Abstract
Magnetic Fe3O4@SiO2/Bi2SiO5 composites with a novel hierarchical nanostructure were synthesized by sol-gel and hydrothermal methods and were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and UV-visible diffuse reflectance spectroscopy (UV-vis DRS). It was found that the introduction of Fe3O4@SiO2 could turn the morphology of Bi2SiO5 from close-grained slab to hollow hierarchical architecture with fabric-structure. The Fe3O4@SiO2/Bi2SiO5 composite showed enhanced photodegradation efficiency for the degradation of reactive brilliant red dye (X-3B) in aqueous solution under simulated sunlight irradiation, as compared with that of commercial P25. In addition, the Fe3O4@SiO2/Bi2SiO5 composite exhibited good magnetic recoverability and excellent photocatalytic stability (no obvious activity loss after recycling tests).
1. Introduction
Photocatalysis has been extensively used for degradation of numerous organic pollutants [1, 2]. In photocatalytic process, the photocatalytic materials play a crucial role in realizing its practical applications. Recently, bismuth-based photocatalysts have been widely reported owing to their excellent photocatalytic performance, such as Bi2GeO5 [3], Bi2MoO6 [4, 5], Bi2WO6 [6], and Bi2SiO5 [7]. Bi2SiO5, one of the family, consists of an interaction of two (Bi2O2)2+ layers and (SiO3)2− pyroxene layers inserted between (Bi2O2)2+ layers. The (Bi2O2)2+ layers are made up of slightly distorted squared oxygen planes. These squares are capped alternatively above and below by the bismuth atoms. The distorted feature of SiO4 tetrahedra could be a benefit for splitting photogenerated electrons and holes. Thus, good photocatalytic activity of the Bi2SiO5 should be expected [8, 9].
Nevertheless, photocatalysts are normally used as suspension in the photocatalytic process. Some processes for separation of suspended catalyst are necessary, such as centrifugation and filtration. And, some loss of catalyst during these separation processes is the major drawback. To overcome this problem, immobilization of photocatalysts on various easily recoverable materials including glass beads [10], glass fibers [11], and ceramic plates [12] has been studied. However, these methods resulted in significant decrease of photocatalytic efficiency because of the decreased surface area of catalyst coated on the support. Thus, the effective removal of nanosized catalyst powders from the treated water suspension is a challenge for recovery of catalyst.
It is well known that magnetic materials could be easily recovered by applying a magnetic field [13, 14]. Thus, if the photocatalyst contains a magnetic material, recovery of the photocatalyst from an aqueous system by applying external magnetic field is attractive for commercial application. In general, the magnetic photocatalysts were composed mainly of catalyst coating, inertial layer (SiO2 or Al2O3), and the magnetic core material (Fe3O4) [15, 16]. The inertial layer (SiO2 or Al2O3) plays a key role to avoid photodissolution phenomenon of magnetic core material, which is an electronic interaction between catalyst coating and the magnetic core.
Herein, magnetic Fe3O4@SiO2/Bi2SiO5 composites with hierarchical nanostructure were fabricated for the improvement of the photocatalytic performance and easy separation. X-3B, a common pollutant in the industry wastewater, was selected as a test substance to evaluate the photocatalytic performance of prepared photocatalysts. Noteworthy, the as-prepared composites not only exhibited excellent photocatalytic activity for the degradation of X-3B but also are easily recycled via an external magnetic field.
2. Experimental Section
2.1. Reagents and Materials
The Bi(NO3)3·5H2O, FeCl3·6H2O, ethylene glycol, NH3·H2O, tetraethyl orthosilicate (TEOS), reactive brilliant Red X-3B dye (X-3B), and Na2SiO3·9H2O were purchased from Tianjin Chemical Reagents Company. All these reagents were of AR grades and were used without further purification.
2.2. Preparation of Samples
2.2.1. Preparation of Fe3O4
The typical synthesis procedure is as follows: 0.1 M FeCl3·6H2O, 0.8 M sodium acetate, and 0.09 M sodium citrate were dispersed in 60 mL ethylene glycol with magnetic stirrer for 1 h at room temperature. The as-obtained mixture was transferred into a Teflon-lined stainless-steel autoclave with a capacity of 100 mL and then heated at 200°C for 12 h. The final product was collected with a magnet and washed with deionized water and anhydrous ethanol for several times and then dried at 60°C for 3 h in air.
2.2.2. Preparation of Fe3O4@SiO2
Firstly, 0.75 g Fe3O4 MNPs was redispersed into 170 mL ethanol. The mixture was homogenized by ultrasonication for 20 min after adding 1 mL ammonium hydroxide (NH3·H2O). After that, as-obtained mixture was vigorously stirred with a mechanical agitator at 30°C for 30 min; then, 1.0 mL tetraethyl orthosilicate (TEOS) was introduced dropwise into the solution. The final product was separated by external magnetic field.
