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Facile synthesis and enhanced photocatalytic activity of hierarchical porous ZnO microspheres
Materials Letters 66 (2012) 72–75
Contents lists available at SciVerse ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Facile synthesis and enhanced photocatalytic activity of hierarchical porous ZnO microspheres Aihua Lei a, Baihua Qu a,⁎, Weichang Zhou a, Yanguo Wang a, Qinglin Zhang a, Bingsuo Zou a, b,⁎ a b
Key Laboratory for Micro-Nano Optoelectronic Devices of Ministry of Education, Hunan University, Changsha 410082, China School of Materials Science and Engineering, Beijing Institute of Technology, Beijing 100081, China
a r t i c l e
i n f o
Article history: Received 22 July 2011 Accepted 4 August 2011 Available online 11 August 2011 Keywords: Semiconductors Microstructure ZnO Porous microspheres Photocatalytic activity
a b s t r a c t In this paper, hierarchical porous ZnO microspheres were successfully synthesized by calcining the microspheric zinc hydroxide carbonate (ZHC) precursor, which were the precipitate products of a hydrothermal reaction by zinc nitrate hexahydrate and urea in the presence of trisodium citrate. The as-prepared ZnO microspheres with diameters of 4–5 μm were assembled by numerous porous nanosheets which had the uniform thickness of about 10 nm. The photocatalytic activity of the ZnO microspheres was evaluated by photodegradation reaction of methylene blue (MB). The as-prepared ZnO microspheres exhibited a significantly enhanced photocatalytic activity than commercial ZnO and TiO2. This was mainly attributed to the larger specific surface area and stability against aggregation. © 2011 Elsevier B.V. All rights reserved.
1. Introduction ZnO, with a wide band gap (3.37 eV) and large exciton binding energy (60 meV), has stimulated intensive research interest due to its specific optical and electrical properties that were useful for nanolasers [1], gas sensors [2], solar cell [3], photocatalyst [4], and so on. Over the past few years, well-defined ZnO one-dimensional (1D) and two-dimensional (2D) nanostructures with various morphologies such as nanorods [5], nanowires [6], nanotubes [7], and nanoplates [8] have been successfully fabricated by a variety of routes. Recently, three-dimensional (3D) hierarchical ZnO architectures which are assembled from zero-dimensional (0D), one-dimensional (1D) and two-dimensional (2D) nanoscaled building blocks have attracted considerable attention [9–11]. Compared with low dimension architectures, such 3D hierarchical architectures combining the features of micrometer and nanometer scale building blocks can provide more chances for exploring novel properties and superior device performances [2,12]. As one of the most important semiconductor photocatalysts, ZnO has attracted considerable interest because of its high photosensitivity and stability [13]. Since a photocatalytic reaction mainly takes place on the surface of the photocatalysts, the photocatalyitc activity of ZnO is strongly dependent on the growth manner and morphology of the
⁎ Corresponding authors at: Key Laboratory for Micro-Nano Optoelectronic Devices of Ministry of Education, Hunan University, Changsha 410082, China. Tel.: +86 731 88820932; fax: +86 731 88822332. E-mail addresses: [email protected] (B. Qu), [email protected] (B. Zou). 0167-577X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2011.08.011
crystal [14]. It has been demonstrated that 3D hierarchical ZnO architectures showed enhanced photocatalytic activity due to their high specific areas and stability against aggregation [15]. Although recent reports have shown that 3D hierarchical ZnO architectures can be prepared through hydrothermal routes [16], the preparation of hierarchical porous ZnO microsphere assembled by nanosheets remains a great challenge. In this paper, novel hierarchical porous ZnO microspheres which are assembled by porous nanosheets are synthesized. The synthesis strategy consists of two steps. First, hierarchical microspheres of zinc hydroxide carbonate [Zn4(CO3)(OH)6] precursor are fabricated in a hydrothermal autoclave. Then, this intermediate precursor converts into the corresponding ZnO with porous structures by a calcination process without significant modification of the morphology. The asprepared hierarchical porous ZnO microspheres show a high specific surface area and exhibit a significantly enhanced photocatalytic activity in the photodegradation of MB.
