Preparation of CoFe 2 O 4 / SiO 2 Nanocomposites at Low Temperatures Using Short Chain Diols

The preparation of 70% CoFe2O4/30% SiO2 (wt%) nanocomposites by sol-gel method using three short chain diols (1,2-ethanediol, 1,3-propanediol, and 1,4-butanediol) as chelators was studied. The Fourier transformed infrared spectra and X-ray diffraction patterns were used to confirm the formation of nanocomposites. The X-ray diffraction analysis showed that the chain length of the carboxylates embedded in the silicamatrix influences the formation of crystallized cobalt ferrite as single phase at low temperatures. The influence of themethylene groups number in the precursors and annealing temperature on the nanocrystallite sizewas revealed. The stability of the obtained compounds was determined by calculation of thermodynamic parameters.


Introduction
During the last decades, the nanocomposites preparation techniques experienced a fast development as these materials have a wide range of applications [1][2][3][4].The properties of nanoparticle-based composites are determined by the material's morphology, which depends on the nanoparticle size and distribution of the nanosized phase in matrix [5].Cobalt ferrite based nanocomposites present unique physicochemical properties that make it an attractive material for catalysis, antenna rods, loading coils, magnetic data storage, sensors, ferrofluids, magnetooptic materials, energy conversion applications, and targeted drug delivery [1][2][3][4][5][6].
The sol was prepared by dissolving Fe(NO 3 ) 3 ⋅9H 2 O and Co(NO 3 ) 2 ⋅6H 2 O in molar ratio of 2 : 1 and the diol (NO 3 − : diol = 1 : 1, molar ratio), at room temperature, in ethanol/HNO 3 solution.An amount of TEOS equal to 70% of the weight of Fe(III) and Co(II) nitrates was added dropwise under continuous stirring, followed by the addition of ethanol until complete dispersion.The resulting clear solution was exposed to open air for slow gelation.The gelation time was 16 days (1,2-ED), 19 days (1,3-PD), and 23 days (1,4-BD), respectively.The gels were heated at 300 ∘ C for 4 hours and afterwards annealed at 500, 700, and 900 ∘ C.
Malonate precursor Succinate precursor 2.2.Characterization.FT-IR spectra were recorded in transmission mode on KBr pellets using a Perkin-Elmer Spectrum BX II FT-IR spectrometer.XRD analysis was performed at room temperature, using a Bruker D8 Advance diffractometer using a CuK  radiation ( = 1.54060Å).Thermogravimetry (TG) and Differential Thermal Analyses (DTA) were performed by a SDT Q600 type instrument from the room temperature up to 900 ∘ C, with a rate of heating of 10 ∘ C/min, in air.For the nanocrystallites' shape and clustering, a Hitachi HD-2700 TEM equipped with digital image recording system and photographic film image with high resolution scanner was used with samples deposited from suspension onto carbon film on 400 mesh copper grids.The SEM measurements were carried out using a Hitachi SU-8230 ultrahigh resolution scanning electron microscope and the samples were sputter-coated with 5 nm gold.

Results and Discussion
3.1.FT-IR and XRD Analysis.The FT-IR spectra (Figure 1) for the gels dried at 40 ∘ C show the presence of nitrates characterized by an intense band at 1384 cm −1 , indicating that the redox reaction was not initiated at this temperature.
In contrast, the absence of this band in gels dried at 140 ∘ C suggests the consumption of nitrates in the redox reaction with formation of the carboxylates [22][23][24].
The FT-IR spectra of gels dried at both 40 and 140 ∘ C show the characteristic bands for the silica matrix: the vibration of Si-O bond at 480 cm −1 , the vibration of SiO 4 tetrahedra at 800 cm −1 , and the stretching vibration of Si-O-Si bonds at 1063 cm −1 [11,[25][26][27].The broad band at 3400-3500 cm −1 was assigned to the vibration of OH groups in water and silica matrix.The vibration bands of bonded Si-OH expected at 3200-3400 cm −1 overlap the broad band of water.In the range of 2900-3000 cm −1 , the characteristic bands for C-H bonds of the methylene groups (CH 2 ) were observed [28][29][30].In all FT-IR spectra, a characteristic band for M-O (M = Fe, Co) vibrations appears at 446 cm −1 .Moreover, the M-OH groups on the surface of the ferrite particles are replaced by M-O-SiO 3 [31].In case of gels dried at 40 ∘ C, the sharp bands at 1650 cm −1 are attributable to the deformation vibration of the H-O-H bond, which indicates the presence of water incorporated in the silica matrix [28,29].By increasing the temperature to 140 ∘ C, the characteristic bands for the carboxylate type ligands at 1617 cm −1 and 1360 cm −1 attributed to asymmetric and symmetric vibration of the COO − groups increase, while characteristic bands for the nitrate decrease in intensity.These results are confirmed by the thermal analysis that indicated the decomposition of nitrates at 80-140 ∘ C. The small peak at 1069 cm −1 was assigned to the C-O stretching, while those from 1000-700 cm −1 were assigned to the C-OH group trapped in the matrix [31].In case of gels dried at 140 ∘ C, the intensity of the bands decreases from oxalate to succinate, probably due to the removal of the two carboxylate groups.The characteristic bands of carboxylates may overlap the bands of the silica matrix [22].bands of the silica matrix, the characteristic bands for cobalt silicate (571 and 870 cm −1 ) [25,26,32,33].The formation of well crystallized single phase CoFe 2 O 4 spinel in the silica matrix occurs at 900 ∘ C.This can be explained by the fact that the reaction between CoO (formed from Co 3 O 4 ) and Fe 2 O 3 is more thermodynamically favored than the reaction between CoO and SiO 2 [22].The FT-IR spectrum shows the characteristic bands for CoFe 2 O 4 (466 and 594 cm −1 ) and the bands of silica matrix, which are more intense than in the previous cases.
Based on the XRD patterns and FT-IR spectra it can be concluded that the longer chain length of the carboxylate embedded in the silica matrix favors the formation of crystallized ferrite cobalt single phase at low temperatures.The average size of CoFe 2 O 4 crystallites was estimated based on the XRD data, using the following Scherrer equation [34]: where  is the average crystallite size,  is the X-ray wavelength,  is the broadening of full width at half maximum (FWHM) intensity of the main intense peak, and  is Bragg angle.
The average crystallite size (Table 1) indicates that the cobalt ferrite was obtained as nanoparticles.The nanocrystallites sizes increase with the number of methylene groups.

