CdS QDs / Bi2WO6 / g-C3N4 double-heterojunction photocatalyst as well as preparation method and application thereof

Abstract

The invention belongs to the technical field of sewage treatment, and particularly relates to a CdS QDs / Bi2WO6 / g-C3N4 double-heterojunction photocatalyst as well as a preparation method and application thereof. The preparation method comprises the following steps: adding a Bi2WO6 / g-C3N4 catalyst and cadmium salt into a solvent for ultrasonic dispersion and stirring, and then adding an alcoholic solution containing a sulfur source for a water bath reaction; and centrifugally collecting the product, washing and drying to obtain the product. The CdS QDs / Bi2WO6 / g-C3N4 double-heterojunction photocatalyst is synthesized by adopting a solvothermal reaction and a chemical in-situ growth technology under a mild condition. The method is easy and convenient to operate, mild in reaction condition and low in cost, the obtained photocatalyst is high in antibiotic visible light degradation activity, good in stability and high in speed and has a good degradation effect on levofloxacin, norfloxacin, oxytetracycline hydrochloride and the like, and the degradation rate is 80% or above.

Description

Technical Field

[0001] The present invention belongs to the technical field of sewage treatment, and specifically relates to a CdS QDs / Bi2WO6 / g-C3N4 double heterojunction photocatalyst and a preparation method and application thereof. Background Art

[0003] Antibiotic wastewater is known for its high organic matter concentration and large fluctuations in water quality and quantity, and is considered one of the most difficult industrial wastewaters to treat. Currently, commonly used treatment methods include biological methods, electrochemical methods, ionizing irradiation (CN201711329143.7), adsorption methods (CN201911182741.5), etc. It is worth noting that simple physical treatment can often only be used as a pretreatment method (CN201710259259.1), aiming to improve the biodegradability of wastewater, but it is difficult to directly make the water quality meet the standards. In addition, photocatalytic degradation has attracted attention due to its relatively low cost. For example, Liu Zhifeng et al. used silver phosphate / bismuth sulfide / bismuth oxide double Z-type photocatalysts to achieve good results in treating antibiotics (CN201811392023.6), but the treatment cycle is relatively long. Although the biological method has a low cost, its long treatment cycle limits its practical application; the adsorption or filtration method can only temporarily collect antibiotics but cannot achieve degradation; the electrochemical method is not effective in treating low-concentration antibiotic wastewater and has high energy consumption. The photocatalytic method uses sunlight to excite semiconductor catalysts to generate photogenerated carriers, which then react with water or dissolved oxygen in water to generate active substances such as hydroxyl radicals and superoxide radicals. These active substances can effectively degrade antibiotic molecules. Relatively speaking, the photocatalytic method is an efficient, low-energy consumption and more thorough method for treating antibiotic wastewater.

[0004] Based on the above background, the present invention has successfully developed a photocatalyst with low cost, low photogenerated carrier recombination rate, excellent photocatalytic performance, strong stability and wide degradation range by constructing a double heterostructure, which is expected to be widely used in the field of industrial antibiotic wastewater treatment. Summary of the invention

[0005] In order to overcome the shortcomings of the prior art, the primary purpose of the present invention is to provide a method for preparing a CdS QDs / Bi2WO6 / g-C3N4 double heterojunction photocatalyst. The preparation process is simple and the reaction conditions are mild. The obtained photocatalyst has a good degradation effect on antibiotics, providing a new solution for the treatment of industrial antibiotic wastewater and has broad application prospects.

[0006] The second purpose of the present invention is to provide a CdS QDs / Bi2WO6 / g-C3N4 double heterojunction photocatalyst, which has a low photogenerated carrier recombination rate, high photocatalytic performance, and good stability, and has a good degradation effect on levofloxacin, norfloxacin, and oxytetracycline hydrochloride.

[0007] The third object of the present invention is to provide an application of the above-mentioned CdS QDs / Bi2WO6 / g-C3N4 double heterojunction photocatalyst.

[0008] In order to achieve the above first purpose, the technical solution adopted by the present invention is:

[0009] A preparation method of a CdS QDs / Bi2WO6 / g-C3N4 double heterojunction photocatalyst comprises the following steps: adding a Bi2WO6 / g-C3N4 catalyst and a cadmium salt into a solvent for ultrasonic dispersion and stirring, then adding an alcohol solution containing a sulfur source for water bath reaction; and collecting the product by centrifugation, washing and drying.

[0010] The present invention synthesizes a CdSQDs / Bi2WO6 / g-C3N4 double heterojunction photocatalyst by adopting solvent thermal reaction and chemical in-situ growth technology under mild conditions. The method is simple to operate, the reaction conditions are mild, and the obtained catalyst has broad application prospects in the field of photocatalysis.

[0011] As a preferred technical solution of the present invention, the dosage ratio of the Bi2WO6 / g-C3N4 catalyst to the cadmium salt is 1g:(3-14)mmol.

