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

Abstract

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 nanoinclusions at both nanorod termini (i.e., dumbbell morphology) are presented. The key step in the synthesis of these heterostructured nanorods was the decoration of CdSe@CdS nanorods with platinum nanoparticle tips, which promoted the deposition of metallic CoNPs onto Pt-tipped CdSe@CdS nanorods. Cobalt nanoparticle tips were then selectively oxidized to afford CdSe@CdS nanorods with cobalt oxide domains at both termini. In the case of longer cobalt-tipped nanorods, heterostructured nanorods were observed to self-organize into complex dipolar assemblies, which formed as a consequence of magnetic associations of terminal CoNP tips. Colloidal polymerization of these cobalt-tipped nanorods afforded fused nanorod assemblies from the oxidation of cobalt nanoparticle tips at the ends of nanorods via the nanoscale Kirkendall effect. Wurtzite CdS nanorods survived both the deposition of metallic CoNP tips and conversion into cobalt oxide phases, as confirmed by both XRD and HRTEM analysis. A series of CdSe@CdS nanorods of four different lengths ranging from 40 to 174 nm and comparable diameters (6-7 nm) were prepared and modified with both cobalt and cobalt oxide tips. The total synthesis of these heterostructured nanorods required five steps from commercially available reagents. Key synthetic considerations are discussed, with particular emphasis on reporting isolated yields of all intermediates and products from scale up of intermediate precursors.

Universal Length Dependence of Rod-to-Seed Exciton Localization Efficiency in Type I and Quasi-Type II CdSe@CdS Nanorods

“…58,59 CdSe QDs of different sizes were used as seeds, and the lengths and diameters of overcoated CdS NRs were controlled by the ratio of CdSe QD seeds to CdS NR precursors. Details can be found in the SI.…”

Colloidal Magnetic Heterostructured Nanocrystals with Asymmetric Topologies: Seeded-Growth Synthetic Routes and Formation Mechanisms

“…For these systems, a high degree of selectivity for metal nucleation at both seed apexes was readily achievable (Figure 7A, path 1) (Habas et al, 2008;Jen-La Plante et al, 2009;Wang et al, 2009d;Nakibli and Amirav, 2016), whereas indiscriminate deposition at random sites was observed when large aspect-ratio nanorods/nanowires were used as starting seeds, or when the synthesis was carried out at exceedingly high reactant-to-seed concentration ratios ( Figure 7A, path 3). However, within the context of delicately manipulated organometallic chemistry, the choice of coordinating ligands and adjustment of their concentration in the liquid media appeared to be critical to ensuring topological selectivity, disclosing the important role of ligandcontrolled kinetic processes (Habas et al, 2008;Maynadiè et al, 2009;Hill et al, 2012;Nakibli and Amirav, 2016).…”

“…This idea has actually been proven to be practically feasible within the framework of rationally designed multiseeding-step reaction schemes (Hill et al, 2012;Pavlopoulos et al, 2016). In one report, Pt-tipped Cd@CdS core@shell nanorods were exploited to direct the otherwise unfeasible overgrowth of Co onto the Pt tips leading to corresponding Pt@Co-CdSe@CdS-Pt@Co dumbbell-like MHNCs with Pt@Co core@shell terminal domains; these MHNCs could be then converted into Pt@Co x O y -CdSe@CdS-Pt@Co x O y dumbbell-like heterostructures with Pt@Co x O y yolk@shell termini ( Figures 7E,F) upon selective oxidation and hollowing of the Co shell of the Pt@Co apexes (Hill et al, 2012).…”

“…This idea has actually been proven to be practically feasible within the framework of rationally designed multiseeding-step reaction schemes (Hill et al, 2012;Pavlopoulos et al, 2016). In one report, Pt-tipped Cd@CdS core@shell nanorods were exploited to direct the otherwise unfeasible overgrowth of Co onto the Pt tips leading to corresponding Pt@Co-CdSe@CdS-Pt@Co dumbbell-like MHNCs with Pt@Co core@shell terminal domains; these MHNCs could be then converted into Pt@Co x O y -CdSe@CdS-Pt@Co x O y dumbbell-like heterostructures with Pt@Co x O y yolk@shell termini ( Figures 7E,F) upon selective oxidation and hollowing of the Co shell of the Pt@Co apexes (Hill et al, 2012). In another protocol (Pavlopoulos et al, 2016), asymmetrically Au-tipped CdSe@CdS tetrapods were first fabricated by accommodating one single Au sphere at one of the four arm termini of preformed Cd@CdS core@shell tetrapods through a photodeposition process that proceeded through the initial attainment of multiply Au-decorated tetrapod arms, followed by intraparticle ripening of the as-nucleated Au patches into a single Au tip ( Figure 7B, paths 1 and 2); subsequently, thermolysis of Co 2 (CO) 8 in the presence of the Au-CdSe@CdS tetrapod heterostructures and of carboxylic-acid-terminated polystyrene ligands in 1,2,4-trichlorobenzene at T = 140°C enabled selective overgrowth of a Co shell around the previously implanted Au domains.…”

“…The inherent magnetic properties were also found to be modified upon heterojunction attainment (Jen-La Plante et al, 2009;Maynadiè et al, 2009). Interestingly, selective anchoring of magnetic-metal sections onto shaped semiconductor NCs was proven to be a valuable strategy toward promoting the self-assembly of corresponding anisotropic MHNCs into nanoheterostructure "polymers" by profiting from tip-to-tip dipolar magnetic interactions (Hill et al, 2012;Pavlopoulos et al, 2016).…”