Microwave synthesis of magnetically separable ZnFe2O4-reduced graphene oxide for wastewater treatment

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CERAMICS INTERNATIONAL

Ceramics International 40 (2014) 7057–7065 www.elsevier.com/locate/ceramint

Microwave synthesis of magnetically separable ZnFe2O4-reduced graphene oxide for wastewater treatment F.A. Jumeria, H.N. Lima,b,n, S.N. Ariffina, N.M. Huangc, P.S. Teoc, S.O. Fatinc, C.H. Chiad, I. Harrisone b

a Department of Chemistry, Faculty of Science, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor Darul Ehsan, Malaysia Functional Device Laboratory, Institute of Advanced Technology, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysia c Low Dimensional Materials Research Centre, Department of Physics, Faculty of Science, University of Malaya, 50603 Kuala Lumpur, Malaysia d School of Applied Physics, Faculty of Science and Technology, Universiti Kebangsaan Malaysia, 43600 Bangi, Selangor, Malaysia e Faculty of Engineering, The University of Nottingham Malaysia Campus, Jalan Broga, 43500 Semenyih, Selangor, Malaysia

Received 2 September 2013; received in revised form 10 December 2013; accepted 10 December 2013 Available online 17 December 2013

Abstract A magnetically separable ZnFe2O4-reduced graphene oxide (rGO) nano-composite was synthesised via a microwave method. Field emission scanning electron microscopy images of the nano-composite showed a uniform dispersion of nanoparticles on the rGO sheets. The performance of the nano-composite in wastewater treatment was assessed by observing the decomposition of methylene blue. The nano-composite showed excellent bifunctionality, i.e. adsorption and photocatalytic degradation of methylene blue, for up to five cycles of water treatment when illuminated with light from a halogen bulb. In contrast, water treatment with the nano-composite without illumination and the illuminated rGO, with no decoration of nanoparticles, diminished significantly after the first treatment. The reclamation of the ZnFe2O4-rGO nano-composite from treated water could be easily achieved by applying an external magnetic field. & 2013 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: Graphene; Nanoparticles; Microwave; Photocatalysis; Wastewater treatment

1. Introduction Organic effluents from industries, agricultural activities and rapidly developing urban areas pose one of the greatest challenges for wastewater treatment. It is estimated that around four billion people worldwide have no or little access to clean and sanitised water supplies, resulting in millions of deaths attributed to waterborne diseases [1]. Photodegradation of organic pollutants has attracted increasing attention during the past decade [2] because it provides a viable decontamination process with widespread applications and does not necessarily require external power sources since the absorption of solar energy is sufficient to drive the process [3]. n Corresponding author at: Department of Chemistry, Faculty of Science, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor Darul Ehsan, Malaysia. Tel.: þ60 16 3301609. E-mail address: [email protected] (H.N. Lim).

The fundamental process behind photo-catalytic degradation is the involvement of charge at the surface of the catalyst in redox reactions, which decompose the contaminant. When light is absorbed by the catalyst, usually a semiconductor nano-crystal, an electron is excited from the valance band to the conduction band, resulting in an electron–hole pair. Although there is an attractive force between the electron and hole, there is a significant probability that the two particles will drift away from each other before they can recombine. In nano-crystals, especially, there is a strong likelihood that these charges will become fixed at surface defects where they can react with absorbed surface species such as H2O to form free dOH radicals; this promotes the oxidation of organic compounds [4]. However, the technological applications of photocatalysts (TiO2, ZnO, ZnS and SnO2) are limited by their band gap (3.0–3.8 eV) and the requirement for ultraviolet (UV) light to create electron–hole pairs. Solar energy is abundant and

0272-8842/$ - see front matter & 2013 Elsevier Ltd and Techna Group S.r.l. All rights reserved. http://dx.doi.org/10.1016/j.ceramint.2013.12.037

