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Rapid microwave-assisted hydrothermal synthesis of Bi12TiO20 hierarchical architecture with enhanced visible-light photocatalytic activities
Journal of Physics and Chemistry of Solids 74 (2013) 1739–1744
Contents lists available at ScienceDirect
Journal of Physics and Chemistry of Solids journal homepage: www.elsevier.com/locate/jpcs
Rapid microwave-assisted hydrothermal synthesis of Bi12TiO20 hierarchical architecture with enhanced visible-light photocatalytic activities Zhuo Yang, Huiqing Fan n, Xin Wang, Changbai Long State Key Laboratory of Solidification Processing, School of Materials Science and Engineering, Northwestern Polytechnical University, Xi'an 710072, China
art ic l e i nf o
a b s t r a c t
Article history: Received 2 December 2012 Received in revised form 25 June 2013 Accepted 28 June 2013 Available online 6 July 2013
Flower-like Bi12TiO20 hierarchical nanostructures composed of numerous nanobelts were synthesized at 180 1C within 1 h by a microwave-assisted hydrothermal method in the presence of cetyltrimethylammonium bromide (CTAB) for the first time. The as-prepared products were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and ultraviolet–visible (UV–vis) absorption spectroscopy. Furthermore, the hierarchical Bi12TiO20 nanostructures exhibited higher photocatalytic activities in the degradation of Rhodamine B under visible-light irradiation than that of the samples prepared without CTAB. In addition, the role of CTAB cationic surfactant has been investigated thoroughly and a possible mechanism is proposed. & 2013 Elsevier Ltd. All rights reserved.
Keywords: A. Nanostructures A. Oxides B. Chemical synthesis D. Optical properties
1. Introduction Since the nanostructured materials with good control on their morphologies, size, and crystallographic structure may differ significantly from the corresponding bulk counterparts, nanostructured semiconductors have attracted a growing interest for their fascinating properties and potential applications in technology [1,2]. Recently, ternary bismuth oxide semiconductors, such as Bi2WO6, BiVO4, Bi2Ti2O7 and Bi12TiO20 have been widely studied as a class of promising photocatalysts with catalytic activity under visible-light irradiation [3–12]. Among them, Bi12TiO20 crystals with sillenite structures is one of the most frequently investigated materials due to its excellent intrinsic properties and the potential applications including optical information processing, optical phase conjugation, photovoltaic cells and photocatalysts [13–16]. Bi12TiO20 micro/nanostructures with different morphologies and sizes have been obtained via solid-state reactions, chemical method, or by conventional hydrothermal synthesis [17]. Those methods mentioned above, however, have obvious disadvantages like the requirement for high-temperature calcination, which often results in large-sized crystallization and hard aggregation, or requiring complicated equipments and long processing time [18]. Microwave irradiation could be used as a heat source for the hydrothermal process. It leads to a rapid heating to attain the desired temperature in a short time and increases the reaction kinetics, compared to the n
Corresponding author. Tel.: +86 29 88494463; fax: +86 29 88492642. E-mail address: [email protected] (H. Fan).
0022-3697/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jpcs.2013.06.020
conventional hydrothermal method. Microwave-assisted reaction is seldom employed to synthesize Bi12TiO20 crystals for photocatalysis although such a method has been used successfully to fabricate various materials [19]. Further, tailoring the architecture of nanomaterials has been of great research interest due to their unique properties are closely related to the size and shape of nanostructures. In terms of morphology control, cetyltrimethylammonium bromide (CTAB) has been generally used to synthesize different nanostructures, such as nanowires, nanotubes, hollow spheres, and 3D hierarchical structures [20,21]. Phuruangrat and co-workers described the synthesis of MoO3 nanowire with unprecedentedly high aspect ratios in the presence of CTAB [22]. Hu et al. synthesized CuO nanotubes and nanorods using CTAB by a hydrothermal process [23]. Multilayered ZnO nanosheets with hierarchically porous structures are successfully synthesized from a hydrothermal preparation and thermal decomposition by using CTAB [24]. However, relatively few studies on the synthesis of hierarchically Bi12TiO20 nanostructures using CTAB have been reported up to now. Herein, a microwave-assisted hydrothermal route was used to synthesize hierarchical nanostructured Bi12TiO20 powders in the presence of cetyltrimethylammonium bromide (CTAB) for the first time. The key features of this method are that it is time-saving, has low energy consumption, and is inexpensive. Moreover, the photodegradation of Rhodamine B (RhB) was employed to evaluate the photocatalytic activities of Bi12TiO20 hierarchical nanostructures under visible-light irradiation. It is worthy to note that the hierarchical Bi12TiO20 materials exhibit enhanced visible-lightdriven photocatalytic performance.
