Rational design of magnetically separable core/shell Fe3O4/ZnO heterostructures for enhanced visible-light photodegradation performance

Magnetically separable core/shell Fe3O4/ZnO heteronanostructures (MSCSFZ) were synthesized by a facile approach, and their application for enhanced solar photodegradation of RhB was studied. The formation mechanism of MSCSFZ was proposed, in which Fe3O4 nanoparticles served as a template for supporting and anchoring the ZnO crystal layer as the shells. The morphology of MSCSFZ can be varied from spherical to rice seed-like structures, and the bandgap was able to be narrowed down to 2.78 eV by controlling the core–shell ratios. As a result, the MSCSFZ exhibited excellent visible-light photocatalytic activity for degradation of rhodamine B (RhB) in aqueous solution as compared to the controlled ZnO nanoparticles. Moreover, MSCSFZ could be easily detached from RhB solution and maintained its performance after 4 cycles of usage. This work provides new insights for the design of high-efficient core/shell recyclable photocatalysts with visible light photocatalytic performance.


Introduction
Utilizing nanomaterials for water treatment technology has been considered as a potential solution for protecting and developing the environment and energy resources. 1 With the rapid development of water treatment nanotechnology, semiconductor nanophotocatalysts have drawn considerable attention owing to their superior efficiency in photocatalytic degradation of stable and toxic organic pollutants in water sources. 2,3 The photocatalysis mechanism is based on light absorption of semiconductor materials to create electrons and holes in the conduction band and valance band. These photoinducted charge carriers can move to the surface of semiconductors and act as strong reductants and oxidants to generate reactive oxygen species for degradation of pollutants. 4 From previous studies, it is evident that photocatalysis possess outstanding advantages compared to other treatment techniques such as (i) utilize solar energy; (ii) suitable operation conditions (temperature and pressure), (iii) high efficiency with completely decomposing the stable and toxic organic substances without generating any secondary pollutants; (iv) low operating cost; (v) save time. [4][5][6] Over the years, a wide range of single semiconductors were used for photocatalytic technology. 7 However, recently, a various type of heterogeneous photocatalysis composed by two or more semiconductors were rapidly developed to overcome draw-backs of single semiconductors as: enhancing visible photocatalytic activity with suitable band edges and improving photo-generated charge carrier separation; easy collecting in the solution. 5,[7][8][9] An alternative to benchmark TiO 2 , ZnO is also an well-known photocatalyst for degradation of organic pollution in aqueous media because of its higher electron mobility, longer lifetime of charge carriers, and especially in low cost and environmental friendliness. 10,11 Besides, the structure-morphology of ZnO can be easily varied by wet-chemistry strategy to obtain the well-dened structures for photocatalytic and sensing applications. Although ZnO has been widely served as an efficient and low-cost photocatalyst in water remediation, however, there are some problems needed to be solved, such as (i) the fast photogenerated electron-hole recombination and (ii) the large bandgap energy of 3.37 eV, resulting in low efficiency of solar radiation absorption (from 3% to 5%); subsequently, the photocatalytic activity is unsatisfactory for the applications in industry. 12 To overcome these limitations, recent efforts have been focused on modifying the electronic band structure of ZnO by doping transition/noble metals or combining ZnO with other materials to form a hybrid photocatalyst system such as low bandgap semiconductors and magnetic nanomaterials. [13][14][15] Currently, the synthesis of heterogeneous structures between ZnO and magnetic nanomaterials has gained some interest in the eld. [16][17][18] The combination of ZnO and several spinel ferrite nanomaterials has created a new hybrid material that is capable to enhance the visible light photocatalytic efficiency and advantageous in recovery and reusability. [19][20][21][22] Typically, nanoscale materials have a primary bottleneck in recovery and reuse since they are in particulate form in solution, which hinders the outstanding performance in practical industrial environment. The nanocomposites of photocatalysts with magnetic-based materials allows detaching the nanocomposites from aqueous treatment solution by simply applying an external magnetic elds. Various metal magnetic oxides, such as FeO, Fe 2 O 3 , Fe 3 O 4 or ferrites of MFe 2 O 4 (M ¼ Co, Mn, Ni) are widely used for incorporating with ZnO to form the magnetically recoverable photocatalysis. 