B. Anasori et al. Nature Reviews Materials
1
2D metal carbides and nitrides (MXenes) for energy storage
Babak Anasori, Maria R. Lukatskaya and Yury Gogotsi
A.J. Drexel Nanomaterials Institute and Department of Materials Science & Engineering,
Drexel University, Philadelphia, PA 19104, USA
Correspondence to Y.G.: gogotsi@drexel.edu
((Web summary, 40 words))
More than twenty 2D carbides, nitrides and carbonitrides of transition metals (MXenes)
have been synthesized and studied, and dozens more predicted to exist. Highly
electrically conductive MXenes show promise in electrical energy storage,
electromagnetic interference shielding, electrocatalysis, plasmonics and other
applications.
Abstract
The family of 2D transition-metal carbides, carbonitrides and nitrides (collectively referred
to as MXenes) has expanded rapidly since the discovery of Ti
3
C
2
in 2011. The materials
reported to date always have surface terminations, such as hydroxyl, oxygen or fluorine,
which impart hydrophilicity to their surfaces. About 20 different MXenes have been
synthesized, and the structures and properties of dozens more have been theoretically
predicted. The availability of solid solutions, control of surface terminations, and a recent
discovery of multi-transition-metal layered MXenes offers the potential for synthesis of
many new structures. MXenes’ versatile chemistry renders their properties tunable for
applications including energy storage, electromagnetic interference shielding, reinforcement
for composites, water purification, bio- and gas-sensors, lubrication, and chemical, photo-
and electro-catalysis. Attractive electronic, optical, plasmonic and thermoelectric properties
have also been shown. In this Review, we present the synthesis, structure and properties of
MXenes, as well as their energy-storage and related applications, and an outlook for future
research.
Cover Article, In Press.
B. Anasori et al. Nature Reviews Materials
2
2D materials have unusual electronic, mechanical and optical properties
1-6
, which have led them
to be extensively studied in the past decade for diverse applications. They can also serve as
convenient building blocks for a variety of layered structures, membranes and composites
7
.
Although several single-element 2D materials have been prepared, such as graphene, silicene
8
,
germanene
9,10
and phosphorene
11,12
, the majority contain two (for example, dichalcogenides and
oxides)
13,14
or more elements (for example, clays)
1
.
Transition metal carbides, carbonitrides and nitrides (MXenes) are among the latest
additions to the 2D world
15-21
. Their general formula is M
n+1
X
n
T
x
(n = 1–3), where M represents
an early transition metal (Sc, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, and so on), X is carbon and/or
nitrogen and T
x
stands for the surface terminations (for example, OH, O or F)
22
. Some examples
include Ti
2
CT
x
16
, Ti
3
C
2
T
x
15
and Nb
4
C
3
T
x
19
. In MXenes, n+1 layers of M cover n layers of X in an
[MX]
n
M arrangement. Structures of M
2
X, M
3
X
2
and M
4
X
3
are shown in FIG. 1. Ti
3
C
2
T
x
was the
first MXene reported in 2011
15
, and 19 different MXene compositions have subsequently been
synthesized (marked with blue in FIG. 1) with dozens more predicted to exist and studied in silico
(marked with grey in FIG. 1)
18,20,23,24
. MXenes with more than one M element also exist in two
forms: solid solutions and ordered phases. In the former, a random arrangement of two different
transition metals is observed in the M layers (marked with green in FIG. 1). By contrast, in the
ordered MXenes, single or double layers of one transition metal (for example, Ti) are sandwiched
between the layers of a second transition metal (for example, Mo) in a 2D carbide structure (third
row in FIG. 1). Density functional theory (DFT) calculations showed that for certain
combinations of transition metals, ordered MXenes are energetically more stable than their solid-
solution counterparts and more than 25 different ordered MXenes have been predicted
20
. These
ordered compositions are marked in purple in FIG. 1. In addition to carbides, 2D transition metal
carbonitrides (that is, Ti
3
CN)
16
and nitrides (that is, Ti
4
N
3
)
25
were also reported, and there have
been numerous predictions of the properties of nitrides, mostly from the M
2
N family
18,26-30
.
