[Image above] Credit: JudeanPeoplesFront; Flickr CC BY-NC-ND 2.0
We’ve written a lot about MXenes here on Ceramic Tech Today (most recently here). That’s because these materials have interesting properties that make them useful for a whole host of applications, from electromagnetic shielding and biomedicine to supercapacitors and water filtration.
Sure you’ve heard the term—but what exactly are MXenes? Simply, MXenes are 2-D materials that are transition metal carbide and nitrides.
The name MXene indicates their composition, where “M” is a transition metal (such as titanium, tantalum, niobium, vanadium, or molybdenum) and “X” is carbon or nitrogen. The “-ene” indicates that the material is 2-D, similar to “graphene.”
MXenes are generally formed by etching materials called MAX phases—also carbides and nitrides, albeit with an extra element crowding their composition. You can think of a MAX phase as an Oreo cookie—in this less tasty version, “M” and “X” are the cookie layers, and “A” is the cream filling.
The filling is made from a group A element from the Periodic Table, such as aluminum, silicon, tin, or indium.
MXenes are generally formed by chemically etching out the “A” layer of a MAX phase—like eating only the cream filling from the Oreo and leaving behind the stacked cookie layers.
Scientists have experimentally synthesized more than 70 MAX phases to date, but most of the 20 or so MXenes that have been made are derived from MAX phases containing aluminum in the filling layer.
One such popular MAX phase, titanium aluminum carbide (Ti3AlC2), however, is neither cheap nor abundant—it’s only produced in small batches on request, driving up costs and limiting large-scale production of titanium carbide MXenes.
In contrast, titanium silicon carbide (Ti3SiC2) is produced in large volumes and thus is much more economical, so it would be a more viable precursor for scaling-up production of MXenes.
But, of course, there’s a reason why the majority of MAX phases used to synthesize MXenes to date have involved aluminum—possibly because aluminum has the lowest reduction potential of the group A elements.
So, it’s a big deal that scientists at Drexel University (Philadelphia, Pa.) now report that they have devised a new method to produce MXenes from MAX phases containing a filling element other than aluminum.
“The key result is that after 7 years of trying to etch elements other than aluminum in the A-layer of MAX phases, we succeeded with silicon,” Yury Gogotsi, professor of materials science and engineering at Drexel, senior author of the new research, and ACerS Fellow, writes in an email. “So this opens new opportunities in MXene synthesis.”
It’s not like they didn’t try before—initial attempts to etch MXenes from titanium silicon carbide failed because the standard MXene hydrofluoric acid etching process can’t break silicon atoms free from their strong bonds.
The team has now found that adding a dash of oxidizing agent (such as nitric acid, hydrogen peroxide, or potassium permanganate) can do the trick, weakening the bonds so that silicon can be etched out. The simple technique can successfully transform titanium silicon carbide into titanium carbide MXene—a big step for potential scale-up of these useful materials.
“The etching process leaves behind stacks of titanium carbide, which can be delaminated to make flakes, which are approximately 1 nanometer in thickness. The researchers used this method to make flexible, electrically conducting titanium carbide films on a relatively large scale,” according to a Wiley news release.
Regarding how the oxidation process may affect the quality of MXene films, Gogotsi says that they’re not yet sure how the process will affect properties, “but somewhat different surface chemistry will certainly lead to different properties.”
So the team’s next step will be to determine the effects of processing parameters on properties—“which useful properties will be enhanced and which ones will be negatively affected,” Gogotsi says.
And with such new avenues for MXene synthesis, the team also plans to investigate whether they can create new MXenes from other aluminum-containing MAX phases as well.
The paper, published in Angewandte Chemie, is “Selective etching of silicon from Ti3SiC2(MAX) to obtain 2D titanium carbide (MXene)” (DOI: 10.1002/anie.201802232).