English

• 0.   Introduction

  Materials of Institute Lavoisier (MIL), one of the categories of metal - organic frameworks (MOF), is a porous nanomaterial formed from terephthalic acid and transition metals, with large specific surface area, high porosity, multi - functionality, tunable structural function, and high stability. Typically, MIL-templatederived materials can inherit the advantages of MIL and have been used in a wide range of excellent applications, such as supercapacitors, lithium-ion batteries, and catalysis^[1].

  Tetracycline (TC), one of the most common antibiotics, is becoming a significant pollutant in the water environment due to its widespread and inappropriate use in human life and pharmaceutical production^[2]. Semiconductor - based photocatalytic degradation technology has attracted a lot of attention in the removal of pollutants from the aqueous environment because of its green - friendly and energy - saving properties^[3]. Most recently, visible-light active BiFeO₃ has drawn growing interest due to its ease of preparation and easy band gap adjustment^[4]. However, the photocatalytic activity of pure BiFeO₃ is still very low because of the low charge transfer efficiency. The development of new and more efficient BiFeO₃-based photocatalysts is essential to meet the requirements of future environmental applications and make better use of solar energy^[5].

  It is widely accepted that strengthening the separation of photogenerated electron - hole pairs and prolonging the visible light reaction are two effective methods^[6]. The recombination of light - generated electronhole pairs can be effectively suppressed once BiFeO₃ is coupled to another semiconductor with a suitable conduction band (CB) or valence band (VB) position^[7]. For example, BiOX@BiFeO₃ and Fe₂O₃@BiFeO₃ binary heterostructures were developed, which showed stronger photocatalytic activity in the oxidative degradation of organic pollutants. Meanwhile, BiOX and Fe₂O₃ were both very sensitive to visible light, which may also be beneficial for the extension of visible light response^[8-9].

  Several new photocatalysts, focusing on metal oxide and oxygen-acid composite semiconductors with narrow band gaps, have now been investigated for photocatalytic applications^[10]. Inorganic BiOI has been reported to be a promising oxy-acid photocatalyst. Among bismuth oxyhalides (BiOX, X=F, Cl, Br, Ⅰ), BiOI with a narrow band gap (1.77 eV) exhibits great photocatalytic performance in practical experiments under visible light irradiation. Fe₂O₃ with a narrow band gap (2.2 eV) is a potential visible light - driven n-type semiconductor due to its low cost, high thermodynamic stability, and environment friendliness^[11-12]. However, the short lifetime of the photogenerated charge carriers and the small hole diffusion length greatly hinder the widespread use of Fe₂O₃ in photocatalysis^[13]. What′s more, when Fe₂O₃ were dispersed onto the surface of a semiconductor with suitable CB edges, such as BiFeO₃, BiOI, WO₃, BiPO₄, or TiO₂, high stability as well as superior visible-light photocatalytic activity of Fe₂O₃ can be attained^[14]. Since both BiOI and Fe ₂ O₃ are suitable for modifying BiFeO₃, it is desirable to devise a ternary composite containing BiOI, Fe₂O₃, and BiFeO₃.

  In this study, new visible-light active BiFeO₃@Fe₂O₃@BiOI ternary composites have been prepared by calcination of MOF precursors and hydrothermal method. The photocatalytic performance of TC degradation with BiFeO₃@Fe₂O₃@BiOI in the absence and presence of organic pollutants was investigated^[15]. In-depth studies were carried out to investigate the effects of changes in complex composition and reaction conditions^[16]. Finally, a capture experiment was carried out to study the factors affecting the photocatalyst.

  1.   Experimental

  1.1   Materials

  The consumable chemicals were absolute ethanol (EtOH), ethylene glycol (EG), iron(Ⅲ) chloride hexahydrate (FeCl₃·6H₂O), N, N-dimethylformamide (DMF), 1, 4-benzene-dicarboxylic acid (C₈H₆O₄), sodium hydroxide (NaOH), hydrogen chloride (HCl), isopropyl alcohol (C₃H₈O), benzoquinone(C₆H ₄ O₂) and sodium oxalate(C₂Na₂O₄), all purchased from Merck Company. The bismuth nitrate pentahydrate (Bi(NO₃)₃·5H₂O), potassium iodide(KI), and tetracycline hydrochloride (C₂₂H₂₅ClN ₂O₈) were purchased from Sigma-Aldrich.

  1.2   Synthesis of MIL-101(Fe)

  Following a reported procedure^[17], 0.675 g (2.45 mmol) FeCl₃ ·6H₂O, 0.206 g (1.24 mmol) 1,4-benzenedicarboxylic acid, and 15 mL DMF was heated in a polytetrafluoroethylene lined hydrothermal reactor at 110 ℃ for 20 h. The orange powder was centrifuged after cooling to room temperature and washed three times with H₂O and ethanol. The orange powder was finally dried at 80 ℃ under a vacuum for 12 h to obtain MIL-101(Fe).

  1.3   Synthesis of Fe₂O₃

  Fe₂O₃ was obtained through calcined in a muffle furnace at 600 ℃ for 5 h and cooled in the furnace to room temperature.

  1.4   Synthesis of MIL-101(Fe)@BiOI

  MIL-101(Fe) (0.711 g, 0.001 mol), Bi(NO₃)₃·5H₂O (0.485 g, 0.001 mol), and 1 g KI were dissolved and suspended in 50 mL of EG and stirred for 20 min. Then, it was added to a polytetrafluoroethylene - lined hydrothermal reactor and reacted at 180 ℃ for 2 h. The suspension obtained was washed three times with water and ethanol and dried in an oven at 80 ℃ for 12 h.

