English

• 0   Introduction

  With increasing environmental problems and the energy crisis, photocatalytic degradation of organic pollutants is one of the most important technologies for further efficient use of solar energy^[1-3]. In this area, the development of high efficient and stable photo-catalysts is one of vital mission to be accomplished^[4-7].

  Bi₂Fe₄O₉ has been considered as one of the most promising semiconductor materials due to its lower band gap, low cost, physical and chemical stability, availability, and can be stimulated by visible light, but, its low adsorption capacity is unfavorable to photocatalysis. In contrast, metal-organic frameworks (MOFs) have high specific surface area^[8-10], excellent adsorption^[11] performance, and can grow on other photocatalytic materials surface to form composite materials^[12-16]. UiO-66, as one extensively investigated MOF, is a rarely observed water stable MOF, and exhibits high hydrothermal stability up to 773 K. Hence, it can be considered as an effective adsorbent to remove dyes from water. G-C₃N₄, as a polymeric semiconductor, has been investigated extensively due to its suitable band gap energy of about 2.70 eV, and has visible-light absorbing ability up to ca. 460 nm, moreover, it has low cost, ease of preparation, good stability, and environmental friendly features. It is noteworthy that when g-C₃N₄ wrinkled sheet is combined with other semiconductor, the resulted composite often exhibits solid-state Z-scheme^[17-19] heterojunction stru-cture, which can enhance separation and the redox ability of the photogenerated electron-hole pairs. These features are favorable for g-C₃N₄ to obtain potential application in photocatalytic degradation of pollutants.

  In this paper, a novel ternary Bi₂Fe₄O₉/g-C₃N₄/UiO-66 composite photocatalyst was prepared. The results of structural characterization and photocatalytic experiments for degradation of RhodamineB dye and colorless phenol solutions show that 2D solid-state Z-scheme Bi₂Fe₄O₉/g-C₃N₄ heterojunction structure is formed between Bi₂Fe₄O₉ nanosheet and g-C₃N₄ wrinkled sheet, which plays a key role for enhanced photocatalytic activity of the binary composite. Furthermore, under the synergistic effect of high adsorption capacity of UiO-66, the resulted Bi₂Fe₄O₉/g-C₃N₄/UiO-66 ternary composite exhibits the highest photocatalytic activity.

  1   Experimental

  1.1   Materials

  Bismuth nitrate pentahydrate, six water ferric chloride, acetone, ammonia, sodium hydroxide, ZrCl₄, dimethylformamide (DMF, 99%), chloroform, benzoic acid, 1, 4-benzenedicarboxylic acid (H₂bdc, 98.9%), nitric acid.

  1.2   Preparation of Bi₂Fe₄O₉ and g-C₃N₄

  Bi₂Fe₄O₉ is produced by hydrothermal synthesis method: [Bi(NO₃)₃·5H₂O] and [FeCl₃·6H₂O] in a stoichiometric ratio (1: 1 in molar ratios), were dissolved in acetone with stirring and ultrasound until comp-letely dissolved. And then distilled water was added into the solution, and ammonia was added slowly until the pH value is up to 10~11. The sediment was washed with distilled water until the pH is neutral.

  The obtained sediment was added into the NaOH (8 mol·L^-1) solution with ultrasonic dispersion for 30 min, then the mixture was transferred to a stainless steel teflon-lined autoclave of 50 mL capacity and maintained at 180 ℃ for 72 h. Black powder was obtained by filtration and washed with water and anhydrous ethanol.

  The g-C₃N₄ power was synthesized according to the literature^[20]. Melamine was heated at 520 ℃ for 4 h in a semi-closed crucible under air condition. The final product was washed with water and ethanol for several times and dried at 80 ℃ for 12 h. Then, g-C₃N₄ was obtained.

  1.3   Synthesis of Bi₂Fe₄O₉/g-C₃N₄ (1: 2) binary composite

  Acid modified g-C₃N₄^[21]: The as-prepared bulk g-C₃N₄ solid (0.500 g) was ground well and then put into 45 mL nitric acid (0.5 mol·L^-1). The resulted solution was transferred to a stainless steel teflon-lined autoclave of 50 mL capacity and maintained at 135 ℃ for 6 h. The solid product was then collected by centrifugation and washed with deionized water for several times. Then, the product was dried at 60 ℃ in a vacuum oven.

