Significantly enhanced photocatalytic hydrogen evolution under visible light over CdS embedded on metal-organic frameworks

Article abstract of DOI:10.1039/c3cc43218a

We demonstrated that embedding of CdS on MOFs could significantly increase the photocatalytic efficiency of CdS for visible light-driven hydrogen production.

Full text of DOI:10.1039/c3cc43218a

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Significantly enhanced photocatalytic hydrogen
evolution under visible light over CdS embedded on
Cite this: DOI: 10.1039/c3cc43218a
metal–organic frameworks†
Received 1st May 2013,
Accepted 11th June 2013
Jiao He, Zhiying Yan, Jiaqiang Wang,* Jiao Xie, Liang Jiang, Yangmei Shi,
Fagui Yuan, Fei Yu and Yuejuan Sun
DOI: 10.1039/c3cc43218a
www.rsc.org/chemcomm
We demonstrated that embedding of CdS on MOFs could significantly particular, chalcogenide nanoparticles embedded in MOFs, to our
increase the photocatalytic efficiency of CdS for visible light-driven knowledge have not been thoroughly investigated.
hydrogen production.
Herein, we report for the first time the fabrication of CdS
embedded MIL-101 (Cr3F(H2O)2O[BDC]3ÁnH2O, BDC = terephthalic
In view of increasingly serious energy and environmental problems, acid). We chose this because this Cr MOF shows good resistance to
considerable efforts have been invested in photocatalytic hydrogen air, water, common solvents, and thermal treatment.6d MIL-101 did
production from water splitting.1 Chalcogenide nanomaterials are not exhibit any photocatalytic activity by itself. However, it is
attractive because they have ideal edge positions of the valence and demonstrated in this research that the photocatalytic activity of
conduction bands for the reduction of water molecules utilizing CdS embedded on MIL-101 for hydrogen evolution under visible-
the abundant visible region of sunlight.1b–d Many approaches to light is significantly enhanced in comparison with bare CdS when
improving the hydrogen production rate and photostability of pure using Pt as a co-catalyst.
CdS particles have been investigated. For example, CdS loaded with
MIL-101 was synthesized first and then CdS nanoparticles
MoS2,2 co-loaded with Pt and PdS,3 incorporation of CdS particles were embedded on it at 0, 5, 10, 20 and 50% CdS by weight,
into the mesopores of Ti-MCM-414 and introduction of graphene herein labelled as CdS/MIL-101(X) where X = 5, 10, 20 and 50%.
nanosheets into CdS5 have been used.
X-ray diffraction (XRD) measurements (ESI,† Fig. S1) proved
On the other hand, metal–organic frameworks (MOFs) have that MIL-101 was successfully synthesized and its structure was
gained widespread attention recently due to their high surface areas, not changed after modification.6d,18 The embedded CdS was
crystalline open structures, tunable pore size, and functionality. They confirmed to be cubic crystalline (JCPDS 80-0019)5 with the
also have a high volume fraction of active metal sites. These average crystallite sizes of between 2 to 3 nm calculated using
properties have inspired many potential applications in catalysis.6 the Scherrer formula for the (111) facet diffraction peak.
Because of the well-defined crystal structures of MOFs and con-
The N2 adsorption–desorption isotherm (ESI,† Fig. S2) of
trollable chromophore distances via crystal engineering, highly MIL-101 is of type I indicating the presence of two kinds of micro-
crystalline MOFs have provided an ideal platform for designing pores as described in the literature.18a A type II isotherm is observed
molecular solids for light harvesting.7 Indeed, UiO-67,8 UiO-66,9 on pure CdS indicating nonporous or macroporous structure.5 When
Pt@UiO MOFs,7c amino-functionalized Ti(IV) MOFs,10 Ru-based CdS was introduced into MIL-101, type I adsorption–desorption
MOFs,11 Zn-MOFs12 and Zn4O-based MOFs13 show intrinsic photo- isotherms were observed but the uptake at P/P0 around 0.1 and 0.2
activities. However they are not as effective as semiconductor damped with increasing amounts of CdS. The Brunauer–Emmett–
catalysts.14 Compared with MOFs, the semiconductor@MOF Teller (BET) surface area (SBET) and the pore volume of MIL-101
heterostructures show great advantages due to their synergistic decrease with the increasing CdS content (Table 1). This is attributed
effect. Particularly, ZnO@ZIF-815 and TiO2@MOF-516 exhibited dis- to pore blockage. Nevertheless, CdS/MIL-101 maintained its porous
tinct optical properties. ZnO@MOF-517 has also been synthesized. structure and high specific surface area.
However, the photocatalytic activities of these heterostructures, in
The morphologies of the samples were further analyzed using
a transmission electron microscope (TEM). It is seen that CdS
