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Synthesis of Bi3NbO7 nanoparticles with a hollow structure and their photocatalytic activity under visible light
Solid State Sciences 13 (2011) 1649e1653
Contents lists available at ScienceDirect
Solid State Sciences journal homepage: www.elsevier.com/locate/ssscie
Synthesis of Bi3NbO7 nanoparticles with a hollow structure and their photocatalytic activity under visible light Jingrui Fang, Junfeng Ma*, Yong Sun, Zhensen Liu, Chang Gao State Key Lab. of Green Building Materials, China Building Materials Academy, Beijing 100024, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 May 2011 Accepted 15 June 2011 Available online 21 June 2011
Bi3NbO7 nanoparticles with a hollow structure can be easily prepared by a facile molten-salt technique, where an active precursor is firstly synthesized, and then calcined in KNO3. The crystallization and development of Bi3NbO7 nanoparticles can be effectively controlled by adjusting such processing parameters as calcining temperature, holding time, and incorporated salt quantity. Without adding KNO3, only large-sized and irregular particles can be obtained, having lower crystallinity. Introduction of KNO3 will densify their microstructure and reduce their particle size, enhancing their crystallinity. At a low temperature (400 C), Bi3NbO7 nanoparticles with a hollow structure can be obtained, exhibiting much better photocatalytic activity under visible light than its normal ones, but the hollow structure is sensitive to calcining temperature; at 500 C, it completely disappears. Ó 2011 Elsevier Masson SAS. All rights reserved.
Keywords: Bi3NbO7 nanoparticles Hollow structure Molten-salt synthesis Photocatalytic degradation Visible light irradiation
1. Introduction Photocatalysis technology has attracted lots of interests as one of the most efficient ways to solve both energy and environmental problems since 1970s [1,2]. TiO2 is a good photocatalyst, but it can only be excited by UV light (less than 5% of solar light energy) for its large band gap, thus its wide applications have been limited [3]. Many efforts have been made for fully utilizing solar energy in photocatalysis field, principally including two aspects: One is on the improvement of TiO2 photocatalytic property by such modifying techniques as doping [4], sensitization [5], or constructing coupled semiconductor [6], but they often bring about some new problemsdinstable products and high recombination rate; another one focuses on the design and development of new type photocatalysts with appropriate band gap [7,8], which can work well under the visible light. Now, Bi3NbO7, Y2FeSbO7, Bi12TiO20, BiVO4, and AgNbO3 have been proved to have a good response to visible light and exhibit excellent photocatalytic activity [9e13]. Bi3NbO7 with an oxygen deficient fluorite structure [14] is generally prepared by wet-chemical methods [9,15,16] because traditional solid-state reaction cannot ensure the formation of its pure phase. Even so, the lowest synthesizing temperature reported was also higher than 500 C [9], and their manipulation process complicated.
* Corresponding author. Tel.: þ86 10 51167477; fax: þ86 10 65761714. E-mail address: [email protected] (J. Ma). 1293-2558/$ e see front matter Ó 2011 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2011.06.017
In addition, there have so far been few reports on the photocatalytic performance of Bi3NbO7. Therefore, it is urgent to explore new methods for preparing Bi3NbO7 photocatalyst and characterize its photocatalytic ability, especially under the visible light. Here, we report on a simple and convenient molten-salt method for preparing Bi3NbO7 powders, especially for Bi3NbO7 nanoparticles with a hollow structure, where an active precursor is firstly produced, and then calcined using KNO3 as a molten-salt medium. The influence of some processing parameters (calcining temperature, holding time, and weight ratio of salt/precursor) on Bi3NbO7 crystallization and its photocatalytic performance under visible light were also investigated. 2. Experimental 2.1. Synthesis of Bi3NbO7 nanoparticles Nb2O5, Sr(NO3)2, Bi(NO3)3$5H2O, KOH, and HF solution (17%) with analytical grade were used as received without further purification. At first, Nb2O5 was weighed and dissolved in HF solution, and then, an appropriate amount of Bi(NO3)3$5H2O was dissolved in deionized water. Two solutions were mixed together with 3:1 mole ratio of Bi/Nb, and added dropwise into KOH solution with strongly magnetic stirring at room temperature, a white precipitate was obtained. After separated by centrifugation and washed with deionized water, the precipitate was dried at 60 C in an oven for 6 h, the resultant can be used as a precursor for Bi3NbO7.
