Fabrication of Titanium Dioxide/Carbon Fiber (TiO2/CF) Composites for Removal of Methylene Blue (MB) from Aqueous Solution with Enhanced Photocatalytic Activity

Guangxi Key Laboratory of Green Processing of Sugar Resources, College of Biological and Chemical Engineering, Guangxi University of Science and Technology, Liuzhou 545006, Guangxi, China Guangxi Key Laboratory of Clean Pulp & Papermaking and Pollution Control, College of Light Industry and Food Engineering, Guangxi University, Nanning 530004, Guangxi, China Province and Ministry Co-sponsored Collaborative Innovation Center of Sugarcane and Sugar Industry, Nanning 530004, Guangxi, China


Introduction
Water pollution has become a problem of concern in many countries [1][2][3][4][5]. Many dyes in wastewater contain aromatic compounds, which are chemically stable and harmful to human health [6][7][8]. Due to the adverse effects of these refractory organic compounds on the environment, it is necessary to develop new methods to degrade them. Photocatalysis is a promising technique for the photodegradation of harmful chemicals in wastewater [9][10][11][12][13][14]. TiO 2 and TiO 2 -based nanostructures are being used for photocatalytic degradation of organic pollutants to protect the environment [15][16][17]. With the advantages of low cost, large specific surface area, high oxidation ability, and good chemical stability, TiO 2 has become one of the most promising candidates for photocatalysis [18]. However, the photocatalytic performance of TiO 2 is severely limited by the large bandgap (3.2 eV) and the high recombination rate of photogenerated electron-hole pairs [19,20]. On the other hand, the industrial treatment of wastewater containing a variety of organic pollutants using TiO 2 -photocatalysts is not common due to the low photocatalytic activity [21]. In order to solve this problem, several methods have been developed to improve the efficiency of the photocatalytic process of TiO 2 , for example, doping, combining with metal oxides, quantum dots, semiconductors, carbon materials, and so on [22][23][24][25][26]. It has been reported that in carbonized PANI/TiO 2 composites, compared with bare anatase TiO 2 nanoparticles, more efficient photo-induced charge separation results in high photogenerated charge carrier mobility and ultimately improves the efficiency of the composite, which can also be explained as the existence of nitrogen in such carbonized nanostructure [27].
During the past twenty years, polyaniline (PANI) has been the most widely studied conductive polymer with good stability, corrosion resistance, nontoxicity, simple and low-cost synthesis method, and high instinctive redox performance, and the research on it eventually won the Nobel Prize Chemistry in 2000 [28]. Particularly, PANI has great potential due to its high absorption coefficients and high mobility of charge carriers. In addition, after the irradiation of light, PANI is not only an electron donor but also an excellent hole acceptor [27]. ese special characteristics of PANI make it an ideal material for improving the efficiency of charge separation in the field of photocatalysis. Recently, more and more attention has been focused on the combination of PANI and semiconductor photocatalyst [27,28]. Zhang et al. and Wang et al. PANI/ semiconductor composites were prepared by chemical adsorption and in situ oxidative polymerization, and it was found that the prepared samples had enhanced photocatalytic activity [11,29].
Composites based on carbonaceous materials and TiO 2 particles usually were synthesized by harsh methods using expensive equipment or at high temperatures [30]. Generally, in these composites, additional interaction of carbonaceous part and TiO 2 nanoparticles was weak, which hinders the effective functionalization of TiO 2 nanoparticles [31]. is article reports for the first time a simple method to synthesize TiO 2 /CF composites under room temperature conditions and its photocatalytic performance, to the best of our knowledge. Our method greatly facilitates the synthesis of very effective photocatalytic composites based on TiO 2 nanoparticles and carbon fiber. e structure, morphology, and optical properties of the prepared materials were studied. e photocatalytic activity of TiO 2 , CF, and synthetic composites in the degradation of methylene blue dyes was investigated under ultraviolet (UV) light in a comparative study. e relationship between the photocatalytic degradation of methylene blue dye and its ratio, contact time, and the amount of catalyst was studied. In addition, the degradation kinetics and mechanism were also discussed. Also, the stability of the catalyst was studied.

