# Preparation of CoFe2O4/SiO2 Nanocomposites at Low Temperatures Using Short Chain Diols

Authors: Thomas Dippong; Erika Andrea Levei; Oana Cadar
Journal: Journal of Chemistry
 (2017)

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## Abstract

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 silica matrix influences the formation of crystallized cobalt ferrite as single phase at low temperatures. The influence of the methylene groups number in the precursors and annealing temperature on the nanocrystallite size was revealed. The stability of the obtained compounds was determined by calculation of thermodynamic parameters.


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## Body

## 1. Introduction

During the last decades, the nanocomposites preparation techniques experienced a fast development as these materials have a wide range of applications [1–4]. 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 [1–6].The preparation methods for CoFe2O4 nanoparticles require special techniques to prevent agglomeration [7, 8]. A high number of methods have been reported previously for the preparation of CoFe2O4 nanoparticles, including microemulsion, thermal decomposition, reverse micelles, coprecipitation, sol-gel, mechanical alloying, combustion, and hydrothermal, electrochemical procedures, and green synthesis [9–18]. However, the sol-gel technique followed by annealing is one of the simplest, most effective, and feasible routes to produce high purity, homogeneous, and crystalline nanoparticles [19–21].In the present study, the influence of the chelator chain length (1,2-ethanediol (1,2-ED), 1,3-propanediol (1,3-PD), and 1,4-butanediol (1,4-BD)) on the precursor formation and decomposition to obtain 70% CoFe2O4/30% SiO2 (wt%) nanocomposites was investigated. The obtained gels were heated to 300°C, annealed at 500, 700, and 900°C, and characterized by thermal analysis (TG and DTA), X-ray diffraction (XRD), Fourier transformed infrared spectrometry (FT-IR), scanning electron microscopy (SEM), and transmission electron microscopy (TEM).


## 2. Materials and Methods

### 2.1. Synthesis

The used reagents were Fe(NO3)3·9H2O as iron source, Co(NO3)2·6H2O as cobalt source, 1,2-ED, 1,3-PD, and 1,4-BD as chelators, tetraethyl orthosilicate (TEOS) as matrix precursor, ethanol as solvent, and HNO3. All reagents were of analytical grade and used as received without further purification.The sol was prepared by dissolving Fe(NO3)3·9H2O and Co(NO3)2·6H2O in molar ratio of 2 : 1 and the diol (NO3- : diol = 1 : 1, molar ratio), at room temperature, in ethanol/HNO3 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.The redox reaction between the nitrates and diol (1,2-ED, 1,3-PD, and 1,4-BD) with formation of the carboxylate precursors takes place according to (1).(1)


### 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 CuKα radiation (λ=1.54060 Å). 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.


## 2.1. Synthesis

The used reagents were Fe(NO3)3·9H2O as iron source, Co(NO3)2·6H2O as cobalt source, 1,2-ED, 1,3-PD, and 1,4-BD as chelators, tetraethyl orthosilicate (TEOS) as matrix precursor, ethanol as solvent, and HNO3. All reagents were of analytical grade and used as received without further purification.The sol was prepared by dissolving Fe(NO3)3·9H2O and Co(NO3)2·6H2O in molar ratio of 2 : 1 and the diol (NO3- : diol = 1 : 1, molar ratio), at room temperature, in ethanol/HNO3 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.The redox reaction between the nitrates and diol (1,2-ED, 1,3-PD, and 1,4-BD) with formation of the carboxylate precursors takes place according to (1).(1)


## 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 CuKα radiation (λ=1.54060 Å). 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.


