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TiO2-loaded carbon fiber: Microwave hydrothermal synthesis and photocatalytic activity under UV light irradiation
Journal of Physics and Chemistry of Solids 136 (2020) 109138
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Journal of Physics and Chemistry of Solids journal homepage: www.elsevier.com/locate/jpcs
TiO2-loaded carbon fiber: Microwave hydrothermal synthesis and photocatalytic activity under UV light irradiation
T
Zhudan Chu, Linlin Qiu, Yue Chen, Zhishan Zhuang, Pingfan Du*, Jie Xiong** Silk Institute, College of Materials and Textile, Zhejiang Sci-Tech University, Hangzhou, 310018, PR China
ARTICLE INFO
ABSTRACT
Keywords: Microwave hydrothermal TiO2 Carbon fiber Photocatalytic activity Recycling
Recycling and rapid synthesis of photocatalysts are of significant importance in wastewater degradation. Herein, a systemic strategy was developed, employing loaded nano-TiO2 on carbon fiber (CF) utilizing microwave hydrothermal method for the fast and efficient preparation of photocatalysts. The prepared TiO2/CF was characterized using a variety of analytical techniques, including scanning electron microscopy (SEM), X-ray diffraction (XRD), energy dispersive spectrometer (EDS), and UV–vis. Compared with pure TiO2 particles, TiO2/CF is easily recycled when employed as a photocatalyst. Nitric acid oxidation treatment of CF generates polar functional groups, which improves the bonding properties between TiO2 and CF. With increasing CF treatment time between 0 and 4 h, the loading rate of TiO2 increases, until TiO2/CF produces optimum photocatalytic activity. Moreover, the experimental results indicate that TiO2/CF possesses favorable photocatalytic activity, achieving 97% degradation rate of Rhodamine B within 1 h of UV light irradiation. Furthermore, even after 10 experimental cycles the photocatalyst still generates 88% degradation rate.
1. Introduction The continuous demand for safe drinking water is of critical importance for all life, this is aspirated by escalating industrial pollution, population growth, climate change, etc. Thus, studies on wastewater treatment and purification have become a hot topic worldwide [1–4]. Organics are the governing pollutants in wastewater, which are extremely difficult to degrade naturally. Furthermore, dyes and pigments account for a large portion of organic pollutants. According to reports, the outstanding global consumption of dyes and pigments is 7 × 105 tons yearly, with approximately two-thirds originating from textile industries [5]. Dyes in wastewater have a complex aromatic structure, which attributes to their degradability into non-toxic materials. Traditional biological and physical treatment methods for removing organic pollutants, which include precipitation, adsorption, flocculation, reverse osmosis and ultrafiltration, are inefficient and unsuitable for industrial applications [6–8]. Therefore, an alternative, effective, lowcost and environmentally friendly method is highly desirable. Advanced oxidation processes (AOPs) are the most promising treatment technologies for the degradation of hazardous pollutants [9–11]. Upon exposure to high temperature, high pressure, electricity, sound, light, catalyst, etc. strong oxidizing hydroxyl radicals (•OH) are produced, allowing AOPs to rapidly oxidize and degrade almost all organic *
pollutants, making them of particular interest to researchers over the past few decades. Since the 70's photocatalytic degradation has been a staple in AOP technology, with a multitude of articles depicting the efficiency of the method in the decomposition of various organic pollutants [12–15]. Semiconductor photocatalyst is the core of photocatalytic technology, in which its band gap, between the valence band (VB) and conduction band (CB), differs from metal [16–19]. Substances possessing oxidative potential above VB can oxidize via photogenerated holes, whereas those having reductive potential below CB can reduce by photogenerated electrons. Although electrons and holes (photogenerated carriers) undergo multiple paths of change after they are produced, the most impactful competing processes on the photocatalytic reaction process are electron-hole capture and recombination. Among various photocatalysts, titanium dioxide (TiO2), discovered by Japanese scientists Fujihima and Honda, is widely used owing to its mild reaction conditions, simple reaction equipment, easy to control operation and highly efficient, and stable, excellent chemical properties, etc. [20–22]. However, recycling pure TiO2 particles is difficult for practical applications, which may cause secondary pollution [23]. Thus, preloading of TiO2 particles on inorganic fibers is an alternative approach. CF is an inorganic polymer fiber with carbon content exceeding 90%. It is also generated via carbonization of the organic
Corresponding author. Corresponding author. E-mail addresses: [email protected] (P. Du), [email protected] (J. Xiong).
**
https://doi.org/10.1016/j.jpcs.2019.109138 Received 28 May 2019; Received in revised form 17 July 2019; Accepted 6 August 2019 Available online 07 August 2019 0022-3697/ © 2019 Elsevier Ltd. All rights reserved.
