Study On The Construction Of Metal Oxide Semiconductor With Hierarchical And Hollow Structure And Its Sensing Properties

As the industrialization and the urbanization has been rapidly advanced,more and more toxic and harmful gases are emitted into the environment,which results in the increasingly serious air pollution.Metal oxide semiconductor-based gas sensor,which can detect target gases in the environment,has attracted people's attention.Because of its low production cost,simple structure,easy integration,and compatibility with the internet of things,metal oxide semiconductor gas sensor is widely used in the gas leakage early warning,indoor/outdoor air quality monitoring,public safety,medical diagnosis,aerospace,and other fields.However,the practical application of metal oxide semiconductor gas sensor is limited due to these shortcomings,such as insufficient selectivity and stability,low sensitivity,and high power consumption.The sensing material is the important component for gas sensors to sense target gas molecules in the environment,and its micro-morphology and structure as well as composition have a significant impact on the gas sensing performance.Therefore,it is particularly important to prepare the sensing materials with excellent properties to further improve the gas sensing performance.In this graduation thesis,for the improvement of the gas sensing performance,metal oxide semiconductors with hierarchical and hollow structure were constructed and the possible sensing process and mechanism are deeply analyzed based on the density functional theory(DFT)calculation and quasi in-situ X-ray photoelectron spectroscopy(XPS).The main research contents are as follows:(1)PdO/ZnO composites with hierarchical and hollow nanocage structure are successfully prepared by using ZIF-8 as sacrificial templated and subsequent thermal annealing process.Compared with simple ZnO,The PdO/ZnO composites show significantly enhanced gas sensing performances.It is found that PdO/ZnO composites have ultra-high response(Ra/Rg=650)toward 5 ppm CH3SH gas at low operating temperature(160?),and can detect CH3SH gas as low as 100 ppb.The PdO/ZnO composites also exhibit excellent selectivity toward CH3SH gas.By the quasi in-situ XPS measurement,it is found that PdO nanoparticles can not only promote the chemisorption of oxygen on the surface of PdO/ZnO composites,but also promote the decomposition of CH3SH gas molecules on the surface,which are the key to improve the sensing response.(2)Thin film composed of porous ferric oxide(a-Fe2O3)hollow sphere array is successfully synthesized in situ on the planar electrode through template-assisted hydrothermal reaction and subsequent calcination.At a low operating temperature(160?),the sensor shows high response(75.8)toward 10 ppm triethylamine(TEA)with a short response time(2 s)and can detect TEA as low as 50 ppb.More importantly,an obvious attenuation characteristic is observed during TEA detection.This distinctive response toward TEA gas is repeatable when the operating temperature is over a threshold(>120?).Moreover,the instantaneous attenuated response is found to be independent of the gas concentration and humidity but much related to specific gases,demonstrating its potential to be used as a characteristic signal to achieve highly selective detection of TEA molecules.Furthermore,based on density functional theory(DFT)calculations,the attenuated response can be ascribed to a special surface dynamic reaction process involving the thermal decomposition of TEA molecules and the formation of nitrogen oxides,which is verified by the quasi in-situ X-ray photoelectron spectroscopy analysis.(3)By using polystyrene colloidal sphere array as the template,combined with subsequent magnetron sputtering and thermal annealing treatment,ZnO@CuO hierarchical heterostructures,which are composed of porous hollow spheres array are in situ prepared on the sensor electrode.Compared with pristine ZnO and CuO sensing film,the ZnO@CuO hierarchical heterostructure shows remarkably enhanced sensing properties toward H2S gas.At a low operating temperature(25?),the ZnO@CuO hierarchical heterostructure has a high response(Ra/Rg=20.7)toward 10 ppm H2S gas,a very low detection limit(20 ppb).And it possesses easy recovery even when it is exposed to a high concentration of H2S gas(100 ppm).In addition,according to the results of density functional theory(DFT)calculations,the adsorption energy of H2S gas molecules on the ZnO@CuO hierarchical heterostructure is obviously lower than that on the pristine ZnO surface,and the energy barrier of the decomposition of H2S gas molecules on the surface of heterostructure is only 0.25 eV.Combined with XPS tests,it can be concluded that the interface-formed p-n heterojunction,the hierarchical heterostructure with large porosity,and the small size of intermediate CuS nanoparticles are the main reasons for improving the sensing performance of gas sensor toward H2S gas.