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J. Richardi et al.
[26], magnetite [28] and Fe 3 O 4 [29] NPs. Even more rarely, only some colloidal
crystals made of MNPs have been, at this day, reported in the literature, which can
be attributed to the difficulty to meet all the conditions to growth them. They have
been obtained using Ni [26], γ-Fe 3 O 4 -Fe 2 O 3 [30] and Fe 3 O 4 [29].
Physico-chemical properties, especially magnetic ones of assemblies of NPs, are
known to be significantly impacted by a set of parameters including (1) MNP size,
(2) nanocrystallity, (3) interparticle gap, (4) degree and type of ordering of the superlattice, (5) morphology. In this chapter, we cover the results obtained in our group
on (1) the colloidal synthesis of uniform and spherical cobalt NPs, (2) their use
for the elaboration of 2D and 3D assemblies by means of spontaneous and assisted
assembling and (3) their magnetic properties. The key factors for the synthesis of
uniform Co NPs and their assemblies will be discussed. We will also present theories
and simulations, which predict the size and shape of mesostructures obtained from
evaporation of MNP solutions under a magnetic field.
8.2 Synthesis of Uniform Spherical Co Nanoparticles
8.2.1 Synthesis by Micellar Approach
8.2.1.1 Cobalt Nanoparticle Size Polydispersity Control
Co NPs are synthesized by solution-phase reduction approach, at room temperature [20]. The precursor used is the cobalt (II) bis(2-ethylhexyl) sulfosuccinate,
Co(AOT) 2 . 5 × 10
–2 M Co(AOT) 2 solution is prepared using isooctane as bulk
solvent [31]. The amount of water concentration defined as w = [H 2 O]/[AOT] = 32.
This prepared microemulsion is vigorously shaken for a few minutes for equilibrium
and form reverse micelles [32]. The reducing agent used is sodium tetrahydroborate,
and its concentration is given by R (R = [NaBH 4 ]/[Co(AOT) 2 ]). R value varies from
0.5 to 8 by varying the volume of a fixed concentration ([NaBH 4 ] = 1 M) added to the
micellar solution. Under vigorous shaking, the solutions instantaneously turn from
pink to black indicating the formation of Co NPs. Above R = 0.5, resulting from
the large amount of water brought by the reducing agent injected, reverse micelles
are instantaneously destroyed [32, 33]. After the synthesis, whatever the R value is,
Co NPs are extracted from the AOT surfactant by adding highly concentrated dodecanoic acid solution ([C 12 H 25 COOH] = 0.2 M). After washing several times with
ethanol, the NPs are dispersed in hexane and then centrifuged to precipitate bulky
material. Only the upper phase containing dodecanoic acid coated NPs is collected.
All the steps are carried out in a nitrogen glove box using deoxygenated solvents
to prevent metal oxidation. Transmission electron microscopy (TEM) investigation
is performed after depositing some drops of the colloidal solutions onto a carbon
TEM grid and complete evaporation of the solvent. As can be observed in TEM
images (Fig. 8.1), the populations of NPs significantly depend on R value. At R =
J. Richardi et al.
[26], magnetite [28] and Fe 3 O 4 [29] NPs. Even more rarely, only some colloidal
crystals made of MNPs have been, at this day, reported in the literature, which can
be attributed to the difficulty to meet all the conditions to growth them. They have
been obtained using Ni [26], γ-Fe 3 O 4 -Fe 2 O 3 [30] and Fe 3 O 4 [29].
Physico-chemical properties, especially magnetic ones of assemblies of NPs, are
known to be significantly impacted by a set of parameters including (1) MNP size,
(2) nanocrystallity, (3) interparticle gap, (4) degree and type of ordering of the superlattice, (5) morphology. In this chapter, we cover the results obtained in our group
on (1) the colloidal synthesis of uniform and spherical cobalt NPs, (2) their use
for the elaboration of 2D and 3D assemblies by means of spontaneous and assisted
assembling and (3) their magnetic properties. The key factors for the synthesis of
uniform Co NPs and their assemblies will be discussed. We will also present theories
and simulations, which predict the size and shape of mesostructures obtained from
evaporation of MNP solutions under a magnetic field.
8.2 Synthesis of Uniform Spherical Co Nanoparticles
8.2.1 Synthesis by Micellar Approach
8.2.1.1 Cobalt Nanoparticle Size Polydispersity Control
Co NPs are synthesized by solution-phase reduction approach, at room temperature [20]. The precursor used is the cobalt (II) bis(2-ethylhexyl) sulfosuccinate,
Co(AOT) 2 . 5 × 10
–2 M Co(AOT) 2 solution is prepared using isooctane as bulk
solvent [31]. The amount of water concentration defined as w = [H 2 O]/[AOT] = 32.
This prepared microemulsion is vigorously shaken for a few minutes for equilibrium
and form reverse micelles [32]. The reducing agent used is sodium tetrahydroborate,
and its concentration is given by R (R = [NaBH 4 ]/[Co(AOT) 2 ]). R value varies from
0.5 to 8 by varying the volume of a fixed concentration ([NaBH 4 ] = 1 M) added to the
micellar solution. Under vigorous shaking, the solutions instantaneously turn from
pink to black indicating the formation of Co NPs. Above R = 0.5, resulting from
the large amount of water brought by the reducing agent injected, reverse micelles
are instantaneously destroyed [32, 33]. After the synthesis, whatever the R value is,
Co NPs are extracted from the AOT surfactant by adding highly concentrated dodecanoic acid solution ([C 12 H 25 COOH] = 0.2 M). After washing several times with
ethanol, the NPs are dispersed in hexane and then centrifuged to precipitate bulky
material. Only the upper phase containing dodecanoic acid coated NPs is collected.
All the steps are carried out in a nitrogen glove box using deoxygenated solvents
to prevent metal oxidation. Transmission electron microscopy (TEM) investigation
is performed after depositing some drops of the colloidal solutions onto a carbon
TEM grid and complete evaporation of the solvent. As can be observed in TEM
images (Fig. 8.1), the populations of NPs significantly depend on R value. At R =