Journal of Physics and Chemistry of Solids 136 (2020) 109138
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matrix fiber under inert atmosphere at temperatures 1000–3000 °C, and possesses good corrosion resistance, high adsorption capacity and high tensile strength. Therefore, CF is an ideal carrier for loading TiO2 particles. The hydrothermal method is an efficient approach for generating well-crystallized and dispersed nanoparticles. However, the main drawback with this method is the required high temperature and timeconsuming process. For example, Xie et al. reported a 3 h reaction time for the preparation of TiO2/ACF material by hydrothermal method. Whereas, Abdalla et al. reported a 6 h reaction time for the preparation of TiO2 nanorods coated onto nylon 6 nanofibers [24,25]. The microwave hydrothermal method is a fast and efficient synthetic approach compared to the classic hydrothermal method. Over the past two decades, microwave assisted technologies have gained considerable attention for heating chemical reactions, and have been successfully employed in multiple areas such as material sciences, nanotechnology, polymer chemistry, and biochemical processes [26–28]. In this study, TiO2 particles are successfully loaded on CF by microwave hydrothermal method using CF as a carrier material, which effectively improves the recycle rate and reduces the economic cost of recovering TiO2 particles. We are convinced that our research will support a new route for microwave-assisted preparation of nanomaterials with high surface area, high crystallization degree and high photocatalytic ability.
dark for 30 min to gain good dispersion and establish an adsorption–desorption equilibrium between the photocatalyst and dye. Then the photocatalytic activity of TiO2/CF was evaluated toward Rhodamine B (RhB) degradation in aqueous solution with a concentration of 10 mg/L under UV light irradiation (500 W high pressure mercury lamp). More specifically, TiO2/CF (1.5 g) was added into a photocatalytic reactor containing aqueous solution of RhB (60 mL). Direct photolysis tests were undertaken in the absence of TiO2/CF using UV irradiation and then sampled with a pipette every 10 min. Finally, the degradation rate was calculated by ultraviolet–visible (UV–vis) analysis and the absorbance of RhB solution. In addition, the recycling experiments of RhB degradation on TiO2/CF were also performed under the same conditions. After each experimental cycle, TiO2/CF was separated from the RhB solution, washed with distilled water and dried at 60 °C for 4 h, then weighed and the weight loss recorded. The stability of TiO2/CF was depicted by the weight loss after each experiment and SEM images after multiple cycles. All of these measurements were carried out at room temperature. 3. Results and discussion The surface morphologies of CF are illustrated in Fig. 1a–c. Fig. 1a shows that the surface of pure CF is smooth, ca. 7 μm in diameter. Fig. 1b demonstrates that a part of the organic protective glue on the surface of CF is removed by acetone, but acetone does not react with the fiber, so the fiber diameter is not significantly reduced and its surface remains smooth [29]. Fig. 1c shows an increase in roughness of CF and decrease in diameter (ca. 6.3 μm) after HNO3 treatment, which is a common liquid phase oxidation method [30]. After HNO3 treatment, the oxygen-containing functional groups and roughness of CF greatly increase, which benefits the bonding force between CF and other materials. In order to further establish the oxidation process, CFs were investigated using EDS and dispersion state in water. As shown in Fig. 1d–f, untreated CF displays organic protective glue that exhibits elements not of CF (Na, Pt and Cl). Furthermore, CF cannot be dispersed in water due to the organic protective glue making CF adhere to each other (Fig. 1d). Removal of some impurities is achieved by reacting with acetone, with the majority of CF being separated in water (Fig. 1e). The organic protective glue is removed post oxidation treatment with HNO3. The energy spectrum image shows that the fiber contains only carbon and oxygen elements, and can be completely dispersed in water (Fig. 1f). The dispersed fibers are beneficial to increase the contact area with the TiO2 precursor solution and improve the loading rate. The oxidation treatment time of CF is an important factor affecting TiO2 loading rate, which requires a satisfactory roughness to obtain sufficient interactions between fiber and TiO2. The surface SEM images of CF after oxidation treatment at various times are shown in Fig. 2a–e (a-1h, b-2h, c-3h, d-4h, e−5h). As the oxidation treatment time increases, the etching effect on the surface of CF is gradually strengthened. When the treatment time is not sufficient, the etching degree of CF is shallow. In contrast, the fiber imposes serious damage after prolonged oxidation treatment time. Hence, 4 h is the optimum oxidation treatment time for the apparent etching of the surface, and the axial grooves of the distribution are widened and deepened. The oxidation treatment increases surface area and oxygen-containing functional groups of the fiber, which is more favorable for TiO2 loading. Fig. 2f shows the FTIR spectra of untreated and HNO3 treated CF. It is clear that the surface of CF displays absorption peaks of –OH at 3442 and 1640 cm−1, and –COOH at 1710 cm−1 after HNO3-treatment. The absorption peaks at 2917 and 2850 cm−1 (-CH) after HNO3-treatment are reduced, whereas the peak at 1710 cm−1 (-COOH) increases. This indicates that HNO3 oxidization of CF generates oxygen-containing polar groups such as –OH and –COOH on the surface. These functional groups are advantageous for increasing the bonding force between the fibers and other materials in the base. The produced –COOH group on the surface of CF is coordinated with Ti4+ in TiO2 precursor solution. TiO2
2. Experimental details 2.1. Materials Titanium sulfate (TiSO4), urea, ethylenediaminetetraacetic acid disodium salt (EDTA-2Na), acetone, concentrated nitric acid (HNO3) and sulfuric acid (H2SO4) were purchased from Aladdin and used without further purification. The CF was purchased from Weihai Guangwei Composite Material Co., Ltd. All reagents were of analytical grade. 2.2. Preparation of TiO2/CF materials First, CF was placed into a round bottom flask and acetone (150 mL) was added. The mixture was heated at 60 °C for 15 min, and then refluxed for 4 h at 115 °C in an oil bath with HNO3 (300 mL, 65%–68%). The fibers were then immersed in distilled water for 12 h and dried at 70 °C. The precursor solution was prepared by adding titanium sulfate (0.24 g) to distilled water (15 mL) with rapid stirring. Then urea (0.14 g) and EDTA-2Na (0.37 g) were added, and the mixture stirred at room temperature for 4 h. Subsequently, the treated fibers (0.2 g) were mixed with the precursor solution in the microwave reactor. The temperature of microwave digestion instrument was kept constant at 200 °C for 30 min. The fibers were rinsed carefully with distilled water and dried at 70 °C for 4 h. Finally, TiO2/CF was obtained after being heattreated at 450 °C for 60 min with a heating rate of 5 °C/min. 2.3. Characterization The shape and crystal structure of the resulting anatase TiO2/CF was investigated by X-ray diffraction (XRD) spectroscopy and spectral pattern. The surface morphologies of oxidation treated and untreated CF were characterized by field emission scanning electron microscopy (FESEM, Hitachi/S4800). The elemental changes in CF surface were analyzed by energy dispersive spectrometer (EDS). The material degradation rate was taken using an ultraviolet–visible (UV–vis) spectrophotometer (MAPADA PC1800) equipped with quartz cuvette. 2.4. Photocatalytic activity test Prior to the photocatalytic reaction, the mixture was kept in the 2
