**6.3 Nano-hematite-based materials prepared by base-catalyzed sol-gel synthesis**

Literature review confirmed that acid-catalyzed sol-gel synthesis enabled the preparation of the samples characterized by various phase transformation routes resulting in the formation of *α-Fe2O3* phase: spinel phase (Fe3O4/γ-Fe2O3) → rhombohedral phase (α-Fe2O3) as well as spinel (Fe3O4/γ-Fe2O3) → orthorhombic (ε-Fe2O3) → rhombohedral (α-Fe2O3) phase. Dependent on the synthesis conditions, different Hmeas values of the samples are recorded [27]. On the other hand, base-catalyzed synthesis in combination with inverse micelle method is characterized by the phase transformation route Fe3O4/γ-Fe2O3 → ε-Fe2O3 → α-Fe2O3 and presents a highly reproducible method for synthesis of high-temperature nano-hematite particles. The influence of the post-annealing treatment onto the Hmeas value of the samples prepared by this type of sol-gel method is investigated in Ref. [63].

In the method described below, nano-hematite particles are obtained after post-annealing treatment of the samples prepared in base-catalyzed sol-gel synthesis in combination with the microemulsion method [64, 65]. Two identical microemulsions, containing water, cetyltrimethyl ammonium bromide (CTAB), buthanol, and n-octan, were mixed in a particular moral ratio. CTAB is an agent which facilitates formation of the matrix pores in the desired size [54]. Octan presents the solvent that enables the mixing of the reactants, while usage of alcohol of the somewhat longer chain (butanol) ensures a shortened time of the condensation reactions. In one microemulsion Sr2+ is added, whose role is the acceleration of the particle growth along one crystallographic axis, resulting in the rod morphology of the nanoparticles [64]. In another microemulsion a base catalyst, ammonia, is added that possesses a significant role in the defining of the SiO2 pore size. Mixing the microemulsions enables stirring of the solution. Afterwards, TEOS is added in the precise stechiometrical ratio. Gao et al. [66] confirmed that the ideal volume ratio of the TEOS and alcohol (desirable in order to shorten the gelation time) is 1:2, while the same effect is achieved by simultaneously mixing the TEOS and NH3 in the volume ratio 2:5.

#### *6.3.1 Variation of the sol-gel synthesis conditions: post-annealing treatment*

Having in mind that the influence of the annealing conditions on the samples synthesized by this method is well-established in literature [64, 65], obtained samples were performed to post-annealing treatment in order to investigate coercivity behavior of the samples post-annealed at low temperature (100°C) and high temperature (1100°C) [63, 67].

The synthesis of the sample comprised the preparation of two identical micelles, consisting of CTAB, isooctane, butanol, and water in the molar ratio— 0.03:0.33:0.12:1.00. Iron (III) nitrate (prepared by dissolving elemental iron in nitric acid and water) is added to the water in the molar ratio 0.00047:1. In the first micelles precursors of the iron and strontium ions in molar ratio 3:1 are added. In another micelle 0.09 mol of ammonia is added. After mixing the micelles, TEOS is dropped into the stirred solution (volume ratio of TEOS and ammonia was 4.5:1.7, while volume ratio of the TEOS and butanol was 4.5:10.7). Solution is stirred for 24 h. Afterward, precipitate is collected and treated with a chloroform and ethanol in order to wash organic moistures, attached to the surface of the precipitated nanoparticles. A coprecipitate has been annealed at 1050°C for 4 h. The same amounts of the sample are performed to the post-annealing treatment [63].

