ZnO nanorod decorated by Au-Ag alloy with greatly increased activity for photocatalytic ethylene oxidation

Article abstract of DOI:10.1016/S1872-2067(19)63473-X

In recent years, the preservation of fruits and vegetables in cold storage has become an issue of increasing concern, ethylene plays a leading role among them. We found ZnO has the effect of degrading gaseous ethylene, however its effect is not particularly satisfactory. Therefore, we used simple photo-deposition procedure and low-temperature calcination method to synthesize Au, Ag, and AuAg alloy supported ZnO to improve the photocatalytic efficiency. Satisfactorily, after ZnO loaded with sole Au or Ag particles, the efficiency of ethylene degradation was 17.5 and 26.8 times than that of pure ZnO, showing a large increase in photocatalytic activity. However, the photocatalytic stability of Ag/ZnO was very poor, because Ag can be easily photooxidized to Ag₂O. Surprisingly, when ZnO was successfully loaded with the AuAg alloy, not only the photocatalytic activity was further improved to 94.8 times than that of pure ZnO, but also the photocatalytic stability was very good after 10 times of cycles. Characterization results explained that the Au-Ag alloy NPs modified ZnO showed great visible-light absorption because of the surface plasmon resonance (SPR) effect. Meanwhile, the higher photocurrent density showed the effective carrier separation ability in AuAg/ZnO. Therefore, the cooperative action of plasmonic AuAg bimetallic alloy NPs and efficient carrier separation capability result in the outstanding photoactivity of ethylene oxidation. At the same time, the formation of the alloy produced a new crystal structure different from Au and Ag, which overcomes the problem of poor stability of Ag/ZnO, and finally obtains AuAg/ZnO photocatalyst with high activity and high stability. This work proposes a new concept of using metal alloys to remove ethylene in actual production.

Full text of DOI:10.1016/S1872-2067(19)63473-X

Chinese Journal of Catalysis 41 (2020) 1613–1621
a v a i l a b l e a t w w w . s c i e n c e d i r e c t . c o m
j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / c h n j c
Article
ZnO nanorod decorated by Au-Ag alloy with greatly increased
activity for photocatalytic ethylene oxidation
Huishan Zhai, Xiaolei Liu, Zeyan Wang, Yuanyuan Liu, Zhaoke Zheng, Xiaoyan Qin, Xiaoyang Zhang,
Peng Wang *, Baibiao Huang #
State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, Shandong, China
A R T I C L E I N F O
A B S T R A C T
Article history:
In recent years, the preservation of fruits and vegetables in cold storage has become an issue of
increasing concern, ethylene plays a leading role among them. We found ZnO has the effect of de-
grading gaseous ethylene, however its effect is not particularly satisfactory. Therefore, we used
simple photo-deposition procedure and low-temperature calcination method to synthesize Au, Ag,
and AuAg alloy supported ZnO to improve the photocatalytic efficiency. Satisfactorily, after ZnO
loaded with sole Au or Ag particles, the efficiency of ethylene degradation was 17.5 and 26.8 times
than that of pure ZnO, showing a large increase in photocatalytic activity. However, the photocata-
lytic stability of Ag/ZnO was very poor, because Ag can be easily photooxidized to Ag2O. Surprising-
ly, when ZnO was successfully loaded with the AuAg alloy, not only the photocatalytic activity was
further improved to 94.8 times than that of pure ZnO, but also the photocatalytic stability was very
good after 10 times of cycles. Characterization results explained that the Au-Ag alloy NPs modified
ZnO showed great visible-light absorption because of the surface plasmon resonance (SPR) effect.
Meanwhile, the higher photocurrent density showed the effective carrier separation ability in
AuAg/ZnO. Therefore, the cooperative action of plasmonic AuAg bimetallic alloy NPs and efficient
carrier separation capability result in the outstanding photoactivity of ethylene oxidation. At the
same time, the formation of the alloy produced a new crystal structure different from Au and Ag,
which overcomes the problem of poor stability of Ag/ZnO, and finally obtains AuAg/ZnO photo-
catalyst with high activity and high stability. This work proposes a new concept of using metal alloys
to remove ethylene in actual production.
Received 17 February 2020
Accepted 26 March 2020
Published 5 October 2020
Keywords:
Surface plasmon resonance
Au-Ag bimetallic alloy nanoparticles
Cooperative action
Effective carrier separation
© 2020, Dalian Institute of Chemical Physics, Chinese Academy of Sciences.
Published by Elsevier B.V. All rights reserved.
