The exceptional electronic, thermal and mechanical properties of graphene are expected to transform numerous technological applications. The self-assembly of these ultrathin, highly ordered sheets of carbon is crucial for their implementation in macroscopic devices. However, conventional synthetic methods for the self-assembly of graphene oxide-based materials, which typically involve successive layer depositions and ‘Langmuir–Blodgett’ deposition techniques, usually result in two-dimensional structures.

Fig. 1: Electron microscopy image showing the microscopic structure of the three-dimensional graphene–palladium assembly.© 2010 Wiley-VCH*

Xun Wang and co-workers from Tsinghua University in Beijing, China, have now developed a method that efficiently produces three-dimensional (3D) macroscopic assemblies of graphene-based layers using noble metal nanocrystals.1 The researchers used a simple hydrothermal technique that involved suspending graphene oxide sheets in water and treating the resulting colloids with a noble metal salt and glucose at high temperature to yield strong cylindrical assemblies.

Wang explains that they opted for noble metal nanocrystals because of their useful optical and catalytic properties. They combined the nanocrystals with graphene oxide to investigate whether this would create attractive composite materials, and unexpectedly obtained 3D structures (pictured) with excellent catalytic properties.

The team discovered that the cylinder initially obtained by the hydrothermal method lost its regular shape upon removal of the nanoparticles. According to Wang, this is because the nanoparticles randomly decorate the graphene oxide sheets and serve as active points that attract another graphene oxide sheet, leading to a multilayered material featuring microscopic pores. “The noble metal nanoparticles act as linkages between two graphene oxide sheets in the 3D porous cylinder, just like screws for an armor plate,” he says.

The macroscopic shape and size of the assemblies can be adjusted using the reaction vessel as a mold, and the microporosity can be controlled by varying the graphene oxide concentration.

When the researchers embedded the cylinders with palladium, which is known to catalyze organic reactions such as carbon–carbon coupling, they observed that the 3D structures exhibited high selectivity and conversion for these reactions. “These novel composites show excellent catalytic properties in cross-coupling reactions,” says Wang.

The researchers are currently trying to improve the electrical properties of their composites for potential device applications and continue to study the catalytic activity of other graphene-based composites.