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Effect of the thermal treatment on the magnetic and structural properties of cobalt ferrite particles
Author’s Accepted Manuscript Effect of the thermal treatment on the magnetic and structural properties of cobalt ferrite particles J. Venturini, D.H. Piva, J.B.M. da Cunha, C.P. Bergmann www.elsevier.com/locate/ceri
PII: DOI: Reference:
S0272-8842(16)30963-4 http://dx.doi.org/10.1016/j.ceramint.2016.06.120 CERI13136
To appear in: Ceramics International Received date: 19 February 2016 Revised date: 24 February 2016 Accepted date: 19 June 2016 Cite this article as: J. Venturini, D.H. Piva, J.B.M. da Cunha and C.P. Bergmann, Effect of the thermal treatment on the magnetic and structural properties of cobalt ferrite particles, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2016.06.120 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Effect of the thermal treatment on the magnetic and structural properties of cobalt ferrite particles J. Venturinia,*, D.H. Pivaa, J.B.M. da Cunhab, C.P. Bergmanna a
Laboratory of Ceramic Materials, Federal University of Rio Grande do Sul, Brazil
b
Institute of Physics, Federal University of Rio Grande do Sul, Brazil
*
corresponding author: [email protected], +55 48 3308 3637 Abstract We herein report a study on the sol-gel synthesis of CoFe2O4 and the effect of
thermal treatment on the product outcome. Xerogels treated at 750, 800 and 850 °C had their structural and magnetic properties thoroughly studied, in order to correlate their synthesis conditions to the positions in which the cations are inserted in the spinel structure. X-ray diffractograms exhibit reflections representative of the spinel structure and demonstrate that the thermal treatment does not affect the lattice parameters of the material. Mossbauer spectroscopy studies indicate a very low inversion degree in the synthesized spinels, which is very unusual for CoFe2O4. A maximum in coercivity of 1405.2 Oersted was achieved for the sample treated at 800 °C. Keywords: sol-gel processing; magnetic properties; ferrites; spinels. 1. Introduction Insulating magnetic materials are in ever increasing demand due to their ability to suppress certain losses, most notably the hysteresis loss[1]. Spinel ferrites (MFe2O4) have hereof been in the spotlight of many research groups for their applicability in modern electric devices,[2] such as sensors[3], memories[4] and in the medical field.[5] Owing to their high coercivity, moderate saturation magnetization,[6] mechanical hardness and excellent chemical stability,[7, 8] these materials are nowadays highly popular for applications in magnetic and magneto-optical devices. Cobalt ferrite (CoFe2O4) is a member of the class of spinel ferrites which has been widely used for 1
information storage applications due to its highly desirable characteristics.[9] Its large magnetocrystalline anisotropy and facile synthesis, coupled to the aforementioned features of ferrites, make this material an ideal candidate for cutting-edge applications in industry.[10] Under ambient conditions, most ferrites crystallize in the spinel space group (
̅ , n°227). This family of compounds presents the formulation A2+B3+2O2-4,
with the anions arranged in an fcc structure, whilst the divalent and trivalent cations occupy 1/8 of the tetrahedral and half of the octahedral sites, respectively.[11] In inverse spinels, the interstitial occupancy is inverted, with the trivalent cation filling the available tetrahedral sites, while the octahedral interstices are populated by the divalent and remaining trivalent ions.[12] This inversion process is usually explained in terms of crystal field stabilization energies (CSFE).[13] The interplay between the distinct preferences of each cation for tetrahedral or octahedral arrangements is one of the factors governing the final configuration of ferrite crystals. Cobalt ferrite, however, displays a mixed structure, with an intermediate character between normal and inverse spinel, albeit with a clear tendency towards the inverse structure; i.e. in the structure Co1-xFex(CoxFe2-x)O4, where x is the inversion degree which ranges from 0 to 1, the variable tends to values close to unity.[14] This degree of mixing is not fixed and can be steered towards each end of the spectrum by the conditions utilized during synthesis. Configurational entropy reaches a maximum with a A1/3B2/3(A2/3B4/3)O4, the most random possible distribution.[15] Nevertheless, kinetic contributions have been demonstrated to influence the final structure more profoundly. Different cooling rates after thermal treatment have previously shown to have a dramatic role in the degree of inversion displayed by CoFe2O4.[16] Given the ferrimagnetic behavior of cobalt ferrite, its net magnetism is strongly influenced by the positioning of its cations. Therefore, the cationic arrangement inside the lattice plays a pivotal role in the determination of the
2
macroscopic magnetism displayed by CoFe2O4.[17] Cobalt ferrite can be synthesized via a wide range of methods. Co-precipitation[18], polyol[19] and solid state methods[20] have already demonstrated to be viable pathways for the production of this material. Nonetheless, one of the most versatile methods for ferrite production is the sol-gel method. This procedure allows for a great control of the various synthetic parameters, which thus translates into a more direct manipulation of the desired product.[21, 22] This work aimed to synthesize CoFe2O4 nanoparticles via the sol-gel method and to analyze the effects of the synthesis parameters on the outcome of the process. Furthermore, we intended to thoroughly characterize the thus-obtained particles, in order to correlate the synthesis parameters to the occupancy of the lattice sites and ultimately to the magnetic behavior of this material. 2. Material and Methods Cobalt ferrite with nominal stoichiometry CoFe2O4 was prepared by a sol-gel method using citric acid as a complexant. Analytical grade Co(NO3)2.6H2O, Fe(NO3)3.9H2O and citric acid in a molar ratio of 1:2:3 were dissolved in 20 mL of deionized water for the production of 10 mmol of product. The solution was heated to 85 ºC for 1.5 hours under magnetic stirring until a dark red gel was formed. The gel was dried at 110 ºC for 12 hours, transferred to an alumina crucible and further treated at 750, 800 or 850 °C for 4 hours. The as-obtained black powder was then subjected to characterization. The crystal structure of the samples was characterized by an X-ray diffractometer (XRD, Philips, X’pert MPD) equipped with a Cu-K source. SEM studies were performed in a JEOL JSM 6060 scanning electron microscope operating at 15kV. The nanoparticles were previously sputtered with Au in order to enhance their electrical conductivity. For the assessment of their surface area, the samples underwent