2.2.3. Praparation of Fe3O4@SiO2/Bi2SiO5
In a typical process [7], a desired amount of Fe3O4@SiO2 MNPs (0, 0.05, 0.1, 0.2, and 0.3 g) was dispersed in 75 mL deionized water under ultrasonication for 20 min. Then, 0.05 M Bi(NO3)3·5H2O and 0.025 M Na2SiO3·9H2O were added into the above suspension. The pH value of solution was adjusted to 9 by adding NH3·H2O. After ultrasonication for 30 min, the mixture was transferred into a Teflon-lined stainless-steel autoclave and then heated at 180°C for 48 h. Finally, the autoclave was cooled down to room temperature naturally. The products were collected and washed with deionized water and anhydrous ethanol several times and then dried at 80°C for 6 h. The sample is labeled as BSO-0, BSO-0.05, BSO-0.1, BSO-0.2, and BSO-0.3. Furthermore, pure Bi2SiO5 for reference was also prepared using the same method without Fe3O4@SiO2 nanoparticle.
2.3. Characterizations
The morphology of the samples was displayed using scanning electron microscopy (SEM) on a JSM 6460LV instrument (JEOL Ltd.) operated at 20 kV and equipped with an energy-dispersive X-ray analyzer (Phoenix Ltd.). The X-ray diffraction (XRD) analysis of as-obtained samples was performed on a Shimadzu XRD-6100 diffractometer (Shimadzu Ltd.) at 40 kV and 40 mA with Cu Ka radiation. Fourier transform infrared spectroscopy (FTIR) of product was recorded on a Shimadzu IRAffinity-1 (Shimadzu Ltd.) with a resolution of 4 cm−1. X-ray photoelectron spectroscopy (XPS) measurement was done with a VGESCALAB 250 spectrometer (ThermoFisher Scientific) equipped with a monochromated Al-Ka radiation source (1486.6 eV). UV-vis diffuse reflectance spectra were recorded on a CARY-100 spectrometer (VARIAN Ltd.) and BaSO4 was used as a reflectance standard.
2.4. Photocatalytic Activity Measurements
The photocatalytic activity of the prepared catalysts was estimated by measuring the degradation rate of X-3B (40 mg/L) in an aqueous solution under sunlight. 0.1 g of photocatalyst was added in quartz reactor containing 100 mL dye aqueous solution with air stirring. The 400 W xenon lamp was used to get simulated sunlight. In the process of degradation, a certain volume of suspension was sampled under interval of 20 min and then centrifuged immediately to remove the particles. The absorbance of solution was measured by a UV-1800PC spectrophotometer (MAPADA, China) at 538 nm, and the degradation rate () of X-3B was calculated by the following expression:in which was the absorbance of initial solution of X-3B and was the absorbance of the solution of X-3B.
3. Results and Discussion
The SEM images of Fe3O4@SiO2 and Fe3O4@SiO2/Bi2SiO5 composites are shown in Figure 1. As shown in Figure 1(a), the Fe3O4@SiO2 particles are spherical with uniform size, and the average particle size is about 500 nm. From Figure 1(b), it can be seen that pure Bi2SiO5 sample exhibits close-grained slab microstructure. Nevertheless, introducing Fe3O4@SiO2 changed obviously the morphology and microstructure of Bi2SiO5 (seeing Figures 1(c)–1(f)). The introduction of Fe3O4@SiO2 turned the morphology of Bi2SiO5 from close-grained slab to hollow hierarchical architecture with fabric structure. Further SEM observation reveals that the hollow hierarchical architecture is constructed by exclusive knitting of a large quantity of irregular nanosheets. In addition, it is also clearly observed that more and more spherical Fe3O4@SiO2 nanoparticles are embedded on the surface of hollow hierarchical Bi2SiO5 architecture with increase of Fe3O4@SiO2 loadings. The hierarchical Bi2SiO5 architectures possess many open pores derived from the self-arrangement of nanosheets, which could promote multiple scattering of the incident light and lead to an enhanced light-harvesting capacity. It is believed that the hollow hierarchical microstructures are ideal materials for potential applications in photocatalytic process.
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Phase structures of the as-prepared samples were examined by powder XRD analysis and are shown in Figure 2. For Figure 1(a), all diffraction peaks can be indexed to the Fe3O4 phase (JCPDS number 15-7609). Figure 2(b–f) depicts XRD patterns of Fe3O4@SiO2/Bi2SiO5 composites with different quantities of Fe3O4@SiO2. As shown in Figure 2(b–f), all the diffraction peaks could be perfectly indexed to the tetragonal phase of Bi2SiO5 (JCPDS 36-0288). No diffraction peak of Fe3O4 was detected, which may be attributed to incorporation of the magnetic cores in Bi2SiO5 microstructures.
