New Insights into the Growth Mechanism of Ultrathin Au Nanowires from Combined in Situ EXAFS and SAXS Studies

Abstract

The synthesis of Au nanowires (NWs) in hexane solution of HAuCl4·3H2O, oleylamine and triisopropylsilane at room temperature was "in situ" monitored by means of X-ray absorption fine structure spectroscopies and small-angle X-ray scattering to determine, under identical synthesis conditions, both the changes in the oxidation state of gold atoms and the evolution of the size and shape of the objects involved in the formation of Au NWs. We propose a multistage process for the formation of the NWs: first, Au(III) atoms form a planar-square geometry complex that is continuously reduced to give Au(I) disk-like structures with diameters bigger than that of the final NWs. In a second stage, characteristic length/thickness ratio of these disk-like objects increases to form cylinders, presumably through aurophilic interactions between Au(I) centers and stacking of the disks. When most of the Au atoms have been reduced to Au(I), the reduction to Au(0) begins (third stage), and the NWs grow to form an hexagonal arrangement, separated by a bilayer of oleylamine molecules (fourth stage). Finally, a slow reduction leads the reaction to the final product, formed by bundles of long, ultrathin Au NWs.