Complexity of Intercalation in MXenes: Destabilization of Urea by Two-Dimensional Titanium Carbide

Article abstract of DOI:10.1021/jacs.8b05913

MXenes are a new class of two-dimensional materials with properties that make them important for applications that include batteries, capacitive energy storage, and electrocatalysis. These materials can be exfoliated and delaminated to create high surface areas with interlayers accessibility. Intercalation is known to be possible, and it is critical for many applications including electrochemical energy storage, water purification, and sensing. However, little is known about the nature of the intercalant and bonding interactions between the intercalant within the MXene. We have investigated urea interaction within a titanium carbide based MXene using inelastic neutron scattering (INS) to probe the state of intercalated species. By comparison with reference materials, we find that under intercalation conditions urea decomposes readily, leading to intercalation of ammonium cations observable by INS and evolving carbon dioxide detected by infrared spectroscopy. Reactive molecular dynamics calculations were conducted to provide atomistic insights about reaction pathways and their energetics. These results have implications for understanding intercalation in active layered materials.

Full text of DOI:10.1021/jacs.8b05913

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Article
Complexity of Intercalation in MXenes: Destabilization
of Urea by Two-Dimensional Titanium Carbide
Steven H. Overbury, Alexander I. Kolesnikov, Gilbert M. Brown, Zhiyong Zhang, Gokul
S. Nair, Robert L. Sacci, Roghayyeh Lotfi, Adri C.T. van Duin, and Michael Naguib
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b05913 • Publication Date (Web): 23 Jul 2018
Downloaded from http://pubs.acs.org on July 23, 2018
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Complexity of Intercalation in MXenes: Destabilization of Urea by Twoꢀ
Dimensional Titanium Carbide
Steven H. Overbury,†,1 Alexander I. Kolesnikov, ‡,* Gilbert M. Brown, †,1 Zhiyong Zhang†,2 Gokul S.
Nair,§ Robert L. Sacci,§ Roghayyeh Lotfi¶, Adri C.T. van Duin¶, Michael Naguibǁ,*
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† Chemical Sciences Division, ‡Neutron Scattering Division, § Materials Science and Technology
Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA.
¶ Department of Mechanical & Nuclear Engineering, Penn State University, University Park,
PA,16802, USA.
Department of Physics and Engineering Physics, Tulane University, New Orleans, LA 70118,
USA.
* Corresponding Authors: kolesnikovai@ornl.gov and naguib@tulane.edu
1 retired from Chemical Sciences Division, Oak Ridge National Laboratory.
2 current address: Department of Chemistry, University of Virginia, Charlottesville, VA.
ǁ
Abstract:
MXenes are a new class of twoꢀdimensional materials with properties that make them important
for applications that include batteries, capacitive energy storage and electrocatalysis. These
materials can be exfoliated to create high surface areas with interlayer accessibility. Intercalation
is known to be possible and it is critical for many applications including electrochemical energy
storage, water purification and sensing. However, little is known about the nature of the
intercalant and bonding interactions between the intercalant within the MXene. We have
investigated urea interaction within a titanium carbide based MXene using inelastic neutron
scattering (INS) to probe the state of intercalated species. By comparison with reference
materials, we find that under intercalation conditions urea decomposes readily, leading to
intercalation of ammonium cations observable by INS and evolving carbon dioxide detected by
infraꢀred spectroscopy. Reactive molecular dynamics calculations were conducted to provide
atomistic insights about reaction pathways and their energetics. These results have implications
for understanding intercalation in active layered materials.
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1. Introduction
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MXenes are a new class of twoꢀdimensional (2D) materials within the family of
transition metal carbides and nitrides.1 They are 2D in the sense that they are crystalline with a
thickness corresponding to few atomic layers and a high aspect ratio. The Ti3C2 MXenes are
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prepared by selective extraction of Al from the crystalline MAX phase Ti
3
AlC using fluoride
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containing aqueous solution (HF,2 NH4HF2,3 or mixture of acid and fluoride salt4). This MAX
phase can be thought of as Ti3C2 metal carbide layers separated by Al atoms.5ꢀ6 In the selective
extraction process the Al group is removed leaving Ti cations which are terminated by F¯, O=,
and/or OH¯ groups producing materials designated as Ti3C2Tz, in which the termination group Tz
= (OH¯ , F¯ or O=). Bonding between the layers is much weaker in the MXene phase than in the
MAX phase and the 2D layers can be easily separated in the former. Extraction of Al and
sonication in an alcohol leads to delamination.2 The extent to which the terminations are
hydroxyl, fluoride or oxygen and how the terminations depend upon detailed synthetic
conditions remains an open question.7
The experimentally measured specific surface area of MXenes (measured using gas
adsorption techniques) is not as high as carbon materials used in supercapacitors, but MXenes
can be intercalated with ions or molecules. Thus, to achieve high capacitance or efficient
electrochemical capability it is beneficial to make use of the interlayer spaces. It is possible that
penetration of molecular reactants in electrochemical processes may be enhanced by the presence
of intercalants which increase the interlayer spacing.8 In addition, intercalation is critical for
other application where MXenes were found promising such as water purification,9ꢀ10 and
sensing.11 Also, intercalation leads to extensive largeꢀscale delamination of the layers.7, 12 Key
questions are how ions and reactive molecules enter into, move through and interact with the
layers and what is the nature of intercalated species. Several reagents have been found to be
effective intercalants including hydrazine hydrate, urea (O=C(NH2)2), dimethylsulfoxide
(DMSO),7 and tetrabutylammonium (TBA). Early transition metal carbides, including Ti, Zr, V,
and NbꢀMo carbides have been reported as catalysts for activation of H2 and CO2,13
hydrodesulfurization / hydrodenitrogenation14 and electrocatalytic reactions such as oxygen
reduction15ꢀ16 so the possibility exists that there may be catalytic activity associated with Ti based
MXenes.
