Microwave Absorption Analysis Of Nitrogen-Doped Two-Dimensional Composites With Magnetic/Dielectric Properties

The rapid advancement of electronic information technology has resulted in the escalation of electromagnetic wave（EMW）pollution,necessitating the development of efficient absorbing materials for mitigating electromagnetic interference in diverse domains such as military,commercial,and civil communications.Two-dimensional（2D）materials,including MXenes and graphene,with nanoscale thin-layer structures,offer promising opportunities to construct multiple loss paths for effective EMW absorption.However,their intrinsic high carrier mobility poses challenges in achieving optimal impedance matching,while the propensity for vertical aggregation limits their practical utility in EMW absorption applications.To address these limitations,the introduction of nitrogen doping defects in 2D materials has emerged as a pivotal approach to reduce carrier mobility and induce dipole polarization.Intercalation of magnetic and dielectric substances offers a promising strategy to ameliorate the stacking propensity of 2D nanosheets,consequently bolstering EMW absorption capabilities.The inclusion of magnetic intercalators facilitates the induction of magnetization loss,encompassing hysteresis loss,natural resonance,and eddy current loss.On the other hand,dielectric intercalators introduce variations in dielectric properties,thereby augmenting dielectric loss.Consequently,the resulting 2D composite materials exhibit a balance between magnetic and dielectric components,leading to enhanced EMW absorption.Exploring the potential of these composite materials has profound implications for advancing EMW absorption technologies.This paper addresses the performance limitations exhibited by current 2D absorbing materials and introduces novel composite materials with the key attributes of wideband capability,high absorption strength,lightweight characteristics,and thin profile.Firstly,this research endeavors involve the modification of the 2D materials Ti₃C₂T_x-MXene and graphene using nitrogen doping techniques.This doping process facilitates the regulation of carrier mobility and induces dipole polarization,consequently amplifying the dielectric loss mechanism.Concurrently,first-principles calculations elucidate the defect state and conductor property under varying nitrogen doping contents.Secondly,annealed ZIF-67@ZIF-8 bimetallic zeolitic imidazolate frameworks（ZIFs）is employed as suitable candidates to be integrated with N-Ti₃C₂T_x.This composite system not only exhibits dielectric loss,but also magnetization loss stemming from the Co phase.The resultant composites demonstrated a remarkable enhancement in EMW absorption performance.To delve into the fundamental principles governing the electromagnetic properties of 2D dielectric and magnetic composite materials,with a specific focus on iron-cobalt-based prussian blue analogues（PBAs）.These PBAs serve as the precursors for derivative nanoparticles,namely Co Fe₂O₄ and Fe₃O₄,which exhibit stronger magnetization loss.PBAs-Fe Co/N-Ti₃C₂T_xcomposites lead to an observed increase in the minimum reflectivity value(R_Lmin)within the lower frequency band.However,the aforementioned materials solely fulfill the"wide"and"strong"characteristics,while the sample loading capacity remains approximately 30–35%and the thickness around 2–3 mm,thereby necessitating further exploration for achieving the attributes of"light"and"thin".Consequently,a dielectric/dielectric composite material is chosen,wherein the lightweight and resilient nitrogen-doped graphene（N-r GO）aerogel is combined with silver nanowires（Ag NWs）to facilitate multiple charge transfer pathways.Notably,despite the slender thickness of approximately 1.5 mm,the dielectric composites successfully achieve the attributes of being lightweight while maintaining the highly efficient and broad EMW absorption characteristics.The research encompasses four primary aspects,as detailed below.