**3. Results and Discussion** *3.1. XRD Analysis*

*3.1. XRD Analysis* Structural and phase identification of the materials were confirmed using X-ray diffraction (XRD) as shown in Figure 2. All exhibited XRD peaks of pure and Zn doped hematite assigned to (012), (104), (110), (113), (024), (116), (214) and (300) planes can be easily indexed to rhombohedral with space group *R3c* phase of hematite (JCPDS card no. 84-0311) [19]. No diffraction peaks other than hematite has been observed, indicating that Zn atoms were incorporated in α-Fe2O<sup>3</sup> matrices. Thus, crystallinity is altered by dopant atoms without disturbing the rhombohedral structure of hematite. A visual inspection of XRD reveals that (104) diffraction peak is shifting towards lower angle up to 4% of Zn doping and then shifts toward higher angle side for 6% Zn concentration. This shifting of XRD peaks result in the variation of lattice parameters (a and c) as shown in Table 1. It is contemplated that lower doping (≤4% Zn) concentration occupies substitutional sites, whereas, higher doping of Zn occupies partial interstitial sites or segregate on the surface which distorts the Structural and phase identification of the materials were confirmed using X-ray diffraction (XRD) as shown in Figure 2. All exhibited XRD peaks of pure and Zn doped hematite assigned to (012), (104), (110), (113), (024), (116), (214) and (300) planes can be easily indexed to rhombohedral with space group *R3c* phase of hematite (JCPDS card no. 84-0311) [19]. No diffraction peaks other than hematite has been observed, indicating that Zn atoms were incorporated in α-Fe2O<sup>3</sup> matrices. Thus, crystallinity is altered by dopant atoms without disturbing the rhombohedral structure of hematite. A visual inspection of XRD reveals that (104) diffraction peak is shifting towards lower angle up to 4% of Zn doping and then shifts toward higher angle side for 6% Zn concentration. This shifting of XRD peaks result in the variation of lattice parameters (a and c) as shown in Table 1. It is contemplated that lower doping (≤4% Zn) concentration occupies substitutional sites, whereas, higher doping of Zn occupies partial interstitial sites or segregate on the surface which distorts the host lattice structure. In other words, higher concentrations of Zn2<sup>+</sup> ions causes non uniform distribution in the host lattice, which plays a dominant role in modifying the various physical properties. Distortion in host matrix is

relation [20],

0

expected due to incorporation of large size Zn2<sup>+</sup> ions in place of smaller size Fe3<sup>+</sup> ions which in turn leads to stress (σ) in the system. This can be obtained using the relation [20], = 226.28 (c − 0) 0 (1) where (c−0)

associated with oxygen vacancy present in the Mg doped ZnO thin films [20]. The crystallite size (D)

represents strain, c<sup>0</sup> and c corresponds to the lattice parameter values from JCPDS card

*Crystals* **2020**, *10*, x FOR PEER REVIEW 4 of 19

of smaller size Fe3+ ions which in turn leads to stress (σ) in the system. This can be obtained using the

$$
\sigma = \frac{226.28 \text{ (c-c\_0)}}{c\_0} \tag{1}
$$

where (c−*c*0) *c*0 represents strain, c<sup>0</sup> and c corresponds to the lattice parameter values from JCPDS card and XRD results, respectively. The obtained negative values of stress indicates the compressive stress in the system. A report by K. Vijayalakshmi et al. stated that compressive stress (negative sign in stress value) may be attributed to zinc interstitials and tensile stress (positive sign in stress values) is associated with oxygen vacancy present in the Mg doped ZnO thin films [20]. The crystallite size (D) of these nanoparticles was calculated from the full-width half maxima (FWHM) of (104) peak using Debye-Scherer formula. It is observed that crystallite size increases up to 4% Zn concentration then decreases for 6% Zn concentration. The enhancement in crystallite size after Zn doping plays an important role in crystal growth and also in crystallization of Fe2O3. The enlargement in size is due to the substitution of Fe3<sup>+</sup> ions with relatively large sized Zn2<sup>+</sup> ions. The obtained trend in crystallite size for higher Zn doped Fe2O<sup>3</sup> samples has a similar trend as also discussed in previous reports for Y doped ZnO, Mn-doped CeO<sup>2</sup> and Mg-doped ZnO samples [21–23]. of these nanoparticles was calculated from the full-width half maxima (FWHM) of (104) peak using Debye-Scherer formula. It is observed that crystallite size increases up to 4% Zn concentration then decreases for 6% Zn concentration. The enhancement in crystallite size after Zn doping plays an important role in crystal growth and also in crystallization of Fe2O3. The enlargement in size is due to the substitution of Fe3+ ions with relatively large sized Zn2+ ions. The obtained trend in crystallite size for higher Zn doped Fe2O<sup>3</sup> samples has a similar trend as also discussed in previous reports for Y doped ZnO, Mn-doped CeO<sup>2</sup> and Mg-doped ZnO samples [21–23]. Additionally, to obtain more information about the defects present in the synthesized samples, dislocation density (δ) is evaluated from = 1 2 . The obtained dislocation density is significantly low for Zn 4% indicating the presence of large number of defects which is helpful in photocatalytic degradation. However, the defects are reduced for Zn 6%. The increase in crystallite size and decrease in dislocation density up to Zn 4% indicates that dopant atoms are entirely included in the lattice. While, in higher Zn dopant concentrations, the decrease in crystallite size and increase in dislocation density infers that dopant atoms occupy interstitial positions in the matrix. This results in a decrease in crystalline order and an increase in dislocation density. The change in dislocation density and stress in synthesized samples confirm the presence of defects in the lattice structure that are responsible for modification in various physical properties.

**Figure 2.** X-ray diffraction pattern of pure Fe2O<sup>3</sup> , Zn 2%, Zn 4% and Zn 6% nanoparticles.

**Table 1.** Structural parameters of pure Fe2O<sup>3</sup> , Zn 2%, Zn 4% and Zn 6% synthesized nanoparticles.


Additionally, to obtain more information about the defects present in the synthesized samples, dislocation density (δ) is evaluated from δ = <sup>1</sup> *D*<sup>2</sup> . The obtained dislocation density is significantly low for Zn 4% indicating the presence of large number of defects which is helpful in photocatalytic degradation. However, the defects are reduced for Zn 6%. The increase in crystallite size and decrease in dislocation density up to Zn 4% indicates that dopant atoms are entirely included in the lattice. While, in higher Zn dopant concentrations, the decrease in crystallite size and increase in dislocation density infers that dopant atoms occupy interstitial positions in the matrix. This results in a decrease

in crystalline order and an increase in dislocation density. The change in dislocation density and stress in synthesized samples confirm the presence of defects in the lattice structure that are responsible for modification in various physical properties. **Samples (Å) Crystallite Size (nm) Dislocation Density (nm)−2 × 10−<sup>4</sup> Stress (GPa) Particle Size From TEM (nm)** *a***–axis** *c***–axis**  Pure Fe2O3 5.035 13.229 15 44.44 −8.41 18

> Zn 2% 5.041 13.242 18 30.86 −8.20 20 Zn 4% 5.043 13.248 21 22.67 −8.10 23

**Lattice Parameter** 

*Crystals* **2020**, *10*, x FOR PEER REVIEW 5 of 19 **Figure 2.** X-ray diffraction pattern of pure Fe2O3, Zn 2%, Zn 4% and Zn 6% nanoparticles. **Table 1.** Structural parameters of pure Fe2O3, Zn 2%, Zn 4% and Zn 6% synthesized nanoparticles.
