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Mesocrystalline Zn-Doped Fe3O4 Hollow Submicrospheres: Formation Mechanism and Enhanced Photo-Fenton Catalytic Performance Xuan Sang Nguyen, Gaoke Zhang, and Xianfeng Yang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b16839 • Publication Date (Web): 24 Feb 2017 Downloaded from http://pubs.acs.org on February 26, 2017

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ACS Applied Materials & Interfaces

Mesocrystalline Zn-doped Fe3O4 Hollow Submicrospheres: Formation Mechanism and Enhanced Photo-Fenton Catalytic Performance Xuan Sang Nguyen a, c, Gaoke Zhang a*, Xianfeng Yang b* a Hubei Key Laboratory of Mineral Resources Processing and Environment, School of Resources and Environmental Engineering, Wuhan University of Technology, 122 Luoshi Road, Wuhan 430070, China. b Analytical and Testing Centre, South China University of Technology, Guangzhou, 510640, China. c Environmental Engineering Institute, Viet Nam Maritime University. *

Correspondence should be addressed to Gaoke Zhang (E-mail: [email protected]) and

Xianfeng Yang ([email protected])

1 Environment ACS Paragon Plus


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ABSTRACT

Uniform and magnetic recyclable mesocrystalline Zn doped Fe3O4 hollow submicrospheres (HSMSs) were successfully synthesized via simple one-pot solvothermal route and were used for efficient heterogeneous photo-Fenton catalyst. XRD, XPS, Raman spectroscopy, Mössbauer spectroscopy, SEM, HRTEM and EDX analyses revealed that the shell of HSMS is highly porous and assembled by oriented attachment magnetite nanocrystal building blocks with Znrich surfaces. Furthermore, a possible formation mechanism of mesocrystalline hollow materials was proposed. Firstly, Fe3O4 mesocrystals were assembled by oriented nanocrystals and a Znrich amorphous shell grew on the surfaces. Then, Zn gradually diffused into Fe3O4 crystals to form Zn-doped Fe3O4 due to Kirkendall effect with increasing the reaction time. Meanwhile, inner nanocrystals would be dissolved and outer particles would grow larger owning to Ostwald repining process leading to form hollow structure with porous shell. The Zn-doped Fe3O4 HSMSs exhibited high and stable photo-Fenton activity for degradation of rhodamine B (RhB) and cephalexin under visible light irradiation in presence of H2O2, which result from their hollow mesocrystal structure and Zn-doping. It could be easily separated and reused by external magnetic field. The results suggested that the as-obtained magnetite hollow mesocrystals could be a promising catalyst in the photo-Fenton process.

KEYWORDS: Zn-doped Fe3O4, mesocrystal, cephalexin, oriented attachment, magnetite, photoFenton activity.


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1. Introduction Mesocrystals are regarded as a new class of materials or non-classical crystallization by oriented attachment of crystalline prime-nanoparticles into a high ordered superstructure on the scale of hundreds nanometers to micrometres.1 Due to the unique characteristics, such as the high internal porosity and single crystalline structure, mesocrystals have been applied for many fields such as photocatalysis, heterogeneous catalysis, adsorption, biological sensing and optical devices.2-4 However, the formation mechanism of mesocrystals is still understood very poorly, to date. There are two possible reasons for this fact. One is that the high lattice energy of crystalline nanoparticle leads to crystallographic fusion, which can make it difficult to confirm the intermediate types of mesocrystals. Another one is that the homogeneous aqueous environment results in that studying the behavior of hydrophilic mesocrystal becomes very hard. In addition, because of inverse relation between surface area, porosity and crystallinity, creation of materials with high porosity and high crystalline structure is still a very difficult and challenging task. Recent years, spinel oxide nanoparticles have attracted much more attention due to their enhanced electrical conductivity, interesting magnetic properties and high photocatalytic activities.5-8 Among the spinel oxides, Fe3O4 nanoparticles with a cubic spinel structure where a half of Fe3+ ions occupy in all the tetrahedral sites, a half of the Fe3+ ions and all the Fe2+ ions local on the octahedral sites, are considered as promising materials in many fields owing to its abundant, low-cost, friendly-environment, easy controllable synthesis and interesting magnetic properties. There are many ways for fabrication of magnetite nanoparticles, such as hydrothermal route,9 ball mill method,10 poly diallyldimethylamonium chloride method,11 co-precipitation, emulsion method,12 solvolthermal,13 and etc. Among these methods, solvothermal method with polyol as solvent has been widely studied and used for preparing magnetite and metal doped



