Formation mechanism of CoFe2O4 magnetic spherical mesoporous assemblies


Figure 1. TEM images of the sample prepared at 80 °C at different times.

Figure 2. SAXS patterns collected during the reaction at a) T = 80°C and at b) T = 50°C.The arrows indicate the q-range where we observe the growth of primary particles, N1, the aggregates N2, and the consumption of the intermediate lamellar phase, L. c) Schematic representation of the multistep hierarchical formation mechanism of CoFe2O4 mesoporous assemblies.
We have combined in-situ time-resolved SAXS, TEM and XRD techniques, to study the hierarchical mechanism of CoFe2O4 spherical mesoporous magnetic assemblies formation obtained with an eco-friendly, surfactant-assisted water-based precipitation approach. We found a lamellar (L) intermediate phase providing active sites for the formation of primary ferrite nanoparticles (N1), which in turn are seeds for the formation of secondary entities (N2) finally forming the stable spherical magnetic mesoporous assemblies.

Mesoporous magnetic particles with high magnetization and surface area values are of particular interest for biomedicine, bioseparation, catalysis and adsorption. Our study proves that the application of an in situ time-resolved SAXS-XRD-TEM approach can provide a powerful and exhaustive tool to clearly visualize the peculiar mechanism leading to the formation of hierarchical organized andstable mesoporous structures obtained with an eco-friendly, surfactant-assisted water-based precipitation approach. Furthermore, we demonstrate the important, multifold role played by the surfactant (sodium dodecylsulphate, SDS) during primary particle formation and their subsequent assembly, affecting crystal size, shape, assembly as well as mesoporous size and pathway.

To study the mechanism involved into the formation of the spherical assemblies, TEM and XRD analyses performed at different times of the reaction have been accompanied by in situ and time- resolved SAXS measurements. TEM and high resolution TEM (HRTEM) analyses on the sample prepared at 80 °C after different minutes of reaction (Fig. 1) show a great deal of hexagonal platelet crystals and at the same timeat their edges, and often at the corners, small assemblies of nanocrystals. The images show the evolution of the reaction; the number of the spherical assemblies increases and, at
 the same time, the hexagonal platelets are worn out gradually up to be almost completely consumed. XRD patterns allows to know the crystallographic phases: typical peaks ascribable to the CoFe2O4 phase coexist with peaks associable to metal hydroxides (β-Co(OH)2 or/and Fe(OH)2 ) and oxy-hydroxides phase (δ-FeOOH). The amount of CoFe2O4 with respect to hydroxides increases with reaction time and after 30 min the hydroxide-oxide transition is completed.

To understand the growth kinetics of the primary particles and their aggregation in mesoporous assemblies, in situ time-resolved SAXS measurements havebeen performed at two differenttemperatures (Fig. 2a and
2b). The evolution with time in SAXS patterns evidence the formation of three different
entities that correspond to the formation of primary CoFe2O4 nanocrystals (N1), secondary nanoparticles made up of three primary nanoparticles (N2)
 grown on the lamellar phase edges, and CoFe2O4 spherical assemblies at the expenses of the lamellar phase (L) that gradually disappears (Fig. 2c). This is in agreement with TEM and XRD measurements that clearly show that the CoFe2O4 forms and grows on the hexagonal platelets starting from their corners or edges (the most reactive zones). In particular, the formation of the spinel
phase probably derives from 
the reaction of iron(III) released from dissolution of δ-FeOOH
in strongly alkaline media with
 the β-Co(OH)2 plates, which, as
 a consequence, are gradually consumed. The surfactant has 
a key role on the creation of mesopores, as confirmed by an experiment carried out in the absence of SDS

Our results suggest that the formation mechanism can be kinetically controlled by the reaction temperature, able to modify the speediness of the reaction, affecting both the size distribution of primary particles and the size of their mesoporous assemblies. The study of the formation mechanism involved into stable spherical mesoporous assemblies made up of primary nanoparticles is a prerequisite that could render this soft chemistry route highly appealing to other magnetic ferrites (MeIIFe2O4, Me= Mn, Fe, Ni, Zn, Cu), a versatile class of materials that find applications in a wide variety of fields.

Retrieve article

Hierarchical Formation Mechanism of CoFe2O4 Mesoporous Assemblies;
C. Cannas, A. Ardu, A. Musinu, L. Suber, G. Ciasca, H. Amenitsch and G. Campi;
ACS Nano 9, 7 (2015). doi: 10.1021/acsnano.5b02145

Last Updated on Tuesday, 14 May 2019 17:05