Br treated graphite nanoplatelets for improved electrical conductivity of polymer composites

doi:10.1016/j.carbon.2006.11.031

Carbon

Volume 45, Issue 4, April 2007, Pages 744-750

https://doi.org/10.1016/j.carbon.2006.11.031 Get rights and content

Abstract

The graphite nanoplatelets (GNP) were treated by vapor-phase bromination. The increase in weight and atomic concentration of Br indicated the bromine uptake. The intercalation of Br between graphene layers of GNP was confirmed by the X-ray diffraction result, showing an increase in the interlayer spacing from 3.342 Å to 3.361 Å. Two types of bonds between C and Br were introduced simultaneously, ionic and covalent bonds, both of them increased with bromination duration. The fraction of ionic bond reached the highest value by 3 h Br exposure, which corresponded to the highest electrical conductivity of GNP. Although the bromination treatment did not change the percolation threshold of composites, it increased the absolute value of electrical conductivity of composites when the filler content was higher than the percolation threshold.

Introduction

Graphite nanoplatelet (GNP) is a nanometer-scale conductive filler produced by exfoliating natural flake graphite, and has attracted significant attention as the low cost and lightweight alternative to metal- and carbon-based electrically conducting reinforcements for conducting polymer composites. GNPs are produced from graphite flakes intercalated with high concentrated acids, which can be expanded up to a few hundreds of times along its c-axis at a high temperature. The exfoliation of expanded graphite results in separation of the graphite sheets into nanoplatelets with a very high aspect ratio [1], [2]. The high aspect ratio and the large surface area of GNPs are responsible for the much lower percolation threshold and better electrical conductivity of conducting polymer composites than micrometer-scale conventional reinforcements. [1], [3]

Conducting polymer composites have many potential applications in electromagnetic interference shielding for electronic devices and electrostatic dissipation, where high electrical conductivity of composite materials is the most critical requirement. Therefore, significant efforts have been directed towards improving the electrical conductivity of conducting composites. It has been reported [4], [5] that intercalation of guest atoms between the graphene planes forming ionic bonds (charge transfer) can enhance the electrical conductivity of graphitic material by adding electrons to its conduction band or holes to the valence band. Meanwhile, the formation of covalent bonds between the guest atoms and graphite can depress the carrier concentration and displace some of the C atoms slightly away from the highly planar structure of pristine graphite, which in turn decreases the electrical conductivity.

Amongst many intercalation species, some compounds such as AsF₅ would most probably generate higher conductivity improvement than bromine [4]. We chose bromine as the model compound for three reasons: (1) bromine is less toxic and easier to intercalate; (2) brominated carbon fibres showed long term stability under ambient condition [4] and a high thermal stability below 150 °C [4], [5]; (3) bromine has intermediate chemical activities in host–guest interactions, compared to iodine which allows a weak host–guest reactivity, thus introducing only weak charge transfer, and fluorine, which is so strong that covalent bonding can be easily formed with host materials.

Bromine intercalation has been successfully used to increase the electrical conductivity of carbon fibres [4], [5], [6]. The charge transfer was the main bonding type between Br and C, and covalent bonding was introduced only after prolonged reactions [5]. A higher level of graphitization of the host fiber induced better intercalation and higher electrical conductivity of doped carbon fibres [4], [6]. Vapor-phase bromination treatment has also been applied to nanometer-scale graphite, such as single wall carbon nanotubes [7] and onion-like nanographite particles [8]. A remarkable 30 time increase in electrical conductivity was reported [7] for single wall carbon nanotubes through the formation of charge transfer complexes. Bromine intercalation showed similar effects on the electronic properties of onion-like nanographite [8].

This paper is part of a larger project on fabrication, properties characterization and applications of conducting GNP composites. In our previous studies [2], [9], the UV/O₃ treatment technique was successfully applied to improve the electrical conductivity as well as the thermo-mechanical and mechanical properties of GNP/epoxy composites. Here, GNPs were treated with bromine to improve the electrical properties of conducting polymer composites made therefrom. With much larger density of open edges than the other carbon materials mentioned above, GNPs would show significant synergy in improving the electrical conductivity after the treatment. X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) analysis and time-of-flight secondary ion mass spectrometry (ToF-SIMS) were employed to evaluate the intercalation states of Br atoms and quantitatively analyze the bonding types between the C and Br atoms.

Section snippets

Sample preparation

GNPs were prepared according to the method described in our previous studies [2], [9]. Graphite intercalation compound (GIC) containing 2.8 wt% of sulfur as intercalant (supplied by Asbury Graphite Mills, USA) was put into an oven that was maintained at 1050 °C and taken out after 30 s to produce expanded graphite. Upon rapid heating the GIC was expanded explosively several hundred times along the thickness direction due to the evaporation of the intercalant and the thermal shock. The expanded

Bromine uptake and intercalation

The uptake of bromine by GNP upon its exposure to molecular Br (generally termed – bromination) is expected to become evident by weight increase, which occurs by two parallel routes forming either intercalation complexes or covalent C–Br groups. Because only the first is expected to affect the π-electron interaction between the bromine and GNP, it is important that in addition to the overall uptake the proportion of the two compounds be determined. Fig. 2 shows the changes in weight and Br

Conclusion

The GNPs were treated by vapor-phase bromination at room temperature for different durations. Weight increase was closely monitored, and the XPS, XRD and ToF-SIMS were employed to evaluate the intercalation of Br atoms and quantitatively analyze the bonding types between C and Br. Electrical conductivities of the brominated GNPs and the epoxy composites were measured and compared between those with and without bromination. Major findings are highlighted as following:

1.
Systematic increases in

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Br treated graphite nanoplatelets for improved electrical conductivity of polymer composites

Abstract

Introduction

Section snippets

Sample preparation

Bromine uptake and intercalation

Conclusion

Polymer

Comp Sci Techol

Mater Chem Phys

Carbon

Scripta Mater

Performance of expanded graphite and expanded milled-graphite fillers in thermosetting resins

Polym Compos