Cactus-Inspired Bimetallic MetalâOrganic Framework-Derived 1Dâ2D

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Functional Nanostructured Materials (including low-D carbon)

Cactus-Inspired Bimetallic Metal-Organic Frameworks Derived 1D-2D Hierarchical Co/ N-decorated Carbon Architecture towards Enhanced Electromagnetic Wave Absorbing Performance Xueqing Xu, Feitian Ran, Zhimin Fan, Hua Lai, Zhongjun Cheng, Tong Lv, Lu Shao, and Yuyan Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b00356 • Publication Date (Web): 18 Mar 2019 Downloaded from http://pubs.acs.org on March 19, 2019

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ACS Applied Materials & Interfaces

Cactus-Inspired Bimetallic Metal-Organic Frameworks Derived 1D-2D Hierarchical Co/N-decorated Carbon Architecture towards Enhanced Electromagnetic Wave Absorbing Performance Xueqing Xu a, Feitian Ran a, Zhimin Fan a, Hua Laia, Zhongjun Cheng b, Tong Lv c, Lu Shao a, Yuyan Liu *a a

MIIT Key Laboratory of Critical Materials Technology for New Energy Conversion and Storage, School of

Chemistry and Chemical Engineering, Harbin Institute of Technology, Harbin, Heilongjiang 150001, P. R. China. b

Natural Science Research Center, Academy of Fundamental and Interdisciplinary National Key Laboratory of

Science and Technology on Advanced Composites in Special Environments, Harbin Institute of Technology, Harbin, Heilongjiang 150090, P. R. China. c

Aerospace Institute of Advanced Material & Processing Technology，Beijing 100074, China

E-mail: [email protected]; Fax: +86 045186402368; Tel: +86 045186413711

Keywords:

Metal-organic

frameworks,

magnetic

metal/carbon

dimensional, CoZn-ZIF-L, electromagnetic wave absorption.

ACS Paragon Plus Environment

nanocomposites,

mixed

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT:

Metal-organic

frameworks

(MOFs)

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derived

magnetic

metal/carbon

nanocomposites have shown tremendous potential for lightweight electromagnetic wave (EMW) absorption. However, it is a challenge, but highly significant to design and construct the mixeddimensional hierarchical architectures with synergistically integrated characteristics from individual MOFs for advancing the EMW absorption performance. Inspired by the structure of cactus, a novel hierarchical 1D-2D mixed dimensional Co/N-decorated carbon architecture comprising of carbon nanotubes grafted on carbon flakes (abbreviated as CoNC/CNTs) has been fabricated by pyrolysis of bimetallic CoZn-ZIF-L. The CoNC/CNTs integrate the advantages of 1D nanotubes for extra polarization of EMW, and 2D nanoflakes with interconnected porous structure for multiple reflection losses of EMW and optimization of impedance matching. The resultant CoNC/CNTs demonstrate excellent EMW absorbing performance. For the optimal EMW absorbing material of CoNC/CNT-3/1, minimum reflection loss reaches −44.6 dB at 5.20 GHz with low filler loading of 15 wt.%. Moreover, the largest effective bandwidth range achieves to 4.5 GHz with a thickness of 1.5 mm and filled ratio of 20 wt.%. These findings indicate that such mixed 1D-2D dimensional hierarchical architecture synergistically enhances EMW absorbing performance. This work sheds light on the rational design of mixed dimensional carbon architecture derived from MOFs for desirable functionalities.

1. INTRODUCTION With the continuous advances in high-tech communication and information technology, electromagnetic interference or pollution has becoming a widespread concern owing to its potential impact on human health and electronic safety.

1-3

Considerable efforts therefore have been

committed to explore electromagnetic wave (EMW) absorbing materials with satisfactory performance. 4-8 Traditional absorbing materials with single magnetic loss or dielectric loss property have resulted unsatisfying EMW absorbing performance owing to the impedance mismatching. 9 Metal-organic frameworks (MOFs), self-assembled by inorganic clusters bridged to organic ligands, 10-12 could be selected as self-sacrificed precursors or templates for in situ transformation into porous magnetic metal (or alloy)/carbon nanostructures.

