Highly Efficient Photocatalytic Degradation of Dyes by a Copper–Triazolate Metal–Organic Framework

Article abstract of DOI:10.1002/chem.201803306

A copper(I) 3,5-diphenyltriazolate metal–organic framework (CuTz-1) was synthesized and extensively characterized by using a multi-technique approach. The combined results provided solid evidence that CuTz-1 features an unprecedented Cu₅tz₆ cluster as the secondary building unit (SBU) with channels approximately 8.3 ? in diameter. This metal–organic framework (MOF) material, which is both thermally and chemically (basic and acidic) stable, exhibited semiconductivity and high photocatalytic activity towards the degradation of dyes in the presence of H₂O₂. Its catalytic performance was superior to that of reported MOFs and comparable to some composites, which has been attributed to its high efficiency in generating ^.OH, the most active species for the degradation of dyes. It is suggested that the photogenerated holes are trapped by Cu^I, which yields Cu^II, the latter of which behaves as a catalyst for a Fenton-like reaction to produce an excess amount of ^.OH in addition to that formed through the scavenging of photogenerated electrons by H₂O₂. Furthermore, it was shown that a dye mixture (methyl orange, methyl blue, methylene blue, and rhodamine B) could be totally decolorized by using CuTz-1 as a photocatalyst in the presence of H₂O₂ under the irradiation of a Xe lamp or natural sunlight.

Full text of DOI:10.1002/chem.201803306

A Journal of
Accepted Article
Title: Highly efficient photocatalytic degradation of dyes by a novel
copper-triazolate metal-organic framework
Authors: Guang Yang, Chen-Xia Liu, Wen-Hua Zhang, Nan Wang,
Penghu Guo, Martin Muhler, Yuemin Wang, Zhongfang Chen,
and Shiru Lin
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To be cited as: Chem. Eur. J. 10.1002/chem.201803306
Link to VoR: http://dx.doi.org/10.1002/chem.201803306
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Highly efficient photocatalytic degradation of dyes by a novel
copper-triazolate metal-organic framework
Chen-Xia Liu,[a] Wen-Hua Zhang,[a] Nan Wang,[a] Penghu Guo,[b] Martin Muhler,[b, c] Yuemin Wang,[d]
Shiru Lin,[e] Zhongfang Chen,[e] Guang Yang * [a]
Abstract:
A
novel CuI 3,5-diphenyltriazolate metal-organic
Metal-organic frameworks (MOFs), developed from
coordination compounds since about two decades ago, have
been recently gaining more and more attention in materials
framework (CuTz-1) was synthesized and extensively characterized
by a multi-technique approach. The combined results provided solid
evidence that CuTz-1 features unprecedented Cu5tz6 cluster as SBU
with channels of ca 8.3 Å diameter. This MOF material, which is both
thermally and chemically (basic and acidic) stable, exhibits semi-
conductivity and high photocatalytic activity towards degradation of
dyes in the presence of H2O2. Its catalytic performance is superior to
that of reported MOFs and comparable to some composites, which
has been attributed to high efficiency of generating •OH – the most
active species for degradation of dyes. It is suggested that the
photogenerated holes are trapped by CuI yielding CuII, the latter of
which behaves as a catalyst for a Fenton-like reaction to produce
extra amount of •OH in addition to that formed via scavenging of
photogenerated electrons by H2O2. Furthermore, it has been shown
that a dye-mixture (methyl orange, methyl blue, methylene blue and
rhodamine B) can be totally decolorized by using CuTz-1 as
photocatalyst in the presence of H2O2 under the irradiation of a Xe
lamp or natural sunlight.
science.[9-13] As
a novel emerging type of materials, the
structures of MOFs can be, at least in part, designed, prepared
and modified for a given purpose not only due to the well-
established construction theory and assembling strategies for
MOF synthesis, but also because of the seemingly infinite
number of choices for the two components of MOFs – metal ions
(clusters) and organic linkers.[14-16]
Adsorptive removal of dyes is regarded as one of the
competitive methods because of its high efficiency and easy
18]
operation.[17,
Several reports appeared using MOFs as
adsorbent to remove dyes from solutions, mostly via ion-
exchange processes.[19-22] Due to the microporosity of MOFs, for
larger dye molecules it was found to be difficult to enter the
pores or inner voids of MOFs, thus resulting in a lower
adsorption capacity. In addition, adsorption by MOFs via ion-
exchange would experience charge selectivity, that is, only
oppositely charged dye ions can be exchanged into the interior
of a charged MOF structure. Although the size- and/or charge-
selectivity are useful for the design of MOF-based adsorbents
for a specific dye molecule, for the treatment of wastewater
containing complex dye compositions these characteristics are
usually unfavorable for the practical use of MOFs in this regard.
On the other hand, photocatalysis has drawn much
attention worldwide in the elimination of contaminants in water,
because it provides the possibility to employ sustainable and
abundant solar energy to promote reactions under mild
conditions.[23, 24] Semiconductors such as TiO2,[25-27] other metal
oxides[28] and metal sulfides[29] have been exploited as
photocatalysts for the purpose of catalytic decomposition
/decolorization of dyes. The catalytic effectiveness of these
semiconductors heavily relies on the photo-induced electrons
and/or holes. It has been found that these catalysts usually
suffer from limitations such as fast recombination of the electron-
hole pairs, low solar energy utilization efficiency, and easy
agglomeration.[30] Therefore, new catalytic materials showing
high photocatalytic performance are desirable.
