A. C. Kleinschmidtab,
R. K. Donato*b,
M. Perchaczc,
H. Benešc,
V. Štengld,
S. C. Amicoa and
H. S. Schrekker*b
aComposite Materials Group – GCOMP, Polymer Materials Laboratory – LAPOL, Universidade Federal do Rio Grande do Sul – UFRGS – Porto Alegre-RS, Brazil
bLaboratory of Technological Process and Catalysis – Tecnocat, Institute of Chemistry, Universidade Federal do Rio Grande do Sul – UFRGS – Porto Alegre-RS, Brazil. E-mail: henri.schrekker@ufrgs.br; donatork@gmail.com
cDepartment of Polymer Processing – Institute of Macromolecular Chemistry – IMC, AS CR v.v.i, Prague, Czech Republic
dDepartment of Solid State Chemistry, Institute of Inorganic Chemistry, AS CR v.v.i., Prague, Czech Republic
First published on 29th August 2014
This work describes a straightforward procedure for the preparation of graphene by opening multi-walled carbon nanotubes (CNT), using ionic liquids (IL) as lubricating and stabilizing agents. The sequential application of vacuum and sonication allows the successful opening and unrolling of the CNT, and the final nanocarbon morphology is IL-dependent. This enabled the preparation of epoxy-based nanocomposites with morphologically distinct carbon nanofillers. The CNT–IL mixtures and nanocomposites obtained were characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM), atomic force microscopy (AFM) and Raman spectroscopy.
Within this class of materials, carbon nanotubes (CNT) were only discovered in 1991 in a Matrioshka doll shape (formed by several concentric tubes) or as bundles (rolled graphene sheets),5 and in 1993 the application of catalysts allowed the formation of CNT constituted of a single wall.6,7 Interestingly, despite being known since the 1940s, graphene (a bidimensional layer of sp2 carbons)1 was considered as an “unrealistic” material due to its production hurdles and low thermodynamic stability at ambient conditions. In 2004, the work of Geim and Novoselov changed this perspective, showing that graphene was not only a “possible” material but also accessible, if alternative approaches were applied for its obtention.4,8,9 Thereafter, the production and use of graphene became of strategic interest, representing a promise of breakthroughs in several areas, especially materials sciences. Despite the excitement caused by the advances in this area and the relative abundance of prime material for its formation, the road to reach the production of graphene based materials as a commodity is still long.10
The biggest challenges for obtaining and using graphene are the low efficiency of the available methods. Several top-down and bottom-up methods have been demonstrated in the literature, but a good balance of productivity, quality and cost is a goal still to be reached.4,10 A feasible option for obtaining graphene nanoribbons with regular sizes is through “unzipping” or “unrolling” of CNT,11–14 but mostly these procedures use a method commonly applied for graphene preparation that involves an aggressive oxidation, creating dispersed graphene oxide (GO) sheets or ribbons (Hummer method).15 Herein, the major drawback is the use of strong acids (H2SO4, HNO3) and oxidizing agents (KMnO4, CrO3) for GO formation, which is then exposed to strong reducing agents (NaBH4), hydrazine vapour or hydrogen at high temperatures (600–800 °C). The inconveniences of this process, together with the formation of oxidation debris, make it unsuitable for large-scale production.16
The search for milder conditions and higher graphene yields led to several procedures,11,14,17,18 among which the irradiation of graphite or CNT dispersions in solvents and/or surfactants presented significant results.19–21 The main parameter for successful exfoliation is in the properties of the solvents/surfactants used, which need to provide enough solvent/surfactant–graphene interaction for the expansion of the graphene layers.22 However, generally the mentioned procedures involve the usage of volatile organic solvents, which present problematic environmental issues as well as technological inconvenience regarding the potential application of graphene for nanocomposite preparations.
Recently, the application of ionic liquids (IL) has shown promising results for structural control of nanocomposites.23,24 IL are organic salts with low melting points; often they are liquid at room temperature and present several intrinsic properties, e.g., low volatility, high thermal and chemical stability, insignificant flammability, good thermal conductivity, high ionic mobility and moisture resistance.25–27 The imidazolium-based IL present these properties together with a strong capacity for interacting with carbon based materials. A “π–π stacking” of the IL cations at the π-electronic surface of the sp2 carbon network allows it to act as both a solvent and a surfactant due to the IL's amphiphilic structure.28 These features make IL ideal for exfoliating/stabilizing graphene through a solution irradiation process, allowing broader experimental conditions, especially as volatile organic solvents have the drawback of limited working temperature ranges.
