MXenes are an emerging class of two-dimensional (2D) ceramic nanomaterials that were first discovered in 2011. Unlike other 2D ceramics like graphene, MXenes uniquely combine metallic conductivity, hydrophilic surfaces, and excellent mechanical properties resembling clay. Owing to this versatile combination, MXenes are being explored for diverse applications from energy storage, electromagnetic shielding, water purification to structural composites.
MAX Phases to MXenes
The name MXene was coined as a wordplay on graphene and the MAX phases they are derived from. MAX phases are layered hexagonal carbides/nitrides of early transition metals, denoted as Mn+1Xn, where:
- M is an early transition metal (Ti, V, Nb etc.)
- X is carbon/nitrogen
- n = 1-3
By selectively etching the A layers from MAX phases, what is left behind are 2D transition metal carbide/nitride layers called MXenes. The etched ceramic layers have surface terminations like -F, -O, -OH denoted as Mn+1XnTx. It is the surface terminations that make MXenes hydrophilic.
Common MAX Phases and the MXenes Derived from Them
| MAX Phase | MXene |
| Ti3AlC2 | Ti3C2Tx |
| Nb2AlC | Nb2CTx |
| Ta4AlC3 | Ta4C3Tx |
| Mo2TiAlC2 | Mo2TiC2Tx |
| Ti2AlN | Ti2NTx |
Exceptional Properties
MXenes present exceptional electrical, optical and surface properties that set them apart and make them promising materials across nanoelectronics, energy storage, composites and purification applications.
Salient properties
- High electrical conductivity (6,000 – 8,000 S/cm)
- Hydrophilic and clay-like surface terminating layers
- Very high volumetric capacitance (>900 F cm−3)
- Broad optical absorption from UV to NIR
- Excellent mechanical properties
This unique combination is not found in other 2D materials.
- Example: Graphene lacks surface functionalization while other ceramic 2D materials have poor electrical conductivity.
Synthesis Techniques
There are three main techniques to synthesize MXenes:
- Wet Chemical Etching: Most common method where a MAX phase is etched by a fluoride salt solution, typically hydrofluoric acid based. The process selectively dissolves A elements, leaves MXene flakes and introduces surface terminations.
- Electrochemical Etching: Involves using an applied voltage to electrochemically etch Al from Ti3AlC2. Allows better control over etching.
- Physical Deposition: Techniques like magnetron sputtering used to deposit MXene thin films on substrates from a MAX phase target.
Of these, wet chemical etching of MAX powders has seen the most progress with over 30 types of MXenes reported. Ti3AlC2 is the most widely etched MAX precursor to produce Ti3C2Tx MXene.
Applications and Commercialization
The applications where MXenes are being researched broadly fall into four domains:
Energy Storage
- Batteries (Li-ion, Li-S, Al-ion)
- Supercapacitors
- Hydrogen storage
Electromagnetic Shielding
- Radar absorption
- 5G communication
Water Purification
- Desalination membranes
- Selective separation
- Pollutant degradation
Composites
- Polymer reinforcement
- Ceramic matrix composites
Global Research Activity
Owing to their broad range of tunable properties, research interest and scientific publications on MXenes have accelerated in recent years across the globe as highlighted in the chart.
China leads in total MXene publications followed closely by the USA. However, the USA has a higher share of highly cited MXene articles reflecting dominant fundamental contributions. Asian countries like India, Iran and Turkey have also seen a sharp increase in quality research on applying MXenes.
Future Outlook
The unique combination of conductivity, hydrophilicity and surface functionalization provided by MXenes make them extremely versatile 2D ceramic materials. As material quality, controlled synthesis and manufacturing processes evolve further, MXenes are primed to enable next-generation innovations at the intersection of material science and engineering across energy storage architecture, electromagnetic and optical applications.