Synthesis of TiO2 nanotubes from ilmenite with CuS nanoparticles as efficient visible-light photocatalyst

Titanium dioxide nanotube (TNT) is one of the most widely used photocatalysts. In this research, TNT was prepared by a facile method using ilmenite (FeTiO3) concentrate as the titanium source. For this purpose, iron was leached out from ilmenite using HCl in assistance with the iron powder as the reducing agent to produce pure TiO2, where consequently, TNT was produced through hydrothermal treatment of the prepared TiO2 in an alkaline solution. CuS quantum dots, using the l-cysteine as a linker, were coated on the TNT to improve TNTs’ photocatalytic properties. Characterization was done using XRD, SEM, FESEM, HRTEM, FT-IR, nitrogen sorption, and band gap measurement. The results revealed the formation of TNT with a star-shaped macrostructure as well as, a good dispersion of uniform CuS quantum dots with an average diameter of a few nanometers on the TiO2 structure. A dye adsorption kinetics study of the TNT and CuS-dopped TNT showed that TNT carries a higher adsorption capacity compared to the CuS-dopped TNT, developed due to its higher surface area and pore volume. Next, the photocatalytic performance (under visible light) of the prepared composite was studied over the methylene blue (MB) and malachite green (MG) dyes, after the determination of the dye adsorption equilibrium point (where the adsorption stops). TNT showed almost no dye degradation while the prepared composite degraded almost 95 % of the dyes as the result of the reduced band gap from 3.21 to 2.67 eV. In this study, for the first time, the TNT was prepared using a mineral source and ilmenite, enhanced in photocatalytic properties, and presented a successful application.

In the meantime, most researchers based their work on the use of high-purity chemicals to produce these nanostructures (Ferraz et al. 2013;Roy et al. 2011;Yang et al. 2010), which can make the possibility of industrial application of these materials as photocatalysts with high investment risk. Looking at the numerous deposits of titanium dioxide in nature such as ilmenite, it seems that developing a technique to use such minerals for the production of nanotubes can make tremendous changes in this industry. In this regard, several attempts have been made to produce titanium dioxide nanostructures from ilmenite, 1 3 among which there are few with reasonable results that can be mentioned: first of all, Sarvi et. al. removed the iron from the ilmenite structure using the HCl leaching process along with the use of surfactants as the precursor to produce titanium dioxide nanoparticles (Gharakhlou and Sarvi 2017). Small nanoparticles with large surface areas and mesoporous structures were detected in the synthesized nanoparticles. Lavasani et al. presented a solvothermal method in the presence of surfactants as a structure-directing agent to prepare mesoporous TiO 2 nanoparticles (Lavasani et al. 2019). In another study, Schaffie et al. used a complex decomposition of ilmenite followed by the leaching of different impurities to precipitate TiO 2 nanoparticles (Kordzadeh-Kermani et al. 2020). Pecharapa et al. introduced a hydrothermal method for the preparation of TiO 2 nanofibers from natural ilmenite (Simpraditpan et al. 2013). But our research indicated that no successful research is done to prepare TNT from ilmenite. Besides, application of quantum dots helps to reduce the band gap of TiO 2 and among different materials tested (Pan et al. 2021;Vu Nu et al. 2022), copper sulfide has been able to improve the photocatalytic efficiency of titanium dioxide nanostructures under visible light (Khaki et al. 2017;Ratanatawanate et al. 2011).
In this study, TNT was successfully synthesized from ilmenite and coated with copper sulfide dots to enhance its photocatalytic activity under visible light. For this purpose, initially, the iron, in the ilmenite structure, was removed using acidic leaching with the help of a reducing agent (iron particles). Then, titanium dioxide nanotubes were synthesized in one step using the hydrothermal technique. Finally, copper sulfide nanoparticles were coated on the TiO 2 nanotubes using a layer-by-layer process, and the degradation of two organic dyes (Ferraz et al. 2013;Guo et al. 2023;Khalil et al. 2019), methylene blue, and malachite green (MG), under visible light was characterized. We believe that this is the first successful report of the synthesis of TNT from an ilmenite source with enhanced pore size, surface area, and photocatalytic properties.