  Acid modified g-C₃N₄ (0.5 g) and Bi₂Fe₄O₉ (1 g) were dispersed in ethanol solution(50 mL) under magnetic stirring for 24 h. The product was dried in an oven^[22].

  1.4   Preparation of ternary composite Bi₂Fe₄O₉/g-C₃N₄/UiO-66

  Bi₂Fe₄O₉/g-C₃N₄/UiO-66 was synthesized as UiO-66^[23]: The content of UiO-66 in composite materials was controlled by the proportion of Bi₂Fe₄O₉/g-C₃N₄ and ZrCl₄. For example binary composite materials Bi₂Fe₄O₉/g-C₃N₄ (0.106 g), ZrCl₄ (0.053 g), terephthalic acid (0.034 g) and benzoic acid (0.530 g) were intro-duced to N, N-dimethyl formamide solution (42 mL) with ultrasonic dispersion for 30 min, then the solution was transferred to a stainless steel teflon-lined autoclave of 50 mL capacity and then maintained at 393 K for 24 h. The ternary composite Bi₂Fe₄O₉/g-C₃N₄/UiO-66 (2: 1) (w_{Bi2Fe4O9/g-C3N4}: w_UiO-66=2: 1) was obtained after filtration, treatment with DMF and CHCl₃ several times and drying under reduced pressure.

  1.5   Photocatalytic experiments

  The photocatalytic degradation of RhB was measured at ambient pressure and 298 K in a set of home-made photochemical reaction equipment. The light source was a PHILIPS 70 W metal halide lamp (λ < 380 nm was filtered out by a cut off filter). 20 mg of photocatalyst was added into 100 mL RhB (initial concentration, C₀=10 mg·L^-1) aqueous solution. Before irradiation, the suspension was stirred continuously for 12 h in the dark in order to reach the adsorption-desorption equilibrium between RhB and the photo-catalyst. The supernatant liquid was obtained through filtration by 0.22 μm filter, and examined using a Shimadzu UV-240 spectrophotometer. In order to confirm that RhB was not photodegraded by itself, control experiments were carried out under the same condition but without irradiation or photocatalyst. For comparison, the photocatalytic activities of the as-prepared samples were also tested to degradation of colorless model pollutant phenol (10 mg·L^-1) under the same conditions with RhB.

  1.6   Photoelectrochemical measurements

  Photocurrent density was measured using a CHI electrochemical analyzer (CH instruments 660e) in a standard three-electrode configuration, with the working electrode (an effective area of 2 cm²), a platinum foil as the counter electrode, and a saturated calomel electrode (SCE) as the reference electrode. Na2SO4 (0.01 mol·L^-1) was used as the electrolyte. The light source was a PHILIPS 70 W metal halide lamp (λ < 380 nm was filtered out by a cut off filter) was used as the visible light source.

  1.7   Photocatalysts characterization

  X-ray diffraction (XRD) patterns of the samples were determined in the range of 2θ=4°~50° by step scanning on a Rigaku D/max-2500V X-ray diffracto-meter using Cu Kα (λ=0.154 nm) radiation (U=40 kV, I=100 mA). The morphological analysis of the samples was studied using a JEM-2100F field emission transmission electron microscopy (FETEM). UV-Vis spectra were recorded at a scanning rate of 3 600 nm·min^-1 in solid-state on a DUV-3700 spectrometer with the double beam, in which spectral resolution is 0.1 nm, and measure range varies from 165 nm to 3 300 nm depending on application and the use of an integrating sphere and the optional direct detection unit. The valence band X-ray photoelectron spectros-copy (XPS) was conducted using an ESCALAB250 spectrometer. Photoluminescence emission spectra (PL) were mea-sured on a PL measurement system (Fluorolog Tau-3) with the excitation wavelength of 320 nm. N₂ adsorption-desorption was performed on a Tristar Ⅱ 3020M surface area and porosity analyzer at 77 K. Before the actual measurements, the sample was degassed at 70 ℃ for 3 h.