23 Among the magnetic-based materials, Fe 3 O 4 possesses the highest saturation magnetization (theoretical value of 93 emu g À1 at room temperature), superior adsorption of heavy metals, as well as the excellent ability to decompose organic pollutants. 24,25 The combination of Fe 3 O 4 and ZnO will formed a heterojuntion in which electrons and holes of two materials can be transfer to others based on the energy band position. These lead to an effective separation of photogenerated charge carrier and, in some cases, extends absorption of visible-region light. 4,5,8 It is expected that the core/shell structured Fe 3 O 4 /ZnO at the nanoscale not only exhibits good magnetic separability, but also suppresses the charge carrier recombination, and achieving a narrow bandgap of ZnO for reaching visible-light-driven photocatalysis. So far, there has been much literature on fabrication of core/shell nanostructured Fe 3 O 4 /ZnO for photocatalytic applications. However, to the best of our knowledge, the previous works mainly reported on the effective degradation of organic pollutants under ultraviolet irradiation, while the visible light-driven photocatalytic performance of core/shell nanostructure is still in its fancy. Additionally, the ratio between Fe : Zn in hybridization is believed to tune the overall bandgap and thus enhance the photocatalytic performance. Nonetheless, there is a gap knowledge in this eld due to the lack of synthesis control over Fe : Zn ratio. Therefore, it is imperative to comprehensively study on the effects of Fe 3 O 4 /ZnO ratio as a core/shell heterostructures for photocatalytic degradation of organic pollutants under the visible light irradiation.
In this work, we reported a facile approach to synthesize core/shell nanostructured Fe 3

Preparation of core/shell Fe 3 O 4 /ZnO heterostructures
In a typical procedure, various amounts of Zn(CH 3 COO) 2 $2H 2 O were dissolved into 100 mL of a solution including 75 mL of distilled water and 25 mL of DEG under mechanical stirring, followed by the addition of 25 mL of EG-CA mixed solution containing the dispersed Fe 3 O 4 NPs at room temperature. Aer stirring for 2 h, Na 2 CO 3 (1.5 M) solution was added dropwise to the above solution, and then continuously stirred for 1 h. Subsequently, the reaction product was separated by centrifugation, washed with copious DI water, and dried in an oven for 24 h at 60 C. The nal product was annealing at 500 C for 2 h with a ramping rate of 5 C min À1 . To investigate the inuence of the core-shell ratio of

Characterization
The crystalline structure of core/shell nanostructured Fe 3 O 4 / ZnO was characterized by X-ray diffraction (XRD) by using a Bruker D8 Advance X-ray diffractometer with Cu-Ka radiation (k ¼ 1.5406Å) at 2q range of 10-70 . The X'pert Highscore Plus program was carried out to evaluate the grain size of the MNPs. The morphological characteristics, shape, and size of the MNPs and the core/shell nanostructure were observed through eld emission scanning electron microscopy (FESEM; Hitachi S-4800) equipped with an energy dispersive X-ray spectrometer, and high-resolution transmission electron microscope (JEM 2100, JEOL). Fourier transform infrared (FTIR) spectroscopy (FTIR-GBC Cintra 40 Nicolet Nexus 670 FTIR), Raman spectra (XploRA, Horiba) and X-ray photoelectron spectroscopy measurement (Mutilab-2000 spectrometer with an Al Ka monochromatized source) were carried out to investigate the interaction between the Fe 3 O 4 MNP core and ZnO shell. The optical properties of samples were characterized by UV-vis-NIR absorption spectroscopy (Hitachi U-4100) and HR photoluminescence system (IHR 500, Jobin Yvon) with an excitation wavelength of 350 nm. A commercial VSM (MicroSence EZ9) was used to observe the magnetic properties and saturation magnetization. Hysteresis loops were determined at an applied eld of up to AE18 kOe at room temperature.

