Engineering heteroatoms with atomic precision in donor-acceptor covalent triazine frameworks to boost photocatalytic hydrogen production

Article abstract of DOI:10.1039/c8ta07391k

Porous organic polymers (POPs) with conjugated structures, high surface areas and variable building blocks are shown to possess enormous potential to be used in photocatalytic hydrogen evolution reaction. However, photocatalytic performances of most POPs are impeded by insufficient charge transfer and rapid charge recombination. Herein, based on covalent triazine frameworks (CTFs), we show a strategy to boost the photocatalytic performance by enhancing charge transfer/separation, where the donors can be easily tailored through substituting different heteroatoms with atomic precision in a series of donor-acceptor CTFs, resulting in optimal energy levels and enhanced charge transfer ability. Among them, the N-doped fluorene (carbazole) bearing the strongest electron donating ability achieves the highest photocatalytic performance among POPs, with an exceptionally high hydrogen evolution rate of 538 μmol h^-1 under visible light irradiation. Photophysical and electrochemical studies reveal that the high photocatalytic performance is attributed to high charge transfer efficiency and low charge recombination in these heteroatom-doped donor-acceptor CTFs. The work demonstrates that adjusting the push-pull interaction between the donor and acceptor via heteroatom engineering is an effective strategy to increase photocatalytic performance.

Full text of DOI:10.1039/c8ta07391k

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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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Engineering Heteroatom with Atomic Precision in Donor-Acceptor Covalent
Triazine Frameworks to Boost Photocatalytic Hydrogen Production
Liping Guoa, Yingli Niub, Haitao Xua, Qingwei Lic, Shumaila Razzaquea, Qi Huanga, Shangbin Jin*a
and Bien Tan*a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Porous organic polymers (POPs) with conjugated structures, high surface areas and variable building blocks are shown to
possess enormous potentials in photocatalytic hydrogen evolution. However, photocatalytic performances of most POPs
are impeded by insufficient charge transfer and rapid charge recommbination. Herein, based on covalent triazine
frameworks (CTFs), we show a strategy to boost the photocatalytic performance by enhancing charge transfer/separation,
where the donors can be easily tailored through substututing different heteroatoms with atomic precision in a series of
donor-acceptor CTFs, resulting in optimal energy levels and enhanced charge transfer ability. Among them, the N-doped
fluorene (carbazole) bearing strongest donor ability achieves the highest photocatlaytic performance among POPs, with an
exceptional high hydrogen evolution rate up to 538 μmol h-1 under visible light irradiation. Photophysical and
electrochemical studies reveal the high photocatalytic performance are attributed to high charge transfer efficency and
low charge recommbination in these heteroatom-doped donor-acceptor CTFs. The work demonstrates that adjusting
push-pull interaction between donor and acceptor via heteroatom engineering is an effective strategy to increase
photocatalytic perofrmance.
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organic frameworks (COFs), and covalent triazine frameworks
(CTFs) are receiving increasing interest as photocatalysts for
PHE, which also partially because of the abundance of organic
Introduction
The utilization of hydrogen energy is a prospective way to
address the environmental and energy issues.1 Among various
approaches to produce hydrogen, photocatalytic water
splitting has been regarded as the one of the most promising
routes. The key for the success relies on design of active
photocatalysts.2,3 Indeed, the concept of photocatalysts is
stemmed from inorganic materials, but many of them possess
certain limitations such as poor absorption of visible light or
scarcity of precursors.4 To overcome these deficiencies,
polymer photocatalysts have been widely investigated because
of the abundance and low cost of the components.5-7 Thus the
first report on photocatalytic hydrogen evolution (PHE) over g-
C3N4 has fuelled up the development of polymer
photocatalysts.8 Recently, porous organic polymers (POPs),
including conjugated microporous polymers (CMPs), covalent
building blocks, tunable semiconductive structures and strong
visible light absorptions.9-15 Moreover, POPs can be controlled
more precisely as compared with g-C3N4, which provide great
opportunities for the design and synthesis of appropriate
photocatalysts. Thus, a lot of works have been devoted to
expatiating the mechanism and exploited the more suitable
POPs in photocatalytic hydrogen evolution.16-19 Up to date, the
highest hydrogen evolution rate (HER) reported for CMPs,
COFs and CTFs are 116 μmol h-1, 20 μmol h-1 and 275 μmol h-1,
respectively.20-22 However, these modest values of
photocatalytic efficiencies are still unsatisfactory. To achieve
the adequate photocatalytic performance of POPs is
challenging task.
