The Effect of Stabiliser s Molarity to the Growth of ZnO Nanorods

Transcription

1 Defect and Diffusion Forum Vols (211) pp Online available since 211/Apr/2 at (211) Trans Tech Publications, Switzerland doi:1.428/ The Effect of Stabiliser s Molarity to the Growth of ZnO Nanorods Zuraida Khusaimi 1,2, a, Mohamad Hafiz Mamat 3,b, Mohamad Zainizan Sahdan 3,c, Norbani Abdullah 4,d, Mohamad Rusop 3,e 1 NANO-SciTech Centre, Institute of Science, 2 Faculty of Applied Sciences, 3 Faculty of Electrical Engineering, Universiti Teknologi MARA (UiTM), 445 Shah Alam, Selangor, Malaysia 4 Chemistry Department, University of Malaya, 563 Kuala Lumpur, Malaysia a zuraidakhusaimi@gmail.com, b hafiz_3@yahoo.com, c zainizno@gmail.com, d norbania@um.edu.my, e rusop8@gmail.com Keywords: ZnO nanorods, aqueous immersion method, Zn 2+ : HMTA molar ratio Abstract A wet chemical approach, originating from sol-gel preparation, was adopted with the intention to develop a low-temperature benign method of preparation. ZnO nanorods are successfully grown in an aqueous medium. The precursor, zinc nitrate hexahydrate (Zn(NO 3 ) 2.6H 2 O), is stabilized by hexamethylene tetraamine (HMTA). The effect of changing the molarity of HMTA to the structural orientation of ZnO nanorods is investigated. X-ray diffraction of the synthesized ZnO shows hexagonal zincite structure. The structural features of the nanocrystalline ZnO were studied by SEM. Structural features, surface morphology and differences in lattice orientation are seemingly influenced by varying the Zn 2+ : HMTA molar ratio. The formation of ZnO nanorods with blunt and sharp tips is found to be significantly affected by this ratio. Introduction Bulk zinc oxide (ZnO) is a white material with high melting point of 1975 C. Its insolubility in water arises from having both ionic and covalent bonds. The material is now fervently studied in nano-sized form as size reduction results in high surface to volume ratio, thus exhibiting highquality structural, optical and electrical properties [1]. Nano-sized ZnO is transparent, with crystals commonly found in hexagonal wurtzite structure. At room temperature, its lattice parameters are a =.325 nm and c =.52 nm. In this structure, Zn is tetrahedrally bonded to O, and the lack of centre of symmetry gives rise to piezoelectricity [2]. ZnO also has semiconducting property with a wide energy bandgap of 3.37 ev and high exciton binding energy of 6 mev [3]. Depending on the intention of its potential use, the growth techniques for nanostructured ZnO can be either through top-down or bottom-up approach, and template-assisted or template-free method. We used a wet chemical approach originating from sol-gel preparation, with growth technique known as deposition-precipitation [4]. The intention is to develop a low-temperature, benign method of preparation and study the effect in the structural and optical properties. This paper reports the effect of varying the molarity of hexamethylenetetramine as a stabilizer, on the growth of ZnO nanorods grown on gold-seeded Si substrate in homogeneous aqueous medium. 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 TTP, (ID: /7/11,18:15:29)

2 1 Diffusion in Solids and Liquids VI Experimental The flowchart for the growth ZnO nanorods by the aqueous-solution method is shown in Figure 1. Silicon (Si) wafer, one-sided polished, p-type (1), was used as a template. After the wafer was cut to the desired dimension, it was ultrasonically cleaned with acetone, methanol, and finally DI water, and then dried. Gold (Au) was sputtered on the cleaned Si in argon plasma. The thickness of gold was set to 6 nm. Figure 1. Flowchart for the growth of ZnO nanorods by the aqueous-solution method. Zinc nitrate hexahydrate (Zn(NO 3 ) 2.6H 2 O) and the stabilizer, hexamethylenetetramine (C 6 H 12 N 4 ; HMTA) were dissolved in DI water. The molar ratio of HMTA:Zn 2+ was varied from 1:2, 1:1, 2:1, 4:1 and 6:1. Aqueous solution of Zn 2+ was colourless and remained so even after stirring and heating at 6 C for 1 hour and left to aged at room temperature for 24 hours. The deposition process has been described elsewhere [5]. Structural properties of ZnO were analysed by X-ray diffractography, using Cu Kα radiation in a continuous scan from 2θ = 2-6 at a scan rate of 2θ s -1. The structural morphology was determined by scanning electron microscopy (SEM) taken at 1, times magnification at 1 kv. Results and Discussion The structural morphologies shown in Figure 2 are SEM micrographs. In general, crystalline ZnO nanorods with sharp tapered ends grew in solution when the molar ratios of HMTA:Zn 2+ were 1:2 and 1:1; the width of the tips was 8-1 nm. At higher molar ratios (higher concentration of HMTA), blunt ends are seen instead. The size of the rods remained similar in all solutions: diameter 3-5 nm and length 7 µm. The role of HMTA is as a stabilizer through its chelating effect. This helps to avoid agglomeration thus preventing the clustering of ZnO precursor molecules thus might reduce the rate of formation of ZnO which lead to the formation of smaller structures. HMTA also acts as the source of hydroxide ions essential for the eventual formation of ZnO.

