Alkadiyne-Pyrene Conjugated Frameworks with Surface Exclusion Effect for Ultrafast Seawater Desalination.

Billions of populations are suffering from the supply-demand imbalance of clean water, resulting in a global sustainability crisis. Membrane desalination is a promising method to produce fresh water from saline waters. However, conventional membranes often encounter challenges related to low water permeation, negatively impacting energy efficiency and water productivity. Herein, we achieve ultrafast desalination over the newly developed alkadiyne-pyrene conjugated frameworks membrane supported on a porous copper hollow fiber. With membrane distillation, the membrane exhibits nearly complete NaCl rejection (>99.9%) and ultrahigh fluxes (∼500 L m-2 h-1) from the seawater salinity-level NaCl solutions, which surpass the commercial polymeric membranes with at least 1 order of magnitude higher permeability. Experimental and theoretical investigations suggest that the large aspect ratio of membrane pores and the high evaporation area contribute to the high flux, and the graphene-like hydrophobic surface of conjugated frameworks exhibits complete salt exclusion. The simulations also confirm that the intraplanar pores of frameworks are impermeable for water and ions.

Double pyramid stacked CoO nano-crystals induced by graphene at low temperatures as highly efficient Fenton-like catalysts.

Transition metal oxides are widely used as Fenton-like catalysts in the treatment of organic pollutants, but their synthesis usually requires a high temperature. Herein, an all-solid-state synthesis method controlled by graphene was used to prepare a double pyramid stacked CoO nano-crystal at a low temperature. The preparation temperature decreased by 200 °C (over 30% reduction) due to the introduction of graphene, largely reducing the reaction energy barrier. Interestingly, the corresponding degradation rate constants (kobs) of this graphene-supported pyramid CoO nano-crystals for organic molecules after their adsorption were over 2.5 and 35 times higher than that before adsorption and that of free CoO, respectively. This high catalytic efficiency is attributed to the adsorption of pollutants at the surface by supporting graphene layers, while free radicals activated by CoO can directly and rapidly contact and degrade them. These findings provide a new strategy to prepare low carbon-consuming transition metal oxides for highly efficient Fenton-like catalysts.

Hydrated cation-π interactions of π-electrons with hydrated Mg2+ and Ca2+ cations.

Hydrated cation-π interactions at liquid-solid interfaces between hydrated cations and aromatic ring structures of carbon-based materials are pivotal in many material, biological, and chemical processes, and water serves as a crucial mediator in these interactions. However, a full understanding of the hydrated cation-π interactions between hydrated alkaline earth cations and aromatic ring structures, such as graphene remains elusive. Here, we present a molecular picture of hydrated cation-π interactions for Mg2+ and Ca2+ by using the density functional theory methods. Theoretical results show that the graphene sheet can distort the hydration shell of the hydrated Ca2+ to interact with Ca2+ directly, which is water-cation-π interactions. In contrast, the hydration shell of the hydrated Mg2+ is quite stable and the graphene sheet interacts with Mg2+ indirectly, mediated by water molecules, which is the cation-water-π interactions. These results lead to the anomalous order of adsorption energies for these alkaline earth cations, with hydrated Mg2+-π < hydrated Ca2+-π when the number of water molecules is large (n ≥ 6), contrary to the order observed for cation-π interactions in the absence of water molecules (n = 0). The behavior of hydrated alkaline earth cations adsorbed on a graphene surface is mainly attributed to the competition between the cation-π interactions and hydration effects. These findings provide valuable details of the structures and the adsorption energy of hydrated alkaline earth cations adsorbed onto the graphene surface.

Multistage Filtration Desalination via Ion Self-Rejection Effect in Cation-Controlled Graphene Oxide Membrane under 1 Bar Operating Pressure.