[0012] By precisely controlling the dosage ratio of Bi2WO6 / g-C3N4 and cadmium salt, the distribution and loading amount of CdS QDs on the Bi2WO6 / g-C3N4 surface can be optimized, thereby forming a more uniform and dense double heterojunction structure, which is beneficial to the effective separation and transmission of photogenerated carriers and improves the photocatalytic efficiency.

[0013] As a preferred technical solution of the present invention, the added amounts of the cadmium salt and the sulfur source are such that the molar ratio of Cd atoms to S atoms is 1:(1.1-1.2).

[0014] By strictly regulating the molar ratio of Cd atoms and S atoms, the generation amount and size distribution of CdS QDs can be precisely controlled, which helps to form uniform and monodisperse CdS QDs, thereby improving their coverage and loading efficiency on the Bi2WO6 / g-C3N4 surface, and further optimizing the structure of the double heterojunction to ensure a close interface contact between CdS and Bi2WO6 / g-C3N4.

[0015] As a preferred technical solution of the present invention, the mass ratio of CdS QDs to Bi2WO6 / g-C3N4 in the CdS QDs / Bi2WO6 / g-C3N4 double heterojunction photocatalyst is (0.5-2):1.

[0016] By controlling the dosage of Bi2WO6 / g-C3N4, cadmium salt and sulfur source in the above technical scheme, the mass ratio of CdS QDs to Bi2WO6 / g-C3N4 in the CdSQDs / Bi2WO6 / g-C3N4 double heterojunction photocatalyst prepared by the present invention is (0.5-2):1, and more preferably 0.5:1, 0.625:1, 0.75:1, 0.875:1, 1:1 and 2:1.

[0017] As a preferred technical solution of the present invention, the cadmium salt is any one of (CH3COO)2Cd·2H2O, CdCl2, and Cd(NO3)2; and the sulfur source is any one of thioacetamide, thiourea, and sodium sulfide.

[0018] As a preferred technical solution of the present invention, the ultrasonic dispersion time is 10-15 minutes; the stirring temperature is 60-90°C and the time is 8-15 minutes.

[0019] The ultrasonic dispersion time is controlled within 10-15min, which can ensure that the cadmium salt and Bi2WO6 / g-C3N4 are evenly dispersed in the solvent, which is conducive to the subsequent uniform loading of CdS QDs on the surface of Bi2WO6 / g-C3N4 and avoids the occurrence of agglomeration, thereby improving the performance of the double heterojunction photocatalyst. Controlling the stirring temperature and time can ensure that Cd 2+ Fully contact with Bi2WO6 / g-C3N4 to prepare for the subsequent sulfurization reaction. Moreover, appropriate ultrasonic dispersion and stirring conditions can accelerate the reaction process and shorten the preparation cycle.

[0020] As a preferred technical solution of the present invention, the temperature of the heating reaction is 60-90°C and the time is 1-2h.

[0021] By controlling the temperature and reaction time, not only can the complete sulfurization reaction and the optimized growth of CdS quantum dots be promoted, but also the interfacial bonding force between CdS quantum dots and Bi2WO6 / g-C3N4 can be enhanced, thereby forming a more compact and uniform double heterojunction structure and improving the degradation efficiency and activity of the photocatalyst.

[0022] As a preferred technical solution of the present invention, the mass ratio of g-C3N4 to Bi2WO6 in the g-C3N4 / Bi2WO6 catalyst is (0.05-2):1; the preparation process of the g-C3N4 / Bi2WO6 catalyst includes: adding tungstate and g-C3N4 to a hexadecyltrimethylammonium bromide solution, adding bismuth salt after ultrasonic dispersion for hydrothermal reaction, and obtaining the catalyst after centrifugation, washing and drying.

[0023] As a preferred technical solution of the present invention, the tungstate is sodium tungstate, potassium tungstate or ammonium tungstate; the bismuth salt is any one of bismuth nitrate, bismuth oxynitrate, bismuth chloride, bismuth oxychloride, bismuth acetate and bismuth formate; the temperature of the hydrothermal reaction is 100-160°C and the time is 12-36h.

[0024] In order to achieve the above second purpose, the technical solution adopted by the present invention is:

[0025] A CdS QDs / Bi2WO6 / g-C3N4 double heterojunction photocatalyst is prepared using the preparation method described in the first objective of the present invention.

[0026] In order to achieve the third purpose above, the technical solution adopted by the present invention is:

[0027] An application of a CdS QDs / Bi2WO6 / g-C3N4 double heterojunction photocatalyst for photocatalytic degradation of antibiotic pollutants in water.

[0028] As a preferred technical solution of the present invention, the antibiotic pollutant can be tetracycline, levofloxacin, norfloxacin or oxytetracycline hydrochloride.

