Abstract
Ordered nanoscale patterns have been observed by atomic force microscopy at graphene–water and graphite–water interfaces. The two dominant explanations for these patterns are that (i) they consist of self-assembled organic contaminants or (ii) they are dense layers formed from atmospheric gases (especially nitrogen). Here we apply molecular dynamics simulations to study the behavior of dinitrogen and possible organic contaminants at the graphene–water interface. Despite the high concentration of N2 in ambient air, we find that its expected occupancy at the graphene–water interface is quite low. Although dense (disordered) aggregates of dinitrogen have been observed in previous simulations, our results suggest that they are stable only in the presence of supersaturated aqueous N2 solutions and dissipate rapidly when they coexist with nitrogen gas near atmospheric pressure. On the other hand, although heavy alkanes are present at only trace concentrations (micrograms per cubic meter) in typical indoor air, we predict that such concentrations can be sufficient to form ordered monolayers that cover the graphene–water interface. For octadecane, grand canonical Monte Carlo suggests nucleation and growth of monolayers above an ambient concentration near 6 μg m−3, which is less than some literature values for indoor air. The thermodynamics of the formation of these alkane monolayers includes contributions from the hydration free-energy (unfavorable), the free-energy of adsorption to the graphene–water interface (highly favorable), and integration into the alkane monolayer phase (highly favorable). Furthermore, the peak-to-peak distances in AFM force profiles perpendicular to the interface (0.43–0.53 nm), agree with the distances calculated in simulations for overlayers of alkane-like molecules, but not for molecules such as N2, water, or aromatics. Taken together, these results suggest that ordered domains observed on graphene, graphite, and other hydrophobic materials in water are consistent with alkane-like molecules occupying the interface.
Highlights
The behavior of graphitic surfaces in various media is important for technological applications of graphite, graphene, and carbon nanotubes
Similar to Peng et al, we found that hemispherical aggregates (Fig. S1 of the Electronic supplementary information (ESI)†) of N2 spontaneously form at the graphene–water interface at relatively low areal densities of N2, using the same N2, water, and graphene models as these authors
The conclusion of this work is that the striped patterns observed by atomic force microscopy (AFM) at the graphene–water interface are likely due to ordered arrangements of hydrocarbons, such as alkanes, that migrate to the interface from the air or from the surfaces of materials used in the experiments
Summary
The behavior of graphitic surfaces in various media is important for technological applications of graphite, graphene, and carbon nanotubes. Contact angle[1,2] and capacitance measurements,[3] infrared spectroscopy,[2] and atomic force microscopy (AFM)[4–11] suggest the presence of contaminants of some nature that rapidly accumulate on even freshly cleaved graphite surfaces exposed to water or air under typical laboratory conditions. The identity of these contaminants remains somewhat unclear and likely depends on the details of the. Volatile organic compounds (VOCs), of both natural and arti cial origin, are present at low concentrations in indoor[12,13] and outdoor air,[14] and emanate from polymeric materials[15] and human breath.[16]. À3 12,13,17 Many of these VOCs have a high affinity for graphitic surfaces and the graphitic–water interface[18–20] and can be expected to reach
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