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Progress in Chemistry 2022, Vol. 34 Issue (7): 1524-1536 DOI: 10.7536/PC220309 Previous Articles   Next Articles

• Review •

Aromatic Rings in Ion Soultions: Two-Dimensional Crystals of Unconventional Stoichiometries and Ferromagnetism

Yizhou Yang1, Bingquan Peng2,3, Xiaoling Lei1, Haiping Fang1,2,3()   

  1. 1. School of Physics, East China University of Science and Technology,Shanghai 200237
    2. Wenzhou Institute, University of Chinese Academy of Sciences,Wenzhou 325001
    3. Oujiang Laboratory,Wenzhou 325000
  • Received: Revised: Online: Published:
  • Contact: Haiping Fang
  • Supported by:
    National Natural Science Foundation of China(11974366); Fundamental Research Funds for the Central Universities.
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Hydrated cation-π interaction, as a kind of non-covalent interaction, is essential to the study of soft condensed matter. This paper reviews the recent progress in the two-dimensional crystals of unconventional stoichiometries induced by the hydrated cation-π interaction, and their distinguished features. These crystals include Na2Cl, Na3Cl and CaCl at ambient conditions. These crystals have abnormal cation-anion ratios different from those of normal three-dimensional crystals and unique electronic structures. Consequently, their physical and chemical properties are usually different from those of normal three-dimensional crystals, including the room temperature ferromagnetism. These ferromagnetic materials of unconventional stoichiometries may provide new insight into biomagnetism, medicine-related magnetism and the design of low-dimensional ferromagnetic materials.

Contents

1 Introduction

2 Two-dimensional crystals of unconventional stoichiometries, Na2Cl and Na3Cl, at ambient conditions

2.1 Kinetic study of the enrichment and crystallization of ions in unsaturated NaCl solution on the graphene surface

2.2 Theoretical study of the crystal structure of two-dimensional Na-Cl of unconventional stoichiometries on graphene

2.3 Experimental confirmation of Na-Cl crystals of unconventional stoichiometries on rGO membranes

3 Two-dimensional crystals of unconventional stoichiometries, CaCl, at ambient conditions

3.1 Cryo-electron microscopy observations of Ca-Cl crystal structure on rGO membrane

3.2 Theoretical study of the crystal structure of two-dimensional Ca-Cl of unconventional stoichiometries on graphene

3.3 Study on the Ca-Cl crystals of unconventional stoichiometries on rGO membranes, ferromagnetism and applications of this ferromagnetic material.

