Highly efficient photoelectrocatalytic reduction of CO2 on the Ti3C2/g-C3N4 heterojunction with rich Ti3+ and pyri-N species

Article abstract of DOI:10.1039/c8ta03315c

Photoelectrocatalytic (PEC) reduction of CO₂ into chemical fuels in water is a promising route to mitigate the energy crisis and global warming issues. Herein, Ti₃C₂/g-C₃N₄ (TCCN) heterojunctions were fabricated by in situ heat treatment and were applied to PEC CO₂ reduction. These heterojunctions have narrow band gaps (2.3-2.6 eV) and rich Ti³⁺ species, which are beneficial to the absorption of solar light and the separation of electrons and holes. Besides this, the abundant pyri-N species in the TCCN heterojunctions can adsorb CO₂ molecules, which is favourable for CO₂ reduction. The deposition of nanometal particles can accelerate charge transfer. In a two-electrode system of M-TCCNBiVO₄, the total formation rate of formate and methanol was as high as 50.2 μM cm^-2 h^-1 (25.1 mM h^-1 g^-1), which is tenfold that of pristine g-C₃N₄. The carbon source of the products was verified by a ¹³CO₂ labeling experiment. These heterojunctions show outstanding PEC performance and stability and are promising candidates in the solar-to-fuel engineering field.

Full text of DOI:10.1039/c8ta03315c

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ISSN 2050-7488
PAPER
Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
MoS2 nanocages as a lithium ion battery anode material
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DOI: 10.1039/C8TA03315C
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Highly efficient photoelectrocatalytic reduction of CO2 on the
Ti3C2/g-C3N4 heterojunction with rich Ti3+ and pyri-N species
Received 00th January 20xx,
Accepted 00th January 20xx
Yanjie Xu,a Shuai Wang,b Jun Yang,*b Bo Han,a Rong Nie,a Jixian Wang,a Yapeng Dong,a Xiaogang
Yu,a Jianguo Wangc and Huanwang Jing*a,c
DOI: 10.1039/x0xx00000x
The photoelectrocatalytic (PEC) reduction of CO2 into chemical fuels in water is conceived as a promising route to mitigate
energy crisis and global warming issues. Herein, Ti3C2/g-C3N4 (TCCN) heterojunctions were fabricated by in-situ heating
treatment and applied to PEC CO2reduction. These heterojunctions have narrow band gaps (2.3-2.6 eV) and rich Ti3+
species, which are beneficial to the absorption of solar light and the separation of electrons and holes. Besides, the
abundant pyri-N species in TCCN heterojunctions could adsorbe CO2 molecules, which is favourable for the CO2 reduction.
The deposition of nanometal particles would accelerate the charge transfer obviously. In the two-electrode system of M-
TCCN||BiVO4, the total formation rate of formate and methanol can be as high as 50.2 μMcm-2h-1 (25.1 mMh-1g-1) that is
tenfold than that of pristine g-C3N4. The carbon source of products was verified by 13CO2 labeling experiment. These
heterojunctions show outstanding PEC performance and stability, which can be used as promising candidates in solar-to-
fuel engineering field.
www.rsc.org/
earth-abundant and physicochemically stable.20-25 However, bulk g-
C3N4 always suffers from low electrical conductivity, low surface
area and high recombination rate of charge carriers, which confines
its applications in photocatalysis.26-29 In order to obtain efficient g-
C3N4-based photocatalysts for solar fuel conversion, many
strategies have been developed, for example, depositing metal
nanoparticles and forming heterojunction with other
semiconductor.30-38
1. Introduction
The renewable energy, such as sunlight, wind and geothermal heat,
is a viable substitute for fossil fuels. Converting CO2 into chemical
fuels using solar energy is considered as a sustainable avenue to
alleviate energy crisis and global warming issues in the future.1-5 In
the natural photosynthesis system, green plants convert CO2 and
H2O into hydrocarbons under the irradiation of sunlight, which
suggests that the CO2 might be transformed into chemical fuels by
imitating the natural photosynthesis.6-8 The photocatalysis and PEC
reduction of CO2 are attractive approaches to convert solar energy
into renewable chemical fuels.9-15 Particularly, the PEC method is
more efficient than photocatalysis because that the external
voltage could accelerate the transfer of charge carries and suppress
the recombination of photo-generated electron-hole pairs.16-19
Therefore, it is feasible to construct PEC system to generate
chemical fuels by achieving the coupling of water splitting and CO2
reduction.
