# Reactivity of Ti3C2Tx MXene with Atomic Hydrogen: Tuning of Surface Terminations by Halogen Removal and Reversible O to OH Conversion

Monika Schied, Hanna Pazniak, Florian Brette, Paolo Lacovig, Michael Paris, Florent Boucher, Silvano Lizzit, Vincent Mauchamp, Rosanna Larciprete

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Monika Schied, Hanna Pazniak, Florian Brette, Paolo Lacovig, Michael Paris, et al.. Reactivity of Ti3C2Tx MXene with Atomic Hydrogen: Tuning of Surface Terminations by Halogen Removal and Reversible O to OH Conversion. Chemistry of Materials, 2024, 10.1021/acs.chemmater.4c02422. [hal-04836598]

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Distributed under a Creative Commons Attribution 4.0 International License
Reactivity of Ti₃C₂Tₓ MXene with
Atomic Hydrogen: Tuning of Surface
Terminations by Halogen Removal and
Reversible O to OH Conversion

Monika Schied,† Hanna Pazniak,‡ Florian Brette,¶,§ Paolo Lacovig,† Michael Paris,§ Florent Boucher,§ Silvano Lizzit,† Vincent Mauchamp,¶ and Rosanna Larciprete*,||

†Elettra Sincrotrone Trieste, S.S. 14 Km 163.5, 34149 Trieste, Italy

‡Université Grenoble Alpes, CNRS, Grenoble INP, LMGP, F, 38000, Grenoble, France

¶Université de Poitiers, CNRS, ISAE-ENSMA, PPRIME, 86073 Poitiers, France

§Nantes Université, CNRS, Institut des Matériaux De Nantes Jean Rouxel, IMN, F-44000 Nantes, France

||CNR-Institute for Complex Systems, Via dei Taurini 19, 00185 Roma, Italy

E-mail: rosanna.larciprete@isc.cnr.it

Abstract

Two-dimensional transition metal carbides or
nitrides, so-called MXenes, hold the prospect
of a proactive emergence as innovative cata-
lysts and device components, owing to the spe-
cific qualities gained from the chemical species
that functionalize the layers. Tuning the na-
ture and the number of the surface termina-
tions becomes the key factor for an effective
use of MXenes in technology. This study ex-
plores the capability of H atoms to modify the
surface composition of Ti₃C₂Tₓ flakes. While
exposing the sample at room temperature to
H atoms, the change of its surface chemical
state is followed by synchrotron radiation x-ray
photoelectron spectroscopy. It turns out that
halogen terminations are progressively and sub-
stantially removed. In parallel the O termina-
tions are partially converted into OH groups,
the O/OH ratio being possibly controlled by
the OH-OH repulsion. The dramatic surface
composition change leaves the valence state of
the Ti atoms almost unchanged. DFT simu-

lations of the valence band spectra for different
Ti₃C₂Tₓ model systems identify all spectral fea-
tures and model the change of the electronic
properties around the Fermi level. Heating the
hydrogenated sample to 400 K removes the OH
groups leaving the MXene surface deprived of
most of the pristine terminations, thus giving
way to new, application-oriented functionaliza-
tion.
Introduction

MXenes are a young family of two-dimensional
(2D) materials whose chemical formula is
M<sub>n+1</sub>X<sub>n</sub>T<sub>x</sub>, where M is an early transition
metal, X is carbon or nitrogen and T rep-
resents various surface terminating groups.<sup>1,2</sup>
MXenes are synthesized from strongly bonded
layered MAX phase precursors (A is an element
from groups 13 and 14 of the Periodic Table)
by selective etching of the A-element. Upon
MAX phase etching, the dangling bonds on the
M<sub>n+1</sub>X<sub>n</sub> surface are spontaneously saturated
by a manifold of chemical species (T) inherited
from the etching environment,<sup>3</sup> whose nature
and surface concentration depend on the MAX
phase precursor, etchant used and delamina-
tion protocols. It has been extensively demon-
strated that chemical nature, coordination and
surface density of the T groups determine the
MXenes electronic structure and properties,
changing their behavior from metallic to semi-
conducting,<sup>4,5</sup> varying their work function in
a wide range<sup>5</sup> and introducing new function-
ality such as magnetism or superconductiv-
ity.<sup>6,7</sup> This makes MXenes attractive as gas
sensors, biosensors, catalysts and for applica-
tions in many technological fields ranging from
optoelectronics to electrochemical energy stor-
age.<sup>8–12</sup> Therefore, understanding and control-
ling the rich MXene surface chemistry is critical
to adapt MXene-based materials to specific ap-
plications.

The most common functionalization of MX-
ene flakes obtained via wet etching procedures
is provided by a mixture of OH, O, F and Cl
terminations, that distribute non-uniformly on
the surface layers and whose concentration ra-
tios depend on the etchant concentration and
preparation conditions.<sup>13–15</sup> One way to tai-
lor the composition of the surface terminations
is the use of post-etching processing. High-
temperature annealing in vacuum, reactive or
inert atmospheres of MXenes flakes with mixed
terminations leads to the desorption of specific
functional groups in certain temperature inter-
vals,<sup>5,16,17</sup> which opens up possibilities for ma-
nipulating their surface functionalization. In
particular, OH terminations are intrinsically

unstable and are desorbed at relatively low tem-
perature (450-470 K), while F terminations,
which are more strongly bonded to the tran-
sition metal, start to desorb at higher temper-
ature (~ 750 K) and are fully removed around
1000 K.⁵,¹⁷ In all cases, F removal is accompa-
nied by a redistribution of the O terminations
that are stable up to 1170 K.¹⁷ Unfortunately,
post-etching high-temperature annealing in in-
ert atmosphere leads to poor control over the
desorption rate and unavoidably leads to the
decomposition of the MXene matrix,¹⁸ while
treatments in oxidizing environment or in the
presence of water vapor tend to induce oxida-
tive reactions in the M and/or X sublattices. It
appears then opportune and valuable setting up
other, more compliant, processing routes. More
complex methods, which include cation interca-
lation coupled with moderate thermal process-
ing¹⁹ or gas hydrolization²⁰,²¹ have been shown
to be effective towards the selective MXene
functionalization while limiting material degra-
dation, but the development of alternative pro-
cessing approaches is still highly desirable.

In this study we investigated the surface re-
activity of Ti₃C₂Tₓ towards atomic hydrogen,
with the perspective to use it for tuning the
Ti₃C₂Tₓ functionalization by modifying nature
and number of the terminating groups. In par-
ticular the conversion of O into OH termina-
tions would increase the concentration of sur-
face hydroxyls beyond the usually low contents
reported from standard chemical etching pro-
cesses.¹³ Such terminations are key to, e.g.,
ultra-low work function layers for Schottky–
barrier-free contacts with other 2D materi-
als,²² heavy metal adsorption for water depo-
lution²³ or key catalytic reactions such as car-
bon dioxide reduction.²⁴ In addition, there are
other relevant issues concerning Ti₃C₂Tₓ hy-
drogenation, an important one being to verify
the real possibility to directly bind H to the
MXene surface, considering that calculations
found Ti₃C₂Hₓ less stable than other Ti₃C₂Tₓ
(T=O, F, OH) structures and that H atoms
have higher adsorption energy than O, OH and
F terminations.²⁵ Furthermore, stable and ef-
ficient chemisorption of H atoms on Ti₃C₂Tₓ
would complement the low temperature ph-
ysisorption of H₂ molecules observed in multi-
layered samples,²⁶ thus increasing the overall
hydrogen gravimetric capacity of this material
and hence reinforcing its appeal for hydrogen
storage.²⁷ Also, there is the general interest for
proton intercalation in MXenes, which can be
an important contribution in the total charge-
storage mechanism.²⁸

