Article pubs.acs.org/cm
Growth Mechanism of Gold Nanorods Kyoungweon Park,†,‡ Lawrence F. Drummy,† Robert C. Wadams,§ Hilmar Koerner,†,‡ Dhriti Nepal,† Laura Fabris,§ and Richard A. Vaia†,* †
Materials and Manufacturing Directorate, Air Force Research Laboratory, Wright-Patterson AFB, Ohio 45433-7702, United States UES, Inc., Dayton, Ohio 45432, United States § Department of Materials Science and Engineering, Rutgers, The State University of New Jersey, Piscataway, New Jersey 08854, United States ‡
S Supporting Information *
ABSTRACT: Gold nanorods (Au NRs) are the archetype of a nanoantenna, enabling the directional capture, routing, and concentration of electromagnetic fields at the nanoscale. Solution-based synthesis methods afford advantages relative to top-down fabrication but are challenged by insufficient precision of structure, presence of byproducts, limited tunability of architecture, and device integration. This is due in part to an inadequate understanding of the early stages of Au NR growth. Here, using phase transfer via ligand exchange with monothiolated polystyrene, we experimentally demonstrate the complete evolution of seed-mediated Au NR growth in hexadecyltrimethylammonium bromide (CTAB) solution. Au NR size and shape progress from slender spherocylinders at short reaction times to rods with a dumbbell profile, flattened end facets, and octagonal prismatic structures at later stages. These evolve from a single mechanism and reflect the majority of reported Au NR morphologies, albeit reflecting different stages. Additionally, the fraction of nonrod impurities in a reaction is related to the initial distribution of the structure of the seed particles. Overall, the observations of early and intermediate stage growth are consistent with the formation of a surfactant bilayer on different crystal facets at different growth stages due to a fine balance between kinetic and thermodynamic factors. KEYWORDS: gold nanorods, growth mechanism, seed-mediated growth, surface reconstruction
■
INTRODUCTION Gold nanorods (Au NRs) are of intense technological interest due to their broadly tunable surface plasmon resonances that provide ways to manipulate electromagnetic fields at the nanoscale.1 Recently, research has demonstrated routes for the assembly of discrete colloidal Au NR clusters, providing even greater control of optical responses and near field coupling via the hybridization of local surface plasmons.2−4 The resulting strong field confinement and nonlinear optical effects are being examined for ultrasensing, imaging, data storage, solar energy conversion, and opto-electronics.5,6 Structural features of the Au NR, such as aspect ratio, shape of the end, and tapering of the sides, are crucial for controlling the intrinsic physicochemical properties as well as the quality of the assembly.7−9 Substantial efforts have therefore been made to develop methods that provide control over the size, shape, and crystallographic facets of colloidal Au NRs with high monodispersity.9,10 Nonetheless, the growth mechanism, especially at the earliest times where a spherical Au seed transforms into a rod, is still poorly understood, and thus limits the ability to refine synthetic procedures and additives to improve the yield of nanostructures with precise architecture and minimal dispersity. © XXXX American Chemical Society
Most of the suggested mineralization mechanisms involve a preferential interaction of surfactant molecules with the crystallographic facets along the rod side, thus providing the rod ends more access to the reaction medium for growth.11 Additives, such as AgNO3, are known to assist in selective binding and packing of hexadecyltrimethylammonium bromide (CTAB) by decreasing the repulsion between the CTAB headgroups.9 UV−vis spectroscopy,12,13 small-angle X-ray scattering,14 atomic force microscopy,15 and transmission electron microscopy (TEM) of particles arrested during growth16 provide some interesting perspectives on the evolution from an isotropic seed to an anisotropic rod. For example, the longitudinal localized surface plasmon resonance (L-LSPR) reaches its lowest energy (longest wavelength) very early in the rod growth (10−20 min) and subsequently blue shifts toward the visible over the next 50−60 min.12 This behavior of the L-LSPR implies that the seed particles rapidly achieve their largest aspect ratio, and as growth continues the aspect ratio decreases. Since the aspect ratio is defined as the ratio of length to width, this implies that the seed grows very Received: November 12, 2012
A
dx.doi.org/10.1021/cm303659q | Chem. Mater. XXXX, XXX, XXX−XXX
Chemistry of Materials
Article
Figure 1. Evolution of Au NR morphology. TEM images of seeds and nanorods arrested and isolated at different times after seed addition. (a) Seeds aged for 5 min prior to addition to growth solution; nanorods arrested at (b) 2 min, (c) 5 min, (d) 7 min, (e) 20 min, (f) 30 min, (g) 45 min, and (h) 24 h. The scale bar is 10 nm for (a) and (b) and 100 nm for the rest. The scale bar in the insets is 5 nm.