Ti3C2, a layered two-dimensional (2D) transition metal carbide
belonging to MXenes, possesses excellent hydrophily, structural
stability, high electrical conductivity and superior light-harvesting
ability.39-44 Nevertheless, there are few reports about the
applications of MXenes in the renewable energy conversion so far.45,
46
Presumably, combining g-C3N4 and Ti3C2 to form heterojunction
might be an effective strategy to fabricate an efficient photocatalyst,
which may exhibit strong ability of light absorption and high
efficiency of charge separation in CO2 reduction reaction.
In this work, a series of Ti3C2/g-C3N4 heterojunctions were prepared
by a facile heating method and used as photocathodes in the CO2
PEC reduction experiments. It is confirmed that these new TCCN
heterojunctions have narrow band gaps, rich Ti3+ and pyri-N species,
which are favourable for the CO2 reduction. Besides, a possible
mechanism was carefully proposed.
Graphitic C3N4 (g-C3N4) has been proved to be a fascinating
semiconductor for photocatalysis since it is facile synthesized,
a.State Key Laboratory of Applied Organic Chemistry, College of Chemistry and
Chemical Engineering, Lanzhou University, Lanzhou 730000, P. R. China. *E-mail:
hwjing@lzu.edu.cn; Fax: 86-931-8912582. Tel: 86-931-8912585.
b.State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics,
Chinese Academy of Sciences, Lanzhou 730000, P. R. China. *E-mail:
jyang@lzb.ac.cn.
2. Experimental
2.1 Preparation of Ti3C2 sheets
c. State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese
Academy of Sciences, Taiyuan 030001, P. R. China.
Commercially available powders of Ti, Al and graphite were used as
raw materials to synthesize Ti3AlC2 powder. The detailed synthesis
method can be found elsewhere.47,48 Then 2D Ti3C2 sheets were
Electronic
Supplementary
Information
(ESI)
available
See
DOI: 10.1039/x0xx00000x
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fabricated by selectively exfoliating the Al atoms from Ti3AlC2 Ltd.). The UV-vis absorption spectra were collected on a UV-vis
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powders in aqueous HF solutions (40 %) for 24 h at room spectrophotometer
(UV-2600,
Shimadzu).
DOI: 10.1T0h39e/C8TeAl0e3c3t1ro5Cn
temperature. The obtained Ti3C2 specimens were rinsed repeatedly paramagnetic resonance (EPR) spectra were recorded by a Bruker
with deionized water to neutral and dried in vacuum.48
ER200DSRC10/12 spectrometer at room temperature. The
photoluminescence (PL) emission spectra were recorded on a
FLS920 spectrofluorometer with an excitation wavelength of 325
nm. The N2 adsorption-desorption and the CO2 sorption isotherms
2.2 Preparation of Ti3C2/g-C3N4 (TCCN) heterojunctions
The TCCN heterojunctions were prepared by a feasible in-situ
heating treatment method. Briefly, 15 g thiourea with different
mass contents of Ti3C2 (50:1, 20:1 and 10:1) was mixed and ground
in an agate mortar for 30 min to obtain homo-disperse mixture.
Subsequently, the mixture was placed into a crucible that was
wrapped tightly with aluminium foil, and then annealed at 500 °C
for 2 h in muffle furnace. The obtained samples were designated as
TCCN1 (50:1), TCCN2 (20:1), TCCN3 (10:1), respectively. The pristine
g-C3N4 was also prepared by the same method using thiourea as
raw material and labelled as CN.
were measured at −196 °C and 25°C, respectively, using
a
micrometrics sorption analyzer (TriStar II 3020). The CO2
temperature programmed desorption (TPD) was conducted on a TP-
5080 catalyst characterization instrument with a TCD detector. Prior
to the measurement, the samples were heated at 300 °C for 1 h in
helium gas environment.
