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nanostructures: Microwave and ultrasonic wave combined synthesis and their visible-light photocatalytic activities
Journal of Alloys and Compounds 551 (2013) 544–550
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Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Monoclinic BiVO4 micro-/nanostructures: Microwave and ultrasonic wave combined synthesis and their visible-light photocatalytic activities Yafang Zhang a,1, Guangfang Li a,1, Xiaohui Yang a, Hao Yang a, Zhong Lu a, Rong Chen a,b,⇑ a Key Laboratory for Green Chemical Process of Ministry of Education and Hubei Novel Reactor & Green Chemical Technology Key Laboratory, Wuhan Institute of Technology, Xiongchu Avenue, Wuhan 430073, PR China b Engineering Research Center of Nano-Geomaterials of Ministry of Education, China University of Geosciences, Lumo Road, Wuhan 430074, PR China
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Article history: Received 15 August 2012 Received in revised form 2 November 2012 Accepted 2 November 2012 Available online 16 November 2012 Keywords: Bismuth vanadate Microwave Ultrasonic wave Photocatalysis
a b s t r a c t Monoclinic bismuth vanadate (m-BiVO4) micro-/nanostructures with different sizes and morphologies were successfully prepared via a facile and rapid microwave and ultrasonic wave combined technique. The obtained BiVO4 products were characterized by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), transmission electron microscopy (TEM) and UV–vis diffuse reflection spectroscopy (DRS). It was found that the solvent and pH value had a significant influence on morphology, size and crystalline structure of the product. Nut-like, potato-like and broccoli-like monoclinic BiVO4 were fabricated in different solvents. The crystal phase could be modulated by varying the pH value of reaction system. The photocatalytic activities of the products were also evaluated by the degradation of Rhodamine B (RhB) under visible light irradiation. The result revealed that the photocatalytic activities of BiVO4 nanostructures were closely related to the crystalline phase, band gap and particle size. Monoclinic BiVO4 nanoparticles with small crystal size and large band gap exhibited remarkable photocatalytic performance. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction As an alternative green technology, photocatalysis has attracted considerable research interest due to its potential applications in renewable energy and environmental remediation [1–5]. Over the past few decades, much effort has been made to develop novel and effective semiconductor photocatalysts, which serve a crucial issue in photocatalytic process [1,6–9]. However, the most widely used photocatalyst TiO2 is only active under ultraviolet (UV) light irradiation which accounts for a very small portion of sunlight spectrum. Therefore, developing visible-light-responsive photocatalysts with a good photocatalytic performance under sunlight irradiation represents an alternative technique in photocatalysis field [10–12]. Bismuth vanadate (BiVO4) micro-/nanostructures are of special attention, as one of the most promising visible-light-driven photocatalysts, by virtue of its fascinating photophysical and photochemical properties. Various BiVO4 nanostructures reported exhibit outstanding photocatalytic activities in degradation of ⇑ Corresponding author at: Key Laboratory for Green Chemical Process of Ministry of Education and Hubei Novel Reactor & Green Chemical Technology Key Laboratory, Wuhan Institute of Technology, Xiongchu Avenue, Wuhan 430073, PR China. Tel.: +86 13659815698; fax: +86 2787195671. E-mail address: [email protected] (R. Chen). 1 These authors contributed equally to this work. 0925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.11.017
different organic pollutants [13–16], water splitting for O2 and H2 evolution [8,17], and destruction of microorganisms [18], under visible light irradiation. Most commonly, BiVO4 has three main crystal phases, including tetragonal zircon, monoclinic and tetragonal scheelite structures [19]. In literatures, it has been well-documented that monoclinic BiVO4 (m-BiVO4) nanostructures displayed much better photocatalytic performance than that of other crystal phases under visible light irradiation [15,20–23]. Furthermore, the shape and size of nanostructures have a significantly direct influence on their photocatalytic activities. Therefore, an increasing interest has been focused on controllable fabrication of m-BiVO4 nanostructures by different preparation routes in recent years [14,24,25]. Unfortunately, the methods mostly reported involved harsh experimental conditions such as solid-state or melting reaction at high temperature, uncertainty of the reaction process, long reaction time, and organic additive or template, which dramatically limited the large-scale production of m-BiVO4. Therefore, it is highly desired to develop a facile and environmentfriendly route for tunable synthesis of m-BiVO4. Recently, microwave assisted heating technique and sonochemistry method are particularly attractive in a range of synthetic fields [26–29], particularly in inorganic materials preparation. Microwave irradiation provides great advantages of uniform and rapid dielectric heating without thermal gradient effect, which greatly promotes the reaction rate and results in a high-throughput capacity. It has been recognized as a facile and mild synthetic
