Mechanistic formulation of inorganic membranes at the air-liquid interface

Freestanding functional inorganic membranes, beyond the limits of their organic and polymeric counterparts¹, may unlock the potentials of advanced separation², catalysis³, sensors^4,5, memories⁶, optical filtering⁷ and ionic conductors^8,9. However, the brittle nature of most inorganic materials, and the lack of surface unsaturated linkages¹⁰, mean that it is difficult to form continuous membranes through conventional top-down mouldings and/or bottom-up syntheses¹¹. Up to now, only a few specific inorganic membranes have been fabricated from predeposited films by selective removal of sacrificial substrates^4-6,8,9. Here we demonstrate a strategy to switch nucleation preferences in aqueous systems of inorganic precursors, resulting in the formation of various ultrathin inorganic membranes at the air-liquid interface. Mechanistic study shows that membrane growth depends on the kinematic evolution of floating building blocks, which helps to derive the phase diagram based on geometrical connectivity. This insight provides general synthetic guidance towards any unexplored membranes, as well as the principle of tuning membrane thickness and through-hole parameters. Beyond understanding  a complex dynamic system, this study comprehensively expands the traditional notion of membranes in terms of composition, structure and functionality.