Janus Membranes via Diffusion-Controlled Atomic Layer Deposition

The first use of atomic layer deposition (ALD) to produce Janus membranes is reported, with an example system consisting of a compositional gradient ranging from hydrophilic Al2O3 on one face to hydrophobic poly(propylene) on the opposite face. Alternating pulses of trimethyl aluminum and water vapor lead to the growth of covalently bonded Al2O3 conforming to the membrane pore surfaces. Precise control of ALD parameters significantly affects the surface wetting of the modified membrane face and the depth of Al2O3 infiltration into the porosity. This depth control derives from slow precursor diffusion through the 200 nm membrane pores compared to much faster ALD surface reactions. For a given precursor exposure and purge time, increasing the number of ALD cycles decreases the water contact angle at the modified surface from hydrophobic to hydrophilic, until the water droplet is completely imbibed by the membrane. To demonstrate the utility of these Janus membranes, a hydrophilic/superaerophobic Janus treatment is shown to greatly reduce the size of air bubbles generated through the membrane, enabling faster mixing. This technique represents the first application of vapor-deposited covalently bonded metal oxides to form Janus membranes. Further opportunities are afforded by the ability to laterally pattern Al2O3 across the membrane surface via physical masking.