Photocatalytic activity experiment
Photocatalytic activity of the as-prepared samples was investigated by the degradation of RhB solution (a test contaminate) under solar light irradiation (AM 1.5G, Newport). Typically, 60 mg of the as-synthesized catalyst was added into 60 mL of RhB solution (10 ppm) under vigorous stirring in the dark for at least 1 h to achieve the adsorption-desorption equilibrium. The mixture was irradiated by solar light, and the distance from the light source to the liquid level of RhB aqueous solution was kept at 15 cm. During irradiation, 2 mL of the reaction mixture was withdrawn at each time interval of 15 min, and photocatalysts were separated from RhB solution by using a magnet. The degradation of RhB solution was monitored by determining the concentration of RhB in solution through absorbance measurements at 554 nm. The degradation efficiency of RhB was calculated using the following equation: where C 0 and C are the initial and real-time concentrations of RhB solution, respectively, and A 0 and A are the initial and realtime absorbance of RhB solution, respectively.

Structural and morphological characterizations
The crystal structure and phase purity of the as-prepared samples were identied by XRD. Fig. 1   According to Bragg's law, the shi in diffraction peaks to lower 2q can be related to the increase in lattice strain, implying the signicant expansion of the Fe 3 O 4 crystal distance in the heterostructure samples. This is due to the hybridization of core/ shell at the interface as a result of the formation of core/shell structure that ZnO shell was grown on the surface of Fe 3 O 4 core. 26 The average crystallite size was performed by X'pert Highscore Plus and shown in Table 1. The calculated results are in good agreement with the above mentioned ndings.
The FE-SEM was used to investigate the morphological features of all samples. As shown in Fig. 2a, the pristine Fe 3 O 4 NPs display a uniform spherical shape with an average size of 15.8 nm, thus making them well-dispersed in the EG-CA solution. Aer being covered by the ZnO shell, the morphology and size of CSFZ (Fig. 2b-d) changed drastically compared to the pristine Fe 3 O 4 NPs. At Fe/Zn ratio of 1 : 5, the core/shell heterostructure retained nearly spherical morphology with a wide size distribution, and the average particle size was estimated to be 39.8 nm. As increasing the Fe : Zn ratios up to 1 : 10 and 1 : 20, the morphology changes from spherical shape of core/ shell structure to rice seed-like structure ( Fig. 2c and d). The rice seed-like shaped nanostructures with an average size of 42 nm in diameter and 6 nm in lengths were observed for the Fe 3 O 4 /ZnO 1 : 10 sample. For the Fe 3 O 4 /ZnO 1 : 20 samples, the formation of the nano rod-like structure of CSFZ was seen with average lengths of 139 nm. It should be noted that all samples were synthesized in the same experimental conditions with the same amount of Fe 3 O 4 core, meanwhile the ZnO shell content varied by adjusting zinc precursor concentration. Thus, the structural evolution of the CSFZ is assigned for the formation of ZnO shell, suggesting the Zn content is crucial for designing the desirable structure. As comparison for ZnO NPs, they displayed a spherical shape with an average diameter size of 87 nm, which is larger than Fe 3 O 4 NPs (Fig. 2e). To further analyze the elemental composition, EDS analysis was carried out on the Fe 3 O 4 /ZnO 1 : 5 and Fe 3 O 4 /ZnO 1 : 20 samples (Fig. 2f)  To gain further insight into the evolution of CSFZ, HR-TEM images of samples with Fe : Zn molar ratio of 1 : 5, 1 : 10 and 1 : 20 were shown in Fig. 3. TEM images shows the features of the core-shell structure formation, in which Fe 3 O 4 core is dark and ZnO shell is light (different brightness). From HR-TEM images, different lattice fringes were observed in the center and the edge region of a particle, suggesting the existence of a crystalline core/shell structure. The lattice spacing of the core and shell correspond to (311) and (222)  To gain in-depth understanding about the chemical bonding and vibration, the