Size and shape-controlled synthesis and characterization of CoFe2O4 nanoparticles embedded in a PVA-SiO2 hybrid matrix
Introduction
Cobalt ferrite (CoFe2O4) is a ferromagnetic oxide with high magnetic anisotropy, high saturation magnetization, high resistivity and good chemical and thermal stability [1]. CoFe2O4 crystallizes into an inverse spinel structure, where one half of the Fe3+ ions occupy the tetrahedral sites, while the other half are located in the octahedral sites together with Co2+ ions [2], [3]. CoFe2O4 in powder or in dispersed fluid form, has received increasing attention due to its catalytic, electrical and magnetic properties that make it suitable for a wide range of applications (transformer cores, recording heads, antenna rods, loading coils, memory, microwave devices, catalysts, ferrofluids, magnetic refrigeration, magnetic sensors, drug delivery, gas detectors, sensors, solar energy conversion, stress and biomedical sensors, cellular therapy, tissue repair, etc.) [3], [4], [5], [6], [7], [8].
As the synthesis method influences the CoFe2O4 morphological and structural features, various synthesis methods, such as sol-gel, co-precipitation, microemulsion, hydrothermal, combustion, sono-chemical, solid-state, complexation, microwave sintering, mechanical alloying, spray pyrolysis, reverse micelle, forced hydrolysis in polyol, pulsed laser deposition and ultrasonic cavitation have been reported [2], [5], [6], [8], [9], [10], [11], [12], [13], [14]. Among them, combustion is a simple, low-cost, fast and high-yielding method leading to superior quality products, while co-precipitation and EDTA complexing routes enable crystallite size control [2], [8]. Microwave sintering allows rapid volumetric heating, high production rates, low energy consumption and low sintering temperature [6]. Although the ceramic method is often used for industrial-scale production, it requires high energy consumption [12]. The sol-gel method is a simple and efficient route to produce materials with high purity, controlled crystallinity and homogenous particles size [4], [5], [7], [10]. The nanoparticles are produced in-situ by decomposition of precursors, while the chelating gel is dried at relatively low temperatures [4].
To stabilize and reduce nanoparticle agglomeration, dispersion in a suitable matrix, such as resin, polymer or silica, can be used [15]. The addition of non-reactive species determines the growth of a solid oxide network around the nanoparticles, forming a doped gel with dispersed nanoparticles embedded in the network. The silica matrix provides a biocompatible, hydrophilic and non-toxic surface and may enhance the formation of single-phase spinel and the magnetic properties of nanocomposites [16], [17]. Polyvinyl alcohol (PVA) is a semi-crystalline, non-toxic, biocompatible and biodegradable polymer with good chemical resistance, excellent film-forming capacity, as well as emulsifying and adhesive properties [11]. It prevents particle agglomeration and influences the particle size, magnetic properties and crystallinity of CoFe2O4 [5], [10], [11]. Hybrid matrices combine the advantages of both inorganic (thermal stability and rigidity, processability, chemical inertness, low cost, biocompatibility with sol-gel transitions) and organic (elasticity, ductility) components [18], [19], [20]. The thermal and mechanical properties of SiO2 and the good flexibility of PVA make SiO2-PVA hybrids important candidates for applications in the fields of catalysis, coatings, adsorption, pre-evaporation, medicine and enzyme immobilization [18], [19], [20], [21], [22]. SiO2-PVA hybrids may embed materials in different forms such as monoliths, powders, tubes and fibers. The formation of the hybrid matrix consists in the embedding of PVA polymer in SiO2, followed by a thermal decomposition of the organic part [23].
Numerous studies present the obtaining and properties of silica/PVA hybrids, CoFe2O4/PVA and CoFe2O4/SiO2 nanocomposites [4], [6], [7], [9], [10], [11], [15], [16], [17], [20], [21], [22], [23], [24], [25], [26]. However, there is scarce data on CoFe2O4/SiO2-PVA nanocomposites. The iron(III) acetylacetonate/silica/PVA nanocomposites were synthetized and characterized by Ianasi et al. [23], while the formation of hybrid gels starting from tetraethyl orthosilicate (TEOS), PVA and 1,3-propanediol and the possibility of obtaining homogenously dispersed cobalt ferrite inside the silica matrix was studied by Stoia et al. [27].
In order to increase the crystallinity and size of nanoparticles, CoFe2O4/SiO2-PVA nanocomposites were synthesized by a three-step sol-gel method. The used synthesis route was adapted after Stoia et al. [27]. The synthesis method consist in: (i) hybrid (SiO2-PVA) matrix formation with embedding of reactants (Co and Fe nitrates, 1,4-butanediol (1,4-BD)), (ii) formation of succinate precursors and (iii) decomposition of succinate precursors with the formation of CoFe2O4 spinel. The advantage of this synthesis method is the decomposition of precursors at low temperatures, producing very reactive, low-crystalline oxide compounds that, by subsequent thermal treatments, leading to the formation of crystalline systems. The thermal behavior of CoFe2O4/SiO2-PVA nanocomposites and the influence of the hybrid matrix and of the CoFe2O4/matrix ratio on the CoFe2O4 nanoparticles size and shape are also discussed and compared to that of PVA or silica matrix.
Section snippets
Materials and methods
Analytical grade reagents (Merck, Germany) were used without further purification. The average molecular weight of PVA was 145.000 g/mol. CoFe2O4/SiO2-PVA nanocomposites were prepared by a sol-gel method. TEOS (Si(OC2H5)4) dissolved in water was added drop-wise under stirring into the mixture of nitrates ((Co(NO3)2·6H2O and Fe(NO3)3·9H2O). Afterwards, 1,4-BD (OHCH2CH2CH2CH2OH), 3% wt. PVA solution and ethanol (CH3CH2OH) were added drop-wise under continuous stirring, at 70 °C (Table 1). HNO3 was
Results and discussion
Fig. 1 shows the TG and DTA curves of gels dried at 40 °C and 200 °C. The DTA curve of SP10 gel at 40 °C displays four effects: (i) an endothermic effect (30–131 °C) with a maximum intensity at 93 °C (8% weight loss) attributed to residual water loss, (ii) an exothermic effect (131–169 °C) with a maximum at 153 °C (2% weight loss) corresponding to succinate precursors formation, (iii) an exothermic effect (169–348 °C) with a maximum at 289 °C (13% weight loss) corresponding to Co and Fe succinates
Conclusions
The obtained results indicate that the PVA-SiO2 hybrid matrix was generated and the formation of succinate precursors from Co and Fe nitrates and 1,4-BD occurs up to 200 °C. The thermal decomposition of succinates results in the embedding of the single-crystalline phase CoFe2O4 in the PVA-SiO2 hybrid matrix, at 400 °C. The exothermic effects of formation and decomposition of succinate precursors embedded in the PVA-SiO2 hybrid matrix are shifted to higher temperatures when compared to the
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