Novel magnetic Fe3O4/ZnO/NiWO4 nanocomposites ...

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Separation and Purification Technology 184 (2017) 334–346

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Novel magnetic Fe3O4/ZnO/NiWO4 nanocomposites: Enhanced visible-light photocatalytic performance through p-n heterojunctions Aziz Habibi-Yangjeh ⇑, Maryam Shekofteh-Gohari Department of Chemistry, Faculty of Science, University of Mohaghegh Ardabili, P.O. Box 179, Ardabil, Iran

a r t i c l e

i n f o

Article history: Received 4 February 2017 Received in revised form 14 March 2017 Accepted 5 May 2017 Available online 6 May 2017 Keywords: Fe3O4/ZnO/NiWO4 Magnetic photocatalyst p-n heterojunction Visible-light-driven

a b s t r a c t Fe3O4/ZnO/NiWO4 nanocomposites with p-n heterojunctions were synthesized through a facile refluxing method at 96 °C. The as-prepared samples were characterized using XRD, EDX, SEM, TEM, UV–vis DRS, FT-IR, PL, and VSM instruments. It was found that in the nanocomposites, Fe3O4, ZnO, and NiWO4 components were uniformly combined to each other. Photocatalytic activity of the nanocomposites was evaluated by degradation of rhodamine B under visible-light irradiation, revealing that the nanocomposites exhibit enhanced photocatalytic activity compared to the Fe3O4/ZnO and Fe3O4/NiWO4 samples. Photocatalytic activity of the Fe3O4/ZnO/NiWO4 (40%) nanocomposite was enhanced 36 and 6.3-times relative to the Fe3O4/ZnO and Fe3O4/NiWO4 samples, respectively. This enhancement was explained by the efficient separation of the photogenerated electron–hole pairs due to formation of p-n heterojunctions between NiWO4 and ZnO semiconductors. Additionally, it was found that h+ and  O 2 species generated in the photocatalytic process played a key role in the degradation reaction. More importantly, the nanocomposite can be separated from the reaction media by applying an external magnetic field and it can be reused for five cycles without significant changes in the degradation efficiency. Ó 2017 Elsevier B.V. All rights reserved.

1. Introduction With the population growth and industrial development in the past decades, the environment has seriously deteriorated [1]. It is well known that water pollution considerably influences on our living standards. According to World Health Organization, about 780 million people have no access to affordable drinking water [2]. On the other hand, sources of fresh water is strongly limited. Hence, purification of wastewaters from industrial, agricultural, and pharmaceutical sources has attracted much attention from the research communities. For this purpose, different strategies such as physical, chemical, and biological methods have been used for decades [3,4]. Heterogeneous photocatalytic processes, as one of the most effective and green methods of advanced oxidation processes, are promising procedures to manipulate different environmental concerns [5]. In photocatalytic processes using semiconductors, one can ideally degrade wide variety of organic pollutants to non-hazardous components. Among various photocatalysts, ZnO has extensively employed, owing to its attractive properties such as nontoxicity, low price, and high stability [6]. Unfortunately, this wide band gap semiconductor mainly absorbs

⇑ Corresponding author. E-mail address: [email protected] (A. Habibi-Yangjeh). http://dx.doi.org/10.1016/j.seppur.2017.05.007 1383-5866/Ó 2017 Elsevier B.V. All rights reserved.

UV fraction of the solar irradiation with nearly 4% of its total energy, restricting its industrial application [7]. Hence, a large number of attempts have been paid to achieve higher visiblelight absorption capacity of ZnO through doping different elements and combining narrow band gap semiconductors [8–19]. In addition, recombination of the photogenerated electron-hole pairs decreases efficiency of the photocatalytic processes. To overcome this problem, an appropriate combination of semiconductors are generally applied [20–25]. Metal tungstate semiconductors with formula of MWO4 (M is cation of transition metals such as Fe, Co, Ni, and Cu with +2 charge) have a wide range of applications including gas sensors, optical fibers, humidity sensors, pigments, and catalysts [26–32]. Nickel tungstate is an intrinsic p-type semiconductor with a narrow band gap of 2.20 eV [27,28]. Very recently this semiconductor was prepared and applied as catalyst, heterogeneous photocatalyst for degradation of pollutants, antimicrobial material, and generation of hydrogen through water splitting [33–35]. As a p-type semiconductor, NiWO4 has a potential to form p-n heterojunction with ZnO. Hence, it seems that by formation of p-n heterojunction between NiWO4 (as a narrow band gap semiconductor) and ZnO (as a wide band gap semiconductor), not only significant numbers of electron-hole pairs could be produced under visible-light irradiation, but also the photogenerated charge carriers could be

