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carbon nanotubes nanocomposites with a threaded structure
Journal of Alloys and Compounds 689 (2016) 366e373
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Synthesis and microwave absorption property of two-dimensional porous nickel oxide nanoflakes/carbon nanotubes nanocomposites with a threaded structure Jie Wang a, 1, Xilai Jia a, 1, Tihong Wang a, Sai Geng a, Chen Zhou a, Fan Yang a, Xiaojuan Tian a, Liqiang Zhang a, Haitao Yang b, **, Yongfeng Li a, * a b
State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing, 102249, PR China Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing, 100190, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 June 2016 Received in revised form 22 July 2016 Accepted 30 July 2016 Available online 31 July 2016
Composite material of NiO/CNTs consisting of two-dimensional porous NiO nanosheets threaded by carbon nanotubes was prepared by thermal treatment of Ni(OH)2/CNTs obtained from an in-situ hydrothermal method. The introduction of CNTs during the synthesis processes can effectively prevented the aggregations of NiO nanosheets, and form interpenetrated composite structure. Therefore, porous NiO nanoflakes are threaded by CNTs, leading to the formation of the lightweight composite with different NiO loadings. The as-prepared NiO/CNTs composite is highly porous, effective for microwave absorption. The minimum reflection loss reaches 25.4 dB when the NiO loading is 85 wt% with thickness of 2.0 mm at 10 GHz. This composite material is expected to be a promising candidate as microwave absorbing materials. © 2016 Elsevier B.V. All rights reserved.
Keywords: NiO nanosheets Carbon nanotubes Hierarchically nanostructure Lightweight composites Microwave absorbing materials
1. Introduction The rapid growth of the information technology has led to the emergence of a large number of electromagnetic (EM) devices in civil and military fields. In order to protect people from harmful electromagnetic interferences, it is necessary to develop highperformance devices of shielding and electromagnetic wave absorption (EA) [1e6]. Furthermore, it is popular that the EA materials have light weight, strong absorption ability, wide absorption frequencies, and thinness. So far, there are many kinds of microwave absorbing materials such as ferrite [7e9], magnetic metal powders [10,11], ceramics [12,13], conducting polymers [14,15], carbonbased materials [16e18] etc. However, the traditional microwave absorbing materials, such as the ferrite, cannot meet the requirements including lightweight, thinness and so on. Therefore, many attempts have been made to fabricate novel microwave absorbing materials with excellent properties.
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (H. Yang), yfl[email protected] (Y. Li). 1 These authors contributed equally to this work. http://dx.doi.org/10.1016/j.jallcom.2016.07.328 0925-8388/© 2016 Elsevier B.V. All rights reserved.
Metal oxides are widely explored for electromagnetic wave absorption. For better performance, low-dimensional oxides and nanostructured oxides are synthesized with enhanced EA performance [19e21]. However, aggregations of low-dimensional oxides usually compromise the performance. Also, their performance is limited by the low dielectric properties. In order to design better structure, those metal oxides can be loaded onto porous nanocarbon materials for better performance. As a lightweight microwave absorber, carbon nanotubes (CNTs) have been considerably researched for their low density [22e24]. It is a very important strategy to introduce metal oxides into/onto CNTs, aiming to improve electromagnetic shielding and attenuation performance by both dielectric and magnetic loss. Hence, composites based on CNTs and various metal oxides are favored for high-performance microwave absorbing materials. For example, several metal/metal oxide nanoparticles such as Fe [25], Ni [26], Co [27] and CoFe2O4 [28] have been compounded with CNTs, to investigate their microwave absorbing properties. The results suggest that the composite strategies are effective for enhancement of EA performance. It is noted that most of the research works have focused on the nanoparticles, while two-dimensional (2D) nanosheets of metal oxides combined with CNTs have rarely been studied as EA
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materials. Recently, 2D oxides show excellent physicochemical properties [29e31], so we hope to synthesize 2D nanosheets for EA materials. Unfortunately, 2D oxide nanosheets usually meet severe aggregations. In this work, we provide an effective method of combination of CNTs and NiO nanoflakes, forming a lightweight NiO/CNTs composite. Importantly, CNT frameworks hinder the aggregation of NiO nanosheets. The composite is highly porous, and featured with a threaded structure. CNTs act as the frameworks for the growth of NiO and porous NiO nanoflakes are threaded by CNTs, therefore the hierarchically structured composite is advantageous for microwave absorption. As the results indicated, as-prepared NiO/CNTs composites show excellent microwave absorption performance. 2. Experimental 2.1. Materials Nickel nitrate (J&K Scientific Ltd.), 1-methyl-2-pyrrolidinone (NMP), sodium dodecyl phenyl sulfate (SDPS) (Aladdin Industrial Corporation), C2H5OH (Beijing Chemical Works), Uera (Sinopharm Chemical Reagent Co., Ltd). All chemicals used were analytical grade. CNTs were produced from a chemical vapor deposition (CVD) method, and were purified and weakly oxided for use [32]. 