Dual-Functionalized Mixed Keggin- and Lindqvist-Type Cu24-Based POM@MOF for Visible-Light-Driven H2 and O2 Evolution

Article abstract of DOI:10.1021/acs.inorgchem.9b00206

The development of logical visible-light-driven heterogeneous photosystems for water splitting is a subject of new research. As the first example of a noble-metal-free photocatalyst for both H₂ and O₂ production, a high-nuclear {Cu

Full text of DOI:10.1021/acs.inorgchem.9b00206

Article
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
pubs.acs.org/IC
Dual-Functionalized Mixed Keggin- and Lindqvist-Type Cu -Based
24
POM@MOF for Visible-Light-Driven H and O Evolution
2
2
Dongying Shi, Rui Zheng, Chun-Sen Liu,* Di-Ming Chen, Junwei Zhao, and Miao Du*,†
†
‡
,†
†
‡
†Henan Provincial Key Laboratory of Surface & Interface Science, Zhengzhou University of Light Industry, Zhengzhou 450002, P.
R. China
‡
Henan Key Laboratory of Polyoxometalate Chemistry, Henan University, Kaifeng 475004, P. R. China
*
S Supporting Information
ABSTRACT: The development of logical visible-light-driven hetero-
geneous photosystems for water splitting is a subject of new research.
As the first example of a noble-metal-free photocatalyst for both H
2
I
and O2 production, a high-nuclear {Cu (μ -Cl) (μ -Cl) }-based
24
3
8
4
6
polyoxometalate (POM)@metal−organic framework (MOF)
ZZULI-1) is rationally designed to serve as a robust dual-
functionalized photocatalyst. ZZULI-1 exhibits highly efficient photo-
(
−
1
−1
catalytic H evolution (6614 μmol g h ) and O evolution (1032
2
2
−
1
I
μmol g calculated for the first 6 min). The {Cu (μ -Cl) (μ -Cl) }
24
3
8
4
6
clusters and mixed POMs not only work as the active units for H and
2
O production, respectively, but also improve the effective electron
2
transfer between the photosensitizer and ZZULI-1. The highly stable
dual-functionalized ZZULI-1 affords new penetrations into the
development of cost-effective high-nuclear cluster-based POM@MOFs for efficient solar-to-fuel generation.
7
INTRODUCTION
application for light-induced processes recently. In our
previous work, we have demonstrated semiconductive
■
Under the double pressure of the energy crisis and environ-
mental pollution, sustainable clean energy is needed
imminently to solve the fuel problem. Photocatalytic water
splitting has been widely researched because it affords an
ecofriendly approach to the generation of H and O , which
could be applied as renewable energy sources. The
decomposition of water contains two half-reactions, known
as water oxidation (producing O ) and water reduction
I
Cu X -based MOFs (X = Cl, Br, or I) for efficient
photocatalytic H evolution without cocatalysts and photo-
sensitizers. The {Cu X } units of Cu-X-bpy work as
photoelectron producers to greatly improve the H generation
activity, offering suitable reaction locations for H production.
If the incorporation of functional oxidative blocks into Cu X -
based MOF materials, {Cu X } clusters, and oxidative units of
m n
2
2
1
2
I
2 2
2
2
2
8
2
2
I
m n
I
2
(
generating H ), and it is inhibited by water oxidation because
modified MOFs would work synergistically to provide dual
functions, then they might be efficient photocatalysts for H2
2
3
of its four-electron and four-proton-transfer processes. As a
rising kind of porous species, metal−organic frameworks
and O evolution. As a huge class of nanosized inorganic units
2
(
MOFs) afford a diversified platform for the development of
with oxygen-rich surfaces, polyoxometalates (POMs) can easily
perform multielectron-transfer courses, therefore exhibiting
water-splitting materials by advisable choices of bridging units
4
9
and metal nodes. In comparison with photocatalytic water
reduction, less research of MOFs has been explored for water
oxidation and very few studies have investigated potential
heterogeneous photocatalysts for both H and O production.