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efficiently separated, resulting in highly enhanced photocatalytic performance [36–39]. On the other hand, the separation and recycling of the applied catalysts are still a serious problem for the heterogeneous photocatalytic processes. Fabrication of magnetically separable photocatalysts are convenient strategy to overcome this problem using an external magnetic field [40]. In this paper, novel magnetic Fe3O4/ZnO/NiWO4 nanocomposites with highly enhanced photocatalytic activity are reported. The as-prepared samples were characterized for the structural, morphological, optical, and magnetic properties using X-ray diffraction (XRD), energy dispersive analysis of X-rays (EDX), scanning electron microscopy (SEM), transmission electron microscopy (TEM), UV–vis diffuse reflectance spectroscopy (DRS), Fourier transform-infrared spectroscopy (FT-IR), photoluminescence spectroscopy (PL), and vibrating sample magnetometer (VSM). For evaluation photocatalytic activity of the prepared nanocomposites under visible-light irradiation, three dye pollutants of rhodamine B (RhB), methylene blue (MB) and methyl orange (MO) were applied. The matching of energy-band structures between ZnO and NiWO4 semiconductors through p-n heterojunctions induced an efficient way for transfer of the photogenerated charge carriers, resulting in the subsequent promotion of photocatalytic activity. Meanwhile, through combining of Fe3O4 nanoparticle with ZnO and NiWO4 semiconductors, the novel ternary photocatalyst showed magnetically separability. In addition, the prepared nanocomposites showed considerably activity in degradation of different dye pollutants under visible-light irradiation. Finally, a plausible mechanism for the enhanced activity was proposed for the ternary nanocomposites. 2. Experimental 2.1. Materials All reagents were at least of analytical grade and used without further purification. Some chemicals, such as zinc nitrate (Zn (NO3)24H2O), ferric chloride (FeCl36H2O), ferrous chloride (FeCl24H2O), sodium hydroxide, and nickel nitrate (Ni(NO3)26H2O) were purchased from Loba Chemie Company. Other chemicals such as hydrochloride acid, RhB, MO, and MB, sodium tungstate (Na2WO4H2O), 2-propanol, ammonium oxalate, benzoquinone, and absolute ethanol were supplied by Merck Company. All experiments were carried out in deionized water. 2.2. Instruments The XRD patterns were recorded by a Philips Xpert X-ray diffractometer with Cu Ka radiation (k = 0.15406 nm), employing scanning rate of 0.04°/s. Surface morphology and distribution of

