COMMUNICATION pubs.acs.org/cm
Shape Transformation of Gold Nanoplates and their Surface Plasmon Characterization: Triangular to Hexagonal Nanoplates Soonchang Hong,† Kevin L. Shuford,*,§ and Sungho Park*,†,‡ †
Department of Chemistry and ‡Department of Energy Science and SKKU Advanced Institute of Nanotechnology, Sungkyunkwan University, Suwon 440-746, South Korea § Department of Chemistry, Drexel University, 3141 Chestnut Street, Philadelphia, Pennsylvania 19104, United States
bS Supporting Information KEYWORDS: hexagonal nanoplate, gold nanoparticle, surface plasmon resonance, nanoprism, shape transformation
B
ecause the optical properties of metal nanoparticles are known to be dependent primarily on their shape and size, one of the desired structural goals for these materials is to obtain a uniform size and morphology in a controlled fashion.13 Many literature reports describe the preparation routes of various nanostructures, including spherical, rodlike, cubic, and planar nanoparticles.46 However, only a few methods have been developed for preparing nanoparticles with good control over their sizes and homogeneities, with which one can systematically study the optical properties of such nanostructures.79 Among the many different shapes of metal nanoparticles, anisotropic nanoparticles are particularly interesting because the decreased symmetry allows one to tune their optical properties to a great extent. For instance, elongated nanostructures (or nanorods) exhibit two types of localized surface plasmon resonance (SPR) modes one oscillating along the short axis and the other along the long axis.1012 These modes can be fine-tuned by controlling geometric parameters such as diameter and length. Also, platelike nanostructures like nanodisks allow a similar tunability in their optical properties resulting from in-plane and out-of-plane modes. Another representative example in this context is the triangular prism (also referred to as nanoprisms) and their synthetic route is well-developed.13 The ratio of surface to bulk atoms of plate-like nanostructures is higher than that of other nanostructures; therefore, it is expected that the SPRs of nanoplates will be very sensitive to the dielectric medium, including binding of adlayer molecules and solvent that surrounds them. The SPR sensitivity to dielectric constant of the medium is important for developing highly efficient chemical and biological sensors based on the SPR shifts. Herein, we present the synthetic route of hexagonal nanoplates and their corresponding optical properties. Our synthetic method relies on the synthesis of triangular nanoplates that is followed by shape transformation under mild reduction conditions, leading to the formation of hexagonal nanoplates.14 The higher-order multipole SPRs of such structures could be resolved and compared with theoretical results for the first time. Figure 1 shows a schematic diagram of the steps involved in the synthesis of hexagonal nanoplates. Their corresponding FESEM images are displayed in Figure 2. First, triangular nanoplates were prepared by following the previously reported method. They show fairly homogeneous size (edge L = 136 r 2011 American Chemical Society
Figure 1. Procedure for the shape transformation from triangular to hexagonal nanoplates. AA is ascorbic acid.
((8) nm, thickness = 8 ((1) nm) and shape distribution. In the synthesis, AuCl4 ions were reduced by ascorbic acid in the presence of CTAB and small amount of iodide ions. The concentration of I played a critical role in determining the shape of resulting nanoparticles, as suggested in the previous publication.15 At a certain concentration of iodide, the reaction predominantly leads to the formation of triangular nanoplates. Second, AuCl4 ions are added to the triangular nanoplate solution in order to etch selectively the tips of triangular nanoplates. The standard redox potential of AuCl4 (AuCl4 þ 2e f AuCl2 þ 2Cl, Eo: 0.926 V) is lower than that of AuCl2 (AuCl2 þ e f Au(s) þ 2Cl, Eo: 1.154 V).16 Therefore, the oxidation of Au(s) is unlikely to occur under given conditions. However, the resulting Au nanoplates initially show sharp vertexes and they become gradually rounded as a function time, indicating that the vertex atoms are more labile than other surface atoms to the chemical change of medium. The selective etching of Au triangular nanoplates indicates that dissimilar crystallographic planes react differently toward the change of chemical environments. Unlike the Au(111) terrace atoms (flat domain), which have a surface coordination number, CN, equal to 9, the vertex sites have surface atoms with low CN (
