Fabrication of Titanium Dioxide/Carbon Fiber (TiO2/CF) Composites for Removal of Methylene Blue (MB) from Aqueous Solution with Enhanced Photocatalytic Activity
TiO2 powder was firstly synthesized and carbon fiber was secondly prepared via the carbonization of polyaniline fiber, and TiO2/carbon fiber composites were lastly synthesized via a simple method at room temperature. The prepared samples are evidently investigated by X-ray powder diffraction, scanning electron microscopy, energy dispersive spectroscopy, ultraviolet-visible diffuse reflectance spectroscopy, photoluminescence spectrum, and X-ray photoelectron spectroscopy, respectively. Using the monochromatic light of ultraviolet, the photocatalytic activity of the TiO2/CF composites was accurately evaluated with respect to the degradation of an aqueous dye (methylene blue) solution. The relationship between the photocatalytic degradation of methylene blue dye and its ratio, contact time, and the amount of catalyst was studied. The kinetics and mechanisms of degradation were discussed. The results show that TiO2/CF composites have good photocatalytic activity and stability. The TiO2/CF2/1 composite was used in effective photocatalytic degradation of methylene blue, the weight ratio of TiO2 to carbon fiber was 2:1, and the degradation rate was obtaining up to 97.7% of degradation during 120 min of reaction. The photocatalytic stability of TiO2/CF composites was dependent on the stability of their structure. After 5 repeated uses, the composite TiO2/CF2/1 still exhibited rather high activity toward the degradation of methylene blue, where the decolorization efficiency of methylene blue achieved 92% and the loss of activity was negligible. Based on radical trapping experiments, the mechanism of TiO2/CF composites on photocatalytic degradation of methylene blue is proposed, which could explain the enhanced photocatalytic activity of the composites better. Superoxide radicals, photogenerated holes, and photogenerated electrons were the main active substances for methylene blue degradation.
Water pollution has become a problem of concern in many countries [1–5]. Many dyes in wastewater contain aromatic compounds, which are chemically stable and harmful to human health [6–8]. 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 [9–14]. TiO2 and TiO2- based nanostructures are being used for photocatalytic degradation of organic pollutants to protect the environment [15–17]. With the advantages of low cost, large specific surface area, high oxidation ability, and good chemical stability, TiO2 has become one of the most promising candidates for photocatalysis . However, the photocatalytic performance of TiO2 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 TiO2-photocatalysts is not common due to the low photocatalytic activity . In order to solve this problem, several methods have been developed to improve the efficiency of the photocatalytic process of TiO2, for example, doping, combining with metal oxides, quantum dots, semiconductors, carbon materials, and so on [22–26]. It has been reported that in carbonized PANI/TiO2 composites, compared with bare anatase TiO2 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 .
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 . 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 . These 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 TiO2 particles usually were synthesized by harsh methods using expensive equipment or at high temperatures . Generally, in these composites, additional interaction of carbonaceous part and TiO2 nanoparticles was weak, which hinders the effective functionalization of TiO2 nanoparticles . This article reports for the first time a simple method to synthesize TiO2/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 TiO2 nanoparticles and carbon fiber. The structure, morphology, and optical properties of the prepared materials were studied. The photocatalytic activity of TiO2, CF, and synthetic composites in the degradation of methylene blue dyes was investigated under ultraviolet (UV) light in a comparative study. The 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.
2. Materials and Methods
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. High-purity deionized water was used throughout all experiments.
2.2. Synthesis of Pure TiO2 Powder
The pure TiO2 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. Then, 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. The 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. The pure TiO2 powder was obtained.
2.3. Preparation of CF
The polyaniline fiber was calcined at 400°C in a N2 atmosphere for 2 h with a heating ramp of 5°C/min, and it was ground and stored.
2.4. Synthesis of TiO2/CF Composites
First, 0.1 g CF was dispersed in a mixture of 20.0 mL ethanol and 5.0 mL DI water. Then, a certain amount of TiO2 (0.05 g, 0.1 g, 0.2 g, 0.3 g, and 0.4 g) was added and the suspension was left for 2 h under stirring. After that, the suspension was dried for 24 h at 80°C. Hence, the as-prepared composites were named TiO2/CFx, where x represented the total weight ratio of TiO2 to CF. Thus, the composites were labeled as follows: TiO2/CF0.5/1, TiO2/CF1/1, TiO2/CF2/1, TiO2/CF3/1, TiO2/CF4/1.
The X-ray powder diffraction (XRD) patterns of TiO2 powder, CF, and the synthetic composites were acquired using a Bruker D8 A25 diffractometer (Cu Kα radiation (λ = 1.5406 Å), operated at 40 mA and 40 kV) in the 2θ range of 10–90° at a scanning speed of 10°/min. Morphology of TiO2 powder, CF, and the prepared TiO2/CF composites were studied using a scanning electron microscope (SEM). Element analysis of the prepared TiO2/CF composites was studied using an energy dispersive spectroscopy (EDS). The UV-visible diffuse reflectance spectroscopy (UV-vis DRS) and photoluminescence (PL) spectrum of TiO2 powder, CF, and the synthetic composites were measured by multifunctional optical fiber spectrometer (QE65 Pro, Ocean Optics, USA). The X-ray photoelectron spectroscopy (XPS) of TiO2 powder, CF, and the synthetic composites were measured by an X-ray photoelectron spectrometer (Thermo escalab 250X).