In this work we present a mild process (relatively low energy irradiations and temperatures) to prepare high quality, oxidative debris free, graphene nanoribbons and sheets from unrolling multiwalled carbon nanotubes (MWCNT) in IL media, which were used to prepare reinforced epoxy–graphene nanocomposites with minimum filler content (∼0.1 wt%). Vadahanambi et al. reported the use of an IL-assisted splitting method under microwave radiation for the production of graphene nanoribbons from multi-wall or single wall carbon nanotubes.29 Differently from our approach, this method is based on the decomposition of the BF4 IL anion, using the generated fluorine as the “splitting agent” for the CNT. The main differences between the two processes are the radiation source (microwave vs. ultrasound) and the nature of the process itself (chemical vs. physical modification). The NTf2 IL anion used in our process is thermally and chemically much more stable, allowing graphene formation and stabilization without decomposition of either CNT or IL.
Four different routes were defined, varying heating time, stirring under vacuum and sonication time of the CNT–IL (1:
10 mass ratio) mixtures. The routes were organized in a way to gradually observe the relation between energy applied and efficiency of CNT unrolling, as well as the individual contributions of temperature, mechanical stirring and ultrasound application: (a) 1 h of heating (100 °C) and mechanical stirring under vacuum, followed by 90 min of ultrasound application at room temperature; (b) 1 h of heating and mechanical stirring under vacuum, followed by 3 h of ultrasound application at room temperature; (c) 3 h of heating and mechanical stirring under vacuum, followed by 90 min of ultrasound application at room temperature; (d) 3 h of heating and mechanical stirring under vacuum, followed by 3 h of ultrasound application at room temperature. All the procedures were divided into 3 cycles for each part, i.e., 3 h heating/vacuum followed by 3 h sonication means 3 cycles of 1 h of heating/vacuum followed by 3 cycles of 1 h of sonication.
The fundamental importance of the IL in this process is evidenced by comparing the TEM images of the original CNT (Fig. 1e) with the ones from a CNT–toluene mixture (Fig. 1f) and CNT–C4MImNTf2 (Fig. 1d), both applied to the optimized procedure.
Toluene was used due to its electronic resemblance with the CNT surface, which could promote the disruption of the π-packing from the CNT bundles. Nevertheless, the CNT–toluene mixture presents no structural difference with the original CNT, despite some partially swelled superficial layers and smaller bundle sizes due to the effect of dilution (Fig. 1e vs. f). This evidences the unique role of the IL in CNT unrolling (Fig. 1e vs. d).
In an attempt to elucidate the process through which the CNT loses its original structure, AFM images of CNT in different unrolling stages were taken. Apparently, a process of CNT swelling–unrolling is taking place during sonication in IL media. The pristine CNT presents a diameter of ∼150 nm (Fig. 2a). Initially the CNT swells to more than twice its diameter (∼400 nm), which forces it to lose the layered structure of the inner walls and an unrolling process is started (Fig. 2b). This swelling could be the result of IL penetration among the CNT walls during the sonication process, leading to the formation of cavitation in the CNT and producing enough energy to separate the previously π-packed graphene layers. As a result of this process, micrometric-sized tactoids consisting of a few layered graphene sheets (∼15 nm thick) are formed (Fig. 2c and d).
Although the CNT–IL interactions are mainly governed by the π–π packing between sp2 carbons from CNT and the IL imidazolium ring,28 the IL anion size and polarity also play important roles in the IL permeation among the highly compacted CNT walls. This phenomenon of CNT unrolling is influenced by the composition of the IL anion, by way of increasing the CNT–IL interaction by polarity tuning, however, and the IL anion size appears to be crucial.
Once the molecule vibrates under ultrasonic influence, larger molecules will form a larger momentum, producing a larger number of cavitation bubbles.30 When C4MImNTf2 is applied, where NTf2− is a large and hydrophobic anion, CNT opening is clearly observed (Fig. 1d). Differently, when using the chloride anion equivalent C4MImCl in this process, no CNT unrolling can be observed (Fig. 3a). Despite this observation, C4MImCl appears to be efficiently disentangling the CNT bundles into only small aggregates, functioning in this case as a type of dispersing-lubricating agent. Such a mechanism of CNT isolation from a bundle using sonication and surfactant (non-covalent) adsorption was previously mentioned in the literature.31 Firstly, the ultrasonic treatment provides high local shear to the CNT bundle-end, which leads to the formation of larger spaces among CNTs. Subsequent surfactant adsorption further enlarges the CNT distances and results in the separation of the individual CNTs from their bundle. It has been proved that π-stacking interactions of the benzene rings (or other highly aromatic molecules) onto the CNT surface increase the adsorption ratio of surfactants.32–34 Similarly in our case, the imidazolium ring of C4MImCl interacts with the CNT surface via π–π electron bonding, which promotes its surfactant effect and dispersion strength. The presence of C4MImCl as supramolecular aggregates all over the CNT surface can be observed, as dark spots, in Fig. 3b.
