Method for preparing nickel-based metal-organic framework / bismuth oxychloride composite for degrading tetracycline

Abstract

A method for preparing a nickel-based metal-organic framework (Ni-MOF) / bismuth oxychloride (BiOCl) composite for degrading tetracycline (TC) comprising: preparing a BiOCl nanosheet; preparing a Ni-MOF

Description

[0001] The present disclosure belongs to the technical field of agriculture, and specifically relates to a method for preparing a nickel-based metal-organic framework (Ni-MOF) / bismuth oxychloride (BiOCl) composite for degrading tetracycline (TC). 15 01 25 BACKGROUND

[0002] Antibiotics have become one of the most effective means of treating human diseases in recent years due to their huge advantages in the field of medicine. However, studies have shown that antibiotics are not easily metabolized by human beings and animals, and are generally discharged into the natural environment to contaminate water sources and soil, posing a huge threat to ecosystems and human health.

[0003] Traditional water treatment technologies, including filtration, adsorption, and biodegradation, could not effectively eliminate these antibiotics due to their high stability and low biodegradability. Therefore, it is highly necessary to develop environmental-friendly and feasible technologies to adsorb and degrade the antibiotics. Semiconductor photocatalysis is considered a promising technology to solve environmental pollutions because of high efficiency and low cost. A key to this technology lies in building photocatalysts with powerful catalytic capabilities. In recent years, various semiconductor photocatalysts have been developed for catalyzing the degradation of antibiotics, such as TiOz, CdS, ZnO, and ZnS. As a layered structure semiconductor, BiOCl has high oxidation ability, desirable stability, and non-toxicity, but shows narrow photoresponse range and low separation efficiency of photogenerated carriers that severely limit its applications in the field of photocatalysis. Accordingly, a series of strategies have been investigated to compensate for the shortcomings of BiOCl. A process for constructing a heterojunction by combining the BiOCl with another semiconductor having matchable energy levels is considered to have great application prospects. In the past, there have been many related studies on BiOCl coupling with other semiconductor materials to form heterojunctions, such as BiOCl / BiOBr, BiOCl / TiO2, PDI / BiOCl, BiOCl / CuBi2O4, and CusO / BiOCl. S-scheme heterojunctions are impressive due to their high carrier separation efficiency and strong redox capability.

[0004] Metal-organic frameworks (MOFs), with high specific surface area, abundant active sites, and tunable hole channels, have recently received widespread attention in the field of photocatalysis. In particular, two-dimensional (2D) MOFs nanosheets have many significant advantages in catalysis. 15 01 25 Due to ultra-low thickness and abundant exposed active sites, this type of nanosheets ensures not only fast electron transfer but also high catalytic activity. Inspired by these significant advantages of 2D MOFs, a 2D / 2D S-scheme heterojunction photocatalyst is constructed by stacking 2D BiOCl nanosheets on the surface of 2D Ni-MOF nanosheets. This process can bring huge advantages: on one hand, this face-to-face approach could promote close and large-area contact between the above two materials, promoting the separation and transfer of photogenerated carriers at an interface of the two nanosheets; on the other hand, the strong electronic coupling between nanosheets also contributes to the improvement of photocatalytic performance of this S-scheme heterojunction by constructing a 2D / 2D Ni-MOF / BiOCl S-scheme heterojunction. In this way, an efficient photocatalytic material could be obtained for the degradation of tetracycline (TC) in aquaculture wastewater.

[0005] In view of the deficiencies of low carrier separation efficiency and low photocatalytic activity of pure BiOCl nanosheets, it is necessary to provide a method for preparing an Ni-MOF / BiOCl composite for degrading TC. SUMMARY

[0006] An object of the present disclosure is to provide a method for preparing an Ni-MOF / BiOCl composite for degrading TC. In the present disclosure, a BiOCl nanosheet and an Ni-MOF nanosheet are combined through self-assembly to prepare a photocatalyst with a 2D / 2D S-scheme heterojunction. A special S-scheme charge transfer mechanism and a 2D / 2D structure of the heterojunction effectively promote the separation / transfer of charges, and also improve an adsorption capacity of the composite to antibiotics, thus promoting the degradation of the antibiotics. Therefore, the present disclosure solves the problems existing in the prior art proposed above that a simple BiOCl nanosheet shows low carrier separation efficiency and low photocatalytic activity.

