X ‑ ray Photoelectron Spectroscopy of Ti 3 AlC 2 , Ti 3 C 2 T z , and TiC Provides Evidence for the Electrostatic Interaction between Laminated Layers in MAX-Phase Materials

: The inherently nanolaminated Ti 3 AlC 2 is one of the most studied MAX-phase materials. MAX-phases consists of two-dimensional M n +1 X n -layers (e.g., T 3 C 2 -layers) with strong internal covalent bonds separated by weakly interacting A-layers (e.g., Al-layers), where the repetitive stacking of the M n +1 X n -layers and the A-layers suggests being the foundation for the unusual but attractive material properties of the MAX-phases. Although being an important parameter, the nature of the bonding between the M n +1 X n -layers and the A-layers has not yet been established in detail. The X-ray photoelectron spectroscopy data presented in this paper suggest that the weak interaction between the Ti 3 C 2 -layers and the Al-layers in Ti 3 AlC 2 is through electrostatic attraction facilitated by a charge redistribution of the delocalized electrons from the Ti 3 C 2 -layers to the Al-layers. This charge redistribution is of the same size and direction as between Ti atoms and Al atoms in TiAl alloy. This ﬁ nding opens up a pathway to predict and improve MAX-phase materials properties through A-layer alloying, as well as to predict new and practically feasible MXene compounds.


INTRODUCTION
MAX-phases are a group of ternary compounds with the general composition M n+1 AX n (n = 1, 2, or 3), where M is an early transition metal, A is an A-group element (mainly group 13 or 14), and X is carbon (C) or nitrogen (N). 1−3 Today, we can find more than 150 reported synthesized MAX-phases, 4 materials that possess characteristics of both metals and ceramics. The MAX-phases are inherently nanolaminated materials of a metal carbide or nitride (M n+1 X n -layers) separated by monolayers of A-atoms (A-layers). The M n+1 X nlayer consists of n + 1 M-monolayers and n X-monolayers stacked in an alternated sequence where the first and the last M-monolayers form the interfaces toward the A-layers on each side of the M n+1 X n -layer. The bond between the M n+1 X n -layers and the A-layers is relatively weak, and through selective etching of the A element, using a suitable etchant, 5 it is possible to exfoliate MAX-phases to form two-dimensional (2D) M n+1 X n (n = 1, 2, or 3) materials. 5−8 These 2D materials are known as MXene, and until today, there are about 30 different MXene compounds reported. 9 Both the MAX-phase materials and the MXene compounds have interesting and tunable properties. Some of the MAXphases show, for example, high resistance of corrosion and oxidation, good electrical and thermal conductivities, and have high stiffness and low tendency to deform under the influence of mechanical stress. 3,4,10 The MXene compounds have characteristics of being a 2D material and have shown great promise in a host of applications, not the least of which is in energy storage. 8,11 All of these properties can be modified by the selection of the M, A, and X elements and, in the case of the MXene compounds, also the termination species (T z ) on the M n+1 X n -surfaces. It is common to add the T z to the MXene formula, i.e., M n+1 X n T z (n = 1, 2, or 3), to emphasize the significance of the termination species for the MXene properties.
Theoretical studies have suggested that the key to the unusual material properties of the MAX-phases is, at least in part, the strong covalent bonding within the M n+1 X n -layer 12 facilitated by the weak interaction between the M n+1 X n -layer and the A-layer. 13 The weak interaction between the M n+1 X nlayers and the A-layers is also the basis of the method employed to synthesize MXene, i.e., removing the A-layers with a suitable etchant without disrupting the bonds in the M n+1 X n -layers. [5][6][7]14 Despite being an important parameter, there are only a few theoretical/experimental studies reported on this topic, 13,15−21 and a fundamental understanding of the interaction between the M n+1 X n -layers and the A-layers is not yet established. Calculations of partial density of states indicate bonding between Ti 3d and Al 3p in Ti 3 AlC 2 , see, e.g., Zhou et al. 13 The strength and nature of this bonding have generally been suggested to be weak and even of weak covalent character; 19,20 however, conclusive evidence thereof remains to be presented.
