Photocatalytic H2O2 production using Ti3C2 MXene as a non-noble metal cocatalyst

Article abstract of DOI:10.1016/j.apcata.2021.118127

Photocatalytic H₂O₂ production by O₂ reduction is an environmental-friendly process for solar light conversion to chemical energy. In this work, Ti₃C₂ MXene was used as a non-noble metal cocatalyst to load on P25 as Ti₃C₂/TiO₂ (TC/TO) photocatalysts for photocatalytic H₂O₂ synthesis. A H₂O₂ formation rate (179.7 μmol L^?1 h^?1) of the optimized 10 %-TC/TO composite was obtained to be over 21 folds as high as that of P25 under UV light. Radical quenching experiments and superoxide radical detection confirmed the superoxide radical as the primary intermediate, suggesting the O₂ reduction in two-step single-electron indirect reaction. The higher activity of TC/TO can be attributed to the functions of Ti₃C₂ MXene in accelerating the separation and transfer of photogenerated electron-hole pairs, suppressing their recombination, and blocking the surface Ti–OOH formation. This work proves the promising roles of Ti₃C₂ MXene in the photocatalytic reaction and further expands their new applications in photocatalysis.

Full text of DOI:10.1016/j.apcata.2021.118127

Applied Catalysis A, General 618 (2021) 118127
Contents lists available at ScienceDirect
Applied Catalysis A, General
journal homepage: www.elsevier.com/locate/apcata
Photocatalytic H2O2 production using Ti3C2 MXene as a non-noble
metal cocatalyst
,
,
,
Yiming Chena, Wenquan Gua b, Li Tana, Zhimin Aoa *, Taicheng Ana, Shaobin Wangc *
a Guangzhou Key Laboratory of Environmental Catalysis and Pollution Control, Guangdong Key Laboratory of Environmental Catalysis and Health Risk Control, School
of Environmental Science and Engineering, Institute of Environmental Health and Pollution Control, Guangdong University of Technology, Guangzhou, 51006, China
b Weng Yuan County Economic Development Zone, Shaoguan, 512600, China
c School of Chemical Engineering and Advanced Materials, University of Adelaide, Adelaide, SA 5005, Australia
A R T I C L E I N F O
A B S T R A C T
Keywords:
Photocatalytic H2O2 production by O2 reduction is an environmental-friendly process for solar light conversion
to chemical energy. In this work, Ti3C2 MXene was used as a non-noble metal cocatalyst to load on P25 as Ti3C2/
TiO2
TiO2 (TC/TO) photocatalysts for photocatalytic H2O2 synthesis. A H2O2 formation rate (179.7
μ
mol Lꢀ 1 hꢀ 1) of
Photocatalysis
Ti3C2 MXene
Cocatalyst
the optimized 10 %-TC/TO composite was obtained to be over 21 folds as high as that of P25 under UV light.
Radical quenching experiments and superoxide radical detection confirmed the superoxide radical as the primary
intermediate, suggesting the O2 reduction in two-step single-electron indirect reaction. The higher activity of TC/
TO can be attributed to the functions of Ti3C2 MXene in accelerating the separation and transfer of photo-
H2O2 production
–
generated electron-hole pairs, suppressing their recombination, and blocking the surface Ti OOH formation.
This work proves the promising roles of Ti3C2 MXene in the photocatalytic reaction and further expands their
new applications in photocatalysis.
1. Introduction
[11–14], g-C3N4 [15–18], CdS [19,20], and BiVO4 [21,22], have been
explored in H2O2 generation via photocatalysis. Due to its stable struc-
Hydrogen peroxide (H2O2) has drawn much attention since it was
first synthesized in 1818 [1]. It is well known that H2O2 is a clean and
ture, low cost and non-toxicity, TiO2 is the widely investigated photo-
catalyst so far [23]. Moreover, TiO2 has a more negative conduction
band potential (ꢀ 0.19 V) than the reduction potential of O2/H2O2
(0.695 V). Thus, in the presence of electron donors (ethanol, isopropanol
or formic acid, etc), oxygen can be reduced to H2O2 by UV light.
Nevertheless, the fast recombination of the photoinduced charge car-
environmental-friendly multifunctional oxidant with water as
a
by-product only. H2O2 has been intensively used in environmental
remediation and chemical industry, such as organic synthesis, pulp
bleaching, disinfection, wastewater treatment and fuel cells [2–5].