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strategy for synthesis of various nanostructures with controllable shape and size [30–35]. And sonochemistry method powered by high-frequency sound waves could produce an exceedingly high transient temperature and pressure, which promotes both physical effects and chemical reactions during the formation process of various nanostructures [36–39]. Based on these work, it is believed that the combination of microwave and ultrasonic wave method could be employed for controllable preparation of nanostructures. As a matter of fact, Poinern et al. have demonstrated that this combined technique could be used to fabricate nano-structured hydroxyapatite [40]. Our previous work also has successfully prepared well-ordered flower-like ZnO nanostructures through the microwave and ultrasonic combined technique [41]. To the best of our knowledge, however, there is no report on the fabrication of BiVO4 nanostructures by microwave and ultrasonic wave combined technique. In this work, m-BiVO4 micro-/nanostructures with different sizes and morphologies were synthesized by a microwave and ultrasonic wave combined technique in different solvents. The photocatalytic performance of the as-synthesized BiVO4 nanostructures was also evaluated by the degradation of Rhodamine B (RhB) under visible light irradiation. 2. Experimental sections 2.1. Chemicals Bismuth nitrate pentahydrate (Bi(NO3)35H2O), sodium bicarbonate (NaHCO3) and Rhodamine B (RhB) were purchased from Aladdin. Ammonium metavanadate (NH4VO3), diethylene glycol (DEG), ethylene glycol (EG), glycerol and mannitol were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). All the reagents were analytical grade and used directly without further purification. 2.2. Preparation In a typical synthesis, 0.485 g Bi(NO3)35H2O and 0.117 g NH4VO3 were dissolved in 15 mL diethylene glycol (DEG) and 15 mL deionized water, respectively. Then the solutions were mixed, and a yellow-milky suspension was formed immediately (pH 1). The mixture was heated to 110 °C within 3 min and then kept at that temperature for 27 min by a microwave and ultrasonic wave combined reactor (XH-300A, Beijing Xianghu Technology Co., Ltd.) under the microwave irradiation power at 500 W and the ultrasonic irradiation power at 800 W (2 s sonication and 1 s interruption) (Fig. S1, Supplementary Materials). After reaction, the solution was cooled down to room temperature naturally. The precipitate was collected by centrifugation and washed with deionized water and absolute ethanol for five times, followed by drying at 60 °C overnight. Finally, the dry sample was calcined at 500 °C for 2 h in a muffle and obtained sample S1. Other BiVO4 samples (S2– S8) were also prepared under the identical conditions by varying the solvent and pH value of the reaction system (adjusted by saturated NaHCO3), respectively. The detailed experimental parameters are listed in Table 1. 2.3. Characterization Powder X-ray diffraction (XRD) was carried out on Bruker axs D8 Discover (Cu Ka = 1.5406 Å) at a scan rate of 2° min 1 in the 2h range from 10° to 80°. Scanning electron microscopy (SEM) images and energy dispersive X-ray (EDX) spectra were taken on a Hitachi S4800 scanning electron microscope operating at 5.0 kV. Transmission electron microscopy (TEM) images were recorded on a Philips Tecnai 20 electron microscope, using an accelerating voltage of 200 kV. UV–vis diffuse reflec-
tance spectra (DRS) were recorded on a UV–vis spectrometer (Shimadzu UV-2550) by using BaSO4 as a reference and were converted from reflection to absorbance by the Kubelka–Munk method. 2.4. Evaluation of photocatalytic activity Photocatalytic activities of the as-synthesized BiVO4 samples were evaluated by the degradation of RhB under visible light irradiation using a 500 W Xe lamp with a 400 nm cut-off filter (Fig. S2, Supplementary Materials). In the typical experiment, the as-prepared BiVO4 sample (0.05 g) was added into 100 mL RhB solution (1 10 5 M). Prior to irradiation, the suspension was sonicated for 10 min and magnetically stirred for 10 min in the dark to ensure the establishment of an adsorption–desorption equilibrium. Then, the solution was exposed to visible light irradiation under magnetic stirring. At each given time interval, 4 mL suspension was sampled and centrifuged to remove the BiVO4 powder. The concentration of RhB during the degradation was determined using a Shimadzu UV2800 spectrophotometer. All the measurements were carried out at room temperature.
3. Results and discussion 3.1. Structure and morphology The phase and composition of the as-synthesized product (S1) was identified by powder X-ray diffraction (XRD) analysis. Fig. 1a showed the typical XRD pattern of the product prepared in DEG/ H2O mixed solvent (pH 1), which could be well indexed to the standard data of the pure monoclinic BiVO4 (JCPDS 14-0688). No impurity peaks were observed in the pattern, indicative of high purity of the product. The intense and sharp diffraction peaks suggested that the as-prepared product was well-crystallized. The size and morphology of the product (S1) were examined by SEM. As shown in Fig. 1b, a large quantity of uniform nut-like BiVO4 microstructures with rough surface and sharp edges were obtained. The high-magnification SEM image in Fig. 1c further demonstrated the product was composed of many aggregated particles, which closely linked together layer by layer. The elemental composition of the as-synthesized nut-like BiVO4 microstructures (S1) was also confirmed by energy dispersive X-ray (EDX) analysis (Fig. 1d). The result observed indicated that the sample was composed of Bi, V, and O elements with the atom ratio of about 1:1:4, which was in good agreement with the stoichiometry of BiVO4. 3.2. Effect of solvents Monoclinic BiVO4 products could also be obtained in other solvents, including ethylene glycol/H2O (EG/H2O) (S2), glycerol/H2O (S3) and 0.1 M mannitol solution (S4). The corresponding XRD patterns (Fig. 2a) indicated that well-crystallized monoclinic BiVO4 were obtained in different solvents under the identical conditions. The typical SEM image (Fig. 2b) showed the formation of nut-like BiVO4 microstructures in EG/H2O mixture (S2). The high-resolution SEM image revealed the smooth surface of the obtained BiVO4 microstructures (inset of Fig. 2b). As shown in Fig. 2c, potato-like BiVO4 microstructures with rough surface were fabricated in glycerol/H2O solution (S3). When mannitol solution was employed as
Table 1 Experimental conditions for the preparation of BiVO4 micro-/nanostructures.a
a
Samples
Bismuth precursor
Vanadium precursor
Solvent
pH
Morphology
S1 S2 S3 S4 S5 S6 S7 S8
Bi(NO3)3 Bi(NO3)3 Bi(NO3)3 Bi(NO3)3 Bi(NO3)3 Bi(NO3)3 Bi(NO3)3 Bi(NO3)3
NH4VO3 NH4VO3 NH4VO3 NH4VO3 NH4VO3 NH4VO3 NH4VO3 NH4VO3
DEG/H2O EG/H2O Glycerol/H2O 0.1 M mannitol DEG/H2O DEG/H2O DEG/H2O DEG/H2O
1 1 1 1 2 4 6 8
Nut-like Nut-like Potato-like Broccoli-like Nanoparticles Nanoparticles Nanoparticles Network-like
The amount of Bi(NO3)3 and NH4VO3 was both 1 mmol. The volume of solvent was 30 mL.
