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BiVO4 composites having enhanced visible light photocatalystic activity
Materials Science in Semiconductor Processing 34 (2015) 198–204
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Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Fabrication of heterojunction SnO2/BiVO4 composites having enhanced visible light photocatalystic activity Jiazhi Yin a,c, Shaobin Huang a,b,c,n, Zicong Jian a,c, Zhixin Wang a,c, Yongqing Zhang a,b a b c
School of Environment and Energy, South China University of Technology, Guangzhou 510006, China State Key Laboratory of Pulp and Paper Engineering, South China University of Technology, Guangzhou 510640, China Guangdong Provincial Key Laboratory of Atmospheric Environment and Pollution Control, Guangzhou 510006, China
a r t i c l e in f o
Keywords: SnO2 BiVO4 Degradation Heterojunction Photocatalytic performance Visible light
abstract SnO2/BiVO4 heterojunction composite photocatalysts with various mole ratios have been prepared via a simple hydrothermal method. The structure, composition and optical properties of the SnO2/BiVO4 composites were determined by X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), Brunauer–Emmett–Teller (BET) surface analysis, X-ray photoelectron spectroscopy (XPS) and UV–vis diffuse reflectance spectroscopy (UV–vis DRS). Photocatalytic activities of the composites were evaluated by studying the degradation of methylene blue (MB) solutions under simulated visible light irradiation (500 W halogen tungsten lamp). The 3:7 mol ratio SnO2/BiVO4 composite exhibited the highest photocatalytic performance, leading to 72% decompositon of MB within 120 min of irradiation. & 2015 Elsevier Ltd. All rights reserved.
1. Introduction In the past few decades, photocatalytic technology has been applied in degradation and decomposition of poisonous and noxious organic compounds and photocatalytic water splitting product hydrogen [1–5]. Titanium dioxid (TiO2) is the most widely used semiconductor photocatalytic material due to its high chemical stability, low cost and low toxicity [6]. However, the practical use of TiO2 is limited by its wide band gap (3.2 eV) which means that it responds only to ultraviolet light that covers 4% of the solar energy spectrum [7–9]. Therefore, there have been many efforts aimed at the development of a visible-lightdriven catalyst in order to effectively utilize the natural solar energy. n Corresponding author at: School of Environment and Energy, South China University of Technology, Guangzhou 510006, China. E-mail address: [email protected] (S. Huang).
http://dx.doi.org/10.1016/j.mssp.2015.02.044 1369-8001/& 2015 Elsevier Ltd. All rights reserved.
Recently, monoclinic bismuth vanadate (m-BiVO4) with a narrow band gap of 2.4 eV has caught considerable attention as a promising a visible-light-driven photocatalyst material and has shown significant visible-light activity for organic photocatalytic degradation [10,11]. On the other hand, the low efficiency of the separation of electron–hole pairs in m-BiVO4 limits its application in the visible region. However, by coupling m-BiVO4 with another semiconductor with appropriate band potentials to form a heterojunction is an effective approach to improving the charge separation, increase the charge carrier lifetime, and thus enhanced photocatalytic activity [12,13]. Moreover, due to the high efficiency of the interfacial charge transfer from catalyst to absorbed substrate, a rapid re-dox process can be obtained easily [14–16]. As a result, photocatalytic properties of BiVO4-based composite materials have been extensively explored. Ji et al. [17] investigated the catalytic performance of C3N4/BiVO4 heterojunction photocatalyst and found that the composite catalysts showed remarkably
J. Yin et al. / Materials Science in Semiconductor Processing 34 (2015) 198–204
Table 1 Surface area and energy gaps of pure BiVO4, pure SnO2 and SnO2/BiVO4 composites with different mole ratios. 2
Sample
Sn:Bi mole ratio
SBET (m g
BiVO4 SnO2 S1B9 S3B7 S5B5 S7B3 S9B1
– – 1:9 3:7 5:5 7:3 9:1
0.30 74.85 0.49 7.60 21.63 37.08 62.24
1
)
Eg (eV) 2.34 3.40 2.28 2.28 2.32 2.29 2.33
improved visible-induced photocatalytic activities in degrading RhB compared with the C3N4 or BiVO4 alone. The increased photocatalytic activity of this composite catalyst was ascribed to the stability of the heterostructure and the energy level match that facilitated the charge separation and transport. Photocatalytic degradation of benzene by BiVO4/TiO2 heterostructure catalyst under visible-light irradiation was also evaluated by Hu’s group [18]. The composition that gave the largest benzene conversion efficiency was BiVO4/TiO2 with the mass ratio of 1:200. Su and Zou [19] synthesized macroporous V2O5/BiVO4 composites with a heterojunction structure using colloidal carbon spheres. The formation of heterojunction structures in the V2O5/ BiVO4 composite plays an important role in the separation, transport and recombination of photogenerated charges by greatly increasing the physical separation and the lifetime of the photogenerated charges formed. He et al. [20] studied the photocatalytic degradation of methyl orange (MO) over a BiOCl/BiVO4 heterojunction composite prepared using a hydrothermal method. The composite markedly improved the efficiency of MO photodegradation compared to results obtained with pure BiVO4, BiOCl, and Degussa P25. The highest activity of BiOCl/BiVO4 heterojunction was obtained in the composite of 13 mol% BiOCl and 87 mol% BiVO4. In related studies, CuO/BiVO4 [21–23], WO3/BiVO4 [24,25], Co3O4/BiVO4 [26–28] and SiO2/BiVO4 [29] heterojunction composites have also been reported to have higher photocatalytic activity than that of BiVO4 alone due to the enhanced efficiency separation of photogenerated electron–hole pairs. These studies show that the improvement in the photosensitivity results from a coupling effect between the two components in the composites that increases the electron transfer and inhibits the recombination of photo-generated charge carriers [30]. In this study, we have successfully developed a series of highly photoactive SnO2/BiVO4 heterojunction catalysts using a simple hydrothermal method. To the best of our knowledge, the photocatalytic activity of SnO2/BiVO4 composites has not been previously reported. The aim of this study is to explore the photocatalytic efficiency of SnO2/ BiVO4 heterojunction composite catalysts with the intent of identifying a promising visible-light driven catalyst. A mechanism is also proposed that leads to improved separation of electrons and holes based on energy band position as well as related characteristics.