Currently, the anthraquinone method, as the main method to synthesize
H2O2 in industry, has some drawbacks, such as high costs, complicated
synthesis routes and toxic by-products, which restrict its applications [1,
6]. Also, the direct synthesis route from O2 and H2 using noble metals
(Pd or Au) or their alloy catalysts inevitably has a hazard of explosion
[7–9]. Besides, electrocatalytic redox reaction synthesis of H2O2 also has
the problem of high energy consumption [10]. Therefore, a facile, low
energy consumption, eco-friendly, and safe alternative way to produce
H2O2 is highly desirable.
–
riers and the Ti OH groups on TiO2 surface severely hinder the for-
mation of H2O2. Therefore, several techniques have been developed to
design high-efficiency photocatalysts based on TiO2, including surface
complexation with ions [24,25], heterojunction construction [26],
quantum-dots (QDs) modification [27,28], coupling with noble metal
nanoparticle cocatalysts [11,29] and nonmetal doping [30], aiming at
facilitating the separation of electron-hole pairs or restraining H2O2
decomposition, thereby improving the efficiency of photocatalytic H2O2
production. The applications of photocatalysis with non-noble metal
cocatalysts were also studied using different metal oxides, metal chal-
cogenides, metal oxysulfide, metal phosphides, and even graphene to
substitute scarce and expensive noble metal cocatalysts [31–35].
MXenes are two-dimensional materials as early transition metal
Owing to the advantages of semiconductor photocatalysis, photo-
catalytic reaction is considered to be a promising approach to produce
H2O2 with molecular oxygen (O2) as the oxygen source and a sacrificial
agent as the hydrogen source. A lot of semiconductor materials, TiO2
* Corresponding authors.
E-mail addresses: zhimin.ao@gdut.edu.cn (Z. Ao), shaobin.wang@adelaide.edu.au (S. Wang).
https://doi.org/10.1016/j.apcata.2021.118127
Received 11 February 2021; Received in revised form 21 March 2021; Accepted 28 March 2021
Available online 2 April 2021
0926-860X/© 2021 Elsevier B.V. All rights reserved.

Y. Chen et al.
Applied Catalysis A, General 618 (2021) 118127
nitrides, carbides, and carbonitrides in a typical molecular structure of
2.3. Preparation of Ti3C2/TiO2 composites
M
n+1XnTx (M stands for an early d-block transition metal, X represents
carbon and/or nitrogen, Tx is the surface termination functional group of
P25 TiO2 (0.3 g) was added in 100 mL water under an ultrasound
treatment for 30 min. Then, 4 mL Ti3C2 solution was added to the above
P25 suspension drop by drop and stirred for another 30 min (the mass
ratio of Ti3C2 MXene to P25 is 2 %). After stirring and standing for 30
– –
–
O, OH and F, and n = 1–3). Currently, they are typically prepared
from the corresponding MAX phase precursors by selectively etching the
A layers (usually Al element) with HF or LiF/HCl solution [36–38]. As a
classic MXene, Ti3C2 has been widely applied in electrochemical energy
storage [39], electromagnetic interference shielding [40], sensors [41]
and cancer therapies [42] due to its excellent structural stability, high
electrical conductivity, and hydrophilicity [43,44]. Apart from that,
Ti3C2 MXene could be used as a noble-metal-free cocatalyst to couple
with other semiconductor-based photocatalysts to form heterojunctions,
which can be widely applied to carbon dioxide reduction reaction
(CO2RR), nitrogen reduction reaction (N2RR), hydrogen evolution re-
action (HER), oxygen evolution reaction (OER), pollutant degradation
and other promising photocatalytic reactions [45,46]. For instance, Su
et al. [47] reported an excellent performance of Ti3C2/g-C3N4 (2D/2D)
◦
min, a gray precipitate was collected and dried at 80 C for 12 h in a
vacuum oven, which was denoted as 2 %-TC/TO. Similarly, other Ti3C2/
TiO2 composites with a different Ti3C2 loading (7 %, 10 %, 15 % and 20
%) were also prepared and were labeled as 7 %-TC/TO, 10 %-TC/TO, 15
%-TC/TO, and 20 %-TC/TO, accordingly.
2.4. Characterizations of catalysts
X-ray diffraction (XRD) patterns of P25 and its composites were
performed by using an X-ray diffractometer with a Cu-K
α radiation
source (λ =1.5218 Å) (Bruker D8 ADVANCE). Transmission electron
microscopy (TEM) images were collected on a Thermo Talos F200S
field-emission transmission electron microscope operated at 200 kV.