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Fig. 1. XRD pattern (a), SEM images (b and c) and EDX spectrum (d) of the as-prepared calcined BiVO4 product in DEG/H2O mixed solvent (S1).
solvent, broccoli-like BiVO4 crystals with pores on the surface were formed (S4, Fig. 2d). The results revealed that the solvent played an important role in the formation of m-BiVO4 crystals with different morphologies. The variation in the morphology of the m-BiVO4 crystals might be attributed to the different physicochemical properties of the solvents [42]. 3.3. Effect of pH value The influence of pH value on the formation of BiVO4 nanostructures was also investigated. Fig. 3 showed the XRD patterns, SEM and TEM images of BiVO4 nanostructures prepared in DEG/H2O solvent at different pH values (S5–S8). It demonstrated that m-BiVO4
could be obtained in DEG/H2O solvent when pH value was 2 (S5), 4 (S6) and 6 (S7), respectively, as confirmed by the XRD patterns (Fig. 3a). However, when the pH value of the reaction system was increased to 8 (S8), orthorhombic Bi4V2O11 structure (JCPDS 42-0135) was formed. The morphology and structure of the BiVO4 samples obtained at different pH values were further characterized by SEM and TEM. Fig. 3b and c showed SEM and TEM images of mBiVO4 prepared at pH value of 2 (S5). It clearly revealed that BiVO4 nanoparticles with an average size of about 150 nm were formed. The SEM and TEM images of the BiVO4 sample prepared at pH value of 4 (S6) were illustrated in Fig. 3d and e, respectively. It was observed that the product also consisted of nanoparticles. Differently, most of BiVO4 nanoparticles connected with each other to
Fig. 2. XRD patterns (a) and SEM images of calcined BiVO4 products synthesized in different solvents: (b) EG/H2O, S2; (c) glycerol/H2O, S3; (d) 0.1 M mannitol, S4.
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form large nanoparticles with an average diameter of about 200 nm. With further increasing pH value to 6 (S7), bulky and relative large nanoparticles with a diameter about 500 nm were fabricated (Fig. 3f and g). The morphology of Bi4V2O11 sample prepared at alkaline condition (pH = 8, S8) was depicted in Fig. 3h and i. Network-like Bi4V2O11 microspheres with a diameter
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about 3 lm were fabricated, which were constructed by numerous inter-crossed nanosheets. It indicated that pH value has a great influence on the crystal phase, shape, and size of the final product, and low pH was preferential for the generation of m-BiVO4 with smaller size. Similar phenomenon was also reported in the literatures which demonstrated that pH value of the reaction system had a significant influence on morphology, structures and the growth processes of BiVO4 nanostructures [7,20,22,43–45]. On the basis of the experimental results, it was proposed that the difference in morphology and size of m-BiVO4 might be ascribed to the variation in the concentration of H+ depending on pH value. At low pH value (S1), for instance, the concentration of H+ was high enough to restrain the hydrolysis of Bi(NO3)3 to BiONO3 [43,46]. Thus, the concentration of free Bi3+ in the reaction system was high, which resulted in the formation of BiVO4 with large particle size due to its rapid crystal growth [17]. With the increase of pH value to 2 (S5), relatively fewer amount of free Bi3+ existed in the reaction system, therefore, smaller BiVO4 nanoparticles were generated due to the fast nucleation and comparatively slow crystal growth rate [47]. On the contrary, high pH (S6, S7) promoted hydrolysis of Bi(NO3)3 and thus led to pretty low Bi3+ concentration in reaction system, which consequently also resulted in the formation of large m-BiVO4 nanocrystals. Nevertheless, our present understanding of the formation and growth of m-BiVO4 at different pH values is limited, and further investigation is still in progress. 3.4. Photophysical property Diffuse reflectance spectroscopy (DRS) is a useful tool for characterizing the optical absorption property of photocatalyst which is viewed as a critical factor to influence its photocatalytic activity [48]. Fig. 4a depicted the DRS spectra of the as-synthesized BiVO4 samples (S5–S8). All the samples showed a strong absorption in visible-light region. Meanwhile, it was found that the samples showed slight red shift with the decreasing of the size of assynthesized m-BiVO4 products (S5–S7), which might be attributed to the quantum-confined Stark effect [49]. As a crystalline semiconductor, the band gap of a semiconductor could be estimated from the formula ahm = A(hm Eg)n/2, where a, m, Eg and A are absorption coefficient, light frequency, band gap energy and a constant, respectively. Among them, n depends on the characteristic of the transition in a semiconductor (i.e. n = 1 for direct transition or n = 4 for indirect transition). As BiVO4 exhibits the characteristic of direct band transition [50], the value of n is 1. The band gap energies of the BiVO4 samples could be thus estimated from a plot of (ahm)2 versus the photon energy (hm). The intercept of the tangents to the x-axis will give a good approximation of the band gap energy for the products. As shown in Fig. 4b, the estimated band gap of the as-synthesized products (S5–S8) was about 2.29, 2.27, 1.85 and 2.01 eV, respectively. It clearly demonstrated that the band gaps of BiVO4 nanoparticles with different sizes and crystal phases were obviously different. The variation in the band gap might lead to different degrees of delocalization of photogenerated electron-hole pair, which probably resulted in different photocatalytic efficiency. However, the obtained m-BiVO4 microstructures with relatively large crystal sizes (S1–S4) displayed similar optical properties, as shown in Fig. S3 (Supplementary Materials). 3.5. Photocatalytic activity
Fig. 3. XRD patterns (a), SEM and TEM images of the calcined BiVO4 samples prepared in DEG/H2O under different pH values: (b and c) pH 2, S5; (d and e) pH 4, S6; (f and g) pH 6, S7; (h and i) pH 8, S8.
