HengAn Wu

Unimpeded permeation of water through helium-leak-tight graphene-based membranes

Porous Membranes Thin semi-permeable membranes are commonly used as chemical barriers or for filtration purposes. While the size of the pores will influence which molecules are able to pass, other factors—including the surface chemistry of the pore walls, electrostatic interactions, and differences in solubility—can also affect the diffusion rates. There is also a trade-off between the thickness of the membrane regarding strength and permeation rates (see the Perspective by Paul). Karan et al. (p. 444) fabricated membranes from amorphous carbon, which showed excellent strength and could be used for filtrations involving organic solvents. Nair et al. (p. 442) observed unusual behavior in graphene-based membranes which were able to prevent the diffusion of many small-molecule gases, including helium, but showed almost barrier-free movement of water. Graphite oxide membranes are impermeable to many liquids, vapors, and gases, including He, but allow evaporation of water. Permeation through nanometer pores is important in the design of materials for filtration and separation techniques and because of unusual fundamental behavior arising at the molecular scale. We found that submicrometer-thick membranes made from graphene oxide can be completely impermeable to liquids, vapors, and gases, including helium, but these membranes allow unimpeded permeation of water (H2O permeates through the membranes at least 1010 times faster than He). We attribute these seemingly incompatible observations to a low-friction flow of a monolayer of water through two-dimensional capillaries formed by closely spaced graphene sheets. Diffusion of other molecules is blocked by reversible narrowing of the capillaries in low humidity and/or by their clogging with water.

Precise and Ultrafast Molecular Sieving Through Graphene Oxide Membranes

Graphene oxide membranes allow only very small hydrated molecules and ions to pass with an accelerated transport rate. [Also see Perspective by Mi] Graphene-based materials can have well-defined nanometer pores and can exhibit low frictional water flow inside them, making their properties of interest for filtration and separation. We investigate permeation through micrometer-thick laminates prepared by means of vacuum filtration of graphene oxide suspensions. The laminates are vacuum-tight in the dry state but, if immersed in water, act as molecular sieves, blocking all solutes with hydrated radii larger than 4.5 angstroms. Smaller ions permeate through the membranes at rates thousands of times faster than what is expected for simple diffusion. We believe that this behavior is caused by a network of nanocapillaries that open up in the hydrated state and accept only species that fit in. The anomalously fast permeation is attributed to a capillary-like high pressure acting on ions inside graphene capillaries. On the Fast Track Membranes based on graphene can simultaneously block the passage of very small molecules while allowing the rapid permeation of water. Joshi et al. (p. 752; see the Perspective by Mi) investigated the permeation of ions and neutral molecules through a graphene oxide (GO) membrane in an aqueous solution. Small ions, with hydrated radii smaller than 0.45 nanometers, permeated through the GO membrane several orders of magnitude faster than predicted, based on diffusion theory. Molecular dynamics simulations revealed that the GO membrane can attract a high concentration of small ions into the membrane, which may explain the fast ion transport.

Self-adaptive strain-relaxation optimization for high-energy lithium storage material through crumpling of graphene

High-energy lithium battery materials based on conversion/alloying reactions have tremendous potential applications in new generation energy storage devices. However, these applications are limited by inherent large volume variations and sluggish kinetics. Here we report a self-adaptive strain-relaxed electrode through crumpling of graphene to serve as high-stretchy protective shells on metal framework, to overcome these limitations. The graphene sheets are self-assembled and deeply crumpled into pinecone-like structure through a contraction-strain-driven crumpling method. The as-prepared electrode exhibits high specific capacity (2,165 mAh g(-1)), fast charge-discharge rate (20 A g(-1)) with no capacity fading in 1,000 cycles. This kind of crumpled graphene has self-adaptive behaviour of spontaneous unfolding-folding synchronized with cyclic expansion-contraction volumetric variation of core materials, which can release strain and maintain good electric contact simultaneously. It is expected that such findings will facilitate the applications of crumpled graphene and the self-adaptive materials.

Joule-heated graphene-wrapped sponge enables fast clean-up of viscous crude-oil spill

The clean-up of viscous crude-oil spills is a global challenge. Hydrophobic and oleophilic oil sorbents have been demonstrated as promising candidates for oil-spill remediation. However, the sorption speeds of these oil sorbents for viscous crude oil are rather limited. Herein we report a Joule-heated graphene-wrapped sponge (GWS) to clean-up viscous crude oil at a high sorption speed. The Joule heat of the GWS reduced in situ the viscosity of the crude oil, which prominently increased the oil-diffusion coefficient in the pores of the GWS and thus speeded up the oil-sorption rate. The oil-sorption time was reduced by 94.6% compared with that of non-heated GWS. Besides, the oil-recovery speed was increased because of the viscosity decrease of crude oil. This in situ Joule self-heated sorbent design will promote the practical application of hydrophobic and oleophilic oil sorbents in the clean-up of viscous crude-oil spills.

