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SrFe12O19: Preparation, characterization, and photocatalytic activity under visible light
Applied Surface Science 273 (2013) 684–691
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Magnetic composite ZnFe2 O4 /SrFe12 O19 : Preparation, characterization, and photocatalytic activity under visible light Taiping Xie a,∗ , Longjun Xu a,∗ , Chenglun Liu b , Yuan Wang b a b
State Key Laboratory of Coal Mine Disaster Dynamics and Control, Chongqing 400044, PR China College of Chemistry and Chemical Engineering, Chongqing University, Chongqing 400044, PR China
a r t i c l e
i n f o
Article history: Received 28 January 2013 Received in revised form 22 February 2013 Accepted 24 February 2013 Available online 4 March 2013 Keywords: Magnetic composite ZnFe2 O4 /SrFe12 O19 One-step chemical coprecipitation Methylene blue Magnetic Photocatalytic
a b s t r a c t One-step chemical coprecipitation with high-temperature sintering method was employed for preparing magnetic composite ZnFe2 O4 /SrFe12 O19 including a hard-magnetic phase (SrFe12 O19 ) and a soft-magnetic phase (ZnFe2 O4 ). The magnetic composite was characterized by FTIR, XRD, SEM, BET, XPS, VSM, and UV–vis. The testing results showed that the saturation magnetization (Ms ), remanent magnetization (Mr ), and coercivity (Hc ) were 34.95 emu/g, 18.31 emu/g, and 2254.54 G, respectively, indicating that the composite possessed excellent magnetic properties and a greater capacity for anti-demagnetize. The properties of the composite were favourable to its separation, recycling, and reuse after reaction. The photocatalytic performance of the composite was studied by the degradation reaction of methylene blue under visible light irradiation. The experimental results revealed that the degradation rate was still more than 70% when the composite was reused for four times. In addition, this research was expected to provide a promising method to prepare various composite materials with multi-functional components. Crown Copyright © 2013 Published by Elsevier B.V. All rights reserved.
1. Introduction Photocatalysts, such as TiO2 [1,2], ZnO [2,3], SrTiO3 [4,5], and BiOCl [2,6] using for environmental remediation have been attracting considerable attention for several decades. Many species of industrial organic dye (wastewater) involving methylene blue (MB), methyl orange (MO), phenol, rhodamine B, and bromophenol blue [2,7], were decomposed over those photocatalysts under ultraviolet or visible radiation. However, it was difficult to separate catalysts from liquid solution after reaction, which probably produced secondary pollution and increased costs. Catalysts with magnetism, namely magnetic catalysts could overcome the two problems. Therefore, it was crucial to find a strategy for making those photocatalysts possess magnetism. In recent years, a kind of soft-magnetic material, ZnFe2 O4 was extensively used for ferrofluids [8], high-density magnetic recording media [9], biomedicine [10], radar absorbing material [11] due to its various advantages, for instance good chemical stability, corrosion resistivity, and superior magnetic properties. Furthermore, ZnFe2 O4 had applications in gas sensor [12,13], semiconductor photocatalysis [14,15], and organic reaction catalyst fields [16–18]. So the potential interest in synthesis and application of ZnFe2 O4
∗ Corresponding author. Tel.: +86 2386809361; fax: +86 2386809361. E-mail addresses: [email protected] (T. Xie), [email protected] (L. Xu).
for many researchers had been revived. In terms of application in photocatalysis, however, owing to its bad valence band potential and inferior nature of photoelectric conversion [19], ZnFe2 O4 was modified with other elements or compounds to improve its photocatalytic activity. ZnFeNdO4 [16], ZnFe2 O4 /Fe3 O4 /Ag [19], ZnFe2 O4 /Fe2 O3 /ZnO [20], ZnO/ZnFe2 O4 [21], TiO2 /Al2 O3 /ZnFe2 O4 [22], ZnFe2 O4 /Graphene [23], and ZnFe2 O4 /TiO2 [24] were synthesized and used as photocatalysts. A kind of hard-magnetic material, SrFe12 O19 could provide various species of non-equivalent sites for various magnetic or non-magnetic cations [25]. SrFe12 O19 had several merits, such as relatively large saturation magnetization, superior coercivity, high uniaxial magnetic crystalline anisotropy, chemical stability, and corrosion resistivity, which stimulated researchers to appreciate its significance in electronic components, magnetic memories, biotechnology, and in magnetic substrate for magnetic catalysts. To the best of our knowledge, magnetic composite ZnFe2 O4 / SrFe12 O19 was synthesized by sol–gel technique [26,27] and twostep coprecipitation method [28,29]. The sol–gel technique was expensive, time consuming, and polluted. The pollution originated from the decomposition of the used organic dispersants in the process of heat treatment. For the two-step coprecipitation method, the two materials were individually prepared by coprecipitation, and then the resultant was obtained by mechanical blending, which complicated the procedures. It was understood that the two abovementioned methods had provided some fundamental studies, but they were not obviously suitable for industrial applications. So it is
0169-4332/$ – see front matter. Crown Copyright © 2013 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.02.113
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still a challenge for chemists to invent an easy and simple process to prepare ZnFe2 O4 /SrFe12 O19 for mass production. In this work, therefore, magnetic composite ZnFe2 O4 /SrFe12 O19 was prepared by one-step chemical coprecipitation with hightemperature sintering method. ZnFe2 O4 was decorated with SrFe12 O19 , which improved magnetic properties of ZnFe2 O4 and would help to enhance its photocatalytic activity. The composite was characterized by FTIR, XRD, SEM, BET, XPS, VSM, and UV–vis. The photocatalytic activity was studied by the degradation of MB under visible light irradiation. 2. Experimental procedures 2.1. Synthesis of ZnFe2 O4 /SrFe12 O19 All reagents were of analytical grade purity and used directly without further purification except that SrCO3 [30] was obtained from industrial strontium residue. The water used was deionized water. SrCO3 , ZnCl2 , and FeCl3 were precisely weighted with a molar ratio of 1:1:14. SrCO3 and FeCl3 were dissolved in suitable HCl aqueous solution and water, respectively. The two homogeneous solutions were completely mixed by ultrasonic-assisting. At the same time, ZnCl2 was added into the mixed solution. The pH of the mixed solution was adjusted to 10 with NaOH solution after ZnCl2 was adequately dissolved. The reactions of NaOH with SrCl2 , FeCl3 and ZnCl2 immediately occurred to generate the brownish yellow slurry of the hybrid complex. The final mixture was transferred into a stainless steel autoclave that maintained at 60 ◦ C for 10 h. The dry block was crushed into a fine powder and sintered at 1000 ◦ C for 2 h. In addition, pure SrFe12 O19 and ZnFe2 O4 were prepared using similar procedure. In the process of preparation of SrFe12 O19 , SrCO3 and FeCl3 were weighted with a molar ratio of 1:12. The precursor was sintered at1000 ◦ C for 2 h. In the process of synthesis of ZnFe2 O4 , ZnCl2 and FeCl3 were weighted with a molar ratio of 1:2. The precursor was sintered at 700 ◦ C for 6 h. 2.2. Characterization Phase identification via X-ray diffraction (XRD) was conducted on an X-ray diffractometer (Bruker Advance D8) using Cu K␣ radiation. Fourier transform infrared spectroscopy (FTIR) spectra of samples were recorded on a 5DX FTIR (5DX, Nicolet Co., USA) spectrometer using KBr powder-pressed pellets. The samples’ morphologies were observed by scanning electron microscopy (SEM, ZEISS, EVO-LS15). The magnetic properties were investigated using a vibrating sample magnetometer (VSM, Lakeshore 7410). The measurement of N2 absorption performance of samples was performed on an adsorption instrument (ASAP-2020, Micromeritics, USA). X-ray photoelectron spectroscopy (XPS) was recorded on an XPS-XSAM800 (Kratos, UK) spectrometer with an achromatic Al K␣ X-ray source and an analytical chamber with a base pressure of 2 × 10−7 Pa. The X-ray gun was operated at 180 W (12 kV, 15 mA). The UV–vis spectra of samples were measured using a UV–vis spectrophotometer (TU1901, China). BaSO4 was used as a reflectance standard in the UV–vis diffuse reflectance experiment. 2.3. Photocatalytic degradation of MB The photocatalytic activity of the ZnFe2 O4 /SrFe12 O19 was studied by the degradation of MB under irradiation of a 500WXe lamp at the natural pH value. A 200 mL of 10 mg/L MB aqueous solution and its corresponding composite dosage of 2 g/L were added into quartz container and stirred for 1 h in the dark. After a given irradiation time, about 3 mL of the mixtures was withdrawn. And
Fig. 1. FTIR spectra of materials.
