Preparation And Gas Sensing Properties Of Bi/Y-doped SnO₂ Hierarchical Nanoflowers And BiFeO₃ Microspheres

Tin dioxide（SnO₂）is a typical n-type IV-VI metal oxide semiconductor material with a direct band gap width of 3.6 eV at room temperature and an exciton binding energy of up to 130 meV.The use of nanomaterials as gas sensing materials has led to the rapid development of gas sensors in the past few decades.However,there are still defects such as poor selectivity,high operating temperature,and inability to respond to low concentrations of gas to be tested in a short period of time.In order to improve its gas sensing performance,the SnO₂ nanomaterial can be modified by constructing a3D hierarchical flower-shaped structure and doping with rare earth and other elements to meet the actual needs.At the same time,it is urgent to explore the application of multi-metal oxide materials in gas sensing to broaden the research field of gas sensing materials.We have prepared the hierarchical flower-like nanostructures of Y and Bi-doped SnO₂ and the multi-metal oxide walnut-shaped BiFeO₃ microspheres by hydrothermal method,and studied their gas sensing properties.The main work of this thesis includes the following three parts:（1）In order to improve the gas sensing performance of traditional SnO₂semiconductornanomaterials,Bi-dopedSnO₂hierarchicalflower-shaped nanostructures were synthesized by a one-step hydrothermal route.Diverse techniques characterization results collectively revealed that Bi ions have been successfully doped into SnO₂ lattice and samples present numerous uniform flower-shaped nanostructures composed of many layered petal-like thin nanosheets.Benefiting from such unique 3D hierarchical flower-shaped nanostructures and Bi doping,the sensors based on Bi-doped SnO₂ exhibited superior gas sensing performance concluding higher response value,faster response/recovery time,good stability and excellent selectivity to formaldehyde at the optimum operating temperature of 170℃.Therefore,Bi-doped SnO₂ hierarchical flower-shaped nanostructures could be used as a candidate semiconductor gas sensing material for manufacturing highly sensitive formaldehyde gas sensors in the future.（2）In this work,we have successfully prepared Y-doped SnO₂ hierarchical flower-like nanostructures through a facile one-step hydrothermal method for the purpose of improving the formaldehyde gas sensing performance of conventional SnO₂ semiconductor nanomaterials.The products were characterized by diverse techniques,which revealed that trivalent Y cations have been introduced into the crystal lattice of SnO₂ and they exhibited lots of regular flower-like nanostructures consisting of several layers of rough and porous flakes.At the optimal working temperature of 180℃,the fabricated gas sensor based on Y-doped SnO₂ shown much better gas sensing properties to formaldehyde compared with SnO₂ due to the combinationofthisdistinctivethree-dimensionalhierarchicalflower-like nanostructures and doping of Y ions.In particular,the detectable formaldehyde minimum limit has been reduced to 1 ppm.Consequently,Y-doped SnO₂ hierarchical flower-like nanostructures are expected to become ideal gas sensing materials of highly sensitive gas sensors for formaldehyde detection due to their excellent gas sensing performance.（3）In this paper,well-crystallized walnut-shaped BiFeO₃ microspheres have been successfully synthesized via a facile hydrothermal process.Meanwhile,the morphology growth process of walnut-shaped microspheres was investigated.The gas sensing properties of fabricated sensor based on BiFeO₃ microspheres to different gases concluding formaldehyde,acetic acid,acetone and ethanol etc.were systematically studied.The results revealed that at the optimum operating temperature of 240℃,the gas sensor exhibited typical p-type semiconductor gas sensing behavior concluding high response value,good repeatability and fast response/recovery time to HCHO,which also provides an experimental basis for bismuth ferrite to become a new multi-metal oxide semiconductor gas sensing materials in the future.