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BiOBr hollow hierarchical microspheres with enhanced activity and stability for RhB degradation under visible light irradiation
Catalysis Communications 42 (2013) 30–34
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Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Short Communication
Ag/AgBr/BiOBr hollow hierarchical microspheres with enhanced activity and stability for RhB degradation under visible light irradiation Tingjiang Yan a,b, Xueyuan Yan a, Ruirui Guo a, Weining Zhang a, Wenjuan Li a, Jinmao You a,⁎ a b
Key Laboratory of Life-Organic Analysis of Shandong Province, College of Chemistry Science, Qufu Normal University, Qufu, 273165, PR China Fujian Provincial Key Laboratory of Photocatalysis-State Key Laboratory Breeding Base, Fuzhou University, Fuzhou, 350002, PR China
a r t i c l e
i n f o
Article history: Received 30 May 2013 Received in revised form 8 July 2013 Accepted 10 July 2013 Available online 18 July 2013 Keywords: Ag/AgBr/BiOBr Hollow Hierarchical microspheres RhB degradation Visible light
a b s t r a c t Ag/AgBr/BiOBr hollow hierarchical microspheres were synthesized through a one-pot solvothermal process. The phase structure, morphology and optical property of the samples were characterized by XRD, SEM, XPS and DRS. Ag/AgBr/BiOBr hollow microspheres exhibited higher photocatalytic activity and improved stability than AgBr/BiOBr nanoplates for rhodamine B degradation under visible light irradiation. The enhanced activities of Ag/AgBr/BiOBr could be attributed to the hollow structure and Ag deposition, which is favorable for adsorption of reactants, enhancement of photoadsorption and transfer of photogenerated carriers. Ag deposition also prevented the decomposition of AgBr under light illumination and contributed to an improved stability. © 2013 Elsevier B.V. All rights reserved.
1. Introduction With urbanization and industrialization, the environmental pollution caused by unabated release of toxic agents into the air and water have become an overwhelming problem all over the world. As one of the most promising methods for solving environmental problems, TiO2-based heterogeneous photocatalysis has attracted great research interest in its fundamental and technological importance [1]. However, the wide band gap of TiO2 renders it only responsive to ultraviolet (UV) light, which accounts for a small portion of sunlight (less than 5%). Although other non TiO2-based semiconductors, such as In1 −xNixTaO4 (x = 0–0.2), [2], BiVO4 [3], Bi2WO6 [4], CaBi2O4 [5], Ag3PO4 [6], g-C3N4 [7], etc., have been found to have strong visible light response, the photocatalytic activity of these non TiO2-based semiconductors is expected to be further promoted to satisfy the large-scale application under visible light radiation. Therefore, the search for visible light driven photocatalysts with high efficiency and good stability is still challenge. Among novel photocatalysts, bismuth oxyhalides (BiOX, X = Cl, Br, I) have attracted increasing interest recently due to their good photocatalytic activities under both ultraviolet and visible light irradiation [8–10]. In view of the better utilization of solar light and increase overall quantum efficiency of unique BiOX material, many attempts have been adopted to enhance its activity by construct of heterojunction with other semiconductors that can absorb visible light, such as Bi2O3/BiOCl [11], Bi2S3/BiOCl [12], Fe3O4/BiOCl [13] and WO3/BiOCl [14]. Besides, AgX (X = Cl, Br, I), ⁎ Corresponding author. Fax: +86 537 4456305. E-mail address: [email protected] (J. You). 1566-7367/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.catcom.2013.07.022
a well-known photosensitive material, has also attracted increasing attentions in the photocatalysis field [15–17]. Due to its photosensitive characteristic, AgX have often been performed as sensitizer to enhance the photocatalytic activity of UV-responding semiconductors (or even insulators) under visible light irradiation. For example, Kakuta et al. [17] demonstrated that AgBr dispersed on silica supports highly improved the visible light photoactivities of the catalysts in hydrogen generation from CH3OH/H2O solution. Kong et al. [18] constructed various AgBr– BiOBr heterojunctions with different AgBr loading through an acidic co-precipitation method and used them for photodegradation of RhB under visible