a
†
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In general, an efficient photocatalyst may have a wide
absorption range of visible light and powerful utilization of
solar light. Moreover, proper band alignment is
a
a
prerequisite, i.e. the energy level of conduction band should
be more negative than the redox potential of H+/H2 and
efficient charge separation/transfer is required to promote the
redox reactions.23-27 Comprehensively, charge transfer and
separation plays a vital role to achieve high photocatalytic
performance. Given the importance of charge separation/
transfer, the PHE could be enhanced by heteroatom doping
strategy.28 However, the doping strategy in the previous work
are mostly by post-modification29, 30, and the heteroatoms are
randomly distributed in the polymers. Consequently, it is
elusive to understand the structure-property relationship, so
as to give guideline for the further study. On the other hand,
incorporating electron donor-acceptor structures in polymers
is also an accessible strategy to enhance the photocatalytic
performance. Due to the presence of electrostatic interactions
between donor and acceptor units, donor-acceptor system
could accelerate the electron migration and therefore lowering
the combination of charges.31,32 Since the above description is
absolutely convincing but the real exploration of pathways to
control the charge recombination/separation/transfer in
photocatalytic water splitting process requires an exquisite
design.
Results and discussion
The CTFs were synthesized by condensation of benzene-1, 4-
dicarboximidamide hydrochloride with 3, 6-dicarbaldehyde-N-
ethylcarbazole, 2, 8-dicarbaldehyde-dibenzothiophe, and 2, 8-
dicarbaldehyde-dibenzofuran, which are denoted as CTF-N,
CTF-S, and CTF-O, respectively (Fig. 1a). From FT-IR spectra of
resulting CTFs (Fig. S1, ESI†), the stretching vibration of C=O
(1693 cm-1) from monomers and the stretching vibration of N-
H (3244 cm-1 and 3047 cm-1) from amidine disappeared. At the
same time, the stretching vibration band of C=N (1526 cm-1)
and stretching vibration band of C-N (1345 cm-1) emerged,
which is attributed to newly formed triazine units. These
spectroscopy features indicate the triazine framework
structures are formed as a result of the polycondensation of
amidine with aldehyde monomers. To further confirm the
triazine units in CTFs, the solid-state 13C-NMR spectrum
presents the chemical shift at 170 ppm (Fig. 1b), which is
Herein, we combine the heteroatom doping with donor-
acceptor pairing and demonstrate a new strategy to maximize
PHE activity by engineering the heteroatoms with atomic
precision in donor-acceptor CTFs. To this end, a series of
fluorene analogue (building blocks), with atomic precisions of
heteroatoms serve as electron donors and the triazine rings as
electron acceptors in CTFs, which have been confirmed by the
experimental and theoretical studies. The fluorene analogues
have large conjugated structures and the donor ability can
readily be tuned by varying the heteroatoms (N, S, O), thus
charge transfer efficiency and energy levels of CTFs can be
finely modulated. Both experimental and theoretical studies
have shown the building blocks based on fluorene analogues
containing heteroatoms can work as efficient strong electron
donating sites, and triazine rings work as electron acceptors.
Photocatalytic experiments revealed these CTFs boost the
photocatalytic hydrogen evolution performance significantly
by varying heteroatom from oxygen to nitrogen, in line with
the donor ability, which is with an exceptional high
photocatalytic HER up to 538 μmol h-1 for nitrogen under
visible light irradiation. This photocatalytic activity is about 7
times of CTF-O and 2 times of CTF-S, and also ranks as one of
the highest values among the POPs.20-22,33 The CTFs also exhibit
high quantum yields, with the highest apparent quantum yield
up to 4.11 % at 420 nm, which is comparable to the most of
other POPs. The photophysical and electrochemical studies
suggest that the high photocatalytic activity of CTFs is
managed by high charge transfer efficiency and low charge
recombination rates. We highlight the importance of the
heteroatom types to the charge transfer efficiency in the
donor-acceptor CTFs and thus give a guideline for the design of
efficient photocatalysts.
