*Article* **Ti2O3/TiO2-Assisted Solar Photocatalytic Degradation of 4-tert-Butylphenol in Water**

**Saule Mergenbayeva <sup>1</sup> , Timur Sh. Atabaev <sup>2</sup> and Stavros G. Poulopoulos 1,\***


**Abstract:** Colored Ti2O<sup>3</sup> and Ti2O3/TiO<sup>2</sup> (mTiO) catalysts were prepared by the thermal treatment method. The effects of treatment temperature on the structure, surface area, morphology and optical properties of the as-prepared samples were investigated by XRD, BET, SEM, TEM, Raman and UV–VIS spectroscopies. Phase transformation from Ti2O<sup>3</sup> to TiO<sup>2</sup> rutile and TiO<sup>2</sup> anatase to TiO<sup>2</sup> rutile increased with increasing treatment temperatures. The photocatalytic activities of thermally treated Ti2O<sup>3</sup> and mTiO were evaluated in the photodegradation of 4-tert-butylphenol (4-t-BP) under solar light irradiation. mTiO heated at 650 ◦C exhibited the highest photocatalytic activity for the degradation and mineralization of 4-t-BP, being approximately 89.8% and 52.4%, respectively, after 150 min of irradiation. The effects of various water constituents, including anions (CO2<sup>−</sup> 3 , NO3, Cl and HCO− 3 ) and humic acid (HA), on the photocatalytic activity of mTiO-650 were evaluated. The results showed that the presence of carbonate and nitrate ions inhibited 4-t-BP photodegradation, while chloride and bicarbonate ions enhanced the photodegradation of 4-t-BP. As for HA, its effect on the degradation of 4-t-BP was dependent on the concentration. A low concentration of HA (1 mg/L) promoted the degradation of 4-t-BP from 89.8% to 92.4% by mTiO-650, but higher concentrations of HA (5 mg/L and 10 mg/L) had a negative effect.

**Keywords:** 4-tert-butylphenol; solar photocatalysis; Ti2O3/TiO<sup>2</sup> ; degradation; mineralization

### **1. Introduction**

Water pollution by a broad category of organic pollutants is a rising issue of worldwide concern [1]. During the last decade, the consumption of personal care products (PPCPs), pharmaceuticals and endocrine-disrupting compounds (EDCs) has increased owing to economic development and population growth [2–5]. Their widespread use has increased their appearance in the aqueous environment, including rivers, lakes and reservoirs, at concentrations starting from several nanograms (ng/L) to several micrograms (µg/L) per liter [6–12]. They can even escape wastewater treatment plants (WWTPs) and drinking water treatment plants (DWTPs), ultimately reaching drinking water sources. These contaminants are termed emerging pollutants (EPs) and can cause severe adverse effects on human health and the aquatic environment [13].

In particular, 4-t-BP is an industrial chemical used as a raw material for the production of synthetic phenol and polycarbonate resins [14,15]. As a representative of EDCs, 4-t-BP has a high estrogenic effect and acute/chronic environmental toxicity [16,17]. Considering its adverse effects on human health and aquatic systems, 4-t-BP, as a highly persistent pollutant, needs to be controlled efficiently.

To date, various methods have been investigated to remove 4-t-BP from water, mainly including advanced oxidation processes (AOPs) and biological processes [16,18–20]. Among them, AOPs have attracted great attention for the removal such contaminants by converting

**Citation:** Mergenbayeva, S.; Sh. Atabaev, T.; Poulopoulos, S.G. Ti2O3/TiO2-Assisted Solar Photocatalytic Degradation of 4-tert-Butylphenol in Water. *Catalysts* **2021**, *11*, 1379. https://doi.org/ 10.3390/catal11111379

Academic Editors: Gassan Hodaifa and Rafael Borja

Received: 30 September 2021 Accepted: 11 November 2021 Published: 16 November 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

them into carbon dioxide and water [21,22]. The high efficiency of the process has mostly been associated with the production of hydroxyl radicals (standard potential, 2.8 V) used as oxidants. AOPs may vary in terms of work conditions, used materials and different paths of hydroxyl radical (OH. ) production [23,24]. Heterogeneous photocatalysis may be considered an economically feasible solution to remove 4-t-BP from water due to the competitive cost of the process and the ambient conditions of temperature and pressure [25–27]. Moreover, it is considered an environmentally friendly oxidation process since it allows not only the degradation of the pollutant from the contaminated system but also its total elimination, without generating any undesired by-products, which could be even more toxic compounds than the parent one [28,29].

Although various photoactive materials have been investigated, TiO2-based photocatalysts remain the most studied ones due to their high photocatalytic oxidation activity, chemical stability and availability [30–32]. The P25 form of TiO<sup>2</sup> is one of the most effective photocatalytic materials, which can be attributed to the combination of anatase and rutile phases [33–35]. However, the high energy band gap of approximately 3.0–3.2 eV limits the application of TiO<sup>2</sup> under solar light. In this context, numerous strategies have been devoted to extending the absorption wavelength to the visible area for the efficient utilization of sunlight. For example, the introduction of Ti3<sup>+</sup> into TiO<sup>2</sup> demonstrated the capacity to extend the light response of TiO2. It has been reported that the formation of Ti3<sup>+</sup> species is accompanied by the generation of oxygen vacancies (Ov), which can favor the separation of electron–hole pairs and thus improve the visible light activity of TiO<sup>2</sup> [36–38]. Moreover, Ti3+ and oxygen vacancies can form localized states below the conduction band (CB), which reduces the band gap of TiO<sup>2</sup> (Figure 1), so that it can distinctly expand the absorption to the visible region [39–43]. The reported methods to prepare structurally defective TiO<sup>2</sup> with Ti3<sup>+</sup> include the partial oxidation of low-valence Ti species (Ti, Ti (II) and Ti (III)), H<sup>2</sup> thermal treatment and the reduction of Ti4<sup>+</sup> to Ti3<sup>+</sup> by a chemical reducing agent (NaBH4), metals (Al, Mg, Li, Zn), etc. [44–49]. Although there are numerous preparation methods available, most of them require high consumption of chemicals, as well as multiple steps using specialized equipment. Therefore, it is of significant importance to develop a facile and feasible method to prepare defective TiO<sup>2</sup> with Ti3+. *Catalysts* **2021**, *11*, x FOR PEER REVIEW 3 of 20

