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Visible light-induced degradation of methylene blue by photocatalyst of bismuth layered Bi7VO13 nanoparticles
Author's Accepted Manuscript
Visible light-induced degradation of methylene blue by photocatalyst of bismuth layered Bi7VO13 nanoparticles Yinfu Pu, Yuze Li, Yanlin Huang, Sun Il Kim, Peiqing Cai, Hyo Jin Seo
www.elsevier.com/locate/matlet
PII: DOI: Reference:
S0167-577X(14)02033-3 http://dx.doi.org/10.1016/j.matlet.2014.11.045 MLBLUE18031
To appear in:
Materials Letters
Received date: 21 October 2014 Accepted date: 12 November 2014 Cite this article as: Yinfu Pu, Yuze Li, Yanlin Huang, Sun Il Kim, Peiqing Cai, Hyo Jin Seo, Visible light-induced degradation of methylene blue by photocatalyst of bismuth layered Bi7VO13 nanoparticles, Materials Letters, http: //dx.doi.org/10.1016/j.matlet.2014.11.045 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Visible light-induced degradation of methylene blue by photocatalyst of bismuth layered Bi7VO13 nanoparticles Yinfu Pu,a Yuze Li,a Yanlin Huang,a Sun Il Kim,b Peiqing Cai,b Hyo Jin Seo b* a
College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, China
b
Department of Physics and Interdisciplinary Program of Biomedical, Mechanical & Electrical Engineering, Pukyong National University, Busan 608-737, Republic of Korea
Abstract A visible-light-driven photocatalyst of Bi7VO13 nanoparticles was prepared by the Pechini method. The nanoparticles were characterized with the measurements such as X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscopy (TEM), and UV-vis absorption spectrum. Bi7VO13 presents the indirect allowed electronic transition with narrow band gap energy of 2.13 eV. The photocatalyst shows an excellent activity on the photo-degradation of methylene blue (MB) under visible-light irradiation. The photocatalysis was discussed on the base of the structure characteristics such as (Bi2O2)2+ layers connected by the isolated VO4 groups and the good conductivity. This vanadate could be a potential photocatalyst.
Keywords: Vanadate; Semiconductor; Photocatalysis; Solar materials 1 Introduction Bismuth (Bi3+) ion has an electronic configuration [Xe]4f145d106s2 with a peculiar 6s2 lone pair (without bonding or sharing with other atoms). The induced repulsive force can apply to other bonding schemes generally causing a heavy structure distortion. Bi-containing materials have been
*
Corresponding authors: E-mail: [email protected] (Hyo Jin Seo) Tel.: +82-51-629 5568; Fax:+82-51-6295549 1
investigated as visible-light driven photocatalysts due to efficient optical absorption and suitable band gaps. One of the important reasons is that Bi3+ ions make band gap narrow utilizing O2 phybridized with metals (e.g. Bi6s) orbitals [1]. Among them, some Bi-layered oxides of Bi2O22+ modules present excellent photcatalytic activities with fascinating physicochemical properties such as BiVO4 [1,2], Bi2O3 [3-7], BiOBr [8], Bi4V2O11−δ [9], Bi2MoO6 [10], and so on. The common structure characteristic is alternated (Bi2O2)2+ layers, which is confirmed to a play important role in photocatalysis [10]. The photogenerated electrons and holes were carried mainly in (Bi2O2)2+ layers, which has a significant impact on the conductivity. Consequently this could induce an internal polar electric field, which facilitates the charge separation due to opposite movements of electrons and holes in electric field, resulting in photocatalytic activity. In this work, Bi7VO13 was selected to investigate its photocatalytic activity. Firstly, Bi7VO13 was reported as structurally related to CaF2 and δ-Bi2O3 [11]. There are dominated Bi2O2 layers in the lattices. Secondly, VO4 in the lattices act as the activated centers to improve its optical absorption. We explore the synthesis Bi7VO13 by the Pechini method. The sample was investigated by XRD, morphologies, and UV–vis absorption. The efficient photocatalytic activity was confirmed by degradation of methylene blue (MB) under visible irradiation.
