The Preparation Of Porous SnO₂ Nanotubes, Nanospheres And Their Gas Sensing Properties

Gas sensor is the device that can detect the type and concentration of gas. With the increasing environmental problems, developing the gas sensor with excellect sensing performance has become one of the important topics of current research. Tin oxide?SnO₂?, a well known n-type wide band gap semiconductor?Eg=3.6 eV, at 300K? with high extion binding energy of 130 me V at room temperature, has been widely used in gas sensors due to its low cost, higher chemical sensitivity, faster gas response and good stability. Gas sensing reaction is one kind of gas-solid chemical reaction,which occurs on the surface of SnO₂ sensing materials. Hence, the microstructure,surface defect and morphological of materials have important influence on gas sensing performance. Study shows that the gas sensing properties of SnO₂ can be enhanced by doping rare earth elements or precious metals, building heterostructure and surface modification. In this study, the Yb-doped SnO₂ nanotubes, porous SnO₂ hollow nanospheres and multishelled SnO₂ hollow microspheres were successfully synthesized by electrospinning and template method, respectively, and their gas sensing proterties were also tested. The detailed information of the dissertation is listed as follows.1. Pure and Yb-doped?0.5wt%, 1.0 wt%, 1.5 wt%? SnO₂ hollow nanofibers are synthesized by single capillary electrospinning followed by calcination. The structure,mophological and elemental composition are investigated by XRD, SEM, TEM, XPS and BET. The results showed that all the samples exhibit the hollow structure, and the nanotubes are composed of a large number of random SnO₂ nanoparticles. Moreover,Yb3+ ions are successfully doped into the SnO₂ lattice, leading to the increasing in the amount of chemisorbed oxygen species and resistance value of Yb-doped SnO₂ samples. The sensor based on 1.0 wt% Yb-doped SnO₂ hollow nanotubes exhibited perfect gas sensing performance toward ethanol at 340°C, due to the unique hollow structure and surface defects.2. Porous SnO₂ hollow nanospheres are successfully synthesized using carbon spheres as a template, which are prepared by hydrohtermal method using glucose and deionized water as raw materials. The average size of carbon spheres are about 320 nm, which are much higher than that of SnO₂ samples. This result may be caused byconstriction in the aging and calcination process. It can be also obtained from TEM images that all of samples are composed of numerous nanoparticles, and the surface are porous. This feature is further validated by BET test, and the average pore size are about 20.49 nm. The pores can provide a more convenient pathway for the diffusion of target gas, make the reaction more effective and enhance the sensitivity of SnO₂.The sensor based on the sample aged 48 h exhibit a high response?673 for 500 ppm?and fast response-recovery?10 s for adsorption and 11 s for desorption? capability to ethanol at 260°C. The large surface-to-volume of porous SnO₂ hollow nanospheres can provide more active sites for gas molecules, and the pores can promote the rate of reaction between surface oxygen species and ethanol molecules. Both of them are beneficial to increase the response value and reduce the optimal working temperature of gas sensor.3. Firstly, in order to prepare an ideal template, the carbon spheres are optimized by adjusting experimental parameters. The average diameter of carbon spheres is increase to 2 ?m, and the dispersion of them is greatly improved by adding trace amounts of ethylene glycol. Secondly, the multishelled hollow SnO₂ microspheres are successfully synthesized by adjusting heating rate. The result of TEM demonstrate that the product have a unique structure of close double-shelled solid core, and both of two shells are about 100 nm in thickness. This structure can provide more surface area for the adsorption of gas molecules, making the optimum temperature of gas sensor shift to the low temperature direction. Finally, the result of gas sensing test shows that the sensor based on the product have a high response?500 ppm, 342?, short response and recovery time?10-12s?, low working temperature?200°C? and long-time stability to acetone, suggesting a promising candidate for the detection of acetone.