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The precursor was well mixed with KNO3 according to 8:1 weight ratio of salt to precursor, and then put into an Al2O3 crucible and calcined at 350 C for 8 h. The product was thoroughly washed by hot deionized water and dried at 60 C for 4 h. The obtained yellow powder was labeled as M3-8-8. Similarly, other samples were produced, as shown in Table 1. 2.2. Characterization XRD analysis was carried out on a D8 advanced diffractometer A) as X-ray (Bruker, Germany), using Cu Ka radiation (l ¼ 1.5406 source with a 2q range from 10 to 80 . TEM observation was conducted by a JEM-2010 electron microscope (JEOL, Japan). Diffuse reflection spectra were obtained by a TU-1901 UVeviseNIR spectrometer (Beijing Purkinje General Instrument Co. Ltd., China), and converted from reflection to absorbance mode by KubelkaeMunk method. 2.3. Evaluation of photocatalytic ability The photocatalytic ability of Bi3NbO7 nanoparticles was evaluated by the degradation of RhB solution (20 mg/l) under visible light at room temperature. 0.20 g sample was added into 200 ml RhB solution, and continuously stirred in the dark for 50 min. Then, the suspension was poured into a photocatalytic reactor with a magnetic stirrer, where irradiation source was a 500 W xenon lamp with a 420 nm cutoff filter. During the irradiation, 5 ml suspension was sequentially taken from the reactor every 15 min, and centrifuged to remove the photocatalyst. The filtrate was analyzed by a spectrophotometer (722-E, Shanghai spectrum Instruments Co., Ltd. China) at its maximum absorption wavelength (554 nm). 3. Result and discussion 3.1. XRD analysis Fig. 1 shows the variation of products’ phase compositions with calcining temperature under KNO3 molten-salt condition. Obviously, the as-prepared precursor is a mixture composed of Bi2O3 (JCPDS: 27-0052), a trace unknown crystalline phase, and amorphous phase (Fig. 1(a)). When KNO3 was added in the preparing process according to 8:1 weight ratio of salt/precursor, even at 350 C, Bi3NbO7 phase (JCPDS: 50-0087) could be easily formed, accompanied by a trace amount of impurity (Fig. 1(b)). Pure-phase Bi3NbO7 can be obtained at a higher calcining temperature (Fig. 1(c) or (d)). Thus, appropriately elevating the calcining temperature will favor the formation and crystallization of Bi3NbO7 phase, herein, 400 C is high enough to obtain well-crystallized Bi3NbO7 phase. Fig. 2 shows the effect of holding time on the crystallization of Bi3NbO7 phase, all samples were produced at 400 C with the same
Fig. 1. XRD patterns of precursor, and samples prepared at different calcining temperatures: (a) as-prepared precursor; (b) 350 C, M3-8-8; (c) 400 C, M4-8-8; and (d) 500 C, M5-8-8 (; Bi3NbO7; A Bi2O3; C trace unknown phase).
weight ratio of salt/precursor (8:1). It can be seen that pure-phase Bi3NbO7 can be readily obtained within 4 h (Fig. 2(a)), and increasing the holding time will enhance their XRD intensity, benefiting the crystallization of Bi3NbO7 phase (Fig. 2(b)), but its extensive increase doesn’t give rise to a significant improvement in crystallinity (Fig. 2(c)). That is also the reason why we chose the moderate holding time of 8 h. The influence of incorporated KNO3 amount upon products can be found in Fig. 3. Obviously, the presence of KNO3 promotes the crystallization and development of Bi3NbO7 phase (Fig. 3(b) and (c)) since the molten salt would greatly speed up diffusing rates of reaction species [17] though our precursor can also be directly crystallized at 400 C into Bi3NbO7 due to its high reactivity (Fig. 3(a)). However, XRD analysis can’t further reveal the difference of above Bi3NbO7 samples in morphology and microstructure, which would often affect their
Table 1 Samples’ name and their preparing conditions. Sample
Calcining temperature ( C)
Holding time (h)
Salt/precursor (weight ratio)
M4-8-0 M4-8-4 M4-8-8 M4-4-8 M4-16-8 M5-8-8 M3-8-8
400 400 400 400 400 500 350
8 8 8 4 16 8 8
0:1 4:1 8:1 8:1 8:1 8:1 8:1
Fig. 2. XRD patterns of Bi3NbO7 samples obtained at 400 C for different holding times: (a) 4 h, M4-4-8; (b) 8 h, M4-8-8; and (c) 16 h, M4-16-8 (salt/precursor: 8:1, ; Bi3NbO7).
