Bimetallic zeolite-imidazole framework-based heterostructure with enhanced photocatalytic hydrogen production activity

Article abstract of DOI:10.1039/d1ra00781e

Bimetallic zeolite-imidazole frameworks with controllable flat band position, band gap and hydrogen evolution reaction characteristics were adopted as a photocatalytic hydrogen production catalyst. Furthermore, the g-C₃N₄-MoS₂2D-2D surface heterostructure was introduced to the ZnM-ZIF to facilitate the separation as well as utilization efficiency of the photo-exited charge carriers in the ZnM-ZIFs. On the other hand, the ZnM-ZIFs not only inhibited the aggregation of the g-C₃N₄-MoS₂heterostructure, but also improved the separation and transport efficiency of charge carriers in g-C₃N₄-MoS₂. Consequently, the optimal g-C₃N₄-MoS₂-ZnNi-ZIF exhibited an extraordinary photocatalytic hydrogen evolution activity 214.4, 37.5, and 3.7 times larger than that of the pristine g-C₃N₄, g-C₃N₄-ZnNi-ZIF and g-C₃N₄-MoS₂, respectively, and exhibited a H₂-evolution performance of 77.8 μmol h^?1g^?1under UV-Vis light irradiation coupled with oxidation of H₂O into H₂O₂. This work will furnish a new MOF candidate for photocatalysis and provide insight into better utilization of porous MOF-based heterostructures for hydrogen production from pure water.

Full text of DOI:10.1039/d1ra00781e

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Bimetallic zeolite-imidazole framework-based
heterostructure with enhanced photocatalytic
hydrogen production activity†
Cite this: RSC Adv., 2021, 11, 9048
*
*
Shiquan Liu
Nayab Arif, Ye-Zhan Lin, Kai Wang, Yi-Chuan Dou, Yu Zhang, Kui Li,
and Fu-Tian Liu
Bimetallic zeolite-imidazole frameworks with controllable flat band position, band gap and hydrogen
evolution reaction characteristics were adopted as a photocatalytic hydrogen production catalyst.
Furthermore, the g-C3N4–MoS2 2D–2D surface heterostructure was introduced to the ZnM-ZIF to
facilitate the separation as well as utilization efficiency of the photo-exited charge carriers in the ZnM-
ZIFs. On the other hand, the ZnM-ZIFs not only inhibited the aggregation of the g-C3N4–MoS2
heterostructure, but also improved the separation and transport efficiency of charge carriers in g-C3N4–
MoS2. Consequently, the optimal g-C3N4–MoS2–ZnNi-ZIF exhibited an extraordinary photocatalytic
hydrogen evolution activity 214.4, 37.5, and 3.7 times larger than that of the pristine g-C3N4, g-C3N4–
ZnNi-ZIF and g-C3N4–MoS2, respectively, and exhibited a H2-evolution performance of 77.8 mmol hꢀ1
gꢀ1 under UV-Vis light irradiation coupled with oxidation of H2O into H2O2. This work will furnish a new
MOF candidate for photocatalysis and provide insight into better utilization of porous MOF-based
heterostructures for hydrogen production from pure water.
Received 29th January 2021
Accepted 22nd February 2021
DOI: 10.1039/d1ra00781e
rsc.li/rsc-advances
applications. Hence, various Zr, Ti, Cr-based MOFs with and
without Pt, small molecular and even polyoxometalates (POMs)
Introduction
have been extensively investigated as photocatalyst for
hydrogen production.25,26,30,31
The direct conversion of sustainable solar power into eco-
friendly energy over high performance photocatalysts has
been extremely signicant for powering human society.1–7 Up to
now, it is universally recognized that the serious recombination,
underutilization and poor transport of photon-generated
carriers weaken the photocatalytic activity of the photo-
catalyst.8–12 Consequently, extensive efforts have been devoted
to constructing heterostructures or phase junctions, which
could increase the specic surface area of the photo-
catalyst,2,13–21 and adopting a suitable cocatalyst for improving
the separation, transport and utilization of the photo-induced
charge carriers.10,11,22,23 However, most of the photocatalyst
systems were composed of inorganic metal oxides/suldes,
which may result in poor transport and separation efficiency
of charge carriers, low porosity, and limited exibility in mate-
rial design. In this instance, metal–organic frameworks (MOFs),
constructed from metal ions/clusters and organic linkers,
exhibited various attractive characteristics such as high specic
surface area, crystalline nature,24–26 and especially structure
diversity for easy tailorability of the functionality,24,27–29 and
have attracted widespread interest in photocatalytic
As one of the most stable metal organic frameworks in
neutral and alkaline solutions,26,32,33 Zeolite Imidazole Frame-
works (ZIFs) and its heterostructure has been extensively
studied for the photocatalytic reaction.33,34 Especially, the
bimetallic ZIFs has been widely investigated because it
possesses various structures, has versatile functions and its
photo-electric properties. Moreover, the bimetallic ZIF derived
nanomaterials have been fabricated and used in the electro-
catalytic energy conversion and storage.25,32,34–36 However, the
bimetallic ZIFs itself with special merits for photocatalysis was
rarely reported as photocatalyst, although this is a stirring work,
and will bring out some new interesting prospects. Moreover,
almost all the MOFs photocatalysts suffered from the problems
of limited photocatalytic hydrogen production activity and poor
photocatalytic stability even in sacricial agent solution owing
to the low photo-response, rapid recombination of photo-exited
electron–hole pairs. Construction of ZIFs-based heterostructure
not only resolved the problems of low porosity of semi-
conductors, but also improved the separation and utilization of
the photo-generated charge carriers.
