NANO LETTERS
CTAB Mediated Reshaping of Metallodielectric Nanoparticles
2003 Vol. 3, No. 12 1707-1711
Carla M. Aguirre,† Tara R. Kaspar,‡ Corey Radloff,‡ and Naomi J. Halas*,†,‡,§ Rice Quantum Institute, Department of Chemistry, Department of Electrical and Computer Engineering, Rice UniVersity, Houston, Texas 77251 Received August 28, 2003; Revised Manuscript Received September 30, 2003
ABSTRACT We report on the dramatic chemical reshaping of metallodielectric nanoparticles upon exposure to cetyltrimethylammonium bromide (CTAB) in aqueous solutions. Metal nanoshells possessing a silica core with a gold shell were observed to undergo a morphological change from their initial shell structure to that of large, elongated gold nanoparticles or toroids. This reaction is dependent on the chemical species available in solution and on the metal shell thickness on the nanostructure. Surprisingly, dissolution of the silica core due to the presence of CTAB was also observed during the reshaping of the nanoshells.
The “bottom-up” approach of nanostructure synthesis/ fabrication from molecular precursors has been very successful. Large quantities of monodisperse nanoparticles having a wide range of compositions and structures can now be synthesized.1-6 Long-range arrays of planar metal lines and metal island structures have also been fabricated using chemical methods.7,8 It remains a major challenge, however, to develop new chemical synthetic approaches to nanoparticle synthesis to obtain a further variety of more complex nanoscale shapes, for an increasingly wide range of applications. One approach has been to use preformed nanoparticles as building blocks to assemble new nanostructures. For example, metal nanoparticles can be assembled onto a variety of nanoparticle substrates to create metallodielectric nanostructures,9-11 including nanostructures of reduced symmetry,12,13 or used as templates for fabricating new nanoparticles.14-16 An alternative approach is to modify the nanoparticle growth reaction in situ to favor geometric and morphological changes in the final nanoparticle. Recent work has demonstrated that it is possible to fabricate branched CdSe nanocrystals of tunable size by controlling the growth kinetics during different phases of the reaction to favor growth along specific crystal facets of the material.17 To create anisotropic metal nanoparticles, a similar in situ growth control can be achieved by the inclusion of surfactants in the growth solution. Metal nanoparticles including nanorods,18-21 nanocubes,6,22 and nanotubes15 are some examples of particles that have been prepared using surfactant-based methods. The size and shape of the surfactant micelles provide control over particle morphology during nanoparticle growth.23 The selective †
Rice Quantum Institute. Department of Chemistry. Department of Electrical and Computer Engineering. * Corresponding author:
[email protected].
‡ §
10.1021/nl034712+ CCC: $25.00 Published on Web 11/12/2003
© 2003 American Chemical Society
adsorption of surfactant molecules and their respective counterions on certain crystallographic facets during particle growth is also believed to affect nanoparticle shape.15,24 Studies that have attempted to characterize the mechanisms controlling the size and shape of metal nanoparticles produced through a surfactant-mediated approach have found the process to be highly dependent on the various chemical species in solution and the thermodynamic stability of the nanoparticle crystalline domains.15,21,25-27 Virtually all prior work in this area has focused on the role of the surfactant in nanoparticle synthesis. The structure and geometry of metal nanoparticles can also be modified after synthesis. Specifically, metal nanoparticles have been converted from one geometry to another using pulsed laser irradiation28-31 and photoinduced and thermalinduced conversion processes.32,33 Very little attention has been paid to the reshaping of nanoparticle morphology through chemical techniques.34,35 In this letter we report a surfactant-mediated reshaping of metallodielectric silicagold core-shell nanostructures, producing new nanostructure morphologies in a highly consistent manner. We have found that the nanoparticle reshaping is dependent upon the presence of CTAB, and that the final morphology depends on the reactant environment of the nanostructure and also on the morphology of the starting nanostructure. Specifically, we have observed the room temperature reshaping of the gold shell to form highly asymmetric gold rod-like or beanlike structures attached to silica nanoparticles, and gold toroids. We also have observed that the silica nanoscale structures present reshape and ultimately dissolve over time. In this study, we have identified several parameters that affect this remarkable chemical reactivity.
Figure 1. SEM image of a typical nanoshell batch consisting of particles having a 122 ( 15 nm diameter silica core and ∼12 nm thick gold shell. Inset: bright and dark field TEM images reveal various facets of the polycrystalline gold shell.
The nanoshells used in these studies were fabricated by a method described in detail elsewhere.9 A number of different nanoshell batches were used in the course of these experiments. The nanoshells consisted of a 122 ( 15 nm diameter silica core with a gold shell that was varied in thickness from 10 ( 1 to 17 ( 2 nm. Figure 1 shows a scanning electron micrograph (SEM) and bright and dark field transmission electron micrographs (TEM) of as-prepared gold nanoshells having a 12 nm gold shell. Small defects can be seen in the SEM image of nanoshells in Figure 1. Various crystalline domains are apparent from the TEM micrographs (inset). The polycrystalline structure of the metallic shell is a result of the colloid-nucleated growth mechanism involved in the fabrication process. After fabrication, all nanoshell batches were centrifuged twice and redispersed in ultrapure water. No particle flocculation was observed during this process, and nanoshells prepared in this manner were observed to be stable for over six months. To eliminate adsorbed species from the surface of the nanoshells, some nanoshell batches were further purified by dialysis. Nanoshell samples cleaned through this process will be referred to as dialyzed nanoshells. In a typical dialysis, 40 mL of nanoshells in aqueous solution
was transferred into a regenerated cellulose membrane dialysis bag (Spectra-Por, 6-8k MWCO). The bag was suspended in a reservoir of ultrapure water and gently stirred overnight. Because of the unique morphology of gold nanoshells, where their plasmon-derived optical resonance is highly dependent upon the relative size of the inner and outer dimensions of the metal layer,9,36,37 any changes in their morphology are easily detected using UV-visible spectroscopy. The precise nature of the shape changes was confirmed through TEM and SEM analysis. Aqueous solutions of gold nanoshells were prepared containing various amounts of the cationic surfactants CTAB and cetyltrimethylammonium choride (CTAC). The typical nanoshell concentration was of 5 × 109 particles/mL. No changes were seen for gold nanoshells suspended in CTAC solutions, regardless of shell thickness or CTAC concentration. Gold nanoshells dispersed in aqueous CTAB solutions underwent a slow reshaping of their gold shell layer. These changes were independent of the surfactant concentration used in the range of 0.1 M to 1 mM. At very low surfactant concentrations (
