Elsevier

Carbon

Volume 45, Issue 7, June 2007, Pages 1558-1565
Carbon

Synthesis of graphene-based nanosheets via chemical reduction of exfoliated graphite oxide

https://doi.org/10.1016/j.carbon.2007.02.034Get rights and content

Abstract

Reduction of a colloidal suspension of exfoliated graphene oxide sheets in water with hydrazine hydrate results in their aggregation and subsequent formation of a high-surface-area carbon material which consists of thin graphene-based sheets. The reduced material was characterized by elemental analysis, thermo-gravimetric analysis, scanning electron microscopy, X-ray photoelectron spectroscopy, NMR spectroscopy, Raman spectroscopy, and by electrical conductivity measurements.

Introduction

Graphite-like nanoplatelets have recently attracted attention as a viable and inexpensive filler in composite materials [1], [2], [3] that can be used in many engineering applications, given the excellent in-plane mechanical, structural, thermal, and electrical properties of graphite [4]. These excellent properties may be relevant at the nanoscale if graphite can be exfoliated into thin nanoplatelets, and even down to the single graphene sheet level [5].

Graphite nanoplatelets have often been made from expanded graphite, which in turn was produced from graphite intercalation compounds via rapid evaporation of the intercalant at elevated temperatures. For example, rapid thermal expansion of sulfuric acid-intercalated graphite, followed by a suitable treatment to produce platelets/nanoplatelets from the expanded material (ball milling or exposure to ultrasound) has been recently demonstrated [6], [7], [8], [9], [10], [11], [12], [13]. Although this simple method has been applied on a large scale to commercially available sulfuric acid-intercalated graphite, it never results in complete exfoliation of graphite to the level of individual graphene sheets. The extent of thermal expansion (and therefore the platelet thickness) is dependent on the type of graphite used and on the intercalation procedure [14], [15]. With few exceptions [3], the graphite nanoplatelets obtained via this process typically consist of hundreds of stacked graphene layers (assuming that the thickness of one layer is equal to the interlayer separation in graphite, 0.34 nm) and average between 30 and 100 nm in thickness.

In addition to the thermal expansion route, the delamination of intercalated graphite can sometimes be achieved by inducing a gas-producing chemical reaction within its interlayer galleries (chemical expansion). For example, a low-temperature chemical expansion route to graphite nanoplatelets and nanoscrolls, based on potassium-intercalated graphite, has been reported [16], [17]. However, this approach could not be reproduced in our laboratory even with a duplication of the expensive ultra-high-intensity ultrasonic equipment reported therein.

Given our interest in the preparation of graphene-based materials [18], [19], [20], we set out to develop a general and reproducible approach for the preparation of graphene sheets from graphite. After numerous failed attempts to create graphene-based sheets via graphite intercalation compounds, we decided to use graphite oxide (GO) as one possible route for meeting this challenge. Our basic strategy involved the complete exfoliation of GO into individual GO sheets followed by their in-situ reduction to produce individual graphene-like sheets [19], [20]. Herein, we describe the detailed process for the reduction of exfoliated GO sheets with hydrazine and the characterization of the resulting material. In particular, we present evidence to support the claim that GO can be completely exfoliated into individual graphene oxide sheets and that chemical reduction of such sheets can furnish graphene-like sheets.

GO is produced by the oxidative treatment of graphite via one of three principal methods developed by Brodie [21], Hummers [22], and Staudenmeier [23], respectively. It still retains a layered structure, but is much lighter in color than graphite due to the loss of electronic conjugation brought about by the oxidation. According to the most recent studies [24], [25], [26], [27], [28], [29], GO consists of oxidized graphene sheets (or ‘graphene oxide sheets’) having their basal planes decorated mostly with epoxide and hydroxyl groups, in addition to carbonyl and carboxyl groups located presumably at the edges (Lerf–Klinowski model). These oxygen functionalities render the graphene oxide layers of GO hydrophilic and water molecules can readily intercalate into the interlayer galleries. GO can therefore be also thought of as a graphite-type intercalation compound with both covalently bound oxygen and non-covalently bound water between the carbon layers. Indeed, rapid heating of GO results in its expansion and delamination caused by rapid evaporation of the intercalated water and evolution of gases produced by thermal pyrolysis of the oxygen-containing functional groups [30]. Such thermal treatment has recently been suggested to be capable of producing individual functionalized graphene sheets [30].

By nature, GO is electrically insulating (see below) and thus cannot be used, without further processing, as a conductive nanomaterial. In addition, the presence of the oxygen functional groups makes GO thermally unstable, as it undergoes pyrolysis at elevated temperatures [31], [32]. Notably, it has been demonstrated that the electrical conductivity of GO (and presumably its thermal stability as well) can be restored close to the level of graphite by chemical reduction [33], [34], [35], [36]. Such reductions of GO, however, have not been studied in great detail. To that end, we have examined the chemical reduction of exfoliated graphene oxide sheets with several reducing agents and found hydrazine hydrate (H2NNH2 · H2O) to be the best one in producing very thin graphene-like sheets, consistent with previous reports [31], [32]. High-resolution scanning electron microscopy (SEM) also provided us with evidence of thin sheets. Here we report a comprehensive study of this reduced material by elemental analysis, X-ray photoelectron spectroscopy (XPS), gas adsorption, solid state NMR spectroscopy, Raman spectroscopy, thermo-gravimetric analysis (TGA), SEM, and electrical conductivity measurements.

Section snippets

Materials and methods

GO was prepared from purified natural graphite (SP-1, 30-μm nominal particle size, Bay Carbon, Bay City, MI) by the Hummers method [22]. SEM images were obtained with a field emission gun scanning electron microscope (LEO1525, Carl Zeiss SMT AG, Oberkochen, Germany). Samples for AFM imaging were prepared by depositing colloidal suspensions of GO on freshly cleaved mica surfaces (Ted Pella Inc., Redding, CA). AFM images were taken on a MultiTask AutoProbe CP/MT Scanning Probe Microscope (Veeco

Exfoliation of GO in water

An important property of GO, brought about by the hydrophilic nature of the oxygenated graphene layers, is its easy exfoliation in aqueous media. As a result, GO readily forms stable colloidal suspensions of thin sheets in water [41], [42]. After a suitable ultrasonic treatment, such exfoliation can produce stable dispersions of very thin graphene oxide sheets in water [18], [19] These sheets are, however, different from graphitic nanoplatelets or pristine graphene sheets due to their low

Conclusion

In conclusion, reduction of exfoliated graphene oxide sheets in water with hydrazine results in a material with graphitic characteristics that are comparable to those of pristine graphite. On the nanoscale, this carbon-based material consists of thin graphene-based sheets and possesses a high specific surface area. The characterization of the reduced GO indicates that the hydrazine treatment results in the formation of unsaturated and conjugated carbon atoms, which in turn imparts electrical

Acknowledgements

Support from NASA (Award # NCC-1-02037) through the University Research, Engineering and Technology Institute (URETI) on Bio-inspired Materials (BiMat) is appreciated. We acknowledge the use of instruments in the Northwestern NUANCE (supported by NSF-NSEC, NSF-MRSEC, Keck Foundation, the state of Illinois, and Northwestern Univ.) facility.

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