English

• 0.   Introduction

  Emerging contaminants, particularly antibiotics, have been extensively documented as ubiquitous in aquatic environments, including municipal wastewater treatment facilities and groundwater systems, which pose significant risks to human health and ecological balance^[1]. Current research reports have outlined methods for treating antibiotic wastewater, including physical removal, biological removal, and chemical degradation^[2-7]. Physical and biological removal methods have their limitations, as antibiotics can persist after such treatments, sometimes even leading to the production of harmful substances, disrupting ecological balance, and requiring significant time for degradation. However, these issues can be effectively addressed through photocatalysis technology, which has emerged as one of the most effective methods for treating antibiotic wastewater today.

  Bismuth-based metal oxides not only have abundant resources and low toxicity but also exhibit excellent photocatalytic performance and high stability^[8-12]. Bismuth-based materials exhibit significant differences in morphology, optical properties, and specific surface area. Among them, multicomponent oxides with unique layered structures, such as Bi₂MoO₆, BiVO₄, and Bi₂CrO₆, are important components of Bi-based materials and have wide application potential in photoelectric, catalytic, and other fields^[13]. Especially, Bi₂CrO₆ has a wide light absorption range, which can reach about 600 nm, indicating that it should be one of the ideal materials for photocatalytic treatment of organic pollutants^[14]. Tao et al.^[15] discovered a novel triclinic phase Bi₂CrO₆, which is a single-layer structure composed of [CrO₄]-[Bi₂O₂]-[CrO₄] sandwich units and the outermost orbital of Bi³⁺-6s²6p⁰ plays a crucial role in shaping the energy bands. This unique layered structure and hybrid orbital mode can facilitate the transportation of photogenerated charges. Unlike other photocatalysts, the low electron transport rate of Bi₂CrO₆ results in many charge carriers that cannot be effectively separated, and thus, its photocatalytic activity is limited^[15-17]. To solve this issue, modification strategies, including heterojunction construction^[18], element doping^[19], and defect engineering^[20], could be effective for bismuth-based materials. Especially, heterojunction construction has been extensively researched in the field of photocatalysis. For example, Tao et al.^[15] successfully coupled Bi₂CrO₆ with SrTiO₃∶Rh photocatalytic material to construct a Z-scheme heterojunction composite photocatalyst, which exhibited a light absorption edge at approximately 650 nm and a band gap of 1.86 eV, achieving efficient water splitting.

  Cu₂O is regarded as a potential visible light- responsive semiconductor catalytic material because of its low toxicity, narrow band gap (2.17 eV), and cost- effectiveness. It has been widely applied in optical, catalytic, and magnetic fields^[21-22]. The photocatalytic performances of Cu₂O with different morphologies are always different because different Cu₂O morphologies expose different crystal planes. For example, its (110) crystal plane energy barrier is low, so it is easy to generate hydroxyl radicals (·OH) and superoxide radicals (·O₂^-) on such a surface, which can be used to degrade organic pollutants^[23-25]. However, pure Cu₂O exhibits rapid recombination of photogenerated electrons (e^-) and holes (h⁺) under visible light excitation, leading to low conductivity and oxidative instability, which limits its long-term photocatalytic performance. So far, for improving the catalytic performance of Cu₂O, many optimization strategies have been explored, among which the creation of a composite structure with bismuth-based materials can construct a heterojunction structure. At present, Z-scheme heterojunction Bi₂Sn₂O₇/Cu₂O composite catalyst^[26] and Cu₂O/BiVO₄ composite photocatalyst^[27] have been successfully fabricated and investigated for dye degradation. The experimental results suggest that the photocatalytic reaction rate can be improved compared to pure substances.

  In this work, a Cu₂O/Bi₂CrO₆ composite photocatalyst was prepared by distributing Cu₂O with a dodecahedral structure into triclinic Bi₂CrO₆ to construct a Z-scheme heterojunction interface, which suppressed the recombination of photogenerated carriers. Meanwhile, the optimized electron migration path within this heterojunction ensured that a higher proportion of the photoexcited e^- was utilized in reduction and oxidation reactions, further improving the photocatalytic efficiency of the Cu₂O/Bi₂CrO₆ system. Herein, triclinic Bi₂CrO₆ and rhombohedral dodecahedron Cu₂O were prepared first, and then the Cu₂O/Bi₂CrO₆ heterojunction composite photocatalyst was successfully obtained. Finally, its photocatalytic performance was evaluated by using tetracycline (TC) as the target degradation material.

