Elsevier

Carbon

Volume 83, March 2015, Pages 136-143
Carbon

High-yield graphene exfoliation using sodium dodecyl sulfate accompanied by alcohols as surface-tension-reducing agents in aqueous solution

https://doi.org/10.1016/j.carbon.2014.11.035Get rights and content

Abstract

The exfoliation of graphite was investigated in aqueous solutions containing sodium dodecyl sulfate (SDS) as a surfactant. The exfoliation was greatly enhanced near the surface aggregation concentration (SAC) of SDS, 2.6 mM, and then decreased for higher SDS contents. However, the flakes exfoliated near the SAC were graphite, whereas graphene was obtained above the critical micelle concentration (CMC). The effect of the use of alcohols as surface-tension-reducing agents (STRAs) on the exfoliation was then investigated. With ethyl alcohol, a dispersion of 2.1 mg ml−1 graphene was achieved from 2.6 mM SDS after only 1 h of sonication, whereas a dispersion of 0.2 mg ml−1 was obtained above the CMC in the absence of STRAs. The results demonstrate that the SDS content near the SAC is highly beneficial for exfoliation as long as the surface tension is maintained near 41.0 mN ml−1. This finding supports the notion that the structure of the adsorbed SDS, depending on its concentration, strongly affects the exfoliation of graphite into graphene.

Introduction

Graphene has a unique, single-atomic-layer 2D structure composed of sp2 carbons bonded together in a honeycomb lattice. The excellent electronic, thermal, and mechanical properties of graphene [1], [2], [3], [4], [5] have attracted great interest due to the material’s enormous potential in many fields, including composite materials [3], [6], electronic devices [7], [8], energy storage, and molecular sensors [9], [10]. However, the practical application of graphene is limited by the lack of cost-effective production methods that allow for high throughput. To overcome this obstacle, the exfoliation of graphite in the liquid phase has been investigated because liquid-phase production methods can be scaled up more easily and inexpensively than vacuum processes, such as chemical vapor deposition.

Recently, the chemical oxidation method, a liquid-phase exfoliation technique, has been intensively studied because it can easily produce high-concentration graphene oxide dispersions [11], [12], [13]. However, it has been reported that the chemical bonding state of graphene exfoliated in this manner changes locally from sp2 to sp3 during oxidation. These deformed states are unable to be recovered entirely, deteriorating the electrical and mechanical properties of graphene [14], [15]. Thus, various exfoliation strategies that do not involve chemical reaction have been developed to produce high-quality graphene, such as solvent-assisted sonication [16], [17], [18], [19], [20], [21], [22], [23]. Previous studies have reported that graphene dispersions of high concentrations above 1 mg ml−1 could be obtained using sonication treatments over several hours in some aromatic solvents, such as N-methyl pyrrolidone or phenol [20], [24]. However, we believe that it is more important to exfoliate graphite into graphene using a non-toxic solvent, such as water, than to obtain a high concentration of graphene because the use of harmful organic solvents should be restricted to protect the environment.

Although the exfoliation of graphite in an aqueous solution is challenging due to the hydrophobic nature of graphene, this hydrophobicity can be managed by surfactant-assisted sonication methods via the electrostatic repulsion of ionic surfactants or the steric stabilization effect of non-ionic surfactants [25], [26], [27]. Previous studies have also reported the positive effect of sodium dodecyl sulfate (SDS), an anionic surfactant, on the conductivity of poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) conducting polymers blended with single-walled carbon nanotubes exfoliated using SDS. The enhanced conductivity was attributed to the existence of SDS molecules reducing the distorted structure of PEDOT chains [28], [29], [30].

On the other hand, it is difficult to obtain high concentrations of graphene in dispersion solutions with ionic surfactants such as SDS because electrostatic repulsion is less effective for exfoliating graphite in an aqueous solution than the steric stabilization effect induced by non-ionic surfactants [12]. The exfoliation of graphite with an anionic surfactant such as sodium dodecyl benzene sulfonate has produced graphene dispersions at typical concentrations of 0.002–0.05 mg ml−1 [27]. Although the dispersed graphene concentration could be increased up to values several times higher by long sonication periods (several hundred hours), such processing may not be viable in commercial production [22].

