贵金属Au-Ag合金修饰ZnO用于光催化高效降解乙烯

Chinese Journal of Catalysis 41 (2020) 1613-1621催化学报2020年第41卷第10期丨
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 Q in, Xiaoyang Zhang, Peng Wang *, Baibiao Huang #
State Key Laboratory o f C rystal 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:
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
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. W e found ZnO has the effect of de­grading gaseous ethylene, however i t s effect i s not particularly satisfactory. Therefore, w e 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 photocata­l y t i c stability of Ag/ZnO was very poor, because Ag can be easily photooxidized to Ag 2〇. Surprising­l y , 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) e f f e c t . Meanwhile, the higher photocurrent density showed the effective carrier separation ability i n 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 n e w 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 st a b i l i t y . This work proposes a ne w concept of using metal alloys to remove ethylene in actual production.
© 2020, Dalian Institute of Chemical Physics, Chinese Academy of Sciences.
Published by Elsevier B.V. All rights reserved.
1. Introduction
In recent years, the noble metals have attracted great atten­tion because of their outstanding physical, chemical and optical performances, which m a d e t h e m suitable as co-catalysts on s o m e photocatalysts [1-8]. Mos t of t h e m can be used as the electron-transfer active sites in photodegradation of dye [9],
photocatalytic H 2 production [10,11], C O 2 reduction [12], transformation of green organic [13], thermocatalytic reduc­tion of nitroaromatics in water [14] and so on. In particular, A u and A g as c o m m o n noble metals in our li f e are also widely in­vestigated in photocatalytic academic research du e to their unique surface plasmon resonance (SPR) [15-19]. T h e Au, A g N P s under SPR-excitation have strong visible-light absorption,
* Corresponding author. E-mail: *******************.cn Corresponding author. E-mail: ***************.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.sciencedirectcom/science/journal/18722067 | Chin. J . C a t a l ., Vol. 41, No. 10, October
2020
1614H uisha n Z h a i e t al. / Chinese J o u rn a l o f C atalysis 41 (2020) 1613-1621
thus they can be applied to solve the challenges in photocata- lytic reactions such as n a r r o w spectral response [15-19]. Fur­thermore, just like a semiconductor heterojunction can pro­m o t e carrier separation [20-23], A u or A g N P s can act as the active sites for capturing electrons, improving the separation of electron and hole pairs in photocatalytic system as well. Therefore, the A u or A g supported materials have been also investigated in solving the problem of low q u a n t u m efficiency [24].
Bimetallic alloy i s c o m p o s e d of t w o metals by a certain method. Recently, s o m e studies have found that the semicon­ductors loaded with bimetallic alloy have m a n y superior prop­erties c o m p a r e d to that loaded with sole metal [25-29]. M o r e importantly, they can c o m p l e m e n t each other to achieve a win-win situation. For example, Au-Cu alloy supported on Ti〇2 i s effective on photocatalytic C〇2 reduction [30]. A m o n g them, the selectivity in C H4conformation o w n e d to the C u bonding to C O2on Ti〇2,while the visible light photoresponse w o u l d be due to the surface plasmon ban d of Au, illustrating A u and C u in the A u-Cu alloy have been put to the best utilization from each other [30]. In addition, Au-Pt alloy loaded on W O3w a s used to strengthen hydrogen and oxygen generation o w i n g to the m u­tual promotion of plasmonic function in A u and catalytic char­acteristic in Pt [15]. As w e al l know, A g has similar plasmonic property as Au. Therefore the A u and A g in A u-A g alloy exhibit outstanding photocatalytic performances and synergistic ef­fects which are superior to the pure metals under visible-light irradiation [31]. A u-A g alloy loaded semiconductors have been applied to restore C O2 to fuels [32], heighten the photoelectro- catalytic properties [33], oxide of methanol selectively [34], and so on. In particular, w e have found A u-A g alloy played an important role in the degradation of the harmful gas ethylene recently.
