MXenes, a class of 2D nanomaterials, are attracting significant interest owing to the large surface area, broad range of tunable properties, and potential to form composite films with various polymers. Polyacrylic acid (PAA), a polyanion rich in carboxyl groups, is expected to enhance interactions with inorganic materials. However, constructing composite films with highly ordered and compact structures using MXene and PAA together is challenging because they are both negatively charged, resulting in electrostatic repulsion. Here, we present MXene and PAA-based layer-by-layer (MXene/PAA LbL) multilayers formed by leveraging hydrogen bonding interactions. We observe drastic changes in film growth and properties depending on the pH of LbL assembly. Under certain assembly pH values, MXene/PAA LbL films exhibit uniform deposition, pin-hole-free, and nacre-like structures. This structure proves beneficial as a barrier to hydrogen gas, in which MXene/PAA LbL films yield high blocking capability and low permeability (0.14 ± 0.012 cc mm m^-²day^-1MPa^-1). Electrical conductivity is empirically correlated to the number of MXene layers per layer pair and the long-term stability in air is also demonstrated. Synchrotron-radiation XPS measurements confirm the initial oxidation mechanisms of MXene in air and the dependency of the multilayer composition on the assembly pH. The responses of MXene/PAA LbL films to several pH-adjusted aqueous solutions are also examined and interestingly, the electrical conductivity of all LbL films increases more than 5 times (up to 370 ± 30 S cm^-1) after immersion into pH 5 aqueous solution due to the high sensitivity of hydrogen bonding interactions to the pH. This study shows that hydrogen bonding interactions can be leveraged to form MXene LbL multilayers as gas barriers, electronically conductive coatings, and deconstructable thin films via pH control.

Hisato Yamaguchi

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