Molten salt synthesis of Bi2WO6 powders with enhanced visible-light-induced photocatalytic activities

Journal of Alloys and Compounds 680 (2016) 301e308

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Molten salt synthesis of Bi2WO6 powders with enhanced visible-lightinduced photocatalytic activities Lin Zhou a, Chuangui Jin a, Yi Yu a, Fangli Chi a, b, Songlin Ran a, *, Yaohui Lv a, ** a b

School of Materials Science and Engineering, Anhui University of Technology, 243002 Ma’anshan, China State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, 130012 Changchun, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 19 October 2015 Received in revised form 14 April 2016 Accepted 16 April 2016 Available online 19 April 2016

Novel visible-light-induced Bi2WO6 photocatalysts were successfully synthesized by a molten salt method. The effects of the calcination temperature and the salt amount were investigated. The results indicated that the presence of molten salts significantly enhanced the surface area of the powders from 7.08 to a maximum of 21.09 m2 g
1, and higher calcination temperature improved the crystallinity but decreased the surface area of the powders. When the precursor powders were calcinated at 300  C for 8 h with the presence of 10 times weight of salts, the as-synthesized Bi2WO6 powders exhibited the highest degradation efficiency in the photodegradation of RhB under the visible light irradiation and the reaction contant was more than 10 times that of the samples synthesized without adding salts. © 2016 Elsevier B.V. All rights reserved.

Keywords: Molten salt method Bi2WO6 Visible-light-induced photocatalyst Rhodamine B

1. Introduction In recent years, semiconductor-based photocatalysis has attracted great attention because it can not only split water into hydrogen and oxygen but also degrade organic compound under light irradiation without any other energy. Both solar energy and hydrogen are known to be renewable, sustainable and nonpolluting energy sources. Therefore, semiconductor-based photocatalysis is considered as a major technology to solve the present energy and environmental problems [1e3]. Among various photocatalysts, Bi2WO6 has attracted considerable attention for its chemical stability, nontoxicity and relatively narrow band gap (2.7 eV). Until now, Bi2WO6 has been reported to have photocatalytic activities for both water splitting (O2 evolution) and organic contaminants decomposition under visible light irradiation [1e3]. However, Bi2WO6 powders synthesized by traditional solid-state reaction exhibited poor photocatalytic activity due to its large particle size and small surface area [4]. In addition, the band gap made Bi2WO6 only be driven by light with wavelength shorter than 450 nm, which limited the utility of the full-spectrum of the solar energy [2]. As a result, improving the photocatalytic

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected], [email protected] (S. Ran), [email protected] (Y. Lv). http://dx.doi.org/10.1016/j.jallcom.2016.04.144 0925-8388/© 2016 Elsevier B.V. All rights reserved.

activity of Bi2WO6 has been an interesting research topic. The most common method is to synthesize Bi2WO6 powders with nanostructures and controlled morphologies. The decrease of particle size greatly enhances the reactive sites for the photogenerated electron-hole pairs and the exposed crystal facets play a critical role in determining the photocatalytic reactivity and efficiency [1]. Another method is to narrow the band gap of Bi2WO6 by energy band engineering such as doping with other elements, forming solid solutions and inducing oxygen vacancies [2,5e8]. Designing and fabricating hierarchical composite nanostructures is also an effective way to enhance the photocatalytic efficiency of Bi2WO6 photocatalyst by promoting carrier separation [3,9e11]. It should be noticed that most of the above approaches were achieved by wet chemistry reaction at relatively low temperatures [1e3,5e10]. The low reactive temperature benefits for the growth of Bi2WO6 particles with nanostructures but not for the crystallinity. It is well known that the crystallinity strongly affects the photocatalytic efficiency of a photocatalyst. The high crystalline quality indicates small amount of defects which operate as trapping and recombination centers between photogenerated electrons and holes [12]. Thus, synthesizing powders with both small particle size and high crystallinity is a potential method to improve the photocatalytic efficiency of Bi2WO6 photocatalyst. Molten salt synthesis (MSS) is a method which allows solid reactants reacting much faster in a molten salt medium. MSS has been a classical method in preparing functional materials due to its

