Two-Dimensional Nanocrystals Produced by
Exfoliation of Ti(3)AlC(2)
Michael Naguib, Murat Kurtoglu, Volker Presser, Jun Lu, Junjie Niu,
Min Heon, Lars Hultman, Yury Gogotsi and Michel W Barsoum
Linköping University Post Print
N.B.: When citing this work, cite the original article.
This is the authors’ version of the article which is published in final form at:
Michael Naguib, Murat Kurtoglu, Volker Presser, Jun Lu, Junjie Niu, Min Heon, Lars
Hultman, Yury Gogotsi and Michel W Barsoum, Two-Dimensional Nanocrystals Produced
by Exfoliation of Ti(3)AlC(2), 2011, Advanced Materials, (23), 37, 4248-4253.
http://dx.doi.org/10.1002/adma.201102306
Copyright: Wiley-VCH Verlag Berlin
http://www.wiley-vch.de/publish/en/
Postprint available at: Linköping University Electronic Press
http://urn.kb.se/resolve?urn=urn:nbn:se:liu:diva-72027
Submitted to
1
DOI: 10.1002/adma.((please add manuscript number))
Two-Dimensional Nanocrystals Produced by Exfoliation of Ti
3
AlC
2
By Michael Naguib, Murat Kurtoglu, Volker Presser, Jun Lu, Junjie Niu, Min Heon, Lars
Hultman, Yury Gogotsi* and Michel W. Barsoum*
[*] Prof. Michel Barsoum and Prof. Yury Gogotsi Corresponding-Authors
Department of Materials Science and Engineering, Y.G. is also a Trustee Chair Director of A.J.
Drexel Nanotechnology Institute. Drexel University, Philadelphia, PA 19104 (USA)
E-mail: (barsoumw@drexel.edu and gogotsi@drexel.edu)
Michael Naguib Author-One, Murat Kurtoglu Author-Two, Dr. Volker Presser Author-
Three, Dr. Junjie Niu Author-Five, Min Heon Author-Six.
Department of Materials Science and Engineering, and A.J. Drexel Nanotechnology Institute.
Drexel University, Philadelphia, PA 19104 (USA)
Jun Lu Author-Four, Prof. Lars Hultman Author-Seven
Department of Physics, IFM
Linkoping University, Linkoping 58183 (Sweden)
Keywords: Nanosheets, MAX phase, Exfoliation, Carbide, Two-dimensional material
Typically two-dimensional 2-D free-standing crystals exhibit properties that differ from those of
their three-dimensional, 3-D counterparts.
[1]
Currently, however, there are relatively few such
atomically layered solids.
[2, 3, 4, 5]
Herein we report on 2-D nanosheets, comprised of a few Ti
3
C
2
layers and conical scrolls produced by the room temperature exfoliation of Ti
3
AlC
2
in
hydrofluoric acid. The large elastic moduli predicted by ab initio simulation, and the possibility
of varying their surface chemistries (herein they are terminated by hydroxyl and/or fluorine
groups) render these nanosheets attractive as polymer composite fillers. Theory also predicts that
their band gap can be tuned by varying the surface terminations. The good conductivity and
ductility of the treated powders suggest uses in Li-ion batteries, pseudocapacitors and other
electronic applications. Since Ti
3
AlC
2
is a member of a 60+ group of layered ternary carbides and
nitrides known as the MAX phases, this discovery opens a door to the synthesis of a large
number of other 2-D crystals.
Submitted to
2
Arguably the most studied freestanding 2-D material is graphene, which was produced by
mechanical exfoliation, into single-layers in 2004
[1]
. Some other layered materials, such as
hexagonal BN,
[2]
transition metal oxides and hydroxides,
[4]
including clays,
[3]
have also been ex-
foliated into 2-D sheets. Interestingly, exfoliated MoS
2
single layers were reported as early as in
1986.
[5]
Graphene is finding its way to applications ranging from supercapacitor electrodes
[6]
to
reinforcement in composites.
[7]
Although graphene has attracted more attention than all other 2-D
materials together, its simple chemistry and the weak van der Waals bonding between layers in
multi-layer structures limit its use. Complex layered structures that contain more than one
element may offer new properties because they provide a larger number of compositional
variables that can be tuned for achieving specific properties. Currently, the number of non-oxide
materials that have been exfoliated is limited to two fairly small groups, viz. hexagonal, van der
Waals bonded structures (e.g. graphene and BN) and layered metal chalcogenides (e.g. MoS
2
,
WS
2
, etc.).
[8]
It is well established that the ternary carbides and nitrides with a M
n+1
AX
n
chemistry - where n =
1, 2, or 3, M is an early transition metal, A is an A-group (mostly groups 13 and 14) element, and
X is C and/or N form laminated structures with anisotropic properties.