3
a desorption pre-treatment of 24 h at 110 °C in a muffle oven and further 2 h at 300 °C under reduced pressure. The measurements were performed in a Nova 1000e device (Quantachrome) using N2 as adsorbate at a temperature of -195.8 °C. BET theory was applied to the isotherms for the retrieval of surface data. Raman studies were performed in an inVia Renishaw Raman spectrometer equipped with a 532 nm laser. Magnetic hysteresis curves were achieved in a vibrating sample magnetometer (Model EZ9, Microsense). Hyperfine studies were performed through 57Fe Mössbauer spectroscopy in transmission mode. Measurements were carried out at room temperature using a 57Co source and the spectra were adjusted based on the discrete Gaussian line for each hyperfine site. 3. Results and Discussion The X-ray diffraction studies of the three samples show that the synthesized powders present the spinel phase as major component (JCPDS 22-1086). The diffractograms of the synthesized ferrites are displayed in Figure 3.1, with the corresponding reflections indexed to the
̅
space group (227). The samples treated
at 750, 800 and 850 ºC present lattice parameters a of 8.3765(7), 8.3709(6) and 8.3760(9), respectively. These values are compatible with standards for the unit cell of cobalt ferrite and rather similar among themselves, indicating that the different thermal treatments applied on the xerogel do not strongly affect the crystalline structure of the products. Nonetheless, the sample that underwent thermal treatment at the intermediate temperature shows a slight deviation towards a more compact unit cell. A second constituent was also detected in the diffractograms. This minor phase was indexed to hematite (-Fe2O3, ̅ ). The sample treated at 800 ºC presents the highest ratio of ferrite/hematite peaks, indicating either a larger fraction of the secondary phase or a lower crystalline character of the major phase. The Scherrer
4
equation applied to the 311 reflection yields a lower limit for crystallite size of 42.1, 39.5 and 42.3 nanometers for the samples treated at 750, 800 and 850 ºC. The results of nitrogen adsorption isotherms are displayed in Table 3.1, along with an approximate aggregate diameter, considering these to be spherical in shape. BET theory yields 4.884, 3.409 and 4.226 m2.g-1 for the samples treated at 750, 800 and 850 ºC. The specific areas are rather low, as expected of a magnetic material, where aggregation cannot be avoided in the absence of separation processes. Nevertheless, the sample treated at the intermediate temperature presents an anomalous behavior. Increasing temperature usually works towards diminishing the available surface due to increased atomic diffusion. This decrease, however, did not occur when increasing the treatment temperature from 800 to 850 ºC. SEM images of the sample treated at 750 ºC are exhibited in Figure 3.2. Its morphology is mostly composed of heterogeneous, highly-agglomerated flattened grains. The extent of aggregation among particles explains the low specific area obtained from the N2 isotherms. Increasing the thermal treatment to 800 ºC does not significantly affect the morphology of the ferrite, as can be seen in Figure 3.3. The SEM images are quite similar to those previously shown. The particles are visibly larger, though, which explains the decrease in specific area observed through the adsorption isotherms. The thermal treatment at 850 ºC for 4 hours brings on marked modifications to the overall configuration of the ferrite particles. While the morphology of some areas of the sample remains agglomerated and heterogeneous, some analyzed regions exhibit spherical, detached particles. The latter morphology can be explained in terms of the Ostwald ripening.[23] In this process, larger particles effectively absorb the smaller ones, decreasing the specific area of the material and therefore its net surface energy. A
5
higher treatment temperature increases the atomic mobility inside the material, which precipitates the diffusion of atoms into the large grains, i.e. the latter grow at the expense of the smaller particles. The homogeneity observed at the right of Figure 3.4 is also a natural consequence of this process. The subsequent cooling of the material might have deterred the steric rearrangement of the particles, thus rendering the material more porous. The presence of these less aggregated regions explains the larger surface area observed in adsorption essays for this temperature. Although an increase in temperature usually induces crystal growth, the latter is countered by the creation of areas where the particles are more separated from each other and, therefore, have a larger surface available for nitrogen adsorption. The synthesized ferrites were also subjected to Raman spectroscopy. The spectra can be seen in Figure 3.5. All samples present very similar Raman profiles, which further confirm the successful synthesis of cobalt ferrite. CoFe2O4 displays five Raman-active vibrational modes (A1g, Eg and 3 T2g). The peaks around 109 and 306 cm-1 are attributed to T2g and Eg modes, respectively. The peak centered on 459 cm-1 is related to the vibration of Fe3+ and O2- in an octahedral site. The A1g doublet around 624 and 680 cm-1, where only one signal would be expected for compounds with higher symmetry, arises due to symmetry reduction operations present in CoFe2O4.[24] The magnetic behavior of the samples has also been analyzed. Once more, the sample treated at the intermediate temperature does not behave linearly when compared to the other ferrites. The data obtained from the magnetic hysteresis curves is displayed on Table 3.2.