Therefore, the development of dual-functionalized MOF-
remarkable candidates as water oxidation photocatalysts.
I
Imbedding inorganic POMs into the pores of Cu X -based
m
n
MOFs is an excellent method to stabilize and heterogenize the
5
I
m
2
2
reductive {Cu X } clusters and oxidative POMs for photo-
n
catalytic H and O production. It was not until 2018 that
2
2
derived photocatalysts for both H and O generation and
2
2
comparatively cost-effective P W Co @MOF was reported for
2
18
4
the fact that the consequent composition has efficient fuel-
forming sustainability are the most demanding and challenging.
MOFs provide a promising configuration for the settling of
functional units to enhance the photocatalytic properties
because of their porous structures and wealthy chemical
10
only O evolution or Ni P @MOF and P W @UiO were
2
4
2
2
18
1
1
reported for only H evolution. However, to our knowledge,
2
no POM@MOF photosystems have been published in the
literature that could photocatalyze water reduction and water
oxidation, respectively (Table S2). Thus, the rational design of
6
I
m
functionality. Cu X -based (X = Cl, Br, or I) MOFs have
n
been as one of the most significant families of hybrid materials
1
0
because of their d electronic configuration and relatively
Received: January 27, 2019
intense luminescent behavior as well as their considerable
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.9b00206
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
photocatalytic systems for both H and O generation by dual-
functionalized POM@MOFs is of great significance.
temperature of 120 °C for 3 days. After the container was cooled to
ambient temperature, dark-green block crystals of ZZULI-1 were
obtained. Yield: ca. 56% based on TPB. EA and inductively coupled
2
2
Bearing all of these in mind, by incorporating the oxidative
8−
2−
plasma. Calcd for C726
H N O383Cl14W114Cu30: C, 20.85; H, 1.39;
504 192
[
W O ] and [W O ] polyoxoanions within the pores of
12 40 6 19
I
m
N, 7.24; Cu, 4.56; W, 50.12. Found: C, 20.94; H, 1.51; N, 7.16; Cu,
Cu X -based MOFs, in this work, we develop a new method
n
4
.49; W, 50.08.
to merge the photocatalytic water oxidation and water
reduction within a novel dual-functionalized POM@MOF
Photocatalytic H Evolution Experiments. Visible-light-driven
2
H2 generation experiments were completed in a 25 mL quartz
reaction tube. Different amounts of the dual-functionalized photo-
catalyst ZZULI-1, the sacrificial electron donor of triethanolamine
(
Scheme 1). We envision that the mixed Keggin- and
(
TEA) and the photosensitizer of fluorescein (Fl) in the mixed
Scheme 1. Dual-Functionalized Mixed Keggin- and
I
solvents of H O/CH CH OH [1:1 (v/v), 3 mL], were added into the
Lindqvist-Type {Cu (μ -Cl) (μ -Cl) }-Based POM@
2
3
2
24
3
8
4
6
reaction tube. The specific pH value of the photocatalytic system was
adjusted with a NaOH or HCl solution. After that, the reaction
solution was degassed by argon for 10 min. The solution was
irradiated using a 300 W xenon lamp light source at ambient
MOFs as Photocatalysts for Both H and O Evolution
2
2
temperature. The amount of H was investigated by a GC 7900
2
instrument using a 5 Å molecular sieve column (0.6 m × 3 mm) as
well as a thermal conductivity detector.
Photocatalytic O Evolution Experiments. Visible-light-driven
2
O generation experiments were similar to those of photocatalytic H
2
2
evolution. Different amounts of the dual-functionalized photocatalyst
ZZULI-1, the [Ru(bpy) ]Cl photosensitizer, and the Na S O
8
3
2
2
2
−2
electron acceptor in a sodium borate buffer (8.0 × 10 M, 10.0
mL) were added into the reaction flask. The solution was degassed by
argon for 10 min. Subsequently, the solution was irradiated by a light-
emitting-diode (LED) lamp (beam diameter = 2 cm; light intensity =
1
6 mW; λ ≥ 420 nm) at room temperature. The amount of O was
2
investigated by a GC 7900 instrument using a 5 Å molecular sieve
column (2 m × 3 mm) as well as a thermal conductivity detector.
