Nanolayered Cobalt@Carbon Hybrids Derived from MetalâOrganic

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Nanolayered Cobalt@Carbon Hybrids Derived from Metal-Organic-Frameworks for Microwave Absorption Yu Zhang, Hao-Bin Zhang, Xinyu Wu, Zhiming Deng, Ergang Zhou, and Zhong-Zhen Yu ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.9b00226 • Publication Date (Web): 22 Mar 2019 Downloaded from http://pubs.acs.org on March 25, 2019

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ACS Applied Nano Materials

Nanolayered Cobalt@Carbon Hybrids Derived from Metal-Organic-Frameworks for Microwave Absorption Yu Zhang†,‡, Hao-Bin Zhang*,†,‡, Xinyu Wu†, Zhiming Deng‡, Ergang Zhou‡, Zhong-Zhen Yu*,‡,§ †

State Key Laboratory of Organic-Inorganic Composites, College of Materials Science and

Engineering, Beijing University of Chemical Technology Beijing 100029, China ‡

Beijing Key Laboratory of Advanced Functional Polymer Composites, Beijing University of

Chemical Technology, Beijing 100029, China §

Beijing Advanced Innovation Center for Soft Matter Science and Engineering, Beijing

University of Chemical Technology, Beijing 100029, China ABSTRACT: High-performance broadband microwave absorbers are greatly required to cope with the increasingly serious microwave radiation pollution. Integration of dielectric and magnetic components is a promising strategy to achieve desirable microwave absorption performance. Herein, we demonstrated a facile and efficient approach to fabricate two-dimensional cobalt@carbon (Co@C) hybrids by solvothermal synthesis of cobalt-metal-organic-framework (Co-MOF) nanosheets and subsequent thermal pyrolysis for efficient microwave absorption application. The resultant nanolayered Co@C hybrids inherit the two-dimensional architecture of the Co-MOF nanosheets. During the thermal pyrolysis of Co-MOF, its organic component is carbonized to amorphous carbon while its cobalt ions are carbothermally reduced to cobalt nanoparticles, thus in-situ integrating the magnetic metal nanocrystal with the dielectric carbon. Consequently, a wide effective absorption bandwidth of 5.44 GHz at a small thickness of 1.76 mm and a minimum reflection loss value of -49.76 dB are achieved for the wax composite with 30 wt% of Co@C hybrid pyrolyzed at 800 °C. The outstanding microwave absorption 1

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performances are attributed to the synergistic effects of the Co@C hybrid nanosheets and their large shape anisotropy, which facilitates the strong dielectric loss and good impedance matching, thus promising the nanolayered Co@C hybrid as a lightweight and high-performance microwave absorber. KEYWORDS: cobalt@carbon hybrid; MOF nanosheets; microwave absorption; dielectric loss; magnetic loss INTRODUCTION Recently, the increasingly serious microwave radiation pollution and stealth technology for military weaponry call for lightweight and high-performance microwave absorbers with broad effective absorption frequency bandwidth.1-4 Generally, traditional microwave absorbers can be divided into two main types: dielectric and magnetic materials, which are distinguished by the electromagnetic wave attenuation mechanisms. Among them, typical dielectric carbon nanomaterials including porous carbon,5 graphene6 and carbon nanotubes1 are widely investigated due to their low density, tunable dielectric constant and high chemical stability. Despite the encouraging advances for carbon absorbers, many methodologies have been proposed to integrate magnetic components with the carbon materials to extend the effective absorption bandwidth and improve the microwave attenuation capability based on the synergistic effect of dielectric and magnetic losses.7-10 However, the traditional methods usually require multiple complicated and tedious processes and the favorable hybrid architectures are difficult to be well controlled. It is thus imperative to develop an efficient and facile approach for the fabrication of carbon/magnetic hybrids with satisfactory microwave absorption performances.

