1. Introduction
Environmental pollution, especially water pollution, caused by rapid industrialization and population growth, poses a serious threat to human civilization [1]. Water is an important resource on which all living beings depend; however, domestic sewage, industrial and agricultural wastewater and other water bodies contain a large number of organic pollutants that are difficult to degrade and will cause a series of environmental pollution problems and damage ecosystems if discharged directly or indirectly without purification and treatment [2]. Therefore, the efficient removal of pollutants from the polluted water has attracted extensive attention from researchers [3]. The most promising approach to solve this problem in various wastewater treatment methods is semiconductor photocatalysis, and the preparation of environmentally efficient photocatalysts is the key to this photocatalytic technology [4].
In recent years, bismuth-based semiconductor photocatalysts have received much attention in the field of photocatalysis because of their unique electronic structure and photoelectric properties [5]. Among them, BiPO4 is favored by researchers due to its good stability and high catalytic activity. However, its application is limited by the shortage of its photoresponse range limited to the UV region and high compounding rate of photogenerated carriers [6]. It has been reported in the literature that the photocatalytic activity can be improved by compounding with visible-light type Bi2WO6 semiconductor that can extend the light absorption range and promote the rapid separation of photogenerated charges [7] [8]. In addition, the preparation of photocatalysts loaded on activated flexible carbon cloth (CC) can not only improve adsorption capacity, increase the contact area between catalysts and pollutants, but also solve the problem of difficult separation of powder catalysts [9]. CC has high corrosion resistance, large specific surface area, excellent electrical conductivity and light transmission, and good stability and easy separation properties, which makes it a good loading material for adsorption and photocatalyst [10].
Therefore, in this paper, activated flexible carbon cloth (CC) was used as the substrate, and BiPO4 and Bi2WO6 were sequentially loaded on the carbon cloth by hydrothermal method to finally obtain CC/BiPO4/Bi2WO6 composite photocatalytic material, and the specific preparation process was shown in Figure 1.
Figure 1. Experimental flowchart of the preparation of CC/BiPO4/Bi2WO6 composite.
2. Experimental
2.1. Reagents
Bismuth nitrate pentahydrate (Bi(NO)3∙5H2O), disodium hydrogen phosphate dodecahydrate (Na2HPO4∙12H2O), sodium tungstate dihydrate (Na2WO6∙2H2O), ammonia, isopropyl alcohol, glacial acetic acid are all analytically pure and purchased from Sinopharm Chemical Reagent Co., Ltd.
2.2. Materials Preparation
Activation of carbon cloth: The 1 cm × 1 cm carbon cloth was placed in acetone, ethanol and water, respectively, and ultrasonically cleaned for 20 min. After cleaning, it was immersed in a mixed acid solution containing 30 mL of concentrated sulfuric acid and concentrated nitric acid 1:3 for 24 h. After completion, the carbon cloth was removed with tweezers, washed to neutral, and dried.
Preparation of CC/BiPO4 composite: The treated carbon was arranged in 18 mL of 0.3 mol/L Bi (NO)3∙5H2O solution containing 2 mL of concentrated nitric acid, and 15 mL of 0.3 mol/L Na2HPO4∙12H2O solution was added dropwise to it under magnetic stirring, and the pH was adjusted to 7 with ammonia, and then it was put into a reaction kettle at 180˚C for reaction for 12 h. After the reaction was completed, the carbon cloth was rinsed with water and ethanol, respectively, and dried at 60˚C. And the solution in the reaction kettle was washed by centrifugation to recover BiPO4 powder and dried for use.
Preparation of CC/BiPO4/Bi2WO6 composite: The prepared CC/BiPO4 composite was immersed in 18 mL of 2 mol/L Bi (NO)3∙5H2O solution containing 6 mL of glacial acetic acid for 12 h. Subsequently, 15 mL of 1 mol/L Na2WO6∙2H2O solution was added drop by drop, and finally the solution was loaded into a reaction kettle at 180˚C for 16 h. After the reaction, the solution was washed by centrifugation and the solid powder was recovered, the carbon cloth was removed and rinsed for use.
