Magnetically Separable Reduced Graphene Oxide/Iron Oxide Nanocomposite Materials for Environmental Remediation

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Introduction
The requirement for fresh water is paramount for all living organisms, including human beings, and its availability is a major problem throughout the world at present.In the future, this issue will become even more pressing, owing to rapid industrialization and population growth.In particular, the rapid growth of the textile and dying industries has caused critical environmental problems, as some of the dye effluents from these industries can pollute ground water resources, and their utilisation has many toxic and harmful effects on human beings.In this respect, different types of photocatalytic materials viz.free-standing photocatalysts, doped photocatalysts, dual semiconductors and noble metal deposited photocatalysts have been developed, for the treatment and purification of dye-contaminated waste water.Among them, very few have been reported that use a physical separation technique, by which the photocatalysts can be separated easily from the reaction medium.The major disadvantage of a colloidal photocatalytic system is the complicated separation of the catalyst from the reaction medium.To overcome this issue, magnetic separation of the photocatalyst has been proposed as a promising solution.Magnetically separable photocatalytic systems are more advantageous than colloidal photocatalytic systems, as they do not require filtration or centrifugation for the removal of the catalyst from the reaction medium.Hence, the physical separation of a photocatalyst can easily be achieved using these magnetically separable photocatalysts.
Graphene, a single layer of sp 2 -bonded carbon atoms arranged in a two-dimensional (2D) honeycomb structure, possesses a high surface area and excellent thermal, mechanical and electrical properties; these make it a good supporting material for inorganic nanoparticles used in various energy and environmental applications. 1,2The combination of graphene with the inorganic nanoparticles liberates new functional hybrid materials, which possess complementary behaviours to each constituent and thus open up new opportunities for the enhancement of wider applications. 3,46][7] Although these graphene-based nanocomposites (e.g.rGO-TiO 2 , rGO-ZnO, etc.) display excellent performances in photocatalytic systems, the problems associated with the recovery, reuse and separation of the catalysts from the reaction medium still exist after the photodegradation process, because the good dispersive properties of these materials means that they are inconvenient to recycle.Hence, the introduction of magnetic nanophotocatalytic materials into the graphene sheets can provide convenient magnetic separation, in order to remove and recycle the magnetic nanocomposite catalysts under an external magnetic field. 8,9ecently, magnetite (Fe 3 O 4 ) nanoparticles have attracted much attention because of their low-cost, eco-friendly and simple preparation, and because they show desirable properties of strong super-paramagnetic and electrical conductivities, as well as optical and chemical properties. 10,11Hence, they find potential applications in the areas of biosensors, 12 catalysis, 13,14 supercapacitors 15 and photocatalysis. 16A few studies have shown that bare Fe 3 O 4 nanoparticles are photocatalytically inactive under solar irradiation, owing to the rapid aggregation caused by their high surface area and magnetic interactions between the particles; this leads to the formation of larger Fe 3 O 4 particles and hence impairs their chemical and photocatalytic properties.The incorporation of these Fe 3 O 4 nanoparticles into graphene sheets is a promising way to overcome this limitation, as it prevents the serious agglomeration of magnetite nanoparticles, 17,18 leading to a high photocatalytic performance under sunlight due to the contribution of improved photo-induced charge separation efficiency within the Fe 3 O 4 nanoparticles. 19n this investigation, we report a green, facile and costeffective method to prepare, at room temperature, rGO/Fe 3 O 4 nanocomposite materials which are magnetically separable and recyclable.The photocatalytic performances of the prepared rGO/Fe 3 O 4 nanocomposite materials were evaluated in the degradation of a model organic dye, methylene blue (MB).The influence of different contents of Fe 3 O 4 nanoparticles in the magnetically separable (rGO/Fe 3 O 4 ) photocatalysts was studied, to optimize the Fe 3 O 4 nanoparticles for maximum photodegradation efficiency.The magnetically separable rGO/Fe 3 O 4 photocatalysts showed better photocatalytic performances when compared to control samples such as rGO and pristine Fe 3 O 4 nanoparticles.Moreover, the separated rGO/Fe 3 O 4 nanoparticles were reused for several photodegradation experiment cycles, indicating their sustainability.