2.2. Preparation of NiO/CNTs composites To prepare composite material of NiO/CNTs, Ni(OH)2/CNTs was first prepared by an in-situ hydro-thermal method. In a typical preparation procedure, 0.4 g of sodium dodecyl phenyl sulfate (SDPS) was diluted into 50 ml super pure water, next 20 mg CNTs were added into SDPS solution under ultrasonic for 0.5 h, and 441 mg of nickel nitrate was mixed well with CNTs dispersion. After that, 3.6 g of urea solution was dropped into this suspension slowly under stirring and the mixture was transferred to a flask for oil bath at 90 C for 5 h, then the mixture was kept at 90 C for 10 h without stirring. After cooling down, the precipitate, that is Ni(OH)2/CNTs, was collected by centrifugation, washed with super pure water for three times, and freeze-dried overnight. Finally NiO/CNTs composite material was obtained from the thermal treatment of Ni(OH)2/CNTs at 400 C for 2 h. The various NiO/CNTs composites can be adjusted by adding different weights of nickel nitrate. And the pure NiO can be obtained by the same method without adding CNTs dispersion. 2.3. Characterizations SEM (scanning electron microscopy) experiments were conducted on a Quanta 200F FE-SEM. TEM (transmission electron microscopy) experiments were conducted on a JEM-2100 instrument operated at 120 kV. X-ray diffraction was conducted on a BRUKER D8 ADVANCE X-ray diffractometer using Cu-Ka radiation (l ¼ 1.54 Å). Nitrogen sorption isotherms were measured at 77 K with a Micromeritics SSA-4200 analyzer of Builder. Thermogravimetric analysis was conducted on a STA7200 (HITACHI) instrument at a ramping rate of 10 C min1 under an O2 flow. Raman spectra were performed on a Horiba Jobin Yvon Lab RAMHR800 Raman spectrometer with HeeNe laser excitation at 633 nm. X-ray photoelectron spectroscopy (XPS) of the nanocomposite was analyzed by a Thermo Fisher spectrometer with an ESCALAB 250Xi spectrometer using MgeK X-rays. 2.4. EM absorption measurement For EM absorption measurement, the samples were prepared by
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mixing NiO/CNTs and paraffin with 20% mass fraction of NiO/CNTs. The mixtures were then pressed into toroidal-shaped samples (Fout ¼ 7.00 mm and Fin ¼ 3.04 mm). The complex permittivity and permeability values were measured in the 2e18 GHz range with the coaxial line method by an Agilent N5224A vector network analyzer. 3. Results and discussion 3.1. Process of synthesis of NiO/CNTs composites The preparation process of NiO/CNTs is illustrated in Scheme 1. To grow the composites, CNTs are dispersed in an aqueous solution to form uniform networks assisted by SDPS surfactant. Then nickel nitrate was added into CNTs suspension to cultivate nanosheet seeds. An in-situ hydro-thermal reaction leaded to the growth of Ni(OH)2 nanoflakes in the CNTs dispersion. Note that this method can enhance the dispersion of CNTs and Ni(OH)2 between each other. After thermal treatment, porous NiO/CNTs composites with a threaded structure were obtained. By changing the concentration of nickel nitrate, the composition of the composites can be tuned, leading to formation of NiO/CNTs composites with various compositions. 3.2. Crystal structure and Raman analysis The XRD pattern of porous NiO/CNTs composites is shown in Fig. 1(a), indicating the crystalline structure of the synthesized sample. The typical peaks of 36.8 , 42.9 , 62.1 could be indexed as the characteristic (111), (220), (200) reflections of the NiO nanocrystal of the composite (JCPDS No. 65-2901), suggesting a successful formation of NiO nanoflakes. Although the typical peaks of CNTs at around 26 in the XRD are not obvious [33], Fig. 1(b) shows the Raman spectrum of the composite, indicating the formation of hybridization additionally. There are the characteristic carbon peaks seen at about 1330 cm1 and 1590 cm1 respectively, which are assigned to the D band for disordered and G band for graphite carbon [10,34]. The composites have a smaller value of the ratio of D-band to G-band intensity (ID/IG) compared with the CNTs, which indicates that NiO/CNTs composites have a more ordered structure with less structure defect. Compared with pure NiO, there is no obvious peak at around 508 cm1 for the NiO/CNTs composites. 3.3. Morphological characterization The morphologies of the NiO/CNTs composites were characterized by the SEM. They were compared with pure NiO nanosheets. As shown in Fig. 2(a), as-prepared pure NiO nanosheets have uniform size; however, they are easily stack together to tight aggregates. Compared with pure NiO, the NiO nanosheets of the nanocomposites have good dispersion morphologies. There are no obvious aggregations of the nanosheets even though the NiO components are increased to 95 wt%, as seen in Fig. 2(b). In Fig. 2(c), it displays clearly the interconnected 3D network microstructure of 85 wt% NiO/CNTs composites. This finding indicates efficient assembly between the NiO flakes and CNTs during the hydro-thermal treatment, which can lead to uniform formation of nanocomposites. When the NiO components are decreased to 75 wt% of Fig. 2(d), the CNT networks are more clear which can enhance the growth of NiO flakes and prevent their stacks [35]. The SEM and TEM images of Fig. 3(a) and (b) reveal that the NiO flakes are threaded by CNTs, forming the integrated structure. Also, the high-resolved TEM image shows that as-formed NiO flakes are porous morphology (Fig. 3(c)). In Fig. 3(d), the clear lattice fringes of NiO nanorings illustrate that the interplanar spacing is about 0.21 nm, corresponding to (200) planes of face-centered cubic NiO.
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Scheme 1. Process of synthesis of NiO/CNTs composites. (a) The CNT dispersion, (b) the formation of Ni(OH)2 flakes threaded by CNTs after in-situ hydro-thermal reaction (step Ⅰ), (c) the porous NiO/CNTs composites after calcination (step Ⅱ).
Fig. 1. XRD patterns (a), Raman spectra (b) of CNTs, NiO and NiO/CNTs composites.
