J Mater Sci (2013) 48:1543–1554 DOI 10.1007/s10853-012-6910-4
Influence of heat-treatment environment on Ni-ferrite nanoparticle formation from coconut water precursor E. P. Muniz • J. R. C. Proveti • R. D. Pereira • B. Segatto • P. S. S. Porto • V. P. Nascimento • M. A. Schettino • E. C. Passamani
Received: 6 June 2012 / Accepted: 19 September 2012 / Published online: 28 September 2012 Ó Springer Science+Business Media New York 2012
Abstract The kinetics of formation of NiFe2O4 nanoparticles were investigated where the nanoparticles were produced by the proteic-sol–gel method using coconut water followed by annealing in (i) air, (ii) air in the presence of boron nitride (BN), or (iii) nitrogen. The sample dried at 473 K for 5.5 h was composed of small disordered NiFe2O4 nanoparticles in a superparamagnetic state as determined from Mo¨ssbauer spectroscopy. In general, heat treatment at high temperature leads to a nanocomposite rich in NiFe2O4. In air, annealing at 1173 K for 8 h favored the formation of the magnetically ordered NiFe2O4 inverse spinel structure, with bulk characteristics and average crystal sizes of approximately 66 nm. In a nitrogen atmosphere and in compacted BN powder under air atmosphere, the NiFe2O4 spinel structure stabilized for temperatures up to 873 and 773 K, respectively, however, decomposition of the NiFe2O4 phase into other undesired structures was observed above 873 K.
Introduction Spinel-type ferrite oxides, with general formula AB2O4, are promising materials in technological applications and are E. P. Muniz (&) J. R. C. Proveti R. D. Pereira B. Segatto Departamento de Cieˆncias Naturais, Universidade Federal do Espı´rito Santo, Sa˜o Mateus, ES 29932-540, Brazil e-mail:
[email protected] P. S. S. Porto Departamento de Engenharias e Computac¸a˜o, Universidade Federal do Espı´rito Santo, Sa˜o Mateus, ES 29932-540, Brazil V. P. Nascimento M. A. Schettino E. C. Passamani Departamento de Fı´sica, Universidade Federal do Espı´rito Santo, Vitoria, ES 29075-910, Brazil
used widely in many electronic and magnetic devices because of their high magnetic permeability and low magnetic losses. Owing to their interesting electrical and magnetic properties, applications of ferrites are found in ferrofluids, radar absorbing coatings, waveguides in the gigahertz region, biomedical and clinical devices, repulsive suspension for levitated systems, gas sensors and magnetic cores of read/write heads for high-speed digital tapes or for disc recording [1]. Among the spinel-type ferrites, nickel ferrite (NiFe2O4) has attracted considerable attention owing to its high magneto-crystalline anisotropy, chemical stability, electrical resistivity, and mechanical hardness [2], which has made this material suitable for magnetic and magnetooptical applications. It is well known that magnetic properties of the inverse spinel-type NiFe2O4 are strongly influenced by the crystallite size. Different magnetic states (ferrimagnetic, FI; superparamagnetic, SPM; or paramagnetic, PM) can exist depending on the crystallite size, as reported in literature [3, 4]. The FI-phase particularly is observed in polycrystalline samples with grain sizes larger than or equal to 15 nm, while the SPM-state has been found in samples with grain sizes smaller than 10 nm. A PM-like state has been associated with non-crystalline samples either in the form of a crystalline anion lattice with disorder among the cationic-sites or more likely in the form of a totally disordered state (amorphous-like material). Several authors reported that besides the super-exchange interaction between metallic ions in tetra- and octahedral sublattices, the magnetic properties of nanostructured NiFe2O4 are strongly dependent on the local magneto-crystalline anisotropies, dipolar interactions between the projected moments on the surface of the nanoparticle and on the oxygen vacancies [4, 5].
