Abstract

1. Introduction

Core-shells are a relatively new family of particles. All micro- or nanoparticle constituted of either an internal particle (core) or a set of internal particles (multicore), and a simple or multiple coating material (shell) of different nature, is called a core-shell structure. A wide range of different structural combinations such as inorganic/inorganic, inorganic/organic, organic/inorganic, and organic/organic materials can be found in core-shell systems¹1 Chaudhuri R, Paria S. Core/Shell Nanoparticles: Classes, Properties, Synthesis Mechanisms, Characterization, and Applications. Chemical Reviews. 2012;112(4):2373-2433.. Core-shell structures often possess superb chemical and physical properties compared to their single-component counterparts²2 Lu W, Guo X, Luo Y, Li Q, Zhu R, Pang H. Core-shell materials for advanced batteries. Chemical Engineering Journal. 2019;355:208-237.. Coating the particles with a thin shell of a compatible material makes it possible to control the inherent properties of powders, and introduces others desirable (physical and chemical) characteristics according to the nature of coating material, thereby improving their functional properties and expanding a broader range of potential application³3 Liu G, Hong G, Sun G. Synthesis and characterization of SiO2/Gd2O3:Eu core-shell luminescent materials. Journal of Colloid and Interface Science. 2004;278(1):133-138.^,44 Li W, Elzatahry A, Aldhayan D, Zhao D. Core-shell structured titanium dioxide nanomaterials for solar energy utilization. Chemical Society Reviews. 2018;47(22):8203-8237.. Core-shell structures are highly functional materials with modifiable surface properties, in some cases decreasing the sintering, increasing the surface area, reactivity and thermal stability, regarding single constituent materials⁵5 Zhang Q, Lee I, Joo JB, Zaera F, Yin Y. Core-Shell Nanostructured Catalysts. Accounts of Chemical Research. 2013;46(8):1816-1824.^,66 Zhang N, Liu S, Xu YJ. Recent progress on metal core@semiconductor shell nanocomposites as a promising type of photocatalyst. Nanoscale. 2012;4(7):2227-2238.. Particularly, the applications of inorganic core-shell particles are found in many fields including biology, chemistry, physics and engineering⁷7 Mélinon P, Begin-Colin S, Duvail JC, Gauffre F, Boime NH, Ledoux G, et al. Engineered inorganic core/shell nanoparticles. Physics Reports. 2014;543(3):163-197., because of their multiple functions such as electrical, optical, magnetic, catalytic and electrocatalytic, among others²2 Lu W, Guo X, Luo Y, Li Q, Zhu R, Pang H. Core-shell materials for advanced batteries. Chemical Engineering Journal. 2019;355:208-237.^,44 Li W, Elzatahry A, Aldhayan D, Zhao D. Core-shell structured titanium dioxide nanomaterials for solar energy utilization. Chemical Society Reviews. 2018;47(22):8203-8237.^,88 Yang S, Tan Y, Yin X, Wang L, Wang H, Zhou Y. A facile and green approach to prepare monodispersion nanonickel nanofluids. Particulate Science and Technology. 2018;36(2):141-145.^,99 Han T, Peng L, Cao S, Zhu D, Tu M, Zhang J. Preparation of a Core-Shell Structured Al2O3/YAG:Ce Phosphor by a Spray Drying Method and Formation Mechanism. Rare Metal Materials and Engineering. 2014;43(10):2311-2315.^,1010 Jiang R, Tung S, Tang Z, Li L, Ding L, Xi X, et al. A review of core-shell nanostructured electrocatalysts for oxygen reduction reaction. Energy Storage Materials. 2018;12:260-276..

Different morphologies of core-shell materials have been reported, such as spheres, wires, rods, tubes, belts, fibers, plates, sheets, cubes, etc.²2 Lu W, Guo X, Luo Y, Li Q, Zhu R, Pang H. Core-shell materials for advanced batteries. Chemical Engineering Journal. 2019;355:208-237.. It is generally accepted that the performance of a catalyst can be strongly affected by its morphology, structure, and composition¹⁰10 Jiang R, Tung S, Tang Z, Li L, Ding L, Xi X, et al. A review of core-shell nanostructured electrocatalysts for oxygen reduction reaction. Energy Storage Materials. 2018;12:260-276.. In heterogeneous catalysis, micro- or nanoparticles with high surface area are preferably used because generally more active catalysts are obtained as result of more dispersed and more exposed active phases. In order to reuse the catalysts and separate the products, solid particles have to be recovered after reaction by unit operations such as either centrifugation or filtration, but these procedures are difficult, impractical or simply inefficient when fine particles are used. For example, in environmental heterogeneous catalysis, recovering small particles from aqueous suspensions is very difficult, and these particles after used are frequently released into the aqueous effluents¹¹11 Torres-Luna JA, Carriazo JG, Sanabria NR. Delaminated montmorillonite with iron(III)-TiO2 species as a photocatalyst for removal of a textile azo-dye from aqueous solution. Environmental Technology. 2016;37(11):1346-1356.. Several scientific works report that fine particles of TiO₂ can penetrate biological membranes of various microorganisms and plants, causing them death and provoking serious environmental damages in aqueous ecosystems¹¹11 Torres-Luna JA, Carriazo JG, Sanabria NR. Delaminated montmorillonite with iron(III)-TiO2 species as a photocatalyst for removal of a textile azo-dye from aqueous solution. Environmental Technology. 2016;37(11):1346-1356.^,1212 Ma H, Brennan A, Diamond S. Phototoxicity of TiO2 nanoparticles under solar radiation to two aquatic species: Daphnia magna and Japanese Medaka. Environmental, Toxicology and Chemistry. 2012;31(7):1621-1629.. Thus, there are several reasons to remove fine particles from reaction media after catalytic processes. The removal of fine particles from suspensions using magnetic fields is effective, and energetically more efficient and often faster than centrifugation or filtration¹³13 Tang H, Yu CH, Oduoro W, He H, Tsang SC. Engineering of a Monodisperse Core-Shell Magnetic Ti-O-Si Oxidation Nanocatalyst. Langmuir. 2008;24(5):1587-1590.. In this sense, fine powders of heterogeneous catalysts designed with ferrimagnetic or superparamagnetic properties that allow their attraction by a magnetic field are enormously appreciated. Several types of core-shell systems have been synthesized using cores of magnetite (Fe₃O₄) and shells of different inorganic materials, to be used as heterogeneous catalysts or catalytic supports. Magnetite is a ferrimagnetic stable oxide¹⁴14 Parkinson GS. Iron oxide surfaces. Surface Science Reports. 2016;71(1):272-365., and thus the core-shell powders synthesized with this oxide are easily separated from liquid media by a magnetic field⁴4 Li W, Elzatahry A, Aldhayan D, Zhao D. Core-shell structured titanium dioxide nanomaterials for solar energy utilization. Chemical Society Reviews. 2018;47(22):8203-8237.. In recent years, magnetite has received great attention and has been enormously studied due to its attractive chemical and magnetic properties, low toxicity and its potential applicability in multidisciplinary fields such as catalysis, separation processes, biomedicine, biosensing, hyperthermia treatments, magnetic resonance imaging, and drug delivery¹⁵15 Fodjo EK, Gabriel KM, Serge BY, Li D, Kong C, Trokourey A. Selective synthesis of Fe3O4AuxAgy nanomaterials and their potential applications in catalysis and nanomedicine. Chemistry Central Journal. 2017;11:58.