J. Fang et al. / Solid State Sciences 13 (2011) 1649e1653
Fig. 3. XRD patterns of Bi3NbO7 samples prepared by calcining different weight ratios of salt/precursor mixtures at 400 C: (a) 0:1, M4-8-0; (b) 4:1, M4-8-4; and (c) 8:1, M48-8 (holding time: 8 h, ; Bi3NbO7).
photocatalytic performance to a certain extent. Therefore, more detailed discussion requires TEM observation of those samples. 3.2. TEM observation Fig. 4(a)e(c) shows the dependence of Bi3NbO7 crystallization on the incorporated KNO3 quantity. Obviously, homogenous and fine nanoparticles could be obtained in the case of adding a large amount of KNO3, while large-sized and irregular shape particles with a loose microstructure were formed in the absence of the salt. Here, the presence of molten KNO3 would densify their
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microstructure and reduce their particle size, resulting in a good crystallinity. Moreover, at such a low temperature as 400 C, obtained products have a special hollow structure (Fig. 4(c)). However, at 500 C, when other experimental conditions remain unchanged, the hollow structure completely disappears (Fig. 4(f)), which is sensitive to calcining temperature. Fig. 4(c)e(e) displays the variation of Bi3NbO7 particle size and morphology with the holding time. All the samples have nearly round particle morphology with the special hollow structure, and their particle sizes and inner voids slightly increase with the holding time. Several mechanisms can be used for explaining the formation of hollow structure, including Kirkendall effect [18,19], Ostwald ripening mechanism [20,21], and oriented attachment mechanism [22,23]. In our case, there exist two different formation mechanisms, corresponding to a low (400 C) and high calcining temperatures (500 C), respectively. In accordance with XRD analysis in Fig. 1(a), HRTEM image of Fig. 5(b) also confirmed the coexistence of Bi2O3 with the amorphous phase. At 400 C, the amorphous phase with a high reactivity more easily dissolves in molten KNO3 medium than Bi2O3, which, therefore, would be surrounded by the molten KNO3 rich in the amorphous phase. As shown in Fig. 6(a), Bi3NbO7 layer firstly forms on the surface of Bi2O3 particles and continues to grow up by the diffusing process of reaction species, while the outward mass transfer of Bi2O3 will leave voids in their cores, similar to Kirkendall effect reported [18]. Without adding KNO3 or incorporating a small amount, the amorphous phase will react with Bi2O3 to form irregular shape aggregates by solid-state reaction. On the other hand, elevating calcining temperature will enhance the solubility of Bi2O3. At 500 C (Fig. 6(b)), both the amorphous phase and crystalline Bi2O3 can dissolve completely in molten KNO3 medium. And once reaching its saturation, Bi3NbO7 phase will precipitate from the liquid medium, which should be ascribed to a dissolutioneprecipitation mechanism [24], homogenous and dense Bi3NbO7 nanoparticles can be obtained. The more detailed study on their formation mechanism is underway.
Fig. 4. TEM images of Bi3NbO7 powders obtained under different molten-salt conditions: (a) M4-8-0, without adding KNO3; (b) M4-8-4; (c) M4-8-8; (d) M4-4-8; (e) M4-16-8; and (f) M5-8-8.
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Fig. 5. TEM (a) and HRTEM (b) images of the as-prepared precursor.
Fig. 6. Schematic illustration of the forming mechanism of Bi3NbO7 crystallites.
3.3. UVevis absorption spectra and photocatalytic ability of Bi3NbO7 samples UVevis absorption spectra of typical Bi3NbO7 samples are given in Fig. 7, where no significant difference can be found from their
Fig. 7. UVevis absorption spectra of Bi3NbO7 samples: (a) M4-8-0; (b) M4-8-8; and (c) M5-8-8.
profiles. Their band gap is about 2.74e2.75 eV, exhibiting a good response to visible light. Fig. 8 shows the degradation rate for RhB solution under visible light by our Bi3NbO7 samples, and blank experimental data with
Fig. 8. Photodegradation rate of RhB solution by Bi3NbO7 samples under visible light: (>) M4-8-8; (,) M4-8-0; (B) M5-8-8; and (7) blank data without adding any photocatalyst.