The photocatalytic activities of different BiVO4 products were evaluated by the degradation of RhB solution upon visible light irradiation. Fig. 5 showed the time-dependent absorption spectra of RhB solution during the photodegradation process in the presence of different BiVO4 samples (S5–S8). The absorption peaks at 554 nm corresponding to RhB diminished gradually as the
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Fig. 4. UV–vis diffuses reflectance spectra (a) and the (ahm)2 versus energy (hm) curve (b) of the products prepared at different pH values.
irradiation time extended. In the presence of S5, in particular, the absorption peak dramatically decreased and completely disappeared after about 5 h. It suggested that S5 exhibited the highest photocatalytic activity compared with other BiVO4 samples. The variation of RhB concentrations (C/C0) with irradiation time over the different BiVO4 samples was shown in Fig. 6, where C0 is the initial concentration of RhB solution before irradiation and C is the concentration of RhB solution at t time. For comparison, direct photolysis of RhB in the absence of BiVO4 and RhB degradation over commercial TiO2 were performed under the identical conditions. It was clearly observed that RhB concentration in the absence of BiVO4 sample and in presence of commercial TiO2 hardly changed with the increase of irradiation time. The as-synthesized BiVO4 products with different crystal phases, morphologies and sizes exhibited remarkable variation in photocatalytic activities for RhB degradation. Noticeably, the m-BiVO4 products (S5–S7) demonstrated better photocatalytic activities than that of Bi4V2O11 product (S8). Among m-BiVO4 products, S5 exhibited superior photocatalytic activity than other m-BiVO4 products (S6 and S7). It was also found that photocatalytic activities decreased with the in-
Fig. 5. UV–vis absorption spectra of the RhB solution (1 10 (a) S5; (b) S6; (c) S7 and (d) S8, respectively.
5
crease of the particle size and the decrease of the band gap. It was probably because the opportunities for recombination of the electrons and holes could be reduced in wide band gap, and the surface area of small particle size provided more active sites during photocatalytic process [47,51]. The superior activity of S5 might be ascribed to its relatively wide band gap, small particle size and monoclinic phase. The orthorhombic Bi4V2O11 (S8) with large band gap possessed the lowest photocatalytic efficiency, similar with the reported literatures [7,44]. Interestingly, it was also found that samples S1–S4 exhibited almost same photocatalytic activities for RhB degradation (Fig. S4, Supplementary Materials), which might mainly result from their pretty similar optical properties. It indicated that the photocatalytic activity of the as-synthesized products was closely related with its crystal phase, size and band gap, and that m-BiVO4 exhibited high visible-light-driven efficiency. Considering the practical application, it is important and necessary to investigate the reusability and stability of a photocatalyst. In this work, four cycles of the photocatalytic experiment for the BiVO4 nanoparticles (S5) were carried out. As shown in Fig. 7, the as-synthesized BiVO4 nanoparticles (S5) exhibited almost no
M, 100 mL) in the presence of 0.05 g of different calcined BiVO4 nanostructures under visible light irradiation:
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Century Excellent Talents in University (NCET-09-0136) and Open Research Fund of Engineering Research Center of Nano-Geomaterials of Ministry of Education, China University of Geosciences (CUGNGM 201202). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jallcom. 2012.11.017. References Fig. 6. Photodegradation of RhB over calcined BiVO4 nanostructures (S5–S8) synthesized under different pH values and commercial TiO2.
Fig. 7. Recycling results of RhB photodegradation over BiVO4 nanostructures (S5).