1. Introduction
photocatalytic H2 production [10,11], CO2 reduction [12],
transformation of green organic [13], thermocatalytic reduc-
In recent years, the noble metals have attracted great atten-
tion because of their outstanding physical, chemical and optical
performances, which made them suitable as co-catalysts on
some photocatalysts [1–8]. Most of them can be used as the
electron-transfer active sites in photodegradation of dye [9],
tion of nitroaromatics in water [14] and so on. In particular, Au
and Ag as common noble metals in our life are also widely in-
vestigated in photocatalytic academic research due to their
unique surface plasmon resonance (SPR) [15–19]. The Au, Ag
NPs under SPR-excitation have strong visible-light absorption,
* Corresponding author. E-mail: pengwangicm@sdu.edu.cn
# Corresponding author. E-mail: bbhuang@sdu.edu.cn
This work was supported by the National Natural Science Foundation of China (51602179, 21333006, 21573135, and 11374190) and the Taishan
Scholars Program of Shandong Province.
DOI: S1872-2067(19)63473-X | http://www.sciencedirect.com/science/journal/18722067 | Chin. J. Catal., Vol. 41, No. 10, October 2020

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Huishan Zhai et al. / Chinese Journal of Catalysis 41 (2020) 1613–1621
thus they can be applied to solve the challenges in photocata-
lytic reactions such as narrow spectral response [15–19]. Fur-
thermore, just like a semiconductor heterojunction can pro-
mote carrier separation [20–23], Au or Ag NPs can act as the
active sites for capturing electrons, improving the separation of
electron and hole pairs in photocatalytic system as well.
Therefore, the Au or Ag supported materials have been also
investigated in solving the problem of low quantum efficiency
[24].
than that of pure ZnO, while loaded by single 0.8 wt% of Au or
Ag was only 17.5 or 26.8 times higher than that of ZnO. These
results confirm the Au-Ag alloy is superior to the single metal
Au or Ag in ethylene-oxidation process. Therefore, the Au-Ag
alloy is the promising cocatalyst for ethylene oxidation to
freshen fruits and vegetables in refrigeration storage.
2. Experimental
Bimetallic alloy is composed of two metals by a certain
method. Recently, some studies have found that the semicon-
ductors loaded with bimetallic alloy have many superior prop-
erties compared to that loaded with sole metal [25–29]. More
importantly, they can complement each other to achieve a
win-win situation. For example, Au-Cu alloy supported on TiO2
is effective on photocatalytic CO2 reduction [30]. Among them,
the selectivity in CH4 conformation owned to the Cu bonding to
CO2 on TiO2, while the visible light photoresponse would be due
to the surface plasmon band of Au, illustrating Au and Cu in the
Au-Cu alloy have been put to the best utilization from each
other [30]. In addition, Au-Pt alloy loaded on WO3 was used to
strengthen hydrogen and oxygen generation owing to the mu-
tual promotion of plasmonic function in Au and catalytic char-
acteristic in Pt [15]. As we all know, Ag has similar plasmonic
property as Au. Therefore the Au and Ag in Au-Ag alloy exhibit
outstanding photocatalytic performances and synergistic ef-
fects which are superior to the pure metals under visible-light
irradiation [31]. Au-Ag alloy loaded semiconductors have been
applied to restore CO2 to fuels [32], heighten the photoelectro-
catalytic properties [33], oxide of methanol selectively [34],
and so on. In particular, we have found Au-Ag alloy played an
important role in the degradation of the harmful gas ethylene
recently.
Ethylene is produced in natural sources, plants and plant
products. It has some harm on the storage life, development
and growth of many ornamental crops, fruits and vegetables at
extremely low concentration [35,36]. Some researchers have
already realized the dangers of ethylene and looked for some
ways to degrade it to keep fresh of fruits and vegetables. In the
field of photocatalysis, many studies were focus on traditional
TiO2 and TiO2-based photocatalysts [37–40], while new mate-
rials were rarely exploited on ethylene degradation. Actually
there were only few new semiconductors had eth-
ylene-oxidation function and their photocatalytic activity were
very poor, such as BiVO4 and In2O3-Ag-Ag3PO4 [41,42]. Recent-
ly, we reported that Fe-doped WO3 has the property of degrad-
ing ethylene [43], and the activity was greatly improved com-
pared with previous studies. However, the ethylene cannot be
completely mineralized to CO2. Therefore, it is urgent to find a
highly active substance to deal with the ethylene. In this paper,
we interestingly found ZnO had capability to oxide ethylene,
but the activity was not so satisfactory. To improve the photo-
catalytic reaction rate greatly, Au-Ag bimetallic alloy NPs were
loaded on the ZnO. We systematically investigated the perfor-
mance of single Au, Ag, and Au-Ag alloy decorated on ZnO, and
we found the photocatalytic ethylene-oxidation activity of ZnO
decorated by 0.8 wt% of Au-Ag alloy was 94.8 times higher
2.1. Materials synthesis
2.1.1. ZnO nanorods
The ZnO nanorods were prepared referring to the simple
hydrothermal method [44]. Particularly, 0.4 g NaOH was added
to 60 mL ethanol under ultrasonic treatment until forming ho-
mogeneous 0.17 mol/L NaOH/ethanol solution. Then the solu-
tion was put into a Teflon tank of 100 mL capacity containing 2
mmol Zn(CH3COO)2·2H2O and stirred for 30 min. The tank was
transferred into a stainless-steel autoclave and heated at 160 °C
for 24 h, the white powder was obtained after suction filtration
with water and dried at 60 °C. Then the ZnO powder was cal-
cined at 400 °C for 1 h to remove the adsorbed ethanol.