Ethylene i s produced in natural sources, plants and plant products. I t has s o m e h a r m o n the storage l i f e, development and growth of m a n y ornamental crops, fruits a n d vegetables at extremely low concentration [35,36]. S o m e researchers have already realized the dangers of ethylene and looked for s o m e w a y s to degrade i t to keep fresh of fruits and vegetables. In the field of photocatalysis, m a n y studies w e r e focus on traditional Ti〇2 and Ti〇2-based photocatalysts [37-40], while n e w m a t e­rials w e r e rarely exploited on ethylene degradation. Actually there w e r e only few n e w semiconductors had eth­ylene-oxidation function a n d their photocatalytic activity w e r e very poor, such as BiVCU and In2〇3-Ag-Ag3P〇4 [41,42]. Recent­l y,w e reported that Fe-doped W O3has the property of degrad- ing ethylene [43], and the activity w a s greatly improved c o m­pared with previous studies. However, the ethylene cannot be completely mineralized to C O2. Therefore, i t is urgent to find a highly active substance to deal with the ethylene. In this paper, w e interestingly found Z n O had capability to oxide ethylene, but the activity w a s not so satisfactory. T o improve the photo­catalytic reaction rate greatly, A u-A g bimetallic alloy N P s w e r e loaded on the ZnO. W e systematically investigated the perfor­m a n c e of single Au, Ag, and A u-A g alloy decorated on ZnO, and w e found the photocatalytic ethylene-oxidation activity of Z n O decorated by 0.8 w t% of Au-A g alloy w a s 94.8 times higher than that of pure ZnO, while loaded by single 0.8 w t%of A u or A g w a s only 17.5 or 26.8 times higher than that of ZnO. These results confirm the A u-A g alloy is superior to the single metal A u or A g in ethylene-oxidation process. Therefore, the A u-Ag alloy i s the promising cocatalyst for ethylene oxidation to freshen fruits and vegetables in refrigeration storage.
2. Experim ental
2.1. M a te ria ls synthesis
2.1.1. ZnO nanorods
T h e Z n O nanorods w e r e prepared referring to the simple hydrothermal m e t h o d [44]. Particularly, 0.4 g N a O H w a s added to 60 m L ethanol under ultrasonic treatment until forming h o­m o g e n e o u s 0.17 m o l/L NaOH/ethanol solution. T h e n the solu­tion w a s put into a Teflon tank of 100 m L capacity containing 2 m m o l Zn(CH3COO)2-2H2〇 and stirred for 30 min. Th e tank w a s transferred into a stainless-steel autoclave and heated at 160 °C for 24 h, the white p o w d e r w a s obtained after suction filtration with water a n d dried at 60 °C. T h e n the Z n O p o w d e r w a s cal­cined at 400°C for 1 h to r e m o v e the adsorbed ethanol.
2.1.2. A u o r Ag NPs loaded on ZnO nanorods
T h e deposition of single A u N P s on the Z n O nanorods w a s synthesized via a photo-reduction method. Briefly, 0.15 g Z n O w a s dispersed in 100 m L H2O in a beaker. T h e n different vol­u m e s of 0.1 m o l/L H A u C U w e r e dripped into the system and stirred for 5 m i n in order to form a h o m o g e n e o u s mixture. B e­fore irradiation, 1 m L methyl alcohol w a s added as the sacrifi­cial agent. T h e n the mixture w a s irradiated for 30 m i n under the Xe lam p of 300W. Finally, the solid w a s filtered, dried at 60 °C and calcined at 400 °C for 1 h to m a k e the A u NP s have a tightly integrated with the surface of Z n O and r e m o v e the a d­sorbed ethanol. T h e deposition of single A g N P s on the Z n O nanorods w a s similar with these processes. T h e only difference w a s that 0.1 m o l/L H A u C U w a s replaced with 0.1 m ol/L A g N〇3.
2.1.
