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PEG induced tunable morphology and band gap of ZnO
Accepted Manuscript PEG Induced Tunable Morphology and Band gap of ZnO Wenji Zheng, Rui Ding, Xiaoming Yan, Gaohong He PII: DOI: Reference:
S0167-577X(17)30694-8 http://dx.doi.org/10.1016/j.matlet.2017.04.133 MLBLUE 22554
To appear in:
Materials Letters
Received Date: Revised Date: Accepted Date:
6 February 2017 16 April 2017 28 April 2017
Please cite this article as: W. Zheng, R. Ding, X. Yan, G. He, PEG Induced Tunable Morphology and Band gap of ZnO, Materials Letters (2017), doi: http://dx.doi.org/10.1016/j.matlet.2017.04.133
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
PEG Induced Tunable Morphology and Band gap of ZnO Wenji Zheng, Rui Ding, Xiaoming Yan, Gaohong He* State Key Laboratory of Fine Chemicals, School of Petroleum and Chemical Engineering, Dalian University of Technology, 2 Dagong Road, Panjin, LN 124221, China. Corresponding Authors *E-mail: [email protected], Tel: +86-0411-84986291. Fax: +86-0411-84986291. Abstract: We demonstrate the fabrication ZnO nanostructures with tunable morphology and band gap by polyethylene glycol (PEG) assisted microemulsion strategy. Rectangular and spindle-like ZnO nanoparticles were obtained for PEG 200, 400 and 1000, while ZnO nanorods were achieved by PEG 10000. The formation mechanism was discussed correspondingly. XRD and PL spectra reveal that the defects in ZnO products are enhanced with PEG molecular weight increasing from 200 to 10000, resulting in the band gap energies decreasing from 3.12eV to 2.95eV, as demonstrated in the UV-Vis absorption spectra. The present research work can provide a new way to tune the optical property of ZnO. Keywords: Semiconductors; Polyethylene glycol; Band gap; Defects; Photoluminescence.
1
1. Introduction ZnO is an important semiconductor with a wide band gap of 3.37 eV and a large exciton binding energy of 60 meV at room temperature [1]. It has been considered as a promising material in the applications of optoelectronic field, such as photocatalysis and solar cells, due to its high activity and low cost. However, because of the wide band gap of ZnO, it can only absorb UV light, which is just 4 to 5% of solar spectrum, and this results in its poor photoelectric conversion efficiency. Therefore, it is critical to reduce the band gap of ZnO to improve its solar light harvesting capability. In fact, ZnO crystals are very rich in defects, and the native and dopant defects were reported to be responsible for the reduction of band gap [2]. Therefore several attempts of narrowing the band gap have been proposed by adding metal or nonmetal impurities [3, 4]. However, by the introduction of dopants, charge carrier recombination centers were produced and photoelectric conversion efficiency was then decreased. Therefore, how to manipulate the native defects becomes significantly important to narrow the band gap of ZnO. Correspondingly, annealing, mechanical milling and ion-beam irradiation were proposed to tune the defects of ZnO, but the large energy consuming triggers more simple ways to be explored. Recently, morphology and size which can be controlled simply by preparation process, are reported to be greatly responsible for the native defects production of ZnO. It has been well accepted that smaller size of ZnO favors more surface oxygen defects due to the higher surface area [5]. Various morphologies of ZnO, such as nanosheets, nanoflowers and nanorods, et al., have been studied for the native defects, but the influence mechanism is still unclear [6, 7]. Therefore, it still remains a big challenge to investigate the defects induced band gap narrowing, in order to improve the visible light utilization of ZnO. PEG has been used to fabricate different morphologies of ZnO. And it was proposed that short-chain PEG could produce rod-like ZnO, while long-chain PEG were favorable for spherical organization of ZnO nanostructures. Different from their reports, in this study, ZnO nanostructures with various morphologies and defects were synthesized by the synergistic action of PEG and microemulsion. We also demonstrate that the defects in ZnO can be adjusted by PEG molecular weight (Mw), which results in the narrowing of ZnO band gaps. 2. Experimental Two microemulsion mixing method was used to synthesize ZnO. Firstly, 0.2 mol/L TritonX-100 oil solution was obtained by dissolving TritonX-100 into the mixture of nheptane and n-hexanol with a mole ratio of 3:1. Then microemulsion 1 and 2
