Design principles in MOF-derived electromagnetic wave absorption materials: Review and perspective

1.   Introduction

Facing the flourishing development of Industry 4.0, the era of intelligence is driving the Fourth Industrial Revolution, while cyber-physical systems based on the network entity system have comprehensively improved the field of electromagnetic (EM) equipment application [1]. However, EM interference is also becoming a new pollution source in the new era. In this case, it is of great significance to eliminate harmful radiation by absorbing the useless EM waves (EMWs) using microwave absorption materials (MAMs) [2]. Nowadays, several nano–micro materials with diverse topological, chemical, and physical properties have been applied to absorb EMWs. In particular, metal–organic frameworks (MOFs) with unique components and constructions have received increasing attention for not only high EMW absorption performance but also deep absorbing mechanism investigation [3].

MOFs are generally constructed by transition metal ions and organic ligands periodically [4]. The fundamental design concept and research orientation of MOF-derived MAMs can be predominantly generalized into the following stages: (1) choosing coordination monomers (organic ligands and metal ions), (2) controlling topologies by the coordinate self-assembly, (3) obtaining relative derivatives by post-processing, and (4) checking EMW absorbability and EM response mechanism [5]. First, monomers (ions and ligands) usually determine the species of MOFs. According to previous reports, there have been more than 20000 kinds of MOFs [6]. Second, the coordination condition during the MOF synthesis may impact the topologies, resulting in MOFs with 1 dimension (1D), 2 dimension (2D), 3 dimension (3D), and even hierarchical structures [7]. Third, the post-processing procedures will derive MOFs into crystalline or amorphous metal and carbon composites in various chemical states [8]. Finally, attributing to their tunable physical properties, such as semiconductivity, magnetism, and conductivity, these MAMs can perform high-efficiency EMW absorption [9].

In the past decades, it has been extensively studied that EMW absorption can contribute to dielectric and magnetic responses [10]. According to the classical EM absorption theory, the EMW propagation behavior with energy transfer can be simulated using a simplified model constituted with excited incident wave, absorber, and perfect electric conductor (metal) support [11]. As there is no energy transmission due to the metal support, the EMW absorption performance can be monitored by detecting the remaining energy of the reflected wave. Therefore, the reflection loss (RL) value will exceed −10 dB when the energy absorption rate is greater than 90%, and the corresponding frequency band is defined as the effective absorption bandwidth (f_E) [12]. Typically, the rational manipulation of impedance matching and attenuation ability has been regarded as the fundamental of surface reflection and internal energy dissipation, respectively [13]. Meanwhile, energy attenuation is mainly attributed to the dielectric and magnetic loss, including conductive loss, polarization relaxation (e.g., dipole polarization and interfacial polarization), and magnetic coupling effect [14].

However, demystifying the principle of EMW absorption regarding the structure–performance relationship at a deeper level still shoulders heavy responsibilities. In this case, we systematically and comprehensively reviewed the basic design methods and goals for MOF-derived MAMs and the EMW absorption structural dependence, including MOFs with diverse ligands, ions, and topologies, MOF derivatives with diverse chemical states, crystal structures, physical properties, and corresponding classical EMW absorption mechanism (Fig. 1). Finally, some urgent challenges and future research perspectives are also proposed.

Fig. 1.  Schematic illustration of the synthesis strategies, structures, and properties of MOF-derived MAMs. The detailed classification shows MOFs with diverse ligands and ions, MOF derivatives with diverse chemical states (Reprinted with permission from X.Q. Xu, F.T. Ran, Z.M. Fan, et al., ACS Appl. Mater. Interfaces, vol. 12, No. 15, 17870-17880 [15]. Copyright 2020, American Chemical Society), physical properties, synthesis strategies of MOFs (Reprinted with permission from P. Miao, Z. Yu, W.X. Chen, et al., Inorg. Chem., vol. 61, 1787-1796 [16]. Copyright 2022, American Chemical Society), diverse topologies and architectures (Reprinted from Chem. Eng. J., 426, W. Wang, H. Zhang, Y.Z. Zhao, et al., A novel MOF-drived self-decomposition strategy for CoO@N/C–Co/Ni–NiCo₂O₄ multi-heterostructure composite as high-performance electromagnetic wave absorbing materials, art. No. 131667, Copyright 2021, and from J. Colloid Interface Sci., 600, Y.H. Cui, Z.H. Liu, X.X. Li, et al., MOF-derived yolk-shell Co@ZnO/Ni@NC nanocage: Structure control and electromagnetic wave absorption performance, 99-110, Copyright 2021, with permission from Elsevier [17–18]), and corresponding EMW absorption mechanism (L. Wang, M.Q. Huang, X. Qian, et al., Small, vol. 17, art. No. e2100970 (2021) [19]. Copyright Wiley-VCH Verlag GmbH & Co. KGaA. Reproduced with permission).

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2.   MOFs with diverse ligands and ions

Generally, MOF family members have exploded up to more than 20 thousand since the first report of a stable MOF synthesis by Furukawa et al. [20]. Attributed to the adjustable crystal structure, large specific surface area, and high porosity features, thousands of MOFs have been employed in various fields for high-performance functional materials, such as energy, environment, medical, intelligent sensing, and EM interference shielding and EMW absorption. Simultaneously, according to the crystal field theory and hard/soft/acid/base theory, most MOFs are built by the Lewis acid (metal ions)–base (ligands) assembly. In this case, researchers [21–26] have explored a huge amount of MOFs with various ligands and ions (Fig. 2). Furthermore, MOFs possessing unique metals and organic ligands can be pyrolyzed into derivative MAMs with specific metal phases and carbon matrices, performing predictable EM characteristics. They provide indispensable prerequisites for further applications of reflection, absorption, or filtering of frequency-domain EMWs.

Fig. 2.  Structures of MOFs commonly used for EMW absorption with diverse ions and ligands. The detailed classification shows structures of ZIF-67 (Reprinted from Chem. Eng. J., 421, Z.G. Gao, J.Q. Zhang, S.J. Zhang, J. Wang, and Y.H. Song, Cationic etching of ZIF-67 derived LaCoO₃/Co₃O₄ as high-efficiency electromagnetic absorbents, art. No. 127829, Copyright 2021, with permission from Elsevier [21]), MIL-88B-Fe (P. Miao, R. Zhou, K.J. Chen, J. Liang, Q.F. Ban, and J. Kong, Adv. Mater. Interfaces, vol. 7, art. No. 1901820 (2020) [22]. Copyright Wiley-VCH Verlag GmbH & Co. KGaA. Reproduced with permission), HKUST-1, PBA, MOF-74 (Reprinted from J. Colloid Interface Sci., 606, W.B. Li, J. Chen, and P. Gao, MOFs-derived hollow copper-based sulfides for optimized electromagnetic behaviors, 719-727, Copyright 2022, with permission from Elsevier [23]), UiO-66 (Reproduced from Ref. [24] with permission from the Royal Society of Chemistry), PCN-415 (Reprinted by permission from Springer Nature: Nanomicro Lett., Non-magnetic bimetallic MOF-derived porous carbon-wrapped TiO₂/ZrTiO₄ composites for efficient electromagnetic wave absorption, J. Qiao, X. Zhang, C. Liu, et al., Copyright 2021 [25]), and PCN-415 (Reprinted with permission from X.B. Liu, T.T. Liang, R.T. Zhang, et al., ACS Appl. Mater. Interfaces, vol. 13, 9643-9655 [26]. Copyright 2021, American Chemical Society).