Journal of Physics and Chemistry of Solids 136 (2020) 109138
Z. Chu, et al.
Fig. 1. FESEM image of CF (a) before treatment; (b) after acetone treatment; (c) after acetone and HNO3 oxidation treatment; EDS image of CF (d) before treatment; (e) after acetone treatment; (f) after acetone and HNO3 oxidation treatment.
is chemically coordinated to the surface of the fiber, which is not embedded on the fiber carrier, and effectively improves the utilization of TiO2. Fig. 3a shows the curve of different oxidation treatment times and the corresponding loading rate of nano-TiO2. By increasing the oxidation treatment time, the loading rate of nano-TiO2 initially increases but after 4 h (27.9%) a decline in loading rate is observed. The average loading rate of TiO2 on CF is directly observed in Fig. 3b. To compare the photocatalytic activities of TiO2-CF with different treatment times, photodegradation experiments of RhB dye were conducted. The sample (1.5 g) was added to RhB solution (60 mL, 10 mg/L) under various photocatalytic conditions. As shown in Table 1, the concentration of RhB solution was 9.8 mg/L according to UV–vis analysis after 1 h CF adsorption in the absence of light. This indicates that the adsorption capacity of CF has little influence on the final degradation effect. When CF oxidation treatment time is between 0 and 4 h, the concentration of RhB decreases under UV light irradiation. However, after 4 h an increase in RhB concentration is observed. In
combination with the results displayed in Fig. 3, we determined that the optimum CF loading for TiO2 is achieved after 4 h CF oxidation treatment time and produces the greatest photocatalytic performance. Fig. 4a–d shows the representative scanning electron microscope (SEM) images of TiO2/CF (Fig. 4b–d show the fiber without any treatment) prepared by microwave hydrothermal method. Fig. 4a shows that TiO2 is tightly agglomerated onto the fiber and difficult to drop (Fig. 4c) after 12 h. Furthermore, TiO2 is only present on the fiber's surface and not bound to the fiber (Fig. 4b). Thereby, TiO2 is very easy to drop from CF after setting for 12 h (Fig. 4d). Fig. 5 shows a schematic illustration of microwave hydrothermal synthesis of TiO2/CF including CF treatment. Post oxidation treatment CF displays a rough surface, which is conducive toward tighter combination of TiO2 and CF. When the fibers are not subjected to oxidation treatment, TiO2 particles, although only distributed on the surface, are not firmly bound. Fig. 6a displays the corresponding XRD pattern of pure CF and synthesized TiO2/CF. TiO2/CF crystallographic structure was confirmed by XRD analysis (Fig. 6a). As shown in Fig. 6a, the diffraction peaks at
Fig. 2. (a–e) FESEM images of CF at different oxidation treatment times (1–5 h); (f) FTIR spectra of untreated and HNO3-treated CF. 3
Journal of Physics and Chemistry of Solids 136 (2020) 109138
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Fig. 3. (a) Loading rate of TiO2 on CF at different oxidation treatment times; (b) average loading rate of TiO2 on CF after 4 h oxidation treatment.
