Nano-sized manganese ferrites Mn Fe О (х =0–1.3) were prepared using contact non-equilibrium plasma (CNP) х 3 − х 4 in two different pH (11.5 and 12.5). The influence of synthesis conditions (e.g., cation ratio and initial pH) on phase composition, crystallite size, and magnetic properties were investigated employing X-ray diffraction (XRD), differential thermal analysis (DTA), Fourier transform infrared (FTIR), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and magnetic measurement techniques. The formation of monodispersed faceted ferrite particles at −1 х =0–0.8 was shown. The FTIR spectra revealed reflection in region 1200–1700 cm caused by the presence of water adsorbed on the surface of Fe Mn O micro-granules or embedded into their crystal lattice. The most sensitivity of 3 − x x 4 −1 reflection spectra to the composition changes takes place within a 400–1200 cm range, typical to the stretching −1 −1 vibrations of Fe(Mn)–O (up to 700 cm ), Fe(Mn)–OH, and Fe(Mn)–OH bonds (over 700 cm ). The XRD results showed that the nanocrystalline Mn Fe О (0 < x < 1.0) had cubic spinel crystal structure with average crystallite х 3 − х 4 size 48–49 A. The decrease of crystalline size with the x increase was also observed. Keywords: MnFe O spinel, Preparation, Combustion, Chemical precipitation, Characterization, CNP 2 4 Background doping [23, 24]. Hydrophase methods allow regulating The ability of nanodispersive spinels with polyvalent composition, crystallinity, and particle morphology. metals to form a number of solid solutions and Such methods have been studied by many compounds gives unlimited possibilities to control researchers and are successfully applied for synthesis technological properties of spinel compounds. For a long of ferrites [9, 25, 26] with particle size of 30–50 nm time, great attention of many researchers has been paid to at 50–150 °С, which is significantly lower than for the investigations of manganese ferrites (Fe O − Mn O ceramic technology. Hydrophase methods, as a rule, 3 4 3 4 system) because of their wide application in industry. include several stages: the first—deposition, the They are widely used in microwave ovens and magnetic second—directly ferrite synthesis, carried out due to storage devices, as well as highly active catalysts in oxidation, aging, etc. The methods for initiation of producing hydrogen via methane dehydrogenation into the second main stage of ferrite synthesis using ethylene or acetylene, adsorbents [1–6]. ultrasound treatment, microwave influence, ultraviolet, The synthesis of manganese ferrite spinel is technologically and various discharges [27–29] have been used complex. Presently, there are few methods for the synthesis recently. During the treatment of solutions with of manganese ferrite particles, such as ceramic , coprecipi- discharge of CNP, a complicated complex of chemical tation [8–12], hydrothermal method , reverse micelle [14, reactions involving radical particles and free electrons 15], sol-gel , combustion method , mechanosynthesis occurs. The main products of such interactions are [18–20], high-energy technologies [21, 22], and mechanical oxygen, hydrogen, and hydrogen peroxide. Oxidative activity of plasmochemically “activated” solutions can be used for synthesis of complex oxide compounds. * Correspondence: email@example.com The emission spectrum [30–32] has shown that the Ukrainian State University of Chemical Technology, Gagarin Ave., 8, Dnipropetrovsk 49005, Ukraine main contributions to the emission spectrum of the water Full list of author information is available at the end of the article © The Author(s). 2017 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Frolova and Derhachov Nanoscale Research Letters (2017) 12:505 Page 2 of 9 vapor plasma are OH, atomic hydrogen, and the oxygen sets of samples were prepared. The first set at initial radical. In the case of the bubble mode, when the рН = 11.5 and the second at 12.5. The coprecipitated streamers fill the entire bubble, a significant emission from compounds were prepared by pouring at continuous the nitrogen second positive system and the nitrogen ion stirring of corresponding mixture of sulfate solutions (first negative system). The discharge operates in two with necessary cation ratio. The further treatment was different modes. For small conductivities of