Hollow-ZIF-templated formation of a ZnO@C-N-Co core-shell nanostructure for highly efficient pollutant photodegradation

Article abstract of DOI:10.1039/c7ta02184d

Semiconducting metal oxides have been considered as effective photocatalysts for the degradation of organic pollutants to decolorize contaminated water. However, the poor performance and difficulty in recycling greatly hinder their practical application. Herein, we designed and fabricated, for the first time, a novel ZnO@C-N-Co core-shell nanocomposite towards efficient and recyclable photocatalysis by directly pyrolyzing a hollow Zn/Co-ZIF matrix consisting of a ZIF-8 shell with some Co-ZIF nanoplates inside the hollow cavity. It was demonstrated that the ZIF-8-shell-derived ZnO nanoparticles could spontaneously agglomerate and move to the hollow cavity while the internal Co NPs transferred inversely to the ligand-derived N-C shell at suitable pyrolysis temperature, resulting in the unique ZnO@C-N-Co core-shell structure. This unique nanostructure possessed the following superior properties: (1) a core-shell structure may make the intermediate ZnO much more stable during the reaction; (2) a porous carbon shell can not only provide a high BET specific area and thus high adsorption capability for reactants, but can also inhibit the recombination of photogenerated electrons and holes; (3) the embedded Co NPs were able to provide richer electron traps that further suppress the recombination of electrons and holes. As exemplified for the degradation of methyl orange, the ZnO@C-N-Co showed a significantly improved performance and excellent recyclability due to the highly synergistic effects between the C-N-Co shell and the robust ZnO. This strategy for the synthesis of MOF-derived core-shell nanomaterials could offer prospects for developing highly efficient photocatalysts.

Full text of DOI:10.1039/c7ta02184d

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ISSN 2050-7488
PAPER
Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
MoS nanocages as a lithium ion battery anode material
2
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DOI: 10.1039/C7TA02184D
Journal Name
ARTICLE
Hollow-ZIFs-Templated Formation of ZnO@C-N-Co Core-Shell
Nanostructure for Highly Efficient Pollutant Photodegradation
Received 00th January 20xx,
Accepted 00th January 20xx
Huirong Chen, Kui Shen,* Junying Chen, Xiaodong Chen and Yingwei Li*
DOI: 10.1039/x0xx00000x
Semiconducting metal oxides have been considered as effective photocatalysts for the degradation of organic pollutants
to decolorize contaminated water. However, the poor performance and difficulty in recycling greatly hinder their practical
applications. Herein, we designed and fabricated, for the first time, a novel ZnO@C-N-Co core-shell nanocomposite
towards efficient and recyclable photocatalysis by directly pyrolyzing a hollow Zn/Co-ZIF matrix consisting of ZIF-8 shell
with some Co-ZIF nanoplates inside the hollow cavity. It was demonstrated that the ZIF-8-shell-derived ZnO nanoparticles
could spontaneously agglomerate and move to the hollow cavity while the internal Co NPs transferred inversely to ligand-
derived N-C shell at suitable pyrolysis temperature, resulting in the unique ZnO@C-N-Co core-shell structure. This unique
nanostructure possessed the following superior properties: (1) a core-shell structure may make the intermediate ZnO
much more stable during the reaction; (2) a porous carbon shell can not only obtain high BET specific area and thus high
adsorption capability for reactants, but also inhibit the recombination of photogenerated electrons and holes; (3) the
embedded Co NPs were able to provide richer electron traps that further suppress the recombination of electrons and
holes. As exemplified for the degradation of methyl orange, the ZnO@C-N-Co showed a significantly improved
performance and excellent recyclability due to the highly synergistic effects from the C-N-Co shell and the robust ZnO. This
strategy for the synthesis of MOF-derived core-shell nanomaterials could offer prospects for developing highly efficient
photocatalysts.
