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BiOCl heterojunction photocatalyst by in-situ transformation method for norfloxacin photocatalytic degradation
Journal of Alloys and Compounds 702 (2017) 68e74
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Preparation of BiVO4/BiOCl heterojunction photocatalyst by in-situ transformation method for norfloxacin photocatalytic degradation Xiumin Ma a, *, Zheng Ma a, b, Tong Liao a, Xuehui Liu a, b, Yaping Zhang c, LeiLei Li a, Weibing Li c, Baorong Hou a a b c
Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, China School of Chemistry and Chemical Engineering, Shandong University, Jinan 250100, China School of Environment and Safety Engineering, Qingdao University of Science and Technology, Qingdao, 266042, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 19 October 2016 Received in revised form 7 January 2017 Accepted 19 January 2017 Available online 23 January 2017
In this study, we employed an in-situ transformation method to prepared a mesoporous spindle like BiVO4/nanosheet BiOCl composite photocatalyst. The nanostructure and content of 2-D BiOCl could be facilely tuned by controlling the concentration of Cl ions in the reaction solution. The as-prepared BiVO4-BiOCl photocatalyst showed an excellent photocatalytic norfloxacin degradation performance, and removed it in 1 h almost completely. Further research results indicated that a well-defined p-n heterojunction interface has been formed between BiVO4 and BiOCl, to enhance the separation efficiency of photogenerated carriers. In addition, after the nanosheets BiOCl interlaced with the mesoporous BiVO4 spindles, since the 2-D structure of the former, the charge transfer capacity of the composite would be improved to prolong the lifetime of the photoinduced carriers. © 2017 Published by Elsevier B.V.
Keywords: BiVO4/BiOCl p-n junction Ion exchange method Photocatalysis Norfloxacin
1. Introduction Monoclinic bismuth vanadate (BiVO4), with a band gap width of 2.4 eV, shows an excellent visible light absorption capacity. So, it is widely used in photocatalytic organic pollution degradation and water oxidation for O2 evolution [1e4]. However, the dramatic recombination of the photogenerated carriers limited to increasing the quantum yield of this semiconductor [5,6]. In order to control this recombination process, except tuning the semiconductor characteristics of BiVO4 [7,8], combining it with other suitable semiconductors to form a heterojunction system, has been widely considered as a simple and effective method [9e15]. Recently, except BiVO4, other Bi-based semiconductors, bismuth oxyhalides (BiOX, X ¼ Cl, Br, I), have drawn considerable research potential in the area of photocatalysis, which is owing to their adjustable band gap width, controllable nanostructure and facile synthetic process [16e23]. In the bismuth oxyhalides group, with the anions diameters decreasing, the band gap with of the BiOX increase gradually, such as BiOCl, a p-type semiconductor with a
* Corresponding author. E-mail address: [email protected] (X. Ma). http://dx.doi.org/10.1016/j.jallcom.2017.01.214 0925-8388/© 2017 Published by Elsevier B.V.
band gap width of 3.5 eV, it just can absorb UV light [24,25]. However, this material shows much higher photocorrosion stability than visible light absorbed BiOBr and BiOI. Furthermore, in its crystal of BiOCl, The [Bi2O2]2þ layers, interleaved by double slabs of Cl atoms, to induce BiOCl crystal growth along [001] to form a two dimensional nanosheet structure [26e28]. Thus, the transfer process of the photogenerated carriers inside the crystal can be accomplished smoothly, to prolong the lifetime of the photoinduced carriers. Composition of n-type BiVO4 and p-type BiOX to form a p-n heterojunction photocatalyst has been widely considerable an effective method to increase the separation efficiency of the photogenerated electrons and holes [29,30]. As we know, the band gap widths of BiOBr and BiOI are smaller than BiOCl, meaning that the redox energy of photogenerated carriers of the BiOBr and BiOI are lower than that of BiOCl. Especially, since BiOBr or BiOI composite with BiVO4 to form a p-n junction system, the corresponding photogenerated carriers redox energy would decrease further, so these composites were only able to treat some easy degradation organics such as RhB, MB etc. In order to treat some more obstinate environmental pollution by photocatalytic methods, such as antibiotic, BiVO4/BiOCl composite is a better choice [31,32]. A favorable interface contact and a unique nanostructure can
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improve the photons adsorption, photogenerated charge separation and migration capacities of the p-n heterojunction photocatalyst. Ion exchange has been confirmed a utilized method to prepare heterojunction materials through a in-situ growth process [33,34]. Generally, a heterojunction photocatalyst with better interface contact and more effective nanostructure could be achieved by this method. Herein, we employed this facile ion exchange method to fabricate a novel BiVO4/BiOCl (mesoporous spindle/ nanosheet) composite. Specially, the micro morphology and the content of BiOCl in this composite can be easily controlled by tuning the Cl ion concentration in the reaction solution. The asprepared BiVO4/BiOCl composite in this work shows an excellent photogenerated carriers separation capacity and keeps a high redox energy of these carriers, to degrade the norfloxacin smoothly. 2. Experimental 2.1. Preparation of spindle like BiVO4 The reagents and solvents used in this experiment were analytical grade and purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China) without further purification. BiVO4 powder was prepared by a hydrothermal method. 1.5 mmol Bi(NO)3$5H2O and 2 g polyvinylpyrrolidone (PVP, MW ~40 K) were dissolved in 50 mL ethylene glycol with sonicating for 20 min, which was marked as solution A. 1.5 mmol of NH4VO3 was dissolved in 20 mL of deionized water with sonicating for 10 min, and marked as solution B. Then, the solution B was added dropwise to solution A with stirring. The resulting mixture was then transferred into a 100 mL Teflon-lined stainless-steel autoclave, and heated at 180 C for 10 h. After that, the liquid mixture was centrifugalized and repeatedly washed with deionized water and ethanol, dried at 60 C for 5 h in air. 2.2. Preparation of BiVO4/BiOBr composite photocatalyst Simply, 1 mmol as-prepared BiVO4 power and 1 g of PVP were mixed into 80 mL of deionized water with sonicating for 20 min. Then, a certain amount of concentrated HCl (0.1, 0.3, 0.5 and 0.7 mL) was added to above mixture under magnetic stirring. After 10 Min, the resulting mixture was transferred into a 100 mL Teflon-lined stainless-steel autoclave and kept at 180 C for 10 h. The final precipitations were collected by centrifugation, and washed by deionized water and ethanol for three times, then dried at 60 C for 5 h. The samples were marked as BiVO4-BiOCl-0.1, BiVO4-BiOCl-0.3, BiVO4-BiOCl-0.5 and BiVO4-BiOCl-0.7, respectively.
