**3.2 Characterization**

The textural and morphological characteristics of the spinel ferrite nanocrystals we prepared were studied with various techniques to determine the influence of calcination temperature on the crystallization, morphology, and particle size distribution of the nanocrystals and to explore other parameters of interest. The characterization of the prepared spinel ferrites nanoparticles were conducted by using various techniques such as to (TGA), X-ray diffraction (XRD), Fourier transforms infrared spectroscopy (FTIR), transmission electron microscopy (TEM), and vibrating sample magnetometer (VSM) verify the particle size and distribution and to explore other parameters of interest.in this section we introduce these techniques.

#### **3.3 Mechanism of interaction of PVP and metal ions in synthesize of Zn, Mn and Ni ferrite nanoparticle by thermal treatment method**

Interactions between the PVP capping agent [25] and metal ions are shown schematically in Figure 15.We have shown the Metal (II) (e.g. Zn, Mn and Ni) and iron (III) ions which are bound by the strong ionic bonds between the metallic ions and the amide group in a polymeric chain. PVP acts as a stabilizer for dissolved metallic salts through steric and electrostatic stabilization of the amide groups of the pyrolidine rings and the methylene groups. Initially, the PVP stabilizer may decompose to as limited extent, thereby producing shorter polymer chains that are capped when they are adsorbed onto the surfaces of metallic ions [26]. The metallic ions, which are well dispersed in the cavities and networks, are created as a result of the shorter polymer chains. These mechanisms continue until they are terminated by the drying step. The influence of PVP is not restricted only to the solution and the drying step; PVP also affects the formation of the nuclei (i.e., nucleation) of the nickel ferrite nanoparticles in the calcination step. In this step, the small nanoparticles with high surface energy levels would become larger via the Ostwald ripening process [27] without the presence of PVP, disrupts steric hindrance, thereby preventing their aggregation. Steric hindrance is a phenomenon that is attributed to large molecular weight (>10,000) and the repulsive forces acting among the polyvinyl groups [28, 29]. These interactions are similar to the stabilization of metallic nanoparticles, i.e., silver and gold [30, 31].

In this study, metal nitrate reagents, poly (vinyl pyrrolidon) (PVP), and deionized water were used as precursors. In addition, a capping agent to control the agglomeration of the particles and a solvent were used. Iron nitrate, Fe(NO3)39H2O, zinc nitrate, Zn(NO3)26H2O, nickel nitrate, Ni(NO3)26H2O , and manganese nitrate Mn(NO3)26H2O , were purchased from Acros Organics with a purity exceeding 99%. PVP (MW = 29000) was purchased from Sigma Aldrich and was used without further purification. An aqueous solution of PVP was prepared by dissolving polymer in deionized water at 363 K, before mixing 0.2 mmol iron nitrate and 0.1 mmol metal nitrate (Fe:M = 2:1) into the polymer solution and constantly stirring for 2 h using a magnetic stirrer until a colorless, transparent solution was obtained. The mixed solution was poured into a glass Petri dish and heated at 353 K in an oven for 24 h to evaporate the water. The dried, orange, solid zinc ferrite that remained was crushed and ground in a mortar to form powder. The calcinations of the powders were conducted at 723, 773, 823, and 873 K for 3 h for the decomposition of organic compounds and the crystallization of the nanocrystals. The processing steps employed separately for the

The textural and morphological characteristics of the spinel ferrite nanocrystals we prepared were studied with various techniques to determine the influence of calcination temperature on the crystallization, morphology, and particle size distribution of the nanocrystals and to explore other parameters of interest. The characterization of the prepared spinel ferrites nanoparticles were conducted by using various techniques such as to (TGA), X-ray diffraction (XRD), Fourier transforms infrared spectroscopy (FTIR), transmission electron microscopy (TEM), and vibrating sample magnetometer (VSM) verify the particle size and distribution and

to explore other parameters of interest.in this section we introduce these techniques.

the stabilization of metallic nanoparticles, i.e., silver and gold [30, 31].

