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Coordination compounds containing urea as precursors for oxides—A new route of obtaining nanosized CoFe2O4
Materials Chemistry and Physics 101 (2007) 142–147
Coordination compounds containing urea as precursors for oxides—A new route of obtaining nanosized CoFe2O4 O. Carp a,∗ , L. Patron a , A. Reller b a
Institute of Physical Chemistry “I.G.Murgulescu” Spl. Independentei no. 202, Sect. 6, Bucharest, Romania b Solid State Chemistry, University of Augsburg, Universit¨ atsstrasse 1, D-86159 Augsburg, Germany Received 10 September 2005; received in revised form 9 March 2006; accepted 10 March 2006
Abstract ˚ and a surface area of 60–67 m2 g−1 have been produced at a low temperature Pure CoFe2 O4 particles with an average size in the range 38–57 A (450 ◦ C) through thermal decomposition of polynuclear coordination compounds containing urea as ligand, with different metal:ligand molar ratios (Fe3+ :Co2+ :urea =2:1:6 and 2:1:8), synthesized by two routes (a precipative and a solid state one). © 2006 Elsevier B.V. All rights reserved. Keywords: Nanosized materials; Magnetic materials; Powder diffraction; M¨ossbauer spectroscopy; CoFe2 O4
1. 1.Introduction Spinel ferrites represent a group of technologically important materials, extensively used in modern electronic tehnologies [1–5], microwave absorbers [6], chemical sensors [7–9], catalysts [10–13] and biomedical applications [14]. For all these applications the fine particle nature of the ferrite is crucial, being usually achieved by “soft chemistry” synthesis methods, respectively low temperature range decomposition of suited precursors. Cobalt ferrite, CoFe2 O4 , is a partially inverse spinel with the formula (Cox Fe1−x )[Co1−x Fe1+x ], where the parentheses and square brackets indicate tetrahedral (A) and octahedral (B) sites, respectively. The site occupancy ratio, Fe(A)/Fe(B), depends on the preparation route. It varies from 0.61 to 0.87 for rapidly quenched and slowly cooled samples [15]. Bulk CoFe2 O4 is characterized by a high coercivity (5400 Oe) and a moderate saturation magnetization (about 72 emu g−1 ) [16]. Most of the synthesis approaches (for both powder and film synthesis) are wet chemical methods such as coprecipitation [17–22], hydrolytic decomposition [23,24], solvothermal [25], sol–gel [26–30] and microemulsion [31–33] procedures. Traditional solid state routes [34,35] and physical techniques, such as mechanochemical [20], sonochemical [36], radio frequency ∗
Corresponding author. Tel.: +40 21 21288 71; fax: +40 21 3121147. E-mail address: [email protected] (O. Carp).
0254-0584/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2006.03.002
(RF) sputtering [37] and molecular beam epitaxy [38] methods, are also used. The synthesis of ferrites through thermal decomposition of pattern polynuclear coordination compounds represents a versatile and predictable method, due to the possibility of controlling the quality (composition and microstructure) of the end products featuring, by selecting suitable ligands and outer coordination sphere ions [39]. Choosing of a pair ligand-outer sphere ion (such as urea—NO3 − ), which decompose or evolves a large amount of volatile products during a short lapse of time, in the same low temperature range, directs the synthesis in obtaining fine oxides particles. In this work, we report the synthesis of nanosized cobalt ferrite through thermal decomposition of some polynuclear coordination compounds with urea as ligand and NO3 − as outer sphere anion. 2. Experimental details In order to prepare the pattern compounds, as starting materials Fe(NO3 )3 ·9H2 O, Co(NO3 )2 ·6H2 O and urea (Merck reagents) were used. Two different synthesis routes (precipitation and solid state methods) were adopted, in which the molar ratios Fe3+ :Co2+ :urea (2:1:6 and 2:1:8) as variable parameter was chosen. Details of the synthesis methods are presented elsewhere [40]. The coordination compounds were identified by quantitative analysis. The metal content was determined by an atomic absorption technique, and the carbon, nitrogen and oxygen contents by a combustion method coupled with a chromatographic technique. Fe2 CoC6 H36 N20 O36 : found/calculated Fe, 10.52/9.84; Co, 5.01/5.19; C, 6.20/6.34; H, 3.28/3.17. Fe2 CoC8 H40 N24 O36 : Found/calculated Fe, 9.78/9.16; Co, 4.25/4.83; C, 7.71/7.88; H, 3.20/3.28; N, 27.50/27.58.