In the light of these diverse reasons, in this
study we investigated the surface reactions oc-
curring in the Ti₃C₂Tₓ surface exposed at room
temperature (RT) to reactive H atoms, in or-
der to identify the hydrogenation routes that
are followed by the different surface termi-
nations and, ultimately, by the MXene lat-
tice itself. This objective was reached by us-
ing high-resolution X-ray photoelectron spec-
troscopy (XPS) to determine chemical nature,
concentration, and bonding state of the ter-
minating groups after Ti₃C₂Tₓ hydrogenation,
to monitor step-by-step the surface reactions
caused by the interaction with H atoms and to
establish the thermal stability of the functional-
ized surface. Actually, since obtaining such in-
formation required punctual analysis and confi-
dent attribution of the spectral features, a pre-
liminary step of this study was to revisit the
assignment of the core level spectra measured
for Ti₃C₂Tₓ before the hydrogenation reaction.
In this respect, as recently reported in Refs.²⁹
and,³⁰ there are several inconsistencies and de-
bated points in the literature, which need fur-
ther elucidation, especially concerning the attri-
bution of the spectral components of the Ti2p
and O1s spectra, which are the most complex to
interpret. Often literature discrepancies can be
traced back to the disadvantage of measuring
the XPS spectra with low energy resolution or
to the improper use of aged samples or of sam-
ples with plenty of spurious phases. Also, a fur-
ther misleading factor resides in the habit to re-
fer, for the assignment of the core level spectra
components, to computational studies regard-
ing Ti₃C₂Tₓ functionalized by a single termi-
nating group, whereas real samples are bonded
to mixed terminations,³¹ which rearrange the
charge distribution and shift in energy the XPS
features with respect to model systems. In this
study, owing to the possibility to acquire spec-

tra with different photon energies coupled with
high energy resolution, and thanks to limited
quantities of spurious phases in the Ti₃C₂Tₓ
flakes and to the additional information gained
by treating the samples with different post-
hydrogenation processing, we could add impor-
tant contributions for a definite assignment of
the XPS spectral features. Turning to the hy-
drogenation, we exploited the capability of core
level spectroscopy³² to monitor the evolution
of surface terminations in the sample exposed
at room temperature to a flux of H atoms and
modeled the electronic structure of different
Ti₃C₂T₂ model systems by density functional
theory (DFT) calculations. Our results demon-
strate that the use of H atoms is a valid and
safe approach to effectively substitute halogen
with hydroxyl terminations at room tempera-
ture. By carrying out post-hydrogenation heat-
ing and further re-hydrogenation we could addi-
tionally change the composition of the Ti₃C₂Tₓ
terminations, demonstrating that a clever use
of these processes, carried out separately or in
a smart sequence, represents a flexible tool to
tune nature and concentration of the functional
groups.

**Results and discussion**

Pristine sample

Figure 1 illustrates the structure of the Ti₃C₂Tₓ
monolayer. In Ti₃C₂ the inner Tₐ atoms are
bonded to six C atoms and each C atom is
bonded to three Tiₐ atoms and to three surface
Ti₉ atoms to form TiC octahedra. The Ti₃C₂
matrix is stabilized when the dangling bonds
of each Ti₉ atom are saturated with T termi-
nations,³ which frequently are O and F atoms
and OH groups, but can also be H, Cl or other
heteroatoms. As shown in Figure 1, the surface
terminations can occupy the threefold hcp sites
above the C atoms of the second layer and the
fcc sites above the Tiₐ atoms of the third layer,
but also adsorb in the bridge sites between two
Ti₉ atoms or between Ti₉ and C atoms and in
the top sites above the Ti₉ atoms. Calculations
found that F, OH and H terminations tend to
adsorb in the surface *fcc* sites,³³,³⁴ whereas *hcp*, *bridge* and especially *top* sites are less favored adsorption sites for all terminations.

<figure><img src="image_3.png" /></figure>

Figure 1: Structure of the Ti₃C₂Tₓ monolayer. The *left* side view and *right* top view show the surface (Ti_b) and middle (Ti_a) titanium atoms and the two carbon layers between them. The terminating T atoms saturating the Ti_b dangling bonds are positioned in the *fcc* (red), *hcp* (yellow), *bridge-1* (between two surface Ti_b atoms, light blue) and *bridge-2* (between Ti_b and C atoms, green) sites.

We have characterized the Ti₃C₂Tₓ films by high resolution core level and valence band XPS. The survey spectra of the as-prepared Ti₃C₂Tₓ films (Figure S1), besides the main Ti and C atoms that form the MXene matrix, showed the presence of other elements, namely O, F and Cl, deriving from the etching reactions used to delaminate the MAX phase and likely terminating the Ti_b dangling bonds. Moreover, the C and O atoms contained in the sample surface were in part due to adventitious contaminants after the exposure to atmosphere. Therefore, before any other experiment, the Ti₃C₂Tₓ samples were treated with a mild annealing to 650 K, needed to free the film surface from the environmental adsorbates. In Figure S2 we show the results obtained while monitoring the evolution of the core level spectra of the Ti₃C₂Tₓ components during the annealing of the as-prepared samples.

Here it is only worth pointing out that during heating up to 650 K the Ti2p spectrum showed a stable line shape confirming that the valence states of the Ti atoms were unaffected by the moderate temperature¹⁷ and finally exhibited only a slight intensity increase due to

the removal of surface adsorbates. Analogously, the core level spectra of the halogen atoms remained stable in line shape and intensity. On the contrary, both the C1s and the O1s spectra were consistently modified, also because the O and C atoms were in part constituents of the surface contaminants removed by the thermal treatment. The trends followed by the Ti2p, F1s, O1s and C1s spectra during sample heating are reported in Figure S2. The high resolution XPS spectra measured after the annealing of the as-prepared samples, which in the following will be considered pristine samples, are shown at the top of Figure 2a-e. Figure S3 shows the wide XPS spectra measured at different photon energies, from where it comes out that the chemical composition of the pristine MXene sample is Ti₃.₀C₃.₅O₂.₂F₀.₇Cl₀.₃ (see Figure S4 and related text in the Supporting Information). The difference from the expected nominal MXene stoichiometry is due to the presence of spurious phases inherited from the MAX phase, developed during the etching processing or due to the exposure to atmosphere.

### Ti2p spectra

The top panel of Figure 2b shows that the Ti2p spectrum measured on the pristine sample is composed by five doublet components, whose high number is indicative of the complex sample composition. The BE positions of the 3/2 spin orbit components of the five doublets are in order Ti₀ (454.8 eV), Ti₁ (455.2 eV), Ti₂ (456.0 eV), Ti₃ (457.4 eV) and Ti₄ (459.3 eV). The different widths of the component peaks reflect the unequal ensemble of configurations related to each of them. In contrast with the spectral decomposition usually reported for the Ti2p spectra acquired with lower energy resolution, which below 455.5 eV identifies a single component attributed to Ti-C bonds and located between 454.8 and 455.2 eV,⁵,¹⁷,³⁵ in our case the presence of two different components Ti₀ and Ti₁ separated by 0.4 eV³⁶ instead of a single one, appeared evident after the analysis of a large set of Ti2p spectra acquired after different sample treatments (Figure S5). In order to identify the origin of Ti₀ and Ti₁, it should be
<figure><img src="image_4.png" /><figcaption>Figure 2: a) Survey, b) Ti2p, c) C1s, d) F1s and e) O1s spectra measured for the Ti₃C₂Tₓ sample before and after the exposure to H atoms for increasing time t₉. The curves in f), g) and h) are the integrated intensities of the Ti2p, F1s and O1s spectra, respectively, as a function of t₉. The inset in g) shows the Cl2p spectra measured for the pristine and H dosed sample and the corresponding integrated intensity vs t₉ is shown in g). The top curve in h) represents the total intensity of the O1s spectra, that is the sum of all O1s component intensities. The intensities of the Ti2p, F1s, O1s, C1s and Cl2p spectra were normalized by the photon beam flux.</figcaption></figure>
considered that the calculated Bader charge³⁰,³⁷ and Mulliken population²⁵ in the Ti₃C₂ compound show that Ti₉ atoms have lower oxidation state than Tiₐ atoms, and therefore are expected to contribute at lower BEs in the Ti2p spectrum.²⁹ Hence, we assign Ti₁ to Ti atoms in the intermediate sublayer Tiₐ and Ti₀ to the unsaturated Ti₉ atoms (not terminated with T groups) present in the surface layers. The assignment of the Ti₁ component to Tiₐ atoms is also in agreement with our recent DFT simulations³⁰ also showing that the Tiₐ BE is insensitive to the nature of T terminations bonded to the Ti₉ atoms, except for Ti₃C₂O₂.

Concerning the Ti2p spectral components appearing at higher BEs, there is a wide consensus about their assignment to Ti atoms in II, III and IV valence states. We can start the discussion by considering the oxidized phase which are usually detected in Ti₃C₂Tₓ.³⁸ In this respect, the Ti2p BEs measured for crystalline and polycrystalline TiO₂ surfaces fall in the range 458.5-459.5 eV,³⁹⁻⁴² whereas BEs around 457.5 eV⁴⁰,⁴¹,⁴³ and in the range 455.6-455.1 eV⁴¹,⁴³ are reported for Ti suboxides with oxidation states III and II, respectively. Therefore, the Ti₂, Ti₃ and Ti₄ components can be assigned in part to TiO, Ti₂O₃ and TiO₂ phases likely formed in the sample during the delamination of the MAX phase or due to the contact of MXenes with air.³⁸ Also, the Ti₂, Ti₃ components might contain the contributions of oxycarbides and halogenated Ti compounds.¹³,²⁹ More intriguing it would be to identify in the Ti2p spectra the real contribution of surface Ti₉ atoms bonded to T groups. In this regard there are several factors to consider. First of all, each Ti₉ atom can be bonded to three different terminations. For Ti₉ atoms fully saturated with a single T group it has been reported that the oxidation state rises in order as Ti₃C₂H₂ < Ti₂C₃F₂ ≈ Ti₃C₂(OH)₂ < Ti₃C₂O₂ and remains anyhow between III and IV.²⁵,³⁷ However, the Ti₃C₂T₂ configurations are not all equally probable, but their stability follows the order Ti₃C₂O₂ > Ti₃C₂F₂ > Ti₃C₂(OH)₂ > Ti₃C₂H₂ > Ti₃C₂.³⁷ Moreover, the pristine samples used in this experiment have a sizable concentration of Cl atoms (Figure

