The stability of graphene oxide films in water is crucial to their use as membranes for aqueous filtration and separation applications. That stability is also something of a mystery. In water, the films become negatively charged, so electrostatic repulsions ought to cause the films to disintegrate. Researchers have now shown that the membranes remain stable owing to metal ion contamination (Nat. Chem. 2015, DOI: 10.1038/nchem.2145). The study suggests ways to dope thin films to customize their properties.
Scientists use vacuum filtration methods to fuse together tiny graphene oxide flakes, forming paperlike membranes. The films are robust and water stable—most of the time. Their counterintuitive stability, along with the occasional report of instability, led Northwestern University materials scientists Che-Ning Yeh and Jiaxing Huang and coworkers to scrutinize the methods and materials used for making the membranes.
They soon zeroed in on porous aluminum oxide filter disks commonly used to process the membranes. Prized for their smooth, rigid surfaces, these disks have been thought to give rise to uniformly structured membranes. Yeh and Huang found, however, that the disks corrode and release aluminum ions during membrane processing. Residual sulfuric acid, which is used to make graphene oxide from graphite, triggers the corrosion. The Al3+ ions turn out to be useful: They cross-link the negatively charged graphene oxide flakes and strengthen the resulting membranes.
Less favored, flimsy polymer filter disks don’t release contaminating metal ions. But the pristine membranes they yield disintegrate rapidly in water.
“This is a very exciting finding,” says Richard B. Kaner, a materials chemist at UCLA. He adds that the idea that Al3+ can leach out of anodized alumina membranes and stabilize graphene oxide membranes is not only surprising, but could lead to improved membranes for many separation applications.
Materials engineer Dan Li of Monash University, in Victoria, Australia, remarks that these well-thought-out experiments pinpoint the underlying science and demonstrate a simple way to engineer graphene oxide paper to suit various applications. “This is an excellent example of how a ‘trivial’ contamination problem can trigger an important discovery,” he says.