Atomic layer deposition of alumina on porous polytetrafluoroethylene membranes for enhanced hydrophilicity and separation performances

Porous polytetrafluoroethylene (PTFE) membranes are seeking extensive applications in gas and liquid purification whereas the strong hydrophobicity of PTFE limits their use in water system. We demonstrate that atomic layer deposition (ALD) is a simple and effective method to upgrade the separation performances of PTFE membranes by improving their surface hydrophilicity. Alumina was ALD-deposited on PTFE membranes following a subsurface nucleation and surface growth mechanism, leading to the formation of fine particulates with lower ALD cycles and continuous thick layers with higher ALD cycles on the surface of PTFE membranes, respectively. Thanking to the nanofibril-containing, porous surface morphology, the adhesion between the deposited alumina and PTFE substrate is strong, which can withstand harsh ultrasonication. The hydrophilicity of the deposited membranes was enhanced progressively with the rise of cycle numbers and the membrane with 500 cycles was completely water-wettable with a water angle less than 20°. Improved hydrophilicity of the alumina-deposited PTFE membranes not only affords remarkably enhanced fouling resistance, but also facilitates water permeation through the membrane, giving higher flux. Filtration experiments indicate that the deposited membranes prepared at optimal conditions possessed an increase of pure water flux of more than 50% and simultaneously an increase of retention of 12.4% compared to the pristine PTFE membrane.