Electrodepositing freestanding ultrathin membranes at the air-water interface

Freestanding ultrathin metallic membranes (FUMMs) with nanometer thickness are essential elements for flexible and wearable devices as electrodes1-4. Conventional press-rolling or melt-spinning methods are impossible to prepare ultrathin metallic membranes5,6. The bottom-up Langmuir-Blodgett self-assembly method can prepare monolayer nanoparticle films at the air-liquid interface, however their weak mechanical strength constrains their application fields7-9. Wet chemical methods have been developed to prepare ultrathin metallic nanosheets, but the at most square micrometer area makes them unsuitable to be used in real macroscopic devices10. Up to now, the only way available to prepare FUMMs is to release the thermally or sputtered metallic films by dissolving the underneath sacrificial substrate1,11,12, which needs vacuum conditions and harmful chemicals, wastes the metal sources, and is time-consuming. Here we demonstrate a strategy to switch the nucleation and growth sites from the conventional electrode surface spontaneously to the air-electrolyte interface where covered by carefully designed multi-lamellar architecture consisting of alternatively stacked metal ion and surfactant ion layers, realizing direct electrodeposition of FUMMs at the air-electrolyte interface by electrochemically reducing the metal ions in place under ambient conditions. The growth speed of the FUMMs can reach > 3 cm2/min with the thickness immediately adjustable by the applied potentials from less than 100 nm to several micrometers. The area of the electrodeposited FUMMs can easily reach 100 cm2 and in principle is only constrained by that of the electrolyte container. The FUMMs at the air-electrolyte interface can be transferred onto any arbitrary solid substrates or liquid surfaces. The FUMMs can regrow for hundreds of times at the air-electrolyte interface provided that the consumed metal ions are replenished in time. More strikingly, the microstructure, the thickness, and in turn the transparency and conductivity of the nanofilms can be immediately designed via varying the applied potentials, allowing us to prepare striped nanofilms by programming the potential waveforms. As preliminary examples, the FUMMs are used to construct high-performance flexible plasmonic nanosensors and sensitive pressure sensors. This study provides a simple but robust electrochemical approach to prepare FUMMs at the air-liquid interface with promising applications in next-generation flexible and wearable electronics.