Fabrication of TiO2 Spheres and a Visible Light Active α-Fe2O3/TiO2-Rutile/TiO2-Anatase Heterogeneous Photocatalyst from Natural Ilmenite

High-purity (98.8%, TiO2) rutile nanoparticles were successfully synthesized using ilmenite sand as the initial titanium source. This novel synthesis method was cost-effective and straightforward due to the absence of the traditional gravity, magnetic, electrostatic separation, ball milling, and smelting processes. Synthesized TiO2 nanoparticles were 99% pure. Also, highly corrosive environmentally hazardous acid leachate generated during the leaching process of ilmenite sand was effectively converted into a highly efficient visible light active photocatalyst. The prepared photocatalyst system consists of anatase-TiO2/rutile-TiO2/Fe2O3 (TF-800), rutile-TiO2/Fe2TiO5 (TFTO-800), and anatase-TiO2/Fe3O4 (TF-450) nanocomposites, respectively. The pseudo-second-order adsorption rate of the TF-800 ternary nanocomposite was 0.126 g mg–1 min–1 in dark conditions, and a 0.044 min–1 visible light initial photodegradation rate was exhibited. The TFTO-800 binary nanocomposite adsorbed methylene blue (MB) following pseudo-second-order adsorption (0.224 g mg–1 min–1) in the dark, and the rate constant for photodegradation of MB in visible light was 0.006 min–1. The prepared TF-450 nanocomposite did not display excellent adsorptive and photocatalytic performances throughout the experiment period. The synthesized TF-800 and TFTO-800 were able to degrade 93.1 and 49.8% of a 100 mL, 10 ppm MB dye solution within 180 min, respectively.


INTRODUCTION
The first discovery of titanium was in 1791 by the British geologist William Gregor. 1 He examined a new element in FeTiO 3 found in black color sand from the Helford River in England. William Gregor produced an impure white metallic oxide from FeTiO 3 by treatment with hydrochloric (HCl) acid followed by washing with sulfuric (H 2 SO 4 ) acid. 2 After 119 years, pure TiO 2 was synthesized in 1910 by M.A. Hunter. 3 Globally, 50% of the titanium that is produced is used for the production of TiO 2 as white pigments, and 20 and 12% are used in paper and plastic industries, respectively. 4 The remaining 18% is used in food, medicines, toothpastes, rubber, floor coverings, and ceramics and as an additive in the cosmetics industry. 4 In recent decades, TiO 2 has been used as catalysts, dielectrics in capacitors, a functional part in some oxygen sensors, a component of surgical implants and prostheses (for bone/medical implant integration), and as a semiconductor. 2 Also, TiO 2 nanocomposites have been developed for use in solar cells and photocatalytic hydrogeneration from water. 5 TiO 2 and TiO 2 modified nanocomposites are used in advanced oxidation processes (AOPs) in water purification. 6,7 Currently, titanium dioxide is produced from titanium ores by traditional processes such as the sulfate and chloride processes. 8,9 Globally, the sulfate process is employed in 40% of total TiO 2 production and the chloride process in 60%. 8 In the sulfate process, low-grade ilmenite (40−60%, TiO 2 ) or titanium slag (72−85%, TiO 2 ) is digested in concentrated sulfuric acid at elevated temperature and pressure. After separating the iron sulfate, the titanium liquid employed by TiO 2 is called a sulfatable titanium slag. The titanium liquid is hydrolyzed to synthesize hydrated titanium dioxide. However, the sulfate process generates huge amounts of iron sulfate byproducts, which are considered a hazardous waste. In the chloride process, either high-grade rutile or titanium slag is mixed with chlorine gas at a temperature of around 900−1000 °C in a fluidized bed reactor in the presence of coke as a reducing agent to produce TiCl 4 vapor. The resulting TiCl 4 vapor is liquefied and converted to titanium dioxide. 10 The chloride process has advantages over the sulfate process, such as the production of high-purity TiO 2 , less environmental impact, and the generation of a limited amount of hazardous waste products. 11,12 Nevertheless, the requirement of highpurity TiO 2 ore for use in the chloride process was a significant hurdle due to the inability to produce pure TiO 2 by leaching ilmenite ore. To improve ilmenite sand purity, low-grade ilmenite ores were subjected to gravity and to magnetic and electrostatic separation processes. 9,10 In general, chloride and sulfate hydrometallurgical processes can be classified as (i) a smelting process followed by either sulfuric or hydrochloric acid leaching at elevated temperatures, (ii) reduction of ilmenite followed by acid leaching, (iii) diminution of the iron content in ilmenite by oxygen and ammonium chloride, (iv) oxidation of ilmenite followed by reduction of oxidized ilmenite and hydrochloric acid leaching, and (v) roasting and magnetic separation followed by hydrochloric acid leaching. 10, 13 These extractive metallurgical processes involve multiple steps of energy and high-temperature treatments over 1000°C, making them expensive. 14 It is essential to develop an alternative process to produce high-quality TiO 2 from low-grade ores by minimizing and environmentally managing hazardous waste products.