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Up to now, the zeolitic imidazolate framework (ZIF) accounts for more than half of the publications (data from Web of Science) among MOFs, which are usually assembled by 2-methylimidazole (Hmim) and Co²⁺ or Zn²⁺. Dating back to 2015, ZIF-67 (Co(mim)₂) was synthesized and applied in EMW absorption by Lü et al. [27] for the first time by coordinating Co²⁺ and Hmim in a methanol solution. By using thermogravimetric (TG) curves, they analyzed the thermal decomposition kinetics of ZIF-67 in air and inert gas, such as Ar, and checked the structure of pyrolysis products at graduating temperatures. Finally, f_E of the best absorber reached 5.80 GHz, which verified that ZIF-67 can be employed as a kind of ideal precursor of MAMs, subsequently attracting several researchers to participate in the study of ZIF-67-derived MAMs. In addition to Co²⁺, Hmim can also coordinate with Zn²⁺ in the form of Zn(mim)₄ to obtain ZIF-8 [28]. Typically, ZIF-8 would transform into dielectric losing MAMs constructed with Zn derivatives and N-doped carbon matrices due to the nonmagnetic property of Zn [29–32]. Aside from diverse ions, some researchers began to explore and try the application of ZIFs synthesized by ligands containing various imidazole structures in the field of EMW absorption. For instance, Wang et al. [33] synthesized a kind of novel ZIF using 1-benzyl-2-methylimidazole as the ligand and Co²⁺ as the center ion with the assistance of dodecyltrimethylammonium bromide. The results indicated that the change in ligands mainly regulated the porous structure of the carbon skeleton, which not only had a significant impact on the dielectric loss during the EMW absorption process but also affected the impedance matching due to the surface structure.

In the Material Institute Lavoisier (MIL) family, MIL-101, MIL-100, MIL-125, MIL-53, MIL-88A, MIL-88B, NH₂-MIL-101, NH₂-MIL-88, and NH₂-MIL-88B have successively appeared as precursors of MAMs. Usually, the ligands of MIL-101, MIL-53, MIL-125, and MIL-88B are terephthalic acid (H₂BDC) [34–36]. The ligands of NH₂-MIL-101, NH₂-MIL-88, and NH₂-MIL-88B are 2-aminoterephthalic acid (NH₂-H₂BDC) [26,37–38]. The ligand of MIL-88A is fumaric acid [39]. The ligand of MIL-100 is 1,3,5-benzenetricarboxylic acid (H₃BTC) [40]. The central ions of MIL are usually occupied by transition metals, such as Fe, Cr, and Ti. The most typical MILs for MAM precursors is MIL-101(Fe), which is generated by the solvothermal process of H₂BDC and Fe³⁺ in N,N-dimethylformamide (DMF) as the solvent. For example, Peng et al. [41] synthesized MIL-101(Fe) and corresponding sintered products under different carbonization temperatures, of which the simultaneous optimization of multi-interfacial polarization, ferromagnetic resonance, conduction loss, and interference cancelation principles finally resulted in the excellent EMW absorbability. As MILs with H₂BDC as ligands do not contain the N element, some research groups have devoted themselves to using N-containing ligands, such as NH₂-H₂BDC, to improve the heteroatom doping structure of pyrolysis products and the polarization loss capability. For example, Zhou et al. [42] synthesized NH₂-MIL-101 by NH₂-H₂BDC, hence creating a strong polarization effect on the final obtained powders. In addition, H₃BTC can be utilized to construct MILs. For example, Zhu et al. [43] synthesized MIL-100(Fe) by coordinating H₃BTC and Fe²⁺ under an alkaline condition. Moreover, diverse coordination ions enriched the types of MILs; for example, Ti can generate MIL-125 (Ti) with H₂BDC [44].

Prussian blue analogs (PBAs) have also been considered excellent candidates as precursors of MAMs. PBAs are a class of cubic MOFs assembled from transition metal hexacyanometallates networks [45], which can usually be prepared by the reactions of ${\mathrm{F}\mathrm{e}\left(\mathrm{C}\mathrm{N}\right)}_{6}^{4-}$, Fe(CN)³⁻, and Co(CN)³⁻ with Fe²⁺, Co²⁺, Ni²⁺, and Mn²⁺. Typically, Liu et al. [46] synthesized Prussian blue (PB) through the self-condensation of K₄Fe(CN)₆, whose derivative MAM obtained a broad f_E of 5.44 GHz at only 1.5 mm. Another method to synthesize PB is the coordination polymerization and deposition of [Fe(CN)₆]³⁺ and Fe²⁺ in an acidic solution [47]. In addition to typical hexacyanoferrate PB with only the Fe metal element, researchers have also developed hexacyanocobaltate networks and PBAs with various metals, such as Co [48], FeCo [49], NiCo [50], and MnFe [51].

In summary, the above three MOFs almost cover most of the publications of MOF-derived MAMs. Apart from the MOFs above, researchers have gradually developed new MOFs from the perspectives of synthesizing new MOFs and adjusting the coordination structure of MOFs in an attempt to apply them to EMW absorption. For instance, 2,5-dihydroxyterephthalic acid (H₄DOBDC) was utilized to synthesize Co-MOF-74 [52–53], Mn-MOF-74 [54], and Cu-MOF-74 [23]. H₂BDC and NH₂-H₂BDC were utilized to synthesize UiO-66 [55] and NH₂-UiO-66 [56], respectively. Under tunable condition, H₂BDC can also be constructed into MOF-5 with Zn²⁺ [57], Fe³⁺ [58–59], PCN-415 with Ti⁴⁺ and Zr⁴⁺ [60], and MOF-53 with Fe³⁺ [61]. H₃BTC can also be employed in the synthesis of HKUST-1 (Hong Kong University of Science and Technology) with Cu²⁺ [62] and BTC (Some MOFs assembled with H₃BTC) with Ni²⁺ [63–64]. Some researchers have attempted to develop new MOFs with new ligands. For example, Che et al. [65] synthesized bimetal Cu_xNi_1−x-MOF with the 5,10,15,20-tetrakis(4-carboxylphenyl)-porphyrin (TCPP) ligand, of which the Cu/Ni/C derivate showed a broad absorption band up to 5.87 GHz.

3.   MOFs with diverse topologies

Owing to the controllability of the coordination structure between ligands and ions and the chemical condition-induced assembly orientation, the symmetric structures of MOFs exhibit diverse structures, which in turn make their topologies ever-changing.

3.1   1D MOFs

In general, 1D MOFs have an overwhelmingly dominant orientation along one direction, so they are usually more difficult to prepare than other MOFs. Typically, MOF-74 with the H₄DOBDC ligand can be constructed into micron-scale rods with high aspect ratios. For instance, Yang et al. [54] synthesized a series of Mn-MOF-74 and corresponding MnO₂- and MnO-doped nanoporous carbon 1D composites. Finally, the results indicated that the 1D topology with a significant anisotropy has a positive effect on the multiple scattering and reflection, conductive loss, and polarization loss, attributed to the intricate wave transmission path, integral migration, and hopping paths for electrons and rich charge accumulation, respectively. In addition, some MOF-74 with Co²⁺ (Fig. 3(a)) [66], Ni²⁺ [67], and Cu²⁺ (Fig. 3(b)) [68] metal ion centers can also be assembled into 1D MOFs.