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2. Experimental section 2.1. Sample synthesis All chemicals applied in experiment were analytical grade and used without further purification. Pure SnO2 nanoparticles were prepared by a hydrothermal method [31] using tin chloride pentahydrate (SnCl4 5H2O) as a tin precursor. The synthesized SnO2 powders were subsequently added to the solution of bismuth nitrate pentahydrate (Bi(NO3)3 5H2O) in 4 M nitric acid (HNO3) and ammonium vanadate (NH4VO3) in 4 M sodium hydroxide (NaOH) (1:1 mol ratio) to form suspensions with different BiVO4:SnO2 mole ratios. The pH of the mixtures was then adjusted to 7 by slowly adding 1 M NaOH solution. After that, the suspension was transferred to a stainless autoclave where the hydrothermal reaction was carried out at 150 1C and maintained at that temperature for 24 h. Finally, the SnO2/BiVO4 composites were collected by filtration and dried at 80 1C for 24 h. As a control, pure BiVO4 photocatalyst was also prepared by the same procedure. As shown Table 1, all of the composite samples were labeled as SxBy in which S and B stand for SnO2 and BiVO4, x and y stand for the respective molar amounts in the composites. 2.2. Material characterization Crystalline phases of the composites were subjected to X-ray diffraction (XRD) analysis (Bruker D8 ADVANCE). The diffraction patterns were recorded in the range of 2θ¼101–801 using Cu Kα radiation (λ¼0.15406 nm). Raman spectra were obtained on a HORIBA JY LabRAM HR Evolution instrument with a 532 nm Nd-YVO4 laser as the excitation source. The morphology and particle size of the samples were examined by scanning electronmicroscopy (SEM) (Model ZEISS, MERLIN Compact). The microstructures of the samples were investigated by transmission electronmicroscopy (TEM) (JEOL, Model JEOL-2100). The specific surface areas of the samples were measured by a Brunauer–Emmett–Teller specific surface area instrument (American Micromeritics Instrument Corporation ASAP 2020N). The UV–vis diffuse reflectance spectra were recorded on a UV–vis spectrophotometer (Shimadzu, Model UV2450) in the wavelength range of 200–800 nm and BaSO4 was used as the reference. X-ray photoelectron spectroscopy (XPS) measurements were take on a British Kratos Axis Ultra DLD system with Al Kα radiation as the excitation source, where the binding energies were calibrated by referencing the C 1 s peak (284.6 eV). 2.3. Photocatalytic activity studies Photocatalytic activities of the samples were determined by decolorization of methylene blue (MB) aqueous solutions under simulated visible radiation. The photocatalytic reaction was conducted in a 100 mL cylindrical glass vessel fixed in the XPA photochemical reactor (Nanjing Xujiang Machineelectronic Plant). The XPA reactor consists of a magnetic stirrer, quartz cool trap, and a condenser to keep the reaction temperature steady and to prevent the
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However, upon increasing the mole ratio of SnO2 in the composites, the characteristic diffraction peaks of tetragonal SnO2 are intensified accordingly. 3.2. Raman spectra measurement
Fig. 1. The XRD patterns of the SnO2/BiVO4 composites compared to those of pure BiVO4 and pure SnO2.
evaporation of water. A 500-W halogen lamp (Foshan Fengjiang Modulator Tube Plant) equipped with a 420 nm cutoff glass filter was used as visible light source in this study. MB solutions with an initial concentration (10 mg L 1) was used as reactant. In a typical run, 0.05 g of a catalyst was added to 100 mL of MB solution. Prior to illumination, the solutions were stirred for 30 min in the dark, in order to reach the adsorption–desorption equilibrium. At a given time intervals, a 5 mL suspension was collected and filtered (MICROPES 0.45 μm membrane filter) to remove the photocatalyst particles. The concentration of MB was then determined by measuring the absorbance at λmax ¼664 nm, using a UV–vis spectrophotometer (Shimadzu, Model UV-2450). 3. Results and discussion 3.1. XRD analysis of SnO2/BiVO4 composites Fig. 1 shows the XRD patterns of the SnO2/BiVO4 composites compared to those of pure BiVO4 and pure SnO2. By comparing the standard card of the Joint Committee Power, the characteristic peaks appearing in the XRD patterns of pure BiVO4 and pure SnO2 can be identified as peaks of monoclinic BiVO4 (JCPDS No. 75-1866) and tetragonal SnO2 (JCPDS No. 70-4177), suggesting that in each sample both BiVO4 and SnO2 solids have been formed with a good crystallinity. However, once the addition of SnO2 began in forming the composites, the monoclinic BiVO4 was accompanied by diffraction peaks assigned to tetragonal BiVO4 (JCPDS No. 14-0133). The diffraction peaks of tetragonal BiVO4 are gradually intensified as the concentration of SnO2 increased from 10% to 50%. However, those peaks disappeared when the concentration of SnO2 exceeded 50%, indicating that a certain amount of SnO2 in SnO2/BiVO4 composites may cause the existence of mixed crystal phase of BiVO4, in which both monoclinic and tetragonal structures are present. Focusing on the impact of the Sn structures, we see that when the mole ratio of SnO2 was 10%, there was no obvious diffraction peak corresponding to tetragonal SnO2.
Fig. 2 presents the Raman spectra of the pure BiVO4, pure SnO2 and SnO2/BiVO4 composite samples with various mole ratios in the 100–1500 cm 1 region. The peaks at 124.32, 210.89, 328.07, 366.54, 712.82 and 827.37 cm 1 correspond to the typical vibrations of pure BiVO4 that have been reported in previous studies [19,32]. The bands at 827.37 and 712.82 cm 1 are assigned to the typical symmetric and asymmetric stretching modes of V–O bonds, while the peaks at 328.07 and 366.54 cm 1 reflect the symmetric and 3 asymmetric bending vibrations of VO4 [19]. Other bands 1 located at 124.32 and 210.89 cm are attributed to external vibration modes [32]. In addition, the peaks at 631.53 and 775.78 cm 1(the left inset in Fig. 2) are associated with the pure SnO2 [33,34], whereas the peak at 576.41 cm 1 could be related to surface modes. Note that there is no obvious diffraction peak corresponding to SnO2 in any of the composite samples, due to the relatively weak Raman signal of tin oxide which is negligible compared with the strong Raman signal of bismuth vanadate. However, with the increase of SnO2 mole percentage from 10% to 90%, the intensity of the BiVO4 vibration bands did not decreased monotonically as expected but initially decreased and subsequently increased, which is contrary to earlier reports [19,32]. From the right inset in Fig. 2, it can be seen that both the pure BiVO4 and all of the SnO2/BiVO4 composites show a peak near 642.87 cm, 1 which may be attributed to the surface modes of BiVO4. Also, with the increased concentration of SnO2, differing amounts of red-shift can be observed which can be attributed to the overlapping of the peak of SnO2 at 631.5 cm 1, thereby providing further structural evidence of the existence of SnO2 in the SnO2/BiVO4 composites.