## 3. Results and Discussion

### 3.1. FT-IR and XRD Analysis

The FT-IR spectra (Figure1) 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 [22–24].Figure 1
FT-IR spectra of the gels obtained using 1,2-ED (a), 1,3-PD (b), and 1,4-BD (c) dried at 40°C and 140°C.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 SiO4 tetrahedra at 800 cm−1, and the stretching vibration of Si-O-Si bonds at 1063 cm−1 [11, 25–27]. 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 (CH2) were observed [28–30]. 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-SiO3 [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].Figures2–4 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; SiO4 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 [11, 25, 26, 31] which indicates the formation of cobalt ferrite, insufficiently crystallized to be noticed in the XRD pattern. At 700°C, the XRD patterns show the formation of poorly crystallized cobalt ferrite (JCPDS File number 42-1467) contaminated with olivine type cobalt silicate (Co2SiO4) (JCPDS File number 87-0053). The formation of olivine at 700°C could be explained by the experimental set-up that inhibits the formation of Co3O4 spinel oxide up to 900°C and favors the formation of CoO at lower temperatures. The formed CoO reacts with the amorphous SiO2 during annealing and forms olivine [22]. The FT-IR spectrum of the gel annealed at 700°C shows, in addition to the characteristic 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 CoFe2O4 spinel in the silica matrix occurs at 900°C. This can be explained by the fact that the reaction between CoO (formed from Co3O4) and Fe2O3 is more thermodynamically favored than the reaction between CoO and SiO2 [22]. The FT-IR spectrum shows the characteristic bands for CoFe2O4 (466 and 594 cm−1) and the bands of silica matrix, which are more intense than in the previous cases.Figure 2
XRD patterns (a) and FT-IR spectra (b) of the gels obtained using 1,2-ED, annealed at 500, 700, and 900°C.
(a)
(b)Figure 3
XRD patterns (a) and FT-IR spectra (b) of the gels obtained using 1,3-PD, annealed at 500, 700, and 900°C.
(a)
(b)Figure 4
XRD patterns (a) and FT-IR spectra (b) of the gels obtained using 1,4-BD, annealed at 500, 700, and 900°C.
(a)
(b)The XRD patterns of the gels obtained using 1,3-PD (Figure3(a)) show the development of crystalline CoFe2O4 (JCPDS File number 42-1467). By annealing at 500°C, poorly crystallized CoFe2O4 is formed. By increasing the annealing temperature to 700 and 900°C, the degree of crystallization increases. At 700°C, the cobalt ferrite formation is more thermodynamically favored than the formation of olivine. The FT-IR spectra (Figure 3(b)) show characteristic bands for CoFe2O4 (466 and 594 cm−1) and silica matrix (480, 798, 1080, 1650, and 3400–3500 cm−1) [25, 26, 31, 32].At all annealing temperatures, the XRD pattern of gels obtained from 1,4-BD (Figure4(a)) shows the formation of crystalline CoFe2O4 as a single phase (JCPDS File number 42-1467). Compared to synthesis using other diols, the synthesis method using 1,4-BD is very attractive, since it allows the obtaining of CoFe2O4 spinel at low temperature. In the FT-IR spectra (Figure 4(b)), the specific bands for CoFe2O4 (466 and 594 cm−1) and silica matrix (480, 798, 1080, 1650, and 3400–3500 cm−1) are present [11, 25, 26, 32].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 CoFe2O4 crystallites was estimated based on the XRD data, using the following Scherrer equation [34]:(2)D=0.9λβcos⁡θ,where D is the average crystallite size, λ is the X-ray wavelength, β is the broadening of full width at half maximum (FWHM) intensity of the main intense peak, and θ is Bragg angle.The average crystallite size (Table1) indicates that the cobalt ferrite was obtained as nanoparticles. The nanocrystallites sizes increase with the number of methylene groups.Table 1
Average diameters of CoFe2O4 crystallites calculated according to the Scherrer equation.
Temperature (°C)
Average size (nm)
1,2-ED
1,3-PD
1,4-BD
500
—
2
5
700
3
5
11
900
11
15
22


### 3.2. Thermal Analysis

The formation and decomposition of carboxylate precursors were investigated by the thermal analysis of gels dried at 40°C.On TG diagram (Figure5), 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: pKa[FeNO33] = 2.22 and pKaCoNO32 = 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.Figure 5
Thermal analysis (TG and DTA) of carboxylate precursors.
(a)
(b)