treated Ti2O3 and Ti2O3/TiO2 (hereinafter denoted as mTiO) catalysts (Figures 2 and 3). The diffraction peaks at 2θ = 23.823° (012), 33.040° (104), 34.836° (110), 40.219° (113), 48.786° (024), 53.692° (116), 61.42° (214) and 62.64° (300) were attributed to Ti2O3 (JCPDS No. 00- 043-1033). With the increase in treatment temperature, the intensity of all characteristic peaks corresponding to Ti2O3 became weaker in both Ti2O3 and mTiO samples. As the temperature further increased to 750 °C, no typical peaks of Ti2O3 were observed, indicating the complete transformation of Ti2O3 to rutile TiO2 (JCPDS No. 00-021-1276) [50,51]. In the Ti2O3-550 sample (Figure 2), apart from the diffraction peaks of TiO2 rutile, peaks attributed to the anatase phase of TiO2 (JCPDS No. 00-021-12-72) also appeared at 2θ = 25.3° (101) and 48.028° (200). These findings reveal that the transformation of Ti2O3 into TiO2 anatase also took place. The results are in good agreement with previously reported ones [52]. These peaks almost completely disappeared at 900 °C, suggesting the

The composition of the catalysts was further investigated by Raman spectroscopy (Figures 4 and 5). The Raman peak at around 143 cm−1 justified the existence of the TiO2 anatase phase in both types of catalysts. For treated Ti2O3 (Figure 4), this peak became more intense with the increase in treatment temperature to 750 °C, confirming the successful transformation of Ti2O3 into TiO2 anatase. In addition, low-intensity peaks corresponding to the TiO2 anatase phase were observed at 196.85 cm−1, 399.57 cm−1 and 514.54 cm−1 in the spectra of Ti2O3-650 and Ti2O3-750, while a further increase in temperature to 900 °C led to an increase in the TiO2 rutile phase. However, no peaks were observed corresponding to Ti2O3, which could be attributed to the low intensities of the Raman bands

**2. Results and Discussion** 

*2.1. Characterization of Photocatalysts* 

transformation of TiO2 anatase into TiO2 rutile.

**Figure 1.** Schematic showing energy levels of TiO2 in the presence of Ti3+ ions. of the Ti2O3 structure. **Figure 1.** Schematic showing energy levels of TiO2 in the presence of Ti3+ ions.

In the present work, Ti2O<sup>3</sup> alone or in combination with TiO<sup>2</sup> (P25) was thermally treated through a simple one-step method, and their photocatalytic performance towards 4-t-BP degradation under simulated solar light was tested. The as-prepared samples were characterized by means of SEM, TEM, BET, XRD, Raman and UV–VIS spectroscopies to study their morphology, textural properties, crystal structure and optical properties. The effects of the presence of humic acid (HA) and inorganic ions (CO2<sup>−</sup> 3 , NO− 3 , Cl− and HCO− 3 ) on 4-t-BP degradation were also investigated.

### **2. Results and Discussion**

### *2.1. Characterization of Photocatalysts*

XRD measurements were conducted to identify the phase structures of the thermally treated Ti2O<sup>3</sup> and Ti2O3/TiO<sup>2</sup> (hereinafter denoted as mTiO) catalysts (Figures 2 and 3). The diffraction peaks at 2θ = 23.823◦ (012), 33.040◦ (104), 34.836◦ (110), 40.219◦ (113), 48.786◦ (024), 53.692◦ (116), 61.42◦ (214) and 62.64◦ (300) were attributed to Ti2O<sup>3</sup> (JCPDS No. 00- 043-1033). With the increase in treatment temperature, the intensity of all characteristic peaks corresponding to Ti2O<sup>3</sup> became weaker in both Ti2O<sup>3</sup> and mTiO samples. As the temperature further increased to 750 ◦C, no typical peaks of Ti2O<sup>3</sup> were observed, indicating the complete transformation of Ti2O<sup>3</sup> to rutile TiO<sup>2</sup> (JCPDS No. 00-021-1276) [50,51]. *Catalysts* **2021**, *11*, x FOR PEER REVIEW 4 of 20

**Figure 2.** XRD patterns of treated Ti2O3 catalysts. **Figure 2.** XRD patterns of treated Ti2O<sup>3</sup> catalysts.

In the Ti2O3-550 sample (Figure 2), apart from the diffraction peaks of TiO<sup>2</sup> rutile, peaks attributed to the anatase phase of TiO<sup>2</sup> (JCPDS No. 00-021-12-72) also appeared at 2θ = 25.3◦ (101) and 48.028◦ (200). These findings reveal that the transformation of Ti2O<sup>3</sup> into TiO<sup>2</sup> anatase also took place. The results are in good agreement with previously reported ones [52]. These peaks almost completely disappeared at 900 ◦C, suggesting the transformation of TiO<sup>2</sup> anatase into TiO<sup>2</sup> rutile.

The composition of the catalysts was further investigated by Raman spectroscopy (Figures 4 and 5). The Raman peak at around 143 cm−<sup>1</sup> justified the existence of the TiO<sup>2</sup> anatase phase in both types of catalysts. For treated Ti2O<sup>3</sup> (Figure 4), this peak became more intense with the increase in treatment temperature to 750 ◦C, confirming the successful

**Figure 3.** XRD patterns of treated mTiO catalysts.

transformation of Ti2O<sup>3</sup> into TiO<sup>2</sup> anatase. In addition, low-intensity peaks corresponding to the TiO<sup>2</sup> anatase phase were observed at 196.85 cm−<sup>1</sup> , 399.57 cm−<sup>1</sup> and 514.54 cm−<sup>1</sup> in the spectra of Ti2O3-650 and Ti2O3-750, while a further increase in temperature to 900 ◦C led to an increase in the TiO<sup>2</sup> rutile phase. However, no peaks were observed corresponding to Ti2O3, which could be attributed to the low intensities of the Raman bands of the Ti2O<sup>3</sup> structure. **Figure 2.** XRD patterns of treated Ti2O3 catalysts.

*Catalysts* **2021**, *11*, x FOR PEER REVIEW 4 of 20

**Figure 3.** XRD patterns of treated mTiO catalysts. **Figure 3.** XRD patterns of treated mTiO catalysts.

**Figure 4.** Raman spectra of treated Ti2O3 catalysts. **Figure 4.** Raman spectra of treated Ti2O<sup>3</sup> catalysts.

