Zn5 (OH) 6 (CO3) 2 / BiVO4 heterojunction photocatalyst as well as preparation method and application thereof

Abstract

The invention discloses a Zn5 (OH) 6 (CO3) 2 / BiVO4 heterojunction photocatalyst as well as a preparation method and application thereof, and belongs to the technical field of environmental functional materials and photocatalysis. The catalyst is formed by uniformly loading BiVO4 nanoparticles on the surface of a Zn5 (OH) 6 (CO3) 2 nanosheet, and a close contact I-type heterojunction structure is formed. The preparation method comprises the following steps: firstly, synthesizing BiVO4 by a hydrothermal method; and then mixing with zinc nitrate and urea in deionized water, and carrying out secondary hydrothermal reaction to obtain the composite material. The photocatalyst shows excellent degradation performance on antibiotic pollutants such as tetracycline under visible light. The generated active species mainly comprise photo-generated holes and superoxide free radicals, and almost no hydroxyl free radicals are generated. The catalyst also has good cycle stability and environmental interference resistance, can efficiently work in natural sunlight and actual water, and is suitable for green treatment of emerging organic pollutants in the water environment.

Description

Technical Field

[0001] This invention belongs to the fields of environmental functional materials and photocatalysis technology, specifically relating to a Zn5(OH)6(CO3)2 / BiVO4 heterojunction photocatalyst, its preparation method, and its application. Background Technology

[0002] Tetracycline is a common antibiotic used in human and animal treatment. Its high hydrophilicity and low volatility lead to its persistence and stability in aquatic environments. After widespread use in human, poultry, and fish farming, it is difficult to completely metabolize in organisms, resulting in its frequent detection in aquatic systems and posing a threat to human health and ecosystem balance. Furthermore, tetracycline can induce various drug-resistant genes and bacteria, posing a significant ecological risk. Therefore, removing tetracycline from the environment has become an extremely urgent problem. Photocatalysis is a green and pollution-free technology. Under light irradiation, semiconductor photocatalysts can effectively degrade stubborn organic macromolecular pollutants. Therefore, the use of photocatalysts to control environmental pollution has attracted great interest.

[0003] BiVO4 is a visible-light-responsive semiconductor photocatalyst with a narrow bandgap (~2.4 eV), but its high recombination rate of photogenerated electron-hole pairs limits its practical applications. Zn5(OH)6(CO3)2 / BiVO4 (zinc hydroxide) is a wide-bandgap semiconductor. Although its photocatalytic activity is limited, its surface is rich in hydroxyl groups, which is beneficial for adsorbing organic pollutants containing functional groups such as carboxyl groups. Moreover, its synthesis is simple and inexpensive. However, how to effectively couple these two materials to construct a highly efficient heterojunction to improve visible-light photocatalytic performance, especially for the degradation of tetracycline antibiotics, remains a technical challenge that urgently needs to be solved.

[0004] In view of this, this invention successfully prepared a novel Zn5(OH)6(CO3)2 / BiVO4 photocatalyst using BiVO4 nanoparticles as a support via in-situ hydrothermal growth. Based on a series of characterization and comparative experiments, the possible transfer and separation behavior of photoinduced charge carriers and the photocatalytic mechanism were explored in depth. This research provides a promising approach for the future design of photocatalysts with special structures. Summary of the Invention

[0005] The purpose of this invention is to propose a method for preparing a Zn5(OH)6(CO3)2 / BiVO4 heterojunction photocatalyst, which can prepare a Zn5(OH)6(CO3)2 / BiVO4 heterojunction photocatalyst with a wide visible light response range, high photogenerated carrier separation efficiency, excellent catalytic activity and stability, and can achieve efficient degradation of organic pollutants such as tetracycline in water.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0007] First, this invention proposes a Zn5(OH)6(CO3)2 / BiVO4 heterojunction photocatalyst, which is composed of BiVO4 nanoparticles and Zn5(OH)6(CO3)2 nanosheets. The BiVO4 nanoparticles are uniformly loaded on the surface or between the layers of the Zn5(OH)6(CO3)2 / BiVO4 nanosheets to form a type I heterojunction structure.

[0008] As a preferred technical solution of the present invention, in the Zn5(OH)6(CO3)2 / BiVO4 heterojunction photocatalyst, the molar ratio between Zn5(OH)6(CO3)2 and BiVO4 is preferably 3:4 to 48, more preferably 1:4.

[0009] Secondly, this invention also proposes a method for preparing a Zn5(OH)6(CO3)2 / BiVO4 heterojunction photocatalyst, the steps of which are as follows:

[0010] (1) Dissolve bismuth nitrate and sodium dodecylbenzenesulfonate in nitric acid solution to obtain solution A, and dissolve ammonium metavanadate in nitric acid solution to obtain solution B; add solution B dropwise to solution A, adjust the pH value and mix evenly, and then carry out hydrothermal reaction. After the reaction is completed, wash and dry to obtain BiVO4 powder.

[0011] (2) The BiVO4 powder, urea and zinc nitrate obtained in step (1) are added to deionized water, stirred evenly and then subjected to hydrothermal reaction. After the reaction is completed, the Zn5(OH)6(CO3)2 / BiVO4 heterojunction photocatalyst is obtained by washing, drying and grinding.

