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# Site-projected electronic structure of two-dimensional Ti₃C₂ MXene: role of the surface functionalization groups

Damien Magne,ᵃ Vincent Mauchamp,ᵃ* Stéphane Célérier,ᵇ Patrick Chartier,ᵃ and Thierry Cabioc'hᵃ

The role of the surface groups T (T = OH, O or F) on the chemical bonding in two-dimensional Ti₃C₂Tₓ MXene is directly evidenced combining electron energy-loss spectroscopy in a transmission electron microscope and simulations based on density functional theory. By focusing on the 1s core electrons excitations of the C and (F,O) atoms, the site projected electronic structure is resolved. The Electron Energy-Loss Near Edge Structures (ELNES) at the C-K edge are shown to be sensitive to the chemical nature and location of the T-groups on the MXene's surface and thereby allow for the characterization of the MXene's functionalization on the nanometre scale. In addition, the ELNES at the C and F-K edges are shown to be determined by the hybridizations of these atoms with the Ti *d* bands: these edges are thus relevant probes of the Ti *d* density of states close to the Fermi level which is of particular interest since it drives most of the Ti₃C₂Tₓ electronic properties. Finally, the crucial role on the MXene's functionalization of the etchant used for its synthesis is evidenced by locally determining the [F]/[O] concentration ratio using the corresponding K edges. This ratio is shown to be drastically increased from 1.4 to 3.5 when using HF or LiF/HCl respectively.

## 1 Introduction

Since the discovery of graphene and its supreme properties,¹ two-dimensional (2D) materials have become a major research interest because of their peculiar properties as compared to their bulk counterpart. In this dynamic context, a new group of 2D transition metal carbides or nitrides, called MXenes, has emerged.²,³ This new class of materials is produced by selective etching of the Al layers from nanolaminated ternary carbides or nitrides called M<sub>n+1</sub>AX<sub>n</sub> phases (M being a transition metal, A being Al, X being either C or N and n = 1,2 or 3) using essentially HF or LiF/HCl as the etchants.⁴ The chemical process leading to the synthesis of the MXenes results in weakly interacting 2D MX sheets passivated by OH, F and/or O surface termination groups T (See fig. 1) giving the following composition: 2,5 M<sub>n+1</sub>X<sub>n</sub>T<sub>x</sub>. Since their discovery, the optical, magnetic, transport and electronic properties

of these systems have been investigated.⁶⁻¹³ Moreover, the MXenes (Ti₃C₂Tₓ in particular) were shown to exhibit outstanding properties for energy storage applications:¹⁴⁻¹⁶ they are considered as strong candidates for electrodes materials¹⁷,¹⁸ and their potential application for Li-ion batteries (LIB),¹⁹ non-LIB¹⁶,²⁰,²¹ and supercapacitors is now widely recognized.¹⁴,²²,²³

It is clear that one major advantage of MXenes over many other 2D materials relies in the possibility to significantly modify their chemical composition, in order to adjust their properties, by changing the MAX phase precursor. Another, maybe more direct approach, is to modify the surface functionalization groups. Indeed, in a very analogous way to graphene derivatives,²⁴ the T surface groups location and/or their chemical nature play a key role in the MXenes' chemical or physical properties. They affect the capacity for energy storage applications,²⁵ change the surface plasmon resonance,⁷ modify their magnetic properties,²⁶,²⁷ open a band gap²⁸ and deeply modify the electron transport properties of the MXenes.¹³,²⁹ These T groups have thus attracted significant attention both experimentally and theoretically.⁹,¹⁶,²⁹⁻³³

In general, two different surface sites are investigated for the T-groups: in the first configuration (labelled C1), the T groups are above the hollow sites of three neighbouring X atoms (i.e pointing towards the volume titanium atoms), and in the second (la-

<footnote>ᵃ Institut Pprime, UPR 3346 CNRS - Université de Poitiers - ISAE-ENSMA, BP 30179, 86962 Futuroscope-Chasseneuil Cedex, France.</footnote>

