Visible-Light-Driven Photocatalytic Activity of Magnetic BiOBr/SrFe₁₂O₁₉ Nanosheets

Abstract

Magnetic BiOBr/SrFe₁₂O₁₉ nanosheets were successfully synthesized using the hydrothermal method. The as-prepared samples were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), and UV-visible diffused reflectance spectra (UV-DRS), and the magnetic properties were tested using a vibration sample magnetometer (VSM). The as-produced composite with an irregular flaky-shaped aggregate possesses a good anti-demagnetization ability (Hc = 861.04 G) and a high photocatalytic efficiency. Under visible light (λ > 420 nm) and UV light-emitting diode (LED) irradiation, the photodegradation rates of Rhodamine B (RhB) using BiOBr/SrFe₁₂O₁₉ (5 wt %) (BOB/SFO-5) after 30 min of reaction were 97% and 98%, respectively, which were higher than that using BiOBr (87%). The degradation rate of RhB using the recovered BiOBr/5 wt % SrFe₁₂O₁₉ (marked as BOB/SFO-5) was still more than 85% in the fifth cycle, indicating the high stability of the composite catalyst. Meanwhile, after five cycles, the magnetic properties were still as stable as before. The radical-capture experiments proved that superoxide radicals and holes were main active species in the photocatalytic degradation of RhB.

1. Introduction

Throughout the last few years, semiconductor photocatalysts, which can utilize the clean, renewable, and most accessible solar energy, have attracted more and more attention in the material science field [1,2]. The photocatalytic degradation of organic pollutants under sunlight irradiation possesses promising applications in water pollutant control. Composite photocatalytic materials are receiving considerable attraction owing to their excellent photocatalytic activity and stability [3,4,5,6,7,8].

Bismuth oxyhalide (BiOX; X = Cl, Br, and I) has been widely studied because of its narrow band gap (E_g = 1.7–3.4 eV) [9,10,11]. In particular, BiOBr, with an intrinsic lamellar structure and outstanding photocatalytic property, is one of the most promising photocatalysts. However, the E_g of BiOBr is 2.8 eV, leading to a low absorption ability of visible light and a high recombination rate of photo-induced electron-hole pairs [12,13]. There are reports on exploring strategies for the improvement of visible light absorption and enhancement of the charge separation efficiency of BiOBr [14,15,16,17]. However, the problem of a low recycling rate after the photocatalytic reaction still exists. The recovered method using an external magnet is a quick and low-cost approach [18], if the catalyst materials hold magnetization.

The development research of magnetic photocatalysts (e.g., Mn_xZn_1-xFe₂O₄/βBi₂O₃, Fe₃O₄/BiOCl, BiOBr/ZnFe₂O₄, BiOBr/CoFe₂O₄, and Ag/AgCl/CoFe₂O₄) [19,20,21,22,23,24] is interesting, because of its easier recyclable and repeatable use. It is worth noting that hard-magnetic strontium ferrite (SrFe₁₂O₁₉) has a large saturation magnetization, superior coercivity, good chemical stability, and corrosion resistivity [25]. It was reported that SrFe₁₂O₁₉ inhibited facet growth [1] and the selective exposure of bismuth-based semiconductor photocatalyst [26]. Consequently, visible light absorption was enhanced by compositing SrFe₁₂O₁₉, involving the low band gap energy and resulting in quick charge separation and increased photocatalytic ability.

The photocatalytic degradation of organic contaminants under sunlight is a bright application prospect in pollution control engineering. However, sunlight is a hot light and easily causes a series of side reactions in the process of the photocatalysis reaction. In our previous studies [19,25], magnetic photocatalysts were prepared by a dip-calcination method. It is noted that light emitting diodes (LEDs) with little heat and good linearity are superior to conventional light sources or to sunlight. Thus, LEDs have been widely used in hydrogen production and organic synthesis over semiconductor photocatalysts [27,28,29,30]. The outstanding advantages of LED light are their long-life time and high controllability in the reaction progress.