16 Carriazo Baños JG, Noval Lara VE, Ochoa-Puentes C. Magnetite (Fe3O4): An inorganic structure with many applications for heterogeneous catalysis. Revista Colombiana de Química. 2017;46(1):42-59.^-1717 Baeza A, Guillena G, Ramón DJ. Magnetite and Metal-impregnated Magnetite Catalysts in Organic Synthesis: A Very Old Concept with New Promising Perspectives. ChemCatChem. 2016;8(1):49-67..

Magnetite (Fe₃O₄) has been used as core in several core-shell systems with coating of silicon dioxide¹⁸18 Cendrowski K, Sikora P, Zielinska B, Horszczaruk E, Mijowska E. Chemical and thermal stability of core-shelled magnetite nanoparticles and solid silica. Applied Surface Science. 2017;407:391-397.

19 Lu CH, Chen GH, Yu B, Cong HL, Kong LM, Guo L. Design and synthesis of Fe3O4@SiO2 core-shell nanomaterials. Integrated Ferroelectrics. 2017;182(1):46-52.^-2020 Wang S, Tang J, Zhao H, Wan J, Chen K. Synthesis of magnetite-silica core-shell nanoparticles via direct silicon oxidation. Journal of Colloid and Interface Science. 2014;432:43-46. or titanium dioxide²¹21 Zhang Q, Meng G, Wu J, Li D, Liu Z. Study on enhanced photocatalytic activity of magnetically recoverable Fe3O4@C@TiO2 nanocomposites with core-shell nanostructure. Optical Materials. 2015;46:52-58.^,2222 Zheng J, Wu Y, Zhang Q, Li Y, Wang C, Zhou Y. Direct liquid phase deposition fabrication of waxberry-like magnetic Fe3O4@TiO2 core-shell microspheres. Materials Chemistry and Physics. 2016;181:391-396., among others. These (Fe₃O₄-SiO₂ or Fe₃O₄-TiO₂) particulate systems are synthesized because of their enormous potential application as catalysts or catalytic supports, in which various linkers or specific chemical groups can be anchored in order to obtain magnetically separable catalysts. In addition, both silica and titania have low toxicity, protect the magnetite particles from thermal effects, chemical oxidation to hematite (a non-ferrimagnetic iron oxide), dissolution by acid attack, and agglomeration¹⁸18 Cendrowski K, Sikora P, Zielinska B, Horszczaruk E, Mijowska E. Chemical and thermal stability of core-shelled magnetite nanoparticles and solid silica. Applied Surface Science. 2017;407:391-397.^,2020 Wang S, Tang J, Zhao H, Wan J, Chen K. Synthesis of magnetite-silica core-shell nanoparticles via direct silicon oxidation. Journal of Colloid and Interface Science. 2014;432:43-46.

21 Zhang Q, Meng G, Wu J, Li D, Liu Z. Study on enhanced photocatalytic activity of magnetically recoverable Fe3O4@C@TiO2 nanocomposites with core-shell nanostructure. Optical Materials. 2015;46:52-58.^-2222 Zheng J, Wu Y, Zhang Q, Li Y, Wang C, Zhou Y. Direct liquid phase deposition fabrication of waxberry-like magnetic Fe3O4@TiO2 core-shell microspheres. Materials Chemistry and Physics. 2016;181:391-396.. Titanium dioxide (TiO₂) is well recognized because of its photocatalytic activity and is widely used as photo-catalyst for degradation of organic pollutants in wastewater treatment.

Evidently, the scientific community has increased its endeavors on the synthesis of magnetite micro/nano-particles and the production of core-shell systems. In the literature, magnetite of high purity (synthesized in laboratories) is always used to synthesize the core-shell systems. However, the synthesis of magnetite is a crucial point for a possible real application of these systems. According to the literature, in all cases, magnetite for core-shell synthesis is prepared from diluted aqueous solutions of chemical reagents such as iron salts, e.g. FeCl₃ and FeCl₂, iron nitrates or sulfates, and bases such as ammonia or alkaline hydroxides¹⁷17 Baeza A, Guillena G, Ramón DJ. Magnetite and Metal-impregnated Magnetite Catalysts in Organic Synthesis: A Very Old Concept with New Promising Perspectives. ChemCatChem. 2016;8(1):49-67.^,2323 Kumar R, Sakthivel R, Behura R, Mishra BK, Das D. Synthesis of magnetite nanoparticles from mineral waste. Journal of Alloys and Compounds. 2015;645:398-404.