The elemental mapping images of Fig. 3(e) further confirm that NiO flakes are penetrated by CNTs, suggesting the good dispersion. 3.4. XPS analysis and BET surface areas The XPS survey spectrum of Fig. 4 shows the C1s, O1s and Ni2p peaks. The typical C1s peak was observed at 283 eV and O1s peak was at 530 eV; while the Ni2p spectrum presents two main structures resulting from the spin-orbit splitting of the p orbital which are assigned as Ni2p3/2 (850e870 eV region) and Ni2p1/2 (870e890 eV region). Ni2p3/2 presents a main peak at ~856 eV with an intense satellite structure at ~862 eV, (~874 eV and ~880 eV respectively for Ni2p1/2) which is characteristic of Ni2þ. All those results suggest that NiO flakes were successfully prepared in the composites. The surface areas and pore sizes of NiO/CNTs nanocomposites were investigated with nitrogen adsorption measurement (Fig. 5). A surface area of 106 m2 g1 can be obtained for pure NiO. It clearly illustrates much larger amount of N2 gas adsorbed by the NiO/CNTs samples with the increasing of the percentage of CNTs (Fig. 5(a)). The Brunauer-Emmett-Teller (BET) surface area of composite increases from 139 m2 g1 to 198 m2 g1 when decreasing the NiO content from 95 wt% to 75 wt%. The increased surface areas of the
composites also suggested the enhanced dispersion of NiO nanoflakes due to the introduction of CNTs. The pore sizes of pure NiO nanosheets were averaged at 2.0 nm; while the nanocomposites of NiO/CNTs exhibit similar pore sizes which are smaller than pure NiO (Fig. 5(b)). It can be verified that all of the composites have micropores with the average radius of 1.7e2.0 nm. The pore structure is expected to enhance the microwave absorption [20,36e38]. 3.5. EM absorption properties of NiO/CNTs composites The EM parameters shown in Fig. 6(a)e(f) (relative complex permittivity εr ¼ ε0 jε00 , and relative complex permeability mr ¼ m0 jm00 ) of the composites with different NiO loading in a wax matrix were measured in the range 2e18 GHz to investigate the microwave absorbing properties of NiO/CNTs composites. As is known, the real permittivity (ε0 ) and the real permeability (m0 ) represent the ability to store electric and magnetic energy, while the imaginary permittivity (ε00 ) and the imaginary permeability (m00 ) represent the dissipation of electric and magnetic energy [39,40]. As shown in Fig. 6(a), the ε0 values of 95 wt% NiO/CNTs show negligible fluctuations in which its ε0 is around 8. However, the ε0 values of 75 wt% and 85 wt% NiO/CNTs decrease with
J. Wang et al. / Journal of Alloys and Compounds 689 (2016) 366e373
Fig. 2. The SEM images of NiO (a), 95 wt% NiO/CNTs composites (b), 85 wt% NiO/CNTs composites (c) and 75 wt% NiO/CNTs composites (d).
Fig. 3. The SEM image (a), the TEM images (b), (c), (d) and elemental mapping image (e) of 85 wt% NiO/CNTs composites.
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Fig. 4. XPS spectra of the NiO/CNTs composites: wide scan (a), Ni2p spectrum (b), O1s spectrum (c) and C1s spectrum (d).
Fig. 5. N2 sorption isotherms and BJH pore size distribution of pure NiO and the NiO/CNTs composites with NiO loading of 75, 85, and 95 wt%.
increasing the frequency or increasing the content of NiO. The ranges of their ε0 are about 20 to 9, which are significantly larger than that of 95 wt% NiO/CNTs nanocomposite. Fig. 6(b) shows the frequency dependence of ε00 . The value of ε00 is obviously enhanced with the decreasing of NiO loading. It can be seen that ε00 values of 95 wt% NiO/CNTs nanocomposite are close to zero, indicating very poor dielectric loss. The range of ε00 for 75 wt% NiO/CNTs and 85 wt% NiO/CNTs is 9.7 to 8.1 and 7.7 to 5.9, respectively. The ε0 decreases with increasing frequency can be explained by Debye theory equation and the decrease of ε0 and ε00 with increasing of NiO loading can be illustrated rationally according to the effective medium theory [41]. Fig. 6(c)e(d) show m0 and m00 of all NiO/CNTs composites in the range of 2e18 GHz. The m0 values of 95 wt% NiO/CNTs are decreased with increasing frequency. But for 75 wt% and 85 wt% NiO/CNTs, they show the slowly decreased m0 values and then the values
abruptly increase. This may be attributed to crystallinity and size of NiO [35]. Meanwhile, the m00 values of all NiO/CNTs composites keep decreasing with increasing frequency. And the m00 are negative in a specific range of frequencies, demonstrating there is a resonance phenomenon between m0 and m00 [42]. The dielectric loss tangent (tandE ¼ ε00 /ε0 ) and the magnetic loss tangent (tandM ¼ m00 /m0 ) are also calculated based on the complex permeability and permittivity measured as above. As shown in Fig. 6(e) and (f), with the decreasing of NiO loading, the dielectric loss factor is improved, indicating that dielectric loss is increasing [43]; but the value of tandM shows a contrary trend to the tandE. On the other hand, the difference between tandE and tandM of 95 wt% NiO/ CNTs is bigger than 75 wt% and 85 wt% NiO/CNTs. Therefore, 75 wt% and 85 wt% NiO/CNTs should have better impedance matching. According to transmission line theory, when EM waves are normally incident on a layer terminated by a conductor, the
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Fig. 6. Frequency dependence of real part (a) and imaginary part (b) of relative complex permittivity, real part (c) and imaginary part (d) of relative complex permeability, and dielectric loss tangent (e) and magnetic loss tangent (f) of NiO/CNTs composites.