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simple, rapid, large scale and low-cost features [13]. In contrast to solid-state method, MSS could decrease the synthesis temperature because it allows faster mass transfer transport in the liquid phase by means of convection and diffusion. In addition, the molten salt could also serve as an inhibitor to limit the grain growth of freshlyproduced particles and result in a smaller particle size [14e16]. In contrast to wet chemistry method, the synthesis temperature of MSS is higher which induces higher crystalline quality for the powder products. Therefore, MSS has potential to prepare powders with both high crystallinity and fine particle size. Although there has a paper concerning the synthesis of Bi2WO6 powders by MSS before [17], the photocatalytic properties of the products have not been investigated. In this study, Bi2WO6 powders were synthesized by MSS. The effect of the temperature and the salt amount on the crystallinity, morphology and particle size were investigated. The photocatalytic activity was also evaluated by the photodegradation of rhodamine B (RhB) under visible light irradiation. It is demonstrated that the Bi2WO6 powders synthesized by MSS method exhibit much enhanced visible photocatalytic efficiency than those synthesized without adding salt. 2. Experimental procedures The synthesis process was similar to the literature [17]. All the chemical reagents used in the experiments were analytic grade without further purification. Bi(NO3)3.5H2O was dissolved in dilute HNO3 solution under heating and constant stirring, then Na2WO4$2H2O was added. The molar ratio of Bi3þ/WO2
4 was 2:1. After the above solution became clear, the ammonia solution was slowly added and a white precipitate was formed. When the pH value reached 9, the suspensions were stirred for another 60 min. The precipitate was filtered and washed several times using distilled water and absolute ethanol, then dried at 80  C for 1 h to get the precursor powders. The mixture of LiNO3 and NaNO3 with a molar ratio of 27:23 was used as molten salts. In a typical synthesis of Bi2WO6 powders, 0.5 g precursor powders were mixed with 0.5*R g LiNO3/NaNO3 salts by hand for 30 min in an agate mortal using a pestle, where R ¼ 0, 5, 10 and 20. The mixture was put into an Al2O3 crucible (20 mL) and heated at 200e350  C for 8 h. After cooling to room temperature, the resulting products were dipped into distilled water to dissolve LiNO3/NaNO3. The pale yellow precipitate was filtered, washed with distilled water and absolute ethanol for several times, and then dried at 80  C overnight. The phase composition of as-prepared powders was measured by X-ray diffraction (XRD, Bruker AXS D8 Discover, Germany) with Cu-Ka radiation. The morphologies and microstructures of the powders were examined by a scan electron microscopy (SEM, FEI QUANTA 250 FEG, USA) and a transmission electron microscopy (TEM, JEM-2100F, JEOL, Japan). Chemical compositions of the powders were analyzed using X-ray photoelectron spectroscopy (XPS) analysis (ESCALAB 250Xi, Thermo Fisher Scientific, USA). UVevis diffuse reflectance spectra of the samples was obtained on an UVevis spectrophotometer (UV-3600, Shimadzu, Japan). The surface area of powders were determined using the BrunauerEmmett-Teller (BET) method to analyze data collected using nitrogen adsorption analysis (Tri-star 3000, Micromeritics Instrument Corp., USA). Photocatalytic activities of the samples were evaluated by the photodegradation of Rhodamine-B (RhB). The light source was a 500 W Xe lamp (l > 420 nm). In a typical process, 250 mg Bi2WO6 powders were added into a beaker containing 250 mL RhB solution (10
5 mol L
1) with a cooling water system to keep the temperature constant (25  C). Before irradiation, the suspension was magnetically stirred in the dark for 30 min to ensure the