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particles were studied by LEO 1430VP SEM, using an accelerating voltage of 15 kV. The purity and elemental analysis of the products were obtained by EDX on the same SEM instrument. The TEM investigations were performed by a Philips CM30 instrument with an acceleration voltage of 150 kV. The DRS spectra were recorded by a Scinco 4100 apparatus. The FT-IR spectra were obtained by a Perkin Elmer Spectrum RX I apparatus. The PL spectra of the samples were studied using a Perkin Elmer (LS 55) fluorescence spectrophotometer with an excitation wavelength of 300 nm. UV–vis spectra for the degradation reaction were studied using a Cecile 9000 spectrophotometer. Magnetic properties of the samples were obtained using a VSM instrument (Meghnatis Kavir Kashan Co., Iran). The ultrasound radiation was performed using a Bandelin ultrasound processor HD 3100 (12 mm diameter Ti horn, 75 W, 20 kHz). The pH of solutions was measured using a Metrohm digital pH meter of model 744. 2.3. Preparation of the samples Nanoparticles of Fe3O4 were prepared using a chemical coprecipitation process described elsewhere [41]. The Fe3O4/ZnO (1:4) nanocomposite was prepared using the method reported by our research group [42]. For combining NiWO4 to Fe3O4/ZnO nanocomposite, 0.3 g of the Fe3O4/ZnO nanocomposite was dispersed in 150 mL of water under ultrasonic irradiation for 5 min. Then, 0.189 g of nickel nitrate was added and the suspension was mechanically stirred 30 min. Afterwards, 0.214 g of sodium tungstate was separately dissolved in water. Then, the solution was added dropwise into the Fe3O4/ZnO suspension under constant stirring for 30 min. Thereafter, the suspension was refluxed at 96 °C for 2 h. The formed Fe3O4/ZnO/NiWO4 nanocomposite was washed with water and ethanol for several times and recovered by magnetic separation. Finally, the dried particles were calcined at 450 °C for 3 h in air atmosphere (Scheme 1). 2.4. Photocatalysis experiments Photocatalysis experiments were performed under a LED lamp of 50 W, as visible-light source. Experiments were carried out in a glass reactor with a circulating water system to maintain the temperature at 25 °C. In each test, 0.1 g of the photocatalyst was added into 250 mL of the solution containing RhB, MB, or MO with concentration of 1  105 M. Then, the suspension was stirred in the dark for 60 min to achieve the adsorption–desorption equilibrium prior to visible-light irradiation. During the light irradiation, about 2 mL of the suspension was taken out and the photocatalyst removed before measurement concentration of the dyes at 553, 664, and 477 nm corresponding to the maximum absorption wavelengths of RhB, MB, and MO, respectively.

Scheme 1. Schematic illustration for the preparation of Fe3O4/ZnO/NiWO4 nanocomposites.

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Fig. 1. XRD patterns for the Fe3O4/ZnO, Fe3O4/NiWO4, and Fe3O4/ZnO/NiWO4 nanocomposites with different weight percents of NiWO4 calcined at 450 °C.

Fig. 2. (a) EDX spectra for the Fe3O4/ZnO, Fe3O4/NiWO4, and Fe3O4/ZnO/NiWO4 (40%) samples. (b)–(g) EDX mapping for the Fe3O4/ZnO/NiWO4 (40%) nanocomposite.

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Fig. 3. SEM images for the (a) Fe3O4/ZnO, (b) Fe3O4/NiWO4, and (c) Fe3O4/ZnO/NiWO4 (40%). (d) TEM image of the Fe3O4/ZnO/NiWO4 (40%) nanocomposite.

3. Results and discussion The phase structure and crystallinity of the as-prepared Fe3O4/ ZnO, Fe3O4/NiWO4, and Fe3O4/ZnO/NiWO4 nanocomposites with different weight percentages of NiWO4 were investigated by XRD analysis and the results are given in Fig. 1. The diffraction peaks of the Fe3O4 nanoparticles consistent with cubic structure (JCPDS file number of 75-1610) [43]. In addition, crystalline phase of ZnO with wurtzite hexagonal structure (JCPDS 75-0457) was obtained [44]. After loading of NiWO4 over the Fe3O4/ZnO nanocomposite, the Fe3O4/ZnO/NiWO4 nanocomposites were successfully fabricated. As can be seen in Fig. 1, all compositions of the Fe3O4/ZnO/NiWO4 nanocomposites exhibit almost the same XRD patterns containing simultaneously cubic Fe3O4, wurtzite ZnO, and monoclinic structure of NiWO4 (JCPDS card No. 150755) [27], indicating coexistence of Fe3O4, ZnO, and NiWO4 in the ternary nanocomposite. To confirm purity of the Fe3O4/ZnO, Fe3O4/NiWO4, and Fe3O4/ ZnO/NiWO4 (40%) samples, their EDX spectra were provided, exhibiting the presence of Fe, O, and Zn elements for the Fe3O4/ ZnO and Fe, O, Ni, and W elements for the Fe3O4/NiWO4 sample and Fe, O, Zn, Ni, and W elements for the Fe3O4/ZnO/NiWO4 (40%) sample. The contents of elements in the Fe3O4/ZnO/NiWO4 (40%) nanocomposite are 34.5 (Zn), 9.79 (Fe), 8.51 (Ni), 27.8 (W), and 19.4% (O). The distribution of elements in the Fe3O4/ZnO/ NiWO4 (40%) nanocomposite is also determined by EDX elemental mapping. As shown in Fig. 2(b–g), the highly uniform distribution of Fe, O, Zn, Ni, and W elements suggests that components of the