limited model that accounted for facets and directional growth by an anisotropic coarsening mechanism.19 Unfortunately, the lack of experimental observation of the transformation from spherical to extended particle limits the confirmation of the various hypotheses revealed by modeling and theory. To address these challenges, we discuss the use of strongly binding polymeric ligands (monothiolated polystyrene, PS-SH) to simultaneously arrest growth and to phase transfer the particles into a nongrowth medium (toluene). Even at the earliest stages of the reaction (0−15 min), the procedure removes CTAB and unreacted precursors, while ensuring Au NR isolation as well as enabling to increase the concentration. Together, this provides a facile and reliable TEM observation. The ligand exchange does not alter the optical spectrum of the Au NRs, and the shifts of the LSPR are simply reflective of the transfer from aqueous to organic media. Overall, the Au NR morphology evolves from a subpopulation of seed particles, presumably those with a mixture of {110} and {111} crystal facets.20 Different growth rates on these facets, mediated by the different stability of adsorbed CTAB,21 account for the sequential emergence of a range of shapes, including slender
rapidly in one direction, followed by a more uniform growth. Zweifel and Wei16 were able to arrest the growth of Au NRs by treatment with Na2S and found that at an intermediate stage (t ∼ 15−30 min) the rod morphology transforms from dumbbell to an oblate shape, consistent with a decreasing aspect ratio at increasing growth time. Early stage analysis (5 min), the bimodal distribution of seed particle size becomes more obvious (Supporting Information Figure S3a−d show SAXS and TEM of 90 min aged seed solution). The population around 1 nm at 5 min increases slightly to 1.2 nm while the population around 1.9 nm shifts to 4.8 nm. The slower growth of the smaller particles again implies that they have crystal facets, such as {100}, with more strongly adsorbed CTAB. We therefore hypothesize that seeds containing {100}/{110} and {111} facets are necessary for directional growth. Thus, during stage I all seeds grow isotropically until a critical size which defines the transition to stage II. The smaller seeds, exhibiting crystalline facets enriched in {100} and {111}, lead F
dx.doi.org/10.1021/cm303659q | Chem. Mater. XXXX, XXX, XXX−XXX
Chemistry of Materials
Article
Scheme 1. Complete Growth Process of Au NR and Impurities during Seed Mediated Growtha
a The growth of Au NR in a CTAB-based growth solution showing the evolution of rods, spheres, and cubes from different crystal structure seed upon the formation of a surfactant bilayer on different crystal facets at different growth stages due to a fine balance between kinetic and thermodynamic factors. The scale bar in the TEM image is 100 nm.