Fig. 2. Raman spectra of the pure BiVO4 and SnO2/BiVO4 composites with different mole ratios; Raman spectra of the pure SnO2 (the left inset); highlight of Sn–O responses for all samples (the right inset).
J. Yin et al. / Materials Science in Semiconductor Processing 34 (2015) 198–204
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Fig. 3. SEM micrographs of (a) pure BiVO4, (b) pure SnO2, and (c) S3B7 sample. TEM images of (d) pure BiVO4, (e) pure SnO2, and (f) (g) S3B7 sample.
3.3. SEM and TEM analysis The morphology and crystal structure of pure BiVO4, pure SnO2, and an S3B7 sample were obtained from SEM and TEM images. The SEM image in Fig. 3a shows that the pure BiVO4 was a sheet-shaped structure with an average width of 400– 600 nm and a thickness of 100 nm. The morphology of pure SnO2 (Fig. 3b) shows it to consist of fused spherical-shaped particles with a diameter in the range of 20–30 nm. Fig. 3c shows the presence of SnO2 nanoparticles deposited on the BiVO4 surface. Note that there were also some BiVO4 particles that were free of any SnO2.
The TEM images of Fig. 3d and e show the morphology of pure BiVO4 and pure SnO2, respectively. It can be seen that the shape of pure BiVO4 was plate-like with a width of 200–500 nm and the shape of pure SnO2 was cone and rod with a diameter about 5–20 nm, which agreed well the SEM image. As shown in Fig. 3f and g, two different crystal structures were observed on the composite surface, which can be assigned to BiVO4 and SnO2. Suggesting that the SnO2/BiVO4 composites were successfully prepared with SnO2 nanoparticles covering the BiVO4 surface. This phenomenon was also consistent with our observations of the SEM image in Fig. 3c.
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Fig. 4. UV–vis diffuse reflectance spectra of pure BiVO4, pure SnO2 and different mole ratios of SnO2/BiVO4. Fig. 5. (a) Typical XPS survey of the S3B7.
3.4. BET analysis The surface areas (SBET) of pure BiVO4, pure SnO2, and various prepared SnO2/BiVO4 composites are listed in Table 1. The SBET of pure BiVO4 and pure SnO2 were 0.3034 and 74.8489 m2 g 1, respectively. All the composite catalysts exhibited a larger BET surface area than the pure BiVO4. Furthermore, the composite catalysts showed a gradual increase in specific surface area with the increased mole ratio of SnO2. 3.5. UV–vis diffuse reflectance spectroscopy Fig. 4 shows the UV–vis diffuse reflectance spectra of BiVO4, SnO2, and a series of SnO2/BiVO4 composites. The absorption edges of the pure BiVO4 and SnO2 occurred at about 529 nm and 365 nm, respectively. The band-gap energies were estimated to be 2.34 eV and 3.40 eV for BiVO4 and SnO2, respectively, and were calculated using the equation [35] 1239:8 λg ¼ Eg
and Sn elements were detected in the sample. The observed peak of C1s at 284.6 eV is attributed to a the signal from carbon in the instrument [19,20,36]. (The other XPS spectrum figures are presented in the Supporting Information.) In the O 1s region (Fig. 5b), there are three types of oxygen energy levels existing in S3B7 composite. These can be attributed to lattice oxygen in metal oxides, such as SnO2 and BiVO4 at 529.68 eV, hydroxyl groups at 530.56 eV, and chemisorbed water at 532.10 eV [37]. The peaks at 158.97 eV and 164.27 eV are assigned to the Bi 4f7/2 and Bi 4f5/2 (Fig. 5c), respectively, confirming that the bismuth species in SnO2/BiVO4 is Bi3 þ [35,38]. The peaks at about 524.18 eV and 516.63 eV (Fig. 5d) correspond to V 2p1/2 and V 2p3/2, respectively [39]. The high resolution XPS spectra for Sn 3d (Fig. 5e) reveal the spin orbital splitting of Sn 3d5/2 (486.89 eV) and Sn 3d3/2 (495.33 eV) core level states were assigned to the lattice SnO2 [34,40]. 3.7. Photocatalytic activity for methylene blue (MB) degradation
ð1Þ
where λg and Eg represent constant, the band-gap wavelength and the band-gap energy, respectively. The bandgap energies of the SnO2/BiVO4 composites shown in Table 1 were calculated in the same way. It can be seen that the band-gap energies of all of the composites are lower than that of pure BiVO4 (2.34 eV) and pure SnO2 (3.40 eV). This observation indicates that the SnO2/BiVO4 composites can lead to higher photocatalytic performance due to the production of more electron–hole pairs under visible-light irradiation [32]. On the other hand, the absorbance in the 373–529 nm range decreased with the increase of the mole concentration of SnO2 in SnO2/BiVO4 composites. 3.6. XPS analysis The chemical states of the atoms in the S3B7 sample were investigated by XPS analysis, as shown in Fig. 5. According to the XPS observations (Fig. 5a), only C, Bi, O, V
The photocatalytic activities of the prepared samples were evaluated by their ability to degrade methylene blue (MB) under simulated visible light. Fig. 6 displays the change in MB concentration (C/C0) with visible-light irradiation time using different composite catalysts. As a comparison, direct photolysis of MB alone (blank), as well as the photocatalytic degradation with pure BiVO4 and SnO2, were performed under identical condition. Fig. 6 shows that the pure SnO2 strongly adsorbed MB during the dark time allowed for reaching adsorption– desorption equilibrium. As expected, the adsorption of MB on the surface of SnO2/BiVO4 composite catalysts was increased with the increasing mole content of SnO2. When the proportion of SnO2 in SnO2/BiVO4 composites increased from 10% to 90%, the concentration variation of MB caused by the initial adsorption of composites to MB in the dark time was also increased from 14% to more than 90%. Therefore, it can be seen that the molar concentration of SnO2 in SnO2/BiVO4 composite catalysts plays an important role in the concentration variation of MB during the
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Fig. 6. Comparison of photodegradation efficiency of MB using pure BiVO4, pure SnO2, and SnO2/BiVO4 composites with various mole ratios.
