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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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Journal of the American Chemical Society

Complexity of Intercalation in MXenes: Destabilization of Urea by TwoDimensional 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,* †

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. ǁ Department of Physics and Engineering Physics, Tulane University, New Orleans, LA 70118, USA. * Corresponding Authors: [email protected] and [email protected] 1 retired from Chemical Sciences Division, Oak Ridge National Laboratory. 2 current address: Department of Chemistry, University of Virginia, Charlottesville, VA. Abstract: 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 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 infra-red 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 MXenes are a new class of two-dimensional (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 prepared by selective extraction of Al from the crystalline MAX phase Ti3AlC2 using fluoride 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.5-6 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,9-10 and sensing.11 Also, intercalation leads to extensive large-scale 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 Nb-Mo carbides have been reported as catalysts for activation of H2 and CO2,13 hydrodesulfurization / hydrodenitrogenation14 and electrocatalytic reactions such as oxygen reduction15-16 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 X-ray diffraction to determine changes in the c-lattice parameter. An increase in the c-lattice 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). Co-intercalation of organic solvents21 and water11 were also reported. Water intercalation in MXenes in presence of ions was the focus of multiple studies.22-23 Ghidiu et al.22 reported step changes in the c-lattice 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 clattice parameter. 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 infra-red (IR) and inelastic neutron spectroscopy (INS). Also, other Ti-urea 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 C-N bonds of the molecule. In contradiction to previous presumptions that urea intercalates in-between 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 3 ACS Paragon Plus Environment


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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’. 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 urea-water 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 ‘u-MX’. Vacuum treatment was done of a portion of the u-MX 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 ‘u-MX 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 (‘a-MX’). 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 ‘a-MX vac ann’ Ti(urea)6 complex: The Ti urea complex material, Ti(urea)6I3, was prepared following a modification of the synthesis described by Hartman et al24 and by Pickering.

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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. Well-formed blue-purple 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 c-lattice 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 c-lattice 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 c-lattice 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.

Figure 1. XRD patterns of the Ti-MXene, (the MX sample (blue-bottom pattern) and the MX after (red-middle pattern) treatment with urea at 60 °C, the u-MX sample and after vaccuum annealing to remove any excess water (orange-top pattern), the u-MX dried sample. Urea is known to form a stable oxygen-bound complex with Ti(III) from an aqueous solution, and it appears to stabilize the Ti(III) oxidation state when compared to H2O bound to 5 ACS Paragon Plus Environment


Ti(III).26-27 The structure of this complex, [Ti(O=C(NH2)2)6]I3, was determined by X-Ray 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 I- anions.26 In general urea is oxygen-bound 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. 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 Keuleers28-29

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 N-H stretching modes, around 3448 and 3345 cm-1 respectively, and including a broad shoulder near 3250 cm-1 assigned to a combination of CO stretch and NH2 deformation or it may be due to two-phonon scattering involving δ (NH2). Features between 1600 and 1700 cm-1 may be assigned to a mixture of C=O stretch and NH2 deformation modes, δ(NH2), while the peak split off at 1462 cm-1 may be assigned primarily to asymmetric CN stretch, νa(CN). It is relevant that the mode at 1462 cm-1 also contains a mixture of NH2 deformation.29 Peaks near 1153 and 1050 cm-1 may be assigned to symmetric and asymmetric NH2 rocking modes, respectively, and a small peak near 1000 cm-1 is due to symmetric CN stretch. Finally, a small peak near 790 cm-1 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 Absorbance (a.u.)

