The explosion of research interest in two-dimensional materials such as graphene and molybdenum disulphide has, to a large extent, been dominated by their physics, and in turn the exploitation of their electronic and optical properties. Researchers have, of course, also explored the chemical and mechanical properties of these materials — and sought applications that principally utilize these attributes — but the results have, arguably, received less attention. One intriguing line of research in this regard is the use of graphene as a nanoporous separation membrane. Here, through a combination of sophisticated fabrication and characterization techniques, unique membranes could be developed for use in critical applications such as gas separation, water purification, and desalination.
Graphene is an attractive material for the development of membranes due to its atomic thickness, mechanical strength and chemical stability. Pristine sheets of graphene are thought to be impermeable to all atoms and molecules. However, by forming nanometre-sized pores in the material, it can potentially act as a filter, allowing molecules smaller than the pores to pass through while excluding larger species. A number of theoretical studies have suggested that the selectivity and permeability of such membranes could be vastly superior to the polymer-based filtration membranes that are typically used today.
Researchers have now shown that a single layer of nanoporous graphene can be used to desalinate water. The pores are created in the graphene layer by exposing the material to short bursts of oxygen plasma, a process that allows holes of precise dimensions to be controllably etched in the layer. By finding just the right plasma conditions — and with the help of aberration-corrected scanning transmission electron microscopy to characterize pores that have sizes of only 0.5–1 nm — membranes can be fabricated that exhibit a salt rejection of nearly 100%, as well as high water fluxes.
As Dong-Yeun Koh and Ryan Lively note in an accompanying News & Views article11, there are many challenges to be addressed before such membranes could be of practical use, including issues related to mechanical stability and membrane fouling. However, these proof-of-concept experiments are an encouraging illustration of the potential of atomically thick membranes.
SOURCE – Nature Nanotechnology