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The X-ray powder diffraction patterns of the mixed oxides end products were measured at laboratory temperature using a Philips PW 1049 diffractometer with ˚ For the determination of a Ni-filtered Cu K␣ radiation (λwavelength = 1.5418 A). the average crystallite size the Scherrer formula D = 0.91λ/(βcosθ) has been used, where D is the crystallite size, λ is the wavelength (Cu K␣), β is the corrected half-width obtained using ␣-quartz as reference and the Warren formula, and θ is the diffraction angle of used diffraction peaks ((1 1 1), (1 1 0) and (1 0 0)) [41]. The infrared (IR) spectra were obtained by KBr disc technique in the range 400–4000 cm−1 using BIO-RAD FTIR 120 infrared spectrophotometer. M¨ossbauer spectroscopy was performed on a HALDER device with a 10 mCi 57 Co (Rh) source at room temperature and 4.2 K. The isomers shifts were referred to ␣-Fe. The magnetization curves of the encapsulated oxides samples were measured at room temperature at a decreasing applied field from 5 T, using a SQUID magnetometer (MPMS-5S, Quantum Design). The thermal decomposition investigations were carried out on a NETZSCH VTA 409 coupled to a BALZERS QMS 421 mass spectrometer, under dynamic air and nitrogen flow (20 cm3 min−1 ), with a sample mass of about 20 mg at a heating rate of 2 K min−1 . The Brunauer-Emmet-Tellers (BET) surface area (N2 , 77 K) was calculated from five-point absorption isotherm. The samples were outgassed at 150 ◦ C for 4 h before the area measurements started.
Fig. 2. Thermoanalytical curves (TG, DTG and DTA) of [CoFe2 (urea)6 ] (NO3 )8 ·6H2 O (air, heating rate 2 K min−1 ). T = the difference temperature between the sample and reference material (Al2 O3 ), endo: endotherm process, exo: exotherm process.
3. Results and discussion
3.2. Thermal decomposition of the polynuclear coordination compounds
3.1. Characterization of the polynuclear coordination compounds Two pattern coordination compounds characterized by the following molecular formula are isolated: [Fe2 Co (urea)6 ](NO3 )8 ·6H2 O and [Fe2 Co(urea)8 ](NO3 )8 ·4H2 O. The IR spectra (Fig. 1) evidenced a coordination of urea through oxygen atom. This coordination mode leads to a decrease of the νCO stretching frequencies in comparison with urea ones (1681 cm−1 /≈1650 cm−1 from urea 6/8 coordination compounds) [42,43]. Bands assigned to uncoordinated NO3 − are also identified at ≈1380 and ≈830 cm−1 .
Fig. 1. IR spectra of pattern coordination compounds: (a) urea, (b) [Fe2 Co(urea)6 ](NO3 )8 ·6H2 O (solid state method), (c) [Fe2 Co(urea)8 ](NO3 )8 ·4H2 O (solid state method), (d) [Fe2 Co(urea)6 ](NO3 )8 ·6H2 O (precipitation method). a.u.: arbitrary units.
The thermal behaviour of the two compounds is similar and practically atmosphere independent, but different with the ones obtained in the case of mononuclear complexes with urea [44,45]. The comments of this paragraph are restricted to the thermal decomposition of the [Fe2 Co(urea)6 ](NO3 )8 ·6H2 O polynuclear coordination compound in air atmosphere flow. The thermogravimetric (TG), derivative thermogravimetric (DTG) and differential thermal analysis (DTA) curves are shown in Fig. 2. The most representative ion intensities are selected in Fig. 3. In the temperature range 45–250 ◦ C the compound undergoes a stepwise decomposition in six distinct stages of weight loss. The observed mass loss (79.1%) is in good agreement with the calculated one (79.3%). Due to the high mass loss, the sample undergoes a volume reduction, spongeous end products being obtained. The first decomposition step corresponding to the evolving of five water molecules is followed by the compound’s melting (87 ◦ C). The second stage consists in a overlapping of at least three decomposition steps: the releasing of the remainder outer coordination sphere water molecule and partial evolving of urea (which in mass spectrometry (MS) analysis conditions cleaves into ionic fragments m/z = 16(NH2 + ) and 44 (NH2 CO+ )), where m is the mass fragment value and z the the ionic charge and nitrate ions (decomposition products NO2 , NO, H2 O). Due to the last process the reaction changes to an exothermic one. The next three decomposition stages (127–205 ◦ C) are associated with the loss of four urea molecules. The experimental findings lead to the following assumptions. The urea release occurs through two different mechanisms, as a whole molecule elimination (ionic fragments m/z = 16 and 44), identified in all three reaction steps and as decomposition products (NH3 and HNCO, reaction (1)), evidenced only in the third one. In the temperature range 205–235 ◦ C two fast, strong exothermic reaction steps are identified. The release of remainder urea
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Fig. 4. X-ray diffractograms of the end products obtained from [CoFe2 (urea)6 ](NO3 )8 ·6H2 O and [Fe2 Co(urea)8 ](NO3 )8 ·4H2 O precursors at different calcination temperatures (calcination time 4 h). c.p.s.: counts per second, ASTM: American Society for Testing Materials.