Introduction
Dye-containing wastewater, largely discharged from textile,
paper or printing industries, is one of the main sources of water
pollution.[1,
The treatment of dye-containing wastewater
2]
presents a tremendous challenge for human beings not only
because of the still rapidly growing discharge amount, but also
due to the stable structures of dye molecules (usually
conjugated), which render them chemical inert to light, heat and
oxidizing agents, and therefore make it rather difficult to
decompose or degrade dyes.[3] Although a number of methods,
such as adsorption, biodegradation, chemical oxidation,
coagulation, flocculation, etc.,[4-7] have been used for the
treatment of dyes, new materials and technologies,[8] with high
efficiency, wide applicability, economic feasibility and simplicity
of design/operation, are still urgently demanded in order to
better cope with this increasingly serious pollution issue,
especially in some developing countries.
More recently MOFs have once again attracted academic
interest in their potential use as a new type of photocatalyst.[30-33]
Qiu et al. found that the efficient degradation of methylene blue
(MLB) can be achieved under irradiation of UV or visible lights
by using MIL-53(Fe) as photocatalyst.[34] Xing’s group recently
[a]
C.-X. Liu, Dr. W.-H. Zhang, N. Wang, Prof. Dr. G. Yang
College of Chemistry and Molecular Engineering
Zhengzhou University, 450001 Zhengzhou, China
E-mail: yang@zzu.edu.cn
P. Guo, Prof. Dr. M. Muhler
Laboratory of Industrial Chemistry
Ruhr-University Bochum, 44780 Bochum, Germany
Prof. Dr. M. Muhler
Max Planck Institute for Chemical Energy Conversion
45470 Mülheim an der Ruhr, Germany
Dr. Y. Wang.
Institute of Functional Interfaces (IFG)
Karlsruhe Institute of Technology (KIT), 76344 Eggenstein-
Leopoldshafen, Germany
[b]
[c]
[d]
reported that NNU-36 is
a highly efficient heterogeneous
photocatalyst for the reduction of aqueous CrVI and degradation
of dyes (rhodamine B: RhB, rhodamine 6G: R6G, MLB).[35]
These research reports, together with others,[36-39] have revealed
the enormous potential held by MOFs for their use in
photocatalytic removal of environmental contaminants.
Inspired by our recent progress in the utilization of the silver
3,5-diphenyltriazolate MOF (previously designated as 1-NO3 and
here denoted as AgTz-1) in CO2 uptake[40, 41] and CrVI removal,[42]
we attempted to extend its application to the sorption of dye
[e]
S. Lin, Prof. Dr. Z. Chen
Department of Chemistry, University of Puerto Rico
Rio Piedras, San Juan, PR 00931, USA
1
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10.1002/chem.201803306
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molecules from aqueous solution. Preliminary test showed that
AgTz-1 can efficiently adsorb methyl orange (MO, a smaller
anionic dye) in water, however, for the other three dye
molecules-methyl blue (MB, larger anionic), MLB (cationic), and
RhB (cationic), this MOF failed to work as well as it for MO,
indicative of the size- and charge-selectivity experienced for ion-
exchange using MOF materials.
Due to the severe thermal disorder, the anions and solvents
cannot be definitely located in the Fourier map, however, the
presence of two HSO4- anions is required by charge balance and
was verified by elemental analysis and IR spectra. (Figure S2).
PLATON calculation gives a porosity of 24.4% for the squeezed
structure of CuTz-1, this value would be lower if the volume of
anions were taken into consideration.
The coordination properties of CuI are similar to those of
AgI in many cases, although copper is more prone to show
CuI/CuII redox chemistry. It is therefore possible to prepare a
CuI-containing MOF while the structure remains similar to that of
AgTz-1. The former would show some new functions like redox
activity, luminescence or catalytic activity and may find
substantial use in the treatment of organic contaminants in
wastewater. Here, we report the synthesis, characterization and
structure of a new MOF based on CuI 3,5-diphenyltriazolate
(denoted as CuTz-1), which is isostructural to our previously
reported AgTz-1, but behaves differently from AgTz-1 in many of
its physical and chemical properties. Photocatalytic performance
and mechanism of CuTz-1 was investigated in detail for the
degradation of dyes in aqueous solution.
Results and Discussion
Characterization and crystal structure of Cu8(3,5-Ph2-
tz)6(HSO4)2·(H2O)5.5 (CuTz-1-HSO4)
Figure 1. (a) The trigonal-prismatic Cu3[Cu5(μ3-3,5-Ph2-tz)6]2+ repeating unit of
the three-dimensional structure, Cu1-N, 1.977(2); Cu2-N, 1.868(2); Cu3-N,
1.876(2) Å; (b) Packing diagram of CuTz-1 viewed along the c-axis, showing
one channel with diameter of ca. 8.3 Å (if the van der Waals’ radius of H is
taken into consideration, the effective diameter of the channels is estimated to
be ca 6.0 Å); (c) The topological net underlying the structure of CuTz-1.
The colorless crystals of CuTz-1-HSO4 were firstly prepared
by a hydrothermal reaction of Cu(NO3)2 with 3,5-diphenyltriazole
in dilute H2SO4 solution in 75% yield. Interestingly, CuII ions
were reduced to CuI under hydrothermal conditions and triazole
was deprotonated during the synthesis, although the reaction
medium is acidic. Copper(II) perchlorate was also used as the
metal source and the same MOF structure can be obtained with
ClO4- as counter anion in a lower yield of 38%.