### 3.3. Thermodynamic Parameters

In order to determine the stability of cobalt ferrite and cobalt silicate, the variation of the standard enthalpy of formation (ΔHo), entropy (So), and molar heat capacity (cpo) and the decomposition temperature (TD) were calculated. For calculations, room temperature (To = 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. cp was calculated according to the following equation [35, 36]: (3)cpo=a+bT+cT2,where a, b, and c are the molar heat capacity coefficients characteristics of each substance and T is the temperature.Table 2
Thermodynamic parameters of the compounds formed during synthesis.
Compound
ΔHo (kcal/mol)
S
o (cal/mol K)
c
p
o
a
b
·
10
3
c
·
10
-
5
CoO
−56923
12.7
10.8
2.6
14.4
Fe2O3
−63200
14.1
12.6
8.5
−0.8
SiO2
−205
10.1
11.2
8.2
−2.7
CoFe2O4
−341548
30.1
39.9
4.2
3.3
Co2SiO4
−346170
38.2
41.1
4.4
−3.6The standard enthalpy of formation at temperatureT (ΔHTo) was calculated using the values of standard enthalpy of formation at 25°C (ΔHoo) (CoO = −60.5 kcal/mol, Fe2O3 = −44.4 kcal/mol, SiO2 = −36.8 kcal/mol, CoFe2O4 = −541.7 kcal/mol, and Co2SiO4 = −55.2 kcal/mol) according to the following equation [35, 36]: (4)ΔHTo=ΔHoo+cpo·T-25.Using the values of the entropy at 25°C (Soo) (CoO = −53,5 cal/mol·K, Fe2O3 = −2588 cal/mol K, SiO2 = −2497 cal/mol K, CoFe2O4 = −1461 cal/mol K, Co2SiO4 = −1495 cal/mol K), the entropy at temperature T (STo) was calculated according to the following equation [36]: (5)STo=Soo+∫25TcpodTT,where Soo is the standard entropy at T = 25°C and cpo is the molar heat capacity.The variation of Gibbs free energy (ΔGTo) in function of the temperature in standard condition was calculated according to the following equation [36]:(6)ΔGTo=ΔHTo-T∗STo,where ΔGTo is the variation of Gibbs free energy, ΔHo is the enthalpy variation at temperature T, T is the temperature, and ΔSTo is the entropy at temperature T.Table3 presents the thermodynamic parameters calculated for CoO, Fe2O3, and SiO2, while Table 4 presents the thermodynamic data calculated for olivine and cobalt ferrite.Table 3
Thermodynamic parameters at different temperatures for CoO, SiO2, and Fe2O3.
T (°C)
c
p
o (cal/mol·K)
ΔHo (kcal/mol)
S
o (cal/mol·K)
ΔGo (kcal/mol)
CoO
Fe2O3
SiO2
CoO
Fe2O3
SiO2
CoO
Fe2O3
SiO2
CoO
Fe2O3
SiO2
25
12.6
2560
2471
−57.1
−58.1
4.7
12.7
31.1
26.5
−60.9
−67.4
−3.2
100
12.6
3409
3291
−55.8
240
293
16.4
884
850
−62.4
−113
−47.1
200
12.7
4258
4111
−54.6
624
663
19.4
1736
1672
−64.1
−244
−173
300
12.9
5107
4931
−53.3
1092
115
21.5
2587
2494
−66.2
−460
−381
400
13.0
5956
5751
−52.0
1645
1649
23.5
3438
3316
−68.4
−761
−672
500
13.2
6805
6571
−50.7
2283
2265
25.3
4288
4137
−70.9
−1148
−1045
600
13.4
7654
7391
−49.3
3006
2963
26.8
5139
4959
−73.5
−1619
−1499
700
13.6
8503
8211
−48.0
3814
3744
28.3
5989
5780
−76.2
−2176
−2036
800
13.8
9352
9031
−46.6
4706
4606
29.6
6839
6601
−79.1
−2817
−2655
900
14.0
10201
9851
−45.2
5684
5550
30.8
7690
7422
−82.2
−3543
−3357Table 4
Thermodynamic parameters at different temperatures for CoFe2O4 and Co2SiO4.
T (°C)
c
p
o (cal/mol·K)
ΔHo⁡ (kcal/mol)
S
o
⁡ (cal/mol·K)
ΔGo⁡ (kcal/mol)
CoFe2O4
Co2SiO4
CoFe2O4
Co2SiO4
CoFe2O4
Co2SiO4
CoFe2O4
Co2SiO4
25
1312
1348
−339
−343
38.8
47.2
−351
−358
100
1736
1785
−187
−187
474
495
−376
−385
200
2151
2221
8.24
13.5
907
940
−445
−457
300
2584
2657
245
257
1339
1384
−558
−573
400
3008
3093
525
545
1769
1826
−713
−733
500
3432
3529
847
876
2198
2268
−911
−938
600
3856
3965
1211
1251
2627
2708
−1153
−1187
700
4280
4401
1618
1669
3055
3149
−1437
−1480
800
4704
4837
2067
2131
3483
3589
−1764
−1816
900
5128
5273
2559
2636
3910
4028
−2133
−2197Considering that the equilibrium between different reaction products CoFe2O4 and Co2SiO4 and their precursors (CoO, Fe2O3, and SiO2) 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).(7)CoFe2O4⟶CoO+Fe2O3The 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 CO2 is equal to 1 atmosphere.The thermodynamic data presented in Tables3 and 4 was used to calculate the reaction enthalpy ΔHR and reaction entropy ΔSR for cobalt ferrite decomposition according to (8) and (9):(8)ΔHR=∑nprod·ΔHprod0-∑nreact·ΔHreact0,where nprod and nreact are the stoichiometric coefficients of reaction products and reactants, respectively, and ΔHprod0 and ΔHreact0 are the variations of enthalpy of reaction products and reactants, respectively.(9)ΔSR=Soo+∫25Tnprod·cp,prododTT-∫25Tnreact·cp,reactodTT,where Soo is the standard entropy at T = 25°C, nprod and nreact are the stoichiometric coefficients of reaction products and reactants, and cp,prodo and cp,reacto are the molar heat capacities of reaction products and reactants, respectively.The variation of Gibbs free energy of the reaction (ΔGR) was calculated according to (6) using calculated ΔHR and ΔSR. 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 ΔGR = 0 and ΔGR can be calculated by interpolation using the function ΔGR=f(T). The decomposition temperature is considered the temperature where ΔHR = 0.Figure 6
Variation of thermodynamic parameters for cobalt ferrite decomposition.Table5 shows the thermodynamic parameters at decomposition temperature (TD = 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.Table 5
Thermodynamic parameters for cobalt ferrite decomposition at the decomposition temperature.
Compound
c
p
o (cal/mol·K)
ΔHo⁡ (kcal/mol)
S
o (cal/mol·K)
ΔGo (kcal/mol)
CoFe2O4
2673
301
1429
−587
CoO
12.9
−53.0
21.9
−66.6
Fe2O3
5285
1201
2766
−517
T
D
2625
847
1359
3.5Similarly, the thermodynamic parameters were calculated for the reaction of olivine decomposition according to (10).(10)Co2SiO4⟶2CoO+SiO2Also, 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 (TD = 370.6°C). The necessary enthalpy to reach the decomposition temperature is 724 kcal/mol.Table 6
Thermodynamic parameters for olivine decomposition at the decomposition temperature.
Compound
c
p
o (cal/mol·K)
ΔHo (kcal/mol)
S
o (cal/mol·K)
ΔGo (kcal/mol)
CoFe2O4
2848
378
1577
−638
CoO
12.9
−52.7
22.4
−67.1
Fe2O3
5290
1339
2854
−499
T
D
2468
855
1321
4.7Figure 7
Variation of thermodynamic parameters for olivine decomposition.