the following degradation routes: blank test (only RhB solution) and photodegradation by TiO2/CF, under UV light irradiation for 1 h. Fig. 7a shows that the absorption spectrum of TiO2/CF significant absorption region at 554 nm for 0, 10, 20, 30, 40, 50 min and 1 h. The photodegradation degradation efficiency of RhB after 1 h UV light irradiation is 97%. The photodegradation degradation efficiency η of RhB was calculated using the following formula:
Table 1 The photocatalytic activity of CF, TiO2-CF after different oxidation treatment times. Condition
Sample
oxidation treatment time(h)
RhB concentration (mg/L)
dark condition under UV light irradiation
CF TiO2-CF TiO2-CF TiO2-CF TiO2-CF TiO2-CF TiO2-CF
0 0 1 2 3 4 5
9.8 8.9 6.4 4.3 2.8 0.3 1.4
=
C0
Ct Co
*100%
(1)
where C0 is the initial concentration of dye and Ct is the concentration of RhB solution after photodegradation. The photodegradable curve demonstrates that degradation in the absence of TiO2/CF is extremely slow under UV light irradiation. Compared with the photocatalytic performance of pure TiO2 and TiO2/CF (the weight of TiO2 is the same) in Fig. 7b, the concentration of RhB after pure TiO2 l h degradation is 1.6 mg/L. Using the formula of Eq. (1), the efficiency of photocatalytic degradation of RhB by pure TiO2 was 84%. Hence, the photocatalytic performance of TiO2/CF is better. This is due to C in CF materials acting as electron traps that effectively promote separation of photogenerated electrons and holes, and significantly improve photocatalytic activity [33,34]. Moreover, even after 10 min the RhB solution showed a
2θ = 25.6°, 38.2°, 48.2°, 54.1°, 55.3°, 63.0°, 69.0°, 70.5°, and 75.3° are, respectively, assigned to the reflections of the (101), (004), (200), (105), (211), (204), (116), (220), and (215) crystal planes of TiO2. All observed XRD peaks are ascribed to pure anatase crystalline phase according to No. 21–1272 JCPDS card [31,32]. EDS spectrum of TiO2/CF further demonstrates that TiO2 is successfully loaded on CF (Fig. 6b). The photocatalytic activity of TiO2/CF was evaluated using RhB concentration 10 mg/L and TiO2/CF dosage 1.5 g and compared with
Fig. 4. (a) FESEM image of TiO2/CF after oxidation treatment; (b) FESEM image of TiO2/CF without oxidation treatment; (c) FESEM image of TiO2/CF after oxidation treatment and allowed to stand at room temperature for 12 h; (d) FESEM image of TiO2/CF without oxidation treatment and allow to stand at room temperature for 12 h.
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Journal of Physics and Chemistry of Solids 136 (2020) 109138
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Fig. 5. Schematic illustration of microwave hydrothermal synthesis of TiO2/CF with CF treatment.
Fig. 6. (a) XRD patterns of pure CF and TiO2/CF; (b) energy spectrum of TiO2/CF.
Fig. 7. (a) UV–vis absorption spectra of RhB solutions using TiO2/CF under UV light irradiation for different times; (b) photodegradation efficiency toward RhB when using TiO2/CF; (c) comparison photo of RhB and blank group after degradation.
Fig. 8. (a) Weight loss after multiple degradation, (b) degradation of RhB with TiO2/CF in different recycling runs under UV light irradiation for 1 h, (c) FESEM image of TiO2/CF after the fifth cycle experiment. 5
Journal of Physics and Chemistry of Solids 136 (2020) 109138
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lightning of color, revealing the photocatalytic speed of the method. Prolonging the time (1 h) results in an almost colorless solution. In the absence of TiO2/CF no significant color change is observed, giving further precedence to the photocatalytic effectiveness of the method. Consequently, Fig. 7a–c further confirms TiO2/CF photocatalytic excellence. According to the reaction mechanism, the photocatalytic degradation of organic substances in the presence of TiO2 is essentially a free radical reaction [35]. The electron transfer process of RhB under UV light irradiation is presented as Eqs. (2)–(6) [36]:
RhB + hv
RhB * + O2
RhB+
(3)
+ TiO2 (e )
+ O2
(4)
TiO2 + O2
(5)
RhB+
TiO2 (e ) + O2
RhB *
(2)
RhB*
RhB * + TiO2
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Rhoda min e
(6)
products *
Excited RhB molecules (RhB ) can inject electrons into the conduction band of TiO2 based on energy matching between TiO2 and RhB, and cationic dye radicals (RhB+) can form (Eq. (3)). The same electron transfer can also occur between RhB* and adsorbed oxygen (Eq. (4)) and between TiO2 and adsorbed oxygen (Eq. (5)) [37]. To further study the catalytic and structural stability of TiO2/CF composite fibers, cyclic photocatalytic experiments were carried out under identical conditions (Fig. 8a–c). After every recycling experiment, recycled TiO2/CF was washed and dried at 60 °C for 4 h, and once again subjected to the photoreaction. Fig. 8a shows the weight loss curve of TiO2/CF after each experimental cycle. After the fifth cycle experiment, the loading quality of TiO2 showed little variation. Fig. 8b shows slight reduction in the photocatalytic efficiency of recycled TiO2/ CF with increasing cycling times, but even after the tenth cycle the degradation rate remains at 88%. SEM analysis of TiO2/CF composite fibers after the catalytic tests were employed to evaluate if TiO2 particles undergo separation from CF. The obtained results (Fig. 8c) were compared with those displayed in Fig. 3c, with no apparent differences being detected other than the agglomeration of TiO2. It shows that TiO2 is partially stripped from CF but remains strongly bound to CF after several photocatalytic processes while maintaining high photocatalytic properties. 4. Conclusion In summary, TiO2/CF photocatalytic material was successfully prepared by microwave hydrothermal method at 200 °C for 30 min, which exhibits superior photocatalytic performance for the degradation of RhB dye. The degradation rate of TiO2/CF was 97% under UV light irradiation after 1 h. After ten cycles TiO2/CF photocatalytic activity was more than 86%, emphasizing its reusability. Our study contributes to a better understanding, as well as providing a strong foundation for energy efficient and greener methodologies for wastewater degradation. Acknowledgments This work was supported by Zhejiang Provincial Natural Science Foundation of China (LY18F050011), the Applied Basic Research Project of China National Textile and Apparel Council (J201801), and the Fundamental Research Funds of Zhejiang Sci-Tech University (2019Q011). References [1] L. Chen, L. Wang, X. Wu, A process-level water conservation and pollution control performance evaluation tool of cleaner production technology in textile industry, J. Clean. Prod. 143 (2017) 1137–1143.