<footnote>ᵇ Institut de Chimie des Milieux et Matériaux de Poitiers (IC2MP), Université de Poitiers, CNRS, F-86073 Poitiers, France.</footnote>

<footnote>* E-mail: vincent.mauchamp@univ-poitiers.fr</footnote>

<footnote>† Electronic Supplementary Information (ESI) available: [details of any supplementary information available should be included here]. See DOI: 10.1039/b000000x/</footnote>
belled C2) the T groups are located above the X atoms (see figure 1 (a)). In multilayer MXenes (ML-MXenes), a third configuration (labelled C3₁ in the present paper) can be considered with a mixture of C1 and C2 configurations (see figure 1(b)). A local probe such as transmission electron microscopy (TEM) is thus a powerful tool to identify the location of these T-groups.³⁴,³⁵ In particular, it allows analysing the MXenes on the nanometre scale thereby avoiding the critical question of impurities which can be numerous in MXene powders. The presence of these impurities significantly complicates the interpretation of data collected with macroscopic probes.³⁶ When combined to electron energy-loss spectroscopy (EELS),³⁷ TEM can also provide information on the electronic structure: (S)TEM-EELS experiments were thus recently used to characterize the optical properties and the T-groups location or local changes in the composition of Ti₃C₂Tₓ.⁷,⁹,³⁵ Besides the characterization of surface groups, which is a very general issue for 2D materials as recently demonstrated with EELS on graphene oxide,³⁸ electron spectroscopies such as EELS or X-ray absorption spectroscopy (XAS) have also been used to characterize the electronic structure of Ti₃C₂Tₓ by focusing essentially on the core-edges (i.e. L₂,₃ or K-edges) of the titanium atoms. These investigations were performed in order to bring insights into changes in the titanium valence charge, either correlated to changes in the T-groups, or to different steps in the charge-discharge process of electrochemical cycles.²⁵,³⁹

<figure><img src="image_1.png" /><figcaption>**Fig. 1** (a) Side view of a Ti₃C₂Tₓ single sheet with the two T surface groups locations labelled C1 and C2. (b) The three most stable configurations corresponding to a multilayer (ML) Ti₃C₂Tₓ system based on the periodic stack of two MXene sheets: C3₁ corresponds to a configuration C1 with one T placed in C2 location. (c) TEM micrograph of a typical ML-Ti₃C₂Tₓ sample on which EELS data were collected.</figcaption></figure>

Although the combination of EELS with first principles calculations proved to give key information on the electronic structure of 2D materials,⁴⁰,⁴¹ to the best of our knowledge no quantitative interpretation EELS data collected on MXenes have been provided so far from a theoretical point of view. In this paper we thus go one step further in the quantitative understanding of the electronic structure of Ti₃C₂Tₓ by investigating the site-projected electronic structure of this MXene using a combination of TEM-EELS and first principles calculations based on the density func-

tional theory (DFT). In order to probe the electronic structure both inside the MXene sheet and on its surface, we focus on the electron energy-loss near edge structures (ELNES) at the carbon and surface elements (i.e. O or F) K-edges. The core-hole in the 1s orbital of the excited element ensures that the local environment around the excited atom is probed. The choice for K-edges is also driven by the fact that they can be interpreted in terms of the site-projected electronic density of states (DOS) of the material, giving direct access to the electronic structure of the investigated sample. This is generally not the case for transition metals L₂,₃ edges, the interpretation of which is not straightforward in terms of DOS since they are dominated by multiplet effects.⁴²

# 2 Methods

## 2.1 Experimental details

The Ti₃C₂Tₓ samples were prepared by etching the Al from Ti₃AlC₂ powders using two different solutions: either hydrofluoric acid ³,⁵ or a milder solution of lithium fluoride with hydrochloric acid.²³ Although the HF-based synthesis was the first to be proposed, the second solution is safer and gives MXenes with much better volumetric capacitance when used as electrodes for electrochemical capacitors.²³ Moreover, the LiF/HCl etching method leads to the formation of MXenes where water can be inserted between the MXene sheets contrary to the MXenes obtained by the HF etching agent, evidencing different hydrophilic behaviours depending of the etching method. The synthesis methods and the samples characterizations are described in ESI.