www.rsc.org/
the photocatalytic performance, structural stability and economic
efficiency of ZnO. One method is to fabricate ZnO with a variety of
Introduction
5
6
7
nanoarchitectures, such as nanoplates, nanowires, microspheres,
In the field of environmental chemistry, a great deal of interest has
been devoted to photocatalytic degradation of organic pollutants
using various inorganic semiconductors such as ZnO, TiO , CdS, ZnS
8
9
nanotubes, and core-shell structures, to enhance its catalytic
activity. Nevertheless, their scale up often suffers from uncontrolled
10
2
crystallization and tedious procedures. Another effective solution
1
and MoS as catalysts. Among these catalysts, the semiconducting
2
is to fabricate composite photocatalysts by immobilizing ZnO on
supporting materials with large surface areas in order to provide
more contact areas between the catalysts and reactants, prevent
the agglomeration of the nanoparticles, and recycle the catalysts
ZnO has been considered as one of the most promising
photocatalytic materials, mainly due to its wide bandgap (3.37 eV)
and high excitation binding energy (～60 meV), which make it a
suitable candidate for short wavelength optoelectronic
11
from aqueous solutions easily. In this regard, porous carbons
2
applications. However, the bulk ZnO shows a low BET surface area
seem to be attractive supports due to their good electrochemical
and relatively high electron/hole recombination rate in
photocatalytic reactions that greatly reduce its photocatalytic
12
stability, super adsorption capability and low cost. Recently, it was
found that surface coating of metals/metal oxides particles with
various carbon layers could maximize the interactions between the
building b1l3ocks, dramatically changing their physicochemical
properties. Thus, we believe that it should be highly desirable to
fabricate ZnO@carbon core-shell hybrids which combine the
3
efficiency. Moreover, since the photocatalytic degradation of
organic pollutants generally occurs in aqueous solution in which the
ZnO powders are suspended, it is also unfavorable for the catalyst
to be recycled and reused.
4
So far two main strategies have been developed to enhance properties of the ZnO core and carbon shell into one material with
enhanced photocatalytic performance. Nevertheless, the
development of general and facile methods to prepare such core-
Key Laboratory of Fuel Cell Technology of Guangdong Province, School of
Chemistry and Chemical Engineering, South China University of Technology,
Guangzhou 510640, china, E-mail: cekshen@scut.edu.cn (K. S.), liyw@scut.edu.cn
shell structures is facing tremendous challenges.
Metal-organic frameworks (MOFs), a new class of porous
materials built up with metal ions/clusters and organic ligands, have
become more and more attractive in a variety of potential
(Y. L.).
Electronic Supplementary Information (ESI) available: Additional SEM and xps
images, etc. See DOI: 10.1039/x0xx00000x
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4
applications. Especially, with tunable structures and compositions, thoroughly with methanol, and dried overnight at 110 °C. XRD
View Article Online
MOFs have been considered as promising candidates to synthesize pattern (Figure 2f) confirmed the formation oDfOpI:u1r0e.1Z0I3F9-8/Cs7tTrAu0c2tu18re4D.
various carbon-based composites directly upon a thermolysis
Synthesis of ZIF-67. ZIF-67 was also prepared according to the
previous reports but with some modifications. In a typical
15
procedure. However, despite a number of nanocarbons/metal
oxides have been successfully prepared and investigated as
photocatalysts, most of them exhibit unsatisfactory catalytic
18
synthesis, Co(NO ) •6H O (1.091 g) was dissolved into methanol (30
3
2
2
mL) to form a solution. 2-methylimidazole (0.616 g) was dissolved in
methanol (20 mL) to generate another clear solution. Then the two
methanolic solutions were mixed together under stirring for 5 min.
The mixture was kept on reacting at room temperature for 12 hours.
The bright purple powders were collected by centrifugation,
washed thoroughly with methanol, and dried overnight at 110 °C
for further use.
