# Preparation of Few-Layer Graphene Dispersions from Hydrothermally Expanded Graphite
## 3 Results and Discussions
Figure 1c shows the stability of graphene dispersions, at the lowest concentration of CTAB, obtained from pretreated and pristine graphite. In both cases, graphene flakes are easy to precipitate, as the storage time increases, however, it is noteworthy that the stability of graphene dispersion obtained from pretreated graphite is higher than from pristine graphite within 12 weeks. These results corroborate the importance of HT. The UV-visible absorption spectra of graphene dispersions, after 3 h of sonication and different CTAB concentrations, are shown in Figure 1d. The spectra are featureless in the visible region as expected for graphene. A prominent peak is found at \(\sim\)266 nm, corresponding to the \(\pi\) to \(\pi^{*}\) transitions of graphene and graphite [29]. Previously, the transformation of the graphite (powder) structure, after heat-treatment in NMP, was effectively demonstrated by SEM, Raman, and X-ray diffraction (XRD) measurements, i.e., the increased interlayer spacing of graphite as a direct consequence of increasing the heat [15]. In Figure 2, we confirm this fact by scrutinizing the morphology of pretreated (natural) graphite flakes considering the different amounts of CTAB. Independently of the CTAB concentration (0.1, 0.5 and 1.0 mg mL\({}^{-1}\)), as shown in Figure 2a-c, HT also increases the interlayer spacing of graphite which is stabilized by surfactant. Therefore, the resulting pretreated graphite is called hydrothermally expanded graphite (HEG). At the highest concentration of CTAB, in Figure 2a, we show the surfactant intercalation (green arrows) and the HEG surface coated by the surfactant molecules (green dashed circle). After 3 h of sonication in the three different CTAB concentrations, Figure 2d-f demonstrate that graphene dispersions are composed of smaller and thinner layers. These layers seem to be semitransparent and surfactant-free, which is interpreted as an important result for future applications in electronic devices. However, by scrutinizing the morphology of layers at the highest concentration of CTAB and by changing the accelerating voltage from 5 to 30 kV, in Supplementary Figure S3, we observe the presence of surfactant which corroborates the statement about the difficulty of the residual surfactant removal and the need to use the proposed method at the lowest concentration of CTAB in order to prevent the intrinsic properties of the obtained layers. Figure 1: Concentration of dispersed graphene as a function of three different hexadecyltrimethylammonium bromide (CTAB) concentrations (0.1, 0.5, 1.0 mg/mL\({}^{-1}\)) at three different sonication times, from (**a**) pristine (untreated) graphite and (**b**) treated graphite by the hydrothermal treatment (HT). (**c**) Stability of the graphene dispersion in 0.1 mg/mL\({}^{-1}\) of the CTAB aqueous solution as a function of the storage time, considering pristine and pretreated graphite source. (**d**) UV-visible spectra of exfoliated graphene in the three different CTAB concentrations after HT and 3 h of sonication. Optical photos of pristine graphite (**a**, inset) and pretreated graphite (**b**, inset). The Raman spectrum of graphite and dispersed graphene (at the lowest concentration of CTAB, after HT and 3 h of sonication) are reported in Figure 3. We detect the three major peaks, i.e., the D peak at \(\sim\)1340 cm\({}^{-1}\), the G peak at \(\sim\)1580 cm\({}^{-1}\) (\(\sim\)572 cm\({}^{-1}\)), and the 2D peak at \(\sim\)2719 cm\({}^{-1}\) (\(\sim\)2711 cm\({}^{-1}\)). The D peak and G peak are attributed to the edge/basal defects and the ordered \(sp^{2}\)-hybridized carbon bonds in the graphene/graphite lattice, respectively. In dispersed graphene, the peak ratio between the intensity of D and G peaks (\(I_{\text{D}}/I_{\text{G}}\)) is 0.28, indicating that some edge/basal defects are induced during the process of ultrasonication [30].