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Direct intertube crosslinking of carbon nanotubes at room temperature Yunxiang Gao, Hongwei Chen, Jun Ge, Jingna Zhao, Qingwen Li, Jian-Xin Tang, Yi Cui, and Liwei Chen Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.6b03184 • Publication Date (Web): 22 Sep 2016 Downloaded from http://pubs.acs.org on September 24, 2016
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Direct intertube crosslinking of carbon nanotubes at room temperature Yunxiang Gao,1, † Hongwei Chen,1,† Jun Ge,1 Jingna Zhao,2 Qingwen Li,2 Jianxin Tang,3 Yi Cui4, 5 and Liwei Chen1, 6* 1
i-Lab, CAS Center for Excellence in Nanoscience, Suzhou Institute of Nano-Tech
and Nano-Bionics (SINANO), Chinese Academy of Sciences, Suzhou 215123, China 2
Division of Advanced Materials, SINANO, Chinese Academy of Sciences, Suzhou
215123, China 3
Institute of Functional Nano and Soft Materials (FUNSOM), Soochow University,
Suzhou 215123, China 4
Departments of Materials Science and Engineering and Department of Chemical
Engineering, Stanford University, Stanford, California 94305, United States 5
Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator
Laboratory, Menlo Park, CA 94025 6
Vacuum Interconnected Nanotech Workstation, SINANO, Chinese Academy of
Sciences, Suzhou 215123, China
† These authors contributed equally to this work * To whom correspondence should be addressed:
[email protected] 1
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KEYWORDS: Carbon nanotubes, Intertube crosslinking, Radical chemistry, Mechanical strength
ABSTRACT: Carbon nanotubes (CNTs) have long been regarded as efficient free radical scavenger because of the large-conjugation system in their electronic structures. Hence, in spite of abundant reports on CNT reacting with incoming free radical species, current research has not seen CNT itself displaying the chemical reactivity of free radicals. Here we show that reactive free radicals can in fact be generated on carbon nanotubes via reductive defluorination of highly fluorinated single-walled carbon nanotubes (FSWNTs). This finding not only enriches the current understanding of carbon nanotube chemical reactivity, but also opens up new opportunities in CNT-based material design. For example, spacer-free direct intertube crosslinking of carbon nanotubes was previously achieved only under extremely high temperature and pressure or electron/ion beam irradiation. With the free radicals on defluorinated FSWNTs, the nanotubes containing multiple radicals on the sidewall can directly crosslink with each other under ambient temperature through intertube radical recombination. It is demonstrated that carbon nanotube fibers reinforced via direct crosslinking displays much improved mechanical properties.
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Carbon nanomaterials have become an extremely active research field. Surface modification and heteroatom doping have resulted in materials with much-enriched properties.1
For example, nitrogen-doped carbon nanotubes (N-CNTs) have shown
electrocatalytic activity for metal-free oxygen reduction reactions (ORR) in fuel cell applications,2 and oxygen modified CNTs (O-CNTs) become active in catalyzing oxidative dehydrogenation of alkanes.3 To a large extent, it is the charge transfer and redistribution due to the doping elements that enable the novel properties of these materials.4 In light of this, fluorinated carbon nanotubes constitutes another type of surface modified carbon nanotube derivatives.5 Previous studies of fluorinated singlewalled carbon nanotubes (FSWNTs) focused on applying them as precursors in further chemical functionalization of CNTs.6-7 Interestingly, the high electronegativity of F on the curved surface of SWNTs leads to relatively low energy barrier for C-F bond dissociation and good leaving ability of fluoride ions.8 This feature may lead to unique properties for future carbon nanomaterials design and application. Crosslinking of CNTs has been a major approach for enhancing mechanical properties of CNT-based materials. Present literature indicates that direct intertube covalent crosslinking of CNTs requires either high pressure,9-10 high temperature and high pressure,11 or strong electron/ion beam irradiations,12-15 some of which even requires a temperature higher than 2000 K16-17 or a pressure higher than 35 GPa.18 While crosslinking of SWNTs has indeed been shown to strengthen the material in its bulk form, 19 due to the highly demanding conditions, it is still not possible to produce other forms of directly crosslinked SWNT materials such as porous films. Thus, it is critically important to achieve direct intertube crosslinking of CNTs under mild conditions.
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In this work, we show that carbon nanotubes can be directly crosslinked via intertube covalent bonds under room temperature through reductive defluorination of FSWNTs. While pristine CNTs have long been regarded as efficient free radical scavengers due to their large delocalized π-electron conjugation systems,20-21 the πconjugation systems on highly fluorinated FSWNTs (with a C:F ratio being close to 2:1) are severely disrupted.22 This feature allows for the generation of localized reactive free radicals on the sidewall of carbon nanotubes; and the CNT free radicals are demonstrated to be capable of initiating radical polymerization and direct intertube crosslinking via radical recombination. CNT nanofibers and films after fluorination and reduction defluorination treatments are shown to have unique directly crosslinked structure and much enhanced mechanical properties. The novel property of FSWNTs provides a new tool in CNT-based materials design. Reductive defluorination of FSWNTs is first observed under UV-irradiation. FSWNT (C:F ≈ 2:1) solutions in various organic solvents with concentration about 0.5~2 mg/ml (depending on the solubility) were irradiated in a micro-photochemical reactor with a UV lamp (Pen-Ray, 5W) under N2 gas protection. After two hours irradiation, CNTs precipitated out of the solution. The precipitated CNTs were collected, rinsed with ethanol and acetone, and dried in vacuum for Fourier transform infrared (FTIR) characterization and elemental analysis with scanning electron microscope (SEM)-energy dispersive X-ray microanalysis (EDX). Compared to the control experiment in which FSWNT materials had gone through the same procedure except for UV irradiation, fluorine content in UVirradiated FSWNT samples decreased greatly according to SEM-EDX (Table 1). Since only a trace of nitrogen was detected for samples irradiated with UV light in N-
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containing solvents, we conclude that most fluorine atoms were eliminated from the nanotubes without substitution of solvent molecules. Table 1. Elemental composition of FSWNT samples treated under various conditions measured with EDX.
Representative Observation
Solvent (DN*)
Conditions
Composition(at%) C
O
F
N
In dark, fresh
63
4
33
-
In dark, a month
68
4
28
-
ACN (14.1)
UV, 2 hrs
77
7
16