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Most previous studies of intercalation in MXenes depended on Xꢀray diffraction to
determine changes in the cꢀlattice parameter. An increase in the cꢀlattice parameter was
attributed to intercalation between the layers. No detailed studies were conducted to reveal the
nature of the intercalant and it was assumed that either intact pristine molecule or cation were
simply intercalated. The pristine molecules are assumed to intercalate when MXene is soaked in
an organic solvent (e.g. DMSO), organic aqueous solution (e.g. urea in water) or inorganic
aqueous solution (e.g. hydrazine in water),7 the cations (e.g. Li+,17 or TBA+ , 12) are assumed to
intercalate when MXene is soaked in ionic aqueous solution (e.g. LiOH or TBAOH solution) or
electrochemically cycled in an organic electrolyte (e.g. LiPF6,18 NaPF6,19 KPF6,20 in carbonates).
Coꢀintercalation of organic solvents21 and water11 were also reported. Water intercalation in
MXenes in presence of ions was the focus of multiple studies.22ꢀ23 Ghidiu et al.22 reported step
changes in the cꢀlattice parameter of ions intercalated MXene as function of relative humidity.
Muckley et al.23 found that water molecules intercalate between the layers in presence of
intercalated ions, such as K+ and Mg2+, and form pillars leading to the step increase in the cꢀ
lattice parameter.
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One of the main objectives of this work is to shed a light on the complexity of
intercalation in MXenes using urea/Ti3C2Tz system as an example. Ti3C2Tz was selected since it
is the most studied MXene so far and urea was selected since it has active vibration modes that
can be studied using vibrational spectroscopy including infraꢀred (IR) and inelastic neutron
spectroscopy (INS). Also, other Tiꢀurea complexes can be synthesized and compared to urea
treated MXene. We find that under the intercalation conditions, the urea undergoes
decomposition, promoted by the MXene, leading to scission of the CꢀN bonds of the molecule.
In contradiction to previous presumptions that urea intercalates inꢀbetween the MXene layers,7
we find that the interlayer expansion is related to the intercalation of the reaction product
ammonium which remains following the decomposition reaction. These results have implications
for the use of intercalant systems in catalytically active 2D materials.
2. Experimental and Computational Methods
Preparation of samples
MX: Ti3C2Tz MXene was synthesized as described previously2 by immersing the MAX phase
Ti3AlC2 powders (ꢀ325 mesh; < 44 µm) in 48% HF, typically using 1 g powder for 10 mL of
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solution. The slurry was held for 18 h with stirring at room temperature (RT) followed by
washing using deionized (DI) water until the pH reached values > 4. The slurry was then filtered
and the solid was left to dry in air at RT yielding the Ti3C2Tz MXene powder that is referred to
below as ‘MX’.
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u-MXene: Urea treatment was carried out by mixing 0.9 g MX powder with 20 g of 50 wt%
aqueous urea solution for 15 h at 60 °C under constant stirring. The pH values for the ureaꢀwater
MXene solution at the beginning and at the end of the treatment was measured using a general
purpose pH electrode (Corning 4136L21). After the 15 h the mixture was centrifuged to isolate
the solid that was rinsed with DI water, and filtered to dry at RT in air. The resulting solid is
referred to as ‘uꢀMX’. Vacuum treatment was done of a portion of the uꢀMX to remove any
“free” water in the samples by heating at 110 °C under dynamic vacuum for 18 h. The dried
black solid is referred to as ‘uꢀMX dried’. A similar vacuum drying at 110 °C was also done for
MX used for INS measurements.
a-MXene: An ammonium treated (intercalated) MXene was produced from the MAX phase by
etching the aluminum using ammonium bifluoride [NH4][HF2] as previously described.3 For this
complex, 3 g of Ti3AlC2 powders (< 45 µm) was immersed in 60 mL of 1M NH4HF2 aqueous
solution at RT for 120 h with stirring. Then it was washed with DI water till the pH reached
values >4. After that, it was filtered and left to dry in air at RT (‘aꢀMX’). Vacuum treatments
were done to remove any “free” water in the samples by heating at 110 °C under dynamic
vacuum for 18 h. The resulting solid is referred to as ‘aꢀMX vac ann’
Ti(urea)6 complex: The Ti urea complex material, Ti(urea)6I3, was prepared following a
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modification of the synthesis described by Hartman et al24 and by Pickering. Commercially
available 20% TiCl3 solution in dilute HCl was added to a large excess of urea dissolved in
water. A large excess of KI was dissolved in the solution, the solution was filtered, and the flask
put in an ice bath to crystalize. Wellꢀformed blueꢀpurple crystals were collected by filtration and
air dried. The structure of this complex has been determined for the iodide salt Davis and
Wood26 and for the perchlorate salt.27 Here we used an iodide salt to decrease interference in the
IR and INS spectra.
Spectroscopic measurements and ReaxFF Simulations details can be found in the Supporting
Information.
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3. Results and Discussion
Intercalation into the MXene resulting from urea treatment was indicated by a shift to
lower angle of a prominent peak in the small angle XRD of the MXene as shown in Figure 1. As
described previously the XRD peak near 2θ of 9° corresponds to the (0002) diffraction feature
of the double layers spacing cꢀlattice parameter of the MXene.2 The shift from about 9.0 to 7.6
degrees 2θ observed following treatment in urea equates to expansion of the cꢀlattice parameter
from about 1.92 to 2.5 nm and is taken as a clear signal that an intercalation took place into the
interlayers spacings of the MXene.7 The intercalant was speculated by Mashtalir et al. to be
urea.7 Conducting the same experiment at room temperature (instead of 60 °C) led to no change
in the cꢀlattice parameter (Figure S1) demonstrating that the urea incorporation is an activated
process consistent with intercalation or reactive intercalation. It is the purpose of this paper to
describe the bonding and interaction of and the nature of the intercalant between the MXene
layers.
