Growth of Colloidal Gold Nanostars and Nanowires Induced by Palladium Doping Olga Krichevski and Gil Markovich* School of Chemistry, Raymond and BeVerly Sackler Faculty of Exact Sciences, Tel AViV UniVersity, Tel AViV 69978, Israel ReceiVed August 24, 2006. In Final Form: October 19, 2006 Gold-palladium nanocrystals with starlike shapes and high aspect ratio nanowires were grown in a surfactant solution. The incorporation of palladium into the growing gold nanostructures induced nanowire formation with high yield. Kinetic control of the metal deposition rate through tuning of the pH value to about 5 was crucial for the nanowire growth. The nanostructures were characterized by high-resolution electron microscopy and energy-dispersive X-ray spectroscopy. The Au-Pd nanowires were deposited on functionalized silicon wafers. Introduction Development and understanding of mechanisms for elongated metal nanorods and nanowires growth are important for various applications where the bottom-up mass production of such nanostructures is desired. 1 In this respect the seeded metal nanorod solution growth process developed by Murphy and co-workers has been an important milestone. 2 Using this process, it has been possible to routinely prepare colloidal gold 3 and silver 4 nanorods of varying aspect ratios and also to modify it to grow the nanorods on various types of solid substrates. 5 However, this process has been suffering from a limited yield, and in spite of the possibility of increasing the aspect ratios or total lengths of the grown nanorods by successive growth steps, the achievable final length of these single-crystal nanorods was limited to ∼1 μm. A possible explanation for this limitation originates in the postulated growth mechanism suggested by several researchers: The nanorods are grown in the presence of a cationic surfactant, hexadecyltrimethylammonium bromide (CTAB), which is be- lieved to adsorb strongly to the {110} and {100} facets appearing at the sides of these penta-prism-shaped nanorods. 6 The CTAB adsorbs to the {111} facets appearing at the edges of the nanorods weakly and, thus, faster growth occurs along the axial [001] direction of the nanorods. However, as the nanorods grow, the effective area of exposed {111} planes is probably diminishing while {100} surfaces, strongly stabilized by the CTAB, expand at the edges, until no further increase in aspect ratio can be achieved. Recently, we have shown that by using radically different growth conditions, it is possible to obtain much longer and thinner Au/Ag nanowires, where the major driving force for the growth of such high aspect ratio metal objects was the confinement of metal deposition to CTAB tubular structures formed in a drying growth solution film. 7 In that process the resulting metal nanowires were highly polycrystalline. In the present paper we describe a different approach to the growth of long, high aspect ratio metal nanowires in bulk solution, which is closer to the original seeded nanorods growth process. In this process 1:2 molar ratio Pd:Au ionic precursors were used in the growth solution. The palladium doping caused the small metal nanocrystals, which were formed at the beginning of the growth process, to develop small pyramidal edges with exposed {111} surfaces, similar to larger pure gold star-shaped particles obtained under different conditions. 8 In addition, the palladium seemed to induce defects during the nanorods growth, which enabled the formation of fresh {111} edges in polycrystalline segments of the wires, leading to nanowire elongation. Experimental Section The nanowire growth solution was prepared by combining Au and Pd precursor ions in a molar ratio of 2:1 in the presence of a high surfactant concentration, reducing agent (ascorbic acid), and pH control by NaOH addition. The standard growth solution contained 10 mL of 0.05 M CTAB with 2.5 μmol of HAuCl 4 and 1.25 μmol of (NH 4 ) 2 PdCl 6 , and 55 μmol of ascorbic acid. To trigger the growth process, 40 μL of 1 M NaOH was added with gentle stirring, increasing the pH of the solution to ∼5. The reaction was accompanied by a color change from yellow-orange to deep brown. Samples for transmission electron microscopy (TEM) were taken at different time intervals after the base addition by dipping carbon-coated copper grids into the solution. Excess solvent was immediately blown with dry nitrogen after dipping. Finally, the substrates were dipped in ethanol for several minutes to remove excess CTAB. The deposition of the nanowires on 1 × 1 cm 2 silicon pieces was achieved by first dipping the wafers in a 0.005% mercaptopropyl- trimethoxysilane (MPTMS) solution in dry toluene at room tem- perature for 15 min. Then the substrates were taken out, put in ethanol and sonicated for several seconds, washed with distilled water, and immediately immersed into a grown Au-Pd nanowire solution for 60 min. After withdrawal of the substrates from the nanowire solution, they were dried with a N 2 stream and washed with ethanol to remove the excess CTAB. These silicon wafers were imaged using a field emission scanning electron microscope (FE- SEM). Results and Discussion A series of TEM images of samples taken out of the gold- palladium nanowire growth solution at various delays after base * To whom correspondence should be addressed. E-mail: gilmar@ post.tau.ac.il. (1) Perez-Juste, J.; Pastoriza-Santos, I.; Liz-Marzan, L. M.; Mulvaney, P. Coord. Chem. ReV. 2005, 249, 1870. (2) Murphy, C. J.; San, T. K.; Gole A. M.; Orendorff, C. J.; Gao, J. X.; Gou, L.; Hunyadi, S. E.; Li, T. J. Phys. Chem. B 2005, 109, 13857. (3) Jana, N. R.; Gearheart, L.; Murphy, C. J. J. Phys. Chem. B 2001, 105, 4065. (4) Jana, N. R.; Gearheart, L.; Murphy, C. J. Chem. Commun. 2001, 617. (5) Taub, N.; Krichevski, O.; Markovich, G. J. Phys. Chem. B 2003, 107, 11579. (6) Gai, P. L.; Harmer, M. A. Nano Lett. 2002, 2, 771. (7) Krichevski, O.; Tirosh, E.; Markovich, G. Langmuir 2006, 22, 867. (8) Burt, J. L.; Elechiguerra, J. L.; Reyes-Gasga, J.; Montejano-Carrizale, J. M.; Jose-Yacaman, M. J. Cryst. Growth 2005, 285, 681. 1496 Langmuir 2007, 23, 1496-1499 10.1021/la062500x CCC: $37.00 © 2007 American Chemical Society Published on Web 11/29/2006