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Polyoxometalate-assisted electrochemical deposition of ZnO spindles in an ionic liquid
Materials Letters 64 (2010) 643–645
Contents lists available at 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
Polyoxometalate-assisted electrochemical deposition of ZnO spindles in an ionic liquid Mingliang Ju, Qiuyu Li, Jianmin Gu, Rui Xu, Yangguang Li, Xinlong Wang, Enbo Wang ⁎ Key Laboratory of Polyoxometalate Science of Ministry of Education, Department of Chemistry, Northeast Normal University, Ren Min Street No. 5268, Changchun, Jinlin 130024, PR China
a r t i c l e
i n f o
Article history: Received 12 August 2009 Accepted 15 December 2009 Available online 24 December 2009 Keywords: Semiconductors Deposition Nanomaterials Luminescence
a b s t r a c t With the assistance of polyoxometalate (POM), ZnO spindles have been successfully synthesized in ionic liquid by electrochemical deposition. The as-obtained ZnO spindles are composed of small nanoparticles and have a porous structure with a specific surface area of 97.95 m2/g. Parallel experiments were also performed to understand the formation mechanism of the spindle-like ZnO. The experimental results showed that POM played a key role for the formation of the spindle-like ZnO. A possible formation mechanism was also proposed. The photoluminescence spectrum of the ZnO spindles exhibits a strong ultraviolet emission at 390 nm and a very weak visible emission at around 560 nm. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Zinc oxide (ZnO) is expected to have potential applications in the fabrication of nanoscale electronic devices and in biotechnology, photocatalysts, gas sensors [1] and electrical and optoelectrical fields [2,3] due to a direct wide band gap (3.37 eV) and large exciton binding energy (60 meV) at room temperature. Up to now, various synthetic methods have been applied to synthesize ZnO nanomaterials, such as thermolysis [4], hydrothermal [5], sol–gel [6] and microwave irradiation methods [7]. Among all the synthetic methods, electrochemical deposition has shown a powerful ability to control nanocrystalline materials. Recently, room temperature ionic liquids (RTILs) as a new solvent system have attracted great interest because of their negligible vapor pressure, high ionic conductivity, and a large electrochemical window [8]. These extraordinary physical properties give RTILs a wide variety of applications including electrodeposition, batteries and separation [9]. Polyoxometalates (POMs) are a unique class of molecular metal– oxygen clusters, which have received wide attention due to their unique molecular structure and electronic characteristics [10]. Recently, POMs were also applied to the morphology-controlled fabrication of nanostructures and has become more and more remarkable in material synthesis field [11–13]. Our group has also reported the POM-assisted synthesis of carbon nanotube and nanobelt [14], hematite hollow microspheres [15], ZnO microspheres [16] and hollow spheres [17].
Herein, by combining advantages of both RTIL and POM, α-Na9H [SiW9O34]·16H2O was introduced into electrochemical system with IL [EMIM]+Br− for morphology-controlled synthesis of ZnO spindles. The role of α-Na9H[SiW9O34] during the formation of ZnO nanostructures was investigated by a series of experiments. Moreover, optical properties of the ZnO powder were also investigated. 2. Experimental 2.1. Synthesis All of the reagents employed were used as received without further purification. [EMIM]+Br− was synthesized according to the literature procedure [18]. Commercial zinc foils were cut into 10 × 15 × 0.4 mm3 pieces and carefully cleaned using acetone followed by deionized water before used as electrodes. A zinc foil served as the working electrode, and a graphite rod was used as the counter electrode. The working and counter electrodes was parallel with each other with a distance set at 3.0 cm. An electrochemical bath consisted of a mixture of 0.2 mL 9 × 10− 3 M α-Na9H[SiW9O34]·16H2O, 6 mL hydrophilic [EMIM]+Br− and 6 mL deionized water. Electrolysis was performed under the cathodic current density of 8.9 mA/cm2 for 30 min at room temperature. ZnO products deposited at the bottom of the electrochemical cell. The product was separated by centrifugation, washed with absolute ethanol three times and finally dried at 60 °C for 2 h under vacuum. 2.2. Characterization
⁎ Corresponding author. Tel./fax: +86 431 85098787. E-mail addresses: [email protected], [email protected] (E. Wang). 0167-577X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2009.12.034
The details are provided in the Supplementary data.