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Metal-organic-frameworks (MOFs) are a series of ordered crystals consisting of periodic coordinated metal ions and organic molecules, which are often used for gas absorption and separation due to their large surface areas.11,12 Interestingly, the MOF’s superlattice is recently considered as an ideal precursor for fabricating carbon/metal hybrids by converting the organic ligands and metal ions to carbon and metallic materials with thermal treatments.13,14 Particularly, with the MOFs containing magnetic metal ions (Co, Fe, Ni) as precursors, the carbon/magnetic hybrids, such as Co/C,15 Co/C/TiO216 and Fe@C nanocubes,17 were fabricated by thermal annealing process, which provided a facile approach for preparing microwave absorbers. For example, Lü et al.15 fabricated a ZIF-67 derived Co/C composite with maximum reflection loss (RL) value of -35.3 dB (4 mm) and effective absorption bandwidth of 5.8 GHz (2.5 mm) at a loading of 60 wt%. Using Fe-MOF-5 as a template, Liu et al.18 developed ZnO/Fe/Fe3C carbon composites and achieved an excellent RL performance of -50.5 dB (2.6 mm) at the filler loading of 60 wt%. Although some attractive performances are achieved for different MOFs-derived hybrids, it is still required to address the issues of high filler loadings and narrow effective absorption bandwidth. Previous results suggested that the design and construction of unique favorable architectures, such as core-shell structure19 and hierarchical urchin-like structure,20 were beneficial for optimizing overall microwave absorption performances. By controlling the crystalline structure of MOFs, zero dimensional,21 one-dimensional,22 two-dimensional (2D)23 and three-dimensional24 architectures have been realized. 2D nanomaterials are expected to exhibit remarkable microwave absorption performances because of their high surface-to-volume ratios and the large shape anisotropy, which helps to realize abundant defects on the

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heterogeneous interfaces and multiple scattering.25-27 However, there are few reports on 2D microwave absorbers originated from MOFs till now.28-32 Herein, we demonstrate an efficient and facile approach to fabricate 2D Co@C hybrids by solvothermal synthesis of Co-MOF nanosheets followed by thermal pyrolysis for efficient microwave absorption applications. The Co-MOF nanosheets are prepared by a bottom-up and surfactant-assistant method, where polyvinyl pyrrolidone (PVP) as the surfactant facilitates the confined crystal growth of pristine Co-MOFs. Subsequent thermal pyrolysis carbonizes the organic components to amorphous carbon and carbothermally reduces the Co ions to zero-valent Co nanoparticles. The combination of the magnetic Co nanoparticles and the dielectric carbon effectively improves the impedance matching of the Co@C hybrids, which helps to introduce more electromagnetic wave into the hybrids. Meanwhile, the unique 2D microstructure provides abundant interfaces and defects to increase the polarization and multiple scattering, further ensuring the competitive microwave absorption performances. In addition, the thermal pyrolysis temperatures are varied to further improve the microwave absorption performances of the asprepared Co@C hybrids by tuning their dielectric properties. The optimized microwave absorption performances including the minimum RL value of -49.76 dB at a 2-mm thickness and the quite wide effective absorption bandwidth (EAB) of 5.44 GHz at a thickness of 1.76 mm are achieved for the wax composite with 30 wt% of the Co@C hybrid prepared at 800 °C. The excellent performances including the large RL and EAB values obtained at such low filler loading and small thickness promise the nanolayered Co@C hybrid as a lightweight, ultrathin and efficient microwave absorber. EXPERIMENTAL SECTION 4


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Materials: Cobalt acetate tetrahydrate (Co(Ac)2∙4H2O), PVP with an average molecular weight of 24,000 g∙mol-1, 1,4-dicarboxybenzene (H2BDC), N, N-dimethylformamide (DMF, 99.8%) and tert-butanol were purchased from Aladdin (China). All chemicals and reagents were used as received without further purification. Synthesis of Co-MOF Nanosheets: Co-MOF nanosheets were synthesized by a bottom-up surfactant-assisted method to control the crystal growth of the pristine Co-MOFs.33,34 Firstly, 0.321 g of Co(Ac)2∙4H2O and 0.5 g of PVP were mixed in 10 mL of DMF, while 0.214 g of H2BDC was dissolved in 20 mL of DMF under stirring. Then, the H2BDC solution was added dropwise into the Co(Ac)2∙4H2O/PVP solution with stirring. After the solution became dark purple, it was sonicated for 10 min and transferred into a 50 mL Teflon-lined stainless-steel autoclave for synthesis of Co-MOF nanosheets at 105 °C for 20 h with a constant magnetic stirring. After the reaction, the Co-MOF nanosheets were collected by centrifugation, washed with DMF and ethanol solvents for several times, dispersed in tert-butanol and finally freeze-dried to obtain light purple MOF powders. Synthesis of Nanolayered Co@C Hybrids: The synthesized Co-MOF nanosheets were calcinated to obtain nanolayered Co@C hybrids at different temperatures of 600, 700 and 800 °C for 80 min under argon atmosphere with the heating rate of 2 °C/min, and the products were correspondingly denoted as Co@C600, Co@C700 and Co@C800 for morphological and performance characterizations. Characterization: Morphologies and microstructures of the Co-MOF nanosheets and Co@C hybrids were observed with a Hitachi SU8020 scanning electron microscope (SEM), a JEOL 5