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norfloxacin solution with a concentration of 5 mg L1 and kept stirring for 30 min under dark. The light source was a 150 W Xenon lamp (PLS-SXE300C, Beijing Bofeilai Co. Ltd., Beijing, China). A 420nm cutoff filter was used to get visible light. The distance between the light source and the liquid level was 10 cm. A circulating water system was employed to control the operation process temperature at 25 C. The concentration of the norfloxacin relative to photocatalysis time were tested by UVeVis DRS. 2.5. Photoelectrochemical measurements Photoelectrochemical measurements were performed in a three-electrode experimental system using CHI660D Electrochemical Workstation (Shanghai Chenhua Instrument Co., Ltd., Shanghai, China). The prepared series photoelectrodes, Ag/AgCl electrode, and Pt electrode acted as the working, reference, and counter electrodes, respectively. The light source was a 150 W Xenon lamp (PLS-SXE300C, Beijing Bofeilai Co. Ltd., Beijing, China). The variations of the photoinduced current density with time (i-t curve) were measured at a bias potential of 1.0 V (vs Ag/AgCl) during a 3-cycle light switching on and off. Electrochemical impedance spectroscopy (EIS) tests were performed at 0 V bias potential over the frequency range between 104 and 101 Hz, with an AC voltage magnitude of 5 mV, using 12 points/decade. All tests are carried out in 0.1 mol L1 Na2SO4 electrolyte. Mott-Schottky plots were measured at the frequency of 1000 Hz with an AC voltage magnitude of 10 mV. 3. Results and discussion The XRD results of the BiVO4 and series of BiVO4-BiOCl composite with different HCl adding contents are shown in Fig. 1. BiVO4 curve shows a typical monoclinic phase. For the sample of BiVO4BiOCl-0.1, except monoclinic phase peaks of BiVO4, a weak diffraction peak is found near 32.6, consistent with the tetragonal BiOCl, indicating that trace content BiOCl had formed in this sample with a low concentration of HCl in the reaction solution. With the HCl adding content increasing, we can find that the tetragonal BiOCl diffraction peaks density increase gradually. This phenomenon demonstrated that the content and crystallinity of the BiOCl in the composite of BiVO4-BiOCl were increased with the HCl
2.3. Characterizations The morphology and the microstructure of the prepared samples were analyzed by using a scanning electron microscope (SEM) (SEM, JSM-6700F, JEOL, Japan) and a high-resolution transmission electron microscope (HRTEM, Tecnai G2 F20, FEI Company, USA). The elemental compositions, the crystalline structures and bonding information of the prepared samples were analyzed by using X-ray diffraction (XRD, D/MAX-2500/PC, Rigaku Co., Tokyo, Japan) and XPS (Axis Ultra, Kratos Analytical Ltd., England). The optical absorption properties were tested by a UV/Vis diffuse reflectance spectrophotometer (UVeVis DRS, UV-2600, Shimadzu, Japan). The fluorescence spectrum (PL, Fluoro Max-4) was employed to characterize the PL density of these photocatalysts. 2.4. Photocatalytic degradation of norfloxacin 0.05 g prepared photocatalyst was added into 100 mL
Fig. 1. XRD results of spindle BiVO4 and series of BiVO4-BiOCl composite with different HCl adding contents.
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concentration increasing in the reaction solution. The microsturcture of these materials were observed by SEM and TEM further, and the results are shown in Fig. 1. The SEM image of pure BiVO4 is shown in Fig. 2A, a close-grained spindle like structure with a length of approximately 1.5 mm can be observed under the SEM. Fig. 2B ~ 2F present the SEM images of BiVO4-BiOCl0.1, BiVO4-BiOCl-0.3, BiVO4-BiOCl-0.5 and BiVO4-BiOCl-0.7. Since the adding content of HCl is low, such as the sample of BiVO4-BiOCl0.1, after the hydrothermal reaction the BiVO4 would be etched to form a mesopourous spindle structure. However, with the concentration HCl adding content exceeded than 0.1 mL, from Fig. 2CeE, we can find that more and more nanosheets interlaced
with the mesopourous spindle structures. Fig. 2F is the corresponding TEM image of Fig. 2D, from this image, we can clearly find that the mesopourous spindle BiVO4 and some nanoparticles are dispersed on the surface of BiOCl sheets. Thus, we proposed the possible reaction process of the BiVO4-BiOCl with different contents of HCl adding in Fig. 2G when the concentration of HCl is low in the reaction solution, it would like to etch the spindle BiVO4 to mesopourous spindle structure only, because the Cl concentration is low at this case. However, with the HCl concentration increasing, more BiVO4 would be dissolved and transfer to [Bi2O2]2þ, and reacted with the Cl to form BiOCl nanosheet under the hydrothermal condition. The SEM elements mapping of the sample
Fig. 2. (A) to (E) show the SEM images of BiVO4 and BiVO4-BiOCl-0.1, BiVO4-BiOCl-0.3, BiVO4-BiOCl-0.5 and BiVO4-BiOCl-0.7, orderly; (F) is the TEM image of BiVO4-BiOCl-0.5; (G) schematic of the nano structure formation process; (H) SEM elements mapping of BiVO4-BiOCl-0.5, including Bi, V, O and Cl.
