- Email: [email protected]
Hydrothermal synthesis of hierarchical flower-like Bi2WO6 microspheres with enhanced visible-light photoactivity
Materials Letters 157 (2015) 158–162
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Hydrothermal synthesis of hierarchical flower-like Bi2WO6 microspheres with enhanced visible-light photoactivity Yumin Liu n, Zhiwei Ding, Hua Lv, Jing Guang, Shuang Li, Juhui Jiang Collaborative Innovation Center of Henan Province for Green Manufacturing of Fine Chemicals, School of Chemistry and Chemical Engineering, Henan Normal University, Xinxiang, Henan 453007, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 31 March 2015 Accepted 10 May 2015 Available online 16 May 2015
Three-dimensional (3D) hierarchical flower-like Bi2WO6 microspheres assembled from 2D nanosheets were successfully prepared by a simple hydrothermal route using the non-ionic surfactant F127 (EO100– PO65–EO100) as the morphology director. On the basis of the evolution of the morphologies as a function of reaction time, a possible formation mechanism for the hierarchical architecture was proposed. The photocatalytic activities of the as-prepared Bi2WO6 samples were evaluated for the degradation of Rhodamine B (RhB) under visible-light irradiation. Due to the combined effects of large surface area and efficient separation of charge carriers, the hierarchical flower-like Bi2WO6 microspheres exhibited improved photocatalytic performances. & 2015 Elsevier B.V. All rights reserved.
Keywords: Bi2WO6 F127 Semiconductor Crystal growth
1. Introduction Over the past few years, visible-light-driven photocatalysts have attracted extensive attention in view of the efficient utilization of solar energy. Bismuth tungstate (Bi2WO6), one of the simplest Aurivillius oxides, was found to possess excellent photocatalytic performance for water splitting and photodegradation of organic contaminants under visible light irradiation [1]. Generally, nanoscale Bi2WO6 photocatalysts exhibit good photocatalytic activity due to their relatively large specific surface areas and low recombination rates of photoinduced charges. Hence, considerable efforts have been devoted to the synthesis of nanosized Bi2WO6 photocatalysts with various nanostructures including nanoplates [2], nanoparticles [3], nanocages [4] etc. Nevertheless, in view of practical applications, how to separate and recycle the nano-photocatalyst effectively after fulfillment of the photocatalysis process has become an issue. Comparatively speaking, 3D hierarchical structures constructed by nanosized building blocks have the advantages of both mico- and nano-sized materials, such as easy separation and recyclability, abundant transport paths for reactant molecules, efficient separation of charge carriers and high photocatalytic performance. Recently, Bi2WO6 with 3D hierarchical structures have been fabricated by several research groups. For example, Qian et al. produced hierarchical Bi2WO6 architectures by a simple inorganic salt-assisted hydrothermal method [5]. Tayade
n
Corresponding author. Tel.: þ 86 373 3326335; fax: þ86 373 3326336. E-mail address: [email protected] (Y. Liu).
http://dx.doi.org/10.1016/j.matlet.2015.05.024 0167-577X/& 2015 Elsevier B.V. All rights reserved.
et al. successfully prepared spherical and flower-like Bi2WO6 architectures with and without SiO2 protected calcination [6]. However, despite of these recent progresses, it still remains a great challenge to develop a simple and reliable method for the synthesis of 3D Bi2WO6 hierarchical structures with high visiblelight-driven photocatalytic activity. Herein, we report the fabrication of hierarchical flower-like Bi2WO6 microspheres assembled from 2D nanosheets by an efficient F127-assisted hydrothermal method. The possible formation mechanism of hierarchical architectures was proposed on the basis of a series of time-dependent experiments. The photodegradation of RhB solution was carried out to evaluate the photocatalytic performances of the as-prepared Bi2WO6 photocatalysts under visible light irradiation.
