Gas Sensors Based On ZnO Nanoparticles

It is well known that the specific surface area of material is inverse proportion to the size of material. When the size of material is at the level of nanometer, the amounts of atoms on the surface of material increase rapidly. Due to the larger number of the surface atoms, deficiency of atom coordination and higher surface energy, these surface atoms are easy to interact with other atoms and keep a steady state. The chemical activities of them are fairly high. The sensing mechanism of Zinc oxide gas sensors is based on the surface reaction of ZnO. Therefore, high surface-volume ratio is critical to its sensing properties. It can be concluded that the gas sensing properties of nano-sized ZnO will be improved by doping.In the dissertation, the ZnO nanoparticles doped with Mo, Fe₂O₃, Mn₃O₄ and CdS nanoparticles have been prepared by a sol-gel method. PVP-modified ZnO nanoparticles with different molar ratios of Zn²⁺:PVP were also prepared by sol-gel method. The structure, morphology and sensing properties have been systematically investigated. Moreover, the mechanisms of the gas sensing properties have been elucidated. The significant results achieved in this dissertation are given as below:The gas sensing behavior of PVP-modified ZnO nanoparticles with different molar ratios of Zn²⁺:PVP has been studied prepared by sol-gel method for the first time. The sensor with a molar ratio of Zn²⁺ :PVP = 1:1 showed uniform morphology and fairly excellent sensitivity and selectivity to TMA. The response and recovery characteristics are almost reproducible and rather quick when exposed to TMA and when exposed again to N₂. Finally, the mechanism for the improvement in the gas sensing properties was discussed.The gas sensing behavior of ZnO nanoparticles doped with Mo with different molar ratios of Mo:Zn has been studied prepared by sol-gel method. The doping concentration of Mo affected the structure and morphology of ZnO nanoparticles. The sensor with a composition of Mo:Zn = 10% showed fairly excellentsensitivity and selectivity to ammonia at room temperature. The response and recovery characteristics are almost reproducible. Finally, the mechanism for the improvement in NH3-sensing properties was discussed.The gas sensing behavior of ZnO nanoparticles doped with Fe2O3 (Mn3O4) nanoparticles with different percentages of Fe:Zn (Mn:Zn) has been studied prepared by sol-gel method. The doping concentration of Fe (or Mn) affected the structure of ZnO nanoparticles. The doping concentration of Fe2O3 changed the morphology of ZnO. But there was little effect of doping concentration of Mn3O4 on the morphology of ZnO. The sensor with a composition of Fe:Zn = 2% (or Mn:Zn =3.5%) showed fairly excellent sensitivity and selectivity to ammonia at room temperature. The response and recovery characteristics are almost reproducible. Finally, the mechanism for the improvement in NH3-sensing properties was discussed.The gas sensing behavior of ZnO nanoparticles doped with CdS nanoparticles with different percentages of CdS:Zn has been studied prepared by sol-gel method. There was little effect of doping concentration of CdS on the morphology of ZnO. The sensor with a composition of CdS:Zn = 3% showed fairly excellent sensitivity and selectivity to ammonia at room temperature. The response and recovery characteristics are almost reproducible. Finally, the mechanism for the improvement in NH3-sensing properties was discussed.