light irradiation. Nevertheless, in their study on AgBrsensitive photocatalysts, Ag0 or Ag2O species were usually formed on AgBr surface since AgBr was vulnerable under light irradiation [17,18]. Recently, many works have demonstrated that Ag/AgX (X = Cl, Br, I) was much efficient and stable photocatalysts under visible light illumination because of the plasmon resonance of Ag nanoparticles and low recombination rate of photogenerated electrons and holes between Ag and AgX [19–21]. Based on the synergetic effect of plasmonic Ag/AgBr and AgBr/BiOBr composite photocatalysts, novel Ag/AgBr/BiOBr hierarchical microspheres or nanoparticles were fabricated by adopting in situ ion exchange reaction followed by light reduction and low-temperature chemical bath method [22,23]. However, to the best of our knowledge, it is still rare in the literature about the photocatalytic stability and the preparation of Ag/AgBr/BiOBr with hollow structures. In the present study, we prepared Ag/AgBr/BiOBr hollow hierarchical microspheres via a facile one-pot solvothermal method. Different from the reported BiOBr hollow microsphere [24], this synthesis for Ag/AgBr/BiOBr hollow hierarchical microspheres was performed
T. Yan et al. / Catalysis Communications 42 (2013) 30–34
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0.01 mmol AgNO3 were dissolved in 30 mL of ethylene glycol (EG) containing 3 mL acetic acid (HAc). The solution was stirred for 30 min and was then added to 30 mL of EG containing 3 mmol cetyltrimethyl ammonium bromide (CTAB). The obtained suspension was transferred into 100 mL Teflon-lined autoclave and maintained at 160 °C for 10 h. The resulting precipitates were washed with ethanol and deionized water and dried at 80 °C. AgBr/BiOBr nanoplates were prepared by a similar procedure except that EG was replaced by deionized water and other conditions were identical. 2.2. Characterization
Fig. 1. XRD patterns of Ag/AgBr/BiOBr hollow hierarchical microspheres and AgBr/BiOBr nanoplates before and after photocatalytic reaction.
in the absence of ionic liquid. The photocatalytic performance of Ag/ AgBr/BiOBr was examined by the degradation of rhodamine B (RhB) under visible light irradiation and was compared with that of AgBr/ BiOBr nanoplates. The photocatalytic stability of Ag/AgBr/BiOBr was also studied. 2. Experimental section 2.1. Synthesis All chemicals were of analytical grade and were used without further purification. Ag/AgBr/BiOBr hollow microspheres were synthesized through a one-pot solvothermal method: 3 mmol Bi(NO3)⋅3.5H2O and
X-ray diffraction (XRD) patterns were performed using a Rigaku 2000 apparatus with Cu Kα irradiation. Morphologies of the products were observed by field emission scanning electron microscopy (FE-SEM) (JEOL JSM-6700), equipped with an energy-dispersive X-ray spectroscopy (EDS). Brunauer–Emmett–Teller (BET) surface areas were determined by nitrogen adsorption–desorption using a Mircromeritics ASAP 2000 analyzer. Optical property was carried out using a Varian Cary 500 Scan UV/Vis system. 2.3. Photocatalytic activity and stability measurements The photocatalytic degradation of RhB was carried out under visible light irradiation from 300 W halogen lamp (Philips Electronics) and a composited cutoff filter that restricted the illumination in a range of 400–800 nm. Briefly, 80-mg catalysts were added to 80 mL of RhB solution (10 ppm). Prior to illumination, the suspension was magnetically stirred in the dark to establish an adsorption-desorption equilibrium. A 3-mL aliquot was taken, centrifuged and analyzed on a Perkin-Elmer UV WinLab Lambda 35 spectrophotometer. The degradation percentage is reported as C/C0, where C is the absorption intensity of RhB for each
Fig. 2. SEM images of Ag/AgBr/BiOBr hollow hierarchical microspheres (a, b and c) and AgBr/BiOBr nanoplates (d).
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3. Results and discussion 3.1. Characterization of the samples
Fig. 3. DRS spectra of Ag/AgBr/BiOBr hollow hierarchical microspheres and AgBr/BiOBr nanoplates.
irradiated time interval at wavelength 554 nm and C0 is the absorption intensity of RhB after adsorption equilibrium. After each photoreaction, the catalyst was filtered, washed and dried at 80 °C for 4 h. Then the regenerated sample was employed to degrade a new 10 ppm RhB aqueous solution for stability test under the same visible light irradiation.