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magnetite materials with high crystallinity, dispersed and high porosity. For instance, Li et al. prepared magnetic single-crystal monodisperse microspheres in size of 200-800 nm via solvothermal process.14 Magnetite monodispersed nanospheres with controllable particle size in a mixed solvent of ethylene glycol/diethylene using polyvinyl pyrrolidone as the surfactant were also reported.15 Uniform magnetite hollowspheres composed of ultrathin nanosheet fabricated via a one-pot solvothermal method using a mixed of glyxeryl, isopropyl and a small amount of water has been reported.16 However, the mesocrystal Fe3O4 hollow spheres are hardly reported, which are expected to overcome the drawbacks of the single-crystal Fe3O4 hollow spheres and Fe3O4 nanoparticles, such as a low relatively magnetic, poor dispersibility, limited adsorption properties and low catalytic activity.17,18 Metal element doping is also a promising approach to improve Fe3O4 activity. For examples, Cu/Fe3O4 prepared by a green route as a magnetically separable catalyst showed high activity for the reduction of nitroarenes.19 Very recently, Zndoped Fe3O4 magnetic nanoparticles with different Zn contents have been achieved through a simple solvothermal method.20 Porous nanoplate Zn doped γ-Fe2O3 single-crystal exhibited a good photo-Feton on RhB.6 However, fabrication of uniform hollow Zn doped Fe3O4 with internal submicrosphere still remain as a challenge. More importantly, the formation mechanism of Zn doped iron oxide mesocrystals have not reported yet, to date. Herein, we successfully prepared uniform mesocrystalline Zn doped Fe3O4 hollow submicrospheres via a simple solvothermal route in absence of any surfactants. The as-obtained mesocrystals showed porosity properties and single crystalline structure and consist of oriented Zn-doped Fe3O4 nanocrystals. Furthermore, the growth mechanism of the as-prepared Zn-doped Fe3O4 hollow submicrosphere mesocrystal was studied. The catalytic property of the as-obtained products in photo-Fenton reaction was evaluated by degradation of rhodamine B (RhB) and


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cephalexin under visible light irradiation. This work could provide an insight into synthesizing Zn-doped Fe3O4 hollowsphere mesocrystals, understanding their formation mechanism and developing efficient photo-Fenton catalysts. 2. Experimental 2.1. Materials All chemicals used in this work were from Sinopharm Chemical Reagent Co., Ltd, China without further treatment and purification. De-ionized water was used in the whole experiments. 2.2. Mesocrystal preparation Zn-doped Fe3O4 hollow submicrosphere mesocrystals were prepared by a simple solvothermal approach. Typically, 0.272 g of ZnCl2 and 1.08 g of FeCl3.6H2O and 2.312 g of amoniacetate (NH4Ac) dissolved in 100 ml ethylene glycol while stirring kept for 0.5h. Sequentially, the solution was transferred into the 100 ml autoclave and heated at 200 °C for 24h after ultrasonic treatment (at 59 Hz) for 30 minutes. After naturally cooled until room temperature, the resulting product was collected by a magnet, washed with ethanol and deionized water for several times. Finally, product powder was dried at 80 ºC in an electric oven for 12h before further use. For comparison, pure Fe3O4 were synthesized in the absence of zinc chlorate though the same procedure. 2.3. Characterization X-ray diffraction (XRD) analysis was carried out using a X-ray powder diffractometer with Cu Kα radiation at 40 kV and 50 mA. The morphology and internal structure of the prepared samples were further checked by transmission electron microcopy (TEM), selected area electron diffraction (SAED) and high-resolution transmission electron microcopy (HRTEM) using a JEOL JEM-2010 HR electron microscope operated at a voltage of 200 kV and equipped with a



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Gatan GIF Tridiem system. The specific surface areas (BET) of products were examined by nitro adsorption on a Micromeritics ASAP 2020 automatic equipment. The element composition of the as-prepared samples was investigated by energy dispersive X-ray (EDX) method with a Zeiss Ultra Plus-43-13 field emission scanning electron microscopy. X-ray photoelectron spectroscopy (XPS) was checked by a Thermo ESCALAB 250XI spectrometer with Al Kα source. The PL spectra of product were measured by a transient fluorescence spectrometer (Shimadzu RF5301PC). 2.4. Photo-Fenton catalytic activity The photo-Fenton catalytic activities of the as-synthesized sample were evaluated by degradation of both RhB and cephalexin solution in the presence of H2O2 under visible light at room temperature, using a 350 W Xe lamp with a cutoff filter (λ > 420 nm). In a typical experiment, 0.10 g of HSMSs catalyst was added in cephalexin or RhB aqueous solution (10 mg/L, 100 mL) and was stirred in the dark for 30 min to disperse the catalyst. After that, 0.1 mL of hydrogen peroxide solution (H2O2, 30 wt %) was added to the suspension under stirring. Then the suspension was irradiated by the lamp. During the visible light irradiation, about 4 mL of the suspension was taken out at given time intervals then centrifuged (5000 rpm, 5 min) to remove the residual catalyst powder. Cephalexin and RhB concentration was analyzed by a UV-visible Spectrophotometer (Shimadzu UV 1750, Japan) with de-ionized water as a reference sample. 3. Results and Discussion 3.1. Characterization of hollowsphere Zn-doped Fe3O4 mesocrystals