**3.3 Mechanism of interaction of PVP and metal ions in synthesize of Zn, Mn and Ni** 

Interactions between the PVP capping agent [25] and metal ions are shown schematically in Figure 15.We have shown the Metal (II) (e.g. Zn, Mn and Ni) and iron (III) ions which are bound by the strong ionic bonds between the metallic ions and the amide group in a polymeric chain. PVP acts as a stabilizer for dissolved metallic salts through steric and electrostatic stabilization of the amide groups of the pyrolidine rings and the methylene groups. Initially, the PVP stabilizer may decompose to as limited extent, thereby producing shorter polymer chains that are capped when they are adsorbed onto the surfaces of metallic ions [26]. The metallic ions, which are well dispersed in the cavities and networks, are created as a result of the shorter polymer chains. These mechanisms continue until they are terminated by the drying step. The influence of PVP is not restricted only to the solution and the drying step; PVP also affects the formation of the nuclei (i.e., nucleation) of the nickel ferrite nanoparticles in the calcination step. In this step, the small nanoparticles with high surface energy levels would become larger via the Ostwald ripening process [27] without the presence of PVP, disrupts steric hindrance, thereby preventing their aggregation. Steric hindrance is a phenomenon that is attributed to large molecular weight (>10,000) and the repulsive forces acting among the polyvinyl groups [28, 29]. These interactions are similar to

**3.1.1 Process of fabrication of metal ferrite nanocrystals** 

synthesis of each ferrite nanoparticles.

**ferrite nanoparticle by thermal treatment method** 

**3.2 Characterization** 

Fig. 15. The proposed mechanism of interactions between PVP and metal ions in the formation of the ferrites nanoparticles.

Crystalization in Spinel Ferrite Nanoparticles 365

average particle size of 12 nm. These results were similar to the results achieved when a PVP concentration of 0.035 gm/ml was used (shown in Figure 17c). But, due to the high concentration of PVP, traces of organic materials were observed at 1254 cm-1, which was attributed to the C–H bending vibration of methylene groups, as shown in Figure 18b while

in concentration of 0.035 gm/mlnickel ferrite nanoparticles were pure (Figure 18a).

Fig. 17. TEM images of nickel ferrite nanoparticles with PVP concentrations of (a) 0,

Fig. 18. FTIR spectra of nickel ferrite nanoparticles with PVP concentration of (a) 0.035 and

So, in fact, it is apparent that the nickel ferrite nanoparticles were contaminated with organic compounds in this case. Therefore, in the thermal treatment method, the optimum concentration of PVP for the synthesis of nickel ferrite nanoparticles is 0.035 gm/ml. That

(b) 0.015, (c) 0.035, and (d) 0.055 gm/ml calcined at 723 K.

0.055 gm/ml calcined at 723 K.

#### **3.4 Determination of range of calcinations temperature for removing of PVP**

Figure 16 shows the simultaneous thermal analyses (TG–DTG) for PVP. It is evident that this polymer exhibited only one mass loss which started at 678 K and its maximum rate decomposition temperature was located at 778 K. This confirms that the majority of the mass loss occurs under 778 K and allows for optimization of the heat treatment program. It is worth noting that several authors reported the thermal degradation of PVP exhibits only one mass loss [32-34].

Fig. 16. Thermogravimetric (TG) and thermogravimetric derivative (DTG) curves for: PVP at a heating rate of 10 k/min.

