**1.9. Thermal plasma**

Thermal plasmas, such as direct current arcs and radio frequency (RF) plasmas, offer unique advantages for the synthesis of ceramic powders due to the easily achievable high temperatures and energy densities. In a RF thermal plasma flame, the gas temperatures may exceed 10<sup>4</sup> K independently of the gas composition. In addition, a high temperature gradient exists between the hot plasma flame and the surrounding gas phase. The resulting rapid quenching rate is favorable for producing fine particles with unstable structures in thermodynamic terms [17].

The large variety of methods used to obtain zinc ferrites nanoparticles reflects on a very scattered scenery in terms of properties of the obtained materials. This is not simply due to the above-mentioned close correlation among functional parameters and morphology, gran size, intentional doping, cation distribution, and nanoparticle architecture. Indeed, it is extremely important to verify and control the magnetic, structural and chemical purity, and homogeneity of nanoparticles because even low amount of extrinsic phases or elements can alter the functional properties. In particular, it is very important to monitor the presence of unwanted iron oxides phases, possible source of extrinsic contribution to the functional properties of zinc ferrites. In addition, several factors such as nucleation, growth, aggregation, and adsorption of impurities can affect the morphology of prepared particles [18]. It is thus evident the need to deeply characterize the materials with a multi-technique approach to reveal even subtle effects due to unwanted alterations of the designed nanoparticles.

cell contains 2 formula units for a total of 14 atoms. By the factor group analysis, it is pos-

where the (R) and (IR) identify Raman-active and infrared-active vibrational species, respectively, and the rest of the modes are silent modes. The Eg,u and F1g,2g,1u,2u modes are doubly and

observed [25]. The three Raman-active F2g modes are labeled F2g(1), F2g(2), and F2g(3), where F2g(1) is the lowest frequency F2g mode and F2g(3) is the highest frequency mode of this

**Figure 2** shows the spectra from two different ZFO prepared by our group. These spectra are reported as an example of the features detected in the Raman spectrum of ZFO. The red spectrum is obtained from pure ZFO, synthesized by solid state reaction (see Section 2.2.), while the black curve is the Raman signal of an Al-doped ZFO sample synthesized by coprecipitation method. In the region 100–800 cm−1, three different spectral intervals can be recognized for both samples: (i) 600–800 cm−1 is the region of A1g modes; (ii) 410–550 cm−1 is the region of F2g(3) modes, and (iii) 260–380 cm−1 should be the region of F2g(2) modes. At

Al-doped sample, narrow extra peaks are detected at about 218, 285, and 395 cm−1. These

**Figure 2.** Raman spectra from two different ZFO prepared by our group: Pure ZFO synthesized by solid state reaction (red curve) and Al-doped ZFO synthesized by conventional co-precipitation method (black curve). The latter will not be

and F2g(1) are sometimes detected. Furthermore in the spectrum from

triply degenerate, respectively. The three acoustic modes belong to the F1u species.

O4

+ 3 *F*2g(*R*) + 2 *A*2*<sup>u</sup>* + 2 *Eu* + 4 *F*1*<sup>u</sup>*

spinel, 3 acoustic modes, and 39 optical

Raman Spectroscopy in Zinc Ferrites Nanoparticles http://dx.doi.org/10.5772/intechopen.72864

(IR) + 2 *F*2*<sup>u</sup>* (1)

229

+ E<sup>g</sup> + 3F2g should be

sible to derive the following 42 modes in ZnFe<sup>2</sup>

(*R*) + *F*1g

Thus in the Raman spectra of zinc ferrites, only five modes A1g

modes [19]:

*A*1g(*R*) + *E*<sup>g</sup>

vibrational species.

lower frequencies, E<sup>g</sup>

further considered in the chapter.

Usually, all the researchers give evidence of the magnetic properties measuring by SQUID magnetometer or Vibrating Sample Magnetometry, Saturation magnetization (M<sup>s</sup> ), Coercivity (H<sup>c</sup> ), and Blocking temperature (TB). The local inspection about the source of magnetic behavior is usually accomplished by combining 57Fe Mossbauer, XANES, XPS, and EPR investigations. Morphology and chemical compositions are usually investigated by TEM and SEM techniques, the latter with the possibility to perform EDX analyses for elemental check. Sometimes, LA-ICP is used to validate the composition. Crystalline structure quality is usually assessed by XRD analyses and micro-Raman measurements, that also allowed to estimate the inversion degree.

