Diameter control of ultrathin zinc oxide nanofibers synthesized by electrospinning
© Liao et al.; licensee Springer. 2014
Received: 22 April 2014
Accepted: 21 May 2014
Published: 29 May 2014
Electrospinning is a versatile technique, which can be used to generate nanofibers from a rich variety of materials. We investigate the variation of a zinc oxide (ZnO)-polyvinylpyrrolidone (PVP) composite structure in morphology by electrospinning from a series of mixture solutions of ZnO sol–gel and PVP. Calcination conditions for the crystallization of ZnO nanofibers and removal of the PVP component from the ZnO-PVP composite nanofibers were also studied. The progression of the ZnO-PVP composite structure from grains to nanofibers was observed, and ZnO-PVP nanofibers as thin as 29.9 ± 0.8 nm on average were successfully fabricated. The size of the resultant ZnO-PVP composite nanofibers was considerably affected by two parameters: the concentrations of zinc acetate and PVP in the precursor solution. The concentration of zinc acetate particularly influenced the diameter distribution of the ZnO-PVP nanofibers. The ZnO-PVP nanofibers could be subsequently converted into ZnO nanofibers of a pure wurtzite phase via calcination in air at 500°C for 2 h.
KeywordsZinc oxide nanofiber Diameter control Crystallization Electrospinning
One-dimensional zinc oxide (ZnO) nanostructures have attracted considerable attention within the last decade because of unique characteristics such as large aspect ratio, high electron mobility, and electrical and optical anisotropy [1, 2]. Their potential applications in various functional devices, including sensors, solar cells, photodetectors, etc., have been noted [3, 4]. Most reported methods for synthesizing one-dimensional ZnO nanostructures follow vapor-solid, vapor-liquid–solid, solution-solid, and solvothermal routes [2, 5, 6].
Electrospinning is a simple and versatile method along the solution-solid route for producing oxide nanofibers [4, 7–10]. Although extensive investigations on the synthesis of ZnO nanofibers by electrospinning, including geometrical directional alignment , hydrophobicity , electrical properties [3, 13], and growth of nanograins , have been reported, size control of ZnO nanofibers, especially on the 10-nm scale, has been seldom addressed. Such research, however, is important not only for understanding the mechanism of the electrospinning process but also for widening the field of geometry-dependent applications of ZnO nanofibers.
In this work, a mixture of ZnO sol–gel solution and polyvinylpyrrolidone (PVP) (Mw = 1,300,000, Aldrich, St. Louis, MO, USA) in ethanol was used for electrospinning [15, 16]. In a typical procedure, 43.9 mg of Zn(CH3COO)2 · 2H2O was first dissolved in a monoethanolamine (MEA)-2-methoxyethanol solution at room temperature. The molar ratio of MEA to zinc acetate was kept at 1.0, and the concentration of zinc acetate was 0.1 mol/L. The resultant mixture was stirred at 60°C for 30 min to obtain a transparent and homogeneous solution. Then an ethanol solution containing 0.2 g PVP was added to the ZnO sol–gel solution, and the mixture was loaded into a glass capillary with a 100-μm inner diameter at the blunt tip.
Stable high voltage between 0 and 20 kV was generated by a power supply (ETM3-20K01PN1, Element, Sagamihara-shi, Kanagawa, Japan) and applied to the solution through a copper wire in the glass capillary. In addition, an indium tin oxide (ITO)-coated glass substrate (25 mm × 25 mm) was placed perpendicular to the axis of the capillary at a distance of 10 cm from its tip as a counter electrode. This counter electrode was connected to the ground along with the high-voltage power supply.
Three groups of samples were electrospun at 6.0 kV from the precursor solutions, which contained 0.1, 0.4, and 0.75 M zinc acetate, respectively. PVP solution was added into the precursor solution before electrospinning at concentrations varying from 0.02 to 0.14 g/mL for each group. A portion of the synthesized ZnO nanofibers were treated at 300°C in air for 10 min, and the others were calcined at 500°C in a programmable furnace for 2 h. Scanning electron microscope (SEM) images were taken using a field-emission SEM (S-4100, HITACHI, Chiyoda-ku, Japan) operated at an accelerating voltage of 15 kV. The diameters of these fibers were quantitatively evaluated using their high-magnification SEM images. Transmission electron microcopy (TEM) images were taken using a Tecnai G2 20 microscope operated at 200 kV. The X-ray diffraction (XRD) pattern was recorded with a D/MAX Ultima III diffractometer (Cu Kα radiation) at a scanning rate of 0.02°/s in 2θ ranging from 20° to 80°.
