Controllable Preparation Of Hierarchical Zinc-aluminum Bimetallic Hydroxides And Research On NO₂ Gas Sensing Properties

To solve the problem of the semiconductor gas sensor such as higher operating temperature and relatively lower sensor sensitivity at room temperature,ZnAl LDHs series materials were prepared used the simple step hydrothermal by adding different nitrogen source and anionic surfactant sodium dodecyl sulfate(SDS)modulation to controllable two-dimensional flake and three-dimensional flower shaped materials.Morphology characterization and properties test showed that 3D flower ZnAl_SDS LDHs materials has higher gas sensitive performance for NO₂compared with the other materials.Firstly,the two-dimensional ZnAl_HMT LDHs nanosheets were prepared by adjusting the amount of zinc and aluminum and using hexamine(HMT)as precipitator through one-step hydrothermal method.The morphology characterization of ZnAl_HMTLDHs series material showed that they were composed of two-dimensional nanometer sheets with hexagonal structure.The ZAH-2 sample with a molar ratio of 2:1 had the best gas sensitivity response to 100 ppm NO_2,it was 8.75 and the minimum detection limit was up to 0.05 ppm,much better than that of ZAH-1 and ZAH-3.At the same time,ZAH-2 sample also had good repeatability,selective and stability.Secondly,ZnAl_urea LDHs materials were prepared by adpoting the one-step hydrothermal method with changing the nitrogen source and use urea as precipitant.By adjusting the ratio amount of zinc and aluminum,ZnAl_urea LDHS material with 3D structure presented 2D nano pieces stacked into a three dimensional flower morphology.The ZAU-2 sample with a molar ratio of 2:1 had the best gas sensitivity response to 100ppm NO₂and the gas sensor response was 13.78(above ZAU-1 and ZAU-3),the minimum detection limit of 0.05 parts per million.The sample also had good repeatbility,selectivity and stability.Compared with the 2D ZnAl_HMT LDHs,the performance of 3D ZnAl_urea LDHs had been raised to a certain extent,but the gassensitivty had not improved much because of 3D flowers were incomplete and agglomeration occured.In order to further improve the performance of the material,on the basis of urea as the nitrogen source and deposite,the anionic surfactant sodium dodecyl sulfate was added to prepare controllable ZnAl_SDSLDHs materials with three-dimensional flower structure.It was found that the flower-like ZnAl_SDS LDHs sample with a Zn:Al molar ratio of 2:1 ZAS-2 not only had the best morphology,but alsoa larger specific surface area(85.7 m²·g^-1)with a thinner nanosheet(3.2 nm),it had a higher chemisorbed oxygen content(63.39%),which was higher than ZAS-1 and ZAS-3.The response of ZAS-2 gas sensor to 100 ppm NO₂ was 28.7,2.1 times the 3D flower ZnAl_urea LDHs,far higer than other samples prepared in the laboratory before,the detection limit is as low as 0.05 ppm,and also presented good selectivity,repeatability and stability.The addition of SDS not only expanded the layer spacing of two-dimensional sheet materials,and the guiding effect of the long-chain anionic surfactant lead 2D structure changed into 3D flower structure,further reduced the on the transmission,at the same time,it also reduced the charge transfer resistance due to the formation of heterogeneous and built a fast channel for electronic transmission,which was the main reason for the further improvement of the gas sensitivity performance of the ultra-thin materials.Therefore,the addition of SDS could be used as an effective method to improve the NO₂ gas sensing performance of ZnAl LDHs materials at room temperature.