Titanium dioxide is the most frequently applied semiconductor photocatalyst in photoinduced degradation processes. Even though TiO 2 has the disadvantage of being activated by ultraviolet (UV) light rather than visible light, it has several advantages, including being chemically and biologically inert, photocatalytically stable, and relatively easy to produce and use. 15−18 The most common polymorphs of TiO 2 , rutile and anatase, exhibit band gaps (E g ) of 3.2 and 3.0 eV, respectively, and they correspond to the wavelengths of 387. 45 and 413.28 nm, respectively. 19 The UV range in which light wavelengths shorter than roughly 400 nm are considered accounts for about 4% of solar radiation. Therefore, many researches have been conducted in the past to improve the photocatalytic efficiency by enhancing the visible light sensitivity such as fabrication of semiconductor heterostructures, 20 metal doping, 21,22 nonmetallic doping, 23−25 surface hybridization, 26 noble metal co-deposition, 26 and coupling with other technologies. 19,27,28 Among these methods, TiO 2 has been doped with metals including Pt, 29 Fe, 30 Cu, 31 V, 32 Cr, 33 Ag, 34 etc., and nonmetals such as C, 35 N, 36−38 and S. 37,39 Furthermore, TiO 2 has been combined with other semiconductors such as Cu 2 O, 40 CdS, 41 and Bi 2 S 3 41 to improve visible light photocatalytic activity by enhancing charge separation. However, the construction of heterostructures is proven to be effective to improve photoinduced charge separation and photocatalytic performance. Similarly, TiO 2 has been combined with other metal oxides such as Fe 2 O 3 , 42 V 2 O 5 , 43 CeO 2 , 44 and CoTiO 3 , 45 yielding heterojunctions in between semiconductors. In this study, we report the synthesis of an anatase-TiO 2 /rutile-TiO 2 /α-Fe 2 O 3 heterojunction. The anatase and rutile binary heterostructure exhibited type II band alignment with the conduction band of rutile residing above that of anatase. The anatase/rutile phases allowed efficient charge separation through the photocatalyst with a minor influence on the hole recombination and strong effect on electron hole transferring. The excitation of α-Fe 2 O 3 resulted due to the irradiation of visible light and electron holes migrated through the heterostructure.
The present work's significant contribution is the novel approach to produce high-purity rutile from ilmenite sand and synthesize high-purity magnetite nanoparticles based on the photocatalyst through a very effective separation technique called facile synthesis. The novel facile synthesis approach is able to utilize natural ilmenite without generating environmentally hazardous waste substances. The first key advantage of the process implemented by this work is the use of commercially available ilmenite sand as a titanium source without additional gravity, magnetic, electrostatic separation, ball milling, and smelting process, making it a straightforward and cost-effective process. The second advantage of the proposed method is that it is nonhazardous to the environment unlike existing hydrometallurgical methods due to effectively converting the byproduct iron-based hydrochloric acid leachate to an efficient photocatalyst. The photocatalyst recovery process makes an extraordinary contribution to a greener environment by effective conversion and use of highly corrosive iron-based hydrochloric acid leachate. TiO 2 Nanoparticles. Dried titanium solid (3 g) residue was placed in a round-bottom flask, and 120 mL of 1 M NaOH solution was added. Afterward, 2 mL of 30% H 2 O 2 was added as an oxidizing reagent, and the slurry was condensed at 40°C for 1 h under reflux. The resulting mixture was centrifuged for 30 min at 6500 rpm, and the supernatant and precipitate were collected separately. The yellowish-green supernatant was called sodium titanate liquor, and it was divided into an equal volume of two portions. The first portion of sodium titanate liquor was treated hydrothermally at 180°C for 2 h. The resulting white dispersion was further diluted with acidified deionized water. The resultant white color dispersion was filtered using a Whatman cellulose nitrate membrane filter paper, and the sedimented residue was thoroughly washed with ultra-pure water and ethanol until the washings are free of hydroxide ions. The successful elimination of hydroxy ions was confirmed by measuring pH. The resulting powder was dried at 80°C and calcined at 800°C to obtain TiO 2 nanoparticles. This material is referred to as TiO 2 -A.

Preparation of Nanomaterials. 2.3.1. Synthesis of
Similarly, to understand the formation of a porous structure during the synthesis of TiO 2 , cetyl trimethyl ammonium bromide (CTAB) was introduced as the structure-directing agent. CTAB was gently dissolved in a 1:1 (v/v) mixture of ethanol and water. The conductivity of ethanol and water mixture was measured during the addition of CTAB. The addition of CTAB into the water−ethanol mixture took place until the conductivity of the solution became constant. The second portion of sodium titanate liquor was added dropwise into the CTAB solution and homogenized for 30 min. This mixture was treated hydrothermally at 180°C for 2 h. The residue was isolated in the same manner described above for the synthesis of TiO 2 without CTAB. This material is referred to as TiO 2 -B .
2.3.2. Synthesis of TiO 2 /Fe 2 TiO 5 Based Nanocomposites. Concentrated ammonium hydroxide was added dropwise to the leachate obtained by acid digestion of ilmenite under vigorous stirring until the pH of the reaction mixture reached 10. The resultant brown color slurry was aged for 12 h and washed thoroughly with ultra-pure water until the washings were free from chloride ions and the solution became neutral. Dried powder (TFTO) was calcined at 800°C. This prepared powder is referred to as TFTO-800.

Preparation of TiO 2 /Fe 3 O 4 and TiO 2 /Fe 2 O 3 Based Nanocomposites.
Oxygen gas was bubbled through the leachate that was obtained from the acid digestion of ilmenite. The obtained solution was transferred to a two-neck roundbottom flask, and nitrogen gas was bubbled into the reaction mixture for 1 h. Then, 0.0237 mol of FeSO 4 ·7H 2 O was added to the reaction mixture in a nitrogen atmosphere. After the complete dissolution of FeSO 4 ·7H 2 O, ammonium hydroxide was added dropwise under vigorous stirring. Nitrogen gas was bubbled continuously until the pH of the reaction mixture reached 10. The resultant black slurry was aged for 12 h and thoroughly washed with ultra-pure water until the washings were free from chloride ions and neutral. The dried black color powder was calcined at 450 and 800°C, separately. The uncalcined powder is referred to as TF, and samples calcined at 450 and 800°C are referred to as TF-450 and TF-800, respectively.

