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Local chemical bonding and structural properties in Ti 3 AlC 2 MAX phase and Ti 3 C 2 T x MXene probed by Ti 1s x-ray absorption spectroscopy


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The chemical bonding within the transition-metal carbide materials MAX phase Ti 3 AlC 2 and MXene Ti 3 C 2 T x is investigated by x-ray absorption near-edge structure (XANES) and extended x-ray absorption fine-structure (EXAFS) spectroscopies. MAX phases are inherently nanolaminated materials that consist of alternating layers of M n+1 X n and monolayers of an A-element from the IIIA or IVA group in the Periodic Table, where M is a transition metal and X is either carbon or nitrogen. Replacing the A-element with surface termination species T x will separate the M n+1 X n-layers forming two-dimensional (2D) flakes of M n+1 X n T x. For Ti 3 C 2 T x the T x corresponds to fluorine (F) and oxygen (O) covering both sides of every single 2D M n+1 X n-flake. The Ti K-edge (1s) XANES of both Ti 3 AlC 2 and Ti 3 C 2 T x exhibit characteristic preedge absorption regions of C 2p-Ti 3d hybridization with clear crystal-field splitting while the main-edge absorption features originate from the Ti 1s → 4p excitation, where only the latter shows sensitivity toward the fcc-site occupation of the termination species. The coordination numbers obtained from EXAFS show that Ti 3 AlC 2 and Ti 3 C 2 T x are highly anisotropic with a strong in-plane contribution for Ti and with a dynamic out-of-plane contribution from the Al monolayers and termination species, respectively. As shown in the temperature-dependent measurements, the O contribution shifts to shorter bond length while the F diminishes as the temperature is raised from room temperature up to 750°C.
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Local chemical bonding and structural properties in Ti3AlC2MAX phase and Ti3C2TxMXene
probed by Ti 1sx-ray absorption spectroscopy
Martin Magnuson *and Lars-Åke Näslund
Department of Physics, Chemistry and Biology (IFM), Linköping University, SE-581 83 Linköping, Sweden
(Received 30 June 2020; accepted 10 September 2020; published 29 September 2020)
The chemical bonding within the transition-metal carbide materials MAX phase Ti3AlC2and MXene Ti3C2Tx
is investigated by x-ray absorption near-edge structure (XANES) and extended x-ray absorption fine-structure
(EXAFS) spectroscopies. MAX phases are inherently nanolaminated materials that consist of alternating layers
of Mn+1Xnand monolayers of an A-element from the IIIA or IVA group in the Periodic Table, where Mis a
transition metal and Xis either carbon or nitrogen. Replacing the A-element with surface termination species
Txwill separate the Mn+1Xn-layers forming two-dimensional (2D) flakes of Mn+1XnTx.ForTi
3C2Txthe Tx
corresponds to fluorine (F) and oxygen (O) covering both sides of every single 2D Mn+1Xn-flake. The Ti K-edge
(1s) XANES of both Ti3AlC2and Ti3C2Txexhibit characteristic preedge absorption regions of C 2p-Ti 3d
hybridization with clear crystal-field splitting while the main-edge absorption features originate from the Ti
1s4pexcitation, where only the latter shows sensitivity toward the fcc-site occupation of the termination
species. The coordination numbers obtained from EXAFS show that Ti3AlC2and Ti3C2Txare highly anisotropic
with a strong in-plane contribution for Ti and with a dynamic out-of-plane contribution from the Al monolayers
and termination species, respectively. As shown in the temperature-dependent measurements, the O contribution
shifts to shorter bond length while the F diminishes as the temperature is raised from room temperature up to
750 °C.
DOI: 10.1103/PhysRevResearch.2.033516
Despite the large interest in graphene [1], which lacks
a natural band gap, it has been difficult to artificially pro-
duce graphene-based materials with suitable band gaps. This
has encouraged researchers to explore other two-dimensional
(2D) materials such as hexagonal boron nitride (h-BN),
molybdenum disulfide (MoS2), tungsten disulfide (WS2), and
MXenes (Mn+1XnTx). The last example is also the latest
2D material, developed in the past decade, and it con-
sists of a family of 2D transition-metal carbides denoted
Mn+1XnTx(n=1,2,3), where Mis a transition metal, Xis
either carbon or nitrogen, and Txdenotes surface termination
species [24]. The layered structures contain more than one
element, thus they offer properties that may be useful for tran-
sistors and spintronics [5], 2D-based electronics and screens
[6], in addition to energy storage systems such as superca-
pacitors [7], Li-ion batteries [8], fuel and solar cells [9], as
well as transparent conductive electrodes [10] and composite
materials with high strength [11].
The parent precursor compounds of MXenes are in-
herently nanolaminated materials known as MAX phases
*Corresponding author:
Published by the American Physical Society under the terms of the
Creative Commons Attribution 4.0 International license. Further
distribution of this work must maintain attribution to the author(s)
and the published article’s title, journal citation, and DOI. Funded
by Bibsam.
(Mn+1AXn,n=1,2,3) [12] (space group P63/mmc), where
Ais a p-element that usually belongs to groups IIIA or IVA
in the Periodic Table. These phases contain more than 150
variants [13], including Ti3AlC2, which is a precursor for
the Ti3C2TxMXene. To make MXene from Mn+1AXn,the
weakly bound A-layers are etched away and replaced by sur-
face termination groups (Tx) in the exfoliation process [2]. The
delamination results in weakly bound stacks of 2D sheets with
Generally, MXenes consist of a core of a few atoms thick
2D Mn+1Xnconductive carbide layer that is crystalline in the
basal plane and a transition-metal surface that can be func-
tionalized for different material properties by changing the
chemistry of the termination species. Layered structures like
MXenes contain more than one element and can therefore of-
fer better variations of physical properties than pure materials,
such as graphene, since they can provide a larger number of
compositional variables that can be tuned for specific prop-
erties. Figure 1shows schematic side views of the Ti3AlC2,
Ti3C2Tx, and TiC structures where the blue and black spheres
are the Ti and C atoms, respectively, with strong covalent
bonds in the conductive carbide core layer. The stacking of
the Ti and C atoms forms three monolayers of Ti and two
monolayers of C in an alternated sequence. In Ti3AlC2the
Ti3C2layers are alternated with monolayers of Al, highlighted
in Fig. 1as yellow spheres, while Ti3C2Txshows purple and
red spheres on both sides of the transition-metal carbide that
are the F and O atoms, respectively, terminating the surfaces.
Two alternatives for the F and O atoms to coordinate on the
Ti3C2surfaces are the threefold hollow face-centered-cubic
(fcc) sites and the threefold hollow hexagonal-close-packed
2643-1564/2020/2(3)/033516(10) 033516-1 Published by the American Physical Society
FIG. 1. Structure of an M3C2TxMXene layer with various ter-
mination sites of Tx:F (purple) and O (red). The F atoms
are adsorbed in a threefold hollow fcc site (A-site). In this model
structure the O atoms are in a bridge site between two Ti and in a
three-fold hollow hcp site (B-site). Blue and black spheres are Ti
and C atoms, respectively, with strong covalent bonds in the M3X2
conductive carbide core layer.
(hcp) sites [1417]; an fcc site is formed by three surface
Ti atoms in a triangular formation where the center of the
triangle is above a Ti atom in the second (middle) Ti mono-
layer, while an hcp site is formed by three surface Ti atoms
in a triangular formation where the center of the triangle is
above a C atom in the adjacent C monolayer. Other alternative
coordination sites are on top of the surface Ti atoms or in
bridge sites between two Ti atoms [14]. The fcc site (also
called A-site) as the preferred site for the termination species
on Ti3C2Txhas been confirmed experimentally using high-
resolution transmission electron microscopy (HRTEM) and
x-ray photoelectron spectroscopy (XPS) [18]. The combined
HRTEM/XPS study found that F only occupies the fcc sites in
competition with O and that O also occupies other sites, e.g.,
bridge sites or on-top sites, but not the hcp sites (also called
Bsites). The study found no terminating species other than F
and O and that F desorbs at elevated temperatures (>550 °C).