Fig. 3.  (a) Schematic illustration of the preparation process of hierarchical nest-like structures of CoFe@C composites (above) and SEM images of CoFe-MOF-74 with different amounts of Co²⁺ (below) (Reprinted from Composites Part A, 135, L. Wang, B. Wen, H.B. Yang, Y. Qiu, and N.R. He, Hierarchical nest-like structure of Co/Fe MOF derived CoFe@C composite as wide-bandwidth microwave absorber, art. No. 105958, Copyright 2020, with permission from Elsevier [66]). (b) (b1) XRD patterns of 1D MOF-74, TEM (b2) and SEM (b3) images of MOF-74 derivatives, and (b4) EMW absorption mechanism (Reprinted from Carbon, 186, Z.H. Zhao, K.C. Kou, L.M. Zhang, and H.J. Wu, Optimal particle distribution induced interfacial polarization in bouquet-like hierarchical composites for electromagnetic wave absorption, 323-332, Copyright 2022, with permission from Elsevier [68]).

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3.2   2D MOFs

The synthesis process of 2D MOFs usually requires a special chemical induction, such as solvents [69], pH conditions [70], and surfactants [71]. To date, very few 2D MOFs have been employed in EMW absorption, such as ZIF and MOF-71. For ZIF, because H₂O molecules and oxygen participate in the coordination of Hmim with Co²⁺ or Zn²⁺, the growth of the (100) lattice face of ZIF-L is greatly encouraged, promoting morphology to be flaky with a 2D topological structure [72]. For example, Xu et al. [73] fabricated a series of bimetallic CoZn-ZIF-L 2D nanoflakes with regulated Co²⁺/Zn²⁺ values, of which pyrolysis products effectively excited electron migration and hopping in or out of the nanoflakes, achieving a considerable RL value of −44.6 dB at 5.20 GHz (Fig. 4(a)). Liu et al. [74] proposed a novel synthesis of 2D ultrathin CoNi-MOF-71 nanosheet arrays involving the synergistic effects of anisotropic, epitaxial, and confined growth (Fig. 4(b)). The results finally confirmed that the 2D features gave rise to a significant shape anisotropy, porosity, and conductivity, which optimized not only multiple attenuation mechanisms but also impedance matching.

Fig. 4.  (a) Schematic illustration for the design of cactus-inspired 1D−2D mixed-dimensional CoNC/CNTs for EMW absorbing materials and RL values of 2D ZIF-L derivatives (Reprinted with permission from X.Q. Xu, F.T. Ran, Z.M. Fan, et al., ACS Appl. Mater. Interfaces, vol. 11, 13563-13573 [73]. Copyright 2019, American Chemical Society). (b) Schematic illustration showing the synthesis of the 2D CoNi-MOF-71 array and the Co/Ni/C derivative (Reprinted with permission from G. Liu, J.Q. Tu, C. Wu, et al., ACS Appl. Mater. Interfaces, vol. 13, 20459-20466 [74]. Copyright 2019, American Chemical Society).

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3.3   3D MOFs

Almost all publications on MOF-derived MAMs are discussed based on 3D MOFs. For 3D MOFs, the EM properties are primarily determined by morphology [75], particle size [76], and porosity [77], among others. The morphological structure can be controlled by not only the types of MOFs but also the synthesis condition. For example, ZIF-67 will be constructed into traditional dodecahedrons in a methanol solution [78], whereas ZIF particles will usually show leafy and cubic structures in aqueous and ethanol solutions, respectively [79–80]. Moreover, the concentration and molar ratio between ligands and metals generate the morphological transformation. As shown in the work from Huang et al. [81], a high molar ratio between Hmim and Co²⁺ brought about cubic ZIFs and small particle sizes (Fig. 5(a)). Furthermore, some works began to explore MOFs with a new morphology with the assistance of surfactants, such as polyvinylpyrrolidone and cetyltrimethylammonium bromide (CTAB) [82] (Fig. 5(b)). For example, Li et al. [83] proposed a novel and innovative CTAB-assisted fabrication strategy of hollow ZIF-67, which dramatically reinforced multiple reflection behaviors. Some 3D MOFs have excellent porosity and large specific surface areas, such as MIL-88B, and researchers have begun to analyze the relationship between the porous structure and EMW absorption [84]. Relying on the Brunauer–Emmett–Teller (BET) characterization, Xu et al. [85] found that MOFs with a high porosity can be converted to 3D interconnected carbon with a high surface area, which will yield multiple transmission paths for charges, finally initiating rich microcurrent and polarization centers.

Fig. 5.  (a) Schematic preparation process of Co@NC composites (M.Q. Huang, L. Wang, K. Pei, et al., Small, vol. 16, art. No. 2000158 (2020) [81]. Copyright Wiley-VCH Verlag GmbH & Co. KGaA. Reproduced with permission). (b) Schematic illustration of the fabrication process of (b1) ZIF-67, (b2) ZIF-67/PVP, (b3) ZIF-67@PVP-PDA, and (b4) concave nanocube particles (Reprinted from Carbon, 135, Y.J. Liu, Z.J. Yao, J.T. Zhou, L.Q. Jin, B. Wei, and X.X. He, Facile synthesis of MOF-derived concave cube nanocomposite by self-templated toward lightweight and wideband microwave absorption, 574-588, Copyright 2022, with permission from Elsevier [82]).

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In summary, topology modulation has a significant impact on the dielectric and magnetic properties. Accordingly, a group of researchers have worked on establishing the relationship between the topology and EMW absorption performance of MOFs and relative derivatives.

4.   MOFs with diverse derivatives

The chemical structures and physical properties of MOF derivatives show a flourishing development trend. After further processing, MOFs are usually converted into a metal/metal alloy, ferrite, metal sulfide, and metal phosphide, with a carbonaceous matrix, which is performed as a semiconductor, magnetic particle, and conductive network.

4.1   Chemical states

4.1.1   Metal or metal alloy/C

Metal alloys are usually obtained by annealing MOFs in an inert atmosphere such as N₂ and Ar. For example, Xu et al. [15] fabricated a pomegranate-like CoNi alloy by annealing CoNi-BTC at 700°C in an N₂ atmosphere for 2 h (Fig. 1). Finally, the RL value of CoNi/C composites reached −61.8 dB at 9.36 GHz. In a previous publication of Shen et al. [86], a series of metal alloy-based MAMs was prepared by annealing CoMn-MOF and FeCoMn-MOF in an N₂ atmosphere (Fig. 6(a)), resulting in an efficient magnetoelectric coupling and even reinforcing EMW absorption enhancement. In addition, a large variety of MOFs has been demonstrated for the preparation of metal/metal alloys, such as ZIF-67 [87], MIL-88B [85], MOF-74 [52], and PBA [48].