Synthesis and characterization of the powdery sample of
CuTz-1
X-ray single crystal structure analysis shows that CuTz-1-
HSO4 crystallizes in the hexagonal space group - P6322, and
exhibits a 3D porous structure. Cationic skeleton {Cu3[Cu5(3,5-
For the further properties investigation of this new MOF, a
large quantity of the sample is needed. A facile, fast，energy-
saving synthesis of this MOF is desired for further study, which
was successfully achieved by simply mixing Cu(CH3CN)4BF4 and
triazole in degassed methanol. The chemical formula of the
powdery sample was evaluated to be Cu8(3,5-Ph2-
tz)6(BF4)2(CH3OH)3 based on the result of CHN elemental
analysis and IR spectroscopy. As shown in Figure S2a, there is
a strong broad band at 3440 cm-1, which can be assigned to the
vibration of O-H. The weak peaks at 2926 cm-1 and 2853 cm-1
are possibly caused by the asymmetric and symmetric C-H
2+
Ph2-tz)6]}n is composed of the trigonal bipyramidal clusters –
[Cu5(3,5-Ph2-tz)6]- (Figure 1a), which are further connected
through the linear coordination of CuI, forming a 3D metal-
organic framework (MOF) structure (Figure 1b). In this structure,
4
all the 3,5-diphenyltriazolate anions act as μ3-Ntz1,Ntz2,Ntz
bridging ligand. On the other hand, there are three
crystallographically independent CuI ions which have different
chemical environments: Cu1 is triply coordinated by N atoms
forming a triangular CuN3 arrangement, and is located at the
apex of a Cu5 cluster; Cu2 adopts a linearly coordination serving
as the equatorial vertex of the Cu5 cluster; the linearly
-
vibrations of methanol molecules. The presence of BF4 (as
counterion) was verified by its characteristic IR peak at 1084 cm-
1 (Figure S2b). Moreover, the structure of the powdery sample is
almost the same as that of CuTz-1-HSO4, as evidenced by
comparison of their PXRD patterns (Figure S3). The notation of
CuTz-1-BF4 was therefore coined for the powdery sample to
show that both crystalline and powdery samples actually
possess the same cationic MOF-skeleton and the main
difference lies in their counterions. For the sake of convenience,
we use an even more succinct notation – CuTz-1 for the
powdery MOF in this paper.
4
coordinated Cu3 functions as a “linker” connecting two Ntz
atoms from a pair of adjacent clusters of [Cu5(3,5-Ph2-tz)6]-.
Within the Cu5 cluster, the distances of Cuax···Cueq, Cuax···Cuax,
Cueq···Cueq are 3.130(1), 3.350(1), and 4.579(1) Å, respectively.
Each Cu5tz6 cluster is connected with six peripheral identical
clusters, in a trigonal prismatic arrangement, by six Cu3 atoms,
and therefore functions as 6-connected trigonal prismatic
secondary building units (SBU) in the structure of CuTz-1. If the
Cu5tz6-SBU is regarded as
a 6-connected node, the net
underlying CuTz-1 is acs (or 49·6 6) (Figure 1c).[43] Extensive π-π
interactions involving phenyl and triazolyl rings were observed in
the crystal structure of CuTz-1, which may also contribute to the
stabilization of the structure. (Figure S1)
The thermal stability of CuTz-1 was evaluated by
thermogravimetric analysis. As shown in Figure S4, the TG
2
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curve in air exhibits two weight loss steps. In the temperature
range of 55‐100 °C, the weight loss (4.22%) is attributed to the
removal of methanol molecules (calculated value: 4.57%). The
second severe weight loss observed between 335‐800 °C is
probably related to the decomposition of the framework. The
final product, accounting for 31.75% of the total mass, is
considered to be CuO according to the calculated residual
weight percentage (32.50% as CuO).
Adsorption of organic dyes on AgTz-1 and CuTz-1
The adsorptive behavior of two MOFs was investigated for
the removal of dyes from aqueous solution, and the results are
listed in Figure 4 and Table S2. As can be seen, both MOFs can
effectively remove smaller-sized anionic MO from water,
however, the removal rate for other dyes is much lower under
the same conditions: 35% for larger-sized anionic MB and less
than 10% for cationic MLB and RhB.
In order to determine the pH stability of CuTz-1, the MOF
samples were suspended in aqueous solutions at different pH
for 24 h, followed by PXRD measurements to monitor the
structural change of the MOF. As shown in Figure S5, the
structure of CuTz-1 remains unchanged in the pH range of 3‐
11.
IR spectra of the MO-adsorbed samples demonstrate the
-
appearance of characteristic peaks of RSO3 , which is
accompanied by a substantial attenuation of the peaks from
-
-
original MOFs (1084 cm-1 for BF4 and 1384 cm-1 for NO3 )
(Figure S6). These findings indicate the occurrence of an anion-
exchange process, which we observed in CrVI removal by AgTz-
1.[42]
The luminescence spectra show that CuTz-1 exhibits a
broad emission with maximum at 525 nm at room temperature
when excited at 325 nm (Figure 2). The lifetime of this emission
was measured to be 33 μs, indicative of phosphorescence.
