Improving photocatalysis and magnetic recyclability in Bi5Fe0.95Co0.05Ti3O15 via europium doping

Accepted Manuscript Improving photocatalysis and magnetic recyclability in Bi5Fe0.95Co0.05Ti3O15 via europium doping Zhu Zhu, Xiaoning Li, Wen Gu, Jianlin Wang, Haoliang Huang, Ranran Peng, XiaoFang Zhai, Zhengping Fu, Yalin Lu PII:

S0925-8388(16)31723-6

DOI:

10.1016/j.jallcom.2016.06.012

Reference:

JALCOM 37881

To appear in:

Journal of Alloys and Compounds

Received Date: 24 March 2016 Revised Date:

29 May 2016

Accepted Date: 3 June 2016

Please cite this article as: Z. Zhu, X. Li, W. Gu, J. Wang, H. Huang, R. Peng, X. Zhai, Z. Fu, Y. Lu, Improving photocatalysis and magnetic recyclability in Bi5Fe0.95Co0.05Ti3O15 via europium doping, Journal of Alloys and Compounds (2016), doi: 10.1016/j.jallcom.2016.06.012. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Improving photocatalysis and magnetic recyclability in

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E-mail: Zhu Zhu : [email protected] Xiaoning Li : [email protected] Wen Ge : [email protected] Jiangling Wang : [email protected] Haoliang Huang : [email protected] Ranran Peng : [email protected] XiaoFang Zhai : [email protected] Zhengping Fu : [email protected] Yalin Lu : [email protected] 1

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CAS Key Laboratory of Materials for Energy Conversion, Department of Materials Science and Engineering, University of Science and Technology of China, Hefei 230026, P. R. China 2 Hefei National Laboratory for Physical Sciences at Microscale, University of Science and Technology of China, Hefei 230026, P. R. China 3 Synergetic Innovation Center of Quantum Information & Quantum Physics, University of Science and Technology of China, Hefei 230026, P. R. China 4 National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei 230029, China 5 Hefei Physical Sciences and Technology Center, CAS Hefei Institutes of Physical Sciences, Hefei 230031, Anhui, China 6 Laser Optics Research Center, US Air Force Academy, Colorado 80840, USA

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Zhu Zhu1, Xiaoning Li1, Wen Gu1, Jianlin Wang3,4, Haoliang Huang1, 3, Ranran Peng1, 3, XiaoFang Zhai2,3, Zhengping Fu1,3,*, Yalin Lu1,2,3,4,5,6*

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Bi5Fe0.95Co0.05Ti3O15 via europium doping

Abstract: Nanomaterials with improved photocatalysis and magnetism may enrich their field

implementation in actual situations where both high light activity and good room-temperature (RT) magnetic recyclability are required. Previously, composite structures formed by at least one magnetic component with a photocatalysis part are mainly used to realize magnetically retrievable photocatalysts. In this work, europium doped Bi5Fe0.95Co0.05Ti3O15, in the format of nanoflowers, exhibiting a 1

ACCEPTED MANUSCRIPT significant ferromagnetism at the room temperature and the notable UV- and visible-light-driven degradation capability, were synthesized by the hydrothermal method. Both ferromagnetism and photocatalysis properties of the resulting Bi5-xEuxFe0.95Co0.05Ti3O15 were greatly improved by optimizing the doped europium content. Recyclability of such nanoflower photocatalysts in water solutions was demonstrated by simply applying a magnetic field at the RT environment, while a

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complete photocatalytic decompose of RhB dye was verified by the Fourier transform infrared spectra.

Keywords: photocatalyst, visible-light, ferromagnetism, recyclability.

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1. Introduction 10

The environment has been facing serious challenges associated with toxic organic

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contaminations, mainly from the widespread abuse and producing of pharmaceuticals, dyes and pesticides.[1, 2] While incineration, filtration, microbiological degradation, and oxidation routes are traditionally adopted to treating such water pollutants, technologies with more feasibility and less costs, such as the photocatalysis using solar energy, has been regarded as 15

economic and energy saving ways.[3, 4] It also has been widely accepted that photocatalysts in

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the nanoscale can improve the photocatalysis efficiency, through increasing the reactive surfaces and reducing the recombination possibility of the photo-generated carriers via shortening the migration distances. Unfortunately, small particles will definitely lead to the

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difficulties in removing them from the solutions after the field implementation, which further 20

causes the unwanted secondary contamination.[5, 6] Photocatalysts with magnetism which can