Fig. 6.  MOF-derived MAMs with diverse chemical states. The detailed classification shows MOF-derived (a) CoFeMn metal alloys (Reprinted from Carbon, 185, Z.J. Shen, H.L. Yang, C.B. Liu, E.W. Guo, S.Y. Huang, and Z.Q. Xiong, Polymetallic MOF-derived corn-like composites for magnetic-dielectric balance to facilitate broadband electromagnetic wave absorption, 464-476, Copyright 2021, with permission from Elsevier [86]), (b) LaCoO₃/Co₃O₄ ferrite (Reprinted with permission from P. Miao, J.Y. Yang, Y.K. Liu, H.Z. Xie, K.J. Chen, and J. Kong, Cryst. Growth Des., vol. 20, 4818-4826 [89]. Copyright 2020, American Chemical Society), (c) metal phosphosulfide (Reprinted from W.J. Ruan, C.P. Mu, B.C. Wang, et al., Nanotechnology, vol. 29, art. No. 405703 (2018) [99]. © IOP Publishing. Reproduced with permission. All rights reserved), and (d) metal sulfide (Reproduced from Composites Part A, 149, G.L. Song, K.K. Yang, L.X. Gai, et al., ZIF-67/CMC-derived 3D N-doped hierarchical porous carbon with in situ encapsulated bimetallic sulfide and Ni NPs for synergistic microwave absorption, art. No. 106584, Copyright 2021, with permission from Elsevier [97]).

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4.1.2   Ferrite/C

Ferrite materials are the main derivatives from MOFs for EMW absorption. Attributed to tunable metal elements, oxidation state, and crystal structure, these ferrites have made great contributions to the development of electromagnetically responsive materials. On the one hand, the metal elements determine the basic chemical states of ferrites. In a previous work of Liu et al. [46], PB microcubes were oxidized into Fe₂O₃ ferrites in air by releasing C and N elements. On the other hand, under special synthesis circumstances, MOFs will convert into ferrite with a specific oxidation and crystal structure, such as spinel and perovskite structural metal oxides [17]. For instance, Yuan et al. [88] concluded that the Co-based ferrite derived from ZIF-67 can be Co₃O₄, Co₃O₄/Co, and Co in 500, 600, and 700°C, respectively, of which the Co₃O₄/Co-based composite showed an optimal EMW absorption. Apart from the calcination temperature, the heating rate during calcination also has a significant impact on the ferrite structure. In the previous work of Miao et al. [89], perovskite-phase LaCoO₃ can be easily obtained from LaCo-bi-MOF with a heating rate of 0.5°C/min, while a heating rate exceeding 1°C/min only brought about Co₃O₄ (Fig. 6(b)).

4.1.3   Metal sulfide and phosphide

After some special hetero-element treatments, MOFs can be transformed into novel chemical derivatives, such as metal sulfides [90], metal phosphides [91], and double-layer hydroxides [92]. For example, Yan et al. [93] converted ZIF-67 into CoS₂ composites using sulfur powder as the sulfur source, indicating that CoS₂ exhibited better dielectric loss compared to ZIF-67-derived Co. In addition, some sulfur compounds can be employed as sulfur sources, such as thiourea (CH₄N₂S) [94–95] and sodium sulfide nonahydrate (Na₂S) [96]. For example, Song et al. [97] proposed an in situ sulfuration strategy using thiourea for the rational construction of Co_xS_y/Ni_xS_y based on ZIF-67, achieving an outstanding EMW absorbability in the X and Ku bands (Fig. 6(d)). Analogously, MOFs can also be converted to metal phosphides by phosphating, such as sodium hypophosphite monohydrate (NaH₂PO₂) [98] and sodium dihydrogen phosphate (NaH₂PO₄) [99]. Typically, as shown in a previous study on Mu [99], a series of ZIF-67 derivatives, including Co, CoS₂, CoP, and CoS_2−xP_x, were simultaneously fabricated for EMW absorption. In this work, carboxymethylcellulose sodium (CMC-Na) The final results indicated that the composites based on hybrid CoS_2−xP_x performed considerable EMW attenuation, whose RL value reached −68 dB with a thickness of only 1.5 mm (Fig. 6(c)) [99].

4.2   Crystal structure

Aside from the chemical states, the effect of the crystal structure on EMW absorption is also indispensable. Generally, the crystal structure is highly related to the dielectric and magnetic properties. In this case, the EMW absorption performance and corresponding mechanism were summarized in MOF-derived MAMs with diverse crystal types, multiphase materials, heteroatoms substitutions, distortions, and defects.

4.2.1   Crystal type

Above all, the crystal type determines the essential EM properties of MOF-derived materials. Generally, metals and metal alloys, especially transition metals, present cubic symmetric crystal structures, whereas metal oxides and metal sulfides show a hexagonal symmetry. Cubic metals usually perform ferromagnetic properties. For example, Ouyang’s group reported a trimetallic FeCoNi alloy derived from FeCoNi-based MOF-74 (Fig. 7(a)) [100]. The MAM with a cubic crystal structure showed an excellent magnetic loss, whose f_E value reached 8.08 GHz. MOF derivatives with a hexagonal symmetry usually attenuate EMWs by the dielectric loss. For example, Liao et al. [101] fabricated ZnO composites using CoZn-MOF, of which RL_min reached −52.6 dB.

Fig. 7.  (a) Cubic-structure FeCoNi alloy (a1); 3D (a2) and 2D (a3) RL values of FeCoNi alloys (Reprinted with permission from J. Ouyang, Z.L. He, Y. Zhang, H.M. Yang, and Q.H. Zhao, ACS Appl. Mater. Interfaces, vol. 11, 39304-39314 [100]. Copyright 2019, American Chemical Society). (b) MAMs with a perovskite structure and oxygen vacancy in LaCoO₃ (Reprinted from Chem. Eng. J., 421, Z.G. Gao, J.Q. Zhang, S.J. Zhang, J. Wang, and Y.H. Song, Cationic etching of ZIF-67 derived LaCoO₃/Co₃O₄ as high-efficiency electromagnetic absorbents, art. No. 127829, Copyright 2021, with permission from Elsevier [21]).

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Furthermore, some special crystal structures have been determined with a great EMW absorption performance by optimizing not only impedance matching but also EM energy attenuation. Nowadays, MOF-derived MAMs based on spinels and perovskites have attracted considerable attention for EMW absorption. Conventional spinels are face-centered cubic metal oxides with an XY₂O₄ structure, where X and Y are divalent and trivalent metals, respectively. Some metal sulfides with XY₂S₄ are also defined as spinels in a broad sense. For example, a series of NiFe₂S₄-based composites were fabricated by Zhang et al. [102], of which the optimal MAM candidate obtained RL_min up to −51.41 dB. After calcination, MOFs with rare earth metals can be converted to ABO₃ perovskite-type oxides, where A is the rare earth/alkaline metal and B is the transition metal [103]. In general, the introduction of the perovskite phase can greatly improve the dielectric loss due to the special charge response properties (Fig. 7(b)).

4.2.2   Multiphase material

To integrate the EM advantages of different types of crystals, several multiphase materials have been employed to prepare composite or hybrid MAMs. Typically, the construction of dielectric and magnetic materials can effectively alleviate the impedance mismatch, thus improving EMW absorption. For example, a CoMn-MIL MOF was utilized to fabricate Co/MnO/C nanocomposites by Miao et al. [16]. Attributed to the magnetic property of Co, the dielectric property of MnO, and the conductive property of carbon, a significant dielectric–magnetic synergistic effect was induced, which facilitated the energy dispersion. Furthermore, the multiphase contributes to the interface effect for EMW absorption. For example, Wang et al. [104] constructed a Schottky contact interface in the Ni@C@ZnO multiphase MAM derived from NiZn-MOF, leading to an effective interfacial polarization (Fig. 8). The RL_min and f_E values reached −55.8 dB and 4.1 GHz, respectively.

