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Paper: SYNTHESIS AND CHARACTERIZATION OF AG/PVA NANORODS BY CHEMICAL REDUCTION METHOD

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Paper: SYNTHESIS AND CHARACTERIZATION OF AG/PVA NANORODS BY CHEMICAL REDUCTION METHOD
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  This article appeared in a journal published by Elsevier. The attachedcopy is furnished to the author for internal non-commercial researchand education use, including for instruction at the authors institutionand sharing with colleagues.Other uses, including reproduction and distribution, or selling orlicensing copies, or posting to personal, institutional or third partywebsites are prohibited.In most cases authors are permitted to post their version of thearticle (e.g. in Word or Tex form) to their personal website orinstitutional repository. Authors requiring further informationregarding Elsevier’s archiving and manuscript policies areencouraged to visit:http://www.elsevier.com/copyright  Author's personal copy Synthesis and characterization of Ag/PVA nanorods bychemical reduction method M.A.S. Sadjadi a,  , Babak Sadeghi a , M. Meskinfam a,b , K. Zare a,b , J. Azizian a,b a Department of Chemistry, Science and Research Campus, Islamic Azad University, Poonak, Tehran, Iran b Department of Chemistry, Faculty of Science, Shahid Beheshti University, Tehran, Iran a r t i c l e i n f o  Article history: Received 28 March 2008Received in revised form19 May 2008Accepted 21 May 2008Available online 28 May 2008 PACS: 81.20.Ka33.20.Rm68.37.Hk Keywords: Chemical synthesisX-ray spectraScanning electron microscopy (SEM)Silver nanorodsPolyvinyl alcoholFace-centered cube a b s t r a c t Silver (Ag) nanorods with the average length of 280nm and diameters of around 25nm weresynthesized by a simple reduction process of silver nitrate in the presence of polyvinyl alcohol (PVA)and investigated by means of scanning electron microscopy (SEM), X-ray diffraction (XRD),transmission-electron microscopy (TEM) and UV–vis spectrum. It was found out that both temperatureand reaction time are the important factors in determining the morphology and aspect ratios of nanorods. TEM images showed the prepared Ag nanorods have a face centered shape (fcc) with fivefoldsymmetry consisting of multiply twinned face centered cubes as revealed in the cross-sectionobservations. The fivefold axis, i.e. the growth direction, normally goes along the (111) zone axisdirection of the basic fcc Ag-structure. Preferred crystallographic orientation along the (111), (200) or(220) crystallographic planes and the crystallite size of the Ag nanorods are briefly analyzed. &  2008 Elsevier B.V. All rights reserved. 1. Introduction Metal or semiconductor nanostructures, in particular, nano-wires and nanorods, may be used in the fabrication of nanode-vices and novel nanocomposites due to their fascinating optical,electronic and magnetic properties [1–4]. Over the past decade,the synthesis strategy and mechanism of one-dimensional (1D)nanomaterials have been greatly developed [5]. As shown in anumber of reported studies,1D nanomaterials can be prepared byvarious chemical routes, such as the polyol-based process [6], wetchemical synthesis [7,8], hydrothermal method [9], ultraviolet irradiation photoreduction technique [10], electrochemical de-position technique [11], DNA or organic molecular template[12,13], porous materials template [14] and anodic alumina membranes (AAMs) [15].Recent advances in soft synthesis of low-dimensional nano-crystals such as nanorods, nanowires, nanotubes, and morecomplex nanostructures, demonstrated that the ‘‘soft’’ approachesare very promising alternative pathways for the synthesis of 1Dnanocrystals under natural/mild conditions in