SrFe12O19 Magnetic Photocatalyst with

Oct 25, 2013 - Novel Heterojunction Bi2O3/SrFe12O19 Magnetic Photocatalyst with. Highly Enhanced Photocatalytic Activity. Taiping Xie,*. ,†. Chenglu...
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Novel Heterojunction Bi2O3/SrFe12O19 Magnetic Photocatalyst with Highly Enhanced Photocatalytic Activity Taiping Xie,*,† Chenglun Liu,*,‡ Longjun Xu,† Jun Yang,† and Wei Zhou‡ †

State Key Laboratory of Coal Mine Disaster Dynamics and Control, Chongqing University, Chongqing 400044, P. R. China College of Chemistry and Chemical Engineering, Chongqing University, Chongqing 400044, P. R. China

‡

ABSTRACT: Magnetic composite photocatalyst Bi2O3/SrFe12O19 was synthesized by hydrolysis with medium temperature sintering method. Xray diffraction (XRD) investigation revealed that introduction of SrFe12O19 did not change the favorite growth direction of Bi2O3, [121] orientation. Micromorphology study indicated that SrFe12O19 was distributed on the surface of Bi2O3 to possess the best possibility of forming some heterojunction structures. Vibrating sample magnetometer (VSM) measurements manifested the composite possessed better magnetic properties, which was conducive to its separation, recycling, and reuse. Three main reasons for the increase in photocatalytic activity of Bi2O3/SrFe12O19 were (1) the formation of p−n-type heterojunction between p-type Bi2O3 and ntype SrFe12O19 semiconductors, (2) magnetic field effect stemming from magnetic composite itself generated a shunt effect for photoexcited electrons, and (3) as a sensitizer absorbing visible light, SrFe12O19 could help Bi2O3 absorb more incident photons and then produce more photoexcited electron−hole pairs. In addition, this work was expected to provide a simple preparation method for various functional materials, especially magnetic functional materials. eV,9,10 which enabled itself to oxidize water and to produce some active species that could initiate oxidation reactions. However, in the practical application of wastewater treatment using photocatalyst, such as Bi2O3, TiO2, ZnO, and BiOCl, chemical scientists found a troublesome problem that the separation of photocatalyst powder from water following treatment was complicated, time-consuming, and high-cost.2,3 Suppose that the used photocatalyst could not be recycled exhaustively, the remaining photocatalyst probably caused secondary pollution, which violated the initial idea for wastewater treatment. It was encouraging to find that making the photocatalyst possess magnetization, namely, magnetic photocatalyst could overcome these problems.11−14 Owing to its various excellent properties, for instance, relatively large magnetization, superior coercivity, a better chemical stability, and corrosion resistivity,15 as a species of hard-magnetic material, SrFe12O19 has also been a focus of considerable attention in recent years. It was pleased to know that SrFe12O19 could be used as magnetic substrate for magnetic catalyst, confirmed by Pullar,16 Ji,17 Aziz,18 and our previous investigation.19 It was more interesting to find that as a kind of n-type semiconductor SrFe12O19 also could be directly used as photodegradation reaction under visible light on account of its narrow band gap, i.e., ca. 1.86 eV.11,20

1. INTRODUCTION Since the photolysis water using a TiO2 semiconductor electrode under UV light irradiation was found by Fujishima and Honda in 1972,1 the photocatalytic techniques and corresponding photocatalytic materials have attracted overwhelming attention for many years, especially in recent years, which is attributable to a fact that the compatibility of photocatalysis with modern technology is excellent. In other words, the photocatalysis can be applied in many fields, such as environmental control or remediation, and energy-related areas. Interestingly, the application of the TiO2 photocatalysis in offset printing was found and reported by Nakata.2,3 As many scientists speculated, the prospective applications of photocatalysis still need to be continuously explored with great effort. In fact, the exploration of new photocatalysts is as important as the development of photocatalysis applications.4,5 So finding new photocatalysts and improving the obtained photocatalyst for better and larger contribution toward their practical applications are still essential. Generally, two strategies can be used to enhance photocatalytic activity of a photocatalyst. One way is to make the photocatalyst absorb more incident photons and subsequently produce more photoexcited electron−hole pairs. The other is to prevent photoproduced electron−hole pairs from recombining, i.e., enhancement of electron−hole separation efficiency.6−8 A kind of p-type Bi2O3 semiconductor was considered as a promising photocatalytic material alternative to other many binary oxides, which was attributable to a fact that Bi2O3 possessed the band gap energy in visible light range, i.e., ca. 2.8 © 2013 American Chemical Society