The XRD patterns of the as-prepared Ag/AgBr/BiOBr and AgBr/BiOBr samples are shown in Fig. 1, in which all the diffraction peaks can be indexed to tetragonal BiOBr (JCPDS file no. 09-0393). No reflections assigned to Ag or AgBr are present, possibly because of the low content (~0.2%) and well dispersion of AgBr [18]. The strong and sharp diffraction peaks of Ag/AgBr/BiOBr relative to AgBr/BiOBr reveal that the former is well crystallized. Further observation shows that the relative intensity ratio of the (110) peak to the (102) peak in Ag/AgBr/BiOBr is dramatically enhanced compared with that in AgBr/BiOBr and JCPDC Card, suggesting a preferential growth along the (110) plane. It is also found that the used Ag/AgBr/BiOBr exhibits similar XRD patterns to the fresh one, which indicates the Ag/AgBr/BiOBr is quite stable. Fig. 2 displays the SEM images of the Ag/AgBr/BiOBr and AgBr/BiOBr samples. It can be seen that the obtained Ag/AgBr/BiOBr is composed of many microspheres with range from 3 to 5 μm in diameters (Fig. 2a). Careful observations reveal that these Ag/AgBr/BiOBr microspheres are hollow structures and assembled with nanosheets (Fig. 2a–c). As compared, the obtained AgBr/BiOBr is plate-like sample with a diameter of approximately 400 nm and a thickness of about 20 nm. The EDS performed on both samples reveals that Bi, Br, O and Ag elements are coexisting (Fig. S1), which further proves the composition of the samples.
Fig. 4. Temporal UV-vis absorption spectral changes during the photocatalytic degradation of RhB in aqueous solution over Ag/AgBr/BiOBr hollow hierarchical microspheres and AgBr/BiOBr nanoplates.
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AgBr/BiOBr was prone to deactivation in photocatalytic reaction since AgBr was vulnerable and transformed to Ag2O under light irradiation [18]. However, in the present study, no obvious decline in RhB degradation over Ag/AgBr/BiOBr after five recycles (Fig. 5). This improved photocatalytic stability of Ag/AgBr/BiOBr may be due to the formation of Z-scheme bridge, which leads to an efficient and high separation rate of photogenerated holes and electrons [23]. The used Ag/AgBr/BiOBr inherited the original phase structure (Fig. 1) and wellfined hollow hierarchical structure after photocatalytic reaction while AgBr/BiOBr nanoplates tightly aggregated (Fig. S2), suggesting that Ag/AgBr/BiOBr is a photocatalyst active and stable under visible light irradiation. 4. Conclusions
Fig. 5. Cycling tests of RhB degradation over Ag/AgBr/BiOBr hollow hierarchical microspheres.
The optical properties of Ag/AgBr/BiOBr hollow microspheres and AgBr/BiOBr nanoplates are shown in Fig. 3, which reveal that the absorption for Ag/AgBr/BiOBr at the range of 400–800 nm is much higher than AgBr/BiOBr. The remarkable absorption enhancement of Ag/AgBr/BiOBr might be attributed to the special hollow hierarchical structure. Moreover, Ag/AgBr/BiOBr displays a new broad absorption band located at about 510 nm, corresponding to the characteristic surface plasmon resonance of silver nanoparticles [22,23,25], which confirms the deposition of metallic Ag nanoparticles on the AgBr particles during solvothermal process. In addition, the BET surface areas of Ag/AgBr/BiOBr hollow microspheres and AgBr/BiOBr nanoplates were measured to be 16.9820 and 6.1872 m2/g. The larger surface area for Ag/AgBr/BiOBr is mainly due to its hollow hierarchical structures.
Ag/AgBr/BiOBr hollow hierarchical microspheres were fabricated by a one-pot solvothermal method. Compared with AgBr/BiOBr nanoplates, the hollow Ag/AgBr/BiOBr possessed larger surface area, increasing visible light absorption and lower recombination rate of photogenerated carriers, which accounted for higher photocatalytic activity. The introduction of Ag component into AgBr/BiOBr suppressed the transformation of AgBr to Ag2O and contributed to an improved photocatalytic stability. Acknowledgments This work is financially supported by the National Natural Science Foundation of China (no. 21103193) and the Scientific Research Foundation of Qufu Normal University. Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.catcom.2013.07.022.