contrast to tradi-tional high-temperature approaches [16]. The soft synthesisroutes, such as solvothermal/hydrothermal processes, solution–-liquid–solid mechanism, capping agents/surfactant-assistedsynthesis, bio-inspired approaches and oriented attachmentgrowth mechanism, open alternative doorways to the low-dimensional nanocrystals and even more complex superstruc-tures. Among these solution strategies, the solvothermal processshows the potential capabilities and versatilities for selectivesynthesis of various important semiconductor nanocrystals withcontrollable shape, size, phase, and dimensionalities under mildconditions.In this work, following the solvothermal process, we employedpolyvinyl alcohol (PVA, Mw ¼ 72,000) as a protecting agent tosynthesize silver (Ag) nanorods with an average length of 280nmand diameter of 25nm by reduction of silver nitrate using DMF asa solvent and reducing agent. Scanning electron microscopy(SEM), transmission electron microscopy (TEM), powder X-raydiffraction (XRD) and UV–visible spectrum measurements werecombined to investigate thus-obtained Ag nanorods, and furtherto elucidate the structure and the growth mechanism of them. ARTICLE IN PRESS Contents lists available at ScienceDirectjournal homepage: www.elsevier.com/locate/physe Physica E 1386-9477/$-see front matter  &  2008 Elsevier B.V. All rights reserved.doi:10.1016/j.physe.2008.05.010  Corresponding author. Tel.: +989121075139; fax: +982144817175. E-mail address:  msadjad@gmail.com (M.A.S. Sadjadi).Physica E 40 (2008) 3183–3186  Author's personal copy 2. Experimental Materials containing a large yield of Ag nanorods weresynthesized by reduction of silver nitrate in the presenceof PVA. The procedure is briefly described as follows: Firstly,5ml  N-N  0 - dimethyl formamide (DMF) was refluxed in a three-necked round-bottom flask at 80 1 C for 2h, then 5ml DMFsolution of 0.02M silver nitrate and 5ml DMF solution of 0.05mM PVA were simultaneously injected dropwise. Whenthe first drops of silver nitrate and PVA/DMF solutions wereadded, the mixture turned yellow immediately. Continuing theinjection, the solution became opaque gradually. By finishing theinjection, the solution turned turbid with a gray color in about15min indicating the appearance of Ag nanoparticles. Thereaction was continued at 80 1 C for 16h. After finishing thereaction and removal of the supernatant, a gray precipitateremained.The XRD pattern was recorded on a Seisert Argon 3003 PTCusing nickel-filtered XD-3a CuK a  radiations ( l ¼ 1.5418A˚ ). TheUV–visible spectrum, in absorbance mode was recorded on anUV–visible Hitachi spectrophotometer model U-2101 PC. Thesolution form of the sample was prepared by suspending asmall amount of powder in ethanol. TEM was performed on aPhilips EM208 and microscope operated at 100kV. Sampleswere prepared by dispersing the powder in ethanol. Imagingwas enabled by depositing few drops of suspension on a carboncoated 400 mesh Cu grid. The solvent was allowed to eva-porate before imaging. SEM images of fabricated Ag nanorodswere obtained using LEO 440i electron microscope. The sampleswere rinsed with distilled water, dried and coated with a thinlayer of gold by evaporation at vacuum to form conductingfilm. 