Research activities

A sieve for molecules

RUB, Kramer

Researchers have long tried to use graphene, which is made of carbon, as a kind of sieve. But it has no pores. Now a team has found an alternative material that provides the holes on its own.

Researchers from Bielefeld, Bochum and Yale have succeeded in producing a layer of two-dimensional silicon dioxide. This contains natural pores and can therefore be used like a sieve for molecules and ions. Scientists have been searching for such materials for some time, as they could help desalinate seawater or be used in new types of fuel cells. The team describes the fabrication process in the journal Nano Letters, published online Jan. 19, 2022. The teams led by Dr. Petr Dementyev of Bielefeld University, Prof. Dr. Anjana Devi of Ruhr University Bochum and Prof. Dr. Eric Altman of Yale University collaborated on the work.

When two-dimensional materials are pierced with high precision, they can be used to screen out specific ions or molecules. Researchers have repeatedly tried to use graphene, a material made of carbon atoms, for this purpose. Since it has no natural pores, they have to be inserted artificially. But it is difficult to create holes of a defined size in graphene without permanently damaging the material, which breaks easily. This is because it loses too much stability due to the perforation. Consequently, an alternative had to be found. In the current work, the research team took advantage of the fact that the crystal lattice of two-dimensional silicon dioxide naturally has openings. They showed that these openings can be used to separate certain gases.

"Silicon dioxide naturally has a very high density of tiny pores that could not be created in artificial membranes," says Petr Dementyev of the Bielefeld-based Physics of Supramolecular Systems and Surfaces group. "Unlike graphene, the pores are all nearly the same size. And there are so incredibly many that the material behaves like a fine-mesh sieve for molecules."

2D silica has been known since 2010. However, its production was very expensive and only possible on a small scale. The researchers from Bochum, Bielefeld and Yale brought together expertise from materials chemistry, chemical engineering and chemical physics to devise a new manufacturing process. They used what is known as atomic layer deposition to deposit a single layer of silicon dioxide on a gold surface. Using a high-pressure process, the researchers transferred the layer to its two-dimensional form and then characterized it in detail spectroscopically and microscopically. They then studied the gas flow through the 2D membrane in a vacuum chamber.

While evaporated water and evaporated alcohol were able to pass through the silica layer, the gases nitrogen and oxygen were retained. "Materials like this with selective permeability are in high demand in industry," says Anjana Devi. However, before the 2D silica can be used in practice, it is important to evaluate exactly how many different molecules can attach to or penetrate the surface of the material. 

"We expect our results to be important for materials science worldwide," sums up Anjana Devi of the Inorganic Materials Chemistry group in Bochum. "Such 2D membranes could help at the forefront of sustainable development, for example in the field of energy conversion or storage."

adapted from RUB, Julia Weiler