24 Chinnaraj K, Manikandan A, Ramu P, Arul Antony S, Neeraja P. Comparative Studies of Microwave- and Sol-Gel-Assisted Combustion Methods of Fe3O4 Nanostructures: Structural, Morphological, Optical, Magnetic, and Catalytic Properties. Journal of Superconductivity and Novel Magnetism. 2015;28(1):179-190.

25 Tombácz E, Turcu R, Socoliuc V, Vékás L. Magnetic iron oxide nanoparticles: Recent trends in design and synthesis of magnetoresponsive nanosystems. Biochemical and Biophysical Research Communications. 2015;468(3):442-453.^-2626 Salviano LB, Cardoso TMS, Silva GC, Dantas MSS, Ferreira AM. Microstructural Assessment of Magnetite Nanoparticles (Fe3O4) Obtained by Chemical Precipitation Under Different Synthesis Conditions. Materials Research. 2018;21(2):20170764.. On a large scale, this synthesis procedure will lead to environmental problems. In order to reduce economic costs and overcome the environmental impacts derived from this step, a new way of synthesis of core-shell systems has to be explored, in which industrially produced magnetite can be used. Green Magnetita S.A.S. is a Colombian company that produces industrial magnetite-rich powders for several uses. This micro-particulate material is industrially obtained from siderurgy wastes generated in iron-steel factories, by thermally assisted reduction processes in solid state, and without use of chemical reagents. This route involves green procedures with friendly outcomes for environment. From an economic point of view, currently 1 kg of this product has a price about USD 2.5, indicating a favorable value compared to the costs yielded from chemical procedures. In this paper, we report the synthesis of core-shell micro-particles from that (green magnetita®) product and making separate covering of both TiO₂ and SiO₂. The aim of this work is to advance on the synthesis procedure of these core-shell powders from an actually commercial, economic, green and industrially available magnetite-rich micro-particulate material. According to our knowledge, this is the first time that core-shell systems have been synthesized from industrially processed magnetite (Fe₃O₄) microparticles or nanoparticles.

2. Materials and Methods

2.1 Materials

Magnetite-rich powder was supplied by Green Magnetita S.A.S (Bogotá-Colombia). This industrial synthetic material was passed through a 325 ASTM mesh to obtain a fraction of particles lesser than 45 μm. The magnetic properties of this powder were preliminary checked using an ordinary magnet, observing that 100% of powder was attracted. Ammonium hydroxide (Merck, 28%), ethyl alcohol (Aldrich, 99.8%), acetone (Aldrich, 99.5%), cyclohexane (Merck, 99%), titanium(IV) butoxide (Aldrich, 97%), tetraethyl orthosilicate (Aldrich, 99%), and span 80 (sorbitan oleate, Merck) were used.

2.2 Synthesis of core-shell particles

Fe₃O₄-TiO₂ and Fe₃O₄-SiO₂ core-shell structures were synthesized using the reverse microemulsion method described in the literature²⁷27 Wu S, Sun A, Lu Z, Cheng C, Gao X. Magnetic properties of iron-based soft magnetic composites with SiO2 coating obtained by reverse microemulsion method. Journal of Magnetism and Magnetic Materials. 2015;381:451-456.

28 Ding HL, Zhang YX, Wang S, Xu JM, Xu SC, Li GH. Fe3O4@SiO2 Core/Shell nanoparticles: The Silica Coating Regulations with a Single Core for Different Core Sizes and Shell Thicknesses. Chemistry of Materials. 2012;24(23):4572-4580.^-2929 Tan TTY, Liu S, Zhang Y, Han MY, Selvan ST. Microemulsion Preparative Methods (Overview). In: Andrews DL, Scholes GD, Wiederrecht GP, eds. Comprehensive Nanoscience and Technology. Volume 5. London: Elsevier; 2011. p. 339-441. with some variations of the typical procedure. Here 10 mL of cyclohexane, 2 mL of Span 80 and 10 mg of commercial magnetite were mixed. This mixture was sonicated (equipment Branson 2510, 40 kHz) at room temperature for 10 minutes. Then, a volume of 0.8 mL of ammonium hydroxide (28%) was added to form the reverse microemulsion in which the microdrops of this solution are dispersed and stabilized by the polar fraction of Span 80 in cyclohexane phase. This microemulsion was subjected to mechanical stirring for 10 minutes in order to suspend the powder microparticles (magnetite) in the polar phase of the microemulsion. After that, a quantity (1.12 mL) of alkoxide (titanium butoxide or tetraethyl orthosilicate, according to the shell to be synthesized) was slowly added to the suspension under continuous stirring in an inert atmosphere, using a polyethylene glove-bag (Sigma-Aldrich, Z106089-1EA) saturated with nitrogen gas. The obtained mixture was left at rest, outer the bag, during 72 hours at room temperature (≈20 ºC). Finally, the solid fraction was separated by centrifugation (3000 rpm, 15 minutes) and purified by three washes with ethanol. The obtained solids were dried at room temperature for 8 hours, and then calcined at 400 ºC for 2 hours in static air atmosphere (a temperature ramp and heating rate of 10 ºC/min were used).

2.3 Characterization of solids

Chemical analysis of the magnetite-rich powder was carried out by X-ray fluorescence (XRF), using a Magix Pro PW-2440 Philips spectrometer with a rhodium tube and a maximum power of 4 kW. Before analysis, a sample of this material was dried at 100 ºC and then pressed at 120 kN on a spectrometric wax (Merck) to yield a standardized tablet of 36 mm diameter. Chemical analysis of the core-shell particles was performed by energy dispersive X-ray analysis (EDX) from SEM exploration. Scanning electron microscopy (SEM) analyses were carried out with a microscope FEI Quanta 200, taking several images and EDX profiles at different points of the solids. The samples were previously metalized with a gold-palladium alloy using the sputtering technique (Quorum Q15OR ES metallizer). On the other hand, X-ray powder diffraction profiles were recorded using a Panalytical X'Pert PRO MPD equipment with copper anode (Cu Kα radiation: λ=1.54056 Å) and Bragg-Brentano configuration. All diffractograms were taken at room temperature, with 0.01º (2θ) step size and 10 s step time. Transmission electron micrographs (TEM analyses) were obtained using an electron microscope FEI TECNAI 20 Twin at 200 kV. The samples were previously dispersed in ethanol and analyzed on a copper grid covered with formvar resin.