reflection loss (RL) can be calculated by the following equations [44,45]:
Zin ¼
rffiffiffiffiffi mr 2p pffiffiffiffiffiffiffiffiffi fd mr εr tanh j εr c
Z 1 RL ¼ 20 log in Zin þ 1
(1)
(2)
where Zin is the input impedance of the absorber, mr and εr are respectively the relative complex permeability and permittivity, f is the frequency of microwave, d is the thickness of the absorber, and c is the velocity of light in free space. Fig. 7 shows the calculated RL of NiO/CNTs composites with different thicknesses. Compared with the pure NiO (Fig. 7(a)), the composite of NiO/CNTs shows much better absorption properties. It can be observed that the frequency at the minimum RL gradually shifts towards lower frequency with increasing thickness. And the relationship between the RL peak frequency and the absorber thickness can be explained by the quarter-wavelength cancellation
[46]. The minimum RL values of 75 wt% NiO/CNTs composites with thicknesses of 2 mm and 95 wt% NiO/CNTs composites with thicknesses of 5 mm are all less than 15 dB, which suggests that 97% absorption of incident microwaves can be achieved. Furthermore, the minimum RL values of 85 wt% NiO/CNTs composites with thicknesses of 2e5 mm are all below 17 dB (98% absorption) and the minimum value is 25.4 dB at 10 GHz with the thickness of 2 mm. The microwave absorption properties of this nanocomposite are compared to those of reported Ni-based composites in Table 1. The nanocomposite materials obtained in this work show reasonable absorption properties. As discussed above, the as-prepared NiO/CNTs composites show excellent absorption ability in the range of 2e18 GHz and the minimum RL can be adjusted to different frequencies by varying the composition and the thickness of the absorber. 3.6. Possible microwave absorbing mechanism The possible microwave absorbing mechanism could be explained by the following factors: dielectric loss and multiple reflections [50e52], as shown in Fig. 8. The strong dielectric loss is
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Fig. 7. Reflection loss of pure NiO (a) and NiO/CNTs nanocomposite with NiO loading of 75 wt% (b), 85 wt% (c), and 95 wt% (d) at different thicknesses from 1 to 5 mm.
Table 1 Properties of microwave absorption of as-prepared NiO/CNTs nanocomposite compared with reported composite materials. Material
Minimum RL value (dB)
d (mm) (RL<10 dB)
Frequency range (GHz) (RL<10 dB)
Ref.
Ni/graphene Ni/MWCNTs Ni/MWCNTs Ni@C Ni/polypyrrole Ni/polyaniline Co/Ni/CNTs NiO/CNTs
11.8 37 37.9 32 15.2 23 24 25.4
3 5.2 4 2 2 1 1.5 2
6e7 1e4 5.7e8.0 12.5e13.5 11e15.4 16e18 12.4e18 8.7e11.4
[35] [10] [43] [47] [48] [49] [27] This work
scattering of the incident microwaves, which may result in the attenuation of electromagnetic (EM) energy [54e56]. Above all, the EM energy could be dissipated rapidly by interference in the form of thermal energy, on account of the high thermal conductivity of the composites. 4. Conclusion
Fig. 8. A possible microwave absorbing mechanism of the NiO/CNTs nanocomposites.
mainly due to electron polarization relaxation and interfacial polarization. The high conductivity of CNTs leads to the formation of more charges and dipoles, and promotes electron polarization and relaxation processes [53]. Meanwhile, NiO sheets threaded by CNTs also play an extremely important role, which can strengthen interfacial polarization between nanosheets and CNTs. In addition, the porous layered NiO brings about multiple reflections and
In summary, the lightweight NiO/CNTs composite has been successfully synthesized via the thermal decomposition of Ni(OH)2/ CNTs made from an in-situ growth method. The as-prepared NiO nanoflakes are porous and penetrated with the CNTs. Based on the rational combination of these two, the composite displayed enhanced microwave absorbing performance, and also had lightweight properties. The minimum RL reaches 25.4 dB when the NiO loading is 85 wt% with thickness of 2.0 mm at 10 GHz. Also, the EM wave absorption property can be adjusted easily by varying the thickness of the samples and the NiO loading. It is believed that NiO/CNTs composites could be a potential kind of excellent microwave absorbing material with lightweight and strong absorption.