*Mineralogy - Significance and Applications*

the sample in detail.

for 2 h.

measurement of this sample is shown in **Figure 5(b)**. Interestingly, magnitude of the measured magnetic field of the sample was ~400 Oe. Since literature data showed that this Fe2O3 polymorph is characterized by high Hci (10–20 kOe) [29–31] or by SPM behavior (Hci ~ 0 Oe) [33], mentioned Hmeas value is not characteristic neither for high coercivity ε-Fe2O3 nor for the SPM ε-Fe2O3 phase. Moreover, obtained value is similar to the case presented in **Figure 3(b)**, where it is observed that the sample, containing the α-Fe2O3 as a dominant phase, showed Hmeas ~ 600 Oe. It is important to notice here that an alcogel of the sample whose diffraction pattern is represented in the **Figure 1(a)**, performed to the thermal treatment under the similar annealing conditions, is characterized by the value of the measured magnetic field of 14.1 kOe, although hematite phase was presented as a dominant [59]. Further research of this sample will be performed by Mossbauer spectroscopy, in order to discuss the observed measured magnetic field behavior of

**6.2 Nano-hematite-based materials prepared by acid-catalyzed sol-gel synthesis**

In order to investigate the influence of the catalyst in the sol-gel synthesis of hematite nanoparticles, a sample is synthesized by acid sol-gel synthesis route [62]. This synthesis method is similar to auto-catalyzed synthesis procedure, with the difference that nitric acid (HNO3) is used as a catalyst. Tetraethyl orthosilicate, ethanol, iron (III) nitrate nonahydrate, and nitric acid were mixed in a molar ratio of 1:3:0.2:10. Solution is magnetically stirred for 1 h at room temperature. Gelation took place for 20 days. Obtained gel is dried at 80°C for 19 h, after which it is subjected to thermal treatment under the air atmosphere at temperature of 800°C

**Figure 6(a)** presents a diffraction pattern of the investigated sample. Pure α-Fe2O3 phase is observed as the only iron oxide phase. Hematite nanoparticles are observed at lower temperature than investigated examples characterized by diffrac-

Corresponding hysteretic curves are shown in **Figure 6(b)**, pointing to the presence of hematite phase as the only iron oxide phase. Measured magnetic field of the α-Fe2O3/SiO2 sample achieved the value of 114 Oe, which is ascribed to the coercivity of hematite nanoparticles. Accordingly, the presence of the catalyst enabling the accelerated formation of the hematite phase at lower temperatures (samples examined in **Figures 1–4** revealed the appearance of hematite phase at

*Sample annealed at 800°C for 2 h (HNO3 used as a catalyst): (a) diffraction pattern [62]; (b) hysteretic curves.*

temperatures higher than the sample presented at **Figure 6** [59, 60]).

tion patterns shown in **Figures 1(a)** and **4(a)**.

**114**

**Figure 6.**

**Figure 7.**

*Sample annealed at 1050°C for 4 h, post-annealed at 100°C for 3 h: (a) diffraction pattern; (b) hysteretic curves [60].*

Diffraction pattern of the sample post-annealed at 100°C is shown in **Figure 7(a)** [67]:

The only noticed phase was the ε-Fe2O3 phase. In order to examine its magnetic behavior, hysteretic curves are measured and shown in the **Figure 7(b)** [67]. Measured magnetic field of the sample was 1611 Oe. Literature data showed that obtained value is not exactly characteristic for epsilon phase coercivity, and it would be more appropriate to ascribe that Hmeas value to hematite phase, then for epsilon. According to **Figure 7(b)** and the literature data [67], it can be concluded that post-annealing treatment at 100°C brings to the drastic drop of measured measured magnetic field value. Before post-annealing treatment, sample annealed at 1050°C for 4 h showed measured magnetic field value of 21.3 kOe [63], although the phase composition of the sample was the same [67]. This fact underlined that sharp changes in the Hmeas value of the nanocomposite samples prepared by sol-gel method could not be ascribed to Fe2O3 polymorph transformations.

To get a better insight in the Hmeas variations initiated by post-annealing treatment, the piece of alcogel annealed at 1050°C for 4 h was performed to the postannealing treatment at 1100°C for 3 h [63].