then the solution and ZnFe2 O4 /SrFe12 O19 particles were separated by an extra magnet. The photocatalytic degradation process of MB was monitored by measuring its absorption with a UV–vis spectrophotometer. The degradation rate of MB was calculated by the following Eq. (1), Degradation rate (%) =
A0 − At × 100% A0
(1)
where A0 represents the initial absorbance of MB, At denotes the variable absorbance of MB at different reaction time. 3. Results and discussion 3.1. FTIR Fig. 1 showed the FTIR spectra of the prepared materials. The peaks of metal-oxygen bonds at 551.5 cm−1 and 434.0 cm−1 were the characteristic absorption peaks of ZnFe2 O4 , which was consistent with previous literatures [31] and [32]. The characteristic peaks of SrFe12 O19 were at 603.0 cm−1 , 553.2 cm−1 , and 451.4 cm−1 [33]. Similarly, the three absorption peaks (at about 593 cm−1 , 549 cm−1 , and 444 cm−1 ) appeared in the spectrum of ZnFe2 O4/ SrFe12 O19 , which demonstrated that the added Zn2+ did not change the intrinsic structure of SrFe12 O19 . The absorption peak of the ZnFe2 O4 at 551.5 cm−1 was not clearly seen in the spectrum of ZnFe2 O4/ SrFe12 O19 , which was ascribed to the fact that the peak of Zn-O bonds in the ZnFe2 O4 overlapped with the peak of SrFe12 O19 at 553.2 cm−1 . It was worth noting that the peaks at about 593 cm−1 , 549 cm−1 , and 444 cm−1 in the spectrum of ZnFe2 O4/ SrFe12 O19 became sharper and shifted to low wavenumber as compared with the characteristic peaks of SrFe12 O19 . On the one hand, the chemical polarization of the internal chemical bonds of SrFe12 O19 was greatly strengthened as the formation of the ZnFe2 O4 . On the other hand, both Fe-O-Zn and Sr-O-Zn bonds were probably formed by introducing the Zn2+ into SrFe12 O19 . Thus, the oxygen atom of Fe(Sr)-O bond might be shared with Zn atom, giving rise to vibrational coupling between Fe(Sr)-O and Zn-O. 3.2. XRD The XRD patterns of ZnFe2 O4 , SrFe12 O19 , and ZnFe2 O4 /SrFe12 O19 were exhibited in Fig. 2. Evidently, every diffraction peak could be fully indexed as ZnFe2 O4 (JCPDS file 01-1109), with
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was at c.a. 529.4 eV, 531.0 eV, and 532.7 eV, respectively. And the peak at 529.4 eV could ascribable to O2− in Fe2 O3 system [38]. Fig. 3(c) exhibited the photoelectron peaks of the Sr 3d5/2 and 3d3/2 at 133.2 eV and 134.9 eV, respectively. The Sr 3d5/2 was assigned to surface Sr–O bonds and the Sr 3d3/2 could verify the presence of Sr2+ [40]. The criteria binding energy of Sr 3d5/2 and 3d3/2 were 133.7 eV and 135.5 eV, respectively. Compared with this criteria binding energy of Sr 3d, the values of the prepared sample were altered, arising out of the formed new chemical bonds, such as Sr–O–Zn and Sr–O–Fe. These inferences were consistent with the XRD and FTIR analyses. 3.4. SEM
Fig. 2. XRD patterns of materials.
characteristic reflection phases (2 2 0), (3 1 1), (2 2 2), (4 0 0), etc. [34], which proved that a pure S-type ZnFe2 O4 phase was prepared. The peaks of the as-prepared SrFe12 O19 were at 2 of 32.5◦ , 34.3◦ , 37.8◦ , 57.4◦ , corresponding to the characteristic (1 0 7), (1 1 4), (1 0 8), (2 1 8) reflection [35]. These peaks could be indexed as the M-type SrFe12 O19 (JCPDS file 24-1207). No impurity peaks were observed, indicating that the pure SrFe12 O19 sample was synthesized. As could be seen, the XRD pattern of ZnFe2 O4 /SrFe12 O19 composite not only exhibited a series of the characteristic diffraction peaks of SrFe12 O19 , but also presented the peaks of ZnFe2 O4 , which signified that the as-prepared composite were definitely composed of hard-magnetic phase (SrFe12 O19 ) and soft-magnetic phase (ZnFe2 O4 ), and that the composite was desirous material, ZnFe2 O4/ SrFe12 O19 . 3.3. XPS Fig. 3 showed the XPS spectra of ZnFe2 O4 /SrFe12 O19 , (a) low resolution for all atoms; (b), (c), and (d) high resolution for Fe, O, and Sr respectively. The low resolution XPS scan (Fig. 3 (a)) was collected the major peaks from Sr 3p, Sr 3d, Fe 2p, Fe 3p, O 1s, and Cl 1s. The high resolution scans were gathered to study the major peaks from the Fe 2p, Sr 3d, and O1s in detail.The peaks of Zn 2p or Zn 3p were not detected via the XPS scan. The content of Zn was not less than the detection limit of a surface concentration of 0.01%. So the probable explanation was that the Zn2+ occupied the 2a sites for Fe3+ that inserted in interstices between the oxygen positions in M-type SrFe12 O19 [36], resulting in no Zn2+ ions were on the external surface of sample. The Fe 2p3/2 and 2p1/2 photoelectron peaks with multiplet splitting phenomenon were observed at 710.9 eV and 724.4 eV (Fig. 3 (b)) respectively, corresponding to Fe2+ (FeO) and Fe3+ (␣Fe2 O3 ) [37,38], which was confirmed by the Fe3p photoelectron peaks at ca. 54∼56 eV (Fig. 3 (a)), namely (55.9 ± 0.2 eV (Fe3+ ) and 53.9 ± 0.2 eV (Fe2+ ) [39]). The O1s spectrum with three components
The different morphologies of the as-prepared materials were depicted in Fig. 4. The (a) sample exhibited octahedral structure (Fig. 4 (a)), which was consistent with XRD analysis. The Fig. 4 (b) presented hexagonal crystal system that was a typical feature of M-type SrFe12 O19 [37]. Fig. 4 (c) illustrated almost uniform particle distribution and uniform octahedral structure. The conformity of surface morphology (Fig. 4 (c)) could probably improve the chemical stability and morphology anisotropy of composite. The octahedral crystal growth of ZnFe2 O4 tended to restrain the growth of hexagonal crystal of SrFe12 O19 . The Zn ions inserted and occupied the non-equivalent sites [36], especially octahedral sites (4f2 or 2a sites) in SrFe12 O19 , which led to a phenomenon that the crystal growth of SrFe12 O19 was probably towed towards octahedral crystal. It was called crystal transformation. Meanwhile, to some extent, the exchange coupling interaction between the two magnetic phases was generated, which was attributable to the formation of the hard- and soft-magnetic phases (Fig. 2) as the Zn ions inserted. The molecular mass of SrFe12 O19 was larger than that of ZnFe2 O4 , which brought about a phenomenon that the SrFe12 O19 particles absorbed more electrons than ZnFe2 O4 [25]. It was certain that the heavier molecular increased in the cross section for elastic scattering, as a result, electrons underwent more elastic scattering per unit distance and the average scattering angle was greater than that of low molecular mass material as well. There was also a possibility that the heavier material showed less bright, and that the low molecular mass material displayed more bright. Therefore, the SrFe12 O19 regions were possibly dark, while the ZnFe2 O4 regions were relatively bright (Fig. 4(c)). 3.5. N2 adsorption–desorption (BET) The nitrogen adsorption–desorption isotherms of ZnFe2 O4 and ZnFe2 O4/ SrFe12 O19 composite and their pore size distribution curves were presented in Fig. 5. On the whole, the nitrogen adsorption–desorption isotherms of pure ZnFe2 O4 and ZnFe2 O4/ SrFe12 O19 composite could be categorized as a typical Type III isotherm which was convex to the p/p0 axis over its entire range [41]. Their most probable pore sizes were 1.91 nm and 2.07 nm, respectively. They therefore could be classified as mesoporous material. The pure ZnFe2 O4 was same porous features as the ZnFe2 O4 /SrFe12 O19 . Both had a large quantity of macropore. The difference between the two materials was that the pure ZnFe2 O4 had more mesopore as compared to the composite, which brought about a phenomenon that the specific surface area of the pure ZnFe2 O4 was in excess of that of the composite (Table 1). It was interesting to note that a discrete curve (p/p0 ∼ 0.6–0.7) appeared in desorption isotherm (Fig. 5 (b)) of the composite, which elucidated that the escape of nitrogen from the sample surface, i.e. nitrogen desorption, was difficult in the p/p0 range of 0.6–0.7, arising from the uniform surface structure of the composite.
