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ArticlePhotocatalytic CO2 Reduction Using Ti3C2Xy (X = Oxo, OH, F, or Cl)
MXene−ZrO2: Structure, Electron Transmission, and the Stability
Published as part of Langmuir virtual special issue “Highlights in Interface Science and Engineering: Reactive
Separations for Carbon Capture”.
Hongwei Zhang,* Ikki Abe, Tomoki Oyumi, Rento Ishii, Keisuke Hara, and Yasuo Izumi*

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sı Supporting Information*

ABSTRACT: CO2 photoreduction using a semiconductor-based
photocatalyst is a promising option for completing a new carbon−
neutral cycle. The short lifetime of charges generated owing to lightenergy is one of the most critical problems in further improving theperformance of semiconductor-based photocatalysts. This study
shows the structure, electron transmission, and stability of Ti3C2Xy(X = oxo, OH, F, or Cl) MXene combined with a ZrO2
photocatalyst. Using H2 as a reductant, the photocatalytic CO
formation rate increased by 6.6 times to 4.6 μmol h−1 gcat−1 using
MXene (3.0 wt %)−ZrO2 compared to that using ZrO2, and thecatalytic route was confirmed using 13CO2 to form 13CO. In clear
contrast, using H2O (gas) as a reductant, CH4 was formed as the
major product using Ti3C2Xy MXene (5.0 wt %)−ZrO2 at the rateof 3.9 μmol h−1 gcat−1. Using 13CO2 and H2O, 12CH4, 12C2H6, and 12C3H8 were formed besides H212CO, demonstrating that the C
source was the partial decomposition and hydrogenation of Ti3C2Xy. Using the atomic force and high-resolution electron
microscopies, 1.6 nm thick Ti3C2Xy MXene sheets were observed, suggesting ∼3 stacked layers that are consistent with the Ti−C
and Ti···Ti interatomic distances of 0.218 and 0.301 nm, respectively, forming a [Ti6C] octahedral coordination, and the major
component as the X ligand was suggested to be F and OH/oxo, with the temperature increasing by 116 K or higher owing to theabsorbed light energy, all based on the extended X-ray absorption fine structure analysis.

■

INTRODUCTION

semiconductor and the EF value of the combined MXene isnear the CB minimum of the semiconductor, the electrons
effectively transfer from the semiconductor to MXene via theband bending (Schottky junction).4,5 Especially, photocatalytic
reduction of CO2 utilizing MXenes is highly expected and has
been reported to generate CO, CH4, and CH3OH.1,6 The
general formula of MXene is Mn+1ZnXy, where Z is C and/or
N; X is oxo, OH, F, or Cl; and n is 1−4. However, a detailed
photocatalytic reaction pathway from CO2 has been rarely
reported except for the evaluation of electron conductivity by
MXenes and implication of photoreduction by the transmitted
electrons via MXenes. In this paper, the structure of MXene−
ZrO2, electron transmission via MXene, and the photocatalytic
reaction pathway starting from CO2 are investigated. The

Photocatalytic CO2 reduction is one of the highly expected
options to complete a new carbon neutral cycle applicable in asustainable society. Various types of photocatalysts have been
researched for this purpose, and among them, semiconductorbased photocatalysts are expected to be stable in comparison
to molecular and/or metal complex photocatalysts.
Semiconductor photocatalysts utilize charge separation
owing to the electron excitation at the band gap for theoxidation reaction utilizing hole(s) at the valence band (VB)
and the reduction reaction utilizing excited electron(s) at the
conduction band (CB). However, the efficiency of thephotocatalytic reaction is determined by how long theseparated charges remain. In this context, nanomaterials used
to transmit such charges quickly have been tested in
combination with semiconductors: MXenes,1 covalent organic
frameworks,2 and graphene.3
Among them, recently, the applications of MXenes tophotocatalysis as nanoscale conductive components have been
intensively studied combined with semiconductors. When the
Fermi level (EF) of MXene is in the band gap of the© 2024 American Chemical Society

Received: December 14, 2023
Revised: January 26, 2024
Accepted: February 1, 2024
Published: February 16, 2024

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ArticleScheme 1. (A) Synthesis Steps of the Composite of Ti3C2Xy MXene with ZrO2 and the Photographs of (B) Ti3C2Xy MXene
Synthesized and (C) Ti3C2Xy Reacted with ZrO2 and the Suspension Being Rested

Photocatalytic CO2 Reduction. The CO2 photoreduction tests
were conducted using 20 mg of the photocatalysts. The photocatalyst
fine powder was placed in a quartz reactor (bottom plate area 9.56
cm2 and internal volume 38 mL) connected to a Pyrex glass
circulation system (100 mL; Perfect 6A, China) and kept undervacuum for 1 h using a rotary pump (10−2 Pa). The reaction gas ofeither CO2 (2.5 kPa), H2 (22.5 kPa), and Ar (25 kPa) or CO2 (2.3
kPa) and H2O (2.3 kPa) was introduced into the glass circulation
system. The photocatalyst was irradiated with Ultraviolet−visible(UV−vis) light from a 300 W xenon lamp (PLS-SXE300UV, Perfect
Light, China), and the distance between the UV−vis light exit and thephotocatalyst was 50 mm. The light intensity was 500 mW cm−2 atthe center of the photocatalyst.
The isotope tracing photocatalytic tests were also performed toinvestigate the photocatalytic reaction path. The isotope labeled
13
CO2 (2.3 kPa, chemical purity >99.9%; 99.0% 13C, 0.1% 17O, 0.7%
18
O, Cambridge Isotope Laboratories, Inc., Tewksbury, MA) and D2O(2.3 kPa; D 99.9%, chemical purity >99.5%, Cambridge Isotope
Laboratories) were used. The U-shaped quartz reactor (internal
volume 46.0 mL) containing the catalyst (20 mg) was irradiated with
UV−vis light using a 500 W Xe arc lamp (SX-UID502XAM, Ushio,
Japan) through a Y-shaped quartz light guide (model 1.2S15-1000F1Q7-SP-RX; 40 cm long fiber and 80 cm-long branches; Optel,
Tokyo, Japan). The distance between the fiber light exit and thephotocatalyst was 20 mm. The light intensity at the center of thesample was 142 mW cm−2. A packed column of 13X-S molecularsieves (3 m length, 3 mm internal diameter; GL Sciences, Inc., Japan)
was used for online gas chromatography−mass spectrometry (GC−MS) analyses (Model JMS-Q1050GC, JEOL, Tokyo, Japan), andhelium (0.40 MPa, purity >99.9999%) was used as the carrier gas.
Characterizations. The X-ray diffraction (XRD) pattern wasmeasured using an Ultima IV (Rigaku, Japan) at 40 kV and 40 mA.
The Cu Kα (wavelength λ = 0.15419 nm) and Ni filter was used as asource light at a Bragg angle (θBragg) of 2θBragg = 5−85° with a scanstep of 0.02° and the scan rate of 1 s per step.
The images of samples were observed by field-emission scanningelectron microscopy (FE-SEM) using a Model Apreo 2C (Thermo
Scientific, Waltham, MA). Atomic force microscopy (AFM) was alsoutilized to monitor the thickness and surface flatness using a Model
Dimension Icon (Bruker, Billerica, MA). The high-resolution
transmission electron microscopy (HR-TEM) image was observed
using a Model Talos F200S (Thermo Scientific). The elemental
mapping was also performed using the energy-dispersive detector
Model SUPER X, which is attached to the Model Talos F200S. The
high-angle annular dark-field−scanning transmission electron microscopy (HAADF−STEM) image was also observed using the Model
Talos F200S.
X-ray photoelectron spectroscopy (XPS) measurements were
performed with a Model ESCALAB XI+ photoelectron spectrometer(Thermo Scientific) using a 150 W X-ray radiation source of Al Kα.