**Figure 5.** Raman spectra of treated mTiO catalysts.

**Figure 4.** Raman spectra of treated Ti2O3 catalysts.

Compared to untreated mTiO, mTiO-900 exhibited a negative shift at 143 cm−<sup>1</sup> (Figure 5), indicating the association of Ti2O<sup>3</sup> with TiO2, while, for mTiO-550 and mTiO-650, a positive shift in this peak could possibly be attributed to the introduction of Ti3+ and oxygen vacancies into the TiO<sup>2</sup> lattice as a result of the thermal treatment [53]. In the photocatalytic process, the presence of such structural defects in TiO<sup>2</sup> can inhibit the recombination of charge carriers and thus improve the photocatalytic activity.

The results from Raman spectroscopy are in general agreement with the ones obtained from XRD analysis, with the exception of the TiO<sup>2</sup> anatase phase, which was detected only with the first technique for the catalysts treated at 900 ◦C.

The textural properties of all catalysts were evaluated by BET N<sup>2</sup> adsorption/desorption measurements. As presented in Figures 6 and 7, all the catalysts revealed a typical type-III isotherm according to the classification of the international union of pure and applied chemistry (IUPAC). Interestingly, Ti2O<sup>3</sup> and mTiO catalysts heated at 650 ◦C exhibited the highest N<sup>2</sup> adsorption capacity and pore volume (Vp). In general, larger values of V<sup>p</sup> can be beneficial for the photocatalytic reaction through providing ionic diffusion and charge transfer on the surface of the photocatalyst [54].

Some other characteristics obtained from the BET analysis are displayed in Table 1, which shows that the treatment temperature had a significant effect on the microstructure of thermally treated Ti2O<sup>3</sup> and mTiO, particularly on the BET surface area (SBET) and pore volume (Vp). It could be noticed that the SBET of treated Ti2O<sup>3</sup> was relatively low compared to that of mTiO. The SBET of treated Ti2O<sup>3</sup> catalysts increased gradually as the treatment temperature increased from 550 ◦C to 750 ◦C, which could be likely associated with the formation of a better crystalline framework. However, a further increase in the treatment temperature to 900 ◦C caused a drastic decrease in SBET due to the phase transformation of TiO<sup>2</sup> anatase to TiO<sup>2</sup> rutile [55].

**Figure 5.** Raman spectra of treated mTiO catalysts.

of charge carriers and thus improve the photocatalytic activity.

charge transfer on the surface of the photocatalyst [54].

tected only with the first technique for the catalysts treated at 900 °C.

Compared to untreated mTiO, mTiO-900 exhibited a negative shift at 143 cm−1 (Figure 5), indicating the association of Ti2O3 with TiO2, while, for mTiO-550 and mTiO-650, a positive shift in this peak could possibly be attributed to the introduction of Ti3+ and oxygen vacancies into the TiO2 lattice as a result of the thermal treatment [53]. In the photocatalytic process, the presence of such structural defects in TiO2 can inhibit the recombination

The results from Raman spectroscopy are in general agreement with the ones obtained from XRD analysis, with the exception of the TiO2 anatase phase, which was de-

The textural properties of all catalysts were evaluated by BET N2 adsorption/desorption measurements. As presented in Figures 6 and 7, all the catalysts revealed a typical type-III isotherm according to the classification of the international union of pure and applied chemistry (IUPAC). Interestingly, Ti2O3 and mTiO catalysts heated at 650 °C exhibited the highest N2 adsorption capacity and pore volume (Vp). In general, larger values of Vp can be beneficial for the photocatalytic reaction through providing ionic diffusion and

**Figure 6.** N **Figure 6.** N2 adsorption/desorption of treated Ti2O3 catalysts. <sup>2</sup> adsorption/desorption of treated Ti2O<sup>3</sup> catalysts.

Some other characteristics obtained from the BET analysis are displayed in Table 1, which shows that the treatment temperature had a significant effect on the microstructure of thermally treated Ti2O3 and mTiO, particularly on the BET surface area (SBET) and pore

The increase in treatment temperature continuously decreased the SBET of treated mTiO. The lowering of SBET can be attributed to the increase in particle size as a result of

**Photocatalyst SBET (m2/g) Vp (cm3/g)**  Ti2O3-550 1.629 0.009 Ti2O3-650 1.985 0.017 Ti2O3-750 2.733 0.014 Ti2O3-900 0.974 0.012 mTiO-550 23.012 0.255 mTiO-650 20.894 0.347 mTiO-750 5.593 0.134 mTiO-900 3.443 0.029

to that of mTiO. The SBET of treated Ti2O3 catalysts increased gradually as the treatment temperature increased from 550 °C to 750 °C, which could be likely associated with the formation of a better crystalline framework. However, a further increase in the treatment temperature to 900 °C caused a drastic decrease in SBET due to the phase transformation of

**Figure 7.** N2 adsorption/desorption of treated mTiO catalysts. **Figure 7.** N<sup>2</sup> adsorption/desorption of treated mTiO catalysts.

TiO2 anatase to TiO2 rutile [55].

**Table 1.** BET surface area and pore volume of as-prepared catalysts.

aggregation [56].


**Table 1.** BET surface area and pore volume of as-prepared catalysts.

The increase in treatment temperature continuously decreased the SBET of treated mTiO. The lowering of SBET can be attributed to the increase in particle size as a result of aggregation [56].

The morphology of the prepared catalysts was examined by SEM and TEM. As can be seen from Figure 8, untreated Ti2O<sup>3</sup> particles exhibited an irregular shape with a smooth continuous morphology. In contrast, heating under different temperatures resulted in the formation of a much rougher surface of Ti2O3, which could be associated with the phase transformation from Ti2O<sup>3</sup> to TiO<sup>2</sup> rutile. Such an increase in surface roughness can increase the surface area of the catalyst and further influence the catalytic activity of the material. These results are consistent with the findings obtained from BET analysis, where the heating of Ti2O<sup>3</sup> up to 750 ◦C was accompanied by an increase in SBET.

SEM images of treated mTiO catalysts clearly revealed that thermal treatment caused a particle size growth in TiO2, well-distributed on the surface of Ti2O<sup>3</sup> (Figure 9). The increase in the size of TiO<sup>2</sup> particles may have resulted in the decrease in SBET.