#### **3.5 Determine of optimum parameters of nickel ferrite nanocrystal**

To investigate the optimum concentration of PVP in the synthesis of nickel ferrite nanoparticles, we synthesized nickel ferrite nanoparticles with others PVP concentrations of 0, 0.015 and 0.055 gm/ml, and the results are shown in the TEM images in Figure 17 and the FT-IR spectra in Figure 18. Also, optimum temperature for calcinations of nickel ferrite nanoparticle was 723 K because, this temperature was minimum temperature that nanoparticles were pure; furthermore, they have also the lowest particle size with a nearly uniform distribution in shapes.Figure 17a shows that nickel ferrite nanoparticles were formed even in the absence of PVP. However, in this case, it was observed that the nanoparticles did not have a uniform distribution of shapes, and they were aggregated extensively and, in some areas, completely disproportionately distributed. Thus, without the use of PVP in the synthesis of nanoparticles, the small nanoparticles aggregate and produce larger nanoparticles [29] due to high surface energy (as shown earlier in Fig15)

When the concentration of PVP was increased to 0.015 gm/ml, the nickel ferrite nanoparticles that were formed became more regular in shape than in the case without PVP (Figure 17b). But, due to the low concentration of PVP, these nanoparticles also aggregated because there was insufficient PVP to cap them well and prevent their agglomeration.By increasing the PVP concentration to 0.055 gm/ml, the nickel ferrite nanoparticles did not agglomerate, and they were nearly uniform in shape, as shown in Figure 17d. However, in this case, the nickel ferrite nanoparticles ranged in size from 9 to 21 nm, with an estimated

Figure 16 shows the simultaneous thermal analyses (TG–DTG) for PVP. It is evident that this polymer exhibited only one mass loss which started at 678 K and its maximum rate decomposition temperature was located at 778 K. This confirms that the majority of the mass loss occurs under 778 K and allows for optimization of the heat treatment program. It is worth noting that several authors reported the thermal degradation of PVP exhibits only

Fig. 16. Thermogravimetric (TG) and thermogravimetric derivative (DTG) curves for: PVP at

To investigate the optimum concentration of PVP in the synthesis of nickel ferrite nanoparticles, we synthesized nickel ferrite nanoparticles with others PVP concentrations of 0, 0.015 and 0.055 gm/ml, and the results are shown in the TEM images in Figure 17 and the FT-IR spectra in Figure 18. Also, optimum temperature for calcinations of nickel ferrite nanoparticle was 723 K because, this temperature was minimum temperature that nanoparticles were pure; furthermore, they have also the lowest particle size with a nearly uniform distribution in shapes.Figure 17a shows that nickel ferrite nanoparticles were formed even in the absence of PVP. However, in this case, it was observed that the nanoparticles did not have a uniform distribution of shapes, and they were aggregated extensively and, in some areas, completely disproportionately distributed. Thus, without the use of PVP in the synthesis of nanoparticles, the small nanoparticles aggregate and produce

When the concentration of PVP was increased to 0.015 gm/ml, the nickel ferrite nanoparticles that were formed became more regular in shape than in the case without PVP (Figure 17b). But, due to the low concentration of PVP, these nanoparticles also aggregated because there was insufficient PVP to cap them well and prevent their agglomeration.By increasing the PVP concentration to 0.055 gm/ml, the nickel ferrite nanoparticles did not agglomerate, and they were nearly uniform in shape, as shown in Figure 17d. However, in this case, the nickel ferrite nanoparticles ranged in size from 9 to 21 nm, with an estimated

**3.5 Determine of optimum parameters of nickel ferrite nanocrystal** 

larger nanoparticles [29] due to high surface energy (as shown earlier in Fig15)

**3.4 Determination of range of calcinations temperature for removing of PVP** 

one mass loss [32-34].

a heating rate of 10 k/min.

average particle size of 12 nm. These results were similar to the results achieved when a PVP concentration of 0.035 gm/ml was used (shown in Figure 17c). But, due to the high concentration of PVP, traces of organic materials were observed at 1254 cm-1, which was attributed to the C–H bending vibration of methylene groups, as shown in Figure 18b while in concentration of 0.035 gm/mlnickel ferrite nanoparticles were pure (Figure 18a).

