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α-Fe2O3 decorated ZnO nanorod-assembled hollow microspheres: Synthesis and enhanced visible-light photocatalysis
Materials Letters ∎ (∎∎∎∎) ∎∎∎–∎∎∎
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Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
α-Fe2O3 decorated ZnO nanorod-assembled hollow microspheres: Synthesis and enhanced visible-light photocatalysis Qiaoqiao Yin, Ru Qiao n, Lanlan Zhu, Zhengquan Li, Miaomiao Li, Wenjie Wu College of Chemistry and Life Sciences, Zhejiang Normal University, Jinhua 321004, China
art ic l e i nf o
a b s t r a c t
Article history: Received 20 June 2014 Accepted 25 July 2014
α-Fe2O3 nanoparticles-decorated ZnO nanorod-assembled hollow spheres were successfully synthesized by a simple acetone-assisted impregnation deposition. SEM images showed a uniform distribution of α-Fe2O3 nanoparticles not only on the surfaces of ZnO spheres but also inside pores of ZnO parallel aligned nanorods. The α-Fe2O3/ZnO binary semiconductor composites exhibited a superior visible-light photocatalytic activity for degradation of rhodamine B (RhB) as compared with pure ZnO spheres. The enhancement was attributed to stronger visible light absorption ability and effective photogenerated charge separation of α-Fe2O3/ZnO composites. & 2014 Published by Elsevier B.V.
Keywords: ZnO α-Fe2O3 Impregnation deposition Semiconductors Microstructure Photocatalysis
1. Introduction In semiconductor photocatalysts, ZnO is usually considered as an alternative photocatalyst to TiO2 [1–4] for use in solar cells, pollutant degradation, photolysis of water, gas sensor and biological application due to its unique physico-chemical properties. However, ZnO cannot be utilized for direct solar irradiation because of its quick recombination of charge carriers, poor response to visible light and critical drawback of photocorrosion [5,6]. To solve these problems, several methods, including developing ZnO-based heterostructures or composites with electron scavenging agents such as metals, metal oxides or organic molecules [7–12] have been made to reinforce the photocatalytic performance by improving the separation of electron– hole pairs and enhancing the photostability of ZnO. α-Fe2O3, a narrow band gap ( E1.9–2.2 eV) n-type semiconductor with a wide photoelectrochemical response under visible light, is a possible candidate which can serve as a sensitizer under sunlight irradiation because it is able to transfer electrons to large band gap semiconductors such as TiO2 and ZnO. Therefore, a superior photocatalyst combining the merits of both ZnO and α-Fe2O3 might be expected through the construction of; α-Fe2O3/ ZnO semiconductor hierarchical structures. Herein, we report our recent efforts on the synthesis of α-Fe2O3 nanoparticles (NPs)-deposited ZnO nanorod-assembled hollow spheres by impregnation deposition. The visible-light photocatalytic activities of the binary composites were evaluated using the
n
Corresponding author. Tel.: þ 86 15888928162; fax: þ86 579 82282269. E-mail address: [email protected] (R. Qiao).
degradation of RhB as a model reaction. It is found that these α-Fe2O3/ZnO heterostructural composites show enhanced visible light absorption and photocatalytic activity for degrading RhB.
2. Experimental Synthesis: ZnO nanorod-assembled hollow microspheres were synthesized using the method reported by Yuan et al. [13] with modification. Briefly, 2 mmol of Zn(Ac)2∙2H2O was dissolved in 5 mL of distilled water and then mixed with 30 mL of ethylene glycol under magnetic stirring to form a clear solution. Subsequently, the solution was transferred to a 50 mL Teflon-lined autoclave and maintained at 150 1C for 2 h. After the experiment, the precipitates were collected, washed with water and ethanol several times, and then dried at 60 1C. α-Fe2O3 NPs-decorated ZnO composites were synthesized by an acetone-assisted impregnation method. Typically, 50 mg of the above ZnO was dispersed in 50 mL of a certain mass concentration of Fe(NO3)3 acetone solution by ultrasonication, followed by vigorous stirring for 2 h. Then, the reaction temperature was elevated to 50 1C and 5 mL of 10 wt% ammonia solution was added dropwise. After 2 h of reaction, the resulting precipitates were collected, washed with acetone, dried at 60 1C, and finally annealed at 400 1C for 3 h in air atmosphere. Four α-Fe2O3/ZnO samples obtained from a series of iron precursor solutions with different concentrations (2 wt%, 5 wt%, 7.5 wt% and 10 wt%) were named FZ-1, FZ-2, FZ-3 and FZ-4, respectively. For comparison, α-Fe2O3 NPs were also synthesized by this impregnation method without adding ZnO products.
http://dx.doi.org/10.1016/j.matlet.2014.07.149 0167-577X/& 2014 Published by Elsevier B.V.