Field emission scanning electron microscopy (FE-SEM) with energy-
dispersive X-ray spectroscopy (EDX) elemental mapping images were
taken by a field-emission electron microscope (Hitachi SU8220) with an
acceleration voltage of 15 kV. The specific surface area (Brunauer-
Emmett-Teller, BET) of materials was measured on a Micromeritics
ASAP2460 equipment using nitrogen adsorption-desorption isotherm.
X-ray photoelectron spectroscopy (XPS) spectra were obtained from a
nanosheets in photocatalytic hydrogen evolution. The hydrogen pro-
ꢀ 1
duction reached 72.3
μ
mol hꢀ 1
g
, 10 times higher compared to pris-
cat
tine g-C3N4. Wang et al. [48] found that Ti3C2 incorporated BiOCl
presented a greatly improved removal of p-nitrophenol pollutant. The
removal efficiency reached 98 % in 50 min and the degradation rate was
nearly 3.3 times as high as that of pure BiOCl. Thus, Ti3C2 MXene was a
superior synergistic catalyst to suppress effectively the recombination of
the photogenerated electron-hole pairs and accelerate the electrons
transfer process, enhancing the overall performance and increasing the
reaction rate of catalysts. It is expected that the application of Ti3C2
MXene toward photocatalytic H2O2 production would be promising.
However, very few investigations have been reported using Ti3C2
MXene as a non-noble metal cocatalyst in photocatalytic H2O2 synthesis
[49]. Herein, Ti3C2/TiO2 photocatalysts were constructed by a simple
impregnation method. Interestingly, Ti3C2 MXene can distinctly boost
the performance of P25 for H2O2 generation under UV light irradiation.
From the photoluminescence spectra, photocurrent analysis, electro-
chemical measurement, radical quenching tests and superoxide radical
examination, we derived the possible mechanism of H2O2 production.
Thermo Fisher Escalab 250Xi spectrometer with Al Kα radiation. Pho-
toluminescence (PL) spectra were obtained using an Edinburgh FS5
fluorescence spectrophotometer under 380 nm excitation. UV–vis
diffuse reflectance spectra (UV–vis DRS) were acquired on a Cary 300
spectrophotometer. The electron paramagnetic resonance (EPR) was
conducted on a Bruker EMXplus-10/12 spectrometer under UV light
irradiation.
2.5. Photocatalytic performance
Typically, a photocatalyst (50 mg) was uniformly dispersed in a
mixture of 45 mL water and 5 mL ethanol in a quartz reactor. Before the
irradiation, an adsorption experiment was conducted in dark for the
adsorption-desorption equilibrium in 30 min and oxygen was continu-
ously bubbled into the suspension. Then it was irradiated for 2 h under
UV light (λ =365 nm) which was provided by a 9 W white lamp (PCX50B
Discover, Beijing Perfectlight Technology Co., Ltd.). During the photo-
catalytic process, 1 mL of the solution was collected and filtered through
2. Experimental section
2.1. Chemicals and materials
Commercial P25 TiO2 was obtained from Degussa. Titanium
aluminum carbon (Ti3AlC2, 200 mesh) powder was bought from Jilin 11
Technology Co., Ltd. Disodium hydrogen phosphate (Na2HPO4) was
provided by Tianjin DaMao Chemical Reagent Factory. Hydrofluoric
acid (HF, 40 wt%) was received from Macklin. Absolute ethanol
(C2H5OH), sulfuric acid (H2SO4), hydrogen peroxide (H2O2, 30 wt%),
and silver nitrate (AgNO3) were obtained from Guangzhou Chemical
Reagent Factory, while other materials and reagents including 5,5-
dimethyl-1-pyrroline-N-oxide (DMPO), horseradish peroxidase (POD,
RZ > 3.0), N,N-diethyl-p-phenylenediamine sulfate salt (DPD), sodium
phosphate monobasic (NaH2PO4), sodium sulfate anhydrous (Na2SO4),
p-benzoquinone (p-BQ), and methanol (CH3OH), were purchased from
Aladdin. All the reagents and chemicals were in a purity of analytical
grade and ultrapure water was used in all experiments.
a PES millipore filter (0.22 μm) to remove the photocatalyst powders at a
given time interval. The quantity of generated H2O2 was analyzed by the
DPD-POD method [51,52]. In this method, 1 mL of sample aliquots were
mixed with 1.12 mL water, 0.4 mL phosphate buffered solution, 0.05 mL
POD (1 mg mLꢀ 1) and 0.05 mL DPD (10 mg mLꢀ 1). After the mixture
was kept stirring vigorously for a while, the absorbance of the mixed
liquid was measured at 551 nm on a multifunctional microplate reader
(Thermo Scientific, Varioskan LUX). The absorbance of H2O2 was
measured three times. The photodecomposition of H2O2 was studied by
adding 50 mg photocatalyst into 50 mL 0.4 mmol Lꢀ 1 H2O2 solution for
irradiation of 2 h under the similar condition as above.
