*3.3. Raman Analysis*

Raman spectroscopy is a fast and non-destructive tool for identifying the vibrational phonon modes that access the clear identification of compounds. Hence, the structural properties of as synthesized samples are further studied using Raman spectroscopy. The optical vibrational modes can be assumed as lattice waves arising due to an out of phase movement of atoms inside the crystal lattice. As these waves can interact with applied external electric field so, it is easy to excite them through conventional spectroscopic techniques. For a particular vibrational mode to be Raman active, it should be accompanied by change in polarizability. Whereas, changes in the dipole moment are required for vibrations to be infrared active. Vibrational modes of α-Fe2O<sup>3</sup> at the first Brillouin zone center are represented by [24]:

$$\mathbf{F} = 2\mathbf{A}\_{1\mathbf{g}} + 2\mathbf{A}\_{1\mathbf{u}} + 3\mathbf{A}\_{2\mathbf{g}} + 2\mathbf{A}\_{2\mathbf{u}} + 5\mathbf{E}\_{\mathbf{g}} + 4\mathbf{E}\_{\mathbf{u}} \tag{2}$$

Among these, the acoustic modes (A1u and A2g) are optically silent, due to an in-phase movement of atoms inside the crystal lattice, and cannot be identified by these techniques, as they propagate with the speed of sound of a much lower frequency. The six antisymmetric modes (2A2u and 4Eu) are infrared active vibrations and seven symmetrical (2A1g and 5Eg) modes are Raman active vibrations. As the rhombohedral crystal structure of α-Fe2O<sup>3</sup> features an inversion center, no modes are both infrared and Raman active.

Raman spectra of Zn doped α-Fe2O<sup>3</sup> in the range 200–800 cm−<sup>1</sup> at room temperature is shown in Figure 4. The assignment of Raman active modes are consistent with the group theory predicted for the space group *R3c* of hematite. Five phonon modes (2 A1g and 3 Eg) of hematite corresponding to transverse optical (TO) modes are detected by group theory at A1g(1) ~215 cm−<sup>1</sup> , Eg(1) ~280 cm−<sup>1</sup> , Eg(2) ~398 cm−<sup>1</sup> , A1g(2) ~492 cm−<sup>1</sup> , Eg(3) ~544 cm−<sup>1</sup> respectively, which are well in agreement with existing literature, thereby confirming the rhombohedral structure of synthesized samples [9]. The expected Raman spectra, corresponding to E<sup>g</sup> modes at ~245 cm−<sup>1</sup> and ~412 cm−<sup>1</sup> is missing in the present case due to crystalline disorder or broadening of peaks. A1g symmetry can be viewed as the movement of Fe atoms along the crystallographic *c-axis* of the unit cell, while E<sup>g</sup> symmetry involves the symmetric breathing mode of O atoms correlated to each iron cation (Fe) in the plane perpendicular to the *c-axis* of the unit cell. It is observed from Figure 4 that peaks shift towards higher wavenumber till 4% Zn doping and then shift towards lower wave number on further doping. This shifting in Raman modes is governed by the change in host lattice strain with the addition of foreign atoms. The observed variation in Raman spectra correlates well with the XRD results of variation in lattice parameter and stress values. Apart from these symmetrical phonon modes, it is observed that there is an additional feature illustrating IR (infrared) - active longitudinal optical (LO) E<sup>u</sup> mode at ~597 cm−<sup>1</sup> which is forbidden in Raman scattering, but is activated by surface defects or disorder in hematite crystalline lattice [25]. The intensity of this mode is maximum for Zn 4% sample. These defects attribute to oxygen vacancies and modify the electronic structure that, in turn, enhances the photocatalytic activity. *Crystals* **2020**, *10*, x FOR PEER REVIEW 6 of 19 **Figure 3.** TEM images of (**a**) pure Fe2O3, (**b**) Zn 2%, (**c**) Zn 4% and (**d**) Zn 6% nanoparticles. *3.3. Raman Analysis* Raman spectroscopy is a fast and non-destructive tool for identifying the vibrational phonon modes that access the clear identification of compounds. Hence, the structural properties of as synthesized samples are further studied using Raman spectroscopy. The optical vibrational modes can be assumed as lattice waves arising due to an out of phase movement of atoms inside the crystal lattice. As these waves can interact with applied external electric field so, it is easy to excite them through conventional spectroscopic techniques. For a particular vibrational mode to be Raman active, it should be accompanied by change in polarizability. Whereas, changes in the dipole moment are required for vibrations to be infrared active. Vibrational modes of α-Fe2O<sup>3</sup> at the first Brillouin zone center are represented by [24]: Γ = 2A1g + 2A1u + 3A2g + 2A2u + 5E<sup>g</sup> + 4Eu. (2) Among these, the acoustic modes (A1u and A2g) are optically silent, due to an in-phase movement of atoms inside the crystal lattice, and cannot be identified by these techniques, as they propagate with the speed of sound of a much lower frequency. The six antisymmetric modes (2A2u and 4Eu) are infrared active vibrations and seven symmetrical (2A1g and 5Eg) modes are Raman active vibrations. As the rhombohedral crystal structure of α-Fe2O<sup>3</sup> features an inversion center, no modes are both