section with facets and relative angles that are on average reflective of distinct crystal planes. In the course of this process, {120} facets initially evolve, followed by {250} facets (Figure 3 f). This is consistent with the evolving cross sectional shapes of rods at stage V (Figure 3g,h). The cross-section gradually changes from truncated square to octagon. Figure 3g shows side facets close to {120} that have developed on top of {110}. The measured angle between the evolving facets is 149° which is close to the theoretical value of 143° for two adjacent {120} facets. Figure 3h shows a cross section of a Au NR at an even later time. The measured angle is 138° which is approaching the theoretical value of an octahedron (135°) and two adjacent {250} facets (133.6° and 136.4°). It is noticeable that the angles in different rods as well as within a rod show slight distribution ( {100} > {111}. Given the ⟨001⟩ axial orientation of the fcc lattice, the {110} facets will decrease while the {100} facets will grow, transforming the cross section from {110} dominant to a {100} dominant truncated square.45 However, an HRTEM image (Figure 3c) of completely grown Au NRs (stage V) reveals an octagonal cross-section with approximately equal angles consistent with {250} side facets. The phase image (Figure 3d) of the thickness profile is also consistent with {250} surfaces, which confirms recent findings from other groups.26,46 These crystallographic features are consistent with structure reorganization occurring by continual adatom migration on surfaces. At the atomic scale, the {250} plane can be thought of as being composed of unit-cell-sized {120} and {130} planes. These are intermediate toward the {100} plane27 (Figure 3e). We speculate that at the final stage the side facets go through surface reconstruction via surface step migration that will progressively transform the cross-section, rather than an abrupt geometric shift in the relative angle of the facets. The local surface step density and distribution of lattice termini will mesoscopically result in a gradually curved crossG
dx.doi.org/10.1021/cm303659q | Chem. Mater. XXXX, XXX, XXX−XXX
Chemistry of Materials
Article
fraction of byproducts. (2) The isotropic particles begin to grow anisotropically due to selective group adsorption of the CTAB micelle on high energy {110}/{100} surfaces when they become comparable to the size of the micelle. (3) The initial lengthwise growth is very rapid (accompanied by a red shift in the L-LSPR) since Au atoms are added to the small area at the rod end. The length at this stage is polydispersed due to rapid addition to these open {111} facets. The rapid growth slows down when the {100} facet at the end becomes large enough to accommodate a CTAB micelle. The relationship between diameter and end facet size is presumably geometric, thus resulting in a convergence of aspect ratio and a decrease of the rod polydispersity. (4) The dumbbell shape is due to the addition of the atoms from the end of the rod, which move toward the center resulting in a signature (1 × 2) surface reconstruction. (5) The emergence of {250} facets of final rods is due to the process of surface reconstruction to minimize thermodynamically unfavorable {110} facets, which transform into {120} and, later, {250} facets. Overall, the thermodynamically driven adsorption of CTAB micelles selectively to {100} and {110} facets promotes kinetic elongation of unstable surfaces due to growth inhibition on those surfaces. Therefore, the morphological evolution is closely related to the time scale of forming a dense bilayer and the amount of adatoms deposited at the rod end. The proposed growth mechanism is based on the rationalization of the evolution of Au NR morphologies with the adsorption kinetics of CTAB and the structural features inferred from the reported experimental studies on 2D microscale substrates20,47 and modeling studies.18 It is noteworthy to address a general challenge to characterize the adsorption kinetics and the structure of organic materials such as CTAB at the nanoparticle surface. We believe that more analytical studies will fortify the proposed mechanism and disclose further structural details of Au NR.
