Local chemical bonding and structural properties in Ti3AlC2 MAX phase and Ti3C2Tx MXene probed by Ti 1s X-ray absorption spectroscopy

The chemical bonding within the transition-metal carbide materials MAX phase Ti3AlC2 and MXene Ti3C2Tx is investigated by X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopies. MAX phases are inherently nanolaminated materials that consist of alternating layers of Mn+1Xn and monolayers of an A-element from the IIIA or IVA group in the periodic table, where M is a transition metal and X is either carbon or nitrogen. Replacing the A-element with surface termination species Tx will separate the Mn+1Xn-layers forming two-dimensional (2D) flakes of Mn+1XnTx. For Ti3C2Tx the Tx corresponds to fluorine (F) and oxygen (O) covering both sides of every single 2D Mn+1Xn-flake. The Ti K-edge (1s) XANES of both Ti3AlC2 and Ti3C2Tx exhibit characteristic pre-edge absorption regions of C 2p - Ti 3d hybridization with clear crystal-field splitting's while the main-edge absorption features originate from the Ti 1s ->4p excitation, where only the latter shows sensitivity towards the fcc-site occupation of the termination species. The coordination numbers obtained from EXAFS show that Ti3AlC2 and Ti3C2Tx are highly anisotropic with a strong in-plane contribution for Ti and with a dynamic out-of-plane contribution from the Al monolayers and termination species, respectively. As shown in the temperature-dependent measurements, the O contribution shifts to shorter bond length while the F diminishes as the temperature is raised from room temperature up to 750 {\deg}C.


Introduction
Despite the large interest in graphene [1], which lacks a natural band gap, it has been difficult to artificially produce graphene-based materials with suitable band gaps.This has encouraged researchers to explore other two-dimensional (2D) materials such as hexagonal boron nitride (h-BN), molybdenum disulphide (MoS2), tungsten disulphide (WS2), and MXenes (Mn+1XnTx).The last example is also the latest 2D material, developed in the last decade, and consists of a family of 2D transition metal carbides denoted Mn+1XnTx (n = 1, 2, 3) where M is a transition metal, X is either carbon or nitrogen, and Tx denotes surface termination species [2][3][4].The layered structures contain more than one element and, thus, offer properties that may be useful for transistors and spintronics [5], 2D-based electronics and screens [6] in addition to energy storage systems such as supercapacitors [7], Li-ion batteries [8], fuel-and solar cells [9] as well as transparent conductive electrodes [10] and composite materials with high strength [11].
The parent precursor compounds of MXenes are inherently nanolaminated materials known as MAX phases (Mn+1AXn, n = 1, 2, 3) [12] (space group P63/mmc), where A is a p-element that usually belongs to groups IIIA or IVA in the periodic table.These phases contain more than 150 variants [13], including Ti3AlC2 that is a precursor for the Ti3C2Tx MXene.To make MXene from Mn+1AXn, the weakly bounded A-layers are etched away and replaced by surface termination groups (Tx) in the exfoliation process [2].The delamination results in weakly bounded stacks of 2D sheets with Mn+1XnTx composition.
Generally, MXenes consist of a core of a few atoms thick 2D Mn+1Xn conductive carbide layer that is crystalline in the basal plane and a transition metal surface that can be functionalized for different material properties by changing the chemistry of the termination species.Layered structures like MXenes contains more than one element and can therefore offer better variations of physical properties than pure materials, such as graphene, since they can provide a larger number of compositional variables that can be tuned for specific properties.Figure 1 shows schematic side views of the Ti3AlC2, Ti3C2Tx, and TiC structures where the blue and the black spheres are the Ti and C atoms, respectively, with strong covalent bonds in the conductive carbide core layer.The stacking of the Ti and C atoms forms three monolayers of Ti and two monolayers of C in an alternated sequence.In the Ti3AlC2 the Ti3C2-layers are alternated with monolayers of Al, highlighted in Fig. 1 as yellow spheres, while the Ti3C2Tx shows purple and red spheres on both sides of the transition metal carbide that are the F and O atoms, respectively, terminating the surfaces.Two alternatives for the F and O atoms to coordinate on the Ti3C2 surfaces are the three-fold hollow face-centered cubic (fcc) sites and the three-fold hollow hexagonal close-packed (hcp) sites [14][15][16][17]; an fcc site is formed by three surface Ti atoms in a triangular formation where the center of the triangle is above a Ti atom in the second (middle) Ti-monolayer while an hcp site is formed by three surface Ti atoms in a triangular formation where the center of the triangle is above a C atom in the adjacent C-monolayer.Other alternative coordination sites are on top of the surface Ti atoms or in bridge sites between two Ti atoms [14].The fcc site (also called A-site) as the preferred site for the termination species on Ti3C2Tx has been confirmed experimentally using high-resolution transmission electron microscopy (HRTEM) and X-ray photoelectron spectroscopy (XPS) [18].The combined HRTEM/XPS study found that F only occupies the fcc sites in competition with O and that O also occupies other sites, e.g.bridge sites or on top sites, but not the hcp sites (also called B-sites).The study found no other terminating species than F and O and that F desorbs at elevated temperatures (>550 °C).