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Owing to the dependence of the properties of NiFe2O4 on grain size and crystalline structure, it is necessary: (i) to develop effective methods that allow for control of these variables and also (ii) to understand the nanostructure formation under different chemical environments. In this work, the temperature and temporal dependence of the evolution of nanostructured NiFe2O4 formation are studied with the objective of understanding the sample crystallization process. Samples are prepared by the proteic sol–gel (PSG) method [6] and then isothermally heat treated in three different environments: air, nitrogen and bore nitrate (BN) pellets. Regarding the chosen sample preparation method, there has been much recent work reported in literature where the PSG-method has been used to produce nanoparticles, with certain emphasis in the ferrite system [7]. Precursors such as gelatin [7–9] and coconut water [6, 10, 11] are most commonly used in the PSG-method to prepare ferrite nanoparticles. The proteins in these precursors act as a catalyst by binding to the metals and helping to approximate metallic atoms [7]. Significant advantages that justify the use of coconut water as precursor are: (i) its naturally large protein chain, which can easily bind metal ions, (ii) its low price in Brazilian markets, (iii) its availability on an industrial scale (it is produced on a large scale, bottled and sold throughout Brazil), and (iv) the simple material production process. Because of the large protein chain, mixed micelles are responsible for synthesis of the nanosized powders and help to control particle morphology [1].
Experimental Samples were prepared from a mixture of coconut water (Cocos nucifera), iron-(III)-nitrate-nonahydrate [Fe(NO3)3 9H2O] and nickel nitrate hexahydrate [Ni(NO3)26H2O] in a procedure similar to that reported in literature [6]. Coconut water was obtained from green dwarf coconuts in the latter (9 months) stages of maturation. The coconut protein content at this stage of maturation varies from 0.72 to 0.52 g/100 g [12] and its concentration ranges between 0.5 and 0.7 % protein in the water. The concentrations in the final sol–gel for the Fe(NO3)39H2O and Ni(NO3)2 6H2O were 0.41 and 0.15 %, respectively, resulting in a large excess of metal compared with initial amount of protein [1]. Consequently, the initial number of generated micelles (seeds of the crystallization process) is small when compared with the total amount of available metallic ions, which in fact guarantees that the generated micelles will have the same composition (uniform local stoichiometry).
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After production, the gel was dried in air at 473 ± 7 K for 5.5 h, forming a brown powder. This dried sample, or proto ferrite, was first analyzed by Mo¨ssbauer spectroscopy (MS) and X-ray absorption spectroscopy (XAS) and thereafter separated into parts that were subjected to heating between 300 and 1223 K in three different atmospheres (air, air compacted with BN powder and nitrogen). Different amounts of the dried sample were treated isothermally in air at 673, 873, 1073, and 1173 K for 2, 4, 6, and 8 h, respectively. The resultant samples were characterized at room temperature by X-ray powder diffraction (XPD), MS, and Fourier Transform Infrared Spectroscopy (FTIRS). Conventional XPD spectra were collected at room temperature using a Rigaku Ultima IV diffractometer with ˚ ) radiation in the 2h range from 20° Cu Ka (k = 1.5409 A to 100°. 57 Fe Mo¨ssbauer spectra were recorded at room temperature in transmission geometry, using a 57Co:Rh c-ray source moving in a triangular wave form. The Mo¨ssbauer spectra were analyzed using the Normos program [13]. The isomer shift values (d) are reported relatively to the a-Fe value recorded at 300 K. XAS was conducted in the XAFS1 beam line of the Brazilian Synchrotron Light Laboratory (LNLS). Room temperature spectra were recorded from 7000 to 8000 eV (Fe-K edge extended region), whereas in the Ni case, the spectra were recorded from 8200 to 9200 eV. The Fourier transform of the magnitudes of v(j) were performed in the ˚ -1, using a Hanning-type window k-range from 2 to 13 A function. FTIR spectra were measured at room temperature in a Nicolet 6700 spectrometer using 500 scans and a resolution of 2 cm-1 in transmission geometry. Spectra were normalized by height of the highest band and the samples were pressed in KBr pellets before measurement. Parts of the dried sample were also submitted for XPD analysis in the high temperature oven of the LNLS XPD beam line to study the crystallization process under nitrogen. The samples were heated from 323 to 1223 K, whilst the patterns were measured in situ at specific chosen temperatures. In both the air or nitrogen annealed atmospheres, the average crystalline grain sizes of the samples were estimated using the Williamson–Hall method modified for Gaussian lines [14, 15]. Finally, portions of the dried samples were dispersed into BN powder and pressed to form pellets, which were submitted for dispersive XAS measurements (DXAS). In situ heat treatment in the LNLS DXAS beam line produced the third different annealing environment. For measurements at the Fe-K edge, pellets were heated to the first target temperature (673 K) at 10 K/min, followed by