Fig. 8.  EMW absorption mechanism (a), energy barriers (b), and space charge zone (c) between Ni@C and ZnO in multiphase MAMs (Reprinted from Chem. Eng. J., 383, L. Wang, X.F. Yu, X. Li, J. Zhang, M. Wang, and R.C. Che, MOF-derived yolk–shell Ni@C@ZnO Schottky contact structure for enhanced microwave absorption, art. No. 123099, Copyright 2020, with permission from Elsevier [104]). W_f represents work function

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4.2.3   Heteroatom substitutions, distortions and defects

Heterogeneous or hybrid structures in crystals can change the EM response characteristics, especially heteroatom substitution, lattice distortion, and defects. Heteroatoms (guest atoms) can substitute the host atoms at the carbon matrices and metal unit cells derived from MOFs. For example, Qiu et al. [105] confirmed in their previous publication that the N heteroatom substitution in carbon materials can influence the degree of graphitization, hence affecting the conductive loss (Fig. 9). The heteroatom substitutions also greatly impact the properties of semiconductors and magnets. Therefore, multi-metal MOFs have been extensively utilized in bimetal or multi-metal MAMs. For example, in our previous work, Zn/Co bimetal MOFs were used to fabricate the Zn/Co phase and ion hybrids, of which the optimal MAMs obtained a high RL_min up to −45.85 dB [106]. In addition, this work systematically studied that distortions and defects in crystals are highly associated with dipole polarization.

Fig. 9.  Schematic description for preparation of CNT/Ni@N-C composite (Reprinted from J. Colloid Interface Sci., 581, Y. Qiu, H.B. Yang, L. Ma, et al., In situ-derived carbon nanotube-decorated nitrogen-doped carbon-coated nickel hybrids from MOF/melamine for efficient electromagnetic wave absorption, 783-793, Copyright 2021, with permission from Elsevier [105]).

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4.3   Physical properties

The physical nature determines the EM response and EMW absorption of materials. Therefore, we summarized several classes of MOF derivatives with classical physical properties, such as semiconductors, magnets, and carbon conductive networks.

4.3.1   Semiconductors

The derivatives of nonmagnetic element-based MOFs are usually presented in the form of semiconductors, which generally have two types: n-type semiconductors and p-type semiconductors, whose charge carriers depend on negatively charged electrons and positively charged holes, respectively [107]. As one of the most promising new materials in the 21st century, contributing to its unique charge response in the alternative EM field, the micro-current in the absorbers will also be regulated accordingly, such as unidirectional current conduction [108], space charge collection and dispersion [109], and rectification effect [110], finally leading to tunable EMW attenuation. Conventionally, on the one hand, Ti- and Zn-based MOFs can be utilized in n-type semiconductor fabrication, such as MIL-125 and ZIF-8, which will deliver TiO₂ and ZnO, respectively. For example, in the previous work of Qiao et al. [25], a series of semiconductor-based MAMs, e.g., TiO₂, ZrO₂, and TiO₂/ZrTiO₄, derived from MIL-125, UiO-66, and PCN-415, respectively, were fabricated. On the other hand, most Ni and Cu MOFs can be employed on p-type semiconductors [111–112]. For instance, Jiao et al. [113] fabricated CuO MAMs based on Cu-BTC, obtaining a wideband f_E of 6.8 GHz (Fig. 10(a)). Semiconductor-based MAMs derived from MOFs are being developed toward diversification and p/n-heterojunctions. Typically, a NiO/ZnO p/n-heterojunction-type semiconductor-based MAM was fabricated via the Ni–Zn bimetal MOF by Thi et al. (Fig. 10(b)) [114]. Finally, the p/n-heterojunction played a critical role on facilitating interfacial polarization relaxation and periodic responses at heterointerfaces, prompting an f_E value as high as 4.5 GHz.

Fig. 10.  MOF-derived MAMs with diverse physical properties. The detailed classification shows (a) MOF-derived p-type semiconductors (Reprinted from Composites Part B, 211, Y.Z. Jiao, S.Y. Cheng, F. Wu, et al., MOF−Guest complex derived Cu/C nanocomposites with multiple heterogeneous interfaces for excellent electromagnetic waves absorption, art. No. 108643, Copyright 2021, with permission from Elsevier [113]), (b) MOF-derived p/n heterojunctions (Reprinted from Mater. Chem. Phys., 266, Q.V. Thi, S. Park, J. Jeong, et al., A nanostructure of reduced graphene oxide and NiO/ZnO hollow spheres toward attenuation of electromagnetic waves, art. No. 124530, Copyright 2021, with permission from Elsevier [114]), (c) synthesis strategy (c1), C₀ value (c2), hysteresis loop (c3), and RL value (c4) of MOF-derived magnetic MAMs (Reprinted from Chem. Eng. J., 410, Q. Wu, B.L. Wang, Y.G. Fu, Z.F. Zhang, P.F. Yan, and T. Liu, MOF-derived Co/CoO particles prepared by low temperature reduction for microwave absorption, art. No. 128378, Copyright 2021, with permission from Elsevier [118]), and (d) structure (d1), polarization loss value (d2), and conductive loss value (d3) of MOF-derived carbon-based MAMs (Reproduced from Ref. [123] with permission from the Royal Society of Chemistry).

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4.3.2   Magnetic particles

To intensify the dielectric–magnetic synergistic effect, several MOFs have been used to prepare derivative MAMs with ferromagnetism. Generally, MOF precursors utilized to prepare magnetic particles are based on Fe [115], Co [116], or Ni coordination metals [117]. For example, the reduced magnetic particles Co/CoO were prepared by Wu et al. [118] based on ZIF-67-derived Co₃O₄ for a tunable magnetic loss (Fig. 10(c)). Accompanied by the reduction reaction proceeding, it can be concluded from the hysteresis loops that the saturation magnetization (M_s) and coercivity (H_c) values of cobalt-based magnetic particles would increase and decrease, respectively, which can be attributed to the comprehensive consequence of multicrystalline phases, stacking fault, and size effect, inducing a higher imaginary part of complex permeability ($\mu''$ ) value and superior magnetic loss.

4.3.3   Carbon conductive networks

Carbon conductive networks are also extremely significant parts of MOF-derived composites, as they play an indispensable role in the dielectric loss, especially the conductive loss [119–120]. First, inheriting the porous structural characteristics of MOFs, these carbon matrices will also exhibit the advantages of high porosity and lightweightness [121]. As shown in a previous work of Wang et al. [53], the pore structure, specific surface area, and EMW absorption performance of Co-MOF-74-derived carbon composites were analyzed via BET characterization. The results indicated that an appropriate calcination temperature will convert MOFs into carbon frameworks with a mesoporous structure, whereas too high temperature will yield macropores, of which mesoporous samples usually possess more prominent RL values benefiting from the multiple scattering and reflection. Second, the graphitization degree of MOF-derived carbon will significantly affect the dielectric properties [122]. Tao et al. [123] reported that a high carbonization temperature can promote the pyrolysis of organic ligands and lead to a high graphitization degree, which is conducive to charge migration, hence improving the electrical conductivity and conductive loss (Fig. 10(d)). Third, the modification of heteroatoms (N, O, and S) also has a non-negligible effect on the dielectric properties of the MOF-derived carbon network [124]. For example, Wen et al. [125] systematically analyzed the contribution of S doping in the carbon framework to the EMW absorption performance by manipulating the ratio of p-toluenesulfonic acid ligands in Co-MOFs. Finally, the S modification not only introduced considerable polarization centers but also effectively optimized the impedance matching. However, the introduction of heteroatoms will usually also lead to the destruction of the integrity of the conductive network, resulting in the weakening of the conductivity loss [126].