3. Au-Ag b im e ta llic a llo y NPs loaded on ZnO nanorods
Th e A u-A g bimetallic alloy N P s supported on Z n O nanorods w e r e conducted with a co-photodeposition method. That i s, a certain volume of H A u C U (0.5 w t% Au) and A g N〇3 (0.3 w t% Ag) w e r e put together into the Z n O suspension under continu­ous stirring, and other reaction conditions w e r e s a m e as single Au/ZnO. T h e optimal A u-A g proportion o n Z n O w a s 0.8 w t%. Therefore, 0.8 w t% A u/Z n O and 0.8 w t% A g/Z n O w e r e also obtained as comparison by using the similar procedure.
2.2. C h a rac terization
Morphologies w e r e investigated by S E M(Hitachi S-4800) equipped with an EDS. X R D patterns w e r e conducted on a Bruker A X S D8 diffractometer equipped with C u Ka radiation to reveal the crystal structure. T h e UV-vis D R S analyzes w e r e rec­orded by Shimadzu U V-2550 spectrophotometer using Ba S〇4 as reflectance standard to explore the optical absorption. T h e T E M and H R T E M tests w e r e performed with a )EOL JEM-2100F
H uishan Z h a i e t al. / Chinese J o u rn a l o f C atalysis 41 (2020) 1613-16211615
microscope to analyze the nanostructure and composition of
the as-prepared A u A g/Z n O photocatalyst. X P S measurements
w e r e obtained using a T h e r m o E S C A L A B 250X1, and the peak
positions of various elements w e r e calibrated by C Is (284.8
eV). Photoelectrochemical tests w e r e measured by CHI-660C
electrochemical workstation using a three-electrode system.
Th e F T O glass coated catalyst w a s served as working electrode,
Pt sheet as counter electrode and Ag/AgCl as reference elec­
trode. 0.2 m o l/L Na2S〇4 solution w a s used as the electrolyte. A
300 W Xe-arc l a m p (CEL-HXF300, Beijing C E A U Light, China) w a s used for a light illuminant. T h e gas mixture w a s researched by m e a n s of the S h i madzu GC-2014C.
2.3. P h o to c atalytic ev alu a tio n
Photocatalytic oxidation of ethylene w a s measured in a quartz-covered reactor with 400 m L volume irradiated by a 300 W X e lamp. 0.12 g photocatalyst w a s dispersed uniformly in the bottom of the container with a rotor. T h e reactor w a s then sealed by the quartz cover and injected into 0.5 m L eth­ylene under stirring. Before turning on the lights, the container w a s stirred in the dark for 2 h to m a k e ethylene and air in the container mix evenly an d attain the adsorption and desorption balance. W h e n the balance w a s achieved, the reactor w a s illu­minated on top of quartz cover and 50 \xL of gas mixture w a s sampled at regular intervals and tested by a gas chromatog­raphy. C/Co indicates the degradation percentage of ethylene, w h e r e C is ethylene concentration at a specific time an d Co i s the initial concentration of ethylene. Stability of A u A g/Z n O product w a s also investigated as follows: after each ethylene oxidation reaction, op e n e d the cover and set aside 30 minutes to allow excess C〇2and C2H4 in the container to diffuse out. T h e n the reactor w a s sealed again to degrade a n e w 0.5 m L ethylene for another test under the s a m e irradiation.
3. Results and discussion
3.1. X-ra y d iffra c tio n research o f a s-prep are d nanocomposites
T h e X R D spectra of obtained products are s h o w n in Fig. 1. As can be seen from the left figure, the peaks of pure Z n O match well with the standard Z n O card (JCPDS, No. 70-2551). All of the diffraction peaks of the as-prepared samples after incorpo­ration of noble metals are very identical to that of pristine ZnO, with only small peak displacement in the part of the dotted line.
