Abstract
Many far-reaching applications of graphene require a deep understanding of the interactions between graphene and other surfaces, including the wetting behaviour of graphene. However, its two-dimensional nature does not allow qualifying graphene as simply hydrophobic or hydrophilic, but instead gives rise to a diversity of interfacial phenomena governing the apparent wettability of graphene. As a result, wide disparities in the wetting properties of graphene have been widely reported. In this review we analyse the wettability of graphene with a special focus on the experimental conditions and on discriminating the causes of the reported inconsistencies. The elimination of the environmental factors causing misleading data is a major challenge. Importantly, progresses made in graphene research yielded new experimental insights and tools enabling the minimization of unwanted effects and, ultimately, the achievement of reliable contact angle measurements. Besides the macroscopic wettability studied using contact angle measurements under ambient conditions or by theoretical modelling, we also analysed correlations with the wettability of graphene at the molecular level in supremely pure environment of ultra-high vacuum.
Highlights
Thermodynamics of graphene wettingThe surface energy of a solid sS, is the interfacial tension of a solid-gas interface sSG, and is defined as an excess energy of its surface compared to the bulk, and is related to the contact angle q with the Young equation (Fig. 1a): sSG - sSL e sLG cosq 1⁄4 0 where sSL is solid-liquid interfacial energy and sLG is liquid-gas interfacial energy (or surface tension of the liquid sL)
Leiden University, Faculty of Science, Leiden Institute of Chemistry, Einsteinweg 55, 2333CC, Leiden, the Netherlands article info
Progresses made in graphene research yielded new experimental insights and tools enabling the minimization of unwanted effects and, the achievement of reliable contact angle measurements
Summary
The surface energy of a solid sS, is the interfacial tension of a solid-gas interface sSG, and is defined as an excess energy of its surface compared to the bulk, and is related to the contact angle q with the Young equation (Fig. 1a): sSG - sSL e sLG cosq 1⁄4 0 where sSL is solid-liquid interfacial energy and sLG is liquid-gas interfacial energy (or surface tension of the liquid sL). The type of interactions between graphene and a wetting liquid can be determined from the contributions of polar (hydrogen bonding, dipole-dipole and dipole-induced dipole) and dispersive (London-van der Waals) interactions to the total surface energy [37], by measuring multiple contact angle measurements with liquids of different polarities as described in Fowkes [38], OwensWendt [39] or Neumann models (Fig. 1) [40e42]. Such an approach yielded more consistent results than determining the total surface energy (i.e. the sum of dispersive and polar contributions): most studies agree, in qualitative terms, on the dominance of the dispersion forces in the surface energy of graphene [3,13,15,18,43]. This hypothesis agrees with a separate study, where graphene was transferred to six different substrates and where e for all samples e the polar component was screened by graphene while the dispersive component increased [13]
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