Carbon-graphene material does not have a well-defined thickness, it consists only of one layer of atoms. Therefore it is often referred to as “two-dimensional matter”. Attempting to make a 3D structure out of it may seem contradictory at first, but it’s an important goal: if the properties of the graphene layer are to be best exploited, the largest possible active surface area should be integrated within a finite volume.
The best way to achieve this goal is to produce graphene on complex nanostructures. This is exactly what the collaboration between CNR Nano in Pisa, TU Wien (Vienna) and the University of Antwerp has now achieved. This could help, for example, to increase the storage capacity per volume of hydrogen or to build chemical sensors with higher sensitivity.
From solid to porous
In the group of Professor Ulrich Schmid (Institute of Sensor and Actuator Systems, TU Wien), years of research have been carried out on how to convert solids such as silicon carbide into extremely fine porous structures in a precisely controlled manner. “If you can control the porosity, then many different material properties can therefore be affected as a result,” explains George Pfosterschmid, one of the authors of the current research.
The technological procedures required to achieve this goal are challenging: “It’s an electrochemical process that consists of several steps,” says Markus Littgebe, a chemist who also works in the Ulrich Schmid research group at TU Wien. “We work with very specific etching solutions, and apply tailored electric current properties in combination with ultraviolet irradiation.” This allows drilling of small holes and channels in certain materials.
Because of this expertise in achieving porous structures, Stefan Heun’s team from the Italian National Research Council’s Institute of Nanosciences CNR turned to their colleagues at TU Wien. The Pisa team was looking for a way to produce graphene surfaces in branched nanostructures to enable larger surface areas of graphene. And the technology developed at TU Wien is well suited to this task.
“The starting material is silicon carbide – a crystal of silicon and carbon,” says Stefano Veronesi, who performed the graphene growth process at CNR Nano in Pisa. “If you heat this material, the silicon evaporates, the carbon remains, and if you do it right, it can form a layer of graphene on the surface.”
Therefore an electrochemical etching process was developed at TU Wien that turns solid silicon carbide into the desired porous nanostructure. About 42% of the volume is removed in this process. The remaining nanostructure was then heated in a high vacuum in Pisa so that graphene formed on the surface. Then the result was examined in detail in Antwerp. This revealed the success of the new process: in fact, a large number of graphene flakes form on the complex surface of the 3D nanostructure.
Lots of surface area in a compact shape
“This allows us to use the advantages of graphene more effectively,” says Ulrich Schmid. “The original motivation for the research project was hydrogen storage: You can temporarily store hydrogen atoms on graphene surfaces and then use them in different processes. The larger the surface, the more hydrogen you can store.” But there are also many other ideas for using 3D graphene structures. The large surface area is also a critical feature in chemical sensors, which, for example, can be used to detect rare materials in gases.
Material provided by Vienna University of Technology. Note: Content can be modified according to style and length.