Because they may provide ultrathin, high-flux, and energy-efficient membranes for precise ionic and molecular sieving in aqueous solution, GO membranes (partially oxidized, stacked sheets of graphene) have shown great potential in water desalination and purification, gas and ion separation, biosensors, proton conductors, lithium-based batteries and super-capacitors. Unlike carbon nanotube (CNT) membranes, in which the nanotube pores have fixed sizes, the pores of GO membranes - the interlayer spacing between GO sheets - are of variable size. This presents a challenge for using GO membranes for filtration. Despite the great efforts to tune and fix the interlayer spacing, it remains difficult both to reduce the interlayer spacing sufficiently to exclude small ions while keeping this separation constant against the tendency of GO membranes to swell when immersed in aqueous solution, which greatly affects the applications of GO membranes. Here, we demonstrate experimentally that highly efficient and selective ion rejection by GO membranes can be readily achieved by controlling the interlayer spacing of GO membranes using cations (K+, Na+, Ca2+, Li+ and Mg2+) themselves. The interspacing can be controlled with precision as small as 1 A, and GO membranes controlled by one kind of cation can exclude other cations with a larger hydrated volume, which can only be accommodated with a larger interlayer spacing. First-principles calculations reveal that the strong noncovalent cation-pi interactions between hydrated cations in solution and aromatic ring structures in GO are the cause of this unexpected behavior. These findings open up new avenues for using GO membranes for water desalination and purification, lithium-based batteries and super-capacitors, molecular sieves for separating ions or molecules, and many other applications.
Recently we have reported the direct observation of two-dimensional (2D) Ca-Cl crystals on reduced graphene oxide (rGO) membranes, in which the calcium ions are only about monovalent (i.e. ~+1) and metallic rather than insulating properties are displayed by those CaCl crystals. Here, we report the experimental observation and demonstration of the formation of graphene-Ca-Cl heterojunction owing to the metallicity of 2D Ca-Cl crystals, unexpected piezoelectric effect, room-temperature ferromagnetism, as well as the distinct hydrogen storage and release capability of the Ca-Cl crystals in rGO membranes. Theoretical studies show that the formation of those abnormal crystals is attributed to the strong cation-pi interactions of the Ca2+ with the aromatic rings in the graphitic surfaces. Since strong cation-pi interactions also exist between other metal ions (such as Mg2+, Fe2+, Co2+, Cu2+, Cd2+, Cr2+ and Pb2+) and graphitic surfaces, similar 2D crystals with abnormal valence state of the metal cations and corresponding abnormal properties as well as novel applications are highly expected. Those findings further 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.
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.
Abstract 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 μ B 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.
Abstract Hydrogels do not have observable interaction with external magnetic fields as they are conventionally thought to be diamagnetic. If hydrogels alone can be magnetically controlled, they can promise wider applications without concerns about side effects of additives. Here we show that calcium cations can induce strong paramagnetism of hydrogels containing structures rich in carbon-oxygen double bonds, including alginate, carboxymethyl chitosan, polyacrylamide, and N-isopropyl acrylamide. Both experiments and computations reveal that the ubiquitous presence of net magnetic moments, the key to paramagnetism, is induced by the unexpected coupling of one calcium cation and one carbon-oxygen double bond. The paramagnetic phenomenon is also observed in the endogenous biomolecule sodium hyaluronate with calcium cations. We further demonstrate safe applications of the strongly paramagnetic alginate-calcium hydrogel as a contrast agent in magnetic resonance imaging and a carrier in magnetic drug delivery. Our findings provide novel insights into the origin of magnetism and advance magnetism-related biomedical innovations.
The sheet size of a graphene oxide (GO) can greatly influence its electrical, optical, mechanical, electrochemical and catalytic property. It is a key challenge to how to control the sheet size during its preparation in different application fields. According to our previous theoretical calculations of the effect of temperature on the oxidation process of graphene, we use Hummers method to prepare GOs with different sheet sizes by simply controlling the temperature condition in the process of the oxidation reaction of potassium permanganate (KMnO4) with graphene and the dilution process with deionized water. The results detected by transmission electron microscopy (TEM) and atomic force microscopy (AFM) show that the average sizes of GO sheets prepared at different temperatures are about 1 μm and 7 μm respectively. The ultraviolet–visible spectroscopy (UV-vis) shows that lower temperature can lead to smaller oxidation degrees of GO and less oxygen functional groups on the surface. In addition, we prepare GO membranes to test their mechanical strengths by ultrasonic waves, and we find that the strengths of the GO membranes prepared under low temperatures are considerably higher than those prepared under high temperatures, showing the high mechanical strengths of larger GO sheets. Our experimental results testify our previous theoretical calculations. Compared with the traditional centrifugal separation and chemical cutting method, the preparation process of GO by temperature control is simple and low-cost and also enables large-size synthesis. These findings develop a new method to control GO sheet sizes for large-scale potential applications.
Nanographene oxide (nGO) flakes-graphene oxide with a lateral size of ≈100 nm or less-hold great promise for superior flux and energy-efficient nanofiltration membranes for desalination and precise ionic sieving owing to their unique high-density water channels with less tortuousness. However, their potential usage is currently limited by several challenges, including the tricky self-assembly of nano-sized flakes on substrates with micron-sized pores, severe swelling in aqueous solutions, and mechanical instability. Herein, the successful fabrication of a robust membrane stacked with nGO flakes on a substrate with a pore size of 0.22 µm by vacuum filtration is reported. This membrane achieved an unprecedented water permeance above 819.1 LMH bar
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