  1.   Experimental

  1.1   Materials

  The main reagents required for the experiment were the bismuth nitrate pentahydrate (Bi(NO₃)₃), potassium chromate (K₂CrO₄), copric chloride dihydrate (CuCl₂·2H₂O), sodium hydroxide (NaOH) and hydroxylammonium chloride (NH₂OH·HCl), which were purchased from Shanghai Aladdin Biochemical Technology Co., Ltd. All the above experimental reagents are analytically pure.

  1.2   Preparation of Bi₂CrO₆

  0.970 1 g of Bi(NO₃)₃·5H₂O and 0.194 2 g of K₂CrO₄ were completely dissolved into 30 mL HNO₃ (2 mol·L^-1) solution by stirring magnetically until a translucent yellow liquid. Then, with vigorous stirring, 2 mol·L^-1 solution of ammonia water was carefully added until the pH of the supernatant was 1.0. After that, the precursor was placed in an autoclave and heated in an oven at 180 ℃ for 6 h. After cooling, a red precipitate was obtained by filtration and washed repeatedly with a mixture of ethanol and distilled water. At last, the above precipitate was dried at 80 ℃ for 24 h and then ground to obtain the Bi₂CrO₆ photocatalyst.

  1.3   Preparation of Cu₂O/Bi₂CrO₆ composite

  A certain amount of Bi₂CrO₆, 83.4 mL of deionized water, 5.0 mL of copper chloride solution (0.5 mol·L^-1), and 20.0 mL of anhydrous ethanol were mixed with stirring for 30 min at 40 ℃. Next, 30 mL of absolute ethanol and 9 mL of NaOH solution (1 mol·L^-1) were continued to drop, and the time of adding one drop was controlled to 3-4 s and 6-7 s, respectively. Upon completing the additions, 9.8 mL 0.5 mol·L^-1 of hydroxylamine hydrochloride solution was rapidly introduced. Then, the mixture was stirred for 10 min and left for 3 h. Finally, the precursor was extracted and filtered to get the filter cake, which was washed with an ethanol aqueous solution several times to get the orange product. After drying at 40 ℃ for 24 h, the Cu₂O/Bi₂CrO₆ photocatalyst was obtained. The photocatalysts prepared according to the above steps with a different amounts of Bi₂CrO₆ were labeled as 5%Cu₂O/Bi₂CrO₆, 10%Cu₂O/Bi₂CrO₆, 15%Cu₂O/Bi₂CrO₆, 20%Cu₂O/Bi₂CrO₆, and 25%Cu₂O/Bi₂CrO₆, respectively, where 5%, 10%, 15%, 20%, and 25% were the ratios of the mass of Cu₂O to that of Bi₂CrO₆ in the system.

  1.4   Characterizations

  The morphology of the photocatalyst was detected by the scanning electron microscope (SEM, TESCANMAIA 3LMH), the transmission electron microscope (TEM, FEI-G20, operating voltage: 200 kV), and the high-resolution transmission electron microscope (HRTEM, JeM-29999FMⅡ apparatus). X-ray diffractometer (XRD, Nikaku D/max 2500) was conducted with Cu Kα as the radiation source (λ=0.151 8 nm), the current was 300 mA, and the working voltage was 60 kV. Meanwhile, the scanning rate was 10 (°)·min^-1 in a scanning range (2θ) of 10°-80°. The UV-Vis diffuse reflectance spectra (UV-Vis DRS) were collected by Jena SPECORD®210 PLUS spectrophotometer. X-ray photoelectron spectroscopy (XPS) was obtained on the Thermo Fisher Scientific Escalab 250Xi to analyze the element composition and chemical status of the Cu₂O/Bi₂CrO₆ photocatalyst. Both photocurrent response and electrochemical impedance (EIS) tests were performed on an electrochemical workstation (CHI760E) and a three-electrode system (working electrode: sample, reference electrode: Ag/AgCl electrode, auxiliary electrode: platinum sheet). The EIS test was processed in a mixed solution (containing 0.1 mol·L^-1 KCl and 2.5 mmol·L^-1 [Fe(CN)₆]^3-/[Fe(CN)₆]^4-). The photocurrent response was processed in 0.2 mol·L^-1 Na₂SO₄ electrolyte. The experiments shown in the Mott-Schottky plots were conducted in 0.5 mol·L^-1 Na₂SO₄ to determine the flat-band potential. Electron paramagnetic resonance (EPR) spectroscopy was performed in the Bruker A300 to test the active species.

