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Morphology and optical properties of ZnO particles modified by diblock copolymer
Materials Letters 64 (2010) 500–502
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
Morphology and optical properties of ZnO particles modified by diblock copolymer N. Samaele a, P. Amornpitoksuk a,⁎, S. Suwanboon b a b
Department of Chemistry and Center for Innovation in Chemistry, Faculty of Science, Prince of Songkla University, Hat Yai, Songkhla 90112, Thailand Department of Materials Science and Technology, Faculty of Science, Prince of Songkla University, Hat Yai, Songkhla 90112, Thailand
a r t i c l e
i n f o
Article history: Received 30 October 2009 Accepted 19 November 2009 Available online 3 December 2009 Keywords: ZnO Diblock copolymer Precipitation Optical properties
a b s t r a c t Biprism, rugby cone and triangle-like shapes of ZnO particles were synthesized directly from an aqueous zinc acetate dihydrate solution in the presence of poly(ethylene oxide)-b-poly(propylene oxide) copolymer and sodium hydroxide at a pH of 8, 10 and 12, respectively. The particle sizes of their ZnO particles decreased with an increase of pH values. Furthermore, it had been found that the estimated band gap value and the emission peak in the UV region showed a blue shift as a dependence on their particle sizes. © 2009 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental
Zinc Oxide (ZnO) is an n-type semiconductor with a wide direct band gap (3.37 eV) and a large exciton binding energy (60 meV). Currently, the morphological control of ZnO particles has been extensively investigated and there are numerous methods in the literature for preparing the ZnO particles including both wet and dry methods [1–3]. Precipitation is one of the most effective methods because it provides a simple and effective way to control the various shapes and sizes of the ZnO particle by starting materials and/or stabilizers without the need for high temperatures and sophisticated equipment. Bahadur et al. [4] reported that the ZnO films grown by the different precursor materials show the different morphological features. The stabilizers have a potential for changing the shape of ZnO particles to nanorod [5,6], nanodisk [6], nanowire [7] hollow microsphere [8], etc. Many di- and triblock copolymers have been used for preparing ZnO particles and thin films [9–12]. However, there has been no report of the effect of poly(ethylene oxide)-b-poly(propylene oxide) (PEO)19-b-(PPO)3. In this present work, we have investigated the effect of poly (ethylene oxide)-b-poly(propylene oxide) (PEO)19-b-(PPO)3 as a new diblock copolymer on the morphological control of ZnO particles and also report the optical properties of ZnO particles prepared from the (PEO)19-b-(PPO)3-assisted Zn2+ solution at pH 8–12 by the precipitation method.
In a typical procedure, 3 mmol PEO-b-PPO (MW 1000, Huntsman, USA) was added in each aqueous Zn(C2H3O2)2·2H2O (Fluka) solution so a mole ratio of Zn2+:PEO-b-PPO was 1:0.1. Finally, 0.2 M NaOH was added dropwise to the PEO-b-PPO-modified Zn2+ precursor solutions until the pH of the solutions reached 8, 10 and 12, respectively, the precipitates were then moderately stirred at 70 °C for 1 h. After being cooled to room temperature, these precipitates were filtered, rinsed with distilled water several times, then collected and dried at 100 °C for 1 h in air. The structural identification of ZnO particles was carried out using an X-ray diffractometer (XRD, X'Pert MPD, Philips) with Cu Kα radiation. Morphological studies used a scanning electron microscope (SEM, JSM-5800, JEOL). The UV–vis diffuse reflectance spectra were recorded on a UV–vis spectrophotometer (UV-2450, Shimadzu). The room temperature photoluminescence (PL) measurement was performed by a luminescence spectrophotometer (LS/55, Perkin Elmer).
