Abstract
Laminar membranes of two-dimensional materials are excellent candidates for applications in water filtration due to the formation of nanocapillaries between individual crystals that can exhibit a molecular and ionic sieving effect, while allowing high water flux. This approach has been exemplified previously with graphene oxide, however these membranes suffer from swelling when exposed to liquid water, leading to low salt rejection and reducing their applicability for desalination applications. Here, we demonstrate that by producing thin (∼5 μm) laminar membranes of exfoliated molybdenum disulfide (MoS2) in a straightforward and scalable process, followed by a simple chemical functionalization step, we can efficiently reject ∼99% of the ions commonly found in seawater, while maintaining water fluxes significantly higher (∼5 times) than those reported for graphene oxide membranes. These functionalized MoS2 membranes exhibit excellent long-term stability with no swelling and consequent decrease in ion rejection, when immersed in water for periods exceeding 6 months. Similar stability is observed when exposed to organic solvents, indicating that they are ideal for a variety of technologically important filtration applications.
Highlights
Environmental pressures, along with the discovery of twodimensional (2D) materials such as graphene, have combined to provide renewed interest in the development of membrane technologies.[1,2] Membrane technology is favored over other purification techniques such as disinfection or distillation due to the lack of chemical additives, energy efficiency, and simplicity of the technology
We show that ion-selective filtration membranes can be fabricated from laminar films of MoS2 and demonstrate that these MoS2 membranes exhibit excellent ionic sieving for all major cationic components commonly found in seawater (e.g., Na+, K+, Ca2+, and Mg2+), while still possessing high water permeation rates
The MoS2 membranes were found to be tolerant to solvent exposure in general, with immersion in solvents of varying polarity having no appreciable effect on the interlayer spacing (Figure S1), unlike graphene oxide (GO) membranes which suffer from various degrees of swelling.[20]
Summary
Environmental pressures, along with the discovery of twodimensional (2D) materials such as graphene, have combined to provide renewed interest in the development of membrane technologies.[1,2] Membrane technology is favored over other purification techniques such as disinfection or distillation due to the lack of chemical additives, energy efficiency, and simplicity of the technology. Recent work has suggested that other 2D layered materials, such as the transition-metal dichalcogenides (TMDs), could be suitable for applications in water filtration.[13−15] Despite this, there have been few reported studies of the filtration properties of these TMD materials. We demonstrate that chemical functionalization is necessary to impart the desired ionic sieving properties by altering the surface charge present on the MoS2 membrane. These chemically functionalized MoS2 membranes exhibit superior long-term stability with no change in the ionic rejection properties even after being continuously immersed in liquid water for over 6 months. This demonstrates that these membranes are suitable for water filtration and have potential in a variety of nonaqueous filtration applications.[20−22] This stability of the MoS2 membranes is in agreement with a recent study which found them to be highly stable in aqueous environments, even with varying pH.[18]
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