By building a thin graphene oxide membrane with Na+ self-rejection ability, high permeability, and multistage filtration strategy, we obtained fresh water from a saline solution under 1 bar of operating pressure. After five and 11 cycles of the multistage filtration, the Na+ concentration decreased from 0.6 to 0.123 mol/L (below physiological concentration) and 0.015 mol/L (fresh water), respectively. In comparison with the performance of commercial reverse osmosis membranes, energy consumption was only 10% and water flux was higher by a factor of 10. Interestingly, the energy consumption of this multistage filtration strategy is close to the theoretical lowest energy consumption. Theoretical calculations showed that such Na+ self-rejection is attributed to the lower transportation rate of the Na+ than that of water within the graphene oxide membrane for the hydrated cation-π interaction. Our findings present a viable desalination strategy for graphene-based membranes and improve the mechanistic understanding of water/ion transportation behaviors in confined spaces.

An anomalous anion transfer order in graphene oxide membranes induced by anion-π interactions.

Selective transport of anions across membranes has become an important goal in chemistry and biology. Here, we found an anomalous anion transfer order within the graphene oxide membrane: Cl- > Br- > F- > I-. This is at odds with the conventional ranking of the transfer order, which usually decreases as the radii of the anions increase, i.e., F- > Cl- > Br- > I-. The abnormal transportation of F- can be ascribed to the strong anion-π interactions between F- and graphene oxide sheets. Such unexpectedly strong anion-π interaction resulted in the lower movement of F- in the graphene oxide membrane and caused the anomalous anion transfer order. Our findings not only provide experimental evidence of anion-π interactions, but also improve our understanding of anion-π interactions in the selective transport of anions across a two-dimensional membrane.

Unexpected higher corrosion in the gas phase region of metals caused by calcium and magnesium ions compared to sodium ions.

In the marine environment, Na+ ions have been the focus of attention owing to their high content, which is one of the important factors causing marine corrosion. With reference to the content of macro ions in seawater, circular iron samples were semi-immersed in 0.04 M MgCl2 and 0.6 M NaCl solutions containing different proportions of ethanol. Unexpectedly, we observed more severe corrosion effects in the gas phase region and at the gas-liquid interface of metal samples semi-immersed in the MgCl2 solution. Although the concentration of the MgCl2 solution was only 1/15 of that of the NaCl solution, the iron corrosion induced by MgCl2 was significantly more severe than that caused by NaCl when the ethanol content was increased. Mg2+ ions outperform Na+ ions in metal gas phase corrosion. Especially in the oxygen content of the gas phase corrosion product, MgCl2 caused an increase by up to 52.7%, while NaCl only resulted in a 10.3% increase. Ethanol is normally regarded as a corrosion inhibitor and exists in the liquid phase. Interestingly, in the gas phase and at the gas-liquid interface, ethanol aggravated rather than reducing iron corrosion, particularly in the presence of Mg2+ ions. In addition, we observed that Ca2+ ions produced more severe corrosion effects.

Ion sieving in graphene oxide membranes via cationic control of interlayer spacing.

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+, Ca2+, Li+ or Mg2+ 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 (Fe2+, Co2+, Cu2+, Cd2+, Cr2+ and Pb2+) 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.

Revisit the Hydrated Cation-π Interaction at the Interface: A New View of Dynamics and Statistics.

Carbon-based matter, such as biomolecules and graphitic structures, often form a liquid-solid/soft matter interface in salt solution and continuously affect the surrounding cations through hydrated cation-π interactions. In this Perspective, we revisit the effect of the hydrated cation-π interactions at the interface using statistical physics, which reveals how hydrated cation-π interactions affect every component dynamically and cause a time-dependent statistical effect at the liquid-solid/soft interface. We also highlight several pieces of experimental evidence from a statistical perspective and discuss the remarkable applications related to environmental protection, industrial manufacturing, and biological sciences.

Unveiling the structure of aqueous magnesium nitrate solutions by combining X-ray diffraction and theoretical calculations.

