Ali et al. [52] reported a one-step

Ali et al. [52] reported a one-step approach for the synthesis of MoS2/ graphene by electrochemical exfoliation of graphite. For performing the anodic electrochemical method, the connected bulk MoS2 crystal, and graphite rod as the anode were used. Also, a platinum wire was used as cathode and the 1 M Na2SO4 solution was the electrolyte, in their study. The electrochemical process was conducted by applying the anodic voltage of 2 V for 5 min, followed by 10 V for min. Similar approaches were carried out for bulk MoS2 crystal and graphite anodes, separately. Fig. 8 compared the XRD patterns of MoS2/ graphene composite, MoS2, and graphene samples. As shown in Fig. 8, the XRD pattern of obtained powder by electrochemical exfoliation of graphite/MoS2 anode is comprised of both diffraction peaks of graphene and MoS2.Fig. 8Download : Download high-res image (301KB)Download : Download full-size imageFig. 8. Comparative XRD patterns MoS2/ graphene composite, MoS2 and graphene samples [52].4. Functionalization of graphene with heteroatomsTo achieve better electrical conductivity and electrochemical behavior, graphene sheets doped with heteroatoms (i.e. S, B, N, F, etc.) have attracted attention. In this section, it is tried to review the literature regarding the one-step electrochemical exfoliation and doping of the graphite with heteroatoms. Some information on in-situ exfoliation and functionalization of graphite with heteroatoms is presented in Table 2.A facile electrochemical method to synthesize sulfur-doped graphene (S-GN) was reported by Parveen et al. [63]. The simultaneous exfoliation and S doping of graphite were the results of their study. For the synthesis of GN and S-GN by the electrochemical exfoliation method, the graphite rods were used as both the anode and cathode materials. The electrolyte was prepared by dissolving the mixture of Na2S2O3 + H2SO4 in H2O for both functionalization and exfoliation of graphite, in this study. Electrochemical exfoliation was done by applying an anodic voltage of 5 V under ultra-sonication bath conditions. Fig. 9 shows the XRD patterns, Raman spectra, ID/IG values, and magnified parts of Raman analyses of the GN and S-GN samples. The XRD pattern of GN and S-GN samples indicated that the better exfoliation of the graphite using the mixture of graphite in Na2S2O3 + H2SO4 electrolyte (Fig. 9a). It is worth noting that the Na2S2O3 in the electrolyte was used for functionalization while H2SO4 was utilized for the exfoliation of graphite. As shown in Fig. 9a, the position of the (002) peak in the S-GN sample is at lower angles compared to the GN sample. This observation proves the presence of oxygen and sulfur functional groups on the surface of graphene sheets. Besides, the lower peak intensity of the S-GN, compared with the GN sample, indicates the higher degree of exfoliation of graphite. The Raman spectra of samples (Fig. 9b–e) showed that the ID/IG value for the S-GN sample was about 0.92 while for GN sample was 0.35. The higher ID/IG value is an indication of the functionalization of the graphene with sulfur.Fig. 9Download : Download high-res image (383KB)Download : Download full-size imageFig. 9. (a) The XRD patterns, (b) Raman spectra, (c) peak shifting of the G band, (d) peak shifting of the 2D band, and (e) ID/IG value of the graphene and sulfur-doped graphene, reproduced from [63].In another similar study, Lee et al. [64] used an electrochemical exfoliation technique to synthesize of sulfur-doped graphene nanosheets (SDGNs). The graphite foil as anode and platinum foil as a counter electrodes were used in their investigation. The used solution mixtures of Na2S2O3 + H2SO4 with different volume ratios of 0:1, 3:1, 5:1, and 10:1; the obtained product of electrochemical method in the mentioned electrolytes was respectively called SDGN (0), SDGN (3), SDGN (5), and SDGN (10). For the electrochemical exfoliation, the anodic voltage of 15 V was applied through the simultaneous sonication for 2 h. Their results demonstrated that the highest degree of sulfur doping was achieved for SDGN (5) sample. The presence of Na2S2O3 in electrolytes act as the sulfur doping source. In other words, the carbon atoms in graphene are replaced with sulfur by adding the Na2S2O3 in electrolytes. Fig. 10 shows the TEM images along with EDS mapping. According to the EDS mapping images (Fig. 10d–f), the successful preparation of the few-layered sulfur-doped graphene in the SDGN (5) sample is demonstrated. Fig. 11 indicates the Raman spectra, ID/IG ratios, and magnified parts of Raman spectra of SDGN (0), SDGN (3), SDGN (5), and SDGN (10) samples. As indicated in Fig. 11b, the highest ID/IG value (1.21) has been obtained for the SDGN (5) sample, indicating the higher level of sulfur doping in this sample.