2.1.2. Au or Ag NPs loaded on ZnO nanorods
The deposition of single Au NPs on the ZnO nanorods was
synthesized via a photo-reduction method. Briefly, 0.15 g ZnO
was dispersed in 100 mL H2O in a beaker. Then different vol-
umes of 0.1 mol/L HAuCl4 were dripped into the system and
stirred for 5 min in order to form a homogeneous mixture. Be-
fore irradiation, 1 mL methyl alcohol was added as the sacrifi-
cial agent. Then the mixture was irradiated for 30 min under
the Xe lamp of 300 W. Finally, the solid was filtered, dried at 60
°C and calcined at 400 °C for 1 h to make the Au NPs have a
tightly integrated with the surface of ZnO and remove the ad-
sorbed ethanol. The deposition of single Ag NPs on the ZnO
nanorods was similar with these processes. The only difference
was that 0.1 mol/L HAuCl4 was replaced with 0.1 mol/L AgNO3.
2.1.3. Au-Ag bimetallic alloy NPs loaded on ZnO nanorods
The Au-Ag bimetallic alloy NPs supported on ZnO nanorods
were conducted with a co-photodeposition method. That is, a
certain volume of HAuCl4 (0.5 wt% Au) and AgNO3 (0.3 wt%
Ag) were put together into the ZnO suspension under continu-
ous stirring, and other reaction conditions were same as single
Au/ZnO. The optimal Au-Ag proportion on ZnO was 0.8 wt%.
Therefore, 0.8 wt% Au/ZnO and 0.8 wt% Ag/ZnO were also
obtained as comparison by using the similar procedure.
2.2. Characterization
Morphologies were investigated by SEM (Hitachi S-4800)
equipped with an EDS. XRD patterns were conducted on a
Bruker AXS D8 diffractometer equipped with Cu Kα radiation to
reveal the crystal structure. The UV-vis DRS analyzes were rec-
orded by Shimadzu UV-2550 spectrophotometer using BaSO4
as reflectance standard to explore the optical absorption. The
TEM and HRTEM tests were performed with a JEOL JEM-2100F

Huishan Zhai et al. / Chinese Journal of Catalysis 41 (2020) 1613–1621
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microscope to analyze the nanostructure and composition of
the as-prepared AuAg/ZnO photocatalyst. XPS measurements
were obtained using a Thermo ESCALAB 250XI, and the peak
positions of various elements were calibrated by C 1s (284.8
eV). Photoelectrochemical tests were measured by CHI-660C
electrochemical workstation using a three-electrode system.
The FTO glass coated catalyst was served as working electrode,
Pt sheet as counter electrode and Ag/AgCl as reference elec-
trode. 0.2 mol/L Na2SO4 solution was used as the electrolyte. A
300 W Xe-arc lamp (CEL-HXF300, Beijing CEAU Light, China)
was used for a light illuminant. The gas mixture was researched
by means of the Shimadzu GC-2014C.
Fig. 1. The XRD patterns of the pure ZnO and Au, Ag, Au-Ag alloy nano-
particles decorated on ZnO samples.
2.3. Photocatalytic evaluation
Photocatalytic oxidation of ethylene was measured in a
quartz-covered reactor with 400 mL volume irradiated by a
300 W Xe lamp. 0.12 g photocatalyst was dispersed uniformly
in the bottom of the container with a rotor. The reactor was
then sealed by the quartz cover and injected into 0.5 mL eth-
ylene under stirring. Before turning on the lights, the container
was stirred in the dark for 2 h to make ethylene and air in the
container mix evenly and attain the adsorption and desorption
balance. When the balance was achieved, the reactor was illu-
minated on top of quartz cover and 50 μL of gas mixture was
sampled at regular intervals and tested by a gas chromatog-
raphy. C/C0 indicates the degradation percentage of ethylene,
where C is ethylene concentration at a specific time and C0 is
the initial concentration of ethylene. Stability of AuAg/ZnO
product was also investigated as follows: after each ethylene
oxidation reaction, opened the cover and set aside 30 minutes
to allow excess CO2 and C2H4 in the container to diffuse out.
