Design and Preparation of Polyimide/TiO2@MoS2 Nanofibers by Hydrothermal Synthesis and Their Photocatalytic Performance

Organic–inorganic nanocomposite fibers can avoid the agglomeration of single nanoparticles and reduce the cost (nanoparticles assembled on the surface of nanofibers), but also can produce new chemical, electrical, optical, and other properties, with a composite synergistic effect. Aromatic polyimide (PI) is a high-performance polymer with a rigid heterocyclic imide ring and an aromatic benzene ring in its macromolecular framework. Due to its excellent mechanical properties, thermal stability, and easy-to-adjust molecular structure, PI has been widely used in electronics, aerospace, automotive, and other industries related to many applications. Here, we report that TiO2 nanorods were grown on polyimide nanofibers by hydrothermal reaction, and MoS2 nanosheets were grown on TiO2 nanorods the same way. Based on theoretical analysis and experimental findings, the possible growth mechanism was determined in detail. Further experiments showed that MoS2 nanosheets were uniformly coated on the surface of TiO2 nanorods. The TiO2 nanorods have photocatalytic activity in the ultraviolet region, but the bandgap of organic/inorganic layered nanocomposites can redshift to visible light and improve their photocatalytic performance.


Introduction
With the increases in social development and people's living standards, environmental pollution has received more and more attention. Indoor and outdoor air, water, and soil pollution have seriously affected people's health and normal lives [1,2]. Using the photocatalytic properties of semiconductor materials, organic pollutants in water or air can be completely degraded into carbon dioxide and water. This method has been widely used for the treatment of wastewater and gas, and has a significant advantage over the electrocatalytic and wet catalytic techniques requiring high temperature for the decomposition of refractory toxic organic compounds [3][4][5]. When the catalyst has a nanostructure, it produces significant surface and size effects. Nanocatalysts have a huge surface area relative to the general size of the catalyst, which expands the surface of the atomic number. With the reduction in size, the pore size channel becomes very short with a large number of edges and steps, which increases the surface activity of the catalyst. However, it easily agglomerates, thereby affecting its dispersibility and utilization, and it is necessary to use carrier materials as a template for uniform dispersion. The electrospun fibers have a small fiber diameter, good flexibility, and ease of operation. Catalytic carriers can produce a strong synergistic effect in the catalyst, which increases the catalytic performance. In addition, some Pt, Au, and other precious metals are expensive as catalysts. The use of electrospun fiber as a template of catalyst can effectively overcome the above shortcomings [6]. Studies showed that the metal oxides made of nanoparticles have an increased quantum size effect, the energy levels of the conduction band and valence band are separate, and they can be

Preparation of Polyimide
In this experiment, polyimide nanofibers were prepared by a two-step method. Both 3.97 g ODA and 4.33 g PMDA were reacted in 35 mL of N,N-dimethylformamide (DMF) by polycondensation, with a solid content of 25%. The solution became highly viscous and we stopped stirring after 8 h. The solution of the precursor of polyamide acid (PAA) was obtained. The PAA nanofibers were prepared by electrospinning the PAA solution (15%) at 15 kV with 15 cm from needle to collector. After electrospinning (KD Scientific 100 and Tianjin Dongwen 30KVDC) the PAA solution, we obtained the electrospun polyamide acid nanofibers, then polyamide acid nanofibers proceeded with thermal imidation. Finally, polyimide nanofibers were prepared. Figure 1 illustrates the electrospinning process in the present work.

Preparation of Polyimide
In this experiment, polyimide nanofibers were prepared by a two-step method. Both 3.97 g ODA and 4.33 g PMDA were reacted in 35 mL of N,N-dimethylformamide (DMF) by polycondensation, with a solid content of 25%. The solution became highly viscous and we stopped stirring after 8 h. The solution of the precursor of polyamide acid (PAA) was obtained. The PAA nanofibers were prepared by electrospinning the PAA solution (15%) at 15 kV with 15 cm from needle to collector. After electrospinning (KD Scientific 100 and Tianjin Dongwen 30KVDC) the PAA solution, we obtained the electrospun polyamide acid nanofibers, then polyamide acid nanofibers proceeded with thermal imidation. Finally, polyimide nanofibers were prepared. Figure 1 illustrates the electrospinning process in the present work.

