Adsorption of Organic Molecules to van der Waals Materials: Comparison of Fluorographene and Fluorographite with Graphene and Graphite

Understanding strength and nature of noncovalent binding to surfaces
imposes significant challenge both for computations and experiments. We explored the
adsorption of five small nonpolar organic molecules (acetone, acetonitrile, dichloromethane,
ethanol, ethyl acetate) to fluorographene and fluorographite using inverse gas
chromatography and theoretical calculations, providing new insights into the strength and
nature of adsorption of small organic molecules on these surfaces. The measured
adsorption enthalpies on fluorographite range from −7 to −13 kcal/mol and are by 1−2
kcal/mol lower than those measured on graphene/graphite, which indicates higher affinity
of organic adsorbates to fluorographene than to graphene. The dispersion-corrected
functionals performed well, and the nonlocal vdW DFT functionals (particularly optB86bvdW)
achieved the best agreement with the experimental data. Computations show that
the adsorption enthalpies are controlled by the interaction energy, which is dominated by
London dispersion forces (∼70%). The calculations also show that bonding to structural
features, like edges and steps, as well as defects does not significantly increase the
adsorption enthalpies, which explains a low sensitivity of measured adsorption enthalpies to coverage. The adopted Langmuir
model for fitting experimental data enabled determination of adsorption entropies. The adsorption on the fluorographene/
fluorographite surface resulted in an entropy loss equal to approximately 40% of the gas phase entropy.