The Quantum Detection Group at the National Physical Laboratory has conducted a study on the hydrophobicity of epitaxial graphene, which could be used in the future to develop graphene coatings suitable for electronics, medicine and more.
The research yielded results that are in contrast to the most common beliefs. They showed that the hydrophobicity of graphene is strongly dependent on its thickness, and found that single-layer graphene is considerably more hydrophilic than thicker varieties.
Graphene is a 2D carbon crystal with a number of potential applications including novel sensors, efficient transistors and flexible electronics. It is necessary to address the problems associated with large-scale production and control of graphene's properties under different environmental conditions so as to promote graphene's use for electrical applications.
The operation of most graphene-based devices is carried out in ambient conditions with non-zero humidity. The performance of graphene can be affected by air humidity due to variations in its mechanical and electrical properties. Hence it is critical to know the water affinity characteristics of graphene.
The research answers the much-debated question of whether graphene is hydrophilic or hydrophobic in nature. Like any other carbon-based materials, graphene is commonly assumed to be hydrophobic. Chemical force microscopy with a hydrophobic probe was used to investigate the adhesion and friction properties of single- and double-layer graphene.
This device is a variation of atomic force microscopy, in which the forces between a probe and a surface are used to study a substrate. The measurement of a larger adhesion force between the probe and double/triple-layer graphene, compared to single-layer graphene, indicates that the double/triple-layer graphene is more hydrophobic. This also shows that the hydrophobicity is based on the graphene layer thickness.
Using the nanoscale mapping of friction forces, the research findings were further confirmed. Hydrophobic domains exhibited a lower friction force, confirming the fact that the varying levels of hydrophobicity can affect the order of surrounding water molecules, and hence, the sliding motion of the probe tip.
This new technique could be of great help in the future to further obtain knowledge on the wetting characteristics of graphene, with an emphasis on the effectiveness of different graphene production methods. The technique specifically holds promise in differentiating graphene-based coatings and modifying them for a particular application.
Thicker coatings, for instance, are suitable for hydrophobic applications like electronic components and medical equipment. By contrast, single-layer graphene coatings could be used in applications that require a hydrophilic surface, for instance, in anti-fog glass and coatings for buildings.
The results of the study, conducted in association with the Naval Research Laboratory, were published in the American Chemical Society journal ACS Nano.