Preparation of Hollow Tubular TiO2/C Photocatalyst and Study on Photocatalytic Degradation Process and Mechanism of Dyes

ABSTRACT Hollow tubular structure TiO2/C photocatalyst was prepared by carbonization at 400℃ with cotton fabric as initial carrier and tetrabutyltitanate as precursor. The structure of the samples was characterized by SEM and XRD. The results show that anatase crystal TiO2 with good crystallization is formed, TiO2 was uniformly distributed on the surface of the hollow tubular carbon material. The photocatalytic degradation performance was measured by using the methylene blue as the target dyes. When the dosage of TiO2/C was 1.7 g/L, the degradation rate of methylene blue solution was high up to 97.5%. The effective degradation of dyes was realized by using the strong adsorption property of hollow carbon materials and the high photocatalytic activity of anatase titanium dioxide. Due to the repeated adsorption of carbon materials to dyes and the continuous degradation of titanium dioxide to dyes, TiO2/C photocatalyst could be regenerated in situ.


Introduction
With the explosion of population and the improvement of industrialization level, the global energy crisis and environmental pollution have become the hot spots of the society. Water pollution, especially the shortage of water resources caused by organic pollutants, has become a major problem threatening the sustainable development of human society (Chen et al. 2020). For textile industry, the discharge of dye wastewater is an important problem that restricts its sustainable development (Patehkhor, Fattahi, and Khosravi-Nikou 2021). There are more than 100,000 commercial dyes, and the total dye consumption in the textile industry worldwide is more than 10,000 tons/year, approximately 100 tons/year of dyes entered into water streams (Chen et al. 2020). Dye wastewater is characterized by high concentration, deep color, and strong toxicity. If it is discharged into rivers without timely treatment, it will cause serious water pollution problems. Dye waste water not only produces certain discomfort in the senses but also the toxicity of fuel waste water may endanger the lives of animals and humans (Chanikya et al. 2021;Kong et al. 2021;Li 2020).
Photo-catalysts have been considered effective in decomposing organic water pollutants (Li et al. 2022;Yao et al. 2019). Semiconductor photocatalysis is a sustainable technology for solving the problems related to environmental pollution and energy crisis, as it can be used to oxidize organic pollutants (Luo et al. 2021). Among them, titanium dioxide (TiO 2 ) photocatalytic oxidation technology is widely studied, and various modification methods, such as constructing step-scheme (S-scheme) heterojunctions (Chen et al. 2020;Dolatabadia, Fattahia, and Nabati 2021;Wang et al. 2021;Zhang, Liao, and Sun 2020) or surface/interfacial modification, have been developed to improve the photocatalytic efficiency (Li, Xi, and Liu 2021;Lu et al. 2020;Mei et al. 2021). Due to easy agglomeration, easy loss, and difficult recovery in water, a large number of researchers have been devoted to developing supported TiO 2 photocatalyst. In 2016, Ponce et al loaded TiO 2 nanoparticles onto polyethylene by different methods and confirmed that TiO 2 nanoparticles have a good application prospect in water purification through photocatalytic performance test. Long et al. 2013 synthesized partially reduced graphene and TiO 2 composite photocatalytic material and proved that it has better degradation effect on dyes than that of pure TiO 2 . Cui et al. (2015) loaded anataseTiO 2 nanoparticles on the surface of fabrics, and the degradation rate of formaldehyde by the composite catalyst could reach 97.0%. Tang et al. (2019) loaded nano-TiO 2 particles on the surface of a polyester fiber, and the results showed that it has a better degradation effect on methylene blue dyes.
In this paper, cheap and easily available cotton fabrics (CFs) were used as initial carriers to load tetrabutyl titanate (TBT). After hydrothermal and carbonization treatment, the CFs were converted into hollow tubular carbon materials, which were the final carrier, while TBT was converted into TiO 2 . Finally, titanium dioxide/carbon composite photocatalyst (TiO 2 /C) was obtained. The adsorption of hollow tubular carbon materials and the photocatalytic capability of TiO 2 have jointly realized efficient and rapid degradation of dyes.

