Modified MAX Phase Synthesis for Environmentally Stable and Highly
Conductive Ti
3
C
2
MXene
Tyler S. Mathis
a
, Kathleen Maleski
a
, Adam Goad
a
, Asia Sarycheva
a
, Mark Anayee
a
, Alexandre C.
Foucher
b
, Kanit Hantanasirisakul
a
, Eric A. Stach
b,c
, Yury Gogotsi
a,*
a
A.J. Drexel Nanomaterials Institute and Department of Materials Science and Engineering, Drexel University, Phil-
adelphia, PA, USA
b
Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, PA, USA
c
Laboratory for Research on the Structure of Matter, University of Pennsylvania, Philadelphia, PA, USA
Keywords: MXene, Ti
3
C
2
, long term stability, oxidation resistant
ABSTRACT: One of the primary factors limiting further research and the commercial use of the two-dimensional (2D)
MXene titanium carbide (Ti
3
C
2
), as well as MXenes in general, is the rate at which freshly made samples oxidize and degrade
when stored as aqueous suspensions. Here, we show that including excess aluminum during synthesis of the Ti
3
AlC
2
MAX
phase precursor leads to the creation of Ti
3
AlC
2
grains with improved stoichiometry and crystallinity. Ti
3
C
2
nanosheets
produced from the improved Ti
3
AlC
2
are of higher quality, as evidenced by their increased resistance to oxidation and an
increase in their electrical conductivity to 20,000 S/cm. Our results indicate that defects created during the synthesis of
Ti
3
C
2
(and by inference, other MXenes) lead to the previously observed instability. We show that by eliminating those
defects results in Ti
3
C
2
that is highly stable in aqueous solutions and in air. Aqueous suspensions of single- to few-layer
Ti
3
C
2
flakes produced from the modified Ti
3
AlC
2
have a shelf life of over ten months, compared to one to two weeks for
Ti
3
C
2
produced from conventional Ti
3
AlC
2
, even when stored in ambient conditions. Freestanding films made from Ti
3
C
2
suspensions stored for ten months show minimal decreases in electrical conductivity and negligible oxidation. Oxidation
of the improved Ti
3
C
2
in air initiates at temperatures that are 100-150°C higher than conventional Ti
3
C
2
. The observed im-
provements in both the shelf life and properties of Ti
3
C
2
will facilitate the widespread use of this material.
Introduction
Exfoliation of layered materials into two-dimensional
(2D) nanosheets can lead to properties that often differ sig-
nificantly from their bulk analogs. This has provided new
building blocks for nanoscale devices, metamaterials, and
composites to meet emerging technological needs.
1
MXenes are a large family of 2D transition metal carbides,
nitrides, and carbonitrides that have the general formula
M
n+1
X
n
T
x
, where M is an early transition metal, X is carbon
and/or nitrogen, and n is an integer from 1 to 4. The T
x
rep-
resents surface terminations (=O, -OH, and -F) that com-
monly result from the wet chemical etching methods used
to produce MXenes from their MAX phase precursors (A is
typically a group 13 or 14 element, e.g. aluminum). More
than 30 stoichiometric MXene structures have been syn-
thesized from the more than 100 predicted compositions,
and there have also been reports on the creation of solid
solution MXenes.
2-3
MXenes have been utilized in various
fields, including energy storage and conversion, electro-
magnetic interference shielding, nanocomposites, sensors,
and biomedical applications.
4-5
As the scope of MXene research has expanded, so too
have studies on improving the quality of MXenes by
exploring new synthesis routes and processing methods in
order to enhance their performance.
6-9
MXenes have sev-
eral significant advantages over graphene and many other
conducting nanomaterials: MXenes form stable colloidal
solutions without additives or surfactants and they can
easily be processed using the cheapest and safest solvent –
water. However, MXenes are quick to oxidize in aqueous
solutions and generally last no more than a few weeks
when stored in aqueous media.
10-14
Some M
2
C MXenes will
even degrade within a day.
10, 15
This has prompted many in-
vestigations into the mechanisms of MXene degradation in
order to prolong their shelf life.