⁎ Corresponding author. Department of Chemistry and Center for Innovation in Chemistry, Faculty of Science, Prince of Songkla University, Hat Yai, Songkhla 90112, Thailand. Tel.: + 66 74 28 84 38; fax: + 66 42 12 918. E-mail addresses: [email protected] (P. Amornpitoksuk), [email protected] (S. Suwanboon). 0167-577X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2009.11.055
3. Results and discussion The as-prepared ZnO particles synthesized from the solutions containing the mole ratio of Zn2+:PEO-b-PPO = 1:0.1 in the pH range of 8–12 were characterized by XRD and the results are displayed in Fig. 1. All diffraction peaks were identified as belonging to the hexagonal or wurtzite ZnO structure in accordance with the JCPDS card number 36-1451. Therefore, it could be concluded that ZnO can be formed at low temperature without a calcining process. The crystallite size calculated from the Scherrer equation [13], decreased from 53.3 nm (pH 8) to 33.3 nm (pH 12) as a function of the OH−
N. Samaele et al. / Materials Letters 64 (2010) 500–502
501
Fig. 1. XRD patterns and SEM images of as-prepared ZnO particle precipitated at mole ratios of Zn2+:PEO-b-PPO = 1:0.1 at pH (a) 8, (b) 10 and (c) 12, respectively.
concentration. The ZnO shape tends to form a biprismatic structure at pH 8, a rugby corn structure at pH 10 and triangle-like shape at pH 12 as shown in Fig. 1. An estimate of the value of the band gap according to the Kubelka– Munk model [14] was calculated from a plot of (αhv)2 vs. photon energy (hv) as shown in Fig. 2(a). In this investigation, the Eg values of 3.194, 3.240 and 3.268 eV were achieved as the pH value was increased from 8 to 12. It is evident that the absorption edge is slightly shifted towards the blue with a decrease of the particle size. The room temperature PL spectra of ZnO particles prepared from the solutions containing a pH in the range of 8 to 12 are presented in Fig. 2(b). The emission peak centered at about 390 nm in the UV region is well understood as being the near-band-edge emission [15]. As the concentration of NaOH was increased, a blue shift of these emissions was observed as shown in the inset of Fig. 2(b). This evidence corresponds with the obtained band gap values of ZnO particles as shown in Fig. 2(a). The broad visible emission originated from structural defects, e.g. oxygen vacancies and zinc interstitials. Based on the deconvolution of this broad band (shown as three gray curves in Fig. 2(b)), a green band (500 nm), a yellow band (620 nm) and an infrared band (740 nm) were revealed. In the case of the ZnO material, the yellow and infrared emissions were reported to be due to oxygen excess or extrinsic defects [15,16] while the origin of the green emission has been attributed to oxygen vacancies [15,17]. In this work, the ZnO prepared from the solution at pH 10 shows a
highest intensity of visible band but a lowest one is the ZnO prepared from the solution at pH 8. Yu et al. [13] reported a progressive increase in the visible emission intensity by the decrease in the aspect ratios of ZnO that implies a higher surface area to volume ratio. ZnO has a lot of defects on or near the surface which can adsorb the O2− and O− ions to form the oxygen vacancies (VO, VO⁎ and VO⁎⁎). The photogenerated holes are trapped into surface defects and can tunnel back into the particle. When it combines with a VO⁎ to form VO⁎⁎ center, it leads to the transition responsible for the visible emission [18–20]. This tunneling rate decreases as the surface to bulk ratio or surface area decreases [18]. The open rugby cone structure has a highest surface area. On the other hand, the surface area of biprism structure is lower than the smaller triangle-like shape.
4. Conclusion With an increase of the pH values, the biprism shape of ZnO particles transforms to the rugby cone structure and then to the small triangle-like shape. The UV–vis spectra show an adsorption band edge shifts to a shorter wavelength in accordance with an increase of band gap value owing to the reduction in the particle size. At pH 10, the asprepared ZnO particles exhibit a strongest PL intensity in visible region. The ruby cone structure has a very high surface area that can increase oxygen defects in ZnO particles.
502
N. Samaele et al. / Materials Letters 64 (2010) 500–502
is gratefully acknowledged. We would like to thank Dr. Brian Hodgson for English corrections.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]
Fig. 2. The estimated band gap (a) and room temperature photoluminescence (b) of asprepared ZnO particles precipitated at various pH values. The gray curves depict a result of fitting analysis using a Gaussian function.