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be recycled via a magnetic field could become an economic and straightforward way to solve this issue. In past decades, researches have been done on synthesizing magnetic heterostructured photocatalysts which consisted of at least one magnetic component and one photocatalysis component. While realizing the magnetic retreatment being successful in most 25

heterostructured cases, [7, 8] the composite structure itself may lead to new problems, including the apparent material instability, high fabrication cost, reduced photocatalysis efficiency, et al. Recently, we have proposed that using multifunctional homogeneous nanomaterials could become an alternative way to realize the efficient magnetic retreatment, as well as the new way to improve the visible light photocatalysis efficiency. Aurivillius phase oxides, with a 2

ACCEPTED MANUSCRIPT general formula of Bim+1Mm-3Ti3O3m+3 (m≥3) (here M is the transition metal atom which can have the 3+ valance, including Fe3+, Co3+, Cr3+, etc.), have a typical layered structure stacked alternatively by two fluorite-type (Bi2O2)2+ and m perovskite-type (Bim-1Mm-3Ti3O3m+1)2- slabs. New functions can be feasibly implemented by modifying both A-sites (Bi) and B-sites (M) 5

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elements inside the structure. As an initial attempt, Aurivillius phase nanoshelves have been synthesized by a facile hydrothermal method, and was demonstrated as visible light-driven photocatalysts for decomposing Rhodamine B (RhB), as well as the co-existed ferroelectricity and ferromagnetism properties.[9] The nanoshelf structure could also suppress the unwanted

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aggregation problem introduced by the existed spontaneous polarization and ensure a high 10

Brunauer–Emmett–Teller surface area. Stability of both photoactivity and magnetic

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recyclability were demonstrated under the temperature higher than room temperature (RT) and the good viscosity conditions, which were intended to simulate the actual industrial environments.

In fact, both photocatalysis and magnetic performances of Aurivillius phase oxides could 15

be further improved by doping rare earth elements according to following reasons: 1) a rare

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earth element could promote the photocatalysis due to that their f-orbitals could form complexes with various Lewis bases;[10,

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2) the rare earth element could improve the

photocatalysis due to the possibly enhanced ferroelectricity through the introduced lattice distortion;[12] 3) morphology of the resulted nanoparticles can be modified by the doping,

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which may improve the specific surface area, as well as the exposed specific crystal facets of preferred;[13] 4) the doping could enhance the magnetism by changing the exchange

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interactions via altering the bonding.[14] Therefore in this work, europium was introduced into Bi5Fe0.95Co0.05Ti3O15 and its

nanoflowers as the preferred morphology were fabricated by the hydrothermal method. Both 25

structures and material properties were carefully investigated, and the obtained results indicated that Eu doping can effectively improve the photocatalysis and RT ferromagnetic performances. A complete photo-decompose of RhB dyes were demonstrated by the Fourier transform

infrared

spectra

(FTIR),

and

a

full

retreatment

of

the

in-solution

Bi5-xEuxFe0.95Co0.05Ti3O15 (BEFCTO) nanoparticles by simply applying a weak magnetic bar. 3

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2. Experimental 2.1. Sample Preparation

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In a typical procedure, hydrothermal syntheses of Bi5-xEuxFe0.95Co0.05Ti3O15, Ti(OC4H9)4 (> 99.7%), Eu2O3, Bi(NO3)3·5H2O (> 99.0%), and Fe(NO3)3·9H2O (> 98.5%) were taken according to the proportion x (x = 0, 0.1, 0.4, 0.7) in the formula. All the chemicals used are analytical grade reagents without further purification, which were purchased from Sinopharm

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Chemical Reagent Co., Ltd. The chemicals were mixed and dissolved into HNO3 (4 M). After being magnetic stirred for 20 minutes, the homogeneous metal-ions solution was added into 1 M NaOH solution. Afterwards, the slurry was transferred into a Teflon-lined stainless steel

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autoclave up to 80% of the total volume. The autoclave was sealed and heated at 200 °C for 72 hours, after that the autoclave was cooled down to RT naturally. Finally, the sediment was washed with water and ethanol for several times and then dried at 60 °C for 8 hours. 2.2. Characterization

The purity and crystallinity of the as-prepared samples were characterized by X-Ray

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powder diffraction (XRD) conducted on a Rigaku-TTR III X-ray diffractometer with the Cu-Kα radiation. Morphologies and structures of the powders were observed by scanning

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electron microscopy (SEM, JSM-6700F) and high resolution transmission electron microscopy (HRTEM, JEM-2010). A UV-Vis spectrophotometer (SOLID3700) was used to measure the light absorption spectroscopy. The Brunauer-Emmett-Teller (BET) specific

