Nanobelts
Bi2MoO6 Nanobelts for Crystal Facet-Enhanced Photocatalysis Jinlin Long,* Shuchao Wang, Hongjin Chang, Baozhou Zhao, Botong Liu, Yangen Zhou, Wei Wei, Xuxu Wang, Ling Huang,* and Wei Huang*
Tailoring the surface structure of inorganic materials at the nanoscale by varying the chemical synthesis conditions, such as the solvent, surfactant, and temperature,[1–3] has long been employed for endowing them distinctive properties and obtaining atomic-level insight into the structure-performance relationship.[4] Recently, considerable attention has been paid to the facet-controlled fabrication of noble metal nanocrystals and single-crystal metal oxides with well-defined morphologies because of their facet-dependent catalytic, photocatalytic, and other surface-related properties.[5–7] Especially for anisotropic semiconducting oxides, the pursue of intentionally-exposed high-energy crystal planes have not only paved a new direction for enhanced photocatalytic performance, but also offered great opportunities to investigate the relationships between the surface structures (the crystal facets) and the photocatalytic performance. Taking anatase titanium oxide (TiO2) as an example, its low-index facets, i.e., (001), (101), and (010) with well-defined geometric and electronic structures, were well-exploited as the active spots for various types of reactions with enhanced photocatalytic activity,[8–10] compared to those with high-index crystalline materials. Besides anatase TiO2, a few single crystal semiconducting nanomaterials including WO3, BiOCl, AgBr, and Ag3PO4 Dr. J. Long,[+] Dr. Y. Zhou, Prof. X. Wang Research Institute of Photocatalysis State Key Laboratory of Photocatalysis on Energy and Environment College of Chemistry Fuzhou University Fuzhou 350116, China E-mail: [email protected] Tel: +86-591-83779121 Fax: +86-591-83779121 H. Chang, B. Zhao, B. Liu, Prof. L. Huang, W. Huang Institute of Advanced Materials Nanjing Tech University Nanjing 210009, China E-mail: [email protected]; [email protected] Tel: +65 63168825 Dr. J. Long, S. Wang,[+] W. Wei, Prof. L. Huang School of Chemical and Biomedical Engineering Nanyang Technological University 637457, Singapore [+]These authors are contributed equally to this work DOI: 10.1002/smll.201302950 small 2014, DOI: 10.1002/smll.201302950
have also been studied theoretically and experimentally in order to discover the facet-dependent photocatalytic properties.[11–14] Undoubtedly, crystal facet engineering of inorganic semiconductors is an exciting direction with great potential to achieve highly active new-generation catalysts, especially the photocatalysts. Bismuth molybdate (Bi2MoO6), an important aurivillius oxide possessing layered structure with the perovskite-like slab of MoO6, has recently become a popular target due to its excellent photocatalytic performance under ≥420 nm (thereafter, visible-light) irradiation,[15–17] and a great deal of effort has focused on the morphology-controlled fabrication and photocatalytic properties of this material with hierarchical nanostructures.[18–21] However, the facet effect on its photocatalytic activity has rarely been investigated. Herein, we first report the controlled synthesis of single crystal Bi2MoO6 nanobelts (BMNBs) with dominant (010) facets via oleyamine-mediated hydrothermal method, and then studied their facet-dependent photocatalysis behavior for the degradation of Rhodamine B dye (RhB) under visiblelight irradiation. More importantly, we have proved that the as-prepared BMNB exhibits higher activity than the counterpart with hierarchical nanorod-based microspheres, and the according mechanisms are further provided. and To synthesize BMNBs, Bi(NO3)3·5H2O (NH4)6Mo7O24·4H2O were first dissolved in the aqueous solution, and then heated in an autoclave at 160 °C, where oleyamine works as surfactant and NH3·H2O for adjusting the pH value of the reaction solution. The series of transmission electron microscopy (TEM) images in Figure S1 show the morphology variation of the BMNBs synthesized at different pH values and it is evident that the optimal pH value is 7.0, where the nanobelts with lengths of 300–600 nm, widths of 20–30 nm, and thicknesses of 5–10 nm (Figure 1A) are the major product. High resolution TEM (HRTEM) image of the nanobelt (Figure 1B) proves its high crystalline characteristics. The clear lattice fringes with an interplanar spacing of 0.275 nm corresponding to the (200) peak in the XRD pattern of γ-Bi2MoO6 (Figure S2) and an interplanar spacing of 0.274 nm corresponding to the (001) atomic planes can be established to be the crystal growth directions. The corresponding selected-area electron diffraction (SAED) pattern indicates the single-crystal structure of the BMNB sample (inset to Figure 1B). On the basis of the above discussion and the symmetry of orthorhombic Bi2MoO6 single crystal,
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Figure 1. (A) TEM and (B) HRTEM images of BMNB prepared at pH = 7.0, atomic structrue of (C) top-view exposed {001} and (D) {010} facets, and (E) atomic construction model of BMNB.