5.   EMW absorption mechanism

Benefiting from the aforementioned diverse structural features and physical properties of MOFs and their derivatives, the EMW absorption of the corresponding MAMs exhibits desirable properties. Herein, the EMW absorption process can be briefly summarized in two parts: (i) the incident wave entering the absorber and (ii) the attenuation and dissipation of EM energy. First, the good impedance matching characteristics of MAMs will ensure that the incident EMW enters the MAMs as much as possible by avoiding surface reflection, which is a decisive prerequisite for the EMW to activate the EM energy-attenuating sites and further dissipation [127]. Second, EM attenuation dominated by dielectric loss and magnetic loss is the core link of EMW absorption, whereas multiple scattering and synergistic effects excited simultaneously will further promote EMW dissipation.

5.1   Impedance matching

Impedance matching can be examined according to the following equation [103]:

where ${\varepsilon }_{\mathrm{r}}= \varepsilon'-\text{j}{\varepsilon}''$, ${\mu}_\text{r}= \mu'-\text{j}{\mu}''$, Z_in, Z₀, f, d, and c are the complex permittivity, complex permeability, input impedance of MAMs, free space impedance, incident EMW frequency, thickness of absorbers, and light velocity, respectively. Based on the simplified transmission line model, impedance matching can be easily obtained by stimulating the input impedance value at the interface between MAMs and air (|Z_in/Z₀|) [128]. In general, a well-matched impedance will be achieved when the |Z_in/Z₀| value is close to 1, which implies an effective avoidance of surface EMW reflections and shielding (Fig. 11(a)) [63]. For example, in a previous work of Guo’s group, the impedance matching of porous hollow Ni/C composites derived from Ni-MOFs was controllably regulated, leading to an optimal RL value high up to −55.4 dB at 1.53 mm [121]. In addition, it can be inferred from Eq. (1) that the closer the ε_r value to the ${\mu}_\text{r}$ value, the better the impedance matching, indicating that a complementary balance between dielectricity and magnetism is extremely profitable for EMW absorption [129]. As is reported in a typical example, the electrical conduction network was combined with a magnetic coupling network by precisely regulating the MOF template and transformation strategy, which achieved compatible confinement of magnetic–dielectric balance in as-prepared Co/ZnO@N-doped carbon composites (Fig. 1) [130].

Fig. 11.  EMW absorption mechanism of MOF-derived MAMs. The detailed classification shows (a) RL values, matching thickness and impedance matching of MOFs derived Ni@C (Reprinted with permission from J. Yan, Y. Huang, Y.H. Yan, L. Ding, and P.B. Liu, ACS Appl. Mater. Interfaces, vol. 11, 40781-40792 [63]. Copyright 2019, American Chemical Society), (b) EM parameters (b1–b4), Cole–Cole curves (b5–b8) of ZIF-67 derived LaCoO₃/Co₃O₄ (Reprinted from Chem. Eng. J., 421, Z.G. Gao, J.Q. Zhang, S.J. zhang, J. Wang, and Y.H. Song, Cationic etching of ZIF-67 derived LaCoO₃/Co₃O₄ as high-efficiency electromagnetic absorbents, art. No. 127829, Copyright 2021, with permission from Elsevier [21]), and (c) hysteresis loop, M_s, H_c, and C₀ values of PBAs derived MAMs (Reprinted from J. Mater. Sci. Technol., 82, Z.G. Gao, Z.H. Zhao, D. Lan, K.C. Kou, J.Q. Zhang, and H.J. Wu, Accessory ligand strategies for hexacyanometallate networks deriving perovskite polycrystalline electromagnetic absorbents, 69-79, Copyright 2021, with permission from Elsevier [140]).

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Moreover, impedance matching is strongly dependent on the thickness of absorbers. Therefore, the quarter-wavelength matching model was usually adopted to normalize the matching thickness (t_m) on the basis of the following equation [131]:

where |ε_r| and |${\mu}_\text{r}$| is the modulus of ε_r and μ_r, respectively. The correlation between t_m and the matching frequency of incident EMWs can be established when the EMW absorption performance is dramatically increased. Research from Chen et al. [132] reported that the matching frequency is inversely proportional to t_m, resulting in the matching frequency shifting to lower frequencies with the increase in t_m. In this case, the peak position of RL is closely correlated with the synergistic effect of normalized impedance and quarter-wavelength cancelation.

5.2   EM attenuation

The evaluation of the EMW attenuation capability is based on dielectric loss and magnetic loss. In general, the dielectric loss tangent ($\mathrm{tan}\,{\delta }_{\varepsilon }=\varepsilon '' /\varepsilon '$) and magnetic loss tangent ($\mathrm{tan}\,{\delta }_{\mu }={\mu }'' /{\mu }'$) are the preliminary references for dielectric loss and magnetic loss evaluation, respectively. For MAMs with dielectric loss and magnetic loss, the attenuation constant (α) is proposed to access the simultaneous attenuation of EM energy, as calculated by the following equation [133]:

Usually, the higher the value of α, the more vigorous the response of the MAMs to the alternating EM field, and the stronger the attenuation of the EM energy.

Taking the detailed dielectric loss mechanism into consideration, ε_r can be analyzed according to the following equation [134]:

where ε_∞, ε_s, τ, and σ are the relative dielectric permittivity at the high-frequency limit, static permittivity, relaxation time, and electrical conductivity, respectively. In Eq. (5), the dielectric loss can be mainly attributed to polarization loss (${\varepsilon }_{\mathrm{p}}''$) and conductive loss (${\varepsilon }_{\mathrm{c}}''$).

For polarization loss, the relaxation phenomena will happen during the “aggregation–dispersion” process of charges or “orientation–deorientation” of polarization sites on various heterogeneous constructions [135]. Hence, the periodical repeating reaction of polarization centers under incident EMWs will transform the EM energy into thermal energy for further attenuation. Conventionally, polarization centers could be built on ions, electrons, dipoles, and heterointerfaces. However, ionic and electronic polarizations are usually excluded as the corresponding relaxation frequency (10³–10⁶ GHz) is much higher than the microwave frequency [136]. Therefore, the polarization relaxation loss (2–18 GHz) can be primarily attributed to dipole polarization and interfacial polarization. Typically, dipole polarization is initiated by defects, impurities, and distortions, whereas interfacial polarization is raised from the deflection or displacement of spatial charges in heterointerfaces. According to the Debye theory, the polarization relaxation behavior can be analyzed and presented by the Cole–Cole plots:

Every Cole–Cole semicircle represents a polarization relaxation process (Fig. 11(b)) [21]. Therefore, the multiple semicircles in Cole–Cole plots always indicate multiple polarization losses.

The conductive loss generally occurs on the local microcurrent network, where aggregation-induced charge transportation will be stimulated under alternating EM fields. The pyrolytic derivatives of MOFs, as metal-decorated carbon topological frameworks, provide plentiful responsive sites for conductive loss, which effectively enhances the EM attenuation capability [137]. In addition, the conductive loss of highly conductive MAMs can be directly associated with ε" according to the free electron theory [138]:

In this case, the dielectric loss mechanism can be initially confirmed based on the Debye theory and free electron theory introduced above.