[0012] As a preferred embodiment of the present invention, in the preparation method, the molar ratio of bismuth nitrate to ammonium metavanadate in step (1) is 0.8~1.2:0.8~1.2, the hydrothermal reaction temperature is 180~220 ℃, and the reaction time is 8~15 h. In step (2), the amount of zinc nitrate added is controlled by the molar ratio of Zn5(OH)6(CO3)2 to BiVO4 in the preparation of the heterojunction photocatalyst being 3:4~48, the hydrothermal reaction temperature is 100~140 ℃, and the reaction time is 10~30 h.

[0013] In addition, this invention also proposes the application of the Zn5(OH)6(CO3)2 / BiVO4 heterojunction photocatalyst in the degradation of organic pollutants in water, especially for the efficient degradation and removal of organic pollutants such as tetracycline, enrofloxacin, ciprofloxacin, norfloxacin, or methylene blue.

[0014] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0015] (1) High efficiency: The type I heterojunction constructed by the in-situ hydrothermal method effectively promotes the spatial separation of photogenerated electrons and holes and significantly suppresses carrier recombination. Under visible light irradiation, the degradation efficiency of tetracycline is as high as 97.9%, and the apparent rate constant is 4.2 times that of pure BiVO4.

[0016] (2) High selectivity: The active species identification experiment confirmed that the catalyst system mainly relies on photogenerated holes (h + ) and superoxide radicals (·O2) - It degrades pollutants with almost no hydroxyl radicals (•OH), and the reaction pathway is clear.

[0017] (3) Strong stability and practicality: The catalyst maintains high activity after multiple cycles and exhibits good tolerance to environmental interference factors such as different pH values, common anions and humic acids. At the same time, the catalyst has a universal degradation ability for a variety of antibiotics and dyes and can work efficiently in real sunlight and actual water bodies (such as lake water and tap water), showing broad prospects for practical application.

[0018] (4) Simple process: The two-step hydrothermal method is adopted, which does not require complex post-processing such as high-temperature calcination. The raw materials are cheap and readily available, making it suitable for large-scale production. Attached Figure Description

[0019] Figure 1 This is a schematic diagram of the synthetic route for Zn5(OH)6(CO3)2 / BiVO4.

[0020] Figure 2 The following are scanning electron microscopes (SEMs) of (a) BiVO4, (b) Zn5(OH)6(CO3)2, (c) BZ-20, (d) regional SEM, (h) elemental superposition, (e~i) EDS elemental distribution, (j) transmission electron microscope, and (k) high-resolution transmission electron microscope.

[0021] Figure 3 It is an X-ray diffraction pattern.

[0022] Figure 4 The following are the (a) photodegradation experiment of tetracycline hydrochloride, (b) reaction kinetics, and (c) mineralization rate of sample BZ.

[0023] Figure 5 The experiments are (a) the effect of pH, (b) the interference of anions, (c) the effect of humic acid, and (d-f) the corresponding reaction kinetics.

[0024] Figure 6 The results are (a) Fourier transform infrared spectrum, (b) Raman spectrum, and (c) N2 adsorption isotherm.

[0025] Figure 7These are XPS spectra: (a) Bi 4f, (b) V 2p, (c) O 1s, (d) Zn 2p.

[0026] Figure 8 The images show (a) the UV-Vis diffuse reflectance spectrum, (b) the Tauc curve, (c) the valence band XPS spectrum, and (d) the band structure diagram.

[0027] Figure 9 This is a diagram showing the flow of type I heterojunction and photogenerated carriers.

[0028] Figure 10 The following are (a) transient photocurrent curves, (b) impedance spectra, (c) photoluminescence spectra, and (d) time-resolved fluorescence spectra.

[0029] Figure 11 (a) Quenching experiment, (b) Kinetic constant, (c, d) EPR spectrum.

[0030] Figure 12 This is an analysis of possible degradation pathways of tetracycline.

[0031] Figure 13 It is (a) Daphnia magna LC 50 (b) Fathead minnow LC 50 (c) Developmental toxicity.

[0032] Figure 14 (a) Five cycles of degradation experiment; BZ-20 sample after reaction: (b) XRD and (c) SEM.

[0033] Figure 15 The experiments include (a) degradation experiments of multiple pollutants, (b) real sunlight experiments, (c) actual water body experiments and their respective kinetics (d~f). Detailed Implementation

[0034] The present invention will be further described in detail below with reference to the embodiments and accompanying drawings.

[0035] Example 1

[0036] First, please refer to Figure 1 As shown, Zn5(OH)6(CO3)2 / BiVO4 was synthesized using a two-step in-situ hydrothermal method.

[0037] 1) Preparation of BiVO4: First, 5 mmol Bi(NO3)3•5H2O (2.4254 g) and 1 mmol SDBS (0.3485 g) were ultrasonically dissolved in 20 mL HNO3 (2 M), denoted as solution A; similarly, 5 mmol NH4VO3 (0.5850 g) was ultrasonically dissolved in 20 mL HNO3 (2 M), denoted as solution B. After stirring magnetically for 10 min each, solution B was added dropwise to solution A using a pear-shaped funnel, and the mixture was magnetically stirred for 20 min. Next, the pH was adjusted to 2 using NH3•H2O and HNO3 (2 M), and the mixture was magnetically stirred for 1 h. Finally, the mixture was hydrothermally reacted for 12 h in a 100 mL PTFE high-pressure reactor at 200 ℃. The mixture was washed three times each with ultrapure water and ethanol by centrifugation, and then dried at 60 ℃ for 8 h. After grinding with an agate mortar, a bright yellow powder precursor was obtained.