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Fig. 3. XPS spectra of ZnFe2 O4/ SrFe12 O19 .
3.6. VSM The magnetic properties of materials were investigated by VSM. The magnetic hysteresis loops were depicted in Fig. 6 and the magnetic parameters were listed in Table 2. It was obvious that ZnFe2 O4 was soft-magnetic material due to its coercivity (Hc ) of zero. In other words, the magnetic hysteresis loop of ZnFe2 O4 passed through the origin of coordinates (Fig. 6(a)). This phenomenon also proclaimed that ZnFe2 O4 was superparamagnetic. Moreover, a sample could not fully saturate at 10 kG, which underlined the presence of superparamagnetic and single domain particles. The SrFe12 O19 possessed an Hc of 1710.10 G and a remanent magnetization (Mr ) of 18.31 emu/g (Table 2), which was a characterization of a typical hard-magnetic material (ferromagnetic property). Similarly, the ZnFe2 O4 /SrFe12 O19 composite was also classified as a hard-magnetic material. Indeed, the pure ZnFe2 O4 normally had superparamagnetic property [42] rather than ferromagnetism. So the strong ferromagnetic property of the composite should be attributed to the hard-magnetic phase (SrFe12 O19 ) component.
Previous investigation pointed out that magnetization of a ferromagnetic material was potentially dependent on the strength of superexchange interaction between Fe3+ cations in octahedral sites [43]. The Ms and Mr of composite were comparatively low as compared with that of SrFe12 O19 , owing to the presence of the softmagnetic phase (ZnFe2 O4 ). First of all, the non-magnetic ion, Zn2+ ions occupied the octahedral 2a sites for Fe3+ ions with a higher magnetic moment (5 B ). Second, Zn2+ ions occupied the octahedral 2a sites for Fe3+ ions in the SrFe12 O19 system, resulting in the valence change of Fe3+ to Fe2+ to maintain the electroneutrality, and then bringing about the decrease in amount of Fe3+ cations, which weakened the strength of exchange interaction between Fe3+ cations. The two above reasons both induced to the decrease in Ms and Mr of the prepared composite. It was worthwhile to note that the composite had a larger coercivity (2254.54 G, Table 2), indicating that it possessed a better capacity of anti-demagnetize, which was helpful to its recyclable, recovery, and reuse.
3.7. UV–vis spectra
Table 1 Specific surface area and most probable pore size of materials. samples
Specific surface area (m2 /g)
Most probable pore size (Å)
ZnFe2 O4 (a) ZnFe2 O4/ SrFe12 O19 composite (b)
8.383 5.115
19.1 20.7
The UV–vis spectra of SrFe12 O19 , ZnFe2 O4 , and ZnFe2 O4 / SrFe12 O19 were shown in Fig. 7. The three samples showed intense absorption in a wide wavelength range from UV to visible light with absorption tail extending into infra red region. It was clear that the absorption spectrum of ZnFe2 O4 /SrFe12 O19 composite contained
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Fig. 4. SEM photographs of materials: (a) ZnFe2 O4 ; (b) SrFe12 O19 ; (c) ZnFe2 O4/ SrFe12 O19 .
Fig. 5. Nitrogen adsorption–desorption isotherms for (a) ZnFe2 O4 , (b) ZnFe2 O4/ SrFe12 O19 . Inset: the corresponding pore size distribution of (a) ZnFe2 O4 , (b) ZnFe2 O4 /SrFe12 O19 . Table 2 Magnetic parameters of materials. Samples
ZnFe2 O4 (a) SrFe12 O19 (b) ZnFe2 O4/ SrFe12 O19 composite (c)
Magnetization (emu/g)
Coercivity Hc (G)
Saturation magnetization (Ms )
Remanent magnetization (Mr )
1.67 49.97 34.95
0 29.84 18.31
0 1710.10 2254.54
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Fig. 8. Degradation rate of MB with different photocatalyst, i.e. pure ZnFe2 O4 and ZnFe2 O4 /SrFe12 O19 . Inset: Degradation rate of MB with ZnFe2 O4/ SrFe12 O19 photocatalyst for five times reuse. Fig. 6. Magnetic hysteresis loops of materials.
the absorptions of ZnFe2 O4 and SrFe12 O19 from UV to visible range. Previous literature [44] reported that the visible light sensitivity was concerned with Fe cations in the system. The Fe3+ ion possessed a 3d5 configuration which had a sextet state in the octahedral crystal field. Meanwhile, very weak crystal field transitions were expected as these transitions were spin, symmetry and parity forbidden [44,45]. In addition, charge transfer transition from O2− to Fe3+ normally gave rise to a strong absorption around 275 nm (see Fig. 7). Therefore, such intense absorption in the visible region was likely due to other types of charge transfer transition. Metal to metal charge transfer transitions (2Fe3+ → Fe2+ + Fe4+ ) had been proposed by Haart and his coworkers [45]. Besides, the Zn2+ inserted and occupied the 2a site for Fe3+ in SrFe12 O19 , leading to the valence change of Fe3+ to Fe2+ to maintain the electroneutrality and the
Fig. 7. UV–vis absorption spectra of (a) SrFe12 O19 , (b) ZnFe2 O4 , and (c) ZnFe2 O4/ SrFe12 O19 .
formation of Zn-O-Fe bonds. In other words, the Zn2+ introduced into the SrFe12 O19 system would cause the amount of Fe3+ cations and Fe-O-Fe bonds to decrease, which rendered metal charge transition more difficult. So absorption of ZnFe2 O4 /SrFe12 O19 composite became weaker as compared with ZnFe2 O4 . In contrast to single phase ZnFe2 O4 , the photoresponse of the composite to the visible range was expanded, which was expedient to the degradation of organic dye over the composite photocatalyst under the visible light. The above observation could also verify the formation of the composite with involving the two phases, i.e. ZnFe2 O4 and SrFe12 O19 .