According to the previous studies on luminescent properties of
CuI complexes, the low-energy emission can be assigned to
The adsorption of MO by AgTz-1 and CuTz-1 is essentially
an anion-exchange process between the anionic MO and the
counter-ions within the cationic skeletons of [M8tz6]n2+. Since the
dimension of MO is small enough to be trapped in the channels
of the MOFs, the observed high removal rate for MO is thus not
unexpected. It should be pointed out that the MOFs still retain
3
3MLCT or MM excited states. In the present case, the shortest
Cu…Cu distance is 3.130(1) Å, which is slightly larger than the
sum of van der Waals radius of the two copper atoms (2.80 Å).[44] their structures after MO-adsorption, as demonstrated by PXRD
It is thus conjectured that the emission originates from a triplet
metal-to-ligand excited state transition (3MLCT), possibly
modified by weak Cu…Cu interactions.[45-51]
measurements (Figure S7).
On the other hand, MB, although anionic, has a much larger
size than MO, which makes it impossible for MB to enter the
inside of the MOF crystal. The much lower removal rate for MB
is probably a result of surface adsorption. As for cationic RhB
and MLB, it is not surprising to find that both MOFs (AgTz-1 and
CuTz-1) failed to absorb them, because the cationic MOF
skeletons cannot hold additional cations if the adsorption is
based on anion exchange.
To summarize, MTz-1 MOFs can be used as a MO
adsorbent, however, for the treatment of complex dye-mixtures,
adsorptive removal by these MOFs suffers from the so-called
“charge-selection” and “size-exclusion” effects, which thus
limiting the wide applicability for this purpose.[20]
Figure 2. The photoluminescence spectrum of CuTz-1.
In order to probe the porosity of this MOF, nitrogen
physisorption was conducted on CuTz-1 at 77 K. Prior to the
measurement, the CuTz-1 sample was heated at 45 °C under
high vacuum for 4h to remove residual methanol molecules. The
N2-adsorption/desorption isotherms obtained follow type
I
adsorption behavior,[52] revealing permanent microporosity of
CuTz-1 (Figure 3). The BET surface area was determined to be
215.5 m2 g-1, and the pore size to approximately 0.64 nm,[53]
which is in good agreement with the crystal structure data.
Figure 4. Percentages of dyes remaining in the aqueous solution monitored
with time in the presence of (a) AgTz-1 and (b) CuTz-1 (MO: methyl orange;
MB: methyl blue; MLB: methylene blue; RhB: rhodamine B ).
Photocatalytic degradation of dyes by CuTz-1
Preliminary test revealed that CuTz-1 can decolorize the
dye solutions in the presence of H2O2 under photoirradiation,
while AgTz-1 is inactive for the photodegradation of dye
molecules. A detailed investigation therefore ensued on the
photocatalytic degradation of dyes by using CuTz-1 as
photocatalyst. The four common dyes were chosen as
representatives of different types of dyes (anionic: MO, MB;
cationic: MLB, RhB; azo dye: MO; triarylmethane dye: MB;
thiazine dye: MLB). To investigate the roles played by different
Figure 3. Nitrogen adsorption (black) and desorption (red) isotherms of CuTz-
1 at 77K. Inset: Pore size distribution calculated from the isotherms.
3
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factors such as H2O2, CuTz-1 and photoirradiation, seven
experimental conditions have been used for each dye: (1)
H2O2/dark; (2) CuTz-1/dark; (3) H2O2/CuTz-1/dark; (4) blank
(irradiation only); (5) H2O2/irradiation; (6) CuTz-1/irradiation; (7)
H2O2/CuTz-1/irradiation. As shown in Figure 5, the best
degradation result can only be achieved when H2O2, CuTz-1 and
irradiation are applied simultaneously to the dye-containing
system. For MO and MB, the observed removal of dye under
CuTz-1/dark can be attributed mainly to the adsorption of the
dyes within the channels or on the surface of MOF. We also
noticed that the degradation performance of H2O2/CuTz-1/dark
is always better than that of CuTz-1/dark for each dye under
investigation. When irradiation is considered, we found that the
irradiated groups exhibit more or less higher removal rate than
the corresponding groups in the dark.
respectively, slightly lower than those obtained before achieving
the adsorption equilibrium.
In order to assess the reusability of CuTz-1, the
photocatalytic degradation of each dye was run 4 cycles. As can
be seen in Figure S14-S17, CuTz-1 still held similar
photocatalytic efficiency even after being reused for 4 times.
Importantly, the PXRD patterns of CuTz-1 after photo-
degradation are almost the same as that of as-synthesized
sample, confirming that the catalyst did not degrade under the
photodegradation reaction condition. From comparison of SEM
images (Figure S18), we noticed that the morphology of CuTz-1
did not change significantly much after 4 cycles reaction,
although the surface of some crystals became coarse and
shrank a little as seen in the SEM images of the recycled
samples.
Table 1. Rate constants for photo-degradation of dyes by CuTz-1 in
comparison with other reported catalysts.
Dye
Catalyst
k (min-1)
Ref.