[0038] 2) Preparation of Zn5(OH)6(CO3)2 / BiVO4: In a 100 mL PTFE liner, 3 mmol BiVO4 (0.972 g) and 20 mmol Urea (1.2 g) were first added to 60 mL of deionized water. Then, 0.2230 g, 0.4463 g, 0.8925 g, 1.3388 g, 1.7850 g, and 2.6775 g of Zn(NO3)2•6H2O were added respectively, and the mixture was magnetically stirred for 30 min. Finally, the liner was placed in a Teflon high-pressure reactor and hydrothermally treated in an oven at 120 ℃ for 24 h. The mixture was washed three times each with ultrapure water and ethanol by centrifugation, and then dried at 60 ℃ for 8 h. After grinding with an agate mortar, the Zn5(OH)6(CO3)2 / BiVO4 composite material was obtained. The prepared products were named BZ-5 (1:16), BZ-10 (1:8), BZ-20 (1:4), BZ-30 (3:8), BZ-40 (1:2), and BZ-60 (3:4) according to the different molar ratios between Zn5(OH)6(CO3)2 and BiVO4.

[0039] Secondly, Zn5(OH)6(CO3)2 was synthesized using an in-situ hydrothermal method.

[0040] Preparation of Zn5(OH)6(CO3)2: 5 mmol Zn(NO3)2•6H2O (1.4875 g) and 20 mmol Urea (1.2 g) were added to 60 mL of deionized water and magnetically stirred for 30 min. Finally, the mixture was hydrothermally treated at 120 °C for 24 h using a 100 mL Teflon autoclave. The mixture was washed three times each with ultrapure water and ethanol by centrifugation, and then dried at 60 °C for 8 h. After grinding with an agate mortar, a white Zn5(OH)6(CO3)2 material was obtained.

[0041] Example 2

[0042] Characterization and performance testing of the prepared product

[0043] 1. Morphological characteristics

[0044] The morphology of the photocatalyst was observed using SEM. For example ( Figure 2 As shown in (a), the BiVO4 material exhibits nanobulk particles with a particle size range of 20~200 nm, as shown in (a). Figure 2 As shown in b), the Zn5(OH)6(CO3)2 material exhibits sheet-like nanoflowers with a lateral width in the micrometer range and an average thickness of approximately 2–10 nm; finally, as shown in ( Figure 2 As shown in (c) and (d), the BZ-20 material exhibits Zn5(OH)6(CO3)2 nanosheets as a substrate, while BiVO4 nanoparticles are attached to the sheets or between the layers, with the two materials in close contact at the interface. Subsequently, the microstructure and elemental composition of BZ-20 were further investigated using TEM and HRTEM. Figure 2 TEM dark-field imaging (j) clearly shows that BiVO4 nanoparticles are uniformly distributed on the surface of Zn5(OH)6(CO3)2 nanosheets; subsequently, as shown in (j) Figure 2 HRTEM of (k) shows that the 0.25 nm and 0.37 nm lattice fringes correspond to the (002) plane of BiVO4 and the (310) plane of Zn5(OH)6(CO3)2, respectively. Furthermore, ( Figure 2 The EDS in e~i) shows that the five elements Bi, V, Zn, C and O are uniformly distributed in the BZ-20 catalyst.

[0045] 2. Structural Characterization

[0046] The crystal structures of a series of composite catalysts with varying loading ratios were analyzed by XRD. For example ( Figure 3As shown in the figure, the catalyst exhibits distinct diffraction peaks and high crystallinity. The XRD diffraction peaks of BiVO4 closely match the standard color chart (JCPDS NO. 14-0688), which corresponds to the monoclinic scheelite phase; while the XRD peaks of Zn5(OH)6(CO3)2 closely match the standard color chart (JCPDS NO. 19-1458). The distinct diffraction peaks of BiVO4 at 18.7°, 19°, 29°, 30.6°, 34.5°, 35.2°, 40°, 42.5°, 47.3°, 50.3°, 53.5°, 58.5°, and 59.3° correspond to (110), (011), (121), (040), (200), (002), (-112), (051), (042), (202), and (310) respectively. The main diffraction peaks of Zn5(OH)6(CO3)2 are at 13°, 24.3°, 28.4°, 31.4°, 34.7°, 36.2°, 39.1°, 58.6°, and 59.7°, corresponding to the (200), (310), (020), (220), (-202), (510), (420), (-622), and (-223) crystal planes. It is clearly observed that the characteristic diffraction peaks of Zn5(OH)6(CO3)2 are the diffraction peaks of the (200), (021), and (002) crystal planes at 13°, 32.9°, and 33.3°, respectively. As the loading ratio increases, the peak intensity increases, indicating that the successful introduction of Zn5(OH)6(CO3)2 forms a heterojunction material with bismuth vanadate.