6
The sample treated at 750 ºC exhibits a coercivity of 1056.0 Oersted, which rises to 1405.2 for the sample treated at 800 ºC and decreases to 1147.9 with a further increase of 50 ºC. This extra difficulty to remove the alignment of the magnetic domains may be related to a lower crystallinity degree. A smaller crystallite size, as indicated previously by the application of the Scherrer equation, also leads to higher coercivities. Literature data indicates that the coercivity maximum of CoFe2O4 is found at a particle diameter of approximately 40 nanometers; little deviation around this value causes large differences in coercivity.[25] The opposite behavior is observed for the remanence of the ferrites. When treated at 750 ºC, CoFe2O4 exhibits remanence (MR) and saturation magnetization (MS) values of 24.07 and 70.90 A.m2.kg-1, respectively. At 800 ºC these values drop to 16.36 and 45.00 A.m2.kg-1. Firing at 850 ºC reverses this decrease tendency, with MR and MS equaling 21.67 and 60.16 A.m2.kg-1, respectively. The inverse relation between coercivity and remanence values is commonly reported for CoFe2O4.[26] The higher Squareness value, a merit figure commonly used for magnetic materials, is exhibited by the sample treated at the intermediate temperature, whose hysteresis curve more closely resembles a square. In order to analyze the site occupancy in the structure of the prepared samples, the latter were subjected to Mossbauer spectroscopy studies. The spectra are displayed in Figures 3.6-8. The curves were fitted using a model with four different iron species. A sextet from hematite can be observed at a hyperfine field of 51.8 Tesla. The octahedral and tetrahedral positions of iron inside the spinel lattice produce signals at 49.1 and 46.5 T, respectively. A minor doublet contribution arising from the presence of
7
superparamagnetic iron can also be observed. The site occupancy for each of the four different iron environments is displayed in Table 3. The proportion of hematitic iron is larger than would be expected solely from analyzing the X-ray diffractograms displayed previously. A fraction of 14 wt.% of the sample treated at 750 ºC is composed of hematite. This ratio increases to 17 and 19 wt.% for the samples treated at 800 and 750 ºC, respectively. Since the reagents were added stoichiometrically, the presence of hematite incurs in two possible scenarios for the chemical composition of the product. The excess cobalt might have been oxidized to one of its oxides. However, no other reflection has been identified in XRD which might points towards the presence of other crystalline materials, neither does it present a background which might indicate the presence of these cobalt oxides in an amorphous structure. On the other hand, a non-stoichiometric ferrite may have been formed. The material could have crystallized in a defect-rich manner, with vacancies on iron positions. The decrease in the iron fraction present in superparamagnetic particles with increasing temperature is seen as a natural consequence of the increasing temperature. Superparamagnetic particles, with sizes on the order of a few nanometers, tend to disappear with an increase in treatment temperature, another consequence of the Ostwald process. The ferrites display unusually low inversion degrees. The samples treated at 750, 800 and 850 ºC exhibit x values of 0.40, 0.44 and 0.25, respectively. All synthesis pathways led to the formation of a normal spinel structure instead of an inverse spinel, as expected for bulk CoFe2O4. Cobalt ferrites with such iron distribution ratios are usually related to substoichiometric cobalt ferrites, with iron vacancies located primarily in octahedral
8
positions. Such structure, CoFe2-yO, explains the formation of hematite along the absence of an oxidized cobalt compound. Considering that cobalt is present solely in the spinel structure, the obtained material is roughly equivalent to CoFe1.5O. The recalculated inversion degrees for the latter structure equal 0.35, 0.39 and 0.23 for the ferrites treated at 750, 800 and 850 ºC, respectively. The magnetic behavior of the sol-gel ferrites can now be related to the cationic occupancy density among tetrahedral and octahedral interstices. Since the magnetic properties of hematite are negligible when compared to cobalt ferrite, we can safely assume that the magnetic behavior of the samples derives from the latter. The larger coercivity displayed by the sample treated at 800 ºC can be interpreted as a macroscopic expression of its larger inversion degree. The Co2+ ion presents a much more pronounced anisotropy when located in octahedral sites, which results in an overall increase of the coercivity of the sample. On the other hand, a higher density of cobalt in tetrahedral positions leads to the loss in coercivity observed for the other samples. The remanence of the samples followed an inverse tendency, as expected of cobalt iron oxide. The material treated at 800 ºC suggests a lower capacity to retain its magnetism in the absence of an applied field. Despite the lower inversion degrees of the sample treated at a higher temperature, a larger mass fraction of hematite might be contributing towards a larger net loss of magnetization, since this value is defined in terms of the mass of the magnetic material.
4. Conclusions The sol-gel method using nitrates as metal sources was successfully applied in the synthesis of CoFe2O4. However, the appearance of hematite as a secondary phase was
9
observed for all utilized thermal conditions. The ferrites synthesized via this method present a rather low inversion degree, unlike the normal behavior of the bulk material. Different thermal treatments affected sensibly the morphological, structural and magnetic properties of the prepared ferrite. A maximum in coercivity of 1405.2 Oersted was achieved for the sample treated at 800 ºC; the largest remanence (24.07 emu.g-1) was obtained when exposing the xerogel to 750 °C.
5. Acknowledgments The authors would like to thank the Coordination of Improvement of Higher Level Personnel (Capes) and the Support Program for Excellence Centers of the Foundation for Research Support of the State of Rio Grande do Sul (PRONEX/FAPERGS) for their financial support and the Electron Microscopy Center of the Federal University of Rio Grande do Sul for their technical support. 6. References [1] R.K. Panda, R. Muduli, S.K. Kar, D. Behera, Dielectric relaxation and conduction mechanism of cobalt ferrite nanoparticles, Journal of Alloys and Compounds, 615 (2014) 899905. [2] A. Rafferty, T. Prescott, D. Brabazon, Sintering behaviour of cobalt ferrite ceramic, Ceramics International, 34 (2008) 15-21. [3] R.S. Gaikwad, Cobalt Ferrite Nanocrystallites for Sustainable Hydrogen Production Application, International Journal of Electrochemistry, 2011 (2011) 729141. [4] X. Gao, L. Liu, B. Birajdar, M. Ziese, W. Lee, M. Alexe, D. Hesse, High-Density Periodically Ordered Magnetic Cobalt Ferrite Nanodot Arrays by Template-Assisted Pulsed Laser Deposition, Advanced Functional Materials, 19 (2009) 3450-3455. [5] S. Amiri, H. Shokrollahi, The role of cobalt ferrite magnetic nanoparticles in medical science, Materials Science and Engineering: C, 33 (2013) 1-8.