13-15

In this context, the well-built

MOFs derived nanostructures for EMW absorbing materials with both magnetic loss and electric loss could optimize impedance matching. 16-19 For instance, Wang et al. prepared the core-shell CoC nanostructure derived from MOF-74 for high performance EMW absorption.20 Fe/C nanocubes


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fabricated by Prussian blue exhibit good microwave attenuation properties. 21 Some other N-doped porous carbon polyhedron or CNTs/Co nanocomposites obtained via carbonization of the ZIF-67 (ZIF, zeolitic imidazolate frameworks) have controllable EMW absorption performance.

4, 22-24

It

should be noted that the morphology and structure of MOFs derived carbon nanomaterials always manifest single dimensional structures rather than unique mixed-dimensional hierarchical architecture. To our knowledge, however, the mixed-dimensional hierarchical architecture from individual MOFs has rarely been explored to fabricate EMW absorbing materials. Generally, the mixed/multi-dimensional hierarchical architecture could realize synergistic integration from different structure units, revealing its unique superiority in applications such as catalysis

15, 25

energy storage

26, 27

and electrochemistry.

28, 29

Nature endows many creatures with

multi-dimensional hierarchical architecture for target functions.30 Cactus could adapt harsh environment owing to its unique structural features. Their thorns can collect water and succulent stem composed of interconnected storage cells can be used to storage water.31 As presented in Figure 1, these teeny 1D thorns and large 2D flakes with abundant interconnected porous could analogize as the hierarchical architecture contained interconnected 2D nanosheets supporting 1D nanotubes (Figure 1a-c). Inspired by above fascinating 1D-2D mixed dimensional hierarchical characteristic, the bimetallic CoZn-ZIF-L was firstly used as the precursor for fabricating the cactus-like architecture with unique 2D structure. Moreover, as known to previous literature, carbon nanotubes (CNTs) can be synthesized by pyrolysis of some specific MOFs.32 Herein, we rationally design and synthesize a mixed-dimensional hierarchical Co/N-decorated carbon architecture constructed with carbon nanotube grafted on carbon nanosheets (here-after denoted as CoNC/CNTs) through the pyrolysis of bimetallic CoZn-ZIF-L. It exhibits a cactus-like structure (2D carbon sheets combined with 1D carbon nanotube) (Figure 1d). Such a mixeddimensional hierarchical architecture can integrate the advantages of interconnected 2D microstructure that enhance multiple reflections and optimize impedance matching (Figure 1e), and 1D carbon nanotubes that provide more multiple polarization (Figure1 f-g). And therefore, the more incident EMW could be attenuated rather than reflected or transmitted (Figure 1h). Consequently, the optimized CoNC/CNTs nanoarchitectures exhibit remarkable absorption performances. The minimum reflection loss reaches −44.6 dB when the loading of absorber is 15 wt.%, and an effective bandwidth below −10 dB achieves to 4.5 GHz when the thickness is only 1.5 mm under absorber



loading of 20 wt.%. These results confirmed that elaborate construction of mixed-dimensional hierarchical architecture with controllable interfacial engineering between CNTs and carbon nanosheets is an effective strategy for boosting the absorption performance of EMWs.

Figure 1 Scheme illustration for design of cactus inspired 1D-2D mixed dimensional CoCN/CNTs for EMW absorbing materials. (a) optical image of cactus, (b) interconnected porous flakes, (c) teeny 1D thorns of cactus, (d) the structure of the CoCN/CNTs, (e) the interconnected networks of 2D nanosheets for enhancing the multiple reflections/scatterings and optimizing impedance matching, (f, g) 1D nanotubes for interfacial and dipole polarization, and (h) primary EMW transmitting process.