J. Fang et al. / Solid State Sciences 13 (2011) 1649e1653
out adding any photocatalyst are also plotted for comparison. Obviously, the photocatalytic activity of M4-8-8 sample with better crystallinity and smaller particle sizes is superior to that of M4-8-0, especially the former has a unique hollow structure, which would be beneficial to its photocatalytic activity due to the increased specific surface area and active surface sites [25,26]. After the irradiation of 60 min, more than 95% RhB can be degraded by using M4-8-8, whereas only 65% and 7% degradation rates can be achieved by using M4-8-0 and M5-8-8. The abrupt decline of M5-8-8 sample in photodegradation ability can be attributed to the growth of Bi3NbO7 grains and complete disappearance of their hollow structure. Therefore, the photocatalytic performance of Bi3NbO7 nanoparticles is affected not only by their particle size and crystallinity but also by their morphology and special microstructure. 4. Conclusion Bi3NbO7 nanoparticles were successfully synthesized by a molten-salt process for the first time, exhibiting excellent photocatalytic performance under visible light. Their crystallization and development can be easily controlled by adjusting such processing parameter as calcining temperature, holding time, and incorporated salt (KNO3) quantity, where a unique hollow structure can be obtained at appropriate conditions. Here, the possible mechanism was proposed to explain the formation of Bi3NbO7 nanoparticles, especially with a hollow structure under the moltensalt condition. The as-prepared Bi3NbO7 nanoparticles with the hollow structure and good crystallinity show much better photocatalytic ability than its normal ones.
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References [1] A. Fujishima, K. Honda, Nature 238 (1972) 37e38. [2] I.S. Cho, D.W. Kim, C.M. Cho, J.S. An, H.S. Roh, K.S. Hong, Solid State Sci. 12 (2010) 982e988. [3] R. Wang, K. Hashimoto, A. Fujishima, M. Chikun, E. Kojima, A. Kitamura, M. Shimohigoshi, T. Watanabe, Nature 388 (1997) 431e432. [4] A. Alem, H. Sarpoolaky, Solid State Sci. 12 (2010) 1469e1472. [5] J. Yu, J. Zhang, M. Jaroniec, Green Chem. 12 (2010) 1611e1614. [6] H. Tian, J. Ma, L. Xie, Z. Zhao, J. Zhou, Y. Wang, J. Tao, X. Zhu, Ceram. Int. 33 (2007) 915e918. [7] Z. Zou, J. Ye, K. Sayama, H. Arakawa, Nature 414 (2001) 625e628. [8] M. Hara, G. Hitoki, T. Takata, J.N. Kondo, H. Kobayashi, K. Domen, Catal. Today 78 (2003) 555e560. [9] G. Zhang, J. Yang, S. Zhang, Q. Xiong, B. Huang, J. Wang, W. Gong, J. Hazard. Mater. 172 (2009) 986e992. [10] J. Luan, L. Zhang, K. Ma, Y. Li, Z. Zou, Solid State Sci. 13 (2011) 185e194. [11] J. Zhou, Z. Zou, A.K. Ray, X.S. Zhao, Ind. Eng. Chem. Res. 46 (2007) 745e749. [12] L. Zhou, W. Wang, S. Liu, L. Zhang, H. Xu, W. Zhu, J. Mol. Catal. A: Chem. 252 (2006) 120e124. [13] A. Kudo, H. Kato, I. Tsuji, Chem. Lett. 33 (2004) 1534e1539. [14] A. Castro, E. Aguado, J.M. Rojo, P. Herrero, R. Enjalbert, J. Galy, Mater. Res. Bull. 33 (1998) 31e41. [15] A. Castro, D. Palem, J. Mater. Chem. 12 (2002) 2774e2780. [16] D. Zhou, H. Wang, X. Yao, J. Am. Ceram. Soc. 90 (2007) 327e329. [17] Y. Wang, J. Ma, J. Tao, X. Zhu, J. Zhou, Z. Zhao, L. Xie, H. Tian, Mater. Lett. 60 (2006) 291e293. [18] H.J. Fan, U. Gösele, M. Zacharias, Small 3 (2007) 1660e1671. [19] R.K. Chiang, R.T. Chiang, Inorg. Chem. 46 (2007) 369e371. [20] X. Tian, J. Li, K. Chen, J. Han, S. Pan, Cryst. Growth Des. 9 (2009) 4927e4932. [21] X.W. Lou, L.A. Archer, Z. Yang, Adv. Mater. 20 (2008) 3987e4019. [22] B. Liu, H.C. Zeng, J. Am. Chem. Soc. 126 (2004) 8124e8125. [23] K. An, T. Hyeon, Nanotoday 4 (2009) 359e373. [24] L. Li, J. Deng, J. Chen, X. Sun, R. Yu, G. Liu, X. Xing, Chem. Mater. 21 (2009) 1207e1213. [25] X. Song, L. Gao, J. Phys. Chem. C 112 (2008) 15299e15305. [26] S. Liu, J. Yu, M. Jaroniec, J. Am. Chem. Soc. 132 (2010) 11914e11916.
