Versatile Electrification of Two-dimensional Nanomaterials in Water

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Versatile Electrification of Two-dimensional Nanomaterials in Water, Nature Communications 10,1656 (2019)


The common wisdom states the chemical inertness of boron-nitride and carbon-based nanomaterials in mild conditions. However, this passive picture is not a dogma and was actually invalidated in various conditions. For example, defect-free graphene basal plane can be activated electronically when supported on metal substrates in ultrahigh vacuum. More puzzling, the chemical reactivity of carbon and boron-nitride two-dimensional materials was assessed in aqueous solvents, thanks to nanofluidic transport measurements. Indeed giant surface charging was highlighted in ionic transport measurement of electrolytes through multiwall nanotubes made from carbon (CNT) or its BN analogue (BNNT), with a surface charge reaching up to 1 C.m−2for BNNT and 0.01 to 0.1 C.m−2 for CNT. This is one to two orders of magnitude larger than any other reported surface charge, e.g., on silica glass. The strong pH dependence for both materials led experimentalists to suggest hydroxide adsorption as a source of surface charge for both materials, although the very different salinity dependence pointed to different mechanisms and strengths of adsorption. This conjecture remains however unsupported up to now in terms of the chemical reactivity of these materials and the fundamental origin of this surface charging remains mysterious. 



This charging mechanism can possibly be further investigated experimentally at the molecular scale by means of sum-frequency generation spectroscopy as the OH remains mainly in the vicinity of the graphene surface. Also, the confinement of the hydroxide at the interface, with proton transfer restrained to the first interfacial water layer raises the question of its validity in extremely confined channels such as the recently elaborated ångstrom slit-pores nanofluidic devices. In fact, only up to two water layers can penetrate in between the graphite layers constituting those channels. As interfacial molecular information on such system is not accessible experimentally, this will be the object of future AIMD studies, in the same spirit of the recent hydroxide solvation modelling in inorganic mackinawite slit-pores. Slit-pores channels made of non-metallic materials could also provide an appropriate substrate to probe the normal orientation of the hydroxide to the surfaces by the above-mentioned interfacial spectroscopy technique.


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Nature Communications 10,1656 (2019)


The recent emergence of nanofluidics has highlighted the exceptional properties of graphene and its boron-nitride counterpart as confining nanomaterials for water and ion transport. Surprisingly, ionic transport experiments have unveiled a consequent electrification of the water/carbon surfaces, with a contrasting response for its water/boron-nitride homologue. In this paper, we report free energy calculations based on ab initio molecular dynamics simulations of hydroxide OH ions in water near graphene and hexagonal boron nitride (h-BN) layers. Our results disclose that both surfaces get charged through hydroxide adsorption, but two strongly different mechanisms are evidenced. The hydroxide species shows weak physisorption on the graphene surface while it exhibits also strong chemisorption on the h-BN surface. Interestingly OH is shown to keep very fast lateral dynamics and interfacial mobility within the physisorbed layer on graphene. Taking into account the large ionic surface conductivity, an analytic transport model allows to reproduce quantitatively the experimental data.



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Versatile Electrification of Two-dimensional Nanomaterials in Water 


Benoît Grosjean, Marie-Laure Bocquet, and Rodolphe Vuilleumier


Nature Communications 10,1656 (2019)


DOI: 10.1038/s41467-019-09708-7