“Before our experiments, previous work on SLGO looked at fabricating graphene-oxide membranes with a lamellar structure and then simply investigating how good these were at purifying water,” explains team member and lead author of the new study Weiwei Xu. “There was no research into how the interlayer nanostructure in the material influenced water permeation. Our new findings show that this is an important factor and may direct future designs of improved SLGO membranes for water purification.”

Graphene is a sheet of carbon just one atom thick in which the atoms are arranged in a honeycomb lattice. Graphene oxide is like ordinary graphene but it is covered with molecules such as hydroxyl groups, which means that it can be readily dispersed in water.

Graphene-oxide sheets can easily be stacked on top of each other to form extremely thin, but mechanically strong membranes. These membranes consist of millions of small flakes of graphene oxide with nanosized empty channels (or capillaries) between the flakes. These channels act as highly efficient sieves that block particles above a certain size and through which water passes almost without any friction at all.

Water molecules pass through much quicker

The researchers, led by Miao Yu, prepared their SLGO membranes using a vacuum filtration process in which the deposition rate of graphene was controlled by the feed liquid volume. “For instance, to prepare a 118 nm thick GO membrane at a slow deposition rate (calculated to be 0.02 nm/s), we dispersed 0.19 mg of SLGO in 250 ml of water,” explains Yu. “We found that it took around 1.5 hours for all the water to filter through this membrane. To prepare a GO membrane with the same thickness at the fast rate of 0.37 nm/s, we dissolved 0.19 mg in just 25 ml of water and the whole filtration process here only took around five minutes.”

These results indicate that GO membranes prepared at a slow deposition rate have more hydrophobic channels of narrower size, he tells nanotechweb.org. These channels allow water molecules to pass through them much quicker, and more selectively.

The team, which also includes researchers from Virginia Tech, says that it now plans to prepare and characterize GO flakes containing different amounts of oxygen-containing functional groups and then make GO membranes from these flakes though self-assembly by slow deposition. “We will then look at how good these membranes are at desalinating and removing small organic contaminants in water,” says Xu.

The new SLGO membranes are detailed in Nano Letters DOI: 10.1021/acs.nanolett.7b00148.