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.
NaCl in a 1:1 stoichiometry is the only known stable form of the Na-Cl crystal under ambient conditions, and non-1:1 Na-Cl species can only form under extreme conditions, such as high pressures. Here we report the direct observation, under ambient conditions, of NaCl and NaCl as two-dimensional (2D) Na-Cl crystals, together with regular NaCl, on reduced graphene oxide membranes and on the surfaces of natural graphite powders from salt solutions far below the saturated concentration. Molecular dynamics and density functional theory calculations suggest that this unconventional crystallization process originates from the cation-π interaction between the ions and the π-conjugated system in the graphitic surface, which promotes the ion-surface adsorption. The strong Na-π interaction and charge transfer lead to stoichiometries with an excess of Na. With unique electron and spin distributions and bonding, the resulting 2D crystals may have unusual electronic, magnetic, optical and mechanical properties.
Expression of PD-L1 and PD-1 could be used as clinical prognostic biomarkers for evaluating CIN and cervical cancer because of its positive correlation with CIN progression and tumor metastasis.
Based on DFT computations, we show that different hydrated cations can precisely control the interlayer spacings between graphene sheets, which are smaller than that between graphene oxide sheets, indicating an ion sieving.
Oxygen vacancies at ceria (CeO 2) surfaces play an essential role in catalytic applications. However, during the past decade, the near-surface vacancy structures at CeO 2 (111) have been questioned due to the contradictory results from experiments and theoretical simulations. Whether surface vacancies agglomerate, and which is the most stable vacancy structure for varying vacancy concentration and temperature, are being heatedly debated. By combining density functional theory calculations and Monte Carlo simulations, we proposed a unified model to explain all conflicting experimental observations and theoretical results. We find a novel trimeric vacancy structure which is more stable than any other one previously reported, which perfectly reproduces the characteristics of the double linear surface oxygen vacancy clusters observed by STM. Monte Carlo simulations show that at low temperature and low vacancy concentrations, vacancies prefer subsurface sites with a local (2 × 2) ordering, whereas mostly linear surface vacancy clusters do form with increased temperature and degree of reduction. These results well explain the disputes about the stable vacancy structure and surface vacancy clustering at CeO 2 (111), and provide a foundation for the understanding of the redox and catalytic chemistry of metal oxides.
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