MLB
MIL-53(Fe)
NNU-36
0.125
0.0315
0.0071
0.01916
0.51
[34]
[35]
[54]
[55]
[56]
[57]
This work
UiO-66-NH2
lp-2
12.5Cu@SBA-15
RGO/Co DND nanocomposite
CuTz-1
0.4627
0.42
RhB
MO
MB
NNU-36
MIL-53(Fe)
MHMCs
RGO/Co DND nanocomposite
Porous Carbon- Fe2O3
Bi(0.25%)/Co(0.25%)-TiO2
Hollow Pt/Ag nanosphere
CuTz-1
0.0468
0.0445
0.0513
0.3367
0.2-0.14
0.00315
0.047
[35]
[58]
[58]
[57]
[59]
[60]
[61]
This work
0.19
ZIF-8-600
ZIF-67-600
ZnO
Pt NP
0.0033
0.0025
0.0066
0.0029
0.0049
0.0283
0.2912
0.17
[62]
[62]
[62]
[63]
[63]
[62]
[57]
This work
Figure 5. Degradation of dyes under different conditions. (1) H2O2/dark; (2)
CuTz-1/dark; (3) H2O2/CuTz-1/dark; (4) blank (irradiation only); (5)
H2O2/irradiation; (6) CuTz-1/irradiation; (7) H2O2/CuTz-1/irradiation (MO:
methyl orange; MB: methyl blue; MLB: methylene blue; RhB: rhodamine B ).
Au NP
ZnO@C-N-Co
RGO/Co DND nanocomposite
CuTz-1
It is noticed that different dyes display different sensitivity to
the degradation conditions used here: the time needed for
complete removal of dyes is 8 min for MLB, 15 min for MO, 22
min for RhB and 24 min for MB (Figure S8). To facilitate the
evaluation of the efficiency of the catalyst and the comparison
with literature data, the concentration change of dye vs. time is
simulated using a first-order kinetic model, which can be written
0.15 *
CuTz-1
0.13
0.12 *
This work
*
Values were obtained after the adsorption-desorption equilibrium was
achieved.
as ln(C/C0)
=
kt. As shown in Figure S9-S12, the
photodegradation of dyes over CuTz-1 follows first-order kinetics,
and the rate constants (k) are 0.42 min-1, 0.17 min-1, 0.19 min-1
and 0.13 min-1 for MLB, MO, RhB and MB, respectively. From
Table 1, one can clearly find that CuTz-1 exhibits high
photocatalytic activity towards dyes in the presence of H2O2, its
efficiency in this regard is higher than those of reported MOFs
and other one-component materials such as nanoparticles, and
comparable to those of composite materials. It should be pointed
out that degradation data for methyl blue (MB) has not yet been
reported in the literature, to the best of our knowledge.
Since the adsorption of dyes may affect the photocatalytic
performance of MOFs, the degradation of MO and MB over
CuTz-1 was conducted after the adsorption-desorption
equilibrium was achieved. As shown in Figure S13 and Table 1,
the rate constants k are 0.15 and 0.12 min-1 for MO and MB,
The degradation of mixed dyes (MLB, RhB, MO and MB) on
CuTz-1 was also studied. CuTz-1(12 mg) and H2O2 (400 L)
were added into 20 mL of the aqueous solution of mixed dyes
(the concentration of each dye is 12 mg/L). Figure 6 shows the
UV-vis spectra for the photodegradation of MLB-MO-RhB-MB
mixture on CuTz-1. Before degradation, the mixture had a dark
brown color, resulting from the simultaneous presence of four
dyes. Under Xe lamp irradiation, the mixture gradually faded
from purple to colorless after 21 min, and the PXRD pattern of
CuTz-1 after photodegradation remained the same as that of
pristine MOF (Figure S19). This experiment demonstrates that
CuTz-1 still retains its high catalytic efficiency for complex dye
mixtures. In order to further evaluate the practicability of this
catalyst, the degradation of mixed dyes was also conducted in
the open air, the natural sunlight acting as the irradiation source.
4
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It is found that the dyes were totally decolorized with 80 min
when the natural sunlight was used instead of Xe lamp.
investigation is required to understand the nature of the unusual
absorption behavior of CuTz-1.
Figure 6. The UV-vis spectral changes of mixed dyes with time. Insert:
Photographs of mixed dyes solutions taken at different time points of
photocatalytic degradation reaction (mixed dyes: methyl orange, methyl blue,
methylene blue and rhodamine B).
Figure 7. UV-vis absorption spectra of CuTz-1 and AgTz-1 at ambient
temperature.
XPS analysis of the catalysts
We further conducted XPS measurements for as-
synthesized and pretreated CuTz-1 catalysts in order to obtain
more information about the chemical and electronic states of
elements, as well as the atomic concentration of different
species (CuI/CuII, O- and F- containing species) on the surface
of the catalyst. The identification and a quantitative analysis of
these species are important for a deep understanding of the
photodegradation mechanism. For a thorough comparison, the
MOF samples were treated under different conditions: (a) freshly
prepared CuTz-1; (b) CuTz-1 pretreated by H2O2; (c) CuTz-1
pretreated under irradiation of Xe lamp; (d) CuTz-1 pretreated by
H2O2 under irradiation of Xe lamp; (e) CuTz-1 after one cycle of
the degradation of MO; (f) CuTz-1 after 4 cycles of the
degradation of MO (Figure S22).