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free, but only 5% of solar energy is in the UV region compared with 45% in the visible region. Therefore, the challenge is to develop photo-catalyst technology such that visible solar light can be used to generate charge at the surface of the catalyst and hence cause the degradation of organic contaminants. ZnFe2O4 is a semiconductor with a spinel crystallographic structure and a narrow band-gap (1.9 eV) [5] when compared with traditional photo-catalysts. Like many spinel structures, the crystal behaves magnetically as a ferrite. It exhibits an excellent visible-light response, good photochemical stability and favourable magnetism. These characteristics make it a potential photo-catalyst that can be magnetically extracted from a suspension system without the addition of magnetic particles. Graphene is a two-dimensional single atomic layer of sp2 carbon atoms densely packed into a honeycomb crystal lattice [6,7]. Graphene has a zero energy band gap and, depending on the position of the Fermi level, which can be adjusted by changing the voltage on a gate, the electron or hole concentration can be made as high as 1013 cm  2 with room temperature mobility around 10,000 cm2/Vs. [8], Mechanically, graphene has a large specific surface area of up to 4 2600 m2/g [7], high mechanical strength, extremely high thermal conductivity [9], and unconventional magnetic properties e.g. it exhibits a fractional quantum Hall effect [10]. These fascinating properties render graphene suitable for many technological applications such as electronics, sensing platforms and energy storage and conversion. Combining the photo-activity of ZnFe2O4 nano-crystals with the absorption properties of graphene into a nano-composite may produce a superior photo-catalyst for the photodegradation of organic effluents in wastewater [11,12]. Metal oxide/graphene nano-composites have been widely reported in the literature, including Ag2O/graphene, CuO/graphene, Cu2O/ graphene, CoO/graphene, Co3O4/graphene, Fe2O3/graphene, Fe3O4/graphene, Mn3O4/graphene, NiO/graphene, SnO2/graphene, TiO2/graphene and ZnO/graphene. These nanocomposites were fabricated using a variety of approaches such as gas/liquid interface reaction, hydrothermal, in situ chemical synthesis, in situ oxidation, laser irradiation, microwave, sonochemical and ultra-sonication [13]. Microwave heating is believed to be more dependent on the molecular properties and the reaction conditions than conventional heating [14]. Microwave synthesis has been increasingly used in the preparation of high mono-dispersity oxide nanoparticles, such as SnO2, CeO2 and ZrO2 [15]. Utilising microwave energy for thermal treatment generally leads to a particle sizes in the range of 15–35 nm because of the shorter synthesis time and highly focused local heating when compared to conventional thermal heating [16]. In this work, a magnetically separable ZnFe2O4-reduced graphene oxide nano-composite was produced using the microwave method. The nano-composites were subjected to wastewater treatment under the illumination of halogen light, where methylene blue (MB) was used as the sacrificial reagent. A 150 W halogen lamp was used as the light source because it has a similar spectrum as sunlight [17,18].

2. Experimental 2.1. Materials Graphite flakes were purchased from Ashbury Carbons NJ, USA. Sulphuric acid (H2SO4, 98%), potassium permanganate (KMnO4, 99.9%), hydrogen peroxide (H2O2, 30%), hydrochloric acid (HCl) and iron (II) and sulphate (FeSO4) were purchased from Systerm, Shah Alam, Malaysia. Sodium hydroxide (NaOH) and zinc chloride (ZnCl2) were purchased from Merck, Malaysia. Methylene blue (MB) was purchased from R&M Chemicals, Malaysia. Deionised water was used throughout the experiment. Reduced graphene oxide (rGO) was synthesised via the simplified Hummers method [19]. 2.2. Preparation of the ZnFe2O4-reduced graphene oxide nano-composite The ZnFe2O4-reduced graphene oxide nano-composite was synthesised using a microwave synthesis technique. 50 ml of deionised water was used to dissolve 0.2 g of NaOH. Then, 2.5 ml of 15 mg/ml of GO, 0.250 g of FeSO4 and 0.0341 g of ZnCl were added one by one into the alkaline solution while stirring. The solution was transferred to a 100 ml Duran bottle and heated using a microwave oven for 30 min. The bottle was open to the atmosphere. The solution was allowed to cool to room temperature and the precipitate (nano-composite) was separated from the solution using a centrifuge. The nanocomposite was washed several times with distilled water to remove the ions before it was characterised. In order to compare the performance of the new nano-composite Fe3O4, ZnO, and ZnFe2O4 nano-crystals, rGO and Fe3O4-rGO and ZnO-rGO nano-composites were also prepared using the same method. 2.3. Characterisations X-ray diffraction patterns were recorded using Cu Kα radiation (λ¼ 1.5405980 A˙ ) at 45 kV and 40 mÅ in the 2θ range of 41–701 using a Philips X'Pert machine. The morphology was characterised by field emission scanning microscopy (FESEM; EI Nova NanoSEM 400) and high resolution transmission electron microscope (HRTEM, JEOL JEM-2100F). An ultraviolet-visible spectrophotometer (UV–vis; UV-1601, Shimadzu) was used to measure the MB solution absorbance. The Raman spectra were recorded on a Renishaw inVia Raman microscope system. Magnetisation measurements were carried out using a vibrating sample magnetometer (VSM). 2.4. Wastewater treatment measurement The treatment of the as-obtained wastewater samples was evaluated by measuring the degradation of MB using the method of Kumar et al. [20]. The suspension was prepared in the dark by adding 10 mg of the nano-composite to 50 ml of MB solution (10 mg/l). The suspension was stirred for 20 min in the dark. The sample was illuminated by placing a 150 W

