ZnO encapsulating carbon spheres with enhanced photocatalytic performance

Accepted Manuscript Title: Facile synthesis of the flower-like ternary heterostructure of Ag/ZnO encapsulating carbon spheres with enhanced photocatalytic performance Authors: Xiaohua Zhao, Shuai Su, Guangli Wu, Caizhu Li, Zhe Qin, Xiangdong Lou, Jianguo Zhou PII: DOI: Reference:

S0169-4332(17)30528-7 http://dx.doi.org/doi:10.1016/j.apsusc.2017.02.155 APSUSC 35264

To appear in:

APSUSC

Received date: Revised date: Accepted date:

10-12-2016 16-2-2017 18-2-2017

Please cite this article as: Xiaohua Zhao, Shuai Su, Guangli Wu, Caizhu Li, Zhe Qin, Xiangdong Lou, Jianguo Zhou, Facile synthesis of the flower-like ternary heterostructure of Ag/ZnO encapsulating carbon spheres with enhanced photocatalytic performance, Applied Surface Science http://dx.doi.org/10.1016/j.apsusc.2017.02.155 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.

Facile synthesis of the flower-like ternary heterostructure of Ag/ZnO encapsulating carbon spheres with

enhanced photocatalytic performance

Xiaohua Zhao1,2, Shuai Su2, Guangli Wu2, Caizhu Li2, Zhe Qin 1, Xiangdong Lou 2, Jianguo Zhou 1*

(1 School of Environment, Key Laboratory of Yellow River and Huai River Water Environment and Pollution

Control (Ministry of Education), Henan Key Laboratory for Environmental Pollution Control, Henan

Engineering Laboratory of Environmental Functional Materials and Pollution Control, Henan Normal

University, Xinxiang 453007, China)

(2 School of Chemistry and Chemical Engineering, Henan Normal University, Xinxiang 453007, China)

*

Corresponding author.

E-mail address: zhoujgwj @163.com (J. Zhou)

1

Graphical abstract

Highlights 

Flower-like Ag/ZnO encapsulating carbon spheres (Ag/ZnO@C) was synthesized.



A green facile synthesis method was used.



Ag/ZnO@C exhibited better photocatalytic performance than Ag/ZnO, ZnO@C, and ZnO.



Dye and metronidazole both can be decomposed by Ag/ZnO@C.

Abstract

To utilize sunlight more effectively in photocatalytic reactions, the flower-like ternary

heterostructure of Ag/ZnO encapsulating carbon spheres (Ag/ZnO@C) was successfully synthesized by a green and facile one-pot hydrothermal method. The carbon spheres (CSs) were wrapped by ZnO nanosheets, forming flower-like microstructures, and Ag nanoparticles (Ag NPs) were deposited on the surface of the ZnO nanosheets. The Ag/ZnO@C ternary heterostructure exhibited enhanced photocatalytic activity compared to those of Ag/ZnO, ZnO@C and pure ZnO for the degradation of Reactive Black GR and metronidazole under sunlight and visible light irradiation. This was attributed to the enhanced visible light absorption and effective charge

2

separation based on the synergistic effect of ZnO, Ag NPs, and CSs. Due to the surface plasmon resonance effect of Ag NPs and surface photosensitizing effect of CSs, Ag/ZnO@C exhibited enhanced visible light absorption. Meanwhile, Ag NPs and CSs can both act as rapid electron transfer units to improve the separation of photogenerated charge carriers in Ag/ZnO@C. The primary active species were determined, and the photocatalytic mechanism was proposed. This work demonstrates an effective way to improve the photocatalytic performance of ZnO and provides information for the facile synthesis of noble metal/ZnO@C ternary heterostructure.

Keywords: Ag/ZnO encapsulating carbon spheres; Flower-like microstructure; Hydrothermal method; Photocatalysis; Organic dye; Metronidazole

1. Introduction Semiconductor-based photocatalysis is a promising technology that allows the utilization of natural sunlight for the degradation of pollutants [1]. Zinc oxide (ZnO) is considered a promising semiconductor photocatalytic material because of its low toxicity, high photocatalytic efficiency, mild synthesis conditions and low cost [2]. However, the narrow adsorption light range (only utilizing ultraviolet (UV) light) and the rapid recombination of photogenerated electron-hole pairs hinder wide applications of ZnO in environmental engineering [3-5]. To solve these drawbacks, several strategies have been proposed to modify ZnO, such as doping with metals or non-metals [2, 6], deposition of noble metals [4, 5, 7], hybridization with carbon materials [8-10] or coupling with narrow band gap semiconductors [3, 11, 12]. Among them, hybridization of ZnO with carbon materials is an effective and promising strategy. Because of their excellent electrical conductivity, the carbon materials in the 3

ZnO-carbon hybrid can behave as a photoelectron reservoir to allow them to be shuttled away from ZnO, resulting in an increase in the electron lifetime. On the other hand, carbon material can absorb visible light and act as a photosensitizer for the generated electrons, thus increasing the number of photoelectrons available to participate in the photoreactions [8-10, 13]. Even so, the photocatalytic performance of the ZnO-carbon hybrid in the visible light region still requires improvement [8, 14]. Deposition of silver nanoparticles (Ag NPs) on ZnO is another effective method because the surface plasmon resonance (SPR) of Ag NPs results in the oscillation of the free electrons at the interface between Ag NPs and ZnO and thus extends the spectral response of ZnO toward the visible light region. Moreover, the Schottky barrier formed between ZnO and the Ag NPs can suppress the recombination of electrons and holes in photocatalytic reactions [4, 7, 15, 16]. Additionally, compared to other noble metals, Ag has been extensively used due to its low cost and non-toxicity, as well as its high electrical and thermal conductivity [7]. Recently, to achieve more efficient charge separation and visible light absorption, series of Ag/ZnO-carbon ternary hybrids, such as Ag/ZnO-graphene [13, 17, 18], Ag/ZnO-carbon dots [19], Ag/ZnO-carbon nanofibers [20], and Ag/ZnO-amorphous carbon layers [21, 22], were synthesized. Based on the synergistic effect among ZnO, Ag NPs, and carbon materials, Ag/ZnO-carbon ternary hybrids exhibited improved photocatalytic activity compared with Ag/ZnO or ZnO-carbon binary hybrids. For example, Ag-graphene quantum dots (GQDs)-ZnO displayed better visible light photocatalytic activity than Ag-ZnO, GQDs-ZnO, and ZnO for the degradation of RhB [13]. Ag/ZnO/C (amorphous carbon layers) exhibited enhanced UV and visible light photocatalytic activity for the degradation of tetracycline hydrochloride compared to ZnO/C and pure ZnO [21]. However, the synthesis procedure for the Ag/ZnO-carbon ternary hybrid often suffers from problems of rigorous experimental conditions, tedious operations, high temperatures, or the use of toxic and harmful reagents. Especially, in the Ag/ZnO-graphene ternary hybrid, the synthesis of graphene from graphite itself is an elaborate process, which 4