2. EXPERIMENTAL SECTION 2.1 Synthesis of CoZn-ZIF-L The bimetallic CoZn-ZIF-L was prepared according to the reported method with some modification.33 In detail, various molar ratios of Co(NO3)2·6H2O and Zn(NO3)2·6H2O (the specific content was shown in the Table S1, Supporting Information), and 2.606 g of 2-Methylimidazole (2MIM) were dissolved in 80 mL deionized (DI) water, respectively. Then, the metal ion solution was quickly poured into the 2-MIM solution, and stirred for 12 h at room temperature. After being centrifuged and washed with DI water, the obtained samples were dried at 80 °C in vacuum. The resultant CoZn-ZIF-L with different molar ratios of Co2+ and Zn2+ were termed as ZIF-1/0, ZIF-3/1, ZIF-1/1, ZIF-1/3, and ZIF-0/1 respectively, corresponding to the molar ratios of Co2+ and Zn2+. 2.2 Synthesis of CoNC/CNTs The as-synthesized CoZn-ZIF-L was placed in porcelain boat and programmatically heated to


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700 °C for 2 h under N2 atmosphere (heating rate of 2 °C min-1) in a tubular furnace. The fabricated carbon architecture derived from ZIF-0/1 and ZIF-1/3 were labeled as NC and Co-NC, respectively. Others labeled as CoNC/CNT-1/1, CoNC/CNT-3/1 and CoNC/CNT-1/0, corresponding to the molar ratios of Co2/Zn2+ in their precursor. As a contrast, the ZIF-1/1 was pyrolyzed in different temperature, and the corresponding carbon architecture were named as ZIF-x (x=600, 700, 800 and 900, x refer to the pyrolysis temperature).

3. RESULTS AND DISCUSSION To obtain the cactus-like 1D-2D hierarchical carbon architecture, the unique 2D leaf-like bimetallic CoZn-ZIF-L microstructure was firstly synthesized by facile one-step reaction approach at ambient temperature. After pyrolysis at 700 oC, the bimetallic CoZn-ZIF-L was evoluted to cactus-like carbon architecture (Figure 2a). Scanning electron microscopy (SEM) images indicate that the obtained ZIF-3/1 has leaf-shaped morphology with length of 7 μm, width of 3 μm, and thickness of about 200 nm, respectively (Figure 2b). The original 2D leaf-like morphology of CoZnZIF-L is maintained after pyrolysis, at the same time, the nanotubular structure were uniformly grafted in the surface of carbon nanoflakes (Figure 2c, d). The transmission electron microscopy (TEM) was applied to elucidate the microstructure of CoNC/CNT-3/1. As presented in Figure 2e and Figure S3, the small nanoparticles with diameter about 7 nm were well distributed over all carbon nanosheets. The high-resolution TEM (HR-TEM) images in Figure 2f, g reveal that the interplanar spacing of 0.35 nm and 0.22 nm are associated with C (002) plane of graphitic carbon and (111) plane of face-centered cubic metallic Co (PDF 15-0806), respectively, indicating that these small nanoparticles are cobalt nanoparticles (Co NPs) which were totally embedded in carbon layers.34 Additionally, the EDS results in Figure 2k demonstrate that the C, N, O and Co elements uniformly distributed in the CoNC/CNT-3/1. Furthermore, in the HR-TEM images, it could also be demonstrated that the interplanar spacing of the nanotubular structure is 0.36 nm, which is assigned to the plane of C (002) (Figure 2j), being indicative of carbon nanotubes (CNTs).

29

The outside

diameter of CNTs is about 12 nm and inner diameter is about 5 nm, signifying the multi-walled structure of CNTs. Moreover, TEM observation also manifest the curving CNTs grown on the carbon nanosheets and Co NPs encased on their top (Figure 2h-i).



