Study on contact angles and surface energy of MXene films

MXene is a growing two-dimensional material family of transition metal carbides and nitrides and showing great promise in various applications, such as energy storage, water treatment, composites, electromagnetic interference shielding, etc. In all these applications, at least one of the MXene flake sides is in contact with a medium of interest. So the wetting behaviors of MXene are critical for the performance of MXene. Although the hydrophilicity of MXene is unquestionable, the reported contact angles of MXene have covered an extensive range. Additionally, the surface energy of MXene flakes is surprisingly poorly known and rarely studied. In this work, the static contact angles of MXene films were studied using water, glycerol, and diiodomethane. The surface energy of MXene films has a range of 49.92 ± 2.01 to 62.44 ± 0.25 mJ m^-2, calculated based on the measured contact angles. The loading, drying and storage condition of the MXene films have various impacts on their contact angles and surface energy. The root cause for the wide-range of contact angles is related to the surface chemistry of the MXene films. Organic contamination and surface oxidation are responsible for the scattering water contact angle. The contact angles are mass loading-independent for MXene films with loadings from 0.3 to 2 mg cm^-2. The surface energy and its acid-base component are sensitive to the delamination methods and MXene compositions, while the dispersion component of the surface energy is stable. These findings will provide valuable insight and guidance for measuring contact angles of MXene films and the rational design and synthesis of MXene-based films, composites, coatings, and energy storage devices.