Synthesis of Hybrid Gold-Gold Sulfide Colloidal Particles Jero ˆme Majimel, † Daniel Bacinello, † Etienne Durand, † Fabrice Valle ´e, ‡ and Mona Tre ´guer-Delapierre* ,† ICMCB-CNRS, UniVersity of Bordeaux I, 87 aV. Dr. Schweitzer, 33608 Pessac Cedex, France, and LASIM-CNRS, UniVersity of Lyon 1, 43 Bd du 11 NoVembre, 69622 Villeurbanne, France ReceiVed September 13, 2007. In Final Form: January 29, 2008 The nucleation and growth mechanism of nanometer size gold onto gold sulfide colloidal particles by irradiation- induced reduction is reported. The process is characterized by ultraviolet-visible spectroscopy, electronic diffraction, and high-resolution transmission electron microscopy, allowing for observation of several key intermediates and characteristics of the growth mechanism. The formation mechanism of gold on the surface of the gold sulfide particles is shown to depend strongly on the deposition rate. At low dose rate, gold nucleates preferentially onto specific gold-rich Au 2 S facets {110}, resulting in epitaxial growth. The gold crystal lattice plastically deforms near the interface to accommodate a substantial lattice mismatch. Upon increasing gold precursor concentration, this low dose rate results in growth of elongated gold island on the gold sulfide surface. At a high dose rate, several randomly oriented gold particles are simultaneously produced on gold sulfide, resulting in a layered structure. The absorption spectra of these particles show a dominant surface plasmon band, whose peak wavelength shifts markedly to the red as layered structure is formed. Introduction Nanostructures composed of multiple materials have recently become the focus of intensive study with particular attention being paid to their optical properties and the enhanced role of the interface between materials. In this context, dielectric- or semiconductor-metal hybrid nanoparticles are particularly in- teresting, since they offer the possibility to combine different type of linear and nonlinear optical responses or to design surface plasmon resonance effects in new frequency ranges. This is in particular the case for nanoshell materials, whose surface plasmon resonance can be tuned over the infrared and visible spectrum offering many applications for biolabeling or plasmon mediated optical response enhancement (Raman scattering or luminescence of adsorbed molecule for instance). In order to manipulate the diverse properties of these materials or to create novel composite materials to meet technological requirements or to perform desired functions, it is necessary to establish control over the interface between the various components. In addition, a thorough understanding of the nature and character of this interface is required. In this work, we discuss the nucleation and growth process of gold nanoparticles onto gold sulfide semiconductor nanocores. Similar metal-semiconductor objects have already found widespread use in applications such as optics, sensors, photography, biomedical contrast imaging, and therapeutics. 1-7 In this study, Au I is reduced onto the surface of the gold sulfide core by a radiolytic method, the mechanism of which is understood and elucidated. The factors that affect the gold deposition, such as diffusion of gold atoms across the semiconductor surface as well as the rate of reduction, are discussed. Special attention is paid to structural characterization of the interface and the plastic deformation of the crystal lattice at the heterointerface. Materials and Methods Materials. Methanol and hydrochloric acid (HCl) were both purchased from J. T. Baker; nitric acid (HNO 3 ) was purchased from Carlo Erba Reagenti, and all materials were used as received. For the synthesis of Au 2 S particles and the Au 2 S/Au composite particles, potassium cyanoaurate (KAu(CN) 2 , 99.98%, Aldrich) was purchased and used as received. Prior to experiments, all glassware was washed thoroughly with aqua regia and rinsed with deionized water. Synthesis of Au 2 S Particles. 8,9 A solution (100 mL) of KAu- (CN) 2 of 0.5 mM is prepared. This solution is diluted to 0.2 mM in water (100 mL) and introduced into a three-neck flask. The solution is degassed by bubbling with Ar(g) for 30 min at 80 °C. The mixture is then bubbled with H 2 S(g) for approximately 80 min at 80 °C with stirring, at which point it is left under Ar(g) (without heating) for approximately 5 h to eliminate any excess H 2 S(g). This procedure yields a pale yellow solution consisting of Au 2 S nanoparticles. The linear absorption spectra of the solution can be used to confirm the elimination of the gold ions and the formation of the Au 2 S particles. The mechanism involved in the formation of the gold sulfide includes the generation of various gold sulfide complexes, such as Au 0 HS aq and Au(HS) 2 -, identified by Renders et al. 10 Synthesis of Au 2 S/Au Composite Particles. The deposition of colloidal gold onto the gold sulfide core is accomplished by radiolytic reduction of Au(CN) 2 - ions. The advantage of using ionizing radiation lies in the fact that the rate of the formation of the reducing radicals is well-known and can easily be regulated by changing the absorbed dose rate. The principle of the method has previously been described by Henglein and Meisel. 11 The desired amount of KAu(CN) 2 is added to the Au 2 S colloid solution along with 0.5 M methanol, and the solution is flushed with nitrous oxide. It is then irradiated either * Corresponding author: E-mail: treguer@icmcb-bordeaux.cnrs.fr. † University of Bordeaux I. ‡ University of Lyon 1. (1) Jain, P. K.; Lee, K. S.; El-Sayed, I. H.; El-Sayed, M. A. J. Phys. Chem. 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