**Figure 8(a)** revealed that the only observed iron oxide phase is pure α-Fe2O3 phase [63], pointing to the ending of the ε-Fe2O3 to α-Fe2O3 phase transformation. In diffraction pattern is in addition noticed change of the silica matrix, converted from amorphous silica to highly crystalline cristoballite (JCPDS card no.: 39-1425) and quartz phase (JCPDS card no.: 46-1045) [56, 63]. Interestingly, hysteretic curves shown in **Figure 8(b)** exerted measured magnetic field value of 1760 Oe

#### **Figure 8.**

*Sample annealed at 1050°C for 4 h, post-annealed at 1100°C for 3 h: (a) diffraction pattern (letter "c" referred to cristoballite SiO2 phase; letter "q" referred to quartz SiO2 phase); (b) hysteretic curves [49].*

**117**

**Figure 9.**

*Preparation and Characterization of Fe2O3-SiO2 Nanocomposite for Biomedical Application*

that is quite similar to the measured value of the sample post-annealed at 100°C (Hmeas ~ 1600 Oe). Noteworthy, **Figures 7(b)** and **8(b)** showed a similar values of measured magnetic field of the samples consisted of obviously different phases (**Figures 7(a)** and **8(a)**), pointing to the potential usage of the samples consisted of α-Fe2O3 and ε-Fe2O3 together (as in the case of "SPION" species), although this

synthesis cause the significant alterations of the phase composition of the investigated samples and differences in Hmeas value (**Figures 1** and **5**). Appearance of the catalyst accelerated phase transformations of the Fe2O3 polymorph and favors the obtaining of

Noteworthy, nanocomposite samples containing different phase compositions could be characterized by a significantly similar measured magnetic field values. This is confirmed by examination of the samples prepared by auto-catalyzed sol-gel synthesis (**Figures 3** and **5**) and samples synthesized by base-catalyzed sol-gel method (**Figures 7** and **8**). An observed feature is pointing to the fact that coercivity of the nanocomposite materials could not be mainly driven by the parameters α and β from Eq. (2). The results represented in this chapter indicated the necessity of taking other parameters [parameters: γ − κ, Eq. (2)] into consideration, in order to

An important issue in the characterization of the nanoparticles presents transmission electron microscopy (TEM). In order to understand the distribution of

*TEM micrographs of the sample: (a) prepared by auto-catalyzed sol-gel synthesis, annealed at 1050°C for 3 h* 

*[52], left; (b) prepared by base-catalyzed sol-gel synthesis, annealed at 1050°C for 4 h [56], right.*

understand properly the coercivity behavior of nanocomposite materials.

precursor and change of the precursor ratio during

claim requires deeper investigation of magnetic properties of the samples. Conclusively, it is important to notice that all hysteretic curves (except **Figure 6(b)**), referring to the sample containing the only hematite phase, observed at lower temperature in comparison with the other investigated samples) could be classified as hysteretic loops having constricted middles (wasp-waisted loops). Generation of wasp-waisting curves appears as a result of two population of the particles characterized by distinct coercivity spectra; numerical simulations reveals that wasp-waisting curves requires and SPM contribution [68], that is confirmed by **Figures 4(b)** and **1(b)** [58]. Experimental results shown in **Figures 2(a)** and **3(a)** revealed that the decrease of measured magnetic field value of the samples is not a certain parameter that indicates vanishing of the other iron oxide polymorphs and the presence of the pure hematite nanoparticles. Moreover, represented results pointed out that ε-Fe2O3 → α-Fe2O3 phase transformation cannot be the decisive factor on the coercivity value of the nanocomposite material (**Figures 1**–**4**) [58]. The variation of the initial iron ion precursor amount [60] enables the alteration of measured magnetic field value without changing the phase composition of the nanocomposite mate-

*DOI: http://dx.doi.org/10.5772/intechopen.81926*

rial. The usage of anhydrous Fe**<sup>3</sup>**<sup>+</sup>

**6.4 TEM measurements**

the pure nano-hematite phase (**Figure 6(a)**).

