Synthesis of CC/BiPO₄/Bi₂WO₆ Composite Material and Its Photocatalytic Performance

Xiangdong Shi¹, Chaoyang Gao², Xiangyu Wei², Qingtao Chen², Fenghua Chen^2*, Guixia Liu^1*

¹School of Chemistry and Environmental Engineering, Changchun University of Science and Technology, Changchun, China.
²School of Materials and Chemical Engineering, Zhengzhou University of Light Industry, Zhengzhou, China.

DOI: 10.4236/opj.2023.136014                

Abstract

Along with the popularity of environmental protection concepts, the environmental treatment of water pollution attracts widespread attention, among which, the research on Bi-based semiconductor photocatalytic degradation technology has made great progress. However, the development of such bismuth-based composites still remains a challenging task due to difficult recovery and low catalytic efficiency. Herein, a novel CC/BiPO₄/Bi₂WO₆ composite was successfully synthesized through two-step hydrothermal method using activated flexible carbon cloth as a substrate. The results of the photocatalytic degradation experiments showed that the obtained CC/BiPO₄/Bi₂WO₆ composites can degrade 92.1% RhB in 60 min under UV-visible light irradiation, which was much higher than that of unloaded BiPO₄ (24.4%) and BiPO₄/Bi₂WO₆ (52.9%), exhibiting a better adsorption-photocatalytic degradation performance than BiPO₄ and BiPO₄/Bi₂WO₆. Photoluminescence spectra indicated that the improved photocatalytic activity was due to the more effective inhibition of photogenerated carrier complexation. Furthermore, the radical capture experiments confirmed that h⁺, ·OH and O₂^- were the main active substances in the photocatalytic degradation process of RhB by the CC/BiPO₄/Bi₂WO₆ composites. More importantly, the prepared CC/BiPO₄/Bi₂WO₆ composite had a simple separation process and good recycling stability, and its photocatalytic degradation efficiency can still reach 53.3% after six cycles of RhB degradation.

Keywords

BiPO₄, Bi₂WO₆, Activated Flexible Carbon Cloth, Photocatalytic Degradation

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 4f_7/2 and Bi 4f_5/2 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 4f_7/2 and W 4f_5/2, respectively (Figure 4(c)). The characteristic peak of P 2p in the composite (Figure 4(d)) was located at E_B = 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. C_t is the RhB concentration at time t, and C₀ that in the initial solution. (b) Plots of ln (C_t/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 (λ_ex = 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⁺, $·{\text{O}}_2^{-}$ 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).

Conflicts of Interest

The authors declare no conflicts of interest regarding the publication of this paper.