changes in photocatalytic activity during each recycle photocatalytic experiment. In addition, the BiVO4 photocatalyst also showed good stability in the photocatalytic reaction, which was confirmed by the XRD pattern and SEM image of the BiVO4 sample (S5) after the photocatalysis. No change was observed in its composition and morphology, as shown in Fig. S5 (Supplementary Materials). It indicated that the as-synthesized BiVO4 photocatalyst possessed favorable recycling characteristics and high stability. 4. Conclusions A facile microwave and ultrasonic wave combined technique has been successfully developed for the synthesis of BiVO4 micro-/nanostructures. m-BiVO4 with various morphologies including nut-like, potato-like and broccoli-like BiVO4 structures were obtained in different solvents. Different crystal phases could be formed at various pH values, and low pH facilitated the generation of m-BiVO4. It demonstrated that the morphology, size and crystal phase of BiVO4 product could be modulated by varying the solvent and pH value of the reaction system. The obtained BiVO4 micro-/ nanostructures possessed different optical properties and photocatalytic activities for the RhB degradation under visible light irradiation. The variation of photocatalytic performance was probably attributed to the difference in band gap, crystalline phase and particle size. This work not only develops a facile, promising method for the synthesis of monoclinic BiVO4 nanostructures, but also provides a new strategy for the preparation of other nanomaterials. Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant 21171136, 20801043), the Program for New
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Monoclinic BiVO4 micro-/nanostructures: Microwave and ultrasonic wave combined synthesis and their visible-light photocatalytic activities Yafang Zhang a,1, Guangfang Li a,1, Xiaohui Yang a, Hao Yang a, Zhong Lu a, Rong Chen a,b,⇑ a Key Laboratory for Green Chemical Process of Ministry of Education and Hubei Novel Reactor & Green Chemical Technology Key Laboratory, Wuhan Institute of Technology, Xiongchu Avenue, Wuhan 430073, PR China b Engineering Research Center of Nano-Geomaterials of Ministry of Education, China University of Geosciences, Lumo Road, Wuhan 430074, PR China
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Article history: Received 15 August 2012 Received in revised form 2 November 2012 Accepted 2 November 2012 Available online 16 November 2012 Keywords: Bismuth vanadate Microwave Ultrasonic wave Photocatalysis
a b s t r a c t Monoclinic bismuth vanadate (m-BiVO4) micro-/nanostructures with different sizes and morphologies were successfully prepared via a facile and rapid microwave and ultrasonic wave combined technique. The obtained BiVO4 products were characterized by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), transmission electron microscopy (TEM) and UV–vis diffuse reflection spectroscopy (DRS). It was found that the solvent and pH value had a significant influence on morphology, size and crystalline structure of the product. Nut-like, potato-like and broccoli-like monoclinic BiVO4 were fabricated in different solvents. The crystal phase could be modulated by varying the pH value of reaction system. The photocatalytic activities of the products were also evaluated by the degradation of Rhodamine B (RhB) under visible light irradiation. The result revealed that the photocatalytic activities of BiVO4 nanostructures were closely related to the crystalline phase, band gap and particle size. Monoclinic BiVO4 nanoparticles with small crystal size and large band gap exhibited remarkable photocatalytic performance. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction As an alternative green technology, photocatalysis has attracted considerable research interest due to its potential applications in renewable energy and environmental remediation [1–5]. Over the past few decades, much effort has been made to develop novel and effective semiconductor photocatalysts, which serve a crucial issue in photocatalytic process [1,6–9]. However, the most widely used photocatalyst TiO2 is only active under ultraviolet (UV) light irradiation which accounts for a very small portion of sunlight spectrum. Therefore, developing visible-light-responsive photocatalysts with a good photocatalytic performance under sunlight irradiation represents an alternative technique in photocatalysis field [10–12]. Bismuth vanadate (BiVO4) micro-/nanostructures are of special attention, as one of the most promising visible-light-driven photocatalysts, by virtue of its fascinating photophysical and photochemical properties. Various BiVO4 nanostructures reported exhibit outstanding photocatalytic activities in degradation of ⇑ Corresponding author at: Key Laboratory for Green Chemical Process of Ministry of Education and Hubei Novel Reactor & Green Chemical Technology Key Laboratory, Wuhan Institute of Technology, Xiongchu Avenue, Wuhan 430073, PR China. Tel.: +86 13659815698; fax: +86 2787195671. E-mail address: [email protected] (R. Chen). 1 These authors contributed equally to this work. 0925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.11.017
different organic pollutants [13–16], water splitting for O2 and H2 evolution [8,17], and destruction of microorganisms [18], under visible light irradiation. Most commonly, BiVO4 has three main crystal phases, including tetragonal zircon, monoclinic and tetragonal scheelite structures [19]. In literatures, it has been well-documented that monoclinic BiVO4 (m-BiVO4) nanostructures displayed much better photocatalytic performance than that of other crystal phases under visible light irradiation [15,20–23]. Furthermore, the shape and size of nanostructures have a significantly direct influence on their photocatalytic activities. Therefore, an increasing interest has been focused on controllable fabrication of m-BiVO4 nanostructures by different preparation routes in recent years [14,24,25]. Unfortunately, the methods mostly reported involved harsh experimental conditions such as solid-state or melting reaction at high temperature, uncertainty of the reaction process, long reaction time, and organic additive or template, which dramatically limited the large-scale production of m-BiVO4. Therefore, it is highly desired to develop a facile and environmentfriendly route for tunable synthesis of m-BiVO4. Recently, microwave assisted heating technique and sonochemistry method are particularly attractive in a range of synthetic fields [26–29], particularly in inorganic materials preparation. Microwave irradiation provides great advantages of uniform and rapid dielectric heating without thermal gradient effect, which greatly promotes the reaction rate and results in a high-throughput capacity. It has been recognized as a facile and mild synthetic