16
performance. Moreover, there are almost no reports on the
fabrication and photocatalytic performance of MOFs-derived metal-
oxide@carbon core-shell nanocomposites so far. In this work, we
present a new and effective strategy to synthesize ZnO@C-N-Co
core-shell hybrids, where uniform ZnO particles are the “core” with
N-doped graphitized carbon embedded with Co NPs as the “shell”
layer, by directly pyrolyzing a hollow Zn/Co ZIF matrix under inert
atmosphere. With many advantages in composition and unique
core-shell structure, the as-prepared ZnO@C-N-Co showed
enhanced activity and good magnetic recyclability in photocatalytic
degradation of methyl orange (MO), as compared to pure ZnO
under the same condition. To the best of our knowledge, this work
presents the first example of fabricating metal oxide/carbon based
core-shell structure by employing hollow ZIFs as the single
precursor.
Synthesis of core-shell ZnO@C-N-Co. In a typical procedure, 0.5 g
of hollow Zn/Co-ZIF was placed in a tubular furnace and then
heated at 600 °C for 2 h with a heating rate of 1 °C/min from room
temperature under argon atmosphere. For comparison purposes,
another two control samples were also prepared by using pure ZIF-
8
and ZIF-67 with similar sizes as single precursor under the same
thermal conditions, which are denoted as ZIF-8-600 and ZIF-67-600,
respectively.
Characterization. Powder X-ray diffraction (XRD) patterns of the
samples were recorded with a Rigaku diffractometer (D/MAX/IIIA, 3
kW) using Cu Kα radiation (40 kV, 30 mA, λ=0.1543 nm). BET surface
Experimental Section
area and pore size were measured with N adsorption/desorption
2
Chemicals. Analytical grade Zinc nitrate hexahydrate, Cobalt nitrate
hexahydrate, 2-methylimidazole, and commercial ZnO were
purchased from Shanghai Aladdin Reagent Co. Ltd, China. All
chemicals were obtained from commercial suppliers and used
without further purification.
isotherms at 77 K on a Micromeritics ASAP 2020M instrument.
Before the measurements, the samples were degassed at 150 °C for
4
hours. X-ray photoelectron spectroscopy (XPS) was performed by
using Kroatos Axis Ultra DLD system with a base pressure of 10-9
Torr. Thermalgravimetric (TG) curves were recorded on a SDT-Q600
instrument from room temperature to 900 °C at a ramping rate of
Synthesis of hollow Zn/Co-ZIF. Hollow Zn/Co ZIF was prepared
according to the procedures developed by Li’s group but with
some modifications. In a typical synthesis, Co(NO ) •6H O (1.098 g),
1
7
1
0 °C/min under N2 flow. The cobalt and zinc contents in the
3
2
2
samples were measured quantitatively by atomic absorption
spectroscopy (AAS) by using a HITACHI Z-2300 instrument. The
surface morphology of the materials was investigated by a high-
resolution scanning electron microscope (SEM, MERLIN of ZEISS)
equipped with energy dispersive X-ray spectrometer (EDS). The
structure and the element distribution were determined by a high
resolution transmission electron microscope (TEM, JEOL, JEM-
2100F) with EDX analysis (Bruker Xflash 5030T) operated at 200 kV.
Zn(NO ) •6H O (1.116 g) and 2-methylimidazole (1.232 g) were first
3
2
2
dissolved in 15 mL methanol, 15 mL methanol, and 30 mL methanol,
respectively, to obtain their methanolic solutions. After stirring for
1
0 min at room temperature, the above methanolic solution of
Co(NO ) •6H O was slowly injected into the methanolic solution of
3
2
2
2
-methylimidazole with continuous stirring at room temperature.
Next, the methanolic solution of Zn(NO ) •6H O was further added
3
2
2
into the above mixed solution. After stirring for another 5 min, the
resulting suspension was transferred to a Teflon-lined stainless-
steel autoclave and hydrothermally treated at 120 C for 4 h. The
formed powders were collected by centrifugation, washed
thoroughly with methanol, and dried overnight at 110 °C for further
use.