RESULTS AND DISCUSSION
Crystal Structure and Characterizations. The dark-
green block crystals of ZZULI-1 were synthesized by the
■
Lindqvist-type POMs may work as oxidative photocatalysts
I
to generate O , whereas the {Cu (μ -Cl) (μ -Cl) } clusters of
2
24
3
8
4
6
the cationic framework can act as photoelectron generators to
hydrothermal reaction of (TBA) [W O ] and CuCl ·2H O
improve the activity of H generation.
4
10 32
2
2
2
with the ligands TPB and TPC in a yield of 56%. The
4
−
EXPERIMENTAL SECTION
[W10
O
32
]
precursors are unstable under solvothermal
■
conditions and favor rebuilding into steady Keggin- or
Materials and Methods. All chemical reagents were obtained
from the merchant channels and utilized without any treatment.
Elemental analysis (EA) experiments were performed with an
elemental analyzer (Flash EA 1112 series) after total combustion at
1
3
Lindqvist-type polyoxoanions. Among the hydrothermal
reaction, the TPC ligand will play a very important role in
the constitution of ZZULI-1 materials. PXRD and EA confirm
the pure phase of ZZULI-1. Single-crystal X-ray analysis
9
00 °C under an O atmosphere. Thermogravimetric analysis (TGA)
2
was measured by a PerkinElmer Pyris Diamond DTA/TG thermal
reveals that ZZULI-1 crystallizes in cubic space group Pm3
̅
m,
system under a N environment. Powder X-ray diffraction (PXRD)
I
2
consisting of 26-hedral {Cu (μ -Cl) (μ -Cl) } cages, mono-
2
4
3
8
4
6
diffractograms were obtained on a Rigaku D/Max-2500 X-ray
diffractometer using a graphite monochromator and a copper-target
tube. Solid-state IR spectra were obtained by a Bruker Tensor 27
spectrometer. Optical diffuse-reflectance spectra were recorded on a
Hitachi UH4150 spectrophotometer. UV−vis absorption spectra were
measured by a Hitachi UH4150 spectrophotometer. Photolumines-
cence spectra were obtained by a Cary Eclipse fluorescence
spectrophotometer (Agilent G9800A). Cyclic voltammetry (CV)
measurements were performed on a ModuLab XM electrochemical
system of the standard three-compartment cell. Transient photo-
current responses were obtained on a ModuLab XM electrochemical
system in a standard three-compartment cell. The elemental mappings
of energy-dispersive X-ray spectroscopy were determined by a JSM-
II
nuclear Cu ions as nodal points, and rigid TPB ligands as
8
−
connectors with two types of POMs, [W O ]
and
1
2
40
2−
[
W O ] , embedded (Figure S1). The asymmetry unit
6
19
comprises two crystallographically independent Cu ions. The
II
I
Cu center and Cu atom adopt five-coordinate square-
pyramidal and four-coordinate tetrahedral geometries, respec-
tively. Typically, 24 symmetry-equivalent Cu ions are ligated
I
I
24
by 8 μ -Cl and 6 μ -Cl to form a unique spherical {Cu (μ -
3
4
3
Cl) (μ -Cl) } cluster (Figure 1a).
8
4
6
I
More interestingly, the 26-hedral {Cu (μ -Cl) (μ -Cl) }
2
4
3
8
4
6
II
cores and mononuclear Cu nodes are connected by TPB
ligands, affording the cationic 3D framework (Figure 1c). The
7
001F field-emission scanning electron microscope.