Received: August 28, 2013 Revised: October 24, 2013 Published: October 25, 2013 24601

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Figure 1. (a−c) Time-dependent UV−vis absorption spectra of the MB in the presence of Bi2O3 prepared at different pH values; (d) photocatalytic degradation ratio of MB versus visible light (≥420 nm) irradiation time by Bi2O3 photocatalyst.

2.1. Preparation of Pure Bi2O3. A certain amount of Bi(NO)3 was weighted and dissolved in 4 mol/L HNO3 solution to form a homogeneous solution. The pH of the prepared homogeneous solution was adjusted to more than 7. The chemical reaction could be obviously observed due to color change. The obtained yellow precipitate by filtrating was washed several times, and then was dried at 60 °C. The dry block was crushed into fine powder and was sintered at 550 °C for 3 h to obtain pure Bi2O3. 2.2. Preparation of Bi2O3/SrFe12O19. Similarly, a certain amount of Bi(NO)3 was weighted and dissolved to form a homogeneous solution (A) using 4 mol/L HNO3 solution. SrFe12O19 with a mass ratio of 20% and a small amount of sodium dodecyl benzene sulfonate (SDBS) were weighted and dissolved under vigorous stirring to form a suspension (B). The whole mixture was obtained by blending A solution and B suspension. The pH of the whole mixture was adjusted to 13. Then the whole mixture was filtrated. The filter residue was washed several times with absolute alcohol to remove the remaining SDBS. The washed filter residue was dried at 60 °C. The composite Bi2O3/SrFe12O19 (20 wt %) was obtained by sintering the dried filter residue at 550 °C for 3 h. The composites Bi2O3/SrFe12O19 (25, 30, 35, and 40 wt %) were prepared by adjusting the different mass ratios of SrFe12O19. 2.3. Material Characterizations. Fourier transform infrared spectroscopy (FTIR) spectra of samples were recorded on a

In light of the aforementioned considerations, therefore, in this work, coupling SrFe12O19 with Bi2O3 was to prepare a novel p−n-type heterojunction Bi2O3/SrFe12O19 between ptype Bi2O3 semiconductor and n-type SrFe12O19 semiconductor. It was well-known that a heterojunction structure with a matching band gap potential might enhance the separation efficiency of photoproduced electron−hole pairs.4 So the introduction of SrFe12O19 could enhance photocatalytic ability of Bi2O3, as a consequence of the formation of a p−n-type heterojunction. Meanwhile, magnetic field stemming from SrFe12O19 could make the composite photocatalyst capable of easier separation and recycling. In this work, therefore, a novel heterojunction, Bi2O3/SrFe12O19 magnetic photocatalyst with the best of both worlds was fabricated and characterized. Its photocatalytic activity was preliminarily evaluated using (methylene blue) MB degradation, and a possible mechanism for the enhanced photocatalytic ability was thoroughly interpreted from three aspects.

2. EXPERIMENTAL SECTION All reagents (such as Bi(NO)3, HNO3, absolute alcohol, and sodium dodecyl benzene sulfonate obtained from Sinopharm Chemical Reagent Co., Ltd.) were of analytical grade purity and used directly without further purification, except SrFe12O1921 was prepared using industrial strontium residue. The water used in all experiment was deionized water. 24602