microemulsion 2 were prepared by dispersing 0.25mol/L Zn(NO3)2 and 0.5mol/L NaOH solution into equal volumes of TritonX-100 oil solution, respectively. After that two microemulsions were mixed and transferred into a 100 ml Teflon-lined stainless steel autoclave for hydrothermal reaction at 140℃ for 14h. The autoclave was cooled naturally to room temperature and the precipitates were collected by centrifugation and washed with absolute ethanol and deionized water alternatively for several times prior to drying in air at 60℃. The concentration of PEG in Zn(NO3)2 solutions was kept at 0.15 mg/L. The crystal structures of ZnO were characterized by X-ray diffraction (XRD, D/MAX2400) with CuKa radiation (λ=0.15418nm). The morphology and size of the products were characterized by Transmission Electron Microscope (TEM, Tecnai-20) operated at accelerating voltage of 200KV. The UV-Vis spectra were obtained using a UV-Visible spectrophotometer (XinMao UV-7504). The photoluminescence (PL) spectra were measured at room temperature using a Hitachi F-7000 Fluorescence spectrophotometer. The excitation wavelength was 325nm. 3. Results and discussion The XRD patterns of ZnO products are shown in Fig.1. All the diffraction peaks can be well indexed to the hexagonal wurtzite structured ZnO and the diffraction data are consistent with the JCPDS file (75-0576). The strong and sharp peaks of the XRD patterns reveal the high purity and crystallinity of the as-prepared ZnO. It is also noted that with PEG Mw increasing, the intensities of XRD diffraction peaks become weaker, especially for ZnO product obtained by PEG 10000. That demonstrates that the crystallinity of ZnO obtained by PEG 10000 is lower than those of the other ZnO products.
Fig.1
3
Different ZnO nanostructures were shown in FE-SEM images in Fig.2. It is clear that with PEG Mw increasing, the size of ZnO increases. When PEG 200, 400 and 1000 are used, rectangular and spindle-like ZnO nanoparticles are obtained, which is attributed to the combined restricting effect of microemulsion interface and PEG molecular on the growth of ZnO crystals [8, 9], as shown in Fig. 3.
Fig.2 ZnO nanorods are achieved by PEG 10000. As is well known, PEG 10000 prefers to be in the form of a long serrated chain in the oil phase of microemulsions, due to its high hydrophobicity, large molecular volume and low concentration [10]. Under the hydrogen bonding interaction between the polar O atoms in the PEG chain and the interfaces of microemulsions, more droplets tend to distribute around the PEG chain and coalesce with each other, resulting in the formation of aggregates, in which ZnO nanorods are formed.
Fig. 3 4
Fig.4a, b show the UV-Vis absorbance spectra of ZnO products at room temperature. An obvious increase in the absorption at wavelengths less than 400 nm can be ascribed to the intrinsic band gap absorption of ZnO. Particularly, an obvious red-shift in the absorption edge is observed for the ZnO products obtained with increasing PEG Mw, which is probably due to their changing morphologies and surface defects. Moreover, as shown in Fig.3b, the estimated band gap energies by K–M model [8] are 3.12eV, 3.08eV, 2.98eV, and 2.95eV for the ZnO products obtained with PEG Mw increasing from 200 to 10000. Such a decrease in the ZnO band gap energy is in good agreement with their corresponding red shift of absorption edge mentioned above.