3. Results and discussion
2.3 Preparation of CN/FTO and M-TCCN/FTO electrodes
3.1 Characterization of photocatalysts and photocathodes
The TCCN heterojunctions were deposited onto clean FTO glasses
by an electrophoretic deposition method.44 A typical procedure was
described as follows: 50 mg different samples and 10 mg iodine
were dispersed into 50 mL acetone; then the particles were
deposited onto the FTO glass under 20 voltage provided by a DC
power supply for 15 min; lastly, the metal (M = Pd, Pt, Au)
nanoparticles (NPs) were deposited by electrodeposition to gain M-
TCCN/FTO electrodes according to our previous report.19
The XRD patterns of photocatalysts are displayed in Fig. 1a. It can
be clearly seen that the XRD pattern of TCCN3 has two distinct
feature peaks around 13.0° and 27.4°, which correspond to the (100)
and (002) planes of g-C3N4, respcectively.21 The (002), (004) and
(110) planes of Ti3C2 are appeared and labelled in Fig. 1a, as well.49
With the increasing Ti3C2 content, the diffraction signals of g-C3N4
nanosheets weaken evidently, while the signals of Ti3C2 strengthen
gradually. As shown in Fig. 1b, the shoulder peak at 25.3°
corresponds to the (101) plane of anatase TiO2,19, 50 revealing that
small amounts of TiO2 was formed during the fabrication processes
of TCCN samples.
2.4 Photoelectrochemical measurement and CO2 reduction
experiments
The photoelectrochemical properties of photocathodes were
characterized by an electrochemical workstation (CHI660E). The M-
TCCN/FTO, BiVO4/FTO and Ag/AgCl were employed as working
electrode, counter electrode and reference electrode, respectively
in
a
tri-electrode system of M-TCCN|Ag/AgCl|BiVO4. The
electrochemical impedance spectroscopy (EIS) and the Mott-
Schottky plots were collected under the dark condition in the tri-
electrode system. The linear sweep voltammetry (LSV) curves and
the amperometric photocurrent responses were detected in the
two-electrode system of M-TCCN||BiVO4.
The PEC reduction of CO2 experiments were performed with the
two-electrode system of M-TCCN||BiVO4 in the electrolyte of
KHCO3 aqueous solution (0.1 M, pH = 6.8, 50 mL). The external
voltages (–0.45 V, –0.65 V, –0.85 V and –1.05 V) were powered by a
Silicon solar cell. The light intensity was 200 mW/cm2 and provided
by a 300 W Xenon lamp (PLS-SXE300/300UV). The liquid products
1
were identified by H NMR (JNM-ECS 400 MHz) spectra with water
suppression, using DMSO (10 mM) and m-trihydroxybenzene (MTB,
50 mM) as internal standards.19 The gas-phase products were
determined by gas chromatography (GC, CP-3380 Varian).
2.5 Morphology and structure characterization
The phase constituents of as-prepared specimens were identified
by XRD (D/max-2400 Rigaku) with Cu K radiation. The
morphologies of CN and TCCN samples were observed by field
emission scanning electron microscope (FESEM, JSM-6701F) and
field emission transmission electron microscope (TEM, Tecnai G2
TF30). The elements chemical states of specimens were identified
by XPS spectra on a Kratos AXIS Nova spectrometer (Shimazu Co.,
Fig. 1. XRD patterns of (a) g-C3N4, Ti3C2 and TCCN3, and (b) different
TCCN samples
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Fig. 4 and S3 demonstrate the XPS spectra of photocathodes.
