**6. α-Fe2O3 nanoparticles prepared by auto-, acid-, and base-catalyzed sol-gel syntheses**

In the following part, synthesis of the samples prepared by auto-, acid-, and base-catalyzed sol-gel methods will be described in detail. X-ray diffraction patterns and hysteretic measurements recorded at 200 K are shown. X-ray diffraction intensity is normalized. In the entire text, the value of the magnetization (M) is normalized so that Ms = 1*.* The normalized values of magnetization were introduced in order to avoid uncertainty in the estimation of the magnetization expressed in emu/g. Having in mind that Hcrit presents magnetic field measured by magnetometer, Hcrit will be labeled as Hmeas in the figures of hysteresis.

#### **6.1 Nano-hematite-based materials prepared by auto-catalyzed sol-gel synthesis**

The auto-catalyzed sol-gel synthesis implied the dissolving of iron (III) nitrate nonahydrate (Fe(NO3)3 × 9H2O) in water in molar ratio 0.013:1 (catalyst solution),

**111**

**Figure 1.**

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

while mixing of tetraethyl orthosilicate (TEOS), ethanol (C2H5OH), and water in molar proportion 1:12:12 enabled formation of alkoxide solution [59]. Solutions were mixed and stirred at room temperature. Gelation occurred during 36 days, afterward alcogel was dried for 5h at room temperature. Thermal treatment is performed in two ways. Alcogel is annealed in the air atmosphere for 3 h at 1050 and 1060°C as well as at 1050°C for 25 h. Investigated samples contained α-Fe2O3 as a dominant phase and smaller amount of the ε-Fe2O3 phase. Variation of the annealing conditions enables observation of the changes in the Hcrit of the prepared

*6.1.1 Variation of the sol-gel synthesis conditions: alteration of the annealing* 

Diffraction pattern of the alcogel annealed at 1050°C for 3 h is presented in

The increase of the annealing temperature for only 10°C (Tann = 1060°C, tann = 3 h) resulted in the sharp decrease of Hmeas (2300 Oe), which is shown in

Hematite nanocrystallites (JCPDS card no.: 72-469) are observed as a dominant phase, while α-Fe2O3 phase (JCPDS card no.: 16-653) is presented in small amount. **Figure 2** presents hysteresis of the same sample [59]. Although the sample showed a higher amount of the α-Fe2O3 phase, measured critical field achieved a value

Observed behavior of measured critical field of the sample has been attributed to the completion of an ε-Fe2O3 → α-Fe2O3 phase transformation [59]; thus the presence of the only one phase—α-Fe2O3 phase—at 1060°C would be expected. With the aim to check the concluded remark regarding the completion of ε-Fe2O3 → α-Fe2O3 phase transformation at depicted temperature, investigation presented in Ref. [59] is continued by measuring the diffraction pattern of the sample annealed at 1060°C 3 h. Diffraction measurement (**Figure 2(b)**) revealed that, although the value of 2300 Oe could be characteristic for nano-sized α-Fe2O3 samples [24, 59], an investigated sample still contained the ε-Fe2O3 phase, although represented in the smaller

On the other hand, published data reported more pronounced sharp decrease in Hmeas value (600 Oe) for the sample performed to annealing at 1050°C for 25 h (**Figure 3(a)**). In order to confirm the completion of the ε-Fe2O3 to α-Fe2O3 phase transformation under these annealing conditions [59], investigation was continued by the measuring diffraction pattern of the mentioned sample (**Figure 3(b)**).

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

samples, which is in detail discussed in Ref. [59].

characteristic for ε-Fe2O3 phase: 14.1 kOe (**Figure 2**) [59].

amount than the sample annealed at 1050°C (**Figure 1(a)**).

*Sample annealed at 1050°C for 3 h (a) diffraction pattern; (b) hysteretic curves [59].*

*conditions (temperature and time)*

**Figure 1(a)** [59].

**Figure 2(a)** [59].

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

while mixing of tetraethyl orthosilicate (TEOS), ethanol (C2H5OH), and water in molar proportion 1:12:12 enabled formation of alkoxide solution [59]. Solutions were mixed and stirred at room temperature. Gelation occurred during 36 days, afterward alcogel was dried for 5h at room temperature. Thermal treatment is performed in two ways. Alcogel is annealed in the air atmosphere for 3 h at 1050 and 1060°C as well as at 1050°C for 25 h. Investigated samples contained α-Fe2O3 as a dominant phase and smaller amount of the ε-Fe2O3 phase. Variation of the annealing conditions enables observation of the changes in the Hcrit of the prepared samples, which is in detail discussed in Ref. [59].

