Facile preparation of large-scale graphene nanoscrolls from graphene oxide sheets by cold quenching in liquid nitrogen
Introduction
Graphene nanoscroll (GNS), a new type of graphene-derivative materials, is a spirally wrapped two-dimensional (2D) graphene sheet (GS) with a 1D tubular structure resembling that of a multiwalled carbon nanotube (MWCNT) [1], [2], [3], [4]. On one hand, this structure allows GNS to inherit some excellent properties of both graphene and CNT. On the other hand, owning to its unclosed topological structure, GNS is expected to possess some unusual electronic and optical properties quite distinct from graphene and CNT [5], [6]. For example, compared with GS, the self-encapsulated structure of GNS means that the electrical transport is significantly affected by the π–π interaction between the inner and outer surfaces of wrapped graphene [2], [3]; compared with MWCNT, the electric current flows within a single scrolled graphene layer rather than through several coaxially nested graphene cylinders [2], [3]. Moreover, in contrast to MWCNT, the diameter of GNS can be varied because of the unclosed topological structure, and the galleries between GNS interlayers can be intercalated with donors and acceptors [4]. This feature has been expected to be exploited for a variety of technological applications, such as hydrogen storage [7], [8], energy storage devices [9] and novel nanodevices [10], [11], [12].
Until now, almost all studies on the applications of GNSs focus on theoretical predictions and calculations. It is mainly restricted from the difficulty in the preparation of GNSs [3], [9]. Although some chemical and physical methods have been explored to prepare GNSs to date, they all have obvious drawbacks. In chemical methods, GNSs can be prepared by the exfoliation-sonication of intercalated graphite [1], [13], [14], but unexpected impurities and defects are simultaneously introduced in GNSs. Similarly, GNSs can be produced by chemical vapor deposition (CVD) with high quality and controllable morphologies [15], [16], but the experimental process is comparatively complicated and the yield is very low. The physical methods mainly include arc-discharge [17], high-energy ball milling of graphite [18], microwave spark assistance [3] and nanowire template [19]. In these methods, GNSs commonly exist in the final products and it is very hard to isolate GNSs from byproducts such as graphite and amorphous carbon. Recently, a new physical method based on the rolling up of graphene on a substrate surface has been reported [2], [20]. Although it is an efficient route for the fabrication of high-quality GNS, the drawback is the extremely low throughput because the monolayer graphene used is obtained in advance by CVD or mechanical exfoliation. Therefore, exploring an ideal technique for the large-scale preparation of GNSs is one of the greatest challenges at present.
In this paper, we demonstrate a novel method for the large-scale preparation of GNSs from GO aqueous suspension by cold quenching in liquid nitrogen, freeze-drying and subsequent thermal reduction. During the cold quenching process, GO sheets are able to roll up into GNSs and the structural conversion is closely correlated with the initial temperature of GO suspension, the size of GO sheets and the immersion way.
Section snippets
Materials
Natural flake graphite, provided by Qingdao Tianheda Graphite Ltd. Co. (Qingdao, China), was washed by water and ethanol. The purity of the natural flake graphite is about 99%. The liquid nitrogen was purchased from Lanzhou Zhongli Chemical Gas Ltd. Co. (Lanzhou, China).
Preparation of GNSs
First, large- and small-size GO sheets were respectively prepared by modified chemical exfoliation [21], using natural flake graphite with different average sizes (32 and 325 mesh). The large-size of GO and the small-size of GO
Results and discussion
Fig. 1a shows the preparation procedure of GNSs, which includes the following key steps: an aqueous suspension of large-size GO sheets (about tens of microns) was prepared by the chemical exfoliation from graphite (32 mesh) according to the reported method [21], [22], [23], the GO aqueous suspension in a plastic box was heated up to 80 °C, the plastic box with the hot GO suspension was put into liquid nitrogen quickly for cold quenching, the frozen suspension was freeze-dried, and finally the
Conclusions
We developed a novel and simple method for the large-scale preparation of GNSs from GO aqueous suspension by cold quenching in liquid nitrogen, freeze-drying and subsequent thermal reduction. During the cold quenching process, large-size GO sheets are able to roll up into GNSs and form a 3D network of GNSs. Both the initial temperature of GO suspension and the size of GO sheets have important effects on this structural conversion. These results may open up the possibility for the large-scale
Acknowledgements
The authors thank Miss L.L. Zhang, Mr. W. Chen, Dr W. Jiao and Dr. Z. Weng for their valuable discussions. This work was supported by the Top Hundred Talents Program of the Chinese Academy of Sciences, Natural Science Foundation of China (No. 21303234), China Postdoctoral Science Foundation (No. 2013M530437) and Ministry of Science and Technology of China (No. 2012AA030303).
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