2.3. Photocatalytic Degradation of CC/BiPO4/Bi2WO6
The prepared CC/BiPO4/Bi2WO6 composite was placed in 10 mg/L RhB solution (40 mL) and stirred for 40 min under light-proof conditions to reach adsorption equilibrium, after which the supernatant was extracted by centrifugation every 10 min under the irradiation of a 500 W xenon lamp, and the photocatalytic process was detected in real time with a UV-Vis spectrophotometer.
2.4. Characterization
The morphology of the samples was characterized using a scanning electron microscope (SEM JSM-6490LV) from JEOL, Japan, and the crystalline structure of the samples was tested using a RIGAKU D/max 2500 X-ray diffractometer with a test voltage of 40 KV, a current of 20 mA, and a scan rate of 2˚/min, using a Cu target Kα1 radiation line (λ = 0.15405 nm). XPS was tested using a Thermo ESCALAB 250Xi tester, USA, with monochromatic Al Ka (hv = 1486.6 eV), power 150 W, 500 μm beam spot, and binding energy calibrated at C1s 284.8. The UV spectra were tested in a Hitachi U-3900H UV-Visible spectrometer, Japan.
3. Results and Discussion
3.1. Crystal Structure, Morphology and Composition
Figure 2(a) and Figure 2(b) showed the SEM images of CC/BiPO4 and CC/BiPO4/Bi2WO6 composites. As illustrated in Figure 2(a), the BiPO4 was a one-dimensional nanorod structure. After loading Bi2WO6, the composite had a distinct layered two-dimensional morphology (Figure 2(b)), which indicated that the prepared CC/BiPO4/Bi2WO6 had a large specific surface area that can provide multiple photocatalytic active sites. The EDS mapping of CC/BiPO4/Bi2WO6 (Figure 2(c)) displayed that the surface of the CC/BiPO4/Bi2WO6 composite contained Bi, P, W, and O elements, indicating that BiPO4 and Bi2WO6 were successfully loaded on the carbon cloth.
Figure 3 showed the XRD patterns of CC, CC/BiPO4 and CC/BiPO4/Bi2WO6. In Figure 3(a), the diffraction peaks located at 25.5˚ and 43.6˚ attributed to the graphitized carbon of carbon cloth. Figure 3(b) displayed the characteristic diffraction peaks of BiPO4 at 2θ = 19.0, 21.7, 29.1, 31.2, 34.4, 36.8, 46.3, 52.8, 56.7 (JCPDS: 80-0287), indicating that the carbon cloth was successfully loaded with BiPO4. Figure 3(c) showed sharp diffraction peaks of Bi2WO6 at 28.3˚, 32.8˚,
Figure 2. SEM images of the prepared CC/BiPO4 (a) and CC/BiPO4/Bi2WO6 (b); EDS mapping of CC/BiPO4/Bi2WO6 (c).
Figure 3. XRD patterns of the prepared CC, CC/BiPO4 and CC/BiPO4/Bi2WO6 photocatalyst.
47.2˚, 55.8˚, 58.6˚, 68.8˚, 75.9˚ and 78.4˚, corresponding to the (113), (200), (220), (313), (226), (400), (139) and (420) crystallographic planes (JCPDS No. 73-1126), respectively, indicating that the CC/BiPO4 composite was further successfully loaded with Bi2WO6.