644
M. Ju et al. / Materials Letters 64 (2010) 643–645
3. Results and discussion Fig. 1a and b are typical TEM images of the ZnO nanostructures with different magnifications showing exclusively spindle-like morphology, with an average diameter of about 300 nm and average length of about 600 nm. Fig. 1c shows a single ZnO spindle, implying that the spindle-like structure is composed of small nanoparticles with 5–10 nm in diameter. A typical HRTEM image of section of a small ZnO spindle is shown in Fig. 1d. The interplane spacing d is determined to be about 0.28 nm, corresponding to the spacing of the (100) planes of wurtzite ZnO. The HRTEM image further indicates the porous framework formed by the nanoparticles. Fig. 1e shows the typical XRD patterns of the obtained ZnO spindles. All of the diffraction peaks can be indexed as pure hexagonal phase of ZnO with a typical wurtzite structure (JCPDS No.36-1451, a = 3.249 Å, c = 5.206 Å). No other peak was observed. As the ZnO spindles are composed of small nanoparticles, some interesting properties might be expected. The N2 adsorption– desorption method was also employed to confirm the porous structure of the ZnO spindles. The N2 adsorption and desorption isotherms (Fig. 2) of the as-prepared ZnO spindles were categorized as type IV isotherm with a H3 type hysteresis loop in the P/P0 range of 0.45–1.0, indicating the characteristic of mesoporous materials. The BET surface area is as high as 97.95 m2/g calculated from the data in the P/P0 region of 0.01–0.30. As can be seen in the inset of Fig. 2, the corresponding BJH analyses exhibit bimodal mesoporous distribution in which the peak values are 2 and 4 nm, respectively. These pores presumably arise from the interstices among the small nanoparticles within the ZnO spindles [19].
Fig. 2. N2 adsorption–desorption isotherms and pore-size distribution curve (inset) of the ZnO spindles.
To study the influence of experimental conditions on the formation of the ZnO nanostructures, parallel experiments were performed. Only nanoparticles with average diameter about 100 nm were obtained (details in the Supplementary data) when SiW9 was absent without changing other conditions. By contrast, spindle-like nanoparticles were easily formed under the existence of polyoxometalates (Fig. 1a). Therefore, SiW9 played a crucial role in the formation process of ZnO spindles. When pure deionized water was used instead of RTILs [Emim]+Br− in the electrochemical environment, no white precipitate was observed in the same reaction time. When the electrolyte time was further prolonged to 60 min, white precipitation appeared. The result suggested that the ionic liquids can shorten the reaction time due to high ionic conductivity. The RTIL [Emim]+Br− consists of cation Emim+ and anion Br−. The good electrical conductivity and a large electrochemical window of Emim+ make it interesting medium for electrodeposition, thus leading to high quality deposits and significantly shorten reaction time [20]. The above results demonstrate that the POM (SiW9) clearly plays a crucial role in the formation of ZnO spindles. According to the above discussion, a possible growth mechanism of the ZnO spindles can be tentatively proposed as shown in Fig. 3. The entire formation of ZnO spindles may follow the growth mechanism involving in ‘nucleation– aggregation–self-assembly’ process [17]. Under the electrolysis, a large quantity of small primary ZnO nuclei are intensively generated by the rapid hydrolysis of Zn2+. Meanwhile, the freshly formed ZnO nuclei are unstable because of their high surface energy and they tend to aggregate rapidly. In the present of the POM (SiW9), the nanocrystals aggregate gradually through assembly and finally form well-defined ZnO spindles. Fig. 4 is the PL spectrum of ZnO spindles, which shows two emission peaks. One strong ultraviolet emission peaks centered at 390 nm, which comes from recombination of excitonic centers [21]. The other emission peaks centered at 560 nm referred to the near band-edge free excitonic emission and the deep level or trap state emission [22]. 4. Conclusion In summary, ZnO nanostructures with spindle-like morphology were successfully synthesized by a polyoxometalate-assisted
Fig. 1. Low-magnification (a) and high-magnification (b) TEM images of the ZnO spindles. (c) TEM image of a single ZnO spindle. (d) HRTEM image, the inset is the corresponding lattice fringe pattern. (e) XRD pattern of the as-prepared ZnO.
