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Ag nanoparticle-functionalized ZnO micro-flowers for enhanced photodegradation of herbicide derivatives
Accepted Manuscript Research paper Ag nanoparticle-functionalized ZnO micro-flowers for enhanced photodegradation of herbicide derivatives Yan Xu, Shumin Wu, Xianliang Li, Hao Meng, Xia Zhang, Zhuopeng Wang, Yide Han PII: DOI: Reference:
S0009-2614(17)30417-7 http://dx.doi.org/10.1016/j.cplett.2017.04.091 CPLETT 34780
To appear in:
Chemical Physics Letters
Received Date: Revised Date: Accepted Date:
10 March 2017 4 April 2017 25 April 2017
Please cite this article as: Y. Xu, S. Wu, X. Li, H. Meng, X. Zhang, Z. Wang, Y. Han, Ag nanoparticle-functionalized ZnO micro-flowers for enhanced photodegradation of herbicide derivatives, Chemical Physics Letters (2017), doi: http://dx.doi.org/10.1016/j.cplett.2017.04.091
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Ag nanoparticle-functionalized ZnO micro-flowers for enhanced photodegradation of herbicide derivatives Yan Xu,1,* Shumin Wu,1 Xianliang Li,2,* Hao Meng,1 Xia Zhang,1,* Zhuopeng Wang,1 and Yide Han1 1
Department of Chemistry, College of Science, Northeastern University,
Shenyang, Liaoning 110819, China. 2
College of Materials Science and Engineering, Shenyang University of
Chemical Technology, Shenyang, Liaoning 110142, China.
Corresponding authors: Yan Xu, E-mail: [email protected] Fax: +86-024-83684533; Tel.: +86-024-83684533
Xianliang Li, E-mail: [email protected] Xia Zhang, [email protected]
1
Abstract: We demonstrate a general strategy to design step by step the Ag nanoparticle-functionalized ZnO micro-flowers (Ag/ZnO composites). XRD patterns confirmed the presence of Ag nanoparticles in ZnO/Ag composites, and the SEM and TEM results further demonstrated that Ag nanoparticles were highly dispersed and anchored onto the surface of each ZnO nanosheets. By using the ZnO/Ag composites, the photodegradation of two herbicide derivatives, metamitron and metribuzin, were studied. The enhanced photocatalytic performance was ascribed to the fact that the Ag deposition could reduce the recombination probability of electron-hole pairs, and the photocatalytic mechanism were also investigated in this paper.
Keywords: ZnO/Ag composites; hydrothermal synthesis; photocatalytic
2
1. Introduction Increasing contamination caused by variety and quantities of agrochemicals poses a significant threat to the environment, as well as the health of living beings.1 For instance, the agrochemicals, such as metamitron and metribuzin, derived from the structure of 4-amino-1,2,4-triazin-5(4H)-one, have been widely used as herbicides, which are chemically stable, and can slowly penetrate through the soil and cause severe contamination of underground resources of drinking water. Hence, they should be adequately treated before they are discharged into the environment. Heterogeneous photocatalysis is regarded as one of the most competitive techniques for remedying environmental pollution.2-4 Up to now, a variety of compounds with enhanced photocatalytic activities, such as TiO2, WO3, CdS, BiOCl and Ag3PO4, have been widely applied in photocatalytic degradation of organic contaminants.5-9 Among them, zinc oxide (ZnO), an important n-type semiconductor with a wide-band-gap of 3.37 eV, is considered to be one of the most promising and active photocatalyst due to its low cost, environmental sustainability, and high photocatalytic activity.10 However, large crystals of ZnO with low specific surface area are often obtained owing to its low crystallization temperature and fast growth rate, so that the photocatalyst could not expose as much active sites as possible. On the other hand, the rapid recombination
of
photogenerated
electron-hole
pairs
usually
weakens
its
photocatalytic efficiency.11 To solve these problems, many efforts have been made to develop ZnO-based hybrids with different morphologies and structures. One of the best methods is to design ZnO/metal heterostructure, where the Schottky barrier 3
formed between metal and ZnO interface restricts the recombination of photogenerated electron-hole pairs.12 This character can inevitably prolong the lifetime of photoexcited electron-hole pairs and ultimately improve the photocatalytic activity. As the cheapest noble metal, Ag is the most promising one to fabricate high-efficiency metal-ZnO photocatalyst. Several Ag functionalized ZnO composites with special morphologies, such as nanorods, radical-shaped, dendrite-like, and worm-like ZnO/Ag composites, have been successfully synthesized by using different methods.13-17 Among them, three-dimensional (3D) ZnO-matrix nanostructures are of great interest to scientists since their novel architectural nanostructures may supply more active sites on the surface, and thus it leads to significantly increased photocatalytic performance.18-19 Our previous work has demonstrated that the flower-like ZnO assembled with numerous of nanosheets possess excellent structural stability, easy recovery, and remarkable photocatalytic activity toward the degradation of metamitron 20. However, the degradation rate is slow, which might be caused by the easy recombination of charge carriers in the photocatalysis process. So it is expected to enhance the photodegradation ability if one can develop an efficient way to produce high dispersed Ag nanoparticles on to the surface of the flower-like ZnO assembled by their nanosheets. Compared to the previously reported work,13-17 we firstly demonstrated a novel two-step synthesis route to prepare the ZnO/Ag composites, which can be general for the preparation of other semiconductor/metal composites. Based on this method, one can easily control the morphologies of the composites from the control of that of 4
semiconductor at the first step. A detailed characterization on the phase structures, morphologies and optical properties of ZnO/Ag composites and their ZnO counterparts has been performed systematically. Compared with pure ZnO, the photocatalytic activity of ZnO/Ag heterostructure toward the degradation of metamitron and metribuzin is significantly improved due to the efficient separation of electron-hole pairs, and the acceleration of charge transfer induced by the deposition of Ag nanoparticles. The potential mechanism of photocatalysis of metamitron and metribuzin is also discussed, and this is the first report on such kind of organic contaminants with enhanced photocatalytic degradation rates.
2. Experimental 2.1. Materials All chemicals were analytical grade reagents and used as received without further
purification.