Despite vast interest in MXenes in general, and in Ti3C2Tx
in particular, there is little known experimentally about the
bonding between the transition metals and the terminating
species, Tx. Previous work of MXenes’ electronic structure
has mainly been based on ground-state density functional
theory (DFT) calculations at 0 K [1417,19]. Many of the
theoretical investigations find no obstacles regarding replac-
ing the inherently formed termination species with others in
the pursuit of tailoring the properties for specific applications.
Yet there is no indisputably experimental evidence showing
that the inherently formed termination species can be replaced
(retermination) [20]. The different theoretical and experimen-
tal results and experiences show how important it is to fully
understand the bonding conditions at the surfaces of the MX-
In this work, we will elucidate the local structural prop-
erties and interactions around the Ti atoms in Ti3AlC2and
Ti3C2Txthrough synchrotron radiation-based Ti K-edge (1s)
x-ray absorption spectroscopy, including both x-ray absorp-
tion near-edge structure (XANES) and extended x-ray absorp-
tion fine structure (EXAFS). XANES probes the unoccupied
density of states of the absorbing atoms and is therefore an
ideal technique for determining the chemical surroundings
and local bonding structure around the transition metal. The 1s
electron transition can, as a consequence of the selection rule,
only occur to molecular orbitals with p-character (electric
dipole transition) and d-character (electric quadrupole tran-
sition), although the probability of the 1s3dtransitions is
about one-thousandth that of the 1snp transitions [21,22].
EXAFS provides information about the coordination num-
bers, atomic distances, and amount of atomic displacements
and disorder around the probed element [23].
Ti 1sXANES and EXAFS spectra of Ti3AlC2and Ti3C2Tx
have been presented previously [2426]. Yet a detailed anal-
ysis of the spectra remains to be performed. Instead of only
comparing the Ti 1sXANES and EXAFS spectra of different
compounds for similarities (or not) or a crude estimate of
the Ti oxidation state, as in previous XANES and/or EXAFS
studies of MAX-phases and MXenes [2430], our aim in the
present work is to learn more about the local bonding around
the probed Ti atoms in both Ti3AlC2and Ti3C2Txthrough
distinguished Ti 1sXAS features.
The Ti3C2Txsamples were fabricated as freestanding foils
through wet etching, which leads to inherent F and O termi-
nations [18]. Through heat treatments, the fcc site coordinated
F will desorb and be replaced by O, which prefers the fcc
site when it is available [18]. The change of the fcc coordi-
nation will induce modifications in the XANES and EXAFS
spectra, thus it will reveal local information on the bonding
situation between the transition-metal atoms and termination
species. Hence, the study demonstrates how x-ray absorption
spectroscopy can be used to probe the MXene surfaces to shed
more light on the local chemical interaction between F, O,
and Ti atoms, which is relevant knowledge when designing
MXenes for sought-after material properties.
A. Sample preparation
Powders of Ti3AlC2were produced starting with a mixture
of TiC (Alfa Aesar, 98+%),Ti(AlfaAesar,98+%), and Al
(Alfa Aesar, 98+%) in 1:1:2 molar ratios. The mixture was
processed in a mortar with a pestle for 5 min and thereafter
inserted in an alumina tube furnace. With a continuous stream
of Ar gas, the furnace was heated at a rate of 5 Cmin
up to 1450 °C and held for 280 min before cooling down to
room temperature. The resulting material is a lightly sintered
Ti3AlC2sample, which is then crushed to a powder with
particle size <60 μm using a mortar and pestle. A few mg
of the Ti3AlC2powder was mixed with polyethylene powder
(Aldrich, 40–48 μm particle size) and thereafter cold-pressed
into an 500-µm-thick pellet for the x-ray absorption spec-
To convert the Ti3AlC2to Ti3C2Txflakes, half a gram of
Ti3AlC2powder was added to a premixed 10 mL aqueous
solution of 12 M HCl (Fisher, technical grade) and 2.3 M
LiF (Alfa Aesar, 98+%) in a Teflon bottle. Prior to adding
the MAX powder to the HCl/LiF(aq) solution, the latter was
placed in an ice bath to avoid the initial overheating that
otherwise can be a consequence of the exothermic reaction
when the MAX power is added. After 0.5 h in the ice bath,
the bottle was placed on a magnetic stirrer hot plate in an oil
bath and held at 35 °C for 24 h. The mixture was thereafter
washed, first through three cycles using 40 mL of 1 M HCl(aq)
and thereafter three cycles using 40 mL of 1 M LiCl (Alfa
Aesar, 98+%). Then, the mixture was washed through cycles
of 40 mL of deionized water until the supernatant reached
a pH of approximately 6. In the end, 45 mL of deionized
water was added, which was de-aerated by bubbling N2gas
through it and sonicated using an ultrasonic bath for 1 h. The
resulting suspension was centrifuged for 1 h at 3500 rpm,
which removed larger particles. The supernatant produced had
3C2Txconcentration of 1 mg mL1. To make freestand-
ing films, 20 mL of the suspension was filtered through a
nanopolypropylene membrane (3501 Coated PP, 0.064 μm
pore size, Celgard, USA). The obtained Ti3C2Txfoil is de-
scribed in detail in Ref. [7].
Moreover, to observe XANES and EXAFS features orig-
inating from termination species, the surfaces must be free
from oxidized material, mainly TiO2[31]. The newly made
Ti3C2Txfoils were therefore stored in an argon (Ar) atmo-
sphere and mounted on the sample holder in a glove-bag filled
with nitrogen gas (N2). A continuous flow of N2protected
the sample from oxidation during measurement. Hence, the
obtained XANES and EXAFS spectra of the Ti3C2Txfoils
have no detectable contribution from TiO2impurities. The in-
significant amount of TiO2impurities and carbon-containing
contamination in the Ti3C2Txsamples was confirmed through
XPS. The XPS further showed a small amount of adsorbed
Cl that desorbed completely at a moderate heat treatment.
In addition, no indication of OH termination was observed,
which agrees with the combined HRTEM/XPS study [18].
The Ti3AlC2(and TiC), on the other hand, showed small
amounts of TiO2impurities in the XPS spectra. Contribution
from TiO2impurities in the Ti 1sXANES and EXAFS spec-
tra of the Ti3AlC2(and TiC), therefore, cannot be excluded.
Nevertheless, the TiO2contribution was shown to be at a
negligible level.
B. XANES and EXAFS analysis
The XANES and EXAFS spectra were measured at the Ti
1sedge using an Si111 crystal in the monochromator at the
wiggler beamline BALDER on the 3 GeV electron storage
ring at MAX IV in Lund, Sweden. The x-ray absorption of the
Ti3C2TxMXene, the Ti3AlC2MAX phase, a TiC reference,
and a Ti metal reference foil were monitored in transmission
mode with ionization chambers (Io: 200 mbar N2and He;
I1: 2 bar N2). The Ti3C2Txsample was positioned in a water-
cooled gas cell (Linkam Scientific Instruments) with a low
N2flow and a heating element that enabled measurements
at different temperatures. The Ti 1sXANES spectra were
obtained at room temperature (RT) and at 250°C after 20 min
heat treatments at 250, 550, and 750 °C. The energy resolution
at the Ti 1sedge of the beamline monochromator was 1.0 eV
with 0.1 eV energy steps for XANES and 0.2 eV steps for
EXAFS. The photon energy scale was calibrated using the
first derivative of a Ti foil absorption spectrum, where the
first inflection point was set to 4.9660 keV. The obtained
spectra were normalized below the absorption edge before the
background intensity subtraction and thereafter normalized
above the absorption edge in the photon energy region of
5.045–5.145 keV. Self-absorption effects were found to be
negligible in the normal incidence geometry for the Ti3C2Tx
and the Ti foils. The spectra of the Ti3AlC2and TiC pellets
showed, on the other hand, some self-absorption effects even
at normal incidence, and they have therefore been corrected
using the simple function AI(ε)[1 BI(ε)]1, where Ais a
scaling factor, Bis the self-absorption compensation factor,
and I(ε)istheTi1sXANES spectrum of Ti3AlC2and TiC,
respectively. Aand Bare adjusted until the absorption features
in the photon energy region of 5.045–5.145 keV have a similar
appearance to the Ti 1sXANES spectrum of Ti3C2Tx.