Curie temperature of magnetite-rich powder was determined by thermogravimetric analysis (TGA) under a magnetic field. This measure was performed using 10 mg of sample in an alumina pan, a thermal analyzer TA Instruments SDT Q 600, nitrogen (N₂) atmosphere, and a magnet certified by TA Instruments, Inc. For this experiment, the magnet was put on the microbalance chamber in order to recording the changes of mass during a thermogravimetric analysis. The equipment was previously calibrated with high purity indium and sapphire. On the other hand, the magnetization curves of both magnetite-rich powder and core-shell solids were measured in a VersaLab Free 3 Tesla Cryogen-Free vibrating sample magnetometer, at 300 K, and between -30 and 30 kOe.

BET surface areas were determined from nitrogen adsorption isotherms. The isotherms were taken at 77 K using a Micromeritic ASAP 2020 adsorption analyzer in the P/P₀ range of 1×10⁻⁵ to 0.99. The samples were previously outgassed at 200 ºC for 8 h. On the other hand, acidity (Brönsted and Lewis) of the samples was qualitatively assessed by in situ diffuse reflectance infrared Fourier-transform spectroscopy (DRIFTS). The assessment was carried out using NH₃ as probe molecule, and an IR Tracer-100 (Shimadzu) FT-IR spectrometer with a Harrick Praying Mantis diffuse reflection spectroscopy accessory coupled to a high temperature reaction chamber³⁰30 Torres-Luna JA, Carriazo JG. Porous aluminosilicic solids obtained by thermal-acid modification of a commercial kaolinite-type natural clay. Solid State Sciences. 2019;88:29-35.. According to a previous procedure described in the literature³⁰30 Torres-Luna JA, Carriazo JG. Porous aluminosilicic solids obtained by thermal-acid modification of a commercial kaolinite-type natural clay. Solid State Sciences. 2019;88:29-35., each sample was preheated at 400 ºC in a flow of N₂ (10 mL/min) for 1 h. Then, after the sample was cooled (30 ºC), a stream (10 mL/min) of gas ammonia (5% in He) was passed through the sample for 15 min. After that, the sample was subjected to desorption for 30 min with a flow of N₂ (10 mL/min) at different temperatures (room temperature, 100 ºC, 200 ºC, 400 ºC). After this desorption at each indicated temperature, the IR spectra were in situ recorded at 30 ºC (150 scans with a resolution of 4 cm⁻¹). IR bands corresponding to N-H vibrations, when NH₃ is linked (coordinated) to acid sites, is used to observe the surface sites retaining ammonia³¹31 Yao X, Zhang L, Li L, Liu L, Cao Y, Dong X, et al. Investigation of the structure, acidity, and catalytic performance of CuO/Ti0.95Ce0.05O2 catalyst for the selective catalytic reduction of NO by NH3 at low temperature. Applied Catalysis B: Environmental. 2014;150-151:315-329.. Brönsted acidity³¹31 Yao X, Zhang L, Li L, Liu L, Cao Y, Dong X, et al. Investigation of the structure, acidity, and catalytic performance of CuO/Ti0.95Ce0.05O2 catalyst for the selective catalytic reduction of NO by NH3 at low temperature. Applied Catalysis B: Environmental. 2014;150-151:315-329. is observed about 1435 cm⁻¹, and Lewis acidity³¹31 Yao X, Zhang L, Li L, Liu L, Cao Y, Dong X, et al. Investigation of the structure, acidity, and catalytic performance of CuO/Ti0.95Ce0.05O2 catalyst for the selective catalytic reduction of NO by NH3 at low temperature. Applied Catalysis B: Environmental. 2014;150-151:315-329. around 1600 cm⁻¹. Prior to ammonia adsorption, a background spectrum was acquired for bare (clean) sample in order to compare the IR bands.

3. Results and Discussion

3.1 Characterization of industrial magnetite-rich powder

Chemical analysis (Table 1) reveals the high content of iron (97.666% reported as Fe₃O₄) in the magnetite-rich powder, indicating a probable high content of magnetite. However, small quantities of other elements such as Si, Mn, Ti and Cu were observed. These elements are contaminants coming from the industrially processed siderurgy wastes. Likewise, EDX analysis (Figure 1a) reveals iron species as the main constituents and small contents of other elements in the surface. SEM and TEM micrographs (Figures 1b and 1c) show varied morphology and size of particles in this powdered material, including nanoscale and microscale levels.

The X-ray diffraction analysis of magnetite-rich powder (Figure 2) revealed that magnetite is the principal component of this material, with other phases of iron oxides, hematite (α-Fe₂O₃) and wustite (FeO). All peaks were identified according to the literature³²32 Jin Y, Zhao C, Lin Y, Wang D, Chen L, Shen Y. Fe-Based Metal-Organic Framework and its Derivatives for Reversible Lithium Storage. Journal of Materials Science & Technology. 2017;33(8):768-774.