FTIR analysis of prepared samples was carried out. Fig. 4 presents the FTIR spectra of Fe 3 O 4 NPs, ZnO NPs and CSFZ with different Fe : Zn molar ratio of 1 : 5, 1 : 10, and 1 : 20. The stretching vibration modes on the FTIR spectra were generally categorized into three wavenumber regions. (i) Table 1 Microstructural parameters of the as-prepared samples: the lattice spacing of (220) plane of Fe 3 O 4 crystal (d 220 (Fe 3 O 4 )), average crystallite size (D XRD ) and particle size estimated from FESEM images (D FESEM )    in the change of bond length and energy. In addition, a new absorption mode was found at 692 cm À1 in core/shell samples, which is associated to the incorporation Fe into the Zn-O lattice due to the hybridization of Fe 3 O 4 core and ZnO shell at the coreshell interface and similar to previous scholarly publications. [31][32][33] In terms of peak intensity located at 692 cm À1 , the core/shell Fe 3 O 4 /ZnO 1 : 5 exhibited the strongest and sharpest among these compared samples, which further conrms the better core/shell formation at a low precursor Fe : Zn ratio. To verify this unique feature, we employed Raman spectra and their results are shown in Fig. 5. The peaks observed at 99.6, 338 and 439 cm À1 were assigned to the E 2 (low) mode, A 1 (TO) mode E 2 (high) mode of ZnO NPs, which is consistent with the previous study. 34 The broad peak centered at 664 cm À1 was attributed to A 1g mode corresponding to the symmetric stretch of oxygen atoms along Fe-O bonds in Fe 3 O 4 NPs. It is wellknown that E 2 (high) and E 2 (low) modes in ZnO NPs corresponded to the vibrations of oxygen atoms and Zn sub-lattice, respectively. 35 Evidence by the previous work of Morozov et al., the alter in Raman peak intensity of E 2 (high) and E 2 (low) related to the defects on the ZnO side of the interface. 35 Karamat et al. revealed that the intensity of the E 2 (low) mode decreased due to phonon vibrations of the existence of Fe dopant. 36 Thus, the decreasing of the intensity of E 2 (high) and E 2 (low) mode observed in Raman spectra of core/shell heteronanostructures conrm again the hybridization of Fe 3 O 4 core and ZnO shell with the Fe ions incorporated in the ZnO crystal lattice at the interface of core-shell. Taken the above discussion, it is strongly conrmed that core/shell nanostructured Fe 3 O 4 /ZnO was successfully prepared.
To further study the surface composition of CSFZ, XPS measurement was conducted for Fe 3 O 4 /ZnO 1 : 5 sample and shown in Fig. 6. From XPS survey spectra of 1-5 sample in Fig. 6a, it can be found that the C, O, Fe and Zn elements coexisted in the sample and the intensity of peak represents for the ZnO phase (Zn 2p) is much larger than that for the Fe 3 O 4 Fig. 4 (a) FTIR spectrums of as-prepared samples and (b) the enlarged spectra with wavenumber in the range of 800 cm À1 to 400 cm À1 . phase (Fe 2p), revealing the formation of the Fe 3 O 4 /ZnO coreshell nanostructure with the elemental signals from the ZnO shell are much stronger than those from the deeper core of Fe 3 O 4 in the sample. The high-resolution Fe 2p spectrum (Fig. 6b) shows main peak at 710.64 eV which could be attributed to Fe 2p3/2 of Fe 2+ spcies, two other main peaks at 724.2 eV and 713.08 eV were correspond to Fe 2p1/2 and Fe 2p3/2 of Fe 3+ species. 37 There is a satellite peak observed at 717.79 eV, con-rming the presence of high crystalline Fe 3 O 4 in the samples. 38 As shown in the high resolution Zn 2p spectrum (Fig. 6c), two main peak of ZnO centered at 1021.58 eV and 1044.58 eV corresponding to Zn 2p3/2 and Zn 2p1/2, suggesting the existence of ZnO. 39 Interestingly, a strong satellite structure was observed in both of Fe 2p and Zn 2p. The reason for the increasing intensity of satellite peaks is most likely due to the defects found in core-shell systems resulting from the interaction between Fe 3 O 4 core and ZnO shell. The incorporation of Fe ion or Zn ion at interfaces creates a greater defect density would increase conductivity within the particle, which in turn, could boosting shake-up processes that lead to a strong satellite structure, 34 conrming the hybridization between Fe 3 O 4 and ZnO phase in the heterostructures. Additionally, O1s spectrum (Fig. 6d) can be tted by the Gaussian function with the coincidence of 3 peaks distributed at binding energy of 529.7 eV, 532.26 eV, and 533.75 eV which able to assigned respectively, to metal-O bonds (Fe-O bonds, Zn-O bonds); oxygen defects and oxygen adsorbed on the surface of samples including O 2 , hydroxyl (OH À ) and carbonate (CO 2 3À ) groups (O 2À ). 