The Ti-Ti, Ti-C, Ti-O, and Ti-F scattering paths obtained
from the effective scattering amplitudes (FEFF) [3234]were
included in the EXAFS fitting using the Visual Processing
in EXAFS Researches (VIPER) software package [35]. The
k2-weighted χEXAFS oscillations were extracted from the
raw absorption data, the average of 10 absorption spectra, after
removing known monochromator-induced glitches and subse-
quent atomic background subtraction and normalization. The
atomic distances (R), number of neighbors (N), Debye-Waller
factors (σ2, representing the amount of disorder), and the re-
duced χ2
ras the squared area of the residual, were determined
by fitting the back-Fourier-transform signal between k=0
and 12.
1originally obtained from the forward Fourier
transform within R=0–3.35 Å of the first coordination shell
using a Hanning window function [3234] with a many-body
factor of S02=0.8. The disorder and high-frequency ther-
mal vibration of the atoms depending on the temperature
was accounted for by an increasing Debye-Waller term σ2=
σ2stat +σ2vib consisting of a static and a vibrational part that
was proportional to the difference of the mean-square atomic
displacements [3638].
Figure 2shows the obtained high-resolution Ti 1sXANES
spectra of Ti-metal, TiC, Ti3AlC2, and Ti3C2Tx. The absorp-
tion spectra consist of two regions: preedge and main edge.
Features in the preedge region of K-edge XANES spectra of
3dtransition-metal compounds are assigned to electric dipole
(1snp) and quadrupole (1s3d) transitions. However,
the intensity contribution of the latter is minor because of
orbital symmetry restrictions [21,22,39], and preedge features
are therefore in most cases assigned to 1selectron excitation
into p-dhybridized orbitals, i.e., the transition into the p-
component of the molecular orbitals that have both p- and
d-character. The intensity of the preedge features depends
mainly on coordination, symmetry, and bond angles [40].
The preedge features in the spectrum of the Ti metal refer-
ence, peaks Aand Bat 4.967 and 4.971 keV, respectively, are
typical examples of 1sp-dhybridized molecular orbital
excitation near the Fermi energy (Ef)[41]. The sharp peak A
near the Fermi energy originates from Ti 1sexcitation into a
local Ti 3d-4pmixing orbital (internal orbital mixing in the
probed element), while peak Boriginates from Ti 1sexcita-
tion into hybridized 3d-4porbital where the 3dcontribution
Intensity [arb. units]
Photon Energy (keV)
4.95 4.96 4.97 4.98 4.99 5.00 5.01 5.02
Ti 1s XAS
250 °C
550 °C
750 °C
FIG. 2. Ti 1sXANES spectra of Ti metal foil, TiC, Ti3AlC2,and
3C2Txspectra are recorded at RT and at 250 °C after
20 min heat treatments at 250, 550, and 750 °C. Note that the red
spectrum of Ti3C2Txheat-treated at 250 °C covers the black spectrum
of nonheated Ti3C2Tx. The difference of the RT Ti3C2Txspectrum
and the spectra obtained after 250, 550, and 750 °C heat treatment
are shown at the bottom.
originates from neighboring Ti atoms [42,43]. The intensity
of the preedge features is, however, reduced because of the
sixfold coordination of the Ti atoms in the metal [41]. The
main-edge region consists of two broad features C(white line)
and Dthat correspond to 1s4pexcitations [43]. With the
Ti 1sXANES spectrum of the Ti metal reference, it is possible
to estimate the Efposition, which is located close to the onset
of peak Aat 4.9642 keV. The full width at half-maximum of
peak Aof the Ti metal reference (1.3 eV) is also an indication
of the good resolution at the Balder beamline.
The XANES spectra of TiC, Ti3AlC2, and Ti3C2Txdis-
play significantly different structures compared with the Ti
metal reference spectrum (see Fig. 2). The preedge region
consists of two features, which are highlighted in Fig. 3.
The TiC and Ti3AlC2spectra show a shoulder at 4.9679 keV
and a peak at 4.9710 and 4.9709 keV, respectively, while
the Ti3C2Txspectrum shows a shoulder at 4.9689 keV and
Intensity [arb. units]
Photon Energy (keV)
4.965 4.970 4.975 4.980
Ti 1s XAS
750 °C
FIG. 3. Curve fitting of the Ti 1sXANES preedge region for TiC,
Ti3AlC2, room-temperature (RT) Ti3C2Tx,andTi
3C2Txafter 750 °C
heat treatment. The dotted lines represent the Ti 1sexcitations into
the t2gand egorbitals, and the dashed lines represent the Ti 1s
4pexcitations (black) and the accumulated intensity from all Ti 1s
excitations (red).
a peak at 4.9711 keV. A basic density-of-states calculation
obtained from a simple Ti3C2Txmodel with Txbeing F on
the fcc sites and O on the bridge sites shows two C 2p
peaks located at 4.2 and 7.1 eV above the Efand two Ti
3dpeaks located at 4.0 and 6.9 eV above Ef. The orbital
mixing provides the C 2p-Ti 3dhybridization and would cor-
respond to 4.968 and 4.971 keV, respectively, in the Ti 1s
XANES, which is close to the two preedge features obtained
in the experimental spectra. Hence, the Ti 1sXANES preedge
features of TiC, Ti3AlC2, and Ti3C2Txare assigned to Ti
1sC2pTi 3dhybridized molecular orbital excitation.
The Ti3C2Txspectrum shows lower intensity in the preedge
region compared to TiC and Ti3AlC2, which suggests that the
Ti 1sC2pTi 3dhybridized molecular orbital excitation
is reduced for Ti3C2Txcompared with both TiC and Ti3AlC2.
The energy difference between the two C 2p-Ti 3dpeaks
corresponds to the crystal-field splitting of the Ti 3dstates
into the t2gand egorbitals [43], thus it can be determined
experimentally to be 3.1, 2.9, and 2.2 eV for TiC, Ti3AlC2,
and Ti3C2Tx, respectively. The absence of a sharp peak near
the Efin the Ti 1sXANES of TiC, Ti3AlC2, and Ti3C2Tx
indicates that there is no local unoccupied Ti 3d-4phybridized
orbital available for electron excitation.
The main-edge region shows two peaks, Cand Din Fig. 2,
which because of the 2D nature are relatively sharp for
Ti3C2Tx. The peaks originate from Ti 1s4pexcitations
[43]. In addition, a closer look at the rising edge, i.e., the steep
increase in intensity at the main edge, reveals a shoulder at
4.9780 keV. The shoulder is more noticeable in the Ti3C2Tx
spectrum, which is because of the high-energy shift of the first
main-edge peak C; the peak Cis at 4.9850 and 4.9847 keV for
the TiC and Ti3AlC2spectra, respectively, while the Ti3C2Tx
spectrum has the peak Cat 4.9861 keV. The position of peak
Cfor the TiC and Ti3AlC2spectra is almost the same, which
suggests that the interaction between the Al layers and the
Ti3C2layers in Ti3AlC2is very weak. The 1.4 eV shift of
the Cpeaks between the Ti3AlC2and Ti3C2Txspectra is a
consequence of the replacement of the weak interacting Al
layers with the stronger interacting termination species Tx,
where the F and O atoms attract charge from the Ti atoms.
