2D material now in three dimensions

This could help, for example, to store hydrogen with the help of graphene or to build chemical sensors

The preparation chamber of the electron microscope, where the graphene is produced

How to fit as much of the 2D material graphene as possible in a limited space? By making it not as a flat surface, but on a 3D nanostructure.

The carbon material graphene has no real thickness, it merely consists of a single layer of atoms. One therefore speaks of a "two-dimensional material". Making a three-dimensional structure out of this may sound counterintuitive at first, but it is an important goal: If the properties of the graphene layer are to be used optimally, then you need as much active surface area as possible in a limited volume.

The best way to achieve this is to produce graphene on complicated, branched nanostructures. Exactly this has now been achieved by a cooperation of CNR Nano in Pisa, the Vienna University of Technology and the University of Antwerp. This could help store hydrogen or build chemical sensors using graphene, for example.

Solid and solid becomes porous and perforated

In the research group of Prof. Ulrich Schmid (Institute for Sensor and Actuator Systems, Vienna University of Technology) has been researching for years how to make solid materials such as z. B. single-crystal silicon carbide, into extremely fine, porous structures in a precisely controlled manner. "If you can control porosity in a targeted way, then many different material properties can be influenced as a result," explains Georg Pfusterschmied, one of the authors of the current paper.

The necessary procedures for this are complicated: "It’s an electrochemical process that consists of several steps," says Markus Leitgeb, a chemist who also works in Ulrich Schmid’s research group at the Faculty of Electrical Engineering and Information Technology at TU Wien. "One works with very specific solvents, with electric current and with UV irradiation." This allows tiny holes and channels to be etched into certain materials.

Because of this expertise in the production of porous structures, Stefan Heun’s team from the Istituto Nanoscienze of the CNR in Pisa turned to the TU Wien. In fact, Pisa was looking for a method to produce graphene surfaces in branched nanostructures in order to use as large a graphene surface area as possible. And this is what the technology of the Vienna University of Technology is made for.

"The starting material is silicon carbide – a crystal of silicon and carbon," says Stefano Veronesi, who conducted the graphene growth experiments at CNR Nano Pisa. "When you heat this material, the silicon evaporates first, the carbon remains, and then, if you do it right, it can become a graphene layer on the surface."

An electrochemical etching process was therefore developed at the Vienna University of Technology that turns solid silicon carbide into the desired porous nanostructure. About 42% of the volume is removed during this process. The remaining nanostructure was then heated in a high vacuum in Pisa, so that graphene formed on the surface. The result was then examined in detail in Antwerp. This revealed the success of the new process: a large number of graphene sections actually form on the intricately shaped surface of the 3D nanostructure.

A lot of interface in a compact form

"It allows you to take advantage of graphene much more effectively," says Ulrich Schmid. "The original motivation for the research project was to store hydrogen: on graphene surfaces, you can temporarily store hydrogen atoms and then reuse them for various processes. The larger the surface area, the larger the amount of hydrogen that can be stored."But there are also many other ideas for using such graphene structures. Also in chemical sensors, with which one would like to prove for instance rare contents materials of gases, a large surface is of crucial advantage.

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