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Self-assembly and enhanced visible-light-driven photocatalytic activities of Bi2MoO6 by tungsten substitution
Applied Surface Science 265 (2013) 424–430
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Self-assembly and enhanced visible-light-driven photocatalytic activities of Bi2 MoO6 by tungsten substitution Hongguang Yu a,∗ , Zhenfeng Zhu a , Jianhong Zhou b , Jing Wang c , Junqi Li a , Yanli Zhang a a b c
School of Materials Science and Engineering, Shaanxi University of Science and Technology, Xi’an 710021, PR China Department of Physics and Information Technology, Baoji University of Arts and Sciences, Baoji 721007, PR China Deparment of Materials Science and Engineering, The University of Sheffield, Sir Robert Hadfield Building, Mappin Street, Sheefield S1 3JD, UK
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Article history: Received 21 February 2012 Received in revised form 3 November 2012 Accepted 5 November 2012 Available online 14 November 2012 Keywords: Bi2 MoO6 Tungsten substitution Photocatalysis Nanosheets Hydrothermal method
a b s t r a c t The different compositions of Bi2 MoO6 by tungsten substitution have been successfully synthesized via a facile hydrothermal process in the absence of a surfactant. All W-doped Bi2 MoO6 samples were composed of nanosheets with similar orthorhombic Aurivillius layered structures. Bi2 MoO6 samples show different phases varied with W content. For the Bi2 MoO6 samples with the different nw values, their band gaps have changed obviously compared with Bi2 MoO6 (2.69 eV) and Bi2 WO6 (2.75 eV). The Bi2 MoO6 by tungsten substitution exhibited good photocatalytic activity in degradation of Rhodamine-B under 500 W Xe lamp light irradiation. When nw value is 0.5, the sample has the highest photocatalytic activity for RhB photodecomposition under visible light irradiation. It shows that photocatalytic activities of Bi2 MoO6 samples by W substitution are relevant to nanosheet morphology and size, the intrinsic layered structure, band gap and the W content. © 2012 Elsevier B.V. All rights reserved.
1. Introduction With the rising level of air and water pollution, researchers are giving more and more attention to environmentally friendly materials, especially photodegradation catalysts. Since the discovery of the photocatalytic splitting of water on the TiO2 electrodes by Fujishima and Honda in 1972 [1], the applications of semiconductor photocatalysts on solar energy conversion and degradation of pollution have received great attention [2]. Because of the good chemical stability, high oxidized activity, nontoxicity and low price, TiO2 has been the most popular photocatalyst for environmental purification [3,4]. Although TiO2 and TiO2 -based materials are the most popular photocatalysts, the band gap of the TiO2 is 3.2 eV and it absorbs only the ultraviolet light ( < 400 nm) which lowers the efficiency of using solar energy and accounts for about 4% of the sunlight [5–11]. In the past decades, single semiconducting photocatalysts have been used, but they are restricted by their structure, morphology and immanence property. Thus the worldwide interest of heterogeneous semiconducting photocatalysts on transforming solar energy into chemical energy and decomposing organic contaminants are taken [12–15]. To improve the efficiency of solar energy
∗ Corresponding author. Tel.: +86 029 86168820; fax: +86 029 86177108. E-mail addresses: [email protected] (H. Yu), [email protected] (J. Zhou), [email protected] (J. Wang). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.11.023
utilization, researchers have been exploring alternative materials for visible-light-responsive photocatalysts such as Bi2 MoO6 and Bi2 WO6 [16–28,2,29], which can be used as excellent photocatalysts and a solar energy transfer materials. Bi2 MoO6 and Bi2 WO6 are two of the simplest members of the Aurivillius oxide family [24]. Compared with Bi2 WO6 , Bi2 MoO6 has a narrower band gap (for example, 2.77 eV for Bi2 WO6 and 2.63 eV for Bi2 MoO6 in Zhou et al.’s report [25]) and thus the ability to harness more sunlight. However, its photocatalytic activity is not as high as that of Bi2 WO6 , so it has attracted less attention. Fortunately, it is possible to integrate the advantages of both high photocatalytic activity of Bi2 WO6 and narrower band gap of Bi2 MoO6 into one compound, W-doped Bi2 MoO6 samples, as it is reasonable to postulate that the properties of both will favorably mix [26]. Additionally, it is possible to improve the photocatalytic properties by utilizing the multiple scattering within the complex to increase the optical path length through the crystals. Moreover, there have been few reports concerning the photocatalytic properties and morphology of Bi2 MoO6 samples by W substitution until now [30–32]. In the present work, we demonstrate the W-doped Bi2 MoO6 samples of the different nw values (nw = 0, 0.25, 0.5, 0.75, 1.0) synthesized by a facile hydrothermal treatment method without a surfactant, and the visible light ( > 400 nm) photocatalytic activities have also been investigated. Our studies show that the introduction of W into Bi2 MoO6 lattices affects not only the morphology but also the band gap as well as yield interesting shaped-associated visible-light-driven photocatalytic activities.
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Fig. 1. XRD pattern of the Bi2 MoO6 samples by different concentrations of tungsten substitution, and the JCPDS cards of Bi2 MoO6 and Bi2 WO6 are also given as comparison. (a) nw = 0, (b) nw = 0.25, (c) nw = 0.5, (d) nw = 0. 75, and (e) nw = 1.
