Stable Layered Semiconductive Cu(I)-Organic Framework for Efficient Visible-Light-Driven Cr(VI) Reduction and H2 Evolution

Article abstract of DOI:10.1021/acs.inorgchem.8b01137

Metal-organic frameworks (MOFs) have gained tremendous attention in the fields of environmental restoration and sustainable energy for their potential use as photocatalyst. Herein, a new two-dimensional (2D) Cu(I)-based MOF material showing a narrow forbi

Full text of DOI:10.1021/acs.inorgchem.8b01137

Article
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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Stable Layered Semiconductive Cu(I)−Organic Framework for
Efficient Visible-Light-Driven Cr(VI) Reduction and H2 Evolution
Di-Ming Chen, Chun-Xiao Sun, Chun-Sen Liu,* and Miao Du*
College of Material and Chemical Engineering, Zhengzhou University of Light Industry, Zhengzhou 450002, P. R. China
S
* Supporting Information
ABSTRACT: Metal−organic frameworks (MOFs) have
gained tremendous attention in the fields of environmental
restoration and sustainable energy for their potential use as
photocatalyst. Herein, a new two-dimensional (2D) Cu(I)-
based MOF material showing a narrow forbidden-band of 2.13
eV was successfully constructed using a visible-light-harvesting
anthracene-based bipyridine ligand. The as-prepared MOF
demonstrates high chemical stability and could be stable in the
pH range 2−13, which is favorable for its potential application
in photocatalysis. Photocatalytic experiments demonstrate that
this Cu(I)-MOF exhibits high reactivity for reduction of Cr(VI)
in water, with 95% Cr(VI) converting to Cr(III) in 10 min by
using MeOH as scavenger under visible-light illumination.
Furthermore, this MOF could behave as a highly active photocatalyst for H2 evolution without additional photosensitizers and
cocatalyst. Remarkably, the as-prepared MOF shows enhanced photocatalytic Cr(VI) reduction and H2 evolution performances
compared with the pristine light-harvesting ligand under the same conditions. In connection to these, the photocatalytic
reaction mechanism has also been probed.
high energy density and renewability. It has been reported that
the photogenerated H2 evaluation is one of the most promising
approaches for the production of H2 from water.14−17
INTRODUCTION
■
With the fast evolution of industry, increasing concerns about
the energy crisis and environmental pollution have been faced
by people all over the world. With the purpose of solving the
increasing demand for sustainable energy and the accompanied
environmental pollutions of modern industry, great endeavors
have been dedicated to the developing new technology using
renewable solar light through photocatalysis.1−6 As a common
heavy metal pollutant in effluent generated by industrial
production such as leather-making, electroplating, and
chemical metallurgy, Cr(VI) is notorious for its nonbiode-
gradation and high toxicity.7−9 The transformation of Cr(VI)
to Cr(III) though reductive reactions is considered to be an
effective approach to eliminate Cr(VI) from wastewater
because Cr(III) is less soluble in water though formation of
the (hydr)oxides and shows less toxicity than Cr(VI).10 In
comparison with chemical reduction and electroreduction, the
visible-light-driven transformation of Cr(VI) to Cr(III) using
photocatalysts has more prospects, because it does not
generate additional harmful chemicals and is low-cost. A series
of semiconductors such as CdS, SnS2, and Ag2S has been used
as photocatalysts for reduction of Cr(VI).11−13 However, the
use of sulfide reagents in the photocatalytic process sometimes
is subjected to photocorrosion and may lead to the generation
of secondary pollutants. In this case, the search for stable
photocatalysts for effective Cr(VI) reduction is of high
importance. On the other hand, H2 is considered as a
promising chemical fuel for future application because of its
Composed of metal nodes and organic linkers, metal−
organic frameworks (MOFs) have emerged as a promising
kind of crystalline material due to their adjustable composi-
tions and designable topological networks.18 The rich selection
of light-harvesting building blocks makes this kind of material
an ideal platform for application in the artificial photosynthetic
systems.19 To date, several MOF-based photocatalysts have
been studied either in aqueous Cr(VI) reduction or H2
evolution such as NH2-MIL-88, NH2-MIL-100 (Fe), NH2-
MIL-125(Ti), and NH2-UiO-66 (Fe).20−24 For example, Wang
and co-workers have demonstrated the employment of the
NH2-MIL-125(Ti) as photocatalyst for effective reduction of
Cr(VI); Gascon et al. have prepared a noble-metal-free Co@
NH2-MIL-125(Ti) composite as recyclable catalyst for water
splitting under visible-light irradiation. However, to our
knowledge, no dual-functional MOF-based photocatalyst for
both Cr(VI) reduction and H2 evolution has been reported,
especially without the presence of cocatalyst and photo-
sensitizer in the reaction systems.
