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.

The use of carbon based materials on the removal of antibiotics with high concentrations has been well studied, however the effect of this removal method is not clear on the actual concentration of environments, such as the hospital wastewater, sewage treatment plants and aquaculture wastewater. In this study, experimental studies on the adsorption of 7 antibiotics in environmental concentration of aqueous solutions by carbon based materials have been observed. Three kinds of carbon materials have shown very fast adsorption to antibiotics by liquid chromatography–tandem mass spectrometry (LC-MS-MS) detection, and the highest removal efficiency of antibiotics could reach to 100% within the range of detection limit. Surprisedly, the adsorption rate of graphene with small specific surface area was stronger than other two biochar, and adsorption rate of the two biochar which have approximate specific surface and different carbonization degree, was significantly different. The key point to the present observation were the π-π interactions between aromatic rings on adsorbed substance and carbon based materials by confocal laser scanning microscope observation. Moreover, adsorption energy markedly increased with increasing number of the π rings by using the density functional theory (DFT), showing the particular importance of π-π interactions in the adsorption process.

Under ambient conditions, the only known valence state of calcium ions is + 2, and the corresponding crystals with calcium ions are insulating and nonferromagnetic. Here, using cryo-electron microscopy, we report the direct observation of two-dimensional (2D) CaCl crystals on reduced graphene oxide (rGO) membranes, in which the calcium ions are only monovalent (i.e. +1). Remarkably, metallic rather than insulating properties are displayed by those CaCl crystals. More interestingly, room-temperature ferromagnetism, graphene–CaCl heterojunction, coexistence of piezoelectricity-like property and metallicity, as well as the distinct hydrogen storage and release capability of the CaCl crystals in rGO membranes are experimentally demonstrated. We note that such CaCl crystals are obtained by simply incubating rGO membranes in salt solutions below the saturated concentration, under ambient conditions. Theoretical studies suggest that the formation of those abnormal crystals is attributed to the strong cation–π interactions of the Ca cations with the aromatic rings in the graphene surfaces. Those findings show the realistically potential applications of such abnormal CaCl material with unusual electronic properties in designing novel transistors and magnetic devices, hydrogen storage, catalyzer, high-performance conducting electrodes and sensors, with a size down to atomic scale.

No abstract

The application of graphene oxide (GO)-based membranes combined with a quartz crystal microbalance (QCM) as a humidity sensor has attracted great interest over the past few years. Understanding the influence of the structure of the GO membrane (GOM) on the adsorption/desorption of water molecules and the transport mechanism of water molecules in the membrane is crucial for development of applications using GOM-based humidity sensors. In this paper, by investigating the effects of oxygen-containing groups, flake size and interlayer spacing on the performance of humidity sensing, it was found that humidity-sensing performance could be improved by rational membrane-structure design and the introduction of magnesium ions, which can expand the interlayer spacing. Therefore, a novel HGO&GO&Mg2+ structure prepared by uniformly doping magnesium ions into GO&HGO thin composite membranes was designed for humidity sensing from 11.3% RH to 97.3% RH. The corresponding sensor exhibits a greatly improved humidity sensitivity (~34.3 Hz/%RH) compared with the original pure GO-based QCM sensor (~4.0 Hz/%RH). In addition, the sensor exhibits rapid response/recovery times (7 s/6 s), low hysteresis (~3.2%), excellent repeatability and good stability. This research is conducive to understanding the mechanism of GOM-based humidity sensors. Owing to its good humidity-sensing properties, the HGO&GO&Mg2+ membrane-based QCM humidity sensor is a good candidate for humidity sensing.

2D materials are promising nanomaterials for future applications due to their predominant quantum effects and unique properties in optics, electrics, magnetics, and mechanics. However, explorations in unique properties and potential applications of novel 2D materials have been hampered by synthesis and their stability under ambient conditions. Recently, in the graphene, 2D β‐CuI is observed experimentally under ambient conditions. Here, it is shown that this 2D β‐CuI@graphene possesses unexpected piezoresistive effect and room‐temperature ferromagnetism. Moreover, this 2D β‐CuI crystal is likely to be stable in a wide range of temperature, that is, below 900 K. Theoretical studies reveal that the unexpected piezoresistive effect is mainly attributable to the convergence of the electrons on Cu and I atoms to the Fermi level with increasing strain. There is a magnetic moment that is ≈0.97 μB on the edge of β‐CuI nanocrystal created by an iodine vacancy, which is considered the origin of such strong room‐temperature ferromagnetism. Clearly, the 2D β‐CuI@graphene provides a promising nanomaterial in the nano‐sensors with low power consumption pressure and magnetic nano‐devices with a size down to atomic scale. The discovery in the present work will evoke various new 2D nanomaterials with novel properties in nanotechnology, biotechnology, sensor materials, and technologies.

The discovery of specific matter phases with abnormal physical properties in low-dimensional systems and/or on particular substrates, such as the hexagonal phase of ice and two-dimensional (2D) CaCl with an abnormal valence state, continuously reveals more fundamental mechanisms of the nature. Alkali halides, represented by NaCl, are one of the most common compounds and usually thought to be well-understood. In the past decades, many theoretical studies suggested the existence of one particular phase, that is, the graphitic-like hexagonal phase of alkali halides at high pressure or in low-dimension states, with the expectation of improved properties of this matter phase but lacking experimental evidence due to severe technical challenges. Here, by optimized cryo-electron microscopy, we report the direct atomic-resolution observation and in situ characterization of the prevalent and stable graphitic-like alkali halide hexagonal phases, which were spontaneously formed by unsaturated NaCl and LiCl solution, respectively, in the quasi-2D confined space between reduced graphene oxide layers under ambient conditions. Combined with a control experiment, density functional theory calculations, and previous theoretical studies, we believe that a delicate balance among the cation−π interaction of the solute and substrate, electrostatic interactions of anions and cations, solute–solvent interactions, and thermodynamics under confinement synergistically results in the formation of such hexagonal crystalline phases. These findings highlight the effects of the substrate and the confined space on the formation of specific matter phases and provide a universal scheme for the preparation of special graphitic-like hexagonal phases of alkali halides.

Concentrations of hexabromocyclododecanes (HBCDs) were determined in surface sediments of Shanghai, China. The concentrations of total HBCD diastereoisomers (ΣHBCD) ranged from 0.01 to 13.70 ng g(-1) dry weight (dw) with a mean value of 3.41 ng g(-1) dw, which was up to several orders of magnitude lower than those reported for sediments from European countries. The ΣHBCD concentrations in sediments from chemical/textile industrial or densely populated areas were generally higher than those from rural or less chemical/textile industrialized areas in Shanghai. A high proportion of α-HBCD was observed in sediment samples and was significantly higher than that of commercial HBCD products. This might be due to thermal isomerization from γ-HBCD to α-HBCD and slower degradation rate of α-HBCD compared to γ-HBCD in anaerobic conditions. The mass inventory of ΣHBCD in surface sediments of Shanghai was estimated at 164.4 kg, representing a significant source of HBCDs to the Shanghai environment. This indicates that further study on potential transfer of HBCDs from sediments to aquatic organisms and ecological risk assessments is required.