ARTICLE pubs.acs.org/Langmuir
How Can the Nanocrystallinity of 7 nm Spherical Co Nanoparticles Dispersed in Solution Be Improved? Manon Cavalier,† Michael Walls,‡ Isabelle Lisiecki,*,† and Marie-Paule Pileni*,† †
Laboratoire des Materiaux Mesoscopiques et Nanometriques, UMR CNRS 7070, Universite Pierre et Marie Curie, B^at. F, 4 Place Jussieu, 75005 Paris, France ‡ Laboratoire de Physique des Solides, UMR 8502, Universite Paris-Sud, B^at. 510, 91405 Orsay Cedex, France ABSTRACT: We report a solution-phase annealing of spherical Co nanocrystals synthesized in reverse micelles and coated with dodecanoic acid. The deposition of a drop of solution on a transmission electron microscope grid shows that a progressive increase in the temperature to 316 °C results in the progressive crystallographic transition from a polycrystalline and probably face-centered cubic Co phase to the single-crystalline hexagonal close-packed (hcp) Co phase. These nanocrystals are highly stable against oxidation and coalescence. We stress that, to our knowledge, this constitutes the first example in the literature of pure hcp-Co spherical single crystals dispersed in solution. These nanocrystals can be freely manipulated and, due to their low size dispersion, can self-organize on various substrates.
1. INTRODUCTION A great expansion in research on inorganic nanocrystals has recently taken place, not only for their fundamental interest but also for their applications in fields such as biology, electronics, transport, and information technology.114 At normal pressure, bulk cobalt exhibits two stable phases: hexagonal close-packed (hcp) Co, which is stable below 425 °C, and face-centered cubic (fcc) Co at higher temperatures. Because of its high anisotropy, hcp-Co could be a good candidate for applications in recording magnetic technology.1517 This explains why a rather large number of groups have succeeded in producing hcp nanocrystals. However, most of the data published in this domain concern the formation of nanocrystals characterized by anisotropic shapes such as disks18 or rods.19,20 Many syntheses of spherical Co nanocrystals gave rise to particles dispersed in a solvent and characterized by three different crystal structures: ε,2123 fcc, and hcp. By thermal decomposition of Co2(CO)8,2426 or by reducing a cobalt salt at high temperature,27 spherical Co nanocrystals in the ε phase have been produced. Multiply twinned fcc nanocrystals have been obtained by thermally decomposing cobalt octacarbonyl.28 The ε and fcc phases require annealing at relatively low temperatures,