In this work, magnetic nanosheet BiOBr/SrFe₁₂O₁₉ (BOB/SFO-5) was prepared using a reasonable fabrication method, and the corresponding photocatalytic property was investigated under visible light and UV-LED (390–410 nm). Further insight into the magnetization and repeatable rate, and the photocatalytic mechanism were researched with different kinds of tests. The photocatalytic mechanism assisted by a magnetic field has been further discussed in this research. These results provide important concepts concerning the synthesis method, and pave the way for the industrial application of magnetic photocatalysts.

2. Experimental Section

All reagents were of analytical grade and were used directly, without further purification. The water was deionized throughout all the experiments.

2.1. Preparation of BiOBr/SrFe₁₂O₁₉

Solution A consisted of 5 mmol of Bi(NO₃)₃·5H₂O and 762 mg of SrFe₁₂O₁₉ (prepared by a hydrothermal method [31]) dissolved in 20 mL of deionized water. Afterwards, 15 mL of NaBr (5 mmol) was slowly dipped into solution A, and the formed suspension was transferred into a 50 mL Teflon-lined stainless-steel autoclave and heated to 160 °C for 12 h. Subsequently, the solid was separated and washed several times with deionized water and absolute alcohol, alternately. Then, the washed solid was dried at 60 °C for 12 h, and the BiOBr/5 wt % SrFe₁₂O₁₉ (marked as BOB/SFO-5) was obtained. A series of BiOBr-based composite products, including 0 wt %, 3 wt %, 7 wt %, 10 wt %, and 15 wt % SrFe₁₂O₁₉, were prepared using the same process, and the gained composites were noted as BOB, BOB/SFO-3, BOB/SFO-7, BOB/SFO-10, and BOB/SFO-15, respectively.

2.2. Material Characterization

Phase identification via X-ray diffraction (XRD) was conducted on an X-ray diffractometer (Bruker Advance D8, Bruker, Germany). The average crystallite sizes of the samples were calculated from the XRD peak (102) using the classical Scherrer equation, D_hkl = Kλ/βcosθ, where D_hkl is the particle diameter (nm), K is the constant (0.943), λ is the X-ray wavelength (0.15405 nm), β is the half-maximum line width, and θ is the diffraction angle. The microstructural morphology was observed by scanning electron microscopy (SEM; EVO-LS15X, ZEISS, Oberkochen, Germany) and high-resolution transmission electron microscopy (HR-TEM). Energy dispersive analysis (EDS) systems were used to verify the element constituents. A vibrating sample magnetometer (VSM; Lakeshore 7410, Lake Shore, Carson, USA) was employed to determine the magnetization. The UV-VIS diffuse reflectance spectra (DRS) were measured using a UV-VIS spectrophotometer (TU1901, Beijing Purkinje General Instrument Co. ltd, Beijing, China).

2.3. Determination of Photocatalytic Property

The photocatalytic performance of the BOB/SFOs was evaluated using the Rhodamine B (RhB) photodegradation under visible light (λ > 420 nm) and UV light-emitting diode (LED) irradiation. The visible light was obtained using a 300 W Xenon lamp with a UV cut-off filter (CEL-HXF300, AULTT, Beijing, China), and the UV light was acquired by the LED under cool light (λ = 390–410 nm; power = 122 μW). In addition, the distance between the light source and the reactor was 10 cm. A 100 mL RhB solution (10 mg/L) and 50 mg catalyst were added into a quartz container and stirred for 30 min in the dark, so that the adsorption equilibrium was reached. The irradiation time was set as 30 min in the process of continuous stirring. Then, 3 mL of the RhB solution was sampled in the setting time interval. The photocatalytic degradation process of the RhB was monitored by measuring the characteristic absorption at 554 nm with a UV-VIS spectrophotometer. Composite catalysts were separated and recovered with an external magnet.