effects and the stability of Ti3C2Xy (X = oxo, OH, F, or Cl)
MXene on CO2 photoreduction were critically evaluated using
H2 or H2O as a reductant. The layered structural unit of Ti3C2
and the X ligand that was introduced for the exfoliation was
analyzed in relation to the promoting effects on thephotocatalytic reduction of CO2 over the ZrO2 surface.

■

MATERIALS AND METHODS

Synthesis of Ti3C2Xy MXene. Ti3C2Xy MXene nanosheets wereprepared using a revised method as reported in the literature.7
Ti3AlC2 (1.0 g) was slowly added to a solution in a Teflon liner,
which consisted of HCl (6.0 mL), HF (1.0 mL), and deionized (DI)water (3.0 mL; <0.055 μS cm−1), for etching. The suspension was
stirred at a rate of 400 rotations per minute (rpm) for 24 h at 314 K.
Then, the resultant solid was washed with DI water and centrifuged(4500 rpm, for 5 min each) several times until the pH of the filtrate
became neutral.
Next, the residue was dispersed in DI water (25 mL) containing
LiCl (1.5 g) and stirred for 2 h at 293 K. Then, the resultant solid waswashed with DI water and centrifuged (10,000 rpm, 10 min) for fivetimes. Subsequently, the separated solid was dispersed again in DI
water (100 mL), and the dispersion was ultrasonicated (360 W, 40
kHz) for 6 h under an argon atmosphere cooled by an ice−water bath(278 K) to achieve exfoliation of MXene layers. Then, the dispersion
in the middle and upper layers were collected and extracted via
centrifugation (13,000 rpm, 30 min). Finally, the thus obtained
dispersion was divided into two parts: one part for vacuum freezedrying to obtain solid MXene powder and the other part for the
preparation of the MXene−ZrO2 composite photocatalysts. The
concentration of the MXene suspension was determined by weighing
the residual solid after drying.
Synthesis of Ti3C2Xy MXene−ZrO2. Scheme 1A illustrates thesynthesis steps of the Ti3C2Xy MXene−ZrO2 composite. ZrO2 (1.0 g;
JRC-ZRO-03, Catalysis Society of Japan, monoclinic phase major,
tetragonal phase minor, and a specific surface area of 94.4 m2 g−1) wassuspended in DI water (200 mL) followed by ultrasonification (360W, 40 kHz) for 10 min and stirring at a rate of 900 rpm for 10 min.
Subsequently, the as-prepared homogeneously distributed Ti3C2Xy
MXene dispersion (Scheme 1B) was slowly dropped onto the ZrO2
suspension and continuously stirred at 900 rpm for 2 h. The upper
layer of the suspension was transparent after resting, as shown in
Scheme 1C, demonstrating that most of the introduced MXene was
supported on ZrO2 (Scheme 1A). The precipitate was filtered, washedseveral times with DI water (200 mL each), and dried by vacuum
freeze-drying. The Ti3C2Xy MXene−ZrO2 composites with Ti3C2Xy
contents of 1.0, 3.0, 5.0, and 10 wt % were denoted MZ-1, MZ-3, MZ5, and MZ-10, respectively. As the Ti3C2Xy MXene loading increased,
the color of the composites changed from gray (1.0 wt %) to black(10 wt %).
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ArticleFigure 1. Photocatalytic reduction tests conducted under (A) CO2 (2.5 kPa) H2 (22.5 kPa), and Ar (25 kPa) and (B) CO2 (2.3 kPa) and H2O (2.3
kPa) for ZrO2 and Ti3C2Xy MXene (1.0−10 wt %)−ZrO2 using UV−vis light irradiation of wavelength in the range of 300−700 nm. The
photocatalyst amount used was 20 mg.
XPS data were analyzed by the software Advantage, and the binding
energies were calibrated by the C 1s line for adventitious C to 284.8
eV for samples of ZrO2, Ti3C2Xy MXene, and Ti3C2Xy MXene−ZrO2
for the C 1s, O 1s, Zr 3d, and Ti 2p electron orbitals.
UV−visible−near-infrared (UV−vis−NIR) spectra were recorded
on a Model LAMBDA 1050+ spectrometer (PerkinElmer, Waltham,MA) using deuterium and tungsten−halogen lamps below and above
340 nm, respectively, equipped with a photomultiplier tube and an
integrated sphere of 150 mm diameter for diffuse reflectancedetection within the range between 200 and 2000 nm. The
Kubelka−Munk function was used to transform the diffuse reflectancespectra. The integrated sphere contains two detectors: a photomultiplier tube for UV−vis light monitoring and an InGaAs
photodiode detector for NIR light monitoring.
The absorption−fluorescence spectra were recorded on FLS1000
fluorescence spectrometer (Edinburgh Instruments, UK) using a 450
W Xe arc lamp equipped with a photomultiplier tube for theexcitation at 200−400 nm within a fluorescence range of 250−600
nm. The incident excitation light from the Xe lamp was monitored bya Si photodiode, and the monitored fluorescence light emitted fromthe sample was normalized based on the incident light intensity ateach wavelength. The excitation spectra were corrected for theincident light intensity, and the emission spectra were corrected forthe spectral sensitivity of the recording system.
In situ Fourier transform infrared (FTIR) spectra were collectedusing a Tensor II NEXUS (Bruker) operating at 1 cm−1 resolution in
the range of 4000−400 cm−1, and the data accumulation included 256
scans. The prepared sample was under vacuum for 1 h to remove thegases adsorbed on the surface, and then, a spectrum was recorded
under vacuum as the background spectrum. Then, a mixed gas
comprising CO2 (5.0 kPa), H2 (45 kPa), and Ar (50 kPa, balance gas)or one comprising CO2 (97 kPa) and H2O (2.3 kPa) was introduced