The TEM samples were prepared by dispersing the Ti₃C₂Tₓ powders before picking them up on a Cu grid covered with a lacy carbon film. The ML-MXene sheets, which are several hundreds of nanometres long (See fig. 1(c)), were probed with the electron beam perpendicular to the sheets. For both types of samples, the EELS spectra were recorded in image mode in a JEOL 2200 FS microscope equipped with an in-column omega filter and operated at 200 kV. The energy resolution determined from the zero-loss peak full width at half maximum was 0.8 eV. Spectra recorded on ML-Ti₃C₂Tₓ with typical thickness of 0.3 times the inelastic free path were selected. The collection angle was 7 mRad. The core-edges were extracted from the background signal using a power law (i.e. A·E⁻⁴, where E is the energy-loss), and were deconvolved from multiple scattering using the Fourier-ratio method.⁴³ The acquisition conditions were optimized in order to minimize irradiation damages which can be important in 2D materials (details will be given elsewhere).

## 2.2 Computational details

Like XAS, EELS gives the response of the sample to the creation of a core-hole (in a 1s orbital in the case of K-edges). The corresponding inelastic scattering cross-section is thus expressed in terms of the dynamic form factor (DFF) which describes the excitation of the system from its initial state |Ψᵢ⟩ of energy Eᵢ to all allowed final states |Ψᵣ⟩ of energies Eᵣ. In the dipole approximation, the DFF can be expressed in the form of the following Fermi golden rule:³⁷
$$S(\mathbf{q}, E) = \sum_{f} |\langle\Psi_{i}|\mathbf{q}|\mathbf{r}|\Psi_{f}\rangle|^2 \delta(E - E_{f} + E_{i}) \quad (1)$$

where $\mathbf{q}$ is the momentum transfer between the electron of the TEM beam and the excited core electron. The delta function is homogeneous to the density of unoccupied states. Due to the selection rules imposed by the matrix element ($\Delta l = \pm 1$ with $l$ the orbital angular momentum quantum number), the fine structures at the K-edge of a given element are proportional to its site projected unoccupied DOS of $p$ symmetry. In addition, the signal acquisition conditions (zone axis, convergence and collection angles) fix the relative orientation between $\mathbf{q}$ and the electronic orbitals of the excited atom (e.g. $p_x$, $p_y$, $p_z$ symmetries) giving some selectivity on the magnetic quantum number.

From the previous considerations, the theoretical ELNES were obtained from the unoccupied DOS of $p$ symmetry of the different elements (the negligible effect of the matrix elements in equation 1 has been checked, see fig.S3 in ESI). In the case of the C-K edge, the $p_x$-$p_y$ and $p_z$ components were properly weighted (85%-15% respectively) according to the theory developed by Le Bosse et al.⁴⁴ Calculations were performed within the framework of DFT performed in the generalized gradient approximation (GGA) as parametrized by Perdew, Burke and Ernzerhof.⁴⁵ We used the Vienna ab initio simulation package (VASP) with projector augmented wave (PAW) potentials.⁴⁶,⁴⁷ The PAW atomic configurations were $2s^2 2p^2$ for C, $2s^2 2p^5$ for F, $2s^2 2p^4$ for O and $3p^6 3d^2 4s^2$ for Ti with an energy cut-off of 750 eV. We used a previously derived structural models as input for the calculations: in brief, $Ti_3C_2T_x$ is described as a periodic stack of two sheets with a slight distortion of the unit cell as compared to the original MAX phase, and unit cell parameters are fixed at the values determined from Rietveld refinements of the XRD data.⁹ The positions of the T-groups in the C1 or C2 configurations on the MXene sheets where optimized with VASP, keeping the experimental unit cell parameters, until forces were converged to better than 0.01 eV/Å. The core-hole created during the excitation process might have dramatic effects on the observed fine structures at a given edge.⁴⁸ We took it into account by replacing the excited atom with its next neighbour in the periodic table, this is the so-called Z+1 approximation (the validity of this approximation as compared to a more “realistic” final state rule approach based on the consideration of an explicit core hole on the carbon 1s orbital has been checked by comparison with all electron calculations - see fig.S3 in ESI). In order to avoid unphysical mutual interactions between adjacent excited atoms, we used a $2*2*1$ supercell and the calculations were performed using a $10*10*2$ k-points sampling of the first Brillouin zone using a Monkhorst-Pack grid. In order to approximately model the excited state life-time broadening, a linearly dependent broadening $\Gamma(E) = 0.1 \times E$, with E the energy above threshold, was applied to the DOS before comparison with the experimental spectra.⁴⁹