The composition and structure of the products were also measured by infrared (IR) spectra. As shown in Figure 3, the typical Fe-O stretching bands at 584 cm−1 can be found in all samples, indicating existence of magnetic core in composites. The Fe3O4@SiO2 sample showed the bending, symmetric and asymmetric stretching vibration of Si-O-Si at 482 cm−1, 790 cm−1, and 1083 cm−1, respectively. Furthermore, the bands attributed to vibration of Si-O around 1224 cm−1 [17] can also be observed in Fe3O4@SiO2 sample. This result indicates that Fe3O4 microspheres were wrapped by SiO2. Moreover, the characteristic bands of Bi2SiO5 can be observed from IR spectrum of Fe3O4@SiO2/Bi2SiO5 composite, such as the Bi-O-Si vibration at 894 cm−1 and 1020 cm−1 originated from (SiO5)6−. Nevertheless, Fe3O4@SiO2/Bi2SiO5 composite still possesses abundant surface O-H bond located at 1637 cm−1, which could enhance the photocatalytic activity owing to the -OH offering larger capacity for oxygen adsorption [18].
The optical properties of Fe3O4@SiO2/Bi2SiO5 hierarchical nanostructure were also investigated by UV-vis DRS. As a comparison, the spectra of Bi2SiO5 and TiO2 were also measured. As shown in Figure 4, the intense absorption of Bi2SiO5 in the UV light regions was similar to that of TiO2, which indicated that the Bi2SiO5 could be used as an effective photocatalyst to degrade various pollutants [7, 19]. It is worth noting that Fe3O4@SiO2/Bi2SiO5 composites showed the stronger absorption in visible light region, as compared with that of sample Bi2SiO5. With the increase of the mass fraction of Fe3O4@SiO2, the absorption intensity of visible light region becomes stronger, which can be attributed to introducing of magnetic cores. Obviously, the strong absorption in visible light region is favorable to enhance utilization efficiency of sunlight.
3.1. Photocatalytic Activity Analysis
The photocatalytic performance of the as-prepared samples was measured under irradiation of simulated solar light. It can be seen from Figure 5 that Bi2SiO5 and Fe3O4@SiO2/Bi2SiO5 composites exhibit the better photocatalytic activity under irradiation of simulated solar light, as compared with P25. Nevertheless, the activity of as-prepared Fe3O4@SiO2/Bi2SiO5 composites did not monotonously increase with increase of Fe3O4@SiO2 content. The BSO-0.2 sample showed best photocatalytic activity, and the photodegradation rate can reach 65% after irradiation for 120 min. The highly enhanced photocatalytic activity of Fe3O4@SiO2/Bi2SiO5 composites under simulated solar light irradiation can be attributed to (1) the introduction of Fe3O4@SiO2 which widened the range of spectral response and (2) the hollow hierarchical structure which could promote multiple scattering of the incident light and lead to an enhanced light-harvesting capacity.
The stability and magnetic recoverability of Fe3O4@SiO2/Bi2SiO5 composite were also investigated. The magnetic recovering test was performed as illustrated in Figure 6. As shown in Figure 6(a), the Fe3O4@SiO2/Bi2SiO5 composite dispersed in an aqueous solution was easily recovered by external magnetic field. This result indicated that Fe3O4@SiO2/Bi2SiO5 composites are magnetically recoverable. Furthermore, the recycled photodegradation experiments were performed using BSO-0.2 owing to its high activity. The sample was recollected by external magnetic field after each cycle. It can be seen from Figure 6(b) that the photocatalytic activity of photocatalyst did not exhibit an obvious loss of activity after five rounds. The results indicated that Fe3O4@SiO2/Bi2SiO5 composite possessed good durability during photocatalytic process.
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4. Conclusions
In summary, the hollow hierarchical Fe3O4@SiO2/Bi2SiO5 composites with magnetic recoverability have been successfully prepared through simple hydrothermal method. Fe3O4@SiO2/Bi2SiO5 composites show highly photocatalytic activity under simulated solar light irradiation, as compared with that of pure Bi2SiO5 and P25. The highly photocatalytic performance of the Fe3O4@SiO2/Bi2SiO5 can be ascribed to the strong light absorption and high light-harvesting efficiency of hollow hierarchical structure. Furthermore, the synthesized magnetic composites can be recovered by external magnetic field, which can enhance the separation efficiency of photocatalyst in wastewater treatment.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.
Acknowledgments
The research was supported by the National Natural Science Foundation of China (Grant no. 21476033) and the Cultivation Program for Excellent Talents of Science and Technology Department of Liaoning Province (no. 201402610).