the bottom and top surfaces of BMNBs are identified as {010} facets and the two lateral surfaces along the growth direction and the two crystal growth planes are {001} and {100} facets, respectively. The percentage of the {010} facet is estimated from the geometric calculation to be ca. 80% for the BMNB sample. The layered structure of γ-Bi2MoO6 can be described as the corner-shared and disordered MoO6 octahedra and planar (Bi2O2)2+ layers[22] as shown in Figure 1C, where we can see the atomic arrangement of the exposed {001} facets of BMNB, and the MoO6 octahedra are sandwiched between the (Bi2O2)2+ layers. The atomic structure of the {010} facet is characterized by the high density of exposed oxygen atoms
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of the (Bi2O2)2+ layers (Figures 1D and 1E). For comparison, γ-Bi2MoO6 hierarchical microstructures were also prepared by reacting Bi(NO3)3·5H2O and (NH4)6Mo7O24·4H2O in ethylene glycol (EG)/ethanol solution and heated in an autoclave. The result shows that in the absence of oleyamine, EG leads to the formation of hierarchical nanorod-based microspheres (BMMSs), rather than the regular nanobelts (Figure 2A). HRTEM image of an individual nanorod (Figure 2B) reveals the lattice fringes with the interplanar spaces of about 0.326 and 0.235 nm corresponding to the {140} and {221} atomic planes, respectively, suggesting no lowindex facets are exposed in the BMMS sample. Therefore, we deduce that the exposure of the dominant {010} facets in
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small 2014, DOI: 10.1002/smll.201302950
Bi2MoO6 Nanobelts for Crystal Facet-Enhanced Photocatalysis
Figure 2. (A) SEM and (B) HRTEM images of the as-prepared BMMS sample. The inset to (A) is the corresponding TEM image of part of the microsphere.
the BMNB sample is because of the adsorption of oleyamine chromospheres proceed synchronously on RhB molecules, molecules on the surface of the nucleating (Bi2O2)2+ layers, suggesting that two kinds of active oxygen species, O2−• and which further regulates the directional growth of the lamellar •OH radicals, are generated under visible–light irradiation in BMNB-catalyzed photo-degradation process. nanobelts. The variation in morphology and exposed crystal facets of To elaborate the detailed photo-degradation process, the BMNB and BMMS samples results in drastically different electron paramagnetic resonance (EPR) was used to detect photoreactivities, which can be evaluated by the decomposi- the formation of these active oxygen species under visible tion of RhB dye in an aqueous solution under visible light light irradiation. Figure 4 shows the change in EPR signals irradiation. Figure 3A demonstrates the temporal evolution of of the BMNB/water suspension with irradiation time. Four the UV-spectral changes collected during the photocatalytic characteristic signals belonging to the DMPO-•OH adduct degradation of RhB dye catalyzed by BMNB. At the reaction appear intermediately in the EPR spectra upon light on, time of 0 min, RhB shows a major absorption band at 552 nm and increases in intensity with irradiation time (Figure 4A), (Figure 3A). Visible light irradiation of the RhB/BMNB dis- suggesting the formation of •OH radical. When the BMNBs persion leads to a rapid decrease of the 552 nm absorption are uniformly suspended into methanol, the DMPO spinwith a concomitant wavelength shift of the feature band to trapping EPR spectra obtained are completely different shorter wavelength, reminiscent of the photodegradation of from those of the aqueous system. As shown in Figure 4B, RhB over CdS, TiO2, and CdS/TiO2 nanocomposites.[23,24] six characteristic signals assigned to DMPO-O2−• adduct are Zhao and co-workers[25–27] have clearly revealed the photo- observed and gradually increase in intensity with irradiation degradation mechanism and that the hypsochromic shift time, indicating the formation of O2−• radicals under visible originates from a stepwise de-ethylation process of the fully light irradiation. Obviously, the •OH radical is originated N,N,N′,N′-tetraethylated rhodamine molecules (552 nm) from the hole oxidation, and the O2−• is due to the electron attacked by •OH radicals, via N,N,N′-triethylated rhoda- reduction. Thus, it can be concluded that the oxidative •OH mine (539 nm), N,N′-diethylated rhodamine (522 nm), N-ethylated rhodamine (510 nm), and finally rhodamine (498 nm). The de-ethylation process can be completed within 120 min for the BMNB photocatalyst, whereas the BMMS photocatalyst takes over 660 min under the same experimental conditions (Figure 3B). While in the absence of γ-Bi2MoO6 photocatalysts, the absorbance decreases only by about 5% after 300 min visible-light irradiation, which is due to the photosensitized self-degradation. These results indicate that BMNB shows a superior photocatalytic activity to the BMMS for RhB dye degradation. Moreover, the spectra evolution of RhB dye in Figure 3A also clearly Figure 3. (A) Photocatalytic decomposition of RhB dye over the BMNB sample under visible demonstrates that the de-ethylation light irradiation. (B) Decoloration rate of the RhB chromophore (a) without catalyst, (b) with and the benzene ring-opening of BMMS, and (c) with BMNB, under visible light irradiation. small 2014, DOI: 10.1002/smll.201302950
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Figure 4. Spin trapping EPR spectra of (a) DMPO-•OH adduct and (b) DMPO-O2• adduct formed upon visible light irradiation of BMNB.