Fig. 4 In order to reveal the defects induced band gap narrowing, the room temperature PL spectra of different ZnO nanostructures are depicted in Fig.4c. All the ZnO products show UV, blue and green emissions, locating at around 400 nm, 468 nm and 530 nm, respectively. Compared with the blue and green emissions, the UV emission is much stronger. Specifically, it exhibits an intensity reduction with the PEG Mw increasing. As is well known, UV emission comes from the recombination of free excitons through an exciton-exciton collision process [11], the intensity of which is proportional to the crystallinity. Therefore it is reasonable that UV emission becomes weaker for ZnO
5
products obtained from PEG 200 to 10000 with the reduction of crystallinity, as supported by the XRD results. The broad blue emission is apparently composed of several sub-bands, which are believed to result from the oxygen or zinc defects of ZnO [12] . Originated from intrinsic oxygen vacancy defects, green emission spreads from 529 to 600 nm and the ratio of UV to visible light intensity decreases with the PEG Mw increasing, revealing more intrinsic oxygen vacancy defects existing in ZnO nanorods. This is probably due to the surface structure induced by PEG. Sakohara [13] reported that the adsorption of acetate groups on surface of ZnO could intensify the visible luminescence. In our study, it is possible that the surface structure can be changed by polar O atoms in PEG chain bonding on the surface of ZnO. That bonding may become more obvious for PEG 10000, thus resulting in more surface defects, as proved in XRD and PL results. Therefore, in the future it is necessary to study the surface structure of ZnO to investigate the detailed mechanism of PEG induced defects change.
4. Conclusions In summary, a facile PEG assisted microemulsion strategy was developed for the fabrication of ZnO nanostructures with tunable morphologies and band gaps. Rectangular and spindle-like ZnO nanoparticles were obtained for PEG 200, 400 and 1000, while ZnO nanorods were acquired for PEG 10000. XRD and PL results indicate the existence of defects in ZnO nanostructures. Moreover, ZnO nanorods exhibit high level of defects, resulting in low band gap, as evidenced in UV-Vis absorption spectra. The band gap energies are 3.12eV, 3.08eV, 2.98eV, and 2.95eV for the ZnO products obtained with PEG Mw increasing from 200 to 10000, indicating a red shift to visible light. The present research work can provide a new way to adjust the morphology and optical property of ZnO. Acknowledgements The work was supported by the National Natural Science Foundation of China (21506028), and Changjiang Scholars Program (T2012049). References [1] K. S. Choi, H. C. Lichtenegger, G. D. Stucky, E. W. McFariand, Electrochemical Synthesis of Nanostructured ZnO Films Utilizing Self-Assembly of Surfactant Molecules at Solid-Liquid Interfaces, J. Am. Chem. Soc. 124 (2002) 12402-12403. [2] M. Guo, A. Ng, F. Liu, A. B. Djurisic, W. Chan, H. Su, Effect of native defects on photocatalytic properties of ZnO, J. Phys. Chem. C 115 (2011) 11095-11101. [3] S. U. Awan, S. K. Hasanain, G. Hassnain Jaffari, D. H. Anjum, U. S. Qurashi, 6
