Hierarchical C/Co3O4 nanoarray on a nickel substrate integrating electromagnetic and thermal shielding

Jin-Cheng Shu , Si-Qi Zhu , Wen-Qiang Cao * and Mao-Sheng Cao *
School of Materials Science and Engineering, Beijing Institute of Technology, Beijing 100081, China. E-mail: caomaosheng@bit.edu.cn; wenqiang_cao@sina.com

Received 15th May 2021 , Accepted 14th July 2021

First published on 14th July 2021


Abstract

A multi-functional electromagnetic (EM) device is an important pillar for promoting the progress of an intelligent society. Herein, a ZIF-67 nanoneedle array assembled on a nickel substrate is used as a precursor to construct a C/Co3O4 nanoarray via a simple thermal treatment approach, integrating the two functions of EM and thermal shielding. Its unique three-dimensional porous structure and integrated components endow this material with an average shielding efficiency of 99.62% and a maximum temperature difference of about 25 K. In addition, this skeleton structure features light weight, low density and small cost, which can be utilized to construct wearable devices to protect human beings from EM and thermal radiation. This study furnishes a novel horizon for the design and construction of multi-functional and wearable EM devices, and promotes the development of the EM field and the progress of multi-disciplinary technologies.


Introduction

The 5G era ignites the spark of an intelligent society, greatly facilitating human life and social operation.1–3 Unfortunately, electromagnetic (EM) and thermal radiation from various electronic devices seriously threatens human health, which has become the focus of social attention.4–6 In order to solve this problem conveniently and efficiently, it is imperative to develop a novel multi-functional EM device integrating excellent EM and thermal shielding.7–9

Metal–organic framework (MOF), featuring light weight, abundant hole and large specific surface area, is a rising star in the fields of microwave absorption and EM interference (EMI) shielding.10–15 In recent years, numerous novel MOF-based absorbers and shielding materials have been reported, displaying meritorious EM response.16–18 For example, Ji et al. exploited a new FeIII-MOF-5-derived carbon fiber composite with the integration of magnetic loss and multiple polarization, achieving a reflection loss of −39.2 dB and a bandwidth of 4.44 GHz.19 Fei et al. synthesized a novel carbon nanofiber@Co/C aerogel from a bacterial cellulose@ZIF-67 aerogel, with an EMI shielding efficiency (SE) of 31.05 dB.20 These MOF-based EM functional materials set off a huge wave in the high-tech field.21–27

Porous structure and large specific surface area dominate the realization of excellent thermal shielding. This indicates that MOFs with abundant apertures also have an important application in thermal shielding. In this case, in order to further improve the thermal shielding performance, the materials with a porous network structure can be selected as the substrate for the growth of MOF monomers. Thus, the nickel skeleton with a three-dimensional (3D) porous structure enters the human field of vision, which can effectively facilitate the reflection of thermal radiation and inhibit heat exchange. In addition, its excellent electrical conductivity enhances the reflection of EM waves, drastically ameliorating EMI shielding performance.28

In this study, a nickel skeleton with abundant apertures is applied as the substrate to grow a one-dimensional (1D) ZIF-67 nanoneedle array. The C/Co3O4 nanoarray is then derived after high-temperature pyrolysis in air. Through an in-depth exploration of the pyrolysis process, the effect of the pyrolysis temperature (723 to 923 K) on the microstructure and properties of the C/Co3O4 nanoarray is revealed. The optimal SE appears in the product pyrolyzed at 823 K, reaching 33 dB, and its maximum temperature difference is close to 25 K. The high-efficiency EM and thermal shielding performance originates from its unique 3D structure and integrated components. This study can inspire the design and fabrication of novel multi-functional materials and devices, and promote the progress of multidisciplinary fields.

Results and discussion

Fig. 1a demonstrates the fabrication process of hierarchical C/Co3O4 nanoarray on a nickel substrate. In an aqueous solution, the dispersed cobalt ions (Co2+) and 2-methylimidazole ligand self-assemble on the surface of the nickel substrate and grow into a 1D ZIF-67 nanoneedle array. After further pyrolysis in air, Co2+ and the ligand are converted into Co3O4 and carbon layer, respectively, to form a hierarchical C/Co3O4 nanoarray with a broken structure. The microstructure at different stages is visually shown in Fig. 1b–e. It can be clearly observed that the fabricated C/Co3O4 nanoarray can maintain a relatively complete microscopic morphology, but abundant holes are generated inside resulting from the loss of the ligand during pyrolysis. The Raman spectrum of the C/Co3O4 nanoarray is shown in Fig. 1f. There are five peaks appearing at 194, 483, 523, 621 and 692 cm−1, which confirm the presence of Co3O4. Notably, no obvious carbon peak is found in the spectrum, suggesting a low carbon content in the product. It originates from the significant loss of carbon to the air during high-temperature pyrolysis.
image file: d1qm00731a-f1.tif
Fig. 1 (a) Schematic of the fabrication process of hierarchical C/Co3O4 nanoarray on the nickel substrate. (b) SEM image of the nickel substrate, (c) ZIF-67 nanoneedle array and (d and e) hierarchical C/Co3O4 nanoarray. (f) Raman spectrum of the C/Co3O4 nanoarray.

