Graphene - a layer of carbon one atom thick - is considered the thinnest and strongest of the measured materials, and it also conducts electricity and heat excellently. It is not surprising that people are trying to adapt it for solar cells, batteries, water filters and sensors. But for such ideas to reach practice, it is necessary to understand how changes in the structure of graphene affect its behavior.
A graduate student, Sakshi, from the University of Saskatchewan (Canada), who works with Dr. Alexander Moewes, took up this question. She combined experiments at the Canadian Light Source (CLS) with computer modeling to study graphene oxide - a form of graphene whose surface has oxygen-containing groups. The results were published in the journal npj 2D Materials and Applications.
Graphene oxide is an important raw material for large-scale production of graphene materials. However, oxygen-containing fragments can attach to different places in the carbon lattice, and where they end up depends on the manufacturing method and changes the properties of the material. In other words, what matters is not only how much oxygen there is, but also where it is located.
Sakshi notes: there are many theoretical models of the structure of graphene oxide, but there are few experimental works that would show where the atoms are actually located. "If it has so many exciting potential applications, why is no one studying the structure itself, when knowledge of the structure at the atomic level - where the atoms are - will suggest how the atoms will behave when moving from one position to another?" the researcher says.
Indeed, a material for energy storage may require different properties than a water filter.
Using powerful X-ray radiation at the CLS, Sakshi studied a sample about the size of a grain of sand. She then used computer models to see how different arrangements of oxygen would affect the material's X-ray "fingerprint." By comparing the models with the real results, the team determined how the oxygen is arranged in the sample.
It turned out that the arrangement of oxygen matters. Two types of oxygen-containing groups - hydroxyls and epoxides - act differently: hydroxyls change the behavior of electric charge, while epoxides weaken some bonds between carbon atoms.
In addition, these groups prefer to attach to only one side of the material. As a result, about 60% of the carbon structure of graphene remains untouched, which helps preserve many of the properties for which graphene is valued.
Currently, Sakshi is studying how different methods of producing graphene oxide affect its structure and properties. If researchers better understand how the production process determines where oxygen will end up, they will be able to control the material's properties more precisely and create graphene materials for specific tasks - from energy storage to sensors and water purification.