10
[6] Y. Fu, H. Chen, X. Sun, X. Wang, Combination of cobalt ferrite and graphene: Highperformance and recyclable visible-light photocatalysis, Applied Catalysis B: Environmental, 111–112 (2012) 280-287. [7] S.H. Xiao, W.F. Jiang, L.Y. Li, X.J. Li, Low-temperature auto-combustion synthesis and magnetic properties of cobalt ferrite nanopowder, Materials Chemistry and Physics, 106 (2007) 82-87. [8] Y. Köseoğlu, F. Alan, M. Tan, R. Yilgin, M. Öztürk, Low temperature hydrothermal synthesis and characterization of Mn doped cobalt ferrite nanoparticles, Ceramics International, 38 (2012) 3625-3634. [9] F. Tourinho, R. Franck, R. Massart, R. Perzynski, Synthesis and mangeitc properties of managanese and cobalt ferrite ferrite ferrofluids, in: P. Bothorel, E.J. Dufourc (Eds.) Trends in Colloid and Interface Science III, Steinkopff1989, pp. 128-134. [10] V. Rusanov, V. Gushterov, S. Nikolov, A.X. Trautwein, Detailed Mössbauer study of the cation distribution in CoFe2O4 ferrites, in: E. Kuzmann, K. Lázár (Eds.) ISIAME 2008, Springer Berlin Heidelberg2009, pp. 397-404. [11] K.E. Sickafus, J.M. Wills, N.W. Grimes, Structure of Spinel, Journal of the American Ceramic Society, 82 (1999) 3279-3292. [12] G. Hu, J.H. Choi, C.B. Eom, V.G. Harris, Y. Suzuki, Structural tuning of the magnetic behavior in spinel-structure ferrite thin films, Physical Review B, 62 (2000) R779-R782. [13] J.K. Burdett, G.D. Price, S.L. Price, Role of the crystal-field theory in determining the structures of spinels, Journal of the American Chemical Society, 104 (1982) 92-95. [14] H.P. Rooksby, B.T.M. Willis, Crystal Structure and Magnetic Properties of Cobalt Ferrite at Low Temperatures, Nature, 172 (1953) 1054-1055. [15] L. Kumar, P. Kumar, A. Narayan, M. Kar, Rietveld analysis of XRD patterns of different sizes of nanocrystalline cobalt ferrite, Int Nano Lett, 3 (2013) 1-12. [16] M.R. De Guire, R.C. O’Handley, G. Kalonji, The cooling rate dependence of cation distributions in CoFe2O4, Journal of Applied Physics, 65 (1989) 3167-3172.
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[17] P.W. Anderson, Ordering and Antiferromagnetism in Ferrites, Physical Review, 102 (1956) 1008-1013. [18] X. Yang, X. Wang, Z. Zhang, Electrochemical properties of submicron cobalt ferrite spinel through a co-precipitation method, Journal of Crystal Growth, 277 (2005) 467-470. [19] L. Zhou, L. Ji, P.-C. Ma, Y. Shao, H. Zhang, W. Gao, Y. Li, Development of carbon nanotubes/CoFe2O4 magnetic hybrid material for removal of tetrabromobisphenol A and Pb(II), Journal of Hazardous Materials, 265 (2014) 104-114. [20] C.O. Augustin, Srinivasan, L.K., Kamaraj, P; and Mani, A., J. Mater. Sci. Technol., 12 (1996). [21] G. Cao, Nanostructures and Nanomaterials: Synthesis, Properties and Applications, Imperial College Press2004. [22] M. Sajjia, M. Oubaha, M. Hasanuzzaman, A.G. Olabi, Developments of cobalt ferrite nanoparticles prepared by the sol–gel process, Ceramics International, 40 (2014) 1147-1154. [23] Z.X. Tang, C.M. Sorensen, K.J. Klabunde, G.C. Hadjipanayis, Preparation of manganese ferrite fine particles from aqueous solution, Journal of Colloid and Interface Science, 146 (1991) 38-52. [24] S. Ayyappan, J. Philip, B. Raj, Effect of Digestion Time on Size and Magnetic Properties of Spinel CoFe2O4 Nanoparticles, The Journal of Physical Chemistry C, 113 (2008) 590-596. [25] C.N. Chinnasamy, M. Senoue, B. Jeyadevan, O. Perales-Perez, K. Shinoda, K. Tohji, Synthesis of size-controlled cobalt ferrite particles with high coercivity and squareness ratio, Journal of Colloid and Interface Science, 263 (2003) 80-83. [26] K. Maaz, A. Mumtaz, S.K. Hasanain, A. Ceylan, Synthesis and magnetic properties of cobalt ferrite (CoFe2O4) nanoparticles prepared by wet chemical route, Journal of Magnetism and Magnetic Materials, 308 (2007) 289-295.
Figure 3.1. Diffractograms of the samples treated at 750 (black), 800 (red) and 850 ºC (blue line). All datasets were normalized to their most intense reflection (311). A secondary phase is indicated by an asterisk (*).
12
Figure 3.2. SEM images of the CoFe2O4 sample treated at 750 ºC at different magnifications. Figure 3.3. SEM images of the CoFe2O4 sample treated at 800 ºC at different magnifications. Figure 3.4. SEM images of the CoFe2O4 sample treated at 850 ºC at different magnifications. To the left, an agglomerated region. To the right, an area where the particles display a higher degree of mutual separation. Figure 3.5. Raman spectra of the samples treated at 750 (black), 800 (red) and 850 °C (blue line). exc = 532 nm. Figure 3.6.
57
Fe Mössbauer spectrum of the sample treated at 750°C. Lines indicate the
results of the fitting procedure. Figure 3.7.
57
Fe Mössbauer spectrum of the sample treated at 800°C. Lines indicate the
results of the fitting procedure. Figure 3.8.
57
Fe Mössbauer spectrum of the sample treated at 850°C. Lines indicate the
results of the fitting procedure.