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Figure 1. (a) SEM images of the as-synthesized sample, TEM images of (b) the typical product at low magnification, (c) at high magnification, (d) a single as-synthesized hollow submicrosphere mesocrystal with wall thickness of 120 nm (red arrow).

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Figure 2. (a), (b) HRTEM images, (c) SAED pattern from the single as-prepared Zn doped Fe3O4 obtained at 200 °C for 24h. Morphology and crystalline structure of the as-prepared products obtained at 200 °C for 24h were investigated by SEM, TEM and HRTEM. As depicted in Figure 1a-c, the products had uniform submicrosphere shape with an average size of 400 nm. The TEM image of an individual Zn-doped Fe3O4 submicrosphere in Figure 1d confirms its hollow and high porosity structure. The submicrospheres are a dense assembly of the small particles from center to out layer and have a mean size of about 400 nm with a shell thickness of 120 nm. In addition, the hollow structure of the as-synthesized product is further verified by HAADF-STEM and EDX element mapping analyses as shown in Figure S1 and Figure S2, respectively. It is obvious to see the presence of nanometer pores with different size located inside the HSMSs. The structure of the as-synthesized hollow submicrosphere was further observed by HRTEM (Figure 2). The lattice fringes of 0.295 nm (Figure 2a) between two adjacent lattice planes of inner core match well with the crystallographic planes (220) of magnetite with cubic spinel structure.21 As shown in


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Figure 2b, the core and shell structure of the samples can be seen obviously in which a shell may be consist of amorphous zinc-rich (showed more below). The SAED patterns in Figure 2c indicate that the as-prepared Zn-doped Fe3O4 hollow submicrospheres are mesocrystals, in which primal nanocrystals were assembled by crystallographically oriented aggregation.

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2 Theta (degree) Figure 3. (a) XRD pattern of pure Fe3O4 and HSMSs obtained at 200 °C for 24 h, (b) the highlighted (311) diffraction peak of pure Fe3O4 and HSMSs obtained at 200 °C for 24 h.



The XRD patterns of the pure Fe3O4 and Zn-doped Fe3O4 hollowsphere mesocrystals prepared at 200 °C for 24 h are showed in Figure 3a. Both the patterns indicate that all reflection peaks could be indexed to magnetite with cubic spinel structure (JCPDS 77-1545). No diffraction peaks of zinc oxide were observed, suggesting that crystalline zinc oxides did not form during the synthesis process. However, it can be learned from Figure 3b that the (311) crystal plane of Zndoped Fe3O4 slightly shifted to smaller angle and the corresponded lattice constant increased from 0.8395 nm to 0.8406 nm, which could be ascribed to the substitution of a small amount of Fe2+ (ion radius =0.61 nm) and Fe3+ (ion radius = 0.49 nm) in the magnetite by Zn2+ with a larger ion radius of 0.74 nm. The results suggest that Zn has been successfully doped into the crystal structure of Fe3O4, in which doped Zn2+ ion may tend to substitute Fe3+ ion in tetrahedral sites and a small amount Fe2+ ion located on octahedral sites.6,22

(a) Relative Transmission

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Figure 4. (a) Room-temperature 57Fe Mössbauer spectra of the as-obtained sample at 200 °C for 24 h; Raman spectra of the samples obtained at 200 °C for different times excited at 633nm (b) with 20%; (c) with 40% power. To get more information on the valance state of iron and the phase composition in the asprepared samples, Mössbauer and Raman spectra analysis were carried out. The room temperature Mössbauer spectra of the as-prepared sample were fitted with Lorentzian-shaped lines using the method of least squares and are depicted in Figure 4a. The spectra of the sample