The photocatalytic mechanism was probed using active species trapping experiments. The degradation rates of the RhB over the photocatalyst were measured in active species scavenger agents, including the hydroxyl radical scavenger of isopropanol (IPA), the hole scavenger of disodium ethylenediaminetetra acetic acid (Na₂-EDTA), and the superoxide radical scavenger of 1,4-benzoquinone (BZQ), respectively.

3. Results and Discussion

A primary analysis of the photodegradation revealed that BOB/SFO-5 was the most efficient in the RhB degradation under UV irradiation.

3.1. Microstructure and Pore Characteristics

Figure 1 shows the XRD patterns of SrFe₁₂O₁₉, BiOBr, and BOB/SFO-5. As we can see from Figure 1a, the pattern of SrFe₁₂O₁₉ revealed a hexagonal primitive crystal structure (JCPDS card no. 33-1340) [32]. This was a member of the space group, P₆₃/mmc, and the lattice parameters were a = b = 5.8868 Ǻ and c = 23.037 Å. The main diffraction peaks of BiOBr (Figure 1c) were located at 2θ = 10.975°, 21.979°, 25.390°, 31.851°, 32.432°, and 57.403°, which matched with the (001), (002), (101), (102), (110), and (212) crystal planes, respectively (JCPDS card no. 85-0862; space group P₄/nmm; a = b = 3.92 Å; c = 8.11 Å). For BOB/SFO-5 (Figure 1b), the diffraction peak of BiOBr was relatively strong and that of SrFe₁₂O₁₉ was relatively weak, and the (002), (110), (111), and (114) peaks of BiOBr overlapped with the (103), (107), (114), and (304) diffraction peaks of SrFe₁₂O₁₉, respectively. Thus, the characteristic peaks of SrFe₁₂O₁₉ in the pattern of BOB/SFO-5 were not distinct.

It is worthwhile mentioning that the amount of magnetic matrix (5 wt %) was relatively low, and no impurity phase was found in BiOBr/SrFe₁₂O₁₉, confirming that there is no appreciable decomposition and perceptible chemical reaction between BiOBr and SrFe₁₂O₁₉.

The average crystallite sizes of BiOBr and BiOBr/SrFe₁₂O₁₉ calculated by the Scherrer equation were 24.26 nm and 19.77 nm, respectively. The small size of BiOBr/SrFe₁₂O₁₉ was possibly due to the growth inhibition of BiOBr by SrFe₁₂O₁₉ [27].

The morphological characteristics (Figure 2) of SrFe₁₂O₁₉, BiOBr, and BOB/SFO-5 were investigated using SEM. As shown in Figure 2a, the SrFe₁₂O₁₉ is a hexagonal-shape, which is consistent with the literature [26]. Figure 2b shows that the BiOBr nanosheets assembled as “petals” and formed a flower-like microstructure, which could provide more active sites for the adsorption of organic pollutants and help to enhance the photocatalytic activity for the pollutants’ decomposition [33]. As for BOB/SFO-5 (Figure 2c), SrFe₁₂O₁₉ nanosheets were inserted into a BiOBr flower-like shape, which indicated that the irregularly flaky-shape aggregated on the surface of BiOBr. The intimate interaction between BiOBr and SrFe₁₂O₁₉ might contribute to the improvement of the electron transfer and separation capacity in the photocatalysis process. Furthermore, the EDS spectra of BOB/SFO-5 is shown in Figure 2d, confirming the existence of the Sr, Fe, O, Br, and Bi elements.

The TEM image of BOB/SFO-5 is given in Figure 3a, where the nanosheet-shaped structure is further proven. Figure 3b shows the HR-TEM image, which demonstrates that the nanosheet was well crystalized, and the interplanar of the (002) plane of the monoclinic BiOBr is 0.247 nm. Furthermore, BiOBr is a p-type semiconductor material and SrFe₁₂O₁₉ is an n-type semiconductor material, leading to a p–n heterojunction structure of BOB/SFO. In this way, the magnetic composite might hold a high photogenerated charge separation ability.