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2. Experimental section 2.1. Synthesis of hierarchical nanostructured Bi12TiO20 powders All reagents were analytically pure, bought from Shanghai Chemical Corp., and used without further purification. Typically, the 0.576 g of Bi(NO3)3d5H2O was dissolved in 30 ml of deionized water with dilute nitric acid under continuous stirring, followed by the addition of stoichiometric amount of titanium butoxide. After stirred for 1 h, 10 ml solution consisting of 0 g, 0.05 g, 0.15 g and 0.30 g of cetyltrimethylammonium bromide (CTAB) was introduced into the reaction mixture. The pH of the final solution was adjusted to 12 using sodium hydroxide. The as-prepared solution was transferred to a polytetrafluoroethylene (PTFE)-lined autoclave, sealed and maintained for microwave irradiation at 180 1C for 1 h, and cooled to room temperature naturally. Then, the resulting precipitate was recovered by centrifugation, washed with deionized water and ethanol thoroughly, and finally dried in an oven at 80 1C. For a comparison, nanostructured TiO2 were also synthesized by the sol–gel method. The preparation followed the procedure of Huang et al. [25].
Fig. 1. XRD patterns of as-prepared Bi12TiO20 (a) without additive and with CTAB before (b) and (c) after photocatalysis.
2.2. Characterization and photocatalytic measurements The structure and morphology of the products were characterized by X-ray diffraction measurement (XRD; X'pert, Philips, The Netherlands) with Cu-Kα radiation (λ¼1.5406 Å), field emitting scanning electron microscopy (FE-SEM; JSM-6701F, JEOL, Japan) with an accelerating voltage of 20 kV and transmission electron microscopy (TEM; JEM-3010, Questar, New Hope, USA) respectively. The photodegradation experiments were performed in a slurry reactor containing 50 mL of a 10 mg L 1 solution of RhB and 0.05 g of catalyst [26]. A 500 W Xe lamp was used as the light source with a 420 nm cutoff filter to provide visible-light irradiation. Prior to irradiation, the suspension was kept in the dark under magnetically stirring for 40 min to ensure the establishing of an adsorption/ desorption equilibrium between the photocatalyst and RhB. At a given time interval, 4 mL suspension was collected and separated by centrifugation at 8000 rpm for 10 min. The filtrates were analyzed using a UV–vis spectrophotometer (UV-3150, Shimadzu, Japan) at its maximum absorption wavelength of 554 nm.
3. Results and discussion 3.1. Characterization of hierarchical Bi12TiO20 architectures The as-prepared products with additive-free and CTAB additive have been structurally characterized by X-ray diffraction, as shown in Fig. 1. All the diffraction peaks can be indexed as the cubic Bi12TiO20 in accordance with the values in the standard card (JCPDS NO.340097) and the sharp peaks indicate 1 h microwave hydrothermal time is enough for the formation of high crystalline Bi12TiO20. No significant differences can be detected for peak positions or FWHM of the Fig. 1b and c, which reveals the crystal structure of the hierarchical nanostructured Bi12TiO20 were very stable. The representative microstructures of the bismuth titanate materials that were synthesized with 0.15 g CTAB at 180 1C for 1 h are presented in Fig. 2. Fig. 2a shows a panoramic TEM image of as-prepared Bi12TiO20 using CTAB, from which uniformly-sized flower-like architectures with diameters of 1–2 μm can be clearly observed. A magnified FE-SEM image of the sample (inset of Fig. 2a) indicates that the Bi12TiO20 nanostructures look like some natural flowers consisting of multitudinous uniformly distributed nanobelts. These nanobelts are intercrossed with each other and form a 3D flower-like structure. The observation from Fig. 2b (the inset) suggests the nanobelts are randomly oriented crystallites
Fig. 2. TEM (a) and HRTEM (b) images of hierarchical flower-like Bi12TiO20 architectures obtained with 0.15 g CTAB at 180 1C for 1 h by microwave hydrothermal process. Inset: corresponding SEM and TEM images.
Z. Yang et al. / Journal of Physics and Chemistry of Solids 74 (2013) 1739–1744
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Fig. 3. SEM images of as-prepared Bi12TiO20 obtained with different CTAB amount: (a) 0 g; (b) 0.05 g; (c) 0.15 g; and (d) 0.30 g.