Fig. 3. Schematic illustration for ZnO spindles growth in the presence of POM.
M. Ju et al. / Materials Letters 64 (2010) 643–645
645
Technology Development Project Foundation of Jilin Province (no. 20060420), the Postdoctoral Station Foundation of Ministry of Education (no. 20060200002), the Testing Foundation of Northeast Normal University and the Program for Changjiang Scholars and Innovative Research Team in University. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.matlet.2009.12.034. References
Fig. 4. Photoluminescence spectrum of the as-prepared ZnO sample.
electrodeposition route in ionic liquid at room temperature. These ZnO spindles composed of small nanoparticles show high surface area and narrow mesoporous pore distribution. The polyoxometalate plays a crucial role in the formation of the ZnO spindles. The present method is a facile and environmentally benign route to prepare oxide nanometerials, which is expected to be applicable to other metal oxides in next step work. Acknowledgments This work was supported by the National Natural Science Foundation of China (nos. 20701005/20701006), the Science and
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22]
Chang S-J, Hsueh T-J, Chen I-C, Huang B-R. Nanotechnology 2008;19:175502–6. Ding Y, Kong XY, Wang ZL. J Appl Phys 2004;95:306–10. Huang MH, Mao S, Feick H, Yan HQ, Wu YY, Kind H. Science 2001;292:1897–9. Kim CG, Sung K, Chung T-M, Jung DY, Kim Y. Chem Commun 2003:2068–9. Tang Q, Zhou WJ, Shen JM, Zhang W, Qian YT. Chem Commun 2004:712–3. Spanhel L, Anderson MA. J Am Chem Soc 1991;113:2826–33. Hu XL, Gong JM, Zhang LZ, Yu JC. Adv Mater 2008;20:1–6. Welton T. Chem Res 1999;99:2071–84. Earl MJ, Seddon KR. Pure Appl Chem 2000;72:1391–8. Pope MT, Müller A. Polyoxometalate Chemistry, The Netherlands; 2001. Mandal S, Selvakannan PR, Pasricha R, Sastry M. J Am Chem Soc 2003;125:8440–1. Keita B, Liu TB, Nadjo L. J Mater Chem 2009;19:19–33. Kang ZH, Tsang CHA, Zhang ZD. J Am Chem Soc 2007;129:5326–7. Kang ZH, Wang EB, Mao BD, Su ZM, Gao L. J Am Chem Soc 2005;127:6534–5. Mao BD, Kang ZH, Wang EB, Tian CG. J Solid State Chem 2007;180:489–95. Li QY, Wang EB, Li SH, Wang CL, Tian CG, Sun GY. J Solid State Chem 2009;182:1149–55. Li QY, Kang ZH, Mao BD, Wang EB. Mater Lett 2008;62:2531–4. Emily RP, Russell EM. Chem Mater 2006;18:4882–7. Zhang QF, Chou TP, Russo B, Jenekhe SA, Cao GZ. Angew Chem Int Ed 2008;47:2402–6. Endres F. ChemPhysChem 2002;3:144–54. Vanheusden KV, Warren WL, Seager CH. J Appl Phys 1996;79:7983–90. Shim M, Guyot-Sionnest P. J Am Chem Soc 2001;123:11651–4.