8−
2−
S y n t h e s i s o f H 4 8 [ C u I ( μ - C l ) ( μ - C l ) ] -
12 40
6
19
oxidative [W O ] /[W O ] polyoxoanions are further
2 4
3
8
4
6
II
(
TPB)24[Cu (CH OH)] [W
O
12 40
] [W O ] (ZZULI-1; CCDC
19 3
12
embedded in the cationic pores of cage-based MOFs via
electrostatic interaction to produce a dual-functionalized
POM@MOF with a 10.0 Å × 8.0 Å channel (Figure 1b,d),
which remarkably has not been reported in the POM@MOF
research field. Because copper(I) halide aggregates have
received extensive attention in photocatalytic water reduc-
3
6
8
6
1
833731). A mixture of (TBA) [W O ] (TBA = tetrabutylam-
4 10 32
monium; 33.2 mg, 0.01 mmol), 5′-(4-(4H-1,2,4-triazol-4-yl)phenyl)-
′′-(4H-1,2,4-triazol-4-yl)-[1,1′:3′,1′′-terpheny]-4-carboxylic acid
TPC;12.1 mg, 0.025 mmol), 1,3,5-tris(3-(1,3,4-triazol-1-yl)phenyl)-
benzene (TPB; 2.5 mg, 0.005 mmol), and CuCl ·2H O (34.1 mg,
4
(
2
2
0
.20 mmol) in mixed solvents of N,N-dimethylformamide (DMF; 2.0
8
,14
I
tion,
the coexistence of both reductive {Cu (μ -Cl) (μ -
mL), methanol (0.25 mL), and acetonitrile (4.0 mL) was stirred and
adjusted to pH = 2.3 using 1.0 mol L− HCl. The above suspension
was placed in a 25 mL reaction container and kept at a reaction
24 3 8 4
1
Cl) } clusters and oxidative POM polyoxoanions would
contribute to a dual oxidation−reduction POM@MOF,
6
B
DOI: 10.1021/acs.inorgchem.9b00206
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Figure 2. Confocal laser scanning microscopy images of empty (a and
b) and soaked (c−l) Fl molecules. Bright-field images (a and c) and
confocal micrographs (b and d−l) were explored at λ = 510−610
em
nm and excited by λ = 488 nm. The confocal images of parts d−l
ex
show nine slices from the top down.
result reveals that ZZULI-1 can be used as an ideal
semiconductive POM@MOF and that such a low-band-gap
19
Eg value is very rare in MOF materials.
Photocatalytic H Evolution. Photocatalytic tests of dual-
Figure 1. Crystal structure of ZZULI-1: (a) {CuI (μ -Cl) (μ -Cl) }
2
2
4
3
8
4
6
functionalized ZZULI-1 for H evolution were investigated
2
cage with an inner diameter of 8.2 Å. (b) 3D framework formed by
CuI (μ -Cl) (μ -Cl) } units, Cu nodes, and TPB linkers with two
II
under visible-light irradiation with TEA as the sacrificial factor
and Fl as the photosensitizer. Under optimized conditions,
{
24 3 8 4 6
types of POMs embedded. (c) Octahedral cage generated from six
{
CuI (μ -Cl) (μ -Cl) } clusters. (d) Connolly surface diagrams
2
2
ZZULI-1 is highly active for H production, with a rate of H
24
3
8
4
6
−
1
−1
showing the 1D channels.
evolution of 6614 μmol g
h
(Figure 4). It should be noted
that no detectable H was detected in the absence of ZZULI-1
2
or in the dark under the same experimental conditions,
indicating that ZZULI-1 plays a very important role in
which is promisingly designed for photocatalytic H and O
2
2
generation.
photocatalytic H generation. Furthermore, the stability of
2
Confocal fluorescence microscopy has been well applied in
the bioimaging field. It can offer a new approach to
investigating thick porous MOF materials because it provides
ZZULI-1 for H evolution could be confirmed by recycling
2
experiments. Solids of ZZULI-1 were separated from the
suspension by simple centrifugation and recycled three times
15
−1 −1
the benefit of increased infiltration depth (>500 mm).