These
analytical
reagents
include
zinc
acetate
(C4H6O4Zn·2H2O, 99.9%, Guangfu, Tianjin, China), silver nitrate (AgNO3, 99.0%, Aladdin, Shanghai, China), sodium citrate (C6H5O7Na3·H2O, 99.0%, Yongda, Tianjin, China), sodium hydroxide (NaOH, 99,5%, Guoyao, Shanghai, China), metamitron and metribuzin (C8H14N4OS and C10H10N4O, Aladdin, Shanghai, China) and pure water (commercial “Wahaha” distilled water, China). 2.2. Synthesis of ZnO micro-flowers
5
The ZnO micro-flowers were prepared by one-step hydrothermal method.19 Briefly, 0.549 g of zinc acetate (2.5 mmol) and 0.735 g of sodium citrate (2.5 mmol) were dissolved into 40 mL deionized water under stirring until a clear aqueous solution was obtained. Then, 4 mL of 4.0 M NaOH was added into the above prepared aqueous solution with rigorous stirring. The mixture was transferred into a 50 mL of Teflon liner stainless steel autoclave and heated at 120ºC for 8 h. After cooling to room temperature, the white precipitation was separated from the solution by centrifugation, washed with distilled water and absolute ethanol several times, and dried under vacuum at 60 ºC for 2 h. 2.3. Synthesis of Ag nanoparticle-functionalized ZnO micro-flowers Photodeposition method was used to modify ZnO micro-flowers with Ag nanoparticles. In detail, 0.2 g of the as-prepared 3D ZnO micro-flowers were dispersed in 300 mL 0.001 M AgNO3 aqueous solution, and stirred for 20 min to achieve adsorption equilibrium of Ag+ ions onto the surface of ZnO. The suspension was then irradiated for 30 min by a high-pressure Hg UV lamp (175 W) under continuously stirring. The as-obtained ZnO/Ag products were collected and washed with distilled water and ethanol for several times to remove the residual Ag+ ions. The product were centrifuged and separated from the reaction solution, washed with pure water and ethanol and dried under vacuum at 60 ºC for 2 h. A series of Ag nanoparticle-functionalized ZnO micro-flowers were obtained using a similar procedure with synthetic molar 6
ratio of AgNO3/ZnO ranged from 0 to 1.2, and the products were denoted as ZnO/Ag -0.2, ZnO/Ag -0.4, ZnO/Ag -0.8 and ZnO/Ag -1.2, respectively. 2.4. Characterization The obtained ZnO/Ag composites were characterized systematically. The phases of the samples were characterized by powder X-ray diffraction (XRD) on a X’Pert Pro MRDDY2094 diffractometer with Cu-Kα radiation (λ = 1.5418 Å). A scan rate of 0.0167º s-1 was applied to record the pattern in the 2θ range of 5–70º. N2 adsorption-desorption isotherm was conducted on a Micrometritics ASAP-2020M volumetric gas adsorption apparatus to investigate the porous feature of ZnO. The morphologies and microstructures of as-prepared ZnO and ZnO/Ag were studied by using Ultra Plus field-emission scanning electron microscope (SEM) and transmission electron microscope (TEM, Tecnai G220). The chemical composition was examined using energy dispersive spectroscopy (EDS) in conjunction with SEM. The UV-Visible adsorption spectra were recorded using a Hitachi U-3010 UV-Visible spectrometer. Photoluminescence spectroscopy was recorded by Shimadzu spectrophotometer (RF-5301 PC). X-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientist Co., Theta Probe) was used to study the surface chemical composition of ZnO/Ag. 2.5. Photodegradation of metamitron and metribuzin A 300W xenon lamp light emitting UV and visible radiation over 300-700 nm was used as the light resource. In a typical test, 100 mg as-prepared photocatalyst powders (ZnO, ZnO/Ag-0.2, ZnO/Ag-0.4, ZnO/Ag-0.8 and 7
ZnO/Ag-1.2) were placed in 100 mL solution containing 13 mg L-1 metamitron or 14 mg L-1 metribuzin. The reaction mixture was stirred for 30 min under darkness to realize the adsorption equilibrium. Then the solution was irradiated in a photochemical reaction chamber with continuous stirring. The distance between the reaction bottle and the light source was maintained at 10 cm. At a certain time of 30 min interval, 3 mL of the treated solution was taken out and centrifuged at 9800 rpm for 3 min. The relative concentration of metamitron or metribuzin in the treated solution was analysed by UV-Vis spectroscopy. The ZnO/Ag-0.4 showed superior photocatalytic activity than ZnO/Ag-0.2, which can be interpreted that more active metal sites are available to accept photogenerated electrons, and restrict the recombination of charge carriers with holes. Furthermore, when the Ag content in ZnO/Ag composites increased (above 0.4), the photocatalytic activity is again decreased due to the aggregation of Ag nanoparticles to form clusters on the surface of ZnO nanosheets, which will again enhance the recombination opportunity between charge carriers with holes.21 With regard to this, ZnO/Ag-0.4 sample with the optimum Ag content was chosen for the overall experiments. A reutilization experiment was performed to evaluate the cyclic stability of ZnO/Ag-0.4 photocatalyst. 3. Results and discussion 3.1. Structure and morphology The phases of the prepared samples were characterized by X-ray 8
diffraction (XRD). As shown in Fig. 1 (a), it can be found that the XRD pattern of as-prepared ZnO micro-flowers (red line) contains a series of diffraction peaks at 30.1º, 35. 5º, 43.2º, 53.6º, 57.0º and 62.7º, which can be indexed well to the simulated XRD pattern of hexagonal wurtzite ZnO (bottom line, JCPDS 36-1451), corresponding to (100), (002), (101), (102), (110) and (103), respectively. When Ag nanoparticles were deposited on the surface of ZnO nanosheets, four additional peaks of 38.1º, 44.3º, 64.4º and 77.4º can be observed (blue line), which can be attributed to the (111), (200), (220) and (311) crystal planes of cubic Ag (JCPDS No. 04-0783), respectively. These results indicated that no other impurities are found in the as-prepared ZnO and ZnO/Ag-0.4 products. Due to the important role of surface area in enhancing photocatalytic activity, quantitative characterization of ZnO and ZnO/Ag-0.4 samples were conducted with N2 adsorption-desorption measurements (Fig. 1b). The obtained ZnO and ZnO/Ag-0.4 had a BET surface area of 18.9 m2 g-1 and 10.5 m2 g-1, respectively. It is observed the deposition of Ag nanoparticles on the surface of ZnO nanosheets enables the slight decrease of the BET surface area, as also observed by Yang, et. al,22 which should be ascribed to partly aggregation of the ZnO nanosheets for ZnO/Ag-0.4 micro-flowers compared to pure ZnO, as shown in Fig. 1c and 1d. It is also found that both as-prepared pure
ZnO
and
ZnO/Ag-0.4
composites
possess
the
micro-flower-like
microstructures, which had average sizes of 3 μm, assembled from numerous nanosheets of about 20 nm in thickness. 9