Chemicals
Ilmenite concentrate was provided by Kahnouj ilmenite mine located in southern Iran, hydrochloric acid (37%, Merck), iron powder (>98%, Merck), sodium hydroxide (>99%, Merck), l-cysteine (99%, Merck), Cu(II) acetate (>99%, Merck), hydrated sodium sulfide (99%, Merck), methylene blue (analytical reagent, Merck), and MG (analytical reagent, Merck) were used as received. Distilled water was used in all experiments. Figure 1 shows the proposed TiO 2 nanotube synthesis process. In summary, initially, the iron content of the ilmenite was removed; for this purpose, the HCl leaching technique was developed. Ilmenite powder was ground for 6 h in a ball mill containing 500 g of ilmenite concentrate, 350 g of water, and 5 kg of steel balls. Then, the ground ilmenite was filtered and dried at 80 °C for 8 h. Then, 20 g of ground ilmenite was mixed with 146 g of 20% (W/W) hydrochloric acid and heated under stirring. As soon as the initial bubbles were formed (at around 100 °C), 1.5 g iron powder was slowly added to the mixture and the mixing was continued for 6 h. Afterward, the mixture was centrifuged and the remaining solids were separated and washed with a 3% hydrochloric acid solution followed with distilled water several times.

Synthesis of TiO 2 nanotubes
For the synthesis of TNT, 2 g of solids prepared in the previous stage were mixed in 100 ml of a 10 M NaOH solution at room temperature for 10 min. Then, the resulting mixture was transferred into a Teflon-lined autoclave and placed in an oven, and heated at 110°C for 24 h. Then, the autoclave was cooled down to room temperature and the solid was separated using a centrifuge. Finally, the resulting solid was washed with a dilute solution of 0.1 M hydrochloric acid several times until the pH of the solution reaches 6, and then, the solid was washed finally using deionized water. The obtained mixture was centrifuged and the resulting solid was dried in an oven at 80°C, overnight.

Synthesis of CuS quantum dots onto TNT
The CuS quantum dots were applied to the TNTs using a modified method reported previously (Ratanatawanate et al. 2011). In summary, initially, 1 g of TNT was added to 0.3 mL of l-cysteine solution (0.3 M) and after adjusting the pH to 4 (using 0.1 M hydrochloric acid), the mixture was kept in the dark without stirring for one hour. Then, the solid was centrifuged, washed with deionized water to remove the unreacted l-cysteines, and dried at 50°C for 8 h. Then, the l-cysteine-modified TNT was mixed with 0.1 mL copper (II) acetate solution (0.1 M) for 15 min, washed with deionized water, centrifuged, and dried at 50°C for 8 h. In the final stage, the solid was mixed with 100 mL of sodium sulfide solution (0.3 M) to achieve the CuS quantum dots on the TNT. The solid from this stage was separated, dried at 50°C for 8 h, and named TNT-QD10.
Also, two other quantum-dotted samples were prepared, in the first one, the concentration of l-cysteine was set at 0.03 M, copper (II) acetate at 0.01 M, and sodium sulfide 0.03 M, and in the second one, the concentration of l-cysteine was set at 0.15 M, copper (II) acetate was set at 0.05 M, and sodium sulfide at 0.15 M, and samples named TNT-QD1 and TNT-QD5, respectively. Figure 2 shows the process of CuS quantum dot formation on the TiO 2 nanotubes.

Kinetics of adsorption and photocatalytic degradation of organic dyes onto CuS QDs@ TiO 2 nanotubes
Kinetics of the absorption of methylene blue and MG onto different adsorbents were performed in the dark. In detail, a concentration of 60 ppm of methylene blue and MG dye solution (100 mL) was prepared and 25 mg of adsorbent (TNT and TNT with quantum dots) were used for absorption. The concentration of the remaining dyes in the solution was measured at intervals of 5, 10,20,30,40,60,120,180,240,360,480,600, and 720 min using a spectrophotometer. After analyzing the equilibrium of adsorption, the photocatalytic degradation tests were done under visible light.
The tests were carried out in a 250 mm quartz container in an incubator at 25°C under visible light using a Philips sun simulator lamp with a cut-off filter for under 420 nm wavelength (a similar set of experiments of similar conditions were continued under dark for comparison). In an interval of 10,20,40,60,70,80,90,100,110,120, and 130 min, the concentration of the dye that remained in the solution was measured at the wavelength of 664 nm and 617 nm for methylene blue and MG, respectively.

Characterization
XRD patterns were obtained by Asenware-AW-DX300 with a copper lamp as the radiation source. For transmission electron microscopy (TEM), Philips EM208S 100KV was used. FEI TECNAI F20 at 200kv was used for HR-TEM images. The scanning electron microscopy (SEM) model PHILIPS XL30 (the energy-dispersive X-ray (EDX) was used the same instrument) and field emission scanning electron microscopy (FESEM) model QUANTA FEG 450 was used to analyze the morphology of the samples. The FT-IR spectrum was obtained using Bruker Tensor 27. A JASCO V_670 spectrophotometer was used to measure the band gap. The dye concentration was calculated using a spectrophotometer model UV-2100 using a standard curve.

