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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 gold3 and silver4 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 believed 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 * 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.
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 HAuCl4 and 1.25 µmol of (NH4)2PdCl6, 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 cm2 silicon pieces was achieved by first dipping the wafers in a 0.005% mercaptopropyltrimethoxysilane (MPTMS) solution in dry toluene at room temperature 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 N2 stream and washed with ethanol to remove the excess CTAB. These silicon wafers were imaged using a field emission scanning electron microscope (FESEM).
Results and Discussion A series of TEM images of samples taken out of the goldpalladium nanowire growth solution at various delays after base (8) Burt, J. L.; Elechiguerra, J. L.; Reyes-Gasga, J.; Montejano-Carrizale, J. M.; Jose-Yacaman, M. J. Cryst. Growth 2005, 285, 681.
10.1021/la062500x CCC: $37.00 © 2007 American Chemical Society Published on Web 11/29/2006
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Figure 1. TEM images of a growth solution taken (a) 10 s, (b) 1 min, (c) 10 min, (d) 60 min, (e) 24 h, and (f) 7 days after the base addition.
Figure 2. TEM image showing the different shapes of seed nanocrystals formed in the initial seconds after base addition. The scale bars of the insets are 5 nm.
addition is shown in Figure 1. The gradual elongation of the nanowires, starting from irregularly shaped seed particles, occurred over ∼1 h and the formed nanowires were stable in solution for many days. An interesting observation that is unique to this Au-Pd mix is the frequent occurrence of multiple wires growing out of the same seed nanocrystal. The nanowires had typically a diameter of the order of 10 nm and lengths in the range of 0.5-10 µm. We believe that the images shown in Figure 1 represent the growth stages occurring in bulk solution as the dipping of the TEM grids in the solution in the early growth stages took several seconds only, followed by almost full removal of excess solution immediately after drawing of the grids out of the solution. Thus, the observed nanostructures were those adsorbed to the carbon
film during dipping with virtually no possibility for continued growth after grid withdrawal. The base addition increased the pH of the growth solution from ∼2 to 5 and triggered the beginning of reduction of the metal ions in solution as follows: The Au3+ ions were quickly reduced to Au+ in the presence of the ascorbic acid but could not be further reduced to metallic gold due to their complexation with the concentrated CTAB. The Pd4+ ions, on the other hand, could be reduced in the same environment only when the oxidation potential of the ascorbic acid was increased by raising the pH above its first pKa (∼4). Consequently, metal particle formation could start only with the pH rise, where small Pd particles initially nucleated. These small seed particles enabled the catalytic reduction of the Au+ to Au0 by ascorbic acid molecules on their surfaces.3 After 10-20 s from base addition the solution mostly contained star-shaped nanocrystals with the majority of the surface area at {111} orientation. Such branched gold particles have been obtained before using different growth conditions without palladium.9 Their structures were found to originate from cubooctahedra having tetrahedral pyramids grown on their {111} faces, exposing more {111} surfaces to the solution, or multiply twinned icosahedra with similar pyramids.8 Fast reduction of gold ions by ascorbic acid was found to be a strong inducer of deposition of gold atoms on the {111} faces of these growing seeds. High concentration of CTAB tends to stabilize other crystal faces such as {100} and {110} and minimize the exposed {111} surfaces. Figure 2 displays the initial seed particles formed at the early growth stages in solution with some magnified images of several typical shapes. Those shapes resemble the cubooctahedralbased star-shaped particles observed by Burt et al.8 and by Sau and Murphy.9 Nanoprobe energy-dispersive X-ray spectroscopy (EDS) measurements of the nanostars revealed that they had an average Pd concentration of ∼25%, which is higher than the concentration found in the wires (5-15%). No nanowire growth was observed without the presence of (9) Sau, T. K.; Murphy, C. J. J. Am. Chem. Soc. 2004, 126, 8648.
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Figure 3. TEM image of Pd particles grown without Au ions.
Figure 5. HRTEM and Fourier transform images of (a) twin defects and kink in a wire and (b) end of a wire. Scale bars are 2 nm.