Our approach provides a facile process to tune both nanostructure and band gap of photocatalysts by simply adjusting the additive concentration, leading to the enhanced visible-light-driven photocatalytic activity. Moreover, our work may be further extended to the design and synthesis of novel and highly efficient visible-lightdriven semiconductors for environmental remediation. 2. Experimental 2.1. Synthesis of Bi2 MoO6 samples by W substitution All chemicals were analytical grade and used as raw materials without further purification, and deionized water was used throughout the experimental process. First, 2 mmol of Bi(NO3 )3 ·5H2 O was mixed with a 40 mL aqueous solution containing 1 mmol of citric acid. After 10 min of magnetic stirring (1 mmol, 0.75 mmol, 0.5 mmol, 0.25 mmol, 0 mmol separately) (NH4 )6 Mo7 O24 ·H2 O and (2 mmol) NaHCO3 were gradually and respectively added into the above mixed solution. And then nw (nw = 0, 0.25, 0.5, 0.75, 1.0) mmol Na2 WO4 ·2H2 O were further mixture with the mixture and sufficiently stirred for another 10 min. When the suspension was transferred into a stainless steel autoclave with a 70 ml Teflon liner, then heated at 200 ◦ C for 24 h and cooled to room temperature, the products were separated centrifugally and washed three times with deionized water and absolute ethanol, respectively. Finally, the final products were obtained by drying under vacuum at 60 ◦ C for 20 h. 2.2. Characterization Powder X-ray diffraction (XRD) was carried out with a D/max2200 X-ray diffractometer (Rigaku, Japan) using Cu K␣ radiation ( = 0.154 nm) in the 2 range of 15–70◦ . Field emission scanning electron microscopy (FE-SEM) images were taken on a JEOL JSM-6700F scanning electron microscope (Japan), and transmission electron microscopy (TEM) observation and selected area electron diffraction (SAED) patterns were performed with a JEM-3010 highresolution transmission electron microscope (HRTEM, Japan), using an accelerating voltage of 200 kV. The optical diffuse reflectance spectra were recorded on a Japan Shimadzu UV-2550 (Japan) using BaSO4 as reference.
Fig. 2. (a) XRD pattern of Bi2 MoO6 samples when nw value is 0.5, and the JCPDS cards of Bi2 MoO6 and Bi2 WO6 . (b) Comparison of 2 peaks from 26◦ to 30◦ for Bi2 MoO6 samples by 0.5 mmol W substitution, the JCPDS cards of Bi2 MoO6 and Bi2 WO6 .
2.3. Photocatalytic test The photocatalytic activity of the W-doped Bi2 MoO6 samples was evaluated by the degradation of RhB under a 500 W Xe lamp light ( > 400 nm). In each experiment, 0.1 g of the photocatalyst was added to 30 mL of RhB solution (10−5 mol/L) in test tube. Before illumination, the suspensions were magnetically stirred in the dark for 3 h to ensure the establishment of an adsorption–desorption equilibrium between the photocatalysts and RhB. And then, the solution under magnetic stirring was exposed to Xe lamp light irradiation. At given interval time, the test tubes were sampled and then centrifuged to remove the photocatalyst particles. Then, the filtrates were analyzed by recording variations of the absorption band maximum (553 nm) in a UV–vis spectra of RhB by using a Shimadzu UV-2550 spectrophotometer (Japan). 3. Results and discussion 3.1. Structure and morphology characterization Fig. 1 gives the XRD patterns of the Bi2 MoO6 samples doped by different concentrations of tungsten, and the JCPDS cards of Bi2 MoO6 and Bi2 WO6 are also given as comparison. As shown in
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Fig. 3. FE-SEM images of the samples. (a) nw = 0; (b) nw = 0.25; (c) nw = 0.5; (d) higher-magnification SEM image of (c); (e) nw = 0.75; (f) higher-magnification SEM image of (e); (g) nw = 1; (h) higher-magnification SEM image of (g).
Fig. 1, there are wide range solid solution between the two end compounds, Bi2 MoO6 and Bi2 WO6 . They exhibit similar XRD patterns to those of orthorhombic Bi2 WO6 (JCPDS Card No. 39-0256) and Bi2 MoO6 (JCPDS No. 21-0102), while their corresponding XRD peaks shift gradually between those of the Bi2 WO6 and Bi2 MoO6 with increase in nw value. And XRD pattern without W doping (nw = 0), can be indexed to orthorhombic ␥(L)-Bi2 MO6 phase. Meanwhile, other peaks cannot be detected, indicating the high purity of the sample. For nw = 1 (Fig. 1, pattern e), high-purity orthorhombic Bi2 WO6 was obtained, and other peaks were also not detected, indicating the high purity of the sample. The broad diffraction peaks imply that the size of the crystalline grain is small and well crystallized. For nw = 0.25, 0.5, and 0.75 (Fig. 1, patterns b–d), the XRD
patterns generally resemble that of Bi2 WO6 and/or ␥(L)-Bi2 MoO6 ; thus, these materials can be referred to as solid solutions. The XRD patterns of Bi2 MoO6 samples when nw value is 0.5 as an example, was given in Fig. 2a and the JCPDS cards of Bi2 MoO6 and Bi2 WO6 was used for comparison. As shown in Fig. 2a, the sample exhibits similar XRD patterns with those of orthorhombic-phase Bi2 MoO6 (JCPDS No. 77-1246) and Bi2 WO6 (JCPDS No.26-1044). Fig. 2b shows the amplified (1 3 1) peak of Bi2 MoO6 samples by 0.5 mmol W substitution. It can be found that the amplified 2 peak around 28◦ for the sample consists of a single XRD peak, instead of the mixture of the two respective peaks of the Bi2 MoO6 and Bi2 WO6 . Therefore, it can be concluded that the sample doped by 0.5 mmol W shows single phase with orthorhombic structure.
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Fig. 4. TEM images of the different nw values samples. (a and b) nw = 0; (c) a SAED pattern of the corner of (b); (d and e) nw = 0.25; (f) a SAED pattern of the corner of (e).