# 3 Results and Discussion

## 3.1 C-K edge

The C-K edges recorded on the $Ti_3C_2T_x$ samples obtained either using HF or LiF/HCl are given in figure 2. These spectra are very similar evidencing the weak impact of the etchant on the electronic structure of the MXene's $Ti_3C_2$ skeleton. These spectra exhibit four main peaks labelled B, C, E, F and three weaker structures labelled A, B' and D (note that structure B' is more clearly resolved in fig. 3 in which an enlargement of the first 30 eV after the edge onset is given). These data are in very good agreement with those obtained by Karlsson et al. at 60 kV on $Ti_3C_2T_x$ samples synthesized using $NH_4HF_2$ as the etchant:³⁵ this further confirms the weak effect of the etchant.

<figure><img src="image_2.png" /><figcaption>**Fig. 2** Comparison between the C-K edges recorded on the HF (black line) and LiF/HCl-prepared samples (grey line). For the comparison, the spectra are normalized on the intensity of peak B and the grey spectrum is shifted vertically. Lines are given as guides to the eyes.</figcaption></figure>

The experimental C-K edges are compared to calculations performed considering the T=OH groups in positions C1 or C2 in figure 3(a) and different T-groups (i.e. OH, F and O) in C1 in figure 3(b). The spectra corresponding to the configuration $C3_1$ are not given since they are very similar to those obtained for the configuration C1. Focusing first on figure 3(a), some similarities can be seen between both configurations: they reproduce the two main structures B and C as well as the shoulders B' and D. One can however notice that the calculation corresponding to configuration C1 is in better agreement with the experiment: the energy position and relative separation between peaks C and D is better reproduced and, although both calculations give an overestimation of the intensity ratio of structures B/C when compared to the experiment, this overestimation is larger for configuration C2. Among the possible reasons explaining this wrong intensity ratio are intrinsic limitations of the Z+1 approximation (and final state rule in general). As evidenced in graphite, the dynamical core-hole screening has to be considered in order to reproduce the $\pi^*/\sigma^*$
intensity ratio at the carbon-K edge.⁵⁰ Finally, the shoulder A, which is clearly evidenced in the experiment, is only reproduced for the T-groups in C1. This structure is thus characteristic of the location of the surface groups and evidences that C1 is the experimentally obtained configuration for ML-Ti₃C₂Tₓ. This confirms previous conclusions obtained from valence electron energy-loss spectroscopy.⁹ Such a fine structure could thus be of great interest for the investigation of T-groups modifications or when following intercalation mechanisms in MXenes using *in-situ* core-loss spectroscopies. It is also worth noticing that the calculated ELNES at the C-K edge are insensitive to the stacking of the MXene as evidenced by the inset of figure 3(a) where the C-K edge obtained from the multilayer model is compared to that obtained considering a single Ti₃C₂(OH)₂ sheet. This result, in line with experiments performed on single or ML-Ti₃C₂Tₓ,³⁵ evidences the very weak coupling between the MXene sheets.

