*2.1. Structural Analysis*

X-ray diffraction (XRD) studies were carried out to determine the structural and crystallite size information of the (TiO2)1−x(Fe2O3)x nanoparticles. Figure 1 displays the typical XRD pattern of Fe2O3–TiO2 nanocomposites, for different concentrations of Fe2O3 (TiO2)1−x(Fe2O3)x, where x = 0, 0.1, 0.5, and 1.0 for pure TiO2 and Fe2O3 contents 0.1, 0.5, and pure Fe2O3 which are denoted as PT, 0.1F, 0.5F, and PF, respectively). Pure TiO2 is found in three dominant crystal structures, anatase, rutile, and brookite in nature [28]. Among them, anatase form is the most common polymorph due to its lower surface energy, especially at the nanoscale [29]. From Figure 1, it is evident that the pure TiO2 is formed in the polycrystalline tetragonal anatase phase with multiple peaks marked with the asterisk (\*) symbol and the planes are marked as (101), (103), (004), (112), (200), (105), (211), (204), (116), (220), (215) and (301) in Figure 1. The peaks are matching well with the ICDD pattern 21-1272. Similarly, the pure Fe2O3 is formed in the rhombohedral hematite structure matching well with the ICDD pattern# 33-0664. Figure 1 also displays the intermediate structures showing systematic changes with the Fe2O3 incorporating on TiO2 lattice, which

is clearly evident from the changes in the peak intensities, emergence of new peaks and peak shifts happening with different dopant levels.

**Figure 1.** XRD analysis of (TiO2)1−x(Fe2O3)x nanoparticles.

Scherrer's formula was employed to calculate the mean crystal size (*D*) of the NPs from the XRD peaks using the following relation [30],

$$D = \frac{0.9\lambda}{\beta \cos(\theta)}\tag{1}$$

where *λ* is the X-ray wavelength, and *β* is the peak width at half maximum. The microstrain (*ε*) and dislocation density (*δ*) values were also calculated using the relations.

$$
\varepsilon = \frac{\beta \cos(\theta)}{4} \tag{2}
$$

and

$$
\delta = \frac{1}{D^2} \tag{3}
$$

The obtained values are tabulated in Table 1. The results clearly show that the Fe2O3 concentration strongly influences the crystallite sizes and so the dislocation values. Microstrain values are almost the same. It can be inferred from the table that the crystallite size increases in general with the Fe2O3 content. The results are matched well with the results obtained by Zhao et al. [30]. Zhao et al. [31] synthesized a set of Fe2O3, TiO2 and TiO2/Fe2O3 multilayered thin films and found that the mean crystalline size increases with the Fe2O3, which is in accord with the present results. Tang et al. [32] report that the formation of Fe2O3 and TiO2 particles which are significantly affected by the concentration of hydrolysis liquid. The intensities of the characteristic peaks of both anatase and rutile phases increased with the hydrolysis liquid concentration.


**Table 1.** Crystallite size (*D*), dislocation density (*δ*) and microstrain (*ε*).

The surface morphology studies and distribution of particles of the nanostructured material provide useful information about the utilization of the sample in various technological important applications. Figure 2 shows the scanning electron microscope (SEM) images of PT, 0.1F, 0.5F, and PF samples. It is quite evident that the Fe content has altered the surface morphology of the samples, notably. The pure TiO2 sample shows an irregular distribution of spherical and rectangular-shaped particles. When Fe is added to TiO2, the size of the particles reduces, and there are small particles observed throughout the surface of the sample. Apart from this, in the case of the pure Fe sample, a group of nanoparticles of larger size was observed. From morphology studies, it is clear that the decrease in grain size results in an increase in the surface area of the synthesized material and produces more active sites for adsorption of target contaminant and thus attacked by reactive oxygen species, which will be discussed later in this study. A similar type of decreasing particles size with an increasing percentage of incorporating was observed by Gareso et al. [18]. Moreover, it is also observed that the decrease in particle size with Fe dopant can make the synthesized catalyst a potential candidate for various photocatalysis and sensing applications.

**Figure 2.** SEM images of (TiO2)1−x(Fe2O3)x nanoparticles.

Figure 3 shows the Raman spectra of PT, 0.1F, 0.5F, and PF samples. In Raman spectra, four main peaks were observed at 145 cm−1, 396 cm−1, 514 cm−1, and 637 cm−1, which belong to Eg, B1g, A1g + B1g, Eg mode, respectively. The main peaks observed in Raman spectra were well correlated to earlier reports confirms the phase of the prepared samples [20,33]. The position of the main peak at 145 cm−<sup>1</sup> is slightly shifted towards a higher wavenumber when Fe was doped in TiO2. The shift in Raman bands is attributed to the incorporation of Fe that causes changes in defect structure and particle size. Moreover, a continuous decrease in the intensity of the Raman band (145 cm−1) was also observed at higher concentrations, suggesting a decrease in the particle size of synthesized photocatalysts with Fe2O3 incorporating. The vibrational properties of materials are significantly

affected when the grain size decreases to the nanometer scale. Due to the size-induced radial strain, a volume contraction occurs primarily within the nanoparticles, which leads to an increase in force constants due to the decrease in interactor pressure. In the case of pure Fe material, the new peak at 1310 cm−<sup>1</sup> reveals the hematite group of α-Fe2O3.