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DOI: 10.1039/C8TA03315C
According to the survey spectrum of TCCN3, the binding energy
peaks of Ti element are appeared in comparison with that of
pristine g-C3N4, indicating that the Ti3C2 species are incorporated
into the g-C3N4 phase (Fig. 4a). As shown in the high resolution
spectrum of Ti 2p, the binding energies at 454.8 and 460.3 eV,
458.2 and 463.7 eV are assigned to Ti-C and Ti-O species,
respectively (Fig. S3).45 For the C 1s spectrum of TCCN3, the peak at
286.0 eV corresponds to C-OH due to the join of Ti3C2.36,51
Furthermore, the high resolution XPS spectra of N 1s in TCCN3 and
CN were analyzed detailedly. The binding energies at 398.5 eV,
399.5 eV and 400.8 eV are assigned to the C-N-C (pyri-N), N-(C)3
and N-H groups, respectively (Fig. 4b).52-54 When the TCCN3
heterojunction was formed, in comparison with CN, the
concentration of pyri-N increased from 45.2 % to 57.2 %. The
presence of abundant pyri-N species in the TCCN could facilitate the
adsorption of CO2,51-53 and enhance the activity of CO2 reduction
consequently.
Fig. 2. FESEM images of (a) Ti3C2, (b) g-C3N4, (c) TCCN and (d) TCCN-
FTO samples
As shown in Fig.2, the as-prepared Ti3C2 sheets (Fig. 2a) has a more
obviously laminate structure than the pristine g-C3N4 (Fig. 2b). It can
be seen from Fig. 2c that the TCCN shows a partially-covered
layered structure, indicating that g-C3N4 is successfully decorated on
the Ti3C2 sheets surface. As shown in Fig. 2d, the TCCN samples still
retain the covered layered structure after electro-depositing onto
the FTO glasses, and the thickness of TCCN film is about 6 μm (Fig.
S1).
Fig. 4. (a) XPS survey spectra of g-C3N4, TCCN3 and Pd-TCCN3
electrodes, (b) high resolution XPS spectra of N 1s in CN and TCCN3
Fig. 3. (a,b) TEM images and (c) EDS mappings of Pd-TCCN3
TEM images of the Pd-TCCN photocathode are displayed in Fig. 3
and Fig. S2. It clearly illustrates a heterojunction interface between
the g-C3N4 and Ti3C2 phases. The high magnification TEM image
demonstrates the diffraction plane (002) of Ti3C2 phase (Fig. 3b).45
Notably, the EDS mappings reveal that the C, N, O, Ti and Pd
elements are well distributed onto the surfaces of Pd-TCCN
electrode, validating that the heterojunctions were formed.
Moreover, after depositing the metal NPs onto the TCCN3
electrodes, the binding energy of Pd, Au and Pt were measured. As
shown in Fig. S3, the peaks at 342.9 and 337.6 eV are assigned to 3d
orbital of Pd,55 peaks at 83.3 and 87.0 eV are belonged to 4f orbital
of Au, and peaks at 75.4 and 72.2 eV are attributed to 4f orbital of
Pt. The above results reveal that the metals NPs were deposited on
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the surface of electrode, and the target M-TCCN electrodes were
fabricated successfully.
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DOI: 10.1039/C8TA03315C
In the EPR spectra (Fig. 5a), the pristine g-C3N4 shows a weak signal
with Landé factor (g) of 2.004, which is derived from the unpaired
electrons on the aromatic rings.20 Obviously, there are two strong
signals with g = 2.004 and g = 1.94 for TCCN heterojunctions, and
the intensities of signals are gradually enhanced with the increase
of Ti3C2 content. It was reported that the signal at g = 1.94 is
assigned to the Ti3+ species that could provide trapping centres for
charge carriers and suppress the recombination of photogenerated
electrons and holes.56-59
Fig. 6. PL spectra of different samples
To investigate the recombination efficiency of photo-carriers under
radiation, the PL spectra of all samples were obtained under a 325
nm excitation (Fig. 6). The PL spectra evidently display an emission
peak at 460 nm on account of the recombination of photo-induced
electron-hole pairs. The peak intensity of TCCN heterojunction
materials decreases rapidly in comparison with CN, suggesting that
the recombination of electron-hole pairs is obviously suppressed.51
As the Ti3C2 content increases, the peak intensity reduces gradually
from TCCN1 to TCCN3, which is possibly attributed to the good
transfer and utilization of charge carriers in the TCCN
heterojunction.