Photograph in Fig. 2a shows the black nickel skeleton of the C/Co3O4 nanoarray with a mass of macroscopic holes, revealing its features of light weight and low density. As observed in Fig. 2b, the 3D nickel skeleton with an open pore structure furnishes a significant advantage for the growth of ZIF-67 and the in situ synthesis of C/Co3O4. The high-resolution SEM image in Fig. 2c is used to characterize the microstructure of the C/Co3O4 nanoarray, which clearly exhibits the vertical growth and uniform decoration of the C/Co3O4 nanoarray on the surface of the nickel skeleton. Fig. 2d and its inset present the microstructure of the nanoarray before and after pyrolysis. From the TEM image, the produced C/Co3O4 nanoarray consists of smaller-sized nanoparticles and is accompanied by the formation of abundant interfaces. Moreover, the structure of ZIF-67 collapses after pyrolysis, causing a decrease in diameter. High-resolution TEM and selected area electron diffraction (SEAD) are adopted to deeply investigate the C/Co3O4 nanoarray, and two lattice fringes with the d-spacings of 0.24 nm and 0.29 nm were found, which correspond to the (311) and (220) planes, as shown in Fig. 2e and f.


image file: d1qm00731a-f2.tif
Fig. 2 (a) Photograph of the nickel skeleton with the C/Co3O4 nanoarray. (b and c) SEM images of the C/Co3O4 nanoarray on the nickel substrate with different resolutions. The inset in Fig. 2(b) exhibits the SEM image of the nickel substrate. (d) TEM images of C/Co3O4 and ZIF-67 (inset). (e) High-resolution TEM image and SAED image (inset) of C/Co3O4. (f) Intensity profiles of the d-spacings of the (311) and (220) crystal planes corresponding to the lattice fringes in Fig. 2(e). (g–k) SEM images of C/Co3O4 nanoarrays at different pyrolysis temperatures. All scale bars are 500 nm.

The effect of pyrolysis temperature in the range of 723 to 923 K on the microstructure of the C/Co3O4 nanoarray is deeply explored. As shown in Fig. 2g–k, the increase in temperature gradually intensifies the structural fracture and shrinkage of the C/Co3O4 nanoarray, effectively adjusting the diameter of the internal hole. Simultaneously, the pyrolysis temperature also has an important effect on the distribution of nanoarrays on the surface of the nickel substrate. High temperatures easily destroy the growth of nanoneedles and cause them to fall off, which affects the EM and thermal shielding performance.

In general, EMI SE in dB can be described as the sum of the contribution of absorption (SEA) and reflection (SER), which can be obtained by the following formula:29

 
SE = SEA + SER(1)

A unique 3D structure and integrated components endow the C/Co3O4 nanoarray on the nickel substrate with outstanding EMI shielding. The EMI SE, SEA and SER of C/Co3O4 nanoarrays with different pyrolysis temperatures are displayed in Fig. 3a and b. The maximum SE of the product pyrolyzed at 823 K reaches 33 dB, and its average SE achieves 24 dB, which is the best among all products. Its SEA is greater than SER. A deep insight is given into the absorption (A) and reflection (R) coefficients to investigate the EM response. As illustrated in Fig. 3c, the large average R coefficient indicates its high conductivity, being able to generate strong reflection, which inhibits most EM waves from entering the material. In addition, under an alternating EM field, the electron transport and polarization behavior also cause conduction loss and relaxation loss.30,31 Moreover, the 3D skeletal structure featuring a big specific surface area stimulates the multiple scattering of EM waves and enhances dissipation.


image file: d1qm00731a-f3.tif
Fig. 3 (a) The maximum and (b) the average EMI SE, SEA and SER of C/Co3O4 nanoarrays pyrolyzed in the temperature range of 723–923 K. (c) The average A and R coefficients of C/Co3O4 nanoarrays at different pyrolysis temperatures. (d) The plots of EMI SE, SEA and SER of three-layer product versus frequency. (e) The evaluation of the maximum and the average EMI SE, SEA and SER of three-layer product. (f) The average SE in dB and the average shielded EM radiation (%) of mono-layer and three-layer products. They are compared to a commercial standard (20 dB). The mono-layer product pyrolyzed at 823 K is used here.