(from 6614 to 6607 μmol g
h
of yield after recycling three
Evaluation of the guest-accessible volume in porous MOFs can
be strictly tested by using a confocal fluorescence microscope
with a tool probe of luminescent dyes with large sizes. Dye
uptake research was tested by soaking ZZULI-1 with a
methanol solution of Fl. The confocal laser scanning
microscopy images (Figure 2) exhibit obvious green
times; Figure 3d). The PXRD patterns and Raman spectra of
ZZULI-1 before and after the H evolution reaction suggest
2
that the crystallinity was well-maintained (Figures 3c and S20).
Heretofore, the high catalytic efficiency of ZZULI-1 for
splitting water into H is one of the highest values among
2
20
MOFs, revealing the excellent advantage of dual-function-
alized ZZULI-1.
fluorescence (λ = 488 nm), which is assigned to the emission
ex
of Fl, confirming the successful uptake of Fl dyes into the pores
Control experiments were investigated to provide insights
for the catalytic mechanism and assess the contribution of
16
of ZZULI-1 materials. Moreover, the very regular distribu-
tion of Fl dyes over the entire crystal confirms that Fl
molecules penetrate deeply into the channels rather than
adhering to the external surface of ZZULI-1. Taking no
account of the guest solvent molecules, the effective guest-
accessible volume of ZZULI-1 was evaluated as 48.3% by
I
{Cu (μ -Cl) (μ -Cl) } clusters and the outer coordination
2
4
3
8
4
6
sphere for H evolution. The H evolution efficiencies of
2
2
CuCl , TPB, POMs, and CuCl + TPB + POMs were tested
2
2
under the same measured conditions as those of ZZULI-1.
The order of the catalytic photoactivity is as follows: TPB <
CuCl ≈ POMs ≈ CuCl + TPB + POMs ≪ ZZULI-1. The
1
7
PLATON software.
2
2
Compared with those of the free POMs and TPB, the UV−
vis spectrum of ZZULI-1 reveals that the adsorption band of
ZZULI-1 exhibits a significant red shift (Figure 3a). This is
possibly related to the impression of coordinated Cu centers
on the excited state of TPB, d−d electronic transitions of Cu
atoms, π−π* interaction of TPB, and ligand-to-metal charge
ligand TPB will be completely inactive because of the lack of
photoactive sites. Neither CuCl nor POMs nor CuCl + TPB
2
2
+ POMs shows meaningful amounts of H production. The
2
importance of the distinct 3D framework of ZZULI-1 is
confirmed by a comparison of CuCl + TPB + POMs with
2
I
2
ZZULI-1. The ordered structural arrangement of {Cu (μ -
4
3
1
8
transfer. The solid-state diffuse-reflectance spectroscopy and
Kubelka−Munk representation of ZZULI-1 were also
Cl) (μ -Cl) } clusters in ZZULI-1 may profit from electron
8 4 6
21
and energy transfer over a long distance. The structural
characteristics would promote the rapid diversion of photo-
generated charges from the excited-state photosensitizer and
investigated (Figure S9). The band-gap energy (E ) of
g
ZZULI-1 determined from the Tauc plot is 1.90 eV. The
C
DOI: 10.1021/acs.inorgchem.9b00206
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Figure 3. (a) Solid-state UV−vis absorption spectra of ZZULI-1. (b) Transient photocurrent profiles of ZZULI-1. (c) PXRD patterns of ZZULI-1.
d) Durability testing for H evolution over ZZULI-1, which was recycled from the reaction solution and used again under the same catalytic
(
2
conditions three times.
(
−0.96 V; vs Ag/AgCl), is competent of reducing protons to
H . The third quasi-reversible redox couple (E = −0.45 V and
2
pc
E = −0.38 V) is probably related to the ligand and metal. The
pa
proton likely bonds to the opened Cu sites to form copper
22
hydride species for the release of H2.
