In November 2012, Materials Today reported [D. Bradley DOI: 10.1016/S1369-7021(12)70209-2] on research into nanoscopic flow of graphene oxide. Jiaxing Huang of Northwestern University and colleagues demonstrated that electrolytes confined to channels with nanoscopic dimensions do not flow in the same manner as their bulk counterparts. In that work, we very carefully embedded GO films in plastic and only exposed the two tiny ends to water to measure ionic conductance through the interlayer spacings in the horizontal direction," Huang says. "We did not do the experiment in the cross-membrane direction because we were concerned that GO films may dissolve!" Now, he and his colleagues have made an intriguing discovery regarding the behavior of GO in water that could change how the material is handled and the products in which it might be exploited.
GO films, it turns out, are paradoxically stable in water. One would expect that upon immersion single layers would become charged and so repel each other leading to their subsequent disintegration. However, the reverse has been observed for many years and it turns out to be due to the ubiquity of a common contaminant that stabilizes the graphene sheets. [Yeh et al, Nature Chemistry (2015); DOI: 10.1038/nchem.2145].
Jiaxing Huang of Northwestern University and colleagues were puzzled by the previously reported behavior of GO, a product of graphite oxidation and itself a common precursor of graphene wherein rather than repelling each other and disintegrating layers of GO appear stable in water.
Huang's team finally realized that the secret of GO's failure to dissolve in water and to exist as intact membranes was due to the unintentional introduction of a contaminant during preparation. A common step in GO film preparation involves passing an acidic dispersion of individual sheets through porous anodized aluminum oxide filter discs. The team discovered that during the filtration process the filter discs corrode, releasing aluminum ions into the acidic water which bond to negatively charged sites on the GO sheets inhibiting repulsion and thus stabilizing the product as intact membranes.
"The puzzle was solved using essentially freshman-level inorganic chemistry," Huang explains. "Now we know that GO films are indeed soluble in water. It's just a matter of sample purity." The team also suggests that other multivalent metal ions, such as manganese, which might also be a byproduct of GO synthesis, can form cross links between the sheets.
The team also showed that clean GO films are not quite as strong as materials scientists had hoped as it is the presence of the aluminum ions that make the films as stiff as is commonly observed; without the ions, the films is three to four times weaker, the team found. "This is also a reminder for anyone using aluminum oxide filter discs," he explains. "People have used it for sample preparation in many areas of materials science and biology. Now we know it's not as clean as we think."
"We are investigating whether these interlayer cation impurities have any impact on molecular transport through the GO films, i.e., whether there is any impact on the 'nano flow', " Huang told Materials Today. "A more accurate understanding of the layer-layer interaction between GO sheets should also be very helpful for us to design GO based structures such as films, fibers and foams."
David Bradley blogs at Sciencebase Science Blog and tweets @sciencebase, he is author of the bestselling book "Deceived Wisdom".