and O2−• radicals are predominantly responsible for the photo-degradation of the dye pollutant. In order to discover the origin of the facet-enhanced photocatalysis, the UV-Vis diffuse reflectance spectra (DRS) of BMNB and BMMS are collected. As shown in Figure S3, both samples have clear absorption at ca. 468 nm, indicating no difference in the light absorption efficiency between them. Based on the equation Eg = 1240/λ, the optical band-gap energy of BMNB and BMMS is equal to 2.65 eV. For a crystalline semiconductor, the optical band-gap energy also follows the equation of F(R)E =A(E-Eg)n/2, where F(R), E, A, n, and Eg are diffuse absorption coefficient, photon energy, proportionality constant, an integer, and band-gap, respectively. The integer n depends on the optical transition (n = 1, 2, 4, 6), and the value of n determined for BMNB is 4, indicating that the semiconductor has an indirect transition.[28] Long et al.[29] determined the flat-band potential of γ-Bi2MoO6 to be −0.32 eV versus NHE at pH = 7.0, which is more negative than the redox potential of O2−•/O2 (0.28 eV).[30] Combined with the equation of EVB = ECB + Eg, where EVB, ECB and Eg are the valence band, conduction band, and bandgap energy of the semiconductor, respectively, the edge
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of the valence band of BMNB is estimated to be +2.33 eV, which is more positive than the redox potential of •OH/H2O (+1.23 eV). This indicates that the electrons and holes generated on BMNBs upon visible light irradiation can react with the adsorbed O2 and H2O to form active oxygen species, O2−• and •OH radicals, respectively. Theoretical calculation[15,16] has already revealed that the visible light absorption of γ-Bi2MoO6 is originated from the transition of the valence band consisting of the O 2p orbitals to the conduction band derived from the Mo 4d orbitals in MoO6 octahedral and the Bi 6p orbitals in the (Bi2O2)2+ layers. The structural analysis of BMNB indicates that majority of the oxygen atoms in the valence band sitting on the {010} facet and the Mo and Bi atoms of the conduction band are placed on the {100} and {001} facets. They are completely exposed to the reacting substrates including O2, H2O, and RhB dye molecules. Consequently, the photogenerated holes located at the valence band are directly available to H2O and RhB molecules, and the photogenerated electrons located at the conduction band are directly available to O2 molecules. It is evident that in BMNB, the charge carriers can be efficiently separated due to the exposure of more photoactive sites. Considering the similar light absorption and band structure of the BMNB and BMMS, we believe that the higher photoreactivity of BMNB is resulted from the faster separation of charge carriers, which is well-known to be one of the important factors that govern the photocatalysis efficiency, and more photoactive sites exposed on the surface of BMNB. In summary, we have successfully prepared γ-Bi2MoO6 single-crystal nanobelts and further demonstrated their crystal facet-enhanced photocatalytic characteristics, which is because of the direct exposure of more photoactive sites to the reacting substrates and sequentially the charge carriers in the nanobelts can be more efficiently utilized. The experimental results have proved that H2O molecules are oxidized by photogenerated holes to •OH radials on the {010} facets, and O2 molecules are reduced by photogenerated electrons to O2−• radials on the {100} and {001} facets. This work has indicated that crystal facet engineering is a promising strategy for deep understanding of morphology-dependent photocatalysis, the development of new semiconducting photocatalysts, as well as the fine manipulation of their photoreactivities.
Experimental Section Preparation of γ-Bi2MoO6 Single-Crystal Nanobelts: The γ-Bi2MoO6 single-crystal nanobelts were prepared using the oleyamine-mediated hydrothermal procedure. Typically, 0.485 g of Bi(NO3)3·5H2O and 0.089 g of (NH4)6Mo7O24·4H2O were completely dissolved in 10 mL of diluted HNO3 (1 mMol/L) and 10 mL of deionized water, respectively, and then 1 mL of oleyamine and the Bi(NO3)3 solution were added to the (NH4)6Mo7O24 aqueous solution under magnetic stirring. The pH value of solution was adjusted by NH3·H2O. Finally, the resultant suspension was transferred into a 50 mL autoclave and heated at 160 °C for 10 h. After this, the autoclave was cooled down to room temperature in air, and the resulting solid was separated by a centrifuge and washed several with ethanol, finally dried at 120 °C overnight.