In Fig. 2there are also Ti 1sXANES spectra of the Ti3C2Tx
sample after it has been heated to 250, 550, and 750 °C, re-
spectively. After each heat treatment (to 550 and 750 °C), the
sample was brought back to 250 °C before XANES-spectrum
recording. As expected, there are no significant changes in the
spectra after the heat treatments to 250 and 550 °C, because
temperatures above 550 °C are needed to alter the termina-
tion of the Ti3C2Txsurfaces [18]. Included in Fig. 2are also
difference spectra that highlight the influence from the heat
treatments of the Ti3C2Txsample. The small deviations from
the zero line are inconsiderable for the temperatures 250 and
550 °C, although there is an indication that 550 °C is the
temperature threshold onset at which to introduce changes
in the Txcoordination, which are then reflected in the Ti
1sXANES spectra. A heat treatment to 750 °C (and sub-
sequent cooling to 250 °C) provides, on the other hand, a
high-energy shift of the peaks in the main-edge region of 0.5
eV, which has the effect that the preedge region appears to
show a slightly reduced intensity. The difference spectrum
for the 750 °C spectrum shows the characteristic variations
common for a main-edge energy shift and not an intensity
redistribution. It is interesting to note that the energy positions
of the features in the preedge region are almost not affected
by the heat treatment; see Fig. 3.Thet2gpeak has the same
position while the egpeak has shifted 0.1 eV, thus widening
the crystal-field splitting slightly. The fact that the preedge
region, which mainly originates from Ti 1sexcitations into
the C 2p-Ti 3dhybridized molecular orbitals in the Ti3C2
layer, is almost unaffected by the heat treatments is supported
by the previous combined HRTEM/XPS study, which found
that while removing F from a Ti3C2Txsample through a heat
treatment, the C 1sXPS carbide peak remained unaffected
[18]. The fact that the crystal-field splitting widens to some
extent suggests a stronger interaction between the O and the
fcc-site compared to F. The main-edge features that originate
from the Ti 1s4pexcitation show, on the other hand, a
higher sensitivity toward the fcc-site occupation.
The XANES spectra of Ti3C2Txshow some similarities
to the XANES spectrum of TiO2[2427,41,43,44]. A direct
comparison shows that the preedge region of Ti3C2Txhas
more intensity and a smaller crystal-field splitting. The ab-
sorption rising edge of the Ti3C2Txspectrum is shifted about
3 eV while the absorption energy shift at peak Cis about
1 eV. A larger difference between the Ti3C2Txand the TiO2
XANES spectra is the shape of peak Dwhere the TiO2shows
strong peak intensity at 5.0035 keV that is absent in the
Ti3C2Txspectrum before the heat treatment, hence there is no
detectable contribution from TiO2impurities in the Ti3C2Tx
sample. The Ti 1sXANES spectra of the 750 °C heat treated
Ti3C2Txshow a trend of changes—slightly larger crystal-field
splitting, 0.5 eV shift of the rising edge and the Cpeak, and
the intensity shift in the Dpeak inducing a shoulder at 5.0035
keV—that suggest a stronger Ti-O interaction in heat treated
Ti3C2Txcompared to the nontreated Ti3C2Tx. Hence, the Ti 1s
XANES supports the observation [18] that F occupies only the
fcc sites and that a heat treatment up to 750 °C removes F and
makes the fcc sites available for O where the Ti-O interaction
becomes stronger. (Experiments with and without the low N2
flow in the Linkam gas cell ensured that no oxidation of the
Ti3C2Txsample occurred in the presented work.)
Figure 4shows the EXAFS structure factor oscillations
of Ti3AlC2and Ti3C2Txin comparison with the Ti metal
reference, obtained from raw data that have not been phase-
shifted. The structure factors χ, which are displayed as a
function of the wave vector k,werek2-weighted to highlight
the higher k-region, where k=¯h1[2m(EEo)] is the wave
vector of the excited electron in the x-ray absorption pro-
cess. The frequency of the oscillations and the intensity of
the EXAFS signal are directly related to the atomic distance
(R) and the number of nearest neighbors (N), respectively; a
higher frequency of the oscillations implies extended Rwhile
an enlarged amplitude implies increased N.
Starting with Ti metal at the top of Fig. 4, we observe the
main oscillations at 4.18, 5.55, and 6.80 Å1where the middle
one is caused by the Ti-Ti in-plane scattering that corresponds
to the distance for the a-axis in the hexagonal crystal structure.
For k-values above 12 Å1, the oscillations are damped out
and simultaneously the noise increases.
For Ti3AlC2and Ti3C2Tx, shown in the middle and bot-
tom of Fig. 4, the main sharp oscillations occur in the
1k-space region. There are also peaks at k=4.63
and 5.83 Å1, i.e., between the main Ti-Ti peaks, that only
appear as weak shoulders in the Ti metal. The positions of the
three main Ti-Ti peaks in Ti3AlC2and Ti3C2Tx(3.93, 5.28,
and 6.50 Å1) are shifted 0.25 to 0.30 Å1in compari-
son to Ti metal (4.18, 5.55, and 6.80 Å1) and similar for
TiC [45,46]. The small features at low-kvalues at 1.58 and
1for the Ti3C2Txare associated with superimposed
oscillations from Ti-O/F scattering.
While the heat treatments do not cause any k-shifts, the
intensities of the oscillations decrease with increasing tem-
perature, except for the oscillation at k=4.63 Å1.The
k2 χ(k) [Å-2]
k (Å-1)
250 °C
550 °C
750 °C
FIG. 4. k2-weighted EXAFS, k2χ(k), as a function of the photo-
electron wave number kof the Ti metal foil, Ti3AlC2,andTi
sample. The Ti3C2Txsamples are recorded at RT and at 250 °C after
20 min heat treatments at 250, 550, and 750 °C. The horizontal arrow
at the top shows the k-window for the most pronounced oscillations,
and the vertical arrows indicate changes in the peak intensities.
oscillation intensity of the double peak at 7.55–7.95 Å1also
decreases with increasing temperature. More interestingly, the
small features at 1.58 and 2.4–2.
1are also affected by
the heat treatment. To analyze the detailed local structure
and atomic distances in the films, Fourier transforms of the
EXAFS data were performed.
Figure 5shows the magnitude of the Fourier transform
obtained from the k2-weighted EXAFS oscillations χ(k)in
Fig. 4by the standard EXAFS procedure [34] related to the
radial distribution function. The horizontal arrow at the top
of Fig. 4indicates the applied k-window. Table Ishows the
final results of the EXAFS fitting using the FEFF scattering
paths of Ti3AlC2and Ti3C2Txand hcp Ti metal as structure
model systems. The obtained radius values are in comparison
to atomic distances determined for lattice parameters from
x-ray diffraction (XRD) in the literature, listed in parenthe-
ses in Table I. The initial crystal structure in the modelling
assumes a Ti3C2F1O2composition in line with previous quan-
titative core-level XPS results [18]. However, the obtained
atomic distances have sources of errors such as photon energy
calibration and dispersion, statistical noise, and inaccuracies
in the ab initio calculations of scattering paths using FEFF.
FIG. 5. Fourier transform obtained from the k2-weighted EX-
AFS oscillations χ(k) in Fig. 4of Ti metal foil, Ti3AlC2,and
The corresponding errors in XRD are in the same order of
magnitude as for EXAFS.
In Ti metal with hcp structure, the main peak consists of
the in-plane Ti-Ti scattering (N=6) at 2.842 Å and (N=6)
of 2.966 Å, where the latter corresponds to the a-axis of
the crystal. For comparison, the peak that consists of the
out-of-plane Ti-Ti scattering path of the c-axis is located at
4.675 Å as a weak feature. The corresponding values obtained
from XRD measurements are 2.897, 2.950, and 4.686 Å,
respectively [47]. The more intense peak in Ti metal ob-
served at 4.7 Å in Fig. 4is because of the superposition
of the three longer direct Ti-Ti scattering paths with atomic
distances 5.079–5.110 Å in the higher-order coordination
For Ti3AlC2, shown in the middle in Fig. 5, there is a slight
shift of the main Ti-Ti peak to larger atomic distance, and a
new peak corresponding to Ti-C bonding appears at 1.66 Å.
In addition, there is a prominent peak at 3.8 Å caused by
Ti-Ti and Ti-Al scattering from the central Ti layer.
The main peak of the Ti3C2Tx, shown at the bottom in
Fig. 5, is dominated (N=4.982) by the in-plane Ti-Ti scat-
tering at 3.016 Å; the in-plane Ti-Ti scattering corresponds to
the a-axis unit-cell edge. The weaker out-of-plane scattering
at 3.072 Å shows a significantly lower intensity (N=1.312),
TABLE I. Structural parameters for Ti3AlC2and Ti3C2Txin comparison to Ti metal reference obtained from fitting of calculated scattering
paths in the first coordination shell. Nis the coordination number, Ris the atomic distance (in Å) for the Ti-Ti and Ti-C scattering paths,
respectively, σis the corresponding Debye-Waller factor representing the amount of atomic displacement and disorder, reduced χ2
ris the
squared area of the residual, Nind is the number of independent points, Pis the number of fitting parameters, and vis the degrees of freedom.