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BiVO4-BiOCl-0.5 shows in Fig. 2H. From this result we can find that there are Bi, V, O and Cl elements distributed on the composite material without any other elements, which is corresponding to the components of BiVO4-BiOCl composite. The XPS was employed to further investigate the surface components and the bond states of the as-prepared BiVO4-BiOCl-0.5 and the results are shown in Fig. 3. Fig. 3A shows the Survey curve of this sample, we only can find the Bi, V, O, Cl elements (C 1s peak was contributed to the background carbon source of the XPS) on the surface, indicating a pure BiVO4-BiOCl composite has been synthesized. Bi 4f spectrum of this sample is shown in Fig. 3B, the core peaks at 158.6 eV and 164.1 eV are corresponding to Bi 4f7/2 and Bi 4f5/2, respectively [31]. The V 2p XPS spectrum is presented in Fig. 3C, the peak at 516.1 eV contributes to the V 2p 3/2 at the surface of BiVO4 [31]. The Cl 2p shown in Fig. 3D resolved into two
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peaks at 197.8 and 199.3 eV, which corresponding to the Cl 2p3/ 2and Cl 2p1/2 states for BiOCl, respectively [35]. The O 1s spectrum is shown in Fig. 3E can be divided into two peaks at 529.3 and 530.0 eV, respectively. The former one contributed to the lattice oxygen of Bi-O bond, and the latter one contributed to the hydroxyl groups which on the surface of BiOCl or BiVO4. The optical absorption characteristics of the pure BiVO4 and BiVO4-BiOCl-0.1, BiVO4-BiOCl-0.3, BiVO4-BiOCl-0.5 and BiVO4-BiOCl-0.7 samples were tested, and the results showed in Fig. 4. The absorption threshold of pure BiVO4 was near at 540 nm, corresponded a band gap width of 2.3 eV. However, after BiVO4-BiOCl composite formation, their absorption thresholds blue shifted to approximately 527 nm, corresponded a 2.35 eV band gap width. As we know, BiOCl is a semiconductor with absorption in UV region, thus, the band gap enlarged after transferring part of BiVO4 to BiOCl.
Fig. 3. XPS result of the sample BiVO4-BiOCl-0.5.
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Fig. 4. UVeVis DRS curves of BiVO4 and BiVO4-BiOCl-0.1, BiVO4-BiOCl-0.3, BiVO4BiOCl-0.5 and BiVO4-BiOCl-0.7.
The photocatalytic abilities of pure BiVO4 and BiOCl-BiVO4 composites were evaluated by degradation of the norfloxacin solution. Norfloxacin is an antibiotic, which is not an easy degradation by microbiological technology because of the growth process of bacterium, would be inhabited since the antibiotic existing in the waste water. Photocatalysis is a potential method to treat norfloxacin, since the redox energy of the radicals produced by photocatalysis process is unselective during degradation the organics. And all the corresponding results are shown in Fig. 5. Fig. 5(A)e(E) present the UVeVis absorption curves of the photocatalytic norfloxacin degradation properties of BiVO4 and series of BiVO4-BiOCl
composites, respectively. From these results, we can find that the characteristic absorption peak of norfloxacin is at near 277 nm, and during the photocatalytic time extending, this peak intensity decreased gradually, indicating that the norfloxacin was degraded by the photocatalysis process. Compared with the photocatalytic properties of these photocatalysts, the absorbance at 277 on these curves have been marked, and conversion by the standard absorption curves of norfloxacin in water, to get the concentration change of norfloxacin during photocatalysis process for each photocatalyst, and the results are shown in Fig. 5F. After stirring for 30 min under dark, just negligible norfloxacin could be adsorbed by these photocatalysts. After light illumination, the sample of pure BiVO4 could degradate 60% norfloxacin in 1 h. For the sample of BiVO4-BiOCl-0.1, it about 80% norfloxacin has been degraded in 1 h, indicating that the photocatalytic property could be improved after compact spindle BiVO4 transfer to mesoporous spindle BiVO4 because of more reaction active sites on the surface of the latter one. With the BiOCl content in the BiVO4-BiOCl composite increasing further, as show in Fig. 5F, the photocatalytic norfloxacin properties of BiVO4-BiOCl-0.3, BiVO4-BiOCl-0.5 and BiVO4-BiOCl0.7 increased greatly than pure BiVO4 and BiVO4-BiOCl-0.1, them could almost completely remove the norfoxacin in 1 h, and the sample of BiVO4-BiOCl-0.5 showed a litter higher property than that of BiVO4-BiOCl-0.3 and BiVO4-BiOCl-0.7. This phenomenon demonstrated that a heterojunction is possibly formed between the BiVO4 and BiOCl nanosheet interface that to improve the separation efficiency of the photoinduced carriers. Blank curve shows a near invariable tendency, indicating that the norfloxacin did not selfdegradation under visible light illumination. As shown in Fig. 5F, BiOCl cannot provide a degradation efficiency for norfloxacin in visible light region. BiOCl is a UV region response semiconductor, meanwhile, norfloxacin also a UV region response organic. So, the norfloxacin cannot photo-sensitized the BiOCl in visible light.
Fig. 5. (A) ~ (E) UVeVis absorption curves of the photocatalytic norfloxacin degradation properties of BiVO4 and series of BiVO4-BiOCl composites, respectively. (F) photocatalytic curve of these materials by fit UVeVis absorption curves at 277 nm from each corresponding UVeVis absorption curve.