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surface area was estimated by using the adsorption data (Tristar II 3020M, Mircomeritics, USA). Magnetic properties were tested by vibrating sample magnetometer (VSM) option of the Quantum Design physical property measurement system (PPMS) (Quantum Design, USA). 25

Photocatalytic activities of all the Bi5-xEuxFe0.95Co0.05Ti3O15 samples were investigated by photo-degradation of RhB with the initial concentration of 5 mg/L in water solution. In a typical test, 50 mg Bi5-xEuxFe0.95Co0.05Ti3O15 powders were suspended in 50 mL RhB solution and were stirred for 30 minutes in the darkness, followed by irradiation of a 20 W ultraviolet lamp (254 nm) or a 20 W fluorescent lamp. A 400 nm long-wave-pass filter was used to 4

ACCEPTED MANUSCRIPT block the light below 400 nm from the fluorescent lamp. 3 mL suspension was sampled at every 1 hour interval and was centrifuged to separate the photocatalyst powder. The concentration of residual RhB in each sampled solution was estimated on the basis of its maximum absorbance at 554 nm from the UV-Vis absorption spectrum. The FTIR of the samples were taken on Nicolet 6700 (Thermo, USA) by mixing the samples into KBr pellet.

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3. Results and Discussion

XRD patterns and a schematic demonstration of the atomic configuration in Fig. 1(a)

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confirm that the samples with different Eu doping levels have the structure of Bi5FeTi3O15 (JCPDS 38-1257). Impurity concentrations are below the detection limit of our XRD

instrument, which is about 5%. In other words, addition of Eu and Co didn’t change their

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crystalline structure, which implies that the Bi5FeTi3O15 host is robust for the incorporation of foreign elements, especially for rare earth elements. Fig. 1(b) displays the schematic of Bi5FeTi3O15 in which the doped elements were designated with different colors. There is a four-layered perovskite unit of (Bi3FeTi3O13)2− sandwiched by two (Bi2O2)2+ layers along the c-axis.[15] The diffraction peaks of (2 0 0), (2 2 0), (2 2 14) crystalline plane move to the

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higher angle when increasing the Eu content. The lattice constant have been calculated accordingly and are listed in Table 1. It is evidence that the lattices shrink with the increase of Eu doping content when keeping the Co doping content constant, which can be interpreted

host Bi3+ (r = 1.17 Å, 12-coordination) ion.[16] Increasing the x value to 1.0 resulted in the present of impurity phase, therefore, only those samples with x values smaller than 1.0 were

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that the Eu3+ (r = 1.03 Å, 12-coordination) ion has a smaller ionic radius than the substituted

discussed below.

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Fig. 1 (a) XRD spectra of BEFCTO-x (x = 0, 0.1, 0.4, 0.7). The peaks that move to the higher angle with the increase of Eu content were designated with the item “
”. (b) The schematic of the BEFCTO-x structures.

Table 1. Evolution of lattice constants with Eu content

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b (nm)

c (nm)

Volume (nm3)

0.5441

4.1488

1.230

0.5435

4.1464

1.228

0.5430

0.5427

4.1422

1.220

0.5418

0.5426

4.0819

1.200

a (nm)

0

0.5447

0.1

0.5446

0.4 0.7

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Eu content (x)

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Typical morphology of the BEFCTO-x with different proportion of x was examined by SEM. Figure 2 shows that the Bi5Fe0.95Co0.05Ti3O15 sample without Eu is with a multilayer laminar nanostructure, while the BEFCTO-x samples with x = 0.1, 0.4, and 0.7 all present the

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nanoflower format. Each slice inside the flower is thin and almost aligned perpendicular to each other, and the average slice thickness matches with the crystalline thickness calculated by the Scherrer's formula, indicating that the direction normal to the slice plane is correctly <001>. Geometrically, the nanoflower structure will have more effective specific surface area 15

when comparing to other normal nanoscale formats, which could enhance the contact area with the reactants when for photocatalysis applications. To verify this statement, BET testes were took and the results indicated that the specific surface area in deed improved substantially when increasing Eu content, in line with the SEM results. The specific surface 6

ACCEPTED MANUSCRIPT area equals to 2.69 m2/g, 9.29 m2/g, 19.91 m2/g, 22.48 m2/g while x = 0, 0.1, 0.4, 0.7, respectively. Thus the Eu doping not only changes the morphology, but also increases the specific surface area. Typical HRTEM lattice fringe image of the BEFCTO-0.1 nanoflower was taken and shown in Fig. 2 (e), displaying clearly four-layered perovskite unit between two bismuth oxide layers, corresponding well with the structure in Fig. 1 (b). SAED pattern in

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Fig. 2 (f) can be identified with [001] zone axis of BEFCTO-0.1. The sharp diffraction spots, in which four weak diffraction spots between every two brightly spots can be clearly recognized, indicate a good crystallinity of the nanoflowers with the four-layered perovskite

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unit.