  2   Results and discussion

  2.1   Characterization

  The microstructure of the as-prepared samples was investigated by FETEM. As shown in Fig. 1(a), UiO-66 has discoid shape with particle size at 15~20 nm. Fig. 1(b) indicated that Bi₂Fe₄O₉ nanosheet was formed by flake structure with size at about 1~2 μm. A typical thick and wrinkled sheet structure was clearly observed in the pristine g-C₃N₄ in Fig. 1(c). Interestingly, it can be seen that the aggregate of g-C₃N₄ and UiO-66 loads on to the surface of Bi₂Fe₄O₉ nanosheet in the Fig. 1(d). The single-crystal nature of the material can be confirmed from the high-resolution TEM image (Fig. 1(e)). Inter-planar spacings of 0.594 nm was calculated corres-ponding to the (001) plane of the orthorhombic Bi₂Fe₄O₉ which implies the nanosheet crystals grow prefere-ntially along the (001) direction.

  图1 TEM images of (a) UiO-66 discoid particles, (b) Bi₂Fe₄O₉ nanosheet, (c) g-C₃N₄ wrinkled sheet and (d) Bi₂Fe₄O₉/g-C₃N₄/UiO-66 ternary composite (e) HRTEM images of the as-prepared sample Bi₂Fe₄O₉/g-C₃N₄/UiO-66 Figure1. TEM images of (a) UiO-66 discoid particles, (b) Bi₂Fe₄O₉ nanosheet, (c) g-C₃N₄ wrinkled sheet and (d) Bi₂Fe₄O₉/g-C₃N₄/UiO-66 ternary composite (e) HRTEM images of the as-prepared sample Bi₂Fe₄O₉/g-C₃N₄/UiO-66

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  To further observe the combination of Bi₂Fe₄O₉, g-C₃N₄ and UiO-66, FETEM (Fig. 2(a)) and the EDS mapping (Fig. 2(b), (c) and (d)) were measured. The N and Zr distribution around the nanosheet shows that the ternary composites are successfully prepared, in which each component is intimately contacted with other two components.

  图2 (a) FETEM of Bi₂Fe₄O₉/g-C₃N₄/UiO-66 (2: 1); (b) Bi, (c) N, (d) Zr distribution by EDS mapping of Bi₂Fe₄O₉/g-C₃N₄/UiO-66 (2: 1) Figure2. (a) FETEM of Bi₂Fe₄O₉/g-C₃N₄/UiO-66 (2: 1); (b) Bi, (c) N, (d) Zr distribution by EDS mapping of Bi₂Fe₄O₉/g-C₃N₄/UiO-66 (2: 1)

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  The XRD patterns of Bi₂Fe₄O₉, g-C₃N₄ and UiO-66 shown in Fig. 3 are corresponds to those reported in the literature^[24-25], and the composite displays the characteristic peaks of Bi₂Fe₄O₉, g-C₃N₄ and UiO-66.This means the composite consists of the three components.

  图3 XRD patterns of Bi₂Fe₄O₉, UiO-66, g-C₃N₄, Bi₂Fe₄O₉/g-C₃N₄ and Bi₂Fe₄O₉/g-C₃N₄/UiO-66 Figure3. XRD patterns of Bi₂Fe₄O₉, UiO-66, g-C₃N₄, Bi₂Fe₄O₉/g-C₃N₄ and Bi₂Fe₄O₉/g-C₃N₄/UiO-66

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  The N₂ adsorption-desorption isotherms of Bi₂Fe₄O₉, g-C₃N₄, UiO-66 and Bi₂Fe₄O₉/g-C₃N₄/UiO-66 (2: 1) are displayed in Fig. 4(a), and the specific surface areas of them are 1, 11, 838 and 488 m²·g^-1, respectively. The N₂ adsorption-desorption isotherms of UiO-66 shows the type Ⅳ behavior, which indicates the presence of mesoporous in the material and that the pore diameter ranges from 2 to 50 nm (Fig. 4(b)). In contrast, Bi₂Fe₄O₉ doesn′t have the mesoporous structure, whereas g-C₃N₄ has it, and exhibits higher BET surface area than Bi₂Fe₄O₉.The pore size distribution of the ternary composite is not as uniform as that of UiO-66, but its pore size is even larger as shown in Fig. 4(c). Higher specific surface area and larger pore size of the structure can increase the adsorption of dye molecules on the active site, which can improve the photocatalytic activity. As known, the recombina-tion of electron-hole pairs will release energy, which can be detected by PL emission. A lower PL intensity is a general indication of a lower recombination of electron-hole pairs, resulting in higher photocatalytic activity, thus PL spectra can be used to detect efficient separation of photogenerated charge carriers. PL spectra of pure materials and composites are showed in Fig. 4(d), and the spectra intensities decrease as follows: UiO-66 > g-C₃N₄ > Bi₂Fe₄O₉/g-C₃N₄/UiO-66 (2: 1) > Bi₂Fe₄O₉/g-C₃N₄ > Bi₂Fe₄O₉. As indicated, UiO-66 and g-C₃N₄ both have the relatively high recombination rate of electron-hole pairs, whereas when binary and ternary composites were constructed by the combination of Bi₂Fe₄O₉, the separation efficiency was greatly enhanced.

