1. No category

Nano Research
Nano Res
DOI
10.1007/s12274-014-0689-3
Facile Fabrication of CdS–Metal-Organic Framework
Nanocomposites with Enhanced Visible-Light
Photocatalytic Activity for Organic Transformation
Fei Ke1,2, Luhuan Wang1 and Junfa Zhu1()
Nano Res., Just Accepted Manuscript • DOI: 10.1007/s12274-014-0689-3
http://www.thenanoresearch.com on December 16 2014
© Tsinghua University Press 2014
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1
TABLE OF CONTENTS (TOC)
Facile Fabrication of CdS–Metal-Organic Framework
Nanocomposites
with
Enhanced
Visible-Light
Photocatalytic Activity for Organic Transformation
Fei Ke1,2, Luhuan Wang1 and Junfa Zhu*1
1National
Synchrotron
Radiation
Laboratory
and
Collaborative Innovation Center of Suzhou Nano Science
and Technology, University of Science and Technology of
China, Hefei 230029, P. R. China.
2Department
of Applied Chemistry, Anhui Agricultural
University, Hefei 230036, P. R. China.
A novel type of CdS-MIL-100(Fe) nanocomposite photocatalysts
which show significantly enhanced photocatalytic activity for the
selective oxidation of benzyl alcohol to benzaldehyde under
visible-light irradiation.
Provide the authors’ webside if possible.
Junfa Zhu, http://staff.ustc.edu.cn/~jfzhu
Nano Research
2
DOI
(automatically inserted by the publisher)
Nano Res.
Research Article
Facile Fabrication of CdS–Metal-Organic Framework
Nanocomposites
with
Enhanced
Visible-Light
Photocatalytic Activity for Organic Transformation
Fei Ke1,2, Luhuan Wang1 and Junfa Zhu1()
Received: day month year
ABSTRACT
Revised: day month year
Visible-light initiated organic transformations have received much attention
because they have advantages in terms of low cost, relative safety, and
environmental friendliness. In this work, we report a novel type of
visible-light-driven photocatalysts, namely, porous CdS nanoparticle decorated
metal-organic framework (MOF) nanocomposites, which were prepared by a
simple solvothermal method in which porous MIL-100(Fe) served as the
support and cadmium acetate (Cd(Ac)2) as the CdS precursor. Using selective
oxidation of benzyl alcohol to benzaldehyde as the probe reaction, the results
show that the introduction of MIL-100(Fe) into the semiconductor CdS can
remarkably enhance the photocatalytic efficiency at room temperature as
compared to that using pure CdS. The enhanced photocatalytic performance
can be attributed to the integrative effects of enhanced light absorption
intensity, more efficient separation of the photogenerated electron-hole pairs,
and increased surface area of CdS due to the presence of MIL-100(Fe). This
work demonstrates that MOF-based materials hold great promises in the
applications of solar energy conversion into chemical energy.
Accepted: day month year
(automatically inserted by
the publisher)
© Tsinghua University Press
and Springer-Verlag Berlin
Heidelberg 2014
KEYWORDS
metal-organic
frameworks;
porous
materials;
nanocomposites;
photocatalysis;
semiconductors
1 Introduction
With the rapid development of industrialization,
environmental pollution and energy shortages have
raised awareness of a potential global crisis [1]. For
this reason, the development of both pollution free
technologies for environmental remediation and
alternative clean energy supplies is an urgent task. In
the last few years, the visible-light photocatalysis has
emerged as a powerful tool in synthetic organic
chemistry, due to its natural abundance, ease of use,
environmental friendliness, and fascinating potential
of applications [2-7]. Semiconductor nanoparticles
(NPs) with high photocatalytic activity and strong
quantum-size effect are considered to be promising
Address correspondence to Junfa Zhu , E-mail: [email protected]
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Nano Res.
photocatalytic materials [8-13]. To date, most
semiconductor materials are based on transition
metal oxides, sulfides, or nitrides with suitable band
gaps and band positions [14]. Among them,
crystalline cadmium sulfide (CdS) has been
extensively studied because of its narrow band gap
(2.4 eV) which corresponds well with the visible
spectral range of solar irradiation, size dependent
optical and electronic properties [15, 16], and diverse
applications, such as solar cells [17], environment
purification [18], hydrogen evolution [19], and
chemical sensors [20]. However, there are still several
issues that limit the utilization of pure CdS NPs,
including aggregation of CdS NPs in reactions, high
recombination rate of photogenerated electron-hole
pairs, and separation/reuse of NPs from
photoreactors [21]. Therefore, it remains an
important challenge to improve the performance of
CdS photocatalysts. Over the years, a number of
technologies have been explored to enhance the
photocatalytic activity of CdS NPs, such as doping
with noble metals [22], synthesis of CdS quantum
dots (QDs) [23], and combination with other
semiconductors [24] or carbon materials [25].
However, there is plenty of room to explore
unfulfilled potentials for fabricating CdS based
photocatalysts.
