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Shape-controlled synthesis of flower-like ZnO microstructures and their enhanced photocatalytic properties
Materials Letters 192 (2017) 1–4
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
Shape-controlled synthesis of flower-like ZnO microstructures and their enhanced photocatalytic properties Minjuan Cao, Fen Wang ⇑, Jianfeng Zhu, Xin Zhang, Yi Qin, Lei Wang School of Materials Science and Engineering, Shaanxi University of Science and Technology, Xi’an 710021, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 20 December 2016 Received in revised form 12 January 2017 Accepted 13 January 2017 Available online 16 January 2017 Keywords: Shape-controlled ZnO Microstructure Structural Photocatalytic activity
a b s t r a c t In this paper, we report a simple one-step method to prepare shape-controlled flower-like ZnO microstructure. The different morphologies of ZnO flowers, including spindle-based, flake-based and rod-based, were synthesized by simply adjusting the Zn2+ in alkaline solution. The photocatalytic activities of the variously structural ZnO were investigated under UV-light irradiation, and the results showed that the flake-based ZnO displayed better in catalytic degradation of dyes than the other two structures. Since the flake-based ZnO featured with small grain size, enormous interface area and abundant mesopores due to its special structures. In brief, the present study provides an efficient method for the enhancement of photocatalytic activity by designing the shape-controlled ZnO flowers, especially the flake-based ZnO. These nanoflakes-assembled ZnO with superior photocatalytic performance are also expected to be used in other application such as gas-sensing. Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction Recently, the growing population and industrial development cause more and more serious water pollution. Organic dye is one of the major sources of water pollution because it is very difficult to be biodegraded, and it has a strong toxicity to aquatic plants as well as a fearful carcinogenicity to the human body. Photocatalytic degradation is an effective solution to eliminate the organic dyes in water [1–3]. Semiconductor-based photocatalyst, an important candidate to degrade organic dyes into nontoxic molecules in aqueous solution, has attracted considerable interest [4,5]. Nanostructured ZnO with a wide direct band gap of 3.37 eV [6] has been recognized as significant photocatalytic materials owning to its high photosensitivity, better exciton ability, nontoxic character, low cost and abundant availability [7,8]. Since the morphology of ZnO has strong influence on the performance of photocatalysis [9,10], developing a shape-controlled ZnO synthesis strategy is absolutely necessary for the application of ZnO with high photocatalytic properties. The difference in the behavior of various ZnO is partially because of the grain size and grain boundaries, and the very small grains and enough grain boundaries are beneficial for the emergence or enhancement of some properties [11,12]. Herein, we propose direct precipitation method to prepare self-assembled ZnO flowers with various morphologies. The ⇑ Corresponding author. E-mail address: [email protected] (F. Wang). http://dx.doi.org/10.1016/j.matlet.2017.01.051 0167-577X/Ó 2017 Elsevier B.V. All rights reserved.
photocatalytic performances of such flower-like ZnO are investigated depending on the different structures. Compared with spindle-based and rod-based ZnO flowers, flake-based flowers with smaller grain size, greater interface area and more mesopores exhibit a greatly enhanced photocatalytic activity in the photodegradation of dyes (such as RhB, MO and MB).
2. Experimental 2.1. Materials All chemicals were of analytic grade and without further purification. Zn(NO3)26H2O and NaOH were purchased from Sinopharm Chemical Reagent Co., Ltd., China. Deionized water was used in all experiments.
2.2. Characterization The morphology and crystal structure of the as-prepared samples were characterized by SEM (Hitachi S4800), TEM (FEI Tecnai G2 F20 S-TWIN) and XRD (Rigaku D/max 2200pc, Cu Ka). The specific surface area and pore size distribution of the simples were measured by ASAP2460 Micromeritics instrument. Optical absorption spectra were measured by a UV–Visible spectrophotometer (YQ2015001210).