establishment of an adsorption-desorption equilibrium between Bi2WO6 and RhB. Then, the solution was exposed to visible light irradiation under magnetic stirring. The average light intensity in the surface of the solution was 42 mW cm
2. The solution (4 mL) was collected every 10 min and centrifuged at 12,000 rpm to remove the Bi2WO6 powders. The concentration change of the RhB was analyzed by recording the variations of the absorbance at 554 nm in the UVevis spectrum using a spectrophotometer (UV2102PC, Beijing Purkinje General Instrument Co., Ltd., China). The photodegradation efficiency of the photocatalyst was evaluated by the value of C/C0 or C/Ca, where C0, Ca and C was the concentration of the starting dye solution, the dye solution after absorptiondesorption equilibrium and the dye solution after irradiation under visible light for a certain time, respectively. The stability of the photodegradation efficiency of Bi2WO6 powders was evaluated by the circulating runs of the above photodegradation process. Bi2WO6 powders were collected after the complement of each photodegradation experiment by filtration and used for the next run. The obtained powders were a litter lighter than those added at the beginning of the photodegradation process due to the inevitable loss during sampling and filtering. At each run, the concentration of Bi2WO6 and RhB were kept at 1.0 g L
1 and 10
5 mol L
1, respectively. The methyl orange (MO) (10
5 mol L
1) was used to further explore the selectivity of the photocatalyst and the experimental procedure was similar. 3. Results and discussion Bi2WO6 powders were synthesized by molten salt method with co-precipitated powders as precursors. Fig. 1 shows the XRD patterns of samples prepared at different temperatures with or without the presence of molten salts, respectively. Without the molten salts, the powder products were amorphous until the temperature reached 300  C (Fig. 1(a)). The addition of molten salts accelerated the crystallization process of the precursor and pure orthorhombic Bi2WO6 (JCPDS card No.39-0256) phase could be obtained at 250  C, as shown in Fig. 1(b). The results indicated that the molten salts played an important role in the formation of Bi2WO6 phase. Fig. 1(c) shows the effects of the salt amount on the crystalline phase and crystallinity of the obtained samples at 300  C. It is clear that the diffraction peaks of Bi2WO6 for samples prepared with molten salts are much wider than that prepared without molten salts, indicating smaller particle size caused by the presence of the molten salts. The BET surface area measurement in Table 1 confirms the result. The BET surface areas of Bi2WO6 powders prepared with molten salts (19.48e21.09 m2 g
1) at 300  C were nearly 3 times higher than that prepared without molten salts (7.08 m2 g
1). The amount of the molten salts had little impact on the BET surface area. With increasing amount of molten salts, the BET surface area of the obtained Bi2WO6 powders slightly decreased. The calcination temperature had obvious impact on the crystallinity of the products. When the calcination temperature increased from 250  C to 350  C, the XRD peaks in Fig. 1(b) is becoming sharper and the BET surface area of the sample decreases from 21.86 to 12.93 m2 g
1, as shown in Table 1. The effects of salt amount and the calcination temperature on the morphology of the as-prepared Bi2WO6 powders were investigated by SEM. Fig. 2(aed) show the morphology evolution of Bi2WO6 powders prepared at 300  C with increasing salt amount. When no salt was introduced, the shape of Bi2WO6 was irregular nanoparticles with a shape of sintered aggregates, and the average particle size was about 90 nm (Fig. 2(a)). The addition of molten salts induced the Bi2WO6 particles to exhibit laminar structures which were composed of nanoplates with very small thickness (Fig. 2(bed)). This kind of structure greatly enlarged the surface

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303

Fig. 1. XRD patterns of the powders synthesized (a) at different temperatures with R ¼ 0, (b) at different temperatures with R ¼ 10 and (c) at 300  C with different values of R.

Table 1 BET surface area of the as-synthesized Bi2WO6 powders. R 

Temperature ( C) Surface area (m2 g
1)

0

5

10

300 7.08

300 21.09

250 21.86

20 300 20.00

350 12.93

300 19.48

area of the powders, as shown in Table 1. The salt amount had little effect on the shape of the as-prepared Bi2WO6 powders, but the decrease of the surface area with increasing salt amount revealed that the increasing salt slightly enhanced the growth of the nanoplates. According to Fig. 2(c,e,f), it could be seen that the calcination temperature had significantly affected the morphology of the asprepared Bi2WO6 powders. When the temperature reached 350  C, the morphology of the Bi2WO6 particles (Fig. 2(f)) is similar to those prepared without molten salt (Fig. 2(a)). The plate-like Bi2WO6 particles were further characterized by TEM and HRTEM. Fig. 3(a) confirmed that the plates had very small thickness. The distance between adjacent lattice fringes in Fig. 3(b) was measured to be about 0.274 nm, corresponding to the (200) plane of orthorhombic Bi2WO6 [3]. The morphologies of Bi2WO6 powders in this study were different from those in the previous report on the synthesis of Bi2WO6 powders with the same process. In the report, the morphology of the Bi2WO6 powders changed from irregular to nanoflakes and finally to nanorods with increasing salt amount [17]. The difference might be due to the different details in the synthesized process such as pH value for the precipitate, salt amount and the size of the crucible. Fig. 3(c) shows an energydispersive spectroscopy (EDS) analysis of a local area of the assynthesized Bi2WO6 plates, revealing that only Bi, W and O element signals existed in the plate. Further quantitative analysis indicated the atomic molar ratio of Bi:W was 1.86:1, which was