  1.5   Photocatalytic degradation

  The photocatalytic performance of Bi₂CrO₆, Cu₂O, and Cu₂O/Bi₂CrO₆ photocatalysts was evaluated by TC degradation with a xenon lamp (CEL-S500, 500 W). Firstly, 100 mg of the photocatalysts was added to a 90 mL 10 mg·L^-1 TC solution, and after stirring in the dark for 50 min, the adsorption equilibrium was achieved. Subsequently, under a xenon lamp, the degradation process lasted 100 min, and 4 mL of the above mixture was obtained every 10 min. Finally, the supernatant was obtained after centrifugation, and the absorbance was measured by a 722E UV-Vis spectrophotometer.

  2.   Results and discussion

  2.1   Characterization analysis

  The morphologies of Bi₂CrO₆ and Cu₂O were confirmed by SEM. Fig. 1a shows that Bi₂CrO₆ exhibited a rod-like structure with a relatively regular shape and mutually interwoven features. Fig. 1b indicated that Cu₂O had a dodecahedral structure and the particle size was 330-380 nm. Fig. 1c shows the TEM image of 20%Cu₂O/Bi₂CrO₆. After the combination of Cu₂O and Bi₂CrO₆, the dodecahedron Cu₂O was dispersed around Bi₂CrO₆. The HRTEM image of 20%Cu₂O/Bi₂CrO₆ showed that Bi₂CrO₆ and Cu₂O were well combined (Fig. 1d). The lattice spacing of 0.232 and 0.262 nm are attributed to the (111) crystal planes of Cu₂O and the (210) crystal planes of Bi₂CrO₆, respectively^[15].

  Figure 1

  

  Figure 1.  SEM images of Bi₂CrO₆ (a) and Cu₂O (b); TEM (c) and HRTEM (d) images of 20%Cu₂O/Bi₂CrO₆

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  The phase structures of Bi₂CrO₆, Cu₂O, and 20%Cu₂O/Bi₂CrO₆ composite photocatalysts were analyzed by XRD (Fig. 2). Cu₂O showed the characteristic peaks at 2θ of 29.62°, 36.43°, 42.24°, 61.32°, and 73.44°, respectively^[28-29] (Cu₂O: PDF No.77-0199). The Bi₂CrO₆ photocatalyst exhibited several characteristic diffraction peaks at 2θ=11.23°, 22.53°, and 34.04° which belong to the (001), (002), and (003) crystal planes of triclinic Bi₂CrO₆, which were consistent with the characteristic diffraction peaks of Bi₂CrO₆ reported in the previous literature^{[15, 30-31]}. For the 20%Cu₂O/Bi₂CrO₆ composite, the position of the characteristic diffraction peak of Bi₂CrO₆ did not shift, implying that the phase structure of the photocatalyst remained unchanged after the combination. The characteristic diffraction peak of Cu₂O was not obvious, which was due to the low doping amount of Cu₂O.

  Figure 2

  

  Figure 2.  XRD patterns of Cu₂O, Bi₂CrO₆, and 20%Cu₂O/Bi₂CrO₆ photocatalysts

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  Fig. 3 presents the UV-Vis DRS and the band gaps of Cu₂O, Bi₂CrO_6, and Cu₂O/Bi₂CrO₆ photocatalysts. From Fig. 3a, it can be seen that the light absorption edges of Cu₂O and Bi₂CrO₆ were located at 680 and 640 nm, respectively, and both showed significant light absorption to visible light. After the combination of Cu₂O and Bi₂CrO₆, the light absorption edges of the 5%Cu₂O/Bi₂CrO₆, 10%Cu₂O/Bi₂CrO₆, 15%Cu₂O/Bi₂CrO₆, 20%Cu₂O/Bi₂CrO₆, and 25%Cu₂O/Bi₂CrO₆ composite photocatalysts were located at 668, 684, 702, 708, and 700 nm, respectively. Except for the 5%Cu₂O/Bi₂CrO₆, which exhibited a red shift only relative to Bi₂CrO₆, the samples with other doping ratios showed a red shift in the light absorption edge compared to both Cu₂O and Bi₂CrO₆. When the mass ratio of Cu₂O to Bi₂CrO₆ was 20%, the composite photocatalyst could absorb visible light of a larger wavelength, resulting in the generation of more photogenerated e^- and h⁺, thus improving the degradation efficiency of the photocatalyst. Fig. 3b shows that the band gap values of Bi₂CrO₆ and Cu₂O were 1.96 and 1.91 eV, respectively, consistent with those reported in previous literature^{[15, 28]}.