The aim of the present work is to increase our understanding of the interaction between the M n+1 X n -layers and the A-layers in M n+1 AX n -phases. With an expanded knowledge about this interaction, it might be possible to predict and improve the properties of a MAX-phase through modifications of the A-layers, e.g., through alloying 22,23 or introducing A 2layers, 24,25 as well as to predict and facilitate the synthesis of novel MXenes. So far, most synthesized MXene compounds originate from MAX-phases with aluminum (Al) as the A element, although they can also be produced from, e.g., a MAX-phase-related material, M n+1 A 2 X n -phase, where the A 2layer is a gallium (Ga) bilayer. 26 In this work, we have investigated Ti 3 AlC 2 , Ti 3 C 2 T z , and TiC by employing X-ray photoelectron spectroscopy (XPS), which provides element-specific information on the electronic structure of the examined components. Through XPS, it is thus possible to gain information about the chemical environment around the probed element, including the nature of the bonding to the nearest-neighbor atoms. XPS measurements on Ti 3 AlC 2 and Ti 3 C 2 T z have been performed previously. 16,17,27,28 Nevertheless, in this study, we have provided extra attention to the binding energy calibration, which is essential in the characterization of the weak interactions between the laminated layers. It is ensured that the XPS binding energy scale is calibrated with the same care and method for all samples, including the reference samples, since comparison with XPS databases and literature values might not be relevant, especially if they are obtained from XPS spectra that are not calibrated or calibrated using the adventitious C 1s of carbon contamination. 29,30 In our study, presented herein, we compare a MAX-phase with Al as the A element (Ti 3 AlC 2 ) with its corresponding MXene compound (Ti 3 C 2 T z ), three-dimensional (3D) cubic MX (TiC), and 99.95% commercially pure aluminum (Al-1050). The results will not only describe the bonding characters between the M n+1 X n -layers and the A-layers in M n+1 AX n -phases but also explain why selective etching of A elements can be of great challenge for some MAX-phases and, in addition, provides a method to screen MAX-phases for the synthesis of practically feasible MXene compounds.

METHODS
2.1. Samples Preparation. Thin films of Ti 3 AlC 2 were deposited on c-axis-oriented sapphire substrates of 10 × 10 cm 2 surface area using direct current magnetron sputtering (DC-MS) in an ultrahigh-vacuum (UHV) system. The depositions were performed using elemental Ti, Al, and C targets with diameters of 75, 50, and 75 mm, respectively. Prior to deposition, the substrates were preheated inside a deposition chamber at 780°C for 1 h. While keeping the substrates at 780°C , the Ti and C targets were ignited with powers of 92 and 142 W, respectively, for 30 s, forming incubation layers of TiC before the Al target was ignited at a power of 26 W. The sample holder, which held six substrates, was continuously rotating at 30 rpm for uniform deposition. The duration of sputtering the three targets was 10 min, which produced Ti 3 AlC 2 films about 30 nm thick. The thin-film TiC was obtained separately when the Al target was not ignited.
Thin-film Ti 3 C 2 T z samples were prepared through immersion of the DC-MS-obtained Ti 3 AlC 2 in 10% concentrated HF(aq) (Sigma-Aldrich, Stockholm, Sweden) for 1 h at room temperature (RT) and were thereafter rinsed in deionized water and ethanol. The formation of Ti 3 C 2 T x was confirmed through the increase in the c lattice parameter, as observed in the diffractograms shown in Figure 1 (the XRD was performed quickly to reduce the sample exposure to the laboratory atmosphere). The size of the XRD peak shift depends on the obtained spacing between the Ti 3 C 2 layers when the Al-layers are removed in the HF(aq) etching process. 11 The amount of impurities, such as TiO 2 and Al 2 O 3 , and contamination, such as hydrocarbon and alcohol compounds, was kept as low as possible. For example, the obtained DC-MS Ti 3 AlC 2 and TiC samples were removed from the UHV system first after cooling down to RT and were exposed to the atmosphere as brief as possible. The Ti 3 C 2 T z samples were prepared immediately and thereafter placed in the XPS instrument shortly after the etching process was performed.
In addition to Ti 3 AlC 2 , this study also includes the following MAX-phases: Ti 2 AlC, V 2 AlC, Nb 2 AlC, (Cr,Mn) 2 AlC, Mo 2 GaC, and the MAX-phase related material Mo 2 Ga 2 C. Ti 2 AlC was commercially obtained (3-ONE-2, Voorhees, NJ, >92 wt % purity), while V 2 AlC and Nb 2 AlC were synthesized by M. W. Barsoum 31 Mo 2 GaC, Mo 2 Ga 2 C, and (Cr,Mn) 2 AlC were synthesized as thin films through magnetron sputtering deposition. 25,32 2.2. Material Characterization. XPS experiments for F 1s, O 1s, Ti 2p, C 1s, and Al 2p were performed with the AXIS Ultra DLD system from Kratos Analytical Ltd. using monochromatic Al Kα radiation and a pass energy (E pass ) of 20 eV. The samples were placed in an analyzer chamber with the surface normal in the direction along the electron lens toward the electron energy analyzer and with the incident X-ray at an angle of 45°relative to the surface normal, which provided an analysis area of 300 × 700 μm 2 on the surface with a photoelectron acceptance angle of ±15°. Figure 1. X-ray diffractograms of Ti 3 AlC 2 thin films before and after immersion in 10% concentrated HF(aq). The diffractograms show the Ti 3 AlC 2 film at 9.65°and the Ti 3 C 2 T z film at 6.60°. X-ray diffraction was performed with an X'Pert Panalytical XRD system using Nifiltered Cu Kα radiation in a normal Bragg−Brentano geometry. The binding energy scale of all XPS spectra, presented herein, was carefully calibrated against the Fermi edge (E F ), which was set to a binding energy of 0.00 ± 0.02 eV. The overall energy resolution obtained for the XPS spectra presented in this report was better than 0.3 eV (see Table  1), as determined through differentiation of the intensity over the Fermi edge. Normalization of all spectra was performed at the background on the low-binding-energy side of the main peak/peaks. No Ar + sputtering was performed prior to XPS acquisition of the Ti 3 AlC 2 , Ti 3 C 2 T z , and TiC samples. The Al-1050 sample was, on the other hand, exposed to a 4 keV Ar + beam at a 25°angle of incidence, relative the surface plane, for 60 s, which removed the protective layer of Al 2 O 3 on the Al sample.