2a) which, in analogy with the F terminations, at least in part are expected to functionalize the Ti₉ atoms. Then, according to the Cl electronegativity intermediate between F and OH and to the C-Cl bond longer than C-F and C-OH bonds,³⁰ for Ti₂C₃Cl₂ it can be assumed an oxidation state of the Ti₉ atoms equivalent or even lower than in Ti₃C₂F₂ and Ti₃C₂(OH)₂. As pointed out above, these considerations apply for Ti₃C₂ fully saturated with a single kind of T terminations. In reality, the oxidation state of Ti₉ atoms changes if they are bonded to three different terminations³⁰,³¹ or are left with unsaturated bonds. For these Ti₉ atoms the number and the nature of the terminal groups changes the Ti2p BE because T electronegativity, bonding configuration and consequent charge transfer influence not only the valence electrons directly involved in the bonding, but also the deep core level electrons. Hence, this complexity makes it difficult to precisely relate the Ti2p components to Ti₉ atoms functionized with specific terminations relying only on the Ti2p measured on the pristine sample. However, as it will be shown below, fundamental information for this analysis is provided by the evolution of the Ti2p spectrum after different surface reactions.

### C1s spectra

Much easier is to assign the C1s spectrum shown in Figure 2c, which contains the component C₀ (281.9 eV) due to C atoms constituents of the MXene octahedra and five additional components, namely C₁ (282.5 eV), C₂ (284.2 eV), C₃ (284.7 eV), C₄ (285.5 eV) and C₅ (286.6 eV). The intense C₂ peak is assigned to C-C bonds in amorphous and graphitic carbon domains which form during the MAX phase delamination process, whereas C₃, which was much more intense in the as-prepared sample and reduced considerably during the preliminary annealing (Figure S2), is attributed to hydrogenated C atoms. The minor C₄ and C₅ components are due to oxidized atoms in C-O, C=O and/or C-F bonds and their minimal intensity reveals the negligible concentration of these spurious phases in the pristine
samples. The small C₁ peak can be attributed
to the interface between carbide and amorphous
C phase or to disorder effects,⁴⁴ but can also be
assigned to the formation of C-Hₓ groups in the
carbide matrix.⁴⁵

F1s and Cl2p spectra

The F1s spectrum of the pristine Ti₃C₂Tₓ sam-
ple shown at the top of Figure 2d exhibits a
strong component F₀ at 684.8 eV arising from F
atoms bonded to Ti₉ atoms and a weaker shoul-
der F₁ at 686.2 eV. The presence of a main int-
tense F1s component has been related to the
occurrence of a single occupation site for the
F atoms, supposed to be the surface fcc site.
However, recent DFT calculations³⁰ show that
the single F1s component rather reflects the in-
sensitivity of the F1s states to their local en-
vironment. As for F₁, it is usually assigned
to F-C bonds in correspondence of lattice de-
fects. The other attribution frequently reported
to Al-F bonds in our case can be reasonably ex-
cluded, since we do not observe the Al spectral
features in the XPS spectra. The inset in Fig-
ure 2g shows the 3/2 and 1/2 components of the
Cl2p spectrum at 199.1 e 200.7 eV. In compar-
ison with the Cl2p BEs lower by ~ 0.5-0.7 eV
measured for Ti₃C₂Cl₂ (198.6 and 200.1 eV)⁴⁶
or for adsorbed TiCl₄ molecules (198.4, 200.0
eV),⁴⁷ the up-shift we find could be due to a
lower charge transfer from the Ti₉ atoms to the
Cl atoms, due to the coexistence of more elec-
tronegative (F and O) terminations, which also
withdraw charge from the Ti₉ atoms.

O1s spectra

The O1s spectrum measured on the pristine
sample and reported at the top of Figure 2e ex-
hibits, in addition to the main O₀ component
(529.7 eV), two minor components O₁ (530.5
eV) and O₂ (531.2 eV). In all cases, the compo-
nent BEs overlap the range typical for the TiOₓ
compounds, since the O1s peak of TiO₂ is re-
ported between 530 and 531 eV,⁴⁰⁻⁴³,⁴⁸,⁴⁹ similar
BE values are found for Ti suboxides,⁴⁰ whereas
a BE slightly higher than 531 eV is reported for
TiO.⁴¹ In order to identify the contribution of
the O terminations, we considered that in the

O1s spectra measured for the Ti₃C₂Tₓ samples
the relative intensity of the O1s components
change with the photon energy, that is with the
sample thickness probed by XPS. This is well
illustrated in Figure 3, which compares the O1s
spectra measured with photon energies of 600,
850 and 1030 eV, that is acquired with XPS
sampling depth increasing from ~ 1.4 to ~ 3
nm.⁵⁰,⁵¹ Note that the photoemission intensity
decays exponentially from the surface towards
the interior of the sample.

<figure><img src="image_5.png" /></figure>

Figure 3: O1s spectra measured for the pristine Ti₃C₂Tₓ sample at photon energies of 600, 850 and 1030 eV. The inset shows the relative intensity of the O₀, O₁ and O₂ components *vs.* the XPS probing depth at the different photon energies.

Hence, the surface localization of the O₀ com-
ponent, deduced from its higher relative inten-
sity at lower photon energies, together with its
dominating intensity in all O1s spectra, sup-
port the prevalent attribution to O termina-
tions bonded to Ti<sub>b</sub> atoms. Concerning the
site occupied by the O terminations, the O1s
BE calculated for O atoms residing in *fcc* and
*hcp* sites differ by less than 0.1 eV<sup>5</sup> or by 0.3
eV<sup>30</sup> and are therefore essentially indistinguish-
able in the measured spectra, whereas the *top*
(and the *bridge*) sites are energetically disadvantaged.⁵ On the other hand, the rise of the relative intensity of the O₁ and O₂ components with increasing photon energy (and sampling depth) indicates that the corresponding O atoms might reside at the MXene surface, but have undoubtedly a prevalent bulk-like origin. As surface terminations, they can be assigned to configurations other than those contributing to the O₀ component. In this respect, our recent calculations show that the photoemitted intensity around 531 can be attributed to surface O atoms with disordered surrounding.³⁰ As bulk-like species they can be mostly related to C-O bonds in the carbon sublattices lying two layers below the O terminations⁵² and representing the counterpart of the C₄ and C₅ components of the C1s spectrum and/or to three-dimensional Ti oxide domains on the basal plane or on the edge of the MXene flakes,³⁸ either produced during the delamination process or inherited from the MAX phase.⁵² This conclusion is supported by the results of additional Ti₃C₂Tₓ oxidation experiments summarized in the Supporting Information (Figure S6).

## Ti₃C₂Tₓ hydrogenation

We first explored the possibility to hydrogenate the Ti₃C₂Tₓ film with molecular hydrogen by exposing the sample to ~1000 L of H₂ at room temperature, but did not observe any change in the XPS spectra (Figure S7). Then the Ti₃C₂Tₓ sample was exposed at RT to a flux of H atoms for increasing dosing time t₉ up to 110 minutes. The survey spectrum measured at photon energy of 600 eV after the highest H dose is displayed at the bottom of Figure 2a. The complete set of survey spectra acquired at different photon energies after increasing t₉ is shown in Figure S3. The XPS spectra measured with photon energies of 1030 eV allow to reliably deduce the MXene chemical composition (for details see the Supporting Information). It comes out that the chemical composition Ti₃.₀C₃.₅O₂.₂F₀.₇Cl₀.₃ of the pristine sample changes to Ti₃.₀C₂.₉O₃.₃F₀.₄Cl₀.₂ after the heaviest hydrogenation. The changes in the concentration of the different elements are discussed

below.

## Core level spectroscopy

More precise information is obtained by measuring during the hydrogenation the high resolution Ti2p, C1s, F1s, Cl2p and O1s XPS spectra, which are shown in Figure 2b-e together with their best-fit curves and spectral components. The integrated intensities of the core level spectra components are plotted as a function of t₉ in Figure 2f-h, whereas their assignments are listed in Table I.

