Copper(II) imidazolate frameworks as highly efficient photocatalysts for reduction of CO2 into methanol under visible light irradiation

Article abstract of DOI:10.1016/j.jssc.2013.04.016

Three copper(II) imidazolate frameworks were synthesized by a hydrothermal (or precipitation) reaction. The catalysts were characterized by X-ray diffraction (XRD), nitrogen adsorption, transmission electron microscopy (TEM), ultraviolet-visible spectroscopy (UV-vis), Fourier transform infrared spectra (FTIR), thermogravimetry (TG). Meanwhile, the photocatalytic activities of the samples for reduction of CO₂ into methanol and degradation of methylene blue (MB) under visible light irradiation were also investigated. The results show that the as-prepared samples exhibit better photocatalytic activities for the reduction of carbon dioxide into methanol with water and degradation of MB under visible light irradiation. The orthorhombic copper(II) imidazolate frameworks with a band gap of 2.49 eV and green (G) color has the best photocatalytic activity for reduction of CO₂ into methanol, 1712.7 μmol/g over 5 h, which is about three times as large as that of monoclinic copper(II) imidazolate frameworks with a band gap 2.70 eV and blue (J) color. The degradation kinetics of MB over three photocatalysts fitted well to the apparent firstorder rate equation and the apparent rate constants for the degradation of MB over G, J and P (with pink color) are 0.0038, 0.0013 and 0.0016 min^-1, respectively. The synergistic effects of smallest band gap and orthorhombic crystal phase structure are the critical factors for the better photocatalytic activities of G. Moreover, three frameworks can also be stable up to 250 °C. The investigation of Cu-based zeolitic imidazolate frameworks maybe provide a design strategy for a new class of photocatalysts applied in degradation of contaminations, reduction of CO₂, and even water splitting into hydrogen and oxygen under visible light.

Full text of DOI:10.1016/j.jssc.2013.04.016

Journal of Solid State Chemistry 203 (2013) 154–159
Contents lists available at SciVerse ScienceDirect
Journal of Solid State Chemistry
journal homepage: www.elsevier.com/locate/jssc
Copper(II) imidazolate frameworks as highly efficient photocatalysts
for reduction of CO2 into methanol under visible light irradiation
n
Jingtian Li, Deliang Luo, Chengju Yang, Shiman He, Shangchao Chen, Jiawei Lin, Li Zhu, Xin Li
Institute of Biomaterial, College of Science, South China Agricultural University, Guangzhou 510642, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Three copper(II) imidazolate frameworks were synthesized by a hydrothermal (or precipitation) reaction.
The catalysts were characterized by X-ray diffraction (XRD), nitrogen adsorption, transmission electron
microscopy (TEM), ultraviolet–visible spectroscopy (UV–vis), Fourier transform infrared spectra (FTIR),
thermogravimetry (TG). Meanwhile, the photocatalytic activities of the samples for reduction of CO2 into
methanol and degradation of methylene blue (MB) under visible light irradiation were also investigated.
The results show that the as-prepared samples exhibit better photocatalytic activities for the reduction of
carbon dioxide into methanol with water and degradation of MB under visible light irradiation. The
orthorhombic copper(II) imidazolate frameworks with a band gap of 2.49 eV and green (G) color has the
best photocatalytic activity for reduction of CO2 into methanol, 1712.7 μmol/g over 5 h, which is about
three times as large as that of monoclinic copper(II) imidazolate frameworks with a band gap 2.70 eV and
blue (J) color. The degradation kinetics of MB over three photocatalysts fitted well to the apparent first-
order rate equation and the apparent rate constants for the degradation of MB over G, J and P (with pink
color) are 0.0038, 0.0013 and 0.0016 min−1, respectively. The synergistic effects of smallest band gap and
orthorhombic crystal phase structure are the critical factors for the better photocatalytic activities of G.
Moreover, three frameworks can also be stable up to 250 1C. The investigation of Cu-based zeolitic
imidazolate frameworks maybe provide a design strategy for a new class of photocatalysts applied in
degradation of contaminations, reduction of CO2, and even water splitting into hydrogen and oxygen
under visible light.
Received 20 November 2012
Received in revised form
11 April 2013
Accepted 14 April 2013
Available online 23 April 2013
Keywords:
Photocatalysts
MOFs
Imidazolate frameworks
Visible-light photocatalysis
Photocatalytic reduction of CO2
& 2013 Elsevier Inc. All rights reserved.