10-14, 16
Recent work has fo-
cused on modifying the synthesis of MAX phases, as this
can profoundly affect the synthesis, quality, and properties
of the resultant MXene.
17
The common assumption is that
phase-pure MAX should lead to the highest quality MXene,
and most studies have utilized established MAX phase syn-
thesis procedures that result in the incorporation of mini-
mal amounts of impurities into the sintered product. We
report here the surprising result that the phase purity of
the as-sintered MAX does not necessarily determine the
quality of the resultant MXene.
2
In this study, we included excess aluminum (A-element)
during the high-temperature synthesis of the MAX phase
Ti
3
AlC
2
9
to create a liquid phase at an early stage of the sin-
tering process. We believe the presence of molten metal
during the sintering reaction enhances the diffusion of re-
actants, resulting in Ti
3
AlC
2
grains with improved struc-
tural ordering and morphology. This MAX phase (which
we will refer to as Al-Ti
3
AlC
2
) was then used for the synthe-
sis of high quality Ti
3
C
2
T
x
nanosheets (subsequently de-
noted as Ti
3
C
2
for simplicity). We find that the aqueous
Ti
3
C
2
solutions produced from the Al-Ti
3
AlC
2
MAX (Al-
Ti
3
C
2
) have an exceptional shelf life (>10 months at ambient
temperature) with only minimal steps taken to protect the
MXene. Freestanding films made from fresh Al-Ti
3
C
2
solu-
tions have electronic conductivities as high as 20,000 S/cm.
These results represent a significant improvement in the
oxidative stability of MXenes and can be expected to sig-
nificantly impact their incorporation into industrial appli-
cations, enhancing their commercial viability.
Results
Al-Ti
3
AlC
2
MAX was produced by pressureless sintering
of a non-stoichiometric mixture of TiC, Ti, and Al powders
that contained excess Al (see Experimental Methods sec-
tion). The as-produced MAX contains intermetallic com-
pounds – namely in the form of TiAl
3
– as seen in the X-ray
diffraction (XRD) pattern of Al-Ti
3
AlC
2
(Fig. 1a, red). The
intermetallic impurities cause the body of the block of
MAX to have a lustrous, metallic sheen when the sintered
block is milled, which is not the case for blocks of Ti
3
AlC
2
produced using conventional synthesis methods (Fig. S1).
9
Generally, the use of excess aluminum in the synthesis of
MAX phases is known to introduce deleterious impurities
into the final sintered product.
18-19
XRD analysis shows
however that these intermetallic impurities can easily be
removed by washing the milled Al-Ti
3
AlC
2
powder in hy-
drochloric acid (HCl) at room temperature (Fig. 1a, blue).
To better understand how the excess aluminum affects the
composition and bonding within the MAX, we further
compared the Al-Ti
3
AlC
2
with conventional Ti
3
AlC
2
using
Raman spectroscopy. The Raman spectra of Al-Ti
3
AlC
2
(red, not acid washed) and conventional Ti
3
AlC
2
(green,
not acid washed) show the presence of TiC in both sam-
ples, along with the MAX phase Ti
3
AlC
2
(Fig. 1b). The vi-
brational spectrum of Ti
3
AlC
2
consists of seven modes: 3 E
2g
+ 2 E
1g
+ 2 A
1g
, where the sharp peak at 201 cm
-1
in the spec-
trum of Ti
3
AlC
2
(green) is assigned to the E
2g
vibration of
Ti, Al and C.
