MXene nanomaterials enter a new dimension

(Press-News.org) Researchers from Drexel University who discovered a versatile type of two-dimensional conductive nanomaterial, called a MXene, nearly a decade and a half ago, have now reported on a process for producing its one-dimensional cousin: the MXene nanoscroll. The group posits that these materials, which are 100 times thinner than human hair yet more conductive than their two-dimensional counterparts, could be used to improve the performance of energy storage devices, biosensors and wearable technology.

Their finding, which was recently published in the journal Advanced Materials, offers a scalable method for producing the nanoscrolls from a MXene precursor with precise control over their shape and chemical structures.

“Two-dimensional morphology is very important in many applications. However, there are applications where 1D morphology is superior,” said Yury Gogotsi, PhD, Distinguished University and Bach professor in Drexel’s College of Engineering, who was a corresponding author of the paper. “It’s like comparing steel sheets to metal pipes or rebar. One needs sheets to make car bodies, but to pump water or reinforce concrete, long tubes or rods are needed.”  

By rolling 2D MXene flakes into 1D scrolls, the team has created a tubular material — ten thousand times thinner than a water pipe — that can reinforce polymers or metals or direct the flow of ions in a battery or a capacitive water desalination membrane with much less resistance.

“With standard 2D MXenes, the flakes lay flat on top of each other, which creates a confined-space and a difficult path for ions or molecules to navigate and move between the layers,” said Teng Zhang, PhD, a postdoctoral researcher in the College of Engineering, who was a co-author of the research. “By converting 2D nanosheets into 1D scrolls, we prevent this nano-confinement effect. The open, tubular geometry effectively creates 'highways' for rapid transport, allowing ions to move freely.”

Similar structures made of graphene sheets, called carbon nanotubes or graphene nanoscrolls, are already well-known and studied. But until now, the potential for producing high-quality 1D scrolls from MXene — which offers richer chemistry, better processability and higher conductivity than graphene — has remained challenging, according to the Drexel researchers, with previous attempts often producing inconsistent results.

The process for making nanoscrolls begins with a multilayer MXene flake as its precursor. By strictly controlling the chemical environment, the researchers use water to alter the surface chemistry of the flakes. This creates an structural asymmetry, called a Janus reaction, that causes lattice strain within the layers of the flakes. Driven by the release of internal strain, the layers peel away and curl into tight tubular scrolls.

The team tested this process with six different types of MXenes — two types of titanium carbide, niobium carbide, vanadium carbide, tantalum carbide and titanium carbonitride — reliably producing 10 grams of nanoscrolls with a controllable chemical composition and physical structure.

In addition to enabling superior electrical conductivity and mechanical strength, the nanoscrolls’ geometry produces unique behaviors that could allow them to be used in chemical sensing and functional composite materials.

“In a standard stacked 2D structure, the active sites for molecular adsorption are often hidden between layers, making it difficult for molecules, especially large biomolecules to reach them,” Gogotsi said. “The open, hollow structure of the scroll solves this by allowing the analytes easy access to the MXene surface. Combining with the material’s high conductivity and mechanical stiffness, this ensures we get a strong, stable signal. Thus, we envision the use of scrolls in biosensing. The same accessible surface of conductive scrolls may be useful for gas sensors, electrochemical capacitors and other devices that require access of ions and molecules to the surfaces.”

In the field of wearable electronics, or ionotronic devices, the researchers posit that MXene scrolls could perform a dual function, serving both as mechanical reinforcement and to enhance conductivity. Because of their rigid structure, they are able to anchor themselves into the soft polymer matrix to provide strength while simultaneously maintaining a robust conductive network. 

This would allow for the creation of stretchable composite materials that withstand the strain of daily movement without losing their electrical connection.

The researchers also found that in the solution, the orientation of the nanoscrolls can be controlled with an electric field. This discovery means they could easily be fabricated to align with the axis of fibers in a functional textile to produce a more durable, conductive coating.

“Imagine manipulating millions of tubules 100 times thinner than a human hair to make them build a wire or stand up vertically to make a brush,” Zhang said. “This is real nanotechnology, as we can manipulate matter at the nanoscale. It is also a critical development for functional textiles, as the scrolls could be incorporated as reinforcement materials in synthetic fibers.”

This controllable behavior is something the team will continue to explore. The team also anticipates further examining the quantum behaviors of the material, particularly its potential for superconductivity.

“Until now, superconductivity in this class of MXenes was limited to pressed pellets of particles and powders, having never been realized in solution-processed films with mechanical flexibility,” Gogotsi said. “By using niobium carbide scrolls, we observed the change of the material enough to enable superconductivity in free-standing, macroscopic films for the first time. The scrolling process introduces specific lattice strain and curvature that are absent in flat sheets. While the exact physical mechanism is still being explored, we hypothesize that this strain, combined with the continuous 1D structure, stabilizes the superconducting state.”

The quantum nature of nanomaterials has led to a number of prominent discoveries in recent years, as the field has gained more attention for its potential to boost computing power and data storage. For the Drexel researchers, this study is a critical breakthrough, because it transforms MXene superconductivity from a laboratory curiosity into a practical property of the nanomaterial.

“Using the methods described in this paper, we can now process superconducting MXenes into flexible films, coatings or wires at room temperature for potential superconducting interconnectors or quantum sensors,” Zhang said. “We expect many other interesting phenomena caused by scrolling and are going to study them.”

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