Preparation of graphene oxide
Graphene oxide (GO) was synthesized from graphite by adopting the simplified Hummer's method. 20Briefly, 3 g of graphite flakes were oxidatively treated with 400 mL of H 2 SO 4 and 18 g of KMnO 4 for 5 min under magnetic stirring, leading to the development of graphite oxide.However, to ensure complete oxidation of the graphite, the solution was stirred for another 3 days.During the oxidation process, a colour change of the solution from dark purplish-green to dark brown was observed.Then, H 2 O 2 solution was added to stop the oxidation process, during which the colour of the solution changed to bright yellow; this indicated the highly oxidized level of the graphite.The obtained graphite oxide was washed 3 times with an aqueous solution containing 1 M of HCl, and this procedure was repeated until the solution pH reached 4-5.At this pH, the graphite oxide experienced exfoliation, which resulted in a thickening of the GO solution and formation of a GO gel.Finally, the GO gel was freezedried to obtain solid GO.

Preparation of rGO/Fe 3 O 4
rGO/Fe 3 O 4 nanocomposites with different weight ratios of rGO and Fe 3 O 4 were prepared by a simple in situ chemical synthesis method.Typically, 25 mg of GO was dispersed in deionized double distilled (DD) water under stirring, and this was then subjected to sonication for 20 min.Then, 25% of NH 4 OH solution was added drop-wise into the GO solution until the pH reached ~11-12.Following this, a specific quantity of FeSO 4 solution was slowly added to the above solution containing GO under magnetic stirring, and the solution was left overnight at room temperature.The obtained black solution containing rGO/Fe 3 O 4 nanocomposites was centrifuged and washed with DD water for 10 min at 4000 rpm, and this procedure was repeated three times, to remove excess NH 4 OH present in the solution.Finally the solution was dried in a vacuum oven.The same protocol was followed to prepare Fe 3 O 4 nanoparticles, in the absence of GO.Meanwhile the rGO was prepared using the same procedure, without adding FeSO 4 solution.The weight ratios of the GO and FeSO 4 are given in Table 1.

Characterization techniques
The size, shape and morphology of the rGO/Fe 3 O 4 nanocomposites were analyzed with a high resolution transmission electron microscope (HR-TEM).Raman and photoluminescence spectral data were collected using a Renishaw 2000 inVia Raman microscope system, with an argon ion laser emitting at 514.5 nm.A Siemens-D5000 X-ray diffractometer with copper Kα radiation (λ = 1.5418Å) at a scan rate of 0.02 degree s −1 was used for X-ray diffraction (XRD) analysis.A Thermo Scientific Evolution-300 UV-vis absorption spectrophotometer was employed for absorption studies, in a spectral range of 190-900 nm.Magnetization measurements were carried out at room temperature using a Lakeshore-736 vibrating sample magnetometer (VSM), with a maximum magnetic field of 10 kOe.

Photocatalytic studies
The photocatalytic performances of the prepared rGO/Fe 3 O 4 samples were evaluated with methylene blue (MB) dye as a model target organic pollutant, under natural sunlight irradiation.The photocatalytic experiments were performed on bright sunny days from 9 a.m. to 2 p.m.All the prepared photocatalyst materials (2 mg) were separately dispersed in 12 mL of MB solution (10 mg L −1 ) under stirring and left overnight (12 h) at room temperature, in order to study their adsorption behaviours.Prior to the sunlight irradiation, the MB solution was stirred for 1 h in the dark, thereby allowing the system to attain an adsorption-desorption equilibrium between the photocatalyst and the MB molecules.At given time intervals of irradiation, 2 mL of irradiated MB dye solution was periodically withdrawn, and at the end of the experiment the photocatalyst was removed from the reaction solution by magnetic separation using a permanent magnet.The equilibrium concentration of MB dye in the reaction solution for each sample was determined with a UV-visible absorption spectrophotometer, by measuring the absorbance intensity at 662 nm during the photocatalytic degradation process.To study the sustainability of the photocatalysts, the rGO/Fe 3 O 4 was collected by applying a magnetic field and washed with DI water, before being re-dispersed into fresh MB solution for the next cycle.
The photodegradation rates and MB dye removal efficiencies under natural sunlight irradiation of different photocatalysts were calculated using the following equations.
where C 0 represents the initial concentration of MB, and C t is the concentration of MB at reaction time 't'.content of Fe 3 O 4 nanoparticles in the rGO sheets.The saturated magnetization value observed for the pristine Fe 3 O 4 nanoparticles was 58.70 emu g −1 , which was higher than that of the magnetic rGO/Fe 3 O 4 nanocomposites.This can be attributed to the presence of graphene in the nanocomposites. 25][29] Among the prepared nanocomposites and rGO, G1F2 has the lowest PL intensity, which indicates that G1F2 efficiently suppresses electron-hole pair recombination and promotes charge separation; these are highly beneficial for photocatalytic applications.The synergistic effect 30 between graphene and Fe 3 O 4 nanoparticles in the G1F2 nanocomposite allows graphene to capture or trap the photo-induced electrons from the conduction band of the Fe 3 O 4 through the extended π-conjugation carbon network, and consequently restrict the flash recombination of electron-hole pair.Therefore, G1F2 is expected to show higher photocatalytic activity when compared to the other nanocomposites. 31However, further increasing the Fe 3 O 4 content in the rGO/Fe 3 O 4 nanocomposite results in a higher PL intensity.The Fe 3 O 4 nanoparticles possess large surface energies, and hence they tend to aggregate to minimize these surface energies.Thus, an excess amount of Fe 3 O 4 nanoparticles in the rGO/Fe 3 O 4 nanocomposite may lead to aggregation of the nanoparticles, resulting in larger particles. 32Consequently, increasing the amount of Fe 3 O 4 nanoparticles in the rGO/Fe 3 O 4 nanocomposite introduces new charge recombination centres for photoinduced charge separation, and as a consequence this decreases the photocatalytic efficiency.