Figure 2 (a) Schematic illustration of the synthesis of CoNC/CNTs, SEM images of (b) ZIF-L-3/1, (c, d) CoNC/CNT-3/1, (e) TEM image of CoNC/CNT-1/3, (f, g) HR-TEM images of the Co NPs embedded in graphitic carbon shells, (h, i) the curving CNTs rooted in the porous carbon nanosheets with Co NPs on their top, (j) HR-TEM image of curving CNTs, and (k) HAADF-STEM image and corresponding elemental mapping of CoNC/CNT-3/1. The X-ray photoelectron spectroscopy (XPS) was utilized to elucidate the surface chemical compositions of CoNC/CNT-3/1. The XPS survey spectrum demonstrates that C, N, O, Co and Zn elements were presented on the surface of CoNC/CNT-3/1 (Figure S4). For the C 1s spectrum (Figure 3a), the peaks located at 285.4 eV and 284.5 eV correspond to the C−C and C=C. The polar groups such as C=O (288.8 eV) and C−O/C−N (286.6 eV) have also been detected. In the fitted N


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1s spectrum (Figure 3b), the binding energies of 398.5 eV, 400.5 eV and 401.1 eV are assigned to pyridinic N (N−6) and pyrrolic N (N−5), and N-oxides (N−O), respectively. 35, 36 For O 1s spectrum (Figure 3c), it is also demonstrated that the polar group C−O−C (533.9 eV), C−OH (532.6 eV), C=O (531.9 eV) and C−O=H (530.7 eV) exist in the CoNC/CNT-3/1. 14 The fitting spectrum of Co 2p exhibits three distinct Co species corresponding to Co, Co2+ and satellites. As presented in Figure 3d, the peaks located at 778.8 eV and 796.2 eV were identified as Co 2p3/2 and Co 2p1/2, respectively. Other peaks correspond to metallic Co (793.8 eV), Co2+ (781.3 eV), and Co−Nx (802.2 eV), verifying the formation of Co−N bonds.

37

The abundant polar groups of cactus-like hierarchical

carbon architecture are beneficial to enhance dipole polarization, thus facilitating the absorption of EMW. Moreover, above mentioned TEM and XPS results synthetically demonstrate that cactuslike 1D-2D hierarchical Co/N-decorated carbon architecture constructed with carbon nanosheets supporting carbon nanotubes (CoNC/CNTs) has been synthesized successfully.

Figure 3 XPS spectra of CoNC/CNT-3/1 for (a) C 1s, (b) N 1s, (c) O 1s and (d) Co 2p. To have better insight to the evolution process of cactus-like carbon architecture, the precursor CoZn-ZIF-L with various molar ratio of Co2+/Zn2+ were pyrolyzed at 700 oC under N2 atmosphere. The crystalline nature of the as-synthesized bimetallic CoZn-ZIF-L were characterized by powder X-ray diffraction (XRD). All of the MOFs have the consistent patterns (Figure S1), indicating these prepared bimetallic CoZn-ZIF-L have similar crystal structure. Besides, these XRD patterns are



identical to simulated one of Co-ZIF-L reported by previous literature, confirming the formation of well crystallographic phase CoZn-ZIF-L.