2. Experimental Materials and synthesis: In a typical procedure, 0.12 g Pluronic F-127 was mixed with 20 mL of 0.001 M (NH4)10W12O41 6H2O solution under magnetic stirring until a homogeneous solution was obtained. Then, 20 mL of 0.024 M Bi(NO3) 5 H2O was added to the above solution dropwise. The as-formed mixture solution was transferred into a 100 mL Teflon-lined autoclave and kept at 180 1C for 24 h. After the autoclave was cooled down to room temperature naturally, the as-obtained white precipitation was separated by centrifugation, washed with distilled water and absolute ethanol and dried under vacuum at 60 1C for 12 h. To remove F127, the as-prepared samples were calcined in muffle furnace at 400 1C for 1 h to obtain the Bi2WO6 microspheres
Y. Liu et al. / Materials Letters 157 (2015) 158–162
(denoted as BWO-F127). For comparison, Bi2WO6 sample was also synthesized without using F127 (denoted as BWO), while the other experimental conditions remained the same. Characterization: Characterization of the samples was carried out by employing X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), UV–vis Diffuse Reflectance Spectroscopy, BET surface area analysis, Fluorescence Spectrophotometer (PL). The transient photocurrent responses of samples were measured under visible light irradiation by electrochemical workstation and the detailed process was described in our previous study [7]. Photocatalytic test: Typically, 0.1 g of Bi2WO6 sample was added into 100 mL RhB solution (5 mg L 1). Before irradiation, the suspension was dispersed in an ultrasonic bath for 10 min and then stirred for 30 min in the dark to establish the adsorption– desorption equilibrium. After that, the solution was irradiated by a 300 W xenon lamp equipped with a UV cut off filter (λ 4400 nm). At given time intervals, 5 mL suspension was taken from the reactor and centrifuged to remove the photocatalyst particles. The concentration of RhB solution was analyzed using an UV–vis spectrophotometer at λmax of 553 nm.
3. Results and discussion The crystal structure and phase purity of the samples were characterized by XRD. As shown in Fig. 1a, the diffraction peaks of both samples can be assigned to the pure orthorhombic phase of Bi2WO6 (JCPDS Card no. 39-0256). No peaks of other impurities or phases can be detected, indicating the high phase purity of the asobtained samples. Moreover, there are no obvious differences in diffraction peak intensity for both samples, indicating that F127 has no effects on the crystallinity of BWO-F127. The typical SEM images of BWO and BWO-F127 are shown in Fig. 1b and c, respectively. The obtained BWO, which was synthesized without F127, was composed of aggregates of irregular flakes
159
and particles. On the other hand, 3D hierarchical microspheres with the average diameters varying from 4 to 5 μm were observed in BWO-F127. These microspheres exhibit a flower-like microstructure and each flower is built from numerously welldeveloped nanosheets as the petals. As illustrated in Fig. 1c, these nanosheets are interwoven with each other to form a porous structure, which can serve as transport paths for reactant molecules and thereby enhance the photocatalytic performances of the samples. Fig. 1d is the TEM image of an individual microsphere, which further proves the flower-like microstructure of asprepared BWO-F127 sample. In the HRTEM image (inset of Fig. 1d) taken from the edge of the individual microsphere, the lattice interplanar spacing is determined as 0.273 nm, corresponding to the (020) plane of orthorhombic Bi2WO6. To reveal the growth process of the flower-like Bi2WO6 microspheres, SEM observations were carried out at different time periods as depicted in Fig. 2. Irregular small Bi2WO6 nanoparticles and their aggregates were firstly developed within the initial 1 h (Fig. 2a). As the reaction time was prolonged to 6 h (Fig. 2b), the Bi2WO6 crystals tended to form sheet-like structures because of the intrinsic anisotropic growth of the Bi2WO6 crystal. After 12 h of reaction time (Fig. 2c), sphere-like structures were obtained through self-assembly of nanosheets with certain crystallographic orientation. Finally, highly regular flower-like Bi2WO6 microspheres were obtained after aging for a longer period up to 24 h (Fig. 2d). On the basis of above results, the formation mechanism of flower-like Bi2WO6 microspheres was proposed to be as follows (Fig. 2e): self-aggregation, anisotropic growth and self-assembly. Optical absorption properties of BWO and BWO-F127 were determined by UV–vis diffuse reflectance spectroscopy. As illustrated in Fig. 3a, both samples exhibit strong absorption in the ultraviolet and visible light regions and the band gaps of BWO and BWO-F127 (inset of Fig. 3a) are calculated to be 2.91 and 2.96 eV, respectively, being a little larger than that of bulk Bi2WO6 (2.60 eV) produced at high temperature [8]. The increase in the band gaps of the as-obtained Bi2WO6 can be attributed to the quantum
Fig. 1. (a) XRD patterns, (b) SEM image of BWO, (c) SEM image of BWO-F127, (d) TEM image of BWO-F127 (inset is the corresponding HRTEM image).