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isothermal treatment over 1 h. Thereafter, the temperature was increased to 773 K and maintained at this value for 2 h. For measurements at the Ni-K edge, the samples were also heated to 1123 K. During the entire process, DXAS spectra were extracted with 10 ms (milliseconds) intervals to gain insight into the time-dependent formation of nanostructured NiFe2O4. To reduce the time necessary to obtain one spectrum; the energy range in the DXAS beam line was reduced. In this case, Fe-K edge spectra were taken from 7050 to 7450 eV, while the energy range for the Ni-spectra varied from 8250 to 8780 eV. The Fourier transform of the magnitudes of v(j) were performed in the ˚ -1, using a Hanning-type window k-range from 2 to 9 A function. Since the spectra obtained in the DXAS beam line have reduced energy ranges, after each thermal analysis, the heat-treated pellets were removed and later submitted for room temperature XAS measurements in the XAFS-1 beam line for more accurate analysis. The spectra were obtained in the same energy range as those for the dried sample.
Results and discussion Dried samples As expected, the X-ray diffraction spectrum of the as-dried sample (not shown) displays a broad halo near the background. This is an indicative of either a non-ordered (amorphous) structure or very small crystallites. Thermogravimetric
Fig. 2 Fe-K edge XAS spectra (a) and the Fourier transform of the magnitudes of v(j) (b) for the liquid and dried sample. Fe-K edge XAS spectrum of bulk standard nickel ferrite was also added for comparison
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Fig. 1 300 K Mossbauer spectrum of the dried sample (dried at 473 K for 5.5 h). Full circles are the experimental data, while the line connecting these points is obtained from the fitting process using Normos program [8]
analysis suggests that this sample loses organic material up to 773 K, whilst FTIRS displays peaks resulting from the organics for samples treated isothermally in air at 773 K even after 24 h of annealing. It should be mentioned that these peaks reduce in intensity with annealing time. Above 773 K, there is no evidence of organic contributions. On the other hand, the 300 K Mo¨ssbauer spectrum, shown in Fig. 1, displays a well-defined doublet characteristic of a non-cubic symmetry for Fe-atoms.
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Fig. 4 X-ray diffraction patterns of the nanostructured NiFe2O4 prepared by sol–gel and isothermally annealed for 8 h at Tan. Simulated diffraction patterns for the bulk NiFe2O4 and NiO phases are also added for comparison
This type of spectrum suggests that the dried material is in a chemically disordered state or is composed of small particles in the SPM-state. The hyperfine parameters extracted from the spectrum fitting are: d = 0.34 mm/s and quadrupolar split (D) equal to 0.75 mm/s; values found in Fe3? state [16]. To better understand the changes in structure during the drying process, XAS spectra were obtained for the liquid
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and as-dried sample. The Fe-K edge spectra (a) and Fourier ˚ -1) (b) are shown in Fig. 2. transforms of v(j) (2–13 A Fitting of the X-ray absorption near edge structure (XANES) spectrum of the liquid sample (Fig. 2) indicates that this material displays characteristics of the Fe(NO3)39H2O precursor. In addition, its Fourier transform does not display evidence of long range crystalline arrangement. However, the presence of the peak at approximately 7148 eV (see arrows in Fig. 2) in the XAS spectrum of the dried sample is an indication for the initiation of the crystallization process (first atomic-coordination spheres), which commenced at 473 K during the drying process. The pre-edge peak (7114 eV) region detached in the inset of Fig. 2a shows that the pre-edge peak intensity of the dried sample spectrum is higher than that for the liquid sample and has the same intensity as in the spectrum of the bulk standard NiFe2O4 (Fig. 2a). For 3d metals, the increase in intensities of the pre-edge peaks has been commonly attributed to the local mixing of 4p and 3d orbitals; an effect that may occur in the tetrahedral symmetry, but is not allowed in the octahedral symmetry of a ferrite with inverse spinel structure [9, 17]. Thus, the fact that the intensity of this peak is similar to that seen for the bulk standard NiFe2O4 is indicative that the inverse spinel structure of the NiFe2O4 phase, where it should be expected that equal numbers of Fe3? atoms occupy tetrahedral and octahedral sites, is already present in the dried sample [9, 17].