Then the reactor was sealed again to degrade a new 0.5 mL
ethylene for another test under the same irradiation.
3.2. EDS and elemental maps of nanocomposites
Fig. 2a reveals the distribution and content of C, O, Zn, Au
and Ag. Among them, the source of C might be conductive plas-
tic or the adsorbed CO2. As can be seen from Fig. 2b, the weight
percentages of Au and Ag are, respectively, 0.55% and 0.21%,
which are close to the actual loading amount of 0.5% and 0.3%.
From Fig. 2c, we can see the overall distribution of Au and Ag
elements are strongly uniform, illustrating that Au and Ag are
evenly loaded on the surface of ZnO.
3.3. UV-vis diffuse spectroscopy
To understand the different absorption of Au, Ag and AuAg
loaded ZnO, the UV-vis diffuse reflectance spectra of 0.8 wt%
Au/ZnO, 0.8 wt% Ag/ZnO and 0.5 wt% Au@0.3 wt% Ag/ZnO
samples, together with that of pure ZnO nanorods, are investi-
gated and exhibited in Fig. 3.
As can be observed in Fig. 3, pure ZnO has no absorption in
the visible range, in agree with the reported band gap of ∼3.1
eV. However, the Au or Ag loaded ZnO displays increased visi-
ble light absorption owned to the surface plasmon resonance
(SPR) effect of the metallic Au and Ag particles. Especially,
there is a characteristic peak at around 550 nm of Au/ZnO
while Ag/ZnO is approximately at 470 nm, which is in accord-
ance with the previous article [31,45,46]. Interestingly, when
Au or Ag is replaced by the same amount of Au-Ag, it shows a
broader and stronger peak at about 510 nm lying between the
characteristic peaks of single Au and Ag. These data further
illustrate that when Au and Ag were co-loaded on ZnO, they
might form the Au-Ag alloy with the synergistic SPR effect,
which can enhance the light absorption at 400–800 nm [32,33].
And it is worth putting forward that the promoted light absorp-
tion generally along with the increased photocatalytic property.
3. Results and discussion
3.1. X-ray diffraction research of as-prepared nanocomposites
The XRD spectra of obtained products are shown in Fig. 1.
As can be seen from the left figure, the peaks of pure ZnO match
well with the standard ZnO card (JCPDS, No. 70-2551). All of
the diffraction peaks of the as-prepared samples after incorpo-
ration of noble metals are very identical to that of pristine ZnO,
with only small peak displacement in the part of the dotted line.
From the magnified figure on the right and the Table 1 below, it
can be seen that the Ag/ZnO has two peaks at 37.99° and
44.21° while the peaks of Au/ZnO at 38.30° and 44.66°, which
are in line with the Ag and Au standard cards (Fig. S1). It should
be pointed out, however, the AuAg/ZnO has peak locations
(38.22° and 44.39°) different from any one of single Au and Ag,
which are located between the peaks of Au and Ag. Similar
consequence of Au-Ag alloy peak-displacement was also found
in Fig. S1. This interesting phenomenon concludes that Au-Ag
alloy might come into being when Au and Ag are co-loaded on
the ZnO and calcined at 400 °C. Certainly, this conclusion needs
to be proved further by the following results.
Table 1
The two XRD peaks in the range of 37°–45° of Ag/ZnO, AuAg/ZnO and
Au/ZnO samples.
Sample
2θ1 (°)
37.99
38.22
38.30
2θ2 (°)
44.21
44.39
44.66
Ag/ZnO
AuAg/ZnO
Au/ZnO

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Huishan Zhai et al. / Chinese Journal of Catalysis 41 (2020) 1613–1621
Fig. 4. (a) SEM of pure ZnO, (b) SEM of AuAg/ZnO, (c) TEM of
AuAg/ZnO, and (d, e, f) HRTEM images of AuAg/ZnO.
0.26 nm (JCPDS, No. 70-2551). After a lot of detection, the
d-spacing values of these small metal NPs (3–7 nm) are 0.235
nm (Au(111) JCPDS 01-1172) and 0.237 nm (Ag(111) JCPDS
01-1164), one of the pictures is presented in Fig. 4d. However,
the d-spacing values of these big metal NPs (10–15 nm) are all
0.236 nm (AgAu(111) JCPDS 65-8424), presented in Fig. 4e and
4f. The size of AuAg NPs coated on ZnO is larger than Au or Ag
NPs due to Au-Ag alloy formation [32].
