Research On The Construction Of Hierarchical Structure Nickel-aluminum-based Nanocomposites And The NO_x Gas-sensing Properties

In view of the generally low response of conventional semiconductor materials for gases at room temperature,zinc-doped nickel-aluminum hydroxide composites?NZA-LDHs?and expanded graphite?EG?-doped nickel-aluminum hydroxide composites?EG/NiAl-LDH?were synthesized by a simple hydrothermal method.The composition,structure and morphology of the composites were characterized by XRD,FT-IR,SEM,TEM,AFM,BET,XPS and Raman.The gas sensing properties of the composites were investigated at room temperature,and the gas sensing mechanism was preliminarily discussed.Firstly,three-dimensional layered flower structure NZA-LDHs series were systhesized by adjusting the doping ratio of Zn²⁺using the simple and low-cost hydrothermal synthesis method with urea as a precipitant and pore former.The lamellar thickness of NZA-LDHs series was about 40-50 nm,and the mesopores were evenly distributed on the sheet.The pore size was concentrated at 2.36 nm,and the specific surface area of the NZA-LDHs sample was between 45-53 m²·g^-1.By comparison,the gas sensing properties of the sensors constructed by pure NiAl-LDH,pure ZnAl-LDH and NZA-LDHs series samples were investigated at room temperature.NZA 1-2 sample had best gas sensing performance for NO_x,with a response of 3.34 for 100 ppm NO_x,a response time of 16 s,and a minimum detection limit of 0.1 ppm.The reason for the enhanced gas sensitivity of NZA 1-2 was that it had a porous layered flower-like structure and a layered polycrystalline composition.Secondly,NiZnAl ternary metal oxide nanocomposite with good dispersibility was successfully synthesized using LDH as a self-sustaining template by pyrolysis treatment.It was found that the three-dimensional flower-like NiZnAl multi-metal oxide nanocomposites were composed of ultra-thin porous nanosheets with a thickness about5.2-7.5 nm,a pore size mainly distributed at 3.03-10.01 nm,and the specific surface area of NiZnAl multimetal oxide between 60-65 m²·g^-1.The gas sensing properties of the gas film sensor constructed by NiZnAl multi-metal oxide series samples were investigated at room temperature.By comparison,NZAO 1-2 sample exhibited best response to 100 ppm NO_x by 9.16,the response time was always within 8 s,and the stability to NO_x was as long as 350 days.The reason for the improvement of the gas sensitivity of the NZAO sensor was attributed to the synergistic effect of the 3D flower-like structure with ultra-thin porous nanosheets and the formation of heterojunction in the composite.Finally,expanded graphite/NiAl-LDH composites were controlled and assembly synthesized by using microwave heating-vacuum-assisted-hydrothermal synthesis method.EG was used as a template to change the urea loading in the hydrothermal reaction,and the EG sheet was extended to provide anchor points for NiAl-LDH nanosheets.Compared to different forms of NiAl-LDH and pure EG,expanded graphite/NiAl-LDH nanowire composites exhibited improved NO_x sensing performance at room temperature.The best sample EG/NA3 had fast response/recovery behavior and good NO_x selectivity.The gas sensitivity response to 100 ppm NO_x was 17.65,the detection limit was as low as 10 ppb.The enhanced gas sensitivity was due to the excellent electrical conductivity of EG,and the synergistic effect of the nanostructure of NiAl-LDH.In this paper,the doping/modification method was used to effectively improve the gas sensing performance of NiAl-LDH based nanocomposites at room temperature for NO_x.This high performance nanocomposite film sensor was expected to be widely used in practical monitoring and commerce.