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5DX FTIR (5DX, Nicolet. Co., USA) spectrometer using KBr powder-pressed pellets. Phase identification via X-ray diffraction (XRD) was conducted on an X-ray diffractometer (Bruker Advance D8) using Cu Kα irradiation at a scanning rate of 4°· min−1 with the 2θ range of 20−70°. X-ray photoelectron spectroscopy (XPS) measurements were carried out on an XPS-XSAM800 (Kratos, U.K.) spectrometer with an achromatic Al Kα 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 magnetic properties were investigated using a vibrating sample magnetometer (VSM, Lakeshore 7410) in applied fields up to 5 T at room temperature. The Brunauer−Emmett−Teller (BET) special surface area was determined through N2 adsorption at 77 K using an adsorption instrument (ASAP-2020, Micromeritics, USA). The samples’ morphologies and microstructures were observed by scanning electron microscopy (SEM, FEI, F50) and transmission electron microscopy (TEM, FEI, Tecnai G2 F20). Meanwhile, Digital Micrograph software was used to analyze highresolution transmission electron microscopy (HRTEM). The UV−vis diffuse reflectance spectra (DRS) of samples were measured using a UV−vis spectrophotometer (TU1901, China). BaSO4 was used as a reflectance standard. 2.4. Evaluation of Photocatalytic Activity. The photocatalytic activity of Bi2O3/SrFe12O19 composite was evaluated by (methylene blue) MB degradation under irradiation of a 500 W Halogen lamp (λ > 420 nm, light intensity was 110.8 mW/ cm2) 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 a quartz container and stirred for 1 h in the dark. After a given irradiation time, about 3 mL of the mixtures was withdrawn. Then the solution and Bi2O3/ SrFe12O19 particles were separated by an extra magnet. The photocatalytic degradation process of MB was monitored by measuring its characteristic absorption at 664 nm with a UV− vis spectrophotometer.

Figure 2. FTIR spectra of pure Bi2O3 and Bi2O3/SrFe12O19 (35 wt %) composite.

451.4 cm−1, respectively.21 Although little shift of the corresponding peaks of both Bi2O3 and SrFe12O19 in the FTIR spectrum of the composite was observed due to vibrational coupling between the peaks of SrFe12O19 and Bi− O bond, their own characteristic peaks could be found in the FTIR pattern of the composite. In addition, the peaks at 3441.7 and 1631.5 cm−1 exhibited the stretching vibration and deformation vibration of hydroxy group (−OH) acquired from wet atmosphere. The XRD spectra of pure Bi2O3 and composite Bi2O3/ SrFe12O19 (35 wt %) were shown in Figure 3. For comparison,

3. RESULTS AND DISCUSSION 3.1. Optimum pH for Preparing Bi2O3. In previous reports, the different pH values, such as 8, 9, and 13,10,22,23 were adopted in the process of preparation of pure Bi2O3. Here, therefore, the influence of pH on photocatalytic activity of pure Bi2O3 was evaluated to find a suitable pH. The results (Figure 1) remarkably showed that Bi2O3 (pH = 13) was superior to both Bi2O3 (pH = 8) and Bi2O3 (pH = 9) in terms of photocatalytic efficiency under identical test conditions. The photocatalytic efficiency of Bi2O3 (pH = 13) reached to 97.4% after 6 h of photodegradation. So the optimal pH value for preparing Bi2O3 was 13. We speculated that the morphologies of Bi2O3 preparing at different pH values were differential. The detail had not been explored because the main aim of this work was the composite preparation and photocatalytic activity evaluation, but the investigation of thorough mechanism about this point is ongoing in our group. Therefore, pH = 13 was accepted in the process of the synthesis composite. 3.2. FTIR, XRD, XPS, VSM, and BET Analyses. Primary analysis of photodegradation revealed that Bi2O3/SrFe12O19 (35 wt %) was the most efficient in the MB degradation. Figure 2 showed FTIR spectra of pure Bi2O3 and composite Bi2O3/SrFe12O19 (35 wt %). The intensive signal around 432.8 and 510.8 cm−1 appeared in IR spectrum of Bi2O3 was ascribable to the stretching vibration of Bi−O bonds. The three characteristic peaks of SrFe12O19 were at 603.0, 553.2, and

Figure 3. XRD spectra of pure Bi2O3, pure SrFe12O19, and composite Bi2O3/SrFe12O19 (35 wt %).