As depicted in Figure 8a, the deconvoluted Cu 2p XPS data
provided solid spectroscopic evidence that the Cu atoms are
almost entirely in +1 valence in the pristine CuTz-1, which is
characterized by the 2p3/2 peak at ~ 933 eV. The weak satellite
at ca. 940 eV may imply the presence of a tiny amount of CuII
species. CuTz-1 seems to be resistant to the oxidation of H2O2
and remains stable under the Xe lamp irradiation, since the
signal at 940 eV is practically unchanged in intensity after the
treatment of CuTz-1 with H2O2 or under photo-irradiation,
revealing that no additional CuII was formed (Figure 8b, c).
Interestingly, when both H2O2 and irradiation were applied to
CuTz-1, a small amount of CuII was formed, as evidenced by the
appearance of an additional weak Cu 2p3/2 signal at 934.7 eV
together with the typical shake-up satellites between 938‐947
eV (8.5%, Figure 8d). Furthermore, the content of CuII continued
to increase when CuTz-1 was used as photocatalyst in the
degradation of dyes; it reached to 14.7% and 18.8% after one
and four reaction cycles, respectively (Figure 8e and f).
Photocatalytic mechanism
Semi-conductivity of CuTz-1
Since CuTz-1 shows high photocatalytic efficiency under
Xe-lamp irradiation, it is hypothesized that this MOF may
possess semi-conductivity. Some recent studies demonstrated
that MOFs constructed by metals with variable valence exhibit
semi-conductivity and act as photocatalyst.[64,
Thus, UV-vis
65]
DRS was conducted to evaluate the optical properties of the
samples.
As shown in Figure 7, AgTz-1 has
a strong broad
absorption band in the range of 200 to 300 nm, the absorbance
dropped sharply beyond 310 nm, resulting in nearly no
absorption in the visible region. On the other hand, CuTz-1
shows a relatively weak absorption peak at 213 nm and two
strong peaks at 267 nm and 300 nm in the UV region.
Interestingly and importantly, the absorbance of CuTz-1 in the
visible region still remains in high (about 3/5 that of highest peak)
throughout the entire visible spectrum up to 800 nm.
The band gap of materials can be estimated from UV-vis
DRS according to the Kubelka-Munk function. As shown in
Figure S20, the band gap (Eg) is obtained as the intersection
point between the energy axis and the line extrapolated from the
linear portion of the adsorption edge in a plot of the Kubelka-
Munk function F against E.[66, 67] The Eg values of CuTz-1 and
AgTz-1 were determined to be 2.4 eV and 4.0 eV, respectively.
Compared to AgTz-1, CuTz-1 has a much narrower energy gap
and is expected to behave as a semiconductor. Theoretical
calculation gave a band gap of 2.21 eV for CuTz-1, in
reasonable agreement with our experimental results (see ESI for
more calculation details).
The presence of O-containing species and BF4- counterions
was also identified by the O1s peak at 531.5 eV and the F1s
peak at 685.0 eV (see Figure S23). In addition, the O1s binding
energy at 531.5 eV is characteristic for hydroxyl (OH-) species. A
quantitative analysis of the O1s and F1s XPS results allowed to
obtain more information about the relative concentration of O
It is noteworthy that CuTz-1 exhibits continuous strong
absorption in the near infrared region (NIR) (Figure S21), which
could be tentatively assigned to the intervalence charge-transfer
bands (IVCT), and to a lesser extent, d-d transition.[68,69] As is
known to all, IVCT is characteristic of mixed-valence complexes.
[70]
In the present case, the structure of CuTz-1 is undoubtedly
and
F species on the surface of CuTz-1 under various
based on CuI. Still and all, XPS analysis suggests that there is a
tiny amount of CuII on the surface of CuTz-1 (see next part for
details on XPS measurement). It is thus supposed that the
absorption in NIR by CuTz-1 could be related to the IVCT band
involving CuI  CuII within the polymeric structure. Further
experimental conditions. Overall, the surficial content of O
remained rising with the proceeding of the catalytic reaction,
while the F content kept dropping.
Based on the results of the XPS study, CuII ions are
assumed to be created upon irradiation on CuTz-1 in the
presence of H2O2. The photo-induced CuII may combine with O-
5
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10.1002/chem.201803306
Chemistry - A European Journal
FULL PAPER
containing ligands (predominantly OH-) to form CuII-O(H)
species on the surface of MOF. Degradation of dyes seems to
be more in favor of the formation of such surficial CuII-O(H)
compounds. The continuous decrease in surficial F content may
be attributed to the replacement of BF4- by O-containing anions
(OH-).
Proposed mechanism
Based on our research results, we propose a possible
mechanism to account for the observed high efficient
photocatalytic degradation of dyes. Illumination of CuTz-1 by
photons with energy equal to or greater than its band gap
excites electrons (e−) from the valence band (VB) to the
conduction band (CB) and leaving holes (h+) in VB. These holes
are possibly located at the metal ions, that is, they are present
as CuII ions in the framework of CuTz-1.[77] This hypothesis
seems to be consistent with the assignment of the luminescence
peak at 525 nm of CuTz-1, which is attributed to a 3MLCT
excited state. Although a widely accepted viewpoint is that the
organic linker is regarded as VB and the metallic cluster plays
78]
the role of CB for MOF-based semiconductors,[55,
for the
present case the situation is probably reversed: Cu ions or
Cu5tz6 clusters possibly act as VB, while triazole ligands as CB.