**Figure 4.** Raman spectrum of pure Fe2O3, Zn 2%, Zn 4% and Zn 6% nanoparticles. **Figure 4.** Raman spectrum of pure Fe2O<sup>3</sup> , Zn 2%, Zn 4% and Zn 6% nanoparticles.

Figure 4. The assignment of Raman active modes are consistent with the group theory predicted for

#### Raman spectra of Zn doped α-Fe2O<sup>3</sup> in the range 200–800 cm−<sup>1</sup> at room temperature is shown in *3.4. FTIR Analysis*

infrared and Raman active.

the space group *R3c* of hematite. Five phonon modes (2 A1g and 3 Eg) of hematite corresponding to transverse optical (TO) modes are detected by group theory at A1g(1) ~215 cm−<sup>1</sup> , Eg(1) ~280 cm−<sup>1</sup> , Eg(2) ~398 cm−<sup>1</sup> , A1g(2) ~492 cm−<sup>1</sup> , Eg(3) ~544 cm−<sup>1</sup> respectively, which are well in agreement with existing literature, thereby confirming the rhombohedral structure of synthesized samples [9]. The expected Raman spectra, corresponding to E<sup>g</sup> modes at ~245 cm−<sup>1</sup> and ~412 cm−1 is missing in the present case due to crystalline disorder or broadening of peaks. A1g symmetry can be viewed as the movement of Fe atoms along the crystallographic *c-axis* of the unit cell, while E<sup>g</sup> symmetry involves the symmetric breathing mode of O atoms correlated to each iron cation (Fe) in the plane perpendicular to the *c-axis*  of the unit cell. It is observed from Figure 4 that peaks shift towards higher wavenumber till 4% Zn doping and then shift towards lower wave number on further doping. This shifting in Raman modes Fourier transform infrared (FTIR) spectroscopy is a powerful method to get the information related to chemical bonds adsorbed on the surface of the material. FTIR spectrum of pure and Zn doped α-Fe2O<sup>3</sup> nanoparticles was recorded in the range 400–4000 cm−<sup>1</sup> given in Figure 5a. Different bands in FTIR spectra arises due to various functional groups. As discussed above in Raman studies, group theory analysis predicts six infrared active modes corresponding to α-Fe2O<sup>3</sup> lattice, out of which two infrared active A2u modes are associated with the vibrations polarized parallel to crystallographic *c-axis*, while other four active E<sup>u</sup> modes are polarized perpendicular to crystallographic *c-axis* [26]. The spectra features two prominent peaks at ~463 cm−<sup>1</sup> and ~551 cm−<sup>1</sup> are assigned to E<sup>u</sup> and A2u + E<sup>u</sup> (overlapping of A2u and Eu) phonon modes, respectively. These sharp and strong intensity bands at ~463 cm−<sup>1</sup> and ~551 cm−<sup>1</sup> indicate the metal oxygen (Fe–O) vibrations in rhombohedral lattice of hematite [27]. Also, these peaks confirm the existence of α-Fe2O<sup>3</sup> and are consistent with XRD

activity.