Please cite this article as: Yin Q, et al. α-Fe2O3 decorated ZnO nanorod-assembled hollow microspheres: Synthesis and enhanced visible-light photocatalysis. Mater Lett (2014), http://dx.doi.org/10.1016/j.matlet.2014.07.149i
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Q. Yin et al. / Materials Letters ∎ (∎∎∎∎) ∎∎∎–∎∎∎
Characterization: The products were characterized by field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), UV–vis spectroscopy and X-ray photoelectron spectroscopy (XPS). In XPS measurements, all the spectra were calibrated to the binding energy of the adventitious C 1s peak at 284.8 eV. Photocatalytic tests: The photocatalytic activities were evaluated by degradation of RhB under visible-light irradiation. A Xenon lamp (HSX-UV300W, NBet) with a UV cut-off 400 nm filter acted as a visible light source. 20 mg of the photocatalysts was added to 50 ml of RhB solution (5 mg L 1). Before irradiation, the suspension was magnetically stirred in dark for 30 min to ensure an adsorption–desorption equilibrium. During the experiment, 5 mL
of the suspension was taken at 30 min intervals and centrifuged for subsequent RhB absorbance analysis. The change of RhB absorbance in the solution was used to monitor the extent of degradation at given time intervals.
3. Results and discussion As shown in Fig. 1a, the ZnO obtained after heating at 150 1C for 2 h consists of a high yield of flowerlike hollow microspheres 2–2.5 μm in size although some of them are broken. The close-up views of cracked spheres (Fig. 1b and its inset) reveal that the ZnO spheres are self-assembled hierarchical structures that are composed
Fig. 1. (a, b) FESEM images of ZnO hollow microspheres. Inset of (b): High-magnification FESEM image of ZnO sphere. (c, d) Low- and (e, f) High-magnification FESEM images of α-Fe2O3/ZnO composites (FZ-3). (g) Low- and (h) High-magnification TEM images of FZ-3 sample.
Please cite this article as: Yin Q, et al. α-Fe2O3 decorated ZnO nanorod-assembled hollow microspheres: Synthesis and enhanced visible-light photocatalysis. Mater Lett (2014), http://dx.doi.org/10.1016/j.matlet.2014.07.149i
Q. Yin et al. / Materials Letters ∎ (∎∎∎∎) ∎∎∎–∎∎∎
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of many elongated ZnO nanorods as building units, and the nanorods align parallel with one another to form a sphere with an interior cavity. These nanorods interweave together, forming discernible pores with irregular size throughout the sphere. The shell wall of individual microsphere is approximately 300–500 nm in uniform thickness. The unique hierarchical structure with interconnected but a periodic pore channels can serve as the transport paths for small molecules, which is an attractive feature for heterogeneous catalysis. Based on the special structure of ZnO hollow spheres, we synthesized α-Fe2O3 NPs-decorated ZnO composites via an
Fig. 2. XRD patterns of the undecorated ZnO microspheres, α-Fe2O3 nanoparticles, and FZ-3 sample.