In addition, the Ag nanoparticles are distributed uniformly on the surface of ZnO micro-flowers observed from TEM as shown in Fig. 2(a). The darker dots and lighter plates correspond to Ag nanoparticles and ZnO nanosheets, respectively. A magnified TEM image of the boundary of the Ag nanoparticles is displayed in Fig. 2(b), indicating the formation of ZnO/Ag heterostructure in as-prepared ZnO/Ag-0.4 sample. The SEM image of a single ZnO/Ag-0.4 micro-flower particle is shown in Fig. 3(a). The elemental mapping of the ZnO/Ag-0.4 microparticle recorded using energy-dispersive spectroscopy are shown in Fig. 3(b-f), where yellow, red, and green refer to the Zn, Ag, and O elements, respectively, indicating that they are uniformly distributed on the surface of the as-prepared ZnO/Ag composites. 3.2. XPS, PL, and solid UV-vis reflectance spectra To further investigate the elemental composition and chemical states of the as-prepared ZnO and ZnO/Ag-0.4 samples, XPS analysis was performed with binding energies calibrated using C1s (284.8 eV) (Fig. 4a). Two peaks in the curves of ZnO and ZnO/Ag-0.4 samples centered at 1020.9 eV and 1043.9 eV can be assigned to Zn 2p3/2 and Zn 2p1/2, respectively (Fig. 4b), indicating that Zn presents mainly in the form of Zn2+ state on the surface of the samples.23 Compared to pure ZnO, the intensity of Zn 2p peak is slightly decreased and shifted, this phenomenon may be caused by the electron transfer from ZnO to Ag nanoparticles in ZnO/Ag-0.4 heterostructure.24 The peaks centered at 368.3 eV and 373.3 eV can be assigned to Ag 3d5/2 and Ag 3d3/2, respectively, 10
confirming that Ag nanoparticles have been successfully deposited on the surface of ZnO nanosheets (Fig. 4c).16 O 1s profile is asymmetric, which can be attributed to the lattice oxygen of ZnO and physical adsorbed oxygen, respectively (Fig. 4d).21 Fig. 5 shows the PL spectra of the as-prepared ZnO and ZnO/Ag-0.4 composites (λex = 308 nm). In general, the lower photoluminescence intensity, the less chance of electron-hole recombination. It can be found that the emission intensity of the PL spectrum of ZnO/Ag-0.4 was lower than that of ZnO, suggesting that the deposition of Ag nanoparticles quenched the fluorescence produced from the electron-hole recombination in ZnO, and hence the photogenerated charge carriers possess a longer lifetime. Thus, it can be predicted that the existence of Ag nanoparticles in ZnO/Ag-0.4 heterostructure may finally improve its photocatalytic activity significantly.25 Moreover, the solid UV-vis absorbance spectra of pure ZnO and ZnO/Ag-0.4 composites are comparatively taken in the range of 200-800 nm and the results are shown in Fig. 6. The adsorption peaks at UV region corresponds to the adsorption of ZnO. It can be found that the ZnO/Ag-0.4 composites shows apparently enhanced UV-Vis adsorption intensity, demonstrating the presence of Ag (0) nanoparticles on ZnO surface.26-27 Therefore, compared to those ZnO/Ag composites published previously,13-17 the Ag-functionalized assembled ZnO/Ag flowers reported herein have large surface area and more active sites exposed during the photocatalytic reaction. These characteristics would effectively 11
accelerate the transfer of photogenerated electron-hole pairs, and finally increase the photocatalytic reaction rate. 3.3. Photocatalytic performance of ZnO/Ag composites photocatalysts The photocatalytic performance of ZnO/Ag-0.4 composites for the degradation of two herbicide derivatives, metamitron and metribuzin, was investigated in detail, and pure micro-flower-like ZnO is used as a photocatalytic reference to qualitatively understand the photocatalytic activity of as-designed ZnO/Ag composites used as catalyst. Two different conditions were conducted in water solution: under light in the absence of catalyst and in the presence of catalyst (ZnO or ZnO/Ag-0.4). The obtained results are shown in Fig. 7a and Fig. 7b. A solution of metamitron and metribuzin in water shows maximum adsorption at 293 nm and 306 nm, respectively. The degradation curves of metamitron and metribuzin were calculated according to the following equations: Ads% =
C o -C × 100 Co
where Co and C are the initial and final concentrations of metamitron or metribuzin vs. time, respectively. As shown in Fig. 7c and 7d, when the photodegradation of metamitron and metribuzin was performed in the presence of the light without catalyst, the degradation rate after 180 min of the irradiation is about 18%, and 10%. If the photocatalysts of ZnO or ZnO/Ag-0.4 were added in the absence of light, no significant change was observed in the degradation of metamitron and metribuzin, indicating that the metamitron and metribuzin 12
are stable in dark. However, when ZnO or ZnO/Ag-0.4 were added into the metamitron or metribuzin solution and irradiated under 300W xenon lamp light, significant degradation of metamitron and metribuzin were observed. Meanwhile, compared with pure ZnO, ZnO/Ag-0.4 heterostructure has higher performance in terms of photocatalytic activity. The maximum degradation efficiency for 100 mL of metamitron (13 mg L-1) and metribuzin (14 mg L-1) on 100 mg of ZnO/Ag-0.4 is nearly 90% in 90 min, while it is only about 35% and 20% if 100 mg pure ZnO micro-flowers were adopted. The time of completely photocatalytic reaction towards metamitron was shortened by nearly 90 min compared to TiO2 material published before.28 3.4. Stability of ZnO/Ag composites photocatalysts Owing to the significance of cycling stability of catalyst in its practical application, reutilization experiments were conducted to evaluate the performance of ZnO/Ag-0.4 heterostructure photocatalyst. All precipitates in each circle were retrieved, washed with ethanol and acetone several times, dried in a vacuum, and allowed for continuous photodegradation experiments. As can be seen in Fig. 8, the photodegradation efficiency is still kept at around 80% after three cycles, demonstrating its good reusability and making it a promising candidate as a highly efficient photocatalyst for the removal of herbicide derivatives. After five runs, the photocatalytic performance of ZnO/Ag-0.4 drops owing to the weight loss of photocatalyst in cycling experiments. 3.5. Mechanism of photocatalytic activity of ZnO/Ag composites 13
It is generally known that photocatalytic acitivity is closely related with the lifetime of photogenerated electron-hole pairs. The schematic illustration of heterogeneous photocatalysis of herbicide derivative in the presence of ZnO/Ag and ZnO is shown in Fig. 9.20,
29
However, the specific photodegradation