Procedures for the photodegradation of MO. Methyl orange (MO)
was used as a probe dye to evaluate the photocatalytic activity of
the as-prepared samples by using the modified procedures. For a
typical photocatalytic test, a certain amount of catalyst (1 mmol Zn
for ZIF-8-600, ZnO@C-N-Co and pure ZnO or 1 mmol Co for ZIF-67-
o
6
00) was suspended in 50 ml of methyl orange aqueous solution
-5
Synthesis of ZIF-8. ZIF-8 was prepared according to the previous
(
5.0×10 M), and then the mixture was put in a quartz tube and
1
8
reports but with some modifications. In a typical synthesis,
Zn(NO ) •6H O (258 mg) was dissolved into methanol (20 mL) to
shaken overnight in the absence of light to enable an equilibrium
adsorption on the catalyst surface. UV irradiation was carried out
using a 300 W Xenon lamp cooled by cycling water. After a given
irradiation time, about 3 ml of the mixture was withdrawn, and the
catalyst was separated from the suspension. The degradation
process was monitored by a Cary-500 spectrometer (measuring the
absorption of MO at 463 nm).
3
2
2
form a solution. 2-methylimidazole (263 mg) was dissolved in
methanol (20 mL) to generate another clear solution. Then the two
methanolic solutions were mixed together under stirring for 5 min.
The mixture was kept on reacting at room temperature for 12 hours.
The white powders were collected by centrifugation, washed
2
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Figure 1. Schematic illustration for the preparation of core-shell ZnO@C-N-Co.
nanosheets were located and encapsulated by the Zn-containing
ZIF-8 shell, in agreement with the XRD results (Figure 2f). Then the
hollow Zn/Co-ZIF was directly pyrolyzed under inert gas to obtain
the desired ZnO@C-N-Co. In our preliminary experiments, we found
that choosing suitable pyrolysis temperature was of vital
importance for the successful preparation of ZnO@C-N-Co. At the
o
low pyrolysis temperature of 550 C, we found that most of the Co
NPs did not distribute in the carbon shell and the formed ZnO NPs
seemed to be aggregated but not able to form a complete core yet
(
Figure S1 and S2). However, when increased the pyrolysis
o
temperature to 700 C, the Zn ions in ZIF-8 shell could be first
reduced by the ligand-derived carbon layers and then completely
evaporated, resulting in the formation of hollow carbon-cobalt-
based structure (Figure S1 and S3). It was interesting that when the
15e
o
pyrolysis temperature was setted at 600 C, the ZnO NPs generated
Figure 2. (a) SEM image, (b) and (c) TEM images, (d) elemental from the hydrolysis of ZIF-8 shell could spontaneously agglomerate
mappings, (e) STEM image and elemental line-scanning profiles of and move to the hollow interior of the parent hollow Zn/Co-ZIF
hollow Zn/Co-ZIF. (f) Wide-angle XRD patterns of the as-sythesized while the internal Co NPs transferred inversely to the N-doped
ZIF-8, ZIF-67, and hollow Zn/Co-ZIF.
carbon shell, affording the final ZnO@C-N-Co core-shell structure.
The morphology and structure of ZnO@C-N-Co can be
revealed by SEM and TEM images. As shown in Figure 3a, the as-
prepared nanocomposites basically maintained the original
rhombododecahedron shape with an average size of 330 nm.
Careful observation by high-magnification SEM (the inset in Figure
Results and discussion
The developed facile hollow-ZIF-template strategy for the
preparation of ZnO@C-N-Co is illustrated in Figure 1. Briefly, a ZIF-
3
a) showed that the surfaces of these rhombododecahedrons were
6
7@ZIF-8 core–shell structure was first synthesized by a sequential
a little crimped and rough. The TEM images (Figure 3b, 3c and S4)
showed that the obtained material had a core-shell structure,
where the uniform ZnO particles with an average size of 150 nm
were the “core” with N-doped graphitized carbon embedded with
Co NPs as the “shell” layer. The HRTEM image (Figure 3d) and the
corresponding FFT image (Figure 3d inset) revealed that the
interplanar spacing of the selected particle in carbon shell was
nucleation method, which was then used as the precursor to
2+
generate a hollow Zn/Co-ZIF structure by treating it with Co in
o
methanol at 120 C for 4 h. The shape of the hollow Zn/Co-ZIF is of
a well-defined rhombic dodecahedron, as demonstrated by the
SEM image (Figure 2a). From TEM images (Figure 2b and 2c) and the
corresponding elemental mappings (Figure 2d) and linescan (Figure
2
e), we confirmed that each of the resultant particles possessed a
dodecahedron-shaped interior hollow cavity, in which the Co-ZIF
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Figure 3. (a) SEM image of the as-sythesized core-shell ZnO@C-N-Co. Inset of (a) shows high magnification SEM image of ZnO@C-N-Co. (b)
and (c) TEM images of ZnO@C-N-Co. (d) HRTEM image of an individual Co nanoparticle. Inset of (d) shows the FFT image of the
corresponding Co nanoparticle. (e-j) EDS mappings, (k) STEM image, and (l) elemental line-scanning profiles of the ZnO@C-N-Co.