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Acknowledgements We gratefully thank for the National Natural Science Foundation of China (Nos. 21202203, 21322609, 21576289 and 51502347), the Science Foundation Research Funds Provided to New Recruitments of China University of Petroleum, Beijing (Nos. 2462014QZDX01, YJRC-2013-31), the National Basic Research Program of China (No. 2012CB933102) and Thousand Talents Program. References [1] H. Sun, R. Che, X. You, Y. Jiang, Z. Yang, J. Deng, L. Qiu, H. Peng, Adv. Mater. 26 (2014) 8120e8125. zquez, M. Prato, ACS Nano 3 (2009) 3819e3824. [2] E. Va [3] B. Wen, M. Cao, M. Lu, W. Cao, H. Shi, J. Liu, X. Wang, H. Jin, X. Fang, W. Wang, Adv. Mater. 26 (2014) 3484e3489. [4] K. Singh, A. Ohlan, V.H. Pham, R. Balasubramaniyan, S. Varshney, J. Jang, S.H. Hur, W.M. Choi, M. Kumar, S. Dhawan, Nanoscale 5 (2013) 2411e2420. [5] D.X. Yan, H. Pang, B. Li, R. Vajtai, L. Xu, P.G. Ren, J.H. Wang, Z.M. Li, Adv. Funct. Mater. 25 (2015) 559e566. [6] G. Wang, Z. Gao, G. Wan, S. Lin, P. Yang, Y. Qin, Nano Res. 7 (2014) 704e716. [7] T. Liu, Y. Pang, H. Kikuchi, Y. Kamada, S. Takahashi, J. Mater. Chem. C 3 (2015) 6232e6239. [8] H.-M. Xiao, X.-M. Liu, S.-Y. Fu, Compos. Sci. Technol. 66 (2006) 2003e2008. [9] M. Meshram, N.K. Agrawal, B. Sinha, P. Misra, J. Magn. Magn. Mater. 271 (2004) 207e214. [10] F. Wen, F. Zhang, Z. Liu, J. Phys. Chem. C 115 (2011) 14025e14030. [11] J. Liu, J. Cheng, R. Che, J. Xu, M. Liu, Z. Liu, ACS Appl. Mater. Inter. 5 (2013) 2503e2509. [12] Y. Qing, X. Wang, Y. Zhou, Z. Huang, F. Luo, W. Zhou, Compos. Sci. Technol. 102 (2014) 161e168. [13] S.-S. Kim, S.-T. Kim, J.-M. Ahn, K.-H. Kim, J. Magn. Magn. Mater. 271 (2004) 39e45. [14] L. Kong, Z. Li, L. Liu, R. Huang, M. Abshinova, Z. Yang, C. Tang, P. Tan, C. Deng, S. Matitsine, Int. Mater. Rev. 58 (2013) 203e259. [15] W.-L. Song, X.-T. Guan, L.-Z. Fan, Y.-B. Zhao, W.-Q. Cao, C.-Y. Wang, M.-S. Cao, Carbon 100 (2016) 109e117. [16] D. Micheli, A. Vricella, R. Pastore, M. Marchetti, Carbon 77 (2014) 756e774. [17] X. Sun, J. He, G. Li, J. Tang, T. Wang, Y. Guo, H. Xue, J. Mater. Chem. C 1 (2013) 765e777. [18] X. Zhang, G. Ji, W. Liu, B. Quan, X. Liang, C. Shang, Y. Cheng, Y. Du, Nanoscale 7 (2015) 12932e12942. [19] H.-J. Yang, W.-Q. Cao, D.-Q. Zhang, T.-J. Su, H.-L. Shi, W.-Z. Wang, J. Yuan, M.S. Cao, ACS Appl. Mater. Inter. 7 (2015) 7073e7077. [20] T. Liu, Y. Pang, X. Xie, W. Qi, Y. Wu, S. Kobayashi, J. Zheng, X. Li, J. Alloys Compd. 667 (2016) 287e296. [21] T.K. Gupta, B.P. Singh, V.N. Singh, S. Teotia, A.P. Singh, I. Elizabeth, S.R. Dhakate, S. Dhawan, R. Mathur, J. Mater. Chem. A 2 (2014) 4256e4263. [22] P. Watts, D. Ponnampalam, W. Hsu, A. Barnes, B. Chambers, Chem. Phys. Lett. 378 (2003) 609e614. ro ^me, I. Molenberg, I. Huynen, [23] J.-M. Thomassin, D. Vuluga, M. Alexandre, C. Je C. Detrembleur, Polymer 53 (2012) 169e174.
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Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Synthesis and microwave absorption property of two-dimensional porous nickel oxide nanoflakes/carbon nanotubes nanocomposites with a threaded structure Jie Wang a, 1, Xilai Jia a, 1, Tihong Wang a, Sai Geng a, Chen Zhou a, Fan Yang a, Xiaojuan Tian a, Liqiang Zhang a, Haitao Yang b, **, Yongfeng Li a, * a b
State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing, 102249, PR China Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing, 100190, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 June 2016 Received in revised form 22 July 2016 Accepted 30 July 2016 Available online 31 July 2016
Composite material of NiO/CNTs consisting of two-dimensional porous NiO nanosheets threaded by carbon nanotubes was prepared by thermal treatment of Ni(OH)2/CNTs obtained from an in-situ hydrothermal method. The introduction of CNTs during the synthesis processes can effectively prevented the aggregations of NiO nanosheets, and form interpenetrated composite structure. Therefore, porous NiO nanoflakes are threaded by CNTs, leading to the formation of the lightweight composite with different NiO loadings. The as-prepared NiO/CNTs composite is highly porous, effective for microwave absorption. The minimum reflection loss reaches 25.4 dB when the NiO loading is 85 wt% with thickness of 2.0 mm at 10 GHz. This composite material is expected to be a promising candidate as microwave absorbing materials. © 2016 Elsevier B.V. All rights reserved.
Keywords: NiO nanosheets Carbon nanotubes Hierarchically nanostructure Lightweight composites Microwave absorbing materials
1. Introduction The rapid growth of the information technology has led to the emergence of a large number of electromagnetic (EM) devices in civil and military fields. In order to protect people from harmful electromagnetic interferences, it is necessary to develop highperformance devices of shielding and electromagnetic wave absorption (EA) [1e6]. Furthermore, it is popular that the EA materials have light weight, strong absorption ability, wide absorption frequencies, and thinness. So far, there are many kinds of microwave absorbing materials such as ferrite [7e9], magnetic metal powders [10,11], ceramics [12,13], conducting polymers [14,15], carbonbased materials [16e18] etc. However, the traditional microwave absorbing materials, such as the ferrite, cannot meet the requirements including lightweight, thinness and so on. Therefore, many attempts have been made to fabricate novel microwave absorbing materials with excellent properties.
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (H. Yang), yfl[email protected] (Y. Li). 1 These authors contributed equally to this work. http://dx.doi.org/10.1016/j.jallcom.2016.07.328 0925-8388/© 2016 Elsevier B.V. All rights reserved.