A UV–VIS absorption study was carried out to assess the light-harvesting ability of the prepared samples (Figures 10 and 11). It can be seen that the temperature variation influenced the light absorption properties of all prepared catalysts. The rise in treatment temperature for Ti2O<sup>3</sup> catalysts from 550 ◦C to 650 ◦C extended the light absorption to the visible region (400–550 nm), while a further increase in the treatment temperature to 900 ◦C lowered the visible light absorption capacity (Figure 10).

In contrast, all prepared mTiO catalysts demonstrated a good light absorption ability within the wavelength range of 300−400 nm, although to different extents (Figure 11). Specifically, catalysts heated at lower temperatures demonstrated stronger absorption, which is in accordance with the expectations based on the color change of the catalysts. At the same time, mTiO-550 and mTiO-650 were found to be absorbing in the 400–550 nm region. This phenomenon may be attributed to the transformation of Ti2O<sup>3</sup> to TiO<sup>2</sup> rutile, containing Ti3+ (oxygen vacancies) sites [50,56]. It is noteworthy that the light absorption of mTiO-650 in the visible region was substantially enhanced compared with mTiO-550, as a result of the higher concentration of oxygen vacancies in the lattice of TiO2. Such an enhancement in the light absorption is favorable for improving the photoactivity of the material. On the other hand, the intensities at wavelengths higher than approximately 550 nm gradually weakened for mTiO-550 and mTiO-650.

Moreover, the band gap values of the prepared catalysts were estimated using the Kubelka–Munk equation and the corresponding Tauc plots. As illustrated in Figure 12, the calculated direct band gap energies were found to be 1.76, 1.75, 1.79 and 2.69 eV for Ti2O3-550, Ti2O3-650, Ti2O3-750 and Ti2O3-900, respectively. Similar variations in band gap energies were obtained for mTiO catalysts, where mTiO-650 had a lower band gap of 2.01 eV compared to mTiO-550, mTiO-750 and mTiO-900 (Figure 13). These results reveal the possible application of the prepared catalysts in solar-light-driven photocatalytic reactions.

Ti2O3-650 (**F**).

The morphology of the prepared catalysts was examined by SEM and TEM. As can be seen from Figure 8, untreated Ti2O3 particles exhibited an irregular shape with a smooth continuous morphology. In contrast, heating under different temperatures resulted in the formation of a much rougher surface of Ti2O3, which could be associated with the phase transformation from Ti2O3 to TiO2 rutile. Such an increase in surface roughness can increase the surface area of the catalyst and further influence the catalytic activity of the material. These results are consistent with the findings obtained from BET analysis, where

SEM images of treated mTiO catalysts clearly revealed that thermal treatment caused

a particle size growth in TiO2, well-distributed on the surface of Ti2O3 (Figure 9). The in-

the heating of Ti2O3 up to 750 °C was accompanied by an increase in SBET.

crease in the size of TiO2 particles may have resulted in the decrease in SBET.

**Figure 8.** SEM images of Ti2O3 untreated (**A**), Ti2O3-550 (**B**), Ti2O3-650 (**C**), Ti2O3-750 (**D**), Ti2O3-900 (**E**) and TEM image of **Figure 8.** SEM images of Ti2O<sup>3</sup> untreated (**A**), Ti2O<sup>3</sup> -550 (**B**), Ti2O<sup>3</sup> -650 (**C**), Ti2O<sup>3</sup> -750 (**D**), Ti2O<sup>3</sup> -900 (**E**) and TEM image of Ti2O<sup>3</sup> -650 (**F**).

**Figure 9.** SEM images of mTiO untreated (**A**), mTiO-550 (**B**), mTiO-650 (**C**), mTiO-750 (**D**), mTiO-900 (**E**) and TEM image of mTiO-650 (**F**). **Figure 9.** SEM images of mTiO untreated (**A**), mTiO-550 (**B**), mTiO-650 (**C**), mTiO-750 (**D**), mTiO-900 (**E**) and TEM image of mTiO-650 (**F**).

A UV–VIS absorption study was carried out to assess the light-harvesting ability of the prepared samples (Figures 10 and 11). It can be seen that the temperature variation influenced the light absorption properties of all prepared catalysts. The rise in treatment temperature for Ti2O3 catalysts from 550 °C to 650 °C extended the light absorption to the visible region (400–550 nm), while a further increase in the treatment temperature to 900

In contrast, all prepared mTiO catalysts demonstrated a good light absorption ability

within the wavelength range of 300−400 nm, although to different extents (Figure 11). Specifically, catalysts heated at lower temperatures demonstrated stronger absorption, which is in accordance with the expectations based on the color change of the catalysts. At the same time, mTiO-550 and mTiO-650 were found to be absorbing in the 400–550 nm region.

ing Ti3+ (oxygen vacancies) sites [50,56]. It is noteworthy that the light absorption of mTiO-650 in the visible region was substantially enhanced compared with mTiO-550, as a result of the higher concentration of oxygen vacancies in the lattice of TiO2. Such an enhancement in the light absorption is favorable for improving the photoactivity of the material.

°C lowered the visible light absorption capacity (Figure 10).

On the other hand, the intensities at wavelengths higher than approximately 550 nm grad-

On the other hand, the intensities at wavelengths higher than approximately 550 nm grad-

**Figure 10.** UV–VIS spectra of treated Ti2O3 catalysts. **Figure 10.** UV–VIS spectra of treated Ti2O<sup>3</sup> catalysts. **Figure 10.** UV–VIS spectra of treated Ti2O3 catalysts.

ually weakened for mTiO-550 and mTiO-650.

ually weakened for mTiO-550 and mTiO-650.

*Catalysts* **2021**, *11*, x FOR PEER REVIEW 10 of 20

**Figure 11.** UV–VIS spectra of treated mTiO catalysts. **Figure 11. Figure 11.** UV–VIS spectra of treated mTiO catalysts. UV–VIS spectra of treated mTiO catalysts.

tions.

tions.