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Fig. 5 a Crystalline grain size (D) evolution for the dried sample thermally treated by 8 h at different temperatures (Tan) in air. b Time evolution for the grain sizes (D) of the sample annealed at 1173 K. Lines connecting the experimental data are guides for the eyes
Fig. 6 300 K Mo¨ssbauer spectra for the nanostrutured NiFe2O4 samples obtained from the annealing of the dried sample for 8 h at the indicated temperatures
Despite this evidence of the beginning of crystallization, the XAS spectra of the dried sample and the bulk standard NiFe2O4 display some differences. Moreover, while the
Fig. 7 Infrared spectra of samples isothermally treated for 8 h (a) 673 K, (b) 873 K, (c) 1073 K, and (d) 1173 K
Fourier transform of v(k) for the dried sample displays a strong peak associated with the first coordination sphere (Fetet–O and Feoct–O) and a small one that could be attributed to other spheres (Feoct–Feoct and Fetet–Feoct), the Fourier transform for the bulk standard NiFe2O4 presents peaks related to all coordination spheres. Thus, it can be inferred that the dried sample is composed of small NiFe2O4 nanoparticles. Therefore, following the core–shell model for nanoparticles (small particles have high surface/core ratio [18]), it can be suggested that there is no contribution to the Fourier transform [17] from the large disordered surface of small particles; consequently the peak intensities are low if one considers only the volume fraction contribution. The existence of small particles is consistent with the X-ray diffraction data (no Bragg peak = amorphous structure) and with the MS (disordered superparamagnetic particle behavior) results discussed above. The Ni-K edge XANES spectrum of the liquid sample (Fig. 3) can be attributed to the Ni(NO3)26H2O, whereas the Ni-K edge XAS spectrum of the dried sample (Fig. 3a), seen from the octahedral Ni-site, already shows the presence of a peak at approximately 8366 eV (see the arrow). This is an indication of the initiation of the crystallization process that already occurred (first atomic-coordination spheres) during the drying of the sample. The Fourier transform of v(j) (Fig. 3b) is also characteristic of small particles in the initial stages of crystallization (large disorder at the surface). In this case, however, it is not possible to have the degree of inversion of the structure, since Ni-atoms occupy only octahedral sites, which do not contribute to the pre-edge peak intensity. For this reason, the comparison of this spectrum with that of the pre-edge peak
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of the bulk standard NiFe2O4 has not been done; consequently the existence of the NiFe2O4 inverse spinel-like structure from the Ni-point of view could not be confirmed. In summary, it can be inferred that the as-dried (473 K for 5.5 h) sample has a distribution of small NiFe2O4 nanoparticles dispersed in an organic matrix. Heat treatment in air XPD spectra of samples annealed isothermally in air at 673, 873, 1073, and 1173 K for 8 h are shown in Fig. 4. It is important to note that Bragg peaks owing to the NiOphase have an angular position similar to those from NiFe2O4; however, their diffraction line intensities are
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different. Comparison of the relative intensities of the Bragg peaks of the simulated and experimental patterns reveals that the sample annealed at 1173 K contains a significant fraction of cubic NiFe2O4 with lattice parameter a = (0.8346 ± 0.0005) nm according to the JCPDC data (Card no. 10-0325). However, it should be mentioned that a contribution from the NiO-phase is quite common in the literature [1, 19]. Also, NiFe2O4 spinel-like nanocomposites generally contain different chemical phases, such as metallic-NiFe, NiO, c-Fe2O3, and a-Fe2O3, when prepared and annealed under different experimental conditions [19, 20]. Except for the NiO-phase that was detected by FTIR spectroscopy, none of these other phases were found for the samples treated in air.
