Delivered by Ingenta to National Chemical Laboratory (cid 111194), Council of Scientific & Industrial Research (cid 290154) IP : 127.0.0.1 Wed, 31 Aug 2005 18:55:29 RESEARCH ARTICLE Copyright © 2005 American Scientific Publishers All rights reserved Printed in the United States of America Journal of Nanoscience and Nanotechnology Vol. 5, 1721–1727, 2005 Synthesis of Gold Nanospheres and Nanotriangles bytheTurkevichApproach S. Shiv Shankar, 1 Suresh Bhargava, 2 and Murali Sastry 1 ∗ 1 Nanoscience Group, Materials Chemistry Division, National Chemical Laboratory, Pune – 411 008, India 2 Department of Applied Chemistry, RMIT University, Melbourne, Victoria 3001, Australia Gold nanoparticles of triangular morphology possess interesting optical properties with potential application in medicine and infrared absorbing coatings, however, little is known about conditions that favor their growth. In this paper, we have reinvestigated a time-tested recipe for the formation of gold nanospheres by citrate reduction of aqueous gold ions under boiling conditions (Turkevich recipe). Our principle findings are that gold nanotriangle formation is kinetically controlled and is highly favored at low temperatures. Furthermore, the presence of chloride ions from the precursor chloroaurate ions plays a major role in promoting the growth of 111 oriented triangular/truncated triangular particles. The presence of bromide and iodide ions that possess the ability to replace surface-bound chloride ions inhibits triangle formation to varying degrees. Keywords: Nanospheres, Gold, Nanotriangles, Crystal Growth, Halides, Temperature Effect. 1. INTRODUCTION It is now generally recognized that the shape of metal nanoparticles plays an important role in modulating their optoelectronic 1 2 and catalytic properties. 3 The emerging field of plasmonics 4 and potential application of shape- modulated metal nanoparticles in areas such as cancer hyperthermia 5 require efficient synthetic protocols for achieving shape and size control. A number of shapes ranging from rods 6 to cubes, 7 disks, 8 and mono-/bi-/tri-/ tetrapod nanostructures 9 may be routinely obtained by solution methods. Nanotriangles and nanoprisms of gold and silver are relatively recent additions to this list and show interesting quadrupole plasmon excitations not observed in spherical particles. 1 10 A large volume of liter- ature is available for nanorod synthesis in solution 6 includ- ing a good understanding of their growth mechanism; 11 in comparison, methods for the synthesis of metal nano- triangles are relatively scarce. 1 10–14 And in addition pos- sess an in-plane surface plasmon resonance (SPR) band that could extend well into the near infrared region of the electromagnetic spectrum 13 with application in architec- tural optical coatings 14 and cancer hyperthemia. ∗ Author to whom correspondence should be addressed. The synthesis of gold nanoparticles by citrate reduction of aqueous gold ions that was proposed by Hauser and Lynn 15 and later well studied by Turkevich is one of the oldest and most popular procedures for obtaining gold nanoparticles of 10–15 nm diameter in water. 16 In this pro- cedure, aqueous chloroaurate ions are reduced by citric acid/sodium citrate under boiling conditions resulting in spherical gold nanoparticles stabilized electrostatically by surface-bound citrate ions. 16 Intrigued by an earlier report on the presence of a very small percentage of triangu- lar particles in citrate-reduced gold solutions 17 and our own experience with the citrate reduction procedure, we have reinvestigated this procedure in detail with an eye on gold nanotriangle formation. We observe that the temper- ature of reaction is a crucial parameter and that slowing reduction rate of gold ions by progressive temperature reduction leads to the growth of gold nanotriangles. Trans- mission electron microscopy (TEM) and UV-Vis-NIR spectroscopic analysis of triangle growth indicate that it is a kinetically controlled process and that multiply twinned gold nanoparticles act as seeds that promote formation of the triangular morphology. We also observe that the pres- ence of halide ions in solution during gold ion reduction critically affects the morphology of the particles. While Cl − ions promote the formation of gold nanotriangles, Br − and J. Nanosci. Nanotech. 2005, Vol. 5, No. 10 1533-4880/2005/5/1721/007 doi:10.1166/jnn.2005.192 1721