  1.5   Synthesis of BiFeO₃

  MIL-101(Fe) (0.711 g, 0.001 mol), Bi(NO₃)₃·5H₂O (0.161 g, 0.00033 mol), and 0.33 g KI were dissolved in 50 mL of EG and stirred for 20 min. Then, it was added to a polytetrafluoroethylene-lined hydrothermal reactor and reacted at 180 ℃ for 2 h. The obtained suspension was aged for 1 h, centrifuged, washed several times with water and ethanol, and dried at 80 ℃ overnight. The brown powder was calcined in a muffle furnace at 600 ℃ for 5 h and cooled in the furnace to obtain BiFeO₃.

  1.6   Synthesis of BiFeO₃@Fe₂O₃

  MIL - 101(Fe)@BiOI in three calculated mass ratios (6∶1, 5∶1, 4∶1) was put into a crucible, wrapped in tin foil, calcined at 600 ℃ for 5 h in a muffle furnace, and cooled in the furnace. Three mass ratios of BiFeO₃ and Fe₂O₃ (1∶1, 1∶2, 2∶1) were synthesized to explore the best ratio for enhancing activity. When the mass ratio of BiFeO₃ and Fe₂O₃ was 1∶1, we denoted it as 1-BF for clear presentation. Similarly, 2-BF and 0.5-BF mean that the mass ratios of BiFeO₃ and Fe ₂O₃ were 2∶1 and 1∶2, respectively.

  1.7   Synthesis of BiFeO₃@Fe₂O₃@BiOI

  BiFeO₃@Fe₂O₃ (0.891 g, 0.001 mol), Bi(NO₃)₃·5H₂O (0.485 g, 0.001 mol), and 1g KI were dissolved in 50 mL EG and stirred for 20 min. Then, it was added to a polytetrafluoroethylene-lined hydrothermal reactor and reacted at 180 ℃ for 2 h. The obtained suspension was aged for 1 h, centrifuged, washed with water and ethanol several times, and dried at 80 ℃ overnight. BiFeO₃@Fe₂O₃@BiOI (1-BFB) indicates that the mass ratio of BiFeO₃, Fe₂O₃, and BiOI was 1∶1∶1. The preparation flow chart of 1-BFB is shown in Fig. 1.

  Figure 1

  

  Figure 1.  Preparation of 1-BFB

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  1.8   Characterisation

  X-ray diffraction (XRD) data were collected with Cu Kα radiation (λ =0.154 nm) on a Shimadzu XRD-6000 instrument (voltage: 40 kV, current: 30 mA, scan range: 5°-80°, scan speed: 5 (°)·min^-1). Scanning electron microscopy (SEM) was performed on a Japan Electronics JSM-6480 microscope to observe the morphology of the photocatalysts. High-resolution transmission electron microscopy (HRTEM) and scanning TEM elemental mapping were achieved by an FEI Tecnai G2 F30 microscope (accelerating voltage: 300 kV). X-ray spectroscopy (XPS) was used to study the binding energies of all the elements of composite materials. FTIR spectra were obtained using an Agilent spectrometer in a wavenumber range of 4 000-400 cm^-1. The UV - Vis absorption spectra were recorded on a Hitachi U4100 UV spectrometer. Fluorescent spectra were obtained on an FS5 spectrofluorometer. The transient photo-current measurements were performed using a DH-7000 Electrochemical Workstation (Jiangsu Donghua Analysis Instruments Co., Ltd., China), equipped with three electrodes including an ITO electrode covered with the samples, Pt and Ag@AgCl electrodes. For the single working electrode, 5 mg of the sample was dispersed in 10 μL Nafion and then added 0.1 mL anhydrous ethanol to make a homogeneous solution. Then, 40 μL of the above solution was dropped onto the ITO conducting glass. The 0.5 mol·L^-1 Na₂SO₄ aqueous solution was used as the electrolyte and exposed with a Xe lamp (250 W, λ > 420 nm). The impedance test was performed in a frequency range of 0.1 Hz-10 kHz with an amplitude of 0.005 V, a silence time of 2 s, and an initial potential of 0.071 V.

  1.9   Photocatalytic activity test

  The photocatalytic degradation of TC under visible light irradiation was studied with the photocatalysts. A 250 W xenon lamp (λ≥420 nm) and an ultraviolet cut-off filter were used as the visible light source. The photocatalyst (50 mg) was dispersed in 100 mL TC aqueous solution (10 mg·L^-1) and stirred for 30 min away from light to ensure the adsorption - desorption equilibrium. After centrifugation of the suspension to remove the catalyst from the aqueous solution, the absorbance of TC was analyzed at 357 nm using a UV-Vis spectrophotometer (Persee TU-1901, China) to detect the concentration of TC in the solution.

  2.   Results and discussion

  2.1   Morphology and structure

  Fig. 2 shows the XRD patterns of composites BiFeO₃, Fe₂O₃, BiOI, BiFeO₃@Fe₂O₃, and BiFeO₃@ Fe₂O₃@BiOI. As shown in Fig. 2, the XRD patterns reflect the tetragonal phase of BiOI (PDF No.73-2062), with the main diffraction peaks at 2θ being 29.73°, 31.72°, 45.46°, and 55.23°^[18]. Moreover, the peaks at 2θ of 22.51°, 32.11°, and 45.97° are indexed to the (101), (110), and (202) planes of BiFeO₃ (PDF No.72-2321). Other diffraction peaks were observed at 2θ = 28.08°, 31.1° and 53.4°^[19-20], which are similar to those of the Fe₂O₃ hematite phases with rhombohedra (PDF No.25-1402), suggesting the successful construction of Fe₂O₃ on BiFeO₃. For 1-BFB, different peaks corresponding to BiFeO₃ and Fe₂O₃, and BiOI can be observed, indicating the ternary composite was successfully synthesized^[21].