makes the entire synthesis procedure for the ternary hybrid much more complicated. Thus, the development of the Ag/ZnO-carbon ternary hybrid photocatalyst with a facile and environmentally friendly synthesis route remains an important topic of investigation. Herein, the flower-like ternary heterostructure composed of Ag/ZnO encapsulating carbon spheres (Ag/ZnO@C) was synthesized by a green facile hydrothermal method. The major differences from the available reports in the literature are: (1) Carbon spheres (CSs) are used as the carbon material in the Ag/ZnO@C ternary heterostructure. Among various types of carbon materials, CSs have been intensively studied recently due to their good electrical conductivity and high chemical stability [23]. CSs can be obtained via a green facile hydrothermal process using glucose as the starting material [24]. Most of all, CSs are a promising candidate as an electron-acceptor/transport material to retard the combination of charge carriers in photocatalysis, or as a photosensitizer to improve the photocatalytic activity of semiconductors. This has been proven in various photocatalyst systems, such as CSs/g-C3N4 [23], CuO-BiVO4@CSs [25], CSs/CdS [26], and Cu2O/CSs [27]. To the best of our knowledge, the ternary heterostructure of Ag/ZnO encapsulating carbon spheres has not been reported. (2) The synthesis method of Ag/ZnO@C is facile and green. After Zn(NO3)2, C6H5Na3O7, CSs, NaOH, and AgNO3 are mixed and sealed for the hydrothermal reaction at 100 °C for 10 h, the flower-like Ag/ZnO@C can be obtained. (3) The three-dimensional (3D) flower-like microstructure composed of nanosheets could provide much more space to deposit Ag NPs, and this microstructure facilitates reflecting and scattering incident light among the nanosheets, thus boosting the light harvest and leading to enhanced photocatalytic performance [28]. The synthesized Ag/ZnO@C ternary heterostructure exhibited better photocatalytic activity than Ag/ZnO, ZnO@C and ZnO for the degradation of Reactive Black GR (GR) and metronidazole under both sunlight and visible light irradiation. This enhanced photocatalytic performance could be attributed to the good visible light absorption capability and high separation efficiency of photogenerated electron-hole pairs based on the 5

synergistic effect of ZnO, Ag NPs, and CSs. A detailed mechanism behind the enhanced photocatalysis performance was proposed. 2. Experimental section 2.1. Materials All chemicals were analytical grade and used as received without further purification. Zinc nitrate hexahydrate (Zn(NO3)2·6H2O), sodium citrate (C6H5Na3O7·2H2O), sodium hydroxide (NaOH), silver nitrate (AgNO3) and glucose (C6H12O6·H2O) were purchased from Shanghai Chemical Industrial Co. Ltd. (Shanghai, China). The chemical structure of the commercial dye Reactive Black GR is shown in Fig. S1 (Supporting Information). Deionized water was used in all experiments. 2.2. Synthesis of Carbon Spheres (CSs) CSs were synthesized via hydrothermal treatment of an aqueous glucose solution, according to the literature [24]. Briefly, 0.5 M glucose solution was transferred into a 100 mL Teflon-sealed autoclave with a fill rate of 70% and maintained at 180 °C for 10 h. After being cooled in air, the CSs were collected by centrifugation, washed with deionized water and absolute ethanol, and finally dried at 60 °C in an oven. 2.3. Synthesis of flower-like Ag/ZnO@C ternary heterostructure The Ag/ZnO@C ternary heterostructure was prepared by a facile hydrothermal process. Firstly, 3.7 g of Zn(NO3)2·6H2O and 1.5 g of C6H5Na3O7·2H2O were dissolved in distilled water under stirring to form a solution. Then, 0.01 g of the as-prepared CSs were dispersed in the above solution with the assistance of sonication for 1 h. Subsequently, 1 M NaOH solution was added dropwise into the dispersion until a suspension of pH 13 was obtained. After stirring for 1 h, 8.71 mL of AgNO3 solution (0.1 M) was dropped into the suspension. 70 mL of the resulting suspension was then sealed into a 100 mL Teflon-lined stainless steel autoclave and heated at 100 ºC for 10 h. The final brown Ag/ZnO@C sample was collected by centrifugation and repeated washing with 6

deionized water and dried in a vacuum oven at 60 °C. For comparison, the sample of Ag/ZnO, ZnO@C or pure ZnO was obtained using a similar procedure without adding CSs or AgNO 3 or both CSs and AgNO3, respectively. 2.4. Characterization The crystallographic structures of the samples were characterized by X-ray diffraction (XRD) (Bruker advance-D8 XRD with Cu Kα radiation, λ=0.154178 nm, the accelerating voltage was set at 40kV with a 100 mA flux). The morphology and microstructure were analysed by field-emission scanning electron microscopy (FESEM, HITACHI SU-8010), scanning electronic microscopy (SEM, JEOL JSM-6390LV) and high-resolution transmission electron microscopy (HRTEM, JEOL JEM-2010), respectively. The surface element compositions over the desired region of samples were detected by an energy dispersive X-ray spectrometer (EDS) attached to the FESEM. Raman analysis was carried out with Fourier transform infrared Raman spectroscopy (Thermo Nicolet, USA) under ambient conditions and the Raman spectra were recorded with a 532 nm laser as the light source. X-ray photoelectron spectroscopy (XPS) measurements were performed with an ESCALab220i-XL electron spectrometer (VG Scientific, England) using monochromatic Al Kα (E=1486.6 eV) radiation as the source. The UV-Visible diffuse reflectance spectra (UV-Vis DRS) were recorded by a UV-Vis spectrophotometer (Lambda 950, PerkinElmer). BaSO4 was used as a reflectance standard in the UV-Visible diffuse reflectance experiment. The photoluminescence (PL) emission spectra were measured on a fluorescence spectrophotometer (Shimadzu RF-5301 PC) with a Xe lamp emitting at 355 nm as the excitation source to investigate the recombination of photogenerated electron-hole pairs in the photocatalysts. The photocurrent was measured on an electrochemical workstation (CHI 660E, ChenHua Instrument Company, Shanghai, China ) in a conventional three-electrode system with a working electrode, a platinum plate counter electrode, and an saturated Ag/AgCl reference electrode. 0.5 M Na2SO4 aqueous solution was used as the electrolyte. A 300 W Xe lamp (Aulight, 7