Atomic distances obtained from lattice parameters in XRD are given in parentheses [39].
System Shell R(Å)aNbσ2)cStatistics
Ti metal (at RT) 6*Ti-Ti 2.842 5.535 0.0042 χ0.95
20 =8.44
(2.897) Nind =28,P=8
6*Ti-Ti: LPd-a2.966 2.237 0.0014 ν=Nind P=20
6*Ti-Ti: LPd-c4.675 2.005 0.0077
Ti3AlC2(at RT) 3 TiII -C 2.160 2.764 0.0025 χ0.95
(2.120) Nind =24,P=20
6TiI-C 2.220 1.560 0.0025 ν=Nind P=4
3TiII -Al 2.708 1.299 0.0015
3*Ti-Ti oope3.029 1.164 0.0011
6*Ti-Ti: LPd-a3.032 5.735 0.0066
Ti3C2Tx(at RT) 1 TiII -OA1.707 0.199 0.0045 χ0.95
10 =8.307
1TiII -Ob1.952 0.292 0.0030 Nind =36,P=26
1TiII -F 2.115 0.245 0.0029 ν=Nind P=10
6TiI-C 2.139 1.992 0.0012
3TiII -C 2.234 2.057 0.0079
6*Ti-Ti: LPd-a3.016 4.982 0.0056
3*Ti-Ti oope3.072 1.312 0.0021
aThe errors in the atomic distances are estimated to be ±0.01 Å.
bThe errors in the coordination numbers are estimated to be ±0.01.
cThe errors in the Debye-Waller factors are estimated to be ±0.001 Å2.
dLattice parameter.
which is not surprising for a 2D material. As also observed
in Fig. 5, the Ti-Ti distances of the in-plane and out-of-plane
contributions of Ti3C2Tx(3.016 and 3.072 Å) are located at
slightly different distances compared to the Ti-Ti atomic dis-
tances in Ti3AlC2(3.029 and 3.032 Å). The value of 3.016 Å
for the in-plane Ti-Ti distance of Ti3C2Txis somewhat smaller
than the calculated in-plane Ti-Ti atomic distance obtained
from the XRD lattice parameter (a=3.075 Å) [47].
The peak feature between 0 and 2 Å in the first coordina-
tion shell of Ti3C2Txis caused by superimposed Ti-O, Ti-F,
and Ti-C scattering, where the Ti-C interaction has two dif-
ferent contributions: TiIfor the inner Ti atoms that bond only
to C, and a second TiII contribution to the outer Ti atoms that
bond both to C and to the termination species. The Ti-C bond
length of the outer Ti atoms is 0.056 Å longer than that for
the inner Ti atoms. Interestingly, the three peaks in the higher
coordination shells between 3 and 6 Å also contain a large
contribution of long inclined single Ti-O and Ti-F scattering
paths from all surface Ti atoms to the termination species Tx,
in addition to Ti-Ti scattering and superimposed multiscatter-
ing paths, e.g., Ti-Ti-C, Ti-C-O, Ti-C-F, and Ti-F-Ti-F.
The Ti-Txscattering paths exhibit a significant tempera-
ture dependence as the data were measured at RT, 250, 550,
and 750 °C. During heating, the peaks become significantly
broadened and less intense as observed by the Debye-Waller
factor, which increases with the temperature, as a consequence
of more atomic vibrations. For the main Ti-Ti scattering, σ2
increases linearly from 0.0066 at room temperature to 0.0082
at 750 °C. A similar trend has been observed in other systems
usingEXAFS[48]. The broadening is also observed as in-
tensity increases between the peaks in the difference spectra.
In addition to the broadening of the peaks, the vibrational
behavior and peak broadening exhibit strong anisotropy of the
Ti–Ti bonds. Therefore, before each measurement the elevated
temperature was stabilized for 20 min and then decreased and
stabilized at 250 °C after which the spectra were recorded.
Figure 6shows the Fourier transform obtained from the
k2-weighted EXAFS oscillations χ(k)ofTi
3C2Txand the
effect of the heat treatment, where arrows indicate the general
trends. Difference spectra that highlight the temperature-
induced changes are shown in the bottom. In the low-radius
region of the first coordination shell of the probed Ti (between
1 and 2 Å), including the bonding of the O and F elements,
an intensity shift is observed corresponding to an 0.2 Å
shorter bond distance. In particular, the F-related intensity de-
creases indicating desorption of the F upon heating to 750 °C.
FT(k2 χ(k))
R (Å)
250 °C
550 °C
750 °C
FIG. 6. Temperature-dependent Fourier transforms obtained
from Ti 1sEXAFS oscillations χ(k)ofTi
3C2Tx. Difference spectra
(x3.5) are shown at the bottom.
Contrary to F, the intensity related to O-bonding slightly
increases and shifts to shorter bond lengths. This behavior
is consistent with previous temperature-dependent core-level
XPS results [18] that showed desorption of F and a change of
bond site for O in this temperature region.
The three peaks observed between 3 and 6 Å in Ti3C2Tx
also occur in EXAFS data of cubic TiC [45,46]. The first peak
at 3.5 Å is mainly ascribed to Ti-Ti and Ti-C scattering in
the second coordination shell. The second peak at 4.5–5 Å
mainly consists of Ti-C-Ti and Ti-Ti-O/F scattering, while the
third peak 5.5–6 Å contain many multiscattering paths such as
Ti-Ti-Ti (5.30 Å) and Ti-Ti-Ti-C (5.31 Å), etc. However, the
intensity of the three peaks in the higher coordination shells
also contains a significant contribution of long inclined single
Ti-O (3.62, 3.73, 4.28, 4.75, 4.79, and 5.04 Å) and Ti-F (3.76,
4.58 Å, 4.78, and 4.86 Å) scattering paths. As observed in the
difference spectra, the contributions of these paths decrease as
the temperature is increased. This is consistent with the fitting
results showing that the Ti-O bond length in the first coordi-
nation shell seems to shorten as the temperature increases to
750 °C.
Yet, after a decade of extensive research activities, there is
new knowledge to gain about MAX phases and MXenes. In
the present study, there are several interesting observations.
For example, through XANES we find that the C 2p-Ti 3d
hybridization is altered when the Ti3AlC2transforms into
Ti3C2Txleading to a smaller crystal-field splitting of the t2g
and egorbitals, which suggests slightly weaker Ti-C bonds
in Ti3C2Txcompared to Ti3AlC2. Another observation is the
1.4 eV energy shift of the main absorption edge for the
Ti3C2Txcompared to Ti3AlC2. A main-edge shift is often
an indication of a charge redistribution, and when the shift
is toward higher energies, the charge transfer is away from
the probed atoms. Hence, replacing the weak interacting Al
layers in Ti3AlC2with chemisorbed F and O in Ti3C2Tx
will withdraw charge from the Ti toward the termination
The additional main-edge energy shift caused by the heat
treatment, however, cannot be a consequence of a further
withdrawal of charge from the Ti atoms, because that would
contradict the previous temperature-programmed XPS study
[18]; the intensity at the high-binding-energy side of the
Ti 2p3/2XPS spectra decreases while F desorbs, indicating
that Ti in Ti3C2Txchemically reduces in a heat treatment.
In addition, the electronegativity of O is lower compared
to F (Pauling scale 3.44 and 3.98, respectively). The Ti 1s
XANES main-edge shift toward higher energy must there-
fore be caused by something else. It can, for example, be
a response to the stronger bonding of the O in the fcc site
compared to F that pushes the unoccupied Ti 4porbitals
toward higher energy. Regardless of the reason, the Ti 1s
XANES shows that orbitals with Ti 4pcharacter are sensitive
to changes of the termination species on the fcc sites.