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Figure 1. XRD patterns of the TiꢀMXene, (the MX sample (blueꢀbottom pattern) and the MX
after (redꢀmiddle pattern) treatment with urea at 60 °C, the uꢀMX sample and after vaccuum
annealing to remove any excess water (orangeꢀtop pattern), the uꢀMX dried sample.
Urea is known to form a stable oxygenꢀbound complex with Ti(III) from an aqueous
solution, and it appears to stabilize the Ti(III) oxidation state when compared to H2O bound to
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Ti(III).26ꢀ27 The structure of this complex, [Ti(O=C(NH2)2)6]I3, was determined by XꢀRay
crystallography, and the stability of the complex in the solid state to air oxidation was attributed
to hydrogen bonding of the amide groups to the Iꢀ anions.26 In general urea is oxygenꢀbound to
transition metal atoms, but it is not generally thought of as a strongly bound ligand. A
comparison of the vibrational spectroscopy of [Ti(O=C(NH2)2)6]I3 with the material produced by
intercalation of Ti3C2Tz with urea provides an aid in understanding the interaction of urea with
the MXene interlayers.
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IR spectra of the reference materials, urea and the Ti(urea)I6 complex are shown in
Figure 2. The spectrum of urea is in agreement with results of Roussseau and Keuleers28ꢀ29
who have published detailed vibrational analysis and assignments of gas phase and solid urea
including various isotopomers. Summarizing the urea spectrum, the high wavenumber region is
dominated by asymmetric and symmetric NꢀH stretching modes, around 3448 and 3345 cmꢀ1
respectively, and including a broad shoulder near 3250 cmꢀ1 assigned to a combination of CO
stretch and NH2 deformation or it may be due to twoꢀphonon scattering involving δ (NH2).
Features between 1600 and 1700 cmꢀ1 may be assigned to a mixture of C=O stretch and NH2
deformation modes, δ(NH2), while the peak split off at 1462 cmꢀ1 may be assigned primarily to
asymmetric CN stretch, νa(CN). It is relevant that the mode at 1462 cmꢀ1 also contains a mixture
of NH2 deformation.29 Peaks near 1153 and 1050 cmꢀ1 may be assigned to symmetric and
asymmetric NH2 rocking modes, respectively, and a small peak near 1000 cmꢀ1 is due to
symmetric CN stretch. Finally, a small peak near 790 cmꢀ1 is associated with a CO wag.28 These
assignments are marked on Figure 2 and the assignments are summarized in Table 1.
Urea
Ti(urea)6I3
υsCN
ω CO
ρNH2
δNH
2
υ
asN-H
υC=O
υsN-H
υaCN
500
1000
1500
2000
2500
3000
3500
4000
Wavenumber (cm-1)
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Figure 2. IR transmission spectra of urea and of the Ti(urea )6I3 complex recorded at room
temperature in KBr wafer. Dashed lines indicate positions of features observed in urea spectrum.
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Table 1. Vibrational frequencies observed for urea and for the Ti complex obtained from both IR
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and INS.
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Urea
IR a
Urea
Ti(urea)6I3
IR a
Ti(urea)6I3
INS b
Vibrational mode (cmꢀ1)
INS b
υas NH2
υs NH2
3448
3345
3476
3379
3489
3383
3400 (br)
Combination ν CO + δ (NH2);
twoꢀphonon δ (NH2)
3298
3213
3250 (br)
1681
1630
1600
3250
n/o
1626
1575
υ CO
n/o
1647
1600
1636
1549
δ NH2
υa CN enhanced by H in NH2
1462
1488
1495
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1032
1500
ρs NH2
ρa NH2
1153
1050
1163
1033 (br)
1156
1053 (br)
υs CN
ω CO
1003
n/o
n/o
790
ꢀꢀ
748
575
765
ꢀꢀ
697
533
a IR spectra at room temperature; b converted to cmꢀ1 from the INS spectra at 6 K;
n/o = not observed; br = broad; sh= shoulder
The features for the Tiꢀcomplex deviate in several details from the spectrum of urea.
Obvious shifts in position are observed in NH stretching region with both the asymmetric and
symmetric modes shifting to higher wavenumber in the complex. The combination mode
observed as a broad shoulder near 3250 cmꢀ1 in urea, resolves into two strong modes at 3298 and
3213 cmꢀ1 in the complex. The C=O stretch that is observed at 1681 cmꢀ1 in pure urea, is either
absent or is red shifted in the complex. Similarly, the positions of the δ(NH2) and υa(CN) are
also shifted compared to urea making exact correlation uncertain without further isotopic data.
The NH2 rocking mode, ρꢀNH2, redꢀshifts only slightly, but it and the symmetric CN stretch are
much clearer in the complex.
Comparison of the IR spectra of urea with the urea complexed to Ti serves to
demonstrate the effects of Tiꢀurea interaction upon the vibrational modes of the urea. It is wellꢀ
known that urea exhibits resonances that lead to zwitterionic intermediates, making it possible
for the urea to interact through either at ꢀNH2 end or the ꢀCꢀO δꢀ end of the zwitterion.30 As
δ+
described by Penland et al, the bonding in urea complex can be distinguished by the shifts in the
features near 1700 cmꢀ1. 31 According to Penland, if the urea molecule is bonded through the O in
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the complex then the C=O stretch will shift to lower frequencies, while bonding through the N
will cause a blue shift in the C=O stretch. In the Tiꢀcomplex there is no evidence in the IR
spectrum of a peak at higher wavenumber than the 1680 cmꢀ1 observed for urea, arguing against
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a blue shift in the C=O stretch and consistent with bonding through the C=O. Similarly, Li
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concluded for the Cl salt of the Ti complex that based upon the disappearance of a peak at 1684
cmꢀ1, the urea bonds in the complex through the formation of C=OꢀTi coordination. In addition,
the facts 1) that the NH2 rocking modes and the CN stretch are relatively unshifted and 2) that
they appear clearly in the complex, are indications that bonding through the N to the Ti does not
occur.