160
Y. Liu et al. / Materials Letters 157 (2015) 158–162
Fig. 2. SEM images of BWO-F127 synthesized for (a) 1 h, (b) 6 h, (c) 12 h, (d) 24 h and (e) schematic illustration of the formation of flower-like microstructures.
confinement effects resulting from the nanosized building blocks [9]. Moreover, the suitable bandgaps indicate the possible application of the as-prepared samples in blue-light photocatalysis. PL spectra are widely used to survey the separation efficiency of the photoinduced electron–hole pairs in a semiconductor. Generally, a lower PL intensity represents a lower recombination rate of photogenerated charge carriers. Fig. 3b shows the PL spectra of BWO and BWO-F127 samples excited at 300 nm. It can be observed that BWO-F127 exhibits significantly decreased PL intensity in comparison with that of BWO, suggesting that the recombination of photogenerated charge carriers is greatly inhibited in the BWO-F127. The lifetime of the charge carriers for BWO and BWO-F127 samples was examined by the time-resolved fluorescence decay spectra and shown in Fig. 3c. Clearly, in contrast to BWO, BOW-F127 exhibits slow decay kinetics. The prolonged lifetimes of charge carriers would lead to the increased possibility of electrons or holes participating in photocatalytic reactions.
To provide further evidence to support the high separation efficiency of photogenerated charge carriers of BWO-F127, the photocurrent responses of BWO-F127 and BWO were recorded under visible light irradiation provided by a 300 W Xe lamp with a UV cut off filter (λ 4400 nm). As shown in Fig. 3d, the photocurrent of BWO-F127 is about 1.7 times higher than that of BWO, implying an enhanced photoinduced charge separation. On the basis of the above results, it can be concluded that the 3D hierarchical flower-like microstructures of BWO-F127 can improve the separation efficiency of photogenerated electron–hole pairs, thereby enhancing the photocatalytic activity. The photodegradation rates of RhB in the presence of different Bi2WO6 samples are displayed in Fig. 3e. The blank test (without any photocatalyst) was also performed and demonstrated that the direct photolysis rate of RhB was only about 12% after 180 min. However, when BWO and BWO-F127 photocatalysts were introduced to the RhB solution, the photodegradation rates were rapidly increased to 63% and 91%, respectively. As is well-known,
Y. Liu et al. / Materials Letters 157 (2015) 158–162
161
Fig. 3. UV–vis diffuse reflectance spectra (a), room-temperature PL spectra (b), time-resolved fluorescence decay spectra (c), transient photocurrent responses (d) and photocatalytic activities (e) of as-prepared samples.
the photocatalytic properties of catalysts are interrelated with many factors such as band gap (optical absorbance), surface area, crystallinity, separation efficiency of photogenerated charge carriers, etc. In the present study, the enhanced photocatalytic activity of BWO-F127 can be ascribed to the synergistic effects of two main factors: (1) the BWO-F127 hierarchical structures possess a large number of pores and exhibit much higher surface area (33.4 cm3/g) than that of BWO (16.4 cm3/g), which can not only provide more active sites for the adsorption of reactant molecules and radicals, but also afford more transport paths for the reactants to reach the reactive sites, thereby leading to a higher photocatalytic activity; (2) the flower-like hierarchical
architectures can improve the separation efficiency of photogenerated electron–hole pairs, which improve the number of photogenerated charge carriers to participate in the photodegradation process.