From the EXAFS we learn that the in-plane Ti-Ti distances
decrease while the out-of-plane Ti-Ti distances increase when
the Ti3AlC2is converted into Ti3C2Tx. Concerning EXAFS,
it probes the local short-order atomic distances between
the absorber atom and the neighboring scatterers using the
constructive and destructive interference in the unoccupied
electronic structure. Since the EXAFS photoelectrons travel
much faster than the speed of the thermal motion of the
atoms, the obtained atom distances are an average of “snap-
shots” that in most cases correspond to the distance between
the average atomic positions as obtained with XRD and
neutron diffraction, which are considered to be long-order
probes. However, if adjacent atoms are moving in an an-
ticorrelated motion, the distance between them will be the
same as the atomic positions distance only when both atoms
are in-plane, while it will become larger when the atoms
are moving in the opposite direction out-of-plane. The atom
distances obtained from EXAFS will then be larger than if
they would be obtained from XRD and neutron diffraction.
The surface Ti atoms and the termination species F and O
are expected to move in an anticorrelated motion, and the
obtained TiII-OA,Ti
II-Ob, and TiII-F distances are proba-
bly larger than the true atomic positions. This discrepancy
between the average atom distances between the Txatoms
and the surface Ti atoms will become larger with increasing
Through a combination of XANES and EXAFS we have
investigated the MAX phase material Ti3AlC2and the MX-
ene material Ti3C2Tx, where the latter was examined before
and after a series of heat treatments. The preedge absorption
region of both Ti3AlC2and Ti3C2Txshows mainly Ti 1sex-
citations into two C 2p-Ti 3dhybridized molecular orbitals
corresponding to the Ti 3dt
2gand egorbitals. The crystal-field
splitting is determined to be 2.9 and 2.2 eV for Ti3AlC2
and Ti3C2Tx, respectively. The main-edge absorption features
originate from the Ti 1s4pexcitation and appear to be
sensitive toward the fcc-site occupation, which led to a 1.4 eV
shift when the Al layers in Ti3AlC2were replaced with the
termination species F and O. The local chemical bonding
structure and structural properties with atomic distances in
Ti3C2TxMXene show a significant temperature dependence.
Heat treatment up to 750 °C removed F and made the fcc sites
available for O occupation, which is manifested as a 0.5 eV
high-energy shift of the peaks in the main-edge absorption
EXAFS shows that the shortest inplane Ti-Ti atomic dis-
tances in Ti3AlC2and Ti3C2Txare 3.032 and 3.016 Å,
respectively, which are longer and shorter than the out-of-
plane distance of 3.029 Å in Ti3AlC2and the corresponding
atomic distance in Ti3C2Txof 3.072 Å. The TiI-C and TiII-C
bond lengths in Ti3AlC2are 2.220 and 2.160 Å, respectively,
while the TiI-C and TiII-C bond lengths in Ti3C2Txare 2.139
and 2.234 Å, respectively. Significant changes in the Ti-O/F
coordination are observed with increasing temperature in the
heat treatment. The TiII-O bond lengths becomes shorter be-
cause of a change in coordination from bridge to fcc facilitated
through the desorption of the F as the F contribution is found
to diminish when the temperature is raised from room tem-
perature up to 750 °C. The significant contribution of long
inclined single Ti-O and Ti-F scattering paths decreases as the
temperature increases.
We acknowledge MAX IV Laboratory for time on
Beamline Balder under Proposals 20190399 and 20191016.
Research conducted at MAX IV, a Swedish national user
facility, is supported by the Swedish Research Council under
contract 2018-07152, the Swedish Governmental Agency for
Innovation Systems under contract 2018-04969, and Formas
under contract 2019-02496. The computations were enabled
by resources provided by the Swedish National Infrastructure
for Computing (SNIC) at the National Supercomputer Centre
(NSC) partially funded by the Swedish Research Council
through Grant Agreement No. 2016-07213. We would also
like to thank the Swedish Research Council (VR) LiLi-NFM
Linnaeus Environment and Project Grant No. 621-2009-5258.
The research leading to these results has received funding
from the Swedish Government Strategic Research Area in
Materials Science on Functional Materials at Linköping Uni-
versity (Faculty Grant SFO-Mat-LiU No. 2009-00971). M.M.
acknowledges financial support from the Swedish Energy Re-
search (Grant No. 43606-1) and the Carl Tryggers Foundation
(CTS16:303, CTS14:310). Most importantly, we thank Dr.
Joseph Halim at Linköping University for preparing the sam-
ples and Kajsa Sigfridsson Clauss at the MAX IV Laboratory
for experimental support.
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Supplementary resource (1)

... However, by employing light with different photon energies it is possible to reduce or enhance structures that originate from separate hybridized orbital states because the UPS/XPS/PES intensity at different photon energies varies with the photoionization cross-sections and resonant phenomena [19,20]. Since the valence band structures are very sensitive to the local environment of the probed species, the photon energy dependent UPS/XPS/PES will provide valuable information regarding bonding conditions as well as structural arrangement of termination species in comparison to a recent Ti K-edge XAS study [21]. ...
... The structural models were 2D slabs that contained 17 Ti atoms, 8 C atoms, and 0-16 T x species (F, O, and OH). The distances between Ti and C in the 2D slab models were based on the bond length determination in a previous Ti K-edge extended xray absorption fine structure study [21]. The distances between the termination species and the surface Ti atoms in the Ti 3 C 2 -layer were optimized to provide acceptable agreement with the experimental UPS/XPS valence band spectra. ...
... Earlier studies of TiC have suggested that Ti 4p hybridizes with C 2s and 2p forming filled 1t 1u and 2t 1u levels, respectively [30], and it is therefore not a far-fetched suggestion that Ti 4p might be involved in bonding with O on the fccsite. The recent Ti K-edge XAS study found that the main-edge features that originate from the Ti 1s → 4p excitation showed sensitivity toward termination species on the fcc-site [21]. Hence, the strong O bonding toward the Ti atoms at the fcc-site involves the O 2p, Ti 3p, Ti 3d, and Ti 4p orbitals. ...
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MXenes are technologically interesting 2D materials that show potential in numerous applications. The properties of the MXenes depend at large extent on the selection of elements that build the 2D MX-layer. Another key parameter for tuning the attractive material properties is the species that terminate the surfaces of the MX-layers. Although being an important parameter, experimental studies on the bonding between the MX-layers and the termination species are few and thus an interesting subject of investigation. Here we show that the termination species fluorine (F) bonds to the Ti 3 C 2 -surface mainly through Ti 3 p —F 2 p hybridization and that oxygen (O) bonds through Ti 3 p —O 2 p hybridization with a significant contribution of Ti 3 d and Ti 4 p . The study further shows that the Ti 3 C 2 -surface is not only terminated by F and O on the threefold hollow face-centered-cubic site. A significant amount of O sits on a bridge site bonded to two Ti surface atoms on the Ti 3 C 2 -surface. In addition, the results provide no support for hydroxide (OH) termination on the Ti 3 C 2 -surface. On the contrary, the comparison of the valence band intensity distribution obtained through ultraviolet- and x-ray photoelectron spectroscopy with computed spectra by density of states, weighed by matrix elements and sensitivity factors, reveals that OH cannot be considered as an inherent termination species in Ti 3 C 2 T x . The results from this study have implications for correct modeling of the structure of MXenes and the corresponding materials properties. Especially in applications where surface composition and charge are important, such as supercapacitors, Li-ion batteries, electrocatalysis, and fuel- and solar cells, where intercalation processes are essential.
... The resultant TiO x /C nanosheets inherit the layered structure and high specific surface area of its Ti 3 C 2 MXene precursor, providing rich active sites for catalytic applications [31]. Since TiO x / C nanosheets are highly active catalysts to thermal decomposition of AP owing to the unoccupied 3d orbital [32] and multi oxidation states (Ti 2þ , Ti 3þ , and Ti 4þ ) of Ti element [33], the Ti 3 C 2 MXene is expected to serve as a high performance reactive catalyst for enhancing the thermal decomposition of AP and controlling the burning rate of ammonium perchlorate based CSP. Herein, Ti 3 C 2 MXene is explored as reactive catalysts for thermal decomposition of AP to realize the intentionally designed reactive combustion catalyst. ...