Fig. 17. TEM images of nickel ferrite nanoparticles with PVP concentrations of (a) 0, (b) 0.015, (c) 0.035, and (d) 0.055 gm/ml calcined at 723 K.

Fig. 18. FTIR spectra of nickel ferrite nanoparticles with PVP concentration of (a) 0.035 and 0.055 gm/ml calcined at 723 K.

So, in fact, it is apparent that the nickel ferrite nanoparticles were contaminated with organic compounds in this case. Therefore, in the thermal treatment method, the optimum concentration of PVP for the synthesis of nickel ferrite nanoparticles is 0.035 gm/ml. That

Crystalization in Spinel Ferrite Nanoparticles 367

XRD and FT-IR were used to characterize the precursors and ferrite nanoparticles calcined at 723, 773, 823 and873 K. The XRD diffraction patterns of the precursor and metal ferrite nanoparticles are shown in Figure20. A broad peak occurred for all samples in the precursor, which does not have sharp diffraction patterns and is still amorphous. The calcined patterns show the reflection planes (111), (220), (311), (222), (400), (331), (422), (511), and (440), which confirm the presence of single-phase in ZnFe2O4 , MnFe2O4 and NiFe2O4 with a face-centered cubic structure [37-39].Except for the impure phases of α-Fe2O3 (for all the metal ferrite nanoparticles shown in Figure 20a, 20b and 20c) and ZnO (for the zinc ferrite nanopaticles shown in Figure20a), which occur naturally as hematite and zincite, respectively [37, 40], the remaining peaks correspond to the standard pattern of ZnFe2O4 (cubic, space group: Fd3m, Z = 8; ICDD PDF: 22-1012), MnFe2O4 (cubic, space group: Fd3m, Z = 8; ICDD PDF: 73-1964) and NiFe2O4 (cubic, space group: Fd3m, Z = 8; ICDD PDF: 44-

**3.6 Phase composition and morphology of metal ferrite nanocrystal** 

1485)[37-39].

concentration, in combination with the optimum temperature (723 K) provided the conditions required to fabricate pure nickel ferrite nanoparticles that have the smallest particle size. So, as discussed earlier in connection with Figure 15 and as demonstrated in the above discussion of the results obtained, PVP plays three crucial roles in synthesizing nickel ferrite nanoparticles, i.e., (1) the control of the growth of the nanoparticles; (2) the prevention of agglomeration of the nanoparticles; and (3) the production of nanoparticles that have a uniform distribution of shapes [35].

Fig. 19. XRD patterns of nickel ferrite nanoparticles with heating rate of calcination of 10, and 20 K/min calcined at 723.

After our examination, for optimum concentration of PVP (0.035 gm/ml) and optimum temperature for calcinations (723 K) of nickel ferrite nanoparticle, we investigated optimum the time of calcinations and the heating rate of calcinations of nickel ferrite nanoparticles as last optimization.In this investigation, the minimum time that allowed the crystallization to be completed was 3 h as lower than 3 h the crystallization was uncompleted and higher than 3 h particles size increased. The heating rate of calcination was 10 K/min for nickel ferrite nanoparticles calcined at 723K, which was an optimum heating rate. By increasing the heating rate of calcination to 20 K/min, the percent of impure phase of α-Fe2O3 increased and the crystallite of nickel ferrite nanoparticles were not as pure as optimum heating rate as shown in Figure19a and Figure 19b.When the heating rate of calcinations was lower than 10 K/min (5 K/min) we wasted long time for calcinations and several neighboring particles fuse together to increase particle sizes by melting their surfaces [36].

Note: We have done exactly these experiments on others ferrites i.e. zinc ferrite and manganese ferrite and have obtained for each of them optimum parameters. But to prevent long or repeated exposures and similar experiments, we reported only the experiments of nickel ferrite nanoparticles and for others ferrites we sufficed only to report of values (Table3).