Raman spectroscopy (RS) has been widely used to study spinel ferrites and in particular zinc ferrites both for routine identification of materials [19] and for very fine investigations concerning basic phenomena. This is due to an interesting overlapping between characteristics of RS and properties of nanosized zinc ferrites: (i) RS sensitivity to cations distribution [20], (ii) RS ability to reveal the presence of extrinsic iron oxides phases like hematite or magnetite [21], (iii) possibility to use RS to monitor stability of spinel ferrites vs. light exposure or thermal treatment [22], (iv) influence of nanometric scaling on Raman lines features—peak energies, widths and shape [23], (v) possibility to evaluate the presence of unwanted impurities. In addition, it is important to note that Raman spectroscopy has been used to study Surface-enhanced Raman scattering (SERS) when ferrites nanoparticles are functionalized for specific biomedical applications [24].

In this chapter, we combine a brief overview on the literature about Raman data from different zinc ferrites nanoparticles to new Raman results by our group on pure and doped zinc ferrites nanoparticles prepared by using the microwave-assisted combustion method. In particular, we present data obtained from undoped ZnFe<sup>2</sup> O4 , Zn1−xCax Fe2 O4 (x = 0.05 and 0.25), ZnFe1.9Gd0.1O4 , Zn0.95Sr0.05Fe2 O4 , and ZnFe1.9Al0.1O4 doped ferrites. The Ca substitution is particularly interesting due to the low toxicity of the substituent; on the other side, Gd ions are commonly used as contrast agents for MRI. In all the cases, the Raman data are accompanied by recalling functional parameters and synthesis procedures. For our samples, a more complete description about synthesis, morphology, and magnetic behavior are outlined allowing a better comprehension about the powerful of Raman spectroscopy in this kind of system.

#### **2. Raman effect in ZnFe<sup>2</sup> O4 nanoparticles**

ZnFe2 O4 spinel has a cubic structure that belongs to the space group Fd3m (O7 <sup>h</sup>) consisting of 8 molecules within the unit cell, for a total of 56 atoms; nevertheless the smallest Bravais cell contains 2 formula units for a total of 14 atoms. By the factor group analysis, it is possible to derive the following 42 modes in ZnFe<sup>2</sup> O4 spinel, 3 acoustic modes, and 39 optical modes [19]:

ferrites. In addition, several factors such as nucleation, growth, aggregation, and adsorption of impurities can affect the morphology of prepared particles [18]. It is thus evident the need to deeply characterize the materials with a multi-technique approach to reveal even subtle

Usually, all the researchers give evidence of the magnetic properties measuring by SQUID

Raman spectroscopy (RS) has been widely used to study spinel ferrites and in particular zinc ferrites both for routine identification of materials [19] and for very fine investigations concerning basic phenomena. This is due to an interesting overlapping between characteristics of RS and properties of nanosized zinc ferrites: (i) RS sensitivity to cations distribution [20], (ii) RS ability to reveal the presence of extrinsic iron oxides phases like hematite or magnetite [21], (iii) possibility to use RS to monitor stability of spinel ferrites vs. light exposure or thermal treatment [22], (iv) influence of nanometric scaling on Raman lines features—peak energies, widths and shape [23], (v) possibility to evaluate the presence of unwanted impurities. In addition, it is important to note that Raman spectroscopy has been used to study Surface-enhanced Raman scattering (SERS) when ferrites nanoparticles are functionalized for specific biomedical

In this chapter, we combine a brief overview on the literature about Raman data from different zinc ferrites nanoparticles to new Raman results by our group on pure and doped zinc ferrites nanoparticles prepared by using the microwave-assisted combustion method.

, and ZnFe1.9Al0.1O4

is particularly interesting due to the low toxicity of the substituent; on the other side, Gd ions are commonly used as contrast agents for MRI. In all the cases, the Raman data are accompanied by recalling functional parameters and synthesis procedures. For our samples, a more complete description about synthesis, morphology, and magnetic behavior are outlined allowing a better comprehension about the powerful of Raman spectroscopy in this

 **nanoparticles**

spinel has a cubic structure that belongs to the space group Fd3m (O7

of 8 molecules within the unit cell, for a total of 56 atoms; nevertheless the smallest Bravais

O4

, Zn1−xCax

Fe2 O4

doped ferrites. The Ca substitution

(x = 0.05 and

<sup>h</sup>) consisting

In particular, we present data obtained from undoped ZnFe<sup>2</sup>

**O4**

O4

, Zn0.95Sr0.05Fe2

), and Blocking temperature (TB). The local inspection about the source of magnetic behavior is usually accomplished by combining 57Fe Mossbauer, XANES, XPS, and EPR investigations. Morphology and chemical compositions are usually investigated by TEM and SEM techniques, the latter with the possibility to perform EDX analyses for elemental check. Sometimes, LA-ICP is used to validate the composition. Crystalline structure quality is usually assessed by XRD analyses and micro-Raman measurements, that also allowed to estimate

), Coercivity

magnetometer or Vibrating Sample Magnetometry, Saturation magnetization (M<sup>s</sup>

effects due to unwanted alterations of the designed nanoparticles.