J Mater Sci (2013) 48:1543–1554 Fig. 9 Isothermal evolution of Ni-K edge normalized absorption (a, b) and Fourier transform magnitudes of v(j) (c, d) of NiFe2O4 particles synthesized by proteic sol–gel method
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For the annealed samples, the Williamson–Hall analysis modified for peaks with Gaussian features has been applied to estimate the average crystalline grain size (D), using this relation reported in literature [14, 15, 21]: 2 K b cosh ¼ 16 e sinh þ D 2
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where k is the wavelength of the X-ray radiation, b is the full width at half maximum of the diffracted peaks, h is the diffraction angle, e is the microstrain factor, D is the average grain diameter, and K is equal to 0.9, assuming
spherical grain shapes. For the analysis, Gaussian curves were fitted to the NiFe2O4 peaks to estimate the experimental widths, bexp. The values of b were calculated excluding the instrumental broadening binst: qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ð2Þ b ¼ b2exp b2inst where the binst value was determined measuring a standard sample of CeO2 using a typical procedure where all b values were plotted as a function of 2h. A general relation was obtained for binst = 0.253(3) - 0.0020(1) 9 2h ? 3.7(1) 9 10-5 9 4h2, assuming the interval 28° \ 2h \ 107°.
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Fig. 10 Time evolution of Ni- and Fe-K edge normalized absorptions in (a) and (c), respectively, and in b and c Ni and Fe-Fourier transform magnitudes of v(j) of the sample annealed at 773 K Fig. 11 a XANES spectrum at the Fe-K edge and b the Fourier transform of the v(j) of the sample heated at 773 K in DXAS beam line. Bulk standard commercial NiFe2O4 spectrum is added to the graph for comparison
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While the D values are shown in Fig. 5a for different annealing temperatures, in Fig. 5b they were obtained for a fixed annealing temperature (Tan = 1173 K) under different annealing times. It can be seen that the D values
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increase substantially with temperature or time. One has to mention that our D values, determined at each temperature, are smaller than those found by other authors for samples prepared in a citric acid solution [22] but are similar to
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Fig. 12 a Isothermal in situ evolution of the X-ray diffraction patterns during the heat treatment applied to the asdried sample. b Amplification of the angular positions near to the main peak (311) of the NiFe2O4 spinel-like phase and the peak (200) of the Ni-phase
those of other authors that also used an aqueous synthesis method [23]. The 300 K Mo¨ssbauer spectra of the samples annealed isothermally at different temperatures are displayed in Fig. 6 (annealing time of 8 h). While the sample annealed at low temperature (Tan = 673 K) has a Mo¨ssbauer spectrum showing a particle distribution effect (particles in magnetic relaxing process), the other samples, annealed at higher temperatures, present spectra of particles in the blocked magnetic state (well-defined sextets). Thus, the 673 K spectrum was fitted with a distribution of magnetic hyperfine fields (BHF). It was possible to distinguish contributions from SPM-NiFe2O4 particles (field regions with values smaller than 10 T) and larger NiFe2O4 particles, blocked magnetically at 300 K, for field regions of BHF 10 T. On the other hand, for the sample annealed at 873 K, the spectrum is better fitted with three well-defined sextets; an effect also observed in NiFe2O4 prepared by mechanosynthesis [24]. Two of these sextets, with BHF of 48 and 52 T and d of 0.25 and 0.37 mm/s, respectively, are related to the tetra- and octahedral Fe-sites commonly found in bulk NiFe2O4 spinel-like structures [25–28]. The third sextet, with BHF of 46 T, has Fe3? features and can be associated with those Fe-ions at the surface/interface of the crystalline grain [26]. The spectrum of the sample annealed at 1073 K for 8 h is fitted to bulk NiFe2O4 spinel hyperfine parameters. The area ratio of these two sextets is approximately equal to 1, which leads us to infer that this spinellike phase is an inverse structure. The sextet component attributed to the Fe-surface atoms is not detected in the Mo¨ssbauer spectra of the sample annealed at high temperatures owing to the fact that particles approximately