CEL-HXF 300) was used as the light source. The working electrodes were prepared as follows: 50 mg of sample was dispersed in 1 mL of ethanol. The obtained slurry was transferred onto a fluorine-doped tin oxide (FTO) glass by spin coating and dried at 60 ℃. The photocurrent responses of the working electrodes were recorded by sudden light on and off at the bias voltage of 0.5 V. 2.5. Evaluation of photocatalytic activity The photocatalytic activities of the samples were evaluated by the degradation of Reactive Black GR and metronidazole. For a typical photocatalytic experiment, 30 mg of photocatalyst was added to 50 mL of Reactive Black GR solution (10 mg/L), or 50 mg of photocatalyst was added in 50 mL of metronidazole solution (5 mg/L). Prior to irradiation, the suspensions containing photocatalyst and organic substances were magnetically stirred in the dark for 30 min to establish adsorption-desorption equilibrium. A 500 W xenon lamp was used as a simulated sunlight source, or used as a visible light source by equipping it with a cutoff filter (λ > 420 nm). During the irradiation, approximately 4 mL of the suspension was taken out at a given time interval and centrifuged to remove the photocatalysts. The concentrations of Reactive Black GR and metronidazole were monitored by the UV−Vis spectrophotometer (T60, Beijing Purkinje General Instrument Co., Ltd, China) at 604 nm (Reactive Black GR) or 318 nm (metronidazole). The percentage of degradation was calculated by C/C0, where C0 is the initial concentration of Reactive Black GR or metronidazole and C is the remaining concentration of Reactive Black GR or metronidazole at a certain reaction time. 3. Results and discussion 3.1. Structure and morphology Fig. 1 shows the XRD patterns of the synthesized ZnO, Ag/ZnO, ZnO@C and Ag/ZnO@C. All samples exhibit the sharp and intense diffraction peaks at 2θ = 31.7, 34.4, 36.2, 47.5, 56.6 and 62.8° (Fig. 1a), assigned to the (100), (002), (101), (102), (110) and (103) planes of hexagonal wurtzite ZnO (JCPDS No. 36-1451), 8

respectively. For Ag/ZnO and Ag/ZnO@C, three additional peaks at 38.1, 44.3 and 64.5° (marked with “”) can be assigned to the (111), (200) and (220) planes of face-centred cubic (fcc) Ag (JCPDS No. 04-0783), indicating the formation of a silver metallic phase on the surface of ZnO. The XRD pattern of the CSs shows a weak and wide peak at approximately 2θ = 21° (Fig. S2a), which indicates that the CSs are nearly amorphous [29]. Hence, for ZnO@C and Ag/ZnO@C, no obvious phases of CSs are found (Fig. 1a). Fig. 1b demonstrates the enlarged diffraction region of corresponding samples between 31 and 37°. The (100), (002) and (101) peaks in ZnO@C and Ag/ZnO@C both shift slightly to lower 2θ values compared with those in ZnO and Ag/ZnO. This may be ascribed to the lattice expansion of ZnO due to the introduction of CSs [25, 30]. The corresponding lattice parameter changes from a = 3.242 Å, c = 5.191 Å (ZnO) to a = 3.247 Å, c = 5.200 Å (ZnO@C) or a = 3.249 Å, c = 5.202 Å (Ag/ZnO@C). Similar results were reported in carbon sphere supported CuO-BiVO4 [25]. Fig. 2 shows the morphologies of the synthesized samples. A pure ZnO microstructure (Fig. 2a) consists of numerous 3D flower-like aggregates with diameters of 2-3 µm. Each of aggregate is assembled from many 2D nanosheets as “petals”, which alternately stack and intercross with one another. After Ag deposition, Ag/ZnO retains its flower-like microstructure with unchanged diameters, as shown in Fig. 2b, but the surface of the ZnO nanosheets become rough since the randomly scattered spherical Ag NPs deposit on the surface of the nanosheets (Fig. 2c). After incorporating CSs with a diameter of approximately 1 µm (the SEM of CSs is presented in Fig. S2b), ZnO@C still maintains the flower-like microstructure, except for the diameter increasing to 3-4 µm (Fig. 2d). The SEM image of the broken ZnO@C illustrates that the ZnO nanosheets directly grow on the surface of the carbon sphere (CS) (Fig. S3). Fig. 2(e,f) present the FESEM images of Ag/ZnO@C; clearly, the coexistence of Ag NPs and CSs does not change the flower-like microstructure. The diameter of Ag/ZnO@C also increases to 3-4 µm, and Ag NPs are observed on the surface of the ZnO nanosheets (Fig. 2f). The TEM images of ZnO, ZnO@C and Ag/ZnO are displayed in Fig. S4. Similar to the results shown by the FESEM 9

