Controlled oxidation of graphite to graphene oxide with novel oxidants in a bulk scale
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In this study, a novel method of graphite chemical exfoliation to create graphene oxide (GO) is reported. Here, new oxidants were examined: a mixture of perchloric and nitric acids and potassium chromate. Furthermore, an effect of oxidation time, temperature of oxidation, and ultrasonication on graphite exfoliation degree was investigated. The obtained GOs were next reduced with glucose, used as a reducing agent. Detailed analysis of the materials indicated that when graphite was oxidized for 24 h at 50 °C, 5-layered graphene was prepared. An effect of sonication process was also examined, and it was found to enhance the exfoliation to bilayer graphene. Furthermore, when time and temperature were increased to 48 h and 100 °C, respectively, graphite was exfoliated to single-layer graphene. Therefore, it is believed that the proposed route can be applied for the preparation of graphene or few-layered graphene with defined number of layers upon the process parameters optimization and in a bulk scale. The materials were characterized with atomic force microscopy, Fourier-transform infrared spectroscopy, Raman spectroscopy, and X-ray diffraction.
KeywordsGraphite Chemical exfoliation Graphene oxide
Graphene shows excellent properties, such as high intrinsic carrier mobility (200,000 cm2/V3 s), superior thermal conductivity, and excellent mechanical strength and elasticity (Bolotin et al. 2008; Balandin et al. 2008; Lee et al. 2008). Since graphene was isolated in 2004 by Geim and coworkers (2004) using a scotch tape method, there have been many processes developed to produce graphene. Although the mechanical exfoliation of graphite leads to production of high-quality and high-mobility graphene flakes, this method is a time consuming process limited to a small scale production. One of the methods is thermal decomposition of SiC in temperature range between 1,000 and 1,500 °C. Here, Si sublimate from the silicon carbide and leave behind a carbon-rich surface (Hass et al. 2008). This method requires controlling the number of graphene layers, repeatability of large area growths, and interface effects with the SiC substrate (Choi et al. 2010). The second method of graphene synthesis is chemical vapor deposition (CVD). This process is promising in large scale production of graphene. Many papers concerning CVD growth of graphene using different catalysts (Ni, Cu, ZnS, Fe) have been reported (An et al. 2011; Wei et al. 2009; Li et al. 2009; Reina et al. 2009; Obraztsov 2009). Graphene can be also synthesized via chemical exfoliation of graphite, where interlayer van der Waals forces are eliminated: chemical derivatization, intercalation, thermal expansion, the use of surfactants, and oxidation–reduction (Chakraborty et al. 2008; Lotya et al. 2009; Lee et al. 2008). The most common route to chemically exfoliate graphite is the use of strong oxidants to produce graphene oxide in a first stage. Graphite oxide was first prepared by B.C. Brodie, where it was treated with a mixture of potassium chlorate and nitric acid (Brodie 1859). Later, Hummers and Offeman (1958) used a mixture of sulfuric acid, sodium nitrate, and potassium permanganate to oxidize graphite. Recently, many papers reporting a modification of the Hummers method have been published. For instance, Marcano et al. (2010) used a mixture of sulfuric and orthophosphoric acids and potassium permanganate in the oxidation process, and found it improved an efficiency of the oxidation. This process is still widely investigated for two reasons: (1) it gives an opportunity to produce large scale graphene or few-layered graphene, (2) it has potential to provide the samples with controlled number of graphene layers upon the process parameters optimization.
In this study, novel oxidants of graphite, toward its chemical exfoliation, were examined: a mixture of perchloric and nitric acids and potassium chromate. Furthermore, an effect of oxidation time, temperature of oxidation and ultrasonication in exfoliation degree were investigated. The obtained materials were characterized with TEM, FT-IR, Raman spectroscopy, and XRD.
Graphite was purchased from Alfa Aesar (synthetic, 99.9995 %, 325 mesh). Perchloric acid, nitric acid, hydrochloric acid, and ethanol were obtained from Chempur. K2CrO4 was bought from POCH.
Here, three types of graphene oxides (GOs) were prepared. In the first procedure of GO1 synthesis, 1 g of graphite was dispersed in a mixture of perchloric and nitric acids (350 mL, 4:3—volume ratio), and next, K2CrO4 (6 g) was added. The mixture was then heated to 50 °C and reaction was operated for 24 h. The obtained mixture was next filtrated through polycarbonate (PC) membrane (Whatman 0.2 μm) and washed three times with ethanol (200 mL) and 10 % hydrochloric acid (200 mL) to remove residual metal ions, and finally with distilled water until pH of the solution was 7. Finally, the material was dried in air at 100 °C for 24 h.
In the second method, prior heating a mixture of graphite, perchloric acid, nitric acid, and K2CrO4, an ultrasonication process was performed for 6 h at room temperature. After oxidation process, the purification, filtration, and drying steps were realized as in the first procedure, and finally GO2 was obtained.
In the third route, the time and the temperature of oxidation process were increased to 48 h and 100 °C, respectively. The sonication, purification, filtration, and drying steps were carried out as in the second procedure to obtain GO3. After the purification process of the each graphene oxide, the content of impurities was determined with thermogravimetric analysis. It was estimated that GO1, GO2, and GO3 contain 0.11, 0.19, and 0.08 wt% of contaminants, respectively (data are not presented here).
Reduced graphene oxide (RGO) was synthesized with glucose using as a reducing agent (Zhu et al. 2010). In a typical procedure, graphene oxides (GO1, GO2, and GO3) were separately dispersed in 50 mL of water (0.5 mg/mL) and ultrasonicated for 2 h. Next, 80 mg of glucose was added to each homogeneous GO dispersion, and the mixtures were stirred for 30 min. Then, 40 μL of ammonia solution was added and reactions were heated to 95 °C, stirring simultaneously for 2 h. Next, the each reaction mixture was filtered through PC membrane (0.2 μm Whatman). The obtained solid material was then washed with water and ethanol (3 times). Finally, the products (RGO1, RGO2, and RGO3) were dried in air at 100 °C for 24 h.
The morphology of the obtained materials was characterized via atomic force microscopy (Nanoscope V MultiMode 8, Bruker). The measurements were done in air under ambient conditions. Raman measurements were performed on an In-Via Raman microscope (Renishaw) with excitation laser wavelengths of 785 nm. Raman spectra were obtained from individual flakes deposited on SiO2/Si wafer (300 nm SiO2) (Liana et al. 2010). The crystallographic structure of the samples were characterized by XRD analysis (X’Pert PRO Philips diffractometer) using a CuKα radiation. FT-IR absorption spectra were recorded on Nicolet 6700 FT-IR Spectrometer.
Results and discussion
In summary, novel method of chemical exfoliation of graphite was presented. Here, novel oxidants were examined: a mixture of perchloric and nitric acids and potassium chromate, and an effect of oxidation time, temperature of oxidation, and ultrasonication in exfoliation degree were investigated. The presented methodology leads to creation of graphene with controlled number of layers: single-, bi-, and 5-layered graphene in a bulk scale.
The authors are grateful for the financial support of National Science Center, nr 2011/01/N/ST5/02912.
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