Material Characterization.
The crystalline structure of the synthesized samples was examined by an Advanced Bruker system X-ray diffraction (XRD) diffractometer. The X-rays were generated at 30 mA current and 40 kV voltage equipped with a Cu Kα anode. The diffractograms were generated by scanning in the 2θ range of 5−80°with a 2°/min scanning rate. The samples were analyzed with X'Pert High Score Plus (PANalytical). The average crystalline size of the nanoparticles was calculated by the Debye−Scherrer equation (eq 1).
where λ is the wavelength of Cu Kα (1.54060 nm), θ is the diffraction angle, and β is the full width at half-maximum in radians.
The chemical composition of the samples was analyzed by HORIBA Scientific XGT-5200 X-ray fluorescence (XRF). The morphology and particle size of the prepared nanoparticles were characterized by a Zeiss field emission scanning electron microscope (FE-SEM) and transmission electron microscope (TEM). A FEI Tecnai F20 Super-Twin equipped with an FEG source (200 kV) was used to examine the TEM morphology of the produced nanocomposites. An Oxford Instruments energy dispersive X-ray (EDX) was used to acquire EDX spectra. The particle size distribution and the average particle size were determined by the ImageJ software. The nitrogen sorption isotherms of the TiO 2 samples were obtained at 77 K using a Quantachrome Autosorb IQ Station 1 analyzer. The samples were dried under a vacuum for 12 h at 393 K before performing the analysis. The ultraviolet−visible diffuse reflectance (UV−vis DRS) of the prepared samples were measured using a Shimadzu 1800 UV/Visible spectrophotometer utilizing a precision Czerny−Turner optical system. The absorbance of MB samples was acquired by a Shimadzu UV-1990 double beam UV−visible spectrophotometer.

Photocatalytic Measurements.
The photocatalytic performance of the synthesized catalysts was evaluated under irradiation generated by a 50 W visible light LED source. In a typical photocatalytic experiment, 200 mg of the photocatalyst (TFTO-800, TF-450, and TF-800) was immersed in 100 mL (10 mg/L) of methylene blue dye solution (MB). Photocatalysts were shaken in the MB solution under dark conditions till adsorption−desorption equilibrium was reached. During the dark adsorption, aliquots were withdrawn at specific times to measure the absorbance readings. The mixture was then exposed to the LED light source, and aliquots were withdrawn at specific time intervals. The absorption spectra were obtained after necessary dilution using a Shimadzu UV-1900 UV−visible spectrophotometer, and maximum absorbance was recorded at 665 nm. Similar to this, the effects of sacrificial agents were investigated using the sacrificial agents IPA, EDTA, and PS. The concentration of the sacrificial agents was maintained at 8 mmol in a 100 mL solution of 10 mg/L MB. Using a Shimadzu UV-1900 UV−visible spectrophotometer, the absorption spectra were acquired following the appropriate dilution and maximum absorbance recorded in 665 nm.

Precipitation of the Rutile Nanoparticles and
Photocatalyst Composites. Ilmenite sand was directly digested in concentrated hydrochloric acid as expressed in eqs 2 and 3. 46 Dissolution of iron is prominent at low ratios of acid to ilmenite, while at high ratios, efficient dissolution of both iron and titanium was obtained. 47 Ilmenite sand was acid leached in five continuous cycles. After each leaching cycle, unreacted ilmenite sand, titanium residue, and acid leachate were separated by a solid−liquid separation technique. As shown in Figure 1a unleached ilmenite settled at the bottom of the funnel, while the leached titanium residue remained in between the aqueous acid leachate and the denser unleached ilmenite layer. Less titanium residue was produced after the first leaching cycle with a highly intense orange color leachate. The color of the leachate decreased with increasing number of the cycle and turned out to be more yellow by the end of the fifth cycle. This suggests that during the first few cycles, more iron was leached out with titanium, while during latter leaching cycles, predominantly titanium leached out with less iron. This is due to the fact that loosely bound iron tends to leach out more easily rather than breaking the stable lattice, which limits the dissolution of titanium.
The white-colored titanium residue solid layer that was obtained is a combination of the precipitated hydrolyzed TiO 2 in the form of TiO 2 ·nH 2 O and H 2 TiO 3 according to eqs 4 and 5. 46 Ilmenite sand consists of a variety of minerals. Hence, the leaching process of the ilmenite with hydrochloric acid was a complex heterogeneous reaction. The white titanium residue synthesized by the hydrochloric acid leaching process suggested the complete separation of pure titanium products by removing iron. However, the presence of quartz, anorthite, and aluminum phosphate in the titanium residue significantly affects the purity of synthetic rutile. It is well-known that titanium peroxide gel Ti(OH) 2 O 2 forms in a reaction of metallic Ti with H 2 O 2. 49 The titanium peroxide gel that is produced decomposes into an amorphous hydrate titania TiO 2 ·nH 2 O layer. 49 Similarly, to increase the purity of synthesized rutile in this study, the titanium residue was treated in a mild alkaline condition using NaOH with H 2 O 2 . The presence of H 2 O 2 is necessary for the formation of high-purity amorphous TiO 2 nanoparticles. As shown in eq 6, under an alkaline environment, the hydrated titania layer may react with NaOH to form NaHTiO 3. 50 The NaHTiO 3 that is formed is expected to be transformed into Na x Ti(O 2 ) y (OH) j via the condensation reactions shown in eqs 7−9. 46 The synthesized Na x Ti-(O 2 ) y (OH) j is transferred to H 2 Ti 3 O 7 when diluted in deionized water. 14  During the formation of Na x Ti(O 2 ) y (OH) j , silica and other impurities remained undissolved. 46 As shown in Figure 1b, Na x Ti(O 2 ) y (OH) j was a greenish-yellow solution named titanium solution (titanium-sol), and it was used to synthesize rutile nanoparticles. Titanium-sol was directly subjected to the hydrothermal treatment at 180°C for 2 h to synthesize amorphous titanium followed by calcination at 800°C to synthesize rutile nanoparticles (TiO 2 -A). TiO 2 -B was synthesized by adding titanium-sol into CTAB dropwise and hydrothermally treating the product followed by calcination at 800°C.
