Zeolitic Imidazolate Framework 8-Derived Au@ZnO for Efficient and Robust Photocatalytic Degradation of Tetracycline

Article abstract of DOI:10.1002/cjoc.201800440

Tetracycline (TC) and other antibiotics accumulated in groundwater and soil pollute ecological environment and threaten human health. Gold nanoparticles doped on photocatalysts are able to enhance the photodegradation efficiency during removing these anti

Full text of DOI:10.1002/cjoc.201800440

Zeolitic Imidazolate Framework 8-derived Au@ZnO for Efficient
and Robust Photocatalytic Degradation of Tetracycline
[
a]
[a]
[a]
[a]
[a]
[a]
[a]
Bijun Gao, Jin Zhou, Hongliang Wang, Guping Zhang, Jinghui He, Qingfeng Xu, Najun Li,
[
a]
[a]
[a]
Dongyun Chen, Hua Li and Jianmei Lu*
a
B. Gao, J. Zhou, H. Wang, G. Zhang, Prof J. He. Q Xu, N. Li, D.
Chen, H, Li, J. Lu，College of Chemistry Chemical Engineering
and Materials Science Soochow U niversity, 199 Ren’ai
Road , Suzhou 215123 , P.R. China ,E-mail:
lujm@suda.edu.cn; dychen@suda.edu.cn
ABSTRACT Tetracycline (TC) and other antibiotics accumulated in groundwater and soil pollutes ecological environment and threaten human health. Gold
nanoparticles doped on photocatalysts are able to enhance the photodegradation efficiency during removing these antibiotics, but preparation of Au
nanoparticles of well-dispersion on photocatalysts remains challenging. In this work, zeolite imidazolate (ZIF-8) was employed as the precursor to prepare
Au@ZnO photocatalyst via impregnation and in-situ reduction method to efficiently degrade the tetracycline in the aqueous solution. Au nanoparticles
are of 10 nm in size and uniformly dispersed on the surfaces of ZnO microstructures. The as-prepared Au@ZnO is able to remove 85.5% of TC of 0.010
mg/mL within two hours, of higher photocatalytic activity than pure ZnO catalyst. Most importantly, the catalyst shows its superior stability after five
cycles without structure and activity changing.The mechanism of the photocatalytic degradation was discussed in detail.
KEYWORDS ZIF-8 • Au@ZnO • tetracycline • photocatalytic • degradation
Introduction
In the 21st century, 40% of the world population are estimated
to be affected by water pollution, which was caused by various
In this paper, we demonstrate that metal organic framework
could be used for gold catalyst support for effective
[
1-2]
photo-degradation of TC. Metal organic frameworks were
reported to be able to prepared noble metal-based catalysts with
organic and inorganic pollutants.
Among them, antibiotics and
their metabolites accumulated in both surface and ground waters
due to their abuse in human therapy, animal farming and
agriculture. Regulation on antibiotics become stricter and stricter
in many countries. For example, antibiotics such as tetracycline
[
20-22]
high photocatalytic performance.
Particularly, zeolitic
imidazolate frameworks (ZIF-8) was synthesized as the precursor
[
23]
to absorb HAuCl
4
.
Gold nanospheres in dimeters of ~10 nm
[
3-4]
were uniformly anchored on zinc oxide supports by an in-situ
（
TC）are frequently used as animal drugs or feed additives.
[
24]
4
reduction of HAuCl @ZIF-8. The obtained Au@ZnO catalyst
Tetracycline present in water or soil could has caused some
[
3]
show more efficient and robust photodegradation of tetracycline
in aqueous solution to both bare ZIF-8 and ZnO without Au doping.
Our result indicated that metal organic frameworks may be a
questions like allergies and toxicity. In some countries where
livestock needs to rely on antibiotics to support intensive farming
operations, such as South Korea, their concentrations of TC in
livestock wastewater were up to 2960 mg/L in influent and 524
[
5]
mg/L in effluents.
Therefore, removal or degradation of TC
appears as significant task in water treatment and
a
environmental protection.
Influenced by its antibacterial properties, traditional biological
[
6]
methods cannot effectively eliminate residual TC in sewage.
[7-10]
Alternatively, numerous studies
show that TC can be
class of good precursors to prepare photocatalysts with
well-dispersed actives sites.
[11]
photodegraded catalyzed by photocatalysts, such as TiO
2
,
ZnO,
[
12]
[13]
[14]
CdS,
BiYO
3
. However, these pure photocatalysts has
simple energetic band structure, leading to the poor utilization of
solar energy from its full spectrum. Such low efficiencies hindered
the practical application of these pure photocatalysts in
degradation of TC.
