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The Effect of Stabilisers Molarity to the Growth of ZnO Nanorods Zuraida Khusaimi1,2, a, Mohamad Hafiz Mamat3,b,Mohamad Zainizan Sahdan3,c, Norbani Abdullah4,d, Mohamad Rusop3,e 1NANO-SciTech Centre, Institute of Science, 2Faculty of Applied Sciences,3Faculty of Electrical Engineering, Universiti Teknologi MARA (UiTM),40450 Shah Alam, Selangor, Malaysia4 Chemistry Department, University of Malaya, 50603 Kuala Lumpur, Malaysia [email protected], [email protected], [email protected], [email protected], [email protected] Keywords: ZnO nanorods, aqueous immersion method, Zn2+: HMTA molar ratioAbstractAwetchemicalapproach,originatingfromsol-gelpreparation,wasadoptedwiththe intentiontodevelopalow-temperaturebenignmethodofpreparation.ZnOnanorodsare successfullygrowninanaqueousmedium.Theprecursor,zincnitratehexahydrate (Zn(NO3)2.6H2O),isstabilizedbyhexamethylenetetraamine(HMTA).Theeffectofchangingthe molarityofHMTAtothestructuralorientationofZnOnanorodsisinvestigated.X-raydiffraction ofthesynthesizedZnOshowshexagonalzincitestructure.Thestructuralfeaturesofthe nanocrystalline ZnO were studied by SEM. Structural features, surface morphology and differences inlatticeorientationareseeminglyinfluencedbyvaryingtheZn2+:HMTAmolarratio.The formationofZnOnanorodswithbluntandsharptipsisfoundtobesignificantlyaffectedbythis ratio. Introduction Bulkzincoxide(ZnO)isawhitematerialwithhighmeltingpointof1975C.Itsinsolubilityin waterarisesfromhavingbothionicandcovalentbonds.Thematerialisnowferventlystudiedin nano-sizedformassizereductionresultsinhighsurfacetovolumeratio,thusexhibitinghigh-quality 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 = 0.325 nm and c = 0.520 nm. In this structure, Zn is tetrahedrally bonded to O, and the lack of centre ofsymmetrygivesrisetopiezoelectricity[2].ZnOalsohassemiconductingpropertywithawide energy bandgap of 3.37 eV and high exciton binding energy of 60 meV [3]. Dependingontheintentionofitspotentialuse,thegrowthtechniquesfornanostructured ZnO can be either through top-down or bottom-up approach, and template-assisted or template-free method.Weusedawetchemicalapproachoriginatingfromsol-gelpreparation,withgrowth techniqueknownasdeposition-precipitation[4].Theintentionistodevelopalow-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 hexamethylenetetramineas astabilizer, on the growth of ZnO nanorods grown on gold-seeded Si substrate in homogeneous aqueous medium. Defect and Diffusion Forum Vols. 312-315 (2011) pp 99-103Online available since 2011/Apr/20 at www.scientific.net (2011) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/DDF.312-315.99All 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,www.ttp.net. (ID: 115.135.236.13-05/07/11,18:15:29)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 (100), 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 thecleaned Si in argon plasma. The thickness ofgold was set to 6 nm. Figure 1. Flowchart for the growth of ZnO nanorods by the aqueous-solution method. Zincnitratehexahydrate(Zn(NO3)2.6H2O)andthestabilizer,hexamethylenetetramine(C6H12N4; HMTA) were dissolved in DI water. The molar ratio of HMTA:Zn2+ was varied from 1:2, 1:1, 2:1, 4:1and6:1.AqueoussolutionofZn2+wascolourlessandremainedsoevenafterstirringand heating at 60C 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 K radiation inacontinuousscanfrom2=20-60atascanrateof2s-1.Thestructuralmorphologywas determined by scanning electron microscopy (SEM) taken at 10,000 times magnification at 10 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:Zn2+ were 1:2 and1:1;thewidthofthetipswas80-100nm.Athighermolarratios(higherconcentrationof HMTA), blunt ends are seen instead. The size of the rods remained similar in all solutions: diameter 300 - 500 nm and length 7 m.The role of HMTA is as a stabilizer through its chelating effect. ThishelpstoavoidagglomerationthuspreventingtheclusteringofZnOprecursormoleculesthus 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. 