1. Introduction
world nowadays. Recently, Zifs have also been used in the photo-
catalytic degradation of organic dye, Isimjan et al. [7] loaded Pt/
Porous metal–organic frameworks (MOFs) have been exten-
sively investigated since 1990 [1,2]. Zeolitic imidazolate frame-
works (ZIFs) are a new class of nanoporous compounds with
tetrahedral networks that resemble those of zeolites: transition
metals (Co, Cu, Zn, etc.) replace tetrahedrally coordinated atoms
(for example, Si), and imidazolate links replace oxygen bridges
[3,4]. As a subfamily of MOFs, ZIFs exhibit the tunable pore size
and chemical functionality of classical MOFs [5]. Meanwhile, they
are chemically and thermally stable, yet have the long-sought-
after design flexibility offered by functionalized organic links
and a high density of transition metal ions [3]. Because of these
combined features, ZIFs have very promising applications in
catalysis [6–8], gas storage [3,9,10], chemical separation [11,12],
molecular recognition [13] and optical [14], electronic [15,16],
photoluminescence and magnetic materials [17], and consequently
have drawn considerable attention from chemists around the
ZIF-8 nano-crystallites onto the surfaces of TiO2 nanotubes and
observed increase in photocatalytic properties of ZIF-8 loaded TiO2
nanotubes by photodegradation of phenol. Yang et al. [18] doped
copper into ZIF-67 and found that the Cu-doped ZIF-67 showed
high gas uptake capacity and highly efficient visible-light-driven
photocatalytic property on the degradation of methyl orange.
The energy shortages and the environmental pollution causing
by the depletion of fossil fuels and the gradual increase of atmo-
spheric carbon dioxide (CO2) concentration are two main chal-
lenges in today's society. The artificial photosynthesis, converting
CO2 and water into valuable energy-bearing compounds (such as
CO, methane, and methanol) by means of solar energy, is one of
the most attractive methods to overcome both global warming
and energy crisis. Since photocatalytic reducduction of CO2 to
formic acid, formaldehyde, methanol, and methane over TiO2
under UV light was discovered by Fujishima and Inoue [19], a
number of semiconductors such as TiO2 nanoparticles or nano-
tubes [20–26], CaFe2O4 [27], ZnGa2O4 [28], ZnGe2O4 [29,30],
Bi2WO6 [31], HNb3O8 [32], InTaO4 [33], Zn–Cu–Ga photocatalyst
[34], and GaP [35] have been reported to be available for CO2
n
Corresponding author.
E-mail address: xinliscau@yahoo.com (X. Li).
0022-4596/$ - see front matter & 2013 Elsevier Inc. All rights reserved.
http://dx.doi.org/10.1016/j.jssc.2013.04.016

J. Li et al. / Journal of Solid State Chemistry 203 (2013) 154–159
155
conversion under light illumination. However, the efficiency
2.3. Photocatalytic reduction of CO2
attained by these studies has been very low due to the large band
gaps or the photocorrosion process. In order to make full use of
solar energy, it is of great importance to develop new visible light
photocatalysts with high activities. Moreover, it is well known that
copper has good selectivities for photo-reduction CO2 into metha-
nol [22,36–38] and that the metal free polymer containing C¼N
band have good photocatalytic activities for hydrogen produced
from water [39,40] and photocatalytic reduction of CO2 [41]. In
particular, to our knowledge, the directly photocatalytic reduction
of CO2 on the photocatalysts of copper-based MOFs or imidazo-
dlate frameworks containing C¼N band under visible light has not
been reported yet until now.
Therefore, the synergistic effects of copper and C¼N band in
imidazodlate frameworks on the photocatalytic activities of semi-
conductors are very interesting research topics. Herein, we
reported for the first time the photocatalytic reduction of CO2 into
methanol on three copper(II) Imidazolate Frameworks, which
named green (G), blue (J) and pink (P), respectively. The photo-
The photocatalytic reduction of CO2 was performed in a
continuous-flow reactor system as shown in our previous study
[24]. A 500 W Xe lamp located in the quartz cool trap was the
irradiation source and the UV light below 400 nm was removed by
a 2.0 M sodium nitrite solution [24,46]. Prior to experiments,
sodium hydroxide (0.80 g) and absolute sodium sulfite (2.52 g)
were dissolved in 200 ml distilled water. This solution was then
put into a photochemical reaction instrument. Before irradiation,
ultrapure CO2 was bubbled through the solution in the reactor for
at least 30 min to ensure that all dissolved oxygen was eliminated,
then, 200 mg of catalyst powder was added into above solution,
and the irradiation lamp was turned on to start the photoreaction.