Photocatalytic activity of magnetically separable rGO/Fe 3 O 4 nanocomposites for the degradation of methylene blue
The photocatalytic performances of the prepared photocatalytic materials viz.rGO, Fe 3 O 4 and magnetically separable rGO/Fe 3 O 4 nanocomposites were separately evaluated for the degradation of a model dye pollutant, methylene blue (MB) under natural sunlight irradiation for 5 h.The results of the photodegradation study are shown in Fig. 7(a).During the photocatalytic experiments, the bare Fe 3 O 4 nanoparticles achieved only 57% photodegradation even after 5 h of sunlight irradiation (Fig. 7(a)).The poor photocatalytic performance of the bare Fe 3 O 4 nanoparticles can be ascribed to the aggregation caused by the high surface area of Fe 3 O 4 nanoparticles and the magnetic interactions between the particles, which lead to the formation of larger sized particles. 33Interestingly, a maximum photodegradation of MB was observed at 1 h light irradiation when a photocatalyst of Fe 3 O 4 nanoparticles, incorporated into reduced graphene oxide sheets, was used.To optimize the Fe 3 O 4 content for maximum photodegradation of MB dye, photocatalytic experiments were carried out with different compositions of rGO and Fe 3 O 4 .Among the photocatalysts, G1F2 exhibited excellent photocatalytic activity; almost 89% of the MB was decolourized after 30 min and 100% after 1 h light irradiation.GIF5, GIF10 and GIF 20 achieved 100% degradation of MB after 2 h of irradiation.Meanwhile, the bare Fe 3 O 4 nanoparticle photocatalyst could achieve only 57% MB dye removal efficiency.The maximum photocatalytic activity exhibited by the rGO/Fe 3 O 4 nanocomposite photocatalysts is due to the emergence of synergistic effects in the rGO/Fe 3 O 4 during the photocatalytic reaction i.e. efficient photogenerated charge transfer from Fe 3 O 4 to the graphene sheets, which facilitates increased electronhole pair separation, and as a consequence a better photocatalytic performance is achieved.
To further understand the enhanced photocatalytic performance of the rGO/Fe 3 O 4 nanocomposites, the MB solution was continuously stirred with the photocatalysts overnight (12 h); the results are shown in Fig. 7(b).The rGO and GIF2 showed maximum adsorptions of ~86 and ~84%, respectively.Further increasing the Fe 3 O 4 content in the nanocomposite led to a decrease in the adsorption of MB.The shadowed area in Fig. 7(a) follows a similar trend to the adsorption behaviour of the photocatalysts.The enhanced MB dye adsorptivity of the rGO/Fe 3 O 4 photocatalysts is due to the large phenyl ring structure of the graphene 34,35 in the nanocomposites.Moreover, graphene is a 2D crystalline structure and has a large surface area, superior electrical conductivity and unique transport properties, making it a great electron-transport material in the process of photocatalysis.When Fe 3 O 4 nanoparticles are anchored on the surface of graphene sheets, the graphene provides more adsorption sites and photocatalytic reaction centres for the MB dye molecules through the π-π conjugation and electrostatic attraction between the MB dye and the aromatic region of the graphene sheets. 36,37These highly exposed surface active reaction sites are beneficial for promoting the generation of hydroxyl radicals for MB adsorption, by redox reactions within the active sites (Fe 2+ /Fe 3+ ).Additionally, the strong Fe-O-C interactions of rGO/Fe 3 O 4 , between the delocalized unpaired π electrons from the π-conjugated carbon network on graphene's basal plane, facilities electron transfer between the rGO sheets and iron centres. 38Notably, the photocatalytic activity is highly dependent on the concentration of photogenerated charge carriers during the reaction. 