Direct observation of water-mediated single-proton transport between hBN surface defects.

Aqueous proton transport at interfaces is ubiquitous and crucial for a number of fields, ranging from cellular transport and signalling, to catalysis and membrane science. However, due to their light mass, small size and high chemical reactivity, uncovering the surface transport of single protons at room temperature and in an aqueous environment has so far remained out-of-reach of conventional atomic-scale surface science techniques, such as scanning tunnelling microscopy. Here, we use single-molecule localization microscopy to resolve optically the transport of individual excess protons at the interface of hexagonal boron nitride crystals and aqueous solutions at room temperature. Single excess proton trajectories are revealed by the successive protonation and activation of optically active defects at the surface of the crystal. Our observations demonstrate, at the single-molecule scale, that the solid/water interface provides a preferential pathway for lateral proton transport, with broad implications for molecular charge transport at liquid interfaces.

Spontaneous liquid water dissociation on hybridised boron nitride and graphene atomic layers from ab initio molecular dynamics simulations.

Two-dimensional materials such as graphene (G) and hexagonal boron nitride (BN) have demonstrated potential applications in membrane science and in particular for the harvesting of blue energy. Although pure G and BN atomic layers are known to remain inert towards neutral water, one may wonder about the aqueous reactivity of hybridized monolayers formed by joining BN and G sheets in a planar fashion. Here, we perform ab initio molecular dynamics calculations of liquid water in contact with all possible planar heterostructures. Remarkably, we could observe the spontaneous chemisorption and dissociation of the interfacial water molecule into its self-ions at one specific and non-standard one-dimensional border. Our simulations predict that this type of heterostructure is prone to ionize liquid water in the absence of any electrical gating.

Versatile electrification of two-dimensional nanomaterials in water.

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.

Chemisorption of Hydroxide on 2D Materials from DFT Calculations: Graphene versus Hexagonal Boron Nitride.

Recent nanofluidic experiments revealed strongly different surface charge measurements for boron-nitride (BN) and graphitic nanotubes when in contact with saline and alkaline water (Nature 2013, 494, 455-458; Phys. Rev. Lett. 2016, 116, 154501). These observations contrast with the similar reactivity of a graphene layer and its BN counterpart, using density functional theory (DFT) framework, for intact and dissociative adsorption of gaseous water molecules. Here we investigate, by DFT in implicit water, single and multiple adsorption of anionic hydroxide on single layers. A differential adsorption strength is found in vacuum for the first ionic adsorption on the two materials-chemisorbed on BN while physisorbed on graphene. The effect of implicit solvation reduces all adsorption values, resulting in a favorable (nonfavorable) adsorption on BN (graphene). We also calculate a pKa ≃ 6 for BN in water, in good agreement with experiments. Comparatively, the unfavorable results for graphene in water echo the weaker surface charge measurements but point to an alternative scenario.