2D Electrolytes: Theory, Modeling, Synthesis, and Characterization

Abstract

A class of compounds sharing the properties of 2D materials and electrolytes, namely 2D electrolytes is described theoretically and demonstrated experimentally. 2D electrolytes dissociate in different solvents, such as water, and become electrically charged. The chemical and physical properties of these compounds can be controlled by external factors, such as pH, temperature, electric permittivity of the medium, and ionic concentration. 2D electrolytes, in analogy with polyelectrolytes, present reversible morphological transitions from 2D to 1D, as a function of pH, due to the interplay of the elastic and Coulomb energies. Since these materials show stimuli-responsive behavior to the environmental conditions, 2D electrolytes can be considered as a novel class of smart materials that expand the functionalities of 2D materials and are promising for applications that require stimuli-responsive demeanor, such as drug delivery, artificial muscles, and energy storage.

Keywords: 2D electrolytes; 2D materials; electrolytes; graphene.

Figure 1

Characterization of different 2D electrolytes. a–d) Optical images of a dispersion containing reduced functionalized graphene oxide (rGO‐SH‐FITC) at pH 3.0, where flat 2D structures are observed (a,b), and the same material in a dispersion at pH 9.0, where scrolls are seen (c,d). e,f) HR‐STEM images for functionalized graphene (G‐COOH) scroll (e), with its interlayer distance (f). AFM images and their respective height profiles of rGO‐SH‐FITC at pH 3.0 (flat) (g,j) and pH 9.0 (scrolls) (h,i,k,l).

Figure 2

a,b) Phase diagrams for 2D electrolytes: a) G‐COOH and b) rGO‐SH in an aqueous dispersion. The experimental points for ζ potential versus pH are shown by the blue squares (2D flat) and the red circles (1D scrolls) depending on the morphological state. The theoretical regions of instability are shown by the green shaded area for a given average flake size (L = 0.48 µm and L = 2.33 µm for G‐COOH and rGO‐SH, respectively). The bending stiffness is 0.025 eV for soft GO‐like materials.^{[ 14 ]} The Hamaker constant is 0.624 eV for graphene‐like materials,^{[ 15 ]} but it is very large for rGO‐SH at pH > 7 to simulate strong covalent S–S bonding formed in alkaline conditions (see the main text).

Figure 3

Morphological configurations as a function of pH for different 2D electrolytes. a–d) SEM images of GO (a), rGO‐SH (b), rGO‐SH‐FITC (c), and G‐COOH (d). The pH for all samples was adjusted using HCl and KOH under sonication.

Figure 4

Statistical analysis of 2D electrolytes: rGO‐SH and G‐COOH. a) Average aspect ratio as a function of pH for rGO‐SH showing the tendency for scroll formation with increasing pH. The red dot indicates the value achieved after pH readjustment, from pH 10.2 to 3.1. b) Statistical distribution of morphologies for rGO‐SH (scrolled—including both twisted and isolated structures, folded, and planar) identified in samples at different pH. c) SEM images demonstrating the reversibility effect of rGO‐SH from 1D scroll structures at pH 10.2 to 2D flat sheets at pH 3.1, respectively. d) Average aspect ratio, showing the increase of 2D flat structures with increasing pH, and e) statistical distribution of structures for G‐COOH. f) STEM images showing the reversibility process for G‐COOH.