Besides, the information crisply exhibits that the AuAg/ZnO
photocatalyst possesses not only a distribution of the Au-Ag
alloy NPs but also the coexistence of unalloyed Au and Ag NPs.
Similar phenomenon is also appeared in Au-Cu alloy loaded on
TiO2, that is, AuCu/TiO2 sample consists of Au-Cu alloy NPs
together with independent Au and Cu NPs, which is difficult to
avoid [30]. Anyway, this result further proves the formation of
Au-Ag alloy NPs in AuAg/ZnO and its photocatalytic activity
and stability are both satisfactory in the following tests.
Fig. 2. (a, b) EDS spectra and (c) EDS elemental mapping analysis of O,
Zn, Au, and Ag in the AuAg/ZnO sample.
3.4. SEM and TEM
Fig. 4a and b are typical SEM images of the as-prepared ZnO
and AuAg/ZnO. As can be seen, ZnO are nanorods with an av-
erage thickness of around 20 nm. After loaded by Au-Ag alloy
NPs, we can see from Fig. 4c that there are some small particles
deposited on the surfaces and edges of nanorods. The key is
that these nanoparticles have uneven size, some are big and
some are small, with an average of ~6 nm by calculating (see
Supporting Information Fig. S2), and they are closely attached
to the ZnO surfaces to have a better contact. Fig. 4d, e and f are
the corresponding HRTEM images of AuAg/ZnO. The detected
lattice fringes of ZnO are matching well with the (002) plane of
3.5. X-ray photoelectron spectroscopy study
To obtain the surface chemical status of AuAg/ZnO before
and after irradiation, X-ray photoelectron spectroscopy was
carried out. Fig. 5a is the full spectra of AuAg/ZnO sample be-
fore and after degradation, where the peaks for Zn 2p and O 1s
can be seen clearly. In Fig. 5b, the two peaks centered at 1021
eV and 1044 eV are corresponding to the Zn 2p3/2 and Zn 2p1/2
.
There is no displacement of the peak position before and after
the reaction, indicating that the chemical state of the Zn ele-
ment has not changed. Similarly, as seen from Fig. 5c, the peaks
of AuAg/ZnO before degradation located at 529.5 eV and 531.4
eV are indexed to lattice oxide and adsorbed oxygen [32]. After
degradation, the peak of lattice oxide has no change, indicating
ZnO did not change before and after reaction. However, the
Fig. 3. UV-vis diffuse reflectance spectra of ZnO, Au/ZnO, Ag/ZnO and
AuAg/ZnO samples.

Huishan Zhai et al. / Chinese Journal of Catalysis 41 (2020) 1613–1621
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Fig. 5. (a) XPS survey spectra and (b–e) high-resolution XPS spectra of Zn 2p, O 1s, Au 4f and Ag 3d for AuAg/ZnO before and after degradation.
peak of adsorbed oxygen exhibits a slightly negative shift, sug-
gesting the chemical reactions of adsorbed O2 might be in-
volved. In Fig. 5d, the diffraction peak located at 83.3 eV and
88.1 eV are corresponded to Au 4f7/2 and Au 4f5/2. It can be seen
that the valence state does not change significantly before and
after illumination, indicating the alloyed Au and unalloyed Au
are very stable. But Fig. 5e shows that before the illumination,
the peak positions are located at 367.1 eV and 373.2 eV, re-
spectively, which are features of Ag0. After the illumination, the
peak position shifts slightly to the high binding energy at 367.4
eV and 373.5 eV and exists a mixed state. This is because the Ag
particles are unstable and easily lose electrons under long-time
illumination. The unalloyed Ag particles lose elections and be-
come Ag+ and thus the peaks exhibited a positive shift [47].
However, the most Ag elements in AuAg alloy are still Ag0,
which proves the superiority of the AuAg alloy. This result also
corresponds to the test results of photocatalytic stability.
has been significantly improved and the reaction rate of 0.5%
Au/ZnO even reaches up to 0.162 g–1min–1, which is 40 times
that of pure ZnO. It can be seen the existence of noble metal Au
has a great role in promoting the activity due to its extended
light absorption, which confirms our speculation exactly. Then
we study the effect of Au-Ag alloy and the optimal loading
amount of Ag (Fig. S3 d–f). We find that when the precursor of
Au (0.5%) and Ag (0.1%–0.7%) are simultaneously added into
the ZnO suspension and reduced to Au, Ag and Au-Ag alloy, the
activity has been further upgraded. When the ratios of Au and
Ag are 0.5% and 0.3% (total amount of noble metal is 0.8%),
the reaction rate reaches the highest. Then we also make a
comparative experiment of 0.8% Au and 0.8% Ag in Fig. 6, both
are less active than AuAg/ZnO. The reaction rate of 0.8%
AuAg/ZnO is approximately 5.41 and 3.54 times more than the
0.8% Au/ZnO and 0.8% Ag/ZnO. Hence, it is clear that the syn-
ergistic effect of Au and Ag has unmatched superiority.