Fig. 5. (a) EPR spectra measured at room temperature, (b) N2
adsorption-desorption isotherms measured at −196 °C, (c) CO2
adsorption isotherms measured at 25 °C and (d) CO2 TPD profiles of
different samples
As shown in the Fig. 5b, the N2 adsorption–desorption isotherms
reveal that the surface areas of TCCN are enlarged gradually as Ti3C2
content increased. The TCCN2 sample possesses the largest surface
area of 28.2 m2 g-1, while the TCCN3 has the smallest surface area
of 17.3 m2 g-1, because the pure Ti3C2 has a relatively small surface
area.60 Moreover, their pore sizes of mesoporous are gradually
narrowed from CN to TCCN3 sample.
The CO2 sorption experiments were conducted to evaluate the CO2
adsorption ability of different samples. It can be seen from Fig. 5c
that the adsorption capacities of TCCN samples are superior to that
of CN, implying that TCCN heterojunctions have strong abilities of
CO2 capture.61 The CO2 TPD profiles were recorded in a helium
stream and shown in Fig. 5d. The peaks appear around 160 °C can
be assigned to the desorption signal of CO2 molecules physically
adsorbed by N species of CN.62, 63 Ti3C2 itself should be inactive in
physical adsorbing of CO2 without peak in this region. In contrast,
TCCN heterojunctions show larger peak areas than CN, which is
consistent with the results of CO2 sorption. Besides, the desorption
signals of adsorbed CO2 appear at high temperature, attributing to
the strong interaction of the basic groups.31 These results suggest
that the incorporation of Ti3C2 could improve the CO2 adsorption
capacities of TCCN heterojunctions.
Fig. 7. (a) UV-vis absorption spectra, and (b) Tauc plots of different
photocathodes
The UV−vis absorption spectra of photocathodes were displayed in
Fig. 7. It can be seen that the absorption ability of TCCN3 is superior
to CN due to the contribution of Ti3C2. After the deposition of Pd
NPs, the absorption ability of Pd-TCCN3 in visible light region is
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obviously improved on account of the surface plasmonic resonance the current of Pd-TCCN photocathodes are higher thVainewtAhratictleoOfnPlinde-
of metal NPs.64 In addition, the corresponding Tauc plots CN. That reveals that the formation of TCCN heterojunctions could
DOI: 10.1039/C8TA03315C
demonstrate that the band gap energies of photocathodes are 2.7 obviously improve the efficiency of charge separation and light
eV (Pd-CN), 2.6 eV (Pd-TCCN1), 2.36 eV (Pd-TCCN2) and 2.3 eV (Pd- conversion (Fig. 8a).
TCCN3), which is favourable for light absorption and photocatalytic
reactions.
The transient photocurrent responses of photocathodes were
tested at −1.0 V in a two-electrode system and illustrated in Fig. 8b.
3.2 Photoelectrochemical properties of photocathodes
When the light was turned on, the current of photocathodes raised
sharply from zero to 0.7-1.0 mA. The Pd-TCCN2 gives the highest
current value of 0.85 mA among the four photocathodes, which
might be ascribed to the proper coupling effect of Ti3C2 and g-C3N4.
The interfacial charge transfer of photocathodes was surveyed by
EIS spectra. In Nyquist plots, a smaller semicircle represents a lower
resistance (Rct) between the photocathode and electrolyte.65
Obviously, the Rct values of M-TCCN3 are much lower than that of
TCCN3 (Fig. 8c), which could be attributed to the good transfer of
charge carriers on the interface of heterojunction with the
existence of metals NPs. The Rct value of Pt-TCCN3 is larger than
that of Pd-TCCN3 and Au-TCCN3 that was in consistent with the
current data. Besides, the positive slope of Mott-Schottky plots
suggests that the heterojunction is n-type semiconductor (Fig. S5).