[0007] To achieve the above object, the present disclosure adopts the following technical solutions.

[0008] Provided is a method for preparing an Ni-MOF / BiOCl composite for degrading TC, including or consisting of the following steps:

[0009] step 1: preparing a BiOCl nanosheet, including

[0010] adding 30 mL of distilled water into a beaker, and adding 2 mmol of bismuth nitrate pentahydrate and 2 mmol of potassium chloride thereto, to obtain a mixed solution;

[0011] stirring the mixed solution for 1.5 h, pouring into a 50 mL polytetrafluoroethylene-lined stainless steel autoclave, and performing a reaction at 160 °C for 24 h, to obtain a reaction system; and cooling the reaction system to room temperature, and centrifuging to obtain a first precipitate, washing the first precipitate with deionized water three times, and drying at 60 °C for 12 h;

[0012] step 2: preparing an Ni-MOF nanosheet, including

[0013] preparing a solution comprising 30 mL of N,N-dimethylformamide, 2 mL of ethanol, and 2 15 01 25 mL of deionized water, adding 125 mg of terephthalic acid thereto while stirring, adding 180 mg of NiCh 6H2O and 1 mL of tri ethylamine thereto in sequence, and stirring a resulting mixture for another 10 min to obtain a colloidal suspension; and

[0014] sealing the colloidal suspension, and sonicating at room temperature for continuous 8 h to obtain a treated product; washing the treated product with ethanol by centrifugation three times, and drying at 60 °C for 12 h; and

[0015] step 3: preparing an Ni-MOF / BiOCl heterojunction, i.e., the Ni-MOF / BiOCl composite, including

[0016] adding the Ni-MOF nanosheet and the BiOCl nanosheet into 20 mL of isopropyl alcohol to obtain a mixture, stirring the mixture for 12 h and centrifugating, discarding a supernatant obtained from centrifugating, to obtain a second precipitate; and

[0017] drying the second precipitate at 60 °C for 12 h, to obtain the Ni-MOF / BiOCl composite.

[0018] In some embodiments, step 1 specifically includes or consists of: adding 30 mL of distilled water into a beaker, and adding 2 mmol of bismuth nitrate pentahydrate and 2 mmol of potassium chloride thereto to obtain a mixed solution; and stirring the mixed solution for 1.5 h, and pouring into a 50 mL polytetrafluoroethylene (PTFE)-lined stainless steel autoclave, and performing a reaction at 160 °C for 24 h to obtain a reaction system; cooling the reaction system to room temperature, and centrifuging to obtain a first precipitate, washing the first precipitate with deionized water three times, and then drying at 60 °C for 12 h.

[0019] In some embodiments, step 2 specifically includes or consists of: preparing a solution including or consisting of 30 mL of N,N-dimethylformamide (DMF), 2 mL of ethanol, and 2 mL of deionized water, adding 125 mg of terephthalic acid thereto under rapid stirring, adding 180 mg of NiCh 6H2O, and 1 mL of tri ethylamine thereto in sequence, and stirring a resulting mixture for another 10 min to obtain a uniform colloidal suspension; and sealing the uniform colloidal suspension, and sonicating at room temperature for continuous 8 h; washing a resulting treated product with ethanol by centrifugation three times, and drying at 60 °C for 12 h.