Metal-organic frameworks (MOFs), also known
as porous coordination polymers (PCPs), are highly
ordered, nanoporous networks that are derived from
metallic centers bonded by terminal organic linkers
[26]. MOFs have attracted a great deal of attention in
both academia and industry due to their numerous
potential applications in gas storage [27], sensing [28],
drug delivery [29], and catalysis [30]. In addition to
these attractive applications, we are particularly
interested in utilizing MOFs as active structures for
light absorption and excited state applications in
energy conversion [31]. When a MOF is treated as a
light-sensitive semiconductor, the metal oxide cluster
center can be considered as a discrete QD analogue,
which is stabilized and interconnected by the organic
linkers acting as the photon antenna [32]. Recently, a
few important studies demonstrating the promise of
MOFs in application of the conversion of solar
energy to electrical or chemical energy have been
reported [33]. For example, MOF-5, also known as
IRMOF-1, has an absorption spectrum with an onset
at 450 nm and can undergo photochemical processes
upon photoexcitation of the organic linker [34]. In
2007, Garcia et al. first reported the photocatalytic
activity of MOF-5 in the reaction of phenols
degradation [35]. Later, Lin and co-workers reported
the fabrication and energy migration dynamics of
metal-bipyridine based MOFs for light harvesting
[36].
Further,
Long
et
al.
reported
an
amine-functionalized zirconium MOF (UiO-66-NH2)
used as a visible-light photocatalyst for selective
aerobic oxygenation of various organic compounds
[37]. However, their photoactivities are not as
effective as inorganic semiconductor NPs. Compared
with traditional semiconductor photocatalysts, the
superiority of MOFs is their ultrahigh surface area
and narrow micropore distribution which may lead
to the formation of monodisperse photoactive species
supported on MOFs [38].
Considering that both semiconductor NPs and
MOFs have their own advantages and disadvantages
in the use of photocatalysts, if one combines them
together, the semiconductor-MOF hybrid materials
are expected possess advanced properties of both
components
and
overcome
their
shortages
mentioned above. To date, there have been a few
pioneering studies on the semiconductor-MOF
hybrid structures for enhanced light harvesting (e.g.
CdSe/ZnS-MOF
[39],
GaN@ZIF-8
[40])
or
size-selective molecular sensor (e.g. CdTe/ZIF-8 [41],
CdSe/CdS/ZnS@MOF-5 [42]) applications. However,
the utilization of semiconductor-MOF hybrid
structures as photocatalysts in organic synthesis has
rarely been investigated. Only very recently, Wu et al.
first reported the application of CdS-UiO-66(NH2)
nanocomposites, synthesized by a photodeposition
technique, in the photocatalytic oxidation of alcohols
[43]. Unfortunately, their photocatalytic activity is
relatively low due to the large CdS nanorod sizes.
Herein, we report a novel CdS-MIL-100(Fe)
nanocomposite that shows promising application in
the reaction of photocatalytic oxidation of benzyl
alcohol under ambient conditions. A series of
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Nano Res.
CdS-MIL-100(Fe) nanocomposites with different
weight addition ratios of MIL-100(Fe) have been
successfully synthesized by a facile solvothermal
method, as illustrated in Scheme 1. The MIL-100(Fe)
supporting matrix remarkably improves the
photostability and the photocatalytic efficiency of the
CdS photocatalyst for the selective oxidation of
benzyl alcohol to benzaldehyde. Therefore, the
present reported CdS-MIL-100(Fe) nanocomposites
are more suitable to be used as photocatalysts in
industry on a large scale.
Scheme 1 Illustration for preparation of CdS-MIL-100(Fe)
nanocomposites via a solvothermal process.
2 Experimental
2.1 Materials
Benzene-1,3,5-tricarboxylic
acid
(H3btc)
was
purchased from Sigma-Aldrich Co. LLC. Ferric
trichloride hexahydrate, cadmium acetate, dimethyl
sulfoxide, nitric acid, and hydrofluoric acid were
purchased from Sinopharm (Shanghai) Chemical
Reagent Co., Ltd., China. All other chemicals used in
this work were of analytical grade, obtained from
commercial suppliers, and used without further
purification unless otherwise noted.
2.2 Synthesis of MIL-100(Fe)
According to the literature [44], MIL-100(Fe) was
prepared by reacting H3btc (0.8466 g) with
FeCl3•6H2O (1.6263 g) in the presence of aqueous HF
(0.213 mL), HNO3 (0.163 mL), and distilled water (30
mL) at 150 °C for 12 h. Then the orange solid was
recovered by filtration, washed with hot water and
ethanol, and finally dried overnight at 150 °C under
vacuum.
2.3 Synthesis of CdS-MIL-100(Fe) nanocomposites
The as-synthesized MIL-100(Fe) with the desired
weight addition was dispersed in 30 mL of DMSO by
ultrasonication to obtain the homogeneous
MIL-100(Fe)-DMSO dispersion. Then 0.106 g of
Cd(CH3COO)2•2H2O was added into the above
solution. After vigorous stirring, the mixture was
transferred into a Teflon-lined stainless steel
autoclave (50 mL) and reacted under 180 °C for 12 h.
After cooling, the obtained products were collected
by centrifugation, washed with acetone and ethanol,
and finally dried overnight at 100 °C under vacuum.
As a control, pure CdS was also synthesized in
DMSO under the same reaction conditions as that of
CdS-MIL-100(Fe) nanocomposites except for the
addition of MIL-100(Fe).
