|Posted on February 10, 2015 at 6:25 PM|
Graphene oxide forms the world's thinnest paper - it can be as thin as one molecular layer. As a single sheet of paper, graphene oxide cannot be used for membrane applications, as the average pore size of the graphene oxide is smaller than the molecular size of most solvents. However, when stacked together, the gap between the 'paper' sheets provides nearly impedance-free paths for the liquid. This phenomenon means that it has great potential for membrane applications.
One of the mysteries surrounding stacks of graphene oxide 'paper' has been its structural stability when placed in water. Graphene oxide is negatively charged; so when it's placed in water, the sheets should repel against each other due to electrostatic forces. Incredibly, it keeps its structural stability in water.
Ms. Che-Ning Yeh, a graduate student in Jiaxing Huang's group at Material Science Engineering department at Northwestern University, led an investigation to find the origins behind this mystery, which was recently published in Nature Chemistry. They showed that the origin of stability arises from impurities that occur in the filtration process during graphene oxide synthesis. Anodized aluminum oxide (AAO) is a popular membrane used for the filtration process, which also happens to give the best stability of graphene oxide membranes. When a solution filters through AAO membrane, the membrane tends to corrode, releasing positively charged aluminum ions which stabilize the negatively charged graphene oxide sheets. This also explains why graphene oxide synthesized through different membranes, such as teflon, would not succeed in making stable graphene oxide sheets.
(Che-Ning at the Skydeck of Willis Tower (Chicago Spirit!)).
Although the long-term stability of graphene oxide in water is still an issue, this study provides an important first step towards utilizing graphene oxide as a membrane for variety of applications. This wok was featured by a number of different media, including Materials360Online, and was selected in the top 2 in a list of Top10 materials news from January 2015 by MRS's Materials Today.
(Photo credits: Provided by and used with permission from Che-Ning Yeh).