#### *Preparation and Characterization of Fe2O3-SiO2 Nanocomposite for Biomedical Application DOI: http://dx.doi.org/10.5772/intechopen.81926*

that is quite similar to the measured value of the sample post-annealed at 100°C (Hmeas ~ 1600 Oe). Noteworthy, **Figures 7(b)** and **8(b)** showed a similar values of measured magnetic field of the samples consisted of obviously different phases (**Figures 7(a)** and **8(a)**), pointing to the potential usage of the samples consisted of α-Fe2O3 and ε-Fe2O3 together (as in the case of "SPION" species), although this claim requires deeper investigation of magnetic properties of the samples.

Conclusively, it is important to notice that all hysteretic curves (except **Figure 6(b)**), referring to the sample containing the only hematite phase, observed at lower temperature in comparison with the other investigated samples) could be classified as hysteretic loops having constricted middles (wasp-waisted loops). Generation of wasp-waisting curves appears as a result of two population of the particles characterized by distinct coercivity spectra; numerical simulations reveals that wasp-waisting curves requires and SPM contribution [68], that is confirmed by **Figures 4(b)** and **1(b)** [58]. Experimental results shown in **Figures 2(a)** and **3(a)** revealed that the decrease of measured magnetic field value of the samples is not a certain parameter that indicates vanishing of the other iron oxide polymorphs and the presence of the pure hematite nanoparticles. Moreover, represented results pointed out that ε-Fe2O3 → α-Fe2O3 phase transformation cannot be the decisive factor on the coercivity value of the nanocomposite material (**Figures 1**–**4**) [58]. The variation of the initial iron ion precursor amount [60] enables the alteration of measured magnetic field value without changing the phase composition of the nanocomposite material. The usage of anhydrous Fe**<sup>3</sup>**<sup>+</sup> precursor and change of the precursor ratio during synthesis cause the significant alterations of the phase composition of the investigated samples and differences in Hmeas value (**Figures 1** and **5**). Appearance of the catalyst accelerated phase transformations of the Fe2O3 polymorph and favors the obtaining of the pure nano-hematite phase (**Figure 6(a)**).

Noteworthy, nanocomposite samples containing different phase compositions could be characterized by a significantly similar measured magnetic field values. This is confirmed by examination of the samples prepared by auto-catalyzed sol-gel synthesis (**Figures 3** and **5**) and samples synthesized by base-catalyzed sol-gel method (**Figures 7** and **8**). An observed feature is pointing to the fact that coercivity of the nanocomposite materials could not be mainly driven by the parameters α and β from Eq. (2). The results represented in this chapter indicated the necessity of taking other parameters [parameters: γ − κ, Eq. (2)] into consideration, in order to understand properly the coercivity behavior of nanocomposite materials.

## **6.4 TEM measurements**

An important issue in the characterization of the nanoparticles presents transmission electron microscopy (TEM). In order to understand the distribution of

#### **Figure 9.**

*TEM micrographs of the sample: (a) prepared by auto-catalyzed sol-gel synthesis, annealed at 1050°C for 3 h [52], left; (b) prepared by base-catalyzed sol-gel synthesis, annealed at 1050°C for 4 h [56], right.*

*Mineralogy - Significance and Applications*

**Figure 7(a)** [67]:

**Figure 7.**

*curves [60].*

Diffraction pattern of the sample post-annealed at 100°C is shown in

*Sample annealed at 1050°C for 4 h, post-annealed at 100°C for 3 h: (a) diffraction pattern; (b) hysteretic* 

behavior, hysteretic curves are measured and shown in the **Figure 7(b)** [67]. Measured magnetic field of the sample was 1611 Oe. Literature data showed that obtained value is not exactly characteristic for epsilon phase coercivity, and it would be more appropriate to ascribe that Hmeas value to hematite phase, then for epsilon. According to **Figure 7(b)** and the literature data [67], it can be concluded that post-annealing treatment at 100°C brings to the drastic drop of measured measured magnetic field value. Before post-annealing treatment, sample annealed at 1050°C for 4 h showed measured magnetic field value of 21.3 kOe [63], although the phase composition of the sample was the same [67]. This fact underlined that sharp changes in the Hmeas value of the nanocomposite samples prepared by sol-gel

method could not be ascribed to Fe2O3 polymorph transformations.

annealing treatment at 1100°C for 3 h [63].