2.4 Photocatalytic test
The photocatalytic activity of the samples was
evaluated by the selective oxidation of benzyl alcohol
to benzaldehyde under visible-light irradiation
which was provided by a 500 W Xe lamp with a
cutoff filter at 420 nm. In a typical process, 16 mg of
as-synthesized photocatalyst was dispersed into 3
mL of toluene, and then 0.2 mmol of benzyl alcohol
was added. Here we chose toluene as the catalysis
reaction solvent. Although a previous study showing
that the conversion rate of benzyl alcohol to
benzaldehyde using benzotrifluoride as solvent is
slightly higher than that using toluene (42% vs 38%)
[21], the price of benzotrifluoride is much higher
than that of toluene. Prior to irradiation, the mixture
was stirred for 30 min to make the photocatalyst
blend evenly in the solution. After reaction, the
mixture was centrifuged at 12000 rmp to completely
remove the photocatalyst particles. The remaining
solution was analyzed by GC and the products were
identified by comparison with authentic samples.
Conversion of benzyl alcohol (BA) and selectivity for
benzaldehyde (BAD) were defined as the follows:
Conversion (%) = [(C0-CBA) / C0] × 100
Selectivity (%) = [CBAD / (C0 - CBA)] × 100
where C0 is the initial concentration of benzyl alcohol,
CBA and CBAD are the concentration of benzyl alcohol
and benzaldehyde, respectively, at a certain time
after the photocatalytic reaction.
2.5 Characterization
The morphologies of the samples were conducted by
a JEM-2100F transmission electron microscope at 200
kV, respectively. The porosity properties of the
catalysts
were
characterized
by
nitrogen
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Nano Res.
adsorption-desorption isotherm at 77 K on a
Micromeritics TriStar II 3020 adsorption analyzer.
The powder X-ray diffraction (PXRD) patterns of the
samples were collected using an X-ray diffractometer
with Cu target (36 kV, 25 mA) from 3 to 65. The
UV-vis diffuse reflectance spectra were recorded at
room temperature on a Shimadzu UV-2550
spectrophotometer, using BaSO4 as a reference. The
electrochemical measurements were carried out in a
conventional three electrode cell, using a Pt plate and
an Ag/AgCl electrode as the counter electrode and
reference electrode, respectively. The working
electrode was prepared on indium-tin oxide (ITO)
glass, which was cleaned by sonication in ethanol for
20 min and dried at 353 K. The 10 mg sample was
dispersed in 1 mL of ethanol by sonication to get
slurry. The slurry was spread onto the pretreated ITO
glass whose boundary was previously protected
using scotch tape. After air drying, the working
electrode was further dried at 393 K for 1 h to
improve adhesion. Then, the scotch tape was unstuck
and the working electrode was obtained for further
measurements. The electrolyte was 0.5 M of aqueous
Na2SO4
solution
without
additive.
The
photoluminescence (PL) spectra for solid samples
were investigated on a Hitachi F-4500 fluorescence
spectrophotometer. The radical species were detected
by electron spin resonance (ESR) spectrometry on a
JES-FA200 instrument. The percentage conversion
and purity of the final products were determined
using GC1690 gas chromatograph with FID detector
and high purity helium as carrier gas. The products
were identified by comparison with authentic
samples.
patterns of CdS-MIL-100(Fe) nanocomposites
synthesized with different contents of MIL-100(Fe).
For comparison, the patterns of the pure CdS and
MIL-100(Fe) as well as simulated MIL-100(Fe) are
also shown. As seen, all of the diffraction peaks from
the CdS-MIL-100(Fe) samples can be indexed to the
hawleyite CdS phase with lattice constant a = 5.818 Ǻ
(JCPDS No. 10-0454) [23] and cubic crystalline
MIL-100(Fe) [45], and no peak assigned to other
impurities is detected. Moreover, with decreasing the
amounts of MIL-100(Fe) in the samples, the
intensities of diffraction peaks for CdS get intensified.
Accordingly, the peak intensities of the MIL-100(Fe)
patterns become weakened, implying that the CdS
content in the nanocomposites gradually increases.
The CdS diffraction peaks are broad due to the small
crystallite sizes of CdS NPs ( < 5 nm, calculated with
the Scherrer formula using the (111) diffraction peak)
in the samples, which are resulted from the slow
release of S2- ions from dimethyl sulfoxide (DMSO),
which serves as the solvent and the source of sulphur,
during the formation of CdS NPs [19].
3 Results and discussion
A series of samples were synthesized by adding
different amounts of MIL-100(Fe). Specifically, 0, 7.5,
15, 30, 60, 120 and 240 mg of MIL-100(Fe) were used,
and the samples were named as CdS,
CdS-MIL-100(Fe)-7.5,
CdS-MIL-100(Fe)-15,
CdS-MIL-100(Fe)-30,
CdS-MIL-100(Fe)-60,
CdS-MIL-100(Fe)-120, and CdS-MIL-100(Fe)-240,
respectively. The crystal structures and phase purities
of the as-synthesized samples were examined by
PXRD patterns. Figure 1 shows the typical PXRD
Figure 1 PXRD patterns of pure MIL-100(Fe), CdS and
CdS-MIL-100(Fe) nanocomposites with different weight addition
ratios. The simulated PXRD pattern from the crystallographic
data of MIL-100(Fe) is shown as the bottom.