2. Experimental Bi7VO13 was prepared by the Pechini method. The raw materials are stoichiometric amounts of Bi(NO3)3·5H2O (99.99%), and NH4VO3 (99.99%). Firstly, the nitrate solutions were complexed by citric acid with double molar weight of the Bi3+ ions. The solution was neutralized by controlled addition of ammonium hydroxide (30% wt) and was promoted at 85oC for 3h. Then, a certain amount of aqueous polyvinylalcohol (PVA) was added to adjust the viscoelastic, which
2
was stirred for 2h to obtain a homogeneous viscous solution for the spin-coating on several clean glasses. The precursor thin film can be obtained by natural withering of the coated glasses. Finally, the films were annealed at 700oC to produce the Bi7VO13 powder. The XRD pattern was collected on a Rigaku D/Max diffractometer operating at 40 kV, 30 mA with Bragg–Brentano geometry using Cu-Kα radiation (λ=1.5405 Å). Diffuse reflection spectra (DRS) were taken on a Cary 5000-UV–Vis-NIR spectrophotometer by using BaSO4 powders as a standard reference. The microstructure morphology was investigated using scanning electron microscopy (JSM-6700F, JEOL). The photocatalytic activity was tested for the efficient degradation of methylene blue dye. The characteristic absorption peak at 665 nm is used to monitor the photocatalytic dye degradation reaction.
3. Results and discussion
Figure 1 The XRD pattern of Bi7VO13 nanoparticles is shown in Fig. 1, which is indexed to PDF2 No:44-0322. No impurity peaks were observed. The result reveals that the photocatalyst crystallized with a pure Bi7VO13 phase. Bi7VO13 has been reported to be in monoclinic symmetry with space group of P21/n in Bi7VO13, which is structurally related to CaF2 and δ-Bi2O3 [11]. The unit parameters are a=3.9867Å, b=3.9247Å, c=5.4365Å, Z=6 and V=85.05Å3. The microstructures were characterized by SEM and TEM images shown in Fig. 2 (a,b). The 3
nanoparticles consist of the irregular grains with average size less than 50nm. The selected area electron diffraction (SAED) data show the presence of diffraction circles indicating the polycrystalline formation (Fig. 3 c). The representative EDS spectrum is shown in Fig. 3 (d). Several specific lines show the signals of Bi, V and O elements. The average Bi/V ratio was calculated to be about 6.5, which is close to the theoretical stoichiometric value of the chemical formulae of Bi7VO13.
Figure 2
The UV-Vis absorption spectrum of Bi7VO13 nanoparticles is shown in Fig. 3. It can be seen that the optical absorption edge is located at about 570 nm. Below the cutoff of the absorption edge the sample shows the broad absorption band at 200-600 nm, which could have two possible attributions, the one is charge transfers (CT) transitions from O2- to V5+ inside VO43− groups [12], and another is the transition of the 6s electrons of Bi3+ to the empty 3d orbitals of V5+ ions. As shown in Fig. 3. Bi7VO13 nanoparticles are bright yellow red. The color should be generally attributed to the contribution of the optical transitions. The orbitals of V5+ with Td symmetry in VO4 are expressed as a state ground 1A1 and excited states 1T1, 1T2, 3T1, and 3T2 [12]. Optical absorption takes place by spin-allowed transitions from 1A1 to 1T2 and 1T1 levels. Usually this absorption is located in the UV wavelength region 4
(250–400 nm) [12]. In Bi-containing transition-metal (M) oxides, transition of 6s electrons of Bi3+ to the empty 3d orbitals of M5+ usually is possible, and its wavelength locates in a longer wavelength than the CT band in (M-O). This has been observed in the monoclinic BiVO4 [1]. Based on the discussion mentioned above, the band structures of Bi7VO13 are described inset Fig. 3b. The conduction bands (CBs) of Bi7VO13 are comprised of the V 3d orbitals, while the valence bands (VB) are formed by not only Bi6s but also O2p, namely a hybrid orbital of Bi6s and O2p. The intrinsic band-gap absorption is due to the electronic transitions from Bi6s/O2p orbitals in the (Bi2O2)2+ layers (VB) to the CB band consisting of V3d orbitals.