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Photoinduced i-t curves and EIS curves were employed to test the carriers separation and transfer capacity of the BiVO4 and the BiVO4-BiOCl composites, and the results are shown in Fig. 6. Photoinduced i-t curve is a method which can reflect the separation efficiency and transfer capacity of the photogenerated carriers indirectly. As shown in Fig. 6A, the photocurrent of BiVO4 in this case was lowest. However, with the HCl concentration in the reaction solution increasing, the photocurrents of the as-prepared BiVO4-BiOCl-0.1, BiVO4-BiOCl-0.3 and BiVO4-BiOCl-0.5 increased gradually, however the sample BiVO4-BiOCl-0.7 decreased. To date, many previous work had been clarified that the p-n junction formed between BiOCl and BiVO4 can improve the photoinduced carriers separation efficiency, since to increase the corresponding photocurrent density. In this case, except this reason, the electron transfer capacity maybe increase with the BiOCl nanosheet formed in the composite. Thus, the EIS method was used to test the electron transfer characteristics of these materials, and the results are shown in Fig. 6B. From these curves, we can find that the order of EIS arcs diameter is BiVO4 > BiVO4-BiOCl-0.1 > BiVO4-BiOCl0.7 > BiVO4-BiOCl-0.3 > BiVO4-BiOCl-0.5, this order indicated that the charge transfer capacity can be improved after BiOCl nanosheets interlacing between mesoporous BiVO4 spindle. However, with the content of BiOCl increasing to a limit, the interface contact would decrease because of the content of BiVO4 decreasing, to enhance the difficulty of charge separation, so the EIS arc of BiVO4BiOCl-0.7 are larger than that of BiVO4-BiOCl-0.5. From the above results, we can concluded that the photocatalytic norfloxacin degradation property not only contribute to the p-n junction form between BiVO4-BiOCl, but also the charge carrier transfer capacity increases with the BiOCl nanosheet interlaced between BiVO4 mesoporous spindles. The PL spectra of these photocatalysts are shown in Fig. S1. Compared with pure BiVO4, the PL peak intensity of BiVO4-BiOCl0.5 is dramatically quenched. Furthermore, the other composited BiVO4-BiOCl photocatalysts present lower PL peak intensity than pure BiVO4 also. This phenomenon means that the BiVO4-BiOCl photocatalysts, especially BiVO4-BiOCl-0.5, possess much higher photogenerated carriers separation capacity than other samples. For further clarify the mechanism of the enhanced photogenerated charge separation, the Mott-Schottky plot method was employed to test the semiconductor type of BiVO4 and BiOCl, and the results are showed in Fig. S2. As shown in Fig. S2A, BiOCl shows a typical p-type semiconductor characteristic, with a 2.42 V (vs Ag/ AgCl) VB flat potential. In addition, from Fig. S2B, the BiVO4 shows a n-type semiconductor characteristic, with a 0.13 V CB flat
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Fig. 7. Possible mechanism of the BiVO4-BiOCl p-n heterojunction composite to enhance the photo-change separation and transfer capacity.
potential. Thus, The possible mechanism of the BiVO4-BiOCl p-n heterojunction composite to enhance the photo-change separation and transfer capacity is shown in Fig. 7. When the p-type BiOCl nanosheet grew in-situ and interlaced with the n-type BiVO4, a p-n junction could form between them. After the Fermi level equilibrium of them, a downward bending would be formed from BiOCl to BiVO4. Thus, under visible light illumination, the photogenerated holes of BiVO4 would transfer to BiOCl, to enhance the charge separation efficiency of this composite. Furthermore, because of the nanosheet structure of the BiOCl, the holes shifting process on the surface would change to more smooth, to prolong the lifetime of the separated carriers. Thus, the photocatalytic norfloxacin degradation performance of the sample BiVO4-BiOCl-0.5 could be improved. 4. Conclusion In this study, we prepared spindle like BiVO4, and transferred part of the BiVO4 into BiOCl by hydrothermal method, carried out in HCl solution. When the concentration of HCl is low, a mesoporous spindle like BiVO4 could be achieved. However, with the concentration of HCl increasing further, a mesoporous spindle like BiVO4/
Fig. 6. (A) photoinduced i-t curves and EIS curves of pure BiVO4 and series of BiVO4-BiOCl composites.
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nanosheet BiOCl composite could be formed in-situ. The sample of BiVO4-BiOCl-0.5 showed the best photocatalytic norfloxacin degradation performance, which could almost degraded the norfloxacin in 1 h. Further research results demonstrated that there are two reasons contributed to the photocatalytic performance of this sample. For the one, a p-n junction could form between BiOCl and BiVO4 to enhance the charge separation efficiency of this composite. Another one is that the nanosheet structure of the BiOCl present in the composite, can decrease the holes transfer energy barrier, to prolong the lifetime of the separated carriers. Acknowledgements This work is financially supported by National Basic Research Program of China (2014CB643304). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jallcom.2017.01.214. References [1] A. Kudo, K. Omori, H. Kato, J. Am. Chem. Soc. 121 (1999) 11459. [2] M. Zhou, H.B. Wu, J. Bao, L. Liang, X.W. Lou, Y. Xie, Angew. Chem. 125 (2013) 8741. [3] W.J. Jo, J.W. Jang, K.J. Kong, H.J. Kang, J.Y. Kim, H. Jun, K.P.S. Parmar, J.S. Lee, Angew. Chem. 124 (2012) 3201. [4] S. Kohtani, M. Koshiko, A. Kudo, K. Tokumura, Y. Ishigaki, A. Toriba, K. Hayakawa, R. Nakagaki, Appl. Catal. B 46 (2003) 573. [5] W. Li, J. Yue, Y. Bu, Z. Chen, RSC Adv. 5 (2015) 77823. [6] S.J. Hong, S. Lee, J.S. Jang, J.S. Lee, Energy Environ. Sci. 4 (2011) 1781. [7] J.K. Cooper, S.B. Scott, Y. Ling, J. Yang, S. Hao, Y. Li, F.M. Toma, M. Stutzmann, K.V. Lakshmi, I.D. Sharp, Chem. Mater 28 (2016) 5761.
