Ag yolk-shell microspheres with enhanced visible-light photocatalytic activity

Materials Letters 190 (2017) 60–63

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Facile photochemical synthesis of ZnWO4/Ag yolk-shell microspheres with enhanced visible-light photocatalytic activity Jinli Zhu, Mengjie Liu, Yanfeng Tang ⇑, Tongming Sun, Jinjin Ding, Liwei Han, Miao Wang ⇑ School of Chemistry and Chemical Engineering, Nantong University, Nantong 226019, PR China

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Article history: Received 23 September 2016 Received in revised form 24 November 2016 Accepted 17 December 2016 Available online 22 December 2016 Keywords: Yolk-shell Microstructure Semiconductors Visible-light Photocatalytic activity

a b s t r a c t ZnWO4/Ag yolk-shell microspheres have been prepared by a simple and cost effective photochemical method. The crystalline phase, physicochemical properties and morphologies of the products were characterized by XRD, UV–vis DRS, SEM, TEM and EDXA. The photocatalytic activity of the as-prepared ZnWO4/Ag samples was evaluated by the degradation of methyl blue (MB) under xenon lamp irradiation. In comparison with pure ZnWO4 yolk-shell microspheres, ZnWO4/Ag yolk–shell microspheres displayed dramatically better visible-light driven photocatalytic performance toward dye photo-removal reaction. In light of its outstanding performance and unique hollow structures, ZnWO4/Ag yolk–shell microspheres exhibit a promising potential application in the development of visible-light photocatalysts. Ó 2016 Elsevier B.V. All rights reserved.

1. Introduction As a semiconductor, ZnWO4 has been widely investigated due to its various potential applications, such as photocatalysts [1–3], gas sensor [4] and photoluminescence [5]. Pure ZnWO4 exhibits relatively low capability photocatalytic activity owing to its low utilization of visible light and the rapid recombination of photogenerated electron-hole pairs, which largely limit its practical applications in visible-light condition. For the sake of further improving the photocatalytic properties and extending the applications of ZnWO4, expanding its optical absorption from UV to visible-light region and hindering the recombination of the photoexcited electron-hole pairs are the two main challenges. Hybridizing Ag into ZnWO4 micro/nanostructures has attracted a lot of attention due to its wide applications in enhanced photocatalytic activities [6,7]. Although various methods have been used to successfully synthesize the ZnWO4/Ag, the design and synthesis of high performance ZnWO4/Ag with specific morphologies and applications is still in great need. From the economical and environmental friendly point of view, developing mild, low-cost strategies for controlling synthesis of ZnWO4/Ag micro/nanostructures is very attractive and highly desirable for the practical applications. Compared with other techniques, photochemical preparation has the advantages of environmental friendliness, energy saving and high reproducibility [8–10]. Recently, we have successfully ⇑ Corresponding authors. E-mail addresses: [email protected] (Y. Tang), [email protected] (M. Wang). http://dx.doi.org/10.1016/j.matlet.2016.12.056 0167-577X/Ó 2016 Elsevier B.V. All rights reserved.

prepared novel yolk-shell ZnWO4 microspheres via L-Asp assisted hydrothermal route [2], which have interior core, void space, and permeable outer shell resulting in great application in photocatalysis. Motivated by the previous work on photochemical deposition of Ag onto the surface of semiconductor, herein, we developed a photochemical method for the controlled preparation of ZnWO4/ Ag yolk–shell microspheres by UV light irradiation to get yolk– shell ZnWO4 microspheres coated Ag nanoparticles. The results showed that, compared with the ZnWO4 microspheres, the ZnWO4/Ag microspheres have enhanced photocatalytic activity on the decomposition of MB under xenon lamp irradiation. 2. Experimental section The yolk-shell ZnWO4 microspheres used in this experiment were synthesized according to a procedure described in our previous work [2]. All chemicals were of analytical grade and used as received. The yolk-shell structured ZnWO4/Ag microspheres were fabricated by a facile photoinduced method. 100 mg ZnWO4 microspheres were dispersed in a mixture solution of 35 mL deionized water and 5 mL glycol under continuous stirring. Then 10 mg AgNO3 powder was poured into the above suspension. Finally, the mixed solution was placed in a quartz bottle with stirring and irradiated under two 250 W high-pressure Hg lamp (kmax = 365 nm) for 30 min. The quartz bottle was loaded with cycled condensate water to prevent the concomitant heat during irradiation. The distance between quartz bottle and light source was 10 cm. The obtained products were collected by centrifugation