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30 nm in size have a bulk contribution dominating the spectra. FTIRS measurements were also conducted to study the spinel-like structure. For ferrites, it is well known that two main broad metal–oxygen stretching bands are observed in the IR-spectra. The highest one, m1, generally seen in the range 550–600 cm-1, corresponds to intrinsic stretching vibrations of the metal at the tetrahedral site, Mtetra $ O, whereas the m2-lowest band, usually observed in the range 385–450 cm-1, is assigned to octahedral-metal stretching, Mocta $ O [29]. Our data, as shown in Fig. 7, suggests that: (i)
As the isothermal treatment progresses, the two bands, m1 and m2, become more defined. This is a clear signature of improvement in the crystal structure. (ii) Above 873 K, the intensity of m1 increases above that of m2. This indicates the transition between mixed and inverse spinel-like structures, since m2 being bigger than m1 is associated with mixed-spinel, while the opposing relation is attributed to the inverse structure [4]. (iii) The measured position of the m1 band has shown a tendency to change with isothermal treatment from 570 cm-1 at 673 K to a range between 586 and 616 cm-1 at and above 873 K, that is, to the wavenumber values reported to result from the nickel ferrite system [17, 19, 22, 29–31]. (iv) For isothermal annealing at 873 K, a shoulder, in m1, around 637 cm-1, becomes visible in the IR-spectra. This shoulder may be associated with vacancies of the ordered type [18, 32]. It is also possible to see a
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shoulder in m2 at 427 cm-1, owing to the presence of the NiO-phase, not observed in Mo¨ssbauer measurements, but commonly found in the NiFe2O4 spinellike structure [1, 19]. It is important to emphasize that two samples were prepared by the SGP process a year apart but isothermally treated in air at 1173 K and analyzed by FTIR in the same period of time and their spectra are similar. This observation suggests that the crystallization process is relatively reproducible and seems to be independent of the intrinsic properties of the coconut water used. So, when annealed in air, the sample evolved to NiFe2O4 ? NiO structures, with a phase transition between a more disordered (or mixed-spinel phase) to an ordered
inverse spinel-like structure around 873 K. The schematic diagram representing the formation of NiFe2O4 nanoparticles in air is in the central part of Fig. 13. Heat treatment in air embedded in BN pellets DXAS spectra at the Fe- and Ni-K edges for samples embedded in BN pellets were acquired in air at the LNLS DXAS beam line. This provided insight into the kinetics of formation of the NiFe2O4 spinel-like structure in a compacted and relatively inert (compared with direct air exposure) environment. The spectra and corresponding Fourier transforms of v(k) are shown in Figs. 8 and 9, respectively.
Fig. 13 Schematic representation of the sample evolution under the three different conditions. Each colored layer represents one temperature of thermal treatment
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Figures 8a, c and 9a, c display a gradual process of particle growth and changes in atomic positions at the surfaces in the temperature interval of 323–523 K. An increase in growth is observed at 573 K. This indicates that the system gains energy to increase the core size rapidly, which favors the growth of the second coordination sphere. In Figs. 8b, d and 9b, d, it is shown that the structure stabilized at 573 K remains up to 773 K. Ni-DXAS spectra recorded up to 1123 K (Fig. 9) indicate the presence of a new Ni-rich phase when the sample is annealed above 873 K, as seen at the right side of Fig. 13. It is also relevant to note that the time evolution of the structure during isothermal annealing at 673 K for 60 min and at 773 K for 120 min indicates that the sample structure does not change substantially, as can be seen from Fig. 10. To check the final phases of the sample after annealing at 773 K, XANES analysis obtained in the XAFS-1 beam line at the Fe-K edge and the correspondent Fourier Transform magnitude of v(j) was conducted with the results displayed in Fig. 11. A XANES spectrum of bulk standard commercial NiFe2O4 phase is also included in this figure for use as a reference sample. It can be concluded that the NiFe2O4 spinel-like structure seems to be stabilized in the core from the chemical point of view, but the particles are still small with a low core/shell ratio and therefore one assumes that the atomic disorder at the surfaces causes the observed difference from the standard sample spectrum. Heat treatment in nitrogen atmosphere Figure 12a, b displays XPD patterns of the as-dried sample subjected to high temperature annealing in the chamber of the LNLS XPD beam line under nitrogen atmosphere. These patterns were recorded between 323 and 1223 K. An amplification of the angular