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Preparation of BiVO4/BiOCl heterojunction photocatalyst by in-situ transformation method for norfloxacin photocatalytic degradation Xiumin Ma a, *, Zheng Ma a, b, Tong Liao a, Xuehui Liu a, b, Yaping Zhang c, LeiLei Li a, Weibing Li c, Baorong Hou a a b c
Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, China School of Chemistry and Chemical Engineering, Shandong University, Jinan 250100, China School of Environment and Safety Engineering, Qingdao University of Science and Technology, Qingdao, 266042, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 19 October 2016 Received in revised form 7 January 2017 Accepted 19 January 2017 Available online 23 January 2017
In this study, we employed an in-situ transformation method to prepared a mesoporous spindle like BiVO4/nanosheet BiOCl composite photocatalyst. The nanostructure and content of 2-D BiOCl could be facilely tuned by controlling the concentration of Cl ions in the reaction solution. The as-prepared BiVO4-BiOCl photocatalyst showed an excellent photocatalytic norfloxacin degradation performance, and removed it in 1 h almost completely. Further research results indicated that a well-defined p-n heterojunction interface has been formed between BiVO4 and BiOCl, to enhance the separation efficiency of photogenerated carriers. In addition, after the nanosheets BiOCl interlaced with the mesoporous BiVO4 spindles, since the 2-D structure of the former, the charge transfer capacity of the composite would be improved to prolong the lifetime of the photoinduced carriers. © 2017 Published by Elsevier B.V.
Keywords: BiVO4/BiOCl p-n junction Ion exchange method Photocatalysis Norfloxacin
1. Introduction Monoclinic bismuth vanadate (BiVO4), with a band gap width of 2.4 eV, shows an excellent visible light absorption capacity. So, it is widely used in photocatalytic organic pollution degradation and water oxidation for O2 evolution [1e4]. However, the dramatic recombination of the photogenerated carriers limited to increasing the quantum yield of this semiconductor [5,6]. In order to control this recombination process, except tuning the semiconductor characteristics of BiVO4 [7,8], combining it with other suitable semiconductors to form a heterojunction system, has been widely considered as a simple and effective method [9e15]. Recently, except BiVO4, other Bi-based semiconductors, bismuth oxyhalides (BiOX, X ¼ Cl, Br, I), have drawn considerable research potential in the area of photocatalysis, which is owing to their adjustable band gap width, controllable nanostructure and facile synthetic process [16e23]. In the bismuth oxyhalides group, with the anions diameters decreasing, the band gap with of the BiOX increase gradually, such as BiOCl, a p-type semiconductor with a
* Corresponding author. E-mail address: [email protected] (X. Ma). http://dx.doi.org/10.1016/j.jallcom.2017.01.214 0925-8388/© 2017 Published by Elsevier B.V.
band gap width of 3.5 eV, it just can absorb UV light [24,25]. However, this material shows much higher photocorrosion stability than visible light absorbed BiOBr and BiOI. Furthermore, in its crystal of BiOCl, The [Bi2O2]2þ layers, interleaved by double slabs of Cl atoms, to induce BiOCl crystal growth along [001] to form a two dimensional nanosheet structure [26e28]. Thus, the transfer process of the photogenerated carriers inside the crystal can be accomplished smoothly, to prolong the lifetime of the photoinduced carriers. Composition of n-type BiVO4 and p-type BiOX to form a p-n heterojunction photocatalyst has been widely considerable an effective method to increase the separation efficiency of the photogenerated electrons and holes [29,30]. As we know, the band gap widths of BiOBr and BiOI are smaller than BiOCl, meaning that the redox energy of photogenerated carriers of the BiOBr and BiOI are lower than that of BiOCl. Especially, since BiOBr or BiOI composite with BiVO4 to form a p-n junction system, the corresponding photogenerated carriers redox energy would decrease further, so these composites were only able to treat some easy degradation organics such as RhB, MB etc. In order to treat some more obstinate environmental pollution by photocatalytic methods, such as antibiotic, BiVO4/BiOCl composite is a better choice [31,32]. A favorable interface contact and a unique nanostructure can
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improve the photons adsorption, photogenerated charge separation and migration capacities of the p-n heterojunction photocatalyst. Ion exchange has been confirmed a utilized method to prepare heterojunction materials through a in-situ growth process [33,34]. Generally, a heterojunction photocatalyst with better interface contact and more effective nanostructure could be achieved by this method. Herein, we employed this facile ion exchange method to fabricate a novel BiVO4/BiOCl (mesoporous spindle/ nanosheet) composite. Specially, the micro morphology and the content of BiOCl in this composite can be easily controlled by tuning the Cl ion concentration in the reaction solution. The asprepared BiVO4/BiOCl composite in this work shows an excellent photogenerated carriers separation capacity and keeps a high redox energy of these carriers, to degrade the norfloxacin smoothly. 2. Experimental 2.1. Preparation of spindle like BiVO4 The reagents and solvents used in this experiment were analytical grade and purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China) without further purification. BiVO4 powder was prepared by a hydrothermal method. 1.5 mmol Bi(NO)3$5H2O and 2 g polyvinylpyrrolidone (PVP, MW ~40 K) were dissolved in 50 mL ethylene glycol with sonicating for 20 min, which was marked as solution A. 1.5 mmol of NH4VO3 was dissolved in 20 mL of deionized water with sonicating for 10 min, and marked as solution B. Then, the solution B was added dropwise to solution A with stirring. The resulting mixture was then transferred into a 100 mL Teflon-lined stainless-steel autoclave, and heated at 180 C for 10 h. After that, the liquid mixture was centrifugalized and repeatedly washed with deionized water and ethanol, dried at 60 C for 5 h in air. 2.2. Preparation of BiVO4/BiOBr composite photocatalyst Simply, 1 mmol as-prepared BiVO4 power and 1 g of PVP were mixed into 80 mL of deionized water with sonicating for 20 min. Then, a certain amount of concentrated HCl (0.1, 0.3, 0.5 and 0.7 mL) was added to above mixture under magnetic stirring. After 10 Min, the resulting mixture was transferred into a 100 mL Teflon-lined stainless-steel autoclave and kept at 180 C for 10 h. The final precipitations were collected by centrifugation, and washed by deionized water and ethanol for three times, then dried at 60 C for 5 h. The samples were marked as BiVO4-BiOCl-0.1, BiVO4-BiOCl-0.3, BiVO4-BiOCl-0.5 and BiVO4-BiOCl-0.7, respectively.