3.3. Characterization of the end products
Fig. 3. Ion intensities curves of the masses m = 17,18, 30, 43, 44, and 46 obtained during the thermal decomposition of [CoFe2 (urea)6 ](NO3 )8 ·6H2 O (air, heating rate 2 K min−1 ).
molecules and nitrate ions is assumed to be the elementary act of the processes. Further, the reaction proceeds through three routes: urea decomposition (reaction (1)), hydrolysis in oxidic medium of the thermal decomposition products (reactions (2) and (3)) and ammonia oxidation (reaction (4)): NH2 CONH2 → NH3 + HNCO,
(1)
NH2 CONH2 + H2 O → [NH4 ]2 CO3 ,
(2)
(NH4 )2 CO3 → 2NH3 + CO2 ,
(3)
x NO2
[O]
+ y NH3 −→(u N2 O + v NO + z N2 ) + 3H2 O,
(4)
where 0.5u + v + 0.5z = x + y.
The X-ray diffraction measurements show that in all decomposition products annealed in the temperature range 350–450 ◦ C, the typical spinel structure (space group Fd3m) of a cobalt ferrite powder is present (Fig. 4). Although cobalt ferrite is identified in all the oxide products, it is evident that there is an evolution of phase composition and oxides properties, dependent mainly on the calcination treatment and the precursor (Table 1). The crystalline lattice is formed at temperatures ≥350 ◦ C (calcination time 4 h). At 350 ◦ C all the oxides contain besides Co3 O4 detected as main product, Fe3 O4 and Fe2 CoO4. At 400 ◦ C, Fe2 CoO4 becomes the majority phase due to the solid state reaction between the two single oxides, but Co3 O4 and ␥-Fe2 O3 are also detected in the oxides derived from both precursors. Pure Fe2 CoO4 is obtained only at 450 ◦ C. The X-ray diffraction patterns consist in fairly broad but still resolved peaks superimposed on a smoothly varying background. This line broadenings decrease (smooth and regular) with decreasing annealing temperature. The considerable broadening of all diffraction peaks, amounting up to 2.2◦ full width half maximum (FWHM) for the most intense reflection in randomly oriented CoFe2 O4 (3 1 1) indicates that the investigated samples consist of quite small crystallites. The mean crystal-
Table 1 X-ray and magnetic characteristics and specific surface areas of some end products Precursor
Phase composition
˚ ao (A)
˚ D (A)
M/g (emu/g)
BET area (m2 /g)
[CoFe2 (urea)6 ](NO3 )8 ·6H2 O 350 ◦ C, 4 h. [CoFe2 (urea)8 ](NO3 )8 ·4H2 O 350 ◦ C, 4 h. [CoFe2 (urea)6 ](NO3 )8 ·6H2 O 400 ◦ C, 4 h. [CoFe2 (urea)8 ](NO3 )8 ·4H2 O 400 ◦ C, 4 h. [CoFe2 (urea)6 ](NO3 )8 ·6H2 O 450 ◦ C, 4 h. [CoFe2 (urea)8 ](NO3 )8 ·4H2 O 450 ◦ C, 4 h.
Co3 O4 , Fe3 O4 , Fe2 CoO4 Co3 O4 , Fe3 O4 , Fe2 CoO4 Fe2 CoO4 Co3 O4 , ␥-Fe2 O3 Fe2 CoO4 , Co3 O4 ,␥-Fe2 O3 Fe2 CoO4 Fe2 CoO4
– – 8.381 8.375 8.372 8.378
– – – – 38 57
– – 28.16 13.16 42.44 42.55
– – 69.19 74.46 60.15 67.21
ao : lattice parameter, D: mean crystallite size, M/g: magnetization/gram.