Results and discussions
TNT synthesis from ilmenite FESEM and N 2 sorption results of samples in different stages of the synthesis process are detailed in Fig. 3. Spherical TiO 2 particles after acid leaching were obtained (Fig. 3a). The acid-leached samples were gone through a hydrothermal process to prepare TNT, and the morphology Fig. 2 The process of formation of CuS quantum dots onto TiO 2 nanotubes of TNT particles was mainly like star-shaped particles with intertwined nanotubes (Fig. 3b). CuS doping did not affect the morphology of the particles, and still, a similar morphology was detected ( Fig. 3c and d). Such morphology comes from the aggregation of tens of bundles of nanotubes during acid washing after a hydrothermal reaction and has not been reported previously. The advantages of such a structure are the hierarchical porousity (large pores between nanotube particles as well as mesopores of the nanotubes) and enhancement of the diffusion rate between the nanotubes as well as improvement of nanotube recovery in different applications such as adsorption and photocatalyst.
The TEM images of the TNT (Fig. 3e) indicated the formation of a tubular morphology of nanotubes after the hydrothermal reaction. The length of the nanotubes is about a few hundred nanometers and many of them are accumulated to form a nanoparticle. Besides, a clear formation of hollow nanotubes proves the formation of the nanotube (and not the fibers which were reported previously (Simpraditpan et al. 2013)) The H1 shape of the nitrogen sorption analysis is another proof of the formation of cylindrical nanotubes (Fig. 3f) and the pores were calculated around 5 to 10 nm showing a mesoporous structure of the TNT samples.
The leaching of Fe from ilmenite was tested using previously reported experiments; however, using just a simple leaching of ilmenite did not remove the Fe, and still, some remained after several leaching processes even in the form of ilmenite (Fig. 4). It is possible to continue the leaching process in order to minimize the existence of the Fe in the remained solids; however, it could make the process longer, with much higher consumption of acid and consequently economically impossible. Hence in a modified method, Fe particle was added to the leaching process and surprisingly almost all Fe was removed from the ilmenite structure in just one step process (Fig. 4).
A clear change has occurred after acid leaching of the ilmenite (Fig. 4, samples a to b and Figures S1 to S5), indicating the removal of FeO from the ilmenite (FeTiO 3 ) structure and the creation of a rutile structure after that. After the hydrothermal process, a phase change was made in titanium dioxide and it converted to anatase. There is also a new peak around 2θ of 10 degrees, which can be related to the formation of nanotubes (Cui et al. 2012b). Besides, the calcination temperature might destroy the TNT structure and a final anatase phase was formed at the calcination temperature of 900 °C (Fig. 4d).
For the rest of the experiments, the calcination temperature was set at 350 °C to avoid any damage to the TNT nanostructures. In Fig. 4d, there is no change in the material phase after doping with CuS, and titanium dioxide was still in the anatase phase with well nanotube formation (similar peaks as the TNT before CuS doping). Besides, the blue lines are the location of CuS-related peaks with the pattern number 06-0464 based on standard ICDD. In comparison to the location of the CuS pattern with Fig. 4d (CuS doped TiO 2 nanotubes), the existence of the CuS peaks in this pattern is obvious Fig. 5. Figure 6 compares the FTIR spectra of TNT at different stages of quantum dot synthesis on nanotubes. In the first step, (Fig. 6a, the TNT sample), the 3389 and 1641cm -1 peaks are related to the O-H bond, which is related to water molecules in the titanium dioxide nanotube sample. In the second step (covering the TNT with l-cysteine, (Fig. 6b)), the formation of a small peak at 2916 cm -1 can be seen, which can be related to the C-H bond, in the l-cysteine. Also, the peak at 1514 cm -1 is related to the N-H bond, related to the amino group in l-cysteine. The location of these peaks is preserved after the addition of copper acetate and sodium sulfide; Fig. 6c and d, respectively, show that l-cysteine remains on the surface during these stages. In the last stage, after the formation of quantum dots on the surface (Fig. 6d), the peak 1114 cm -1 sharpened, relating to the S=O bond, and shows that the sulfur present in the CuS quantum dots has been transferred into oxide form. Figure 7 shows HRTEM images of TNT and TNT covered with CuS quantum dots. In these images, the formation of nanotubes (Fig. 7a 1 , a 2 , and a 3 ), the walls of the tubes, and their elongation and extension can be observed. In higher magnifications, the layered structure of nanotubes can be seen, which indicates the lamination of the plane network in the synthesis process. Such a tubular layered structure, whose influence on the XRD results was previously confirmed, indicates the formation of titanium dioxide nanotubes. The elongation of titanium dioxide tubes extends to several tens of nanometers (a 2 ) and the size of the holes of these structures can be roughly estimated by several nanometers, but an accurate measurement of the size of the holes will be reported in detail using nitrogen sorption analysis.
In the case of TNT containing CuS quantum dots (Figs. 7b 1 , b 2 , and b 3 ), the stretching of nanotubes up to several tens of nanometers is visible. In these images, CuS quantum dots formed on the surface of the tubes can be observed with the size of approximately several nanometers. The balanced and scattered placement of these particles and their lack of coagulation are other interesting points in these images. These images are consistent with the previously reported results about the synthesis of such structures (Ratanatawanate et al. 2011). Figure 8 shows the EDX pattern of TiO 2 nanotubes and CuS-doped TNT. The main elements shown in the diagram for the TNT sample are titanium, oxygen, silicon, and iron. Titanium and oxygen, clearly relate to the titanium dioxide, and Fe and Si represent the existence of some minor impurities in the sample after the leaching process. After the CuS doping, a peak corresponding to the copper element was also clearly visible in the results, which is another proof of CuS particles on the TNT. Detailed results of the EDX analysis indicating the amount of Cu element (representing the CuS-dopped particles) on the TNT samples are provided in Fig. 8, and the amount of Cu on samples was as follows: TNT-QD-10 > TNT-QD-5 > TNT-QD-1.
To find out the details of the porous structure characteristics of the nanotubes, the nitrogen sorption test was performed. Nitrogen absorption-desorption diagrams and pore size distributions based on the BJH theory for TNT and TNT doped with different amounts of CuS are given in Fig. 9 and Table 1. The general shape of the nitrogen sorption graphs is H3-IUPAC type (Haul 1982), which shows a porous structure with two pore size ranges, one representing a few nanometers (increase in nitrogen absorption amount in the range of P/P0 less than 0.4) and the other is of inter-structural larger pores caused by the formation of the star-like shape (increasing the nitrogen absorption amount in the range of P/P 0 more than 0.9) (Mousavi Elyerdi et al. 2019). In the case of CuS doped samples, the decrease in the amount of specific surface area was seen by increasing the amount of CuS coating, also, the total volume of pores has decreased, which is due to the deposition of CuS on the surface of TNT and consequently, increase in the mass of the sample and decrease in the surface area and pore volume Pore size (nm) Fig. 9 Nitrogen sorption isotherms per gram. By increasing the amount of CuS coating, the diameter of the holes has not been affected, hence the pores are not plugged or reduced in pore sizes. The data in Table 1 indicate a high specific surface area for TNT and CuS-doped nanotubes.