Defects induced luminescence and tuning of bandgap energy narrowing in ZnO nanoparticles doped with Li ions, J. Appl. Phys. 116 (2014) 083510. [4] Y. Liu, H. Liu, Y. Yu, Q. Wang, Y. Li, Z. Wang, Structural and optical properties of ZnO thin films with heavy Cu-doping prepared by magnetron co-sputtering, Mater. Lett. 143 (2015) 319-321. [5] S. Dutta, S. Chattopadhyay, A. Sarkar, M. Chakrabarti, D. Sanyal, D. Jana, Role of defects in tailoring structural, electrical and optical properties of ZnO, Prog. Mater. Sci. 54 (2009) 89-136. [6] X. Zhang, J. Qin, Y. Xue, P. Yu, B. Zhang, L. Wang L, Effect of aspect ratio and surface defects on the photocatalytic activity of ZnO nanorods, Sci. Rep. 4 (2014) 4596. [7] Z. Yang, Z. Ye, Z. Xu, Effect of the morphology on the optical properties of ZnO nanostructures, Physica E 42 (2009) 116-119. [8] X. Li, G. He, G. Xiao, H. Liu, M. Wang, Synthesis and morphology control of ZnO nanostructures in microemulsions, J. Colloid Interface Sci. 333 (2009) 465-473. [9] Z. Li, Y. Xiong, Y. Xie, Selected-control synthesis of ZnO nanowires and nanorods via a PEG-assisted route, Inorg. Chem. 42 (2003) 8105-8109. [10] G. A. Krei GA, H. Hustedt, Extraction of enzymes by reverse micelles, Chem. Eng. Sci. 47 (1992) 99-111. [11] Q. Ahsanulhaq, A. Umar, Y. B. Hahn, Growth of aligned ZnO nanorods and nanopencils on ZnO/Si in aqueous solution: growth mechanism and structural and optical properties, Nanotechnology 18 (2007) 115603. [12] E. Rauwel, A. Galeckas, P. Rauwel, M. F. Sunding, H. Fjellvåg, Precursor-dependent blue-green photoluminescence emission of ZnO nanoparticles, J. Phys. Chem. C 115 (2011) 25227-25233. [13] S. Sakohara, L. D. Tickanen, M. A. Anderson, Luminescence properties of thin zinc oxide membranes prepared by the sol-gel technique: change in visible luminescence during firing, J. Phys. Chem. C 96 (1992) 11086-11091. Figure captions Fig.1. XRD patterns for ZnO nanostructures: (a) PEG 200, (b) PEG 400, (c) PEG 1000, and (d) PEG 10000 Fig.2. FE-SEM images of ZnO nanostructures: (a) PEG 200, (b) PEG 400, (c) PEG 1000, and (d) PEG 10000 （Inserted are TEM images） Fig 3. Formation mechanism of ZnO nanostructures Fig.4. (a) UV–Vis spectra, (b) Band gaps, (c) PL spectra of ZnO nanostructures
7
Highlights
ZnO with tunable morphology and band gap is achieved by PEG assisted microemulsion strategy. A high level of defects are demonstrated to exist in ZnO nanorods. The band gap energies of ZnO decrease with PEG molecular weight increasing. .
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S0167-577X(17)30694-8 http://dx.doi.org/10.1016/j.matlet.2017.04.133 MLBLUE 22554
To appear in:
Materials Letters
Received Date: Revised Date: Accepted Date:
6 February 2017 16 April 2017 28 April 2017
Please cite this article as: W. Zheng, R. Ding, X. Yan, G. He, PEG Induced Tunable Morphology and Band gap of ZnO, Materials Letters (2017), doi: http://dx.doi.org/10.1016/j.matlet.2017.04.133
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
PEG Induced Tunable Morphology and Band gap of ZnO Wenji Zheng, Rui Ding, Xiaoming Yan, Gaohong He* State Key Laboratory of Fine Chemicals, School of Petroleum and Chemical Engineering, Dalian University of Technology, 2 Dagong Road, Panjin, LN 124221, China. Corresponding Authors *E-mail: [email protected], Tel: +86-0411-84986291. Fax: +86-0411-84986291. Abstract: We demonstrate the fabrication ZnO nanostructures with tunable morphology and band gap by polyethylene glycol (PEG) assisted microemulsion strategy. Rectangular and spindle-like ZnO nanoparticles were obtained for PEG 200, 400 and 1000, while ZnO nanorods were achieved by PEG 10000. The formation mechanism was discussed correspondingly. XRD and PL spectra reveal that the defects in ZnO products are enhanced with PEG molecular weight increasing from 200 to 10000, resulting in the band gap energies decreasing from 3.12eV to 2.95eV, as demonstrated in the UV-Vis absorption spectra. The present research work can provide a new way to tune the optical property of ZnO. Keywords: Semiconductors; Polyethylene glycol; Band gap; Defects; Photoluminescence.