The only noticed phase was the ε-Fe2O3 phase. In order to examine its magnetic

To get a better insight in the Hmeas variations initiated by post-annealing treatment, the piece of alcogel annealed at 1050°C for 4 h was performed to the post-

**Figure 8(a)** revealed that the only observed iron oxide phase is pure α-Fe2O3 phase [63], pointing to the ending of the ε-Fe2O3 to α-Fe2O3 phase transformation. In diffraction pattern is in addition noticed change of the silica matrix, converted from amorphous silica to highly crystalline cristoballite (JCPDS card no.: 39-1425) and quartz phase (JCPDS card no.: 46-1045) [56, 63]. Interestingly, hysteretic curves shown in **Figure 8(b)** exerted measured magnetic field value of 1760 Oe

*Sample annealed at 1050°C for 4 h, post-annealed at 1100°C for 3 h: (a) diffraction pattern (letter "c" referred* 

*to cristoballite SiO2 phase; letter "q" referred to quartz SiO2 phase); (b) hysteretic curves [49].*

**116**

**Figure 8.**

nano-hematite particles, TEM micrographs of the chosen nanocomposite samples annealed at 1050°C, obtained by auto-catalyzed and base-catalyzed sol-gel synthesis routes, are shown in **Figure 9(a)** and **(b)**.

Detailed TEM analysis is given in Ref. [59, 63]. Quantitative description of morphological properties of the investigated particles is performed by measuring ellipticity. The results of the analysis showed that the shape of observed Fe2O3 nanoparticles (ε-Fe2O3 and α-Fe2O3) varies from ellipticity to circularity. In other words, **Figure 9** confirms the presence of nonideally spherical particles, whose shape deviates from circularity in a different measure [59]. Fe2O3 particle sizes, presented in **Figure 9(a)**, are ranging between 10 and 20 nm, while the sample presented in **Figure 9(b)** showed a wider particle size distribution, from 4 to 50 nm, and the same variations from ellipticity to circularity. Wide size distribution leads to the presence of different particle shapes during the annealing treatment, elliptic/spherical (**Figure 9(a)**). This feature appeared as a consequence of the fact that sol-gel method consisted of coprecipitation of the particles within the SiO2 pores (coprecipitated samples are characterized by wide particle size distribution) [53]. The best way to overcome the mentioned problem is the coating of the nanoparticles within the SiO2 pores [69].

Notwithstanding, a significant difference between micrographs is the presence of nanorod particles within the sample synthesized by base-catalyzed sol-gel synthesis (**Figure 9(b)**). Rod-like morphology appeared as a consequence of the participation of group II element, Sr2+, in the synthesis procedure (**Figure 9(b)**). The addition of Sr2+ ions accelerated the growth of the ε-Fe2O3 particles in one crystallographic axis [63], inducing more pronounced shape variations and formation of rod-like nanoparticles. If we recall the fact that TEM image shown in **Figure 9(b)** revealed the presence of the only γ-Fe2O3 and ε-Fe2O3 nanoparticles in the investigated sample (discussed in more detail in the Ref. [63]), as well as having in mind that α-Fe2O3 formation occurs as a consequence of phase transformations ε-Fe2O3 → α-Fe2O3, it can be assumed that the sample presented in **Figure 8** contained rod-like α-Fe2O3 nanoparticles.

It is important to note that the origin of dependence of Hmeas behavior on the synthesis conditions of the samples investigated in this chapter is found in quantum mechanics.