image, pure ZnO consisted of numerous nanosheets (Fig. S4a). CS is encapsulated by ZnO nanosheets in ZnO@C (Fig. S4b), and the dark dots corresponding to Ag NPs are observed on the surface of the ZnO nanosheets in Ag/ZnO (Fig. S4c). Fig. 2g is the TEM image of Ag/ZnO@C; it is also observed that CS is wrapped by ZnO nanosheets, and Ag NPs deposit on the surface of the nanosheets. The HRTEM image of Ag/ZnO@C (Fig. 2h), taken from the side of one nanosheet with nanoparticles on it, further reveals that the lattice spacing of 0.235 nm for nanoparticles agrees well with the (111) plane of metallic Ag, and 0.279 nm on the nanosheet corresponds to the (100) plane of wurtzite ZnO [21]. The elemental mapping images of Ag/ZnO@C are presented in Fig. 2i, which clearly demonstrate the presence of Zn, O, and Ag; the signal of C scattered on the periphery comes from the carbon substrate. The Raman spectra were used to further prove the presence of CSs in the Ag/ZnO@C ternary heterostructure. As shown in Fig. 3, two significant peaks, correlating with the G band at 1590 cm-1 and the D band at 1360 cm-1, are both observed in the spectra of the CSs (Fig. 3a) and Ag/ZnO@C (Fig. 3b), suggesting the presence of CSs in the flower-like Ag/ZnO@C ternary heterostructure. The D band is assigned to the A1g mode due to the existence of disordered amorphous carbon, which contains abundant defects and vacancies. The G band is ascribed to the E2g mode of the graphite lattice [29]. The presence of graphitic carbon as well as some amorphous carbon in the CSs may favour the separation of photogenerated electron-hole pairs in the photocatalytic process. X-ray photoelectron spectra (XPS) of the Ag/ZnO@C ternary heterostructure are also reported in Fig. 4. The fully scanned spectrum (Fig. 4a) demonstrates that Zn, O, C, and Ag exist in Ag/ZnO@C. The high resolution spectra of Zn 2p, O 1s, C 1s and Ag 3d are shown in Fig. 4(b-e), respectively. In Fig. 4b, the peak of Zn 2p1/2 and Zn 2p3/2, respectively located at 1044.7 and 1021.6 eV, indicate the presence of ZnO in the sample [31]. In Fig. 4c, the peak centred at 530.4 eV originates from the lattice oxygen of ZnO, and the peak at 531.5 eV 10

is assigned to chemisorbed oxygen caused by the surface hydroxyl and oxygen making single bonds to carbon (C–O) [2, 31]. As shown in Fig. 4d, the C 1s peak is divided into three peaks at 284.8, 286.6, and 288.8 eV. The peak at 284.8 eV is ascribed to adventitious carbon contamination [31, 32], whereas the peaks at 286.6 and 288.8 eV are attributed to surface C–O and C=O groups, respectively [32-34]. In Fig. 4e, the peaks at 367.6 and 373.6 eV correspond to the Ag 3d5/2 and Ag 3d3/2 of metallic silver, respectively. The binding energies of the Ag 3d shift to lower values compared to those of bulk Ag, which are approximately 368.2 eV for 3d5/2 and 374.2 eV for 3d3/2 [4, 35]. This observed shift is often ascribed to the interaction between Ag and ZnO [4, 35]. 3.2. Formation mechanism of the flower-like Ag/ZnO@C ternary heterostrucure Based on the results of characterization described above, we propose a possible formation mechanism of the flower-like Ag/ZnO@C ternary heterostructure, as illustrated in Scheme 1. First, a Zn-citrate complex formed after Zn(NO3)2·6H2O and sodium citrate were mixed in solution, decreasing the free Zn2+ ion concentration in the solution. When CSs were dispersed in above solution, the free Zn 2+ ion could be adsorbed on the surface of the CSs with the help of its functional groups (e.g., –OH, –COO-). Second, after NaOH was added, Zn(OH)42complexes formed in-situ on the surface of CSs by OH ions reacting with the adsorbed Zn2+ ion. Then, these Zn(OH)42- complexes decomposed into ZnO nuclei on the surface of the CSs (Eqs. (1) – (3)) [16]. At the same time, part of the Zn-citrate complex hydrolysed to Zn(OH)42- complexes and citrate ions. Third, ZnO crystals are polar, with a positive polar (0001) plane rich in Zn2+ cations and a negative polar (000 1 ) plane rich in O2 anions [36], showing a growth rate v with ν[0001] >> ν[01 1 0] >> ν[000 1 ] [28]. However, in this work, the high concentration of OH ion (pH 13) and citrate ions adsorbed on the positively charged Zn-(0001) plane in competition with Zn(OH)42, which allowed ZnO fast growth along the [01 1 0] direction on the surface of CSs because of the growth rate along the c-axis direction being suppressed to some extent. So, CSs were wrapped by ZnO nanosheets with a {2 1 1 0}-plane [28]. Finally, after AgNO3 was added dropwise into the above 11

suspension, the grey Ag2O gradually formed on the surface of ZnO by a driving force of Weak van der Waals force or electrostatic interactions [16]. During the hydrothermal process, Ag2O was reduced to metallic Ag NPs by the citrate ions. The flower-like ternary heterostructure of Ag/ZnO encapsulating carbon spheres was thus formed. Zn2+ + 2OH → Zn(OH)2 Zn(OH)2 + 2OH → [Zn(OH)4]2[Zn(OH)4]2- → ZnO +H2O + OH

(1) (2) (3)

3.3. Optical and photoelectrochemical properties Fig. 5a shows the UV-Visible DRS of the samples. All samples show an obvious absorption in the UV range, which is assigned to the intrinsic band gap absorption of ZnO [4]. Compared to pure ZnO, the ZnO@C, Ag/ZnO and Ag/ZnO@C additionally show absorption in the visible range. For ZnO@C, this extension of the absorption range could be attributed to the surface photosensitizing effect of CSs [25]. For Ag/ZnO, the absorption in the range of 410-480 nm could be attributed to SPR effect of Ag NPs [4]. As for the strong visible light absorption for Ag/ZnO@C, the synergistic effect of CSs and Ag NPs should be the main reason. The PL emission spectra are often used to disclose the immigration, transfer, and recombination processes of the photogenerated electron-hole pairs in semiconductor materials, and it is generally believed that a weaker PL intensity means a lower recombination probability of the photogenerated charge [2, 25]. Fig. 5b presents the photoluminescence spectra of the samples excited with a wavelength of 355 nm. The PL intensity decreases as: ZnO > ZnO@C > Ag/ZnO > Ag/ZnO@C, indicating that the recombination of the photogenerated charge is greatly inhibited in the ZnO-based composites. In ZnO@C, the excited electrons in ZnO can be transferred to the CSs through an interfacial charge transfer process, reducing the recombination of photogenerated charge carriers, which is agreement with the previous studies [25, 37, 38]. In Ag/ZnO, Ag NPs can act as electron sinks to hinder 12

