A new way to make two-dimensional conductors on the MX-scene

In efforts to build ultrathin electronic circuits—for applying to cell phones, human skin, and more—the conductive material of choice is often graphene. (See, for example, Physics Today, September 2022, page 17 .) Electrons flow freely through the honeycomb carbon sheets, just as they do through graphene’s bulk parent material, graphite.

But graphene is a semimetal, not a metal. Rather than a continuous partially filled band of plentiful electronic states, graphene has valence and conduction bands that touch at just a few discrete points. It has no bandgap, but the quantum states that contribute to charge transport aren’t as plentiful as they might be.

Although true metals are rare among two-dimensional materials, a notable exception is a class of materials called MXenes (pronounced “Maxines,” like the name), with the general formula M₂X, where M is a transition metal and X is carbon or nitrogen. (Thicker structures of the form M₃X₂ or M₄X₃ are also possible.) In addition to being electrically conductive, MXenes are valued for their chemical properties, with applications in catalysis, energy storage, and more.

Despite their promise, MXenes have been difficult to make cleanly. But the University of Chicago’s Dmitri Talapin and colleagues are working to change that, with not one but two new MXene synthesis routes that could help streamline basic research and open the door to more practical industrial production.

Since MXenes’ discovery in 2011, most researchers have made them from bulk precursor materials called MAX phases, in which MXene sheets are interspersed with layers of a third element, denoted A. Bathing the MAX phase in hydrofluoric acid removes the A atoms and leaves behind a stack of MXene sheets ready to be peeled off and used.

But hydrofluoric acid is an extremely harsh chemical, and it doesn’t always limit its attack to the sacrificial A atoms. It can also eat away at the MXenes themselves to create defect-riddled materials that are hard to reliably characterize.

In search of an alternative, Talapin and colleagues were guided by the principle of atom economy, the philosophy of optimizing reactions so that most, if not all, of the atoms present in the reactants find their way to the products. (The wasted A atoms from the MAX phases, for example, wouldn’t qualify.) They found, as shown below, that they could make the Ti₂C MXene by cooking together titanium metal, graphite, and titanium tetrachloride. The product was capped with chlorine atoms on both sides, but that’s a feature, not a bug: MXenes’ ability to form chemical bonds with other atoms and molecules is one of their desirable characteristics, and the Cl atoms can be swapped out for other functional groups.

The synthesis is surprisingly robust. Apart from forming a varying amount of titanium carbide by-product, the reaction proceeded as planned under a wide variety of reaction conditions. That’s evidence that MXenes are a thermodynamically favored structure—something that couldn’t have been known from the MAX-phase-etching method.

But the bigger surprise came when the researchers took the reaction into the gas phase by reacting TiCl₄ and methane with a Ti metal surface. Such a reaction would normally be expected to be self-limiting: When the reactants run out of surface, the reaction should stop. But instead, more MXene kept forming, eventually buckling up from the surface to form the micron-sized pom-pom-like structures shown in the micrograph above. The pom-poms aren’t just nice to look at; because of their high surface area, they’re also ideal for energy storage, as lithium ions, for example, can readily be held between the MXene sheets.

Talapin credits his success to the new perspective he brought to the field. MXene research has largely been the domain of chemical engineers, whereas he and his colleagues are more traditional chemists and physicists. “There’s nothing we did that couldn’t have been discovered many years ago,” he says. “It was just a matter of getting the right people together.” (D. Wang et al., Science 379, 1242, 2023 .)