2.3. XPS Binding Energy Calibration. To be able to compare features in the obtained XPS spectra, it is important to calibrate the binding energy scale using a consistent procedure. In this work, the binding energy scale of all XPS spectra was calibrated against E F of each sample; Fermi levels of all samples were aligned with the electron energy analyzer. The accuracy of the E F being equal to 0.00 eV for each sample was controlled by comparison with the E F of an Ag reference, which was set to a binding energy of 0.00 ± 0.01 eV using the following procedure. The photoelectron intensity over the E F of the Ag reference sample was differentiated, and because of the line shape of the E F , the differentiated intensity curve shows a peak with the shape of a Gaussian function. The center of the Gaussian-shaped peak defines the 0 eV binding energy reference point and could be determined within ±0.01 eV accuracy. The E F values of all samples were thereafter shifted in binding energy to fit the E F of the well-calibrated Ag reference sample. Through this procedure, the binding energy scale of all XPS spectra was calibrated within ±0.02 eV accuracy.
The Ag 3d 5/2 was also recorded before the measurements of Ti 3 AlC 2 , Ti 3 C 2 T z , and TiC, which were loaded into the XPS system together on the same sample holder, and before the measurement of Al-1050. The peak position for the two Ag 3d 5/2 spectra could be determined within ±0.01 eV. The binding energy for the Ag 3d 5/2 was 368.33 ± 0.02 eV at both measurements, i.e., the binding energy scale between E F and Ag 3d 5/2 was stable in the time period for the present study.
Using the binding energy calibration procedure described above, the XPS spectra for F 1s, O 1s, Ti 2p, C 1s, and Al 2p could be recorded with a high precision. The binding energies for the Ti−C and the Al components in the Ti 2p, C 1s, and Al 2p XPS spectra were, thus, determined within ±0.03 eV, i.e., the sum of the precision for the Ag E F , which defined the 0.00 eV binding energy, the binding energy shift of E F for each sample, and the Ti 2p, C 1s, and Al 2p peak positions, respectively; the Ti 2p, C 1s, and Al 2p peak positions were determined through curve fitting of each spectrum. The binding energies for other components, such as the TiO 2 components in the Ti 2p spectra, the Al 2 O 3 components in the Al 2p spectra, and the graphite-like (C−C), hydrocarbons (CH x ), alcohol (C−OH), and carboxyl (COO) components in the C 1s spectra, are on the other hand determined within ±0.1 eV. In the O 1s spectra, the TiO 2 components and the adsorbed O on Ti 3 C 2 T z could be determined within ±0.05 eV, while features at higher O 1s binding energies were determined within ±0.1 eV.
2.4. XPS Spectrum Curve Fitting. The background contributions are represented by Shirley functions, which were subtracted from the XPS spectra before curve fitting. The curve fitting of the O 1s, Ti 2p, C 1s, and Al 2p XPS spectra for Ti 3 AlC 2 , Ti 3 C 2 T z , TiC, and Al-1050 was performed using asymmetric Gaussian−Lorentzian curves. All four samples possess metallic conductivity, i.e., they all have delocalized electrons. These free-moving electrons can flow toward the created photoelectron core holes with the purpose to screen them and will, thus, interact with the escaping photoelectrons, i.e., reducing the kinetic energy of the photoelectrons, which will appear as tails on the high binding energy side of the photoelectron main intensity peaks. The Ti−C components in the Ti 2p and C 1s spectra and the Al components in the Al 2p spectra were therefore represented by asymmetric Gaussian− Lorentzian curves with tails toward higher binding energies. The TiO 2 and the Al 2 O 3 components, which are isolators and therefore do not have free-moving electrons, were best fitted by Gaussian−Lorentzian curves with small tails; oxidized Ti and Al are not only present on the surface but also embedded in the Ti 3 AlC 2 and TiC samples, e.g., in grain boundaries, and some of the Ti 2p and Al 2p photoelectrons from TiO 2 and Al 2 O 3 impurities must then penetrate and interact with the conductive Ti−C materials before they escape the sample. The carbon-based and oxygen-based contamination at the surfaces, i.e., the noncarbide components C−C, CH x , C−OH, and COO in the C 1s spectra and a hydroxide (OH) component in the O 1s spectra, were fitted by symmetric Gaussian− Lorentzian curves.