Contents lists available at ScienceDirect
Solid State Sciences journal homepage: www.elsevier.com/locate/ssscie
Synthesis of Bi3NbO7 nanoparticles with a hollow structure and their photocatalytic activity under visible light Jingrui Fang, Junfeng Ma*, Yong Sun, Zhensen Liu, Chang Gao State Key Lab. of Green Building Materials, China Building Materials Academy, Beijing 100024, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 May 2011 Accepted 15 June 2011 Available online 21 June 2011
Bi3NbO7 nanoparticles with a hollow structure can be easily prepared by a facile molten-salt technique, where an active precursor is firstly synthesized, and then calcined in KNO3. The crystallization and development of Bi3NbO7 nanoparticles can be effectively controlled by adjusting such processing parameters as calcining temperature, holding time, and incorporated salt quantity. Without adding KNO3, only large-sized and irregular particles can be obtained, having lower crystallinity. Introduction of KNO3 will densify their microstructure and reduce their particle size, enhancing their crystallinity. At a low temperature (400 C), Bi3NbO7 nanoparticles with a hollow structure can be obtained, exhibiting much better photocatalytic activity under visible light than its normal ones, but the hollow structure is sensitive to calcining temperature; at 500 C, it completely disappears. Ó 2011 Elsevier Masson SAS. All rights reserved.
Keywords: Bi3NbO7 nanoparticles Hollow structure Molten-salt synthesis Photocatalytic degradation Visible light irradiation
1. Introduction Photocatalysis technology has attracted lots of interests as one of the most efficient ways to solve both energy and environmental problems since 1970s [1,2]. TiO2 is a good photocatalyst, but it can only be excited by UV light (less than 5% of solar light energy) for its large band gap, thus its wide applications have been limited [3]. Many efforts have been made for fully utilizing solar energy in photocatalysis field, principally including two aspects: One is on the improvement of TiO2 photocatalytic property by such modifying techniques as doping [4], sensitization [5], or constructing coupled semiconductor [6], but they often bring about some new problemsdinstable products and high recombination rate; another one focuses on the design and development of new type photocatalysts with appropriate band gap [7,8], which can work well under the visible light. Now, Bi3NbO7, Y2FeSbO7, Bi12TiO20, BiVO4, and AgNbO3 have been proved to have a good response to visible light and exhibit excellent photocatalytic activity [9e13]. Bi3NbO7 with an oxygen deficient fluorite structure [14] is generally prepared by wet-chemical methods [9,15,16] because traditional solid-state reaction cannot ensure the formation of its pure phase. Even so, the lowest synthesizing temperature reported was also higher than 500 C [9], and their manipulation process complicated.
* Corresponding author. Tel.: þ86 10 51167477; fax: þ86 10 65761714. E-mail address: [email protected] (J. Ma). 1293-2558/$ e see front matter Ó 2011 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2011.06.017
In addition, there have so far been few reports on the photocatalytic performance of Bi3NbO7. Therefore, it is urgent to explore new methods for preparing Bi3NbO7 photocatalyst and characterize its photocatalytic ability, especially under the visible light. Here, we report on a simple and convenient molten-salt method for preparing Bi3NbO7 powders, especially for Bi3NbO7 nanoparticles with a hollow structure, where an active precursor is firstly produced, and then calcined using KNO3 as a molten-salt medium. The influence of some processing parameters (calcining temperature, holding time, and weight ratio of salt/precursor) on Bi3NbO7 crystallization and its photocatalytic performance under visible light were also investigated. 2. Experimental 2.1. Synthesis of Bi3NbO7 nanoparticles Nb2O5, Sr(NO3)2, Bi(NO3)3$5H2O, KOH, and HF solution (17%) with analytical grade were used as received without further purification. At first, Nb2O5 was weighed and dissolved in HF solution, and then, an appropriate amount of Bi(NO3)3$5H2O was dissolved in deionized water. Two solutions were mixed together with 3:1 mole ratio of Bi/Nb, and added dropwise into KOH solution with strongly magnetic stirring at room temperature, a white precipitate was obtained. After separated by centrifugation and washed with deionized water, the precipitate was dried at 60 C in an oven for 6 h, the resultant can be used as a precursor for Bi3NbO7.
