Colloidal Properties and Stability of 2D Ti3C2 and Ti2C MXenes in Water

While MXenes have been utilized in many applications, their surface properties in water systems are still unexplored, especially in the aspect of colloidal properties, which highly limits their practical use. In this study, the colloidal properties and stabilities of 2D Ti3C2 and Ti2C MXenes were investigated using time-resolved dynamic light scattering and zeta potential over a wide range of waters (deionized water, tap water and physiological NaCl solution of 0.9%) relevant to natural and engineered systems. Our results indicate that pH has a significant influence on Ti3C2 and Ti2C stabilities from pH 4 to 10. The type of water environment also affects the stability of Ti3C2 and Ti2C due to electrical double layer compression. The aggregation and stabilities of Ti3C2 and Ti2C in the aquatic systems followed colloidal theories, even though Ti3C2 and Ti2C flakes shape is not spherical. The presence of NaCl stabilized more the 2D Ti3C2 than the 2D Ti2C. Dispersions prepared when using tap water were more stable in the case of Ti2C compared to the NaCl solution. The opposite effect was found for Ti3C2. This may be due to the binding capacity of Ca2+ ions with hydroxyl and carbonyl functional groups whose amounts vary on the surface of Ti3C2 and Ti2C. In general, our study demonstrates that the 2D flakes of Ti3C2 MXenes are highly stable in NaCl, although they settle more quickly in tap water. The 2D flakes of Ti2C MXenes are more stable in distilled water and less stable in the physiological NaCl solution.