Constructing macroporous C/Co composites with tunable interfacial polarization toward ultra-broadband microwave absorption

Recent Advances in Hierarchical Porous Engineering of MOFs and Their Derived Materials for Catalytic and Battery: Methods and Application

Hierarchical porous materials have attracted the attention of researchers due to their enormous specific surface area, maximized active site utilization efficiency, and unique structure and properties. In this context, metal-organic frameworks (MOFs) offer a unique mix of properties that make them particularly appealing as tunable porous substrates containing highly active sites. This review focuses on recent advances in the types and synthetic strategies of hierarchical porous MOFs and their derived materials. Furthermore, it highlights the relationship between the mass diffusion and transport of hierarchical porous structures and the pore size with examples and simulations, while identifying their potential and limitations. On this basis, how the synthesis conditions affect the structure and electrochemical properties of MOFs based hierarchical porous materials with different structures is discussed, highlighting the prospects and challenges for the synthetization, as well as further scientific research and practical applications. Finally, some insights into current research and future design ideas for advanced MOFs based hierarchical porous materials are presented.

Microstructure optimization strategy of ZnIn2S4/rGO composites toward enhanced and tunable electromagnetic wave absorption properties

Although microstructure optimization is an effective strategy to improve and regulate electromagnetic wave (EMW) absorption properties, preparing microwave absorbents with enhanced EMW absorbing performance and tuned absorption band by a simple method remains challenging. Herein, ZnIn2S4/reduced graphene oxide (rGO) composites with flower-like and cloud-like morphologies were fabricated by a convenient hydrothermal method. The ZnIn2S4/rGO composites with different morphologies realize efficient EMW absorption and tunable absorption bands, covering a wide frequency range. The flower-like structure has an optimal reflection loss (RL) of up to -49.2 dB with a maximum effective absorption bandwidth (EAB) of 5.7 GHz, and its main absorption peaks are concentrated in the C and Ku bands. The minimal RL of the cloud-like structure can reach -36.3 dB, and the absorption peak shifts to the junction of X and Ku bands. The distinguished EMW absorption capacity originates from the uniquely optimized microstructure, complementary effect of ZnIn2S4 and rGO in dielectric constant, and synergy of various loss mechanisms, such as interfacial polarization, dipole polarization, conductive loss, and multiple reflections. This study proposes a guide for the structural optimization of an ideal EMW absorber to achieve efficient and tunable EMW absorption performance.

Designing flower-like MOFs-derived N-doped carbon nanotubes encapsulated magnetic NiCo composites with multi-heterointerfaces for efficient electromagnetic wave absorption

In order to acquire exceptional electromagnetic wave absorption properties, the microstructure design and component modification of composites are essential. Metal-organic frameworks (MOFs), due to the unique metal-organic crystalline coordination, tunable morphology, high surface area, and well-defined pores, have been regarded as promising electromagnetic wave absorption materials precursors. However, the inadequate contact abilities between adjacent MOFs nanoparticles endow it with undesirable electromagnetic wave dissipation capacity at a low filler loading, which is a great challenge to break size effect of nanoparticles to achieve efficient absorption. Herein, NiCo-MOFs derived N-doped carbon nanotubes encapsulated with NiCo nanoparticles anchored on flowers-like composites (denoted as NCNT/NiCo/C) were successfully prepared through facile hydrothermal method followed by thermal chemical vapor deposition with melamine-assisted catalyst. By controlling the Ni/Co ratio in precursor, the tunable morphology and microstructure of MOFs are achieved. Most importantly, the derived N-doped carbon nanotubes tightly connect the adjacent nanosheets to construct the special 3D interconnected conductive network, which effectively accelerates the charge transfer and improves the conduction loss. And notably, the NCNT/NiCo/C composite delivers excellent electromagnetic wave absorption performance with minimum reflection loss of -66.1 dB and wide effective absorption bandwidth up to 4.64 GHz when the Ni/Co ratio is 1:1. This work provides a novel method for the preparation of morphology controllable MOFs-derived composites and realizes high-performance electromagnetic wave absorption properties.