One of the key novel outcomes of this synthesis method is the ease in which valuable photocatalysts (TF-450, TF-800, and TFTO-800) were produced, which were synthesized effectively by utilizing environmentally hazardous iron-rich acid leachate. TF-450 composed of the anatase phase of TiO 2 and Fe 3 O 4 is magnetically separable. Also, TF-800 is not magnetically separable as it consists of a mixture of anatase and rutile phases of TiO 2 with α-Fe 2 O 3 . The acid leachate obtained after the acid digestion of ilmenite contains iron in ferrous (Fe 2+ ) and ferric (Fe 3+ ) forms in addition to the Ti 4+ . However, it is virtually impossible to determine the exact Fe 2+ and Fe 3+ ion concentration of acid leachate as Fe 2+ readily oxidizes to Fe 3+ in the presence of O 2 dissolved in the medium and that is present in the atmosphere as shown in eq 11. The molar ratio of Fe 2+ and Fe 3+ was very important in determining the magnetism of the synthesized magnetite nanoparticles. 51 Therefore, to completely oxidize Fe 2+ to Fe 3+ , oxygen gas was bubbled through the acid leachate, and the resulting solution contained Fe 3+ as FeCl 3 Figure 2 shows the detailed XRD analysis of the synthesized materials. As shown in Figure  2a, the XRD pattern of ilmenite sand matched with the ilmenite (ICDD no. 01-075-0519), hematite (01-073-0603), rutile (00-034-0180), and silica (01-085-0335) phases, suggesting these as the major phases with high crystallinity. The chemical composition of the ilmenite sand analyzed by XRF is tabulated in Table 1. When a sample is bombarded with X-rays, the material in the specimen emits additional X-rays. After that, a detector records the X-rays. The resulting spectrum characterizes the chemical components that make up the substance by oxide-analysis detection mode. XRF analysis revealed that ilmenite sand consisted of relatively high proportions of titanium, iron, silica, and vanadium content (48.87, 42.81, 3.94, and 1.21%, respectively) compared to the other impurities present in low quantities.

XRD and XRF Analysis.
As shown in Figure      (211), (220), and (002) crystalline planes of rutile. 20, 52 The diffraction peak at 68.87°corresponds to the diffractions of the (031) plane of synthetic quartz. 53 The XRD pattern of amorphous titanium is shown in Figure  2c where no clear diffractions peaks were observed. As shown in Figure 2d,e, upon calcination of amorphous titanium at 800°C , diffraction peaks corresponding to the crystalline rutile phase of titanium dioxide (ICDD DB card no. 01-076-1939) appeared in TiO 2 -A and TiO 2 -B, respectively. The average crystallite size calculated from Scherrer's equation using the (110) crystalline plane for both TiO 2 -A and TiO 2 -B was 100.9 and 131.0 nm, respectively. The crystallite size of TiO 2 synthesized in the presence of CTAB (TiO 2 -B) is higher than that of TiO 2 -A, which was synthesized without CTAB. In the case without the CTAB surfactant, the Ti(OH) 4 chains polymerize. Polymerized chains could interact with other chains by interchain hydrogen bonding, so aggregation of TiO 2 particles and TiO 2 clusters may be formed. 54 Also, the reduction of TiO 2 cluster aggregation occurs with the presence of the CTAB surfactant in critical micelle concentration. The hydrogen atoms of the polymeric Ti(OH) 4 interacted with (C 16 H 33 )(CH 3 ) 3 N + cationic hydrophilic groups of CTAB micelles. These hydrophilic groups are removed upon calcination, resulting in less aggregated TiO 2 particles and clusters. 53 (215) crystalline planes of TiO 2 (anatase), respectively. 60 Figure 2j shows the XRD pattern of the nanocomposite calcined at 800°C . The diffraction pattern matches with ICDD DB card nos. 01-079-1741, 01-088-1173, and 01-075-1537, indicating the presence of hematite, rutile, and anatase, respectively. 60 The diffraction peaks at 23  The process is called oxygen dissociation and can occur at temperatures starting from 700 to 1000°C, but the exact temperature varies due to composition and oxygen partial pressure. A quick hysteresis loop and back field demagnetization curve (to get coercivity and coercivity of remanence, respectively) should give a quick  (101) and (110) crystalline planes of anatase and rutile, respectively. 66 Table 2 summarizes the detailed analysis of Fe 2 TiO 5 , TiO 2 (rutile), Fe 3 O 4 , TiO 2 (anatase), and α-Fe 2 O 3 .

Raman Analysis.
