Two-dimensional (2D) transition metal carbides and nitrides, known as MXenes, have attracted attention in academia and industry due to their attractive electronic, electrochemical, chemical, and optical properties. Advanced thermal gravimetric analysis of these materials enables scientists to better understand their thermal properties and devise more efficient synthesis methods.
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In the past decade, 2D materials, like graphene and molybdenum disulfide, have become central to material research owing to their unique electronic, optical, mechanical, and thermal properties that are different from their bulk counterparts.
The properties of the 2D materials are the result of quantum confinement effects within the atomically-thin layers and are strongly dependent on the layer thickness and composition. Moreover, the properties of these materials are highly tunable by chemical doping and application of external fields (electric or magnetic), thus enabling precise control of the material properties.
What Are MXenes and How Are They Made?
A new class of 2D materials, known as MXenes, has gained a lot of interest from academia and industry after discovering the 2D titanium carbide (Ti3C2Tx) in 2011. The term MXene describes a 2D structure consisting of transition metal carbides, carbonitrides, and nitrides with a formula Mn+1XnTx, where M represents a transition metal, X is carbon and/or nitrogen (n = 1, 2, or 3), and Tx stands for different surface functional terminations, such as hydroxyls, oxygen, and fluorine.
So far, more than 30 different MXenes have been synthesized (and several additional types predicted computationally), rendering these materials one of the most diverse, versatile, and fastest-growing families of 2D materials. MXenes have already shown promising performance in energy storage, electromagnetic interference shielding, antennas, water desalination, and optoelectronics.
Typically, MXenes are made by selective etching of A elements from Mn+1AXn compounds, where A is a group IIIA to VIA element. Many research groups currently explore other precursors and synthesis methods. Owing to their composition, MXenes exhibit a unique combination of metallic conductivity (resulting from the free electrons of the transition metal carbide or nitride backbone structure) and tunable hydrophilicity resulting from the different functional terminations on the surface of the layered material.
Analyzing the Properties of MXenes
The emerging industrial applications of MXene require rapid, reliable, and cost-effective analytical techniques for in-depth characterization of these materials. Most of the advanced characterization techniques applicable to 2D materials are localized characterization methods, such as scanning and transmission electron microscopy, atomic force microscopy, and nano-Raman spectroscopy, which can probe the properties of MXene materials only over relatively small areas.
Thermogravimetric Analysis for Structural and Chemical Composition Studies
Thermogravimetric analysis (TGA) is a thermal analysis technique used to determine the changes in the physical and chemical properties of a wide range of materials.
The TGA involves measuring the sample mass loss either as a function of temperature increase (at a constant heating rate) or as a function of time at a constant temperature.
By performing the TGA, various physical and chemical processes, such as phase transitions, evaporation, desorption, decomposition, dehydration, and others, can be studied.
The resulting mass loss or mass gain of the material under investigation can result from the sample's decomposition, degradation, and oxidation or the loss of volatile compounds. Besides, the technique enables scientists to investigate the thermal stability of various materials in terms of the resistance toward thermal decomposition and degradation.
TGA apparatus typically consists of a highly sensitive thermally-isolated balance (to measure mass changes) and a programmable furnace for precise temperature control of the sample.
The TGA instruments can be combined with an infrared spectrometer or a mass spectrometer to enable the analysis and identification of gases and volatile compounds produced by the degradation of the sample. A modern TGA apparatus can heat the sample to temperatures above 1000 °C in a controlled environment (air or inert gas) at heating rates in the range of 0.1-200 °C/min. The balance sensitivity is typically better than 0.1 μg.
TGA Reveals Surface Chemistry Evolution of MXenes
Recently, researchers at Drexel University in Philadelphia, USA employed a combination of TGA and mass spectrometry to explore the changes in the surface chemical composition of titanium carbide MXenes.
The surface chemistry of the MXenes is one of the keys to tuning the material properties for applications such as heterogeneous catalysis, electrochemical energy storage, and others.
The research team at Drexel employed simultaneous TGA and mass spectrometry to systematically study the thermal properties of Ti3C2Tx, Nb2CTx, and Mo2CTx MXenes at temperatures ranging from ambient to 1500 °C in a helium atmosphere.
The researchers found that the thermal stability of the materials was strongly dependent on their chemical composition (the type of transition metals in the backbone) and the surface chemistry. More importantly, it was found that the material synthesis conditions during the etching and delamination processes greatly affected the surface chemistry.
Increasing the concentration of the hydrofluoric acid used as an etchant from 5 to 30 wt% resulted in a larger amount of water trapped between the MXene layers together with an increase in the fluorine-to-oxygen ratio on the material surface.
In contrast, using a mixture of acids as an etchant, either combination of hydrofluoric and hydrochloric or hydrofluoric and sulfuric acids, decreased the interlayer water and the number of hydroxyl groups on the surface of the MXene. These results indicate that MXene produced from the same Mn+1AXn precursor using different etchants can have very different thermochemical properties due to their surface chemistry, thus enabling fine-tuning of the material properties.
Continue reading: How MXene Nanomaterials Are Unlocking Future Nanotechnologies
References and Further Reading
De, S., et al. (2021) Current trends in MXene research: properties and applications. Mater. Chem. Front. 5, 7134-7169. Available at: https://doi.org/10.1039/D1QM00556A
Farivar F., et al. (2021) Thermogravimetric Analysis (TGA) of Graphene Materials: Effect of Particle Size of Graphene, Graphene Oxide and Graphite on Thermal Parameters. Journal of Carbon Research 7(2), 41. Available at: https://doi.org/10.3390/c7020041
Hart, J.L. et al. (2019) Control of MXenes' electronic properties through termination and intercalation. Nat Commun 10, 522. Available at: https://doi.org/10.1038/s41467-018-08169-8
Yury Gogotsi, Y. and Anasori, B. (2019) The Rise of MXenes. ACS Nano 13 (8), 8491-8494. Available at: https://doi.org/10.1021/acsnano.9b06394
Seredych, M., at al. (2019) High-Temperature Behavior and Surface Chemistry of Carbide MXenes Studied by Thermal Analysis. Chemistry of Materials 31 (9), 3324-3332. Available at: https://doi.org/10.1021/acs.chemmater.9b00397