about 0.21 nm, which coincided with (111) plane spacing of Co Co-MOF-derived Co NPs. No other residues or contaminants were
19
NPs, indicating that Co (Ⅱ) in Co-ZIF had been reduced to metallic observed, indicating the high purity of this sample. The intensive
Co that spontaneously migrated to the ligand-derived carbon shell and sharp peaks also confirmed its good crystallinity. For
during the thermolysis process. The EDS mapping images (Figure comparison, ZIF-67-600 (Figure S5) and ZIF-8-600 (Figure S6) were
3e-j) of an individual ZnO@C-N-Co particle, along with the also prepared by the pyrolysis of solid ZIFs, and pure ZnO (Figure S7)
corresponding STEM image (Figure 3k) and linescan data (Figure 3l), was obtained from commercial source. Both ZIF-67-600 and ZIF-8-
clearly revealed that C, N and Co elements were homogeneously 600 showed totally different XRD diffraction patterns from the
distributed in the shell layer while Zn and O elements were mainly ZnO@C-N-Co, indicating their different compositions. The XRD of
distributed in the core section of the product. Although these ZIF-67-600 only showed two diffraction peaks at around 44.4° and
21
results undoubtedly confirmed a ZnO@C-N-Co core/shell structure 51.6°, which are characteristics of metallic Co. The XRD pattern of
for the final thermolysis product, it was worth noting that there ZIF-8-600 showed two peaks at around 7.3° and 12.6°, implying that
22
were also very few ZnO NPs that existed in the porous carbon shell the ZIF-8 precursor had not fully decomposed yet, which can also
of ZnO@C-N-Co and Co NPs on the surface of the ZnO core, as be confirmed by the TG curve (Figure S8). The reason why they had
revealed by Figure 3f-g.
such a remarkably different composition after the same calcination
treatment can be attributed to their different composition and
morphology. Obviously, it is the unique hollow structure of the
Zn/Co-ZIF precursor that leads to the formation of the ZnO@C-N-Co
core-shell structure.
X-ray diffraction (XRD) patterns can provide more crystallinity
and phase information of the synthesized materials. As shown in
Figure 4a, the main ZnO@C-N-Co diffraction peaks were in good
20
consistency with ZnO (JCPDS card NO. 36-1451) and metallic Co
21
(
JCPDS No. 15-0806). This result suggested that the synthesized
The carbonization of the hollow Zn/Co-ZIF generated abundant
ZnO@C-N-Co was a composite consisting of ZIF-8-deived ZnO and
4
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Figure 4. (a) XRD patterns and (b) N adsorption/desorption isothermes of ZIF-67-600, ZIF-8-600 and ZnO@C-N-Co. The Inset of (b) shows
2
the pore-size distribution of the samples. XPS spectra of the Co 2p narrow scan (c) and the Zn 2p narrow scan (d) of the samples.