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small 2014, DOI: 10.1002/smll.201302950
Bi2MoO6 Nanobelts for Crystal Facet-Enhanced Photocatalysis
Preparation of γ-Bi2MoO6 Nanorod-Based Microspheres: The γ-Bi2MoO6 nanorod-based microspheres were prepared using the EG-mediated solvothermal procedure. Typically, 0.210 g of Bi(NO3)3·5H2O and 0.0382 g of (NH4)6Mo7O24·4H2O were completely dissolved in 15 mL of EG and 25 mL ethanol, respectively, and then the two solutions were mixed together under magnetic stirring. The resulting clear solution was transferred to a 50 mL autoclave, which was heated at 160 °C and maintained for 24 h. Then, the autoclave was cooled down to room temperature in air, and the resulted solid was separated by centrifugation, washed several times with ethanol, and finally dried at 120 °C overnight. Catalyst Characterizations: XRD measurements were performed on a Bruker D8 Advance X–ray diffractometer using Cu Kα1 radiation (λ = 1.5406 Å). The accelerating voltage and the applied current were 40 kV and 40 mA, respectively. HRTEM images were obtained by a JEOL model JEM 2010 EX instrument at an accelerating voltage of 200 kV. EPR spectra were recorded on a Bruker A-300-EPR X-band spectrometer. The UV/Vis diffuse reflectance spectrum was done on Perkin Elmer Lambda 950 UV-Vis-NIR system. Photocatalytic Testing: The photocatalytic activities for RhB degradation under visible light irradiation were performed in a 250 mL beak. The visible light source was a 300 W xenon lamp with a cut-off filter (λ ≥ 420 nm) positioned above a cylindrical reaction vessel. 0.05 g of the catalyst was suspended in 100 mL water containing 10 ppm RhB dye. The suspensions were prestirred magnetically in the dark for 1 h to ensure the equilibrium of the work solution. At given time intervals, 2 mL aliquots were sampled and filtered to remove the catalyst. The degraded solutions were analyzed using a Varian Cary 50 Scan UV-Vis spectrophotometer, and the absorption peak at 522 nm was monitored.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements This work was financially supported by the NSFC (Grant Nos. 21373051 and U1305242), National Basic Research Program of China (973 Program, No. 2012CB722607), and the Science and Technology Project of Education Office of Fujian Province, P. R. China (JA12017), LH thanks the financial support from NTU Start-Up Grant and Tier 1 Grant (RG20/09) from Ministry of Education, Singapore., and the financial support from the NSFC (Grant No. 21371095).
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[1] S. Sun, Adv. Mater. 2006, 18, 393. [2] T. Hyeon, Chem. Commun. 2003, 39, 927. [3] Y. Xia, P. Yang, Y. Sun, B. Mayers, B. Gates, Y. Yin, F. Kim, H. Yan, Adv. Mater. 2003, 15, 353. [4] A. T. Bell, Science 2003, 299, 1688. [5] X. Xie, Y. Li, Z.-Q. Liu, M. Haruta, W. Shen, Nature 2009, 458, 746. [6] K. Zhou, Y. Li, Angew. Chem. Int. Ed. 2012, 51, 602. [7] G. Liu, J. C. Yu, G. Q. Lu, H.-M. Cheng, Chem. Commun. 2011, 47, 6763. [8] H. G. Yang, C. H. Sun, S. Z. Qiao, J. Zou, G. Liu, S. C. Smith, H.-M. Cheng, G. Q. Lu, Nature 2008, 453, 638. [9] J. Pan, G. Liu, G. Q. Lu, H.-M. Cheng, Angew. Chem. Int. Ed. 2011, 50, 2133. [10] N. Wu, J. Wang, D. N. Tafen, H. Wang, J.-G. Zheng, J. P. Lewis, X. Liu, S. S. Leonard, A. Manivannan, J. Am. Chem. Soc. 2010, 132, 6679. [11] J. Jiang, K. Zhao, X. Xiao, L. Zhang, J. Am. Chem. Soc. 2012, 134, 4473. [12] Y. P. Xie, G. Liu, L. Yin, H.-M. Cheng, J. Mater. Chem. 2012, 22, 6746. [13] Y. Bi, S. Ouyang, N. Umezawa, J. Cao, J. Ye, J. Am. Chem. Soc. 2011, 133, 6490. [14] H. Wang, J. Yang, X. Li, H. Zhang, J. Li, L. Guo, Small 2012, 8, 2802. [15] J. Bi, L. Wu, J. Li, Z. Li, X. Wang, X. Fu, Acta Mater. 2007, 55, 4699. [16] Y. Shimodaira, H. Kato, H. Kobayashi, A. Kudo, J. Phys. Chem. B 2006, 110, 17790. [17] M. Zhang, C. Shao, J. Mu, X. Huang, Z. Zhang, Z. Guo, P. Zhang, Y. Liu, J. Mater. Chem. 2012, 22, 577. [18] G. Tian, Y. Chen, W. Zhou, K. Pan, Y. Dong, C. Tian, H. Fu, J. Mater. Chem. 2011, 21, 887. [19] M. Shang, W. Wang, J. Ren, S. Sun, L. Zhang, Nanoscale 2011, 3, 1474. [20] L. Zhang, T. Xu, X. Zhao, Y. Zhu, Appl. Catal. B: Environ. 