the recombination of photogenerated electron-hole pairs [16, 39]. In the Ag/ZnO@C ternary heterostructure, the separation rates of photogenerated electron-hole pairs are further improved by the respective electron transfer to the CSs and Ag NPs, leading to the weakest emission intensity in the PL spectra. To further confirm the separation rate of photogenerated electron-hole pairs, the transient photocurrent responses of the samples are examined (Fig. 5c). The photocurrent responses of the samples follow the order of Ag/ZnO@C > Ag/ZnO > ZnO@C > ZnO, suggesting that the incorporation of CSs or the deposition of Ag NPs benefits the separation of photogenerated electrons and holes in ZnO. Especially, Ag/ZnO@C exhibits improved photocurrent response compared to Ag/ZnO and ZnO@C, disclosing that more efficient separation and transportation of the photogenerated charges are realized. This could be attributed to the synergistic effect of the CSs and Ag NPs. The photocurrent response results were consistent with those of the PL measurements. The photocatalytic activity of Ag/ZnO@C is thus expected to be higher than those of Ag/ZnO, ZnO@C and ZnO. 3.4. Photocatalytic activity To explore the photocatalytic ability of the samples, the photodegradation of Reactive Black GR and metronidazole under sunlight and visible light irradiation were conducted. Under sunlight or visible light irradiation, no obvious change was observed for the concentration of Reactive Black GR without a photocatalyst, indicating that the Reactive Black GR solution is very stable (Fig. 6(a,b)). Pure ZnO exhibits a low photocatalytic activity under sunlight irradiation and even poor photocatalytic activity under visible light irradiation, e.g., the degradation rate of Reactive Black GR is 45.9% in 40 min of sunlight exposure and only 24.4% in 120 min of visible light exposure. A possible reason for these low rates is that ZnO photocatalysis proceeds only at UV light due to its wide band gap energy. After the introduction of CSs, ZnO@C displays improved photocatalytic activity under sunlight and visible light irradiation, which is attributed to the extended light absorption range and fast separation of photogenerated charge carriers by the CSs. In addition, the presence 13

of CSs benefits the ZnO@C dispersion in Reactive Black GR solution because they are very light. As for Ag/ZnO, Ag NPs could help the ZnO harvest the visible light energy by their SPR effect and enhance photogenerated charge carriers separation by serving as an electron reservoir [4, 40]; thus, it also shows higher sunlight and visible light degradation efficiency than ZnO. When the effects of CSs and Ag NPs are combined, the light absorption intensity and the separation efficiency of photogenerated charge carriers of Ag/ZnO@C are further enhanced. So the Ag/ZnO@C ternary heterostructure shows the best photocatalytic activity and the corresponding degradation rate can reach ~100% (sunlight, 40 min) and 95.8% (visible light, 120 min), respectively. The photocatalytic degradation of Reactive Black GR can be assigned to a reaction with pseudo-first-order kinetics with a simplified Langmuir–Hinshelwood model: ln(C0/C)= kt, where k is the apparent pseudo-first-rate kinetics constant and t is the irradiation time [1]. The photodegradation curves of Reactive Black GR in the form of ln(C0/C) versus time are shown in Fig. S5, and the k values were determined by linear fitting for different samples as shown in Fig. 6c. Clearly, the Ag/ZnO@C gives the highest apparent rate constant in all of samples, which is approximately 8.1 and 11.7 times higher than that of pure ZnO under sunlight and visible light irradiation, respectively. The apparent rate constants of Ag/ZnO and ZnO@C were approximately 5.7 (9.6) and 4.1 (5.4) times higher than that of pure ZnO under sunlight (visible light) irradiation, respectively. The time-dependent absorbance spectra of the Reactive Black GR solution over the Ag/ZnO@C photocatalyst under sunlight irradiation are shown in Fig. 6d. The characteristic absorption peaks corresponding to Reactive Black GR decrease rapidly as the exposure time increases, indicating the decomposition of Reactive Black GR. In contrast to the dye solution, metronizadole neither absorbs visible light nor has a sensitization effect. Here, we use metronizadole as a model reaction to exclude the photosensitization effect. Fig. 7(a,b) display the photocatalytic activity of the samples for the degradation of metronizadole under sunlight and visible light 14

irradiation, respectively. In the absence of a photocatalyst, the self-degradation of metronidazole is almost negligible. The photocatalysed degradation for metronizadole is similar to Reactive Black GR, and the Ag/ZnO@C ternary heterostructure shows the highest photocatalytic degradation activity among the samples. The kinetic data curves for metronizadole degradation in the form of ln(C0/C) versus time are shown in Fig. S6, and the corresponding apparent rate constants k are illustrated in Fig. 7c. Ag/ZnO@C has the highest rate constant, which is approximately 6.7 and 7.1 times higher than that of pure ZnO under sunlight irradiation and visible light irradiation, respectively. In the presence of the Ag/ZnO@C photocatalyst, the absorption spectra of metronidazole solutions under sunlight irradiation (Fig. 7d) decrease rapidly as the exposure time increases, indicating the decomposition of metronidazole. 3.5. Photocatalytic stability of the flower-like Ag/ZnO@C ternary heterostructure The recycling experiments were carried out by the degradation of Reactive Black GR solution over Ag/ZnO@C under sunlight and visible light irradiation. All operations were exactly same. As shown in Fig. 8, the photocatalytic activity of Ag/ZnO@C has no obvious loss after five recycling tests whether under sunlight or under visible light. In addition, the crystal structure as well as the morphology of Ag/ZnO@C has no discernible change before and after the five recycling tests under sunlight irradiation (Fig. S7). So, the Ag/ZnO@C ternary heterostructure is stable for the photodegradation of organic pollutants. 3.6. Photodegradation Mechanism The photodegradation of pollutants can occur via either direct oxidation by photogenerated holes or reactions with generated intermediate oxygen species [41]. A series of quenchers were employed to ascertain the dominant active species during the photodegradation processes. Benzoquinone (BQ), disodium salt ethylenediaminetetraacetic acid (EDTA-2Na), and isopropanol (IPA) were used as scavengers for •O2−, photogenerated holes, and •OH in the degradation of Reactive Black GR, respectively. As shown in Fig. 9a, 15

under sunlight irradiation, the photocatalytic activity of Ag/ZnO@C is greatly suppressed by the addition of BQ or EDTA-2Na, suggesting that both photogenerated holes and •O2− are the main oxidative species. The addition of IPA only causes a limited decrease in the photodegradation of Reactive Black GR, indicating that •OH plays an assistant role. While under visible light irradiation (Fig. 9b), the photocatalytic activity of Ag/ZnO@C is greatly suppressed by the addition of BQ and partly suppressed by the addition of EDTA-2Na or IPA in the reaction system, suggesting that •O2− is the main oxidative species, while photogenerated holes and •OH play assistant roles.