The transient photocurrent−time experiments under visible-
light irradiation illustrate that the electrode modified by
ZZULI-1, ZZULI-1 + Fl, or ZZULI-1 + [Ru(bpy) ]Cl
3
2
provides the minutiae on the efficient charge separation
Figure 3b). When the xenon lamp is turned on, the
(
photocurrent signal quickly rises to a certain value, whereas
when the xenon lamp is turned off, the photocurrent signal
rapidly returns to zero. Pristine ZZULI-1 has a small
photocurrent response (1.0 μA cm ) in a KCl solution,
which evidences the charge transport occurring from ZZULI-1
to the glassy carbon electrode. This result is confirmed with
the relatively low photocatalytic activity without any photo-
sensitizer. However, strong photocurrent responses were
obtained with the electrode modified by ZZULI-1 + Fl (4.0
−
2
Figure 4. Kinetics of H evolution over ZZULI-1 in the photo-
2
catalytic system. Conditions: ZZULI-1 (3 mg), Fl (5 mg), and TEA
(
10% v/v); a mixed solvent of H O/CH CH OH [1:1 (v/v), 3 mL,
2 3 2
pH = 13.0]; with or without xenon-lamp irradiation. Inset: Proposed
mechanism for photocatalytic H evolution.
2
−
2
−2
μA cm ) or ZZULI-1 + [Ru(bpy) ]Cl (2.0 μA cm ). These
3
2
enhance the charge separation efficiency, which would figure
out the completely different activities proven by ZZULI-1 and
results are confirmed by the improvement of electron transfer
occurring from the excited state of [Ru(bpy) ]Cl or Fl to
3
2
CuCl + TPB + POMs.
ZZULI-1 and, subsequently, to the outside of the working
2
23
The CV experiment for ZZULI-1 was investigated in a KOH
solution at pH = 13.0 (Figure S10). ZZULI-1 undergoes
multiple redox processes, and the cyclic voltammogram shows
the first nearly irreversible redox couple at a midpoint potential
of −1.12 V (vs Ag/AgCl), which could be distributed to the
electrode. In addition, when ZZULI-1 was added to a
CH CH OH/H O [1:1 (v/v)] solution of Fl (10 μM),
3
2
2
emission quenching was observed (Figure S11). The
quenching can be assigned to the photoinduced charge
transport from the excited state of Fl to ZZULI-1, which
also offers the possible excited state of Fl to activated ZZULI-1
for proton reduction. The findings of transient photocurrent
tests and quenching experiments provide direct evidence of
charge transport from the excited state of [Ru(bpy) ]Cl or Fl
I
0
Cu /Cu couple. The second quasi-reversible reduction curve
occurs at E = −0.96 V, whereas the corresponding oxidation
pc
curve shifts to E = −0.93 V, which can be attributed to the
pa
II
I
Cu /Cu redox processes. Therefore, the reduction potential of
3
2
II
I
the excited state of Fl* can feasibly reduce Cu or Cu in
to ZZULI-1.
0
I
0
ZZULI-1 to Cu . Furthermore, Cu /Cu , with a more negative
On the basis of the results of transient photocurrent tests
and quenching experiments, a possible reaction mechanism is
reduction potential than reduction of the protons at pH = 13.0
D
DOI: 10.1021/acs.inorgchem.9b00206
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Inorganic Chemistry
Article
proposed (Figure 4, inset). Photogenerated electrons of Fl*
effectively transfer to ZZULI-1 under visible-light irradiation,
and then the conjugated packing and long-length order of TPB
conditions (Table S3, entry 5). No O will be generated in the
2
absence of light or without the catalyst of ZZULI-1 (Table S3,
entries 6 and 7). Furthermore, it could be found that the O2
evolution efficiency for (TBA) W O or Na W O ·H O is
I
transfer electrons to the coordinately unsaturated {Cu (μ -
2
4
3
2
6
19
6
12 39
2
24
Cl) (μ -Cl) } clusters, where H generation reactions occur.
significantly weaker than that of ZZULI-1 (Table S3, entries
8
4
6
2
+
·
The sacrificial agent TEA supplements the holes in Fl to
recover the excited state of Fl* to the ground state. These
1−3). It can be concluded that the high O evolution efficiency
2
is dominated by the synergistic catalysis between the two types
of POMs and the spacious environment of the channels in
ZZULI-1.