INS investigation of MXene intercalation
IR spectra of the uꢀMX could not be obtained because the MXene materials are black and
were strongly absorbing. Instead, INS spectra of the intercalated MXene, as well as the reference
materials, were obtained using neutrons with various incident energies. The full INS spectra
across all recorded incident energies are merged together in Figure S2 for comparison, but we
will now focus on each of the incident energies separately.
The highest wavenumber region was examined using 600 meV neutrons, making visible
the region in the NH stretches. The INS spectra in this region are shown in Figure 3. Solid urea
exhibits a strong feature centered at 3400 cmꢀ1, but the feature is broadened compared to the IR
spectra and fails to resolve the symmetric and asymmetric stretches, in part due to the lower
resolution of the INS measurement. The INS features of the Ti(urea)6 complex are blueꢀshifted
and sharpened compared to the urea, a trend also observed in the IR spectra (Figure 2). It is
therefore surprising that the uꢀMX does not exhibit more clearly resolved peaks matching the NH
stretching features of the Tiꢀcomplex. Instead, as shown in Figure 3, a broad feature is observed
in the region from 3100 to 3600 cmꢀ1 which is similar to the feature seen for the untreated MX
containing no urea. This broad feature therefore may be associated in part with OꢀH stretching
modes of water, intercalated or adsorbed, or to terminal OH groups or counterꢀions within the
Ti
3
C2
Tz itself. Upon drying the uꢀMX, the broad feature changes shape and decreases in
intensity. The INS spectra of both urea and the Tiꢀcomplex also exhibit intense combination
bands due to twoꢀphonon scattering, located near 2200, 2750 and 4000 cmꢀ1. For example, the
peak near 2750 cmꢀ1 is a combination of strong modes at 1600 and 1163 cmꢀ1 (see below). These
combination modes are not visible in the spectrum of the uꢀMX.
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4
urea
5
Ti(urea)6I3
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MX (x10)
u-MX (x10)
u-MX dried (x10)
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0.0
2000
2500
3000
3500
4000
Energy transfer (cm-1)
Figure 3. INS spectrum obtained using 600 meV incident neutrons, recorded at 6 K. Dashed
lines indicate the location of the νs and νa NꢀH stretches of urea observed in the IR spectrum of
urea. For comparison with IR data, neutron energy transfer is converted from meV to cmꢀ1. For
clarity, curves are offset from zero.
The INS spectra recorded using incident 250 meV neutrons is shown in Figure 4. Using
this incident energy, the region dominated by NH2 deformation and rocking modes and by the Cꢀ
N stretching modes are clearly seen and are assigned as labeled in Figure 4. The INS spectrum
is dominated by features involving H vibrations because of the high H neutron scattering cross
section. As such the CO stretch observed in the IR urea spectrum at 1681 cmꢀ1 (Figure 2) is not
expected to be strong in INS because of the low neutron scattering cross sections of C and O
atoms and weak coupling between the C=O stretch and the H located on the NH2 adjacent to the
C. No peak near at 1681 cmꢀ1 is observed in the INS urea spectrum, or it is buried in the shoulder
of the much stronger NH2 deformation modes, and similarly for the Ti complex. Strong coupling
between N and H may serve to enhance the intensity of the CN stretches in INS. The mode
observed at 1488 cmꢀ1 in INS matches the IR mode described as being primarily due to υCN and
may be enhanced by contributions from H covalently bound to N. The weak υsCN mode
observed in IR at 1003 cmꢀ1 is not detectable under the much stronger NH2 rocking modes.
Below 1000 cmꢀ1, there are intense bands observed for the urea spectrum which may be assigned
to phonon bands of the crystalline urea as observed in previous INS measurements and well
described by Johnson et al.33 Taking these considerations into account, there is good
correspondence between the urea INS spectrum and the urea IR spectrum (Figure 2). In addition,
the changes observed between the urea and the Tiꢀurea complex observed by IR are mimicked in
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the INS spectrum, namely: NH2 deformations and the NH2 rock are redꢀshifted in the complex
compared to the solid urea and the CN stretches are slightly blueꢀshifted in the complex
compared to solid urea. Some of the phonon peaks (below 900 cmꢀ1) observed prominently in the
INS spectrum of urea can be seen in the IR spectrum, but obviously these states are better seen in
the INS.
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urea
Ti(urea)6I3
δNH2
MX (x10)
u-MX (x10)
u-MX dried (x10)
ρsNH2
0.002
0.001
0.000
δaNH2
v
ρaNH2
+
CN
500
1000
1500
Energy transfer (cm-1)
Figure 4. INS spectrum obtained using 250 meV incident neutrons, recorded at 4 K. Dashed
lines guide the eye through features observed in the urea (INS) spectrum and listed in Table 1.
For comparison with IR data, neutron energy transfer is converted from meV to cmꢀ1. For clarity,
curves are offset from zero.
Once again, the features observed for the uꢀMX do not well match the expectations for
intercalated molecular urea. In particular, the rocking mode ρꢀNH2, seen clearly for urea in the
INS spectrum, are completely absent from the uꢀMX spectrum. In addition, there is not a clear
correspondence between the νaCN and δNH2 modes, clearly observed in urea and the Tiꢀurea
complex, and the peaks observed for the uꢀMX in this range of energy transfers. Yet, the
features present in this region (near 1460 and 1700 cmꢀ1) for the uꢀMX indicate the presence of
molecular features that are not observed in the MX. The intense phonon features observed for the
urea (400ꢀ900 cmꢀ1) are absent in both the uꢀMX and the MX spectra, where only broad features,
likely associated with water, are observed. Similarly, a broad feature near 1650 cmꢀ1 that is most
evident for the MX, may result in part from water deformation mode for water trapped within the
MXene sheets. A decrease in the intensity of this feature after vacuum drying the uꢀMX is
consistent with this conclusion. The persistence of the features near 1460 and 1700 cmꢀ1 indicate
that a molecular species remains in the dried uꢀMX, although it is not urea.