### Halogen spectra

It is worth starting the discussion from the evolution of the F1s spectra with the H dose, that together with the trend exhibited by the Cl2p doublet, represents one of the most relevant results of this study. Figures 2d and 2g show that the hydrogenation at RT effectively removes the terminating halogen atoms, whose coverages in the near surface layer after the highest H dose we used are reduced to ~40% of the initial values. Fluorine and chlorine desorption has to be ascribed to the formation of volatile HF and HCl molecules consequent to the interaction of H atoms with the halogen terminations. It is worth pointing out that in order to remove substantially F and Cl terminations from Ti₃C₂Tₓ, it is necessary to heat the sample to ~1000 K.⁵,¹⁷,⁵³ The reactions determined by the interaction with H atoms impact minimally on the F1s spectrum of the F atoms still bonded to the MXene surface, which during hydrogenation still consists of a single main F₀ peak that finally appears only slightly broadened. Therefore F atoms members of homogeneous or mixed terminations triples, or even bonded to a partially unsaturated Ti atom cannot be distinguished in the F1s spectrum. This finding is in close agreement with the DFT predictions.³⁰ Note that the up-shift of +0.1 eV, evident mainly for F₀, has to be attributed to the Fermi level shift into the conduction band that will be discussed below. The incomplete halogen removal after the hydrogenation cycles we carried out can be in part ascribed to the
limited hydrogen penetration depth in the MXene film, which might be lower than the XPS sampling depth. Hence, some of the halogen atoms detected after hydrogenation could belong to sample layers not reached by H atoms. Also, the remaining F and Cl atoms could be bound to grain boundaries and/or contained in stable halogenated Ti oxide and carboxide phases. Longer hydrogenation times, possibly performed at temperature slightly higher than RT to increase H diffusivity, might effectively remove F and Cl also from MXene subsurface layers and/or from mixed phases.

## O1s spectra

The depletion of halogen atoms has to be evaluated together with the evolution of the O1s spectra, since it has often been reported that O atoms tend to occupy the surface sites set free by the removal of F terminations.¹⁷,⁵⁴ The modification of the O1s spectrum induced by the exposure to H atoms is summarized in Figure 2e. The comparison with the pristine O1s spectrum displayed at the top of the figure points out the appearance of the components O₃ and O₄ and makes evident the remarkable increase of the overall spectral intensity. The components O₃ can be straightforwardly assigned to OH terminal groups³⁰,⁴⁰ formed after the bonding of impinging H atoms to O terminations. The component O₄ is attributed to adsorbed water molecules,⁴⁰,⁴³ respectively. The trend of O₄, closely replicating the O₃ behavior, indicates that the adsorbed water molecules derive from the hydrogenation of OH terminations. It is worth noting that the O₃ component, identified as spectral fingerprint of OH terminations, is also present in the O1s spectra of the as-prepared samples (Figure S2), which means that, in contrast with the conclusions of Ref.,⁵⁵ Ti₃C₂Tₓ flakes obtained by wet etching do have hydroxyls bonded to the surface. Figure 2e shows that the O₀ peak after exposure to H atoms is up-shifted by ~0.1 eV, whereas O₃ progressively moves to lower BEs and for t_d=110 minutes the shift reaches -0.2 eV. The first shift, which is common to all core level spectra, is due to the Fermi level shift conse-

quent to hydrogenation and will be discussed in the next paragraph. In case of O₃ this shift overlaps with a larger shift (~-0.3 eV) in the opposite direction, likely caused by charge rearrangement in the hydroxyl groups due to the interaction with an increasing number of O terminations.

The evolution of the F1s, Cl2p and O1s spectra in Figure 2 makes clear that H atoms interact with halogen terminations determining their desorption and with the O termination converting them into OH groups. Furthermore, the rise of the O1s intensity (Figure 2h) supports the hypothesis that the surface sites previously occupied by F and Cl terminations and made free by their desorption, are refilled by O atoms. It has been observed that O adatoms are highly mobile on Ti₃C₂ surfaces even at ambient temperature,⁵⁶ but their surface diffusion cannot account for the increased O1s intensity. Actually, the observed increase could be explained by the migration to the MXene surface of O atoms which had a lower or null spectral weight in the pristine sample. This means O atoms that were bonded to C or Ti atoms of the intermediate layers or were terminating the opposite Ti₃C₂Tₓ surface or, else, belonged to the three-dimensional oxidized phases which decorate the MXene flakes.³⁸ The O transport in the material⁵⁷ could be driven by the structural and electrostatic instabilities determined by the formation of halogen vacancies at the surface⁵⁸ and take place along lattice defect paths and flake edges. Also, the interaction between the different oxidized phases and the incoming H atoms, which possibly diffuse also below the surface and release their energy to the surrounding, could activate bond dissociation and O transport in the material.⁵⁷ The evidence that the oxygen increase is opposite to the decay of F and Cl coverages (Figures 2g and 2h), indicates that when the rate of the halogen removal drops, the force driving the migration of O atoms to the surface vanishes. In the first hydrogenation steps (t_d ≤10 minutes) the rapid growth of O₃ (and of the weaker O₄ component) occurs at the expenses of O₀ (Figures 2h and S8), because the H atoms which arrive first, bind to the available O terminations. As
quickly as the halogen sites become free, they
are refilled by new O atoms and so gradually
the number of O and OH terminations rises.
For long H exposures, O₀ recovers and exceeds
its initial intensity, whereas O₃ saturates and at
the end moderately decreases.

It is less straightforward to explain the be-
havior of the O₁ and O₂ components, which
grow together with quite stable rates. Also for
hydrogenated Ti₃C₂Tₓ the O1s spectra mea-
sured with different photon energies incontro-
vertibly establish that the atoms contributing
to O₀, and now also to O₃ and O₄, are predom-
inantly located at the surface sites with respect
to O₁ and O₂ (Figure S8 and S9). Then, possi-
ble explanations for their growth could be that
the O atoms migrating towards the surface get
trapped at lattice defects and locally form ad-
ditional Ti oxidized phases,³⁸ or also that the
oxycloride and oxyfluoride phases in the pris-
tine sample were saturated with halogen atoms,
whose desorption via the recombination with H
atoms render their surfaces TiOₓ terminated.
In addition, since as reported above, O₁ and O₂
could also arise from O termination with disor-
dered surrounding, their increase, especially in
the first hydrogenation stage, could be a conse-
quence of the dramatic halogen desorption that
rises the number of O atoms neighboring sur-
face vacancies.³⁰

A point worth discussing concerns the ev-
idence that in several hydrogenation experi-
ments we never observed the O₃ component
more intense than O₀, that is hydroxyl termi-
nations more numerous than O terminations.
The curves in Figures 2h and S8 indicate that
only in the very first hydrogenation stage the
O₀ decrease and the O₃ increase compensate
each other, whereas, in correspondence of t₉ ~
10 minutes, O₀ starts to grow again. Initially
the hydrogenation is so efficient to deplete sig-
nificantly the populations of O terminations,
thus producing hydroxyl groups, while at later
stages the rate of OH formation diminishes and
the surface density of O termination is restored.
The saturation of the O₀ → O₃ conversion
might be attributed to the OH-OH repulsion,
which regulates the surface density of the newly
formed hydroxyls in order to keep them distant

from each other. For the hydrogenation of the
Ti₃C₂O₂ surface to Ti₃C₂(OH)₂ it was found
that the Gibbs energy (for 0 V electrode poten-
tial) has a minimum when ~35% of the O atoms
are protonated,²⁸ which corresponds to a ratio
of ~ 1.8 between the numbers of O and OH ter-
minations. The curves in Figure 2h show that,
leaving out the initial hydrogenation stage, the
ratio between O₀ and O₃ slowly rises up to ~
1.5 (see also Figure S10b), a value not very dif-
ferent from the calculated one,²⁸ if we consider
that the presence of residual halogen atoms
and probably of H terminations contribute to
separate the OH groups from each other. We
also observed that after the permanence of sev-
eral hours in UHV of the highly hydrogenated
Ti₃C₂Tₓ film, the O₃ component decreased sig-
nificantly, demonstrating the instability of high
hydroxyl concentrations, in agreement with the
tendency reported for OH-terminated MXenes
to transform into O-terminated MXenes and H₂
gas.⁵⁹