On the basis of the characterization and experimental results mentioned above, the flower-like Ag/ZnO@C ternary heterostructure shows enhanced photocatalytic performance attributed to the good light absorption capability and high separation efficiency of photogenerated electron-hole pairs based on the synergistic effect among ZnO, Ag NPs and CSs. The possible photocatalytic mechanisms are illustrated in Scheme 2. Under sunlight irradiation, the localized SPR of Ag NPs could excite the surrounding semiconductor to produce more photogenerated electrons and holes [42]. Thus, more electrons (e−) in ZnO could be excited to the conduction band (CB) with simultaneous generation of the same amount of holes (h +) in the valence band (VB) (Scheme 2a). Subsequently, a portion of the photogenerated electrons transfer quickly from the CB of ZnO to the Ag NPs through the Schottky barrier at Ag-ZnO interface and are trapped by the Ag NPs [15, 16, 39]. The remaining photogenerated electrons in the CB of ZnO flow to the CSs [27, 38, 43]. Thus, the separation of the photogenerated charge is increased. Then, photogenerated electrons accumulated on the surface of the Ag NPs react with dissolved oxygen molecules to yield superoxide radical anions (•O2¯), which dominate the degradation of organic pollutants in this case. Some •O2¯ may proceed to form active •OH to be involved in the degradation of organic pollutants [44]. Meanwhile, the photogenerated holes, another strong oxidizing agent in this case, 16

migrate to the surface of ZnO and directly oxidize the organic pollutants. Some holes may be captured by adsorbed OH− species to form •OH to degrade organic pollutants. For the visible light driven photocatalysis process (Scheme 2b), the electrons (e−) in the VB of ZnO could hardly be excited to the CB, so the as-obtained visible light photocatalysis is attributed to the SPR effect of the Ag NPs and the surface photosensitizing effect of the CSs. Upon visible light irradiation, the photogenerated electrons generated from Ag NPs are injected quickly into the CB of ZnO due to the SPR effect, leaving behind the photogenerated holes on the Ag NPs [4, 16, 45]. Simultaneously, the photogenerated electrons generated from CSs are also injected into the CB of ZnO due to the surface photosensitizing effect [25, 46]. Then, those photogenerated electrons react with absorbed oxygen molecules on the surface of ZnO to form •O2−, which dominates the degradation of organic pollutants. Some •O2− radicals react with H2O molecules to produce •OH, which takes part in the degradation of organic pollutants. The photogenerated holes on the Ag NPs also take part in the degradation of organic pollutants by directly oxidizing organic molecules. 4. Conclusion The flower-like Ag/ZnO@C ternary heterostructure was successfully synthesized via a green facile hydrothermal approach, and the possible formation mechanism was discussed. Significant visible light absorption capability and high separation efficiency of photogenerated electron-hole pairs were observed in the Ag/ZnO@C ternary heterostructure based on the synergetic effect of ZnO, CSs, and Ag NPs. Compared with binary heterostructure materials (ZnO@C, Ag/ZnO), Ag/ZnO@C further improved the sunlight and visible light photocatalytic performance in the degradation of Reactive Black GR and metronizadole. Ag/ZnO@C was photostable and reusable even after five cycles, which would be beneficial for practical applications. Moreover, our findings are helpful for synthesizing the noble metal/ZnO@C ternary heterostructure for different applications. 17

Acknowledgements The authors are grateful for the financial support from the Major Science and Technology Program for Water Pollution Control and Treatment, PR China (Grant Nos. 2015ZX07204-002), the Basic Scientific and Technological Frontier Project of Henan Province, PR China (Grant Nos. 152300410087 and 132300410286), the Key Scientific Research Project for Colleges and Universities of Henan Province, PR China (Grant Nos. 16A150032), and the Key Science and Technology Program of Henan Province, PR China (Grant Nos. 122102210233 and 132102210439).

References [1] Z.M. Yang, G.F. Huang, W.Q. Huang, J.M. Wei, X.G. Yan, Y.Y. Liu, C. Jiao, Z. Wan, A. Pan, Novel Ag3PO4/CeO2 composite with high efficiency and stability for photocatalytic applications, J. Mater. Chem. A, 2 (2014) 1750-1756. [2] F. Wang, L. Liang, L. Shi, M. Liu, J. Sun, CO2-assisted synthesis of mesoporous carbon/C-doped ZnO composites for enhanced photocatalytic performance under visible light, Dalton. Trans., 43 (2014) 16441-16449. [3] S. Le, T. Jiang, Y. Li, Q. Zhao, Y. Li, W. Fang, M. Gong, Highly efficient visible-light-driven mesoporous graphitic carbon nitride/ZnO nanocomposite photocatalysts, Appl. Catal. B: Environ., 200 (2017) 601-610. [4] S.A. Ansari, M.M. Khan, M.O. Ansari, J. Lee, M.H. Cho, Biogenic Synthesis, Photocatalytic, and Photoelectrochemical Performance of Ag–ZnO Nanocomposite, J. Phys. Chem. C, 117 (2013) 27023-27030. [5] D.R. Hang, S.E. Islam, C.H. Chen, K.H. Sharma, Full Solution-Processed Synthesis and Mechanisms of a Recyclable and Bifunctional Au/ZnO Plasmonic Platform for Enhanced UV/Vis Photocatalysis and Optical Properties, Chem. Eur. J. 22 (2016), 1-13. [6] C. Abed, C. Bouzidi, H. Elhouichet, B. Gelloz, M. Ferid, Mg doping induced high structural quality of sol–gel ZnO nanocrystals: Application in photocatalysis, Appl. Surf. Sci., 349 (2015) 855-863. [7] X. Zhang, Y. Wang, F. Hou, H. Li, Y. Yang, X. Zhang, Y. Yang, Y. Wang, Effects of Ag loading on structural and photocatalytic properties of flower-like ZnO microspheres, Appl. Surf. Sci., 391 (2017) 476-483. [8] C. Han, M.Q. Yang, B. Weng, Y.J. Xu, Improving the photocatalytic activity and anti-photocorrosion of 18