Autogenous and Tunable CNTs for Enhanced Polarization and Conduction Loss Enabling Sea Urchin-Like Co3ZnC/Co/C Composites with Excellent Microwave Absorption Performance

ZIF-67-derived magnetic metal/carbon composites are considered prospective candidates for use as microwave absorption (MA) materials owing to their magnetoelectric synergy. However, the structure of ZIF-67-derived MA materials mainly depends on the morphology and composition of pristine metal-organic frameworks (MOFs), and their microstructures lack a rational design. Herein, a multidimensional sea urchin-like carbon nanotubes (CNTs)-grafted carbon polyhedra-encapsulated Co₃ZnC/Co nanoparticle composite was prepared by one-step catalytic pyrolysis of ZIF-67/ZnO using a rational structural design. The autogenous and tunable CNTs obtained with the assistance of zinc evaporation not only overcome the limitation of homogeneous dispersion but also endow the Co₃ZnC/Co/C composite with outstanding MA properties owing to the conduction loss provided by CNTs, polarization loss caused by multiple components, and electromagnetic wave trap composed of a special sea urchin-like structure. Consequently, the minimum reflection loss of ZZ_0.1-600 reaches -60.3 dB at 1.6 mm, the maximum absorption bandwidth of ZZ_0.05-600 is 6.24 GHz (covering nearly the entire Ku band) at 1.9 mm, and the structure has a low weight ratio (30 wt %). Compared with Z-600 and pure ZnO, the MA performance of the sea urchin-like Co₃ZnC/Co/C composite obtained by rational structural design has been greatly improved; this strategy offers a new approach for optimizing the MA performance of materials according to their structural design.

Ion-Exchange Strategy for Metal-Organic Frameworks-Derived Composites with Tunable Hollow Porous and Microwave Absorption

Hollow metal-organic frameworks (MOFs) with careful phase engineering have been considered to be suitable candidates for high-performance microwave absorbents. However, there has been a lack of direct methods tailored to MOFs in this area. Here, a facile and safe Ni²⁺ -exchange strategy is proposed to synthesize graphite/CoNi alloy hollow porous composites from Ni²⁺ concentration-dependent etching of Zeolite imidazole frame-67 (ZIF-67) MOF and subsequent thermal field regulation. Such a special combination of hollow structure and carefully selected hybrid phase are with optimized impedance matching and electromagnetic attenuation. Especially, the suitable carrier transport model and the rich polarization site enhance the dielectric loss, while more significant hysteresis loss and more natural resonance increase the magnetic loss. As a result, excellent microwave absorbing (MA) performances of both broadband absorption (7.63 GHz) and high-efficiency loss (-63.79 dB) are obtained. Moreover, the applicability and practicability of the strategy are demonstrated. This work illustrates the unique advantages of ion-exchange strategy in structure design, component optimization, and electromagnetic regulation, providing a new reference for the 5G cause and MA field.

Metal-organic framework-derived Co/CoO nanoparticles with tunable particle size for strong low-frequency microwave absorption in the S and C bands

Nowadays, constructing strong absorption materials addressing the low-frequency electromagnetic radiation (S and C bands) from electronic devices remains a significant challenge. In this work, size-tunable Co/CoO nanoparticles (NPs) are fabricated by decomposing zeolitic imidazolate framework (ZIF-67) precursors and subsequent hydrogen reduction. All samples show obvious low-frequency attenuation in the S and C bands. At a thin thickness of 2.3 mm, the minimum reflection loss (RL) value for the Co/CoO NPs of 30 nm reaches up to -90.3 dB at 4.4 GHz, and the corresponding effective absorption bandwidth (EAB) of RL ≤ -10 dB ranges from 3.8 to 5.4 GHz. Notably, 90 % of the electromagnetic waves can be absorbed in the frequency range of 2.3-13.2 GHz, covering almost the entire S, C, and X bands at a thickness of 1.0-4.0 mm. The strong low-frequency absorption performance is attributed to the nano-porous structure, high conduction loss, tunable dielectric/magnetic loss, as well as optimized impedance matching. These Co/CoO NPs are promising candidates for high-efficient microwave absorbers in the low-frequency application.