Table 3.1. Specific area and average aggregate size of the cobalt ferrite samples. Temperature (ºC)
Specific area (m2.g-1)
Average aggregate size (nm)
750
4.884
231
800
3.409
332
850
4.226
265
13
Table 3.2. Magnetic properties of the synthesized ferrites. Tcalc (ºC)
Coercivity (Oe)
Remanence
Saturation
(A.m2.kg-1)
magnetization
Squareness
(A.m2.kg-1) 750
1056.0
24.07
70.90
0.339
800
1405.2
16.36
45.00
0.381
850
1147.9
21.67
60.16
0.360
Table 3.3. Site occupancy for the different iron environments (FeOh – octahedral site, FeTd – tetrahedral site, Fe/Fe2O3 – iron in hematite, Fe SP – iron in superparamagnetic particles). Tcalc (°C)
FeOh
FeTd
Fe/Fe2O3
Fe SP
FeOh/FeTd
750
0.6202
0.1525
0.1924
0.0348
4.06
800
0.5834
0.1640
0.2273
0.0253
3.56
850
0.6337
0.0909
0.2551
0.0202
6.97
14
15
16
17
18
PII: DOI: Reference:
S0272-8842(16)30963-4 http://dx.doi.org/10.1016/j.ceramint.2016.06.120 CERI13136
To appear in: Ceramics International Received date: 19 February 2016 Revised date: 24 February 2016 Accepted date: 19 June 2016 Cite this article as: J. Venturini, D.H. Piva, J.B.M. da Cunha and C.P. Bergmann, Effect of the thermal treatment on the magnetic and structural properties of cobalt ferrite particles, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2016.06.120 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Effect of the thermal treatment on the magnetic and structural properties of cobalt ferrite particles J. Venturinia,*, D.H. Pivaa, J.B.M. da Cunhab, C.P. Bergmanna a
Laboratory of Ceramic Materials, Federal University of Rio Grande do Sul, Brazil
b
Institute of Physics, Federal University of Rio Grande do Sul, Brazil
*
corresponding author: [email protected], +55 48 3308 3637 Abstract We herein report a study on the sol-gel synthesis of CoFe2O4 and the effect of
thermal treatment on the product outcome. Xerogels treated at 750, 800 and 850 °C had their structural and magnetic properties thoroughly studied, in order to correlate their synthesis conditions to the positions in which the cations are inserted in the spinel structure. X-ray diffractograms exhibit reflections representative of the spinel structure and demonstrate that the thermal treatment does not affect the lattice parameters of the material. Mossbauer spectroscopy studies indicate a very low inversion degree in the synthesized spinels, which is very unusual for CoFe2O4. A maximum in coercivity of 1405.2 Oersted was achieved for the sample treated at 800 °C. Keywords: sol-gel processing; magnetic properties; ferrites; spinels. 1. Introduction Insulating magnetic materials are in ever increasing demand due to their ability to suppress certain losses, most notably the hysteresis loss[1]. Spinel ferrites (MFe2O4) have hereof been in the spotlight of many research groups for their applicability in modern electric devices,[2] such as sensors[3], memories[4] and in the medical field.[5] Owing to their high coercivity, moderate saturation magnetization,[6] mechanical hardness and excellent chemical stability,[7, 8] these materials are nowadays highly popular for applications in magnetic and magneto-optical devices. Cobalt ferrite (CoFe2O4) is a member of the class of spinel ferrites which has been widely used for 1
information storage applications due to its highly desirable characteristics.[9] Its large magnetocrystalline anisotropy and facile synthesis, coupled to the aforementioned features of ferrites, make this material an ideal candidate for cutting-edge applications in industry.[10] Under ambient conditions, most ferrites crystallize in the spinel space group (
̅ , n°227). This family of compounds presents the formulation A2+B3+2O2-4,
with the anions arranged in an fcc structure, whilst the divalent and trivalent cations occupy 1/8 of the tetrahedral and half of the octahedral sites, respectively.[11] In inverse spinels, the interstitial occupancy is inverted, with the trivalent cation filling the available tetrahedral sites, while the octahedral interstices are populated by the divalent and remaining trivalent ions.[12] This inversion process is usually explained in terms of crystal field stabilization energies (CSFE).[13] The interplay between the distinct preferences of each cation for tetrahedral or octahedral arrangements is one of the factors governing the final configuration of ferrite crystals. Cobalt ferrite, however, displays a mixed structure, with an intermediate character between normal and inverse spinel, albeit with a clear tendency towards the inverse structure; i.e. in the structure Co1-xFex(CoxFe2-x)O4, where x is the inversion degree which ranges from 0 to 1, the variable tends to values close to unity.[14] This degree of mixing is not fixed and can be steered towards each end of the spectrum by the conditions utilized during synthesis. Configurational entropy reaches a maximum with a A1/3B2/3(A2/3B4/3)O4, the most random possible distribution.[15] Nevertheless, kinetic contributions have been demonstrated to influence the final structure more profoundly. Different cooling rates after thermal treatment have previously shown to have a dramatic role in the degree of inversion displayed by CoFe2O4.[16] Given the ferrimagnetic behavior of cobalt ferrite, its net magnetism is strongly influenced by the positioning of its cations. Therefore, the cationic arrangement inside the lattice plays a pivotal role in the determination of the
2
macroscopic magnetism displayed by CoFe2O4.[17] Cobalt ferrite can be synthesized via a wide range of methods. Co-precipitation[18], polyol[19] and solid state methods[20] have already demonstrated to be viable pathways for the production of this material. Nonetheless, one of the most versatile methods for ferrite production is the sol-gel method. This procedure allows for a great control of the various synthetic parameters, which thus translates into a more direct manipulation of the desired product.[21, 22] This work aimed to synthesize CoFe2O4 nanoparticles via the sol-gel method and to analyze the effects of the synthesis parameters on the outcome of the process. Furthermore, we intended to thoroughly characterize the thus-obtained particles, in order to correlate the synthesis parameters to the occupancy of the lattice sites and ultimately to the magnetic behavior of this material. 2. Material and Methods Cobalt ferrite with nominal stoichiometry CoFe2O4 was prepared by a sol-gel method using citric acid as a complexant. Analytical grade Co(NO3)2.6H2O, Fe(NO3)3.9H2O and citric acid in a molar ratio of 1:2:3 were dissolved in 20 mL of deionized water for the production of 10 mmol of product. The solution was heated to 85 ºC for 1.5 hours under magnetic stirring until a dark red gel was formed. The gel was dried at 110 ºC for 12 hours, transferred to an alumina crucible and further treated at 750, 800 or 850 °C for 4 hours. The as-obtained black powder was then subjected to characterization. The crystal structure of the samples was characterized by an X-ray diffractometer (XRD, Philips, X’pert MPD) equipped with a Cu-K source. SEM studies were performed in a JEOL JSM 6060 scanning electron microscope operating at 15kV. The nanoparticles were previously sputtered with Au in order to enhance their electrical conductivity. For the assessment of their surface area, the samples underwent
