Optical Properties of Gold Clusters Precipitated on Zirconia Particles
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ABSTRACT We have studied precipitation of noble metals with a view to extending these synthetic processes to other metals. In this paper we discuss the synthesis of gold clusters on the surface of oxide fine particles (in the range 60-100nm). The influence of pH of the solution, the concentration of metal and the reducing agent are discussed. The size distribution of the precipitated gold clusters, determined by transmission electron microscopy, are around 2 -10 nm. The interface between the gold clusters and oxide has been investigated by HRTEM. Optical measurements of the zirconia supported gold clusters shows a significant shift in the plasmon resonance. A model based on effective medium theory is proposed taking into account the high refractive index of zirconia.
INTRODUCTION Clusters or fine metallic particles deposits on ceramic powders are of great interest in the domain of catalysis and may also have promissing applications in microelectronics due to the inherent non-linear electronic and optical properties [1]. Theoretical calculations show that nonlinearity can be enhanced by increasing the refractive index of the host material [2]. This can be achieved by using for example titania or zirconia. These oxides have been extensivly used for many years as support for catalysis applications. Some authors have allready reported modification of light absorption of noble metals supported by these oxides. In this work, we focussed on the optical properties of such nano-composites which are strongly dependent on the nature of the interface nano-metal/insulator (oxide) [3]. However, the controlled synthesis of these nano-structured materials requires proper understanding of the chemical reactions. EXPERIMENT Initial zirconia powder (commercial 3Y-TZP, Tosoh) was heated at 600'C under oxidizing atmosphere during five hours in order to prepare the surface of grains. The isoelectic point (4.6-4.8) of the calcined powder was deduced from zeta potential values mesured as a function of the pH of the aqueous suspension with constant salt concentration ([NaCl]=10 3 mol.l'). For preparation of gold clusters deposited on zirconia, aqueous solution of gold chloride was added to zirconia suspension with 0.5% mol. Au content. The pH of the suspension was adjusted at different pH values by addition of calcium carbonate. The suspension was stired and the reduction of gold was obtained by addition of formaldehyde. The same process was used for cooled down suspensions (between 0°C and 25'C). The suspension was then filtred, washed with deionised water and dried at room temperature. Optical measurements were performed using a Lamba-19 Perkin-Elmer UV-Vis spectrophotometer with a sphere of integration for analysis of the diffuse reflexion. For TEM observations we used an Hitachi FEG-HF2000 with conventional transmission and dark-STEM mode imaging. The so-prepared powder was deposited on carbon coated copper grids and introduced in the microscope using sample holder with Gatan cold stage. Very local analyses of the chemical elements (f
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