Constructing ordered macropores in hollow Co/C polyhedral nanocages shell toward superior microwave absorbing performance

Rational design of porous carbon architecture is essential for superior microwave absorbing performance. Herein, we report a new type of hollow porous Co/C polyhedral nanocages with ordered macropores of  ∼60 nm (HP-Co/C) as microwave absorber, which were readily manufactured by epitaxial growth of ZIF-67/SiO₂ nanolayers on the surfaces of polyhedral ZIF-8 nanoparticle, and followed by simple calcination in Ar atmosphere and subsequent removal of SiO₂ with HF. The ordered macropores can effectively tune the electromagnetic parameters of HP-Co/C, affording the obtained HP-Co/C composites strong attenuation capability and excellent impedance matching characteristics for electromagnetic wave (EMW) absorption. As a result, the reflection loss (RL) and effective absorption bandwidth (EAB) of HP-Co/C prepared under pyrolysis temperature of 600 °C can reach up to -66.5 dB and 8.96 GHz, respectively, at filler fraction of only 15 wt%. Together, this study offers a new design philosophy to make lightweight and broadband microwave absorbent and can be extended to other types of microwave absorbers, significantly enriching the categories of the efficient microwave absorbing materials.

Electrochemically enhanced adsorption of organic dyes from aqueous using a freestanding metal-organic frameworks/cellulose-derived porous monolithic carbon foam

Monolithic carbon foams are promising materials for adsorption due to the easy recyclability and without secondary-pollution. However, poor adsorption efficiency for organic pollutants limits its practical application. Hence, this work proposed a novel monolithic porous carbon foam by a facile carbonization approach as freestanding electrodes to remove the organic dyes. The prepared carbon foam derived from waste cigarette filters and zeolitic-imidazolate frameworks-8 with well-developed pores, and the calculated surface area is 1457 m²·g^-1, and exhibited an outstanding removal efficiency for methylene blue in aqueous. The maximum adsorption capacity for methylene blue can reach up to 1846.7 mg·g^-1 under the applied voltage of -1.2 V. Importantly, as-prepared carbon foams possessed excellent stability, and the removal efficiency can remain above 85% after 5 cycles. Thus, obtained porous carbon foams in this paper as a free standing electrode is expected to be promising materials of adsorbent besides supercapacitors.

Synthesis of hierarchical porous nitrogen-doped reduced graphene oxide/zinc ferrite composite foams as ultrathin and broadband microwave absorbers

Magnetic graphene foams with three-dimensional (3D) porous structure, low bulk density and multiple electromagnetic loss mechanisms have been widely recognized as the potential candidates for lightweight and high-efficiency microwave attenuation. Herein, zinc ferrite hollow microspheres decorated nitrogen-doped reduced graphene oxide (NRGO/ZnFe₂O₄) composite foams were prepared via a solvothermal and hydrothermal two-step method. Results demonstrated that the attained magnetic composite foams possessed the ultralow bulk density (12.9-13.5 mg·cm^-3) and 3D hierarchical porous netlike structure constructed through stacking of lamellar NRGO. Moreover, the microwave dissipation performance of binary composite foams could be notably improved through annealing treatment and further elaborately regulating the annealing temperature. Remarkably, the attained composite foam with the annealing temperature of 300.0 °C presented the integrated excellent microwave attenuation capacity, i.e. the strongest reflection loss reached -40.2 dB (larger than 99.99% absorption) and broadest bandwidth achieved 5.4 GHz (from 12.4 GHz to 17.8 GHz, covering 90.0% of Ku-band) under an ultrathin thickness of only 1.48 mm. Furthermore, the probable microwave dissipation mechanisms were illuminated, which derived from the optimized impedance matching, strengthened dipole polarization, interfacial polarization and multiple reflection, notable conduction loss, natural resonance and eddy current loss. Results of this work would pave the way for developing graphene-based 3D lightweight and high-efficiency microwave absorption composites.

Surface modification of helical carbon nanocoil (CNC) with N-doped and Co-anchored carbon layer for efficient microwave absorption

Surface modification and composition control for nanomaterials are effective strategies for designing high-performance microwave absorbing materials (MWAMs). Herein, we have successfully fabricated Co-anchored and N-doped carbon layers on the surfaces of helical carbon nanocoils (CNCs) by wet chemical and pyrolysis methods, denoted as Co@N-Carbon/CNCs. It is found that pure CNCs show a very good microwave absorption performance under a filling ratio of only 6%, which is attributed to the uniformly dispersed conductive network and the cross polarization induced by the unique chiral and spiral morphology. The coating of N-doped carbon layers on CNCs further enriches polarization losses and the uniform anchoring of Co nanoparticles in these layers generates magnetic losses, which enhance the absorption ability and improve the low frequency performance. As compared with the pure CNCs-filling samples, the optimized Co@N-Carbon/CNCs-2.4 enhances the absorption capacity in the lower frequency range under the same thickness, and realizes the decreased thickness from 3.2 to 2.8 mm in the same X band, as well as the decreased thickness from 2.2 to 1.9 mm in the Ku band. Resultantly, a specific effective absorption wave value of 22 GHz g^-1 mm^-1 has been achieved, which enlightens the synthesis of ultrathin and light high-performance MWAMs.

