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Excellent Microwave Absorbing Property of MWCNTs with SkinCore Heterostructure Formed by Outer Dominated Fluorination Yang Liu, Yichun Zhang, Xu Wang, Zaoming Wang, Wenchuan Lai, Xiaojiao Zhang, and Xiangyang Liu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b10819 • Publication Date (Web): 21 Feb 2018 Downloaded from http://pubs.acs.org on March 1, 2018
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The Journal of Physical Chemistry
Excellent Microwave Absorbing Property of MWCNTs with Skin-Core Heterostructure Formed by Outer Dominated Fluorination Yang Liu,a Yichun Zhang,a Xu Wanga, Zaoming Wang,a Wenchuan Lai,a Xiaojiao Zhanga and Xiangyang Liua* a
College of Polymer Science and Engineering, State Key Laboratory of Polymer Material and
Engineering, Sichuan University, Chengdu 610065, People's Republic of China. ABSTRACT: It is attractive that excellent microwave absorbing materials with novel structure is prepared. In this paper, we present a novel skin-core strctured fluorinated multiwalled carbon nanotubes (MWCNTs) microwave absorbing material through selective fluorination of outer shell of MWCNTs. Fluorine gas (F2) can attack defect sites to form CF2 at defect sites at low temperature. The preformed CF2 bond can prevent the permeation of F2 into inner tubes, bringing about the mere fluorination of outer skin with inner tubes intact at high temperature (the sample is denoted as F-h-MWCNTs). As a result, composed of outer insulating skin and inner conductive tubes, F-h-MWCNTs behaves skin-core heterostructure and shows excellent microwave absorbing property with minimal reflection loss up to -69 dB at thickness of 1.5 mm when the filler loading is only 9.1 wt.%. Based on the investigation of electromagnetic behavior of fluorinated single-walled carbon nanotube (SWCNT), the strong microwave absorption capacity of F-h-MWCNTs was attributed to the skin-core heterostructure. The outer skin is fluorinated layer behaving good impedance matching, which is conducive to the transmission of
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electromagnetic wave. The inner tubes maintain intact after direct heating fluorination and show strong ability of attenuating electromagnetic wave. Meanwhile, F-h-MWCNTs possess better dispersibility in epoxy, making F-h-MWCNTs as proper candidate to prepare advanced composite with strong absorption and good mechanical property. 1. INTRODUCTION In the past decades, serious electromagnetic wave pollution especially in the gigahertz (GHz) band ranges has aroused great concern in civil and military fields on account of the widespread utilization of electronic device and communication facilities.1 The electromagnetic wave radiation and interference not only disturb normal communication, but also pose a great threat to health of human body.2-4 As a consequence, microwave absorption (MA) materials have received increasing attention. Traditional MA materials such as ferroelectric, ferrites and carbides are restricted by the deficiency of high density, large thickness and high concentration.5 As a result, the design and fabrication of effective absorbers with low density, low thickness and strong absorption are fascinating. Carbon nanotubes (CNTs) are one-dimensional quantum wire with remarkable electrical conductivity, exceptional thermal conductivity and superior mechanical properties.6 Due to its unique tubular nanostructure with extremely large aspect ratio, CNTs possess larger excluded volume, smaller percolation threshold and lower additive amount to achieve the same conductivity level compared with carbon black.7 These advantages endow CNTs with great potential as lightweight microwave absorbers. Specially, multi-walled carbon nanotubes (MWCNTs) are multilayer coaxial tubes with more complicated structure. It can be viewed as several shells in parallel, inducing crosstalk effect between adjacent shells which is favorable to the MA property.8
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According to MA theory, MA property is highly dependent on impedance matching between absorbing materials and free space, which is generally indicated by impedance matching ratios Z defined as Zvalue=(µr/εr)^1/2.3 To realize impedance matching, it is critical to achieve the match between complex permeability (µr) and permittivity (εr) of absorber. Pure CNTs with single composition and structure, however, behave improper electrical conductivity and poor impedance matching, leading to limited MA property with reflection loss value always higher than -10dB.9 As a result, further modification of CNTs is always necessary to enhance MA property. Most attentions have been focused on construction of heterostructure by decorating CNTs with magnetic particles. The modification methods include filling10-15 or coating16-24 hollow CNTs with nanoparticles or nanowires. Some special structure such as flower-like,23 grape-like,5 tree-like25 and rambutan-like26 heterostructure have also been reported. The introduced nanoparticles or nanowires can induce multiple scattering and interfacial polarizations, thus leading to the enhanced MA property. It's worth noting that most researches in regard to hybrid methods show minimal reflection loss in the range between -20 and -55 dB, while few reports can achieve more superior MA property. Besides, the hybrid methods need sophisticated structure control and complicated process,27 and the unfirmed hybrid structure may probably be destroyed by harsh conditions such as ultrasonication and high temperature.7 These imperfections limited their practical applications in MA field. Another approach was focused on assemblies and orientation of CNTs reported by Peng recently. Aligned CNT films are used as MA absorbers and the MA property can be regulated by varying the intersectional angles and stacked number of aligned CNT films.28 Theoretically, MWCNTs behaves good electrical conductivity and it brings about high permittivity and poor impedance matching. When incident microwave arrive at the outer layer of