F r o m the magnified figure on the right and the Table 1 below, i t can be seen that the A g/Z n O has t w o peaks at 37.99° and 44.21° while the peaks of A u/Z n O at 38.30° and 44.66°, which are in line with the A g and A u standard cards (Fig. SI). I t should be pointed out, however, the A u A g/Z n O has peak locations (38.22° and 44.39°) different from any one of single A u and Ag, which are located betw e e n the peaks of A u and Ag. Similar consequence of A u-A g alloy peak-displacement w a s also found in Fig. SI. This interesting p h e n o m e n o n concludes that A u-Ag alloy might c o m e into being w h e n A u and A g are co-loaded on the Z n O and calcined at 400°C. Certainly, this conclusion needs to be proved further b y the following results.
20 30 40 50 60 70 80 38 40 42 44 46
2 Theta(deg.)
Fig. 1.The X R D patterns of the pure ZnO and Au, Ag, Au-Ag alloy nano- particles decorated on ZnO samples.
3.2. EDS a n d e le m e n ta l m aps o f n anocom posites
Fig. 2a reveals the distribution an d content of C, 0, Zn, A u and Ag. A m o n g them, the source of C might be conductive plas­tic or the adsorbed C O2.As can be seen from Fig. 2b, the weight percentages of A u an d A g are, respectively, 0.55% an d 0.21%, which are close to the actual loading a m o u n t of 0.5%and 0.3%.
F r o m Fig. 2c, w e can see the overall distribution of A u and A g elements are strongly uniform, illustrating that A u an d A g are evenly loaded on the surface of ZnO.
3.3. UV-vis diffuse spectroscopy
T o understand the different absorption of Au, A g and A u A g loaded ZnO, the UV-vis diffuse reflectance spectra of 0.8 w t% Au/ZnO, 0.8 w t% A g/Z n O and 0.5 w t% A u@0.3 w t% A g/Z n O samples, together with that of pure Z n O nanorods, are investi­gated and exhibited in Fig. 3.
As can be observed in Fig. 3, pure Z n O has no absorption in the visible range, in agree with the reported b a n d gap of 〜3.1 eV. However, the A u or A g loaded Z n O displays increased visi­ble light absorption o w n e d to the surface plasmon resonance (SPR) effect of the metallic A u and A g particles. Especially, there i s a characteristic peak at around 550n m of A u/Z n O while A g/Z n O is approximately at 470 n m; which i s in accord­ance with the previous article [31,45,46]. Interestingly, w h e n A u or A g is replaced by the s a m e a m o u n t of Au-Ag, i t s h o w s a broader and stronger peak at about 510 n m lying betw e e n the characteristic peaks of single A u and Ag. These data further illustrate that w h e n A u and A g w e r e co-loaded on ZnO, they might form the A u-A g alloy with the synergistic S P R effect, which can enhance the light absorption at 400-800 n m [32,33].
A n d i t i s worth putting forward that the p r o m o t e d light absorp­tion generally along with the increased photocatalytic property.
Table 1
The two X R D peaks in the
Au/ZnO samples.
range of 37°-45° of Ag/ZnO, AuAg/ZnO and
Sample20i 〇202(〇)
Ag/ZnO37.9944.21
AuAg/ZnO38.2244.39
Au/ZnO38.3044.66
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1616H uisha n Z h a i e t al. / Chinese J o u rn a l o f C atalysis 41 (2020) 1613-1621
a )
Element
Weight%Atomic%C 3.7711.25o
21.3747.89Zn 74.1040.65Au 0.550.18Ag
0.21
0.04
T o ta ls
100.00
3.4.
