Michel Barsoum takes pains to find the right analogy for the discovery he and his team at Drexel published last month in the journal Science Advances. He wants a strong, visual analogy because the discovery—a new behavior exhibited by MXenes, the two-dimensional, metallic material uncovered by Drexel researchers seven years ago—is tough to apprehend. It can’t be observed, as it takes place on the atomic scale. It will not have a discernible impact on anyone’s life anytime soon. The behavior itself is the very definition of counterintuitive.
And yet, it is a wonder.
After shuffling through a couple of choices, Barsoum, PhD, distinguished professor in Materials Science and Engineering, settles on the prosaic analogy of a book to describe how MXenes operate in the presence of water when they are compressed. Against all suppositions, they expand.
“Take a book,” says Barsoum. “If you now sit on it, you would expect the distance between the pages to shrink. That is what we find here, but only in the absence of water. If you now sit on a book in a full bathtub, you will find that the layers will actually expand; i.e. the book will swell. Here is the kicker: if you simply sit on a book in water the effect will be quite small. However, if you were to slide the layers of the book relative to each other, water will rush between the layers and the book will expand quite a bit. It is really extremely counterintuitive.
“We take wet, multilayered MXene powders and we apply a compressive stress in a die, and the water that surrounds these layers decides, oh boy, it’s more fun to be between the layers, or ‘pages,’ so it rushes in. That’s why it expands. And it expands significantly.”
The Science Advances paper details how the behavior increases dramatically with the application of shear, one of two ways to apply stress to a material. One way is hydrostatic pressure, which is brought to bear from all directions and is compressive, as if the book were being squashed from all four sides. The MXene expansive behavior occurs in this instance, but only to a small degree. But the effect increases significantly when shear is applied, or, when the ‘pages’ are made to slide against each other.
“Imagine when you have these multilayers that there are bottlenecks around them that will not let water go in,” says Barsoum, “and as soon as you shear them, you break that barrier and water flows in and pushes the layers apart.”
The team’s paper, “Pressure-induced shear and interlayer expansion in Ti3C2MXene in the presence of water,” came online in mid-January as a lead story for Science Advances, a peer-reviewed, multidisciplinary, open-access journal published by the American Association for the Advancement of Science and a sister publication of Science. The paper was co-authored by Barsoum; Michael Ghidiu, PhD, and Sankalp Kota, graduate students and members of the Layered Solids Group in the Department of Materials Science and Engineering, Drexel; and Vadym Drozd, of the Department of Mechanical and Materials Engineering, Center for the Study of Matter at Extreme Conditions, Florida International University.
MXenes are promising materials in a large host of applications, such as energy storage, optoelectronics and electromagnetic shielding, to name a few. This latest discovery will contribute to a better understanding of how they can be processed, says Barsoum, and opens a new avenue of study surrounding them. Researchers at Drexel have been calling MXenes “conductive clays” almost since discovery in 2011 because of their similarities to clay. This recent finding further strengthens the link between these classes of 2D layered materials.
Because MXenes are layered materials, researchers can insert things—like water—between the layers and also peel them apart to yield individual sheets, says Ghidiu, who successfully defended his thesis last month on the subject of MXenes. The materials consist of a metal, titanium, and carbon bonded together to form the sheets. The thickness of the sheets is about 1/1,000,000,000th or 1 one-billionth of a meter.
The clay-like behavior was first observed in MXenes through a series of routine experiments in 2014. So, the team has known about the behavior for some time. However, it took a few years to analyze it through X-ray diffraction. The expansion behavior has been detailed in other materials like clays and graphite oxide. But there is little consensus, adds Ghidiu, on how the effect works on the atomic scale. And the expansion observed with MXenes is “generally greater” than that observed in the other materials.
“It may actually have practical applications,” Ghidiu says. “But this could be, in a sense, another option for more experimental variation in later experiments. This is common in other fields of research where there may not be a practical benefit to the layman, but it can help to advance a next set of discoveries.
“The discovery is more incremental,” he adds. “But it still provides valuable new knowledge, and will likely be more important backstage in the processing steps of making the MXene material, or in fine-tuning its properties.”
The MXene material is based on work supported by the NSF Graduate Research Fellowship Program under grant no. 283036-3304 (to M.G.). Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the NSF. M.W.B. was supported by the Swedish Research Council (621-2014-4890).