4. Conclusion 3D Hierarchical flower-like Bi2WO6 microspheres self-assembled from 2D nanosheets were synthesized by a facile and effective F127-assisted hydrothermal approach. Based on the SEM observation, the formation mechanism of hierarchical architectures involved
162
Y. Liu et al. / Materials Letters 157 (2015) 158–162
a three-step process: self-aggregation, anisotropic growth and selfassembly. Due to the high specific surface area and high separation efficiency of photogenerated charge carriers, the flower-like Bi2WO6 microspheres (BWO-F127) exhibited superior photocatalytic activities than that BWO. Acknowledgments The authors gratefully acknowledge financial support from National Natural Science Foundation of China (Grant no. U1204503) and Key projects of science and technology of Henan Educational Committee (Grant no. 14B150049).
References [1] Zhu G, Liang J, Hojamberdiev M, Aldabe Bilmes S, Wei X, Liu P, et al. Mater Lett 2014;122:216–9. [2] Mann AKP, Skrabalak SE. Chem Mater 2011;23:1017–22. [3] Yu JG, Xiong JF, Cheng B, Yu Y, Wang JB. J Solid State Chem 2005;178:1968–72. [4] Shang M, Wang W, Xu H. Cryst Growth Des 2008;9:991–6. [5] Chen Z, Qian L, Zhu J, Yuan Y, Qian X. Cryst Eng Commun 2010;12:2100–6. [6] Sivakumar Natarajan T, Bajaj HC, Tayade RJ. Cryst Eng Commun 2015;17:1037–49. [7] Lv H, Liu Y, Hu J, Li Z, Lu Y. RSC Adv 2014;4:63238–45. [8] Huang Y, Ai Z, Ho W, Chen M, Lee S. J Phys Chem C 2010;114:6342–9. [9] Zhang C, Zhu Y. Chem Mater 2005;17:3537–45.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Hydrothermal synthesis of hierarchical flower-like Bi2WO6 microspheres with enhanced visible-light photoactivity Yumin Liu n, Zhiwei Ding, Hua Lv, Jing Guang, Shuang Li, Juhui Jiang Collaborative Innovation Center of Henan Province for Green Manufacturing of Fine Chemicals, School of Chemistry and Chemical Engineering, Henan Normal University, Xinxiang, Henan 453007, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 31 March 2015 Accepted 10 May 2015 Available online 16 May 2015
Three-dimensional (3D) hierarchical flower-like Bi2WO6 microspheres assembled from 2D nanosheets were successfully prepared by a simple hydrothermal route using the non-ionic surfactant F127 (EO100– PO65–EO100) as the morphology director. On the basis of the evolution of the morphologies as a function of reaction time, a possible formation mechanism for the hierarchical architecture was proposed. The photocatalytic activities of the as-prepared Bi2WO6 samples were evaluated for the degradation of Rhodamine B (RhB) under visible-light irradiation. Due to the combined effects of large surface area and efficient separation of charge carriers, the hierarchical flower-like Bi2WO6 microspheres exhibited improved photocatalytic performances. & 2015 Elsevier B.V. All rights reserved.