### 3.4. SEM and TEM Analysis

Figure8 shows the SEM images of CoFe2O4 nanocrystallites embedded in the silica matrix. The SEM images revealed spherical particles assembled in high agglomerations of irregular shape.Figure 8
SEM images of gels obtained using 1,2-ED, 1,3-PD, and 1,4-BD, annealed at 700 and 900°C.
(a)
(b)
(c)
(d)
(e)
(f)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.The TEM images (Figure9) 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.Figure 9
TEM images of gels obtained using 1,2-ED, 1,3-PD, and 1,4-BD, annealed at 700 and 900°C.
(a)
(b)
(c)
(d)
(e)
(f)


## 3.1. FT-IR and XRD Analysis

The FT-IR spectra (Figure1) 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 [22–24].Figure 1
FT-IR spectra of the gels obtained using 1,2-ED (a), 1,3-PD (b), and 1,4-BD (c) dried at 40°C and 140°C.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 SiO4 tetrahedra at 800 cm−1, and the stretching vibration of Si-O-Si bonds at 1063 cm−1 [11, 25–27]. 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 (CH2) were observed [28–30]. 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-SiO3 [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].Figures2–4 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; SiO4 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 [11, 25, 26, 31] which indicates the formation of cobalt ferrite, insufficiently crystallized to be noticed in the XRD pattern. At 700°C, the XRD patterns show the formation of poorly crystallized cobalt ferrite (JCPDS File number 42-1467) contaminated with olivine type cobalt silicate (Co2SiO4) (JCPDS File number 87-0053). The formation of olivine at 700°C could be explained by the experimental set-up that inhibits the formation of Co3O4 spinel oxide up to 900°C and favors the formation of CoO at lower temperatures. The formed CoO reacts with the amorphous SiO2 during annealing and forms olivine [22]. The FT-IR spectrum of the gel annealed at 700°C shows, in addition to the characteristic 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 CoFe2O4 spinel in the silica matrix occurs at 900°C. This can be explained by the fact that the reaction between CoO (formed from Co3O4) and Fe2O3 is more thermodynamically favored than the reaction between CoO and SiO2 [22]. The FT-IR spectrum shows the characteristic bands for CoFe2O4 (466 and 594 cm−1) and the bands of silica matrix, which are more intense than in the previous cases.Figure 2
XRD patterns (a) and FT-IR spectra (b) of the gels obtained using 1,2-ED, annealed at 500, 700, and 900°C.
(a)
(b)Figure 3
XRD patterns (a) and FT-IR spectra (b) of the gels obtained using 1,3-PD, annealed at 500, 700, and 900°C.
(a)
(b)Figure 4
XRD patterns (a) and FT-IR spectra (b) of the gels obtained using 1,4-BD, annealed at 500, 700, and 900°C.
(a)
(b)The XRD patterns of the gels obtained using 1,3-PD (Figure3(a)) show the development of crystalline CoFe2O4 (JCPDS File number 42-1467). By annealing at 500°C, poorly crystallized CoFe2O4 is formed. By increasing the annealing temperature to 700 and 900°C, the degree of crystallization increases. At 700°C, the cobalt ferrite formation is more thermodynamically favored than the formation of olivine. The FT-IR spectra (Figure 3(b)) show characteristic bands for CoFe2O4 (466 and 594 cm−1) and silica matrix (480, 798, 1080, 1650, and 3400–3500 cm−1) [25, 26, 31, 32].At all annealing temperatures, the XRD pattern of gels obtained from 1,4-BD (Figure4(a)) shows the formation of crystalline CoFe2O4 as a single phase (JCPDS File number 42-1467). Compared to synthesis using other diols, the synthesis method using 1,4-BD is very attractive, since it allows the obtaining of CoFe2O4 spinel at low temperature. In the FT-IR spectra (Figure 4(b)), the specific bands for CoFe2O4 (466 and 594 cm−1) and silica matrix (480, 798, 1080, 1650, and 3400–3500 cm−1) are present [11, 25, 26, 32].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 CoFe2O4 crystallites was estimated based on the XRD data, using the following Scherrer equation [34]:(2)D=0.9λβcos⁡θ,where D is the average crystallite size, λ is the X-ray wavelength, β is the broadening of full width at half maximum (FWHM) intensity of the main intense peak, and θ is Bragg angle.The average crystallite size (Table1) indicates that the cobalt ferrite was obtained as nanoparticles. The nanocrystallites sizes increase with the number of methylene groups.Table 1
Average diameters of CoFe2O4 crystallites calculated according to the Scherrer equation.
Temperature (°C)
Average size (nm)
1,2-ED
1,3-PD
1,4-BD
500
—
2
5
700
3
5
11
900
11
15
22


## 3.2. Thermal Analysis

The formation and decomposition of carboxylate precursors were investigated by the thermal analysis of gels dried at 40°C.On TG diagram (Figure5), 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: pKa[FeNO33] = 2.22 and pKaCoNO32 = 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.Figure 5
Thermal analysis (TG and DTA) of carboxylate precursors.
(a)
(b)