## *6.1.1 Variation of the sol-gel synthesis conditions: alteration of the annealing conditions (temperature and time)*

Diffraction pattern of the alcogel annealed at 1050°C for 3 h is presented in **Figure 1(a)** [59].

Hematite nanocrystallites (JCPDS card no.: 72-469) are observed as a dominant phase, while α-Fe2O3 phase (JCPDS card no.: 16-653) is presented in small amount. **Figure 2** presents hysteresis of the same sample [59]. Although the sample showed a higher amount of the α-Fe2O3 phase, measured critical field achieved a value characteristic for ε-Fe2O3 phase: 14.1 kOe (**Figure 2**) [59].

The increase of the annealing temperature for only 10°C (Tann = 1060°C, tann = 3 h) resulted in the sharp decrease of Hmeas (2300 Oe), which is shown in **Figure 2(a)** [59].

Observed behavior of measured critical field of the sample has been attributed to the completion of an ε-Fe2O3 → α-Fe2O3 phase transformation [59]; thus the presence of the only one phase—α-Fe2O3 phase—at 1060°C would be expected. With the aim to check the concluded remark regarding the completion of ε-Fe2O3 → α-Fe2O3 phase transformation at depicted temperature, investigation presented in Ref. [59] is continued by measuring the diffraction pattern of the sample annealed at 1060°C 3 h. Diffraction measurement (**Figure 2(b)**) revealed that, although the value of 2300 Oe could be characteristic for nano-sized α-Fe2O3 samples [24, 59], an investigated sample still contained the ε-Fe2O3 phase, although represented in the smaller amount than the sample annealed at 1050°C (**Figure 1(a)**).

On the other hand, published data reported more pronounced sharp decrease in Hmeas value (600 Oe) for the sample performed to annealing at 1050°C for 25 h (**Figure 3(a)**). In order to confirm the completion of the ε-Fe2O3 to α-Fe2O3 phase transformation under these annealing conditions [59], investigation was continued by the measuring diffraction pattern of the mentioned sample (**Figure 3(b)**).

**Figure 1.** *Sample annealed at 1050°C for 3 h (a) diffraction pattern; (b) hysteretic curves [59].*

*Mineralogy - Significance and Applications*

(parameter β)

matrix (parameter γ)

a.Synthesis conditions (parameter α)

The main influence on the coercivity value arises from:

e.Particle size and shape distribution (parameter ε)

g.Nanoparticle structural defects (denoted as parameter θ)

nanoparticle system (parameter η)

h.Anisotropy field (parameter ι)

i. Surface effects (κ)

samples will be represented.

**sol-gel syntheses**

b.Presence of different iron oxide species in the investigated sample

d.Angular distribution of the nanoparticle orientations (parameter δ)

f. An interplay between different inter- and intra-particle interactions in the

In the next section, the impact of the parameters α and β on the measured magnetic field, that is in literature labeled as coercivity field, of the synthesized nanocomposite materials will be considered. X-ray diffraction and hysteretic measurements were performed in order to investigate the influence of the variation of synthesis parameters onto the formation of hematite phase as well as on the measured magnetic field value of the samples containing the pure hematite phase and hematite phase (as a dominant phase) in combination with the epsilon phase (appeared in traces). Few examples of the peculiar Hcrit behavior of the investigated

**6. α-Fe2O3 nanoparticles prepared by auto-, acid-, and base-catalyzed** 

In the following part, synthesis of the samples prepared by auto-, acid-, and base-catalyzed sol-gel methods will be described in detail. X-ray diffraction patterns and hysteretic measurements recorded at 200 K are shown. X-ray diffraction intensity is normalized. In the entire text, the value of the magnetization (M) is normalized so that Ms = 1*.* The normalized values of magnetization were introduced in order to avoid uncertainty in the estimation of the magnetization expressed in emu/g. Having in mind that Hcrit presents magnetic field measured by magnetometer, Hcrit will be labeled as Hmeas in the figures of

**6.1 Nano-hematite-based materials prepared by auto-catalyzed sol-gel** 

The auto-catalyzed sol-gel synthesis implied the dissolving of iron (III) nitrate nonahydrate (Fe(NO3)3 × 9H2O) in water in molar ratio 0.013:1 (catalyst solution),

c.Contribution originated from the physical and chemical properties of the SiO2 matrix, such as pore size distribution or the flow of different gases through the

**110**

hysteresis.

