Communication pubs.acs.org/crystal
Cite This: Cryst. Growth Des. XXXX, XXX, XXX−XXX
Co/C Composite Derived from a Newly Constructed Metal−Organic Framework for Effective Microwave Absorption Bao-Yong Zhu,†,§ Peng Miao,‡,§ Jie Kong,*,‡ Xiu-Ling Zhang,† Guang-Yin Wang,† and Kai-Jie Chen*,‡ ‡
Department of Applied Chemistry, School of Science, Northwestern Polytechnical University, Xi’an, Shaanxi 710072, China School of Chemistry and Chemical Engineering, Dezhou University, Dezhou, 253023, China
†
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ABSTRACT: The discovery of efficient electromagnetic wave absorbing materials for solving serious electromagnetic wave pollution is an urgent task. By rational design and assembly, a new three-dimensional metal−organic framework (MOF, CPT-1-Co) with the Co ions was constructed and employed as the precursor for the fabrication of Co/C composites via pyrolysis at high temperatures. The resultant Co/C composite pyrolyzed at 700 °C can absorb more than 90% of an EM wave at 2−18 GHz by the alternation of fabrication thickness. The effective absorption bandwidth can reach as high as 5.4 GHz at a thin thickness of 1.7 mm, which is superior to that of most EM wave absorbers. The bottom-up protocol in this report represents the first attempt to pyrolyze a new MOF for microwave absorber fabrication, which will provide valuable insight for designing more advanced MOFs and their applications in efficient electromagnetic wave absorption.
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In this context, metal−organic frameworks (MOFs)31 and metal−organic materials (MOMs)32 with highly ordered and diverse structure, tunable pore geometry,33−41 and an abundant metal/carbon source have emerged as great candidates for fabricating such metal/carbon composites via pyrolysis for catalysis and energy-related applications. The advantage of MOF is that the pyrolysis template relies on the inheritance of the porous architectural nature with regard to light density and more cavities. After annealing under mild condition, the ferromagnetic metal particles or domains can be well distributed in the resultant carbon matrix or network. A few superior MOFs (e.g., ZIF-67, HKUST-1, MOF-5, MIL125-Ti, and Co-MOF-74) have been utilized for the synthesis of these metal/carbon composites, which perform quite well with high reflection coefficients and broad absorption bandwidths.42−51 However, exquisite control of the resultant composition and three-dimensional porous skeleton of metal/ C composites for efficient EM wave absorption by scarfing MOF precursors is still being developed. Exploring more benchmark MOFs, especially new MOFs with preorganized porous structures, seems to be a necessity for achieving this goal. Herein, we present the design and construction of a new Co-based MOF with high porosity, namely, [Co 2 O(cptpy)2(DMF)] (CPT-1-Co, Hcptpy = 4′-(4-carboxyphenyl)-4,2’:6′,4′′-terpyridine, DMF = N,N-dimethylformamide), by reacting a multidentate ligand, 4′-(4-carboxyphenyl)-
ith the rapid development of the electrical industry, more and more electrical devices are used in our daily life. Meanwhile, electromagnetic (EM) wave radiation and pollution have become more serious with regard to our safety and health.1,2 Though some advanced EM-absorbing materials can achieve remarkable performance and have already applied in the real market,3,4 the discovery of EM wave absorbers/ materials with “wide absorption bandwidth, strong absorption capacity, low thickness and density” is still a great challenge. Typically, the EM-wave-absorbing mechanism relies on magnetic loss and dielectric loss and their impedance matching. In the early stage, ferrites and metals were used in this field and showed large magnetic ferromagnetic losses and minor dielectric losses.5−7 Meanwhile, carbon materials and electronically conductive polymers were also employed for their significant dielectric loss and low density.8−13 The hybridization of carbon material with a Si-based ceramic component can also realize the enhancement of EM wave absorption.14−19 To combine their magnetic and dielectric loss advantages for better EM-wave-absorbing performance, carbon and metal/metal oxide composites or complexes were fabricated and applied for efficient EM wave absorption.20−26 Though considerable progress has been achieved by multiple synthesis methods (e.g., plasma spray,27 chemical vapor deposition,28 solid-phase synthesis,29 and electrospinning30), a simple and efficient protocol for preparing such composites with low-density, highly tunable structure composition, welldistributed metal particles in a carbon matrix, and mild fabrication conditions is still in high demand. © XXXX American Chemical Society
Received: January 15, 2019 Revised: February 12, 2019 Published: February 21, 2019 A
DOI: 10.1021/acs.cgd.9b00064 Cryst. Growth Des. XXXX, XXX, XXX−XXX
Crystal Growth & Design
Communication
Scheme 1. Synthesis Routine of CPT-1-Co-Derived EM Absorbers
4,2′:6′,4′′-terpyridine (Hcptpy), with Co(OAc)2 salt. By the pyrolysis of CPT-1-Co at 700 °C, the resultant Co/C composite (Co/C-700) was fabricated and behaves as an effective microwave absorber with a broad absorption bandwidth of 5.4 GHz (Scheme 1). To the best of our knowledge, this work represents the first example of utilizing a newly constructed MOF for fabricating effective EM wave absorbers. The CPT-1-Co sample was synthesized by the solvothermal reaction of Co(OAc)2 and Hcptpy with a high yield of 65%. The crystal data of CPT-1-Co was determined by a singlecrystal X-ray diffraction experiment. CPT-1-Co is a 3D framework crystallizing in the tetragonal system with an I41/ a space group. The detailed single-crystal data after refinement is summarized in Table S1. Every dinuclear Co2+ cluster is coordinated by two carboxylate anions of two organic ligands, four nitrogen atoms from four