close to the stoichiometric proportion of Bi2WO6. X-ray photoelectron spectroscopy (XPS) was used to characterize the surface composition and chemical state of the assynthesized plate-like Bi2WO6 powders, as shown in Fig. 4. The overall XPS spectra in Fig. 4(a) indicate that only peaks of Bi, W, O and C elementals are observed. The presence of C is due to the adventitious carbon-based contaminant, and the binding energy of C 1s peak at 284.6 eV is used as a reference for calibration. The results revealed that the as-synthesized Bi2WO6 powders were pure and no contaminant was derived from the adopted synthetic procedure. Fig. 4(bed) display the high-resolution spectrum for Bi, W and O species, respectively. The main peaks in Fig. 4(b), having a W 4f7/2 at 34.5 eV and a W 4f5/2 at 36.6 eV, are attributed to the W atoms being in a 6 þ oxidation state. The binding energies at 158.3 and 163.6 eV in Fig. 4(c) are indexed as Bi 4f7/2 and Bi 4f5/2, respectively, which reveal a trivalent oxidation state for Bi. In Fig. 4(d), the peak at 529.4 eV is owing to the oxygen species in the lattice oxygen and the peaks at 531.8 eV is associated with the hydroxyl groups on the surfaces of the powders [5,6,8]. UVevis diffuse reflectance (DR) of the samples were measured to evaluate the band gap of the as-prepared Bi2WO6 powders. All samples presented nearly the same optical absorption. Fig. 5 shows the DR spectra of the Bi2WO6 powders prepared at different temperatures. All Bi2WO6 have intense absorption bands with steep edges in the visible light region with a wavelength shorter than 470 nm, which is in accord with the reports in the literature [18e20]. The previous research revealed that the RhB could be degraded efficiently by Bi2WO6 under visible light in the range 400e490 nm [21]. The steep edges indicate that the visible light absorption bands of the Bi2WO6 are not due to the transition from impurity level but the band-gap transition [22]. The UVevis absorption for a crystalline semiconductor near the band edge follows the equation [23].

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Fig. 2. SEM images of the Bi2WO6 powders prepared at (a) 300  C, R ¼ 0, (b) 300  C, R ¼ 5, (c) 300  C, R ¼ 10, (d) 300  C, R ¼ 20, (e) 250  C, R ¼ 10 and (f) 350  C, R ¼ 10.

ahn ¼ A hn
Eg


n

(1)

where a, h, n, A and Eg are the absorbance, Planck’s constant, photon frequency and a constant and photonic energy band gap, respectively. The parameter n depends on whether the transition is direct (n ¼ 1/2) or indirect (n ¼ 2). According to the literature [20], the optical transition of Bi2WO6 is indirectly allowed and thus the value of n for Bi2WO6 is 2. The band gap of the Bi2WO6 could be estimated from tangent lines in the plots of the square root of the KubelkaMunk functions ((ahn)1/2) against the photon energy (hn), as shown in the inset of Fig. 5. The band gaps of the samples were 2.64, 2.66 and 2.68 eV from the onset of the absorption edge, corresponding to a temperature of 250  C, 300  C and 350  C, respectively. The increasing temperature slightly enhanced the band gap of the photocatalyst. In this study, Bi2WO6 powders were synthesized by molten salt method. As discussed above, the calcination temperature and the salt amount were the main factors to affect the particle size and the morphology of the product. To compare the photocatalytic activities of the Bi2WO6 powders synthesized at different temperatures with different salt amounts, the photocatalytic activities of the assynthesized samples were evaluated through the degradation of RhB under visible light irradiation, as shown in Fig. 6(aeb).