Acknowledgments Financial support from the Center for Innovation in Chemistry (PERCH-CIC), Commission on Higher Education, Ministry of Education
Wand D, Zhang J, Hu Y, Li G, Bi Z, Zhang X, et al. Mater Lett 2009;63:2157. Sun XM, Chen X, Deng X, Li D. Mater Chem Phys 2006;78:99. Mndai S, Goswami MLN, Das K, Dhar A, Ray SK. Thin Solid Films 2008;516:8702. Bahadur H, Srivastava AK, Sharma RK, Chandra S. Nanoscale Res Lett 2007;2:469. Volk J, Nagata T, Erdelyi R, Barsony I, Toth AL, Lukacs IE, et al. Nanoscale Res Lett 2009;4:699. Long T, Yin S, Tkabatake K, Zhnag P, Sato T. Nanoscale Res Lett 2009;4:247. Qurashi A, Tabet N, Faiz M, Yamzaki T. Nanoscale Res Lett 2009;4:948. Yan C, Xue D. J Phys Chem B 2006;110:11076. Ali HA, Iliadis AA. Thin Solid Films 2004;469–470:425. Zhang Z, Mu J. J Colloid Interface Sci 2007;307:79. Tao J, Chen X, Sun Y, Shen Y, Dai N. Colloids Sur A 2008;330:67. Pal E, Oszko A, Mela P, Moller M, Dekany I. Colloids Sur A 2008;331:213. Weller MT. Inorganic Material Chemistry. UK: Oxford Chemistry; 1996. p. 15. Yu J, Li C, Liu S. J Colloid Interface Sci 2008;326:433. Hung Z, Yan D, Yang M, Liao X, Kang Y, Yin G, et al. J Colloid Interface Sci 2008;325:356. Zhao X, Chen Z, Luo Y, Wng L. Solid State Comm 2008;147:447. Hernaadez GM, Morles AE, Pal U. Crystal Growth & Design 2009;9:297. Sun G, Cao M, Wang Y, Hu C, Liu Y, Ren L, et al. Mat Lett 2006;60:2777. Gong Y, Andelman T, Neumark GF, O'Brien S, Kuskovsky IL. Nanoscale Res Lett 2007;2:297. Li H, Liang C, Zhong K, Liu M, Hope GA, Tong Y, et al. Nanoscale Res Lett 2009;4:1183.
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
Morphology and optical properties of ZnO particles modified by diblock copolymer N. Samaele a, P. Amornpitoksuk a,⁎, S. Suwanboon b a b
Department of Chemistry and Center for Innovation in Chemistry, Faculty of Science, Prince of Songkla University, Hat Yai, Songkhla 90112, Thailand Department of Materials Science and Technology, Faculty of Science, Prince of Songkla University, Hat Yai, Songkhla 90112, Thailand
a r t i c l e
i n f o
Article history: Received 30 October 2009 Accepted 19 November 2009 Available online 3 December 2009 Keywords: ZnO Diblock copolymer Precipitation Optical properties
a b s t r a c t Biprism, rugby cone and triangle-like shapes of ZnO particles were synthesized directly from an aqueous zinc acetate dihydrate solution in the presence of poly(ethylene oxide)-b-poly(propylene oxide) copolymer and sodium hydroxide at a pH of 8, 10 and 12, respectively. The particle sizes of their ZnO particles decreased with an increase of pH values. Furthermore, it had been found that the estimated band gap value and the emission peak in the UV region showed a blue shift as a dependence on their particle sizes. © 2009 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental
Zinc Oxide (ZnO) is an n-type semiconductor with a wide direct band gap (3.37 eV) and a large exciton binding energy (60 meV). Currently, the morphological control of ZnO particles has been extensively investigated and there are numerous methods in the literature for preparing the ZnO particles including both wet and dry methods [1–3]. Precipitation is one of the most effective methods because it provides a simple and effective way to control the various shapes and sizes of the ZnO particle by starting materials and/or stabilizers without the need for high temperatures and sophisticated equipment. Bahadur et al. [4] reported that the ZnO films grown by the different precursor materials show the different morphological features. The stabilizers have a potential for changing the shape of ZnO particles to nanorod [5,6], nanodisk [6], nanowire [7] hollow microsphere [8], etc. Many di- and triblock copolymers have been used for preparing ZnO particles and thin films [9–12]. However, there has been no report of the effect of poly(ethylene oxide)-b-poly(propylene oxide) (PEO)19-b-(PPO)3. In this present work, we have investigated the effect of poly (ethylene oxide)-b-poly(propylene oxide) (PEO)19-b-(PPO)3 as a new diblock copolymer on the morphological control of ZnO particles and also report the optical properties of ZnO particles prepared from the (PEO)19-b-(PPO)3-assisted Zn2+ solution at pH 8–12 by the precipitation method.