Preparation of FeNi/C composite derived from metal-organic frameworks as high-efficiency microwave absorbers at ultrathin thickness

Developing metal-organic frameworks (MOFs) derived microwave absorbers with the merits of thin matching thickness, broad bandwidth and strong absorption still remains a big challenge in the electromagnetic absorption field. Herein, FeNi-MOFs derived magnetic-carbon composites were fabricated via a solvothermal and pyrolytic two-step strategy. It was found that the micromorphology of carbon frameworks could be regulated from the regular octahedron to spherical shape through facilely adjusting the molar ratios of Fe³⁺ to Ni²⁺ in the precursors. Furthermore, results revealed that the molar ratios of Fe³⁺ to Ni²⁺ had notable effects on the electromagnetic parameters and microwave attenuation capacity of attained composites. Significantly, the obtained FeNi/C composite with the molar ratio of Fe³⁺ to Ni²⁺ of 1:0.5 showed the comprehensively optimal electromagnetic attenuation performance, i.e. the reflection loss achieved -40.2 dB (larger than 99.99% absorption) and absorption frequency band was as high as 5.8 GHz (from 11.9 to 17.7 GHz, covering 96.7% of Ku-band) under an ultrathin thickness of 1.65 mm. Besides, the probable microwave dissipation mechanisms were clarified, which mainly derived from the optimized impedance matching, strengthened interfacial polarization and dipole polarization relaxation, enhanced conduction loss and natural resonance effect. Therefore, our results would be helpful for designing and developing high-performance microwave absorbing composites derived from MOFs.

Synthesis of three-dimensional porous netlike nitrogen-doped reduced graphene oxide/cerium oxide composite aerogels towards high-efficiency microwave absorption

Three-dimensional (3D) graphene aerogels with porous structure and lightweight feature have been regarded as promising candidates for microwave attenuation. Herein, nitrogen-doped reduced graphene oxide/cerium oxide (NRGO/CeO₂) composite aerogels were fabricated via a hydrothermal route. The obtained composite aerogels possessed low bulk density and unique 3D porous netlike structure constructed by the stacking of lamellar NRGO. Moreover, it was found that the microwave dissipation performance of NRGO aerogel could be notably improved through complexing with CeO₂ nanoparticles and carefully regulating the contents of CeO₂ in the composite aerogels. Remarkably, the attained NRGO/CeO₂ composite aerogel with the content of CeO₂ of 44.11 wt% presented the comprehensively excellent microwave attenuation capacity, i.e. the optimal reflection loss reached -50.0 dB (larger than 99.999% absorption) at a thickness of 4.0 mm and wide bandwidth achieved 5.7 GHz (from 12.3 GHz to 18.0 GHz, covering 95.0% of Ku-band) under an ultrathin thickness of only 1.9 mm. Furthermore, the probable microwave dissipation mechanisms of as-synthesized composite aerogels were clarified, which included the optimized impedance matching, strengthened interfacial polarization and dipole polarization relaxation, notable oxygen vacancy effect and enhanced conduction loss. This work could shed light on developing graphene-based 3D broadband microwave absorption composites.

Cubic-like Co/NC composites derived from ZIF-67 with a dual control strategy of size and graphitization degree for microwave absorption

MOFs with high tunability are considered ideal candidates as microwave-absorbing materials. Strict experimental conditions can ensure the repeatability and maximize the potential of such materials. In this study, cubic ZIF-67 carbides synthesized at different solution temperatures showed an adjustable average size, and then by adjusting the calcination temperature we could control the degree of graphitization, so as to explore the synergistic effect of these two aspects to achieve an in-depth understanding of the electromagnetic properties and microwave absorption properties. The results showed that sample 30-600 (with the former number referring to the synthesis temperature and the latter to the calcination temperature) showed the widest effective absorption bandwidth (5.75 GHz, 1.8 mm) and the optimal reflection loss (-56.92 dB, 2.1 mm). The best matching electromagnetic parameters were obtained under the synergistic action of a smaller particle size and appropriate degree of graphitization, so as to achieve strong attenuation characteristics under low electromagnetic wave reflection. The microwave loss mechanism of the sample mainly involved dielectric losses, such as from conductance loss, dipole polarization, and interface polarization. Starting from the experimental details, this work proposes a dual control strategy for developing microwave-absorbing materials with both simplicity and practicability, which provides a useful reference for other microwave absorbents synthesized at room temperature.