3.8. The photocatalytic properties To compare the photocatalytic activities of pure ZnFe2 O4 and ZnFe2 O4/ SrFe12 O19 composite, they were used for photocatalytic degradation of MB under identical test condition. The experimental results were shown in Fig. 8. It was evident that the ZnFe2 O4 /SrFe12 O19 composite surpassed the pure ZnFe2 O4 in degradation efficiency after 2 h reaction. These results were in good agreement with the deduction of UV–vis spectra. Since its bad valence band potential and inferior nature of photoelectric conversion [19], there was need to decorate ZnFe2 O4 in order to improve its photocatalytic activity. In this work, ZnFe2 O4 was modified with a kind of hard-magnetic material, SrFe12 O19 . It was found through the above analyses that the Zn2+ inserted and occupied the 2a site for Fe3+ in SrFe12 O19 , which resulted in the valence change of Fe3+ to Fe2+ to maintain the electroneutrality. And Fe2+ not only enhanced the interfacial polarization due to the Fe2+ was easier polarized [46], but also elevated the electrons hopping between Fe2+ and Fe3+ . In addition, the Zn2+ at 2a sites and Fe3+ formed the electric dipoles with surrounding negative O2− [47], which improved the capacity of electron transport, and then enhanced the photocatalytic activity of composite. The stability of the photocatalysis of the ZnFe2 O4 /SrFe12 O19 composite was confirmed by repeating the decomposition processes for four times. Here, to obtain the largest degradation rate, the irradiation time of all the recycle experiments was 5 h. The results were shown in Fig. 8 (see inset). After the degradation reaction, the XRD pattern of the recycled composite was collected to
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T. Xie et al. / Applied Surface Science 273 (2013) 684–691 + ·O− 2 + H → ·HO2
(4)
+ ·O− 2 / · HO2 /hVB + MB → CO2 + H2 O
(5)
In fact, there was need to further research for whether the degradation rate was influenced by the magnetic field (Lorentz force) stemming from the remanent magnetization of magnetic catalyst or not. And there was also need further investigation for whether the presence of SrFe12 O19 prevented recombination between the photogenerated electrons and holes in the VB. 4. Conclusions
Fig. 9. XRD patterns of (a) ZnFe2 O4 /SrFe12 O19 and (b) recycled ZnFe2 O4 /SrFe12 O19 sample after being used for photocatalysis experiments.
use for confirming its intrinsic crystal structure. The XRD patterns of fresh state sample and recycled sample were displayed in Fig. 9. When the composite was reused for four times, the degradation rate was still more than 70%, though the degradation rate decreased with its reuse. The composite could keep higher degradation efficiency, because its intrinsic crystal structure was still reserved after reuse. In other words, no change in structure of composite before and after reaction was observed, declaring that its better stability (Fig. 9). It was worth mentioning that the reduction of degradation efficiency derived from activity loss of little amount of the recycled composite. On the one hand, the partially destruction of the mesostructure in the original sample was generated due to the stirring process prior to degradation reaction. On the other hand, the surface of the composite absorbed some micromolecules or ions originating from the decomposition of MB. On the whole, the composite had an excellent photocatalytic activity and stability. And its superior magnetic properties conduced its recycling through the agency of an extra magnet. A possible mechanism for the photodegradation of MB over ZnFe2 O4 /SrFe12 O19 was proposed, as illustrated by Eqs. (2)–(5). From the above study, especially in analyses of UV–vis spectra and related literature [19–24], it was reported that the ZnFe2 O4 was a potential photocatalyst in decomposing organics under UV light irradiation. Thus, in the ZnFe2 O4 /SrFe12 O19 composite system, ZnFe2 O4 acted as a main photocatalyst, while SrFe12 O19 was a sensitizer absorbing of visible light. Under visible light irradiation, some electrons (e− ) in the valence band (VB) was excited to the conduction band (CB) causing the generation of holes (h+ ) in the VB simultaneously (Eq. (2)). A portion of the photogenerated electrons would recombine with holes in the VB, while others transferred to the surface and reacted with the adsorptive oxygen molecule to yield • O2 − (Eq. (3)). The generated • O2 − would further combined with H+ to produce • HO2 (Eq. (4)). The reactive species, such as • O − , • HO , and h+ , all could oxidize MB (Eq. (5)). 2 2 VB
− + h+ ZnFe2 O4/SrFe12 O19 + hv → ZnFe2 O4 /SrFe12 O19 eCB Vb − + O2 → ZnFe2 O4 /SrFe12 O19 + ·O− ZnFe2 O4/SrFe12 O19 eCB 2
(2) (3)
Magnetic composite ZnFe2 O4 /SrFe12 O19 was prepared by onestep chemical coprecipitation with high-temperature sintering method. The composite was characterized by FTIR, XRD, XPS, SEM, BET, VSM, and UV–vis. And the photocatalytic activity of the composite was probed through decomposition of MB by photocatalysis. The results manifested that the composite possessed better magnetic properties, conformity surface structure, a better photocatalytic activity and stability. The photocatalytic test revealed that the degradation rate was still more than 70% when the composite was reused for four times. In the ZnFe2 O4 /SrFe12 O19 composite system, ZnFe2 O4 worked as main photocatalyst while SrFe12 O19 acted as sensitizer absorbing for visible light and magnetic source. This research was hoped to provide simple synthesis technique for multi-phase or multi-functional composite materials. Acknowledgements We would like to appreciate our associates, especially Jingrong He, Wenli Wu, Jun Yang, for their valuable contributions to our research program. We want to thank the financial support from the program of Chongqing Science and Technology Commission (Nos. 2010AC7180, 2011AC4070). We acknowledge gratefully many important contributions from the researchers of all literatures cited in our paper. References [1] M. Pelaez, N.T. Nolan, S.C. Pillai, M.K. Seery, P. Falaras, A.G. Kontos, P.S.M. Dunlop, J.W.J. Hamilton, J.A. Byrne, K. OShea, M.H. Entezari, D.D. Dionysiou, A review on the visible light active titanium dioxide photocatalysts for environmental applications, Applied Catalysis B 125 (2012) 331–349. [2] A.D. Paola, E. García-López, G. Marcì, L. Palmisano, A survey of photocatalytic materials for environmental remediation, Journal of Hazardous Materials 211–212 (2012) 3–29. [3] A. Moezzi, A.M. McDonagh, M.B. Cortie, Zinc oxide particles: synthesis, properties and applications, Chemical Engineering Journal 185–186 (2012) 1–22. [4] S. Song, L.J. Xu, Z.Q. He, H.P. Ying, J.M. Chen, X.Z. Xiao, B. Yan, Photocatalytic degradation of C.I. Direct Red 23 in aqueous solutions under UV irradiation using SrTiO3 /CeO2 composite as the catalyst, Journal of Hazardous Materials 152 (2008) 1301–1308. [5] M. Tsuchiya, S.K.R.S. Sankaranarayanan, S. Ramanathan, Photon-assisted oxidation and oxide thin film synthesis: a review, Progress in Materials Science 54 (2009) 981–1057. [6] F. Dong, Y.J. Sun, M. Fu, Z.B. Wu, S.C. Lee, Room temperature synthesis and highly enhanced visible light photocatalytic activity of porous BiOI/BiOCl composites nanoplates microflowers, Journal of Hazardous Materials 219–220 (2012) 26–34. [7] E. Casbeer, V.K. Sharma, X.Z. Li, Synthesis and photocatalytic activity of ferrites under visible light: a review, Separation and Purification Technology 87 (2012) 1–14. [8] K. Raj, R. Moskowitz, Commercial applications of ferrofluids, Journal of Magnetism and Magnetic Materials 85 (1990) 233–245. [9] A. Moser, K. Takano, D.T. Margulies, M. Albrecht, Y. Sonobe, Y. Ikeda, S. Sun, E.E. Fullerton, Magnetic recording: advancing into the future, Journal of Physics D: Applied Physics 35 (2002) R157–R167. [10] J.M. Bai, J.P. Wang, High-magnetic-moment core–shell-type FeCo-Au/Ag nanoparticles, Applied Physics Letters 87 (2005) 152502. [11] R.C. Che, L.M. Peng, X.F. Duan, Q. Che, X.L. Liang, Microwave absorption enhancement and complex permittivity and permeability of Fe encapsulated within carbon nanotubes, Advanced Materials 16 (2004) 401–405.