Synthesis of the PI/TiO2 Nanofibers
In the hydrothermal reaction autoclave, 33 mL water solution with 1 mol/L hydrochloric acid was added, then 0.04~0.05 g of titanium powders was added; finally, the polyimide fibers (0.1 g) were added. We then sealed it and set it aside for 3 h. The reaction kettle was put into the oven, heated to 160 °C ,and reacted for 16 h. After the reaction, we removed the product, washed it with deionized water, and dried it at 60 °C.

Synthesis of the PI/TiO2@MoS2 Nanofibers
A small amount of sodium molybdate and thioacetamide was added to the deionized water, and the solution was stirred and put into to the hydrothermal reaction autoclave. The PI/TiO2 fibers were then added into the autoclave reactor; the autoclave reactor was sealed and put it in the oven heated to 200 °C and left to react for 24 h. Then, the product of the reaction was washed with deionized water several times and put in the oven at 60 °C to dry.

Characterization
The morphologies of the as-obtained samples were observed using field-emission scanning electron microscopy (FE-SEM, Carl Zeiss Merlin Compact and Hitachi S4800). The crystal structure of the products was characterized by X-ray diffraction (XRD, Bruker D8 Advance diffractometer) using CuKa1 radiation (λ = 0.15406 nm). UV-Vis absorption spectra were obtained using a UV-Vis spectrophotometer (UV-3600, Shimadzu, Japan). FTIR spectra were recorded on a Varian 670-IR spectrometer. The UV lamp was from Shanghai Guanghao ZF-2 (365 nm). The UV-Vis diffuse reflectance spectra (DRS) were recorded using UV-V is spectrophotometer (V-650.Jasco). X-ray photoelectron spectroscopy (XPS) measurements were carried out with an ESCALAB 250 Xi photoelectron spectrometer using Al Ka radiation (ThermoFisher Scientific, Waltham, MA, USA).

Synthesis of the PI/TiO 2 Nanofibers
In the hydrothermal reaction autoclave, 33 mL water solution with 1 mol/L hydrochloric acid was added, then 0.04~0.05 g of titanium powders was added; finally, the polyimide fibers (0.1 g) were added. We then sealed it and set it aside for 3 h. The reaction kettle was put into the oven, heated to 160 • C, and reacted for 16 h. After the reaction, we removed the product, washed it with deionized water, and dried it at 60 • C.

Synthesis of the PI/TiO 2 @MoS 2 Nanofibers
A small amount of sodium molybdate and thioacetamide was added to the deionized water, and the solution was stirred and put into to the hydrothermal reaction autoclave. The PI/TiO 2 fibers were then added into the autoclave reactor; the autoclave reactor was sealed and put it in the oven heated to 200 • C and left to react for 24 h. Then, the product of the reaction was washed with deionized water several times and put in the oven at 60 • C to dry.

Characterization
The morphologies of the as-obtained samples were observed using field-emission scanning electron microscopy (FE-SEM, Carl Zeiss Merlin Compact and Hitachi S4800). The crystal structure of the products was characterized by X-ray diffraction (XRD, Bruker D8 Advance diffractometer) using CuKa1 radiation (λ = 0.15406 nm). UV-Vis absorption spectra were obtained using a UV-Vis spectrophotometer (UV-3600, Shimadzu, Japan). FTIR spectra were recorded on a Varian 670-IR spectrometer. The UV lamp was from Shanghai Guanghao ZF-2 (365 nm). The UV-Vis diffuse reflectance spectra (DRS) were recorded using UV-V is spectrophotometer (V-650.Jasco). X-ray photoelectron spectroscopy (XPS) measurements were carried out with an ESCALAB 250 Xi photoelectron spectrometer using Al Ka radiation (ThermoFisher Scientific, Waltham, MA, USA).

