High quality graphene sheets from graphene oxide by hot-pressing
Introduction
Since its successful isolation by mechanical exfoliation [1], [2], graphene has attracted a strong recent interest. It is a promising material for energy-storage, interfacing to biological materials, for electronic and optical devices, and other applications [3], [4] due to its unique physical, chemical and mechanical properties, which include high values of Young’s modulus (∼1100 GPa) [5], [6], specific surface area (calculated value, 2630 m2 g−1) [7], thermal conductivity (∼5000 Wm−1 K−1) [8], mobility of charge carriers (exceeding 200,000 cm2/V s) at room temperature [9], saturation velocity (4.5 × 107 cm/s) [10], and critical current densities (∼3 × 109 A/cm2) [11]. Methods of obtaining C-pure graphene sheets is thus of strategic interest.
A variety of methods exist for synthesizing graphene sheets with either top-down or bottom-up approaches. Graphene or chemically modified graphene can be made from four different methods including mechanical exfoliation, chemical exfoliation, epitaxial growth on SiC and chemical vapor deposition on metal surfaces [12], [13], [14], [15], [16]. Mechanical exfoliation, epitaxial growth on SiC, and chemical vapor deposition yield ‘C-pure’ graphene sheets, which are very useful as attractive electronic materials for further micro- and nano-electronics. However, the relatively small yield and high costs have limited its development in electrodes of lithium ion batteries and supercapacitors, hydrogen storage, composites reinforcement, catalysis, and so on. ‘Chemical exfoliation’ is employed for ‘large scale’ (gram-scale and larger) production of ‘graphene’ sheets. However, the defects and some chemical functional groups such as hydroxyl (COH), carboxyl (CO, OCOH), and epoxide group (COC) could be introduced on the graphene sheets inevitably, which alters the physical and chemical properties. It should be noted that so far the reduction of oxygen content in reduced graphene oxide (RGO) is still very difficult [17]. After thermal or chemical treatment, the CO and OCOH could be partially removed or converted to a new chemical species (COH), whereas the remaining COH or COC in RGO could not be reduced easily, leading to the degradation of electrical properties of the graphene sheets. Scalable conversion of chemically modified graphenes to C-pure graphene sheets still remains a central challenge.
Herein, we report a simple and effective route to convert graphene oxide (GO) sheets to graphene sheets using hot pressing. The product materials were assessed by scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS) as well as Raman spectroscopy. Significantly, no appreciable oxygen content was observed from XPS, and in the Raman spectrum, no D peak was detected, while the G and 2D peaks characteristic of highly crystalline graphene were readily observed. Moreover, the product graphene sheets had much higher electron mobility (1000 cm2 V−1 S−1) than other chemically modified graphenes [18], [19], [20]. We believe that, highly crystalline graphene sheets from hot pressing treatment will provide an opportunity and a possibility to radically overcome the barrier for further application of graphene.
Section snippets
Materials
Graphite powder, natural, ∼325 mesh, 99.95% was purchased from Alfa Aesar. P2O5, K2S2O4, and KMnO4 with analytical grade and 98% H2SO4, 30% H2O2 aqueous solution were purchased from Shanghai Chemical Reagents Company, and were used directly without further purification.
Preparation and thermal reduction of GO
Graphite oxide was synthesized from natural graphite by a modified Hummers method [21]. The product (graphite oxide) was then purified by dialysis to completely remove residual salts and acids. After ultrasonication for 1 h and
Results and discussion
The conversion process consists of four steps, shown in Fig. 1a. Our novel contribution is that the RGO sheets were then treated by hot pressing at 1500 °C and 40 MPa uniaxial pressures for 5 min in vacuum. The RGO sheets were thereby converted from a pile of powder into a compacted lamellar material with tight combination (see Fig. S1). From the characterizations studies (see Fig. S2), the hot pressing did not change the original morphologies of the RGO sheets (such as size and number of layers),
Summary
The perfectly structural integrity and gram-scale production of graphene have been physically produced using hot pressing with high temperature and moderate pressure. The process is simple and effective, meeting the industrial level requirement of graphene applications. Besides the chemical produced graphene sheets, the graphene sheets from other processes may also be treated with hot pressing for further getting a perfect and highly crystalline graphene. This process could provide a
Acknowledgements
This research was supported by the National Natural Science Foundation of China (No. 11174227), National Basic Research Program of China (973 Program) (No. 2009CB939705), academic award for excellent Ph.D. candidates funded by Ministry of Education of China, and the Fundamental Research Fund for the Central Universities (2011202020003). L. Liao acknowledges the MOE NCET-10-0643 and NSFC Grant (Nos. 11104207, 91123009 and 10975109), Hubei Province Natural Science Foundation (2011CDB271), the
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