Preparation of TiO 2 /C
The CFs were cut into 1 cm × 1 cm, and then 1.6 g of the cut CFs were immersed in 8 mL of TBT for 24 h. The impregnated CFs were then taken out and immersed in 40 mL of deionized water at 80℃ for 30 min. Subsequently, hydrothermal treatment was carried out at 150℃ for 10 h. The products were placed in a tube furnace, heated to the target temperature (400℃, 500℃, 600℃) (nitrogen atmosphere) at a heating rate of 5 ℃/min, and kept at the target temperature for 2 h, then cooled to room temperature to obtain titanium dioxide/carbon composite photocatalyst (TiO 2 /C-X, X is the calcination temperature (℃)). The simulation diagram of the preparation process of TiO 2 /C is shown in Figure 1.

Photocatalytic degradation of MB
MB was selected as model chemicals to evaluate the photocatalytic performance of TiO 2 /C. 0.01 g of TiO 2 /C samples were added in 60 mL of MB dye solution (10 mg/L). Before irradiation, the solutions were stirred (500 r/min) in the dark for 30 min to achieve the adsorption equilibrium; then, the mixed solutions were exposed to UV radiation with the wavelength of 365 nm (UV-365) maintaining vigorous stirring (500 r/min). The MB solutions were taken out at regular intervals, and the absorbance was measured by a 721 visible spectrophotometer, at the maximum absorption wavelength of 664 nm (Figure 2). Figure 3(a) is the scanning electron microscopy (SEM) image of the original cotton fabric fiber. Cotton fibers have a natural warp morphology, which is beneficial to the loading of TBT and the in-situ synthesis of TiO 2 . Figures 3b-d is SEM images of TiO 2 /C. Cotton fibers have changed greatly after high-temperature carbonization, cotton fibers were broken and transformed into fiber rod-like carbon materials; the fibers were severely shrunk and the surface was obviously wrinkled, but TiO 2 was still uniformly and firmly distributed on the surface of the fibers.

Microstructure of TiO 2 /C
The qualitative analysis of elements is carried out by EDS, and the results are shown in Figures 3e-h. From the energy spectrum, it is found that the three TiO 2 /C samples prepared contain C, O and Ti elements. The existence of Ti element further confirms the success of TiO 2 particle loading.

