Delivered by Publishing Technology to: McMaster University IP: 180.159.91.236 On: Fri, 01 Jan 2016 11:52:38 Copyright: American Scientific Publishers Copyright © 2007 American Scientific Publishers All rights reserved Printed in the United States of America Journal of Nanoscience and Nanotechnology Vol. 7, 2808–2817, 2007 Synthesis of Gold Nanorods in Organic Media S. Prathap Chandran 1 , Renu Pasricha 1 , Umananda M. Bhatta 2 , P. V. Satyam 2 , and Murali Sastry 1 ∗† 1 Nanoscience Group, Materials Chemistry Division, National Chemical Laboratory, Pune 411008, India 2 Institute of Physics, Bhubaneshwar 751005, India A seed mediated approach for the synthesis of anisotropic rod shaped gold nanoparticles in organic media (toluene) is demonstrated. Pre-formed gold nanoparticles stabilized in toluene by 4-hexadecylaniline (HDA) are used as seeds. These when reacted with 1-octadecylamine (ODA) hydrophobised chloroaurate ions in toluene lead to the formation of gold nanorods. ODA or alky- lamines of different chain lengths which are the chloroaurate ion phase transfer agent have been found to play a key role in the formation of the nanorods. The gold nanorods that have a five-fold symmetry evolve from multiply twinned particles and are bound at the tips by {111} faces and at the sides by {100} faces. The gold nanorods have been shown to grow under the shape directing effect of the alkylamines which stabilize the high energy {100} faces. The concentration of the alkylamines has been found to play a critical role in the formation of the gold nanorods. Higher concentrations of the alkylamines lead to formation of spherical particles, at times of narrow size distribution. Keywords: Gold Nanorods, Seed Mediated Growth, Organic Media, HRTEM, Electron Diffraction. 1. INTRODUCTION Synthesis of noble metal nanocrystals have been an impor- tant area of research since the time of Faradays semi- nal work. 1 A variety of synthetic procedures have been explored and developed since then. The Turkevich method for the synthesis of gold nanoparticles, 2 borohydride reduc- tion, 3 reduction using amino acids, 4 and Brust’s two phase reduction method 5 are some of the important recipes for the synthesis of noble metal nanoparticles. Excellent con- trol over the size and dispersity has been demonstrated using these methods both in aqueous and organic media. Organic media based procedures have attracted attention due to tailorability of particle size, surface functionality, redispersibility in a variety of solvents and film form- ing applications including Langmuir Blodgett films. 6 More recently, equal importance is given to shape control in addition to size control. The drastically different optical properties of anisotropic particles have provided an added impetus for research in this direction. 7 However anisotropic nanoparticle synthesis has predominantly been performed in aqueous media. This has been mainly due to the seed mediated growth procedures that have been developed in ∗ Author to whom correspondence should be addressed. † Present address: Tata Chemicals Innovation Centre, Anmol Pride, Baner, Pune 411045, India. water. The CTAB based seeded growth procedure has been used for the synthesis of gold nanorods, nanotriangles, and nanocubes. 8 Similarly, Xia and co-workers have demon- strated the synthesis of a variety of anisotropic silver nano- structures like nanowires, nanocubes, and nanotriangles. 9 A recent report that describes an elegant procedure for the synthesis of gold crystals of different shapes albeit micron sized, in toluene is an exception in this direction. 10 Moreover protocols for the synthesis of anisotropic nano- structures of gold such as nanowires in the aqueous media followed by their phase transfer to organic media do exist. 11 In this paper we explore a hitherto unreported seed mediated growth procedure in organic media with mor- phology control as the objective. The procedure leads to the formation of nanorods along with spherical parti- cles. 4-hexadecylaniline which has been reported previ- ously to be a facile reducing agent of gold ions 12 has been used for the phase transfer of preformed gold nanoparti- cles to toluene. These on reaction with phase transferred chloroaurate ions lead to the formation of gold nanorods due to seeded growth. Varying the chain length of the chloroaurate ion phase transfer agent led to change in the yield and aspect ratio of the nanorods formed. Our experiments indicate that alkylamines (chloroaurate ion phase transfer agents) are important for the rod forma- tion process. The crystallography of the nanorods has 2808 J. Nanosci. Nanotechnol. 2007, Vol. 7, No. 8 1533-4880/2007/7/2808/010 doi:10.1166/jnn.2007.636