Figure 1 | MXenes reported to date. MXenes can have at least three different formulas, M
2
C, M
3
C
2
and M
4
C
3
, where M is
an early transition metal and X is C and/or N. They can be made in three different forms: mono-M elements (for example,
Ti
2
C, Nb
4
C
3
); solid solution of at least two different M elements (for example, (Ti,V)
3
C
2
, (Cr,V)
3
C
2
); or ordered double-M
elements, in which one transition metal occupies the perimeter layers and another fills the central M layers (for example,
Mo
2
TiC
2
, Mo
2
Ti
2
C
3
, in which the outer M layers are Mo and the central M layers are Ti). Solid solutions on the X site
produce carbonitrides.
B. Anasori et al. Nature Reviews Materials
3
Synthesis of MXenes
MXenes are made by selective etching of certain atomic layers from their layered
precursors, such as MAX phases. MAX phases are a very large family of ternary carbides and
nitrides with more than 70 reported to date, in addition to numerous solid solutions and ordered
double transition metal structures
22,31-34
. They are made of layers of transition metal carbides or
nitrides (M
n+1
X
n
) that are interleaved with layers of A-element atoms (mostly group 13 and 14
elements of the periodic table). A list of known MAX phases and their structures are provided in
Supplementary information S1, table and figure. Because the M–A bond is metallic, it has not
been possible to separate the M
n+1
X
n
layers and make MXenes by mechanical shearing of MAX
phases. However, M–A bonds are more chemically active than the stronger M–X bonds, which
makes selective etching of A layers possible. Highly selective etching is the key condition for
making MXenes.
Aqueous fluoride-containing acidic solutions have been predominantly used to
selectively etch the A layers from MAX phases to synthesize MXenes (FIG. 2), either by using
aqueous hydrofluoric acid (HF)
22
or in situ formation of HF via a reaction of hydrochloric acid
(HCl) and fluoride (for example, lithium fluoride (LiF))
35
. Ammonium hydrogen bifluoride
(NH
4
HF
2
) and ammonium fluoride were also successfully applied for Ti
3
C
2
synthesis from
Ti
3
AlC
2
36-38
. From about 10 different A elements of group 13 and 14 (Supporting information S1,
table), only Al has been successfully etched from MAX phases to form MXenes.
It is also possible to synthesize MXenes from non-MAX phase precursors
39-41
. Mo
2
CT
x
is
the first MXene of this kind that was made by etching Ga layers from Mo
2
Ga
2
C
39,40
. This phase,
despite its similarity to MAX phases, has two A-element layers (Ga) separating the carbide
layers. Zr
3
C
2
T
x
was synthesized from another non-MAX-phase precursor by selectively etching
aluminium carbide (Al
3
C
3
) layers from Zr
3
Al
3
C
5
41
, instead of just the Al layers. Zr
3
Al
3
C
5
belongs
to a family of layered ternary transition metal carbides with general formulas of M
n
Al
3
C
n+2
and
M
n
Al
4
C
n+3
, where M is a transition metal, typically Zr or Hf, and n = 1 – 3. In these phases, a
carbon layer separates each metal later. Thus, Al-C units, instead of A-element layers, separate
the M
2
C or M
3
C
2
layers
42,43
. It was recently shown that it is energetically more favourable to etch
the Al-C units than just the Al layers in Zr
3
Al
3
C
5
41
. This finding may enable the synthesis of new
MXenes from non-MAX precursors, for example, by etching Al
3
C
3
from U
2
Al
3
C
4
44
to form
U
2
CT
x
and other structures. Ultrathin MoN nanosheets have also been fabricated via liquid
exfoliation of the bulk nitride
45
.
MXenes can also be made by high-temperature etching of MAX phases, as was recently
demonstrated by treating Ti
4
AlN
3
in a molten fluoride salt mixture at 550 ºC under an argon
atmosphere to form Ti
4
N
3
MXene
25
. Before the discovery of MXenes, gaseous etchants (halides)
were used to etch MAX phases at elevated temperatures, but their selectivity was not sufficient
and removed both A and M elements, leading to the formation of carbide-derived carbon
46,47
.
There were also reports of high-temperature (>800 ºC) removal of the A-element layers from
MAX phases using molten salt
48,49
, and evaporating the A layer in a vacuum
50
. However, the
resulting carbides were cubic, not 2D, owing to the specific treatment conditions (for example,
the temperature and gas environment). For example, in 2011, a layered titanium carboxyfluoride
structure with cubic TiC
x
layers was prepared by heating Ti
3
AlC
2
in molten LiF in air at 900 ºC
51
.