[9][10]
These, so called
MAX, phases are layered hexagonal (space group P6
3
/mmc), with two formula units per unit cell
(Figure 1a). Near close-packed M-layers are interleaved with pure A-group element layers, with
the X-atoms filling the octahedral sites between the former. One of the most widely studied and
promising members of this family is Ti
3
AlC
2
.
[11, 12]
(Fig. 1a). Over 60 MAX phases are currently
known to exist.
[9]
The M
n+1
X
n
layers are chemically quite stable. By comparison, because the A-group atoms are
relatively weakly bound, they are the most reactive species. For example, heating Ti
3
SiC
2
in a C-
rich atmosphere results in the loss of Si and the formation of TiC
x
.
[13]
When the same compound
Submitted to
3
is placed in molten cryolite,
[14]
or molten Al,
[15]
essentially the same reaction occurs: the Si
escapes and a TiC
x
forms. In the case of cryolite, the vacancies that form lead to the formation of
a partially ordered cubic TiC
0.67
. In both cases, the high temperatures led to a structural
transformation from a hexagonal to a cubic lattice and a partial loss of layering. In some cases,
such as Ti
2
-group element and
TiC
x
formation.
[16]
Removing of both the M and A elements from MAX structure by high
temperature chlorination results in a porous carbon known as carbide derived carbon with useful
and unique properties.
[17, 18]
Mechanical deformation of the MAX phases which is mediated by basal dislocations and is
quite anisotropic - can lead to partial delamination and formation of lamellas with thicknesses
that range from tens to hundreds of nanometers.
[19]
However, none of MAX phases has ever been
exfoliated into a few nanometer thick, crystalline layers reminiscent of graphene. Furthermore, as
far as we are aware, there are no reports on the selective room, or even moderate, temperature
liquid or gas phase extraction - of the A-group layers from the MAX phases and/or their
exfoliation. Herein we report the extraction of the Al from Ti
3
AlC
2
, and formation of a new of 2-
-like
morphology.
Based on the results presented below it is reasonable to conclude that the following simplified
reactions occur when Ti
3
AlC
2
is immersed in HF:
Ti
3
AlC
2
+ 3HF = AlF
3
+ 3/2 H
2
+ Ti
3
C
2
(1)
Ti
3
C
2
+ 2H
2
O = Ti
3
C
2
(OH)
2
+ H
2
(2)
Ti
3
C
2
+ 2HF = Ti
3
C
2
F
2
+ H
2
(3)
Reaction (1) is essential and is followed by reaction (2) and/or (3). In the remainder of this paper
we present evidence for the aforementioned reactions and that they result in the exfoliation of 2-
Submitted to
4
D Ti
3
C
2
layers, with OH and/or F surface groups (Figs. 1b and c). Reactions (2) and (3) are
simplified in that they assume the terminations are OH or F, respectively, when in fact they most
probably are a combination of both.
XRD spectra of the initial Ti
2
AlC-TiC mixture after heating to 1350°C for 2 h resulted in
peaks that corresponded mainly to Ti
3
AlC
2
(bottom curve in Fig. 2a). When the Ti
3
AlC
2
powders
were placed into the HF solution, bubbles, presumed to be H
2
, were observed suggesting a
chemical reaction. Ultrasonication of the reaction products in methanol for 300 s resulted in
significant weakening of the peaks and the appearance of an amorphous broad band around 24°
(top spectrum in Fig. 2a). In other words, exfoliation leads to a loss of diffraction signal in the
out-of-plane direction, and the non-planar shape of the nanosheets results in broadening of peaks
corresponding to in-plane diffraction. When the same powders were cold pressed at 1 GPa, into
free-standing, 300 µm thick and 25 mm diameter discs (Fig. 2e), their XRD showed that most of
the non-basal plane peaks of Ti
3
AlC
2
- most notably the most intense peak at - disappear
(middle curve in Fig. 2a). On the other hand, the (00l) peaks, such as the (002), (004) and (0010),
broadened, lost intensity, and shifted to lower angles compared to their location before treatment.
Using the Scherrer formula
[20]
the average particle dimension in the [000l] direction after
treatment is estimated to be 11±3nm, which corresponds to roughly ten Ti
3
C
2
(OH)
2
layers. To
identify the peaks we simulated XRD patterns of hydroxylated, viz. Ti
3
C
2
(OH)
2
, (red curve in
center of Fig. 2a) and fluorinated, Ti
3
C
2
F
2
, structures (gold curve in center of Fig. 2a). Clearly,
both were in good agreement with the XRD patterns of the pressed sample (purple curve in Fig.
2a), the agreement was better with the former. The disappearance of the most intense diffraction
peak of Ti
3
AlC
2
at 39° and the good agreement between the simulated XRD spectra for
Ti
3
C
2
(OH)
2
and the experimental results provides strong evidence of the formation of the latter.
The presence of OH groups after treatment was confirmed by FTIR.