Scheme 1. Schematic illustration of the fabrication process of Au@ZnO
photo-catalyst.
Results and Discussion
Instead, gold nanoparticles doped on photo-catalysts are able
to enhance the photodegradation efficiency during removing
Preparation and characterization of the Au@ZnO catalysts
[
15]
antibiotics . However, the catalytic activity of gold nanoparticle
Herein, Au@ZIF-8 intermediate was synthesized by
impregnating the synthesized ZIF-8 rystal (0.06 g/mL ZIF-8 in 4.4
[
16]
is highly size-sensitive.
When gold is usually loaded onto the
supports via impregnation method, it is crucial to make the
photocatalysts (or inactive support) of high-density of active sites
to anchor and stabilize the gold nanospheres to obtain a uniform
mL methanol) with 0.8 mL of HAuCl
mixture was stirred for 3 hours and followed by filtration. The
sample was finally annealed under a flow of 3% H in Ar at 400 ℃
for 5 h. After reduction, the dark powder of Au@ZnO was
4
in methanol (0.2 g/mL). The
2
[
17-19]
size distribution.
This usually required delicate and complex
[25]
process to nanostructuring the photocatalysts, rendering the
production difficult to control and less cost-effective.
successful synthesized.
The simple process was shown in
Scheme 1. In order to control the size of the nanoparticles, we
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adjusted the reaction temperature and reactant concentration.
We synthesized Au@ZnO350 at 350 °C and Au@ZnO450 at 450 °C
by controlling the reaction temperature. By controlling the
In order to study the morphology and the detailed structure of
the prepared nanocrystals, we performed TEM and SEM tests.
Figure 1b shows a typical TEM image of ZIF-8 nanocrystals.
Semi-transparent hexagonal profiles of ~200 nm in diameter are
decorated by several straight ridges, suggesting its cubic nature,
concentration of HAuCl
g/mL HAuCl and Au2@ZnO with 0.4 g/mL HAuCl
reaction can be found in the Experimental.
4
, we synthesized Au0.5@ZnO with 0.1
4
4
. The detailed
[
28]
which could be confirmed by their SEM images in Figure 1d . As
seen in Figure 1b and c, both ZIF-8 and Au@ZIF-8 showed a
similar shape, but the surface on Au@ZIF-8 was rougher than that
of pure ZIF-8, which was caused by the gold of ~20 nm in
diameter grown on the surface of ZIF-8. After calcination of
Au@ZIF-8 under H2 atmosphere at 400 ℃, the Au element
mapping shows that gold nanospheres still uniformly disperse on
ZnO surface (Figure 1f). The TEM image shows that the previous
ZIF-8 nanocubes collapse and shrink to smaller amorphous ZnO
while gold nanospheres did not aggregate too much. The average
diameter of Au nanoparticles sustain at ~10 nm (Figure 1h), which
As shown in Figure 1a, the successful preparation of ZIF-8,
ZIF-8@Au, ZnO and Au@ZnO were confirmed by their XRD
patterns. ZIF-8 powder has a characteristic peak in the small angle
o
regime (7.4 ), corresponding to its well-known (011) lattice
[
26]
plane. Furthermore, the XRD pattern in Figure 1a shows that
the addition of Au did not collapse the framework of ZIF-8,
because Au@ZIF-8 exhibited the same diffraction peaks of ZIF-8o.
Meanwhile, three broad peaks (38.1°, 45.0° and 65.3°) after 30
could be assigned to the (111), (200), (220) crystal facets of Au
[
16, 29-31]
is sufficiently active for photocatalysis.
All above the
(
JCPDS 00-004-0784), indicating that Au nanoparticles were
characterization indicates that ZIF-8 is a good catalyst support
probably thanks to its multiple defective sites on surface, which
help to stabilize the Au particles during annealing and reaction
successful doped on ZIF-8. Surprisingly, there are extra peaks
rather than from Au or ZIF-8, but from ZnO (marked by asterisks).
2
After annealing in H atmosphere, all ZIF-8 peaks disappeared and
the XRD pattern of Au@ZnO is analogous to that of pure ZnO,
indicating the complete conversion of ZIF-8 into ZnO, in consistent
[
27]
with the reference
.
Figure 1. XRD patterns (a) of the synthesized samples. TEM images of ZIF-8 (b)and Au@ZIF-8 (c). SEM images of ZIF-8 (d). Mapping images of ZnO (e) and
the Au element mapping shows in (f). TEM images of Au@ZnO (g), and particle size distribution of Au in Au@ZnO are provided in (h).