[0047] 3. Photodegradation performance

[0048] To investigate the efficiency of photocatalysts in the oxidative degradation of antibiotics, this invention conducted experiments on the visible light degradation of tetracycline hydrochloride. Figure 4 a) indicates that in the blank experiment without photocatalysts, TC hardly degrades under visible light irradiation, which excludes the interference of TC self-degradation on the photocatalytic degradation reaction. In the 6-h visible light degradation experiment, among a series of photocatalysts, BZ-20 showed the highest TC removal rate (97.9%), which was 1.75 times, 1.05 times, 1.02 times, 1.02 times, 1.07 times, 1.58 times, and 1.49 times higher than that of BiVO4 (55.9%), BZ-5 (92.9%), BZ-10 (95.5%), BZ-40 (96%), BZ-60 (91.7%), Zn5(OH)6(CO3)2 (61.8%), and P25 (71.0%), respectively. To further investigate the reaction kinetics of TC degradation under visible light, a pseudo-first-order kinetic model was used. By fitting the data from the first 4 hours of the degradation experiment, the apparent kinetic constant (k) could be obtained. obsThis parameter was used to evaluate the degradation rate of TC by the photocatalytic oxidation of the catalyst. Figure 4 b) It can be seen that, among the synthesized Zn5(OH)6(CO3)2 / BiVO4 samples, the photodegradation efficiency of TC by BZ samples showed a volcanic model trend of first increasing and then decreasing with the increase of Zn5(OH)6(CO3)2 loading. Among them, BZ-20 showed the highest photodegradation efficiency, and the k of BZ-20 was... obs (h) -1 The concentrations were 0.63, which were 4.2 times, 1.31 times, 1.19 times, 1.17 times, 1.5 times, 3.94 times, and 3.32 times that of BiVO4 (0.15), BZ-5 (0.48), BZ-10 (0.53), BZ-40 (0.54), BZ-60 (0.42), Zn5(OH)6(CO3)2 (0.16), and P25 (0.19), respectively. Finally, to investigate the degree of oxidation of antibiotics by photocatalysis, the same amount of catalyst was added, and a photodegradation experiment was conducted under visible light irradiation. Figure 4 c). The comparison shows that BZ-20 has the highest degree of mineralization, and the mineralization rate first increases and then decreases with the loading of Zn5(OH)6(CO3)2, which is similar to the kinetic trend of the degradation experiment.

[0049] 4. Environmental factors

[0050] This invention tested five different pH values, six anions, and four different concentrations of humic acid to investigate the interference of environmental factors in water on the photocatalytic degradation of antibiotics. (By...) Figure 5 (a) and (d) show that the photocatalytic TC degradation rate of sample BZ-20 first increases and then decreases with increasing pH value. pH=9 is the optimal pH value for TC photocatalytic degradation rate (0.87 h). -1 ), other pH values ​​k obs (h) -1 The pH values ​​were 3 (0.12), 5 (0.31), 7 (0.62), and 11 (0.65), respectively. In the anion interference experiment, 1 mM Cl was added... - NO3 - HCO3 - SO4 2- CO3 2- HPO4 2- Afterwards, the TC removal rates of BZ-20 over 6 hours were 96.4%, 97.1%, 96%, 97.5%, 99.8%, and 100%, respectively. Figure 5 b). From the apparent rate constant of the degradation kinetics, it can be known that ( Figure 5 e), Cl - NO3- HCO3 - SO4 2- It has almost no effect on its degradation rate, while HPO4 2- and CO3 2- This promotes photodegradation efficiency due to the increase in pH after hydrolysis and the addition of CO3. 2- It will also replenish the interlayer CO3 consumed by Zn5(OH)6(CO3)2. 2- In addition, h + It may also be related to CO3 2- and HPO4 2- HPO4• - and CO3• - The free radicals are reduced, thereby improving photocatalytic activity. To investigate the effect of natural organic matter (NOM) on photocatalytic activity, different concentrations of humic acid (HA) were added to conduct TC photodegradation experiments. After adding HA at 0, 5, 10, 15, and 20 ppm, the TC removal rates of BZ-20 at 6 h were 97.9%, 92.1%, 87.2%, 84.5%, and 84.4%, respectively. Figure 5 c). Based on the apparent rate constant of the degradation kinetics, it can be seen that even after the addition of HA, there is still a reasonable TC removal rate ( Figure 5 f).

[0051] 5. Bond state characterization

[0052] The structure, chemical composition, and bonding of the synthesized material were further analyzed using FTIR and Raman spectral characterization. In FTIR (… Figure 6 a), 3312~3518 cm -1 The broadband at that point corresponds to the tensile vibration of surface OH or water, regarding the free CO3 in Zn5(OH)6(CO3)2. 2- The internal modes are as follows: symmetric OCO stretching mode ν1, out-of-plane OCO bending mode ν2, asymmetric OCO stretching mode ν3, and asymmetric OCO bending mode ν4, at 1510 and 1388 cm. -1 Two strong spectral bands appear at 1045 cm⁻¹, belonging to the double degenerate asymmetric stretching mode ν3, followed by a band at 1045 cm⁻¹. -1 A weak spectral band associated with vibrational mode ν1 appears at 837 cm⁻¹, while at 837 cm⁻¹... -1 The band at that location corresponds to the ν2 mode. Meanwhile, the band at 739 cm⁻¹ in BiVO₄... -1 The strong spectral bands are due to the asymmetric and symmetric stretching vibration peaks of its VO bonds, while the presence of adsorbed hydroxyl vibration peaks and the characteristic peaks of both in BZ-20 indicates that the materials have been successfully composited.

[0053] As shown by Raman spectroscopy ( Figure 6 b), at 710 and 638 cm -1 Weak asymmetric stretching bands of VO were observed at 327 and 367 cm⁻¹. -1 Asymmetric and symmetric bending vibrations of the VO4 tetrahedron were observed at [location missing]. Notably, the strong vibrations of BiVO4 are attributed to the VO4 symmetric stretching mode; after loading Zn5(OH)6(CO3)2, a Raman displacement of 820 cm⁻¹ was observed. -1 Increased to 826 cm -1 This may be due to the decrease in VO bond length.

