(397j) Molten Droplet Synthesis of CdSe Hollow Nanoparticles

Many colloidal synthesis routes are not scalable to high production rates, especially for nanoparticles of complex shape or composition, due to precursor expense and hazards, low yields, and the large number of processing steps. The recognition that optical, electronic, catalytic, and other material properties of nanoparticles can be manipulated by shape, in addition to size, drives the current interest in devising new synthesis methods to non-spherical nanoparticles. A new strategy has been devised to readily synthesize hollow nanoparticles (HNPs) out of metal chalcogenides, based on the slow heating of a low-melting-point metal salt, an elemental chalcogen, and an alkylammonium surfactant in octadecene solvent. Colloidally stable and monodisperse CdSe HNPs with an outer diameter of 15.6±3.5 nm and a shell thickness of 5.4±0.9 nm are characterized extensively using UV-visible spectroscopy, high resolution TEM, X-Ray diffraction, dynamic light scattering and electron energy loss spectroscopy. The HNP synthesis is proposed to proceed with the formation of alkylammonium-stabilized nano-sized droplets of molten cadmium salt, which then come into contact with dissolved selenium species to form a CdSe shell at the droplet surface. In a reaction–diffusion mechanism similar to the nanoscale Kirkendall effect it is speculated that the cadmium migrates outwardly through this shell to react with more selenium, causing the CdSe shell to thicken. The proposed CdSe HNP structure comprises a polycrystalline CdSe shell coated with a thin layer of amorphous selenium. Photovoltaic device characterization indicates that HNPs have improved electron transport characteristics compared to standard CdSe quantum dots, possibly due to this selenium layer. The HNPs are colloidally stable in organic solvents even though carboxylate, phosphine, and amine ligands are absent; stability is attributed to octadecene-selenide species bound to the particle surface. This scalable synthesis method presents opportunities to generate hollow nanoparticles with increased structural and compositional variety.