Graphene oxide (GO) films are known to be highly stable in water and this property has made their use in membrane applications in solution possible. However, this state of affairs is somewhat counterintuitive because GO sheets become negatively charged on hydration and the membrane should disintegrate owing to electrostatic repulsion. We have now discovered a long-overlooked reason behind this apparent contradiction. Our findings show that neat GO membranes do, indeed, readily disintegrate in water, but the films become stable if they are crosslinked by multivalent cationic metal contaminants. Such metal contaminants can be introduced unintentionally during the synthesis and processing of GO, most notably on filtration with anodized aluminium oxide filter discs that corrode to release significant amounts of aluminium ions. This finding has wide implications in interpreting the processing-structure-property relationships of GO and other lamellar membranes. We also discuss strategies to avoid and mitigate metal contamination and demonstrate that this effect can be exploited to synthesize new membrane materials.

Enthused by the fascinating properties of graphene, we have prepared graphene analogues of BN by a chemical method with a control on the number of layers. The method involves the reaction of boric acid with urea, wherein the relative proportions of the two have been varied over a wide range. Synthesis with a high proportion of urea yields a product with a majority of 1-4 layers. The surface area of BN increases progressively with the decreasing number of layers, and the high surface area BN exhibits high CO(2) adsorption, but negligible H(2) adsorption. Few-layer BN has been solubilized by interaction with Lewis bases. We have used first-principles simulations to determine structure, phonon dispersion, and elastic properties of BN with planar honeycomb lattice-based n-layer forms. We find that the mechanical stability of BN with respect to out-of-plane deformation is quite different from that of graphene, as evident in the dispersion of their flexural modes. BN is softer than graphene and exhibits signatures of long-range ionic interactions in its optical phonons. Finally, structures with different stacking sequences of BN have comparable energies, suggesting relative abundance of slip faults, stacking faults, and structural inhomogeneities in multilayer BN.

Electrolytes confined in nanochannels with characteristic dimensions comparable to the Debye length show transport behaviors deviating from their bulk counterparts. Fabrication of nanofluidic devices typically relies on expensive lithography techniques or the use of sacrificial templates with sophisticated growth and processing steps. Here we demonstrate an alternative approach where unprecedentedly massive arrays of nanochannels are readily formed by restacking exfoliated sheets of layered materials, such as graphene oxide (GO). Nanochannels between GO sheets are successfully constructed as manifested by surface-charge-governed ion transport for electrolyte concentrations up to 50 mM. Nanofluidic devices based on reconstructed layer materials have distinct advantages such as low cost, facile fabrication, ease of scaling up to support high ionic currents, and flexibility. Given the rich chemical, physical, and mechanical properties of layered materials, they should offer many exciting new opportunities for studying and even manufacturing nanofluidic devices.

Exfoliated two-dimensional (2D) sheets can readily stack to form flexible, free-standing films with lamellar microstructure. The interlayer spaces in such lamellar films form a percolated network of molecularly sized, 2D nanochannels that could be used to regulate molecular transport. Here we report self-assembled clay-based 2D nanofluidic channels with surface charge-governed proton conductivity. Proton conductivity of these 2D channels exceeds that of acid solution for concentrations up to 0.1 M, and remains stable as the reservoir concentration is varied by orders of magnitude. Proton transport occurs through a Grotthuss mechanism, with activation energy and mobility of 0.19 eV and 1.2 Â 10 À 3 cm 2 V À 1 s À 1 , respectively. Vermiculite nanochannels exhibit extraordinary thermal stability, maintaining their proton conduction functions even after annealing at 500°C in air. The ease of constructing massive arrays of stable 2D nanochannels without lithography should prove useful to the study of confined ionic transport, and will enable new ionic device designs.

A new analogue of graphene containing boron, carbon and nitrogen (BCN) has been obtained by the reaction of high-surface-area activated charcoal with a mixture of boric acid and urea at 900 degrees C. X-ray photoelectron spectroscopy and electron energy-loss spectroscopy reveal the composition to be close to BCN. The X-ray diffraction pattern, high-resolution electron microscopy images and Raman spectrum indicate the presence of graphite-type layers with low sheet-to-sheet registry. Atomic force microscopy reveals the sample to consist of two to three layers of BCN, as in a few-layer graphene. BCN exhibits more electrical resistivity than graphene, but weaker magnetic features. BCN exhibits a surface area of 2911 m(2) g(-1), which is the highest value known for a B(x)C(y)N(z) composition. It exhibits high propensity for adsorbing CO(2) ( approximately 100 wt %) at 195 K and a hydrogen uptake of 2.6 wt % at 77 K. A first-principles pseudopotential-based DFT study shows the stable structure to consist of BN(3) and NB(3) motifs. The calculations also suggest the strongest CO(2) adsorption to occur with a binding energy of 3.7 kJ mol(-1) compared with 2.0 kJ mol(-1) on graphene.

Hydrothermal carbonization (HTC) of biomass such as glucose and cellulose typically produces micrometer-sized carbon spheres that are insulating. Adding a very small amount of Graphene oxide (GO) to glucose (e.g., 1:800 weight ratio) can significantly alter the morphology of its HTC product, resulting in more conductive carbon materials with higher degree of carbonization. At low mass loading level of GO, HTC treatment results in dispersed carbon platelets of tens of nanometers in thickness, while at high mass loading levels, free-standing carbon monoliths are obtained. Control experiments with other carbon materials such as graphite, carbon nanotubes, carbon black, and reduced GO show that only GO has significant effect in promoting HTC conversion, likely due to its good water processability, amphiphilicity, and two-dimensional structure that may help to template the initially carbonized materials. GO offers an additional advantage in that its graphene product can act as an in situ heating element to enable further carbonization of the HTC products very rapidly upon microwave irradiation. Similar effect of GO is also observed for the HTC treatment of cellulose.

MoSx is grown on crumpled graphene particles supported on carbon cloth substrates for the hydrogen evolution reaction. Modifying carbon cloth with crumpled graphene allows for higher loading levels of MoSx and thus significantly enhances its electrocatalytic activity. Measurements yield a current density of –220 mA cm−2 at an overpotential of 0.3 V (before iR correction) for the crumpled‐graphene‐modified carbon cloth.

BCN nanotube brushes have been obtained by the high temperature reaction of amorphous carbon nanotube (a-CNT) brushes with a mixture of boric acid and urea. The a-CNT brushes themselves were obtained by the pyrolysis of glucose in a polycarbonate membrane. The BCN nanotubes have been characterized by EELS, XPS, electron microscopy, Raman spectroscopy and other techniques. The composition of these nanotubes is found to be BC 4 N. The nanotubes, which are stable up to 900 uC, are insulating and nonmagnetic. They exhibit a selective uptake of CO 2 up to 23.5 wt%. In order to understand the structure and properties, we have carried out first-principles density functional theory based calculations on (6,0), (6,6) and (8,0) nanotubes with the composition BC 4 N. While (8,0) BC 4 N nanotubes exhibit a semiconducting gap, the (6,0) BC 4 N nanotube remains metallic if ordered BN bonds are present in all the six-membered rings. The (6,6) BC 4 N nanotubes, however, exhibit a small semiconducting gap unlike the carbon nanotubes. The most stable structure is predicted to be the one where BN 3 and NB 3 units connected by a B-N bond are present in the graphite matrix, the structure with ordered B-N bonds in the six-membered rings of graphite being less stable. In the former structure, (6,0) nanotubes also exhibit a gap. The calculations predict BC 4 N nanotubes to be overall nonmagnetic, as is indeed observed.