3.2. Photocatalytic activity and stability
References
Fig. 4a and b display the absorption spectra of RhB aqueous solution after adsorption equipment and during the photocatalytic process. Ag/AgBr/BiOBr exhibited much excellent adsorption capability than AgBr/BiOBr. Within 30 min, about 87% of RhB solution was adsorbed by Ag/AgBr/BiOBr while it was 52% for AgBr/BiOBr. This strong adsorption capacity of Ag/AgBr/BiOBr could be attributed to its hollow structure and large BET surface area. During photocatalytic process for Ag/AgBr/BiOBr, the main absorption peak of RhB at 554 nm fast disappeared within 6 min. However, the peak in the presence of AgBr/BiOBr shifted and diminished gradually as reaction time increased. Fig. 4c depicts the comparative degradation rate of RhB. It can be observed that no RhB was degraded by photolysis and the activity of Ag/AgBr/BiOBr is much higher than that of AgBr/BiOBr. The photocatalytic degradation of RhB over Ag/AgBr/BiOBr and AgBr/BiOBr followed the pseudo-first-order kinetics (Fig. 4d). The k of RhB removal in the photocatalytic process with AgBr/BiOBr was 0.26669 min−1, while that over Ag/AgBr/BiOBr was 1.39705 min−1, which was 5.2 times that with AgBr/BiOBr. The superior photocatalytic activity of Ag/AgBr/BiOBr can be firstly explained in terms of its larger surface area, which favors the adsorption of RhB and then reacting with the photogenerated radicals on such catalyst surface [26]. Secondly, Ag/AgBr/BiOBr has an increased visible light absorption due to the surface plasmon resonance by metallic Ag and the hollow hierarchical structure. This means there are more photons absorbed by the catalysts and more photogenerated carriers participating in the RhB degradation process. Lastly, the introduction of Ag component in AgBr/BiOBr could low the recombination rate for electrons and holes as a Z-scheme bridge is formed in the Ag/AgBr/BiOBr system [23].
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Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Short Communication
Ag/AgBr/BiOBr hollow hierarchical microspheres with enhanced activity and stability for RhB degradation under visible light irradiation Tingjiang Yan a,b, Xueyuan Yan a, Ruirui Guo a, Weining Zhang a, Wenjuan Li a, Jinmao You a,⁎ a b
Key Laboratory of Life-Organic Analysis of Shandong Province, College of Chemistry Science, Qufu Normal University, Qufu, 273165, PR China Fujian Provincial Key Laboratory of Photocatalysis-State Key Laboratory Breeding Base, Fuzhou University, Fuzhou, 350002, PR China
a r t i c l e
i n f o
Article history: Received 30 May 2013 Received in revised form 8 July 2013 Accepted 10 July 2013 Available online 18 July 2013 Keywords: Ag/AgBr/BiOBr Hollow Hierarchical microspheres RhB degradation Visible light
a b s t r a c t Ag/AgBr/BiOBr hollow hierarchical microspheres were synthesized through a one-pot solvothermal process. The phase structure, morphology and optical property of the samples were characterized by XRD, SEM, XPS and DRS. Ag/AgBr/BiOBr hollow microspheres exhibited higher photocatalytic activity and improved stability than AgBr/BiOBr nanoplates for rhodamine B degradation under visible light irradiation. The enhanced activities of Ag/AgBr/BiOBr could be attributed to the hollow structure and Ag deposition, which is favorable for adsorption of reactants, enhancement of photoadsorption and transfer of photogenerated carriers. Ag deposition also prevented the decomposition of AgBr under light illumination and contributed to an improved stability. © 2013 Elsevier B.V. All rights reserved.