to the reaction chamber, followed by the light irradiation from a 300
W Xe arc lamp, and the in situ FTIR spectra were collected in timecourse to monitor the intermediate species and the further conversion
toward the products under UV−vis light irradiation.
Titanium K-edge X-ray absorption fine structure (XAFS) spectra
were measured on beamlines 9C and 12C at the Photon Factory,
High Energy Accelerator Research Organization (KEK, Tsukuba,
Japan). The spectra for 30 wt % Ti3C2Xy MXene−ZrO2 samples in a
Pyrex glass cell (Φ = 20 mm, t = 2.0 mm) equipped with polyethene
terephthalate film windows (Toyobo Film Solutions, Japan, a modelG2; 38 μm thick) for both UV−vis and X-ray transmission were
measured in the transmission mode at 295 K. The storage ring energy
was 2.5 GeV, and the ring current was 449.9−450.0 mA. A Si (1 1 1)
double-crystal monochromator and a Rh-coated focusing bent
cylindrical mirror were inserted into the X-ray beam path. A
piezotransducer was used to detune the X-ray to two-thirds of the
maximum intensity to suppress the higher harmonics. The Ti K-edge
absorption energy was calibrated at 4964.5 eV using the spectrum of
the Ti metal (5.0 μm thick).
The obtained Ti K-edge XAFS data were analyzed using the XDAP
software package.8 The pre-edge background was approximated with amodified Victoreen function, C2/E2 + C1/E + C0, where E is thephoton energy and C0, C1, and C2 are constants. Multiple-shell curve
fit analyses were performed with the data obtained from the Fourierfiltered angular wavenumber k3-weighted extended XAFS (EXAFS)
using the empirical amplitude extracted from the EXAFS data for the
Ti metal. The fit parameters for the EXAFS data were theoreticallygenerated using the ab initio multiple-scattering calculation code,
FEFF8.40,9 with the interatomic distance (R) and coordination
number (N) values for the Ti−C, Ti···Ti, Ti−O, Ti−F, and Ti−Cl
interatomic pairs in Ti3C2X2 (X = O, F, or Cl) model structures set to0.2164 nm and 3 (in the top and bottom Ti layers) and 6 (in the6332

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middle Ti layer),10 0.30604 nm and 9 (in the top and bottom Ti
layers) and 12 (in the middle Ti layer),10,11 0.2164 nm and 3 (in the
top and bottom Ti layers),10 0.190 nm and 3 (in the top and bottom
Ti layers),12 and 0.218 nm and 3 (in the top and bottom Ti layers),13
respectively. The Debye temperature of Ti3C2X2 (X = O, F, or Cl)
was approximated to that of TiC (863.3 K)14 for the calculation using
FEFF8.40.
Density Functional Theory Calculations. Periodic density
functional theory (DFT) calculations were conducted using theVienna ab initio simulation package code version 6.4.115 computed on
a VT64 Server XS2-2TI comprising four units of Intel Xeon Platinum
9242 processors (2.3 GHz, 48 cores; Visual Technology, Tokyo,
Japan). The projector augmented wave method was employed at the
DFT-D3 level16 to incorporate the van der Waals interaction. The
generalized gradient approximation-revised Perdew−Burke−Ernzerhof (RPBE) exchange−correlation functional17 was used for structural
optimization, with the cutoff set at 500 eV and adding Hubbard U(4.0 eV for both Ti and Zr).18,19 The calculations of electronic states
including the density of states (DOS) and the band structure were
performed using the HSE06 hybrid functional for the structures
optimized using RPBE + U. A 3 × 3 × 1 k-point mesh was used forthe structural optimization, while a 1 × 1 × 1 k-point mesh was used
for the calculations of electronic states. The Ti3C2Xy hexagonal fivelayers model20 exposing the X ligand (X = OH, F, or Cl) on bothsides of the Ti3C2 layer was used combined with the Zr11O22 cluster
based on the monoclinic phase ZrO221 as a slab model with a vacuum
spacing of 2.0 nm between the slabs in the direction perpendicular to
the Ti3C2Xy layer (Chart S1).

Articleand unsupported Ti3C2Xy MXene to selectively form CH4, but
the rate was only 0.45 μmol h−1 gcat−1 (Figure S1B).
The above-described MXene-promoted photocatalytic formation was confirmed by a 13C-labeled CO2 photoreduction
study. Using 13CO2 (2.3 kPa), H2 (21.7 kPa), Ti3C2Xx MXene(1.0 wt %)−ZrO2, and UV−vis light irradiation, CO wasformed at a rate of 0.78 μmol h−1 gcat−1 (Figure 2A). The

Figure 2. Photocatalytic reduction tests using Ti3C2Xy MXene (1.0 wt%)−ZrO2 (20 mg) and the irradiation of UV−vis light under (A)13
CO2 (2.3 kPa) and H2 (21.7 kPa) and (B) 13CO2 (2.3 kPa) andH2O (2.1 kPa).