  Figure 2

  

  Figure 2.  XRD patterns of selected samples

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  To study the surface morphology of the synthesized samples, they were characterized using SEM images. As shown in Fig. 3b, BiOI presented a uniform 3D microsphere structure, uniformly distributed on the surface of MIL-101(Fe). In Fig. 4a, BiFeO₃ particles synthesized by calcination were agglomerated. Fe₂O₃ was attached to the surface of BiFeO₃ through a calcination reaction (Fig. 4b). After being decorated with BiOI, Fe₂O₃ and BiOI nanoparticles could be observed to anchor on the surface of ball-like BiFeO₃, where the hollow structure was collapsed (Fig. 4c and 4d) ^[22-23]. This indicates that ternary composites with a heterojunction structure have been successfully synthesized. Fig. 4e-4h presents the element mapping and energy dispersive spectrum of 1-BFB for elements O, I, Fe, and Bi. No other impurities were discovered.

  Figure 3

  

  Figure 3.  SEM images of (a) BiOI and (b) MIL-101(Fe)@BiOI

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  Figure 4

  

  Figure 4.  SEM images of (a) BiFeO₃, (b) 1-BF, (c, d) 1-BFB and corresponding elemental mappings of O (e), I(f), Fe(g) and Bi(h)

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  The HRTEM image (Fig. 5b) showed a lattice spacing of 0.285 nm which corresponds to the (110) plane of BiFeO₃, a lattice fringe at 0.276 nm which is ascribed to the (104) plane of Fe₂ O₃, and a lattice fringe at 0.204 nm which is ascribed to the (113) plane of BiOI. The FFT (fast Fourier transform) image has been derived by selecting the square area in Fig. 5c. The FFT image gave three groups of adjacent symmetrical points, which can be indexed to the planes of (110) of BiFeO₃, (104) of Fe₂O₃ and (113) of BiOI (Fig. 5d). This also verifies the successful preparation of 1-BFB. All the above results confirm the successful formation of as-prepared heterojunctions.

  Figure 5

  

  Figure 5.  (a) TEM and (b, c) HRTEM images of 1-BFB; (d) FFT image in the red square box in c

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  XPS spectra of 1-BFB are displayed in Fig. 6a, for analyzing the surface element compositions and the chemical states of all elements in as-prepared photocatalysts. The binding energy of the obtained samples was calibrated by the C1s peak at 284.7 eV (Fig. 6c). The high-resolution XPS spectrum of Bi4f revealed that two peaks presented at 159.04 and 164.7 eV are assigned to Bi4 f_7/2 and Bi4f_5/2 spin states, which are corresponding to the oxidation state of Bi³⁺ in BiFeO₃ and BiOI (Fig. 6b). The binding energy of O peak at 530.2 eV corresponding to O1s can represent for O^2- in BiFeO₃, BiOI, and Fe₂O₃ (Fig. 6d). The binding energies of I peaks at 619.2 and 630.7 eV corresponding to I3d_5/2 and I3d_3/2 can represent for I^- in BiOI (Fig. 6e). The binding energies of Fe peaks at 711.6 and 725.0 eV corresponding to Fe2p_3/2 and Fe2p_1/2 can represent for Fe³⁺ in BiFeO₃ and Fe₂O₃ (Fig. 6f).

  Figure 6

  

  Figure 6.  XPS spectra of 1-BFB: (a) survey, (b) Bi4f, (c) C1s, (d) O1s, (e) I3d, and (f) Fe2p

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  The UV-Vis absorption spectra of BiFeO₃, Fe₂O₃, 0.5-BF, 1-BF, 2-BF, and 1-BFB are shown in Fig. 7. The band edges of the light absorption coefficients follow the Tauc plot as a function of (αhν)²=A(hν-E_g), where E_g, ν, h, and A are the band gap, optical frequency, Planck′s constant, and constant, respectively^[24]. The E_g of the photocatalyst was evaluated by plotting (αhν)^1/2 vs the optical energy (hν) ^[25]. The estimated band gaps for BiFeO₃, Fe₂O₃, 1-BF, and 1-BFB were about 2.20, 1.88, 1.71, and 1.68 eV, respectively. The reduction in band gap results in better visible light response and enhanced photocatalytic activity.

  Figure 7

  

  Figure 7.  UV-Vis absorption spectra and Tauc plot of (αhν)² vs hν for the samples

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  2.2   Photocatalytic performance

  The photocatalytic performance of the prepared photo-catalyst was assessed by the degradation of TC under visible light irradiation (Fig. 8). When BiFeO₃, 1-BF, 0.5-BF, 2-BF, and 1-BFB were induced as photocatalysts, 40%, 53%, 52%, 51% and 96% of TC can be removed respectively. The enhanced removal of this material can be attributed to the powerful adsorption capacity of the hollow flower-like structure derived from BiOI^[26].

  Figure 8

  

  Figure 8.  (a) Degradation efficiencies and (b) fitted rate constants for photocatalytic degradation of TC (10 mg·L^-1) by as-prepared samples

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  1-BFB shows higher photocatalytic activity compared with BiFeO₃ and BiFeO₃@Fe₂O₃ composites, indicating improved charge transfer in 1-BFB^[27]. The photodegradation of organic pollutants usually follows a pseudo-first-order equation:

  $ \ln \left(c_0 / c\right)=k t $

  where c (mol·L^-1) is the transient concentration of the TC concentration in solution at time t, c₀ is reactant concentration at t=0 and k is the epiphenomenal pseudo-first-order rate constant^[28]. The linear fitting results are shown in Fig. 8b. The results show that the charge transfer efficiency of BiFeO₃@Fe₂O₃ composites can be improved by combining BiOI to create effective heterogeneous functional structures. Due to the charge potential difference and interfacial interactions between BiFeO₃, Fe₂O₃, and BiOI, BiOI can serve as a good platform to facilitate the separation and transfer of electron-hole pairs^[29]. Table 1 shows that 1-BFB can be an excellent photocatalyst compared to the materials reported^[30-35].