20
This vibration has a larger full width at half
maximum and lower intensity for Al-Ti
3
AlC
2
(red). It is
worth noting that this is the only observable vibration that
involves Al atoms. The broadening and diminishing of this
peak in Al-Ti
3
AlC
2
suggests some structural changes in the
Figure 1. (a) X-ray diffraction patterns of Al-Ti
3
AlC
2
before (red) and after (blue) HCl washing. (b) Polarized Raman spectra of Al-
Ti
3
AlC
2
(red, not acid washed) and conventional Ti
3
AlC
2
(green). (c) Scanning electron microscopy (SEM) image of a hexagonal
grain of HCl washed Al-Ti
3
AlC
2
. (d) SEM image of a hexagonal, single layer flake of Al-Ti
3
C
2
produced via HF/HCl etching and
LiCl delamination (supported on an anodic aluminum oxide membrane). (e) High-angle annular dark-field scanning transmission
electron microscopy (STEM) images of Ti
3
C
2
flake edges produced from conventional Ti
3
AlC
2
(top) and Al-Ti
3
AlC
2
(bottom). Both
types of MAX were HCl washed prior to etching. Inset in (e) shows an atomic-resolution cross-sectional TEM image from an Al-
Ti
3
C
2
flake.
3
Al layer. The out-of-plane peaks – A
1g
symmetric and asym-
metric – are present in both spectra. However, in the case
of Al-Ti
3
AlC
2
, the symmetric peak shifted slightly from 270
to 274 cm
-1
and the asymmetric peak shifted from 659 to
661 cm
-1
. The positions of the corresponding peaks in Ti
3
C
2
are located at 200 and 723 cm
-1
, respectively.
21
The 300 – 500
cm
-1
region has previously been attributed to impurities,
but the exact origin of these peaks has yet to be deter-
mined.
20
There is also a peak at approximately 549 cm
-1
which is present only together with the Ti
3
AlC
2
, which sug-
gests that the MAX phase is the origin of this peak. The
acid washed Al-Ti
3
AlC
2
MAX also has well-shaped, hexag-
onal grains. We believe this to be the result of enhanced
diffusion of the reactants during the sintering process
caused by the presence of molten aluminum (Fig. 1c and
Fig. S2).
We etched the HCl washed Al-Ti
3
AlC
2
using a mixture of
hydrofluoric and hydrochloric acids (HF/HCl etching) and
then delaminated the MXene by stirring the etched Al-
Ti
3
C
2
in an aqueous solution of LiCl. This procedure yields
suspensions of delaminated Al-Ti
3
C
2
flakes that largely re-
tain the shape of the starting Al-Ti
3
AlC
2
MAX particles
(Fig. 1d). High-resolution scanning transmission electron
microscopy (HRSTEM) images of the edges of conven-
tional Ti
3
C
2
(Fig. 1e, top) and Al-Ti
3
C
2
(Fig. 1e, bottom)
flakes show that the edges of Al-Ti
3
C
2
are smoother, with-
out any of the protuberances seen in the conventional
Ti
3
C
2
. Images of the basal planes of both flakes look very
similar, however (Fig. S3).
One of the characteristic properties of MXenes, and Ti
3
C
2
in particular, is the high electronic conductivity of films
produced from solutions of single- or few-layer MXene
flakes. Freestanding films made by vacuum filtering Al-
Ti
3
C
2
solutions have conductivities ranging from slightly
higher than 10,000 S/cm up to values exceeding 20,000
S/cm (Fig. 2a). It is important to note, that the electronic
conductivity of a MXene film depends not only on the
quality of the MXene, but also on the film structure and
morphology. Features such as flake alignment, film rough-
ness, and interflake distance influence the conductivity of
MXene films. The conductivities of the films produced in
this study (>20,000 S/cm) exceed the values reported in re-
cent years for Ti
3
C
2
freestanding films and coatings (rang-
ing from 8,000 to 15,000 S/cm).
9, 22-24
The conductivity of the Al-Ti
3
C
2
films varies slightly de-
pending on the quantity of water used during the delami-
nation process (Fig. 2a). Since the concentration of the de-
laminated Al-Ti
3
C
2
colloidal solutions is also dependent on
the quantity of water used during delamination (Fig. S4),
it is likely that the highest quality single-layer flakes de-
laminate first, leading to the highest quality films. How-
ever, traces of LiCl present in the MXene solutions in the
initial stages of delamination may also influence the prop-
erties of the final films. We find that as the delamination
process continues, the remaining LiCl is removed (Fig. S5).