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show well-resolved Zeeman slitting, which could be fitted with two sextets, typical of inverse spinel structure of magnetite.23-25 One sextets has hyperfine fields (Hhf) of 486.00 kOe and isomer shift (IS) of 0.29mm/s and another is of 453.97 kOe and 0.64 mm/s, which reveal that the Fe3+ ions occupied on the tetrahedral sites and mixed valance (Fe2+, Fe3+) ions located on the octahedral sites, respectively (Table S1). The results are also in good agreement with the Mössbauer spectra of magnetite nanoparticles observed at room temperature as reported by the other researchers,23-25 which indicates that the resulting product is only Fe3O4 phase. The results also reveal that zinc is mainly on the surface of Fe3O4 nanoparticles and a small amount of zinc diffused into Fe3O4 nanocrystals. Mössbauer analysis results also imply the ferromagnetic behavior of the as-prepared Zn-doped sample.21 Raman spectroscopy was conducted at excited 633nm with 20% and 40% power to further confirm the phase composition of the as-prepared hollow submicrospheres. Figure 4b shows the Raman spectra of the samples obtained at 200 ºC for different times excited at 633nm with 20% power. The band at 670 cm-1 of all the samples could be ascribed to the characteristic peak of magnetite, which corresponds to a symmetric Fe−O breathing A1g mode.26,27 No bands of ZnO were observed, which is well consistent with the results of XRD and Mössbauer analyses. Figure 4c gives the Raman spectra of the samples obtained at 200 ºC for different times excited at 633nm with 40% power. Interestingly, we found that the remarkable phase transition occurred at the surface of the samples when the samples were excited at 633nm with 40% power. It is found that new bands appeared at 220, 284, 396, 485, 601, 655 and 1300 cm-1 could be the characteristic of α–Fe2O3. The peaks seen at 220 cm-1 and 485 cm are assigned to the A1g modes. The remaining five peaks at 284, 396, 604, and 654 cm-1 are ascribed to the Eg modes. The band at 720 cm-1 corresponds to the characteristic of γ-Fe2O3 and the band at 670 cm-1 could be


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ascribed to the characteristic peak of magnetite.26-28 The band at 893 cm-1 may correspond to vibrations of oxo-bridge di-iron O−Fe−O bonds at intermediate phase.29 No apparent bands of ZnO were observed. It is reported that the main characteristic band of ZnO appear at 437 cm−1.30,31 These results indicate the magnetite was degraded under laser irradiation of 633nm with 40% power.32 It is worth mentioning that the symmetric stretch of Zn-O in Raman mode of Zn ferrite is at 650 cm-1 33, which was not observed in both the samples excited at 633 nm with 20% and 40% power, suggests the as-prepared sample is Zn doped Fe3O4.

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Binding Energy (eV) Figure 5. XPS spectra of the as-obtained Zn doped Fe3O4 hollow submicrosphere mesocrystals: (a) survey spectrum, (b) Fe 2p spectra where “1∼3”and “5∼7”are Fe2+ (B-site), Fe3+ (A-sites) and Fe3+ (B-sites) on Fe 2p3/2 and Fe 2p1/2, respectively. “4” and “8” are satellite peaks associated with Fe 2p3/2 and Fe 2p1/2, respectively (c), Zn 2p spectrum. 14 Environment ACS Paragon Plus

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To further confirm the element composition and valence state on the surface of the Zn-doped Fe3O4 hollow submicrosphere mesocrystals, the sample was examined by X-ray photoelectron method within a range of binding energies of 0–1200 eV. The survey spectrum of the samples in Figure 5a indicates that the Zn, Fe, O are main elements in the product. Generally, in Fe3O4 with spinel structure, due to Fe cation at both A sites and B sites, the binding energies of corresponding Fe 2p electron will be not same. As Mössbauer spectra shown above, Fe cations are mixed valence states (Fe2+, Fe3+) and lead to the emitted photoelectrons with different energies. Thus, it suggests that the XPS of Fe 2p electron in the as-prepared Zn-doped Fe3O4 hollowsphere mesocrystals should be fitted into several of peaks. As depicted in Figure 5b, the Fe 2p3/2 peaks of the Zn-doped Fe3O4 mesocrystal submicrospheres can be resolved into four sub-peaks noted with “1-4”, where “1” at 708.9 eV is for Fe2+ (octahedral-sites) sub-peak, while “2” and “3” at 710.1 and 712.2 eV are Fe3+ sub-peaks at tetrahedral and octahedral sites, respectively and along with associated satellite “4” at 718.3 eV.33,34 The peak at 1021.2 eV (Figure 5c) is ascribed to the binding energy of Zn2+ 2p3/2.34 It is worthy to note that the Zn/Fe ratio (1.25) from XPS is much larger than that (0.06) from EDX in the resulting product. Based on this, it can be further confirmed that Zn is mainly on the surface of the particles due to the signal of XPS is mainly derived from the surface and that of EDX is mainly from the whole particle. The results are also in good agreement with that of the XRD, Raman and Mössbauer analyses. The porosity properties of the as-prepared hollow submicrosphere mesocrystals were measured by nitrogen sorption method. As shown in Figure S3, Zn-doped Fe3O4 hollow submicrosphere mesocrystals have the dominance of mesoporous structure with the surface area value 15.01 m2/g. The pore size distribution curve was identified from desorption branch by the Barret-Joyner-