To confirm the crystallographic structure suggested by XRD, Raman spectroscopic analysis of synthesized materials was conducted. Figure 2 shows the Raman spectra of TFTO-800, TF-450, TF-800, and TiO 2 -A materials respectively. As shown in Figure 3a Figure 3b, the Raman spectra of the FT-450 nanocomposite consist of peaks located at 318, 517, and 671 cm −1 . Those peaks are attributed to the E g , T 2g , and A 1g mode of Fe 3 O 4. 60 To verify whether there is a phase transformation from Fe 3 O 4 to γ-Fe 2 O 3 , the corresponding Raman spectra were intensively analyzed. These did not contain any apparent peak related to the γ-Fe 2 O 3 or α-Fe 2 O 3 . This result gives the evidence for the existence of Fe 3 O 4 . The Raman band at 154 cm −1 is very intense and sharp. Four characteristic Raman active modes of anatase TiO 2 with symmetries E g , B 1g , A 1g , and E g were observed at 154, 382, 481, and 628 cm −1 . 67−69 These characteristic vibration frequencies confirmed the phase pure anatase TiO 2 in the TF-450 nanocomposite. An additional broad and intense Raman scattering effect was observed at 1416 cm −1 due to the 2LO Raman scattered intensities. 20 These Raman data confirm the formation of the pure phase of anatase and Fe 3 O 4 in the TF-450 nanocomposite. Figure 3c shows the Raman spectra of the 800°C annealed TF-800 nanocomposite. As shown in Figure 3c, the sharp Raman vibration positions at 243 cm −1 were assigned to the A 1g and those at 290, 410, and 610 cm −1 were assigned to the E g vibration modes of α-Fe 2 O 3. 70,71 Similarly, the rutile TiO 2 exhibited characteristic stretching peaks at 140 and 596 cm −1 with relatively less intensity and correspond to the B 1g and A 1g vibration modes with respective intensity. 52 Raman vibration modes of anatase TiO 2 with symmetries E g , B 1g , A 1g , and E g were observed at 123, 375, 500, and 618 cm −1 wavenumbers with relatively less intensity. 52 These characteristic vibrational wavenumbers and their intensity ratios confirmed the presence of the α-Fe 2 O 3 , anatase TiO 2 , and rutile TiO 2 , respectively. Figure 3d shows the 800°C annealed TiO 2 -A sample, and Raman vibration positions were centered at 142, 445, and 611 cm −1 , respectively. These fundamental Raman vibrations are attributed to the B 1g , E g , and A 1g modes of rutile, accordingly. 72 A noticeable broad band at 235 cm −1 was observed in TiO 2 -A, which did not coincide with any theoretical calculations for the fundamental Raman modes of rutile. However, the broad peak suggests that it might be either second-order or disorder induced photon scattering effect.     rutile, corresponding to interlayer distances of 0.24 and 0.32 nm, respectively.
Bright-field TEM images of TF-800 showed aggregated nanoparticles with spherical and irregular shapes, as shown in  Consequently, different exposed crystalline planes provided diverse electrochemical and catalytic properties. The (014) crystal plane of α-Fe 2 O 3 nanocrystals have a higher surface energy and are more responsive to surface defects. As a result, photocatalytic activity might be affected by different exposed crystal surfaces. As shown in Figure 5i−m, images of EDX elemental maps exhibit the constituent elemental distribution of the TF-800 heterostructure. The elements Fe, Ti, and O are dispersed uniformly throughout the material. This result demonstrates the homogenous distribution of α-Fe 2 O 3 and TiO 2 phases through the surface of TF-800. 3.6. XPS Analysis. X-ray photoelectron spectroscopy (XPS) was employed to study the surface composition and valence state of the synthesized materials. Figure 6 shows the high-resolution spectra of Ti 2p, Fe 2p, O 1s, and C 1s and survey spectra. High-resolution spectra of Ti 2p of TiO 2 -A, TF-800, TF-450, and TFTO-800 are shown in Figure 6a,d. The high-resolution spectrum of Ti of TiO 2 -A (Figure 6a) shows three peaks. Peaks at 458.97, 464.66, and 472.19 eV corresponded to the 2p 3/2 , 2p 1/2 , and satellite peak of 2p 3/2 of Ti 4+ in the TiO 2 lattice, respectively. 20, 74 The satellite peak was due to a sudden change in Coulombic potential as the photo ejected electron passed through the valence band. 75 Figure 6b shows the high-resolution spectrum of Ti 2p of TF-800 with peaks at 458.48 and 464.15 eV corresponding to the 2p 3/2 and 2p 1/2 of Ti 4+ in the TiO 2 lattice, respectively. 20 Figure 6c shows the high-resolution spectrum of Ti 2p of TF-450. The four peaks appeared at 458.81, 459. 68, 464.65, and 472.2 eV, which corresponded to the 2p 3/2 , 2p 3/2 , 2p 1/2 , and satellite peak of 2p 3/2 of Ti 4+ in the TiO 2 lattice, respectively. 20 However, the 459.68 eV binding energy corresponded to the distortion of the Ti 4+ in the TiO 6 octahedral from TiO 2. 76 Figure 6d shows the high-resolution spectrum of Ti 2p of TFTO-800. The peaks centered at 464.49 and 458.79 eV were ascribed to Ti 2p 3/2 and Ti 2p 1/2 , respectively, revealing that Ti elements exist mainly in the form of the Ti 4+ oxidation state. 74 The high-resolution spectra of Fe 2p of the synthesized materials are shown in Figure 6e−h. Figure 6e shows the highresolution spectrum of Fe 2p of TiO 2 -A, and the entire spectrum consists of noises without any sharp peaks related to the Fe 2p 3/2 and Fe 2p 1/2 oxidized states, revealing the absence of Fe in TiO 2 -A. 53 Figure 6f shows the high-resolution spectrum of Fe 2p of TF-800. As shown in Figure 6f, the peaks of Fe 2p centered at 725.61 and 711.83 eV were attributed to 2p 1/2 and 2p 3/2 of Fe 3+ , respectively. 77,78 Also, the peaks at 734.30 and 719.86 eV correspond to the satellite peaks of Fe 2p 1/2 and Fe 2p 3/2 , indicating the oxidation state of Fe 3+ in the TF-800. 78 The Fe 2p XPS spectrum of the TF-450 material is shown in Figure 6g. The XPS Fe 2p core-level spectra of α-  Figure 6q, the TiO 2 -A nanomaterial consisted of Ti and O as the major constituents. However, the presence of Mn, Ca, and Si in minor concentrations in other photocatalysts was observed, while Fe and Ti were the major constituents of the photocatalyst material. The Ti/Fe ratios calculated from the area under the curves of TF-800, TF-450, and TFTO-800 were 1.25, 0.29, and 3.33, respectively. Also, the C/Ti ratio calculated for TiO 2 -A nanomaterial was 0.08.

