140
H. Khurshid et al.
are produced and the hollow nanostructures are obtained by oxidizing them [45].
Kirkendall effect was first reported by Kirkendall and Smigelkas in [46] and is based
on the different diffusion rates of core and shell materials. This will be analyzed in
more detail later in the book chapter.
It is to be noted that in these core/shell MNPs, the core is metallic while the
shell is an oxidized form of the core material, being the end product a metal-oxide
core/shell MNP [47]. These core/shell structures can be initially synthesized either by
physical or chemical routes. For example, core/shell structured (Ni 33 Fe 67 )/(NiFe 2 O 4 )
nanoparticles synthesized by the inert gas condensation method were used as the
seeds to obtain NiFe 2 O 4 hollow MNPs [20]. To obtain the hollow morphology, the
core/shell particles were annealed above 350◦52C. Later CoFe 2 O 4 nanoparticles
were obtained by following similar physical synthesis routes, that is by annealing
Co 33 Fe 67 /CoFe 2 O 4 (core/shell) nanoparticles [48].
In order to obtain iron-oxide-based hollow MNPs, thermal decomposition
synthesis routes have been frequently employed. A typical setup for thermal decomposition is presented in Fig. 6.2a. During a thermal decomposition process, core/shell
nanoparticles are synthesized by thermally decomposing organometallic compounds
at high temperature in the presence of organic solvents and hydrophobic surfactants,
oleic acid (OA), oleyl amine (OY), trioctyle phosphine, octanoic acid, etc. [49]. These
surfactants play a very important role in stabilizing as well as in obtaining monodisperse and controlled particle size of core/shell nanoparticles that will eventually
be transformed into hollow. Thermal decomposition of Fe(CO) 5 in the presence of
organic surfactants is a common synthesis technique used to obtain to obtain ironbased core/shell MNPs [50]. The resultant core/shell MNP is usually composed of
iron and iron oxide (either maghemite or magnetite) [51]. During a second step, the
reaction product is heated again under flow of oxygen, aiding the Kirkendall effect
and hence leading to a hollow morphology.
As an illustrative example, in Fig. 6.2b, we present the synthesis route for hollow
γ-Fe 2 O 3 (maghemite) nanoparticles [16]. Briefly, a three-necked flask was charged
with oleylamine, 70%, and 1-octadecene, 90%, and the mixture was stirred at 140 °C
under a mixture of 95% Ar + 5% H 2 gases for two hours to make sure that there
was no trace of moisture or air in the flask. The temperature was raised subsequently
to 220 °C where iron pentacarbonyl, Fe (CO 2 ) 5 , was injected and left to reflux
for 20 min. After injection, the iron pentacarbonyl immediately decomposed into
iron fragments, which are the onset for nanoparticle formation (black precipitate).
Acetone and/or CO gas formed in the reaction vessel (white smoke) and the reaction
temperature raised a few degrees because of its exothermic nature. The injection
temperature is important to have a narrow size distribution. Following reflux, the
sample was cooled down to room temperature. The average particle size of the
core/shell nanoparticles can be controlled by varying the injection temperature and/or
amount of oleylamine. The obtained MNPs consist of a Fe core and a γ-Fe 2 O 3 shell.
To create hollow γ-Fe 2 O 3 MNPs, the core/shell sample was annealed at 180 °C for
one hour under a flow of oxygen. Both core/shell and hollow nanoparticles were
washed with a mixture of 3 ml hexane, 95%, and 97 ml ethanol, ≥99.5%.
H. Khurshid et al.
are produced and the hollow nanostructures are obtained by oxidizing them [45].
Kirkendall effect was first reported by Kirkendall and Smigelkas in [46] and is based
on the different diffusion rates of core and shell materials. This will be analyzed in
more detail later in the book chapter.
It is to be noted that in these core/shell MNPs, the core is metallic while the
shell is an oxidized form of the core material, being the end product a metal-oxide
core/shell MNP [47]. These core/shell structures can be initially synthesized either by
physical or chemical routes. For example, core/shell structured (Ni 33 Fe 67 )/(NiFe 2 O 4 )
nanoparticles synthesized by the inert gas condensation method were used as the
seeds to obtain NiFe 2 O 4 hollow MNPs [20]. To obtain the hollow morphology, the
core/shell particles were annealed above 350◦52C. Later CoFe 2 O 4 nanoparticles
were obtained by following similar physical synthesis routes, that is by annealing
Co 33 Fe 67 /CoFe 2 O 4 (core/shell) nanoparticles [48].
In order to obtain iron-oxide-based hollow MNPs, thermal decomposition
synthesis routes have been frequently employed. A typical setup for thermal decomposition is presented in Fig. 6.2a. During a thermal decomposition process, core/shell
nanoparticles are synthesized by thermally decomposing organometallic compounds
at high temperature in the presence of organic solvents and hydrophobic surfactants,
oleic acid (OA), oleyl amine (OY), trioctyle phosphine, octanoic acid, etc. [49]. These
surfactants play a very important role in stabilizing as well as in obtaining monodisperse and controlled particle size of core/shell nanoparticles that will eventually
be transformed into hollow. Thermal decomposition of Fe(CO) 5 in the presence of
organic surfactants is a common synthesis technique used to obtain to obtain ironbased core/shell MNPs [50]. The resultant core/shell MNP is usually composed of
iron and iron oxide (either maghemite or magnetite) [51]. During a second step, the
reaction product is heated again under flow of oxygen, aiding the Kirkendall effect
and hence leading to a hollow morphology.
As an illustrative example, in Fig. 6.2b, we present the synthesis route for hollow
γ-Fe 2 O 3 (maghemite) nanoparticles [16]. Briefly, a three-necked flask was charged
with oleylamine, 70%, and 1-octadecene, 90%, and the mixture was stirred at 140 °C
under a mixture of 95% Ar + 5% H 2 gases for two hours to make sure that there
was no trace of moisture or air in the flask. The temperature was raised subsequently
to 220 °C where iron pentacarbonyl, Fe (CO 2 ) 5 , was injected and left to reflux
for 20 min. After injection, the iron pentacarbonyl immediately decomposed into
iron fragments, which are the onset for nanoparticle formation (black precipitate).
Acetone and/or CO gas formed in the reaction vessel (white smoke) and the reaction
temperature raised a few degrees because of its exothermic nature. The injection
temperature is important to have a narrow size distribution. Following reflux, the
sample was cooled down to room temperature. The average particle size of the
core/shell nanoparticles can be controlled by varying the injection temperature and/or
amount of oleylamine. The obtained MNPs consist of a Fe core and a γ-Fe 2 O 3 shell.
To create hollow γ-Fe 2 O 3 MNPs, the core/shell sample was annealed at 180 °C for
one hour under a flow of oxygen. Both core/shell and hollow nanoparticles were
washed with a mixture of 3 ml hexane, 95%, and 97 ml ethanol, ≥99.5%.