Crystal structure and particle size calculation
X Ray Diffraction (×RD) is used to characterize the crystal structure of the products after hightemperature carbonization of cotton fabric fibers and the crystal structure of TiO 2 particles in TiO 2 /C, as shown in Figure 4. Then, according to Scherrer formula, the particle size of TiO 2 is calculated by XRD pattern.
From Figure 4(a), it can be clearly seen that only partial characteristic diffraction peaks (37.2° and 47.6°) of cotton fibers exist in the products (C-400, C-500 and C-600). The characteristic diffraction peaks of the cotton fabric fibers at 15.6°, 16.8°, and 23.1° are completely disappeared, which indicates that the crystalline region in the cotton fabric fiber structure has been completely destroyed. There is a wide diffraction peak in the range of 2θ = 10°−40°, which indicates that the cotton fabric fibers are converted into amorphous carbon after high-temperature calcination. From Figure 4(b), it can be obviously observed that the peak positions of the five groups of diffraction peaks are basically same (25.6°, 38.3°, 48.4°, 55.0°, and 63.4°), corresponding to (101), (112), (200), (211), and (204) crystal planes of anatase TiO 2 ,respectively. In addition, sharp characteristic diffraction peaks appear around 25.6°, indicating that anatase crystal TiO 2 with good crystallization is formed. When the calcination temperature is increased from 400℃ to 500℃ and 600℃, the diffraction peaks of TiO 2 remain unchanged, indicating that the influence of calcination temperature on the crystal structure of TiO 2 particles could be ignored. According to Scherer formulafor calculation, the particle sizes of TiO 2 in TiO 2 /C-400, TiO 2 /C-500 and TiO 2 /C-600 are about 5.6 nm, 6.3 nm, and 5.5 nm, respectively. Figure 5 shows the influence of calcination temperature on photocatalytic effect and the test conditions: dark adsorption for 30 min, UV lamp irradiation for 6 h, MB solution concentration of 10 mg/L, and TiO 2 /C dosage of 0.17 g/L. From the three curves in Figure 4, it is obvious that the degradation rate of MB by TiO 2 /C increases with the extension of irradiation time. After irradiation for 1.6 h, the MB degradation rate of TiO 2 /C-400 is higher than that of TiO 2 /C-600 and TiO 2 /C-500, which may be related to the particle size of TiO 2 . The average grain sizes of TiO 2 in TiO 2 /C-400, TiO 2 /C-600 and TiO 2 /C-500 are about 5.6 nm, 5.5 nm, and 6.3 nm, respectively. This indicates that TiO 2 with smaller particle size has higher photocatalytic activity. In addition, the average particle size of TiO 2 in the prepared TiO 2 /C does not continuously increase with the increase in temperature. The reason may be related to the presence of appropriate carbon, which can inhibit the growth and agglomeration of TiO 2 crystals. Considering comprehensively, the optimum calcination temperature in the preparation of TiO 2 /C is determined to be 400℃. Figure 6 shows the influence of dosage of TiO 2 /C-400 on photocatalytic effect under the above conditions. When the dosage of TiO 2 /C-400 is 0.17 g/L, 1.7 g/L, and 2.16 g/L, the degradation rate of MB is 15.9%, 97.5%, and 97.3%, respectively. The results show that the degradation rate of MB can be significantly increased by increasing the amount of TiO 2 /C-400, which increases the number of active sites on the surface of TiO 2 /C-400. However, the area of TiO 2 /C-400 irradiated by UV light is limited, so excessive TiO 2 /C-400 will not significantly improve the degradation rate of MB. Therefore, when the dosage of TiO 2 /C-400 is more than 1.7 g/L, the degradation rate of MB is not significantly improved. In addition, too many TiO 2 /C-400 will block light, which is not conducive to the photocatalytic reaction.

Photocatalytic degradation performance test
Therefore, the optimal dosage of TiO 2 /C-400 is 1.7 g/L during the photocatalytic experiment. In this work, under the conditions of calcination temperature of 400℃, UV −365 irradiation for 6 h, MB solution concentration of 10 mg/L, and TiO 2 /C dosage of 1.7 g/L, the degradation of MB by TiO 2 /C could reach 97.5%. Shown as Figure 7, by comparing the degradation of MB with other reported literatures (Bubacz et al. 2010;Rammohan et al. 2014;Chuan et al. 2008;Lin et al. 2012) under different irradiation times, it could be seen that the TiO 2 /C prepared in this experiment had significantly higher degradation effect on MB. However, compared with reference (Qin et al. 2019), the photocatalytic performance needs to be further improved.
Regardless of how efficiently an adsorbent can remove pollutants from water, the reusability and regeneration of the adsorbents are of great essence in terms of the economic and industrial practicality of the adsorption process (Mahmoodi, Fattahi, and Motevasse 2021). Figure 8 shows the effect of cycle times on photocatalytic activity (TiO 2 /C-400, 1.7 g/L). When the cycle times of powder photocatalyst were 1, 2, 3, 4, and 5, the degradation rates of methylene blue were 96.4%, 94.1%, 94.2%, 93.5%, and 92.1%, respectively. After five cycles, the degradation rate of MB was still above 92.0%, indicating that the prepared TiO 2 /C-400 composite photocatalyst could be reused and could maintain high photocatalytic activity, and the good repeatability and stability of TiO 2 /C-400 are beneficial to reduce water treatment costs and avoid secondary pollution.