  图4 (a) N₂ adsorption-desorption isotherms of Bi₂Fe₄O₉, g-C₃N₄, UiO-66 and Bi₂Fe₄O₉/g-C₃N₄/UiO-66 (2: 1); BJH pore diameter distribution curves of UiO-66 (b) and Bi₂Fe₄O₉/g-C₃N₄/UiO-66 (2: 1) (c); (d) PL spectra of Bi₂Fe₄O₉, g-C₃N₄, UiO-66, Bi₂Fe₄O₉/g-C₃N₄ and Bi₂Fe₄O₉/g-C₃N₄/UiO-66 (2: 1) Figure4. (a) N₂ adsorption-desorption isotherms of Bi₂Fe₄O₉, g-C₃N₄, UiO-66 and Bi₂Fe₄O₉/g-C₃N₄/UiO-66 (2: 1); BJH pore diameter distribution curves of UiO-66 (b) and Bi₂Fe₄O₉/g-C₃N₄/UiO-66 (2: 1) (c); (d) PL spectra of Bi₂Fe₄O₉, g-C₃N₄, UiO-66, Bi₂Fe₄O₉/g-C₃N₄ and Bi₂Fe₄O₉/g-C₃N₄/UiO-66 (2: 1)

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  Fig. 5 shows that the Bi₂Fe₄O₉/g-C₃N₄/UiO-66 (2: 1) electrode has a strong instant photoresponse to the visible light irradiation. The short-circuit photocurrent density of the Bi₂Fe₄O₉/g-C₃N₄/UiO-66 (2: 1) electrode is as great as 3 times that of the UiO-66 electrode. This demonstrates that the separation rate of photogenerated holes and electrons increases because of the loading of g-C₃N₄ and UiO-66. More photogenerated electrons collected from the Bi₂Fe₄O₉/g-C₃N₄/UiO-66 (2: 1) electrode suggest that more photogenerated holes survive from recombination or from the longer lifetime that the holes had.

  图5 Short-circuit photocurrent density versus time plotted (0 V versus SCE) for Bi₂Fe₄O₉/g-C₃N₄/ UiO-66 (2: 1), Bi₂Fe₄O₉, g-C₃N₄ and UiO-66 electrodes in 0.01 mol·L^-1 Na₂SO₄ solution under visible light irradiation Figure5. Short-circuit photocurrent density versus time plotted (0 V versus SCE) for Bi₂Fe₄O₉/g-C₃N₄/ UiO-66 (2: 1), Bi₂Fe₄O₉, g-C₃N₄ and UiO-66 electrodes in 0.01 mol·L^-1 Na₂SO₄ solution under visible light irradiation

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  Fig. 6(a) shows the UV-Vis absorption spectra of Bi₂Fe₄O₉, g-C₃N₄ and UiO-66. Two absorption bands of Bi₂Fe₄O₉ are observed in the visible region, indicating that the sample can serve as a photocatalyst driven by visible light. The first absorption from 400 to 600 nm primarily results from two types of excitations overlapping each other. The first excitation process is due to the pair excitation process: ⁶A_1g+⁶A_1g → ⁴T_1g(⁴G)+⁴T_1g(⁴G) and the second one is due to the excitation from ⁶A_1g to ⁴E_g, ⁴A_1g(⁴G) ligand field transitions (octah-edral coordination) and ⁶A₁ to ⁴T₂(⁴G) ligand field tran-sitions (tetrahedral coordination) as well as the charge transfer band tail. The second absorption from 610 to 770 nm can then be assigned to the d-d transitions of Fe^3+[26]. There are two absorption bands of UiO-66: the first absorption in the ultraviolet region primarily results from the direct transition of valence electrons and the second absorption from 380 to 600 nm can then be assigned to the crystal lattice defects. G-C₃N₄ wrinkled sheet can absorb light of less than 460 nm and the peak intensity is weaker in ultraviolet region. The band gap energy E_g of Bi₂Fe₄O₉, g-C₃N₄ and UiO-66 can be determined using the following equation: αhν=A(hν-E_g)^n/2, where α, ν, E_g and A are absorption coefficient, light frequency, band gap and a constant, respectively. Among them, n depends on the transition process (n=1 for direct transition or n=4 for indirect transition^[27-28]). Bi₂Fe₄O₉ has two low band gap E_g: 1.84 eV and 1.49 eV, which indicates the response to visible light. The E_g of g-C₃N₄ and UiO-66 are 2.81^[29] and 3.49 eV (Fig. 6(b)), respectively.

