Ultrathin Membranes: A New Opportunity for Ultrafast and Efficient Separation

Abstract

membrane thickness L. However, mass transfer in a nonporous membrane is often described by solution diffusion theory, expressed by the integration of Fick's first law [8] J P p L = ∆ /where the permeation flux J of a dense membrane is directly proportional to the membrane permeability P and the net effective pressure difference (the hydraulic pressure difference minus the osmotic pressure difference) Δp, while inversely proportional to the membrane thickness L. Hence, a feasible concept for obtaining a high permeance membrane is to reduce the membrane thickness. This observation has driven much of the development of ultrathin membranes. An ideal ultrathin membrane or active layer should be as thin as possible to reduce the transfer path length and transfer resistance simultaneously without sacrificing its effective pore size. [9,10] Advanced membranes integrating both ultrathin membranes and large effective pore sizes to achieve ultrahigh permeance, excellent selectivity, and good mechanical stability are highly desired and the subject of extensive research.The rapid development of nanomaterials has provided new avenues toward ultrathin membranes construction. For instance, by using a porous nanostrand membrane as a sacrificial layer, polyamide (PA) membranes as thin as 8.4 nm and diamond-like carbon (DLC) membranes with thickness down to 10 nm were fabricated via interfacial polymerization (IP) reaction and chemical vapor deposition (CVD), respectively. [2,11] Moreover, a series of thin-film composite (TFC) membranes with a PA active layer less than 15 nm were fabricated via IP reaction on porous nanostrands or nanotube support membranes. [12][13][14][15][16][17][18][19] 1D (such as nanotubes, [20][21][22][23][24] nanowires, [25,26] and nanostrands [27][28][29][30] ) and 2D nanomaterials (such as graphene nanosheet derivatives [31][32][33][34][35][36][37][38] and metal sulfide nanosheets [39,40] ) have also been widely adopted to fabricate ultrathin network or laminar membranes with thickness of tens of nanometers via vacuum filtration. The thickness and effective pore size of these nanomaterial-based membranes can be controlled to some extent by tuning the filtrated volume of the nanomaterial dispersion solution.In this progress report, some promising ultrathin membranes are highlighted for liquid filtration or gas separation. In each section, the designs, fabrication techniques, and separation performances of these membranes are introduced. The Developing advanced membranes with both high permeance and selectivity to enable ultrafast and efficient separation is a persistent pursuit by materials scientists. In recent years, diverse ultrathin membranes with nanometer-scale thickness and unique membrane structures have been developed. These materials show ultrahigh permeance and excellent selectivity in water desalination, dye rejection, oil-water purification, and gas separation applications. Herein, current state-of-the-art ultrathin membranes are introduced, including polyamide membranes, diamond-like carbon m...

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