Table 3. Summary of optimum parameters of metal ferrite nanocrystal prepared by thermal treatment method

concentration, in combination with the optimum temperature (723 K) provided the conditions required to fabricate pure nickel ferrite nanoparticles that have the smallest particle size. So, as discussed earlier in connection with Figure 15 and as demonstrated in the above discussion of the results obtained, PVP plays three crucial roles in synthesizing nickel ferrite nanoparticles, i.e., (1) the control of the growth of the nanoparticles; (2) the prevention of agglomeration of the nanoparticles; and (3) the production of nanoparticles

Fig. 19. XRD patterns of nickel ferrite nanoparticles with heating rate of calcination of 10,

fuse together to increase particle sizes by melting their surfaces [36].

Optimum calcinations temperature (K)

After our examination, for optimum concentration of PVP (0.035 gm/ml) and optimum temperature for calcinations (723 K) of nickel ferrite nanoparticle, we investigated optimum the time of calcinations and the heating rate of calcinations of nickel ferrite nanoparticles as last optimization.In this investigation, the minimum time that allowed the crystallization to be completed was 3 h as lower than 3 h the crystallization was uncompleted and higher than 3 h particles size increased. The heating rate of calcination was 10 K/min for nickel ferrite nanoparticles calcined at 723K, which was an optimum heating rate. By increasing the heating rate of calcination to 20 K/min, the percent of impure phase of α-Fe2O3 increased and the crystallite of nickel ferrite nanoparticles were not as pure as optimum heating rate as shown in Figure19a and Figure 19b.When the heating rate of calcinations was lower than 10 K/min (5 K/min) we wasted long time for calcinations and several neighboring particles

Note: We have done exactly these experiments on others ferrites i.e. zinc ferrite and manganese ferrite and have obtained for each of them optimum parameters. But to prevent long or repeated exposures and similar experiments, we reported only the experiments of nickel ferrite nanoparticles and for others ferrites we sufficed only to report of values (Table3).

> Optimum concentration of PVP (gm/ml)

**ZnFerrite** 873 0.030 10 3 **MnFerrite** 873 0.030 10 3 **NiFerrite** 723 0.035 10 3

Table 3. Summary of optimum parameters of metal ferrite nanocrystal prepared by thermal

Optimum Heating rate (K/min)

Optimum Calcinations Time (h)

that have a uniform distribution of shapes [35].

and 20 K/min calcined at 723.

**Pure** 

**metal ferrite nanoparticles** 

treatment method

#### **3.6 Phase composition and morphology of metal ferrite nanocrystal**

XRD and FT-IR were used to characterize the precursors and ferrite nanoparticles calcined at 723, 773, 823 and873 K. The XRD diffraction patterns of the precursor and metal ferrite nanoparticles are shown in Figure20. A broad peak occurred for all samples in the precursor, which does not have sharp diffraction patterns and is still amorphous. The calcined patterns show the reflection planes (111), (220), (311), (222), (400), (331), (422), (511), and (440), which confirm the presence of single-phase in ZnFe2O4 , MnFe2O4 and NiFe2O4 with a face-centered cubic structure [37-39].Except for the impure phases of α-Fe2O3 (for all the metal ferrite nanoparticles shown in Figure 20a, 20b and 20c) and ZnO (for the zinc ferrite nanopaticles shown in Figure20a), which occur naturally as hematite and zincite, respectively [37, 40], the remaining peaks correspond to the standard pattern of ZnFe2O4 (cubic, space group: Fd3m, Z = 8; ICDD PDF: 22-1012), MnFe2O4 (cubic, space group: Fd3m, Z = 8; ICDD PDF: 73-1964) and NiFe2O4 (cubic, space group: Fd3m, Z = 8; ICDD PDF: 44- 1485)[37-39].

Crystalization in Spinel Ferrite Nanoparticles 369

The results obtained from XRD were analyzed using the Chekcell program, which calculated lattice parameters of the samples calcined at 723, 773, 823, and 873 K (Table 4).