  图6 (a) UV-Vis absorption spectra of Bi₂Fe₄O₉, UiO-66 and g-C₃N₄; (b) Band-gaps of Bi₂Fe₄O₉, UiO-66 and g-C₃N₄ Figure6. (a) UV-Vis absorption spectra of Bi₂Fe₄O₉, UiO-66 and g-C₃N₄; (b) Band-gaps of Bi₂Fe₄O₉, UiO-66 and g-C₃N₄

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  The valence band of UiO-66 is 2.49 eV^[30], and those of g-C₃N₄ and Bi₂Fe₄O₉ are 1.51 and 2.51 eV, respectively according to Wang^[31]. Combined with the energy gap and valence band, the CB positions of UiO-66, g-C₃N₄ and Bi₂Fe₄O₉ can be calculated to be -1.00, -1.30 and 0.67 eV, respectively. The middle band of Bi₂Fe₄O₉ is 1.02 eV. CB positions for g-C₃N₄ and UiO-66 are more negative than O₂/·O₂^- (-0.33 eV) and O₂/HOO· (-0.037 eV), indicating that it is possible for the transfer of photogenerated electrons from CB to O₂ to form ·O₂^-, and then become ·OH free radicals.

  2.2   Adsorption activity of Bi₂Fe₄O₉, g-C₃N₄, UiO-66 and the composites

  UiO-66 and Bi₂Fe₄O₉/g-C₃N₄/UiO-66 (2: 1) display high adsorption capacity, and rapid adsorption process happens in the first three hours, and then reaches to balance (Fig. 7(a)). Bi₂Fe₄O₉ and g-C₃N₄ display poor adsorption capacity of RhB in contrast with UiO-66 which is due to a large BET surface area of UiO-66. As shown in Fig. 7(b), the adsorption activities of com-posites are between those of Bi₂Fe₄O₉ and UiO-66, and increase with the increasing of the UiO-66 content.

  图7 (a) Adsorption capacity of RhB onto Bi₂Fe₄O₉, g-C₃N₄, UiO-66, Bi₂Fe₄O₉/g-C₃N₄ and Bi₂Fe₄O₉/ g-C₃N₄/UiO-66 (2: 1); (b) Adsorption capacity of RhB onto the ternary composites with different mass ratios of Bi₂Fe₄O₉/g-C₃N₄ to UiO-66 Figure7. (a) Adsorption capacity of RhB onto Bi₂Fe₄O₉, g-C₃N₄, UiO-66, Bi₂Fe₄O₉/g-C₃N₄ and Bi₂Fe₄O₉/ g-C₃N₄/UiO-66 (2: 1); (b) Adsorption capacity of RhB onto the ternary composites with different mass ratios of Bi₂Fe₄O₉/g-C₃N₄ to UiO-66

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  RhB dye was used as probing molecules to explore the photocatalytic degradation property of the as-prepared samples. For low concentration pollutants, the pseudo-first order kinetic model can be adopted according to the following equation:

  where C_t is the concentration of pollutant (mg·L^-1) in the time of t, C₀ is the adsorption equilibrium concen-tration of pollutant before irradiation (mg·L^-1), t is the reaction time (min), and k is the reaction rate constant (min^-1). As shown in Fig. 8(a), the photocatalytic activities are increased as the following order: g-C₃N₄ < Bi₂Fe₄O₉ < UiO-66 < Bi₂Fe₄O₉/g-C₃N₄ < Bi₂Fe₄O₉/g-C₃N₄/UiO-66 (2: 1). The photocatalytic performance of ternary composites with the different ratios was also investi-gated, and they generally display better photodegra-dation activity than pure materials and binary com-posite (Fig. 8(b)). It can be seen that the mass ratio of 2: 1 is the optimum value to achieve high photode-gradation activity.