Fe3O4/ZnO/NiWO4 (40%) nanocomposite have homogeneously distributed in this sample, confirming formation of the ternary nanocomposite. The morphologies of Fe3O4/ZnO, Fe3O4/NiWO4, and Fe3O4/ZnO/ NiWO4 (40%) samples were analysed by SEM and TEM instruments. It could be clearly seen that morphology and aggregation of these samples are considerably different from each other. In addition, TEM image (Fig. 3d) is given for further proving the morphology of the Fe3O4/ZnO/NiWO4 (40%) nanocomposite. It is evident that in the Fe3O4/ZnO/NiWO4 (40%) nanocomposite, particles of Fe3O4 and NiWO4 have distributed over spindle-like particles of ZnO. It is well known that optical absorption properties of photocatalysts play a significant role in their photocatalytic activities [5,7]. Thus, UV–vis absorption spectra of the Fe3O4/ZnO, Fe3O4/NiWO4, and Fe3O4/ZnO/NiWO4 nanocomposites with different weight percentages of NiWO4 were measured by UV–vis DRS spectra in the wavelength range of 270–770 nm (Fig. 4a). Compared with the Fe3O4/ZnO nanocomposite, the Fe3O4/ZnO/NiWO4 nanocomposites show a broader absorption in the visible region and the increased absorption intensity in this range. When the absorption intensity increases, the formation rate of electron-hole pairs on the photocatalyst surface also increases, leading to the higher photocatalytic activity [5,7]. The red-shift of the absorption threshold may be related to the formation of heterojunctions between counterparts of the ternary nanocomposite. FT-IR spectroscopy was also used to investigate formation of the photocatalysts. The FT-IR spectra of the synthesized Fe3O4/ZnO, Fe3O4/NiWO4, and Fe3O4/ZnO/NiWO4 (40%) samples are shown in

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Fig. 4. (a) UV–vis DRS spectra for the Fe3O4/ZnO, Fe3O4/NiWO4, and Fe3O4/ZnO/NiWO4 nanocomposites with different weight percents of NiWO4. (b) FT-IR spectra for the Fe3O4/ZnO, Fe3O4/NiWO4, and Fe3O4/ZnO/NiWO4 (40%) samples.

Fig. 4b. As could be seen in these spectra, all samples show wide absorption peaks at 3300–3500 cm1 and small peak at 1660 cm1, which they are related to the stretching and bending vibrations of OAH groups of adsorbed water molecules over the samples, respectively. The bands appeared at 876 and 820 cm1 arise from vibration of the WO2 entity present in the W2O8 groups [33]. In addition, the characteristic absorption peaks at 430 and 512 cm1 can be assigned to the FeAO stretching vibrations [45]. Finally, ZnO exhibits characteristic absorption peak at 602 cm1 which are assigned to ZnAO vibration, confirming the presence of ZnO in the nanocomposites [46]. The magnetic properties of Fe3O4 and Fe3O4/ZnO/NiWO4 (40%) samples were displayed in Fig. 5. The saturation magnetizations of the Fe3O4 and Fe3O4/ZnO/NiWO4 (40%) samples were 53.4 and 5.88 emu/g, respectively. It was shown that saturation magnetizations of the Fe3O4 nanoparticles was significantly higher than that of the Fe3O4/ZnO/NiWO4 (40%) nanocomposite, which was attributed to the presence of the nonmagnetic materials in the Fe3O4/ZnO/ NiWO4 (40%) nanocomposite. However, saturation magnetization of the nanocomposite was good enough to easily recover it from

