Study On The Gas Sensing Properties Of ZnO Thin Films Optimized On Microstructure And Defects Control

ZnO-based semiconductor gas sensor has attracted wide attention because of its good stability,high sensitivity and low cost.The performance of gas sensor mainly depends on the material of the sensitive layer.Therefore,an effective control of the microstructure and defect state within the sensing material is essential to obtain high quality devices.In this paper,thin films of ZnO were selected as research target.Their response to low concentration NO₂,ethanol and other gases are improved by controlling the size of particles,morphology as well as surface defect characteristics.The corresponding mechanism is verified by means of calculation and simulation.The main research contents and conclusions are as listed follows:1)Aerosol-assisted chemical vapor deposition technology was used to prepare particulate,pyramid-shaped and plate-shaped ZnO thin films.The microstructure,morphology and defect state of the materials were characterized by SEM,XRD and PL,and the growth mechanism of ZnO thin films was described.The results show that,without chemical surfacant in the zinc precursor solution,the growth rate of different ZnO crystal surface is similar,and there is no optimal growth direction,so the particles are agglomerated into particles.When modified by acetic acid,it first grows into a sheet along the a-axis direction,and then grows into a cone along the c-axis direction.When acetylacetone and deionized water were co-modified,the deposited ZnO grew into sheet,indicating that the ZnO grows preferentially along the a-axis direction.2)ZnO thin films with different thickness,morphology and defect state were prepared into gas sensors and their response to NO₂,ethanol and other gases were tested.The results show that the optimum operating temperature of particulate ZnO thinfilms sensor is 225?,and the maximum response value to 1.0 ppm NO₂ is 41.The optimum working temperature of quasi-pyramid ZnO is 350?,and the maximum response to100 ppm ethanol gas is 236.The optimal working temperature of the plate-shaped ZnO sensor for ethanol gas is 400?,and the response is 43.0.The results of gas sensing mechanism analysis show that the size of ZnO particulate is 17 nm,which is close to the total debye length of 15.6 nm,indicating that the sensor's response performance depends on the particle size.There is a high content of interstitial zinc defects in quasi-pyramid shape ZnO thin films.The first-principles calculation results show that the existence of Zn_i can increase the oxygen adsorption capacity of the materials,thus improving the sensitivity to ethanol gas.There are defects of oxygen vacancy and gap Zn in plate-shaped ZnO thin films,which play an important role in gas sensing performance.3)In order to improve the gas sensing performance,Ag nanoparticles were loaded on the surface of ZnO films by thermal evaporation process.The response of the prepared Ag-ZnO-based sensor to 100 ppm ethanol is about 125.6,which is nearly 3times higher than that of the pure plate-shaped ZnO sensor.Ag has good catalytic activity and increases the surface depletion layer as a donor defect in the reaction process,which is the main reason to enhance the gas sensing properties of the materials.