Visible-light-driven photocatalysts of metal-organic frameworks derived from multi-carboxylic acid and imidazole-based spacer

Article abstract of DOI:10.1021/cg2016512

Five new metal-organic frameworks [M(btec)_0.5(bimb)]_n (1) (M = Co (1), Ni (2), Cu (3), Zn (4)) and [Cd(btec)_0.5(bimb) _0.5]_n (5), were obtained by reactions of the conjugated 1,2,4,5-benzenetetracarbox

Full text of DOI:10.1021/cg2016512

Article
pubs.acs.org/crystal
Visible-Light-Driven Photocatalysts of Metal−Organic Frameworks
Derived from Multi-Carboxylic Acid and Imidazole-Based Spacer
Lili Wen,* Jinbo Zhao, Kangle Lv, Yuhui Wu, Kejian Deng, Xiaoke Leng, and Dongfeng Li*
,
†
†
‡
§
‡
†
,†
†Key Laboratory of Pesticide & Chemical Biology of Ministry of Education, College of Chemistry, Central China Normal University,
Wuhan, 430079, P. R. China
‡
Key Laboratory of Catalysis and Materials Science of the State Ethnic Affairs Commission & Ministry of Education, South-Central
University for Nationalities, Wuhan 430074, P. R. China
§
School of Material Science and Engineering, Changchun University of Science and Technology, Changchun, 130022, P. R. China
*
S Supporting Information
ABSTRACT: Five new metal−organic frameworks [M-
btec) (bimb)] (1) (M = Co (1), Ni (2), Cu (3), Zn (4))
(
0.5
n
and [Cd(btec) (bimb) ] (5), were obtained by reactions of
0
.5
0.5 n
the conjugated 1,2,4,5-benzenetetracarboxylic acid (H btec) and
4
4,4′-bis(1-imidazolyl)biphenyl (bimb) with corresponding metal
salts under hydrothermal conditions, respectively. MOFs 1-5
show different structures and topologies: compounds 1 and 4
are isomorphic, which possess typical PtS 3D nets; compound
2
, 3 and 5 exhibit 2D layer structure, NbO 3D network and
(
4,6)-connected 3D binodal topology, respectively. Notably,
compounds 1, 2, and 5 represent the rare example of MOFs-
based visible-light-driven photocatalysts and show good stability
toward photocatalysis. Furthermore, compound 5 is photo-
catalytically more active than 1 and 2 because of the relatively narrower band gap calculated from LMCT transitions. In addition,
the formation rate of •OH radicals on compound 5/H O interface via photocatalytic reactions is much higher than that of 1 and
2
2
, implying that the formation rate of •OH radicals during photocatalysis is in agreement with photocatalytic activity and the
formation rate of •OH radicals is an important factor influencing photocatalytic performance.
5
INTRODUCTION
aesthetic structures and topological features, as well as their
promising applications in molecular magnetism, molecular
sensor, heterogeneous catalysis, drug delivery, molecular
■
Photocatalysis is a “green” technology for the treatment of all
kinds of contaminants, which has many advantages over other
treatment methods, for instance, the use of the environmentally
6
−10
sorption and so on.
Most recently, considerable attention
has been paid to developing new photocatalytic materials based
on MOFs. The advantages of MOFs as photocatalyst lie in the
fact that the presence of organic linkers and transition metal
centers, resulting in different ligand-to-metal charge-transfer
friendly oxidant O , the ambient temperature reaction
2
condition, and oxidation of the organics compounds, even at
1
low concentrations. To date, TiO has undoubtedly proven to
2
be the most excellent photocatalyst for the oxidative
decomposition of many organic compounds under UV
irradiation. However, the relatively wide band gap limits further
application of the material in the visible-light region (λ > 400
(
LMCT) transitions, makes MOFs versatile and potentially
tunable photocatalysts. Furthermore, MOFs usually exhibit
absorption bands in the visible region, which indicate MOFs
may undergo photochemical processes and exhibit responses
upon visible-light excitation. Chen and co-workers have
reported the synthesis and characterization of new uranium-
containing materials, which show rich structural features and
encouraging photophysical properties. Garcia and co-workers
have studied MOF-5 as a semiconductor for photodegradation
of phenol in aqueous solutions and water stable Zr−
benzenedicarboxylate MOF as photocatalysts for hydrogen
2
nm). In view of the efficient utilization of visible light, the
largest proportion of the solar spectrum and artificial light
sources, the development of a photocatalyst with high activity
under a wide range of visible-light irradiation is indispensable.
Currently, there are two ways to exploit the photocatalysts
responsive to visible-light irradiation: the first involves the
11
modification of TiO ; the second is the development of new
2
materials. The former has been largely investigated by doping
or ion-implanting methods to affect photocatalysis under
visible-light irradiation. On the other hand, there have only
been a few reports on the development of new materials.