formation rate was higher using CO2 and H2 in the previous
paragraph because of the greater light intensity (500 mW
cm−2) versus that using 13CO2 and H2 (142 mW cm−2). The
12
CO ratio in the total CO (13CO and 12CO) was 30%,
significantly higher than the 12C ratio in the reactant 13CO2(1.0%), suggesting that the reaction intermediate comprising
12
C was concentrated during the photocatalytic test startingfrom 12CO2 adsorbed on surface O vacancy (VO••) sites fromair.21−23
Using 13CO2 (2.3 kPa), H2O (2.2 kPa), Ti3C2Xy MXene(1.0 wt %)−ZrO2, and UV−vis light irradiation, photocatalytic
13
CO generation was only at the rate of 0.004 μmol h−1 gcat−1
compared to the reported rate of 0.12 μmol h−1 gcat−1 using
ZrO2.22 Instead, H212CO, 12CO, 12CH4, 12C2H6, and 12C3H8
were formed at lower rates of 0.19, 0.032, 0.25, 0.037, and
0.019 μmol h−1 gcat−1, respectively (Figure 2B). The
corresponding 13C-containing hydrocarbons and formaldehyde
were negligibly formed, strongly suggesting that these wereformed by the decomposition of Ti3C2Xy MXene under H2O
and the irradiation of UV−vis light. This result was consistent
with the quite poor photoactivity using CO2, H2O, andTi3C2Xy to form CH4 as described above (Figure S1B).
Ti3C2Xy decomposition was effectively accelerated by theseparated charges originating from ZrO2 under light [Figure
1B(c)].
A blank photocatalytic test using H2O (2.1 kPa), Ti3C2Xy
MXene (1.0 wt %)−ZrO2, and UV−vis light irradiation was
also performed (not shown). H212CO, 12CO, 12CH4, 12C2H6,
and 12C3H8 were formed at the rates of 0.001, 0.033, 0.039,
0.013, and 0.012 μmol h−1 gcat−1, respectively, also supporting
the partial decomposition of MXene under H2O and the lightirradiation. The product distribution was compared between
the conditions of under 13CO2 and H2O and under H2O. In
the absence of CO2, the decomposition of Ti3C2Xy MXene
possibly at the edge section became slower because theintermediate CO was stably adsorbed at the MXene site andwas not further hydrogenated, while CO2 was adsorbed at thesite in the presence of CO2.

■

RESULTS AND DISCUSSION
Photocatalytic CO2 (13CO2) Reduction. Photocatalytic
CO2 reduction was tested using H2 and Ti3C2Xy MXene−ZrO2
photocatalysts in comparison to using ZrO2 (Figure 1A). In all
the tests using the photocatalysts, CO was the major productand the activity continued for more than 30 h of the reaction[Figure 1A(b)]. Some amounts of CH4 were also formed as
the byproduct [Figure 1A(a)]. The CO formation rate was the
maximum using Ti3C2Xy MXene (3.0 wt %)−ZrO2 at 4.6 μmol
h−1 gcat−1, which was an increase by 6.6 times compared to therate using ZrO2 (0.70 μmol h−1 gcat−1), indicating the electronconducting effect of MXene to promote the CO2 photoreduction over the ZrO2 surface. Unsupported Ti3C2Xy MXene(20 mg) exhibited only poor activity for CO formationcompared to ZrO2, and the rate was 0.021 μmol h−1 gcat−1(Figure S1A), suggesting that Ti3C2Xy MXene itself did notcomprise an independent major photocatalytic active site forCO2 reduction.
The dependence of the CO formation rate on the ratio of
the MXene content in the composite photocatalyst was a
volcano-like peak at MXene 3.0 wt % [Figure 1A(b)],suggesting the counterbalance of the electron-conducting
effect versus the blocking of exciting light by MXene layers
over the ZrO2 surface.
Next, photocatalytic CO2 reduction was tested using H2O
and Ti3C2Xy MXene−ZrO2 (Figure 1B). In clear contrast to
using H2 (Figure 1A), methane was the major product and theformation rate was the maximum using Ti3C2Xy MXene (5.0
wt %)−ZrO2 at 3.9 μmol h−1 gcat−1 [Figure 1B(c)]. The rate
was comparable to those using the composites of MXene 1.0
and 3.0 wt %; however, volcano-like dependence on the
MXene content was evident. As a byproduct, the CO
photoformation rate also followed volcano-like dependence,and the maximum rate was 1.1 μmol h−1 gcat−1 [Figure 1B(d)].
The photocatalytic CO2 reduction was also tested using H2O
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ArticleFigure 3. (A) XRD patterns of (a) Ti3AlC2, (b) Ti3C2Xy MXene, (c) ZrO2, and (d−g) Ti3C2Xy MXene−ZrO2 composite. The MXene content ineach sample was 1.0, 3.0, 5.0, or 10 wt %. (B) FE-SEM (a,b) and AFM (c,d) images of Ti3C2Xy (a,c) and the 3.0 wt % Ti3C2X MXene−ZrO2
composite (b,d). The yellow inset and the scale show the cutaway height for the Ti3C2Xy sample observed in (c).

Figure 4. (a,b) TEM, (c−e) HR-TEM, (f) HAADF−STEM, and (g) corresponding STEM energy-dispersive elemental mapping for Ti Kα, Zr Kα,
O Kα, C Kα, Cl Kα, and F Kα for (a) Ti3C2Xy MXene and (b−g) Ti3C2Xy MXene (3.0 wt %)−ZrO2 samples.

XRD Analysis. The crystal structure during the sample
synthesis was studied by XRD. The Ti3AlC2 phase exhibits
intense peaks [Figure 3A(a)] in accord with the previous
report.24 In the first step of synthesis, after etching treatmentswith HCl and HF (see the Materials and Methods section), theAl content in Ti3AlC2 was eliminated, and intense diffraction
peaks [(0 0 2) and (1 0 4) diffractions] for Ti3AlC2

disappeared. A broad (0 0 2) diffraction peak and weaker (00 4) and (1 0 4) diffraction peaks appeared [Figure 3A(b)],demonstrating that Ti3AlC2 was successfully transformed into
the thinner layered Ti3C2Xy compound (Scheme 1A, right).
Furthermore, the diffraction peak of Ti3C2Xy (0 0 2) shiftedfrom 2θBragg = 10.1−8.1° after the treatment with LiCl [Figure
3A(b)], indicating an expansion of the interlayer distance of6334