[0054] The specific surface area of ​​all samples was evaluated using N2 adsorption-desorption isotherms. Figure 6 c). All samples exhibited Type IV adsorption isotherms, a characteristic of mesoporous materials. The shape of the hysteresis loops was related to Type H3; this type of loop is given by non-rigid aggregates of plate-like particles, creating slit-like pores, which matches the morphology observed through SEM images. The mesoporous features likely originate from their nanosheet / nanocryst spacing. Calculations using the BET model yielded specific surface areas of 2.69 m² for BiVO₄, BZ-20, and Zn₅(OH)₆(CO₃)₂. 2 / g, 8.74 m 2 / g、16 m 2 / g. The results showed that modification with Zn5(OH)6(CO3)2 rich in hydroxyl groups increased the specific surface area of ​​BiVO4, and more adsorption sites meant that more organic molecules could be adsorbed on the catalyst surface.

[0055] 6. Chemical valence state analysis

[0056] To investigate the electron transfer direction in the heterostructure formation and photodegradation reaction, the surface composition and chemical state of the samples were determined by XPS spectroscopy. In the fine spectrum, the spectral peak of Bi4f in BiVO4 (… Figure 7 a) Typical spin-orbit bimodal splitting is observed at 159.35 eV and 164.65 eV, corresponding to Bi 4f, respectively. 7 / 2 and Bi 4f 5 / 2 Orbital; spectral peak of V 2p ( Figure 7 b) The spin-orbit splitting peaks at 517.09 eV and 524.46 eV correspond to V 2p, respectively. 3 / 2 and V 2p 1 / 2 ; while Zn5(OH)6(CO3)2 has Zn 2p 3 / 2 and Zn 2p 1 / 2 orbital splitting peak ( Figure 7d) The XPS peaks at 1022.06 eV and 1045.07 eV are observed. After BiVO4 is combined with Zn5(OH)6(CO3)2, the XPS peaks of Bi 4f, V 2p, and lattice O shift towards lower binding energies, indicating a decrease in electron cloud density around Bi, V, and O, and a corresponding increase in valence states. Conversely, the XPS peak of Zn 2p shifts towards higher binding energies, indicating an increase in electron cloud density around Zn, and a corresponding decrease in Zn valence states. Furthermore, after photocatalysis, the XPS peak of Zn 2p in BZ-20 shifts towards higher binding energies, while the XPS peak of lattice O shifts towards lower binding energies, revealing an electron flow shift from Zn to O. Figure 7 c).

[0057] 7. Bandwidth Testing

[0058] The band structure theory of semiconductors can well explain the photocatalytic mechanism, and the measurement of its band gap, conduction band potential, and valence band potential is particularly crucial. For example ( Figure 8 As shown in a), Zn5(OH)6(CO3)2 has the lowest visible light absorbance, and after modification, the absorbance of BiVO4 is dominant. Band gap formula: α(hν) = A(hν-E) g ) 1 / n , where α, hν, E g Let A and B represent the absorption coefficient, photon energy, band gap, and absorbance, respectively. Since BiVO4 is an indirect band gap semiconductor, its n value is 2; while Zn5(OH)6(CO3)2 is a direct band gap semiconductor, its n value is 1. After the Mott-Schottky transformation, the Tauc diagram is obtained (…). Figure 8 (b) The band gap values ​​were derived as follows: BiVO4 (2.36 eV), BZ-20 (2.31 eV), and Zn5(OH)6(CO3)2 (5.51 eV). The results show that the band gap of the BiVO4 sample decreases with recombination with Zn5(OH)6(CO3)2. Next, the VB position was determined by XPS spectroscopy. Figure 8 c), BiVO4 is 2.03 eV, while Zn5(OH)6(CO3)2 is 3.12 eV.

[0059] Combining formula E CB = E VB - E g The calculated band structure (CB) values ​​for BiVO4 and Zn5(OH)6(CO3)2 are -0.33 eV and -2.39 eV, respectively. From this, the band structure diagram can be obtained (…). Figure 8 d) BiVO4 is a narrow bandgap semiconductor, while Zn5(OH)6(CO3)2 is a wide bandgap semiconductor. This band structure may form a type I heterojunction.

[0060] 8. Photocatalytic mechanism: Type I heterojunction

[0061] Based on the previously measured valence band and conduction band potentials of two semiconductors, BiVO4 and Zn5(OH)6(CO3)2, a type I heterojunction was constructed. Figure 9 The photocatalytic mechanism is as follows: First, both semiconductors absorb visible light radiation energy. Electrons in the low-energy valence band layer transition to the high-energy conduction band layer, forming photogenerated electrons, while photogenerated holes remain in situ in the valence band layer. At the tightly contacted interface, electrons in the high-potential conduction band layer of Zn5(OH)6(CO3)2... - Transported to the low-potential conduction band layer of BiVO4, and simultaneously to the high-potential valence band layer of Zn5(OH)6(CO3)2. + The electron migration mechanism involves electrons migrating to the low-potential valence band of BiVO4 to form a type I heterojunction. This is due to the interaction of H2O / •OH and OH-. - The standard redox potentials of / •OH are 2.34 V and 1.99 V, respectively, while the valence band potential of α-Fe2O3 (1.93 V) is more negative than both potentials. This indicates that theoretically h + Insufficient to combine H2O and OH - It is oxidized to •OH. Conversely, due to the conduction band potential of BiVO4 (-0.33 V) and O2 / •O2... - The standard redox potential (-0.33 V) is exactly the same, which indicates that theoretically e - Capable of oxidizing O2 to •O2 - .