Nitrogen Sorption Analysis.
To distinguish the effect of the CTAB surfactant for the formation of a porous structure, TiO 2 samples were subjected to Brunauer−  Table 3. Figure 7c−e. In Figure 7c, the Kubelka−Munk function was plotted against wavelength. The TiO 2 -A solid reveals an abrupt drop of absorption at about 400 nm, indicating an increase in radiation absorption below this wavelength. Also, it can be seen that TiO 2 exhibits a typical high absorption in the UV region and almost no absorption in the visible region. The adsorption edge for TiO 2 -A was approximately 425 nm. This event is associated with the electron transition from the valence band to the conduction band in the rutile structure. On the other hand, diffuse reflectance UV−vis data were used to calculate the band gap energy. The band gap energy was calculated from the plot of [F(R)hν] n vs hν (photon energy). The value of n used for the direct and indirect allowed transitions is 2 and 1 / 2 , respectively. The Kubelka−Munk function, F(R), is given by eq 18, and the band gap was determined using to the Tauc plot. 89 19) where α = absorption coefficient (a.u), S = dispersion factor, R = diffuse reflectance of the sample divided by the reflectance of the reference material (BaSO 4 ), h = Planck's constant (4.1357 × 10 −15 eV s), E g = band gap (eV), and A = proportional constant. According to Figure 7d,e, the calculated direct and indirect band gap energies for the synthesized TiO 2 -A are 2.91 and 2.88 eV, respectively. The band gap that resulted from the direct transition was chosen, and it was lesser than the theoretical band gap value reported for rutile (3.0 eV). 90 According to the XRF analysis, vanadium (V) is present at a weight percentage of 1.22%. Vanadium easily gets doped to the TiO 2 lattice as the ionic radius of vanadium is quite similar to that of titanium and thus lowers the band gap of rutile by creating an energy level below the conduction band of TiO 2. 56,91 However, according to the XRD, Raman, and XPS analysis of TiO 2 -A and TiO 2 -B, the crystalline parameters related to the standard rutile TiO 2 are precisely equivalent to those in the TiO 2 in TiO 2 -A and TiO 2 -B. These results suggested that V doped into the TiO 2 lattices and it may not deposit on the surface of TiO 2 as heterostructures.

Optical Adsorption Properties. The UV−vis DRS optical absorption properties of the prepared TiO 2 -A are shown in
Furthermore, Figure 7f shows the Kubelka−Munk function plotted against the wavelength of TF-800, TF-450, and TFTO-800. As shown in Figure 7f, TF-800 and TFTO-800 reveal an abrupt drop of absorption at the visible light region. However, TF-450 shows an adsorption edge far away from the TF-800 and TFTO-800 photocatalysts. The direct and indirect band gap responsible for the TF-800 and TFTO-800 is illustrated in Figure 7g,h, respectively. The indirect band gap for TF-800, TF-450, and TFTO-800 can be assigned as 2.01, 2.00, and 2.26 eV, respectively. In addition to that, the direct band gap for the direct transcription of TF-800, TF-450, and TFTO-800 can be assigned as 2.02, 2.04, and 2.28 eV, respectively.
3.9. Photocatalytic Degradation. The photocatalytic activities of the prepared TFTO-800, TF-450, and TF-800 samples were evaluated based on degradation of 10 mg/L methylene blue dye (MB) under visible light irradiation (Figure 8). The experiments were performed under the following conditions: C 0 = 10 mg/L, T = 25°C, and pH = 7. C t is the absorbance at different time intervals, and C 0 is the initial MB concentration.
The photodegradation of MB under visible light is shown in Figure 8a. Catalysts were shaken with the MB solution under dark conditions for 60 min until adsorption−desorption was reached as shown in Figure 8a.
TF-450 did not show a significant capacity to adsorb MB. However, TFTO-800 and TF-800 did adsorb MB, where a significant adsorption was observed during the first 15 min because, initially, the surface of all the catalysts was vacant. With time, the progress rate of adsorption decreased, reaching an adsorption−desorption equilibrium at 60 min. As adsorption sites became occupied, available vacant sites for MB molecules became limited, resulting in low adsorption. Moreover, though adsorption sites may be available, incoming molecules cannot reach those sites due to the high steric hindrance of the preadsorbed MB molecules. 92, 93 To study the adsorption kinetics, data obtained under dark conditions were fitted to pseudo-first-order and pseudo-second-order kinetics models. The best fitting model was selected based on the linear regression correlation coefficient (r 2 ) values. A linear form of the pseudo-first-order and pseudo-second-order adsorption models could be expressed by eqs 20 and 21. 94 The plot for the pseudo-first-order kinetics model for all three catalysts is shown in Figure 8b. The plot that represents the pseudosecond-order kinetic model of TF-450 is given in Figure 8c, and that of TFTO-800 and TF-800 is shown in Figure 8d. Corresponding kinetic parameters are summarized in Table 4. The reusability of the TF-800 photocatalyst was evaluated for 3 h to determine how effective the catalysts would be when used multiple times. The initial concentration and volume of MB were 10 mg/L and 100 mL, respectively. The efficiency of TF-800 in photodegrading MB under visible light dropped from 98.32% at the first cycle to 94.25% at the fifth cycle, as illustrated in Figure 8i. There was only a 4.07% decrease in conversion of MB from cycle 1 to cycle 5, which may have been caused by an accumulation of MB molecules in the porous structure that slows the movement of fresh MB molecules to the catalytically active sites. where q e = amount of adsorbed (adsorbate) at equilibrium per unit mass of adsorbent (mg/g), q t = amount of adsorbed (adsorbate) at time t per unit mass of adsorbent (mg/g), k 1 = pseudo-first-order adsorption rate constant (1/min), t = time (min), and k 2 = pseudo-second-order adsorption rate constant (g/min * mg).