1
1. Introduction ZnO is an important semiconductor with a wide band gap of 3.37 eV and a large exciton binding energy of 60 meV at room temperature [1]. It has been considered as a promising material in the applications of optoelectronic field, such as photocatalysis and solar cells, due to its high activity and low cost. However, because of the wide band gap of ZnO, it can only absorb UV light, which is just 4 to 5% of solar spectrum, and this results in its poor photoelectric conversion efficiency. Therefore, it is critical to reduce the band gap of ZnO to improve its solar light harvesting capability. In fact, ZnO crystals are very rich in defects, and the native and dopant defects were reported to be responsible for the reduction of band gap [2]. Therefore several attempts of narrowing the band gap have been proposed by adding metal or nonmetal impurities [3, 4]. However, by the introduction of dopants, charge carrier recombination centers were produced and photoelectric conversion efficiency was then decreased. Therefore, how to manipulate the native defects becomes significantly important to narrow the band gap of ZnO. Correspondingly, annealing, mechanical milling and ion-beam irradiation were proposed to tune the defects of ZnO, but the large energy consuming triggers more simple ways to be explored. Recently, morphology and size which can be controlled simply by preparation process, are reported to be greatly responsible for the native defects production of ZnO. It has been well accepted that smaller size of ZnO favors more surface oxygen defects due to the higher surface area [5]. Various morphologies of ZnO, such as nanosheets, nanoflowers and nanorods, et al., have been studied for the native defects, but the influence mechanism is still unclear [6, 7]. Therefore, it still remains a big challenge to investigate the defects induced band gap narrowing, in order to improve the visible light utilization of ZnO. PEG has been used to fabricate different morphologies of ZnO. And it was proposed that short-chain PEG could produce rod-like ZnO, while long-chain PEG were favorable for spherical organization of ZnO nanostructures. Different from their reports, in this study, ZnO nanostructures with various morphologies and defects were synthesized by the synergistic action of PEG and microemulsion. We also demonstrate that the defects in ZnO can be adjusted by PEG molecular weight (Mw), which results in the narrowing of ZnO band gaps. 2. Experimental Two microemulsion mixing method was used to synthesize ZnO. Firstly, 0.2 mol/L TritonX-100 oil solution was obtained by dissolving TritonX-100 into the mixture of nheptane and n-hexanol with a mole ratio of 3:1. Then microemulsion 1 and 2
microemulsion 2 were prepared by dispersing 0.25mol/L Zn(NO3)2 and 0.5mol/L NaOH solution into equal volumes of TritonX-100 oil solution, respectively. After that two microemulsions were mixed and transferred into a 100 ml Teflon-lined stainless steel autoclave for hydrothermal reaction at 140℃ for 14h. The autoclave was cooled naturally to room temperature and the precipitates were collected by centrifugation and washed with absolute ethanol and deionized water alternatively for several times prior to drying in air at 60℃. The concentration of PEG in Zn(NO3)2 solutions was kept at 0.15 mg/L. The crystal structures of ZnO were characterized by X-ray diffraction (XRD, D/MAX2400) with CuKa radiation (λ=0.15418nm). The morphology and size of the products were characterized by Transmission Electron Microscope (TEM, Tecnai-20) operated at accelerating voltage of 200KV. The UV-Vis spectra were obtained using a UV-Visible spectrophotometer (XinMao UV-7504). The photoluminescence (PL) spectra were measured at room temperature using a Hitachi F-7000 Fluorescence spectrophotometer. The excitation wavelength was 325nm. 3. Results and discussion The XRD patterns of ZnO products are shown in Fig.1. All the diffraction peaks can be well indexed to the hexagonal wurtzite structured ZnO and the diffraction data are consistent with the JCPDS file (75-0576). The strong and sharp peaks of the XRD patterns reveal the high purity and crystallinity of the as-prepared ZnO. It is also noted that with PEG Mw increasing, the intensities of XRD diffraction peaks become weaker, especially for ZnO product obtained by PEG 10000. That demonstrates that the crystallinity of ZnO obtained by PEG 10000 is lower than those of the other ZnO products.
