Design And Modification Of Tin Oxide Hollow Spheres For Gas Sensing

The quantities of hazardous and harmful gases are increasing rapidly in the environment with the development of industry and the consumption of fossil fuels, which not only causes a series of environmental problems, such as acid rain and smog, but also does harm to the safty of industrial production and the health of humanbeings. Therefore, it is considerably important to monitor the hazardous and harmful gases in environment real-timely, and then take effective measurements to avoid the damage. Semiconductor gas sensors are widely used in the area of gas monitoring due to their advantages of low cost, fast response and easy operations. It is well known that the performances of semiconductor gas sensors mainly depend on the structure and composition of semiconductor material. Hence it is significant to design and tailor the structure and composition of oxide semiconductor materials in aims of improving the sensing performances of the gas sensors.SnO2 has been widely used as semiconductor sensing materials because of its high thermal and chemical stability, and appropriate forbidden band. The merits of high specific surface area and open pore structure of metal oxide hollow spheres have given hollow spheres the advantages of high sensing sensitivity and fast response. However, the relatively large crystal size and thick shell of SnO2 hollow spheres reported previously hinder the diffusion of target gases and have an adverse effect on the sensing performance. In order to overcome the shortcomings of SnO2 hollow spheres, controllable preparation of SnO2 hollow spheres with uniform diameter, small crystalline size, and thin shell had been carried out in this thesis, and the acetone sensing performances of these hollow spheres were also studied. Noble metal nanoparticles have been used to modify the SnO2 hollow spheres to further enhance their sensing performances toward acetone by taking advantage of the catalytical effects of noble metals. The concrete study results are as follows:1. SnO2 hollow microspheres with diameters of 200 to 700 nm and shell thickness of 7 to 33 nm were prepared through a simple method by using SiO2 microspheres with size from 200 to 700 nm as template and controlling the deposition amounts of SnO2. Acetone sensing tests demonstrated that thin shell of microspheres facilitates the diffusion of acetone, and the smaller crystal size of SnO2 can improve the sensing response to acetone. Therefore, the sensing performances of SnO2 hollow microspheres to acetone are markedly improved by decreasing the diameter and shell thickness of microspheres, and the size of SnO2 crystallite.2. Monodispersed Ag (4.86 nm), Pd (2.59 nm) and Au (3.55 nm) nanoparticles were respectively prepared by PVA-NaBH4 method at first, and then Ag, Pd, and Au nanoparticles modified SnO2 hollow microspheres were obtained through sol immobilizing noble metal nanoparticles on the surface of SnO2 hollow spheres. Acetone sensing tests showed that Au modified SnO2 hollow spheres has the best sensing performance among the three noble metal modified SnO2 hollow spheres. Au nanoparticles modified SnO2 hollow spheres has a response of 110 to acetone, which is about 7 times higher than that of SnO2 hollow spheres. It was also found that modification with Au nanoparticles can effectively reduce the best operating temperature of the sensor to 150? as a result of the high catalytic activity of Au nanoparticles.3. AgAu (2.37 nm) and AuPd (2.34 nm) nanoalloys with different molar rations were prepared through the co-reduction method, and then loaded on the surface of SnO2 hollow spheres by sol immobilization method to prepare AgAu and AuPd nanoalloys modified SnO2 hollow spheres. Acetone sensing assays illustrated that the sensing response of AuPd nanoalloys modified SnO2 hollow spheres can be as high as 247, which is about 2.5 and 7.1 times of that of Au and Pd nanoparticles modified SnO2 hollow spheres, respectively. The improvement of the sensing performance of AuPd nanoalloys modified SnO2 hollow spheres could be attributed to the synergy effect between Au and Pd.