（1）Acetonitrile is employed as a liquid nitrogen source to facilitate nitrogen doping in Ti₃C₂T_x and graphene materials.The nitrogen substitution process involved replacing 5.3 at%of carbon atoms in Ti₃C₂T_x with nitrogen atoms.First-principles calculations revealed that this substitution did not fundamentally alter the metallic conductor behavior of Ti₃C₂T_x.The resulting nitrogen-doped N-Ti₃C₂T_x structure maintains its original parallelogram shape,albeit with deviated symmetry centers of carbon and nitrogen atoms,giving rise to dipole polarization.Additionally,nitrogen doping replaces 2.1 at%of carbon atoms in graphene,retaining its hexagonal honeycomb network structure.The introduction of sp³ defects and disordered sites through nitrogen doping significantly improves the coupling properties of graphene,leading to pronounced polarization effects.First-principles calculations demonstrate that the semi-metallic conductor graphene transformes into a semiconductor with an indirect band gap of 0.25 e V.Nitrogen doping-induced dipole polarization in these 2D materials effectively enhanced relevant EMW absorption performance.（2）The synthesis of ZIF-67@ZIF-8/N-Ti₃C₂T_x composites via electrostatic self-assembly is present.The resultant composites exhibit an advantageous combination of dielectric and magnetic properties,leading to excellent impedance matching.By integrating replaceable metallic nodes with regular organic frameworks,the polar units within the composites are significantly augmented.Notably,the newly formed Co/C or Co/N-Ti₃C₂T_x interfaces within ZIF-67@ZIF-8/N-Ti₃C₂T_x composites induce electron rearrangement,resembling a capacitor-like structure and resulting in dielectric loss.Simultaneously,diverse factors such as magnetization loss,multipolar units,and defects contribute to the efficient multi-band EMW absorption performance.ZIF-67@ZIF-8/N-Ti₃C₂T_x composites achieve the maximum effective absorption bandwidth（EAB）of 5.36 GHz,accompanied by an impressive R_Lmin value of–65.03d B（2.273 mm,at 13.00 GHz）,symbolizing strong EMW absorption capability.（3）The synthesis of FeCo-PBAs/N-Ti₃C₂T_x composites using electrostatic self-assembly is focused.A novel approach of introducing Co groups into co-coordinated PBAs has been implemented to enhance the polar units,resulting in increased magnetization loss upon annealing.Within Fe Co-PBAs/N-Ti₃C₂T_x composites,the newly formed Fe Co/C or Fe Co/N-Ti₃C₂T_x interface facilitates the rearrangement of electrons into a capacitor-like structure,thereby achieving dielectric loss.The composites exhibit efficient continuous multi-band（the lower frequency）EMW absorption properties,which can be attributed to several contributing factors,including enhanced magnetization loss,multipolar units,and defects.Fe Co-PBAs/N-Ti₃C₂T_xcomposites demonstrate a wide EAB of approximately 4.10 GHz,accompanying a notable R_Lmin value of–51.15 d B（1.776 mm,at 12.30 GHz）.Fascinatingly,Fe Co-PBAs/N-Ti₃C₂T_x composites exhibit multiple low-frequency R_Lmin values,indicating the potential for flexible adjustment of the designated frequency range in dielectric/magnetic composite materials.（4）Ag NWs/N-rGO aerogel,a dielectric/dielectric composite material,exhibits a self-supporting sponge-like structure comprising silver nanowires（Ag NWs）and N-r GO sheets,offering a unique combination of"wide,""strong,"and"light"characteristics for efficient EMW absorption.Notably,even at an ultra-small density range of 8–12 mg/m L,Ag NWs/N-r GO aerogels maintain exceptional toughness.Ag NWs serve as elastic supports and conductive units,establishing multiple charge transfer paths with N-r GO.This collaboration results in a heterogeneous dielectric structure,inducing multi-dimensional interface polarization and significantly enhancing dielectric loss effect.At an ultra-low sample loading of 5 wt%,Ag NWs/N-r GO aerogels achieve an EAB of 3.50 GHz（2.660 mm,at 9.52 GHz）and exhibits the strongest R_Lmin value of–79.99 d B,all while maintaining a thinness of approximately1.5 mm.This exceptional combination of"light"and"thin"characteristics further enhances the practical applications.Moreover,Ag NWs/N-r GO aerogels demonstrate high resilience,flame retardant ability,and hydrophobicity,ensuring application stability,even under deformation and thermal attack conditions.