According to Fig. 6(a), Bi2WO6 powders synthesized at 300  C had the highest photocatalytic activity. It is well-known that the photocatalytic activities are closely related to their band gaps and surface areas. A narrower band gap is easier to be illuminated to generate electron-hole pairs in the particles and a larger surface area provides more active sites in the photocatalytic process [12]. As a result, the Bi2WO6 sample synthesized at 350  C had lower photocatalytic activity due to its broader band gap and smaller surface area (Fig. 5 and Table 1). It should be noted that the Bi2WO6 powders synthesized at 250  C performed the worst photocatalytic activity although it had the narrowest band gap and the highest surface area. Fig. 5 and Table 1 indicated that the differences of band gap and surface area between the samples synthesized at 250  C and 300  C were very slight and could be ignored. Therefore, these two factors were not consequentially in association with the photocatalytic activity. The crystallinity of a semiconductor also has significant effects on their photocatalytic activities due to fewer defects acting as electro-hole recombination centers. Higher calcination temperature benefited to the crystallinity of the samples, as shown in Fig. 1(b). Compared to the sample synthesized at 250  C, the sample synthesized at 300  C had a better crystallinity and a comparable surface area. When compared to the sample synthesized at 350  C, the sample synthesized at 300  C had a larger surface area. These factors contributed to the highest photocatalytic

L. Zhou et al. / Journal of Alloys and Compounds 680 (2016) 301e308

Fig. 3. (a) TEM image, (b) HRTEM image and (c) EDS spectrum of the Bi2WO6 plates prepared at 300  C with R ¼ 10.

Fig. 4. XPS spectra of the Bi2WO6 powders synthesized at 300  C with R ¼ 10: (a) survey, (b) W 4f, (c) Bi 4f and (d) O 1s.

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Fig. 5. UVevis diffuse reflectance spectra of Bi2WO6 samples prepared at different temperatures.

activity of the sample synthesized at 300  C. As indicated in Fig. 6(b), the samples synthesized by molten salt method obviously exhibited superior photocatalytic activities over the sample synthesized without the presence of the molten salts, which could be attributed to the great enhancement of surface area. The increasing salt amount enhanced the crystallinity, which could be deduced from the decrease of the surface area (see Table 1). The improved crystallinity was due to the faster mass transfer transport in the liquid phase by means of convection and diffusion [14e16]. Fig. 6(c) compares the apparent photodegradation rate constants of the samples synthesized at different conditions. The photodegradation rate constants (k) were calculated assuming a first-order reaction kinetics i.e. ln(C/Ca) ¼
kt, although this might not always be the case. The value of k was 0.0847 min
1 for the sample synthesized by molten salt method at 300  C with R ¼ 10, more than 10 times that (0.0082 min
1) for the sample synthesized at 300  C with R ¼ 0, suggesting the excellent photocatalytic

Fig. 6. Photocatalytic degradation curves of RhB by Bi2WO6 prepared (a) at different temperatures, (b) with different values of R; (c) the linear fitting of ln(C/Ca) and irradiation time for the photocatalytic degradation of RhB with different values of R; (d) UVevis spectra of RhB at different irradiation times and (e) photocatalytic degradation curves of RhB and MO in the presence of Bi2WO6 powders prepared at 300  C with R ¼ 10.

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307

Fig. 7. Photocatalytic degradation curves of RhB by Bi2WO6 powders prepared at 300  C with R ¼ 10: (a) effect of RhB concentration and (b) the linear fitting of ln(C/Ca) and irradiation time.