photocatalytic process. The SBET date from BET analysis also provides the strong support for this conclusion. As shown in Fig. 6, the blank experiment in the absence of photocatalyst demonstrates that there is only a minor decrease in MB concentration during the irradiation process alone. Given the concentration variation of MB resulted from the initial adsorption of composites, the decrease of MB concentration in the presence of the pure BiVO4 is actually about 53% after 120 min of irradiation, while the decrease of MB concentration in the presence of SnO2 alone was negligible due to the original strong adsorption of MB. Furthermore, it is obvious that the photocatalytic activity of the coupled SnO2–BiVO4 catalysts within 120 min of irradiation increases and reaches a maximum value and then quickly decreases as the proportion of SnO2 increases from 0% to 90%, suggesting that there is exists an optimal mole ratio for SnO2/BiVO4 composites. In fact, the mole ratio of SnO2/BiVO4 at 3:7 is found to have the highest photocatalytic activity, exhibiting the highest MB degradation of 72% was obtained after 120 min of irradiation. This enhancement in the photocatalytic performance for SnO2/BiVO4 composites can be attributed to the presence of the heterojunction structures at the interface of SnO2 and BiVO4, where the photogenerated electron–hole pairs can have improved charge separation efficiency and increased charge carrier lifetime that result in enhanced photocatalytic activity [17,19]. It is known that the surface area of a photocatalyst is an important factor that influences its photocatalytic performance. From the SBET date in Table 1, we see that the greater the proportion of SnO2 in SnO2/BiVO4 composites, the greater specific surface area. When the proportion of SnO2 in SnO2/BiVO4 composites surpasses the optimal ratio, the degradation rates of MB by the composites decreases significantly, even though the specific surface area of the composites increased simultaneously. It seems that, in higher SnO2-content composites, the excess SnO2 which has a low photoactivity in the visible region, may account for this result. Thus, the role of SBET in the enhancement of photocatalytic performance is limited. For the heterogeneous SnO2/BiVO4 composites, the conduction band (CB) and valence band (VB) of semiconductors
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Fig. 7. Possible photocatalytic mechanism for degradation of MB over SnO2/BiVO4 composite catalysts under simulated visible light irradiation.
at the point of zero charge can be estimated using the following equation [21,41] EVB ¼ X–Ee þ0:5Eg
ð2Þ
ECB ¼ EVB –Eg
ð3Þ
where EVB is the VB potential, X is the absolute electronegativity of the semiconductor (X is 6.04 eV [41] and 6.52 eV [42] for BiVO4 and SnO2, respectively), Ee is the energy of free electrons on the hydrogen scale (4.5 eV), and Eg is the band gap energy of the semiconductor. The band gap values of SnO2 and BiVO4 are 3.40 eV and 2.34 eV, respectively. From the above equation, the calculated results were EVB (SnO2)¼3.72 eV, ECB (SnO2)¼0.32 eV and EVB (BiVO4)¼2.71 eV, ECB (BiVO4)¼0.27 eV. Fig. 7 is a schematic diagram of energy band structures of SnO2/BiVO4, which is similar to that of the well-studied SnO2/SnS2 [43] and In2O3/TiO2 [44] composites. According to Fig. 7, when the SnO2/BiVO4 composites were irradiated by visible light (λ 4420 nm), SnO2 had no response ability due to its wide band gap. However, the electrons in the VB of BiVO4 were excited to its CB, leading to the generation of holes in its VB. The CB of SnO2 was more positive than that of BiVO4, resulting in a local electric field [19–21,45]. As a consequence, the excited electrons tend to transfer from the CB of BiVO4 to the CB of SnO2 at the interface, whereas the generated holes remained in the VB of BiVO4. Thus, the photo-induced charge carriers in BiVO4 can be effectively separated and their recombination inhibited [43]. The efficient charge separation can enhance the lifetime of the charge carriers and improve the efficiency of the interfacial charge transfer to adsorbed substrates. Therefore, it can be concluded that the SnO2/BiVO4 composites with appropriate contents of SnO2 (such as S3B7) can demonstrate higher photocatalytic efficiencies than BiVO4 nanoparticles by themselves. On the other hand, the poorer photocatalytic activity of SnO2/BiVO4 composites (such as S5B5, S7B3, and S9B1) may be explained by the following aspects. First, SnO2 itself has no photocatalytic activity under visible light irradiation due to its wide band gap energy. The higher mole ratio of SnO2 means a lower molarity of the photocatalytically
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active BiVO4 in the composite. The lower amounts of the photocatalytic active BiVO4 involved in the photocatalytic processes would then bring about lower photocatalytic efficiencies. Second, a significant amount of the SnO2 and BiVO4 nanoparticles in SnO2/BiVO4 composite were not attached to each other (Fig. 3c) which would eliminate the interfacial electron transfer. Third, excessive amounts of SnO2 in the SnO2/BiVO4 composites have a large negative impact on the photocatalytic process of MB, because the material possess a large specific surface area and strong adsorption capacity for MB, which would reduce the contact of BiVO4 with MB. In addition, the presence of so much MB to the SnO2 could prevent the visible light from reaching the surface of BiVO4. 4. Conclusion Novel SnO2/BiVO4 heterojunction photocatalysts with different mole ratios have been successfully synthesized via a simple hydrothermal method. Through the photocatalytic experiments using aqueous MB as a target contaminant under simulated visible light irradiation using a 500 W halogen tungsten lamp, it was found that: (1) pure SnO2 has a very strong adsorption capacity for MB; (2) the mole ratio of SnO2 to BiVO4 in the SnO2/BiVO4 composites affects the degradation of MB under simulated visible light irradiation; and (3) the mole ratio of SnO2/BiVO4 at 3:7 exhibits the highest photocatalytic activity for degradation MB. According to the relevant characterization results and energy band positions of SnO2/BiVO4 composites, it may be concluded that the enhancement of photocatalytic performance of SnO2/BiVO4 composites (S3B7), compared with that of pure BiVO4, is mainly attributable to the formed heterogeneous structures at the interface of SnO2 and BiVO4 and the lower band gap energy of SnO2/BiVO4 composites for pure BiVO4 also can enhanced its photocatalytic activity. Acknowledgements This research was financially supported by National Natural Science Foundations of China (Grant nos. 51378217, U1360101), Research Project of Guangdong Provincial Department of Science and Technology (Grant no.2012A010800006), and Guangdong Natural Science Foundation (Grant no. S2012020010887). Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j. mssp.2015.02.044.