Contents lists available at 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
Polyoxometalate-assisted electrochemical deposition of ZnO spindles in an ionic liquid Mingliang Ju, Qiuyu Li, Jianmin Gu, Rui Xu, Yangguang Li, Xinlong Wang, Enbo Wang ⁎ Key Laboratory of Polyoxometalate Science of Ministry of Education, Department of Chemistry, Northeast Normal University, Ren Min Street No. 5268, Changchun, Jinlin 130024, PR China
a r t i c l e
i n f o
Article history: Received 12 August 2009 Accepted 15 December 2009 Available online 24 December 2009 Keywords: Semiconductors Deposition Nanomaterials Luminescence
a b s t r a c t With the assistance of polyoxometalate (POM), ZnO spindles have been successfully synthesized in ionic liquid by electrochemical deposition. The as-obtained ZnO spindles are composed of small nanoparticles and have a porous structure with a specific surface area of 97.95 m2/g. Parallel experiments were also performed to understand the formation mechanism of the spindle-like ZnO. The experimental results showed that POM played a key role for the formation of the spindle-like ZnO. A possible formation mechanism was also proposed. The photoluminescence spectrum of the ZnO spindles exhibits a strong ultraviolet emission at 390 nm and a very weak visible emission at around 560 nm. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Zinc oxide (ZnO) is expected to have potential applications in the fabrication of nanoscale electronic devices and in biotechnology, photocatalysts, gas sensors [1] and electrical and optoelectrical fields [2,3] due to a direct wide band gap (3.37 eV) and large exciton binding energy (60 meV) at room temperature. Up to now, various synthetic methods have been applied to synthesize ZnO nanomaterials, such as thermolysis [4], hydrothermal [5], sol–gel [6] and microwave irradiation methods [7]. Among all the synthetic methods, electrochemical deposition has shown a powerful ability to control nanocrystalline materials. Recently, room temperature ionic liquids (RTILs) as a new solvent system have attracted great interest because of their negligible vapor pressure, high ionic conductivity, and a large electrochemical window [8]. These extraordinary physical properties give RTILs a wide variety of applications including electrodeposition, batteries and separation [9]. Polyoxometalates (POMs) are a unique class of molecular metal– oxygen clusters, which have received wide attention due to their unique molecular structure and electronic characteristics [10]. Recently, POMs were also applied to the morphology-controlled fabrication of nanostructures and has become more and more remarkable in material synthesis field [11–13]. Our group has also reported the POM-assisted synthesis of carbon nanotube and nanobelt [14], hematite hollow microspheres [15], ZnO microspheres [16] and hollow spheres [17].
Herein, by combining advantages of both RTIL and POM, α-Na9H [SiW9O34]·16H2O was introduced into electrochemical system with IL [EMIM]+Br− for morphology-controlled synthesis of ZnO spindles. The role of α-Na9H[SiW9O34] during the formation of ZnO nanostructures was investigated by a series of experiments. Moreover, optical properties of the ZnO powder were also investigated. 2. Experimental 2.1. Synthesis All of the reagents employed were used as received without further purification. [EMIM]+Br− was synthesized according to the literature procedure [18]. Commercial zinc foils were cut into 10 × 15 × 0.4 mm3 pieces and carefully cleaned using acetone followed by deionized water before used as electrodes. A zinc foil served as the working electrode, and a graphite rod was used as the counter electrode. The working and counter electrodes was parallel with each other with a distance set at 3.0 cm. An electrochemical bath consisted of a mixture of 0.2 mL 9 × 10− 3 M α-Na9H[SiW9O34]·16H2O, 6 mL hydrophilic [EMIM]+Br− and 6 mL deionized water. Electrolysis was performed under the cathodic current density of 8.9 mA/cm2 for 30 min at room temperature. ZnO products deposited at the bottom of the electrochemical cell. The product was separated by centrifugation, washed with absolute ethanol three times and finally dried at 60 °C for 2 h under vacuum. 2.2. Characterization
⁎ Corresponding author. Tel./fax: +86 431 85098787. E-mail addresses: [email protected], [email protected] (E. Wang). 0167-577X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2009.12.034
The details are provided in the Supplementary data.
