Graphene oxide membranes-partially oxidized, stacked sheets of graphene-can provide ultrathin, high-flux and energy-efficient membranes for precise ionic and molecular sieving in aqueous solution. These materials have shown potential in a variety of applications, including water desalination and purification, gas and ion separation, biosensors, proton conductors, lithium-based batteries and super-capacitors. Unlike the pores of carbon nanotube membranes, which have fixed sizes, the pores of graphene oxide membranes-that is, the interlayer spacing between graphene oxide sheets (a sheet is a single flake inside the membrane)-are of variable size. Furthermore, it is difficult to reduce the interlayer spacing sufficiently to exclude small ions and to maintain this spacing against the tendency of graphene oxide membranes to swell when immersed in aqueous solution. These challenges hinder the potential ion filtration applications of graphene oxide membranes. Here we demonstrate cationic control of the interlayer spacing of graphene oxide membranes with ångström precision using K, Na, Ca, Li or Mg ions. Moreover, membrane spacings controlled by one type of cation can efficiently and selectively exclude other cations that have larger hydrated volumes. First-principles calculations and ultraviolet absorption spectroscopy reveal that the location of the most stable cation adsorption is where oxide groups and aromatic rings coexist. Previous density functional theory computations show that other cations (Fe, Co, Cu, Cd, Cr and Pb) should have a much stronger cation-π interaction with the graphene sheet than Na has, suggesting that other ions could be used to produce a wider range of interlayer spacings.

Graphene nanosheet has exhibited an increasing prospect in various biomedical applications because of its extraordinary properties. Meanwhile, recent experiments have shown that graphene has antibacterial activity or cytotoxicity and can cause cell membrane damage. Therefore, it is necessary to understand the interactions between graphene and cell membrane to avoid its adverse effects. Here, we use molecular dynamics simulation to explore these interactions. The results show that pristine graphene (PG) can readily penetrate into the bilayer and has no effect on the integrity of membrane. When graphene oxide (GO) is embedded in the membrane, several lipids are pulled out of the membrane to the surface of GO, resulting in the pore formation and water molecules flowing into the membrane. The difference between PG and GO in the membrane originates from GO's oxygen-contained groups, which enhance the adsorption of the lipids on GO surface. However, the main interactions between GO and membrane are still determined by the strong dispersion interactions between its hydrophobic domains and the lipid tails of the bilayer. Therefore, the toxicity of coated GO can be weakened, since its hydrophobic domains are screened by polymers. The findings may offer new perspective for better designing GO based nanocarrier or antibiotics and other biomedical applications.

No abstract

Using molecular dynamics simulations, we show that the evaporation of nanoscale water on hydrophobic-hydrophilic patterned surfaces is unexpectedly faster than that on any surfaces with uniform wettability. The key to this phenomenon is that, on the patterned surface, the evaporation rate from the hydrophilic region only slightly decreases due to the correspondingly increased water thickness; meanwhile, a considerable number of water molecules evaporate from the hydrophobic region despite the lack of water film. Most of the evaporated water from the hydrophobic region originates from the hydrophilic region by diffusing across the contact lines. Further analysis shows that the evaporation rate from the hydrophobic region is approximately proportional to the total length of the contact lines.

No abstract

An approximate expression of a Bessel-Gaussian beam (BGB) with desired topological charge is introduced using a coherence superposition of decentered Gaussian beams (dGBs). And based on such an expression and the extended Huygens-Fresnel principle, the propagation properties of BGBs traveling in turbulent atmosphere are explored. An analytical expression of the average intensity of a BGB with phase singularity propagating through turbulent atmosphere is obtained and analyzed numerically. It is found that intensity profiles of BGBs experienced successive variations and the phase singularity rapidly fades away during propagating in turbulent atmosphere.

Intermetallic compounds are atomically ordered inorganic materials containing two or more transition metals and main-group elements in unique crystal structures. Intermetallics based on group 10 and group 14 metals have shown enhanced activity, selectivity, and durability in comparison to simple metals and alloys in many catalytic reactions. While high-temperature solid-state methods to prepare intermetallic compounds exist, softer synthetic methods can provide key advantages, such as enabling the preparation of metastable phases or of smaller particles with increased surface areas for catalysis. Here, we study a generalized family of heterobimetallic precursors to binary intermetallics, each containing a group 10 metal and a group 14 tetrel bonded together and supported by pincer-like pyridine-2-thiolate ligands. Upon thermal decomposition, these heterobimetallic complexes form 10-14 binary intermetallic nanocrystals. Experiments and density functional theory (DFT) computations help in better understanding the reactivity of these precursors toward the synthesis of specific intermetallic binary phases. Using Pd2Sn as an example, we demonstrate that nanoparticles made in this way can act as uniquely selective catalysts for the reduction of nitroarenes to azoxyarenes, which highlights the utility of the intermetallics made by our method. Employing heterobimetallic pincer complexes as precursors toward binary nanocrystals and other metal-rich intermetallics provides opportunities to explore the fundamental chemistry and applications of these materials.

No abstract