its suspension by applying an external magnetic field, as shown in the inset of Fig. 5. Hence, the nanocomposite can be almost completely recovered with a minimum loss from the treated systems. The photocatalytic activity of the as-prepared samples was evaluated by degradation aqueous solution of RhB under visiblelight irradiation. As can be seen, in absence of any photocatalyst (photolysis condition), the concentration of RhB changes a little, indicating stability of RhB under visible light. Prior to the illumination, adsorption of RhB over the photocatalysts was checked for 60 min and it was found to be negligible. For the Fe3O4/ZnO and Fe3O4/NiWO4 samples, only 14.1 and 52.3% of RhB were degraded during the visible-light irradiation for 300 min. Interestingly, it was found that the Fe3O4/ZnO/NiWO4 nanocomposites showed enhanced photodegradation activity compared to the Fe3O4/ZnO and Fe3O4/NiWO4 samples under visible-light irradiation. The optimal photocatalytic activity was exhibited by the Fe3O4/ZnO/NiWO4 (40%) nanocomposite with 97.9% of RhB degradation during the light irradiation for 300 min. UV–vis spectral data taken during the degradation reactions (Fig. 6b–e) show that the main characteristic absorption band of

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Fig. 5. Magnetization curves for the Fe3O4 nanoparticles and Fe3O4/ZnO/NiWO4 (40%) nanocomposite. Inset of the figure shows the photograph of a separation process by using a magnet.

RhB has located at 553 nm. As can be seen, the absorbance at this wavelength gradually decrease and there are not any hypsochromic shifts. The hypsochromic shift is observed in the case of N-deethylation of RhB during its photocatalytic degradation reaction [47,48]. Hence, it was concluded that the degradation reaction of RhB over the prepared nanocomposites takes place through the cleavage process of whole conjugated chromophore. To quantify the degradation activity of the prepared samples, the degradation kinetics of RhB over the photocatalysts was investigated by fitting the experimental data to the LangmuirHinshelwood model:

Rate ¼ 

d½RhB kK½RhB ¼ dt 1 þ K½RhB

ð1Þ

In which, K and k are thermodynamic adsorption constant and degradation rate constant of RhB over the photocatalysts, respectively. Due to low initial concentration of RhB, we have K[RhB]  0. Hence, Eq. (1) is converted to a pseudo first-order reaction:

ln½RhB=½RhBo ¼ kobs t

ð2Þ

where [RhB]0 and [RhB] are RhB concentrations at times t = 0 and t = t, respectively, and kobs is the pseudo-first-order rate constant. The degradation rate constants of RhB over the Fe3O4/ZnO, Fe3O4/ NiWO4, and Fe3O4/ZnO/NiWO4 (40%) were 3.76  104, 22.0  104, and 120  104 min1, respectively. Thus, it is found that photocatalytic activity of the Fe3O4/ZnO/NiWO4 (40%) nanocomposite is nearly 32 and 5.4 times of the Fe3O4/ZnO and Fe3O4/NiWO4 nanocomposites, respectively, which indicates that the introduction of NiWO4 over the Fe3O4/ZnO nanocomposite could strongly enhance the photocatalytic activity under visible light. It is well known that interactions between components of a photocatalyst is important for the formation of heterojunctions in the prepared nanocomposite and these heterojunctions have a major role in separation of the photogenerated electron–hole pairs [5,7]. In the case of the Fe3O4/ZnO/NiWO4 (40%) nanocomposite, the enhanced separation efficiency of electron–hole pairs relative to the other samples was confirmed using PL spectra. Fig. 7b shows the PL spectra for the Fe3O4/ZnO, Fe3O4/NiWO4, Fe3O4/ZnO/NiWO4 (40%), and Fe3O4/ZnO/NiWO4 (50%) samples to verify the aforementioned

