Researchers from the University of Vienna, South Korea's Institute for Basic Science (IBS) and the Chinese Academy of Sciences (CAS) have reported an ultrathin covalent organic framework (COF) that was constructed homogeneously on the surface of a graphene template via a facile hydrothermal method.

The composite material of COF and graphene reportedly shows high and rapid adsorption capacity for organic pollutants.

“There are several ways, including activated carbon filters, to purify water today, but there is still room for improvement in the efficiency or adsorption capacity of the applications,” says first author and postdoctoral researcher Changxia Li.

Freddy Kleitz’s group at the Institute of Inorganic Chemistry – Functional Materials at the University of Vienna is developing novel nanoporous materials. Porous materials have a much larger total surface area than a non-porous material for the same volume and can thus accumulate a particularly large number of molecules on the surfaces in the course of adsorption.

Covalent organic frameworks (COFs) are a relatively novel class of materials. They are particularly porous, while at the same time being low-density and lightweight.

The dyes the researchers studied in their aqueous model solution were about 0.8 to 1.6 nanometer in size. “We developed a novel method to form COF in a comparatively environmentally friendly way, using water. As such, we were able to develop small ‘sponges’, with designed pore sizes and pore shapes in the nanometer range, as well as a tuned negative surface charge, that was very selective in pulling the positively charged target molecules, i.e., our dyes, out of the water,” the researchers said, “Just like the sponge soaks up the water, only in our case it’s the pollutants.”



When using bulk COF powder, the inner pores of the material are often no longer accessible to pollutants due to pore blockage at the outer edge, especially for large pollutant molecules. The novel composite material developed by the researchers offers a thoroughly permeable structure: For this purpose, the researchers grew COF on thin-layered graphene nanosheets. The combination of graphene – in itself already a 2D layer of carbon atoms – and the layer of COF, which is up to two nanometers thick, resulted in a compact, open 3D structure. The ultrathin COF layer could expose more adsorption sites than the bulk COF powder.

On the other hand, the larger, honeycomb-like pores of the graphene network support the transport of water through the filter material. “The large pores of the graphene network in combination with the ultrathin COF layer with large amount of adsorption sites therefore enable particularly fast as well as efficient wastewater treatment,” the researchers said. Due to the comparatively low material input of graphene as well as the possibility to reuse the composite material – after the pollutants have been washed out – as a filter, the development is also relatively cost-efficient, they said.

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