The 2p feature of a component consists of two peaks, which is because of the atomic spin−orbit interaction that splits the 2p XPS features into two peaks, 2p 3/2 and 2p 1/2 . Two examples of 2p 3/2 and 2p 1/2 spin−orbit split are shown in the Ti 2p and Al 2p XPS spectra of Ti metal and Al metal, respectively, presented in Figure 2. The intensity ratio between the 2p 3/2 and 2p 1/2 XPS peaks is 2:1, and it is therefore important to include a restriction in the curve fitting procedure that keeps the XPS intensity ratio equal to 2:1 for the two p-orbital components. It was therefore included in the curve fitting procedure that the integrated intensity for the curve that corresponds to the Ti 2p 3/2 component must be twice as large as the corresponding integrated curve intensity for the Ti 2p 1/2 component. The curve fitting parameters of the 2p 3/2 and 2p 1/2 XPS peaks, obtained for the Ti 2p and Al 2p spectra shown in Figure 2, are presented in Table 2.

RESULTS
Through the extra attention toward the binding energy calibration, the obtained Ti 2p, C 1s, and Al 2p features of Ti 3 AlC 2 , Ti 3 C 2 T z , TiC, and Al-1050 could be determined with a high precision. Hence, differences in the obtained binding 0.17 ± 0.03 a All spectra were obtained using E pass = 20 eV, except for the Al highresolution spectrum, which was obtained using E pass = 10 eV. b The Al 2p 3/2 and 2p 1/2 spin−orbit split of 0.4 eV is resolved with this resolution.
The Journal of Physical Chemistry C pubs.acs.org/JPCC Article energies for the corresponding core-level XPS spectra can therefore be related to charge redistribution in the studied compounds. 33 3.1. Ti 2p XPS. Figure 3 presents the Ti 2p XPS spectra of Ti 3 AlC 2 , Ti 3 C 2 T z , and TiC. The three spectra are fitted with two asymmetric Gaussian−Lorentzian curves representing the 2p 3/2 and 2p 1/2 spin−orbit split Ti−C components. The Ti 3 AlC 2 and TiC spectra are also fitted with two asymmetric Gaussian−Lorentzian curves representing features identified as TiO 2 components, 34−36 which is absent in the Ti 3 C 2 T z spectrum. Instead, the Ti 3 C 2 T z spectrum has additional two pairs of asymmetric Gaussian−Lorentzian curves representing Ti−C components where the binding energy position depends on the local bonding to the termination species fluorine (F) and oxygen (O). The results obtained from temperatureprogrammed XPS (TP-XPS), presented in a recent work, 28 were combined with atomically resolved images of single 2D Ti 3 C 2 T z sheets attained from an in situ scanning transmission electron microscope (STEM). The conclusions were that the Ti 3 C 2 surfaces are terminated by F and O, exclusively, and that both elements prefer the face-centered cubic (fcc) site, which is the hollow site 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 of the 2D Ti 3 C 2 sheet. 28 Adsorbed O also accepts a second site, which was not identified. However, while heating the Ti 3 C 2 T z sample up to 750°C F desorbs and O migrates from the second site to occupy the vacated fcc-sites. These processes were monitored  The Ti 2p was obtained using E pass = 20 eV, while the Al 2p was obtained using E pass = 10 eV. b Full width at half-maximum (fwhm) for the 2p 3/2 and 2p 1/2 curves. c Binding energy difference (ΔBE) between the 2p 1/2 and 2p 3/2 components. The combined TP-XPS and high-resolution STEM study 28 showed that Ti 3 C 2 T z with O occupying the fcc-sites, after that F has desorbed from the Ti 3 C 2 surface, is well defined and the Ti 2p 3/2 peak at 455.1 eV is, thus, the most suitable for the comparison with the obtained Ti−C peaks for Ti 3 AlC 2 and TiC.
The curve fitting parameters of the Ti−C components of Ti 3 AlC 2 , Ti 3 C 2 T z , and TiC, obtained for the Ti 2p spectra shown in Figure 3, are presented in Table 3. The main 2p 3/2 Ti−C components are located at 454.75 ± 0.03, 455.06 ± 0.03, and 454.85 ± 0.03 eV for Ti 3 AlC 2 , Ti 3 C 2 T O , and TiC, respectively, which are about 1 eV higher than that for Ti metal; the obtained binding energies for Ti 2p 3/2 and Ti 2p 1/2 of Ti metal are 453.76 ± 0.03 and 459.94 ± 0.03 eV, respectively (see Figure 2). The Ti 2p 3/2 binding energy positions for the three Ti−C features indicate different degrees of charge depletion at the Ti sites compared to Ti metal. It is interesting to note, however, that the chemical shift of the Ti 3 AlC 2 and TiC is very similar, while it is significantly larger for the Ti 3 C 2 T z features.