*Catalysts* **2021**, *11*, x FOR PEER REVIEW 11 of 20

Moreover, the band gap values of the prepared catalysts were estimated using the Kubelka–Munk equation and the corresponding Tauc plots. As illustrated in Figure 12, the calculated direct band gap energies were found to be 1.76, 1.75, 1.79 and 2.69 eV for Ti2O3-550, Ti2O3-650, Ti2O3-750 and Ti2O3-900, respectively. Similar variations in band gap energies were obtained for mTiO catalysts, where mTiO-650 had a lower band gap of 2.01 eV compared to mTiO-550, mTiO-750 and mTiO-900 (Figure 13). These results reveal the possible application of the prepared catalysts in solar-light-driven photocatalytic reac-

Moreover, the band gap values of the prepared catalysts were estimated using the Kubelka–Munk equation and the corresponding Tauc plots. As illustrated in Figure 12, the calculated direct band gap energies were found to be 1.76, 1.75, 1.79 and 2.69 eV for Ti2O3-550, Ti2O3-650, Ti2O3-750 and Ti2O3-900, respectively. Similar variations in band gap energies were obtained for mTiO catalysts, where mTiO-650 had a lower band gap of 2.01 eV compared to mTiO-550, mTiO-750 and mTiO-900 (Figure 13). These results reveal the possible application of the prepared catalysts in solar-light-driven photocatalytic reac-

**Figure 12. Figure 12.** Tauc plot of treated Ti Tauc plot of treated Ti 2O3 catalysts. <sup>2</sup>O<sup>3</sup> catalysts. **Figure 12.** Tauc plot of treated Ti2O3 catalysts.

**Figure 13.** Tauc plot of treated mTiO catalysts.

*2.2. Photocatalytic Degradation of 4-t-BP in Aqueous Solution*

The photocatalytic activity of all the prepared catalysts was evaluated by the photodegradation of 4-t-BP under solar light irradiation, and the results are shown in Figure 14. In the absence of a catalyst, the decomposition of 4-t-BP observed after 150 min of irradiation was only 8.3%.

**Figure 13.** Tauc plot of treated mTiO catalysts.

ation was only 8.3%.

*2.2. Photocatalytic Degradation of 4-t-BP in Aqueous Solution* 

The photocatalytic activity of all the prepared catalysts was evaluated by the photodegradation of 4-t-BP under solar light irradiation, and the results are shown in Figure 14. In the absence of a catalyst, the decomposition of 4-t-BP observed after 150 min of irradi-

**Figure 14.** The 4-t-BP degradation under solar light irradiation in the presence of prepared catalysts. Reaction conditions: [4-t-BP]0 = 5 mg/L, [photocatalyst] = 200 mg/L. **Figure 14.** The 4-t-BP degradation under solar light irradiation in the presence of prepared catalysts. Reaction conditions: [4-t-BP]<sup>0</sup> = 5 mg/L, [photocatalyst] = 200 mg/L.

Treated Ti2O3 showed low photocatalytic activity and approximately 13%, 12.7%, 10.5% and 14% of 4-t-BP was decomposed by Ti2O3-550, Ti2O3-650, Ti2O3-750 and Ti2O3- 900, respectively, after 150 min of irradiation. In contrast, mTiO exhibited much higher photocatalytic degradation efficiency. Among mTiO catalysts, mTiO-650 showed the highest photocatalytic activity, achieving 89.8% of 4-t-BP degradation. These findings are consistent with the results obtained from physico-chemical characterization, where mTiO-650 exhibited better optical properties and a lower band gap and pore volume as compared to mTiO-550, mTiO-750 and mTiO-900, indicating the importance of the treatment temperature on the optical properties and photocatalytic activity of the catalyst [57]. Treated Ti2O<sup>3</sup> showed low photocatalytic activity and approximately 13%, 12.7%, 10.5% and 14% of 4-t-BP was decomposed by Ti2O3-550, Ti2O3-650, Ti2O3-750 and Ti2O3- 900, respectively, after 150 min of irradiation. In contrast, mTiO exhibited much higher photocatalytic degradation efficiency. Among mTiO catalysts, mTiO-650 showed the highest photocatalytic activity, achieving 89.8% of 4-t-BP degradation. These findings are consistent with the results obtained from physico-chemical characterization, where mTiO-650 exhibited better optical properties and a lower band gap and pore volume as compared to mTiO-550, mTiO-750 and mTiO-900, indicating the importance of the treatment temperature on the optical properties and photocatalytic activity of the catalyst [57].

The mineralization efficiency of a photocatalyst is an important indicator for assessing its practical application. Thus, the mineralization of 4-t-BP was evaluated via total organic carbon (TOC) measurements (Figure 15). As in the case of 4-t-BP photodegradation, the treated mTiO catalysts exhibited higher TOC removal than Ti2O3 catalysts. In particular, 54.2% of TOC removal was obtained in 150 min using mTiO-650 under solar light The mineralization efficiency of a photocatalyst is an important indicator for assessing its practical application. Thus, the mineralization of 4-t-BP was evaluated via total organic carbon (TOC) measurements (Figure 15). As in the case of 4-t-BP photodegradation, the treated mTiO catalysts exhibited higher TOC removal than Ti2O<sup>3</sup> catalysts. In particular, 54.2% of TOC removal was obtained in 150 min using mTiO-650 under solar light irradiation. In the same reaction time, 32.5%, 12.4%, 12.2%, 11.2%, 10%, 7.8% and 5.4% of TOC removal was obtained for mTiO-550, mTiO-900, mTiO-750, Ti2O3-750, Ti2O3-900, Ti2O3-650 and Ti2O3-550. The observed photocatalytic efficiency of the catalysts tested for TOC removal was in accordance with the 4-t-BP photodegradation results.

**Figure 15.** TOC removal under solar light irradiation in the presence of prepared catalysts. [4-t-BP]0 = 5 mg/L, [photocatalyst] = 200 mg/L. **Figure 15.** TOC removal under solar light irradiation in the presence of prepared catalysts. [4-t-BP]<sup>0</sup> = 5 mg/L, [photocatalyst] = 200 mg/L.

