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D.F. Carrasco, J.I. Paredes, S. Villar-Rodil et al. Carbon 195 (2022) 57e68 Fig. 2. Microscopic characterization of different holey graphene precursors. (aeb) Representative field emission scanning electron microscopy (FE-SEM) images of the dry powder of electrochemically derived, highly oxidized graphene. (c) Digital pictures of the previous material in concentrated (left) and diluted (right) aqueous suspension upon ultrasonication. (d) Representative atomic force microscopy (AFM) image of a nanosheet of anodic graphene and histograms of (e) thickness and (f) lateral size. (g) AFM image and histograms of (h) thickness and (i) lateral size for mildly reduced graphene oxide. The histograms were measured on ~60 objects from the AFM images. domains [35,36]. For example, it is located at ~230 nm in unreduced graphene oxides, which only possess extremely small (~1e3 nm) aromatic domains, but shifts to ~270 nm for well-reduced graphene oxides, which exhibit larger domains, as well as for pristine gra- phene (indefinitely large domains) [26,35,37]. Consistent with the substantial degree of oxidation of the present electrochemically derived graphene, the absorption peak of EOG lay in-between these two extremes at a similar position to that of MRGO (see Fig. 3a, orange trace). More significantly, a shoulder around 300 nm, which is assigned to n/p* transitions of C1⁄4O bonds also appeared for EOG but was absent for MRGO. Thus, even though the overall oxidation level of EOG and MRGO was very much the same, the oxidized domains where more noticeable in the former graphene. Further evidence in support of this conclusion was collected from the observation of the p plasmon band in EOG by X-ray photoelectron spectroscopy (XPS). As expected, the high resolution C 1s XPS envelope of EOG (Fig. 3b, red trace) was dominated by a component located at ~284.6 eV, arising from sp2-based carbon atoms in unaltered aromatic structures. A strong component at ~286.5 eV associated to carbon atoms in an oxidation state of þ1 (e.g., in hydroxyl and/or epoxy groups) was also present [23,25], in agreement with the high oxygen content of this graphene. In addition, a broad, very weak but nonetheless discernible compo- nent above ~291 eV could be seen in the C 1s envelope of EOG. This feature corresponds to the well-known p/p* shake-up satellite band characteristic of delocalized electrons in sp2-hybridized car- bon structures [38], and its presence was a clear indication of (at least) a relatively well developed aromatic character in the carbon material. Notably, the C 1s spectrum of MRGO lacked such a satellite peak, i. e., no intensity was detected above 291 eV (Fig. 3c, orange trace). This result implied that EOG boasted more abundant, larger aromatic domains compared to those of MRGO, despite their having the same extent of oxidation. The Raman spectrum of EOG (Fig. 3d, red trace) displayed the typical features of graphite/graphene materials, namely, the D and G bands in the first-order region (1100-1700 cm1) at 1344 and 1590 cm1, respectively, together with some additional bands in the second-order region (2300-3500 cm1), most notably the 2D band (~2690 cm1) [39,40]. The G band is due to in-plane bond- stretching motions (E2g mode) of sp2-hybridized carbon atoms, whether they are in aromatic or aliphatic structures, whereas the D band arises from breathing vibrations (A1g mode) of six-fold aro- matic rings lying in close proximity to any defect or discontinuity that breaks the ideal periodicity of the graphite/graphene lattice [41,42]. For this reason, the intensity ratio of the D and G bands (ID/ IG ratio) is sensitive to the average lateral size of the crystallites/ aromatic domains, La, present in such materials. More to the point, as La progressively decreases across the nanometer size regime (i.e., from several hundred down to just very few nanometers), ID/IG increases as ID/IG f 1/La (nanocrystallization regime). However, upon further reducing La the overall number of six-fold aromatic rings that contribute to the D band starts to decrease, so that ID/IG decreases as ID/IG f L2a (amorphization regime) [41]. The ID/IG ratio for EOG was measured to be 1.04 ± 0.02. Coincidentally, a very similar value was obtained in the case of MRGO (1.070 ± 0.003; see Fig. 3e, orange trace). At first sight this result seemed to imply that 61PDF Image | electrochemical route to holey graphene nanosheets
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