Two-dimensional (2D) nanocrystals can be used in many technology applications thanks to their exceptional chemical and physical properties that are not seen in their bulk counterparts. Some famous examples of 2D materials include graphene (a carbon sheet just one atom thick) and the transition metal dichalcogenides, which have the formula MX2, where M is a transition metal (such as Mo or W) and X is a chalcogen (such as S, Se and Te). Mixed transitional metal oxide (MTMO) nanomaterials, for their part, are promising for electro- and photocatalysis, energy storage and conversion. This is because of their mixed valance states and the fact that they undergo rich redox reactions owing to their abundant active sites, on which electrochemical reactions can take place, and shortened distances over which ions can diffuse.

MTMOs that are fabricated into 2D nanostructures are expected to have better properties, but until now researchers have mainly focused on studying MTMOs in the form of 0D nanoparticles, 1D nanotubes or nanowires and 3D nanoclusters or microspheres. The problem here is that MTMOs are in fact non-layered materials that cannot be mechanically or chemically exfoliated to 2D nanostructures – like graphene and other 2D materials can. It is thus not possible to produce them using a top-down method.

General two-step strategy

A team led by Guihua Yu is now reporting on a general two-step bottom-up strategy to synthesize a series of holey MTMO nanosheets in which the pore sizes can be readily tuned. The researchers made the sheets using graphene oxide as a sacrificial template to grow various MTMO precursors on its surface. They followed this step with a post-thermal treatment (calcination) to transform the MTMO precursors into 2D holey MTMO nanosheets while removing the GO template.

Yu and colleagues used their technique to synthesize nanosheets of simple oxides, including Fe2O3, Co3O4, Mn2O3, and mixed oxides, such as ZnMn2O4 (ZMO), ZnCo2O4 (ZCO) and CoFe2O4 (CFO).

Promising candidates for many energy-related applications

The holey structures are good for a number of reasons, says Yu. “First, they have large surface areas and short diffusion lengths for effective ion transport and storage. Second, they minimize the restacking of 2D nanosheets and provide more active sites for alkali-ion storage. They can thus help accommodate the natural volume changes that take place during lithiation/delithiation or other electrochemical reactions, and thus help maintain the structural integrity of electrode materials while battery cycling to achieve a longer life cycle.”

And that is not all: the sacrificial-template strategy developed in this work may be generalized to synthesize other materials beyond oxides, such as phosphides, sulphides and selenides, and those with intrinsically non-layered structures, he adds. “Thanks to the synergetic effects of tunable porosity and their inherently strong mechanical stability, these 2D holey nanosheets could be promising candidates for many energy-related applications, such as energy storage and conversion systems, and electrocatalysis,” he tells nanotechweb.org.

The research is detailed in Nature Communications doi:10.1038/ncomms15139.