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Halenda (HJH) of Zn doped Fe3O4 HSMSs (Inserted in Figure S3) and displays mesopores structure with an average pore with of approximate 30 nm and a small part located at about 5nm. The results suggest that there are the interspaces of the constituent particles and the void in the hollow sphere. It can be found that the as-prepared Zn-doped Fe3O4 hollow submicrosphere mesocrystals exhibited high porosity hollowsphere architecture. 3.2. The possible formation mechanism of Zn-doped Fe3O4 hollowsphere mesocrystals

Figure 6. TEM images of the samples prepared at 200 °C for 10h (a), for 18h (b), for 24h (c), for 28h (d) with coresponding SAED pattern inserted. To understand the formation mechanism of the as-prepared Zn-doped Fe3O4 hollow submicrosphere mesocrystals, a series of experiments were carried out at 200 ºC for different reaction times to capture the intermediate states of the crystal growth and the as-obtained products were characterized in detail by XRD, electron microscopy and EDX mapping. XRD


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patterns of the products obtained at 200 ºC for different reaction time are showed in Figure S4a. It is obviously observed that all the patterns are consistent with that of magnetite with cubic spinel structure (JCPDS 77-1545). No impurities were detected. However, Figure S4b shows that the (311) diffraction peak of the as-obtained products shifted slightly to smaller angle. Detailed XRD analysis revealed that corresponding lattice parameters increased with increasing the hydrothermal reaction time from 10h to 28h (Table S2). The results suggest that Zn had been continuously doped in the crystal structure of Fe3O4.6,22,35 The results from ICP test also show that doped Zn content increased with increasing the reaction time (Table S3). Figure 6 presented TEM images of the products prepared at 200 ºC for 10h, 18h, 24h and 28h, in which corresponding SAED pattern inserted. It is found that there is no significant change in size with the increase of reaction time while obviously changing for the interior cavity of these samples can be observed. According to Figure 6a, loosed spheres products with a diameter of about 400500 nm formed when the reaction time was 10 h. With increasing reaction time to 18h, these loosed solid sphere particles changed into tighter porous sphere morphology with a diameter of about 450 nm. When the reaction time was further prolonged to 24h, inner crystals would be dissolved to form internal cave and high regular Zn-doped Fe3O4 hollow submicrosphere mesocrystals formed. After 28h of solvothermal treatment, evacuation process apparently occurred, which resulted in larger holes in the products. Obviously, growth of larger particles results from the dissolution of smaller one, so-called Ostwald ripening growth was dominant in this state. These processes have been successfully used for explaining fabrication mechanism of many nanomaterial systems with hollow interiors.36,37 Interestingly, almost of all the samples in the time-dependent experiments possess single mesocrystal structure, indicating the ordered assembly of Fe3O4 nanocrystals. However, SAED pattern reveal clearly that the sample obtained



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at 10 h and 18h has some particles disordered from perfect alignment between nanocrystallites in the spheres. These miss-ordered particles (red rectangle and circle) and amorphous ones (black arrow) were further proved at HRTEM image as shown at Figure S5, suggesting slight disordering by Zn doping.38 Recently, Kirkendall effect, a classical phenomenon in metallurgy, has been applied to fabricate monocrystalline spinel nanotube, hollow spherical, polyhedral, capsule-like porous hollow nanoparticles.39,40 It was reported that the formation of zinc cobalt oxide hollow nanocubes is because of ion-exchange by a unique Kirkendall effect.41 Herein, it is believed that there are two processes leading to form Zn doped Fe3O4 product. One is the replacement of Fe3+ ion with Zn2+ in tetrahedral sites due to Zn2+ ion usually preferring tetrahedral sites.42 Another is the ion-exchange based on Kirkendall effect may favorably occur between Zn2+ ion and Fe2+ ion in Fe3O4 mesocrystal, which could be attributed to similar ion radii of zinc ions in tetrahedral environment and divalent iron ion radius in octahedral [(rZn2+)terta= 0.60 nm and (rFe2+)octa= 0.61 nm].41 Doping effect was further confirmed by EDX mapping images. As depicted at Figure 7, Fe3O4 mesocrystals and amorphous zinc-rich shell could be obtained after reaction for 10h. Then Zn in the shell gradually diffused into Fe3O4 nanocrystals with prolonging reaction time. Base on the above evidence and analysis, a possible formation mechanism for Zn-doped Fe3O4 hollowsphere mesocrystals was proposed (see Figure 7). According to this, Fe3O4 mesocrystal covered by an amorphous zinc-rich shell was firstly formed. Then doping effect will be dominant due to Zn2+ ion diffusing into Fe3O4 crystal via ionexchanged Kirkendall effect. At following state, Zn-doped effect was successively, meanwhile the growth of larger particles by dissolving smaller ones which have high surface energy and soluble capacity. As the result, Zn-doped Fe3O4 hollow submicrosphere mesocrystals formed. When increasing the reaction time, both oriented attachment and Ostwald repining mechanisms