[0062] TC degradation mechanism: A large amount of e- ions accumulate in the BiVO4 conduction band. - Combining with O2 to generate •O2 - Finally, •O2 - and h + As active species, they co-catalyze the oxidation of tetracycline hydrochloride, degrading it into smaller organic molecules and even mineralizing it. This conclusion is consistent with the results of subsequent quenching experiments and active species capture experiments.

[0063] 9. Photoelectric Performance Analysis

[0064] First, in the photocurrent response diagram ( Figure 10 a) The composite material BZ-20 exhibits the highest average photocurrent density under visible light irradiation (262.25 nA•cm). -2 This indicates its strong ability to separate photogenerated carriers induced by visible light. Compared to BiVO4 (205.63 nA•cm⁻¹), this demonstrates its superior ability. -2 ) and Zn5(OH)6(CO3)2 (16.58 nA• cm -2Compared to the previous method, the average photocurrent density of BZ-20 increased by 1.28 times and 15.82 times, respectively. Then, electrochemical impedance spectroscopy was used to explore the electron migration efficiency within the semiconductor. Here, charge transfer resistance (Rct) reflects the charge transfer resistance between the electrode surface and the solution, and is related to the electron transport rate. The results show ( Figure 10 (b) BZ-20 possesses the smallest Nyquist radius, and therefore the smallest charge transfer resistance, consistent with the trend in photocurrent response. Compared to BiVO4 and Zn5(OH)6(CO3)2, BZ-20 exhibits a significantly lower impedance curvature radius. A smaller charge transfer resistance is beneficial for the transfer and transport of photogenerated electrons. BZ-20 exhibits the highest photocurrent response intensity and the smallest impedance curvature radius, suggesting that photogenerated carriers are more likely to separate and migrate at the interface, suppressing electron transport. - - h + The composite probability.

[0065] Besides photoelectric testing, the fluorescence quenching phenomenon caused by photoluminescence can also illustrate the recombination probability of photogenerated carriers. Therefore, photoluminescence spectroscopy was used for this purpose. In the steady-state PL spectrum ( Figure 10 c), the BiVO4 sample exhibited a maximum emission wavelength of 622 nm at an excitation wavelength of 507 nm. The relative fluorescence intensity of BiVO4 decreased by 35% after modification with Zn5(OH)6(CO3)2. In the transient TRPL spectrum ( Figure 10 In step d), the same excitation and emission wavelengths were selected, and the decay curves were fitted using a double exponential model to obtain the decay kinetics diagram. The average fluorescence lifetime of the BZ-20 composite sample (0.55 ns) was extended to 1.25 times that of BiVO4 (0.44 ns), indicating that the separation efficiency of photogenerated electron-hole pairs generated by the heterojunction is higher than that of the single-component material. BZ-20 reduced the PL intensity and extended the carrier lifetime, suggesting that the construction of the heterojunction led to the separation of photogenerated carriers in the interface space, suppressing the emission of electrons and holes. - - h + The composite probability.

[0066] 10. Identification of active species

[0067] To differentiate the effects of different active species on the photocatalytic degradation of BPA, quenching experiments were conducted using different scavengers at 1 mM concentrations: TBA, EDTA-2Na, and p-BQ. (For example...) Figure 11 As shown in a), the degradation efficiency was almost unaffected after the addition of TBA, indicating that •OH had little effect on the removal of TC. When EDTA-2Na (h + When quenching agents are used, the degradation rate is almost completely inhibited. Similarly, p-BQ (•O2)- Quenching agents can also significantly inhibit the degradation of TC. This was demonstrated by a pseudo-first-order kinetic model. Figure 11 b) Evaluate the degradation dynamics of the scavenger. The degree of decrease in different k values ​​demonstrates the different contributions of each active species, indicating that h + It is the most important active species, while •O2 - It participated in the photocatalytic process, but •OH contributed almost nothing to the reaction. To further verify the presence of active species in the reaction system, an active species capture experiment was conducted using EPR technology. Figure 11 (c, d) Comparing the EPR signal intensity before and after illumination, the EPR signal intensity after adding TEMPO decreased significantly after illumination, indicating the presence of photogenerated holes during photocatalysis; while the addition of DMPO and illumination resulted in the appearance of characteristic signals of superoxide radicals. The EPR test results were consistent with previous active species scavenging experiments, verifying the reliability of the active species.

[0068] 11 TC Degradation Pathway

[0069] The intermediate products of TC in the photocatalytic reaction system were identified by liquid chromatography-mass spectrometry (LC-MS). Based on the identified intermediate products, two possible pathways for the degradation of TC by BZ-20 under visible light were proposed. Figure 12 ). In h + and •O2 - In its presence, the degradation of TC mainly involves oxidation and substitution reactions that break the benzene ring. The C-C bond connecting the two benzene rings is easily attacked, leading to the breakage of the C-C bond and the formation of a compound with a single aromatic ring.

[0070] In pathway I, TC loses two methyl groups to generate P1 (m / z = 417), then loses one amino group and one hydroxyl group to generate P2 (m / z = 385), then loses three hydroxyl groups to generate P3 (m / z = 335), and then loses one amide bond to generate P4 (m / z = 292). It is also possible that P5 loses one hydroxyl group to generate P5 (m / z = 275), or it may undergo C=C and C=C cleavage to generate P9 (m / z = 163). Furthermore, P5 may also undergo C=C cleavage to open the ring and generate P11.