Keywords: Bi2WO6 F127 Semiconductor Crystal growth
1. Introduction Over the past few years, visible-light-driven photocatalysts have attracted extensive attention in view of the efficient utilization of solar energy. Bismuth tungstate (Bi2WO6), one of the simplest Aurivillius oxides, was found to possess excellent photocatalytic performance for water splitting and photodegradation of organic contaminants under visible light irradiation [1]. Generally, nanoscale Bi2WO6 photocatalysts exhibit good photocatalytic activity due to their relatively large specific surface areas and low recombination rates of photoinduced charges. Hence, considerable efforts have been devoted to the synthesis of nanosized Bi2WO6 photocatalysts with various nanostructures including nanoplates [2], nanoparticles [3], nanocages [4] etc. Nevertheless, in view of practical applications, how to separate and recycle the nano-photocatalyst effectively after fulfillment of the photocatalysis process has become an issue. Comparatively speaking, 3D hierarchical structures constructed by nanosized building blocks have the advantages of both mico- and nano-sized materials, such as easy separation and recyclability, abundant transport paths for reactant molecules, efficient separation of charge carriers and high photocatalytic performance. Recently, Bi2WO6 with 3D hierarchical structures have been fabricated by several research groups. For example, Qian et al. produced hierarchical Bi2WO6 architectures by a simple inorganic salt-assisted hydrothermal method [5]. Tayade
n
Corresponding author. Tel.: þ 86 373 3326335; fax: þ86 373 3326336. E-mail address: [email protected] (Y. Liu).
http://dx.doi.org/10.1016/j.matlet.2015.05.024 0167-577X/& 2015 Elsevier B.V. All rights reserved.
et al. successfully prepared spherical and flower-like Bi2WO6 architectures with and without SiO2 protected calcination [6]. However, despite of these recent progresses, it still remains a great challenge to develop a simple and reliable method for the synthesis of 3D Bi2WO6 hierarchical structures with high visiblelight-driven photocatalytic activity. Herein, we report the fabrication of hierarchical flower-like Bi2WO6 microspheres assembled from 2D nanosheets by an efficient F127-assisted hydrothermal method. The possible formation mechanism of hierarchical architectures was proposed on the basis of a series of time-dependent experiments. The photodegradation of RhB solution was carried out to evaluate the photocatalytic performances of the as-prepared Bi2WO6 photocatalysts under visible light irradiation.
2. Experimental Materials and synthesis: In a typical procedure, 0.12 g Pluronic F-127 was mixed with 20 mL of 0.001 M (NH4)10W12O41 6H2O solution under magnetic stirring until a homogeneous solution was obtained. Then, 20 mL of 0.024 M Bi(NO3) 5 H2O was added to the above solution dropwise. The as-formed mixture solution was transferred into a 100 mL Teflon-lined autoclave and kept at 180 1C for 24 h. After the autoclave was cooled down to room temperature naturally, the as-obtained white precipitation was separated by centrifugation, washed with distilled water and absolute ethanol and dried under vacuum at 60 1C for 12 h. To remove F127, the as-prepared samples were calcined in muffle furnace at 400 1C for 1 h to obtain the Bi2WO6 microspheres
Y. Liu et al. / Materials Letters 157 (2015) 158–162
(denoted as BWO-F127). For comparison, Bi2WO6 sample was also synthesized without using F127 (denoted as BWO), while the other experimental conditions remained the same. Characterization: Characterization of the samples was carried out by employing X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), UV–vis Diffuse Reflectance Spectroscopy, BET surface area analysis, Fluorescence Spectrophotometer (PL). The transient photocurrent responses of samples were measured under visible light irradiation by electrochemical workstation and the detailed process was described in our previous study [7]. Photocatalytic test: Typically, 0.1 g of Bi2WO6 sample was added into 100 mL RhB solution (5 mg L 1). Before irradiation, the suspension was dispersed in an ultrasonic bath for 10 min and then stirred for 30 min in the dark to establish the adsorption– desorption equilibrium. After that, the solution was irradiated by a 300 W xenon lamp equipped with a UV cut off filter (λ 4400 nm). At given time intervals, 5 mL suspension was taken from the reactor and centrifuged to remove the photocatalyst particles. The concentration of RhB solution was analyzed using an UV–vis spectrophotometer at λmax of 553 nm.