The structure of aqueous magnesium nitrate solution is gaining significant interest among researchers, especially whether contact ion pairs exist in concentrated solutions. Here, combining X-ray diffraction experiments, quantum chemical calculations and ab initio molecular dynamics simulations, we report that the [Mg(NO3)2] molecular structure in solution from the coexistence of a free [Mg(H2O)6]2+ octahedral supramolecular structure with a free [NO3(H2O)n]- (n = 11-13) supramolecular structure to an [Mg2+(H2O)n(NO3-)m] (n = 3, 4, 5; m = 3, 2, 1) associated structure with increasing concentration. Interestingly, two hydration modes of NO3--the nearest neighbor hydration with a hydration distance less than 3.9 Å and the next nearest neighbor hydration with hydration distance ranging from 3.9 to 4.3 Å-were distinguished. With an increase in the solution concentration, the hydrated NO3- ions lost outer layer water molecules, and the hexagonal octahedral hydration structure of [Mg(H2O)62+] was destroyed, resulting in direct contact between Mg2+ and NO3- ions in a monodentate way. As the concentration of the solution further increased, NO3- ions replaced water molecules in the hydration layer of Mg2+ to form three-ion clusters and even more complex chains or linear ion clusters.

Enhancement of the Water Affinity of Histidine by Zinc and Copper Ions.

Histidine (His) is widely involved in the structure and function of biomolecules. Transition-metal ions, such as Zn2+ and Cu2+, widely exist in biological environments, and they are crucial to many life-sustaining physiological processes. Herein, by employing density function calculations, we theoretically show that the water affinity of His can be enhanced by the strong cation-π interaction between His and Zn2+ and Cu2+. Further, the solubility of His is experimentally demonstrated to be greatly enhanced in ZnCl2 and CuCl2 solutions. The existence of cation-π interaction is demonstrated by fluorescence, ultraviolet (UV) spectroscopy and nuclear magnetic resonance (NMR) experiments. These findings are of great importance for the bioavailability of aromatic drugs and provide new insight for understanding the physiological functions of transition metal ions.

Hydrated cation-π interactions of π-electrons with hydrated Li+, Na+, and K+ cations.

Cation-π interactions are essential for many chemical, biological, and material processes, and these processes usually involve an aqueous salt solution. However, there is still a lack of a full understanding of the hydrated cation-π interactions between the hydrated cations and the aromatic ring structures on the molecular level. Here, we report a molecular picture of hydrated cation-π interactions, by using the calculations of density functional theory (DFT). Specifically, the graphene sheet can distort the hydration shell of the hydrated K+ to interact with K+ directly, which is hereafter called water-cation-π interactions. In contrast, the hydration shell of the hydrated Li+ is quite stable and the graphene sheet interacts with Li+ indirectly, mediated by water molecules, which we hereafter call the cation-water-π interactions. The behavior of hydrated cations adsorbed on a graphene surface is mainly attributed to the competition between the cation-π interactions and hydration effects. These findings provide valuable details of the structures and the adsorption energy of hydrated cations adsorbed onto the graphene surface.

Unconventional Atomic Structure of Graphene Sheets on Solid Substrates.

The atomic structure of free-standing graphene comprises flat hexagonal rings with a 2.5 Å period, which is conventionally considered the only atomic period and determines the unique properties of graphene. Here, an unexpected highly ordered orthorhombic structure of graphene is directly observed with a lattice constant of ≈5 Å, spontaneously formed on various substrates. First-principles computations show that this unconventional structure can be attributed to the dipole between the graphene surface and substrates, which produces an interfacial electric field and induces atomic rearrangement on the graphene surface. Further, the formation of the orthorhombic structure can be controlled by an artificially generated interfacial electric field. Importantly, the 5 Å crystal can be manipulated and transformed in a continuous and reversible manner. Notably, the orthorhombic lattice can control the epitaxial self-assembly of amyloids. The findings reveal new insights about the atomic structure of graphene, and open up new avenues to manipulate graphene lattices.

Precise control of the interlayer spacing between graphene sheets by hydrated cations.