In this work, a new 2D Cu(I)-based MOF material showing
a narrow forbidden-band of 2.13 eV was successfully
constructed by employing a visible-light-harvesting anthra-
Received: April 25, 2018
© XXXX American Chemical Society
A
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Photocatalytic Experiments. The reduction of Cr(VI) was
conducted in a 100 mL quartz reactor at ambient temperature. Into a
quartz reactor was added a mixture of 40 mL of K2Cr2O7 aqueous
solution (10 ppm) and 15 mg of 1. The pH values of the solution
were modulated using 0.2 M NaOH and 0.2 M H2SO4. The mixture
was stirred, and then MeOH was introduced. After being mixed for 80
min to achieve the desorption−adsorption balance in the dark, the
quartz reactor was exposed to light illumination (Xe lamp) at room
temperature. During each interval, about 1.5 mL of solution was taken
from the quartz reactor, and the insoluble solid samples of 1 were
separated by centrifugation. The diphenylcarbazide method (DPC)
was used to get the absorbance of Cr(VI) in the solution.
cene-based bipyridine ligand. The as-prepared MOF demon-
strates high chemical stability which could be stable in the pH
range 2−13. Photocatalytic studies demonstrate that this
Cu(I)-based MOF exhibits high reactivity for reduction of
Cr(VI) in water, with 95% Cr(VI) converting to Cr(III) in 10
min by using MeOH as scavenger (pH = 3.09) under visible-
light irradiation. Furthermore, this MOF could behave as a
highly active photocatalyst for H2 evolution without adding
any photosensitizers and cocatalyst. In connection to these, the
possible photocatalytic mechanism has also been discussed.
Photocatalytic water splitting experiments were performed in a 25
mL quartz reaction flask. A 2 mg portion of 1 was added into a
mixture of ethanol, deionized water, and triethylamine. The pH value
of this suspension was modulated using HCl or NaOH and monitored
with a pH meter. The flask was sealed with a septum, and the
suspension was degassed by bubbling argon for 5 min. A 500 W Xe
lamp was used to irradiate the suspension. The GC 7890T instrument
was used to characterize the photoproduct of H2, and the external
standard method was used to determine the amount of H2.
EXPERIMENTAL SECTION
■
Chemicals and Testing Methods. All the reagents were bought
commercially and used as received. The Vario EL III Elementar
analyzer was used to get the elemental analysis results of the C, H, and
N content. The Rigaku model Ultima IV diffractometer with Cu Kα
radiation was employed to collect the PXRD curves at room
temperature. The TGA curve in the range 25−800 °C was obtained
on the Netzsch STA-409CD thermal analyzer. The Mott−Schottky
plots were measured on a three-electrode electrochemical workstation
(Solartron ModuLab XM Instruments). The luminescent spectra were
obtained from the fluorescence spectrophotometer (Cary Eclipse
Agilent). X-ray photoelectron spectroscopy (XPS) analysis was
conducted on the AXIS HIS 165 spectrometer (Kratos Analytical,
Manchester, UK).
Synthesis of [Cu2I2(BPEA)](DMF)4 (1). Into a Teflon vessel (25
mL) was added a mixture of BPEA (0.02 mmol, 7.6 mg), CuI (0.04
mmol, 7.6 mg), EtOH (2 mL), HCOOH (0.1 mL), DMF (3 mL),
and CH3CN (5 mL). The vessel was heated at 423 K for 48 h. After
slowly cooling the reaction system to ambient temperature, yellow
crystals for 1 were harvested, washed with DMF (2 × 3 mL), and
dried at room temperature. Yield: 32% (on the basis of the BPEA
ligand used). Anal. Found (Calcd) for C40H44Cu2I2N6O4: H, 4.56
(4.21); C, 45.62 (45.59); N, 7.49% (7.98%).
X-ray Crystallography. The X-ray single-crystal data were
measured on the Oxford SuperNova diffractometer using Cu Kα
radiation. The initial structure model was derived from the SHELXT
program and then refined using the SHELXL program based on the
least-squares technique.25 All non-H atoms were treated anisotropi-
cally, and all hydrogen atoms are generated geometrically. The lattice
DMF guests are highly disordered and difficult to locate in the
refinements, so their electronic contributions are eliminated by the
SQUEEZE manipulation in PLATON.26 Table 1 shows the further
details of structural refinement parameters of 1.