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υ s CN ω CO

ρNH 2

δNH 2

υ asN-H

υC=O

υsN-H

υaCN

500

1000

1500

2000

2500

3000

3500

-1

Wavenumber (cm )


4000

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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. Table 1. Vibrational frequencies observed for urea and for the Ti complex obtained from both IR and INS. Vibrational mode (cm-1) υas NH2 υs NH2 Combination ν CO + δ (NH2); two-phonon δ (NH2) υ CO δ NH2 υa CN enhanced by H in NH2 ρs NH2 ρa NH2 υs CN ω CO

Urea IR a 3448

Urea INS b

Ti(urea)6I3 IR a 3476

Ti(urea)6I3 INS b 3489

3345

3400 (br)

3379 3298 3213 1636 1549

3383

3250 (br) 1681 1630 1600 1462 1153 1050 1003

n/o 1647 1600 1488 1163 1033 (br) n/o

790 --

748 575

3250

1032

n/o 1626 1575 1500 1156 1053 (br) n/o

765 --

697 533

1495 1147

a

IR spectra at room temperature; b converted to cm-1 from the INS spectra at 6 K; n/o = not observed; br = broad; sh= shoulder The features for the Ti-complex 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 cm-1 in urea, resolves into two strong modes at 3298 and 3213 cm-1 in the complex. The C=O stretch that is observed at 1681 cm-1 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, ρ-NH2, red-shifts 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 Ti-urea interaction upon the vibrational modes of the urea. It is wellknown that urea exhibits resonances that lead to zwitterionic intermediates, making it possible for the urea to interact through either at -NH2δ+ end or the -C-O δ- end of the zwitterion.30 As described by Penland et al, the bonding in urea complex can be distinguished by the shifts in the features near 1700 cm-1. 31 According to Penland, if the urea molecule is bonded through the O in 7 ACS Paragon Plus Environment


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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 Ti-complex there is no evidence in the IR spectrum of a peak at higher wavenumber than the 1680 cm-1 observed for urea, arguing against 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 cm-1, the urea bonds in the complex through the formation of C=OTi 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 u-MX 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 cm-1, 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 blue-shifted and sharpened compared to the urea, a trend also observed in the IR spectra (Figure 2). It is therefore surprising that the u-MX does not exhibit more clearly resolved peaks matching the NH stretching features of the Ti-complex. Instead, as shown in Figure 3, a broad feature is observed in the region from 3100 to 3600 cm-1 which is similar to the feature seen for the untreated MX containing no urea. This broad feature therefore may be associated in part with O-H stretching modes of water, intercalated or adsorbed, or to terminal OH groups or counter-ions within the Ti3C2Tz itself. intensity.

Upon drying the u-MX, the broad feature changes shape and decreases in

The INS spectra of both urea and the Ti-complex also exhibit intense combination

bands due to two-phonon scattering, located near 2200, 2750 and 4000 cm-1. For example, the peak near 2750 cm-1 is a combination of strong modes at 1600 and 1163 cm-1 (see below). These combination modes are not visible in the spectrum of the u-MX. 8 ACS Paragon Plus Environment

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i

5.0x10-4 urea Ti(urea)6I3

MX (x10) u-MX (x10) u-MX dried (x10)

G (E) (arb units)

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0.0 2000

2500

3000

3500

4000

-1

Energy transfer (cm )

Figure 3. INS spectrum obtained using 600 meV incident neutrons, recorded at 6 K. Dashed lines indicate the location of the νs and νa N-H stretches of urea observed in the IR spectrum of urea. For comparison with IR data, neutron energy transfer is converted from meV to cm-1. 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 CN 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 cm-1 (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 cm-1 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 cm-1 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 cm-1 is not detectable under the much stronger NH2 rocking modes. Below 1000 cm-1, 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 Ti-urea complex observed by IR are mimicked in 9 ACS Paragon Plus Environment


the INS spectrum, namely: NH2 deformations and the NH2 rock are red-shifted in the complex compared to the solid urea and the CN stretches are slightly blue-shifted in the complex compared to solid urea. Some of the phonon peaks (below 900 cm-1) 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. urea Ti(urea)6I3

G (E) (arb units)