The designed construction of metal−organic frameworks
MOFs) has been of intense interest, due to their intriguing
1
2,13
generation.
Preliminary ab initio results by Civalleri et al.
3
4
Received: December 13, 2011
Revised: January 10, 2012
Published: January 20, 2012
(
©
2012 American Chemical Society
1603
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Crystal Growth & Design
Article
suggest a significant decrease in the band gap of MOF-5 when
changing the benzenedicarboxylic acid linker for larger organic
molecules. Our recent study has been mainly focused on the
syntheses and construction of new MOFs from accessible
the vessel was sealed and heated at 160 °C for 2 days. After the
mixture was slowly cooled to room temperature, purple crystals of 1
were obtained (yield = 42% based on Co). Anal. Calcd For
C H CoN O : C, 58.72; H, 3.22; N, 11.92%. Found: C, 58.75; H,
23
15
4
4
−1
14
3.30; N, 11.94%. IR spectrum (cm ): 3425w, 3157w, 2955s, 2925s,
multicarboxylic acids and imidazole-based spacers. A Cd-
containing MOF, a Gd-containing 3D supramolecular frame-
work and two isostructural trinodal 4-connected 3D frame-
works were found to be photocatalytically active, which
prompted us to synthesize new water-insoluble MOFs with
improved photocatalytic performance, especially visible-light-
driven MOF-based photocatalysts. As an extension of our work,
in order to examine the photoactivity of the MOFs by varying
the metal centers incorporated in the structures, the conjugated
2
1
8
854s, 1604s, 1567s, 1517s, 1489 m, 1420s, 1374s, 1329 m, 1302s,
253 m, 1127 m, 1104w, 1070s, 1007w, 961 m, 941w, 873 m, 853 m,
24 m, 765w, 729 m, 688w, 654 m, 626w, 600 m, 543w, 521w, 450w.
Synthesis of [Ni(btec)0.5(bimb)·(H O)]n (2). A mixture of
2
NiCl ·6H O (23.8 mg, 0.10 mmol), H btec (21.8 mg, 0.10 mmol),
2
2
4
bimb (28.6 mg, 0.10 mmol), and NaOH (0.10 mol/L, 3 mL) was
heated at 140 °C for 2 days in an analogous to procedure for 1. Pale
green crystals of 2 were obtained (yield = 33% based on Ni). Anal.
Calcd For C H N NiO : C, 56.67; H, 3.52; N, 11.50%. Found: C,
23 17
4
5
−
1
56.70; H, 3.56; N, 11.52%. IR spectrum (cm ): 3421w, 3164w,
3075w, 2925 m, 2855w, 1608s, 1520s, 1483 m, 1413w, 1344s, 1314s,
1256 m, 1198 m, 1151w, 1116 m, 1066s, 962 m, 943 m, 868 m, 820s,
800 m, 696 m, 647 m, 616w, 582 m, 508 m, 462 m, 424w.
1
,2,4,5-benzenetetracarboxylic acid (H btec) and 4,4′-bis(1-
4
imidazolyl)biphenyl (bimb) were chosen as the starting
materials together with different divalent metal ions. Utilizing
hydrothermal technique, five new MOFs, [M(btec) (bimb)]
0.5
n
Synthesis of [Cu(btec)0.5(bimb)] (3). A mixture of CuCl ·2H O
n
2
2
(
1) (M = Co (1), Ni (2), Cu (3), Zn (4)) and
(17.0 mg, 0.10 mmol), H btec (21.8 mg, 0.1 mmol), bimb (28.6 mg,
4
[
Cd(btec) (bimb) ] (5) have been obtained. Interestingly,
0.10 mmol), HCl (6 mol/L, a drop), and deionized water (3 mL) was
heated at 160 °C for 2 days in an analogous to procedure for 1. Purple
crystals of 3 were obtained (yield = 19% based on Cu). Anal. Calcd
For C H CuN O : C, 58.17; H, 3.18; N, 11.80%. Found: C, 58.15;
H, 3.24; N, 11.82%. IR spectrum (cm ): 3434w, 3132 m, 2925 m,
2854w, 1613s, 1586s, 1519s, 1417 m, 1376s, 1350 m, 1310 m, 1296 m,
1
8
4
0.5
0.5 n
compounds 1, 2, and 5 represent the rare examples of MOFs
with high photocatalytic activity for dye degradation under
visible light and good stability toward photocatalysis.
Compared to the MOFs constructed from 1,4-benzenedicar-
23 15
4
4
−1
14
boxylate acid and bimb in our previous work, the more
delocalized π electrons of 1,2,4,5-benzenetetracarboxylic acid
may facilitate the LMCT transitions and decrease electronic
band gap of the MOFs 1−5, which can be useful for the
development of visible-light-driven photocatalysts of MOFs.