activity enhanced by the present of molten salts. Fig. 6(d) displays the temporal UVevis spectral changes of RhB aqueous solution during the photodegradation process for the samples with best photocatalytic activity. With increasing time of light irradiation, the absorption of RhB at 554 nm decreased markedly with an absorption band shifted to shorter wavelengths and turned broadened at the same time. The inset in Fig. 6(d) shows the corresponding color changes of RhB solution. With increasing irradiation time, the bright red starting RhB solution gradually turns to colorless. According to Fig. 6(a), the photodegradation efficiency(C/Ca) of RhB reached 92% at 30 min and 97% at 40 min, respectively. However, obvious absorption at 498 nm still presented in the corresponding times in Fig. 6(d). After 60 min of irradiation, there had almost no absorption. The blue shifts of the wavelength in Fig. 6(d) were related to the two competitive degradation process of RhB. The first one is the de-ethylation process due to the attack by one of the active oxygen species on the N-ethyl group. Deethylation of the fully N,N,N0 ,N0 -tetraethylated rhodamine molecule (i.e., RhB) changes the wavelength position of its major absorption band from 554 nm to 539 nm for N,N,N0 -tri-ethylated rhodamine, 522 nm for N,N0 -di-ethylated rhodamine, 510 nm for Nethylated rhodamine and finally 498 nm for rhodamine, respectively. Another is the cleavage of the RhB chromophore ring structure. In Fig. 6(d), the maximum absorption band shifts to 532 nm after 20 min of irradiation, suggesting the existence of other rhodamine species mentioned above. The de-ethylation process predominated at this period. After the RhB solution was irradiated for 30 min, the maximum absorption band was at 498 nm, indicating the main species in the solution was rhodamine. With increasing irradiation time, the maximum absorption band was kept at 498 nm while the absorption decreased continually and reached zero after 60 min, revealing the complete degradation of rhodamine. During this period, the cleavage of conjugated chromophore structure predominated. According to the literature, the photocatalyst may have a certain selectivity for the degradation of dyes. In this study, the photocatalytic efficiency of RhB and MO with the same concentration (10
5 mol/L) were compared, as shown in Fig. 6(e). Although the dark absorption of MO was higher than RhB, the photodegradation efficiency of RhB was much higher than MO. RhB could be degraded completely after 40 min irradiation under visible light while only 25% MO(C/Ca) was degraded even after 60 min irradiation. The selectivity of a particular photocatalyst on the dyes largely depends on the molecular structure of the dye. The degradation pathways of organic dyes are different according to the chemical structure and functional groups, resulting in different adsorption characteristics and difference in susceptibility to photodegradation. Similar

phenomena were reported on the Bi2WO6/TiO2 and AgBr/Ag3PO4 heterojunctions [24,25]. The effect of initial concentration of RhB on the photodegradation efficiency was studied by varying the initial concentration from 10
5 mol/L to 3  10
5 mol/L, as shown in Fig. 7. Fig. 7(b) shows that the ln(C/Ca) vs. t curves present good linearity; however, the obtained rate constants decrease with increasing initial concentration of RhB. According to the literature [26e28], there are two explanations for this exist: (1) the concentration dependence of the photodegradation rate constant has been previously modeled by the integrated form of the Langmuir adsorption isotherm:

t¼

1 Ca 1 ln þ ðCa
CÞ kK k C

(2)

Fig. 8. (a) Cycling test for the photocatalytic degradation of RhB, (b) XRD patterns of Bi2WO6 powders before and after cycling tests.

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where t is the time in minutes for the initial concentration of the dye, Ca, to decrease to C and k, K are constants for the photodegradation reaction and adsorption of the solute; (2) the decreasing of dye concentration increases the path length of photons entering the solution and thereby increases the number of photon absorption by the catalyst, which finally enhances the photodegradation efficiency. As shown in Fig. 7(b), when the concentration of RhB solute was diluted by 3 times, the first-order rate constant was enhanced 2.9 times. Moreover, the profiles in Fig. 7(b) tends to be deflexed as photodegradation proceeds for all concentrations. These phenomena indicated that the second explanation might be predominant in this system. The stability of an ideal photocatalytic is very important for its practical application. Herein, the stability of Bi2WO6 synthesized by molten salt method was evaluated by two experiments: (1) five cycling tests for the photocatalytic degradation of RhB and (2) the XRD comparison between photocatalyst before and after cycling tests. As shown in Fig. 8(a), after five recycles for the photocatalytic degradation of RhB, the catalyst did not exhibit a significant loss of activity. The kinetic constants obtained in recycle tests were calculated based on the photodegradation experiment in 30 min and the values were 8.12  10
2, 7.68  10
2, 7.42  10
2, 7.41  10
2 and 7.33  10
2, respectively. As the cycle number increased, the photocatalytic activity of the photocatalyst slightly decreased. The XRD patterns in Fig. 8(b) also illustrates that the crystal structure of the Bi2WO6 photocatalyst did not change after the photocatalytic tests. Both experimental results confirming that the Bi2WO6 powders synthesized by molten salt in this study were relatively stable as a photocatalyst.