Adsorption and photocatalytic degradation of dyes
The nanotubes can adsorb different pollutants, to accurately measure the photocatalysis performance of the samples initially the adsorption capacity of the nanotubes should be analyzed , and then, its photocatalytic performance should be checked. Hence, to design the photocatalytic experiments, initially, the kinetics of absorption of two types of dye on all adsorbents was tested in the dark (TNT, QD1-TNT, QD5-TNT, and QD10-TNT). The final equilibrium absorption point in the dark was set as the start of the photocatalytic process in the presence of visible light. Figure 10a and a′ show the kinetic of the absorption results of MB and MG dyes on different absorbents. With the increase in the quantum dots concentration on the surface of the sample, the amount of dye absorption has decreased, which can be due to the decrease in the porosity of the adsorbent as well as the increase in its specific weight due to the loading of CuS on it. The highest absorption capacity is related to the TNT sample with the absorption capacity of 136 mg of MB and 167 mg of MG per gram of absorbent and the lowest absorption amount was related to the QD10-TNT; the absorption capacity of 105 mg of methylene blue and 148 mg of MG per gram of the sample. Figure 10b and b′ show the interparticle diffusion model (Ramazani Afarani et al. 2018;Wu et al. 2009) fitted on the adsorption kinetic results. Both graphs consist of three regions, the first region which has the highest slope is related to the fast absorption of dye on the external surface of the absorbents, and the second region, which has a lower slope compared to that of the first region, is related to the diffusion of dye into the absorbent nanotubes. The third region, which has the lowest slope compared to the previous two stages, indicates the completion of dye absorption. Figure 10c and c′ show the results of the pseudo-second-order model fitted to the absorption kinetic data (Chahardahmasoumi et al. 2022;Ho and McKay 1999). The graphs show that this model is well fitted to the data and for both dyes, with the increase in the density of quantum dots, the slope of the graph increased showing that the absorption capacity decreases. Table 2 shows the calculated parameters of the kinetic models fitted to the results. The kinetics of adsorption results in the dark indicated that the adsorption of dyes onto samples reached equilibrium after 700 min. Hence, for the photocatalysis experiment, all samples were initially mixed with dye in the dark for 700 min, and after that, the photocatalysis experiments were started immediately in the presence of visible light. Figure 11 shows the results of dye degradation tests in the presence of visible light. Figure 11a is related to the degradation of methylene blue dye (MB). The highest degradation of MB is related to the QD10-TNT sample where a higher concentration of dye remained. This sample was able to degrade up to 76% of the dye that remained in the solution. Also, QD5-TNT, QD1-TNT, and An-TNT samples were able to degrade 59, 43, and 21% of this dye, respectively. Figure 10b shows the degradation of MG dye under visible light in the presence of the TiO 2 nanotubes which is an indication of higher performance of the photocatalysts (Vu Nu et al. 2022). In this graph, again, the highest degradation was for the QD10-TNT sample with 96% degradation of the remained MG under visible light. Also, QD5-TNT, QD1-TNT, and An-TNT samples were able to destroy 76, 54, and 16% of the dye, respectively.
The band gap of the samples was analyzed to justify the photodegradation of the samples (Fig. 12). The results showed that by applying CuS quantum dots on titanium dioxide nanotubes the band gap has reduced from 3.21 to 2.67 eV. This decrease in bandgap can increase the range of light absorption by titanium dioxide nanotubes that have CuS quantum dots. In short, the results of photocatalytic tests showed that adding quantum dots to TNT increases the photocatalytic performance and increases the dye degradation under visible light. The reason for improving and increasing this property by adding quantum dots to TNTs is due to the change in the bandgap of the photocatalyst, which increases the sensitivity of the photocatalyst to visible light and as a result, increases the photocatalytic reactions (Moradeeya et al. 2022).