<figure><img src="image_3.png" /><figcaption>**Fig. 3 (a)** Comparison between the experimental C-K edge and calculations performed considering the T=OH group in configurations C1 or C2. Inset: comparison between the C-K edges calculated considering a multilayer system or a single Ti₃C₂(OH)₂ sheet (T-groups in C1). (b) Comparison between the experimental C-K edge and calculations performed considering different termination groups in configuration C1: OH (black line), F (dashed black lines), O (grey dashed line). The energy scale is taken with respect to the theoretical Fermi energy (set to zero) and the experimental spectra were aligned with the calculations using the peak B as a reference. Lines are given as guides to the eyes.</figcaption></figure>

In Figure 3(b), the effect of the T-groups chemical nature on the C-K edge is investigated: from bottom to top, the three theoretical spectra corresponding to O, F or OH terminations (all relaxed

in configuration C1) are compared to the experiment. It appears clearly from these results that OH and F T-groups give very similar ELNES at the C-K edge: this is probably an evidence of the fact that they play similar roles on the electronic structure of the MX-part of the MXene. On the contrary, the ELNES corresponding to a T=O surface group exhibit strong differences with the other calculations and with the experiment: none of the obtained fine structures are at the good energy position when compared to the experiment. The here-presented calculations do not take into account the fact that MXenes are functionalized with a mixture of -OH, -O and -F groups. However, given the similarity between the C-K edges obtained considering -OH or -F surface groups, such a mixture would probably have significant effects only when considering -O groups. As a first estimate of the influence of the mixing between -O and (-OH and -F) groups, the C-K edges obtained with T=-OH and -O have been averaged considering different proportions of -O groups ranging from 10 to 90 %. Such an average gives rise to two main changes at the C-K edge: the increase in the energy separation $\Delta E_{BC}$ between structures B and C and the broadening of the first peak (fig. S4-a of ESI). The $\Delta E_{BC}$ measured on thirteen different experimental spectra is 9.35 eV with a standard deviation of 0.2 eV: by comparison with the $\Delta E_{BC}$ extracted from the calculations (see Fig. S4-b of ESI), we conclude that for HF and LiF/HCl, the surface T-groups are predominantly OH and/or F with probably less than 20% of -O type surface groups: this conclusion, consistent with previous reports,⁶,²⁵ is in very good agreement with the compositions obtained recently by Wang *et al.* using atomic pair distribution function. Moreover, the fact that the electronic structure of Ti₃C₂Tₓ (and particularly the Ti₃C₂ part) is deeply modified when going from -OH/F to -O surface groups is also very consistent with the measurements published by Lai *et al.* since they evidenced important changes in the resistivity vs temperature behaviour of Ti₂CT₂ when going from -F or OH to -O surface groups.²⁹ In addition, these results also support the calculations of G.R. Berdiyorov who showed that Ti₃C₂O₂ behaves very differently from Ti₃C₂(OH)₂ and Ti₃C₂F₂ in terms of electronic transport properties.⁵¹ Given that many properties of the MXenes (e.g. energy storage applications, electronic properties, etc) are significantly modified when considering O or OH surface groups,¹⁶,²⁵,²⁶,²⁸,⁵¹ the here-presented results give an important step towards the characterization of these surface groups at the nanometre scale and evidence the C-K edge as a very suitable spectroscopic signature for the investigation of T-groups in MXenes.

The sensitivity of the C-K edge to the position and chemical nature of the surface groups can be understood from the analysis of the site-projected unoccupied densities of states given in figure 4. Focusing first on the top part of the figure, it appears clearly that the *d*-states of the surface titaniums (Tiₓ) dominate the electronic structure of Ti₃C₂(OH)₂ up to 2.5 eV above the Fermi level. This is a major element explaining the high sensitivity of the MXenes' electronic structure to surface groups: it is now well known that band gaps can be opened or modified in MXene by changing the T-groups. Above 3 eV, the volume and surface titanium atoms' *d* states alternatively dominate the electronic structure up to 7 eV after what all atoms more or less equally contribute to the elec-
tronic structure.

