3.5.1. Photocatalytic Activity

The photocatalytic performance of Zn doped α-Fe2O<sup>3</sup> (catalyst) was investigated by recording the time-dependent degradation of RB dye (as a contaminant). Figure 6a–d showing the change in absorption spectra over time for RB dye solution with catalysts under the presence of UV light irradiation. Figure 6a demonstrates that pure α-Fe2O<sup>3</sup> shows poor performance, compared to Zn doped (2% and 4%) samples as the high recombination rate between electrons and holes, and which cannot be easily separate out due to the short hole diffusion length in case of pure α-Fe2O3. However, the addition of Zn as dopant is a useful tactic to introduce localized electronic band structure which improves the charge separation efficiency. The appearance of no new absorption peak during whole process indicates the degradation in presence of proposed photocatalyst. The characteristic absorption peak intensity of RB dye gradually decreases with increasing exposure time from 0 min to 90 min. The intense absorption peak of RB dye around 562 nm decreases much faster in the presence of catalyst (Zn 4%) compared to other synthesized samples. The photodegradation activity increases with Zn dopant concentration of α-Fe2O<sup>3</sup> in the following order: Zn 6% < pure Fe2O<sup>3</sup> < Zn 2% < Zn 4% as shown in Figure 6a–d. It is well-established that synthesized samples in nano-region exhibit unique surface chemical reactivity for photocatalytic activity. There are several factors that influence the photocatalytic activity, such as type of dopant, recombination of electron hole pairs and band gap of semiconductors. Researchers have claimed that Cu2<sup>+</sup> doping in α-Fe2O<sup>3</sup> creates a trap state (separate band) which controls the electron hole recombination in photocatalytic process [1]. In the present study, Zn2<sup>+</sup> forms a trap state in the band gap of α-Fe2O3, i.e., a separate band between conduction band and valence band. The trap state induces defect state/impurity level, which entraps the charge carriers, as soon as they have been generated by UV light illumination, and inhibits the recombination so that charge carriers can be used for the redox process. The band gap decreases in Zn doping (up to 4% concentration), resulting in further surface defects (as clearly seen in Raman spectra), as well as delaying the recombination of charge carriers also which yields better catalyst for the degradation of RB dye.

the degradation of RB dye.

cannot be easily separate out due to the short hole diffusion length in case of pure α-Fe2O3. However, the addition of Zn as dopant is a useful tactic to introduce localized electronic band structure which improves the charge separation efficiency. The appearance of no new absorption peak during whole process indicates the degradation in presence of proposed photocatalyst. The characteristic absorption peak intensity of RB dye gradually decreases with increasing exposure time from 0 min to 90 min. The intense absorption peak of RB dye around 562 nm decreases much faster in the presence of catalyst (Zn 4%) compared to other synthesized samples. The photodegradation activity increases with Zn dopant concentration of α-Fe2O3 in the following order: Zn 6% < pure Fe2O3 < Zn 2% < Zn 4% as shown in Figure 6a–d. It is well-established that synthesized samples in nano-region exhibit unique surface chemical reactivity for photocatalytic activity. There are several factors that influence the photocatalytic activity, such as type of dopant, recombination of electron hole pairs and band gap of semiconductors. Researchers have claimed that Cu2+ doping in α-Fe2O<sup>3</sup> creates a trap state (separate band) which controls the electron hole recombination in photocatalytic process [1]. In the present study, Zn2+ forms a trap state in the band gap of α-Fe2O3, i.e., a separate band between conduction band and valence band. The trap state induces defect state/impurity level, which entraps the charge carriers, as soon as they have been generated by UV light illumination, and inhibits the recombination so that charge carriers can be used for the redox process. The band gap decreases in

**Figure 6.** Time-dependent UV–Vis absorption spectra for RB dye in the presence of Catalyst: (**a**) pure Fe2O3, (**b**) Zn 2%, (**c**) Zn 4% and (**d**) Zn 6%. **Figure 6.** Time-dependent UV–Vis absorption spectra for RB dye in the presence of Catalyst: (**a**) pure Fe2O<sup>3</sup> , (**b**) Zn 2%, (**c**) Zn 4% and (**d**) Zn 6%.

