The Atomic Layer Deposition (ALD) Route to Upgrade and Functionalize Separation Membranes

Atomic layer deposition (ALD) technique is based on the self-limiting reaction of two precursors, to which the samples to be deposited are exposed alternately. The reaction always happens on sample surfaces and since both precursors are vaporized the reaction is taking place in gas phase. Inner surfaces of very small pores with size in the range of nanometer scale are accessible and will be conformally deposited. The thickness of the deposited layer can be tuned with the accuracy of angstrom scale simply by changing the deposition cycles. Oxides, metals and even polymers can be deposited on different substrates via the ALD method. There is a huge arena for ALD to display its unique advantages in the field of separation membranes. In this work, we demonstrated the multiple functions of ALD in the pore size tuning and surface modification of separation membranes with different chemical nature and pore structures. Polycarbonate track-etched membranes, PVDF ultrafiltration membranes, PTFE and PP microfiltration membranes, and ceramic microfiltration membranes were explored as substrate membranes on which alumina and titania were deposited by ALD. ALD coating is first a well-controlled and predictable way to change the rejection of the substrate membranes as a single cycle produces a constant thickness of oxide layer conformally coated on the pore wall. The deposited oxides endowed the membrane greatly improved hydrophilicity and consequently a good anti-fouling property. Moreover, at a balanced ALD cycle number, the deposited membranes displayed simultaneously enhanced rejection and water flux. In addition, because of the presence of oxide on the membrane surface working as a barrier layer, the ALD treated membranes showed much better resistance to acids, bases as well as organic solvents, compared to the pristine membranes. Last but not least, the thermal and mechanical stability of the membranes were enhanced significantly in some cases.