Aspect-Ratio Dependence on Formation Process of Gold Nanorods Studied by Time-Resolved Distance Distribution Functions

Gold nanorods with aspect ratios of 2, 4, and 6 were generated by a seed-mediated growth method developed by Jana et al. and improved by Nikoobakht and El-Sayed. Nanorod formation during synthesis was investigated by time-resolved measurements of small-angle X-ray scattering (SAXS) and in situ UV−vis−NIR absorption. A sample holder made entirely of titanium was newly constructed for the simultaneous measurements. Distance distribution functions were calculated from the SAXS profiles, and the growth in nanorod length as a function of reaction time was determined without having to approximate or simplify the actual nanorod shape. Structural anisotropy, which corresponds to aspect ratio of nanorods, was also evaluated from the functions. For formed nanorods with aspect ratios in the range of 2−4, the time dependence of anisotropy shows maxima. In contrast, for formed aspect-ratio 6 nanorods, a maximum is not observed, and anisotropy continuously increases until an early reaction-equilibrium is reached. Growth kinetics for the variation in the aspect ratios of the formed nanorods is evaluated based on the length growth rate and the width growth rate, each considered individually. Finally, structural changes of nanorod end-caps during synthesis are discussed.