with 50–100 nm in width and several micrometers in length. More importantly, it is impossible that the aggregation resulted in the 3D architectures, because long time ultrasonic treatment could not destroy the nanostructures. The corresponding high-resolution TEM image in Fig. 2b shows the fringe spacing of 1.023 nm roughly corresponds to that of the lattice space of (100) of cubic Bi12TiO20, which is consistent with previous reports [9]. To study the influence of CTAB on the final structure and morphology of the products, the Bi12TiO20 materials were synthesized by adding to various amounts of CTAB when keeping other conditions unchanged. Remarkable differences in the morphologies of the products were observed in Fig. 3. As can be seen in Fig. 3a, the SEM image of powder prepared without CTAB is present as nano sized irregular nanorods and spherical grains. When introduction of a low CTAB content (such as 0.05 g), these aggregates made up of many small nanoparticles are the major products (Fig.3b). Increasing the amount to 0.15 g, hierarchical flower-like Bi12TiO20 architectures consisting of nanobelts were obtained on a large scale, as shown in Fig. 3c. The unique assembled superstructure, which can provide much more effective surface area for absorption, can be used in the photodecomposition of organic compounds and catalyst support [27]. Further increase of CTAB to 0.30 g, however, numerous nanobelts are intercrossed with each other to form the Bi12TiO20 networks structures and some irregular agglomerates sprinkled on the nanobelts. The reason that the hierarchical flower-like architectures can only be obtained in a appropriate CTAB amount is due to the fact that the CTAB can determines the precipitation speed. When the precipitation speed was appropriate, that nanobelts can be obtained and then assembled into 3D architectures. It is considered that CTAB not only accelerates the reaction as a transporter of the growth units, but also leads to form an almost perfect oriented structure. 3.2. Formation mechanism of Bi12TiO20 architectures In order to investigate the formation mechanism of the hierarchical flower-like Bi12TiO20 prepared with 0.15 g CTAB at 180 1C, timedependent experiments were carefully conducted. Fig. 4 show the TEM images of the products obtained at different solvothermal reaction stages, reflecting the morphology evolution process of the
products clearly. Fig. 4a shows that the nanoparticles are mainly produced in the reaction system at the initial stage. When the reaction time is prolonged to 20 min, some nanoparticles aggregated and aligned into undeveloped sphere-like frames, coexisting with the nanoparticles (Fig. 4b). With increasing the reaction time to 40 min, the size of the spherical particles increases obviously, but they were morphologically rough and structurally loose. With a further increase in the reaction time to 60 min, the flower-like superstructure appeared as the result of self-assembly of nanobelts, as shown in Fig. 4d. On the basis of the above discussion, the overall formation process (as schematically illustrated in Fig. 5.) of 3D flower-like Bi12TiO20 architectures can be summarized as follows: (a) the high reactants' concentration lead to the burst of initial Bi12TiO20 nucleation. (b) the supersaturated Bi12TiO20 nuclei would aggregate together driven by reducing the surface energy of the nanoparticles. (c) the preferential growth directions caused by additive growth inhibition effects would form thin elongated nanobelts on the surface of the initially formed Bi12TiO20 aggregation. Finally more and more nanobelts interlace and overlap with each other into a hierarchical Bi12TiO20 flower-like microstructures. As for the formation of the hierarchical flower-like Bi12TiO20 architectures, the role of surfactants should be taken into consideration. In the CTAB-assisted hydrothermal process, the surface tension of solution is reduced, which is conducive to the formation of Bi12TiO20 crystal in a lower supersaturation. In addition, CTAB used in our experiment as a shape-controlled agent can be adsorbed on the surface of Bi12TiO20 nuclei due to the existence of electrostatic and stereochemical effects [28]. It is proposed that CTAB adsorbed on the capped oxide nuclei tend to form active sites, which will then strongly affect the growth speed and orientation of crystals [29]. Under the efficient heating effect of microwave irradiation, nanobelts can grow on those active sites along the oriented direction on the surface of the initially formed Bi12TiO20 aggregations, resulting in the formation of the flower-like Bi12TiO20 hierarchical nanostructures. 3.3. Photocatalytic properties The photocatalytic activities of Bi12TiO20 hierarchical nanostructures were investigated by the photodegradation of Rhodamine B (RhB) in aqueous solution under visible-light irradiation.
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Fig. 4. TEM images of the Bi12TiO20 samples prepared with 0.15 g CTAB at 180 1C for different reaction times: (a) 5 min; (b) 20 min; (c) 40 min; and (d) 60 min.
Fig. 5. Possible mechanism for the formation of hierarchical flower-like Bi12TiO20 architectures.
Fig. 6 displays the changes of RhB relative concentration C/C0 as a function of time in presence of different photocatalysts (where C0 is the original concentration of the RhB and C is the real concentration at different time). RhB has a strong absorption at 554 nm, but visible-light irradiation of aqueous RhB/Bi12TiO20 dispersions leads to a decrease in absorption intensity and a blue-shift of the wavelength as the RhB degrades, which was plotted in the UV–vis spectra in the insets of Fig. 6. A blank experiment in the absence of the photocatalyst showed that only a small quantity of RhB was degraded (Fig. 6a). Further, TiO2 with many advantages of chemical stability, low cost and good photocatalytic activity has been put into commercial use [30]. In this regard, the contrast experiments were carried out between Bi12 TiO20 and TiO2. From Fig. 6, we could see that the Bi12TiO20 nanomaterial exhibites higher photocatalytic activities in the degradation of Rhodamine B under visible-light irradiation compared to
Fig. 6. Photodegradation of Rhodamine B by different photocatalysts versus time under visible-light irradiation and in dark for 40 min before: (a) RhB solution, (b) TiO2, as-prepared Bi12TiO20 (c) with no CTAB and (d) 0.15 g CTAB. The inset is the temporal evolution of the absorption spectra of the RhB solution in the presence of nanostructured Bi12TiO20 prepared with 0.15 g CTAB.
