title: Time Evolution of the Wettability of Supported Graphene under Ambient Air Exposure.
abstract: The wettability of graphene is both fundamental and crucial for interfacing in most applications, but a detailed understanding of its time evolution remains elusive. Here we systematically investigate the wettability of metal-supported, chemical vapor deposited graphene films as a function of ambient air exposure time using water and various other test liquids with widely different surface tensions. The wettability of graphene is not constant, but varies with substrate interactions and air exposure time. The substrate interactions affect the initial graphene wettability, where, for instance, water contact angles of ∼85 and ∼61° were measured for Ni and Cu supported graphene, respectively, after just minutes of air exposure. Analysis of the surface free energy components indicates that the substrate interactions strongly influence the Lewis acid-base component of supported graphene, which is considerably weaker for Ni supported graphene than for Cu supported graphene, suggesting that the classical van der Waals interaction theory alone is insufficient to describe the wettability of graphene. For prolonged air exposure, the effect of physisorption of airborne contaminants becomes increasingly dominant, resulting in an increase of water contact angle that follows a universal linear-logarithmic relationship with exposure time, until saturating at a maximum value of 92-98°. The adsorbed contaminants render all supported graphene samples increasingly nonpolar, although their total surface free energy decreases only by 10-16% to about 37-41 mJ/m2. Our finding shows that failure to account for the air exposure time may lead to widely different wettability values and contradicting arguments about the wetting transparency of graphene.
authors.affiliation: Division of Electrical Engineering and Division of Mechanics, Materials and Design, Department of Engineering, University of Cambridge , Cambridge, United Kingdom CB2 1PZ.
authors.forename: Adrianus I
authors.initials: AI
authors.lastname: Aria
authors.affiliation: Division of Electrical Engineering and Division of Mechanics, Materials and Design, Department of Engineering, University of Cambridge, Cambridge, United Kingdom CB2 1PZ; Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139-4307, United States.
authors.forename: Piran R
authors.initials: PR
authors.lastname: Kidambi
authors.affiliation: Division of Electrical Engineering and Division of Mechanics, Materials and Design, Department of Engineering, University of Cambridge, Cambridge, United Kingdom CB2 1PZ; Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States.
authors.forename: Robert S
authors.initials: RS
authors.lastname: Weatherup
authors.affiliation: Division of Electrical Engineering and Division of Mechanics, Materials and Design, Department of Engineering, University of Cambridge , Cambridge, United Kingdom CB2 1PZ.
authors.forename: Long
authors.initials: L
authors.lastname: Xiao
authors.affiliation: Division of Electrical Engineering and Division of Mechanics, Materials and Design, Department of Engineering, University of Cambridge , Cambridge, United Kingdom CB2 1PZ.
authors.forename: John A
authors.initials: JA
authors.lastname: Williams
authors.affiliation: Division of Electrical Engineering and Division of Mechanics, Materials and Design, Department of Engineering, University of Cambridge , Cambridge, United Kingdom CB2 1PZ.
authors.forename: Stephan
authors.initials: S
authors.lastname: Hofmann