Recently, we have demonstrated that highly efficient ion rejection by graphene oxide membranes can be facilely achieved using hydrated cations to control the interlayer spacing in GO membranes. By using density functional theory calculations, we have shown that different hydrated cations can also precisely control the interlayer spacings between graphene sheets, which are smaller than graphene oxide sheets; this indicates ion sieving. The interlayer distances are 9.35, 8.96 and 8.82 Å for hydrated Li+, Na+ and K+, respectively. Since the radii of the hydrated Na+ and Li+ ions are larger than that of hydrated K+, graphene membranes controlled by the hydrated K+ ion can exclude K+ and the other two cations with larger hydrated volumes. Further analysis of charge transfer and orbit analysis showed that this type of control by the hydrated cations is attributed to the strong hydrated cation-π interactions; moreover, when soaked in a salt solution, graphene membranes adsorb hydrated Na+ and Li+ and form intercalation compounds. However, it is hard to find K-doped intercalation compounds in the inner part of graphene.

Janus effect of antifreeze proteins on ice nucleation.

The mechanism of ice nucleation at the molecular level remains largely unknown. Nature endows antifreeze proteins (AFPs) with the unique capability of controlling ice formation. However, the effect of AFPs on ice nucleation has been under debate. Here we report the observation of both depression and promotion effects of AFPs on ice nucleation via selectively binding the ice-binding face (IBF) and the non-ice-binding face (NIBF) of AFPs to solid substrates. Freezing temperature and delay time assays show that ice nucleation is depressed with the NIBF exposed to liquid water, whereas ice nucleation is facilitated with the IBF exposed to liquid water. The generality of this Janus effect is verified by investigating three representative AFPs. Molecular dynamics simulation analysis shows that the Janus effect can be established by the distinct structures of the hydration layer around IBF and NIBF. Our work greatly enhances the understanding of the mechanism of AFPs at the molecular level and brings insights to the fundamentals of heterogeneous ice nucleation.

Unexpectedly High Salt Accumulation inside Carbon Nanotubes Soaked in Dilute Salt Solutions.

We experimentally demonstrate the formation of salt aggregations with unexpectedly high concentration inside multiwalled carbon nanotubes (CNTs) soaked only in dilute salt solution sand even in solutions containing only traces of salts. This finding suggests the blocking of fluid across CNTs by the salt aggregations when CNTs are soaked in a dilute salt solution with the concentration of seawater or even lower, which may open new avenues for the development of novel CNT-based desalination techniques. The high salt accumulation of CNTs also provides a new CNT-based strategy for the collection or extraction of noble metal salts in solutions containing traces of noble metal salts. Theoretical analyses reveal that this high salt accumulation inside CNTs can be mainly attributed to the strong hydrated cation-π interactions of hydrated cations and π electrons in the aromatic rings of CNTs.

Water flow in carbon-based nanoporous membranes impacted by interactions between hydrated ions and aromatic rings.

Carbon-based nanoporous membranes, such as carbon nanotubes (CNTs), graphene/graphene oxide and graphyne, have shown great potential in water desalination and purification, gas and ion separation, biosensors, and lithium-based batteries, etc. A deep understanding of the interaction between hydrated ions in an aqueous solution and the graphitic surface in systems composed of water, ions and a graphitic surface is essential for applications with carbon-based nanoporous membrane platforms. In this review, we describe the recent progress of the interaction between hydrated ions and aromatic ring structures on the carbon-based surface and its applications in the water flow in a carbon nanotube. We expect that these works can be extended to the understanding of water flow in other nanoporous membranes, such as nanoporous graphene, graphyne and stacked sheets of graphene oxide.

Accelerated evaporation of water on graphene oxide.