2. Experimental section All the reagents were of analytical grade and used without further purification. In a typical synthesis process, 1.5 mmol zinc nitrate hexahydrate (Zn(NO 3 ) 2·6H2 O), 3 mmol urea (CO(NH2 ) 2 ) and 0.15 mmol trisodium citrate were dissolved in 30 mL deionized water to form a clear solution, the mixed solution was transferred into a 50 mL Teflon-lined stainless steel autoclave and heated at 120 °C for 6 h. The precipitate was centrifuged and washed several times with deionized water and absolute ethanol, and then dried at 60 °C for 12 h. Finally,
A. Lei et al. / Materials Letters 66 (2012) 72–75
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Pyrex beaker (100 mL), and the solution was magnetically stirred in the dark for 1 h to reach the adsorption equilibrium and then exposed to light from a 6 W UV lamp with wavelength centered at 365 nm. Solutions were collected every 20 min and centrifuged to remove the catalysts and were analyzed on a TU-1901 UV–vis spectrophotometer. 3. Results and discussion
Fig. 1. XRD patterns of (a) the ZHC precursor and (b) porous ZnO microspheres.
hierarchical porous ZnO microspheres were obtained by calcining the ZHC precursor at 300 °C for 2 h in air. The crystal structure of the products were characterized by a power X-ray diffraction (XRD, Siemens D-5000 diffractometer with Cu Kα irradiation (λ = 1.5406 Å)).The morphologies and microstructures of the products were investigated by field-emission scanning electron microscopy (FE-SEM Hitachi S4800) and transmission electron microscope (TEM JEOL 2010). The Brunauer–Emmett–Teller (BET) specific surface area of the products was analyzed by nitrogen adsorption in a Micromeritics ASAP 2020 nitrogen adsorption apparatus. For photocatalytic measurement, a mixture of 20 mg ZnO products and 50 mL 1 × 10 −5 M MB solution (C16H18ClN3S·3H2O) was put in a
Fig. 1 shows the X-ray diffraction patterns of the ZHC precursor and porous ZnO microspheres calcined at 300 °C. All the peaks in Fig. 1a can be indexed as monoclinic zinc hydroxide carbonate [Zn4(CO3)(OH)6] (JCPDS 19–1458). All the peaks in Fig. 1b are in good agreement with the hexagonal wurtzite structure of ZnO (JCPDS 36–1451). No other diffraction peaks were detected, indicating that no impurity existed and the precursor had completely transformed into the ZnO phase. Fig. 2 shows the typical SEM and TEM images of the precursor and the samples calcined at 300 °C in air for 2 h. Fig. 2a shows the general morphology of the precursor. It reveals that the precursor consists of relatively uniform microspheres with diameters in the range of 4–6 μm. An enlarged SEM image of an individual microsphere is shown in Fig. 2b. It indicates that the microsphere was constructed by many nanosheets, which are intercrossed with each other to form a big sphere. Fig. 2c and d show the SEM images of the samples calcined at 300 °C in air for 2 h. It can be seen that the structure of the microspheres and the nanosheets is retained after calcinations and the average thickness of the nanosheets is approximately 10 nm. However, as shown in Fig. 2d many pores are formed in the nanosheets, which could be attributed to the loss of volatile gas such as H2O and CO2 during the heat treatment. The porous
Fig. 2. (a, b) SEM images of the ZHC precursor; (c, d) SEM images of samples calcined at 300 °C; (e) TEM images porous ZnO nanosheets. Inset: corresponding SAED pattern, (f) HRTEM image of ZnO nanosheets.
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A. Lei et al. / Materials Letters 66 (2012) 72–75
Fig. 3. Typical nitrogen adsorption–desorption isotherm and BJH pore-size distribution curve (inset) of the porous ZnO microspheres.