  图8 Kinetics of RhB photodegradation on: (a) Bi₂Fe₄O₉, g-C₃N₄, UiO-66, Bi₂Fe₄O₉/g-C₃N₄ and Bi₂Fe₄O₉/ g-C₃N₄/UiO-66; (b) Ternary composites with the different ratios; Kinetics of phenol (10 mg·L^-1) photodegradation on: (c) Bi₂Fe₄O₉, g-C₃N₄, UiO-66, Bi₂Fe₄O₉/g-C₃N₄ and Bi₂Fe₄O₉/g-C₃N₄/UiO-66 (2: 1) Figure8. Kinetics of RhB photodegradation on: (a) Bi₂Fe₄O₉, g-C₃N₄, UiO-66, Bi₂Fe₄O₉/g-C₃N₄ and Bi₂Fe₄O₉/ g-C₃N₄/UiO-66; (b) Ternary composites with the different ratios; Kinetics of phenol (10 mg·L^-1) photodegradation on: (c) Bi₂Fe₄O₉, g-C₃N₄, UiO-66, Bi₂Fe₄O₉/g-C₃N₄ and Bi₂Fe₄O₉/g-C₃N₄/UiO-66 (2: 1)

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  These behaviors indicate that Bi₂Fe₄O₉, g-C₃N₄ and UiO-66 have the lower photodegradation efficien-cies, which can be interpreted as their lower adsorption ability or due to the higher recombination rate of electron-hole pairs, resulting in poor photocatalytic degradation efficiency.

  In addition, phenol, as a colorless substance without absorbing visible-light, was used as the second model pollutant to further evaluate the visible light photocatalytic performance of the as-prepared samples under visible light, and the obtained results are illustrated in Fig. 8(c). It is clear that the photode-gradation efficiency follows the same order: g-C₃N₄ < Bi₂Fe₄O₉ < UiO-66 < Bi₂Fe₄O₉/g-C₃N₄ < Bi₂Fe₄O₉/g-C₃N₄/UiO-66 (2: 1), as compared to photodegradation of RhB. After 180 min irradiation, about 65% percent of phenol was removed over Bi₂Fe₄O₉/g-C₃N₄/UiO-66 (2: 1) com-posites. By comparing Fig. 7(a) with Fig. 7(c), one can see that the photocatalytic activity for phenol is lower than that for RhB. This phenomenon is often observed for photocatalytic study.

  The regeneration of the photocatalyst is one of the important steps for considering in practical applications. The stability of Bi₂Fe₄O₉/g-C₃N₄/UiO-66 (2: 1) was investigated, and after each photodegrada-tion, it was separated from solution by centrifuge, and can be reused without considerable amount of mass loss. As shown in Fig. 9(a), (b), after three cycles of degradation RhB, the photodegradation kinetics constant of Bi₂Fe₄O₉/g-C₃N₄/UiO-66 (2: 1) is 87% of the first cycle and after three cycles of photodegradation of phenol (10 mg·L^-1), the k value stabilizes at about 0.119 min^-1, which is 85.6% of the first cycle. indica-ting that composite material has the good regeneration performance.

  图9 (a) Kinetics of RhB photodegradation on the recycled Bi₂Fe₄O₉/g-C₃N₄/UiO-66 (2: 1), (b) Kinetics of phenol photodegradation on the recycled Bi₂Fe₄O₉/g-C₃N₄/UiO-66 (2: 1) Figure9. (a) Kinetics of RhB photodegradation on the recycled Bi₂Fe₄O₉/g-C₃N₄/UiO-66 (2: 1), (b) Kinetics of phenol photodegradation on the recycled Bi₂Fe₄O₉/g-C₃N₄/UiO-66 (2: 1)

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  2.3   Photocatalytic mechanism of Bi₂Fe₄O₉/g-C₃N₄/UiO-66

  To investigate the active ingredient in photocatal-ytic process of Bi₂Fe₄O₉/g-C₃N₄/UiO-66, EDTA-2Na, t-BuOH and BQ were as the probe molecules^[32-33]. After adding each probe molecule, the photodegradation rate of the ternary composite is generally decreased, and the inhibition degree is decreased in the following order: EDTA-2Na > BQ > t-BuOH. As shown in Fig. 10, h⁺ radicals are the main active oxygen species in the degradation of RhB, followed by ·O₂^- radicals and ·OH radicals^[34-36].