In a typical procedure, 3 mmol PEO-b-PPO (MW 1000, Huntsman, USA) was added in each aqueous Zn(C2H3O2)2·2H2O (Fluka) solution so a mole ratio of Zn2+:PEO-b-PPO was 1:0.1. Finally, 0.2 M NaOH was added dropwise to the PEO-b-PPO-modified Zn2+ precursor solutions until the pH of the solutions reached 8, 10 and 12, respectively, the precipitates were then moderately stirred at 70 °C for 1 h. After being cooled to room temperature, these precipitates were filtered, rinsed with distilled water several times, then collected and dried at 100 °C for 1 h in air. The structural identification of ZnO particles was carried out using an X-ray diffractometer (XRD, X'Pert MPD, Philips) with Cu Kα radiation. Morphological studies used a scanning electron microscope (SEM, JSM-5800, JEOL). The UV–vis diffuse reflectance spectra were recorded on a UV–vis spectrophotometer (UV-2450, Shimadzu). The room temperature photoluminescence (PL) measurement was performed by a luminescence spectrophotometer (LS/55, Perkin Elmer).
⁎ Corresponding author. Department of Chemistry and Center for Innovation in Chemistry, Faculty of Science, Prince of Songkla University, Hat Yai, Songkhla 90112, Thailand. Tel.: + 66 74 28 84 38; fax: + 66 42 12 918. E-mail addresses: [email protected] (P. Amornpitoksuk), [email protected] (S. Suwanboon). 0167-577X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2009.11.055
3. Results and discussion The as-prepared ZnO particles synthesized from the solutions containing the mole ratio of Zn2+:PEO-b-PPO = 1:0.1 in the pH range of 8–12 were characterized by XRD and the results are displayed in Fig. 1. All diffraction peaks were identified as belonging to the hexagonal or wurtzite ZnO structure in accordance with the JCPDS card number 36-1451. Therefore, it could be concluded that ZnO can be formed at low temperature without a calcining process. The crystallite size calculated from the Scherrer equation [13], decreased from 53.3 nm (pH 8) to 33.3 nm (pH 12) as a function of the OH−
N. Samaele et al. / Materials Letters 64 (2010) 500–502
501
Fig. 1. XRD patterns and SEM images of as-prepared ZnO particle precipitated at mole ratios of Zn2+:PEO-b-PPO = 1:0.1 at pH (a) 8, (b) 10 and (c) 12, respectively.
concentration. The ZnO shape tends to form a biprismatic structure at pH 8, a rugby corn structure at pH 10 and triangle-like shape at pH 12 as shown in Fig. 1. An estimate of the value of the band gap according to the Kubelka– Munk model [14] was calculated from a plot of (αhv)2 vs. photon energy (hv) as shown in Fig. 2(a). In this investigation, the Eg values of 3.194, 3.240 and 3.268 eV were achieved as the pH value was increased from 8 to 12. It is evident that the absorption edge is slightly shifted towards the blue with a decrease of the particle size. The room temperature PL spectra of ZnO particles prepared from the solutions containing a pH in the range of 8 to 12 are presented in Fig. 2(b). The emission peak centered at about 390 nm in the UV region is well understood as being the near-band-edge emission [15]. As the concentration of NaOH was increased, a blue shift of these emissions was observed as shown in the inset of Fig. 2(b). This evidence corresponds with the obtained band gap values of ZnO particles as shown in Fig. 2(a). The broad visible emission originated from structural defects, e.g. oxygen vacancies and zinc interstitials. Based on the deconvolution of this broad band (shown as three gray curves in Fig. 2(b)), a green band (500 nm), a yellow band (620 nm) and an infrared band (740 nm) were revealed. In the case of the ZnO material, the yellow and infrared emissions were reported to be due to oxygen excess or extrinsic defects [15,16] while the origin of the green emission has been attributed to oxygen vacancies [15,17]. In this work, the ZnO prepared from the solution at pH 10 shows a
highest intensity of visible band but a lowest one is the ZnO prepared from the solution at pH 8. Yu et al. [13] reported a progressive increase in the visible emission intensity by the decrease in the aspect ratios of ZnO that implies a higher surface area to volume ratio. ZnO has a lot of defects on or near the surface which can adsorb the O2− and O− ions to form the oxygen vacancies (VO, VO⁎ and VO⁎⁎). The photogenerated holes are trapped into surface defects and can tunnel back into the particle. When it combines with a VO⁎ to form VO⁎⁎ center, it leads to the transition responsible for the visible emission [18–20]. This tunneling rate decreases as the surface to bulk ratio or surface area decreases [18]. The open rugby cone structure has a highest surface area. On the other hand, the surface area of biprism structure is lower than the smaller triangle-like shape.