2.2. Synthesis of Ti3C2 MXene
2.6. Photocurrent and electrochemical measurements
According to the previous literature [50], 1 g Ti3AlC2 powders were
slowly added to 60 mL HF solution in batches and the mixture was
stirred at room temperature for 72 h. Then Ti3C2 was separated by a
centrifuge and washed continuously several times using water and
ethanol. The solid powders were collected and dried at 60 ◦C for 24 h in
a vacuum oven. Finally, 30 mg of the obtained sample was dispersed and
sonicated in 20 mL water for 1 h to obtain Ti3C2 solution at a concen-
The transient photocurrent analysis and electrochemical measure-
ment were conducted on an electrochemical workstation (CHI 660E,
Shanghai Chenhua, China) in a standard three-electrode system. The
catalyst powder coated on a fluorine-doped tinoxide (FTO) glass sub-
strate (1 × 1.5 cm2) was applied as the working electrode. A Pt foil was
used as the reference and a saturated Ag/AgCl electrode was employed
as the counter electrode. The working electrode was synthesized as
tration of 1.5 mg mLꢀ 1
.
follows: 250 μL absolute ethanol and 12.5 μL Nafion solution (5 wt%)
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Y. Chen et al.
Applied Catalysis A, General 618 (2021) 118127
were mixed with 5 mg catalyst by ultrasound-treated for 20 min. Then,
Ti, O elements and relatively few F element from the surface-terminating
functional groups of Ti3C2 MXene. The homogeneous dispersion of these
elements in the 10 %-TC/TO photocatalyst indicates the successful
preparation of the TC/TO nanocomposite.
12.5 μL catalyst suspension was uniformly dispersed on a FTO substrate
and then dried at room temperature. The measurements were carried
out in Na2SO4 electrolyte solution (0.2 M) under ambient condition. A
50 W lamp (PLS-FX300HU, Beijing Perfectlight Technology Co., Ltd.)
was used in the photocurrent analysis.
The detailed morphology of 10 %-TC/TO was further observed from
its TEM and high resolution TEM (HRTEM) images (Fig. 3). The size of
P25 particles and the thickness of Ti3C2 flakes are at a nanoscaled level
(Fig. 3a). From the HRTEM image in Fig. 3b, the obvious lattice fringes
of P25 and Ti3C2 MXene can be clearly seen. The lattice spacing values of
0.27 and 0.35 nm are corresponding to the (0110) plane of Ti3C2 MXene
and the (101) plane of anatase TiO2, respectively, indicating the suc-
cessful synthesis of the 10 %-TC/TO composite and consistence with the
SEM observations.
3. Results and discussion
3.1. Characterizations of materials
The crystal structure and phase of the photocatalysts were studied by
XRD patterns (Fig. 1a). After HF etching, the strongest diffraction peak
(104) of Ti3AlC2 at about 39◦ disappeared. Due to the increase of the
interlayer spacing, the (002) and (004) peaks of Ti3C2 MXene became
broader and shifted to a lower angle compared to those of Ti3AlC2,
indicating the removal of Al element from Ti3AlC2 to form Ti3C2 MXene
[53]. Fig. 1b shows that all of the TC/TO materials present similar
diffraction patterns to rutile TiO2 (JCPDS: 21-1276) and anatase TiO2
(JCPDS: 21-1272). Compared with the XRD pattern of P25, a very weak
peak of Ti3C2 MXene appeared at about 9.8◦ after loading of Ti3C2,
which was caused by the low content of Ti3C2 in the TC /TO composites.
With the increased loading of Ti3C2, the characteristic peak of Ti3C2
becomes clearer in 20 %-TC/TO. In addition, no other new diffraction
peaks can be detected, indicating no effect of Ti3C2 introduction on the
crystal structure of P25 [54]. These results prove that Ti3C2 MXene was
successfully compounded with P25.
In general, the photocatalytic reaction mainly occurs on the photo-
catalyst surface and the surface area ultimately affects its photocatalytic
performance. N2 adsorption-desorption experiments (Fig. S1) presented
the BET surface areas of the photocatalysts. The BET specific surface
areas of P25 and 10 %-TC/TO are 48.5 and 56.0 m2 gꢀ 1, respectively,
indicating that the introduction of Ti3C2 can improve the surface area of
10 %-TC/TO composite. The larger surface area of 10 %-TC/TO shows
that the composite photocatalyst can provide more active sites for the
reaction, which is expected to enhance the H2O2 production.
