Figure 3. Effect of additives on CMG-CNs microstructure and crystallinity.

SEM images of: GO-CNs produced without (3.3 mg cm⁻³ density, 65 μm average cell size) (a) and with the addition of 5 wt% organic additives (PVA:sucrose, 1:1 wt%) in 0.65 wt% GO-sus (15 mg cm⁻³ density, 40 μm average cell size) (b); rGO-CNs thermal treated at 1,000 °C in Ar/H₂ atmosphere produced without (1 mg cm⁻³ density, 63 μm average cell size) (c) and with the addition of 5 wt% organic additives (112 mg cm⁻³ density, 7 μm average cell size) (d). Shrinkage during the thermal treatment results in highly wrinkled rGO at the cell walls (d). The corresponding Raman spectra (514 nm laser) and C1s XPS of rGO-CNs (prepared without and with organic additives (SEM in c,d)) are represented in e and f, respectively. The letter ‘D’ and ‘G’ stand for two characteristic Raman active modes for graphene and other carbon allotropes and the D/G ratio is a measure of the density of defects present in the carbon material28. The C1s XPS spectra (hν=1,253.6 eV) collected on rGO-CN with 5 wt% additives is indicated in red colour whereas the rGO-CN produced without organic additives appears in blue colour. The C1s spectrum collected on rGO-CN with 5 wt% additives was fit by Doniach–Sunjic function after subtracting a Shirley background as indicated in the lowermost spectrum. The different components related to various chemical shifts of carbon bonds are indicated. The component at 284.6 eV is due to the sp² carbon, the peak at 286.5 eV is related to the remaining C–O bonding and possible carbon sp³ defects, and the component at 287.8 eV is related to residual C=O bonding24. The full width at half maximum (FWHM) for the C1s core level for rGO-CNs, with initially 5 wt% additives is narrower than for rGO-CNs without additives. This reflects the different sp² content, which is about 82% in rGO-CNs, with initially 5 wt% additives (before reduction) and about 80% in rGO-CNs, produced without additives. Scale bars, 100 μm (a,b), 10 μm (c,d) and 2 μm (insert image in d). a.u., arbitrary unit.