S E M a n d T E M
Fig. 4a and b are typical S E M images of the as-prepared Z n O and A u Ag/ZnO. As can be seen, Z n O are nanorods with an av­erage thickness of around 20 nm. After loaded by A u -A g alloy NPs, w e can see from Fig. 4c that there are s o m e small particles deposited on the surfaces and edges of nanorods. T h e key i s that these nanoparticles have uneven size, s o m e are big and s o m e are small, with an average of 〜6 n m by calculating (see Supporting Information Fig. S2), and they are closely attached to the Z n O surfaces to have a better contact. Fig. 4d, e and f are the corresponding H R T E M images of A u Ag/ZnO. T h e detected lattice fringes of Z n O are matching well with the (002) plane of
Fig. 4. (a) S E M of pure ZnO, (b) S E M of AuAg/ZnO, (c) T E M of
AuAg/ZnO, and (d , e , f ] H R T E M images of AuAg/ZnO.
0.26 n m (JCPDS, No. 70-2551). After a lot of detection, the d-spacing values of these small metal N P s (3-7 n m ) are 0.235 n m (Au(lll) JCPDS 01-1172) and 0.237 n m (Agflll) JCPDS 01-1164), on e of the pictures i s presented in Fig. 4d. However, the (/-spacing values of these big metal N P s (10-15 n m ) are a l l 0.236 n m (AgAu(lll) ]CPDS 65-8424), presented in Fig. 4e and
4f. T h e size of A u A g N P s coated on Z n O i s larger than A u or A g NP s due to A u -A g alloy formation [32].
Besides, the information crisply exhibits that the A u A g /Z n O photocatalyst possesses not only a distribution of the Au-Ag alloy NP s but also the coexistence of unalloyed A u and A g NPs. Similar p h e n o m e n o n i s also appeared in Au-Cu alloy loaded on Ti〇2, that i s , A u C u /T i〇2 sample consists of A u-Cu alloy NPs together with independent A u an d C u NPs, which i s difficult to avoid [30]. Anyway, this result further proves the formation of Au-Ag alloy N P s in A u A g /Z n O a n d i t s photo c atalytic activity and stability are both satisfactory in the following tests.
3.5.
X -ra y photoelectron spectroscopy study
Fig. 3. UV-vis diffuse reflectance spectra of ZnO, Au/ZnO, Ag/ZnO and
AuAg/ZnO samples.
T o obtain the surface chemical status of A u A g /Z n O before and after irradiation, X-ray photoelectron spectroscopy w a s carried out. Fig. 5a is the full spectra of A u A g /Z n O sample be­fore and after degradation, w h e r e the peaks for Zn 2p and 0 Is can be seen clearly. In Fig. 5b, the t w o peaks centered at 1021 eV and 1044 e V are corresponding to the Z n 2p3/2 and Zn 2pi/2. There i s no displacement of the peak position before and after the reaction, indicating that the chemical state of the Zn ele­m e n t has not changed. Similarly, as seen from Fig. 5c, the peaks of A u A g /Z n O before degradation located at 529.5 e V and 531.4 eV are indexed to lattice oxide and adsorbed oxygen [32]. After
degradation, the peak of lattice oxide has no change, indicating Z n O
did not change before and after reaction. However, the
Huishan Zhai et a l / Chinese Journal o f C atalysis 41 (2020) 1613-16211617
peak of adsorbed oxygen exhibits a slightly negative shift, sug­gesting the chemical reactions of adsorbed 〇2 might be in­volved. In Fig. 5d, the diffraction peak located at 83.3 eV and 88.1 eV are corresponded to Au 4/7/2 and Au 4/5/2. 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, re­spectively, which are features of Ag°. 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 be­come Ag+ and thus the peaks exhibited a positive shift [47]. However, the most Ag elements in AuAg alloy are still Ag°, which proves the superiority of the AuAg alloy. This result also corresponds to the test results of photocatalytic stability.
3.6. Photocatalytic C 2H 4 oxidation and s t a b i l i t y t e s t of
as-obtained samples
Ethylene is used as an objective organic pollutant to m eas­ure 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 corre­sponding kinetics curves and reaction rate constants. It is ob­vious that pure ZnO has a poor reaction rate constant of 0.004 g -^i r r 1. After loading with a small amount of Au, the activity
has been significantly improved and the reaction rate of 0.5% Au/ZnO even reaches up to 0.162 g ^m in 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 syn­ergistic effect of Au and Ag has unmatched superiority.