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clearly deriving from the triazine carbon, and the other fluorene-like structures have lower HOMO energy levels than
chemical shifts can also be assigned to the corresponding that of triazine block. Therefore, it suggests that fluorene-like
carbon in CTF-N, CTF-S and CTF-O, respectively. In addition, X- units can be excited easily to generate electrons and work as
ray photoelectron spectroscopy (XPS) results (Fig. S2, ESI†) electron donors. The change in the electronic properties was
were given to explore the chemical state of elements on the observed by heteroatoms doping (N, S, O) in fluorene-like
surface of CTFs. From the results of high-resolution spectra of structures. A gradual increase in HOMO energy levels was
N1s of these polymers, the binding energy at 398.7eV/398.8eV observed in the order of N < S < O, suggesting the gradual
also proved the existence of triazine ring (C=N-C) in CTFs. decrease in electron donor ability. The charge distribution at
Elemental analysis showed the composition of these CTFs are different electronic status simulated using oligomer model
close to the theoretically calculated results (Table S1, ESI†) structures (Fig. 3b) further suggests the triazine can function as
which evidences that the CTFs are forming large network the electron acceptors and the fluorene-like building blocks
structures. In particular, the N, S and O elements found in the can act as electron donors, and the benzene ring work as the
CTFs indicates the corresponding building blocks are contained bridge to transfer electron from donor to acceptor with the aid
in the resulting polymers. The CTFs are also showing high of the push-pull interaction. The donor can push the electrons
thermal stability as revealed by thermogravimetric analysis to move away, and at the same time the acceptor can attract
(Fig. S3, ESI†). The CTFs are exhibiting lamellar structures as and pull the electron to near close. The path of photoexcited
observed by TEM images (Fig. 2a-c). These lamellar electron in donor-acceptor CTFs transfer was shown in Fig.3c.
morphologies endow CTFs with good dispersibility in aqueous Under visible light irradiation, the excited electrons are more
solution, which might be beneficial to charge and mass easily migrated from fluorene-like units (donor, blue part), and
transfer in photoredox reactions. The XRD patterns reveals the then inclined to migrate to triazine rings (acceptor, green
low crystallinity nature of CTFs (Fig. S4, ESI†). Due to the
limited crystallinity, very few and weak diffraction peaks has
been observed in powder X-ray diffraction curves. Although it
is difficult to make a decisive structure from these weak
diffraction peaks, but compared with the CTF-1, the peak
positioning at 7.6 degree indicated the partial formation of
CTF-1 structures in these CTFs.34 The textural properties of
CTFs were investigated by nitrogen absorption and desorption
isotherm at 77K (Fig. 2d). The results showed the CTFs are
highly porous and possess high Brunauer-Emmett-Teller (BET)
surface areas. The BET surface areas of CTF-N, CTF-S and CTF-O
are calculated to be 650 m2 g-1, 533 m2 g-1 and 569 m2 g-1,
respectively. The pore size distribution showed the CTFs are
mainly microporous, but some mesoporous structures can also
be observed. Such hierarchical porosity endows CTFs more
active surface for the guest molecules to access and may
enhance mass transfer process as well.
Theoretical calculations are made to estimate the electronic
structure and confirm the donor-acceptor pairing in CTFs using
Gaussian 09 program package35. Initially we carried out the
calculation for the orbital energies of building blocks and the
charge distribution in different electronic state based on
oligomer structures of CTFs as models. According to the
theoretical calculation (Fig. 3a and Table S2, ESI†), the
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part).
From the UV-Visible diffuse reflectance spectra (DRS) (Fig.
4a), all the polymers have wide absorption in visible light
region and red-shift starts from CTF-O, CTF-S to CTF-N,
resulting with the largest absorption up to 550 nm. The
different absorption performance of CTFs are in accordance
with the colour of CTF samples. The optical band gaps of CTFs
were calculated by Tauc-plot method (Fig. 4a, inset).
Accordingly, the band gaps of CTF-N, CTF-S and CTF-O are
calculated as 2.17 eV, 2.47 eV and 2.67 eV, respectively. The
tendency of band gaps calculated from experimental
measurements are consistent with the theoretical vertical
excited energies using time-dependent density functional
theory (TDDFT),36-38 based on model structures both at
vacuum and in solution (Table S3†).39-42 It is noteworthy that
CTF-N has the narrowest band gap and can absorb more visible
light and generate more charges in photocatalytic process.