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Contents lists available at ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Fabrication of heterojunction SnO2/BiVO4 composites having enhanced visible light photocatalystic activity Jiazhi Yin a,c, Shaobin Huang a,b,c,n, Zicong Jian a,c, Zhixin Wang a,c, Yongqing Zhang a,b a b c
School of Environment and Energy, South China University of Technology, Guangzhou 510006, China State Key Laboratory of Pulp and Paper Engineering, South China University of Technology, Guangzhou 510640, China Guangdong Provincial Key Laboratory of Atmospheric Environment and Pollution Control, Guangzhou 510006, China
a r t i c l e in f o
Keywords: SnO2 BiVO4 Degradation Heterojunction Photocatalytic performance Visible light
abstract SnO2/BiVO4 heterojunction composite photocatalysts with various mole ratios have been prepared via a simple hydrothermal method. The structure, composition and optical properties of the SnO2/BiVO4 composites were determined by X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), Brunauer–Emmett–Teller (BET) surface analysis, X-ray photoelectron spectroscopy (XPS) and UV–vis diffuse reflectance spectroscopy (UV–vis DRS). Photocatalytic activities of the composites were evaluated by studying the degradation of methylene blue (MB) solutions under simulated visible light irradiation (500 W halogen tungsten lamp). The 3:7 mol ratio SnO2/BiVO4 composite exhibited the highest photocatalytic performance, leading to 72% decompositon of MB within 120 min of irradiation. & 2015 Elsevier Ltd. All rights reserved.
1. Introduction In the past few decades, photocatalytic technology has been applied in degradation and decomposition of poisonous and noxious organic compounds and photocatalytic water splitting product hydrogen [1–5]. Titanium dioxid (TiO2) is the most widely used semiconductor photocatalytic material due to its high chemical stability, low cost and low toxicity [6]. However, the practical use of TiO2 is limited by its wide band gap (3.2 eV) which means that it responds only to ultraviolet light that covers 4% of the solar energy spectrum [7–9]. Therefore, there have been many efforts aimed at the development of a visible-lightdriven catalyst in order to effectively utilize the natural solar energy. n Corresponding author at: School of Environment and Energy, South China University of Technology, Guangzhou 510006, China. E-mail address: [email protected] (S. Huang).
http://dx.doi.org/10.1016/j.mssp.2015.02.044 1369-8001/& 2015 Elsevier Ltd. All rights reserved.
Recently, monoclinic bismuth vanadate (m-BiVO4) with a narrow band gap of 2.4 eV has caught considerable attention as a promising a visible-light-driven photocatalyst material and has shown significant visible-light activity for organic photocatalytic degradation [10,11]. On the other hand, the low efficiency of the separation of electron–hole pairs in m-BiVO4 limits its application in the visible region. However, by coupling m-BiVO4 with another semiconductor with appropriate band potentials to form a heterojunction is an effective approach to improving the charge separation, increase the charge carrier lifetime, and thus enhanced photocatalytic activity [12,13]. Moreover, due to the high efficiency of the interfacial charge transfer from catalyst to absorbed substrate, a rapid re-dox process can be obtained easily [14–16]. As a result, photocatalytic properties of BiVO4-based composite materials have been extensively explored. Ji et al. [17] investigated the catalytic performance of C3N4/BiVO4 heterojunction photocatalyst and found that the composite catalysts showed remarkably
J. Yin et al. / Materials Science in Semiconductor Processing 34 (2015) 198–204
Table 1 Surface area and energy gaps of pure BiVO4, pure SnO2 and SnO2/BiVO4 composites with different mole ratios. 2
Sample
Sn:Bi mole ratio
SBET (m g
BiVO4 SnO2 S1B9 S3B7 S5B5 S7B3 S9B1
– – 1:9 3:7 5:5 7:3 9:1
0.30 74.85 0.49 7.60 21.63 37.08 62.24
1
)
Eg (eV) 2.34 3.40 2.28 2.28 2.32 2.29 2.33
improved visible-induced photocatalytic activities in degrading RhB compared with the C3N4 or BiVO4 alone. The increased photocatalytic activity of this composite catalyst was ascribed to the stability of the heterostructure and the energy level match that facilitated the charge separation and transport. Photocatalytic degradation of benzene by BiVO4/TiO2 heterostructure catalyst under visible-light irradiation was also evaluated by Hu’s group [18]. The composition that gave the largest benzene conversion efficiency was BiVO4/TiO2 with the mass ratio of 1:200. Su and Zou [19] synthesized macroporous V2O5/BiVO4 composites with a heterojunction structure using colloidal carbon spheres. The formation of heterojunction structures in the V2O5/ BiVO4 composite plays an important role in the separation, transport and recombination of photogenerated charges by greatly increasing the physical separation and the lifetime of the photogenerated charges formed. He et al. [20] studied the photocatalytic degradation of methyl orange (MO) over a BiOCl/BiVO4 heterojunction composite prepared using a hydrothermal method. The composite markedly improved the efficiency of MO photodegradation compared to results obtained with pure BiVO4, BiOCl, and Degussa P25. The highest activity of BiOCl/BiVO4 heterojunction was obtained in the composite of 13 mol% BiOCl and 87 mol% BiVO4. In related studies, CuO/BiVO4 [21–23], WO3/BiVO4 [24,25], Co3O4/BiVO4 [26–28] and SiO2/BiVO4 [29] heterojunction composites have also been reported to have higher photocatalytic activity than that of BiVO4 alone due to the enhanced efficiency separation of photogenerated electron–hole pairs. These studies show that the improvement in the photosensitivity results from a coupling effect between the two components in the composites that increases the electron transfer and inhibits the recombination of photo-generated charge carriers [30]. In this study, we have successfully developed a series of highly photoactive SnO2/BiVO4 heterojunction catalysts using a simple hydrothermal method. To the best of our knowledge, the photocatalytic activity of SnO2/BiVO4 composites has not been previously reported. The aim of this study is to explore the photocatalytic efficiency of SnO2/ BiVO4 heterojunction composite catalysts with the intent of identifying a promising visible-light driven catalyst. A mechanism is also proposed that leads to improved separation of electrons and holes based on energy band position as well as related characteristics.