The adsorption data of TFTO-800 and TF-800 fitted well with the pseudo-second-order model with a linear regression correlation coefficient of 0.99. Also, as shown in Table 4, the experimental equilibrium adsorption capacity (q e, exp ) of TFTO-800 and TF-800 is well aligned with the theoretical equilibrium adsorption capacities calculated from the pseudosecond-order model. Furthermore, the k 2 value of TFTO-800 was higher than the k 2 value of TF-800, indicating that the rate of adsorption of MB onto TFTO-800 is greater than TF-800, which could be due to the high surface area and pore volume.
The photocatalytic activity of the synthesized catalysts was evaluated based on the degradation of MB upon exposure to visible light. The photodegradation efficiency of the catalysts is shown in Figure 8e. The percentage decolorization was calculated by eq 22. The maximum percentage decolorization (88.6%) occurred when TF-800 was used as the photocatalyst followed by TFT0-800 (33.1%) and TF-450 (13.5%). The efficient photodegradation removal percentage of TF-800 due to the maximization of charge separation through the heterostructure explained in Section3.9. i k j j j j j y where A 0 = absorbance of MB at zero time and A = absorbance of MB at time t. Photocatalytic degradation kinetics of organic molecules usually follows the Langmuir−Hinshelwood kinetics mechanism, and the simplified equation can be expressed as in eq 23. 95 Table 5.
where y = (ln( ) and x = exposure time (t)/(min). Table 5 shows that the initial rate constant for the photodegradation of MB in the presence of TF-800 (0.044 min −1 ) was higher than that of TFT0-800 (0.006 min −1 ) and TF-450 (0.002 min −1 ). When the catalysts are illuminated by photons with energy higher than the band gap energy, photo-generated electrons are excited from the valence band (VB) to the conduction band (CB), leaving holes in the VB. When the surface of TFTO-800, TF-450, and TF-800 photocatalysts is illuminated by visible light, TiO 2 is not expected to show a significant contribution to the photocatalytic activity because the energy of the incoming photons is insufficient to promote an electron from the VB to the CB. However, in the presence of only visible light, TiO 2 merely serves as an electron transport layer. 97 The electron transport layer can selectively collect photogenerated charge carriers produced in the visible light sensing absorber material and limit the recombination of carriers at interfaces, thus ensuring a high photocatalytic efficiency. Synthesized nanocomposites consisted of a visible light active component The TF-450 nanocomposite exhibited the lowest photoactivity. As revealed by the XRD analysis, TF-450 consisted of approximately 65.7% of Fe 3 O 4 and 34.2% of TiO 2 . This decrease in photocatalytic activity could have occurred because of the photo-dissolution of iron oxide induced by photoactive TiO 2 due to electronic interactions at the phase junction. 101,102 As confirmed by the XRD analysis, Fe 3 O 4 has a spinel cubic structure that has a band gap of 0.14 eV. 98 The TFTO-800 binary nanocomposite consisted of 71.1% Fe 2 TiO 5 and 28.9% TiO 2 . Fe 2 TiO 5 facilitated the initial excitation of electrons from CB to VB in the TFTO-800 binary nanocomposite. As reported in our previous study, Fe 2 TiO 5 / TiO 2 shows a type I band alignment. 74 Therefore, as the CB of TiO 2 lies above the CB of Fe 2 TiO 5 , migration of photogenerated electrons from Fe 2 TiO 5 to TiO 2 would not take place. Hence, electron−hole pair recombination is possible at Fe 2 TiO 5 . Therefore, the photocatalytic activity of TFTO-800 is greater than TF-450 but less than TF-800. The initial rate constant for the photodegradation of MB in the presence of TF-800 was 6 times higher than TFTO-800 and 22 times faster than TF-450.
The TF-800 nanocomposite consisted of 56% α-Fe 2 O 3 , 26% anatase TiO 2 , and 17% rutile TiO 2 . The band gap energy of rutile and anatase polymorphs of TiO 2 is at 3.03 and 3.2 eV, respectively. The CB of anatase lies 0.17 eV above that of rutile TiO 2 . This type of band alignment could facilitate an excellent transfer of electrons from anatase to rutile, producing a higher photocatalytic activity. As shown in Figure 9a, upon expose to the visible light irradiation, excited electrons in α-Fe 2 O 3 transferred to the CB of anatase and followed by CB of rutile and the reduction of O 2 taken place at the CB of rutile. On the First-order deterioration is depicted in Figure S1. The linear first-order rate constant calculated for P-25 was 0.0001 min −1 .
The photocatalytic decomposition of MB in the presence of TF-800, TFTO-800, and TF-450 catalysts was 440, 60, and 20 times faster than that of P-25, respectively. There is a lack of research work to evaluate kinetic parameters of dye degradation by photocatalyst materials synthesized from ilmenite sand. However, the photocatalytic performance of the TF-800 nanocomposite for MB degradation was compared with the first-order degradation rate of other ilmenite sand-based photocatalyst materials (Table 6).
Based on the information available, TF-800 showed higher initial degradation rate compared to the other materials.