The photo-redox properties of CTFs are also affected by the
electronic configuration. In donor-acceptor system, the
electron donor units determine the HOMO energy level while
the acceptor determine the LUMO energy level.43 The
simulated results show that CTF-N has the highest LUMO level
and smaller HOMO level than that of the other two CTFs (Table
S3, ESI†). To verify the result, we determined the LUMO
energy levels of the CTFs by measuring Mott-Schottky plots
(Fig. 4b and Fig. S5, ESI†). The tendency of the reducibility and
driving force for hydrogen production in different CTFs will not
change with different reference energy levels in principle44.
Thus, the investigation only focuses on the tendency of the
relative values of energy levels between theoretical calculation
and experimental results. As revealed, the LUMO energy level
of CTF-N, both of theoretical and experiment results, is higher
than CTF-S and CTF-O, which means CTF-N has the largest
driving force in photocatalytic hydrogen production. At the
same time, the strongest donor ability of carbazole in CTF-N
could be inferred from the smallest HOMO energy level of CTF-
N. The electron donor ability of dibenzothiophene and
dibenzofuran are inferior to that of carbazole, which can be
deduced from high HOMO energy level of CTF-S and CTF-O.
These results comprehended the theoretical feasibility of using
donor-acceptor CTFs for photocatalytic hydrogen evolution
reaction. Therefore, the photocatalytic performance efficiency
of the CTFs were evaluated by measuring the hydrogen
evolution in aqueous solution. The photocatalytic hydrogen
evolution experiments were carried out under visible light
(>420 nm), with the addition of triethanolamine (TEOA) as
sacrificial agent and Pt as co-catalyst. 45,46 The Pt nanoparticles
were uniformly distributed after reduction in CTFs with
average particle size of 5 nm as revealed by TEM image (Fig.
S6, ESI†) and the amount of Pt are determined by inducꢀvely
coupled plasma (ICP) spectrometer to be 2.11 wt% for CTF-N,
1.91 wt% for CTF-S and 1.93 wt% for CTF-O, respectively. After
visible light irradiation for five hours, the average HER of CTF-
N, CTF-S and CTF-O can reach 538 μmol h-1, 266 μmol h-1 and
72 μmol h-1, respectively (Fig. 4c). The photocatalytic efficiency
of CTFs improve as the donor ability of building blocks
enhance. The HER of CTF-N is about 7 times higher than that of
CTF-O and 2 times of CTF-S. And the HER of CTF-N is around
double of that of the state-of-the-art CTF photocatalyst.22
Besides CTFs, the photocatalytic hydrogen evolution activity of
CTF-N far surpasses most of the POPs in the literatures (Table
S4, ESI†), and, what’s more, is also about 78 ꢀmes higher than
bulk g-C3N4 (7 μmol h-1) measured under the identical
condition (Fig. S7, ESI†). The CTFs are also showing high
quantum efficiency in photocatalytic hydrogen evolution,
which is comparable to other POPs (Table S4, ESI†). The
apparent quantum yields (AQY) of CTF-N, CTF-S and CTF-O are
measured to be 4.07 %, 4.11 % and 2.10 % at 420 nm, and are
2.79 %, 0.19 % and 0.10 % at 500 nm, respectively.47 To
confirm the hydrogen are photo-catalytically produced by the
CTFs, control experiments were carried out. The obtained
results showed that no hydrogen was detected when the
reaction solution was kept in dark or in absence of CTFs. The
stability of the CTF-N was measured by repeating the
experiment over 5 cycles (Fig. 4d). The results showed that the
average HER of CTF-N remained almost constant with a
negligible decrease and the structures of CTFs were still intact
after photocatalytic cycles as shown by FT-IR spectrum (Fig. S8,
ESI†).
To further understand mechanism and the influence of the
push-pull interaction in donor-acceptor system on the
photocatalysis activity of these CTFs, we studied photophysical
spectroscopies and electrochemical measurements. According
to the steady-state photoluminescence spectroscopy (Fig. 5a),
CTF-N showed the lowest emission intensity as compared to
CTF-S and CTF-O, which indicates CTF-N has a lowest charge
recombination rate and is advantageous for photoredox
processes. Because of the strong push-pull interaction
between donor and acceptor in CTFs, the excited electrons are
preferred to transfer from donor to acceptor. Therefore, it
increases the spatial separation of electrons and holes and
benefit to decrease the recombination of charges. Time-
resolved photoluminescence spectroscopy is employed to
elucidate the charge transfer efficiency (Fig. 5b). The decay
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curves of CTFs fit well with a biexponential function and the recombination rate of electrons and holes in the donor-
calculated values of lifetimes are summarized in Table S3 acceptor CTF. The present work provides a fundamental
(ESI†). By comparison, the photoluminescence lifetime (τ) of insight into the structure-property relationship for
CTF-N is only 0.36 ns, much shorter than that of CTF-S (τ = 0.62 photocatalysis applications of CTFs and gives guideline for the
ns) and CTF-O (τ = 2.02 ns). For the purpose of comparison, design of efficient photocatalysts for hydrogen evolution in
CTF-C was selected as a reference (Fig. S9-S11, ESI†), which has future.