mesopores, leading to a different pore structure from the ZIF-8-600 both the ZnO@C-N-Co and ZIF-67-600 showed two distinct peaks at
and ZIF-67-600. As shown in Figure 4b, both ZIF-67-600 and ZIF-8- 793.8 eV for Co 2p1/2 and at 778.7 eV for Co 2p3/2, further
6
00 mainly exhibited a typical type-I isotherm, corresponding to confirming that Co(II) ions in Co-MOF were reduced to Co(0) by the
23
25
their microporous structures. The pore size distribution curve in situ formed carbon. In the Zn 2p XPS spectra of ZnO@C-N-ZnO
further confirmed that the ZIF-67-600 did not show any and ZIF-8-600 (Figure 4d), two distinct peaks appeared at binding
mesoporous nature, while the ZIF-8-600 had only
a little energies of 1044.1 eV for Zn 2p1/2 and 1021.2 eV for Zn 2p3/2, both
mesoporous size distribution. In contrast, the ZnO@C-N-Co of which were close to, but lower by ca. 0.2 eV than that of ZnO.
exhibited a typical adsorption curve of type I plus type IV with an Such a peak shift to lower values reflected the increase in electron
obvious hysteresis loop at P/P of 0.4-1.0, indicating the remarkable density, which may be attributed to carbon layer coated around the
0
24
26
micro/mesoporous structure of the synthesized core-shell sample.
surface of the ZnO. The Zn content in the ZnO@C-N-Co was
Correspondingly, the pore size distribution curve by the BJH method determined to be ca. 25.7 wt% by AAS analysis (Table S1)
using the adsorption branch of the isotherms revealed that the
It is noteworthy that the thermal treatment converted the
mesopores were in the range of 4-30 nm with a mean size of about
hollow Zn/Co ZIF matrix into ZnO@C-N-Co framework instead of
1
0 nm. In addition, the BET specific surface area of ZnO@C-N-Co
discrete ZnO nanoparticles embedded in a carbon matrix. Thus, to
further understand the formation process of the unique core-shell
structure of ZnO@C-N-Co, a time-dependent experiment was
conducted and the intermediate products were characterized by
2
-1
was calculated to be 188.7 m g and total pore volum was 0.36
cm g (Table S1). It is noteworthy that a significant portion of the
total pore volume was contributed by mesopores, with the value
3
-1
3
-1
reaching 0.34 cm g (Table S1). We expect this core-shell structure
with abundant mesopores can effectively enhance the mass
transfer of guest molecules through the channel, ensuring
outstanding photocatalytic performance on the ZnO@C-N-Co
material.
2+
TEM. As shown in Figure 5a-e and Figure S10, Zn in the ZIF-8 shell
as the coordination skeleton was in situ converted into ZnO NPs in
the first 10 minutes of pyrolysis time, which were uniformly
distributed in the amorphous carbon shell. In the next 10 minutes
the above-formed ZnO NPs seemed to aggregate spontaneously to
Important information on the electronic structure and produce bigger particles and tended to migrate to the hollow
chemical valence of ZnO@C-N-Co was further investigated by X-ray interiors. After 30 minutes, most of the ZnO NPs have moved to the
photoelectron spectroscopy (XPS). The survey XPS spectrum of center cavity and formed a poor-developed ZnO sphere, while a
ZnO@C-N-Co showed the typical characteristic peaks of Zn, Co, C, N small amount of ZnO still remained in the shell section. In the
and O (Figure S9), confirming the existence of these elements in the meantime, the Co ions that were originally distributed in the hollow
composite as expected. As shown in Co 2p XPS spectra (Figure 4c), interior migrated inversely to the ligand-derived carbon shell,
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Figure 5. (a-e) TEM images of the products collected at different pyrolysis times. (f) Schematic illustration of the forming process of the
core-shell ZnO@C-N-Co.
where they were reduced to Co NPs. After 60 minutes, all the ZnO needed to fully understand how these Co-based species determines
NPs were concentrated and occupied the hollow interior, which the shape transformation of the ZnO@C-N-Co core-shell structure.