RESULTS AND DISCUSSION
■
Characterization of 1. Solvothermal reaction of CuI and
BPEA ligand in a mixed solvent of DMF/EtOH/CH3CN/
HCOOH generates the yellow crystalline products of 1 with
the formula [Cu2I2(BPEA)](DMF)4. An X-ray structural
analysis demonstrates that complex 1 locates in the monoclinic
I2/a space group with one I− ion, one Cu(I) ion, and half a
BPEA ligand in the building unit. The distorted tetrahedral
geometry of the four-coordinated Cu(I) ion is shaped by three
I− ions and one pyridyl N donor from the BPEA ligand (Figure
1a). The adjacent Cu(I) ions are connected with each other via
the μ3-bridge I− ions to shape a 1D ladder-like building unit
along the b axis, which is further extended into a 2D layered
structure by the BPEA ligand along the a axis (Figure 1b). The
2D layers are further stacked along the c axis to afford a 3D
supramolecular structure (Figure 1c,d). There is a weight loss
of 37.1% from 25 to 150 °C as revealed by the TGA curve,
which might be ascribed to the escape of four lattice DMF
molecules (Figure S1). The PXRD measurements were
conducted to validate the phase purity of 1, and they show a
good match between the measured diffraction peaks and
calculated ones from the crystal data, confirming the high
phase purity of 1.
Table 1. Crystalline Data and Refinement Parameters for 1
Considering that the following photocatalysis studies were
performed in water, the aqueous stability of 1 was checked
correspondingly. The PXRD measurements show that complex
1 not only could be stable in aqueous solutions with pH values
ranging from 2 to 13 but also could keep its framework
integrity in boiling water for 1 day, showing its high water
stability (Figure 2a and Figure S2). The optical absorbance of
1 was investigated by diffuse reflectance UV−vis analysis, and
the corresponding result is depicted in Figure 2b. MOF 1
demonstrates significant visible-light absorption ability, and the
absorption edge was found to be around 610 nm, which
indicates its potential use as visible-light photocatalysis.27 On
the basis of the equation of αhν = (hν − Eg)1/2, the calculated
band gap value of 1 is 2.13 eV, implying that 1 could show a
response to visible light (Figure 2c). In comparison, the BPEA
ligand shows visible-light absorption with the edge at about
570 nm, which is an obvious blue-shift compared to that of 1,
indicating a broadening band gap (2.3 eV estimated from the
UV−vis spectrum, Figure S3). The above results also
illuminate that the HOMO−LUMO energy gap is reduced
formula
fw
temp (K)
cryst syst
space group
a (Å)
b (Å)
c (Å)
β (deg)
V (Å3)
Z
Dc (g cm−3
μ (mm−1
C14H8CuIN
380.65
293(2)
monoclinic
I2/a
18.9903(7)
4.18100(10)
30.8429(12)
101.511(4)
2399.62(14)
8
)
2.107
22.539
0.0418
2123
)
Rint
reflns collected/unique
GOF on F2
R1, wR2 [I > 2σ(I)]
R1, wR2 (all data)
largest peak/hole (e Å−3
1.064
0.0378, 0.1037
0.0394, 0.1060
1.07/−1.08
)
B
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Figure 1. Views for (a) coordination surrounding the Cu(I) ion of 1, (b) the connection of the adjacent CuI chains via the BPEA ligand, (c) the
2D layered network of 1, and (d) the stacking of the 2D layers along the c axis.
Figure 2. (a) pH-dependent PXRD patterns for 1, (b) UV−vis spectrum of 1, (c) plot of (αhv)2 versus photon energy (hv) of 1, and (d) Mott−
Schottky plots of 1.
though incorporation of the organic ligand BPEA into the
framework of 1, which makes the photoinduced electron-
transfer (PET) process favorable.28,29 In order to evaluate the
semiconductor property of 1, Mott−Schottky plots were
measured in 0.2 M Na2SO4 aqueous solution with the pH
value of 6.8. Figure 2d displays that the flat-band potential of 1
derived from Mott−Schottky plots is −1.02 V versus the Ag/
AgCl electrode, corresponding to a redox potential of the
conduction band (CB) of −0.82 V versus normal hydrogen
electrode (NHE) according to the Nernst equation, and this is
more negative than the Cr(VI)/Cr(III) potential (0.51 V and
pH = 6.8).30 The above results indicate that 1 has the potential
for reduction of Cr(VI) to Cr(III). On the basis of the
forbidden-band calculation discussed above, the valence band
(VB) of 1 is 1.49 V vs NHE.
