hydrated protonic compounds occur. These compounds are later substituted with orga­

noammonium ions that led to the expansion of interlayer spacing between layers of bulk

material. The expansion causes the exfoliation of the layered metal oxides with a positive

surface charge. In the case of layered metal phosphorus trichalcogenides, the exfoliation

process is slightly different. Firstly, metal phosphorus trichalcogenides are immersed in

an alkali-based solution to replace the metal ions with K⁺ ions to form intermediate

compounds. Secondly, interlayer spacing is increased by exchanging K⁺ ions with Li-ions.

Finally, the exfoliation of metal phosphorus trichalcogenides is achieved. Furthermore,

the anion exchange method is used to exfoliate layered double hydroxide [41]. The in­

terlayers of one anion are exchanged with the other anion, resulting in the expansion of

interlayer distance. Later, 2D materials are obtained from exchanged layered double

hydroxides by sonicating or heating in organic solvents. Through this method, a high

yield of 2D material can be achieved. Consequently, liquid exfoliation through the ion

exchange method is suitable for the large-scale production of 2D materials.

3.3.1.4 Liquid Exfoliation Through Oxidation and Reduction

The preparation of 2D materials through liquid exfoliation by oxidation is extensively

studied. The oxidation is performed by employing modified Hummer’s method [42]. In

the process, strong oxidizing agents are used to oxidized bulk material to obtain 2D

materials. For instance, graphite is oxidized by using the mixture of KMnO4 and H2SO4

to obtain graphene. The oxidation process led to the production of various oxygen-based

functional groups including hydroxyl, carboxyl, and epoxy groups attached to the

surface of graphene layers. These functional groups promote the expansion of the in­

terlayer and weaken the van der Waals forces between graphite layers. Later, sonication

is used to convert the expanded graphite oxide into mono- or few-layered graphene

oxide sheets

FIGURE 3.8

Electrochemical-assisted liquid exfoliation of graphite to obtain graphene. Adapted with permission [ 39].

Copyright (2020) Elsevier.

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