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Phonon peaks in the urea and Ti(urea)6 complex are even more pronounced at lower incident
neutron energies. These are shown in Figure 5a and 5b for incident energies of 110 meV and 30
meV respectively. Molecular vibrational modes in the range 320 to 800 cmꢀ1 are clear and strong
for urea and are present but shifted in the Tiꢀcomplex (Figure 5a). These are intraꢀmolecule
librations of NH2 groups and some may be described as deformation modes involving the NꢀCꢀO
bond angle, and they are dispersed in the urea due the strong intermolecular interactions via
hydrogen bonds, resulting in the coupled motion of the NH2 groups on neighboring molecules.33
They are sensitive to the molecular environments, as have been described previously from
analysis of vibrational spectra of gas phase and crystalline urea,28 and they are strongly dispersed
in INS by hydrogen bonding in the crystal.33 These modes are not generally identifiable for uꢀ
MX where two broad peaks at 370 and 470 cmꢀ1 are superꢀimposed upon a broad background
that roughly matches that of the unintercalated MXene (supposedly originated from librational
band of the confined water). The two broad peaks are absent for the MX and indicate the
presence of a molecular species in the uꢀMX, but they do not seem to be assignable to urea.
Strong intermolecular vibrational bands are observed for both urea and the Ti(urea) complex
below 200 cmꢀ1, as seen in Figure 5b. Modes between 100 – 200 cmꢀ1 may be associated with
whole molecule librations while those below 100 cmꢀ1 may be due to translations.34 It is
noteworthy that a peak at about 120 cmꢀ1 identifiable as due to transition from a para to ortho H2
is observed in the MXene sample35 likely resulting from the reductive acid treatment used to etch
Al from the MAX phase. This peak is absent in the intercalated MXene, confirming increase of
interlayer distances which allows molecular hydrogen to escape.
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a.
Ei = 110 meV
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0.004
0.003
0.002
0.001
0.000
δN-C-N
urea
Ti(urea)6I3
δN-C-O
MX (x10)
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u-MX (x10)
u-MX dried (x10)
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Energy transfer (cm-1)
b.
Ei = 30 meV
urea
0.006
0.005
0.004
0.003
0.002
0.001
0.000
Ti (urea)6
MX (x10)
u-MX (x10)
u-MX dried (x10)
para H2
50
100
150
200
Energy transfer (cm-1)
Figure 5. INS spectrum obtained using 110 meV (a) and 30 meV (b) incident neutrons, recorded
at 6 K. Dashed lines to guide the eye through features observed in urea spectrum. Neutron
energy transfer is converted from meV to cmꢀ1. For clarity, curves are offset from zero.
Decomposition and hydrolysis of urea
The discrepancy between the spectra of the reference compounds and the uꢀMX suggests
the possibility that although the urea treatment leads to intercalation, as indicated by the cꢀlattice
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parameter expansion in Figure 1, the intercalated species may not be molecular urea but may
instead be that of a decomposition product. Decomposition of urea is a wellꢀstudied reaction and
the mechanisms and kinetics for the acid and base catalyzed hydrolysis of urea has been studied
extensively. Shaw and Bordeaux measured the rate of decomposition of urea in aqueous solution
and reported first order reaction rates of 2x10ꢀ7 sꢀ1 independent of pH and at 60°C, the
temperature used in this work for the urea treatment.36 More recent computational work clarifies
that the reaction occurs by intramolecular elimination reaction, assisted by water, to form
ammonia and isocyanic acid Eq. (1). 37
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ꢂ ꢃ
2
ꢀ ꢁ
1
ꢀ
ꢁ
NH
C = O
→ NH + OCNH
2 2
3
Pathways for hydrolysis, in which the urea experiences electrophilic attack to form carbamate or
carbamic acid, are generally found to be far slower in the absence of a catalyst but are acid/base
catalyzed.37ꢀ39 In water, the product ammonia may evolve or be protonated to ammonium, and
the OCNH may deprotonate to cyanate, depending upon pH. In acid, cyanate is rapidly
converted to ammonium and carbonic acid by Eq. (2). 36, 40
(2)
CNO¯ + 2H3O+ ꢀ NH4 + + H2CO3
Depending upon details of the solution conditions especially pH, HCO3¯ formation or
subsequent evolution of CO2 by Eq (3) may be a result.
(3)
H2CO3 ꢀ H2O + CO2
Whether urea decomposition occurred in the present case, notably with MXene present,
was determined by monitoring gas production during a urea treatment. In a sealed system
(isochoric and isothermic) equilibrium between CO2 dissolved in water and present in the
headspace can be estimated using Henry’s Law. CO2 solubility in water at 60 °C is 0.015 mM at
0.1 bar of CO2 partial pressure,41 and therefore if urea decomposes to ammonium and H2CO3,
then CO2 should be detectable in the cell’s head space. As shown in Figure 6, the evolution of
CO2 was observed and tracked using IR spectroscopy of the headspace over the reaction mixture.