Photocatalytic activity generally includes the partial/complete degradation of organic waste dyes with the assistance of active species existing on the surface of the catalyst. When the catalyst is exposed to UV light, the photogenerated electrons (e−) are excited from top of valence band to the bottom of conduction band, leaving behind the holes in valence band. This lead to positive holes and negative electrons on the catalyst surface. The photogenerated holes interact with adsorbed water present on the surface of catalyst to generate reactive hydroxyl free radical (**∙**OH), while O<sup>2</sup> acts as an electron acceptor to form a superoxide (O<sup>2</sup> **<sup>∙</sup>**−) anion radical which on protonation yields HOO**<sup>∙</sup>** in the presence of water [36]. Further, the O<sup>2</sup> **<sup>∙</sup>**<sup>−</sup> can act as an oxidizing agent or as an additional source of Photocatalytic activity generally includes the partial/complete degradation of organic waste dyes with the assistance of active species existing on the surface of the catalyst. When the catalyst is exposed to UV light, the photogenerated electrons (e−) are excited from top of valence band to the bottom of conduction band, leaving behind the holes in valence band. This lead to positive holes and negative electrons on the catalyst surface. The photogenerated holes interact with adsorbed water present on the surface of catalyst to generate reactive hydroxyl free radical (·OH), while O<sup>2</sup> acts as an electron acceptor to form a superoxide (O<sup>2</sup> ·−) anion radical which on protonation yields HOO· in the presence of water [36]. Further, the O<sup>2</sup> ·− can act as an oxidizing agent or as an additional source of OH· radicals. These hydroxyl radicals are, thus, more efficient for degradation of RB dye into some non-toxic organic compounds, such as CO<sup>2</sup> and H2O, as shown in Figure 7. The oxidative (using holes) and reductive (using electrons) pathway, followed by the degradation process, are summarized as follows [33,37]:

$$\text{Zn} - (\text{a} - \text{Fe}\_2\text{O}\_3) + \text{hv} \rightarrow \text{Zn} - (\text{a} - \text{Fe}\_2\text{O}\_3) + \text{e}\_{\text{CB}}^{-} + \text{h}\_{\text{VB}}^{+} \tag{5}$$

$$e\_{\rm CB}^{-} + \rm O\_{2} \rightarrow \rm O\_{2}^{-} \tag{6}$$

$$+O\_2^{-} + H\_2O \rightarrow HOO^{\cdot} + OH^- \tag{7}$$

$$h\_{VB}^{+} + OH^{-} \rightarrow OH^{\cdot} \tag{8}$$

$$\rm{OH}^{\cdot} + \rm{O}\_{2}^{\cdot -} + \rm{R} \,\rm{d}y \,\rm{e}^{\cdot} \,\rm{intermediate} \rightarrow \rm{CO}\_{2} + \rm{H}\_{2}\rm{O} \tag{9}$$

This is in accordance with significant activity of samples which is attributed to the effective inhibition of (e−/h <sup>+</sup>) recombination and migrates to the photocatalyst surface to generate highly reactive free radicals that in turn oxidize RB dye (Figure 7).

Figure 8 displaying the experimental and linear plot of −ln(c/c0) versus time (t) for RB dye with different Zn concentration in hematite. It suggests that photodegradation of RB molecules by catalyst follows the pseudo-first-order kinetics [38]:

$$-\ln(\mathbb{C}/\mathbb{C}\_0) = kt \tag{10}$$

$$t\_{1/2} = \ln 2 / k \tag{11}$$

where, C<sup>0</sup> is the initial concentration of pollutant (RB dye) when the UV light is turned on, while C is the real-time concentration of pollutant under UV light irradiation, and k is the apparent rate summarized as follows [33,37]:

ܼ݊ − (ߙ − ܨ݁ଶܱଷ) + ℎ

OH**<sup>∙</sup>**

constant of pseudo-first-order equation, t is the irradiation time. The half-life time (t1/2) is defined as the time required to degrade 50% of initial RB dye concentration. The slope of the plot −ln(C/C0) with irradiation time provides the estimated apparent rate constant as given in Table 3. The observed degradation rate constant of RB dye in the presence of a catalyst Zn 4% is 0.02277 min−<sup>1</sup> , which is significantly larger than other synthesized samples. ℎ (8) .ܪܱ → ିܪܱ + ା ଶܱ + .ܪܱ (9) ܱଶܪ + ଶܱܥ → ݏ݁ݐ݉݁݀݅ܽݎ݁ݐ݅݊ ݁ݕ݀ ܤܴ + .ି This is in accordance with significant activity of samples which is attributed to the effective inhibition of (e−/h+) recombination and migrates to the photocatalyst surface to generate highly reactive free radicals that in turn oxidize RB dye (Figure 7).