3
impregnation deposition. Fig. 1c and d is SEM images of the α-Fe2O3/ZnO composite products. Fig. 1e and f clearly indicates that small Fe2O3 NPs have been grown in situ and homogeneously deposited not only on ZnO spherical surface but also inside pores of ZnO self-assembled nanorods, meanwhile, the hierarchical structure of ZnO is still maintained. The morphology features of as-obtained FZ-3 sample were further investigated by TEM. A close observation of a single α-Fe2O3/ZnO sphere clearly shows the decoration of Fe2O3 NPs on ZnO sphere (Fig. 1g). Besides, the interplanar spacings of 0.28 and 0.27 nm obtained by HRTEM (Fig. 1h) can be indexed to the (100) plane of ZnO and (104) plane of α-Fe2O3, respectively. XRD analysis was carried out to investigate the crystal phases of the as-synthesized products, shown in Fig. 2. The green curve shows two sets of diffraction peaks for FZ-3 sample well indexed to hexagonal wurtzite ZnO (JCPDS no. 36-1451) and rhombohedral hematite α-Fe2O3 (JCPDS no. 33-0664). No additional peaks are observed, which confirms that the sample only contained nanocrystalline ZnO and α-Fe2O3. The chemical composition of FZ-3 sample was further analyzed by XPS. The survey spectrum (Fig. 3a) shows the presence of Zn, Fe, O, C elements. Fig. 3b is the high resolution spectrum of Zn, the peaks at 1022.0 eV and 1045.0 eV correspond to Zn 2p3/2 and 2p1/2, respectively. The O 1s peak (Fig. 3c) could be resolved into three peaks by the XPS peak fitting program. The peak at 530.2 eV and 530.5 eV can be attributed to oxygen atoms in the lattice bounding to Zinc (Zn–O) in hexagonal wurtzite ZnO [14] and iron atoms (Fe O) in rhombohedral hematite α-Fe2O3, respectively. And the other peak centered at 531.7 eV could be ascribed to adsorbed hydroxyl species [15]. In the XPS spectra of Fe 2p core level (Fig. 3d), besides two main peaks at 710.6 eV and 724.2 eV corresponding to Fe 2p3/2 and 2p1/2 of α-Fe2O3, the shakeup
Fig. 3. XPS spectra of FZ-3 sample: (a) survey spectrum, (b)–(d) high-resolution binding energy spectra of Zn 2p, O 1s and Fe 2p, respectively. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
Please cite this article as: Yin Q, et al. α-Fe2O3 decorated ZnO nanorod-assembled hollow microspheres: Synthesis and enhanced visible-light photocatalysis. Mater Lett (2014), http://dx.doi.org/10.1016/j.matlet.2014.07.149i
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Q. Yin et al. / Materials Letters ∎ (∎∎∎∎) ∎∎∎–∎∎∎
Fig. 4. (a) UV–vis diffuse reflectance spectra of the undecorated ZnO microspheres and FZ-3 sample; (b) Photodegradation of RhB under visible light irradiation by blank, ZnO hollow microspheres, α-Fe2O3 nanoparticles and α-Fe2O3/ZnO composites with increasing amounts of α-Fe2O3.
satellite structures are also observed at the higher binding energy sides of the main peaks, indicated by arrows. Those satellite peaks are the finger prints of the electronic structures of Fe3 þ [16], confirming the formation of α-Fe2O3 in FZ-3 composite. The XPS analysis result is in accordance with the result of the XRD pattern. The optical properties of the pure ZnO hollow microspheres and FZ-3 sample were then investigated by UV vis diffuse reflectance (Fig. 4a). It is seen that the wavelength of the absorption edge for pure ZnO is located at 387 nm, which can be assigned to the intrinsic absorption band derived from the band gap transition. An obvious red-shift of 15 nm is observed in the absorption edge of α-Fe2O3/ZnO in comparison with pure ZnO. This “red-shift” can be attributed to formation of Fe–O–Zn bond after Fe2O3 coating formed. In addition, the absorption plot of FZ-3 is much higher than that of pure ZnO, indicating the improved lightabsorption ability of the composites. Therefore, after coupling with the narrow band gap of α-Fe2O3, α-Fe2O3/ZnO hierarchical structures increase the utilization of visible light, which could accordingly lead to a higher visible-light photocatalytic activity. The degradation of RhB under visible-light irradiation was used to evaluate the photocatalytic activity of pure ZnO spheres, α-Fe2O3 nanoparticles, and α-Fe2O3/ZnO composites with varying α-Fe2O3 mass loading. All α-Fe2O3/ZnO binary composite samples exhibit higher photocatalytic activity than ZnO and α-Fe2O3 (Fig. 4b), and FZ-3 has the best photocatalytic performance among them. The enhancement of the photocatalytic efficiency of FZ-3 could be ascribed to favorable synergistic effect between α-Fe2O3 and ZnO. The position of ZnO and α-Fe2O3 bands has a type-II alignment where the conduction band (CB) edge of ZnO is located between the CB and the valence band (VB) of α-Fe2O3 [17]. In this configuration, once the electrons in the VB of α-Fe2O3 are excited to the CB under irradiation, the photo-induced electrons will transfer to the CB of ZnO, living photo-induced holes on the VB of α-Fe2O3. Therefore, the electron hole pairs could be separated more efficiently at the interface of the heterostructure, which increases the lifetime of charge carriers and enhances the interfacial charge transfer, resulting in high activity of the α-Fe2O3/ZnO photocatalysts.