[0020] In some embodiments, step 3 specifically includes or consists of: adding the Ni-MOF nanosheet and the BiOCl nanosheet each in an appropriate amount into 20 mL of isopropyl alcohol to obtain a mixture, stirring the mixture for 12 h and centrifugating, discarding a supernatant obtained from centrifugating to obtain a second precipitate; and drying the second precipitate at 60 °C for 12 h, to obtain the Ni-MOF / BiOCl heterojunction, wherein a mass percentage of the Ni-MOF nanosheet in the composite is 10%, 30%, and 50%, the Ni-MOF / BiOCl composite being expressed as 0.1Ni-MOF / BiOCl, 0.3Ni-MOF / BiOCl, and 0.5Ni-MOF / BiOCl, respectively; and a mass percentage of Ni-MOF in the Ni-MOF / BiOCl heterojunction is 10%, 30%, and 50%, the Ni-MOF / BiOCl heterojunction being expressed as O.lNi-MOF / BiOCl, 0.3Ni-MOF / BiOCl, and 0.5Ni-MOF / BiOCl, 15 01 25 respectively.

[0021] In some embodiments, the method further includes step 4: characterizing a crystal structure of a power sample of the Ni-MOF / BiOCl heterojunction prepared in step 3 by X-ray diffraction

[0022] In some embodiments, step 4 specifically includes or consists of: : determining an XRD pattern of the powder sample with a D / Max2200PC X-ray diffractometer, wherein XRD patterns of obtained pure BiOCl, pure Ni-MOF, and the Ni-MOF / BiOCl composite are shown in FIG. 1; the pure BiOCl has obvious diffraction peaks at 11.96°, 24.12°, 25.88°, 32.52°, 33.52°, 36.56°, 49.72°, 55.16°, and 58.64°, assigned to planes (001), (002), (101), (110) (102), (003), (113), (104), and (212) of a tetragonal structure (JCPDS No. 06 to No. 0249), respectively; the pure Ni-MOF has diffraction peaks that are consistent with a standard pattern (CCDC NO.638866); different Ni-MOF / BiOCl composites have generally the same XRD patterns, and have a diffraction peak similar to that of the pure BiOCl; however, a diffraction peak of the Ni-MOF is difficult to be observed in XRD pattern of the Ni-MOF / BiOCl composite since the diffraction peak of the Ni-MOF has a low diffraction intensity; and successful combination of the BiOCl nanosheet and the Ni-MOF nanosheet is determined and further confirmed by subsequent characterizations.

[0023] In some embodiments, the method further includes step 5: characterizing appearances and microstructures of the Ni-MOF nanosheet, the BiOCl nanosheet, and the Ni-MOF / BiOCl composite separately by transmission electron microscopy (TEM).

[0024] In some embodiments, in step 5, the Ni-MOF appears as a thinner flexible micron sheet structure (FIGs. 2A and 2B); FIG. 2C shows that the BiOCl exhibits a rectangular nanosheet structure with a transverse size of 1 pm to 3 pm; a TEM image of the Ni-MOF / BiOCl heterojunction (FIG. 2D) shows that the BiOCl nanosheet with a 2D rectangular sheet shape is stacked on the Ni-MOF nanosheet, indicating successful construction of a 2D / 2D Ni-MOF / BiOCl heterojunction; a TEM image of FIG. 2E shows that there is a clear contact interface between the BiOCl nanosheet and the Ni-MOF nanosheet; a lattice structure of the Ni-MOF / BiOCl composite is further revealed by a high-resolution TEM; and a clear lattice fringe with a spacing of 0.335 nm is attributed to the plane (101) of the BiOCl (FIG. 2F), all of which demonstrate the successful construction of the 2D / 2D Ni-MOF / BiOCl heterojunction.

[0025] In some embodiments, electron spin resonance (ESR) testing was conducted using 5,5-dimethyl-1-pyrroline N-oxide (DMPO) as a trapping agent to detect spin-active -OH and -O2' species during the photodegradation of the TC. FIGs. 3 A and 3B show that no ESR signal is detected at 0 min in the dark; and four obvious characteristic peaks of DMP0--02’ with a relative intensity of 1:2:2:1 and a characteristic signal of DMP0--0H are observed under simulated visible light after the trapping agent DMPO is added, and a signal intensity gradually increases over time. 15 01 25