XRD pattern of pure SrFe12O19 was added into Figure 3. The 2θ values of SrFe12O19 were at 30.4, 32.3, 34.2, 37.1, and 55.2° corresponding to (110), (107), (114), (203), and (217) diffraction phases.21 The peaks observed at 27.0, 27.5, 28.1, 33.7, and 46.5° corresponded to (112), (121), (012), (1̅12), and (041) diffraction planes of Bi2O3 (No. PDF 7-0398) that was a member of space group P21/C(14). The lattice parameters of the prepared Bi2O3 were a = 5.830 Å, b = 8.148 Å, and c = 7.480 Å. Even if the intensity of diffraction peaks of Bi2O3 was relatively stronger and the counterpart of SrFe12O19 was relatively weaker, the characteristics peaks of the two materials in XRD pattern of composite could be observed. At the same time, there was no remarkable shift of diffraction peaks and no other crystalline impurities. The above-mentioned 24603

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Figure 4. XPS analysis of composite Bi2O3/SrFe12O19 (35 wt %).

of Bi2O3 and SrFe12O19, which was consistent of XRD and FTIR investigations. The analysis of magnetic properties for magnetic composite Bi2O3/SrFe12O19 (35 wt %) was indispensable. The magnetic hysteresis loop of the prepared composite was illustrated in Figure 5. Similarly, the magnetic hysteresis curve of SrFe12O19 was inserted for comparison. Their magnetic parameters were listed in Table 1.

analyses indicated that the as-prepared composite was desirous material, namely, composite photocatalyst Bi2O3/SrFe12O19. It was worthwhile noting that the peaks at 27.5° for both pure Bi2O3 and Bi2O3/SrFe12O19 were sharper and stronger. The relatively high intensity of the (121) peak was indicative of anisotropic growth and implied a preferred orientation of the crystallites. Indeed, Bi2O3 should favor growth along the [121] orientation.24 This meant that the introduction of SrFe12O19 did not change the preferred orientation of the crystallites of Bi2O3 and its crystal structure. To find out the presence of elements in Bi2O3/SrFe12O19 and to determine their valence states, we carried out XPS study. The binding energy peaks of Bi, Sr, Fe, and O were analyzed. The corresponding high-resolution spectra were shown in Figure 4. In Figure .4a, the peaks located at 158.6 and 163.9 eV were ascribed to Bi 4f7/2 and Bi 4f5/2, respectively, confirming that bismuth species in Bi2O3/SrFe12O19 composite were Bi3+ cations.9 In Figure 4b, the Sr 3d profile was asymmetric and could be fitted to two symmetrical peaks located at 133.2 and 134.9 eV corresponding to the photoelectron peaks of the Sr 3d5/2 and Sr 3d3/2, respectively. The previous literature reported that the Sr 3d5/2 was assigned to surface Sr−O bonds, and the Sr 3d3/2 could verify the presence of Sr2+.11 Figure 4c showed the Fe 2p peaks were at binding energies of 711.3 eV (Fe 2p3/2) and 725.1 eV (Fe 2p1/2), which was consistent with photoelectron peaks of Fe3+ in SrFe12O19 system.11 Similarly, O1s spectrum (Figure 4d) was also asymmetric and could be fitted to three symmetrical peaks situating at 529.4, 531.0, and 532.7 eV, which were ascribable to O2− in SrFe12O19 system, Bi−O bonds, and chemisorbed oxygen,7 respectively, revealing that three kinds of O species in the composite. The XPS analyses indicated that the presences

Figure 5. Magnetic hysteresis loop of composite Bi2O3/SrFe12O19 (35 wt %).

Like the SrFe12O19 itself, the composite also possessed a high coercivity (Hc), i.e., 337.06 kA·m−1, revealing the composite was also a kind of hard-magnetic material that was blessed with good antidemagnetize ability,15 which was favorable toward its reuse as photocatalyst. 24604

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3.3. SEM Images. Figure 7 showed SEM images of pure Bi2O3 and composite Bi2O3/SrFe12O19 (35 wt %). The grain

Table 1. Magnetic Parameters of Composite Bi2O3/ SrFe12O19 (35 wt %) and Pure SrFe12O19 magnetization (A·m2·kg−1) sample SrFe12O19 Bi2O3/ SrFe12O19

saturation magnetization

remanent magnetization

coercivity (kA·m−1)