the liquid, the carried out using CNP. discharge is a direct liquid streamer discharge (liquid The treatment was conducted in a cylindrical reactor mode). This mode is similar to the typical so-called corona with inner diameter of 45 mm and height of 85 mm. discharges in water. For conductivities above typically The reaction mixture was cooled by the continuous −1 45 μScm a large vapor bubble is formed. In the bubble circulation of cold water in the outer jacket. One of mode, the streamers are located at the bubble–liquid stainless steel electrodes (diameter 4 mm) was located in interface. The hydrogen peroxide formation efficiency is the lower part of the reactor, and the other (diameter dependent on power with a maximum for intermediate 2.4 mm) was located 10 mm above the surface of the powers. The hydrogen peroxide formation efficiency is solution. The initial voltage was delivered to the step-up significantly smaller in the bubble mode than in the liquid transformer. The ac current from the secondary coil was mode. In the work , the kinetic parameters of delivered to the bridge rectifier and then, now pulsating electrons for the dielectric barrier discharge with a liquid voltage, was delivered via a ballast resistor to the reactor electrode at atmospheric pressure have been estimated. electrodes. The igniting unit was additionally connected Thus, we may suppose that the CNP will possess chemical to the anode. This unit formed pulses with amplitude of activity with respect to its application in realization of the up to 15 kV at a width of 1.5 ms. The pulses were strictly different oxidative-reductive processes. synchronized with the phase of the pulsating voltage. At Our preliminary studies of plasma treatment of solu- the instant when the igniting pulse was formed, there tions have shown that the composition of synthesized was a breakdown between the reactor electrodes in the oxidizer solution depends on a wide range of factors . vacuum space created by rarefaction to 0.06–0.08 MPa. The use of CNP guarantees high degree of homogeneity The resistance sharply dropped, and an anode current in component distribution both in the initial solution and started to flow thereby creating a discharge. The dis- in the product formed during oxidation, which stimulates charge burning voltage remained nearly unchanged at effective interaction between them with formation of fer- 750–900 V. The current in the discharge gap was deter- rites with homogeneous structure and composition. mined by the plasma resistance and the voltage applied The aim of the work is to study the possibility of obtain- to the system formed by the plasma discharge and the ing nano-sized Mn Fe О spinel from aqueous solu- ballast controller. The voltage was controlled by the Х 3 − Х 4 tions using contact non-equilibrium plasma. Since ferrites phase method principle, i.e., the average anode voltage are solid solutions, it is important to establish the degree applied to the reactor depended on the phase of the pul- of their structural and concentration homogeneity under sating voltage at the anode and on the instant at which the selected synthesis conditions. The experimental an ignition pulse was delivered. method consisted in comparison of ferrospinel obtained The plasma appeared at the ignition instant and was from manganese and iron sulfates at different cation ratio. extinguished when the anode voltage pulsations termi- Such comparative research of the samples allow estab- nated (Fig. 1). The repetition frequency of the process lishing the influence of the chemical composition of the was 100 Hz. The discharge current was controlled by initial solution and the synthesis conditions on the changing the instant of ignition relative to the phase of structural-phase state of the compounds prepared using anode voltage pulsations with a synchronizing device. CNP treatment. The duration of plasma treatment varied from 10 to 40 min. All precipitates were washed until negative Methods reaction on sulfate-ion. The washed and filtered precipi- For the synthesis of manganese ferrite, the authors have tates were dried at 150 °С. Relative magnetic properties used aqueous solutions of FeSO ·7H O, MnSO ·5H O, (of saturation magnetization I (emu /g), coercive force 4 2 4 2 S and aqueous solution of NaOH was used as a precipi- Нс (Oe)) were evaluated by magnetometer . 