3. Results and discussion The crystal structure and phase purity of the samples were characterized by XRD. As shown in Fig. 1a, the diffraction peaks of both samples can be assigned to the pure orthorhombic phase of Bi2WO6 (JCPDS Card no. 39-0256). No peaks of other impurities or phases can be detected, indicating the high phase purity of the asobtained samples. Moreover, there are no obvious differences in diffraction peak intensity for both samples, indicating that F127 has no effects on the crystallinity of BWO-F127. The typical SEM images of BWO and BWO-F127 are shown in Fig. 1b and c, respectively. The obtained BWO, which was synthesized without F127, was composed of aggregates of irregular flakes
159
and particles. On the other hand, 3D hierarchical microspheres with the average diameters varying from 4 to 5 μm were observed in BWO-F127. These microspheres exhibit a flower-like microstructure and each flower is built from numerously welldeveloped nanosheets as the petals. As illustrated in Fig. 1c, these nanosheets are interwoven with each other to form a porous structure, which can serve as transport paths for reactant molecules and thereby enhance the photocatalytic performances of the samples. Fig. 1d is the TEM image of an individual microsphere, which further proves the flower-like microstructure of asprepared BWO-F127 sample. In the HRTEM image (inset of Fig. 1d) taken from the edge of the individual microsphere, the lattice interplanar spacing is determined as 0.273 nm, corresponding to the (020) plane of orthorhombic Bi2WO6. To reveal the growth process of the flower-like Bi2WO6 microspheres, SEM observations were carried out at different time periods as depicted in Fig. 2. Irregular small Bi2WO6 nanoparticles and their aggregates were firstly developed within the initial 1 h (Fig. 2a). As the reaction time was prolonged to 6 h (Fig. 2b), the Bi2WO6 crystals tended to form sheet-like structures because of the intrinsic anisotropic growth of the Bi2WO6 crystal. After 12 h of reaction time (Fig. 2c), sphere-like structures were obtained through self-assembly of nanosheets with certain crystallographic orientation. Finally, highly regular flower-like Bi2WO6 microspheres were obtained after aging for a longer period up to 24 h (Fig. 2d). On the basis of above results, the formation mechanism of flower-like Bi2WO6 microspheres was proposed to be as follows (Fig. 2e): self-aggregation, anisotropic growth and self-assembly. Optical absorption properties of BWO and BWO-F127 were determined by UV–vis diffuse reflectance spectroscopy. As illustrated in Fig. 3a, both samples exhibit strong absorption in the ultraviolet and visible light regions and the band gaps of BWO and BWO-F127 (inset of Fig. 3a) are calculated to be 2.91 and 2.96 eV, respectively, being a little larger than that of bulk Bi2WO6 (2.60 eV) produced at high temperature [8]. The increase in the band gaps of the as-obtained Bi2WO6 can be attributed to the quantum
Fig. 1. (a) XRD patterns, (b) SEM image of BWO, (c) SEM image of BWO-F127, (d) TEM image of BWO-F127 (inset is the corresponding HRTEM image).
160
Y. Liu et al. / Materials Letters 157 (2015) 158–162
Fig. 2. SEM images of BWO-F127 synthesized for (a) 1 h, (b) 6 h, (c) 12 h, (d) 24 h and (e) schematic illustration of the formation of flower-like microstructures.
confinement effects resulting from the nanosized building blocks [9]. Moreover, the suitable bandgaps indicate the possible application of the as-prepared samples in blue-light photocatalysis. PL spectra are widely used to survey the separation efficiency of the photoinduced electron–hole pairs in a semiconductor. Generally, a lower PL intensity represents a lower recombination rate of photogenerated charge carriers. Fig. 3b shows the PL spectra of BWO and BWO-F127 samples excited at 300 nm. It can be observed that BWO-F127 exhibits significantly decreased PL intensity in comparison with that of BWO, suggesting that the recombination of photogenerated charge carriers is greatly inhibited in the BWO-F127. The lifetime of the charge carriers for BWO and BWO-F127 samples was examined by the time-resolved fluorescence decay spectra and shown in Fig. 3c. Clearly, in contrast to BWO, BOW-F127 exhibits slow decay kinetics. The prolonged lifetimes of charge carriers would lead to the increased possibility of electrons or holes participating in photocatalytic reactions.