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M. Cao et al. / Materials Letters 192 (2017) 1–4
2.3. Preparation of flower-like ZnO with various morphologies First, a certain concentration of Zn(NO3)26H2O was dissolved into distilled water under stirring. Specifically, the concentration of Zn(NO3)26H2O in aqueous solution was 0.001–0.004, 0.01– 0.025 and 0.06–0.1 M, respectively, for spindle-based, flake-based and rod-based ZnO. Then, NaOH (Zn2+/OH = 1:10 molar ratio) was slowly added into the Zn(NO3)26H2O aqueous solution. After stirring for 0.5 h at room temperature, the above mixture solution was heated at 95 °C for 5 h. Finally, the resulting precipitate was washed and dried at 60 °C for 8 h. 2.4. Photocatalytic activity measurement In a specific procedure, 50 mg ZnO sample was immersed into 50 mL of dye aqueous solution (10 mg/L). Before irradiation, the mixture containing the photocatalyst and dye was stirred in the dark for 1 h to establish adsorption–desorption equilibrium. The solutions were then exposed to UV-light irradiation with a mercury lamp (500 W). The tested dye solution of 4 mL was taken out every 20 min and analyzed by collecting the maximum absorption band using a 2600 UV/Vis spectrophotometer. 3. Results and discussion SEM and TEM images of the ZnO based on different concentration of Zn2+ are presented in Fig. 1: 0.001–0.004 M (a)–(c), 0.01– 0.025 M (d)–(f), 0.06–0.1 M (g)–(i). It is evidently seen that all the ZnO crystals exist as flower architectures with highly branched. In Fig. 1a, the flower-like ZnO are assembled by spindles and the mean size of the flowers is approximately 0.5–0.8 lm. In the higher magnification SEM image of Fig. 1b, the spindles are estimated to be 300 nm diameters and 350 nm lengths. Furthermore,
each spindle consists of many nanoparticles with size of 40 nm. When the concentration of Zn2+ is set to 0.01–0.25 M, such ZnO flowers with size of 2–3 lm are organized by nanoflakes with thickness of 30 nm, as shown in Fig. 1(d)–(e). When increasing Zn2+ concentration to 0.06–0.1 M, rod-based flowers, in the form of rods of 150 nm diameters and of 2 lm lengths stretching radially from center are obviously seen in Fig. 1(g)–(h). Consistently, flower-like ZnO structures are obviously observed on TEM images (Fig. 1(c), (f) and (i)), and such flowers are assembled by several spindles (c), flakes (f) and rods (i). Evidently, in comparison with nanospindles and nanorods, the ultrathin nanoflakes possess the smallest grain size and most interfacial boundaries. Thus, the flake-based ZnO are expected to absorb more dyes and harvest improved UV-light, resulting in the enhanced photocatalytic activities. Fig. 2(a) exhibits the XRD patterns of as-prepared ZnO simples. It is noteworthy that all the diffraction peaks can be indexed as (0 0 2), (1 0 0), (1 0 1), (1 0 2), (1 0 3), (1 1 0), (1 1 2), (2 0 0) and (2 0 1) crystal planes of the hexagonal wurtzite structure of ZnO (JCPDS 36-1451). No extra peak is detected by XRD, indicating the superior purity of the ZnO simples. Fig. 2(b) shows the UV–vis absorption spectra of the different ZnO flowers. Clearly, all the products exhibit a broad and strong absorption in the UV-light range (lower than 400 nm), which comes from the wide-band gap of ZnO. It is noteworthy that the flake-based ZnO has the stronger absorption intensity among these three architectures, which may reveal more light-harvesting and better photocatalytic behavior under UV-light irradiation. The nitrogen adsorption and desorption isotherms of the as-synthesized ZnO powders are shown in Fig. 2(c). The specific surface area for spindle-based, flake-based and rod-based ZnO flowers are, 9.34, 21.55 and 2.49 m2/g, respectively. The much larger specific surface area of flake-based ZnO structure is may
Fig. 1. SEM images of ZnO simples formed in different concentration of Zn2+: 0.001–0.004 M (a)–(b), 0.01–0.025 M (d)–(e), 0.06–0.1 M (g)–(h); TEM images of ZnO simples formed in different concentration of Zn2+: 0.001–0.004 M (c), 0.01–0.025 M (f), 0.06–0.1 M (i).