Thermal Analysis.
The formation and decomposition of carboxylate precursors were investigated by the thermal analysis of gels dried at 40 ∘ C.
On TG diagram (Figure 5), the weight losses were 51% for 1,2-ED, 58% for 1,3-PD, and 69% for 1,4-BD, respectively, increasing with the increase of the carboxylate precursor chain length, as a consequence of the additional loss of a methylene or ethylene group in the case of malonic and succinic precursors.After 300 ∘ C, the mass slowly decreases up to 900 ∘ C due to the dehydroxylation of the silica matrix.In all cases, the DTA diagram (Figure 5) shows (i) two endothermic effects corresponding to the redox reaction between nitrates and diol with the formation of carboxylate anions that coordinate to the metallic ions and (ii) two exothermic effects corresponding to the oxidative decomposition of the precursor (oxalate, malonate, and succinate) and the combustion of organic chains intercalated in the silica network.The two exothermic effects at 80-140 ∘ C suggest that Fe(III) and Co(II) nitrates react separately with the diol due to the difference of the aqua cations acidity: pK a[Fe(NO 3 ) 3 ] = 2.22 and pK a[Co(NO 3 ) 2 ] = 12.2 [22,23].Thermal behavior of the metal nitrates, diol solutions, suggests the formation of a homogeneous mixture of homonuclear Fe(III) and Co(II) carboxylates.The two endothermic effects at 200-300 ∘ C on DTA curves of gels show that the decomposition of precursors takes place in two stages indicating that both Fe(III) and Co(II) carboxylic compounds are formed separately.Thus, the first exothermic effect corresponds to decomposition of cobalt oxalates (200 ∘ C), malonates (224 ∘ C), and succinates (243 ∘ C), while the second exothermic effect corresponds to the decomposition of iron oxalates (250 ∘ C), malonates (284 ∘ C), and succinates (300 ∘ C).The decomposition of gel obtained from 1,4-BD occurs at the highest temperatures, indicating that, by increasing the number of methylene groups, the thermic effect increases and shifts toward higher temperatures.