For
those
already
extensively
studied
MOF-based
semiconductors, the metal ions are either in their higher valence
34, 79-82]
Figure 8. Cu 2p region of the deconvoluted XPS for six samples (a) freshly
prepared CuTz-1; (b) CuTz-1 pretreated by H2O2; (c) CuTz-1 pretreated under
irradiation of Xe lamp; (d) CuTz-1 pretreated by H2O2 under irradiation of Xe
lamp; (e) CuTz-1 after one cycle of the degradation of MO; (f) CuTz-1 after 4
cycles of the degradation of MO. The formation of CuII species was also
confirmed by their characteristic satellite peaks between 938-947 eV (MO:
methyl orange).
(e.g. FeIII in MIL-53(Fe), ZrIV in UiO-66)[30,
or in very
stable valence (e.g. ZnII in MOF-5),[83, 84] and therefore it is very
difficult for these metal ions to lose electrons. For the present
case, CuTz-1 contains only CuI clusters in the framework and
CuI can be easily transformed to CuII, as indicated by the
standard electrochemical potential of CuII/CuI (0.159 V). Our
hypothesis is fully supported by the XPS observation that CuII
ions appeared and increased in concentration when irradiating
CuTz-1 in the presence of H2O2 (and dyes).
Effect of the presence of •OH scavenger – TBA
The photo-induced electrons and holes are easily
recombined, therefore no CuII can be detected on the surface of
CuTz-1 after irradiation under Xe-lamp in the absence of H2O2
and dyes, as verified by XPS analysis (Figure 8c).
Our experimental results showed that the addition of H2O2 is
necessary to achieve a high efficiency of the photocatalytic
degradation over CuTz-1. As generally accepted, hydroxyl
radicals (•OH) are the active species in the H2O2-involved
When H2O2, an efficient electron scavenger, was added, it
can combine with an electron to produce •OH and OH- (equation
2). Furthermore, photogenerated holes (CuII) can be stabilized
within the framework of CuTz-1 through bonding with hydroxides
(generated from equation 2) and/or other O-containing species
in the reaction system. The presence of CuII-O(H) species on the
surface of catalyst was verified by XPS analysis for irradiated
samples in the presence of H2O2 (Figure 8d). About 8.5%
surficial Cu atoms were identified to be in the oxidation state +2
for this sample.
74]
photocatalytic reactions over MOFs.[71,
To verify the major
active species in our system, tert-butyl alcohol (TBA) was used
as scavenger of hydroxyl radicals in the degradation
a
reaction.[35, 75, 76] As illustrated in Figure 9, the addition of TBA
greatly depressed the photocatalytic efficiency for the
degradation of RhB. In the presence of 100 L (or 200 L) of
TBA, 70.2% (or 64.2%) of RhB was degraded after 22 min, and
the first-order reaction rate constant dropped to only ca 1/4 of
the original value (0.190 min−1 vs. 0.056 (or 0.044) min−1). It is
therefore concluded that the major active species may be •OH
for the present system, the degradation performance would thus
heavily rely on the efficiency of generating •OH during the
reaction.
The in-situ formed CuII species can undergo Fenton-like
reactions to produce •OH, as shown in equations. 3-a and 3-b.
When dyes were added to the photocatalytic system,
degradation of dyes consumes •OH, which seems to promote
the reactions of equations. 1, 2 and 3 and accelerate the
formation of CuII and hydroxyl (OH-) species, in accordance with
the XPS analysis on the samples after 1 or 4 cycles of dye
degradation (see Figure 8e, f and Figure S23e, f ).
The CuII-O(H) species are considered to exist only on the
surface of CuTz-1. The presence of CuII on the surface, as
determined by XPS, cannot be applied automatically to the inner
part of CuTz-1, since XPS is a surface analysis technique. The
PXRD patterns of CuTz-1 after several cycles of dye-
degradation remained almost identical to that of pristine MOF. If
more than 10% of total Cu atoms in the MOF were in oxidation
state of +2, the structure of CuTz-1 would experience a
remarkable change, which is obviously not true for the present
case. In our previous paper, we already showed that hydroxide
ions fail to enter the channels of the MOF by anion-exchange,[42]
because anions with higher hydration energy (OH-) would be
Figure 9. Photocatalytic degradation of RhB over CuTz-1 with or without TBA;
inset: comparison of the apparent reaction rate constants in the
presence/absence of TBA (black: without TBA, red: 100 L TBA, blue: 200 L
TBA, RhB: rhodamine B, TBA: tert-butyl alcohol).
6
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10.1002/chem.201803306
Chemistry - A European Journal
FULL PAPER
energetically unfavorable to replace anions of lower hydration
different degradation times were taken for mass spectrometry
detection, and their MS patterns are shown in Figure S24-S27.
Based on MS measurements, the main metabolites resulting
from degradation of dyes were deduced and logically reported
according to their molecular weight (Scheme S1-S4), which
indicates that the present catalytic system can effectively
degrade or decompose the dyes under investigation.
-
-
energy (NO3 , BF4 ) located within the frames of the MOF. The
same argument can be applied to the present case, that is, it
would be rather difficult for hydroxides (or other smaller-sized,
highly charged O-containing anions) to enter the channels of
CuTz-1 via anion-exchange. The photogenerated CuII inside of
the MOF would not be stabilized by OH- and probably be
transformed back to CuI quickly.
