Two-Dimensional Oxide and Hydroxide Nanosheets: Controllable High-Quality Exfoliation, Molecular Assembly, and Exploration of Functionality

Abstract

CONSPECTUS: Two-dimensional (2D) materials, represented by graphene, have attracted tremendous interest due to their ultimate structural anisotropy and fascinating resultant properties. The search for 2D material alternatives to graphene, molecularly thin with diverse composition, structure, and functionality, has become a hot research topic. A wide variety of layered metal oxides and hydroxides have been exfoliated into the form of individual host layers, that is, 2D nanosheets. This Account presents an overview of 2D oxide and hydroxide nanosheets on the following subtopics: (1) controllable preparation of high-quality nanosheets and (2) molecular assembly and the exploration of functionality of the nanosheets. High-quality exfoliation is generally achieved via a multistep soft chemical process, comprised of ion-exchange, osmotic swelling, and exfoliation. A high degree of hydration-induced swelling, typically triggered by intercalation of organo-ammonium ions, is a critical stage leading to the high-yield production of molecularly thin nanosheets. Recent studies reveal that massive swelling, an astounding ∼100 times the original size, can be induced by a range of amine solutions. The degree of swelling is controlled by the balance of osmotic pressure between the inner gallery and the outer electrolyte solution, which is strongly influenced by amine molarity. Conversely, the stability of the resultant swollen structure is dependent on the chemical nature of the amine/ammonium ions. Particular species of lower polarity and bulky size, for example, quaternary ammonium ions, are beneficial in promoting exfoliation. Rational design and tuning of the lateral dimension, chemical composition, and structure of nanosheets are vital in exploring diverse functionalities. The lateral dimension of the nanosheets can be tuned by controlling the crystal size of the parent layered compounds, as well as the kinetics of the exfoliating reaction, for example, the type of amine/ammonium ions, their concentration, and the mode of exfoliation (manual versus mechanical shaking, etc.). Employing optimum conditions enables the production of high-quality nanosheets with a lateral size as large as several tens of micrometers. A couple of examples tailoring the nanosheets have been demonstrated with a highlight on a novel class of 2D perovskite-type oxide nanosheets with a finely tuned composition and a progressively increasing thickness at a step of 0.4-0.5 nm (corresponding to the height of the MO6 octahedron). The charge-bearing nanosheets can be organized through solution-based molecular assembly techniques (e.g., electrostatic layer-by-layer deposition, Langmuir-Blodgett method) to produce highly organized nanofilms, superlattices, etc., the exploration of which holds great potential for the development of various electronic and optical applications, among others.

Highly selective charge-guided ion transport through a hybrid membrane consisting of anionic graphene oxide and cationic hydroxide nanosheet superlattice units

“…The LDH crystallites can be exfoliated into the ultimate 2D NS with unilamellar thickness and high positive charge density, which represent another new type of 2D nanomaterial. [39][40][41][42][43][44][45] Because of the in-plane positive charges of LDH-NS, superlattice composites can be expected via alternative face-to-face assembly of cationic LDH-NS and anionic GO-NS on a molecular scale. 46,47 With these superlattice units, a macroscopic membrane may be constructed, in which the cationic LDH-NS are intercalated uniformly into the GO galleries, acting as both strengthening phases for electrostatic interactions and interlayer phases for modifying the physicochemical properties of the nanochannels.…”

Ionic solutions of two-dimensional materials

“…Common routes to forming such liquids involve adding functional groups 3,4 , ligands 5,6,7 surfactants 8,9,10 or charge 7,13,14 to the nanomaterials' surfaces in order to decrease inter-particle attraction and thus increase the metastable lifetime of the resulting dispersion. These approaches are traditionally guided by classical models developed for colloidal systems, which balance attractive and repulsive forces between the dispersed species, treating the solvent as a uniform continuum 7,15 .…”

Tuning the interlayer of transition metal oxides for electrochemical energy storage

“…18 A recent review described synthesis methods for controlling nanosheet size, composition, and structure. 104 Such exfoliated layered oxide nanosheets have primarily been investigated for use as electrochemical capacitor electrodes, and specifically as pseudocapacitors. 25 This is because the nanosheet architecture would ideally expose all of the redox active sites directly to the electrolyte and enable rapid charge-transfer reactions with minimal solid state diffusion.…”