Briefly, the quantity that strongly affects the shape of hysteresis loop is magnetic anisotropy [parameter ι, in Eq. (3)]. For the most simplest case, in crystal systems whose symmetry is determined by a single axis of high symmetry (uniaxial symmetry), anisotropy energy is defined as:

$$\mathbf{E\_{z}} \sim \mathbf{KVsin}^{2} \mathbf{0} \tag{4}$$

**119**

*Preparation and Characterization of Fe2O3-SiO2 Nanocomposite for Biomedical Application*

initiated by thermal treatment, caused the crystallization of the amorphous matrix and changes in the size of the pores. Coalescing of the pores resulted in the alterations of the distance between magnetic nanoparticles within the pores [59], conse-

The main message of this chapter was to emphasize the importance of the investigation of the influence of the synthesis parameter variations onto the magnetic properties of the composite materials containing nano-hematite particles that could be used as a starting material for preparation of multifunctional nanoparticles, used in different areas of biomedicine. Since coercivity field presents a parameter of importance for application of this type of materials, alterations of measured measured Hcrit value, initiated by changing the synthesis parameters, are discussed. To get a better insight into relation between synthesis conditions and magnetic properties of composites containing α-Fe2O3 nanoparticles, sol-gel synthesis is recognized as a suitable preparation method. Alterations of measured Hcrit value of the samples are driven by the variation of the pH of the performed sol-gel synthesis

the iron precursor, and annealing conditions (T and t) and by performing postannealing treatment. The author expected that this chapter will facilitate a current and objective evaluation of the knowledge regarding the search for the exact mathematical expression of the measured intrinsic coercivity field value of the composite nanomaterials containing nano-hematite phase, which is of significance for improvement of the preparation of a high-quality nano-α-Fe2O3 particles for

The research was carried out thanks to the support of the Ministry of Education, Science and Technology Development, Republic of Serbia (Project No. III 45015). The author gratefully acknowledges Dr. Vojislav Spasojević for the magnetic mea-

Laboratory for Theoretical Physics and Condensed Matter Physics, Institute of

© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

Nuclear Sciences Vinča, University of Belgrade, Serbia

\*Address all correspondence to: violeta@vinca.rs

provided the original work is properly cited.

and Si4+ precursor ratio, amount of

*DOI: http://dx.doi.org/10.5772/intechopen.81926*

(auto-, acid-, or base-catalyzed), initial Fe**<sup>3</sup>**<sup>+</sup>

biomedical application.

**Acknowledgements**

surement (**Figure 5(a)**).

**Author details**

Violeta N. Nikolić

**7. Conclusion**

quently influencing the magnetic behavior of the samples.

where K is the anisotropy constant, V is the volume, and θ is the angle between two spins with respect to each other [17]. The overall magnetic anisotropy energy is dependent on the symmetry of the investigated systems and defined by various contributions, such as magnetocrystalline anisotropy, shape anisotropy, surface anisotropy, strain anisotropy and stress anisotropy.

Anisotropy energy appeared as a consequence of spin-orbit interaction and the partial quenching of the angular momentum [17]. From the aspect of nanomaterial preparation and dependence of samples of magnetic properties on synthesis conditions (annealing temperature and time), it is important to emphasize that the anisotropy constant is strongly temperature dependent [17]. Independent of the presence of the same or different iron oxide polymorph phases within the sample, differences in the structure and morphology characteristics of each individual nanoparticle resulted in the changes in a magnetic anisotropy.

Noteworthy, alteration of the SiO2 matrix during the annealing treatment impacts magnetic properties of the samples [59]. Gas diffusion in the SiO2 matrix, initiated by thermal treatment, caused the crystallization of the amorphous matrix and changes in the size of the pores. Coalescing of the pores resulted in the alterations of the distance between magnetic nanoparticles within the pores [59], consequently influencing the magnetic behavior of the samples.