than that of TiO2. Furthermore, the photocatalytic degradation efficiency of RhB by the samples synthesized using 0.15 g CTAB reaches nearly 94% after 6 h irradiation, while the products without additive only reach 67% of the total decomposition. The enhanced photocatalytic activity can be attributed to the fact that the samples synthesized with additive showed much higher specific surface, which can provide more active sites for the
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Bi 6s and O 2p), and while the holes form in the VB when Bi12TiO20 semiconductor is irradiated by light. The generated conduction band electrons (e ) probably reacted with dissolved oxygen molecules to yield superoxide radical anions O2 , which on protonation generated the hydroperoxy radicals HO2̇ , producing the hydroxyl radical OH, which was a strong oxidizing agent to decompose the organic dye [33,34]. The reaction possibly includes the following in the photosensitized process. RhB-RhB*(hν) Bi12TiO20+RhB*-RhB++Bi12TiO20 (e ) Bi12TiO20 (e )+O2-Bi12TiO20+O2 RhB++O2/O2 -degraded products Fig. 7. Kinetic linear simulation curves of three time cycling in the photocatalytic degradation of RhB by nanostructured Bi12TiO20 obtained with 0.15 g CTAB and the corresponding RhB photodegradation curves of C/C0 versus irradiation time by different reaction cycles (inset).
Namely, the RhB dye first absorbed the incident photon flux. Then, the photogenerated electrons were transferred to the excited state of the dye owing to the intramolecular π–π* transition and the dyes were oxidized. The photoelectrons of the excited state were immediately injected into the CB of Bi12TiO20. The photoelectrons in the CB were then captured by O2, and the succeeding reactions can lead to the mineralization of the dyes.
4. Conclusions
Fig. 8. Schematic diagram of photocatalytic mechanism of RhB over Bi12TiO20 under visible-light irradiation.
photocatalytic reaction as well as promote the efficiency of the electron-hole separation. In addition, the space between the intermeshed nanobelts structures is in favor of the transfer of electrons and holes generated inside the crystal to the surface, and facilitated the degradation of RhB [31]. The stability of a photocatalyst is important to its industrial application in eliminating organic pollutants from wastewater. After three recycles for the photodegradation of RhB with flower-like Bi12TiO20 samples, the catalyst did not exhibit any significant loss of activity, as shown in the insets of Fig. 7. Furthermore, the reaction of the photodegradation of RhB by Bi12TiO20 is close to the pseudofirst-order reaction kinetics between ln(C/C0) and the irradiation time t, which can be seen from the linearized kinetic data curves of three time cycling in RhB photocatalytic degradation in Fig. 7. The determined reaction rate constant (k) with different reaction cycles are 0.0081, 0.0079, 0.0078 min 1, respectively, confirming Bi12TiO20 is not photocorroded during the photocatalytic oxidation of the pollutant molecules. Combined with the XRD patterns (Fig. 1c), all information demonstrates that the Bi12TiO20 with high photocatalytic stability can be easily recycled by a simple filtration. The possible visible-induced degradation mechanism of RhB over Bi12TiO20 is demonstrated in Fig. 8. It is known that the photocatalytic oxidation of organic compounds is mainly controlled by the following processes [32]: (1) the light absorption of the semiconductor catalyst, (2) the transportation and separation of the photogenerated electron and hole, (3) the utilization of the charge carriers by the reactants. In the photocatalytic process, the photoinduced electrons can be transferred to the conduction band (the Ti 3d orbital) from the valence band (the hybrid orbitals of
In summary, high crystalline Bi12TiO20 hierarchical nanostructures are produced by microwave-assisted hydrothermal method at a lower heating temperature and shorter reaction time, relative to the conventional hydrothermal process. In the case of morphology control, assembly of the nanobelts into flower-like hierarchical structures has been realized with the assistance of CTAB. Compared to the samples with no additive, the Bi12TiO20 photocatalyst prepared using CTAB showed much higher photocatalytic performance for the degradation of RhB due to their hierarchical architecture with special hierarchical porous structure, good permeability and large surface area.