The surface chemical composition and state of the catalysts were
measured from XPS. Fig. 4a shows the XPS survey spectra patterns of
Ti3C2 MXene, 10 %-TC/TO and P25. The peak at around 75 eV corre-
sponds to Al element in the survey spectra of Ti3C2, implying that a small
amount of Al may remain in Ti3C2. The remaining Al may promote the
adsorption of ethanol [55]. The peaks of Ti, C, O and F can be observed
clearly in the spectra of 10 %-TC/TO, which was consistent with the EDX
elemental mapping. F element related to the termination functional
groups of Ti3C2 can be seen in 10 %-TC/TO, suggesting that the Ti3C2
was successfully incorporated into P25. Since the P25 occupies the main
content in 10 %-TC/TO sample, the 10 %-TC/TO presented a similar XPS
survey spectrum to that of P25. The C 1s XPS spectra of Ti3C2 MXene, 10
%-TC/TO and P25 are exhibited in Fig. 4b, while those of Ti 2p and O 1s
in these samples are presented in Fig. 4c and d, respectively. For the C 1s
XPS spectra, the peak at 284.8 eV is referred to the adventitious
elemental carbon [56]. Generally, the C 1s spectrum of Ti3C2 shows
three other peaks at 288.8, 286.2 and, 281.6 eV, which are ascribed to
The texture and morphology of Ti3AlC2, Ti3C2 MXene, P25 TiO2 and
10 %-TC/TO were obtained through SEM measurements. As shown in
Fig. 2a, bulky Ti3AlC2 exhibits a compact and layered ternary carbide
structure. After HF etching treatment, the Al layers were disappeared
and 2D accordion-like layered stacked structure of Ti3C2 MXene can be
apparently found (Fig. 2b). Meanwhile, it can be observed that multiple
monolayers are stacked in Ti3C2 MXene caused by ultrasonication and
the thickness of the monolayer is about 5ꢀ 20 nm. Fig. 2c presents the
nanoparticles of P25 TiO2 with a uniform size, which are gathered
together. Compared with Fig. 2b, the surface of Ti3C2 MXene flakes in
Fig. 2d becomes rough and agglomerates with many nanoparticles.
Interestingly, P25 nanocrystals are distributed evenly on the surface of
Ti3C2 nanosheets and partially embedded in the lamella without severe
aggregation, indicating that there is a strong bonding force between the
P25 nanoparticles and Ti3C2 nanosheets. Ti3C2 MXene is wrapped by
P25 nanoparticles in the composite material, thus ensuring that P25 can
capture more light and facilitate the rapid transfer of charges from P25
to Ti3C2. From the HAADF-SEM and EDX elemental mapping images
(Fig. 2e), it can be found that 10 %-TC/TO contains a large quantity of C,
– – –
the C F, C O, C Ti bonds, respectively [57]. While 10 %-TC/TO
exhibits two more characteristic peaks at 288.9 and 286.3 eV, which can
– – –
O Ti and C O bonds, respectively. Then for Ti
be assigned to the C
2p XPS spectra, two typical peaks at 458.4 and 464.1 eV (Fig. 4c) are
–
corresponding to the Ti 2p3/2 and Ti 2p1/2 of the Ti O bond in P25 [58],
while they are presented at 458.6 and 464.3 eV in 10 %-TC/TO, which
exhibit a positive shift of 0.2 eV in comparison with that in P25. Five
Fig. 1. XRD patterns of Ti3C2 MXene, Ti3AlC2 (a), and P25 as well as composites (b).
3

Y. Chen et al.
Applied Catalysis A, General 618 (2021) 118127
Fig. 2. SEM images of (a) Ti3AlC2, (b) Ti3C2 MXene, (c) P25 TiO2 and (d) 10 %-TC/TO. (e) HAADF-SEM image and EDX elemental mapping of 10 %-TC/TO.
Fig. 3. TEM (a) and HRTEM images (b) of 10 %-TC/TO.
peaks at 454.8, 456.2, 459.1, 461.0 and 464.9 eV can be deconvoluted in
around 400 nm, attributing to the intrinsic light-harvesting capability of
P25 [61]. Pure Ti3C2 does not show an absorption band due to its
metallic nature [62]. Besides, it presents strong absorption in the entire
wavelength range (200ꢀ 800 nm), which is caused by its darker color.
