The wetting of two dimensional nanomaterials and their industrial applications

Abstract

Two dimensional atomic crystals such as graphene and transition metal dichalcogenides (TMD's), when synthesized or exfoliated from their layered counterparts, have been shown to possess unique nanoscale attributes. Ballistic transport of charges, tunable band-gaps, optical transparency and high surface area, in addition to their strength and flexibility, make these 2D nanomaterials very desirable. Their impact spans multiple industries including energy storage, semiconductors, photovoltaics and composites among others.

As such, it is critical to understand how these monolayer and few layer materials interact with water at a molecular level. Their extreme thinness allows for wetting transparent behavior; wherein water may see through these sheets and interact with the underlying substrate. The behavior of wetting transparent graphene sheets on planar substrates, as well as roughened underlying architectures is thoroughly investigated. Non-conformal graphene coatings reduce surface friction experienced by moving droplets, while conformal graphene coatings retain their wetting transparency even on highly hydrophobic surfaces. Finally, 3-dimensional, freestanding macrostructures of graphene are synthesized to behave as super-hydrophobic foams, showing extreme water repellency. Beyond graphene, the transparency effect is studied for TMD's as well. It is found that all 2D layered materials when aged for some time, adsorb airborne contaminants that affect their observable wettability when compared with fresh samples. With the understanding developed through these fundamental studies, two distinct industrial applications are explored. Given the wide use of graphite and metal sulfides in lubrication, variably sized graphene sheets are stably dispersed as additives in semi-synthetic and plant derived metal cutting oils for micro-machining applications. Finally, a novel roll-to-roll electrodeposition process is devised using surface charges and functionalization to manipulate graphene oxide sheets in an aqueous dispersion, enabling the large scale manufacturing and assembly of graphene-based electrodes for high performance lithium-ion batteries.