2D MXenes: the lowest-emissivity black materials

Black materials with low infrared absorption/emission are rare in nature but highly desired in numerous areas, such as solar-thermal energy harvesting, infrared camouflage, and anti-counterfeiting. Due to the lack of spectral selectivity in intrinsic materials, such counter-intuitive properties are generally realized by constructing complicated subwavelength artificial nanostructures with precise nanofabrication techniques. Here, we report that 2D Ti3C2Tx MXenes embrace both a low emissivity (down to 10%) and a high solar absorptance (up to 90%), yielding the best spectral selectivity (8.2) and the highest solar-thermal efficiency among the reported intrinsic solar absorbing materials. We demonstrate their appealing potentials in the aforementioned areas. Moreover, the spectral selectivity relies on both the nanoflake orientations and terminal groups, providing great tunability. First-principles calculations suggest more potential low-emissivity MXenes such as Ti2CTx, Nb2CTx, and V2CTx. This work opens the avenue to further exploration of a family of low-emissivity black materials with over 70 members.

Identifying and quantifying amorphous and crystalline content in complex powdered samples: application to archaeological carbon blacks

Carbon black materials have been frequently used from prehistory as pigments for drawings and paintings and also as dyes, inks and cosmetics, since they are easy to make by burning organic matter. However, the carbonaceous phases they form are often ill-ordered and not easy to characterize. Five carbon black Roman micro samples found in vessels in houses in Pompeii were studied. These precious powders correspond to mixed phase samples that contain both crystalline and ill-ordered components. Here, a methodological approach that accomplishes the identification, quantification and mapping of the different phases in these heterogeneous samples using synchrotron-based techniques is proposed. The results were compared with those from scanning electron microscopy. Information about the nature of the mixtures and the origin of carbon black pigments is obtained. The use of charred vegetable materials is concluded, independently of the shape and the nature of the container.