38-40

It could be observed from the SEM images that

various molar ratio of Co2/Zn2+ have no obvious effect on the morphology of CoZn-ZIF-L, and all of them have uniform size and leaf-shaped morphologies (Figure S2). However, the corresponding carbon materials have different morphologies (Figure S5), indicating that the Co/Zn molar ratios in precursors are crucial factor for generating cactus-like carbon architecture. When the molar ratio of Co2+/Zn2+ is lower than 1, the original morphology of CoZn-ZIF-L was destroyed in certain degree, and no CNTs were formed. The ZIF-0/1 was turned into N-doped carbon sheets (NC) (Figure 4a, S5f). ZIF-1/3 was transformed into Co NPs and N co-doped carbon sheets (Co-NC) (Figure 4b, S5e). To further increase the Co/Zn molar ratio to 1/1, 3/1 and 1/0, the CNTs could be grown on the surface of carbon nanosheets. Interestingly, the length and the relative contents of CNTs increase as the molar ratios of Co2+/Zn2+ increase (Figure 4c-e, S5a-d). These results show that the higher Co2+ content can efficiently facilitate the formation of CNTs. Its formation mechanism is similar to the previous literatures that highly crystallographic MOFs self-assembled by hexatomic or Nheterocyclic organic ligands and transition metal clusters could be in situ transformed into the CNTs by pyrolysis. In this evolution, the hexatomic or N-heterocyclic organic ligands serve as the basic units for forming the CNTs, while the metal clusters become metal NPs (such as Co, Fe and Ni), which play a catalytic role for the formation of CNTs. 32, 41 In this work, the 2-MIM basic units have been evolved into CNTs under the catalysis of Co NPs in the pyrolysis process. Additionally, pyrolysis temperature of CoZn-ZIF-L has also been systematically investigated. After treatment at 600 oC, the CoZn-ZIF-L precursors could not generate CNTs, and the original morphology of CoZn-ZIF-L was not maintained (Figure S6b, S7). When the temperature increases to 800 oC even 900 oC, the CNTs could be formed, nevertheless, the original morphology of CoZnZIF-L was destroyed seriously (Figure S6e-f). As mentioned above, only under the pyrolysis temperature of 700 oC, the original morphology of CoZn-ZIF-L also was maintained along with the formation of CNTs. Therefore, controlled pyrolysis temperature is also crucial for obtaining such cactus-like carbon architecture. For the XRD patterns of N-doped carbon architecture (Figure 4f), the peaks at about 22.8° correspond to the C (002) plane, indicating the existence of graphitic carbon, which is consistent with the results of TEM. Meanwhile, other diffraction peaks at about 44.3°, 52.3° and 76.1° are


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attributed to (110) (200) and (220) crystalline planes of metallic-Co (JCPDS No.15-0806) as presented in the XRD patterns of CoNC/CNTs and Co-NC. Their diffraction intensity became stronger as increasing the Co content in the corresponding precursors. Although CoNC/CNT-3/1, CoNC/CNT-1/1, Co-NC and NC contained a certain content of Zn in the MOFs precursors, it could not observe any diffraction pattern of zinc, zinc oxide or CoZn alloy in their patterns. It is well known that the melting point and boiling point of Zn are 420 oC and 907 oC, respectively, while both of the melting point and boiling point of Co are higher than 1400 oC. Therefore, the possible reason of it is that the metal Zn in the CoZn-ZIF-L has been volatilized under high temperature or presented an amorphous state. 22 Moreover, for further expound the evolution of the CoZn-ZIF-L, the XRD patterns of samples with different pyrolysis temperature were also investigated. When the pyrolysis temperature is 600 oC, the Co3ZnC has been generated. However, the Zn3Co is not stable at higher temperature and decomposed into Co NPs, so the Co3ZnC phase was fade away under 700 oC

to 900 oC (Figure S8).

Raman spectra of the CoNC/CNTs reveal the characteristic structure of carbon nanocomposite. The D bands at around 1300 cm−1 are associated with the defect/disordered carbon atoms, while the G bands located at about 1580 cm−1 are caused by the in-plane stretching vibration of graphitic sp2carbon. The intensity ratio (IG/ID) can be applied to indicate the graphitization degree of carbon material. 42, 43 As shown in Figure 4g, the intensity ratio (IG/ID) of CoNC/CNTs, CN and Co-CN are inferior to 0.90, revealing the domination of short-range order graphitic structure. 29 Moreover, both the D band and G bands are blue shifted in wavelength with the Co amount increased, indicating possibly strong electronic interaction between carbon and Co NPs. However, a shoulder peak appears at around 1450 cm−1 and its intensity increase with the order for NC, Co-NC, CoNC/CNTs1/1, CoNC/CNTs-3/1, and CoNC/CNT-1/0, which may be related to the increase of the disordered degree of carbon framework. The porous characteristics and specific surface areas of the Co N-codoped carbon architecture are characterized using N2 absorption-desorption measurements. As observed in Figure 4h, all the N2 absorption-desorption curves have sharp uptakes at low relative pressure (

Cactus-Inspired Bimetallic MetalâOrganic Framework-Derived 1Dâ2D

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