# Facile Preparation of Multilayered Graphene with CO\({}_{2}\) as a Carbon Source
## 3 Results
### Multilayered Graphene under Different Concentrations of HCl Solution for Sample Post-Processing  
Note that the sample should be treated with HCl solution to remove the reaction products MgO and the remaining metal Mg that did not react with CO\({}_{2}\). In order to study the effect of different concentrations of HCl solution for sample post-processing, a range of concentrations of the HCl solution (0.5 M, 1.0 M, and 2.0 M) were used in the post-processing process. TEM was performed to observe the morphology of the carbon-based sample produced. EDS was also performed to determine the content of Mg in the multilayered graphene sample. The results are shown in Figures 5-8. The morphology of the sample treated with 1.0 M concentration of HCl solution is shown in Figure 6. The image in Figure 6a reveals a silky-smooth satin ribbon dotted with some obstacles with the scale ranging from ten to a hundred nanometers (Figure 6b). As seen from the magnified TEM images in Figure 6c, a typical graphene structure with graphitic lattices occurred on the surface of  
Figure 4: The yield under different reaction times prepared with an Mg ribbon and Mg powder. Figure 5: Transmission electron microscopy (TEM) images of multilayered graphene treated with 0.5 M concentration of HCl. (**a**) Thin graphene sheets aggregates. (**b**) The composition of multilayered graphene. (**c**) Typical stripe structures that occurred on the surface of the sample. (**d**) A high-resolution TEM image of multilayered graphene. the sample. The image in Figure 6d indicates a three-to-nine-layer graphene with a lattice space of about 3.5 A. The morphology of the sample treated with a 2.0 M concentration of HCl solution is shown in Figure 7. Compared with the sample treated with 1.0 M HCl, a similar morphology was obtained with the sample treated by 2.0 M HCl. From Figure 7, a typical strips pattern can be seen and proved to be multilayered graphene with the lattice space of 3.5 A. To determine the content of Mg in the samples, the result of EDS is shown in Figure 8. It can be observed that the sample treated with 0.5 M HCl solution showed a high Mg content (the Mg was 56.52 (norm wt%) and O was 40.27 (norm wt%)), which proved that Mg atoms were left in the sample after 0.5 M HCl treatment. This further demonstrated that the 0.5 M concentration of HCl could not remove the Mg atoms in the sample. This is consistent with the result of TEM mentioned above. The sample treated with 1.0 M HCl solution showed a rare Mg existence (0.01 norm wt%), and the purity of  
Figure 6: TEM images of multilayered graphene treated with 1.0 M concentration of HCl. (**a**) Thin graphene sheets with occasional folding. (**b**) The composition of multilayered graphene. (**c**) Typical graphene structures occurred on the surface of the sample. (**d**) High-resolution TEM image of multilayered graphene. Figure 7: TEM images of multilayered graphene treated with a 2.0 M concentration of HCl. (**a**) Typical stripe structures that occurred on the surface of the sample. (**b**) A high-resolution TEM image of multilayered graphene. the carbon-based materials obtained reached up to 98.61 (norm wt%). Therefore, it can be deduced that almost no Mg atoms existed in the sample treated with 1.0 M HCl. Moreover, the EDS pattern of the sample treated with the 2.0 M HCl solution demonstrated that the amounts of C were nearly 98.6% (norm wt%), which showed a similar trend with that of the 1.0 M HCl solution. Therefore, 1.0 M HCl as the optimal concentration can be selected for sample post-processing. As stated above, 1.0 M HCl can be selected for post-processing of the samples. Hence, the optimum experimental condition used in this study was as follows: velocity of CO\({}_{2}\) of 60 mL/min, reaction temperature of 700 \({}^{\circ}\)C, reaction time of 30 min, and a sample post-processing solution of 1.0 M HCl. In the above conditions, the product prepared was 233 mg/g Mg. This showed that the yield rate almost reached 94.2% according to Equation (2).