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lite size is dependent on the urea content of the precursor. A higher amount of urea leads to a smaller value of this parameter. Fe2 CoO4 obtained at 450 ◦ C is characterized by a mean crystal˚ corresponding to 6/8-urea precursor. These lite size of 38/57 A findings agree well with the BET results (Table 1); the oxides derived from 8 urea precursors presenting higher specific areas. The Fd3m space group, in which are the spinal structures included, is characterized by four IR active vibration modes [46–49]. The two higher frequency bands are assigned to A–O–B3 (≈650 cm−1 ) and B–O–B2 (≈550 cm−1 ) stretching vibrations in the spinel lattice. The other two bands of lower frequency and intensities (≈450 and ≈400 cm−1 ) are interpreted as due to the vibrations of tetrahedral and octahedral cations relative to each other [46]. The IR bands of the obtained oxides display broad bands (constituted by a number of bands that may not be clearly distinguished in all cases) typical for spinel mixed oxides with cationic vacancies [50] (Fig. 5). Although the main contribution to both vibrational modes belongs to the octahedral coordinated cations, the domination of the 650 cm−1 bands in the case of the oxides obtained at 350 ◦ C indicates a phase in which the occupation of tetrahedral sites is prevailing (Fig. 5a and b). These two bands reach comparable intensities at higher calcination temperatures (Fig. 5c–f). The M¨ossbauer spectra of the calcinated residua obtained at 450 ◦ C after 4 h taken at 4.2 K and room temperature from the two precursors are presented in Fig. 6. According to the crystal field stabilization energies, cobalt (II) has a stronger preference for octahedral coordination than Fe(III), [51] but not enough to reach complete inversion of the spinel as in Fe2 NiO4 [52]. Thus, a partly inverse spinel distribution is anticipated. The spectra could be resolved by considering two sextets, corresponding to the tetrahedral (A) and octahedral magnetic (B) sublattices characteristic to the spinel structure. The refined values of the magnetic fields corresponding to Fe3+ in tetrahedral and octahedral sites are 51.2 and 49.5 T (6 urea) and 51.5 and 48.4 T (8 urea) respectively, values significantly lower than those observed in bulk samples [15]. The distribution of the iron cations among
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Fig. 5. IR spectra of the oxides obtained from [Fe2 Co(urea)6 ](NO3 )8 ·6H2 O and [Fe2 Co(urea)8 ](NO3 )8 ·4H2 O at different calcination temperatures (calcination time 4 h). a.u.: arbitrary units.
octahedral and tetrahedral sites evaluated from the areas of the spectrum performed at 4.2 K is practically precursor independent, leading to the following cation distribution of the cobalt ferrites synthesized at 450 ◦ C: (Fe0.80 Co0.20 )[Fe1.2 Co0.80 ]O4 – derived from the precursor with 6 urea, (Fe0.82 Co0.18 )[Fe1.18 Co0.82 ]O4 – derived from the precursor with 8 urea. At room temperature the M¨ossbauer spectra consist of a weak central doublet (due to the small particle superparamagnetism [53]) superposed on a distinct sextet pattern. The population of nanoparticles with superparamagnetic behaviour
Fig. 6. M¨ossbauer spectra of the oxides obtained at 450 ◦ C after 4 h: (a) 4.2K-6 urea, (b) 4.2K-8 urea, (c) room temperature-6 urea, (d) room temperature-8 urea.
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References
Fig. 7. Magnetic data for some end products.
increases at higher urea content of the precursor, the ratio magnetic/superparamagnetic is 0.516 (8 urea) comparative with 2.125 (6 urea). The variation of the saturation magnetization with the precursors’ calcination temperature is shown in Fig. 7. The saturation magnetization increase with increasing annealing temperature, and while at 400 ◦ C is precursor dependent, at 450 ◦ C both precursors lead to the same value. The obtained ferrite is characterized by a significantly low maximal magnetization at 450 ◦ C, namely 42.55 emu g−1 compared to that of the bulk one (72 emu g−1 ). This reduction of magnetization could be related to different reasons. The poor crystallinity can lead to some additional degree of frustation and local canting of magnetic moments [54]. On the other hand, the magnetic state of the ions in the particle surface area are different from the state in the bulk, and since the surface of so small particles is very large the contribution of this ‘surface’ spins to the magnetic properties measured becomes substantial. 4. Conclusions Polynuclear coordination compounds containing as ligand urea and as outer sphere NO3 − represent a suitable route (efficient, cheap, time sparing) in synthesis of cobalt ferrite. The evolution of large amount of gases during the thermal decomposition (H2 O, NH3 , CO2 , HNCO and nitrogen oxides) helps to dissipate the heat thereby preventing the oxides sintering. The amount of urea of the coordination compound influence the particle size of final ferrite. CoFe2 O4 particles with a mean ˚ (6 urea/8 urea) and specific areas size crystallite of 38 and 57 A 60–67 m2 g−1 (6 urea/8 urea) are obtained after a heat treatment at 450 ◦ C for 4 h. The urea content of the precursor does not play an important role in cation distribution between tetrahedral and octahedral sites and the magnetic properties of the final oxides, but strongly influence the mean crystallite size and specific area.