to increase the control over structure and impurity content in Au NR synthesis and, thus, improve the opto-electronic performance of individual Au NRs as well as the quality of their self-assembled structures.
CONCLUSIONS We experimentally demonstrate the complete evolution of seed-mediated Au NR growth in CTAB solution and bridge the evolution of morphology at each stage by a few self-consistent kinetic and thermodynamic factors. We extend our understanding to problems such as the exact cause of the initial symmetry-breaking and the parameters which control the growth by connecting the intermediate morphology of evolving rods to the kinetics of surfactant adsorption. Also, previous efforts to modulate the aspect ratio of NRs through additional additives such as benzyldimethylhexadecylammonium chloride, AgNO3, and small aromatic molecules9,28 can be related to their impact on the construction of a more secure and stable bilayer earlier during the growth process. Based on these experimental findings, various routes to manipulate the morphology of Au NRs can be understood from a common framework, including the roles of costabilizers in promoting or inhibiting adsorption and growth at different stages, as well as modulating the reduction rate during growth to fine-tune the amount of adatoms on the surface. The precise control over structure and size becomes imperative to determine the successful application of Au NRs in many fields. The shape and size of rods should be carefully selected to optimize the plasmon resonance wavelength, and the relative contribution of absorption and scattering to the total extinction coefficient are critical for sensing, imaging, medical diagnostics, therapeutic, and energy harvesting technologies. We believe that our findings will allow
(1) Paolo, B.; Jer-Shing, H.; Bert, H. Rep. Prog. Phys. 2012, 75 (2), 024402. (2) Funston, A. M.; Davis, T. J.; Novo, C.; Mulvaney, P. Philos. Trans. R. Soc., A 2011, 369 (1950), 3472−3482. (3) Halas, N. J.; Lal, S.; Chang, W.-S.; Link, S.; Nordlander, P. Chem. Rev. 2011, 111 (6), 3913−3961. (4) Woo, K. C.; Shao, L.; Chen, H. J.; Liang, Y.; Wang, J. F.; Lin, H. Q. ACS Nano 2011, 5 (7), 5976−5986. (5) Zijlstra, P.; Chon, J. W. M.; Gu, M. Nature 2009, 459 (7245), 410−413. (6) Zijlstra, P.; Orrit, M. Rep. Prog. Phys. 2011, 74 (10), 55. (7) Chan, Y.-T.; Li, S.; Moorefield, C. N.; Wang, P.; Shreiner, C. D.; Newkome, G. R. Chem.−Eur. J. 2010, 16 (14), 4164−4168. (8) Liu, K.; Nie, Z.; Zhao, N.; Li, W.; Rubinstein, M.; Kumacheva, E. Science 2010, 329 (5988), 197−200. (9) Ye, X.; Jin, L.; Caglayan, H.; Chen, J.; Xing, G.; Zheng, C.; DoanNguyen, V.; Kang, Y.; Engheta, N.; Kagan, C. R.; Murray, C. B. ACS Nano 2012, 6 (3), 2804−2817. (10) Vigderman, L.; Khanal, B. P.; Zubarev, E. R. Adv. Mater. 2012, 24 (36), 4811−4841. (11) Murphy, C. J.; Thompson, L. B.; Chernak, D. J.; Yang, J. A.; Sivapalan, S. T.; Boulos, S. P.; Huang, J. Y.; Alkilany, A. M.; Sisco, P. N. Curr. Opin. Colloid Interface Sci. 2011, 16 (2), 128−134. (12) Seo, S.; Wang, X.; Murray, D. Ionics 2009, 15 (1), 67−71. (13) Sau, T. K.; Murphy, C. J. Langmuir 2004, 20 (15), 6414−6420. (14) Morita, T.; Tanaka, E.; Inagaki, Y.; Hotta, H.; Shingai, R.; Hatakeyama, Y.; Nishikawa, K.; Murai, H.; Nakano, H.; Hino, K. J. Phys. Chem. C 2010, 114 (9), 3804−3810. (15) Wei, Z.; Qi, H.; Li, M.; Tang, B.; Zhang, Z.; Han, R.; Wang, J.; Zhao, Y. Small 2012, 8 (9), 1331−1335.
■
ASSOCIATED CONTENT
S Supporting Information *
The detailed UV−vis−NIR spectra of gold NR solution, the TEM images of aged seed and fully grown gold NRs, and smallangle X-ray scattering data. This material is available free of charge via the Internet at http://pubs.acs.org.
■