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Magnetic composite ZnFe2 O4 /SrFe12 O19 : Preparation, characterization, and photocatalytic activity under visible light Taiping Xie a,∗ , Longjun Xu a,∗ , Chenglun Liu b , Yuan Wang b a b
State Key Laboratory of Coal Mine Disaster Dynamics and Control, Chongqing 400044, PR China College of Chemistry and Chemical Engineering, Chongqing University, Chongqing 400044, PR China
a r t i c l e
i n f o
Article history: Received 28 January 2013 Received in revised form 22 February 2013 Accepted 24 February 2013 Available online 4 March 2013 Keywords: Magnetic composite ZnFe2 O4 /SrFe12 O19 One-step chemical coprecipitation Methylene blue Magnetic Photocatalytic
a b s t r a c t One-step chemical coprecipitation with high-temperature sintering method was employed for preparing magnetic composite ZnFe2 O4 /SrFe12 O19 including a hard-magnetic phase (SrFe12 O19 ) and a soft-magnetic phase (ZnFe2 O4 ). The magnetic composite was characterized by FTIR, XRD, SEM, BET, XPS, VSM, and UV–vis. The testing results showed that the saturation magnetization (Ms ), remanent magnetization (Mr ), and coercivity (Hc ) were 34.95 emu/g, 18.31 emu/g, and 2254.54 G, respectively, indicating that the composite possessed excellent magnetic properties and a greater capacity for anti-demagnetize. The properties of the composite were favourable to its separation, recycling, and reuse after reaction. The photocatalytic performance of the composite was studied by the degradation reaction of methylene blue under visible light irradiation. The experimental results revealed that the degradation rate was still more than 70% when the composite was reused for four times. In addition, this research was expected to provide a promising method to prepare various composite materials with multi-functional components. Crown Copyright © 2013 Published by Elsevier B.V. All rights reserved.
1. Introduction Photocatalysts, such as TiO2 [1,2], ZnO [2,3], SrTiO3 [4,5], and BiOCl [2,6] using for environmental remediation have been attracting considerable attention for several decades. Many species of industrial organic dye (wastewater) involving methylene blue (MB), methyl orange (MO), phenol, rhodamine B, and bromophenol blue [2,7], were decomposed over those photocatalysts under ultraviolet or visible radiation. However, it was difficult to separate catalysts from liquid solution after reaction, which probably produced secondary pollution and increased costs. Catalysts with magnetism, namely magnetic catalysts could overcome the two problems. Therefore, it was crucial to find a strategy for making those photocatalysts possess magnetism. In recent years, a kind of soft-magnetic material, ZnFe2 O4 was extensively used for ferrofluids [8], high-density magnetic recording media [9], biomedicine [10], radar absorbing material [11] due to its various advantages, for instance good chemical stability, corrosion resistivity, and superior magnetic properties. Furthermore, ZnFe2 O4 had applications in gas sensor [12,13], semiconductor photocatalysis [14,15], and organic reaction catalyst fields [16–18]. So the potential interest in synthesis and application of ZnFe2 O4
∗ Corresponding author. Tel.: +86 2386809361; fax: +86 2386809361. E-mail addresses: [email protected] (T. Xie), [email protected] (L. Xu).
for many researchers had been revived. In terms of application in photocatalysis, however, owing to its bad valence band potential and inferior nature of photoelectric conversion [19], ZnFe2 O4 was modified with other elements or compounds to improve its photocatalytic activity. ZnFeNdO4 [16], ZnFe2 O4 /Fe3 O4 /Ag [19], ZnFe2 O4 /Fe2 O3 /ZnO [20], ZnO/ZnFe2 O4 [21], TiO2 /Al2 O3 /ZnFe2 O4 [22], ZnFe2 O4 /Graphene [23], and ZnFe2 O4 /TiO2 [24] were synthesized and used as photocatalysts. A kind of hard-magnetic material, SrFe12 O19 could provide various species of non-equivalent sites for various magnetic or non-magnetic cations [25]. SrFe12 O19 had several merits, such as relatively large saturation magnetization, superior coercivity, high uniaxial magnetic crystalline anisotropy, chemical stability, and corrosion resistivity, which stimulated researchers to appreciate its significance in electronic components, magnetic memories, biotechnology, and in magnetic substrate for magnetic catalysts. To the best of our knowledge, magnetic composite ZnFe2 O4 / SrFe12 O19 was synthesized by sol–gel technique [26,27] and twostep coprecipitation method [28,29]. The sol–gel technique was expensive, time consuming, and polluted. The pollution originated from the decomposition of the used organic dispersants in the process of heat treatment. For the two-step coprecipitation method, the two materials were individually prepared by coprecipitation, and then the resultant was obtained by mechanical blending, which complicated the procedures. It was understood that the two abovementioned methods had provided some fundamental studies, but they were not obviously suitable for industrial applications. So it is
0169-4332/$ – see front matter. Crown Copyright © 2013 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.02.113
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still a challenge for chemists to invent an easy and simple process to prepare ZnFe2 O4 /SrFe12 O19 for mass production. In this work, therefore, magnetic composite ZnFe2 O4 /SrFe12 O19 was prepared by one-step chemical coprecipitation with hightemperature sintering method. ZnFe2 O4 was decorated with SrFe12 O19 , which improved magnetic properties of ZnFe2 O4 and would help to enhance its photocatalytic activity. The composite was characterized by FTIR, XRD, SEM, BET, XPS, VSM, and UV–vis. The photocatalytic activity was studied by the degradation of MB under visible light irradiation. 2. Experimental procedures 2.1. Synthesis of ZnFe2 O4 /SrFe12 O19 All reagents were of analytical grade purity and used directly without further purification except that SrCO3 [30] was obtained from industrial strontium residue. The water used was deionized water. SrCO3 , ZnCl2 , and FeCl3 were precisely weighted with a molar ratio of 1:1:14. SrCO3 and FeCl3 were dissolved in suitable HCl aqueous solution and water, respectively. The two homogeneous solutions were completely mixed by ultrasonic-assisting. At the same time, ZnCl2 was added into the mixed solution. The pH of the mixed solution was adjusted to 10 with NaOH solution after ZnCl2 was adequately dissolved. The reactions of NaOH with SrCl2 , FeCl3 and ZnCl2 immediately occurred to generate the brownish yellow slurry of the hybrid complex. The final mixture was transferred into a stainless steel autoclave that maintained at 60 ◦ C for 10 h. The dry block was crushed into a fine powder and sintered at 1000 ◦ C for 2 h. In addition, pure SrFe12 O19 and ZnFe2 O4 were prepared using similar procedure. In the process of preparation of SrFe12 O19 , SrCO3 and FeCl3 were weighted with a molar ratio of 1:12. The precursor was sintered at1000 ◦ C for 2 h. In the process of synthesis of ZnFe2 O4 , ZnCl2 and FeCl3 were weighted with a molar ratio of 1:2. The precursor was sintered at 700 ◦ C for 6 h. 2.2. Characterization Phase identification via X-ray diffraction (XRD) was conducted on an X-ray diffractometer (Bruker Advance D8) using Cu K␣ radiation. Fourier transform infrared spectroscopy (FTIR) spectra of samples were recorded on a 5DX FTIR (5DX, Nicolet Co., USA) spectrometer using KBr powder-pressed pellets. The samples’ morphologies were observed by scanning electron microscopy (SEM, ZEISS, EVO-LS15). The magnetic properties were investigated using a vibrating sample magnetometer (VSM, Lakeshore 7410). The measurement of N2 absorption performance of samples was performed on an adsorption instrument (ASAP-2020, Micromeritics, USA). X-ray photoelectron spectroscopy (XPS) was recorded on an XPS-XSAM800 (Kratos, UK) spectrometer with an achromatic Al K␣ X-ray source and an analytical chamber with a base pressure of 2 × 10−7 Pa. The X-ray gun was operated at 180 W (12 kV, 15 mA). The UV–vis spectra of samples were measured using a UV–vis spectrophotometer (TU1901, China). BaSO4 was used as a reflectance standard in the UV–vis diffuse reflectance experiment. 2.3. Photocatalytic degradation of MB The photocatalytic activity of the ZnFe2 O4 /SrFe12 O19 was studied by the degradation of MB under irradiation of a 500WXe lamp at the natural pH value. A 200 mL of 10 mg/L MB aqueous solution and its corresponding composite dosage of 2 g/L were added into quartz container and stirred for 1 h in the dark. After a given irradiation time, about 3 mL of the mixtures was withdrawn. And
Fig. 1. FTIR spectra of materials.