Figure 3 Band gap energy (Eg) is given by the relation of αhυ ∝ ( hυ − E g ) k , where α is absorbance, h is Planck constant, ν is frequency, k is a constant associated to different types of electronic transitions (k=1/2, 2 for direct allowed and indirect allowed, respectively). The best linear relation with k=2 indicates this is an indirect allowed electronic transition. The band gap of Bi7VO13 is calculated to be 2.13 eV, which is significantly narrower than the reported Bi3+-containing catalysts such as F-doped Bi2O3 (2.68-2.74eV) [5], α-Bi2O3 (2.8eV) [6], β-Bi2O3 (3.4eV) [7], tetragonal BiVO4 (2.9eV), monoclinic BiVO4 (2.4eV) [1], α-Bi2Mo3O12(2.92eV), γ-Bi2MoO6 (2.56eV), and β-Bi2Mo2O9 (3.06eV) [10]. This indicates that Bi7VO13 has a reduced the band gap, which is beneficial to the improvement of the photocatalytic activity. 5
The photocatalytic activity of Bi7VO13 nanoparticles was tested for the degradation of a pollutant. The MB degradation was monitored by recording the decrease in the UV-visible absorption (λmax=665nm) as shown in Fig. 4a. The spectra intensity decreased with increasing irradiation time, suggesting that the solution had been decolorized. Besides, no new bands appear during the degradation process. The MB degradation efficiency versus the time is shown in Fig. 4b. A blank experiment was investigated as a background check under the same conditions. It shows that only a very small quantity of dye was degraded for the blank experiment within 120min. In contrast, the MB shows a fast degradation in the presence of Bi7VO13, which is decreased to 90% in 120min. The kinetic constant was determined from the pseudo-first-order reaction rate equation of ln(C0/Ct)=kt, where C0 is initial MB concentration, Ct is concentration of MB at time t, k is kinetic constant. Inset Fig. 4b shows the plot of ln(C0/Ct) vs. The linear fit indicates that the kinetics of the degradation reaction is controlled by a pseudo-first-order reaction with a constant 0.0263min−1. The pseudo-first order constants of MB degradation by bismuth photocatalysts have been reported such as Bi4V1.6Ga0.4O11−δ (6.9×10−3min−1) [9], porous BiVO4 film (0.0138min−1) [13], nano-BiVO4 (0.629×10-3 min-1) and 1.4%-Cu2O/BiVO4 (18.2×10-3min-1) [14]. This shows that Bi7VO13 nanoparticles are more effective for the MB photodegradation.
6
The structure characteristic of Bi7VO13 is responsible for its photocatalysis. Firstly, there are dominated (Bi2O2)2+ layers in Bi7VO13 connected via VO4 layers [11]. (Bi2O2)2+ units play important roles in photocatalysis in Bi3+-containing materials, where the photogenerated electrons and holes are carried. Secondly the polar VO4 with the long V–V distance reduces the recombination probability of excitons. In fact, both [VO4]3− and Bi3+ are well-known luminescence centers. However, no luminescence is detected indicating an extremely low recombination rate of charge carriers. Thirdly, Bi7VO13 is known for its high ion conductivity. This is beneficial for the photocatalysis by providing an easy move for the carriers.
5 Conclusions A visible-light-driven photocatalyst was developed in Bi7VO13 nanoparticles by the Pechini method. The nanoparticles have an average diameter less than 50 nm. The crystal structure of Bi7VO13 nanoparticles has a pure crystal formation. Bi7VO13 shows an indirect allowed electronic transition with a narrow band gap of 2.13eV. The band gap was assigned to the transitions from a VB formed by a hybrid orbital of Bi6s and O2p to a CB of empty V3d. The MB dyes can be efficiently degraded under visible light irradiation in the presence of Bi7VO13. The photocatalytic ability could be related to its special crystal structure such as the dominated (Bi2O2)2+ layers, the polar VO4 groups and the good conductivity. The obtained nanoparticles could be expected to have a potential application in environment protection technology.
7
Acknowledgements This work was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF-2013RA1A2009154) and by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), China.
