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CARBON 47 (2009) 493–499 497 Fig. 6 – Statistical distributions of the thickness and the number of layers of the graphenes prepared from different starting graphites. (a) Histograms of the thickness distribution of graphene derived from HOPG, NFG, KG, FGP, and AG, obtained from 100 graphenes for each sample. (b) Histogram of the distribution of the number of graphene layers, obtained from the corresponding thickness distribution in (a). on a Gaussian fit, indicate that the graphenes prepared from AG, FGP, KG, NFG and HOPG have a mean thickness of 1.30, 1.87, 2.05, 5.75 and 6.19 nm, respectively. Consequently, the layer-number distribution of our graphene can be obtained, as shown in Fig. 6b. It is interesting to find that 80% of the final products are single-layer, single- and double-layer, dou- ble- and triple-layer, and few-layer (4–10 layers) graphene, respectively, from AG, FGP, KG, and NFG as starting materials, while a mixture of few-layer (4–10 layers) and thick graphene (>10 layers) is obtained when HOPG is used. These results sug- gest that the number of layers for chemically derived graph- ene can be tuned by selecting suitable starting graphite materials. AG, FGP and KG with low crystallinity and/or small lateral size are suitable to prepare graphenes with 1 to 3 layers. From the above results, we suggest that the selectivity of the number of graphene layers is intrinsically attributed to the different structure of the starting graphite materials. It is well accepted that GO formation involves the reaction of graphite with strong oxidants [15–17], and the exposed carbon atoms, in particular those on the edges of the graphitic layers, are most likely to be attacked by the oxidants during the oxi- dation stage [25]. The starting graphite with a low crystallinity has weak interlayer interaction, easily causing the increase of d-spacing and configuration change from a planar sp2-hybrid- ized to a distorted sp3-hybrized geometry. This increases the possibility of the diffusion of oxidants between graphitic lay- ers. Moreover, because the diffusion route is shorter for the graphite with smaller lateral size, the oxidation and intercala- tion occur more easily. As a result, graphite with a small lat- eral size and low crystallinity can easily form well-oxidized GO under the same conditions. McAllister et al. [17] proposed that exfoliation upon rapid heating takes place only when the decomposition rate of the epoxy and hydroxyl sites of GO ex- ceeds the diffusion rate of the evolved gases. Therefore, the exfoliation degree of TEGO derived from different starting graphites is different due to the different oxidation degree of the corresponding GO. This result can be proved by the dif- ferent surface areas of TEGO derived from different starting graphites, which were 75, 50, 152, 137, and 351 m2/g, respec- tively for HOPG, NFG, KG, FGP, and AG. Since the number of layers in the obtained graphene is mainly determined by the exfoliation process, we suggest that the synthesis of graphene with selected number of layers is essentially attrib- uted to the different oxidation degrees of the starting graph- ites, caused by their different lateral size and crystallinity. Normally, chemically derived graphene is electrically insu- lating due to the heavy oxygenation of graphene sheets, which can not be used as conductive materials and electronic devices without further processing [15]. Recent studies have demonstrated that the electrical conductivity of graphene can be dramatically improved by chemical reduction or ther- mal treatment [6,15,22,26]. Considering the similar atomic C/ O ratio of our graphene to that of chemically reduced graph- ene sheets, therefore, it is reasonable to expect that the Fig. 7 – Electrical conductivity measurement of the graphene derived from AG. (a) TEM image of an individual single-layer graphene during electric conductivity measurements, using Au as electrode and tungsten as tip. (b) Typical I–V curve of the graphene in (a).PDF Image | graphene with a pre-determined number of layers
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