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Fig. 1. XRD patterns of (a) rGO, GO and graphite; (b) Fe3O4 and Fe3O4-rGO; (c) ZnO and ZnO-rGO; and (d) ZnFe2O4 and ZnFe2O4-rGO.

halogen lamp (Halloid) in close proximity to the Pyrex beaker. After a set time interval, a 2 mL aliquot was taken from the bottle and the concentration of the dye in the sample was monitored by measuring the absorbance of light at 664 nm. All samples were measured for a total of 2 h. To study the recyclability, the nano-composite was recovered from the used solution by applying a magnetic field and cleansed three times with deionised water before being re-dispersed into a fresh dye solution so that its subsequent performance could be assessed. 3. Results and discussion Fig. 1a shows the XRD patterns of graphite, graphene oxide and reduced graphene oxide. The peaks located at 26.41 and 55.31 correspond to the (002) and (004) indices for graphite. The characteristic diffraction peak of GO was observed at (001). This result suggests that the graphite was fully oxidised [21] and the distance between the carbon sheets had increased due to insertion of inter-planar groups. Consequently, the distance between the GO sheets was greater than that of the graphene sheets by 0.632 nm because of the presence of covalently bound oxygen atoms and the displacement of the sp3-hybridised carbon atoms above and below the graphene sheets [22]. The rGO profile shows two peaks at 261 and 431, suggesting restacking of rGO sheets. However, the

distinguishable and broad peak at 141 indicates that GO was only partially reduced after the microwave reaction. The XRD spectra of the Fe3O4 and Fe3O4-rGO samples contain the five typical reflection peaks of Fe3O4. These peaks are at 301, 35.51, 441, 571 and 631 and can be indexed to reflections from the (220), (311), (400), (511) and (440) crystal planes (JCPDF no. 01-075-0033) (Fig. 1b). However, the rGO peak at 141 was not present in the XRD spectrum of Fe3O4-rGO sample; this is explained by the redox reaction between Fe2 þ and GO during the reaction [23]. Meanwhile, ZnO and ZnO-rGO have typical reflection peaks of ZnO (JCPDS no. 01-079-0207) at 31.71, 34.31, 36.21, 47.51, 56.41, 62.81, 67.81 and 68.91, which correspond to ZnO (100), (002), (101), (102), (110), (103), (112) and (201) with hexagonal wurtzite structures (Fig. 1c). The typical peak of GO was also missing for ZnO-rGO, indicating the reduction of GO. The XRD spectra of ZnFe2O4 and ZnFe2O4-rGO are presented in Fig. 1d. The peaks at 301, 35.31, 42.91, 53.31, 56.81 and 62.41 are assigned to reflections from the (220), (311), (400), (422), (511) and (440) crystal planes of cubic spinel ZnFe2O4 (JCPDS no. 01077-0011). The broad peak of GO was again not detected, which implies that GO was reduced during the reaction. The Raman spectra of rGO and all the nano-composites display two prominent peaks at 1350 cm  1 and 1590 cm  1, which are the D and G bands, respectively (Fig. 2a). The D band corresponds to structural defects and disorders of the