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hypothesis. It is evident that these samples exhibit broad band emission appearing around 350–460 nm. As can be observed, the emission intensity of the Fe3O4/ZnO nanocomposite is very higher than those of the Fe3O4/NiWO4, Fe3O4/ZnO/NiWO4 (40%), and Fe3O4/ZnO/NiWO4 (50%) samples. In addition, emission intensity of the Fe3O4/ZnO/NiWO4 (40%) nanocomposite is lower than that of the Fe3O4/NiWO4 nanocomposite. Hence, formation of heterojunctions between the counterparts is effective in separation of the photogenerated charge carriers in the ternary Fe3O4/ZnO/ NiWO4 (40%) nanocomposite relative to the corresponding binary samples. Interestingly, it was observed that with loading more NiWO4 over the Fe3O4/ZnO nanocomposite, more decrease for separation of the electron-hole pairs was not observed. For this reason, photocatalytic activity of the Fe3O4/ZnO/NiWO4 (40%) nanocomposite is higher than that of the Fe3O4/ZnO/NiWO4 (50%) nanocomposite. It is known that photocatalytic activity of photocatalysts mainly depends on extent of electron–hole pairs production and separation efficiency of these charge carriers. Hence, in order to increase photocatalytic activity, two important strategies should be considered. The first one is increasing the number of photogenerated electron-hole pairs and the second one is enhancing separation efficiency of the photoexcited charges [5,7]. Interestingly, both of the above-mentioned strategies take place in the Fe3O4/ZnO/ NiWO4 nanocomposites via the proposed mechanism as shown in Fig. 8. The conduction band (CB) and valence band (VB) potentials for counterparts of the Fe3O4/ZnO/NiWO4 nanocomposites were estimated using the following equations [49]:

ECB ¼ v  Ee  0:5Eg

ð3Þ

EVB ¼ ECB þ Eg

ð4Þ

where EVB and ECB are the VB and CB potentials, respectively. Moreover, Ee is the energy of free electrons vs. hydrogen (4.5 eV). In addition, v is electronegativity of the semiconductor and it was calculated by Eq. (5):

v ¼ ½xðAÞa xðBÞb xðCÞc 

1=ðaþbþcÞ

ð5Þ

In which a, b, and c are the number of atoms in the compounds [49]. The values of Eg and v for ZnO are 3.20 and 5.76 eV, respectively. Hence, the ECB and EVB of ZnO were calculated to be 0.34 and 2.86 eV versus normal hydrogen electrode (NHE). In addition, the calculated values of ECB and EVB for NiWO4 are 0.684 and 2.88 eV. For p and n-type semiconductors, the Fermi levels are close to VB and CB levels, respectively. Hence, when ZnO particles are coupled with NiWO4 semiconductor, p-n heterojunctions are formed at the interfaces and electrons transfer from ZnO to NiWO4 until their Fermi levels align [36–39]. In the equilibrium state, the p-type NiWO4 has a negative charge while the n-type ZnO has a positive charge. Consequently, an inner electrostatic field is created at the junction interface between these semiconductors. Under visible-light illumination, NiWO4 is excited to produce electrons and holes, due to its narrow band gap. After that, the photogenerated electrons are easily transfer from the CB of NiWO4 to that of ZnO under the force of the produced electric field. Thereafter, the photogenerated electrons react with adsorbed oxygen and water molecules on the surface of the nanocomposite to produce reactive  species of O 2 and OH [50]. Meanwhile, the photogenerated holes are trapped by adsorbed H2O and OH to produce OH species. Based on the results obtained from PL spectra, it was concluded that separation of the photogenerated electron-hole pairs are enhanced through heterojunctions formed between ZnO and NiWO4 counterparts of the nanocomposite, leading to production of more reactive species have vital role in the degradation reaction.

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Fig. 6. (a) Photodegradation of RhB over the Fe3O4, Fe3O4/ZnO, Fe3O4/NiWO4, and Fe3O4/ZnO/NiWO4 nanocomposites with different weight percents of NiWO4. UV–Vis spectra for degradations of RhB under visible-light irradiation over the (b) Fe3O4, (c) Fe3O4/ZnO, (d) Fe3O4/NiWO4, and (e) Fe3O4/ZnO/NiWO4 (30%) nanocomposites.

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Fig. 7. (a) The degradation rate constants of RhB over different samples. (b) PL spectra for the Fe3O4/ZnO, Fe3O4/NiWO4, and Fe3O4/ZnO/NiWO4 (40%) samples.

Fig. 8. Proposed mechanism for the enhanced photocatalytic activity of Fe3O4/ZnO/NiWO4 nanocomposites.

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Fig. 9. (a) The degradation rate constant of RhB over the Fe3O4/ZnO/NiWO4 (40%) nanocomposite calcined at different temperatures. (b) XRD patterns for the Fe3O4/ZnO/ NiWO4 (40%) nanocomposite calcined at different temperatures.