Characterization of the As-Prepared Photocatalysts
The morphology and nanostructure of the PI nanofibers were characterized by fieldemission scanning electron microscopy (FESEM) observations ( Figure 2). The surface morphology of the rod-like TiO 2 with a length ranging from 800 to 1000 nm and a width ranging from 50 to 200 nm could be clearly observed, as shown in Figure 3a,b. From the figure, we can see that the TiO 2 nanorods were very dense in the distribution of polyimide fibers. Figure 3c,d show that MoS 2 nanospheres were grown in situ on the surface of the TiO 2 nanorods. We can see that the MoS 2 nanospheres were evenly distributed on the surface of TiO 2 nanorods, which notably increased the specific surface area. The morphology and nanostructure of the PI nanofibers were characterized by fieldemission scanning electron microscopy (FESEM) observations ( Figure 2). The surface morphology of the rod-like TiO2 with a length ranging from 800 to 1000 nm and a width ranging from 50 to 200 nm could be clearly observed, as shown in Figure 3a,b. From the figure, we can see that the TiO2 nanorods were very dense in the distribution of polyimide fibers. Figure 3c,d show that MoS2 nanospheres were grown in situ on the surface of the TiO2 nanorods. We can see that the MoS2 nanospheres were evenly distributed on the surface of TiO2 nanorods, which notably increased the specific surface area.    The morphology and nanostructure of the PI nanofibers were characterized by fieldemission scanning electron microscopy (FESEM) observations ( Figure 2). The surface morphology of the rod-like TiO2 with a length ranging from 800 to 1000 nm and a width ranging from 50 to 200 nm could be clearly observed, as shown in Figure 3a,b. From the figure, we can see that the TiO2 nanorods were very dense in the distribution of polyimide fibers. Figure 3c,d show that MoS2 nanospheres were grown in situ on the surface of the TiO2 nanorods. We can see that the MoS2 nanospheres were evenly distributed on the surface of TiO2 nanorods, which notably increased the specific surface area.    The PI/TiO 2 @MoS 2 nanocomposite was characterized by powder X-ray diffraction (XRD), as shown in Figure   To further confirm the surface layer was MoS2, energy dispersive X-ray spectrometry (EDS) mapping ( Figure 5) analysis of the PI/TiO2@ MoS2 nanocomposites was conducted. The EDS pattern of the TiO2@MoS2 nanorods heterostructures showed that the product nanorods were composed of Ti, O, Mo, and S. The EDS elemental mapping supports our argument that the outer layer was MoS2 nanosheets and the inner layer was TiO2 nanorods. The above experimental results proved that only a few layers of MoS2 were wrapped on the surface of the ultrafine TiO2 nanobelts to form TiO2@MoS2 nanorod heterostructures. The chemical composition and valence state were characterized by X-ray photoelectron spectroscopy (XPS). The full-range XPS spectra of PI/TiO2@MoS2 (0-1050 eV) are shown in Figure 6a. Figure 6b shows that the binding energies (BE) of Ti 2p3/2 and Ti 2p1/2 were 459.3 and 464.7 eV, respectively, which we ascribed to the Ti 4+ oxidation state. In Figure 6c, O1s is shown, and it is useful for identifying the core levels. In Figure 6d, the high-resolution XPS spectra show that the binding energy of Mo 3d3/2 and Mo 3d5/2 peaks in the TiO2@MoS2 heterostructures located at 229.5 and 232.8 eV, respectively, indicating that the Mo element was present in the Mo 4+ chemical state. Figure 6e shows that the binding energies of S 2p3/2 and S 2p1/2 were 162.3 and 163.8 eV, respectively. By comparison to the NIST X-ray Photoelectron Spectroscopy Database, we identified the S element corresponding to producing the material for MoS2.
PAA was analyzed in our previous report [34]. The FTIR of PI is shown without the -OH peak in Figure 6f  To further confirm the surface layer was MoS 2 , energy dispersive X-ray spectrometry (EDS) mapping ( Figure 5) analysis of the PI/TiO 2 @ MoS 2 nanocomposites was conducted. The EDS pattern of the TiO 2 @MoS 2 nanorods heterostructures showed that the product nanorods were composed of Ti, O, Mo, and S. The EDS elemental mapping supports our argument that the outer layer was MoS 2 nanosheets and the inner layer was TiO 2 nanorods. The above experimental results proved that only a few layers of MoS 2 were wrapped on the surface of the ultrafine TiO 2 nanobelts to form TiO 2 @MoS 2 nanorod heterostructures.  To further confirm the surface layer was MoS2, energy dispersive X-ray spectrometry (EDS) mapping ( Figure 5) analysis of the PI/TiO2@ MoS2 nanocomposites was conducted. The EDS pattern of the TiO2@MoS2 nanorods heterostructures showed that the product nanorods were composed of Ti, O, Mo, and S. The EDS elemental mapping supports our argument that the outer layer was MoS2 nanosheets and the inner layer was TiO2 nanorods. The above experimental results proved that only a few layers of MoS2 were wrapped on the surface of the ultrafine TiO2 nanobelts to form TiO2@MoS2 nanorod heterostructures. The chemical composition and valence state were characterized by X-ray photoelectron spectroscopy (XPS). The full-range XPS spectra of PI/TiO2@MoS2 (0-1050 eV) are shown in Figure 6a. Figure 6b shows that the binding energies (BE) of Ti 2p3/2 and Ti 2p1/2 were 459.3 and 464.7 eV, respectively, which we ascribed to the Ti 4+ oxidation state. In Figure 6c, O1s is shown, and it is useful for identifying the core levels. In Figure 6d, the high-resolution XPS spectra show that the binding energy of Mo 3d3/2 and Mo 3d5/2 peaks in the TiO2@MoS2 heterostructures located at 229.5 and 232.8 eV, respectively, indicating that the Mo element was present in the Mo 4+ chemical state. Figure 6e shows that the binding energies of S 2p3/2 and S 2p1/2 were 162.3 and 163.8 eV, respectively. By comparison to the NIST X-ray Photoelectron Spectroscopy Database, we identified the S element corresponding to producing the material for MoS2.
PAA was analyzed in our previous report [34]. The FTIR of PI is shown without the -OH peak in Figure 6f. The PI/TiO2 nanorods showed bands around 3426 and 1648 cm −1 , corresponding to the stretching and bending vibrations of hydroxyl groups on the surface of the TiO2 nanorod surface, respectively (Figure 6f). The strong absorption band between 800 and 400 cm −1 was attributed to the Ti-O and Ti-O-Ti vibrations. The strong characteristic absorption peaks at 3426, 1720, 1648, and 1498 cm −1 in Figure 6f also indicated the presence of amide groups, while the absorption peak at 1648 cm −1 represented the tertiary The chemical composition and valence state were characterized by X-ray photoelectron spectroscopy (XPS). The full-range XPS spectra of PI/TiO 2 @MoS 2 (0-1050 eV) are shown in Figure 6a. Figure 6b shows that the binding energies (BE) of Ti 2p 3/2 and Ti 2p 1/2 were 459.3 and 464.7 eV, respectively, which we ascribed to the Ti 4+ oxidation state. In Figure 6c, O1s is shown, and it is useful for identifying the core levels. In Figure 6d, the high-resolution XPS spectra show that the binding energy of Mo 3d 3/2 and Mo 3d 5/2 peaks in the TiO 2 @MoS 2 heterostructures located at 229.5 and 232.8 eV, respectively, indicating that the Mo element was present in the Mo 4+ chemical state. Figure 6e shows that the binding energies of S 2p 3/2 and S 2p 1/2 were 162.3 and 163.8 eV, respectively. By comparison to the NIST X-ray Photoelectron Spectroscopy Database, we identified the S element corresponding to producing the material for MoS 2 .
imide ring, respectively, which create the characteristic peaks of polyimide groups. The bands near 1648 and 1117 cm −1 were also assigned to the Ti-O and Ti-O-C stretching modes, respectively. The FTIR results indicated that the TiO2 nanoparticles were successfully coated on the polyimide matrix. In MoS2, FTIR peaks exist in the range from 1008 to 1648, 2925, and 3426 cm −1 . The strong O-H peak and water bonding are indicated by 3426 and 603 cm −1 , respectively. The peaks situated at 1008 and 1243 cm −1 occurred due to the formation of complex sulfur with the active sites in MoS2. To evaluate the effects of MoS2 nanoparticles onto the TiO2 nanorod support on the optical properties of PI/TiO2@MoS2 nanocomposite, UV-Vis diffuse absorbance spectra (DRS) analysis was performed. Figure 7a shows the corresponding UV-Vis DRS spectra for the PI/TiO2@MoS2 samples. The absorption thresholds for the samples were obtained from the UV-Vis DRS curves by extrapolating the tangent lies of the spectra. In general, bare TiO2 nanoparticles show absorption in the UV region without any absorption in the visible range owing to its wide band gap (~3.2 eV). As shown in Figure 7a, the threshold wavelength for the synthesized PI/TiO2@MoS2 sample is about 450 nm. Compared with pure TiO2, a red shift in the absorption edges toward the visible region was observed in PAA was analyzed in our previous report [34]. The FTIR of PI is shown without the -OH peak in Figure 6f. The PI/TiO 2 nanorods showed bands around 3426 and 1648 cm −1 , corresponding to the stretching and bending vibrations of hydroxyl groups on the surface of the TiO 2 nanorod surface, respectively (Figure 6f). The strong absorption band between 800 and 400 cm −1 was attributed to the Ti-O and Ti-O-Ti vibrations. The strong characteristic absorption peaks at 3426, 1720, 1648, and 1498 cm −1 in Figure 6f also indicated the presence of amide groups, while the absorption peak at 1648 cm −1 represented the tertiary amide, which indicated that imidization was completed. The peaks at 1405 and 725 cm −1 corresponded to the asymmetric stretching vibration of C-N and the deformation of the imide ring, respectively, which create the characteristic peaks of polyimide groups. The bands near 1648 and 1117 cm −1 were also assigned to the Ti-O and Ti-O-C stretching modes, respectively. The FTIR results indicated that the TiO 2 nanoparticles were successfully coated on the polyimide matrix. In MoS 2 , FTIR peaks exist in the range from 1008 to 1648, 2925, and 3426 cm −1 . The strong O-H peak and water bonding are indicated by 3426 and 603 cm −1 , respectively. The peaks situated at 1008 and 1243 cm −1 occurred due to the formation of complex sulfur with the active sites in MoS 2 .
To evaluate the effects of MoS 2 nanoparticles onto the TiO 2 nanorod support on the optical properties of PI/TiO 2 @MoS 2 nanocomposite, UV-Vis diffuse absorbance spectra (DRS) analysis was performed. Figure 7a shows the corresponding UV-Vis DRS spectra for the PI/TiO 2 @MoS 2 samples. The absorption thresholds for the samples were obtained from the UV-Vis DRS curves by extrapolating the tangent lies of the spectra. In general, bare TiO 2 nanoparticles show absorption in the UV region without any absorption in the visible range owing to its wide band gap (~3.2 eV). As shown in Figure 7a, the threshold wavelength for the synthesized PI/TiO 2 @MoS 2 sample is about 450 nm. Compared with pure TiO 2 , a red shift in the absorption edges toward the visible region was observed in the nanocomposite. This was induced by the strong optical absorption of black MoS 2 in the visible-light region, which illustrated that there were interactions between TiO 2 and MoS 2 on the interface. The band gap energies (Eg) of the samples were calculated using the equation (Ahν) 2 = K(hν−Eg), where hν is the energy of a photon (eV), A is the absorption coefficient, K is a constant, and Eg is the band gap. The band gap was calculated by extrapolating the linear part of the spectra in a diagram of (Ahν) 2 versus the photon energy (Figure 7b). The band gap energy value for the PI/TiO 2 @MoS 2 nanocomposite was calculated as 2.7 eV, implying that MoS 2 nanospheres on a TiO 2 nanorod support decreased the optical band gap energy. Therefore, the electron-hole separation was relatively better in the PI/TiO 2 @MoS 2 nanocomposite.
R PEER REVIEW 7 of 11 the nanocomposite. This was induced by the strong optical absorption of black MoS2 in the visible-light region, which illustrated that there were interactions between TiO2 and MoS2 on the interface. The band gap energies (Eg) of the samples were calculated using the equation (Ahν) 2 = K(hν-Eg), where hν is the energy of a photon (eV), A is the absorption coefficient, K is a constant, and Eg is the band gap. The band gap was calculated by extrapolating the linear part of the spectra in a diagram of (Ahν) 2 versus the photon energy (Figure 7b). The band gap energy value for the PI/TiO2@MoS2 nanocomposite was calculated as 2.7 eV, implying that MoS2 nanospheres on a TiO2 nanorod support decreased the optical band gap energy. Therefore, the electron-hole separation was relatively better in the PI/TiO2@MoS2 nanocomposite.