2010, 98, 138. [21] C. Guo, J. Xu, S. Wang, L. Li, Y. Zhang, X. Li, CrystEngComm 2012, 14, 3602. [22] A. Ayame, K. Uchida, M. Iwataya, M. Miyamoto, Appl. Catal. A 2002, 227, 7. [23] L. Wu, J. C. Yu, X. Fu, J. Mol. Catal. A: Chem. 2006, 244, 25. [24] J. Zhuang, W. Dai, Q. Tian, Z. Li, L. Xie, J. Wang, P. Liu, X. Shi, D. Wang, Langmuir 2010, 26, 9686. [25] T. Wu, G. Liu, J. Zhao, H. Hidalka, N. Serpone, J. Phys. Chem. B 1998, 102, 5845. [26] J. Zhao, T. Wu, K. Wu, K. Oikawa, H. Hidalka, N. Serpone, Environ. Sci. Technol. 1998, 32, 2394. [27] C. Chen, W. Ma, J. Zhao, Chem. Rev. Soc. 2010, 39, 4206. [28] Y. Jin, Z. Zou, J. Ye, J. Phys. Chem. B 2003, 107, 4936. [29] M. Long, W. Cai, H. Kisch, Chem. Phys. Lett. 2008, 461, 102. [30] A. Fujishima, T. N. Rao, D. A. Tryk, J. Photochem. Photobiol. C 2001, 1, 1.
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Bi2MoO6 Nanobelts for Crystal Facet-Enhanced Photocatalysis Jinlin Long,* Shuchao Wang, Hongjin Chang, Baozhou Zhao, Botong Liu, Yangen Zhou, Wei Wei, Xuxu Wang, Ling Huang,* and Wei Huang*
Tailoring the surface structure of inorganic materials at the nanoscale by varying the chemical synthesis conditions, such as the solvent, surfactant, and temperature,[1–3] has long been employed for endowing them distinctive properties and obtaining atomic-level insight into the structure-performance relationship.[4] Recently, considerable attention has been paid to the facet-controlled fabrication of noble metal nanocrystals and single-crystal metal oxides with well-defined morphologies because of their facet-dependent catalytic, photocatalytic, and other surface-related properties.[5–7] Especially for anisotropic semiconducting oxides, the pursue of intentionally-exposed high-energy crystal planes have not only paved a new direction for enhanced photocatalytic performance, but also offered great opportunities to investigate the relationships between the surface structures (the crystal facets) and the photocatalytic performance. Taking anatase titanium oxide (TiO2) as an example, its low-index facets, i.e., (001), (101), and (010) with well-defined geometric and electronic structures, were well-exploited as the active spots for various types of reactions with enhanced photocatalytic activity,[8–10] compared to those with high-index crystalline materials. Besides anatase TiO2, a few single crystal semiconducting nanomaterials including WO3, BiOCl, AgBr, and Ag3PO4 Dr. J. Long,[+] Dr. Y. Zhou, Prof. X. Wang Research Institute of Photocatalysis State Key Laboratory of Photocatalysis on Energy and Environment College of Chemistry Fuzhou University Fuzhou 350116, China E-mail: [email protected] Tel: +86-591-83779121 Fax: +86-591-83779121 H. Chang, B. Zhao, B. Liu, Prof. L. Huang, W. Huang Institute of Advanced Materials Nanjing Tech University Nanjing 210009, China E-mail: [email protected]; [email protected] Tel: +65 63168825 Dr. J. Long, S. Wang,[+] W. Wei, Prof. L. Huang School of Chemical and Biomedical Engineering Nanyang Technological University 637457, Singapore [+]These authors are contributed equally to this work DOI: 10.1002/smll.201302950 small 2014, DOI: 10.1002/smll.201302950
have also been studied theoretically and experimentally in order to discover the facet-dependent photocatalytic properties.[11–14] Undoubtedly, crystal facet engineering of inorganic semiconductors is an exciting direction with great potential to achieve highly active new-generation catalysts, especially the photocatalysts. Bismuth molybdate (Bi2MoO6), an important aurivillius oxide possessing layered structure with the perovskite-like slab of MoO6, has recently become a popular target due to its excellent photocatalytic performance under ≥420 nm (thereafter, visible-light) irradiation,[15–17] and a great deal of effort has focused on the morphology-controlled fabrication and photocatalytic properties of this material with hierarchical nanostructures.