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494 CARBON 47 (2009) 493–499 epoxide reduces interlayer interactions and results in an in- crease in the d-spacing of GO, thereby promoting complete exfoliation of single GO layers in some specific conditions. By using the chemical exfoliation method, single-layer graph- ene has been prepared. For example, Ruoff and co-workers prepared single-layer graphene by exfoliation of GO via ultra- sonic treatment, followed by chemical reduction with hydra- zine hydrate to modify their transport properties [15]. Schniepp et al. prepared functionalized graphene by oxida- tion of graphite and subsequent thermal expansion/exfolia- tion of GO with rapid heating [16]. Very recently, Li et al. fabricated ultra-smooth graphene nanoribbons by combining thermal exfoliation of expandable graphite with chemome- chanical breaking of the resulting graphene sheets by sonica- tion [19]. However, the selective production of high-quality graphene with a selected number of layers in a large quantity still remains a significant challenge. We have studied the effect of the lateral size and crystal- linity of starting graphite materials on the number of graph- ene layers by the chemical exfoliation method. It is interesting to find that both the lateral size and the crystallin- ity of the starting graphite materials play important roles in the number of graphene layers produced, and the majority of graphene can be tuned to a specific number of layers by selecting suitable starting graphite. For example, artificial graphite, a graphite with a small lateral size and low crystal- linity, is suitable for the production of single-layer graphene. Also, we found that this graphene has high electrical conductivity. 2. Experimental 2.1. Materials Five types of graphite materials: highly-oriented pyrolytic graphite (HOPG), natural flake graphite (NFG, Qingdao Black Dragon Graphite Co., Ltd), Kish graphite (KG, Sinopharm Chemical Reagent Co., Ltd), flake graphite powder (FGP, Sinop- harm Chemical Reagent Co., Ltd) and artificial graphite (AG, Qingdao Black Dragon Graphite Co., Ltd), were used as the starting materials to demonstrate the production of graphene with a selected number of layers using chemical exfoliation. 2.2. Preparation of graphene The preparation of graphene involves three key steps, (i) oxi- dation of the starting graphite to synthesize GO, (ii) thermal expansion/exfoliation of the as-prepared GO to obtain ther- mally expanded GO (TEGO), and (iii) reduction and dispersion of the resulting TEGO to produce graphene. It should be emphasized that, for comparison, all the experimental proce- dures related to the synthesis of GO, TEGO and graphene from the five starting graphites were performed under the same conditions. 2.2.1. Synthesis of GO We used the Hummer method [21] to oxidize the different starting graphites for the synthesis of GO. First, 2 g graphite, 1 g sodium nitrate and 46 mL of sulfuric acid were mixed and strongly stirred at 0 °C for 15 min in a 500 mL reaction flask immersed in a water-glycol bath (DFY-5 L/25). Then 6 g potassium permanganate was added slowly to the above solution and cooled for 15 min. After this, the suspended solution was stirred continuously for 1 h, and 92 mL of water was added slowly to the suspension for 10 min. Subsequently, the suspension was diluted by 280 mL of warm water and treated with 10 mL of H2O2 (30%) to reduce residual perman- ganate to soluble manganese ions. Finally, the resulting sus- pension was filtered, washed with water, and dried in a vacuum oven at 60 °C for 24 h to obtain GO. 2.2.2. Thermal expansion/exfoliation of GO The as-prepared GO was thermally expanded to synthesize TEGO by rapidly heating it in a Lindberg tube furnace. Gener- ally, the as-prepared GO was first loaded in a quartz boat of length 100 mm and diameter 20 mm, which was then in- serted into a 1.5 m-long quartz tube with inner diameter of 22 mm and outer diameter of 25 mm. After the tube furnace was heated to 1050 °C and argon was flowed though the tube for 10 min, with a flow rate of 200 mL/min, the sample boat of GO placed in the quartz tube was rapidly moved into the mid- dle heating zone of the furnace and kept there for 30 s, before being quickly removed from the heating zone. 2.2.3. Reduction and dispersion of TEGO The resulting TEGO was first reduced by H2 for 2 h at 450 °C in a gas flow of H2 (100 mL/min) and argon (100 mL/min). Then, the reduced TEGO platelets were stirred for 1 h in a solution of N-methylpyrolidone (NMP) before being dispersed for 2 h at 40 °C by sonication to form a homogenous suspension. Final- ly, centrifugation was used to remove thick multilayer pieces and not fully exfoliated graphite flakes from the TEGO-gener- ation process, and to retain the thin graphene sheets in the supernatant. 2.3. Material characterization X-ray photoelectron spectroscopy (XPS, Escalab 250, Al ka), atomic force microscopy (AFM, Veeco MultiMode/NanoScope IIIa), scanning electron microscopy (SEM, LEO, Supra 35, 15 kV), high-resolution transmission electron microscopy (HRTEM, JEOL JEM-2010, 200 kV, and Technai F30, 300 kV), and nitrogen cryosorption (Micromeritics, ASAP2010M) were used to characterize the GO, TEGO and graphene. It is neces- sary to point out that the supernatant was dropped onto grids with a holey carbon film for SEM investigations. 2.4. Electrical conductivity measurement The electrical conductivity of graphene was measured inside a JEOL JEM-2010 HRTEM equipped with a Nanofactory TEM- scanning tunneling microscopy (STM) system (ST1000), which integrates a fully functional STM into a HRTEM. The STM probe was controlled by a piezo-manipulator that can ap- proach individual nanostructures inside the HRTEM. The ob- tained supernatant after centrifugation treatment was filtered and then dried in a vacuum oven at 60 °C for 24 h. The resulting graphene powders were attached to an Au elec- trode, and the tungsten STM tip was controlled precisely inPDF Image | graphene with a pre-determined number of layers
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