100 Diffusion in Solids and Liquids VI(a)1:2 (b) 1:1 (c)2:1 (d) 4:1 (e)6:1 Figure 2. SEM micrographs of ZnO nanorods at 10,000 xmag., 10 kV, and varying molar ratios of HMTA:Zn2+. The X-ray 2 scan patterns of Zno nanorods show only diffraction lines for ZnO and gold. Figure 3 shows the scan in a smaller range between 30 to 40 in order to study three strongest peaks at (100), (002)and(101)whichcorrespondtohexagonalwurtzitestructureofZnO.Thea-andc-axesof orientationarecorrelatedto(100)and(002)peaks.InthinZnOfilms,(002)planeistheusual 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 theseplanes.(100)planegivesthehighestintensityfollowedby(101)andthelowestintensityis (002) (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 (100)planeisshowninFigure4(a)(e)ofZnOnanorodsgrownfromvariousHMTAmolarity have been characterized. Figure 5 shows dependence of XRD peak position of (100) planes versus HMTA:Zn2+molarratioinZnOnanorods.Itcanbeseenthatthe2valuesofthe(100)peak remain approximately similar (at 31.72 to 31.75 ) at low molar HMTA and increase up to 31.95 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 (100) peak versus molarratioofHMTA:Zn2+.FWHMshowslittlevariation,indicatingthatdifferentamountof HMTA in the starting solution gives similar lattice defects to the nanorods. Defect and Diffusion Forum Vols. 312-315 10130 400100020002:11:11:21:41:6Intensity (a.u.)2theta(degree) Figure 3. X-ray diffraction pattern of ZnO nanorods grown on gold-seeded Si substrate by the aqueous-solution method. Table 1.Intensity of (100), (002) and (101) planes shown as a function of HMTA:Zn2+ molar ratio 2 / deg.(hkl) 1:21:12:14:16:1 31.904 158325166387977(100) 34.556 80164117314672(002) 36.409 1994132194891364(101) (a) 2:1 31.0 31.2 31.4 31.6 31.8 32.0 32.2 32.40100200300400500 2to131.7150.341 (b) 1:1 31.0 31.2 31.4 31.6 31.8 32.0 32.2 32.40100200300400500 1to131.7510.395 (c) 2:1 31.0 31.2 31.4 31.6 31.8 32.0 32.2 32.40100200300400500600 1to231.7170.320 (d) 4:1 31.0 31.2 31.4 31.6 31.8 32.0 32.2 32.40100200300400500600700 1to431.7340.393 (e) 6:1 31.0 31.2 31.4 31.6 31.8 32.0 32.2 32.4 32.6 32.8 33.002004006008001000 1to631.9050.349 Figure 4. Full width at half maximum (FWHM) of (100) XRD peaks as a function of HMTA: Zn2+ ratio. 102 Diffusion in Solids and Liquids VI0 1 2 3 4 5 631.7031.7531.8031.8531.9031.95X-ray peak position (deg.)Molar ratio ofHMTA : Zn2+ 0 2 4 60.200.250.300.350.400.450.50FWHM (deg.)Molar ratio of HMTA : Zn2+ Figure 5. Variation of the (100) peak position as a function of HMTA: Zn2+ ratio. Figure 6. Variation of the FWHM of (100) peak as a function of HMTA: Zn2+ ratio. Conclusion ZnOnanorodsweresuccessfullygrownbyusingabenignlow-temperatureaqueous-solution method. It was interesting to find that sharp tip nanorods were formed at lower HMTA: Zn2+ molar ratios(1:2and1:1),whilenanorodswithbluntendswereformedathigherratios(2:16:1). However, the size of the nanorods was not significantly affected by this variation. X-ray diffraction peaks of three intense planes of (100), (002) and(101) showed that their intensity increases as the HMTA: Zn2+ 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 (2008), p. 57. [2]C.Jagadish,S.J.Pearton:ZincOxide-Bulk,ThinFilmsandNanostructures-Properties and Applications, Elsevier, Amsterdam, 2006. [3]L. Schmidt-Mende, J.L. MacManus-Driscoll: Materials Today Vol. 10 (2007), p. 40. [4]J.L.G.Fierro:MetalOxides-ChemistryandApplication,CRCpress,TaylorandFrancis Group Boca Raton, Fl, 2006. [5]Z. Khusaimi, S. Amizam, H.A. Rafaie, M.H. Mamat, M.Z.Sahdan, N. Abdullah, M. Rusop: AIP Conf. Proc. Vol. 1136 (2009), p. 867. [6]J. Prywer: Progress in Crystal Growth and Characterization of Materials Vol. 50 (2005), p. 1. [7]Z.Sofiani,B.Derkowska,P.D.ski,M.Wojdya,S.Dabos-Seignon,M.A.Lamrani,L. Dghoughi,W.Baa,M.Addou,B.Sahraoui:OpticsCommunicationsVol.267(2006),p. 433. Defect and Diffusion Forum Vols. 312-315 103Diffusion in Solids and Liquids VI doi:10.4028/www.scientific.net/DDF.312-315 The Effect of Stabilisers Molarity to the Growth of ZnO Nanorods doi:10.4028/www.scientific.net/DDF.312-315.99