4. Conclusions In summary, α-Fe2O3 NPs decorated ZnO nanorod-assembled hollow microspheres were prepared by a simple acetone-assisted impregnation method. The α-Fe2O3/ZnO composites exhibit an excellent photocatalytic activity for RhB degradation under visible light irradiation. This behavior was attributed to enhanced visible light absorption ability and efficient separation of charge carriers.
Acknowledgments This work is supported by NSFC (Grant nos. 21201151, 21273203) Q2 and Open Research Fund of Jiangsu Key Laboratory of Environmental Material and Environmental Engineering (Grant no. K13069). References [1] Zhang H, Chen G, Bahnemann DW. J Mater Chem 2009;19:5089–121. [2] Hernandez-Alonso MD, Fresno F, Sareza S, Coronado JM. Energy Environ Sci 2009;2:1231–7. [3] Hu JL, Qian HS, Li JJ, Hu Y, Li ZQ, Yu SH. Part Part Syst Charact 2013;30:306–10. [4] Hu JL, Qian HS, Hu Y, Li ZQ, Tong GX, Ying TK, et al. CrystEngComm 2012;141:7118–22. [5] Fox MA, Dulay MT. Chem Rev 1993;93:341–57. [6] De Jongh PE, Meulenkamp EA, Vanmaekelbergh D, Kelly JJ. J Phys Chem B 2000;104:7686–93. [7] Deng Q, Duan XW, HL Ng D, Tang HB, Yang Y, Kong MG, et al. Appl Mater Interfaces 2012;4:6030–7. [8] Fu HB, Xu TG, Zhu SB, Zhu YF. Environ Sci Technol 2008;42:8064–9. [9] Zhang ZY, Shao CL, Li XH, Zhang L, Xue HM, Wang CH, et al. J Phys Chem C 2010;114:7920–5. [10] Cho S, Jang JW, Lee JS, Lee KH. Nanoscale 2012;4:2066–71. [11] Khanchandani S, Kundu S, Patra A, Ganguli KA. J Phys Chem C 2012;116:23653–62. [12] Balachandran S, Swaminathan M. J Phys Chem C 2012;116:26306–12. [13] Hu P, Zhang X, Han N, Xiang WC, Cao YB, Yuan FL. Cryst Growth Des 2011;11:1520–6. [14] Yu J, Yu X. Environ Sci Technol 2008;42:4902–7. [15] Ballerini G, Ogle K, Labrousse MGB. Appl Surf Sci 2007;253:6860–7. [16] Fujii T, de Groot FMF, Sawatzky GA, Voogt FC, Hibma T, Okada K. Phys Rev B 1999;59:3195–202. [17] Wu W, Zhang SF, Xiao XH, Zhou J, Ren F, Sun LL, et al. Appl Mater Interfaces 2012;4:3602–9.
Please cite this article as: Yin Q, et al. α-Fe2O3 decorated ZnO nanorod-assembled hollow microspheres: Synthesis and enhanced visible-light photocatalysis. Mater Lett (2014), http://dx.doi.org/10.1016/j.matlet.2014.07.149i
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Q1 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
α-Fe2O3 decorated ZnO nanorod-assembled hollow microspheres: Synthesis and enhanced visible-light photocatalysis Qiaoqiao Yin, Ru Qiao n, Lanlan Zhu, Zhengquan Li, Miaomiao Li, Wenjie Wu College of Chemistry and Life Sciences, Zhejiang Normal University, Jinhua 321004, China
art ic l e i nf o
a b s t r a c t
Article history: Received 20 June 2014 Accepted 25 July 2014
α-Fe2O3 nanoparticles-decorated ZnO nanorod-assembled hollow spheres were successfully synthesized by a simple acetone-assisted impregnation deposition. SEM images showed a uniform distribution of α-Fe2O3 nanoparticles not only on the surfaces of ZnO spheres but also inside pores of ZnO parallel aligned nanorods. The α-Fe2O3/ZnO binary semiconductor composites exhibited a superior visible-light photocatalytic activity for degradation of rhodamine B (RhB) as compared with pure ZnO spheres. The enhancement was attributed to stronger visible light absorption ability and effective photogenerated charge separation of α-Fe2O3/ZnO composites. & 2014 Published by Elsevier B.V.