Despite vast interest in MXenes in general, and in Ti3C2TX in particular, there is little known experimentally about the bonding between the transition metals and the terminating species, Tx.Previous work of MXenes' electronic structure has mainly been based on ground-state density functional theory (DFT) calculations at 0 K [14][15][16][17]19].Many of the theoretical investigations find no obstacles regarding replacing the inherently formed termination species with others in the pursuit of tailoring the properties for specific applications.Yet there are no indisputably experimental evidences showing that the inherently formed termination species can be replaced (re-termination) [20].The different theoretical and experimental results and experiences show how important it is to fully understand the bonding conditions at the surfaces of the MXenes.
In this work, we will elucidate the local structural properties and interactions around the Ti atoms in Ti3AlC2 and Ti3C2Tx through synchrotron radiation-based Ti K-edge (1s) X-ray absorption spectroscopy, including both X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS).XANES probes the unoccupied density of states of the absorbing atoms and is therefore an ideal technique for determining the chemical surroundings and local bonding structure around the transition metal.The 1s electron transition can, as a consequence of the selection rule, only occur to molecular orbitals with p-character (electric dipole transition) and d-character (electric quadrupole transition), although the probability of the 1s → 3d transitions are about one thousandth of 1s → np transitions [21,22].EXAFS provides information about the coordination numbers, atomic distances, and amount of atomic displacements and disorder around the probed element [23].
Ti 1s XANES and EXAFS spectra of Ti3AlC2 and Ti3C2Tx have been presented previously [24][25][26].Yet a detailed analysis of the spectra remains to be performed.Instead of only comparing the Ti 1s XANES and EXAFS spectra of different compounds for similarities (or not) or a crude estimate of the Ti oxidation state, as in previous XANES and/or EXAFS studies of MAXphases and MXenes [24][25][26][27][28][29][30], the present work aims to learn more about the local bonding around the probed Ti atoms in both Ti3AlC2 and Ti3C2Tx through distinguished Ti 1s XAS features.
The Ti3C2Tx samples were fabricated as freestanding foils through wet etching, which leads to inherent F and O terminations [18].Through heat treatments the fcc site coordinated F will desorb and be replaced by O, which prefer the fcc site when it is available [18].The change of the fcc coordination will induce modifications in the XANES and EXAFS spectra and, thus, reveal local information of the bonding situation between the transition metal atoms and termination species.Hence, the study demonstrates how X-ray absorption spectroscopy can be used to probe the MXene surfaces to shed more light onto the local chemical interaction between F, O, and Ti atoms, which is relevant knowledge when tailor designing MXenes for after-sought material properties.

Sample preparation
Powders of Ti3AlC2 were produced starting with a mixture of TiC (Alfa Aesar, 98+%), Ti (Alfa Aesar, 98+%), and Al (Alfa Aesar, 98+%) of 1:1:2 molar ratios.The mixture was processed in a mortar with a pestle for 5 min and thereafter inserted in an alumina tube furnace.While a continuous stream of Ar gas, the furnace was heated with a rate of 5 °C min -1 up to 1450 °C and held for 280 min before cooled down to room temperature.The resulting material is a lightly sintered Ti3AlC2 sample, which is then crushed to powder of particle size < 60 microns using mortar and pestle.A few mg of the Ti3AlC2 powder was mixed with polyethylene powder (Aldrich, 40-48 µm particle size) and thereafter cold pressed into a ~500 µm thick pellet for the X-ray absorption spectroscopy.
To convert the Ti3AlC2 to Ti3C2Tx flakes, half a gram of Ti3AlC2 powder was added to a premixed 10 ml aqueous solution of 12 M HCl (Fisher, technical grade) and 2.3 M LiF (Alfa Aesar, 98+%) in a Teflon bottle.Prior to adding the MAX powder to the HCl/LiF(aq) solution, the latter was placed in an ice bath to avoid the initial overheating that otherwise can be a consequence of the exothermic reaction when the MAX power is added.After 0.5 h in the ice bath the bottle was placed on a magnetic stirrer hot plate in an oil bath and held at 35 °C for 24 h.The mixture was thereafter washed, first through 3 cycles using 40 ml of 1 M HCl(aq) and thereafter 3 cycles using 40 ml of 1 M LiCl (Alfa Aesar, 98+%).Then, the mixture was washed through cycles of 40 ml of deionized water until the supernatant reached a pH of approximately 6.In the end, 45 ml of deionized water were added, which was deaerated by bubbling N2 gas through it and sonicated using an ultrasonic bath for 1 h.The resulting suspension was centrifuged for 1 h at 3500 rpm, which removed larger particles.The supernatant produced had a Ti3C2Tx concentration of 1 mg ml -1 .To make freestanding films, 20 ml of the suspension were filtered through a nanopolypropylene membrane (3501 Coated PP, 0.064 µm pore size, Celgard, USA).The obtained Ti3C2Tx foil is in detail described in Ref. [7].