***Ti2p spectra***

The removal of more than half of the halogen
atoms and the formation of a high number of
Ti-OH bonds and possibly also of Ti-H bonds,
induce only moderate changes in the Ti2p spec-
trum. Figure 2b shows that the Ti2p spectra
measured after the exposure to H atoms only
exhibit some modification of the relative inten-
sities of the pristine spectral components. In
particular, as illustrated by the curves in Fig-
ure 2f, Ti₁ loses some intensity for t > 20 min-
utes, Ti₀ and Ti₂ manifest only a limited de-
crease during the whole hydrogenation, whereas
Ti₃ and Ti₄ rise progressively. The two latter
components can be basically considered as the
counterparts of the O₂ and O₁ components of
the O1s spectrum, respectively, as attested also
by the very similar behavior vs. t₉ (compare
Figures 2f abd 2h) which can be explained with
the same arguments exposed above to explain
the increase of O₁ and O₂. The depletion of the
T₁ component, i.e. of the number of Tiₐ atoms,
can be related to the intercalation of H atoms
below the surface and the subsequent insertion
into the Ti–C bonds. Instead, the mild decrease
<table><caption>Table 1: Binding energies and assignments for the core level spectra components.</caption><thead><tr><th>Component</th><th>BE(eV)</th><th>Assignment</th></tr></thead><tbody><tr><td>Cl2p<sub>3/2</sub></td><td>199.1</td><td>mainly Cl surface terminations</td></tr><tr><td rowspan="6">C1s</td><td>C<sub>0</sub></td><td>281.9</td><td>Ti<sub>3</sub>C<sub>2</sub></td></tr><tr><td>C<sub>2</sub></td><td>282.5</td><td>carbide/a-C interface, C-H<sub>x</sub> in carbide, disorder effects</td></tr><tr><td>C<sub>1</sub></td><td>284.2</td><td>C-C bonds in amorphous and graphitic domains</td></tr><tr><td>C<sub>3</sub></td><td>284.7</td><td>C-H bonds</td></tr><tr><td>C<sub>4</sub></td><td>285.5</td><td>C-O, C-F compounds</td></tr><tr><td>C<sub>5</sub></td><td>286.6</td><td>C=O, C-F compounds</td></tr><tr><td rowspan="5">Ti2p</td><td>Ti<sub>0</sub></td><td>454.8</td><td>unsaturated Ti<sub>b</sub> atoms</td></tr><tr><td>Ti<sub>1</sub></td><td>455.2</td><td>Ti<sub>a</sub> atoms</td></tr><tr><td>Ti<sub>2</sub></td><td>456.0</td><td>Ti<sub>b</sub> atoms bonded to mixed surface terminations, TiO, Ti oxy-carbides also halogenated</td></tr><tr><td>Ti<sub>3</sub></td><td>457.4</td><td>Ti<sub>2</sub>O<sub>3</sub>, Ti oxycarbides also halogenated</td></tr><tr><td>Ti<sub>4</sub></td><td>459.3</td><td>mainly TiO<sub>2</sub></td></tr><tr><td rowspan="5">O1s</td><td>O<sub>0</sub></td><td>529.7</td><td>O surface terminations</td></tr><tr><td>O<sub>1</sub></td><td>530.5</td><td>TiO<sub>2</sub>, C=O bonds</td></tr><tr><td>O<sub>2</sub></td><td>531.2</td><td>TiO<sub>x</sub>, C-O bonds, O terminations with disorder surrounding</td></tr><tr><td>O<sub>3</sub></td><td>532.0</td><td>OH terminations</td></tr><tr><td>O<sub>4</sub></td><td>533.1</td><td>adsorbed water molecules</td></tr><tr><td rowspan="2">F1s</td><td>F<sub>0</sub></td><td>684.8</td><td>mainly F surface terminations</td></tr><tr><td>F<sub>1</sub></td><td>686.2</td><td>F-C bonds</td></tr></tbody></table>

of Ti$_{0}$ i.e. of the number of unsaturated Ti$_{b}$ atoms, agrees with the formation of new Ti-T bonds. More interestingly, the marginal decrease of the Ti$_{2}$ intensity indicates that the number of Ti atoms with oxidation state II is not substantially modified after the heavy modification of the chemical composition of MXene surface. This can be understood by assuming that the substitution of F and Cl with O, OH and probably H terminations and their consequent rearrangement on the MXene surface leave the valence state of the Ti$_{b}$ atoms basically unchanged. If so, we can conclude that in general Ti$_{b}$ atoms bonded to mixed groups of surface terminations, which are the most favored arrangements,³¹ have a predominant oxidation state of ~ II and in the Ti2p spectrum are mainly represented by the Ti$_{2}$ component. The assignment of Ti$_{2}$ to Ti$_{b}$ atoms bonded to mixed terminations is also in agreement with the Ti2p BEs calculated for Ti$_{3}$C$_{2}$T$_{2}$ (T=F, Cl, O, OH) MXenes, which range between 455 and 456.9 eV.³⁰

### C1s spectra

As for the C1s spectra, Figure 2c shows that the main modification determined by the interaction with H atoms is the conversion of the component C$_{2}$ into C$_{3}$, which supports the assignment to non-hydrogenated and hydrogenated C-C bonds, respectively. Afterwards, the prolonged exposure to H atoms removes most of the C$_{3}$ intensity, likely because of the formation of small volatile hydrocarbons, but also slightly reduces the intensity of C$_{0}$. Whereas the loss of C$_{3}$ does not impact on the Ti2p spectrum, the decrease of C$_{0}$ parallels the equivalent decrease of Ti$_{1}$, manifesting some decomposition of the Ti-C lattice caused by the extended interaction with H atoms. Finally, although H atoms cannot be revealed by XPS, we could tentatively relate the moderate growth of C$_{1}$ to the bonding of H atoms to the C atoms in the MXene octahedra.⁴⁵
Valence band spectroscopy

Deeper insights complementing the analysis of
the core level spectra were obtained by studying
the valence band region. Figure 4a,b shows the
VB spectra measured on the Ti₃C₂Tₓ sample in
its pristine state and after the subsequent hy-
drogenation steps described above. Eight main
spectral features marked in Figure 4 as V1–
V8 were identified to facilitate the VB anal-
ysis. In order to separate the contributions
of the uppermost from the underlying sample
layers, in each case the VB spectra were ac-
quired at photon energies of 140 eV (Figure 4a;
more surface sensitive) and 600 eV (Figure 4b;
more bulk sensitive). Note that the energy of
the incident x-ray beam not only determines
the probed depth, but also the atomic pho-
toionization cross sections σ, as already demon-
strated by Näslund et al.⁵⁵ From the experi-
mental point of view only, it is not straightfor-
ward to discriminate between these two effects,
which can lead to different evolutions in terms
of intensity variations. Figure 4c displays the
VB spectra calculated for pristine and function-
alized Ti₃C₂ sheets, the latter being obtained
by saturating all the MXene surface Ti₉ atoms
with F, Cl, OH and O terminations in the *fcc*
sites. Calculations were carried out for photon
energies of 140 and 600 eV, by taking into ac-
count, for all the different elements, the energy
dependent photoionization cross sections σ (see
Experimental Section), that modulate the peak
intensities.

The spectral intensity in the region close to
the Fermi level mainly originates from Ti3d
states responsible for the metallic-like conduc-
tivity of this material³⁷ (in Figure 4c see the
blue areas in the simulated spectra for Ti₃C₂).
The comparison between the VB spectra mea-
sured for pristine and heavily hydrogenated
Ti₃C₂Tₓ shows that the DOS at the Fermi en-
ergy N(E<sub>F</sub>) increases after hydrogenation. Ac-
cording to the simulated VB spectra (Figure 4c)
N(E<sub>F</sub>) rises in order for O, F/Cl and OH ter-
minated Ti₃C₂ surface,⁶⁰,⁶¹ and is the largest
for unterminated Ti₃C₂ layers. Hence the ob-
served trend can be due both to the formation
of OH terminations and to the Ti₉ atoms re-

mained unsaturated after the F/Cl desorption
(Figure 2g).

Figures 4a,b and 5a show also that, after hy-
drogenation, all spectral features move by ~
+0.15 eV, a trend already observed, to a lesser
extent, for the core level spectra. This result,
which in the rigid band approximation⁶²,⁶³ can
be read as a shift of the Fermi level into the con-
duction band, is consistent with the simulated
spectra (Figure 4c) showing that the peaks V1
and V3 (the latter marked for Ti₃C₂O₂, but also
observed for Ti₃C₂(OH)₂) move by ~+0.4 and
~+0.6 eV, respectively, when replacing O ter-
minations with OH terminations. That the ex-
perimental shifts are lower than the calculated
values is not surprising, since the mixed ter-
mination arrangements present in the real sur-
faces lead to a reduction of the effective peak
shifts. These findings are also consistent with
other calculations, showing that the Fermi level
moves into the conduction (valence) band when
the concentration of the OH terminations (O
terminations) increases, still remaining in the
Ti-d band.³¹,⁶⁴ For a better visualization of the
DOS increase at the Fermi energy and of the
high BE shift of the VB spectra see also Figure
6a).

