# Preparation of Few-Layer Graphene Dispersions from Hydrothermally Expanded Graphite
## 4 Conclusions  
In this study, a simple method to prepare graphene dispersions has been demonstrated. Natural graphite flakes were effectively exfoliated into FLG under HT and liquid-phase exfoliation in a surfactant aqueous solution. The cationic surfactant, CTAB, was used as a dispersant and a stabilizer. First, pristine graphite was hydrothermally expanded in the CTAB aqueous solution reacted at 180 \({}^{\circ}\)C for 15 h. Subsequently, pretreated graphite (denominated as hydrothermally expanded graphite, HEG) was exfoliated into FLG by sonication for 3 h, producing stable and well-dispersed graphene dispersions. The expansion of interlayer spacing of graphite was demonstrated using SEM measurements. The efficiency of HT was estimated by measuring the concentration of the dispersed graphene which was shown to be comparable to graphene dispersions reported with a long sonication time [22; 27]. Depending on the amount of the CTAB, sonication time, and HT, the concentration of dispersed graphene ranged from 40 \(\upmu\)g mL\({}^{-1}\) to 60 \(\upmu\)g mL\({}^{-1}\). The obtained graphene dispersions from pretreated graphite were shown to have good stability within 12 weeks. Using Raman analyses, the obtained FLG presented lower (edge/basal) defects, and most importantly, based on the FWHM analysis, we confirmed the presence of FLG, likely, no more than five layers. This novel approach (but non-optimized) may be explored in the future for cationic/anionic surfactants (or biosurfactants) in order to improve its efficiency. Figure 3: Raman spectra of (**a**) dispersed graphene after hydrothermal treatment and 3 h of sonication in 0.1 mg mL\({}^{-1}\) CTAB aqueous solution, fabricated by drop casting of dispersion, and (**c**) pristine graphite. (**b–d**) Fitting of the 2D peaks with two Lorentzian functions. The intensity was normalized by the G peak. **Supplementary Materials**: The following are available online at [[http://www.mdpic.com/2076-3417/9/12/2539/s1](http://www.mdpic.com/2076-3417/9/12/2539/s1)]([http://www.mdpic.com/2076-3417/9/12/2539/s1](http://www.mdpic.com/2076-3417/9/12/2539/s1)), Figure S1: Obtained graphene dispersions (12 weeks stored) after hydrothermal treatment (HT), 3 h of sonication and centrifugation, in the three different concentrations of CTAB (0.1, 0.5, and 1.0 mg mL\({}^{-1}\)) reported in the main text. After sonication, the obtained suspensions were left to stand overnight and then centrifugated for 90 min at 2000 rpm., Figure S2: Schematic route of the preparation process for FLG dispersions. Natural graphite flakes are added into CTAB aqueous solution (at different surfactant concentrations: 0.1, 0.5, and 1.0 mg mL\({}^{-1}\)). The resulting mixture is reacted at 180 \({}^{\circ}\)C for 15 h. The obtained pretreated graphite is immediately dispersed in the very same CTAB aqueous solution by sonication, Figure S3: Scanning electron microscopy (SEM) morphology of dispersed graphene layer in 1.0 mg mL\({}^{-1}\) of CTAB aqueous solution, by changing the accelerating voltage from 5 to 30 kV. Semitransparent layer extracted from supernatant after HT, 3 h of sonication, centrifugation for 90 min at 2000 rpm, and dried at 235 \({}^{\circ}\)C, Figure S4: Raman spectrum of the 2D peak of dispersed graphene in 0.1 mg mL\({}^{-1}\) of CTAB aqueous solution and fitting of the 2D peaks with two Lorentzian functions. The w values represent the respective full width at half maximum (FWHM) values, Figure S5: Raman spectrum of the 2D peak of dispersed graphene in 0.1 mg mL\({}^{-1}\) of CTAB aqueous solution and fitting of the 2D peaks with one Lorentzian function. The w value represents the respective FWHM value. **Author Contributions:** All authors contributed to designing and performing measurements, data analysis, scientific discussions, and writing the article. **Funding:** This research received no external funding. **Acknowledgments:** We acknowledge [PERSON] for helping us with Raman measurements, as well as [PERSON], [PERSON], and [PERSON] for the stimulating discussions.