Thermodynamic Parameters.
In order to determine the stability of cobalt ferrite and cobalt silicate, the variation of the standard enthalpy of formation (Δ  ), entropy (  ), and molar heat capacity (   ) and the decomposition temperature (  ) were calculated.For calculations, room temperature (  = 25 ∘ C) was considered as reference.The thermodynamic data of various reactants and products is presented in the literature [35].The thermodynamic parameters of the compounds formed during synthesis are listed in Table 2.   was calculated according to the following equation [35,36]: where , , and  are the molar heat capacity coefficients characteristics of each substance and  is the temperature.The standard enthalpy of formation at temperature  (Δ   ) was calculated using the values of standard enthalpy of formation at 25 ) was calculated according to the following equation [36]: where    is the standard entropy at  = 25 ∘ C and    is the molar heat capacity.
The variation of Gibbs free energy (Δ   ) in function of the temperature in standard condition was calculated according to the following equation [36]: where Δ   is the variation of Gibbs free energy, Δ  is the enthalpy variation at temperature ,  is the temperature, and Δ   is the entropy at temperature .
Table 3 presents the thermodynamic parameters calculated for CoO, Fe 2 O 3 , and SiO 2 , while Table 4 presents the thermodynamic data calculated for olivine and cobalt ferrite.
Considering that the equilibrium between different reaction products CoFe 2 O 4 and Co 2 SiO 4 and their precursors (CoO, Fe 2 O 3 , and SiO 2 ) is influenced by the value of the thermodynamic parameters, the variation of these parameters was calculated.The decomposition of cobalt ferrite takes place according to (7).
The calculations were performed using the thermal decomposition of oxalates (in case of using 1,2-EG as chelator) which corresponds to the temperature at which the pressure of CO 2 is equal to 1 atmosphere.The thermodynamic data presented in Tables 3 and 4 was used to calculate the reaction enthalpy (Δ  ) and reaction entropy (Δ  ) for cobalt ferrite decomposition according to (8) and ( 9): where  prod and  react are the stoichiometric coefficients of reaction products and reactants, respectively, and Δ 0 prod and Δ 0 react are the variations of enthalpy of reaction products and reactants, respectively.
where    is the standard entropy at  = 25 ∘ C,  prod and  react are the stoichiometric coefficients of reaction products and reactants, and   ,prod and   ,react are the molar heat capacities of reaction products and reactants, respectively.
The variation of Gibbs free energy of the reaction (Δ  ) was calculated according to (6) using calculated Δ  and Δ  .In the case of cobalt ferrite decomposition, the reaction enthalpy and entropy increase with the increase of temperature and decrease with the decrease of free enthalpy (Figure 6).There is a temperature range where Δ  = 0 and Δ  can be calculated by interpolation using the function Δ  = ().The decomposition temperature is considered the temperature where Δ  = 0.
Table 5 shows the thermodynamic parameters at decomposition temperature (  = 348 ∘ C) for cobalt ferrite.The necessary enthalpy to reach the decomposition temperature is 642 kcal/mol.The decomposition temperature is reached when the lattice energy of the reaction products is equal to the lattice energy of cobalt ferrite.If the lattice energy of cobalt ferrite is lower, it tends to pass into a more stable form.In our case, the lattice energy of cobalt ferrite is lower than that of the two reaction products.Similarly, the thermodynamic parameters were calculated for the reaction of olivine decomposition according to (10).
Also, in the case of olivine decomposition, the reaction enthalpy and entropy increase with increase of temperature and decrease with the decrease of free enthalpy (Figure 7).Table 6 shows the thermodynamic parameters at the decomposition temperature for olivine (  = 370.6 ∘ C).The necessary enthalpy to reach the decomposition temperature is 724 kcal/mol.The SEM images revealed spherical particles assembled in high agglomerations of irregular shape.Using 1,2-ED and 1,4-BD for the carboxylate precursor obtaining, larger agglomerates were observed.The agglomerates' size increases also with the annealing temperature.

SEM and TEM Analysis
The TEM images (Figure 9) show that the size of the nanoparticle spheres increases with the number of methylene groups of the carboxylate precursor.The size of nanocrystallites obtained from the Scherrer equation was confirmed by the nanoparticle size obtained from TEM images.In the case of gels annealed at 900 ∘ C, nanoparticles of 10 nm to 23 nm diameters were obtained.

Conclusions
The embedding of the reactants in the silica matrix followed by the redox reaction with formation of carboxylate type precursors (oxalate, malonate, and succinate, respectively) and their thermal decomposition allowed the obtaining of 70% CoFe 2 O 4 /30% SiO 2 (wt%) nanocomposites.Longer

Figures 2 -
Figures 2-4 present the XRD patterns (a) and FT-IR spectra (b) of the gels obtained using 1,2-ED, 1,3-PD, and 1,4-BD, respectively, annealed at 500, 700, and 900 ∘ C. The diffraction pattern of gels obtained using 1,2-ED (Figure 2(a)) annealed at 500 ∘ C did not show the presence of crystalline phase, while the FT-IR spectrum (Figure 2(b)) showed both the characteristic bands for the silica matrix (Si-O bonds vibration at 480 cm −1 ; SiO 4 tetrahedron vibration at 798 cm −1 and Si-O-Si bonds stretching vibration at 1080 cm −1 ; H-OH bond deformation vibration at 1650 cm −1 ; vibration of OH groups in water and silica matrix at 3400-3500 cm −1 ) and the characteristic bands for the M-O bond at 400-500 cm −1

Figure 8
shows the SEM images of CoFe 2 O 4 nanocrystallites embedded in the silica matrix.

Table 1 :
Average diameters of CoFe 2 O 4 crystallites calculated according to the Scherrer equation.

Table 3 :
Thermodynamic parameters at different temperatures for CoO, SiO 2 , and Fe 2 O 3 .

Table 4 :
Thermodynamic parameters at different temperatures for CoFe 2 O 4 and Co 2 SiO 4 .

Table 5 :
Thermodynamic parameters for cobalt ferrite decomposition at the decomposition temperature.

Table 6 :
Thermodynamic parameters for olivine decomposition at the decomposition temperature.