The surface compositions and chemical state of the prepared CC/BiPO4/Bi2WO6 were confirmed by XPS spectra, which was illustrated in Figure 4. The survey spectrum of CC/BiPO4/Bi2WO6 in Figure 4(a) clearly identified the existence of Bi, C, W, P, O. In Figure 4(b), two characteristic peaks with binding energies of 159.12 and 164.47 eV belonged to Bi 4f7/2 and Bi 4f5/2 components, defining the existence of Bi3+ in the CC/BiPO4/Bi2WO6 composite. The high resolution XPS of W 4f with binding energies located at 35.34 eV and 37.47 eV attributed to W 4f7/2 and W 4f5/2, respectively (Figure 4(c)). The characteristic peak of P 2p in the composite (Figure 4(d)) was located at EB = 133.51 eV and the XPS profile of C 1s (Figure 4(e)) showed two characteristic peaks with binding energies at 284.17 eV and 285.54 eV corresponding to the C-C and C = C characteristic peaks of carbon cloth and the characteristic peak of activated carbon cloth, respectively.
3.2. UV Diffuse Reflectance Spectra
The UV diffuse reflectance spectra of CC, CC/BiPO4 and CC/BiPO4/Bi2WO6 as well as pure BiPO4 and Bi2WO6 catalysts were shown in Figure 5. The absorption band edge of pure BiPO4 is about 300 nm due to its large band gap energy (Figure 5(d)), while the optical response of the pure Bi2WO6 powder (Figure 5(e)) catalyst extended from the UV region to the visible region with band edge of 450 nm. After the powder sample was loaded on carbon cloth, the composite photocatalyst had absorption in the whole UV-visible region (200 - 800 nm) and enhanced the absorption intensity of visible light, which was more favorable for photocatalytic degradation of pollutants.
Figure 4. Survey XPS spectrum (a) and the high-resolution Bi 4f (b), W 4f (c), P 2p (d), and C1s (e) peaks of the prepared CC/BiPO4/Bi2WO6 samples.
Figure 5. UV-vis diffuse reflection spectra of the prepared CC (a), CC/BiPO4 (b), CC/BiPO4/Bi2WO6 (c), BiPO4 (d) and Bi2WO6 (e) samples.
3.3. Photocatalytic Performance
The photocatalytic degradation performance of the materials was investigated using BiPO4, BiPO4/Bi2WO6 and CC/BiPO4/Bi2WO6 as catalysts, respectively, and the results were shown in Figure 6. Figure 6(a) displayed that adsorption equilibrium was reached at 40 min of stirring under the dark, at which the adsorption amounts of RhB molecules by BiPO4, BiPO4/Bi2WO6 and CC/BiPO4/Bi2WO6 were 10.9%, 19.2% and 62.9%, respectively, and it can be concluded that the specific surface adsorption of RhB molecules by the composites increased
Figure 6. (a) Variations of RhB concentration as a function of irradiation time (with the time of light on set as 0) using BiPO4, BiPO4/Bi2WO6 and CC/BiPO4/Bi2WO6 as photocatalysts. Ct is the RhB concentration at time t, and C0 that in the initial solution. (b) Plots of ln (Ct/C0) versus reaction time using BiPO4, BiPO4/Bi2WO6 and CC/BiPO4/Bi2WO6 as photocatalysts.
after loading the sheet layer Bi2WO6. The removal rate of RhB by CC/BiPO4/Bi2WO6 composite after UV-vis irradiation for 60 min was 92.1%, which was significantly higher than BiPO4 (24.4%) and BiPO4/Bi2WO6 (52.9%), and the removal effect originated from both of adsorption and catalytic degradation of RhB by CC/BiPO4/Bi2WO6 composite. The absorption peak intensity of RhB in the UV spectrum was proportional to its concentration [11], so the reaction rate constant k was derived by fitting the reactant concentration as a function of time, and the results were shown in Figure 6(b). Figure 6(b) showed that the reactions of photocatalytic degradation over RhB by BiPO4, BiPO4/Bi2WO6 and CC/BiPO4/Bi2WO6 were all consistent with the first-order kinetic, and the reaction rate constants k for CC/BiPO4/Bi2WO6 were 0.0257 min−1, which was 9.5 and 2.9 times that of BiPO4 (k = 0.0089 min−1) and BiPO4/Bi2WO6 (k = 0.0027 min−1), respectively.