#### *2.3. Effect of HA and Coexisting Ions (*ܥܱଷ ଶି, ܱܰଷ ଷܱܥܪ ܽ݊݀ ݈ିܥ ି, ି*) on the Degradation of 4-t-BP 2.3. Effect of HA and Coexisting Ions (CO*2<sup>−</sup> 3 *, NO*− 3 *, Cl*− *and HCO*− 3 *) on the Degradation of 4-t-BP*

The widespread water constituents in wastewater, including natural organic matter (NOM) and inorganic ions (COଷ ଶି, NOଷ ି, Clି and HCOଷ ି), could significantly affect the performance of the reaction system towards the degradation and mineralization of the target pollutants [58,59]. The widespread water constituents in wastewater, including natural organic matter (NOM) and inorganic ions (CO2<sup>−</sup> 3 , NO− 3 , Cl− and HCO− 3 ), could significantly affect the performance of the reaction system towards the degradation and mineralization of the target pollutants [58,59].

irradiation. In the same reaction time, 32.5%, 12.4%, 12.2%, 11.2%, 10%, 7.8% and 5.4% of TOC removal was obtained for mTiO-550, mTiO-900, mTiO-750, Ti2O3-750, Ti2O3-900, Ti2O3- 650 and Ti2O3-550. The observed photocatalytic efficiency of the catalysts tested for TOC

removal was in accordance with the 4-t-BP photodegradation results.

NOM is considered an integral part of natural water bodies and wastewater, and it is mainly composed of humic compounds and proteins [60,61]. In this study, HA was used as a model NOM compound and the effects of different concentrations of HA (1 mg/L, 5mg/L and 10 mg/L) on the degradation of 4-t-BP were investigated. As shown in Figure 16, the presence of HA in the mTiO-650/solar light system could promote or hinder the degradation of 4-t-BP, depending on its concentration. The presence of a relatively low concentration (1 mg/L) of HA increased the degradation efficiency of 4-t-BP from 89.8% to 92.4%, while higher concentrations (5mg/L and 10 mg/L) of HA decreased the degradation efficiency of 4-t-BP to 84.6% and 70.8%, respectively. The enhanced degradation of 4-t-BP in the presence of HA was also observed for the degradation of Bisphenol A [62] and dimethoate [63] by TiO2 photocatalytic degradation. The positive effect of HA at low concentrations might be ascribed to the photosensitization of HA, which would produce extra electrons, leading to an improvement in the photocatalytic degradation of organic NOM is considered an integral part of natural water bodies and wastewater, and it is mainly composed of humic compounds and proteins [60,61]. In this study, HA was used as a model NOM compound and the effects of different concentrations of HA (1 mg/L, 5 mg/L and 10 mg/L) on the degradation of 4-t-BP were investigated. As shown in Figure 16, the presence of HA in the mTiO-650/solar light system could promote or hinder the degradation of 4-t-BP, depending on its concentration. The presence of a relatively low concentration (1 mg/L) of HA increased the degradation efficiency of 4-t-BP from 89.8% to 92.4%, while higher concentrations (5mg/L and 10 mg/L) of HA decreased the degradation efficiency of 4-t-BP to 84.6% and 70.8%, respectively. The enhanced degradation of 4-t-BP in the presence of HA was also observed for the degradation of Bisphenol A [62] and dimethoate [63] by TiO<sup>2</sup> photocatalytic degradation. The positive effect of HA at low concentrations might be ascribed to the photosensitization of HA, which would produce extra electrons, leading to an improvement in the photocatalytic degradation of organic pollutants [62,64,65]. On the other hand, at higher concentrations, HA adsorbed on the surface of the catalyst could compete with 4-t-BP for active sites, resulting in a reduction in degradation efficiency [66,67].

in degradation efficiency [66,67].

**Figure 16.** Effects of HA on 4-t-BP degradation under solar irradiation in the presence of mTiO-650. Reaction conditions: [4-t-BP]0 = 5 mg/L, [photocatalyst] = 200 mg/L. **Figure 16.** Effects of HA on 4-t-BP degradation under solar irradiation in the presence of mTiO-650. Reaction conditions: [4-t-BP]<sup>0</sup> = 5 mg/L, [photocatalyst] = 200 mg/L.

The presence of COଷ ଶି, NOଷ ି, Clି and HCOଷ ି anions in the concentration of 100 mg/L had dual effects on the degradation of 4-t-BP over mTiO-650. COଷ ଶି and NOଷ ି ions resulted in a certain degree of negative effect with respect to the degradation of 4-t-BP. As shown in Figure 17, the 4-t-BP degradation decreased from 89.8% to 87.3% and 70.3% in the presence of nitrate and carbonate, respectively. The inhibition effect of COଷ ଶି and NOଷ ି was due to: (1) the quenching of oxidizing species, such as hydroxyl radicals (OH<sup>∙</sup> ), and positive holes (hା) by anions (Equations (1)–(3)); (2) anions could compete with 4-t-BP molecules for the available active sites of the catalyst surface, which further affects the degradation process [68–71]. Several studies have highlighted that NOଷ ି ions are usually weakly adsorbed on the surface of the catalyst and, thus, they slightly inhibit photodegradation reactions [69,72]. The presence of CO2<sup>−</sup> 3 , NO− 3 , Cl− and HCO− 3 anions in the concentration of 100 mg/L had dual effects on the degradation of 4-t-BP over mTiO-650. CO2<sup>−</sup> 3 and NO− 3 ions resulted in a certain degree of negative effect with respect to the degradation of 4-t-BP. As shown in Figure 17, the 4-t-BP degradation decreased from 89.8% to 87.3% and 70.3% in the presence of nitrate and carbonate, respectively. The inhibition effect of CO2<sup>−</sup> 3 and NO− <sup>3</sup> was due to: (1) the quenching of oxidizing species, such as hydroxyl radicals (OH. ), and positive holes (h <sup>+</sup>) by anions (Equations (1)–(3)); (2) anions could compete with 4-t-BP molecules for the available active sites of the catalyst surface, which further affects the degradation process [68–71]. Several studies have highlighted that NO− 3 ions are usually weakly adsorbed on the surface of the catalyst and, thus, they slightly inhibit photodegradation reactions [69,72].

pollutants [62,64,65]. On the other hand, at higher concentrations, HA adsorbed on the surface of the catalyst could compete with 4-t-BP for active sites, resulting in a reduction

$$\text{CO}\_3^{2-} + \text{OH}^\cdot \rightarrow \text{OH}^- + \text{CO}\_3^- \tag{1}$$

$$\begin{array}{ccccc} \text{NO}\_3^- + \text{h}\_{\text{VB}}^+ \rightarrow \text{NO}\_3^- & & & \text{(2)}\\ \cdots & \cdots & \cdots & \cdots \end{array} \tag{2}$$

$$\text{NO}\_3^- + \text{OH}^- \rightarrow \text{NO}\_3^- + \text{OH}^- \tag{3}$$

$$\begin{array}{ccccc}\text{NO}\_3^- + \text{OH}^- \rightarrow \text{NO}\_3^- + \text{OH}^- & & & \text{(3)}\\\text{N} & \text{H}\_2 & \text{N} & \text{N} & \text{N} \\\text{N} & \text{N} & \text{N} & \text{N} \end{array} \tag{4}$$