The magnetic loss mainly includes the hysteresis loss, domain-wall motion, eddy current, natural resonance, and exchange resonance [139]. The hysteresis loss refers to the irreversible conversion of a portion of EM energy into thermal energy, with magnetization lagging behind the strength of the magnetic field (Fig. 11(c)) [140]. However, in the study of EMW absorption, the magnetic field is too weak to realize the hysteresis loss, so the hysteresis loss is usually negligible [141]. The EM attenuation during domain-wall motion depends on magnetic-domain variations and spin rotations. The response frequency of the domain-wall resonance is generally at low frequency [142]. When the magnetic MAM is in a time-varying or inhomogeneous magnetic field, magnetoelectric conversion will be initiated, promoting energy dissipation by the eddy current loss. Typically, the frequency-domain eddy current loss can be evaluated by the C₀ parameters [143]:

C₀ will be expressed as a constant when the eddy current loss plays the dominant role in the magnetic loss. Multiple resonances, such as natural resonance (2–10 GHz) and exchange resonance (10–18 GHz), are regarded as another significant magnetic loss mechanism in the microwave frequency range. When natural resonance occurs, the moment-damped motion in the magnetic anisotropy field will be accompanied by energy decay. Therefore, magnetic resonance will be strongly influenced by size, morphology, and crystallographic anisotropy. For example, the application of soft magnetic materials, such as Fe, Co, and Ni metals and alloys and highly anisotropic magnetic ferrites, will generate a multi-resonance behavior, which can promote the elimination of Snoek’s limit, thus effectively alleviating impedance mismatch, especially at high frequencies [144].

5.3   EMW absorption mechanism of MOF-derived MAMs

According to the discussion on MOF-derived MAMs with diverse chemical states, chemical structures, and physical properties, corresponding EMW absorption mechanisms can be summarized. First, the metals and ligands of the precursor will determine the composition and structure of the metal product and carbon of the derivative, which determines the fundamental EM characteristics. Second, benefiting from the high porosity of MOFs, these MAMs also achieve a high specific surface area, which is beneficial for multiple reflection and scattering. Third, the moderate conductivity of carbon materials results in an excellent conductivity loss, and the polarization loss can be attributed to heterointerfaces, defects, and hybrids. Transition metal magnetic products are often accompanied by magnetic loss.

In addition, the design of special structures of MOF-derived MAMs, such as porous materials, core–shell, and yolk–shell, enhances the massive scattering of incident EMWs, which further promotes EM attenuation. For example, a series of yolk–shell structure Co@ZnO/Ni@N-doped carbon nanocages were fabricated by the bimetallic core–shell structure ZIF-67@ZIF-8 as a template [18]. Finally, highly porous carbon polyhedrons obtained outstanding advantages in multi-scattering and reflection of EMWs, leading to a high RL value of up to −55 dB at 8.2 GHz. Finally, in view of the flexible structural controllability of MOFs and their derivatives, more detailed and novel EMW absorption mechanism models are being established from different aspects.

6.   Summary and perspectives

MOFs have been regarded as one of the most promising precursors for high-performance MAMs, attributed to their flexible regulation of topologies, chemical states, and physical properties. As listed in Table 1, we systematically summarize MOF-derived MAMs from a comprehensive design perspective in this review. First, based on the coordination activity between ligands and metal ions, a large amount of MOFs have been prepared and applied to EMW absorption, and the number of their species is still increasing. Second, reasonable regulation of synthesis conditions can greatly influence the formation of MOF nucleation and grain growth, hence finally making the deposited MOFs exhibit 1D, 2D, 3D, and even hierarchical structures. Third, the chemical states and physical properties of the calcined products determine the EM properties and EMW absorption performance of MOF-derived MAMs. Despite the remarkable achievements, many challenges still need to be confronted toward a better EMW absorption performance, clearer EMW absorption mechanism, and higher practical application abilities.

Table  1.  Representative MAMs derived from MOFs designed with diverse ligands/ions, topologies, chemical states, and physical properties

MAMs	MOFs	Ligands/ions	Topologies	Chemical states	Physical properties	RLmin / dB	fE / GHz	Refs.
Co/C	ZIF-67	Hmim/Co2+	3D	Metal	Magnetic particles	−49.61	4.40	[145]
CoO/Co@C	ZIF-67	1-benzyl-2-methylimidazole/Co2+	3D	Metal/metal oxide	Magnetic particles	−38.46	4.80	[33]
LaCoO3/ Co3O4	ZIF-67	Hmim/Co2+	3D	Metal oxide	Semiconductor/Magnetic particles	−38.40	6.88	[21]
Cu/Co/C	BTC	H3BTC/Cu2+,Co2+	3D	Metal alloy	Magnetic particles	−25.0	5.68	[146]
Fe3O4/C	MIL-88B	H2BDC/Fe3+	3D	Metal oxide	Magnetic particles		X band	[34]
TiO2/C	MIL-125	H2BDC/Tetrabutyl titanate	3D	Metal oxide	Semiconductor	−64.4	3.90	[35]
Fe3C/C	MIL-88	Fumaric acid/Fe3+	3D	Metal carbide	Magnetic particles	−24.71	5.28	[39]
Fe3C/Fe/C	MIL-101	H2BDC/Fe3+	3D	Metal carbide	Magnetic particles	−20.31		[41]
PP@Fe	NH2-MIL-101	NH2-H2BDC/Fe3+	3D	Metal	Magnetic particles	−60		[42]
Fe@Fe3O4	MIL-100	H3BTC/Fe3+	3D	Metal/Metal oxide	Magnetic particles	−75.30	4.00	[43]
Fe2O3	PB	K4Fe(CN)6	3D	Metal oxide	Magnetic particles	−16.91	5.44	[46]
Fe/C	PB	Fe(CN)${}_6^{3-}$/Fe3+	3D	Metal	Magnetic particles	−44.26	3.68	[47]
CoCN@CNT	PBA	Co(CN)${}_6^{3-}$/Co2+	3D	Metal/Carbon	Magnetic particles/conductive carbon networks	−29.05	6.35	[48]
FeCo@C	PBA	Fe(CN)${}_6^{3-}$/Co2+	3D	Metal alloy	Magnetic particles	−67.80	5.30	[49]
Co/C	MOF-74	H4DOBDC/Co2+	3D	Metal	Magnetic particles	−62.10	11.85	[52]
ZrO2/C	UiO-66	H2BDC/Zr4+	3D	Metal oxide	Semiconductor	−58.70	5.50	[24]
Co/ZrO2/C	NH2-UiO-66	NH2-H2BDC/Zr4+	3D	Metal/Metal oxide	Semiconductor/Magnetic particles	−57.20	11.90	[56]
ZnO@MWCNTs	MOF-5	H2BDC/Zn2+	3D	Metal oxide	Semiconductor	−34.40	3.70	[57]
Fe/Fe3O4	MOF-5	H2BDC/Fe3+	3D	Metal/Metal oxide	Magnetic particles	−39.20	4.44	[58]
TiO2/ZrTiO4	PCN-415	Ti(OiPr)4/ZrCl4	3D	Metal oxide	Semiconductor	−67.80	5.90	[25]
MOF-53/ RGO	MOF-53	H2BDC/Fe3+	3D			−25.80	5.90	[61]
Cu/C	HKUST-1	H3BTC/Cu2+	3D	Metal	Conductor	−60.80	5.60	[62]
Co/Ni/C	MOF-71	H2BDC/Co2+,Ni2+	2D	Metal/Carbon	Magnetic particles/conductive carbon networks	−49.80	7.60	[74]
ZnO/Fe	ZIF-67	Hmim/Co2+	2D	Metal/Metal oxide	Conductor/Semiconductor	−33.36	4.88	[147]
Co/C	ZIF-67	Hmim/Co2+	2D	Metal	Magnetic particles	−39.30	5.10	[79]
Co/CNTs	ZIF-67	Hmim/Co2+	2D	Metal/Carbon	Magnetic particles/conductive carbon networks	−51.80	5.90	[69]
CoNC/CNTs	ZIF-67	Hmim/Co2+	2D	Metal/Carbon	Magnetic particles/conductive carbon networks	−44.60	4.50	[73]
CoFe@C	MOF-74	H4DOBDC/Co2+,Fe3+	1D	Metal alloy	Magnetic particles	−61.80	9.20	[66]
C@Cu2−xS	MOF-74	H4DOBDC/Co2+	1D	Metal sulfide	Semiconductor	−33.50	7.60	[68]
CoS2	ZIF-67	Hmim/Co2+	3D	Metal sulfide	Semiconductor	−65.00	6.20	[93]
CoS2−xPx	ZIF-67	Hmim/Co2+	3D	Metal phosphosulfide	Semiconductor	−68.00	4.60	[99]
Co/SC	Co-MOF	p-toluenesulfonic acid/Co2+	2D	Metal/S doped Carbon	Magnetic particles/conductive carbon networks	−72.30	6.00	[125]