644
M. Ju et al. / Materials Letters 64 (2010) 643–645
3. Results and discussion Fig. 1a and b are typical TEM images of the ZnO nanostructures with different magnifications showing exclusively spindle-like morphology, with an average diameter of about 300 nm and average length of about 600 nm. Fig. 1c shows a single ZnO spindle, implying that the spindle-like structure is composed of small nanoparticles with 5–10 nm in diameter. A typical HRTEM image of section of a small ZnO spindle is shown in Fig. 1d. The interplane spacing d is determined to be about 0.28 nm, corresponding to the spacing of the (100) planes of wurtzite ZnO. The HRTEM image further indicates the porous framework formed by the nanoparticles. Fig. 1e shows the typical XRD patterns of the obtained ZnO spindles. All of the diffraction peaks can be indexed as pure hexagonal phase of ZnO with a typical wurtzite structure (JCPDS No.36-1451, a = 3.249 Å, c = 5.206 Å). No other peak was observed. As the ZnO spindles are composed of small nanoparticles, some interesting properties might be expected. The N2 adsorption– desorption method was also employed to confirm the porous structure of the ZnO spindles. The N2 adsorption and desorption isotherms (Fig. 2) of the as-prepared ZnO spindles were categorized as type IV isotherm with a H3 type hysteresis loop in the P/P0 range of 0.45–1.0, indicating the characteristic of mesoporous materials. The BET surface area is as high as 97.95 m2/g calculated from the data in the P/P0 region of 0.01–0.30. As can be seen in the inset of Fig. 2, the corresponding BJH analyses exhibit bimodal mesoporous distribution in which the peak values are 2 and 4 nm, respectively. These pores presumably arise from the interstices among the small nanoparticles within the ZnO spindles [19].
Fig. 2. N2 adsorption–desorption isotherms and pore-size distribution curve (inset) of the ZnO spindles.
To study the influence of experimental conditions on the formation of the ZnO nanostructures, parallel experiments were performed. Only nanoparticles with average diameter about 100 nm were obtained (details in the Supplementary data) when SiW9 was absent without changing other conditions. By contrast, spindle-like nanoparticles were easily formed under the existence of polyoxometalates (Fig. 1a). Therefore, SiW9 played a crucial role in the formation process of ZnO spindles. When pure deionized water was used instead of RTILs [Emim]+Br− in the electrochemical environment, no white precipitate was observed in the same reaction time. When the electrolyte time was further prolonged to 60 min, white precipitation appeared. The result suggested that the ionic liquids can shorten the reaction time due to high ionic conductivity. The RTIL [Emim]+Br− consists of cation Emim+ and anion Br−. The good electrical conductivity and a large electrochemical window of Emim+ make it interesting medium for electrodeposition, thus leading to high quality deposits and significantly shorten reaction time [20]. The above results demonstrate that the POM (SiW9) clearly plays a crucial role in the formation of ZnO spindles. According to the above discussion, a possible growth mechanism of the ZnO spindles can be tentatively proposed as shown in Fig. 3. The entire formation of ZnO spindles may follow the growth mechanism involving in ‘nucleation– aggregation–self-assembly’ process [17]. Under the electrolysis, a large quantity of small primary ZnO nuclei are intensively generated by the rapid hydrolysis of Zn2+. Meanwhile, the freshly formed ZnO nuclei are unstable because of their high surface energy and they tend to aggregate rapidly. In the present of the POM (SiW9), the nanocrystals aggregate gradually through assembly and finally form well-defined ZnO spindles. Fig. 4 is the PL spectrum of ZnO spindles, which shows two emission peaks. One strong ultraviolet emission peaks centered at 390 nm, which comes from recombination of excitonic centers [21]. The other emission peaks centered at 560 nm referred to the near band-edge free excitonic emission and the deep level or trap state emission [22]. 4. Conclusion In summary, ZnO nanostructures with spindle-like morphology were successfully synthesized by a polyoxometalate-assisted
Fig. 1. Low-magnification (a) and high-magnification (b) TEM images of the ZnO spindles. (c) TEM image of a single ZnO spindle. (d) HRTEM image, the inset is the corresponding lattice fringe pattern. (e) XRD pattern of the as-prepared ZnO.
