Role of Copper Sulfide Seeds in the Growth Process of CuInS2 Nanorods and Networks

CuInS2 nanorods and networks are interesting candidates for applications requiring efficient charge transport, such as solar energy conversion, because of the increased electrical conductivity in elongated or interconnected nanocrystals, compared to isolated, quasi-spherical ones. However, little is known about the growth mechanisms involved in the formation of this kind of nanostructures, yet. Here, CuInS2 nanorods and networks were synthesized through a facile low-cost and phosphine-free method. Copper and indium sources were added together in the presence of oleylamine and oleic acid. Changing the amount of oleic acid present in the reaction solution influenced the reactivity of the monomers, and consequently, the size of copper sulfide seeds formed in situ after the injection of tert-dodecanethiol, serving as the source of sulfur. Two different growth mechanisms of CuInS2 nanorods were observed, depending on the size of the copper sulfide seeds. Larger seeds (8 nm), which were generated with relatively small amounts of oleic acid, resulted in the formation of hybrid copper sulfide-copper indium disulfide nanocrystals as intermediates in the growth process of the nanorods, while smaller seeds (4 nm) obtained with relatively large amounts of oleic acid were gradually converted to copper indium sulfide nanorods. At longer reaction times, these nanorods formed network structures. The reaction between oleylamine and oleic acid at high temperature turned out to be the crucial factor to induce the attachment of nanorods to multipods and networks.