Materials.
All chemicals applied in this experiment including tetrabutyl titanate (TBT, 99%; Shandong West Asia Chemical Industry Co., Ltd.), acetic acid glacial, polyethylene glycol 400 (PEG 400), and ethanol were purchased and used without any further purification. Highpurity deionized water was used throughout all experiments.

Synthesis of Pure TiO 2 Powder.
e pure TiO 2 powder was prepared according to a sol-gel method. In a typical procedure, 10 mL acetic acid glacial, 3.2 mL deionized (DI) water, and 2 mL PEG 400 were firstly dissolved in 20 mL ethanol at room temperature. en, 20 mL tetrabutyl titanate was secondly dissolved in 40.0 mL ethanol, and it was added dropwise to the above solution under vigorous stirring. Next, the final suspension solution was maintained at 35°C for 2 h under vigorous stirring. e obtained solution was aged at room temperature for 24 h and it was dried at 100°C in an air oven for 10 h. Finally, the obtained sample was ground and calcined at 450°C in an air atmosphere for 2 h with a heating ramp of 10°C/min. e pure TiO 2 powder was obtained.

Preparation of CF.
e polyaniline fiber was calcined at 400°C in a N 2 atmosphere for 2 h with a heating ramp of 5°C/ min, and it was ground and stored.

Characterization.
e X-ray powder diffraction (XRD) patterns of TiO 2 powder, CF, and the synthetic composites were acquired using a Bruker D8 A25 diffractometer (Cu Kα radiation (λ �1.5406Å), operated at 40 mA and 40 kV) in the 2θ range of 10-90°at a scanning speed of 10°/min. Morphology of TiO 2 powder, CF, and the prepared TiO 2 /CF composites were studied using a scanning electron microscope (SEM). Element analysis of the prepared TiO 2 /CF composites was studied using an energy dispersive spectroscopy (EDS). e UV-visible diffuse reflectance spectroscopy (UV-vis DRS) and photoluminescence (PL) spectrum of TiO 2 powder, CF, and the synthetic composites were measured by multifunctional optical fiber spectrometer (QE65 Pro, Ocean Optics, USA). e X-ray photoelectron spectroscopy (XPS) of TiO 2 powder, CF, and the synthetic composites were measured by an X-ray photoelectron spectrometer ( ermo escalab 250X).

Photocatalytic Activity Test.
e photocatalytic activity of the photocatalysts was tested by photocatalytic degradation of MB. e photocatalytic tests were carried out under UV-light irradiation at room temperature in an air atmosphere. e TiO 2 / CF composites were invited as photocatalysts. e UV light source was generated by a 300 W Mercury lamp. e distance between the photocatalysts and lamp was 10 cm. Before photocatalytic reaction, 10 mg of prepared photocatalysts was suspended in 50 mL of MB aqueous solution of 5 mg/L. Prior to irradiation, the suspension solutions were stirred magnetically in the dark for 1 h at room temperature in order to reach adsorption-desorption equilibrium between the photocatalysts and MB. After each 20 min irradiation time interval, 4 mL solution of reaction was taken from the suspensions and the photocatalyst was removed by filtered via a 0.45 μm filter membrane (Nylon) for analysis. A UV-vis spectrophotometer at its characteristic wavelength (λ � 664 nm) was used to measure the absorbance of the residual MB solutions in the solution of reaction. e degradation rate was calculated by C/C 0 , where C was the concentration of MB in each time period, and C 0 was the concentration of MB after dark adsorption. In the durability testing, five successive cycles were performed. After each cycle, the photocatalysts were washed with ethanol and DI water carefully and then dried at 60°C for 12 h. en, the fresh MB dye aqueous solution of 5 mg/L was mixed with the used photocatalysts to carry out the next photocatalytic activity testing.
To identify the generated active species during the reaction process, ammonium oxalate (AO), 1, 4-benzoquinone (BQ), and 2-propanol (IPA) were used as the hole (h + ) scavenger, superoxide radical (O 2 ·−) scavenger, and hydroxyl radical (•OH) scavenger, respectively. e right amount of scavenger was added to the MB dye aqueous solution to probe the active species through variation of degradation rate.