249 m, 1128 m, 1101w, 1070 m, 1005w, 962w, 949w, 871 m, 833 m,
22 m, 779w, 743 m, 732w, 688w, 667w, 658w, 615w, 581 m, 536w,
55w, 432w, 406w.
Synthesis of [Zn(btec)0.5(bimb)] (4). The synthesis was similar
n
to that described in 1 except using Zn(Ac) ·2H O (0.10 mmol) for 4
2
2
instead of Co(Ac) ·4H O. Colorless crystals of 4 were obtained (yield
2
2
=
36% based on Zn). Anal. Calcd For C H N O Zn: C, 57.94; H,
EXPERIMENTAL SECTION
23 15 4 4
■
3
.17; N, 11.75%. Found: C, 57.90; H, 3.23; N, 11.77%. IR spectrum
Materials and Measurements. Reagents and solvents employed
−1
(cm ): 3424 m, 3161w, 3119 m, 2924s, 2854 m, 1624s, 1559w,
were commercially available and used as received. Ligand bimb was
1
1
5
520s, 1488 m, 1368s, 1333s, 1302 m, 1273w, 1186w, 1131s, 1103w,
066 m, 1004w, 960w, 942w, 850 m, 828w, 813s, 730 m, 685w, 651 m,
54w, 538 m, 465w, 419w.
15
prepared by literature methods. C, H, and N microanalyses were
carried out with a Perkin-Elmer 240 elemental analyzer. IR spectra
were recorded on KBr discs on a Bruker Vector 22 spectrophotometer
Synthesis of [Cd(btec)0.5(bimb)
]
(5). A mixture of Cd-
−
1
0.5 n
in 4000−400 cm
region. Thermogravimetric analyses were
(NO ) ·4H O (30.8 mg, 0.10 mmol), H btec (21.8 mg, 0.10 mmol),
3 2 2 4
performed on a simultaneous SDT 2960 thermal analyzer under
flowing N2 with a heating rate of 10 °C/min between ambient
temperature and 700 °C. The powder XRD data were collected on a
Siemens D5005 diffractometer with Cu Kα radiation (λ = 1.5418 Å)
over the 2θ range of 5−50° at room temperature. Fluorescence
measurements were recorded with a Hitachi 850 fluorescence
spectrophotometer. The solid-state diffuse-reflectance UV/vis spectra
for powder samples were recorded on a Perkin-Elmer Lambda 35 UV/
vis spectrometer equipped with an integrating sphere by using BaSO4
as a white standard, whereas the UV−vis spectra for solution samples
were obtained on a Shimadzu UV 2450 spectrometer. Photocatalytic
experiments: the visible light source was a 500 W halogen lamp with
UV cutoff filter (providing visible light with λ > 420 nm) and the
simulated solar light was a 350 W xenon lamp. A suspension of
powdered catalyst (50 mg) in fresh aqueous solution of X3B (50 mL, 6
bimb (28.6 mg, 0.10 mmol), NaOH (0.10 mol/L, 1 mL), and
deionized water (3 mL) was heated at 180 °C for 2 days analogous to
the procedure for 1. Colorless crystals of 5 were obtained (yield = 33%
based on Cd). Anal. Calcd For C H CdN O : C, 44.18; H, 2.12; N,
14
8
2
4
−1
7
3
.36%; found: C, 44.19; H, 2.14; N, 7.39%. IR spectrum (cm ):
440w, 3122w, 2923w, 2853w, 1594s, 1557s, 1515 m, 1488 m, 1413
m, 1368s, 1317 m, 1290w, 1262w, 1243w, 1125w, 1105w, 1066 m,
9
4
63w, 938w, 865w, 819 m, 768w, 745w, 661w, 584 m, 552w, 453w,
12w.
X-ray Crystallography. Suitable single crystals of 1−5 were
selected and mounted in air onto thin glass fibers. X-ray intensity data
were measured at 293 K on a Bruker SMART APEX CCD-based
diffractometer with graphite-monochromatic Mo Kα radiation (λ =
0
.71073 Å). Data reductions and absorption corrections were
performed with the SAINT and SADABS software packages,
×
10−6 mol/L) at pH 6 were first sonicated for 5 min, and shaken at a
16
respectively. All structures were solved by a combination of direct
constant rate in the dark overnight (to establish an adsorption/
desorption equilibrium of X3B on the sample surface). At given
irradiating intervals, a series of suspension of a certain volume were
collected and filtered through a membrane filter (pore size, 0.45 μm)
to remove suspended catalyst particles, and the filtrate was analyzed on
the UV−vis spectrometer. The concentration of the dye was measured
by the absorbance at 510 nm, which directly relates to the structure
change of its chromophore. The degradation of phenol was carried out
by the similar measurements. The initial concentration of phenol was
2
methods and difference Fourier syntheses and refined against F by the
17,18
full-matrix least-squares technique.