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The precursor was well mixed with KNO3 according to 8:1 weight ratio of salt to precursor, and then put into an Al2O3 crucible and calcined at 350 C for 8 h. The product was thoroughly washed by hot deionized water and dried at 60 C for 4 h. The obtained yellow powder was labeled as M3-8-8. Similarly, other samples were produced, as shown in Table 1. 2.2. Characterization XRD analysis was carried out on a D8 advanced diffractometer A) as X-ray (Bruker, Germany), using Cu Ka radiation (l ¼ 1.5406 source with a 2q range from 10 to 80 . TEM observation was conducted by a JEM-2010 electron microscope (JEOL, Japan). Diffuse reflection spectra were obtained by a TU-1901 UVeviseNIR spectrometer (Beijing Purkinje General Instrument Co. Ltd., China), and converted from reflection to absorbance mode by KubelkaeMunk method. 2.3. Evaluation of photocatalytic ability The photocatalytic ability of Bi3NbO7 nanoparticles was evaluated by the degradation of RhB solution (20 mg/l) under visible light at room temperature. 0.20 g sample was added into 200 ml RhB solution, and continuously stirred in the dark for 50 min. Then, the suspension was poured into a photocatalytic reactor with a magnetic stirrer, where irradiation source was a 500 W xenon lamp with a 420 nm cutoff filter. During the irradiation, 5 ml suspension was sequentially taken from the reactor every 15 min, and centrifuged to remove the photocatalyst. The filtrate was analyzed by a spectrophotometer (722-E, Shanghai spectrum Instruments Co., Ltd. China) at its maximum absorption wavelength (554 nm). 3. Result and discussion 3.1. XRD analysis Fig. 1 shows the variation of products’ phase compositions with calcining temperature under KNO3 molten-salt condition. Obviously, the as-prepared precursor is a mixture composed of Bi2O3 (JCPDS: 27-0052), a trace unknown crystalline phase, and amorphous phase (Fig. 1(a)). When KNO3 was added in the preparing process according to 8:1 weight ratio of salt/precursor, even at 350 C, Bi3NbO7 phase (JCPDS: 50-0087) could be easily formed, accompanied by a trace amount of impurity (Fig. 1(b)). Pure-phase Bi3NbO7 can be obtained at a higher calcining temperature (Fig. 1(c) or (d)). Thus, appropriately elevating the calcining temperature will favor the formation and crystallization of Bi3NbO7 phase, herein, 400 C is high enough to obtain well-crystallized Bi3NbO7 phase. Fig. 2 shows the effect of holding time on the crystallization of Bi3NbO7 phase, all samples were produced at 400 C with the same
Fig. 1. XRD patterns of precursor, and samples prepared at different calcining temperatures: (a) as-prepared precursor; (b) 350 C, M3-8-8; (c) 400 C, M4-8-8; and (d) 500 C, M5-8-8 (; Bi3NbO7; A Bi2O3; C trace unknown phase).
weight ratio of salt/precursor (8:1). It can be seen that pure-phase Bi3NbO7 can be readily obtained within 4 h (Fig. 2(a)), and increasing the holding time will enhance their XRD intensity, benefiting the crystallization of Bi3NbO7 phase (Fig. 2(b)), but its extensive increase doesn’t give rise to a significant improvement in crystallinity (Fig. 2(c)). That is also the reason why we chose the moderate holding time of 8 h. The influence of incorporated KNO3 amount upon products can be found in Fig. 3. Obviously, the presence of KNO3 promotes the crystallization and development of Bi3NbO7 phase (Fig. 3(b) and (c)) since the molten salt would greatly speed up diffusing rates of reaction species [17] though our precursor can also be directly crystallized at 400 C into Bi3NbO7 due to its high reactivity (Fig. 3(a)). However, XRD analysis can’t further reveal the difference of above Bi3NbO7 samples in morphology and microstructure, which would often affect their
Table 1 Samples’ name and their preparing conditions. Sample
Calcining temperature ( C)
Holding time (h)
Salt/precursor (weight ratio)
M4-8-0 M4-8-4 M4-8-8 M4-4-8 M4-16-8 M5-8-8 M3-8-8
400 400 400 400 400 500 350
8 8 8 4 16 8 8
0:1 4:1 8:1 8:1 8:1 8:1 8:1
Fig. 2. XRD patterns of Bi3NbO7 samples obtained at 400 C for different holding times: (a) 4 h, M4-4-8; (b) 8 h, M4-8-8; and (c) 16 h, M4-16-8 (salt/precursor: 8:1, ; Bi3NbO7).