(H<sup>c</sup>

228 Raman Spectroscopy

the inversion degree.

applications [24].

0.25), ZnFe1.9Gd0.1O4

kind of system.

ZnFe2 O4

**2. Raman effect in ZnFe<sup>2</sup>**

$$A\_{\rm 1g}(\text{R}) + E\_{\rm g}(\text{R}) + F\_{\rm 1g} + 3 \, F\_{\rm 2g}(\text{R}) + 2 \, A\_{\rm 2u} + 2 \, E\_u + 4 \, F\_{\rm 1u}(\text{IR}) + 2 \, F\_{\rm 2u} \tag{1}$$

where the (R) and (IR) identify Raman-active and infrared-active vibrational species, respectively, and the rest of the modes are silent modes. The Eg,u and F1g,2g,1u,2u modes are doubly and triply degenerate, respectively. The three acoustic modes belong to the F1u species.

Thus in the Raman spectra of zinc ferrites, only five modes A1g + E<sup>g</sup> + 3F2g should be observed [25]. The three Raman-active F2g modes are labeled F2g(1), F2g(2), and F2g(3), where F2g(1) is the lowest frequency F2g mode and F2g(3) is the highest frequency mode of this vibrational species.

**Figure 2** shows the spectra from two different ZFO prepared by our group. These spectra are reported as an example of the features detected in the Raman spectrum of ZFO. The red spectrum is obtained from pure ZFO, synthesized by solid state reaction (see Section 2.2.), while the black curve is the Raman signal of an Al-doped ZFO sample synthesized by coprecipitation method. In the region 100–800 cm−1, three different spectral intervals can be recognized for both samples: (i) 600–800 cm−1 is the region of A1g modes; (ii) 410–550 cm−1 is the region of F2g(3) modes, and (iii) 260–380 cm−1 should be the region of F2g(2) modes. At lower frequencies, E<sup>g</sup> and F2g(1) are sometimes detected. Furthermore in the spectrum from Al-doped sample, narrow extra peaks are detected at about 218, 285, and 395 cm−1. These

**Figure 2.** Raman spectra from two different ZFO prepared by our group: Pure ZFO synthesized by solid state reaction (red curve) and Al-doped ZFO synthesized by conventional co-precipitation method (black curve). The latter will not be further considered in the chapter.

Raman lines are associated to the presence of hematite inside the irradiated volume [21]. At higher energies, in the region 1000–1800 cm−1 only second-order features can be revealed, with a more intense feature just above 1300 cm−1 in the Al-doped sample due to second-order signal from hematite.

Briefly, we recall that micro-Raman measurements were carried out at RT by using a Labram Dilor spectrometer equipped with an Olympus microscope HS BX40. The 632.8 nm light from He-Ne laser was employed as excitation radiation. The samples, mounted on a motorized *xy* stage, were tested with a 100× objective and with a laser spot of 1 μm of diameter. The spectral resolution was about 1 cm−1. Neutral filters with different optical density were used to irradiate the samples at different light intensities leading to power density values from 5 × 10<sup>3</sup> to 5 × 10<sup>5</sup> W/cm<sup>2</sup> . A cooled CCD camera was used as a detector and the typical integration times were about 2 min. The sample phase homogeneity was verified by mapping the Raman spectra from different regions of each sample. The parameters of the Raman spectra were extracted by using best fitting procedures based on Lorentzian functions. In this way, the frequency, full width at half maximum, intensity, and integrated intensity of the peaks were determined.