[0026] In some embodiments, the method further includes conducting transient photocurrent response and electrochemical impedance spectroscopy (EIS) tests to further determine a chargecarrier transfer / separation efficiency in the Ni-MOF / BiOCl heterojunction, wherein FIG. 4A shows transient photocurrent responses of the BiOCl nanosheet, the Ni-MOF nanosheet, and the Ni-MOF / BiOCl separately; the Ni-MOF / BiOCl heterojunction has a higher photocurrent intensity compared with the BiOCl nanosheet and the Ni-MOF nanosheet, proving that the Ni-MOF / BiOCl composite shows a significantly-improved electron transfer / separation efficiency; the O.lNi-MOF / BiOCl has a maximum photocurrent response and thus exhibits optimal photocatalytic performance; and FIG. 4B shows an electrochemical impedance spectrum of samples; and a smaller arc radius of a Nyquist curve generally means a higher photogenerated carrier transfer / separation efficiency.

[0027] In some embodiemnts, the method further includes step 6: confirming that the Ni-MOF / BiOCl heterojunction obtained in step 3 has a great potential to photodegrade the TC.

[0028] In some embodiments, step 6 further includes evaluating an activity of a prepared photocatalyst by a degradation efficiency of the TC under UV light irradiation, which includes dispersing 50 mg of the photocatalyst in 20 mL of a TC solution with a concentration of 10 ppm, and stirring a resulting mixed solution in the dark for 30 min to achieve an adsorption-desorption equilibrium; placing the resulting mixed solution under a UV lamp while maintaining continuous magnetic stirring, and collecting 1 mL of a system sample every 30 min; performing centrifugal separation on collected sample to obtain a clear liquid; and detecting a TC concentration in the clear liquid by high-performance liquid chromatography-mass spectrum (HPLC-MS, SCIEX Triple Quad™ 4500), wherein a degradation rate (X) of the TC is calculated according to Equation:

[0029] X(%)=^ X 100%,

[0030] where Co represents an initial concentration of the TC solution, and C represents a concentration of the TC solution at time t.

[0031] In some embodiments, a photocatalytic degradation experiment of the TC is performed under ultraviolet by using the Ni-MOF / BiOCl heterojunction as a photocatalyst; FIG. 5A shows that the TC concentration does not change significantly when not adding photocatalyst, thus excluding an influence of TC self-degradation; the BiOCl nanosheet has a poor photocatalytic effect due to a poor photoresponse; and the heterojunction obtained by combining the BiOCl nanosheet with the Ni-MOF nanosheet has significantly-improved photocatalytic performance.

[0032] Technical effects and advantages:

[0033] compared with the existing technology, the method for preparing an Ni-MOF / BiOCl composite for degrading TC according to some embodiments of the present disclosure has the 15 01 25 following advantages:

[0034] In the present disclosure, a BiOCl nanosheet and an Ni-MOF nanosheet are combined through self-assembly to obtain a 2D / 2D S-scheme heterojunction photocatalyst. The heterojunction has a special S-scheme charge transfer mechanism and a 2D / 2D structure, which effectively promote the separation / transfer of charges and improve an adsorption capacity of the composite to antibiotics, thus promoting the degradation of antibiotics. A 2D / 2D Ni-MOF / BiOCl S-scheme heterojunction photocatalyst is successfully prepared through electrostatic self-assembly, and has excellent carrier separation efficiency and photocatalytic activity. The photocatalyst could be used to degrade TC in agricultural aquaculture water, and has a simple preparation method with a strong controllability, thereby easily achieving large-scale production.

[0035] Other features and advantages of the present disclosure will be illustrated in the following description, and some of these will become apparent from the description or be understood by implementing the present disclosure. The object and other advantages of the present disclosure may be realized and obtained by the structure specifically indicated in the specification and drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] FIG. 1 shows comparisons among the XRD patterns of the Ni-MOF / BiOCl heterojunction photocatalysts according to embodiments of the present disclosure.