Fig. 3. Schematic illustration for ZnO spindles growth in the presence of POM.
M. Ju et al. / Materials Letters 64 (2010) 643–645
645
Technology Development Project Foundation of Jilin Province (no. 20060420), the Postdoctoral Station Foundation of Ministry of Education (no. 20060200002), the Testing Foundation of Northeast Normal University and the Program for Changjiang Scholars and Innovative Research Team in University. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.matlet.2009.12.034. References
Fig. 4. Photoluminescence spectrum of the as-prepared ZnO sample.
electrodeposition route in ionic liquid at room temperature. These ZnO spindles composed of small nanoparticles show high surface area and narrow mesoporous pore distribution. The polyoxometalate plays a crucial role in the formation of the ZnO spindles. The present method is a facile and environmentally benign route to prepare oxide nanometerials, which is expected to be applicable to other metal oxides in next step work. Acknowledgments This work was supported by the National Natural Science Foundation of China (nos. 20701005/20701006), the Science and
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22]
Chang S-J, Hsueh T-J, Chen I-C, Huang B-R. Nanotechnology 2008;19:175502–6. Ding Y, Kong XY, Wang ZL. J Appl Phys 2004;95:306–10. Huang MH, Mao S, Feick H, Yan HQ, Wu YY, Kind H. Science 2001;292:1897–9. Kim CG, Sung K, Chung T-M, Jung DY, Kim Y. Chem Commun 2003:2068–9. Tang Q, Zhou WJ, Shen JM, Zhang W, Qian YT. Chem Commun 2004:712–3. Spanhel L, Anderson MA. J Am Chem Soc 1991;113:2826–33. Hu XL, Gong JM, Zhang LZ, Yu JC. Adv Mater 2008;20:1–6. Welton T. Chem Res 1999;99:2071–84. Earl MJ, Seddon KR. Pure Appl Chem 2000;72:1391–8. Pope MT, Müller A. Polyoxometalate Chemistry, The Netherlands; 2001. Mandal S, Selvakannan PR, Pasricha R, Sastry M. J Am Chem Soc 2003;125:8440–1. Keita B, Liu TB, Nadjo L. J Mater Chem 2009;19:19–33. Kang ZH, Tsang CHA, Zhang ZD. J Am Chem Soc 2007;129:5326–7. Kang ZH, Wang EB, Mao BD, Su ZM, Gao L. J Am Chem Soc 2005;127:6534–5. Mao BD, Kang ZH, Wang EB, Tian CG. J Solid State Chem 2007;180:489–95. Li QY, Wang EB, Li SH, Wang CL, Tian CG, Sun GY. J Solid State Chem 2009;182:1149–55. Li QY, Kang ZH, Mao BD, Wang EB. Mater Lett 2008;62:2531–4. Emily RP, Russell EM. Chem Mater 2006;18:4882–7. Zhang QF, Chou TP, Russo B, Jenekhe SA, Cao GZ. Angew Chem Int Ed 2008;47:2402–6. Endres F. ChemPhysChem 2002;3:144–54. Vanheusden KV, Warren WL, Seager CH. J Appl Phys 1996;79:7983–90. Shim M, Guyot-Sionnest P. J Am Chem Soc 2001;123:11651–4.