Solvothermal synthesis of bi2wo6 and its photocatalytic activity under visible light irradiation - Trinh Duy Nguyen

Journal of Science and Technology 54 (4B) (2016) 42-47 SOLVOTHERMAL SYNTHESIS OF BI2WO6 AND ITS PHOTOCATALYTIC ACTIVITY UNDER VISIBLE LIGHT IRRADIATION Trinh Duy Nguyen1, *, Van Thi Thanh Ho2, Long Giang Bach1 1 NTT Institute of High Technology, Nguyen Tat Thanh University, 298-300A, Nguyen Tat Thanh, Ho Chi Minh City, Vietnam 2Hochiminh City University of Natural Resources and Environment, Vietnam *Email: nguyenduytrinh86@gmail.com Received: 15 August 2016; Accepted for publication: 10 November 2016 ABSTRACT Flower-like Bi2WO6 were successfully synthesized using the solvothermal method at different temperatures and characterized by XRD, FE-SEM, and DRS. We also investigated the photocatalytic activity of Bi2WO6 for the decomposition of rhodamine B under visible light irradiation. From XRD and SEM results, the reaction temperature has significant effects on the morphologies of the samples. From DRS results, Bi2WO6 samples displayed the absorption spectrum up to the visible region and then they showed the high photocatalytic activity under visible light irradiation, as a comparison with TiO2-P25. Keywords: Bi2WO6, solvothermal method, rhodamine B, visible light irradiation. 1. INTRODUCTION Bismuth tungstate (Bi2WO6), an Aurivillius-phase perovskite, has been extensively used in photocatalytic applications [1 - 3]. Bi2WO6 is a typical n-type direct band gap semiconductor with a band gap of 2.75 eV, and thus it exhibits high photooxidative capacity as a catalyst for water splitting processes and degradation of organic pollutions under visible light irradiation [1]. It is a well-known fact that Bi2WO6 has two types of crystal structures: orthorhombic (space group B2cb, with a = 5.457, b = 5.436, c = 16.427Å, Z = 4) and monoclinic (space group structure. Monoclinic structure existed at a high-temperature phase (> 960 oC) while orthorhombic structures existed at a low and intermediate temperatures (< 960 oC) [4 – 6]. However, the orthorhombic phase was usually employed for the presently studied Bi2WO6 photocatalyst. In the photocatalytic process of TiO2 and another semiconductor, the main active species is ●OH. However, Bi2WO6 cannot generate ●OH in the photocatalytic process. The main active species were reported to be photogenerated holes (h+), conduction band electrons (eCB−) and superoxide radical (O2•−) [3, 7 – 9]. In our study, Bi2WO6 photocatalyst was synthesized by solvothermal method. The effects of temperature reaction on morphologies were investigated. We also investigated the Solvothermal synthesis of Bi2WO6 and its photocatalytic activity under visible light irradiation 43 photocatalytic activity of Bi2WO6 in the decomposition of rhodamine B (RhB) under visible light irradiation. 2. MATERIALS AND METHODS All chemicals were used as received without further purification and analytical grade. The syntheses of Bi2WO6 photocatalysts were prepared using a solvothermal method in the mixed solvent of ethylene glycol monomethyl ether (EGME) and water. Typically, Bi(NO3)3 (5 mmol, 2.475 g) was dissolved in 50 mL EGME. A solution of Na2WO4.H2O (2.5 mmol, 0.833 g) in 50 mL H2O was added into the above solution. The mixture was stirred for 1 h before being transferred into a Teflon-lined stainless steel autoclave and heated at 160-240 oC for 12 h. After each the reaction, the obtained suspension was centrifuged at 10000 rpm for 10 minutes, and the Bi2WO6 solids at the bottom of the tube were rinsed with water and ethanol for five times, dried at 60 oC overnight, calcined at 300 oC in air for 3 h. The crystal phase was examined by powder X-ray diffraction (XRD) patterns with Cu Kα radiation (Rigaku Co. Model DMax). Surface morphologies of the products were observed by scanning electron microscopy (SEM, JEOL JSM6700F) at an accelerating voltage of 3 kV. The optical properties of the products were recorded on a Varian Cary 100 UV-vis spectrophotometer using polytetrafluoroethylene (PTFE) as a standard. Photocatalytic activities of the samples were calculated by the photocatalytic decomposition of RhB under visible region with a 300 W Xe-arc lamp (Oriel) and a 410 nm cut- off filter. The light was passed through a 10 cm IR water filter and then focused onto a 150 mL Pyrex with a quartz window. In all catalytic activity of experiments, the reactor was filled with a mixture of RhB aqueous solution (10-5 M, 100 mL) and the given photocatalyst (100 mg). Before lighting on, the solution was magnetically stirred in the dark for 60 minutes to establish adsorption-desorption equilibrium between the photocatalyst surface and organic molecules. At given time intervals, 3 mL of the suspension was withdrawn and then filtered through a 0.22 μm membrane filter to get the clear solution. A decrease in the concentration of RhB solution was measured with a UV-vis spectrophotometer (Mecasys Optizen Pop) at λ = 554 nm. 3. RESULTS AND DISCUSSION Figure 1. XRD patterns of Bi2WO6 samples prepared with different synthesis temperatures: room temperature (a), 160 oC (b), 180 oC (c), 200 oC (d), and 240 oC (e). Trinh Duy Nguyen, Van Thi Thanh Ho, Long Giang Bach 44 XRD patterns of the as-synthesized Bi2WO6 samples prepared at