individual cptpy− anions, and two oxygen atoms from two DMF molecules. Moreover, there is a μ2-O bridge linking two Co2+ ions in this cluster (Figure 1a). In the structure, there are three types of cages constructed from such metal clusters and organic ligands. Three types of cages are interconnected by such metal clusters, in which cages A and B are connected in the edge-shared mode and cages A and C are linked in a vertex-shared way (Figure 1c). Each cage is surrounded by 12 neighboring cages to form a threedimensional framework. This porous structure possesses large three-dimensional intercrossing cavities as shown in Figure S1. The internal diameter of the cages is ca. 12 Å (Figure 1b). The porosity of CPT-1-Co is calculated with PLATON52 to be 68.8%. Topology analysis showed that CPT-1-Co is a binodal 3,6-connected net with a point symbol of {4.62}2{42.67.86}, if treating the dinuclear cobalt cluster as the six-connected node and the cptpy− anion as the three-connected node (Figure 1d). The phase purity of the CPT-1-Co sample was verified by a powder X-ray diffraction (PXRD) measurement in which the experimental pattern is consistent with the calculated one from the single-crystal structure (Figure S2). In order to investigate the thermal stability, a TGA experiment was carried out under a N2 atmosphere in the temperature range of 25−700 °C. The CPT-1-Co sample starts losing DMF guest molecules rapidly upon heating and tends to be stable between 200 and 350 °C. After that, framework decomposition starts and ends at ca. 450 °C (Figure. S4). To verify the porosity of CPT-1-Co, a 195 K CO2 sorption experiment was conducted on an activated sample. As shown in Figure S5, the CO2 adsorption isotherm at 195 K reveals typical type-I sorption behavior, proving the existence of microporosity. The Brunauer−Emmett−Teller (BET) surface area of CPT-1-Co is calculated to be 705.2 m2/ g. The pore size distribution analysis from the density function theory (DFT) model revealed that the pore size of CPT-1-Co
Figure 1. Coordination environment of a newly constructed CPT-1Co (a); view of the octahedral cage and three adjacent cages connected by a dinuclear Co2+ cluster (b, c); topological analysis of CPT-1-Co, where a magenta distorted octahedron represents a 6-c dinuclear Co2+ cluster and the 3-c green triangle represents the ligand (d).
is predominantly around 14 Å, which is consistent with the internal diameter of ca. 12 Å observed in the single crystal structure. To obtain Co-nanoparticle-embedded carbon material, CPT-1-Co was subjected to the pyrolysis process at multiple temperatures (500, 600, 700, 800, and 900 °C) under an Ar atmosphere to harvest Co/C-X composites (where X stands for the corresponding pyrolysis temperature). A detailed pyrolysis procedure has been provided in the experimental section. After systematic optimization of the pyrolysis temperature effect on the resultant microwave absorption performance (Figure S7), 700 °C was chosen as the optimal pyrolysis temperature. Therefore, the characterization and discussion are solely based on Co/C-700. The morphology and microstructure of CPT-1-Co and Co/C-700 were observed by scanning electron microscopy (SEM) and transmission B
DOI: 10.1021/acs.cgd.9b00064 Cryst. Growth Des. XXXX, XXX, XXX−XXX
Crystal Growth & Design
Communication
Figure 2. SEM images of CPT-1-Co (a) and CPT-1-Co after pyrolysis (inset figure); selected-area electron diffraction (SAED) analysis (c) and TEM graphs with a high resolution of Co/C-700 (b, d); element mapping (C and Co) of Co/C-700.
Figure 3. Experimental and calculated PXRD patterns of Co/C-700 and two metallic Co phases, respectively (a); Raman spectrum (b); XPS general spectra (c) and Co 2p (d) and C 1s (e) spectra at high resolution; and magnetic hysteresis loop of Co/C-700 (f).
electron microscopy (TEM). As seen in Figures 2a and S8, the particle surface of the CPT-1-Co crystal after the pyrolysis process becomes less smooth than the surface of the pristine CPT-1-Co crystal, originating from the loss of organic ligands. Moreover, some bulky crystals broke into smaller pieces, but without severe collapse. The morphology of most CPT-1-Co crystals with an octahedral shape under investigation was maintained after pyrolysis under an Ar atmosphere on the basis of the magnified SEM images of Co/C-700. For a detailed analysis, TEM images with high resolution were captured and showed that well-distributed metal nanoparticles were generated in the dark. The particle size of these nanoparticles
is located in a narrow region from 10 to 50 nm (Figure 2b), and all of the particles are fully covered with a thin carbon layer (ca. 5 nm). Expected Co and C contents evenly locate whole composite particles, as evidenced by elemental mapping of the Co/C-700 sample (Figure 2). To verify the existing state of Co in these particles, the PXRD pattern of Co/C-700 was collected and clearly proves the absence of cobalt oxides (Figure 3a). Three main diffraction peaks were found with 2θ values of 44.2, 51.5, and 75.9°, which is highly consistent with that of the face-centered cubic phase of Co (reference pattern from JCPDS no. 15-0806). Meanwhile, two small peaks at 41.7 and 47.5° can be attributed to the existence of a trace impurity C
DOI: 10.1021/acs.cgd.9b00064 Cryst. Growth Des. XXXX, XXX, XXX−XXX
Crystal Growth & Design
Communication
Figure 4. Real and imaginary parts of the complex permittivity (a) and complex permeability (b) vs frequency and frequency-dependent tan δε and tan δμ values (c) of Co/C-700; apparent magnetic behavior exhibition of CPT-1-Co (red powder) and Co/C-700 (black powder) under the stimulus of a magnet and digital photographs of a wax/Co/C-700 sample (d); and 3D representation of EM wave absorption performance for CPT-1-Co (e) and Co/C-700 (f), where the selected area in the black-lined circle represents a reflection coefficient of