Metal oxides are widely explored for electromagnetic wave absorption. For better performance, low-dimensional oxides and nanostructured oxides are synthesized with enhanced EA performance [19e21]. However, aggregations of low-dimensional oxides usually compromise the performance. Also, their performance is limited by the low dielectric properties. In order to design better structure, those metal oxides can be loaded onto porous nanocarbon materials for better performance. As a lightweight microwave absorber, carbon nanotubes (CNTs) have been considerably researched for their low density [22e24]. It is a very important strategy to introduce metal oxides into/onto CNTs, aiming to improve electromagnetic shielding and attenuation performance by both dielectric and magnetic loss. Hence, composites based on CNTs and various metal oxides are favored for high-performance microwave absorbing materials. For example, several metal/metal oxide nanoparticles such as Fe [25], Ni [26], Co [27] and CoFe2O4 [28] have been compounded with CNTs, to investigate their microwave absorbing properties. The results suggest that the composite strategies are effective for enhancement of EA performance. It is noted that most of the research works have focused on the nanoparticles, while two-dimensional (2D) nanosheets of metal oxides combined with CNTs have rarely been studied as EA
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materials. Recently, 2D oxides show excellent physicochemical properties [29e31], so we hope to synthesize 2D nanosheets for EA materials. Unfortunately, 2D oxide nanosheets usually meet severe aggregations. In this work, we provide an effective method of combination of CNTs and NiO nanoflakes, forming a lightweight NiO/CNTs composite. Importantly, CNT frameworks hinder the aggregation of NiO nanosheets. The composite is highly porous, and featured with a threaded structure. CNTs act as the frameworks for the growth of NiO and porous NiO nanoflakes are threaded by CNTs, therefore the hierarchically structured composite is advantageous for microwave absorption. As the results indicated, as-prepared NiO/CNTs composites show excellent microwave absorption performance. 2. Experimental 2.1. Materials Nickel nitrate (J&K Scientific Ltd.), 1-methyl-2-pyrrolidinone (NMP), sodium dodecyl phenyl sulfate (SDPS) (Aladdin Industrial Corporation), C2H5OH (Beijing Chemical Works), Uera (Sinopharm Chemical Reagent Co., Ltd). All chemicals used were analytical grade. CNTs were produced from a chemical vapor deposition (CVD) method, and were purified and weakly oxided for use [32]. 2.2. Preparation of NiO/CNTs composites To prepare composite material of NiO/CNTs, Ni(OH)2/CNTs was first prepared by an in-situ hydro-thermal method. In a typical preparation procedure, 0.4 g of sodium dodecyl phenyl sulfate (SDPS) was diluted into 50 ml super pure water, next 20 mg CNTs were added into SDPS solution under ultrasonic for 0.5 h, and 441 mg of nickel nitrate was mixed well with CNTs dispersion. After that, 3.6 g of urea solution was dropped into this suspension slowly under stirring and the mixture was transferred to a flask for oil bath at 90 C for 5 h, then the mixture was kept at 90 C for 10 h without stirring. After cooling down, the precipitate, that is Ni(OH)2/CNTs, was collected by centrifugation, washed with super pure water for three times, and freeze-dried overnight. Finally NiO/CNTs composite material was obtained from the thermal treatment of Ni(OH)2/CNTs at 400 C for 2 h. The various NiO/CNTs composites can be adjusted by adding different weights of nickel nitrate. And the pure NiO can be obtained by the same method without adding CNTs dispersion. 2.3. Characterizations SEM (scanning electron microscopy) experiments were conducted on a Quanta 200F FE-SEM. TEM (transmission electron microscopy) experiments were conducted on a JEM-2100 instrument operated at 120 kV. X-ray diffraction was conducted on a BRUKER D8 ADVANCE X-ray diffractometer using Cu-Ka radiation (l ¼ 1.54 Å). Nitrogen sorption isotherms were measured at 77 K with a Micromeritics SSA-4200 analyzer of Builder. Thermogravimetric analysis was conducted on a STA7200 (HITACHI) instrument at a ramping rate of 10 C min1 under an O2 flow. Raman spectra were performed on a Horiba Jobin Yvon Lab RAMHR800 Raman spectrometer with HeeNe laser excitation at 633 nm. X-ray photoelectron spectroscopy (XPS) of the nanocomposite was analyzed by a Thermo Fisher spectrometer with an ESCALAB 250Xi spectrometer using MgeK X-rays. 2.4. EM absorption measurement For EM absorption measurement, the samples were prepared by
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mixing NiO/CNTs and paraffin with 20% mass fraction of NiO/CNTs. The mixtures were then pressed into toroidal-shaped samples (Fout ¼ 7.00 mm and Fin ¼ 3.04 mm). The complex permittivity and permeability values were measured in the 2e18 GHz range with the coaxial line method by an Agilent N5224A vector network analyzer. 3. Results and discussion 3.1. Process of synthesis of NiO/CNTs composites The preparation process of NiO/CNTs is illustrated in Scheme 1. To grow the composites, CNTs are dispersed in an aqueous solution to form uniform networks assisted by SDPS surfactant. Then nickel nitrate was added into CNTs suspension to cultivate nanosheet seeds. An in-situ hydro-thermal reaction leaded to the growth of Ni(OH)2 nanoflakes in the CNTs dispersion. Note that this method can enhance the dispersion of CNTs and Ni(OH)2 between each other. After thermal treatment, porous NiO/CNTs composites with a threaded structure were obtained. By changing the concentration of nickel nitrate, the composition of the composites can be tuned, leading to formation of NiO/CNTs composites with various compositions. 3.2. Crystal structure and Raman analysis The XRD pattern of porous NiO/CNTs composites is shown in Fig. 1(a), indicating the crystalline structure of the synthesized sample. The typical peaks of 36.8 , 42.9 , 62.1 could be indexed as the characteristic (111), (220), (200) reflections of the NiO nanocrystal of the composite (JCPDS No. 65-2901), suggesting a successful formation of NiO nanoflakes. Although the typical peaks of CNTs at around 26 in the XRD are not obvious [33], Fig. 1(b) shows the Raman spectrum of the composite, indicating the formation of hybridization additionally. There are the characteristic carbon peaks seen at about 1330 cm1 and 1590 cm1 respectively, which are assigned to the D band for disordered and G band for graphite carbon [10,34]. The composites have a smaller value of the ratio of D-band to G-band intensity (ID/IG) compared with the CNTs, which indicates that NiO/CNTs composites have a more ordered structure with less structure defect. Compared with pure NiO, there is no obvious peak at around 508 cm1 for the NiO/CNTs composites. 3.3. Morphological characterization The morphologies of the NiO/CNTs composites were characterized by the SEM. They were compared with pure NiO nanosheets. As shown in Fig. 2(a), as-prepared pure NiO nanosheets have uniform size; however, they are easily stack together to tight aggregates. Compared with pure NiO, the NiO nanosheets of the nanocomposites have good dispersion morphologies. There are no obvious aggregations of the nanosheets even though the NiO components are increased to 95 wt%, as seen in Fig. 2(b). In Fig. 2(c), it displays clearly the interconnected 3D network microstructure of 85 wt% NiO/CNTs composites. This finding indicates efficient assembly between the NiO flakes and CNTs during the hydro-thermal treatment, which can lead to uniform formation of nanocomposites. When the NiO components are decreased to 75 wt% of Fig. 2(d), the CNT networks are more clear which can enhance the growth of NiO flakes and prevent their stacks [35]. The SEM and TEM images of Fig. 3(a) and (b) reveal that the NiO flakes are threaded by CNTs, forming the integrated structure. Also, the high-resolved TEM image shows that as-formed NiO flakes are porous morphology (Fig. 3(c)). In Fig. 3(d), the clear lattice fringes of NiO nanorings illustrate that the interplanar spacing is about 0.21 nm, corresponding to (200) planes of face-centered cubic NiO.
