Novel 2D CaCl crystals with metallicity, room-temperature ferromagnetism, heterojunction, piezoelectricity-like property and monovalent calcium ions

Abstract

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.

Anomalous Phase Behaviors of Monolayer NaCl Aqueous Solutions Induced by Effective Coulombic Interactions within Angstrom-Scale Slits

“…24−26 Interestingly, Fang and co-workers have even observed the formation of the unconventional Na 2 Cl, Na 3 Cl, and CaCl crystals on graphene surfaces. 27,28 The effective charges of sodium ions seem to be reduced to 0.5e (Na 2 Cl) or 0.333e (Na 3 Cl), and the charges of calcium ions are reduced to 1e for CaCl, due to partial electron transfer from the graphene to the cations. Moreover, previous theoretical and experimental studies have demonstrated that the static dielectric constant of water in confined systems is much smaller than that of the bulk water.…”

“…37−48 Therefore, this seminal idea of scaled charge ionic model is quite helpful to understand thermodynamic behavior of ionic aqueous solutions. However, the full understanding of aqueous electrolytes in nanoscale confinement is still scarce due to the charge transfer, 27,28 as well as the anomalously low dielectric constant of confined solutions. 29−32 In this study, we have carried out systematic microsecondscale MD simulations to study the structure and thermodynamic properties of monolayer aqueous electrolyte solutions in angstrom-scale confinement (with the width of nanoconfinement D = 0.8 nm) by using the electronic continuum correction (see method details and the simulation system (i.e., Figure S1) in the Supporting Information).…”

Pure Graphene Oxide Vertical p–n Junction with Remarkable Rectification Effect

“…Both graphene p-n junctions and heterojunctions have demonstrated the rectification effect [15,16]. Recently, we have reported the direct observation of Na-Cl crystals/Graphene heterojunction [17] and Ca-Cl crystals/Graphene heterojunction [18] with rectification effect in rGO films. However, these graphene p-n junctions and heterojunctions reported are fabricated via chemical doping [19][20][21] or electrostatic doping [22][23][24], and most of them are lateral p-n junction [25][26][27] and heterojunctions [17,18].…”

“…Recently, we have reported the direct observation of Na-Cl crystals/Graphene heterojunction [17] and Ca-Cl crystals/Graphene heterojunction [18] with rectification effect in rGO films. However, these graphene p-n junctions and heterojunctions reported are fabricated via chemical doping [19][20][21] or electrostatic doping [22][23][24], and most of them are lateral p-n junction [25][26][27] and heterojunctions [17,18]. Chemical doping is a method wherein nitrogen [28], potassium [29], and other elements are doped into graphene to control the semiconductor type of graphene.…”

Graphene Oxide-Assisted Growth of Ultralong and Soft Single-Crystalline NaCl Ionic Nanowires for Potential Optical Nanodevices

“…Recently, graphene, graphene oxide (GO), and reduced GO have been successfully used as an excellent synthesis platform for fabricating zero-dimensional, one-dimensional, and two-dimensional normal materials, including novel abnormally stoichiometric two-dimensional Na 2 Cl and Na 3 Cl crystals with CaCl, , control nucleation, and growth of ice − and zeolites . In this study, using GO as an assist, we experimentally obtained ultralong and soft NaCl ionic nanowires on a MCE membrane, which are up to 4000 μm with diameters from 270 to 600 nm and can be naturally bent 180°.…”