 D=0.9ߣ/) βcosθ), (4) where D is the crystallite size (nm), β is the full width of the diffraction line at half the maximum intensity measured in radians, λ is X-ray wavelength, and θ is the Bragg angle [41]. This formula was used to estimate the average particle sizes, which ranged in Table 4.

> Average particle size TEM (nm)

**ZnFerrite 1** 723 21 17±7 0.8498 4.49 negligible **ZnFerrite 2** 773 24 22±2.5 0.8468 2.66 negligible **ZnFerrite 3** 823 31 27±5 0.8471 1.81 negligible **ZnFerrite 4** 873 33 31±11 0.8479 0.74 negligible **MnFerrite 1** 723 15 12±4 0.8524 3.06 negligible **MnFerrite 2** 773 17 15±2 0.8577 6.31 negligible **MnFerrite 3** 823 20 17±5 0.8558 7.96 negligible **MnFerrite 4** 873 23 22±4 0.8537 15.78 negligible **NiFerrite 1** 723 15 12±3 0.8368 21.37 150(-148) **NiFerrite 2** 773 27 22±12 0.8373 26.67 107(-112) **NiFerrite 3** 823 51 47±11 0.8418 29.05 51(-43) **NiFerrite 4** 873 69 67±8 0.8402 34.19 32(-38) Table 4. Summery of variation of particle sizes, lattice parameters, saturation magnetization and coercivity field with temperature calcinations for metal ferrite nanoparticles calcined at

Figure 21 shows the FT-IR spectrum of the precursor and calcined samples in the wavenumber range between 280 and 4000 cm-1.The IR spectra of all calcined samples show the two principle absorption bands in the range of 300-600 cm-1 .These two vibration bands Fe↔O and M↔O are corresponded to the intrinsic lattice vibrations of octahedral and tetrahedral coordination compounds in the spinel structure, respectively [42].The bands with peaks around 670 and 850 cm-1 were assigned to the formation vibration of C-N=O bending and the C-C ring. The bands in the range of 1200 to 1250 cm-1 was associated with C-N stretching vibration, and the appearance of the bands in the range of 1350 to 1450 cm-1 was attributed to C-H bending vibration from the methylene groups. Finally, there were bands in the region 1600 to1800 cm-1 and around 3400 to 3500 cm-1, which were associated with C=O stretching vibration and N-H or O-H stretching vibration, respectively [43].

The vibrational spectra of the absorption bands of pure ZnFe2O4 and MnFe2O4 nanoparticles were observed at 388, and 541 cm-1, and at 404, 502, and 556 cm-1for the samples calcined at 873 K (shown in Figure 21a and 21b). In these two ferrite nanoparticles, at the lower temperature of 873 K, however, there was still traces of broadband absorption peaks at 1497,

Lattice Parameter (nm)

Saturation magnetization Ms (emu/g)

Coercivity field Hc (Oe)

XRD results were analyzed by the Scherer formula:

Average particle size XRD (nm)

Calcination temperature (K)

**Specimens MeFe2O4**

723, 773, 823 and 873 K.

Fig. 20. XRD patterns of precursors and metal ferrite nanoparicles of (a) ZnFe2O4, (b) MnFe2O4 and (c) NiFe2O4 calcined at 723, 773, 823 and 873 K.

Fig. 20. XRD patterns of precursors and metal ferrite nanoparicles of (a) ZnFe2O4,

(b) MnFe2O4 and (c) NiFe2O4 calcined at 723, 773, 823 and 873 K.

The results obtained from XRD were analyzed using the Chekcell program, which calculated lattice parameters of the samples calcined at 723, 773, 823, and 873 K (Table 4). XRD results were analyzed by the Scherer formula:

$$\mathcal{D} \models 0.9\lambda / \,\langle \\$\text{cos}\theta \rangle. \tag{4}$$

where D is the crystallite size (nm), β is the full width of the diffraction line at half the maximum intensity measured in radians, λ is X-ray wavelength, and θ is the Bragg angle [41]. This formula was used to estimate the average particle sizes, which ranged in Table 4.