References

[1]	Bibi, S., Ahmad, A., Anjum, M.A.R., Haleem, A., Siddiq, M. and Shah, S.S. (2021) Photocatalytic Degradation of Malachite Green and Methylene Blue over Reduced Graphene Oxide (rGO) Based Metal Oxides (rGO-Fe3O4/TiO2) Nanocomposite under UV-Visible Light Irradiation. Journal of Environmental Chemical Engineering, 9, Article ID: 105580. https://doi.org/10.1016/j.jece.2021.105580
[2]	Hunge, Y.M., Yadav, A.A., Khan, S., Takagi, K., Suzuki, N. and Teshima, K. (2021) Photocatalytic Degradation of Bisphenol A Using Titanium Diox ide@nanodiamond Composites under UV Light Illumination. Journal of Colloid and Interface Science, 582, 1058-1066. https://doi.org/10.1016/j.jcis.2020.08.102
[3]	Tang, Y.D., Li, T., Xiao, W.X., Huang, Z.T., Wen, H.C., Situ, W. and Song, X.L. (2023) Degradation Mechanism and Pathway of Tetracycline in Milk by Heterojunction N-TiO2-Bi2WO6 Film under Visible Light. Food Chemistry, 401, Article ID: 134082. https://doi.org/10.1016/j.foodchem.2022.134082
[4]	Zhang, B., He, X., Yu, C.Z., Liu, G.C., Ma, D., Cui, C.Y., Yan, Q.H., Zhang, Y.J., Zhang, G.S. and Ma, J. (2022) Degradation of Tetracycline Hydrochloride by Ultrafine TiO2 Nanoparticles Modified g-C3N4 Heterojunction Photocatalyst: Influencing Factors, Products and Mechanism Insight. Chinese Chemical Letters, 33, 1337-1342. https://doi.org/10.1016/j.cclet.2021.08.008
[5]	Liu, L., Liu, J., Sun, K., Wan, J., Fu, F. and Fan, J. (2021) Novel Phosphorus-Doped Bi2WO6 Monolayer with Oxygen Vacancies for Superior Photo Catalytic Water Detoxication and Nitrogen Fixation Performance. Chemical Engineering Journal, 411, Article ID: 128629. https://doi.org/10.1016/j.cej.2021.128629
[6]	Tang, M., Li, X., Deng, F., Han, L., Xie, Y. and Huang J. (2023) BiPO4/Ov-BiOBr High-Low Junctions for Efficient Visible Light Photocatalytic Performance for Tetracycline Degradation and H2O2 Production. Catalysts, 13, 634. https://doi.org/10.3390/catal13030634
[7]	Kumar, R., Raizada, P., Khan, A.A.P., Nguyen, V.H., Van, L.Q. and Ghotekar, S. (2022) Recent Progress in Emerging BiPO4-Based Photocatalysts: Synthesis, Properties, Modification Strategies, and Photocatalytic Applications. Journal of Materials Science & Technology, 108, 208-225. https://doi.org/10.1016/j.jmst.2021.08.053
[8]	Zhu, Y., Wang, Y., Ling, Q. and Zhu, Y. (2017) Enhancement of Full-Spectrum Photocatalytic Activity over BiPO4/Bi2WO6 Composites. Applied Catalysis B: Environmental, 200, 222-229. https://doi.org/10.1016/j.apcatb.2016.07.002
[9]	Shao, M., Feng, X., Liu, D. and Zhang, Y. (2023) A Layer by Layer Strategy for the TiO2/CuxO/CeO2 Hierarchical Structure Supported on Carbon Cloth as a Photocarrier-Assisted Photothermal Catalyst with Fast Visible Light Response. Materials Chemistry Frontiers, 7, 745-752. http://dx.doi.org/10.1039/D2QM01189A
[10]	Jiang, L.C., Gao, X.Y., Chen, S.L., Ashok, J. and Kawi, S. (2021) Oxygen-Deficient WO3/TiO2/CC Nanorod Arrays for Visible-Light Photocatalytic Degradation of Methylene Blue. Catalysts, 11, 1349. https://doi.org/10.3390/catal11111349
[11]	Wang, Y., Rao, L., Wang, P., Shi, Z. and Zhang, L. (2020) Photocatalytic Activity of N-TiO2/O-Doped N Vacancy g-C3N4 and the Intermediates Toxicity Evaluation under Tetracycline Hydrochloride and Cr (VI) Coexistence Environment. Applied Catalysis B: Environmental, 262, Article ID: 118308. https://doi.org/10.1016/j.apcatb.2019.118308
[12]	Tang, T., Yin, Z.L., Chen, J.R., Zhang, S., Sheng, W.C., Wei, W.X., Xiao, Y.G., Shi Q.Y. and Cao, S.S. (2021) Novel p-n Heterojunction Bi2O3/Ti3+-TiO2 Photocatalyst Enables the Complete Removal of Tetracyclines under Visible Light. Chemical Engineering Journal, 417, Article ID: 128058 https://doi.org/10.1016/j.cej.2020.128058
[13]	Chen, F.H., Liang, W.W., Qin, X.Y., Jiang, L.Y., Zhang, Y.H., Fang, S.M. and Luo, D. (2021) Ag@AgCl Photocatalyst Loaded on the 3D Graphene/PANI Hydrogel for the Enhanced Adsorption-Photocatalytic Degradation and In Situ SERS Monitoring Properties. Chemistryselect, 6, 4166-4177. https://doi.org/10.1002/slct.202100580
[14]	Ni, J.X., Wang, W., Liu, D.M., Zhu, Q., Jia, J.L., Tian J.Y., Li, Z.Y., Wang, X. and Xing, Z.P. (2021) Oxygen Vacancy-Mediated Sandwich-Structural TiO2-x/Ultrathin g-C3N4/TiO2-x Direct Z-Scheme Heterojunction Visible-Light-Driven Photocatalyst for Efficient Removal of High Toxic Tetracycline Antibiotics. Journal of Hazardous Materials, 408, Article ID: 124432. https://doi.org/10.1016/j.jhazmat.2020.124432
[15]	Hieu, V.Q., Phung, T.K., Nguyen, T.Q., Khan, A., Doan, V.D. and Tran, V.A. (2021) Photocatalytic Degradation of Methyl Orange Dye by Ti3C2-TiO2 Heterojunction under Solar Light. Chemosphere, 276, Article ID: 130154. https://doi.org/10.1016/j.chemosphere.2021.130154