Transmission electron microscopy (TEM) images,
which were taken to directly monitor the
morphologies of the samples and the influence of
MIL-100(Fe) addition on the nanocomposite sample
microscopic structures, are displayed in Figure 2 and
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Figure S1. As seen in Figure 2a, the small CdS NPs
are aggregated into larger nanospheres which have
diameter of ~150 nm. When an appropriate amount
of MIL-100(Fe) was added during the synthesis of
CdS-MIL-100(Fe) nanocomposites, the morphology
of as-formed CdS-MIL-100(Fe) is quite different from
that of pure CdS. It is apparent that MIL-100(Fe)
serves as a support and its characteristic structure is
retained. As can be seen from Figure 2c-f, the CdS
NPs are well separated from each other with much
smaller sizes and well spread out on the surface of
MIL-100(Fe). As can be seen from Figure S1, the
average size of the CdS NPs in CdS-MIL-100(Fe) is
around 5 nm and there is no apparent aggregation of
CdS NPs on MIL-100(Fe), indicating that MIL-100(Fe)
interacts strongly with CdS NPs and thus inhibits
their aggregation. The good distribution of CdS NPs
on
MIL-100(Fe)
guarantees
the
efficient
photocatalytic properties of CdS-MIL-100(Fe).
Furthermore, it can be seen that the areas of the
MIL-100(Fe) with no CdS NPs decoration markedly
reduces as the amount of MIL-100(Fe) decreases.
However, when the amounts of MIL-100(Fe) are
decreased to 15 mg or 7.5 mg, the small CdS NPs
self-assemble into spherical particles again, as shown
in Figure 2g and h. These results indicate that the
amount of MIL-100(Fe) added has a great influence
on the morphologies of the as-synthesized
CdS-MIL-100(Fe)
samples.
Comparing
the
morphologies of CdS-MIL-100(Fe) samples with
different amounts of MIL-100(Fe), it is clear that the
introduction of an appropriate amount of MIL-100(Fe)
during the synthesis of the CdS-MIL-100(Fe)
nanocomposites is of great importance to achieve a
good and homogeneous distribution of CdS NPs on
the MIL-100(Fe) matrix. To further obtain the
chemical
composition
information
of
the
CdS-MIL-100(Fe) nanocomposites, energy dispersive
X-ray spectroscopy (EDX) was carried out. It can be
seen from Figure S2 that the nanocomposites are
composed of C, O, Fe, S, and Cd elements. With
decreasing the amounts of MIL-100(Fe), the signals of
S and Cd elements in the CdS-MIL-100(Fe)
nanocomposites increase gradually, while those of C,
O, and Fe elements of MIL-100(Fe) decrease, which is
consistent with the results of TEM analysis.
Figure 2 TEM images of the as-synthesized nanocomposites: (a)
pure CdS, (b) MIL-100(Fe), (c) CdS-MIL-100(Fe)-240, (d)
CdS-MIL-100(Fe)-120,
(e)
CdS-MIL-100(Fe)-60,
(f)
CdS-MIL-100(Fe)-30, (g) CdS-MIL-100(Fe)-15, and (h)
CdS-MIL-100(Fe)-7.5.
Porosity and Brunauer-Emmett-Teller (BET)
surface area of the as-synthesized samples were
examined
by
nitrogen
adsorption-desorption
isotherms at 77 K (Figure S3). It can be seen that the
introduction of MIL-100(Fe) clearly affects the BET
surface area of the samples. Compared with pure
CdS, the CdS-MIL-100(Fe) nanocomposites exhibit a
significant increase in the amount of adsorbed
nitrogen. As shown in Table 1, the BET surface area
(SBET) of samples gradually increases with increasing
MIL-100(Fe) content, from 66.1 m2 g-1 to 1536.2 m2 g-1.
It is expected that larger surface area of
photocatalysts can supply more surface active sites
and make charge carriers transport easier, leading to
an enhancement of the photocatalytic performance
[19, 43]. Therefore, MIL-100(Fe) may play a vital role
in enhancing the photocatalytic activity.
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Figure 3 UV-vis DRS (a) and plots of (Ahv)2 vs. photon energy
(b) of pure CdS, MIL-100(Fe) and CdS-MIL-100(Fe)
nanocomposites with different weight addition ratios.
UV-vis diffuse reflectance spectra (DRS) were
employed to characterize the optical properties of the
samples. The absorption of pure CdS at wavelengths
shorter than 500 nm can be assigned to the bandgap
absorption of CdS [38]. The absorption band
positions of MIL-100(Fe) in the UV region ascribed to
the -* transitions of the trimesate ligands, whereas
the absorption band in the visible region belong to
the ligand-to-metal charge transfer transitions
(LMCT) from trimesate ions to Fe3+ centres (Figure 3a)
[38, 46]. The absorption bands of CdS-MIL-100(Fe)
nanocomposites undergo a red shift compared with
pure CdS. Furthermore, it can be seen from Figure
3a that the introduction of different contents of
MIL-100(Fe) significantly affects the optical property
of light absorption for the as-synthesized
CdS-MIL-100(Fe) nanocomposites. Compared with
pure CdS, all of the MIL-100(Fe) supported CdS
samples show enhanced absorbance in the
visible-light region (>500 nm). When the amount of
MIL-100(Fe) is increased to above 60 mg, the
absorbance drops. The same phenomenon has also
been observed in the previous report [25]. Therefore,
optimizing the MIL-100(Fe) content in the
CdS-MIL-100(Fe) nanocomposites is important to get
maximum visible light absorption. The band gap
energy can be estimated from the intercept of the
tangents to the plots of (Ahv)2 vs. photon energy [47].