In order to compare the photocatalytic effects of our syn-
thesized photocatalysts with other photocatalysts, the photo-
catalytic degradation activity of AuAg/ZnO sample was com-
pared with that of other photocatalysts such as Pt-TiO2,
Pt@Fe-WO3, Ag/AgCl/TiO2, Ag-ZnO, BiVO4/P25, P25/Bi2WO6
and In2O3-Ag-Ag3PO4, the detailed results are shown in Table
S1. It can be seen from the roughly calculated reaction rate
(ppm g–1 min–1) that the AuAg/ZnO has the highest activity
among many photocatalysts, which proves that AuAg/ZnO
sample has a good application prospect. In addition, we have
done similar experiments using nanoflower-like ZnO and found
that the morphology of ZnO did not affect the experimental
rules. That is, the regularity in C2H4 degradation of nanoflow-
er-like ZnO was similar to that of the nanorod-like ZnO, indi-
cating that the regularity of AuAg/ZnO in degradation of eth-
ylene is universal. The related comparison experiment results
3.6. Photocatalytic C2H4 oxidation and stability test of
as-obtained samples
Ethylene is used as an objective organic pollutant to meas-
ure the photocatalytic activity of the as-prepared products at
15 °C. The total experimental results are shown in Fig. S3 and
Fig. 6. Firstly, the blank test of ethylene without photocatalyst is
performed and we find that the pure ethylene can hardly be
decomposed in the absence of photocatalyst. Therefore, all the
following degradation of ethylene is due to the presence of the
photocatalyst. Fig. S3a illustrates the degradation curves over
pure ZnO and Au decorated ZnO. Fig. S3b and c are the corre-
sponding kinetics curves and reaction rate constants. It is ob-
vious that pure ZnO has a poor reaction rate constant of 0.004
g–1min–1. After loading with a small amount of Au, the activity

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Huishan Zhai et al. / Chinese Journal of Catalysis 41 (2020) 1613–1621
Fig. 6. (a) Comparative photodegradation activities of C2H4, (b) the corresponding kinetics curves, (c) the reaction rate constants of the total contrast
activities of pure ZnO, 0.8% AuAg/ZnO, 0.8% Au/ZnO and 0.8% Ag/ZnO under (UV-vis) light illumination, (d) the recyclability for the photocatalytic
degradation of C2H4 in the presence of 0.8% AuAg/ZnO composite.
are presented in Fig. S4.
changes.
In order to verify this conjecture, the stability of single
Besides the photocatalytic behavior, the stability of the
photocatalyst is another vital character in practical application.
To investigate the stability of 0.8% AuAg/ZnO, ten-test cycles
were conducted under the same condition. In detail, Fig. 6d is
photodegradation kinetic constant of C2H4 of these ten-time
tests. As can be seen, the photocatalytic C2H4 oxidation over
0.5%Au@0.3%Ag/ZnO shows a slight downward trend after
one cycle and the rate constant drops from 0.379 g–1min–1 to
0.343 g–1min–1. However, there is no significant decrease of
activity from the second to the tenth cycle of photocatalytic
measurements, that is, the corresponding k constant is 0.343,
0.324, 0.335 and 0.325 g–1min–1, separately. The reason for this
phenomenon, in our opinion, may be due to that part of the
separate Ag is oxidized to Ag2O. After one-time irradiation, the
all or most isolated Ag is become Ag2O because of its unstable
chemical properties. But the Au-Ag alloy and separate Au have
no change because they are quite stable. Thus, after the first
little decline, the succeeding photocatalytic activities show little
Au/ZnO and Ag/ZnO has also been studied (Fig. 7a and b). It
can be seen that the Au/ZnO has very great stable activity. After
ten times of circulations, the reaction rates are almost un-
changed. However, it is interesting to see that the Ag/ZnO has a
relatively poor stability. The reaction rate drops to almost half
after the first cycle and attenuates in the following tests. When
going to the tenth cycle, the reaction rate is close to one tenth of
the first. This result just verifies our conjecture that isolated Ag
is not stable while Ag in the Au-Ag alloy is quite stable. These
results demonstrate the Au-Ag alloy exhibits good stability
compared to the separate Ag and exhibits great activity com-
pared to the separate Au. Above all, the Au-Ag alloy supported
on the ZnO nanorods has unparalleled excellence.