Figure 2. Nitrogen adsorption and desorption isotherms of Au@ZnO
(black) and ZIF-8 (red).
Photocatalytic degradation of TC
As shown in Figure 2, both ZIF-8 and Au@ZnO mainly showed
We first compared the photocatalytic degradation performance
of TC at pH=5 using Au@ZnO with pure ZnO by varying catalysis
dosage and initial TC concentrations. Figure 3a and 3b displayed
the UV-vis spectra of TC after degradation by Au@ZnO and ZnO
for different photocatalytic periods, respectively. With the
exposure time for all samples increasing, the concentration of TC
gradually decreased.
microporous structures, which corresponded to type-I isotherm.
2
−1
Additionally, the specific surface area of ZIF-8 was 1761.9 m g
computed by the BET test. It is worth noting that the specific
surface area of the Au@ZnO catalyst obtained after
2
−1
high-temperature calcination is only 26.814 m g , which means
that ZIF-8 undergoes a phase transition after calcination at high
temperature, resulting in collapse of pores.
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We performed a kinetic analysis of the degradation behavior of
the catalyst and the results are shown in Figure 3c. The pure ZnO
exhibited a relatively lower TC removal efficiency of only ca.
similar Zero-order behavior occurs in the first 30 minutes of
photodegradation using Au@ZnO. When TC decrease to 0.005
mg/mL, the reaction shift to a second order kinetics. The TC
diffusion rate from bulk solution to the active Au@ZnO surface is
lower than the catalytic rate, therefore the degradation process is
mainly controlled by diffusion. Thus, the change of reaction
kinetics also prove Au@ZnO has the higher photocatalytic activity.
During photodegradation, the pH of the solution slightly increases
displaying in Figure S1, which suggests that the substance in the
solution is changing, in line with the UV−vis DRS spectra.
4
3.9% decay in 120 minutes under visible light, while the
Au@ZnO had a higher removal efficiency (ca. 85.5%) under the
same condition. The decay of TC concentration catalyzed using
ZnO obeys the zero-order kinetics with a kinetic rate constant
k=0.004, indicating that the degradation rate was determined by
the ZnO catalyst rather than TC concentration (Figure 3c). This
further confirms the lower activity of pure ZnO. In comparison,
Figure 3. UV−vis DRS spectra of TC solution photodegraded by Au@ZnO (a) and ZnO (b). Photocatalytic degradation of TC (0.010 mg/mL) in pH=5 using
Au@ZnO (red) and ZnO (blue).
Figure 4a showed the degradation of Au@ZnO nanocomposite at
different initial TC concentrations. It detected that the initial TC
the surface of the catalyst. The experiments were inducted to see
the decomposition of TC in the range of 10-70 mg catalyst dosage.
Depicting in Figure 4b, when the dosage of catalyst was increased
from 10 to 70 mg, the degradation efficiency of TC raised from
14.4% to 85.4%. The information suggested that the dosage of
photo-catalyst plays a necessary role in obtaining excellent
photocatalytic properties. The degradation rate of TC was mainly
controlled by diffusion.
concentration have
a
large effect during the overall
photo-degradation process, wherein 26.5% and 85.5% of TC were
removed, when the initial TC concentration was 0.005 and 0.020
mg/mL, respectively. At low concentrations, the kinetic behavior
of tetracycline decomposition is in line with the first order
reaction. In contrast, at high concentrations, the degradation of
tetracycline is only related to the concentration of tetracycline on
Figure 4. Effects of (a) initial TC concentrations for the catalytic results; (b) influence of catalysis dosage for the degradation of TC based Au@ZnO as
catalysis.
Considering that the size of gold nanoparticles affects the
catalytic degradation efficiency, we prepared Au@ZnO350,
Au@ZnO450, Au0.5@ZnO and Au2@ZnO by controlling the
reaction conditions. As it can be seen from Figure 5a-d, the
morphology of the Au@ZnO photo-catalysts vary when the
in the obtained Au0.5@ZnO (Figure 5c) was ~10 nm. Similar in
Au2@ZnO (Figure 5d), the average size of Au nanoparticles was
~10 nm. Therefore, the size of the Au nanoparticles is
independent of the amount of HAuCl
reaction temperature.
4
, only related to the
4
reaction temperature and the dosage of HAuCl were changed. It
showed in Figure 5a that many smaller gold nanoparticles present
in Au@ZnO350, but the Au nanoparticles are not uniform in size.