NOଷ → NOଷ It is noteworthy that both Clି and HCOଷ ି accelerated the degradation of 4-t-BP by mTiO-650. The addition of Clି and HCOଷ ି to the system resulted in 92.5% and 100% degradation after 150 min of solar light irradiation. Clିanions reacting with hydroxyl radicals can produce ClOH∙ି and subsequently transform into Cl∙ (Equations (4) and (5)) [73]. The It is noteworthy that both Cl− and HCO− 3 accelerated the degradation of 4-t-BP by mTiO-650. The addition of Cl− and HCO− 3 to the system resulted in 92.5% and 100% degradation after 150 min of solar light irradiation. Cl− anions reacting with hydroxyl radicals can produce ClOH.<sup>−</sup> and subsequently transform into Cl. (Equations (4) and (5)) [73]. The generated active chlorine species can selectively attack electron-rich organic compounds [74]. The complete 4-t-BP degradation in the presence of HCO− 3 could be most likely attributed to the generated alkaline condition or the formation of more selective radicals (CO.<sup>−</sup> 3 ) by

the reaction of HCO− <sup>3</sup> with OH. (Equation (6)), which can promote the degradation of 4-t-BP [69,75]. (COଷ ∙ି) by the reaction of HCOଷ ି with OH<sup>∙</sup> (Equation (6)), which can promote the degradation of 4-t-BP [69,75].

[74]. The complete 4-t-BP degradation in the presence of HCOଷ

*Catalysts* **2021**, *11*, x FOR PEER REVIEW 15 of 20

$$\rm Cl^{-} + OH^{-} \rightarrow OH^{-} + ClOH^{-} \tag{4}$$

ି could be most likely at-

$$\rm \rm H^{+} + \rm ClOH^{-} \rightarrow \rm Cl^{-} + \rm H\_{2}O \tag{5}$$

generated active chlorine species can selectively attack electron-rich organic compounds

tributed to the generated alkaline condition or the formation of more selective radicals

$$\rm{HCO\_3^- + OH^- \to CO\_3^- + H\_2O} \tag{6}$$

**Figure 17.** Effects of anions (COଷ ଶି, NOଷ ି, Clି and HCOଷ ି) on 4-t-BP degradation under solar light irradiation in the presence of mTiO-650. Reaction conditions: [4-t-BP]0 = 5 mg/L, [photocatalyst] = 200 mg/L. **3. Materials and Methods Figure 17.** Effects of anions (CO2<sup>−</sup> 3 , NO− 3 , Cl− and HCO− 3 ) on 4-t-BP degradation under solar light irradiation in the presence of mTiO-650. Reaction conditions: [4-t-BP]<sup>0</sup> = 5 mg/L, [photocatalyst] = 200 mg/L.

#### *3.1. Materials*  **3. Materials and Methods**

### *3.1. Materials*

size, purity ≥ 99.5% trace metals basis, P25), titanium (III) oxide (Ti2O3, 100 mesh, 99.9% trace metals basis), Na2CO3, NaNO3, NaCl, NaHCO3 and HA were purchased from Sigma Aldrich. All chemicals were of analytical grade and used as purchased. All aqueous solutions were prepared with ultrapure water (UPW) using a Milli-Q System (18.2 MΩ. cm). *3.2. Preparation of the Photocatalysts*  The 4-t-BP (99%), titanium (IV) oxide (TiO2, nanopowder, 21 nm primary particle size, purity ≥ 99.5% trace metals basis, P25), titanium (III) oxide (Ti2O3, 100 mesh, 99.9% trace metals basis), Na2CO3, NaNO3, NaCl, NaHCO<sup>3</sup> and HA were purchased from Sigma Aldrich. All chemicals were of analytical grade and used as purchased. All aqueous solutions were prepared with ultrapure water (UPW) using a Milli-Q System (18.2 MΩ. cm).

The 4-t-BP (99%), titanium (IV) oxide (TiO2, nanopowder, 21 nm primary particle

#### The preparation process was the same for both Ti2O3 alone and mTiO. At first, Ti2O3 *3.2. Preparation of the Photocatalysts*

or a mixture of Ti2O3 and TiO2 was crushed into a fine powder and then heated in a muffle furnace at 550 °C, 650 °C, 750 °C or 900 °C for 3 h in air. For mTiO, the weight ratio between The preparation process was the same for both Ti2O<sup>3</sup> alone and mTiO. At first, Ti2O<sup>3</sup> or a mixture of Ti2O<sup>3</sup> and TiO<sup>2</sup> was crushed into a fine powder and then heated in a muffle furnace at 550 ◦C, 650 ◦C, 750 ◦C or 900 ◦C for 3 h in air. For mTiO, the weight ratio between Ti2O<sup>3</sup> and TiO<sup>2</sup> was 1:1. The final products were denoted as Ti2O3-X and mTiO-X (with X being the temperature of the thermal treatment), respectively. The detailed synthesis process is illustrated in Figure 18.

**Figure 18.** Preparation of thermally treated Ti2O3 and mTiO catalysts. **Figure 18.** Preparation of thermally treated Ti2O<sup>3</sup> and mTiO catalysts.

### *3.3. Characterization of the Photocatalysts*

cess is illustrated in Figure 18.