The quality of CNM is widely characterized by Raman spectroscopy, providing information especially about the tangential G band derived from the in-plane vibration of the sp2 carbon atoms and the disorder-induced D band.11,35,36 The ratio of the D to G band intensities (ID/IG) is related to the density of defects and edge smoothness of the graphene.37
The ID/IG ratio of the CNT–C4MImNTf2 mixture is ∼0.2, much lower than the ones presented by GO layers unzipped from CNT by solution-phase oxidation (ID/IG > 1)12 or by mechanical sonication (ID/IG ∼ 0.4),19 which indicates the high quality of the graphene layers unrolled from the CNT using IL. Despite the good values for the ID/IG ratio, a drawback on the use of Raman spectroscopy of IL–CNM mixtures is the overlap of the D band from the CNM (Fig. 4a) with the Raman active bands of the imidazolium cation (Fig. 4b and c).38 This means that the values for the D band could be even lower, consequently affecting the ID/IG ratio and representing a false positive for CNT/graphene defects. This effect is not so evident in the CNT–C4MImNTf2 mixture, where the D band is quite small in relation to the G band (Fig. 4b), but for the CNT–C4MImCl mixture it turns to be completely unreliable as the most prominent band is the one from the IL cation (Fig. 4c). Such a difference between the two applied IL is a probable consequence of the less homogeneous distribution of C4MImCl over the CNT surface, as observed (Fig. 3b).
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Fig. 5 TEM images of the nanocomposites based on (a) CNT–epoxy without IL, (b) CNT–C4MImCl–epoxy and (c and d) CNT–C4MImNTf2–epoxy systems. |
TEM images collected from these nanocomposites allowed observation of the different stages of CNT opening, with the presence of C4MImNTf2, until forming completely unrolled graphene sheets or tactoids (Fig. 6). The apparent mechanism corresponds to unrolling of graphene sheets from a parallel axis. Initially a highly packed and electron dense CNT structure is observed (Fig. 6a). With temperature and application of ultrasound this structure's outer layers eventually swell and unglue from the main axis (Fig. 6b and c), followed by the complete disconnection of the sheet from the main axis (Fig. 6d) and formation of the graphene domains (Fig. 6e), which corroborates the previously displayed AFM results.
Evaluating the size of the final graphene sheets formed, it is clear that this material was ripped at some point of the process. Comparing Fig. 1 and 2 vs. 5, the final fillers formed are from different orders of magnitude. Most likely, two different events could explain this increase in filler-size: (i) filler ripping during the network formation process; and (ii) filler ripping during the nanocomposite ultramicrotomy procedure for TEM specimen preparation (as also evidenced by the CNT tip rupture). To investigate these two possibilities, the nanocomposites were submitted to cryogenic fracturing and SEM was used to evaluate their fractured regions. This method was chosen due to the filler exposition at the fracture surface, revealing the real structure of the final fillers formed and eliminating the ultramicrotomy step from TEM analysis.
Fig. 7a presents the relatively smooth and filler free surface of the neat epoxy matrix, which was used as a reference. The IL free epoxy–CNT nanocomposite presented a similar surface fracture structure, but with the presence of randomly dispersed CNT (Fig. 7b), indicating the low CNT–epoxy interaction. The CNT–epoxy nanocomposite containing IL C4MImCl presents a quite different fracture structure, exposing epoxy-coated CNT structures at the surface (Fig. 7c).
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Fig. 7 SEM images of (a) neat epoxy matrix (DGEBA–ethylene tetramine) and nanocomposites (b) epoxy–CNT without IL, (c) epoxy–CNT with IL C4MImCl and (d) epoxy–CNT with IL C4MImNTf2. |
This could be a result of local plasticization of the epoxy network, avoiding the brittle break at the epoxy–CNT interphase, as this sample presents IL aggregation at the CNT surface (Fig. 3b). In the case of the epoxy–CNT nanocomposite with CNT–C4MImNTf2, its fracturing changed drastically. This nanocomposite presents fractures in the shape of sheets, suggesting an epoxy–graphene interface fracture (Fig. 7d). Furthermore, also unopened CNT were identified in this nanocomposite, indicating that the CNT opening process was not quantitative.
The detailed characterization and discussion of the thermo-mechanical properties, together with the influence of the different CNM morphologies and CNM–epoxy interfaces on the nanocomposite properties, will be the subject of our next paper.
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