## 3.3. Thermodynamic Parameters

In order to determine the stability of cobalt ferrite and cobalt silicate, the variation of the standard enthalpy of formation (ΔHo), entropy (So), and molar heat capacity (cpo) and the decomposition temperature (TD) were calculated. For calculations, room temperature (To = 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. cp was calculated according to the following equation [35, 36]: (3)cpo=a+bT+cT2,where a, b, and c are the molar heat capacity coefficients characteristics of each substance and T is the temperature.Table 2
Thermodynamic parameters of the compounds formed during synthesis.
Compound
ΔHo (kcal/mol)
S
o (cal/mol K)
c
p
o
a
b
·
10
3
c
·
10
-
5
CoO
−56923
12.7
10.8
2.6
14.4
Fe2O3
−63200
14.1
12.6
8.5
−0.8
SiO2
−205
10.1
11.2
8.2
−2.7
CoFe2O4
−341548
30.1
39.9
4.2
3.3
Co2SiO4
−346170
38.2
41.1
4.4
−3.6The standard enthalpy of formation at temperatureT (ΔHTo) was calculated using the values of standard enthalpy of formation at 25°C (ΔHoo) (CoO = −60.5 kcal/mol, Fe2O3 = −44.4 kcal/mol, SiO2 = −36.8 kcal/mol, CoFe2O4 = −541.7 kcal/mol, and Co2SiO4 = −55.2 kcal/mol) according to the following equation [35, 36]: (4)ΔHTo=ΔHoo+cpo·T-25.Using the values of the entropy at 25°C (Soo) (CoO = −53,5 cal/mol·K, Fe2O3 = −2588 cal/mol K, SiO2 = −2497 cal/mol K, CoFe2O4 = −1461 cal/mol K, Co2SiO4 = −1495 cal/mol K), the entropy at temperature T (STo) was calculated according to the following equation [36]: (5)STo=Soo+∫25TcpodTT,where Soo is the standard entropy at T = 25°C and cpo is the molar heat capacity.The variation of Gibbs free energy (ΔGTo) in function of the temperature in standard condition was calculated according to the following equation [36]:(6)ΔGTo=ΔHTo-T∗STo,where ΔGTo is the variation of Gibbs free energy, ΔHo is the enthalpy variation at temperature T, T is the temperature, and ΔSTo is the entropy at temperature T.Table3 presents the thermodynamic parameters calculated for CoO, Fe2O3, and SiO2, while Table 4 presents the thermodynamic data calculated for olivine and cobalt ferrite.Table 3
Thermodynamic parameters at different temperatures for CoO, SiO2, and Fe2O3.
T (°C)
c
p
o (cal/mol·K)
ΔHo (kcal/mol)
S
o (cal/mol·K)
ΔGo (kcal/mol)
CoO
Fe2O3
SiO2
CoO
Fe2O3
SiO2
CoO
Fe2O3
SiO2
CoO
Fe2O3
SiO2
25
12.6
2560
2471
−57.1
−58.1
4.7
12.7
31.1
26.5
−60.9
−67.4
−3.2
100
12.6
3409
3291
−55.8
240
293
16.4
884
850
−62.4
−113
−47.1
200
12.7
4258
4111
−54.6
624
663
19.4
1736
1672
−64.1
−244
−173
300
12.9
5107
4931
−53.3
1092
115
21.5
2587
2494
−66.2
−460
−381
400
13.0
5956
5751
−52.0
1645
1649
23.5
3438
3316
−68.4
−761
−672
500
13.2
6805
6571
−50.7
2283
2265
25.3
4288
4137
−70.9
−1148
−1045
600
13.4
7654
7391
−49.3
3006
2963
26.8
5139
4959
−73.5
−1619
−1499
700
13.6
8503
8211
−48.0
3814
3744
28.3
5989
5780
−76.2
−2176
−2036
800
13.8
9352
9031
−46.6
4706
4606
29.6
6839
6601
−79.1
−2817
−2655
900
14.0
10201
9851
−45.2
5684
5550
30.8
7690
7422
−82.2
−3543
−3357Table 4
Thermodynamic parameters at different temperatures for CoFe2O4 and Co2SiO4.
T (°C)