M. Cao et al. / Materials Letters 192 (2017) 1–4
3
Fig. 2. XRD patterns (a) and UV–visible absorbance spectra (b) of ZnO simples; Nitrogen adsorption–desorption isotherms (c) and pore size distributions (d) of ZnO simples.
be due to the flourish, ultrathin nanoflakes as well as the smaller grain size and the more interfacial boundaries. All the products exhibit distinct hysteresis loops in the range of 0.45–1.0 P/P0, which suggests the presence of mesoporous structures. Pore size distributions of three samples are displayed in Fig. 2(d), and the pores distribute ranging from 3 to 65 nm. As observed in the pore size distributions of spindle-based ZnO, the smaller pores with a sharp peak at about 4 nm may be generated between the nanoparticles on the spindles. The larger pores with a wide distribution in all simples can be attributed to the spaces between the intercrossed ZnO nanobranches. It is note mentioning that the flakebased ZnO possess much more mesopores than others, which could
provide improved transfer paths for light-generated charge carriers and further facilitate the photocatalytic reactions in the degradation processes. The degradation rates of the ZnO simples are displayed in Fig. 3. Remarkably, the flake-based ZnO exhibits superior catalytic efficiencies than other structures on degrading dyes of RhB (Fig. 3 (a)), MO (Fig. 3(b)) and MB (Fig. 3(c)). Under UV-light irradiation of 40 min, the flake-based ZnO successfully degraded 95% of the RhB, while 72% and 68% of the RhB was degraded for spindlebased ZnO and rod-based ZnO, respectively. Similarly, the flake-based ZnO also exhibits the most impressive activities on degrading MO and MB. For flake-based ZnO, the concentration of
Fig. 3. Photocatalytic rates of ZnO simples on degrading RhB (a), MO (b) and MB (c).
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M. Cao et al. / Materials Letters 192 (2017) 1–4
MO and MB decreased 79% and 98%, respectively, under UV-light irradiation of 40 min. On the basic of the photocatalytic test results, the excellent photocatalytic activities of the flake-based ZnO can be attributed to three parts: Firstly, the very fined grains and plenty of boundaries leads to the enhanced dye-adsorption and light-harvesting. Secondly, the numerous nanograins and interfaces can further promote photogenerated electrons/holes diffusion through the whole hierarchical architecture. Thirdly, the rich mespores could provide additional routes for light-generated charge transferring in the degradation processes. The photocatalytic degradation reaction of dyes with ZnO as catalyst is depicted as follows. Firstly, generation of holes (h+VB) in the VB and electrons (e CB) in the CB through absorption of adequate photon energy, as listed in Eq. (1). Secondly, h+VB will react with water or hydroxyl groups to generate OH (Eqs. (2) and (3)). Thirdly, hydroxyl radical OH can react with RhB molecules and degrade them into nontoxic products (CO2 and H2O) (Eq. (4))
ZnO þ hm ! ZnO eCB þ hVB þ
ð1Þ
þ
ð2Þ
hVB þ OH ! OH
þ
ð3Þ
ð4Þ
hVB þ H2 O ! Hþ þ OH
OH þ RhB ! H2 O þ CO2
4. Conclusions In summary, the shape-controlled ZnO flowers of spindle-based, flake-based and rod-based were successfully fabricated by a low temperature wet chemical method. Among these three different
architectures of flower-like ZnO, the flake-based ZnO exhibits the most favorable photocatalytic properties. The improved efficiency is mainly attributed to the very fined grains, enormous interface area and abundant mesopores, which lead to substantial dyeadsorption, effective light-harvesting and fast light-generated charge transferring in the degradation processes. Acknowledgment This work was supported by the National Natural Science Foundation of China (51472153, 51171096). References [1] M.R. Hoffmann, W. Choi, D.W. Bahnemann, Chem. Rev. 95 (1) (1995) 69–96. [2] S. Cao, J. Low, J. Yu, M. Jaroniec, Adv. Mater. 