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can work simultaneously to form hollowsphere mesocrystals with shell of more thickness where the overall surface energy of the crystals decreased and has more stable energy state. Finally, with continuously ripening process, the core interior space in the microspheres was further increased while ordered aggregate process could be slowed down.

Figure 7. Schematic of the formation process for Zn doped Fe3O4 hollow submicrosphere mesocrystals. 3.3. Photo-Fenton catalytic activities The photo-catalytic performance of HSMSs was investigated by degradation of RhB and cephalexin solution in the presence of H2O2, respectively. To further check the catalytic activity of Zn-doped Fe3O4 hollowsphere mesocrystals, the pure Fe3O4 like-spheres and ZnxFe3-xO4 (x=1/3) nanoparticles synthesized by the chemical coprecipitation method34 and was also used as catalyst in the photo-Fenton reaction. The degradation of cephalexin and RhB solution under different conditions is shown in Figure 8 and Figure S6, respectively. The negligible degradation



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was observed in only photolysis process or only H2O2 or only with catalyst. Zn-doped Fe3O4 hollow submicrosphere mesocrystals showed much high efficiency than the pure Fe3O4 for degradation of both cephalexin and RhB with H2O2 under visible light. As observed from the Figure S6 and Figure 8a, 97% of RhB and about 90% of cephalexin were degraded after visible light irradiation for 60 and 180 minutes in the presence of Zn-doped Fe3O4 hollowsphere mesocrystals, respectively. And only 26 % of RhB and 18% of cephalexin were removed by the pure Fe3O4 catalysts under the same conditions, respectively. With Zn doped Fe3O4 nanoparticles as the catalyst, the photo-Fenton degradation of RhB and cephalexin reached only 19% and 12%, respectively. These results indicated that Zn doped Fe3O4 hollowsphere mesocrystals exhibited the highest photo-Fenton activity as compared to Zn doped Fe3O4 nanoparticles and pure Fe3O4. The adsorption of RhB and cephalexin on the Zn-doped Fe3O4 hollow submicrosphere mesocrystals in dark was also investigated. The adsorption of RhB and cephalexin on HSMSs was negligible, indicating that the removal of dye and pharmaceutical contaminant was only achieved by the catalyst/H2O2/visible light system. The absorption spectra of the cephalexin solution during photo-Fenton degradation are shown in Figure 8b. A decrease of the characteristic peak at 266 nm of cephalexin can be observed with increasing visible light irradiation time. The mineralization of cephalexin was also tested as shown in Figure S7. The results imply that the TOC removal of cephalexin and RhB under Zn doped Fe3O4 HSMSs/H2O2/light systems reached 62 % and 70%, respectively, suggesting Zn doped Fe3O4 asprepared is high efficient for the mineralization of contaminant. Effect of dose catalyst was also investigated as depicted in Figure S8. The results show the degradation ratio of cephalexin increased with an increase in catalyst dosage and slightly decreased with further increasing the catalyst dosage. Based on the experiment, 0.1 mg/L of catalyst is suitable for the photo-Fenton


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reaction. To further prove high catalytic properties in the photo-Fenton, our systems have been compared to other zinc iron oxide catalysts reported in the literatures as shown in Table S4. For explaining the higher catalytic activity of the Zn doped Fe3O4, SEM image, BET analysis and UV-vis spectra of both pure Fe3O4 and HSMSs were carried out. SEM images (Figure S9). The results imply that pure Fe3O4 product are not uniform and are lower dispersed than asprepared Zn doped Fe3O4. As depicted in Figure S3, pure Fe3O4 showed lower surface area (11.5 m2/g) than Zn doped Fe3O4 (15.2 m2/g). In addition, the pore size distribution curve was calculated from desorption branch by the Barret-Joyner-Halenda (HJH) of pure Fe3O4 (Inserted in Figure S3) exhibited mesopores structure with an average pore of 7.5 nm while that of Zn doped Fe3O4 showed an average pore with of approximate 30 nm and a small part located at about 5nm. The results suggest that there are the interspaces of the constituent particles and the void in Zn doped Fe3O4 hollow sphere and exhibited higher porosity hollowsphere architecture than pure Fe3O4. The optical absorption spectra of both Zn doped Fe3O4 HSMSs and pure Fe3O4 are depicted in Figure S10. The results imply that the Zn doped Fe3O4 exhibited a stronger Vis light absorption than pure Fe3O4 in the range of 400-700 nm. This can be attributed to the hollow and void space in the structure of Zn doped Fe3O4, which result in a better utilization of light energy.18 A following photocatalytic mechanism suggested for understanding photo-Fenton reaction in the present of Zn doped Fe3O4 HSMSs catalyst.43 Under visible light irradiation, electron/hole pairs can be photogenerated in the catalyst. Then, photogenerated electrons can be trapped by H2O2 leading to •OH. Simultaneously, they can be trapped by Fe3+ on surface of catalyst forming Fe2+. Then more •OH can be produced that resulting in reaction between formed Fe2+ with H2O2. Catalyst + hυ → Catalyst (e-cb, h+vb)