then the solution and ZnFe2 O4 /SrFe12 O19 particles were separated by an extra magnet. The photocatalytic degradation process of MB was monitored by measuring its absorption with a UV–vis spectrophotometer. The degradation rate of MB was calculated by the following Eq. (1), Degradation rate (%) =
A0 − At × 100% A0
(1)
where A0 represents the initial absorbance of MB, At denotes the variable absorbance of MB at different reaction time. 3. Results and discussion 3.1. FTIR Fig. 1 showed the FTIR spectra of the prepared materials. The peaks of metal-oxygen bonds at 551.5 cm−1 and 434.0 cm−1 were the characteristic absorption peaks of ZnFe2 O4 , which was consistent with previous literatures [31] and [32]. The characteristic peaks of SrFe12 O19 were at 603.0 cm−1 , 553.2 cm−1 , and 451.4 cm−1 [33]. Similarly, the three absorption peaks (at about 593 cm−1 , 549 cm−1 , and 444 cm−1 ) appeared in the spectrum of ZnFe2 O4/ SrFe12 O19 , which demonstrated that the added Zn2+ did not change the intrinsic structure of SrFe12 O19 . The absorption peak of the ZnFe2 O4 at 551.5 cm−1 was not clearly seen in the spectrum of ZnFe2 O4/ SrFe12 O19 , which was ascribed to the fact that the peak of Zn-O bonds in the ZnFe2 O4 overlapped with the peak of SrFe12 O19 at 553.2 cm−1 . It was worth noting that the peaks at about 593 cm−1 , 549 cm−1 , and 444 cm−1 in the spectrum of ZnFe2 O4/ SrFe12 O19 became sharper and shifted to low wavenumber as compared with the characteristic peaks of SrFe12 O19 . On the one hand, the chemical polarization of the internal chemical bonds of SrFe12 O19 was greatly strengthened as the formation of the ZnFe2 O4 . On the other hand, both Fe-O-Zn and Sr-O-Zn bonds were probably formed by introducing the Zn2+ into SrFe12 O19 . Thus, the oxygen atom of Fe(Sr)-O bond might be shared with Zn atom, giving rise to vibrational coupling between Fe(Sr)-O and Zn-O. 3.2. XRD The XRD patterns of ZnFe2 O4 , SrFe12 O19 , and ZnFe2 O4 /SrFe12 O19 were exhibited in Fig. 2. Evidently, every diffraction peak could be fully indexed as ZnFe2 O4 (JCPDS file 01-1109), with
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was at c.a. 529.4 eV, 531.0 eV, and 532.7 eV, respectively. And the peak at 529.4 eV could ascribable to O2− in Fe2 O3 system [38]. Fig. 3(c) exhibited the photoelectron peaks of the Sr 3d5/2 and 3d3/2 at 133.2 eV and 134.9 eV, respectively. The Sr 3d5/2 was assigned to surface Sr–O bonds and the Sr 3d3/2 could verify the presence of Sr2+ [40]. The criteria binding energy of Sr 3d5/2 and 3d3/2 were 133.7 eV and 135.5 eV, respectively. Compared with this criteria binding energy of Sr 3d, the values of the prepared sample were altered, arising out of the formed new chemical bonds, such as Sr–O–Zn and Sr–O–Fe. These inferences were consistent with the XRD and FTIR analyses. 3.4. SEM
Fig. 2. XRD patterns of materials.
characteristic reflection phases (2 2 0), (3 1 1), (2 2 2), (4 0 0), etc. [34], which proved that a pure S-type ZnFe2 O4 phase was prepared. The peaks of the as-prepared SrFe12 O19 were at 2 of 32.5◦ , 34.3◦ , 37.8◦ , 57.4◦ , corresponding to the characteristic (1 0 7), (1 1 4), (1 0 8), (2 1 8) reflection [35]. These peaks could be indexed as the M-type SrFe12 O19 (JCPDS file 24-1207). No impurity peaks were observed, indicating that the pure SrFe12 O19 sample was synthesized. As could be seen, the XRD pattern of ZnFe2 O4 /SrFe12 O19 composite not only exhibited a series of the characteristic diffraction peaks of SrFe12 O19 , but also presented the peaks of ZnFe2 O4 , which signified that the as-prepared composite were definitely composed of hard-magnetic phase (SrFe12 O19 ) and soft-magnetic phase (ZnFe2 O4 ), and that the composite was desirous material, ZnFe2 O4/ SrFe12 O19 . 3.3. XPS Fig. 3 showed the XPS spectra of ZnFe2 O4 /SrFe12 O19 , (a) low resolution for all atoms; (b), (c), and (d) high resolution for Fe, O, and Sr respectively. The low resolution XPS scan (Fig. 3 (a)) was collected the major peaks from Sr 3p, Sr 3d, Fe 2p, Fe 3p, O 1s, and Cl 1s. The high resolution scans were gathered to study the major peaks from the Fe 2p, Sr 3d, and O1s in detail.The peaks of Zn 2p or Zn 3p were not detected via the XPS scan. The content of Zn was not less than the detection limit of a surface concentration of 0.01%. So the probable explanation was that the Zn2+ occupied the 2a sites for Fe3+ that inserted in interstices between the oxygen positions in M-type SrFe12 O19 [36], resulting in no Zn2+ ions were on the external surface of sample. The Fe 2p3/2 and 2p1/2 photoelectron peaks with multiplet splitting phenomenon were observed at 710.9 eV and 724.4 eV (Fig. 3 (b)) respectively, corresponding to Fe2+ (FeO) and Fe3+ (␣Fe2 O3 ) [37,38], which was confirmed by the Fe3p photoelectron peaks at ca. 54∼56 eV (Fig. 3 (a)), namely (55.9 ± 0.2 eV (Fe3+ ) and 53.9 ± 0.2 eV (Fe2+ ) [39]). The O1s spectrum with three components
The different morphologies of the as-prepared materials were depicted in Fig. 4. The (a) sample exhibited octahedral structure (Fig. 4 (a)), which was consistent with XRD analysis. The Fig. 4 (b) presented hexagonal crystal system that was a typical feature of M-type SrFe12 O19 [37]. Fig. 4 (c) illustrated almost uniform particle distribution and uniform octahedral structure. The conformity of surface morphology (Fig. 4 (c)) could probably improve the chemical stability and morphology anisotropy of composite. The octahedral crystal growth of ZnFe2 O4 tended to restrain the growth of hexagonal crystal of SrFe12 O19 . The Zn ions inserted and occupied the non-equivalent sites [36], especially octahedral sites (4f2 or 2a sites) in SrFe12 O19 , which led to a phenomenon that the crystal growth of SrFe12 O19 was probably towed towards octahedral crystal. It was called crystal transformation. Meanwhile, to some extent, the exchange coupling interaction between the two magnetic phases was generated, which was attributable to the formation of the hard- and soft-magnetic phases (Fig. 2) as the Zn ions inserted. The molecular mass of SrFe12 O19 was larger than that of ZnFe2 O4 , which brought about a phenomenon that the SrFe12 O19 particles absorbed more electrons than ZnFe2 O4 [25]. It was certain that the heavier molecular increased in the cross section for elastic scattering, as a result, electrons underwent more elastic scattering per unit distance and the average scattering angle was greater than that of low molecular mass material as well. There was also a possibility that the heavier material showed less bright, and that the low molecular mass material displayed more bright. Therefore, the SrFe12 O19 regions were possibly dark, while the ZnFe2 O4 regions were relatively bright (Fig. 4(c)). 3.5. N2 adsorption–desorption (BET) The nitrogen adsorption–desorption isotherms of ZnFe2 O4 and ZnFe2 O4/ SrFe12 O19 composite and their pore size distribution curves were presented in Fig. 5. On the whole, the nitrogen adsorption–desorption isotherms of pure ZnFe2 O4 and ZnFe2 O4/ SrFe12 O19 composite could be categorized as a typical Type III isotherm which was convex to the p/p0 axis over its entire range [41]. Their most probable pore sizes were 1.91 nm and 2.07 nm, respectively. They therefore could be classified as mesoporous material. The pure ZnFe2 O4 was same porous features as the ZnFe2 O4 /SrFe12 O19 . Both had a large quantity of macropore. The difference between the two materials was that the pure ZnFe2 O4 had more mesopore as compared to the composite, which brought about a phenomenon that the specific surface area of the pure ZnFe2 O4 was in excess of that of the composite (Table 1). It was interesting to note that a discrete curve (p/p0 ∼ 0.6–0.7) appeared in desorption isotherm (Fig. 5 (b)) of the composite, which elucidated that the escape of nitrogen from the sample surface, i.e. nitrogen desorption, was difficult in the p/p0 range of 0.6–0.7, arising from the uniform surface structure of the composite.
