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D.F. Carrasco, J.I. Paredes, S. Villar-Rodil et al. Carbon 195 (2022) 57e68 atmosphere, whereby the amount of released CO and CO2 was measured as a function of heating temperature in the 30e950 C range. Fig. 3f shows the corresponding TPD profiles (CO2: red solid trace; CO: red dotted trace), which revealed that a large fraction of the carbon oxides were released at rather low temperatures (~100e300 C, with maxima below 200 C). This low-temperature behavior is quite atypical in carbon materials (except maybe for the evolution of carboxylic acid groups as CO2) [46e48], although it has been previously observed in standard graphene oxides [49e51], and is a clear sign of the abundant presence of readily removable, hole-forming oxygen groups. Extensive theoretical work on gra- phene oxide has identified the latter as mainly epoxide and hy- droxyl moieties bound to the basal plane, rather than to edges, of the NSs and lying in close proximity to each other [52e55], which is the expected configuration of the highly oxidized domains in gra- phene oxides [18]. Accordingly, the present XPS results (Fig. 3b, red trace) indicated that epoxides and/or hydroxyls are the dominant oxygen-containing groups also in EOG. Furthermore, qualitatively similar TPD profiles were recorded in the case of MRGO (see Fig. 3g; CO2: orange solid trace; CO: orange dotted trace). Indeed, the overall evolution of oxygen in the form of CO and CO2 was very similar for both oxidized graphenes, i.e., 9617 (EOG) vs. 9707 (MRGO) mmol g1, in agreement with the fact that they exhibited the same level of oxidation. However, some differences between the TPD profiles of EOG and MRGO were also apparent. First, the amount of carbon oxides released in the lower temperature range (100e300 C) was some- what larger in the EOG sample [2931 (CO2) and 748 (CO) mmol g1 vs. 2144 (CO2) and 508 (CO) mmol g1 for MRGO], whereas the opposite was the case at higher temperatures [985 (CO2) and 1000 (CO) mmol g1 vs. 1633 (CO2) and 1590 (CO) mmol g1 for MRGO]. Second, the evolution of both CO2 and CO in the 100e300 C range peaked at a noticeably lower temperature (~30e40 C lower) in EOG compared to MRGO. These results implied that the oxygen groups in EOG tend to be more labile than those in MRGO. As a plausible hypothesis to account for such a difference, we propose that the density of oxygen groups in the highly oxidized domains of EOG is, on average, higher than that of MRGO. Because the lability of such groups, at least in terms of evolving carbon oxides, is directly related to their spatial proximity to each other on the graphene lattice [53,55], higher oxygen group densities should lead to higher labilities for these groups. At the same time, having denser oxidized domains in EOG implies that its NSs should bear larger fractions of non-oxidized areas compared to the case of MRGO, since both samples possess the same overall level of oxidation. Hence, the aromatic domains of the former graphene should be larger than those of the latter, which would be consistent with the spectro- scopic results discussed above. To obtain holey graphene from EOG, the material was subjected to hydrothermal treatment at 100 C in a Teflon-lined autoclave in the presence of a small amount of hydrogen peroxide (see Experi- mental section for details). In standard graphene oxides, this type of treatment is known to etch away their highly reactive, heavily oxidized domains, leaving behind nanometer-sized holes in the NSs [11e15]. Fig. 4 shows transmission electron microscopy (TEM) im- ages of the starting EOG NSs (a and b) and their hydrogen peroxide- treated counterpart (c and d; sample EOG-H). Different from the starting graphene, the EOG-H NSs were decorated with a moderate density of small holes typically between 4 and 8 nm in diameter, suggesting that this chemical etching treatment was effective on the electrochemically derived graphene. By contrast, the same treatment applied to MRGO (Fig. 4e and f) led to much more extensively etched NSs (Fig. 4g and h, sample denoted as MRGO-H). In this case, a high density of relatively large holes (up to 125 nm in size) was generated. The distinct evolution of EOG and MRGO upon Fig. 4. Transmission electron microscopy (TEM) images of (a,b) the starting EOG nanosheets and (c,d) their hydrogen peroxide-treated counterpart, EOG-H, as well as (e,f) the MRGO and (g,h) MRGO-H nanosheets. Inset to c: hole size histogram deter- mined from the TEM images. chemical etching generally agreed with their structural differences as highlighted above. Specifically, the higher fraction of aromatic areas in EOG should make this graphene less reactive overall to- wards hydrogen peroxide etching than MRGO, because such areas are expected to exhibit a relatively low reactivity compared with that of the heavily oxidized, structurally defective domains [56,57]. Consequently, etched NSs with smaller and/or less abundant holes should be expected in the case of EOG, as it was indeed observed (see Fig. S2 in the ESM for a comparison of the hole size distribution for the two materials). Performing the hydrothermal treatment in the presence of a higher concentration of hydrogen peroxide led to the generation of larger holes than using the standard procedure (see Fig. S4 of the ESM), but still far below the sizes attained for MRGO-H. The heterogeneity of the hole size distribution is also half way between that for EOG-H and MRGO-H. We note that no damage artifactually leading to pore generation was induced by TEM imaging of the samples. Indeed, the starting graphene mate- rials (EOG and MRGO) did not show any porosity even after repeatedly imaging the same zone to magnify certain areas. Neither did any of the graphene materials (whether precursors or holey graphenes derived from them) vary in any detectable way after 63PDF Image | electrochemical route to holey graphene nanosheets
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