Moreover, the (1 1 3) diffraction peak is located in a bigger 2 angle region than the one of Bi2 MoO6 , which indicates the smaller crystal cell volume of the samples by 0.5 mmol W substitution, because of smaller ionic radius of W6+ (0.062 nm) compared to Mo6+ (0.065 nm). FE-SEM images of the as-synthesized samples are given in Fig. 3. For Bi2 MoO6 (nw = 0), lots of irregular nanosheets can be observed (Fig. 3a). These nanosheets randomly aggregated together so that the grain size was up to several micrometers, and there was a small fraction of nanosheets intergrowth. With increasing the value of nw , the nanosheets enlarged gradually, and the thickness of nanosheets diminished as shown in Fig. 3b. Moreover, there were a few of small cubic columnar structures. The low-magnification FE-SEM image (Fig. 3c) of nw = 0.5 shows that the samples were consisted of a large quantity of broom-like structures with a length of about 4 m and the width of 1.5 m. The higher-magnification SEM image of a broom-like individual demonstrates that the broom structure sample was made of many nanoplates with a thickness of ∼60 nm and a length of several hundred nanometers. For the sample with nw = 0.75 (Fig. 3e and f), the aggregated cluster was composed of square nanoplates that were regular in shape and uniform in size, with the length of 500 nm and the thickness of 20 nm, which were observed in higher-magnification SEM image of e. This sample also exhibited greater inter-growth than those in Fig. 3a. Finally, the low-magnification FE-SEM image of Bi2 WO6 (Fig. 3g) shows that all the products are nest-like microspheres. In addition, it can be seen that, from higher-magnification SEM image of an individual nested superstructure, these nest-like Bi2 WO6 structure microspheres had a 2∼3 m average diameter and an umbilicate cavity on every microsphere. The surfaces of these microspheres were coarse with multi-microstructures on them. TEM was employed to understand the detailed morphological and structural characteristics of the doped-Bi2 MoO6 samples. Fig. 4a shows an overall morphology of the Bi2 MoO6 (nw = 0.5) nanosheets with the length of 100–200 nm, the width of 100–300 nm and the thickness of 50 nm. Most of the nanosheets accumulate together and lie flat on the copper grid. Fig. 4b displays a higher-magnification TEM image of a Bi2 MoO6 nanosheet. The lattice distance (0.276 nm which corresponds to the D(0 0 2)
value determined from the XRD pattern) is further proved and corresponds to the (0 0 2) lattice fringes of orthorhombic Bi2 MoO6 . According to the selected area electron diffraction (SAED) pattern in Fig. 4c, the set of diffraction spots can be indexed to orthorhombic Bi2 MoO6 on the (0 0 2) crystal plane, indicating the single-crystalline nature of the nanosheet. In addition, in the strong diffraction spots, there were some weak diffraction spots observed in forbidden sites, which might be due to the high-order Laue zone caused by the combined effects of the elongation of the diffraction spots along the normal of the nanosheet and the narrowed Laue zone along the (0 0 2) direction. Zhou’s group [33] has also found similar phenomena when studying WO3 ·0.33H2 O nanosheets. The TEM image of the nanosheets (nw = 0.25) in Fig. 4d revealed that all the products were nanosheets with length of ∼150 nm, it produces a similar single-crystalline configuration to the Bi2 MoO6 nanosheets. As shown in Fig. 4e, the lattice distance is 0.275 nm which corresponds to (0 0 2) lattice crystal plane of russellite. The sample was further investigated by SAED, Fig. 4f, which confirmed that the nanoplates had a single crystal structure. An individual broom-like hierarchical architecture (nw = 0.5) with the length of about 3–4 m and the width of 1.5 m is shown in Fig. 5a. From a higher-magnification TEM image of the superstructure (Fig. 5b), it can be found that the superstructure is assembled from densely accumulated rectangular nanosheets in a parallel hierarchical manner which corresponds with the picture displayed in Fig. 3d. By analysing pictures (Figs. 5b and 3d), they are shown that this nanosheet is actually composed of two nanosheets sharing part of their top and/or bottom surfaces. In Fig. 5c, it is a higher-magnification TEM image of Fig. 5b, and two sets of atomic spacings (0.274 nm and 0.275 nm) can be distinguished, which correspond to the (2 0 0) and (0 0 2) lattice crystal plane of russellite (corresponding to D(2 0 0) and D(0 0 2) values determined from the XRD pattern), respectively. The SAED pattern (Fig. 5d) taken from a corner of the nanosheet indicates the single-crystalline structure of the nanosheet. The same as Fig. 4c, some weak diffraction spots might be caused by the high-order Laue zone observed at those forbidden sites (indicated by obscuration spots in Fig. 5d). In Fig. 6a, it displays an individual sample (nw = 0.75) approximate square nanoplate with the edge length of ∼1.2 m and the
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Fig. 5. TEM images and SAED of nw = 0.5. (a–c) TEM images of the samples. (d) SAED of the samples.
thickness of ∼6 nm. And also there are many smaller nanoplates dispersed and agglomerated on the surface of the nanoplate. The nanoplate is not perfectly flat, but it exhibits intrinsic out-ofplane wrinkles which indicate the high crystallinity and ultrathin nature of the nanosheet, corresponding with the display of XRD
and TEM patterns (Figs. 1d and 6b, respectively). From a highermagnification TEM image of the superstructure (Fig. 6b), there are obvious lattice fringes, and their spacings are 0.273 nm. Furthermore, the SAED pattern of a corner of the nanosheet (Fig. 6c) indicates the single-crystalline structure of the nanosheet. It also
Fig. 6. TEM images of the different samples. (a and b) nw = 0.75; (c) a SAED pattern of the corner of (b); (d and e) nw = 1; (f) a SAED pattern of the corner of (e).