References [1] Kudo A, Omori K, Kato H. J Am Chem Soc 1999;121:11459-11467 [2] Huang J, Tan G, Zhang L, Ren H, Xia A, Zhao C. Mater Lett 2014;133:20-23 [3] Cheng L, Liu X, Kang Y. Mater Lett 2014;134:218-221 [4] Hao Y, Li F, Chen F, Chai M, Liu R, Wang X. Mater Lett 2014;124:1-3 [5] Jiang HY, Liu J, Cheng K, Sun W, Lin J. J Phys Chem C 2013;117:20029-20036 [6] Hu J, Li H, Huang C, Liu M, Qiu X. Appl Catal B 2013;142-143:598-603 [7] Brezesinski K, Ostermann R, Hartmann P, Perlich J, Brezesinski T. Chem Mater 2010;:3079-3085 [8] Dai K, Li D, lv J, Lu L, Liang C, Zhu G. Mater Lett 2014;136:438-440 [9] Thakral V, Uma S. Mater Res Bull 2010;45:1250-1254. [10] Li HH, Li KW, Wang H. Mater Chem Phys 2009;116:134-142, [11] Jie YC, Eysel W. Powder Diffraction 1995;10:76-80. [12] Nakajima T, Isobe M, Tsuchiya T, Ueda Y, Kumagai T. J Lumin 2009;129:1598-1601 [13] Huo T, Zhang X, Dong X, Zhang X, Ma C, et al. J Mater Chem A 2014;2:17366-17370 [14] Li H, Hong W, Cui Y, Hu X, Fan S, Zhu L. Mater Sci Eng B 2014;181:1-8
8
Figure Captions Fig. 1 X-ray diffraction pattern of Bi7VO13 nanoparticles indexed to PDF2# No: 44-0322. Fig. 2 SEM (a), TEM (b) photos, SAED pattern (c), and the EDX (d) of Bi7VO13 nanoparticles Fig. 3 UV–vis absorption spectrum of Bi7VO13 nanoparticles; Inset the left is the band-gap structure; Inset the right is the estimation for the band-gap energy (Eg) and the picture. Fig. 4 (a): UV/vis adsorption spectra of MB-Bi7VO13 solution on irradiation, (b): the MB photocatalytic degradation; Inset is the degradation kinetics relation plotting ln(C0/C) vs time.
Highlights ►A novel visible-light-driven photocatalyst Bi7VO13 nanoparticle was firstly developed by the Pechini method. ►Bi7VO13 nanoparticle shows high absorption in UV-vis wavelength region with a narrow band gap of 2.13eV. ►Bi7VO13 nanoparticle shows high activity in the MB degradation with a pseudo-first-order reaction with a constant 0.0263min−1. ►The dominated (Bi2O2)2+ layers, the polar VO4 groups and the good conductivity are the structural superiority for photocatalytic capacity.
9
Visible light-induced degradation of methylene blue by photocatalyst of bismuth layered Bi7VO13 nanoparticles Yinfu Pu, Yuze Li, Yanlin Huang, Sun Il Kim, Peiqing Cai, Hyo Jin Seo
www.elsevier.com/locate/matlet
PII: DOI: Reference:
S0167-577X(14)02033-3 http://dx.doi.org/10.1016/j.matlet.2014.11.045 MLBLUE18031
To appear in:
Materials Letters
Received date: 21 October 2014 Accepted date: 12 November 2014 Cite this article as: Yinfu Pu, Yuze Li, Yanlin Huang, Sun Il Kim, Peiqing Cai, Hyo Jin Seo, Visible light-induced degradation of methylene blue by photocatalyst of bismuth layered Bi7VO13 nanoparticles, Materials Letters, http: //dx.doi.org/10.1016/j.matlet.2014.11.045 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Visible light-induced degradation of methylene blue by photocatalyst of bismuth layered Bi7VO13 nanoparticles Yinfu Pu,a Yuze Li,a Yanlin Huang,a Sun Il Kim,b Peiqing Cai,b Hyo Jin Seo b* a
College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, China
b
Department of Physics and Interdisciplinary Program of Biomedical, Mechanical & Electrical Engineering, Pukyong National University, Busan 608-737, Republic of Korea
Abstract A visible-light-driven photocatalyst of Bi7VO13 nanoparticles was prepared by the Pechini method. The nanoparticles were characterized with the measurements such as X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscopy (TEM), and UV-vis absorption spectrum. Bi7VO13 presents the indirect allowed electronic transition with narrow band gap energy of 2.13 eV. The photocatalyst shows an excellent activity on the photo-degradation of methylene blue (MB) under visible-light irradiation. The photocatalysis was discussed on the base of the structure characteristics such as (Bi2O2)2+ layers connected by the isolated VO4 groups and the good conductivity. This vanadate could be a potential photocatalyst.