The comparison between the calculated and
experimental curves allows to attribute the fea-
ture V1 to hybridized C2p—Ti3d states<sup>55</sup> (see
Figure 4c, blue and dark gray areas in the DOS
calculated for Ti<sub>3</sub>C<sub>2</sub>), which form the covalent
Ti-C bonds.<sup>64,65</sup> Carbon electronic states also
contribute to the feature V8, which can be eas-
ily identified as mainly due to C2s states. As
opposed to the feature V1, which shows little
sensitivity to the photon energy, the theoret-
ical and the experimental intensity of feature
V8 rises with photon energy. When consider-
ing the contributions of the MXene termina-
tions a first indication is provided by the fact
that peaks related to the halogen atoms should
vanish with the progress of hydrogenation. At
a glance, it is evident that the structures V2,
V4 and V6 tend to disappear with increasing
H dose, this effect being more evident in the
surface sensitive spectra (Figure 4a). The cal-
culated Ti<sub>3</sub>C<sub>2</sub>F<sub>2</sub> spectra show a main peak at
around 7.5 eV, owing to the F2p states, which
<figure><img src="image_6.png" /><figcaption>Figure 4: Valence band spectra measured with photon energies of a) 140 eV and b) 600 eV for the Ti₃C₂Tₓ sample before and after the exposure to H atoms for increasing time t₉. The spectra were normalized to have the same intensity in the background region around 15 eV. Specific spectral features are indicated with labels V1-V8. c) photon energy dependent VB spectra, calculated using the mBJ potential, for Ti₃C₂ and for functionalized Ti₃C₂T₂ with T = O, F, Cl or OH in order from bottom to the top. The calculations were performed considering the photoionization cross sections of the elements at the two photon energies used experimentally, i.e. 140 eV (full line) and 600 eV (dashed lines). The Ti3d (for both Tiₐ and Ti₉ atoms), C2s and C2p DOS are shown in the Ti₃C₂ spectrum. The termination contributions (O, F, Cl, OH) as represented by their PDOS are shown in the corresponding MXenes spectra for the two photon energies to highlight their energy dependence. The spectra calculated at each photon energy were normalized to have the same intensity at the Fermi level (set to the origin of the x-axis).</figcaption></figure>

can be undoubtedly assigned to the experimen-
tal feature V6, appearing at 8.6 eV.³⁶,⁵⁵ At vari-
ance with the calculations, in the spectra of
the pristine sample, peak V6 is higher at 140
eV than at 600 eV, a discrepancy not referable
to cross section effects, but to the higher con-

centration of the F terminations at the surface
of the MXene flakes producing higher spectral
weight at lower photon energy. The BE posi-
tion of 7.4 eV calculated for the F2p states is
~ 1.2 eV lower than that (8.6 eV) observed in
the experimental spectra. In this respect, it can
be noted that Näslund et al.⁵⁵ and Schultz et al.,⁵ both using the HSE06 exchange correlation functional, which is more computationally expensive than the mBJ potential we used, found the F2p states at 9.8 eV and 8.1 eV respectively, none of which matching the experimental value. Such discrepancy could imply that the position of the F2p states is highly dependent on the structural model of Ti₃C₂F₂ used for the simulation. For Ti₃C₂Cl₂ the calculated Cl3p states consist of two peaks centered at around 4.2 and 6.0 eV, both being more intense when excited with 600 eV photons because for Cl-p states σ increases with photon energy. The first peak optimally corresponds to the feature V2 appearing at 4.2 eV in the experimental spectra. However, due to the close vicinity of the calculated C2p-Ti3d hybridized states, the experimental V2 feature cannot be unequivocally considered as only being due to the Cl3p states. Indeed, in Ref.,⁵⁵ a feature at 4.2 eV observed in a Cl-free Ti₃C₂Tₓ sample was attributed to hybridized (Ti3d+Ti3p+C2p) states. Differently, in our case the evidence that the feature V2 loses intensity with increasing H dose both in the surface sensitive and bulk sensitive spectra, whereas the other Ti–C related bands remain stable, indicates that it carries also the contribution of the Cl terminations. The second peak calculated for the Cl3p states at 6 eV corresponds to the experimental feature V4 appearing at 6.5 eV. This attribution is firmly supported by the higher intensity observed in the spectrum measured at 600 eV, as expected based on the σ values, and by the peak decrease under hydrogenation observed in Figure 4a,b, consistent with the desorption of the Cl terminations.

The experimental feature V3 at ~5.3 eV derives from the O2p orbitals, as indicated by the comparison with the simulated Ti₃C₂O₂ spectra, and its intensity rises at higher H doses, reproducing the overall increase of the O1s intensity. The VB spectra calculated for Ti₃C₂O₂ with the O terminations in fcc, hcp and bridge sites gives similar results in the three cases (Figure S11), indicating that different O configurations cannot be distinguished spectroscopically.⁵ It is worth noting that the feature V3

increases with H dose in the surface sensitive, but not in the bulk sensitive VB spectra, showing that, while the number of O atoms rises at the MXene surface, it remains almost constant in the thicker layer probed at 600 eV. This outcome reproduces with more clarity the trend shown by the O1s spectra measured with photon energy of 1030 eV which, during hydrogenation, increase in intensity much less than those measured at 600 eV (Figure S8).

More interesting is to identify the contribution of the OH terminations. According to the VB spectra calculated for Ti₃C₂(OH)₂ (Figure 4c), states related to OH terminations appear as a peak at 5.8 eV with a shoulder at 6.9 eV and as a second peak deeper in the VB at around 10 eV. The latter peak, due to the mixing between O-p and H-s orbitals (see the detailed PDOS of Ti₂C₂(OH)₂ in Figure S12) is separated from other VB states and can be straightforwardly related to the experimental feature V7 appearing at 10.5 eV, which therefore involves only states due to O-H bonds. In fact, the surface sensitive VB spectra show that the feature V8 arising from the C2s states, during hydrogenation is progressively surmounted by the more intense feature V7 due to the formation of hydroxyl groups. This behavior is less evident in the bulk sensitive spectra because of the high σ of the C2s states at high photon energy, which masks the appearance of the weaker OH band. The calculated OH peak at 5.8 eV is difficult to identify in the experimental spectra because of the overlap with the O-2p band responsible for the feature V3. Finally, the other OH calculated feature at around 6.9 eV can be identified as the experimental feature V5, that is with the spectral intensity which rises at 7.5 eV and progressively fills the dip between V4 and V6. The assignment of the features V5 and V7 to OH terminations is thus strongly supported by the VB spectra simulations as well as by the results obtained in the re-hydrogenation experiment described in one of the next paragraphs. The assignment for all V1-V8 structures are listed in Table II.
<figure><img src="image_7.png" /><figcaption>Figure 5: Annealing of the Ti₃C₂Tₓ sample exposed to H atoms for 110 minutes. a) C1s and c) Ti2p spectra measured for the hydrogenated sample at RT and after annealing to 550 and 770 K; b) O1s spectra measured for the hydrogenated sample before (top) and after annealing at 770 K (bottom). The central panel shows the 2D intensity plot of the spectra acquired during sample heating; d) valence band spectra measured before and after sample annealing and normalized to have the same intensity in the background region around 15 eV. The VB spectrum measured for the hydrogenated sample is shifted by -0.15 eV to align the spectral features. The bottom curve represents the difference between the spectra measured for the hydrogenated (misaligned) and annealed sample and the red (grey) areas indicate the regions where the difference is positive (negative). The misalignment between the VB spectra causes the negative peak around 0 eV in the difference curve which is not real; e-f) integrated intensity of the e) O1s and f) Ti2p components vs. annealing temperature.</figcaption></figure>
<figure><img src="image_8.png" /></figure>

Figure 6: Comparison between the low energy region of the valence band spectra measured for Ti₃C₂Tₓ in the pristine state, after heavy hydrogenation (exposure to H atoms at RT for 110 minutes), after annealing to 550 K and 770 K and after re-hydrogenation (re-exposure to H atoms at RT for 60 minutes). The spectra are plotted in couples to highlight separately the spectral changes induced by each treatment. The compared spectra were normalized to have the same intensity in the background region around 15 eV.

<table><caption>Table 2: Experimental binding energies and assignments for the valence band spectra features.</caption><thead><tr><td>Structure</td><td>BE(eV)</td><td>Assignment</td></tr></thead><tbody><tr><td>V<sub>1</sub></td><td>3.1</td><td>C2p-Ti3d states</td></tr><tr><td>V<sub>2</sub></td><td>4.3</td><td>Cl3p, C2p-Ti3d states</td></tr><tr><td>V<sub>3</sub></td><td>5.4</td><td>O2p states</td></tr><tr><td>V<sub>4</sub></td><td>6.5</td><td>Cl3p states</td></tr><tr><td>V<sub>5</sub></td><td>7.3</td><td>OH states</td></tr><tr><td>V<sub>6</sub></td><td>8.6</td><td>F2p states</td></tr><tr><td>V<sub>7</sub></td><td>10.4</td><td>OH states</td></tr><tr><td>V<sub>8</sub></td><td>11.2</td><td>C2s states</td></tr></tbody></table>

## Thermal stability of hydrogenated Ti₃C₂Tₓ

The thermal stability of the hydrogenated Ti₃C₂Tₓ was explored by following the core level and the valence band spectra while heating the sample up to 770 K. Results are summarized in Figures 5 and 6. It is worth describing the experimental results starting from Figure 5a, which shows the evolution of the C1s spectra measured for the hydrogenated sample, and after intermediate temperature (550 K) and high temperature annealing (770 K). In this case the main changes determined by the high temperature are the progressive decay of the C₃ peak, signaling the dissociation of the C-H bonds, and the dramatic increase of the C₂ component after heating to 770 K, that indicates the segregation

of C atoms at the sample surface, where they mainly form amorphous C–C phases. This last trend parallels the decrease of the C₀ component, indicative of some decomposition of the Ti₃C₂ matrix.

