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Directing the Deposition of Ferromagnetic Cobalt onto Pt-tipped CdSe@CdS Nanorods: Synthetic and Mechanistic Insights


Willinger,  Marc Georg
Inorganic Chemistry, Fritz Haber Institute, Max Planck Society;

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Hill, L. J., Bull, M. M., Sung, Y., Simmonds, A. G., Dirlam, P. T., Richey, N. E., et al. (2012). Directing the Deposition of Ferromagnetic Cobalt onto Pt-tipped CdSe@CdS Nanorods: Synthetic and Mechanistic Insights. ACS Nano, 6(10), 8632-8645. doi:10.1021/nn3019859.

A methodology providing access to dumbbell-tipped, metal-semiconductor and metal oxide-semiconductor heterostructured nanorods has been developed. The synthesis and characterization of CdSe@CdS nanorods incorporating ferromagnetic cobalt domains at both nanorod termini (dumbbell morphology) are presented. Cobalt nanoparticle tips are then selectively oxidized to afford CdSe@CdS nanorods with cobalt oxide domains at both termini. In the case of longer nanorods, self assembly of dipolar cobalt domains prior to oxidation resulted in the colloidal polymerization of the ferromagnetic cobalt domains to afford cobalt oxide domains which were fused due to the nanoscale Kirkendall effect. Intact wurtzite CdS nanorod domains were confirmed via XRD and HRTEM power spectrum analysis. Four lengths of CdSe@CdS nanorods with comparable diameters (6-7 nm) and lengths ranging from 40-174 nm were modified in this fashion. The total synthesis required five steps from commercially available reagents. Key synthetic considerations are discussed, with particular emphasis on isolation of intermediates, yields, and increasing scale of intermediate reactions. We show that installing platinum at CdSe@CdS nanorod termini effectively activates these nanorods towards facile cobalt deposition using our polymeric ligand methodology. Exposing unmodified CdSe@CdS nanorods to identical cobalt deposition conditions resulted only in the formation of free cobalt nanoparticles (CoNPs) and unmodified nanorods. We find that the enabling platinum-tipping step provides a mixture of matchstick and dumbbell PtNP-tipped nanorods. Kinetic investigations and control experiments reveal that other reagents in this key step (besides platinum) can activate CdSe@CdS nanorods towards Co deposition to a lesser extent. A mechanism involving both Pt-assisted Co deposition, and nanorod activation by chemical etching/ligand exchange is proposed.