3
a desorption pre-treatment of 24 h at 110 °C in a muffle oven and further 2 h at 300 °C under reduced pressure. The measurements were performed in a Nova 1000e device (Quantachrome) using N2 as adsorbate at a temperature of -195.8 °C. BET theory was applied to the isotherms for the retrieval of surface data. Raman studies were performed in an inVia Renishaw Raman spectrometer equipped with a 532 nm laser. Magnetic hysteresis curves were achieved in a vibrating sample magnetometer (Model EZ9, Microsense). Hyperfine studies were performed through 57Fe Mössbauer spectroscopy in transmission mode. Measurements were carried out at room temperature using a 57Co source and the spectra were adjusted based on the discrete Gaussian line for each hyperfine site. 3. Results and Discussion The X-ray diffraction studies of the three samples show that the synthesized powders present the spinel phase as major component (JCPDS 22-1086). The diffractograms of the synthesized ferrites are displayed in Figure 3.1, with the corresponding reflections indexed to the
̅
space group (227). The samples treated
at 750, 800 and 850 ºC present lattice parameters a of 8.3765(7), 8.3709(6) and 8.3760(9), respectively. These values are compatible with standards for the unit cell of cobalt ferrite and rather similar among themselves, indicating that the different thermal treatments applied on the xerogel do not strongly affect the crystalline structure of the products. Nonetheless, the sample that underwent thermal treatment at the intermediate temperature shows a slight deviation towards a more compact unit cell. A second constituent was also detected in the diffractograms. This minor phase was indexed to hematite (-Fe2O3, ̅ ). The sample treated at 800 ºC presents the highest ratio of ferrite/hematite peaks, indicating either a larger fraction of the secondary phase or a lower crystalline character of the major phase. The Scherrer
4
equation applied to the 311 reflection yields a lower limit for crystallite size of 42.1, 39.5 and 42.3 nanometers for the samples treated at 750, 800 and 850 ºC. The results of nitrogen adsorption isotherms are displayed in Table 3.1, along with an approximate aggregate diameter, considering these to be spherical in shape. BET theory yields 4.884, 3.409 and 4.226 m2.g-1 for the samples treated at 750, 800 and 850 ºC. The specific areas are rather low, as expected of a magnetic material, where aggregation cannot be avoided in the absence of separation processes. Nevertheless, the sample treated at the intermediate temperature presents an anomalous behavior. Increasing temperature usually works towards diminishing the available surface due to increased atomic diffusion. This decrease, however, did not occur when increasing the treatment temperature from 800 to 850 ºC. SEM images of the sample treated at 750 ºC are exhibited in Figure 3.2. Its morphology is mostly composed of heterogeneous, highly-agglomerated flattened grains. The extent of aggregation among particles explains the low specific area obtained from the N2 isotherms. Increasing the thermal treatment to 800 ºC does not significantly affect the morphology of the ferrite, as can be seen in Figure 3.3. The SEM images are quite similar to those previously shown. The particles are visibly larger, though, which explains the decrease in specific area observed through the adsorption isotherms. The thermal treatment at 850 ºC for 4 hours brings on marked modifications to the overall configuration of the ferrite particles. While the morphology of some areas of the sample remains agglomerated and heterogeneous, some analyzed regions exhibit spherical, detached particles. The latter morphology can be explained in terms of the Ostwald ripening.[23] In this process, larger particles effectively absorb the smaller ones, decreasing the specific area of the material and therefore its net surface energy. A
5
higher treatment temperature increases the atomic mobility inside the material, which precipitates the diffusion of atoms into the large grains, i.e. the latter grow at the expense of the smaller particles. The homogeneity observed at the right of Figure 3.4 is also a natural consequence of this process. The subsequent cooling of the material might have deterred the steric rearrangement of the particles, thus rendering the material more porous. The presence of these less aggregated regions explains the larger surface area observed in adsorption essays for this temperature. Although an increase in temperature usually induces crystal growth, the latter is countered by the creation of areas where the particles are more separated from each other and, therefore, have a larger surface available for nitrogen adsorption. The synthesized ferrites were also subjected to Raman spectroscopy. The spectra can be seen in Figure 3.5. All samples present very similar Raman profiles, which further confirm the successful synthesis of cobalt ferrite. CoFe2O4 displays five Raman-active vibrational modes (A1g, Eg and 3 T2g). The peaks around 109 and 306 cm-1 are attributed to T2g and Eg modes, respectively. The peak centered on 459 cm-1 is related to the vibration of Fe3+ and O2- in an octahedral site. The A1g doublet around 624 and 680 cm-1, where only one signal would be expected for compounds with higher symmetry, arises due to symmetry reduction operations present in CoFe2O4.[24] The magnetic behavior of the samples has also been analyzed. Once more, the sample treated at the intermediate temperature does not behave linearly when compared to the other ferrites. The data obtained from the magnetic hysteresis curves is displayed on Table 3.2.