This is in agreement with the transition metal carbide nonstoichiometric phase diagrams, in which
ordered nonstoichiometric carbides (for example, Ti
2
C, Ti
3
C
2
, which have similar formulae to
MXenes) are stable below certain temperatures (~800 ºC) depending on the phase
52
. Therefore, it
is reasonable to assume that synthesis and annealing of MXenes must be performed below those
temperatures and in a controlled atmosphere; otherwise, a cubic phase will form preferentially.
Taken together, these results and the recent synthesis of Ti
4
N
3
MXene suggest that the molten-salt
approach is effective for the synthesis of new MXenes.
Bottom-up synthesis methods, such as chemical vapour deposition (CVD), should also be
possible for MXene synthesis
21,53
. In 2015, ultrathin (a few nanometres) α-Mo
2
C orthorhombic
B. Anasori et al. Nature Reviews Materials
4
2D crystals with up to 100-µm lateral size were produced by CVD from methane on a bilayer
substrate of copper foil sitting on a molybdenum foil
53
. Using the same method, other transition
metals, such as W and Ta, were made into ultrathin WC and TaC crystals
53
. This method yields
MXenes with a large lateral size and few defects, facilitating the study of their intrinsic
properties
21,53
. The synthesis of MXene monolayers with this method is still to be demonstrated
and bottom-up synthesis options should be further explored.
We focus on wet etching in this Review, because this is the most widely used method to
fabricate MXenes.
Etching with hydrofluoric acid
Various MXenes can be produced by HF etching, as shown in FIG. 2, from room
temperature to 55 ºC by controlling the reaction time and HF concentration
22,54
. HF is a highly
selective etchant that is even capable of selectively removing different polytypes of SiC
55
.
The etching conditions for Al-containing MAX phases (FIG. 2b) vary from one transition
metal to another, depending on the material’s structure, atomic bonding and particle size. Etching
conditions for every MXene synthesized to date are provided in Supplementary information S2
(table). On the basis of experimental findings, increasing the atomic number of M requires a
longer time and stronger etching. This can be related to M–Al bonding
22
— knowing that M–Al
bonding is metallic, we speculate that a larger number of M valence electrons requires stronger
etching.
Etching is a kinetically controlled process and each MXene needs a different etching time
to achieve complete conversion. Usually MXenes with larger n in M
n+1
C
n
T
x
require stronger
etching and/or a longer duration. For example, Mo
2
Ti
2
AlC
3
(n = 4) requires an etching time twice
as long as its n = 3 counterpart (that is, Mo
2
TiAlC
2
) under the same etching conditions
20,56
(Supplementary information S2, table). In general, every MXene can be made under different
etching conditions, which lead to different quality (concentration of defects and surface
chemistry), as will be discussed in the following sections.
Etching in the presence of a fluoride salt
Instead of HF, a mixture of a strong acid and a fluoride salt can be used to synthesize
MXenes
35,40,57
. HCl and LiF react to form HF in situ, which selectively etches the A atoms.
Recently, using a mixture of HF and LiCl, similar etching results were achieved, suggesting that
the presence of protons and fluoride ions is a necessary condition for etching and MXene ‘clay’
formation
58
. Etching in the presence of a metal halide leads to intercalation of cations (for
example, Li
+
) and water, which results in an increased spacing and thus weakened interaction
between MXene layers. This is one of the advantages of this method over pure HF etching,
because MXene can be delaminated with no additional step, simply after washing to a pH value
of about 6 to achieve single- or few-layer flakes (for example, Ti
3
C
2
T
x
59
), as we discuss in the
following section.