Fig.2 (a) - (d) SEM images of BEFCTO-x with x = 0, 0.1, 0.4, 0.7 respectively; (e) HRTEM image and (f) the SAED pattern of the typical BEFCTO-0.1 sample. The UV-Vis diffuse reflectance spectra of all the samples are shown in Fig 3(a). As can be observed, the absorption in visible light region (400 to 1000 nm) of Eu doped samples 15

were greatly improved with the increase of Eu content, and the optical absorption in the 7

ACCEPTED MANUSCRIPT visible light range (400 to 1000 nm) rose with the increase of x from 0 to 0.7, demonstrating that substituting Bi by rare earth element Eu indeed enhanced the light absorption, especially in the visible light region. For a crystalline semiconductor, the optical absorption near the band edge follows the equation

n/2

, where α, ν, A, and Eg are the

absorption coefficient, light frequency, proportionality constant, and band gap, respectively.

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: αhν = A(hν - Eg)

[17]

The index n depends on whether the inter band transition is direct (n = 1) or indirect (n = 4). The well linear characters of the (αhν)2 - hν curves in Fig 3(b) showed that the value of n is 1, indicating the direct band structure of the samples. The band gap of each samples are

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estimated from the fitting and shown as the insert in Fig 3(b), which are 2.75 eV, 2.73 eV, 2.68 eV, and 2.65 eV respectively for the samples with x = 0, 0.1, 0.4, and 0.7. The band gap

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of the sample decreased with respect to the increasing of Eu doping concentration. There may be two mechanisms in improving the visible light absorption with Eu doping. One apparent mechanism is that the transitions in Eu3+ ions will exhibit the 458 nm (7F0 → 5D2) and 536 nm (7F0 → 5D1) absorption bands.[18] Another possible mechanism would be that the substitution 15

of Bi with Eu may create localized states in the band gap of Bi5Fe0.95Co0.05Ti3O15 host, which

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also decreased the band gap, just as the case in BiFeO3.[19] However, this needs further investigation that is beyond the scope of the current work. The photocatalytic activity of BEFCTO-x powders with different Eu content was tested by the photo-degradation of RhB under a visible light source and a UV light source

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respectively. According to Lambert-Beer theory, a plot of (1-C/C0) depending on the

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irradiation time was drawn in Fig. 3(c) and Fig. 3(d). The photo-degradation results after the irradiation for 3 hours with UV and Vis light are also summarized for comparison and presented in Fig. 3(e). As shown in Fig. 3 (c) - (e), about 52.4%, 62.3%, 86.5%, and 90.0% of RhB pollutant in the solution have been decomposed by BEFCTO-x photocatalysts with x = 0, 25

0.1, 0.4, 0.7 after being irradiated with ultraviolet light for 3 hours respectively. The degradation efficiency of 90% for the sample with x = 0.7 is quite remarkable as compared to previous works,

[20]

in which the degradation rate of Bi5FeTi3O15 nanoflower is 76%.

Excitingly, under the irradiation with visible light from 400 to 720 nm for 3 hours, the photo-degradation efficiencies still keep high, which are about 36.8%, 60.5%, 79.2%, 79.4% 30

for the samples with varied Eu content. The photocatalysis efficiency in Vis region is exactly 8

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The enhanced photo absorption, as well as the increased specific surface area, were suggested as the mainly reasons for the greatly improved photocatalysis efficiency.

The RhB solution was complete decolorized with the visible-light irradiation for 5 hours by using the BEFCTO-0.7 photocatalyst. To check whether the RhB dye was complete

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photo-decomposed or just transformed to other colorless organic products, the FTIR of the origin RhB solution, the final colorless RhB solution after 5 hours photocatalytic reaction, and

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the BEFCTO-0.7 powders were taken by mixing them into KBr pellet.. It can be seen from Fig. 3(f) that the characteristic absorption bands of RhB among 500 ~ 1700 cm-1 were not observed in the final solution, as well as that the spectrum of the final solution is almost identical to that of pure KBr pellet, indicating that there is no organic left in the colorless final solution. These results strongly support that RhB was really degraded by the photocatalysis

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reaction.