[0037] FIGs. 2A to 2F show TEM images of the Ni-MOF / BiOCl heterojunction photocatalysts according to embodiments of the present disclosure.

[0038] FIGs. 3A to 3B show ESR signal diagrams of the Ni-MOF / BiOCl heterojunction photocatalysts according to embodiments of the present disclosure.

[0039] FIGs. 4A to 4B show transient photocurrent response and electrochemical impedance spectroscopy (EIS) diagrams of the Ni-MOF / BiOCl heterojunction photocatalysts according to embodiments of the present disclosure.

[0040] FIGs. 5Ato 5B show results of photocatalytic degradation experiment of TC under ultraviolet by using the Ni-MOF / BiOCl heterojunction as a catalyst according to an embodiment of the present disclosure as a photocatalyst.

[0041] FIG. 6 shows a flowchart of the method according to an embodiment of the present disclosure. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0042] The technical solutions of the embodiments of the present disclosure are clearly and completely described below with reference to the drawings in the embodiments of the present disclosure. Apparently, the described embodiments are merely a part rather than all of the embodiments of the present disclosure. The described embodiments are merely used to explain the 15 01 25 present disclosure and are not intended to limit the present disclosure. All the other embodiments derived by those of ordinary skill in the art based on the embodiments of the present disclosure without creative efforts shall fall within the scope of the present disclosure.

[0043] The present disclosure provides a method for preparing an Ni-MOF / BiOCl composite for degrading TC shown in FIG. 1 to FIG. 6, including or consisting of the following steps:

[0044] step 1: preparing a BiOCl nanosheet, including

[0045] adding 30 mL of distilled water into a beaker, and adding 2 mmol of bismuth nitrate pentahydrate and 2 mmol of potassium chloride thereto, to obtain a mixed solution;

[0046] stirring the mixed solution for 1.5 h, pouring into a 50 mL polytetrafluoroethylene-lined stainless steel autoclave, and performing a reaction at 160 °C for 24 h, to obtain a reaction system; and cooling the reaction system to room temperature, and centrifuging to obtain a first precipitate, washing the first precipitate with deionized water three times, and drying at 60 °C for 12 h;

[0047] step 2: preparing an Ni-MOF nanosheet, including

[0048] preparing a solution comprising 30 mL of N,N-dimethylformamide, 2 mL of ethanol, and 2 mL of deionized water, adding 125 mg of terephthalic acid thereto while stirring, adding 180 mg of NiCh 6H2O and 1 mL of tri ethylamine thereto in sequence, and stirring a resulting mixture for another 10 min to obtain a colloidal suspension; and

[0049] sealing the colloidal suspension, and sonicating at room temperature for continuous 8 h to obtain a treated product; washing the treated product with ethanol by centrifugation three times, and drying at 60 °C for 12 h; and

[0050] step 3: preparing an Ni-MOF / BiOCl heterojunction, i.e., the Ni-MOF / BiOCl composite, including

[0051] adding the Ni-MOF nanosheet and the BiOCl nanosheet into 20 mL of isopropyl alcohol to obtain a mixture, stirring the mixture for 12 h and centrifugating, discarding a supernatant obtained from centrifugating, to obtain a second precipitate; and

[0052] drying the second precipitate at 60 °C for 12 h, to obtain the Ni-MOF / BiOCl composite.

[0053] In some embodiments of the present disclosure, step 1 specifically includes or consists of: adding 30 mL of distilled water into a beaker, and then adding 2 mmol of bismuth nitrate pentahydrate and 2 mmol of potassium chloride thereto, to obtain a mixed solution; and stirring the mixed solution thoroughly for 1.5 h, and pouring into a 50 mL PTFE-lined stainless steel autoclave, and performing a reaction at 160 °C for 24 h to obtain a reaction system; cooling a reaction system to room temperature, centrifuging to obtain a first precipitate, and washing the first precipitate with deionized water three times, and then drying at 60 °C for 12 h.

