MIPT has uncovered the secret behind the high water permeability of graphene oxide.

The work is published in the journal Computational Materials Science. We spoke with the first author of the paper, Anastasia Zelennina, a PhD student at MIPT and project leader, as well as Nikita Orechov, the deputy head of the Laboratory of Computational Materials Design at MIPT.

“In October, we celebrated the anniversary of graphene – the main character of the two-dimensional revolution in the materials world,” says Anastasia Zelennina. “Twenty years ago, on October 22, 2004, an article titled Electric field effect in atomically thin carbon films was published in the journal Science, which has since garnered over seventy thousand citations. Although graphene itself, contrary to early predictions, cannot yet boast a large number of practical applications, it has significantly broadened scientists' understanding of materials and paved the way for entire families of 2D structures. Among them are maxenes (MXenes), dichalcogenides, graphynes, diamane, and derivatives of graphene itself, including graphene oxide (GO).”

For a long time, graphene oxide remained overshadowed by its more renowned two-dimensional counterparts: it was viewed merely as an intermediate product arising during the synthesis of graphene. The reason is that to obtain graphene chemically, one must first take pyrolytic graphite and place it in acid. During oxidation, it begins to break down into individual monolayers – that is, the graphene oxide. Then, these monolayers must be stripped of oxygen groups, i.e., reduced, to produce graphene.

However, over time, scientists began to pay attention to the properties of graphene oxide itself, particularly its unusually high water permeability. While pure graphene is extremely hydrophobic, graphene oxide, on the other hand, can form hydrogen bonds with water molecules due to the presence of oxygen groups on its surface. This property makes it a promising material for selective membranes that purify water from ions, macromolecules, and even bacteria. Yet, although the high permeability of graphene oxide has long been known from experiments, the exact mechanisms of water diffusion through this material are still a matter of debate.

When water molecules are trapped in a narrow gap between the planes of graphene oxide, their dynamic properties change significantly. This situation is referred to as “confinement” in jargon. In our study, we used supercomputer atomic modeling methods to analyze whether the nanostructure of graphene oxide influences the nature of water diffusion.

Atomic modeling allows for the numerical description and prediction of the behavior of each individual atom in a certain, usually very small, volume of substance. From a computational standpoint, such methods are extremely resource-intensive and require the use of high-performance machines that can engage hundreds, and sometimes thousands, of individual processors simultaneously for solving a single problem, known as supercomputers.

The oxygen groups in graphene oxide do not have to be evenly distributed across its surface; on the contrary, according to our recent results published in the journal Surfaces and Interfaces, they tend to cluster together. As a result, the surface of the material can form a complex mosaic of alternating oxidized and reduced nanoscale regions. Water diffusion along the boundaries between these regions turns out to be surprisingly high, much greater than within each region alone, and water molecules experience virtually no resistance, gliding like a speed skater on ice.

While previous computational studies primarily focused on diffusion within graphene or oxidized regions, our results emphasize the critical role of the boundary between these regions. Diffusion along such boundaries represents a previously unreported mechanism that could provide a plausible explanation for the high mobility of water observed in graphene oxide.

“The potential role of graphene oxide is not limited to membrane functions,” explains Nikita Orechov. “Our co-authors from Singapore have found another application niche for it – they are creating flexible actuators that respond to laser heating. These are strips of material capable of ‘grabbing’ something or, conversely, ‘releasing’ it when subjected to an external impulse. The reaction speed of such actuators depends on how quickly they can absorb water from the atmosphere when cooling or, conversely, release it back when heating. Thus, the mechanisms of water diffusion in the interlayer space are again at the core of the effect.”

This work was supported by the Russian Science Foundation.