  图10 Species trapping experiments for degradation of RhB over Bi₂Fe₄O₉/g-C₃N₄/UiO-66 (2: 1) Figure10. Species trapping experiments for degradation of RhB over Bi₂Fe₄O₉/g-C₃N₄/UiO-66 (2: 1)

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  The ESR technique was further used to detect the presence of ·OH and ·O₂^- radicals in the Bi₂Fe₄O₉ /g-C₃N₄/UiO-66 (2: 1) photocatalytic reaction systems under visible light. As shown in Fig. 10, for Bi₂Fe₄O₉/g-C₃N₄/UiO-66 (2: 1) samples, the four characteristic peaks of the DMPO-·OH adducts (Fig. 11(a)) and six characteristic peaks of DMPO-·O₂^- adducts (Fig. 11(b)) are observed, Therefore, according to the above results of the active species trapping experiments and this ESR analysis, it can be inferred that the ·O₂^- played a major role in the Bi₂Fe₄O₉/g-C₃N₄/UiO-66 (2: 1) photo-catalytic reactions rather than the ·OH.

  图11 DMPO spin-trapping ESR spectra for Bi₂Fe₄O₉/ g-C₃N₄/UiO-66 (2: 1): (a) in aqueous dispersion for DMPO-·OH, (b) in methanol dispersion for DMPO-·O₂^- Figure11. DMPO spin-trapping ESR spectra for Bi₂Fe₄O₉/ g-C₃N₄/UiO-66 (2: 1): (a) in aqueous dispersion for DMPO-·OH, (b) in methanol dispersion for DMPO-·O₂^-

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  In summary, a possible mechanism of photocatal-ysis is proposed. As shown in Fig. 12, Bi₂Fe₄O₉ and g-C₃N₄ were inspired by the visible light, and photopro-duction electrons are transferred from valence band (VB) via the calculated band-gap to the conduction band (CB). The VB edges of g-C₃N₄ is 1.51 eV which had no sufficient force to drive -OH oxidation to form ·OH, however the CB of Bi₂Fe₄O₉ is 0.67 eV, then the dissolved O₂ can not capture the photogenerated electron at CB of Bi₂Fe₄O₉ to yield the superoxide radical anion, ·O₂^-, to degrade RhB. On the other hand, the h⁺ radicals on VB of g-C₃N₄ have too low oxidation capacity to degrade RhB. However, Bi₂Fe₄O₉/ g-C₃N₄ may act as a solid Z-scheme photocatalyst, in which the photogenerated electrons on the CB of Bi₂Fe₄O₉ can combine with h⁺ on the VB of g-C₃N₄, as a result, VB of Bi₂Fe₄O₉ for h⁺ is more positive than VB of g-C₃N₄, meanwhile CB of g-C₃N₄ for e^- is more negative than CB of Bi₂Fe₄O₉. In other words, h⁺ left on VB of Bi₂Fe₄O₉ has higher oxidation capacity to degrade RhB, and e^- left on CB of g-C₃N₄ can be more readily trapped by O₂ to yield ·O₂^-. After forming ternary composite, e^- on the CB of g-C₃N₄ transfers to the CB of UiO-66, and the dissolved O₂ captures e^- on the CB of UiO-66 to yield the superoxide radical anion, ·O₂^-, and then the HOO· radical upon protonation. The ·OH radical can be produced from the trapped electron after formation of the HOO· radical by the following equations^[37].

  图12 Mechanism diagram of the RhB photodegradation on Bi₂Fe₄O₉/g-C₃N₄/UiO-66 ternary composite Figure12. Mechanism diagram of the RhB photodegradation on Bi₂Fe₄O₉/g-C₃N₄/UiO-66 ternary composite

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  The active oxygen species ·O₂^-, HOO· and ·OH radicals have taken part in the degradation of RhB, meanwhile, the photogenerated holes in the VB of Bi₂Fe₄O₉ play a major role, and can directly destroy the adsorbed RhB or react with H₂O to yield ·OH radicals.

  3   Conclusions

  In summary, Bi₂Fe₄O₉/g-C₃N₄/UiO-66 ternary composites were synthesized by a facile solvothermal method, and exhibit higher visible light photocatalytic activities for degradation of RhB dye and colorless phenol solutions as compared with purity materials and binary composite materials. The synergistic effect of photocatalysis is due to big adsorption capacity of UiO-66 and the effective separation of electron-hole pair and their strong redox capability generated by Z-scheme Bi₂Fe₄O₉/g-C₃N₄ heterojunction B.