3.2. C 1s XPS. Figure 4 presents the C 1s XPS spectra of Ti 3 AlC 2 , Ti 3 C 2 T z , and TiC. The spectra are fitted with asymmetric Gaussian−Lorentzian curves representing the Ti− C component and symmetric Gaussian−Lorentzian curves representing the noncarbide components C−C and CH x , C− OH, and COO. The obtained curve fitting parameters are presented in Table 4.
The C impurities and contamination are mainly graphite-like carbon formed at the growth of the T 3 AlC 2 and TiC thin films and hydrocarbons (CH x ) from the laboratory atmosphere. The different binding energies obtained for the C−C components are because of different ratios between the sp 2 -and sp 3bonding, which are located at 284.3 and 285.2 eV, respectively, 37 and different amounts of CH x component around 285.5 eV, which are not resolved. There is also some intensity around 286.5 and 289.8 eV, which are alcohol and carboxyl compounds from the laboratory atmosphere. 38 3.3. Al 2p XPS. Figure 5 presents the Al 2p XPS spectra of Ti 3 AlC 2 and commercially pure Al-1050. Both the metallic Al and the Al 2 O 3 compounds are curve-fitted with two asymmetric Gaussian−Lorentzian curves representing the 2p 3/2 and 2p 1/2 spin−orbit split, as described in Section 2. The obtained curve fitting parameters of the Al components are presented in Table 5. The binding energies of the metallic Al for both Ti 3 AlC 2 and commercially pure Al-1050 are confirmed through high-resolution measurements (overall energy resolution of 0.17 eV), where the spin−orbit splits  The O 1s XPS spectrum of Ti 3 C 2 T z has been investigated previously. 28 The conclusion was that the two features observed in the O 1s XPS spectra originate from the termination species O sitting on the fcc-site (O fcc ) and on a not yet identified site (O ad ). 28 In addition, the study concluded that the termination species F was adsorbed only at the fcc-site. The F 1s and O 1s XPS spectra of the Ti 3 C 2 T z sample in the present study are very similar to the corresponding spectra in ref 28. The intensity ratio between the F, O fcc , and O ad components depends very much on the preparation routine, and since the Ti 3 C 2 T z sample in the present study was produced through the same routine as in ref 28, the obtained F 1s and O 1s XPS spectra are almost identical. Hence, a detailed investigation of the F 1s and O 1s XPS spectra obtained from Ti 3 C 2 T z is therefore already presented. 28 Further, since only the Ti 3 C 2 T z sample contains F-components, the F 1s XPS is omitted in the present work.
A comparison between the O 1s XPS spectra of the Ti 3 AlC 2 , Ti 3 C 2 T z , and TiC samples is, on the other hand, motivated, although Ti 3 AlC 2 and TiC do not contain O inherently. Instead, O-components are expected in the form of TiO 2 , because the samples were exposed to the laboratory atmosphere when they were transported from the deposition chamber to the XPS system. For the same reason, an Al 2 O 3 component is anticipated in the Ti 3 AlC 2 sample. Features from TiO 2 and Al 2 O 3 contribute to the O 1s XPS spectrum around 530.5 and 532.3 eV, respectively. 39,40 Figure 6 presents the O 1s XPS spectra of Ti 3 AlC 2 , Ti 3 C 2 T z , and TiC. The spectra are fitted with asymmetric Gaussian− Lorentzian curves representing the TiO 2 , Al 2 O 3 , O ad , and O fcc components. The obtained curve fitting parameters are presented in Table 6. Although it cannot be excluded that the surface of Ti 3 AlC 2 has some small amount of O ad and O fcc , the curve fitting performed better without those components. Instead, there is a component in the Ti 3 AlC 2 spectrum at 531.9 eV that also is present in the TiC spectrum. This feature corresponds to adsorbed hydroxide on TiO 2 (OH−TiO 2 ) 40 and was best represented by a symmetric Gaussian−Lorentzian curve, which suggests that the OH−TiO 2 components are not embedded in the conductive Ti 3 AlC 2 and TiC. Since the Ti 3 C 2 T z sample did not have any TiO 2 impurities (see Figure  3b), the OH−TiO 2 component was not applicable in the Ti 3 C 2 T z spectrum curve fitting. The amounts of other Ocontaining components, such as the C−OH and COO contamination observed in Figure 4, are too small to be detected in the O 1s spectra in Figure 6.