Photocatalytic Reduction of Cr(VI). The photocatalytic
performance toward Cr(VI) reduction over 1 has been studied
under visible-light conditions. Before the photocatalytic
reduction, the K2Cr2O7 solution was stirred in the dark for
ca. 80 min to realize the desorption−adsorption balance, and
the amount of Cr(VI) was monitored via the UV−vis
adsorption spectra. To explain the photocatalytic character of
C
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Figure 3. (a) Cr(VI) reduction reactions with different systems, (b) time-dependent adsorption spectra for DPC−Cr(VI) complex solutions, (c)
pH-dependent photocatalytic Cr(VI) reduction over 1, and (d) three runs of Cr(VI) reduction over 1 at pH 3.09.
Figure 4. (a) Comparison study of photocatalytic H2 evolution over 1, BPEA ligand, and CuI. (b) Three cycles of photocatalytic H2 evolution over
1.
the Cr(VI) reduction, control tests were first conducted. As
displayed in Figure 3a, it could be observed that the reduction
of Cr(VI) hardly reacts without the presence of photocatalyst
or light. In contrast, the amount of Cr(VI) is obviously
decreased with the presence of hole scavenger and photo-
catalyst under visible-light illumination. The reduction ratio of
Cr(VI) is 95% after visible illumination for 9 min, which is
more efficient than those shown by NNU-36, NH2-MIL-
125(Ti), and UiO-66(NH2) under similar conditions.20,27,31
The above results indicate that the Cr(VI) reduction reaction
is a light-dependent process in which the hole scavenger and
photocatalyst play vital roles. It has been reported that the pH
value of the solution could greatly influence the reduction rate
of aqueous Cr(VI) over photocatalyst, and in this case,
controlled experiments have been conducted under different
pH conditions.32−34 Figure 3b shows the time-dependent
photocatalytic Cr(VI) reduction over 1 at different pH values
adjusted by 0.2 M H2SO4. It could be found that the best pH
value is 3.09 in the controlled experiments. The observed
Cr(VI) reduction performance of 1 was further compared with
that of the BPEA ligand at the optimized condition. The results
of the time-dependent photocatalytic Cr(VI) reduction
reactions over the two systems are shown in Figure 3c. It is
obvious that the Cu(I)-MOF possesses much higher photo-
catalytic Cr(VI) reduction activity than the BPEA ligand.
From the economic point of view, a good photocatalyst
should have considerable stability and recycling. The durability
of the photocatalytic activity of 1 in the Cr(VI) reduction
process under visible-light illumination has been checked by
the recycling experiments. The used photocatalyst was
separated by filtration, washed with water, and used in the
next cycle of experiment. As shown in Figure 3d, the
photocatalytic activity of 1 shows no obvious decrease after
three cycles of experiments, suggesting the high stability of 1 in
the Cr(VI) reduction process. Furthermore, the PXRD
patterns of 1 after three cycles of experiments reveal that the
D
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Figure 5. (a) Photocurrent response for the bare electrode, BPEA ligand, and 1 in 0.1 M KCl (aq). (b) Possible mechanism of Cr(VI) reduction
and H2 evolution over 1 exposed to visible-light irradiation.
framework keeps its integrality (Figure S5). The ICP analysis
results demonstrate that there are no Cu(I) ions leaching from
1 during the reaction. In addition, the XPS results of the
samples reveal that the valence state of Cu ion remains
unchanged in the reduction process, and the weak peak located
at 576.2 eV could be assigned to the Cr 2p3/2, which is typically
attributed to Cr3+, illuminating the reduction of Cr(VI) which
occurs on the surface of 1 (Figures S6 and S7).35 These results
illuminate that both reusability and stability could be observed
for 1 in this photocatalytic reaction.