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2. Experimental section 2.1. Sample synthesis All chemicals applied in experiment were analytical grade and used without further purification. Pure SnO2 nanoparticles were prepared by a hydrothermal method [31] using tin chloride pentahydrate (SnCl4 5H2O) as a tin precursor. The synthesized SnO2 powders were subsequently added to the solution of bismuth nitrate pentahydrate (Bi(NO3)3 5H2O) in 4 M nitric acid (HNO3) and ammonium vanadate (NH4VO3) in 4 M sodium hydroxide (NaOH) (1:1 mol ratio) to form suspensions with different BiVO4:SnO2 mole ratios. The pH of the mixtures was then adjusted to 7 by slowly adding 1 M NaOH solution. After that, the suspension was transferred to a stainless autoclave where the hydrothermal reaction was carried out at 150 1C and maintained at that temperature for 24 h. Finally, the SnO2/BiVO4 composites were collected by filtration and dried at 80 1C for 24 h. As a control, pure BiVO4 photocatalyst was also prepared by the same procedure. As shown Table 1, all of the composite samples were labeled as SxBy in which S and B stand for SnO2 and BiVO4, x and y stand for the respective molar amounts in the composites. 2.2. Material characterization Crystalline phases of the composites were subjected to X-ray diffraction (XRD) analysis (Bruker D8 ADVANCE). The diffraction patterns were recorded in the range of 2θ¼101–801 using Cu Kα radiation (λ¼0.15406 nm). Raman spectra were obtained on a HORIBA JY LabRAM HR Evolution instrument with a 532 nm Nd-YVO4 laser as the excitation source. The morphology and particle size of the samples were examined by scanning electronmicroscopy (SEM) (Model ZEISS, MERLIN Compact). The microstructures of the samples were investigated by transmission electronmicroscopy (TEM) (JEOL, Model JEOL-2100). The specific surface areas of the samples were measured by a Brunauer–Emmett–Teller specific surface area instrument (American Micromeritics Instrument Corporation ASAP 2020N). The UV–vis diffuse reflectance spectra were recorded on a UV–vis spectrophotometer (Shimadzu, Model UV2450) in the wavelength range of 200–800 nm and BaSO4 was used as the reference. X-ray photoelectron spectroscopy (XPS) measurements were take on a British Kratos Axis Ultra DLD system with Al Kα radiation as the excitation source, where the binding energies were calibrated by referencing the C 1 s peak (284.6 eV). 2.3. Photocatalytic activity studies Photocatalytic activities of the samples were determined by decolorization of methylene blue (MB) aqueous solutions under simulated visible radiation. The photocatalytic reaction was conducted in a 100 mL cylindrical glass vessel fixed in the XPA photochemical reactor (Nanjing Xujiang Machineelectronic Plant). The XPA reactor consists of a magnetic stirrer, quartz cool trap, and a condenser to keep the reaction temperature steady and to prevent the
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However, upon increasing the mole ratio of SnO2 in the composites, the characteristic diffraction peaks of tetragonal SnO2 are intensified accordingly. 3.2. Raman spectra measurement
Fig. 1. The XRD patterns of the SnO2/BiVO4 composites compared to those of pure BiVO4 and pure SnO2.
evaporation of water. A 500-W halogen lamp (Foshan Fengjiang Modulator Tube Plant) equipped with a 420 nm cutoff glass filter was used as visible light source in this study. MB solutions with an initial concentration (10 mg L 1) was used as reactant. In a typical run, 0.05 g of a catalyst was added to 100 mL of MB solution. Prior to illumination, the solutions were stirred for 30 min in the dark, in order to reach the adsorption–desorption equilibrium. At a given time intervals, a 5 mL suspension was collected and filtered (MICROPES 0.45 μm membrane filter) to remove the photocatalyst particles. The concentration of MB was then determined by measuring the absorbance at λmax ¼664 nm, using a UV–vis spectrophotometer (Shimadzu, Model UV-2450). 3. Results and discussion 3.1. XRD analysis of SnO2/BiVO4 composites Fig. 1 shows the XRD patterns of the SnO2/BiVO4 composites compared to those of pure BiVO4 and pure SnO2. By comparing the standard card of the Joint Committee Power, the characteristic peaks appearing in the XRD patterns of pure BiVO4 and pure SnO2 can be identified as peaks of monoclinic BiVO4 (JCPDS No. 75-1866) and tetragonal SnO2 (JCPDS No. 70-4177), suggesting that in each sample both BiVO4 and SnO2 solids have been formed with a good crystallinity. However, once the addition of SnO2 began in forming the composites, the monoclinic BiVO4 was accompanied by diffraction peaks assigned to tetragonal BiVO4 (JCPDS No. 14-0133). The diffraction peaks of tetragonal BiVO4 are gradually intensified as the concentration of SnO2 increased from 10% to 50%. However, those peaks disappeared when the concentration of SnO2 exceeded 50%, indicating that a certain amount of SnO2 in SnO2/BiVO4 composites may cause the existence of mixed crystal phase of BiVO4, in which both monoclinic and tetragonal structures are present. Focusing on the impact of the Sn structures, we see that when the mole ratio of SnO2 was 10%, there was no obvious diffraction peak corresponding to tetragonal SnO2.
Fig. 2 presents the Raman spectra of the pure BiVO4, pure SnO2 and SnO2/BiVO4 composite samples with various mole ratios in the 100–1500 cm 1 region. The peaks at 124.32, 210.89, 328.07, 366.54, 712.82 and 827.37 cm 1 correspond to the typical vibrations of pure BiVO4 that have been reported in previous studies [19,32]. The bands at 827.37 and 712.82 cm 1 are assigned to the typical symmetric and asymmetric stretching modes of V–O bonds, while the peaks at 328.07 and 366.54 cm 1 reflect the symmetric and 3 asymmetric bending vibrations of VO4 [19]. Other bands 1 located at 124.32 and 210.89 cm are attributed to external vibration modes [32]. In addition, the peaks at 631.53 and 775.78 cm 1(the left inset in Fig. 2) are associated with the pure SnO2 [33,34], whereas the peak at 576.41 cm 1 could be related to surface modes. Note that there is no obvious diffraction peak corresponding to SnO2 in any of the composite samples, due to the relatively weak Raman signal of tin oxide which is negligible compared with the strong Raman signal of bismuth vanadate. However, with the increase of SnO2 mole percentage from 10% to 90%, the intensity of the BiVO4 vibration bands did not decreased monotonically as expected but initially decreased and subsequently increased, which is contrary to earlier reports [19,32]. From the right inset in Fig. 2, it can be seen that both the pure BiVO4 and all of the SnO2/BiVO4 composites show a peak near 642.87 cm, 1 which may be attributed to the surface modes of BiVO4. Also, with the increased concentration of SnO2, differing amounts of red-shift can be observed which can be attributed to the overlapping of the peak of SnO2 at 631.5 cm 1, thereby providing further structural evidence of the existence of SnO2 in the SnO2/BiVO4 composites.