DISCUSSION
The Ti 2p XPS spectra of TiC and Ti 3 AlC 2 in Figure 3 show the importance of carefully prepared samples. Although a short exposure to the laboratory atmosphere, about 2 h, the TiC and Ti 3 AlC 2 samples have oxidized and recognizable TiO 2 features are present in the Ti 2p XPS spectra. However, the amount is low and the TiO 2 intensity contribution to the Ti 2p XPS spectra is well separated from the Ti−C components and does not introduce difficulties in the XPS spectrum curve fitting procedures. The Ti 2p XPS spectrum of the Ti 3 C 2 T z shows, on the other hand, no intensity contribution from TiO 2 . Instead, the main features are broadened, which is because of the termination species T F and T F,O as shown in a recent TP-XPS study. 28 The Ti 2p XPS curve fitting of the carefully prepared samples required only a few curves, i.e., the essential contributions to the Ti 2p XPS spectra of Ti 3 AlC 2 , Ti 3 C 2 T z , and TiC.
The Ti 2p XPS spectra of TiC and Ti 3 AlC 2 are very similar. The small Ti 2p 3/2 binding energy difference between the Ti− C components in TiC and Ti 3 AlC 2 , which is only −0.10 eV, The Ti−C components are well-resolved features and can be determined with a high precision. b The C−C + CH x , C−OH, and COO components are determined within ±0.1 eV. c Full width at halfmaximum (fwhm) for the C 1s curves.  Also, the metallic Al contribution in the Al 2p of Ti 3 AlC 2 shows a negative shift in comparison to Al metal. The Al 2p binding energy shift is significantly larger compared to the Ti 2p and C 1s binding energy shifts between Ti 3 AlC 2 and TiC, which suggests that the Al in Ti 3 AlC 2 has gained charge compared to Al metal. The shift is −0.63 eV and is on the same order as the Al 2p binding energy shift of TiAl alloy; 41 the Al 2p 3/2 binding energy of TiAl is 72.3 eV, i.e., a shift of −0.5 eV compared to commercial pure Al metal (Al-1050).
The size of the Al 2p shift is too small to represent an asymmetric distribution of electrons between the Ti and Al atoms that are characteristic for a covalent bond, e.g., between the Ti 3 C 2 layer and the Al-layer, because Al 2p 3/2 core-level shifts between pure Al metal and Al-containing compounds are normally larger than ±1 eV, whereas Al-containing alloys normally show Al 2p 3/2 core-level shifts less than ±1 eV. 42 Instead, the sizes of the Ti 2p, C 1s, and Al 2p core-level shifts and that all shifts are in negative direction suggest a redistribution of the delocalized electrons from the Ti 3 C 2layers to the Al-layers, 33 similar to the observed charge redistribution from Ti atoms toward the Al atoms in the TiAl alloy. 41 This redistribution of delocalized electrons in the Ti 3 C 2 -layers resembles the response obtained when an ideal stoichiometric cubic MX compound is introduced with the appropriate amount and distribution of vacancies at the X sites, i.e., the net flow of delocalized electrons toward the created vacancies results in a more dense electron cloud around the defect to screen it. 33,43,44 Hence, the redistribution of the delocalized electrons will cause a modification of the occupied valence orbital configurations in the MX compound, which in turn leads to a strengthening of the covalent Ti−C bonds. 33,43,44 Similar to the case with vacancies in cubic MX The binding energy for the Al metal agrees well with previously reported values of freshly cleaned Al(111) (Al 2p 1/2,3/2 = 72.8 eV). 39 b The Al component can be determined with a high precision and is confirmed through high-resolution measurements where the spin−orbit splits are well resolved. c The Al 2 O 3 components are determined within ±0.1 eV. d Full width at half-maximum (fwhm) for the 2p 3/2 and 2p 1/2 curves. e Binding energy difference (ΔBE) between the 2p 1/2 and 2p 3/2 components.  The Journal of Physical Chemistry C pubs.acs.org/JPCC Article compounds, the inclusion of Al-monolayers between Ti 3 C 2layers will redistribute the delocalized electrons and thus enhance the core hole screening of the Ti 2p and the C 1s, as suggested by the negative core-level shifts, 33 which successively leads to a strengthening of the covalent Ti−C bonds also in Ti 3 AlC 2 . The fact that the redistribution of the delocalized electrons, caused by the alternating stacking of Ti 3 C 2 -layers and Al-monolayers, will lead to a strengthening of the covalent Ti−C bonds is demonstrated when the Al-monolayer is alloyed with the more electronegative element silicon (Si); the electronegativity for Al and Si is 1.61 and 1.90, respectively, in the Pauling scale. As the amount of Si increases in Ti 3 Al 1-n Si n C 2 , the mechanical strength, as obtained through Vickers hardness measurements, increases significantly, 22 which indicates a strengthening of the covalent Ti−C bonds. 45 In addition, the redistribution of the delocalized electrons will provide bonding between the Ti 3 C 2 -layers and the Allayers that are electrostatic between slightly positive Ti 3 C 2layers and slightly negative Al-layers.