[0054] In some embodiments of the present disclosure, step 2 specifically includes or consists of: preparing a solution including or consisting of 30 mL of DMF, 2 mL of ethanol, and 2 mL of deionized 15 01 25 water, adding 125 mg of terephthalic acid thereto under rapid stirring, adding 180 mg of NiClybHjO and 1 mL of tri ethylamine thereto in sequence, and stirring a resulting mixture for another 10 min to obtain a uniform colloidal suspension; and sealing the uniform colloidal suspension, and sonicating at room temperature for continuous 8 h; washing a resulting treated product with ethanol by centrifugation for three times, and drying at 60 °C for 12 h.

[0055] In some embodiments of the present disclosure, step 3 specifically includes or consists of: adding the Ni-MOF nanosheet and the BiOCl nanosheet separately each in an appropriate amount into 20 mL of IPA to obtain a resulting mixture, stirring the resulting mixture for 12 h and centrifugating, discarding a supernatant obtained from centrifugating to obtain a second precipitate; and drying the second precipitate at 60 °C for 12 h, to obtain the Ni-MOF / BiOCl heterojunction, wherein a mass percentage of the Ni-MOF nanosheet in the composite is 10%, 30%, and 50%, the Ni-MOF / BiOCl composite being expressed as 0.1 Ni-MOF / BiOCl, 0 3Ni-MOFZBiOCl, and 0.5Ni-MOF / BiOCl, respectively; and a mass percentage of Ni-MOF in the Ni-MOF / BiOCl heterojunction is 10%, 30%, and 50%, the Ni-MOF / BiOCl heterojunction being expressed as O.lNi-MOF / BiOCl, 0.3Ni-MOF / BiOCl, and 0.5Ni-MOF / BiOCl, respectively.

[0056] In some embodiments, the method further includes step 4: characterizing a crystal structure of a power sample of the Ni-MOF / BiOCl heterojunction prepared in step 3 by X-ray diffraction (XRD).

[0057] In some embodiments of the present disclosure, step 4 specifically includes or consists of: determining an XRD pattern of the powder sample with a D / Max2200PC X-ray diffractometer, wherein XRD patterns of obtained pure BiOCl, pure Ni-MOF, and the Ni-MOF / BiOCl composite are shown in FIG. 1; the pure BiOCl has obvious diffraction peaks at 11.96°, 24.12°, 25.88°, 32.52°, 33.52°, 36.56°, 49.72°, 55.16°, and 58.64°, assigned to planes (001), (002), (101), (110) (102), (003), (113), (104), and (212) of a tetragonal structure (JCPDS No. 06 to No. 0249), respectively; the pure Ni-MOF has diffraction peaks that are consistent with a standard pattern (CCDCNO.638866); different Ni-MOF / BiOCl composites have generally the same XRD patterns, and have a diffraction peak similar to that of the pure BiOCl; however, a diffraction peak of the Ni-MOF is difficult to be observed in XRD pattern of the Ni-MOF / BiOCl composite since the diffraction peak of the Ni-MOF has a low diffraction intensity; and successful combination of the BiOCl nanosheet and the Ni-MOF nanosheet is determined and further confirmed by subsequent characterizations.

[0058] In some embodiments, the method further includes step 5: characterizing appearances and microstructures of the Ni-MOF nanosheet, the BiOCl nanosheet, and the Ni-MOF / BiOCl composite separately by transmission electron microscopy (TEM).

[0059] In some embodiments of the present disclosure, in step 5, the Ni-MOF appears as a thinner flexible micron sheet structure (shown in FIGs. 2A and 2B); FIG. 2C shows that the BiOCl exhibits 15 01 25 a rectangular nanosheet structure with a transverse size of 1 um to 3 pm; a TEM image of the Ni-MOF / BiOCl heterojunction (FIG. 2D) shows that the BiOCl nanosheet with a 2D rectangular sheet shape is stacked on the Ni-MOF nanosheet, indicating successful construction of a 2D / 2D Ni-MOF / BiOCl heterojunction; a TEM image of FIG. 2E shows that there is a clear contact interface between the BiOCl nanosheet and the Ni-MOF nanosheet; a lattice structure of the Ni-MOF / BiOCl composite is further revealed by a HRTEM; and a clear lattice fringe with a spacing of 0.335 nm is attributed to the plane (101) of the BiOCl (FIG. 2F), all of which demonstrate the successful construction of the 2D / 2D Ni-MOF / BiOCl heterojunction.

