Atomic layer deposition for membrane modification, functionalization and preparation: A review

Membrane separation is playing an increasingly important role in providing clean water and other resources to our thirsty globe. In addition to preparation of new membranes, modification and functionalization to existing membranes are frequently desired to maximize their performance. Among different strategies, the newly emerged atomic layer deposition (ALD) is distinguished for its universality in upgrading of almost all types of membranes independent of membrane structure and chemistry, as well as its striking capability of simultaneously enhancing selectivity and permeability. ALD is based on alternative reactions between gaseous precursors on solid surfaces including pore walls, and it deposits uniform and defect-free, angstrom-scale thin coatings conformally along the substrate surface. In this Review, we first discuss the basic principles of ALD and the early history of utilizing ALD to modify inorganic membranes for gas separation, and then analyze the advantages of applying ALD to upgrade membranes for water purification and gas separation. By directly ALD depositing metal oxides onto membrane surface, the hydrophilicity, fouling resistance and other properties are evidently improved. The (photo)catalysis, adsorption, antibacterial property, tunable wettability and other new functions can be integrated into membrane separation by ALD. Alternatively, some inert polymeric membranes are first subjected to surface activation, which greatly facilitating ALD processes and lowering the threshold for ALD to break the trade-off effect. For inorganic membranes, ALD has also been used to adjust the pore sizes, or to establish new separation layers to enhance the selectivity. We highlight recent progresses in ALD of polymeric materials on inorganic and polymeric substrates, producing advanced membranes with new configurations. Utilization of ALD to prepare or to functionalize new membranes, innovative ALD devices and processes, which are essential for the mass production of ALD-upgraded membranes, are also discussed. We conclude this review by discussing further development, challenges, and limitations of ALD-enabled membrane modification, functionalization, and preparation. As ALD is originally designed for microelectronics, and is not known for long and for many in the membrane community, we expect this review to inspire further attentions and research efforts tackling urgent problems of membrane separation. Also, it may spark new wave of studies on ALD and other advanced deposition technology toward next-generation membranes.