Two-dimensional (2D) transition metal carbides and nitrides, known as MXenes, have attracted the attention of academia and industry due to their attractive electronic, electrochemical, chemical, and optical properties. Advanced thermal gravimetric analysis of these materials allows scientists to better understand their thermal properties and design more efficient synthesis methods.
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Over the past decade, 2D materials, such as graphene and molybdenum disulfide, have become central to materials research due to their unique electronic, optical, mechanical, and thermal properties that are different from their plastic counterparts. bulk.
The properties of 2D materials are the result of quantum confinement effects in the atomically thin layers and strongly depend 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 allowing precise control of the properties of the materials.
What are MXenes and how are they made?
A new class of 2D materials, known as MXenes, has attracted a lot of interest in academia and industry after the discovery of 2D titanium carbide (Ti3VS2JX) in 2011. The term MXene describes a 2D structure consisting of carbides, carbonitrides and nitrides of transition metals with the formula Mn+1XnotJXwhere M represents a transition metal, X is carbon and/or nitrogen (n = 1, 2 or 3), and TX represents different surface functional terminations, such as hydroxyl, oxygen and fluorine.
To date, over 30 different MXenes have been synthesized (and several additional types predicted by computation), making these materials one of the most diverse, versatile, and fastest growing families of 2D materials. MXenes have already shown promising performance in energy storage, EMI shielding, antennas, water desalination and optoelectronics.
Typically, MXenes are made by selective etching of A elements from Mn+1CHOPPEDnot compounds, where A is a group IIIA to VIA element. Many research groups are currently exploring other precursors and synthetic methods. Due to their composition, MXenes exhibit a unique combination of metallic conductivity (resulting from the free electrons of the transition metal carbide or nitride skeleton structure) and tunable hydrophilicity resulting from the different functional terminations at the surface of the material. in layers.
Analysis of the properties of MXenes
Emerging industrial applications of MXene require fast, 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 cannot probe the properties of MXene materials. only in relatively small areas.
Thermogravimetric analysis for structural and chemical composition studies
Thermogravimetric analysis (TGA) is a thermal analysis technique used to determine changes in the physical and chemical properties of a wide range of materials.
TGA consists of measuring the mass loss of the sample either as a function of increasing temperature (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 gain of the material under study may result from sample decomposition, degradation, and oxidation or loss of volatile compounds. Additionally, the technique allows scientists to study the thermal stability of various materials in terms of resistance to thermal breakdown and degradation.
The TGA apparatus typically consists of a highly sensitive thermally insulated balance (for measuring changes in mass) and a programmable oven for precise sample temperature control.
TGA instruments can be combined with an infrared spectrometer or a mass spectrometer to allow the analysis and identification of gases and volatile compounds produced by sample degradation. A modern TGA device can heat the sample to temperatures above 1000°C in a controlled environment (air or inert gas) at heating rates between 0.1 and 200°C/min. The sensitivity of the balance is generally better than 0.1 μg.
TGA Reveals the Evolution of the Surface Chemistry of MXenes
Recently, researchers at Drexel University in Philadelphia, USA used a combination of TGA and mass spectrometry to explore changes in the surface chemical composition of titanium carbide MXenes.
The surface chemistry of MXenes is one of the keys to tuning the properties of materials for applications such as heterogeneous catalysis, electrochemical energy storage and others.
The Drexel research team used TGA and mass spectrometry simultaneously to systematically study the thermal properties of Ti3VS2JXnb2CTXand Mo2CTX MXene at temperatures ranging from room temperature to 1500°C in a helium atmosphere.
The researchers found that the thermal stability of materials was highly dependent on their chemical composition (the type of transition metals in the skeleton) and surface chemistry. More importantly, the material synthesis conditions during the etching and delamination processes were found to greatly affect the surface chemistry.
Increasing the concentration of hydrofluoric acid used as an etchant from 5 to 30% by weight resulted in more water being trapped between the MXene layers as well as an increase in the fluorine/oxygen ratio at the material surface.
In contrast, the use of a mixture of acids as an etchant, either a combination of hydrofluoric and hydrochloric or hydrofluoric and sulfuric acids, decreased the interlayer water and the number of hydroxyl groups on the surface of MXene. These results indicate that MXene produced from the same Mn+1CHOPPEDnot precursor using different etchants can have very different thermochemical properties due to their surface chemistry, thus allowing fine tuning of material properties.
Continue reading: How MXene nanomaterials are unlocking future nanotechnology
References and further reading
From., 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 Graphene, Graphene Oxide and Graphite Particle Size on Thermal Parameters. Carbon Research Journal 7(2), 41. Available at: https://doi.org/10.3390/c7020041
Hart, JL et al. (2019) Control of electronic properties of MXenes by termination and intercalation. Nat Common 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 MXene carbides studied by thermal analysis. Materials chemistry 31 (9), 3324-3332. Available at: https://doi.org/10.1021/acs.chemmater.9b00397