2.6. Photocatalytic Activity Test
The photocatalytic activity of the photocatalysts was tested by photocatalytic degradation of MB. The photocatalytic tests were carried out under UV-light irradiation at room temperature in an air atmosphere. The TiO2/CF composites were invited as photocatalysts. The UV light source was generated by a 300 W Mercury lamp. The 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. The degradation rate was calculated by C/C0, where C was the concentration of MB in each time period, and C0 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. Then, 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 (O2·−) scavenger, and hydroxyl radical (•OH) scavenger, respectively. The right amount of scavenger was added to the MB dye aqueous solution to probe the active species through variation of degradation rate.
3. Results and Discussion
3.1. Structural Properties
The XRD patterns of TiO2 powder, CF, and the prepared TiO2/CF composites are shown in Figure 1. The TiO2 powder, Figure 1, is composed of anatase as the dominant phase. The main characteristic peaks of TiO2 anatase at 2theta 25.3°, 37.8°, and 48° are related to (101), (004), and (200) crystal planes, respectively (JCPDS card No. 21–1272) . In addition, other relevant peaks appear at 55° (211) and 62.6° (204) . It is generally known that TiO2 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, TiO2 with anatase crystal form has a low dielectric constant, low mass density, and high electron mobility . For CF, the broad peaks at 2θ = 17° and 43° are designated as characteristic peaks of carbon . For the TiO2/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. This is mainly due to the coverage of anatase crystals on the carbon . In short, a series of composites with anatase crystals were successfully synthesized, indicating that the composites have high photocatalytic activity.
3.2. Morphological Characterization
The surface morphology of TiO2 powder (a), CF (b), and the prepared TiO2/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 TiO2 particles was about 100–200 nm. TiO2 were spherical particles and they were agglomerated. At the same time, it could be seen that TiO2 was dispersed on CF. The elemental composition of the TiO2/CF composite was characterized by EDS as shown in Figure 2(d). It could be seen from Figure 2(d) that the prepared TiO2/CF composite contains Ti, O, and C elements, which were consistent with the results of XPS analysis.
3.3. Optical Properties
The research on the optical properties of TiO2 powder, CF, and the prepared TiO2/CF composites is one of the most important parameters affecting the applications of synthetic products. The UV-vis light absorption spectra of TiO2 powder, CF, and the prepared TiO2/CF composites are shown in Figure 3. All samples show absorption bands in the visible light region of the spectrum. The 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 . In addition, the absorbance in the visible light band increased as the ratio of CF to TiO2 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 . 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 . Moreover, it was noted that the intensity of the emission band decreased significantly as the content of TiO2 increases. According to reports, the decrease of PL intensity was mainly due to three factors : (1) in the presence of TiO2/CF, photogenerated electrons can be efficiently transferred from TiO2 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. The substantial decrease in the intensity of the PL peak indicates that CF provides the potential to inhibit the recombination of photoelectrons and holes.
3.4. Chemical Compositions
In order to study the chemical composition and interaction between TiO2 and CF, XPS analysis was used, as shown in Figure 5.
Figure 6 shows the high-resolution XPS spectrum of the C 1s region. The 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 . 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 . The C 1s spectrum of TiO2/CF2/1 is shown in Figure 6(c). The peak intensity at 286.1 eV increases significantly, indicating that C-O bonds were formed in TiO2/CF2/1. Carbon atoms were bonded to the interstitial positions of the TiO2 lattice .
Figure 7 shows the high-resolution XPS spectra of the O 1s region of TiO2 (a) and TiO2/CF2/1 (b). For pure TiO2, C-O and O-H bonds centered at 532.8 eV and 531.2 eV were found, respectively . The O 1s spectrum of TiO2/CF2/1 was shown in Figure 7(b). The peak intensities at 532.8 eV and 531.2 eV increase significantly, indicating that C-O and O-H bonds were formed in TiO2/CF2/1; that is, oxygen atoms were incorporated into the TiO2 crystal lattice the gap position .
Figure 8 shows the high-resolution XPS spectra of the N 1s region of CF (a) and TiO2/CF2/1 (b). For the CF in Figure 8(a), there were three corresponding characteristic peaks. The 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 . Therefore, 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 TiO2/CF2/1 as shown in Figure 8(b).