Figure 4. TEM image showing that the nanowires consist of straight segments and highly curved parts.
either palladium or gold ions in solution (see Figure 3). The presence of palladium ions in the growth process was crucial for the nanowire formation. A closer inspection of the Au/Pd nanowires, as in Figure 4, taken 24 h after base addition, reveals that they consisted of relatively straight segments and highly curved parts. High-resolution TEM micrographs show that the thin straight segments in the wires were single crystals, similar to the gold nanorods grown by Murphy and co-workers (Figure 5),10 while the curved parts were polycrystalline of relatively small grain size. As in the gold nanorods, the {111} faces were typically oriented at a small angle to the growth direction, which had a [100] orientation. Figure 5a demonstrates a connection between two straight single-crystalline parts through a twin defect. Such structures were never observed with pure gold nanorods and were thus facilitated by the incorporation of palladium in the growth process. (10) Johnson, C. J.; Dujardin, E.; Davis, S. A.; Murphy, C. J.; Mann, S. J. Mater. Chem. 2002, 12, 1765.
Figure 6. FE-SEM image of the nanowires deposited on a silicon substrate.
The control of the rate of metal deposition was very important for the preparation of the nanowires, as in the case of Au-Ag nanowires.7 This was achieved by tuning the reduction potential of the ascorbic acid through control of the pH value of the solution and by adjusting the concentration of the various components. In cases where the pH value was too low or too high only spherical
Growth of Colloidal Gold Nanostars and Nanowires
particles were formed. A high pH value would lead to a very high reduction rate of palladium and too high concentration of small spherical palladium seeds, while at lower pH values less Pd would be reduced, leading to very small Pd concentrations in the nanoparticles and consequently to a lower yield of elongated particles. The Au/Pd nanowire growth mechanism is probably a hybrid of the gold nanorods growth mechanism6,10 and the Au-Ag nanowire thin film growth process.7 While the nanorod growth occurs along a well-defined crystal axis, due to the difference in surfactant adsorption on different faces of the Au crystal, the Au-Ag nanowires were polycrystalline and probably templated by tubular CTAB nanostructures formed on the substrate. The {111} surface-oriented star-shaped seeds formed in the initial stages induced elongated growth through preferential deposition of Au on the {111} faces. The growing edges may serve as nucleation centers for the formation of tubular CTAB structures, as in the zipping mechanism suggested by Gao et al.11 These tubular CTAB structures can further template the elongation of the nanowire even if their crystalline orientation is modified by a defect. While in the case of the pure gold nanorods the growth is terminated as the area of the {111} surfaces at the tips is diminished, it appears that in the presence of palladium the {111} surface exposure becomes more favorable in the concentrated CTAB environment. It is yet to be established whether Pd enhances the binding of CTAB to the {111} edges or if it stabilizes (11) Gao, J.; Bender, C. M.; Murphy, C. J. Langmuir 2003, 19, 9065.
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the {100} and/or {110} faces to reduce their high affinity toward CTAB adsorption. To further use the nanowires for various applications, it would be beneficial to deposit them on various substrates. We were able to deposit the nanowires on silicon substrates by simply dipping the MPTMS-functionalized substrates for a period of 1 h in the nanowire solution, followed by an ethanol wash. Figure 6 displays a FE-SEM image of such a substrate, showing the random adsorption of the nanowires on the functionalized surface. Without the mercaptosilane layer on the substrate no nanowires were observed on the surface after dipping in the nanowire solution and washing with ethanol.
Conclusion A modification of the colloidal gold nanorods growth process was achieved by adding palladium and controlling the metal deposition kinetics through control of the pH value. It led to the growth of high aspect ratio Au/Pd nanowires, which were highly stable in bulk solution. Unlike the gold nanorods growth process, the present one does not require preformed seed particles, but the seed particles form in the early growth stages. In addition, due to the star-shaped seed particles, multiple nanowires growing out of the same seed frequently occur. Further development of this nanowire growth strategy may be useful for obtaining high surface area electrodes for various electrochemical and catalytic applications. LA062500X