27 (13) (2015) 2150–2176. [3] H. Zhou, Y. Qu, T. Zeid, X. Duan, Energy Environ. Sci. 5 (5) (2012) 6732– 6743. [4] H. Xu, S. Ouyang, L. Liu, P. Reunchan, N. Umezawa, J. Ye, J. Mater. Chem. A 2 (32) (2014) 12642–12661. [5] Y. Hu, X. Gao, L. Yu, Y. Wang, J. Ning, S. Xu, X.W. Lou, Angew. Chem. Int. Ed. 52 (21) (2013) 5636–5639. [6] D. Hong, W. Zang, X. Guo, Y. Fu, H. He, J. Sun, L. Xing, B. Liu, X. Xue, ACS Appl. Mater. Interfaces 8 (33) (2016) 21302–21314. [7] M. Yoon, J.E. Lee, J.J. Yu, L.J. Won, A. Rani, H.K. Dong, ACS Appl. Mater. Interfaces 7 (38) (2015) 21073–21081. [8] A. Ghosh, P. Guha, A.K. Samantara, B.K. Jena, R. Bar, S.K. Ray, P.V. Satyam, ACS Appl. Mater. Interfaces 7 (18) (2015) 9486–9496. [9] H. Lu, S. Wang, L. Zhao, J. Li, B. Dong, Z. Xu, J. Mater. Chem. 21 (12) (2011) 4228–4234. [10] Y.Y. Chen, C.C. Kuo, B.Y. Chen, P.C. Chiu, P.C. Tsai, J. Polym. Sci., Part B Polym. Phys. 53 (4) (2015) 262–269. [11] B.B. Straumal, A.A. Mazilkin, S.G. Protasova, S.V. Stakhanova, P.B. Straumal, M. F. Bulatov, G. Schütz, T. Tietze, E. Goering, B. Baretzky, Rev. Adv. Mater. Sci. 41 (1) (2015) 61–71. [12] B.B. Straumal, S.G. Protasova, A.A. Mazilkin, E. Goering, G. Schütz, P.B. Straumal, B. Baretzky, Beilstein J. Nanotechnol. 7 (2016) 1936–1947.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
Shape-controlled synthesis of flower-like ZnO microstructures and their enhanced photocatalytic properties Minjuan Cao, Fen Wang ⇑, Jianfeng Zhu, Xin Zhang, Yi Qin, Lei Wang School of Materials Science and Engineering, Shaanxi University of Science and Technology, Xi’an 710021, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 20 December 2016 Received in revised form 12 January 2017 Accepted 13 January 2017 Available online 16 January 2017 Keywords: Shape-controlled ZnO Microstructure Structural Photocatalytic activity
a b s t r a c t In this paper, we report a simple one-step method to prepare shape-controlled flower-like ZnO microstructure. The different morphologies of ZnO flowers, including spindle-based, flake-based and rod-based, were synthesized by simply adjusting the Zn2+ in alkaline solution. The photocatalytic activities of the variously structural ZnO were investigated under UV-light irradiation, and the results showed that the flake-based ZnO displayed better in catalytic degradation of dyes than the other two structures. Since the flake-based ZnO featured with small grain size, enormous interface area and abundant mesopores due to its special structures. In brief, the present study provides an efficient method for the enhancement of photocatalytic activity by designing the shape-controlled ZnO flowers, especially the flake-based ZnO. These nanoflakes-assembled ZnO with superior photocatalytic performance are also expected to be used in other application such as gas-sensing. Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction Recently, the growing population and industrial development cause more and more serious water pollution. Organic dye is one of the major sources of water pollution because it is very difficult to be biodegraded, and it has a strong toxicity to aquatic plants as well as a fearful carcinogenicity to the human body. Photocatalytic degradation is an effective solution to eliminate the organic dyes in water [1–3]. Semiconductor-based photocatalyst, an important candidate to degrade organic dyes into nontoxic molecules in aqueous solution, has attracted considerable interest [4,5]. Nanostructured ZnO with a wide direct band gap of 3.37 eV [6] has been recognized as significant photocatalytic materials owning to its high photosensitivity, better exciton ability, nontoxic character, low cost and abundant availability [7,8]. Since the morphology of ZnO has strong influence on the performance of photocatalysis [9,10], developing a shape-controlled ZnO synthesis strategy is absolutely necessary for the application of ZnO with high photocatalytic properties. The difference in the behavior of various ZnO is partially because of the grain size and grain boundaries, and the very small grains and enough grain boundaries are beneficial for the emergence or enhancement of some properties [11,12]. Herein, we propose direct precipitation method to prepare self-assembled ZnO flowers with various morphologies. The ⇑ Corresponding author. E-mail address: [email protected] (F. Wang). http://dx.doi.org/10.1016/j.matlet.2017.01.051 0167-577X/Ó 2017 Elsevier B.V. All rights reserved.