semiconductor ZnO by coupling with versatile carbon, Phys. Chem. Chem. Phys., 16 (2014) 16891-16903. [9] Y. Feng, N. Feng, Y. Wei, G. Zhang, An in situ gelatin-assisted hydrothermal synthesis of ZnO–reduced graphene oxide composites with enhanced photocatalytic performance under ultraviolet and visible light, RSC Adv., 4 (2014) 7933-7943 . [10] C. Yu, Y. Wang, Y. Liu, C. Guo, Y. Hu, Facile growth of ZnO nanocrystals on nitrogen-doped carbon nanotubes for visible-light photodegradation of dyes, Mater. Lett., 100 (2013) 278-281. [11] G. Yang, W. Yan, Q. Zhang, S. Shen, S. Ding, One-dimensional CdS/ZnO core/shell nanofibers via single-spinneret electrospinning: tunable morphology and efficient photocatalytic hydrogen production, Nanoscale, 5 (2013) 12432-12439. [12] M.T. Qamar, M. Aslam, I.M. Ismail, N. Salah, A. Hameed, Synthesis, Characterization, and Sunlight Mediated Photocatalytic Activity of CuO Coated ZnO for the Removal of Nitrophenols, Acs Appl. Mater. Interfaces, 7 (2015) 8757-8769. [13] J. Wang, Y. Li, J. Ge, B.P. Zhang, W. Wan, Improving photocatalytic performance of ZnO via synergistic effects of Ag nanoparticles and graphene quantum dots, Phys. Chem. Chem. Phys., 17 (2015) 18645-18652. [14] A. Meng, J. Shao, X. Fan, J. Wang, Z. Li, Rapid synthesis of a flower-like ZnO/rGO/Ag micro/nano-composite with enhanced photocatalytic performance by a one-step microwave method, RSC Adv., 4 (2014) 60300-60305. [15] S. Kuriakose, V. Choudhary, B. Satpati, S. Mohapatra, Facile synthesis of Ag-ZnO hybrid nanospindles for highly efficient photocatalytic degradation of methyl orange, Phys. Chem. Chem. Phys., 16 (2014) 17560-17568. [16] P. Fageria, S. Gangopadhyay, S. Pande, Synthesis of ZnO/Au and ZnO/Ag nanoparticles and their photocatalytic application using UV and visible light, RSC Adv., 4 (2014) 24962-24972. [17] J. Qin, R. Li, C. Lu, Y. Jiang, H. Tang, X. Yang, Ag/ZnO/graphene oxide heterostructure for the removal of rhodamine B by the synergistic adsorption–degradation effects, Ceram. Int., 41 (2015) 4231-4237. [18] M. Ahmad, E. Ahmed, Z.L. Hong, N.R. Khalid, W. Ahmed, A. Elhissi, Graphene–Ag/ZnO nanocomposites as high performance photocatalysts under visible light irradiation, J. Alloy. Compd., 577 (2013) 717-727. [19] X.Y. Zhang, Y.J. Deng, J.K. Liu, Y. Lu, X.H. Yang, Mass preparation and novel visible light photocatalytic activity of C and Ag Co-modified ZnO nanocrystals, J. Colloid Interface Sci., 459 (2015) 1-9. [20] B. Pant, M. Park, H.Y. Kim, S.J. Park, Ag-ZnO photocatalyst anchored on carbon nanofibers: Synthesis, characterization, and photocatalytic activities, Synth. Met., 220 (2016) 533-537. [21] J. Xue, S. Ma, Y. Zhou, Z. Zhang, P. Jiang, Synthesis of Ag/ZnO/C plasmonic photocatalyst with enhanced adsorption capacity and photocatalytic activity to antibiotics, RSC Adv., 5 (2015) 18832-18840. [22] Z. Shen, P. Liang, S. Wang, L. Liu, S. Liu, Green Synthesis of Carbon- and Silver-Modified Hierarchical ZnO with Excellent Solar Light Driven Photocatalytic Performance, ACS Sustainable Chemistry & Engineering, 3 (2015) 1010-1016. [23] K. Li, X. Xie, W.D. Zhang, Photocatalysts based on g-C3N4-encapsulating carbon spheres with high visible light activity for photocatalytic hydrogen evolution, Carbon, 110 (2016) 356-366. [24] X. Sun, Y. Li, Colloidal carbon spheres and their core/shell structures with noble-metal nanoparticles, Angew. Chem. Int. Ed. Engl., 43 (2004) 597-601. [25] W. Zhao, Y. Wang, Y. Yang, J. Tang, Y. Yang, Carbon spheres supported visible-light-driven CuO-BiVO4 heterojunction: Preparation, characterization, and photocatalytic properties, Appl. Catal. B: Environ., 115-116 (2012) 90-99. [26] Q. Wang, J. Lian, Q. Ma, S. Zhang, J. He, J. Zhong, J. Li, H. Huang, B. Su, Preparation of carbon spheres supported CdS photocatalyst for enhancement its photocatalytic H2 evolution, Catal. Today, 281 (2017) 19