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Ti3C2Xy from 0.874 to 1.09 nm. At the final step of thesynthesis of Ti3C2Xy MXene, by the centrifugation at 13,000
rpm, even thinner layers of Ti3C2Xy nanosheets weresuccessfully achieved, and the layer interval further increased
to 1.26 nm based on the XRD peak at 2θBragg = 7.0° (notshown).
For all the MXene−ZrO2 composites (MXene 1.0−10 wt%), the XRD peaks were exclusively ascribed to the monoclinic
phase ZrO2, while no obvious diffraction peaks attributable to
the Ti3C2Xy moiety were found [Figure 3A(c−g)]. It was dueto the low content and/or higher dispersion of Ti3C2Xy in thecomposites, as supported by TEM micrographs (see thefollowing Microscopy Observation section for HR-TEM
analysis).
Microscopy Observation. The microstructure andmorphology of the samples were investigated by FE-SEM
and AFM. The Ti3C2Xy prepared via centrifugation at the rateof 13000 rpm at the final step of synthesis (see the Materials
and Methods section) exhibits a sheet-like morphology [Figure
3B(a)], and the cutaway height overdrawn on the image in[Figure 3B(c)] demonstrated a mean 1.6 nm thickness of thenanosheets, which corresponds to three layers of Ti3C2Xy units,in accordance with the previous report.25 In contrast, when thecentrifugation rate was reduced from 13,000 to 4000 rpm,
incompletely exfoliated thick layers of Ti3C2Xy are observed,
for which the AFM images showed multiple stacked layers 50−
100 nm thick (not shown), demonstrating that sufficiently highcentrifugation rates, e.g., 13,000 rpm, are required for
obtaining ultrathin Ti3C2Xy nanosheets.
In the FE-SEM image for the 3.0 wt % MXene−ZrO2
composite, the Ti3C2Xy portion tightly adhered to the ZrO2
surface [Figure 3B(b)]. Furthermore, the height analysis
showed that most of the ZrO2 nanoparticles (length ∼20nm) adhered on the MXene nanosheets, while no obvious
stacking of ZrO2 nanoparticles was observed [Figure 3B(d)],demonstrating that the two-dimensional MXene contributes tothe higher dispersion of ZrO2 nanoparticles.
Furthermore, the morphology of Ti3C2Xy and Ti3C2Xy (3.0
wt %) complexed with ZrO2 was analyzed by TEM and HRTEM. As shown in Figure 4a, Ti3C2Xy MXene exhibits adistinct two-dimensional nanosheet structure with a flattenedsurface in the range of a few hundred nanometers without
stacking, and the edge length of the Ti3C2Xy nanosheets was
about 100−300 nm.
For the 3.0 wt % MXene−ZrO2 composite, as shown in
Figure 4b,c, ZrO2 nanoparticles of size 10−20 nm were welldispersed and densely anchored on Ti3C2Xy nanosheets, which
was further confirmed by the elemental mapping analysis(Figure 4g). By HR-TEM observation (Figure 4d,e), latticefringes attributed to monoclinic ZrO2 (1 1 1), (1̅ 1 1), (0 1 1),
and (1 1 2) facets were clearly observed at 0.284, 0.317, 0.36,
and 0.213 nm, respectively, and the Ti3C2Xy interplanar
spacing for the edge cross-section view of nanosheets was 1.22
nm corresponding to Ti3C2Xy (0 0 2) crystal planes and 0.243
nm for the major exposed (1 0 4) crystal planes (Figure 4e,
inset), with a layer thickness of about 1.6 nm due to the threelayers (Figure 4d), which was consistent with the AFM
observation.
The dispersion of ZrO2 nanoparticles was more clearlyobserved as the contrast of Zr (Z = 40) compared to that of Ti(Z = 22) due to the Z2 dependence of HAADF−STEM(Figure 4f). Furthermore, the elemental mapping revealed thepresence of the constituent elements derived from reagents

Articleused for the synthesis (Ti, Zr, O, and C) as well as surface
terminal groups (Cl and F) derived from solvents used during
the synthesis (Figure 4g). Consequently, the terminationgroups of the Ti3C2Xy MXene were identified as X = oxo, OH,
Cl, and/or F and especially F and oxo/OH as the major
components, consistent with the EXAFS analysis discussed
below.
XPS Analysis. The surface chemical states of the preparedTi3C2Xy were analyzed by XPS. As shown in Figure 5A(a), thewide scan XPS spectrum identified that the major elements at
the MXene surface included Ti, C, F, and O as well as a not-Figure 5. (A) (a) Wide scan and (b−d) high-resolution XPS spectra
for Ti3C2Xy MXene at the binding energy of (b) C 1s, (c) O 1s, and(d) Ti 2p and (B) (a) wide scan and (b−d) high-resolution XPS
spectra for ZrO2 at the binding energy of (b) C 1s, (c) O 1s, and (d)Zr 3d.
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ArticleFigure 6. (a) Wide scan and (b−e) high-resolution XPS spectra of Ti3C2Xy (3.0 wt %)−ZrO2 for (b) O 1s, (c) C 1s, (d) Zr 3d, and (e) Ti 2p.

Figure 7. Diffuse reflectance UV−vis−NIr absorption spectra of (a) Ti3AlC2, (b) Ti3C2Xy, (c) ZrO2, and (d−g) Ti3C2Xy−ZrO2. The Ti3C2Xy
content in the sample was 1.0, 3.0, 5.0, and 10 wt %.

negligible Cl peak, consistent with the STEM energy-dispersive
elemental mapping (Figure 4g). Only a very weak Al peakremained, demonstrating that essentially all the Al in the raw
Ti3AlC2 material was effectively removed during the MXene
synthesis, resulting in a well-structured Ti3C2Xy MXene.
Furthermore, high-resolution XPS spectra of Ti3C2Xy
MXene are shown in Figure 5A(b−d). The fit to the data in
the C 1s region revealed four peaks at 282.3, 284.8, 286.0, and289.5 eV owing to C atoms forming C−Ti, C−C, C−O, andC−F/C�O bonds, respectively [Figure 5A(b)]. Three typicalpeaks at 530.1, 531.6, and 534.7 eV were fit to the data in the
O 1s region owing to O atoms forming O−Ti, O(surface)−Ti,
and HO−Ti bonds [Figure 5A(c)]. In the Ti 2p region, fourtypes of Ti atoms were observed owing to the formed Ti−C
(two types), Ti−Cl, and Ti−O bonds [Figure 5A(d)].26,27

In contrast, XPS data for the Ti3C2Xy (3.0 wt %)−ZrO2
composite demonstrated the presence of lattice oxygen (OL)and oxygen vacancy (Ov) (Figure 6b) owing to ZrO2 [Figure
5B(b)].28 In fact, The O 1s and Zr 3d peak positions owing to
ZrO2 negligibly changed upon composite formation with 3.0
wt % of Ti3C2Xy [Figures 5B(c,d) and 6(b,d)]. Furthermore,
two Ti 2p1/2 peaks owing to the Ti−C bond at 461.1 and 462.2
eV for Ti3C2Xy MXene [Figure 5A(d)] effectively shifted
downward to 460.2 and 461.9 eV, respectively, for theTi3C2Xy−ZrO2 composite (Figure 6d), strongly suggesting
the electron transfer from ZrO2 to Ti3C2Xy.
UV−Vis−NIR and Fluorescence Excitation Spectroscopy. The UV−vis−NIR light absorption of Ti3AlC2, Ti3C2Xy,
ZrO2, and Ti3C2Xy−ZrO2 composites were measured in the
wavelength range of 200−850 nm (UV−vis) and 900−2000
nm (NIR). As shown in Figure 7A(a) and B(a), extensive
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ArticleFigure 8. FTIR spectra of Ti3C2Xy (3.0 wt %)−ZrO2 (20 mg) introducing (a) CO2 (5.0 kPa), H2 (45 kPa), and Ar (50 kPa) and (b) CO2 (97 kPa)
and H2O (2.3 kPa), under UV−vis light irradiation from the Xe lamp for 60 min, and the light was turned off for 20 min.

