A new method for rolling graphene into scrolls could have major implications for the performance of Lithium-ion batteries.
Researchers at Umeå University in Sweden, together with researchers at Uppsala University and Stockholm University, have shown how grapheme, in which some of the carbon atoms have been replaced with nitrogen atoms, can be rolled into perfect Archimedean nanoscrolls by adhering magnetic iron oxide nanoparticles on the surface of the graphene sheets.
The nanoscrolls can be visualized as traditional "Swiss rolls" where the sponge-cake represents the graphene, and the creamy filling is the iron oxide nanoparticles. The graphene nanoscrolls are however around one million times thinner.
And the process may have commercial value as it is extraordinary efficient – almost 100 per cent of the graphene sheets are scrolled – and the material produce is believed to have excellent properties for use as electrodes.
"Besides adding valuable fundamental understanding in the physics and chemistry of graphene, nitrogen-doping and nanoparticles we have reasons to believe that the iron oxide decorated nitrogen doped graphene nanoscrolls have very good properties for application as electrodes in for example Lithium-ion batteries, one of the most important batteries in daily life electronics, " says Thomas Wågberg senior lecturer at the university’s Department of Physics.
In a report published in the journal Nature Communications the researchers describe how doping the graphene with nitrogen atoms creates obtain anchoring sites for the iron oxide nanoparticles that are decorated onto the graphene sheets via a solution.
In the decoration process it is possible to control the type of iron oxide nanoparticles that are formed on the graphene surface, so that they either form so called hematite (the reddish form of iron oxide often found in nature) or maghemite, a less stable and more magnetic form of iron oxide.
“Interestingly we observed that when the graphene is decorated by maghemite, the graphene sheets spontaneously start to roll into perfect Archimedean nano scrolls, while when decorated by the less magnetic hematite nanoparticles the graphene remain as open sheets,” said Wågberg
The researchers say the results are conceptually as they show that the magnetic interaction between the iron oxide nanoparticles is one of the main effects behind the scroll formation.
They also shows that the nitrogen defects in the graphene lattice are necessary for both stabilizing a sufficiently high number of maghemite nanoparticles, and also responsible for "buckling" the graphene sheets and thereby lowering the formation energy of the nanoscrolls.
Moreover, they showed that by removing the iron oxide nanoparticles by acid treatment the nanoscrolls again open up and go back to single graphene sheets.