[2] [3]

[4] [5]

[6]

[7] [8]

[9]

[10]

[11]

[12] [13] [14] [15] [16]

4. Conclusions [17]

Bi2WO6 photocatalyst powders were synthesized by a novel molten salt method with precipitates from mixing solutions containing Bi3þ and WO2
4 in aqueous ammonia medium as precursors, and LiNO3/NaNO3 mixtures as salts, respectively. The results indicated that calcination temperature and the salt amount greatly affected the crystallinity, morphology and photocatalytic activity of the as-synthesized Bi2WO6 powders. When the precipitates were calcinated at 300  C for 8 h with the presence of 10 times weight of salts, the as-synthesized Bi2WO6 powders exhibited the highest degradation efficiency under the visible light irradiation, which could completely bleach 10
5 mol/L RhB solution effectively and the removal efficiency (C/Ca) was 97% in 40 min. The present work indicates that the Bi2WO6 photocatalyst with high visible-lightinduced photocatalytic activity could be synthesized by molten salt method, which has practical significations in the commercialization of the photocatalysts.

[18] [19] [20]

[21]

[22]

[23] [24]

[25]

Acknowledgements This research was financially supported by the Natural Science Foundation of Anhui Provincial Education Department (KJ2015A085). Lin Zhou and Yi Yu thank the support from the Graduate Innovation Foundation (2014086) and the Student Research Training Program (SRTP) (201510360144) of Anhui University of Technology, respectively. References [1] G. Zhang, Z. Hu, M. Sun, Y. Liu, L. Liu, H. Liu, C.P. Huang, J. Qu, J. Li, formation of

[26]

[27]

[28]