39.57 25.42

23.92 15.20

387.91 337.06

Although the saturation magnetization (Ms) and remanent magnetization (Mr) of composite decreased by 35.8% and 36.5%, respectively, compared to pure SrFe12O19, because of a decrease in the amount of SrFe12O19 per gram of composite, the magnetic properties of the composite were still relatively excellent. The Ms and Mr of the composite were 25.42 and 15.20 A·m2·kg−1, respectively, which was beneficial to its separation from liquid solution and its recycling from reaction solution using an extra magnet after use. From the aforementioned analyses, we could speculate that the synthesis process of composite did not alter crystal structure of SrFe12O19. Moreover, we could verify the presence of SrFe12O19 in the composite, which further confirmed that the magnetic composite Bi2O3/SrFe12O19 was successfully prepared. The pore structure of composite Bi2O3/SrFe12O19 (35 wt %) was investigated by N2 adsorption−desorption isotherms. The corresponding pore size distribution and specific surface area were determined using the BJH method and BET method, respectively. The N2 adsorption−desorption isotherms and pore size distribution curve were shown in Figure 6. This

Figure 7. SEM images of pure Bi2O3 (a) and composite Bi2O3/ SrFe12O19 (35 wt %) (b).

sizes of pure Bi2O3 and composite Bi2O3/SrFe12O19 were in the range of ca. 0.3−1.0 μm and 0.1−0.5 μm, respectively, indicating that the growth of Bi2O3 was confined due possibly to SrFe 12 O 19 possessing a large molecule mass. The morphology for pure Bi2O3 was rod-like and double irregular sphere-like, but in the composite system, the morphology of Bi2O3 was blocks of irregular shape. This demonstrated that the morphology of Bi 2 O 3 was different before and after introduction of SrFe12O19. SrFe12O19 changed the morphology of Bi2O3 but did not alter its preferential growth direction. Interestingly, a phenomenon that the lamellae-like SrFe12O19 inserted into Bi2O3 was observed. In addition, the surface for both pure Bi2O3 and Bi2O3 in the composite system was smooth and no remarkable defects on the surface were found. It was known that the surface defect of photocatalyst usually was a recombination center for photoproduced electron and holes.5 Therefore, the smooth surface with defects was conducive to photocatalysis. 3.4. TEM and HRTEM Images. The irregular block-shaped morphology of pure Bi2O3 was shown in Figure 8a. The morphology of composite Bi2O3/SrFe12O19 (35 wt %) was presented in Figure 8b. It was notable to see that some

Figure 6. N2 adsorption−desorption isotherm of Bi2O3/SrFe12O19 (35 wt %). Inset: the corresponding pore size distribution.

isotherm could be categorized as a typical Type III isotherm, which was convex to the P/P0 axis over its entire range,11 indicating that the as-prepared composite Bi2O3/SrFe12O19 belonged to nonporous structure. This sharp increase in the adsorption isotherm was attributed to the macropore size. The most probable pore size of composite Bi2O3/SrFe12O19 was 4.49 nm. In addition, the single-point adsorption total pore volume of pores less than 396.320 Å radius at P/P0 = 0.9750 was 0.001256 cm3·g−1. These results further indicated the nonporous structure. The specific surface area of this composite was given by BET measurement as 1.12 m2·g−1. So we could ignore the absorption when the composite was used to decompose MB due to nonporous structure and small specific surface area.

Figure 8. TEM images of pure Bi2O3 (a) and composite Bi2O3/ SrFe12O19 (35 wt %) (b); HRTEM image (c,d) at different positions for the composite. 24605

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Figure 9. UV−vis diffuse reflectance spectra (DRS) of composites. Inset: the corresponding band gap energy.

different mass ratios were displayed in Figure 9. A same phenomenon for both pure Bi2O3 and Bi2O3/SrFe12O19 composites observed was a strong absorption in UV light range of 200−400 nm. It was inspiring to see that the composites showed intense absorption in a wide wavelength range from UV to visible light with absorption tail extending into infrared region, compared to the absorption spectrum of the pure Bi2O3. Meanwhile, the intensity of absorption of visible light increased along with the increase in the amount of SrFe12O19, i.e., its corresponding mass ratio. The abovementioned analyses revealed that SrFe12O19 enhanced the absorption of Bi2O3 in visible light region, namely, photoresponse for visible light. The optical band gap energy (Eg) of a crystalline semiconductor was estimated by the formula Ahv = (hv −Eg)n/2.5,25 The band gap energies for the prepared materials were determined from (Ahv)2 versus hv plots, as also shown in Figure 9 (see insets). The Eg of pure Bi2O3 was 2.83 eV, which was in good agreement with the previous literature,7,9,26 further confirming the valid synthesis procedure for syntheses of Bi2O3