c
p
o (cal/mol·K)
ΔHo⁡ (kcal/mol)
S
o
⁡ (cal/mol·K)
ΔGo⁡ (kcal/mol)
CoFe2O4
Co2SiO4
CoFe2O4
Co2SiO4
CoFe2O4
Co2SiO4
CoFe2O4
Co2SiO4
25
1312
1348
−339
−343
38.8
47.2
−351
−358
100
1736
1785
−187
−187
474
495
−376
−385
200
2151
2221
8.24
13.5
907
940
−445
−457
300
2584
2657
245
257
1339
1384
−558
−573
400
3008
3093
525
545
1769
1826
−713
−733
500
3432
3529
847
876
2198
2268
−911
−938
600
3856
3965
1211
1251
2627
2708
−1153
−1187
700
4280
4401
1618
1669
3055
3149
−1437
−1480
800
4704
4837
2067
2131
3483
3589
−1764
−1816
900
5128
5273
2559
2636
3910
4028
−2133
−2197Considering that the equilibrium between different reaction products CoFe2O4 and Co2SiO4 and their precursors (CoO, Fe2O3, and SiO2) 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).(7)CoFe2O4⟶CoO+Fe2O3The 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 CO2 is equal to 1 atmosphere.The thermodynamic data presented in Tables3 and 4 was used to calculate the reaction enthalpy ΔHR and reaction entropy ΔSR for cobalt ferrite decomposition according to (8) and (9):(8)ΔHR=∑nprod·ΔHprod0-∑nreact·ΔHreact0,where nprod and nreact are the stoichiometric coefficients of reaction products and reactants, respectively, and ΔHprod0 and ΔHreact0 are the variations of enthalpy of reaction products and reactants, respectively.(9)ΔSR=Soo+∫25Tnprod·cp,prododTT-∫25Tnreact·cp,reactodTT,where Soo is the standard entropy at T = 25°C, nprod and nreact are the stoichiometric coefficients of reaction products and reactants, and cp,prodo and cp,reacto are the molar heat capacities of reaction products and reactants, respectively.The variation of Gibbs free energy of the reaction (ΔGR) was calculated according to (6) using calculated ΔHR and ΔSR. 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 ΔGR = 0 and ΔGR can be calculated by interpolation using the function ΔGR=f(T). The decomposition temperature is considered the temperature where ΔHR = 0.Figure 6
Variation of thermodynamic parameters for cobalt ferrite decomposition.Table5 shows the thermodynamic parameters at decomposition temperature (TD = 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.Table 5
Thermodynamic parameters for cobalt ferrite decomposition at the decomposition temperature.
Compound
c
p
o (cal/mol·K)
ΔHo⁡ (kcal/mol)
S
o (cal/mol·K)
ΔGo (kcal/mol)
CoFe2O4
2673
301
1429
−587
CoO
12.9
−53.0
21.9
−66.6
Fe2O3
5285
1201
2766
−517
T
D
2625
847
1359
3.5Similarly, the thermodynamic parameters were calculated for the reaction of olivine decomposition according to (10).(10)Co2SiO4⟶2CoO+SiO2Also, 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 (TD = 370.6°C). The necessary enthalpy to reach the decomposition temperature is 724 kcal/mol.Table 6
Thermodynamic parameters for olivine decomposition at the decomposition temperature.
Compound
c
p
o (cal/mol·K)
ΔHo (kcal/mol)
S
o (cal/mol·K)
ΔGo (kcal/mol)
CoFe2O4
2848
378
1577
−638
CoO
12.9
−52.7
22.4
−67.1
Fe2O3
5290
1339
2854
−499
T
D
2468
855
1321
4.7Figure 7
Variation of thermodynamic parameters for olivine decomposition.