Keywords: Vanadate; Semiconductor; Photocatalysis; Solar materials 1 Introduction Bismuth (Bi3+) ion has an electronic configuration [Xe]4f145d106s2 with a peculiar 6s2 lone pair (without bonding or sharing with other atoms). The induced repulsive force can apply to other bonding schemes generally causing a heavy structure distortion. Bi-containing materials have been
*
Corresponding authors: E-mail: [email protected] (Hyo Jin Seo) Tel.: +82-51-629 5568; Fax:+82-51-6295549 1
investigated as visible-light driven photocatalysts due to efficient optical absorption and suitable band gaps. One of the important reasons is that Bi3+ ions make band gap narrow utilizing O2 phybridized with metals (e.g. Bi6s) orbitals [1]. Among them, some Bi-layered oxides of Bi2O22+ modules present excellent photcatalytic activities with fascinating physicochemical properties such as BiVO4 [1,2], Bi2O3 [3-7], BiOBr [8], Bi4V2O11−δ [9], Bi2MoO6 [10], and so on. The common structure characteristic is alternated (Bi2O2)2+ layers, which is confirmed to a play important role in photocatalysis [10]. The photogenerated electrons and holes were carried mainly in (Bi2O2)2+ layers, which has a significant impact on the conductivity. Consequently this could induce an internal polar electric field, which facilitates the charge separation due to opposite movements of electrons and holes in electric field, resulting in photocatalytic activity. In this work, Bi7VO13 was selected to investigate its photocatalytic activity. Firstly, Bi7VO13 was reported as structurally related to CaF2 and δ-Bi2O3 [11]. There are dominated Bi2O2 layers in the lattices. Secondly, VO4 in the lattices act as the activated centers to improve its optical absorption. We explore the synthesis Bi7VO13 by the Pechini method. The sample was investigated by XRD, morphologies, and UV–vis absorption. The efficient photocatalytic activity was confirmed by degradation of methylene blue (MB) under visible irradiation.
2. Experimental Bi7VO13 was prepared by the Pechini method. The raw materials are stoichiometric amounts of Bi(NO3)3·5H2O (99.99%), and NH4VO3 (99.99%). Firstly, the nitrate solutions were complexed by citric acid with double molar weight of the Bi3+ ions. The solution was neutralized by controlled addition of ammonium hydroxide (30% wt) and was promoted at 85oC for 3h. Then, a certain amount of aqueous polyvinylalcohol (PVA) was added to adjust the viscoelastic, which
2
was stirred for 2h to obtain a homogeneous viscous solution for the spin-coating on several clean glasses. The precursor thin film can be obtained by natural withering of the coated glasses. Finally, the films were annealed at 700oC to produce the Bi7VO13 powder. The XRD pattern was collected on a Rigaku D/Max diffractometer operating at 40 kV, 30 mA with Bragg–Brentano geometry using Cu-Kα radiation (λ=1.5405 Å). Diffuse reflection spectra (DRS) were taken on a Cary 5000-UV–Vis-NIR spectrophotometer by using BaSO4 powders as a standard reference. The microstructure morphology was investigated using scanning electron microscopy (JSM-6700F, JEOL). The photocatalytic activity was tested for the efficient degradation of methylene blue dye. The characteristic absorption peak at 665 nm is used to monitor the photocatalytic dye degradation reaction.
3. Results and discussion
Figure 1 The XRD pattern of Bi7VO13 nanoparticles is shown in Fig. 1, which is indexed to PDF2 No:44-0322. No impurity peaks were observed. The result reveals that the photocatalyst crystallized with a pure Bi7VO13 phase. Bi7VO13 has been reported to be in monoclinic symmetry with space group of P21/n in Bi7VO13, which is structurally related to CaF2 and δ-Bi2O3 [11]. The unit parameters are a=3.9867Å, b=3.9247Å, c=5.4365Å, Z=6 and V=85.05Å3. The microstructures were characterized by SEM and TEM images shown in Fig. 2 (a,b). The 3
nanoparticles consist of the irregular grains with average size less than 50nm. The selected area electron diffraction (SAED) data show the presence of diffraction circles indicating the polycrystalline formation (Fig. 3 c). The representative EDS spectrum is shown in Fig. 3 (d). Several specific lines show the signals of Bi, V and O elements. The average Bi/V ratio was calculated to be about 6.5, which is close to the theoretical stoichiometric value of the chemical formulae of Bi7VO13.