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TiO2-loaded carbon fiber: Microwave hydrothermal synthesis and photocatalytic activity under UV light irradiation
T
Zhudan Chu, Linlin Qiu, Yue Chen, Zhishan Zhuang, Pingfan Du*, Jie Xiong** Silk Institute, College of Materials and Textile, Zhejiang Sci-Tech University, Hangzhou, 310018, PR China
ARTICLE INFO
ABSTRACT
Keywords: Microwave hydrothermal TiO2 Carbon fiber Photocatalytic activity Recycling
Recycling and rapid synthesis of photocatalysts are of significant importance in wastewater degradation. Herein, a systemic strategy was developed, employing loaded nano-TiO2 on carbon fiber (CF) utilizing microwave hydrothermal method for the fast and efficient preparation of photocatalysts. The prepared TiO2/CF was characterized using a variety of analytical techniques, including scanning electron microscopy (SEM), X-ray diffraction (XRD), energy dispersive spectrometer (EDS), and UV–vis. Compared with pure TiO2 particles, TiO2/CF is easily recycled when employed as a photocatalyst. Nitric acid oxidation treatment of CF generates polar functional groups, which improves the bonding properties between TiO2 and CF. With increasing CF treatment time between 0 and 4 h, the loading rate of TiO2 increases, until TiO2/CF produces optimum photocatalytic activity. Moreover, the experimental results indicate that TiO2/CF possesses favorable photocatalytic activity, achieving 97% degradation rate of Rhodamine B within 1 h of UV light irradiation. Furthermore, even after 10 experimental cycles the photocatalyst still generates 88% degradation rate.
1. Introduction The continuous demand for safe drinking water is of critical importance for all life, this is aspirated by escalating industrial pollution, population growth, climate change, etc. Thus, studies on wastewater treatment and purification have become a hot topic worldwide [1–4]. Organics are the governing pollutants in wastewater, which are extremely difficult to degrade naturally. Furthermore, dyes and pigments account for a large portion of organic pollutants. According to reports, the outstanding global consumption of dyes and pigments is 7 × 105 tons yearly, with approximately two-thirds originating from textile industries [5]. Dyes in wastewater have a complex aromatic structure, which attributes to their degradability into non-toxic materials. Traditional biological and physical treatment methods for removing organic pollutants, which include precipitation, adsorption, flocculation, reverse osmosis and ultrafiltration, are inefficient and unsuitable for industrial applications [6–8]. Therefore, an alternative, effective, lowcost and environmentally friendly method is highly desirable. Advanced oxidation processes (AOPs) are the most promising treatment technologies for the degradation of hazardous pollutants [9–11]. Upon exposure to high temperature, high pressure, electricity, sound, light, catalyst, etc. strong oxidizing hydroxyl radicals (•OH) are produced, allowing AOPs to rapidly oxidize and degrade almost all organic *
pollutants, making them of particular interest to researchers over the past few decades. Since the 70's photocatalytic degradation has been a staple in AOP technology, with a multitude of articles depicting the efficiency of the method in the decomposition of various organic pollutants [12–15]. Semiconductor photocatalyst is the core of photocatalytic technology, in which its band gap, between the valence band (VB) and conduction band (CB), differs from metal [16–19]. Substances possessing oxidative potential above VB can oxidize via photogenerated holes, whereas those having reductive potential below CB can reduce by photogenerated electrons. Although electrons and holes (photogenerated carriers) undergo multiple paths of change after they are produced, the most impactful competing processes on the photocatalytic reaction process are electron-hole capture and recombination. Among various photocatalysts, titanium dioxide (TiO2), discovered by Japanese scientists Fujihima and Honda, is widely used owing to its mild reaction conditions, simple reaction equipment, easy to control operation and highly efficient, and stable, excellent chemical properties, etc. [20–22]. However, recycling pure TiO2 particles is difficult for practical applications, which may cause secondary pollution [23]. Thus, preloading of TiO2 particles on inorganic fibers is an alternative approach. CF is an inorganic polymer fiber with carbon content exceeding 90%. It is also generated via carbonization of the organic
Corresponding author. Corresponding author. E-mail addresses: [email protected] (P. Du), [email protected] (J. Xiong).
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https://doi.org/10.1016/j.jpcs.2019.109138 Received 28 May 2019; Received in revised form 17 July 2019; Accepted 6 August 2019 Available online 07 August 2019 0022-3697/ © 2019 Elsevier Ltd. All rights reserved.