(1)



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e- + H2O2 →•OH + OH-

(2)

e- + Fe3+ → Fe2+

(3)

Fe2+ + H2O2 → Fe3+ + •OH + OH-

(4)

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However, using as-prepared Zn-doped Fe3O4 hollow submicrosphere mesocrystals, the photoFenton efficiency can be intensity enhanced. This could be ascribed to unique structure as well as metal doping effect of the as-prepared sample. The Zn-doped Fe3O4 hollow submicrosphere mesocrystals simultaneously possess both porosity hollow and high ordered crystallographic structure. High porosity will lead to accommodation of large number of active sites and accumulation of charge carries, thus adsorption of reactants on surface and transport of products become easier. In addition, the unique hollow structure with submicrospheres and void space will improve visible-light absorption.44 The mesocrystal structure of submicrospheres could eliminate grain boundary as electron trapping center and delayed the combination process of photogenerared electron/hole pairs. The as-obtained hollow submicrosphere mesocrystals display high crystalline structure and thus, could improve the transfer of reactive radicals generated during photo-Fenton catalytic reaction and further enhanced the efficiency of photo-Fenton catalysis.45 Furthermore, the improvement of the photo-Fenton reaction of Zn-doped Fe3O4 could be attributed to that metal doping can make the electron transfer process accelerated due to improving the interface between Fe3+ and H2O2, which result in more ·OH radical from high rate of decomposition H2O2.7,46,47 To further confirm the superior performances of mesocrystal structure materials as compared to their original structure, a summary on the catalytic performances of mesocrystals reported by other groups is shown in Table S5. The results in Table S5 show that the catalytic activities of those mesocrystals are much higher than that of their ordinary products.


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In addition, the stability and reusability of HSMSs in the photo-Fenton process were also investigated. The results show that the catalyst was easily separated by an external magnetic field (Insert Figure 8b) and their photo-Fenton efficiency for cephalexin degradation has no significant change even after ten successive cycles, indicating high stability of the catalyst (Figure 8c). In order to further confirm the stability of HSMSs in the photo-Fenton, leaching iron and zinc also was investigated by AAS method. The results showed that the leaching Zn and Fe content were 0.0102 mg/L and 0.0016 mg/L, respectively. The results indicate that metal content leaching in solution is insignificant. FTIR analysis of Zn doped Fe3O4 HSMSs before and after the photoFenton process also was carried out. As seen from Figure S11, the FTIR spectrum of Zn doped Fe3O4 show peak at 581 cm-1 and 430 cm-1 corresponded to Zn-O stretching vibration in tetrahedral and octahedral bonding of Fe-O, respectively.7 The band at 1541 cm-1 can be attributed to absorbance of CO2 in the ambient air.48 The band at 3474 cm-1 and 1647 cm-1 present the stretching vibration of H2O and OH group.49 It can see clearly that the FTIR spectra of Zn doped Fe3O4 before and after the photo-Fenton reaction has no significant changes, indicating the high stability of the catalyst. In addition, XPS spectra of Fe 2p and SEM image of recovered Zn doped Fe3O4 after ten runs also are investigated and shown in Figure S12 and S13. The results showed that the morphology and iron valence states of Zn doped Fe3O4 had no significant changes after ten runs. These properties will play a very important role in application for water treatment at industry scale.



a

0.8

light off

1.0

Ct/Co

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0.6 0.4 0.2 0.0 -50

Visible light H2O2 Zn doped Fe3O4 HSMSs+ light Zn doped Fe3O4 HSMSs+ H2O2 Zn doped Fe3O4 nanoparticles Pure Fe3O4 Zn doped Fe3O4 HSMSs+ light+ H2O2

0

50

100

Time (min.)