products for metamitron or metribuzin still need to be studied in detail in the future. When the ZnO/Ag-0.4 photocatalyst is illuminated by UV light, valence band electrons are promoted to the conduction band leaving a hole behind. Due to the equilibrium Fermi energy level (Ef) of Ag nanoparticles is lower than the bottom energy level of the conduction band (CB) of pure ZnO, most photogenerated electrons will transfer from ZnO to Ag nanoparticles driven by the potential energy, while the holes can be remained on ZnO surface because of the Schottky barrier formed at the interface between Ag and ZnO particles.30 Electron in the conduction band at the catalyst surface can reduce oxygen molecular to superoxide anion. The holes at the ZnO valence band can oxidize the adsorbed water to produce hydroxyl radicals. The superoxide anion and hydroxyl radicals will then attack organic metamitron or metribuzin compound to produce the main product of CO2, H2O and other intermediates. The recombination of electron-hole pairs is greatly reduced owing to the modification of Ag nanoparticles, and their lifetime is prolonged, which make their photocatalytic activities promoted significantly. Additionally, the quencher of •OH (3 mM of isopropyl alcohol, IPA), O2•(0.05 mM carnosine) and h+ (3.0 mM triethanolamine, TEOA) were added to 14
the photocatalytic system in order to investigate the reaction mechanism of the degradation of metamitron and metribuzin over ZnO/Ag-0.4, respectively.31-32 The experimental results are given in Fig. 10, it is found that the photocatalytic degradation of metamitron and metribuzin on ZnO/Ag can be well described by the pseudo-first-order reaction kinetic, ln (Co/C) = kt, with all of the squares of linear correlation coefficients R2 larger than 0.92, where Co and C are the initial concentration of the investigated herbicide derivatives and their value at time t, respectively.33 k is a pseudo-first-rate kinetic constant, representing the rate of the degradation reaction. As also shown in Fig. 10, the degradation of metamitron and metribuzin decreased in the presence of these quenchers, and the addition of TEOA and IPA into the photocatalytic reaction system resulted in an even more significant decreasing compared with the addition of carnosine. Thus, it can be concluded that •OH and h+ play more important roles among all of the three active intermediate species for the photodegradation of metamitron and metribuzin. 4. Conclusion In summary, the ZnO/Ag composites were successfully prepared via the combination of hydrothermal synthesis of flower-like ZnO and photochemical deposition of Ag nanoparticles on each ZnO nanosheets of the matrix, and such a general two-step synthesis route can be used to guide the preparation of other semiconductor/metal composites. On the basis of the microstructural characterization and photocatalytic results, the ZnO/Ag-0.4 composites 15
exhibited significantly improved photocatalytic activity as well as high stability and recyclability in degradation of two herbicide derivatives metamitron and metribuzin under UV light irradiation, which may be attributed to the efficient electron transfer between ZnO nanosheets and Ag nanoparticles. Thus, the excellent chemical stability, enhanced photocatalytic performance, and easily accessible synthetic route enable the as-developed ZnO/Ag-0.4 composites as potential candidates in the field agrochemical treatment.
Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No. 21401018) and the Fundamental Research Funds for the Central Universities (No. N130305003).
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The figure captions are as follows, Fig. 1 (a) XRD patterns of as-prepared ZnO and ZnO/Ag-0.4 micro-flowers, and the stimulated ZnO pattern is given as a comparison; (b) N2 adsorption and desorption of as-prepared ZnO and ZnO/Ag-0.4 micro-flowers; typical SEM image of the as-prepared (c) ZnO and (d) ZnO/Ag-0.4. Fig. 2 (a) TEM image of as-prepared ZnO/Ag-0.4 micro-flowers, and (b) HRTEM image of Ag nanoparticle attached on the surface of ZnO plate. Fig. 3 (a) SEM image and (b) EDS spectrum of ZnO/Ag-0.4 composites and its corresponding elemental mapping images for (c) Zn, (d) Ag and (e) O elements. Fig. 4 (a) Full range XPS spectra of ZnO and ZnO/Ag-0.4; (b-d) High resolution XPS spectra of Zn 2P, Ag 3d and O 1s for ZnO/Ag composites. Fig. 5 Photoluminescence (PL) spectra of the as-prepared ZnO and ZnO/Ag-0.4 photocatalysts. (λmax = 308 nm) Fig. 6 Solid UV-Vis absorbance spectra of as-prepared ZnO and ZnO/Ag-0.4 photocatalysts. Fig. 7 UV-vis spectra of degradation of (a) metamitron and(b) metribuzin as a function of irradiation time in the presence of ZnO/Ag-0.4 composites; comparison of self- degradation and photocatalytic degradation of (c) metamitron and (d) metribuzin with ZnO and ZnO/Ag-0.4. Fig. 8 Five repeated process by using ZnO/Ag-0.4 composites as photocatalyst for degradation of (a) metamitron and (b) metribuzin before and after light irradiation (t = 150 min). Fig. 9 A schematic illustration of ZnO/Ag composites for the photodegradation mechanism to metribuzin and metamitron. 20
Fig. 10 Photodegradation ratio and the linear relationship between ln (Co/C) and irradiation time for metamitron (a-b) and metribuzin (c-d).
21
Fig. 1 (a) XRD patterns of as-prepared ZnO and ZnO/Ag-0.4 micro-flowers, and the stimulated ZnO pattern is given as a comparison; (b) N2 adsorption and desorption of as-prepared ZnO and ZnO/Ag-0.4 micro-flowers; typical SEM image of the as-prepared (c) ZnO and (d) ZnO/Ag-0.4.
22
Fig. 2 (a) TEM image of as-prepared ZnO/Ag-0.4 micro-flowers, and (b) HRTEM image of Ag nanoparticle attached on the surface of ZnO plate.
23
Fig. 3 (a) SEM image and (b) EDS spectrum of ZnO/Ag-0.4 composites and its corresponding elemental mapping images for (c) Zn, (d) Ag and (e) O elements.
24
Fig. 4 (a) Full range XPS spectra of ZnO and ZnO/Ag-0.4; (b-d) High resolution XPS spectra of Zn 2P, Ag 3d and O 1s for ZnO/Ag composites.
25
Fig. 5 Photoluminescence (PL) spectra of the as-prepared ZnO and ZnO/Ag-0.4 photocatalysts. (λmax = 308 nm)
26
Fig. 6 Solid UV-Vis absorbance spectra of as-prepared ZnO and ZnO/Ag-0.4 photocatalysts.
27
Fig. 7 UV-vis spectra of degradation of (a) metamitron and(b) metribuzin as a function of irradiation time in the presence of ZnO/Ag-0.4 composites; comparison of self- degradation and photocatalytic degradation of (c) metamitron and (d) metribuzin with ZnO and ZnO/Ag-0.4.
28
Fig. 8 F Fiv ve repe r eated pro oceess by usiing g Zn nO//Ag g-0 0.4 com mp posiitess ass ph hoto ocaatallystt fo or degr d rad dation of (a)) meta m amiitro on aand d (b b) metr m ribu uziin befo b oree an nd afte a er liigh ht irrrad diatiion n (t = 150 1 0 min) m ).
29 9
Fig. 9 A scheematicc illlusstraatio on of o ZnO Z O/A Ag ccom mposiitess for th he pho p otod deg grad dattion n meech haniism m to o metr m ribu uzin n and meetam mittron n.
30 0
Fig. 10 Photodegradation ratio and the linear relationship between ln (Co/C) and irradiation time for metamitron (a-b) and metribuzin (c-d).
31
Graphical abstract
32
Highlights
Ag nanoparticle-functionalized ZnO micro-flowers (Ag/ZnO composites) have been fabricated via a two-step route.