Table 4. Summery of variation of particle sizes, lattice parameters, saturation magnetization and coercivity field with temperature calcinations for metal ferrite nanoparticles calcined at 723, 773, 823 and 873 K.

Figure 21 shows the FT-IR spectrum of the precursor and calcined samples in the wavenumber range between 280 and 4000 cm-1.The IR spectra of all calcined samples show the two principle absorption bands in the range of 300-600 cm-1 .These two vibration bands Fe↔O and M↔O are corresponded to the intrinsic lattice vibrations of octahedral and tetrahedral coordination compounds in the spinel structure, respectively [42].The bands with peaks around 670 and 850 cm-1 were assigned to the formation vibration of C-N=O bending and the C-C ring. The bands in the range of 1200 to 1250 cm-1 was associated with C-N stretching vibration, and the appearance of the bands in the range of 1350 to 1450 cm-1 was attributed to C-H bending vibration from the methylene groups. Finally, there were bands in the region 1600 to1800 cm-1 and around 3400 to 3500 cm-1, which were associated with C=O stretching vibration and N-H or O-H stretching vibration, respectively [43].

The vibrational spectra of the absorption bands of pure ZnFe2O4 and MnFe2O4 nanoparticles were observed at 388, and 541 cm-1, and at 404, 502, and 556 cm-1for the samples calcined at 873 K (shown in Figure 21a and 21b). In these two ferrite nanoparticles, at the lower temperature of 873 K, however, there was still traces of broadband absorption peaks at 1497,

Crystalization in Spinel Ferrite Nanoparticles 371

with the calcinations temperature and they had good agreement with XRD results (Table 4).This suggested that several neighboring particles fused together to increase the particle size by the melting of their surfaces [46]. Particle size enlargement due to grain growth has been observed previously in zinc.manganese and nickel ferrite systems at higher calcination

Fig. 22. TEM images of zinc ferrite nanoparticles calcined at (a) 723 (b) 773 (c) 823 (d) 873 K.

Fig. 23. TEM images of manganese ferrite nanoparticles calcined at (a) 723 (b) 773 (c) 823

temperatures [35,44,45].

(d) 873 K.

1761 and 3504 cm-1 due to traces of adsorbed or atmospheric CO2 and O-H stretching vibration, respectively while in NiFe2O4 nanoparticles (Figure 21c), at the lower temperature of 723 K, there was still a trace of a broadband absorption peaks due to ester formation as consequence of the scission of the CO2 and O-H stretching vibration(This is not shown in the Figure21). This suggests that, in thermal treatment method, the calcination temperature of pure nickel ferrite nanoparticles is lower than pure zinc and manganese ferrite nanoparticles [35,44,45].This IR analysis was very useful for establishing the calcination temperature because it removed unwanted ions that may pollute the crystal lattice during preparation.

Fig. 21. FT-IR spectra of precursors and metal ferrite nanoparicles of (a) ZnFe2O4, (b) MnFe2O4 and (c) NiFe2O4 calcined at 723, 773, 823 and 873 K.

The TEM images (Figures 22-24) show the size, shape, and distribution of ZnFe2O4 , MnFe2O4 and NiFe2O4 nanoparticles at different calcination temperatures from 723 to 873 K. The results indicate that the samples prepared by the thermal treatment method were uniform in morphology and particle size distribution. The average particle size of the ZnFe2O4, MnFe2O4 and NiFe2O4 nanoparticles were determined by TEM which increased

1761 and 3504 cm-1 due to traces of adsorbed or atmospheric CO2 and O-H stretching vibration, respectively while in NiFe2O4 nanoparticles (Figure 21c), at the lower temperature of 723 K, there was still a trace of a broadband absorption peaks due to ester formation as consequence of the scission of the CO2 and O-H stretching vibration(This is not shown in the Figure21). This suggests that, in thermal treatment method, the calcination temperature of pure nickel ferrite nanoparticles is lower than pure zinc and manganese ferrite nanoparticles [35,44,45].This IR analysis was very useful for establishing the calcination temperature because it removed unwanted ions that may pollute the crystal lattice during preparation.