  Figure 3

  

  Figure 3.  UV-Vis DRS (a) and band gap plots (b) of Cu₂O, Bi₂CrO₆, and 20%Cu₂O/Bi₂CrO₆

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  Fig. 4 presents the Mott-Schottky plots of Cu₂O and Bi₂CrO₆; the positive slope of the curve demonstrated that both Cu₂O and Bi₂CrO₆ fall into the category of p-type semiconductors. As seen in Fig. 4, the E_fb (the flat-band potential) of Cu₂O and Bi₂CrO₆ were -0.34 and -0.78 V (vs Ag/AgCl), respectively, and the formula can be used to obtain their potentials relative to a normal hydrogen electrode (E)^[32]:

  $ E=E^{\prime}+0.059 \mathrm{pH}+E^{\ominus} $

  Figure 4

  

  Figure 4.  Mott-Schottky plots of (a) Cu₂O and (b) Bi₂CrO₆

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  where E′ is the potential relative to Ag/AgCl and E^$\ominus$ is the standard Ag/AgCl electrode potential. In particular, the pH of 0.5 mol·L^-1 Na₂SO₄ solution was 6.8 and E^$\ominus$=0.197 V^[33-34]. Therefore, the E_fb of Cu₂O and Bi₂CrO₆ were 0.26 V (vs NHE) and -0.18 V (vs NHE), respectively. In general, for n-type semiconductors, the minimum E_CB [conduction band (CB) potential] was negative 0.3 V compared to E_fb, so the E_CB of Cu₂O and Bi₂CrO₆ was -0.04 V (vs NHE) and -0.48 V (vs NHE), respectively^[32].

  The elemental composition and valence of 20%Cu₂O/Bi₂CrO₆ composite photocatalyst were studied by XPS. The results showed that Cr, Bi, Cu, and O were present in the samples. Fig. 5a showed that the binding energies of 578.94 and 587.53 eV were the electron orbitals of Cr2p_1/2 and Cr2p_3/2, attributed to Cr⁶⁺ in bismuth chromate. In Fig. 5b, the peaks at 159.03 and 164.33 eV can be attributed to Bi4f_7/2 and Bi4f_5/2, respectively. Hence, Bi³⁺ existed in the Cu₂O/Bi₂CrO₆ composite photocatalyst^[15]. The energies of 932.48 and 952.38 eV (Fig. 5c) are attributed to Cu2p_3/2 and Cu2p_1/2, respectively^[35], that is, Cu₂O existed in the photocatalyst. In Fig. 5d, for O1s, the peaks at 529.90 and 531.90 eV are attributed to lattice oxygen (Cr—O, Cu—O, Bi—O) and absorbed —OH on the surface of Cu₂O/Bi₂CrO₆ photocatalyst, respectively^[15].

  Figure 5

  

  Figure 5.  XPS spectra of Cr2p (a), Bi4f (b), Cu2p (c), and O1s (d) of 20%Cu₂O/Bi₂CrO₆

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  The photoelectric properties of Bi₂CrO₆ and Cu₂O/Bi₂CrO₆ composite photocatalysts were analyzed through photocurrent response curves and electrochemical impedance spectra. As depicted in Fig. 6a, in contrast to Bi₂CrO₆, the 20%Cu₂O/Bi₂CrO₆ composite photocatalyst exhibited the highest photocurrent density, suggesting that the heterojunction structure formed between Cu₂O and Bi₂CrO₆ facilitated the separation of e^--h⁺ pairs. The EIS in Fig. 6b revealed that compared with Cu₂O and Bi₂CrO₆, the 20%Cu₂O/Bi₂CrO₆ composite photocatalyst had the smallest arc radius, indicating that its interfacial resistance transfer resistance was the lowest, and e^--h⁺ pairs possessed a faster separation rate. Consequently, an interfacial electric field was formed within the Cu₂O/Bi₂CrO₆ heterojunction photocatalyst, which can generate more photogenerated e^- and h⁺.