6
The sample treated at 750 ºC exhibits a coercivity of 1056.0 Oersted, which rises to 1405.2 for the sample treated at 800 ºC and decreases to 1147.9 with a further increase of 50 ºC. This extra difficulty to remove the alignment of the magnetic domains may be related to a lower crystallinity degree. A smaller crystallite size, as indicated previously by the application of the Scherrer equation, also leads to higher coercivities. Literature data indicates that the coercivity maximum of CoFe2O4 is found at a particle diameter of approximately 40 nanometers; little deviation around this value causes large differences in coercivity.[25] The opposite behavior is observed for the remanence of the ferrites. When treated at 750 ºC, CoFe2O4 exhibits remanence (MR) and saturation magnetization (MS) values of 24.07 and 70.90 A.m2.kg-1, respectively. At 800 ºC these values drop to 16.36 and 45.00 A.m2.kg-1. Firing at 850 ºC reverses this decrease tendency, with MR and MS equaling 21.67 and 60.16 A.m2.kg-1, respectively. The inverse relation between coercivity and remanence values is commonly reported for CoFe2O4.[26] The higher Squareness value, a merit figure commonly used for magnetic materials, is exhibited by the sample treated at the intermediate temperature, whose hysteresis curve more closely resembles a square. In order to analyze the site occupancy in the structure of the prepared samples, the latter were subjected to Mossbauer spectroscopy studies. The spectra are displayed in Figures 3.6-8. The curves were fitted using a model with four different iron species. A sextet from hematite can be observed at a hyperfine field of 51.8 Tesla. The octahedral and tetrahedral positions of iron inside the spinel lattice produce signals at 49.1 and 46.5 T, respectively. A minor doublet contribution arising from the presence of
7
superparamagnetic iron can also be observed. The site occupancy for each of the four different iron environments is displayed in Table 3. The proportion of hematitic iron is larger than would be expected solely from analyzing the X-ray diffractograms displayed previously. A fraction of 14 wt.% of the sample treated at 750 ºC is composed of hematite. This ratio increases to 17 and 19 wt.% for the samples treated at 800 and 750 ºC, respectively. Since the reagents were added stoichiometrically, the presence of hematite incurs in two possible scenarios for the chemical composition of the product. The excess cobalt might have been oxidized to one of its oxides. However, no other reflection has been identified in XRD which might points towards the presence of other crystalline materials, neither does it present a background which might indicate the presence of these cobalt oxides in an amorphous structure. On the other hand, a non-stoichiometric ferrite may have been formed. The material could have crystallized in a defect-rich manner, with vacancies on iron positions. The decrease in the iron fraction present in superparamagnetic particles with increasing temperature is seen as a natural consequence of the increasing temperature. Superparamagnetic particles, with sizes on the order of a few nanometers, tend to disappear with an increase in treatment temperature, another consequence of the Ostwald process. The ferrites display unusually low inversion degrees. The samples treated at 750, 800 and 850 ºC exhibit x values of 0.40, 0.44 and 0.25, respectively. All synthesis pathways led to the formation of a normal spinel structure instead of an inverse spinel, as expected for bulk CoFe2O4. Cobalt ferrites with such iron distribution ratios are usually related to substoichiometric cobalt ferrites, with iron vacancies located primarily in octahedral
8
positions. Such structure, CoFe2-yO, explains the formation of hematite along the absence of an oxidized cobalt compound. Considering that cobalt is present solely in the spinel structure, the obtained material is roughly equivalent to CoFe1.5O. The recalculated inversion degrees for the latter structure equal 0.35, 0.39 and 0.23 for the ferrites treated at 750, 800 and 850 ºC, respectively. The magnetic behavior of the sol-gel ferrites can now be related to the cationic occupancy density among tetrahedral and octahedral interstices. Since the magnetic properties of hematite are negligible when compared to cobalt ferrite, we can safely assume that the magnetic behavior of the samples derives from the latter. The larger coercivity displayed by the sample treated at 800 ºC can be interpreted as a macroscopic expression of its larger inversion degree. The Co2+ ion presents a much more pronounced anisotropy when located in octahedral sites, which results in an overall increase of the coercivity of the sample. On the other hand, a higher density of cobalt in tetrahedral positions leads to the loss in coercivity observed for the other samples. The remanence of the samples followed an inverse tendency, as expected of cobalt iron oxide. The material treated at 800 ºC suggests a lower capacity to retain its magnetism in the absence of an applied field. Despite the lower inversion degrees of the sample treated at a higher temperature, a larger mass fraction of hematite might be contributing towards a larger net loss of magnetization, since this value is defined in terms of the mass of the magnetic material.
4. Conclusions The sol-gel method using nitrates as metal sources was successfully applied in the synthesis of CoFe2O4. However, the appearance of hematite as a secondary phase was
9
observed for all utilized thermal conditions. The ferrites synthesized via this method present a rather low inversion degree, unlike the normal behavior of the bulk material. Different thermal treatments affected sensibly the morphological, structural and magnetic properties of the prepared ferrite. A maximum in coercivity of 1405.2 Oersted was achieved for the sample treated at 800 ºC; the largest remanence (24.07 emu.g-1) was obtained when exposing the xerogel to 750 °C.
5. Acknowledgments The authors would like to thank the Coordination of Improvement of Higher Level Personnel (Capes) and the Support Program for Excellence Centers of the Foundation for Research Support of the State of Rio Grande do Sul (PRONEX/FAPERGS) for their financial support and the Electron Microscopy Center of the Federal University of Rio Grande do Sul for their technical support. 6. References [1] R.K. Panda, R. Muduli, S.K. Kar, D. Behera, Dielectric relaxation and conduction mechanism of cobalt ferrite nanoparticles, Journal of Alloys and Compounds, 615 (2014) 899905. [2] A. Rafferty, T. Prescott, D. Brabazon, Sintering behaviour of cobalt ferrite ceramic, Ceramics International, 34 (2008) 15-21. [3] R.S. Gaikwad, Cobalt Ferrite Nanocrystallites for Sustainable Hydrogen Production Application, International Journal of Electrochemistry, 2011 (2011) 729141. [4] X. Gao, L. Liu, B. Birajdar, M. Ziese, W. Lee, M. Alexe, D. Hesse, High-Density Periodically Ordered Magnetic Cobalt Ferrite Nanodot Arrays by Template-Assisted Pulsed Laser Deposition, Advanced Functional Materials, 19 (2009) 3450-3455. [5] S. Amiri, H. Shokrollahi, The role of cobalt ferrite magnetic nanoparticles in medical science, Materials Science and Engineering: C, 33 (2013) 1-8.