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strategy for synthesis of various nanostructures with controllable shape and size [30–35]. And sonochemistry method powered by high-frequency sound waves could produce an exceedingly high transient temperature and pressure, which promotes both physical effects and chemical reactions during the formation process of various nanostructures [36–39]. Based on these work, it is believed that the combination of microwave and ultrasonic wave method could be employed for controllable preparation of nanostructures. As a matter of fact, Poinern et al. have demonstrated that this combined technique could be used to fabricate nano-structured hydroxyapatite [40]. Our previous work also has successfully prepared well-ordered flower-like ZnO nanostructures through the microwave and ultrasonic combined technique [41]. To the best of our knowledge, however, there is no report on the fabrication of BiVO4 nanostructures by microwave and ultrasonic wave combined technique. In this work, m-BiVO4 micro-/nanostructures with different sizes and morphologies were synthesized by a microwave and ultrasonic wave combined technique in different solvents. The photocatalytic performance of the as-synthesized BiVO4 nanostructures was also evaluated by the degradation of Rhodamine B (RhB) under visible light irradiation. 2. Experimental sections 2.1. Chemicals Bismuth nitrate pentahydrate (Bi(NO3)35H2O), sodium bicarbonate (NaHCO3) and Rhodamine B (RhB) were purchased from Aladdin. Ammonium metavanadate (NH4VO3), diethylene glycol (DEG), ethylene glycol (EG), glycerol and mannitol were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). All the reagents were analytical grade and used directly without further purification. 2.2. Preparation In a typical synthesis, 0.485 g Bi(NO3)35H2O and 0.117 g NH4VO3 were dissolved in 15 mL diethylene glycol (DEG) and 15 mL deionized water, respectively. Then the solutions were mixed, and a yellow-milky suspension was formed immediately (pH 1). The mixture was heated to 110 °C within 3 min and then kept at that temperature for 27 min by a microwave and ultrasonic wave combined reactor (XH-300A, Beijing Xianghu Technology Co., Ltd.) under the microwave irradiation power at 500 W and the ultrasonic irradiation power at 800 W (2 s sonication and 1 s interruption) (Fig. S1, Supplementary Materials). After reaction, the solution was cooled down to room temperature naturally. The precipitate was collected by centrifugation and washed with deionized water and absolute ethanol for five times, followed by drying at 60 °C overnight. Finally, the dry sample was calcined at 500 °C for 2 h in a muffle and obtained sample S1. Other BiVO4 samples (S2– S8) were also prepared under the identical conditions by varying the solvent and pH value of the reaction system (adjusted by saturated NaHCO3), respectively. The detailed experimental parameters are listed in Table 1. 2.3. Characterization Powder X-ray diffraction (XRD) was carried out on Bruker axs D8 Discover (Cu Ka = 1.5406 Å) at a scan rate of 2° min 1 in the 2h range from 10° to 80°. Scanning electron microscopy (SEM) images and energy dispersive X-ray (EDX) spectra were taken on a Hitachi S4800 scanning electron microscope operating at 5.0 kV. Transmission electron microscopy (TEM) images were recorded on a Philips Tecnai 20 electron microscope, using an accelerating voltage of 200 kV. UV–vis diffuse reflec-
tance spectra (DRS) were recorded on a UV–vis spectrometer (Shimadzu UV-2550) by using BaSO4 as a reference and were converted from reflection to absorbance by the Kubelka–Munk method. 2.4. Evaluation of photocatalytic activity Photocatalytic activities of the as-synthesized BiVO4 samples were evaluated by the degradation of RhB under visible light irradiation using a 500 W Xe lamp with a 400 nm cut-off filter (Fig. S2, Supplementary Materials). In the typical experiment, the as-prepared BiVO4 sample (0.05 g) was added into 100 mL RhB solution (1 10 5 M). Prior to irradiation, the suspension was sonicated for 10 min and magnetically stirred for 10 min in the dark to ensure the establishment of an adsorption–desorption equilibrium. Then, the solution was exposed to visible light irradiation under magnetic stirring. At each given time interval, 4 mL suspension was sampled and centrifuged to remove the BiVO4 powder. The concentration of RhB during the degradation was determined using a Shimadzu UV2800 spectrophotometer. All the measurements were carried out at room temperature.
3. Results and discussion 3.1. Structure and morphology The phase and composition of the as-synthesized product (S1) was identified by powder X-ray diffraction (XRD) analysis. Fig. 1a showed the typical XRD pattern of the product prepared in DEG/ H2O mixed solvent (pH 1), which could be well indexed to the standard data of the pure monoclinic BiVO4 (JCPDS 14-0688). No impurity peaks were observed in the pattern, indicative of high purity of the product. The intense and sharp diffraction peaks suggested that the as-prepared product was well-crystallized. The size and morphology of the product (S1) were examined by SEM. As shown in Fig. 1b, a large quantity of uniform nut-like BiVO4 microstructures with rough surface and sharp edges were obtained. The high-magnification SEM image in Fig. 1c further demonstrated the product was composed of many aggregated particles, which closely linked together layer by layer. The elemental composition of the as-synthesized nut-like BiVO4 microstructures (S1) was also confirmed by energy dispersive X-ray (EDX) analysis (Fig. 1d). The result observed indicated that the sample was composed of Bi, V, and O elements with the atom ratio of about 1:1:4, which was in good agreement with the stoichiometry of BiVO4. 3.2. Effect of solvents Monoclinic BiVO4 products could also be obtained in other solvents, including ethylene glycol/H2O (EG/H2O) (S2), glycerol/H2O (S3) and 0.1 M mannitol solution (S4). The corresponding XRD patterns (Fig. 2a) indicated that well-crystallized monoclinic BiVO4 were obtained in different solvents under the identical conditions. The typical SEM image (Fig. 2b) showed the formation of nut-like BiVO4 microstructures in EG/H2O mixture (S2). The high-resolution SEM image revealed the smooth surface of the obtained BiVO4 microstructures (inset of Fig. 2b). As shown in Fig. 2c, potato-like BiVO4 microstructures with rough surface were fabricated in glycerol/H2O solution (S3). When mannitol solution was employed as
Table 1 Experimental conditions for the preparation of BiVO4 micro-/nanostructures.a
a
Samples
Bismuth precursor
Vanadium precursor
Solvent
pH
Morphology
S1 S2 S3 S4 S5 S6 S7 S8
Bi(NO3)3 Bi(NO3)3 Bi(NO3)3 Bi(NO3)3 Bi(NO3)3 Bi(NO3)3 Bi(NO3)3 Bi(NO3)3
NH4VO3 NH4VO3 NH4VO3 NH4VO3 NH4VO3 NH4VO3 NH4VO3 NH4VO3
DEG/H2O EG/H2O Glycerol/H2O 0.1 M mannitol DEG/H2O DEG/H2O DEG/H2O DEG/H2O
1 1 1 1 2 4 6 8
Nut-like Nut-like Potato-like Broccoli-like Nanoparticles Nanoparticles Nanoparticles Network-like
The amount of Bi(NO3)3 and NH4VO3 was both 1 mmol. The volume of solvent was 30 mL.