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Fig. 2. Raman spectra of (a) ZnFe2O4-rGO, ZnO-rGO, Fe3O4-rGO and rGO in the range of 900 nm–1700 nm, (b) ZnO, ZnO-rGO, (c) Fe3O4, Fe3O4-rGO and (d) ZnFe2O4, ZnFe2O4-rGO in the range of 150 nm–800 nm.

hexagonal graphitised structure and the G band is assigned to the sp2 carbon-type structure [24]. The calculated ID/IG of Fe3O4-rGO, ZnO-rGO and ZnFe2O4-rGO are 0.88, 0.86 and 0.93, respectively, as compared with 0.95 for rGO. The decrease in the ID/IG ratio for the graphene-based nanocomposite indicates an increase in the average size of the sp2 domains, fewer defects and less disorder in the aromatic carbon atoms upon the reduction of GO [25,26]. The higher ID/IG of ZnFe2O4-rGO compared with the individual metal oxides decorating the rGO clearly suggests that ZnFe2O4 contributed the largest amount of nanoparticles on the rGO. In Fig. 2b, the Raman spectrum of the ZnO shows a peak at 390 cm  1 that corresponds to scattering by A1 transverse optical (TO) phonons [27]. Meanwhile, the peak at around 438 cm  1 is assigned to the E2 high mode phonon. These peaks are characteristic of wurzite ZnO. Lin et al. [28] observed a red shift in the E2 high mode phonon as the size of the ZnO quantum dots decreased from 12 nm to 3.5 nm. Since no shift was observed in Fig. 2b, the implied size of the nano-particles from Raman is greater than 12 nm. The confirmation of the formation of Fe3O4 (magnetite) nanoparticles by Raman is complicated since Raman lines

originating from both phases (hermatite, α and maghemite, γ) of Fe2O3 occur in the same spectral region. The absence of a peak at 225 cm  1 provides evidence for discounting the formation of hermatite (Fig. 2c). A strong Raman peak in the vicinity of 670 cm  1 could provide evidence for the formation of both Fe3O4 or γ–Fe203 nanoparticles. However, the lack of a Raman peak at 715 cm  1 indicates that Fe3O4 was formed. Like the ZnO-rGO sample, there were no peaks in the Raman spectra in Fe3O4-rGO sample that could be attributed to Fe3O4 nanoparticles [29,30]. Fig. 2d shows the Raman spectra of the ZnFe2O4 and ZnFe2O4-rGO samples. The ZnFe2O4 sample has characteristic Raman peaks at 350, 451 and 647 cm  1 [31]. There is no conclusive evidence for these peaks in the ZnFe2O4 sample; however, there could be a weak broad signal centred at 350 cm  1. The Raman spectra below 300 cm  1 does however have some similarity to the Raman spectra of the samples of Bo et al. [32]. This spectrum was characterised by two strong Raman peaks below 300 cm  1 and attributed to α-Fe2O3. Unlike the ZnO-rGO and Fe2O4-rGO samples, the Raman spectrum of the ZnFe2O4-rGO sample shows Raman peaks associated with the metal oxides. The spectrum contains peaks at

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Fig. 3. FESEM images of rGO.