relatively small SrFe12O19 laminar-shaped structures were loaded onto the surface of Bi2O3, which would result in the formation of heterostructured Bi2O3/SrFe12O19. The reported hydrolysis with sintering method here was a simple process without rigorous conditions, and hence, it was a low-cost and convenient method to prepare a magnetic composite Bi2O3/ SrFe12O19 with heterojunctions. To further confirm this structure, HRTEM observation was carried out. From the HRTEM images (Figure 8b), we could clearly find SrFe12O19 was attached and distributed on the surface of Bi2O3. The interplanar spacing was ca. 0.332 and 0.403 nm, which also corresponded to the (112) and (020) planes of Bi2O3, respectively. Because Bi2O3 and SrFe12O19 were p-type and n-type semiconductors, respectively, the heterojunction could be considered to be a well-defined and well-formed p−n-type heterojunction. 3.5. Photocatalytic Activity and Stability. 3.5.1. UV−vis DRS Analysis. The optical properties of the heterojunction Bi2O3/ SrFe12O19 were explored by UV−vis diffuse reflectance. The DRS of pure Bi2O3 and Bi2O3/SrFe12O19 composites with 24606

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Figure 10. (a−e) Time-dependent UV−vis absorption spectra of the MB in the presence of various composite photocatalysts; (f) photocatalytic degradation ratio of MB versus visible light (≥420 nm) irradiation time using photocatalysts.

3.5.2. Photocatalytic Activity. The photocatalytic activities of composites were tested by MB degradation under visible light irradiation for 4 h. The UV−vis absorption spectra of the MB were obtained using UV−vis spectrophotometer after a reaction interval of 1 h. The results were shown in Figure 10a− e. The intensity absorbance of MB at 664 nm gradually decreased along with extension of reaction time. The peaks at 664 and 288 nm did not shift, and no other peak appeared, which indicated that only the pure photochemical reaction was generated. Figure 10f showed the photocatalytic degradation ratio of MB. The photocatalytic activity of pure Bi2O3 was superior to that of Bi2O3/SrFe12O19 (20 and 30 wt %), which revealed that in the composite system the main active central would gather on the surface of Bi2O3. However, the photocatalytic activity of

and composites. The Egs of composites gradually decreased with the increase in mass ratio of SrFe12O19, which was consistent with their corresponding diffuse reflectance spectra. All in all, the prepared composites not only could be considered as a UV-light-driven semiconductor photocatalyst, but also could be used as a visible-light excitation semiconductor photocatalyst. According to intense absorption of Bi2O3 in UV light range and some previous investigations,9,22 we might know that Bi2O3 had a better photocatalytic activity under UV light irradiation. In this work, therefore, preparing heterojunction was to make good use of sunlight by extending the absorbing light range and further to enhance the possibility of practical application. So the photocatalytic abilities of asprepared composites were evaluated only under the visible light irradiation. 24607

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higher magnetization (Table 1). The photocatalysis stability of the Bi2O3/SrFe12O19 (35 wt %) was confirmed by repeating the decomposition process for four times, as shown in Figure 12.

Bi2O3/SrFe12O19 (35 wt %) was better than the counterpart of Bi2O3/SrFe12O19 (45 wt %). Theoretically, Egs of Bi2O3/ SrFe12O19 (35 wt %) and Bi2O3/SrFe12O19 (45 wt %) were 2.74 and 2.72 eV, respectively. Bi2O3/SrFe12O19 (45 wt %) under visible light could generate more photoinduced electrons than Bi2O3/SrFe12O19 (35 wt %), and the photocatalytic ability of Bi2O3/SrFe12O19 (45 wt %) was better than Bi2O3/SrFe12O19 (35 wt %). Actually, this theoretical speculation was opposite to the practical results. On the one side, this phenomenon still could reveal Bi2O3 acted as a main photocatalyst in the composite system. Moreover, only suitable mass ratio of SrFe12O19 (here, it is 35 wt %) in the p−n-type heterojunction structure possessed the best photocatalytic ability. The above results indicated that an insufficient SrFe12O19 amount in Bi2O3/SrFe12O19 heterojunction structures could not effectively separate photogenerated electrons and holes from heterojunction structures, giving rise to a low photocatalytic activity. On the other side, an excess SrFe 12 O 19 amount in heterojunction structures could also decrease the photocatalytic activity because excess SrFe 12 O 19 might offer as the recombination centers of electron−hole pairs, leading to a lower photocatalytic activity. Provided that the photodegradation process was a pseudofirst-order reaction, the apparent reaction rate constant (k) could be fitted by the following equation: −ln(C /C0) = kt

