Materials Science Forum ISSN: 1662-9752, Vol. 888, pp 309-313 doi:10.4028/www.scientific.net/MSF.888.309 © 2017 Trans Tech Publications, Switzerland
Submitted: 2016-10-20 Revised: 2016-10-25 Accepted: 2016-10-25 Online: 2017-03-06
Fabrication and Characterization of ZnO Nanofibers by Electrospinning Nur Ubaidah SAIDIN1,a*, Thye Foo CHOO1,b, Kuan Ying KOK1,c, Mohd Reusmaazran YUSOF1,d and Inn Khuan NG1,e 1
Materials Technology Group, Industrial Technology Division, Malaysian Nuclear Agency, Bangi 43000 Kajang, Selangor, MALAYSIA
a
b
c
d
[email protected],
[email protected],
[email protected],
[email protected], e
[email protected]
Keywords: Electrospinning, Calcination, ZnO
Abstract. ZnO nanofibers were successfully prepared by electrospinning a precursor mixture of polyvinylpyrrolidone (PVP)/zinc acetate, followed by calcination treatment of the electrospun composite nanofibers. The effect of applied voltage to the morphology of nanofibers was studied. Both PVP/Zn acetate and ZnO nanofibers were characterized by FESEM and XRD. The results found that the diameter of the nanofibers changed with applied voltage and also show that the optimum calcination temperature was 500 °C to produce continuous ZnO nanofibers. Introduction Zinc oxide (ZnO), a II-VI compound semiconductor, has a wide energy band gap of 3.37 eV, and large exciton binding energy of 60 meV. It has a hexagonal wurtzite-type structure and is non-toxic and relatively cheap. ZnO in one-dimensional (1-D) nanostructures offer extra characteristics such as high aspect ratios, high electron mobility and possess both electrical and optical anisotropy [1, 2]. These unique multiple characteristics make ZnO suitable for various applications including solar cell [3], gas sensor [4] and photodetector [5]. There are various methods reported in fabricating 1-D ZnO such as sol-gel, electrodeposition, solvothermal routes, vapor-solid, vapor-liquid-solid, solution-solid and electrospinning. Electrospinning is one of the simple and versatile methods among others. It is a method of producing fiber from polymer solutions and melts with diameter ranging from nano to micro scale [6]. Processing parameters of electrospinning were divided into three broad groups: (1) solution parameters (solution viscosity, solution concentration and surface tension), (2) processing parameters (applied voltage, spinning distance and nozzle radius) and (3) environmental parameter (temperature, humidity and atmosphere pressure) [7]. In this work, the effect of applied voltage on the morphology and diameter of the fibers and the effect of calcination temperature on the formation of ZnO nanofibers were studied. Materials and Method 1.5 M Zn(CH3COO)2.2H2O was mixed with 10% PVP dissolved in ethanol with a volume ratio of 1:9 was used for electrospinning. The mixture was stirred for 1 h and sonicated to remove any bubbles
trapped in the solution. The solution was then loaded into a plastic syringe and connected to a highvoltage power supply. A voltage ranges between 10 - 20 kV was applied between the collector and the syringe separated at a distance of 16 cm. The solution flow rate was maintained at 0.03 ml/min. Mild steel plate (5 cm x 6 cm) covered with aluminum foil was used as the collector. Fig. 1 shows the electrospinning setup. The fibers were transferred to silicon wafers and calcination was performed in air at temperature ranging from 400 to 800 °C for 4 h to remove the organic constituents of the fibers and to form crystalline ZnO nanofibers. Both the structures of PVP/zinc acetate and ZnO fibers were characterized by X-ray diffraction (XRD) using PANAlytical XPert Pro MPD with Cu Kα irradiation. The morphologies of the nanofibers were examined by field emission scanning electron microscope (FESEM) Carl Zeiss GeminiSEM 500. All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, www.ttp.net. (#73059842-17/01/17,02:42:53)
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High voltage power supply
Syringe pump
PVP/zinc acetate jet
PVP/zinc acetate solution
Grounded collector
Syringe
Fig. 1 Schematic diagram of the electrospinning setup Results and Discussion Fig. 2 shows the FESEM micrographs of PVP/zinc acetate fibers obtained. The electrospun PVP/zinc acetate fibers give a smooth surface without any beads-in-a-string. The diameter distribution of the fibers is important for certain applications. This was analyzed using ImageJ software and Origin2015 as shown in Fig. 3 and Fig. 4, respectively. Evidently, the diameter of the fibers decreased steadily after the voltage increased from 10 kV to 16 kV. However, the diameter increased after the voltage increased to 18 and 20 kV. (a)
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1 µm
1 µm (d)
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Fig. 2 FESEM morphology for PVP/zinc acetate nanofibers (a) 10 kV, (b) 12 kV, (c) 14 kV, (d) 16 kV, (e) 18 kV, and (f) 20 kV The uniformity of the fibers’ diameter increased as the voltage increased to 14 kV. It can be seen clearly from Fig. 3, which showed that the normal distribution for the diameters improved when 14 kV voltage was applied and the standard deviation decreased by 50% from 122.8 nm to 62.5 nm when the voltage increased from 10 kV to 14 kV. The standard deviation increased after the voltage continues to increase until 20 kV (Figs. 3 and 4). These results suggested that applied voltage at 14 kV is the best for the desirable distributions of the fibers with average diameters of 186 ± 62 nm.
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(a)
(b)
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(d)
(e)
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Fig. 3 Diameter distributions for nanofibers prepared at (a) 10 kV, (b) 12 kV, (c) 14 kV, (d) 16 kV, (e) 18 kV, and (f) 20 kV
Fig. 4 Diameters of PVP/zinc acetate fibers obtained at different applied voltage After calcined at 500 °C, the fibers remained continuous (Fig. 5a) but the diameter was thinner, indicating that shrinkage of fibers occurred during calcination. Fig. 6 shows the effect of the calcination temperatures on the fibers. XRD results indicate that electrospun fibers calcined at 500 °C were successfully converted to ZnO nanofibers. Besides, results proved that calcination at 500 °C can maintain the structure of fiber while the fibers developed into dendritic structures after calcined at higher temperatures.
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(a)
(b)
2 µm (c)
2 µm (d)
2 µm
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Fig. 5 FESEM morphology after calcination of fibers spin at 14 kV at different temperature (a) 500 °C, (b) 600 °C, (c) 700 °C, and (d) 800 °C
Fig. 6 X-ray diffractograms for as-spun and calcined fibers Summary The effect of applied voltage and calcination temperature on the morphology and diameters of the electrospun PVP/zinc acetate fibers was investigated. Voltage plays an important role in controlling the uniform distributions of the fibers’ diameter. It was found that uniform PVP/zinc acetate fibers were successfully electrospun at 14 kV and the optimum temperature for calcination was 500 °C to produce continuous crystalline ZnO fibers of diameters (180 + 38) nm.
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