Graphene, a two-dimensional sheet of carbon, has been the subject of much research since it was discovered in 2004. Its basic properties are fairly well documented, and papers are appearing about possible applications, for example, as supercapacitor electrodes or composite reinforcement. Some novel ideas are emerging as a recent paper on graphene-based artificial muscles illustrates.
Graphene is interesting stuff, but its range of properties is limited by its super-simple chemistry. In multilayer form, weak van der Waals bonding between layers is a limiting factor, too.
However, if two-dimensional materials with more complex chemistries could be made, the door would be opened to tune properties and engineer materials for specific applications.
Well, count among the door-openers Drexel University professors and ACerS Fellows Yury Gogotsi and Michel Barsoum, who describe a process for synthesizing such materials in a new paper, “Two-Dimensional Nanocrystals Produced by Exfoliation of Ti3AlC2,” in Advanced Materials (doi: 10:1002/adma.201102306).
As their paper states, “Complex, layered structures that contain more than one element may offer new properties because they provide a larger number of compositional variables that can be tuned for achieving specific properties.”
Barsoum was among the first to work on the so-called MAX phases, which are layered ternary carbides or nitrides. MAX refers to the material’s chemistry: “M” is an early transition metal (Ti, Ta, etc.), “A” is an A-group metal (Al, In, Si, etc.) and “X” is carbon or nitrogen. So far, more than 60 MAX compounds have been identified.
The layered morphology gives them some interesting physical properties that can be metal-like or ceramic-like, but their structure and chemistry also make them good precursor materials for carbide-derived carbons, which are nanostructured porous materials. CDCs are synthesized by removing the “M” and “A” with hydrofluoric acid, and we wrote about some of their anomalous supercapicitance properties in an earlier post.
Wondering whether a hybrid MAX-CDC material could be synthesized, the Drexel team began experimenting with selective removal of “A” elements. MX compounds are chemically stable, and the “A” elements tend to be weakly bonded and are more reactive.
The process was surprisingly simple: They synthesized Ti3AlC2 by first ball milling, and then immersing the resultant powders in a concentrated HF solution at room temperature; next, they rinsed and centrifuged the material. Finally, they used cold pressing to align flakes. They characterized the flakes with XRD, SEM and TEM, and determined the chemistries with X-ray energy dispersive spectrometry in the TEM.
By removing the aluminum (“A” element), they discovered they had formed a new two-dimensional material with the composition Ti3C2. Because its morphology is similar to graphene, the team refers to this class of materials as “MXene.” They report having formed nanosheets (a few layers thick) and conical scrolls.
Gogotsi says they have demonstrated the ability to synthesize MX compounds through exfoliation on a wide range of MAX compounds, including carbo-nitrides. According to the paper they already have “solid evidence for the exfoliation of Ta4AlC3 into Ta4C3 flakes,” but offered no information on the material properties of these latter two compounds.
Given that the MAX compounds comprise a well-defined family of materials, they seem to be good candidates for the Materials Genome Initiative concept. Gogotsi confirmed that they are. “These materials are a perfect case for computational materials engineering. It’s a much better and more efficient way to go after the structures of this family of materials.”
In a NanoWerk article, Gogotsi says, “We are talking about a large family of 2D metal carbides and nitrides, so exploring different structures to find the optimum chemistry for each application is the next step in our work,” plus property characterization and controlling the surface chemistries.
The potential applications of MXene materials is wide. Ab initio simulations predict that they will have large elastic moduli. By varying their surface chemistries (for example, in the paper, the surfaces are terminated by hydroxl and/or fluorine groups) interfaces and bandgaps can be tuned. The large surface areas and layered structure make these materials interesting candidates for Li-ion battery electrodes, pseudocapacitors, polymer composite fillers and other energy and electronic devices.