40,41 Optical and magnetic properties To determine the band gap of the as-prepared samples, UV-vis absorption spectroscopy was conducted. Fig. 7a shows the UVvis absorption spectra of prepared samples calculated from the diffuse reectance by Kubelka-Munk theory. 42 Fig. 7a shows that the control ZnO NPs absorbed light in the UV range; the wavelength corresponding to one set of the absorption edge around 386 nm was consistent with the literature. 43,44 For the core-shell heterostructures, the absorption spectra showed a shoulder in the region from 400 nm to 450 nm, resulting in double absorption edges. The rst absorption edge corresponded to the ZnO absorption edge and another was related to the presence of magnetic NPs due to the forming of core-shell  structure with a red shi absorption edges. The appearance of two absorption edges was also observed in previous studies on the composites of semiconductor (ZnO and TiO 2 ) and narrow bandgap materials (a-Fe 2 O 3 and graphene). 45,46 As shown in Fig. 7a, the second absorption edges shied toward a longer wavelength with increasing Fe 3 O 4 ratio. The formation of the core/shell structure with Fe 3 O 4 as the core and ZnO as the shell offered to couple between two oxides, thereby altering the electronic band structure of ZnO. Combined with these results and the obtained results from FTIR studies, we suggested that the hybridization of Fe 3 O 4 and ZnO at the core-shell interface created a buffer region in which Fe 3+ ions were incorporated into the ZnO lattice. The appearance of a shoulder in the absorption spectra of heterostructure samples may be evidence for the existence of the Fe ion-doped ZnO crystal shell. Despite evidence of Fe ions incorporated into the ZnO shell in composite samples from FTIR and UV-vis spectra, the shi in the ZnO diffraction peak was not observed in XRD patterns. Therefore, the core-shell hybridization of Fe 3 O 4 NPs and ZnO shell formed a Fe ion-doped buffer region at the core-shell interface. Similar results were also found in Fe 3 O 4 /TiO 2 coreshell NPs as revealed in Stefan's report. 47 The formation of an impurity level near the bottom of the conduction band CB or the top of the valance band by doping anions or cations enables the second absorption of visible light in core-shell heterostructure samples. 44,[46][47][48] In addition, a strong UV emission band assigned to the near band edge emission in ZnO was observed at 390 nm in the photoluminescence spectra of composite samples (as shown in Fig. 7b). Another broad visible emission (from 500 nm to 900 nm), which was related to impurity energy in the ZnO crystal, shied to long wavelengths (inset of Fig. 7b) as the Fe 3 O 4 core ratio increased. These obtained results may lead to an assumption that the optical properties of the heterostructure samples were simultaneously contributed by pure ZnO material, the buffer region, and Fe 3 O 4 NPs. The interaction of ZnO and Fe 3 O 4 NPs at the core-shell interface resulted in the Fedoped ZnO buffer region. The formation of Fe s-levels below the conduction band edge of ZnO effectively extended the absorption edge into the visible region. 46 To understand the results better, this feature should be studied further in future work. For further understanding optical properties of the coreshell heterostructures, bandgap values and the possible charge transfer mechanism were explored. The band gap (E g ) was estimated by employing the Kubelka-Munk equation. 49 where, X is Mulliken's absolute electronegativity (5.73 eV for   incorporated of Fe 2+ , Fe 3+ ions which may lead to extend the bottom of ZnO conduction band.

