Synthesis And Gas Sensitive Properties Of Porous ZnO-based Heterogeneous Composites

With the development of society,gas pollution is causing great concern.Formaldehyde,acetone,SO₂and other toxic and harmful gases can cause great damage to the human immune system and respiratory system.Therefore,the detection device with simple,convenient,low-cost and high-performance needs to be developed.In this need,researchers have developed gas sensors,among which the performance of metal oxide semiconductor gas sensors are the most outstanding.ZnO,a typical n-type semiconductor with a direct bandgap of 3.37 e V,has excellent electric conductivity and chemical stability.Although the sensing performance of ZnO sensors has been greatly improved,the sensitivity,recovery/response time,optimal temperature and other aspects still need to be improved to meet the urgent needs of detecting trace volatile toxic gases.An effective way to improve the gas sensing behaviors is to dop other semiconductor metal oxides with ZnO.In order to impover the properties of ZnO,three kinds of gas-sensitive composites with high performance are prepared by doping Cu,Ni and Co with ZnO by co-precipitation method,respectively.The specific research process is as follows:（1）An urchin-like CuO/ZnO with porous nanostructure is obtained via a simple solution method followed by a calcination process.There are a large quantity of pores formed on the surfaces of the nanowires due to the thermal decomposition of copper-zinc hydroxide carbonate.The BET surface area of the as-prepared CuO/ZnO sample is determined to be 31.3m²g^-1.The gas-sensing performance of porous urchin-like CuO/ZnO nanostructure sensor is studied by the exposure of volatile organic compound vapors.With contrast to pure porous urchin-like ZnO sensor,the porous urchin-like CuO/ZnO sensor shows superior gas-sensing behaviors towards acetone,formaldehyde,methanol,toluene,isopropanol and ethanol.It exhibits a high response of 52.6 towards 100 ppm acetone gas,with a response time of 7 s and a recovery time of 8 s.The superior gas-sensing behaviors of the urchin-like CuO/ZnO composite are mainly ascribed to the porous nanowire-assembled structure with abundant p-n heterojunctions.（2）Porous sea-urchin-like nickel-doped ZnO with various nickel contents and high specific surface area is synthesized using a solution method followed by calcination.The nickel-doped ZnO products consisted of numerous porous nanoleaves.The Ni content in products ranged from 5%to 20%.The Ni dopants in the ZnO lattice is verified by X-ray diffraction and X-ray photoelectron spectroscopy.The sensors based on nickel-doped ZnO sea urchins shows superior sensing performance for some volatile organic compounds（VOCs）.ZnO sea urchins with 10%nickel doping exhibited the best gas-sensing performance,including low working temperature,short response/recovery time,and high sensor response.In particular,the 10%Ni-doped ZnO sea-urchin sensor towards 100 ppm formaldehyde vapor exhibited a response of 84.4 with response/recovery times of 17/20 s.These superior sensing behaviors are attributed mainly to the suitable Ni content with high content of oxygen defects,small nanocrystals,and the porous hierarchical structure with a high specific surface area.（3）Using ZnO as the base material,the effects of adding different amounts of Co element on the morphology and properties of the material are investigated.The morphologies and properties of Co-ZnO composites with different doping amounts are investigated by analytical instruments.Studies shows that 10%doping amount of Co is the largest,whether the oxygen vacancy content,gas sensitivity test results also proved that 10%doping amount of Co-ZnO sensor performance is the superior.Finally,the mechanism of Co₃O₄/ZnO nanomaterials on VOC gas is also studied.