Photocatalytic Activity the As-Prepared Photocatalysts
The photocatalytic performance of PI/TiO2@MoS2 was evaluated for the photodegradation of methylene blue (MB) at room temperature. The decay of the characteristic absorption peak of MB at 663 nm was followed every 30 min by UV-Vis spectrophotometry. Before UV exposure, the solution was kept in the dark for 30 min to build the adsorption/desorption equilibrium between the dye and surface. We found a gradual decrease in the main absorption peak, because of the adsorption of MB molecules on the surface of the sample. Usually, it is difficult to degrade pure MB and PI nanofiber in UV [35,36]. Figure 8 (left) shows the UV-Vis spectra of MB (5 mg/L) after ultraviolet light (λ = 365 nm) irradiation in the presence of PI/TiO2@MoS2 (0.02 g). With increasing irradiation time, the intensity of the characteristic absorption band of MB at 663 nm markedly reduced. In the meantime, the color of the solution changed, turning from blue to colorless after 180 min with irradiation, thus indicating the gradual decomposition of MB molecules during ultraviolet-light irradiation. The degradation efficiency is reported as C/C0, where C is the absorption of the main peak at 663 nm of MB at time t, and C0 corresponds to the initial concentration (after achievement of adsorption/desorption equilibrium (30 min)). As shown in Figure 8 (right), we observed that the dye degradation rate of the prepared samples varied with the same irradiation rate of UV at the same time. The results showed that PI/TiO2@MoS2 composites showed enhanced photocatalytic activity compared with