To examine the mineralization ratio of ethylene oxidation,
the photocatalytic measurement over the AuAg/ZnO product is
further conducted in Fig. 8. At beginning, the concentration of
C2H4 and CO2 is 1250 ppm and 0 ppm (the 0 ppm is after de-
Fig. 7. The recyclability for the photocatalytic degradation of C2H4 in the presence of (a) single Au and (b) single Ag under (UV-visible) light illumina-
tion.

Huishan Zhai et al. / Chinese Journal of Catalysis 41 (2020) 1613–1621
1619
the function to accelerate the separation of charge carriers fur-
ther more. The above results show that the Au-Ag alloy has
good synergy effect to have a prolonged recombination time
and enhanced photocurrent density.
3.8. Mechanism study
Based on the above results, a feasible reaction mechanism is
proposed in Fig. 10. As we know, ZnO is a semiconductor with a
wide band gap of 3.1 eV, which can only absorb the UV light.
Thus, the ZnO cannot be excited by visible light. Nevertheless,
Au and Ag have good absorption in the visible light area due to
their special surface plasmon resonance (SPR) [15–19]. Be-
cause the conduction band position of ZnO (–0.31 eV vs NHE) is
Fig. 8. Photocatalytic C2H4 degradation and CO2 generation of
AuAg/ZnO.
–
more negative than E0 (O2/•O2 ) (–0.046 eV vs NHE), the pro-
ducting the CO2 content in the air), respectively. When the light
is turned on, the concentration of C2H4 rapidly decreases to
zero in 1 h, meanwhile, the concentration of CO2 increases with
two times of C2H4 reducing. Finally, the concentration of CO2 is
2467 ppm (about 2500 ppm). The result confirms that ethylene
oxidation is truly driven by a photocatalytic process, and C=C
bond in C2H4 is almost broken into two times of O=C=O bond.
The mineralization ratio of ethylene is about 100% in this reac-
tion.
cess of the single-electron reduction of oxygen can proceed and
–
generate •O2 [50]. Hence, the plasmon-induced electrons in
the Au, Ag and Au-Ag alloy NPs can migrate to the CB of ZnO
through the metals-ZnO interface. These electrons can produce
–
•O2 and be used for the degradation of C2H4. Details of the
mineralization process of ethylene and the reactions are given
in Fig. 10. It is worth mentioning that the reaction efficiency
under visible light is very low because the number of plas-
mon-induced electrons is very small compared to those photo-
excited electrons in ZnO. The corresponding irradiations were
performed with filtered visible light (λ > 420 nm) in Fig. S5. We
find that there is no capacity of pure ZnO to degradate C2H4
under the visible light while the AuAg/ZnO has a certain per-
formance that can degrade half of the ethylene in 24 h. When
the light source is UV-visible light, the AuAg/ZnO can absorb
both visible and UV light. On the one hand, Au, Ag and Au-Ag
can absorb the visible light and induce the plasmon-excited
electrons e–AuAg. On the other hand, the ZnO body can be excited
3.7. Photoelectrochemical measurements
The strong capacity of charge migration can be certified by
the enlarged photocurrent [49,50]. Just like the results of pho-
tocatalytic activities, the photocurrents of these products have
the same discipline. As shown in Fig. 9, pure ZnO has a very
poor photoelectric response, which means that the electrons
and holes in ZnO are easily recombined under light irradiation.
Surprisingly, Au/ZnO and Ag/ZnO all have the enhanced pho-
tocurrent as expected, which signifies that Au and Ag acting as
electronic capture centers are beneficial to effective
charge-transfer process. On the other hand, by combining Au
and Ag NPs with ZnO, the plasmon-excited electrons can be
injected into ZnO and make it visible light active, resulting in
stronger photocurrent. Then just as we thought, the photocur-
rent intensity of AuAg/ZnO is futher increased compared with
that of single Au or Ag deposited on ZnO, indicating the synergy
effect of Au-Ag alloy can excite more electrons and they have
by ultraviolet light to produce photogenerated electrons e–
.
CB
More importantly, the Au, Ag and Au-Ag NPs serve as electronic
capture center, which can receive electrons from the conduc-
tion band of ZnO and prolong the life of electrons. These two
types of electrons work together so that the reaction rate at full
arc light is much greater than that under visible light. In addi-
tion, the synergy of Au and Ag is another important reason of
Fig. 10. Pictographic representation and the possible mechanisms of
the excitation of surface plasmon and electron transfer process during
the irradiation of UV-visible light.
Fig. 9. Transient photocurrent response of pure ZnO, Au/ZnO, Ag/ZnO
and the AuAg/ZnO composites under (UV-visible) light illumination.