Structural Properties.
e XRD patterns of TiO 2 powder, CF, and the prepared TiO 2 /CF composites are shown in Figure 1. e TiO 2 powder, Figure 1, is composed of anatase as the dominant phase.
e main characteristic peaks of TiO 2 anatase at 2theta 25.3°, 37.8°, and 48°are related to (101), (004), and (200) crystal planes, respectively (JCPDS card No. 21-1272) [32]. In addition, other relevant peaks appear at 55°(211) and 62.6°(204) [32]. It is generally known that TiO 2 possesses three types of crystal structure: anatase, brookite, and rutile. Among them, anatase crystals have the highest photocatalytic activity because the oxygen vacancies of anatase crystals are larger than that of brookite crystals and rutile crystals. Furthermore, TiO 2 with anatase crystal form has a low dielectric constant, low mass density, and high electron mobility [33]. For CF, the broad peaks at 2θ � 17°and 43°are designated as characteristic peaks of carbon [27]. For the TiO 2 /CF composites, the characteristic peaks of anatase still exist with high intensity, but the characteristic peaks of carbon are not visible in the XRD patterns.
is is mainly due to the coverage of anatase crystals on the carbon [28]. In short, a series of composites with anatase crystals were successfully synthesized, indicating that the composites have high photocatalytic activity.

Morphological Characterization.
e surface morphology of TiO 2 powder (a), CF (b), and the prepared TiO 2 /CF composites (c) were characterized by SEM images as shown in Figure 2. It could be clearly seen that although some of them were anatase, the size of the obtained TiO 2 particles was about 100-200 nm. TiO 2 were spherical particles and they were agglomerated. At the same time, it could be seen that TiO 2 was dispersed on CF. e elemental composition of the TiO 2 / CF composite was characterized by EDS as shown in Figure 2(d). It could be seen from Figure 2(d) that the prepared TiO 2 /CF composite contains Ti, O, and C elements, which were consistent with the results of XPS analysis.

Optical Properties.
e research on the optical properties of TiO 2 powder, CF, and the prepared TiO 2 /CF composites is one of the most important parameters affecting the applications of synthetic products. e UV-vis light absorption spectra of TiO 2 powder, CF, and the prepared TiO 2 /CF composites are shown in Figure 3. All samples show absorption bands in the visible light region of the spectrum. e composites showed an obvious enhancement of absorption in the visible light band owing to the presence of CF, which helps to enhance the photocatalyst activity [34]. In addition, the absorbance in the visible light band increased as the ratio of CF to TiO 2 increased.
Photoluminescence (PL) spectrum analysis was commonly employed to investigate the separation efficiency of electron-hole pairs. Generally, high photoluminescence intensity corresponds to high electron-hole pairs recombination efficiency, indicating low photocatalytic activity [35]. As shown in Figure 4, when excited with 210 nm UV light, all photocatalysts showed similar emission spectra and had the strongest peak at about 390 nm, which was mainly due to the emission of edgeless excitons [36]. Moreover, it was noted that the intensity of the emission band decreased significantly as the content of TiO 2 increases. According to reports, the decrease of PL intensity was mainly due to three factors [37]: (1) in the presence of TiO 2 /CF, photogenerated electrons can be efficiently transferred from TiO 2 to CF, which improves the separation efficiency of electron-hole pairs; (2) the absorbance of photoluminescence caused by CF leads to quenching effect; (3) low-electron light excitation may lead to low-charge recombination. e substantial decrease in the intensity of the PL peak indicates that CF provides the potential to inhibit the recombination of photoelectrons and holes.

