2D MXenes: Visible Black but Infrared White Materials

Black materials with low infrared absorption/emission (or IR white) are rare in nature but highly desired in numerous areas, such as solar-thermal energy harvesting, multispectral camouflage, thermal insulation, and anti-counterfeiting. Due to the lack of spectral selectivity in intrinsic materials, such counter-intuitive properties are generally realized by constructing complicated subwavelength metamaterials with costly nanofabrication techniques. Here we report the low mid-IR emissivity (down to 10%) of 2D Ti3C2Tx MXenes. Associated with a high solar absorptance (up to 90%), they embrace the best spectral selectivity among the reported intrinsic black solar absorbing materials. Their appealing potentials in several aforementioned areas are experimentally demonstrated. First-principles calculations reveal that the IR emissivity of MXenes relies on both the nanoflake orientations and terminal groups, indicating great tunability. The calculations also suggest that more MXenes including Ti2CTx, Nb2CTx, and V2CTx are also potential low-emissivity materials. This work opens the avenue to further exploration of a family of intrinsically low-emissivity materials with over 70 members.