[18–21] However, the facet effect on its photocatalytic activity has rarely been investigated. Herein, we first report the controlled synthesis of single crystal Bi2MoO6 nanobelts (BMNBs) with dominant (010) facets via oleyamine-mediated hydrothermal method, and then studied their facet-dependent photocatalysis behavior for the degradation of Rhodamine B dye (RhB) under visiblelight irradiation. More importantly, we have proved that the as-prepared BMNB exhibits higher activity than the counterpart with hierarchical nanorod-based microspheres, and the according mechanisms are further provided. and To synthesize BMNBs, Bi(NO3)3·5H2O (NH4)6Mo7O24·4H2O were first dissolved in the aqueous solution, and then heated in an autoclave at 160 °C, where oleyamine works as surfactant and NH3·H2O for adjusting the pH value of the reaction solution. The series of transmission electron microscopy (TEM) images in Figure S1 show the morphology variation of the BMNBs synthesized at different pH values and it is evident that the optimal pH value is 7.0, where the nanobelts with lengths of 300–600 nm, widths of 20–30 nm, and thicknesses of 5–10 nm (Figure 1A) are the major product. High resolution TEM (HRTEM) image of the nanobelt (Figure 1B) proves its high crystalline characteristics. The clear lattice fringes with an interplanar spacing of 0.275 nm corresponding to the (200) peak in the XRD pattern of γ-Bi2MoO6 (Figure S2) and an interplanar spacing of 0.274 nm corresponding to the (001) atomic planes can be established to be the crystal growth directions. The corresponding selected-area electron diffraction (SAED) pattern indicates the single-crystal structure of the BMNB sample (inset to Figure 1B). On the basis of the above discussion and the symmetry of orthorhombic Bi2MoO6 single crystal,
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Figure 1. (A) TEM and (B) HRTEM images of BMNB prepared at pH = 7.0, atomic structrue of (C) top-view exposed {001} and (D) {010} facets, and (E) atomic construction model of BMNB.
the bottom and top surfaces of BMNBs are identified as {010} facets and the two lateral surfaces along the growth direction and the two crystal growth planes are {001} and {100} facets, respectively. The percentage of the {010} facet is estimated from the geometric calculation to be ca. 80% for the BMNB sample. The layered structure of γ-Bi2MoO6 can be described as the corner-shared and disordered MoO6 octahedra and planar (Bi2O2)2+ layers[22] as shown in Figure 1C, where we can see the atomic arrangement of the exposed {001} facets of BMNB, and the MoO6 octahedra are sandwiched between the (Bi2O2)2+ layers. The atomic structure of the {010} facet is characterized by the high density of exposed oxygen atoms
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of the (Bi2O2)2+ layers (Figures 1D and 1E). For comparison, γ-Bi2MoO6 hierarchical microstructures were also prepared by reacting Bi(NO3)3·5H2O and (NH4)6Mo7O24·4H2O in ethylene glycol (EG)/ethanol solution and heated in an autoclave. The result shows that in the absence of oleyamine, EG leads to the formation of hierarchical nanorod-based microspheres (BMMSs), rather than the regular nanobelts (Figure 2A). HRTEM image of an individual nanorod (Figure 2B) reveals the lattice fringes with the interplanar spaces of about 0.326 and 0.235 nm corresponding to the {140} and {221} atomic planes, respectively, suggesting no lowindex facets are exposed in the BMMS sample. Therefore, we deduce that the exposure of the dominant {010} facets in
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small 2014, DOI: 10.1002/smll.201302950
Bi2MoO6 Nanobelts for Crystal Facet-Enhanced Photocatalysis
Figure 2. (A) SEM and (B) HRTEM images of the as-prepared BMMS sample. The inset to (A) is the corresponding TEM image of part of the microsphere.