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Fig. 7. UV–vis diffusive reflectance spectra of the Bi2 MoO6 samples by different concentrations of tungsten substitution.
exhibits an individual Bi2 WO6 microsphere that a cavity is across the center of the nest by the contrasting between the dark edge and the relatively bright center in Fig. 6d. The nanoplates of the nest-like Bi2 WO6 hierarchical structures are regular in shape and uniform in size with a length of about 150–200 nm (as shown in Fig. 6e). The sample was also further investigated by SAED, Fig. 3f, which confirmed that the nanoplates had a single crystal structure. 3.2. Optical properties of the W-doped Bi2 MoO6 samples In Fig. 7, it displays the diffuse reflectance spectra of the W-doped Bi2 MoO6 samples. The abilities of absorption of all samples have been increased when wavelengths is lower than about 460 nm, indicating the nature of the band-to-band transition. Additionally, the absorption bands in the region of longer wavelength (>500 nm) are also observed and it may be caused by remnant organic impurities. For Bi2 MoO6 (nw = 0), the band gap is ∼2.69 eV, for the samples (nw = 0.25, 0.5, 0.75), the band gaps are in the range of 2.43, 2.53, 2.72, respectively, and for Bi2 WO6 (nw = 1), the band gap is ∼2.75 eV. So it is observed that with increasing of the
Fig. 9. (a) The temporal evolution of the absorption spectrum of the RhB solution of Bi2 WO6 under 500 W Xe lamp light irradiation. Inset: original solution of RhB and after 2.5 h of irradiation. (b) Digital photo of the sample with different irradiated time.
W contribution it has a significant blue shifts of the absorbance band. This result might be related to the conduction band of the photocatalysts. W6+ substituting can increase the conduction band level of the as-synthesized Bi2 MoO6 samples. The overall band gap of the W-doped Bi2 MoO6 samples compound can be thought to be related closely with two factors [32,34,35]: (i) the degree of Mo 4d and W 5d orbitals being involved in the conduction band of the W-doped Bi2 MoO6 samples and (ii) the degree of delocalization of excitation energy due to the distortion of the crystal structure arising from W substitution. Although the change of the band gap energy is not necessarily proportional to the amount of W substitution, the absorbance band has a blue shifts with increasing of the W contribution. Therefore, the results indicate that the W-doped Bi2 MoO6 samples have a suitable band gap for photocatalytic decomposition of organic contaminants under visible-light irradiation. 3.3. Photocatalytic performance of the W-doped Bi2 MoO6 samples
Fig. 8. Photocatalytic degradation of RhB solution by the Bi2 MoO6 samples by different concentrations of tungsten substitution.
The photocatalytic activities of RhB mediated by the different photocatalysts of the W-doped Bi2 MoO6 samples as well as without photocatalyst (the blank test in the inset of Fig. 9a) under visible light irradiation ( > 400 nm) are displayed in Fig. 8. The blank test demonstrates that the degradation of RhB is extremely few without
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a photocatalyst under visible light irradiation (Fig. 9a). As shown in Fig. 8, visible-light irradiation of the aqueous RhB/photocatalyst dispersions led to vastly different photodegradation rates of RhB, and it is obvious that the photodegradation efficiency of RhB by Bi2 MoO6 with 0.5 mmol tungsten is the highest, reaching 95% after 140 min irradiation (Fig. 8) it is found that the photocatalytic performances of all samples are greatly different and are strongly dependent on shape, size, and structure. Additionally, it may integrate the advantages of both Bi2 WO6 and Bi2 MoO6 as a solid solution to improve the photocatalytic performance. To explain the photocatalytic performance of photodegradation of RhB, we took photodegradation of RhB by Bi2 WO6 (nw = 1) as an example. In Fig. 9a, it displays the temporal evolution of the spectra during the photodegradation of RhB at 20 minute intervals. RhB was used as a probe because the dark adsorption (on the surface of semiconductor oxide), decoloration and decomposition can be monitored via its visible-light absorption signature at = 553 nm. And when the exposure time was extended, the position of the absorption peak was shifted to shorter wavelengths and the strength of the peak was reduced. It also shows that RhB is de-ethylated via a stepwise process and followed a destruction of the conjugated structure. After 140 min, the absorption peak completely disappeared and the pink color of the starting RhB solution faded. The inset of Fig. 9b shows a digital photo of the sample with different irradiated time, and the fist tube exhibits original solution of RhB and the others give different irradiated time of RhB/Bi2 WO6 from left to right. Meanwhile, it can be seen that the color fading of RhB is a stepwise process. 4. Discussions The W-doped Bi2 MoO6 samples (nw = 0, 0.25, 0.50, 0.75, and 1.00) have been successfully synthesized through a facile hydrothermal process in the absence of a surfactant at 200 ◦ C for 24 h. With increasing of the contribution of W from nw = 0 to nw = 1, the pure ␥(L)-Bi2 MoO6 transform into orthorhombic Bi2 WO6 with high crystallinity. SEM and TEM indicate that Bi2 MoO6 samples by W substitution are composed of the nanosheets of similar orthorhombic Aurivillius layered structures. The band gap of the Bi2 MoO6 samples with doping W substitution (nw = 0.25, 0.5, 0.75) has an obvious change between Bi2 WO6 and Bi2 MoO6 . Under 500 W Xe lamp light irradiation, all samples could effectively degrade RhB dye by photocatalysis. Furthermore, Bi2 MoO6 with nw = 0.5 shows the highest photocatalytic activity for RhB photodecomposition under visible light irradiation. It may be closely related to shape, size, and structure. Therefore, this work not only provides an example of synthesizing photocatalyst of the nanosheet-based homogeneous Bi2 MoO6 with W substitution but also opens new possibilities to design ideal that integrate the advantages of semiconductor materials for future applications.