  Figure 6

  

  Figure 6.  Photocurrent response curves (a) and EIS (b) of the samples

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  2.2   Photocatalytic performance

  Fig. 7a shows the performance of Bi₂CrO₆, Cu₂O, and Cu₂O/Bi₂CrO₆ photocatalyst for TC degradation under visible light, and only 49.62% of TC was degraded by Bi₂CrO₆. With the rise of Cu₂O loading, the photodegradation performance of Cu₂O/Bi₂CrO₆ initially increased and subsequently decreased. The 20%Cu₂O/Bi₂CrO₆ composite photocatalyst exhibited the optimal degradation performance, being capable of degrading 87.5% of TC within 100 min, which was about 1.8 times and 1.3 times higher than that of Bi₂CrO₆ and Cu₂O, respectively. The observed phenomenon is mainly attributed to the increase of Cu₂O loading, which facilitates the rapid transfer of photogenerated e^- on the surface of the composite material and inhibits the recombination of photogenerated e^- and h⁺, thereby enhancing the photocatalytic degradation performance^[36]. However, when the mass ratio reached 25%, excessive doping led to the aggregation of Cu₂O on the surface of Bi₂CrO₆, which covers the active sites and consequently reduces the photocatalytic activity^[37]. The photocatalytic performance of the Cu₂O/Bi₂CrO₆ composite material is superior to that of Cu₂O and Bi₂CrO₆. This finding is consistent with the photoelectric current response and electrochemical impedance spectra, indicating that a heterojunction is formed between Cu₂O and Bi₂CrO₆. The heterojunction structure formed by Cu₂O with Bi₂CrO₆ enhanced the interfacial charge migration rate between Cu₂O and Bi₂CrO₆, thus improving the photocatalytic performance. Fig. 7b presents the results of 20%Cu₂O/Bi₂CrO₆ after 10 cycles of TC degradation, and its light degradation efficiency remained above 83.4%, suggesting that Cu₂O/Bi₂CrO₆ possessed excellent stability and could be reused. The comparison of catalytic performance of Cu₂O/Bi₂CrO₆ catalyst with other Bi-based catalysts is shown in Table 1. From Table 1, it can be seen that the obtained Cu₂O/Bi₂CrO₆ photocatalyst had higher photocatalytic activity than previously reported catalysts.

  Figure 7

  

  Figure 7.  Performance curves of Bi₂CrO₆ and Cu₂O/Bi₂CrO₆ composite photocatalyst for photocatalytic degradation of TC under visible light (a); Cyclic test results of photocatalytic degradation of TC by 20%Cu₂O/Bi₂CrO₆ (b)

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  Table 1

  

  Table 1.  Comparison of catalytic performance of Cu₂O/Bi₂CrO₆ photocatalyst with other Bi-based catalysts

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  Catalyst	Band gap / eV	Model organic pollutanta	Light source	Experimental conditionsb	Degradation efficiency / %	Number of cycles	Ref.
  AgBr/Bi2CrO6	2.05	TCH	250 W xenon (λ≥420 nm)	100 mg, 100 mL, 10 mg·L-1, 60 min	80	5	[14]
  Bi2CrO6/CuO	2.05	MB	52 W LED lamp	1.5 g·L-1, 150 mL, 20 mg·L-1, 120 min	85	5	[38]
  Cu2O/BiOBr	2.56	TC	300 W xenon (λ < 400 nm)	30 mg, 50 mL, 30 mg·L-1, 60 min	85	5	[39]
  Cu2O/Bi2CrO6	1.96	TC	500 W xenon	100 mg, 90 mL, 10 mg·L-1, 100 min	87.5	10	This work
  a TCH=tetracycline hydrochloride, MB=methylene blue; b The four parameters in the experimental conditions refer to the catalyst mass or concen-tration, target pollutant concentration, degradant volume, and irradiation duration, respectively.

  2.3   Photocatalytic mechanism

  The types of free radicals generated in 20%Cu₂O/Bi₂CrO₆ during the TC degradation were measured by EPR-5, 5-dimethyl-1-pyrroline-N-oxide (DMPO) to investigate the mechanism of the photocatalytic reaction. Fig. 8a and 8b show the EPR signals of DMPO- ·O₂^- and DMPO-·OH of 20%Cu₂O/Bi₂CrO₆ composite photocatalysts, respectively. Under dark, the EPR signals of 20%Cu₂O/Bi₂CrO₆ were not detected, while the characteristic peak was obvious under light conditions. Moreover, the strongest characteristic peak can be found after 10 min illumination, indicating that 20%Cu₂O/Bi₂CrO₆ can produce more ·O₂^- and ·OH. Therefore, ·O₂^- and ·OH were the main active species in the reaction.