In addition, the adsorption–desorption isotherms and the pore size distribution curves for BOB/SOF-5 were tested using an ASAP 2010 instrument (micromeritics with a surface area deviation of 1%) (ASAP-2010, Micromeritics, Norcross, GA, USA). The results are shown in Figure S1. The most probable pore radius was 7.7 nm, which indicates that the composite belongs to a mesopore material that could provide more active sites for photocatalytic experiments.

3.2. Optical Properties

The optical absorption property plays a critical role in the photocatalytic performance. The UV-VIS DRS spectra of the BiOBr and BOB/SFO-5 composite are shown in Figure 4. There is an obvious difference in the light absorption abilities of the two samples. It is clear that both the pure BiOBr and the composite present a strong absorption in the UV light region (wavelength of 200–400 nm). However, it is worth noting that the composite showed a strong absorption in a wide wavelength range from UV to visible light, compared with that of pure BiOBr. Therefore, the visible light absorption ability of BiOBr was enhanced for the BOB/SFO-5 composite, and it can be adopted as a visible light-driven catalyst. According to the formula, $\mathsf{\alpha }\mathrm{h}\mathsf{\upsilon }=\mathrm{A}(\mathrm{h}\mathsf{\upsilon }-{\mathrm{E}}_{\mathrm{g}}){}^{n/2}$, the band gap energy (E_g) of BOB, SFO, and BOB/SFO-5 were estimated from the ($\mathsf{\alpha }$hυ)^n/2 versus photo energy (hυ). Generally speaking, the E_g values can be obtained from the intercept of the tangent to the absorption curves. The estimated E_g values of BOB, SFO, and BOB/SFO-5 are 2.80 eV, 1.86 eV, and 2.67 eV, respectively. In addition, the estimated E_g values are listed in Table S1. Obviously, the decrease in the E_g value is due to the introduction of SrFe₁₂O₁₉. The results show that a new defective energy level formed between the band gap of BiOBr and SrFe₁₂O₁₉.

3.3. Magnetic Property

The magnetic hysteresis loops of BOB/SFO-5 and SrFe₁₂O₁₉ are depicted in Figure 5, revealing the typical feature of hard-magnetic materials [34]. Table S2 lists the magnetic parameters of the samples. The saturation magnetization (Ms) of BOB/SFO-5 was 11.3% c.a. lower than that of SrFe₁₂O₁₉, and the remnant magnetization (Mr) was significantly cut down because of the decreased amount of magnetic component (SrFe₁₂O₁₉) per gram. Furthermore, it was confirmed that the BiOBr/SrFe₁₂O₁₉ was successfully synthesized without any negative changes to the magnetization. The results show that the BiOBr/SrFe₁₂O₁₉ microspheres displayed a good magnetic performance and were easily separated and recovered after the photocatalytic reaction (Figure 4 inset).

3.4. Photocatalytic Property

The photocatalytic performances of the as-prepared photocatalysts were investigated with RhB photodegradation. Figure S2 shows the absorption curves of RhB in BiOBr/SrFe₁₂O₁₉ at different times under visible light (λ > 420 nm) irradiation. The key peak intensity (λ_max = 554 nm) of RhB declined gradually and reached zero after 30 min of the reaction, indicating the complete photodegradation of RhB using BOB/SFO-5 after only 30 min. Thus, the optimization reaction time was set at 30 min. For more details, a series of tests are shown in Figure 6 that were employed to test the effect of the matrix amount on the degradation rate. In view of the low degradation rate in the blank test, it was necessary to boost the degradation rate with suitable catalysts. Further insights demonstrated that the photocatalytic activity of BOB/SFO-5 was indeed higher than that of pure BiOBr. Under visible light (λ > 420 nm) irradiation, the photodegradation rate of RhB using BOB/SFO-5 after 30 min of reaction could reach 97%, which was higher than that using BiOBr (86%). Compared with the results in the literature [26], the photodegradation rate was obviously enhanced because of a UV cut-off filter that was equipped to ensure a single visible light without UV light from the Xe light resource. When the Xe lamp was replaced with a UV LED (cool light), the degradation rate of the RhB was 98% and 87%, respectively, under the same condition. The results above revealed that BOB/SFO-5 was extremely efficient in RhB degradation under visible light and UV LED irradiation. It can be deduced that there was a strong interaction between the BiOBr nanoparticles and SrFe₁₂O₁₉ in the hydrothermal reaction, which played a crucial role in the transfer and separation of photogenerated carriers. Particularly, SrFe₁₂O₁₉ provided a high mobility of the charge carrier through the longitudinal direction, owing to the intimate interaction between BiOBr and SrFe₁₂O₁₉. Thus, the photogenerated electrons can easily be transferred to the BiOBr nanoflakes’ moiety, leading to the efficient separation and slow recombination of electron-hole pairs. Therefore, BOB/SFO-5 possesses an excellent visible-light-driven catalytic property and can be utilized in subsequent experiments.