Graphic carbon nitride (g-C3N4), which is composed of the
earth abundant carbon and nitrogen atoms through strong
covalent bonds, possesses enhanced stability in solutions over
School of Materials Science and Engineering, University of Jinan, Jinan 250022, China.
E-mail: mse_lik@ujn.edu.cn; vctrliu@hotmail.com
† Electronic supplementary information (ESI) available. See DOI:
10.1039/d1ra00781e
a
wide pH range, and suitable for the visible light
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absorption.37–40 The separation and utilization efficiency of the TEM (HRTEM) images were recorded on JEOL-2100F appa-
photo-generated charge carriers in g-C3N4 is very limited, ratus at an accelerating voltage of 200 kV. The atomic structure
resulting in poor photocatalytic activity.9,23,41 Consequently, of the g-C3N4–MoS2-MOF heterojunction was characterized
a large number of cocatalysts were fabricated, and one of the using an ARM-200CF (JEOL, Tokyo, Japan) transmission elec-
most extensively investigated cocatalyst is MoS2.9,42–45 Due to the tron microscope operated at 200 kV and equipped with double
different work function of MoS2 compared with the semi- spherical aberration correctors. Surface morphologies of the
conductor components, and its low hydrogen evolution reaction phase junction materials were examined by a scanning electron
(HER) overpotential, the photo-exited electrons and holes in g- microscope (SEM, JSM-7600F) at an acceleration voltage of 10
C3N4–MoS2 2D–2D surface heterostructure could be trans- kV. The UV-Vis absorption and diffused reectance spectra were
formed into different semiconductor components with high recorded using a Cary 5000 UV-Vis spectrometer (Viarian, USA)
utilization efficiency. Therefore g-C3N4–MoS2 heterostructure with BaSO4 as a reectance standard. Nitrogen adsorption–
was adopted to incorporate on ZIFs-based heterostructure that desorption isotherms were measured at 77 K on a Quantach-
not only resolved the problems of low porosity, but also rome Instruments Autosorb AS-6B. The pore size distributions
improved the separation and utilization of the photo-generated were measured by the Barrett–Joyner–Halenda (BJH) method.
charge carriers between g-C3N4–MoS2–ZnM-ZIF. Simulta- Steady photoluminescence (PL) emission spectra were tested by
neously, ZnM-ZIF could restrain the agglomeration of g-C3N4– a luminescence spectrophotometer (QM-400, PTI) with 350 nm
MoS2 during the subsequent post annealing and photocatalytic excitation wavelength. The transient photocurrent responses
reactions.9,46,47 However, during the fabrication process and/or experiments were carried out at room temperature under visible
the photocatalytic reaction, the g-C3N4–MoS2 nanosheet may light irradiation using a conventional three-electrode system
aggregate occur and thus deteriorate its photocatalytic with a glassy carbon electrode (3 mm in diameter, sheet resis-
hydrogen production activity.34,48 Moreover, the photocatalytic tance 20–25 U per square) as the working electrode, a carbon
hydrogen evolution in pure water for g-C3N4–MoS2 and espe- rod as the auxiliary electrode, and an Ag/AgCl electrode as the
cially MOFs photocatalyst is important and it also faces huge reference electrode. All the samples were dispersed in deionized
challenges.49
water with a concentration of 1.5 mg mLꢀ1 and deposited on the
Herein, bimetallic ZIFs (ZnM-ZIF, M ¼ Co, Ni) with glassy carbon electrode and dried under visible light irradia-
controllable band were synthesized and adopted as the photo- tion. The electrocatalytic hydrogen evolution reaction was
catalytic hydrogen production catalyst with a considerable measured in 1 M KOH solution. EIS data were recorded using
photocatalytic HER activity. The g-C3N4–MoS2 nanorods were electrochemical (Bio-Logic SP-150) workstation in Na2SO4 (0.5
prepared by one-step hydrothermal method. Further the g- M) solution. The transient photocurrent responses measure-
C3N4–MoS2–ZnM-ZIFs (M ¼ Co, Ni) were prepared using the ments were performed with a CHI 660E electrochemical station
similar method with g-C3N4–MoS2 as precursor. The introduc- (Shanghai Chenhua Co. Ltd, China) under irradiation in 0.5 M
tion of g-C3N4–MoS2 onto the surface of the ZnM-ZIF not only Na2SO4 solutions.
improved the utilization efficiency of photo-exited charge
carriers by MoS2, but also inhibited the accumulation of g- Synthesis of g-C3N4
C3N4–MoS2 heterostructure. Hence, the optimal g-C3N4–MoS2–
The pure g-C3N4 photocatalyst was prepared by a traditional
thermal polymerization strategy. In a typical procedure, 5 g of
ZnNi-ZIF (CNMZN) exhibited the extraordinary photocatalytic
HER activity of 30.1 mmol hꢀ1, being 214.4, 37.5, and 3.7 times
C3N3(NH2)3 was placed in a crucible with a cover, and then it
larger than that of the pristine g-C3N4 (CN), g-C3N4–ZnNi-ZIF
(CNZN) and g-C3N4–MoS2 (CNM), respectively. More impor-
tantly, even in pure water, the optimal ternary heterostructures
exhibited considerable hydrogen production rate. This work
provided innovation that can hopefully help in designing
a novel high-performance MOF-based heterostructure photo-
catalyst for solar conversion.