3.4 Valence analysis on cations in two-dimensional crystals of unconventional stoichiometries

4 Conclusion and outlook

Key words: hydrated cation-π interaction, aromatic ring, two-dimensional crystal, unconventional stoichiometries
Fig. 1 Cation-π interaction between a benzene molecule and a K+ cation. Color changing from blue to red represents the electrostatic potential changing from positive to negative, balls represent C, H atoms and the K+ cation
Fig. 2 Distribution of ions and water molecules in unsaturated NaCl solution (3.0 mol/L) on the graphene sheet: (a) and (b) show the simulation snapshots at 0 and 40 ns, respectively. The cyan, white, and red balls represent carbon, hydrogen, and oxygen atoms, respectively, and the blue and green balls represent sodium and chloride ions
Fig. 3 Probability density distribution of Cl- anions (black dashed lines) and Na+ cations (black solid lines) in unsaturated NaCl solution (3.0 mol/L), and the average hydrated water number of Na+ cations (blue lines) with the change curve of height (Z) from the graphene surface
Fig. 4 Theoretical prediction structure of two-dimensional Na3Cl, Na2Cl and NaCl crystals on graphene and their mean cohesive energies
Fig. 5 Theoretical calculations of Na-Cl distance distributions of different Na-Cl crystal structures. The red dotted line shows the location of the peak of the Na-Cl distance of a conventional three-dimensional regular NaCl crystal formed by conventional saturated crystals. The curves, from top to bottom, represent the calculated results of: (1) two-dimensional Na3Cl crystals between the graphene layers, (2) two-dimensional Na2Cl crystals between the graphene layers, (3) two-dimensional Na2Cl crystals on the graphene surface; (4) two-dimensional NaCl crystals between the graphene layers; (5) two-dimensional NaCl crystals on the graphene surface, and (6) Na-Cl distance distribution of the three-dimensional regular NaCl crystals of saturated crystals, respectively
Fig. 6 (a) XRD diffraction spectra of samples obtained by rGO membrane immersion in different concentrations of NaCl solution. The concentrations of NaCl solution shown on the line, from top to bottom, are: saturation concentrations of 4.0, 3.0, 2.5 mol/L; and XRD diffraction spectra of samples obtained by graphite immersion in 3.0 mol/L NaCl solution. (b) is a locally enlarged view near 32°
Fig. 7 Distribution of the Na/Cl elemental ratio for Na-Cl crystals on the rGO membrane measured by EDS
Fig. 8 EDS measurement results of the element content and proportion on the Na-Cl crystal on the rGO membrane. The detection area is shown in the red circle, and the statistical results are listed in the table
Fig. 9 Comparison between diffraction spots for samples of Na-Cl@rGO membranes and predicted Na-Cl structures. The colored dots in (a~d) are the fitting results of the two-dimensional Na3Cl, two-dimensional Na2Cl, two-dimensional NaCl and conventional three-dimensional regular NaCl crystal structures, respectively; the white dots in the figure are experimental results
Fig. 10 (a) Experimental preparation process of two-dimensional Ca-Cl crystals in unsaturated salt solution; (b-i) Cryo-EM images of CaCl crystals in rGO membranes with water; while the illustration is a high-resolution image magnification and the crystal structure of the CaCl theoretical model, where blue-green and green globules represent Ca and Cl atoms, respectively. (b-ii) Diffraction spotted pattern of typical CaCl crystals under cryo-EM, which is a hexagonal diffraction spot with a first-degree diffraction point at (1±0.03)/4.29 Å-1. (b-iii) Fourier transform results of the entire brightfield image, which show the same as the hexagonal diffraction spot in (b-ii). (c) Content ratio of Ca to Cl at different depths within the Ca-Cl@rGO membranes
Fig. 11 Five stable two-dimensional structures of CaCl@graphene, and the corresponding cohesion energies by theoretical calculation
Fig. 12 Hysteresis loop images for dry samples of rGO membranes and Ca-ClrGO membranes (a), and dry samples of Cu-ClrGO membranes (b). The magnetic field is perpendicular to the sample surface. The illustration is a magnification of the measurements of residual magnetism and coercivity in each sample
Fig. 13 ICP-OES results for samples of Ca-Cl@rGO membrane, and rGO membrane samples
Fig. 14 Crystal cluster configurations of (CaCl)N on the same graphene substrate with different number (N), based on the structure 1
Table 1 Magnetic moment for (CaCl)N crystal clusters and the mean magnetic moment for each CaCl unit, respectively
CaCl (CaCl)2 (CaCl)3 (CaCl)4 (CaCl)5 (CaCl)6 (CaCl)7 (CaCl)8
Total moment
M (μB)		 0.7677 1.4706 0.4659 1.4136 0.8665 0.9768 1.5687 1.3728
M/N (μB) 0.7677 0.7353 0.1553 0.3534 0.1733 0.1628 0.2241 0.1716
Fig. 15 Electrical resistivities of samples: Ca-Cl@GO membranes and Ca-Cl@rGO membranes, and the corresponding control groups
Fig. 16 Resistance comparison of graphene sheets with different thicknesses and Ca-Cl@graphene samples with corresponding thicknesses
Fig. 17 Electronic structures of the theoretical models. Element energy bands and electron state density diagram of (a) two-dimensional Ca-Cl crystals without graphene; (b) complete CaCl@graphene model (Structure 1). The size of the dots in figures (a) and (b) represents the contribution weights of the element. The illustration shows the path of the highly symmetrical q-spot in Brillouin. EF stands for Fermi energy level
Fig. 18 Electron state density images of the three structures near the Fermi energy level: a two-dimensional Ca-Cl crystal model (CaCl alone) without graphene substrate, a single-layer graphene model (Graphene), and CaCl@graphene model (Structure 1), represented by blue, black, and red, respectively
Fig. 19 Piezoelectric characteristics of Ca-ClrGO membrane: typical voltage response of dry Ca-Cl@rGO membrane and rGO membrane samples, under periodic strain under periodic strain, and with the condition that the bending angle changes with time and the maximum angle is 90°
Fig. 20 Current-voltage curve of Ca-ClrGO membrane (red) and rGO membrane (blue). The illustration is a schematic diagram of two copper foil electrodes connected to the rGO membrane, to measure the current-voltage curve
Fig. 21 Calcium near-edge X-ray absorption fine structure spectra (NEXAFS) of two sampling areas of the regular calcium metal, the regular CaCl2 crystals and the Ca-Cl crystals in rGO membrane are represented by blue, red, and black solid lines, respectively
Fig. 22 NEXAFS spectra of four Cu-Cl compounds at the Cu L3, 2 edges. Black: the rGO membrane; blue: regular CuCl2 crystals; red: the Cu-Cl crystals in the dried Ca-Cl@rGO membrane; maroon: regular CuCl crystals
Fig. 23 X-ray photoelectron spectroscopy (XPS) of Cu-Cl crystals in the dried rGO membranes. Red and green: the dried rGO membranes with Cu-Cl crystals had been prepared for 10 days and 3 days, respectively. Maroon: CuCl crystals; blue: regular CuCl2 crystals
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