Organic solvent-free process has been adopted to disperse the Ag nanoparticles onto ZnO nanosheets with enhanced photocatalytic activity.
The photodegradation of two herbicide derivatives, metamitron and metribuzin, were studied in detail.
33
S0009-2614(17)30417-7 http://dx.doi.org/10.1016/j.cplett.2017.04.091 CPLETT 34780
To appear in:
Chemical Physics Letters
Received Date: Revised Date: Accepted Date:
10 March 2017 4 April 2017 25 April 2017
Please cite this article as: Y. Xu, S. Wu, X. Li, H. Meng, X. Zhang, Z. Wang, Y. Han, Ag nanoparticle-functionalized ZnO micro-flowers for enhanced photodegradation of herbicide derivatives, Chemical Physics Letters (2017), doi: http://dx.doi.org/10.1016/j.cplett.2017.04.091
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Ag nanoparticle-functionalized ZnO micro-flowers for enhanced photodegradation of herbicide derivatives Yan Xu,1,* Shumin Wu,1 Xianliang Li,2,* Hao Meng,1 Xia Zhang,1,* Zhuopeng Wang,1 and Yide Han1 1
Department of Chemistry, College of Science, Northeastern University,
Shenyang, Liaoning 110819, China. 2
College of Materials Science and Engineering, Shenyang University of
Chemical Technology, Shenyang, Liaoning 110142, China.
Corresponding authors: Yan Xu, E-mail: [email protected] Fax: +86-024-83684533; Tel.: +86-024-83684533
Xianliang Li, E-mail: [email protected] Xia Zhang, [email protected]
1
Abstract: We demonstrate a general strategy to design step by step the Ag nanoparticle-functionalized ZnO micro-flowers (Ag/ZnO composites). XRD patterns confirmed the presence of Ag nanoparticles in ZnO/Ag composites, and the SEM and TEM results further demonstrated that Ag nanoparticles were highly dispersed and anchored onto the surface of each ZnO nanosheets. By using the ZnO/Ag composites, the photodegradation of two herbicide derivatives, metamitron and metribuzin, were studied. The enhanced photocatalytic performance was ascribed to the fact that the Ag deposition could reduce the recombination probability of electron-hole pairs, and the photocatalytic mechanism were also investigated in this paper.
Keywords: ZnO/Ag composites; hydrothermal synthesis; photocatalytic
2
1. Introduction Increasing contamination caused by variety and quantities of agrochemicals poses a significant threat to the environment, as well as the health of living beings.1 For instance, the agrochemicals, such as metamitron and metribuzin, derived from the structure of 4-amino-1,2,4-triazin-5(4H)-one, have been widely used as herbicides, which are chemically stable, and can slowly penetrate through the soil and cause severe contamination of underground resources of drinking water. Hence, they should be adequately treated before they are discharged into the environment. Heterogeneous photocatalysis is regarded as one of the most competitive techniques for remedying environmental pollution.2-4 Up to now, a variety of compounds with enhanced photocatalytic activities, such as TiO2, WO3, CdS, BiOCl and Ag3PO4, have been widely applied in photocatalytic degradation of organic contaminants.5-9 Among them, zinc oxide (ZnO), an important n-type semiconductor with a wide-band-gap of 3.37 eV, is considered to be one of the most promising and active photocatalyst due to its low cost, environmental sustainability, and high photocatalytic activity.10 However, large crystals of ZnO with low specific surface area are often obtained owing to its low crystallization temperature and fast growth rate, so that the photocatalyst could not expose as much active sites as possible. On the other hand, the rapid recombination
of
photogenerated
electron-hole
pairs
usually
weakens
its
photocatalytic efficiency.11 To solve these problems, many efforts have been made to develop ZnO-based hybrids with different morphologies and structures. One of the best methods is to design ZnO/metal heterostructure, where the Schottky barrier 3
formed between metal and ZnO interface restricts the recombination of photogenerated electron-hole pairs.12 This character can inevitably prolong the lifetime of photoexcited electron-hole pairs and ultimately improve the photocatalytic activity. As the cheapest noble metal, Ag is the most promising one to fabricate high-efficiency metal-ZnO photocatalyst. Several Ag functionalized ZnO composites with special morphologies, such as nanorods, radical-shaped, dendrite-like, and worm-like ZnO/Ag composites, have been successfully synthesized by using different methods.13-17 Among them, three-dimensional (3D) ZnO-matrix nanostructures are of great interest to scientists since their novel architectural nanostructures may supply more active sites on the surface, and thus it leads to significantly increased photocatalytic performance.18-19 Our previous work has demonstrated that the flower-like ZnO assembled with numerous of nanosheets possess excellent structural stability, easy recovery, and remarkable photocatalytic activity toward the degradation of metamitron 20. However, the degradation rate is slow, which might be caused by the easy recombination of charge carriers in the photocatalysis process. So it is expected to enhance the photodegradation ability if one can develop an efficient way to produce high dispersed Ag nanoparticles on to the surface of the flower-like ZnO assembled by their nanosheets. Compared to the previously reported work,13-17 we firstly demonstrated a novel two-step synthesis route to prepare the ZnO/Ag composites, which can be general for the preparation of other semiconductor/metal composites. Based on this method, one can easily control the morphologies of the composites from the control of that of 4
semiconductor at the first step. A detailed characterization on the phase structures, morphologies and optical properties of ZnO/Ag composites and their ZnO counterparts has been performed systematically. Compared with pure ZnO, the photocatalytic activity of ZnO/Ag heterostructure toward the degradation of metamitron and metribuzin is significantly improved due to the efficient separation of electron-hole pairs, and the acceleration of charge transfer induced by the deposition of Ag nanoparticles. The potential mechanism of photocatalysis of metamitron and metribuzin is also discussed, and this is the first report on such kind of organic contaminants with enhanced photocatalytic degradation rates.
2. Experimental 2.1. Materials All chemicals were analytical grade reagents and used as received without further
purification.
