Graphene-based membranes promise selective sieving

Two recent advances promise to expand the possible uses of graphene oxide (GO)-based membranes in purification and filtration technologies. In one report, researchers demonstrate, for the first time, that GO-based membranes can be used for the fast and efficient filtration of organic solvents [Yang et al., Nature Materials (2017), DOI: 10.1038/nmat5025]. Meanwhile, another team has demonstrated how the spacing between GO layers can be fixed with a metal cation to control permeability [Chen et al., Nature (2017), DOI: 10.1038/nature24044].

GO membranes are attracting interest for purification and filtration applications because of their unique molecular sieving properties and fast permeation but, until now, could only be used with water-based liquids.

“GO membranes were previously shown to be completely impermeable to all solvents except for water, a phenomenon that is not yet fully understood,“ explains Yang Su of the University of Manchester, who undertook the work along with colleagues from York University, Imdea Nanociencia, Southwest Jiaotong University and the University of Science and Technology of China. But using very thin membranes made from large (10–20 m) flakes of GO, known as highly laminated GO (HLGO) membranes, the researchers have shown otherwise.

“Our ultrathin GO membranes are highly permeable to organic solvents and show molecular sieving properties at 1 nm,” says Su.

The researchers believe that tiny pinholes in the flakes connected by graphene channels just 1 nm wide enable the permeation and sieving of organic solvents once the membrane reaches a critical thickness (8 nm). If the thickness is increased, the membrane becomes increasingly impermeable to solvents. Water, however, continues to permeate through the membranes as normal.

“The membrane could be used for organic solvents nanofiltration (OSN), a process widely used in many chemical manufacturing industries,” suggests Su. “Conventional polymeric membranes are highly unstable in organic solvents while ceramic inorganic membranes are costly and lack separation efficiency.”

Compared with state-of-the-art polymeric membranes for organic solvent nanofiltration, the GO-based membranes show nearly 100% filtration of smaller molecules while allowing ten times greater flux.

“The chemical stability of GO membrane in a wide range of organic solvents opens many new opportunities for this technology,” adds Su.

Meanwhile, the team from Shanghai University, Shanghai Institute of Applied Physics, Zhejiang A&F University, and Nanjing Tech University has shown that the spacing between layers of GO can be fixed using cations such as K⁺, Na⁺, Ca⁺, Li⁺ and Mg²⁺.

“Despite the great efforts to tune and fix the interlayer spacing, to date it has remained a great challenge to control the interlayer spacing effectively enough to enable highly efficient separation of small ions and molecules,” explains Haiping Fang.

To overcome this limitation, the researchers simply immersed layered GO membranes in salt solutions. Depending on the type of salt, the interlayer spacing can be fixed at different values according to the size of the cation.

“We experimentally achieved facile and precise control of the interlayer spacing in GO membranes as small as ∼1 nm and with precision of 1Å in solution,” reports Fang. “Our theoretical calculations reveal that this unexpected behavior is due to hydrated cation adsorption in regions of oxidized groups and aromatic rings, which bind the GO sheets together.”

Once the interlayer spacing has been set by the cation, the membrane will exclude all other cations with larger hydrated sizes because they cannot pass through the gap. The approach could be used to sieve out ions from water in desalination processes. 

“Compared with conventional membrane materials, the GO membranes allow ultra-fast water flow through the low-friction 2D channels,” says Fang.

The membranes could also be useful for gas purification, solvent dehydration, molecular sieving, and in lithium-based batteries and supercapacitors, point out the researchers.

Both studies are significant and advance the field of GO membranes believes Qilei Song of Imperial College London.

“Fang's work demonstrates a simple and novel approach to adjusting the interlayer spacings of GO membranes, which are crucial for achieving precise molecular and ion sieving in desalination and other applications. Meanwhile, Su et al. have demonstrated a new application of ultrathin graphene oxide membranes for nanofiltration in organic solvent systems,” he comments.

More in-depth studies of the GO membrane structure are necessary, he cautions, such as investigation of the size and distribution of defects and pinholes in GO layers. The scale-up of the manufacture of the membranes towards modules will also be needed to realize practical industrial applications points out Song.

This article was originally published in Nano Today (2018), doi: 10.1016/j.nantod.2017.12.002.