The effect of cation distribution and heat treatment temperature on the structural, surface, morphological and magnetic properties of MnxCo1−xFe2O4@SiO2 nanocomposites
Graphical Abstract
Introduction
In the field of science and technology, the nanomaterials offer unique, advantageous applications due to their different properties compared to the bulk materials [1], [2], [3], [4], [5], [6]. Due to their terrestrial abundance and low toxicity, Fe-based materials have been greatly utilized as catalysts in many organic reactions [7]. Nanoferrites and their composites have attracted considerable attention over the past few decades due to their unique and promising electrical, optical, and magnetic properties [6]. It is well-known that the properties of ferrites depend on various parameters that includes the particle size, ion substitution, synthesis route, heat treatment conditions, etc. [2], [8].
Cobalt ferrite (CoFe2O4) is a ferrimagnetic oxide with a cubic inverse spinel structure [9], [10], [11] and notable features such as large magnetocrystalline anisotropy [9], [10], [11], [12], [13], [14], highest magnetostriction coefficient [14], moderate saturation magnetization (MS) [9], [10], [13], high coercive field (HC) [9], [10], [11], [13], high Curie temperature [11], excellent chemical stability [11], [12], [13], high wear resistance [9], good electromagnetic properties [12], outstanding corrosion resistance [12], good electrical insulation [9], [13], significant mechanical hardness [10], [11], [12], [13], high resistivity [10] and easy processability [12]. CoFe2O4 received considerable interest in the last years, due to its numerous applications such as drug delivery [15], magnets [15], catalysts, electronics [15], microwave devices [14], [15], [16], high-density magnetic recording media [14], [16], data storage devices [15], spin electronic devices [16], gas sensors [16], magnetoelastic stress sensors [13] and high sensitivity sensor [14].
The structural, magnetic and electrical properties of the spinel type ferrites are strongly dependent on their preparation method, chemical composition, crystallinity, size, shape and size distribution of nanoparticles [9]. In this regard, the substitution of CoFe2O4 by various cations could modify/ enhance its properties [11], [12], [17]. Recently, the interest has also shifted towards Mn-substituted Co ferrites (MnxCo1−xFe2O4) nanoparticles due to enhanced and emergent properties than that of individually Mn and Co ferrites, making them good candidates for new, various applications [13], [15], [18]. The high surface area of magnetic nanoparticles causes the particle coarsening and aggregation [20]. This could be the result of the high-volume fraction of atoms located at the grain boundaries with special properties like surface anisotropy, surface spin canting and disorder, dislocations and superparamagnetic behavior [9].
Spinel compounds (AB2O4) have two kinds of oxygen polyhedra, i.e. tetrahedral (A corresponds to Co2+ ions and Mn2+ ions) - and octahedral (B corresponds to Fe3+ ion) sites and their magnetization originates from the difference in the magnetic moments of the cations distributed at sublattices A- and B-sites [8], [9], [14], [21], [22]. The cation distribution on different sites plays an important role in defining the behavior of spinel structure [22]. Crystallite size and cation distribution among tetrahedral (A) and octahedral (B) sites strongly affects the physical and chemical properties of ferrite nanoparticles [8], [22]. The spinel structure allows the introduction of various metallic ions, which can noticeably change its magnetic and electrical properties [19].
Commonly, two types of strain can be induced on the crystal lattice. The substitution with cations of different size compared to host atoms can induce strain, i.e. bigger size cations induce tensile strain, while smaller size cations induce compressive strain in the host lattice. Also, the lattice strain derives from the distribution of lattice constants originating from crystal imperfections. Beside the point defects, dislocation density, contact or sinter stress and stacking faults can induce strain [8]. The spin rotation is managed mainly by the crystalline field effect of the substituting (Mn2+) ion and if the magnetic field is strong enough, the spins are forced to dispose along the local anisotropy axis. The preference of Mn2+, Co2+ and Fe3+ ions for tetrahedral (A) - or octahedral (B) sites in the spinel lattice may result in a charge transfer between cations of different valences at equivalent crystallographic lattice points [14], [23]. The anisotropy (K) coefficient decreases with increasing Mn2+ substitution, as a result of the site occupancy by Mn2+ ions in the cubic spinel lattice [13]. Due to the possibility to optimize the Curie temperature and magnetostriction by adjusting the substitution levels, MnxCo1−xFe2O4 systems are promising candidates for magnetic stress sensors (e.g. toxic gases such as liquefied petroleum gas, SO2, NH3, NO2 and H2S) and non-contact torque sensing and embedded stress-sensing applications [9], [17], [19], [24]. Besides, MnFe2O4 nanoparticles are cheap, biocompatible, air-stable and magnetically recoverable, and can be used in applications such as hyperthermia, magnetic resonance imaging (MRI) contrasting agents and drug delivery [7]. Agglomeration is one of the key limitations in the synthesis of magnetic nanoparticles and can be decreased by their incorporation into a polymeric matrix or by using a surfactant to cover the nanoparticles surfaces and to prevent the agglomeration [15]. SiO2 is commonly used as surface coating material for Fe oxide nanoparticles as it avoids the particles agglomeration, improves the chemical stability and it is biocompatible, inert and non-toxic. In addition, the non-magnetic SiO2 matrix does not affect the magnetic behavior of the nanoparticles, nor their dielectric properties due to its lower dielectric constant [25]. Moreover, the SiO2 shell can act as a physical barrier to prevent the magnetic dipolar attraction between magnetic nanoparticles, resulting in improved optical properties of the nanoparticles. The significant effect of SiO2 coating on the band gap (Eg) values is due to the large band gap energy in bulk SiO2 (∼11 eV) [20].