Acknowledgments This work was financially supported by the National Nature Science Foundation (51172187), the SRPDF (20116102130002) and 111 Program (B08040) of MOE, the Xi'an Science and Technology Foundation (CX1261-2, CX1261-3, CX12174)), and the Shaanxi Province Science Foundation (2013KW12-02) of China. References [1] F. Favier, E.C. Walter, M.P. Zach, T. Benter, R.M. Penner, Science 293 (2001) 2227–2231. [2] H.E. Wang, L.X. Zheng, C.P. Liu, Y.K. Liu, C.Y. Luan, H. Cheng, Y.Y. Li, L. Martinu, J.A. Zapien, L. Bello, J. Phys. Chem. C. 115 (2011) 10419–10425. [3] H. Fu, C. Pan, W. Yao, Y. Zhu, J. Phys. Chem. B. 109 (2005) 22432–22439. [4] S. Kohtani, J. Hiro, N. Yamamoto, A. Kudo, K. Tokumura, R. Nakagaki, Catal. Commun. 6 (2005) 185–189. [5] W.F. Yao, H. Wang, X.H. Xu, J.T. Zhou, X.N. Yang, Y. Zhang, S.X. Shang, Appl. Catal. A. 259 (2004) 29–33. [6] W.F. Yao, H. Wang, X.H. Xu, X.F. Cheng, J. Huang, S.X. Shang, X.N. Yang, M. Wang, Appl. Catal. A. 243 (2003) 185–190. [7] H.F. Cheng, B.B. Huang, Y. Dai, X.Y. Qin, X.Y. Zhang, Z.Y. Wang, M.H. Jiang, J. Solid State Chem. 182 (2009) 2274–2278. [8] S.H. Xu, W.F. Shangguan, J. Yuan, J.W. Shi, M.X. Chen, Mater. Sci. Eng. B. 137 (2007) 108–111. [9] J.G. Hou, Y.F. Qu, D. Krsmanovic, C. Ducati, D. Eder, R.V. Kumar, Chem. Commun. 26 (2009) 3937–3939. [10] J.G. Hou, Y.F. Qu, D. Krsmanovic, C. Ducati, D. Eder, R.V. Kumar, J. Mater. Chem. 20 (2010) 2418–2423. [11] X. Lin, P. Lv, Q.F. Guan, H.B. Li, H.J. Zhai, C.B. Liu, Appl. Surf. Sci. 258 (2012) 7146–7153.
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Contents lists available at ScienceDirect
Journal of Physics and Chemistry of Solids journal homepage: www.elsevier.com/locate/jpcs
Rapid microwave-assisted hydrothermal synthesis of Bi12TiO20 hierarchical architecture with enhanced visible-light photocatalytic activities Zhuo Yang, Huiqing Fan n, Xin Wang, Changbai Long State Key Laboratory of Solidification Processing, School of Materials Science and Engineering, Northwestern Polytechnical University, Xi'an 710072, China
art ic l e i nf o
a b s t r a c t
Article history: Received 2 December 2012 Received in revised form 25 June 2013 Accepted 28 June 2013 Available online 6 July 2013
Flower-like Bi12TiO20 hierarchical nanostructures composed of numerous nanobelts were synthesized at 180 1C within 1 h by a microwave-assisted hydrothermal method in the presence of cetyltrimethylammonium bromide (CTAB) for the first time. The as-prepared products were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and ultraviolet–visible (UV–vis) absorption spectroscopy. Furthermore, the hierarchical Bi12TiO20 nanostructures exhibited higher photocatalytic activities in the degradation of Rhodamine B under visible-light irradiation than that of the samples prepared without CTAB. In addition, the role of CTAB cationic surfactant has been investigated thoroughly and a possible mechanism is proposed. & 2013 Elsevier Ltd. All rights reserved.
Keywords: A. Nanostructures A. Oxides B. Chemical synthesis D. Optical properties
1. Introduction Since the nanostructured materials with good control on their morphologies, size, and crystallographic structure may differ significantly from the corresponding bulk counterparts, nanostructured semiconductors have attracted a growing interest for their fascinating properties and potential applications in technology [1,2]. Recently, ternary bismuth oxide semiconductors, such as Bi2WO6, BiVO4, Bi2Ti2O7 and Bi12TiO20 have been widely studied as a class of promising photocatalysts with catalytic activity under visible-light irradiation [3–12]. Among them, Bi12TiO20 crystals with sillenite structures is one of the most frequently investigated materials due to its excellent intrinsic properties and the potential applications including optical information processing, optical phase conjugation, photovoltaic cells and photocatalysts [13–16]. Bi12TiO20 micro/nanostructures with different morphologies and sizes have been obtained via solid-state reactions, chemical method, or by conventional hydrothermal synthesis [17]. Those methods mentioned above, however, have obvious disadvantages like the requirement for high-temperature calcination, which often results in large-sized crystallization and hard aggregation, or requiring complicated equipments and long processing time [18]. Microwave irradiation could be used as a heat source for the hydrothermal process. It leads to a rapid heating to attain the desired temperature in a short time and increases the reaction kinetics, compared to the n
Corresponding author. Tel.: +86 29 88494463; fax: +86 29 88492642. E-mail address: [email protected] (H. Fan).