662-668. [27] K. Zhou, Y. Shi, S. Jiang, Y. Hu, Z. Gui, Facile preparation of Cu 2O/carbon sphere heterostructure with high photocatalytic activity, Mater. Lett., 98 (2013) 213-216. [28] B. Li, Y. Wang, Facile synthesis and enhanced photocatalytic performance of flower-like ZnO hierarchical microstructures, J. Phys. Chem. C, 114 (2009) 890-896. [29] Q. Chen, Q. Wu, Preparation of carbon microspheres decorated with silver nanoparticles and their ability to remove dyes from aqueous solution, J. Hazard. Mater., 283 (2015) 193-201. [30] S. Khanchandani, P.K. Srivastava, S. Kumar, S. Ghosh, A.K. Ganguli, Band Gap Engineering of ZnO using Core/Shell Morphology with Environmentally Benign Ag2S Sensitizer for Efficient Light Harvesting and Enhanced Visible-Light Photocatalysis, lnorg. Chem., 53 (2014) 8902-8912. [31] J. Zhai, L. Wang, D. Wang, Y. Lin, D. He, T. Xie, UV-illumination room-temperature gas sensing activity of carbon-doped ZnO microspheres, Sens. Actuators B: Chem., 161 (2012) 292-297. [32] L. Pan, T. Muhammad, L. Ma, Z.F. Huang, S. Wang, L. Wang, J.J. Zou, X. Zhang, MOF-derived C-doped ZnO prepared via a two-step calcination for efficient photocatalysis, Appl. Catal. B: Environ., 189 (2016) 181-191. [33] J. Dou, S. Yin, J.Y. Chong, B. Zhang, J. Han, Y. Huang, R. Xu, Carbon spheres anchored Co3O4 nanoclusters as an efficient catalyst for dye degradation, Appl. Catal. A: Gen., 513 (2016) 106-115. [34] M. Zhou, X. Gao, Y. Hu, J. Chen, X. Hu, Uniform hamburger-like mesoporous carbon-incorporated ZnO nanoarchitectures: One-pot solvothermal synthesis, high adsorption and visible-light photocatalytic decolorization of dyes, Appl. Catal. B: Environ., 138-139 (2013) 1-8. [35] X.H. Wang, H.L. Liu, W.X. Zhang, W.Z. Cheng, X. Liu, X.M. Li, J.H. Wu, Synthesis and characterization of polymer-coated AgZnO nanoparticles with enhanced photocatalytic activity, RSC Adv., 4 (2014) 44011-44017. [36] W. Peng, S. Qu, G. Cong, Z. Wang, Synthesis and structures of morphology-controlled ZnO nano-and microcrystals, Cryst. Growth Des., 6 (2006) 1518-1522. [37] A. Boudjemaa, A. Rebahi, B. Terfassa, R. Chebout, T. Mokrani, K. Bachari, N.J. Coville, Fe2O3/carbon spheres for efficient photo-catalytic hydrogen production from water and under visible light irradiation, Sol. Energy Mater. Sol. Cells, 140 (2015) 405-411. [38] J. Li, X. Liu, X. Piao, Z. Sun, L. Pan, Novel carbon sphere@ Bi 2MoO6 core–shell structure for efficient visible light photocatalysis, RSC Adv., 5 (2015) 16592-16597. [39] H.R. Liu, G.X. Shao, J.F. Zhao, Z.X. Zhang, Y. Zhang, J. Liang, X.G. Liu, H.S. Jia, B.S. Xu, Worm-Like Ag/ZnO Core–Shell Heterostructural Composites: Fabrication, Characterization, and Photocatalysis, J. Phys. Chem. C, 116 (2012) 16182-16190. [40] K. Awazu, M. Fujimaki, C. Rockstuhl, J. Tominaga, H. Murakami, Y. Ohki, N. Yoshida, T. Watanabe, A plasmonic photocatalyst consisting of silver nanoparticles embedded in titanium dioxide, J. Am. Chem. Soc., 130 (2008) 1676-1680. [41] D. Chen, Z. Wang, T. Ren, H. Ding, W. Yao, R. Zong, Y. Zhu, Influence of Defects on the Photocatalytic Activity of ZnO, J. Phys. Chem. C, 118 (2014) 15300-15307. [42] J. Long, H. Chang, Q. Gu, J. Xu, L. Fan, S. Wang, Y. Zhou, W. Wei, L. Huang, X. Wang, P. Liu, W. Huang, Gold-plasmon enhanced solar-to-hydrogen conversion on the {001} facets of anatase TiO2 nanosheets, Energy & Environmental Science, 7 (2014) 973-977. [43] Y. Chen, X. Cao, J. Kuang, Z. Chen, J. Chen, B. Lin, The gas-phase photocatalytic mineralization of benzene over visible-light-driven Bi2WO6@C microspheres, Catal. Commun., 12 (2010) 247-250. [44] Y. Park, Y. Na, D. Pradhan, B.K. Min, Y. Sohn, Adsorption and UV/Visible photocatalytic performance of 20

BiOI for methyl orange, Rhodamine B and methylene blue: Ag and Ti-loading effects, CrystEngComm, 16 (2014) 3155. [45] J. Shu, Z. Wang, G. Xia, Y. Zheng, L. Yang, W. Zhang, One-pot synthesis of AgCl@Ag hybrid photocatalyst with high photocatalytic activity and photostability under visible light and sunlight irradiation, Chem. Eng. J., 252 (2014) 374-381. [46] Y. Hu, Y. Liu, H. Qian, Z. Li, J. Chen, Coating colloidal carbon spheres with CdS nanoparticles: microwave-assisted synthesis and enhanced photocatalytic activity, Langmuir, 26 (2010) 18570-18575.

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Fig. 1. (a) XRD patterns and (b) the expanded view of the XRD patterns in the range of 2θ from 31 to 37°.

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Fig. 2. FESEM images of (a) ZnO, (b,c) Ag/ZnO, (d) ZnO@C, (e,f) Ag/ZnO@C, TEM (g) and HRTEM (h) images of Ag/ZnO@C, (i) the

elemental mapping images of Ag/ZnO@C.

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Fig. 3. Raman spectra of (a) CSs and (b) the Ag/ZnO@C ternary heterostructure.

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Fig. 4. XPS spectra of the Ag/ZnO@C ternary heterostructure: (a) XPS fully scanned spectrum, (b) Zn 2p, (c) O 1s, (d) C1s, (e) Ag 3d.

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Fig. 5. (a) UV-Visible DRS, (b) Room-temperature PL spectra and (c) Transient photocurrent responses of the samples.

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Fig. 6. Photocatalytic degradation of Reactive Black GR over the samples under (a) sunlight and (b) visible light irradiation, (c) The apparent

rate constants for the photodegradation of Reactive Black GR over the samples, (d) UV–Vis spectra of Reactive Black GR solutions at

different sunlight irradiation times in the presence of the Ag/ZnO@C ternary heterostructure.

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Fig. 7. Photocatalytic degradation of metronizadole over the samples under (a) sunlight irradiation and (b) visible light irradiation, (c) The

apparent rate constants for the photodegradation of metronizadole over the samples, (d) UV–Vis spectra of metronizadole solutions at

different sunlight irradiation times in the presence of the Ag/ZnO@C ternary heterostructure.

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Fig. 8. Cycling tests in the photodegradation of Reactive Black GR using the Ag/ZnO@C photocatalyst under (a) sunlight and (b) visible

light irradiation.

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Fig. 9. Controlled experiments using different active species scavengers for the photodegradation of Reactive Black GR over the flower-like

Ag/ZnO@C ternary heterostucture under (a) sunlight and (b) visible light irradiation.

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Scheme 1. Schematic illustration of the formation process of the flower-like Ag/ZnO@C ternary heterostructure.

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Scheme 2. Postulated photocatalytic mechanism of the Ag/ZnO@C ternary heterostructure under (a) sunlight and (b) visible light irradiation.

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