Adsorption degradation process and mechanism
The surface functional groups of TiO 2 /C-400 before and after photocatalytic reaction were characterized by Fourier Infrared (FT-IR) spectrometer, and the photocatalytic degradation process of MB by TiO 2 /C-400 was further analyzed by comparing with MB.   Figure 9 shows the absorption peak in the 800-500 cm −1 , which is attributed to Ti-O-Ti vibration and is the characteristic absorption peak of TiO 2 (Han et al. 2010;Mathumba et al. 2017;Vinícius et al. 2013). Combined with the previous SEM and XRD analysis, it can be further confirmed that anatase TiO 2 particles have been successfully synthesized in TiO 2 /C-400. The bending vibration absorption peaks at 1634 cm −1 and 3435 cm −1 are related to-OH and adsorbed H 2 O on the surface of TiO 2 /C-400 (ZhVignesh, Vijayalakshmi, and Karthikeyan 2016;Zhang and Yang 2012).
After dark adsorption for 30 min, the absorption peak at 3435 cm −1 is stronger than that of the original TiO 2 /C-400. This indicates that besides the existence of absorption peaks of -OH and H 2 O, the C-H stretching vibration peak on benzene ring is concealed in this broad peak. The absorption peak at 1594 cm-1 is also enhanced, which is caused by the vibration of benzene ring skeleton C=C   ; new in-plane bending vibration peak of -CH 3 and out-of-plane bending vibration absorption peak of C-H on benzene ring also appear at 1373 cm −1 and 762 cm −1 . According to the above information, it can be determined that MB is adsorbed on the surface of TiO 2 /C-400. After photocatalytic degradation for 6 h, it can be seen that the absorption peaks at 1373 cm −1 and 762 cm −1 disappear, and the absorption peak at 1594 cm −1 is severely weakened, indicating that MB has undergone photochemical reaction, that is, MB is first adsorbed on the surface of TiO 2 /C-400 and then degraded. At the same time, the position of the absorption peak of the photodegraded TiO 2 /C-400 is basically same as that of the original TiO 2 /C-400. Therefore, it can be inferred that the photodegraded TiO 2 /C-400 has basically returned to its original state and has realized in-situ regeneration.
In order to further explore the influence of the micro-morphology of TiO 2 /CFs-400 on improving its photocatalytic performance, SEM photos were taken at different magnifications. From Figure 10(a), it can be found that the morphology of cotton fiber changes obviously at the calcination temperature of 400℃ and presents a short rod-like carbon material. From Figs. 10(b, c), the hollow tubular structure of cotton fiber can be clearly observed, and a small amount of TiO 2 nanoparticles are dispersed inside. Such a hollow tubular structure plays a positive role in adsorption of MB solution and provides a good place for photocatalytic reaction.
The photocatalytic performance of TiO 2 /C-400 is higher than that of TiO 2 /C-500 and TiO 2 /C-600, one of the reasons is that TiO 2 /C-400 has more hollow tubular structures. From Figure 11, it can be found that under high-temperature calcination, the morphology of cotton fibers all changed obviously, appearing as short rod-shaped carbon materials. With the increase in calcination temperature, the  cotton fiber is more and more seriously damaged; the short rod-shaped carbon material gradually disintegrates; the tubular structure of the cotton fiber gradually collapses; more and more fragments are scattered, so the side effects gradually increase. Therefore, it is not that the higher the calcination temperature is, the better the photocatalytic effect is. Only at an appropriate temperature can the tubular structure of cotton fibers be more maintained and more MB dyes be adsorbed, thus accelerating the photocatalytic reaction.
Pore structure plays an important role in photocatalysis. N 2 adsorption-desorption analysis results are shown in Figure 12. The adsorption-desorption isotherm of TiO 2 /C-400 can be classified as a type IV isotherm with an H1-type hysteresis loop, which is characteristic of mesopores, which will provide the enhanced capability for pollutant adsorption.
The degradation process of MB by TiO 2 /C-400 is as follows (Pang et al. 2018): TiO 2 /C-400 adsorbs MB dye in the dark, then MB dyes migrate to the surface of TiO 2 , through continuous decomposition of TiO 2 and repeated adsorption of carbon materials, and degradation of MB dyes and in-situ regeneration of TiO 2 /C-400 are finally realized. In the photocatalytic experiment, the dark adsorption time of 30 min is to exclude the effect of adsorption on the degradation rate and ensure that the active center of titanium dioxide is surrounded by a high concentration of organic pollutants, which provide conditions for timely and rapid photocatalytic reactions.
