Oxidative exfoliation: the process
Also called Hummers’ method, the oxidative exfoliation process is one of the most ancient ones to produce graphene, and is still widely used. Indeed, its industrialization is relatively straightforward, since the industrial technique is well known.
For this exfoliation method, graphite is commonly oxidized under harsh conditions to obtain graphite oxide. This material can be dispersed by sonication in polar solvents, i.e. water, to form graphene oxide (GO) layers.
Advantages and drawbacks
The oxidation introduces a large amount of oxygen containing functional groups which allows the exfoliation and obtention of SLG graphene and ensures the stability in dispersion without the need of further additives.
However, these functional groups interrupt the honeycomb structure of graphene which is responsible for its extraordinary properties. Therefore, this family of graphene materials can have lowered thermal or electrical conductivity. In order to resolve this problem, graphene oxide is most commonly reduced with agents, like hydrazine, or through electrochemical approaches to form so-called reduced graphene oxide (rGO). However, this reduction process is never 100% complete.
What to remember about the process of oxidative exfoliation ?
Even if rGO is still slightly oxidized compared to pristine graphene, it has been used for the development of many applications and has shown its convenience.
The GOs and rGOs available in powder form raise the question of the reaggregation of graphene sheets within these powders, as well as the impact of their use on the respiratory system.