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D.F. Carrasco, J.I. Paredes, S. Villar-Rodil et al. Carbon 195 (2022) 57e68 sample under the same conditions as those used for electrode preparation (i.e., 1 ton for a few seconds in the hydraulic press). Then, from the known weight and dimensions of the pellet (the thickness was estimated with a digital micrometer), its density could be ascertained. Typical values were around 1.6 g cm3. 3. Results and discussion 3.1. Structural and chemical characteristics of the electrochemically derived, highly oxidized graphene Our aim was to prepare holey graphene from an alternative precursor that offered advantages with respect to the typically used standard graphene oxide. As mentioned above, oxidized graphenes obtained by electrochemical exfoliation of graphite have been found to possess higher electrical conductivity and overall struc- tural quality than those of their (reduced) standard graphene oxide counterparts of similar oxidation degree. This fact pointed to the presence of larger aromatic domains in the former material, which for a similar oxidation degree, would imply smaller, denser oxidized domains than in standard graphene oxide. As the starting point for this work, we hypothesized that selective etching of the oxidized domains in both types of materials would yield holey graphenes with distinct porosities. Specifically, holey graphene with smaller and more uniform porosity would be expected when using the structurally and chemically more homogeneous electrochemically-derived material as precursor. Fig. 1 illustrates the central idea of this work. Highly oxidized graphene was obtained from graphite foil by way of an electrolytic process carried out in two sequential steps, namely, (1) anodic intercalation of the graphite foil in highly concentrated sulfuric acid and (2) anodic delamination and oxidation of the intercalated graphite in a more diluted sulfuric acid solution (see Experimental section for details) [25]. After a washing and drying work-up procedure, this electrolytic treatment yielded a loose, fluffy powder as the primary product, which was made up of highly expanded and elongated particles, as noticed from field emission scanning electron microscopy (FE-SEM) imaging (Fig. 2a). Closer inspection of the particles (Fig. 2b) revealed them to be comprised of very thin, rippled/wrinkled sheets separated by micrometer- and submicrometer-sized voids. Such a morphology was very similar to that seen in efficient, single-step processes of anodic exfoliation of graphite to give graphene with aqueous electrolytes (e.g., sulfate-based salts), which do not include an anodic intercalation step in concentrated acid [29,30]. This indi- cated that a successful delamination was also attained in the pre- sent case, as will be shown below. For single-step processes of anodic exfoliation, individual single/ few-layer graphene NSs can be readily extracted from the primary product and colloidally dispersed in certain organic solvents and water/surfactant solutions with the aid of sonication or shear forces, but usually not in pure water [29,31,32]. The latter can be ascribed to the relatively limited extent of oxidation (typical O/C atomic ratios ~0.10e0.20), and thus limited hydrophilicity and ionizability, of the graphene product. On the other hand, stable colloidal suspensions of well-exfoliated NSs could be easily ob- tained in neat water, i.e., in the absence of any dispersant, from the product of the present two-step protocol. A digital picture of the suspension (both concentrated and diluted) and an atomic force microscopy (AFM) image of the corresponding NSs are shown in Fig. 2c and d, respectively, as well as thickness (Fig. 2e) and lateral size (Fig. 2f) histograms derived from the AFM images. The colloidal stability of the material in water strongly suggested that the pre- sent product was, as anticipated, substantially more oxidized than common anodic graphenes derived from single-step processes. Indeed, elemental analysis of our product yielded an O/C atomic ratio of ~0.31 [composition in at%: 65.1 (C), 13.0 (H), 20.2 (O), 0.1 (N) and 1.6 (S)], which was comparable to values typical of just slightly reduced graphene oxides obtained from standard protocols (e.g., Hummers method) [26,33,34]. To facilitate comparisons with the latter, more common type of highly oxidized graphenes, we pre- pared a Hummers-based mildly reduced graphene oxide (MRGO) having the same O/C ratio as that of the electrochemically derived oxidized graphene (referred to as EOG). The elemental composition of MRGO (in at%) was the following: 58.0 (C), 20.4 (H), 17.9 (O), 3.6 (N) and 0.1 (S). Details of the preparation of MRGO are given in the Electronic Supplementary Material (ESM), together with digital photographs of the corresponding aqueous suspension (both concentrated and diluted, Fig. S1). An AFM image of the MRGO NSs, together with thickness and lateral size histograms derived from the AFM images, are shown in Fig. 2g, h and i, respectively. Fig. 3a shows the UVeVis absorption spectrum of EOG (black trace), which featured a prominent peak centered at a wavelength of 256 nm in combination with decreasing but still strong absor- bance at longer wavelengths up to 1000 nm. This absorption peak arises from p/p* transitions in the aromatic, electronically con- jugated domains of graphenic and other sp2-based carbon mate- rials, and its position is known to red-shift with the size of the Fig. 1. Schematic of the preparation of holey graphene from structurally/chemically different precursors. (A colour version of this figure can be viewed online.) 60PDF Image | electrochemical route to holey graphene nanosheets
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