In fact, we investigated some of the references reported in the past 10 years in order to compare them with different photocatalysts under visible light irradiation in terms of the RhB photodegradation ratio. The results are listed in Table S3. It was worth mentioning that the photocatalytic activity of BOB/SFO-5 for RhB photodegradation under UV LED irradiation was outstanding. To the best of our knowledge, the photocatalytic rate reached 97% after only 30 min of photocatalytic reaction under UV LED irradiation, the efficiency of which was superior to that of the existing literature reports.

3.5. Stability and Recycle Property

The photocatalytic stability of BiOBr/SrFe₁₂O₁₉ was confirmed by the recycling tests. The irradiation time of the four subsequent times cycles was set as 60 so that the degradation rate change was clearly detected. The results are shown in Figure 7. The degradation rate of the RhB in the recovered catalyst was above 85% in the fifth cycle. After five cycles, the BOB/SFO-5 was separated and recovered with an external magnet, and the XRD patterns of the samples were collected, as shown in Figure 8. The same intrinsic crystal structure of the BOB/SFO-5 and recovered BOB/SFO-5 was further confirmed. In addition, the magnetic property of the recovered BOB/SFO-5 was examined and is shown in Figure 9. The Ms, Mr, and Hc of the recovered sample were 4.38 emu/g, 1.18 emu/g, and 861.04 G, respectively. Compared with the parameters of the original BOB/SFO-5, there was almost no decrease of the Ms and coercivity, while the Mr appeared to have little increase. The results indicated that BOB/SFO-5 possessed rather stable magnetic properties. The above achievements illustrated that BOB/SFO-5 exhibited a good stability and high repeatable ability, which overcame the recycling difficulty in the photocatalytic application.

3.6. Photocatalytic Mechanism

It was important to detect the main active species in the photocatalytic process in order to know how to improve the photocatalytic property. Figure 10 shows the RhB degradation rates over BOB/SFO-5 under different active radical species scavengers. It can be seen from Figure 10 that the introduction of the superoxide radical (O₂⁻) scavenger, BZQ, caused degradation rate declination, namely, the photocatalytic rate was directly proportional to the amount of O₂⁻, proving the dominant role of O₂⁻ in the photocatalytic process, which was identified by the EPR spectra for DMPO, and O₂⁻ acted as the most active species [33,34]. The RhB degradation rates in the hole (h⁺) scavenger, Na₂-EDTA, were slightly larger than those in BZQ. It can be deduced that the effect of the photo-generated holes was lower than that of O₂⁻, although the hole was also one of the main oxidation species. Furthermore, the degradation rate of the hydroxyl radical (·OH) scavenger, IPA, was similar to that of BZQ, indicating the smaller difference for the amount change of ·OH. The results above manifest that the photocatalytic reaction of BiOBr/SrFe₁₂O₁₉ was mainly affected by the O₂⁻ radicals though the presence of holes. Figure 11 shows the photocatalytic mechanism scheme of BiOBr/SrFe₁₂O₁₉ under visible light or LED irradiation.