## 3.4. SEM and TEM Analysis

Figure8 shows the SEM images of CoFe2O4 nanocrystallites embedded in the silica matrix. The SEM images revealed spherical particles assembled in high agglomerations of irregular shape.Figure 8
SEM images of gels obtained using 1,2-ED, 1,3-PD, and 1,4-BD, annealed at 700 and 900°C.
(a)
(b)
(c)
(d)
(e)
(f)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.The TEM images (Figure9) 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.Figure 9
TEM images of gels obtained using 1,2-ED, 1,3-PD, and 1,4-BD, annealed at 700 and 900°C.
(a)
(b)
(c)
(d)
(e)
(f)


## 4. 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% CoFe2O4/30% SiO2 (wt%) nanocomposites. Longer chain diols resulted in higher weight losses in the decomposition process of the precursors and higher decomposition temperature. Longer chain precursors embedded in the silica matrix favored the formation of single phase cobalt ferrite, at lower temperatures: Co and Fe succinates allow the obtaining of crystalline cobalt ferrite at 500°C, while Co and Fe oxalates give amorphous cobalt ferrite at 500°C, poorly crystalline cobalt ferrite with traces of olivine at 700°C, and single phase crystalline cobalt ferrite at 900°C. The average nanocrystallites size of cobalt ferrite ranges from 11 to 22 nm in case of annealing at 900°C, while in case of using 1,4-BD the average nanocrystallite size can reach 5 nm after annealing at 500°C. The nanocrystallites’ size increases with the increase of the methylene groups in the precursors and the annealing temperature. The enthalpy and entropy of the cobalt ferrite and olivine decomposition reaction increase with the increase of annealing temperature. The presented synthesis method offers a viable alternative for obtaining CoFe2O4/SiO2 nanocomposites with applications in the field of catalysis and magnetic materials.



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*Source: 7943164-2017-03-15.xml*