Keywords: ZnO α-Fe2O3 Impregnation deposition Semiconductors Microstructure Photocatalysis
1. Introduction In semiconductor photocatalysts, ZnO is usually considered as an alternative photocatalyst to TiO2 [1–4] for use in solar cells, pollutant degradation, photolysis of water, gas sensor and biological application due to its unique physico-chemical properties. However, ZnO cannot be utilized for direct solar irradiation because of its quick recombination of charge carriers, poor response to visible light and critical drawback of photocorrosion [5,6]. To solve these problems, several methods, including developing ZnO-based heterostructures or composites with electron scavenging agents such as metals, metal oxides or organic molecules [7–12] have been made to reinforce the photocatalytic performance by improving the separation of electron– hole pairs and enhancing the photostability of ZnO. α-Fe2O3, a narrow band gap ( E1.9–2.2 eV) n-type semiconductor with a wide photoelectrochemical response under visible light, is a possible candidate which can serve as a sensitizer under sunlight irradiation because it is able to transfer electrons to large band gap semiconductors such as TiO2 and ZnO. Therefore, a superior photocatalyst combining the merits of both ZnO and α-Fe2O3 might be expected through the construction of; α-Fe2O3/ ZnO semiconductor hierarchical structures. Herein, we report our recent efforts on the synthesis of α-Fe2O3 nanoparticles (NPs)-deposited ZnO nanorod-assembled hollow spheres by impregnation deposition. The visible-light photocatalytic activities of the binary composites were evaluated using the
n
Corresponding author. Tel.: þ 86 15888928162; fax: þ86 579 82282269. E-mail address: [email protected] (R. Qiao).
degradation of RhB as a model reaction. It is found that these α-Fe2O3/ZnO heterostructural composites show enhanced visible light absorption and photocatalytic activity for degrading RhB.
2. Experimental Synthesis: ZnO nanorod-assembled hollow microspheres were synthesized using the method reported by Yuan et al. [13] with modification. Briefly, 2 mmol of Zn(Ac)2∙2H2O was dissolved in 5 mL of distilled water and then mixed with 30 mL of ethylene glycol under magnetic stirring to form a clear solution. Subsequently, the solution was transferred to a 50 mL Teflon-lined autoclave and maintained at 150 1C for 2 h. After the experiment, the precipitates were collected, washed with water and ethanol several times, and then dried at 60 1C. α-Fe2O3 NPs-decorated ZnO composites were synthesized by an acetone-assisted impregnation method. Typically, 50 mg of the above ZnO was dispersed in 50 mL of a certain mass concentration of Fe(NO3)3 acetone solution by ultrasonication, followed by vigorous stirring for 2 h. Then, the reaction temperature was elevated to 50 1C and 5 mL of 10 wt% ammonia solution was added dropwise. After 2 h of reaction, the resulting precipitates were collected, washed with acetone, dried at 60 1C, and finally annealed at 400 1C for 3 h in air atmosphere. Four α-Fe2O3/ZnO samples obtained from a series of iron precursor solutions with different concentrations (2 wt%, 5 wt%, 7.5 wt% and 10 wt%) were named FZ-1, FZ-2, FZ-3 and FZ-4, respectively. For comparison, α-Fe2O3 NPs were also synthesized by this impregnation method without adding ZnO products.
http://dx.doi.org/10.1016/j.matlet.2014.07.149 0167-577X/& 2014 Published by Elsevier B.V.