Journal of Physics and Chemistry of Solids 136 (2020) 109138
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matrix fiber under inert atmosphere at temperatures 1000–3000 °C, and possesses good corrosion resistance, high adsorption capacity and high tensile strength. Therefore, CF is an ideal carrier for loading TiO2 particles. The hydrothermal method is an efficient approach for generating well-crystallized and dispersed nanoparticles. However, the main drawback with this method is the required high temperature and timeconsuming process. For example, Xie et al. reported a 3 h reaction time for the preparation of TiO2/ACF material by hydrothermal method. Whereas, Abdalla et al. reported a 6 h reaction time for the preparation of TiO2 nanorods coated onto nylon 6 nanofibers [24,25]. The microwave hydrothermal method is a fast and efficient synthetic approach compared to the classic hydrothermal method. Over the past two decades, microwave assisted technologies have gained considerable attention for heating chemical reactions, and have been successfully employed in multiple areas such as material sciences, nanotechnology, polymer chemistry, and biochemical processes [26–28]. In this study, TiO2 particles are successfully loaded on CF by microwave hydrothermal method using CF as a carrier material, which effectively improves the recycle rate and reduces the economic cost of recovering TiO2 particles. We are convinced that our research will support a new route for microwave-assisted preparation of nanomaterials with high surface area, high crystallization degree and high photocatalytic ability.
dark for 30 min to gain good dispersion and establish an adsorption–desorption equilibrium between the photocatalyst and dye. Then the photocatalytic activity of TiO2/CF was evaluated toward Rhodamine B (RhB) degradation in aqueous solution with a concentration of 10 mg/L under UV light irradiation (500 W high pressure mercury lamp). More specifically, TiO2/CF (1.5 g) was added into a photocatalytic reactor containing aqueous solution of RhB (60 mL). Direct photolysis tests were undertaken in the absence of TiO2/CF using UV irradiation and then sampled with a pipette every 10 min. Finally, the degradation rate was calculated by ultraviolet–visible (UV–vis) analysis and the absorbance of RhB solution. In addition, the recycling experiments of RhB degradation on TiO2/CF were also performed under the same conditions. After each experimental cycle, TiO2/CF was separated from the RhB solution, washed with distilled water and dried at 60 °C for 4 h, then weighed and the weight loss recorded. The stability of TiO2/CF was depicted by the weight loss after each experiment and SEM images after multiple cycles. All of these measurements were carried out at room temperature. 3. Results and discussion The surface morphologies of CF are illustrated in Fig. 1a–c. Fig. 1a shows that the surface of pure CF is smooth, ca. 7 μm in diameter. Fig. 1b demonstrates that a part of the organic protective glue on the surface of CF is removed by acetone, but acetone does not react with the fiber, so the fiber diameter is not significantly reduced and its surface remains smooth [29]. Fig. 1c shows an increase in roughness of CF and decrease in diameter (ca. 6.3 μm) after HNO3 treatment, which is a common liquid phase oxidation method [30]. After HNO3 treatment, the oxygen-containing functional groups and roughness of CF greatly increase, which benefits the bonding force between CF and other materials. In order to further establish the oxidation process, CFs were investigated using EDS and dispersion state in water. As shown in Fig. 1d–f, untreated CF displays organic protective glue that exhibits elements not of CF (Na, Pt and Cl). Furthermore, CF cannot be dispersed in water due to the organic protective glue making CF adhere to each other (Fig. 1d). Removal of some impurities is achieved by reacting with acetone, with the majority of CF being separated in water (Fig. 1e). The organic protective glue is removed post oxidation treatment with HNO3. The energy spectrum image shows that the fiber contains only carbon and oxygen elements, and can be completely dispersed in water (Fig. 1f). The dispersed fibers are beneficial to increase the contact area with the TiO2 precursor solution and improve the loading rate. The oxidation treatment time of CF is an important factor affecting TiO2 loading rate, which requires a satisfactory roughness to obtain sufficient interactions between fiber and TiO2. The surface SEM images of CF after oxidation treatment at various times are shown in Fig. 2a–e (a-1h, b-2h, c-3h, d-4h, e−5h). As the oxidation treatment time increases, the etching effect on the surface of CF is gradually strengthened. When the treatment time is not sufficient, the etching degree of CF is shallow. In contrast, the fiber imposes serious damage after prolonged oxidation treatment time. Hence, 4 h is the optimum oxidation treatment time for the apparent etching of the surface, and the axial grooves of the distribution are widened and deepened. The oxidation treatment increases surface area and oxygen-containing functional groups of the fiber, which is more favorable for TiO2 loading. Fig. 2f shows the FTIR spectra of untreated and HNO3 treated CF. It is clear that the surface of CF displays absorption peaks of –OH at 3442 and 1640 cm−1, and –COOH at 1710 cm−1 after HNO3-treatment. The absorption peaks at 2917 and 2850 cm−1 (-CH) after HNO3-treatment are reduced, whereas the peak at 1710 cm−1 (-COOH) increases. This indicates that HNO3 oxidization of CF generates oxygen-containing polar groups such as –OH and –COOH on the surface. These functional groups are advantageous for increasing the bonding force between the fibers and other materials in the base. The produced –COOH group on the surface of CF is coordinated with Ti4+ in TiO2 precursor solution. TiO2
2. Experimental details 2.1. Materials Titanium sulfate (TiSO4), urea, ethylenediaminetetraacetic acid disodium salt (EDTA-2Na), acetone, concentrated nitric acid (HNO3) and sulfuric acid (H2SO4) were purchased from Aladdin and used without further purification. The CF was purchased from Weihai Guangwei Composite Material Co., Ltd. All reagents were of analytical grade. 2.2. Preparation of TiO2/CF materials First, CF was placed into a round bottom flask and acetone (150 mL) was added. The mixture was heated at 60 °C for 15 min, and then refluxed for 4 h at 115 °C in an oil bath with HNO3 (300 mL, 65%–68%). The fibers were then immersed in distilled water for 12 h and dried at 70 °C. The precursor solution was prepared by adding titanium sulfate (0.24 g) to distilled water (15 mL) with rapid stirring. Then urea (0.14 g) and EDTA-2Na (0.37 g) were added, and the mixture stirred at room temperature for 4 h. Subsequently, the treated fibers (0.2 g) were mixed with the precursor solution in the microwave reactor. The temperature of microwave digestion instrument was kept constant at 200 °C for 30 min. The fibers were rinsed carefully with distilled water and dried at 70 °C for 4 h. Finally, TiO2/CF was obtained after being heattreated at 450 °C for 60 min with a heating rate of 5 °C/min. 2.3. Characterization The shape and crystal structure of the resulting anatase TiO2/CF was investigated by X-ray diffraction (XRD) spectroscopy and spectral pattern. The surface morphologies of oxidation treated and untreated CF were characterized by field emission scanning electron microscopy (FESEM, Hitachi/S4800). The elemental changes in CF surface were analyzed by energy dispersive spectrometer (EDS). The material degradation rate was taken using an ultraviolet–visible (UV–vis) spectrophotometer (MAPADA PC1800) equipped with quartz cuvette. 2.4. Photocatalytic activity test Prior to the photocatalytic reaction, the mixture was kept in the 2
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Fig. 1. FESEM image of CF (a) before treatment; (b) after acetone treatment; (c) after acetone and HNO3 oxidation treatment; EDS image of CF (d) before treatment; (e) after acetone treatment; (f) after acetone and HNO3 oxidation treatment.