In this work, we investigated the exfoliation of graphite using SDS as a surfactant and focused on increasing the concentration of graphene in SDS aqueous solutions. Our study of the enhancement of the exfoliation of graphite is expected to facilitate the fabrication of highly conductive graphene–SDS/PEDOT:PSS composite films.

Most previous studies have reported, based on experimental results, that graphite can be successfully exfoliated to graphene at a surfactant concentration above the critical micelle concentration (CMC) [27], at which the surfactant molecules form spherical micelles. In contrast, small SDS monomers may be more effective for intercalating into graphite than large SDS micelles (>1.7 nm [31]) due to the narrow spacing between graphene sheets (∼0.34 nm). Another important factor affecting the intercalation of surfactants into graphite is the macroscopic surface tension of aqueous dispersion solutions, which can be related to the spatial structures and intramolecular bonds of solvent molecules on a microscopic scale. In the present study, the performances of low and high concentrations of SDS as a surfactant were compared, and the effect of surface-tension-reducing agents (STRAs) such as methyl alcohol (MeOH), ethyl alcohol (EtOH) and isopropyl alcohol (i-PrOH) on exfoliation was investigated in terms of the concentration and microstructure of graphene.

Section snippets

Preparation of graphene dispersions

The exfoliation of graphite was carried out using surfactant-assisted sonication. All of the chemical reagents, including graphite flakes (>100 mesh) and SDS, were purchased from Sigma–Aldrich. The initial concentration of graphite was 100 mg ml−1, and exfoliation was performed over a wide concentration range of SDS (1.3–35 mM). After sonication (Daihan WUC-D06H, 665 W, 40 kHz) for 1 h, the dispersion solutions were centrifuged (Hanil combi-514R) at 500 rpm for 15 min to sediment aggregated flakes of

Results and discussion

First, the exfoliation of graphite was carried out with SDS concentrations ranging from 1.3 to 35 mM to investigate the dependence of exfoliation on SDS concentration. The sonication time was 1 h, and centrifugation was carried out at 500 rpm for 15 min. As shown in Fig. 1a, the highest concentration of exfoliated flakes, 2.3 mg ml−1, was obtained using 2.6 mM SDS; the concentration of exfoliated flakes then decreased considerably as the SDS concentration increased further. SDS monomers in solution

Conclusion

The results of the present work demonstrate that higher-CG dispersions could be obtained with a low surfactant concentration near the SAC instead of a concentration above the CMC, in contrast to previously reported findings. To provide high-CG dispersions with a low surfactant concentration, it is essential to adjust the surface tension using STRAs. The results also support the notion that the structure of adsorbed SDS molecules on the graphite surface, which depends on the SDS concentration,

Acknowledgments

This work was supported by the New and Renewable Energy Research Projects of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) Grant funded by the Korea Ministry of Trade, Industry and Energy. (Nos. 20118520010040 and 20123010010150).

References (43)

  • S. Stankovich et al.

    Graphene-based composited materials

    Nature

    (2006)
  • J.C. Meyer et al.

    The structure of suspended graphene sheets

    Nature

    (2007)
  • W. Du et al.

    From graphite to graphene: direct liquid-phase exfoliation of graphite to produce single- and few-layered pristine graphene

    J Mater Chem A

    (2013)
  • T. Ramanathan et al.

    Functionalized graphene sheets for polymer nanocomposites

    Nat Nanotechnol

    (2008)
  • J. Du et al.

    25th anniversary article: carbon nanotube- and graphene-based transparent conductive films for optoelectronic devices

    Adv Mater

    (2014)
  • S. De et al.

    Flexible, transparent, conducting films of randomly stacked graphene from surfactant-stabilized, oxide-free graphene dispersions

    Small

    (2010)
  • H. Jiang

    Chemical preparation of graphene-based nanomaterials and their applications in chemical and biological sensors

    Small

    (2011)
  • E. Yoo et al.

    Large reversible Li storage of graphene nanosheet families for use in rechargeable lithium ion batteries

    Nano Lett

    (2008)
  • M. Cai et al.

    Methods of graphite exfoliation

    J Mater Chem

    (2012)
  • J.N. Coleman

    Liquid-phase exfoliation of nanotubes and graphene

    Adv Funct Mater

    (2009)
  • U. Khan et al.

    High-concentration solvent exfoliation of graphene

    Small

    (2010)
  • Cited by (0)

    View full text