Thermogravimetric analysis (TGA) of delaminated film
and multilayer powder Ti
3
C
2
samples conducted in air
shows that Al-Ti
3
C
2
has significantly improved oxidation
stability versus Ti
3
C
2
produced from conventional Ti
3
AlC
2
(Fig. 2b). During the initial stage of heating (below 200
C), each sample shows mass loss due to the removal of
water that was intercalated between the layers or adsorbed
on the surfaces of the MXene samples. Weight gain due to
oxidation begins at ~150 C higher for the delaminated Al-
Ti
3
C
2
versus the conventional Ti
3
C
2
. Oxidation of the Al-
Ti
3
C
2
multilayer powder, where the flake edges are exposed
and no continuous protective oxide can form, occurs at a
much slower rate than the conventional Ti
3
C
2
. This shows
that the oxidation stability of MXenes is improved both as
aqueous suspensions, and as solid films and powders in air.
Moreover, the high-temperature resistance of the delami-
nated Al-Ti
3
C
2
in air is improved by approximately 200 C
over literature reports, up to over 450 C.
25
This can poten-
tially expand the use of MXenes to applications requiring
operation at elevated temperatures in air, such as sensors
or electronics operating near hot engines or electrical com-
ponents.
Figure 2. (a) Electronic conductivity of freestanding films
produced by vacuum-assisted filtration of Al-Ti
3
C
2
suspen-
sions at different stages of the delamination process. (b) Ther-
mogravimetric analysis in air for delaminated ((d), top) and
multilayer ((ML), bottom) Ti
3
C
2
produced from Al-Ti
3
AlC
2
and conventional Ti
3
AlC
2
. Both types of MAX were washed us-
ing HCl prior to etching.
4
The most notable property of the Al-Ti
3
C
2
produced
from HCl-washed Ti
3
AlC
2
MAX is its remarkable shelf life
as an aqueous colloidal suspension. To test the long-term
stability of the Al-Ti
3
C
2
solutions, we took the minimum
amount of precautions to protect the Al-Ti
3
C
2
flakes, as to
simulate the most typical laboratory storage conditions.
Delaminated Al-Ti
3
C
2
solutions were degassed by bubbling
argon through the solutions at the as-produced concentra-
tion directly after centrifugation before the solutions were
transferred to sealed, argon-filled vials and then stored
away from light in a laboratory bench drawer at room tem-
perature. This is a common way of preparing colloidal so-
lutions for shipment or storage that requires no specialized
equipment, deep refrigeration, or stabilizing additives.
Changes in the suspension’s absorbance over time based
on UV-Vis measurements recorded periodically during the
storage period show that the concentration of the suspen-
sion remains relatively unchanged (Fig. 3a). In addition,
no noticeable changes in the UV-vis spectra of the stored
samples occurred until the 4-month mark (Fig. 3b), where
a slight red-shift of the 768 nm peak to 780 nm occurs. Red-
shifts of this peak have been shown to be caused by
changes in the oxidation state of the Ti in Ti
3
C
2
.
26
However,
when a film was made from the solution that was stored
for 4 months, the conductivity was still over 10,000 S/cm,
well within the range of measurements made from films
directly after delamination (Fig. 3c). After 6 months of
storage, the UV-vis spectra of the Al-Ti
3
C
2
solution still had
only a slight red-shift in the ~780 nm peak, but the
conductivity of the film made from the 6-month-old solu-
tion dropped to just over 6000 S/cm. The Raman spectra
of the Al-Ti
3
C
2
films made from fresh, 4-month-old, and 6-
month-old solutions are identical. No photoluminescent
background is present, meaning there was no titanium ox-
ide formation during storage (Fig. 3d).