ZnO nanosheets are further characterized by the TEM and Highresolution TEM (HRTEM) and selected-area electron diffraction (SAED). Fig. 2e shows the TEM image of porous ZnO nanosheets. It is clearly observed that many irregular pores of 5–40 nm are randomly distributed in the nanosheets. The corresponding SAED pattern in the inset of Fig. 2e indicates the single crystalline nature of the porous ZnO nanosheets. The HRTEM image shown in Fig. 2f exhibits clear and coherent lattice fringes with spacing of 0.26 nm between adjacent lattice planes, which is in good agreement with the interplanar spacing of the (002) plane. To obtain further information about the specific surface area and the pore sizes distribution of the as-prepared ZnO microspheres, BET N2 adsorption–desorption analysis was performed. As shown in Fig. 3, the nitrogen adsorption–desorption isotherm belongs to type IV, indicating the existence of abundant mesopores (pores 2–50 nm in diameter) in the ZnO microspheres. The corresponding pore diameter distribution curve (the inset in Fig. 3) reveals that the size of mesopores is not uniform which fits well with the TEM results. Most of the pores fall into a size range of 20–60 nm. The specific surface area of the ZnO microspheres is calculated to be 39.6 m 2/g by the BET equation, which is higher than that of the reported nanostructured ZnO [15]. The photocatalytic activity of the porous ZnO microspheres was evaluated by photodegradation of MB. Fig. 4a shows the UV–vis absorption spectrum of the aqueous solution of MB (initial concentration: 1 × 10 −5 M, 50 mL) with 20 mg of porous ZnO microspheres under exposure to the ultraviolet light lamp (6 W) for various durations. It can be seen that the characteristic absorption of MB at 663 nm diminishes rapidly with increasing the exposure time and completely disappears after 80 min. No new absorption bands appear in the whole spectrum, which indicate the total decomposition of MB
aqueous solution during the reaction. As a comparison, the photocatalytic activities of the commercial ZnO and TiO2 were also investigated under the same conditions. Fig. 4b shows the degradation rate of MB over the hierarchical porous ZnO microspheres, commercial TiO2 powder and commercial ZnO powder. Without any catalyst, only a slow decrease in the concentration of MB was detected under UV irradiation. However, the degradation efficiencies of MB obviously increased with the addition of catalyst. The hierarchical porous ZnO microspheres showed the highest catalytic activity, followed by commercial TiO2 and commercial ZnO. The significant improvement of photocatalytic activity of the porous ZnO microspheres can be attributed to their special structural features. As mentioned above, the as-prepared ZnO microspheres have hierarchical porous structure, which can effectively prevent aggregation during the photodegradation. The large surface area and porous surface supply more active site to adsorb MB, and then facilitate the diffusion and mass transportation of MB molecules and hydroxyl radicals during the photochemical reaction. 4. Conclusions In summary, hierarchical porous ZnO microspheres have been successfully synthesized from a hydrothermal preparation and thermal decomposition of zinc hydroxide carbonate precursor. The porous ZnO microspheres are assembled by numerous porous nanosheets. The photocatalytic activity of the ZnO microspheres was evaluated by photodegradation of MB. The as-prepared ZnO microspheres exhibit a significantly enhanced photocatalytic activity than commercial ZnO and TiO2. This is mainly attributed to the larger specific surface area and stability against aggregation. These porous ZnO microspheres are also expected to be useful for other applications such as dye-sensitized solar cells and gas sensing. Moreover, this facile and economical hydrothermal route has great potential to be extended to the synthesis of other metal-oxide with hierarchical porous microstructure. Acknowledgments We thank the financial support of the National Natural Science Foundation of China (grant nos. 90606001 and 20873039). References [1] [2] [3] [4]
Huang MH, Mao S, Feick H, Yan HQ, Wu YY, Kind H. Science 2001;292:1897–9. Li J, Fan HQ, Jia XH. J Phys Chem C 2010;114:14684–91. Lee YJ, Ruby DS, Peter DW, McKenzie BB, Hsu JWP. Nano Lett 2008;8:1501–5. Wang XJ, Zhang QL, Wan Q, Dai GZ, Zhou CJ, Zou BS. J Phys Chem C 2011;115: 2769–75. [5] Liu B, Zeng HC. J Am Chem Soc 2003;125:4430–1.