Fig. 2. Raman spectra of the pure BiVO4 and SnO2/BiVO4 composites with different mole ratios; Raman spectra of the pure SnO2 (the left inset); highlight of Sn–O responses for all samples (the right inset).
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Fig. 3. SEM micrographs of (a) pure BiVO4, (b) pure SnO2, and (c) S3B7 sample. TEM images of (d) pure BiVO4, (e) pure SnO2, and (f) (g) S3B7 sample.
3.3. SEM and TEM analysis The morphology and crystal structure of pure BiVO4, pure SnO2, and an S3B7 sample were obtained from SEM and TEM images. The SEM image in Fig. 3a shows that the pure BiVO4 was a sheet-shaped structure with an average width of 400– 600 nm and a thickness of 100 nm. The morphology of pure SnO2 (Fig. 3b) shows it to consist of fused spherical-shaped particles with a diameter in the range of 20–30 nm. Fig. 3c shows the presence of SnO2 nanoparticles deposited on the BiVO4 surface. Note that there were also some BiVO4 particles that were free of any SnO2.
The TEM images of Fig. 3d and e show the morphology of pure BiVO4 and pure SnO2, respectively. It can be seen that the shape of pure BiVO4 was plate-like with a width of 200–500 nm and the shape of pure SnO2 was cone and rod with a diameter about 5–20 nm, which agreed well the SEM image. As shown in Fig. 3f and g, two different crystal structures were observed on the composite surface, which can be assigned to BiVO4 and SnO2. Suggesting that the SnO2/BiVO4 composites were successfully prepared with SnO2 nanoparticles covering the BiVO4 surface. This phenomenon was also consistent with our observations of the SEM image in Fig. 3c.
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Fig. 4. UV–vis diffuse reflectance spectra of pure BiVO4, pure SnO2 and different mole ratios of SnO2/BiVO4. Fig. 5. (a) Typical XPS survey of the S3B7.
3.4. BET analysis The surface areas (SBET) of pure BiVO4, pure SnO2, and various prepared SnO2/BiVO4 composites are listed in Table 1. The SBET of pure BiVO4 and pure SnO2 were 0.3034 and 74.8489 m2 g 1, respectively. All the composite catalysts exhibited a larger BET surface area than the pure BiVO4. Furthermore, the composite catalysts showed a gradual increase in specific surface area with the increased mole ratio of SnO2. 3.5. UV–vis diffuse reflectance spectroscopy Fig. 4 shows the UV–vis diffuse reflectance spectra of BiVO4, SnO2, and a series of SnO2/BiVO4 composites. The absorption edges of the pure BiVO4 and SnO2 occurred at about 529 nm and 365 nm, respectively. The band-gap energies were estimated to be 2.34 eV and 3.40 eV for BiVO4 and SnO2, respectively, and were calculated using the equation [35] 1239:8 λg ¼ Eg
and Sn elements were detected in the sample. The observed peak of C1s at 284.6 eV is attributed to a the signal from carbon in the instrument [19,20,36]. (The other XPS spectrum figures are presented in the Supporting Information.) In the O 1s region (Fig. 5b), there are three types of oxygen energy levels existing in S3B7 composite. These can be attributed to lattice oxygen in metal oxides, such as SnO2 and BiVO4 at 529.68 eV, hydroxyl groups at 530.56 eV, and chemisorbed water at 532.10 eV [37]. The peaks at 158.97 eV and 164.27 eV are assigned to the Bi 4f7/2 and Bi 4f5/2 (Fig. 5c), respectively, confirming that the bismuth species in SnO2/BiVO4 is Bi3 þ [35,38]. The peaks at about 524.18 eV and 516.63 eV (Fig. 5d) correspond to V 2p1/2 and V 2p3/2, respectively [39]. The high resolution XPS spectra for Sn 3d (Fig. 5e) reveal the spin orbital splitting of Sn 3d5/2 (486.89 eV) and Sn 3d3/2 (495.33 eV) core level states were assigned to the lattice SnO2 [34,40]. 3.7. Photocatalytic activity for methylene blue (MB) degradation
ð1Þ
where λg and Eg represent constant, the band-gap wavelength and the band-gap energy, respectively. The bandgap energies of the SnO2/BiVO4 composites shown in Table 1 were calculated in the same way. It can be seen that the band-gap energies of all of the composites are lower than that of pure BiVO4 (2.34 eV) and pure SnO2 (3.40 eV). This observation indicates that the SnO2/BiVO4 composites can lead to higher photocatalytic performance due to the production of more electron–hole pairs under visible-light irradiation [32]. On the other hand, the absorbance in the 373–529 nm range decreased with the increase of the mole concentration of SnO2 in SnO2/BiVO4 composites. 3.6. XPS analysis The chemical states of the atoms in the S3B7 sample were investigated by XPS analysis, as shown in Fig. 5. According to the XPS observations (Fig. 5a), only C, Bi, O, V
The photocatalytic activities of the prepared samples were evaluated by their ability to degrade methylene blue (MB) under simulated visible light. Fig. 6 displays the change in MB concentration (C/C0) with visible-light irradiation time using different composite catalysts. As a comparison, direct photolysis of MB alone (blank), as well as the photocatalytic degradation with pure BiVO4 and SnO2, were performed under identical condition. Fig. 6 shows that the pure SnO2 strongly adsorbed MB during the dark time allowed for reaching adsorption– desorption equilibrium. As expected, the adsorption of MB on the surface of SnO2/BiVO4 composite catalysts was increased with the increasing mole content of SnO2. When the proportion of SnO2 in SnO2/BiVO4 composites increased from 10% to 90%, the concentration variation of MB caused by the initial adsorption of composites to MB in the dark time was also increased from 14% to more than 90%. Therefore, it can be seen that the molar concentration of SnO2 in SnO2/BiVO4 composite catalysts plays an important role in the concentration variation of MB during the
J. Yin et al. / Materials Science in Semiconductor Processing 34 (2015) 198–204
Fig. 6. Comparison of photodegradation efficiency of MB using pure BiVO4, pure SnO2, and SnO2/BiVO4 composites with various mole ratios.