33 Komarneni S, Hu W, Noh YD, Van Orden A, Feng S, Wei C, et al. Magnetite syntheses from room temperature to 150 ºC with and without microwaves. Ceramics International. 2012;38(3):2563-2568.^-3434 Cornell RM, Schwertmann U. The Iron Oxides: Structure, Properties, Reactions, Occurences and Uses. Weinheim: Wiley-VCH Verlag; 2003.. The peaks at 2θ (º) = 18.4, 30.1, 35.5, 37.2, 43.2, 53.5, 57.0, 62.6, and 74.0 are assigned to magnetite (Fe₃O₄). All the XRD signals are indexed in Table 2, including the d_(hkl) interplanar spacing values, and the corresponding peaks are labeled in Figure 2. On the basis of the crystalline structures identified by XRD, a semiquantitative estimation of these phases leads to the following approximation: 68% of Fe₃O₄, 8% α-Fe₂O₃ and 24% of FeO. Although other iron oxides different to magnetite are contained in this powder, it is possible to consider that these phases are intimately integrated because all of the powder (100% of the grains) is attracted by a regular magnet.

The Magnetization-demagnetization of magnetite-rich powder (Figure 3a) displays the superparamagnetic character of this material, since the magnetization curve does not show a hysteresis loop³⁵35 Yang M, Gao L, Liu K, Luo C, Wang Y, Yu L, et al. Characterization of Fe3O4/SiO2/Gd2O(CO3)2 core/shell/shell nanoparticles as T1 and T2 dual mode MRI contrast agent. Talanta. 2015;131:661-665.^,3636 Lu AH, Salabas EL, Schüth F. Magnetic Nanoparticles: Synthesis, Protection, Functionalization, and Application. Angewandte Chemie International Edition. 2007;46(8):1222-1244., thus a very low coercive force (approximately zero) is obtained. Bulk magnetite is typically a ferrimagnetic material with both saturation magnetization (85-100 emu/g) and coercivity higher than those of Fe₃O₄ fine powders³⁵35 Yang M, Gao L, Liu K, Luo C, Wang Y, Yu L, et al. Characterization of Fe3O4/SiO2/Gd2O(CO3)2 core/shell/shell nanoparticles as T1 and T2 dual mode MRI contrast agent. Talanta. 2015;131:661-665.^,3737 Zhang DE, Zhang XJ, Ni XM, Song JM, Zheng HG. Fabrication and Characterization of Fe3O4 Octahedrons via an EDTA-assisted Route. Crystal Growth & Design. 2007;7(10):2117-2119., but the superparamagnetic behavior of this material is a result of small particle sizes in fine powder (micro to nanoparticles). A superparamagnet is defined as an assembly of non-interacting giant magnetic moments³⁶36 Lu AH, Salabas EL, Schüth F. Magnetic Nanoparticles: Synthesis, Protection, Functionalization, and Application. Angewandte Chemie International Edition. 2007;46(8):1222-1244., which is frequently encountered in magnetite fine powders. This magnetic behavior of very low coercive force is actually interesting because no remaining magnetization is obtained when a magnetic field is retired, which avoids the particle agglomeration. Thus, it is important to stand out that the largest magnetite nanoparticles that still show superparamagnetic properties are the best to be used in adsorption and catalysis systems²⁶26 Salviano LB, Cardoso TMS, Silva GC, Dantas MSS, Ferreira AM. Microstructural Assessment of Magnetite Nanoparticles (Fe3O4) Obtained by Chemical Precipitation Under Different Synthesis Conditions. Materials Research. 2018;21(2):20170764.. A saturation magnetization of 49 emu/g was measured, which is in the range of values (30 to 80 emu/g) reported in the literature (at 300 K) for synthetic magnetite fine powders²³23 Kumar R, Sakthivel R, Behura R, Mishra BK, Das D. Synthesis of magnetite nanoparticles from mineral waste. Journal of Alloys and Compounds. 2015;645:398-404.^,3636 Lu AH, Salabas EL, Schüth F. Magnetic Nanoparticles: Synthesis, Protection, Functionalization, and Application. Angewandte Chemie International Edition. 2007;46(8):1222-1244.^,3838 Ristic M, Fujii T, Hashimoto H, Opacak I, Music S. A novel route in the synthesis of magnetite nanoparticles. Materials Letters. 2013;100(1):93-97.. The saturation magnetization is influenced by particle size, morphology, shape anisotropy³⁷37 Zhang DE, Zhang XJ, Ni XM, Song JM, Zheng HG. Fabrication and Characterization of Fe3O4 Octahedrons via an EDTA-assisted Route. Crystal Growth & Design. 2007;7(10):2117-2119., and impurities in the sample. On the other hand, Figure 3b shows the Curie temperature (567.6 ºC) determined by TGA under a magnetic field. At temperature values higher than 567.6 ºC this magnetite-rich powder loses its magnetic properties and is not attracted by the magnetic field put above, thus an abrupt gain of mass is registered by the microbalance. According to the literature, Curie temperature is around 584 ºC³⁹39 Harrison RJ, Putnis A. Magnetic properties of the magnetite-spinel solid solution: Curie temperatures, magnetic susceptibilities, and cation ordering. American Mineralogist. 1996;81(3-4):375-384.^,4040 Wu CC, Mason TO. Thermopower Measurement of Cation Distribution in Magnetite. Journal of the American Ceramic Society. 1981;64(9):520-522., although other works have reported 577 ºC³⁸38 Ristic M, Fujii T, Hashimoto H, Opacak I, Music S. A novel route in the synthesis of magnetite nanoparticles. Materials Letters. 2013;100(1):93-97., 565 ºC⁴¹41 Levy D, Giustetto R, Hoser A. Structure of magnetite (Fe3O4) above the Curie temperature: a cation ordering study. Physics and Chemistry of Minerals. 2012;39(2):169-176. and 440 ºC⁴²42 Manohar A, Krishnamoorthi C. Low Curie-transition temperature and superparamagnetic nature of Fe3O4 nanoparticles prepared by colloidal nanocrystal synthesis. Materials Chemistry and Physics. 2017;192:235-243. depending on the synthesis procedures and structural and morphological variations. Curie temperature is a very important parameter because several (magnetic) applications at high temperature may be restricted. At this value of temperature, a ferromagnetic, ferrimagnetic or superparamagnetic solid changes to be merely a paramagnetic material.

