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Fengbin Li, Yang Yang, Yiqun Fan, Weihong Xing, Yong Wang
Journal of Membrane Science, Volumes 397–398, April 2012
Abstract
The technique of atomic layer deposition (ALD) is applied to the structural modification and pore tailoring of ceramic membranes. Ceramic microfiltration membranes with an average pore size of 50 nm are used as substrates, on which Al2O3 is deposited by ALD with the purpose to tailor their pore size. The thickness of the deposited Al2O3 layer increases with the number of ALD cycles, and it is confirmed by scanning electron microscopy that the pore size of the ceramic membrane decreases with increasing ALD cycles until the pores are completely sealed. It is found that an ultrathin selective layer with a gradient porous structure is formed, whose thickness can be tuned by varying the exposure time of percursors. With the increased ALD cycles, the Al2O3-deposited membranes have a decreasing water flux, and an increasing retention to bovine serum albumin. The membrane subjected to 600 ALD cycles has a reduced pure water flux from 1698 L (m2 h bar)−1 for an undeposited membrane to 118 L (m2 h bar)−1, while its BSA retention increases significantly from 2.9% to 97.1%.
Atomic layer deposition finds its potentials in liquid-separation membranes
Atomic layer deposition (ALD) is traditionally used in microelectronics to deposit high-k gate oxides. A group in Nanjing University of Technology led by Prof. Yong Wang pioneered the application of ALD in the field of membranes for liquids separation. Wang et. al demonstrate that ALD is very promising both in the pore size tuning and surface modification of porous membranes. Their work is of great significance to the membrane community because ALD seems to be capable of effectively modifying every type of membranes with different material properties and pore structures. ALD utilizes self- limiting gas-solid surface reactions to produce high-quality thin deposition layers, and is distinctive in its extremely high uniformity and conformality of the deposited layers and precise control in the film thickness. Through ALD coating, membranes obtained a thin coating of oxides, showing improved hydrophilicity, fouling and solvent resistance. More importantly, the membrane pore size can be tuned continuously at sub-angstrom preciseness simply by altering the numbers of ALD cycles. Recently, they showed ALD was also capable of effectively upgrade the separation performances of polytetrafluoroethylene (PTFE) membranes which are hard to be modified because of the chemical inertness and extremely hydrophobic nature of PTFE. 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 20o. 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.
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