Ultrapure CO2 was continuously bubbled through the above
solution in the reactor during the whole irradiation. The radiation
time was 5 h. A needle-type probe was inserted into the solution
in the reactor to withdraw a small liquid sample at 1, 2, 2.5, 3, 3.5,
4, 4.5 and 5 h, respectively. The concentration of methanol in the
catalytic activities of the photocatalysts were studied in
a
samples was analyzed using a GC9560 gas chromatograph
continuous-flow reactor system under visible light irradiation.
The relations between the structures, visible light absorption
capabilities and the CO2 conversion efficiency under visible light
irradiation were reported.
equipped with a flame ionization detector and a stainless steel
packed column (Porapak-Q, 2 mm ꢁ 3 m). The blank test was
performed in the dark with the catalyst and CO2 or in the light
without CO2 under the same other experimental conditions.
The methanol has not been detected.
2. Experimental
2.4. Photocatalytic activity for the degradation of methylene blue.
2.1. Preparation of copper(II) imidazolate frameworks
Visible light photocatalytic activities of G, J, and P were also
evaluated through the degradation of methylene blue (MB) solu-
tion with an initial MB concentration of 10 mg/l. A 500 W Xe lamp
located in the quartz cool trap was the irradiation source, and the
UV light below 400 nm was removed by a 2.0 M sodium nitrite
solution [24,46]. In a typical run, aqueous slurries were prepared
by adding 0.2 g photocatalyst to 600 ml methyl orange aqueous
solution. After that, the suspension was stirred for 30 min in the
dark to ensure adsorption/desorption equilibrium before light
illumination. During the irradiation procedure, the reaction sam-
ple was collected at 20-min intervals and centrifuged to remove
photocatalyst particles. The filtrates were analyzed with a spectro-
photometer by measuring its absorbance at 662 nm.
The crystal of P was synthesized according to the procedure
reported previously [42], whereas the crystal of G and J were
synthesized by a modified procedure reported previously [42].
Briefly, For P, NaOH (0.1 M) was added to a 50 ml of mixture of
imidazole (2 g, 29.4 mmol) and Cu(NO3)2 ꢀ 3H2O (2.50 mmol), the
obtained reddish-purple powder was washed with ethanol and
dried in vacuo at 80 1C for J, after 12.5 ml of CuSO4 ꢀ 5H2O solution
(0.8 M) was added dropwise to a 50 ml of mixture of imidazole
and NaHCO3 at 80 1C, the mixture was left under room tempera-
ture for 2 h. Then, the blue compound was filtered off, washed
with water, and dried at 80 1C overnight in an oven. For G, after
15 ml of imidazole solution (0.245 M) was added to the 25 ml of
mixture of CuSO4 ꢀ 5H2O and NH4OH, the mixture was hydrother-
mally treated at 110 1C in a Teflon-lined stainless steel autoclave
for 48 h. After reaction, the product was collected by filtration and
then washed 3–4 times with distilled water. Finally, the samples
were dried at 80 1C overnight in an oven.
3. Results and discussion
3.1. XRD analysis
All three as-prepared samples were studied by powder XRD to
identify the phase structures. Fig. 1 shows the XRD patterns of
different samples. As observed in Fig. 1, all the diffraction peaks of
G and J matches well with those of G (green) and J (blue)
simulated from the single-crystal structure reported in Ref. [42],
and no other crystallite phases were observed. Thus, it is easy to
find that the G shows orthorhombic structure with distorted
square-planar coordination (trans N–Cu–N angles of 1541 for
Cu1 and 1421 and 1381 for Cu2), and the room temperature lattice
parameters are a¼21.139 Å, b¼19.080 Å and c¼9.2843 Å;
whereas, the results also revealed that the as-synthesized sample
J was monoclinic phase with the unit cell parameters, a ¼11.75 Å,
b¼14.07 Å and c¼8.77 Å. Moreover, although P does not match
well with any structural model due to its complexity and broad
peak features, the XRD patterns of P are consistent with the
observed one of the P phase in Ref [42]. In conclusion, the above
analyses and XRD patterns evidence further confirm that three
different phase structures of G, J and P were successfully prepared.
