Mass-production of highly-crystalline few-layer graphene sheets by arc discharge in various H2–inert gas mixtures
Graphical abstract
Highlights
► Gram-scale graphene sheets were produced by using hydrogen arc discharge method. ► Few-layered graphene sheets have high crystalline structure. ► Arc-discharged graphene sheets were used as anodes of lithium-ion batteries. ► The graphene sheets anodes show excellent cyclic performance.
Introduction
Graphene is a single layer of carbon atoms in a two-dimensional honeycomb lattice. It can exist as a free-standing form and exhibits many unusual and intriguing properties [1], [2]. Graphene has been studied intensively due to its unique electrical, mechanical, optical, thermal and other properties [3], [4], [5]. Several methods have been developed to produce graphene, including micromechanical cleavage of highly oriented pyrolytic graphite [1], epitaxial growth on a substrate such as SiC [6], [7], chemical vapor deposition (CVD) [8], reduction of graphene oxide [9], [10], [11] and arc discharge [12], [13], [14], [15], [16].
The arc discharge method has been widely used to produce carbon-based nanostructures, such as fullerenes and carbon nanotubes (CNTs) [17], [18]. As early as 1997, two of the present authors, Ando, Zhao, and coworkers [12], [18] serendipitously observed the petal-like graphene sheets deposited on the cathode surface by hydrogen arc discharge. In 2009, Rao et al. produced graphene sheets with 2–4 layers by arc discharge in a H2–He gas mixture [13]. Different discharge atmospheres may induce different chemical bonds that cause the prepared graphene sheets to have different properties. Li et al. prepared N-doped graphene sheets by using NH3–He mixtures as discharge atmospheres [14]. Wu et al. synthesized graphene sheets with high electrical conductivity and dispersivity by introducing C−O and OCO bonds in CO2–He gas mixtures [15].
In the present Letter, we report on the large-scale production of highly-crystalline and high-purity few-layer graphene sheets by arc discharge in various H2–inert gas mixtures. In hydrogen arc discharge, hydrogen can inhibit the rolling and closing of graphene sheets by terminating the dangling carbon bonds.
Lithium-ion batteries currently are ubiquitous power sources for portable electronics. Their energy density and cycling performance largely depend on the physical and chemical properties of cathode and anode materials. Graphene sheets synthesized by various methods have so far been reported as the anode materials for lithium-ion batteries [19], [20], [21]. To our knowledge, however, the lithium storage performance of graphene sheets produced by arc discharge method has not yet been reported. Accordingly, we have also evaluated the electrochemical performance in coin-type cells of the arc-discharged graphene sheets versus metallic lithium.
Section snippets
Experimental
To produce the graphene sheets, direct current (DC) arc discharge evaporation was carried out in a homemade water-cooled stainless steel chamber; see Figure 1. The DC arc discharge was generated by applying 150 A current in an atmosphere of H2 and inert gas mixture (H2–He, Ar, N2) at 400 Torr. Two pure graphite-rod electrodes with the diameter of 10 mm were adjusted to maintain a constant distance of ∼2 mm. The volume ratio of H2 to inert gas was 1:1, and the typical synthesis time was 20 min.
SEM and HRTEM
Figure 2a shows a typical SEM image of loose graphene sheet powder mass-produced by arc discharge in a H2–Ar gas mixture. The morphology of graphene sheets prepared in a H2–He or H2–N2 gas mixture is similar to that of samples obtained in H2–Ar. The appearance of graphene sheets resembles naturally crumpled and curly petals. The graphene sheet petals are observed to agglomerate probably because of their small size. In Figure 2b, we can see large-area graphene sheets with sizes of 100–200 nm.
Conclusions
DC hydrogen arc discharge evaporation of pure graphite electrodes in various gas mixtures can be used to mass-produce high-crystalline few-layer graphene sheets. The few-layer graphene sheets are found to possess high purity, adequate thermal stability, wide surface area, and show excellent Li+ storage performance. It has been confirmed that the first discharge capacity can reach as high as 1332 mA h g−1 at a current density of 50 mA g−1 and the discharge capacity remains at 323 mA h g−1 after 350
Acknowledgments
This work is supported by National Natural Science Foundation of China (No. 10974131). We thank Instrumental Analysis and Research Center of Shanghai University for the guidance and use of equipment. Special thanks are due to Mr. D.G. Elliston for English language proof-reading.
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