As shown in Figure 3b, the band gap values of the
samples are about 1.80, 2.44, 2.37, 2.22, 2.16, 2.23, 2.28,
and 2.31 eV for pure MIL-100(Fe), CdS,
CdS-MIL-100(Fe)-7.5,
CdS-MIL-100(Fe)-15,
CdS-MIL-100(Fe)-30,
CdS-MIL-100(Fe)-60,
CdS-MIL-100(Fe)-120, and CdS-MIL-100(Fe)-240
nanocomposites, respectively. This indicates that the
introduction of MIL-100(Fe) into the CdS-MIL-100(Fe)
nanocomposities can narrow the band gap of the
semiconductor CdS. This may arise from the
chemical bonding between CdS and the MIL-100(Fe)
support. Similar phenomenon was also observed for
other graphene-based semiconductor photocatalysts
[21, 24]. When the amount of MIL-100(Fe) support in
the
nanocomposites
becomes
much
more
(CdS-MIL-100(Fe)-60, CdS-MIL-100(Fe)-120, and
CdS-MIL-100(Fe)-240),
the
absorbance
drops,
resulting in the increased band gap values of CdS in
the CdS-MIL-100(Fe) nanocomposities. This can be
ascribed to the fact that the high weight ratio of
MIL-100(Fe) in CdS-MIL-100(Fe) nanocomposites
results in increased diffuse reflectivity and
deteriorates the light absorption ability of the
nanocomposites. The same phenomenon has also
been observed in the previous report [25].
Given its favorable structure, the photocatalytic
activity has been evaluated by the model reaction of
selective oxidation of benzyl alcohol to benzaldehyde
under visible-light irradiation. As shown in Figure 4,
when we take a view of the overall activities in the
presence of different photocatalysts with the same
total amount of photocatalysts (16 mg), an interesting
trend is found. For pure CdS NPs, as expected, a
relatively low photocatalytic activity (40%) is
observed due to the rapid recombination of
photogenerated electron-hole pairs [19]. It can be
seen that when a small amount of MIL-100(Fe) is
added,
compared
with
pure
CdS,
the
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CdS-MIL-100(Fe) nanocomposites exhibit enhanced
photocatalytic performance toward the selective
oxidation of benzyl alcohol to benzaldehyde on both
of the conversion of benzyl alcohol and selectivity to
benzaldehyde (Table 1). In particular, when the
CdS-MIL-100(Fe)-60 nanocomposites was used in this
reaction, the optimal photocatalytic performance is
obtained, for which the conversion reaches the
highest value (54.3%) among all the samples, and
notably, the selectivity to benzaldehyde is nearly
100%. However, it is worthy to mention that the
photocatalytic
activity
gained
by
the
CdS-MIL-100(Fe)-60 nanocomposites is only 1.4 times
than that for pure CdS NPs. At first glance, one may
argue that this improvement is not significant, so that
the photocatalytic activity of catalysts maybe entirely
due to the presence of CdS particles. However, we
should keep in mind that this activity value was
measured based on the same total amount of
catalysts used, which is 16 mg (see Table 1). In fact,
the actual content of CdS in the CdS-MIL-100(Fe)-60
nanocomposites is only 7.84 mg, which is only 49% of
the pure CdS used. If we scale up to the same
amount of CdS used or use the same amount of CdS
for comparison, the photocatalytic activity will be
significantly higher after introduction of porous
MIL-100(Fe) into the CdS NPs. A similar study
reported previously for the photocatalytic activity of
CdS-UiO-66(NH2) catalysts [43], where the
conversion rate of benzyl alcohol to benzaldehyde
reached 31% in the solvent of benzotrifluoride for 6
h.
Further increase of the MIL-100(Fe) weight ratio
in CdS-MIL-100(Fe) leads to a deterioration of the
photocatalytic activity, which is also observed in
other carbon or graphene-based CdS nanocomposites
[19, 21, 24, 25]. Considering that the CdS NPs in the
CdS-MIL-100(Fe) nanocomposites (CdS-MIL-100(Fe)30, CdS-MIL-100(Fe)-60, CdS-MIL-100(Fe)-120, and
CdS-MIL-100(Fe)-240) have similar dispersivity and
almost the same size, the different conversion rate of
these nanocomposites clearly demonstrate that
MIL-100(Fe) plays a vital role in enhancing the
photocatalytic activity.
The introduction of
appropriate amount of MIL-100(Fe) to the CdS NPs
Figure 4 Conversion rate of photocatalytic selective oxidation of
benzyl alcohol to benzaldehyde in the presence of different
photocatalysts with same total weight (16 mg) under the
visible-light irradiation (λ > 420 nm) for 5 h: (a) MIL-100(Fe), (b)
CdS, (c) CdS-MIL-100(Fe)-7.5, (d) CdS-MIL-100(Fe)-15, (e)
CdS-MIL-100(Fe)-30,
(f)
CdS-MIL-100(Fe)-60,
(g)
CdS-MIL-100(Fe)-120, and (h) CdS-MIL-100(Fe)-240.
can remarkably improve the photocatalytic efficiency.