350 and 650 cm  1 which could be caused by F2g(2) and A1g phonons in ZnFe2O4. However, the sample also contains peaks caused by Raman scattering in α-Fe2O3, indicating that the ZnFe2O4-rGO nano-composite also contains α-Fe2O3 impurities. The FESEM image of rGO is almost translucent with signs of wrinkles but free from particles (Fig. 3). The dark areas on the sheets portray the presence of another sheet or folding of the sheets. Fig. 4a, c and e shows Fe3O4, ZnO and ZnFe2O4 of irregular shapes and sizes. Since rGO has a large number of uniformly distributed negatively charged oxide functional groups on the surface of the graphene, it is expected that the metal oxide nanoparticles will be evenly distributed because of the attraction between the positively charged metal ions and the functional groups on the surface. The results in Fig. 4b, d and f confirms this hypothesis. Elemental mapping (Fig. 5) was undertaken to further investigate the formation of ZnFe2O4-rGO. There was a significant correlation between the Fe and Zn signal maps, which provides evidence for the formation of an oxide alloy. The atomic ratio of Fe:Zn from energy dispersive X-ray (EDX) is 2.38. This result, coupled with the XRD result, provides evidence for the presence of ZnFe2O4 on the surface of the rGO. There are areas where there is more Fe, which may account for the Fe2O3 signal in the Raman spectra. Fig. 6a shows the HRTEM image of Fe3O4-rGO, in which the rGO sheets are densely packed with Fe3O4 nanoparticles with an average size of 25 nm. Meanwhile, Fig. 6b shows the ZnO particles with undefined morphology populating the rGO sheets. The size of the particles was greater than 12 nm, which agrees with the prediction from Raman spectroscopy. The average particle size of the ZnFe2O4 nanoparticles (Fig. 6c) decorating the rGO sheets was approximately 20 nm, which is smaller than the Fe3O4 nanoparticles on rGO. Fig. 7a shows that the lattice fringe spacing of 0.297 nm coincides with the (220) plane of Fe3O4, whereas in Fig. 7b, the ZnO nanostructure is evident with a lattice fringe spacing of 0.282 nm assigned to the (100) plane. In Fig. 7c, different

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crystal structures with lattice fringe spacing of 0.252 nm and 0.487 nm are observed on the two sides of the boundary, which coincide with the (311) and (111) planes of ZnFe2O4 [33]. These results are in agreement with the XRD results. Since the aim of this work was to fabricate a photo-catalyst, which could be extracted from a solution magnetically, the magnetic properties of the nano-composites are important. As expected, the ZnO and the rGO samples show no magnetic behaviour in contrast with the samples containing Fe. This is clearly seen in Fig. 8 which presents the VSM results of all samples. The Fe containing samples are super-para-magnetic and have a very low coercivity and remnant magnetisation [34]. The saturation magnetisation of Fe3O4-rGO nanocomposites is 27.4 emu/g, which is higher than Fe3O4 nanoparticles of 18.4 emu/g. Similarly, the saturation magnetisation of ZnFe2O4-rGO nano-composites (24.2 emu/g) is also higher than ZnFe2O4, (16.3 emu/g). This is in contrast with the results of Lu et al. [35], who found that the saturation magnetisation of a nano-composite is lower than that of nano-crystals. These results clearly demonstrate that ZnFe2O4-rGO nanocomposites can be manipulated by an external magnetic field and hence extracted from solution and therefore have applicability in wastewater treatment. The removal of MB from water is shown in Fig. 9a. This figure shows how the concentration, obtained from the absorption at 664 nm, varied with time. After 2 h, the amount of MB removed by Fe3O4 was 7%, 15% by ZnFe2O4, 19% by ZnO, 29% by ZnO-rGO, 60% by Fe3O4-rGO, and 82% by the ZnFe2O4-rGO nano-composite, which was similar to the 83% removal of MB by rGO alone. Initially, the Fe3O4-rGO and ZnFe2O4-rGO nano-composites showed similar performance, but after 40 min, the performance of the ZnFe2O4-rGO sample was superior. Over time, the rGO sample was the best performing sample, although after 100 min, the performance of the ZnFe2O4-rGO nano-composite was similar to the rGO sample. The removal of MB from solution by GO is known to be attributed to the π–π stacking interaction between MB and the π-conjugation regions of the rGO sheets [36]. Since the nanoparticles were absorbed onto the surface of the rGO, there would be fewer receptive sites on the rGO for MB to interact, with thereby reducing the absorption of MB and thus the removal effectiveness of the rGO nano-composite. This was observed in both the Fe3O4-rGO and ZnFe2O4-rGO nanocomposite samples. To investigate the reuse of rGO and the ZnFe2O4-rGO nanocomposite in wastewater treatment, several repeat experiments were performed, and the ability to remove MB from water was evaluated. To ensure a fair comparison was made, fresh “standard” MB solution was used each time. In addition, the effect of illumination on the performance of the ZnFe2O4-rGO nano-composite was also investigated. These results are shown in the bar chart presented in Fig. 9b. Although the performance for the fresh rGO sample was the best, the removal performance of rGO dropped significantly after the second use. This was not unexpected, since the rGO absorbs MB onto the surface and does not break the dye down. Consequently, when all the absorption sites are full, no more MB can be removed.