To provide further evidence to support the high separation efficiency of photogenerated charge carriers of BWO-F127, the photocurrent responses of BWO-F127 and BWO were recorded under visible light irradiation provided by a 300 W Xe lamp with a UV cut off filter (λ 4400 nm). As shown in Fig. 3d, the photocurrent of BWO-F127 is about 1.7 times higher than that of BWO, implying an enhanced photoinduced charge separation. On the basis of the above results, it can be concluded that the 3D hierarchical flower-like microstructures of BWO-F127 can improve the separation efficiency of photogenerated electron–hole pairs, thereby enhancing the photocatalytic activity. The photodegradation rates of RhB in the presence of different Bi2WO6 samples are displayed in Fig. 3e. The blank test (without any photocatalyst) was also performed and demonstrated that the direct photolysis rate of RhB was only about 12% after 180 min. However, when BWO and BWO-F127 photocatalysts were introduced to the RhB solution, the photodegradation rates were rapidly increased to 63% and 91%, respectively. As is well-known,
Y. Liu et al. / Materials Letters 157 (2015) 158–162
161
Fig. 3. UV–vis diffuse reflectance spectra (a), room-temperature PL spectra (b), time-resolved fluorescence decay spectra (c), transient photocurrent responses (d) and photocatalytic activities (e) of as-prepared samples.
the photocatalytic properties of catalysts are interrelated with many factors such as band gap (optical absorbance), surface area, crystallinity, separation efficiency of photogenerated charge carriers, etc. In the present study, the enhanced photocatalytic activity of BWO-F127 can be ascribed to the synergistic effects of two main factors: (1) the BWO-F127 hierarchical structures possess a large number of pores and exhibit much higher surface area (33.4 cm3/g) than that of BWO (16.4 cm3/g), which can not only provide more active sites for the adsorption of reactant molecules and radicals, but also afford more transport paths for the reactants to reach the reactive sites, thereby leading to a higher photocatalytic activity; (2) the flower-like hierarchical
architectures can improve the separation efficiency of photogenerated electron–hole pairs, which improve the number of photogenerated charge carriers to participate in the photodegradation process.
4. Conclusion 3D Hierarchical flower-like Bi2WO6 microspheres self-assembled from 2D nanosheets were synthesized by a facile and effective F127-assisted hydrothermal approach. Based on the SEM observation, the formation mechanism of hierarchical architectures involved
162
Y. Liu et al. / Materials Letters 157 (2015) 158–162
a three-step process: self-aggregation, anisotropic growth and selfassembly. Due to the high specific surface area and high separation efficiency of photogenerated charge carriers, the flower-like Bi2WO6 microspheres (BWO-F127) exhibited superior photocatalytic activities than that BWO. Acknowledgments The authors gratefully acknowledge financial support from National Natural Science Foundation of China (Grant no. U1204503) and Key projects of science and technology of Henan Educational Committee (Grant no. 14B150049).
References [1] Zhu G, Liang J, Hojamberdiev M, Aldabe Bilmes S, Wei X, Liu P, et al. Mater Lett 2014;122:216–9. [2] Mann AKP, Skrabalak SE. Chem Mater 2011;23:1017–22. [3] Yu JG, Xiong JF, Cheng B, Yu Y, Wang JB. J Solid State Chem 2005;178:1968–72. [4] Shang M, Wang W, Xu H. Cryst Growth Des 2008;9:991–6. [5] Chen Z, Qian L, Zhu J, Yuan Y, Qian X. Cryst Eng Commun 2010;12:2100–6. [6] Sivakumar Natarajan T, Bajaj HC, Tayade RJ. Cryst Eng Commun 2015;17:1037–49. [7] Lv H, Liu Y, Hu J, Li Z, Lu Y. RSC Adv 2014;4:63238–45. [8] Huang Y, Ai Z, Ho W, Chen M, Lee S. J Phys Chem C 2010;114:6342–9. [9] Zhang C, Zhu Y. Chem Mater 2005;17:3537–45.