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J. Richardi et al.
8.2.1.3 Cobalt Nanoparticle Crystalline Structure Control
In order to improve the crystalline structure (nanocrystallinity) of the as-synthesized
dodecanoic acid coated Co NPs (fcc polycrystals) obtained by the micellar approach,
annealing treatments by (1) dry- and (2) solution-phase protocols are performed.
(1) Dry annealing is performed on 7.2 nm-NPs deposited on a TEM grid. The grid
is placed in a close quartz ampule with a nitrogen atmosphere, finally placed
in a furnace at various temperatures (250, 300 and 350 °C) for 15 min [23].
Electron diffraction and HRTEM (Fig. 8.4a, 4-1) reveal that the as-synthesized
Co NPs are nearly amorphous with few ordered fcc domains less than 1 nm in
size. After annealing at 250 °C, the structure is mainly hcp with some remaining
fcc (Fig. 8.4b, 4-2). After annealing at 300 °C, pure hcp-Co NPs are obtained
(Figs. 8.4c, 4-3), the defects of which tend to disappear at 350 °C (Figs. 8.4d,
4-4). Within error, the average diameter of the annealed NPs is unchanged.
(2) Recrystallization of fcc-Co into hcp-Co can be also driven by the solution-phase
heating protocol. Using this novel strategy and conversely to the previous one,
annealed NPs can be freely manipulated. Different protocols exist, always taking
place in a refluxing bath, under nitrogen flux [2]. For instance, as-synthesized
Co NPs are dispersed in high boiling point solvent, i.e., octyl ether. The solution
is heated at 220 °C with a heating rate of 10 °C per minute up to 140 °C then
more slowly with a heating rate of 2 °C per minute. Once the temperature is
reached, NPs are maintained in the hot solution without aging or aged for 30,
90 and 180 min before a cooling step. Electron diffraction and HRTEM studies
indicate the disappearance of the cfc phase just after reaching 220 °C, in favor
of hcp-Co. After aging for 180 min, hcp-Co single crystals are obtained with a
slight decrease in the mean diameter (6.5 nm against 7.2 nm for the native NPs).
Due to the high thermal stability of the dodecanoic acid coating, we show that
dry- and solution-phase annealing treatments give rise to the structural transition
from fcc-Co polycrystals to hcp-Co single crystals. No trace of Co oxide is detected.
It is noticeable that reports in the literature of colloidal hcp-Co single crystals are
very rare [14, 18].
8.3 Synthesis by Organometallic Approach
Besides the micellar soft chemical approach developed above, polyol synthesis of
NPs appears to be one of the “simplest” routes to prepare NPs involving reduction of
inorganic salt at high temperature (depending on the polyol) [39–41]. This is also the
most representative approach of a complicated mixture synthesis of NPs as variations
on the polyols, cobalt salt, surfactant, even bi-surfactant mixture, ruthenium seeds
additive allow to reach different shapes, sizes and phases (mainly fcc-Co and hcpCo) of nanocrystals [42–44]. As mentioned above, two other processes have been
developed to produce monodispersed Co NPs in milder conditions using Co(0) or (I)
J. Richardi et al.
8.2.1.3 Cobalt Nanoparticle Crystalline Structure Control
In order to improve the crystalline structure (nanocrystallinity) of the as-synthesized
dodecanoic acid coated Co NPs (fcc polycrystals) obtained by the micellar approach,
annealing treatments by (1) dry- and (2) solution-phase protocols are performed.
(1) Dry annealing is performed on 7.2 nm-NPs deposited on a TEM grid. The grid
is placed in a close quartz ampule with a nitrogen atmosphere, finally placed
in a furnace at various temperatures (250, 300 and 350 °C) for 15 min [23].
Electron diffraction and HRTEM (Fig. 8.4a, 4-1) reveal that the as-synthesized
Co NPs are nearly amorphous with few ordered fcc domains less than 1 nm in
size. After annealing at 250 °C, the structure is mainly hcp with some remaining
fcc (Fig. 8.4b, 4-2). After annealing at 300 °C, pure hcp-Co NPs are obtained
(Figs. 8.4c, 4-3), the defects of which tend to disappear at 350 °C (Figs. 8.4d,
4-4). Within error, the average diameter of the annealed NPs is unchanged.
(2) Recrystallization of fcc-Co into hcp-Co can be also driven by the solution-phase
heating protocol. Using this novel strategy and conversely to the previous one,
annealed NPs can be freely manipulated. Different protocols exist, always taking
place in a refluxing bath, under nitrogen flux [2]. For instance, as-synthesized
Co NPs are dispersed in high boiling point solvent, i.e., octyl ether. The solution
is heated at 220 °C with a heating rate of 10 °C per minute up to 140 °C then
more slowly with a heating rate of 2 °C per minute. Once the temperature is
reached, NPs are maintained in the hot solution without aging or aged for 30,
90 and 180 min before a cooling step. Electron diffraction and HRTEM studies
indicate the disappearance of the cfc phase just after reaching 220 °C, in favor
of hcp-Co. After aging for 180 min, hcp-Co single crystals are obtained with a
slight decrease in the mean diameter (6.5 nm against 7.2 nm for the native NPs).
Due to the high thermal stability of the dodecanoic acid coating, we show that
dry- and solution-phase annealing treatments give rise to the structural transition
from fcc-Co polycrystals to hcp-Co single crystals. No trace of Co oxide is detected.
It is noticeable that reports in the literature of colloidal hcp-Co single crystals are
very rare [14, 18].
8.3 Synthesis by Organometallic Approach
Besides the micellar soft chemical approach developed above, polyol synthesis of
NPs appears to be one of the “simplest” routes to prepare NPs involving reduction of
inorganic salt at high temperature (depending on the polyol) [39–41]. This is also the
most representative approach of a complicated mixture synthesis of NPs as variations
on the polyols, cobalt salt, surfactant, even bi-surfactant mixture, ruthenium seeds
additive allow to reach different shapes, sizes and phases (mainly fcc-Co and hcpCo) of nanocrystals [42–44]. As mentioned above, two other processes have been
developed to produce monodispersed Co NPs in milder conditions using Co(0) or (I)