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Fig. 4. FESEM images of (a) Fe3O4, (b) Fe3O4-rGO, (c) ZnO, (d) ZnO-rGO, (e) ZnFe2O4 and (f) ZnFe2O4-rGO.

Fig. 5. Elemental mapping of ZnFe2O4-rGO based on the image.

For the ZnFe2O4-rGO nano-composite, this was after the fourth use. A similar effect was also seen with the ZnFe2O4rGO nano-composite tested in the dark. Again, this can be

explained by the saturation of absorption sites. In contrast, the performance of the ZnFe2O4-rGO nano-composite in breaking down MB only dropped slightly when sample was under illumination. After five cycles, the reduction of MB was more than 70%. This result indicates that the ZnFe2O4-rGO nanocomposite functioned as a photo-catalyst. These results are similar to recent work [35]. The Fe:Zn atomic ratio of the samples used for five times was investigated by EDX; the ratio had increased to 3.31. Since the removal efficiency of the ZnFe2O4-rGO nano-composite was better than Fe2O4-rGO nano-composite, the removal of Zn would most likely reduce the removal efficiency. However, the mechanism by which the Zn was removed from the nano-composite is not clear. The decrease in removal efficiency of the ZnFe2O4-rGO

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Fig. 6. HRTEM images of (a) Fe3O4-rGO, (b) ZnO-rGO, and (c) ZnFe2O4-rGO.

Fig. 7. Lattice fringe spacing of (a) Fe3O4-rGO, (b) ZnO-rGO, and (c) ZnFe2O4-rGO.

the source of these radicals was the H2O2 added to the solution. However, the results demonstrate that effective decomposition of MB requires both the ZnFe2O4-rGO nanocomposite and light. The first stage of the proposed mechanism is the absorption of the photon by the ZnFe2O4 nano-crystal, forming an electron–hole pair. This is represented by Eq. (1). ZnFe2O4 þ hv-ZnFe2O4 (hþ e)

(1)

Numerical simulations on ZnO-graphene interfaces have shown that photo-generated electrons mainly occupy the π orbits of graphene [37], and a similar effect could occur in ZnFe2O4-rGO nano-composites (Eq. (2)). ZnFe2O4 (e) þ rGO-rGO (e)

Fig. 8. Magnetic hysteresis curves of nano-composites.

nano-composite could also be explained by the leaching of nanoparticles from the surface of the rGO sheets, suggesting that the adhesion of the formed solid solution on the rGO sheets requires improvement. The Raman spectra also provide evidence that the nano-composite contained α-Fe2O3 nanoparticles. Leaching could explain the increase in the EDX Fe:Zn ratio if the binding of α-Fe2O3 nanoparticles to rGO is stronger than binding of ZnFe2O4 nanoparticles. The above analysis explicitly assumes all the magnetic nano-composites were recovered. This may not be case, and the result would be a reduction in the MB removal efficiency with the number of cycles. The hydroxyl radical (dOH) is a very strong oxidising agent and is known to decompose MB. In this series of experiments,

(2)

The additional electron on the π-orbitals will then react with H2O2 to produce dOH radicals. rGO (e)þ H2O2-dOH þ OH 

(3)

ZnFe2O4 (h) þ OH  -dOH

(4)

The generation of the dOH radicals by Eqs. (3) and (4) occurs on the rGO where the MB has been absorbed. The close proximity of the MB to the generation site of the dOH radical will increase the reaction rate and hence the decomposition of MB to carbon dioxide, water and acids. 4. Conclusion A magnetically separable ZnFe2O4-graphene nano-composite was successfully prepared using a microwave method.

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Fig. 9. (a) Comparison of photocatalytic degradation of MB by Fe3O4, ZnFe2O4, ZnO, ZnO-rGO, Fe3O4-rGO, ZnFe2O4-rGO and rGO. (b) Photodegradation of MB solution for five cycles using rGO, ZnFe2O4-rGO with light and ZnFe2O4-rGO without light.

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