Fig. 4. (a) Absorption spectrum of MB in the presence of porous ZnO microspheres under UV light. (b) MB concentration changes over photocatalyst-free solution, commercial ZnO, commercial TiO2, and porous ZnO microspheres.
A. Lei et al. / Materials Letters 66 (2012) 72–75 [6] Zou BS, Liu RB, Wang FF, Pan AL, Cao L, Wang ZL. J Phys Chem B 2006;110: 12865–73. [7] Yu HD, Zhang ZP, Han MY, Hao XT, Zhu FR. J Am Chem Soc 2005;127:2378–9. [8] Zhang JH, Liu HY, Wang ZL, Ming NB, Li ZR, Biris AS. Adv Funct Mater 2007;17: 3897–905. [9] Lu F, Cai WP, Zhang YG. Adv Funct Mater 2008;18:1047–56. [10] Li BX, Wang YF. J Phys Chem C 2010;114:890–6.
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[11] Kim D, Huh YD. Mater Lett 2011;65:2100–3. [12] Li HX, Xia MX, Dai GZ, Yu HC, Zhang QL, Pan AL, et al. J Phys Chem C 2008;112: 17546–53. [13] Lu WW, Gao SY, Wang JJ. J Phys Chem C 2008;112:16792–800. [14] Jang ES, Won JH, Hwang SJ, Choy JH. Adv Mater 2006;18:3309–12. [15] Lu HB, Wang SM, Zhao L, Li JC, Dong BH, Xu ZX. J Mater Chem 2011;21:4228–34. [16] Wu DP, Bai ZY, Jiang K. Mater Lett 2009;63:1057–60.
Contents lists available at SciVerse ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Facile synthesis and enhanced photocatalytic activity of hierarchical porous ZnO microspheres Aihua Lei a, Baihua Qu a,⁎, Weichang Zhou a, Yanguo Wang a, Qinglin Zhang a, Bingsuo Zou a, b,⁎ a b
Key Laboratory for Micro-Nano Optoelectronic Devices of Ministry of Education, Hunan University, Changsha 410082, China School of Materials Science and Engineering, Beijing Institute of Technology, Beijing 100081, China
a r t i c l e
i n f o
Article history: Received 22 July 2011 Accepted 4 August 2011 Available online 11 August 2011 Keywords: Semiconductors Microstructure ZnO Porous microspheres Photocatalytic activity
a b s t r a c t In this paper, hierarchical porous ZnO microspheres were successfully synthesized by calcining the microspheric zinc hydroxide carbonate (ZHC) precursor, which were the precipitate products of a hydrothermal reaction by zinc nitrate hexahydrate and urea in the presence of trisodium citrate. The as-prepared ZnO microspheres with diameters of 4–5 μm were assembled by numerous porous nanosheets which had the uniform thickness of about 10 nm. The photocatalytic activity of the ZnO microspheres was evaluated by photodegradation reaction of methylene blue (MB). The as-prepared ZnO microspheres exhibited a significantly enhanced photocatalytic activity than commercial ZnO and TiO2. This was mainly attributed to the larger specific surface area and stability against aggregation. © 2011 Elsevier B.V. All rights reserved.
