2D Ti₃C₂T_x MXenes: Visible Black but Infrared White Materials

Black inorganic 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, the intrinsically low mid-IR emissivity (down to 10%) of the 2D Ti₃C₂T_x MXene is reported. Associated with a high solar absorptance (up to 90%), it embraces the best spectral selectivity among the reported intrinsic black solar-absorbing materials. Its appealing potential in several of the aforementioned areas is experimentally demonstrated. First-principles calculations reveal that the IR emissivity of MXene relies on both the nanoflake orientations and terminal groups, indicating great tunability. The calculations also suggest more potential low-emissivity MXenes including Ti₂CT_x, Nb₂CT_x, and V₂CT_x. This work opens the avenue to further exploration of a family of intrinsically low-emissivity materials with over 70 members.