Fig. 21. FT-IR spectra of precursors and metal ferrite nanoparicles of (a) ZnFe2O4,

The TEM images (Figures 22-24) show the size, shape, and distribution of ZnFe2O4 , MnFe2O4 and NiFe2O4 nanoparticles at different calcination temperatures from 723 to 873 K. The results indicate that the samples prepared by the thermal treatment method were uniform in morphology and particle size distribution. The average particle size of the ZnFe2O4, MnFe2O4 and NiFe2O4 nanoparticles were determined by TEM which increased

(b) MnFe2O4 and (c) NiFe2O4 calcined at 723, 773, 823 and 873 K.

with the calcinations temperature and they had good agreement with XRD results (Table 4).This suggested that several neighboring particles fused together to increase the particle size by the melting of their surfaces [46]. Particle size enlargement due to grain growth has been observed previously in zinc.manganese and nickel ferrite systems at higher calcination temperatures [35,44,45].

Fig. 22. TEM images of zinc ferrite nanoparticles calcined at (a) 723 (b) 773 (c) 823 (d) 873 K.

Fig. 23. TEM images of manganese ferrite nanoparticles calcined at (a) 723 (b) 773 (c) 823 (d) 873 K.

Crystalization in Spinel Ferrite Nanoparticles 373

interactions, and it does not have intra-sub-lattice (A-A) exchange interactions or inter-sublattice (A-B) super-exchange interactions [48]. Inter-sub-lattice (A-B) super-exchange interactions of the cations are much stronger than the (A-A) and (B-B) interactions [18].Due to the cation inversion, which originates from thermal and mechanical treatment [40], the structure of ZnFe2O4 transfers from a normal spinel structure to a mixed spinel structure [48]. This cation inversion causes the zinc ferrite nanoparticles to experience inter-sub-lattice (A-B) super-exchange interactions and intra-sub-lattice (A-A) exchange interactions in addition to intra-sub-lattice (B-B) exchange interactions. But, due to the degree of inversion, which is large for smaller size particles, inter-sub-lattice (A-B) super-exchange interactions in smaller size particles occur to a greater extent than in larger size particles. Hence, saturation magnetization increases for smaller size particles [49], using Mossbauer's experiment, showed that the degree of inversion is large in the case of smaller size particles. Also, an impure α-Fe2O3 phase was detected by XRD (Figure20a), the heating rate of calcinations and the surface spin structure can be an influence that increases the saturation

Fig. 25. Magnetization curves at room temperature for precursor and zinc ferrite

Figure 26 shows the curves of magnetization of precursor and MnFe2O4 nanoparticles which exhibited a typical super-paramagnetic behavior. Table 4 depicts the values of saturation magnetization (Ms) of different samples. When the calcination temperature increased from 723 K to 873 K, the saturation magnetization increased from 3.06 to 15.78 emu/g. This can be attributed to spin canting and surface spin disorder that occurred in these nanoparticles [50].The interactions between the A and B sub-lattices in the spinel lattice system (AB2O4) consist of inter-sub-lattice (A-B) super-exchange interactions and intra-sub-lattice (A-A) and (B-B) exchange interactions. Inter-sub-lattice super-exchange interactions of the cations on the

nanoparticles calcined at (a) 723 (b) 773 (c) 823 (d) 873 K.

**3.7.2 Manganese ferrite nanoparticles** 

magnetization in smaller size particles [37].

Fig. 24. TEM images of nickel ferrite nanoparticles calcined at (a) 723 (b) 773 (c) 823 (d) 873 K.