1. Introduction With urbanization and industrialization, the environmental pollution caused by unabated release of toxic agents into the air and water have become an overwhelming problem all over the world. As one of the most promising methods for solving environmental problems, TiO2-based heterogeneous photocatalysis has attracted great research interest in its fundamental and technological importance [1]. However, the wide band gap of TiO2 renders it only responsive to ultraviolet (UV) light, which accounts for a small portion of sunlight (less than 5%). Although other non TiO2-based semiconductors, such as In1 −xNixTaO4 (x = 0–0.2), [2], BiVO4 [3], Bi2WO6 [4], CaBi2O4 [5], Ag3PO4 [6], g-C3N4 [7], etc., have been found to have strong visible light response, the photocatalytic activity of these non TiO2-based semiconductors is expected to be further promoted to satisfy the large-scale application under visible light radiation. Therefore, the search for visible light driven photocatalysts with high efficiency and good stability is still challenge. Among novel photocatalysts, bismuth oxyhalides (BiOX, X = Cl, Br, I) have attracted increasing interest recently due to their good photocatalytic activities under both ultraviolet and visible light irradiation [8–10]. In view of the better utilization of solar light and increase overall quantum efficiency of unique BiOX material, many attempts have been adopted to enhance its activity by construct of heterojunction with other semiconductors that can absorb visible light, such as Bi2O3/BiOCl [11], Bi2S3/BiOCl [12], Fe3O4/BiOCl [13] and WO3/BiOCl [14]. Besides, AgX (X = Cl, Br, I), ⁎ Corresponding author. Fax: +86 537 4456305. E-mail address: [email protected] (J. You). 1566-7367/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.catcom.2013.07.022
a well-known photosensitive material, has also attracted increasing attentions in the photocatalysis field [15–17]. Due to its photosensitive characteristic, AgX have often been performed as sensitizer to enhance the photocatalytic activity of UV-responding semiconductors (or even insulators) under visible light irradiation. For example, Kakuta et al. [17] demonstrated that AgBr dispersed on silica supports highly improved the visible light photoactivities of the catalysts in hydrogen generation from CH3OH/H2O solution. Kong et al. [18] constructed various AgBr– BiOBr heterojunctions with different AgBr loading through an acidic co-precipitation method and used them for photodegradation of RhB under visible light irradiation. Nevertheless, in their study on AgBrsensitive photocatalysts, Ag0 or Ag2O species were usually formed on AgBr surface since AgBr was vulnerable under light irradiation [17,18]. Recently, many works have demonstrated that Ag/AgX (X = Cl, Br, I) was much efficient and stable photocatalysts under visible light illumination because of the plasmon resonance of Ag nanoparticles and low recombination rate of photogenerated electrons and holes between Ag and AgX [19–21]. Based on the synergetic effect of plasmonic Ag/AgBr and AgBr/BiOBr composite photocatalysts, novel Ag/AgBr/BiOBr hierarchical microspheres or nanoparticles were fabricated by adopting in situ ion exchange reaction followed by light reduction and low-temperature chemical bath method [22,23]. However, to the best of our knowledge, it is still rare in the literature about the photocatalytic stability and the preparation of Ag/AgBr/BiOBr with hollow structures. In the present study, we prepared Ag/AgBr/BiOBr hollow hierarchical microspheres via a facile one-pot solvothermal method. Different from the reported BiOBr hollow microsphere [24], this synthesis for Ag/AgBr/BiOBr hollow hierarchical microspheres was performed
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0.01 mmol AgNO3 were dissolved in 30 mL of ethylene glycol (EG) containing 3 mL acetic acid (HAc). The solution was stirred for 30 min and was then added to 30 mL of EG containing 3 mmol cetyltrimethyl ammonium bromide (CTAB). The obtained suspension was transferred into 100 mL Teflon-lined autoclave and maintained at 160 °C for 10 h. The resulting precipitates were washed with ethanol and deionized water and dried at 80 °C. AgBr/BiOBr nanoplates were prepared by a similar procedure except that EG was replaced by deionized water and other conditions were identical. 2.2. Characterization
Fig. 1. XRD patterns of Ag/AgBr/BiOBr hollow hierarchical microspheres and AgBr/BiOBr nanoplates before and after photocatalytic reaction.