0022-3697/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jpcs.2013.06.020
conventional hydrothermal method. Microwave-assisted reaction is seldom employed to synthesize Bi12TiO20 crystals for photocatalysis although such a method has been used successfully to fabricate various materials [19]. Further, tailoring the architecture of nanomaterials has been of great research interest due to their unique properties are closely related to the size and shape of nanostructures. In terms of morphology control, cetyltrimethylammonium bromide (CTAB) has been generally used to synthesize different nanostructures, such as nanowires, nanotubes, hollow spheres, and 3D hierarchical structures [20,21]. Phuruangrat and co-workers described the synthesis of MoO3 nanowire with unprecedentedly high aspect ratios in the presence of CTAB [22]. Hu et al. synthesized CuO nanotubes and nanorods using CTAB by a hydrothermal process [23]. Multilayered ZnO nanosheets with hierarchically porous structures are successfully synthesized from a hydrothermal preparation and thermal decomposition by using CTAB [24]. However, relatively few studies on the synthesis of hierarchically Bi12TiO20 nanostructures using CTAB have been reported up to now. Herein, a microwave-assisted hydrothermal route was used to synthesize hierarchical nanostructured Bi12TiO20 powders in the presence of cetyltrimethylammonium bromide (CTAB) for the first time. The key features of this method are that it is time-saving, has low energy consumption, and is inexpensive. Moreover, the photodegradation of Rhodamine B (RhB) was employed to evaluate the photocatalytic activities of Bi12TiO20 hierarchical nanostructures under visible-light irradiation. It is worthy to note that the hierarchical Bi12TiO20 materials exhibit enhanced visible-lightdriven photocatalytic performance.
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2. Experimental section 2.1. Synthesis of hierarchical nanostructured Bi12TiO20 powders All reagents were analytically pure, bought from Shanghai Chemical Corp., and used without further purification. Typically, the 0.576 g of Bi(NO3)3d5H2O was dissolved in 30 ml of deionized water with dilute nitric acid under continuous stirring, followed by the addition of stoichiometric amount of titanium butoxide. After stirred for 1 h, 10 ml solution consisting of 0 g, 0.05 g, 0.15 g and 0.30 g of cetyltrimethylammonium bromide (CTAB) was introduced into the reaction mixture. The pH of the final solution was adjusted to 12 using sodium hydroxide. The as-prepared solution was transferred to a polytetrafluoroethylene (PTFE)-lined autoclave, sealed and maintained for microwave irradiation at 180 1C for 1 h, and cooled to room temperature naturally. Then, the resulting precipitate was recovered by centrifugation, washed with deionized water and ethanol thoroughly, and finally dried in an oven at 80 1C. For a comparison, nanostructured TiO2 were also synthesized by the sol–gel method. The preparation followed the procedure of Huang et al. [25].
Fig. 1. XRD patterns of as-prepared Bi12TiO20 (a) without additive and with CTAB before (b) and (c) after photocatalysis.
2.2. Characterization and photocatalytic measurements The structure and morphology of the products were characterized by X-ray diffraction measurement (XRD; X'pert, Philips, The Netherlands) with Cu-Kα radiation (λ¼1.5406 Å), field emitting scanning electron microscopy (FE-SEM; JSM-6701F, JEOL, Japan) with an accelerating voltage of 20 kV and transmission electron microscopy (TEM; JEM-3010, Questar, New Hope, USA) respectively. The photodegradation experiments were performed in a slurry reactor containing 50 mL of a 10 mg L 1 solution of RhB and 0.05 g of catalyst [26]. A 500 W Xe lamp was used as the light source with a 420 nm cutoff filter to provide visible-light irradiation. Prior to irradiation, the suspension was kept in the dark under magnetically stirring for 40 min to ensure the establishing of an adsorption/ desorption equilibrium between the photocatalyst and RhB. At a given time interval, 4 mL suspension was collected and separated by centrifugation at 8000 rpm for 10 min. The filtrates were analyzed using a UV–vis spectrophotometer (UV-3150, Shimadzu, Japan) at its maximum absorption wavelength of 554 nm.
3. Results and discussion 3.1. Characterization of hierarchical Bi12TiO20 architectures The as-prepared products with additive-free and CTAB additive have been structurally characterized by X-ray diffraction, as shown in Fig. 1. All the diffraction peaks can be indexed as the cubic Bi12TiO20 in accordance with the values in the standard card (JCPDS NO.340097) and the sharp peaks indicate 1 h microwave hydrothermal time is enough for the formation of high crystalline Bi12TiO20. No significant differences can be detected for peak positions or FWHM of the Fig. 1b and c, which reveals the crystal structure of the hierarchical nanostructured Bi12TiO20 were very stable. The representative microstructures of the bismuth titanate materials that were synthesized with 0.15 g CTAB at 180 1C for 1 h are presented in Fig. 2. Fig. 2a shows a panoramic TEM image of as-prepared Bi12TiO20 using CTAB, from which uniformly-sized flower-like architectures with diameters of 1–2 μm can be clearly observed. A magnified FE-SEM image of the sample (inset of Fig. 2a) indicates that the Bi12TiO20 nanostructures look like some natural flowers consisting of multitudinous uniformly distributed nanobelts. These nanobelts are intercrossed with each other and form a 3D flower-like structure. The observation from Fig. 2b (the inset) suggests the nanobelts are randomly oriented crystallites
Fig. 2. TEM (a) and HRTEM (b) images of hierarchical flower-like Bi12TiO20 architectures obtained with 0.15 g CTAB at 180 1C for 1 h by microwave hydrothermal process. Inset: corresponding SEM and TEM images.