Because the porous MIL-100(Fe) can enhance the
adsorption capability for benzyl alcohol, the benzyl
alcohol molecules can be enriched on the surface of
CdS NPs, thus resulting in the acceleration of
photocatalytic reactions. However, relatively high
weight ratio of MIL-100(Fe) in CdS-MIL-100(Fe)
nanocomposites reduces the contact surface of
semiconductor CdS NPs with the light illumination
and results in a rapid decrease in photogenerated
charges, ultimately reducing the photocatalytic
activity [21]. Thus, there should be an optimal weight
ratio
of
MIL-100(Fe)
in
CdS-MIL-100(Fe)
nanocomposites. Moreover, in the blank experiments
without our photocatalysts or light, the oxidation
process was not observed. This result demonstrates
that the photocatalyst and light irradiation are
indispensable for the effective photocatalytic
selective oxidation of benzyl alcohol.
Since the nanocomposites of CdS-MIL-100(Fe)-60
show the best catalytic performance in the selective
oxidation of benzyl alcohol to benzaldehyde under
the visible-light irradiation, here we chose this
material to evaluate the stability and reusability of
the CdS-MIL-100(Fe) catalysts. After the reaction, the
photocatalysts were separated from the reaction
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9
Nano Res.
solution by centrifugation and washed with ethanol
and then dried at 75 °C. After this treatment, the
photocatalysts were used again for other consecutive
runs under the same reaction conditions. As shown
in Figure 5, no significant losses in the conversion of
benzyl alcohol can be observed in the five successive
photocatalytic
cycles,
indicating
that
the
CdS-MIL-100(Fe) photocatalysts possess excellent
long-term catalytic stability and reactivity.
Figure 5 Five cycles of the photocatalytic oxidation of benzyl
alcohol to benzaldehyde using sample CdS-MIL-100(Fe)-60 as
the photocatalysts under the visible-light irradiation (λ > 420 nm)
for 5 h.
In order to further check whether MIL-100(Fe)
plays an entire role in the photocatalysis, we carried
out additional comparable experiments for the
reaction of the selective oxidation of benzyl alcohol to
benzaldehyde
with
pure
CdS
NPs
and
CdS-MIL-100(Fe)-60 nanocomposites under the
visible-light irradiation which was provided by a 500
W Xe lamp with a cutoff filter at 520 nm. As can be
seen from Figure S5, the photocatalytic results show
that under the same reaction conditions, the pure
CdS NPs only shows a very low photocatalytic
activity (6.2%) under the visible-light irradiation (λ >
520 nm) for 5 h toward the reaction, while in the case
of
CdS-MIL-100(Fe)-60
nanocomposites,
the
conversion rate reaches 15.4%, which is 2.5 times
than that of pure CdS. Therefore, these results clearly
demonstrate that MIL-100(Fe) plays an important
role in the enhancement of the photocatalytic activity
of the CdS-MIL-100(Fe) nanocomposites.
To understand the role of MIL-100(Fe) in the
CdS-MIL-100(Fe) nanocomposites on enhancement of
photoactivity, we have performed some other
characterizations, including electrochemical analysis
and PL spectroscopy.
Figure S6 exhibits the
Mott-Schottky plots for the MIL-100(Fe). The
negative slope of C-2-E plot indicates that the pure
MIL-100(Fe) is p-type semiconductor. The enhanced
photocatalytic activity of the CdS-MIL-100(Fe)
nanocomposites compared to the pure CdS can be
ascribed to formation of the p-n junction between
p-type MIL-100(Fe) and n-type CdS semiconductors.
It can be obtained that the flat band potential (EFB) of
the p-type MIL-100(Fe) is around 1.15 V vs Ag/AgCl
(equivalent to 1.35 V vs normal hydrogen electrode,
NHE). The calculated conduction band potential (E CB)
is -0.45 V vs NHE. Because the calculated ECB of the
n-type CdS is –0.52 V vs NHE [21, 24], which is more
negatitive than the ECB of MIL-100(Fe) (–0.45 V vs
NHE), the photogenerated electrons could transfer to
MIL-100(Fe) from the CB of CdS NPs under
visible-light irradiaton. At the same time, the holes
could transfer to CdS NPs from the valence band (VB)
of MIL-100(Fe) (Figure 7, inset). Therefore, the
separation of the photogenerated electron-hole pairs
is facilitated, which can be confirmed by the transient
photocurrent response. Figure S7 shows the transient
photocurrent responses of the same amount of pure
CdS and CdS-MIL-100(Fe)-60 nanocomposites under
intermittent visible-light irradiation. It can be seen
clearly that the photocurrent of CdS-MIL-100(Fe)-60
is enhanced significantly compared to that of pure
CdS. It is well known that the photocurrent is formed
mainly by the diffusion of the photogenerated
electrons to the back contact, meanwhile the
photoinduced holes are taken up by the hole acceptor
in the electrolyte [21, 43]. Thus, the enhanced
photocurrent obtained on the CdS-MIL-100(Fe)-60
can be attributed to more efficient separation of the
photogenerated electron-hole pairs than that in pure
CdS NPs. This is also confirmed by the results of PL
spectra. As shown in Figure S8, the PL intensity of
CdS-MIL-100(Fe)-60 appears much weaker than that
of pure CdS, indicating that the recombination of
photogenerated electron-hole pairs is hampered in
the nanocomposite system. The efficient separation
of the photogenerated electron-hole pairs of
CdS-MIL-100(Fe)-60 nanocomposites are beneficial
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10
Nano Res.
for photocatalytic process and should contribute to
its enhaced photocatalytic activity. Similar findings
are also reported in previous research works [21, 24].