[0071] In pathway II, the loss of the cyclohexanone group and its associated groups generates P6 (m / z = 295), followed by the removal of one hydroxyl group to generate P7 (m / z = 279). P7 may undergo C=C cleavage to remove the phenolic hydroxyl group and the cyclohexanone group and its associated groups to generate P13 (m / z = 114), or it may lose one hydroxyl group followed by C=C cleavage to remove 1,2-dihydroxypropane to generate P8 (m / z = 190), which then undergoes dehydration to generate P9 (m / z = 163), followed by demethylation to generate P10 (m / z = 149), followed by ring-opening to remove one C and one O to generate P11 (m / z = 118), and C=C cleavage to remove one carbon to generate P12 (m / z = 106). Furthermore, P14 (m / z = 72) may be generated from P13 through demethylation and C=C cleavage, or it may be generated from P10 through C=C cleavage to remove the phenolic group. Finally, the unstable intermediate product undergoes further oxidation and decomposition, mineralizing into NH4. + Small molecule inorganic substances such as CO2 and H2O.

[0072] Table 1. Mass-to-charge ratio of degradation product intermediates and TC

[0073] Products Compounds Mass (m / z) Theoretical Mass (m / z) Experimental Tetracycline hydrochloride (TC) 445 445.12 P1 417 415.21 P2 385 385.68 P3 335 335.28 P4 292 291.25 P5 275 274.27 P6 295 295.07 P7 279 279.03 P8 190 190.98 P9 163 163.04 P10 149 149.02 P11 118 119.09 P12 106 105.96 P13 114 113.96 P14 82 81.52

[0074] 12 QSAR Toxicological Analysis

[0075] Three toxicological analyses of TC degradation products were performed using TEST (Toxicity Estimation Software Tool). Figure 13 TEST identifies potential toxic groups in target compounds through extensive data processing and statistical analysis. It can also rapidly analyze the toxicity of compounds with similar structures to the target substance, further comprehensively predicting the toxicity of the target substance. The TEST toxicity assessment tool can be used to predict physicochemical, health toxicology, and ecotoxicological properties. The software integrates multiple QSAR (quantitative structure-activity relationship) models constructed using various algorithms and can provide a comprehensive prediction result based on the suitability of the substance in each model.

[0076] The 96-hour Fathead minnow LC50, 48-hour Daphnia magna LC50, and developmental toxicity results of these two acute toxicological analyses showed that the intermediates in route II were less toxic than those in route I. Among them, P3 and P4, which do not involve ring-opening reactions, showed high toxicity in all three indicators.

[0077] Secondly, the intermediates in pathway II tend to undergo ring-opening and defunctionalization reactions. As the degradation level increases, the toxicity of the degradation products decreases. Among them, P12, P13, and P14 all showed low toxicity in all three indicators. The tetracyclic products of pathway I are unstable and will continue the degradation pathway in pathway II, leading to a decrease in the toxicity of the final degradation products.

[0078] Table 2 Toxicological Indicators and Parameters of TEXT

[0079] Intermediates <![CDATA[Fathead minnow LC 50 (96 hr)Unit:mg / L]]> <![CDATA[Daphnia magnaLC 50 (48 hr)Unit:mg / L]]> Developmental Toxicity P1 0.47 3.6 0.92 P2 0.66 4.09 0.88 P3 0.03 0.61 0.99 P4 0.05 2.12 1 P5 0.08 1.32 0.83 P6 24.21 62.84 0.67 P7 7.75 121.44 0.73 P8 3.54 6.52 0.87 P9 2.52 13.85 0.62 P10 1.28 14.1 0.61 P11 3.85 7.9 0.49 P12 21.98 15.37 0.24 P13 170.28 42.23 0.31 P14 15.40 103.69 0.58 TC 0.90 5.44 0.86

[0080] 13 Catalyst stability

[0081] To further explore the catalytic stability and practical application potential of the BZ-20 photocatalyst, Na2CO3 was added after each cycle to compensate for the interlayer CO3 consumed in the photocatalytic reaction. 2- This can effectively ensure cycle stability, due to ( Figure 14 (a) It can be seen that the degradation rate decreased by only 5% after 5 cycles, indicating good cycle stability. In the XRD patterns before and after the reaction ( Figure 14 (b) The diffraction pattern of the BZ-20 sample still mainly consists of the characteristic diffraction peaks of ms BiVO4, and the intensity and position of the diffraction peaks have not changed significantly.

[0082] After 6 hours of photodegradation, the catalyst washed by centrifugation was imaged by SEM. It was found that there was no significant difference in morphology compared with the BZ-20 sample before the reaction. The morphology was still mainly composed of nanoflowers encapsulating nanoparticles, indicating that the material has a certain degree of chemical stability in the photocatalytic reaction. Figure 14 c).

[0083] 14. Catalyst Practicality

[0084] To explore the universality of BZ-20 catalyst for visible light degradation of organic pollutants, with the aim of its application in practical advanced wastewater oxidation engineering, this invention includes four antibiotics and one dye, ranked by removal rates for different pollutants (…). Figure 15 a): ENR > CIP > TC > NOR > MB. Furthermore, the apparent kinetic constants fitted by the pseudo-first-order reaction equation show the same trend in degradation efficiency and removal rate for different pollutants ( Figure 15 d).

[0085] In addition, to verify the effect of real sunlight on pollutant degradation, a 3-hour outdoor solar radiation experiment was conducted. (For example...) Figure 15b and e show that the degradation efficiency of the five different pollutants was greatly improved, which may be due to the presence of ultraviolet radiation in sunlight and the stronger radiation intensity than the visible light of the photoreactometer.