photocatalytic performances of such flower-like ZnO are investigated depending on the different structures. Compared with spindle-based and rod-based ZnO flowers, flake-based flowers with smaller grain size, greater interface area and more mesopores exhibit a greatly enhanced photocatalytic activity in the photodegradation of dyes (such as RhB, MO and MB).
2. Experimental 2.1. Materials All chemicals were of analytic grade and without further purification. Zn(NO3)26H2O and NaOH were purchased from Sinopharm Chemical Reagent Co., Ltd., China. Deionized water was used in all experiments.
2.2. Characterization The morphology and crystal structure of the as-prepared samples were characterized by SEM (Hitachi S4800), TEM (FEI Tecnai G2 F20 S-TWIN) and XRD (Rigaku D/max 2200pc, Cu Ka). The specific surface area and pore size distribution of the simples were measured by ASAP2460 Micromeritics instrument. Optical absorption spectra were measured by a UV–Visible spectrophotometer (YQ2015001210).
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M. Cao et al. / Materials Letters 192 (2017) 1–4
2.3. Preparation of flower-like ZnO with various morphologies First, a certain concentration of Zn(NO3)26H2O was dissolved into distilled water under stirring. Specifically, the concentration of Zn(NO3)26H2O in aqueous solution was 0.001–0.004, 0.01– 0.025 and 0.06–0.1 M, respectively, for spindle-based, flake-based and rod-based ZnO. Then, NaOH (Zn2+/OH = 1:10 molar ratio) was slowly added into the Zn(NO3)26H2O aqueous solution. After stirring for 0.5 h at room temperature, the above mixture solution was heated at 95 °C for 5 h. Finally, the resulting precipitate was washed and dried at 60 °C for 8 h. 2.4. Photocatalytic activity measurement In a specific procedure, 50 mg ZnO sample was immersed into 50 mL of dye aqueous solution (10 mg/L). Before irradiation, the mixture containing the photocatalyst and dye was stirred in the dark for 1 h to establish adsorption–desorption equilibrium. The solutions were then exposed to UV-light irradiation with a mercury lamp (500 W). The tested dye solution of 4 mL was taken out every 20 min and analyzed by collecting the maximum absorption band using a 2600 UV/Vis spectrophotometer. 3. Results and discussion SEM and TEM images of the ZnO based on different concentration of Zn2+ are presented in Fig. 1: 0.001–0.004 M (a)–(c), 0.01– 0.025 M (d)–(f), 0.06–0.1 M (g)–(i). It is evidently seen that all the ZnO crystals exist as flower architectures with highly branched. In Fig. 1a, the flower-like ZnO are assembled by spindles and the mean size of the flowers is approximately 0.5–0.8 lm. In the higher magnification SEM image of Fig. 1b, the spindles are estimated to be 300 nm diameters and 350 nm lengths. Furthermore,
each spindle consists of many nanoparticles with size of 40 nm. When the concentration of Zn2+ is set to 0.01–0.25 M, such ZnO flowers with size of 2–3 lm are organized by nanoflakes with thickness of 30 nm, as shown in Fig. 1(d)–(e). When increasing Zn2+ concentration to 0.06–0.1 M, rod-based flowers, in the form of rods of 150 nm diameters and of 2 lm lengths stretching radially from center are obviously seen in Fig. 1(g)–(h). Consistently, flower-like ZnO structures are obviously observed on TEM images (Fig. 1(c), (f) and (i)), and such flowers are assembled by several spindles (c), flakes (f) and rods (i). Evidently, in comparison with nanospindles