J. Fang et al. / Solid State Sciences 13 (2011) 1649e1653
Fig. 3. XRD patterns of Bi3NbO7 samples prepared by calcining different weight ratios of salt/precursor mixtures at 400 C: (a) 0:1, M4-8-0; (b) 4:1, M4-8-4; and (c) 8:1, M48-8 (holding time: 8 h, ; Bi3NbO7).
photocatalytic performance to a certain extent. Therefore, more detailed discussion requires TEM observation of those samples. 3.2. TEM observation Fig. 4(a)e(c) shows the dependence of Bi3NbO7 crystallization on the incorporated KNO3 quantity. Obviously, homogenous and fine nanoparticles could be obtained in the case of adding a large amount of KNO3, while large-sized and irregular shape particles with a loose microstructure were formed in the absence of the salt. Here, the presence of molten KNO3 would densify their
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microstructure and reduce their particle size, resulting in a good crystallinity. Moreover, at such a low temperature as 400 C, obtained products have a special hollow structure (Fig. 4(c)). However, at 500 C, when other experimental conditions remain unchanged, the hollow structure completely disappears (Fig. 4(f)), which is sensitive to calcining temperature. Fig. 4(c)e(e) displays the variation of Bi3NbO7 particle size and morphology with the holding time. All the samples have nearly round particle morphology with the special hollow structure, and their particle sizes and inner voids slightly increase with the holding time. Several mechanisms can be used for explaining the formation of hollow structure, including Kirkendall effect [18,19], Ostwald ripening mechanism [20,21], and oriented attachment mechanism [22,23]. In our case, there exist two different formation mechanisms, corresponding to a low (400 C) and high calcining temperatures (500 C), respectively. In accordance with XRD analysis in Fig. 1(a), HRTEM image of Fig. 5(b) also confirmed the coexistence of Bi2O3 with the amorphous phase. At 400 C, the amorphous phase with a high reactivity more easily dissolves in molten KNO3 medium than Bi2O3, which, therefore, would be surrounded by the molten KNO3 rich in the amorphous phase. As shown in Fig. 6(a), Bi3NbO7 layer firstly forms on the surface of Bi2O3 particles and continues to grow up by the diffusing process of reaction species, while the outward mass transfer of Bi2O3 will leave voids in their cores, similar to Kirkendall effect reported [18]. Without adding KNO3 or incorporating a small amount, the amorphous phase will react with Bi2O3 to form irregular shape aggregates by solid-state reaction. On the other hand, elevating calcining temperature will enhance the solubility of Bi2O3. At 500 C (Fig. 6(b)), both the amorphous phase and crystalline Bi2O3 can dissolve completely in molten KNO3 medium. And once reaching its saturation, Bi3NbO7 phase will precipitate from the liquid medium, which should be ascribed to a dissolutioneprecipitation mechanism [24], homogenous and dense Bi3NbO7 nanoparticles can be obtained. The more detailed study on their formation mechanism is underway.
Fig. 4. TEM images of Bi3NbO7 powders obtained under different molten-salt conditions: (a) M4-8-0, without adding KNO3; (b) M4-8-4; (c) M4-8-8; (d) M4-4-8; (e) M4-16-8; and (f) M5-8-8.
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Fig. 5. TEM (a) and HRTEM (b) images of the as-prepared precursor.
Fig. 6. Schematic illustration of the forming mechanism of Bi3NbO7 crystallites.
3.3. UVevis absorption spectra and photocatalytic ability of Bi3NbO7 samples UVevis absorption spectra of typical Bi3NbO7 samples are given in Fig. 7, where no significant difference can be found from their
Fig. 7. UVevis absorption spectra of Bi3NbO7 samples: (a) M4-8-0; (b) M4-8-8; and (c) M5-8-8.
profiles. Their band gap is about 2.74e2.75 eV, exhibiting a good response to visible light. Fig. 8 shows the degradation rate for RhB solution under visible light by our Bi3NbO7 samples, and blank experimental data with
Fig. 8. Photodegradation rate of RhB solution by Bi3NbO7 samples under visible light: (>) M4-8-8; (,) M4-8-0; (B) M5-8-8; and (7) blank data without adding any photocatalyst.
