# Preparation of Few-Layer Graphene Dispersions from Hydrothermally Expanded Graphite
## 3 Results and Discussions  
To begin, we briefly analyze the hydrothermal conditions, the sonication process, the surfactant effect, and the yield. Supplementary Figure S2 shows the schematic route of the preparation process for graphene dispersions. Under hydrothermal conditions, various factors affect the graphite crystal structure which include temperature and pressure. Temperature generates water vapor, and pressure origins changes in the kinetics of reactions affecting the graphite structure [24]. Therefore, temperature and pressure create conditions for the intercalation of the CTAB (and water) molecules between graphene layers [25]. The intercalation increases the interlayer distance of graphite which is crucial to the improvement of the subsequent ultrasonication process [15]. Ultrasonication is described by the effect of acoustic cavitation of high frequency ultrasound in the formation, growth, and collapse of microbubbles in solution, which induces shock waves on the graphite surface, producing exfoliation of graphite into FLG [26]. Notwithstanding, the direct exfoliation of graphite in water is possible and the resulting exfoliated graphene flakes are not stable. The CTAB molecules have a cationic ammonium head group and an extended alkyl chain as a tail [26]. It is expected that the CTAB molecules are adsorbed onto the graphene surface through electrostatic interactions [25]. The hydrophobic interaction of the alkyl chains of the CTAB molecules on the graphene surface prevents the re-staking and agglomeration of exfoliated flakes. The yield of this approach, based on the absorption coefficient of [10], was \(\sim\)5% and needs to be optimized. At this point, the apparent low concentration of dispersed graphene in comparison with the relative amount of CTAB, is the result of using the absorbance to calculate the concentration by the Beer-Lambert law. Since this approach is usually used to obtain the concentration of dispersed graphene [10; 22; 23], it is employed to emphasize the role of hydrothermal treatment instead of its final yield. Attempts to improve the yield are currently in progress using the gravimetric method and extending the study for cationic/anionic surfactants. Pristine (gray) graphite and pretreated (light-yellow) graphite are shown in Figure 1a,b, respectively. The optical change is due to the interaction of graphite with the surfactant, and therefore the obtained light-yellow graphite is due to the presence of molecular bromine because the resulting pretreated graphite was not washed after HT, which is verified by the surfactant bubbles in Figure 1b. Since pretreated graphite is dispersed in the very same CTAB aqueous solution, sonication is immediately used to simplify the method. In order to understand the role of HT, Figure 1a,b present the concentration of dispersed graphene considering different amounts of CTAB and different sonication times. Again, we stress that this study is limited to 3 h of sonication, however it is well-known that the exfoliation yield is increased by increasing the sonication time, and a suitable sonication time is 10 h [15]. We show, however, that the concentration of dispersed graphene from pretreated graphite is significantly enhanced, suggesting that HT plays a crucial role. Therefore, depending on the amount of CTAB, sonication time, and HT, the concentration of dispersed graphene ranges from 40 \(\mu\)g mL\({}^{-1}\) to 60 \(\mu\)g mL\({}^{-1}\). These concentrations are comparable to those obtained in early studies by exfoliating graphite with a long sonication time [22; 27]. CTAB seems to be an efficient dispersant at high concentrations (0.5, 1.0 mg mL\({}^{-1}\)). However, high quantities of surfactant may have a significant impact on the electrical properties of graphene because the residual surfactant is difficult to remove [28]. To circumvent this, we focus on graphene dispersions obtained at the lowest concentration of CTAB (0.1 mg mL\({}^{-1}\)) and after 3 h of sonication.