#### **2.1. Brief overview from the literature**

Even if in the literature the assignment of the specific atomic motions within the spinel lattice during the Raman-active vibrations is controversial, it is common to explain the vibrational dynamic in term of normal modes inside the two sub-units within the spinel unit cell: the tetrahedral unit, AO<sup>4</sup> is comprised of the cation at the center of a cube and four oxygen atoms in the nonadjacent corners; the octahedral unit consists of a cation surrounded by six oxygen atoms, two along each dimensional axis, to form a BO6 octahedron. It is accepted that the highest-frequency A1g mode is assigned to the symmetric breathing mode of the AO<sup>4</sup> unit within the spinel lattice [26]. The oxygen atoms move away from the tetrahedral cations along the direction of the bonds, with all the cations at rest. For all the other Raman modes, there are some controversies about their assignments. In [27], all the other low frequency modes are attributed to vibrations inside the octahedral sites (BO6 ). Besides the highest-frequency Raman-active F2g(3) mode is alternatively attributed to the antisymmetric breathing of the AO4 unit [28], or to the asymmetric bending motion of the oxygens bonded to the tetrahedral cations [29].

At first, one can notice the absence of a clear correlation between grain size and peak positions of A1g modes. Looking at the spectra, the effect of grain size reduction causes as expected an asymmetric broadening of Raman bands and a decreasing of Raman intensities [44, 45]. A1g mode is mainly the most intense Raman feature; while in [27, 37, 46], the highest Raman signal is the Eg mode. Besides, in ZFO, the nanometric regime is primarily the cause of inversion, with marked influence on Raman spectra. Indeed in tetrahedra and octahedra a mixing between A and B cations takes place and this mixing reflects on Raman modes due to different cations

The data are listed at variance with synthesis method and grain size. For syntheses, we used the following acronyms: S MC for soft mechanochemical, SG C for sol-gel combustion, TD for thermal decomposition, CG for citrate-gel, CP for

**Main Raman modes peak energy (cm−1) References**

720 (Fe-O)

653 (Fe-O)

670(Fe-O)

700 (Fe-O)

688.6

— [35, 36]

[37]

231

[38]

221, 286, 403 *α*-Fe2 O3

Raman Spectroscopy in Zinc Ferrites Nanoparticles http://dx.doi.org/10.5772/intechopen.72864

*A*1g maghemite

— [40]

— [43]

721

**F2g(1) Eg F2g(2) F2g(3) A1g (1) Other signals**

In **Table 2**, we summarize some results from nanocrystalline doped zinc ferrites and other

Both for pure and doped nanoparticles zinc ferrites, it is difficult to derive general rules for Raman modes behavior. Different parameters (synthesis, grain size, composition, and chemical purity) can give rise to competing effects with opposite effects on Raman features. In addition, it is evident from the reported data in **Tables 1** and **2** that even the attribution of a certain

even the attribution of A1g mode has been reconsidered. Some authors [50, 51] claimed that

modes, the analyses are complicated by the usually lower intensities. Recently,

modes. Furthermore

Raman signal to a specific mode is uncertain, in particular for F2g and E<sup>g</sup>

involved for the same vibrational modes [35].

common ferrites.

**Method Average** 

Honey-mediated

SG C

**size (nm)**

S MC 3–25 150 265 350–400 450–500 630 (Zn-O)

S MC 20 160 265 370 450 637(Zn-O)

CG 13 — 351 452 490 650 (Zn-O)

SG C 19.6 174.9 340.3 491 — 652.7

co-precipitation, SG AC for sol-gel autocombustion and HT F for high temperature flux.

**Table 1.** Overview on data from literature on Raman modes in pure ZFO nanoparticles.

10–20 241 351 498 438 605 (Zn-O)

TD 4.7 159 254 364 459 665 — [39]

CP 5 — 330 480 — 660 — [41] SG AC 21 262 357 497 — 682 — [42]

HT F — 221 246 355 451 647 800 noise [27] CP 7 235.56 339.61 487.4 — 664.97 — [23]

for F2g and E<sup>g</sup>

The F2g(2) mode should be due to the opposite motion of cation and oxygen along one direction of the lattice [30]. In [30], the E<sup>g</sup> mode is assigned to the symmetric bending motion of the oxygen anions within the AO<sup>4</sup> unit, in agreement to other researchers [28]. Finally, more agreement is found for F2g(1) Raman mode, the lowest frequency one, due to the complete translation of the AO4 unit within the spinel lattice [31–34].

The controversial landscape of Raman modes in ZFO is summarized in **Table 1**, where we report a brief overview on Raman results from nanosized pure zinc ferrites obtained by different methods.


The data are listed at variance with synthesis method and grain size. For syntheses, we used the following acronyms: S MC for soft mechanochemical, SG C for sol-gel combustion, TD for thermal decomposition, CG for citrate-gel, CP for co-precipitation, SG AC for sol-gel autocombustion and HT F for high temperature flux.