in the absence of ionic liquid. The photocatalytic performance of Ag/ AgBr/BiOBr was examined by the degradation of rhodamine B (RhB) under visible light irradiation and was compared with that of AgBr/ BiOBr nanoplates. The photocatalytic stability of Ag/AgBr/BiOBr was also studied. 2. Experimental section 2.1. Synthesis All chemicals were of analytical grade and were used without further purification. Ag/AgBr/BiOBr hollow microspheres were synthesized through a one-pot solvothermal method: 3 mmol Bi(NO3)⋅3.5H2O and
X-ray diffraction (XRD) patterns were performed using a Rigaku 2000 apparatus with Cu Kα irradiation. Morphologies of the products were observed by field emission scanning electron microscopy (FE-SEM) (JEOL JSM-6700), equipped with an energy-dispersive X-ray spectroscopy (EDS). Brunauer–Emmett–Teller (BET) surface areas were determined by nitrogen adsorption–desorption using a Mircromeritics ASAP 2000 analyzer. Optical property was carried out using a Varian Cary 500 Scan UV/Vis system. 2.3. Photocatalytic activity and stability measurements The photocatalytic degradation of RhB was carried out under visible light irradiation from 300 W halogen lamp (Philips Electronics) and a composited cutoff filter that restricted the illumination in a range of 400–800 nm. Briefly, 80-mg catalysts were added to 80 mL of RhB solution (10 ppm). Prior to illumination, the suspension was magnetically stirred in the dark to establish an adsorption-desorption equilibrium. A 3-mL aliquot was taken, centrifuged and analyzed on a Perkin-Elmer UV WinLab Lambda 35 spectrophotometer. The degradation percentage is reported as C/C0, where C is the absorption intensity of RhB for each
Fig. 2. SEM images of Ag/AgBr/BiOBr hollow hierarchical microspheres (a, b and c) and AgBr/BiOBr nanoplates (d).
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3. Results and discussion 3.1. Characterization of the samples
Fig. 3. DRS spectra of Ag/AgBr/BiOBr hollow hierarchical microspheres and AgBr/BiOBr nanoplates.
irradiated time interval at wavelength 554 nm and C0 is the absorption intensity of RhB after adsorption equilibrium. After each photoreaction, the catalyst was filtered, washed and dried at 80 °C for 4 h. Then the regenerated sample was employed to degrade a new 10 ppm RhB aqueous solution for stability test under the same visible light irradiation.
The XRD patterns of the as-prepared Ag/AgBr/BiOBr and AgBr/BiOBr samples are shown in Fig. 1, in which all the diffraction peaks can be indexed to tetragonal BiOBr (JCPDS file no. 09-0393). No reflections assigned to Ag or AgBr are present, possibly because of the low content (~0.2%) and well dispersion of AgBr [18]. The strong and sharp diffraction peaks of Ag/AgBr/BiOBr relative to AgBr/BiOBr reveal that the former is well crystallized. Further observation shows that the relative intensity ratio of the (110) peak to the (102) peak in Ag/AgBr/BiOBr is dramatically enhanced compared with that in AgBr/BiOBr and JCPDC Card, suggesting a preferential growth along the (110) plane. It is also found that the used Ag/AgBr/BiOBr exhibits similar XRD patterns to the fresh one, which indicates the Ag/AgBr/BiOBr is quite stable. Fig. 2 displays the SEM images of the Ag/AgBr/BiOBr and AgBr/BiOBr samples. It can be seen that the obtained Ag/AgBr/BiOBr is composed of many microspheres with range from 3 to 5 μm in diameters (Fig. 2a). Careful observations reveal that these Ag/AgBr/BiOBr microspheres are hollow structures and assembled with nanosheets (Fig. 2a–c). As compared, the obtained AgBr/BiOBr is plate-like sample with a diameter of approximately 400 nm and a thickness of about 20 nm. The EDS performed on both samples reveals that Bi, Br, O and Ag elements are coexisting (Fig. S1), which further proves the composition of the samples.
Fig. 4. Temporal UV-vis absorption spectral changes during the photocatalytic degradation of RhB in aqueous solution over Ag/AgBr/BiOBr hollow hierarchical microspheres and AgBr/BiOBr nanoplates.