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norfloxacin solution with a concentration of 5 mg L1 and kept stirring for 30 min under dark. The light source was a 150 W Xenon lamp (PLS-SXE300C, Beijing Bofeilai Co. Ltd., Beijing, China). A 420nm cutoff filter was used to get visible light. The distance between the light source and the liquid level was 10 cm. A circulating water system was employed to control the operation process temperature at 25 C. The concentration of the norfloxacin relative to photocatalysis time were tested by UVeVis DRS. 2.5. Photoelectrochemical measurements Photoelectrochemical measurements were performed in a three-electrode experimental system using CHI660D Electrochemical Workstation (Shanghai Chenhua Instrument Co., Ltd., Shanghai, China). The prepared series photoelectrodes, Ag/AgCl electrode, and Pt electrode acted as the working, reference, and counter electrodes, respectively. The light source was a 150 W Xenon lamp (PLS-SXE300C, Beijing Bofeilai Co. Ltd., Beijing, China). The variations of the photoinduced current density with time (i-t curve) were measured at a bias potential of 1.0 V (vs Ag/AgCl) during a 3-cycle light switching on and off. Electrochemical impedance spectroscopy (EIS) tests were performed at 0 V bias potential over the frequency range between 104 and 101 Hz, with an AC voltage magnitude of 5 mV, using 12 points/decade. All tests are carried out in 0.1 mol L1 Na2SO4 electrolyte. Mott-Schottky plots were measured at the frequency of 1000 Hz with an AC voltage magnitude of 10 mV. 3. Results and discussion The XRD results of the BiVO4 and series of BiVO4-BiOCl composite with different HCl adding contents are shown in Fig. 1. BiVO4 curve shows a typical monoclinic phase. For the sample of BiVO4BiOCl-0.1, except monoclinic phase peaks of BiVO4, a weak diffraction peak is found near 32.6, consistent with the tetragonal BiOCl, indicating that trace content BiOCl had formed in this sample with a low concentration of HCl in the reaction solution. With the HCl adding content increasing, we can find that the tetragonal BiOCl diffraction peaks density increase gradually. This phenomenon demonstrated that the content and crystallinity of the BiOCl in the composite of BiVO4-BiOCl were increased with the HCl
2.3. Characterizations The morphology and the microstructure of the prepared samples were analyzed by using a scanning electron microscope (SEM) (SEM, JSM-6700F, JEOL, Japan) and a high-resolution transmission electron microscope (HRTEM, Tecnai G2 F20, FEI Company, USA). The elemental compositions, the crystalline structures and bonding information of the prepared samples were analyzed by using X-ray diffraction (XRD, D/MAX-2500/PC, Rigaku Co., Tokyo, Japan) and XPS (Axis Ultra, Kratos Analytical Ltd., England). The optical absorption properties were tested by a UV/Vis diffuse reflectance spectrophotometer (UVeVis DRS, UV-2600, Shimadzu, Japan). The fluorescence spectrum (PL, Fluoro Max-4) was employed to characterize the PL density of these photocatalysts. 2.4. Photocatalytic degradation of norfloxacin 0.05 g prepared photocatalyst was added into 100 mL
Fig. 1. XRD results of spindle BiVO4 and series of BiVO4-BiOCl composite with different HCl adding contents.
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concentration increasing in the reaction solution. The microsturcture of these materials were observed by SEM and TEM further, and the results are shown in Fig. 1. The SEM image of pure BiVO4 is shown in Fig. 2A, a close-grained spindle like structure with a length of approximately 1.5 mm can be observed under the SEM. Fig. 2B ~ 2F present the SEM images of BiVO4-BiOCl0.1, BiVO4-BiOCl-0.3, BiVO4-BiOCl-0.5 and BiVO4-BiOCl-0.7. Since the adding content of HCl is low, such as the sample of BiVO4-BiOCl0.1, after the hydrothermal reaction the BiVO4 would be etched to form a mesopourous spindle structure. However, with the concentration HCl adding content exceeded than 0.1 mL, from Fig. 2CeE, we can find that more and more nanosheets interlaced
with the mesopourous spindle structures. Fig. 2F is the corresponding TEM image of Fig. 2D, from this image, we can clearly find that the mesopourous spindle BiVO4 and some nanoparticles are dispersed on the surface of BiOCl sheets. Thus, we proposed the possible reaction process of the BiVO4-BiOCl with different contents of HCl adding in Fig. 2G when the concentration of HCl is low in the reaction solution, it would like to etch the spindle BiVO4 to mesopourous spindle structure only, because the Cl concentration is low at this case. However, with the HCl concentration increasing, more BiVO4 would be dissolved and transfer to [Bi2O2]2þ, and reacted with the Cl to form BiOCl nanosheet under the hydrothermal condition. The SEM elements mapping of the sample
Fig. 2. (A) to (E) show the SEM images of BiVO4 and BiVO4-BiOCl-0.1, BiVO4-BiOCl-0.3, BiVO4-BiOCl-0.5 and BiVO4-BiOCl-0.7, orderly; (F) is the TEM image of BiVO4-BiOCl-0.5; (G) schematic of the nano structure formation process; (H) SEM elements mapping of BiVO4-BiOCl-0.5, including Bi, V, O and Cl.
