Ti 2 O 3 /TiO 2 -Assisted Solar Photocatalytic Degradation of 4-tert-Butylphenol in Water

: Colored Ti 2 O 3 and Ti 2 O 3 /TiO 2 (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 Ti 2 O 3 to TiO 2 rutile and TiO 2 anatase to TiO 2 rutile increased with increasing treatment temperatures. The photocatalytic activities of thermally treated Ti 2 O 3 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 (CO 2 − 3 , NO 3 , 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.


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][3][4][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][7][8][9][10][11][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][19][20]. Among them, AOPs have attracted great attention for the removal such contaminants by converting

Characterization of Photocatalysts
XRD measurements were conducted to identify the phase structures of the thermally treated Ti2O3 and Ti2O3/TiO2 (hereinafter denoted as mTiO) catalysts (Figures 2 and 3). The  In the present work, Ti 2 O 3 alone or in combination with TiO 2 (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 (CO 2− 3 , NO − 3 , Cl − and HCO − 3 ) on 4-t-BP degradation were also investigated.

Characterization of Photocatalysts
XRD measurements were conducted to identify the phase structures of the thermally treated Ti 2 O 3 and Ti 2 O 3 /TiO 2 (hereinafter denoted as mTiO) catalysts (Figures 2 and 3). The diffraction peaks at 2θ = 23.823 • (012), 33   In the Ti 2 O 3 -550 sample (Figure 2), apart from the diffraction peaks of TiO 2 rutile, peaks attributed to the anatase phase of TiO 2 (JCPDS No. 00-021-12-72) also appeared at 2θ = 25.3 • (101) and 48.028 • (200). These findings reveal that the transformation of Ti 2 O 3 into TiO 2 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 2 anatase into TiO 2 rutile.
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 TiO 2 anatase phase in both types of catalysts. For treated Ti 2 O 3 (Figure 4), this peak became more intense with the increase in treatment temperature to 750 • C, confirming the successful transformation of Ti 2 O 3 into TiO 2 anatase. In addition, low-intensity peaks corresponding to the TiO 2 anatase phase were observed at 196.85 cm −1 , 399.57 cm −1 and 514.54 cm −1 in the spectra of Ti 2 O 3 -650 and Ti 2 O 3 -750, while a further increase in temperature to 900 • C led to an increase in the TiO 2 rutile phase. However, no peaks were observed corresponding to Ti 2 O 3 , which could be attributed to the low intensities of the Raman bands of the Ti 2 O 3 structure.       Compared to untreated mTiO, mTiO-900 exhibited a negative shift at 143 cm −1 ( Figure 5), indicating the association of Ti 2 O 3 with TiO 2 , while, for mTiO-550 and mTiO-650, a positive shift in this peak could possibly be attributed to the introduction of Ti 3+ and oxygen vacancies into the TiO 2 lattice as a result of the thermal treatment [53]. In the photocatalytic process, the presence of such structural defects in TiO 2 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 2 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 2 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, Ti 2 O 3 and mTiO catalysts heated at 650 • C exhibited the highest N 2 adsorption capacity and pore volume (V p ). In general, larger values of V p 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 Ti 2 O 3 and mTiO, particularly on the BET surface area (S BET ) and pore volume (Vp). It could be noticed that the S BET of treated Ti 2 O 3 was relatively low compared to that of mTiO. The S BET of treated Ti 2 O 3 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 S BET due to the phase transformation of TiO 2 anatase to TiO 2 rutile [55].
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 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 Ti2O3 and mTiO, particularly on the BET surface area (SBET) and pore volume (Vp). It could be noticed that the SBET of treated Ti2O3 was relatively low compared to that of mTiO. The SBET of treated Ti2O3 catalysts increased gradually as the treatment  The increase in treatment temperature continuously decreased the S BET of treated mTiO. The lowering of S BET 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 Ti 2 O 3 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 Ti 2 O 3 , which could be associated with the phase transformation from Ti 2 O 3 to TiO 2 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 Ti 2 O 3 up to 750 • C was accompanied by an increase in S BET .
SEM images of treated mTiO catalysts clearly revealed that thermal treatment caused a particle size growth in TiO 2 , well-distributed on the surface of Ti 2 O 3 ( Figure 9). The increase in the size of TiO 2 particles may have resulted in the decrease in S BET .
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 Ti 2 O 3 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 Ti 2 O 3 to TiO 2 rutile, containing Ti 3+ (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 TiO 2 . 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. crease 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 Ti2O3 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 Ti2O3 (Figure 9). The increase in the size of TiO2 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 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 °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 Ti2O3 to TiO2 rutile, containing Ti 3+ (oxygen vacancies) sites [50,56]. It is noteworthy that the light absorption of mTiO-  On the other hand, the intensities at wavelengths higher than approximately 550 nm gradually weakened for mTiO-550 and mTiO-650.   On the other hand, the intensities at wavelengths higher than approximately 550 nm gradually weakened for mTiO-550 and mTiO-650.   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.  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.