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Fig. 3. XPS spectra of ZnFe2 O4/ SrFe12 O19 .
3.6. VSM The magnetic properties of materials were investigated by VSM. The magnetic hysteresis loops were depicted in Fig. 6 and the magnetic parameters were listed in Table 2. It was obvious that ZnFe2 O4 was soft-magnetic material due to its coercivity (Hc ) of zero. In other words, the magnetic hysteresis loop of ZnFe2 O4 passed through the origin of coordinates (Fig. 6(a)). This phenomenon also proclaimed that ZnFe2 O4 was superparamagnetic. Moreover, a sample could not fully saturate at 10 kG, which underlined the presence of superparamagnetic and single domain particles. The SrFe12 O19 possessed an Hc of 1710.10 G and a remanent magnetization (Mr ) of 18.31 emu/g (Table 2), which was a characterization of a typical hard-magnetic material (ferromagnetic property). Similarly, the ZnFe2 O4 /SrFe12 O19 composite was also classified as a hard-magnetic material. Indeed, the pure ZnFe2 O4 normally had superparamagnetic property [42] rather than ferromagnetism. So the strong ferromagnetic property of the composite should be attributed to the hard-magnetic phase (SrFe12 O19 ) component.
Previous investigation pointed out that magnetization of a ferromagnetic material was potentially dependent on the strength of superexchange interaction between Fe3+ cations in octahedral sites [43]. The Ms and Mr of composite were comparatively low as compared with that of SrFe12 O19 , owing to the presence of the softmagnetic phase (ZnFe2 O4 ). First of all, the non-magnetic ion, Zn2+ ions occupied the octahedral 2a sites for Fe3+ ions with a higher magnetic moment (5 B ). Second, Zn2+ ions occupied the octahedral 2a sites for Fe3+ ions in the SrFe12 O19 system, resulting in the valence change of Fe3+ to Fe2+ to maintain the electroneutrality, and then bringing about the decrease in amount of Fe3+ cations, which weakened the strength of exchange interaction between Fe3+ cations. The two above reasons both induced to the decrease in Ms and Mr of the prepared composite. It was worthwhile to note that the composite had a larger coercivity (2254.54 G, Table 2), indicating that it possessed a better capacity of anti-demagnetize, which was helpful to its recyclable, recovery, and reuse.
3.7. UV–vis spectra
Table 1 Specific surface area and most probable pore size of materials. samples
Specific surface area (m2 /g)
Most probable pore size (Å)
ZnFe2 O4 (a) ZnFe2 O4/ SrFe12 O19 composite (b)
8.383 5.115
19.1 20.7
The UV–vis spectra of SrFe12 O19 , ZnFe2 O4 , and ZnFe2 O4 / SrFe12 O19 were shown in Fig. 7. The three samples showed intense absorption in a wide wavelength range from UV to visible light with absorption tail extending into infra red region. It was clear that the absorption spectrum of ZnFe2 O4 /SrFe12 O19 composite contained
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Fig. 4. SEM photographs of materials: (a) ZnFe2 O4 ; (b) SrFe12 O19 ; (c) ZnFe2 O4/ SrFe12 O19 .
Fig. 5. Nitrogen adsorption–desorption isotherms for (a) ZnFe2 O4 , (b) ZnFe2 O4/ SrFe12 O19 . Inset: the corresponding pore size distribution of (a) ZnFe2 O4 , (b) ZnFe2 O4 /SrFe12 O19 . Table 2 Magnetic parameters of materials. Samples
ZnFe2 O4 (a) SrFe12 O19 (b) ZnFe2 O4/ SrFe12 O19 composite (c)
Magnetization (emu/g)
Coercivity Hc (G)
Saturation magnetization (Ms )
Remanent magnetization (Mr )
1.67 49.97 34.95
0 29.84 18.31
0 1710.10 2254.54
T. Xie et al. / Applied Surface Science 273 (2013) 684–691
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Fig. 8. Degradation rate of MB with different photocatalyst, i.e. pure ZnFe2 O4 and ZnFe2 O4 /SrFe12 O19 . Inset: Degradation rate of MB with ZnFe2 O4/ SrFe12 O19 photocatalyst for five times reuse. Fig. 6. Magnetic hysteresis loops of materials.
the absorptions of ZnFe2 O4 and SrFe12 O19 from UV to visible range. Previous literature [44] reported that the visible light sensitivity was concerned with Fe cations in the system. The Fe3+ ion possessed a 3d5 configuration which had a sextet state in the octahedral crystal field. Meanwhile, very weak crystal field transitions were expected as these transitions were spin, symmetry and parity forbidden [44,45]. In addition, charge transfer transition from O2− to Fe3+ normally gave rise to a strong absorption around 275 nm (see Fig. 7). Therefore, such intense absorption in the visible region was likely due to other types of charge transfer transition. Metal to metal charge transfer transitions (2Fe3+ → Fe2+ + Fe4+ ) had been proposed by Haart and his coworkers [45]. Besides, the Zn2+ inserted and occupied the 2a site for Fe3+ in SrFe12 O19 , leading to the valence change of Fe3+ to Fe2+ to maintain the electroneutrality and the
Fig. 7. UV–vis absorption spectra of (a) SrFe12 O19 , (b) ZnFe2 O4 , and (c) ZnFe2 O4/ SrFe12 O19 .
formation of Zn-O-Fe bonds. In other words, the Zn2+ introduced into the SrFe12 O19 system would cause the amount of Fe3+ cations and Fe-O-Fe bonds to decrease, which rendered metal charge transition more difficult. So absorption of ZnFe2 O4 /SrFe12 O19 composite became weaker as compared with ZnFe2 O4 . In contrast to single phase ZnFe2 O4 , the photoresponse of the composite to the visible range was expanded, which was expedient to the degradation of organic dye over the composite photocatalyst under the visible light. The above observation could also verify the formation of the composite with involving the two phases, i.e. ZnFe2 O4 and SrFe12 O19 .