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Self-assembly and enhanced visible-light-driven photocatalytic activities of Bi2 MoO6 by tungsten substitution Hongguang Yu a,∗ , Zhenfeng Zhu a , Jianhong Zhou b , Jing Wang c , Junqi Li a , Yanli Zhang a a b c
School of Materials Science and Engineering, Shaanxi University of Science and Technology, Xi’an 710021, PR China Department of Physics and Information Technology, Baoji University of Arts and Sciences, Baoji 721007, PR China Deparment of Materials Science and Engineering, The University of Sheffield, Sir Robert Hadfield Building, Mappin Street, Sheefield S1 3JD, UK
a r t i c l e
i n f o
Article history: Received 21 February 2012 Received in revised form 3 November 2012 Accepted 5 November 2012 Available online 14 November 2012 Keywords: Bi2 MoO6 Tungsten substitution Photocatalysis Nanosheets Hydrothermal method
a b s t r a c t The different compositions of Bi2 MoO6 by tungsten substitution have been successfully synthesized via a facile hydrothermal process in the absence of a surfactant. All W-doped Bi2 MoO6 samples were composed of nanosheets with similar orthorhombic Aurivillius layered structures. Bi2 MoO6 samples show different phases varied with W content. For the Bi2 MoO6 samples with the different nw values, their band gaps have changed obviously compared with Bi2 MoO6 (2.69 eV) and Bi2 WO6 (2.75 eV). The Bi2 MoO6 by tungsten substitution exhibited good photocatalytic activity in degradation of Rhodamine-B under 500 W Xe lamp light irradiation. When nw value is 0.5, the sample has the highest photocatalytic activity for RhB photodecomposition under visible light irradiation. It shows that photocatalytic activities of Bi2 MoO6 samples by W substitution are relevant to nanosheet morphology and size, the intrinsic layered structure, band gap and the W content. © 2012 Elsevier B.V. All rights reserved.
1. Introduction With the rising level of air and water pollution, researchers are giving more and more attention to environmentally friendly materials, especially photodegradation catalysts. Since the discovery of the photocatalytic splitting of water on the TiO2 electrodes by Fujishima and Honda in 1972 [1], the applications of semiconductor photocatalysts on solar energy conversion and degradation of pollution have received great attention [2]. Because of the good chemical stability, high oxidized activity, nontoxicity and low price, TiO2 has been the most popular photocatalyst for environmental purification [3,4]. Although TiO2 and TiO2 -based materials are the most popular photocatalysts, the band gap of the TiO2 is 3.2 eV and it absorbs only the ultraviolet light ( < 400 nm) which lowers the efficiency of using solar energy and accounts for about 4% of the sunlight [5–11]. In the past decades, single semiconducting photocatalysts have been used, but they are restricted by their structure, morphology and immanence property. Thus the worldwide interest of heterogeneous semiconducting photocatalysts on transforming solar energy into chemical energy and decomposing organic contaminants are taken [12–15]. To improve the efficiency of solar energy
∗ Corresponding author. Tel.: +86 029 86168820; fax: +86 029 86177108. E-mail addresses: [email protected] (H. Yu), [email protected] (J. Zhou), [email protected] (J. Wang). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.11.023
utilization, researchers have been exploring alternative materials for visible-light-responsive photocatalysts such as Bi2 MoO6 and Bi2 WO6 [16–28,2,29], which can be used as excellent photocatalysts and a solar energy transfer materials. Bi2 MoO6 and Bi2 WO6 are two of the simplest members of the Aurivillius oxide family [24]. Compared with Bi2 WO6 , Bi2 MoO6 has a narrower band gap (for example, 2.77 eV for Bi2 WO6 and 2.63 eV for Bi2 MoO6 in Zhou et al.’s report [25]) and thus the ability to harness more sunlight. However, its photocatalytic activity is not as high as that of Bi2 WO6 , so it has attracted less attention. Fortunately, it is possible to integrate the advantages of both high photocatalytic activity of Bi2 WO6 and narrower band gap of Bi2 MoO6 into one compound, W-doped Bi2 MoO6 samples, as it is reasonable to postulate that the properties of both will favorably mix [26]. Additionally, it is possible to improve the photocatalytic properties by utilizing the multiple scattering within the complex to increase the optical path length through the crystals. Moreover, there have been few reports concerning the photocatalytic properties and morphology of Bi2 MoO6 samples by W substitution until now [30–32]. In the present work, we demonstrate the W-doped Bi2 MoO6 samples of the different nw values (nw = 0, 0.25, 0.5, 0.75, 1.0) synthesized by a facile hydrothermal treatment method without a surfactant, and the visible light ( > 400 nm) photocatalytic activities have also been investigated. Our studies show that the introduction of W into Bi2 MoO6 lattices affects not only the morphology but also the band gap as well as yield interesting shaped-associated visible-light-driven photocatalytic activities.
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Fig. 1. XRD pattern of the Bi2 MoO6 samples by different concentrations of tungsten substitution, and the JCPDS cards of Bi2 MoO6 and Bi2 WO6 are also given as comparison. (a) nw = 0, (b) nw = 0.25, (c) nw = 0.5, (d) nw = 0. 75, and (e) nw = 1.
