(Left) Schematic of IR reflection and emission with MXene. (Right) IR emissivity versus thickness of MXene films and other films (from the literature), including metal films and transparent conductive films. MXenes can cover a very broad range of properties, from a very low emissivity at the level of polished metals, to a very high emissivity required for efficient radiative heat release (cooling). MXenes allow thermal management using very thin films.
(Left) Schematic of IR reflection and emission with MXene. (Right) IR emissivity versus thickness of MXene films and other films (from the literature), including metal films and transparent conductive films. MXenes can cover a very broad range of properties, from a very low emissivity at the level of polished metals, to a very high emissivity required for efficient radiative heat release (cooling). MXenes allow thermal management using very thin films.

The MXene family of two-dimensional transition metal carbides and carbonitrides possess unique optical properties in the visible and infrared range that could be ideal for thermal management applications from camouflage to radiation heating. Researchers from Drexel, Pennsylvania, and Fudan Universities have systematically explored the infrared (IR) emission properties of typical MXenes [Han et al., Materials Today (2023), https://doi.org/10.1016/j.mattod.2023.02.024].

“We measured the ability of ten different MXene compositions to help or hinder the passage of infrared radiation — or infrared emissivity — which correlates with their ability to passively capture or dissipate ambient heat transmitted as infrared radiation,” explains Meikang Han of Fudan University and co-first author of the study.

Thermal management by controlling radiative heat flow is challenging as this is a passive process. While the manipulation of infrared (IR) emission is crucial for thermal management systems, effective thin film materials are difficult to manufacture. By contrast, MXene carbides and carbonitrides can be readily fabricated as thin layers on different substrates, including fabric, and demonstrate a broad range of IR emissivities, arising from the ability of highly mobile electrons in the materials to oscillate freely.

The researchers found that the emissivity values of the ten MXene nanolayers investigated varied from 0.06 to 0.59, depending on the composition. These results indicate that MXenes can be used as both low- and high-IR emission materials simply by changing the combination of metals in the MXene lattice.

“[Our] results show that MXenes can be tuned to emit specific levels of infrared radiation,” says PhD student and co-first author Danzhen Zhang.  “The ease of processing coupled with the natural hydrophilicity and flexibility of MXenes facilitates the deposition of MXene films on a wide variety of surfaces, enabling fast and easy coating of surfaces for thermal management, such as selective heating/cooling and IR camouflage,” he adds.

The team tested the versatility of MXenes by coating objects and fabrics with different compositions. The unique IR response to combinations of different MXene compositions can be used to identify objects securely or for thermal imaging. Meanwhile, coating fabrics with MXenes of specific IR emissivity could create flexible or wearable devices or garments capable of keeping the wearer warm or cool, as required.

“We expect to use MXenes to achieve broad-range, green, and precise thermal management, which addresses both, radiative heating and cooling,” says Yury Gogotsi, who led the study. “Moreover, MXenes have diverse optical properties, leading to colorful MXene films… satisfying both energy-saving and aesthetic demands.”