2+ tant. We used 0.5 M solutions of iron and manganese The concentration of Mn in the samples obtained salts. All the chemicals and solvents employed for the was determined complexometrically. The concentration synthesis were of analytical grade and used as received of iron was determined using permanganate and bichro- without further purification. Deionized water was used mate methods. In order to monitor the reaction process, as solvent in whole procedure. the reactor was equipped with an electrode system. The 2+ 2+ Preliminary studies  showed that at pH < 11 non- [Fe ]/[Mn ] ratio in Mn Fe О compound was х 3 − х 4 magnetic oxides and oxyhydroxides were formed, so two calculated according to formula: Frolova and Derhachov Nanoscale Research Letters (2017) 12:505 Page 3 of 9 cations, and character of their location in site of spinel 2+ 2+ crystal lattice. It is known that divalent cations (Zn ,Mn ) are mostly located in tetrahedral positions and triva- 3+ lent (Fe )—in octahedral positions of spinel crystal lattice. According to Néel relaxation theory, such ar- rangement provides maximum value of material magnetization. During preparation of ferrites, oxidation of 2+ 3+ Mn to Mn is possible, which can be accompanied by 3+ 2+ reduction of Fe to Fe and rearrangement of cations in 2+ sublattices, with partial transfer of Fe into tetrahedral 3+ and Mn —into octahedral nodes of crystal lattice, which negatively impacts magnetic properties of ferrites. 2+ Fig. 1 The pillar of contact non-equilibrium plasma between the Oxidation of Mn occurs at highest rate at 900–1000 °С, electrode in the gas phase and the surface of the liquid and optimal condition for sintering of manganese ferrites for ceramic technology—1000–1200 °С. The data available in various literature sources on discussion of magnetic C x Mn structure and properties of manganese ferrites are contra- C 3−x Fe dictory, which is likely related to variation in arrangement and values were equal of х = 0; 0.2, 0.4, 0.6, 0.8, 0.9, 1, of iron and manganese ions and their polyvalency. The 1.1, 1.2, and 1.3 were chosen. Fourier transform infrared data of the dependence of lattice parameter on the value reflection spectra of manganese ferrites Mn Fe O of х in case of various technologies make it possible to as- x 3 − x 4 (x = 0.0, 0.2, 0.4, 0.6, 0.8, 0.9, 1.0, 1.1, 1.2, 1.3) were mea- sume the character of cation arrangement in the lattice. −1 sured within a 400–4000 cm range by employing a The assessment results of prepared samples can be for- Fourier transform infrared (FTIR) spectrometer Nicolet mulated as follows: all samples include chemically bound iS10. To study the transformations occurring upon heat- water in various amounts. In both sets, the highest water ing of obtained powders, we used differential thermal content is found in samples with х = 0.4, 1.1…1.3. The analysis (DTA) and differential thermogravimetric ana- first set showed weak magnetic properties (Figs. 2 and 3), lysis (DTG). DTA, mass loss TG, and mass loss rate and so it was not considered in detail. DTG curves were recorded on Derivatograph Q-1500D As can be seen from Fig. 1, there are some differences in (F. Paulik, J. Paulik, and L. Erdey). The temperature was saturation magnetization of both sets. The highest value for varied in range 20–1000 °C at heating rate of 10°/min. set 1 corresponds to ratio 1,1 Mn Fe Mn О .The 1.0 0.9 0.1 4 γ-Al O was used as a reference. The mass of each highest value is achieved at рН =12.5and ratioof х =0.8 2 3 sample was 200 mg. The morphology of the ferrite (Mn Fe Fe О ). This ratio is different from stoichiomet- 0.8 0.2 2 4 powders and the particle size were studied using ric manganese ferrite. scanning electron microscopy. The phase composition By assessing saturation magnetization, it can be said (XRD) and the structure of the ferrite samples were that sample nos. 1, 2, 3, and 8 have lower values due to studied using X-ray diffractometer DRON-2 in mono- chromatized Cо-K radiation. The crystallite size and the degree of microstrains were calculated using Is , emu/g approximation method. The size and the shape of the particles were determined using electron Microscope “Jem 1010” (JEOL) at working value voltage of 200 kV. Scanning electron microscopy with X-ray microanalysis was carried out using REMMA-102 (SELMI, Ukraine). Results and Discussion Properties of magnetic materials based on manganese ferrites depend on their structural and phase state. The 40 synthesis of such ferrites call for preparation of single- phase product with spinel structure not having residual iron oxide or other phases, which are intermediate 00.4 0.8 1.2 products of ferrite formation from oxides. On par with Fig. 2 Dependence of saturation magnetization on cation molar ratio phase composition, the magnetic properties are signifi- at different рН:1—рН = 11.5 and 2—рН =12.5 cantly influenced by oxidation of iron and manganese Frolova and Derhachov Nanoscale Research Letters (2017) 12:505 Page 4 of 9 and gradual substitution of iron cations with manganese Hc, Oe cations leads to reduction of magnetic properties to ratio 0.4 (the first peak) following 1-1,1, corresponding to stoichiometric manganese ferrite. Analysis of Figs. 2 and 3 makes it possible to establish that the formation of compounds in the second set occurs according to the 50 maghemite formation mechanism. 40 As stated in Table 1, ferrite Mn Fe О was ob- Х 3 − X 4 30 tained in the nanorange. The average crystallite size of nanoparticles Mn Fe О were ranged from 5 to Х 3 − X 4 8 nm and reached a maximum at x = 0. The calculated Mn Fe О crystalline size exceeded ferrite crystallite Х 3 − X 4 00.4 0.8 1.2 x size in TEM image in four times due to aggregation of nanoparticles. Fig. 3 Dependence of coercive force on cation molar ration at different рН:1—рН = 11.5 and 2—рН = 12.5 Also, Table 1 shows the variation of Curie temperature, lattice parameter with ratio x in their amorphous structure and presence of non- FeMn O . Curie temperature decreases as cations 2 − x 4 magnetic phases. manganese content increases. As it can be known, Curie Figure 4 shows XRD patterns of samples from the sec- temperature is mainly determined by the strongest super ond set. The XRD patterns may be divided into two exchange interaction in ferrites. The factors decreasing categories—first samples 6–10 that have monophase this interaction lead to a decrease in Curie temperature. crystal structure corresponding to spinel phase ferrite With an increase in the manganese content, the lattice (JCPDS 10-0467). Relatively sharp and intense lines of parameter increases (Table 1). This leads to an increase spinel phase ferrites can be observed on XRD patterns in ionic distances as well as decrease in Curie of the samples. The lines related to oxide phases of temperature. Fe O and MnO are absent on XRD patterns (Fig. 4). The present assumption requires additional study. 2 3 x The second set has less crystalline with few phases Analysis of derivatography patterns indicates on present. On XRD patterns of the samples, prepared with formation of manganese ferrite in sample nos. 4 and 5 higher manganese content, the lines are slightly broad- and isomorphism of properties for samples 5–10 (Fig. 6). ened, which may indicate changes in its structure in The compounds of various composition are formed in comparison with the stoichiometric sample. Presence of samples 1–5. The lowest mass losses are also observed other phases in case of higher manganese content was for stoichiometric compositions. The first regions of found using XRD method. The broad peaks can be derivatography patterns demonstrate various endo- and observed on XRD patterns, which can be indexed as exothermic effects corresponding to oxidation of (311) of ferrite’s spinel phase (JCPDS 74-2403). In the manganese and iron cations. High temperature region cor- region of angles corresponding to the highest intensity responds to rearrangement of crystal lattice (endo-effects lines for Fe O and Mn O , a halo of low intensity is without changing the mass). 3 4 3 4 observed, which can indicate the presence of these DTG curves demonstrate that for all compositions the oxides in the samples. There is a clear correlation main mass loss corresponds to the loss of free water at between the magnetic characteristics and the degree of 100 °С and bound at 160 °С. For composition 4, crystallinity and homogeneity of the product. corresponding to stoichiometric ferrite, exothermic 2+ Taking into account that Mn cations are the largest peaks are observed, which correspond to oxidation of of all, it could be assumed that as the value of x manganese cation to various oxidation states. In the increases, it is possible to increase the lattice parameter. work , the authors have presented the following set The analysis of XRD patterns (Fig. 5) shows that the par- of reaction occurring at various temperatures. ameter of crystal lattice а = 8.4196 А (for stoichiometric 2+ 3+ tetragonal manganese ferrite MnFe O а = 8.51 А). 