3. Results and discussion A typical XRD pattern of as prepared Ag nanorods shows thepresence of the diffraction peaks corresponding to the (111),(200), (220), (311) planes. All diffraction peaks in this XRDpattern (Fig.1) can be well indexed as face-centered cubic (fcc) Agwith peak positions, indicating that the fcc structure of the Ag ispreserved in these nanorods (JCPDS, File No. 4-0783) [17]. Theseresults indicate a polycrystalline structure for Ag nanorods withan intense (111) preferred orientation. Secondary peaks at (200)and (220), corresponding to the high-angle XRD pattern(2 y ¼ 30  60), supports the presence of Ag in nanocompositeform. The peaks appearing at (2 y ¼ 38.4) and (2 y ¼ 44.6)correspond to the (111) and (200) planes of the Ag lattice,respectively.The dimensions of the crystallites of which the Ag nanorodsare composed can be estimated from the Scherrer formula [18].When the term ‘‘crystallite size’’ is used, we will be referring tothe dimensions of the coherent diffracting domain. This equationis applicable to samples where lattice strain is absent. Nanorodscan posses some strains, which could also be a factor contributingto the width of the peaks, thereby affecting the estimates of thecrystallite size of the nanorods. Moreover, the grain size of the Agnanorods estimated from the XRD peak width (and usingScherrer ’ s formula), was greater than 40nm. However, theDebye–Scherrer formula calculations wereapproximate, and moreaccurate for spherical particles. The results were not comparablewith TEM results [19]. Thus, grain sizes were larger than themaximum critical acceptable values that can be deduced for theseXRD measurements. Therefore, the size of the crystalline domainsdetermined from the XRD peak widths will be used only as acomparative measure among samples.The TEM images of the Ag nanorods as-prepared by thecontrolled-concentration and temperature are shown in Fig. 2(a,b), revealing the as prepared individual Ag nanorods and theirconstituent nanospheres. Fig. 2(c) exhibits the cross section of theprepared multiply fivefold twinned fcc Ag crystallite of the shapeof rod and nanometer dimension nanorods. These pentagonalnanorods show aspect ratio between 3.3 and 12 with the length of their fivefold axis ranging from 45 to 280nm.The SEM images of the as-prepared Ag nanorods are shown inFig. 3. The morphology of the resulting surfaces, indicates that theas-synthesized materials contain a high concentration of non-homogeneous Ag nanorods (Fig. 3(a)). Fig. 3(b, c) illustrates the presence of Ag nanorods with a random orientation in thesamples.Fig. 4 represents the UV–visible spectrum of as prepared Ag/PVA nanorods. This figure shows a broad absorption band at  l max 458nm. This characteristic peak is due to the oscillation of conduction band electrons of Ag known as the surface plasmon ARTICLE IN PRESS Fig. 1.  XRD pattern of Ag/PVA nanocomposite indicative the fcc structure. Fig. 2.  Transmission electron microscopy (TEM) images of (a, b) individual and (c) cross section of Ag/PVA nanorods. M.A.S. Sadjadi et al. / Physica E 40 (2008) 3183–3186  3184  Author's personal copy resonance [20–24]. The position of plasmon absorption banddepends on particle size, aspect ratio and diameter of nanorods ornanowires [25]. The broad nature of the absorption band in thiscase is indicative of the presence of both spheres and rods asreported in citrate reduction method [26].As a result, we conclude that formation of Ag nanorods in thismodel is based on the reduction of Ag (I) ions by DMF. In thisprocess, the DMF serves both as reductant and solvent. In the realreaction, two processes may occur simultaneously. At first, Agnanoparticles are formed through homogenous nucleation andgrow into multiply twinned particles (MTPs) with their surfacesbounded by the lowest energy {111} faces. Silver nanoparticlesforming at the surface of the protecting agent or coming from theouter solution phase through Brownian motion aggregate into a1D arrangement like a bunch of pearls, and then grow intonanorods through Ostwald ripening. 