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• 

  Figure 1  TEM images of (a) UiO-66 discoid particles, (b) Bi₂Fe₄O₉ nanosheet, (c) g-C₃N₄ wrinkled sheet and (d) Bi₂Fe₄O₉/g-C₃N₄/UiO-66 ternary composite (e) HRTEM images of the as-prepared sample Bi₂Fe₄O₉/g-C₃N₄/UiO-66

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  Figure 2  (a) FETEM of Bi₂Fe₄O₉/g-C₃N₄/UiO-66 (2: 1); (b) Bi, (c) N, (d) Zr distribution by EDS mapping of Bi₂Fe₄O₉/g-C₃N₄/UiO-66 (2: 1)

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  Figure 3  XRD patterns of Bi₂Fe₄O₉, UiO-66, g-C₃N₄, Bi₂Fe₄O₉/g-C₃N₄ and Bi₂Fe₄O₉/g-C₃N₄/UiO-66

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  Figure 4  (a) N₂ adsorption-desorption isotherms of Bi₂Fe₄O₉, g-C₃N₄, UiO-66 and Bi₂Fe₄O₉/g-C₃N₄/UiO-66 (2: 1); BJH pore diameter distribution curves of UiO-66 (b) and Bi₂Fe₄O₉/g-C₃N₄/UiO-66 (2: 1) (c); (d) PL spectra of Bi₂Fe₄O₉, g-C₃N₄, UiO-66, Bi₂Fe₄O₉/g-C₃N₄ and Bi₂Fe₄O₉/g-C₃N₄/UiO-66 (2: 1)

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  Figure 5  Short-circuit photocurrent density versus time plotted (0 V versus SCE) for Bi₂Fe₄O₉/g-C₃N₄/ UiO-66 (2: 1), Bi₂Fe₄O₉, g-C₃N₄ and UiO-66 electrodes in 0.01 mol·L^-1 Na₂SO₄ solution under visible light irradiation

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  Figure 6  (a) UV-Vis absorption spectra of Bi₂Fe₄O₉, UiO-66 and g-C₃N₄; (b) Band-gaps of Bi₂Fe₄O₉, UiO-66 and g-C₃N₄

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  Figure 7  (a) Adsorption capacity of RhB onto Bi₂Fe₄O₉, g-C₃N₄, UiO-66, Bi₂Fe₄O₉/g-C₃N₄ and Bi₂Fe₄O₉/ g-C₃N₄/UiO-66 (2: 1); (b) Adsorption capacity of RhB onto the ternary composites with different mass ratios of Bi₂Fe₄O₉/g-C₃N₄ to UiO-66

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  Figure 8  Kinetics of RhB photodegradation on: (a) Bi₂Fe₄O₉, g-C₃N₄, UiO-66, Bi₂Fe₄O₉/g-C₃N₄ and Bi₂Fe₄O₉/ g-C₃N₄/UiO-66; (b) Ternary composites with the different ratios; Kinetics of phenol (10 mg·L^-1) photodegradation on: (c) Bi₂Fe₄O₉, g-C₃N₄, UiO-66, Bi₂Fe₄O₉/g-C₃N₄ and Bi₂Fe₄O₉/g-C₃N₄/UiO-66 (2: 1)

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  Figure 9  (a) Kinetics of RhB photodegradation on the recycled Bi₂Fe₄O₉/g-C₃N₄/UiO-66 (2: 1), (b) Kinetics of phenol photodegradation on the recycled Bi₂Fe₄O₉/g-C₃N₄/UiO-66 (2: 1)

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  Figure 10  Species trapping experiments for degradation of RhB over Bi₂Fe₄O₉/g-C₃N₄/UiO-66 (2: 1)

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  Figure 11  DMPO spin-trapping ESR spectra for Bi₂Fe₄O₉/ g-C₃N₄/UiO-66 (2: 1): (a) in aqueous dispersion for DMPO-·OH, (b) in methanol dispersion for DMPO-·O₂^-

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  Figure 12  Mechanism diagram of the RhB photodegradation on Bi₂Fe₄O₉/g-C₃N₄/UiO-66 ternary composite

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