**Table 1.** Overview on data from literature on Raman modes in pure ZFO nanoparticles.

Raman lines are associated to the presence of hematite inside the irradiated volume [21]. At higher energies, in the region 1000–1800 cm−1 only second-order features can be revealed, with a more intense feature just above 1300 cm−1 in the Al-doped sample due to second-order

Briefly, we recall that micro-Raman measurements were carried out at RT by using a Labram Dilor spectrometer equipped with an Olympus microscope HS BX40. The 632.8 nm light from He-Ne laser was employed as excitation radiation. The samples, mounted on a motorized *xy* stage, were tested with a 100× objective and with a laser spot of 1 μm of diameter. The spectral resolution was about 1 cm−1. Neutral filters with different optical density were used to irradiate the samples at different light intensities leading to power density values from

tion times were about 2 min. The sample phase homogeneity was verified by mapping the Raman spectra from different regions of each sample. The parameters of the Raman spectra were extracted by using best fitting procedures based on Lorentzian functions. In this way, the frequency, full width at half maximum, intensity, and integrated intensity of the peaks

Even if in the literature the assignment of the specific atomic motions within the spinel lattice during the Raman-active vibrations is controversial, it is common to explain the vibrational dynamic in term of normal modes inside the two sub-units within the spinel unit cell: the

in the nonadjacent corners; the octahedral unit consists of a cation surrounded by six oxygen

within the spinel lattice [26]. The oxygen atoms move away from the tetrahedral cations along the direction of the bonds, with all the cations at rest. For all the other Raman modes, there are some controversies about their assignments. In [27], all the other low frequency modes

Raman-active F2g(3) mode is alternatively attributed to the antisymmetric breathing of the

The F2g(2) mode should be due to the opposite motion of cation and oxygen along one direc-

agreement is found for F2g(1) Raman mode, the lowest frequency one, due to the complete

The controversial landscape of Raman modes in ZFO is summarized in **Table 1**, where we report a brief overview on Raman results from nanosized pure zinc ferrites obtained by different

unit within the spinel lattice [31–34].

unit [28], or to the asymmetric bending motion of the oxygens bonded to the tetrahedral

highest-frequency A1g mode is assigned to the symmetric breathing mode of the AO<sup>4</sup>

. A cooled CCD camera was used as a detector and the typical integra-

is comprised of the cation at the center of a cube and four oxygen atoms

octahedron. It is accepted that the

). Besides the highest-frequency

mode is assigned to the symmetric bending motion of

unit, in agreement to other researchers [28]. Finally, more

unit

signal from hematite.

230 Raman Spectroscopy

to 5 × 10<sup>5</sup> W/cm<sup>2</sup>

**2.1. Brief overview from the literature**

tion of the lattice [30]. In [30], the E<sup>g</sup>

the oxygen anions within the AO<sup>4</sup>

translation of the AO4

atoms, two along each dimensional axis, to form a BO6

are attributed to vibrations inside the octahedral sites (BO6

5 × 10<sup>3</sup>

were determined.

tetrahedral unit, AO<sup>4</sup>

AO4

cations [29].

methods.

At first, one can notice the absence of a clear correlation between grain size and peak positions of A1g modes. Looking at the spectra, the effect of grain size reduction causes as expected an asymmetric broadening of Raman bands and a decreasing of Raman intensities [44, 45]. A1g mode is mainly the most intense Raman feature; while in [27, 37, 46], the highest Raman signal is the Eg mode. Besides, in ZFO, the nanometric regime is primarily the cause of inversion, with marked influence on Raman spectra. Indeed in tetrahedra and octahedra a mixing between A and B cations takes place and this mixing reflects on Raman modes due to different cations involved for the same vibrational modes [35].

In **Table 2**, we summarize some results from nanocrystalline doped zinc ferrites and other common ferrites.

