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496 CARBON 47 (2009) 493–499 Fig. 4 – Tapping-mode AFM images and the corresponding height profiles of the graphenes derived from (a) and (d) KG, (b) and (e) FGP, and (c) and (f) AG. The thicknesses of the graphenes are 1.9–2.3 nm, 1.3–2.1 nm, and 1.1–1.3 nm, respectively, for KG, FGP, and AG as starting materials. caused by the presence of epoxy and hydroxyl groups on both sides of the surface [16,17]. This result has also been experi- mentally proved by many groups based on AFM observations, and the thickness of a chemically derived single-layer graph- ene was found to be 1.1 nm [15–17,22,23]. In order to obtain the structural information of our graphene, we performed a great deal of HRTEM observations. Fig. 5 shows the typical HRTEM images of the obtained graphenes with different number of layers. The folded structure of graphene edges al- lows for the evaluation about their thickness and interlayer spacing. It is worth noting that, different from graphite, the adjacent graphene layers of double- and triple-layer graph- enes derived from chemical exfoliation are not parallel but uneven. The maximum thicknesses of single-, double-, and triple-layer graphenes derived from HRTEM images are 0.57, 1.25, and 1.83 nm, respectively. Based on the above XPS analyses, we believe that there exist some residual func- tional groups on the surface of graphene although most of them have been removed during the thermal expansion and H2 reduction processes, and these functional groups can not be directly probed by HRTEM. According to the measured thicknesses of single-, double-, and triple-layer graphenes, the interlayer spacing can be calculated to be in the range of 0.55–0.70 nm. Obviously, this value is much larger than that of graphite, normally 0.335 nm. This spacing increase, 0.21–0.36 nm, is consistent with that theoretically predicted for functional groups on the graphene surface [16,17]. As a consequence, the real thickness of the obtained graphene should be the sum of the observed thickness from HRTEM and the thickness increase caused by the presence of epoxy and hydroxyl groups on both sides of the surface. With 0.36 nm thickness increase, the real thickness of single-, dou- ble-, and triple-layer graphenes was evaluated to be 0.93, 1.61 and 2.19 nm, respectively. Taking into account of the in- creased spacing or instrumental offset between the substrate and graphene sheet [16,17,24], in our calculations, we adopt 1.1 nm and 0.7 nm as the average thickness of single-layer graphene and the interlayer spacing, respectively. The topo- graphic height distributions (Fig. 6a), derived from AFM mea- surements on 100 sheets for each starting material and based Fig. 5 – Typical HRTEM images of (a) single-, (b) double-, and (c) triple-layer graphene sheets with folded edges obtained by chemical exfoliation.PDF Image | graphene with a pre-determined number of layers
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