150

200

c

10th run

0.8

9th run

8th run

7th run

6th run

5th run

4th run

3rd run

1st run

1.0

Ct/Co

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2nd run

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0.6 0.4 0.2 0.0 0

180 360 540 720 900 108012601440162018001980

Time (min.)

Figure 8. (a) The photo-Fenton degradation of cephalexin (CEX) under different conditions; (b) UV-Vis absorption spectra of cephalexin aqueous solution during the heterogeneous photoFenton degradation; the recovery of the used catalysts under an external magnetic field (insert); (c) The regeneration and reutilization of HSMSs for the heterogeneous photo-Fenton degradation of cephalexin. 4. Conclusions Uniform and magnetic recyclable Zn doped Fe3O4 hollow submicrosphere mesocryastals were fabricated via a facile solvothermal process in EG solvent. Zn was doped into Fe3O4 and no ZnO crystalline phase formed. The as-prepared mesocrystals are uniform hollowsphere with high porosity and ferromagnetic behavior, which have an ordered arrangement structure with Zndoped Fe3O4 and zinc-rich shell. The Fe3+ ions in the products occupied on the tetrahedral sites and mixed valance (Fe2+, Fe3+) ions located on the octahedral sites, respectively. The diffuse based on ion-exchange Kirkendall effect, the cooperation oriented attachment and Ostwald repining are responsible for the formation process of Zn doped Fe3O4 hollowsphere mesocrystals.



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The Zn-doped Fe3O4 hollowsphere mesocrystals exhibited high photo-Fenton catalytic efficiency for degradation of RhB and cephalexin in water in the presence of H2O2 and under visible light irradiation, which is ascribed to their hollow mesocrystal structure and Zn-doping. The hollowsphere mesocrystal catalyst has high stability and is easily separated and recycled by external magnetic field and could be promising photo-Fenton catalysts for the degradation of dye and pharmaceutical contaminants.

ASSOCIATED CONTENT Supporting Information The supporting information is available free of charge via the internet at http://pubs.acs.org. Table S1. Hyperfine parameters (IS – isomer shift, H – magnetic hyperfine) and relative area of Zn-doped Fe3O4 HSMSs at 200 °C for 24h; Table S2. Detailed XRD results for lattice parameter of the samples prepared at 200 °C for different reaction time. Table S3. The content of Zn and Fe in the as-prepared product at 200 °C for different reaction time tested by ICP; Table S4. Efficiency of zinc iron oxide catalysts reacted with dyes in the photo-Fenton reaction; Table S5. Summary on the catalytic performance of mesocrystals reported by other groups. Figure S1. HAADF-STEM images of (a) the as-synthesized product, (b) a single Zn-doped Fe3O4 HSMSs; Figure S2. (a) DF-TEM image of the single Zn-doped Fe3O4 HSMSs, (f) EDX elemental mapping of all Zn, Fe, O elements in hollowsphere mesocrystal, (b-e) EDX elemental mapping of individual Zn, Fe and O atom; Figure S3. Nitrogen adsorption-desorption isotherms and the pore size distribution curve (inset) of Pure Fe3O4 and Zn-doped Fe3O4 HSMSs; Figure S4. (a) XRD pattern of Zn doped Fe3O4 hollowsphere mesocrystals obtained at 200 °C for different reaction time; (b) the highlighted (311) diffraction peak corresponding; Figure S5. HRTEM


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images of the samples obtained at 200 °C for 10h (a); for 18h (b); Figure S6. The degradation of RhB under different conditions; Figure S7. The photo-Fenton degradation and TOC removal of (a) Cephalexin, (b) RhB under Zn doped Fe3O4 HSMSs / H2O2 / visible light system.; Figure S8. Effect of contents of Zn doped Fe3O4 HSMSs on the photo-Fenton degradation of cephalexin; Figure S9. SEM image of (a) pure Fe3O4, (b) the as-prepared Zn doped Fe3O4 HSMSs; Figure S10. UV-Vis absorption spectra of pure Fe3O4 and Zn-doped Fe3O4 hollowsphere mesocrystals obtained at 200 °C; Figure S11. FTIR spectra of the original and recovered Zn doped Fe3O4 HSMSs obtained at 200 °C for 24h; Figure S12. SEM image of the as-obtained Zn doped Fe3O4 HSMSs after ten runs; Figure S13. XPS spectra of the as-obtained Zn doped Fe3O4 submicroshperes before and after ten runs. AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] ( Gaoke Zhang) *E-mail: [email protected] ( Xianfeng Yang) Tel: 86-27-87651816; fax: 86-27-87887445. Notes The authors declare no competing financial interests. ACKNOWLEDGMENTS This work was supported by NSFC (No.51472194) and National Program on Key Basic Research Project of China (973 Program) 2013CB632402.



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