Both for pure and doped nanoparticles zinc ferrites, it is difficult to derive general rules for Raman modes behavior. Different parameters (synthesis, grain size, composition, and chemical purity) can give rise to competing effects with opposite effects on Raman features. In addition, it is evident from the reported data in **Tables 1** and **2** that even the attribution of a certain Raman signal to a specific mode is uncertain, in particular for F2g and E<sup>g</sup> modes. Furthermore for F2g and E<sup>g</sup> modes, the analyses are complicated by the usually lower intensities. Recently, even the attribution of A1g mode has been reconsidered. Some authors [50, 51] claimed that


the Raman band at around 650 cm−1 is associated with the presence of Zn2+ ions at B-sites. This conclusion seemed to be supported by the dependence of its intensity with Zn content in

the mass of the tetrahedral cations is effective in the A1g lineshape with different contribu

observed when ZFO NP is functionalized for specific biomedical applications [53].

overlapping of three contributions due to vibrations inside (i) ZnO

tion clearly detectable, as expected in a simple "mass on a spring" model; the reduced grain size causes a decrease in the total Raman yield; all the synthesis methods seem to be able to produce nanoparticles with good crystallinity even if sometimes the Raman signals are quite weak and broadened [22, 52]. Further worsening in the quality of Raman spectra has been

In the Raman spectra of nanocrystalline, both pure and doped zinc ferrites or other ferrites, it is common to recognize different contributions for each Raman mode, just due to different cations involved in a specific vibration [47]. This behavior, the so-called *two-mode* behavior, is particu

larly evident for the most intense A1g mode above 600 cm−1. If doping is considered, the Raman lineshape is affected both by the inversion on A and B sites and by the substitution of divalent or trivalent cations. Thus, the Raman signal in the more complex case could be the result of the

the lower frequency modes; where in some cases, a one-mode behavior cannot be excluded [54]. The Raman studies on ZFO NP have usually the goal to characterize the structure of the samples in view of the magnetic response, being the last affected by inversion and/or cationic substitutions with magnetic ions. Raman data, and in particular the A1g modes, are usually used to perform best-fitting analyses using lorenztian curves and derive the integrated intensities of the two (or three if substituents are considered) different contributions. With the empirical

**Figure 3.** Simulated Raman spectra for ZFO, hematite and maghemite. The intensities are scaled to match the

and by Mossbauer data. Nevertheless, some common features can be derived:

4 , (ii) FeO 4

Raman Spectroscopy in Zinc Ferrites Nanoparticles http://dx.doi.org/10.5772/intechopen.72864

Fe)], one can derive the inversion degree; the lorentzian intensities

due to substitution. The two mode behavior is sometimes less detectable in


233


, due to inversion,

Mg1−xZn x Fe 2 O 4

and (iii) the MeO

formula xFe(A)

4

= [1 − I Zn/(IZn + I

experimental evidences on pure micrometric powders.

the Raman band at around 650 cm−1 is associated with the presence of Zn2+ ions at B-sites. This conclusion seemed to be supported by the dependence of its intensity with Zn content in Mg1−xZnx Fe2 O4 and by Mossbauer data. Nevertheless, some common features can be derived: the mass of the tetrahedral cations is effective in the A1g lineshape with different contribution clearly detectable, as expected in a simple "mass on a spring" model; the reduced grain size causes a decrease in the total Raman yield; all the synthesis methods seem to be able to produce nanoparticles with good crystallinity even if sometimes the Raman signals are quite weak and broadened [22, 52]. Further worsening in the quality of Raman spectra has been observed when ZFO NP is functionalized for specific biomedical applications [53].

In the Raman spectra of nanocrystalline, both pure and doped zinc ferrites or other ferrites, it is common to recognize different contributions for each Raman mode, just due to different cations involved in a specific vibration [47]. This behavior, the so-called *two-mode* behavior, is particularly evident for the most intense A1g mode above 600 cm−1. If doping is considered, the Raman lineshape is affected both by the inversion on A and B sites and by the substitution of divalent or trivalent cations. Thus, the Raman signal in the more complex case could be the result of the overlapping of three contributions due to vibrations inside (i) ZnO4 , (ii) FeO<sup>4</sup> , due to inversion, and (iii) the MeO<sup>4</sup> due to substitution. The two mode behavior is sometimes less detectable in the lower frequency modes; where in some cases, a one-mode behavior cannot be excluded [54]. The Raman studies on ZFO NP have usually the goal to characterize the structure of the samples in view of the magnetic response, being the last affected by inversion and/or cationic substitutions with magnetic ions. Raman data, and in particular the A1g modes, are usually used to perform best-fitting analyses using lorenztian curves and derive the integrated intensities of the two (or three if substituents are considered) different contributions. With the empirical formula xFe(A) = [1 − I Zn/(IZn + I Fe)], one can derive the inversion degree; the lorentzian intensities

**Figure 3.** Simulated Raman spectra for ZFO, hematite and maghemite. The intensities are scaled to match the experimental evidences on pure micrometric powders.