Achieving higher energy storage and controllable energy release processes is the major research goal in solid propellants. Combustion catalysts are applied to modify the energy release behavior of solid propellants but at the expense of lowering the total energy density of the propellants owing to their non-energetic feature. Here, with ammonium perchlorate (AP) based propellant as proof-of-concept material, we demonstrate a reactive combustion catalyst strategy based on the clever use of the novel structure of Ti3C2 MXene, including appropriate reactivity, high energy storage, and large specific surface area. It is found that the Ti3C2 MXene could first react with AP as fuel, releasing large amount of heat and in-situ generating TiOx/C nanosheets. The resulting TiOx/C nanosheets can further catalyze the thermal decomposition of AP by accelerating both electron and proton transfer during AP decomposition. As expected, the Ti3C2 MXene not only improves the pyrolysis reaction kinetics of AP based composites, but also increases the energy output of the propellants. The development of reactive catalysts with excellent catalytic activity and increased energy release provides a new strategy for the design of solid propellants.
MXenes are an interesting family of 2D materials that have the potential to meet challenges in many applications. A useful tool in the work of understanding the nature of the MXenes, as well as exploring their capabilities, is X-ray photoelectron spectroscopy. In analyzing XPS spectra it might be necessary to use curve fitting to extract valuable information. However, approaches toward the curve fitting procedure have been different in many studies and introductions of questionable assumptions, unverified feature assignments, and inconsistent curve fitting have led to contrasting conclusions from XPS analysis. It is therefore motivated to show curve fittings of F 1s, O 1s, Ti 2p, and C 1s XPS spectra obtained from high quality Ti3C2Tx that are based on fundamental knowledge applied step by step through the strategy of first principles thinking. With the use of first principles thinking the curve fittings and the subsequent analysis became more realistic compared to what have been presented in recent studies. The results of the curve fittings presented in this work are well founded and can be used as a model for future curve fittings of MXenes. The strategy of first principles thinking is advantageous in XPS curve fittings in general.
Continuous discoveries in the field of metallic conductive MXenes have shown their feasibility as electrode materials, but their employment remains impeded by low surface area and inhomogeneous edge terminations generated by hazardous HF etching. To solve these problems, for the first time, a eutectic mixture etching strategy is utilized to accomplish one‐step synthesis of Cl‐terminated MXene (Ti3C2Cl2) with tunable in‐plane porosity from a MAX precursor (Ti3AlC2) through manipulating the phase transition of the selected salt melt. Specifically, the temperature and composition of the NaCl/ZnCl2 salt mixture are controlled to initiate a mechanism that creates and critically preserves the MXene pore structure, leading to substantial increment in material mesoporosity and a fourfold increase in surface area. Moreover, X‐ray spectroscopy analyses reveal increased TiC6 octahedral symmetry and density functional theory (DFT) modeling suggests a lower Li diffusion barrier, which imply high suitability for ion transport. Benefiting from these optimizations, mesoporous Ti3C2Cl2 delivers a high capacity of 382 mAh g−1 at 0.1 A g−1 as a dual‐ion battery anode, with capacity retention over 89% after 1000 cycles at 2.0 A g−1. Overall, this study presents a green chemistry approach that enables direct synthesis of MXenes with optimal porosity and surface termination for electrochemical applications, providing fresh insights for targeted structure modifications. An environmentally friendly eutectic etching method is developed to incorporate in‐plane porosity onto Ti3C2Cl2 MXene sheets by manipulating treatment temperature and eutectic mixture composition. For the first time, this fluorine free strategy enables direct synthesis of high surface area Cl‐terminated MXene with superior electrochemical performance when applied as a dual‐ion battery anode.
MXenes (Ti3C2Tx) with –F surface terminations have a negative impact on electrochemical properties when used as potential electrodes in supercapacitors. In this study, Ti3C2Tx with –F surface terminations was one-step treated in LiCl-KCl-K2CO3 molten salt at atmospheric pressure to replace –F by –O surface terminations and simultaneously introduce the intercalation of potassium. Various potassium oxygenated complexes were intercalated into the interlayer of the Ti3C2Tx, resulting in the expansion of d-spacing from 0.96 to 1.05–1.21 nm, the decrease of F content from 11.23 to 3.43 at%, and the increase of O content from 0.79 to 24.18 at%. The modified Ti3C2Tx electrode (KM-Ti3C2Tx) showed an improved specific capacity of 323.6 F g⁻¹ at 1 A g⁻¹ in 1 M H2SO4 solution and excellent capacitance retention (97% after 10000 charging–discharging cycles at 10 A g⁻¹). The storage mechanism is attributed to the reversible conversion of Ti3C2O2 / Ti3C2(OH)2 during the insertion/extraction of hydronium (H⁺). Therefore, the removal of –F by –O surface terminations can form more Ti3C2O2, leading to an increase in conductivity and electrochemical active surface area.
The interlayer regulation of layered environmental adsorption materials such as two-dimensional early transition metal carbides and carbonitrides (MXenes) plays an important role in their purification performance for specific pollutants. Here the enhanced uptake of Th IV by multilayered titanium carbides (Ti 3 C 2 T x ) through a hydrated intercalation strategy is reported. Th IV adsorption behaviors of three Ti 3 C 2 T x samples with different c lattice parameters were studied as a function of contact time, pH, initial concentration, temperature and ion strength in batch experiments. The results indicated that the Th IV uptake was pH and ionic strength dependent, and the adsorption process followed the pseudo-second-order kinetics and the heterogeneous isotherm (Freundlich) model. Thermodynamic data suggested that the adsorption process of all MXene samples was a spontaneous endothermic reaction. The dimethyl sulfoxide intercalated hydrated Ti 3 C 2 T x featured the largest interlayer space and exhibited the highest Th IV adsorption capacity (162 mg g ⁻¹ at pH 3.4 or 112 mg g ⁻¹ at pH 3.0), reflecting the significant increase in available adsorption sites from Ti 3 C 2 T x interlayers. The adsorption mechanism has been clarified based on adsorption experiments and spectroscopic characterizations. An ion exchange process was proposed for the interaction between hydrated MXenes and Th IV , where H ⁺ from surface [Ti−O] ⁻ H ⁺ groups were the primary active sites on Ti 3 C 2 T x . Extended X-ray absorption fine structure (EXAFS) fitting results, in combination with X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) analyses, clearly indicated that Th IV mainly formed the outer-sphere complexes on Ti 3 C 2 T x surface through electrostatic interaction under strong acid conditions, while at pH > 3.0 the adsorption mechanism was determined by inner-sphere coordination and electrostatic interaction together.
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Anatase TiO2 (a-TiO2) exhibits a strong x-ray absorption linear dichroism in the pre-edge, the XANES and the EXAFS at the titanium K edge. In the pre-edge region, the behavior of the A1–A3 and B peaks originating from the 1s−3d transitions is due to the strong p-orbital polarization and strong p−d orbital mixing. An unambiguous assignment of the pre-edge peak transitions is made in the monoelectronic approximation with the support of ab initio finite difference method calculations and spherical tensor analysis in quantitative agreement with the experiment. Our results suggest that several previous studies relying on octahedral crystal field splitting assignments are in accurate due to the significant p-d orbital hybridization induced by the broken inversion symmetry in a-TiO2. It is found that A1 is mostly an on-site 3d−4p hybridized transition, while peaks A3 and B are nonlocal transitions, with A3 being mostly dipolar and influenced by the 3d−4p intersite hybridization, while B is due to interactions at longer range. Peak A2, which was previously assigned to a transition involving pentacoordinated titanium atoms, is shown to exhibit a quadrupolar angular evolution with incidence angle, which implies that its origin is primarily related to a transition to bulk energy levels of a-TiO2 and not to defects, in agreement with theoretical predictions [Vorwerk et al., Phys. Rev. B 95, 155121 (2017)]. Finally, ab initio calculations show that the occurence of an enhanced absorption at peak A2 in defect-rich a-TiO2 materials originates from defect-related p density of states due to the formation of doubly ionized oxygen vacancies. The formation of peak A2 at almost the same energy for single crystals and nanomaterials is a coincidence while the origin is different. These results pave the way to the use of the pre-edge peaks at the Ti K edge of a-TiO2 to characterize the electronic structure of related materials and in the field of ultrafast x-ray absorption spectroscopy where the linear dichroism can be used to compare the photophysics along different axes.