I
2
results highlight the great importance of {Cu (μ -Cl) (μ -
4
3
8
4
Cl) } clusters, functional ligands, coordination environments,
6
I
2
and regular solid-state structures. Notably, {Cu (μ -Cl) (μ -
Photocatalytic O2 evolution with [Ru(bpy) ]Cl as the
4
3
8
4
3
2
I
Cl) } clusters provide an excellent place to improve the Cu
photosensitizer and Na S O as the electron acceptor is
6
2 2 8
hydride interactions, which rapidly give H evolution and
recover the starting ZZULI-1.
Photocatalytic O2 Evolution. Photocatalytic tests of
demonstrated, which shows facile and reversible O−O bond
2
generation (Figure 5, inset). During the photocatalytic process,
2
+
two [Ru(bpy)3] are excited by visible light to generate two
2
+
2+
ZZULI-1 for O generation were tested in a borate buffer
excited states of [Ru*(bpy) ] , whereafter two [Ru*(bpy) ]
2
3
3
3
+
solution (pH = 8.0) with [Ru(bpy) ]Cl as the photosensitizer
are further photooxidized to form [Ru(bpy) ] by the electron
3
2
3
and Na S O as the electron acceptor. Dual-functionalized
acceptor of Na S O . The course occurs via Na S O
2
2
8
2
2
8
2
2
8
ZZULI-1 shows a wonderfully catalytic performance for O
quenching of the accostable metal-to-ligand charge-transfer
2
−
1
2+
26
generation with a rate of O generation of 1032 μmol g
excited state of [Ru*(bpy) ] , which has been well-proven.
2
3
At the same time, 2SO4• in situ formation rapidly oxidizes
−
calculated for the first 6 min (Figure 5). To confirm the nature
2
+
3+
two equivalent [Ru(bpy)3] to generate other [Ru(bpy) ] .
3
Subsequently, ZZULI-1 is oxidized to high oxidation states by
3
+
four [Ru(bpy) ] . During O generation, mixed POMs of
3
2
ZZULI-1 are used to promote effective charge transport from
3
+
the dual-functionalized ZZULI-1 to oxidized [Ru(bpy) ] ,
3
3
+
where the oxidized [Ru(bpy)3] are regenerated to the
starting state.
CONCLUSION
■
In a word, we have successfully demonstrated the conception
that photocatalytic H and O evolution over POM@MOFs
2
2
can be performed for the first time under visible-light
irradiation. ZZULI-1 exhibits highly efficient photocatalytic
H and O evolution. The high H and O evolution rate over
2
2
2
2
ZZULI-1 might be ascribed to its multiple active units derived
Figure 5. Kinetics of O2 generation over ZZULI-1 in the
from the excellent structural characteristics and its reduction−
photocatalytic system. Conditions: [Ru(bpy) ]Cl (7.7 mg),
I
2
3
2
oxidation behavior of {Cu (μ -Cl) (μ -Cl) } and two types of
4
3
8
4
6
ZZULI-1 (2 mg), and Na S O (11.9 mg); a sodium borate buffer
2
2
8
POM components, which results in effective charge transport
during photocatalytic reaction. It is noteworthy that this work
promotes a perfect insight for the design of highly efficient and
noble-metal-free photocatalysts that could functionally imitate
both photosystems I and II of bionic water-splitting photo-
catalysts.