Photocatalytic performance
The photocatalytic activity of all the core-shell photocatalysts was examined by monitoring degradation kinetic of aqueous RhB solution under simulated sunlight irradiation. Typically, the experiments were conducted in dark for 1 h to achieve the adsorption equilibrium. Fig. 10 shows the absorption spectra of RhB aer photodegradation under the presence of photocatalysts with different irradiation time intervals. The efficiency of photocatalysts and the RhB self-degradation against the reaction time are shown in Fig. 11 53 Thus, the buffer region acted as a reservoir for the photoelectrons and extended the lifetime of the photogenerated charges due to the reduction of Fe +3 to Fe +2 . 21,54 Thus, the enhanced photocatalytic activity can be achieved in the core-shell Fe 3 O 4 / ZnO nanocomposite, especially in the sample with a core-shell ratio of 1 : 5. It is well known that the photocatalytic process to degrade organic pollutants involved three reactions: (i) generating electrons and hole at conduction band and valence band of photocatalyst by light absorption; (ii) electrons and hole move to the surface of photocatalyst and take reaction with O 2 and H 2 O absorbed on photocatalyst's surface to create reactive species; (iii) reactive species will degrade organic pollutants through strong oxidation and reduction reaction.
As shown in Fig. 9, it can be seen that the potential of conduction band of ZnO is more negative than the O 2 /O 2 % À potential (À0. 33     . cOH and h + were found as the main oxidizing species for the photocatalytic process of the ZnO-based composites. 55,56 Magnetic properties and reusability of the catalyst The separation and recyclability of a magnetic-based photocatalyst depend strongly on its magnetic properties. Fig. 12a shows the magnetization curves of Fe 3 O 4 NPs and core/shell  Fig. 12a). Reusability is an essential feature of composites for organic pollutant photodegradation in practice. The recycling and sustainability of Fe 3 O 4 /ZnO 1 : 5 were further evaluated with four continuous cycles (Fig. 12b). For each cycle, the photocatalyst in solution was rapidly separated by a magnet with surface magnetic eld strength of 4 kOe. The solar light photocatalytic activities of sample remained good aer four recycles. These results showed that core/shell Fe 3 O 4 /ZnO heteronanostructure could be an excellent candidate for practical wastewater treatment.

Conclusion
Magnetically separable core-shell Fe 3 O 4 /ZnO heteronanostructures for enhanced solar photocatalytic activity were successfully fabricated by ethylene glycol-citric acid assisted the effective sol-gel process. By controlling the core-shell molar ratio during synthesis, the structure-morphology of the Fe 3 O 4 / ZnO heterostructures varied from nearly spherical to rice seedlike. The formation of the core-shell structure and the hybridization of Fe 3 O 4 and ZnO at the core-shell interface created a buffer region, in which Fe ions were incorporated into the ZnO lattice. Compared with pristine ZnO, the band gap of the coreshell heterostructures could be signicantly modied which enhancing the sunlight harvesting ability. The minimum band gap energy was 2.783 eV, which belonged to the spherical nanocomposite with the core-shell ratio of 1 : 5. In comparison with ZnO NPs, the core-shell nanocomposites showed the higher sunlight photocatalytic activity. Efficient sunlight harvesting and reduced photogenerated electron-hole recombination rate should be the main factors to enhance the sunlight photocatalytic performance of Fe 3 O 4 /ZnO nanocomposites. Our results revealed that core-shell Fe 3 O 4 /ZnO heterostructure with Fe : Zn molar ratio is 1 : 5 showed an excellent sunlight photocatalytic performance, efficient magnetic separation, and recyclability during four cycles. Therefore, this photocatalysts can be excellent candidate for visible-light-driven photocatalysis applications.

Conflicts of interest
There are no conicts to declare.