 | Show Table

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(1) New architectures. The microscopic, mesoscopic, and macroscopic structural diversities of MOFs should be studied more extensively to establish their relationship with EMW absorption. The monomers of MOFs, including ligands and central ions, need to continue to be enriched. The texture regulation of MOFs and the preparation of composites are two fundamental ways for the explosion of structural diversity. For example, the construction of MOF-on-MOFs can be a promising strategy for MAMs with various heterointerfaces and core–shell structures.

(2) New preparation methods. The development of synthetic methods for MOF-derived MAMs is still in its infancy. In this case, it is of high urgency to develop novel MOF synthesis methods. Recently, the etching and exchange of ligands or ions and heteroatom self-assembly have been paid increasing attention. Basically, the detailed factors in the synthesis process, such as solvent, temperature, pressure, and surfactant, will further determine the structure of MOFs.

(3) EMW absorption mechanism. MOF-derived MAMs with different compositions and architectures are carriers for the study of EMW absorption mechanisms. On the one hand, it is the key for the EMW absorption mechanism to establish the relationship between the EM response and structure based on the physical properties of materials. On the other hand, some advanced characterization techniques are used to analyze the EMW absorption mechanism. For example, the visible charge density distribution and charge density distribution can be two of the ideal candidates for the confined magnetic–dielectric balance analysis (Fig. 12) [19]. Based on various structural and property characterization, the EMW absorption models can be built, including electron transfer, impedance matching, Mo defects and charge switching, and interfacial charge polarizations (Fig. 13) [148]. Recently, some researchers also tried to build a relationship between electrochemical properties with the EMW absorption mechanism (Fig. 14) [130]. In Fig. 14, |Z| is the impedance modulus; R_sol represents the solution resistance; R_por and CPE_por represent the resistance and constant phase element of the porous layer, respectively; R_ct and CPE_dl are the charge-transfer resistance and double-layer constant phase element, respectively; R_LG and CPE_LG can be assigned to the resistance and constant phase element of the NiAl-LDH/G film, respectively.

Fig. 12.  Advanced EMW absorption models. The detailed classification shows a confined magnetic–dielectric balance where the dielectric response included sub-images are (a1) the schematic diagram of ZnO-N doped carbon-Co interfaces, (a2) HRTEM image, (a3) holograph image, (a4) reconstructed phase image, and (a5, a6) corresponding charge density distribution; the magnetic response includes (b1) magnetic flux line distribution for Co@N doped carbon-ZnO composite, (b4) randomly distributed Co@N doped carbon magnetic particles, and the magnetic flux line distribution of area-1 (b2), area-2 (b3), and area-3 (b5, b6) in (b1) (L. Wang, M.Q. Huang, X. Qian, et al., Small, vol. 17, art. No. e2100970 (2021) [19]. Copyright Wiley-VCH Verlag GmbH & Co. KGaA. Reproduced with permission).

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Fig. 13.  (a) HRTEM images, (b, c) MoC/NC hetero-interface, (d) charge distribution on the MoC(−)/carbon(+) interface, (e) two-step synthetic route and chemical etching mechanism (insert), and (f) illustration schemes of four electromagnetic wave attenuation mechanism (Reproduced from Ref. [148] with permission from the Royal Society of Chemistry).

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Fig. 14.  (a) Nyquist plots, (b) impedance–frequency Bode plots, (c) phase–frequency Bode plots of pure epoxy-coated steel, (d) Nyquist plots, (e) impedance–frequency Bode plots, and (f) phase–frequency Bode plots of 7wt% NiAl-LDH/G/epoxy-coated steel. (g) Equivalent circuit used to simulate the EIS data of coated steel, (h) R_ct values, and (i) Tafel plots recorded in a 3.5wt% NaCl solution for bare steel, pure epoxy-coated steel, and 7wt% NiAl-LDH/G/epoxy-coated steel (X.F. Xu, S.H. Shi, Y.L. Tang, et al., Adv. Sci., vol. 8, art. No. 2002658 (2021) [130]. Copyright Wiley-VCH Verlag GmbH & Co. KGaA).

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(4) Smart devices. The ultimate mission of functional materials, such as MAMs, is to achieve practical applications. In this case, smart devices of MAMs should be fabricated with not only a high EMW absorption performance but also applicability to harsh conditions, environmental friendliness, and multi-function integration. Nowadays, the synthesis of MOF-derived MAMs is still experimental, so their high-yield production is a crucial challenge. Faced with the disadvantage of poor mechanical properties of most MOFs, preparing composites, such as polymer films, hydrogels, and foams, will be a promising field, thereby simultaneously ensuring the intrinsic EM properties and multifunctional advantages of MOF-derived MAMs, such as corrosion resistance, ultra-high- and low-temperature resistance, biocompatibility, and antibacterial property.

(5) Industrial-scale applications. Aside from the EMW absorption mechanism analysis, achieving large-scale industrial production is the ultimate goal. At present, we are facing many challenges, such as high industrial costs, high processing difficulty, low stability and yield, non-toxic and non-polluting to the human body, and environmental friendliness. Regarding the MOF-derived MAMs, we are expecting to fabricate magnetic–dielectric composite materials to gradually meet the above challenges.

In summary, outstanding achievements have been made in the composition and structural design of MOF-derived MAMs. However, in the face of rapid industrial structure upgrading, intensive efforts are expected to be devoted to new material exploration and mechanism investigation. Ultimately, we firmly predict that MOFs will achieve major breakthroughs in the field of EMW absorption and even the environment, energy, and biomedical fields.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (Nos. 51872238, 21806129, and 52074227), the Fundamental Research Funds for the Central Universities (Nos. 3102018zy045 and 3102019AX11), and the Natural Science Basic Research Plan in Shaanxi Province of China (Nos. 2020JM-118 and 2017JQ5116). The authors acknowledge the support from The Analytical & Testing Center of Northwestern Polytechnical University.

Conflict of Interest

The authors declare that they have no conflict of interest.