JEM-1200EX transmission electron microscope (TEM), a Bruke DMFASTSCAN2-SYS atomic force microscope (AFM) and a FEI Tecnai G2 F20 S-TWIN field-emission TEM. Crystal structures of Co-MOF nanosheets and Co@C hybrids were characterized on a Bruke D8 Advance X-ray diffractometer (XRD) with Cu Kα radiation (λ = 0.152 nm) and Co Kα radiation (λ = 0.179 nm), respectively. Nitrogen adsorption-desorption isotherms were obtained by a Micromeritics ASAP 2460 Instrument, and the specific surface area and pore distribution were respectively determined by the Brunauer-Emmett-Teller (BET) method and the Barrett-Joyner-Halenda (BJH) method. Magnetic hysteresis loops were measured with a Versalab vibration sample magnetometer and Raman spectra were obtained on a Renishaw inVia Raman spectroscopy with an excitation line of 633 nm. Microwave Absorption Performance Measurements: Electromagnetic parameters including relative permittivity and relative permeability of samples were characterized by an Agilent HP8722ES network analyzer in the frequency range of 1-18 GHz. Co@C hybrids were mixed with wax at a mass percentage of 30%, and the wax composites were molded into toroidal rings with an outer diameter of 7.00 mm and an inner diameter of 3.04 mm for microwave absorption performance tests. RESULTS AND DISCUSSION Figure 1 illustrates the solvothermal synthesis and subsequent pyrolysis process for the fabrication of nanolayered Co@C hybrids derived from their precursor of Co-MOF nanosheets. Co-MOF nanosheets are fabricated by a PVP-assistant crystal growth method with Co(Ac)2∙4H2O and H2BDC as raw materials. The PVP molecules with strong affinity to Co ions play an important role in guiding the confined crystal growth to form MOF nanosheets during the crystal growth process by selectively attaching on the Co ion clusters. The subsequent thermal pyrolysis process is also critical to convert the Co-MOF nanosheets to nanolayered Co@C hybrids by carbonizing 6


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the organic component to amorphous carbon matrix and carbothermally reducing the cobalt ions to zero-valent Co nanoparticles. The pyrolysis temperatures are also tuned to optimize the microstructures and performances of the nanolayered Co@C hybrids.

Figure 1. Schematic illustrating the solvothermal synthesis of Co-MOF nanosheets and subsequent pyrolysis for fabricating nanolayered Co@C hybrids.

The microstructures of the Co-MOF nanosheets are revealed with SEM (Figure S1), AFM and TEM images. It is seen that the as-prepared light purple Co-MOF nanosheets (Inset of Figure 2c) consist of stacked ultrathin transparent nanosheets with obvious wrinkles (Figure 2a), and their AFM image confirms that the Co-MOF nanosheets have an average thickness of ~3.5 nm and a lateral size of ~300 nm (Figure 2b). Moreover, XRD patterns (Figure 2c) demonstrate the successful fabrication of Co-MOF nanosheets. The main characteristic peaks of the resultant CoMOF nanosheets agree well with the standard pattern of MOF-71.34 The highly porous Co-MOF nanosheets exhibit a large BET surface area of ~110.9 m² g-1 with a typical isotherm and an average pore size of 1.59 nm (Figure 2d). All these results indicate the successful synthesis of the Co-MOF nanosheets. For comparison, the Co-MOFs without the assistance of PVP are also prepared. The pristine Co-MOFs also show the sheet-like morphology but with a much larger 7



thickness of 50-80 nm (Figure S2). In fact, the Co-MOF crystals intend to grow into the laminate structures, but PVP can effectively hinder the crystal growth along the thickness direction, hence leading to the formation of ultrathin Co-MOF nanosheets with the thickness of a few nanometers.