39Therefore, the strong attachment of Fe 3 O 4 on the electron carries of the rGO sheets gives rise to a enhanced migration of photoexcited electrons from the conduction band of Fe 3 O 4 to the rGO sheets.The fast electrontransfer kinetics of rGO/Fe 3 O 4 helps to improve the interfacial charge transfer process, 40  The sustainability of a photocatalyst is one of the most important requirements for successful practical applications.In this respect, the reusabilities of the rGO and rGO/Fe 3 O 4 photocatalysts were investigated using the same photocatalyst for 8 sets of experiments, with fresh MB solution for each experiment, keeping all other experimental parameters constant.After each photodegradation experiment, the photocatalyst was removed from the photolysis cell using an external magnetic field and washed with high pure DI water, to remove the presence of any MB associated organic impurities.The magnetic separation technique represents an easy and convenient way to remove or recycle the photocatalyst.This can be achieved by placing a magnet close to the sample bottle, which causes the rGO/Fe 3 O 4 photocatalyst to move towards the external magnetic field and be attracted to the side of the bottle, leaving behind a clear reaction solution (Fig. 7(d)).Therefore, the rGO/Fe 3 O 4 can be easily reused and recycled after the photocatalytic process.The MB dye molecules could be effectively photodecomposed in each experimental cycle and no significant change in the photocatalytic activity of the G1F2 nanocomposite was observed during the repeated photocatalytic experimental cycles (Fig. 7(c)).
Among the different photocatalysts, the G1F2 nanocomposite exhibited the best stability during photocatalytic degradation of MB dye; hence it can be applied as a recyclable photocatalyst.The lower the content of Fe 3 O 4 nanoparticles in the nanocomposites, the smaller the particle sizes are, with a high surface area and without heavy aggregation of Fe 3 O 4 nanoparticles.Therefore they can offer excellent decolourization activity.Hence, G1F2 contains the lowest amount of Fe 3 O 4 nanoparticles, and so has smaller particle sizes with high specific surface area, offering a number of active sites for the adsorption and subsequent desorption of MB molecules in the nanocomposite.This favours the facile transport of photoexcited electrons to reach the surface reaction sites more easily 42 and thereby efficiently inhibits the recombination of photo-induced electron-hole pairs during the electrontransfer process.Thus, the higher photocatalytic activity and sustainability of this magnetically separable rGO/Fe 3 O 4 nanocomposite is greatly beneficial for industrial waste water treatment processes.
The schematic representation of MB degradation in the presence of the magnetically separable rGO/Fe 3 O 4 photocatalyst is shown in Fig. 8. Upon light irradiation, the Fe 3 O 4 present on the rGO sheet surface undergoes charge separation that leads to promotion of valence band (VB) electrons into the conduction band (CB), leaving a hole in the VB (eqn (1)), whereas the MB molecules are excited to cationic MB radicals (MB*) (eqn (2)).These photogenerated electrons in the conduction band are instantaneously transferred to rGO sheets (eqn (3)), and are consequently captured by dissolved O 2 to generate reactive oxidation species such as ˙OH and O 2 ˙− (eqn (4)).On the other hand, the photoinduced holes are crucial for the oxidation process and adsorbents are effectively oxidized; usually the Fe 3 O 4 (h + ) can react with adsorbed H 2 O/OH − to form strong hydroxyl radicals (˙OH) (eqn (5)).Finally, these ˙OH radicals oxidize the MB molecules adsorbed to CO 2 and H 2 O (eqn (6)) 43 via the π-π stacking/ electrostatic interactions on the active sites of the rGO/Fe 3 O 4 nanocomposites.