After the combination of Ti3C2 with P25, the light absorption intensity of
the composite photocatalysts are higher than that of P25. Moreover, the
increased loading of Ti3C2 in the composites leads to a color change from
light to dark grey (Fig. S2) and the absorption intensity is greatly
improved. The significantly enhanced light absorption of the TC/TO
composite photocatalysts would facilitate the photocatalytic reaction of
H2O2 production.
–
the Ti 2p pattern of Ti3C2, which can be assigned to C Ti (Ti 2p3/2),
–
– – –
O
Ti (Ti 2p3/2), C Ti (Ti 2p1/2), O Ti (Ti 2p1/2) and F Ti bonds,
–
respectively [59,60]. After compounding with P25, the C Ti (Ti 2p3/2
)
–
and C Ti (Ti 2p1/2) of 10 %-TC/TO show the similar trend and their
binding energies shift to higher positions (458.3 and 463.9 eV). Finally,
for O 1s XPS spectra, Fig. 4d presents that the O 1s region of P25 is
– –
– –
O H) at 529.6 and 531.1
composed of two species (Ti
O
Ti and Ti
eV, respectively. However, they shift to 529.8 and 531.3 eV in 10
–
%-TC/TO, which also shows a positive shift of 0.2 eV (like Ti O bonds).
The changes of the bonding energy revealed that the electrons would be
transferred from P25 to the Ti3C2 surface in 10 %-TC/TO, which was due
to the strong interfacial interaction between the layered Ti3C2 and P25.
The UV–vis DRS spectra of various photocatalysts are described in
Fig. 5. All of the photocatalysts based on P25 have an absorption edge
The PL technique is used to examine the separation efficiency of the
photogenerated charge carriers on the semiconductor photocatalysts.
Normally, the higher of the PL fluorescence intensity, the higher of the
electron-hole recombination rate is [18,63]. From Fig. 6a, all of the PL
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Y. Chen et al.
Applied Catalysis A, General 618 (2021) 118127
Fig. 4. (a) XPS survey spectra of Ti3C2 MXene, 10 %-TC/TO and P25, (b) High-resolution XPS C 1s spectra of Ti3C2 MXene and 10 %-TC/TO, (c) High-resolution XPS
Ti 2p spectra of Ti3C2 MXene, 10 %-TC/TO and P25, and (d) High-resolution XPS O 1s spectra of 10 %-TC/TO and P25.
spectra show a same emission peak at about 468 nm and 10 %-TC/TO
displays a lower PL intensity than P25, demonstrating that the intro-
duction of Ti3C2 can markedly decrease the recombination of the pho-
togenerated electron-hole pairs and accelerate their separation. To
further illustrate the spatial separation and transfer of the charge car-
riers in the photocatalytic process, some photoelectrochemical tests
were performed [64,65]. The electrochemical impedance spectra (EIS)
and transient photocurrent response curves were employed to evaluate
the electronic conductivity and the charge carriers transfer ability on the
semiconductor materials [66]. Fig. 6b suggests that all of the TC/TO
composites have a higher photocurrent response compared with pristine
P25 and 10 %-TC/TO sample exhibits the highest photocurrent among
the composites, implying the better charge carriers transfer behavior of
the 10 %-TC/TO sample. Generally, the smaller semicircle diameter of
the Nyquist plots, the smaller the charge transfer resistance of the
photocatalysts is. Fig. 6c illustrates clearly that 10 %-TC/TO composite
material has the smallest semicircle radius among all the samples, sug-
gesting that it owns the most efficient migration ability of the charge
Fig. 5. UV–vis DRS spectra of P25, Ti3C2 MXene, and their composites.
carriers. These results manifest that Ti3C2 MXene is an excellent cocat-
alyst to enhance the separation efficiency of the charge carriers and
Fig. 6. (a) PL spectra of P25, Ti3C2 MXene, and 10 %-TC/TO, (b) The transient photocurrent response curves of P25 and its Ti3C2 MXene composites, and (c) The
electrochemical impedance spectra of P25, and its Ti3C2 MXene composites.
5

Y. Chen et al.
Applied Catalysis A, General 618 (2021) 118127
–
accelerate the migration of the spatial charge, thereby improving the
photocatalytic activity.
introduction of Ti3C2 may reduce the formation of Ti OOH complexes
on the surface of the TC/TO photocatalysts.
3.2. Evaluation of photocatalytic H2O2 production
3.3. The mechanism of enhanced photocatalytic performance
The photocatalytic performance of the TC/TO composites for H2O2
production was tested by O2 reduction in ethanol solution under UV
light. The standard curve (Fig. S3) indicates a good linear relationship
between the concentration of H2O2 and its absorbance (R2 = 0.991).