Moreover, to observe XANES and EXAFS features originating from termination species, the surfaces must be free from oxidized material, mainly TiO2 [31].The new-made Ti3C2Tx foils were therefore stored in argon (Ar) atmosphere and mounted on the sample holder in a glove-bag filled with nitrogen gas (N2).A continuous flow of N2 protected the sample from oxidation during measurement.Hence, the obtained XANES and EXAFS spectra of the Ti3C2Tx foils have no detectable contribution from TiO2 impurities.The insignificant amount of TiO2 impurities and carbon containing contamination in the Ti3C2Tx samples was confirmed through XPS.The XPS further showed a small amount of adsorbed Cl that desorbed completely at a moderate heat treatment.In addition, no indication of OHtermination was observed, which agrees with the combined HRTEM/XPS study [18].The Ti3AlC2 (and TiC), on the other hand, showed small amounts of TiO2 impurities in the XPS spectra.Contribution from TiO2 impurities in the Ti 1s XANES and EXAFS spectra of the Ti3AlC2 (and TiC) can therefore not be excluded.Nevertheless, the TiO2 contribution showed to be at a negligible level.

XANES and EXAFS analysis
The XANES and EXAFS spectra were measured at the Ti 1s edge using a Si111 crystal in the monochromator at the wiggler beamline BALDER on the 3 GeV electron storage ring at MAX IV in Lund, Sweden.The X-ray absorption of the Ti3C2Tx MXene, the Ti3AlC2 MAX phase, a TiC reference, and a Ti metal reference foil were monitored in transmission mode with ionization chambers (Io: 200 mbar N2 and He; I1: 2 bar N2).The Ti3C2Tx sample was positioned in a water-cooled gas cell (Linkam Scientific Instruments) with a low N2 flow and a heating element that enabled measurements at different temperatures.The Ti 1s XANES spectra were obtained at room temperature (RT) and at 250 °C after 20 minutes heat treatments at 250, 550, and 750 °C.The energy resolution at the Ti 1s edge of the beamline monochromator was ~1.0 eV with 0.1 eV energy steps for XANES and 0.2 eV steps for EXAFS.The photon energy scale was calibrated using the first derivative of a Ti foil absorption spectrum, where the first inflection point was set to 4.9660 keV.The obtained spectra were normalized below the absorption edge before the background intensity subtraction and thereafter normalized above the absorption edge in the photon energy region of 5.045-5.145keV.Self-absorption effects were found to be negligible in the normal incidence geometry for the Ti3C2Tx and the Ti foils.The spectra of the Ti3AlC2 and TiC pellets showed, on the other hand, some self-absorption effects even at normal incidence and have therefore been corrected using the simple function AI(e)(1-BI(e)) -1 , where A is a scaling factor, B is the self-absorption compensation factor, and I(e) is the Ti 1s XANES spectrum of Ti3AlC2 and TiC, respectively.A and B are adjusted until the absorption features in the photon energy region of 5.045-5.145keV have similar appearance as for the Ti 1s XANES spectrum of Ti3C2Tx.The Ti-Ti, Ti-C, Ti-O and Ti-F scattering paths obtained from the Effective Scattering Amplitudes (FEFF) [32][33][34] were included in the EXAFS fitting using the Visual Processing in EXAFS Researches (VIPER) software package [35].The k 2 -weighted χ EXAFS oscillations were extracted from the raw absorption data, the average of 10 absorption spectra, after removing known monochromator-induced glitches and subsequent atomic background subtraction and normalization.The atomic distances (R), number of neighbors (N), Debye-Waller factors (σ 2 , representing the amount of disorder) and the reduced χr 2 as the squared area of the residual, were determined by fitting the back-Fourier-transform signal between k=0-12.5 Å −1 originally obtained from the forward Fourier-transform within R=0-3.35Å of the first coordination shell using a Hanning window function [32][33][34] with a many body factor of S0 2 =0.8.The disorder and high-frequency thermal vibration of the atoms depending on the temperature was accounted for by an increasing Debye-Waller term s 2 =s 2 stat+s 2 vib consisting of a static and a vibrational part that was proportional to the difference of the mean square atomic displacements [36][37][38].