2.2. Characterization
Nitrogen adsorption measurements were carried out at 77 K by
using an accelerated surface area and porosimetry system (ASAP
2010, Micromeritics) equipped with commercial software of
calculation and analysis for determination of the textural properties
of the photocatalyst studied [24,43–45]. The UV–Vis spectroscopy in
the 200–800 nm was measured with a Shimadzu UV-2550PC
diffuse reflectance spectroscopy. The transmission electron micro-
scopy (TEM) was performed on a JEM-2100HR (200 kV, Japan)
operated at 120 kV. The scanning electron microscope (SEM) was
done on a LEO 1530VP field emission scanning electron microscope
(LEO Electron Microscopy Inc., Germany). The XRD patterns were
obtained at room temperature using a MSAL-XD2 diffractometer
with CuKα radiation (operated at 36 kV and 30 mA, λ¼0.15406 nm).
The Fourier transform infrared spectra (FTIR) of the samples were
recorded using a Nicolet 510 P spectrometer.

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J. Li et al. / Journal of Solid State Chemistry 203 (2013) 154–159
3.3. Nitrogen sorption
Nitrogen adsorption studies were also conducted to determine
the specified surface areas of the samples. Fig. 3 displays nitrogen
sorption properties of three copper(II) imidazolate frameworks.
According to the Brunauer–Emmett–Teller (BET) equation, the BET
surface areas of P, G and J were calculated to be 11.92, 3.99 and
0.68 m2/g, respectively. It is clear to see that J exhibits the smallest
BET surface among three samples. The possible reasons may be
that J is the one showing the highest density (1.824 g/cm3) [42].
Meanwhile, it is also observed that P has the largest BET surface
among three copper(II) imidazolate frameworks.
3.4. TEM analysis
Fig. 1. XRD patterns of different photocatalysts. (For interpretation of the refer-
ences to color in this figure legend, the reader is referred to the web version of this
article.)
The further morphologic and structural characterization was
investigated by a typical transmission electron microscopy (TEM)
image. Fig. 4 shows the TEM micrographs of G as an example. As
presented in Fig. 4(a), it is found that the irregular particles of
G with sizes from 100 to 300 nm were obtained via the hydro-
thermal synthesis. Furthermore, it is also seen from Fig. 4(b) that
the primary G nanoparticles are inclined to aggregate to form larger
secondary particles. The aggregates vary in sizes from less than
1 μm to as large as 5 μm with sheet-like or rod-like morphologies.
3.5. FTIR analysis
Fourier transform infrared spectroscopy (FTIR) spectra were
measured to identify organic functional-groups in the materials,
and the corresponding results were shown in Fig. 5. As presented
in Fig. 5, the broad band around 3446 cm−1 is usually assigned to
the O–H vibration adsorbed water, H–OH [53–55]. One new peak is
distinguished at 3100 cm−1 which is attributed to N–H vibrations
[56]. Furthermore, all the peaks for G in the 1700–400 cm−1 region
match those reported for copper imidazolate complexes with
ancillary ligands [42]. The peaks at 1646 cm−1 result from the
stretching vibration of C¼N in the imidazole ring, whereas, the
peaks at 939 and 1340 cm−1 can be ascribed to the stretching
vibration C–N in the imidazole ring [56]. The appearance of
vibration bands due to C–N and C¼N groups indicates that the
imidazole molecule exists in three copper imidazolate complexes.
Fig. 2. UV–vis absorption spectra of different photocatalysts.
3.2. UV–vis analysis
The optical properties of three copper(II) imidazolate frame-
works were probed by using UV−vis diffuse reflectance spectra.
The corresponding optical absorption of three copper(II) imidazo-
late frameworks is shown in Fig. 2. As can be seen, all three
samples display strong absorption in the visible light region above
400 nm and their optical absorbance edges are between 450 and
550 nm. Furthermore, the band gap energy of the synthesized
materials can be calculated by the plots of the transformed
Kubelka–Munk function versus the wavelength [47–49]. The plots
of (αhν)2/n as a function of hν are in accordance with the
theoretical Eq. (1) [24,46,50–52],
3.6. Thermogravimetry (TG) analysis
The thermal stability of the sample was studied by thermo-
gravimetry (TG) analysis, and the corresponding results were
shown in Fig. 6. As can be seen in Fig. 6, the sample J shows
weight loss from 100 to 140 1C, which may be due to the
desorption of surface bound water. Meanwhile, three samples of
n=2
αhν ¼ Aðhv−EgÞ
ð1Þ
where a, ν, A, and Eg are the absorption coefficient, light frequency,
proportionality constant, and band gap, respectively. In the equa-
tion, n decides the characteristics of the transition in a semicon-
ductor, i.e., direct transition (n¼1) or indirect transition (n¼4). For
three samples, the band-gap energy in our experiment could be
estimated from a plot of (αhν)2 versus photon energy hν. As
observed in the inset of Fig. 2, the direct band gaps for G, P and
J are estimated to be 2.49, 2.81 and 2.70 eV, respectively. The
UV–vis absorption spectra studies clearly indicate that the crystal
structures have significant influence on the onset of absorption, of
which an obvious shift can be observed due to the change
of crystal phases. The results also suggest that G with orthorhom-
bic phase structures can be photoexcited to generate more
electron−hole pairs than J with monoclinic phase and P under
visible-light irradiation, which could result in higher photocataly-
tic performance.