ꢁ
was annealed at 650 C for 2 h in the argon. The nal yellow
powder was then collected.
Synthesis of the g-C3N4–MoS2
The g-C3N4–MoS2 nanorods were prepared using H3P(Mo3-
O10)4$xH2O, CH4N2S as precursors by one-step hydrothermal
method. In a typical procedure, 200 mg g-C3N4, suitable amount
of H3P(Mo3O10)4$xH2O (with Mo content of 6 wt% (19 mg) in
comparison with g-C3N4) and CH4N2S (the corresponding ratios
of Mo/S equal to 1 : 2) were dissolved in 30 mL distilled water.
The solutions were subsequently transferred into 50 mL Teon-
lined autoclave and maintained for 24 h at 160 ꢁC. The nal
black products were rinsed three times with distilled water and
ethanol respectively, and dried at 60 ꢁC for overnight in
a vacuum oven to evaporate the solvent.
Experimental section
Materials and general methods
C3N3(NH2)3, Zn(NO3)2$6H2O, Ni(NO3)2$6H2O, Co(NO3)2$6H2O,
C4H6N2, H3P(Mo3O10)4$xH2O and CH4N2S (Shanghai Macklin
Biochemical Technology Co., Ltd) are analytical grade and used
as received without further purication. The powder X-ray
diffraction (PXRD) patterns were recorded on a D/max 2500
VL/PC diffractometer (Japan) equipped with graphite mono-
Synthesis of the ZnNi-MOF, ZnCo-MOF and ZIF-8
˚
chromatized Cu Ka radiation (l ¼ 1.54060 A). Corresponding
work voltage and current is 40 kV and 100 mA, respectively. The The ZnNi-MOF nanorods were prepared using Zn(NO3)2$6H2O,
transmission electron microscopy (TEM) and high-resolution Ni(NO3)2$6H2O and C4H6N2 as precursors, an amount of
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Zn(NO3)2$6H2O and Ni(NO3)2$6H2O (the corresponding ratios
of Zn/Ni equal to 1 : 9, 3 : 7, 5 : 5, 7 : 3, 9 : 1) were dissolved into
20 mL distilled water. The mole number of C4H6N2 was 10 times
larger than that of the sum molar number of Zn(NO3)2$6H2O
and Ni(NO3)2$6H2O, these precursors were dissolved into 10 mL
distilled water. Thereaer, the C4H6N2 solution was added
dropwise into the above mixture solution while stirring. The
nal mixture was stirred for 24 h at room temperature. The
precipitates were rinsed with deionized water and anhydrous
ethanol several times, and then dried in a vacuum oven at 60 ꢁC
for 12 h. The ZIF-8 and ZnCo-MOF could be obtained by
replacing the Ni(NO3)2$6H2O with Zn(NO3)2$6H2O and
Co(NO3)2$6H2O, respectively.
Synthesis of the g-C3N4–ZnM-ZIFs and g-C3N4–MoS2–ZnM-
ZIFs
Suitable amount of Zn(NO3)2$6H2O and Ni(NO3)2$6H2O (the
corresponding ratios of Zn/Ni equal to 5 : 5) and as-prepared g-
C3N4 were added into 20 mL distilled water. C4H6N2 with 10
mole ratios of the amount of Zn(NO3)2$6H2O and Ni(NO3)2-
$6H2O were dissolved into 10 mL distilled water. Thereaer, the
C4H6N2 solution was added dropwise into the above mixture
solution while stirring. The nal mixture was stirred for 24 h at
room temperature. The precipitates were rinsed with deionized
water and anhydrous ethanol several times, and then dried in
a vacuum oven at 60 ꢁC for 12 h. The g-C3N4–ZIF-8 and g-C3N4–
ZnCo-ZIF could be obtained by replacing the Ni(NO3)2$6H2O by
Co(NO3)2$6H2O. The g-C3N4–MoS2–ZnM-ZIFs were prepared
using the similar method with g-C3N4–MoS2 as precursor as
shown in Fig. 1a.
Fig. 1 (a) Scheme illustrating the synthesis of the ternary g-C3N4–
MoS2–ZnNi-ZIF heterostructure. (b) PXRD patterns of the g-C3N4, g-
C3N4–MoS2, g-C3N4–MoS2-ZIFs and ZIF-8 (c) Mo and metals
concentrations in the g-C3N4–MoS2 and g-C3N4–MoS2-ZIFs from ICP
spectroscope. TEM images of the (d) g-C3N4, (e) ZnNi-ZIF (inset
showing the SEM image), (f) g-C3N4–ZnNi-ZIF, (g) g-C3N4–MoS2, (h)
g-C3N4–MoS2–ZnNi-ZIF, and (i) HRTEM image of the g-C3N4–MoS2–
ZnNi-ZIF.