Results and Discussion
Figure 2 shows the obtained high-resolution Ti 1s XANES spectra of Ti-metal, TiC, Ti3AlC2, and Ti3C2Tx.The absorption spectra consist of two regions: pre-edge and main-edge.Features in the pre-edge region of K-edge XANES spectra of 3d transition metal compounds are assigned to electric dipole (1s → np) and quadrupole (1s → 3d) transitions.However, the intensity contribution of the latter is minor, because of orbital symmetry restrictions [21,22,39], and pre-edge features are therefore in most cases assigned to 1s electron excitation into p-d hybridized orbitals, i.e. the transition into the p-component of the molecular orbitals that have both p-and d-character.The intensity of the pre-edge features depends mainly on coordination, symmetry, and bond angles [40].
The pre-edge features in the spectrum of the Ti metal reference, peaks A and B at 4.967 and 4.971 keV, respectively, are typical examples of 1s → p-d hybridized molecular orbitals excitation near the Fermi energy (Ef) [41].The sharp peak A near the Fermi energy originates from Ti 1s excitation into a local Ti 3d -4p mixing orbital (internal orbital mixing in the probed element), while peak B originates from Ti 1s excitation into hybridized 3d -4p orbital where the 3d contribution originates from neighboring Ti atoms [42,43].The intensity of the pre-edge features is, however, reduced because of the six-fold coordination of the Ti atoms in the metal [41].The main-edge region consists of two broad features C (white line) and D that corresponds to 1s → 4p excitations [43].With the Ti 1s XANES spectrum of the Ti metal reference it is possible to estimate the Ef position, which is located close to the on-set of peak A at 4.9642 keV.The full width at half maximum of peak A of the Ti metal reference (1.3 eV) is also an indication of the good resolution at the Balder beamline.The XANES spectra of TiC, Ti3AlC2, and Ti3C2Tx display significantly different structures compared with the Ti metal reference spectrum (see Fig. 2).The pre-edge region consists of two features, which is highlighted in Fig. 3.The TiC and Ti3AlC2 spectra show a shoulder at 4.9679 keV and a peak at 4.9710 and 4.9709 keV, respectively, while the Ti3C2Tx spectrum shows a shoulder at 4.9689 keV and a peak at 4.9711 keV.A basic density of states calculation obtained from a simple Ti3C2Tx model with Tx being F on the fccsites and O on bridge sites shows two C 2p peaks located at 4.2 and 7.1 eV above the Ef and two Ti 3d peaks located at 4.0 and 6.9 eV above Ef.The orbital mixing provides the C 2p -Ti 3d hybridization and would correspond to ~4.968 and ~4.971 keV, respectively, in the Ti 1s XANES, which is close to the two pre-edge features obtained in the experimental spectra.Hence, the Ti 1s XANES pre-edge features of TiC, Ti3AlC2, and Ti3C2Tx are assigned to Ti 1s → C 2p -Ti 3d hybridized molecular orbitals excitation.The Ti3C2Tx spectrum shows lower intensity in the preedge region compared to TiC and Ti3AlC2, which suggests that the Ti 1s → C 2p -Ti 3d hybridized molecular orbitals excitations are reduced for Ti3C2Tx compared with both TiC and Ti3AlC2.The energy difference between the two C 2p -Ti 3d peaks corresponds the crystal-field splitting of the Ti 3d states into the t2g and eg orbitals [43] and can, thus, experimentally be determined to be 3.1, 2.9, and 2.2 eV for TiC, Ti3AlC2, and Ti3C2Tx, respectively.The absence of a sharp peak near the Ef in the Ti 1s XANES of TiC, Ti3AlC2, and Ti3C2Tx indicates that there is no local unoccupied Ti 3d-4p hybridized orbital available for electron excitation.
The main-edge region shows two peaks, C and D in Fig. 2, which because of the 2D nature is relatively sharp for Ti3C2Tx.The peaks originate from Ti 1s → 4p excitations [43].In addition, a closer look at the rising edge, i.e. the steep intensity increase at the main-edge, reveals a shoulder at 4.9780 keV.The shoulder is more noticeable in the Ti3C2Tx spectrum, which is because of the high-energy shift of the first main-edge peak C; the peak C is at 4.9850 and 4.9847 keV for the TiC and Ti3AlC2 spectra, respectively, while the Ti3C2Tx spectrum has the peak C at 4.9861 keV.The position of peak C for the TiC and Ti3AlC2 spectra is almost the same, which suggests that the interaction between the Al-layers and the Ti3C2-layers in Ti3AlC2 is very weak.The 1.4 eV shift of the C peaks between the Ti3AlC2 and Ti3C2Tx spectra is a consequence of the replacement of the weak interacting Al-layers with the stronger interacting termination species Tx, where the F and O atoms attract charge from the Ti atoms.