1. Introduction ZnO, with a wide band gap (3.37 eV) and large exciton binding energy (60 meV), has stimulated intensive research interest due to its specific optical and electrical properties that were useful for nanolasers [1], gas sensors [2], solar cell [3], photocatalyst [4], and so on. Over the past few years, well-defined ZnO one-dimensional (1D) and two-dimensional (2D) nanostructures with various morphologies such as nanorods [5], nanowires [6], nanotubes [7], and nanoplates [8] have been successfully fabricated by a variety of routes. Recently, three-dimensional (3D) hierarchical ZnO architectures which are assembled from zero-dimensional (0D), one-dimensional (1D) and two-dimensional (2D) nanoscaled building blocks have attracted considerable attention [9–11]. Compared with low dimension architectures, such 3D hierarchical architectures combining the features of micrometer and nanometer scale building blocks can provide more chances for exploring novel properties and superior device performances [2,12]. As one of the most important semiconductor photocatalysts, ZnO has attracted considerable interest because of its high photosensitivity and stability [13]. Since a photocatalytic reaction mainly takes place on the surface of the photocatalysts, the photocatalyitc activity of ZnO is strongly dependent on the growth manner and morphology of the
⁎ Corresponding authors at: Key Laboratory for Micro-Nano Optoelectronic Devices of Ministry of Education, Hunan University, Changsha 410082, China. Tel.: +86 731 88820932; fax: +86 731 88822332. E-mail addresses: [email protected] (B. Qu), [email protected] (B. Zou). 0167-577X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2011.08.011
crystal [14]. It has been demonstrated that 3D hierarchical ZnO architectures showed enhanced photocatalytic activity due to their high specific areas and stability against aggregation [15]. Although recent reports have shown that 3D hierarchical ZnO architectures can be prepared through hydrothermal routes [16], the preparation of hierarchical porous ZnO microsphere assembled by nanosheets remains a great challenge. In this paper, novel hierarchical porous ZnO microspheres which are assembled by porous nanosheets are synthesized. The synthesis strategy consists of two steps. First, hierarchical microspheres of zinc hydroxide carbonate [Zn4(CO3)(OH)6] precursor are fabricated in a hydrothermal autoclave. Then, this intermediate precursor converts into the corresponding ZnO with porous structures by a calcination process without significant modification of the morphology. The asprepared hierarchical porous ZnO microspheres show a high specific surface area and exhibit a significantly enhanced photocatalytic activity in the photodegradation of MB.
2. Experimental section All the reagents were of analytical grade and used without further purification. In a typical synthesis process, 1.5 mmol zinc nitrate hexahydrate (Zn(NO 3 ) 2·6H2 O), 3 mmol urea (CO(NH2 ) 2 ) and 0.15 mmol trisodium citrate were dissolved in 30 mL deionized water to form a clear solution, the mixed solution was transferred into a 50 mL Teflon-lined stainless steel autoclave and heated at 120 °C for 6 h. The precipitate was centrifuged and washed several times with deionized water and absolute ethanol, and then dried at 60 °C for 12 h. Finally,
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Pyrex beaker (100 mL), and the solution was magnetically stirred in the dark for 1 h to reach the adsorption equilibrium and then exposed to light from a 6 W UV lamp with wavelength centered at 365 nm. Solutions were collected every 20 min and centrifuged to remove the catalysts and were analyzed on a TU-1901 UV–vis spectrophotometer. 3. Results and discussion
Fig. 1. XRD patterns of (a) the ZHC precursor and (b) porous ZnO microspheres.