Photocatalytic Activity the As-Prepared Photocatalysts
The photocatalytic performance of PI/TiO 2 @MoS 2 was evaluated for the photodegradation of methylene blue (MB) at room temperature. The decay of the characteristic absorption peak of MB at 663 nm was followed every 30 min by UV-Vis spectrophotometry. Before UV exposure, the solution was kept in the dark for 30 min to build the adsorption/desorption equilibrium between the dye and surface. We found a gradual decrease in the main absorption peak, because of the adsorption of MB molecules on the surface of the sample. Usually, it is difficult to degrade pure MB and PI nanofiber in UV [35,36]. Figure 8 (left) shows the UV-Vis spectra of MB (5 mg/L) after ultraviolet light (λ = 365 nm) irradiation in the presence of PI/TiO 2 @MoS 2 (0.02 g). With increasing irradiation time, the intensity of the characteristic absorption band of MB at 663 nm markedly reduced. In the meantime, the color of the solution changed, turning from blue to colorless after 180 min with irradiation, thus indicating the gradual decomposition of MB molecules during ultraviolet-light irradiation. The degradation efficiency is reported as C/C 0 , where C is the absorption of the main peak at 663 nm of MB at time t, and C 0 corresponds to the initial concentration (after achievement of adsorption/desorption equilibrium (30 min)). As shown in Figure 8 (right), we observed that the dye degradation rate of the prepared samples varied with the same irradiation rate of UV at the same time. The results showed that PI/TiO 2 @MoS 2 composites showed enhanced photocatalytic activity compared with PI/TiO 2 for the degradation of methylene blue under UV irradiation. The PI/TiO 2 @MoS 2 composites decomposed about 95% methylene blue within 180 min under UV irradiation. Compared with a single PI/TiO 2 composite fiber or PAN nanofibers with MoS 2 -TiO 2 surface-loaded by vacuum filtration [37], a certain amount of molybdenum dioxide increased the efficiency of photodegradation. It may promote the light absorption efficiency of TiO 2 particles that decreases the electron-hole recombination and enhances the photogenerated charge separation.  Figure 9 shows a schematic diagram of the formation mechanism of TiO2 nanorods by the "dissolve and grow" progress, which we describe using the following chemical reaction:

Possible Mechanism of the Experiment
2Ti + 6HCl → 2TiCl 3 + 3H 2 (g) At the beginning, Ti powders react with H + at high temperature and gradually dissolve in the solution of HCL, continuously releasing the Ti(III) precursors into the reaction solution. Due to the instability of Ti(III) in aqueous solution, Ti(III) is hydrolyzed to TiOH 2+ . According to Fujihara et al. [38], TiOH 2+ is oxidized to Ti(IV) by reacting with dissolved oxygen. Therefore, the formation mechanism of rutile TiO2 nanorods using Ti(IV) complex ions as growth units can be described as follows: For rutile TiO2, TiO6 octahedra are first formed by bonding Ti atoms and six oxygen atoms. Then, the TiO2 octahedron shares a couple of opposite edges with the next octahedron, forming a catenarian structure. The growth of rutile nanorods follows the sequence (110) < (100) < (101) < (001), because the growth rate of different crystal planes depends on the number of coordinated polygon body corners and edges. Therefore, rutile TiO2 nanorods grown in the [001] direction were formed [39,40].  Figure 9 shows a schematic diagram of the formation mechanism of TiO 2 nanorods by the "dissolve and grow" progress, which we describe using the following chemical reaction:  Figure 9 shows a schematic diagram of the formation mechanism of by the "dissolve and grow" progress, which we describe using the follo reaction:

Possible Mechanism of the Experiment
2Ti + 6HCl → 2TiCl 3 + 3H 2 (g) At the beginning, Ti powders react with H + at high temperature and solve in the solution of HCL, continuously releasing the Ti(III) precursors in solution. Due to the instability of Ti(III) in aqueous solution, Ti(III) is TiOH 2+ . According to Fujihara et al. [38], TiOH 2+ is oxidized to Ti(IV) by dissolved oxygen. Therefore, the formation mechanism of rutile TiO2 n Ti(IV) complex ions as growth units can be described as follows: For rutile tahedra are first formed by bonding Ti atoms and six oxygen atoms. Then hedron shares a couple of opposite edges with the next octahedron, formi structure. The growth of rutile nanorods follows the sequence (110) < (100) because the growth rate of different crystal planes depends on the number polygon body corners and edges. Therefore, rutile TiO2 nanorods grown rection were formed [39,40]. According to the previous experimental results, a promotional mecha in Figure 10. Under UV-light irradiation, TiO2 absorbs photons and create pairs. Because the conduction band (CB) of TiO2 is higher than that of M photoelectrons generated by CB of TiO2 are easily transferred to MoS2, w the separation of photogenerated electron-hole pairs and enhances the At the beginning, Ti powders react with H + at high temperature and gradually dissolve in the solution of HCL, continuously releasing the Ti(III) precursors into the reaction solution. Due to the instability of Ti(III) in aqueous solution, Ti(III) is hydrolyzed to TiOH 2+ . According to Fujihara et al. [38], TiOH 2+ is oxidized to Ti(IV) by reacting with dissolved oxygen. Therefore, the formation mechanism of rutile TiO 2 nanorods using Ti(IV) complex ions as growth units can be described as follows: For rutile TiO 2 , TiO 6 octahedra are first formed by bonding Ti atoms and six oxygen atoms. Then, the TiO 2 octahedron shares a couple of opposite edges with the next octahedron, forming a catenarian structure. The growth of rutile nanorods follows the sequence (110) < (100) < (101) < (001), because the growth rate of different crystal planes depends on the number of coordinated polygon body corners and edges. Therefore, rutile TiO 2 nanorods grown in the [001] direction were formed [39,40].
According to the previous experimental results, a promotional mechanism is shown in Figure 10. Under UV-light irradiation, TiO 2 absorbs photons and creates electron-hole pairs. Because the conduction band (CB) of TiO 2 is higher than that of MoS 2 [41][42][43], the photoelectrons generated by CB of TiO 2 are easily transferred to MoS 2 , which improves the separation of photogenerated electron-hole pairs and enhances the photocatalytic activity of PI/TiO 2 @MoS 2 heterostructures. The separated electrons react with dissolved O 2 to produce O 2 .− radicals on the surface of MoS 2 nanosheets. Next, they combine with H + to produce H 2 O 2 , and finally decompose into ·OH. In the meantime, the cumulative holes on the surface of TiO 2 are trapped by OHor H 2 O to form hydroxyl radicals, ·OH. Finally, the oxidation of organic dyes mainly occurs due to the involvement of holes, ·OH, and O 2 .− radicals. The main reactions in our study are described by equations:  Figure 10. Assumed mechanism of the photodegradation of dyes with PI/TiO2@MoS2 heterostructures.