In Fig. 2 there are also Ti 1s XANES spectra of the Ti3C2Tx sample after it has been heated to 250, 550 and 750 °C, respectively.After each heat treatment (to 550 and 750 °C) the sample was brought back to 250 °C before XANES-spectrum recording.As expected there are no significant changes in the spectra after the heat treatments to 250 and 550 °C, because temperatures above 550 °C are needed to alter the termination of the Ti3C2Tx surfaces [18].Included in Fig. 2 are also difference spectra that highlight the influence from the heat treatments of the Ti3C2Tx sample.The small deviations from the zero line are inconsiderable  for the temperatures 250 and 550 °C, although there is an indication of that 550 °C is the temperature threshold onset when to introduce changes in the Tx coordination, which then are reflected in the Ti 1s XANES spectra.A heat treatment to 750 °C (and subsequent cooling to 250 °C) provides, on the other hand, a high-energy shift of the peaks in the main-edge region of 0.5 eV, which has the effect that the pre-edge region appears to show a slightly reduced intensity.The difference spectrum for the 750 °C spectrum shows the characteristic variations common for a main-edge energy shift and not an intensity redistribution.It is interesting to note that the energy positions of the features in the pre-edge region are almost not affected by the heat treatment, see Fig. 3.The t2g peak has the same position while the eg peak has shifted 0.1 eV and, thus, widen the crystal-field splitting slightly.That the pre-edge region, which mainly originates from Ti 1s excitations into the C 2p -Ti 3d hybridized molecular orbitals in the Ti3C2 layer, is almost unaffected by the heat treatments is suported by the previous combined HRTEM/XPS study that found that while removing F from a Ti3C2Tx sample through a heat treatment the C 1s XPS carbide peak remained unaffected [18].That the crystal-field splitting to some extent widens suggest a stronger interaction between the O and the fcc-site compared to F. The main-edge features that originate from the Ti 1s → 4p excitation show, on the other hand, higher sensitivity toward the fcc-site occupation.
The XANES spectra of Ti3C2Tx show some similarities with the XANES spectrum of TiO2 [24][25][26][27]41,43,44]. A direct comparison shows that the pre-edge region of Ti3C2Tx has more intensity and a smaller crystal-field splitting.The absorption rising edge of the Ti3C2Tx spectrum is shifted about -3 eV while the absorption energy shift at peak C is about -1 eV.A larger difference between the Ti3C2Tx and the TiO2 XANES spectra is the shape of peak D where the TiO2 shows strong peak intensity at 5.0035 keV that is absent in the Ti3C2Tx spectrum before the heat treatment; hence, no detectable contribution from TiO2 impurities in the Ti3C2Tx sample.The Ti 1s XANES spectra of the 750 °C heat treated Ti3C2Tx show a trend of changes -slightly larger crystal-field splitting, 0.5 eV shift of the rising edge and the C peak, and the intensity shift in the D peak inducing a shoulder at 5.0035 keV -that suggest a stronger Ti-O interaction in heat treated Ti3C2Tx compared to the non-treated Ti3C2Tx.Hence, the Ti 1s XANES supports the observatiion [18] of that F occupies only the fcc-sites and that a heat treatment up to 750 °C removes F and make the fcc-sites available for O where the Ti-O interaction becomes stronger.(Experiments with and without the low N2 flow in the Linkam gas cell ensured that no oxidation of the Ti3C2Tx sample occurred in the presented work.) Figure 4 shows the EXAFS structure factor oscillations of Ti3AlC2 and Ti3C2Tx in comparison with the Ti metal reference, obtained from raw data that has not been phase shifted.The structure factors χ are displayed as a function of the wave vector k, that were k 2 -weighted to highlight the higher k-region, where k = ℏ −1 sqrt[2m(E -Eo)] is the wave vector of the excited electron in the X-ray absorption process.The frequency of the oscillations and the intensity of  the EXAFS signal are directly related to the atomic distances (R) and the number of nearest neighbors (N), respectively; a higher frequency of the oscillations implies extended R while an enlarged amplitude implies increased N.
Starting with Ti metal at the top of Fig. 4, we observe the main oscillations at 4.18, 5.55, 6.80 Å -1 where the middle one is caused by the Ti-Ti in-plane scattering that corresponds to the distance for the a-axis in the hexagonal crystal structure.For k-values above 12 Å -1 , the oscillations are damped out and simultaneously the noise increases.
For Ti3AlC2 and Ti3C2Tx, shown in the middle and bottom of Fig. 4, the main sharp oscillations occur in the 3.4-7.0Å -1 k-space region.There are also peaks at k=4.63 and 5.83 Å -1 , i.e. between the main Ti-Ti peaks, that only appear as weak shoulders in the Ti metal.The positions of the three main Ti-Ti peaks in Ti3AlC2 and Ti3C2Tx (3.93, 5.28, 6.50 Å -1 ) are shifted -0.25 to -0.30 Å -1 in comparison to Ti metal (4.18, 5.55, 6.80 Å -1 ) and similar as for TiC [45,46].The small features at low-k values at 1.58 and 2.4-2.7 Å -1 for the Ti3C2Tx are associated with superimposed oscillations from Ti-O/F scattering.