hierarchical porous ZnO microspheres were obtained by calcining the ZHC precursor at 300 °C for 2 h in air. The crystal structure of the products were characterized by a power X-ray diffraction (XRD, Siemens D-5000 diffractometer with Cu Kα irradiation (λ = 1.5406 Å)).The morphologies and microstructures of the products were investigated by field-emission scanning electron microscopy (FE-SEM Hitachi S4800) and transmission electron microscope (TEM JEOL 2010). The Brunauer–Emmett–Teller (BET) specific surface area of the products was analyzed by nitrogen adsorption in a Micromeritics ASAP 2020 nitrogen adsorption apparatus. For photocatalytic measurement, a mixture of 20 mg ZnO products and 50 mL 1 × 10 −5 M MB solution (C16H18ClN3S·3H2O) was put in a
Fig. 1 shows the X-ray diffraction patterns of the ZHC precursor and porous ZnO microspheres calcined at 300 °C. All the peaks in Fig. 1a can be indexed as monoclinic zinc hydroxide carbonate [Zn4(CO3)(OH)6] (JCPDS 19–1458). All the peaks in Fig. 1b are in good agreement with the hexagonal wurtzite structure of ZnO (JCPDS 36–1451). No other diffraction peaks were detected, indicating that no impurity existed and the precursor had completely transformed into the ZnO phase. Fig. 2 shows the typical SEM and TEM images of the precursor and the samples calcined at 300 °C in air for 2 h. Fig. 2a shows the general morphology of the precursor. It reveals that the precursor consists of relatively uniform microspheres with diameters in the range of 4–6 μm. An enlarged SEM image of an individual microsphere is shown in Fig. 2b. It indicates that the microsphere was constructed by many nanosheets, which are intercrossed with each other to form a big sphere. Fig. 2c and d show the SEM images of the samples calcined at 300 °C in air for 2 h. It can be seen that the structure of the microspheres and the nanosheets is retained after calcinations and the average thickness of the nanosheets is approximately 10 nm. However, as shown in Fig. 2d many pores are formed in the nanosheets, which could be attributed to the loss of volatile gas such as H2O and CO2 during the heat treatment. The porous
Fig. 2. (a, b) SEM images of the ZHC precursor; (c, d) SEM images of samples calcined at 300 °C; (e) TEM images porous ZnO nanosheets. Inset: corresponding SAED pattern, (f) HRTEM image of ZnO nanosheets.
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Fig. 3. Typical nitrogen adsorption–desorption isotherm and BJH pore-size distribution curve (inset) of the porous ZnO microspheres.
ZnO nanosheets are further characterized by the TEM and Highresolution TEM (HRTEM) and selected-area electron diffraction (SAED). Fig. 2e shows the TEM image of porous ZnO nanosheets. It is clearly observed that many irregular pores of 5–40 nm are randomly distributed in the nanosheets. The corresponding SAED pattern in the inset of Fig. 2e indicates the single crystalline nature of the porous ZnO nanosheets. The HRTEM image shown in Fig. 2f exhibits clear and coherent lattice fringes with spacing of 0.26 nm between adjacent lattice planes, which is in good agreement with the interplanar spacing of the (002) plane. To obtain further information about the specific surface area and the pore sizes distribution of the as-prepared ZnO microspheres, BET N2 adsorption–desorption analysis was performed. As shown in Fig. 3, the nitrogen adsorption–desorption isotherm belongs to type IV, indicating the existence of abundant mesopores (pores 2–50 nm in diameter) in the ZnO microspheres. The corresponding pore diameter distribution curve (the inset in Fig. 3) reveals that the size of mesopores is not uniform which fits well with the TEM results. Most of the pores fall into a size range of 20–60 nm. The specific surface area of the ZnO microspheres is calculated to be 39.6 m 2/g by the BET equation, which is higher than that of the reported nanostructured ZnO [15]. The photocatalytic activity of the porous ZnO microspheres was evaluated by photodegradation of MB. Fig. 4a shows the UV–vis absorption spectrum of the aqueous solution of MB (initial concentration: 1 × 10 −5 M, 50 mL) with 20 mg of porous ZnO microspheres under exposure to the ultraviolet light lamp (6 W) for various durations. It can be seen that the characteristic absorption of MB at 663 nm diminishes rapidly with increasing the exposure time and completely disappears after 80 min. No new absorption bands appear in the whole spectrum, which indicate the total decomposition of MB