Fig.1
3
Different ZnO nanostructures were shown in FE-SEM images in Fig.2. It is clear that with PEG Mw increasing, the size of ZnO increases. When PEG 200, 400 and 1000 are used, rectangular and spindle-like ZnO nanoparticles are obtained, which is attributed to the combined restricting effect of microemulsion interface and PEG molecular on the growth of ZnO crystals [8, 9], as shown in Fig. 3.
Fig.2 ZnO nanorods are achieved by PEG 10000. As is well known, PEG 10000 prefers to be in the form of a long serrated chain in the oil phase of microemulsions, due to its high hydrophobicity, large molecular volume and low concentration [10]. Under the hydrogen bonding interaction between the polar O atoms in the PEG chain and the interfaces of microemulsions, more droplets tend to distribute around the PEG chain and coalesce with each other, resulting in the formation of aggregates, in which ZnO nanorods are formed.
Fig. 3 4
Fig.4a, b show the UV-Vis absorbance spectra of ZnO products at room temperature. An obvious increase in the absorption at wavelengths less than 400 nm can be ascribed to the intrinsic band gap absorption of ZnO. Particularly, an obvious red-shift in the absorption edge is observed for the ZnO products obtained with increasing PEG Mw, which is probably due to their changing morphologies and surface defects. Moreover, as shown in Fig.3b, the estimated band gap energies by K–M model [8] are 3.12eV, 3.08eV, 2.98eV, and 2.95eV for the ZnO products obtained with PEG Mw increasing from 200 to 10000. Such a decrease in the ZnO band gap energy is in good agreement with their corresponding red shift of absorption edge mentioned above.
Fig. 4 In order to reveal the defects induced band gap narrowing, the room temperature PL spectra of different ZnO nanostructures are depicted in Fig.4c. All the ZnO products show UV, blue and green emissions, locating at around 400 nm, 468 nm and 530 nm, respectively. Compared with the blue and green emissions, the UV emission is much stronger. Specifically, it exhibits an intensity reduction with the PEG Mw increasing. As is well known, UV emission comes from the recombination of free excitons through an exciton-exciton collision process [11], the intensity of which is proportional to the crystallinity. Therefore it is reasonable that UV emission becomes weaker for ZnO
5
products obtained from PEG 200 to 10000 with the reduction of crystallinity, as supported by the XRD results. The broad blue emission is apparently composed of several sub-bands, which are believed to result from the oxygen or zinc defects of ZnO [12] . Originated from intrinsic oxygen vacancy defects, green emission spreads from 529 to 600 nm and the ratio of UV to visible light intensity decreases with the PEG Mw increasing, revealing more intrinsic oxygen vacancy defects existing in ZnO nanorods. This is probably due to the surface structure induced by PEG. Sakohara [13] reported that the adsorption of acetate groups on surface of ZnO could intensify the visible luminescence. In our study, it is possible that the surface structure can be changed by polar O atoms in PEG chain bonding on the surface of ZnO. That bonding may become more obvious for PEG 10000, thus resulting in more surface defects, as proved in XRD and PL results. Therefore, in the future it is necessary to study the surface structure of ZnO to investigate the detailed mechanism of PEG induced defects change.