*3.4. FTIR Analysis*

the band at ⁓2929 cm−<sup>1</sup>

strong intensity bands at ⁓463 cm−<sup>1</sup> and ⁓551 cm−<sup>1</sup>

adsorbed CO<sup>2</sup> and peak centered at ⁓1633 cm−<sup>1</sup>

consistent with XRD data. In addition, peak observed at 1095 cm−<sup>1</sup>

observed at ⁓3413 cm−<sup>1</sup> corresponds to the presence of hydroxyl group [30].

data. In addition, peak observed at 1095 cm−<sup>1</sup> is attributed to the presence of adsorbed CO<sup>2</sup> and peak centered at ~1633 cm−<sup>1</sup> is assigned to O-H bending of water [28,29]. Further, the band at ~2929 cm−<sup>1</sup> is attributed to CH symmetric stretching vibrations and very broad peak observed at ~3413 cm−<sup>1</sup> corresponds to the presence of hydroxyl group [30]. side up to Zn 4% and then shift to higher wavenumber for Zn 6% due to variation in cation-oxygen bond length [31]. Also, it is well-established that bond length is inversely proportional to wavenumber or frequency. The shifting in these bands are analogous to the change in lattice parameter values analyzed through XRD measurements and reveals the strengthening of metal oxygen bond with the change in Zn content in the host matrix. Moreover, intensity of peaks increases

up to Zn 4% doping and decreases for Zn 6% which is in accordance with crystallinity of XRD pattern.

particle size and the presence of impurities. As discussed above, bands at ⁓463 cm−<sup>1</sup> and ⁓551 cm−<sup>1</sup> are related with Fe–O stretching vibrations and these bands are shifting toward lower wavenumber

rhombohedral lattice of hematite [27]. Also, these peaks confirm the existence of α-Fe2O<sup>3</sup> and are

*Crystals* **2020**, *10*, x FOR PEER REVIEW 7 of 19 is governed by the change in host lattice strain with the addition of foreign atoms. The observed variation in Raman spectra correlates well with the XRD results of variation in lattice parameter and stress values. Apart from these symmetrical phonon modes, it is observed that there is an additional feature illustrating IR (infrared) - active longitudinal optical (LO) E<sup>u</sup> mode at ~597 cm-1 which is forbidden in Raman scattering, but is activated by surface defects or disorder in hematite crystalline lattice [25]. The intensity of this mode is maximum for Zn 4% sample. These defects attribute to oxygen vacancies and modify the electronic structure that, in turn, enhances the photocatalytic

Fourier transform infrared (FTIR) spectroscopy is a powerful method to get the information related to chemical bonds adsorbed on the surface of the material. FTIR spectrum of pure and Zn doped α-Fe2O<sup>3</sup> nanoparticles was recorded in the range 400–4000 cm−<sup>1</sup> given in Figure 5a. Different bands in FTIR spectra arises due to various functional groups. As discussed above in Raman studies, group theory analysis predicts six infrared active modes corresponding to α-Fe2O<sup>3</sup> lattice, out of which two infrared active A2u modes are associated with the vibrations polarized parallel to crystallographic *c-axis*, while other four active E<sup>u</sup> modes are polarized perpendicular to crystallographic *c-axis* [26]. The spectra features two prominent peaks at ⁓463 cm−<sup>1</sup> and ⁓551 cm−<sup>1</sup> are assigned to E<sup>u</sup> and A2u + E<sup>u</sup> (overlapping of A2u and Eu) phonon modes, respectively. These sharp and

indicate the metal oxygen (Fe–O) vibrations in

is assigned to O-H bending of water [28,29]. Further,

is attributed to CH symmetric stretching vibrations and very broad peak

is attributed to the presence of

**Figure 5.** (**a**) FTIR transmittance (%) spectra and (**b**) Tauc plot of pure Fe2O3, Zn 2%, Zn 4% and Zn 6% nanoparticles. **Figure 5.** (**a**) FTIR transmittance (%) spectra and (**b**) Tauc plot of pure Fe2O<sup>3</sup> , Zn 2%, Zn 4% and Zn 6% nanoparticles.

It has been demonstrated that band positions in FTIR spectra are sensitive to lattice parameters, particle size and the presence of impurities. As discussed above, bands at ~463 cm−<sup>1</sup> and ~551 cm−<sup>1</sup> are related with Fe–O stretching vibrations and these bands are shifting toward lower wavenumber side up to Zn 4% and then shift to higher wavenumber for Zn 6% due to variation in cation-oxygen bond length [31]. Also, it is well-established that bond length is inversely proportional to wavenumber or frequency. The shifting in these bands are analogous to the change in lattice parameter values analyzed through XRD measurements and reveals the strengthening of metal oxygen bond with the change in Zn content in the host matrix. Moreover, intensity of peaks increases up to Zn 4% doping and decreases for Zn 6% which is in accordance with crystallinity of XRD pattern.