J. Zhu et al. / Materials Letters 190 (2017) 60–63

and washed with deionized water and ethanol for three times. The final products were dried at 60 °C for 4 h. Similar procedures were performed under the same reaction condition except using the mixture aqueous (35 mL deionized water, 5 mL ethanol) or 40 mL water, which was labelled as SE and Sw, respectively. The photocatalytic activity of the as-prepared yolk-shell ZnWO4/Ag microspheres was evaluated by photocatalytic decolorization of MB aqueous solution under simulated solar irradiation. It was conducted in XPA-7 photochemical reactor equipped with 500 W Xe lamp. An electric fan and cycled condensate water were used to prevent thermal catalytic effects. The suspension was vigorously stirred during the process and the temperature of suspension was maintained at 20 ± 2 °C. Typically, 16 mg of ZnWO4/Ag photocatalyst was introduced into a series of quartz cuvette containing 20 mL of MBaqueous solution (20 mg/L) at room temperature, respectively. Before light was turned on, the solution was continuously stirred for 30 min in dark to reach an adsorption-desorption equilibrium. After that, the solution were irradiated by Xe lamp light under magnetic stirring. During irradiation, the quartz cuvettes were taken from the reactor in proper order at given time intervals. The photocatalyst powders and the MB solution were separated by centrifugation. The concentration of MB solution was analyzed through Shimadzu UV-2401PC spectrophotometer. The crystalline phases of the products were analyzed by XRD on a Bruker D8-advance powder X-ray diffractometer (Cu Ka radiation k = 0.15418 nm). SEM images were obtained on a Japan Hitachi S-4800 field emission scanning electron microscopy. TEM and EDXA images were measured on FEI F-30 transmission electron microscopy. UV–vis diffuse reflectance spectra were obtained on an America Varian Cary 5000 spectrophotometer.

3. Results and discussion Fig. 1A(a) shows the XRD pattern of the samples prepared in the typical procedure. All of the diffraction peaks can be indexed to the tetragonal phase of ZnWO4 by comparison with the data from JCPDS card No. 15-0744. The strong and sharp peaks suggest that the products were well-crystallized. There is no obvious change in the crystallinity and particle sizes of ZnWO4. It is noted that the XRD patterns of the silver deposited samples illustrate that no any diffraction peaks of Ag species can be observed, which may attribute to their small crystallite size or low concentration of Ag, similar results were reported by Yu [1]. Secondly, the peaks of Ag were always in the same position of ZnWO4. Therefore, no diffraction peaks of Ag was found in the XRD pattern.