It is known that BiOBr and SrFe₁₂O₁₉ are the p-type and n-type semiconductors, respectively. The intimate contact effect of BiOBr/SrFe₁₂O₁₉ is analogous to a p–n heterojunction structure in the photocatalytic performance, which was absent in the single BiOBr. In addition, the conduction band (CB) and valence band (VB) potentials of the p-type semiconductor, BiOBr, were 0.30 and 3.10 eV [35,36], respectively. Meanwhile, the CB and VB of n-type SrFe₁₂O₁₉ were 0.20 and 2.06 eV, respectively [37,38]. Thus, the photogenerated electrons in SrFe₁₂O₁₉ easily transferred to BiOBr, because of the lower CB potential position of BiOBr than that of SrFe₁₂O₁₉, and the VB holes in BiOBr spontaneously moved to SrFe₁₂O₁₉ (shown in Equation (1)). The transfer of the electron and hole effectively reduced the charge recombination rate, and resulted in a superior photocatalytic performance. Moreover, the photogenerated electrons inhibited the formation of·HO₂ (shown as Equations (2) and (3)). As a result, O₂⁻ directly caused RhB molecule oxidation in the simultaneous reaction of holes in Equation (4).

$$\mathrm{BiOBr}/{\mathrm{SrFe}}_{12}{\mathrm{O}}_{19}+\mathrm{h}\mathsf{\nu }(\ge \mathrm{Eg})\to \mathrm{BiOBr}({\mathrm{e}}^{-})/{\mathrm{SrFe}}_{12}{\mathrm{O}}_{19}({\mathrm{h}}^{+}),$$

(1)

$$\mathrm{BiOBr}({\mathrm{e}}^{-})+{\mathrm{O}}_2\to \mathrm{BiOBr}+{\mathrm{O}}_2{\cdot }^{-},$$

(2)

$${\mathrm{O}}_2{\cdot }^{-}+{\mathrm{H}}_2\mathrm{O}\to \cdot {\mathrm{HO}}_2+{\mathrm{OH}}^{-},$$

(3)

$${\mathrm{SrFe}}_{12}{\mathrm{O}}_{19}({\mathrm{h}}^{+}),{\mathrm{O}}_2{\cdot }^{-}+\mathrm{RhB}\to \mathrm{deradation} \mathrm{product}.$$

(4)

According to the previous study using GC-MS and FT-IR measurements to discuss the mechanism of RhB oxidation [39,40,41], according to the processing steps in the photocatalytic reaction, the RhB was changed from the primary complex macromolecular organic matter to several small organic matters, like phthalic acid [39]. Finally, all of the matter would be degraded into CO₂ and H₂O, achieving complete mineralization [40,41]. So, the phenomenon where the blue shift (Figure S2) of the absorption peaks in the ultraviolet-visible absorption spectra appeared in the photodegradation process was easier to understand.

4. Conclusions

(1)

Nanosheet and irregular flaky-shaped BOB/SrFe₁₂O₁₉ was successfully prepared by a facile hydrothermal method. The as-prepared composite presented good magnetic properties (Hc = 861.04 G). The recycling experiments proved that the degradation rate of the RhB of the recovered photocatalyst still maintained at 85% in the fifth cycle. The magnetic photocatalyst was conducive to separation and reuse using an external magnet, so that the recycling problem in the industrial application was easily solved.

(2)

The excellent visible-light-driven catalytic activity of BOB/SFO-5 was investigated. An Xe lamp (λ > 420 nm) equipped with a UV cut-off filter used as the light resource provided the visible light without UV light. An LED was employed as a cool light resource. The photodegradation rate of the RhB over the BOB/SFO-5 was 97% and 98% at 30 min, which was higher than that (86% and 87%, respectively) over BiOBr.

(3)

The superoxide radicals and holes were demonstrated to illustrate the main active species in the photocatalytic reaction of BiOBr/SrFe₁₂O₁₉. The stable magnetic property of the composite photocatalyst prompted visible light absorption and utilization, producing photogenerated e⁻ and h⁺. SrFe₁₂O₁₉ further strengthened the light response and enhanced the photocatalytic property of BiOBr by means of absorbing a great number of photons.