3.4. Photoluminescence Spectrum
The emission intensity of photoluminescence (PL) spectrum reflects the compounding efficiency of photoelectron-hole pairs, and the lower the emission intensity of PL spectrum, the lower the compounding rate of photogenerated e− and h+, and the higher the photocatalytic activity of the photocatalyst [12] [13]. Figure 7 showed the PL spectra of BiPO4, BiPO4/Bi2WO6 and CC/BiPO4/Bi2WO6 photocatalysts (λex = 350 nm), from which it can be seen that the CC/BiPO4/Bi2WO6 composite had the weakest luminescence peak intensity, indicating that loading the powdered BiPO4/Bi2WO6 composite photocatalyst on carbon cloth can accelerate the photogenerated carrier mobility, slow down the compounding of photogenerated electron-hole pairs, and further improve the photocatalytic activity.
3.5. Radical Capture Experiments
To investigate the major active species of CC/BiPO4/Bi2WO6 composites in the photocatalytic degradation of RhB by radical capture experiments [14], EDTA-2Na, p-benzoquinone (BQ) and isopropyl alcohol (IPA) were used as radical sacrificial agents for h+, ・O2− and ・OH, respectively. As can be seen from Figure 8, the catalytic activity of the CC/BiPO4/Bi2WO6 catalyst was significantly reduced after the introduction of IPA, BQ and EDTA-2Na compared to the blank control without sacrificial agent, indicating that h+, ・O2− and ・OH were the main active species in the degradation process of RhB.
3.6. Cycle Stability Performance Test
The recyclability stability of photocatalysts plays a significant role in the practical application of photocatalysts [15]. When the photocatalytic reaction is finished, the prepared cloth-like CC/BiPO4/Bi2WO6 composite can be easily removed from the solution with tweezers and rinsed several times with ultrapure water for the next catalytic degradation experiment. As shown in Figure 9, after
Figure 7. PL spectra of BiPO4, BiPO4/Bi2WO6 and CC/BiPO4/Bi2WO6 photocatalysts (λex = 350 nm).
Figure 8. Photodegradation of RhB over the prepared CC/BiPO4/Bi2WO6 photocatalyst in the presence of different scavengers (1 mL, 4 mM).
Figure 9. Relationship between the photocatalytic degradation efficiency of the prepared CC/BiPO4/Bi2WO6 photocatalyst and cycle times.
the catalyst was recycled six times, the photocatalytic degradation efficiency was still up to 53.3% due to the shedding of a small amount of BiPO4 and Bi2WO6, which had a certain stability of recycling, and CC/BiPO4/Bi2WO6 composite reduced tedious steps such as centrifugal washing and recovery, compared to powder materials.
4. Conclusion
In this paper, CC/BiPO4/Bi2WO6 composite was successfully synthesized through two-step hydrothermal method. The results of photocatalytic degradation experiments showed that CC/BiPO4/Bi2WO6 had better adsorption-photocatalytic degradation performance than BiPO4 and BiPO4/Bi2WO6, and the removal rate of RhB by UV-visible light irradiation for 60 min was 92.1%, which was much higher than the powder unloaded BiPO4 (24.4%) and BiPO4/Bi2WO6 (52.9%). Photoluminescence spectra indicated that the improved photocatalytic activity was due to the more effective inhibition of photogenerated carrier complexes. Furthermore, the radical trapping experiments showed that h+,
and ・OH were the main active species in the photocatalytic degradation process of RhB. More importantly, CC/BiPO4/Bi2WO6 composites had a simple separation process and good recycling stability, and the photocatalytic degradation efficiency can still reach 53.3% after six cycles.
Acknowledgements
This work was supported financially by the Science and Technology Development Project of Jilin Province (20220203170SF).