21
Minor oxidation
begins after approximately 4 months for these storage con-
ditions, as determined by the decrease in electronic con-
ductivity. Comparison of TEM images of fresh Al-Ti
3
C
2
flakes and Al-Ti
3
C
2
flakes stored for 10 months exhibit re-
markably few pinholes (commonly observed in samples
stored for extended periods
13
(Fig. 3e, f) and very few TiO2
crystals after nearly a year of storage (Fig. S6). From the
core level X-ray photoelectron spectra (XPS), there were
negligible differences in the chemical environments of Ti,
C, and Al between the Al-Ti
3
AlC
2
and the conventional
Ti
3
AlC
2
MAX phases (Fig. S7). However, close inspection
of the O region reveals that there is less oxygen in the Al-
Ti
3
AlC
2
(potentially in the form of oxycarbides). This may
contribute to the improved oxidation stability of the result-
ing Al-Ti
3
C
2
. The straight edges of the Al-Ti
3
C
2
flakes show
no traces of oxides after being exposed to air for a few days
prior to the TEM measurements, which is a sign that the
Al-Ti
3
C
2
is highly stable. It is known the that oxidation of
Ti
3
C
2
starts from point defects and edges and it was pro-
posed that stabilization of the edges of the flakes by ad-
sorbed species can improve the oxidation stability of
Ti
3
C
2
.
27
Figure 3. (a) Absorbance changes over time for the stored Al-Ti
3
C
2
solution calculated from the UV-vis spectra in (b). The grey
region corresponds to suspension concentrations of 1.5 – 1.8 mg/mL. (b) UV-vis spectra recorded over time for an aqueous Al-
Ti
3
C
2
solution stored in ambient conditions. (c) Electronic conductivity of freestanding Al-Ti
3
C
2
films made from solutions stored
for different periods of time. (d) Raman spectra of films made from solutions stored for different periods of time. TEM images of
a fresh Al-Ti
3
C
2
flake (e) and an Al-Ti
3
C
2
flake from a ten-month-old solution (f). The red circles mark all the observable pinholes
in the flake.
5
Conventionally, aqueous solutions of Ti
3
C
2
will be com-
pletely oxidized after just a few weeks of storage in ambient
conditions.
10-11, 13
Therefore, if further steps were taken to
optimize the storage conditions for Al-Ti
3
C
2
solutions,
such as storing the samples at temperatures near or below
freezing to slow oxidation or by concentrating the Al-Ti
3
C
2
solutions to concentrations of tens or even hundreds of
mg/mL by high speed centrifugation to reduce the total
amount of water in the solutions, we speculate that the
shelf life of Al-Ti
3
C
2
solutions would be increased to years.
Recent results show that freezing Ti
3
C
2
solutions allows for
storage for multiple years
28
, however, we can now achieve
similar results under ambient conditions with Al-Ti
3
C
2
.
Our current results suggest that the improved oxidation
stability of Al-Ti
3
C
2
is most likely due to a reduction in the
number of defects in the MAX synthesized with excess alu-
minum, resulting in MXene flakes that should be less de-
fective and have improved Ti:C stoichiometry (Fig. S7e). It
has been reported that Al monovacancies (V
Al
), Al divacan-
cies (2V
Al–Al
), and divacancies composed of Al and C atoms
(2V
Al–C
) are the most easily formed vacancies in Ti
3
AlC
2
.
29
Therefore, while it may seem counter-intuitive, the pres-
ence of excess aluminum should minimize carbon vacan-
cies and reduce the associated loss of Ti atoms near carbon
vacancies after etching, leading to fewer defects in Ti
3
C
2
.
More in-depth studies will be needed to understand how
the use of excess aluminum, or other molten fluxes
30-31
, dur-
ing MAX phase synthesis affects the atomic structure, com-
position, and growth of MAX phases and how these
changes can be utilized for the synthesis of high-quality
MXene. However, even without a complete understanding
of the exact origin of the dramatic improvement in the sta-
bility of Ti
3
C
2
, the results presented in this study will allow
the MXene community to begin utilizing highly stable
MXenes.
In prior work, researchers selecting MAX phase precur-
sors for MXene synthesis were solely concerned with the
phase purity of the MAX. Our results show that the opti-
mization of MAX phase synthesis should consider the
properties of the resulting MXenes. As of now, the crystal-
linity and M:X stoichiometric ratio of the MAX appear to
be the main factors. Finding optimal precursor ratios and
synthesis conditions for non-Ti
3
AlC
2
MAX phases that will
not produce mixed compositions (i.e., mixed M
3
AlC
2
and
M
2
AlC phases) or introduce impurities that cannot be
readily removed will be the key challenges moving forward.
