Effective disentangling method of bundled multi-walled carbon nanotubes into individual multi-walled carbon nanotubes by magnetic-field induction
Graphical abstract
Introduction
Following the discovery of carbon nanotubes in 1991 by Iijima [1], multi-walled carbon nanotubes (MWCNTs) have been developed and are considered as promising electro-materials, because of their intrinsic low density and superior electron-transfer properties [2], [3], [4], [5], [6]. Although carbon nanotube (CNT) materials have many properties that make them ideal for use in next-generation electronics, they have the disadvantage of being difficult to isolate. This is because MWCNTs synthesized by common chemical vapor deposition (CVD) methods agglomerate very easily and randomly due to the van der Waals interactions among them [7]. Individually separated MWCNTs might be of use for electron-transfer polymeric composites [8] because of their high conductivity and strong tensile strength. Generally, bundled CNT agglomerates cause several defects in such complexes, which can adversely affect their physiochemical properties [9]. The rapid development of advanced separation tool kits has allowed integrated compartments to be selectively isolated under several physiochemical conditions. Many studies have focused on obtaining individual nanotubes [2], [3], [4], [5], [6].
Various CNT isolation methods have been developed to address this issue [10], [11], [12], [13], [14], most of which are based on simple physical and chemical methods such as ultra-sonication, grinding, or chemical acidic and chaotropic treatments. Ball milling has been shown to minimize the length and diameter of MWCNTs, resulting in the enhancement of the gas absorption capabilities [15], [16]. In addition, most chemical treatments improve the dispersion of the MWCNTs due to the strong repulsion among the various functional groups on their surfaces [17], [18], [19]. Some researchers have used physical and chemical methods simultaneously. However, none of these approaches has resulted in the large-scale production of intact individual MWCNTs for commercial applications. Ball milling and ultrasonic waves have been shown to have severe adverse effects on the electrical and mechanical properties of MWCNTs. Chemical treatment methods, such as the addition of strong acids, destroy the surface conformations of the MWCNTs, resulting in a sudden reduction in their electric performance [11]. Surfactants used as chaotropics that have not been removed make it difficult to isolate single MWCNTs with high purity, whereas MWCNTs mixed together with polymer bases such as epoxy are only partially distributed in solution [20]. A recent study reported that MWCNT agglomerates placed in liquid crystals and exposed to electric currents separated into strands along the applied electric current [21]. These MWCNT agglomerates were further exposed to excessive critical voltages to supplement the strong tensile forces, and the CNT agglomerates started to elongate above a certain field and continued to elongate up to nearly 400% in the linear region with a large electro-active constant of 70 (V/μm)−1. Furthermore, some individual CNTs have been extracted from the MWCNT clusters above the breakdown field [21]. However, the liquid crystal ingredients used in the previous report [21] did not yield a large quantity of individual CNTs, were expensive at 3 USD per gram, and could not easily be reused. In the study of magnetic field dispersion and magnetic field induced alignment after mixing CNTs which have functional group introduced liquid crystal [23], and in a study to obtain the optimum ratio of enthalpy and entropy by exposing composites to a magnetic field, the composite was synthesized by mixing polyester with CNTs which were dispersed by sodium dodecyl sulphate [SDS] and coated Fe2CO3 by adding FeCl3 and NH3 [24]. Bagheri et al. [25], [26] conducted a study on the facile stripping voltammetric determination of a specific medicine using a high performance magnetite/carbon nanotube paste electrode; however, this cannot be realized as a commercial CNTs dispersion technique. Therefore, in the present investigation, we aim to present a novel and effective method of disentangling MWCNT bundles using a magnetic field induction technique [22].
Section snippets
Theoretical background
Fig. 1 shows a diagram of the CNT disentanglement principle. When magnetic particles adhere to the CNT agglomerates located along the direction of the magnetic field, they are exposed to a force in the same direction. The force affecting a single paramagnetic particle in a magnetic field according to electromagnetic field theory can be described by Eq. (1).In this formula, U = −m·B, is the energy between the magnetic field and the magnetic dipole, B is the intensity of the magnetic force
Preparation of MWCNTs
We used the CVD method to synthesize the MWCNTs, because this method yields uniform MWCNTs with characteristic temperature distributions and a high heat-transfer efficiency. The mechanisms of the adhesion of magnetic particle-CNTs to the radical initiator, 4,4′-azobis(4-cyanovaleric acid) (Aldrich Chemistry, CAS:2638-94-0, V-501) have previously been described in detail [31]. The magnetic particles used in our experiments were prepared using ferrous chloride and ferric chloride, and had an
Results and discussion
Fig. 6 shows the force measurement results obtained from separating a single CNT strand on the agglomerate surface and a sub-agglomerate with a diameter of 20 μm from the agglomerate to collect the basic data required to calculate the force needed to disentangle the CNT agglomerates. The maximum force that could be used to separate a single strand from the agglomerate was 40 nN and that which could be employed to split the sub-agglomerate from the agglomerate was 8 μN. The single strand was not
Conclusions
We developed a feasible, cost-effective method of disentangling as-synthesized CNT agglomerates involving exposure to a 1 T magnetic field. Using microscopy, we demonstrated that the CNTs were separated from the bundles at a working force of around 40 nN, preventing damage to the individual CNTs. This experimental result was supported by advanced Opera 3D simulation results. We speculated that the single MWCNTs synthesized in this way could have potential to serve as anti-electromagnetic
Acknowledgements
This work was funded by a grant from the National Research Foundation of Korea (NRF), funded by the Korean Government (MEST) (No. 20120002260).
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