Table 1. Curve Fit Analysis Results for EXAFS Measured for Ti3C2Xy MXene (30 wt %)−ZrO2 Photocatalysts
distance (nm) Coordination number ΔE0 (eV) Debye−Waller factor
(nm)
entry

ambient gas

conditions

a

air

fresh

b

CO2 (2.3 kPa) and H2O (2.3 kPa)

before light irradiationcunder light irradiationdlight offemodels generated using FEFF8.40

Ti−F

Ti−C

Ti···Ti

goodness of fit

0.151 (±0.00035)
0.6 (±0.025)
3.3 (±0.965)
0.0039 (±0.0004)
0.154 (±0.00015)
0.7 (±0.015)−3.6 (±0.515)
0.0096 (<± 0.0001)
0.159 (±0.0005)
0.3 (±0.01)−12.9 (±0.6)0.0070 (±0.0002)
0.154 (±0.00005)
0.4 (±0.02)
0.03 (±0.29)
0.0073 (±0.0004)
0.1901220.00550.214 (±0.00055)
2.4 (±0.055)
5.6 (±2.81)
0.0043 (±0.0002)
0.222 (±0.00265)
1.3 (±0.235)
1.2 (±2.815)
0.0032 (±0.0017)
0.209 (±0.0035)
1.1 (±0.52)
1.2 (±1.87)
0.0056 (<± 0.0001)
0.211 (±0.0003)
0.8 (±0.5)
11.3 (±1.805)
0.0093 (±0.0025)
0.216410
40.00720.302 (±0.00015)
7.1 (±0.24)
12.3 (±0.235)
0.0092 (±0.0001)
0.301 (±0.0055)
6.9 (±0.51)
12.5 (±0.095)
0.0091 (±0.0003)
0.298 (±0.00035)
7.0 (±0.44)
11.2 (±0.02)
0.0102 (±0.0003)
0.301 (±0.0001)
7.2 (±0.375)
13.6(±0.07)
0.0099 (±0.0002)
0.3060410,11
100.00498.3 × 105

absorption was observed for Ti3AlC2 up to 2000 nm. In
contrast, the UV−vis absorption on etched and exfoliatedTi3C2Xy was significantly suppressed [Figure 7A(b)] but
extending to the NIR wavelength region [Figure 7B(b)].
The absorption edge of ZrO2 was located at 248 nm,
correlated to a band gap of 5.0 eV, indicating that the ZrO2
primarily absorbed UV light to undergo charge separation22,23
while no visible and infrared light absorption exists [Figure
7A(c) and B(c)].
Then, for the MXene−ZrO2 samples, extensive lightabsorption was observed across the entire UV, vis, and NIR
regions, and the absorbance gradually increased as the Ti3C2Xyloading increased [Figure 7A(d−g) and B(d−g)]. It should be
noted that the same amount of the sample (100 mg) was used
for the spectral measurements, but the absorbance of Ti3C2Xy(3.0 wt %)−ZrO2, Ti3C2Xy (5.0 wt %)−ZrO2, and Ti3C2Xy (10wt %)−ZrO2 (e−g) was significantly greater than the weighted
convolution of the absorbance of ZrO2 (a) and Ti3C2Xy (b),demonstrating that the Ti3C2Xy moiety was well dispersed in

2.1 × 104

5.2 × 105

4.8 × 105

the ZrO2 matrix and the light absorption efficiency significantly
increased. This may also be attributed to light scattering by
ZrO2 and subsequent absorption by Ti3C2Xy, leading toincreased light absorbance.
The fluorescence excitation spectra are depicted in Figure
S2. When the excitation wavelength was 200 nm, thefluorescence intensity in the range of 250−450 nm waseffectively suppressed for the Ti3C2Xy (3.0 wt %)−ZrO2
sample compared to that for the ZrO2 sample, demonstrating
the charge separation effects of combined Ti3C2Xy MXene.
FTIR Spectroscopy. FTIR spectra were monitored for
Ti3C2Xy (3.0 wt %)−ZrO2 under photocatalytic CO2 reduction
conditions (Figure 8). When the reactant mixture of CO2 andH2 was introduced to the photocatalyst, antisymmetric andsymmetric O−C−O stretching vibration [νas(OCO) andνs(OCO)] and C−H bending vibration (δ(CH)) owing tobidentate bicarbonate species were observed at 1607, 1424,
and 1269 cm−1, respectively. These peaks became intense andmore resolved under the light irradiation; however, the6337

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intensity negligibly changed during the light irradiation. The
bicarbonate species should be in dynamic equilibrium present
under CO2, H2, and the UV−vis light irradiation, and thesepeaks became weaker when the light was turned off (Figure
8a).
FTIR spectra under CO2 and H2O gas were also monitored.
The peaks owing to bidentate bicarbonate species also
appeared (Figure 8b). These peaks became intense and more
resolved under the light irradiation; however, the intensity
negligibly changed during the light irradiation. The intensity
was relatively stronger than those under CO2 and H2, andadditional peaks of unidentate and/or bridging bidentate
species also appeared, primarily owing to the higher reactivityof H2O than that of H2 over the ZrO2 surface.29
In the photocatalytic point of view, the adsorbed
bicarbonate species both under CO2 and H2 and under CO2
and H2O gas were not in accord with the critical difference ofthe photocatalytic product: CO, especially 13CO from 13CO2
and H2 (Figures 1A and 2A), and CH4, especially 12CH4 from13
CO2 and H2O (Figures 1B and 2B). Thus, bicarbonate
species seems to be not the intermediate to CO but justaccumulated species over the ZrO2 surface during thephotocatalytic test. 12CH4 was likely derived from MXene
decomposition and not the photocatalytic product. True
intermediate species, e.g., O−C−OH,21,30 was not detectedin the FTIR spectrum (Figure 8a) due to the relatively short
lifetime on the photocatalyst surface.
The negative peak at 1652 cm−1 appeared when the UV−vis
light was turned on, and the negative intensity remained
essentially constant under the irradiation of light. As no
obvious peaks appeared by the introduction of CO2 andmoisture (Figure 8b) before the light irradiation, the negative
peak seems to be not the desorption of the adsorbent but
suggested the partial decomposition of the CO part of Ti3C2Xy(X = oxo and/or OH) MXene by the effects of moisture,probably at the edge section of the three layers of MXene, e.g.,terminal C atom oxidized to CO.31
EXAFS Analyses. EXAFS curve fit analyses for the fresh
Ti3C2Xy MXene−ZrO2 composite sample demonstrated acompact [Ti6C] octahedral coordination of the octahedronside length of 0.302 nm and the C−Ti length of 0.214 nm(Table 1a and Figure 9A) connected at the point forming
Ti3C2 three layers (Scheme 1). The complete Ti3C2X2 model
structure should comprise the mean coordination numbers of2
1
×3+ 3 ×6=4
Ti−C:
and
Ti···Ti:
3