While the heat treatments do not cause any k-shifts, the intensities of the oscillations decrease with increasing temperature, except for the oscillation at k=4.63 Å -1 .The oscillation intensity of the double peak at 7.55-7.95Å -1 also decreases with increasing temperature.More interestingly, the small features at 1.58 and 2.4-2.7 Å -1 are also affected by the heat treatment.In order to analyze the detailed local structure and atomic distances in the films, Fourier transforms of the EXAFS data were performed.
Figure 5 shows the magnitude of the Fourier transform obtained from the k 2 -weighted EXAFS oscillations χ(k) in Fig. 4 by the standard EXAFS procedure [34] related to the radial distribution function.The horizontal arrow at the top of Fig. 4 indicates the applied k-window.Table I shows the final results of the EXAFS fitting using the FEFF scattering paths of Ti3AlC2 and Ti3C2Tx and hcp Ti metal as structure model systems.The obtained radius values are in comparison to atomic distances determined for lattice parameters from X-ray diffraction (XRD) in the literature, listed in parenthesis in Table I.The initial crystal structure in the modelling assumes a Ti3C2F1O2 composition in line with previous quantitative core-level XPS results [18].However, the obtained atomic distances have sources of errors such as photon energy calibration and dispersion, statistical noise, and inaccuracies in the ab initio calculations of scattering paths using FEFF.The corresponding errors in XRD are in the same order of magnitude as for EXAFS.
In Ti metal with hcp structure, the main peak consists of the in-plane Ti-Ti scattering (N=6) at 2.842 Å and (N=6) of 2.966 Å, where the latter corresponds to the a-axis of the crystal.For comparison, the peak that consists of the out-of-plane Ti-Ti scattering path of the c-axis is FIG. 5. Fourier transform obtained from the k 2 -weighted EXAFS oscillations χ(k) in Fig. 4 of Ti metal foil, Ti3AlC2, and Ti3C2Tx.
Ti-C Ti-Ti Ti-Al located at 4.675 Å as a weak feature.The corresponding values obtained from XRD measurements are 2.897, 2.950, and 4.686 Å, respectively [47].The more intense peak in Ti metal observed at ~ 4.7 Å in Fig. 4 is because of the superposition of the three longer direct Ti-Ti scattering paths with atomic distances 5.079-5.110Å in the higher order coordination shells.
For Ti3AlC2, shown in the middle in Fig. 5, there is a slight shift of the main Ti-Ti peak to larger atomic distances and a new peak corresponding to Ti -C bonding appears at ~1.66 Å.In addition, there is a prominent peak at ~3.8 Å caused by Ti -Ti and Ti -Al scattering from the central Ti layer.
The main peak of the Ti3C2Tx, shown at the bottom in Fig. 5, is dominated (N=4.982) by the in-plane Ti-Ti scattering at 3.016 Å; the in-plane Ti-Ti scattering corresponds to the a-axis unit cell-edge.The weaker out-of-plane scattering at 3.072 Å shows a significantly lower intensity (N=1.312), which is not surprising for a 2D material.As also observed in Fig. 5 the Ti-Ti distances of the in-plane and out-of-plane contributions of Ti3C2Tx (3.029 and 3.032 Å) are located at slightly different distances compared to the Ti-Ti atomic distances in Ti3AlC2 (3.040 and 3.111 Å).The value of 3.016 Å for the in-plane Ti-Ti distance of Ti3C2Tx is somewhat smaller than the calculated inplane Ti-Ti atomic distances obtained from the XRD lattice parameter (a=3.075Å) [47].
The peak feature between 0 and 2 Å in the first coordination shell of Ti3C2Tx is caused by superimposed Ti-O, Ti-F and Ti-C scattering where the Ti-C interaction has two different contributions; TiI for the inner Ti atoms that bond only to C and a second TiII contribution to the outer Ti atoms that bond both to C and the termination species.The Ti-C bond length of the outer Ti atoms is 0.056 Å longer than that for the inner Ti atoms.Interestingly, the three peaks in the higher coordination shells between 3-6 Å also contain a large contribution of long inclined single Ti-O and Ti-F scattering paths from all surface Ti atoms to the termination species Tx, in addition to Ti-Ti scattering and superimposed multi-scattering paths e.g., Ti-Ti-C, Ti-C-O, Ti-C-F and Ti-F-Ti-F.