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Fig. 2 shows the SEM and TEM images of the as-obtained ZnWO4/Ag prepared in the typical procedure. As shown in Fig. 2a, there are lots of aggregated yolk–shell microspheres with cores and shells are separated. The magnified SEM image of a single yolk–shell microsphere (Fig. 2b) reveals that the outer diameter of the spheres is about 1.5 lm and the inner hollow core diameter is about 1 lm. The core and the shell of the microsphere are not smooth, actually, it consists of a lot of tiny nanoparticles with mean size of 100 nm. Fig. 2c-d shows the TEM images of ZnWO4/ Ag (10 wt%). It can be clearly found that samples are surfacerough microspheres with diameter about 1.5 lm, and there is a strongly contrastive difference between the core (dark) and the shell (bright). The core was not fixed in the center of the microsphere, as shown in the close-up image (Fig. 2d), which is consistent with the corresponding SEM images. The HRTEM image (Fig. 2e) shows that some small spherical particles (with mean diameter about 10 nm) were observed on the rough surface, indicating that the silver particles were deposited on the surface of ZnWO4. The EDXA spectrum (Fig. 1b) showed peaks for O, Zn, W and Ag. Therefore, the yolk-shell ZnWO4 can be transformed to ZnWO4/Ag while the yolk-shell structures are perfectly retained via photochemical deposition of Ag onto the surface of ZnWO4. A series of controllable experiments were carried out to obtain a better understanding of the formation process of ZnWO4/Ag yolk– shell microspheres. Keeping all parameters constant except the amount of AgNO3 and solvent, the corresponding XRD patterns of the products are shown in Fig. 1A(c–e). In our case, all silver deposited samples are tetragonal phase of ZnWO4. The crystallinity and particle sizes of SE and Sw do not show obvious change. The UV–vis diffuse reflectance spectra of the as-prepared samples are shown in Fig. 3a. The absorption from 220 to 350 nm is assigned to the absorption of ZnWO4 semiconductor. Compared with the absorption spectra of pure ZnWO4, strong peaks centered at 350–400 nm are observed over the ZnWO4/Ag sample, which is the characteristic of surface plasma absorption corresponding to Ag nanoparticle of the sample [1,8–10]. This result further verifies the formation of Ag nanoparticle in ZnWO4. From the UV–vis DRS spectra, theoretically, the as-obtained ZnWO4/Ag sample might have photocatalytic activities to organic dyes under visible-light irradiation. The photocatalytic properties of ZnWO4/Ag were investigated by the decomposition of MB under 500 W Xe lamp irradiation. MB concentration ratio (C/C0) as a function of irradiation time for SE, SW and pure ZnWO4 yolk-shell microspheres under the same condition are plotted as Fig. 3b. The degradation of MB is very slow without any photocatalyst. Compared with photocatalytic activities of other four samples, SE, ZnWO4/10%Ag shows the best

Fig. 1. XRD patterns (A) and EDXA spectrum (B) of the products.

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J. Zhu et al. / Materials Letters 190 (2017) 60–63

Fig. 2. SEM (a–b) and TEM images (c–e) of the products obtained from the typical process.

a

pure ZnWO4

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without catalyst ZnWO4

Sw,ZnWO4/5%Ag Sw,ZnWO4/10%Ag

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SE,ZnWO4/5%Ag

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300

400

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600

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700

800

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SE,ZnWO4/10%Ag 0

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Fig. 3. The UV–vis diffuse reflectance spectra (DRS) of the yolk–shell ZnWO4 and ZnWO4/Ag microspheres (a), the photodegradation efficiencies of MB as a function of irradiation time (b).

efficiency, implying this kind of yolk-shell structured microspheres have excellent photocatalytic activity under visible-light irradiation. The enhanced photocatalytic activity of the ZnWO4/10%Ag is the synergistic results of many factors. Firstly, it is well known that surface hydroxyl radical (OH) is an important factor to affect the photocatalytic activity. The greater number of surface hydroxyl groups present on the photocatalyst surface, the faster the photocatalytic reaction takes place. Secondly, when Ag is introduced into the ZnWO4 samples, the Ag nanoparticles can act as electron traps promoting the electron–hole separation and subsequent transfer the trapped electron to the adsorbed O2, which acting as an electron acceptor on the surface of the ZnWO4 [1,10]. The generated  O2 could also effectively oxidize the MB. Finally, the retained porous surface and the hollow interior cavity of the yolk–shell ZnWO4/Ag microspheres play a crucial role in the improvement of the photocatalytic activities.

photocatalytic activity. The obtained ZnWO4/Ag microspheres exhibited greatly enhanced visible-light photocatalytic activity for photodegradation of MB solution due to the synergistic effect of yolk-shell structure with large surface area, porous hollow sphere structure and the hybridized Ag nanoparticles, which suggests that the yolk-shell ZnWO4/Ag microspheres photocatalyst is promising for practical application. This work provides an intriguing way in designing low-cost, high-performance photocatalytic materials through noble metal doping.

4. Conclusions

References

Yolk-shell ZnWO4/Ag microspheres with different Ag doping amount were successfully synthesized through a simple photochemical process. The 10% Ag doped sample exhibited the best

Acknowledgments This work was supported by National Natural Science Foundation of China (Nos. 21501100, 21376124, 21476117), Innovative Training Program for undergraduates (No. 201610304018Z) and Qinglan Project of Jiangsu Province.

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