Study On The Preparation Of ZnO@ZIFs Composites And Their Gas Sensor Detection Characteristics

Gas sensing detection is the key to detect the fault characteristic gases dissolved in transformer oil.Continuous research on gas detection sensor technology to improve the reliability and stability of sensing technology is of great significance to the long-term reliable detection of transformer fault characteristic gases.Although pure ZnO semiconductor material is sensitive to all fault characteristic gases,its sensitivity,selectivity,stability and other gas-sensitive properties need to be improved urgently.Compositing is an effective means to greatly improve the performance of gas sensitive materials.The study on the gas sensitive performance of ZnO based gas sensor which composites with ZIFs material is of great significance to improve the performance of ZnO gas sensor and accelerate its application.ZnO microrod?ZnO microball?ZnO@ZIF-8 microrod?ZnO@ZIF-8 microball?ZnO@ZIF-71 microball gas sensing materials were prepared in this paper.X-ray diffraction?XRD?,scanning electron microscope?SEM?,transmission electron microscope?TEM?,X-ray electron energy spectrum?XPS?,X-ray energy dispersive spectroscopy?EDS?were applied to analyse the phase composition and microstructure of this five kinds of materials.After that,five planar side-heating gas sensitive sensors were prepared based these materials and their gas sensitive performance were tested.At last,we analyzed the reasons for the improvement of the gas sensitive performance of ZnO@ZIFs composites.The main work and conclusions of this paper are as follows:?1?The ZnO seed layer was prepared on the conductive glass substrate,and then the pure ZnO microrod was prepared based seed layer by hydrothermal method;Pure ZnO microball was prepared by a simple one-step hydrothermal method;ZIF-8 and ZIF-71 were grown on the surface of pure ZnO microrod or microball,which were as the template and Zn²⁺source,by hydrothermal reaction,and we got ZnO@ZIF-8microrod?ZnO@ZIF-8 microball?ZnO@ZIF-71 microball;The above materials were characterized by XRD,and the phase compositions of wurtzite zinc oxide,zinc oxide and ZIF-8,zinc oxide and ZIF-71 were determined.It was found that the pure ZnO microrod mainly exposed?100?crystal plane,and the pure ZnO microball mainly exposed?002?crystal plane;The SEM,TEM,XPS and other characterization results show that:the diameter of ZnO microrod is 320⁵00 nm,and the ZnO microball has internal cavity and loose porous,and the ZnO@ZIF-8 microrod is made up of dozens of pure ZnO rods,and the ZIF-8 wrapped pure ZnO to be as shell,so its diameter increased to micrometers.The outer shell of capsule-shap ZnO@ZIF-8 microrod is ZIF-8,and inter core is pure ZnO.Analogously,the cores of ZnO@ZIF-8 and ZnO@ZIF-71 microball are all pure ZnO,and the shells are ZIF-8 and ZIF-71respectively.?2?Five kinds of materials including ZnO microrod?ZnO microball?ZnO@ZIF-8microrod?ZnO@ZIF-8 microball?ZnO@ZIF-71 microball were prepared into planar side-heating gas sensitive sensors and their gas sensitive properties were tested.Operating temperature-sensitivity:The optimum operating temperature of pure ZnO is reduced by compositing ZIFs;Gas concentration-sensitivity:ZnO@ZIFs composites have better sensitivity than pure ZnO and better linearity of curve in low concentration region;Gas response-recovery time:ZnO@ZIFs composites have a longer response-recovery time than pure ZnO,which is mainly caused by the hindrance of the shell;Selectivity:As the main exposed crystal surface is different,pure ZnO microrod and ZnO microball are more sensitive to H₂ and C₂H₂,respectively.Due to the difference in core material,ZIFs shell aperture size and ZIFs gas adsorption characteristics,ZnO@ZIF-8 microrod has better selectivity for H₂,ZnO@ZIF-8microball has better selectivity for C₂H₂,and ZnO@ZIF-71 microball has poor gas selectivity.?3?The reasons for the improvement on sensitivity of pure ZnO by compositing ZIFs are as follows:ZIFs are directly regrown on the surface of ZnO,and the lattice dislocation causes more defects in the interface of ZnO@ZIFs composite materials,so that the amount of adsorbed oxygen is increased;The high porosity and internal chemical bond characteristics of ZIFs make it has excellent enrichment effect on test gases,especially H₂;The material size of micron reduces the initial resistance of sensors;Compositing bends the interfacial energy band and reduces the gap width of the junction;Catalytic effect of ZIFs on REDOX reaction.The reasons for the improvement on selectivity of ZnO@ZIFs composites:ZnO microrod and microball are more sensitive to H₂ and C₂H₂,respectively,because of the difference of main exposured crystal surface;Due to the different channel size and operating temperature,Rod-shaped zno@zif-8 has better selectivity for H₂,ZnO@ZIF-8 microrod has better selectivity for H₂,ZnO@ZIF-8 microball has better selectivity for C₂H₂,and ZnO@ZIF-71 microball has poor selectivity because of its large aperture.?4?The aging test of pure ZnO microball gas sensors were carried out,and it was found that with the deepening of aging,the sensitive film changed from white to gray-yellow and gray-black.Through SEM and XPS characterization results of the aged sensor substrate,it is found that the reasons for aging of ZnO sensor include particle agglomeration,change of gas-sensitive film morphology,irreversible adsorption of polluted gas on the material surface,besides that air humidity will also affect the stability of the sensor.Finally,this paper analyzes the reasons for higher stability of ZnO@ZIFs composites:The hydrophobic property of ZIFs prevents the material from being affected by moisture and reduces the humidity change influence on the gas sensitivity;In addition,compositing ZIFs reduces the working temperature of the sensors,which also helps to delay the material aging.