Hence, the photocatalytic activity is greatly enhanced in the Fe3O4/ ZnO/NiWO4 nanocomposites than the binary Fe3O4/ZnO and Fe3O4/ NiWO4 samples. Calcination temperature could remarkably affect phase structure, crystallinity, and grain size of photocatalysts, resulting in considerable changes of photocatalytic activity. To achieve high photocatalytic activity of the Fe3O4/ZnO/NiWO4 nanocomposite, the effect of calcination temperature on the photocatalytic activity was evaluated by the degradation of RhB over the Fe3O4/ZnO/ NiWO4 (40%) nanocomposite calcined at different temperatures and the results are shown in Fig. 9a. It can be seen that the calcination temperature has a significant contribution to the photocatalytic activity of this nanocomposite. With increasing the calcination temperature, the photocatalytic activity considerably increases and reaches maxima at 450 °C. The effects of the calcination temperature on the phase structure and crystallinity of the Fe3O4/ZnO/NiWO4 (40%) nanocomposite were examined using

XRD analysis and the results are shown in Fig. 9b. It is obvious that by calcination of the nanocomposite at temperatures higher than 350 °C, the nanocomposite is formed by high crystallinity. Furthermore, intensity of the XRD peaks remarkably increases by increasing the calcination temperature. Hence, it was concluded that crystallinity of the nanocomposite enhances with increasing calcination temperature. Enhancement of the photocatalytic activity up to 450 °C is ascribed to increase of the photocatalyst crystallinity. But with further increase of the calcination temperature, a slight decrease in the photocatalytic activity was observed. This decrease could be related to destruction of the p-n heterojunctions formed between NiWO4 and ZnO semiconductors, leading to decrease of the charge carrier separations efficiency. But, by calcination at 250 °C, WO3 is formed instead of NiWO4. Hence, it was concluded that for preparation of the Fe3O4/ZnO/NiWO4 nanocomposites, a calcination process at temperatures higher than 250 °C are needed [51].

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Fig. 10. (a) The degradation rate constants of RhB over the Fe3O4/ZnO/NiWO4 (40%) nanocomposite prepared at different refluxing times. (b) PL spectra for the Fe3O4/ZnO/ NiWO4 (40%) nanocomposite prepared at different refluxing times.

In order to explore the effect of another preparation condition on the photocatalytic activity of the Fe3O4/ZnO/NiWO4 (40%) nanocomposite, the photocatalyst prepared by refluxing for 30, 60, 120, and 240 min and the degradation reactions were explored over them (Fig. 10a). It is obviously clear that photocatalytic activity of the nanocomposite prepared by refluxing for 60 min is higher than that of the sample prepared in 30 min. Interestingly, by more increase of the preparation time, the degradation rate constant continuously decreases. In order to investigate the effect of preparation time on separation of the charge carriers, PL spectra of the nanocomposite prepared in different refluxing times were provided (Fig. 10b). As can be observed, the Fe3O4/ZnO/NiWO4 nanocomposite prepared at 60 min showed the weakest PL intensity, indicating that it has the best charge carrier separation efficiency, leading to enhanced photocatalytic activity for the sample prepared by refluxing at this time. The reactive species trapping experiments were performed to investigate the role of active species in the photocatalytic

degradation reaction of RhB over the Fe3O4/ZnO/NiWO4 (40%) nanocomposite. In these experiments, some scavengers were added to the RhB solution in order to suppress the photocatalytic reaction. The extent of decrease in the degradation reaction caused by any scavenger indicates the importance of the corresponding reactive species. As shown in Fig. 11, the addition of benzoquinone + (BQ, quencher of O 2 ) and ammonium oxalate (AO, quencher of h ) lead to considerable decrease of the degradation rate constant, + indicating that O 2 and h are the main active species in the photocatalytic process. Meanwhile, 2-propanol (2-PrOH, quencher of  OH) shows small effect on the reaction of RhB degradation, implying that OH species do not have strong effect of the degradation reaction. In the present study, to display ability of the ternary nanocomposite to degrade different pollutants, we examined photocatalytic performance of the Fe3O4/ZnO/NiWO4 (40%) prepared by refluxing for 60 min, for the degradations of MB and MO and the results along with those of RhB are shown in Fig. 12. For RhB, MB, and

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Fig. 11. The degradation rate constants of RhB over the Fe3O4/ZnO/NiWO4 (40%) nanocomposite in presence of various scavengers.

