operating temperatures shows that the sensitivity of 5at%CeO2-added WO3 nanoparticles is higher than that pure WO3 ones to xylene, toluene and benzene. The analysis with Field-Emission SEM reveals that the average grain size decrease with increasing CeO2, i.e. CeO2 inhibit the grain growth, Complex impedance spectroscopy analysis shows that grain-boundary resistance increases and grain-boundary capacitance decreases with the increasing concentration of CeO2, which means that the Ce ions exist mainly in the WO3 grain-boundaries and helps to improve the microstructure. Using complex impedance to define the sensitivity of the sample, the new sensitivity is about three times higher than that of the traditional d.c. method.Thirdly, the effect of ruthenium oxide (RuO2) on the gas-sensing properties of WO3 to VOC gases was investigated. The sensitivity of the samples exposed to 50 vol ppm of xylene, toluene, acetone, benzene and ethanol under different operating temperatures shows that the sensitivity of RuO2-added WO3 nanoparticles is higher than that pure WO3 ones. The sensitivity of the 1wt.%-RuO2-added WO3 sample to xylene and toluene at 4.5V heating voltage is 39.5, 29.9, respectively, and three times higher than that pure WO3 ones. The sensitivity of the 5wt.%-RuO2-added WO3 sample to acetone and ethanol at 4.5V heating voltage is 75.4, 28.4 respectively, and seven times higher than that pure WO3 ones. And the RuO2-added WO3 sample to benzene have good selectivity.Lastly, the double Schottky barrier comes into being when the chemisorption of oxygen takes place. It was obtained that the relation of the surface band bending with the concentration of the reducing analyte gases by the mass action law and the depleted layer approximation. The thermoeletromic emission theory applied for the case, and then the relation of the sensitivity of gas sensor with the concentration of the reducing analyte gases was obtained.
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