These
analytical
reagents
include
zinc
acetate
(C4H6O4Zn·2H2O, 99.9%, Guangfu, Tianjin, China), silver nitrate (AgNO3, 99.0%, Aladdin, Shanghai, China), sodium citrate (C6H5O7Na3·H2O, 99.0%, Yongda, Tianjin, China), sodium hydroxide (NaOH, 99,5%, Guoyao, Shanghai, China), metamitron and metribuzin (C8H14N4OS and C10H10N4O, Aladdin, Shanghai, China) and pure water (commercial “Wahaha” distilled water, China). 2.2. Synthesis of ZnO micro-flowers
5
The ZnO micro-flowers were prepared by one-step hydrothermal method.19 Briefly, 0.549 g of zinc acetate (2.5 mmol) and 0.735 g of sodium citrate (2.5 mmol) were dissolved into 40 mL deionized water under stirring until a clear aqueous solution was obtained. Then, 4 mL of 4.0 M NaOH was added into the above prepared aqueous solution with rigorous stirring. The mixture was transferred into a 50 mL of Teflon liner stainless steel autoclave and heated at 120ºC for 8 h. After cooling to room temperature, the white precipitation was separated from the solution by centrifugation, washed with distilled water and absolute ethanol several times, and dried under vacuum at 60 ºC for 2 h. 2.3. Synthesis of Ag nanoparticle-functionalized ZnO micro-flowers Photodeposition method was used to modify ZnO micro-flowers with Ag nanoparticles. In detail, 0.2 g of the as-prepared 3D ZnO micro-flowers were dispersed in 300 mL 0.001 M AgNO3 aqueous solution, and stirred for 20 min to achieve adsorption equilibrium of Ag+ ions onto the surface of ZnO. The suspension was then irradiated for 30 min by a high-pressure Hg UV lamp (175 W) under continuously stirring. The as-obtained ZnO/Ag products were collected and washed with distilled water and ethanol for several times to remove the residual Ag+ ions. The product were centrifuged and separated from the reaction solution, washed with pure water and ethanol and dried under vacuum at 60 ºC for 2 h. A series of Ag nanoparticle-functionalized ZnO micro-flowers were obtained using a similar procedure with synthetic molar 6
ratio of AgNO3/ZnO ranged from 0 to 1.2, and the products were denoted as ZnO/Ag -0.2, ZnO/Ag -0.4, ZnO/Ag -0.8 and ZnO/Ag -1.2, respectively. 2.4. Characterization The obtained ZnO/Ag composites were characterized systematically. The phases of the samples were characterized by powder X-ray diffraction (XRD) on a X’Pert Pro MRDDY2094 diffractometer with Cu-Kα radiation (λ = 1.5418 Å). A scan rate of 0.0167º s-1 was applied to record the pattern in the 2θ range of 5–70º. N2 adsorption-desorption isotherm was conducted on a Micrometritics ASAP-2020M volumetric gas adsorption apparatus to investigate the porous feature of ZnO. The morphologies and microstructures of as-prepared ZnO and ZnO/Ag were studied by using Ultra Plus field-emission scanning electron microscope (SEM) and transmission electron microscope (TEM, Tecnai G220). The chemical composition was examined using energy dispersive spectroscopy (EDS) in conjunction with SEM. The UV-Visible adsorption spectra were recorded using a Hitachi U-3010 UV-Visible spectrometer. Photoluminescence spectroscopy was recorded by Shimadzu spectrophotometer (RF-5301 PC). X-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientist Co., Theta Probe) was used to study the surface chemical composition of ZnO/Ag. 2.5. Photodegradation of metamitron and metribuzin A 300W xenon lamp light emitting UV and visible radiation over 300-700 nm was used as the light resource. In a typical test, 100 mg as-prepared photocatalyst powders (ZnO, ZnO/Ag-0.2, ZnO/Ag-0.4, ZnO/Ag-0.8 and 7
ZnO/Ag-1.2) were placed in 100 mL solution containing 13 mg L-1 metamitron or 14 mg L-1 metribuzin. The reaction mixture was stirred for 30 min under darkness to realize the adsorption equilibrium. Then the solution was irradiated in a photochemical reaction chamber with continuous stirring. The distance between the reaction bottle and the light source was maintained at 10 cm. At a certain time of 30 min interval, 3 mL of the treated solution was taken out and centrifuged at 9800 rpm for 3 min. The relative concentration of metamitron or metribuzin in the treated solution was analysed by UV-Vis spectroscopy. The ZnO/Ag-0.4 showed superior photocatalytic activity than ZnO/Ag-0.2, which can be interpreted that more active metal sites are available to accept photogenerated electrons, and restrict the recombination of charge carriers with holes. Furthermore, when the Ag content in ZnO/Ag composites increased (above 0.4), the photocatalytic activity is again decreased due to the aggregation of Ag nanoparticles to form clusters on the surface of ZnO nanosheets, which will again enhance the recombination opportunity between charge carriers with holes.21 With regard to this, ZnO/Ag-0.4 sample with the optimum Ag content was chosen for the overall experiments. A reutilization experiment was performed to evaluate the cyclic stability of ZnO/Ag-0.4 photocatalyst. 3. Results and discussion 3.1. Structure and morphology The phases of the prepared samples were characterized by X-ray 8
diffraction (XRD). As shown in Fig. 1 (a), it can be found that the XRD pattern of as-prepared ZnO micro-flowers (red line) contains a series of diffraction peaks at 30.1º, 35. 5º, 43.2º, 53.6º, 57.0º and 62.7º, which can be indexed well to the simulated XRD pattern of hexagonal wurtzite ZnO (bottom line, JCPDS 36-1451), corresponding to (100), (002), (101), (102), (110) and (103), respectively. When Ag nanoparticles were deposited on the surface of ZnO nanosheets, four additional peaks of 38.1º, 44.3º, 64.4º and 77.4º can be observed (blue line), which can be attributed to the (111), (200), (220) and (311) crystal planes of cubic Ag (JCPDS No. 04-0783), respectively. These results indicated that no other impurities are found in the as-prepared ZnO and ZnO/Ag-0.4 products. Due to the important role of surface area in enhancing photocatalytic activity, quantitative characterization of ZnO and ZnO/Ag-0.4 samples were conducted with N2 adsorption-desorption measurements (Fig. 1b). The obtained ZnO and ZnO/Ag-0.4 had a BET surface area of 18.9 m2 g-1 and 10.5 m2 g-1, respectively. It is observed the deposition of Ag nanoparticles on the surface of ZnO nanosheets enables the slight decrease of the BET surface area, as also observed by Yang, et. al,22 which should be ascribed to partly aggregation of the ZnO nanosheets for ZnO/Ag-0.4 micro-flowers compared to pure ZnO, as shown in Fig. 1c and 1d. It is also found that both as-prepared pure
ZnO
and
ZnO/Ag-0.4
composites
possess
the
micro-flower-like
microstructures, which had average sizes of 3 μm, assembled from numerous nanosheets of about 20 nm in thickness. 9
In addition, the Ag nanoparticles are distributed uniformly on the surface of ZnO micro-flowers observed from TEM as shown in Fig. 2(a). The darker dots and lighter plates correspond to Ag nanoparticles and ZnO nanosheets, respectively. A magnified TEM image of the boundary of the Ag nanoparticles is displayed in Fig. 2(b), indicating the formation of ZnO/Ag heterostructure in as-prepared ZnO/Ag-0.4 sample. The SEM image of a single ZnO/Ag-0.4 micro-flower particle is shown in Fig. 3(a). The elemental mapping of the ZnO/Ag-0.4 microparticle recorded using energy-dispersive spectroscopy are shown in Fig. 3(b-f), where yellow, red, and green refer to the Zn, Ag, and O elements, respectively, indicating that they are uniformly distributed on the surface of the as-prepared ZnO/Ag composites. 3.2. XPS, PL, and solid UV-vis reflectance spectra To further investigate the elemental composition and chemical states of the as-prepared ZnO and ZnO/Ag-0.4 samples, XPS analysis was performed with binding energies calibrated using C1s (284.8 eV) (Fig. 4a). Two peaks in the curves of ZnO and ZnO/Ag-0.4 samples centered at 1020.9 eV and 1043.9 eV can be assigned to Zn 2p3/2 and Zn 2p1/2, respectively (Fig. 4b), indicating that Zn presents mainly in the form of Zn2+ state on the surface of the samples.23 Compared to pure ZnO, the intensity of Zn 2p peak is slightly decreased and shifted, this phenomenon may be caused by the electron transfer from ZnO to Ag nanoparticles in ZnO/Ag-0.4 heterostructure.24 The peaks centered at 368.3 eV and 373.3 eV can be assigned to Ag 3d5/2 and Ag 3d3/2, respectively, 10