[0086] In addition, to investigate the effect of actual water bodies on the degradation of tetracycline, different water samples were collected from deionized water (control group), tap water, Chaohu Lake, and Emerald Lake in Hefei City, Anhui Province. After filtration to remove suspended solids, the water samples were prepared into 40 mg / L TC solutions for degradation experiments. (For example...) Figure 15 c) and f) show that the degradation efficiency of TC in the three different water bodies was promoted, which may be due to the presence of anions and the weakly alkaline pH value in the water.

[0087] In summary, this invention synthesized a series of composite materials with different molar ratios of Zn5(OH)6(CO3)2 / BiVO4 using an in-situ hydrothermal method. The morphology, structure, composition, and photoelectric properties of a series of Zn5(OH)6(CO3)2 / BiVO4 catalysts were explored using various characterization techniques. Furthermore, the mechanism of photocatalytic degradation of TC by type I heterojunctions was proposed, and the conclusions are as follows:

[0088] 1) BiVO4 particles with nano-sized particles were synthesized by hydrothermal method, and Zn5(OH)6(CO3)2 nanoflowers with nano-thickness were synthesized in situ by uniform precipitation method. By controlling the addition ratio of precursor and zinc source, Zn5(OH)6(CO3)2 / BiVO4 materials with different loading ratios can be prepared.

[0089] 2) Photocatalytic degradation experiments of TC showed that BZ-20 exhibited the highest photodegradation efficiency, with BZ-20 having the highest k0. obs (h) -1 The concentration of 0.63 was 4.2 times that of BiVO4 and 3.94 times that of Zn5(OH)6(CO3)2.

[0090] 3) Experiments on the quenching and capture of active species show that h + and •O2 - It is the main active substance for the visible light degradation of TC, and •OH is almost non-existent in this system.

[0091] 4) Nitrogen adsorption and photoelectrochemical tests showed that BiVO4 loaded with Zn5(OH)6(CO3)2 increased the BET specific surface area, reduced the band gap width and charge transfer resistance, thus effectively promoting the separation efficiency of photogenerated carriers.

[0092] 5) Using VB-XPS spectroscopy and Tauc plots, the valence band potential and conduction band potential of BiVO4 and Zn5(OH)6(CO3)2 were derived, and a type I heterojunction was constructed from the band structure. The migration direction of photogenerated carriers and the generation of active species were reasonably explained, and a complete mechanism for visible light degradation of TC by Zn5(OH)6(CO3)2 / BiVO4 was proposed.

[0093] 6) The BZ-20 catalyst exhibits excellent performance in both catalytic stability and anti-interference properties. It is not only universally applicable to the degradation of organic pollutants under visible light, but also maintains good photodegradation efficiency in various real water bodies. The catalytic stability of the photocatalyst in aquatic environments indicates its promising practical application prospects in engineering projects.

Claims

1. A Zn5(OH)6(CO3)2 / BiVO4 heterojunction photocatalyst, characterized in that, Composed of BiVO4 nanoparticles and Zn5(OH)6(CO3)2 nanosheets, the BiVO4 nanoparticles are uniformly loaded on the surface or between the layers of Zn5(OH)6(CO3)2 / BiVO4 nanosheets to form a type I heterojunction structure.

2. The Zn5(OH)6(CO3)2 / BiVO4 heterojunction photocatalyst as described in claim 1, characterized in that, The molar ratio between Zn5(OH)6(CO3)2 and BiVO4 is 3:4~48.

3. The Zn5(OH)6(CO3)2 / BiVO4 heterojunction photocatalyst as described in claim 2, characterized in that, The molar ratio between Zn5(OH)6(CO3)2 and BiVO4 is 1:

4.

4. The preparation method of the Zn5(OH)6(CO3)2 / BiVO4 heterojunction photocatalyst as described in claim 1, characterized in that, The steps are as follows: (1) Dissolve bismuth nitrate and sodium dodecylbenzenesulfonate in nitric acid solution to obtain solution A, and dissolve ammonium metavanadate in nitric acid solution to obtain solution B; add solution B dropwise to solution A, adjust the pH value and mix evenly, and then carry out hydrothermal reaction. After the reaction is completed, wash and dry to obtain BiVO4 powder. (2) The BiVO4 powder, urea and zinc nitrate obtained in step (1) are added to deionized water, stirred evenly and then subjected to hydrothermal reaction. After the reaction is completed, the Zn5(OH)6(CO3)2 / BiVO4 heterojunction photocatalyst is obtained by washing, drying and grinding.

5. The preparation method according to claim 4, characterized in that, In step (1), the molar ratio between bismuth nitrate and ammonium metavanadate is 0.8~1.2:0.8~1.2, the hydrothermal reaction temperature is 180~220 ℃, and the reaction time is 8~15 h.

6. The preparation method according to claim 4, characterized in that, In step (2), the amount of zinc nitrate added is controlled by the molar ratio of Zn5(OH)6(CO3)2 to BiVO4 in the preparation of heterojunction photocatalyst being 3:4~48, the hydrothermal reaction temperature being 100~140 ℃, and the reaction time being 10~30 h.

7. The application of the Zn5(OH)6(CO3)2 / BiVO4 heterojunction photocatalyst as described in claim 1 in the degradation of organic pollutants in water, characterized in that, The organic pollutants are tetracycline, enrofloxacin, ciprofloxacin, norfloxacin, or methylene blue.

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