В МФТИ раскрыли тайну высокой водопроницаемости графенового оксида.

The article has been published in the journal Computational materials science. We spoke with the first author of the study, Anastasia Zelenskaya, a PhD student at MIPT and the project leader, Nikita Orekhov, the deputy head of the laboratory for computational materials design at MIPT.

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

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

However, over time, researchers began to pay attention to the properties of graphene oxide itself, particularly its unusually high permeability to water. While pure graphene is extremely hydrophobic, graphene oxide, on the contrary, 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. Although the high permeability of graphene oxide has been known from experiments for quite some time, the exact mechanisms of water diffusion through this material are still debated.

When water molecules are trapped in a narrow gap between the planes of graphene oxide, their dynamic properties change significantly. In jargon, this situation is referred to as “confinement.” In our work, using supercomputing atomic modeling methods, we analyzed whether the nanostructure of graphene oxide affects the nature of water diffusion.

Atomic modeling allows for the numerical description and prediction of the behavior of each individual atom within a typically very small volume of substance. From a computational standpoint, such methods are extremely resource-intensive and require the use of high-performance machines capable of employing hundreds, and sometimes thousands, of individual processors simultaneously for a single task – the so-called supercomputers.

The oxygen groups in graphene oxide do not have to be uniformly distributed across its surface: on the contrary, according to our recent results published in the journal Surfaces and Interfaces, they tend to cluster. As a result, the surface of the material can represent a complex mosaic of alternating oxidized and reduced nanoscale regions. The diffusion of water along the boundaries between these regions proves to be surprisingly high, much greater than in each of them individually, and water molecules experience virtually no resistance, gliding like speed skaters 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 areas. Diffusion along such boundaries represents a previously unreported mechanism that could plausibly explain the high mobility of water observed in graphene oxide.

“The potential role of graphene oxide is not limited to membrane functions,” explains Nikita Orekhov. “Our collaborators from Singapore have found another practical niche for it – they are creating flexible actuators based on it that respond to laser heating. These strips of material can ‘grasp’ or ‘release’ something 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 during heating. Thus, the effect is again based on the mechanisms of water diffusion in the interlayer space.”

This work was carried out with the support of the Russian Science Foundation.