3.8. The photocatalytic properties To compare the photocatalytic activities of pure ZnFe2 O4 and ZnFe2 O4/ SrFe12 O19 composite, they were used for photocatalytic degradation of MB under identical test condition. The experimental results were shown in Fig. 8. It was evident that the ZnFe2 O4 /SrFe12 O19 composite surpassed the pure ZnFe2 O4 in degradation efficiency after 2 h reaction. These results were in good agreement with the deduction of UV–vis spectra. Since its bad valence band potential and inferior nature of photoelectric conversion [19], there was need to decorate ZnFe2 O4 in order to improve its photocatalytic activity. In this work, ZnFe2 O4 was modified with a kind of hard-magnetic material, SrFe12 O19 . It was found through the above analyses that the Zn2+ inserted and occupied the 2a site for Fe3+ in SrFe12 O19 , which resulted in the valence change of Fe3+ to Fe2+ to maintain the electroneutrality. And Fe2+ not only enhanced the interfacial polarization due to the Fe2+ was easier polarized [46], but also elevated the electrons hopping between Fe2+ and Fe3+ . In addition, the Zn2+ at 2a sites and Fe3+ formed the electric dipoles with surrounding negative O2− [47], which improved the capacity of electron transport, and then enhanced the photocatalytic activity of composite. The stability of the photocatalysis of the ZnFe2 O4 /SrFe12 O19 composite was confirmed by repeating the decomposition processes for four times. Here, to obtain the largest degradation rate, the irradiation time of all the recycle experiments was 5 h. The results were shown in Fig. 8 (see inset). After the degradation reaction, the XRD pattern of the recycled composite was collected to
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(4)
+ ·O− 2 / · HO2 /hVB + MB → CO2 + H2 O
(5)
In fact, there was need to further research for whether the degradation rate was influenced by the magnetic field (Lorentz force) stemming from the remanent magnetization of magnetic catalyst or not. And there was also need further investigation for whether the presence of SrFe12 O19 prevented recombination between the photogenerated electrons and holes in the VB. 4. Conclusions
Fig. 9. XRD patterns of (a) ZnFe2 O4 /SrFe12 O19 and (b) recycled ZnFe2 O4 /SrFe12 O19 sample after being used for photocatalysis experiments.
use for confirming its intrinsic crystal structure. The XRD patterns of fresh state sample and recycled sample were displayed in Fig. 9. When the composite was reused for four times, the degradation rate was still more than 70%, though the degradation rate decreased with its reuse. The composite could keep higher degradation efficiency, because its intrinsic crystal structure was still reserved after reuse. In other words, no change in structure of composite before and after reaction was observed, declaring that its better stability (Fig. 9). It was worth mentioning that the reduction of degradation efficiency derived from activity loss of little amount of the recycled composite. On the one hand, the partially destruction of the mesostructure in the original sample was generated due to the stirring process prior to degradation reaction. On the other hand, the surface of the composite absorbed some micromolecules or ions originating from the decomposition of MB. On the whole, the composite had an excellent photocatalytic activity and stability. And its superior magnetic properties conduced its recycling through the agency of an extra magnet. A possible mechanism for the photodegradation of MB over ZnFe2 O4 /SrFe12 O19 was proposed, as illustrated by Eqs. (2)–(5). From the above study, especially in analyses of UV–vis spectra and related literature [19–24], it was reported that the ZnFe2 O4 was a potential photocatalyst in decomposing organics under UV light irradiation. Thus, in the ZnFe2 O4 /SrFe12 O19 composite system, ZnFe2 O4 acted as a main photocatalyst, while SrFe12 O19 was a sensitizer absorbing of visible light. Under visible light irradiation, some electrons (e− ) in the valence band (VB) was excited to the conduction band (CB) causing the generation of holes (h+ ) in the VB simultaneously (Eq. (2)). A portion of the photogenerated electrons would recombine with holes in the VB, while others transferred to the surface and reacted with the adsorptive oxygen molecule to yield • O2 − (Eq. (3)). The generated • O2 − would further combined with H+ to produce • HO2 (Eq. (4)). The reactive species, such as • O − , • HO , and h+ , all could oxidize MB (Eq. (5)). 2 2 VB
− + h+ ZnFe2 O4/SrFe12 O19 + hv → ZnFe2 O4 /SrFe12 O19 eCB Vb − + O2 → ZnFe2 O4 /SrFe12 O19 + ·O− ZnFe2 O4/SrFe12 O19 eCB 2
(2) (3)
Magnetic composite ZnFe2 O4 /SrFe12 O19 was prepared by onestep chemical coprecipitation with high-temperature sintering method. The composite was characterized by FTIR, XRD, XPS, SEM, BET, VSM, and UV–vis. And the photocatalytic activity of the composite was probed through decomposition of MB by photocatalysis. The results manifested that the composite possessed better magnetic properties, conformity surface structure, a better photocatalytic activity and stability. The photocatalytic test revealed that the degradation rate was still more than 70% when the composite was reused for four times. In the ZnFe2 O4 /SrFe12 O19 composite system, ZnFe2 O4 worked as main photocatalyst while SrFe12 O19 acted as sensitizer absorbing for visible light and magnetic source. This research was hoped to provide simple synthesis technique for multi-phase or multi-functional composite materials. Acknowledgements We would like to appreciate our associates, especially Jingrong He, Wenli Wu, Jun Yang, for their valuable contributions to our research program. We want to thank the financial support from the program of Chongqing Science and Technology Commission (Nos. 2010AC7180, 2011AC4070). We acknowledge gratefully many important contributions from the researchers of all literatures cited in our paper. References [1] M. Pelaez, N.T. Nolan, S.C. Pillai, M.K. Seery, P. Falaras, A.G. Kontos, P.S.M. Dunlop, J.W.J. Hamilton, J.A. Byrne, K. OShea, M.H. Entezari, D.D. Dionysiou, A review on the visible light active titanium dioxide photocatalysts for environmental applications, Applied Catalysis B 125 (2012) 331–349. [2] A.D. Paola, E. García-López, G. Marcì, L. Palmisano, A survey of photocatalytic materials for environmental remediation, Journal of Hazardous Materials 211–212 (2012) 3–29. [3] A. Moezzi, A.M. McDonagh, M.B. Cortie, Zinc oxide particles: synthesis, properties and applications, Chemical Engineering Journal 185–186 (2012) 1–22. [4] S. Song, L.J. Xu, Z.Q. He, H.P. Ying, J.M. Chen, X.Z. Xiao, B. Yan, Photocatalytic degradation of C.I. Direct Red 23 in aqueous solutions under UV irradiation using SrTiO3 /CeO2 composite as the catalyst, Journal of Hazardous Materials 152 (2008) 1301–1308. [5] M. Tsuchiya, S.K.R.S. Sankaranarayanan, S. Ramanathan, Photon-assisted oxidation and oxide thin film synthesis: a review, Progress in Materials Science 54 (2009) 981–1057. [6] F. Dong, Y.J. Sun, M. Fu, Z.B. Wu, S.C. Lee, Room temperature synthesis and highly enhanced visible light photocatalytic activity of porous BiOI/BiOCl composites nanoplates microflowers, Journal of Hazardous Materials 219–220 (2012) 26–34. [7] E. Casbeer, V.K. Sharma, X.Z. Li, Synthesis and photocatalytic activity of ferrites under visible light: a review, Separation and Purification Technology 87 (2012) 1–14. [8] K. Raj, R. Moskowitz, Commercial applications of ferrofluids, Journal of Magnetism and Magnetic Materials 85 (1990) 233–245. [9] A. Moser, K. Takano, D.T. Margulies, M. Albrecht, Y. Sonobe, Y. Ikeda, S. Sun, E.E. Fullerton, Magnetic recording: advancing into the future, Journal of Physics D: Applied Physics 35 (2002) R157–R167. [10] J.M. Bai, J.P. Wang, High-magnetic-moment core–shell-type FeCo-Au/Ag nanoparticles, Applied Physics Letters 87 (2005) 152502. [11] R.C. Che, L.M. Peng, X.F. Duan, Q. Che, X.L. Liang, Microwave absorption enhancement and complex permittivity and permeability of Fe encapsulated within carbon nanotubes, Advanced Materials 16 (2004) 401–405.
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