Figure 2
The UV-Vis absorption spectrum of Bi7VO13 nanoparticles is shown in Fig. 3. It can be seen that the optical absorption edge is located at about 570 nm. Below the cutoff of the absorption edge the sample shows the broad absorption band at 200-600 nm, which could have two possible attributions, the one is charge transfers (CT) transitions from O2- to V5+ inside VO43− groups [12], and another is the transition of the 6s electrons of Bi3+ to the empty 3d orbitals of V5+ ions. As shown in Fig. 3. Bi7VO13 nanoparticles are bright yellow red. The color should be generally attributed to the contribution of the optical transitions. The orbitals of V5+ with Td symmetry in VO4 are expressed as a state ground 1A1 and excited states 1T1, 1T2, 3T1, and 3T2 [12]. Optical absorption takes place by spin-allowed transitions from 1A1 to 1T2 and 1T1 levels. Usually this absorption is located in the UV wavelength region 4
(250–400 nm) [12]. In Bi-containing transition-metal (M) oxides, transition of 6s electrons of Bi3+ to the empty 3d orbitals of M5+ usually is possible, and its wavelength locates in a longer wavelength than the CT band in (M-O). This has been observed in the monoclinic BiVO4 [1]. Based on the discussion mentioned above, the band structures of Bi7VO13 are described inset Fig. 3b. The conduction bands (CBs) of Bi7VO13 are comprised of the V 3d orbitals, while the valence bands (VB) are formed by not only Bi6s but also O2p, namely a hybrid orbital of Bi6s and O2p. The intrinsic band-gap absorption is due to the electronic transitions from Bi6s/O2p orbitals in the (Bi2O2)2+ layers (VB) to the CB band consisting of V3d orbitals.
Figure 3 Band gap energy (Eg) is given by the relation of αhυ ∝ ( hυ − E g ) k , where α is absorbance, h is Planck constant, ν is frequency, k is a constant associated to different types of electronic transitions (k=1/2, 2 for direct allowed and indirect allowed, respectively). The best linear relation with k=2 indicates this is an indirect allowed electronic transition. The band gap of Bi7VO13 is calculated to be 2.13 eV, which is significantly narrower than the reported Bi3+-containing catalysts such as F-doped Bi2O3 (2.68-2.74eV) [5], α-Bi2O3 (2.8eV) [6], β-Bi2O3 (3.4eV) [7], tetragonal BiVO4 (2.9eV), monoclinic BiVO4 (2.4eV) [1], α-Bi2Mo3O12(2.92eV), γ-Bi2MoO6 (2.56eV), and β-Bi2Mo2O9 (3.06eV) [10]. This indicates that Bi7VO13 has a reduced the band gap, which is beneficial to the improvement of the photocatalytic activity. 5
The photocatalytic activity of Bi7VO13 nanoparticles was tested for the degradation of a pollutant. The MB degradation was monitored by recording the decrease in the UV-visible absorption (λmax=665nm) as shown in Fig. 4a. The spectra intensity decreased with increasing irradiation time, suggesting that the solution had been decolorized. Besides, no new bands appear during the degradation process. The MB degradation efficiency versus the time is shown in Fig. 4b. A blank experiment was investigated as a background check under the same conditions. It shows that only a very small quantity of dye was degraded for the blank experiment within 120min. In contrast, the MB shows a fast degradation in the presence of Bi7VO13, which is decreased to 90% in 120min. The kinetic constant was determined from the pseudo-first-order reaction rate equation of ln(C0/Ct)=kt, where C0 is initial MB concentration, Ct is concentration of MB at time t, k is kinetic constant. Inset Fig. 4b shows the plot of ln(C0/Ct) vs. The linear fit indicates that the kinetics of the degradation reaction is controlled by a pseudo-first-order reaction with a constant 0.0263min−1. The pseudo-first order constants of MB degradation by bismuth photocatalysts have been reported such as Bi4V1.6Ga0.4O11−δ (6.9×10−3min−1) [9], porous BiVO4 film (0.0138min−1) [13], nano-BiVO4 (0.629×10-3 min-1) and 1.4%-Cu2O/BiVO4 (18.2×10-3min-1) [14]. This shows that Bi7VO13 nanoparticles are more effective for the MB photodegradation.