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BiVO4-BiOCl-0.5 shows in Fig. 2H. From this result we can find that there are Bi, V, O and Cl elements distributed on the composite material without any other elements, which is corresponding to the components of BiVO4-BiOCl composite. The XPS was employed to further investigate the surface components and the bond states of the as-prepared BiVO4-BiOCl-0.5 and the results are shown in Fig. 3. Fig. 3A shows the Survey curve of this sample, we only can find the Bi, V, O, Cl elements (C 1s peak was contributed to the background carbon source of the XPS) on the surface, indicating a pure BiVO4-BiOCl composite has been synthesized. Bi 4f spectrum of this sample is shown in Fig. 3B, the core peaks at 158.6 eV and 164.1 eV are corresponding to Bi 4f7/2 and Bi 4f5/2, respectively [31]. The V 2p XPS spectrum is presented in Fig. 3C, the peak at 516.1 eV contributes to the V 2p 3/2 at the surface of BiVO4 [31]. The Cl 2p shown in Fig. 3D resolved into two
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peaks at 197.8 and 199.3 eV, which corresponding to the Cl 2p3/ 2and Cl 2p1/2 states for BiOCl, respectively [35]. The O 1s spectrum is shown in Fig. 3E can be divided into two peaks at 529.3 and 530.0 eV, respectively. The former one contributed to the lattice oxygen of Bi-O bond, and the latter one contributed to the hydroxyl groups which on the surface of BiOCl or BiVO4. The optical absorption characteristics of the pure BiVO4 and BiVO4-BiOCl-0.1, BiVO4-BiOCl-0.3, BiVO4-BiOCl-0.5 and BiVO4-BiOCl-0.7 samples were tested, and the results showed in Fig. 4. The absorption threshold of pure BiVO4 was near at 540 nm, corresponded a band gap width of 2.3 eV. However, after BiVO4-BiOCl composite formation, their absorption thresholds blue shifted to approximately 527 nm, corresponded a 2.35 eV band gap width. As we know, BiOCl is a semiconductor with absorption in UV region, thus, the band gap enlarged after transferring part of BiVO4 to BiOCl.
Fig. 3. XPS result of the sample BiVO4-BiOCl-0.5.
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Fig. 4. UVeVis DRS curves of BiVO4 and BiVO4-BiOCl-0.1, BiVO4-BiOCl-0.3, BiVO4BiOCl-0.5 and BiVO4-BiOCl-0.7.
The photocatalytic abilities of pure BiVO4 and BiOCl-BiVO4 composites were evaluated by degradation of the norfloxacin solution. Norfloxacin is an antibiotic, which is not an easy degradation by microbiological technology because of the growth process of bacterium, would be inhabited since the antibiotic existing in the waste water. Photocatalysis is a potential method to treat norfloxacin, since the redox energy of the radicals produced by photocatalysis process is unselective during degradation the organics. And all the corresponding results are shown in Fig. 5. Fig. 5(A)e(E) present the UVeVis absorption curves of the photocatalytic norfloxacin degradation properties of BiVO4 and series of BiVO4-BiOCl
composites, respectively. From these results, we can find that the characteristic absorption peak of norfloxacin is at near 277 nm, and during the photocatalytic time extending, this peak intensity decreased gradually, indicating that the norfloxacin was degraded by the photocatalysis process. Compared with the photocatalytic properties of these photocatalysts, the absorbance at 277 on these curves have been marked, and conversion by the standard absorption curves of norfloxacin in water, to get the concentration change of norfloxacin during photocatalysis process for each photocatalyst, and the results are shown in Fig. 5F. After stirring for 30 min under dark, just negligible norfloxacin could be adsorbed by these photocatalysts. After light illumination, the sample of pure BiVO4 could degradate 60% norfloxacin in 1 h. For the sample of BiVO4-BiOCl-0.1, it about 80% norfloxacin has been degraded in 1 h, indicating that the photocatalytic property could be improved after compact spindle BiVO4 transfer to mesoporous spindle BiVO4 because of more reaction active sites on the surface of the latter one. With the BiOCl content in the BiVO4-BiOCl composite increasing further, as show in Fig. 5F, the photocatalytic norfloxacin properties of BiVO4-BiOCl-0.3, BiVO4-BiOCl-0.5 and BiVO4-BiOCl0.7 increased greatly than pure BiVO4 and BiVO4-BiOCl-0.1, them could almost completely remove the norfoxacin in 1 h, and the sample of BiVO4-BiOCl-0.5 showed a litter higher property than that of BiVO4-BiOCl-0.3 and BiVO4-BiOCl-0.7. This phenomenon demonstrated that a heterojunction is possibly formed between the BiVO4 and BiOCl nanosheet interface that to improve the separation efficiency of the photoinduced carriers. Blank curve shows a near invariable tendency, indicating that the norfloxacin did not selfdegradation under visible light illumination. As shown in Fig. 5F, BiOCl cannot provide a degradation efficiency for norfloxacin in visible light region. BiOCl is a UV region response semiconductor, meanwhile, norfloxacin also a UV region response organic. So, the norfloxacin cannot photo-sensitized the BiOCl in visible light.
Fig. 5. (A) ~ (E) UVeVis absorption curves of the photocatalytic norfloxacin degradation properties of BiVO4 and series of BiVO4-BiOCl composites, respectively. (F) photocatalytic curve of these materials by fit UVeVis absorption curves at 277 nm from each corresponding UVeVis absorption curve.