Our approach provides a facile process to tune both nanostructure and band gap of photocatalysts by simply adjusting the additive concentration, leading to the enhanced visible-light-driven photocatalytic activity. Moreover, our work may be further extended to the design and synthesis of novel and highly efficient visible-lightdriven semiconductors for environmental remediation. 2. Experimental 2.1. Synthesis of Bi2 MoO6 samples by W substitution All chemicals were analytical grade and used as raw materials without further purification, and deionized water was used throughout the experimental process. First, 2 mmol of Bi(NO3 )3 ·5H2 O was mixed with a 40 mL aqueous solution containing 1 mmol of citric acid. After 10 min of magnetic stirring (1 mmol, 0.75 mmol, 0.5 mmol, 0.25 mmol, 0 mmol separately) (NH4 )6 Mo7 O24 ·H2 O and (2 mmol) NaHCO3 were gradually and respectively added into the above mixed solution. And then nw (nw = 0, 0.25, 0.5, 0.75, 1.0) mmol Na2 WO4 ·2H2 O were further mixture with the mixture and sufficiently stirred for another 10 min. When the suspension was transferred into a stainless steel autoclave with a 70 ml Teflon liner, then heated at 200 ◦ C for 24 h and cooled to room temperature, the products were separated centrifugally and washed three times with deionized water and absolute ethanol, respectively. Finally, the final products were obtained by drying under vacuum at 60 ◦ C for 20 h. 2.2. Characterization Powder X-ray diffraction (XRD) was carried out with a D/max2200 X-ray diffractometer (Rigaku, Japan) using Cu K␣ radiation ( = 0.154 nm) in the 2 range of 15–70◦ . Field emission scanning electron microscopy (FE-SEM) images were taken on a JEOL JSM-6700F scanning electron microscope (Japan), and transmission electron microscopy (TEM) observation and selected area electron diffraction (SAED) patterns were performed with a JEM-3010 highresolution transmission electron microscope (HRTEM, Japan), using an accelerating voltage of 200 kV. The optical diffuse reflectance spectra were recorded on a Japan Shimadzu UV-2550 (Japan) using BaSO4 as reference.
Fig. 2. (a) XRD pattern of Bi2 MoO6 samples when nw value is 0.5, and the JCPDS cards of Bi2 MoO6 and Bi2 WO6 . (b) Comparison of 2 peaks from 26◦ to 30◦ for Bi2 MoO6 samples by 0.5 mmol W substitution, the JCPDS cards of Bi2 MoO6 and Bi2 WO6 .
2.3. Photocatalytic test The photocatalytic activity of the W-doped Bi2 MoO6 samples was evaluated by the degradation of RhB under a 500 W Xe lamp light ( > 400 nm). In each experiment, 0.1 g of the photocatalyst was added to 30 mL of RhB solution (10−5 mol/L) in test tube. Before illumination, the suspensions were magnetically stirred in the dark for 3 h to ensure the establishment of an adsorption–desorption equilibrium between the photocatalysts and RhB. And then, the solution under magnetic stirring was exposed to Xe lamp light irradiation. At given interval time, the test tubes were sampled and then centrifuged to remove the photocatalyst particles. Then, the filtrates were analyzed by recording variations of the absorption band maximum (553 nm) in a UV–vis spectra of RhB by using a Shimadzu UV-2550 spectrophotometer (Japan). 3. Results and discussion 3.1. Structure and morphology characterization Fig. 1 gives the XRD patterns of the Bi2 MoO6 samples doped by different concentrations of tungsten, and the JCPDS cards of Bi2 MoO6 and Bi2 WO6 are also given as comparison. As shown in
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Fig. 3. FE-SEM images of the samples. (a) nw = 0; (b) nw = 0.25; (c) nw = 0.5; (d) higher-magnification SEM image of (c); (e) nw = 0.75; (f) higher-magnification SEM image of (e); (g) nw = 1; (h) higher-magnification SEM image of (g).
Fig. 1, there are wide range solid solution between the two end compounds, Bi2 MoO6 and Bi2 WO6 . They exhibit similar XRD patterns to those of orthorhombic Bi2 WO6 (JCPDS Card No. 39-0256) and Bi2 MoO6 (JCPDS No. 21-0102), while their corresponding XRD peaks shift gradually between those of the Bi2 WO6 and Bi2 MoO6 with increase in nw value. And XRD pattern without W doping (nw = 0), can be indexed to orthorhombic ␥(L)-Bi2 MO6 phase. Meanwhile, other peaks cannot be detected, indicating the high purity of the sample. For nw = 1 (Fig. 1, pattern e), high-purity orthorhombic Bi2 WO6 was obtained, and other peaks were also not detected, indicating the high purity of the sample. The broad diffraction peaks imply that the size of the crystalline grain is small and well crystallized. For nw = 0.25, 0.5, and 0.75 (Fig. 1, patterns b–d), the XRD
patterns generally resemble that of Bi2 WO6 and/or ␥(L)-Bi2 MoO6 ; thus, these materials can be referred to as solid solutions. The XRD patterns of Bi2 MoO6 samples when nw value is 0.5 as an example, was given in Fig. 2a and the JCPDS cards of Bi2 MoO6 and Bi2 WO6 was used for comparison. As shown in Fig. 2a, the sample exhibits similar XRD patterns with those of orthorhombic-phase Bi2 MoO6 (JCPDS No. 77-1246) and Bi2 WO6 (JCPDS No.26-1044). Fig. 2b shows the amplified (1 3 1) peak of Bi2 MoO6 samples by 0.5 mmol W substitution. It can be found that the amplified 2 peak around 28◦ for the sample consists of a single XRD peak, instead of the mixture of the two respective peaks of the Bi2 MoO6 and Bi2 WO6 . Therefore, it can be concluded that the sample doped by 0.5 mmol W shows single phase with orthorhombic structure.
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Fig. 4. TEM images of the different nw values samples. (a and b) nw = 0; (c) a SAED pattern of the corner of (b); (d and e) nw = 0.25; (f) a SAED pattern of the corner of (e).