6
The structure characteristic of Bi7VO13 is responsible for its photocatalysis. Firstly, there are dominated (Bi2O2)2+ layers in Bi7VO13 connected via VO4 layers [11]. (Bi2O2)2+ units play important roles in photocatalysis in Bi3+-containing materials, where the photogenerated electrons and holes are carried. Secondly the polar VO4 with the long V–V distance reduces the recombination probability of excitons. In fact, both [VO4]3− and Bi3+ are well-known luminescence centers. However, no luminescence is detected indicating an extremely low recombination rate of charge carriers. Thirdly, Bi7VO13 is known for its high ion conductivity. This is beneficial for the photocatalysis by providing an easy move for the carriers.
5 Conclusions A visible-light-driven photocatalyst was developed in Bi7VO13 nanoparticles by the Pechini method. The nanoparticles have an average diameter less than 50 nm. The crystal structure of Bi7VO13 nanoparticles has a pure crystal formation. Bi7VO13 shows an indirect allowed electronic transition with a narrow band gap of 2.13eV. The band gap was assigned to the transitions from a VB formed by a hybrid orbital of Bi6s and O2p to a CB of empty V3d. The MB dyes can be efficiently degraded under visible light irradiation in the presence of Bi7VO13. The photocatalytic ability could be related to its special crystal structure such as the dominated (Bi2O2)2+ layers, the polar VO4 groups and the good conductivity. The obtained nanoparticles could be expected to have a potential application in environment protection technology.
7
Acknowledgements This work was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF-2013RA1A2009154) and by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), China.
References [1] Kudo A, Omori K, Kato H. J Am Chem Soc 1999;121:11459-11467 [2] Huang J, Tan G, Zhang L, Ren H, Xia A, Zhao C. Mater Lett 2014;133:20-23 [3] Cheng L, Liu X, Kang Y. Mater Lett 2014;134:218-221 [4] Hao Y, Li F, Chen F, Chai M, Liu R, Wang X. Mater Lett 2014;124:1-3 [5] Jiang HY, Liu J, Cheng K, Sun W, Lin J. J Phys Chem C 2013;117:20029-20036 [6] Hu J, Li H, Huang C, Liu M, Qiu X. Appl Catal B 2013;142-143:598-603 [7] Brezesinski K, Ostermann R, Hartmann P, Perlich J, Brezesinski T. Chem Mater 2010;:3079-3085 [8] Dai K, Li D, lv J, Lu L, Liang C, Zhu G. Mater Lett 2014;136:438-440 [9] Thakral V, Uma S. Mater Res Bull 2010;45:1250-1254. [10] Li HH, Li KW, Wang H. Mater Chem Phys 2009;116:134-142, [11] Jie YC, Eysel W. Powder Diffraction 1995;10:76-80. [12] Nakajima T, Isobe M, Tsuchiya T, Ueda Y, Kumagai T. J Lumin 2009;129:1598-1601 [13] Huo T, Zhang X, Dong X, Zhang X, Ma C, et al. J Mater Chem A 2014;2:17366-17370 [14] Li H, Hong W, Cui Y, Hu X, Fan S, Zhu L. Mater Sci Eng B 2014;181:1-8
8
Figure Captions Fig. 1 X-ray diffraction pattern of Bi7VO13 nanoparticles indexed to PDF2# No: 44-0322. Fig. 2 SEM (a), TEM (b) photos, SAED pattern (c), and the EDX (d) of Bi7VO13 nanoparticles Fig. 3 UV–vis absorption spectrum of Bi7VO13 nanoparticles; Inset the left is the band-gap structure; Inset the right is the estimation for the band-gap energy (Eg) and the picture. Fig. 4 (a): UV/vis adsorption spectra of MB-Bi7VO13 solution on irradiation, (b): the MB photocatalytic degradation; Inset is the degradation kinetics relation plotting ln(C0/C) vs time.
Highlights ►A novel visible-light-driven photocatalyst Bi7VO13 nanoparticle was firstly developed by the Pechini method. ►Bi7VO13 nanoparticle shows high absorption in UV-vis wavelength region with a narrow band gap of 2.13eV. ►Bi7VO13 nanoparticle shows high activity in the MB degradation with a pseudo-first-order reaction with a constant 0.0263min−1. ►The dominated (Bi2O2)2+ layers, the polar VO4 groups and the good conductivity are the structural superiority for photocatalytic capacity.
9