Graphene Nanoscrolls Form Spontaneously When Maghemite Applied
A new nanomaterial that could have an application as electrodes in for example Li-ion batteries has been demonstrated by researchers at Umeå University, Uppsala University and Stockholm University.
Graphene doped with nitrogen can be rolled into perfect nano scrolls by adhering magnetic iron oxide nanoparticles on the surface of the graphene sheets. Ordinary graphene is made of carbon sheets a single or few atomic layers thick.
In this study, the researchers modified graphene by substituting some of the carbon atoms with nitrogen atoms. This makes anchoring sites for the iron oxide nanoparticles that are decorated onto the graphene sheets in a solution process.
Maghemite and Hematite Nanoparticles
In the decoration process they can manipulate the type of iron oxide nanoparticles that are formed on the graphene surface. The nanoparticles either form so hematite, the reddish form of iron oxide found in nature, or maghemite, a more magnetic but less stable 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”, says Thomas Wågberg of Umeå University.
The nanoscrolls could be visualized as Swiss roll spongecakes. Just imagine the sponge-cake as the graphene, and the creamy filling the iron oxide nanoparticles. (I’ll wait here while you go to the kitchen and grab some cake.) The difference is that graphene nanoscrolls are around one million times thinner, so not so deliciously filling.
The results are conceptually important for a few reasons. First, they show that the magnetic interaction between the iron oxide nanoparticles is one of the main effects behind the scroll formation.
The results also show that the nitrogen defects in the graphene lattice are required for both stabilizing a sufficiently high number of maghemite nanoparticles, while also being to blame for buckling the graphene sheets and so lowering the formation energy of the nanoscrolls.
The process is surprisingly efficient, as nearly 100 percent of the graphene sheet is scrolled. After the decoration with maghemite particles the research team was not able to observe any open graphene sheets.
Researchers also showed that by removing the iron oxide nanoparticles with an acid treatment, the nanoscrolls open up and go back to single flat graphene sheets.
“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 Li-ion batteries, one of the most important batteries in daily life electronics, “ said Thomas Wågberg.