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Fig. 1. XRD pattern (a), SEM images (b and c) and EDX spectrum (d) of the as-prepared calcined BiVO4 product in DEG/H2O mixed solvent (S1).
solvent, broccoli-like BiVO4 crystals with pores on the surface were formed (S4, Fig. 2d). The results revealed that the solvent played an important role in the formation of m-BiVO4 crystals with different morphologies. The variation in the morphology of the m-BiVO4 crystals might be attributed to the different physicochemical properties of the solvents [42]. 3.3. Effect of pH value The influence of pH value on the formation of BiVO4 nanostructures was also investigated. Fig. 3 showed the XRD patterns, SEM and TEM images of BiVO4 nanostructures prepared in DEG/H2O solvent at different pH values (S5–S8). It demonstrated that m-BiVO4
could be obtained in DEG/H2O solvent when pH value was 2 (S5), 4 (S6) and 6 (S7), respectively, as confirmed by the XRD patterns (Fig. 3a). However, when the pH value of the reaction system was increased to 8 (S8), orthorhombic Bi4V2O11 structure (JCPDS 42-0135) was formed. The morphology and structure of the BiVO4 samples obtained at different pH values were further characterized by SEM and TEM. Fig. 3b and c showed SEM and TEM images of mBiVO4 prepared at pH value of 2 (S5). It clearly revealed that BiVO4 nanoparticles with an average size of about 150 nm were formed. The SEM and TEM images of the BiVO4 sample prepared at pH value of 4 (S6) were illustrated in Fig. 3d and e, respectively. It was observed that the product also consisted of nanoparticles. Differently, most of BiVO4 nanoparticles connected with each other to
Fig. 2. XRD patterns (a) and SEM images of calcined BiVO4 products synthesized in different solvents: (b) EG/H2O, S2; (c) glycerol/H2O, S3; (d) 0.1 M mannitol, S4.
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form large nanoparticles with an average diameter of about 200 nm. With further increasing pH value to 6 (S7), bulky and relative large nanoparticles with a diameter about 500 nm were fabricated (Fig. 3f and g). The morphology of Bi4V2O11 sample prepared at alkaline condition (pH = 8, S8) was depicted in Fig. 3h and i. Network-like Bi4V2O11 microspheres with a diameter
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about 3 lm were fabricated, which were constructed by numerous inter-crossed nanosheets. It indicated that pH value has a great influence on the crystal phase, shape, and size of the final product, and low pH was preferential for the generation of m-BiVO4 with smaller size. Similar phenomenon was also reported in the literatures which demonstrated that pH value of the reaction system had a significant influence on morphology, structures and the growth processes of BiVO4 nanostructures [7,20,22,43–45]. On the basis of the experimental results, it was proposed that the difference in morphology and size of m-BiVO4 might be ascribed to the variation in the concentration of H+ depending on pH value. At low pH value (S1), for instance, the concentration of H+ was high enough to restrain the hydrolysis of Bi(NO3)3 to BiONO3 [43,46]. Thus, the concentration of free Bi3+ in the reaction system was high, which resulted in the formation of BiVO4 with large particle size due to its rapid crystal growth [17]. With the increase of pH value to 2 (S5), relatively fewer amount of free Bi3+ existed in the reaction system, therefore, smaller BiVO4 nanoparticles were generated due to the fast nucleation and comparatively slow crystal growth rate [47]. On the contrary, high pH (S6, S7) promoted hydrolysis of Bi(NO3)3 and thus led to pretty low Bi3+ concentration in reaction system, which consequently also resulted in the formation of large m-BiVO4 nanocrystals. Nevertheless, our present understanding of the formation and growth of m-BiVO4 at different pH values is limited, and further investigation is still in progress. 3.4. Photophysical property Diffuse reflectance spectroscopy (DRS) is a useful tool for characterizing the optical absorption property of photocatalyst which is viewed as a critical factor to influence its photocatalytic activity [48]. Fig. 4a depicted the DRS spectra of the as-synthesized BiVO4 samples (S5–S8). All the samples showed a strong absorption in visible-light region. Meanwhile, it was found that the samples showed slight red shift with the decreasing of the size of assynthesized m-BiVO4 products (S5–S7), which might be attributed to the quantum-confined Stark effect [49]. As a crystalline semiconductor, the band gap of a semiconductor could be estimated from the formula ahm = A(hm Eg)n/2, where a, m, Eg and A are absorption coefficient, light frequency, band gap energy and a constant, respectively. Among them, n depends on the characteristic of the transition in a semiconductor (i.e. n = 1 for direct transition or n = 4 for indirect transition). As BiVO4 exhibits the characteristic of direct band transition [50], the value of n is 1. The band gap energies of the BiVO4 samples could be thus estimated from a plot of (ahm)2 versus the photon energy (hm). The intercept of the tangents to the x-axis will give a good approximation of the band gap energy for the products. As shown in Fig. 4b, the estimated band gap of the as-synthesized products (S5–S8) was about 2.29, 2.27, 1.85 and 2.01 eV, respectively. It clearly demonstrated that the band gaps of BiVO4 nanoparticles with different sizes and crystal phases were obviously different. The variation in the band gap might lead to different degrees of delocalization of photogenerated electron-hole pair, which probably resulted in different photocatalytic efficiency. However, the obtained m-BiVO4 microstructures with relatively large crystal sizes (S1–S4) displayed similar optical properties, as shown in Fig. S3 (Supplementary Materials). 3.5. Photocatalytic activity
Fig. 3. XRD patterns (a), SEM and TEM images of the calcined BiVO4 samples prepared in DEG/H2O under different pH values: (b and c) pH 2, S5; (d and e) pH 4, S6; (f and g) pH 6, S7; (h and i) pH 8, S8.