positions close to the (311) most intense Bragg peak of the NiFe2O4 spinel-like phase is displayed in Fig. 12b. From this figure, it is clear that: (i) Bragg peaks related to the inverse Ni-ferrite phase are observed only above 773 K and (ii) with increase in annealing temperature, Bragg peaks associated with the spinel-like structure shift to lower angular positions concomitantly with a reduction of their relative intensities for temperatures higher than 973 K. The former result is unexpected; usually when the particle size increases (owing to the annealing), the respective peak intensities are enhanced and the angular positions shift to higher angles [33]. The reduction in peak intensities can be explained by the appearance of a new diffraction line correlated to the (200) metallic Ni peak above 973 K (see Fig. 12b). This observation suggests that there is a large fraction of
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segregated metallic Ni. On the other hand, the shift in diffracted peaks to lower angles can be attributed to a segregated Fe3O4 phase, which has a structure similar to that of the NiFe2O4 phase, but their corresponding Bragg peaks are positioned at slightly lower angles. The superposition of the Fe3O4 and NiFe2O4 peaks results in peaks shifted to lower angles. The 300 K XRD spectra (not shown), obtained after measurement at 1223 K, shows the presence of Fe3O4, NiFe2O4, and Ni phases and confirms the latter discussion. Analyzing only the NiFe2O4 peaks yields a D value of 140 ± 7 nm. This value is twice as large as that estimated for samples annealed under air atmosphere at 1173 K (66 ± 5 nm). Thus, it can be inferred that after the NiFe2O4 phase formation, if the temperature increases, the nitrogen atmosphere favors the NiFe2O4 phase decomposition into Fe3O4 and metallic Ni, as suggested at the left side of Fig. 13.
Conclusions We have shown that nickel ferrite (NiFe2O4) nanoparticles can be produced by the SGP-method. The formation of spinel-like NiFe2O4 nanoparticles was investigated using a dried sample (dried at 473 K for 5.5 h) annealed under three different environments: in air, in air while in a pellet with BN powder, and in a nitrogen atmosphere. The asdried sample is composed mostly of small superparamagnetic NiFe2O4 nanoparticles dispersed in an organic matrix. On the other hand, annealed samples display nanocomposite behavior, i.e., they generally contain different chemical phases with grain size in the nanometric scale. For samples annealed in air, only a minority NiO-phase was detected besides the NiFe2O4 spinel-like structure. Up to 873 K, MS has shown NiFe2O4 nanoparticles with a significant contribution from Fe-ions at the surface or interfaces of the crystalline grains, while IR data indicate the presence of vacancies. Above 873 K, the sample structure seems to be similar to that of the bulk NiFe2O4 spinel phase. The NiFe2O4 crystalline grain size (estimated by the Williamson–Hall method) increases substantially with increase in temperature and/or time and reaches a value of approximately 66 nm at the highest annealing temperature (1173 K). For samples compacted with BN and annealed in air, the gradual growth of particles and/or changes in atomic positions at the surfaces was observed from 323 to 523 K. An increase in growth is observed at 573 K. This observation indicates that the system gains energy to increase the core size rapidly, which favors the growth of the second coordination sphere. The NiFe2O4 exists up to 773 K, however, above 873 K, undesirable phases are segregated.
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Finally, samples annealed in a nitrogen atmosphere have displayed the formation of NiFe2O4 nanoparticles for temperatures at 773 K, but temperatures higher than 873 K induced segregation of the metallic Ni and Fe3O4 phases. In summary, a decomposition of the NiFe2O4 structure was observed for annealing in a nitrogen atmosphere and for samples compacted with BN powder. However, no segregation was detected when the as-dried sample were annealed in air. As expected, this result indicates the strong influence of external oxygen in the crystallization of the NiFe2O4 structure or, at least, in the final stage of particle growth. When annealing is conducted in an O2-rich atmosphere, the as-dried material converts to a blend of the NiFe2O4 and minority NiO-phase. Acknowledgements We acknowledge the financial support of UFES, CNPq, FAPES and Brazilian Synchrotron Light Laboratory/ LNLS, under Proposals XPD-10022, XAFS1-10171, and DXAS9331. Also the authors thank the LNLS staff for their prompt assistance at the XAFS1, DXAS, and XPD beam lines.
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