a photoluminescence lifetime τ0 = 2.82 ns (Fig. S12, ESI†), and
then the charge transfer efficiency was estimated by using an
48. Accordingly, the corresponding charge
Experimental
equation of
Φ=1−τ /τ0
transfer efficiencies are calculated to be 87 % for CTF-N, 78 %
for CTF-S and 28 % for CTF-O, respectively. In short, CTF-N has
the highest charge transfer efficiency due to strong push-pull
interaction given by the strongest donor ability of N in
carbazole. Electrochemical impedance spectroscopy (EIS) is
exploited to evaluate electrical conductivity of CTFs (Fig. 5c).
As the results, CTF-N has the smallest Nyquist plot radius
diameter, which means it has the smallest resistance. The
small resistance indicates the interfaces of CTFs have high
charge migration ability, which can be favourable to charge
transfer in photocatalytic process.
General synthetic procedures of CTFs.
Taking CTF-N as an example, the mixture of 3,6-
dicarbaldehyde-N-ethylcarbazole (0.39 g, 1.0 mmol),
terephthalamidine dihydro-chloride (0.47 g, 2.0 mmol) and
Cs2CO3 (1.31 g, 4.0 mmol) were dissolved into 40.0 mL DMSO,
and then stirred at 60 °C for 12 h, 80 °C for 12 h, 100 °C for 12
h, 120 °C for 36 h and 150 °C for 36 h, respectively. After
reaction completed, the precipitation was collected by
filtration, washed with water, DMF and then freeze-dried. The
final product CTF-N was bright yellow powder (0.43 mg, 63%).
CTF-S, CTF-O and CTF-C were prepared using the same
procedure as the CTF-N. CTF-S was yellow powder (68%), CTF-
O was light yellow powder (63%) and CTF-C was yellow powder
(71%).
The high charge transfer efficiency and conductivity of CTFs
are also supported by high photoconductivity, since
photocurrent is the collection and migration of the charges
generated by intermittent visible light irradiation. We can
observe the CTFs are displaying high photocurrent values
when the light was switched on (Fig. 5d). Among them, CTF-N
displays the highest photocurrent response in these CTFs. The
intensity of photocurrent of CTFs are also in the order of CTF-N
> CTF-S > CTF-O. The high photocurrent indicates these CTFs
have low charge recombination rates and high charge transfer
efficiency. As above, the photocatalytic activity of CTFs (CTF-N
> CTF-S > CTF-O) is consistent with the tendency of charge
transfer efficiency and charge recombination rate. Considering
the similar BET surface area and pore size distribution of CTFs,
we may conclude the high performance of the photocatalytic
hydrogen evolution is mainly contributed by the donor-
acceptor systems, which resulted in enhancement of charge
transfer and suppression of charge recombination.
Characterizations
Fourier-transformed infrared (FT-IR) spectra were collected on
KBr disks in transmission mode using a Bruker Vertex 70 FTIR
spectrometer. X-ray photoelectron spectroscopy (XPS) analysis
was conducted on Thermo Scientific ESCALAB 250xi instrument
with the step size as 0.05eV. The porous properties of polymer
were measured by nitrogen adsorption and desorption at 77 K
using Micromeritics ASAP 2010M surface area and porosity
analyzer. X-ray diffraction (XRD) patterns of the CTF samples
were obtained on an X-ray diffractometer with Cu ka radiation
source at 40 KV. Solid-state NMR experiments were carried out
to check chemical structure of CTF-X using a 400MHz Bruker
Avance II with a standard 4 mm magic angle spinning double
resonance probe head. Monomer structures were checked
with 1H NMR and 13C NMR using 400MHz Bruker Avance III.