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Photoinduced i-t curves and EIS curves were employed to test the carriers separation and transfer capacity of the BiVO4 and the BiVO4-BiOCl composites, and the results are shown in Fig. 6. Photoinduced i-t curve is a method which can reflect the separation efficiency and transfer capacity of the photogenerated carriers indirectly. As shown in Fig. 6A, the photocurrent of BiVO4 in this case was lowest. However, with the HCl concentration in the reaction solution increasing, the photocurrents of the as-prepared BiVO4-BiOCl-0.1, BiVO4-BiOCl-0.3 and BiVO4-BiOCl-0.5 increased gradually, however the sample BiVO4-BiOCl-0.7 decreased. To date, many previous work had been clarified that the p-n junction formed between BiOCl and BiVO4 can improve the photoinduced carriers separation efficiency, since to increase the corresponding photocurrent density. In this case, except this reason, the electron transfer capacity maybe increase with the BiOCl nanosheet formed in the composite. Thus, the EIS method was used to test the electron transfer characteristics of these materials, and the results are shown in Fig. 6B. From these curves, we can find that the order of EIS arcs diameter is BiVO4 > BiVO4-BiOCl-0.1 > BiVO4-BiOCl0.7 > BiVO4-BiOCl-0.3 > BiVO4-BiOCl-0.5, this order indicated that the charge transfer capacity can be improved after BiOCl nanosheets interlacing between mesoporous BiVO4 spindle. However, with the content of BiOCl increasing to a limit, the interface contact would decrease because of the content of BiVO4 decreasing, to enhance the difficulty of charge separation, so the EIS arc of BiVO4BiOCl-0.7 are larger than that of BiVO4-BiOCl-0.5. From the above results, we can concluded that the photocatalytic norfloxacin degradation property not only contribute to the p-n junction form between BiVO4-BiOCl, but also the charge carrier transfer capacity increases with the BiOCl nanosheet interlaced between BiVO4 mesoporous spindles. The PL spectra of these photocatalysts are shown in Fig. S1. Compared with pure BiVO4, the PL peak intensity of BiVO4-BiOCl0.5 is dramatically quenched. Furthermore, the other composited BiVO4-BiOCl photocatalysts present lower PL peak intensity than pure BiVO4 also. This phenomenon means that the BiVO4-BiOCl photocatalysts, especially BiVO4-BiOCl-0.5, possess much higher photogenerated carriers separation capacity than other samples. For further clarify the mechanism of the enhanced photogenerated charge separation, the Mott-Schottky plot method was employed to test the semiconductor type of BiVO4 and BiOCl, and the results are showed in Fig. S2. As shown in Fig. S2A, BiOCl shows a typical p-type semiconductor characteristic, with a 2.42 V (vs Ag/ AgCl) VB flat potential. In addition, from Fig. S2B, the BiVO4 shows a n-type semiconductor characteristic, with a 0.13 V CB flat
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Fig. 7. Possible mechanism of the BiVO4-BiOCl p-n heterojunction composite to enhance the photo-change separation and transfer capacity.
potential. Thus, The possible mechanism of the BiVO4-BiOCl p-n heterojunction composite to enhance the photo-change separation and transfer capacity is shown in Fig. 7. When the p-type BiOCl nanosheet grew in-situ and interlaced with the n-type BiVO4, a p-n junction could form between them. After the Fermi level equilibrium of them, a downward bending would be formed from BiOCl to BiVO4. Thus, under visible light illumination, the photogenerated holes of BiVO4 would transfer to BiOCl, to enhance the charge separation efficiency of this composite. Furthermore, because of the nanosheet structure of the BiOCl, the holes shifting process on the surface would change to more smooth, to prolong the lifetime of the separated carriers. Thus, the photocatalytic norfloxacin degradation performance of the sample BiVO4-BiOCl-0.5 could be improved. 4. Conclusion In this study, we prepared spindle like BiVO4, and transferred part of the BiVO4 into BiOCl by hydrothermal method, carried out in HCl solution. When the concentration of HCl is low, a mesoporous spindle like BiVO4 could be achieved. However, with the concentration of HCl increasing further, a mesoporous spindle like BiVO4/
Fig. 6. (A) photoinduced i-t curves and EIS curves of pure BiVO4 and series of BiVO4-BiOCl composites.
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nanosheet BiOCl composite could be formed in-situ. The sample of BiVO4-BiOCl-0.5 showed the best photocatalytic norfloxacin degradation performance, which could almost degraded the norfloxacin in 1 h. Further research results demonstrated that there are two reasons contributed to the photocatalytic performance of this sample. For the one, a p-n junction could form between BiOCl and BiVO4 to enhance the charge separation efficiency of this composite. Another one is that the nanosheet structure of the BiOCl present in the composite, can decrease the holes transfer energy barrier, to prolong the lifetime of the separated carriers. Acknowledgements This work is financially supported by National Basic Research Program of China (2014CB643304). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jallcom.2017.01.214. References [1] A. Kudo, K. Omori, H. Kato, J. Am. Chem. Soc. 121 (1999) 11459. [2] M. Zhou, H.B. Wu, J. Bao, L. Liang, X.W. Lou, Y. Xie, Angew. Chem. 125 (2013) 8741. [3] W.J. Jo, J.W. Jang, K.J. Kong, H.J. Kang, J.Y. Kim, H. Jun, K.P.S. Parmar, J.S. Lee, Angew. Chem. 124 (2012) 3201. [4] S. Kohtani, M. Koshiko, A. Kudo, K. Tokumura, Y. Ishigaki, A. Toriba, K. Hayakawa, R. Nakagaki, Appl. Catal. B 46 (2003) 573. [5] W. Li, J. Yue, Y. Bu, Z. Chen, RSC Adv. 5 (2015) 77823. [6] S.J. Hong, S. Lee, J.S. Jang, J.S. Lee, Energy Environ. Sci. 4 (2011) 1781. [7] J.K. Cooper, S.B. Scott, Y. Ling, J. Yang, S. Hao, Y. Li, F.M. Toma, M. Stutzmann, K.V. Lakshmi, I.D. Sharp, Chem. Mater 28 (2016) 5761.
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