Surface Energy of Supported Graphene—The Effects of Chemical Functionalization and Adsorption of Volatile Organic Compounds

The novel properties of 2D materials, such as graphene, are proposed to enable exciting applications in diverse areas such as sensing, electronics, and mechanics. However, the thinness of 2D materials and their sensitivity to environmental conditions present challenges when examining the fundamental liquid/surface interactions. For instance, it has been shown that the wettability, surface friction, and electrical conductivity of graphene on silicon dioxide (SiO₂) can be tuned by adding bond terminations like hydrogen and fluorine to the graphene lattice. However, there have been few studies examining the surface energy of these chemically functionalized forms of graphene. These studies are important to conduct as they would indicate the types of interactions (dispersive and/or polar) the samples are capable of participating in and the strength of the interaction potential. Further, graphene's wettability, typically characterized using water contact angle measurements, has also been shown to vary with its supporting substrate—a phenomenon known as wetting transparency. This wetting transparency can further be complicated by the adsorption of airborne volatile organic compounds (VOCs) that lower the surface energy of the sample, raising the contact angle, leading to irreproducible results. Thus, to fully leverage the exciting properties of 2D materials for novel applications, we must develop a more rigorous understanding of the interactions between the 2D material, its supporting substrate, and its surroundings.<br/>In this work, we used microgoniometry to examine the wettability of supported bare, fluorinated, and hydrogenated graphene. Using contact angle measurements with deionized water and diiodomethane, we used Fowkes’s method to determine the dispersive and polar surface energies of all samples tested. The graphene samples were functionalized with hydrogen or fluorine bond terminations by indirect exposure to hydrogen plasma and xenon difluoride (XeF₂). The graphene samples were supported by two substrates: Parylene C, a low surface energy polymer commonly used in biomedical and printed circuit board applications, and annealed SiO₂, which has a higher surface energy. We used a dry transfer process combined with thermal annealing to ensure that the samples were cleaned properly to minimize the effects of contaminants on the measurements. The effect of the cleaning and quality of the graphene was verified using XPS and Raman spectroscopy. To limit the effect of VOC adsorption on the contact angle measurements, we stored and transported our samples in an airtight vessel filled with adsorbents and pressurized with argon. We systematically examined the effect of VOCs by exposing the samples to ambient conditions for specific amounts of time. The results indicate that the surface energy (polar and dispersive components) and wetting transparency of the samples depends on the specific bond termination, underlying substrate, and exposure to VOCs. Our results offer guidance to those seeking to use graphene in novel applications where the behavior of the liquid/surface interaction is of paramount importance, such as liquid phase sensors, actuators, and surface modifiers.