Figure 2. (a) TEM and (b) AFM images of Co-MOF nanosheets. (c) XRD patterns of Co-MOF nanosheets (with inserted digital image) and simulated MOF-71, and (d) nitrogen adsorptiondesorption curves of Co-MOF nanosheets. The Co-MOF nanosheets were thermally annealed at different temperatures to fabricate Co@C hybrids. In fact, the as-prepared Co@C hybrid powder prepared at 380 °C roughly retains the original morphology of Co-MOF nanosheets just with some small nanoparticles (Figure S3), whereas the large flakes collapse into small ones at high temperatures. Figures 3 a and b show the morphology of Co@C800 hybrid powder under low magnifications, and it is seen that the 8


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resultant hybrid consists with numerous small microspheres wrapped by membrane-like carbon and some litchi-like microspheres. The resultant Co@C hybrids prepared at 600 and 700 °C show similar microstructures (Figure S4). However, further observation with TEM under high magnifications is conducted for the Co@C800 hybrid dispersed in ethanol under ultrasonication. The images reveal that the large particle-like aggregates and flakes are composed of small nanosheets anchored with numerous nanoparticles with a statistically mean diameter of 80 nm (Figures 3c and S5). The high-resolution TEM (HRTEM) observation also reveals that, on the amorphous carbon sheets, the Co nanoparticles are wrapped with few-layer-graphitic carbon shell to form the core-shell nanostructures (Figure 3d). In detail, the lattice fringe with a typical dspacing of ~0.2 nm can be reasonably assigned to the (111) plane of metallic Co, and the d-spacing of ~0.34 nm corresponds well to the (002) plane of graphitic carbon. Furthermore, the generated Co nanoparticles and the graphitic carbon shells are confirmed by their selected area electron diffraction (SAED) patterns, which can be indexed to (111) and (200) planes of face-centeredcubic (fcc) Co and (002) plane of graphitic carbon. Additionally, the sharp X-ray diffraction peaks at 51.9o and 60.7° (Figure 3e) are well indexed to the (111) and (200) planes of fcc-cobalt (PDF#15-0806). Meanwhile, the weak diffraction peak for Co@C700 and Co@C800 at 30.4o is attributed to the (002) plane of graphitic carbon (PDF#75-1621). As reported,22,52 it is reasonable that the Co nanoparticles are generated from their Co moiety by carbothermal reduction with the carbon component during the thermal treatment process, while the graphitic carbon derives from the amorphous carbon that is catalyzed by the encapsulated Co nanoparticles, leading to the Co@graphitic carbon core-shell architecture.4 The energy dispersive X-ray spectra (EDS) (Figure S6) and the elemental mapping (Figure S7) verify the evenly distributed Co and C elements coexisting in the nanolayered Co@C hybrids, which inherits from the regular crystal nature of the Co-MOF nanosheets. The EDS results indicate that the Co content in Co@C800 is 40.38 at.%. The Raman results (Figure 3f) indicate the defects of the carbon structure in the Co@C hybrid, as 9



evidenced by the typical peaks of carbon materials at ~1330 (D band) and 1585 cm-1 (G band). With the increase of pyrolysis temperatures, the intensity ratio (ID/IG) calculated by the ratio of peak areas decreases from 1.63 for Co@C600 to 1.25 for Co@C800, suggesting the removal of the oxygen-containing groups and the defects from the carbon structures, facilitating the improvements in both electrical conductivity and the dielectric loss of the Co@C hybrid.

Figure 3. (a, b) SEM, (c) TEM, (d) HRTEM with a SAED pattern, (e) XRD patterns, and (f) Raman spectra of the Co@C hybrids.

In addition, the pore structures of nanolayered Co@C hybrids are studied by N2-adsorptiondesorption curves. In Figure S8, all the samples show the typical Ⅳ isotherms and the hysteresis loops are located at around 0.4-0.9, which is related to the mesoporous structure. The BET surface areas of the Co@C hybrids show certain decreases from 183.8 to 114.3 m2/g as the treatment temperature increases from 600 to 800 °C, indicating the structural change happens indeed. Most

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pore sizes are below 20 nm and the distribution peaks appear at around 1.8-2.3 nm, which inherits from the Co-MOF nanosheet precursor. As demonstrated above, the Co@C hybrids with nanolayered structures were obtained, and the embedded metallic Co nanocrystals endow them with the magnetic properties (Figure S9, and the related values are listed in Table S1). All the samples present typical ferromagnetic characteristics with a maximum saturation magnetization (Ms) of ~105 emu g-1, which is lower than that of the bulk cobalt (164.8 emu g-1 at 300 K) due to the existed nonmagnetic carbon component.36 The Co@C600 exhibits the smallest values of Ms (90.2 emu g-1) and coercivity (Hc) (283 Oe), because the relatively low crystallinity usually causes spin disorder. Higher pyrolysis temperatures result in lower coercivity values, which may be explained by the spin-canted effect on the surface layer of Co nanoparticles. The effect is related to the slightly increased size of the Co nanoparticles at the elevated pyrolysis temperature.20 The electromagnetic wave absorption performances of the Co@C hybrids were tested according to the transmission line theory, and the reflection loss (RL) was calculated by the following formulas: 37 𝑍in = 𝑍0