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Coordination compounds containing urea as precursors for oxides—A new route of obtaining nanosized CoFe2O4 O. Carp a,∗ , L. Patron a , A. Reller b a
Institute of Physical Chemistry “I.G.Murgulescu” Spl. Independentei no. 202, Sect. 6, Bucharest, Romania b Solid State Chemistry, University of Augsburg, Universit¨ atsstrasse 1, D-86159 Augsburg, Germany Received 10 September 2005; received in revised form 9 March 2006; accepted 10 March 2006
Abstract ˚ and a surface area of 60–67 m2 g−1 have been produced at a low temperature Pure CoFe2 O4 particles with an average size in the range 38–57 A (450 ◦ C) through thermal decomposition of polynuclear coordination compounds containing urea as ligand, with different metal:ligand molar ratios (Fe3+ :Co2+ :urea =2:1:6 and 2:1:8), synthesized by two routes (a precipative and a solid state one). © 2006 Elsevier B.V. All rights reserved. Keywords: Nanosized materials; Magnetic materials; Powder diffraction; M¨ossbauer spectroscopy; CoFe2 O4
1. 1.Introduction Spinel ferrites represent a group of technologically important materials, extensively used in modern electronic tehnologies [1–5], microwave absorbers [6], chemical sensors [7–9], catalysts [10–13] and biomedical applications [14]. For all these applications the fine particle nature of the ferrite is crucial, being usually achieved by “soft chemistry” synthesis methods, respectively low temperature range decomposition of suited precursors. Cobalt ferrite, CoFe2 O4 , is a partially inverse spinel with the formula (Cox Fe1−x )[Co1−x Fe1+x ], where the parentheses and square brackets indicate tetrahedral (A) and octahedral (B) sites, respectively. The site occupancy ratio, Fe(A)/Fe(B), depends on the preparation route. It varies from 0.61 to 0.87 for rapidly quenched and slowly cooled samples [15]. Bulk CoFe2 O4 is characterized by a high coercivity (5400 Oe) and a moderate saturation magnetization (about 72 emu g−1 ) [16]. Most of the synthesis approaches (for both powder and film synthesis) are wet chemical methods such as coprecipitation [17–22], hydrolytic decomposition [23,24], solvothermal [25], sol–gel [26–30] and microemulsion [31–33] procedures. Traditional solid state routes [34,35] and physical techniques, such as mechanochemical [20], sonochemical [36], radio frequency ∗
Corresponding author. Tel.: +40 21 21288 71; fax: +40 21 3121147. E-mail address: [email protected] (O. Carp).
0254-0584/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2006.03.002
(RF) sputtering [37] and molecular beam epitaxy [38] methods, are also used. The synthesis of ferrites through thermal decomposition of pattern polynuclear coordination compounds represents a versatile and predictable method, due to the possibility of controlling the quality (composition and microstructure) of the end products featuring, by selecting suitable ligands and outer coordination sphere ions [39]. Choosing of a pair ligand-outer sphere ion (such as urea—NO3 − ), which decompose or evolves a large amount of volatile products during a short lapse of time, in the same low temperature range, directs the synthesis in obtaining fine oxides particles. In this work, we report the synthesis of nanosized cobalt ferrite through thermal decomposition of some polynuclear coordination compounds with urea as ligand and NO3 − as outer sphere anion. 2. Experimental details In order to prepare the pattern compounds, as starting materials Fe(NO3 )3 ·9H2 O, Co(NO3 )2 ·6H2 O and urea (Merck reagents) were used. Two different synthesis routes (precipitation and solid state methods) were adopted, in which the molar ratios Fe3+ :Co2+ :urea (2:1:6 and 2:1:8) as variable parameter was chosen. Details of the synthesis methods are presented elsewhere [40]. The coordination compounds were identified by quantitative analysis. The metal content was determined by an atomic absorption technique, and the carbon, nitrogen and oxygen contents by a combustion method coupled with a chromatographic technique. Fe2 CoC6 H36 N20 O36 : found/calculated Fe, 10.52/9.84; Co, 5.01/5.19; C, 6.20/6.34; H, 3.28/3.17. Fe2 CoC8 H40 N24 O36 : Found/calculated Fe, 9.78/9.16; Co, 4.25/4.83; C, 7.71/7.88; H, 3.20/3.28; N, 27.50/27.58.