The EMI shielding performance of the three-layer product is analyzed, as shown in Fig. 3d–f. The three products pyrolyzed at 723, 773 and 823 K are assembled into a multi-layer shielding material. By applying the product pyrolyzed at 723 K with the lowest R coefficient as the first interface, the incident EM wave can be absorbed as much as possible rather than reflected. Fig. 3d exhibits the variation of its SE, SEA and SER with frequency. The drastic fluctuations of SE, SEA and SER with frequency may be ascribed to the uneven distribution of its internal structure. The maximum SE and the average SE attain 41 dB and 33 dB, respectively (Fig. 3e). In general, the SE of commercially available EM shielding materials is 20 dB. Obviously, the SE of both mono-layer and three-layer products satisfies this requirement, shielding 99.62% and 99.95% of EM radiation, respectively, which can serve as a potential candidate for the application of anti-electromagnetism interference.

The thermal shielding function can prevent devices and human beings from thermal radiation. In order to explore the thermal shielding performance, the sample is placed on a 358 K heating platform, and the thermal infrared images are captured every 5 min for 40 min using a thermal infrared imager. As observed in Fig. 4a, the obvious color difference between the sample and the heating platform suggests the outstanding thermal shielding capability, and the maximum temperature difference is close to 25 K. The histogram of temperature difference under different heating time periods is exhibited in Fig. 4b to evaluate the thermal shielding performance. With the extension of heating time, the detected upper surface temperature increases slightly, but still maintains a stable state.


image file: d1qm00731a-f4.tif
Fig. 4 (a) Thermal infrared images of the C/Co3O4 nanoarray on the nickel substrate after heating for 5, 10, 15, 20, 25, 30, 35 and 40 min. (b) Histogram of temperature difference between the sample and the heating platform under different heating time periods. (c) Thermal infrared images of the back of hand before and after placing the sample. (d) Schematic of the thermal shielding mechanism.

In order to excavate its potential application in human protection, the sample is placed on the back of the hand to investigate the thermal shielding performance. From the captured thermal infrared image view, the sample blocks the thermal radiation, exhibiting good thermal shielding (Fig. 4c). The mechanism of thermal shielding is illustrated in Fig. 4d. In general, thermal conductivity depends on gas conduction, solid conduction and radiation.32 The 3D skeleton structure and C/Co3O4 nanoarray can reflect and scatter infrared radiation to achieve thermal insulation. Simultaneously, the porous structure contributes to the thermal shielding because air with extremely low thermal conductivity in the void is difficult to exchange heat with the environment. The abundant micropores and mesopores inside the sample inhibit the flow of air, thereby significantly reducing the thermal conductivity.

Conclusions

In summary, the C/Co3O4 nanoarray is successfully fabricated using the ZIF-67 precursor, integrating excellent EM shielding and thermal shielding performance. Through the adjustment of the pyrolysis temperature, the microstructure of the C/Co3O4 nanoarray is effectively tailored, thereby optimizing the EM properties. The product pyrolyzed at 823 K emerges the maximum SE of 32 dB, and the temperature difference is close to 25 K, which is believed to benefit from the 3D porous structure and integrated components. This product can protect human beings from EM and thermal radiation, and guide the research and development of novel multi-functional EM materials and devices.

Experimental

Materials and reagents

Nickel foam and cobalt nitrate hexahydrate (Co(NO3)6H2O) were purchased from Aladdin. 2-Methylimidazole was purchased from Innochem. None of the reagents were purified further.

Fabrication of the Co3O4/C nanoarray on the nickel substrate

In a typical experiment, after cleaning with deionized water and acetone, the tailored nickel foam (1.5 × 3.0 cm) was immersed into 40 mL of an aqueous solution containing Co2+ (0.4 M) and 2-methylimidazole (25 × 10−3 M) for 4 h to obtain a ZIF-67 nanoneedle array grown on nickel substrate. The above product was washed with deionized water and dried at 330 K for 12 h.

The resulting products were pyrolyzed in air at 723, 773, 823, 873 and 923 K, respectively, for 30 min (heating rate of 1 K min−1). Finally, the products with different microstructures were obtained.

Characterization and measurements

Scanning electron microscopy (SEM, JSM 7100F, JEOL) and transmission electron microscopy (TEM, FEI, Tecnai G2 F30) were adopted to characterize the micromorphology and microstructure of the material, respectively. Raman spectrum was recorded using an invia Raman microscope with a 532 nm laser beam as the light source.

The Co3O4/C nanoarray on the nickel substrate was cut into 22.86 mm × 10.16 mm as the test sample. The EM parameters (complex permittivity and complex permeability) in the X band (8.2–12.4 GHz) were measured using a vector network analyzer (VNA Anritsu 37269D).

Conflicts of interest

The authors declare no conflict of interest.

Acknowledgements

Financial support from the National Natural Science Foundation of China (Grant No. 51977009, 11774027, and 51132002).

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