is chemically coordinated to the surface of the fiber, which is not embedded on the fiber carrier, and effectively improves the utilization of TiO2. Fig. 3a shows the curve of different oxidation treatment times and the corresponding loading rate of nano-TiO2. By increasing the oxidation treatment time, the loading rate of nano-TiO2 initially increases but after 4 h (27.9%) a decline in loading rate is observed. The average loading rate of TiO2 on CF is directly observed in Fig. 3b. To compare the photocatalytic activities of TiO2-CF with different treatment times, photodegradation experiments of RhB dye were conducted. The sample (1.5 g) was added to RhB solution (60 mL, 10 mg/L) under various photocatalytic conditions. As shown in Table 1, the concentration of RhB solution was 9.8 mg/L according to UV–vis analysis after 1 h CF adsorption in the absence of light. This indicates that the adsorption capacity of CF has little influence on the final degradation effect. When CF oxidation treatment time is between 0 and 4 h, the concentration of RhB decreases under UV light irradiation. However, after 4 h an increase in RhB concentration is observed. In
combination with the results displayed in Fig. 3, we determined that the optimum CF loading for TiO2 is achieved after 4 h CF oxidation treatment time and produces the greatest photocatalytic performance. Fig. 4a–d shows the representative scanning electron microscope (SEM) images of TiO2/CF (Fig. 4b–d show the fiber without any treatment) prepared by microwave hydrothermal method. Fig. 4a shows that TiO2 is tightly agglomerated onto the fiber and difficult to drop (Fig. 4c) after 12 h. Furthermore, TiO2 is only present on the fiber's surface and not bound to the fiber (Fig. 4b). Thereby, TiO2 is very easy to drop from CF after setting for 12 h (Fig. 4d). Fig. 5 shows a schematic illustration of microwave hydrothermal synthesis of TiO2/CF including CF treatment. Post oxidation treatment CF displays a rough surface, which is conducive toward tighter combination of TiO2 and CF. When the fibers are not subjected to oxidation treatment, TiO2 particles, although only distributed on the surface, are not firmly bound. Fig. 6a displays the corresponding XRD pattern of pure CF and synthesized TiO2/CF. TiO2/CF crystallographic structure was confirmed by XRD analysis (Fig. 6a). As shown in Fig. 6a, the diffraction peaks at
Fig. 2. (a–e) FESEM images of CF at different oxidation treatment times (1–5 h); (f) FTIR spectra of untreated and HNO3-treated CF. 3
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Fig. 3. (a) Loading rate of TiO2 on CF at different oxidation treatment times; (b) average loading rate of TiO2 on CF after 4 h oxidation treatment.