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The X-ray powder diffraction patterns of the mixed oxides end products were measured at laboratory temperature using a Philips PW 1049 diffractometer with ˚ For the determination of a Ni-filtered Cu K␣ radiation (λwavelength = 1.5418 A). the average crystallite size the Scherrer formula D = 0.91λ/(βcosθ) has been used, where D is the crystallite size, λ is the wavelength (Cu K␣), β is the corrected half-width obtained using ␣-quartz as reference and the Warren formula, and θ is the diffraction angle of used diffraction peaks ((1 1 1), (1 1 0) and (1 0 0)) [41]. The infrared (IR) spectra were obtained by KBr disc technique in the range 400–4000 cm−1 using BIO-RAD FTIR 120 infrared spectrophotometer. M¨ossbauer spectroscopy was performed on a HALDER device with a 10 mCi 57 Co (Rh) source at room temperature and 4.2 K. The isomers shifts were referred to ␣-Fe. The magnetization curves of the encapsulated oxides samples were measured at room temperature at a decreasing applied field from 5 T, using a SQUID magnetometer (MPMS-5S, Quantum Design). The thermal decomposition investigations were carried out on a NETZSCH VTA 409 coupled to a BALZERS QMS 421 mass spectrometer, under dynamic air and nitrogen flow (20 cm3 min−1 ), with a sample mass of about 20 mg at a heating rate of 2 K min−1 . The Brunauer-Emmet-Tellers (BET) surface area (N2 , 77 K) was calculated from five-point absorption isotherm. The samples were outgassed at 150 ◦ C for 4 h before the area measurements started.
Fig. 2. Thermoanalytical curves (TG, DTG and DTA) of [CoFe2 (urea)6 ] (NO3 )8 ·6H2 O (air, heating rate 2 K min−1 ). T = the difference temperature between the sample and reference material (Al2 O3 ), endo: endotherm process, exo: exotherm process.
3. Results and discussion
3.2. Thermal decomposition of the polynuclear coordination compounds
3.1. Characterization of the polynuclear coordination compounds Two pattern coordination compounds characterized by the following molecular formula are isolated: [Fe2 Co (urea)6 ](NO3 )8 ·6H2 O and [Fe2 Co(urea)8 ](NO3 )8 ·4H2 O. The IR spectra (Fig. 1) evidenced a coordination of urea through oxygen atom. This coordination mode leads to a decrease of the νCO stretching frequencies in comparison with urea ones (1681 cm−1 /≈1650 cm−1 from urea 6/8 coordination compounds) [42,43]. Bands assigned to uncoordinated NO3 − are also identified at ≈1380 and ≈830 cm−1 .
Fig. 1. IR spectra of pattern coordination compounds: (a) urea, (b) [Fe2 Co(urea)6 ](NO3 )8 ·6H2 O (solid state method), (c) [Fe2 Co(urea)8 ](NO3 )8 ·4H2 O (solid state method), (d) [Fe2 Co(urea)6 ](NO3 )8 ·6H2 O (precipitation method). a.u.: arbitrary units.
The thermal behaviour of the two compounds is similar and practically atmosphere independent, but different with the ones obtained in the case of mononuclear complexes with urea [44,45]. The comments of this paragraph are restricted to the thermal decomposition of the [Fe2 Co(urea)6 ](NO3 )8 ·6H2 O polynuclear coordination compound in air atmosphere flow. The thermogravimetric (TG), derivative thermogravimetric (DTG) and differential thermal analysis (DTA) curves are shown in Fig. 2. The most representative ion intensities are selected in Fig. 3. In the temperature range 45–250 ◦ C the compound undergoes a stepwise decomposition in six distinct stages of weight loss. The observed mass loss (79.1%) is in good agreement with the calculated one (79.3%). Due to the high mass loss, the sample undergoes a volume reduction, spongeous end products being obtained. The first decomposition step corresponding to the evolving of five water molecules is followed by the compound’s melting (87 ◦ C). The second stage consists in a overlapping of at least three decomposition steps: the releasing of the remainder outer coordination sphere water molecule and partial evolving of urea (which in mass spectrometry (MS) analysis conditions cleaves into ionic fragments m/z = 16(NH2 + ) and 44 (NH2 CO+ )), where m is the mass fragment value and z the the ionic charge and nitrate ions (decomposition products NO2 , NO, H2 O). Due to the last process the reaction changes to an exothermic one. The next three decomposition stages (127–205 ◦ C) are associated with the loss of four urea molecules. The experimental findings lead to the following assumptions. The urea release occurs through two different mechanisms, as a whole molecule elimination (ionic fragments m/z = 16 and 44), identified in all three reaction steps and as decomposition products (NH3 and HNCO, reaction (1)), evidenced only in the third one. In the temperature range 205–235 ◦ C two fast, strong exothermic reaction steps are identified. The release of remainder urea
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Fig. 4. X-ray diffractograms of the end products obtained from [CoFe2 (urea)6 ](NO3 )8 ·6H2 O and [Fe2 Co(urea)8 ](NO3 )8 ·4H2 O precursors at different calcination temperatures (calcination time 4 h). c.p.s.: counts per second, ASTM: American Society for Testing Materials.