aggregates can be formed on the external surface of MIL-101
octahedral crystals (Fig. 1). High-resolution TEM reveals a lattice
spacing of 0.33 nm corresponding to the interplanar distance
between adjacent (111) crystallographic planes of cubic CdS.
Fig. 2a shows a comparison of the UV-vis absorbance spectra
of the samples. The significant absorption of pure CdS at
The Universities’ Center for Photocatalytic Treatment of Pollutants in Yunnan
Province, School of Chemical Sciences & Technology, Yunnan University,
Kunming 650091, P. R. China. E-mail: jqwang@ynu.edu.cn;
Fax: +86 871 65031567; Tel: +86 871 65031567
† Electronic supplementary information (ESI) available: Synthetic procedures,
characterization and methods of photocatalytic hydrogen production. See DOI:
10.1039/c3cc43218a
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Table 1 Summary of textural properties and activities for photocatalytic H2
production of the samples
Pore
SBET
volume Pore
Activity
À1
Samples
(m2
g
À1) (cm3 À1) size (nm) (mmol gCdS hÀ1
g
)
MIL-101
2909.85 1.34
2449.12 1.13
2340.33 1.07
1913.11 0.89
1167.32 0.56
131.83 0.12
3.3
3.9
3.5
4.3
4.8
6.0
—
—
CdS/MIL-101(5)
CdS/MIL-101(10)
CdS/MIL-101(20)
CdS/MIL-101(50)
CdS
22.2
75.5
31.5
11.6
33.8
14.1
CdS mixed MIL-101a
—
—
a
The weight ratio of CdS is 10 wt%, the same as CdS/MIL-101(10).
Fig. 3 H2 production over MIL-101 embedded with different amounts of CdS
under visible light (20 mg of the catalyst, 0.5 wt% Pt loaded).
after incorporation of CdS. Furthermore, the absorption edges
of the CdS embedded MIL-101 showed clear red-shifts towards
the absorption edge of pure CdS. Fig. 2b shows the corres-
ponding photos of all the samples, indicating that their colors
change from green to yellow with increasing amounts of CdS.
The CdS/MIL-101 assemblies were photodeposited with Pt
particles to obtain Pt@CdS/MIL-101 and then examined for their
photocatalytic activities for hydrogen evolution in lactic acid aqueous
solution21 under visible-light (l > 420 nm) irradiation.22 As shown in
Fig. 3, no appreciable H2 evolution was detected over MIL-101 in the
absence of CdS. This indicates that MIL-101 did not act as an
effective semiconductor photocatalyst by itself. The TEM images of
Pt@CdS/MIL-101 (ESI,† Fig. S3) show many small Pt nanoparticles
deposited on the CdS surface but not on MIL-101 after in situ
photoreduction. After embedding only 5 wt% of CdS on MIL-101,
the rate of H2 evolution increased to 22 mmol hÀ1 (Fig. 3). The
maximum H2 evolution was found at 10 wt% (10% w/w) CdS. In the
presence of the same amount of CdS (2 mg), the H2 production rate
increased in the following order: CdS mixed MIL-101 o bare CdS o
CdS/MIL-101(10) (Table 1). The photostability of CdS/MIL-101(10)
was investigated over four consecutive runs of totally 5 h. The long-
term durability of the catalysts was also satisfactory (ESI,† Fig. S4).
The XRD patterns of the fresh and used CdS/MIL-101(10) were
almost similar implying that CdS/MIL-101(10) is stable during the
photocatalytic reaction (ESI,† Fig. S5).
Fig. 1 (a) The TEM image of CdS/MIL-101(10) and (b) the HRTEM image of
embedded CdS aggregates.
wavelengths shorter than 500 nm can be attributed to the
intrinsic bandgap absorption of cubic CdS. The band positions
of MIL-101 in the UV region can be assigned to p–p* transitions
of ligands19 and the bands in the visible region belong to the
d–d spin-allowed transition of the Cr3+ (d5).20 The intensity of
the two bands of MIL-101 in the UV and visible range decreased
The success in enhancement of the photocatalytic H2 production
activity by using CdS/MIL-101 encouraged us to extend this approach
to other MOFs or mesoporous materials. MOF-5, the most-studied
MOF with respect to its activity as a semiconductor,23 and MCM-41
were selected. Although the experimental evidence supporting the
behavior of MOF-5 as a semiconductor has been provided,23a,24
different from MIL-101, CdS modified MOF-5 exhibited negligible
activity for the photocatalytic hydrogen evolution under our experi-
mental conditions. The H2 production observed increased in the
following order: CdS embedded on MCM-41 o bare CdS o CdS
embedded on MOF-5 o CdS/MIL-101 (ESI,† Fig. S6). On top of this,
MOF-5 is not stable when exposed to air, water and thermal
treatment. So the activity of CdS embedded on MOF-5 is not so
effective. This is probably due to the collapse of the framework after
CdS incorporation (ESI,† Table S1). In comparison with MIL-101 and
MOF-5, MCM-41 showed decreased H2 production although it
Fig. 2 (a) UV-vis diffuse reflectance spectra and (b) photographs of pure MIL-101,
CdS/MIL-101(X) (X = 5, 10, 20, 50%) and pure CdS.
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