Preparation And Gas-sensing And Electromagnetic Wave Absorption Performances Of MOFs-derived Nanocomposites

Metal Organic Frameworks(MOFs)are polymer hybrids with large surface areas and abundant pore structures,which are self-assembled by the central metal ions and organic ligands through coordination.There are a wide variety of MOFs materials,and various composites can also be obtained by different directions of derivation,which can realize the multi-field application of MOFs materials.For example,Metal Oxide Semiconductors(MOSs)materials derived from MOFs by air calcination have achieved good applications in gas sensing.Owing to the significant casualties and property losses caused by natural gas leakage and gas explosion,it is an important measure to develop an efficient methane gas sensor for protecting the safety of people.Human health and military safety are threatened by electromagnetic pollution,so it is urgent to develop high-performance wave absorbing materials.Besides,the ideas to design electromagnetic wave absorbing materials are also expanded by carbon-based composites derived through carbonization.In this paper,different types of derivatives were obtained by various subsequent processes,taking MOFs materials as sacrificial templates.The methane sensitive properties and sensing mechanism of MOFs-derived MOSs material were studied.The wave absorbing performance and mechanism of MOFs-derived metal sulfide composites were also discussed.The main research contents and results of this paper are as follows:(1)Palladium nanoparticles were synthesized by thermochemical method,and then imbedded in MIL-68(In)to form core-shell Pd@MIL-68(In)precursors.After calcination and hydrogen reduction,Pd@In2O3 porous hexagonal hollow tubes were obtained.Pd@In2O3-2 showed the best methane sensing performances,and we explored the gas-sensitive enhancement mechanism of Pd@In2O3 porous materials.The MOF-derived materials with abundant porous structure,increased oxygen vacancies,and catalytic effect and spill-over effect of noble metals played as important roles to dramatically enhance the methane sensing performances.The response of Pd@In2O3-2 sample to 5000 ppm methane at 370℃ was as high as 23.2,and its response/recovery time was only 7 s/5 s.Moreover,after the modification of Pd nanoparticles,the Pd@In2O3-2 sensor can exhibit outstanding anti-interference capacity.Even though the concentration of methane was as low as 100 ppm,the Pd@In2O3-2 sensor still showed an effective response,which was able to meet the requirement in methane detection.(2)ZIF-8@ZIF-67 precursors were synthesized by the two-step precipitation method at room temperature.Firstly,ZIF-67 shells were coated on the surface of ZIF-8,and the core-shell ZIF-8@ZIF-67 precursors with the morphology of rhombic dodecahedron were obtained.After vulcanization,the ZnS@CoS2/C core-shell structured composites were acquired,and the ZC-1 sample showed the strongest absorption of-61.97 dB at 8.4 GHz with the matching thickness of 2.07 mm.The maximum effective absorption bandwidth of 4.08 GHz was reached at 1.25 mm.ZnS@CoS2/C core-shell structured composites can combine the advantages of multiple components.The introduction of a certain amount of CoS2 adjusted the dielectric constants and increased the conductivity loss,and the multiple-phase interfaces can effectively improve the polarization loss,thus the absorbing performances were greatly improved.(3)ZnCo bimetallic MOFs were firstly prepared by room-temperature precipitation method,and ZnS/CoS2/C heterostructured hybrids were obtained after the vulcanization process.Cobalt ion partially replaced zinc ion in the process of synthesis to form ZnCo-MOF,which led to morphological deformation due to the different sizes of the two metal ions.The differences of wave absorption properties between MOFs-derived bimetal sulfides heterostructured composites and monometallic MOF-derivatives were studied.The electromagnetic wave absorption performances of the Z1C2 sample were significantly enhanced by combining the excellent impedance matching characteristic of ZnS/C with the dielectric loss of CoS2/C.The minimum reflection loss value reached-60.31 dB at 6.64 GHz with the matching thickness of 2.73 mm.Due to the improved structure,the increased microscopic phase interfaces enhanced the polarization loss and widened the maximum effective absorption bandwidth.At the thickness of 1.34 mm,the EAB was about 4.48 GHz.The promoted wave absorbing performances of ZnS/CoS2/C heterostructured-absorber can be attributed to the modified microstructure,impedance matching and dielectric loss.