(Figure 5a). When the light is turned on, 1 presents an
obviously enhanced photocurrent intensity compared with that
of the BPEA ligand. The average photocurrent intensity of 1 is
about 4 times higher than that of the BPEA ligand. The higher
photocurrent response of 1 might be explained by the long
lifetime of the photogenerated hole−electron pairs in the 2D
layered framework. Therefore, the photocurrent results
demonstrate that the electron mobility of the BPEA ligand is
greatly improved by forming MOF, resulting in the decreased
recombination of photoinduced e−/h+, giving rise to the
enhanced photocatalytic activity. According to the literature,
the Cu(I)-based MOFs, containing Cu(I)−I chains as the
building unit, have been demonstrated to have semiconductor
properties.39 To study the electron transfer in 1, its
(photoluminescence) PL spectra were collected at room
temperature. As displayed in Figure S10, the PL spectrum of
the BPEA ligand revealed a peak at 565 nm upon excitation at
300 nm, and this peak can be assigned to the intraligand
emission. However, no PL signal could be observed for 1
around 565 nm with a new peak emerging around 490 nm,
which indicates there is the ligand-to-metal charge-transfer.40
According to the above analysis, the possible mechanism for
the Cr(VI) reduction and evolution of H2 over 1 has been
proposed. In these processes, the BPEA ligands could behave
as light-absorbing antennae, and the excited electrons from the
ligand are able to transfer to the Cu(I) center. Upon light
illumination, the excited electron on the CB of BPEA ligands
jumps to the VB in the Cu(I) center, and then transfers to the
Cr(VI) ion adsorbed on the MOF surface or H+ ion to form
the Cr(III) ion or H2, while the hole scavengers adsorbed on
the MOF surface are oxidized by the photogenerated holes to
form CO2 and H2O (Figure 5b).
Photocatalytic H2 Evolution. The photocatalytic H2
evolution activities of 1 were studied in the presence of
triethanamine (TEA) as the hole scavenger. The reaction
system is deoxygenated and exposed to visible-light illumina-
tion. The photocatalytic H2 evolution amount is negligible in
the presence of the ligand or CuI, demonstrating that neither
the BPEA ligand nor the CuI could act as an efficient
photocatalyst (Figure 4a). After the introduction of 1, the H2
evolution activity of the system is significantly enhanced. The
amount of H2 generated from the system increases with time
and reaches about 75.89 mmol/g within 18 h irradiation.
According to the literature, the pH value of the reaction system
plays a vital effect on photocatalytic performances.36−38 In this
case, 5 h of H2 evolution over the as-prepared 1 at pH from 10
to 12.5 (adjusted by HCl and NaOH) was studied, and the
results are shown in Figure S8, which indicates that the
increase of pH value from 10.0 to 12.0 could boost the H2
evolution amount, although a further increase in pH (greater
than 12.0) results in a decrease of the H2 evolution rate. The
above experimental results indicate that the mild basic
conditions might be propitious to the H2 evolution reaction
of 1, and the optimized pH value is 12.0. To probe the
framework stability of 1 in the photocatalytic process, the
recycling experiments were carried out, and the results are
shown in Figure 4b. The result reveals that the photocatalytic
activity of 1 shows no obvious decrease after three recycles of
experiments. Furthermore, the PXRD measurements also
confirm the framework integrity of 1 after three recycles of
experiments (Figure S9).
CONCLUSION
■
In summary, a visible-light active layered Cu(I)-MOF material
showing a narrow forbidden-band of 2.13 eV was successfully
prepared by using a visible-light-harvesting anthracene-based
bipyridine ligand. The as-prepared material not only exhibited
superior chemical stability which could keep its framework
integrity in the wide pH range 2−13 but also demonstrates
excellent photocatalytic activity for reduction of Cr(VI) and H2
evaluation in water. Furthermore, such photocatalytic
processes do not need the aid of additional photosensitizers
and cocatalyst, which is quite rare among MOF-based
Photocatalytic Mechanism. To probe the plausible
mechanism for 1 mediated photocatalysis, transient photo-
current responses have been carried out to assess the
separation efficiency of photoinduced electron−hole pairs as
well as to understand the interface charge transport behavior
E
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ASSOCIATED CONTENT
■
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Accession Codes
CCDC 1827405 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.
AUTHOR INFORMATION
Corresponding Authors
*E-mail: chunsenliu@zzuli.edu.cn.
*E-mail: dumiao@zzuli.edu.cn.
■
ORCID
Chun-Sen Liu: 0000-0002-5095-7359
Miao Du: 0000-0002-1029-1820
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
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The authors gratefully acknowledge the support from the
National Natural Science Foundation of China (Nos.
21601160, 21571158 and 21471134) and the Doctoral
Scientific Research Foundation of Zhengzhou University of
Light Industry (No.2015BSJJ042).
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