confirming that Ag nanoparticles have been successfully deposited on the surface of ZnO nanosheets (Fig. 4c).16 O 1s profile is asymmetric, which can be attributed to the lattice oxygen of ZnO and physical adsorbed oxygen, respectively (Fig. 4d).21 Fig. 5 shows the PL spectra of the as-prepared ZnO and ZnO/Ag-0.4 composites (λex = 308 nm). In general, the lower photoluminescence intensity, the less chance of electron-hole recombination. It can be found that the emission intensity of the PL spectrum of ZnO/Ag-0.4 was lower than that of ZnO, suggesting that the deposition of Ag nanoparticles quenched the fluorescence produced from the electron-hole recombination in ZnO, and hence the photogenerated charge carriers possess a longer lifetime. Thus, it can be predicted that the existence of Ag nanoparticles in ZnO/Ag-0.4 heterostructure may finally improve its photocatalytic activity significantly.25 Moreover, the solid UV-vis absorbance spectra of pure ZnO and ZnO/Ag-0.4 composites are comparatively taken in the range of 200-800 nm and the results are shown in Fig. 6. The adsorption peaks at UV region corresponds to the adsorption of ZnO. It can be found that the ZnO/Ag-0.4 composites shows apparently enhanced UV-Vis adsorption intensity, demonstrating the presence of Ag (0) nanoparticles on ZnO surface.26-27 Therefore, compared to those ZnO/Ag composites published previously,13-17 the Ag-functionalized assembled ZnO/Ag flowers reported herein have large surface area and more active sites exposed during the photocatalytic reaction. These characteristics would effectively 11
accelerate the transfer of photogenerated electron-hole pairs, and finally increase the photocatalytic reaction rate. 3.3. Photocatalytic performance of ZnO/Ag composites photocatalysts The photocatalytic performance of ZnO/Ag-0.4 composites for the degradation of two herbicide derivatives, metamitron and metribuzin, was investigated in detail, and pure micro-flower-like ZnO is used as a photocatalytic reference to qualitatively understand the photocatalytic activity of as-designed ZnO/Ag composites used as catalyst. Two different conditions were conducted in water solution: under light in the absence of catalyst and in the presence of catalyst (ZnO or ZnO/Ag-0.4). The obtained results are shown in Fig. 7a and Fig. 7b. A solution of metamitron and metribuzin in water shows maximum adsorption at 293 nm and 306 nm, respectively. The degradation curves of metamitron and metribuzin were calculated according to the following equations: Ads% =
C o -C × 100 Co
where Co and C are the initial and final concentrations of metamitron or metribuzin vs. time, respectively. As shown in Fig. 7c and 7d, when the photodegradation of metamitron and metribuzin was performed in the presence of the light without catalyst, the degradation rate after 180 min of the irradiation is about 18%, and 10%. If the photocatalysts of ZnO or ZnO/Ag-0.4 were added in the absence of light, no significant change was observed in the degradation of metamitron and metribuzin, indicating that the metamitron and metribuzin 12
are stable in dark. However, when ZnO or ZnO/Ag-0.4 were added into the metamitron or metribuzin solution and irradiated under 300W xenon lamp light, significant degradation of metamitron and metribuzin were observed. Meanwhile, compared with pure ZnO, ZnO/Ag-0.4 heterostructure has higher performance in terms of photocatalytic activity. The maximum degradation efficiency for 100 mL of metamitron (13 mg L-1) and metribuzin (14 mg L-1) on 100 mg of ZnO/Ag-0.4 is nearly 90% in 90 min, while it is only about 35% and 20% if 100 mg pure ZnO micro-flowers were adopted. The time of completely photocatalytic reaction towards metamitron was shortened by nearly 90 min compared to TiO2 material published before.28 3.4. Stability of ZnO/Ag composites photocatalysts Owing to the significance of cycling stability of catalyst in its practical application, reutilization experiments were conducted to evaluate the performance of ZnO/Ag-0.4 heterostructure photocatalyst. All precipitates in each circle were retrieved, washed with ethanol and acetone several times, dried in a vacuum, and allowed for continuous photodegradation experiments. As can be seen in Fig. 8, the photodegradation efficiency is still kept at around 80% after three cycles, demonstrating its good reusability and making it a promising candidate as a highly efficient photocatalyst for the removal of herbicide derivatives. After five runs, the photocatalytic performance of ZnO/Ag-0.4 drops owing to the weight loss of photocatalyst in cycling experiments. 3.5. Mechanism of photocatalytic activity of ZnO/Ag composites 13
It is generally known that photocatalytic acitivity is closely related with the lifetime of photogenerated electron-hole pairs. The schematic illustration of heterogeneous photocatalysis of herbicide derivative in the presence of ZnO/Ag and ZnO is shown in Fig. 9.20,
29
However, the specific photodegradation
products for metamitron or metribuzin still need to be studied in detail in the future. When the ZnO/Ag-0.4 photocatalyst is illuminated by UV light, valence band electrons are promoted to the conduction band leaving a hole behind. Due to the equilibrium Fermi energy level (Ef) of Ag nanoparticles is lower than the bottom energy level of the conduction band (CB) of pure ZnO, most photogenerated electrons will transfer from ZnO to Ag nanoparticles driven by the potential energy, while the holes can be remained on ZnO surface because of the Schottky barrier formed at the interface between Ag and ZnO particles.30 Electron in the conduction band at the catalyst surface can reduce oxygen molecular to superoxide anion. The holes at the ZnO valence band can oxidize the adsorbed water to produce hydroxyl radicals. The superoxide anion and hydroxyl radicals will then attack organic metamitron or metribuzin compound to produce the main product of CO2, H2O and other intermediates. The recombination of electron-hole pairs is greatly reduced owing to the modification of Ag nanoparticles, and their lifetime is prolonged, which make their photocatalytic activities promoted significantly. Additionally, the quencher of •OH (3 mM of isopropyl alcohol, IPA), O2•(0.05 mM carnosine) and h+ (3.0 mM triethanolamine, TEOA) were added to 14
the photocatalytic system in order to investigate the reaction mechanism of the degradation of metamitron and metribuzin over ZnO/Ag-0.4, respectively.31-32 The experimental results are given in Fig. 10, it is found that the photocatalytic degradation of metamitron and metribuzin on ZnO/Ag can be well described by the pseudo-first-order reaction kinetic, ln (Co/C) = kt, with all of the squares of linear correlation coefficients R2 larger than 0.92, where Co and C are the initial concentration of the investigated herbicide derivatives and their value at time t, respectively.33 k is a pseudo-first-rate kinetic constant, representing the rate of the degradation reaction. As also shown in Fig. 10, the degradation of metamitron and metribuzin decreased in the presence of these quenchers, and the addition of TEOA and IPA into the photocatalytic reaction system resulted in an even more significant decreasing compared with the addition of carnosine. Thus, it can be concluded that •OH and h+ play more important roles among all of the three active intermediate species for the photodegradation of metamitron and metribuzin. 4. Conclusion In summary, the ZnO/Ag composites were successfully prepared via the combination of hydrothermal synthesis of flower-like ZnO and photochemical deposition of Ag nanoparticles on each ZnO nanosheets of the matrix, and such a general two-step synthesis route can be used to guide the preparation of other semiconductor/metal composites. On the basis of the microstructural characterization and photocatalytic results, the ZnO/Ag-0.4 composites 15
exhibited significantly improved photocatalytic activity as well as high stability and recyclability in degradation of two herbicide derivatives metamitron and metribuzin under UV light irradiation, which may be attributed to the efficient electron transfer between ZnO nanosheets and Ag nanoparticles. Thus, the excellent chemical stability, enhanced photocatalytic performance, and easily accessible synthetic route enable the as-developed ZnO/Ag-0.4 composites as potential candidates in the field agrochemical treatment.
Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No. 21401018) and the Fundamental Research Funds for the Central Universities (No. N130305003).
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The figure captions are as follows, Fig. 1 (a) XRD patterns of as-prepared ZnO and ZnO/Ag-0.4 micro-flowers, and the stimulated ZnO pattern is given as a comparison; (b) N2 adsorption and desorption of as-prepared ZnO and ZnO/Ag-0.4 micro-flowers; typical SEM image of the as-prepared (c) ZnO and (d) ZnO/Ag-0.4. Fig. 2 (a) TEM image of as-prepared ZnO/Ag-0.4 micro-flowers, and (b) HRTEM image of Ag nanoparticle attached on the surface of ZnO plate. Fig. 3 (a) SEM image and (b) EDS spectrum of ZnO/Ag-0.4 composites and its corresponding elemental mapping images for (c) Zn, (d) Ag and (e) O elements. Fig. 4 (a) Full range XPS spectra of ZnO and ZnO/Ag-0.4; (b-d) High resolution XPS spectra of Zn 2P, Ag 3d and O 1s for ZnO/Ag composites. Fig. 5 Photoluminescence (PL) spectra of the as-prepared ZnO and ZnO/Ag-0.4 photocatalysts. (λmax = 308 nm) Fig. 6 Solid UV-Vis absorbance spectra of as-prepared ZnO and ZnO/Ag-0.4 photocatalysts. Fig. 7 UV-vis spectra of degradation of (a) metamitron and(b) metribuzin as a function of irradiation time in the presence of ZnO/Ag-0.4 composites; comparison of self- degradation and photocatalytic degradation of (c) metamitron and (d) metribuzin with ZnO and ZnO/Ag-0.4. Fig. 8 Five repeated process by using ZnO/Ag-0.4 composites as photocatalyst for degradation of (a) metamitron and (b) metribuzin before and after light irradiation (t = 150 min). Fig. 9 A schematic illustration of ZnO/Ag composites for the photodegradation mechanism to metribuzin and metamitron. 20
Fig. 10 Photodegradation ratio and the linear relationship between ln (Co/C) and irradiation time for metamitron (a-b) and metribuzin (c-d).
21
Fig. 1 (a) XRD patterns of as-prepared ZnO and ZnO/Ag-0.4 micro-flowers, and the stimulated ZnO pattern is given as a comparison; (b) N2 adsorption and desorption of as-prepared ZnO and ZnO/Ag-0.4 micro-flowers; typical SEM image of the as-prepared (c) ZnO and (d) ZnO/Ag-0.4.
22
Fig. 2 (a) TEM image of as-prepared ZnO/Ag-0.4 micro-flowers, and (b) HRTEM image of Ag nanoparticle attached on the surface of ZnO plate.
23
Fig. 3 (a) SEM image and (b) EDS spectrum of ZnO/Ag-0.4 composites and its corresponding elemental mapping images for (c) Zn, (d) Ag and (e) O elements.
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Fig. 4 (a) Full range XPS spectra of ZnO and ZnO/Ag-0.4; (b-d) High resolution XPS spectra of Zn 2P, Ag 3d and O 1s for ZnO/Ag composites.
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Fig. 5 Photoluminescence (PL) spectra of the as-prepared ZnO and ZnO/Ag-0.4 photocatalysts. (λmax = 308 nm)
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Fig. 6 Solid UV-Vis absorbance spectra of as-prepared ZnO and ZnO/Ag-0.4 photocatalysts.
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Fig. 7 UV-vis spectra of degradation of (a) metamitron and(b) metribuzin as a function of irradiation time in the presence of ZnO/Ag-0.4 composites; comparison of self- degradation and photocatalytic degradation of (c) metamitron and (d) metribuzin with ZnO and ZnO/Ag-0.4.
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Fig. 8 F Fiv ve repe r eated pro oceess by usiing g Zn nO//Ag g-0 0.4 com mp posiitess ass ph hoto ocaatallystt fo or degr d rad dation of (a)) meta m amiitro on aand d (b b) metr m ribu uziin befo b oree an nd afte a er liigh ht irrrad diatiion n (t = 150 1 0 min) m ).
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Fig. 9 A scheematicc illlusstraatio on of o ZnO Z O/A Ag ccom mposiitess for th he pho p otod deg grad dattion n meech haniism m to o metr m ribu uzin n and meetam mittron n.
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Fig. 10 Photodegradation ratio and the linear relationship between ln (Co/C) and irradiation time for metamitron (a-b) and metribuzin (c-d).
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Graphical abstract
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Highlights
Ag nanoparticle-functionalized ZnO micro-flowers (Ag/ZnO composites) have been fabricated via a two-step route.
Organic solvent-free process has been adopted to disperse the Ag nanoparticles onto ZnO nanosheets with enhanced photocatalytic activity.
The photodegradation of two herbicide derivatives, metamitron and metribuzin, were studied in detail.
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