Silica-coated Silver Nanoparticles
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1208-O09-01
Silica-coated silver nanoparticles Georgios A. Sotiriou and Sotiris E. Pratsinis Particle Technology Laboratory, Institute of Process Engineering Department of Mechanical and Process Engineering Swiss Federal Institute of Technology Zurich (ETH Zurich) ABSTRACT Silver (Ag) nanoparticles dispersed in an amorphous silica (SiO2) matrix or coated by a SiO2 layer were synthesized by flame spray pyrolysis (FSP). The coated nanoparticles were produced by using a modified enclosed FSP setup, in which the SiO2 precursor was injected through a ring above the FSP nozzle at various burner-ring-distances (BRDs), after the core Ag nanoparticles had been formed. The produced nanoparticles were characterized by XRD, BET, TEM and UV/vis analysis. The Ag particle size was possible to be controlled by tuning the FSP parameters. For the SiO2 coated nanoparticles, larger Ag core sizes were obtained for higher BRDs. All the produced nanoparticles exhibited the characteristic plasmon resonance frequency of Ag nanoparticles. INTRODUCTION Silver (Ag) nanoparticles play an important role in photography, catalysis, biological labeling, photonics, optoelectronics and surface enhanced Raman scattering [1]. This is attributed to the unique optical properties of Ag that originate from the collective oscillations of conduction electrons, termed as surface plasmon polariton resonances [2]. This intrinsic property of Ag nanoparticles is mainly determined by their size, shape, composition, crystallinity, and structure. In principle, one could control any one of these parameters to fine-tune the corresponding optical properties. An attractive way to do this is by tuning the dielectric medium which surrounds them [3]. Such a dielectric medium is silica (SiO2), with which the unique shape- and size-dependent optical properties of Ag nanoparticles can be tailored. Coating Ag with SiO2 can also enhance the stability of Ag nanoparticles and provide tunable solubility in various solvents [4]. Monodispersed SiO2-coated Ag particles readily self-assemble into photonic crystals with an optical band gap in the visible region [5]. Moreover, SiO2 shells can be easily functionalized, allowing the nanoparticles to be conjugated to other substrates or molecules, a property extremely useful in bio-labeling [6]. There have been reported various wet techniques to encapsulate fine metal nanoparticles, such as Ag, in a SiO2 shell. These methods are effective but time-consuming and difficult to control [4]. Additionally, most of these methods can only be used for coating large metallic particles (~50 nm in diameter) because small particles are unstable and tend to aggregate in the alcoholic solutions typically used for wet methods [4]. Therefore, it is attractive to make Ag nanoparticles coated with a layer of SiO2, by flame spray pyrolysis (FSP), a dry, faster, more versatile and with a proven scalability process [7, 8]. Recently, Teleki et al. [9] reported the in-situ coating of TiO2 nanoparticles with an ultrathin SiO2 layer, using a modified FSP unit, in
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