Delamination
In general, delamination of any 2D material is a necessary step in exploring its properties
in the 2D state. Because multilayered MXenes have two- to sixfold stronger interlayer
interactions than that in graphite and bulk MoS
2
60
, simple mechanical exfoliation provides a low
yield of single layers. There are only two reports of Scotch tape exfoliation of multilayer MXene
into single flakes
61,62
and the remainder have been delaminated via intercalation (FIG. 2d). A
complete list of MXene intercalants reported to date is presented in Supplementary information
S2. MXenes can be intercalated with a variety of polar organic molecules, such as hydrazine, urea
and dimethyl sulfoxide (DMSO), isopropylamine or large organic base molecules such as
tetrabutylammonium hydroxide (TBAOH), choline hydroxide or n-butylamine
63
. MXene
intercalation with these molecules, followed by mechanical vibration or sonication in water, leads
to a colloidal solution of single- and few-layer MXenes. Filtering then results in freestanding
MXene ‘paper’ (FIG. 2d)
20,63-65
.
MXenes (for example, Ti
3
C
2
T
x
) can be intercalated with different metal cations, using
B. Anasori et al. Nature Reviews Materials
5
aqueous solutions of ionic compounds, such as halide salts or metal hydroxides
58,66
. When etching
with a fluoride salt mixed with an acid (for example, HCl and LiF), no additional molecule is
needed, because etched MXene is intercalated with metal cations (Supplementary information S2
(table)). In-situ delamination of MXenes can be achieved by raising the pH to almost neutral and
with very mild mechanical vibration (for example, by hand shaking the solution)
67,68
. In general,
the resulting aqueous colloidal MXene suspensions are stable (FIG. 2d) and do not aggregate
owing to the negative zeta potential of the MXene flakes
69
.
Figure 2 | Synthesis and characterization of MXenes. a | When a layered ternary carbide MAX powders (M
3
AlC
2
here)
is placed into a HF containing acidic aqueous solution (for example, HF or HCl-LiF), Al layer is selectively etched and
replaced with OH, O or F surface terminations (T
x
), forming multilayer M
3
C
2
T
x
MXenes. Intercalation of water, cations,
DMSO, TBAOH and so on into the interlayer spacing, followed by sonication makes it possible to delaminate MXenes to
produce single flake suspensions. b | Images of MAX structures (for example, M
3
AC
2
). From left to right: schematic of the
atomic structure, digital photograph of Ti
3
AlC
2
powder, low magnification and higher magnification SEM images of Ti
3
AlC
2
,
HR STEM image of Mo
2
TiAlC
2
20
. c | Illustrations of multilayered MXene. From left to right: schematic of the atomic
structure, digital photograph of Ti
3
C
2
T
x
powder, low magnification and higher magnification SEM images of Ti
3
C
2
T
x
, HR
STEM of Mo
2
TiC
2
T
x
20
. d | Illustrations of delaminated MXene. From left to right: schematic of the atomic structure, digital
photograph of 400 ml of delaminated Ti
3
C
2
T
x
in water, digital photograph of a Mo
2
TiC
2
T
x
film made by vacuum-assisted
filtration, cross-sectional SEM image
20
of a Mo
2
TiC
2
T
x
film, TEM image of a single-layer Ti
3
C
2
T
x
flake.
SEM, scanning
electron microscope; HR STEM, high-resolution scanning transmission electron microscope; TEM, transmission electron
microscope. HR STEM images in panels b and c and SEM of panel d adapted with permission from Ref.
20
, American
Chemical Society.
Structure and properties
Structure of the MXene layer
Similar to their MAX precursors, M atoms in MXenes are arranged in a close-packed structure
and X atoms fill the octahedral interstitial sites. Three packing arrangements are possible: BγA-
AγB (M
2
X-M
2
X), BγAβC-CγAβB (M
3
X
2
-M
3
X
2
) and BαCβAγB-BγAβCαB (M
4
X
3
-M
4
X
3
). Here,
the capital Roman and Greek letters correspond to the M and X positions, respectively. The
lowercase Greek letters represent the X octahedral interstitial sites corresponding to their Roman
letter counterpart positions (that is, α, β and γ correspond to A, B and C sites, respectively);
Supplementary information S2 (figure). MXenes overall crystal is a hexagonal close packed
structure. However, the ordering of M atoms changes from M
2
X to M
3
X
2
and M
4
X
3
. In M
2
X, M
atoms follow ABABAB ordering (hexagonal close-packed stacking), whereas in M
3
C
2
and M
4
C
3
,
M atoms have ABCABC ordering (face-centred cubic stacking). This atomic ordering (see
Supplementary information S2 (figure)) becomes very important for the synthesis of MXenes