Graphene Oxide (GO): mechanism and methods of synthesis

Strong acids along with highly reactive oxidizing agents are used during the synthesis procedure of graphene oxide. Thereby, optimization of reaction conditions of the system, such as temperature and amount of chemicals used is mandatory to acquire appropriate product. Temperature of various steps of synthesis procedure is conspicuously different; therefore, it is one of the most important parameter to be considered. Mishandling of chemicals and non-uniform conditions of provided temperature can drastically affects the efficiency of synthesized product by leaving defects in the structure of graphene oxide sheets.

1. Formation mechanism:

Mechanism of graphene oxide synthesis is based on two main steps using graphite flakes as precursor raw material which makes it cost effective. The first step attributes to oxidation of graphite to graphite oxide by introducing oxygenated groups to the surface edges and also in the basal planes of GO. These oxygenated species include different functional groups possessing oxygen like hydroxyl group (OH^-), carbonyl group (C=O), epoxy group (C-O-C) and carboxylic group (COOH^-). Oxidation of graphite is followed by the exfoliation of oxidized graphite oxide in second step using sonication or any other shearing approach which results in graphene oxide in suspension form. The suspension of graphene oxide is then further treated in centrifuge machine and electric oven to get final GO sheets. Figure 1 shows the formation mechanism of  graphene oxide from graphite.

  Figure. 1. Formation mechanism of GO

2. Synthetic approaches

Various methods are used for the synthesis of highly demanding material of graphene oxide. Graphite is the main precursor material which is treated usually with strong acids and oxidizing agents at diverse conditions of temperature and stirring time. General techniques used for the synthesis of graphene oxide sheets are

1.      Brodie method of synthesis

2.      Staudenmaier method of synthesis

3.      Hoffman method of synthesis

4.      Hummers method of synthesis

Among aforementioned synthetic approaches first two methods are rarely used nowadays, while Hoffman and Hummers methods are most commonly used synthetic protocols which are being followed in lab and commercial scale synthesis of graphene oxide. However, detail of most commonly used methods are discussed below.

2.1. Hoffman method:

This method follows the use of two strong acids in the preparation of suspension containing graphite oxide. These acids include sulphuric acid (H₂SO₄) and nitric acid (HNO₃) in which graphite powder is added at a temperature below 4^oC. Ice bath is used around reaction flask to keep the temperature at this range. Oxidizing agent used in this method is potassium perchlorate. Agitation is employed to reaction mixture for two to three days for thorough mixing and to get a homogeneous mixture, which followed by the addition of diluted HCl. Ultimately, the final product is washed and separated using centrifuge treatment and then dried at the end.  

2.2. Hummer’s method:

This is the most appropriate method of synthesis, which permits to use least amount of strong acids during the synthetic procedure. Another edge of this method among others is that it inhibits the use of mixtures of strong inorganic acids, as it only relies on the use of sulphuric acid (H2_SO₄) along with sodium nitrate (NaNO₃) and graphite in powdered form to make the suspension of graphite oxide. Oxidizing agent utilized in this reaction is potassium permanganate (KMnO₄), which is added to the suspension using ice bath, to maintain the temperature at 0^oC. The whole mixture is kept at magnetic stirrer for two days, afterwards, hydrogen peroxide (H₂O₂) is added to stop the reaction. The mixture is then washed, separated and dried using the same criteria as mentioned in Hoffman method of synthesis. Figure 2 shows schematic diagram of synthetic approaches of graphene oxide.   

Figure 2. Schematic diagram of synthetic approaches of GO