Using molecular dynamics simulations, we show that the evaporation of nanoscale volumes of water on patterned graphene oxide is faster than that on homogeneous graphene oxide. The evaporation rate of water is insensitive to variation in the oxidation degree of the oxidized regions, so long as the water film is only distributed on the oxidized regions. The evaporation rate drops when the water film spreads onto the unoxidized regions. Further analysis showed that varying the oxidation degree observably changed the interaction between the outmost water molecules and the solid surface, but the total interaction for the outmost water molecules only changed a very limited amount due to the correspondingly regulated water-water interaction when the water film is only distributed on the oxidized regions. When the oxidation degree is too low and some unoxidized regions are also covered by the water film, the thickness of the water film decreases, which extends the lifetime of the hydrogen bonds for the outmost water molecules and lowers the evaporation rate of the water. The insensitivity of water evaporation to the oxidation degree indicates that we only need to control the scale of the unoxidized and oxidized regions for graphene oxide to regulate the evaporation of nanoscale volumes of water.

Mechanism of degradation of a nitrogenous heterocycle induced by a reductive radical: decomposition of a sym-triazine ring.

Cyanuric acid is a major component of many materials and chemicals, and is the most important intermediate in the degradation processes of sym-triazine compounds in the natural environment, as well as being used for water treatment. However, the degradation mechanism of cyanuric acid is still unclear in various advanced oxidation processes (AOPs), where ˙OH is usually regarded as the dominant radical. Here, using a combination of density functional theory calculations and experimental observations, we unexpectedly show that the sym-triazine ring structure is broken efficiently by reductive free radicals - hydrogen radicals (˙H), rather than traditional ˙OH. The energy barrier of cyanuric acid reacting with ˙H to form the -NH2 group and break the sym-triazine ring is only 4.96 kcal mol-1, which is clearly lower than that of cyanuric acid reacting with ˙OH (13.32 kcal mol-1). Our theoretical predictions are further confirmed by γ photon irradiation experiments, which show that when ˙H is present in the reaction, the nitrogen in cyanuric acid (or other nitrogenous compounds including primidone and bezafibrate) rapidly degrades into NH4+. In contrast, when ˙H is scavenged, cyanuric acid stops degrading into NH4+. Our results provide new insight for understanding the decomposition of nitrogenous materials, and we are the first to shed light on the key role of ˙H in organic transformation processes.

Graphene Oxide Facilitates Solvent-Free Synthesis of Well-Dispersed, Faceted Zeolite Crystals.

Zeolites with molecular dimension pores are widely used in petrochemical and fine-chemical industries. While traditional solvothermal syntheses suffer from environmental, safety, and efficiency issues, the newly developed solvent-free synthesis is limited by zeolite crystal aggregation. Herein, we report well-dispersed and faceted silicalite ZSM-5 zeolite crystals obtained using a solvent-free synthesis facilitated by graphene oxide (GO). The selective interactions between the GO sheets and different facets, which are confirmed by molecular dynamics simulations, result in oriented growth of the ZSM-5 crystals along the c-axis. More importantly, the incorporation of GO sheets into the ZSM-5 crystals leads to the formation of mesopores. Consequently, the faceted ZSM-5 crystals exhibit hierarchical pore structures. This synthetic method is superior to conventional approaches because of the features of the ZSM-5 zeolite.

Graphene Oxide Restricts Growth and Recrystallization of Ice Crystals.

We show graphene oxide (GO) greatly suppresses the growth and recrystallization of ice crystals, and ice crystals display a hexagonal shape in the GO dispersion. Preferred adsorption of GO on the ice crystal surface in liquid water leads to curved ice crystal surface. Therefore, the growth of ice crystal is suppressed owing to the Gibbs-Thompson effect, that is, the curved surface lowers the freezing temperature. Molecular dynamics simulation analysis reveals that oxidized groups on the basal plane of GO form more hydrogen bonds with ice in comparison with liquid water because of the honeycomb hexagonal scaffold of graphene, giving a molecular-level mechanism for controlling ice formation. Application of GO for cryopreservation shows that addition of only 0.01 wt % of GO to a culture medium greatly increases the motility (from 24.3 % to 71.3 %) of horse sperms. This work reports the control of growth of ice with GO, and opens a new avenue for the application of 2D materials.