  Table 1

  

  Table 1.  Comparison with reported BiFeO₃-based photocatalysts for degradation of TC

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  Sample	mcat/mg	ρTC/(mg·L-1)	Removal efficiency/%	Time/min	Ref.
  BiFeO3/Bi2O3	20	30	80.0	120	[30]
  BiFeO3/TiO2	50	10	67.9	180	[31]
  BiFeO3/Bi2WO6/g-C3N4	10	10	83.7	45	[32]
  BiFeO3/BNQDs	20	10	80.0	180	[33]
  BiFeO3/MnFeO3	20	10	91.9	180	[34]
  BiFeO3/Er	200	30	57.8	180	[35]
  BiFeO3@Fe2O3@BiOI	50	10	96	90	This work

  In addition, the TOC (total organic carbon) removal efficiency during the photocatalytic TC removal process was investigated to determine the degree of mineralization of the pollutant over 1-BFB. The TOC removal efficiencies at 0 and 60 min were 41% and 75%, respectively. The TOC measurement can detect the intermediates of TC in the photocatalytic process, resulting in a lower removal efficiency than the UV-Vis measurement.

  The pH is considered to have a significant influence on the TC degradation activity. Fig. 9 shows that the degradation rate constant was highest at pH=7 and slightly decreased under acidic (pH=4) or alkaline (pH =13) conditions. However, it is easy to see from Fig. 9 that the effect on the degradation activity of 1-BFB is limited at different pH environments, indicating that the material has good stability. In acidic or alkaline environments, TC molecules generate an electrical charge creating an electrostatic effect on the composite surface, thus enhancing the adsorption capacity of the composite^[36-37].

  Figure 9

  

  Figure 9.  Photocatalytic degradation of TC (10 mg·L^-1) by 1-BFB in the solutions with different pH values: (a) degradation efficiencies and (b) fitted rate constants

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  The ionic strength is another major factor affecting the TC degradation activity. The ionic strength has an effect on the sorption of TC in the dark. When the KCl concentration was increased from 0 to 15 mg·L^-1 (Fig. 10), the adsorption capacity of 1-BFB increased slightly, which may be due to the electrostatic interaction after the addition of KCl^[38].

  Figure 10

  

  Figure 10.  Photocatalytic degradation of TC (10 mg·L^-1) by 1-BFB in the solutions containing KCl with different concentrations: (a) degradation efficiencies and (b) fitted rate constants

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  Under the environment of TC concentration of 10-40 mg·L^-1, the effect of different initial TC concentrations on the degradation was studied. The TC concentration has an important influence on the adsorption and photocatalytic processes (Fig. 11). As the TC concentration increased, the degradation efficiency of the catalyst gradually decreased. The sharp drop in the degradation rate constant should be due to because the TC concentration is too high, the excess TC molecules collect and adsorb on the surface of 1-BFB, which impedes the transfer of visible light^[39].

  Figure 11

  

  Figure 11.  Photocatalytic degradation of TC (10 mg·L^-1) by 1-BFB in the solutions with different initial TC concentrations: (a) degradation efficiencies and (b) fitted rate constants

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  Taking 1-BFB as an example, reactive radical capture tests were carried out on the degradation reactions of TC. Isopropyl alcohol (IPA), p-benzoquinone (BQ), and sodium oxalate (SO) were selected as scavengers for ·OH, ·O₂^-, and h^{+ [40]}. As shown in Fig. 12, when BQ was added, the degradation efficiency of TC decreased, indicating ·O₂^- is the major active substance in the degradation process.

  Figure 12

  

  Figure 12.  Photocatalytic degradation of TC (10 mg·L^-1) by 1-BFB in the presence of different inhibitors: (a) degradation efficiencies and (b) fitted rate constants

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  The transfer and generation of light-excited charge carriers have been studied by transient photocurrent reactions in the presence or absence of visible illumination. Stronger photocurrent intensities tend to reveal higher efficiency in the separation of holes and electrons^[41]. Fig. 13 illustrates the instantaneous photocurrent curve of BiFeO₃, Fe₂O₃, 1-BF, 0.5-BF, 2-BF, and 1-BFB. 1-BFB showed the highest photocurrent response with intensities 3.02 and 2.10 times those of pristine BiFeO₃ and Fe₂O₃, respectively. This indicates that a more efficient electron and hole separation efficiency occurs in the 1 -BFB heterojunction, leading to a significant increase in photocatalytic activity^[42].

  Figure 13

  

  Figure 13.  Photocurrent responses of the monomers and composites

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  To further confirm the separation of the effective charge in the material, the samples were analyzed by fluorescence spectroscopy. An emission peak could be seen near 546 nm due to the recombination of photogenerated electrons and valence band holes (Fig. 14). The peak intensity of BiFeO₃ was 3.02 times that of 1-BFB, which is consistent with the photocurrent test results. The composites had lower emission intensity, suggesting that they are more conducive to charge separation, which inhibits the recombination of electronhole pairs.

  Figure 14

  

  Figure 14.  Emission fluorescence spectra of the monomers and composites

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  The charge transfer efficiency at semiconductor phototube interfaces was verified by electrochemical impedance spectroscopy (EIS). Fig. 15 displays the EIS Nyquist plots of BiFeO₃, Fe₂O₃, BiFeO₃@Fe₂O₃, and 1-BFB composites. The curve arc of 1-BFB was significantly lower than the other samples, indicating that the transfer resistance of the charge carriers at the surface of 1-BFB is smaller^[43]. Fig. 16 shows the resistance R_ct values for each sample, with the phase angle changes concentrated in the low - frequency region, indicating that the response of the material to the circuit is mainly in the low-frequency region.