To determine further the advantage of the
CdS-MIL-100(Fe) nanocomposites over pure CdS
NPs, we have also performed the measurement of
electrochemical impedance spectroscopy (EIS)
Nyquist plots. As shown in Figure S9, the Nyquist
plots of pure CdS and CdS-MIL-100(Fe)-60 electrode
materials cycled in 0.5 M Na2SO4 electrolyte solution
both show semicycles at high frequencies.
Considering the preparation of the electrodes and
electrolyte used are the same, the high frequency
semicircle is relevant to the resistance of the
electrodes [21]. It is known that the high frequency
arc in the Nyquist plots corresponds to the charge
transfer limiting process and can be attributed to the
double-layer capacitance (Cdl) in parallel with the
charge transfer resistance (Rct) at the contact interface
between electrode and electrolyte solution [22, 24].
Notably, the introduction of MIL-100(Fe) leads to
dramatic decrease of the arc as compared to pure
CdS, suggesting that the much more efficient transfer
of charge over CdS-MIL-100(Fe)-60 nanocomposites
than that over pure CdS. Thus, the separation of the
photogenerated electron-hole pairs is facilitated
significantly, which is consistent with the results of
transient photocurrent responses analysis.
Additionally,
ESR
analysis
of
the
CdS-MIL-100(Fe)-60 nanocomposites was carried out
to study the involvement of active radical species in
the photocatalytic process. As can be seen from
Figure 6, the characteristic peaks belong to
superoxide radicals (O2•–) are observed but no
nonselective hydroxyl radical signals can be detected
in
the
CdS-MIL-100(Fe)-60
nanocomposites
suspension in the solvent of toluene under the
visible-light irradiation [22, 48], whereas no signals
can be detected without irradiation. Because the
standard oxygen reduction potential (-0.15 V vs NHE)
[49] is less negative than the conduction band edge of
CdS (-0.52 V vs NHE) [24], the formation of
superoxide radicals is thermodynamically favorable.
From these results, we can conclude that the sample
can avoid the formation of strongly oxidative
Figure 6 ESR spectra of radical adduct trapped by DMPO
(DMPO-O2•–) over the CdS-MIL-100(Fe)-60 nanocomposites
suspension in the toluene solution without or with the
visible-light irradiation.
Figure 7 Proposed mechanism for selective oxidation of benzyl
alcohol to benzaldehyde over the as-synthesized photocatalysts
of CdS-MIL-100(Fe) nanocomposites under the irradiation of
visible light; the upper right inset demonstrates the energy band
of CdS-MIL-100(Fe) nanocomposites.
hydroxyl radicals under such reaction conditions,
thus, the photogenerated hole and O2•– can oxidize
benzyl alcohol with high product selectivity. Based
on these observations, we propose a simple
mechanism, displayed in Figure 7, for photocatalytic
selective oxidation of benzyl alcohol over the
CdS-MIL-100(Fe) nanocomposites: Under the
visible-light irradiation, the electrons are excited
from VB of CdS semiconductor to its CB, leaving the
holes in the VB. The photogenerated electrons of CdS
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11
Nano Res.
can transfer to MIL-100(Fe) owing to their intimate
interfacial contact [24, 43]. According to the values of
energy band (Figure 7, inset), this process is
thermodynamically permissible; then the electrons
react with the adsorbed oxygen to form superoxide
radicals, whose existence in the system has been
confirmed by the results of ESR spectra. At the same
time, the adsorbed benzyl alcohol interacts with the
holes to form the corresponding radical cation, which
further reacts with oxygen or superoxide radical
species to given rise to the product of benzaldehyde
[21, 22, 50]. As a result, the efficiency toward
photocatalytic redox process is improved.
Table 1 Porosity properties and photocatalytic activity of pure
CdS and CdS-MIL-100(Fe) nanocomposites in the selective
oxidation of benzyl alcohol to benzaldehyde under visible light
irradiation.
SBET
V
Conversion
Selectivity
[m2 g-1]
[cm3 g-1]
[%]
[%]
CdS
66.1
0.07
40
95.5
MIL-100(Fe)
1598.8
0.84
3.3
100
CdS-MIL-100(Fe)-7.5
71.3
0.12
42.5
97.4
CdS-MIL-100(Fe)-15
74.9
0.14
45.8
98.1
CdS-MIL-100(Fe)-30
95.3
0.19
46.7
98.5
CdS-MIL-100(Fe)-60
326.9
0.23
54.3
99.2
CdS-MIL-100(Fe)-120
847.6
0.50
44
97.8
CdS-MIL-100(Fe)-240
1536.2
0.78
39.9
97.3
Catalyst
On the basis of the results reported in the
present work, the primary role of introducting an
appropriate amount of porous MIL-100(Fe) in the
CdS-MIL-100(Fe)
nanocomposites
can
be
summarized as follows: (i) MIL-100(Fe) is able to
control the morphology of the CdS-MIL-100(Fe)
nanocomposites during the synthesis process. (ii) The
visible light absorption intensity is improved. (iii)
The separation of the photogenerated electron-hole
pairs is facilitated significantly. All of these
integrative factors account for the observation of
enhanced
photoactivity
of
CdS-MIL-100(Fe)
nanocomposites as compared to pure CdS toward the
selective oxidation of benzyl alcohol to benzaldehyde
under the visible-light irradiation. It is expected that
our present work may provide useful information for
the fabrication of other MOF-based semiconductor
nanocomposites, as well as open a new doorway of
utilizing semiconductor-MOF nanocomposites to
visible-light-driven
photocatalysis
in
organic
synthesis.