the following degradation routes: blank test (only RhB solution) and photodegradation by TiO2/CF, under UV light irradiation for 1 h. Fig. 7a shows that the absorption spectrum of TiO2/CF significant absorption region at 554 nm for 0, 10, 20, 30, 40, 50 min and 1 h. The photodegradation degradation efficiency of RhB after 1 h UV light irradiation is 97%. The photodegradation degradation efficiency η of RhB was calculated using the following formula:
Table 1 The photocatalytic activity of CF, TiO2-CF after different oxidation treatment times. Condition
Sample
oxidation treatment time(h)
RhB concentration (mg/L)
dark condition under UV light irradiation
CF TiO2-CF TiO2-CF TiO2-CF TiO2-CF TiO2-CF TiO2-CF
0 0 1 2 3 4 5
9.8 8.9 6.4 4.3 2.8 0.3 1.4
=
C0
Ct Co
*100%
(1)
where C0 is the initial concentration of dye and Ct is the concentration of RhB solution after photodegradation. The photodegradable curve demonstrates that degradation in the absence of TiO2/CF is extremely slow under UV light irradiation. Compared with the photocatalytic performance of pure TiO2 and TiO2/CF (the weight of TiO2 is the same) in Fig. 7b, the concentration of RhB after pure TiO2 l h degradation is 1.6 mg/L. Using the formula of Eq. (1), the efficiency of photocatalytic degradation of RhB by pure TiO2 was 84%. Hence, the photocatalytic performance of TiO2/CF is better. This is due to C in CF materials acting as electron traps that effectively promote separation of photogenerated electrons and holes, and significantly improve photocatalytic activity [33,34]. Moreover, even after 10 min the RhB solution showed a
2θ = 25.6°, 38.2°, 48.2°, 54.1°, 55.3°, 63.0°, 69.0°, 70.5°, and 75.3° are, respectively, assigned to the reflections of the (101), (004), (200), (105), (211), (204), (116), (220), and (215) crystal planes of TiO2. All observed XRD peaks are ascribed to pure anatase crystalline phase according to No. 21–1272 JCPDS card [31,32]. EDS spectrum of TiO2/CF further demonstrates that TiO2 is successfully loaded on CF (Fig. 6b). The photocatalytic activity of TiO2/CF was evaluated using RhB concentration 10 mg/L and TiO2/CF dosage 1.5 g and compared with
Fig. 4. (a) FESEM image of TiO2/CF after oxidation treatment; (b) FESEM image of TiO2/CF without oxidation treatment; (c) FESEM image of TiO2/CF after oxidation treatment and allowed to stand at room temperature for 12 h; (d) FESEM image of TiO2/CF without oxidation treatment and allow to stand at room temperature for 12 h.
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Fig. 5. Schematic illustration of microwave hydrothermal synthesis of TiO2/CF with CF treatment.
Fig. 6. (a) XRD patterns of pure CF and TiO2/CF; (b) energy spectrum of TiO2/CF.
Fig. 7. (a) UV–vis absorption spectra of RhB solutions using TiO2/CF under UV light irradiation for different times; (b) photodegradation efficiency toward RhB when using TiO2/CF; (c) comparison photo of RhB and blank group after degradation.
Fig. 8. (a) Weight loss after multiple degradation, (b) degradation of RhB with TiO2/CF in different recycling runs under UV light irradiation for 1 h, (c) FESEM image of TiO2/CF after the fifth cycle experiment. 5
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lightning of color, revealing the photocatalytic speed of the method. Prolonging the time (1 h) results in an almost colorless solution. In the absence of TiO2/CF no significant color change is observed, giving further precedence to the photocatalytic effectiveness of the method. Consequently, Fig. 7a–c further confirms TiO2/CF photocatalytic excellence. According to the reaction mechanism, the photocatalytic degradation of organic substances in the presence of TiO2 is essentially a free radical reaction [35]. The electron transfer process of RhB under UV light irradiation is presented as Eqs. (2)–(6) [36]:
RhB + hv
RhB * + O2
RhB+
(3)
+ TiO2 (e )
+ O2
(4)
TiO2 + O2
(5)
RhB+
TiO2 (e ) + O2
RhB *
(2)
RhB*
RhB * + TiO2
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Rhoda min e
(6)
products *
Excited RhB molecules (RhB ) can inject electrons into the conduction band of TiO2 based on energy matching between TiO2 and RhB, and cationic dye radicals (RhB+) can form (Eq. (3)). The same electron transfer can also occur between RhB* and adsorbed oxygen (Eq. (4)) and between TiO2 and adsorbed oxygen (Eq. (5)) [37]. To further study the catalytic and structural stability of TiO2/CF composite fibers, cyclic photocatalytic experiments were carried out under identical conditions (Fig. 8a–c). After every recycling experiment, recycled TiO2/CF was washed and dried at 60 °C for 4 h, and once again subjected to the photoreaction. Fig. 8a shows the weight loss curve of TiO2/CF after each experimental cycle. After the fifth cycle experiment, the loading quality of TiO2 showed little variation. Fig. 8b shows slight reduction in the photocatalytic efficiency of recycled TiO2/ CF with increasing cycling times, but even after the tenth cycle the degradation rate remains at 88%. SEM analysis of TiO2/CF composite fibers after the catalytic tests were employed to evaluate if TiO2 particles undergo separation from CF. The obtained results (Fig. 8c) were compared with those displayed in Fig. 3c, with no apparent differences being detected other than the agglomeration of TiO2. It shows that TiO2 is partially stripped from CF but remains strongly bound to CF after several photocatalytic processes while maintaining high photocatalytic properties. 4. Conclusion In summary, TiO2/CF photocatalytic material was successfully prepared by microwave hydrothermal method at 200 °C for 30 min, which exhibits superior photocatalytic performance for the degradation of RhB dye. The degradation rate of TiO2/CF was 97% under UV light irradiation after 1 h. After ten cycles TiO2/CF photocatalytic activity was more than 86%, emphasizing its reusability. Our study contributes to a better understanding, as well as providing a strong foundation for energy efficient and greener methodologies for wastewater degradation. Acknowledgments This work was supported by Zhejiang Provincial Natural Science Foundation of China (LY18F050011), the Applied Basic Research Project of China National Textile and Apparel Council (J201801), and the Fundamental Research Funds of Zhejiang Sci-Tech University (2019Q011). References [1] L. Chen, L. Wang, X. Wu, A process-level water conservation and pollution control performance evaluation tool of cleaner production technology in textile industry, J. Clean. Prod. 143 (2017) 1137–1143.
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