3.1. TNT Synthesis from ilmenite
FESEM and N2 sorption results of samples in different stages of the synthesis process are detailed in Fig. 3. Spherical TiO2 particles after acid leaching were obtained (Fig. 3a). The acid-leached samples were gone through a hydrothermal process to prepare TNT, and the morphology 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 3d). 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 porous structure (large pores between nanotube particles as well as mesopores of the nanotubes) enhances the diffusion rate between the nanotubes as well as improvement of nanotubes recovery in different applications such as adsorption and photocatalyst.
The TEM images of the TNT (Fig. 3e) clearly indicated the formation of a tubular morphology of nanotubes after the hydrothermal reaction. The length of the nanotubes are about a few hundred of nanometers and many of them are accumulated to form a nanoparticle. Besides, a clear formation of hollow nanotubes proves the formation of nanotube (and not the fibers which was 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 experiment, however, using just a simple leaching of ilmenite did not remove the Fe and still some was remained after several leaching process 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, sample a to b and Figure S1 to S5), indicating the removal of FeO from the ilmenite (FeTiO3) 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 experiments the calcination temperature was set at 350°C to avoid any damages 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 dopping). 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 TiO2 nanotubes), the existence of the CuS peaks in this pattern is obvious.
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 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; Figs. 6c and 6d respectively, show that L-cysteine still 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. 7a1, 7a2, and 7a3), the walls of the tubes, and their elongation and extension can clearly 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 (a2) 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 (Fig. 7b1, 7b2, and 7b3), the stretching of nanotubes up to several tens of nanometers is clearly 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 EDS pattern of TiO2 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 EDS 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.
In order 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 interstructural larger pores caused by the formation of the star-like shape (increasing the nitrogen absorption amount in the range of P/P0 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 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.
Table 1
Details of Nitrogen sorption results
Sample | BET surface area (m2 g− 1) | Pore volume (cm3(STP) g− 1) | Pore size (nm) |
TNT | 259 | 59.7 | 3 & 9.2 |
TNT-QD-1 | 241 | 50.1 | 2.7 & 7.1 |
TNT-QD-5 | 225 | 41.4 | 3.1 & 6.9 |
TNT-QD-10 | 207 | 32.1 | 3.5 & 7.9 |
3.2. Adsorption and photocatalytic degradation of dyes
The nanotubes are able to 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 10a' 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 10b' shows 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. Figures 10c and 10c' show the results of the pseudo-second order model fitted to the absorption kinetic data (Chahardahmasoumi et al. 2022, Ho &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 dark indicated that the adsorption of dyes onto samples reached equilibrium after 700 minutes. Hence for the photocatalysis experiment, all samples were initially mixed with dye in dark for 700 minutes and after that, the photocatalysis experiments were started immediately in presence of visible light.
Table 2
Details of different models fitted to the kinetics of adsorption results
sample | | | Pseudo-second-order | | Intraparticle diffusion |
| | k | R2 | | k1 | R12 | k2 | R22 | k3 | R32 |
TNT | MB | | 0.0075 | 0.99 | | 17.8 | 1 | 5.5 | 0.99 | 1.1 | 0.95 |
TNT-QD1 | 0.0081 | 0.99 | | 11.2 | 0.98 | 4.4 | 0.99 | 1.2 | 0.97 |
TNT-QD5 | 0.0094 | 0.99 | | 10.2 | 0.98 | 3.1 | 0.98 | 0.3 | 0.98 |
TNT-QD10 | 0.0098 | 0.99 | | 9.1 | 0.99 | 2.9 | 1 | 0.7 | 0.99 |
TNT | MG | | 0.0060 | 0.99 | | 18.5 | 1 | 7.3 | 0.98 | 0.8 | 0.96 |
TNT-QD1 | 0.0060 | 0.99 | | 16.8 | 1 | 6.5 | 0.98 | 0.1 | 0.97 |
TNT-QD5 | 0.0067 | 0.99 | | 16.2 | 1 | 6.2 | 0.98 | 1.0 | 0.95 |
TNT-QD10 | 0.0069 | 0.99 | | 15.1 | 1 | 5.2 | 0.97 | 1.0 | 0.98 |
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 TiO2 nanotubes. 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).