The obtaining conditions (required heat-treatment temperature and time), morphology (crystal shape and size), as well as the cation distribution in A and B sites are the key parameters responsible for magnetic and electrical properties of CoFe2O4 nanoparticles. Incorporation of magnetic or non-magnetic metal ions such as Mn2+ leads to a consistent migration of cations between the above-mentioned lattice sites and, accordingly, different physical properties of the product [26]. Various routes were formerly used to obtain mixed Mn-Co ferrites, including sol-gel, hydrothermal, thermal decomposition, microemulsion and laser pyrolysis process [9], [10], [11], [12], [13], [14], [15]. Among these, the sol-gel route is an advantageous method due to its low cost, simplicity and good control over the structural, physical-chemical and magnetic properties [27], [28], [29]. Though, irregular shaped nanoparticles may form as a result of the gases release during the thermal decomposition of organic solvents that spread into nanopores due to the capillary forces. The modified sol-gel method involves the following steps: mixing of reactants with tetraethylorthosilicate (TEOS), gelation of the SiO2 network, formation of glyoxylate precursors and their decomposition into simple or mixed-oxide systems [27], [28]. The formation of an inactive coating of inert materials (i.e. SiO2) on the surfaces of iron oxide nanoparticles could avoid their aggregation and improves their chemical stability. Moreover, the SiO2 shell can act as a physical barrier to control the magnetic dipolar contact attraction between the magnetic nanoparticles [20], [30], [31]. In case of pure and mixed ferrites, the modified sol-gel method allows the obtaining of homogeneous nanoparticles and the incorporation of both organic and inorganic molecules. Alongside simplicity and effectiveness, it presents reduced time and energy and short gelation time. Besides, the variation of the synthesis parameters (i.e. pH, time, temperature) permits enhanced control over the nucleation and growth rates [27], [28], [29]. However, the main shortcomings remain the presence of amorphous phases at low annealing temperatures and secondary phases at high annealing temperatures.
The purpose of this study was to investigate the relationships between the structural (crystallite size), surface (specific surface area, porosity), morphological (particle size, roughness and height) and magnetic (remanent magnetization, saturation magnetization, coercive field, magnetic anisotropy) characteristics and the increasing Mn content in CoFe2O4 embedded in SiO2 matrix, when applying different heat-treating temperatures. A special attention is also paid to the Fe occupancy ratio between tetrahedral and octahedral sites of NCs and to the phase evolution, and their effect on the magnetic properties.
Section snippets
Experimental part
All chemical were of analytical grade (Merck) and were used without further purification. The MnxCo1−xFe2O4 @SiO2 NCs (50% wt. ferrite, 50% wt. SiO2) were prepared by sol-gel route using different Mn:Co:Fe molar ratios: 0:1:2 (x = 0.00), 0.25:0.75:2 (x = 0.25), 0.50:0.50:2 (x = 0.50), 0.75:0.25:2 (x = 0.75) and 1:0:2 (x = 1.00). The sols were prepared by mixing ferric nitrate nonahydrate (Fe(NO3)3∙9 H2O), cobalt nitrate hexahydrate (Co(NO3)2∙6 H2O) and manganese nitrate trihydrate (Mn(NO3)2∙3 H2
Results and discussion
The XRD patterns of MnxCo1−xFe2O4@SiO2 NCs (x = 0, 0.25, 0.50, 0.75, 1.00) heat-treated at 200, 500, 800 and 1200 °C are presented in Fig. 1. The observed diffraction peaks confirm the formation of mixed spinel ferrites with high crystallinity [5]. The relative intensities and the signal-to-noise ratio were the only parameters that visibly changed, indicating distinct levels of crystallinity or different crystallite sizes. In case of CoFe2O4@SiO2 (x = 0.00), for all heat treatment temperatures,
Conclusions
The influence of Mn2+ substitution in CoFe2O4 embedded in SiO2 matrix and heat treatment temperature on the structural, surface, morphological and magnetic characteristics of the MnxCo1−xFe2O4@SiO2 NCs was investigated. For all heat treatment temperatures, in case of NCs with high Co content, the single-phase CoFe2O4 was remarked. With increasing Mn content, beside the main MnFe2O4, mixed Mn-Co ferrite phases, α-Fe2O3 (for heat treatment at 800 °C) and supplementary phase SiO2 (for heat
CRediT authorship contribution statement
Thomas Dippong: Conceptualization, Investigation, Writing, Supervision, Reviewing and Editing. Mihaela Diana Lazar: Methodology, Formal analysis BET and Porosity, Investigation, Reviewing. Iosif Grigore Deac: Methodology, Formal analysis VSM, Writing, Reviewing. Petru Palade: Methodology, Formal analysis Mossbauer, Investigation. Ioan Petean: Methodology, Formal analysis AFM, Investigation. Gheorghe Borodi: Methodology, Formal analysis XRD, Investigation. Oana Cadar: Writing, Visualization,
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was supported by the Romanian Ministry of Research and Innovation through CCCDI-UEFISCDI, project number PN-III-P2-2.1-PED-2019-0844 within PNCDI III, Complex Projects of Frontier Research [PN-III-P4-ID-PCCF-2016-0112] and Core Program projects PN 030101 (contract number 21N/2019) and PN 19 35 02 02 (contract number 36N/2019) of the Romanian Ministry of Research and Innovation.
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