photocatalytic process. The SBET date from BET analysis also provides the strong support for this conclusion. As shown in Fig. 6, the blank experiment in the absence of photocatalyst demonstrates that there is only a minor decrease in MB concentration during the irradiation process alone. Given the concentration variation of MB resulted from the initial adsorption of composites, the decrease of MB concentration in the presence of the pure BiVO4 is actually about 53% after 120 min of irradiation, while the decrease of MB concentration in the presence of SnO2 alone was negligible due to the original strong adsorption of MB. Furthermore, it is obvious that the photocatalytic activity of the coupled SnO2–BiVO4 catalysts within 120 min of irradiation increases and reaches a maximum value and then quickly decreases as the proportion of SnO2 increases from 0% to 90%, suggesting that there is exists an optimal mole ratio for SnO2/BiVO4 composites. In fact, the mole ratio of SnO2/BiVO4 at 3:7 is found to have the highest photocatalytic activity, exhibiting the highest MB degradation of 72% was obtained after 120 min of irradiation. This enhancement in the photocatalytic performance for SnO2/BiVO4 composites can be attributed to the presence of the heterojunction structures at the interface of SnO2 and BiVO4, where the photogenerated electron–hole pairs can have improved charge separation efficiency and increased charge carrier lifetime that result in enhanced photocatalytic activity [17,19]. It is known that the surface area of a photocatalyst is an important factor that influences its photocatalytic performance. From the SBET date in Table 1, we see that the greater the proportion of SnO2 in SnO2/BiVO4 composites, the greater specific surface area. When the proportion of SnO2 in SnO2/BiVO4 composites surpasses the optimal ratio, the degradation rates of MB by the composites decreases significantly, even though the specific surface area of the composites increased simultaneously. It seems that, in higher SnO2-content composites, the excess SnO2 which has a low photoactivity in the visible region, may account for this result. Thus, the role of SBET in the enhancement of photocatalytic performance is limited. For the heterogeneous SnO2/BiVO4 composites, the conduction band (CB) and valence band (VB) of semiconductors
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Fig. 7. Possible photocatalytic mechanism for degradation of MB over SnO2/BiVO4 composite catalysts under simulated visible light irradiation.
at the point of zero charge can be estimated using the following equation [21,41] EVB ¼ X–Ee þ0:5Eg
ð2Þ
ECB ¼ EVB –Eg
ð3Þ
where EVB is the VB potential, X is the absolute electronegativity of the semiconductor (X is 6.04 eV [41] and 6.52 eV [42] for BiVO4 and SnO2, respectively), Ee is the energy of free electrons on the hydrogen scale (4.5 eV), and Eg is the band gap energy of the semiconductor. The band gap values of SnO2 and BiVO4 are 3.40 eV and 2.34 eV, respectively. From the above equation, the calculated results were EVB (SnO2)¼3.72 eV, ECB (SnO2)¼0.32 eV and EVB (BiVO4)¼2.71 eV, ECB (BiVO4)¼0.27 eV. Fig. 7 is a schematic diagram of energy band structures of SnO2/BiVO4, which is similar to that of the well-studied SnO2/SnS2 [43] and In2O3/TiO2 [44] composites. According to Fig. 7, when the SnO2/BiVO4 composites were irradiated by visible light (λ 4420 nm), SnO2 had no response ability due to its wide band gap. However, the electrons in the VB of BiVO4 were excited to its CB, leading to the generation of holes in its VB. The CB of SnO2 was more positive than that of BiVO4, resulting in a local electric field [19–21,45]. As a consequence, the excited electrons tend to transfer from the CB of BiVO4 to the CB of SnO2 at the interface, whereas the generated holes remained in the VB of BiVO4. Thus, the photo-induced charge carriers in BiVO4 can be effectively separated and their recombination inhibited [43]. The efficient charge separation can enhance the lifetime of the charge carriers and improve the efficiency of the interfacial charge transfer to adsorbed substrates. Therefore, it can be concluded that the SnO2/BiVO4 composites with appropriate contents of SnO2 (such as S3B7) can demonstrate higher photocatalytic efficiencies than BiVO4 nanoparticles by themselves. On the other hand, the poorer photocatalytic activity of SnO2/BiVO4 composites (such as S5B5, S7B3, and S9B1) may be explained by the following aspects. First, SnO2 itself has no photocatalytic activity under visible light irradiation due to its wide band gap energy. The higher mole ratio of SnO2 means a lower molarity of the photocatalytically
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active BiVO4 in the composite. The lower amounts of the photocatalytic active BiVO4 involved in the photocatalytic processes would then bring about lower photocatalytic efficiencies. Second, a significant amount of the SnO2 and BiVO4 nanoparticles in SnO2/BiVO4 composite were not attached to each other (Fig. 3c) which would eliminate the interfacial electron transfer. Third, excessive amounts of SnO2 in the SnO2/BiVO4 composites have a large negative impact on the photocatalytic process of MB, because the material possess a large specific surface area and strong adsorption capacity for MB, which would reduce the contact of BiVO4 with MB. In addition, the presence of so much MB to the SnO2 could prevent the visible light from reaching the surface of BiVO4. 4. Conclusion Novel SnO2/BiVO4 heterojunction photocatalysts with different mole ratios have been successfully synthesized via a simple hydrothermal method. Through the photocatalytic experiments using aqueous MB as a target contaminant under simulated visible light irradiation using a 500 W halogen tungsten lamp, it was found that: (1) pure SnO2 has a very strong adsorption capacity for MB; (2) the mole ratio of SnO2 to BiVO4 in the SnO2/BiVO4 composites affects the degradation of MB under simulated visible light irradiation; and (3) the mole ratio of SnO2/BiVO4 at 3:7 exhibits the highest photocatalytic activity for degradation MB. According to the relevant characterization results and energy band positions of SnO2/BiVO4 composites, it may be concluded that the enhancement of photocatalytic performance of SnO2/BiVO4 composites (S3B7), compared with that of pure BiVO4, is mainly attributable to the formed heterogeneous structures at the interface of SnO2 and BiVO4 and the lower band gap energy of SnO2/BiVO4 composites for pure BiVO4 also can enhanced its photocatalytic activity. Acknowledgements This research was financially supported by National Natural Science Foundations of China (Grant nos. 51378217, U1360101), Research Project of Guangdong Provincial Department of Science and Technology (Grant no.2012A010800006), and Guangdong Natural Science Foundation (Grant no. S2012020010887). Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j. mssp.2015.02.044.
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