  Figure 8

  

  Figure 8.  EPR signals of DMPO-·O₂^- and DMPO-·OH over 20%Cu₂O/Bi₂CrO₆

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  To get insight into the photocatalysis mechanism of TC degradation over the 20%Cu₂O/Bi₂CrO₆ photocatalyst, the active species trapping experiments were performed. 1 mmol·L^-1 of AO (ammonium oxalate) and 1 mmol·L^-1 of IPA (isopropyl alcohol) were used as trapping agents for h⁺ and ·OH, respectively. As shown in Fig. 9, after adding AO and IPA, the photocatalytic efficiency was changed obviously. This specified that h⁺ and ·OH radicals played a significant role in the photocatalytic reaction. Besides, photocatalytic activity decreased under N₂ purging, demonstrating that ·O₂^- was generated during the photocatalytic degradation of TC. Thus, the ·O₂^-, ·OH, and h⁺ could be the major active species responsible for the Cu₂O/Bi₂CrO₆ photocatalyst system.

  Figure 9

  

  Figure 9.  Active species trapping experiments with 20%Cu₂O/Bi₂CrO₆ under visible light irradiation

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  Fig. 10 depicts a schematic diagram for the photodegradation of TC by a 20%Cu₂O/Bi₂CrO₆ photocatalyst under visible irradiation. Based on the interlaced band relationship between Cu₂O and Bi₂CrO₆, it can be deduced that the reaction mechanism might be either a type-Ⅱ heterostructure or Z-scheme heterostructure^[40]. For the electron migration mechanism of type-Ⅱ heterostructure, under visible light, a great many of e^- and h⁺ were produced in Cu₂O and Bi₂CrO₆, the e^- accumulated in the CB of Bi₂CrO₆ moved to the CB of Cu₂O, and the h⁺ in the valence band (VB) of Cu₂O moved to the VB of Bi₂CrO₆. Eventually, many e^- and h⁺ accumulated in Cu₂O and Bi₂CrO_6, respectively. Based on the results of Mott-Schottky plots, the E_CB [-0.04 V (vs NHE)] of Cu₂O was higher than the redox potential of O₂/·O₂^- [-0.33 V (vs NHE)]. Consequently, the e^- within the CB of Cu₂O cannot reduce the dissolved O₂ in the solution to ·O₂^-, but in the EPR test of 20%Cu₂O/Bi₂CrO₆, ·O₂^- exhibited a strong current signal, which was contrary to the EPR detection results. Similarly, the E_VB [1.48 V (vs NHE)] of Bi₂CrO₆ was below the redox potential of H₂O/·OH [2.4 V (vs NHE)], thus the VB of Bi₂CrO₆ cannot generate ·OH, and its electron transfer mechanism does not comply with the electron transfer mechanism of the type-Ⅱ heterostructure. According to the Z-scheme heterostructure mechanism, under visible light, e^- in the CB of Cu₂O are transferred to the VB of Bi₂CrO₆, causing a significant accumulation of numerous e^- and h⁺ in the CB of Bi₂CrO₆ and the VB of Cu₂O, respectively. Because the E_CB of Bi₂CrO₆ [-0.48 V (vs NHE)] is less than [-0.33 V (vs NHE)], the e^- in the VB of Bi₂CrO₆ can react with O₂ in the solution to ·O₂^-, which was consistent with the EPR detection result. Nevertheless, the E_VB [1.87 V (vs NHE)] of Cu₂O is less than 2.4 V (vs NHE), so the h⁺ in the VB of Cu₂O cannot produce ·OH, and the EPR detection results indicate that ·OH has a strong current signal. Therefore, it can be deduced that a portion of ·O₂^- produced by the CB of Bi₂CrO₆ is converted to ·OH, and finally, TC is degraded into small molecules^[41].