The negative flat band potential signifies that photoelectrons in
conduction band of TCCN heterojunctions have
a strong
reducibility.51 Furthermore, the stability of Pd-TCCN3 photocathode
was examined by cyclic voltammetry (Fig. S6). After 100 cycles of
continuous cyclic voltammetry, the current is slightly fluctuated,
indicating that the Pd-TCCN3 photocathode is fairly stable.66
3.3 Photoelectrocatalytic reduction of CO2
Initially, the PEC experiments were carried out with the two-
electrode system of Pd-CN||BiVO4 at an external voltage of −0.85 V
(Fig. 9a). Only small amounts of HCOO− and MeOH were examined.
When the Pd NPs modified TCCN heterojunctions (Pd-TCCN) were
used as photocathodes, the yields of HCOO− and MeOH were
speeded up evidently. Clearly, as the Ti3C2 content increases in
TCCN heterojunction, the photocatalytic activity of photocathodes
follows the order of Pd-TCCN3 > Pd-TCCN2 > Pd-TCCN1. Among all
the photocathodes, Pd-TCCN3 exhibits the highest activity of CO2
reduction to hydrocarbons in a rate of 50.2 μM cm-2 h-1 (equal to
25.1 mM h-1 g-1) that is about tenfold of the Pd-CN. The apparent
quantum efficiency of Pd-TCCN3||BiVO4 cell is estimated to be 1.3 %
that is two times of Pd-CN. The remarkable improvement might be
attributed to the synergistic effects of pyri-N and Ti3+ species in
TCCN3 heterojunction.
In order to investigate the influence of external voltage, the PEC
experiments were carried out at various voltages using optimal Pd-
TCCN3 as the photocathode. As shown in Fig. 9b, when the voltages
are varied, the activity for the hydrocarbons evolution is in the
order of −0.85 V > −1.05 V > −0.65 V > −0.45 V. However, the
activity at −1.05 V is inferior to that of −0.85 V, because more H2
releases at more negative voltages (Fig. S7). These results reveal
that a proper voltage can accelerate the CO2 reduction process, and
contribute to the coupling of CO2 reduction and water splitting to
generate hydrocarbons. Additionally, the selectivity of liquid
products was calculated and listed in Table S3. When the higher
voltage was applied to the PEC cell, the higher selectivity for formic
acid could be achieved in the region of −0.45 V  −1.05 V.
Fig. 8. (a) LSV curves, (b) transient photocurrent responses, and (c)
EIS Nyquist plots of different photocathodes
The photoelectrochemical performances of photocathodes were
displayed in Fig. 8 and Fig. S4. The LSV curves demonstrate that the
current values of photocathodes are almost zero under the dark
conditions. In contrast, the photocurrent boosts evidently with the
escalation of external voltages under the light irradiation, indicating
that all photocathodes have excellent photosensitivity. Furthermore,
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Furthermore, a control experiment was conducted atV−ie0w.8A5rticVle uOsnilninge
DOI: 10.1039/C8TA03315C
the TCCN photocathode. As shown in Fig. 10, certain amounts of
hydrocarbons and H2 could be detected, illustrating that the
heterojunction of TCCN has an eigen activity in CO2 reduction and
water splitting. Moreover, the performance of PEC cell of Pd-
TCCN3||BiVO4 retains in the same level after five-cycle assays (20
h), which indicates that the electrode is highly stable (Fig. S9).
3.4 13CO2 labelling experiments
In order to identify the carbon source of hydrocarbons, 13CO2
labelling experiments were conducted in the same cell. The isotopic
1
liquid products were detected by H NMR and 13C NMR spectra. As
shown in Fig. 11 and S10, the doublet peak at 8.08 and 8.57 ppm is
assigned to the proton of formate coupled by the 13C nuclei. The
distinct 13C NMR signal at 160.4 ppm corresponds to the carbon of
H13COO− (Fig. S10).68 These results validate that the carbon source
of HCOO− is derived from CO2.
