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.

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.

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.

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.

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˚,

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.

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.

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

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.

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.

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.

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

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.