When the temperature rises to 450 °C (Figure 5b), the Au
nanoparticles agglomerate and it is difficult to distinguish the size
of the Au nanoparticles. The average size of the Au nanoparticles
In addition, we used Au@ZnO350, Au@ZnO450, Au0.5@ZnO
and Au2@ZnO to degrade TC while the results are shown in the
Figure S2-S5. When the reaction temperature is different, we
found in Figure 5e that Au@ZnO prepared at 400 °C has the
highest degradation efficiency. Although the average size of gold
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nanoparticles is smaller in Au@ZnO350, the degradation
efficiency is not obviously improving due to the uneven size of Au
nanoparticles. By comparing the degradation efficiencies of
Au0.5@ZnO, Au@ZnO and Au2@ZnO in Figure 5f, the
degradation efficiency of Au@ZnO is still the highest. The
degradation efficiency of Au0.5@ZnO may be due to the relatively
small proportion of Au nanoparticle, and the degradation
efficiency of Au2@ZnO may be due to the high concentration of
Au nanoparticle, which almost cover the surface of ZnO, and the
synergistic effect between Au nanoparticle and ZnO is partially
inhibited.
Figure 5. TEM images of Au@ZnO350 (a), Au@ZnO450 (b), Au0.5@ZnO (c), Au2@ZnO (d). Photodegradation efficiency of Au@ZnO350, Au@ZnO and
Au@ZnO450(e), Au0.5@ZnO, Au@ZnO and Au2@ZnO (f).
Figure 6. (a) Cycling of Au@ZnO for TC degradation (a), and photodegradation efficiency of cycling was showed in figure 6b.
The chemical stability and reusability of the photocatalysts also
play important roles for practical applications. Thus, the
photocatalytic stability is investigated by repeating TC
degradation using the same Au@ZnO sample for five times.
Before next run for each cycle, the catalyst was gathering by
filtration, washing and drying. The degradation efficiency of
Au@ZnO after five cycles shown in Figure 6a remained 97.4%
compared to the first run (Figure 6b), suggesting its good
reusability.
Photocatalytic mechanism in reaction system
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To understand the synergetic effect between ZIF-8-derived ZnO
and Au nanocrystal, the band gap of Au@ZnO is researched by
UV-Vis spectrophotometry in Figure 7a. Sample ZnO exhibits a
sharp band on setting from 418 nm, which corresponds to ZnO
optical band gap of 2.97 eV. For Au@ZnO, a broadband centered
at 537 nm representing the surface plasmon resonance of Au
nanoparticles appears together with an unshifted band at 418 nm
for ZnO support.
Before we explored the photocatalytic mechanism of the reaction
system, we tested the photocatalytic degradation of tetracycline
without the catalyst but with light irradiation first. The
experimental data is shown in Figure 7b, in the absence of
photocatalysts, the concentration of tetracycline hardly changed
throughout the process. This indicates that tetracycline aqueous
solution did not spontaneously undergo photolysis reaction under
visible light irradiation, and the photocatalyst played an
important role in the reduction of tetracycline aqueous solution
Figure 7. (a) UV−vis DRS absorption of ZnO and Au@ZnO samples. (b) Photocatalytic degradation of TC (0.010 mg/mL) without photocatalyst
underidentical light irradiation.
The results of as-obtained visible light catalysis confirm that
Our Au@ZnO catalysts convert TC into free radicals, and then the
the catalytic effect of Au@ZnO is better than that of ZnO particles, free radicals were oxidized to intermediate compounds under the
which can be explained by work function matching. Scheme 2
proposes an overall photocatalytic degradation mechanism under
irradiation with visible light illumination. The work function values
action of reactive oxygen species.
[31-33]
of ZnO and Au are reported to be 5.2 eV, and 5.0 eV.
Typically, a Schottky barrier is formed when metal/semiconductor
contacts. When irradiated with visible light, the electrons in EEq
were rapidly injected into the conduction band (CB) of ZnO by a
surface phonon resonance (SPR) mechanism, leaving holes on the
metal surface. The electrons injected in CB react with dissolved
-
oxygen to produce superoxide anion radicals (·O
subsequently react with H
a continuous reaction pathway.
2
), which can
2
O to produce hydroxyl radicals (·OH) in
[34-35]
These superoxide radical
anions and hydroxyl radicals are the orgin of the decomposition
of TC under visible light irradiation.
-
+
Scheme 2. Photocatalytic Mechanism of the Photodegradation of TC
over Au@ZnO under Visible-Light Irradiation.