Conclusions
For the first time, titanium dioxide nanotubes were successfully synthesized from ilmenite through the removal of Fe ion from the structure followed by a hydrothermal reaction in a strongly alkaline solution. The tubular structure of the TiO2 nanotubes with the star-shaped formation of nanotubes together was detected. Then the CuS quantum dots were applied to the nanotubes and the photocatalytic performance of the synthesized samples was analyzed. According to SEM, HR-TEM, XRD, FT-IR, and EDX results, the formation of titanium dioxide nanotubes and CuS quantum dots (spread uniformly on the TiO2 nanotubes with the size of less than 10 nm) on the surface of the nanotubes was confirmed. The combination of nanotubes formed star-shaped particles, with large surface areas. Although the deposition of quantum dots on the surface of titanium dioxide nanotubes reduces the specific surface area of these photocatalysts, the band gap of the samples is reduced dramatically. The degradation tests using CuS-dopped nanotubes as photocatalysts in the presence of visible light also showed that the addition of quantum dots to the surface of nanotubes increases the optical degradation of the color, which increases with the  and b′) and pseudo-second-order (c and c′) models fitted to the data increase in quantum dot content. Also, to check the optical properties, the bandgap of the produced nanocomposites was measured, which showed that the increase in the content of quantum dots decreased the bandgap from 3.2 to 2.67 eV, which increases the range of wavelengths that can be absorbed by the nanocomposite. This increase in absorption range widens the usable wavelength range for photocatalytic applications.
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Ethical approval Hereby, I "Mehdi Nasiri Sarvi" consciously assure that for the manuscript "Synthesis of TiO2 Nanotubes from Ilmenite with CuS nanoparticles as Efficient Visible-Light Photocatalyst," the following is fulfilled: This material is the authors' own original work, which has not been previously published elsewhere. The paper is not currently being considered for publication elsewhere. The paper reflects the authors' own research and analysis in a truthful and complete manner.  The paper properly credits the meaningful contributions of co-authors and co-researchers. The results are appropriately placed in the context of prior and existing research. All sources used are properly disclosed (correct citation). Literally copying of text must be indicated as such by using quotation marks and giving proper reference. All authors have been personally and actively involved in substantial work leading to the paper, and will take public responsibility for its content.
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