[0060] In some embodiments of the present disclosure, ESR testing was conducted using DMPO as a trapping agent to detect spin-active -OH and -O2’ species during the photodegradation of the TC. FIGs. 3A and 3B show that no ESR signal is detected at 0 min in the dark. While after adding the trapping agent DMPO, four obvious characteristic peaks of DMPO--O2’ with a relative intensity of 1:2:2:1 and a characteristic signal of DMPO--OH are observed under simulated visible light, and a signal intensity gradually increases over time. The above results verify the types of free radicals generated during the degradation, indicating that superoxide radicals ¢02-) and hydroxyl radicals (■OH) are generated during the photocatalysis. Moreover, a longer illumination time results in a greater number of free radicals, indicating that the composite has excellent photocatalytic degradation performance.

[0061] In some embodiments of the present disclosure, the method further includes conducting transient photocurrent response and electrochemical impedance spectroscopy (EIS) tests to further determine charge-carrier transfer / separation efficiency in the Ni-MOF / BiOCl heterojunction. FIG. 4A shows transient photocurrent responses of the BiOCl nanosheet, the Ni-MOF nanosheet, and the Ni-MOF / BiOCl separately. As the results shows, the Ni-MOF / BiOCl heterojunction has a higher photocurrent intensity compared with the BiOCl nanosheet and the Ni-MOF nanosheet, proving that the Ni-MOF / BiOCl composite shows a significantly-improved electron transfer / separation efficiency. Further, the O.lNi-MOF / BiOCl has a maximum photocurrent response and thus exhibits optimal photocatalytic performance. FIG. 4B shows an electrochemical impedance spectrum of samples. Generally, a smaller arc radius of the Nyquist curve means a higher photogenerated carrier transfer / separation efficiency. A semicircle radius of the Nyquist curve of the Ni-MOF / BiOCl heterojunction is smaller compared with the BiOCl nanosheet and the Ni-MOF nanosheet, indicating that the composite shows a smaller charge transfer resistance, which is beneficial to improving the charge transfer / separation efficiency.

[0062] In some embodiemnts, the method further includes step 6: confirming that the Ni-MOF / BiOCl heterojunction obtained in step 3 has a great potential to photodegrade the TC.

[0063] In some embodiments of the present disclosure, step 6 further comprises evaluating an 15 01 25 activity of a prepared photocatalyst by a degradation efficiency of TC under UV light irradiation, which comprises dispersing 50 mg of the photocatalyst in 20 mL of a TC solution with a concentration of 10 ppm, and stirring a resulting mixed solution in the dark for 30 min to achieve an adsorptiondesorption equilibrium; placing the resulting mixed solution under a UV lamp while maintaining continuous magnetic stirring, and collecting 1 mL of a system sample every 30 min, performing centrifugal separation on collected sample to obtain a clear liquid; and detecting a TC concentration in the clear liquid by high-performance liquid chromatography-mass spectrum (HPLC-MS, SCIEX Triple Quad™ 4500), wherein a degradation rate (X) of the TC is calculated according to Equation:

[0064] k(%)=^ X 100%, co

[0065] where Co represents an initial concentration of the TC solution, and C represents a concentration of the TC solution at time t.