  Figure 15

  

  Figure 15.  EIS Nyquist plots of the monomers and composites

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  Figure 16

  

  Figure 16.  Equivalent circuit diagram (Inset) and R_ct and C_d values for the monomers and composites

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  The charge density in a semiconductor is verified by testing the Mott-Schottky curves. Fig. 17 shows the Mott-Schottky curves for the monomers and composites, and the positive slope of the Mott-Schottky plot indicates that both 1-BFB and BiFeO₃ are n-type semiconductors^[44]. The slopes of the Mott-Schottky curves of all composite samples were higher than those of BiFeO₃, Fe₂O₃, and BiOI, implying that the carrier densities of the composite samples are higher than those of the monomer samples. In particular, 1-BFB had the best carrier density. The carrier improvement can be attributed to two factors. Firstly, the compounding of the three semiconductors decreases the band gap width, which improves the absorption efficiency of light and facilitates the production of photoelectrons and holes. Secondly, the forming of heterojunctions facilitates the separation of photoelectrons and holes and inhibits the recombination of photogenerated carriers, thus extending their lifetimes^[45].

  Figure 17

  

  Figure 17.  Mott-Schottky curves of the monomers and composites

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  The energy level diagram of the material ground charge separation and transfer between BiFeO₃, Fe₂O₃, and BiOI catalyst surfaces under optical excitation is shown in Fig. 18. The valence band (VB) and conduction band (CB) potentials of BiFeO₃, Fe₂O₃, and BiOI were determined from the literature^[46]. The VB and CB energetic potentials were about 2.44 and 0.64 eV for BiFeO₃, 1.195 and -0.635 eV for Fe₂O₃, and 1.59 and -0.25 eV for BiOI. The electrons on the CB of Fe₂O₃ can interact with the absorbed O₂ to create reactive species superoxide radicals (·O₂^-), which contributes to the degradation of TC^[47-48]. Meanwhile, the holes on the VB of BiOI and BiFeO₃ migrated to the VB of Fe₂O₃ with weaker oxidative activity because of the lower energetic potential of 1.195 eV, which is in accordance with the reactive radical capture test.

  Figure 18

  

  Figure 18.  Energy band structure and charge separation map of 1-BFB and possible ways of photodegradation of pollutants

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  3.   Conclusions

  A ternary composite of BiFeO₃@Fe₂O₃@BiOI was prepared by calcination and hydrothermal methods. As compared to BiFeO₃, Fe₂O₃, and BiOI monomers, the formation of BiFeO₃@Fe₂O₃@BiOI heterojunction enhances the photocatalytic activity of photocatalyst. The trapping experiment showed that ·O₂^- is the prime active molecule. BiFeO₃@Fe₂O₃@BiOI displays n-type heterojunction, which can improve the separation efficiency of electrons and holes, reduce carrier recombination and produce an increased number of active ·O₂^- molecules. Therefore, BiFeO₃@Fe₂O₃@BiOI is very promising as a photocatalyst for the degradation of TC.

  
  Acknowledgments: This work was partially supported by the Natural Science Foundation of Jiangsu Province (Grant No. BK20190245), the Qinglan Project of Jiangsu Province, the Research Foundation for Talented Scholars of Taizhou University (Grant No. QD2016006), the Technology Support Program (agriculture) of Taizhou (Grant No.TN202122).
• 1. [1]

     Mehraj O, Pirzada B M, Mir N A, Khan M Z, Sabir S. A highly efficient visible-light-driven novel p-n junction Fe₂O₃/BiOI photocatalyst: Surface decoration of BiOI nanosheets with Fe₂O₃ nanoparticles[J]. Appl. Surf. Sci., 2016, 387(30):  642-651.

  2. [2]

     Wang Q, Shi X, Liu E, C J, Crittenden , Ma X J, Zhang Y, Cong Y Q. Facile synthesis of AgI/BiOI-Bi₂O₃ multi - heterojunctions with high visible light activity for Cr(Ⅵ) reduction[J]. J. Hazard. Mater., 2016, 317:  8-16. doi: 10.1016/j.jhazmat.2016.05.044

  3. [3]

     Chang M J, Wang H, Li H L, Liu J, Du H L. Facile preparation of novel Fe₂O₃/BiOI hybrid nanostructures for efficient visible light photocatalysis[J]. J. Mater. Sci. Technol., 2017, 53(5):  3682-3691.

  4. [4]

     Zhang Y, Li Y, Sun W N, Yuan C X, Wang B X, Zhang W, Song X M. Fe₂O₃/BiOI - based photoanode with n-p heterogeneous structure for photoelectric conversion[J]. Langmuir, 2017, 33(43):  12065-12071. doi: 10.1021/acs.langmuir.7b02969

  5. [5]

     Bera S, Ghosh S, Shyamal S, Bhattacharya C, Basu R N. Photocatalytic hydrogen generation using gold decorated BiFeO₃ heterostructures as an efficient catalyst under visible light irradiation[J]. Sol. Energy Mater. Sol. Cells, 2019, 194:  195-206. doi: 10.1016/j.solmat.2019.01.042

  6. [6]

     Wang C C, Cai M J, Liu Y P, Yang F, Zhang H Q, Liu J S, Li S J. Facile construction of novel organic-inorganic tetra(4-carboxyphenyl) porphyrin/Bi₂MoO₆ heterojunction for tetracycline degradation: Performance, degradation pathways, intermediate toxicity analysis, and mechanism insight[J]. J. Colloid Interface Sci., 2022, 605:  727-740. doi: 10.1016/j.jcis.2021.07.137

  7. [7]

     Di L J, Yang H, Xian T, Liu X Q, Chen X J. Photocatalytic and photo-Fenton catalytic degradation activities of Z-scheme Ag₂S/BiFeO₃ heterojunction composites under visible-light irradiation[J]. Nanomaterials, 2019, 9(3):  399. doi: 10.3390/nano9030399

  8. [8]

     Hu X J, Wang W X, Xie G Y, Wang H, Tan X F, Jin Q, Zhou D X, Zhao Y L. Ternary assembly of g-C₃N₄/graphene oxide sheets/BiFeO₃ heterojunction with enhanced photoreduction of Cr(Ⅵ) under visible-light irradiation[J]. Chemosphere, 2019, 216:  733-741. doi: 10.1016/j.chemosphere.2018.10.181

  9. [9]

     Huo H W, Hu X J, Wang H, Li J, Xie G Y, Tan X F, Jin Q, Zhou D X, Li C, Qiu G Q, Liu Y G. Synergy of photocatalysis and adsorption for simultaneous removal of hexavalent chromium and methylene blue by g-C₃N₄/BiFeO₃/carbon nanotubes ternary composites[J]. Int. J. Environ. Res. Public Health, 2019, 16(17):  3219. doi: 10.3390/ijerph16173219

  10. [10]

      Shang J, Chen H G, Chen T Z, Wang X W, Feng G, Zhu M W, Yang Y X, Jia X S. Photocatalytic degradation of rhodamine B and phenol over BiFeO₃/BiOCl nanocomposite[J]. Appl. Phys. A-Mater. Sci. Process., 2019, 125(2):  1-7.