4. Conclusions In summary, a facile PEG assisted microemulsion strategy was developed for the fabrication of ZnO nanostructures with tunable morphologies and band gaps. Rectangular and spindle-like ZnO nanoparticles were obtained for PEG 200, 400 and 1000, while ZnO nanorods were acquired for PEG 10000. XRD and PL results indicate the existence of defects in ZnO nanostructures. Moreover, ZnO nanorods exhibit high level of defects, resulting in low band gap, as evidenced in UV-Vis absorption spectra. The band gap energies are 3.12eV, 3.08eV, 2.98eV, and 2.95eV for the ZnO products obtained with PEG Mw increasing from 200 to 10000, indicating a red shift to visible light. The present research work can provide a new way to adjust the morphology and optical property of ZnO. Acknowledgements The work was supported by the National Natural Science Foundation of China (21506028), and Changjiang Scholars Program (T2012049). References [1] K. S. Choi, H. C. Lichtenegger, G. D. Stucky, E. W. McFariand, Electrochemical Synthesis of Nanostructured ZnO Films Utilizing Self-Assembly of Surfactant Molecules at Solid-Liquid Interfaces, J. Am. Chem. Soc. 124 (2002) 12402-12403. [2] M. Guo, A. Ng, F. Liu, A. B. Djurisic, W. Chan, H. Su, Effect of native defects on photocatalytic properties of ZnO, J. Phys. Chem. C 115 (2011) 11095-11101. [3] S. U. Awan, S. K. Hasanain, G. Hassnain Jaffari, D. H. Anjum, U. S. Qurashi, 6
Defects induced luminescence and tuning of bandgap energy narrowing in ZnO nanoparticles doped with Li ions, J. Appl. Phys. 116 (2014) 083510. [4] Y. Liu, H. Liu, Y. Yu, Q. Wang, Y. Li, Z. Wang, Structural and optical properties of ZnO thin films with heavy Cu-doping prepared by magnetron co-sputtering, Mater. Lett. 143 (2015) 319-321. [5] S. Dutta, S. Chattopadhyay, A. Sarkar, M. Chakrabarti, D. Sanyal, D. Jana, Role of defects in tailoring structural, electrical and optical properties of ZnO, Prog. Mater. Sci. 54 (2009) 89-136. [6] X. Zhang, J. Qin, Y. Xue, P. Yu, B. Zhang, L. Wang L, Effect of aspect ratio and surface defects on the photocatalytic activity of ZnO nanorods, Sci. Rep. 4 (2014) 4596. [7] Z. Yang, Z. Ye, Z. Xu, Effect of the morphology on the optical properties of ZnO nanostructures, Physica E 42 (2009) 116-119. [8] X. Li, G. He, G. Xiao, H. Liu, M. Wang, Synthesis and morphology control of ZnO nanostructures in microemulsions, J. Colloid Interface Sci. 333 (2009) 465-473. [9] Z. Li, Y. Xiong, Y. Xie, Selected-control synthesis of ZnO nanowires and nanorods via a PEG-assisted route, Inorg. Chem. 42 (2003) 8105-8109. [10] G. A. Krei GA, H. Hustedt, Extraction of enzymes by reverse micelles, Chem. Eng. Sci. 47 (1992) 99-111. [11] Q. Ahsanulhaq, A. Umar, Y. B. Hahn, Growth of aligned ZnO nanorods and nanopencils on ZnO/Si in aqueous solution: growth mechanism and structural and optical properties, Nanotechnology 18 (2007) 115603. [12] E. Rauwel, A. Galeckas, P. Rauwel, M. F. Sunding, H. Fjellvåg, Precursor-dependent blue-green photoluminescence emission of ZnO nanoparticles, J. Phys. Chem. C 115 (2011) 25227-25233. [13] S. Sakohara, L. D. Tickanen, M. A. Anderson, Luminescence properties of thin zinc oxide membranes prepared by the sol-gel technique: change in visible luminescence during firing, J. Phys. Chem. C 96 (1992) 11086-11091. Figure captions Fig.1. XRD patterns for ZnO nanostructures: (a) PEG 200, (b) PEG 400, (c) PEG 1000, and (d) PEG 10000 Fig.2. FE-SEM images of ZnO nanostructures: (a) PEG 200, (b) PEG 400, (c) PEG 1000, and (d) PEG 10000 （Inserted are TEM images） Fig 3. Formation mechanism of ZnO nanostructures Fig.4. (a) UV–Vis spectra, (b) Band gaps, (c) PL spectra of ZnO nanostructures
7
Highlights
ZnO with tunable morphology and band gap is achieved by PEG assisted microemulsion strategy. A high level of defects are demonstrated to exist in ZnO nanorods. The band gap energies of ZnO decrease with PEG molecular weight increasing. .
8