Abstract
Graphene oxide (GO) is an attractive alternative to graphene for many applications due to its captivating optical, chemical, and electrical characteristics. In this work, GO powders with a different amount of surface groups were synthesized from graphite via an electrochemical two-stage process. Many synthesis conditions were tried to maximize the oxidation level, and comprehensive characterization of the resulting samples was carried out via elemental analysis, microscopies (TEM, SEM, AFM), X-ray diffraction, FT-IR and Raman spectroscopies as well as electrical resistance measurements. SEM and TEM images corroborate that the electrochemical process used herein preserves the integrity of the graphene flakes, enabling to obtain large, uniform and well exfoliated GO sheets. The GOs display a wide range of C/O ratios, determined by the voltage and time of each stage as well as the electrolyte concentration, and an unprecedented minimum C/O value was obtained for the optimal conditions. FT-IR evidences strong intermolecular interactions between neighbouring oxygenated groups. The intensity ratio of D/G bands in the Raman spectra is high for samples prepared using concentrated H2SO4 as an electrolyte, indicative of many defects. Furthermore, these GOs exhibit smaller interlayer spacing than that expected according to their oxygen content, which suggests predominant oxidation on the flake edges. Results point out that the electrical resistance is conditioned mostly by the interlayer distance and not simply by the C/O ratio. The tuning of the oxidation level is useful for the design of GOs with tailorable structural, electrical, optical, mechanical, and thermal properties.
Highlights
Graphene oxide (GO), which is the oxidized form of graphene, is currently attracting a lot of interest due to its unique chemical, optical, and electronic properties that make it suitable for a broad range of uses including supercapacitors, solar cells, fuel cells, lithium batteries, biomedicine, polymer nanocomposites, and more [1,2,3,4,5,6]
Elemental analysis was used to determine the absolute amounts of C, H, N, and S elements of the synthesized Exfoliated Graphene Oxides (EGOs) [33], and to calculate their carbon/oxygen (C/O) atomic ratio, which is indicative of the degree of oxidation
The EGOs display a wide range of carbon/oxygen atomic ratio (C/O) ratios (1.46–2.96, Table 1), which indicates that some of them present higher and other lower degrees of oxidation than the reference, which corroborates the strong influence of the synthesis conditions on the level of oxidation of the final sample
Summary
Graphene oxide (GO), which is the oxidized form of graphene, is currently attracting a lot of interest due to its unique chemical, optical, and electronic properties that make it suitable for a broad range of uses including supercapacitors, solar cells, fuel cells, lithium batteries, biomedicine, polymer nanocomposites, and more [1,2,3,4,5,6] It presents several surface oxygen-containing groups, mainly COOH moieties on the layer edges as well as C-O-C and OH on the basal planes (Scheme 1). The hydroxyl groups can bond to form epoxy groups, which leads to a distorted tetrahedron with four carbon atoms on the six-membered ring of the carbon plane This provokes the flat graphene network to bend. GO is largely used for the synthesis of a number of derivatives to be used in environmental and energy-related applications [10]
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