Based on the arguments above, one would expect that the Ti 2p core level would shift back slightly toward higher binding energies when the Ti 3 AlC 2 has the Al-layers removed, e.g., through HF(aq) etching, to form the 2D Ti 3 C 2 T z . However, because of termination species (T z ), in the form of chemisorbed O, 28 the Ti 2p core-level shift is 0.32 ± 0.06 eV as a consequence of a partial charge transfer from the Ti atoms in the Ti 3 C 2 -layer to the chemisorbed O. When the termination species also includes the chemisorbed F, which is more electronegative compared to O, the core-level shift is even larger (see Table 3). While the Ti 3 AlC 2 shows a Ti 2p core-level shift of −0.10 ± 0.06 eV, compared to TiC, the Ti 2p core-level shift between the Ti 3 AlC 2 and Ti 3 C 2 T z is three times larger. Hence, in contrast to the electrostatic interaction between the Ti 3 C 2 -layer and the Al-layer in the Ti 3 AlC 2 , the bonding interaction between the Ti 3 C 2 -layer and the chemisorbed species in Ti 3 C 2 T z is significantly stronger. This difference in bonding strength might be the driving force when MAX-phases are converted to MXene through an etching process.
The information presented in this work suggests that it should be possible to tune the electrostatic interaction between the A-layer and the M n+1 X n -layer through A-layer alloying, which could be beneficial for sought-after properties such as improved corrosive resistance, electric and thermal conductivity, mechanical properties, and thermal expansion.
Direct and practical information can also be gained through the A element core-level shift between a MAX-phase material and the pure A compound in predicting the ease by which MXene compounds can be synthesized. For example, XPS acquisitions (spectra not shown) of the Al 2p core level of Ti 3 AlC 2 , Ti 2 AlC, V 2 AlC, and Nb 2 AlC show that, compared to the pure Al metal reference, there are shifts of −0.63 ± 0.06, −1.27 ± 0.18, −0.39 ± 0.12, and −0.36 ± 0.10 eV, respectively. Based on this information only, it can be predicted that it requires more effort to remove Al from Ti 2 AlC compared to Ti 3 AlC 2 but easier from V 2 AlC and Nb 2 AlC. The syntheses of Ti 3 C 2 T z , Ti 2 CT z , V 2 CT z , and Nb 2 CT z have been performed successfully through selective etching of the Al-layers by immersing the corresponding MAXphases in, e.g., hydrofluoric acid (HF), at room temperature. 6,7,11,31 The effort necessary for converting the MAXphases to MXene did, in fact, follow the trend predicted by the obtained sizes of the Al 2p core-level shift. 11 Predicting possible conversion of MAX-phases to MXene compounds through estimation of the XPS core-level shift between the A element in a MAX-phase and a pure A element phase is not limited to A equal to Al. It can, for example, also be applied when the A element is Ga, although Ga 3p is less sensitive to the local environment compared to Al 2p. One example is the promising candidate for a high-performance thermoelectric material, the 2D-layered molybdenum carbide (Mo 2 CT z ) MXene. 46,47 A Ga 3p XPS investigation showed a core-level shift of −0.14 ± 0.06 eV between a pure Ga metal spectrum and a Mo 2 GaC spectrum, but only a few hundredth of an eV (less than −0.04 eV shift) for the corresponding comparison with the atomic laminate material Mo 2 Ga 2 C. 25 This implies a significantly reduced electrostatic interaction between the Mo 2 C-layers and the Ga 2 -layers in Mo 2 Ga 2 C compared to the Ga-layers in Mo 2 GaC. Based on the significantly smaller Ga 3p core-level shift for the Ga 2 -layer in the Mo 2 Ga 2 C, compared to pure Ga metal, it can be predicted that it is more feasible to synthesize Mo 2 CT z MXene from Mo 2 Ga 2 C instead of Mo 2 GaC, which in fact is consistent with the reported synthesis. 26 Confident in the method of predicting practically feasible MXene compounds, it should be equally challenging to remove Al from (Cr,Mn) 2 AlC as it is from Ti 3 AlC 2 because the corelevel shift between (Cr,Mn) 2 AlC (spectra not shown) and pure Al metal is −0.47 ± 0.12 eV. The process to form the not yet synthesized (Cr,Mn) 2 CT z MXene compound can, however, be more dependent on other parameters, such as stability of the (Cr,Mn) 2 C-layers against selected etchant. If that would be the case, then the challenge is to find a new suitable etchant rather than to only facilitate removal of the Al-layers.
To summarize, the curve fitting of pure Ti 3 AlC 2 requires only, in total, five curves representing Ti 2p 3/2 , Ti 2p 1/2 , C 1s, Al 2p 2/3 , and Al 2p 1/2 . All additional curves correspond to impurities, such as TiO 2 , Al 2 O 3 , and graphite-like carbon and contaminations, such as hydrocarbon, alcohol, and carboxyl compounds, which are well separated from the Ti−C components in the Ti 2p, C 1s, and Al 2p spectra but only if the amounts of impurities and contaminations are kept to a minimum. It is therefore very important to handle and store MAX-phase samples properly. Ar + sputtering prior to XPS acquisition is not an option since preferential sputtering removes O from TiO 2 and Al 2 O 3 forming metallic Ti and Al that will show metallic features superimposed on the Ti 3 AlC 2 features in the Ti 2p and Al 2p spectra.