*3.3. Characterization of the Photocatalysts*  The crystallographic properties and XRD patterns of the prepared catalysts were acquired at 2θ of 20–80° on an X-ray diffraction (XRD, Rigaku Smartlab) system. Raman spectra were recorded with the help of a Raman spectrometer (Horiba, LabRam HR evolution) and excitation energy was λ = 532 nm. Textural properties, including specific surface area (SBET) and pore volume (Vp), were measured using an automated gas sorption analyzer (Autosorb iQ, Quantachrome, USA) by the Brunauer–Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH) method, respectively. Surface morphology was observed by a Scanning Electron Microscope (SEM, Auriga CrossBeam 540, Carl Zeiss) and Transmission Electron Microscope (TEM, JEOL JEM-1400 Plus). UV–VIS spectroscopy of samples was implemented on a Thermo Scientific Genesys 150 UV–Visible spectrophotome-The crystallographic properties and XRD patterns of the prepared catalysts were acquired at 2θ of 20–80◦ on an X-ray diffraction (XRD, Rigaku Smartlab) system. Raman spectra were recorded with the help of a Raman spectrometer (Horiba, LabRam HR evolution) and excitation energy was λ = 532 nm. Textural properties, including specific surface area (SBET) and pore volume (Vp), were measured using an automated gas sorption analyzer (Autosorb iQ, Quantachrome, Boynton Beach, FL, USA) by the Brunauer–Emmett– Teller (BET) and Barrett–Joyner–Halenda (BJH) method, respectively. Surface morphology was observed by a Scanning Electron Microscope (SEM, Auriga CrossBeam 540, Carl Zeiss) and Transmission Electron Microscope (TEM, JEOL JEM-1400 Plus). UV–VIS spectroscopy of samples was implemented on a Thermo Scientific Genesys 150 UV–Visible spectrophotometer.

Ti2O3 and TiO2 was 1:1. The final products were denoted as Ti2O3-X and mTiO-X (with X being the temperature of the thermal treatment), respectively. The detailed synthesis pro-

#### ter. *3.4. Photodegradation Tests*

*3.4. Photodegradation Tests*  The photocatalytic performance of the prepared catalysts was evaluated through the experiments of 4-t-BP degradation under simulated solar light irradiation. First, 100 mg of catalyst was added to 500 mL 4-t-BP (5 mg/L) aqueous solution. Prior to irradiation, the mixture was kept in the dark for 15 min under stirring to reach the adsorption/desorption equilibrium. Then, while stirring, the suspension was exposed to the simulated solar irradiation produced by a 100 W Xenon lamp with an AM1.5G filter (LCS-100 solar simulator). During the experiment, 20 mL of reaction solution was extracted at regular time in-The photocatalytic performance of the prepared catalysts was evaluated through the experiments of 4-t-BP degradation under simulated solar light irradiation. First, 100 mg of catalyst was added to 500 mL 4-t-BP (5 mg/L) aqueous solution. Prior to irradiation, the mixture was kept in the dark for 15 min under stirring to reach the adsorption/desorption equilibrium. Then, while stirring, the suspension was exposed to the simulated solar irradiation produced by a 100 W Xenon lamp with an AM1.5G filter (LCS-100 solar simulator). During the experiment, 20 mL of reaction solution was extracted at regular time intervals and filtered by a 0.22 µm Millex syringe filter to remove the photocatalyst for further analysis.

tervals and filtered by a 0.22 µm Millex syringe filter to remove the photocatalyst for further analysis. The concentration of 4-t-BP was analyzed by a high-performance liquid chromatography instrument (HPLC, Agilent 1290 Infinity II, USA) equipped with an SB-C8 column (2.1 mm × 100 mm, 1.8 µm). The mobile phase composition was methanol and UPW (50:50, *v*/*v*), which were mixed to compose the mobile phase. The mineralization of 4-t-BP solution was monitored from the decay of TOC content, measured by a TOC analyzer (Multi The concentration of 4-t-BP was analyzed by a high-performance liquid chromatography instrument (HPLC, Agilent 1290 Infinity II, Santa Clara, CA, USA) equipped with an SB-C8 column (2.1 mm × 100 mm, 1.8 µm). The mobile phase composition was methanol and UPW (50:50, *v*/*v*), which were mixed to compose the mobile phase. The mineralization of 4-t-BP solution was monitored from the decay of TOC content, measured by a TOC analyzer (Multi N/C 3100, Analytic Jena, Jena, Germany).

#### N/C 3100, Analytic Jena, Jena, Germany). **4. Conclusions**

In summary, Ti2O<sup>3</sup> and mTiO photocatalysts were prepared via a one-step synthesis method and further characterized by different tests. The effect of treatment temperature on the physicochemical properties and photocatalytic performance of the prepared catalysts in the degradation of 4-t-BP under simulated solar light irradiation was investigated. Based

on the results obtained, the increase in treatment temperature from 550 ◦C to 650 ◦C caused an increase in the pore volume and enhanced light absorbance in the visible region (400–550 nm) for both Ti2O<sup>3</sup> and mTiO photocatalysts. The improved textural and optical properties related to the anatase to rutile ratio and specific surface area contributed to the enhanced performance of mTiO-650, which exhibited the highest photocatalytic activity at the dosage of 0.2 mg/L, achieving 89.8% degradation and 54.2% mineralization of 4-t-BP after 150 min. The effect of treatment temperature on the catalytic performance of the treated Ti2O<sup>3</sup> catalysts was almost negligible and resulted in 13%, 12.7%, 10.5% and 14% 4-t-BP degradation by Ti2O3-550, Ti2O3-650, Ti2O3-750 and Ti2O3-900, respectively. In addition, the effects of the presence of HA and various inorganic ions, including CO2<sup>−</sup> 3 , NO− 3 , Cl− and HCO− 3 on the photodegradation of 4-t-BP by mTiO-650 were also studied. At relatively low concentrations, HA could act as a photosensitizer and therefore promoted the degradation of 4-t-BP, whereas higher concentrations inhibited the degradation. The presence of Cl− and HCO− 3 exhibited a positive influence on 4-t-BP degradation, resulting from the favorable formation of additional reactive species, while the presence of NO− 3 and CO2<sup>−</sup> 3 slightly inhibited the reaction.

**Author Contributions:** Conceptualization, S.M. and S.G.P.; methodology, S.M. and S.G.P.; validation, T.S.A.; investigation, S.M.; resources, T.S.A. and S.G.P.; writing—original draft preparation, S.M.; writing—review and editing, S.M., T.S.A. and S.G.P.; supervision, T.S.A. and S.G.P.; project administration, S.G.P.; funding acquisition, S.G.P. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the Nazarbayev University project "Cost-Effective Photocatalysts for the Treatment of Wastewaters containing Emerging Pollutants", Faculty Development Competitive Research Grants Program for 2020–2022, Grant Number 240919FD3932, awarded to S.G. Poulopoulos.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** The authors acknowledge funding support from Nazarbayev University. The technical support of the Core Facilities of Nazarbayev University is greatly acknowledged.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

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**Photocatalytic Activity**

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