Results and discussion
In summary, we have demonstrated that the diameter of electrospun ZnO-PVP composite nanofibers can be controlled in the range from hundreds of nanometers down to less than 30 nm. The effects of two key factors, the molar concentration of zinc acetate in the ZnO sol–gel solution and the concentration of PVP in the precursor solution, on the morphology and diameter of the electrospun fibers were discussed, and the calcination condition for generating pure crystalline ZnO nanofibers was also investigated. Pure wurtzite-phase ZnO nanofibers with a clear lattice image in the TEM observation were formed after calcination at 500°C for 2 h. We hope to apply these results to the manufacture of ultrathin ZnO nanofibers for solar cells with increased contacting area and better charge collection efficiency, which is currently underway in our laboratory. We believe that the diameter control method described here may extend the application of ZnO nanofibers to more diameter-dependent devices.
indium tin oxide
scanning electron microscope
transmission electron microcopy
The authors gratefully acknowledge the support by the Frontier Photonics Project of the Ministry of Education, Culture, Sports, Science and Technology, Japan.
- Park JA, Moon J, Lee SJ, Lim SC, Zyung T: Fabrication and characterization of ZnO nanofibers by electrospinning. Curr Appl Phys 2009, 9: S210-S212. 10.1016/j.cap.2009.01.044View Article
- Yi GC, Wang CR, Park WI: ZnO nanorods: synthesis, characterization and applications. Semicond Sci Technol 2005, 20: S22-S34. 10.1088/0268-1242/20/4/003View Article
- Stafiniak A, Boratynski B, Baranowska-Korczyc A, Macherzynski W, Fronc K, Paszkiewicz R, Tlaczala M, Elbaum D: Technology of ZnO nanofibers based devices. Mater Sci Eng B-Adv 2012, 177: 1299–1303. 10.1016/j.mseb.2012.03.013View Article
- Thavasi V, Singh G, Ramakrishna S: Electrospun nanofibers in energy and environmental applications. Energ Environ Sci 2008, 1: 205–221. 10.1039/b809074mView Article
- Fan ZY, Lu JG: Zinc oxide nanostructures: synthesis and properties. J Nanosci Nanotechnol 2005, 5: 1561–1573. 10.1166/jnn.2005.182View Article
- Gomez JL, Tigli O: Zinc oxide nanostructures: from growth to application. J Mater Sci 2013, 48: 612–624. 10.1007/s10853-012-6938-5View Article
- Li D, McCann JT, Xia YN: Electrospinning: a simple and versatile technique for producing ceramic nanofibers and nanotubes. J Am Ceram Soc 2006, 89: 1861–1869. 10.1111/j.1551-2916.2006.00989.xView Article
- Wu H, Pan W: Preparation of zinc oxide nanofibers by electrospinning. J Am Ceram Soc 2006, 89: 699–701. 10.1111/j.1551-2916.2005.00735.xView Article
- Li D, Xia YN: Fabrication of titania nanofibers by electrospinning. Nano Lett 2003, 3: 555–560. 10.1021/nl034039oView Article
- Ramaseshan R, Sundarrajan S, Jose R, Ramakrishna S: Nanostructured ceramics by electrospinning. J Appl Phys 2007, 102: 111101–1-111101–17.View Article
- Haider S, Al-Zeghayer Y, Ali FAA, Haider A, Mahmood A, Al-Masry WA, Imran M, Aijaz MO: Highly aligned narrow diameter chitosan electrospun nanofibers. J Polym Res 2013, 20: 105–1-105–11.View Article
- Ding B, Ogawa T, Kim J, Fujimoto K, Shiratori S: Fabrication of a super-hydrophobic nanofibrous zinc oxide film surface by electrospinning. Thin Solid Films 2008, 516: 2495–2501. 10.1016/j.tsf.2007.04.086View Article
- Park JY, Kim JJ, Kim SS: Electrical transport properties of ZnO nanofibers. Microelectron Eng 2013, 101: 8–11.View Article
- Park JY, Kim SS: Growth of nanograins in electrospun ZnO nanofibers. J Am Ceram Soc 2009, 92: 1691–1694. 10.1111/j.1551-2916.2009.03119.xView Article
- O'Brien S, Koh LHK, Crean GM: ZnO thin films prepared by a single step sol–gel process. Thin Solid Films 2008, 516: 1391–1395. 10.1016/j.tsf.2007.03.160View Article
- Ohyama M, Kozuka H, Yoko T: Sol–gel preparation of ZnO films with extremely preferred orientation along (002) plane from zinc acetate solution. Thin Solid Films 1997, 306: 78–85. 10.1016/S0040-6090(97)00231-9View Article
- Li D, Xia YN: Electrospinning of nanofibers: reinventing the wheel? Adv Mater (Weinheim, Ger) 2004, 16: 1151–1170. 10.1002/adma.200400719View Article
- Mali SS, Kim H, Jang WY, Park HS, Patil PS, Hong CK: Novel synthesis and characterization of mesoporous ZnO nanofibers by electrospinning technique. ACS Sustain Chem Eng 2013, 1: 1207–1213. 10.1021/sc400153jView Article
This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly credited.