<figure><img src="image_4.png" /><figcaption>**Fig. 4** Top: Site-projected density of states of Ti₃C₂(OH)₂ with the OH group in C1: surface Ti (Tiᵢ) in thin dashed black lines, volume Ti (Tiᵣ) in full black line, C in blue line, O in red dotted lines, and H green dot-dashed line. Inset: similar quantities for the OH groups in C2. Bottom: m-decomposed projected unoccupied *p*-DOS of the excited carbon atom (C*): total *p* DOS in thin black line, *p*<sub>*z*</sub> DOS in blue line, and *p*<sub>*x*</sub> + *p*<sub>*y*</sub> DOS in red dashed line. The hybridizations involved in the structures A to C of the C-K edge are highlighted in red. The Fermi energy is set to 0.</figcaption></figure>

The origin of the ELNES at the C-K edge can be analyzed from the bottom part of figure 4 where the *m*-decomposed unoccupied *p*-DOS of the excited C atom is given: the peaks in the DOS giving rise the different fine structures A to C are highlighted in red. It appears clearly from this figure that the origin of structure A is due to the hybridization of the C *p*<sub>*z*</sub> states with an intense peak in the surface Ti *d* states. These states, of *d*<sub>*z*2</sub> symmetry, vanish when the T-group is in configuration 2 (see the orange arrow in the inset of figure 4), explaining the absence of structure A in the corresponding calculated C-K edge. Note that beyond bringing information on the T-group positions, having information on the transition metal *d* orbitals around the Fermi energy could be crucial for many applications including half-metallicity, as recently reported in Cr₂C.²⁷

Contrary to structure A, peaks B, B' and C correspond to hy-
bridizations which involve a significant (if not dominating) part
of the volume titanium: the sensitivity of these fine structures to
the position of the surface group is then significantly reduced. As
a consequence, both configurations C1 and C2 give very similar
ELNES in the corresponding energy range. Structural or chemical
modifications within the MX sheets (as recently reported with the
synthesis of double transition MXenes⁵²) would probably lead to
significant changes in this part of the spectrum.

**3.2 Surface groups: O-K and F-K edges**

To get further understanding of the local electronic structure of Ti₃C₂Tₓ, the O-K and F-K edges (around 535 and 685 eV respectively) have also been investigated. The ELNES obtained at these two edges are compared in figure 5 for the HF or LiF/HCl-prepared samples. For the sake of comparison, the spectra were normalized with respect to their first peak intensity (labelled A), shifted vertically and were aligned in energy by setting the position of peak A to zero.

<figure><img src="image_5.png" /><figcaption>**Fig. 5** Representative O-K (bottom) and F-K (top) edges recorded on the HF or LiF/HCl-prepared Ti₃C₂Tₓ samples. Lines are given as guides to the eyes.</figcaption></figure>

From a general point of view, these spectra exhibit similar fine
structures with three main peaks labelled A to C, the ELNES at
the O-K edge being sharper than those at the F-K edge. The here-
presented O-K edges are in very good agreement with those pub-
lished by Karlsson *et al*.³⁵ Having a closer look at fig. 5, some
difference appear between the O-K and F-K edges : the splitting
between the A and B structure is significantly larger for O, their
relative intensities is different (peak A being lower than B for the
F-K edges whereas they have rather similar intensities at the O-K
edges). Finally, the energy position of structure C changes be-
tween the two types of edges as well as a function of the etchant
for a given edge. The F-K edge ELNES are however less affected
by the etchant and/or the F surrounding since they only weakly
change when modifying the etchant. This is not the case for the
O-K edge for which the position of structure C can change for dif-
ferent areas in the same sample and is also modified when chang-
ing the etchant as illustrated in figure 5.