Curve fitting of pure Ti 3 C 2 T z requires additional curves that correspond to the termination species and the effect they have on the Ti 2p. It is therefore crucial that the handling and storage of MXene samples are carefully monitored before an XPS investigation because features from impurities, such as TiO 2 , can overlap the features from the termination species. Another reason to reduce the amount of impurities to a minimum is that they can adsorb molecules from the atmosphere. Especially, TiO 2 is known to adsorb H 2 O that dissociates into OH 40,48−53 and a significant amount of TiO 2 in a MAX-phase sample or a MXene sample will therefore show a significant amount of OH contribution in an XPS study or, actually, in any study where the selected technique is sensitive toward OH, e.g., nuclear magnetic resonance (NMR). 54 An early study of O adsorption on TiC showed that TiO 2 is formed after a large exposure to O 2 at room temperature, 55 which is also observed in the present study through the wellresolved Ti 2p 3/2 and O 1s peaks at 458.8 and 530.4 eV, The Journal of Physical Chemistry C pubs.acs.org/JPCC Article respectively, as shown in Figures 3c and 6c. Further, the O 2 exposure experiment showed that only a single feature around 530.4 eV is generated. 55 Yet Figure 6c shows a distinct feature at 531.8 eV, which then must originate from another Ocontaining species, such as adsorbed OH or H 2 O. However, the Ti 2p spectrum in Figure 3c or the C 1s spectrum in Figure  4c does not show any features indicating OH or H 2 O adsorption on the TiC surface. A feature at 531.8 eV is, on the other hand, observed when OH is adsorbed on TiO 2 , 40 and it is therefore reasonable to conclude that both the TiC and Ti 3 AlC 2 samples contain TiO 2 impurities that are covered with OH. The Ti 3 C 2 T z spectra in Figures 3b and 6b show, on the other hand, no indication of TiO 2 or OH, which suggests that the Ti 3 C 2 T z sample, to some extent, is protected from TiO 2 formation by the termination species O and F, although probably only for a limited exposure time. 56 The Al 2 O 3 components in the Ti 3 AlC 2 spectra shown in Figures 5a and 6a are also because of the exposure to the atmosphere prior to the XPS investigation. In addition, the TiC sample shows some small Al 2 O 3 contribution, which is because of the sapphire substrate that was used when the Ti 3 AlC 2 and TiC samples were produced. The magnetron sputtering technique removes some Al atoms from the substrate that are back-scattered onto the sample surface. (There might also be other sources of Al in the system after previous depositions using Al targets.) The insignificant amount of Al 2 O 3 in the Ti 3 C 2 T z sample is, on the other hand, a residuum from the etching process of Ti 3 AlC 2 .
The comparison between XPS data of TiC, Ti 3 AlC 2 , Ti 3 C 2 T z , and commercially pure Al metal provides evidence of electrostatic interaction between the Al-layers and the 2D Ti 3 C 2 -layers in the laminated MAX-phase material. The redistribution of the delocalized electrons from the 2D Ti 3 C 2 -layers to the Al-monolayers will cause changes in the Ti and C valence orbital configurations that will strengthen the covalent Ti−C bonds. The driving force to form the 2D MXene material Ti 3 C 2 T z from the MAX-phase Ti 3 AlC 2 is that the termination species can promote more efficient bonding configuration in the Ti 3 C 2 T z . The efficiency of the A-layers to attract the delocalized electrons from the M n+1 X n -layers determines the difficulty to remove the A-layers from the MAX-phase material. Determination of the binding energy shift between the A-layer core-level position and the corresponding core-level position of the pure A material is, thus, a comparatively easy method to predict new practically feasible MXene compounds.

CONCLUSIONS
Through comparison of Ti 3 AlC 2 MAX-phase with Ti 3 C 2 T z MXene, cubic TiC, and commercially pure Al-1050, the presented XPS investigation shows a redistribution of the delocalized electrons from the Ti 3 C 2 -layers to the Al-layers in Ti 3 AlC 2 . The size of the charge redistribution is comparable to the charge rearrangement between Ti atoms and Al atoms in TiAl alloy. Hence, the laminated layers in the Ti 3 AlC 2 are held together through electrostatic interaction between the slightly positively charged Ti 3 C 2 -layers and the slightly negatively charged Al-layers. This may be one of the properties facilitating MXene synthesis through selective etching of the MAX-phase A-layers. For comparison, the XPS investigation shows that the interaction between the Ti 3 C 2 -layer and the chemisorbed termination species F and O in 2D Ti 3 C 2 T z is significantly larger. The study further shows that through the determination of the Al 2p and Ga 3d core-level shifts, compared to pure Al and Ga metals, it is possible to predict practically feasible MXene compounds and estimate the challenge to remove the A-layers.