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Scheme 1. Process of synthesis of NiO/CNTs composites. (a) The CNT dispersion, (b) the formation of Ni(OH)2 flakes threaded by CNTs after in-situ hydro-thermal reaction (step Ⅰ), (c) the porous NiO/CNTs composites after calcination (step Ⅱ).
Fig. 1. XRD patterns (a), Raman spectra (b) of CNTs, NiO and NiO/CNTs composites.
The elemental mapping images of Fig. 3(e) further confirm that NiO flakes are penetrated by CNTs, suggesting the good dispersion. 3.4. XPS analysis and BET surface areas The XPS survey spectrum of Fig. 4 shows the C1s, O1s and Ni2p peaks. The typical C1s peak was observed at 283 eV and O1s peak was at 530 eV; while the Ni2p spectrum presents two main structures resulting from the spin-orbit splitting of the p orbital which are assigned as Ni2p3/2 (850e870 eV region) and Ni2p1/2 (870e890 eV region). Ni2p3/2 presents a main peak at ~856 eV with an intense satellite structure at ~862 eV, (~874 eV and ~880 eV respectively for Ni2p1/2) which is characteristic of Ni2þ. All those results suggest that NiO flakes were successfully prepared in the composites. The surface areas and pore sizes of NiO/CNTs nanocomposites were investigated with nitrogen adsorption measurement (Fig. 5). A surface area of 106 m2 g1 can be obtained for pure NiO. It clearly illustrates much larger amount of N2 gas adsorbed by the NiO/CNTs samples with the increasing of the percentage of CNTs (Fig. 5(a)). The Brunauer-Emmett-Teller (BET) surface area of composite increases from 139 m2 g1 to 198 m2 g1 when decreasing the NiO content from 95 wt% to 75 wt%. The increased surface areas of the
composites also suggested the enhanced dispersion of NiO nanoflakes due to the introduction of CNTs. The pore sizes of pure NiO nanosheets were averaged at 2.0 nm; while the nanocomposites of NiO/CNTs exhibit similar pore sizes which are smaller than pure NiO (Fig. 5(b)). It can be verified that all of the composites have micropores with the average radius of 1.7e2.0 nm. The pore structure is expected to enhance the microwave absorption [20,36e38]. 3.5. EM absorption properties of NiO/CNTs composites The EM parameters shown in Fig. 6(a)e(f) (relative complex permittivity εr ¼ ε0 jε00 , and relative complex permeability mr ¼ m0 jm00 ) of the composites with different NiO loading in a wax matrix were measured in the range 2e18 GHz to investigate the microwave absorbing properties of NiO/CNTs composites. As is known, the real permittivity (ε0 ) and the real permeability (m0 ) represent the ability to store electric and magnetic energy, while the imaginary permittivity (ε00 ) and the imaginary permeability (m00 ) represent the dissipation of electric and magnetic energy [39,40]. As shown in Fig. 6(a), the ε0 values of 95 wt% NiO/CNTs show negligible fluctuations in which its ε0 is around 8. However, the ε0 values of 75 wt% and 85 wt% NiO/CNTs decrease with
J. Wang et al. / Journal of Alloys and Compounds 689 (2016) 366e373
Fig. 2. The SEM images of NiO (a), 95 wt% NiO/CNTs composites (b), 85 wt% NiO/CNTs composites (c) and 75 wt% NiO/CNTs composites (d).
Fig. 3. The SEM image (a), the TEM images (b), (c), (d) and elemental mapping image (e) of 85 wt% NiO/CNTs composites.
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Fig. 4. XPS spectra of the NiO/CNTs composites: wide scan (a), Ni2p spectrum (b), O1s spectrum (c) and C1s spectrum (d).
Fig. 5. N2 sorption isotherms and BJH pore size distribution of pure NiO and the NiO/CNTs composites with NiO loading of 75, 85, and 95 wt%.