implied that the final product was formed completely. Based on the
Prompted by the unique structural advantages of ZnO@C-N-Co,
above results, the fabrication process of ZnO@C-N-Co was
we further examined its photodegradation performance with
described by schematic illustration as shown in Figure 5f. To the
methyl orange (MO) as a model organic pollutant. For comparison,
best of our knowledge, this is the first example of successful
photocatalytic tests using ZIF-67-600, ZIF-8-600, and pure ZnO as
fabrication of 3D core-shell structures composed of metal oxide
catalysts were also performed (Figure S11). Blank experiments in
core and carbon-based shell by thermal decomposition of hollow
the absence of any catalysts or illumination revealed that there was
MOFs. We suggest that the Co-based species produced after
little changes in the absorbance peaks of MO at 460 nm (Figure S12),
thermolysis of the Co-ZIF might have played an important role in
indicating that the catalyst and light were essential for
migrating ZnO phase to the hollow cavity, since the direct
photocatalytic degradation. In addition, a set of adsorption
pyrolyzation of pure hollow ZIF-8 just led to ZnO NPs that were
experiment were also carried out to compare the adsorption
27
highly embedded in carbon shell. Future researches would be
capability of different samples (Figure S13). The results showed that
Figure 6. (a) Photodegradation curves of MO over various catalyst under irridiation with UV-Vis light. (b) Calculated degradation rate
constants of MO degradation. Inset of (b) shows the 퐥퐧(푪/푪 ) vs. time curves of MO degradation. (c) Magnetic seperation of ZnO@C-N-Co
ퟎ
by using a magnet. (d) Recyclability of the ZnO@C-N-Co nanocomposites in the MO degradation. (e) TEM image of ZnO@C-N-Co sample
after reaction. (f) XRD patterns of ZnO@C-N-Co sample before and after reaction.
6
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Figure 7. Proposed mechanism for photodegradation of organic pollutants using the as-prepared core-shell ZnO@C-N-Co as catalyst.
the ZIF-8-600 achieved the highest adsorption of 81.8 mg•g-1 of 2-methylimidazole provided a large specific surface area.
(
about 10.5 % of the overall MO quantity in the solution), while Consequently, the photo-generated electron/hole pairs could easily
ZnO@C-N-Co, ZIF-67-600 and ZnO showed much lower adsorption react with the MO molecules adsorbed on the surface of the porous
,
29
of 6.5%, 5.2%, 1.2 %, respectively, probably because ZIF-8-600 had structure so as to obtain excellent photocatalytic performance.
the highest BET surface area. Therefore it was obvious that the Last but not least, the embedded cobalt and porous carbon shell
graphited porous carbon of ZnO@C-N-Co could provide a large can constitute double inhibitation effect, which effectively separate
specific surface area and thus facilitate the adsorption capability of the photogenerated electrons and thus further suppress the
the samples. Figure 6a shows the time profiles of C/C under UV-vis recombination of electrons and holes by providing richer electron
0
irradiation, where C is the reaction concentration of MO and C is traps. As a result, the photo-generated electrons are more likely to
0
the initial concentration after the equilibrium adsorption of the flow onto the surface of the cobalt nanoparticles with lower Fermi
photocatalyst before irradiation, calculated from their level, which may be another important cause of the excellent
30
corresponding time-dependent UV-vis absorption spectra. After 2.5 photoactivity of ZnO@C-N-Co. This possible mechanism of the
hours of irradiation, it was found that the ZnO@C-N-Co exhibited enhancement of the photocatalytic activity is illustrated in Figure 7.
much higher degradation percentage (99.5%) than those on ZIF-67-
The recyclability of a potential photocatalyst is also crucial for
6
00 (29.4%), ZIF-8-600 (41.9%) and pure ZnO (60.2%). Remarkably,
its practical applications. Thus, finally we investigated the recycling
performance of the ZnO@C-N-Co. The irradiation time for each test
was 2.5 hours. After the reaction, the ZnO@C-N-Co could be easily
separated from the reaction solution within 15 seconds with the
assistance of a magnet (Figure 6c), since the Co NPs with good
magnetic property were wrapped in the carbon shell. As shown in
Figure 6d, the photocatalytic efficiency of ZnO@C-N-Co showed
negligible changes after five recycles. The TEM image (Figure 6e)
the degradation rate for the ZnO@C-N-Co could be well fitted by
using pseudo-first-order kinetics equation ln(C/C )=-kt (Figure 6b).