1. 3 Fe →2Fe +□temperature 280 °C 2 4 3+ 4+ Noticeably smaller value of lattice parameter can be 2. 4 Mn →3Mn +□temperature 330 °C 2+ 3+ explained with formation of manganese ferrite at 3. 3 Mn →2Mn +□temperature 360 °C 4+ 3+ рН = 12.5 following magnetite formation mechanism. 4. 3Mn +□→4Mn temperature 420 °C 2+ 2+ 3+ Upon oxidation of Mn : 5. 2Mn →Mn temperature 600 °C 2+ 3+ 3Mn →2Mn +□ vacancies are formed, which facilitate reduction of lat- Upon heating to 450–500 °С a structure of γ-Fe O 2 3 tice parameters. Magnetite is formed in the second set, type is formed. Frolova and Derhachov Nanoscale Research Letters (2017) 12:505 Page 5 of 9 Fig. 4 XRD patterns of ferrite obtained at different ratios of components (Table 1): A—Fe O ,B—MnFe O ,C—Mn O , and D—β-MnO 3 4 2 4 3 4 2 It can be assumed, that peaks at 600 °С correspond to oxidation and reduction of iron and manganese cations. Further oxidation is accompanied with transi- a,nm tion from cubic to rhombohedral lattice, in which all 8.51 cations are trivalent. The formation of α-Fe O and 2 3 8.49 α-Mn O occurs in range from 600 to 1000 °С.XRD 2 3 8.47 analysis of products obtained after heating to 1000 °С 8.45 indicates the presence of magnetic phase of rhombo- 8.43 hedral manganese ferrite for the samples with stoichiometric ratio of iron and manganese formed 8.41 from iron and manganese oxides. 8.39 In addition, upon heading samples 1–10 to 1000 °С 8.37 (Table 1), the formation of complex iron and manganese 8.35 oxide occurs via similar mechanism. The formed 00.2 0.4 0.6 0.81 x compounds have similar peaks regardless of the initial Fig. 5 Dependence of crystal lattice parameter on cation ratio х composition. This is related to rhombohedral structure, Frolova and Derhachov Nanoscale Research Letters (2017) 12:505 Page 6 of 9 Table 1 Dependence of the main characteristics of products on composition Sample number х Composition а, А Crystallite size, А T ,°С Mass loss, % Literature value а, А 1 1.3 Mn Fe О Amorphous 315 11.5 1.3 1.7 4 2 1.2 Mn Fe О Amorphous 315 19.6 1.2 1.8 4 3 1.1 Mn Fe О Amorphous 305 16.4 1.1 1.9 4 4 1 MnFe О 8.4184 48.8 320 13.4 8.51 2 4 5 0.9 Mn Fe О 8.4148 52.3 280 6.7 0.9 2.1 4 6 0.8 Mn Fe О 8.4148 60.9 315 7.2 0.8 2.2 4 7 0.6 Mn Fe О 8.4313 66.1 315 5.3 0.6 2.4 4 8 0.4 Mn Fe О Amorphous 320 15.4 0.4 2.6 4 9 0.2 Mn Fe О 8.4075 66.3 580 6.2 0.2 2.8 4 10 0 Fe О 8.3750 72.3 520 0 8.397 3 4 in which all cations are trivalent. Since hematite and position of the most intensive band is varied with the −1 hausmannite have similar structure, all XRD patterns x changing. Its most shift, from 715 cm in the −1 have similar character. Fe O spectrum (x =0.0)upto688 cm , occurs for 3 4 According to TEM results, all samples synthesized thesamplewith x = 0.8. The broadening of this band using CNP method are composed of particle with with the x increase is also observed (Fig. 8). −1 regular faceted shape, with size ranging from 50 to Moreover, a new band at 445 cm is confidently de- 100 nm (Fig. 7). The product is monodisperse with tected in the spectra of samples with x = 0.8 and 0.9. average particle size 70–80 nm. The observed faceted In addition to these features, we should mention a particles are polycrystalline. Data acquired using SEM significant spectral redistribution in the x =0.4 −1 confirm that large ferrite particles are composed of very spectrum, as a result of the raising of 1039 cm re- −1 small primal particles and their size is not in agreement flection band relatively to the band at 715 cm in with values calculated using crystallite size (Table 1). the x =0.0 spectrum. It is known from literature sources that in IR pat- In accordance with crystallographic data, metal (Mn, terns of γ-Fe O and Fe O , there are two main Fe) ions may occupy positions with tetrahedral and 2 3 3 4 groups of characteristic lines allowing to judge intri- octahedral oxygen neighboring . The most probable cate structural differences. These are lines related to positions for manganese ions at concentration of x < 1.3 2+ vibrations of М–О and М–О–Н bonds. Introduction are the tetrahedral positions corresponding to the Mn of different metal ions into iron oxide, causing charge state. The appearance of the octahedral- symmetry distortion of coordination environment of coordinated manganese ions with the same charge state 3+ Fe or changes in Fe–O bond constant, can lead to is detected for the values of x within a 0.8–1.2 range. 