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Huishan Zhai et al. / Chinese Journal of Catalysis 41 (2020) 1613–1621
2019, 243, 381–385.
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of Au-Ag NPs loaded ZnO in gaseous pollutants oxidation can be
ascribed to efficient visible light absorption by Au-Ag alloy NPs,
synergistic effects of SPR excitation and electronic captures of
Au-Ag NPs.
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Graphical Abstract
Chin. J. Catal., 2020, 41: 1613–1621 doi: S1872-2067(19)63473-X
ZnO nanorod decorated by Au-Ag alloy with greatly increased
activity for photocatalytic ethylene oxidation
Huishan Zhai, Xiaolei Liu, Zeyan Wang, Yuanyuan Liu, Zhaoke
Zheng, Xiaoyan Qin, Xiaoyang Zhang, Peng Wang *, Baibiao Huang *
Shandong University
Au-Ag alloy NPs were loaded on ZnO by photo-deposition proce-
dure with high photocatalytic activity and stability, which can be
ascribed to efficient visible light absorption by AuAg alloy NPs and
synergistic effects of LSPR excitation.

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贵金属Au-Ag合金修饰ZnO用于光催化高效降解乙烯
翟慧珊, 刘小磊, 王泽岩, 刘媛媛, 郑昭科, 秦晓燕, 张晓阳, 王 朋*, 黄柏标#
山东大学晶体材料国家重点实验室, 山东济南250100
摘要: 乙烯是一种植物激素, 可以促进水果和蔬菜在生长过程中成熟, 然而在水果和蔬菜成熟之后, 源源不断产生的乙烯
会加速它们的衰老和腐烂, 不利于水果和蔬菜的贮存和保鲜. 考虑到现实因素, 直接在储存环境中去除乙烯很有必要, 而
光催化技术操作简单易行, 在降解乙烯方面具有应用前景. 我们发现ZnO具有降解乙烯的作用, 但纯ZnO由于光吸收范围
窄及载流子分离效率低, 使得其降解乙烯的效果不佳. 而金、银纳米颗粒由于其表面的等离子体共振效应, 可以增强半导
体的光吸收, 同时贵金属颗粒还可以充当捕获电子的活性位点, 加快电子和空穴分离, 提高光催化效率. 因此, 将金或银作
为ZnO的助催化剂可能是提高ZnO催化性能的有效途径. 一些研究还发现, 使用双金属合金作为助催化剂可以达到比单一
金属更优越的效果. 因此在本论文中, 我们采用简单的光沉积工艺和低温煅烧方法合成了Au, Ag和Au-Ag合金负载的ZnO,
研究了它们对ZnO光催化效率的促进作用.
活性测试表明, 在ZnO负载了单独的Au或Ag颗粒后, 乙烯降解效率分别是纯ZnO的17.5和26.8倍, 光催化活性大幅增
加. 而当ZnO成功负载Au-Ag合金后, 光催化活性进一步提高到纯ZnO的94.8倍. 紫外-可见光谱结果表明, 由于表面等离子
体共振(SPR)效应, Au-Ag合金改性后的ZnO显示出很强的可见光吸收. 同时, 较高的光电流密度表明AuAg/ZnO具有有效
的载流子分离能力. 因此, 等离子体Au-Ag双金属合金纳米粒子的协同作用和有效的载流子分离能力共同带来了乙烯氧化
的优异光催化活性. 催化剂稳定性测试表明, Ag/ZnO的光催化稳定性非常差, 在10次循环后活性下降很多, 而Au/ZnO和
AuAg/ZnO在10次循环后光催化稳定性非常好. 结合XPS分析可知, Ag单质颗粒可以容易地光氧化成为Ag2O, 造成活性降
低, 而Au-Ag由于形成了合金结构, 不易被氧化, 因而变得异常稳定. 最终我们得到了高活性、高稳定性的AuAg/ZnO光催化
剂. 这项工作提出了在实际生产中使用金属合金作为助催化剂去除乙烯的新思路, 具有良好的应用前景.
关键词: 等离子体共振效应; Au-Ag合金; 协同效应; 高效载流子分离
收稿日期: 2020-02-17. 接受日期: 2020-03-26. 出版日期: 2020-10-05.
*通讯联系人. 电话: (0531)88366139; 传真: (0531)88365969; 电子信箱: pengwangicm@sdu.edu.cn
#通讯联系人. 电子信箱: bbhuang@sdu.edu.cn
基金来源: 国家自然科学基金(51602179, 21333006, 21573135, 11374190); 泰山学者特聘专家计划.
本文的电子版全文由Elsevier出版社在ScienceDirect上出版(http://www.sciencedirect.com/science/journal/18722067).