A self-seeding synthesis of Ag microrods of tuned aspect ratio: ascorbic acid plays a key role

Abstract

Control of the shape and size of nanoparticles is crucial for using them as labels or as building blocks in nanotechnology. In fact, silver has so far been considered as having the widest variety of different morphologies at the nano-scale and micro-scale levels. To make progress in these criteria, in our paper we have synthesized highly reproducible silver (Ag) microrods of controlled aspect ratios through a rapid self-seeding method. The Ag nano seeds are formed via the reduction of Ag ions in hot ethylene glycol by ascorbic acid, and the subsequent growth of microrods is controlled by further deposition of Ag atoms in the presence of poly(vinylpyrrolidone). Moreover, ascorbic acid is exclusively responsible for the rod morphology, as we describe here in detail. A very low concentration of ascorbic acid forms very few Ag microrods along with a majority of Ag colloidal particles, while random overgrowth on the surfaces is observed for higher concentrations. The critical reaction condition has been found in that the aspect ratio of the Ag microrods can be systematically tuned between 4 and 90. Also, the method in which ascorbic acid is added to the reaction medium plays a key role in controlling the aspect ratio of the Ag microrods. The non-monotonic dependence of the length and the diameter of the Ag microrods has been described by an empirical equation. The function can be interpreted by means of concentration-dependent competition between the adsorption of ligands and metal deposition.