3.2 Magnetic core-shell structures

Taking into account the good magnetic properties of the magnetite-rich powder, this industrial product was used as core in the synthesis of Fe₃O₄-TiO₂ and Fe₃O₄-SiO₂ core-shell powders by using the reverse microemulsion method. Efficiency of the synthesis was determined as 76.3% for Fe₃O₄-TiO₂ and 86.2 for Fe₃O₄-SiO₂. The TiO₂ and SiO₂ contents were also stoichiometrically calculated. The content of TiO₂ in Fe₃O₄-TiO₂ core-shells is 20.0 wt.%, whereas the SiO₂ content in Fe₃O₄-SiO₂ core-shells is 22.9 wt.%. Because the densities of TiO₂ and SiO₂ (powders) are considerably lower than that of magnetite, the percentages of titania and silica incorporated in the solids represent an important covering volume (shell). At this point, it is important to mention that a comparison with literature results is very difficult because the percentages of silica and titania incorporated in the core-shells are not frequently indicated. Instead of that, the literature reports the thickness of shells. The morphology (SEM analysis) of microparticles obtained by coating magnetite-rich powder with TiO₂ is observed in Figure 4. Rugose particles with irregular morphology (like raisins) and size about 100 (m, and less than that, were observed by SEM. Energy dispersive X-ray analysis (EDX) revealed the high content of titanium in the shell of particles with no signal of iron on the most of these particles. However, for some particles, a small signal of iron was detected as consequence of an irregular coating (Figure 4). Non-spherical morphology for the Fe₃O₄-TiO₂ system is probably a result of the high reactivity of titanium alkoxide in aqueous (ammonia solution) micromicelles formed by the reverse microemulsion method. Titanium alkoxides are more reactive than silicon or aluminum alkoxides. The very fast hydrolysis of titanium alkoxide (titanium tetrabutoxide) led to rugose surface coating. In addition, TEM images (Figure 5) reveal nanometric domains with nanoparticles agglomeration. Irregular morphology is again identified at nanometric scale, confirming the formation of nanoparticles of Fe₃O₄-TiO₂ with core-shell structures.

On the other hand, the synthesis of Fe₃O₄-SiO₂ led to the formation of perfectly spherical microparticles (Figure 6) below 50 (m in size. The more regular and homogeneous covering of this core-shell system perhaps is consequence of the lower reactivity of the silicon alkoxide (silicon tetraethoxide), in the aqueous micromicelles, regarding that of titanium alkoxides. According to the literature, the hydrolysis rate constant for a Ti(OR)₄ ( k_h = 10^-3 M^-1 S^-1) is more than five order of magnitude greater than that of Si(OEt)₄ ( k_h = 5x10^-9 M^-1 S^-1) at pH 7⁴³43 Brinker CJ, Scherer GW. Sol-Gel Science. The Physics and Chemistry of Sol-Gel Processing. San Diego: Academic Press; 1990.. The lower hydrolysis rate for silicon alkoxides allows building well distributed microshell structures around the cores, yielding core-shell microparticles with homogeneous surface. The EDX analyses on the Fe₃O₄-SiO₂ particles (Figure 6) revealed the high content of silicon in the shell of particles with no signal of iron for any particle, which indicates that iron oxide is completely hidden. Additionally, TEM images (Figure 7) confirm the formation of spherical and discrete nanoparticles with size less than 150 nm. This result ratifies the suitable formation of both micro and nanometric Fe₃O₄-SiO₂ core-shells from magnetite-rich powder and the reverse microemulsion method.

The magnetization curves at 300 K for the Fe₃O₄-TiO₂ and Fe₃O₄-SiO₂ core-shell particles (Figure 8) confirm the superparamagnetic performance of these materials, since no hysteresis loop (zero coercive force) can be observed²⁸28 Ding HL, Zhang YX, Wang S, Xu JM, Xu SC, Li GH. Fe3O4@SiO2 Core/Shell nanoparticles: The Silica Coating Regulations with a Single Core for Different Core Sizes and Shell Thicknesses. Chemistry of Materials. 2012;24(23):4572-4580.^,3535 Yang M, Gao L, Liu K, Luo C, Wang Y, Yu L, et al. Characterization of Fe3O4/SiO2/Gd2O(CO3)2 core/shell/shell nanoparticles as T1 and T2 dual mode MRI contrast agent. Talanta. 2015;131:661-665.^,4444 Zhang X, Wang J. Preparation of carbon coated Fe3O4 nanoparticles for magnetic separation of uranium. Solid State Sciences. 2018;75:14-20.. This behavior is coherent and is a result derived from the magnetic properties of magnetite-rich material introduced as cores in the particles. The saturation magnetization values of Fe₃O₄-TiO₂ and Fe₃O₄-SiO₂ (1.37 emu/g and 1.40 emu/g, respectively) are significantly lower than that showed for magnetite-rich powder, which is consequence of the covering (shell) with non-magnetic materials (TiO₂ or SiO₂). However, these saturation magnetization values are sufficient to give a suitable response under a moderate magnetic field, allowing well separation of particles from liquid media, such as it was experimentally observed (Figure 8). Recent studies have demonstrated that the saturation magnetization of silica-coated Fe₃O₄ nanoparticles decreases up to 1.9 emu/g as the silica shell thickness increases to 18.5 nm²⁸28 Ding HL, Zhang YX, Wang S, Xu JM, Xu SC, Li GH. Fe3O4@SiO2 Core/Shell nanoparticles: The Silica Coating Regulations with a Single Core for Different Core Sizes and Shell Thicknesses. Chemistry of Materials. 2012;24(23):4572-4580.. Zhang and Wang⁴⁴44 Zhang X, Wang J. Preparation of carbon coated Fe3O4 nanoparticles for magnetic separation of uranium. Solid State Sciences. 2018;75:14-20. reported a strong decrease from 70 to 8 emu/g in magnetite coated with carbon shells, and Yang et al.³⁵35 Yang M, Gao L, Liu K, Luo C, Wang Y, Yu L, et al. Characterization of Fe3O4/SiO2/Gd2O(CO3)2 core/shell/shell nanoparticles as T1 and T2 dual mode MRI contrast agent. Talanta. 2015;131:661-665. observed a reduction from 56 to 1.4 emu/g as result of coating Fe₃O₄ with SiO₂. Extending this conclusion to the present work, a well coating for the iron oxide is expected in both core-shells Fe₃O₄-TiO₂ and Fe₃O₄-SiO₂.