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Supported nanoparticles are broadly employed in industrial catalytic processes, where the active sites can be tuned by metal-support interactions (MSIs). Although it is well accepted that supports can modify the chemistry of metal nanoparticles, systematic utilization of MSIs for achieving desired catalytic performance is still challenging. The developments of supports with appropriate chemical properties and identification of the resulting active sites are the main barriers. Here, we develop two-dimensional transition metal carbides (MXenes) supported platinum as efficient catalysts for light alkane dehydrogenations. Ordered Pt3Ti and surface Pt3Nb intermetallic compound nanoparticles are formed via reactive metal-support interactions on Pt/Ti3C2Tx and Pt/Nb2CTx catalysts, respectively. MXene supports modulate the nature of the active sites, making them highly selective toward C–H activation. Such exploitation of the MSIs makes MXenes promising platforms with versatile chemical reactivity and tunability for facile design of supported intermetallic nanoparticles over a wide range of compositions and structures.
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The reactive metal–support interaction (RMSI) offers electronic, geometric and compositional effects that can be used to tune catalytic active sites. Generally, supports other than oxides are disregarded as candidates for RMSI. Here, we report an example of non-oxide-based RMSI between platinum and Nb2CTx MXene—a recently developed, two-dimensional metal carbide. The surface functional groups of the two-dimensional carbide can be reduced, and a Pt–Nb surface alloy is formed at a moderate temperature (350 °C). Such an alloy exhibits weaker CO adsorption than monometallic platinum. Water-gas shift reaction kinetics reveals that the RMSI stabilizes the nanoparticles and creates alloy–MXene interfaces with higher H2O activation ability compared with a non-reducible support or a bulk niobium carbide. This RMSI between platinum and the niobium MXene support can be extended to other members of the MXene family and opens new avenues for the facile design and manipulation of functional bimetallic catalysts. Reactive metal–support interactions can tune the activity of heterogeneous catalysts, but have mainly been reported for oxide supports. Now, the metal–support interaction of platinum with MXenes at moderate temperature is reported, using the water-gas shift reaction as an example to showcase the properties of a representative catalyst.
MXenes constitute a family of two-dimensional transition metal carbides, carbonitrides and nitrides. Discovered in 2011, the number of MXenes has expanded significantly and more than 20 different MXenes have been synthesized, with many more predicted from theoretical calculations. MXenes constitute an exceptional family of materials based on their availability for elemental alloying and control of surface terminations, which enables synthesis of a range of structures and chemistries. Consequently, the MXenes exhibit an unparalleled potential for tuning of the materials properties for a wide range of applications. At present, MXenes have emerged with astonishing electronic, optical, plasmonic and thermoelectric properties. This has resulted in a global surge of research around a wide variety of applications, including but not limited to energy storage, carbon capture, electromagnetic interference shielding, reinforcement for composites, water filtering, sensors, and photo-, electro- and chemical catalysis etc. In this review, we present the available state of the art tailoring of the MXene properties owing to recent advances in structural ordering and tuning of surface terminations.
MXenes, a recently discovered family of two-dimensional (2D) materials, are promising catalysts and supports for applications in heterogeneous catalysis, however, the thermal stability of MXenes and their surface chemistry are not fully explored. Here we report that 2D molybdenum carbide Mo2CTx remains stable and shows no appreciable sintering up to ca. 550-600 °C in reducing environment, as assessed by a combined in situ X-ray absorption near edge spectroscopy (XANES) and powder X-ray diffraction (XRD) study during a temperature programmed reduction (TPR) experiment. At higher temperatures, the passivating oxo, hydroxy and fluoro groups de-functionalize the molybdenum-terminated surface, inducing a transformation to bulk -Mo2C that is complete at ca. 730 °C. We demonstrate that Mo2CTx is a highly stable and active catalyst for the water gas shift (WGS) reaction with a selectivity > 99 % toward CO2 and H2 at 500°C. The conversion of carbon monoxide on Mo2CTx starts to decline at temperatures that are associated with the decrease of the interlayer distance between the carbide sheets, as determined by the XRD-probed TPR, indicative of increasing mass transfer limitations at these conditions. Our results provide an insight on the thermal stability and reducibility of Mo2CTx and serve as a guideline for its future catalytic applications
The Mn+1AXn, or MAX, phases are nanolayered, hexagonal, machinable, early transition-metal carbides and nitrides, where n = 1, 2, or 3, M is an early transition metal, A is an A-group element (mostly groups 13 and 14), and X is C and/or N. These phases are characterized by a unique combination of both metallic and ceramic properties. The fact that these phases are precursors for MXenes and the dramatic increase in interest in the latter for a large host of applications render the former even more valuable. Herein we describe the structure of most, if not all, MAX phases known to date. This review covers ~155 MAX compositions. Currently, 16 A elements and 14 M elements have been incorporated in these phases. The recent discovery of both quaternary in-and out-of-plane ordered MAX phases opens the door to the discovery of many more. The chemical diversity of the MAX phases holds the key to eventually optimizing properties for prospective applications. Since many of the newer quaternary (and higher) phases have yet to be characterized, much work remains to be done.
A central topic in single atom catalysis is to build strong interactions between single atoms and the support for stabili-zation. Herein we report the preparation of stabilized single atom catalysts via a simultaneous self-reduction-stabilization process under room-temperature using ultrathin two-dimensional Ti3-xC2Ty MXene nanosheets character-ized by abundant Ti-deficit vacancy defects and high reducing capability. The single atoms therein form strong metal-carbon bonds with the Ti3-xC2Ty support, and are therefore stabilized onto the sites previously occupied by Ti. The Pt-based single atom catalyst (SAC) Pt1/Ti3-xC2Ty offers a green route to utilize the greenhouse gas CO2, via formylation of amines, as a C1 source in organic synthesis. DFT calculations reveal that, compared to Pt nanoparticles, the single Pt atoms on Ti3-xC2Ty support feature partial positive charges and atomic dispersion, which helps to significantly decrease the adsorption energy and activation energy of silane, CO2 and aniline, thereby boosting catalytic performance. We believe that these results would open up new opportunities for the fabrication of SACs and the applications of MXenes in organic synthesis.
Two-dimensional MXenes hold promises in a variety of applications in which their functional groups are indispensable. These functional groups are spontaneously bonded to the atomic MX slabs through competitive adsorption of active species during the acid etching process of nanolaminated MAX phases. Nevertheless, the knowledge of proportion and distribution of functional groups on MXenes, i.e., surface structures, is still limited. By high-throughput computation screening, ground-state stable structures of four kinds of typical MXenes - Ti2CTx, Ti3C2Tx, Nb2CTx and Nb4C3Tx (T = O, F, and OH) with mixed functional groups are established for the first time. The multi-component functional group patterns definitely demonstrate an obvious feature of spatial mixing at a given component. However, the heterogeneous structure has a near linear dependence on the functional group components in terms of free energy. Most of functionalized MXenes are dynamically stable except for Nb2CF2 and Nb2C(OH)2 due to their competing displacive counterparts. Last but not the least, Raman spectra of the four kinds of MXenes experimentally confirm the theoretically established stable surface structures of MXenes. This study provides a clear fundamental basis for understanding the surface structures and surface-related applications of MXenes.
The chemical bonding in the carbide core and the surface chemistry in a new group of transition-metal carbides Tin+1Cn-Tx (n=1,2) called MXenes have been investigated by surface-sensitive valence band X-ray photoelectron spectroscopy. Changes in band structures of stacked nano sheets of different thicknesses are analyzed in connection to known hybridization regions of TiC and TiO2 that affect elastic and transport properties. By employing high excitation energy, the photoelectron cross-section for the C 2s - Ti 3d hybridization region at the bottom of the valence band is enhanced. As shown in this work, the O 2p and F 2p bands strongly depend both on the bond lengths to the surface groups and the adsorption sites. The effect of surface oxidation and Ar⁺ sputtering on the electronic structure is also discussed.