Photocatalytic hydrogen production
parameters. Every 12 h the sacricial regent was renewed for the
evaluation of the photocatalytic stability in vacuum.
The photocatalytic H2-production experiments were performed
via photocatalytic H2-production activity evaluation system
(CEL-SPH2N, CEAU-Light, China) in a 300 mL Pyrex ask, and
the ask openings were sealed with silicone rubber septum. A
Results and discussion
300 W xenon arc lamp was used as all light source, and was The powder X-ray diffraction patterns (PXRD) of g-C3N4 with
positioned 13 cm away from the reaction solution to trigger the MoS2 and/or ZnM-ZIF is demonstrated in Fig. 1b. The charac-
photocatalytic reaction. A 300 W xenon arc lamp through a UV- teristic diffraction peaks of g-C3N4–MoS2-ZIFs could be clearly
cutoff lter (l ¼ 420–800 nm) was used as a visible light source, observed. For comparison, g-C3N4-based samples with and
and was positioned 13 cm away from the reaction solution to without MoS2 is shown in Fig. 1b, the diffraction peak of the
trigger the photocatalytic reaction. FZ-A visible-light radiometer MoS2 (around 9ꢁ) shied to lower theta values (around 1ꢁ) due
(made in the photoelectric instrument factory of Beijing Normal to the larger interplanar spacing, verifying the successful
University, China) was used to measure the focused intensity on fabrication of ternary g-C3N4–MoS2–ZnM-ZIF heterostructure as
the ask (i.e., 200 mW cmꢀ2). In a typical photocatalytic H2- shown in Fig. 1b and S1,† and has been previously reported.50
production experiment, 20 mg of the as-prepared photocatalyst Notably, PXRD pattern justied the successful g-C3N4–ZnM-
was suspended in 50 mL of mixed aqueous solution of 5 mL ZIF–MoS2 heterostructure formation at rst and then reduction
C6H15NO3 (AR, 98%). To ensure the anaerobic condition in the of peak shows that material aer hetero-structure composite is
reactor before irradiation, the system was vacuumed for 5 min 2D material which is amorphous in nature. Furthermore,
by the use of vacuum pump for the entire removal of dissolved Fourier Transform Infrared Spectroscopy (FTIR) spectra were
oxygen. The photocatalyst particles were suspended throughout further employed for conrming the formation of g-C3N4–ZnM-
the experiments by employing a continuous magnetic stirrer at ZIF heterostructures. As shown in Fig. S2a,† the typical peak of
the bottom of the reactor. H2 content was analyzed by gas ZnNi-ZIF appeared at 422 cmꢀ1 belonged to the Ni–N stretching
chromatography (GC-7900, CEAU-Light, China). All glassware vibration, the peaks at 2960 and 2925 cmꢀ1 could be attributed
were carefully rinsed with DI water prior to usage. The photo- to the asymmetric absorption vibration of the C–H bond in the
catalytic stability was also performed in the same processing –CH3 group, the absorption peaks at 748 cmꢀ1 was caused by
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stretching vibration of the C]N bond in 2-methylimidazole,
and the intense absorption peaks at 1000 cmꢀ1 were derived
from the plane bending vibration of the imidazole ring.51
Distinctly, the characteristic FTIR peaks of ZnM-ZIF and g-C3N4
could also be clearly observed in the g-C3N4–MoS2–ZIF-8
(CNMZ) and g-C3N4–MoS2–ZnCo-ZIF (CNMZC) samples
(Fig. S2b†), verifying the existence of ZnM-ZIF in the g-C3N4–
MoS2–ZnM-ZIF. The concentrations of Mo and the metals in
ZnM-ZIFs in the g-C3N4–MoS2 with and without ZnM-ZIFs were
investigated via inductively coupled plasma (ICP) spectroscope
(Fig. 1c). The Mo content in the relevant samples was very close
to the theory predicted value. Whereas, the metals content in
the ZnM-ZIFs was lower than the given value (10 wt%), which
may be attributed to the underreaction between the metals and
ligand. Moreover, the molar ratio of the Zn/Co and Zn/Ni was
close the supposed value, which indicates that the synthesized
bimetallic ZIF meets expectations.
Transmission electron microscopy (TEM) was employed for
investigating the morphology of the g-C3N4 embedded with
ZnM-ZIF and/or MoS2. The dense nanosheet morphology of
pristine g-C3N4 could be observed in Fig. 1d, and compared with
ZIF-8 and ZnNi-ZIFs, the as-prepared ZnNi-ZIF exhibited regular
truncated rhombic dodecahedron structure as conrmed by the
TEM and scanning electron microscope (SEM) images (Fig. 1e
and S3a–c†). For the CNZN, the microstructure of g-C3N4
nanosheet enwrapped MOF blocks is clearly observable, proved
the formation of CNZN surface heterostructure (Fig. 1f), which
was further conrmed by the SEM images of g-C3N4–ZnM-ZIFs
as depicted in Fig. S3d–f.† The CNM was a 2D–2D surface het-
erostructure (Fig. 1g) as reported in the previous publication.50
Notably, the CNM nanosheet was distributed uniformly over the
surface of ZnNi-ZIF as demonstrated in Fig. 1h, and the (002)
lattice plane of MoS2 could be clearly observed from CNMZN
(Fig. 1i), which has an enlarged interlayer spacing (0.95 nm).