Ethanol was chosen as the final sacrificial agent because the yield of
H2O2 was the highest in ethanol solution within 2 h (Fig. S4). Fig. 7a
displays the time courses for H2O2 production of different photo-
catalysts. It can be clearly observed that bare Ti3C2 can hardly generate
H2O2 and pure P25 also exhibits a poor activity toward H2O2 production
Generally, the photocatalytic production of H2O2 over TiO2 photo-
catalysts can be achieved by a one-step two-electron direct reduction or
a two-step sequential single-electron indirect reduction route. Apart
from that, H2O2 can be generated through the reaction of the hydroxyl
radicals formed by the oxidation of H2O. To elucidate H2O2 generation
pathway in the photocatalytic reaction, we conducted some control
experiments. As exhibited in Fig. 8a, no production of H2O2 takes place
in the absence of UV light irradiation, suggesting that the generation of
H2O2 is accomplished by the photocatalysis. In addition, the amount of
H2O2 generated is almost zero under N2 atmosphere, indicating that this
system cannot produce H2O2 in the absence of O2. Similarly, H2O2 can
hardly be detected without ethanol, which shows the role of ethanol as a
sacrificial agent. These results confirm that the formation of H2O2 is
generally derived via the photoreduction of O2 instead of holes induced
H2O oxidation [69].
due to its wide band gap, only 17.19
μ
mol Lꢀ 1 of H2O2 detected after 2 h
of UV light irradiation. Apparently, compounding Ti3C2 with P25 can
dramatically improve the H2O2 generation. From Fig. 7b, 10 %-TC/TO
photocatalyst possesses the best photocatalytic performance for H2O2
production among all the samples, and the concentration of H2O2 can
reach a maximum of 359.43
μ
mol Lꢀ 1 during the photocatalytic process,
which is about 21 folds more than that of P25. Furthermore, the
enhanced photocatalytic activity of 10 %-TC/TO exhibits higher H2O2
production than most of other TiO2 based photocatalysts reported before
(Table S1), indicating that the TC/TO composite is a promising photo-
catalyst for H2O2 generation. Besides, the recycling experiments were
performed to determine the stability of 10 %-TC/TO for photocatalytic
H2O2 production and the results are shown in Fig. S5. Unfortunately,
due to the exposure of high proportion of metal atoms on the surface of
Ti3C2 MXene, it is unstable in air (especially in oxygen), resulting in poor
stability of 10 %-TC/TO. Further increasing loading of Ti3C2 made the
yield of H2O2 gradually decrease. This phenomenon can be explained by
the presence of excessive Ti3C2 in the composite materials, which can
scatter light and lead to the light shielding effect.
In order to further prove whether H2O2 is produced through a
sequential two-step single-electron O2 reduction route or a direct two-
electron O2 reduction, quenching experiments were carried out
(Fig. 8b). The photocatalytic H2O2 generation was depressed and only
46.63
μ
mol Lꢀ 1 of H2O2 can be detected in 2 h in the existence of 4 mM
of AgNO3, which usually acts as an efficient electron acceptor. However,
the photocatalytic H2O2 generation was quenched completely with 4
mM p-BQ addition as a superoxide radical scavenger. A larger concen-
tration of AgNO3 (4 mol L-1) was added and it was found that H2O2 could
not be produced when electrons were completely quenched (Fig. S6).
These results indicated that the photocatalytic generation of H2O2 in this
system may be occurred via a sequential two-step single-electron indi-
rect reduction route, where superoxide radical is an inevitable inter-
mediate product of the photocatalytic reaction.
Generally, formation and decomposition of H2O2 occur simulta-
neously during the photocatalytic reaction process. Therefore, we also
investigated the photocatalytic H2O2 decomposition under various
conditions, at an initial H2O2 concentration of 0.4 mmol Lꢀ 1 (Fig. 7c).
H2O2 decomposes very slowly under UV light without any photocatalyst.
In the presence of P25, H2O2 can be decomposed completely in 20 min.