4. Conclusion With an increase of the pH values, the biprism shape of ZnO particles transforms to the rugby cone structure and then to the small triangle-like shape. The UV–vis spectra show an adsorption band edge shifts to a shorter wavelength in accordance with an increase of band gap value owing to the reduction in the particle size. At pH 10, the asprepared ZnO particles exhibit a strongest PL intensity in visible region. The ruby cone structure has a very high surface area that can increase oxygen defects in ZnO particles.
502
N. Samaele et al. / Materials Letters 64 (2010) 500–502
is gratefully acknowledged. We would like to thank Dr. Brian Hodgson for English corrections.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]
Fig. 2. The estimated band gap (a) and room temperature photoluminescence (b) of asprepared ZnO particles precipitated at various pH values. The gray curves depict a result of fitting analysis using a Gaussian function.
Acknowledgments Financial support from the Center for Innovation in Chemistry (PERCH-CIC), Commission on Higher Education, Ministry of Education
Wand D, Zhang J, Hu Y, Li G, Bi Z, Zhang X, et al. Mater Lett 2009;63:2157. Sun XM, Chen X, Deng X, Li D. Mater Chem Phys 2006;78:99. Mndai S, Goswami MLN, Das K, Dhar A, Ray SK. Thin Solid Films 2008;516:8702. Bahadur H, Srivastava AK, Sharma RK, Chandra S. Nanoscale Res Lett 2007;2:469. Volk J, Nagata T, Erdelyi R, Barsony I, Toth AL, Lukacs IE, et al. Nanoscale Res Lett 2009;4:699. Long T, Yin S, Tkabatake K, Zhnag P, Sato T. Nanoscale Res Lett 2009;4:247. Qurashi A, Tabet N, Faiz M, Yamzaki T. Nanoscale Res Lett 2009;4:948. Yan C, Xue D. J Phys Chem B 2006;110:11076. Ali HA, Iliadis AA. Thin Solid Films 2004;469–470:425. Zhang Z, Mu J. J Colloid Interface Sci 2007;307:79. Tao J, Chen X, Sun Y, Shen Y, Dai N. Colloids Sur A 2008;330:67. Pal E, Oszko A, Mela P, Moller M, Dekany I. Colloids Sur A 2008;331:213. Weller MT. Inorganic Material Chemistry. UK: Oxford Chemistry; 1996. p. 15. Yu J, Li C, Liu S. J Colloid Interface Sci 2008;326:433. Hung Z, Yan D, Yang M, Liao X, Kang Y, Yin G, et al. J Colloid Interface Sci 2008;325:356. Zhao X, Chen Z, Luo Y, Wng L. Solid State Comm 2008;147:447. Hernaadez GM, Morles AE, Pal U. Crystal Growth & Design 2009;9:297. Sun G, Cao M, Wang Y, Hu C, Liu Y, Ren L, et al. Mat Lett 2006;60:2777. Gong Y, Andelman T, Neumark GF, O'Brien S, Kuskovsky IL. Nanoscale Res Lett 2007;2:297. Li H, Liang C, Zhong K, Liu M, Hope GA, Tong Y, et al. Nanoscale Res Lett 2009;4:1183.