Moreover, the (1 1 3) diffraction peak is located in a bigger 2 angle region than the one of Bi2 MoO6 , which indicates the smaller crystal cell volume of the samples by 0.5 mmol W substitution, because of smaller ionic radius of W6+ (0.062 nm) compared to Mo6+ (0.065 nm). FE-SEM images of the as-synthesized samples are given in Fig. 3. For Bi2 MoO6 (nw = 0), lots of irregular nanosheets can be observed (Fig. 3a). These nanosheets randomly aggregated together so that the grain size was up to several micrometers, and there was a small fraction of nanosheets intergrowth. With increasing the value of nw , the nanosheets enlarged gradually, and the thickness of nanosheets diminished as shown in Fig. 3b. Moreover, there were a few of small cubic columnar structures. The low-magnification FE-SEM image (Fig. 3c) of nw = 0.5 shows that the samples were consisted of a large quantity of broom-like structures with a length of about 4 m and the width of 1.5 m. The higher-magnification SEM image of a broom-like individual demonstrates that the broom structure sample was made of many nanoplates with a thickness of ∼60 nm and a length of several hundred nanometers. For the sample with nw = 0.75 (Fig. 3e and f), the aggregated cluster was composed of square nanoplates that were regular in shape and uniform in size, with the length of 500 nm and the thickness of 20 nm, which were observed in higher-magnification SEM image of e. This sample also exhibited greater inter-growth than those in Fig. 3a. Finally, the low-magnification FE-SEM image of Bi2 WO6 (Fig. 3g) shows that all the products are nest-like microspheres. In addition, it can be seen that, from higher-magnification SEM image of an individual nested superstructure, these nest-like Bi2 WO6 structure microspheres had a 2∼3 m average diameter and an umbilicate cavity on every microsphere. The surfaces of these microspheres were coarse with multi-microstructures on them. TEM was employed to understand the detailed morphological and structural characteristics of the doped-Bi2 MoO6 samples. Fig. 4a shows an overall morphology of the Bi2 MoO6 (nw = 0.5) nanosheets with the length of 100–200 nm, the width of 100–300 nm and the thickness of 50 nm. Most of the nanosheets accumulate together and lie flat on the copper grid. Fig. 4b displays a higher-magnification TEM image of a Bi2 MoO6 nanosheet. The lattice distance (0.276 nm which corresponds to the D(0 0 2)
value determined from the XRD pattern) is further proved and corresponds to the (0 0 2) lattice fringes of orthorhombic Bi2 MoO6 . According to the selected area electron diffraction (SAED) pattern in Fig. 4c, the set of diffraction spots can be indexed to orthorhombic Bi2 MoO6 on the (0 0 2) crystal plane, indicating the single-crystalline nature of the nanosheet. In addition, in the strong diffraction spots, there were some weak diffraction spots observed in forbidden sites, which might be due to the high-order Laue zone caused by the combined effects of the elongation of the diffraction spots along the normal of the nanosheet and the narrowed Laue zone along the (0 0 2) direction. Zhou’s group [33] has also found similar phenomena when studying WO3 ·0.33H2 O nanosheets. The TEM image of the nanosheets (nw = 0.25) in Fig. 4d revealed that all the products were nanosheets with length of ∼150 nm, it produces a similar single-crystalline configuration to the Bi2 MoO6 nanosheets. As shown in Fig. 4e, the lattice distance is 0.275 nm which corresponds to (0 0 2) lattice crystal plane of russellite. The sample was further investigated by SAED, Fig. 4f, which confirmed that the nanoplates had a single crystal structure. An individual broom-like hierarchical architecture (nw = 0.5) with the length of about 3–4 m and the width of 1.5 m is shown in Fig. 5a. From a higher-magnification TEM image of the superstructure (Fig. 5b), it can be found that the superstructure is assembled from densely accumulated rectangular nanosheets in a parallel hierarchical manner which corresponds with the picture displayed in Fig. 3d. By analysing pictures (Figs. 5b and 3d), they are shown that this nanosheet is actually composed of two nanosheets sharing part of their top and/or bottom surfaces. In Fig. 5c, it is a higher-magnification TEM image of Fig. 5b, and two sets of atomic spacings (0.274 nm and 0.275 nm) can be distinguished, which correspond to the (2 0 0) and (0 0 2) lattice crystal plane of russellite (corresponding to D(2 0 0) and D(0 0 2) values determined from the XRD pattern), respectively. The SAED pattern (Fig. 5d) taken from a corner of the nanosheet indicates the single-crystalline structure of the nanosheet. The same as Fig. 4c, some weak diffraction spots might be caused by the high-order Laue zone observed at those forbidden sites (indicated by obscuration spots in Fig. 5d). In Fig. 6a, it displays an individual sample (nw = 0.75) approximate square nanoplate with the edge length of ∼1.2 m and the
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Fig. 5. TEM images and SAED of nw = 0.5. (a–c) TEM images of the samples. (d) SAED of the samples.
thickness of ∼6 nm. And also there are many smaller nanoplates dispersed and agglomerated on the surface of the nanoplate. The nanoplate is not perfectly flat, but it exhibits intrinsic out-ofplane wrinkles which indicate the high crystallinity and ultrathin nature of the nanosheet, corresponding with the display of XRD
and TEM patterns (Figs. 1d and 6b, respectively). From a highermagnification TEM image of the superstructure (Fig. 6b), there are obvious lattice fringes, and their spacings are 0.273 nm. Furthermore, the SAED pattern of a corner of the nanosheet (Fig. 6c) indicates the single-crystalline structure of the nanosheet. It also
Fig. 6. TEM images of the different samples. (a and b) nw = 0.75; (c) a SAED pattern of the corner of (b); (d and e) nw = 1; (f) a SAED pattern of the corner of (e).