Fig. 9. The formation rate of hydrocarbons in PEC cells: (a) for four
different photocathodes at −0.85 V; (b) for Pd-TCCN3 photocathode
at voltages of −0.45  −1.05 V in the two-electrode system
Fig. 11. 1H NMR spectra of the H13COO− and H12COO− yielded in the
PEC cell
To study the influence of different metals in CO2 reduction reaction,
the M-TCCN3 electrodes were utilized as photocathodes. Even
though the Pt-TCCN3 photocathode has larger current than Pd-
TCCN3 in LSV curves (Fig. 8a), it produces less hydrocarbons
because of more H2 evolution (Fig. S8).67 Therefore, Pd-TCCN3
photocathodes show higher activity in the hydrocarbons formation
than that of Pt- and Au-TCCN3 due to the good matching of water
splitting and CO2 reduction (Fig. 10). These results reveal that metal
Pd is more suitable for catalysis reduction of CO2 into hydrocarbons
than Pt and Au.
3.5 Possible mechanism of PEC CO2 reduction
Scheme 1. A proposed mechanism for PEC reduction of CO2 into
chemical fuels
Based on the above characterizations and experimental results, a
possible mechanism was proposed for PEC reduction of CO2 into
chemical fuels (Scheme 1). Initially, driven by the low external
voltage, electrons could move to the Ti3C2 and transfer to the
conduction band (CB) of g-C3N4 that are trapped by the protons on
the surfaces of Pd NPs. Under the light irradiation, the electrons in
Fig. 10. The evolution rate of hydrocarbons for TCCN3 and M-TCCN3
photocathodes at −0.85 V
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9 S. Xie, Q. Zhang, G. Liu and Y. Wang, Chem. Commun., 2016,
the valence band (VB) of g-C3N4 could get energy from photons and
jump to the CB. Subsequently, these photoelectrons could combine
with protons to generate active hydrogen atoms that directly
convert CO2 into hydrocarbons.19, 61 In addition, the oxygen could
release on the BiVO4 photoanode.
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52, 35-59.
10 B. Kumar, M. Llorente, J. Froehlich, T. DDOaIn: g10, .A10.3S9a/Cth8rTuA0m33a1n5Cd
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Conclusions
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In summary, new heterojunctions of Ti3C2/g-C3N4 were 14 X. Chang, T. Wang and J. Gong, Energy. Environ. Sci., 2016,
9,
2177-2196.
prepared and further modified by various metals, which was
utilized as the photocathode in the M-TCCN||BiVO4 cell to
reduce CO2. The TCCN heterojunctions have narrow band gaps
and exhibit excellent ability of light harvesting. The Ti3+ species
15 J. L. White, M. F. Baruch, J. E. Pander Iii, Y. Hu, I. C. Fortmeyer,
J. E. Park, T. Zhang, K. Liao, J. Gu, Y. Yan, T. W. Shaw, E.
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12935.
could suppress the recombination of photogenerated 16 K. Sivula and R. van de Krol, Nat. Rev. Mater., 2016,
1
, 15010.
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electrons and holes in heterojunctions, thereby enhancing the
efficiency of photocatalysis. Besides, the Pd NPs could in-situ
capture protons and pyri-N species could adsorb CO2
molecules efficiently. These advantages of heterojunctions are
favourable for the coupling of CO2 reduction and water
splitting to generate chemical fuels. To the best of our
knowledge, the photoelectrocatalytic properties of Ti3C2/g-
C3N4 heterojunction for CO2 reduction have not been reported
to date. This work suggests that the MXene family materials
4
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Conflicts of interest
25 M. Shalom, S. Inal, C. Fettkenhauer, D. Neher and M.
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There are no conflicts to declare.
26 S. Ye, R. Wang, M. Wu and Y. Yuan, Appl. Surf. Sci., 2015, 358
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27 S. Tonda, S. Kumar, S. Kandula and V. Shanker, J. Mater.
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This work was supported by the National Natural Science
Foundation of China (NSFC 21173106), the Foundation of State
Key Laboratory of Coal Conversion (J17-18-913-2), and Natural
Science Foundation of Gansu Province (17JR5RA212).
29 W. Yu, D. Xu and T. Peng, J. Mater. Chem. A, 2015, 3, 19936-
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Graphical abstract
The new heterojunction of Ti3C2/g-C3N4 with rich Ti3+ and
pyri-N species could efficiently photoelectrocatalytic convert
CO2 into hydrocarbons.