The Evolution of MXenes Conductivity and Optical Properties Upon Heating in Air

MXenes, a family of 2D transition-metal carbides and nitrides, have excellent electrical conductivity and unique optical properties. However, MXenes oxidize in ambient conditions, which is accelerated upon heating. Intercalation of water also causes hydrolysis accelerating oxidation. Developing new tools to readily characterize MXenes' thermal stability can enable deeper insights into their structure-property relationships. Here, in situ spectroscopic ellipsometry (SE) is employed to characterize the optical properties of three types of MXenes (Ti3C2Tx, Mo2TiC2Tx, and Ti2CTx) with varied composition and atomistic structures to investigate their thermal degradation upon heating under ambient environment. It is demonstrated that changes in MXene extinction and optical conductivity in the visible and near-IR regions correlate well with the amount of intercalated water and hydroxyl termination groups and the degree of oxidation, measured using thermogravimetric analysis. Among the three MXenes, Ti3C2Tx and Ti2CTx, respectively, have the highest and lowest thermal stability, indicating the role of transition-metal type, synthesis route, and the number of atomic layers in MXene flakes. These findings demonstrate the utility of SE as a powerful in situ technique for rapid structure-property relationship studies paving the way for the further design, fabrication, and property optimization of novel MXene materials.

Automated summary

MXenes are 2D transition-metal carbides and nitrides with excellent electrical conductivity and unique optical properties. However, they tend to oxidize under ambient conditions, especially when heated. The intercalation of water also accelerates oxidation through hydrolysis. By using in situ spectroscopic ellipsometry (SE), the optical properties of three types of MXenes (Ti3C2Tx, Mo2TiC2Tx, and Ti2CTx) with different composition and atomic structures are characterized to investigate their thermal degradation. It is found that changes in MXene extinction and optical conductivity in the visible and near-IR regions correlate well with the amount of intercalated water and hydroxyl termination groups, as well as the degree of oxidation.