X-ray diffraction (XRD) analysis of the Fe₃O₄-TiO₂ powder reveals the characteristic signals of (TiO₂) anatase phase (Figure 9). The peaks at 2-theta positions (º) of 25.2, 37.9, 48.0, 54.4, 62.6. 70.0 and 75.5 clearly confirm the successful formation of a TiO₂ shell (anatase phase) on the iron oxide. All of these diffraction peaks were confirmed by literature²¹21 Zhang Q, Meng G, Wu J, Li D, Liu Z. Study on enhanced photocatalytic activity of magnetically recoverable Fe3O4@C@TiO2 nanocomposites with core-shell nanostructure. Optical Materials. 2015;46:52-58.^,2222 Zheng J, Wu Y, Zhang Q, Li Y, Wang C, Zhou Y. Direct liquid phase deposition fabrication of waxberry-like magnetic Fe3O4@TiO2 core-shell microspheres. Materials Chemistry and Physics. 2016;181:391-396.^,4545 Huang F, Li Q, Thorogood GJ, Cheng YB, Caruso RA. Zn-doped TiO2 electrodes in dye-sensitized solar cells for enhanced photocurrent. Journal of Materials Chemistry. 2012;22(33):17128-17132.. On the other hand, the Fe₃O₄-SiO₂ powder shows an XRD amorphous profile (Figure 9) with a broad signal about 2θ = 23º (between 15 and 30º). This is a typical profile of silica (SiO₂), and confirms the successful formation of SiO₂ shell on the magnetite-rich particles. In both XRD profiles, very small signals of the highest magnetite peaks (2θ = 35.5 and 62.6º) were observed. Similar results have been obtained by other researchers for Fe₃O₄-SiO₂ core-shells²⁰20 Wang S, Tang J, Zhao H, Wan J, Chen K. Synthesis of magnetite-silica core-shell nanoparticles via direct silicon oxidation. Journal of Colloid and Interface Science. 2014;432:43-46.^,2828 Ding HL, Zhang YX, Wang S, Xu JM, Xu SC, Li GH. Fe3O4@SiO2 Core/Shell nanoparticles: The Silica Coating Regulations with a Single Core for Different Core Sizes and Shell Thicknesses. Chemistry of Materials. 2012;24(23):4572-4580. and Fe₃O₄-TiO₂ core shells²¹21 Zhang Q, Meng G, Wu J, Li D, Liu Z. Study on enhanced photocatalytic activity of magnetically recoverable Fe3O4@C@TiO2 nanocomposites with core-shell nanostructure. Optical Materials. 2015;46:52-58.^,4646 Ma P, Jiang W, Wang F, Li F, Shen P, Chen M, et al. Synthesis and photocatalytic property of Fe3O4@TiO2 core/shell nanoparticles supported by reduced graphene oxide sheets. Journal of Alloys and Compounds. 2013;578:501-506. using pure magnetite synthesized in laboratories. XRD signals corresponding to magnetite (core of the systems) are difficult to be observed because a good covering of TiO₂ or SiO₂ was formed. It is well known that the shielding or absorption effects by coating structures lead to the weakening of the XRD intensities coming from the reflections of cores²²22 Zheng J, Wu Y, Zhang Q, Li Y, Wang C, Zhou Y. Direct liquid phase deposition fabrication of waxberry-like magnetic Fe3O4@TiO2 core-shell microspheres. Materials Chemistry and Physics. 2016;181:391-396.^,4646 Ma P, Jiang W, Wang F, Li F, Shen P, Chen M, et al. Synthesis and photocatalytic property of Fe3O4@TiO2 core/shell nanoparticles supported by reduced graphene oxide sheets. Journal of Alloys and Compounds. 2013;578:501-506.. Core-shell formation is verified because these particulate systems have superparamagnetic behavior, and a magnet equally attracts all the particles. The reason whereby these titania-rich or silica-rich particles are attracted under a magnetic field is that they have magnetite cores.

All the basic aforementioned characterizations confirm the effectiveness of the reverse microemulsion method to suspend relatively large particles (microparticles) of a dense materials such as magnetite. One of the difficulties found in previous assays focused to extend the magnetite core-shell synthesis to practical situations has been the suspension of large particles, so the synthesis of magnetite for this purpose is usually restricted to obtain nanoparticles. The present study demonstrates that the reverse microemulsion method is suitable for overcoming this drawback.