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Fig. 3. SEM images of as-prepared Bi12TiO20 obtained with different CTAB amount: (a) 0 g; (b) 0.05 g; (c) 0.15 g; and (d) 0.30 g.
with 50–100 nm in width and several micrometers in length. More importantly, it is impossible that the aggregation resulted in the 3D architectures, because long time ultrasonic treatment could not destroy the nanostructures. The corresponding high-resolution TEM image in Fig. 2b shows the fringe spacing of 1.023 nm roughly corresponds to that of the lattice space of (100) of cubic Bi12TiO20, which is consistent with previous reports [9]. To study the influence of CTAB on the final structure and morphology of the products, the Bi12TiO20 materials were synthesized by adding to various amounts of CTAB when keeping other conditions unchanged. Remarkable differences in the morphologies of the products were observed in Fig. 3. As can be seen in Fig. 3a, the SEM image of powder prepared without CTAB is present as nano sized irregular nanorods and spherical grains. When introduction of a low CTAB content (such as 0.05 g), these aggregates made up of many small nanoparticles are the major products (Fig.3b). Increasing the amount to 0.15 g, hierarchical flower-like Bi12TiO20 architectures consisting of nanobelts were obtained on a large scale, as shown in Fig. 3c. The unique assembled superstructure, which can provide much more effective surface area for absorption, can be used in the photodecomposition of organic compounds and catalyst support [27]. Further increase of CTAB to 0.30 g, however, numerous nanobelts are intercrossed with each other to form the Bi12TiO20 networks structures and some irregular agglomerates sprinkled on the nanobelts. The reason that the hierarchical flower-like architectures can only be obtained in a appropriate CTAB amount is due to the fact that the CTAB can determines the precipitation speed. When the precipitation speed was appropriate, that nanobelts can be obtained and then assembled into 3D architectures. It is considered that CTAB not only accelerates the reaction as a transporter of the growth units, but also leads to form an almost perfect oriented structure. 3.2. Formation mechanism of Bi12TiO20 architectures In order to investigate the formation mechanism of the hierarchical flower-like Bi12TiO20 prepared with 0.15 g CTAB at 180 1C, timedependent experiments were carefully conducted. Fig. 4 show the TEM images of the products obtained at different solvothermal reaction stages, reflecting the morphology evolution process of the
products clearly. Fig. 4a shows that the nanoparticles are mainly produced in the reaction system at the initial stage. When the reaction time is prolonged to 20 min, some nanoparticles aggregated and aligned into undeveloped sphere-like frames, coexisting with the nanoparticles (Fig. 4b). With increasing the reaction time to 40 min, the size of the spherical particles increases obviously, but they were morphologically rough and structurally loose. With a further increase in the reaction time to 60 min, the flower-like superstructure appeared as the result of self-assembly of nanobelts, as shown in Fig. 4d. On the basis of the above discussion, the overall formation process (as schematically illustrated in Fig. 5.) of 3D flower-like Bi12TiO20 architectures can be summarized as follows: (a) the high reactants' concentration lead to the burst of initial Bi12TiO20 nucleation. (b) the supersaturated Bi12TiO20 nuclei would aggregate together driven by reducing the surface energy of the nanoparticles. (c) the preferential growth directions caused by additive growth inhibition effects would form thin elongated nanobelts on the surface of the initially formed Bi12TiO20 aggregation. Finally more and more nanobelts interlace and overlap with each other into a hierarchical Bi12TiO20 flower-like microstructures. As for the formation of the hierarchical flower-like Bi12TiO20 architectures, the role of surfactants should be taken into consideration. In the CTAB-assisted hydrothermal process, the surface tension of solution is reduced, which is conducive to the formation of Bi12TiO20 crystal in a lower supersaturation. In addition, CTAB used in our experiment as a shape-controlled agent can be adsorbed on the surface of Bi12TiO20 nuclei due to the existence of electrostatic and stereochemical effects [28]. It is proposed that CTAB adsorbed on the capped oxide nuclei tend to form active sites, which will then strongly affect the growth speed and orientation of crystals [29]. Under the efficient heating effect of microwave irradiation, nanobelts can grow on those active sites along the oriented direction on the surface of the initially formed Bi12TiO20 aggregations, resulting in the formation of the flower-like Bi12TiO20 hierarchical nanostructures. 3.3. Photocatalytic properties The photocatalytic activities of Bi12TiO20 hierarchical nanostructures were investigated by the photodegradation of Rhodamine B (RhB) in aqueous solution under visible-light irradiation.
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Fig. 4. TEM images of the Bi12TiO20 samples prepared with 0.15 g CTAB at 180 1C for different reaction times: (a) 5 min; (b) 20 min; (c) 40 min; and (d) 60 min.
Fig. 5. Possible mechanism for the formation of hierarchical flower-like Bi12TiO20 architectures.