ି + ܱଶ → ܱଶ

݁

ܱଶ

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

 radicals. These hydroxyl radicals are, thus, more efficient for degradation of RB dye into some non-toxic organic compounds, such as CO2 and H2O, as shown in Figure 7. The oxidative (using holes) and reductive (using electrons) pathway, followed by the degradation process, are

→ ܼ݊ − (ߙ − ܨ݁ଶܱଷ) + ݁

ି + ℎ

(7) ିܪܱ + ܱܱ.ܪ → ܱଶܪ + .ି

.ି (6)

ା (5)

**Figure 7.** Proposed photocatalytic mechanism in α-Fe2O3 for degrading RB dye. **Figure 7.** Proposed photocatalytic mechanism in α-Fe2O<sup>3</sup> *Crystals* **2020** for degrading RB dye. , *10*, x FOR PEER REVIEW 11 of 19

**Figure 8.** Experimental and linear plot of −ln(C/C0) versus irradiation time for pure Fe2O3, Zn 2%, Zn 4% and Zn 6% nanoparticles. **Figure 8.** Experimental and linear plot of −ln(C/C<sup>0</sup> ) versus irradiation time for pure Fe2O<sup>3</sup> , Zn 2%, Zn 4% and Zn 6% nanoparticles.

The percentage degradation of RB dye, using pure hematite as a catalyst, is 63% after UV **Table 3.** Calculated photodegradation parameters of pure Fe2O<sup>3</sup> , Zn 2%, Zn 4% and Zn 6% nanoparticles.


**Figure 9.** Bar diagram of (**a**) % degradation and (**b**) electricity cost in Indian rupees for degradation

Cost evaluation is one of the most important factors in waste water treatment. As saving energy (electricity) benefits the world at large scale. The main reason behind saving electricity is that burning of fossil fuels in plants causes several environmental issues, such as global warming and the greenhouse effect, which directly affect human life. Our present study aims to reduce energy to

of RB dye with pure Fe2O3, Zn 2%, Zn 4% and Zn 6% nanoparticles.

3.5.2. Electricity Cost

The percentage degradation of RB dye, using pure hematite as a catalyst, is 63% after UV irradiation for 90 min. Degradation % increases with an increase in Zn content up to Zn 4% and reached 87% as shown in Figure 9a. Further increase in Zn content decrease the degradation efficiency towards RB dye. Notably, the degradation rate of Zn 6% is even less than that of pure Fe2O3, due to the fact that Zn ions occupy interstitials site in the host matrix for this concentration responsible for the enhanced recombination rate between electrons and holes. 4% and Zn 6% nanoparticles. The percentage degradation of RB dye, using pure hematite as a catalyst, is 63% after UV irradiation for 90 min. Degradation % increases with an increase in Zn content up to Zn 4% and reached 87% as shown in Figure 9a. Further increase in Zn content decrease the degradation efficiency towards RB dye. Notably, the degradation rate of Zn 6% is even less than that of pure Fe2O3, due to the fact that Zn ions occupy interstitials site in the host matrix for this concentration responsible for the enhanced recombination rate between electrons and holes.

**Figure 8.** Experimental and linear plot of −ln(C/C0) versus irradiation time for pure Fe2O3, Zn 2%, Zn

*Crystals* **2020**, *10*, x FOR PEER REVIEW 11 of 19

**Figure 9.** Bar diagram of (**a**) % degradation and (**b**) electricity cost in Indian rupees for degradation of RB dye with pure Fe2O3, Zn 2%, Zn 4% and Zn 6% nanoparticles. **Figure 9.** Bar diagram of (**a**) % degradation and (**b**) electricity cost in Indian rupees for degradation of RB dye with pure Fe2O<sup>3</sup> , Zn 2%, Zn 4% and Zn 6% nanoparticles.

#### 3.5.2. Electricity Cost 3.5.2. Electricity Cost

Cost evaluation is one of the most important factors in waste water treatment. As saving energy (electricity) benefits the world at large scale. The main reason behind saving electricity is that burning of fossil fuels in plants causes several environmental issues, such as global warming and the greenhouse effect, which directly affect human life. Our present study aims to reduce energy to Cost evaluation is one of the most important factors in waste water treatment. As saving energy (electricity) benefits the world at large scale. The main reason behind saving electricity is that burning of fossil fuels in plants causes several environmental issues, such as global warming and the greenhouse effect, which directly affect human life. Our present study aims to reduce energy to mitigate the effects of greenhouse gases. The power consumption can be estimated using the following relation [39],

$$t\_{90} = \ln 10/k \tag{12}$$

$$E\_c = \frac{P \times t\_{90} \times 4.68}{1000 \times 60} \tag{13}$$

where, t<sup>90</sup> signifies the time taken by any dye to be degraded 90% of its initial concentration, E<sup>C</sup> is electricity cost, P is power consumed (in Watt) of UV light source. Power consumers consuming a maximum 500 units of electricity per month pay INR 4.68 per unit in our locality, as shown in Figure 9b. The electricity cost is also found to be minimum for 4% Zn doped sample which has maximum % degradation.