A typical spectrum is shown in Figure S2 (see SI) obtained about 2 hours into the reaction,
characterized by CO2 peaks and smaller peaks that may be attributed in part to NH3. The
CO2 signal at ~2300 cmꢀ1 that immediately showed up after MXene/solution mixing was likely
due to desorption of CO2 on the MXene, as this signal was seen in a control experiment where
pure H2O is added to MXene (Figure 6a red curve). In less than 15 minutes after adding urea
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solution, CO2 signal began to increase before reaching a gaseous concentration of 1.1 mM after 6
h when continuous monitoring was stopped. After 16 h, the full length of the urea treatment, the
CO2 concentration was measured at 1.4 mM. Considering the cell volume (34 mL) this
corresponds to about 0.05 mmole of CO2 is evolved. A much smaller amount of CO2 production
was detected from the urea solution without MXene (Figure 6a black curve). Based upon the
sample mass (0.2 g) and using the theoretical structure which assumes that 2/3 of all Ti is
available for adsorption on this 2D material (in each M3X2 layer there is one M layer buried in
the middle and only two M layers are exposed to the surface), 42 the number of Ti sites available
for adsorption is about 2.0 mmole, about 40 times the CO2 evolved. If MXene is able to
hydrolyze urea into ammonia and CO2, there is less than about one turnover per site.
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Figure 6. (a) IR spectra of the CO2(g) region of the headspace over the MXene/urea solution
slurry at different times (top) along with the MXene/H2O (red, 6 h) and only the urea solution
(black, 2 h) ; (b) the transient of the integrated intensity of the CO2 peak for the first 6 h, when
continuous monitoring was stopped. Cell temperature was 60 °C for all measurements.
In a separate experiment the pH during the urea treatment was measured. The initial urea
solution was measured at pH 6.8, consistent with a 50% aqueous solution of urea in laboratory
air. Addition of the MXene to the solution initially decreased the pH to about 6 consistent with
the slightly acidic surface of the washed MXene (the point of zero charge for Ti3C2Tz is around
2.4)43. As the treatment progressed, the solution pH gradually increased reaching a pH of 3.8
after 12 hours. These pH results are fully consistent with the decomposition of urea by the
MXene leading to the evolution of CO2 and the carbonation of the aqueous solution. Evidently
the ammonia decomposition product is primarily trapped by the MXene (see below).
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These IR experiments indicate CO2 evolution over time scales of roughly 103 ꢀ 104 s,
considerably faster than time scale suggested by the decomposition rates found by Shaw and
Bordeaux (1/ (2x10ꢀ7 sꢀ1)).36 Consistent with the conclusion of the offset blank experiment in Fig
6a (black curve), we conclude that the MXene is promoting the rate of urea decomposition
compared to reaction in water alone. To further probe the nature of this promotion, simulation of
the urea reaction was performed using ReaxFF. First, we performed an energy barrier calculation
for the reaction of one urea molecule on the surface of the MXene sheet as represented in Fig 7a.
It shows reaction to an adsorbed isocyanic acid with an overall barrier of 21.8 kcal/mol,
indicating considerable promotion compared to the experimental value of 32.4 kcal/mol for urea
decomposition in water.36 Interestingly, the urea interacts initially with the ꢀO and ꢀOH terminal
groups, rather than Ti3+ sites, weakening the H2NꢀC bond so that its cleavage precedes
protonation of the H2Nꢀ group by surface ꢀOH, and so there is no Zwitterionic intermediate.37
This surface mediated Hꢀtransfer pathway is distinct from the waterꢀmediated Hꢀtransfer (in the
absence of a surface) found in DFT computations37, 39 and reproduced in our own simulations .
For comparison, our simulations yield activation barriers for urea decomposition and for water
promoted urea decomposition of 42.4 and 29.3 kcal/mole, compared to corresponding DFT
barriers of 46 and 23 kcal/mole, respectively.
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(a)
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0
-5
-10
Reaction Coordinate
(c)
(d)
H2O
H2O
H2O
H2O
Figure 7. (a) Energy barrier diagram computed for the reaction of urea on the surface of MXene,
(b) Initial configuration for the system containing MXene structure with urea and water
molecules, (c) Comparison on the rate of urea dissociation in three different systems, (d)
Comparison on the rate of NH3 formation in three different systems (Ti: brown, C: green, O: red,
H: white, N: blue).
Next, a molecular dynamics simulation was carried out for a single Ti3C2Tz MXene sheet
with dimension of 5.32×6.14×5.0 nm, terminated with 50% each of randomly distributed OH and
O terminal groups, immersed in a bath of 70 molecules each of urea and water. An initial
configuration is shown in Fig 7b. We performed the simulation with a temperature ramp of
0.005 K/iteration and a time step of 0.1 fs. Due to timeꢀscale limitation of molecular dynamics,
in comparison to experimental conditions, higher temperatures are used to accelerate the
dynamics. In order to evaluate the effect of MXene surface on the urea dissociation, we repeated
the simulation for two other systems, one consisting of 70 urea molecules and the other one
containing 70 urea and 70 water molecules. Fig 7c compares the rate of urea dissociation in three
different systems. As indicated, at the end of simulation, 16 urea molecules are dissociated in the
system containing only urea molecules, while for the ureaꢀwater system, 29 urea molecules are
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dissociated. Presence of MXene sheet accelerates the urea dissociation significantly, as 48 urea
molecules are dissociated. Fig 7d compares the rate of ammonia formation in three different
systems. As shown, for the system with only urea molecules, 7 ammonia molecules are formed at
the end of the simulation, while for waterꢀurea and MXeneꢀwaterꢀurea systems,19 and 35
ammonia molecules are formed at the end of the simulation, respectively. Moreover, note that
presence of MXene structure decreases the initial temperature for the formation of ammonia.
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Combination of the energy barrier calculation and the molecular dynamics simulation
together predict that the progress of reaction (1) is enhanced by the presence of MXene,
mediated by the surface terminal groups. Simulation of the full reaction pathways to the
intercalated products and gas phase CO2 are beyond the scope of this paper. But, it is reasonable
to speculate that the nature of the surface terminal groups, especially the amount of available H
is important in controlling the available reactive sites and protonationꢀdeprotonation of reaction
products.
Although preparation of the MXene from MAX phase is performed in strong acid, the
product was washed to remove remaining acid prior to treatment with urea (see above).