3.10. Determination of Band Alignment in Photocatalysts. The mechanism of charge separation and the band where X = absolute electronegativity of the semiconductor, which is defined as the geometric mean of the absolute electronegativity of the constituent atoms; E C = energy of free electrons on the hydrogen scale; E g = band gap of the semiconductor; E CB = conduction band (CB) position; and E VB = valance band (VB) position. The absolute electronegativity values for anatase-TiO 2 , rutile-TiO 2 , and α-Fe 2 O 3 are 5.81, 5.81, and 4.76 eV, respectively. 20 Also, the energy of free electrons on the hydrogen scale is 4.5 eV vs NHE. The parameters toward the calculation of VB and CB energy position have been tabulated in Table 7.
XPS data were further analyzed to provide additional evidence for the electron transfer mechanism of the synthesized TF-800 composite. The binding energy of Fe 2p 3/2 of pure α-Fe 2 O 3 is reported to be at 709.32 eV. 20 The binding energy of Fe 2p 3/2 in TF-800 appeared at 711.95 eV, suggesting a decrease in the electron concentration at α-Fe 2 O 3 of TF-800. 112 The degree of delocalization and/or electronic coupling between TiO 2 empty electronic states can be demonstrated by proving that α-Fe 2 O 3 impacts electronic states around the Fermi level. 113 As a result of the low electron density at iron-based species, photogenerated electrons are readily taken up by TiO 2 , resulting in the production of • O 2 − , which degrades MB as proposed by the mechanism. The binding energy of Ti 2p 3/2 in pure TiO 2 is reported to be at 459.36 eV, 20 while Ti 2p 3/2 of TF-800 appeared at 458.48 eV, indicating a 0.88 eV decrease in binding energy and suggesting that TiO 2 is enriched with electrons. Therefore, it is evident that there is a net electron flow from Fe 2 O 3 to TiO 2 in TF-800. As a result, O 2 •− radicals are formed at the CB band of TiO 2 , while • OH radicals are generated at the VB band of Fe 2 O 3. 3.11. Sacrificial Agent Study of TF-800. Radical scavenging tests were carried out to determine the primary active radical species for the degradation of MB in the presence of TF-800. In summary, EDTA, IPA, and PS were introduced to the degrading system to determine the effect of h + , • OH, and • S 2 O 8 − , respectively. 110 All the parameters were kept constant where 8 mmol of the above reagents was added just before switching on the light. Readings were collected for 2 h under illumination. Figure 10a shows the reduction of the MB concentration with time. The PS added system showed a rapid reduction in the MB concentration where the concentration reached zero in 30 min. Figure 10b exhibits the first-order kinetics, while the rate constants calculated are expressed in Figure 10c. No sacrificial agent or PS was used in the control experiment. Table 8 summarizes the rate constants and the R 2 values. Linear first-order degradation rate constants for the systems with MB and TF-800 only and for the systems with IPA, EDTA, and PS additionally were 0.023, 0.007, 0.008, and 0.079 min −1 , respectively. The PS system was 3.43 times faster than the system with MB and TF-800 only, which acted as the control.
As shown in Figure 10d, the concentration of MB after the degradation process in which EDTA, IPA, and PS were used was 2.82, 2.97, and 0.11 mg/L, respectively, and the calculated removal efficiency for EDTA, IPA, and PS was 72.67, 71.22, and 99.12%, respectively, while that in the control experiment was 93.02%. There was no difference in the photocatalytic degradation of MB when EDTA and IPA were introduced, indicating that the effect of adding both reagents is the same.
Several tests were carried out to investigate the active species involved in the MB degradation process of TF-800. Generally, IPA is considered as an excellent • OH scavenger, while EDTA is a h + scavenger. 111 IPA and EDTA substantially inhibited the ultimate breakdown efficiency of MB as shown in Figure 10 almost equally, suggesting that both • OH and h + are responsible for the degradation of MB. Further, • OH is generated by the action of h + . Therefore, when h + is scavenged automatically, the • OH concentration also declines, further reducing the MB degradation. PS accelerated the visible light driven photocatalysis by yielding SO 4 •− radicals according to eqs 29−31. Further, S 2 O 8 •− , the source of SO 4 •− , is also generated when OH − is present in the medium, further increasing the rate of the reaction.

CONCLUSIONS
Highly pure TiO 2 spheres were successfully synthesized using natural ilmenite sand as the raw material via a combination of acid digestion and hydrothermal synthesis pathways. Synthesized TiO 2 spheres passivated with CTAB possess a higher pore volume and contain more than 98.5% TiO 2 , with vanadium being the only impurity. The band gap that resulted from the direct transition of TiO 2 is 2.91 eV, which is less than that of rutile (3.0 eV) due to the presence of V, which has doped to TiO 2 , lowering the band gap. The corrosive and ecologically harmful acid leachate produced during the leaching of ilmenite sand was successfully converted into highly efficient visible light active photocatalysts, such as TFTO-800, TF-450, and TF-800. Among them, TF-800 nanocomposite was very efficient and showed the highest rate for the photodegradation of methylene blue (0.044 min −1 ), while rate constants of TFTO-800 and TF-450 are 0.006 and 0.002 min −1 , respectively. The TF-800 nanocomposites absorb visible light due to the presence of α-Fe 2 O 3 , and anatase-TiO 2 and rutile-TiO 2 band alignment promotes excellent charge separation throughout the heterojunction. The synthesized TF-800 ternary nanocomposite showed pseudosecond-order kinetics for the adsorption of MB in the dark with a rate of 0.126 g mg −1 min −1 , revealing the chemisorption

Notes
The authors declare no competing financial interest.

■ ACKNOWLEDGMENTS
Authors acknowledge the Sri Lanka Institute of Nanotechnology and Industrial Technology Institute for providing the instrument facilities.