4. Conclusion It was concluded that PVA is a remarkably powerful cappingagent for Ag ions. In this process, the DMF serves both asreductant and solvent [25,27]. The UV–visible spectra indicatewell-defined absorption bands for Ag nanoparticles and nanorodsdue to the surface plasmon resonance phenomena. The XRD dataconfirmed that the Ag nanorod is crystalline with fcc structurehaving a preferred crystallographic orientation along the {220}direction, and a straight, continuous, dense Ag nanorod has beenobtained with a diameter 25nm.SEM and TEM observations along a series of relevant directionsshowed that the silver nanorods have an average length of 280nmand diameters of around 25nm. TEM observations from crosssection of nanorods suggest that the transformation of Agnanospheres to Ag nanorods is achieved by the oriented attach-ment of several spherical particles followed by their fusion.Resulting Ag nanorods have a twinned fcc structure, and theyappear in a pentagonal shape with fivefold twinning. The fivefoldaxis, i.e. the growth direction, normally goes along the (110) zoneaxis direction of the fcc cubic structure.  Acknowledgments We thank the Research Vice Presidency of Science and researchcampus, Islamic Azad University and Executive Directory of Iran-Nanotechnology Organization (Govt. of Iran) for their encourage-ment, permission and financial support. References [1] Z. Zhong, D. Wang, Y. Cui, M.W. Bockrath, C.M. Lieber, Science 302 (2003)1377.[2] F. Favier, E.C. Walter, M.P. Zach, T. Benter, R.M. Penner, Science 293 (2001)2227.[3] D.S. Hopkins, D. Pekker, P.M. Goldbart, A. Bezryadin, Science 308 (2005)1762.[4] R. Adelung, O. Aktas, J. Franc, A. Biswas, R. Kunz, M. Elbahri, J. Kanzow,U. Schrmann, F. Faupel, Nat. Mater. 3 (2004) 375.[5] Y. Xia, P. Yang, Y. Sun, Y. Wu, B. Mayers, B. Gates, Y. Yin, F. Kim, H. Yan, Adv.Mater. 15 (2003) 353.[6] Y. Sun, Y. Xia, Science 298 (2002) 2176.[7] N.R. Jana, L. Gearheart, C.J. Murphy, Chem. Commun. 7 (2001) 617.[8] K.K. Caswell, C.M. Bender, C. Murphy, Nano. Lett. 3 (2003) 667.[9] Z. Wang, J. Liu, X. Chen, J. Wan, Y. Qian, Chem. Eur. J. 11 (2005) 160.[10] Y. Zhou, S.H. Yu, C.Y. Wang, X.G. Li, Y.R. Zhu, Z.Y. Chen, Adv. Mater. 11 (1999)850.[11] L. Huang, H. Wang, Z. Wang, A. Mitra, K.N. Bozhilov, Y. Yan, Adv. Mater. 14(2002) 61.[12] K. Keren, M. Krueger, R. Gilad, G. Ben-Yoseph, U. Sivan, E. Braun, Science 297(2002) 72.[13] C.F. Monson, A.T. Woolley, Nano. Lett. 3 (2003) 359.[14] M. Steinhart, M. Steinhart, J.H. Wendorff, A. Greiner, R.B. Wehrspohn,K. Nielsch, J. Schilling, J. Choi, U. Gosele, Science 296 (2002) 1997.[15] X.J. Xu, G.T. Fei, X.W. Wang, Z. Jin, W.H. Yu, L.D. Zhang, Mater. Lett. 61 (2007)19.[16] H. Chen, Y. Gao, H. Yu, H. Zhang, L. Liu, Y. Shi, H. Tian, S. Xie, J. Li, Micron 35(2004) 469.[17] JCPDS Silver, File 04-0783.[18] B.D. Cullity, Elements of X-ray Diffraction, second ed., Addison-Wesley,Reading, MA, 1978.[19] R.V. Kumer, Y. Koltypin, J. Appl. Phys. 89 (2001) 6324.[20] T. Itakura, K. Tsrcoe, K. Esumi, Langmuir 11 (1995) 4129. ARTICLE IN PRESS Fig. 3.  (a, b, c) SEM image showing high concentrated distribution of Ag/PVA nanorods. Fig. 4.  UV–vis absorption spectrum of Ag/PVA nanocomposite prepared by usingDMF solution. M.A.S. Sadjadi et al. / Physica E 40 (2008) 3183–3186   3185  Author's personal copy [21] C.A. Foss, G.L. Hornyak, J.A. Stockert, C.R. Martin, J. Phys. Chem. 98 (1994)2963.[22] S. Link, M.B. Mohamed, M.A. El-Sayed, J. Phys. Chem. B 103 (1999) 3073.[23] N.R. Jana, L. Gearheart, C.J. Murphy, J. Phys. Chem. B 105 (2001) 4065.[24] V.M. Cepak, C.R. Martin, J. Phys. Chem. B 102 (1998) 9985.[25] P.K. Khanna, N. Singh, S. Charan, V.V.V.S. Subbarao, R. Gokhale, U.P. Mulik,Mater. Chem. Phys. 93 (2005) 117.[26] G. Zhoua, M. Lu, Z. Yanga, H. Zhanga, Y. Zhoua, S. Wanga, S. Wanga, A. Zhanga, J. Cryst. Growth 289 (2006) 255.[27] I. Pastoriza-Santoz, M. Luiz, L. Marzan, Nano. Lett. 2 (2002) 903. ARTICLE IN PRESS M.A.S. Sadjadi et al. / Physica E 40 (2008) 3183–3186  3186
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