aqueous solution during the reaction. As a comparison, the photocatalytic activities of the commercial ZnO and TiO2 were also investigated under the same conditions. Fig. 4b shows the degradation rate of MB over the hierarchical porous ZnO microspheres, commercial TiO2 powder and commercial ZnO powder. Without any catalyst, only a slow decrease in the concentration of MB was detected under UV irradiation. However, the degradation efficiencies of MB obviously increased with the addition of catalyst. The hierarchical porous ZnO microspheres showed the highest catalytic activity, followed by commercial TiO2 and commercial ZnO. The significant improvement of photocatalytic activity of the porous ZnO microspheres can be attributed to their special structural features. As mentioned above, the as-prepared ZnO microspheres have hierarchical porous structure, which can effectively prevent aggregation during the photodegradation. The large surface area and porous surface supply more active site to adsorb MB, and then facilitate the diffusion and mass transportation of MB molecules and hydroxyl radicals during the photochemical reaction. 4. Conclusions In summary, hierarchical porous ZnO microspheres have been successfully synthesized from a hydrothermal preparation and thermal decomposition of zinc hydroxide carbonate precursor. The porous ZnO microspheres are assembled by numerous porous nanosheets. The photocatalytic activity of the ZnO microspheres was evaluated by photodegradation of MB. The as-prepared ZnO microspheres exhibit a significantly enhanced photocatalytic activity than commercial ZnO and TiO2. This is mainly attributed to the larger specific surface area and stability against aggregation. These porous ZnO microspheres are also expected to be useful for other applications such as dye-sensitized solar cells and gas sensing. Moreover, this facile and economical hydrothermal route has great potential to be extended to the synthesis of other metal-oxide with hierarchical porous microstructure. Acknowledgments We thank the financial support of the National Natural Science Foundation of China (grant nos. 90606001 and 20873039). References [1] [2] [3] [4]
Huang MH, Mao S, Feick H, Yan HQ, Wu YY, Kind H. Science 2001;292:1897–9. Li J, Fan HQ, Jia XH. J Phys Chem C 2010;114:14684–91. Lee YJ, Ruby DS, Peter DW, McKenzie BB, Hsu JWP. Nano Lett 2008;8:1501–5. Wang XJ, Zhang QL, Wan Q, Dai GZ, Zhou CJ, Zou BS. J Phys Chem C 2011;115: 2769–75. [5] Liu B, Zeng HC. J Am Chem Soc 2003;125:4430–1.
Fig. 4. (a) Absorption spectrum of MB in the presence of porous ZnO microspheres under UV light. (b) MB concentration changes over photocatalyst-free solution, commercial ZnO, commercial TiO2, and porous ZnO microspheres.
A. Lei et al. / Materials Letters 66 (2012) 72–75 [6] Zou BS, Liu RB, Wang FF, Pan AL, Cao L, Wang ZL. J Phys Chem B 2006;110: 12865–73. [7] Yu HD, Zhang ZP, Han MY, Hao XT, Zhu FR. J Am Chem Soc 2005;127:2378–9. [8] Zhang JH, Liu HY, Wang ZL, Ming NB, Li ZR, Biris AS. Adv Funct Mater 2007;17: 3897–905. [9] Lu F, Cai WP, Zhang YG. Adv Funct Mater 2008;18:1047–56. [10] Li BX, Wang YF. J Phys Chem C 2010;114:890–6.
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[11] Kim D, Huh YD. Mater Lett 2011;65:2100–3. [12] Li HX, Xia MX, Dai GZ, Yu HC, Zhang QL, Pan AL, et al. J Phys Chem C 2008;112: 17546–53. [13] Lu WW, Gao SY, Wang JJ. J Phys Chem C 2008;112:16792–800. [14] Jang ES, Won JH, Hwang SJ, Choy JH. Adv Mater 2006;18:3309–12. [15] Lu HB, Wang SM, Zhao L, Li JC, Dong BH, Xu ZX. J Mater Chem 2011;21:4228–34. [16] Wu DP, Bai ZY, Jiang K. Mater Lett 2009;63:1057–60.