Anisotropic displacement
parameters were refined for all non-hydrogen atoms except for the
disordered atoms. The relevant crystallographic data are presented in
Table S1, while the selected bond lengths and angles are given in
Table S2 in the Supporting Information.
RESULTS AND DISCUSSION
■
4
30 μM, and the filtrate was analyzed on a Dionex P680 HPLC.
Synthesis of [Co(btec)0.5(bimb)]n (1). A mixture of Co-
Description of the Crystal Structures. Similar cell
parameters with the same space group Pbcn (Supporting
Information Table S1), and the results of crystallographic
analysis confirm that 1 and 4 are isostructural with each other
(
Ac) ·4H O (24.9 mg, 0.10 mmol), H btec (21.8 mg, 0.10 mmol),
2 2 4
bimb (28.6 mg, 0.10 mmol), and NaOH (0.10 mol/L, 3 mL) was
placed in a parr Teflon-lined stainless steel vessel (25 cm ), and then
3
1
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Figure 1. (a) Coordination environments of Co atoms with hydrogen atoms omitted for clarity. (b) 2D layer constructed from btec4− and Co . (c)
The complicated 3D structure of 1. (d) A schematic representation of PtS network structure of 1: blue spheres represent Co1 nodes and black
spheres represent btec4 nodes.
2+
−
Figure 2. (a) Coordination environments of Ni1 atoms with hydrogen atoms omitted for clarity. (b) The 2D layer structure of 2.
1
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Figure 3. (a) Coordination environments of Cu atoms with hydrogen atoms omitted for clarity. (b) The complicated 3D structure of 3. (c) A
schematic representation of NbO network structure of 3: aqua spheres represent Cu nodes and purple spheres represent btec4 nodes.
−
though they possess different metal centers; therefore, we
analyze the structure of 1 in detail.
bimb ligands in a tetrahedral geometry and can be regarded as a
tetrahedral node; each btec4 anion is linked to four planar
Co(II) ions and thus can be considered as a four-connected
‑
In compound 1, each Co(II) ion is in a distorted trigonal
bipyramid geometry, coordinated by one carboxylate oxygen
atom (O2) and two different bimb nitrogen atoms (N1 and
N4#2) at the basal positions, as well as two carboxylate oxygen
4‑
planar node; both btec and bimb bridges can be simplified to
be connectors. In such a case, the framework structure of 1 can
19
̈
be represented to be a cooperite PtS net with the Schlafli
4
−
2 4
atoms (O1 and O4#) from two distinct btec anions at the
apical positions (Figure 1a). In 1, each fully deprotonated
btec4 moiety bridges four Co(II) centers, with two para-
carboxylate groups in adopting monodentate coordination
modes and the other two para-carboxylate groups in bidentate
chelate fashions, producing 2D square grids (Figure 1b).
Meanwhile, the 2D layers are connected via bimb spacers with
Co···Co separations of 17.57 Å, resulting in the formation of a
final complicated 3D structure (Figure 1c).
symbol of (4 8 ), as displayed in Figure 1d.
In crystal of 4, although the most structural features are
similar to those in 1, the distinct difference is that each Zn(II)
ion is in a distorted tetrahedron sphere, surrounded by two
−
4
‑
carboxylate oxygen atoms from two separate btec anions and
two bimb nitrogen atoms, as illustrated in Supporting
Information Figure S1.
As shown in Figure 2a, the Ni(II) center is in a slightly
distorted octahedral environment, the equatorial plane of which
comprises three carboxylate oxygen atoms (O1, O3, O3#1)
from two inequivalent btec4 anions and one terminal
coordination water (O5); two nitrogen atoms(N1, N3) from
A better insight into the nature of this intricate framework
can be acquired by using topological analysis. As depicted
−
2+
4‑
above, each Co ion is connected by two btec anions and two
1
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4
‑
2+
Figure 4. (a) Coordination environments of Cd atoms with hydrogen atoms omitted for clarity. (b) 2D layer constructed from btec and Cd . (c)
The complicated 3D structure of 5. (d) A schematic representation of (4,6)-connected network structure of 5: green spheres represent Cd1 nodes,
purple spheres represent btec4 nodes and bimb bridges are shown as green bonds.
−
two different bimb ligands occupying the apical sites. Each
btec4 anion coordinates to four Ni(II) atoms, with
and Cu2 atoms may also be regarded as in a distorted
octahedron environment.