The Ti-Tx scattering paths exhibit a significant temperature dependence as the data was measured at RT, 250, 550, and 750 °C.During heating, the peaks become significantly broadened and less intense as observed by the Debye-Waller factor that increases with the temperature, as a consequence of more atomic vibrations.For the main Ti-Ti scattering, s 2 increases linearly from 0.0066 at room temperature to 0.0082 at 750 °C.A similar trend has been observed in other systems using EXAFS [48].The broadening is also observed as increasing intensity between the peaks in the difference spectra.In addition to the broadening of the peaks, the vibrational behavior and peak broadening exhibit strong anisotropy of the Ti-Ti bonds.Therefore, before each measurement the elevated temperature was stabilized for 20 min and then decreased and stabilized at 250 °C after which the spectra were recorded.shorter bond distance.In particular, the F-related intensity decreases indicating desorption of the F upon heating to 750 °C.Contrary to F, the intensity related to O-bonding slightly increases and shifts to shorter bond lengths.This behavior is consistent with previous temperaturedependent core-level XPS results [18] that showed desorption of F and a change of bond site for O in this temperature region.
The three peaks observed between 3-6 Å in Ti3C2Tx also occur in EXAFS data of cubic TiC [45,46].The first peak at ~3.5 Å is mainly ascribed to Ti-Ti and Ti-C scattering in the second coordination shell.The second peak at 4.5-5 Å mainly consists of Ti-C-Ti and Ti-Ti-O/F scattering while the third peak 5.5-6 Å contain many multi-scattering paths such as Ti-Ti-Ti (5.30Å) and Ti-Ti-Ti-C (5.31 Å) etc.However, the intensity of the three peaks in the higher coordination shells also contains a significant contribution of long inclined single Ti-O (3.62, Table I: Structural parameters for Ti3AlC2 and Ti3C2Tx in comparison to Ti metal reference obtained from fitting of calculated scattering paths in the first coordination shell.N is the coordination number, R is the atomic distance (in Å) for the Ti-Ti and Ti-C scattering paths, respectively, σ is the corresponding Debye-Waller factor representing the amount of atomic displacement and disorder, reduced χr2 as the squared area of the residual, Nind is the number of independent points, P is the number of fitting parameters, and n is the degrees of freedom.Atomic distances obtained from lattice parameters in XRD are given in parenthesis [39].As observed in the difference spectra, the contribution of these paths decrease as the temperature is increased.This is consistent with the fitting results showing that the Ti-O bond length in the first coordination shell seems to shorten as the temperature increases to 750 °C.

System
Yet, after a decade of extensive research activities, there is new knowledge to gain about MAX phases and MXenes.In the present study there are several interesting observations.For example, through XANES we find that the C 2p -Ti 3d hybridization is altered when the Ti3AlC2 transforms into Ti3C2Tx leading to a smaller crystal-field splitting of the t2g and eg orbitals, which suggests slightly weaker Ti-C bonds in Ti3C2Tx compared to Ti3AlC2.Another observation is the 1.4 eV energy shift of the main absorption edge for the Ti3C2Tx compared to Ti3AlC2.A main-edge shift is often an indication of a charge redistribution and when the shift is toward higher energies the charge transfer is away from the probed atoms.Hence, replacing the weak interacting Al-layers in Ti3AlC2 with chemisorbed F and O in Ti3C2Tx will withdraw charge from the Ti toward the termination species.
The additional main-edge energy shift caused by the heat treatment can, however, not be a consequence from a further withdrawal of charge from the Ti atoms, because that would contradict the previous temperature-programmed XPS study [18]; the intensity at the high binding energy side of the Ti 2p3/2 XPS spectra decreases while F desorbs indicating that Ti in Ti3C2Tx chemically reduces in a heat treatment.In addition, the electronegativity of O is lower compared to F (Pauling scale 3.44 and 3.98, respectively).The Ti 1s XANES main-edge shift toward higher energy must therefore be caused by something else.It can, for example, be a response to the stronger bonding of the O in the fcc site compared to F that pushes the unoccupied Ti 4p orbitals toward higher energy.Regardless of the reason, the Ti 1s XANES shows that orbitals with Ti 4p character are sensitive to changes of the termination species on the fcc-sites.
From the EXAFS we learn that the in-plane Ti-Ti distances decreases while the out-of-plane Ti-Ti distances increases when the Ti3AlC2 is converted into Ti3C2Tx.Concerning EXAFS, it probes the local short-order atomic distances between the absorber atom and the neighboring scatterers using the constructive and destructive interference in the unoccupied electronic structure.Since the EXAFS photoelectrons travel much faster than the speed of the thermal motion of the atoms the obtained atom distances are an average of "snapshots" that in most cases correspond to the distance between the average atomic positions as obtained with XRD and neutron diffraction, which are considered to be long-order probes.However, if adjacent atoms are moving in an anti-correlated motion the distance between them will be the same as the atomic positions distance, only when both atoms are in-plane, while it will become larger when the atoms are moving in opposite direction out-of-plane.The atom distances obtained from EXAFS will then be larger than if they would be obtained from XRD and neutron diffraction.The surface Ti atoms and the termination species F and O are expected to move in an anti-correlated motion and the obtained TiII-OA, TiII-Ob, and TiII-F distances are probably larger than the true atomic positions.This discrepancy between the average atom distances between the Tx atoms and the surface Ti atoms will become larger with increasing temperature.

