By Akshatha ChandrashekarReviewed by Frances BriggsOct 23 2025New research clarifies how synthesis methods influence the oxidation behavior of Ti3AlC2 MAX phases and their resulting MXenes, revealing a disconnect in how oxygen content affects each.
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The study, published in Materials & Design, reveals an unexpected distinction between oxygen incorporation in MAX phases and the thermal stability of resulting MXenes. While oxygen content significantly affects the oxidation behavior of Ti3AlC2 MAX phases, it has far less influence on the thermal degradation of Ti3C2 MXenes derived from those phases.
This work adds nuance to a long-standing question in MXene research: to what extent does the composition of the MAX precursor determine the thermal resilience of its 2D offspring? The findings suggest that post-synthesis chemistry and processing play a larger role in governing MXene stability than the oxygen content of the starting material.
MXenes are a class of two-dimensional materials known for their layered graphene-like structure, high electrical conductivity, and hydrophilic nature. They are most often produced by selectively etching the “A” layer from ternary carbides or nitrides, such as Ti3AlC2 MAX phases, to yield 2D Ti3C2-based flakes.
Despite their promising properties, MXenes remain vulnerable to chemical and thermal degradation, limiting their long-term performance in real-world applications such as energy storage and electronics. One hypothesis has been that oxygen substitution in the MAX phase, specifically, oxygen replacing carbon in the lattice, leads to the formation of oxycarbide structures, which degrade MXene performance.
This study explored how the incorporation of oxygen during MAX phase synthesis impacts the thermal stability of both the parent MAX phase and its resulting MXenes.
Experimental Design Focused on Oxygen Incorporation
The researchers synthesized three Ti3AlC2 MAX phase variants using two ball milling techniques: high-energy planetary milling and lower-energy roll milling. Each variant was prepared with carefully controlled ratios of titanium, aluminum, and carbon sources, either graphite or TiC powders.
After this mechanical activation, the powders were annealed at 1450 °C in an argon atmosphere and acid-treated to remove intermetallic impurities. The resulting powders were subsequently converted into MXenes through mild fluoride-based etching and delamination.
To investigate the relationships between structure and properties, the team used a number of different advanced characterization tools.
Using X-ray diffraction in transmission and reflection modes provided lattice parameter measurements and impurity identification. Scanning electron microscopy revealed flake morphology and elemental distribution, while transmission electron microscopy and selected area electron diffraction gave high-resolution insights into crystallinity.
Thermogravimetric analysis measured oxidation onset temperatures, and in situ TEM heating allowed direct observation of nanoscale oxidation behavior.
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MAX Phases Show Oxygen Sensitivity
The three MAX samples showed clear differences in microstructure and impurity levels.
The planetary-milled MAX1 sample exhibited larger agglomerates and higher levels of impurities, including corundum and TiAl3. The team attributed these results to the aggressive nature of planetary milling, which likely led to aluminum loss and increased oxygen uptake.
In contrast, MAX2 and MAX3, produced using roll milling, displayed more uniform hexagonal grains with significantly fewer impurities. MAX2 in particular, synthesized with higher aluminum content, showed the lowest oxygen incorporation.
Lattice parameter analysis confirmed the influence of oxygen. MAX1 and MAX3 had reduced a and c values, consistent with oxycarbide formation, while MAX2’s larger lattice parameters aligned with a more complete carbide phase and lower internal oxygen content.
These differences directly impacted thermal performance. Thermogravimetric analysis showed that MAX1 and MAX3 began oxidizing near 450 °C, whereas MAX2 resisted oxidation until about 780 °C, indicating superior thermal stability.
MXenes Tell a Different Story
Despite the pronounced effect of oxygen on MAX phase oxidation, the trend did not carry over to the MXenes.
All three Ti3C2 MXene samples began oxidizing at roughly the same temperature, around 460 °C, regardless of their precursor’s oxygen content. This was further supported by in situ TEM heating, which revealed oxidation onset between 350 °C and 420 °C. The team observed the formation of anatase TiO2 during this transition and noted that the slight discrepancy between TEM and TGA results stems from differences in measurement sensitivity.
These findings suggest that the oxygen content of the MAX precursor, while important for MAX phase integrity, does not meaningfully affect the oxidation resistance of the derived MXene. Instead, other factors appear to dominate, particularly the chemistry of the etchants used, the quality of delamination, surface functional groups, and residual salts introduced during processing.
One of the most valuable technical contributions of the study was the use of transmission-mode X-ray diffraction. This method enabled more accurate detection of non-basal lattice parameters in the MXenes and successfully revealed trace amounts of residual salts such as LiF and LiCl, impurities that conventional reflection-mode XRD failed to detect.
The ability to spot these remnants could help to improve MXene purity and consistency, especially in large-scale production settings.
Transmission-mode XRD also yielded clearer separation of diffraction peaks, allowing more precise analysis of structural parameters. This was essential for confirming that the a lattice values of MXenes contracted uniformly during etching, independent of the starting MAX phase composition.
Implications for Synthesis and Future Research
The study concludes that controlling oxygen incorporation is crucial for producing high-quality, thermally stable MAX phases. In particular, using roll ball milling and increasing aluminum content can reduce oxygen uptake and enhance phase purity.
However, for MXenes, the key to stability is not in the precursor’s oxygen content but in the processing chemistry that follows. Etchant selection, washing protocols, and storage conditions all play a decisive role in determining how long MXene materials retain their integrity.
The authors emphasized that future research should focus on refining etching techniques, improving post-synthesis treatments, and developing a better understanding of oxidation pathways at the flake level. They also highlighted the value of transmission-mode XRD as a diagnostic tool that could be widely adopted to monitor synthesis quality and optimize performance.
Journal Reference
Hettige Dharmasiri, C. D., et al. (2025). Influence of synthesis routes on oxygen content, crystallography, and thermal stability of Ti3AlC2 MAX phases and resulting MXenes. Materials & Design, 259, 114729. DOI: 10.1016/J.matdes.2025.114729
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