and nanorods, the ultrathin nanoflakes possess the smallest grain size and most interfacial boundaries. Thus, the flake-based ZnO are expected to absorb more dyes and harvest improved UV-light, resulting in the enhanced photocatalytic activities. Fig. 2(a) exhibits the XRD patterns of as-prepared ZnO simples. It is noteworthy that all the diffraction peaks can be indexed as (0 0 2), (1 0 0), (1 0 1), (1 0 2), (1 0 3), (1 1 0), (1 1 2), (2 0 0) and (2 0 1) crystal planes of the hexagonal wurtzite structure of ZnO (JCPDS 36-1451). No extra peak is detected by XRD, indicating the superior purity of the ZnO simples. Fig. 2(b) shows the UV–vis absorption spectra of the different ZnO flowers. Clearly, all the products exhibit a broad and strong absorption in the UV-light range (lower than 400 nm), which comes from the wide-band gap of ZnO. It is noteworthy that the flake-based ZnO has the stronger absorption intensity among these three architectures, which may reveal more light-harvesting and better photocatalytic behavior under UV-light irradiation. The nitrogen adsorption and desorption isotherms of the as-synthesized ZnO powders are shown in Fig. 2(c). The specific surface area for spindle-based, flake-based and rod-based ZnO flowers are, 9.34, 21.55 and 2.49 m2/g, respectively. The much larger specific surface area of flake-based ZnO structure is may
Fig. 1. SEM images of ZnO simples formed in different concentration of Zn2+: 0.001–0.004 M (a)–(b), 0.01–0.025 M (d)–(e), 0.06–0.1 M (g)–(h); TEM images of ZnO simples formed in different concentration of Zn2+: 0.001–0.004 M (c), 0.01–0.025 M (f), 0.06–0.1 M (i).
M. Cao et al. / Materials Letters 192 (2017) 1–4
3
Fig. 2. XRD patterns (a) and UV–visible absorbance spectra (b) of ZnO simples; Nitrogen adsorption–desorption isotherms (c) and pore size distributions (d) of ZnO simples.
be due to the flourish, ultrathin nanoflakes as well as the smaller grain size and the more interfacial boundaries. All the products exhibit distinct hysteresis loops in the range of 0.45–1.0 P/P0, which suggests the presence of mesoporous structures. Pore size distributions of three samples are displayed in Fig. 2(d), and the pores distribute ranging from 3 to 65 nm. As observed in the pore size distributions of spindle-based ZnO, the smaller pores with a sharp peak at about 4 nm may be generated between the nanoparticles on the spindles. The larger pores with a wide distribution in all simples can be attributed to the spaces between the intercrossed ZnO nanobranches. It is note mentioning that the flakebased ZnO possess much more mesopores than others, which could
provide improved transfer paths for light-generated charge carriers and further facilitate the photocatalytic reactions in the degradation processes. The degradation rates of the ZnO simples are displayed in Fig. 3. Remarkably, the flake-based ZnO exhibits superior catalytic efficiencies than other structures on degrading dyes of RhB (Fig. 3 (a)), MO (Fig. 3(b)) and MB (Fig. 3(c)). Under UV-light irradiation of 40 min, the flake-based ZnO successfully degraded 95% of the RhB, while 72% and 68% of the RhB was degraded for spindlebased ZnO and rod-based ZnO, respectively. Similarly, the flake-based ZnO also exhibits the most impressive activities on degrading MO and MB. For flake-based ZnO, the concentration of
Fig. 3. Photocatalytic rates of ZnO simples on degrading RhB (a), MO (b) and MB (c).