**Figure 3.** Raman spectra for (TiO2)1−x(Fe2O3)x nanoparticles.

Figure 4 shows the Fourier-transform infrared spectroscopy (*FT-IR*) spectra of PT, 0.1F, 0.5F, and PF samples. From Figure 4, the band observed at around 3412 cm−<sup>1</sup> was attributed to the presence of the stretching vibrations of the O-H groups of H2O molecules physically adsorbed on the surface of TiO2. This band is gradually shifted to a lower wavelength due to incorporating concentration, suggesting the crystal structure of Fe2O3 was distorted [34]. It is also observed that at higher incorporating concentration and in pure Fe material, the intensity of band is increased. Some weak bands are also observed at around 2919 cm−<sup>1</sup> attributed to different vibrational modes of TiO2. Zhang et al. [34] observed peaks at 2928 cm−1, 2845 cm−1, 1502 cm−1, 1421 cm−<sup>1</sup> and 1364 cm−1. These peaks are assigned to sp3 and sp2 C-H, C=O, unsaturated C-H and C-OH bonds, indicating the existence of carbon quantum dots (CQDs) in the composites [34]. In pure Fe, this band disappears, clearly suggesting the other vibration modes of TiO2. The stretching vibrations of the O-H groups were also observed at around 1645 cm−1. Wu et al. also observed the same peaks close to 1630 cm−<sup>1</sup> [35]. The first one is attributed to the stretching vibration of the corresponding –OH derived from the hydroxyl radical or the adsorbed water on the TiO2 surface. The second peak close to 1630 cm−<sup>1</sup> corresponds to the bending vibration of the H–O–H bond of the adsorbed water on the TiO2 surface. These results are a common feature of semiconductor oxides and a basic condition for photocatalysis. There are some other bands such as 1033, 1439, 557, 469 cm−<sup>1</sup> also observed in the case of pure Fe and incorporating are attributed to vibration modes of Fe2O3 and can indicate the iron oxide formation at the structure of Fe2O3–TiO2 nanocomposites [36].

**Figure 4.** IR transmittance spectra for (TiO2)1−x(Fe2O3)x nanoparticles.

### *2.2. Optical Properties*

Diffuse reflectance spectroscopy (DRS) is a unique technique to study the electronic structure of nanostructured materials. The non-destructive method of this technique allows us to measure exact values of the bandgap of powdered materials by a mirror-like reflection from the loaded samples by diffuse illumination. In literature, this method is well studied by the incident light is partially absorbed and scattered [37]. In the present case, pure TiO2, pure Fe2O3, and different concentrations of Fe2O3 doped TiO2 were subjected to DRS analysis and the corresponding spectra are shown in Figure 5. The optical bandgap of the present nanostructured material was determined from the following Kubelka–Munk model equations [38]:

$$\text{F(R)} = \frac{\left(1 - \text{R}^2\right)}{\text{2R}} \tag{4}$$

**Figure 5.** Diffuse reflectance UV–visible spectra of (TiO2)1−x(Fe2O3)x nanoparticles.

F(R) is the Kubelka–Munk function, and R is the absolute reflectance. For calculating the (α), the Equation (4) is modified in terms of F(R) as [39,40]:

$$\alpha = \frac{\text{absorance}}{\text{t}} = \frac{\text{F}(\text{R})}{\text{t}} \tag{5}$$

where t is the height of the sample holder, which is equal to 2 mm, and the optical bandgap is calculated from Equation (6)

$$\alpha \mathbf{h} \boldsymbol{\nu} = \left( \frac{(\mathbf{F}(\mathbf{R}) \mathbf{h} \boldsymbol{\nu})}{\mathbf{t}} \right)^{\mathbf{n}} = \mathbf{A} \left( \mathbf{h} \boldsymbol{\nu} - \mathbf{E}\_{\mathbf{g}} \right)^{\mathbf{n}} \tag{6}$$

where α absorption coefficient, Eg is bandgap, hν is the absorbed energy, A is the parameter that is related to the effective mass associated with the valence and conduction bands, and n (n = <sup>1</sup> <sup>2</sup> for direct bandgap) is an optical transition. From Figure 6, with increasing the Fe2O3 incorporating concentration, the bandgap values are decreasing from 3.15 eV to 1.91 eV (see Table 2) as a result of incorporating Fe2O3 into the TiO2 lattice. The decreasing band gap with increasing incorporating concentration indicates that the present samples found huge applications in optoelectronic devices.

**Figure 6.** A plot of transferred Kubelka–Munk versus the energy of the light absorbed of (TiO2)1−x(Fe2O3)x nanoparticles.

**Table 2.** The bandgap values of (TiO2)1−x(Fe2O3)x nanoparticles.