Molecular transport through capillaries made with atomic-scale precision

Nanometre-scale pores and capillaries have long been studied because of their importance in many natural phenomena and their use in numerous applications. A more recent development is the ability to fabricate artificial capillaries with nanometre dimensions, which has enabled new research on molecular transport and led to the emergence of nanofluidics. But surface roughness in particular makes it challenging to produce capillaries with precisely controlled dimensions at this spatial scale. Here we report the fabrication of narrow and smooth capillaries through van der Waals assembly, with atomically flat sheets at the top and bottom separated by spacers made of two-dimensional crystals with a precisely controlled number of layers. We use graphene and its multilayers as archetypal two-dimensional materials to demonstrate this technology, which produces structures that can be viewed as if individual atomic planes had been removed from a bulk crystal to leave behind flat voids of a height chosen with atomic-scale precision. Water transport through the channels, ranging in height from one to several dozen atomic planes, is characterized by unexpectedly fast flow (up to 1 metre per second) that we attribute to high capillary pressures (about 1,000 bar) and large slip lengths. For channels that accommodate only a few layers of water, the flow exhibits a marked enhancement that we associate with an increased structural order in nanoconfined water. Our work opens up an avenue to making capillaries and cavities with sizes tunable to ångström precision, and with permeation properties further controlled through a wide choice of atomically flat materials available for channel walls.

Super-elastic and fatigue resistant carbon material with lamellar multi-arch microstructure.

Low-density compressible materials enable various applications but are often hindered by structure-derived fatigue failure, weak elasticity with slow recovery speed and large energy dissipation. Here we demonstrate a carbon material with microstructure-derived super-elasticity and high fatigue resistance achieved by designing a hierarchical lamellar architecture composed of thousands of microscale arches that serve as elastic units. The obtained monolithic carbon material can rebound a steel ball in spring-like fashion with fast recovery speed (∼580 mm s−1), and demonstrates complete recovery and small energy dissipation (∼0.2) in each compress-release cycle, even under 90% strain. Particularly, the material can maintain structural integrity after more than 106 cycles at 20% strain and 2.5 × 105 cycles at 50% strain. This structural material, although constructed using an intrinsically brittle carbon constituent, is simultaneously super-elastic, highly compressible and fatigue resistant to a degree even greater than that of previously reported compressible foams mainly made from more robust constituents.

Interlayer shear effect on multilayer graphene subjected to bending

The effect of interlayer shear on bending property of multi-layer graphene is investigated using molecular dynamics simulations. Interlayer shear modulus is three orders lower than in-plane Young’s modulus, thus the bending behavior is dominated by interlayer shear. Continuum theory and atomistic simulations show that bending rigidity is proportional to layer number when the layer number exceeds 5. The small difference of these two proportional constants results from the stacking pattern, and turbostratic stacking decreases the interlayer shear modulus. For layer number smaller than 4, intrinsic ripples also play an important role and further decrease the interlayer shear modulus.

Enhanced oil droplet detachment from solid surfaces in charged nanoparticle suspensions

The removal of oil droplets from solid surfaces is a key aspect in oil production and environmental protection. Recent progress shows that nanofluids exhibit distinct dynamic spreading behaviors compared with fluids without nanoparticles. Here, we investigated oil droplet detachment from solid surfaces immersed in charged nanoparticle suspensions via molecular dynamics simulations. Our simulated results demonstrate a significant enhancement of the oil removal efficiency using nanofluids of charged nanoparticles. When the charge on each particle exceeds a threshold value, the complete detachment of the oil droplet occurs spontaneously. Our results indicated that the surface wettability of the nanoparticles plays an essential role in oil removal processes. An increase in the interactions between nanoparticles and water molecules would obstruct the oil droplet detachment. The oil droplet detachment in the nanofluid flooding was also studied. Based on our findings, suspensions of charged hydrophobic nanoparticles can be considered to be high-performance agents in removing oil droplets from solid surfaces.

Pseudoelasticity of Cu-Zr nanowires via stress-induced martensitic phase transformations

Atomistic simulations were performed to investigate the pseudoelastic effects induced by martensitic phase transformation from body-centered cubic (B2) to body-centered tetragonal (BCT) lattice in Cu–Zr nanowires. The phase transformation occurs through nucleation and propagation of {100} twin boundary, which differs from the {101} twin boundary for B2 Ni–Al nanowires. During unloading, extension strain up to 45% can be fully recovered through inverse phase transformation. The BCT lattice has also been verified to be metastable for Cu–Zr nanowires with an energy analysis along the epitaxial Bain path. Our work implies Cu–Zr nanowires may be excellent functional components for nanoelectromechanical systems.

Strength and Fracture of Single Crystal Metal Nanowire

Numerical simulations have been carried out to determine the mechanical property of single crystal copper nanowire subjected to tension using the molecular dynamics method. The mechanism of deformation, strength and fracture are elucidated based on these numerical simulations. No strengthening is found after yielding of the single crystal nanowire. The simulation results show that the strength of copper nanowire is far greater than that of realistic polycrystalline bulk copper. By decreasing the size of the nanowires cross-section, which leads to an increase of the ratio of surface atoms, the yield stress is increased. The strain rate has an influence on strength, and mechanism of deformation and fracture. When the strain rate is comparatively low, plastic deformation arises from dislocation slips and twins. However, when the strain rate is sufficiently high, amorphization is a dominant factor of plastic deformation and super-plasticity is found. The fracture process is demonstrated using the atomic images.