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Graphene surface structure in aqueous media: Evidence for an air-bubble layer and ion adsorption

Zhou, Liangzhi, Islas, Luisa, Taylor, Nicholas, Bikondoa, Oier, Robles, Eric, Briscoe, Wuge H.
Carbon 2019 v.143 pp. 97-105
X-ray reflectivity, adsorption, air, bubbles, contact angle, energy, graphene, image analysis, phosphates, silica, soaking, submergence, temperature, wettability
Understanding graphene surface structure in aqueous media is essential for its biotechnological applications. Here, using synchrotron X-ray reflectivity (energy 14 keV), AFM imaging, and contact angle measurements, we have investigated the surface structure of CVD graphene on SiO2/Si in water and phosphate buffered saline (PBS) at 25–60 °C. We found a diffuse layer immediately adjacent to graphene with a scattering length density (SLD) of 6.72 × 10−6 Å−2, attributed to the presence of air bubbles on graphene under water. AFM imaging was indicative of interfacial inhomogeneity, but did not provide conclusive topography information on the bubble-covered graphene-water interface. The diffuse layer diminished after the graphene was submerged in water for 24 h at 25 °C. This is also evident from its enhanced wettability, with the water contact angle on graphene decreasing from 84.9 ± 0.4° to 55.6 ± 0.4° after submergence. An additional layer atop graphene appeared after soaking, with a thickness 10.1 Å, and a higher SLD of 19.5 × 10−6 Å−2 at 25 °C, which increased to 11.8 Å and 21.8 × 10−6 Å−2 at 60 °C in PBS, respectively. We discuss this observation in terms of ion mobility, possible formation of a silanol layer on the SiO2 substrate, and water structure disruption at higher temperatures.