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out adding any photocatalyst are also plotted for comparison. Obviously, the photocatalytic activity of M4-8-8 sample with better crystallinity and smaller particle sizes is superior to that of M4-8-0, especially the former has a unique hollow structure, which would be beneficial to its photocatalytic activity due to the increased specific surface area and active surface sites [25,26]. After the irradiation of 60 min, more than 95% RhB can be degraded by using M4-8-8, whereas only 65% and 7% degradation rates can be achieved by using M4-8-0 and M5-8-8. The abrupt decline of M5-8-8 sample in photodegradation ability can be attributed to the growth of Bi3NbO7 grains and complete disappearance of their hollow structure. Therefore, the photocatalytic performance of Bi3NbO7 nanoparticles is affected not only by their particle size and crystallinity but also by their morphology and special microstructure. 4. Conclusion Bi3NbO7 nanoparticles were successfully synthesized by a molten-salt process for the first time, exhibiting excellent photocatalytic performance under visible light. Their crystallization and development can be easily controlled by adjusting such processing parameter as calcining temperature, holding time, and incorporated salt (KNO3) quantity, where a unique hollow structure can be obtained at appropriate conditions. Here, the possible mechanism was proposed to explain the formation of Bi3NbO7 nanoparticles, especially with a hollow structure under the moltensalt condition. The as-prepared Bi3NbO7 nanoparticles with the hollow structure and good crystallinity show much better photocatalytic ability than its normal ones.
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References [1] A. Fujishima, K. Honda, Nature 238 (1972) 37e38. [2] I.S. Cho, D.W. Kim, C.M. Cho, J.S. An, H.S. Roh, K.S. Hong, Solid State Sci. 12 (2010) 982e988. [3] R. Wang, K. Hashimoto, A. Fujishima, M. Chikun, E. Kojima, A. Kitamura, M. Shimohigoshi, T. Watanabe, Nature 388 (1997) 431e432. [4] A. Alem, H. Sarpoolaky, Solid State Sci. 12 (2010) 1469e1472. [5] J. Yu, J. Zhang, M. Jaroniec, Green Chem. 12 (2010) 1611e1614. [6] H. Tian, J. Ma, L. Xie, Z. Zhao, J. Zhou, Y. Wang, J. Tao, X. Zhu, Ceram. Int. 33 (2007) 915e918. [7] Z. Zou, J. Ye, K. Sayama, H. Arakawa, Nature 414 (2001) 625e628. [8] M. Hara, G. Hitoki, T. Takata, J.N. Kondo, H. Kobayashi, K. Domen, Catal. Today 78 (2003) 555e560. [9] G. Zhang, J. Yang, S. Zhang, Q. Xiong, B. Huang, J. Wang, W. Gong, J. Hazard. Mater. 172 (2009) 986e992. [10] J. Luan, L. Zhang, K. Ma, Y. Li, Z. Zou, Solid State Sci. 13 (2011) 185e194. [11] J. Zhou, Z. Zou, A.K. Ray, X.S. Zhao, Ind. Eng. Chem. Res. 46 (2007) 745e749. [12] L. Zhou, W. Wang, S. Liu, L. Zhang, H. Xu, W. Zhu, J. Mol. Catal. A: Chem. 252 (2006) 120e124. [13] A. Kudo, H. Kato, I. Tsuji, Chem. Lett. 33 (2004) 1534e1539. [14] A. Castro, E. Aguado, J.M. Rojo, P. Herrero, R. Enjalbert, J. Galy, Mater. Res. Bull. 33 (1998) 31e41. [15] A. Castro, D. Palem, J. Mater. Chem. 12 (2002) 2774e2780. [16] D. Zhou, H. Wang, X. Yao, J. Am. Ceram. Soc. 90 (2007) 327e329. [17] Y. Wang, J. Ma, J. Tao, X. Zhu, J. Zhou, Z. Zhao, L. Xie, H. Tian, Mater. Lett. 60 (2006) 291e293. [18] H.J. Fan, U. Gösele, M. Zacharias, Small 3 (2007) 1660e1671. [19] R.K. Chiang, R.T. Chiang, Inorg. Chem. 46 (2007) 369e371. [20] X. Tian, J. Li, K. Chen, J. Han, S. Pan, Cryst. Growth Des. 9 (2009) 4927e4932. [21] X.W. Lou, L.A. Archer, Z. Yang, Adv. Mater. 20 (2008) 3987e4019. [22] B. Liu, H.C. Zeng, J. Am. Chem. Soc. 126 (2004) 8124e8125. [23] K. An, T. Hyeon, Nanotoday 4 (2009) 359e373. [24] L. Li, J. Deng, J. Chen, X. Sun, R. Yu, G. Liu, X. Xing, Chem. Mater. 21 (2009) 1207e1213. [25] X. Song, L. Gao, J. Phys. Chem. C 112 (2008) 15299e15305. [26] S. Liu, J. Yu, M. Jaroniec, J. Am. Chem. Soc. 132 (2010) 11914e11916.