Controlling Length of Gold Nanorods and Monitoring Their Growth Mechanism Using X-ray Absorption Spectroscopy
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0921-T05-09
Controlling Length of Gold Nanorods and Monitoring Their Growth Mechanism Using Xray Absorption Spectroscopy R. S. Liu1, H. M. Chen1, and S. F. Hu2 1 Department of Chemistry, National Taiwan University, Taipei, 106, Taiwan 2 National Nano Device Laboratories, Hsinchu, 300, Taiwan
ABSTRACT A new approach to fabricate long length of gold nanorods by controlling the volume of growth solution will be reported. The shape evolutions ranging from fusiform nanoparticles to 1D rods was observed. Increasing the addition of growth solution can control the length of nanorods. The length of rods can be extended to 2 μm, and nanorods with aspect ratios of up to ~ 70 could be obtained. Moreover, X-ray absorption spectroscopy (XAS) is applied herein to elucidate the growth mechanism of gold nanorods. The gold ions were directly reduced to gold atoms by ascorbic acid during the reaction, and then gold atoms were deposited on the surface of gold seeds that were introduced into the reaction. Extended X-ray absorption fine structure (EXAFS) confirmed the growth of gold and the environment around Au atoms over the reaction. The XAS are expected to have wide applications in the growth of gold and other related materials. INTRODUCTION Many characteristics of nano-materials are size- and shape-dependent, including their catalytic, optical, and physical properties [1]. Spherical particles can be prepared easily by wet chemical methods. Recent studies of spherical metallic particles investigate the bimetallic systems such as Au/Pd, Pd/Pt, Cu/Pd, Pt/Ru, Co/Pt, and Au/Pt [2]. Anisotropic metal nanoparticles have been prepared by using electrochemical, photochemical reduction method in aqueous system, bubbling hydrogen with capping polymer, and polyol process [3]. The metal nanoparticles with variable shapes can be fabricated via these methods, such as cubes, boxes, rods, prisms, multi-pod, and triangular rings [4]. The studies in one dimension nanorods draw a lot of attention in these variable shape nanoparticles. Short aspect ratio Au nanorods are especially interesting because of their optical properties. They exhibit transverse and longitudinal plasmon bands in the visible region of the spectrum, making them to apply in sensing and imaging fields. Both the electrochemical method and seed-mediated growth method used a cetyltrmethylammonium bromide (CTAB) as soft template to fabricate gold nanrods dispersed in aqueous solution. A number of reports have studied the growth mechanism of 1-D gold nanoparticles in the presence of CTAB. The growth direction of gold nanorods in the presence of CTAB have been confirmed by analyzing the electron diffraction pattern and HRTEM images [5]. The surface atomic defect structure and thermal reshaping of gold nanorods have been studied, and it was observed that the mean aspect ratio of the nanorods in solution decreases with increasing temperature while the average width remains constant. The thermal stability of the nanorod shape itself was examined, and it was found to be stable until the temp
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