Moreover, the g-C3N4–MoS2–ZnM-ZIFs exhibited similar
morphology on SEM images (Fig. S3g–i†). Further adopted the
elemental mapping to investigate the distribution of different
components in the ternary CNMZN (Fig. S4†). The result
conrmed that all the C, N, Zn, Ni, S and Mo uniformly
dispersed. Further verifying both the MoS2 and g-C3N4 distrib-
uted evenly on the skeleton of ZnNi-ZIF.
The visible light absorption and the bandgap of the as-
prepared g-C3N4-based heterostructures were investigated via
the UV-Vis diffuse absorption spectra (Fig. 2a). Aer embedding
ZnM-ZIFs, the absorption of the g-C3N4 in the visible light
region has a little enhancement (Fig. S5†), while for the g-C3N4–
MoS2, the absorption rate is signicantly improved (Fig. S6†). It
is worth noting that the co-loading of the MoS2 and ZnM-ZIF
exhibited a synergistic effect for the absorption of visible
light, and the absorption intensity in the visible light region was
in the order of CNMZC > CNMZN > ZNMZ. The bandgaps of g-
C3N4 and MoS2 calculated according to the Kubelka–Munk (KM)
method are 2.74 and 1.67 eV (Fig. 2b).52,53 As displayed in Fig. 2c,
simultaneous doping of Co and Ni could increase the visible
light absorption capacity of ZIF-8, as further conrmed by the
bandgaps for ZIF-8, ZnCo-ZIF and ZnNi-ZIF of 5.1, 2.3 and 2.0,
respectively (Fig. 2d). To determine the band structure
Fig. 2 The UV-Vis absorption spectra and (ahn)2 versus hn curves of (a
and b) g-C3N4 loaded with MoS2 and/or ZnM-ZIF, and (c and d) ZnM-
ZIFs. The (e) Mott–Schottky plots of the g-C3N4, MoS2 and the ZnM-
ZIF, and (f) the band structure matching in the ternary g-C3N4–MoS2–
ZnNi-ZIF heterostructure.
matching and charge separation mechanism among g-C3N4,
MoS2, and ZnM-ZIFs, the Mott–Schottky plot measurements
were used to investigate the at band potential of them. As
shown in Fig. 2e, the ZIF-8, ZnCo-ZIF, and ZnNi-ZIF exhibited
at-band potentials of ꢀ0.89, ꢀ1.24 and ꢀ1.28 V vs. Ag/AgCl,
respectively, which were lower than that of the pristine g-C3N4
(ꢀ1.46 V), while higher than that of MoS2 (ꢀ0.19 V). Therefore,
the band structure matching in the ternary g-C3N4–MoS2–ZnNi-
ZIF heterostructure could be inferred in Fig. 2f. This band
structure indicated the transfer of photo-generated electron
from the conduction band (CB) of g-C3N4 and ZnNi-ZIFs to that
of MoS2. On the other hand, the holes were transferred into the
valence band (VB) of ZnNi-ZIF, where the H2O2 was preferen-
tially formed efficiently owing to the highly porous morphology
of ZnNi-ZIF. The band structure matching of the g-C3N4–ZnNi–
MoS2 not only facilitated the separation and utilization effi-
ciency, but also improved the visible light absorption between g-
C3N4, ZnNi-ZIF and MoS2.
Notably, in sharply contrast to the negligible photocatalytic
hydrogen production activity of ZIF-8 because of its large
bandgap (5.1 eV), the bare bimetallic ZnM-ZIFs exhibited
sizable increase in hydrogen evolution activity as displayed in
Fig. 3a. The optimal photocatalytic HER performance occurs in
the Zn0.5Ni0.5-ZIF, achieved 0.92 mmol hꢀ1 under UV-Vis light
irradiation, which could be comparable to that of g-C3N4
(0.71 mmol hꢀ1). Moreover, the photocatalytic HER performance
of Zn0.5Co0.5-ZIF slightly lower than that of the g-C3N4, which
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MoS2 showed obvious synergistic effect for increasing the
photocatalytic HER activity. Specically, the CNZN and the
CNM exhibited photocatalytic HER performance of 5.7 and 64.2
times larger than that of pristine g-C3N4, indicating the ZnNi-
ZIF and MoS2 played an important role on improving the
number of exposed active sites and utilization efficiency of the
charge carriers, respectively. It was worth noting that the pho-
tocatalytic hydrogen production rate of ternary CNMZN,
CNMZC and CNMZ was 3.7, 1.9 and 1.2-fold in comparison with
sum of photocatalytic hydrogen production of CNM and the
corresponding g-C3N4–ZnM-ZIFs (Fig. S10†), which conrmed
the synergistic effect of the embedding of MoS2 and ZnM-ZIF
and facilitate the transportation and utilization efficiency of
photo-generated charge carriers.
The large specic surface area, caused by improved number
of exposed active sites, was also an important element for the
high photocatalytic activity of CNMZN,5,54 and the nitrogen
adsorption/desorption isotherms (Fig. 3c) was employed to
reveal the vital function of ZnNi-ZIF on increasing the porosity
of the ternary heterostructure. Adopt MoS2 could increase the
specic surface area (SBET) of pure g-C3N4 to a certain extent.