**Sample** ZnFe1.8 La0.2 O4

SG AC

10

250

343

487

—

663

—

[42]

[38, 47]

Ni0.5Zn0.5Fe

Mn0.45Ni0.05Zn0.5Fe

Mn0.05Ni0.45Zn0.5Fe

Mn0.65Zn0.35Fe

Mn0.6Zn0.4Fe

Co0.5Zn0.5Fe

NiFe

CoFe

MnFe

FeFe

O2 4 microemulsion.

**Table 2.**

Overview on data from literature on Raman modes in doped ZFO nanoparticles.

TD

4.9

190

290

453 The data are listed at variance with synthesis method and grain size. For the acronyms, see the caption of **Table 1**. In addition, C stays for citrate and DM for double

535

671

720, 604, 496, 386 maghemite

 [39]

O2 4

SG C

8.5

220.6

277

370.9

507.4

593

240, 300

[43]

638.7

*α*-Fe2O3

O2 4

DM

6

547.7

293.2

460.7

190.2

674.8 A1g (1)

—

[20]

604.9 A1g (2)

O2 4

S MC

10

188

323

480

550

670 (Fe-O)

721

[38]

689 (Ni-O)

*A*1g maghemite

O2 4

CP

—

207.7

314.5

468.5

551.7

632.7 A1g (1)

—

[49]

693.2 A1g (2)

O2 4

SG C

9.4

217.7

275.3

386.5

490.8

591.3

240, 300

[43]

645.7

*α*-Fe

O2 3

O2 4

CG

9

—

350

—

540

610 (Mn-O)

—

[40]

650 (Zn-O)

690 (Fe-O)

O2 4

C

12

—

339

484

550

650 (Zn-O)

—

[48]

695 (Fe-O)

O2 4

C

9

—

336

482

540

636 (Zn-O)

—

[48]

680 (Fe-O)

O2 4

S MC

20

125 (Zn-O)

256 (Zn-O)

362 (Zn)

370 (Zn)

637(Zn-O)

721

200 (Ni-O)

323 (Ni-O)

480 (Ni)

480 (Ni)

670(Fe-O)

*A*1g maghemite

689(Ni-O)

**Method/**

**Average** 

**Main Raman modes peak energy (cm−1)**

**References**

232 Raman Spectroscopy

**size (nm)**

**F2g(1)**

**E<sup>g</sup>**

**F2g(2)**

**F2g(3)**

**A1g (1)**

**Other signals**

**SynthesIs**

can be multiplied by corresponding force constants even if it is not simply to derive a reliable value [39, 47].

*2.2.1. Synthesis*

Sr.(NO<sup>3</sup> )2

the ZnFe2

O4

ric ratio to obtain ZnFe2

, Gd(NO<sup>3</sup>

*2.2.2. XRD and SEM analyses*

and ZnFe1.9Al0.1O4

Al01, respectively.

)3 6H<sup>2</sup>

For comparison, an undoped ZnFe<sup>2</sup>

the desired stoichiometry Zn0.95Ca0.05Fe2

For MW, the starting reagents were Zn(NO<sup>3</sup>

O4

reduce the nitrates oxidant amount is calculated as 2.22.

O, and Al(NO<sup>3</sup>

the XRD patterns of the ZnFe-SS and ZnFe undoped ZnFe<sup>2</sup>

The samples were synthesized by microwave-assisted combustion methodology (MW), a rapid and green method as previously described, and by a conventional high energy ball milling.

fuel, as calculated from the propellant chemistry theory [60]. In brief, by taking into account the reducing and oxidizing valences of the involved elements (Zn = +2, N = 0, O = −2, Fe = +3, C = 4, H = 1), we can calculate a global valence value of −15, −10, and +18 for iron nitrate, zinc nitrate, and citric acid, respectively. We can write the balanced chemical reaction for the ferrite formation as 2·(−15) + 1·(−10) + x·(+18) = 0 and the mol amount of citric acid necessary to

The mixture was placed in a ceramic crucible in a microwave oven for 30 min at 800 W: this power ensures a temperature inside the oven between 450 and 500°C. This undoped sam-

explained. In the following, these samples will be named Ca005, Ca025, Sr005, Gd01, and

by ball milling in tungsten jars for 6 h at 500 rpm with intermediate periods of last. The mixture was then treated in oven in air at 650°C for 12 h (heating rate 5°C/min, spontaneous cool-