The morphology of CTFs were observed by Tecnai G20S−Twin
transmission electron microscopy. Thermogravimetric analysis
Conclusions
In summary, we showed
a new strategy to boost (TGA) was conducted using a TG 209 instrument with a heating
photocatalytic hydrogen evolution, by engineering donor rate at 10 °C/min from 25 to 800 °C under N2 atmosphere. UV-
ability based on CTFs through design of fluorene-based Visible diffuse reflectance spectra (DRS) were recorded in the
building blocks with heteroatoms precisely positioned in the range from 900 nm to 200 nm using Shimadzu UV-3600. The
structures. By tailoring the electronic donors through changing photoluminescence (PL) spectra were measured by an
the heteroatoms in the structures, the photocatalytic Edinburgh FLS-960 spectrophotometer with an excitation
hydrogen evolution performances can be dramatically wavelength at 365 nm in DMF solution (0.5 mg/mL). Time-
enhanced. Both theoretical calculations and experimental resolved PL spectra were measured in the solid state with the
studies reveal the strongest donor ability of N doping in the same instrument. The excitation wavelength was 371.2 nm
corresponding CTF-N exhibited the best performance, and the with picosecond pulsed diode laser, and the data were
HER of CTF-N is up to 538 μmol h-1, which is about 2 times of collected at the emission peak of the corresponding CTFs.
the-state-of-art CTFs. To the best of our knowledge, it also Electrochemical measurements were carried out in three-
exhibits as one of the highest values among POPs and about 78 electrode cell system by using CHI760E electrochemical
times higher than the HER of g-C3N4 measured under the workstation (Chenhua, Shanghai), where ITO deposited with
identical condition. The high performance can be attributed to sample as the working electrode, Ag/AgCl electrode as the
the enhanced charge transfer efficiency, and low reference electrode, Pt flake as the counter electrode. The
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3
4
5
Y. Zheng, L. Lin, B. Wang, X. Wang, Angew. Chem. Int. Ed.,
2015, 54, 12868-12884.
samples were prepared by mixing ground polymer with 5 wt%
Nafion. The Mott-Schottky tests were carried out in 0.1 M
Na2SO4 solution as supporting electrolyte and the pH value is
7. LUMO levels were calculated from the Mott-Schottky
measurements, in which the flat band potential can be first
obtained according to the potential intercept, which was
reversible Ag/AgCl electrode. Then the LUMO values of CTFs
were transformed to reversible hydrogen electrode (RHE)
T. Hisatomi, J. Kubota, K. Domen, Chem. Soc. Rev., 2014, 43
7520-7535.
,
R. Sprick, B. Bonillo, R. Clowes, P. Guiglion, N. Brownbill, B.
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6
7
8
9
,
according to the Nernst equation LUMORHE
0.059*pH + 0.2. The final experimental HOMO value can be
calculated according to the equation EBandgap = HOMORHE
LUMORHE
= EAg/AgCl +
-
.
Adams, A. Cooper, Chem. Commun., 2016, 52, 10008-10011.
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Gaussian 09 program package was employed for calculating
the electronic structures of ground and excited states of the
model structures.35 The geometries of the ground states were
optimized with DFT, and TDDFT was applied for the calculation
of vertical excited energies.36-38 The B3LYP functional and 6-
311G (d, p) basis set were used in the calculation. The
Polarizable Continuum Model (PCM) was used to simulate the
solvation effect.39-42 The dielectric constant of the solvent was
set to be 73.
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Photocatalytic measurements.
The photocatalytic experiments were conducted with 300W Xe
lamp (Perfectlight, PLS-SXE300) under visible light irradiation (
> 420 nm). 50.0 mg Photocatalyst loading with Pt and 10 mL
TEOA were mixed in 100 mL aqueous solution. Pt nanoparticles
were deposited first in CTFs by chemical reduction method
using NaBH4. The temperature of the solution was maintained
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,
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
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We are grateful for the funding from National Natural Science
Foundation of China (Grant No. 21604028, 21474033), the
International S&T Cooperation Program of China (Grant No.
2016YFE0124400), and the Program for HUST Interdisciplinary
Innovation Team (Grant No. 2016JCTD104). We thank Dr. Yong
Zhong from Key Laboratory for Special Functional Materials of
the Ministry of Education, Henan University, for his helpful 27 X. Wang, K. Maeda, X. Chen, K. Takanabe, K. Domen, Y. Hou,
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