Stability of Graphene Oxide Membranes in Water: The Mystery, the Answer, and the Implications.Public Deposited
Graphene oxide (GO), a product of oxidative exfoliation of graphite powders, has received significant attention due to its excellent solution dispersibility, rich functionality, and ease of conversion to chemically modified graphene (also known as œreduced graphene oxide or œr-GO). These properties make GO an attractive building block for constructing various forms of bulk graphene-based materials. Layer-to-layer stacking, either between sheets or in the form of folds, are the fundamental structural feature in all graphene-based bulk materials. Research in this dissertation clarifies a misunderstanding of the inter-layer interactions in stacked GO sheets and demonstrates the implications of the new insights in better understanding of GO properties and new ways to use this material. ', 'First, a mystery about the stability of GO membranes in water is presented - GO membranes are known to be highly stable in water, but they should disintegrate since individual GO sheets dissolve in water very well. We discovered a long-overlooked factor behind this puzzling contradiction. Our findings show that pristine GO membranes indeed disintegrate in water; however, ionic contaminants unintentionally introduced during synthesis and processing of GO cross-link the sheets and make the film stable in water. One of the overlooked sources of contamination is the porous anodized aluminum oxide (AAO) filter membrane, which can readily corrode during filtration of GO dispersions to release Al3+.', 'This new insight is crucial to understanding the intrinsic properties of GO-like lamellar membranes. First, it explains why some earlier reports could use GO papers as separation membranes in water. And it reminds researchers that it is the multivalent ionic contaminants that hold the GO membranes together in water; the presence of which should be considered when interpreting the ionic transport results. Second, it resets the baseline understanding of the mechanical properties of GO papers. In many previous studies, the Youngs modulus values of GO papers were actually measured with Al3+ cross-linked GO papers. Therefore, the stiffness of neat GO papers should be significantly smaller. ', 'The insight also leads to the development of a œcut-and-paste approach to turn GO papers into complex and even dynamic three-dimensional structures. Since the layer-to-layer interactions can be weakened by introducing water, it suggests that all pristine GO structures are reversible (i.e., they can be re-dispersed in water). Indeed, water was found to be able to heal damaged GO papers, reconnect separated GO pieces, and release internal stress in strained GO papers to fix their shapes. Such approach is complementary to origami and pop-up-based fabrication techniques for creating functional 3D structures from paper-like precursors. ', 'Lastly, a GO-water continuum with continuous transitions between states of a dilute dispersion, a thick gel, a malleable dough, and a dense solid is demonstrated, as the water content decreases. The GO dough is found to be highly processable and exhibit super extensibility. Dense solids with isotropically packed sheets, lamellar films, and porous foams can be readily fabricated from the dough state. Overall, the GO dough shows its potential as a versatile starting material to construct bulk GO, and potentially graphene materials with tunable shapes and microstructures.