Oxidation-Driven Degradation of Ti ₂ CT _x MXenes Revealed by X-Ray Spectromicroscopy and Micro-Imaging Ellipsometry

Ti2CTx is a structurally simple yet chemically fragile MXene whose practical use is limited by rapid and poorly understood oxidation and strong sensitivity to synthesis route, flake thickness, and surface chemistry. Here, we characterize the oxidation behavior of Ti2CTx synthesized via conventional chemical etching (Ch-) and fluoride-free molten salt-shielded synthesis (MS3-) routes by combining X-ray absorption spectroscopy (XAS), scanning X-ray microscopy (SXM), and micro-imaging spectroscopic ellipsometry (μISE). XAS and SXM reveal a higher and spatially heterogeneous Ti oxidation state in MS3-Ti2CTx, while Ch-Ti2CTx exhibits a more uniform and reduced Ti state. μISE allows μm-resolved quantitative extraction of optical constants from individual flakes, indicating a high flake thickness dependence of UV−Vis resonances and conductivity, with thicker flakes showing increased metallicity and stability in Ch-Ti2CTx. Ellipsometry also demonstrates that ultrathin flakes undergo rapid oxidation-driven electronic degradation within days, while thicker flakes maintain their optical and electrical properties over extended periods of several weeks. Together, these results establish a framework linking surface chemistry, oxidation heterogeneity, and flake thickness-dependent electronic and optical properties, providing guidance for improving their durability in applications such as nanoelectronics, energy storage, and sensing.