Fig. 6 displays the changes of RhB relative concentration C/C0 as a function of time in presence of different photocatalysts (where C0 is the original concentration of the RhB and C is the real concentration at different time). RhB has a strong absorption at 554 nm, but visible-light irradiation of aqueous RhB/Bi12TiO20 dispersions leads to a decrease in absorption intensity and a blue-shift of the wavelength as the RhB degrades, which was plotted in the UV–vis spectra in the insets of Fig. 6. A blank experiment in the absence of the photocatalyst showed that only a small quantity of RhB was degraded (Fig. 6a). Further, TiO2 with many advantages of chemical stability, low cost and good photocatalytic activity has been put into commercial use [30]. In this regard, the contrast experiments were carried out between Bi12 TiO20 and TiO2. From Fig. 6, we could see that the Bi12TiO20 nanomaterial exhibites higher photocatalytic activities in the degradation of Rhodamine B under visible-light irradiation compared to
Fig. 6. Photodegradation of Rhodamine B by different photocatalysts versus time under visible-light irradiation and in dark for 40 min before: (a) RhB solution, (b) TiO2, as-prepared Bi12TiO20 (c) with no CTAB and (d) 0.15 g CTAB. The inset is the temporal evolution of the absorption spectra of the RhB solution in the presence of nanostructured Bi12TiO20 prepared with 0.15 g CTAB.
than that of TiO2. Furthermore, the photocatalytic degradation efficiency of RhB by the samples synthesized using 0.15 g CTAB reaches nearly 94% after 6 h irradiation, while the products without additive only reach 67% of the total decomposition. The enhanced photocatalytic activity can be attributed to the fact that the samples synthesized with additive showed much higher specific surface, which can provide more active sites for the
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Bi 6s and O 2p), and while the holes form in the VB when Bi12TiO20 semiconductor is irradiated by light. The generated conduction band electrons (e ) probably reacted with dissolved oxygen molecules to yield superoxide radical anions O2 , which on protonation generated the hydroperoxy radicals HO2̇ , producing the hydroxyl radical OH, which was a strong oxidizing agent to decompose the organic dye [33,34]. The reaction possibly includes the following in the photosensitized process. RhB-RhB*(hν) Bi12TiO20+RhB*-RhB++Bi12TiO20 (e ) Bi12TiO20 (e )+O2-Bi12TiO20+O2 RhB++O2/O2 -degraded products Fig. 7. Kinetic linear simulation curves of three time cycling in the photocatalytic degradation of RhB by nanostructured Bi12TiO20 obtained with 0.15 g CTAB and the corresponding RhB photodegradation curves of C/C0 versus irradiation time by different reaction cycles (inset).
Namely, the RhB dye first absorbed the incident photon flux. Then, the photogenerated electrons were transferred to the excited state of the dye owing to the intramolecular π–π* transition and the dyes were oxidized. The photoelectrons of the excited state were immediately injected into the CB of Bi12TiO20. The photoelectrons in the CB were then captured by O2, and the succeeding reactions can lead to the mineralization of the dyes.
4. Conclusions
Fig. 8. Schematic diagram of photocatalytic mechanism of RhB over Bi12TiO20 under visible-light irradiation.
photocatalytic reaction as well as promote the efficiency of the electron-hole separation. In addition, the space between the intermeshed nanobelts structures is in favor of the transfer of electrons and holes generated inside the crystal to the surface, and facilitated the degradation of RhB [31]. The stability of a photocatalyst is important to its industrial application in eliminating organic pollutants from wastewater. After three recycles for the photodegradation of RhB with flower-like Bi12TiO20 samples, the catalyst did not exhibit any significant loss of activity, as shown in the insets of Fig. 7. Furthermore, the reaction of the photodegradation of RhB by Bi12TiO20 is close to the pseudofirst-order reaction kinetics between ln(C/C0) and the irradiation time t, which can be seen from the linearized kinetic data curves of three time cycling in RhB photocatalytic degradation in Fig. 7. The determined reaction rate constant (k) with different reaction cycles are 0.0081, 0.0079, 0.0078 min 1, respectively, confirming Bi12TiO20 is not photocorroded during the photocatalytic oxidation of the pollutant molecules. Combined with the XRD patterns (Fig. 1c), all information demonstrates that the Bi12TiO20 with high photocatalytic stability can be easily recycled by a simple filtration. The possible visible-induced degradation mechanism of RhB over Bi12TiO20 is demonstrated in Fig. 8. It is known that the photocatalytic oxidation of organic compounds is mainly controlled by the following processes [32]: (1) the light absorption of the semiconductor catalyst, (2) the transportation and separation of the photogenerated electron and hole, (3) the utilization of the charge carriers by the reactants. In the photocatalytic process, the photoinduced electrons can be transferred to the conduction band (the Ti 3d orbital) from the valence band (the hybrid orbitals of
In summary, high crystalline Bi12TiO20 hierarchical nanostructures are produced by microwave-assisted hydrothermal method at a lower heating temperature and shorter reaction time, relative to the conventional hydrothermal process. In the case of morphology control, assembly of the nanobelts into flower-like hierarchical structures has been realized with the assistance of CTAB. Compared to the samples with no additive, the Bi12TiO20 photocatalyst prepared using CTAB showed much higher photocatalytic performance for the degradation of RhB due to their hierarchical architecture with special hierarchical porous structure, good permeability and large surface area.
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