Conclusions
By modifying the synthesis of Ti
3
AlC
2
to produce a more
stoichiometric MAX phase with improved structure, we
have significantly improved the quality of the resulting
Ti
3
C
2
MXene flakes, thereby markedly improving the shelf
life and chemical stability of the MXene. Doing so signifi-
cantly improves both the commercial viability of MXenes
and the ease with which MXenes can be studied. Storage
of the improved Ti
3
C
2
in closed vials at room temperature
for 10 months with minimal degradation has been demon-
strated. Additionally, the improved flake quality resulted
in MXene films with higher electronic conductivity,
approaching 20,000 S/cm – the highest value reported for
any solution processable 2D material reported thus far. The
oxidation stability of the MXene in air was also signifi-
cantly improved, increasing the onset of high-temperature
oxidation in air by ~150 C. We anticipate that this new
methodology will be used as a guide to improve the oxida-
tion stability and electronic conductivity of a large variety
of carbide MXenes.
Experimental Methods
Al-Ti
3
AlC
2
MAX Synthesis
A 2:1:1 (mass ratio) mixture of TiC, Ti, and Al powders
was ball milled using zirconia balls for 18 h at 70 rpm. A 2:1
mass ratio of zirconia balls to powder mixture was used.
The ball milled precursor powder was then packed into an
alumina crucible and covered with graphite foil and placed
into a tube furnace. The furnace was purged with argon for
30 min at room temperature. After purging, the precursor
powders were heated to 1380 °C and held for 2 h under a
constant argon flow at ~ 100 sccm. The heating and cooling
rates were both 3 °C/min. The sintered block of Al-Ti
3
AlC
2
was then milled using a TiN coated milling bit to produce
MAX powder which was subsequently washed using 9 M
HCl (Fisher Scientific, USA). Typically, 500 mL of 9 M HCl
is sufficient to wash upwards of 50 to 60 g of Al-Ti
3
AlC
2
.
The MAX was washed until the evolution of gas bubbles
from the solution stopped. Four hours is the minimum
washing time that has been tested so far. Acid washing of
the Al-Ti
3
AlC
2
MAX results in ca. 20-30% loss of mass (Fig.
S1b) which is primarily attributed to the removal of inter-
metallic impurities. The acid washed MAX was then neu-
tralized by filtering the Al-Ti
3
AlC
2
/HCl mixture though a
vacuum filtration unit followed by repeated filtration of DI
water through the Al-Ti
3
AlC
2
deposit. The pore size of the
filter membrane used was 5 μm. During neutralization of
the acid washed Al-Ti
3
AlC
2
, the acidic supernatant has a
deep purple color (Fig. S1c). The neutralized MAX was
then dried in a vacuum oven for at least 6 h at 80 °C. The
dried Al-Ti
3
AlC
2
was then sieved through a 450-mesh (32
μm) particle sieve. The washed, dried, and sieved Al-
Ti
3
AlC
2
was then etched to produce MXene. The same pro-
cedure was used to synthesize Ti
3
AlC
2
with the conven-
tional amount of Al by using precursor ratios reported pre-
viously in the literature.
9
Ti
3
C
2
MXene Synthesis
Typically, 1 g of Al-Ti
3
AlC
2
was mixed with 20 mL of etch-
ant and stirred at 400 rpm for 24 h at 35 ⁰C. The etchant
was a 6:3:1 mixture (by volume) of 12 M HCl, DI water, and
50 wt.% HF (Acros Organics, Fair Lawn, NJ, USA). A
loosely capped 60 mL high density polyethylene bottle was
used as the etching container. The etched Al-Ti
3
C
2
was
washed with DI water via repeated centrifugation and de-
cantation cycles until the supernatant reached pH ~6 using
a 175 mL centrifuge tube. Once the MXene was neutralized,
one more additional wash cycle was performed to ensure
the washing process was complete. 5 wash cycles using a
single 175 mL centrifuge tube are typically enough for 1 g of
MAX etched using 20 mL of etchant. The etched Al-Ti
3
C
2