  11. [11]

      Xie Y Y, Zhang C S, Wang D T, Lu J F, Wang Y H, Wang J, Zhang L Z, Zhang R Q. Catalytic performance of a Bi₂O₃-Fe₂O₃ system in soot combustion[J]. New J. Chem., 2019, 43(38):  15368-15374. doi: 10.1039/C9NJ03419F

  12. [12]

      Yang Q Q, Deng J X, Wang G S, Deng Q S, Zhao J L, Dai Y X, Duan P, Cui M, Kong L, Gao H L, Nie R J, Wang F. The physical properties and microstructure of BiFeO₃/YBCO heterostructures[J]. Vacuum, 2019, 167:  313-318. doi: 10.1016/j.vacuum.2019.06.025

  13. [13]

      Li S J, Shen X F, Liu J S, Zhang L S. Synthesis of Ta₃N₅/Bi₂MoO₆ core-shell fiber-shaped heterojunctions as efficient and easily recyclable photocatalysts[J]. Environ. Sci.-Nano, 2017, 4(5):  1155-1167. doi: 10.1039/C6EN00706F

  14. [14]

      Bai P J, Li Y T, Wang G, Han J, Wei Y X, Li M W, Mao D, Zeng Y M. Fabrication of Al₂O₃-coated BiFeO₃ particles and fine - grained ceramics with improved electric properties[J]. J. Mater. Sci. - Mater. Electron., 2020, 31(23):  21723-21731. doi: 10.1007/s10854-020-04685-w

  15. [15]

      Li S J, Wang C C, Cai M J, Yang F, Liu Y P, Chen J L, Zhang P, Li X, Chen X B. Facile fabrication of TaON/Bi₂MoO₆ core - shell S-scheme heterojunction nanofibers for boosting visible-light catalytic levofloxacin degradation and Cr(Ⅵ) reduction[J]. Chem. Eng. J., 2022, 428:  131158. doi: 10.1016/j.cej.2021.131158

  16. [16]

      Guo Y H, Zhou S H, Sun X K, Yuan H L. Synthesis and photocatalytic activity of BiFeO₃ and Bi/BiFeO₃ cubic microcrystals[J]. J. Am. Ceram. Soc., 2020, 103(8):  4122-4128. doi: 10.1111/jace.17083

  17. [17]

      Huang Y X, Lin H, Zhang Y H. Synthesis of MIL-101(Fe)/SiO₂ composites with improved catalytic activity for reduction of nitroaromatic compounds[J]. J. Solid State Chem., 2019, 283:  121150.

  18. [18]

      Lei Y X, Zhang Y P, Ding W M, Yu L Q, Zhou X P, Wu C M. Preparation and photoelectrochemical properties of BiFeO₃/BiOI composites[J]. RSC Adv., 2020, 10(45):  26658-26663. doi: 10.1039/D0RA02457K

  19. [19]

      Mansingh S, Sultana S, Acharya R, Ghosh M K, Parida K M. Correction to efficient photon conversion via double charge dynamics CeO₂-BiFeO₃ p-n heterojunction photocatalyst promising toward N₂ fixation and phenol-Cr(Ⅵ) detoxification[J]. Inorg. Chem., 2020, 59(9):  6646. doi: 10.1021/acs.inorgchem.0c00981

  20. [20]

      Margha F H, Radwan E K, Badawy M I, Gad-Allah T A. Bi₂O₃-BiFeO₃ glass - ceramic: Controllable β-/γ-Bi₂O₃ transformation and application as magnetic solar - driven photocatalyst for water decontamination[J]. ACS Omega, 2020, 5(24):  14625-14634. doi: 10.1021/acsomega.0c01307

  21. [21]

      Rusly S N A, Ismail I, Matori K A, Abbas Z, Shaari , A H, Ibrahim I R. A study of multiferroic BiFeO₃/epoxy resin composite as potential coating materials for microwave absorption[J]. Solid State Phenomena, 2020, 307:  20-25. doi: 10.4028/www.scientific.net/SSP.307.20

  22. [22]

      Zhang X Y, Wang X, Chai J N, Xue S, Wang R X, Jiang L, Wang J, Zhang Z H, Dionysiou D D. Construction of novel symmetric double Z-scheme BiFeO₃/CuBi₂O₄/BaTiO₃ photocatalyst with enhanced solar-light-driven photocatalytic performance for degradation of norfloxacin[J]. Appl. Catal. B-Environ., 2020, 272:  119017. doi: 10.1016/j.apcatb.2020.119017

  23. [23]

      Li H J, Zhuang J, Bokov A A, Zhang N, Zhang J, Zhao J Y, Ren W, Ye Z G. Evolution of relaxor behavior in multiferroic Pb (Fe_2/3W_1/3)O₃-BiFeO₃ solid solution of complex perovskite structure[J]. J. Eur. Ceram. Soc., 2021, 41(1):  310-318. doi: 10.1016/j.jeurceramsoc.2020.07.068

  24. [24]

      Su G, Liu L H, Liu X, Zhan L X, Xue J R, Tang A P. Magnetic Fe₃O₄@SiO₂@BiFeO₃/rGO composite for the enhanced visible - light catalytic degradation activity of organic pollutants[J]. Ceram. Int., 2021, 47(4):  5374-5387. doi: 10.1016/j.ceramint.2020.10.118