ArticleFigure 9. Ti K-edge EXAFS best fit results for the Ti3C2Xy (30 wt%)−ZrO2 photocatalyst. (A) Fresh, (B−D) under CO2 (2.3 kPa) andH2O (2.3 kPa) before (B), during (C), and after UV−vis lightirradiation (D). Transformed data (black lines) and fits (red lines) to(1) (angular wavenumber k)3 × χ function and (2) its associatedFourier transform.
2

1

tion number of Ti−X: 3 × 3 + 3 × 0 = 2 (see the Materials
and Methods section). Compared to the mean value, thevalues for the fresh Ti3C2Xy MXene−ZrO2 sample and the one
under CO2 and H2O were 0.6−0.7 (Table 1a,b), suggestingη1−X coordination rather than η3−X coordination.
Under UV−vis light irradiation, the Debye−Waller factors(σ) for the interatomic pairs of Ti−C and Ti···Ti significantly
increased by 0.0013−0.0024 nm and 0.0010−0.0011 nm,
respectively, compared to those for samples fresh and before
the light irradiation, whereas the σ value for the interatomic
pair of Ti−F did not increase under the light irradiation (Table
1a−c).
Based on the correlated Debye model,22,23 these trends
strongly suggested the warming of Ti3C2 MXene layers underUV−vis light, and the temperature change of the photocatalyst
was evaluated assuming that the Debye temperature for
Ti3C2Xy was identical to that for TiC (863.3 K)14 and using acode FEFF8.40.9 The σ values for Ti−C and Ti···Tiinteratomic pairs increased from 0.0043 and 0.0092 nm for

2
1
× 9 + 3 × 12 = 103(see the Materials and Methods
section). Compared to the mean values, the values for the
fresh Ti3C2Xy MXene−ZrO2 sample and one under CO2 andH2O before light irradiation were 2.4−1.3 and 7.1−6.9 (Table
1a,b), respectively, suggesting the distorted Ti3C2 and/or
defective layers.
The ligands on both sides of the MXene layer were in
relatively close position at the interatomic distance between Ti
and X of 0.151 nm (Table 1a), suggesting that F and/or oxo
were the major component for MXene in this study, consistent
with the STEM energy-dispersive elemental mapping (Figure4g) and XPS results (Figure 5). Apparently, the Ti3C2 layer
was responsive for electron transfer via the Schottky barrier
from the semiconductor ZrO2 during photocatalysis, as
supported by the metallic nature based on DFT calculations(see the following DFT Calculations section). The complete
Ti3C2X2 model structure should comprise the mean coordina6338

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ArticleChart 1. (A) Band Structure for Ti3C2F2 MXene and (B) DOS Distribution for Ti3C2F2 MXene Combined with the Zr11O22
Cluster

Chart 2. Plane-Averaged Electrostatic Potential Plotted along the Direction Perpendicular to the Vacuum Layer, Where theZero in the Coordinate Corresponds to the Fermi Energy for (A) Ti3C2F2 MXene and (B) Ti3C2F2 MXene Combined with the
Zr11O22 Cluster.

DFT Calculations. Chart 1A shows the band structure
calculated for Ti3C2F2 MXene based on DFT. At the K pointin the Brillouin zone, the electronic configuration of thesemiconductor was obtained, while the conduction levels
shifted to a lower energy, resulting in the conduction level at−0.2 eV below the EF at the Γ point, suggesting a more
semimetal state. The conduction level became even lower at−2.4 eV at the M point. These configurations suggested asemimetal nature rather than a semiconductor nature for
Ti3C2Xy MXene and an indirect transition from the −0.2 eVlevel at the Γ point to the conduction level at the K point and/or from the conduction level between K and Γ points at −0.7
eV to −0.2 eV level at the Γ point (Chart 1A). This result wasin accord with the discussion for enhanced electron transmission in Ti3C2F2 MXene layers when the light energy wastransformed into heat, as monitored by the Debye−Waller
factor increase in Ti K-edge EXAFS measurements.
Chart 1B shows the DOS distribution for Ti3C2F2 MXene
combined with the Zr11O22 cluster. The ZrO2 band gap isshown as the difference between the VB maximum comprising
O 2p orbitals and the CB minimum comprising Zr 4d orbitals.
Below and near the EF, the DOS of major Ti and minor C
components was clearly present, suggesting the photoexcitedelectrons from O 2p at the VB for ZrO2 were band gap excitedby the UV light, and then, the electrons excited to the CB
could be injected into the Ti3C2 MXene layer for electrontransmission to assist the charge separation for ZrO2. The
DOS distribution negligibly changed when the ligand of
MXene was replaced to OH or Cl instead of F except for thethe fresh sample to 0.0056 and 0.0102 nm, respectively, under
the UV−vis light irradiation (Table 1a,c and Figure 9A,C),
corresponding to the temperature increase of 116 K or evenhigher.
Compared to the evaluation, the light intensity used for thisstudy (13CO2 photoreduction) was 0.17 W cm−2,22 thestandard reaction enthalpy change for CO formation was
41.16 kJ mol−1, and the heat capacity of the photocatalyst was
assumed to the weighted average of values for TiC (0.569 J
K−1 g−1)32 and ZrO2 (0.456 J K−1 g−1).33 The endothermic
reaction reduced the temperature of the system at the rate of
0.39 K h−1 at the maximum, while light energy should
sufficiently warm the photocatalyst at the rate as high as 1.1 K
s−1 at the maximum. In contrast, the σ value was essentiallyconstant throughout the light irradiation, demonstrating theequilibrium of transformed heat from light and the heatdissipation to the sample cell.22,23 Judging from the DFT
calculations for Ti3C2Xy MXene (Chart 1), Ti3C2Xy is
semimetal-like, and the electron conductivity is expected toincrease by the thermal excitation of VB electrons to the CB
rather than the thermal inhibiting effects of free electrons of
the metal.
After the UV−vis light was turned off, the σ values did notdecrease to the ones before the light irradiation (Table 1), as
expected to by the monitoring of warming and cooling of Ag,
Au, and Ni nanoparticles,22,23 probably because the increase ofdisorder and/or defective sites under the irradiation of lightand water canceled the effect of cooling on the σ values.
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difference of F 2p, O 2p, and Cl 3p population (Charts 2 andS2). Such comparisons were already reported34 for MXene not
combined with the Zr11O22 cluster.
Chart 2 shows the plane-averaged electrostatic potential
plotted along the direction perpendicular to the vacuum layer.
The work function of Ti3C2F2 MXene was calculated to be
4.83 eV, which was comparable to values in the previous
report.35 The work function value of Ti3C2F2 MXene
combined with the Zr11O22 cluster was calculated to be 4.70
eV, similar to the change in the Ti3C2OH MXene−TiO2
system.35 The reason for the shift by −0.13 eV was due tothe formation of dipoles at the interface and the EF shifted
relative to the band edge of ZrO2, similar to the reported
situation.35 Combined with the calculated positions of the VB
maximum (−2.0 eV) and CB minimum (2.8 eV) relative to the
EF (Chart 1B) as well as the electrostatic potential for the
Zr11O22 cluster mostly populated above the EF for Ti3C2F2(Chart 2B), the work function value was located between theband gap of the Zr11O22 cluster with reference to the vacuum
level, and thus, the light-excited electrons at ZrO2 were
injected into Ti3C2F2 MXene.