Please cite this article as: Yin Q, et al. α-Fe2O3 decorated ZnO nanorod-assembled hollow microspheres: Synthesis and enhanced visible-light photocatalysis. Mater Lett (2014), http://dx.doi.org/10.1016/j.matlet.2014.07.149i
2
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66
Q. Yin et al. / Materials Letters ∎ (∎∎∎∎) ∎∎∎–∎∎∎
Characterization: The products were characterized by field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), UV–vis spectroscopy and X-ray photoelectron spectroscopy (XPS). In XPS measurements, all the spectra were calibrated to the binding energy of the adventitious C 1s peak at 284.8 eV. Photocatalytic tests: The photocatalytic activities were evaluated by degradation of RhB under visible-light irradiation. A Xenon lamp (HSX-UV300W, NBet) with a UV cut-off 400 nm filter acted as a visible light source. 20 mg of the photocatalysts was added to 50 ml of RhB solution (5 mg L 1). Before irradiation, the suspension was magnetically stirred in dark for 30 min to ensure an adsorption–desorption equilibrium. During the experiment, 5 mL
of the suspension was taken at 30 min intervals and centrifuged for subsequent RhB absorbance analysis. The change of RhB absorbance in the solution was used to monitor the extent of degradation at given time intervals.
3. Results and discussion As shown in Fig. 1a, the ZnO obtained after heating at 150 1C for 2 h consists of a high yield of flowerlike hollow microspheres 2–2.5 μm in size although some of them are broken. The close-up views of cracked spheres (Fig. 1b and its inset) reveal that the ZnO spheres are self-assembled hierarchical structures that are composed
Fig. 1. (a, b) FESEM images of ZnO hollow microspheres. Inset of (b): High-magnification FESEM image of ZnO sphere. (c, d) Low- and (e, f) High-magnification FESEM images of α-Fe2O3/ZnO composites (FZ-3). (g) Low- and (h) High-magnification TEM images of FZ-3 sample.
Please cite this article as: Yin Q, et al. α-Fe2O3 decorated ZnO nanorod-assembled hollow microspheres: Synthesis and enhanced visible-light photocatalysis. Mater Lett (2014), http://dx.doi.org/10.1016/j.matlet.2014.07.149i
Q. Yin et al. / Materials Letters ∎ (∎∎∎∎) ∎∎∎–∎∎∎
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 Q3 66
of many elongated ZnO nanorods as building units, and the nanorods align parallel with one another to form a sphere with an interior cavity. These nanorods interweave together, forming discernible pores with irregular size throughout the sphere. The shell wall of individual microsphere is approximately 300–500 nm in uniform thickness. The unique hierarchical structure with interconnected but a periodic pore channels can serve as the transport paths for small molecules, which is an attractive feature for heterogeneous catalysis. Based on the special structure of ZnO hollow spheres, we synthesized α-Fe2O3 NPs-decorated ZnO composites via an
Fig. 2. XRD patterns of the undecorated ZnO microspheres, α-Fe2O3 nanoparticles, and FZ-3 sample.
3
impregnation deposition. Fig. 1c and d is SEM images of the α-Fe2O3/ZnO composite products. Fig. 1e and f clearly indicates that small Fe2O3 NPs have been grown in situ and homogeneously deposited not only on ZnO spherical surface but also inside pores of ZnO self-assembled nanorods, meanwhile, the hierarchical structure of ZnO is still maintained. The morphology features of as-obtained FZ-3 sample were further investigated by TEM. A close observation of a single α-Fe2O3/ZnO sphere clearly shows the decoration of Fe2O3 NPs on ZnO sphere (Fig. 1g). Besides, the interplanar spacings of 0.28 and 0.27 nm obtained by HRTEM (Fig. 1h) can be indexed to the (100) plane of ZnO and (104) plane of α-Fe2O3, respectively. XRD analysis was carried out to investigate the crystal phases of the as-synthesized products, shown in Fig. 2. The green curve shows two sets of diffraction peaks for FZ-3 sample well indexed to hexagonal wurtzite ZnO (JCPDS no. 36-1451) and rhombohedral hematite α-Fe2O3 (JCPDS no. 33-0664). No additional peaks are observed, which confirms that the sample only contained nanocrystalline ZnO and α-Fe2O3. The chemical composition of FZ-3 sample was further analyzed by XPS. The survey spectrum (Fig. 3a) shows the presence of Zn, Fe, O, C elements. Fig. 3b is the high resolution spectrum of Zn, the peaks at 1022.0 eV and 1045.0 eV correspond to Zn 2p3/2 and 2p1/2, respectively. The O 1s peak (Fig. 3c) could be resolved into three peaks by the XPS peak fitting program. The peak at 530.2 eV and 530.5 eV can be attributed to oxygen atoms in the lattice bounding to Zinc (Zn–O) in hexagonal wurtzite ZnO [14] and iron atoms (Fe O) in rhombohedral hematite α-Fe2O3, respectively. And the other peak centered at 531.7 eV could be ascribed to adsorbed hydroxyl species [15]. In the XPS spectra of Fe 2p core level (Fig. 3d), besides two main peaks at 710.6 eV and 724.2 eV corresponding to Fe 2p3/2 and 2p1/2 of α-Fe2O3, the shakeup