4 Conclusion
We have reported a facile solvothermal method to
synthesize
a
series
of
CdS-MIL-100(Fe)
nanocomposites with different weight addition ratios
of MIL-100(Fe). In the synthesis process, porous
MIL-100(Fe) can function as an excellent supporting
matrix for the CdS NPs evenly overspreading. The
introduction of MIL-100(Fe) has been found to have a
significant effect on the morphology, porosity, and
optical nature of the CdS NPs. The resulting
CdS-MIL-100(Fe) nanocomposites exhibit excellent
photostability and enhanced photocatalytic activity
for the selective oxidation of benzyl alcohol to
benzaldehyde under visible-light irradiation. The
improved photocatalytic activity can be ascribed to
coupling interaction of the enhanced light absorption
intensity, more efficient separation of the
photogenerated electron-hole pairs and larger
surface area. These results demonstrate the
significant potential of scale-up synthesis of such
CdS-MOF nanocomposites and exploring their
applications in heterogeneous photocatalysis.
Acknowledgements
This work is supported by the National Natural
Science Foundation of China (Grant Nos. U1232102),
National Basic Research Program of China
(2010CB923302, 2013CB834605) and the Specialized
Research Fund for the Doctoral Program of Higher
Education (SRFDP) of Ministry of Education (Grant
No. 20113402110029).
Electronic Supplementary Material: Supplementary
material (TEM images, EDX spectra and nitrogen
adsorption-desorption isotherms of CdS-MIL-100(Fe)
nanocomposites, Mott-Schottky plot, photocurrent
response, PL spectra, and Nyquist impedance plots
of
pure
CdS
and
CdS-MIL-100(Fe)-60
nanocomposites) is available in the online version of
this
article
at
www.theNanoResearch.com∣www.Springer.com/journal/12274 | Nano
Research
12
Nano Res.
http://dx.doi.org/10.1007/s12274-***-****-*
(automatically inserted by the publisher).
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Nano Res.
Electronic Supplementary Material
Facile Fabrication of CdS–Metal-Organic Framework
Nanocomposites
with
Enhanced
Visible-Light
Photocatalytic Activity for Organic Transformation
Fei Ke1,2, Luhuan Wang1 and Junfa Zhu1()
Supporting information to DOI 10.1007/s12274-****-****-* (automatically inserted by the publisher)
Figure S1 TEM images of the as-synthesized nanocomposites:
CdS-MIL-100(Fe)-120, (c) CdS-MIL-100(Fe)-60, and (d) CdS-MIL-100(Fe)-30.
(a)
CdS-MIL-100(Fe)-240,
www.theNanoResearch.com∣www.Springer.com/journal/12274 | Nano
(b)
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Figure S2 EDX spectra of (a) CdS-MIL-100(Fe)-240, (b) CdS-MIL-100(Fe)-120, (c) CdS-MIL-100(Fe)-60, (d)
CdS-MIL-100(Fe)-30, (e) CdS-MIL-100(Fe)-15, and (f) CdS-MIL-100(Fe)-7.5 nanocomposites.
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Figure S3 Representative nitrogen adsorption-desorption isotherms of pure CdS and CdS-MIL-100(Fe)
nanocomposites with different weight addition ratios measured at 77 K. Solid and empty markers represent the
adsorption (ads) and desorption (des) isotherms, respectively.
Figure S4 Time-dependent photocatalytic selective oxidation of benzyl alcohol to benzaldehyde over the
MIL-100(Fe), CdS and CdS-MIL-100(Fe)-60 nanocomposites under the irradiation of visible-light (λ > 420 nm)
at room temperature.
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Figure S5 Conversion rate of photocatalytic selective oxidation of benzyl alcohol to benzaldehyde over the pure
CdS (a) and CdS-MIL-100(Fe)-60 nanocomposites (b) with same total weight (16 mg) under the irradiation of
visible-light irradiation (λ > 520 nm) for 5 h.
Figure S6 Mott-Schottky plot of MIL-100(Fe) in 0.5 M Na2SO4 aqueous solution (PH = 6.0).
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Figure S7 Transient photocurrent response of pure CdS and CdS-MIL-100(Fe)-60 nanocomposites in 0.5 M
Na2SO4 aqueous solution without bias versus Ag/AgCl under the irradiation of visible-light.
Figure S8 PL spectra of pure CdS and CdS-MIL-100(Fe)-60 nanocomposites.
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Figure S9 Nyquist impedance plots of pure CdS and CdS-MIL-100(Fe)-60 nanocomposites under the
irradiation of visible-light.
Address correspondence to Junfa Zhu , E-mail: [email protected]
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