Graphene carbon has been studied intensively because of its unique electronic properties, yet although it is possible to build various structures from graphene, such as nanoribbons and nanotubes, only a limited range of shapes can be formed directly from graphene sheets. Huaihe Song and co-workers from the Beijing University of Chemical Technology in China1 have now developed a method for creating hollow spheres of graphene oxide — a convenient precursor for the synthesis of graphene.

Fig. 1: Scanning electron microscopy image of hollow microspheres self-assembled from graphene oxide.

The team's approach relies on the surfactant-like hydrophilic nature of graphene oxide, which allows it to self-assemble in a water-in-oil emulsion. The researchers first formed graphene oxide nanosheets by oxidizing graphite, which exfoliates the layers. After drying the nanosheets into a powder and suspending them in aqueous ammonia, the mixture was poured into hot olive oil while stirring vigorously. The water-in-oil emulsion that formed eventually separated, and the oil phase containing the graphene oxide spheres could be obtained and cooled. With the aid of a centrifuge and some washing and drying, the researchers were left with hollow microspheres of just 2–10 μm in size with a wall thickness of about 1 μm (Fig. 1).

The morphology of the microspheres was found to be affected by the duration of the graphite oxidation step — a longer time gave more uniform spheres. The team concluded that this relates to an increase in the number of hydrophilic functional groups as oxidation and exfoliation proceeds. The more of these groups there are, the easier it is for the graphene oxide to disperse in water and self-assemble in oil to form spheres.

When used as an anode material in a lithium-ion battery, the carbonized microspheres provided significantly better energy storage capacity compared with graphene nanosheets. The researchers believe that the hollow space inside the microspheres provides additional volume for the storage of lithium ions. Transmission electron microscopy observations revealed that the microsphere walls were pitted with pores of about 1 μm in diameter, which would allow the easy transport of the ions into and out of the spheres.

The researchers think that the microspheres could find uses as catalyst supports, as supercapacitors and even in drug delivery. Investigations are now underway to refine the preparation methods in order to better control the diameter and wall thickness of the microspheres with a view to further enhancing their functional properties.