  25. [25]

      Zhu Y, Zhu M, Lv H, Zhao S, Shen X R, Zhang Q Y, Zhu W F, Li B D. Coating BiOCl@g-C₃N₄ nanocomposite with a metal organic framework: Enhanced visible light photocatalytic activities[J]. J Solid State Chem., 2020, 292:  121641. doi: 10.1016/j.jssc.2020.121641

  26. [26]

      Kadi M W, Mohamed R M, Ismail A A. Facile synthesis of mesoporous BiFeO₃/graphene nanocomposites as highly photoactive under visible light[J]. Opt. Mater., 2020, 104:  109842. doi: 10.1016/j.optmat.2020.109842

  27. [27]

      Shi Y H, Li J S, Wan D J, Huang J H, Liu Y D. Peroxymonosulfate-enhanced photocatalysis by carbonyl - modified g-C₃N₄ for effective degradation of the tetracycline hydrochloride[J]. Sci. Total Environ., 2020, 749:  142313. doi: 10.1016/j.scitotenv.2020.142313

  28. [28]

      Liu N, Tang M Q, Jing C W, Huang W Y, Tao P, Zhang X D, Lei J Q, Tang L. Synthesis of highly efficient Co₃O₄ catalysts by heat treatment ZIF-67 for CO oxidation[J]. J. Sol-Gel Sci. Technol., 2018, 88:  163-171. doi: 10.1007/s10971-018-4784-x

  29. [29]

      Li S J, Wang C C, Liu Y P, Xue B, Jiang W, Liu Y, Mo L Y, Chen X B. Photocatalytic degradation of antibiotics using a novel Ag/Ag₂S/Bi₂MoO₆ plasmonic p-n heterojunction photocatalyst: Mineralization activity, degradation pathways and boosted charge separation mechanism[J]. Chem. Eng. J., 2021, 415:  128991. doi: 10.1016/j.cej.2021.128991

  30. [30]

      Duan F, Ma Y, Lv P, Sheng J L, Zhu H, Du M L, Chen X, Chen M Q. Oxygen vacancy - enriched Bi₂O₃/BiFeO₃ p-n heterojunction nanofibers with highly efficient photocatalytic activity under visible light irradiation[J]. Appl. Surf. Sci., 2021, 562:  150171. doi: 10.1016/j.apsusc.2021.150171

  31. [31]

      Liao X L, Li T T, Ren H T, Mao Z Y, Zhang X F, Lin J H. Enhanced photocatalytic performance through the ferroelectric synergistic effect of p-n heterojunction BiFeO₃/TiO₂ under visible-light irradiation[J]. Ceram. Int., 2021, 47(8):  10786-10795. doi: 10.1016/j.ceramint.2020.12.195

  32. [32]

      Wang T Y, Bai Y C, Si W, Mao W, Gao Y H, Liu S X. Heterogeneous photo-Fenton system of novel ternary Bi₂WO₆/BiFeO₃/g-C₃N₄ heterojunctions for highly efficient degrading persistent organic pollutants in wastewater[J]. J. Photochem. Photobiol. A-Chem., 2021, 404:  112856. doi: 10.1016/j.jphotochem.2020.112856

  33. [33]

      Balta Z, Simsek E B. Uncovering the systematical charge separation effect of boron nitride quantum dots on photocatalytic performance of BiFeO₃ perovskite towards degradation of tetracycline antibiotic[J]. J. Environ. Chem. Eng., 2021, 9(6):  106567. doi: 10.1016/j.jece.2021.106567

  34. [34]

      Balta Z, Simsek E B. Understanding the structural and photocatalytic effects of incorporation of hexagonal boron nitride whiskers into ferrite type perovskites (BiFeO₃, MnFeO₃) for effective removal of pharmaceuticals from real wastewater[J]. J. Alloy. Compd., 2022, 898:  162897. doi: 10.1016/j.jallcom.2021.162897

  35. [35]

      Zhou J B, Jiang L D, Chen D, Liang J H, Qin L S, Bai L Q, Sun X G, Huang Y X. Facile synthesis of Er-doped BiFeO₃ nanoparticles for enhanced visible light photocatalytic degradation of tetracycline hydrochloride[J]. J. Sol-Gel Sci. Technol., 2019, 90(3):  535-546. doi: 10.1007/s10971-019-04932-5

  36. [36]

      Zhu Y, Han Z G, Zhao S Y, Zhang Q Y, Shen X R, Lv H, Liu J, Li B D. In-situ growth of Ag/AgBr nanoparticles on a metal organic framework with enhanced visible light photocatalytic performance[J]. Mater. Sci. Semicond. Process, 2021, 133:  105973. doi: 10.1016/j.mssp.2021.105973

  37. [37]

      Zhu M, Chen H M, Dai Y, Wu X Y, Han Z G, Zhu Y. Novel n-p-n heterojunction of AgI/BiOI/UiO-66 composites with boosting visible light photocatalytic activities[J]. Appl. Organomet. Chem., 2021, 35(5):  e6186.

  38. [38]

      Wang H Y, Fu W Y, Chen Y W, Xue F Y, Shan G Y. ZIF-67-derived Co₃O₄ hollow nanocage with efficient peroxidase mimicking characteristic for sensitive colorimetric biosensing of dopamine[J]. Spectroc. Acta Pt. A-Molec. Biomolec. Spectr., 2021, 246:  119006. doi: 10.1016/j.saa.2020.119006

  39. [39]

      Hu L X, Hu H M, Lu W C, Lu Y S, Wang S Q. Novel composite Bi-FeO₃/ZrO₂ and its high photocatalytic performance under white LED visible-light irradiation[J]. Mater. Res. Bull., 2019, 120:  110605. doi: 10.1016/j.materresbull.2019.110605

  40. [40]

      Li Z D, Cheng L, Zhang K, Wang Z H. Enhanced photocatalytic performance by Y-doped BiFeO₃ particles derived from MOFs precursor based on band gap reduction and oxygen vacancies[J]. Appl. Organomet. Chem., 2021, 35(3):  6113.