More obviously, CNMZN exhibited much larger nitrogen uptake
Fig. 3 The photocatalytic H2-production activities of (a) ZnM-ZIF with
different molar ratio of Zn to metals. The comparison results of (b) the
photocatalytic hydrogen production activity of the pristine (A) g-C3N4,
(B) ZnNi-ZIF, (C) g-C3N4–ZnNi-ZIF, (D) g-C3N4–MoS2 and (E) ternary
g-C3N4–MoS2–ZnNi-ZIF under UV-Vis and visible light irradiation. (c)
Nitrogen adsorption desorption curve of g-C3N4, g-C3N4–MoS2,
ZnNi-ZIF and the optimal ternary heterostructure. (d) Photocatalytic
H2-production stability of the ternary g-C3N4–MoS2–ZnM-ZIFs. (e)
Comparison results of the photocatalytic activity of the g-C3N4–
MoS2–ZnNi-ZIF with the previously reported g-C3N4 (CN) and MOFs-
based heterostructures.
and SBET of 142.73 m2 gꢀ1 than that of CNM (33.89 m2 gꢀ1
)
owing to the highly porous ZnNi-ZIF (501.34 m2 gꢀ1). In addi-
tion to the excellent photocatalytic hydrogen production
activity, the CNMZN and ZNMZC exhibited very good photo-
catalytic stability aer 30 h continuous irradiation (Fig. 3d). In
comparison with the pristine ZnM-ZIFs and their hetero-
structures, the characteristic FTIR peaks of ZnM-ZIF could also
be clearly observed in the ZnNi-ZIF and CNMZN heterostructure
aer the photocatalytic reaction, which further indicated the
excellent photocatalytic stability of the ternary CNMZN hetero-
structure (Fig. S11a†), and the unchanged FTIR spectra of
CNMZ as well as CNMZC were also displayed in Fig. S11b.† To
further verify the improved transport, separation and utilization
efficiency of the photo-generated charge carrier in g-C3N4
caused by the adoption of MoS2 and ZnM-ZIFs, the photo-
catalytic hydrogen production performance of the optimal
CNMZN (30.1 mmol hꢀ1) was compared with the photocatalytic
hydrogen production activity of g-C3N4, or MOF-based photo-
catalysts in previous publication under visible light irradiation.
As displayed in Fig. 3e, the optimal CNMZN exhibited very
competitive photocatalytic activity in comparison with those of
semiconductor-MOFs heterostructures,2,14,35,55–57 g-C3N4-loaded
with different types of cocatalysts,9–11,22,23,54 and g-C3N4-based
heterostructures.9,58
was 0.51 mmol hꢀ1, which. The photocatalytic HER performance
of the CNZN with different weight ratio of ZnNi-ZIF (Zn : Ni ¼
1 : 1) were compared in Fig. S7,† the optimal HER performance
was obtained in the heterostructure sample containing 10 wt%
ZnNi-ZIF. Notably, the further adoption of the MoS2 (optimal
dosage of 6 wt%, Fig. S8†) cocatalyst remarkably improved the
photocatalytic HER activity under UV-Vis (Fig. S9a†) and visible
light irradiation (Fig. S9b†).
In order to better evaluate the important role of the MoS2 as
well as the ZnM-ZIF, the photocatalytic hydrogen production
activity is improved aer embedded g-C3N4 with MoS2 and then
with ZnM-ZIF. The systematic comparison of photocatalytic
hydrogen production activity g-C3N4, g-C3N4–MoS2, ZnM-ZIF is
showed in Fig. 3b and S10.† Under UV-Vis light irradiation, the
pristine g-C3N4 and ZnNi-ZIF exhibited very limited hydrogen
production rate of only 0.71 and 0.92 mmol hꢀ1, respectively,
and the adoption of MoS2 and ZnNi-ZIF both dramatically
increased the hydrogen production rate of g-C3N4 due to the
utilization and transport efficiency improving of the charge
carriers, respectively. Co-embedding of ZnNi-ZIF and MoS2
enhanced the photocatalytic activity of 124.02 mmol hꢀ1, which
was 167.4 times larger than that of the pristine g-C3N4, and was
34.9 and 4.6-fold of that of the g-C3N4 singly loaded with ZnNi-
ZIF and MoS2, respectively. Hence, the photocatalytic hydrogen
production results proved that the embedding of ZnNi-ZIF and
The optimal g-C3N4–MoS2–ZnNi-ZIF heterostructure exhibi-
ted an excellent apparent quantum yield (AQY) value of 22.0% at
420 nm. The AQY and photocatalytic hydrogen production rate
of optimal g-C3N4–MoS2–ZnNi-ZIF under different specic
wavelength illuminations was evaluated and exhibited (Fig. 4a
and Table S1†) with the irradiation wavelengths of 420, 475, 550
and 650 (ꢂ8) nm, the HER values 18.18, 9.22, 2.76 and 1.42
mmol hꢀ1 (Table S1†), respectively, which indicated the HER of
the optimal g-C3N4–MoS2–ZnNi-ZIF (CNMZN) photocatalyst was
associated with its UV-visible diffuse reection spectra, and the
electron-promoted photocatalytic hydrogen evolution reaction.