the BMNB sample is because of the adsorption of oleyamine chromospheres proceed synchronously on RhB molecules, molecules on the surface of the nucleating (Bi2O2)2+ layers, suggesting that two kinds of active oxygen species, O2−• and which further regulates the directional growth of the lamellar •OH radicals, are generated under visible–light irradiation in BMNB-catalyzed photo-degradation process. nanobelts. The variation in morphology and exposed crystal facets of To elaborate the detailed photo-degradation process, the BMNB and BMMS samples results in drastically different electron paramagnetic resonance (EPR) was used to detect photoreactivities, which can be evaluated by the decomposi- the formation of these active oxygen species under visible tion of RhB dye in an aqueous solution under visible light light irradiation. Figure 4 shows the change in EPR signals irradiation. Figure 3A demonstrates the temporal evolution of of the BMNB/water suspension with irradiation time. Four the UV-spectral changes collected during the photocatalytic characteristic signals belonging to the DMPO-•OH adduct degradation of RhB dye catalyzed by BMNB. At the reaction appear intermediately in the EPR spectra upon light on, time of 0 min, RhB shows a major absorption band at 552 nm and increases in intensity with irradiation time (Figure 4A), (Figure 3A). Visible light irradiation of the RhB/BMNB dis- suggesting the formation of •OH radical. When the BMNBs persion leads to a rapid decrease of the 552 nm absorption are uniformly suspended into methanol, the DMPO spinwith a concomitant wavelength shift of the feature band to trapping EPR spectra obtained are completely different shorter wavelength, reminiscent of the photodegradation of from those of the aqueous system. As shown in Figure 4B, RhB over CdS, TiO2, and CdS/TiO2 nanocomposites.[23,24] six characteristic signals assigned to DMPO-O2−• adduct are Zhao and co-workers[25–27] have clearly revealed the photo- observed and gradually increase in intensity with irradiation degradation mechanism and that the hypsochromic shift time, indicating the formation of O2−• radicals under visible originates from a stepwise de-ethylation process of the fully light irradiation. Obviously, the •OH radical is originated N,N,N′,N′-tetraethylated rhodamine molecules (552 nm) from the hole oxidation, and the O2−• is due to the electron attacked by •OH radicals, via N,N,N′-triethylated rhoda- reduction. Thus, it can be concluded that the oxidative •OH mine (539 nm), N,N′-diethylated rhodamine (522 nm), N-ethylated rhodamine (510 nm), and finally rhodamine (498 nm). The de-ethylation process can be completed within 120 min for the BMNB photocatalyst, whereas the BMMS photocatalyst takes over 660 min under the same experimental conditions (Figure 3B). While in the absence of γ-Bi2MoO6 photocatalysts, the absorbance decreases only by about 5% after 300 min visible-light irradiation, which is due to the photosensitized self-degradation. These results indicate that BMNB shows a superior photocatalytic activity to the BMMS for RhB dye degradation. Moreover, the spectra evolution of RhB dye in Figure 3A also clearly Figure 3. (A) Photocatalytic decomposition of RhB dye over the BMNB sample under visible demonstrates that the de-ethylation light irradiation. (B) Decoloration rate of the RhB chromophore (a) without catalyst, (b) with and the benzene ring-opening of BMMS, and (c) with BMNB, under visible light irradiation. small 2014, DOI: 10.1002/smll.201302950
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Figure 4. Spin trapping EPR spectra of (a) DMPO-•OH adduct and (b) DMPO-O2• adduct formed upon visible light irradiation of BMNB.
and O2−• radicals are predominantly responsible for the photo-degradation of the dye pollutant. In order to discover the origin of the facet-enhanced photocatalysis, the UV-Vis diffuse reflectance spectra (DRS) of BMNB and BMMS are collected. As shown in Figure S3, both samples have clear absorption at ca. 468 nm, indicating no difference in the light absorption efficiency between them. Based on the equation Eg = 1240/λ, the optical band-gap energy of BMNB and BMMS is equal to 2.65 eV. For a crystalline semiconductor, the optical band-gap energy also follows the equation of F(R)E =A(E-Eg)n/2, where F(R), E, A, n, and Eg are diffuse absorption coefficient, photon energy, proportionality constant, an integer, and band-gap, respectively. The integer n depends on the optical transition (n = 1, 2, 4, 6), and the value of n determined for BMNB is 4, indicating that the semiconductor has an indirect transition.[28] Long et al.[29] determined the flat-band potential of γ-Bi2MoO6 to be −0.32 eV versus NHE at pH = 7.0, which is more negative than the redox potential of O2−•/O2 (0.28 eV).[30] Combined with the equation of EVB = ECB + Eg, where EVB, ECB and Eg are the valence band, conduction band, and bandgap energy of the semiconductor, respectively, the edge
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of the valence band of BMNB is estimated to be +2.33 eV, which is more positive than the redox potential of •OH/H2O (+1.23 eV). This indicates that the electrons and holes generated on BMNBs upon visible light irradiation can react with the adsorbed O2 and H2O to form active oxygen species, O2−• and •OH radicals, respectively. Theoretical calculation[15,16] has already revealed that the visible light absorption of γ-Bi2MoO6 is originated from the transition of the valence band consisting of the O 2p orbitals to the conduction band derived from the Mo 4d orbitals in MoO6 octahedral and the Bi 6p orbitals in the (Bi2O2)2+ layers. The structural analysis of BMNB indicates that majority of the oxygen atoms in the valence band sitting on the {010} facet and the Mo and Bi atoms of the conduction band are placed on the {100} and {001} facets. They are completely exposed to the reacting substrates including O2, H2O, and RhB dye molecules. Consequently, the photogenerated holes located at the valence band are directly available to H2O and RhB molecules, and the photogenerated electrons located at the conduction band are directly available to O2 molecules. It is evident that in BMNB, the charge carriers can be efficiently separated due to the exposure of more photoactive sites. Considering the similar light absorption and band structure of the BMNB and BMMS, we believe that the higher photoreactivity of BMNB is resulted from the faster separation of charge carriers, which is well-known to be one of the important factors that govern the photocatalysis efficiency, and more photoactive sites exposed on the surface of BMNB. In summary, we have successfully prepared γ-Bi2MoO6 single-crystal nanobelts and further demonstrated their crystal facet-enhanced photocatalytic characteristics, which is because of the direct exposure of more photoactive sites to the reacting substrates and sequentially the charge carriers in the nanobelts can be more efficiently utilized. The experimental results have proved that H2O molecules are oxidized by photogenerated holes to •OH radials on the {010} facets, and O2 molecules are reduced by photogenerated electrons to O2−• radials on the {100} and {001} facets. This work has indicated that crystal facet engineering is a promising strategy for deep understanding of morphology-dependent photocatalysis, the development of new semiconducting photocatalysts, as well as the fine manipulation of their photoreactivities.