Fig. 3. XPS spectra of FZ-3 sample: (a) survey spectrum, (b)–(d) high-resolution binding energy spectra of Zn 2p, O 1s and Fe 2p, respectively. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
Please cite this article as: Yin Q, et al. α-Fe2O3 decorated ZnO nanorod-assembled hollow microspheres: Synthesis and enhanced visible-light photocatalysis. Mater Lett (2014), http://dx.doi.org/10.1016/j.matlet.2014.07.149i
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Q. Yin et al. / Materials Letters ∎ (∎∎∎∎) ∎∎∎–∎∎∎
Fig. 4. (a) UV–vis diffuse reflectance spectra of the undecorated ZnO microspheres and FZ-3 sample; (b) Photodegradation of RhB under visible light irradiation by blank, ZnO hollow microspheres, α-Fe2O3 nanoparticles and α-Fe2O3/ZnO composites with increasing amounts of α-Fe2O3.
satellite structures are also observed at the higher binding energy sides of the main peaks, indicated by arrows. Those satellite peaks are the finger prints of the electronic structures of Fe3 þ [16], confirming the formation of α-Fe2O3 in FZ-3 composite. The XPS analysis result is in accordance with the result of the XRD pattern. The optical properties of the pure ZnO hollow microspheres and FZ-3 sample were then investigated by UV vis diffuse reflectance (Fig. 4a). It is seen that the wavelength of the absorption edge for pure ZnO is located at 387 nm, which can be assigned to the intrinsic absorption band derived from the band gap transition. An obvious red-shift of 15 nm is observed in the absorption edge of α-Fe2O3/ZnO in comparison with pure ZnO. This “red-shift” can be attributed to formation of Fe–O–Zn bond after Fe2O3 coating formed. In addition, the absorption plot of FZ-3 is much higher than that of pure ZnO, indicating the improved lightabsorption ability of the composites. Therefore, after coupling with the narrow band gap of α-Fe2O3, α-Fe2O3/ZnO hierarchical structures increase the utilization of visible light, which could accordingly lead to a higher visible-light photocatalytic activity. The degradation of RhB under visible-light irradiation was used to evaluate the photocatalytic activity of pure ZnO spheres, α-Fe2O3 nanoparticles, and α-Fe2O3/ZnO composites with varying α-Fe2O3 mass loading. All α-Fe2O3/ZnO binary composite samples exhibit higher photocatalytic activity than ZnO and α-Fe2O3 (Fig. 4b), and FZ-3 has the best photocatalytic performance among them. The enhancement of the photocatalytic efficiency of FZ-3 could be ascribed to favorable synergistic effect between α-Fe2O3 and ZnO. The position of ZnO and α-Fe2O3 bands has a type-II alignment where the conduction band (CB) edge of ZnO is located between the CB and the valence band (VB) of α-Fe2O3 [17]. In this configuration, once the electrons in the VB of α-Fe2O3 are excited to the CB under irradiation, the photo-induced electrons will transfer to the CB of ZnO, living photo-induced holes on the VB of α-Fe2O3. Therefore, the electron hole pairs could be separated more efficiently at the interface of the heterostructure, which increases the lifetime of charge carriers and enhances the interfacial charge transfer, resulting in high activity of the α-Fe2O3/ZnO photocatalysts.
4. Conclusions In summary, α-Fe2O3 NPs decorated ZnO nanorod-assembled hollow microspheres were prepared by a simple acetone-assisted impregnation method. The α-Fe2O3/ZnO composites exhibit an excellent photocatalytic activity for RhB degradation under visible light irradiation. This behavior was attributed to enhanced visible light absorption ability and efficient separation of charge carriers.
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Please cite this article as: Yin Q, et al. α-Fe2O3 decorated ZnO nanorod-assembled hollow microspheres: Synthesis and enhanced visible-light photocatalysis. Mater Lett (2014), http://dx.doi.org/10.1016/j.matlet.2014.07.149i