Conclusions
Through a combination of XANES and EXAFS we have investigated the MAX phase material Ti3AlC2 and the MXene material Ti3C2Tx, where the latter was examined before and after a series of heat treatments.The pre-edge absorption region of both Ti3AlC2 and Ti3C2Tx shows mainly Ti 1s excitations into two C 2p -Ti 3d hybridized molecular orbitals corresponding to the Ti 3d t2g and eg orbitals.The crystal-field splitting is determined to be 2.9 and 2.2 eV for the Ti3AlC2 and Ti3C2Tx, respectively.The main-edge absorption features originate from the Ti 1s → 4p excitation and appears to be sensitive towards the fcc-site occupation, which lead to a 1.4 eV shift when the Al-layers in Ti3AlC2 were replaced with the termination species F and O.The local chemical bonding structure and structural properties with atomic distances in Ti3C2Tx MXene shows significant temperature-dependence.Heat treatment up to 750 °C removed F and made the fcc-sites available for O occupation, which is manifested as a 0.5 eV high-energy shift of the peaks in the main-edge absorption region.
EXAFS shows that the shortest in-plane Ti-Ti atomic distances in Ti3AlC2 and Ti3C2Tx is 3.032 and 3.016 Å, respectively, which are longer and shorter than the out-of-plane distance of 3.029 Å in Ti3AlC2 and the corresponding atomic distance in Ti3C2Tx is 3.072 Å.The TiI-C and TiII-C bond lengths in Ti3AlC2 are 2.220 and 2.160 Å, respectively, while the TiI-C and TiII-C bond lengths in Ti3C2Tx are 2.139 and 2.234 Å, respectively.Significant changes in the Ti-O/F coordination are observed with increasing temperature in the heat treatment.The TiII-O bond lengths becomes shorter because of a change in coordination from bridge to fcc facilitated through the desorption of the F as the F contribution is found to diminish when the temperature is raised from room temperature up to 750 °C.Significant contribution of long inclined single Ti-O and Ti-F scattering paths decrease as the temperature is increases.

FIG. 1 .
FIG. 1. Structure of an M3C2Tx MXene layer with various termination sites of -Tx: -F (purple) and -O (red).The F atoms are adsorbed in a three-fold hollow fcc site (Asite).In this model structure, the O atoms are in a bridge site between two Ti and in a three-fold hollow hcp site (B-site).Blue and black spheres are Ti and C atoms, respectively, with strong covalent bonds in the M3X2 conductive carbide core layer.

FIG. 2 .
FIG.2.Ti 1s XANES spectra of Ti metal foil, TiC, Ti3AlC2, and Ti3C2Tx.The Ti3C2Tx spectra are recorded at RT and at 250 °C after 20 minutes heat treatments at 250, 550, and 750 °C.Note that the red spectrum of Ti3C2Tx heat-treated at 250 °C covers the black spectrum of non-heated Ti3C2Tx.The difference of the RT Ti3C2Tx spectrum and the spectra obtained after 250, 550, and 750 °C heat treatment are shown at the bottom.

FIG. 3 .
FIG.3.Curve fitting of the Ti 1s XANES pre-edge region for TiC, Ti3AlC2, room temperature (RT) Ti3C2Tx, and Ti3C2Tx after 750 °C heat treatment.The dotted lines represent the Ti 1s excitations into the t2g and eg orbitals and the dashed lines represents the Ti 1s → 4p excitations (black) and the accumulated intensity from all Ti 1s excitations (red).

FIG. 4
FIG. 4. k 2 -weighted EXAFS, k 2 χ(k), as a function of the photoelectron wave number k of the Ti metal foil, Ti3AlC2, and Ti3C2Tx sample.The Ti3C2Tx samples are recorded at RT and at 250 °C after 20 minutes heat treatments at 250, 550, and 750 °C.The horizontal arrow at the top shows the kwindow for the most pronounced oscillations and the vertical arrows indicate changes in the peak intensities.

Figure 6
Figure6shows the Fourier transform obtained from the k 2 -weighted EXAFS oscillations χ(k) of Ti3C2Tx and the effect of the heat treatment, where arrows indicate the general trends.Difference spectra that highlight the temperature-induced changes are shown in the bottom.In the low-radius region of the first coordination shell of the probed Ti (between 1-2 Å), including the bonding of the O and F elements, an intensity shift is observed corresponding to a ~0.2 Å .73, 4.28, 4.75, 4.79, and 5.04 Å) and Ti-F (3.76, 4.58 Å, 4.78, and 4.86 Å) scattering paths.
a the error in the atomic distances are estimated to be ±0.01Å.b the error in the coordination numbers are estimated to be ±0.01.c the error in the Debye-Waller factors are estimated to be ±0.001Å 2 .dLattice parameter.eout-off plane.3