**synthesis**

**Figure 2.** *Sample annealed at 1060°C for 3 h: (a) hysteretic curves [59]; (b) diffraction pattern.*

**Figure 3.** *Sample annealed at 1050°C for 25 h: (a) hysteretic curves [59]; (b) diffraction pattern.*

In spite of a very long annealing treatment (25 h), the traces of the epsilon phase is still presented (2θ = 30.25°) at the diffraction pattern presented in **Figure 3(b)**, although Hmeas was very low for the epsilon phase, 600 Oe (**Figure 3(a)**).

Observed results pointed to the often mistakes in the scientific literature, where is sharp decrease in the measured magnetic field (Hmeas) of the samples containing hematite and epsilon phases attributed to the finish of the ε-Fe2O3 → α-Fe2O3 phase transformation. In other words, vanishing of the huge value of the measured field (which is in the articles denoted as Hc) of the probed samples is obviously not a conclusive evidence that is pointing to the absence of epsilon phase in the sample and which could confirms completion of the ε-Fe2O3 → α-Fe2O3 transformation.

### *6.1.2 Variation of the sol-gel synthesis conditions: alteration of the iron precursor initial amount*

Further research of the variation of the sol-gel synthesis parameters directed the investigation in the course of altering of the amount of the iron precursor, Fe(NO3)3·9H2O. The sample is synthesized by the identical auto-catalyzed sol-gel procedure used for the preparation of the former discussed samples, with the only difference that the molar ratio of Fe(NO3)3·9H2O and water was 0.017:1 [60]. Alcogel is annealed at 1030°C for 3 h in the air atmosphere. Diffraction pattern is shown in **Figure 4(a)** [60].

**Figure 4(a)** revealed that α-Fe2O3 phase is recognized as a dominant phase, and ε-Fe2O3 phase as an impurity. Noteworthy, comparison of **Figures 4(a)** and **1(a)** pointed to pronounced similarities between phase compositions of the investigated samples. Surprisingly, hysteretic measurements revealed that, in spite of nearly the

**113**

**Figure 5.**

*(b) hysteretic curves.*

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

same diffraction patterns, the examined sample showed significantly lower value of measured magnetic field, 7.5 kOe [60]. Deeper analysis, that is out of the objective of this chapter but in detail explained elsewhere [60], showed that the origin of the Hmeas value variations occurred as a consequence of the increased amount of the

*Sample annealed at 1050°C for 3 h (reduced content of the iron precursor): (a) diffraction pattern;* 

*6.1.3 Variation of the sol-gel synthesis conditions: alteration of the iron (III) nitrate* 

Hydrated iron (III) nitrate shows high non-stability and tendency to absorb humidity from the air [61]. To ensure avoidance of the contact with the air, the best way is to prepare iron (III) nitrate directly from the elemental iron and nitric acid solution. In order to investigate behavior of measured magnetic field of the sample prepared by auto-catalyzed sol-gel synthesis with the usage of anhydrous iron nitrate (instead of nonahydrated iron (III) nitrate) as a precursor, the next procedure is performed: catalyst solution is prepared by the dissolving of iron (III) nitrate in water in molar ratio 0.007:1. Alkoxide solution is prepared by mixing tetraethyl orthosilicate (TEOS), ethanol (C2H5OH), and water in molar proportion 1:6:6. Mixed solutions are stirred for 5 h. Gelation takes place in 20 days. Gel is dried

**Figure 5(a)** depicted the complete absence of the α-Fe2O3 phase and presence of the ε-Fe2O3 phase as the only observed iron oxide phase in the sample. Hysteretic

*Sample annealed at 1030°C for 3 h (anhydrous Fe(NO3)3 used as a precursor): (a) diffraction pattern;* 

SPM nanoparticles within the investigated sample [60].

for 19 h at 80°C and afterward is annealed at 1030°C for 3 h. Diffraction pattern of the sample is shown in **Figure 5(a)**.