ZnO + hv (UV) → ZnO (eCB +hVB
)
Au@ZnO + hv (visible) → Au@ZnO* (e-)
The LC-MS technique in Figure 8 was performed to reveal
more intermediates of TC degradation catalyzed by Au@ZnO.
Since the bond energy of N−C is low, the N- demethylation
process is apt to occur, and then the product TC 1 (m/z 430) and
TC 2 (m/z 410) appear. The original TC molecule undergoes this
deamination process, thus resulting in the formation of TC 3 (m/z
-
-
Au@ZnO* (e ) → ZnO (eCB ) + Au
-
-
e
CB +O
2
→ ·O
2
4
26). The production of TC 4 (m/z 368) can also be attributed to
+
-
h
VB +OH → ·OH
the deamidation reaction. On the other hand, when the produced
TC 4 (m/z 368) undergo dehydroxylation process, the product (TC
5, m/z 352) can be obtained. The product TC 5 can convert to TC 6
-
+
·
O
2
+2H → ·OH
(m/z 324) by further removing one aldehyde group, followed by
further degradation.
Under visible light, the oxidative holes and hydroxyl radicals
produced from Au@ZnO nanomaterials react with tetracycline.
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The Au-0.5@ZIF-8 intermediate was synthesized by impregnating the
synthesized ZIF-8 crystals (0.06 g/mL ZIF-8 in 4.4 mL methanol) with 0.8
mL of HAuCl
and then filtered. Finally, the sample was annealed in Ar at a flow of 3%
at 400 ℃ for 5 h. After reduction, the Au-0.5@ZnO was synthesized.
4
in methanol (0.1 g/ mL). The mixture was stirred for 3 hours
H
2
The Au-2@ZIF-8 intermediate was synthesized by impregnating the
synthesized ZIF-8 crystals (0.06 g/mL ZIF-8 in 4.4 mL methanol) with 0.8
mL of HAuCl
and then filtered. Finally, the sample was annealed in Ar at a flow of 3%
at 400 ℃ for 5 h. After reduction, the Au-2@ZnO was synthesized.
4
in methanol (0.4 g/ mL). The mixture was stirred for 3 hours
H
2
Photocatalytic experiments. The photocatalytic activity of Au@ZnO
was evaluated by TC decomposition under visible light irradiation. We
use a 300 W Xe lamp as the light source. In a typical experiment, 50 mg
of Au@ZnO photocatalyst was dispersed in an aqueous solution of TC
(
100 mL, 10 mg/L). Prior to the photocatalytic performances test, the
Figure 8. Suggested pathways for TC photodegradation.
suspension was stirred in the dark for 30 minutes to ensure an
adsorption-desorption equilibrium between the catalyst and TC solution.
During the irradiation, the solution (3 mL) was sampled at intervals of time,
then centrifuged at 7000 rpm for 8 min, and filtrated through a Millipore
filter (Pore size: 0.22 μm) to remove residual particles. The
corresponding TC concentration was monitored by its characteristic
absorption band of 357 nm using a UV–vis spectrophotometer.
Conclusions
In summary, a metal-organic framework-based, a simple and
convenient preparation method of Au@ZnO photocatalysts is
developed. Au nanoparticles with uniform size and dispersity are
stabilized on the surface of ZnO. The Au@ZnO catalyst is able to
efficiently photodegrade tetracycline in presence of visible light
with good recyclability and much high efficiency than the pure
ZnO. While the synergetic effect between the surface phonon
resonance from Au and photoactivity of semiconducting metal
oxide, our results provide a new way to control the size and load
shape-dependent metal nanoparticles of ZnO surface, which
hopefully can be used for more photo-degradation
Acknowledgement
This work was financially supported by the Natural Science
Foundation of China (21336005, 21603158), the major research
project of Natural Scientific Research Foundation of the Higher
Education Institutions in Jiangsu Province (15KJA150008,
Experimental
1
7KJA150010), Suzhou Science and Technology Bureau Project
Materials. Zinc nitrate hexahydrate [Zn(NO
3
)
2
·6H
2
O], 2-methylimidazole
OH) and
(
SYG201524) and the Priority Academic Program Development of
[
Hmim], aurichlorohydric acid [HAuCl ], methanol (CH
4
3
tetracycline (TC) were purchased from Alfa Aesar, TCI and Sinopharm
Chemical Reagent Co., Ltd, respectively. All chemicals were used
without further purification. Throughout the experiment, deionized water
was used as the solvent.
Jiangsu Higher Education Institutions (PAPD).
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