Scientists at The National Physical Laboratory's (NPL) have been investigating the hydrophobicity of epitaxial graphene, which could be used in the future to better tailor graphene coatings to applications in medicine, electronics and more. Contrary to popular beliefs, the scientists' findings indicate that graphene's hydrophobicity is strongly thickness-related, with single-layer graphene being significantly more hydrophilic than its multi-layered graphene.
As graphene-based devices will have to operate in ambient conditions with existing (and unmonitored) humidity, it may be troublesome that such conditions can affect graphene's performance through changes in its mechanical and electrical properties; The new study, conducted in collaboration with the Naval Research Laboratory, addresses the question of whether graphene is hydrophobic or hydrophilic. The common assumption is that graphene is hydrophobic, but it seems that the results of this study prove the question more complex than previously thought.
The adhesion and friction properties of single- and double-layer graphene were studied using chemical force microscopy with a hydrophobic probe - a variant of AFM where a substrate is studied using the forces between a probe and a surface. A larger adhesion force was measured between the probe and double/triple-layer graphene compared to single-layer graphene, showing that double/triple-layer graphene is more hydrophobic. This suggests that the hydrophobicity depends on the thickness of graphene layers. These results were also further confirmed by the nanoscale mapping of friction forces.
The techniques demonstrated by NPL could be used in the future to further understanding of graphene's wetting behaviour, with a particular focus on the effects of different graphene production methods. In particular, it paves the way to differentiating graphene-based coatings and tailoring them to a specific application. Thicker coatings, for example, may be more suited for hydrophobic applications, such as medical equipment and electronic components. On the other hand, single-layer graphene coatings could be used where a hydrophilic surface is required, as for example in anti-fog glass and coatings for buildings.