3.3. Characterization of the end products
Fig. 3. Ion intensities curves of the masses m = 17,18, 30, 43, 44, and 46 obtained during the thermal decomposition of [CoFe2 (urea)6 ](NO3 )8 ·6H2 O (air, heating rate 2 K min−1 ).
molecules and nitrate ions is assumed to be the elementary act of the processes. Further, the reaction proceeds through three routes: urea decomposition (reaction (1)), hydrolysis in oxidic medium of the thermal decomposition products (reactions (2) and (3)) and ammonia oxidation (reaction (4)): NH2 CONH2 → NH3 + HNCO,
(1)
NH2 CONH2 + H2 O → [NH4 ]2 CO3 ,
(2)
(NH4 )2 CO3 → 2NH3 + CO2 ,
(3)
x NO2
[O]
+ y NH3 −→(u N2 O + v NO + z N2 ) + 3H2 O,
(4)
where 0.5u + v + 0.5z = x + y.
The X-ray diffraction measurements show that in all decomposition products annealed in the temperature range 350–450 ◦ C, the typical spinel structure (space group Fd3m) of a cobalt ferrite powder is present (Fig. 4). Although cobalt ferrite is identified in all the oxide products, it is evident that there is an evolution of phase composition and oxides properties, dependent mainly on the calcination treatment and the precursor (Table 1). The crystalline lattice is formed at temperatures ≥350 ◦ C (calcination time 4 h). At 350 ◦ C all the oxides contain besides Co3 O4 detected as main product, Fe3 O4 and Fe2 CoO4. At 400 ◦ C, Fe2 CoO4 becomes the majority phase due to the solid state reaction between the two single oxides, but Co3 O4 and ␥-Fe2 O3 are also detected in the oxides derived from both precursors. Pure Fe2 CoO4 is obtained only at 450 ◦ C. The X-ray diffraction patterns consist in fairly broad but still resolved peaks superimposed on a smoothly varying background. This line broadenings decrease (smooth and regular) with decreasing annealing temperature. The considerable broadening of all diffraction peaks, amounting up to 2.2◦ full width half maximum (FWHM) for the most intense reflection in randomly oriented CoFe2 O4 (3 1 1) indicates that the investigated samples consist of quite small crystallites. The mean crystal-
Table 1 X-ray and magnetic characteristics and specific surface areas of some end products Precursor
Phase composition
˚ ao (A)
˚ D (A)
M/g (emu/g)
BET area (m2 /g)
[CoFe2 (urea)6 ](NO3 )8 ·6H2 O 350 ◦ C, 4 h. [CoFe2 (urea)8 ](NO3 )8 ·4H2 O 350 ◦ C, 4 h. [CoFe2 (urea)6 ](NO3 )8 ·6H2 O 400 ◦ C, 4 h. [CoFe2 (urea)8 ](NO3 )8 ·4H2 O 400 ◦ C, 4 h. [CoFe2 (urea)6 ](NO3 )8 ·6H2 O 450 ◦ C, 4 h. [CoFe2 (urea)8 ](NO3 )8 ·4H2 O 450 ◦ C, 4 h.
Co3 O4 , Fe3 O4 , Fe2 CoO4 Co3 O4 , Fe3 O4 , Fe2 CoO4 Fe2 CoO4 Co3 O4 , ␥-Fe2 O3 Fe2 CoO4 , Co3 O4 ,␥-Fe2 O3 Fe2 CoO4 Fe2 CoO4
– – 8.381 8.375 8.372 8.378
– – – – 38 57
– – 28.16 13.16 42.44 42.55
– – 69.19 74.46 60.15 67.21
ao : lattice parameter, D: mean crystallite size, M/g: magnetization/gram.