Chemical Compositions.
In order to study the chemical composition and interaction between TiO 2 and CF, XPS analysis was used, as shown in Figure 5. Figure 6 shows the high-resolution XPS spectrum of the C 1s region. e binding energy of 284.8 eV is the typical peak position of the indeterminate carbon pollution absorbed from the surrounding environment and cannot be eliminated [38]. In addition, the deconvolution peaks centered on the binding energies of 286.1 and 288.7 eV were attributed to C-O and C�O oxygen-containing carbon bands, respectively [39]. e C 1s spectrum of TiO 2 /CF2/1 is shown in Figure 6(c). e peak intensity at 286.1 eV increases significantly, indicating that C-O bonds were formed in TiO 2 /CF2/1. Carbon atoms were bonded to the interstitial positions of the TiO 2 lattice [39].  Figure 7 shows the high-resolution XPS spectra of the O 1s region of TiO 2 (a) and TiO 2 /CF2/1 (b). For pure TiO 2 , C-O and O-H bonds centered at 532.8 eV and 531.2 eV were found, respectively [40]. e O 1s spectrum of TiO 2 /CF2/1 was shown in Figure 7(b). e peak intensities at 532.8 eV and 531.2 eV increase significantly, indicating that C-O and O-H bonds were formed in TiO 2 /CF2/1; that is, oxygen atoms were incorporated into the TiO 2 crystal lattice the gap position [40]. Figure 8 shows the high-resolution XPS spectra of the N 1s region of CF (a) and TiO 2 /CF2/1 (b). For the CF in Figure 8(a), there were three corresponding characteristic peaks. e peak at 398.5 eV was attributed to the C�N-C group, while the other peaks at 399.6 eV and 400.9 eV could be attributed to N-(C) 3 and the N-H group [41]. erefore, it shows that the double bond of N�C had been broken, and both the nitrogen atom and the carbon atom were bonded to the titanium atom, resulting in the formation of the N-Ti-C group. In comparison with CF, there were only three peaks attributed to CF in TiO 2 /CF2/1 as shown in Figure 8 As shown in Figure 9, the formation of C-Ti bonds and N-Ti bonds in TiO 2 /CF2/1 also could be further checked and confirmed by analyzing the Ti 2p core level of XPS. For pure TiO 2 , Ti 2p 3/2 and Ti 2p 1/2 with 458.8 eV and 464.5 eV as the centers were found, which were assigned to the Ti-O bond [42]. In Figure 9(b), in addition to the two characteristic peaks of TiO 2 , which were 458.8 (Ti 2p 3/2 ) and 464.5 eV (Ti 2p 1/2 ), two other peaks could be found, located at 462.1 eV and 456.9 eV, respectively and were determined to be caused by the N-Ti bond, and the remaining peak at 460.8 eV was attributed to the C-Ti bond [43]. is indicates that there were N-Ti bonds and C-Ti bonds in TiO 2 /CF2/1.
Combined with the above analysis, two additional bonds (N-Ti bond and C-Ti bond) were formed in TiO 2 /CF2/1, which indicates that TiO 2 nanoparticles were chemically bonded to CF in TiO 2 /CF2/1. Figure 10(a) shows the photocatalytic performance of TiO 2 powder, CF, and the prepared TiO 2 /CF composites based on MB degradation under UV light irradiation.