Figure 12. Recycle experiments of degrading MB on the composite Bi2O3/SrFe12O19 (35 wt %) under visible light irradiation.

The experimental results revealed the composite could be reused at least four times with no significant decrease in activity, which was indicative of a higher stability. In fact, in the experimental process, we found the composite dispersibility was increasingly good after reuse. 3.6. Possible Photocatalysis Mechanism. To explain the enhanced photocatalytic activity of the Bi 2O3/SrFe12O19 composite, we proposed a possible mechanism in Figures 13

(1)

where k was the apparent reaction constant, C0 was the initial concentration of MB, and C was the MB concentration at different reaction time. The k values were shown in Figure 11.

Figure 13. Schematic diagram of charge transfer between p-type Bi2O3 and n-type SrFe12O19, i.e., p−n-type heterojunction. Figure 11. Pseudo-first-order rate constants of decomposition of MB over pure Bi2O3 and composites Bi2O3/SrFe12O19 with different mass ratios.

Among these k values, the k for Bi2O3/SrFe12O19 (35 wt %) was the largest. The k of Bi2O3/SrFe12O19 (35 wt %) (0.882 h−1) was about 2.8 times as high as that of pure Bi2O3 (0.318 h−1). These calculated results were consistent with the abovementioned experimental results that the degradation ratios of MB for pure Bi2O3 and Bi2O3/SrFe12O19 (35 wt %) were 97.4% after 6 h reaction and 97.7% after 4 h reaction, respectively. 3.5.3. Stability and Recycle of Bi2O3/SrFe12O19. Besides the higher activity, the photostability and reusability were also indispensable for photocatalysts, especially for the magnetic photocatalyst. The prepared magnetic composite was easily recycled with an extra magnet, which was attributable to the fact that the composite (Bi2O3/SrFe12O19 35 wt %) possessed a

Figure 14. Schematic diagram for Eg matching and flow of photoinduced electrons for the prepared samples under visible light irradiation. 24608