3+ the splitting or shifting of characteristic lines of Fe–O The filling of octahedral positions with Mn ions starts bond vibrations. In case of homogeneous distribution at x = 1.0, and the part of them at x = 1.3 is no more of ion of different nature in crystal lattice of spinel than 23% from total quantity of manganese ions . structure, we can usually observe only shift of This is the reason to explain the changes observed in absorption line’s maxima of characteristic oscillations. the x = 0.8 spectrum by starting the filling of octahedral 2+ Figure 8 shows IR spectra of the studied samples. The positions with Mn ions. −1 −1 spectral distribution over 1200 cm is quite independ- The raising of the 1039 cm band in the x = 0.4 ent of the sample composition (Fig. 8). spectrum may be related to the structural variations in Reflection in this region is caused by the presence of water the metal (Mn, Fe) ions neighboring, that results in di- that is adsorbed on the surface of Fe Mn O micro- pole momentum changing. 3 − x x 4 granules or embedded into their crystal lattice. The bands More detailed analysis is, unfortunately, complicated −1 within a 1200–1700 cm range are related to the bending by essential overlapping of broadened bands that is −1 H–O–H vibrations, and those within a 2400–3700 cm typical for solid solutions containing tetrahedral and range are due to the stretching vibrations of O–H bonds. octahedral complexes with central atoms whose masses The most sensitivity of reflection spectra to the are close to each other. composition changes takes place within a 400– −1 1200 cm range, typical to the stretching vibrations Conclusions −1 of Fe(Mn)–O (up to 700 cm ), Fe(Mn)–OH and In thepresent work,wehavefound a new route for the −1 Fe(Mn)–OH bonds (over 700 cm ). The spectral synthesis of ultrafine manganese ferrite of type 2 Frolova and Derhachov Nanoscale Research Letters (2017) 12:505 Page 7 of 9 Fig. 6 Derivatography patterns of samples synthesized at рН = 12.5 2+ Mn Fe О in a wide Mn substitution range from x by pH = 11.5 occurs by the mechanism of the formation of Х 3 − X 4 coprecipitation with CNP treatment. Coprecipitation maghemite. High magnetic properties exhibited nanodis- followed by CNP treatment is an effective method for the persed ferrite obtained at pH = 12.5, x =0.6–0.8. The preparation of manganese ferrite powder. The magnetic average crystallite size ranged from 50 to 80 A. The nano- properties of Mn Fe О samples were increased with dispersed ferrites had a faceted shape and uniform particles. Х 3 − X 4 increasing pH values. Ferritization process was effective XRD pattern indicates the single spinel phase nanocrystals only at pH = 12.5. The formation of compounds at with cubic spinel structure at 0 < x <0.8. Frolova and Derhachov Nanoscale Research Letters (2017) 12:505 Page 8 of 9 Fig. 7 TEM image (a) and SEM image (b) of sample no. 4 set 2 Fig. 8 IR reflection spectra of samples with synthesized рН = 12.5 at different cation ratio Frolova and Derhachov Nanoscale Research Letters (2017) 12:505 Page 9 of 9 FTIR spectroscopy confirmed the results of magnetic 13. Reddy P, Zhou X, Yann A, Sa D, Huang Q et al (2015) Low temperature hydrothermal synthesis, structural investigation and functional properties of measurements. The decrease in the value of magnetic Co Mn Fe O (0≤x≤1.0) nanoferrites. Superlattice Microst 81:233–242 x 1−x 2 4 saturation beginning with x = 1.0 is due to the filling of 14. Misra RDK, Gubbala S, Kale A, Egelhoff WF (2004) A comparison of the 2+ octahedral positions with Mn ions. magnetic characteristics of nanocrystalline nickel, zinc, and manganese ferrites synthesized by reverse micelle technique. Mat Sci Eng B-Solid 111:164–174 15. 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Nanoscale Research Letters – Springer Journals
Published: Aug 23, 2017
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