Graphene oxide (GO): structure, synthesis and properties

Graphene oxide starts from the graphene honeycomb, but oxidation bonds oxygen groups to many of its carbon atoms. In the model most used today, from solid-state NMR of graphite oxide (Lerf et al. 1998; reviewed by Dreyer et al. 2010):

• Epoxides (an oxygen bridging two neighbouring carbons, C–O–C) and hydroxyls (C–OH) sit on both faces of the basal plane.
• Carboxyls (–COOH) and carbonyls (C=O) sit at the sheet edges and at holes.
• The carbons carrying a group become sp³ and pucker out of the plane; between them remain unoxidised sp² patches.

¹³C NMR puts numbers on this: epoxide carbons at 61 ppm, C–OH at 70 ppm and sp² carbon at 134 ppm, with only about 24 % of the carbon still sp² in the GO measured (Brisebois et al. 2020). Lactols have also been identified in graphite oxide (Gao et al. 2009). Because the groups and their amounts depend on the oxidation, GO has no fixed formula.

Stacked, the sheets form graphite oxide: the groups and water between the layers push them apart, from 3.37 Å in graphite to 8.85 Å in GO (Zainab et al. 2023).

1. Oxidation. In the Hummers method, 100 g graphite and 50 g NaNO₃ are stirred in 2.3 L concentrated H₂SO₄ and 300 g KMnO₄ is added while the mixture is kept below 20 °C; it is then held at about 35 °C (Hummers & Offeman 1958). Marcano et al. (2010) left out the NaNO₃, used more KMnO₄ and ran the reaction in 9:1 H₂SO₄/H₃PO₄, which oxidises graphite more efficiently and releases no toxic gas.
2. Quench and wash. Water is added, which heats the mixture to about 98 °C, and hydrogen peroxide removes the leftover permanganate (Hummers & Offeman 1958). The graphite oxide is washed free of acid and salts.
3. Exfoliation. Ultrasonication in water splits graphite oxide into single GO sheets, a brown colloid.
4. Reduction (optional). Hydrazine hydrate (Stankovich et al. 2007), sodium borohydride (Shin et al. 2009) or heat removes most of the oxygen groups and gives reduced graphene oxide (rGO); the reduced sheets aggregate in water. Films reduced with NaBH₄ have a much lower sheet resistance than films reduced with hydrazine (Shin et al. 2009).

• Disperses in water. Its oxygen groups make GO hydrophilic, so it can be processed from water into films, papers and composites (Dreyer et al. 2010).
• Mostly sp³. With only about a quarter of its carbon sp² (Brisebois et al. 2020), GO has lost much of graphene’s π network; reduction restores part of it (75 % sp² after SnCl₂).
• Reduced GO conducts. Films of rGO can be tuned over several orders of magnitude in their opto-electronic properties: the thinnest show graphene-like ambipolar transistor behaviour, thicker ones behave as graphite-like semimetals (Eda et al. 2008). rGO keeps defects and some oxygen, so it is not the same as pristine graphene (Luong et al. 2020).
• Layer spacing. XRD follows the oxidation and reduction: 3.37 Å (graphite), 8.85 Å (GO) and 3.43 Å after thermal reduction (Zainab et al. 2023; Muzyka et al. 2018).
• Disorder. The Raman D band at 1348 cm⁻¹ is about as strong as the G band at 1589 cm⁻¹, I(D)/I(G) = 1.04 (Islam et al. 2024).

Graphene oxide is the most common route to graphene-like sheets in bulk: it starts from cheap graphite, uses batch wet chemistry, and gives sheets that disperse in water.

It also made graphene composites practical: a polystyrene–graphene composite made by way of graphite oxide conducted electricity at room temperature above about 0.1 vol % (Stankovich et al. 2006).

• Membranes. GO laminates in water block solutes with hydrated radii above 4.5 Å while smaller ions pass thousands of times faster than by simple diffusion (Joshi et al. 2014).
• Transparent and flexible electronics. rGO thin films from one to several layers deposited over large areas from solution (Eda et al. 2008).
• Composites. Conducting polymer composites at low loading (Stankovich et al. 2006).
• Films and inks. Single GO layers assembled into Langmuir–Blodgett films (Cote, Kim & Huang 2009).
• Energy storage. Reduced graphite oxide and graphene oxide as anodes for lithium- and sodium-ion batteries (Sun et al. 2017).

For a paper, slide or poster, draw a graphene sheet with epoxides and hydroxyls on both faces and carboxyls at the edges, in patches rather than evenly spread, and if needed the same sheet after reduction with few groups and small holes. The synthesis and characterization figure above has the model, the route and the spectra, the Hummers figure shows graphite to GO to rGO, and the characterization figure shows XPS, FTIR, NMR, XRD, Raman and NEXAFS; each opens as an editable LabCanvas figure. The structure drawing is a high-resolution SVG of the figure editor’s graphene oxide sheet, where you can change the amount of oxidation and the edge groups. The 3D model opens in the LabCanvas 3D Studio, where you can reduce it, stack layers and export a high-resolution image. GO has no single crystal structure, so there is no crystal file for it.