0
The apparent rate constant of MO photodegradation was about
-
1
0
.0283 min for ZnO@C-N-Co, which was at least four-times higher
-1
-1
than that for ZIF-67-600 (0.0025 min ), ZIF-8-600 (0.0033 min ) and
pure ZnO (0.0066 min ), further demonstrating its significantly
improved photocatalytic activity for this degradation reaction.
-
1
Considering the unique structure and composition, the and XRD patterns (Figure 6f) of ZnO@C-N-Co after the
excellent photocatalytic performance of ZnO@C-N-Co could be photocatalytic reaction indicated good stability of the material,
related to the following possible aspects. First, the resulting ZnO which should be due to highly protective effect of carbon shell for
and C-N-Co hybrids were in a good shape of core-shell structure, in the robust ZnO core in reaction process. Thus, it is obvious that the
which graphited carbon could provide a lot of electronic traps. core-shell ZnO@C-N-Co nanocomposite in our work would be an
These electronic traps made the ZnO-photogenerated electrons ideal catalyst for photodegradation of organic pollutants due to its
more easily flow to the porous carbon surface, where they can excellent activity, good recycling performance and easy separation
-
combine with O to afford abundant highly-active ·O radicals for from the reaction system.
2
2
2
8
the MO degradation. Meanwhile, the grievous consumption of
electrons can also inhibit the recombination of the electron/hole
pairs to some extent. These reserved holes would oxide the H O Conclusions
2
into ·OH radicals that contributed much to the MO degradation.
Second, the graphited porous carbon produced by the pyrolyzation
In summary, we have successfully demonstrated an effective
strategy to fabricate porous ZnO@C-N-Co core-shell nanostructure,
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 7
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J Po lue r an sael od fo Mn aot et rai ad l js u Cs ht emm ai rs gt ri yn sA
Page 8 of 9
ARTICLE
Journal Name
Yousefi, S. P. Jahromi, Rsc Adv., 2015, 5, 21888-21896.
where uniform ZnO particles are the “core” with N-doped
graphitized carbon embedded with Co NPs as the “shell” layer. In
this strategy, a hollow Zn/Co-ZIF consisting of ZIF-8 shell with some
Co-ZIF nanoplates inside the hollow cavity was first synthesized and
then used as the sacrificial template to prepare the ZnO@C-N-Co
via thermolysis under inert atmosphere. Benefiting from the highly
synergistic effects between the C-N-Co shell and the robust ZnO,
the ZnO@C-N-Co showed a remarkable photocatalytic activity and
excellent recyclability for the degradation of methyl orange (MO).
The hollow-MOF-templated strategy demonstrated here provides a
new and facile way to synthesize metal oxide@carbon core/shell
nanostructures for a multitude of potential applications, especially
for photocatalysis.
View Article Online
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Acknowledgements
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We thank the National Natural Science Foundation of China
(
21322606, 21436005, 21576095, and 21606087) and
Guangdong Natural Science Foundation (2014A030310445,
016A050502004 and 2013B090500027) for financial support.
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| J. Name., 2012, 00, 1-3
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Page 9 of 9
Journal of Materials Chemistry A
View Article Online
DOI: 10.1039/C7TA02184D
Graphical Abstract
Hollow-ZIFs-Templated Formation of ZnO@C-N-Co Core-Shell Nanostructure
for Highly Efficient Pollutant Photodegradation
Huirong Chen, Kui Shen,* Junying Chen, Xiaodong Chen and Yingwei Li*
We designed and fabricated a novel ZnO@C-N-Co core-shell nanocomposite towards efficient and
recyclable photocatalysis by the direct pyrolysis of a hollow Zn/Co-ZIF matrix consisting of ZIF-8
shell with some Co-ZIF nanoplates inside the hollow cavity. It showed a significantly improved
performance for the model pollutant photodegradation due to the highly synergistic effects from the
C-N-Co shell and the robust ZnO.