Synthesis of Two-Dimensional Materials by Selective Extraction

Abstract

CONSPECTUS: Two-dimensional (2D) materials have attracted much attention in the past decade. They offer high specific surface area, as well as electronic structure and properties that differ from their bulk counterparts due to the low dimensionality. Graphene is the best known and the most studied 2D material, but metal oxides and hydroxides (including clays), dichalcogenides, boron nitride (BN), and other materials that are one or several atoms thick are receiving increasing attention. They may deliver a combination of properties that cannot be provided by other materials. The most common synthesis approach in general is by reacting different elements or compounds to form a new compound. However, this approach does not necessarily work well for low-dimensional structures, since it favors formation of energetically preferred 3D (bulk) solids. Many 2D materials are produced by exfoliation of van der Waals solids, such as graphite or MoS2, breaking large particles into 2D layers. However, these approaches are not universal; for example, 2D transition metal carbides cannot be produced by any of them. An alternative but less studied way of material synthesis is the selective extraction process, which is based on the difference in reactivity and stability between the different components (elements or structural units) of the original material. It can be achieved using thermal, chemical, or electrochemical processes. Many 2D materials have been synthesized using selective extraction, such as graphene from SiC, transition metal oxides (TMO) from layered 3D salts, and transition metal carbides or carbonitrides (MXenes) from MAX phases. Selective extraction synthesis is critically important when the bonds between the building blocks of the material are too strong (e.g., in carbides) to be broken mechanically in order to form nanostructures. Unlike extractive metallurgy, where the extracted metal is the goal of the process, selective extraction of one or more elements from the precursor materials releases 2D structures. In this Account, in addition to graphene and TMO, we focused on MXenes as an example for the use of selective extraction synthesis to produce novel 2D materials. About 10 new carbides and carbonitrides of transition metals have been produced by this method in the past 3 years. They offer an unusual combination of metallic conductivity and hydrophilicity and show very attractive electrochemical properties. We hope that this Account will encourage researchers to extend the use of selective extraction to other layered material systems that in turn will result in expanding the world of nanomaterials in general and 2D materials in particular, generating new materials that cannot be produced by other means.

Penta-graphene: A new carbon allotrope

“…In fact, a monolayer can be fabricated from either a layered structure or a nonlayered structure (19). For van der Waals coupled layered structures like graphite, mechanical exfoliation (3) is sufficient to obtain the monolayer sheet (graphene), whereas for chemically bonded bulk phases like MAX (a family of transition metal carbides or nitrides), chemical exfoliation technique has been developed to extract a single layer (MXene) (20).…”

Not just graphene: The wonderful world of carbon and related nanomaterials

“…51 We call these materials MXenes because they are built of M (early transition metal) and X (carbon or nitrogen) atoms and are produced by selective etching of a monoatomic layer of A-group metal (e.g., aluminum) from MAX phases (MAX to MXene). 24 The as-produced fl aky MXenes resemble thermally expanded graphite or some clays.…”

“…Thus, selective extraction alone 24 can generate a large variety of carbons, ranging from amorphous porous CDCs to graphene and nanotubes, 18 but numerous methods for making carbon materials are available today that can produce almost any structure needed.…”

Electronic properties and applications of MXenes: a theoretical review

“…For the experimental progresses on MXenes, one can find comprehensive reviews in Refs. [80][81][82][83][84]. The rest of this review is organized as follows.…”