Figure 5b displays in the upper and bottom panels the O1s spectra measured for the hydrogenated and annealed Ti₃C₂Tₓ sample, respectively, and in the middle panel the 2D intensity plot of the spectra acquired during heating. The O1s component intensities as a function of the temperature are shown in Figure 5c. The O1s component that diminishes first is O₄, whose decay around 350-400 K reveals the desorption of water molecules. Starting from 400 K, the decrease of O₃ and the increase of O₀ indicate the desorption/decomposition of the hydroxyl terminal groups, which are mostly converted into O terminations. Around 500 K the curves relative to O₁ and O₂ indicate the conversion from TiOₓ to TiO₂. Above 600 K the rise of O₀ and O₁ reverses, likely because it is counteracted by the screening effect due to the C-C phase segregated at the sample surface. In Figure 5c the Ti2p spectra measured on the hydrogenated sample and after intermediate and high temperature annealing above 400 K show the increase of Ti₂ and to a lesser extent of the other Ti2p components except for Ti₁, which manifests the opposite trend. The general intensity raise is likely the consequence of the des-
orption of H₂O and possibly H atoms, which at RT screened the signal of the surface atoms. The decrease of Ti₁ is probably due to the onset of the Ti₃C₂ matrix decomposition, as indicated above by the loss of the C₀ intensity in the C1s spectrum. The common decrease above 600 K is due, as in the case of the O1s components, to the surface segregation of carbon.

The comparison between the VB spectra measured for the hydrogenated sample before and after the annealing to 770 K (Figure 6b) shows that, in the second case, the DOS in the Fermi level region recovers the pre-hydrogenation intensity and that the Fermi level moves back to the original position. Indeed, Figure S13 shows that for this to happen, heating at 550 K is sufficient, due to the desorption of most OH groups. In Figure 5d the two VB spectra are superimposed on each other, with the one measured for the hydrogenated sample shifted by -0.15 eV, to align the spectral features in the two spectra, and subtracted from the other one. The difference curve plotted at the bottom of the figure, is negative between 7 and 11 eV, in correspondence of the V5 and V7 features assigned to OH terminations, and is positive between 3.7 and 6.5 eV in correspondence of the O2p (V3) and C2p—Ti3d (V1) bands, in agreement with the increase of the O terminations and the modification of the Ti-C lattice described above. The stable line shape in correspondence of the F2p (V6) and Cl3p (V2) states indicates that the residual halogen terminations are basically unaffected by the annealing to 770 K.

## Ti₃C₂Tₓ re-hydrogenation

In order to achieve further information about the direct interaction between the Ti₃C₂ matrix and the H atoms, the sample annealed to 550 K was re-exposed at RT to the flux of H atoms for 60 minutes. The O1s, C1s and VB spectra measured before and after the re-hydrogenation are shown in Figure 7a-c. The O1s spectra (Figure 7a) indicate, as in the previous experiment, the rise of the number of OH terminations and of the adsorbed water molecules at the expenses of the O terminations, trends illustrated by the increase (decrease) of the O₃

and O₄ (O₀) components. It is worth noting that in the re-hydrogenated sample the ratio between the number of OH and O terminations is ~ 2, a value close to that expected by the calculations for the Ti₃C₂Oₓ(OH)₂⁻ₓ MXene.²⁸ Figure 7a shows that, while O₀ diminishes, O₁ and O₂ slightly rise. It can also be noted that the loss of spectral intensity at 4.4 eV and 8.6 eV in the VB spectrum (Figure 7c) indicates the removal of the residual halogen atoms. Whenever the F and Cl atoms still present in the annealed sample were located, they essentially disappears after the extra-hydrogenation. In particular, if contained in halogenated carboxides phases, their loss could explain the increase of the C₄ and C₅ components (Figure 7b) that, as O₁ and O₂, would gain spectral intensity after their removal from those mixed compounds. The VB spectra measured before and after the re-hydrogenation are compared in Figure 6c and in Figure 7c. In the first case the comparison clearly shows a DOS increase in the Fermi level region comparable to that observed after the first hydrogenation (Figure 6a), but a higher up-shift of the Fermi energy. Figure 7c shows at the bottom the difference curve between the VB spectra measured for the re-hydrogenated (shifted by -200 meV) and annealed sample. In agreement with the O1s spectra of Figure 7b the difference curve is positive in correspondence of the OH bands (V5 and V7) and negative in correspondence of the O2p band (V3). However, differently from the result of the first hydrogenation, in this case also the C2p—Ti3d band at low BE (V1) is significantly depleted (see also Figure 6c). This result, which is in line with the decrease of the C₀ peak in the C1s spectrum, suggests that during the re-hydrogenation the H atoms impact more severely on the Ti₃C₂ matrix. By summarizing the experimental findings, it can be concluded that H atoms might bind to the Ti₉ atoms enriching the MXene surface of H terminations responsible of the extra-Fermi level shift, but also intercalate below and perturb the MXene octahedra, possibly dissociating the Ti-C bonds. Further experiments are needed to better elucidate the interaction of H atoms with partly defunctionalized Ti₃C₂Tₓ and to better define
<figure><img src="image_9.png" /></figure>

Figure 7: a) O1s, b) C1s spectra measured on the Ti₃C₂Tₓ sample *top*) heated at 550 K after having been hydrogenated for 110 minutes (see Figure 5) and *bottom*) then re-exposed at RT to H atoms for 60 minutes. c) Valence band spectra measured on the annealed and re-hydrogenated sample, normalized to have the same intensity in the background region around 15 eV. The VB spectrum measured for the re-hydrogenated sample is shifted by -0.2 eV to align the spectral features. The bottom curve in c) represents the difference between the VB spectra measured after (misaligned) and before the re-hydrogenation and the blue (grey) areas indicate the regions where the difference is positive (negative). The misalignment between the VB spectra causes the peak around 0 eV in the difference curve which is not real.

the surface composition achievable by exploit-
ing the re-hydrogenation reaction. For the sake
of completeness we mention that the sample
treated with post-hydrogenation annealing, and
therefore deprived of most of the halogen and
OH terminations, was also exposed to molecular
oxygen at 850 K (see Figure S7). As expected,
the material started to convert into TiO₂, as
confirmed by the VB spectrum shown in Figure
S7c. In this case one aspect worth investigating
further is whether the oxidation rate is faster in
the defunctionalized than in the pristine MXene
flakes, which would facilitate the production of
TiO₂ nanoparticles supported on Ti₃C₂Tₓ lay-
ers, reported to have interesting catalytic per-
formances.<sup>66-68</sup>

Conclusions

For MXenes, the terminating groups on one
side enrich their functionalities and on the other
represent a factor of thermal and chemical in-
stability. By leveraging on this point we used

H atoms to modify the surface termination of
Ti₃C₂Tₓ flakes at room temperature, and more
broadly, to study the reactivity of this MXene
with atomic hydrogen. We found that the in-
teraction with reactive H atoms removes the
halogen terminations from the Ti₃C₂Tₓ surface.
Hence hydrogenation acts as a valid route alter-
native to the prolonged high temperature an-
nealing necessary to obtain F and Cl desorp-
tion. In parallel with halogen removal, the
O terminations are in part readily converted
into OH groups, with the constrain that the
OH/O ratio is regulated by the OH-OH repul-
sion. The substantial change of the surface
chemical composition does not modify signifi-
cantly the valence state of the T terminated Ti
atoms, which in general maintain the pristine
valence state of II, determined by the bond-
ing to mixed termination groups. In the hy-
drogenated, as well in the pristine samples, by
varying the probed sample depth we could sep-
arate the spectral contribution of the O atoms
terminating the MXene surface from that of the
oxidized phases distributed below the surface
or arranged in three-dimensional domains. The simulation of the valence band spectra of different $Ti_3C_2T_2$ model systems by DFT calculations allowed the assignment of all spectral features to Cl, F, O and OH terminations, and elucidated the change of the electronic properties induced by hydrogenation in the Fermi level region. Hydroxyls start to desorb from the sample at around 400 K. The hydrogenated MXene, deprived of the halogen and hydroxyl terminations interacts more dramatically with H atoms that have easier access to its lattice and, after prolonged exposure, start to dissociate the $TiC$ bonds. Besides elucidating the interaction of H atoms with $Ti_3C_2T_x$, our results show that hydrogenation, coupled with moderate thermal treatment, can be taken as a clever mean to remove the pristine, in general unwanted, surface terminations from $Ti_3C_2T_x$ flakes, thus paving the way to new, application-oriented MXene functionalization. Additional experiments including the $Ti_3C_2T_x$ hydrogenation at different temperatures, performed also for different classes of samples (non-delaminated multilayers, very thin films, crystalline samples) would expand and enrich the information we now have, provide deeper understanding of the hydrogenation reactions and open up new possibilities for their use in MXene processing.