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MWCNTs, it is reflected without propagating into the inner tubes, meaning that the inner tubes make little contribution to dissipate electromagnetic wave. As a result, the realization of impedance matching of outer layer of MWCNTs on the premise of keeping inner tubes intact may be a possible way to enhance the overall absorption features. Direct fluorination is a powerful method to modify CNTs due to its convenient, economic and efficient preparation process. The attachment of fluorine atoms onto MWCNTs provides a pathway to tailor electrical conductivity and permittivity.29-31 Especially, our recent result manifested that fluorination can reduce dielectric constant of carbon nanomaterials and their polymer composite, which is beneficial to the improvement of impedance match.32 As a result, direct fluorination has great potential as novel modified method to enhance the impedance match of MWCNTs. In general, fluorination can be carried out effectively utilizing atomic fluorine (XeF2) or molecular fluorine (F2) as fluorination agents. F2 always result in fluorinated domains less homogeneously distributed on fluorinated CNTs than XeF2.33-34 As a result, in terms of controllable fluorination, the use of F2 is more feasible. Fluorination utilizing F2 is highly dependent on temperature. Generally, high temperature is always necessary to attain high fluorination degree, a guarantee of low permittivity and consequently high impedance match. For example, fluorination at temperature 150 °C results in C10F ratio. The higher temperatures to 250 °C can increase fluorination degree to C2F.35 However, fluorination at high temperature as 400-500 °C brings about dramatic changes in structure and ultimately destruction of the nanotubes.36 As a result, excessive temperature is not beneficial to the maintainment of inner tubes in the process of fluorination. Meanwhile, fluorine addition patterns are also closely related to the quality of staring material. Although Kawasaki and Muramatsu claim that only outer side of tube wall or outer tube can be under attack by F2 for
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perfect single-wall carbon nanotube (SWCNT) or double-walled
carbon nanotubes
(DWCNTs),37-38 in many cases, defects intrinsically exist in CNTs. For traditional fluorination approach conducted only at high temperature, F2 can permeate into inner tubes along defects,39 leading to the fluorination of inner side of tube wall in SWCNT.40 For example, S. Kawasaki found that F2 can access inner side of open-end SWCNT and realize the fluorination of inner side of SWCNTs.37 S. Kawasaki even reported endo-addition pattern of open-end SWCNT at low temperature of fluorination.41 Thus, it is challenging and meaningful to realize selective fluorination of outer shell of MWCNTs with high content of fluorine (F-content) due to the existence of defects in MWCNTs and high activity of fluorine gas. Our previous study found that defects in MWCNTs and graphene are more vulnerable to F2 in relative to aromatic benzene ring at low temperature.42-43 Consequently, carbon atoms at defect sites can be easily transformed to CF2 even at room temperature of fluorination, resulting in the narrowed defect size and decrease of specific surface area.44 Based on these results, in this work, a two stage of direct heating fluorination process was presented to realize the controllable fluorination of outer shell of MWCNTs. At low temperature stage, low active F2 can identify and attack defect sites. At high temperature stage, the preformed CF2 bond at defect sites can prevent the permeation of F2 into inner tubes, resulting in mere fluorination of outer skin with the inner tubes intact. As comparison, direct isothermal fluorination of CNT at high temperature was also conducted. In this case, high active F2 can pass through tube wall, leading to the simultaneous fluorination of outer shell and inner tubes. Benefiting from the special skin-core heterostructure that is composed of outer fluorinated shell and intact inner tubes, the direct heating-fluorinated sample behave excellent MA property, which is superior to its counterpart prepared through direct isothermal fluorination. The function of fluorinated outer shell and intact inner tubes were
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investigated by exploring the electromagnetic behavior of fluorinated SWCNT (F-SWCNT). Besides, the improved dispersibility of F-MWCNTs in epoxy makes it possible to prepare advanced composite with strong absorption and good mechanical property. 2. EXPERIMENT 2.1. Materials. SWCNT (