In order to compare the photocatalytic effects of our syn­thesized photocatalysts with other photocatalysts, the photo­catalytic degradation activity of AuAg/ZnO sample was com- pared with that of other photocatalysts such as Pt-Ti 〇2,
P t@F e-W 03, Ag/AgCl/TiOz, Ag-ZnO, BiV04/P 25, P 25/B i2W 〇6 and In2〇3-Ag-Ag3P 〇4, the detailed results are shown in Table SI. It can be seen from the roughly calculated reaction rate (ppm g_1 m iir1) 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, indi­cating that the regularity of AuAg/ZnO in degradation of eth­ylene is universal. The related comparison experiment results
82 84
86 88 90 92 94
Binding energ>(e\')
365 370
375
Binding enerK>(eV)
Fig. 5. (a) XPS survey spectra and (b-e) high-resolution XPS spectra of Zn 2p, O Is, Au 4/and Ag 3d for AuAg/ZnO before and after degradation.
400 800Binding encrR>(eV)
d )
530
Binding cnerg>(eV)
{.n -B ).r 'l s u i ;u
l
1618Huishan Zhai et al. / Chinese Journal o f C atalysis 41 (2020) 1613-1621
cycle times
Fig. 6. (a) Comparative photodegradation activities of C 2H 4, (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 C 2H 4 in the presence of 0.8% AuAg/ZnO composite.
are presented in Fig. S4.
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 w ere conducted under the same condition. In detail, Fig. 6d is photodegradation kinetic constant of C 2H 4 of these ten-time tests. As can be seen, the photocatalytic C 2H 4 oxidation over 0.5%******%Ag/ZnO shows a slight downward trend after one cycle and the rate constant drops from 0.379 g -^i n -1 to 0.343 g ^m irr1. 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 -'m iir1, separately. The reason for this phenomenon, in our opinion, may be due to that part of the separate Ag is oxidized to Ag 2〇. After one-time irradiation, the all or most isolated Ag is become Ag2〇 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
changes.
In order to verify this conjecture, the stability of single 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 un­changed. 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 com­pared 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 CO 2 is 1250 ppm and 0 ppm (the 0 ppm is after de-
1
2
34
4 (10)
cycle times
210
Fig. 7. The recyclability for the photocatalytic degradation of C 2H 4 in the presence of (a) single Au and (b) single Ag under (UV-visible) light illumina­
tion.
Huishan Zhai et al. / Chinese Journal o f C atalysis 41 (2020) 1613-16211619
0 50 100
150 200
250 300
time (sec.)
Fig. 9. Transient photocurrent response of pure ZnO, Au/ZnO, Ag/ZnO
and the AuAg/ZnO composites under (UV-visible) light illumination.
Fig.
8
. Photocatalytic C 2H 4 degradation and CO 2 generation of
AuAg/ZnO.
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 C 2H 4 is almost broken into two times of 0=C =0 bond. The mineralization ratio of ethylene is about 100% in this reac­tion.
3.7. Photoelectrochemical measurements
The strong capacity of charge migration can be certified by the enlarged photocurrent [49,50]. Just like the results of pho­tocatalytic 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 pho­tocurrent 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 photocur­rent 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
the function to accelerate the separation of charge carriers fur­ther 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]. Be­cause the conduction band position of ZnO (-0.31 eV vs NHE) is more negative than E 〇(〇2/*〇2~) (-0.046 eV vs NHE), the pro­cess of the single-electron reduction of oxygen can proceed and generate *〇2~ [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
•〇2~ 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 plas­mon-induced electrons is very small compared to those photo- excited electrons in ZnO. The corresponding irradiations were performed with filtered visible light (A > 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 per­formance 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-A u A g . On the other hand, the ZnO body can be excited 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 conduc­tion 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 addi­tion, 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.
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