  41. [41]

      Li S J, Wang C C, Liu Y P, Cai M J, Wang Y N, Guo Y, Zhao W, Wang Z H, Chen X B. Photocatalytic degradation of tetracycline antibiotic by a novel Bi₂Sn₂O₇/Bi₂MoO₆ S-scheme heterojunction: Performance, mechanism insight, and toxicity assessment[J]. Chem. Eng. J., 2022, 429:  132519. doi: 10.1016/j.cej.2021.132519

  42. [42]

      Bai Y, Ye L Q, Chen T, Wang L, Shi X, Zhang X, Chen D. Facet-dependent photocatalytic N₂ fixation of bismuth-rich Bi₅O₇I nanosheets[J]. ACS Appl. Mater. Interfaces, 2016, 8(41):  27661-27668. doi: 10.1021/acsami.6b08129

  43. [43]

      Chao X J, Yang Z P, Kang C, Gu R. Effect of BiFeO₃ addition on Bi₂O₃-ZnO-Nb₂O₅ based ceramics[J]. Curr. Appl. Phys., 2010, 10(1):  26-30. doi: 10.1016/j.cap.2009.04.007

  44. [44]

      Zhu Y, Wang Y M, Liu P, Xia C K, Wu Y L, Lu X Q, Xie J M. Two chelating-amino-functionalized lanthanide metal-organic frameworks for adsorption and catalysis[J]. Dalton Trans., 2015, 44(4):  1955-1961. doi: 10.1039/C4DT02048K

  45. [45]

      Li Z D, Zhang S L, Xu R C, Zhang Q W, Wang Z H, Fu C L. Photocatalytic performance of BiFeO₃ based on MOFs precursor[J]. Appl. Organomet. Chem., 2019, 33(10):  e5105.

  46. [46]

      Li S J, Chen J L, Hu S W, Wang H L, Jiang W, Chen X B. Facile construction of novel Bi₂WO₆/Ta₃ N₅ Z-scheme heterojunction nanofibers for efficient degradation of harmful pharmaceutical pollutants[J]. Chem. Eng. J., 2020, 402:  12616.

  47. [47]

      Lam S M, Sin J C, Mohamed A R. A newly emerging visible light-responsive BiFeO₃ perovskite for photocatalytic applications: A mini review[J]. Mater. Res. Bull., 2017, 90:  15-30. doi: 10.1016/j.materresbull.2016.12.052

  48. [48]

      Li S J, Wang C C, Liu Y P, Xue B, Chen J L, Wang H W, Liu H. Facile preparation of a novel Bi₂WO₆/calcined mussel shell composite photocatalyst with enhanced photocatalytic performance[J]. Catalysts, 2020, 10(10):  1166. doi: 10.3390/catal10101166

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  Figure 1  Preparation of 1-BFB

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  Figure 2  XRD patterns of selected samples

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  Figure 3  SEM images of (a) BiOI and (b) MIL-101(Fe)@BiOI

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  Figure 4  SEM images of (a) BiFeO₃, (b) 1-BF, (c, d) 1-BFB and corresponding elemental mappings of O (e), I(f), Fe(g) and Bi(h)

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  Figure 5  (a) TEM and (b, c) HRTEM images of 1-BFB; (d) FFT image in the red square box in c

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  Figure 6  XPS spectra of 1-BFB: (a) survey, (b) Bi4f, (c) C1s, (d) O1s, (e) I3d, and (f) Fe2p

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  Figure 7  UV-Vis absorption spectra and Tauc plot of (αhν)² vs hν for the samples

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  Figure 8  (a) Degradation efficiencies and (b) fitted rate constants for photocatalytic degradation of TC (10 mg·L^-1) by as-prepared samples

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  Figure 9  Photocatalytic degradation of TC (10 mg·L^-1) by 1-BFB in the solutions with different pH values: (a) degradation efficiencies and (b) fitted rate constants

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  Figure 10  Photocatalytic degradation of TC (10 mg·L^-1) by 1-BFB in the solutions containing KCl with different concentrations: (a) degradation efficiencies and (b) fitted rate constants

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  Figure 11  Photocatalytic degradation of TC (10 mg·L^-1) by 1-BFB in the solutions with different initial TC concentrations: (a) degradation efficiencies and (b) fitted rate constants

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  Figure 12  Photocatalytic degradation of TC (10 mg·L^-1) by 1-BFB in the presence of different inhibitors: (a) degradation efficiencies and (b) fitted rate constants

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  Figure 13  Photocurrent responses of the monomers and composites

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  Figure 14  Emission fluorescence spectra of the monomers and composites

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  Figure 15  EIS Nyquist plots of the monomers and composites

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  Figure 16  Equivalent circuit diagram (Inset) and R_ct and C_d values for the monomers and composites

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  Figure 17  Mott-Schottky curves of the monomers and composites

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  Figure 18  Energy band structure and charge separation map of 1-BFB and possible ways of photodegradation of pollutants

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  Table 1.  Comparison with reported BiFeO₃-based photocatalysts for degradation of TC

  Sample	mcat/mg	ρTC/(mg·L-1)	Removal efficiency/%	Time/min	Ref.
  BiFeO3/Bi2O3	20	30	80.0	120	[30]
  BiFeO3/TiO2	50	10	67.9	180	[31]
  BiFeO3/Bi2WO6/g-C3N4	10	10	83.7	45	[32]
  BiFeO3/BNQDs	20	10	80.0	180	[33]
  BiFeO3/MnFeO3	20	10	91.9	180	[34]
  BiFeO3/Er	200	30	57.8	180	[35]
  BiFeO3@Fe2O3@BiOI	50	10	96	90	This work

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