Spontaneous MXene monolayer assembly at the liquid-air interface

Here we report on the spontaneous assembly of Ti₃C₂T_x MXene flakes into monolayer films at the liquid-air interface. According to X-ray reflectivity and grazing incidence X-ray fluorescence both the structure of the layers and assembly kinetics depends on the pH value of the solution. At pH > 4 MXene flakes form a single ∼1.5 nm thick layer carrying a negative charge, while in the acidic medium the layer contains coordinated anions with the formation of the Br_aq^-/Ti₃C₂T_x/subphase interface. The surface layer compression allows the assembling of MXene flakes into a dense monolayer films with the surface coverage of up to 96% and surface pressure exceeding 40 mN m^-1 in the case of the acidic subphase. The films can readily be transferred onto solid substrates by the conventional Langmuir-Blodgett approach or modified by surfactants to form MXene/surfactant composite films.