Before going into the analysis of the ELNES, the EEL spectra
were used to estimate the relative amount of O and F species cov-
ering the Ti₃C₂Tₓ samples. The standard procedure implemented
in the digital micrograph software has been used (details con-
cerning the quantification procedure can be found in textbooks
such as reference⁴³). In brief, the O/F ratio in the area sampled by the electron beam can be deduced from the ratios of the integrated intensities under the O-K and F-K edges over a given energy window, weighted by the corresponding atomic inelastic cross-sections (for the present purpose, Hartree-Slater atomic cross-sections were used). Assuming that the (OH,F) coverage on the MXene is disordered, we focus on relatively large areas (typically 100 nm²), free of impurities, to get an average information. Moreover, given that the O/F ratio can be modified by ageing,³⁶ the quantifications were performed on samples synthesized at the very same period and stored in vacuum. We evidence a dramatic influence of the etchant on the MXene surface composition since the ratio varies from O/F ≈ 1.4 to ≈ 3.5 (±0.2) for the HF and LiF/HCl-prepared samples respectively. Although our quantifications are probably approximate since the extraction of the O-K edge is critical because of its superimposition to the Ti L edges (the procedure that has been used here is similar to that used by Zhang *et al.* to quantify local changes in the [Ti]/[O] ratio in BaTiO₃/SrTiO₃ multilayers⁵³), the observed evolution is meaningful. Moreover, the ratio obtained for the HF-prepared samples is very similar to that reported by Wang *et al.*⁵⁴: O/F = 1.4. Given their dramatic effect on the MXenes properties and in particular on their hydrophilicity, such a dependence of the T-group on the etchant is of great importance regarding the possibility to tune the MXenes properties. This point has for instance clearly been evidenced by Ghidiu *et al.* when focusing on the morphology and volumetric capacitance of Ti₃C₂Tₓ.²³

<figure><img src="image_6.png" /><figcaption>**Fig. 6** Comparison between the experimental F-K edge recorded on the HF-prepared sample (top - red) and simulations considering the F atoms in C1 (middle - black) or C2 (bottom - grey). The spectra were shifted vertically for comparison and aligned on peak A. Full circles give the position of the Fermi level in the calculation. Lines are given as guides to the eyes.</figcaption></figure>

Given their structure, the ELNES presented in figure 5 carry
significant information on the chemical bonding between the O

<figure><img src="image_7.png" /><figcaption>**Fig. 7** [a] Comparison between the simulated F-K edge in C1 (bold black line), the unoccupied *p*-DOS of the excited F atom (F* - thin black line), the surface titanium (Tiₛ) *d*-DOS (red line) and the volume titanium (Tiᵥ) *d*-DOS (blue dashed line). For ease of reading, the Tiₛ and Tiᵥ DOS were divided by 500 and 250 respectively in order to be plotted on the same scale as the F* DOS. Enlargements of the titanium unoccupied *d*-DOS considering the F atoms in C1 and C2 are given in [b] and [c] respectively.</figcaption></figure>

or F atoms and the Ti₃C₂ skeleton. However, it has been demon-
strated in diverse materials that the O-K edge corresponding to
-OH groups is very sensitive to the TEM electron beam which can
induce dehydration:⁵⁵ for this reason, we focus on the F-K edge.
A detailed analysis of the O-K edge ELNES, including the effects
of irradiation damage, will be presented elsewhere. A comparison
between the experimental F-K edge recorded on the HF-prepared
sample and the simulations obtained with the F atoms in C1 and
C2 is given in figure 6. On the overall, the calculations reproduce
qualitatively the experiment: structures A, B and C are repro-
duced. A fourth structure (labelled D) is predicted in the calcu-
lation but not observed experimentally: the energy resolution at
this edge is probably not good enough to allow a clear identifica-
tion of this structure.