In 3, each fully deprotonated btec moiety bridges four
Cu(II) centers, with two para-carboxylate groups in adopting
monodentate coordination modes and the other two para-
carboxylate groups in bidentate chelate fashions, giving rise to
‑
2
2
0
4−
deprotonated carboxylate groups adopting μ -η :η and
monodentate coordination modes, generating a 1D infinite
chain along b axis. The 1D chain is further stacked with
bidentate bimb spacers acting as double bridges, with Ni···Ni
distance of 17.97 Å, therefore, giving rise to the formation of a
2
D square grids similar to that of compound 1 (Supporting
2D layer (Figure 2b).
Information Figure S2). Furthermore, the 2D layers are
connected via bimb spacers with Cu···Cu separations of 17.57
Å, resulting in the formation of a complicated 3D structure
As displayed in Figure 3a, both Cu1 and Cu2 are situated on
an inversion center with occupancy of 0.5. The Cu1 and Cu2
atoms are located in the similar distorted square sphere,
(
Figure 3b).
coordinated to two carboxylate oxygen atoms from inequivalent
btec4− ligands and two bimb nitrogen atoms. In addition, Cu1−
Thus, from the point of view of structural description of the
compound, Cu1 and Cu2 were considered as equivalent nodes
due to the same coordination configuration; in addition, both
the Cu(II) atoms and the btec4 anions may be considered
equivalent to 4-connecting square-planar nodes in the ratio 1:1,
O2 and Cu1−O2#1 distances are 2.936(3) Å, Cu2−O3 and
Cu2−O3#2 separations of 2.743(3) Å, suggesting non-
negligible interactions between copper atoms and the
carboxylate oxygen atoms, which can be described as a
−
20
21
semichelating coordination mode. Consequently, both Cu1
precisely equivalent to the NbO network (Figure 3c). Such a
1
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net structure can be characterized by a short vertex symbol of
ascribed to π−π* transitions of the ligands, whereas the main
UV absorption bands at 327, 344(393), 317, and 325 nm for 1,
2, 4, and 5 can be attributed to ligand-to-metal charge transfer
(LMCT). It should be noted that the absorption band
occurring in the visible region at 476 nm for 5 also could be
assigned to LMCT. In the case of 1 and 2, additional clear
peaks in the visible region observed at 508(606) and 647 nm,
which probably respectively originate from the d-d spin-allowed
4
2
(
6 8 ).
Though the ligands are the same as those in 1−4, the
structure of 5 is significantly different from 1−4. Compound 5
is a most remarkable and unique three-dimensional coordina-
tion polymer. As shown in Figure 4a, the Cd(II) center is in a
severely distorted octahedral environment, the equatorial plane
of which comprises three carboxylate oxygen atoms (O2, O1#1,
4
−
2+
7
2+
8
O4#1) from two inequivalent btec anions and one bimb
transition of the Co (d ) and Ni (d ) ions. The absorption
of 4 in the visible region is not as distinct as that of other
samples, which may result from the d-d spin transition of the
nitrogen atom (N1); two carboxylate oxygen atoms (O1,
4−
O3#2) from two different btec moieties occupying the apical
2+
10
sites.
Zn (d ) ion. The presence of visible regions transitions
motivated us to explore applications of 1, 2, and 5 in
heterogeneous photocatalysis. To obtain the precise values of
band gap from the absorption edges, the point of inflection in
the first derivatives of the absorption spectrum was used. The
values of the band gap for 1, 2, and 5 obtained from
corresponding LMCT transitions are 2.68, 2.63, and 2.32 eV,
respectively. Clearly, the band gaps of 1, 2, and 5 follow the
order 1 ≈ 2 > 5.
Each btec4 anion coordinates to six Cd(II) centers, with two
−
2
2
1
para-carboxylate groups in adopting μ -η :η coordination
modes and another two para-carboxylate groups in μ -η :η
2
1
1
4−
fashions. The btec ligands thus link Cd ions to form a 2D
layer structure in ab plane, as illustrated in Figure 4b. The 2D
layers were further stacked with bidentate bimb spacers with
Cd···Cd separations of 17.90 Å, giving rise to the formation of a
3
D framework. By closer inspection of the structure 5, if the Cd
4−
centers act as nodes, btec anions and bimb bridges serve as
linkers, each Cd center can be simply considered as linking
Herein, we selected an anionic organic dye X3B (Scheme 1),
as a target pollutant for degradation experiments to evaluate the
three btec4 moieties and one Cd atom whereas each of btec
−
4−
anion acts as connecting six Cd atoms. Remarkably, compound
exhibits (4,6)-connected 3D binodal topology, which can be
Scheme 1. Molecular Structure of X3B
5
3
3
6 6 3
described by the Schlafli symbol of (4 6 ) (4 6 8 ) with the
̈
2
4−
vertex symbols for the 4-connected btec and 6-connected Cd
nodes, as illustrated in Figure 4d.