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AgBr/BiOBr was prone to deactivation in photocatalytic reaction since AgBr was vulnerable and transformed to Ag2O under light irradiation [18]. However, in the present study, no obvious decline in RhB degradation over Ag/AgBr/BiOBr after five recycles (Fig. 5). This improved photocatalytic stability of Ag/AgBr/BiOBr may be due to the formation of Z-scheme bridge, which leads to an efficient and high separation rate of photogenerated holes and electrons [23]. The used Ag/AgBr/BiOBr inherited the original phase structure (Fig. 1) and wellfined hollow hierarchical structure after photocatalytic reaction while AgBr/BiOBr nanoplates tightly aggregated (Fig. S2), suggesting that Ag/AgBr/BiOBr is a photocatalyst active and stable under visible light irradiation. 4. Conclusions
Fig. 5. Cycling tests of RhB degradation over Ag/AgBr/BiOBr hollow hierarchical microspheres.
The optical properties of Ag/AgBr/BiOBr hollow microspheres and AgBr/BiOBr nanoplates are shown in Fig. 3, which reveal that the absorption for Ag/AgBr/BiOBr at the range of 400–800 nm is much higher than AgBr/BiOBr. The remarkable absorption enhancement of Ag/AgBr/BiOBr might be attributed to the special hollow hierarchical structure. Moreover, Ag/AgBr/BiOBr displays a new broad absorption band located at about 510 nm, corresponding to the characteristic surface plasmon resonance of silver nanoparticles [22,23,25], which confirms the deposition of metallic Ag nanoparticles on the AgBr particles during solvothermal process. In addition, the BET surface areas of Ag/AgBr/BiOBr hollow microspheres and AgBr/BiOBr nanoplates were measured to be 16.9820 and 6.1872 m2/g. The larger surface area for Ag/AgBr/BiOBr is mainly due to its hollow hierarchical structures.
Ag/AgBr/BiOBr hollow hierarchical microspheres were fabricated by a one-pot solvothermal method. Compared with AgBr/BiOBr nanoplates, the hollow Ag/AgBr/BiOBr possessed larger surface area, increasing visible light absorption and lower recombination rate of photogenerated carriers, which accounted for higher photocatalytic activity. The introduction of Ag component into AgBr/BiOBr suppressed the transformation of AgBr to Ag2O and contributed to an improved photocatalytic stability. Acknowledgments This work is financially supported by the National Natural Science Foundation of China (no. 21103193) and the Scientific Research Foundation of Qufu Normal University. Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.catcom.2013.07.022.
3.2. Photocatalytic activity and stability
References
Fig. 4a and b display the absorption spectra of RhB aqueous solution after adsorption equipment and during the photocatalytic process. Ag/AgBr/BiOBr exhibited much excellent adsorption capability than AgBr/BiOBr. Within 30 min, about 87% of RhB solution was adsorbed by Ag/AgBr/BiOBr while it was 52% for AgBr/BiOBr. This strong adsorption capacity of Ag/AgBr/BiOBr could be attributed to its hollow structure and large BET surface area. During photocatalytic process for Ag/AgBr/BiOBr, the main absorption peak of RhB at 554 nm fast disappeared within 6 min. However, the peak in the presence of AgBr/BiOBr shifted and diminished gradually as reaction time increased. Fig. 4c depicts the comparative degradation rate of RhB. It can be observed that no RhB was degraded by photolysis and the activity of Ag/AgBr/BiOBr is much higher than that of AgBr/BiOBr. The photocatalytic degradation of RhB over Ag/AgBr/BiOBr and AgBr/BiOBr followed the pseudo-first-order kinetics (Fig. 4d). The k of RhB removal in the photocatalytic process with AgBr/BiOBr was 0.26669 min−1, while that over Ag/AgBr/BiOBr was 1.39705 min−1, which was 5.2 times that with AgBr/BiOBr. The superior photocatalytic activity of Ag/AgBr/BiOBr can be firstly explained in terms of its larger surface area, which favors the adsorption of RhB and then reacting with the photogenerated radicals on such catalyst surface [26]. Secondly, Ag/AgBr/BiOBr has an increased visible light absorption due to the surface plasmon resonance by metallic Ag and the hollow hierarchical structure. This means there are more photons absorbed by the catalysts and more photogenerated carriers participating in the RhB degradation process. Lastly, the introduction of Ag component in AgBr/BiOBr could low the recombination rate for electrons and holes as a Z-scheme bridge is formed in the Ag/AgBr/BiOBr system [23].
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