Fig. 3. N2 adsorption isotherms of different photocatalysts.

J. Li et al. / Journal of Solid State Chemistry 203 (2013) 154–159
157
Fig. 4. TEM images of different photocatalysts.
Fig. 7. Methanol yields of different photocatalysts.
Fig. 5. FTIR spectrum of different photocatalysts.
was analyzed using a gas chromatography (GC). Fig. 7 shows the
methanol yields over various samples under visible light irradia-
tion. As seen in Fig.7, the largest yields of methanol on G, P and J
photocatalysts under visible light irradiation for 5 h were 1712.7,
745.1, 581 μmol/g, respectively. The results clearly indicate that the
sample G with orthorhombic phase exhibits the best photocataly-
tic performance though the surface area of G is smaller than that
of P. The methanol yield over the sample G is also higher than that
of P25 (91 μmol/g) [24],CdS/Bi2S3 heterostructure (613 μmol/g)
[57] and Bi2S3/TiO2 nanotubes (224.6 μmol/g) [24] under the same
conditions. There are possible several reasons for the highly
efficient photocatalytic activities. The main reason may be that
G has the smallest band gap, which means that the responding
performance towards visible light of G is the best in the three
samples. Furthermore, the orthorhombic phase structure of
G maybe play an important role for the high photocatalytic
activity. In addition, the Cu4N cluster in the core of MOFs is similar
with the inorganic semiconductor, Cu3N, which has been proven to
have good photocatalytic activity for the reduction of CO2 [58].
What is more, the C¼N band in the imidazole ring may have the
same function like the C¼N band in the C3N4 polymer [39,40].
Therefore, the synergistic effects of smallest band gap and orthor-
hombic crystal phase structure are the critical factors for the better
photocatalytic activities of G. Meanwhile, the Cu4N cluster and
C¼N band in the imidazole also play important roles for the
enhanced photocatalytic activities of three Cu-based zeolitic
imidazolate frameworks.
Fig. 6. Thermogravimetric (TG) curves of different photocatalysts.
G, J and P were also highly thermally stable below 250 1C. The
weight loss from 250 to 400 1C could be attributed to the
decomposition of the imidazole molecule in the composites.
All the results further suggest that three copper imidazolate
complexes are highly thermally stable below 250 1C though the
sample J contains much water molecular adsorbed in its micro-
porous structure.
3.7. Photocatalytic activity for reducing CO2
3.8. Photocatalytic performance for degradation of MB
The photocatalytic activities of three samples for reducing CO2
to CH3OH under visible light irradiation were evaluated in a
continuous-flow reactor system under visible light irradiation
(4400 nm) [24]. The concentration of methanol in the liquid
To further test the photocatalytic activities of as-prepared
photocatalysts for degradation of pollutants in the liquid phase,
the performance of three photocatalysts for degradation of MB has

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J. Li et al. / Journal of Solid State Chemistry 203 (2013) 154–159
Fig. 8. (a) Photocatalytic activities of the G, J and P samples for the degradation of MB under visible-light irradiation. (b) ln(C0/C) vs. time for the G, J and P samples.
also been investigated. Fig. 8(A) shows photocatalytic activities of
the G, J and P samples for the degradation of MB under visible-
light irradiation. It is clearly observed that the sample G exhibited
the best visible-light-driven catalytic performance for degradation
of MB. For comparison purposes, the rate constants have also been
calculated according to an apparent first order in agreement with a
generally observed Langmuir–Hinshelwood model [59–61]:
Acknowledgments
The work is supported by the National Natural Science Founda-
tion of China (Grant No.20906034) and the key Academic Program
of the 3rd phase “211 Project” of South China Agricultural
University(Grant No.2009B010100001).
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In summary, three copper(II) imidazolate frameworks were
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demonstrate that the Cu-based zeolitic imidazolate frameworks
can be directly used as visible-light driven photocatalysts. At this
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