Mesoporous Cobalt Ferrite Nanosystems Obtained by Surfactant-Assisted Hydrothermal Method: Tuning Morpho-structural and Magnetic Properties via pH-Variation

Abstract

A facile and cheap surfactant-assisted hydrothermal method was used to prepare mesoporous cobalt ferrite nanosystems with BET surface area up to 151 m 2 /g. These mesostructures with high BET surface areas and pore sizes are made from assemblies of nanoparticles (NPs) with average sizes between 7.8 and 9.6 nm depending on the initial pH conditions. The pH proved to be the key factor for controlling not only NP size, but also the phase purity and the porosity properties of the mesostructures. At pH values lower than 7, a parasite hematite phase begins to form. The sample obtained at pH = 7.3 has magnetization at saturation M s = 38 emu/g at 300 K (54.3 emu/g at 10 K) and BET surface area S BET = 151 m 2 /g, whereas the one obtained at pH = 8.3 has M s = 68 emu/g at 300 K (83.6 emu/g at 10 K) and S BET = 101 m 2 /g. The magnetic coercive field values at 10 K are high at up to 12,780 Oe, with a maximum coercive field reached for the sample obtained at pH = 8.3. Decreased magnetic performances are obtained at pH values higher than 9. The iron occupancies of the tetrahedral and octahedral sites belonging to the cobalt ferrite spinel structure were extracted through decomposition of the Mössbauer patterns in spectral components. The magnetic anisotropy constants of the investigated NPs were estimated from the temperature dependence of the hyperfine magnetic field. Taking into consideration the high values of BET surface area and the magnetic anisotropy constants as well as the significant magnetizations for saturation at ambient temperature, and the fact that all parameters can be adjusted through the initial pH conditions, these materials are very promising as recyclable anti-polluting agents, magnetically separable catalysts, and targeted drug delivery vehicles.

Structural, morphological and magnetic investigations on cobalt ferrite nanoparticles obtained through green synthesis routes

“…The highest value of saturation magnetization for this type of material was obtained on bulk (80.8 emu/g at room temperature and 93.9 emu/g at 5 K) [52]. The decrease of 13.9 emu/g of the saturation magnetization at room temperature compared to the one recorded at 5 K in bulk materials was attributed to the variation of the electronic populations on CoFe 2 O 4 orbitals with temperature [19]. For our samples, CF and TCF, the difference between M s recorded at 300 K and that at 5 K is smaller than in bulk, 5.08 and 6.83 emu/g, respectively.…”

“…For our samples, CF and TCF, the difference between M s recorded at 300 K and that at 5 K is smaller than in bulk, 5.08 and 6.83 emu/g, respectively. This smaller decrease of M s for nanoparticles compared with the bulk could be explained by the thermal fluctuations of the magnetic moment in a single energy minimum when temperature increase [19]. At low temperature, when spin orientation thermal fluctuations are negligible, the difference between the M s of our samples and that of bulk materials can be explained by the presence of an outer layer belonging to the nanoparticles surface which has an insignificant contribution to M s [19].…”

“…This smaller decrease of M s for nanoparticles compared with the bulk could be explained by the thermal fluctuations of the magnetic moment in a single energy minimum when temperature increase [19]. At low temperature, when spin orientation thermal fluctuations are negligible, the difference between the M s of our samples and that of bulk materials can be explained by the presence of an outer layer belonging to the nanoparticles surface which has an insignificant contribution to M s [19]. A similar trend was also found in the values of the remanent magnetizations and coercive magnetic fields, respectively.…”

“…Several factors such as particle size, shape and porosity that are closely related to the synthesis method, have significant impact on the cobalt ferrite properties [12]. Among the chemical methods, special attention is given to coprecipitation and Pechini methods [13,14], sol-gel process [15,16], hydrothermal/solvothermal technique [1,[17][18][19], precursor method [20], combustion method and microwave-assisted combustion method [21][22][23][24], microemulsion method [7] and polyol process [25]. Das et al [17] reported a significant modification in the size of the particles and magnetic properties when adding the CTAB surfactant in the synthesis of CoFe 2 O 4 nanoparticles through the hydrothermal method.…”

Silica supported spinel structured cobalt ferrite multifunctional nanocatalyst for hydration of nitriles and oxidative decarboxylation of phenylacetic acids

“…However, many of these methods used toxic organic reagents such as ethylene glycol, [ 18 ] polyethylene glycol, [ 19 ] and so forth, organic acids as capping agent (citric acid, [ 20 ] sulfonic acid, [ 21 ] nicotinic acid, [ 22 ] lauric acid, [ 23 ] etc. ), large amount of surfactants, [ 24 ] high temperature, [ 25 ] and so forth, that is why it suffered from drawbacks. We have chosen the cheap and simple coprecipitation route without using any expensive equipment and led to desired product.…”