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[6] Y. Fu, H. Chen, X. Sun, X. Wang, Combination of cobalt ferrite and graphene: Highperformance and recyclable visible-light photocatalysis, Applied Catalysis B: Environmental, 111–112 (2012) 280-287. [7] S.H. Xiao, W.F. Jiang, L.Y. Li, X.J. Li, Low-temperature auto-combustion synthesis and magnetic properties of cobalt ferrite nanopowder, Materials Chemistry and Physics, 106 (2007) 82-87. [8] Y. Köseoğlu, F. Alan, M. Tan, R. Yilgin, M. Öztürk, Low temperature hydrothermal synthesis and characterization of Mn doped cobalt ferrite nanoparticles, Ceramics International, 38 (2012) 3625-3634. [9] F. Tourinho, R. Franck, R. Massart, R. Perzynski, Synthesis and mangeitc properties of managanese and cobalt ferrite ferrite ferrofluids, in: P. Bothorel, E.J. Dufourc (Eds.) Trends in Colloid and Interface Science III, Steinkopff1989, pp. 128-134. [10] V. Rusanov, V. Gushterov, S. Nikolov, A.X. Trautwein, Detailed Mössbauer study of the cation distribution in CoFe2O4 ferrites, in: E. Kuzmann, K. Lázár (Eds.) ISIAME 2008, Springer Berlin Heidelberg2009, pp. 397-404. [11] K.E. Sickafus, J.M. Wills, N.W. Grimes, Structure of Spinel, Journal of the American Ceramic Society, 82 (1999) 3279-3292. [12] G. Hu, J.H. Choi, C.B. Eom, V.G. Harris, Y. Suzuki, Structural tuning of the magnetic behavior in spinel-structure ferrite thin films, Physical Review B, 62 (2000) R779-R782. [13] J.K. Burdett, G.D. Price, S.L. Price, Role of the crystal-field theory in determining the structures of spinels, Journal of the American Chemical Society, 104 (1982) 92-95. [14] H.P. Rooksby, B.T.M. Willis, Crystal Structure and Magnetic Properties of Cobalt Ferrite at Low Temperatures, Nature, 172 (1953) 1054-1055. [15] L. Kumar, P. Kumar, A. Narayan, M. Kar, Rietveld analysis of XRD patterns of different sizes of nanocrystalline cobalt ferrite, Int Nano Lett, 3 (2013) 1-12. [16] M.R. De Guire, R.C. O’Handley, G. Kalonji, The cooling rate dependence of cation distributions in CoFe2O4, Journal of Applied Physics, 65 (1989) 3167-3172.
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[17] P.W. Anderson, Ordering and Antiferromagnetism in Ferrites, Physical Review, 102 (1956) 1008-1013. [18] X. Yang, X. Wang, Z. Zhang, Electrochemical properties of submicron cobalt ferrite spinel through a co-precipitation method, Journal of Crystal Growth, 277 (2005) 467-470. [19] L. Zhou, L. Ji, P.-C. Ma, Y. Shao, H. Zhang, W. Gao, Y. Li, Development of carbon nanotubes/CoFe2O4 magnetic hybrid material for removal of tetrabromobisphenol A and Pb(II), Journal of Hazardous Materials, 265 (2014) 104-114. [20] C.O. Augustin, Srinivasan, L.K., Kamaraj, P; and Mani, A., J. Mater. Sci. Technol., 12 (1996). [21] G. Cao, Nanostructures and Nanomaterials: Synthesis, Properties and Applications, Imperial College Press2004. [22] M. Sajjia, M. Oubaha, M. Hasanuzzaman, A.G. Olabi, Developments of cobalt ferrite nanoparticles prepared by the sol–gel process, Ceramics International, 40 (2014) 1147-1154. [23] Z.X. Tang, C.M. Sorensen, K.J. Klabunde, G.C. Hadjipanayis, Preparation of manganese ferrite fine particles from aqueous solution, Journal of Colloid and Interface Science, 146 (1991) 38-52. [24] S. Ayyappan, J. Philip, B. Raj, Effect of Digestion Time on Size and Magnetic Properties of Spinel CoFe2O4 Nanoparticles, The Journal of Physical Chemistry C, 113 (2008) 590-596. [25] C.N. Chinnasamy, M. Senoue, B. Jeyadevan, O. Perales-Perez, K. Shinoda, K. Tohji, Synthesis of size-controlled cobalt ferrite particles with high coercivity and squareness ratio, Journal of Colloid and Interface Science, 263 (2003) 80-83. [26] K. Maaz, A. Mumtaz, S.K. Hasanain, A. Ceylan, Synthesis and magnetic properties of cobalt ferrite (CoFe2O4) nanoparticles prepared by wet chemical route, Journal of Magnetism and Magnetic Materials, 308 (2007) 289-295.
Figure 3.1. Diffractograms of the samples treated at 750 (black), 800 (red) and 850 ºC (blue line). All datasets were normalized to their most intense reflection (311). A secondary phase is indicated by an asterisk (*).
12
Figure 3.2. SEM images of the CoFe2O4 sample treated at 750 ºC at different magnifications. Figure 3.3. SEM images of the CoFe2O4 sample treated at 800 ºC at different magnifications. Figure 3.4. SEM images of the CoFe2O4 sample treated at 850 ºC at different magnifications. To the left, an agglomerated region. To the right, an area where the particles display a higher degree of mutual separation. Figure 3.5. Raman spectra of the samples treated at 750 (black), 800 (red) and 850 °C (blue line). exc = 532 nm. Figure 3.6.
57
Fe Mössbauer spectrum of the sample treated at 750°C. Lines indicate the
results of the fitting procedure. Figure 3.7.
57
Fe Mössbauer spectrum of the sample treated at 800°C. Lines indicate the
results of the fitting procedure. Figure 3.8.
57
Fe Mössbauer spectrum of the sample treated at 850°C. Lines indicate the
results of the fitting procedure.
Table 3.1. Specific area and average aggregate size of the cobalt ferrite samples. Temperature (ºC)
Specific area (m2.g-1)
Average aggregate size (nm)
750
4.884
231
800
3.409
332
850
4.226
265
13
Table 3.2. Magnetic properties of the synthesized ferrites. Tcalc (ºC)
Coercivity (Oe)
Remanence
Saturation
(A.m2.kg-1)
magnetization
Squareness
(A.m2.kg-1) 750
1056.0
24.07
70.90
0.339
800
1405.2
16.36
45.00
0.381
850
1147.9
21.67
60.16
0.360
Table 3.3. Site occupancy for the different iron environments (FeOh – octahedral site, FeTd – tetrahedral site, Fe/Fe2O3 – iron in hematite, Fe SP – iron in superparamagnetic particles). Tcalc (°C)
FeOh
FeTd
Fe/Fe2O3
Fe SP
FeOh/FeTd
750
0.6202
0.1525
0.1924
0.0348
4.06
800
0.5834
0.1640
0.2273
0.0253
3.56
850
0.6337
0.0909
0.2551
0.0202
6.97
14
15
16
17
18