MXenes, two-dimensional transition metal carbides or nitrides, have electronic properties rivalling graphene, often considered the nanomaterial of the future. As a result, these nanosheets also have many applications, such as energy storage, conductive coatings, filtration membranes, and electromagnetic interference shielding. However, little is known about the mechanical properties of MXenes, an important criterion when considering usage within these applications.

Lipatov et al. address this issue by directly measuring the Young’s modulus of monolayer solution-processed titanium carbide (Ti₃C₂T_x) MXene using atomic force microscopy (AFM) nanoindentation. Comparison to graphene oxide (GO) and other similar 2D solution-processed nanomaterials reveals the strong potential of Ti₃C₂T_x.

Novel Nanoindentation

As its name implies, nanoindentation AFM uses an AFM tip to apply a known force to a membrane sample. To use nanoindentation AFM, the synthesized MXene flakes must be placed over microwells. “The AFM tip was positioned directly in the centre of a selected well and slowly moved downward, providing controlled stretching of a MXene flake,” describes Research Assistant Professor  Alexey Lipatov. The tip force is incrementally increased and the deflection of the sample is measured to yield force over deflection curves.

Elastic sheets trump graphene oxide

Using these data along with the known Poisson ratio and sheet thickness for monolayer Ti₃C₂T_x, researchers under the direction of Alexander Sinitskii and Yury Gogotsi calculated the Young’s modulus – the ratio of applied stress to resulting elastic strain a material can endure.  They found the solution-processed Ti₃C₂T_x had a Young’s modulus of 333 GPa, which compares favourably with similar solution-processed nanosheets, such as MoS₂, as well as graphene oxide and reduced graphene oxide.  They also report excellent reproducibility, as values ranged from 278 N/m to 393 N/m over 36 data points.

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Interestingly, the measured Young’s modulus is significantly lower than the theoretical limit of 502 GPa for Ti₃C₂. Lipatov states, “As expected, the experimentally determined value… is lower because of surface functionalization and the presence of defects. However, the difference in the Young’s moduli of the ‘ideal’ Ti₃C₂ and the experimentally realized Ti₃C₂T_x is not as dramatic as in the case of graphene and graphene oxide (1050 GPa versus 210 GPa).” He adds that, given the known defects introduced during solution-based synthesis, “There is potential to develop methods to synthesize Ti₃C₂T_x flakes of higher quality to reach a larger Young’s modulus close to the theoretical value.” He also suggests that there is great potential for MXenes within many applications, particularly given that Ti₃C₂T_x is just one of about 30 synthesized MXenes, and MXenes with a different number of atomic layers or a different transition metal may have a higher elasticity.

Solution synthesis but dry transfer

The researchers exploited the scalable solution-based synthesis to generate Ti₃C₂T_x flake samples by using an acidic solution to etch away aluminium from the Ti₃AlC₂ ‘MAX’ phase, where MAX refers to a large family of hexagonal layered ternary transition metal carbides, carbonitrides and nitrides with the composition M_n+1AX_n. This leaves high-quality Ti₃C₂T_x MXene sheets up to 10 μm in size suspended in water.

The researchers then deposited MXene flakes onto a silica-coated silicon microwell plate by first drop casting the dispersion onto a PDMS substrate. They rinsed the substrate with the MXene sample to remove the remaining contaminating salts. They could then place the PDMS-MXene stack MXene-side down onto the microwell plate and manually remove the PDMS to reveal MXene nanosheets tightly suspended over the microwells.

Lipatov notes, “The rationale behind this technique is that hydrophilic MXene flakes should have a stronger attractive interaction with the hydrophilic silica surface than with the hydrophobic PDMS.” Indeed, such a transfer method “consistently produced MXene membranes of excellent quality.”

Full details are reported in Science Advances.