Understanding the intrinsic water wettability of graphiteKozbial, Andrew (2016) Understanding the intrinsic water wettability of graphite. Doctoral Dissertation, University of Pittsburgh. (Unpublished)
AbstractDecades of research since the 1940s has substantiated graphite as a low surface energy material. Its chemical structure led researchers to believe that airborne hydrocarbon contamination was inconsequential and contradictory reports were not convincing. Graphite gained renewed interest when graphene was first isolated in 2004. Being an atomically thin material, the surface properties of graphene are critical to its performance, thus elucidating surface properties of graphene and graphite became important topics in fundamental and applied research. This work began with the realization that fresh graphene and graphite are mildly hydrophilic and approach their established hydrophobicity upon exposure to ambient air. Hydrocarbons in ambient air adsorb onto the fresh surface and cause it to appear hydrophobic. This work was first published in 2013 (doi: 10.1038/nmat3709) and provided the basis for further exploration of the intrinsic chemical nature of graphene, graphite, and MoS2. Fresh graphite is shown to be mildly hydrophilic and becomes hydrophobic upon exposure to ambient air. Similar behaviour was observed for graphene and MoS2. Ellipsometry showed growth of an adsorptive layer on the fresh (clean) surface and ATR-FTIR indicated that the adsorptive layer was airborne hydrocarbon. Theoretical calculation further confirmed that adsorption of only a monolayer of hydrocarbon is enough to reproduce the hydrophobic behavior previously observed on HOPG. Surface energy of fresh CVD graphene was calculated to be 62.2 ± 3.1 mJ/m2 (Fowkes), 53.0 ± 4.3 mJ/m2 (Owens-Wendt), and 63.8 ± 2.0 mJ/m2 (Neumann), which decreased to 45.6 ± 3.9 mJ/m2, 37.5 ± 2.3 mJ/m2, and 57.4 ± 2.1 mJ/m2, respectively, after 24 hours of air exposure. Similar behaviour also occurred for HOPG and MoS2. The fresh surface exhibits highest surface energy which decreases upon adsorption of airborne hydrocarbons. Results also indicate that the fresh surface is mildly polar. Analysis based on defect density and dynamic contact angle measurements determine that the intrinsic WCA of fresh sp2-hybridized carbon is 70.0° ± 1.5°. Current understanding of wetting models show that roughness and chemical heterogeneity do not cause the intrinsic hydrophilicity. This work unequivocally shows that fresh graphitic surfaces are mildly hydrophilic. Share
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