However, the decomposition rate of H2O2 under UV light was signifi-
cantly decreased after Ti3C2 MXene loading. Owing to the few content of
Ti3C2 in the 2 %-TC/TO samples, the decomposition of H2O2 is the same
as that of pure P25. As the content of Ti3C2 increasing, the concentration
of H2O2 decreased more slowly. The H2O2 decomposes at about 36.4 %
on 10 %-TC/TO after UV irradiation of 2 h, far lower than that of P25. In
the presence of P25, the decomposition rate of H2O2 is very fast, which is
To confirm the presence of superoxide radical in the photocatalytic
process, EPR tests were performed using DMPO as the spin trap. The EPR
spectra were acquired with P25 and 10 %-TC/TO photocatalysts under
UV light in methanol solution (Fig. 8c). It is noticeable that the typical
•
EPR signal of DMPO- Oꢀ2 with six peaks appeared after UV light illumi-
nation, suggesting the reduction of O2 by the photo-generated electrons
from P25 and 10 %-TC/TO photocatalysts. In addition, due to a good
•
selectivity of nitroblue tetrazolium (NBT) toward Oꢀ2 , it was chosen as a
•
chemical probe to quantitatively analyze the production of Oꢀ2 by
detecting the decrease in its concentration at a wavelength of about 259
nm [30,70]. After the addition of 10 %-TC/TO, the concentration of NBT
decreased more than that of P25 (Fig. S7), suggesting that the yield of
•
mainly due to the Ti OH groups on the P25 surface. The Ti OH groups
Oꢀ2 produced by 10 %-TC/TO photocatalyst is much more than that of
–
–
can immediately react with H2O2 to form the peroxide complexes
P25, attributing to the enhanced separation and transfer of the photo-
generated charges.
–
(Ti OOH) by the light excitation and the Tiꢀ OOH complexes can be
–
decomposed to Ti OH groups by electrons [67,68]. The coupling of
Based on the aforementioned results, we proposed a possible mech-
anism of photocatalytic H2O2 production over the Ti3C2/TiO2 and
schematically illustrated in Fig. 9. Photoexcitation of P25 TiO2 can
produce the pairs of electron (eꢀ ) and hole (h+) under UV light. The
Ti3C2 with P25 enables the photogenerated electrons generated from
P25 to be rapidly transferred to Ti3C2. Thus, to some extent, the presence
of Ti3C2 can inhibit the photodecomposition of H2O2, because the
Fig. 7. (a) Photocatalytic H2O2 production profiles, (b) Maximum yield of H2O2 production, and (c) Photodecomposition of H2O2 on various catalysts.
6

Y. Chen et al.
Applied Catalysis A, General 618 (2021) 118127
Fig. 8. (a) Photocatalytic H2O2 productions on 10 %-TC/TO under various conditions, (b) The quenching experiments of 10 %-TC/TO sample in H2O2 production,
•
and (c) EPR spectra of DMPO- Oꢀ2 adduct obtained with P25 and 10 %-TC/TO photocatalysts under UV light irradiation in methanol solution.
reaction pathway was the two-step single-electron indirect O2 reduction.
Credit authorship contribution statement
Yiming Chen: Investigation, Data analysis, Writing - original draft.
Wenquan Gu: Discussing - reviewing & editing. Li Tan: Data analysis.
Zhimin Ao: Conceptualization, Supervision, Funding acquisition,
Writing - review & editing. Taicheng An: Supervision. Shaobin Wang:
Conceptualization, Supervision, Writing - reviewing & editing.
Declaration of Competing Interest
The authors report no declarations of interest.
Fig. 9. The possible mechanism of photocatalytic H2O2 production from Ti3C2/
TiO2 photocatalyst.
Acknowledgements
electrons are transferred from the conduction band (CB) of TiO2 to Ti3C2
MXene due to the dissimilar Fermi levels of the Ti3C2 and TiO2. The
holes oxidize ethanol to provide the protons and the electrons react with
This work is financially supported by the National Natural Science
Foundation of China (Grant No. 21777033), Science and Technology
Program of Guangdong Province (2017B020216003), and Innovation
Team Project of Guangdong Provincial Department of Education
(2017KCXTD012).
•
O2 to produce Oꢀ2 and then H2O2 through a successive two-step single-
electron O2 reduction reaction (Eqs. 1–3). The photocatalytic H2O2
production reaction can be described as follows.
Appendix A. Supplementary data
CH3CH2OH +2 h+ → CH3CHO + 2H+
(1)
(2)
(3)
O2 + eꢀ → •Oꢀ2
Supplementary material related to this article can be found, in the
online version, at doi:https://doi.org/10.1016/j.apcata.2021.118127.
•Oꢀ2 + 2H+ + eꢀ → H2O2
On one hand, the loading of Ti3C2 on P25 can boost the separation of
the photogenerated electrons from holes and their transfer. The increase
of BET specific surface area will also provide more sites for H2O2 gen-
eration. On the other hand, the presence of Ti3C2 can prevent the
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