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ternary heterostructures exhibited considerable H2-evolution
performance of 26.7, 36.6 and 77.8 mmol hꢀ1 gꢀ1 under UV-Vis
light irradiation with the considerable H2O2 production rate
(Fig. 4d). For deeply investigate the formation process of H2O2,
using 5,5-dimethyl-1-pyrroline N-oxide (DMPO) as the spin
trapping agent, Electron Paramagnetic Resonance (EPR)
responses were adopted to test the oxidation products during
the photocatalytic reaction in pure water of CNMZN. As
exhibited in Fig. 4e, the characteristic signals of –OH could be
observed in the DMPO aqueous solution aer the visible light
irradiation, indicating the formation of –OH through singe
electron oxidation of water. On the base of the above-mentioned
band structure and photocatalytic activity results, the mecha-
nism of action of photo-exited charge carriers in the CNMZN
photocatalyst was proposed in Fig. 4f.
The photo-induced electrons transferred from the CB of g-
C3N4 and ZnNi-ZIFs to that of MoS2, being benecial for the
photocatalytic hydrogen production owing to the low HER
overpotential and large current density of MoS2. Simulta-
neously, the photo-exited holes were transferred into the VB of
ZnNi-ZIF, where the holes could be timely consumed by sacri-
cial agent due to the highly porous morphology of the ZnNi-
ZIF. For investigating the important role of ZnM-ZIF and/or
MoS2 on suppressing the recombination of the photo-
generated electron–hole pairs of the g-C3N4 based hetero-
structure, the room temperature photoluminescence (PL)
spectra excited at 360 nm (Fig. 5a) was employed. In contrast to
the very strong PL peak of the pristine g-C3N4 (around 450 nm),
the dramatically quenched PL signals were observed against the
g-C3N4–ZnM-ZIF (M ¼ Co, Cu, Ni) and CNM (Fig. S12†), owing to
Fig. 4 (a) Wavelength dependence of the apparent quantum effi-
ciencies for the ternary g-C3N4–MoS2–ZnNi-ZIF heterostructures. (b)
Photocatalytic H2-evolution rate of the optimal g-C3N4–MoS2–ZnNi-
ZIF under different atmosphere in TEOA mixture solution. (c) Photo-
catalytic hydrogen and (d) H2O2 production activity of the g-C3N4–
MoS2–ZnNi-ZIF in pure water under UV-Vis light irradiation. (e) EPR
responses of the DMPO–OH spin adduct in g-C3N4–MoS2–ZnNi-ZIF.
(f) Possible separation and utilization mechanism of the photo-
generated charge carriers in the optimal g-C3N4–MoS2–ZnNi-ZIF
heterostructure.
Different wavelengths further assured the photocatalytic activity
of the optimal g-C3N4–MoS2–ZnNi-ZIF heterostructure. The
considerable photocatalytic hydrogen production in optimal g-
C3N4–MoS2–ZnNi-ZIF even with the irradiation wavelengths of
650 (ꢂ8) nm also veried that g-C3N4–MoS2 not only acted as
the promoter of the charge carriers and high-performance
catalyst, but also facilitated the absorption capacity of visible
light. These results are consistent with the DRS and PL results of
the g-C3N4–MoS2 and ZnNi-ZIF-based heterostructure.
Under UV-Vis light irradiation the photocatalytic hydrogen
production of the optimal ternary CNMZN heterostructure was
evaluated at different atmosphere for comprehensively assess-
ing its practical applications value (Fig. 4b). The photocatalytic
HER activity of the optimal ternary heterostructure under N2
atmosphere decreased slightly due to the easier hydrogen
release under vacuum. It should be noted that even in air
atmosphere with oxygen facilitating the backward reaction,13
the optimal CNMZN exhibited considerably photocatalytic
hydrogen evolution activity, this result revealed that the
CNMZN ternary heterostructure was suitable for practical
applications.
Impressively, as demonstrated in Fig. 4c, sharp contrast to
the very weak hydrogen evolution of the g-C3N4–MoS2 without
sacricial agent (pure water), the CNMZ, CNMZC and CNMZN
Fig. 5 Effect of different type ZnM-ZIF and the adoption of MoS2 on
(a) the PL emission spectra, (b) transient photocurrent responses, (c)
EIS spectra, (d) the Mott–Schottky plots, and electrocatalytic (e) HER
and (f) OER response of the g-C3N4-based heterostructures.
