KAIST
BREAKTHROUGHS
Research Webzine of the KAIST College of Engineering since 2014
Fall 2026 Vol. 27Breaking Crystal Symmetry: A MAX-Phase Precursor for Janus MXenes
Using high-entropy materials design, a KAIST NQe team synthesized the first experimentally verified asymmetrically ordered MAX phase (a precursor candidate for Janus MXenes), opening a pathway toward materials for radionuclide capture and electromagnetic shielding.
A 3D conceptual illustration showing how high-entropy-driven asymmetric o-MAX ordering enables conversion into Janus MXene with chemically distinct surfaces.
MXenes are two-dimensional transition-metal carbides and nitrides known for high electrical conductivity and highly reactive surfaces. Because of this combination, they are widely studied for energy storage, sensors, electromagnetic shielding, and selective radionuclide capture. MXenes are typically produced by chemically removing the A-element layers from a layered ceramic precursor known as a MAX phase. A MAX phase generally has the formula Mn+1AXn, where M is a transition metal, A is a Group 13–16 element, and X is carbon or nitrogen.
Figure 1 SEM-EDS image (left) and X-ray diffraction pattern (right) of the synthesized multi-component MAX phase, where the absence of the (004) peak serves as direct evidence of specific out-of-plane ordering (Ao-MAX).
Most MXenes reported so far are symmetric: the two sides expose essentially the same surface chemistry. Janus MXenes are different. Like the two-faced Roman god Janus, they are envisioned to have chemically distinct surfaces on opposite sides. That asymmetry could enable properties inaccessible to symmetric MXenes, including the Rashba effect, piezoelectricity, and photocatalysis. The problem has been the starting material. Although Janus MXenes have been predicted in simulations, the symmetry-breaking MAX-phase precursor needed to make them had not been obtained experimentally.
A KAIST team led by Professor Ho Jin Ryu has now addressed this bottleneck using a high-entropy approach. In work that began from efforts to develop materials for radioactive-species adsorption, the team mixed six elements (titanium, zirconium, hafnium, tantalum, aluminum, and tin) to obtain an asymmetrically out-of-plane ordered MAX phase. Its composition, (Ti3/11Zr2/11Hf3/11Ta3/11)3(Al2/3Sn1/3)C2, shows different chemical ordering in the outer metal layers, consistent with atomic-size effects among the mixed metal species.
Figure 2 HAADF-STEM image showing asymmetric atomic ordering along the c-axis driven by compositional differences in the transition-metal layers.
Because MAX phases can be converted to MXenes by selective etching, this asymmetric layered ceramic provides an experimentally accessible precursor route toward Janus MXenes. The result clarifies how symmetry can be broken in MAX phases and points to future materials for radionuclide-capture filters, electromagnetic-wave absorbers and shields, sensors, and piezoelectric devices.
“Through high-entropy materials design, we realized an asymmetric atomic structure that conventional crystallography could not easily deliver,” Professor Ryu said. He added that the approach could develop into a platform technology for safety and environmental applications such as radionuclide capture and electromagnetic shielding.
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