A closer look shows however differences between the theoreti-
cal spectra. The splitting between structures A and B is different
between the two simulations: it is around 5.9 eV for C2 whereas
it is between 6.3 and 8.7 eV in C1, depending on which structure
is considered in the broad peak B. The position of peak B is thus
in much better agreement with the experiment when considering
C1, a result confirming the conclusions obtained from the analy-
sis of the C-K edge. In addition, the intensity ratio between the
peaks A and B is also in better agreement with the experiment
when considering C1.
A rationalization of the F-K edge ELNES in terms of the hy-
bridizations between the F atoms and the MX skeleton can be ex-
tracted from a comparison between the simulated spectrum and
the unoccupied *p*-DOS projected on the excited F atom. This com-
parison, including the titanium *d* states which dominate the elec-
tronic structure around the Fermi level, is given in figure 7. Fo-
cusing first on figure7 (a), it appears clearly that the structures
B, C and D arise mainly from the F *p*-bands with the particularity
that the two sharp peaks constituting peak B exhibit $p_x+p_y$ and
$p_z$ splitting (these three kinds of orbitals are mixed at higher en-
ergies). From these considerations, it is not surprising that one
of the main differences between the simulated edges of figure 6
is observed on the $p_z$-related part of peak B, these $p_z$ orbitals be-
ing affected by the relative positions of the F-groups facing each
other in the ML-Ti₃C₂F₂ structural model. Interestingly, the ori-
gin of peak A is completely different since it comes from the hy-
bridization between the F *p*-states and some of the Ti₃C₂ d-states.
More precisely, it involves the Ti *d*-states at the onset of the unoc-
cupied part of the Ti *d*-band (see the red arrow in figure 7 [b]).
This structure is thus relevant in the study of the effect of sur-
face groups on the surface titanium *d*-bands which, given their
importance around the Fermi level, govern most of the electronic
properties of Ti₃C₂Tₓ. As an illustration, the reduced energy sep-
aration between the peaks A and B with F in C2 when compared
to C1 (see figure 6) is partly explained by the shift of the first
maximum of the Ti₃C₂ d-DOS, indicated by red arrows in figures 7
[b] and [c], in this configuration. In configuration C2, this max-
imum is at higher energy with respect to the Fermi level (1.6 eV
instead of 0.8 eV), thereby reducing the energy gap between the
structures A and B. This gap can thus in principle be used to probe
the position of the Ti₃C₂ *d* band maximum with respect to the Fermi
energy.

4 conclusion

The electronic structure of ML-Ti₃C₂Tₓ has been investigated us-
ing a combination of TEM-EELS and DFT calculations. Focusing
on the C and F-K edges, clear evidences of the surface groups lo-
cation are reported: these T-groups are on the so-called C1 site
of the MXene surface, a conclusion confirming a previous report
based on valence EELS. The ELNES at the C-K edge are also shown
to be highly sensitive to the chemical nature of the T-groups. Con-
sidering that the C-K edge is to a good approximation a probe of
the unoccupied C-*p* DOS, our results show that the site-projected
electronic structure on the C atoms (i.e. within the MX skeleton)
is clearly modified when changing the OH/F groups with O. This
result, in line with reported changes in the electronic transport
properties of divers MXenes carbides,²⁹,⁵¹ identifies the C-K edge
as a relevant probe of the chemical nature of T and allows us to
conclude that our ML-Ti₃C₂Tₓ are predominantly functionalized
with OH and F groups.

More generally, careful inspections of the ELNES at the C and
F-K edges show that the peaks at the edge onset arise from hy-
bridizations with the *d* states of either the surface or volume ti-
tanium atoms: the X and T element K-edges can thus be used to
indirectly probe the Ti *d* DOS at the nanometre scale using TEM-
EELS. Given the crucial role of these states on the electronic prop-

erties of the MXene (optical,⁹ magnetic,²⁷, etc), we expect these
features to be useful signatures of the MXene's functionalization
for diverse applications. Finally, by comparing the ELNES at the
C, F and O-K edges recorded on samples synthesized using either
HF or LiF/HCl, we show that the etchant has little effect on the
electronic structure of the MXene's MX part but changes signifi-
cantly the [F]/[O] ratio. Given, the crucial role of the T-groups
on the MXenes' properties in general, these results can be used
to choose the appropriate synthesis route regarding the desired
applications.

Besides providing deep insights into the electronic structure
of Ti₃C₂Tₓ, the here-presented spectra and their analysis can
be used as references for the investigation of intercalation
effects and surface groups modifications in these materials at
the nanometre scale. Such investigation are now possible thanks
to the development of *in situ* or environmental transmission
electron microscopy for instance.⁵⁶,⁵⁷

5 Acknowledgements

The "Agence Nationale de la Recherche" is gratefully acknowl-
edged for financial support: ANR project n° 13-BS09-0024-02.
Computations have been performed on the supercomputer facili-
ties of the "Mésocentre de calcul de Poitou Charentes". This work
has been supported by the "Région Poitou-Charentes".

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