[0066] In some embodiments of the present disclosure, a photocatalytic degradation experiment of TC is performed under ultraviolet by using the Ni-MOF / BiOCl heterojunction as a photocatalyst. FIG. 5A shows that the TC concentration does not change significantly when not adding the photocatalyst, thus excluding an influence of TC self-degradation. The BiOCl nanosheet has a poor photocatalytic effect due to a poor photoresponse. After combining the BiOCl nanosheet with the Ni-MOF nanosheet, the photocatalytic performance of the resulting heterojunction is significantly improved. It is notable that, among the Ni-MOF / BiOCl heterojunctions with different Ni-MOF contents, the O.lNi-MOF / BiOCl has the best photocatalytic degradation effect, with a degradation rate of TC reaching 40.55% within 180 min. In order to quantitatively evaluate the degradation performance of the photocatalyst to TC, the experimental data are fitted with a first-order kinetic equation, as shown in FIG. 5B. The results have confirmed that the 0. INi-MOF / BiOCl has the highest apparent rate constant (k=0.16416 min'1), which is 7.22 times that of the BiOCl nanosheet (k=0.02275 min'1), indicating that the 0. INi-MOF / BiOCl heterojunction shows a great potential for photodegradation of TC.

[0067] Working principle: a BiOCl nanosheet and an Ni-MOF nanosheet are combined through selfassembly to obtain a photocatalyst with a 2D / 2D S-scheme heterojunction. The heterojunction has a special S-scheme charge transfer mechanism and a 2D / 2D structure, which effectively promote the separation / transfer of charges, and improve an adsorption capacity of the composite to antibiotics, thus promoting the degradation of antibiotics. A 2D / 2D Ni-MOF / BiOCl S-scheme heterojunction photocatalyst is also successfully prepared through electrostatic self-assembly, and has excellent carrier separation efficiency and photocatalytic activity. The photocatalyst could be used to degrade TC in agricultural aquaculture water, and has a simple preparation method with a strong controllability, thereby easily achieving large-scale production.

Claims

16 09 241. A method for preparing a nickel-based metal-organic framework (Ni-MOF) / bismuth oxychloride (BiOCl) composite for degrading tetracycline (TC), comprising the steps ofstep 1: preparing a BiOCl nanosheet, comprisingadding 30 mL of distilled water into a beaker, and adding 2 mmol of bismuth nitrate pentahydrate and 2 mmol of potassium chloride thereto, to obtain a mixed solution;stirring the mixed solution for 1.5 h, pouring into a 50 mL polytetrafluoroethylene-lined stainless steel autoclave, and performing a reaction at 160 °C for 24 h, to obtain a reaction system; andcooling the reaction system to room temperature, and centrifuging to obtain a first precipitate, washing the first precipitate with deionized water three times, and drying at 60 °C for 12 h;step 2: preparing an Ni-MOF nanosheet, comprisingpreparing a solution comprising 30 mL of N,N-dimethylformamide, 2 mL of ethanol, and 2 mL of deionized water, adding 125 mg of terephthalic acid thereto while stirring, adding 180 mg of NiCh 6H2O and 1 mL of triethylamine thereto in sequence, and stirring a resulting mixture for another 10 min to obtain a colloidal suspension; andsealing the colloidal suspension, and sonicating at room temperature for continuous 8 h to obtain a treated product; washing the treated product with ethanol by centrifugation three times, and drying at 60 °C for 12 h,and step 3: preparing an Ni-MOF / BiOCl heterojunction, i,e., the Ni-MOF / BiOCl composite, comprisingadding the Ni-MOF nanosheet and the BiOCl nanosheet into 20 mL of isopropyl alcohol to obtain a mixture, stirring the mixture for 12 h and centrifugating, discarding a supernatant obtained from centrifugating, to obtain a second precipitate; anddrying the second precipitate at 60 °C for 12 h, to obtain the Ni-MOF / BiOCl composite.

2. The method as claimed in claim 1, wherein a mass percentage of the Ni-MOF nanosheet in the Ni-MOF / BiOCl composite is 10%, 30%, or 50%.

3. A method for degrading tetracycline, comprisingusing the nickel-based metal-organic framework / bismuth oxychloride composite prepared by the method as claimed in claim 1 as a photocatalyst while exposing to ultraviolet.

4. The method as claimed in claim 3, wherein a degradation rate of tetracycline reaches 40.55% within 180 min.16 09 24

Images