Production of aqueous colloidal dispersions of carbon nanotubes

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Abstract

Stable homogeneous dispersions of carbon nanotubes (CNTs) have been prepared by using sodium dodecyl sulfate (SDS) as dispersing agent. To our knowledge, it is the first report to quantitatively characterize colloidal stability of the dispersions by UV–vis spectrophometric measurements. When the sediment time reaches 500 h, the supernatant CNT concentration drops as much as 50% for the bare CNT suspension, compared to 15% with the addition of SDS. Furthermore, after 150 h, no precipitation is found for CNT/SDS dispersions, exhibiting an extreme stability. Zeta potential, auger electron microscopy, and FTIR analysis are employed to investigate the adsorption mechanism in detail. It has been concluded that the surfactant containing a single straight-chain hydrophobic segment and a terminal hydrophilic segment can modify the CNTs–suspending medium interface and prevent aggregation over long periods. The morphology of the CNT dispersions is observed with optical microscopy. An intermediate domain of homogeneously dispersed nanotubes exhibits an optimum at 0.5 wt% CNTs and 2.0 wt% SDS.

Introduction

The discovery of carbon nanotubes (CNTs) and carbon-nanotube-based materials has inspired scientists for a range of potential applications [1], [2], [3], [4]. The carbon nanotubes have high mechanical strength (strength and flexibility), making them ideal reinforcing fibers in nanocomposites. Therefore, it is generally expected that CNTs can be used as additives to reinforce composite materials, such as epoxy [5], petroleum pitch [6], PMMA [7], and alumina composites [8]. However, fabrication of homogeneous nanocomposites with carbon nanotubes remains a technical challenge [9]. When added to an alumina composite by the hot press and hot extrusion method [8], the mechanical strength of the alumina composite is not enhanced, a fact attributed to aggregation (or inhomogeneous dispersion) of CNTs in the composite materials. CNTs always form aggregates owing to very strong van der Waals interactions. To obtain stable dispersions of CNTs in water indeed is a significant problem, and it is also a prerequisite for its application as additives for reinforcement of composite materials.

The possibility of producing an aqueous C60 suspension has been reported [10], and suspensions have been obtained by means of chemical modification, by using colloidal surfactants, phospholipids, or polyvinyl pyrrolidine (PVP) as solubilizing or/and dispersing agents [11], [12], [13], or with the help of supramolecular complexes “host-guest” formation with γ-cyclodextrin [11], [12], [13], [14]. Dispersing agents, such as surface-active agents, have been used to disperse fine particles of hydrophobic materials in aqueous solution [15], [16]. In general, there are three principles for dispersing fine particles in water [17]: (i) the repulsion between the particles with their zeta potentials, (ii) the steric hindrance of the adsorption layer, and (iii) the reduction of hydrophobic linkages among dispersed particles. Commercially available original CNTs are generally too long to be dispersed in solution and have no well-defined chemical function groups to modify [18]. According to previous reports [19], surfactants can modify the particles–suspending medium interface and prevent aggregation over long time periods. However, how surfactant molecules help dispersion of CNTs is still not clear [20], [21], [22]. It is essential to have a more complete understanding of the dispersion mechanism.

In this paper, an attempt to prepare aqueous dispersions of CNTs in the presence of sodium dodecyl sulfate (SDS) is made, and this resulted in the generation of stable homogeneous dispersions. UV–vis spectrophometric measurements are used to quantitatively characterize colloidal stability of the dispersions as a new method. The stabilization mechanisms of the CNT dispersions are discussed by zeta potential, FTIR, and auger electron spectroscopy (AES) studies. It is expected that the results can help provide a guideline for choosing suitable dispersants. Optical microscopy is employed to characterize the aggregate state of the CNT suspension on a micrometer scale. A phase diagram of the CNT/SDS aqueous system allows the optimum amount for obtaining homogeneous dispersion to be deduced.

Section snippets

Materials

CNTs were kindly provided by Shenzhen Nanotech Port Ltd. Co. (Shenzhen, China) and were used as received. According to thermogravimetric analysis (STA 449C, NETZSCH, Selb, Germany), CNTs contained about 45 wt% water, after being purified by the standard acid treatments. A TEM micrograph (Model 200CX, JEOL, Tokyo, Japan) of raw CNTs is shown in Fig. 1. Distilled water was used in all studies. The concentrations of the CNTs (wt%) in this paper are given as the weight concentration of the pristine

Colloidal stability of the CNT dispersions

A new method introduced in this paper can be used to calculate quantitatively and accurately the suspension concentration with increasing sediment time. Figure 4 illustrates the CNT concentration of the supernatant suspension versus the sediment time without and with SDS, respectively. For the bare CNT suspension, the fastest settling occurs. At 500 h, the supernatant CNT concentration drops as much as 50%. Additionally, without the dispersant, the CNT concentration still reveals a decreasing

Discussion

With the aid of SDS, the formation of homogeneous CNT dispersions in an aqueous medium is no surprise. However, a more complete understanding of the interfacial chemistry and the dispersion mechanism can provide a guideline for better choices of dispersant types.

TEM micrographs of raw CNTs (Fig. 1) display the long tubes attached together; that is, even after ultrasonication the CNTs used here is shaped like a bundle. The magnitude of the ζ potential for the SDS-adsorbed CNTs is higher than

Conclusions

Stable aqueous colloidal dispersions of CNTs are obtained with the aid of SDS. UV–vis spectroscopy is used to quantitatively characterize the stability of the dispersions for the first time. The CNT/SDS dispersion exhibits extreme stability, with the supernatant CNT concentration decreasing only 15% compared with a decrease of 50% for the bare CNTs. The interaction between CNTs and SDS through the hydrophobic segment causes a higher negative surface charge and steric repulsion, which improves

References (31)

  • Z. Jia et al.

    Mater. Sci. Eng. A

    (1999)
  • D.M. Guldi et al.

    Chem. Phys. Lett.

    (1994)
  • S. Ijjima

    Nature

    (1991)
  • A.G. Rinzler et al.

    Science

    (1995)
  • W.A. de Heer et al.

    Science

    (1995)
  • P.G. Collins et al.

    Science

    (1997)
  • L.S. Schadler et al.

    Appl. Phys. Lett.

    (1998)
  • R. Andrews et al.

    Appl. Phys. Lett.

    (1999)
  • T. Kuzumaki et al.

    J. Mater. Res.

    (1998)
  • H.D. Wagner et al.

    Appl. Phys. Lett.

    (1998)
  • W.A. Scrivens et al.

    J. Am. Chem. Soc.

    (1994)
  • Y.N. Yanakoshi et al.

    J. Chem. Soc. Chem. Commun.

    (1994)
  • H. Hungerbühler et al.

    J. Am. Chem. Soc.

    (1993)
  • K.I. Prigadarsini et al.

    Fullerene Sci. Technol.

    (1995)
  • H.W. Sophie et al.

    J. Colloid Interface Sci.

    (2001)
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