4
M. Cao et al. / Materials Letters 192 (2017) 1–4
MO and MB decreased 79% and 98%, respectively, under UV-light irradiation of 40 min. On the basic of the photocatalytic test results, the excellent photocatalytic activities of the flake-based ZnO can be attributed to three parts: Firstly, the very fined grains and plenty of boundaries leads to the enhanced dye-adsorption and light-harvesting. Secondly, the numerous nanograins and interfaces can further promote photogenerated electrons/holes diffusion through the whole hierarchical architecture. Thirdly, the rich mespores could provide additional routes for light-generated charge transferring in the degradation processes. The photocatalytic degradation reaction of dyes with ZnO as catalyst is depicted as follows. Firstly, generation of holes (h+VB) in the VB and electrons (e CB) in the CB through absorption of adequate photon energy, as listed in Eq. (1). Secondly, h+VB will react with water or hydroxyl groups to generate OH (Eqs. (2) and (3)). Thirdly, hydroxyl radical OH can react with RhB molecules and degrade them into nontoxic products (CO2 and H2O) (Eq. (4))
ZnO þ hm ! ZnO eCB þ hVB þ
ð1Þ
þ
ð2Þ
hVB þ OH ! OH
þ
ð3Þ
ð4Þ
hVB þ H2 O ! Hþ þ OH
OH þ RhB ! H2 O þ CO2
4. Conclusions In summary, the shape-controlled ZnO flowers of spindle-based, flake-based and rod-based were successfully fabricated by a low temperature wet chemical method. Among these three different
architectures of flower-like ZnO, the flake-based ZnO exhibits the most favorable photocatalytic properties. The improved efficiency is mainly attributed to the very fined grains, enormous interface area and abundant mesopores, which lead to substantial dyeadsorption, effective light-harvesting and fast light-generated charge transferring in the degradation processes. Acknowledgment This work was supported by the National Natural Science Foundation of China (51472153, 51171096). References [1] M.R. Hoffmann, W. Choi, D.W. Bahnemann, Chem. Rev. 95 (1) (1995) 69–96. [2] S. Cao, J. Low, J. Yu, M. Jaroniec, Adv. Mater. 27 (13) (2015) 2150–2176. [3] H. Zhou, Y. Qu, T. Zeid, X. Duan, Energy Environ. Sci. 5 (5) (2012) 6732– 6743. [4] H. Xu, S. Ouyang, L. Liu, P. Reunchan, N. Umezawa, J. Ye, J. Mater. Chem. A 2 (32) (2014) 12642–12661. [5] Y. Hu, X. Gao, L. Yu, Y. Wang, J. Ning, S. Xu, X.W. Lou, Angew. Chem. Int. Ed. 52 (21) (2013) 5636–5639. [6] D. Hong, W. Zang, X. Guo, Y. Fu, H. He, J. Sun, L. Xing, B. Liu, X. Xue, ACS Appl. Mater. Interfaces 8 (33) (2016) 21302–21314. [7] M. Yoon, J.E. Lee, J.J. Yu, L.J. Won, A. Rani, H.K. Dong, ACS Appl. Mater. Interfaces 7 (38) (2015) 21073–21081. [8] A. Ghosh, P. Guha, A.K. Samantara, B.K. Jena, R. Bar, S.K. Ray, P.V. Satyam, ACS Appl. Mater. Interfaces 7 (18) (2015) 9486–9496. [9] H. Lu, S. Wang, L. Zhao, J. Li, B. Dong, Z. Xu, J. Mater. Chem. 21 (12) (2011) 4228–4234. [10] Y.Y. Chen, C.C. Kuo, B.Y. Chen, P.C. Chiu, P.C. Tsai, J. Polym. Sci., Part B Polym. Phys. 53 (4) (2015) 262–269. [11] B.B. Straumal, A.A. Mazilkin, S.G. Protasova, S.V. Stakhanova, P.B. Straumal, M. F. Bulatov, G. Schütz, T. Tietze, E. Goering, B. Baretzky, Rev. Adv. Mater. Sci. 41 (1) (2015) 61–71. [12] B.B. Straumal, S.G. Protasova, A.A. Mazilkin, E. Goering, G. Schütz, P.B. Straumal, B. Baretzky, Beilstein J. Nanotechnol. 7 (2016) 1936–1947.