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Fig. 3 (a) Typical UV-Vis diffuse reflectance spectra of the BEFCTO-x samples; (b) The

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(αhν)2- hν relationships and the calculated band gap values (insert). (c) Time courses of RhB concentration under the fluorescent lamp (400 - 720 nm); (d) Time-dependent evolution of 5

RhB concentration under a UV lamp (254 nm) with different photocatalysts; (e) the

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comparison of photo-degradation efficiencies in UV light and visible light at the time of 3 hours for different samples; (f) The infrared spectroscopy (Pressing Potassium Bromide

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Troche) of powder, origin and the final solution

The magnetism properties of the samples were further investigated to explore the possible

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retreatment of the photocatalyst by magnetic field. The M-H curves in Fig. 4(a), as well as the evolution of the saturation magnetization (2Ms) and the remnant magnetization (2Mr) with Eu content in Fig. 4(b), demonstrate that the partial substitution of Bi in Bi5Fe0.95Co0.05Ti3O15 with the rare-earth element Eu can greatly enhance the ferromagnetism of the powders. The more Eu is incorporated, the stronger the magnetism becomes. The value of 2Ms was improved

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from 0.111 emu/g to 1.056 emu/g, and 2Mr was increased from 0.015 emu/g to 0.275 emu/g. The magnetism of Bi5Fe0.95Co0.05Ti3O15 could be ascribed to the super-exchange interaction 10

ACCEPTED MANUSCRIPT between Fe and Co ions via bonding with oxygen.[22] As shown in Figure 1 and Table 1, the presence of Eu3+ in Bi3+ sites decreased the cell volume, which may result in the enhanced magnetism of BEFCTO-x. Meanwhile, the morphology evolution, as well as the exchange interaction between Eu 4f electrons and Fe 3d electrons, may also contribute the magnetism increase. [23] Though the above results incontrovertibly demonstrated that the ferromagnetism

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of Bi5Fe0.95Co0.05Ti3O15 have been successfully achieved by doping Eu, just as the previous reports, [24-25] however, much more works are to be done to clarify the detailed mechanism.

As a demonstration of the magnetic recovery of the photocatalyst, BEFCTO-0.7 powders

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which is with the best photocatalytic performance have been separated from the suspension by a bar magnet with magnetic field strength of about 0.5 Tesla. As shown in Fig. 4(c), the

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particles were attracted and agglomerated near the magnet, and the muddy suspension became clear and transparent, which indicates that the powder is easy to recycle. Taking the advantage of this retrievable function, a cycling test was also performed. After the 3 hours reaction, the BEFCTO-0.7 photocatalyst was retrieved with the bar magnet, and then the activity of the retrieved photocatalyst was studied again. Results shown in Fig. 4(d) suggested that the

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photocatalytic activity was well maintained after several cycles.

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Fig. 4 (a) Room temperature M-H hysteresis loops of BEFCTO-x with different value of Eu content; (b) The variation of saturation magnetization (2Ms) and the remnant 11

ACCEPTED MANUSCRIPT magnetization (2Mr) with Eu content; (c) Photograph of the liquid dispersed evenly (right) after ultrasonic several minutes and the same liquid (left) placed nearby a bar magnet several minutes later; (d) Cycling experiment of a typical full visible light photocatalytic degradation of BEFCTO-0.7 (50 mg)/RhB (5 mg/L, 50 mL) in 3 hours.

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4. Conclusion

The doping of Eu in Bi5Fe0.95Co0.05Ti3O15 has been proved to be effective to improve the photocatalysis and ferromagnetism. The morphology, light absorption, band gap, as well as

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the magnetic properties are tailored correspondingly by adjusting Eu doping amount. The combined contributions of these improvements gave rise to the efficient photocatalytic

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performance for the sample with x = 0.7 in both ultraviolet and visible region. The high photo-degradation performance and the magnetic recyclability may enable the practical applications in environment management.

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Acknowledgements

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This work was financially supported by the National Basic Research Program of China (2012CB922000), the Provincial Natural Science Research Project of Anhui Colleges (KJ2014ZD40), Key Research Program of Chinese Academy of Sciences (KGZD-EW-T06)

(211134KYSB20130017).

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and External Cooperation Program of BIC, Chinese Academy of Sciences

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ACCEPTED MANUSCRIPT Highlights • Bi5-xEuxFe0.95Co0.05Ti3O15

(BEFCTO)

nanoflowers

were

synthesized

with

hydrothermal way. • BEFCTO nanoflowers show notable UV- and Vis- driven fully photo-degradation of RhB dye.

RI PT

• Ferromagnetism of BEFCTO nanoflowers enable the feasible recovery from water suspension with magnet.

• Properties of Bi5-xEuxFe0.95Co0.05Ti3O15 were improved by optimizing the europium

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