0.002

δNH 2


ρ s NH 2

δ aNH 2 v

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ρ a NH 2

+ CN

0.001

0.000

500

1000

1500 -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 cm-1. For clarity, curves are offset from zero. Once again, the features observed for the u-MX do not well match the expectations for intercalated molecular urea. In particular, the rocking mode ρ-NH2, seen clearly for urea in the INS spectrum, are completely absent from the u-MX spectrum. In addition, there is not a clear correspondence between the νaCN and δNH2 modes, clearly observed in urea and the Ti-urea complex, and the peaks observed for the u-MX in this range of energy transfers. Yet, the features present in this region (near 1460 and 1700 cm-1) for the u-MX indicate the presence of molecular features that are not observed in the MX. The intense phonon features observed for the urea (400-900 cm-1) are absent in both the u-MX and the MX spectra, where only broad features, likely associated with water, are observed. Similarly, a broad feature near 1650 cm-1 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 u-MX is consistent with this conclusion. The persistence of the features near 1460 and 1700 cm-1 indicate that a molecular species remains in the dried u-MX, although it is not urea. 10 ACS Paragon Plus Environment

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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 cm-1 are clear and strong for urea and are present but shifted in the Ti-complex (Figure 5a). These are intra-molecule librations of NH2 groups and some may be described as deformation modes involving the N-C-O 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 uMX where two broad peaks at 370 and 470 cm-1 are super-imposed 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 u-MX, 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 cm-1, as seen in Figure 5b. Modes between 100 – 200 cm-1 may be associated with whole molecule librations while those below 100 cm-1 may be due to translations.34 It is noteworthy that a peak at about 120 cm-1 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.



a.

E i = 110 meV

0.004

δN-C-N

urea Ti(urea)6I3

G (E) (arb units)

0.003

δN-C-O


0.002

0.001

0.000

200

400

600

800

-1


b.

E i = 30 meV

0.006

urea Ti (urea)6 MX (x10) u-MX (x10) u-MX dried (x10)

0.005

G (E) (arb units)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.004 0.003 0.002 0.001 0.000

para H 2

50

100

150

200

-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 cm-1. For clarity, curves are offset from zero.

Decomposition and hydrolysis of urea The discrepancy between the spectra of the reference compounds and the u-MX suggests the possibility that although the urea treatment leads to intercalation, as indicated by the c-lattice 12 ACS Paragon Plus Environment

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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 well-studied 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 2x10-7 s-1 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 1 NH22C = O 2 → NH3 + OCNH 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.37-39 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 cm-1 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 13 ACS Paragon Plus Environment


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.

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/ (2x10-7 s-1)).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 H2N-C bond so that its cleavage precedes protonation of the H2N- group by surface -OH, and so there is no Zwitterionic intermediate.37 This surface mediated H-transfer pathway is distinct from the water-mediated H-transfer (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.



(a)

(b)

25

Energy (kcal/mol)

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20 15 10 5 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 time-scale 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 urea-water system, 29 urea molecules are 16 ACS Paragon Plus Environment

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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 water-urea and MXene-water-urea 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. 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 protonation-deprotonation 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 clay-like Ti-MXene, produced by a LiF-HCl etching process, there remain concentrations as high as 3x10-2 moles of exchangeable H+ per mole of MXene.44 H+ at these levels may be present in the current Ti-MX, and could facilitate the surface-mediated 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 u-MX 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 17 ACS Paragon Plus Environment


