Due to the strong in-plane but weak out-of-plane bonding, it is relatively easy to separate nanosheets of two-dimensional (2D) materials from their respective bulk crystals. This exfoliation of 2D materials can yield large 2D nanosheets, hundreds of micrometers wide, that can be as thin as one or a few atomic layers thick. However, the underlying physical mechanisms unique to each exfoliation technique can produce a wide distribution of defects, yields, functionalization, lateral sizes, and thicknesses, which can be appropriate for specific end applications. The five most commonly used exfoliation techniques include micromechanical cleavage, ultrasonication, shear exfoliation, ball milling, and electrochemical exfoliation. In this review, we present an overview of the field of 2D material exfoliation and the underlying physical mechanisms with emphasis on progress over the last decade. The beneficial characteristics and shortcomings of each exfoliation process are discussed in the context of their functional properties to guide the selection of the best technique for a given application. Furthermore, an analysis of standard applications of exfoliated 2D nanosheets is presented including their use in energy storage, electronics, lubrication, composite, and structural applications. By providing detailed insight into the underlying exfoliation mechanisms along with the advantages and disadvantages of each technique, this review intends to guide the reader toward the appropriate batch-scale exfoliation techniques for a wide variety of industrial applications.
2D
materials are well-known for their low-friction behavior by
modifying the interfacial forces at atomic surfaces. Of the wide range
of 2D materials, MXenes represent an emerging material class but their
lubricating behavior has been scarcely investigated. Herein, the friction
mechanisms of 2D Ti3C2T
x
MXenes are demonstrated which are attributed to their surface
terminations. We find that Ti3C2T
x
MXenes do not exhibit the well-known frictional
layer dependence of other 2D materials. Instead, the nanoscale lubricity
of 2D MXenes is governed by the termination species resulting from
synthesis. Annealing the MXenes demonstrate a 7% reduction in OH termination
which translates to a 16–57% reduction of friction in agreement
with DFT calculations. Finally, the stability of MXene flakes is demonstrated
upon isolation from their aqueous environment. This work indicates
that MXenes can provide sustainable lubricity at any thickness which
makes them uniquely positioned among 2D material lubricants.
Carbon
nanothreads (NTs) are ultrathin materials synthesized by
solid-state reaction of crystalline benzene or pyridine under high
pressure. Recent experimental studies show that the sp2–sp3 conversion in C–C or C–N bonds
toward NT formation is not always complete, typically resulting in
samples constituted by a mixture of both partially and fully saturated
structures. The objective of this study is to use density functional
theory calculations to compute the mechanical and electronic properties
of partially saturated carbon and carbon nitride nanothreads and analyze
how they differ from those of conventional fully saturated NTs. The
results show that partially saturated NTs have lower ideal strengths
and stiffness compared to their fully saturated versions, but they
are still remarkably strong. The electronic behavior varies from semiconducting
to insulating, with band gaps in the range ∼1.8–4.0
eV, while fully saturated NTs usually have wider gaps (>4.0 eV).
These
results show that partially saturated nanothreads can be used for
the same applications previously suggested for fully saturated NTs
on the basis of their outstanding mechanical strength, and novel applications
may be envisioned due to their wider range of possible band gaps.
Structural, mechanical and electronic properties of carbon nanothreads derived from five-membered ring heterocyclic compounds are presented and discussed, demonstrating their enhanced stability and promising set of features.
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