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Phone: 937-255-9209. Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS This work was completed at Air Force Research Laboratory (AFRL) at Wright Patterson Air Force Base with funding from Materials and Manufacturing Directorate, as well as Air Force Office of Scientific Research. We would like to thank Dr. Alexander Hexemer and Dr. Eric Schaible for guidance, setup and data collection at beamline 7.3.3 at Advanced Light Source/Lawrence Berkley National Laboratory. The Advanced Light Source is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.
■
■
H
REFERENCES
dx.doi.org/10.1021/cm303659q | Chem. Mater. XXXX, XXX, XXX−XXX
Chemistry of Materials
Article
(16) Zweifel, D. A.; Wei, A. Chem. Mater. 2005, 17 (16), 4256−4261. (17) Edgar, J. A.; McDonagh, A. M.; Cortie, M. B. ACS Nano 2012, 6 (2), 1116−1125. (18) Grochola, G.; Snook, I. K.; Russo, S. P. J. Chem. Phys. 2007, 127 (19), 194707−13. (19) Seyed-Razavi, A.; Snook, I. K.; Barnard, A. S. Cryst. Growth Des. 2011, 11 (1), 158−165. (20) Tyrode, E.; Rutland, M. W.; Bain, C. D. J. Am. Chem. Soc. 2008, 130 (51), 17434−17445. (21) Feng, J.; Pandey, R. B.; Berry, R. J.; Farmer, B. L.; Naik, R. R.; Heinz, H. Soft Matter 2011, 7 (5), 2113−2120. (22) Yasun, E.; Gulbakan, B.; Ocsoy, I.; Yuan, Q.; Shukoor, M. I.; Li, C.; Tan, W. Anal. Chem. 2012, 84 (14), 6008−6015. (23) Jain, T.; Roodbeen, R.; Reeler, N. E. A.; Vosch, T.; Jensen, K. J.; Bjornholm, T.; Norgaard, K. J. Colloid Interface Sci. 2012, 376, 83−90. (24) Wang, Z. L.; Mohamed, M. B.; Link, S.; El-Sayed, M. A. Surf. Sci. 1999, 440 (1−2), L809−L814. (25) Goris, B.; Bals, S.; Van den Broek, W.; Carbó-Argibay, E.; Gómez-Graña, S.; Liz- Marzán, L. M.; Van Tendeloo, G. Nat. Mater. 2012, 11 (11), 930−935. (26) Carbó-Argibay, E.; Rodríguez-González, B.; Gómez-Graña, S.; Guerrero-Martínez, A.; Pastoriza-Santos, I.; Pérez-Juste, J.; Liz-Marzán, L. M. Angew. Chem., Int. Ed. 2012, 49 (49), 9397−9400. (27) Katz-Boon, H.; Rossouw, C. J.; Weyland, M.; Funston, A. M.; Mulvaney, P.; Etheridge, J. Nano Lett. 2011, 11 (1), 273−278. (28) Nikoobakht, B.; El-Sayed, M. A. Chem. Mater. 2003, 15 (10), 1957−1962. (29) Yang, J.; Wu, J. C.; Wu, Y. C.; Wang, J. K.; Chen, C. C. Chem. Phys. Lett. 2005, 416 (4−6), 215−219. (30) Molina, L. M.; Hammer, B. Chem. Phys. Lett. 2002, 360 (3−4), 264−271. (31) Soria, L. A.; Zampieri, G.; Martiarena, M. L. J. Phys. Chem. C 2011, 115 (19), 9587−9592. (32) Day, P. N.; Nguyen, K. A.; Pachter, R. Modeling the optical properties of small gold nanoclusters using time-dependent density functional theory. In Proceedings of Quantum Dots, Particles, and Nanoclusters VI, San Jose, CA, USA, 2009; SPIE: San Jose, CA, USA, 2009; pp 722418-1−722418-9. (33) Negishi, Y.; Nobusada, K.; Tsukuda, T. J. Am. Chem. Soc. 2005, 127 (14), 5261−5270. (34) Barnard, A. S. Rep. Prog. Phys. 2010, 73 (8), 086502. (35) Aastuen, D. J. W.; Clark, N. A.; Cotter, L. K.; Ackerson, B. J. Phys. Rev. Lett. 1986, 57 (14), 1733−1736. (36) Goodman, C. H. L. Crystal growth: theory and techniques; Plenum Press: London, 1974; Vol. 1. (37) Pagac, E. S.; Prieve, D. C.; Tilton, R. D. Langmuir 1998, 14 (9), 2333−2342. (38) Notley, S. M. J. Phys. Chem. B 2009, 113 (42), 13895−13897. (39) Valden, M.; Lai, X.; Goodman, D. W. Science 1998, 281 (5383), 1647−1650. (40) Hubert, F.; Testard, F.; Rizza, G.; Spalla, O. Langmuir 2010, 26 (10), 6887−6891. (41) Hubert, F.; Testard, F.; Thill, A.; Kong, Q.; Tache, O.; Spalla, O. Cryst. Growth Des. 2012, 12 (3), 1548−1555. (42) Treiner, C. Encyclopedia of Surface and Colloid Science; Marcel Dekker: New York, 2002; Vol. 4, p 5152. (43) Wang, Z. L.; Gao, R. P.; Nikoobakht, B.; El-Sayed, M. A. J. Phys. Chem. B 2000, 104 (23), 5417−5420. (44) Keul, H. A.; Moller, M.; Bockstaller, M. R. Langmuir 2007, 23 (20), 10307−10315. (45) Wang, Y.; Teitel, S.; Dellago, C. Nano Lett. 2005, 5 (11), 2174− 2178. (46) Katz-Boon, H.; Rossouw, C. J.; Weyland, M.; Funston, A. M.; Mulvaney, P.; Etheridge, J. Nano Lett. 2011, 11 (1), 273−278. (47) Qi, L.; Bi, Z. Colloid J. 2008, 70 (2), 178−183.
I
dx.doi.org/10.1021/cm303659q | Chem. Mater. XXXX, XXX, XXX−XXX