Obviously, from the crystal structures described above, the
different coordination geometry around the central metal atom
resulted in the completely different structures of MOFs. The
results imply that the metal center has a great impact on the
structures of the complexes. The phase purity of the bulk
material was independently confirmed by powder X-ray
diffraction (PXRD). The PXRD patterns of all as-synthesized
products closely match the simulated ones from the single-
crystal data, indicating that products are in a pure phase
photocatalytic performance of compounds 1, 2, and 5,
considering that X3B is commonly used as a representative of
widespread organic dyes that are very difficult to decompose in
waste streams under UV or visible-light irradiation. It is hard
for us to evaluate the photocatalytic performance of compound
22
3
because of its extremely low yield. The photodegradation
(
Supporting Information Figure S9−S13).
experiment under visible irradiation was carried out after the
dark adsorption−desorption equilibrium achieved. In addition,
control experiments on the photodegradation of X3B have been
carried out. Obviously, under dark conditions without light
illumination, the concentration of X3B almost does not change
for every measurement in the presence of compounds 1, 2, and
Photocatalytic Activities. The diffuse-reflectance UV−vis
spectra reveal the absorption features of compounds 1, 2, 4, and
5
(Figure 5), and all spectra consist of absorption components
5
. Illumination in the absence of compounds 1, 2, or 5 does not
result in the photocatalytic decolorization of X3B (Supporting
Information Figure S3−S5). Therefore, the presence of both
illumination and compounds 1, 2 or 5 is necessary for the
efficient degradation of X3B. The distinctly shortened
degradation time compared with the control experiments
indicates that both catalyst 1, 2, and 5 are active for the
decomposition of X3B in the presence of visible-light
irradiation. Figure 6 presents the comparison of photocatalytic
profiles of the samples and commercial TiO (Degussa P-25)
2
under visible irradiation. Apparently, Degussa P-25 showed
only slight photocatalytic activity. On the contrary, the as-
prepared samples of 1, 2, and 5 exhibit higher photocatalytic
efficiencies under the same condition. Notably, a very fast
degradation of X3B for compound 5 was achieved, but only at
the beginning of irradiation. Compound 5 showed the highest
photocatalytic activity among all the photocatalysts, with a
conversion of more than 80% within 5 h. The kinetic data for
the degradation of X3B can be well fitted by the apparent first-
Figure 5. UV−vis diffuse-reflectance spectra of compounds 1, 2, 4, and
with BaSO as background.
5
4
in the UV and Vis regions. In all cases, the intense absorption
peaks at 257, 261, 255, and 262 nm for 1, 2, 4, and 5 can be
1
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indicate that the more delocalized π electrons of 1,2,4,5-
benzenetetracarboxylic acid may facilitate the LMCT tran-
sitions and decrease electronic band gap of the MOFs, which
contribute greatly to the enhanced photocatalytic rate.
Therefore, it can be concluded that the semiconductor
properties of the obtained MOFs strongly depend on the
13
resonance effects in the organic linker.
To make clear what active species are involved in the
photocatalytic process occurring on compound 5, the formation
of hydroxyl radicals (•OH) on the surface of visible-illuminated
compound 5 was detected by the photoluminescence (PL)
technique using terephthalic acid as a probe molecule (Scheme
24−26
2
).
For comparison, compounds 1 and 2 were also
Scheme 2. Formation of 2-Hydroxyterephthalic Acid in the
Reaction of Terephthalic Acid with Hydroxyl Radicals
Figure 6. Degradation profiles of X3B under visible light irradiation in
the presence of (a) without catalyst, (b) Degussa P-25, (c) 1, (d) 2,
and (e) 5.