−
2
(
8.0 × 10 M, 10 mL, pH = 8); with or without LED-lamp
irradiation. Inset: Proposed mechanism for photocatalytic O2
evolution.
of heterogeneous catalysis for ZZULI-1, the supernatant
solution (after separation of ZZULI-1) was carefully tested,
and filtration exhibits no detection of O evolution at the same
2
25
ASSOCIATED CONTENT
photocatalytic conditions (Table S3, entry 4). Furthermore,
ZZULI-1 was isolated and reused for visible-light-driven O2
evolution at least three times (Figure S21). The PXRD
patterns of ZZULI-1 isolated after three cycles are similar to
those of pristine ZZULI-1, which confirms that ZZULI-1 is
well-maintained before and after photocatalysis (Figure S22).
The photocatalytic activities of ZZULI-1 for water oxidation
mainly depend on the POMs embedded in the POM@MOF
materials, which is confirmed by the demand of injecting
multiple electrons.
■
* Supporting Information
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.9b00206.
Additional crystallographic data, structural details, and
experimental results, such as photocatalytic experiments,
durability testing, diffuse-reflectance spectrum, and
elemental mappings (PDF)
Photocatalytic O2 evolution over ZZULI-1 was further
Accession Codes
demonstrated by control experiments. To prove that O was
2
not produced from any other source such as a sodium borate
buffer and an oxygen impurity in the photocatalytic system, the
GC profiles of standard O , O evolution, and air were tested
CCDC 1833731 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
2
2
(Figure S18). Also, we have performed a controlled experiment
using dried acetonitrile instead of a borate buffer, which shows
no detection of O evolution at the same photocatalytic
2
E
DOI: 10.1021/acs.inorgchem.9b00206
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Li, D.-S.; Sun, C.; Feng, P.; Bu, X. Stable Bimetal-Organic
Hierarchical Nanostructures as High-Performance Electrocatalysts
for Oxygen Evolution Reaction. Angew. Chem., Int. Ed. 2019, 58,
AUTHOR INFORMATION
Corresponding Authors
E-mail: chunsenliu@zzuli.edu.cn.
E-mail: dumiao@zzuli.edu.cn.
ORCID
Chun-Sen Liu: 0000-0002-5095-7359
Junwei Zhao: 0000-0002-7685-1309
Miao Du: 0000-0002-1029-1820
■
*
4
(
227−4231.
4) (a) Zhang, T.; Lin, W. Metal-Organic Frameworks for Artificial
Photosynthesis and Photocatalysis. Chem. Soc. Rev. 2014, 43, 5982−
993. (b) Wu, H. B.; Xia, B. Y.; Yu, L.; Yu, X.-Y.; Lou, X. W. Porous
*
5
Molybdenum Carbide Nano-Octahedrons Synthesized via Confined
Carburization in Metal-Organic Frameworks for Efficient Hydrogen
Production. Nat. Commun. 2015, 6, 6512. (c) Zeng, L.; Guo, X.; He,
C.; Duan, C. Metal-Organic Frameworks: Versatile Materials for
Heterogeneous Photocatalysis. ACS Catal. 2016, 6, 7935−7947.
Notes
The authors declare no competing financial interest.
(
5) (a) Jiao, L.; Wang, Y.; Jiang, H.-L.; Xu, Q. Metal-Organic
Frameworks as Platforms for Catalytic Applications. Adv. Mater. 2018,
0, 1703663. (b) Wang, W.; Xu, X.; Zhou, W.; Shao, Z. Recent
ACKNOWLEDGMENTS
We are grateful for financial support from the National Natural
Science Foundation of China (Grants 21571158 and
1701147), the Key Research Project of University of Henan
Province (Grant 19zx004), and the Startup Fund for PhDs of
Natural Scientific Research of Zhengzhou University of Light
Industry (Grant 2016BSJJ026).
■
3
Progress in Metal-Organic Frameworks for Applications in Electro-
catalytic and Photocatalytic Water Splitting. Adv. Sci. 2017, 4,
2
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Zhang, X.; Whangbo, M.-H.; Huang, B. Ni Coordination to Al-Based
Metal-Organic Framework Made from 2-Aminoterephthalate for
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