As revealed above, the hydroxyl radical (•OH) as active
species plays a key role in the degradation of organic dyes,
which is also verified and widely accepted by the academic
Conclusions
community.
photogenerated holes are not very effective in producing •OH
and thus the pristine MOFs usually show moderate
For
MOF-based
semiconductors,
the
A novel MOF (CuTz-1) was successfully constructed from
CuI and 3,5-diphenyltriazolate, extensively characterized by
SXRD, PXRD, IR, UV-vis DRS, TGA, emission spectra, SEM
and XPS. CuTz-1, isostructural to our previously reported AgTz-
1, features an unprecedented Cu5tz6 cluster as SBU in the 3D
polymeric structure and a porous cationic framework with ca
0.83 nm diameter channels along the crystallographic c-axis. In
stark contrast to its silver analog, CuTz-1 exhibits
semiconductor-like behavior and high efficiency in photocatalytic
degradation of dyes of various structural types (cationic, anionic,
azo-type and triarylmethane type) in the presence of H2O2. It has
been suggested that photo-irradiation in the presence of H2O2
leads to the formation of CuII species as the photogenerated
holes that are stabilized by O-containing species (predominantly
OH-), as evidenced by the XPS results. The superior
photocatalytic performance of CuTz-1 is ascribed to two
effective pathways to produce potent oxidant – •OH: (1) H2O2
scavenges a photogenerated electron to produce •OH; (2) a
Fenton-like CuI/CuII redox reaction can also efficiently generate
•OH. It has to be pointed out that a dye-mixture (MLB, MO, RhB,
MB) was totally decolorized by using CuTz-1 and H2O2 under a
Xe lamp or natural sunlight. The superior properties of CuTz-1,
such as high efficiency (comparable to those of composites),
wide applicability (4 different types of dyes under investigation),
economic feasibility (facile and large-scale preparation), simple
operation and reusability, make it a kind of heterogeneous
photocatalyst with potential practical value for the treatment of
dye-containing wastewater. Future studies are underway on the
modification of the MOF catalysts to achieve an even higher
catalytic activity and we also foresee the application of CuTz-1
and its analogs to other important catalytic reactions.
a
photocatalytic activity towards organics.[30] In some recent
reports, this drawback has been (at least partially) overcome by
forming composites with other types of materials.[85-88] In this
work, we demonstrate that CuTz-1 can generate •OH under
irradiation and in the presence of H2O2 through two pathways,
as shown in equations. 2 and 3. Consequently, CuTz-1 is highly
efficient in producing •OH and achieved a superior photocatalytic
activity towards dyes. It should be mentioned here that one of
the advantages of Cu-based Fenton-like catalysts is that they
can be used in neutral solution, just as shown in Table S3, which
recorded the pH values of the reaction solution before and after
the degradation of dyes.[82]
The proposed main photocatalytic mechanism is illustrated
in Figure 10. Other mechanisms are also possible, such as (1)
homolytic cleavage of H2O2 under irradiation; (2) Oxidation of
the surficial CuI to CuII by H2O2, and thus facilitating a Fenton-
like reaction; (3) involvement of O2 from air, which are believed
to be less efficient as compared to the proposed mechanism
(equations. 1-3), and may account for the lower removal rates of
dyes when the degradation was conducted in the dark or without
H2O2.
Acknowledgements
This work has been supported in China by the National Natural
Science Foundation of China (Grant No. 21471132) and in USA
by NSF-CREST Center for Innovation, Research and Education
in Environmental Nanotechnology (CIRE2N) (Grant Number
HRD-1736093) and NASA (Grant 17-EPSCoRProp-0032). P.
Guo acknowledges the funding from the European Union’s
Horizon 2020 research and innovation program under the Marie
Sklodowska-Curie grant agreement No. 641887.
Figure 10. Proposed mechanism of the photocatalytic degradation over CuTz-
1.
CuTz-1 + hν → CuTz-1 [e- + h+(CuII)] (1)
H2O2 + e- → •OH + OH-
(2)
(3-a)
(3-b)
CuII + H2O2 → CuI + •O2H + H+
CuI + H2O2 → •OH + OH- + CuII
Keywords: Metal-organic framework
•
Semiconductor
•
Product analysis of photocatalytic degradation of dyes
Photocatalyst • Fenton-like reaction • Dye degradation
The final products and intermediates generated during the
degradation process were analyzed by LC/MS and by
interpretation of their fragment ions in the mass spectra. The
supernatants of the dye solutions (MLB, RhB, MO and MB) at
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A thermally and chemically stable
CuI triazolate MOF (CuTz-1) has
been prepared and demonstrated
to exhibit high photocatalytic
activity towards degradation of
dyes in the presence of H2O2.
CuTz-1 can be used as a catalyst
at least 4 cycles without significant
change of the crystallinity and
catalytic activity. Shown here are
the photos taken before and after
Chen-Xia Liu,[a] Wen-Hua Zhang,[a]
Nan Wang,[a] Penghu Guo,[b] Martin
Muhler,[b, c] Yuemin Wang,[d] Shiru
Lin,[e] Zhongfang Chen,[e]Guang
Yang * [a]
Page No. – Page No.
Highly efficient photocatalytic
degradation of dyes by a novel
copper-triazolate metal-organic
framework
degradation of
a
dye-mixture
under natural sunlight by using
CuTz-1 as photocatalyst (mixed
dyes: methyl orange, methyl blue,
methylene blue and rhodamine B).
10
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