8 compares the INS spectrum for 250 meV neutrons for the ammonium intercalated MXene, aMX, along with the u-MX. There is a close correspondence between the features seen in the uMX and the a-MX. It is remarkable that all of the most prominent features in the u-MX spectrum, i.e. those near 1460, 1630 and 1690 cm-1, are closely matched by features in the a-MX. These features correspond to fundamental modes of ammonium, so 1460 cm-1 and 1690 cm-1 may be assigned to the υ4 and ν2 modes of tetrahedral ammonium ion45-46. Neither of these modes match the features in the molecular urea or in the Ti(urea) complex (Figure 4). Their close match to the a-MX is an indication that much, or all, of the urea in u-MX has decomposed to ammonium ions which remain intercalated. With this identification, it is possible to estimate the amount of ammonium in the dried u-MX sample to be about 0.77 mmole NH4+ per gram u-MX. 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 u-MX). 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 c-lattice parameter as indicated in Figure S1. In addition, the water intramolecular deformation modes, expected near 1620 cm-1 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 c-lattice parameter increase for u-MX 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 clattice 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 a-MX sample treated in aqueous ammonium fluoride, although some differences are observed and expected due to the differences in counterions. The a-MX exhibits additional features in the INS that are absent from the u-MX spectrum. These are the modes at 735 cm-1, 1060 cm-1 and a peak at 1300 cm-1 which rides on a broad shoulder extending down to 1200 cm-1 (Figure 8). These modes may be a result of the 18 ACS Paragon Plus Environment

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bifluoride anion from the ammonium bifluoride intercalant. In matrix IR studies of ammonium bifluoride and of ammonia-HF co-deposition, primary vibrations at 1093 cm-1 are reported that are assigned to the υ2c vibration of the ammonia sub-molecule in the HF-NH3 complex.47 This mode may account for the peak observed near 1060 cm-1 in the a-MX. Cote and Thompson

48

reported spectra of solid potassium bifluoride in which the H-F-H fundamental deformation mode, υ2 is found to be at about 1250 cm-1, but is split by disorder in the crystal into two components at 1225 and 1274 cm-1. These approximately match to the shoulder observed below the 1300 cm-1 peak. Johnson also has pointed out that bands at 1008 and 1247cm-1 from ammonium bifluoride grow when the matrix is enriched with HF. 47 These comparisons although approximate, do suggest that the features observed in the a-MX that are not seen in the u-MX 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 a-MX 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

G(E) (arb units)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.0004

0.0002

0.0000 500

1000

1500 -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 cm-1. 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 19 ACS Paragon Plus Environment


strong COH bending mode (ν4), in the range of 1175 - 1225 cm-1 and the CO2 symmetric stretch (ν3) in the range 1288 to 1364 cm-1, with the observed ranges for both modes depending upon the degree of H2O solvation.

49

No such modes are observed in the INS spectrum of the u-MX.

Similarly, if the reaction yields cyanate, then it should be observable through an intense vibrational peak near 2160 cm-1. 50 Once again, no such mode is observed from u-MX in Figure 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 H-carbonates adsorbed on oxides, of which CeO2 is a well-studied example. IR spectrum of the carbonates and HO-COx 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 HO-COx for two ν(CO) modes bands ranging from 1390-1413 cm-1 and 1025-1045 cm-1 while they computed values of 1374 -1408 cm-1 and 1000-1017 cm-1 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 1173-1185 cm-1 for various bonding configurations of HO-COx, matching the experimentally observed v4 mode for H2O - HCO3¯ described above. The conclusion is that if HCO3¯ were present within the u-MX, it should give rise to an observable δ(COH) mode in the INS around 1175 cm-1. No such mode is observed, or any other in the range 900 to 1400 cm-1. 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 c-lattice 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 four-fold 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 physio-chemical and electrochemical properties of these active 20 ACS Paragon Plus Environment

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layered materials, will require better defining the nature of each intercalant, its stability, transport and its bonding environment.

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 Ti3C2Tz MXene using inelastic neutron scattering. Although characteristic vibrational modes are clearly observable and identifiable in urea and a well-known Ti(urea) complex, modes clearly identifiable with intercalated urea are not seen in the u-MX. 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 Ti3C2Tz -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 UT-Battelle, LLC under Contract No. DE-AC0500OR22725 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 non-exclusive, paid-up, irrevocable, world-wide license to publish or 21 ACS Paragon Plus Environment


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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/doe-public-access-plan).

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 NH4-HF2 intercalation upon the c-lattice 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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