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%.

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%. 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].
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 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 Ti 2 O 3 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, Ti 2 O 3 -750, Ti 2 O 3 -900, Ti 2 O 3 -650 and Ti 2 O 3 -550. The observed photocatalytic efficiency of the catalysts tested for TOC removal was in accordance with the 4-t-BP photodegradation results.

Effect of HA and Coexisting Ions ( , , ) on the Degradation of 4-t-BP
The widespread water constituents in wastewater, including natural organic matter (NOM) and inorganic ions (CO , NO , Cl and HCO ), could significantly affect the performance of the reaction system towards the degradation and mineralization of the target pollutants [58,59].
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  The widespread water constituents in wastewater, including natural organic matter (NOM) and inorganic ions (CO 2− 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].
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 2 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]. 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]. The presence of CO , NO , Cl and HCO anions in the concentration of 100 mg/L had dual effects on the degradation of 4-t-BP over mTiO-650. CO and NO 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 CO and NO was due to: (1) the quenching of oxidizing species, such as hydroxyl radicals (OH • ), and positive holes (h ) 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][69][70][71]. Several studies have highlighted that NO ions are usually weakly adsorbed on the surface of the catalyst and, thus, they slightly inhibit photodegradation reactions [69,72].

CO + OH • → OH + CO
(1) It is noteworthy that both Cl and HCO accelerated the degradation of 4-t-BP by mTiO-650. The addition of Cl and HCO 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 • and subsequently transform into Cl • (Equations (4) and (5)) [73]. The  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 CO 2− 3 and NO − 3 was due to: (1) the quenching of oxidizing species, such as hydroxyl radicals (OH . ), and positive holes (h + ) 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][69][70][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].
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 .− 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 .− 3 ) by the reaction of HCO − 3 with OH . (Equation (6)), which can promote the degradation of 4-t-BP [69,75].
Catalysts 2021, 11, x FOR PEER REVIEW 15 of generated active chlorine species can selectively attack electron-rich organic compoun [74]. The complete 4-t-BP degradation in the presence of HCO could be most likely a tributed to the generated alkaline condition or the formation of more selective radica (CO • ) by the reaction of HCO with OH • (Equation (6)), which can promote the degrad tion of 4-t-BP [69,75].

Preparation of the Photocatalysts
The preparation process was the same for both Ti2O3 alone and mTiO. At first, Ti2 or a mixture of Ti2O3 and TiO2 was crushed into a fine powder and then heated in a muf furnace at 550 °C, 650 °C, 750 °C or 900 °C for 3 h in air. For mTiO, the weight ratio betwe

Preparation of the Photocatalysts
The preparation process was the same for both Ti 2 O 3 alone and mTiO. At first, Ti 2 O 3 or a mixture of Ti 2 O 3 and TiO 2 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 Ti 2 O 3 and TiO 2 was 1:1. The final products were denoted as Ti 2 O 3 -X and mTiO-X (with X being the temperature of the thermal treatment), respectively. The detailed synthesis process is illustrated in Figure 18. 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 process is illustrated in Figure 18.

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 spectrophotometer.

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 intervals 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 N/C 3100, Analytic Jena, Jena, Germany).

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 (S BET ) and pore volume (V p ), 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.

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 intervals 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, 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).

Conclusions
In summary, Ti 2 O 3 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 Ti 2 O 3 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 Ti 2 O 3 catalysts was almost negligible and resulted in 13%, 12.7%, 10.5% and 14% 4-t-BP degradation by Ti 2 O 3 -550, Ti 2 O 3 -650, Ti 2 O 3 -750 and Ti 2 O 3 -900, respectively. In addition, the effects of the presence of HA and various inorganic ions, including CO 2− 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 CO 2− 3 slightly inhibited the reaction.