As shown in Figure 9, the formation of C-Ti bonds and N-Ti bonds in TiO2/CF2/1 also could be further checked and confirmed by analyzing the Ti 2p core level of XPS. For pure TiO2, Ti 2p3/2 and Ti 2p1/2 with 458.8 eV and 464.5 eV as the centers were found, which were assigned to the Ti-O bond . In Figure 9(b), in addition to the two characteristic peaks of TiO2, which were 458.8 (Ti 2p3/2) and 464.5 eV (Ti 2p1/2), 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 . This indicates that there were N-Ti bonds and C-Ti bonds in TiO2/CF2/1.
Combined with the above analysis, two additional bonds (N-Ti bond and C-Ti bond) were formed in TiO2/CF2/1, which indicates that TiO2 nanoparticles were chemically bonded to CF in TiO2/CF2/1.
3.5. Photocatalytic Activity
Figure 10(a) shows the photocatalytic performance of TiO2 powder, CF, and the prepared TiO2/CF composites based on MB degradation under UV light irradiation. The results show that pure TiO2 has photocatalytic activity under UV light irradiation. Moreover, all TiO2/CF composites exhibit excellent photocatalytic activity, and their activity was higher than that of a single component. When the mass ratio of TiO2/CF was 2 : 1 (TiO2/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/C0) = kappt. Figure 10(b) shows the linear relationship between ln (C/C0) and time, where C/C0 was normalized to the MB concentration, t was the reaction time, and k was the reaction rate constant (10−3 min−1). Figure 10(c) shows the reaction rate constants for TiO2 powder, CF, and the prepared TiO2/CF composites. The rate constant of TiO2/CF2/1 was calculated to be 0.03289 min−1, which was 2.9 times and 4.7 times higher than that of TiO2 and CF, respectively. Therefore, 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. The by-products were always adsorbed on the surface active sites of the photocatalyst. The photocatalytic activity will drop sharply after a short period of exposure time. In order to evaluate the stability of the prepared TiO2/CF2/1, a cycle experiment was carried out under the same conditions. According to the results shown in Figure 10(d), TiO2/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 TiO2/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. The 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. This indicates other active sites of the photocatalyst for MB adsorption. Using CF to functionalizing TiO2 into composites will increase the adsorption capacity of TiO2 because of its extra surface area.
Although the physical and chemical interaction between MB and TiO2/CF2/1 may increase its surface coverage, after reaching the optimal dose, its particles will gather in TiO2/CF2/1 clusters, shielding UV light from interacting with the active surface of the photocatalyst. Therefore, 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. Therefore, 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 TiO2 and CF was the main factor for improving the photocatalytic activity. As shown in Scheme 1, when the TiO2/CF composites were irradiated with UV light, electrons were excited from the VB to the CB of TiO2 and CF, leaving holes in the VB. Since the CB of CF was much higher than that of TiO2, and the VB of CF was located at a higher position than that of TiO2, the excited electrons were transferred from the CB of CF to the CB of TiO2, and the holes migrate in the opposite direction. Moreover, CF loading with TiO2 could facilitate charge transport, hinder the recombination of electron-hole pairs, and further increase the number of charge carriers in the reaction . Therefore, the TiO2/CF composites exhibit better photocatalytic activity.
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 O2 in CB of TiO2. And •OH was formed by the further reaction of O2·−, e−, and 2H+. The photocatalytic process could be represented by the following process: TiO2/CF composites + UV ⟶ CF (h+) + TiO2 (e−) O2 + e− ⟶ O2·− O2·− + e− + 2H+ ⟶ H2O2; H2O2 + e− ⟶ •OH + OH− + O2 O2·−/•OH/h+ + MB ⟶ CO2 + H2O/degraded products
In summary, the TiO2/CF composites with high photocatalytic activity were successfully synthesized by a simple method at room temperature. The MB photodegradation performance of TiO2/CF composites was better than that of TiO2 or CF under the same conditions in the photocatalytic system under UV light irradiation. And the optimum mass ratio of TiO2 to CF was 2 : 1. The stability test of the photocatalyst shows that TiO2/CF2/1 had high stability and the photodegradation efficiency after the fifth cycle was 92%. SEM showed that TiO2 particles grew well on the surface of CF. XPS shows that TiO2 particles were chemically bonded to CF, which was confirmed by the formation of C-Ti bonds and N-Ti bonds. This chemical bonding structure was conducive to the formation of an effective heterojunction, which might lead to the synergistic combination of TiO2 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 TiO2/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. The existence of these free radicals was confirmed by free radical suppression tests using different scavengers for different free radicals. This study shows that simple TiO2/CF composites were efficient and promising photocatalysts for dye degradation in water.
All data used to support this study are included within the paper.
Conflicts of Interest
The authors declare no conflicts of interest.
Hao Cheng and Wenkang Zhang authors contributed equally to this work.
This work was supported by the National Natural Science Foundation of China (no. 21968005), the Opening Project of Guangxi Key Laboratory of Clean Pulp and Papermaking and Pollution Control (KF201812-4), and the High Levels of Innovation Team and Excellence Scholars Program in Colleges of Guangxi.
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