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Fig. 7. UV–vis diffusive reflectance spectra of the Bi2 MoO6 samples by different concentrations of tungsten substitution.
exhibits an individual Bi2 WO6 microsphere that a cavity is across the center of the nest by the contrasting between the dark edge and the relatively bright center in Fig. 6d. The nanoplates of the nest-like Bi2 WO6 hierarchical structures are regular in shape and uniform in size with a length of about 150–200 nm (as shown in Fig. 6e). The sample was also further investigated by SAED, Fig. 3f, which confirmed that the nanoplates had a single crystal structure. 3.2. Optical properties of the W-doped Bi2 MoO6 samples In Fig. 7, it displays the diffuse reflectance spectra of the W-doped Bi2 MoO6 samples. The abilities of absorption of all samples have been increased when wavelengths is lower than about 460 nm, indicating the nature of the band-to-band transition. Additionally, the absorption bands in the region of longer wavelength (>500 nm) are also observed and it may be caused by remnant organic impurities. For Bi2 MoO6 (nw = 0), the band gap is ∼2.69 eV, for the samples (nw = 0.25, 0.5, 0.75), the band gaps are in the range of 2.43, 2.53, 2.72, respectively, and for Bi2 WO6 (nw = 1), the band gap is ∼2.75 eV. So it is observed that with increasing of the
Fig. 9. (a) The temporal evolution of the absorption spectrum of the RhB solution of Bi2 WO6 under 500 W Xe lamp light irradiation. Inset: original solution of RhB and after 2.5 h of irradiation. (b) Digital photo of the sample with different irradiated time.
W contribution it has a significant blue shifts of the absorbance band. This result might be related to the conduction band of the photocatalysts. W6+ substituting can increase the conduction band level of the as-synthesized Bi2 MoO6 samples. The overall band gap of the W-doped Bi2 MoO6 samples compound can be thought to be related closely with two factors [32,34,35]: (i) the degree of Mo 4d and W 5d orbitals being involved in the conduction band of the W-doped Bi2 MoO6 samples and (ii) the degree of delocalization of excitation energy due to the distortion of the crystal structure arising from W substitution. Although the change of the band gap energy is not necessarily proportional to the amount of W substitution, the absorbance band has a blue shifts with increasing of the W contribution. Therefore, the results indicate that the W-doped Bi2 MoO6 samples have a suitable band gap for photocatalytic decomposition of organic contaminants under visible-light irradiation. 3.3. Photocatalytic performance of the W-doped Bi2 MoO6 samples
Fig. 8. Photocatalytic degradation of RhB solution by the Bi2 MoO6 samples by different concentrations of tungsten substitution.
The photocatalytic activities of RhB mediated by the different photocatalysts of the W-doped Bi2 MoO6 samples as well as without photocatalyst (the blank test in the inset of Fig. 9a) under visible light irradiation ( > 400 nm) are displayed in Fig. 8. The blank test demonstrates that the degradation of RhB is extremely few without
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a photocatalyst under visible light irradiation (Fig. 9a). As shown in Fig. 8, visible-light irradiation of the aqueous RhB/photocatalyst dispersions led to vastly different photodegradation rates of RhB, and it is obvious that the photodegradation efficiency of RhB by Bi2 MoO6 with 0.5 mmol tungsten is the highest, reaching 95% after 140 min irradiation (Fig. 8) it is found that the photocatalytic performances of all samples are greatly different and are strongly dependent on shape, size, and structure. Additionally, it may integrate the advantages of both Bi2 WO6 and Bi2 MoO6 as a solid solution to improve the photocatalytic performance. To explain the photocatalytic performance of photodegradation of RhB, we took photodegradation of RhB by Bi2 WO6 (nw = 1) as an example. In Fig. 9a, it displays the temporal evolution of the spectra during the photodegradation of RhB at 20 minute intervals. RhB was used as a probe because the dark adsorption (on the surface of semiconductor oxide), decoloration and decomposition can be monitored via its visible-light absorption signature at = 553 nm. And when the exposure time was extended, the position of the absorption peak was shifted to shorter wavelengths and the strength of the peak was reduced. It also shows that RhB is de-ethylated via a stepwise process and followed a destruction of the conjugated structure. After 140 min, the absorption peak completely disappeared and the pink color of the starting RhB solution faded. The inset of Fig. 9b shows a digital photo of the sample with different irradiated time, and the fist tube exhibits original solution of RhB and the others give different irradiated time of RhB/Bi2 WO6 from left to right. Meanwhile, it can be seen that the color fading of RhB is a stepwise process. 4. Discussions The W-doped Bi2 MoO6 samples (nw = 0, 0.25, 0.50, 0.75, and 1.00) have been successfully synthesized through a facile hydrothermal process in the absence of a surfactant at 200 ◦ C for 24 h. With increasing of the contribution of W from nw = 0 to nw = 1, the pure ␥(L)-Bi2 MoO6 transform into orthorhombic Bi2 WO6 with high crystallinity. SEM and TEM indicate that Bi2 MoO6 samples by W substitution are composed of the nanosheets of similar orthorhombic Aurivillius layered structures. The band gap of the Bi2 MoO6 samples with doping W substitution (nw = 0.25, 0.5, 0.75) has an obvious change between Bi2 WO6 and Bi2 MoO6 . Under 500 W Xe lamp light irradiation, all samples could effectively degrade RhB dye by photocatalysis. Furthermore, Bi2 MoO6 with nw = 0.5 shows the highest photocatalytic activity for RhB photodecomposition under visible light irradiation. It may be closely related to shape, size, and structure. Therefore, this work not only provides an example of synthesizing photocatalyst of the nanosheet-based homogeneous Bi2 MoO6 with W substitution but also opens new possibilities to design ideal that integrate the advantages of semiconductor materials for future applications.
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