different temperatures are shown in Figure 1. In the first stage of the reaction, when we added the 2-4WO solution to the Bi3+ solution, a white precipitate was rapidly formed. From the XRD result (Figure 1(a)), we conclude that the starting precipitate mostly had an amorphous phase. The amorphous phase was transformed into the orthorhombic Bi2WO6 after the 12 hours of the solvothermal process. However, when the reaction temperature was 160 oC, the XRD peaks are indexed to an amorphous state (Figure 1(b)) and thus Bi2WO6 could not be formed. After increasing the reaction temperature up to 180 oC, all the XRD peaks are well indexed to orthorhombic Bi2WO6 (JCPDS No. 73-1126) [4, 5, 10, 11]. No peak for tungsten oxide and bismuth oxide phase or other impurities were detected, which indicated the high purity of the product. Figure 2. SEM images of Bi2WO6 samples prepared with different synthesis temperatures: 160 0C (a), 180 oC (b), 200 oC (c), and 240 oC (d). The morphologies of the as-prepared Bi2WO6 were examined by SEM analysis and the results are shown in Figure 2. As shown in Figure 2, when the sample was treated at a lower temperature, the amorphous phase was observed. As the reaction temperature increased to 180 °C, flower-like Bi2WO6 super structure formed. As the reaction temperature extended to 200 and 240 oC, aggregates of Bi2WO6 nanoparticles were produced. From these images, we can come to a conclusion that the reaction temperature has significant effects on the morphologies of the samples. Solvothermal synthesis of Bi2WO6 and its photocatalytic activity under visible light irradiation 45 Figure 3. UV-vis DRS of Bi2WO6 catalysts prepared using different synthesis temperatures: 180 0C (a), 200 0C (b) and 240 0C (c). The light absorption properties of the photocatalysts were examined using UV–vis diffuse reflectance spectroscopy. Figure 3 shows the UV–vis DRS of the Bi2WO6 samples prepared ư different synthesis temperatures. As shown in Figure 3, the spectra of Bi2WO6 samples showed intensive absorption bands in the visible light region. This result suggests that Bi2WO6 samples can be used as potential visible-light-driven photocatalysts. The indirect band gap energy (Eg) of all samples were determined from the tangent line in the plots of the modified Kubelka–Munk function [F(R’∞)hυ]1/2 versus photon energy. The band gap values of the various samples are shown in Table 1. The band gap of Bi2WO6 nanoparticles is shifted from 2.81 to 2.86 eV. Table 1. The physical properties and photocatalytic activity of the as-prepared samples. No Sample Solvothermal temperature (0 C) SBET (mg/m2) Eg (eV) k (x10-3min-1) 1 P-25 TiO2 - 3.2 1.8 2 Bi2WO6 Room temperature - - 2.1 3 Bi2WO6 160 - - 0.7 4 Bi2WO6 180 322.413 2.81 10.7 5 Bi2WO6 200 252.565 2.82 8.3 6 Bi2WO6 240 203.337 2.86 6.4 Trinh Duy Nguyen, Van Thi Thanh Ho, Long Giang Bach 46 Figure 4. Variation of RhB concentration against irradiation time using Bi2WO6 samples prepared using different synthesis temperatures: room temperature (b), 160 0C (c), 180 0C (d), 200 0C (e) and 240 0C (f), and without catalyst under visible light (a). The photocatalytic activity for the decomposition of RhB on P-25 TiO2 and Bi2WO6 samples prepared at different reaction temperatures under visible light irradiation is shown in Figure 4. The photodecomposition rate constant (k) of RhB over samples, as calculated from a pseudo-first order reaction kinetic model: ln(Co/C) = kt. The results are reported in Table 1. When a blank test was carried out in the absence of the photocatalyst, about 1 % of the RhB was decomposed after 240 min by the photolysis reaction. As shown in Figure 4 and Table 1, Bi2WO6 catalysts showed higher photocatalytic activity compared to P-25 TiO2 catalyst. It is well known that the photocatalytic activity is related to the photoabsorption. Especially, this photocatalytic decomposition of RhB is carried out under visible light irradiation. Therefore, the amount of photo absorption in the visible region plays an important role on the photocatalytic activity. As shown in Figure 3, the spectra of Bi2WO6 samples showed intensive absorption bands in the visible light region and the higher photocatalytic activity. Moreover, Bi2WO6 catalyst at 180 oC showed the highest photocatalytic activity. It is thought that this photocatalytic reaction has a high surface area effect, wherein the photocatalytic activity increases with an increase of surface area. 4. CONCLUSIONS We have prepared Bi2WO6 using the solvothermal method at different temperatures. The phase structure and morphology of as-prepared Bi2WO6 samples were characterized by XRD, SEM, and DRS. We have also investigated the photocatalytic activity of these materials for the decomposition of RhB under visible light irradiation. From DRS results, Bi2WO6 samples showed the absorption spectrum up to the visible region and then their photocatalytic activity was shown higher than commercial P-25 TiO2 materials. Bi2WO6 sample prepared at 180 oC showed the highest photocatalytic activity due to high surface area effect. Acknowledgements. This research is funded by Foundation for Science and Technology Development Nguyen Tat Thanh University, Ho Chi Minh City, Vietnam. 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