Preparation Of Intercalated Cobalt (nickel) Aluminum LDHs Materials And Research On NO_x Gas Sensing Properties

To solve the problems of low sensitivity,high cost,poor stability and short service life of gas sensitive sensing materials,one step hydrothermal synthesis method combined with anion exchange method were adopted to synthesize CoAlLDHs and Ni Al LDHs materials with a hierarchical structures by using urea as precipitant,CO₃^2-and NO₃^-as interlayer anions under the protection of inert gas(N₂).The composition,structure and morphology of LDHs materials were analyzed by XRD,FT-IR,SEM,TEM,AFM,BET and XPS and the NO_x gas sensitive properties of the synthesized materials were studied at room temperature,and the gas sensitive mechanism was preliminarily discussed.Firstly,CoAlLDHs series materials with unique two-dimensional hexagonal nanosheet structure were synthesised through one-step hydrothermal synthesis method by adjusting the molar ratio of metal elements Co and Al in the main board with urea as pore-forming agent and precipitator.SEM and TEM tests showed that the specific surface area of CoAlLDHs series samples were between 35-42 m²·g^-1,and the best sample,CA2-1,with a mole ratio of 2:1,was about 19.30 nm in thickness and 0.767 nm in layer spacing.A large number of mesopores exist in the lamellar layers,and the pore size was concentrated at 2.35 nm.At room temperature,the results of the gas sensitivity of CoAlLDHs series samples with different molar ratios for NO_x showed that the best sample,CA 2-1,had a good sensing performance for NO_x,the response sensitivity to100 ppm NO_x was 17.09,the response/recovery time was the shortest(4.27/38.93 s,respectively),and the minimum detection limit was 0.01 ppm.This was due to its unique hexagonal nanosheet structure and its large specific surface area.Secondly,Ni Al LDHs series materials with unique 3D flower-like nanostructures were prepared by changing the metal elements of the motherboard to Ni and Al and adjusting the molar ratio of metal elements by the same synthesis method.SEM and TEM results showed that the specific surface area of Ni Al LDHS series samples was between 42-55 m²·g-1,and the lamellar thickness was about 16.30 nm.Among them,the best sample CA 2-1 with a molar ratio of 2:1 had a lamellar spacing of 0.754 nm,and there were a large number of mesopores exist in the lamellar layer,the pore size was concentrated at 2.36 nm.The gas sensitivity of samples with different molar ratios to NO_x at room temperature showed that Ni Al LDHs had a shorter gas response/recovery time than that of CoAlLDHs,the sensitivity to 100 ppm NO_x was14.27,the response time was 3.72 s,the recovery time was 37.95 s,and the minimum detection limit was up to 0.01 ppm.SEM,TEM,BET and other characterizations showed that the best proportion of the sample,NA 2-1,possessed excellent gas sensitive properties because of its unique three-dimensional nanometer plate structure and its large specific surface area.Thirdly,CoAlLDHs-NO₃^-and Ni Al LDHs-NO₃^-(CAN and NAN)with NO₃^-intercalated hierarchical structure were prepared by ion exchange method with the best molar ratios of CoAlLDHs-NO₃^-and Ni Al LDHs-NO₃^-(CA 2-1 and NA 2-1)as self-sacrificial templates under the protection of N₂.SEM and TEM results showed that the CAN samples intercalated with NO₃^-presented a uniform and complete hexagonal nanosheet structure,and there were several well-dispersed nanosheets with the thickness of 15.3-17.5 nm,and the spacing of the layers was 0.804 nm.And NAN samples showed uniformly dispersed 3D flower-like structures,the thickness of nanosheets was about 9.5 nm,the layer spacing was 0.789 nm.and their specific surface areas were 59m²g^-1 and 65 m²g^-1,respectively.At room temperature,NO₃^-intercalated LDHs presented the better gas sensing performance than CO₃^2-intercalated LDHs.The gas-sensitive response value of CAN to 100 ppm NO_x was 29.34,which is significantly higher than that of NO_x to CA 2-1 at the same concentration(17.09).The gas response sensitivity of NAN sample(25.59)was significantly higher than that of NA 2-1(14.27),and the response time of NAN sample was also relatively shorter than that of unintercalated NA 2-1(3.32 s).The high gas sensitivity properties of intercalated LDHs samples were mainly due to their larger specific surface area combined with the intercalation of anions,which could be not only increased the adsorption activity site but also led the interlayer spacing increased and effectively increased the fasting charge transfers in the channel and speeded up the reaction speeding with the target gas endly.In this paper,hierarchical structure materials LDHs were synthesized using one-step hydrothermal synthesis combined with ion exchange reaction by adjusting the metal ions and interlayer anions of the motherboard.The NO_xgas-sensitive sensing performances of the as synthesized materials were effectively improved due to the diffusion limit of molecules reduced with the adsorption and diffusion channels of two-dimensional LDHs materialseffectively connected at room temperature Therefore,the thin film sensor made of this nanocomposite materials was expected to be applied in gas monitoring and the other fields.