The graphene-covered nanostructures imaged with an optical microscope (bottom) and an electron microscope (top). Image: Stefano Veronesi.The carbon material graphene consists of one single layer of atoms, and is therefore often referred to as a ‘two-dimensional material’. Trying to make a three-dimensional structure out of it may sound contradictory at first, but it is an important goal: if the properties of the graphene layer are to be exploited fully, then as much active surface area as possible must be integrated within a limited volume.
The best way to achieve this goal is to coat graphene onto complex branched nanostructures, which is exactly what a cooperation between researchers at the Vienna University of Technology (TU Wien) in Austria, CNR Nano in Pisa, Italy, and the University of Antwerp in Belgium has now achieved. The resulting graphene-covered nanostructures could be used, for example, to increase the storage capability per volume for hydrogen or build chemical sensors with higher sensitivity.
In Ulrich Schmid’s group in the Institute for Sensor and Actuator Systems at TU Wien, research has been conducted for several years into transforming solid materials such as silicon carbide into extremely fine, porous structures in a precisely controlled way. "If you can control the porosity, then many different material properties can be influenced as a result," explains Georg Pfusterschmied, one of the authors of a paper on this work in Carbon.
The technological procedures required to achieve this goal are challenging: "It is an electrochemical process that consists of several steps," says Markus Leitgeb, a chemist who also works in Schmid's group. "We work with very specific etching solutions, and apply tailored electric current characteristics in combination with UV irradiation." This allows them to etch tiny holes and channels into certain materials.
Because of this expertise in fabricating porous structures, Stefan Heun's team at the Nanoscience Institute of the Italian National Research Council (CNR) approached their colleagues at TU Wien. The Italian team was looking for a method to fabricate graphene on branched nanostructures, as way to produce larger graphene surface areas. And the technology developed at TU Wien is perfectly suited for this task.
"The starting material is silicon carbide – a crystal of silicon and carbon," explains Stefano Veronesi, who performed the graphene growth 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 graphene layer on the surface."
An electrochemical etching process was therefore developed at TU Wien to turn solid silicon carbide into the desired porous nanostructure, which results in the removal of around 42% of the volume of the material. The remaining nanostructure was then heated in a high vacuum in Pisa, so that graphene formed on the surface, with the result examined in detail in Antwerp. This revealed the success of the new process: a large number of graphene flakes formed on the intricately shaped surface of the 3D nanostructure.
"This allows us to use the advantages of graphene much more effectively," says Schmid. "The original motivation for the research project was to store hydrogen: you can temporarily store hydrogen atoms on graphene surfaces and then use them for various processes. The larger the surface, the larger the amount of hydrogen you can store."
But there are also many other ideas for using such 3D graphene structures. A large surface area is also a decisive advantage in chemical sensors, which, for example, can be used to detect rare substances in gases.
This story is adapted from material from TU Wien, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.