After UV irradiation, the photogenerated hole can react with the electron donor adsorbed on the surface of nano-titanium dioxide to generate the radical •OH. Photogenerated electrons can react with the electron acceptor adsorbed on the surface of nano-sized titanium dioxide to generate •O 2radicals. These free radicals have strong oxidative activity and gradually degrade organic pollutants around titanium dioxide. At the same time, the organic pollutants adsorbed on the carbon material can be continuously migrated around TiO 2 through diffusion. The carbon material adsorption and TiO 2 degradation of organic pollutants were carried out simultaneously, and finally the carbon material was regenerated in situ. The simulation of degradation process of methylene blue solution by TiO 2 /C-400composite photocatalyst is shown in Figure 13.
The mechanism of MB degradation by TiO 2 /C-400 is shown in Figure 14. MB is a cationic dye and is more easily degraded by TiO 2 under neutral or alkaline conditions. The total reaction equation can be expressed as follows (Mills and Mcfarlane 2007): )Initially, the TiO 2 is attached to the surface, and under UV-vis irradiation, the MB absorbs photons to form an excited state and inject electrons into the TiO 2 conduction band. The ·O 2 − and ·OH are formed on the surface of TiO 2 . Then, the reactive oxygen species (·O 2 −) electrophilically attack the dyes to form hydroxylated oxidation by-products. Subsequently, the MB is degraded into biodegradable oxidation.
According to references (Yu and Chuang 2007), at the initial stage of photocatalytic oxidation, the C-S(=O)-C were formed due to the generation of ·OH that electrophilically attacked the cleavage of the C -S+=C of the MB (Houas et al. 2001); meanwhile, the N=C double bond in the paraposition of the central aromatic ring is induced to break, forming imino -NH 2 . Under the continuous attack of ·OH, sulfone compound were formed, andthen are attacked by ·OH again to form sulfonic acid -SO 3 H and finally form stable sulfate ion SO 4 2- (Liu et al. 2015). The -NH 2 can be substituted by ·OH to generate corresponding phenol substances, while ammonia NH 3 is released or ammonium ion NH 4+ is generated (Houas et al. 2001). Through the continuous attack of ·OH, the -N(CH 3 ) 2 undergoes a gradual oxidative degradation process from alcohol oxidation to aldehyde reoxidation to acid acidacid, and finally, the carboxylic acid will be removed to generate CO 2 and -N-is converted to nitrate by oxidation. Usually, the aromatic rings will be converted to CO 2 and H 2 O under the action of ·OH though hydroxylation to form phenolic compounds and then oxidation (Zhang et al. 2013).
In the process of photocatalytic degradation of dyes, the synergistic effect of titanium dioxide and carbon is also one of the reasons for enhancing the photocatalytic activity ( Figure 15). Firstly, after calcination at high temperature, carbon formed by cotton fibers has tubular structures with large specific surface area, which can achieve stronger adsorption of MB solution. Secondly, carbon formed by high-temperature calcination can be used as an effective electron transfer station because carbon has a high ability to capture electrons and low Fermi level, which can promote electron transfer from the conduction band of titanium dioxide to the surface of carbon. Thus, the Schottky barrier is able to form at the interface between carbon and TiO 2 , and photogenerated electrons can move freely toward the carbon surface, while the holes left behind can move to the valence band of TiO 2 , thus achieving the separation of electrons and holes. This process effectively inhibited the recombination of photogenerated electrons and holes and improved the photocatalytic activity of TiO 2 /C-400 composite photocatalyst.

Conclusion
TiO 2 /C photocatalyst with uniform and complete anataseTiO 2 was synthesized by hydrothermal treatment and high-temperature calcination. The optimal calcination temperature was 400℃, and there were more hollow tubular structures at this time. The degradation rate of MB solution could reach 97.5% under UV light irradiation for 6 h, 10 mg/L MB solution, and 1.7 g/L TiO 2 /C-400. Through repeated adsorption of hollow tubular carbon materials and continuous decomposition of TiO 2 , MB dye degradation and in-situ regeneration of photocatalysts were finally realized.