MO degradations, the corresponding degradation rate constants over the Fe3O4/ZnO/NiWO4 (40%) nanocomposite were calculated to be 138  104, 85.5  104, and 61.3  104 min1, which are faster than those of the Fe3O4/ZnO (3.76  104, 4.24  104, and 2.89  104 min1) and the Fe3O4/NiWO4 samples (22.0  104, 14.6  104, and 9.67  104 min1). As a result, activity of the ternary nanocomposite in degradations of RhB is almost 36 and 6.3-folds, in degradations of MB is nearly 20 and 5.8-folds and finally in of degradations of MO is about 21 and 6.3-folds higher than those of the Fe3O4/ZnO and Fe3O4/NiWO4 samples, respectively. Hence, the results indicate that the ternary Fe3O4/ZnO/ NiWO4 (40%) nanocomposite has interestingly high photocatalytic activity in degradations of different dye pollutants. Recycling experiments are important to show possibility of long time performance of photocatalysts. Therefore to investigate the regeneration and reusability of the Fe3O4/ZnO/NiWO4 (40%) nanocomposite, RhB degradation reactions over this photocatalyst were carried out for five successive runs. The photocatalyst was collected by an external magnet after each recycling runs, and then

140

RhB

80

MB

kobs × 10-4 (min-1)

kobs × 10-4 (min-1)

120 100 80 60

60

40

40 20 20 0

0

60

MO

kobs × 10-4 (min-1)

50 40 30 20 10 0

Fig. 12. The degradation rate constants of RhB, MB, and MO dyes over the Fe3O4/ZnO, Fe3O4/NiWO4, and Fe3O4/ZnO/NiWO4 (40%) samples under visible-light irradiation.

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Fig. 13. Reusability of the Fe3O4/ZnO/NiWO4 (40%) nanocomposite in degradation of RhB in four successive runs.

washed with water and ethanol for two times (Fig. 13). It can be seen from this figure that activity of the ternary nanocomposite maintained even after five recycling runs, indicating no considerable deactivation. Therefore, it can be concluded that the magnetically separable ternary photocatalyst is highly stable and it can be used in treatment of wastewaters containing various dye pollutants. 4. Conclusions This paper reports the fabrication of ternary Fe3O4/ZnO/NiWO4 nanocomposites using refluxing method and they were effectively used as magnetically separable visible-light-induced photocatalyst. The as-synthesized Fe3O4/ZnO/NiWO4 (40%) nanocomposite showed greatly enhanced photocatalytic activity for degradations of three dye pollutants (RhB, MB, and MO) under visible-light irradiation in comparison to the binary Fe3O4/ZnO and Fe3O4/NiWO4 nanocomposites. The enhanced activity of the ternary nanocomposite in degradations of MB is about 20 and 5.8-folds higher than those of the Fe3O4/ZnO and Fe3O4/NiWO4 nanocomposites, whereas it is nearly 21 and 6.3-folds in degradations of MO, respectively. It was confirmed that formation of p-n heterojunctions between NiWO4 and ZnO semiconductors played a major role in efficient separation and transfer of photogenerated charge carrier, leading to remarkably elevation of photocatalytic activity. Acknowledgements The authors gratefully acknowledge University of Mohaghegh Ardabili-IRAN, for financial support of this work. References [1] W. Xiao Ping, S. Dian Chao, Y. Tan Dong, Climate change and global cycling of persistent organic pollutants: A critical review, Sci. China 59 (2016) 1899– 1911. [2] UNICEF, Niang, World Health Organization, and UNICEF. WHO (2012). Progress on Drinking Water and Sanitation, 2012. [3] C.R. Holkar, A.J. Jadhav, D.V. Pinjari, N.M. Mahamuni, A.B. Pandit, A critical review on textile wastewater treatments: Possible approaches, J. Environ. Manage. 182 (2016) 351–366.

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