However, it has been reported from calorimetric measurements that in clayꢀlike TiꢀMXene,
produced by a LiFꢀHCl etching process, there remain concentrations as high as 3x10ꢀ2 moles of
exchangeable H+ per mole of MXene.44 H+ at these levels may be present in the current TiꢀMX,
and could facilitate the surfaceꢀmediated urea decomposition, hydrolysis of cyanate by Eq (2) or,
+
importantly, protonation of NH3 to NH4 . In the absence of Brønsted acid functionality, the
reaction may occur at Ti3+ Lewis acid sites assisted by the terminal groups. With respect to
reaction completion, vacuum drying at elevated 110 °C has the effect of decreasing the observed
features from the uꢀMX as seen in Figure 3 and Figure 5. These changes may not be due
entirely to loss of water but could result partly from evolution of CO2 or NH3 as reaction
proceeds in the absence of water.
These facts naturally suggest the possibility that decomposition of the urea leads to
ammonia and its protonation to ammonium. To further test this possibility, the MXene was
treated with ammonium bifluoride to produce intercalation of ammonium ions into the MXene.3
Treatment with ammonium bifluoride also results in an expansion in the interlayer spacing, and
therefore intercalation, as indicated by the XRD (Supplemental Information Figure S1). Figure
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8 compares the INS spectrum for 250 meV neutrons for the ammonium intercalated MXene, aꢀ
MX, along with the uꢀMX. There is a close correspondence between the features seen in the uꢀ
MX and the aꢀMX. It is remarkable that all of the most prominent features in the uꢀMX
spectrum, i.e. those near 1460, 1630 and 1690 cmꢀ1, are closely matched by features in the aꢀMX.
These features correspond to fundamental modes of ammonium, so 1460 cmꢀ1 and 1690 cmꢀ1
may be assigned to the υ4 and ν2 modes of tetrahedral ammonium ion45ꢀ46. Neither of these modes
match the features in the molecular urea or in the Ti(urea) complex (Figure 4). Their close
match to the aꢀMX is an indication that much, or all, of the urea in uꢀMX has decomposed to
ammonium ions which remain intercalated. With this identification, it is possible to estimate the
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+
amount of ammonium in the dried uꢀMX sample to be about 0.77 mmole NH4 per gram uꢀMX.
This estimate is based upon quantifiability of INS and is described in the Supplemental
Information. This should be compared with the IR estimates of CO2 evolved during a 15 h
+
treatment (0.25 mmoles CO2 per gm uꢀMX). Considering the speciation of NH4 / NH3 and CO2
/ H2CO3 between gas, solution and solid phases the ratio of intercalated ammonium to gas phase
CO2 is remarkably close to 2:1 expected from urea decomposition.
Lattice expansion may be in part a result of water incorporation. Vacuum annealing leads
to a decrease in the cꢀlattice parameter as indicated in Figure S1. In addition, the water
intramolecular deformation modes, expected near 1620 cmꢀ1 are decreased in intensity, (Figure
8) although the INS features associated with ammonium are not much affected. It is possible that
water incorporates readily with the ammonium but is not as strongly held within the interlayers.
The water may be responsible for much of the cꢀlattice parameter increase for uꢀMX and we
conclude that the broadening and shift to higher angle observed in XRD upon vacuum drying
(Figure 1 and Figure S1) may be attributed to loss of water from within the interlayers. The cꢀ
lattice parameter does not return to the position for untreated MX because the ammonium
remains intercalated after vacuum annealing, as demonstrated by the INS. Similar changes in
both XRD (Figure S1) and INS (Figure 8) are observed upon vacuum annealing the aꢀMX
sample treated in aqueous ammonium fluoride, although some differences are observed and
expected due to the differences in counterions.
The aꢀMX exhibits additional features in the INS that are absent from the uꢀMX
spectrum. These are the modes at 735 cmꢀ1, 1060 cmꢀ1 and a peak at 1300 cmꢀ1 which rides on a
broad shoulder extending down to 1200 cmꢀ1 (Figure 8). These modes may be a result of the
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bifluoride anion from the ammonium bifluoride intercalant. In matrix IR studies of ammonium
bifluoride and of ammoniaꢀHF coꢀdeposition, primary vibrations at 1093 cmꢀ1 are reported that
c
are assigned to the υ2 vibration of the ammonia subꢀmolecule in the HFꢀNH3 complex.47 This
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mode may account for the peak observed near 1060 cmꢀ1 in the aꢀMX. Cote and Thompson
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reported spectra of solid potassium bifluoride in which the HꢀFꢀH fundamental deformation
mode, υ2 is found to be at about 1250 cmꢀ1, but is split by disorder in the crystal into two
components at 1225 and 1274 cmꢀ1. These approximately match to the shoulder observed below
the 1300 cmꢀ1 peak. Johnson also has pointed out that bands at 1008 and 1247cmꢀ1 from
ammonium bifluoride grow when the matrix is enriched with HF. 47 These comparisons although
approximate, do suggest that the features observed in the aꢀMX that are not seen in the uꢀMX
may be explained by the bifluoride counter ion as altered by their interaction with the ammonium
ions and the MXene interlayer environment. The presence of these peaks in the aꢀMX does not
alter our conclusion that the urea decomposes to ammonium (or ammonia subsequently
protonated) within the interlayers.
u-MX (x10)
a-MX
a-MX vac ann
0.0006
0.0004
0.0002
0.0000
500
1000
1500
Energy transfer (cm-1)
Figure 8. INS spectrum compares the urea intercalated and the ammonium intercalate MXenes,
obtained using 250 meV neutrons. Dashed lines guide the eye through the most prominent
features. For comparison with IR data, neutron energy transfer is converted from meV to cmꢀ1.
