Control of the anisotropic shape of cobalt nanorods in the liquid phase: from experiment to theory… and back

Abstract

The polyol process is one of the few methods allowing the preparation of metal nanoparticles in solution. Hexagonal close packed monocrystalline Co nanorods are easily obtained in basic 1,2-butanediol at 448 K after a few minutes using a Co(II) dicarboxylate precursor. By using a combined experimental and theoretical approach, this study aims at a better understanding of the growth of anisotropic cobalt ferromagnetic nanoparticles by the polyol process. The growth of Co nanorods along the c axis of the hexagonal system was clearly evidenced by transmission electron microscopy, while the mean diameter was found to be almost constant at about 15 nm. Powder X-ray diffraction data showed that metallic cobalt was generated at the expense of a non-reduced solid lamellar intermediate phase which can be considered as a carboxylate ligand reservoir. Density functional theory calculations combined with a thermodynamic approach unambiguously showed that the main parameter governing the shape of the objects is the chemical potential of the carboxylate ligand: the crystal habit was deeply modified from rods to platelets when increasing the concentration of the ligand, i.e. its chemical potential. The approach presented in this study could be extended to a large number of particle types and growth conditions, where ligands play a key role in determining the particle shape.

Controlled metal nanostructures: Fertile ground for coordination chemists

“…This suggests that, while the amine occupies (0001) ones, it doesn't significantly diminish their surface energy. Interestingly the aspect ratio of the rods increased with the length of the alkyl chain in agreement with the calculations made by Atmane et al [60].…”

“…2; section 2.1.2). Theoretical as well as experimental results from Atmane et al[60] further demonstrate that the shape of Co NPs can be tuned from platelets to rods by adjusting the chemical potential of the carboxylic acid (the ligand) in the reaction medium: coordination of carboxylic acid is more favorable onto {1010} facets but the packing density is much less on these facets than on (0001) ones. So at low ligand concentration, preferential adsorption starts at {1010} facets but the ligand coverage is too low to reverse the relative surface energies of the {1010} and (0001) facets.…”

“…For instance, G. Viau et al developed a "polyol" method using carboxylate precursors to obtain Co nanorods and nanodisks [60] or [Co x Ni 1-x ] (x=0.5-1) nanowires and nanorods [61]. M a n u s c r i p t 15 The key benefits result from the high reactivity of the M-N bond, absence of oxygen atoms (no risk of metal oxide formation) and facile thermal decomposition and reduction.…”

“…Results from theoretical calculations are thus of prime importance to get a better understanding of the systems [206,60,207]. Indeed, preferential adsorption of the ligand on some facets or selected topological sites has consequences on the final morphology and reactivity of the NPs (vide infra) and is of outmost importance for catalytic applications (Fig.…”

Co nanocrystals engineering

“…The size and shape of the nanoparticles depend on the Co/acid ratio and concentration of free acid in the medium as confirmed by theoretical calculations [39]. It is well known from literature results that in such conditions, the nanoparticles are stabilized by a double layer coating of oleic acid [40] as depicted in figure 4.…”