Gold Nanostructures DOI: 10.1002/anie.200703259 Chemical Sharpening of Gold Nanorods: The Rod-to-Octahedron Transition** Enrique Carbó-Argibay, Benito Rodríguez-Gonzµlez, Jessica Pacifico, Isabel Pastoriza-Santos,* Jorge PØrez-Juste, and Luis M. Liz-Marzµn* Morphology control at the nanoscale is a hot topic because of the spectacular effects that small changes in the shape of nanoparticles can have on a variety of physical properties of the material. [1] In particular, metal nanoparticles have been shown to display interesting optical properties related with the oscillation of conduction electrons, in resonance with incoming light, the so-called surface plasmon resonances. [2] The specific resonance frequencies of plasmon modes in metal nanoparticles are strongly dependent on their mor- phology, [3] which has driven intensive research towards their sizeandshapecontrol. [4] Alargenumberofsyntheticmethods based on colloid chemistry have been developed for the production of particles with different shapes, and there is a strong debate regarding the specific mechanisms involved in anisotropic growth. [5] In general, metals (most of them with a cubic structure) tend to nucleate and grow into thermody- namically stable nanoparticles with their surfaces bound by the low-energy facets so as to minimize the total surface energy. [6] Nevertheless, highly anisotropic shapes, not favor- able from the perspective of thermodynamics, have been obtained by the introduction of capping agents which can alter the surface energies for the different crystallographic planes. [7] Herein, we present evidence of tip sharpening and reshaping during the growth of gold nanorods, with a complete conversion from spherically capped cylinders into single-crystal octahedrons. A detailed study of the growth process reveals a gradual change in the morphology of the particles directly related with the faster growth of certain crystallographic facets. Fine-tuning of the reaction conditions allows the isolation of nanoparticle colloids with a variety of intermediate shapes, in turn leading to a variation of the relative intensities and positions of longitudinal and trans- verse surface plasmon modes. The chemical process involved in the reshaping process starts from single-crystal gold nanorods synthesized by seed- mediated growth in aqueous solution. These rods are in turn used as seeds for further growth in the ultrasound-induced reduction of HAuCl 4 by N,N-dimethylformamide (DMF), in the presence of poly(vinylpyrrolidone) (PVP), as previously described for the growth of regular gold nanodecahedra on premade, small seeds. [8] Although PVP has been often claimed to be a shape-directing agent, [9] a variety of mor- phologieshavebeenobtainedwiththispolymer.However,we have demonstrated in our previous work that the seed not only serves as a catalyst for gold reduction but also determines the crystallographic structure and geometry of the final particles, so that pentagonal bipyramids (decahe- drons) were obtained from penta-twinned Au seeds, whereas the growth of single-crystal Pt seeds yielded perfect octahe- drons. This observation was in agreement with previous reports from a number of groups [5b,10] using a variety of preparationprotocols.Theuseofnanorodsasseedsarisesasa possibility to manipulate their dimensions in a controlled manner, with the aim of fine-tuning their optical response. The single-crystal nanorods used here were synthesized by seeded growth in aqueous cetyltrimethylammonium bromide (CTAB) solution [5b,11] (see Experimental Section for details) and had an average aspect ratio of 4.2 [(60.8 Æ 8.7)nm (15.1 Æ 3.6) nm]. Prior to the growth experiment, CTAB was exchanged with poly(vinylpirrolidone) (PVP) as previously reported [12] to allow transfer into DMF with no aggregation. As the Au nanorods are known to reshape at moderate temperatures, [13] control heating experiments were carried out to avoid significant thermal reshaping during growth for which the reaction temperature was set around 75 8C (see Supporting Information for further details on thermal reshap- ing). Figure 1 shows the time evolution of the UV/Vis spectra during the growth of Au nanorods (2.92 mmol Au atoms) in 15 mL of a DMF solution containing HAuCl 4 (2.5 mm) and PVP (2.5 mm)previouslysonicatedfor1htoreduceAu III into Au I (see Experimental Section). The extinction spectrum of the initial nanorod dispersion displays a longitudinal surface plasmon resonance (SPR) band centered at 861 nm and a weak transverse SPR band at 513 nm. During the process, the longitudinal SPR band gradually undergoes a blue shift, while the transverse SPR undergoes a red shift and is significantly enhanced. Gradually, both bands merge into a single band centered at 570 nm that subsequently undergoes a red shift and gain in intensity. Transmission electron microscopy (TEM) analysis of aliquots withdrawn at different reaction times reveals that the observed spectral evolution arises from [*] E. Carbó-Argibay, Dr. B. Rodríguez-Gonzµlez,Dr. J. Pacifico, Dr. I. Pastoriza-Santos, Dr. J. PØrez-Juste, Prof. L.M. Liz-Marzµn Departamento de Química Física and Unidad Asociada CSIC Universidade de Vigo 36310, Vigo (Spain) Fax: (+ 34)986-812-556 E-mail: pastoriza@uvigo.es lmarzan@uvigo.es [**] I.P.-S. acknowledges the Isidro Parga Pondal Program (Xunta de Galicia, Spain). J.P.-J. acknowledges the Santiago Ramón y Cajal Program (MEC, Spain). This work was supported by the Spanish Ministerio de Educación y Ciencia (grant MAT2003-02991), Xunta de Galicia, and COST Action D43. Supporting information for this article is available on the WWW under http://www.angewandte.org or from the author. Angewandte Chemie 8983 Angew. Chem. Int. Ed. 2007, 46, 8983–8987 # 2007 Wiley-VCH Verlag GmbH & Co. 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