X-ray diffraction analysis was used to determine the sample purity, in particular to control the effective cation substitution, the crystallite sizes and the eventual inversion degree of the spinel by means of the structural and profile refinement based on the Rietveld method. In **Figure 4**,

with Ca005 one. In all the cases, a pure ferrite sample is formed, whose peaks well agree with

broadening and crystallinity between MW and SS samples suggesting markedly different particle sizes, as can be expected due to the different experimental synthesis methodologies. For the doped samples, XRD patterns demonstrate that the doping successfully occurred, because neither traces of unreacted reagents nor phase impurities are present: only for Sr005, some low peaks reveal the presence of small traces of strontium nitrate, as a residual of the reagent. The structural refinement on the basis of the Rietveld method was performed on all the patterns by using the known cubic spinel model: the main refined structural parameters are reported in **Table 3**. It can be seen that the crystallite sizes are in the nanometer range: the values for the microwave synthesis are all lower than 11 nm, while for ZnFe-SS a value of 22 nm is found.

cubic spinel structure (PDF card N. 89-7412). It is well evident a different peaks

, Zn0.75Ca0.25Fe2

ple will be named ZnFe. To obtain the doped samples, proper amount of Ca(NO<sup>3</sup>

O4

)3 9H<sup>2</sup>

O4

solid-state synthesis, starting from a stoichiometric mixture of ZnO and Fe<sup>3</sup>

ing to 25°C) and ground after cooling. This sample will be named ZnFe-SS.

O and Fe(NO<sup>3</sup>

and mixed in an agate mortar with a proper amount of citric acid as

O4

. The citric acid amount to add to the various mixtures was calculated as

)3 9H<sup>2</sup>

O were added to the previous reagents to obtain

O4

O4

samples are shown together

, Zn0.95Sr0.05Fe2

sample was also synthesized by means of a classical

O4

O taken in stoichiomet-

Raman Spectroscopy in Zinc Ferrites Nanoparticles http://dx.doi.org/10.5772/intechopen.72864

> )2 4H<sup>2</sup> O,

, ZnFe1.9Gd0.1O4

oxides ground

,

235

)2 6H<sup>2</sup>

For this reason, an important issue in ZFO NP studies is to reveal in synthesized samples the presence of iron oxides, like hematite and maghemite [5]. These oxides can infer an effective but extrinsic magnetic behavior with problems in terms of stability and aggregation if used in biomedical applications. To characterize the spin ordering and understand the nature of magnetic behavior, Mossbauer and neutron diffraction studies have been helpfully used [55]. Nevertheless, in this frame also the Raman spectroscopy can play an important role. Hematite (*α*-Fe2 O3 ) belongs to the rhombohedral system while maghemite (γ-Fe<sup>2</sup> O3 ) has a cubic inverted spinel structure, thus from the point of view of Raman activity equivalent to ZFO. These differences reflect on the Raman spectra. In **Figure 3**, we report the simulated spectra for ZFO, hematite, and maghemite.

The Raman spectra of ZFO and maghemite are obviously quite similar with a total Raman yield greater for the former. The main difference is the peak position of the A1g. In ZFO, the ZnO<sup>4</sup> vibrations are dominant and the peak is around 650 cm−1, while in maghemite only the iron is present and the energy peak is usually observed around or just above 700 cm−1. On the contrary, the Raman spectrum of hematite is markedly different having the main peaks in the lower energy region. It is important to note that the Raman yield of hematite is approximately 10–20 times higher than that of spinel-type ferrites, thus a small amount of hematite is enough to give clear signatures in Raman spectrum. It is difficult to quantify this small amount because it is often reported that for ZFO nanometric grains, surface shell of hematite or maghemite can be formed [37, 56]. In this case, the surface sensitivity of the Raman technique can hinder a proper estimation. A more difficult goal is to reveal the presence of few amounts of maghemite in ZFO by Raman spectroscopy. Detailed best-fitting analyses are usually performed on the A1g data, and it is possible to infer about the presence of maghemite [38]; but the variability of peak energies found both for pure and doped ZFO and the weak weight of maghemite contribution in Raman signal hinder a reliable attribution simply based on Raman data. But, RS allows to monitor the stability of maghemite to laser irradiation. Indeed, it is well-known that in the oxidation route of iron ions maghemite is an intermediate metastable phase while hematite is the terminal and stable one [57]. Using this approach, the presence of maghemite in ZFO can be indirectly evaluated by the insurgence of hematite Raman signals under laser irradiation [58].