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and 14. Three main reasons for the increase in the photocatalytic efficiency of Bi2O3/SrFe12O19 were (1) the formation of p−n-type heterojunction between p-type Bi2O3 and n-type SrFe12O19 semiconductors, (2) magnetic field effect stemming from magnetic composite itself, and (3) as a sensitizer absorbing visible light, SrFe12O19 could help Bi2O3 absorb more incident photons. The three factors might generate synergistic effect for the enhancement of photocatalytic ability. The following is a detailed interpretation of the main reasons. (1) The enhanced photocatalytic activity of Bi2O3/SrFe12O19 heterogeneous structures compared to the pure Bi2O3 was partly ascribed to the formation of the p−n-type heterojunction between p-type Bi2O3 and n-type SrFe12O19 semiconductors (see Figure 13). The conduction band (CB) and valence band (VB) potentials of the p-type Bi2O3 semiconductor were 0.30 and 3.13 eV,7,9,10 respectively. The CB and VB for n-type SrFe12O19 were 0.20 and 2.06 eV,20 respectively. Before contact of p-type Bi2O3 semiconductor and n-type SrFe12O19 semiconductor, the CB potential of n-type SrFe12O19 was more negative than that of p-type Bi2O3, and the VB potential of ptype Bi2O3 was more positive than that of n-type SrFe12O19. Thus, photoexcited electrons on SrFe12O19 underwent vertical transfer to Bi2O3, whereas holes on Bi2O3 migrated to SrFe12O19, which would produce an inner electric field.5 In turn, the migrations of photogenerated electrons and holes would be promoted by the formed inner electric field,27 which facilitated the charge separation and a higher photocatalytic ability. (2) The enhanced photocatalytic activity of the composite magnetic photocatalyst Bi2O3/SrFe12O19 compared to the pure Bi2O3 was possibly partly attributed to magnetic field effect originating from the magnetic photocatalyst itself. The remanent magnetization (Mr) of composite magnetic photocatalyst Bi2O3/SrFe12O19 (35 wt %) was 15.20 A·m2·kg−1, which manifested the Bi2O3/SrFe12O19 (35 wt %) could generate a stable magnetic field. It was well-known that charges in stable magnetic field would travel in uniform motion along the direction of magnetic field or move in uniform circular motion around the magnetic field direction.28,29 A part of photoproduced electrons moved from VB to CB along magnetic field direction, which kept the original motion direction of photoinduced electron, like in pure Bi2O3 (Figure 14). Other part of photoinduced electrons traveling in uniform circular motion spiralled upward. The magnetic field was conducive to the motion of photoinduced electrons from VB to CB with two different directions, which was tantamount to the increase in motion velocity of photoinduced electrons. Thus, more photoinduced electrons and holes would be produced and accumulated at the conduction band and valence band, respectively. In other words, the magnetic field could enhance charge collection efficiency. Meanwhile, the recombination between photoinduced charges and holes could be inhibited through this magnetic field. Thus, the separation efficiency of electrons and holes was improved, which subsequently enhanced photocatalytic activity. (3) Owing to the color of SrFe12O19 itself (black), SrFe12O19 could absorb some parts of the solar spectrum.11 Moreover, the Eg of SrFe12O19 was 1.86 eV, indicating that it could absorb light in ≤670 nm wavelength ranges, namely, almost all the visible light ranges. In the composite photocatalyst Bi2O3/ SrFe12O19 system, Bi2O3 was a main photocatalyst, while SrFe12O19 acted as a sensitizer absorbing visible light. SrFe12O19

could help Bi2O3 increase its photoresponse of visible light and absorb more photons and then produce more photoexcited electron−hole pairs.30 Thus, the composite photocatalyst Bi2O3/SrFe12O19 could receive more incident photons and produce more photoexcited electrons than pure Bi2O3 under identical light irradiation. So the electrons and holes were free to initiate reactions with the reactants adsorbed on the photocatalyst surface, leading to the enhanced photocatalytic activity.

4. CONCLUSIONS The magnetic composite photocatalyst Bi2O3/SrFe12O19 was synthesized by hydrolysis with medium temperature sintering method. The synthesis method facilitated mass production as a result of its simple and low-cost procedure. This work was expected to provide a simple preparation method for various functional materials. In addition, this novel magnetic composite might find important applications in relevant fields. XRD study revealed that the introduction of SrFe12O19 did not change the favorite growth direction of Bi2O3, [121] orientation. Micromorphology investiagtion indicated that SrFe12O19 was distributed on the surface of Bi2O3 to form some heterojunction structures. VSM measurements manifested the composite possessed better magnetic properties, which was conducive to its separation, recycling, and reuse. Three main reasons for the increase in the photocatalytic efficiency of Bi2O3/SrFe12O19 were (1) the formation of p−ntype heterojunction between p-type Bi2O3 and n-type SrFe12O19 semiconductors, (2) magnetic field effect stemming from magnetic composite itself generated a shunt effect for photoexcited electrons, and (3) as a sensitizer absorbing visible light, SrFe12O19 could help Bi2O3 absorb more incident photons. The common aim for the three factors was that more photoproduced electron−hole pairs were generated and less recombination for electron−hole pairs could come about prior to participation in oxidation−reduction reaction. Thus, more effective degradation reaction would take place to enhance photocatalytic efficiency.

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AUTHOR INFORMATION

Corresponding Authors

*(T.X.) Tel/Fax: +86 2386809361. E-mail: deartaiping@163. com. *(C.L.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We would like to thank our associates, especially Ting Liao and Wenli Wu, for their valuable contributions to our research program. We want to thank the financial support from the National Science Foundation of China (NSFC, 51374259) and Scientific Research Foundation of State Key Laboratory of Coal Mine Disaster Dynamics and Control (2011DA105287KF201310). We gratefully acknowledge many important contributions from scientists of all reports cited in our article.

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