This image shows the effect of doping on the wettability of graphene. Image: University of Illinois.
This image shows the effect of doping on the wettability of graphene. Image: University of Illinois.

Researchers from the University of Illinois at Urbana-Champaign have demonstrated the doping-induced tunable wetting and adhesion of graphene, revealing new and unique opportunities for advanced coating materials and transducers.

"Our study suggests for the first time that the doping-induced modulation of the charge carrier density in graphene influences its wettability and adhesion," explained SungWoo Nam, an assistant professor in the Department of Mechanical Science and Engineering at Illinois. "This work investigates this new doping-induced tunable wetting phenomena, which is unique to graphene and potentially other 2D materials in complementary theoretical and experimental investigations."

Optically transparent and with superior electrical and mechanical properties, graphene could revolutionize the fields of surface coatings and electrowetting displays, according to the researchers. A material's wettability (i.e. interaction with water) is typically constant in the absence of an external influence, and can be classified as either water-loving (hydrophilic) or water-repelling (hydrophobic). In electrowetting, the hydrophilic characteristics of a material are enhanced by applying an electric current.

"What makes graphene special is that, unlike conventional bulk materials, it displays tunable surface wetting characteristics due to a change in its electron density, or by doping," said Ali Ashraf, a graduate student researcher and first author of a paper on this work in Nano Letters. "Our collaborative research teams have discovered that while graphene behaves typically as a hydrophobic material (due to the presence of strongly held air-borne contamination on its surface), its hydrophobicity can be readily changed by changing electron density.

"Our study shows for the first time that graphene demonstrates tunable wettability – switchable hydrophobic and hydrophilic behavior – when its electron density is changed by subsurface charged polymers and metals (a.k.a. doping). This finding sheds lights on previous unclear links between quantum-level charge transfer and macroscopic surface wettability for graphene. This exciting finding opens new doors of possibility for tunable surface coating and electrowetting displays without continuous external electric current supply, which will translate into significant energy savings."

"In addition, we investigated another closely related property – surface adhesion," Nam said. "We observed changes in electron density of graphene leads to a change in adhesion, which determines how graphene interacts with other hydrophobic and hydrophilic molecules, which is important for graphene-based chemical [sensors] and biosensors. Our finding suggests that it is possible to make reusable, self-cleaning graphene sensors that can first interact with hydrophobic molecules for detection, and then separates from them (i.e. cleans itself) by enhanced hydrophilicity via electron density modulation."

This story is adapted from material from the University of Illinois at Urbana-Champaign, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.