The photocatalytic activities of different BiVO4 products were evaluated by the degradation of RhB solution upon visible light irradiation. Fig. 5 showed the time-dependent absorption spectra of RhB solution during the photodegradation process in the presence of different BiVO4 samples (S5–S8). The absorption peaks at 554 nm corresponding to RhB diminished gradually as the
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Fig. 4. UV–vis diffuses reflectance spectra (a) and the (ahm)2 versus energy (hm) curve (b) of the products prepared at different pH values.
irradiation time extended. In the presence of S5, in particular, the absorption peak dramatically decreased and completely disappeared after about 5 h. It suggested that S5 exhibited the highest photocatalytic activity compared with other BiVO4 samples. The variation of RhB concentrations (C/C0) with irradiation time over the different BiVO4 samples was shown in Fig. 6, where C0 is the initial concentration of RhB solution before irradiation and C is the concentration of RhB solution at t time. For comparison, direct photolysis of RhB in the absence of BiVO4 and RhB degradation over commercial TiO2 were performed under the identical conditions. It was clearly observed that RhB concentration in the absence of BiVO4 sample and in presence of commercial TiO2 hardly changed with the increase of irradiation time. The as-synthesized BiVO4 products with different crystal phases, morphologies and sizes exhibited remarkable variation in photocatalytic activities for RhB degradation. Noticeably, the m-BiVO4 products (S5–S7) demonstrated better photocatalytic activities than that of Bi4V2O11 product (S8). Among m-BiVO4 products, S5 exhibited superior photocatalytic activity than other m-BiVO4 products (S6 and S7). It was also found that photocatalytic activities decreased with the in-
Fig. 5. UV–vis absorption spectra of the RhB solution (1 10 (a) S5; (b) S6; (c) S7 and (d) S8, respectively.
5
crease of the particle size and the decrease of the band gap. It was probably because the opportunities for recombination of the electrons and holes could be reduced in wide band gap, and the surface area of small particle size provided more active sites during photocatalytic process [47,51]. The superior activity of S5 might be ascribed to its relatively wide band gap, small particle size and monoclinic phase. The orthorhombic Bi4V2O11 (S8) with large band gap possessed the lowest photocatalytic efficiency, similar with the reported literatures [7,44]. Interestingly, it was also found that samples S1–S4 exhibited almost same photocatalytic activities for RhB degradation (Fig. S4, Supplementary Materials), which might mainly result from their pretty similar optical properties. It indicated that the photocatalytic activity of the as-synthesized products was closely related with its crystal phase, size and band gap, and that m-BiVO4 exhibited high visible-light-driven efficiency. Considering the practical application, it is important and necessary to investigate the reusability and stability of a photocatalyst. In this work, four cycles of the photocatalytic experiment for the BiVO4 nanoparticles (S5) were carried out. As shown in Fig. 7, the as-synthesized BiVO4 nanoparticles (S5) exhibited almost no
M, 100 mL) in the presence of 0.05 g of different calcined BiVO4 nanostructures under visible light irradiation:
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Century Excellent Talents in University (NCET-09-0136) and Open Research Fund of Engineering Research Center of Nano-Geomaterials of Ministry of Education, China University of Geosciences (CUGNGM 201202). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jallcom. 2012.11.017. References Fig. 6. Photodegradation of RhB over calcined BiVO4 nanostructures (S5–S8) synthesized under different pH values and commercial TiO2.
Fig. 7. Recycling results of RhB photodegradation over BiVO4 nanostructures (S5).
changes in photocatalytic activity during each recycle photocatalytic experiment. In addition, the BiVO4 photocatalyst also showed good stability in the photocatalytic reaction, which was confirmed by the XRD pattern and SEM image of the BiVO4 sample (S5) after the photocatalysis. No change was observed in its composition and morphology, as shown in Fig. S5 (Supplementary Materials). It indicated that the as-synthesized BiVO4 photocatalyst possessed favorable recycling characteristics and high stability. 4. Conclusions A facile microwave and ultrasonic wave combined technique has been successfully developed for the synthesis of BiVO4 micro-/nanostructures. m-BiVO4 with various morphologies including nut-like, potato-like and broccoli-like BiVO4 structures were obtained in different solvents. Different crystal phases could be formed at various pH values, and low pH facilitated the generation of m-BiVO4. It demonstrated that the morphology, size and crystal phase of BiVO4 product could be modulated by varying the solvent and pH value of the reaction system. The obtained BiVO4 micro-/ nanostructures possessed different optical properties and photocatalytic activities for the RhB degradation under visible light irradiation. The variation of photocatalytic performance was probably attributed to the difference in band gap, crystalline phase and particle size. This work not only develops a facile, promising method for the synthesis of monoclinic BiVO4 nanostructures, but also provides a new strategy for the preparation of other nanomaterials. Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant 21171136, 20801043), the Program for New
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