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lite size is dependent on the urea content of the precursor. A higher amount of urea leads to a smaller value of this parameter. Fe2 CoO4 obtained at 450 ◦ C is characterized by a mean crystal˚ corresponding to 6/8-urea precursor. These lite size of 38/57 A findings agree well with the BET results (Table 1); the oxides derived from 8 urea precursors presenting higher specific areas. The Fd3m space group, in which are the spinal structures included, is characterized by four IR active vibration modes [46–49]. The two higher frequency bands are assigned to A–O–B3 (≈650 cm−1 ) and B–O–B2 (≈550 cm−1 ) stretching vibrations in the spinel lattice. The other two bands of lower frequency and intensities (≈450 and ≈400 cm−1 ) are interpreted as due to the vibrations of tetrahedral and octahedral cations relative to each other [46]. The IR bands of the obtained oxides display broad bands (constituted by a number of bands that may not be clearly distinguished in all cases) typical for spinel mixed oxides with cationic vacancies [50] (Fig. 5). Although the main contribution to both vibrational modes belongs to the octahedral coordinated cations, the domination of the 650 cm−1 bands in the case of the oxides obtained at 350 ◦ C indicates a phase in which the occupation of tetrahedral sites is prevailing (Fig. 5a and b). These two bands reach comparable intensities at higher calcination temperatures (Fig. 5c–f). The M¨ossbauer spectra of the calcinated residua obtained at 450 ◦ C after 4 h taken at 4.2 K and room temperature from the two precursors are presented in Fig. 6. According to the crystal field stabilization energies, cobalt (II) has a stronger preference for octahedral coordination than Fe(III), [51] but not enough to reach complete inversion of the spinel as in Fe2 NiO4 [52]. Thus, a partly inverse spinel distribution is anticipated. The spectra could be resolved by considering two sextets, corresponding to the tetrahedral (A) and octahedral magnetic (B) sublattices characteristic to the spinel structure. The refined values of the magnetic fields corresponding to Fe3+ in tetrahedral and octahedral sites are 51.2 and 49.5 T (6 urea) and 51.5 and 48.4 T (8 urea) respectively, values significantly lower than those observed in bulk samples [15]. The distribution of the iron cations among
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Fig. 5. IR spectra of the oxides obtained from [Fe2 Co(urea)6 ](NO3 )8 ·6H2 O and [Fe2 Co(urea)8 ](NO3 )8 ·4H2 O at different calcination temperatures (calcination time 4 h). a.u.: arbitrary units.
octahedral and tetrahedral sites evaluated from the areas of the spectrum performed at 4.2 K is practically precursor independent, leading to the following cation distribution of the cobalt ferrites synthesized at 450 ◦ C: (Fe0.80 Co0.20 )[Fe1.2 Co0.80 ]O4 – derived from the precursor with 6 urea, (Fe0.82 Co0.18 )[Fe1.18 Co0.82 ]O4 – derived from the precursor with 8 urea. At room temperature the M¨ossbauer spectra consist of a weak central doublet (due to the small particle superparamagnetism [53]) superposed on a distinct sextet pattern. The population of nanoparticles with superparamagnetic behaviour
Fig. 6. M¨ossbauer spectra of the oxides obtained at 450 ◦ C after 4 h: (a) 4.2K-6 urea, (b) 4.2K-8 urea, (c) room temperature-6 urea, (d) room temperature-8 urea.
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References
Fig. 7. Magnetic data for some end products.
increases at higher urea content of the precursor, the ratio magnetic/superparamagnetic is 0.516 (8 urea) comparative with 2.125 (6 urea). The variation of the saturation magnetization with the precursors’ calcination temperature is shown in Fig. 7. The saturation magnetization increase with increasing annealing temperature, and while at 400 ◦ C is precursor dependent, at 450 ◦ C both precursors lead to the same value. The obtained ferrite is characterized by a significantly low maximal magnetization at 450 ◦ C, namely 42.55 emu g−1 compared to that of the bulk one (72 emu g−1 ). This reduction of magnetization could be related to different reasons. The poor crystallinity can lead to some additional degree of frustation and local canting of magnetic moments [54]. On the other hand, the magnetic state of the ions in the particle surface area are different from the state in the bulk, and since the surface of so small particles is very large the contribution of this ‘surface’ spins to the magnetic properties measured becomes substantial. 4. Conclusions Polynuclear coordination compounds containing as ligand urea and as outer sphere NO3 − represent a suitable route (efficient, cheap, time sparing) in synthesis of cobalt ferrite. The evolution of large amount of gases during the thermal decomposition (H2 O, NH3 , CO2 , HNCO and nitrogen oxides) helps to dissipate the heat thereby preventing the oxides sintering. The amount of urea of the coordination compound influence the particle size of final ferrite. CoFe2 O4 particles with a mean ˚ (6 urea/8 urea) and specific areas size crystallite of 38 and 57 A 60–67 m2 g−1 (6 urea/8 urea) are obtained after a heat treatment at 450 ◦ C for 4 h. The urea content of the precursor does not play an important role in cation distribution between tetrahedral and octahedral sites and the magnetic properties of the final oxides, but strongly influence the mean crystallite size and specific area.
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