Bi2WO6 bipyramids with vacancy pairs for enhanced solar-driven photoactivity, Adv. Funct. Mater. 25 (2015) 3726e3734. Y. Lv, W. Yao, R. Zong, Y. Zhu, Fabrication of wide-range-visible photocatalyst Bi2WO6-x nanoplates via surface oxygen vacancies, Sci. Rep. 6 (2016) 19347. Y. Zhou, Y. Zhang, M. Lin, J. Long, Z. Zhang, H. Lin, J.C.-S. Wu, X. Wang, Monolayered Bi2WO6 nanosheets mimicking heterojunction interface with open surfaces for photocatalysis, Nat. Commun. 6 (2015) 8340. J. Tang, Z. Zou, J. Ye, Photocatalytic decomposition of organic contaminants by Bi2WO6 under visible light irradiation, Catal. Lett. 92 (2004) 53e56. N. Tian, Y. Zhang, H. Huang, Y. He, Y. Guo, Influences of Gd substitution on the crystal structure and visible-light-driven photocatalytic performance of Bi2WO6, J. Phys. Chem. C 118 (2014) 15640e15648. H. Huang, K. Liu, K. Chen, Y. Zhang, Y. Zhang, S. Wang, Ce and F comodification on the crystal structure and enhanced photocatalytic activity of Bi2WO6 photocatalyst under visible light irradiation, J. Phys. Chem. C 118 (2014) 14379e14387. Z. Zhu, Y. Wang, H. Yu, J. Li, Flabellate Bi2Mo0.5W0.5O6 solid solution synthesized via PEG6000-assisted hydrothermal process, J. Nano Res. 20 (2012) 1e9. J. Tian, Y. Sang, G. Yu, H. Jiang, X. Mu, H. Liu, A Bi2WO6-based hybrid photocatalyst with broad spectrum photocatalytic properties under UV, visible, and near-infrared irradiation, Adv. Mater. 25 (2013) 5075e5080. J. Zhang, L. Huang, L. Yang, Z. Lu, X. Wang, G. Xu, E. Zhang, H. Wang, Z. Kong, J. Xi, Controllable synthesis of Bi2WO6(001)/TiO2(001) heterostructure with enhanced photocatalytic activity, J. Alloys Compd. 676 (2016) 37e45. Z. Zhu, Y. Yan, J. Li, Preparation of flower-like BiOBr-WO3-Bi2WO6 ternary hybrid with enhanced visible-light photocatalytic activity, J. Alloys Compd. 651 (2015) 184e192. X. Liu, Q. Lu, J. Liu, Electrospinning preparation of one-dimensional ZnO/ Bi2WO6 heterostructured sub-microbelts with excellent photocatalytic performance, J. Alloys Compd. 662 (2016) 598e606. H. Tong, S. Ouyang, Y. Bi, N. Umezawa, M. Oshikiri, J. Ye, Nano-photocatalytic materials: possibilities and challenges, Adv. Mater. 24 (2012) 229e251. X. Liu, N. Fechler, M. Antonietti, Salt melt synthesis of ceramics, semiconductors and carbon nanostructures, Chem. Soc. Rev. 42 (2013) 8237e8265. Y. Wei, Z. Huang, L. Zhou, S. Ran, Novel borothermal synthesis of VB2 powders, Int. J. Mater. Res. 106 (2015) 1206e1208. H. Sun, W. Cao, S. Ran, Y. Wei, Y. Lv, Synthesis of TiB2 powders by molten salt method, J. Synth. Cryst. 44 (2015) 2513e2517. S. Ran, H. Sun, Y.n. Wei, D. Wang, N. Zhou, Q. Huang, Low-temperature synthesis of nanocrystalline NbB2 powders by borothermal reduction in molten salt, J. Am. Ceram. Soc. 97 (2014) 3384e3387. L. Xie, J. Ma, J. Zhou, Z. Zhao, H. Tian, Y. Wang, J. Tao, X. Zhu, Morphologiescontrolled synthesis and optical properties of bismuth tungstate nanocrystals by a low-temperature molten salt method, J. Am. Ceram. Soc. 89 (2006) 1717e1720. H. Fu, C. Pan, W. Yao, Y. Zhu, Visible-light-induced degradation of rhodamine B by nanosized Bi2WO6, J. Phys. Chem. B 109 (2005) 22432e22439. C. Zhang, Y. Zhu, Synthesis of square Bi2WO6 nanoplates as high-activity visible-light-driven photocatalysts, Chem. Mater. 17 (2005) 3537e3545. X. Song, Y. Zheng, R. Ma, Y. Zhang, H. Yin, Photocatalytic activities of Modoped Bi2WO6 three-dimensional hierarchical microspheres, J. Hazard. Mater. 192 (2011) 186e191. H. Fu, S. Zhang, T. Xu, Y. Zhu, J. Chen, Photocatalytic degradation of RhB by fluorinated Bi2WO6 and distributions of the intermediate products, Environ. Sci. Technol. 42 (2008) 2085e2091. A. Kudo, I. Tsuji, H. Kato, AgInZn7S9 solid solution photocatalyst for H2 evolution from aqueous solutions under visible light irradiation, Chem. Commun. (2002) 1958e1959. M. Butler, Photoelectrolysis and physical properties of the semiconducting electrode WO2, J. Appl. Phys. 48 (1977) 1914e1920. C. Yang, Y. Huang, F. Li, T. Li, One-step synthesis of Bi2WO6/TiO2 heterojunctions with enhanced photocatalytic and superhydrophobic property via hydrothermal method, J. Mater. Sci. 51 (2016) 1032e1042. J. Zhang, T. Zhang, Preparation and characterization of highly efficient and stable visible-light-responsive photocatalyst AgBr/Ag3PO4, J. Nanomater. 2013 (2013) 1e11. B. Neppolian, H.C. Choi, S. Sakthivel, B. Arabindoo, V. Murugesan, Solar/UVinduced photocatalytic degradation of three commercial textile dyes, J. Hazard. Mater. 89 (2002) 303e317. R.W. Matthews, Photocatalytic oxidation and adsorption of methylene blue on thin films of near-ultraviolet-illuminated TiO2, J. Chem. Soc. Faraday Trans. I 85 (1989) 1291e1302. R.J. Davis, J.L. Gainer, G. O’Neal, I.-W. Wu, Photocatalytic decolorization of wastewater dyes, Water Environ. Res. 66 (1994) 50e53.