Stabilizing Ti3C2Tx MXene flakes in air by removing confined water.

MXenes have demonstrated potential for various applications owing to their tunable surface chemistry and metallic conductivity. However, high temperatures can accelerate MXene film oxidation in air. Understanding the mechanisms of MXene oxidation at elevated temperatures, which is still limited, is critical in improving their thermal stability for high-temperature applications. Here, we demonstrate that Ti[Formula: see text]C[Formula: see text]T[Formula: see text] MXene monoflakes have exceptional thermal stability at temperatures up to 600[Formula: see text]C in air, while multiflakes readily oxidize in air at 300[Formula: see text]C. Density functional theory calculations indicate that confined water between Ti[Formula: see text]C[Formula: see text]T[Formula: see text] flakes has higher removal energy than surface water and can thus persist to higher temperatures, leading to oxidation. We demonstrate that the amount of confined water correlates with the degree of oxidation in stacked flakes. Confined water can be fully removed by vacuum annealing Ti[Formula: see text]C[Formula: see text]T[Formula: see text] films at 600[Formula: see text]C, resulting in substantial stability improvement in multiflake films (can withstand 600[Formula: see text]C in air). These findings provide fundamental insights into the kinetics of confined water and its role in Ti[Formula: see text]C[Formula: see text]T[Formula: see text] oxidation. This work enables the use of stable monoflake MXenes in high-temperature applications and provides guidelines for proper vacuum annealing of multiflake films to enhance their stability.

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 (Ti3 C2 Tx , Mo2 TiC2 Tx , and Ti2 CTx ) 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, Ti3 C2 Tx and Ti2 CTx , 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.

Efficient mercury removal from aqueous solutions using carboxylated Ti3C2Tx MXene.

Water supplies contaminated with heavy metals are a worldwide concern. MXenes have properties that make them attractive for the removal of metal ions from water. This work presents a simple one-step method of Ti3C2Tx carboxylation that involves the use of a chelating agent with a linear structure, providing strong carboxylic acid groups with high mobility. The carboxylation decreases the zeta-potential of Ti3C2Tx by ~16 to ~18 mV over a pH range of 2.0-8.5 and improves Ti3C2Tx stability in the presence of molecular oxygen. pH in the range of 2-6 has a negligible effect on the adsorption capacity of Ti3C2Tx and COOH-Ti3C2Tx. Compared to Ti3C2Tx, COOH-Ti3C2Tx has a slightly higher and much faster mercury uptake, and the concentration of mercury ions leached out from COOH-Ti3C2Tx is lower. For both Ti3C2Tx and COOH-Ti3C2Tx, the leached mercury ion concentration is far below the U.S.-EPA maximum level. At an initial Hg2+ concentration of 50 ppm and pH of 6, COOH-Ti3C2Tx has the equilibrium adsorption capacity of 499.7 mg/g and removes 95% of Hg2+ in less than 1 min. Moreover, it has an equilibrium time of 5 min, which is significantly shorter than that of Ti3C2Tx (~ 60 min). Finally, its mercury-ion uptake capacity is higher than commercially available adsorbents reported in the literature. Its mercury removal is mainly via chemisorption and monolayer adsorption.