On the other hand, the N₂ adsorption isotherms of magnetite-rich powder and core-shells (Figure 10) give valuable information about the surface and porosity of these solids. Clearly, magnetite-rich powder is a non-porous material due to its negligible N₂-adsorption level, whereas the Fe₃O₄-TiO₂ and Fe₃O₄-SiO₂ core-shells increased the porosity and as consequence of that they significantly improved their adsorption capacity. BET surface area of magnetite-rich powder (Fe₃O₄) is insignificant (1 m²/g), but increased to 95 m²/g and 46 m²/g for Fe₃O₄-TiO₂ and Fe₃O₄-SiO₂, respectively. This result indicates that covering magnetite with TiO₂ or SiO₂ is very important to yield large surface areas. Thus, the core-shell systems obtained in the present work are magnetic solids with potential value to be used as catalysts or catalytic supports. The higher surface area value for Fe₃O₄-TiO₂ core-shell is directly related to the higher roughness of the particles, which was verified by SEM. These BET surface areas are comparable with those obtained by other authors for these core-shell systems. Costa and coworkers⁴⁷47 Costa AL, Ballarin B, Spegni A, Casoli F, Gardini D. Synthesis of nanostructured magnetic photocatalyst by colloidal approach and spray-drying technique. Journal of Colloid and Interface Science. 2012;388(1):31-39. reported 141 m²/g for Fe₃O₄-SiO₂-TiO₂ (a core-shell-shell system) and Zhang et al.²¹21 Zhang Q, Meng G, Wu J, Li D, Liu Z. Study on enhanced photocatalytic activity of magnetically recoverable Fe3O4@C@TiO2 nanocomposites with core-shell nanostructure. Optical Materials. 2015;46:52-58. reported 40 m²/g for Fe₃O₄-C-TiO₂, whereas Lee et al.⁴⁸48 Lee J, Lee Y, Youn JK, Na HB, Yu T, Kim H, et al. Simple Synthesis of Functionalized Superparamagnetic Magnetite/Silica Core/Shell Nanoparticles and their Application as Magnetically Separable High-Performance Biocatalysts. Small. 2008;4(1):143-152. synthesized Fe₃O₄-SiO₂ core-shells with BET surface area of 41 m²/g.

In situ DRIFTS analyses of NH₃ adsorbed and desorbed at different temperatures revealed acidity generation (Brönsted and Lewis sites) after covering of magnetite-rich powder with TiO₂ or SiO₂. DRIFTS spectra of the solids with ammonia adsorbed at room temperature and after heating at 100 ºC, 200 ºC and 400 ºC are showed in Figure 11. It is clear that magnetite-rich powder does not adsorb ammonia (Figure 11a), indicating that it does not have acid sites. However, DRIFTS spectra of both Fe₃O₄-TiO₂ and Fe₃O₄-SiO₂ core-shells (Figures 11b-c) clearly show a broad band between 3000 and 3500 cm^-1, which is attributed to the NH₃ adsorbed to acid sites⁴⁹49 Liu D, Yuan P, Liu H, Cai J, Qin Z, Tan D, et al. Influence of heating on the solid acidity of montmorillonite: A combined study by DRIFT and Hammett indicators. Applied Clay Science. 2011;52(4):358-363.

50 Liu J, Li X, Zhao Q, Ke J, Xiao H, Lv X, et al. Mechanistic investigation of the enhanced NH3-SCR on cobalt-decorated Ce-Ti mixed oxide: In situ FTIR analysis for structure-activity correlation. Applied Catalysis B: Environmental. 2017;200:297-308.^-5151 Wang L, Li W, Schmieg SJ, Weng D. Role of Brønsted acidity in NH3 selective catalytic reduction reaction on Cu/SAPO-34 catalysts. Journal of Catalysis. 2015;324:98-106.. This band is reduced according to the heating, as consequence of releasing chemisorbed NH₃ from the solids. The bands about 1600 cm^-1 and 1435 cm^-1 are typically assigned to vibrations of NH3 retained on Lewis and Brönsted acid sites³⁰30 Torres-Luna JA, Carriazo JG. Porous aluminosilicic solids obtained by thermal-acid modification of a commercial kaolinite-type natural clay. Solid State Sciences. 2019;88:29-35.^,3131 Yao X, Zhang L, Li L, Liu L, Cao Y, Dong X, et al. Investigation of the structure, acidity, and catalytic performance of CuO/Ti0.95Ce0.05O2 catalyst for the selective catalytic reduction of NO by NH3 at low temperature. Applied Catalysis B: Environmental. 2014;150-151:315-329.^,5050 Liu J, Li X, Zhao Q, Ke J, Xiao H, Lv X, et al. Mechanistic investigation of the enhanced NH3-SCR on cobalt-decorated Ce-Ti mixed oxide: In situ FTIR analysis for structure-activity correlation. Applied Catalysis B: Environmental. 2017;200:297-308.. These bands also decrease during heating, in response to the acid strength of the sites. From the spectra of Fe₃O₄-SiO₂ it is evident that the intensity of all the bands is strongly reduced after heating at 100 ºC, which allows concluding that this solid has acid sites weaker than Fe₃O₄-TiO₂.

4. Conclusions

Fe₃O₄-TiO₂ and Fe₃O₄-SiO₂ core-shell particles were successfully synthesized from a commercial and industrially produced magnetite-rich powder. Industrial fine powder was carefully characterized revealing the predominant composition and behavior of magnetite having a wide range of particle size (from micrometers to nanometers) and superparamagnetic performance (no coercivity, saturation magnetization of 49 emu/g and Curie temperature of 567.6 ºC). The core-shell systems were synthesized by using the reverse microemulsion method. SEM, TEM, XRD and magnetization curves confirm the formation of superparamgnetic core-shells (no coercivity, and saturation magnetization around 1.4 emu/g) at level of micro and nanoparticles, with coating of TiO₂ (anatase phase) and SiO₂ (amorphous silica). In addition, all these results confirm the appropriate performance of coating method (reverse microemulsion) used here for the core-shell synthesis, demonstrating its efficiency to suspend microparticles with large size of dense materials such as magnetite. The formation of a microemulsion with large size particles of magnetite, more than nanoparticles, is also a challenge for real industrial process. The covering of magnetite-rich powder with TiO₂ and SiO₂ led to the formation magnetic core-shell systems with important surface area values and acidity.

5. Acknowledgements

The authors gratefully acknowledge the (U. N.) Universidad Nacional de Colombia (Bogotá, D.C.). This work was developed at the 125-Lab (Lab-DRES: Laboratorio de Diseño y Reactividad de Estructuras Sólidas) of the Faculty of Science (U.N.-Bogotá). The authors also thank the company Green Magnetita S.A.S (Bogotá-Colombia) for providing the industrial magnetite samples.

6. References

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