Photocatalytic Activity.
e results show that pure TiO 2 has photocatalytic activity under UV light irradiation. Moreover, all TiO 2 /CF composites exhibit excellent photocatalytic activity, and their activity was higher than that of a single component. When the mass ratio of TiO 2 /CF was 2 : 1 (TiO 2 / CF2/1), the photocatalytic activity was the best, and the MB removal rate reaches 97.7% after 2 hours of reaction.
Photocatalytic degradation follows first-order kinetics. Kinetics can be expressed as follows:−ln (C/C 0 ) � k app t.    Journal of Chemistry to be 0.03289 min −1 , which was 2.9 times and 4.7 times higher than that of TiO 2 and CF, respectively. erefore, it was believed that effectively separating electron-hole pairs through chemical bond connection can greatly improve the photocatalytic activity of UV light photocatalyst.
For the practical application of the photocatalyst, the stability of the photocatalyst was one of the key issues. e by-products were always adsorbed on the surface active sites of the photocatalyst. e photocatalytic activity will drop sharply after a short period of exposure time. In order to evaluate the stability of the prepared TiO 2 /CF2/1, a cycle experiment was carried out under the same conditions. According to the results shown in Figure 10(d), TiO 2 /CF2/1 still showed exhibits excellent photocatalytic performance in the fifth cycle.
In this study, the effect of the dosage on the MB removal efficiency of the TiO 2 /CF composites was studied at the same MB concentration by varying its doses from 5 to 20 mg or 0.15 to 0.25 g/L. Figure 11 shows that using an appropriate amount of photocatalyst can increase the removal rate of MB and then reach equilibrium. e degradation efficiency of MB increased significantly to 97.7% as the dose increased from 5 to 20 mg or 0.15 to 0.25 g/L after 2 hours of reaction.
is indicates other active sites of the photocatalyst for MB adsorption. Using CF to functionalizing TiO 2 into composites will increase the adsorption capacity of TiO 2 because of its extra surface area.
Although the physical and chemical interaction between MB and TiO 2 /CF2/1 may increase its surface coverage, after reaching the optimal dose, its particles will gather in TiO 2 / CF2/1 clusters, shielding UV light from interacting with the active surface of the photocatalyst. erefore, this situation reduces the generation of ·OH. Other studies had also reported that a dose of 10 mg or 0.20 g/L was optimum for MB degradation [44,45].
In order to further explore the role of photo-active species in the reaction process, the relevant scavengers were added to the MB solution by comparing the changes in the degradation rate. As shown in Figure 12, it clearly shows that the degradation rate was significantly reduced after adding AO, BQ, and IPA. And the degradation rate follows the following order: no scavenger > AO > IPA > BQ. erefore, it can be known that h + , O2·− and •OH, which were active substances, were generated in the photocatalytic reaction, and O2·− plays a very important role.
3.6. Mechanism Discussion. Based on the above analysis and discussion, it could be concluded that the improvement of light collection between TiO 2 and CF was the main factor for improving the photocatalytic activity. As shown in Scheme 1, when the TiO 2 /CF composites were irradiated with UV light, electrons were excited from the VB to the CB of TiO 2 and CF, leaving holes in the VB. Since the CB of CF was much higher than that of TiO 2 , and the VB of CF was located at a higher position than that of TiO 2 , the excited electrons were transferred from the CB of CF to the CB of TiO 2 , and the holes migrate in the opposite direction. Moreover, CF loading with TiO 2 could facilitate charge transport, hinder the recombination of electron-hole pairs, and further increase the number of charge carriers in the reaction [28]. erefore, the TiO 2 /CF composites exhibit better photocatalytic activity.  Journal of Chemistry Based on the scavenger experiment, combined with the above-mentioned mechanism analysis, O2·− was the main role in the photocatalytic reaction produced by the reduction of O 2 in CB of TiO 2 . And •OH was formed by the further reaction of O2·−, e − , and 2H + . e photocatalytic process could be represented by the following process:

Conclusions
In summary, the TiO 2 /CF composites with high photocatalytic activity were successfully synthesized by a simple method at room temperature. e MB photodegradation performance of TiO 2 /CF composites was better than that of TiO 2 or CF under the same conditions in the photocatalytic system under UV light irradiation. And the optimum mass ratio of TiO 2 to CF was 2 : 1. e stability test of the photocatalyst shows that TiO 2 /CF2/1 had high stability and the photodegradation efficiency after the fifth cycle was 92%. SEM showed that TiO 2 particles grew well on the surface of CF. XPS shows that TiO 2 particles were chemically bonded to CF, which was confirmed by the formation of C-Ti bonds and N-Ti bonds.
is chemical bonding structure was conducive to the formation of an effective heterojunction,  which might lead to the synergistic combination of TiO 2 and CF, thereby significantly reducing the recombination of electron-hole pairs and enhancing the separation of photogenerated carriers. It was also proved by the PL spectrum. Since O2·− and ·OH free radicals were generated in the TiO 2 / CF/UV system, it could degrade up to 97.70% of MB. In addition, scavenger experiments show that O2·− was the main role participating in photocatalytic activity. e existence of these free radicals was confirmed by free radical suppression tests using different scavengers for different free radicals. is study shows that simple TiO 2 /CF composites were efficient and promising photocatalysts for dye degradation in water.

Data Availability
All data used to support this study are included within the paper.

Conflicts of Interest
e authors declare no conflicts of interest.

Authors' Contributions
Hao Cheng and Wenkang Zhang authors contributed equally to this work.