𝜇𝑟 𝜀𝑟

tanh[𝑗(

2𝜋𝑓𝑑 𝑐

) 𝜇𝑟 𝜀𝑟]

𝑍in ― 𝑍0

RL = 20lg(|(𝑍0 + 𝑍in|)

(1) (2)

where Zin is the input impedance of the absorber, d is the absorber thickness, c is the electromagnetic wave velocity in free space, and εr and μr are the relative complex permittivity and relative complex permeability, respectively. Figure 4 shows the 3D forms and RL curves of the Co@C hybrids prepared at different pyrolysis temperatures. The area enclosed within the dashed lines indicates that the RL values are lower than -10 dB, representing the effective 11



electromagnetic wave absorption performance. It is seen that the Co@C600 has poor microwave absorption performances (Figure 4a, b). When the thickness of the wax composite with the Co@C hybrid is less than 6 mm, there is no an effective absorption bandwidth (RL< -10 dB). At a large thickness of 7 mm, the absorber exhibits a minimum RL value of -22 dB at 16.9 GHz and an effective absorption band of 3.23 GHz. Similarly, the Co@C700 also presents modest microwave absorption performances with a minimum RL value of -16.3 dB at 2.87 GHz and relatively narrow EAB at a large thickness of 8 mm (Figure 4c, d). Interestingly, the Co@C800 exhibits a much better microwave absorption capability (Figure 4e, f), giving the largest minimum RL value of 49.76 dB at 12.56 GHz at a small thickness of 2 mm and a remarkable bandwidth of 5.44 GHz at a thin thickness of 1.76 mm. Besides, the microwave absorption performance of Co@C900 is also studied (Figure S10). Compared to Co@C800, Co@C900 shows a minimum RL value of -24.45 dB with a thickness of 2 mm at 15 GHz, and the strongest absorption RL reaches -45.12 dB at 6.44 GHz with a thickness of 4.1 mm. The effective absorption bandwidth of Co@C900 ranges from 7.97 to 11.37 GHz, almost covering the whole X band at the thickness of 3 mm. It is thus expected that the pyrolysis conditions strongly influence the absorption performances of the Co@C hybrids.

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Figure 4. RL images of (a, b) Co@C600, (c, d) Co@C700, and (e, f) Co@C800.

To highlight the advantages of the nanolayered Co@C hybrids, the microwave absorption performances of Co@C800 are compared with those of other MOFs-based nanomaterials reported previously in terms of filler loading, thickness, minimum RL value, and the EAB (Table 1). 13



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Clearly, the wax composite with 30 wt% of the Co@C hybrids yields a high RL value (-49.76 dB) at a sample thickness of 2 mm and a broad EAB (5.44 GHz) at a small thickness (1.76 mm), indicating its great potential as high-performance microwave absorbing materials. Table 1. Microwave absorption performance of absorbers derived from different MOFs. Maximum RL

Absorbers TiO2/C ZnO/Fe/Fe3C/C Fe/Co/C ZnO/Co/C CuO/C Co/C/TiO2 Ni/C ZIF-67 derived Co/C PB derived Fe/C Co@ZIF-67 Fe/C CoZn/C MOF-74 derived Co/C CNTs/Co S-GA S-Co/C Co/NPC@Void@CI Nanolayered Co@C hybrids

Loading (wt%) Thickness (mm) 60 1.6 60 2.6 50 1.2 50 2.5 50 1.55 50 1.65 40 2.6 60 4 40 2 25 3 15 2.5 30 4.5 30 2.4 20 1.81 10 2.65 50 1.6 40 2.2 30 2.0

Value (dB) -49.6 -50.5 -21.7 -45 -57.5 -51.7 -51.8 -35.3 -22.6 -30.31 -29.5 -59.7 -62.12 -60.4 -46.2 19.86 -49.2 -49.76

Maximum EAB (RL

Nanolayered Cobalt@Carbon Hybrids Derived from MetalâOrganic

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