Experimental evidence of structural evolution in ultrafine cobalt particles stabilized in different polymers—From a polytetrahedral arrangement to the hexagonal structure

Abstract

Ultrafine cobalt particles have been reproducibly synthesized by decomposition of an organometallic precursor in the presence of a stabilizing polymer. The size of the stable monodisperse colloids thus obtained is seen to strongly depend on the nature of the polymer: around 4.2 nm diameter in polyphenylenoxide ͑PPO͒ and around 1.4 nm diameter in polyvinylpyrrolidone ͑PVP͒. Investigations by wide angle x-ray scattering ͑WAXS͒ and high-resolution transmission electron microscopy ͑HRTEM͒ give evidence for a size dependence of the structural organization, and hence for a close relationship between structure and synthesis conditions. Co/PPO particles exhibit a hexagonal compact structure with the metal-metal bond length of the bulk material while Co/PVP ones display an original structure. We show that the unusual features of the experimental data in Co/PVP clearly point to a nonperiodic polytetrahedral structure. Successful simulations of the HRTEM and WAXS results have been obtained using models built on the basis of a polytetrahedral arrangement.

Structural study of bimetallicCoxRh1−xnanoparticles: Size and composition effects

“…The structure of pure cobalt particles with same size and synthesized by the same chemical route has already been proposed as a well-defined polytetrahedral arrangement. 5 Alloying with rhodium, as alloying with platinum that we studied in a previous work, 9 seems to introduce structural disorder in the Co-rich particles, at least at the surface, which impedes their unambiguous structural determination.…”

“…3 In this previous paper, we reported the synthesis and magnetic study of bimetallic Co 0.5 Rh 0.5 particles with diameters lying in the 1 -3 nm range. The synthesis process, already successfully followed for cobalt clusters, 4,5 was based on the decomposition of organometallic precursors in mild conditions of pressure and temperature in the presence of a polymer, the polyvinylpyrrolidone (PVP). Both the magnetic and the structural results obtained on cobalt particles demonstrated that they behave as free particles, with very weak or none interaction with the PVP polymer.…”

Organometallic Nanoparticles of Metals or Metal Oxides

“…When using PVP as a stabilizer, nanoparticles of 1.6 or 2.0 nm mean size were obtained as a function of the precursor concentration whereas with PPO larger particles (4 nm) form [59]. The particles of small size (1.6 or 2 nm) adopt a non-periodic polytetrahedral structure whereas the larger ones (4 nm) adopt the hcp structure of bulk cobalt [60]. Interestingly, and somewhat surprisingly, these particles were demonstrated to display the same magnetic properties as particles of same size prepared in ultra high vacuum in the gas phase by time-of-flight experiments [61].…”

“…These species are well defined and relatively robust [39]. It may therefore be interesting to emphasize their similarity with the molecular cluster Pd 145 (CO) 60 (PEt 3 ) 30 recently characterized by Dahl. The size of the metal core of this cluster is estimated to be of the same order of magnitude as these platinum phosphine colloids and it is noteworthy that this Pd 145 cluster adopts a non-periodic structure [40].…”

Analysis and simulation of the structure of nanoparticles that undergo a surface-driven structural transformation

“…Fourier analysis of the WAXS data provides the real-space pair distribution function (PDF). PDF analysis is not yet widely used in nanoscale systems, yet provides a view of longer-range correlations [17,18]. Applied to powder samples, both these experimental methods yield average structural information that do not precisely reveal the specific modifications that arise due to small particle size.…”