# Experimental Section

## $Ti_3C_2T_x$ films preparation

In this study, we used $Ti_3C_2T_x$ MXenes synthesized by wet chemical etching of $Ti_3AlC_2$ phase using the minimally intensive layer delamination (MILD) method. The synthesis procedure included etching $Ti_3AlC_2$ in a mixture of LiF and 9M HCl for 24 h at 40 °C. This allowed us to use soft shaking to delaminate MXenes into single layers and produce less defective $Ti_3C_2T_x$ sheets with increased lateral size.⁶⁹ The delaminated $Ti_3C_2T_x$ MXenes were re-dispersed in water to obtain a suspension with a concentration of 3 mg/ml. After this, the suspension was dropped onto a $SiO_2$/Si substrate and left to dry overnight resulting in a film with an average thickness of 400 nm.

## Sample hydrogenation

The hydrogenation experiment was performed at the SuperESCA beamline of the synchrotron radiation source Elettra (Trieste, Italy). The $Ti_3C_2T_x$ films supported on the $SiO_2$/Si substrates were mounted on a manipulator capable of providing sample heating by W filaments placed behind the sample. After a preliminary annealing to 650 K to remove the surface contaminants due to the exposure to atmosphere, the $Ti_3C_2T_x$ films were exposed at room temperature to the hydrogen flux, at a pressure of ~ $2 \times 10^{-6}$ mbar. During exposure, the $H_2$ flux passed through a tungsten capillary held at temperature of ~ 3000 K to achieve thermal cracking. The end of the cracker was positioned at a distance of 5 cm from the sample and was partially shuttered to prevent the sample from being directly exposed to the flux of H atoms and to the radiation emitted by the hot cracker filament. H atoms produced in the hot capillary were estimated to have 0.25 eV of kinetic energy⁷⁰ and the fraction $f$ of dissociated molecules at the sample surface was estimated to be in the range $0.7 \le f \le 0.9$. Due to the uncertainty on the $H/H_2$ ratio in the gas flux impinging on the sample, throughout this work data are plotted as a function of the exposure time to H atoms $t_d$, since the dosing conditions were kept stable during the whole experiment. During all hydrogenation experiments the sample was kept at RT and hydrogen was dosed for $t_d = 20, 50$ and $110$ minutes. The thermal stability of the hydrogenated $Ti_3C_2T_x$ was investigated by annealing the sample dosed for $110$ minutes to $550$ and $700$ K.

## XPS spectroscopy

High resolution core level spectra were measured at different photon energies to have similar kinetic energies of the photoemitted electrons. When not differently specified, high resolution C1s and F1s spectra were measured with photon energy of 400 and 850 eV and energy resolution of 60 and 120 meV, respectively.
tively, whereas for the acquisition of Ti2p and O1s spectra the photon energy was 600 eV and the energy resolution was 80 meV. With the selected photon energies, the photoelectron kinetic energies corresponded to inelastic mean free paths of the order of 0.5 nm<sup>50,51</sup> and to XPS sampling depths around 1.5 nm. Valence band (VB) spectra were measured at 140 and 600 eV (XPS sampling depth of about 1.5 and 3.5 nm, respectively).<sup>50,51</sup> It should be considered that the Ti<sub>3</sub>C<sub>2</sub>T<sub>x</sub> crystalline thickness is ~ 1 nm<sup>71</sup> and that the presence of terminating groups expands non uniformly the interlayer spacing.<sup>72,73</sup> For each XPS spectrum, the binding energy was calibrated by measuring the Fermi level position on an Au substrate positioned close to the Ti<sub>3</sub>C<sub>2</sub>T<sub>x</sub> sample. The measurements were performed with the photon beam impinging on the sample at grazing incidence (70°), while photoelectrons were collected at normal emission angle. The core level spectra were best-fitted with Doniach-Sunjić functions convoluted with Gaussians. For C1s, F1s and O1s spectra a linear background was subtracted, whereas the Ti2p spectra were best-fitted after the subtraction of a Shirley background. In this case the spin orbit splitting between the 3/2 and the 1/2 components was 5.80±0.05 eV for low valence state components (in the following Ti<sub>0</sub> and Ti<sub>1</sub>) and 5.60±0.05 for higher valence state components (in the following Ti<sub>2</sub>, Ti<sub>3</sub> and Ti<sub>4</sub>).<sup>39,74</sup> The intensities of the core level and valence band spectra were normalized by the photon beam flux.

## DFT calculations

XPS valence band spectra calculations were performed with the all-electron full potential linearized augmented plane wave (FLAPW) method as implemented in the WIEN2k code,<sup>75,76</sup> the LAPW treatment being one of the most accurate methods for electronic structure calculations. Calculations presented in this paper were performed in the scalar relativistic framework. Exchange and correlation effect were treated in the Generalized Gradient Approximation (GGA) using the Perdew-Burke-Ernzerhof functional, especially for the

structural relaxations of MXene layers (see below).<sup>77</sup> Since GGA calculations were found to underestimate the positions in energy of some of the valence band spectral features, the modified Becke-Johnson (mBJ) potential by Tran *et al.*<sup>78</sup> was also used in order to improve agreement between simulations and experiment. This approach has already been demonstrated to improve the description of spectroscopic data in oxides, and the comparison between GGA and mBJ simulations of the VB spectra is given in Figure S14.<sup>79</sup> The plane wave basis set was defined using a RK-max value of 7.5, except for (OH) terminations where it was reduced to 3.5 due to the shorter O-H bonds. The muffin-tin radii associated to the different elements of the material under consideration were obtained from the automated routine provided with WIEN2k. The full Brillouin zone was sampled using 5000 k-points and a Monkhorst-Pack scheme k-points grids.<sup>80</sup> To analyze the experimental XPS valence band spectra, we conducted simulations of energy dependent valence band spectra for photon energies of 140 and 600 eV (i.e. corresponding to the experimental values), using the PES module<sup>81</sup> as implemented in the WIEN2k code. The PES module computes XPS spectra using a combination of partial density of states (PDOS – including a correction accounting for the too small PDOS inside the atomic spheres for delocalized states) times excitation–energy dependent atomic orbital cross sections σ<sub>tl</sub> (from Tables in Refs.<sup>82,83</sup>). Calculations were performed considering an unpolarized X ray source, and a Gaussian broadening of 0.4 eV was applied to the simulated spectra for an easier visualization.

Ti<sub>3</sub>C<sub>2</sub>T<sub>2</sub> MXene structures were described as periodic stacks, the unit cell of which are built on two MXene layers (i.e., following the MAX phase precursors), and described in the P6<sub>3</sub>/mmc space group (Figure S15). Fixed unit cell parameters were used with *a* = *b* =3.058 Å (corresponding to the experimental parameters obtained in Ref.<sup>64</sup>) and a *c* parameter of 25 Å. For all the investigated MXenes, atomic positions were relaxed keeping the unit cell parameters fixed and considering the T-groups in
their most stable position, *i.e.* the *fcc* position. In order to investigate a potential effect of the position of surface groups, simulations were also carried out (Figure S11) considering the so-called *hcp* and *bridge* positions for the O-terminations (Figure 1). For these calculations, the MXenes were described as single layers separated by 18 Å vacuum.

## Associated content

The Supporting Information is available free of charge at https://pubs.acs.org/doi/....

Annealing of as-prepared Ti₃C₂Tₓ; wide XPS spectra of pristine and hydrogenated Ti₃C₂Tₓ at different photon energies; chemical composition of MXenes derived from the XPS spectra; further details of Ti2p and O1s spectroscopy; oxidation of Ti₃C₂Tₓ flakes; exposure of Ti₃C₂Tₓ to molecular hydrogen; Ti₃C₂O₂ valence band spectra calculated for different O adsorption sites; valence band spectra of the hydrogenated sample annealed at different temperatures; partial densities of states calculated for Ti₃C₂(OH)₂; comparison between GGA-PBE and mBJ DFT calculations of Ti₃C₂O₂ and Ti₃C₂F₂ valence band spectra.

## Acknowledgments

R.L. thanks Elettra Sincrotrone Trieste for providing access to its synchrotron radiation facilities and for financial support. F.B., M.P., F.B. and V.M. acknowledge financial support from the “Agence National de la Recherche” (reference ANR-18-CE08-014, MXENECAT project). The Ph.D. thesis of F.B. was funded by the MXENECAT project (reference ANR-18-CE08-014). The computational part of this work was supported by the French government programs “Investissements d’Avenir” (EUR INTREE, Reference ANR-18-EURE-0010 and Labex INTERACTIFS, Reference ANR-11-LABX-0017-01). Computations have been performed on the supercomputer facilities of the “Mésocentre de calcul de Poitou Charentes”.

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<figure><img src="image_10.png" /><figcaption>F1s</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>Binding energy (eV)</figcaption></figure>

<figure><img src="image_12.png" /><figcaption>Dosing time</figcaption></figure>

<figure><img src="image_13.png" /><figcaption>Binding energy (eV)</figcaption></figure>

<figure><img src="image_14.png" /><figcaption>Dosing time</figcaption></figure>

<figure><img src="image_15.png" /><figcaption>H dose</figcaption></figure>