NiCo alloy/C nanocomposites derived from a Ni-doped ZIF-67 for lightweight microwave absorbers

In this work, we prepared NiCo alloy/C with rhombic dodecahedron structure and superior microwave absorption performance by using ZIF-67 as the raw material. The rhombic dodecahedron NiCo alloy/C was with rough particles on the surface was photographed by field emission scanning electron microscopy. By adjusting the doping amount of Ni and the temperature of pyrolysis, improved the impedance matching of NiCo alloy/C. Specifically, NiCo alloy/C exhibits a minimum reflection loss of -65.48 dB at 13.48 GHz, while the thickness is 1.63 mm. Defects introduced in the Ni doping process and the special rhombic dodecahedral structure can cause multiple loss mechanisms. Therefore, this NiCo alloy/C composite has the potential to be a potential microwave absorber material with lightweight and high microwave absorption properties.

Fabrication of three-dimensional nitrogen-doped reduced graphene oxide/tin oxide composite aerogels as high-performance electromagnetic wave absorbers

Developing light-weight and high-efficiency electromagnetic wave (EMW) absorbers has been considered as an effective strategy to resolve the electromagnetic radiation pollution problem. Herein, nitrogen-doped reduced graphene oxide/tin oxide (NRGO/SnO₂) composite aerogels were facilely prepared through the hydrothermal process and subsequent lyophilization treatment. Morphological characterization results manifested that the attained NRGO/SnO₂ composite aerogels possessed unique three-dimensional (3D) porous network structure constituted by the tiny SnO₂ nanoparticles decorated wrinkled surfaces of flake-like NRGO. Moreover, excellent EMW absorption performance could be achieved through facilely regulating the additive volumes of ethylenediamine and filler contents. Impressively, the composite aerogel with a doped nitrogen concentration of 6.5 wt% displayed the optimal minimum reflection loss of -62.3 dB at a matching thickness of 3.5 mm and the broadest effective absorption bandwidth of 5.1 GHz under an ultrathin thickness of merely 1.6 mm. Furthermore, the as-synthesized composite aerogels showed a light-weight characteristic with the low bulk density of 19.9-25.7 mg·cm^-3. Additionally, the potential EMW absorption mechanisms of obtained composite aerogels were revealed, which were mainly ascribed to the unique 3D porous network structure, synergistic effects between conduction loss and polarization loss, as well as the balanced attenuation loss and impedance matching. This work could be valuable for the structural design and fabrication of 3D graphene-based dielectric composites as light-weight and high-efficiency EMW absorbers.

Constructing a nitrogen-doped carbon and nickel composite derived from a mixed ligand nickel-based a metal-organic framework toward adjustable microwave absorption

The rational design of nanostructures for absorbers has great potential in the microwave absorption field. In this work, a mixed ligand nickel metal-organic framework (ML-Ni MOF) was first prepared by the self-assembly of pyrazine and 1,3,5-benzenetricarboxylic acid with nickel ions. Then, the as-prepared ML-Ni MOF was used as a precursor to fabricate a nitrogen-doped carbon and nickel composite (ML-Ni/C). With the molar ratio of pyrazine and 1,3,5-benzenetricarboxylic acid of 1 : 1, the flower-like ML-Ni MOF was obtained. After pyrolysis, the ML-Ni MOF-derived ML-Ni/C composite showed an optimal reflection loss value of -65.33 dB with a thickness of 2.4 mm and a corresponding effective absorbing bandwidth (EAB, RL ≤ -10 dB) of 5.1 GHz. Besides, the broadest EAB of 7.6 GHz was achieved when the thickness was about 2.8 mm. This strategy paves a new way to design novel MOFs as precursors for fabricating absorbers.