Conclusion
We have reported the preparation of photocatalysts based on magnetically separable rGO/Fe 3 O 4 nanocomposite materials,

3. 1 .
Morphological characterization of rGO/Fe 3 O 4 nanocompositesThe TEM images of Fe 3 O 4 and rGO/Fe 3 O 4 nanocomposite materials before and after the 8 cycles of photocatalytic experiments are shown in Fig.1and 2(a-e), respectively.The TEM images clearly indicate that the Fe 3 O 4 nanoparticles are uniformly embedded on the surface of the rGO sheets in all the nanocomposite materials, and that no significant changes in the morphology of the rGO/Fe 3 O 4 nanocomposite are observed before and after the photocatalytic studies (Fig.1 and 2).The decreasing tendency of the Fe 3 O 4 to agglomerate, and the restacking of the rGO sheets, are vital in determining the photocatalytic activity of the rGO/Fe 3 O 4 nanocomposite.The lattice-resolved TEM image of the rGO/Fe 3 O 4 nanocomposite reveals the presence of clear atomic lattice-fringes of Fe 3 O 4 nanoparticles on the surface of the rGO sheets (Fig.2f).The estimated lattice-fringe or fringe values of the magnetic Fe 3 O 4 nanoparticles are 2.533 Å and 1.643 Å, which can be respectively indexed to the (311) plane (2.530 Å) and (511) plane (1.614 Å); this is compatible with the XRD observations.The increase in concentration of the FeSO 4 nanoparticles during the preparation of the nanocomposite led to an increase in the particle sizes of Fe 3 O 4 , in the order of G1F2 < G1F5 < G1F10 < G1F20 (Fig.1(b-f)).This indicates that the aggregation tendency of the Fe 3 O 4 nanoparticles in the rGO/Fe 3 O 4 nanocomposite becomes greater with an increasing FeSO 4 precursor concentration.Among the different nanocomposite materials, G1F20 showed the most aggregation of Fe 3 O 4 nanoparticles (Fig.1(f)), while G1F2 exhibited a good distribution of Fe 3 O 4 nanoparticles on the surface of the rGO sheets (Fig.1(c)).

3. 5 .
Photoluminescence (PL) was used to study the electronic and optical properties of the nanocomposites, including the migration, transfer and recombination of electron-hole pair of the photoinduced semiconductor.Fig.6shows the room temperature PL spectra of rGO, the rGO/Fe 3 O 4 nanocomposites and Fe 3 O 4 .It was observed from the PL spectra that the rGO and rGO/Fe 3 O 4 nanocomposites exhibited lower PL intensities than that of the bare Fe 3 O 4 nanoparticles.Fe 3 O 4 is an indirect band gap semiconductor with a narrow optical gap value of 1.4 eV.26This narrowness value arises from the d orbitals, suggesting that Fe 3 O 4 exhibits a high electrical conductivity with an almost metallic nature at room temperature, but that the low charge carrier (electron and hole) mobility in Fe 3 O 4 may lead to an increase in electronhole recombination.Therefore, the higher PL intensity of the bare Fe 3 O 4 is due to the recombination of excited electrons and holes, whereas the lower PL emission intensities of rGO and the Fe 3 O 4 /rGO nanocomposites are due to the lower charge recombination rates.This suggests that graphene has a tendency to greatly influence the PL intensities of rGO/Fe 3 O 4 nanocomposites, owing to its 2D hexagonal π-conjugation structure and excellent electronic conductivity.The high charge mobility of graphene means that it acts as an electron acceptor for the photo-excited electrons from Fe 3 O 4 , leading to a low charge recombination rate.[27][28][29]Among the prepared nanocomposites and rGO, G1F2 has the lowest PL intensity, which indicates that G1F2 efficiently suppresses electron-hole pair recombination and promotes leading to enhanced photocatalytic activity.On the other hand, the strong anisotropic dipolar interactions of Fe 3 O 4 in aqueous phase 41 are likely to diminish or restrict its catalytic activity, and thus the Fe 3 O 4 nanoparticles are prone to aggregate into larger Fe 3 O 4 particles (Fig. 1(b)), leading to decreased MB decolourization efficiency.

Fig. 8
Fig. 8 Schematic representation of the photocatalytic degradation of MB in the rGO/Fe 3 O 4 nanocomposites under natural sunlight irradiation.

Table 1
Weight ratios of GO and FeSO 4 used for the preparation of rGO/Fe 3 O 4