3 Defect and Diffusion Forum Vols (a) 1:2 (b) 1:1 (c) 2:1 (d) 4:1 (e) 6:1 Figure 2. SEM micrographs of ZnO nanorods at 1, x mag., 1 kv, and varying molar ratios of HMTA:Zn 2+. The X-ray 2θ scan patterns of Zno nanorods show only diffraction lines for ZnO and gold. Figure 3 shows the scan in a smaller range between 3 to 4 in order to study three strongest peaks at (1), (2) and (11) which correspond to hexagonal wurtzite structure of ZnO. The a- and c-axes of orientation are correlated to (1) and (2) peaks. In thin ZnO films, (2) plane is the usual preferential plane due to lower surface energy [6]. However, our result shows that all three phases produce approximately equal strong peaks suggesting that slow growth formation was favoured for these planes. (1) plane gives the highest intensity followed by (11) and the lowest intensity is (2) (Refer to Table 1). Increased intensity as the molarity of HMTA increases was found to be the common trend for all three planes. The comparison of full width at half maximum (FWHM) of the (1) plane is shown in Figure 4(a) (e) of ZnO nanorods grown from various HMTA molarity have been characterized. Figure 5 shows dependence of XRD peak position of (1) planes versus HMTA: Zn 2+ molar ratio in ZnO nanorods. It can be seen that the 2θ values of the (1) peak remain approximately similar (at to ) at low molar HMTA and increase up to which indicate the presence of some defects in the obtained rods at ratio 6:1. This shift arises from an increase of local strain associated with the rods to surface of gold-seeded Si substrate [7]. Figure 6 shows independence of the full width at half maximum (FWHM) value of the (1) peak versus molar ratio of HMTA: Zn 2+. FWHM shows little variation, indicating that different amount of HMTA in the starting solution gives similar lattice defects to the nanorods.

4 12 Diffusion in Solids and Liquids VI 2 2:1 Intensity (a.u.) 1 1:2 1:1 1: theta (degree) 1:6 Figure 3. X-ray diffraction pattern of ZnO nanorods grown on gold-seeded Si substrate by the aqueous-solution method. Table 1. Intensity of (1), (2) and (11) planes shown as a function of HMTA:Zn 2+ molar ratio 2θ / deg. (hkl) 1:2 1:1 2:1 4:1 6: (1) (2) (11) (a) 2:1 (b) 1:1 (c) 2:1 (d) 4:1 (e) 6: to to to to to Figure 4. Full width at half maximum (FWHM) of (1) XRD peaks as a function of HMTA: Zn 2+ ratio.

5 Defect and Diffusion Forum Vols X-ray peak position (deg.) FWHM (deg.) Molar ratio of HMTA : Zn 2+ Molar ratio of HMTA : Zn Figure 5. Variation of the (1) peak position as a function of HMTA: Zn 2+ ratio. Figure 6. Variation of the FWHM of (1) peak as a function of HMTA: Zn 2+ ratio. Conclusion ZnO nanorods were successfully grown by using a benign low-temperature aqueous-solution method. It was interesting to find that sharp tip nanorods were formed at lower HMTA: Zn 2+ molar ratios (1:2 and 1:1), while nanorods with blunt ends were formed at higher ratios (2:1 6:1). However, the size of the nanorods was not significantly affected by this variation. X-ray diffraction peaks of three intense planes of (1), (2) and (11) showed that their intensity increases as the HMTA: Zn 2+ molar ratio increases. Acknowledgement We would like to thank Universiti Teknologi MARA (UiTM) and Ministry of Higher Education of Malaysia for the scholarship. Thank you also to Microwave Technology Centre of UiTM for the use of scanning electron microscope. References [1] P. Tonto, O. Mekasuwandumrong, S. Phatanasri, V. Pavarajarn, P. Praserthdam: Ceramics International Vol. 34 (28), p. 57. [2] C. Jagadish, S.J. Pearton: Zinc Oxide - Bulk, Thin Films and Nanostructures - Properties and Applications, Elsevier, Amsterdam, 26. [3] L. Schmidt-Mende, J.L. MacManus-Driscoll: Materials Today Vol. 1 (27), p. 4. [4] J.L.G. Fierro: Metal Oxides - Chemistry and Application, CRC press, Taylor and Francis Group Boca Raton, Fl, 26. [5] Z. Khusaimi, S. Amizam, H.A. Rafaie, M.H. Mamat, M.Z.Sahdan, N. Abdullah, M. Rusop: AIP Conf. Proc. Vol (29), p [6] J. Prywer: Progress in Crystal Growth and Characterization of Materials Vol. 5 (25), p. 1. [7] Z. Sofiani, B. Derkowska, P.D. ski, M. Wojdyła, S. Dabos-Seignon, M.A. Lamrani, L. Dghoughi, W. Bała, M. Addou, B. Sahraoui: Optics Communications Vol. 267 (26), p. 433.

6 Diffusion in Solids and Liquids VI doi:1.428/ The Effect of Stabiliser s Molarity to the Growth of ZnO Nanorods doi:1.428/