Study On Defect Control And Gas Sensing Performance Of SnO₂-Based Nanomaterials

In recent years,air pollution has become increasingly serious and has caused great harm to human health.It is an potential"invisible killer".Gas sensors have great application prospects in the detection,monitoring and alarming of toxic and harmful gases.How to improve gas-sensing performance and gas selectivity is an urgent problem for researchers.At present,the main means to enhance gas-sensing sensitivity and selectivity include the regulation of the morphology and crystal plane of gas-sensing materials,addition of dopants,and surface modification.In this thesis,we take use SnO₂ as sensing materials to investigate the gas performance.By adjusting the type of defect,noble metal gold loading and metal nickel doping,the SnO₂ is modified to improve the gas sensing performance and improve the gas selectivity.By analyzing the relationship between the structural properties and gas sensitivity of SnO₂,the gas sensitivity mechanism was explained.The main research contents of this thesis include the following aspects:?1?SnO₂ nanoparticles with different defect types were successfully prepared by changing the calcination atmosphere.The type and concentration of defects in SnO₂nanoparticles were analyzed by electron paramagnetic resonance spectroscopy?ESR?and positron annihilation spectroscopy?PALS?.The prepared material was tested for gas sensitivity and it was found that the type of defects is closely related to the gas sensitivity of the gas.SnO₂ nanoparticles with oxygen vacancies(V_O^··)exhibited high response and selectivity to ethanol,while SnO₂ with V'_S''_n'V_O^··V'_S''_n'defect cluster to formaldehyde respond is fast with high selectivity.Through in-situ infrared technology,it was found that the target gas was finally oxidized to CO₂ gas.In addition,the energy band structure of SnO₂ with different defects was analyzed by means of Mott-Schottky plots and UV-vis diffuse-reflectance spectra.It is found that different types of defects lead to changes in the band structure of SnO₂,which changes the selectivity of gases.Therefore,when the oxidation potential of the gas molecules can match the conduction band of the defective SnO₂,the target gas will be easily oxidized to CO₂ gas.Because different gas molecules have different oxidation potentials,they cause different degrees of difficult in being oxidized to CO2 gas.In addition,there are different types of defects in the SnO₂ nanoparticles prepared by calcination in different atmospheres,ie,their energy band structures are different.As a result,SnO₂ nanoparticles exhibited different gas-sensitivity responses when exposed to VOC gases,ie,differences in selectivity.?2?The flower-like SnO₂ micro-nanomaterials was successfully synthesized by hydrothermal method.At the same time,Au nanoparticles were modified to obtain the flower-like Au-SnO₂ composite material.The prepared Au-SnO₂ composites were characterized by X-ray diffraction?XRD?,scanning electron microscopy?SEM?and X-ray photoelectron spectroscopy?XPS?.The results show that the material of flower-like structure is self-assembled by SnO₂ nanosheets with a thickness of 30⁶0 nm and its size is 1²?m.The gas sensitivity test of 1-500 ppm formaldehyde was performed with flower-like SnO₂ and Au-modified flower-like SnO₂ composites.The results showed that the gas-sensitive properties of formaldehyde were greatly improved by Au modification.When the formaldehyde concentration is 200 ppm,the sensitivity?Ra/Rg?was about 120,which was 8 times more than that of the unmodified flower-like SnO₂ sensitive material.In addition,Au-modified flower-like SnO₂ composite sensors exhibit good selectivity to formaldehyde.This may be attributed to the high catalytic activity of Au nanoparticles which promotes the diffusion of formaldehyde gas and the adsorption of O₂,thus making it exhibit better gas sensitivity.?3?Ni-doped SnO₂ nanoflowers were prepared by hydrothermal method.The SnO₂ nanoflowers and Ni-doped SnO₂ nanoflowers were analyzed by electron paramagnetic resonance?ESR?.The results showed that the concentration of oxygen vacancies in SnO₂ nanoflowers was increased by Ni doping.The prepared material were tested with gas sensitivity,and the appropriate amount of Ni-doped SnO₂ was found to exhibit an outstanding response to SO₂ gas.