increasing the frequency or increasing the content of NiO. The ranges of their ε0 are about 20 to 9, which are significantly larger than that of 95 wt% NiO/CNTs nanocomposite. Fig. 6(b) shows the frequency dependence of ε00 . The value of ε00 is obviously enhanced with the decreasing of NiO loading. It can be seen that ε00 values of 95 wt% NiO/CNTs nanocomposite are close to zero, indicating very poor dielectric loss. The range of ε00 for 75 wt% NiO/CNTs and 85 wt% NiO/CNTs is 9.7 to 8.1 and 7.7 to 5.9, respectively. The ε0 decreases with increasing frequency can be explained by Debye theory equation and the decrease of ε0 and ε00 with increasing of NiO loading can be illustrated rationally according to the effective medium theory [41]. Fig. 6(c)e(d) show m0 and m00 of all NiO/CNTs composites in the range of 2e18 GHz. The m0 values of 95 wt% NiO/CNTs are decreased with increasing frequency. But for 75 wt% and 85 wt% NiO/CNTs, they show the slowly decreased m0 values and then the values
abruptly increase. This may be attributed to crystallinity and size of NiO [35]. Meanwhile, the m00 values of all NiO/CNTs composites keep decreasing with increasing frequency. And the m00 are negative in a specific range of frequencies, demonstrating there is a resonance phenomenon between m0 and m00 [42]. The dielectric loss tangent (tandE ¼ ε00 /ε0 ) and the magnetic loss tangent (tandM ¼ m00 /m0 ) are also calculated based on the complex permeability and permittivity measured as above. As shown in Fig. 6(e) and (f), with the decreasing of NiO loading, the dielectric loss factor is improved, indicating that dielectric loss is increasing [43]; but the value of tandM shows a contrary trend to the tandE. On the other hand, the difference between tandE and tandM of 95 wt% NiO/ CNTs is bigger than 75 wt% and 85 wt% NiO/CNTs. Therefore, 75 wt% and 85 wt% NiO/CNTs should have better impedance matching. According to transmission line theory, when EM waves are normally incident on a layer terminated by a conductor, the
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Fig. 6. Frequency dependence of real part (a) and imaginary part (b) of relative complex permittivity, real part (c) and imaginary part (d) of relative complex permeability, and dielectric loss tangent (e) and magnetic loss tangent (f) of NiO/CNTs composites.
reflection loss (RL) can be calculated by the following equations [44,45]:
Zin ¼
rffiffiffiffiffi mr 2p pffiffiffiffiffiffiffiffiffi fd mr εr tanh j εr c
Z 1 RL ¼ 20 log in Zin þ 1
(1)
(2)
where Zin is the input impedance of the absorber, mr and εr are respectively the relative complex permeability and permittivity, f is the frequency of microwave, d is the thickness of the absorber, and c is the velocity of light in free space. Fig. 7 shows the calculated RL of NiO/CNTs composites with different thicknesses. Compared with the pure NiO (Fig. 7(a)), the composite of NiO/CNTs shows much better absorption properties. It can be observed that the frequency at the minimum RL gradually shifts towards lower frequency with increasing thickness. And the relationship between the RL peak frequency and the absorber thickness can be explained by the quarter-wavelength cancellation
[46]. The minimum RL values of 75 wt% NiO/CNTs composites with thicknesses of 2 mm and 95 wt% NiO/CNTs composites with thicknesses of 5 mm are all less than 15 dB, which suggests that 97% absorption of incident microwaves can be achieved. Furthermore, the minimum RL values of 85 wt% NiO/CNTs composites with thicknesses of 2e5 mm are all below 17 dB (98% absorption) and the minimum value is 25.4 dB at 10 GHz with the thickness of 2 mm. The microwave absorption properties of this nanocomposite are compared to those of reported Ni-based composites in Table 1. The nanocomposite materials obtained in this work show reasonable absorption properties. As discussed above, the as-prepared NiO/CNTs composites show excellent absorption ability in the range of 2e18 GHz and the minimum RL can be adjusted to different frequencies by varying the composition and the thickness of the absorber. 3.6. Possible microwave absorbing mechanism The possible microwave absorbing mechanism could be explained by the following factors: dielectric loss and multiple reflections [50e52], as shown in Fig. 8. The strong dielectric loss is
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Fig. 7. Reflection loss of pure NiO (a) and NiO/CNTs nanocomposite with NiO loading of 75 wt% (b), 85 wt% (c), and 95 wt% (d) at different thicknesses from 1 to 5 mm.
Table 1 Properties of microwave absorption of as-prepared NiO/CNTs nanocomposite compared with reported composite materials. Material
Minimum RL value (dB)
d (mm) (RL<10 dB)
Frequency range (GHz) (RL<10 dB)
Ref.
Ni/graphene Ni/MWCNTs Ni/MWCNTs Ni@C Ni/polypyrrole Ni/polyaniline Co/Ni/CNTs NiO/CNTs
11.8 37 37.9 32 15.2 23 24 25.4
3 5.2 4 2 2 1 1.5 2
6e7 1e4 5.7e8.0 12.5e13.5 11e15.4 16e18 12.4e18 8.7e11.4
[35] [10] [43] [47] [48] [49] [27] This work
scattering of the incident microwaves, which may result in the attenuation of electromagnetic (EM) energy [54e56]. Above all, the EM energy could be dissipated rapidly by interference in the form of thermal energy, on account of the high thermal conductivity of the composites. 4. Conclusion
Fig. 8. A possible microwave absorbing mechanism of the NiO/CNTs nanocomposites.
mainly due to electron polarization relaxation and interfacial polarization. The high conductivity of CNTs leads to the formation of more charges and dipoles, and promotes electron polarization and relaxation processes [53]. Meanwhile, NiO sheets threaded by CNTs also play an extremely important role, which can strengthen interfacial polarization between nanosheets and CNTs. In addition, the porous layered NiO brings about multiple reflections and
In summary, the lightweight NiO/CNTs composite has been successfully synthesized via the thermal decomposition of Ni(OH)2/ CNTs made from an in-situ growth method. The as-prepared NiO nanoflakes are porous and penetrated with the CNTs. Based on the rational combination of these two, the composite displayed enhanced microwave absorbing performance, and also had lightweight properties. The minimum RL reaches 25.4 dB when the NiO loading is 85 wt% with thickness of 2.0 mm at 10 GHz. Also, the EM wave absorption property can be adjusted easily by varying the thickness of the samples and the NiO loading. It is believed that NiO/CNTs composites could be a potential kind of excellent microwave absorbing material with lightweight and strong absorption.
J. Wang et al. / Journal of Alloys and Compounds 689 (2016) 366e373
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