Experimental Section Preparation of γ-Bi2MoO6 Single-Crystal Nanobelts: The γ-Bi2MoO6 single-crystal nanobelts were prepared using the oleyamine-mediated hydrothermal procedure. Typically, 0.485 g of Bi(NO3)3·5H2O and 0.089 g of (NH4)6Mo7O24·4H2O were completely dissolved in 10 mL of diluted HNO3 (1 mMol/L) and 10 mL of deionized water, respectively, and then 1 mL of oleyamine and the Bi(NO3)3 solution were added to the (NH4)6Mo7O24 aqueous solution under magnetic stirring. The pH value of solution was adjusted by NH3·H2O. Finally, the resultant suspension was transferred into a 50 mL autoclave and heated at 160 °C for 10 h. After this, the autoclave was cooled down to room temperature in air, and the resulting solid was separated by a centrifuge and washed several with ethanol, finally dried at 120 °C overnight.
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
small 2014, DOI: 10.1002/smll.201302950
Bi2MoO6 Nanobelts for Crystal Facet-Enhanced Photocatalysis
Preparation of γ-Bi2MoO6 Nanorod-Based Microspheres: The γ-Bi2MoO6 nanorod-based microspheres were prepared using the EG-mediated solvothermal procedure. Typically, 0.210 g of Bi(NO3)3·5H2O and 0.0382 g of (NH4)6Mo7O24·4H2O were completely dissolved in 15 mL of EG and 25 mL ethanol, respectively, and then the two solutions were mixed together under magnetic stirring. The resulting clear solution was transferred to a 50 mL autoclave, which was heated at 160 °C and maintained for 24 h. Then, the autoclave was cooled down to room temperature in air, and the resulted solid was separated by centrifugation, washed several times with ethanol, and finally dried at 120 °C overnight. Catalyst Characterizations: XRD measurements were performed on a Bruker D8 Advance X–ray diffractometer using Cu Kα1 radiation (λ = 1.5406 Å). The accelerating voltage and the applied current were 40 kV and 40 mA, respectively. HRTEM images were obtained by a JEOL model JEM 2010 EX instrument at an accelerating voltage of 200 kV. EPR spectra were recorded on a Bruker A-300-EPR X-band spectrometer. The UV/Vis diffuse reflectance spectrum was done on Perkin Elmer Lambda 950 UV-Vis-NIR system. Photocatalytic Testing: The photocatalytic activities for RhB degradation under visible light irradiation were performed in a 250 mL beak. The visible light source was a 300 W xenon lamp with a cut-off filter (λ ≥ 420 nm) positioned above a cylindrical reaction vessel. 0.05 g of the catalyst was suspended in 100 mL water containing 10 ppm RhB dye. The suspensions were prestirred magnetically in the dark for 1 h to ensure the equilibrium of the work solution. At given time intervals, 2 mL aliquots were sampled and filtered to remove the catalyst. The degraded solutions were analyzed using a Varian Cary 50 Scan UV-Vis spectrophotometer, and the absorption peak at 522 nm was monitored.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements This work was financially supported by the NSFC (Grant Nos. 21373051 and U1305242), National Basic Research Program of China (973 Program, No. 2012CB722607), and the Science and Technology Project of Education Office of Fujian Province, P. R. China (JA12017), LH thanks the financial support from NTU Start-Up Grant and Tier 1 Grant (RG20/09) from Ministry of Education, Singapore., and the financial support from the NSFC (Grant No. 21371095).
small 2014, DOI: 10.1002/smll.201302950
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