Conclusions
PI/TiO2@MoS2 heterostructures were successfully fabricated by the assembly of MoS2 nanosheets and TiO2 nanorods on electrospun polyimide nanofibers using a simple hydrothermal method. Our innovation is in the successful structure of nanofiber-nanorodnanosheet multilevel nanostructure of PI/TiO2@MoS2 composite fibers. Compared with the usual nanoparticles on the surface of electrospun nanofibers, the functionalized application of composite nanofibers is expected. In this study, nanocomposite fibers with a multistage structure were proposed, which improves the performance of a single composite and can realize various functional applications. This multistage structure not only improves photocatalytic performance, but also the choice of gas adsorption, gas separation, supercapacitors, bi-sensing, etc. These nanocomposite fibers will have a wide range of applications, which is our next research direction.

Conclusions
PI/TiO 2 @MoS 2 heterostructures were successfully fabricated by the assembly of MoS 2 nanosheets and TiO 2 nanorods on electrospun polyimide nanofibers using a simple hydrothermal method. Our innovation is in the successful structure of nanofiber-nanorodnanosheet multilevel nanostructure of PI/TiO 2 @MoS 2 composite fibers. Compared with the usual nanoparticles on the surface of electrospun nanofibers, the functionalized application of composite nanofibers is expected. In this study, nanocomposite fibers with a multistage structure were proposed, which improves the performance of a single composite and can realize various functional applications. This multistage structure not only improves photocatalytic performance, but also the choice of gas adsorption, gas separation, supercapacitors, bi-sensing, etc. These nanocomposite fibers will have a wide range of applications, which is our next research direction.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.

Conflicts of Interest:
The authors declare no conflict of interest.