The use of intense chemical etching for MXene synthesis has been shown to result in many flaws or voids on the exterior of the fabricated MXene wafers.
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Faulty areas are susceptible to oxidative deterioration processes with oxygen or water molecules, which degrade MXene inherent characteristics. Recent research has provided an insight into the nature of oxidative deterioration of MXenes, paving the way for advances in managing the oxidative processes of MXenes using several potential strategies.
An Introduction to MXenes
A collection of early transition metallic carbides or carbonitrides dubbed MXenes has lately emerged as a novel and possibly revolutionary type of two-dimensional material. These are 2D nanosheets created by etching the A layers out of MAX phases.
MAX phases are named from their chemical formula: Mn+1AXn, where M is an early transition metal, A is primarily a group 13 or 14 element, X is carbon and, or, nitrogen, and n = 1, 2, or 3. About 60 distinct pure MAX phases are known, with several more possibilities.
Features and Applications of MXenes
In contrast with other 2D materials, MXenes have exceptional electrical, electrolytic, photonic, and mechanical capabilities, prompting researchers to investigate additional MXenes with varied compositions of elements.
MXenes exhibit a wide range of inherent features, including good electrical conductance owing to a surplus of electron density at the Fermi level, hydrophilicity due to an abundance of water-attracted polar surface terminations, and quick processability in solutions without the requirement of dispersion agents.
These distinguishing characteristics make them suitable for a wide range of possible uses, including terahertz shielding, electromagnetic interference (EMI) insulation, electrolytic energy storage, photonics, transparent pliable electrodes, sensor systems, heating elements, LEDs, and antimicrobial coatings.
The Problem of Oxidative Degradation
MXenes are prone to severe oxidative deterioration, which significantly degrades all of their properties and limits future uses. Flawed areas at the corner or on the exterior of MXene wafers, resulting from the intense and corrosive chemical etching fabrication process, behave as active regions for the oxidative deterioration process, causing highly crystalline MXenes to morph into oxides of transition metals such as TiO2 and amorphous carbon.
Inherent characteristics of MXenes diminish due to oxidative deterioration. Improving the oxidative stability of MXenes to extend their shelf life for commercial uses is quite challenging.
Details of MXene Synthesis
MXene fabrication is complex, but MXenes with the desired characteristics may be obtained by meticulously regulating all synthesis conditions.
Each stage in the fabrication of MXenes has an impact on their finished framework, physiochemical characteristics, and oxidation resistance. MXenes are produced via topochemical extraction from their source MAX phases; hence, the framework and characteristics of MXenes are heavily influenced by the MAX phase quality.
Quality of the MAX phase is influenced by the precursors used. Raw components of the MAX phase directly determine the characteristics of the resulting MXenes.
The very first MXene (Ti3C2Tx) was produced in 2011 by chemically etching Ti3AlC2 MAX in concentrated hydrofluoric acid. Following HF etching, many techniques which utilize or form HF have been documented for the fabrication of Ti3C2Tx and other MXenes.
Substantial improvements were achieved by utilizing less severe etching parameters for the delamination of Ti3AlC2 MAX to Ti3C2Tx Mxene. Here, a combination of LiF and hydrochloric acid produced HF acid on-site, resulting in minimum intensive layer delamination (MILD).
Synthesis Strategies to Minimize Oxidative Degradation
Iqbal et al. reported that the acidic etchant concentration determines the kind and quantity of atomic imperfections that initiate oxidation processes. A weak acidic mixture may aid in the preservation of the MXene wafers. Pristine MAX phases produced in stoichiometric proportions often yield exceptional-quality MXenes post-etching.
One more variable that influences the extent of oxidative deterioration is the wafer size of the prepared MXene. Along with acidic etchant concentration, sonication is a method that regulates the size of MXene flake. It reduces multilayered constructions into extremely thin layers by using ultrasonic waves to break down bigger flakes into tiny ones, which are then retrieved using a centrifugal process.
Bigger flakes exhibit better stability and resistance to oxidation than small ones; the more the exposed surface area of the corners in the small flakes, the quicker the oxidation.
Utilizing a MILD method results in MXenes with larger flake sizes, which have greater oxidation resistance than smaller flakes owing to a lower density of flawed areas such as faults and corners. A suitable fabrication procedure may aid in eliminating the sonication stage, which crumbles MXene flakes and decreases their average lateral size.
The use of ordinary polymers may help minimize the oxidation extent of MXenes while increasing their mechanical characteristics for practical uses. Thin MXene films may also be placed between polymeric films to meet the demands of pliable electronics with increased oxidation stability and mechanical strength.
The Last Word
The nature and quality of the precursors used in the fabrication of MAX and MXene directly impact the characteristics of the resulting MXene. After etching in moderate acidic settings, a good quality MAX phase produced by pressureless sintering would provide a high-quality MXene dispersion, ensuring an extended shelf life.
Despite encouraging research progress, the mechanism of the oxidative deterioration process of aqueous MXene dispersions remains unknown. Completely inhibiting MXene oxidation is also difficult, particularly when they are moved from labs to factories for commercial product development. As a result, MXenes have spurred scientists to study their oxidation kinetics and processes and create innovative techniques to increase their oxidation stability.
Continue reading: How MXene Nanomaterials Are Unlocking Future Nanotechnologies.
References and Further Reading
Gogotsi, Y., & Anasori, B. (2019). The Rise of MXenes. ACS Nano, 13(8). Available at: https://doi.org/10.1021/acsnano.9b06394
Iqbal, A., Hong, J., Ko, T. Y. et al. (2021). Improving oxidation stability of 2D MXenes: synthesis, storage media, and conditions. Nano Convergence, 8. Available at: https://doi.org/10.1186/s40580-021-00259-6
Naguib, M., Mochalin, V. N. et al. (2014). 25th Anniversary Article: MXenes: A New Family of Two-Dimensional Materials. Advanced Materials, 26(7), 992-1005. Available at: https://doi.org/10.1002/adma.201304138