order rate equation, ln(C/C ) = kt, where k is the rate constant,
0
C and C are the concentration of X3B at irradiation time t = 0
0
and t, respectively. The rate constant under visible-light
irradiation was found to be 0.18, 0.19, 0.30, and 0.04 h−1
separately for that of 1, 2, 5, and P-25. The enhanced
photocatalytic activity of 5 may be attributed to the combined
effects of two factors: first, the band gap of 5 is obviously lower
than that of other samples, therefore, the charge transfer for 5,
which is from oxygen and (or) nitrogen 2p bonding orbitals
examined under the same conditions (Supporting Information
Figure S6−S7). Figure 7a shows the changes in the PL spectra
for 5 × 10− M terephthalic acid solution in 2 × 10 M NaOH
with irradiation time in the presence of compound 5. As can be
clearly seen from this figure, a gradual increase in the PL
intensity of photogenerated 2-hydroxyterephthalic acid at about
425 nm is observed with increasing irradiation time. However,
no PL intensity increase is observed in the absence of
compound 5, which suggests that the fluorescence is caused
by chemical reactions of terephthalic acid with •OH formed at
the MOF/water interface via photocatalytic reactions. Figure 7b
shows a comparison of the induced PL intensity at 425 nm for
different samples with irradiation time. It can be seen that the
PL intensity of photogenerated 2-hydroxyterephthalic acid
increases linearly with time for all samples. Consequently, it can
be inferred that •OH radicals produced at the catalyst surface
are proportional to the light irradiation time obeying zero-order
4
−3
(
valence band) to empty Cd orbitals (conduction band), easily
takes place on photoexcitation. According to CASTEP
2
3
calculations (Supporting Information Figure S14), the solid-
state compound 5 thus shows a semiconducting character with
a band gap of 2.96 eV, which is comparable with the
experimental value. Second, by careful comparison, it could
be noted that the charge-transfer transition of 5 could occurs in
the visible region (476 nm), whereas that of other samples lie in
the UV region, which may give the reasons for the higher
visible-light responsive catalytic activities for compound 5. To
exclude the possibility that the photocatalytic properties of 1, 2,
and 5 result from dissolved molecular or oligomeric fragments
of solid catalysts in the photocatalytic process, another control
experiments were conducted. The reaction suspensions after 10
h of irradiation were filtered to remove the solid catalyst
particles, and fresh X3B was added into the respective filtrates
for catalysis testing. Without solid catalyst in the reaction
system, the fresh X3B was not degraded during another 10 h of
irradiation under halogen lamp, which indicates that the
solution contains no photocatalytically active fragments. Clearly
the photocatalytic activities arise solely from the solid 1, 2, and
27
reaction rate kinetics. The formation rates of •OH radicals
can be expressed by the slope of these lines shown in Figure 7b.
Obviously, the formation rate of •OH radicals on compound 5
is much higher than that of 1 and 2, implying that the
formation rate of •OH radicals during photocatalysis is in
agreement with photocatalytic activity and the formation rate of
•OH radicals is an important factor influencing photocatalytic
activity.
The photocatalytic degradation kinetics and •OH radical
detection experiments provide valuable mechanistic informa-
tion. Consequently, a simplified model of photocatalytic
reaction mechanism was proposed as depicted in Supporting
Information Scheme S1. Because the HOMO is mainly
contributed by oxygen and (or) nitrogen 2p bonding orbitals
(valence band, VB) and the LUMO by empty transition metal
orbitals (conduction band, CB). Under visible light irradiation,
5. After photocatalysis, compounds 1, 2, 5 were recycled by
filtration and powder XRD for the catalysts were checked to
evaluate the stability toward photocatalysis. The patterns were
found to be nearly identical to those of the parent compounds,
which demonstrate that these complexes are rather stable
during photocatalysis (Supporting Information Figure S9−
S13).
−
electrons (e ) in the HOMO (VB) of MOF were excited to its
+
In our earlier work, the MOFs with photocatalytic activity are
constructed from 1,3,5-benzenetricarboxylate acid (btc) and
bimb combined with different metal ions. The degradation rate
constants under visible light irradiation were 0.073 and 0.13 h−1
for isostructural compounds [Mn (btc) (bimb) ·(H O) ] and
LUMO (CB), with same amount of holes (h ) left in VB. The
HOMO strongly demands one electron to return to its stable
state. Therefore, one electron was captured from water
molecules, which was oxygenated into •OH active species.
−
Meanwhile, the electrons (e ) in LUMO could be combined
3
2
2
2
4 n
[
Co (btc) (bimb) ·(H O) ] , respectively, which are obviously
with the oxygen adsorbed on the surfaces of MOF to form
3
2
2
2
4 n
14a
−
slowly than those of compounds 1, 2, and 5. The results
•O , then they might transform to the hydroxyl radicals
2
1
609
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Crystal Growth & Design
Article
Figure 7. (a) PL spectral changes observed during illumination of compound 5 in a 5 × 10−4 M basic solution of terephthalic acid (excitation at 315
nm). Each fluorescence spectrum was recorded every 30 min of visible illumination. (b) Comparison of the induced PL intensity at 425 nm for
compounds 1, 2, and 5.
Figure 8. Concentration changes of X3B as a function of irradiation time for complexes 1 (a), 2 (b), and 5 (c), respectively. Conditions: (I) H O /
2
2
1
(2, 5)/dark, (II) 1(2, 5)/visible light, (III) H O /visible light, (IV) H O /1(2, 5)/visible light.
2 2 2 2
(
•OH). Then the formed •OH radicals could cleave X3B
conductance band would also produce hydroxyl radical. Even if
H O was not reduced at the conductance band, it could accept
an electron from superoxide to give rise to hydroxyl radical (eq
2). Third, the self-decomposition by illumination would also
produce hydroxyl radical (eq 3).
effectively to complete the photocatalytic process. The
proposed photocatalytic degradation mechanism was very
similar to those of the previously reported MOFs.