■AUTHOR INFORMATION

Corresponding Authors

Hongwei Zhang − Chengdu Biogas Institute, Ministry ofAgriculture and Rural Affairs, Key Laboratory ofDevelopment and Application of Rural Renewable Energy,
Ministry of Agriculture and Rural Affairs, Chengdu 610041,
People’s Republic of China; orcid.org/0000-0002-67738769; Phone: +86-28-8523-5478; Email: zhw9590@
163.com; Fax: +86-28-8526-0330
Yasuo Izumi − Department of Chemistry, Graduate School of
Science, Chiba 263-8522, Japan; orcid.org/0000-00018366-1864; Phone: +81-43-290-3696; Email: yizumi@
faculty.chiba-u.jp; Fax: +81-43-290-2783
Authors

Ikki Abe − Department of Chemistry, Graduate School of
Science, Chiba 263-8522, Japan; orcid.org/0009-00089732-6846
Tomoki Oyumi − Department of Chemistry, Graduate Schoolof Science, Chiba 263-8522, Japan
Rento Ishii − Department of Chemistry, Graduate School of
Science, Chiba 263-8522, Japan
Keisuke Hara − Department of Chemistry, Graduate School of
Science, Chiba 263-8522, Japan
Complete contact information is available at:https://pubs.acs.org/10.1021/acs.langmuir.3c03883

CONCLUSIONSThe photocatalytic effects for CO2 reduction, nanoscale andsite coordination structures, and electron transmission of
Ti3C2Xy (X = oxo, OH, F, or Cl) MXene were investigated. As
the active site for CO2 reduction, ZrO2 was used in
combination with Ti3C2Xy MXene. The CO photogeneration
rate using Ti3C2Xy (3.0 wt %)−ZrO2 and H2 (4.6 μmol h−1
gcat−1) was increased by 6.6 times compared to the rate using
ZrO2 and H2, and the reaction route was confirmed bymonitoring 13CO2 photoconversion into 13CO. In contrast,
using Ti3C2Xy (3.0 wt %)−ZrO2 and moisture, CO photogeneration became negligible and CH4 formation was found to
be due to the partial decomposition of Ti3C2Xy under theirradiation of UV−vis light and moisture based on the
predominant 12CH4 generation under the reactant comprising
13
CO2. FTIR spectroscopy, XRD, FE-SEM, AFM, HR-TEM,
and HAADF−STEM analyses confirmed that around threelayers of Ti3C2Xy MXene and 10−20 nm size ZrO2 particles
were attached to the flat MXene layers. The major X ligands
seemed to be F, oxo, and/or OH based on STEM energydispersive elemental mapping, XPS, and Ti K-edge EXAFS
analysis. Ti K-edge EXAFS demonstrated compact Ti−C and
Ti···Ti coordination, and the Debye−Waller factor increased
under the irradiation of UV−vis light, indicating the warmingof MXene by ∼100 K to enable accelerated electrontransmission in photocatalytic CO2 reduction over ZrO2, in
accord with the semimetal-like electronic configuration of
Ti3C2Xy MXenes based on DFT calculations.

■

ArticleAuthor Contributions

The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS
The authors are grateful for the financial support from the
National Natural Science Foundation of China (22209086,
HZ), the Central Public Interest Scientific Institution Basal
Research Fund (1610012022006_03104, HZ), the YouthInnovation Program of Chinese Academy of Agricultural
Sciences (Y2023QC33, HZ), CAAS-ASTIP-2021-BIOMA, theSichuan Science and Technology Program (2023YFH0056,
HZ), the Sichuan Province Science and Technology Activities
Funding for Returned Overseas Scholars (no. 13, HZ), theOpen Funding Project of the Jiangxi Province Key Laboratoryof Cleaner Production of Rare Earths (no. E03MYB0310, HZ),
the Grant-in-Aid for Scientific Research B (20H02834, YI)
from the Japan Society for the Promotion of Science, and theEstablishment of University Fellowships toward the Creationof Science and Technology Innovation from the Japan Scienceand Technology Agency (JPMJFS2107, K.H.). X-ray absorption experiments were performed with the approval of the
Photon Factory Proposal Review Committee (2022G527).
Part of the calculations was executed at the Supercomputer
Center, the Institute for Solid State Physics, the University ofTokyo, Japan.

ASSOCIATED CONTENT

sı Supporting Information*

The Supporting Information is available free of charge athttps://pubs.acs.org/doi/10.1021/acs.langmuir.3c03883.
The following files are available free of charge at ACS
Web site. The illustration of the optimized structure andDOS based on DFT calculations for Ti3C2X2 (X = O, Cl,or F) MXene combined with Zr11O22; photocatalytic
reduction tests using Ti3C2Xy MXene; and fluorescenceemission spectra for ZrO2 and the Ti3C2Xy MXene−
ZrO2 composite (PDF)■

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