If urea decomposes to form ammonium then it may be expected that other products may
be present. Carbamic acid, an intermediate in urea hydrolysis is expected to be unstable and to
react rapidly to CO2 which could be rapidly solvated to bicarbonate or related hydrogen
carbonates. Vibrational features for H2O ꢀ HCO3¯ have been studied previously are typified by
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strong COH bending mode (ν4), in the range of 1175 ꢀ 1225 cmꢀ1 and the CO2 symmetric stretch
(ν3) in the range 1288 to 1364 cmꢀ1, with the observed ranges for both modes depending upon
49
the degree of H2O solvation. No such modes are observed in the INS spectrum of the uꢀMX.
8
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Similarly, if the reaction yields cyanate, then it should be observable through an intense
vibrational peak near 2160 cmꢀ1. 50 Once again, no such mode is observed from uꢀMX in Figure
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3.
But, bicarbonate residing within the interlayers are likely to interact with the Ti or
through terminal groups of the Ti3C2Tz ꢀMXene, modifying the position of the expected modes.
The extent to which the modes may be modified can be estimated from the location of modes
observed for Hꢀcarbonates adsorbed on oxides, of which CeO2 is a wellꢀstudied example. IR
spectrum of the carbonates and HOꢀCOx adsorbed on CeO2 clusters have been summarized for
CeO2 by Vayssilov et al51 who compared experimental frequencies with those computed for
various minimal energy bonding configurations. They cite experimental IR spectra for HOꢀCOx
for two ν(CO) modes bands ranging from 1390ꢀ1413 cmꢀ1 and 1025ꢀ1045 cmꢀ1 while they
computed values of 1374 ꢀ1408 cmꢀ1 and 1000ꢀ1017 cmꢀ1 for these same two v(CO) modes,
indicating the quality of agreement between computed and experimental values. These modes
may be more apparent for IR spectra than for corresponding INS spectra where the H coupled
δ(COH) mode is expected to be more prominent. Vayssilov et al. predict the δ(COH) mode to be
at 1173ꢀ1185 cmꢀ1 for various bonding configurations of HOꢀCOx, matching the experimentally
observed v4 mode for H2O ꢀ HCO3¯ described above. The conclusion is that if HCO3¯ were
present within the uꢀMX, it should give rise to an observable δ(COH) mode in the INS around
1175 cmꢀ1. No such mode is observed, or any other in the range 900 to 1400 cmꢀ1. Evidently
decomposition of the urea occurs readily, but the products containing the C=O portion of the
urea are readily removed, presumably as the CO2 observed by IR.
Intercalation in MXenes and increasing the cꢀlattice parameter were previously found to
affect the electrical resistivities of MXenes. For example, hydrazine treatment results in
increasing the c-lattice parameter by 30% and about fourꢀfold increase in the resistivity7 and to
increases in volumetric and gravimetric capacitance.8 Halim et al.3 reported that ammonium
intercalation results in increasing both the optical transparencies and electrical resistivities
compared to pristine MXene with no intercalants. Understanding why intercalation of ionic and
molecular species affects the physioꢀchemical and electrochemical properties of these active
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layered materials, will require better defining the nature of each intercalant, its stability, transport
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4. Conclusions
MXenes are a new class of 2D materials that have been identified as possible systems for
multiple applications including capacitive energy storage. We have now investigated the reported
intercalation of urea within Ti
3
C
2
T MXene using inelastic neutron scattering. Although
z
characteristic vibrational modes are clearly observable and identifiable in urea and a wellꢀknown
Ti(urea) complex, modes clearly identifiable with intercalated urea are not seen in the uꢀMX.
Instead, modes associated with ammonium are identified by comparison with a corresponding
ammonium intercalated MXene. We conclude that urea is decomposed, partly in the synthesis
solution but also by either Bronsted or Lewis acid sites within the interlayers of the MXene.
Decomposition of the urea by the Ti
3
C
2
Tz ꢀMXene and the fate of the reaction products
has implications for the use of these families of 2D materials for the development in many
applications. Given that the material induces the reactions of the urea, the possibility of profound
changes in the nature of any other intercalated system must be considered. Any intercalant used
to alter the MXene performance in an application may be destabilized by the environment of the
intercalating layers, especially if the layers include active metal sites or strongly acidic/basic
moieties that could serve as catalytic centers. It is also possible that, for a given redox active
intercalant, the stability may depend upon the charge state of the redox couple. A designer must
beware that such destabilization may occur spontaneously during synthesis or result in a pathway
for degradation of performance of the capacitor system.
Acknowledgements
This manuscript has been authored by UTꢀBattelle, LLC under Contract No. DEꢀAC05ꢀ
00OR22725 with the U.S. Department of Energy. The United States Government retains and the
publisher, by accepting the article for publication, acknowledges that the United States
Government retains a nonꢀexclusive, paidꢀup, irrevocable, worldꢀwide license to publish or
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reproduce the published form of this manuscript, or allow others to do so, for United States
Government purposes. The Department of Energy will provide public access to these results of
federally sponsored research in accordance with the DOE Public Access
Plan(http://energy.gov/downloads/doeꢀpublicꢀaccessꢀplan).
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This work was supported as part of the Fluid Interface Reactions, Structures and Transport
(FIRST) Center, an Energy Frontier Research Center funded by the U.S. Department of Energy,
Office of Science, Office of Basic Energy Sciences. This research at ORNL’s Spallation
Neutron Source was sponsored by the Scientific User Facilities Division, Office of Basic
Energy Sciences, U.S. Department of Energy. The authors wish to thank Yury Gogotsi for
helpful comments on the manuscripts.
Supporting Information.
Details for the INS spectroscopic measurements, headspace IR and ReaxFF simulations; XRD
patterns showing the effect of urea and NH4ꢀHF2 intercalation upon the cꢀlattice parameter; full
INS spectra across all recorded incident energies; IR spectrum of headspace during urea
treatment of MXene; details of quantification of INS spectra are supplied as Supporting
Information.
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