2
2
1
4
Limitation to the rate of photocatalytic degradation had been
attributed to the recombination of photogenerated hole−
CB−
−
2
8−30
e
+ H
2
O
2
→ OH + •OH
(1)
(2)
(3)
electron pairs.
The addition of inorganic oxidant, such as
H O , may play important roles in accelerating the degradation
−
−
2
2
•
O
+ H O → OH + •OH + O
2
2 2
2
rate of azo dyes. It has been found that H O could increase the
2
2
rate of hydroxyl radical formation through three ways. First, it
could act as an alternative electron acceptor to oxygen (eq 1),
which might restrain the recombination of the photogenerated
electrons and holes. Second, the reduction of H O at the
H O + hν → 2•OH
2
2
Here, experiments were also conducted to study the synergistic
effect of H O and MOF on the photodegradation of X3B.
2
2
2
2
1
610
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Crystal Growth & Design
Article
Figure 8c shows the degradation of X3B in the presence of
model pollutant which is recognized as being difficult to
decompose. Furthermore, compound 5 is photocatalytically
more active than 1 and 2 because of the relatively narrower
band gap calculated from LMCT transitions. The presence of
•OH radicals, which can effectively oxide the dye molecules,
have been detected by PL technique. In addition, the formation
H O (10 mL) and compound 5 (50 mg) under different
2
2
conditions. The degradation of X3B is slow in the dark (23.4%
of X3B is degraded after 5 h, curve I in Figure 8c). Visible-light
irradiation greatly accelerates the photodegradation of X3B
(
94.1% of X3B is photodegraded after visible irradiation for 5 h,
curve II in Figure 8c). It can be seen that, the presence of H O2
rate of •OH radicals on 5/H O interface via photocatalytic
2
2
can greatly enhance the photodegradation of X3B on 5. The1
photocatalytic degradation rate constant of X3B was 0.56 h−
on 5 in the presence of H O (curve IV in Figure 8c), which is
reactions is much higher than that of 1 and 2, implying that the
formation rate of •OH radicals during photocatalysis is in
agreement with photocatalytic activity and the formation rate of
•OH radicals is an important factor influencing photocatalytic
performance. Meanwhile, the synergistic effect of H O and
2
2
−1
1
.8 times higher than that in the absence of H O (0.31 h )
2 2
under visible illumination. Similar experimental results were
2
2
also obtained for 1 and 2 photocatalysts after addition of H O .
MOF on the photodegradation of X3B has been studied, which
could obviously enhance the degradation rate of X3B under
visible light. The successful synthesis of 1, 2, and 5 provides
access to a promising path in the search for stable new visible-
light-driven photocatalysts.
2
2
The corresponding rate constant for the degradation of X3B on
−1
1
a1nd 2 increased from 0.12 to 0.20 h and from 0.13 to 0.39
−
h , respectively.
Considering that the dye can also be degraded through
photosensitized pathway, colorless molecule, phenol, was
therefore selected to test the photocatalytic activity of
compound 5. Figure 9 shows the concentration profiles of
ASSOCIATED CONTENT
■
*
S
Supporting Information
Crystallographic data, selected bond lengths and angles,
additional crystal figures, control experiments on the photo-
degradation of X3B, PL spectral changes observed during
illumination of compound 1 or 2 in a basic solution of
terephthalic acid, TGA, PXRD, and computational procedures.
This information is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*
E-mail: wenlili@mail.ccnu.edu.cn (L.W.); dfli@mail.ccnu.edu.
cn (D.L.). Fax: + 86 27 67867232. Phone: +86 27 67862900.
Notes
The authors declare no competing financial interest.
Figure 9. Photocatalytic degradation of phenol in the absence of 5
under simulated solar light.
ACKNOWLEDGMENTS
■
This work was financially supported by the National Nature
Science Foundation of China (Nos. 201171062 and 20801021),
Program for New Century Excellent Talents in University of
China (NCET-10-040), Program for Distinguished Young
Scientist of Hubei Province (2010CDA087), Self-determined
research funds of CCNU from the colleges’ basic research and
operation of MOE (CCNU10A01008, CCNU09A02002,
CCNU11C01002), Program for Innovation Group of Hubei
Province of China (2009CDA048), and Program for
Changjiang Scholars and Innovative Research Team in
University (PCSIRT, No. IRT0953).
phenol and the evolution of intermediates as a function of the
irradiation time under simulated solar light. Experimental
results show that the concentration of phenol decreases, while
ortho- and para-intermediates increases with light irradiation
time (the conversion of the phenol is 40.13% and the selectivity
for the intermediate catechol is 45.32% after 9 h irradiation).
Control experiments (without catalyst or in the dark) show that
no obvious phenol degradation was observed. Therefore, it can
be safe to draw a conclusion that compound 5 is an efficient
3
1,32
visible-responsible photocatalyst.
CONCLUSION
■
REFERENCES
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