  Figure 10

  

  Figure 10.  Interfacial charge transfer diagram for degradation of TC by Z-scheme mechanism of 20%Cu₂O/Bi₂CrO₆ photocatalyst under visible light

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  3.   Conclusions

  In this study, triclinic Bi₂CrO₆ and dodecahedral Cu₂O were prepared by the hydrothermal method and the co-precipitation method, respectively. Then, the Cu₂O/Bi₂CrO₆ photocatalyst was successfully synthesized, and the TC degradation investigated the photocatalytic performance of Cu₂O/Bi₂CrO₆. The results showed that, under visible light irradiation, 20%Cu₂O/Bi₂CrO₆ exhibited the optimal catalytic performance and 87.5% TC could be degraded within 100 min, which was about 1.8 times and 1.3 times higher than that of pure Bi₂CrO₆ and pure Cu₂O, respectively. The degradation efficiency of the 20%Cu₂O/Bi₂CrO₆ composite photocatalyst remained above 83.4% after being reused 10 times, and the photocatalyst presented excellent stability. The best degradation performance of 20%Cu₂O/Bi₂CrO₆ is attributed to the formation of a Z-scheme heterojunction structure between Bi₂CrO₆ and Cu₂O, which enhances the interfacial charge transfer rate and can generate more ·O₂^-, ·OH, and h⁺, thereby decomposing TC into CO₂, H₂O, and other small molecules.

  
  Acknowledgments: This study was supported by the National Natural Science Foundation of China (Grants No.21908135, 21975146, 52203266), the Provincial Research Projects for Returned Overseas Chinese Scholars Funded by Shanxi Province (Grants No.2020-134, 2022-173), the Fund Program for the Scientific Activities of Selected Returned Overseas Professionals in Shanxi Province (Grant No.20240027) and the General Program of the National Natural Science Foundation (Grant No.202403021211022).
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  Figure 1  SEM images of Bi₂CrO₆ (a) and Cu₂O (b); TEM (c) and HRTEM (d) images of 20%Cu₂O/Bi₂CrO₆

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  Figure 2  XRD patterns of Cu₂O, Bi₂CrO₆, and 20%Cu₂O/Bi₂CrO₆ photocatalysts

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  Figure 3  UV-Vis DRS (a) and band gap plots (b) of Cu₂O, Bi₂CrO₆, and 20%Cu₂O/Bi₂CrO₆

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  Figure 4  Mott-Schottky plots of (a) Cu₂O and (b) Bi₂CrO₆

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  Figure 5  XPS spectra of Cr2p (a), Bi4f (b), Cu2p (c), and O1s (d) of 20%Cu₂O/Bi₂CrO₆

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  Figure 6  Photocurrent response curves (a) and EIS (b) of the samples

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  Figure 7  Performance curves of Bi₂CrO₆ and Cu₂O/Bi₂CrO₆ composite photocatalyst for photocatalytic degradation of TC under visible light (a); Cyclic test results of photocatalytic degradation of TC by 20%Cu₂O/Bi₂CrO₆ (b)

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  Figure 8  EPR signals of DMPO-·O₂^- and DMPO-·OH over 20%Cu₂O/Bi₂CrO₆

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  Figure 9  Active species trapping experiments with 20%Cu₂O/Bi₂CrO₆ under visible light irradiation

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  Figure 10  Interfacial charge transfer diagram for degradation of TC by Z-scheme mechanism of 20%Cu₂O/Bi₂CrO₆ photocatalyst under visible light

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  Table 1.  Comparison of catalytic performance of Cu₂O/Bi₂CrO₆ photocatalyst with other Bi-based catalysts

  Catalyst	Band gap / eV	Model organic pollutanta	Light source	Experimental conditionsb	Degradation efficiency / %	Number of cycles	Ref.
  AgBr/Bi2CrO6	2.05	TCH	250 W xenon (λ≥420 nm)	100 mg, 100 mL, 10 mg·L-1, 60 min	80	5	[14]
  Bi2CrO6/CuO	2.05	MB	52 W LED lamp	1.5 g·L-1, 150 mL, 20 mg·L-1, 120 min	85	5	[38]
  Cu2O/BiOBr	2.56	TC	300 W xenon (λ < 400 nm)	30 mg, 50 mL, 30 mg·L-1, 60 min	85	5	[39]
  Cu2O/Bi2CrO6	1.96	TC	500 W xenon	100 mg, 90 mL, 10 mg·L-1, 100 min	87.5	10	This work
  a TCH=tetracycline hydrochloride, MB=methylene blue; b The four parameters in the experimental conditions refer to the catalyst mass or concen-tration, target pollutant concentration, degradant volume, and irradiation duration, respectively.

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