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the increased porosity and separation as well as utilization HER and OER capacity of the g-C3N4-based heterostructure with
efficiency of the charge carriers, respectively and the relative MoS2 and/or ZnM-ZIF were evaluated using HER and OER
order of PL peaks for g-C3N4–ZnM-ZIF samples was found to be; measurements. As could be observed from Fig. 5e and f, the
g-C3N4 > g-C3N4–ZnCo > g-C3N4–ZnCu > g-C3N4–ZnNi. Notably, embedding of both MoS2 and ZnNi-ZIF could dramatically
the ternary samples, co-embedded with ZnM-ZIF and MoS2, promote the electrocatalytic HER and OER performance, and
showed very weak PL peaks, and the relative order of PL the co-embedding of MoS2 and ZnM-ZIF over g-C3N4 further
response intensity for the samples with different types of ZnM- facilitated the electrocatalytic HER and OER performance, and
ZIF was found to be: CNMZ > CNMZC > CNMZN, which was the ternary CNMZN displayed the smallest HER and OER
consistent with the trend of the photocatalytic hydrogen overpotential, and the largest current density, being consistent
evolution activity.
with the photocatalytic HER performance of the g-C3N4–MoS2–
The g-C3N4 based photocatalysts were investigated through ZnM-ZIFs.
the transient photocurrent and electrochemical impedance
spectroscopy (EIS) measurements to explore the transport and
Conclusions
excitation behavior of the photo-exited charge carriers. As
shown in Fig. 5b, with the repeated on/off cycles of UV-Vis light
irradiation, all the g-C3N4 based samples exhibited the steady
and reproducible photocurrent responses. The rapid recombi-
nation of charge carriers made the photocurrent density of the
pristine g-C3N4 very weak. Both the g-C3N4–ZnM-ZIF (Fig. S13†)
and CNM demonstrated stronger photocurrent response owing
to the increased specic surface area and separation as well
rapid consumed electrons, respectively. The relative order for
the photocurrent density for the g-C3N4–ZnM-ZIF samples was
found to be; g-C3N4 > g-C3N4–ZnCo > g-C3N4–ZnCu > g-C3N4–
ZnNi, which showed that aer incorporation of ZnM-ZIF (M ¼
Cu, Co, Ni) the photocurrent density increase that was consis-
tent with photocatalytic HER activity. Impressively, for the
ternary heterostructure, the co-loading of ZnM-ZIF and MoS2
exhibited the synergistic effect for facilitating the photocurrent
response, and the value of the photo-current for the ternary
heterostructures with different type of ZnM-ZIF was consistent
with the photocatalytic HER activity.
The EIS results of the g-C3N4, ZnM-ZIF g-C3N4–ZnM-ZIF and
g-C3N4–MoS2–ZnM-ZIF were exhibited in Fig. 5c, the ternary g-
C3N4–MoS2–ZnM-ZIF displayed much smaller semicircle radius
than that of and pristine g-C3N4, ZnNi-ZIF, g-C3N4–ZnCo-ZIF
and g-C3N4–ZnCu-ZIF (Fig. S14†), indicating that the interfa-
cial transfer of the charge carriers was dramatically enhanced as
veried by the smaller arc radius, which is normally the
evidence of a lower electron transfer resistance. Notably, the
CNMZN owning the optimal photocatalytic HER performance
showed the smallest arc radius. In addition, the hydrogen
evolution behaviors were evaluated by the Mott–Schottky plots
and electrocatalytic HER and OER measurements. As depicted
in Fig. 5d, the CNM displayed a more negative at-band
potentials than that of the pristine g-C3N4, indicating its
improved reduction capacity. Notably, aer the embedding of
ZnM-ZIF, the sample exhibited higher reduction capacity as
conrmed by their further upward shi in the at-band
potential, and the CNMZN with the optimal photocatalytic
activity possessed the most negative at-band potential. It was
extensively reported that the cocatalyst with low electrocatalytic
HER and/or oxygen evolution reaction (OER) overpotential and
high current density could boost the utilization efficiency of
charge carriers, thereby improve the photocatalytic activity.
Moreover, the HER and OER cocatalyst could play a synergistic
effect by co-loading to enhance the photocatalytic activity.59 The
Herein, we have successfully designed and fabricated various
bimetallic zeolite imidazole framework (ZnM-ZIF, M ¼ Co, Ni)
as the photocatalytic hydrogen production catalyst with
considerable photocatalytic HER activity. Furthermore, photo-
catalytic HER activity of zeolite imidazole framework (ZnM-
ZIF, M ¼ Co, Ni) was enhanced by the introduction of g-C3N4–
MoS2 2D–2D heterostructure. g-C3N4–MoS2 2D–2D hetero-
structure facilitate the separation as well as utilization efficiency
of the photo-generated charge carriers due to the lower HER
at-band potential of MoS2. ZnM-MOF not only inhibited the
aggregation of the CNM heterostructure, but also further
improved the separation and transport efficiency of the charge
carriers in the ternary g-C3N4–MoS2–ZnM-ZIF. Consequently,
the optimal g-C3N4–MoS2–ZnNi-ZIF exhibited the extraordinary
photocatalytic HER activity, which is 214.4, 37.5, and 3.7 times
larger than that of the pristine g-C3N4, g-C3N4–ZnNi-ZIF and g-
C3N4–MoS2, respectively, and exhibited a H2-evolution perfor-
mance of 77.8 mmol hꢀ1 gꢀ1 under UV-Vis light irradiation
coupled with oxidation of H2O into H2O2. This work would
furnish an innovative path for MOF candidate and open a new
method for the better utilization of porous MOF-based hetero-
structure for photocatalyst.
Conflicts of interest
There are no conicts to declare.
Acknowledgements
This work was nancially supported by the National Natural
Science Foundation of China (NSFC) (No. 22071081, 21601063),
and the Opening Project of Key Laboratory of Inorganic Func-
tional Materials and Devices, Chinese Academy of Sciences
(Grant No. KLIFMD202007).
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