*precursor*

*(b) hysteretic curves [59].*

**Figure 4.**

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

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

**Figure 4.**

*Mineralogy - Significance and Applications*

**Figure 2.**

**Figure 3.**

In spite of a very long annealing treatment (25 h), the traces of the epsilon phase is still presented (2θ = 30.25°) at the diffraction pattern presented in **Figure 3(b)**,

Observed results pointed to the often mistakes in the scientific literature, where is sharp decrease in the measured magnetic field (Hmeas) of the samples containing hematite and epsilon phases attributed to the finish of the ε-Fe2O3 → α-Fe2O3 phase transformation. In other words, vanishing of the huge value of the measured field (which is in the articles denoted as Hc) of the probed samples is obviously not a conclusive evidence that is pointing to the absence of epsilon phase in the sample and which could confirms completion of the ε-Fe2O3 → α-Fe2O3 transformation.

although Hmeas was very low for the epsilon phase, 600 Oe (**Figure 3(a)**).

*Sample annealed at 1050°C for 25 h: (a) hysteretic curves [59]; (b) diffraction pattern.*

*Sample annealed at 1060°C for 3 h: (a) hysteretic curves [59]; (b) diffraction pattern.*

*6.1.2 Variation of the sol-gel synthesis conditions: alteration of the iron precursor* 

Further research of the variation of the sol-gel synthesis parameters directed the investigation in the course of altering of the amount of the iron precursor, Fe(NO3)3·9H2O. The sample is synthesized by the identical auto-catalyzed sol-gel procedure used for the preparation of the former discussed samples, with the only difference that the molar ratio of Fe(NO3)3·9H2O and water was 0.017:1 [60]. Alcogel is annealed at 1030°C for 3 h in the air atmosphere. Diffraction pattern is

**Figure 4(a)** revealed that α-Fe2O3 phase is recognized as a dominant phase, and ε-Fe2O3 phase as an impurity. Noteworthy, comparison of **Figures 4(a)** and **1(a)** pointed to pronounced similarities between phase compositions of the investigated samples. Surprisingly, hysteretic measurements revealed that, in spite of nearly the

**112**

*initial amount*

shown in **Figure 4(a)** [60].

*Sample annealed at 1050°C for 3 h (reduced content of the iron precursor): (a) diffraction pattern; (b) hysteretic curves [59].*

same diffraction patterns, the examined sample showed significantly lower value of measured magnetic field, 7.5 kOe [60]. Deeper analysis, that is out of the objective of this chapter but in detail explained elsewhere [60], showed that the origin of the Hmeas value variations occurred as a consequence of the increased amount of the SPM nanoparticles within the investigated sample [60].

### *6.1.3 Variation of the sol-gel synthesis conditions: alteration of the iron (III) nitrate precursor*

Hydrated iron (III) nitrate shows high non-stability and tendency to absorb humidity from the air [61]. To ensure avoidance of the contact with the air, the best way is to prepare iron (III) nitrate directly from the elemental iron and nitric acid solution. In order to investigate behavior of measured magnetic field of the sample prepared by auto-catalyzed sol-gel synthesis with the usage of anhydrous iron nitrate (instead of nonahydrated iron (III) nitrate) as a precursor, the next procedure is performed: catalyst solution is prepared by the dissolving of iron (III) nitrate in water in molar ratio 0.007:1. Alkoxide solution is prepared by mixing tetraethyl orthosilicate (TEOS), ethanol (C2H5OH), and water in molar proportion 1:6:6. Mixed solutions are stirred for 5 h. Gelation takes place in 20 days. Gel is dried for 19 h at 80°C and afterward is annealed at 1030°C for 3 h.

Diffraction pattern of the sample is shown in **Figure 5(a)**.

**Figure 5(a)** depicted the complete absence of the α-Fe2O3 phase and presence of the ε-Fe2O3 phase as the only observed iron oxide phase in the sample. Hysteretic

#### **Figure 5.**

*Sample annealed at 1030°C for 3 h (anhydrous Fe(NO3)3 used as a precursor): (a) diffraction pattern; (b) hysteretic curves.*

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 the sample in detail.

#### **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 for 2 h.

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

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

**Figure 6.** *Sample annealed at 800°C for 2 h (HNO3 used as a catalyst): (a) diffraction pattern [62]; (b) hysteretic curves.*

**115**

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

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 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

*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

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].

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

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

**synthesis**

in Ref. [63].

and NH3 in the volume ratio 2:5.

temperature (1100°C) [63, 67].