[0029] As a preferred technical solution of the present invention, the concentration of the antibiotic pollutants in water is 10-80 mg / L, and the addition ratio of the CdS QDs / Bi2WO6 / g-C3N4 double heterojunction photocatalyst in the antibiotic wastewater is (0.5-1) g / L.

[0030] As a preferred technical solution of the present invention, the light intensity of the photocatalytic degradation is 300-500mW / cm 2 , temperature is 25-35℃, time is 10-60min.

[0031] Through the above technical scheme, the CdS QDs / Bi2WO6 / g-C3N4 double heterojunction photocatalyst of the present invention is mixed with antibiotic wastewater, stirred under dark conditions, and a photocatalytic reaction is carried out under light conditions after reaching adsorption equilibrium, thereby completing the degradation treatment of antibiotic pollutants.

[0032] The present invention has the following effects compared with the prior art:

[0033] 1. The present invention synthesizes a CdS QDs / Bi2WO6 / g-C3N4 double heterojunction photocatalyst by using solvent thermal reaction and chemical in-situ growth technology under mild conditions. Specifically, the Bi2WO6 / g-C3N4 catalyst and an appropriate amount of cadmium salt are added to a solvent and dispersed evenly, and then an alcohol solution containing a sulfur source is added to generate CdSQDs in situ under mild conditions, and form a tight double heterojunction structure with the Bi2WO6 / g-C3N4 catalyst. The method is simple to operate, has mild reaction conditions, and is low in cost. The obtained photocatalyst has a good degradation effect on antibiotics, provides a new solution for the treatment of industrial antibiotic wastewater, and has broad application prospects.

[0034] 2. The CdS QDs / Bi2WO6 / g-C3N4 double heterojunction photocatalyst synthesized by the present invention has high activity, good stability and fast speed in the degradation of antibiotics under visible light, and has a good degradation effect on levofloxacin, norfloxacin, oxytetracycline hydrochloride and tetracycline, with a degradation rate of more than 80%.

[0035] 3. The CdS QDs / Bi2WO6 / g-C3N4 double heterojunction photocatalyst synthesized in the present invention has good universality for antibiotic degradation, and provides a new idea and method for the research and development of new high-efficiency visible light photocatalysts. It has very important practical application prospects in the field of photocatalytic treatment of antibiotic wastewater. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 TEM image of the CdS QDs / Bi2WO6 / g-C3N4 double heterojunction photocatalyst of the present invention;

[0037] Figure 2 The free radical capture activity of the CdS QDs / Bi2WO6 / g-C3N4 double heterojunction photocatalyst for tetracycline degradation of the present invention;

[0038] Figure 3 Mott-schottky curves of Bi2WO6, CdS QDs and g-C3N4, where (a), (b), and (c) correspond to Bi2WO6, CdS QDs, and g-C3N4, respectively;

[0039] Figure 4 The photocatalytic mechanism diagram of the CdS QDs / Bi2WO6 / g-C3N4 double heterojunction photocatalyst of the present invention;

[0040] Figure 5 The degradation activity of tetracycline by the CdS QDs / Bi2WO6 / g-C3N4 double heterojunction photocatalyst obtained in Example 1 of the present invention and the comparative catalysts obtained in Comparative Examples 1-3 is compared;

[0041] Figure 6 The photodegradation activity of the CdS QDs / Bi2WO6 / g-C3N4 double heterojunction photocatalyst obtained in Example 1 of the present invention towards various antibiotics;

[0042] Figure 7 The absorbance variation of each antibiotic over time when the CdS QDs / Bi2WO6 / g-C3N4 double heterojunction photocatalyst of the present invention degrades various antibiotics, where (a), (b), (c), and (d) correspond to oxytetracycline hydrochloride, norfloxacin, levofloxacin, and tetracycline, respectively. DETAILED DESCRIPTION

[0043] The technical scheme of the present invention is further described below in conjunction with specific embodiments. However, it should be understood by those skilled in the art that the following examples are only used to illustrate the present invention and should not be regarded as limiting the present invention. The specific conditions not specified in the examples are carried out according to conventional conditions or conditions recommended by the manufacturer. The reagents or instruments used, unless otherwise specified, are conventional products obtained through commercial channels.

[0044] Example 1

[0045] A method for preparing a CdS QDs / Bi2WO6 / g-C3N4 double heterojunction photocatalyst comprises the following steps:

[0046] (1) Melamine and NH4Cl were mixed in a mass ratio of 1:1, ground thoroughly, placed in a 50 mL crucible and placed in a muffle furnace, heated from room temperature to 550°C and kept warm for 4 h (heating rate of 5°C / min). After cooling to room temperature and grinding, the crucible containing the powder was placed in a muffle furnace, heated to 550°C again (heating rate of 5°C / min) and kept warm for 2 h. After the powder was cooled to room temperature, it was washed alternately with distilled water and anhydrous ethanol several times, placed in an oven and dried at 60°C to obtain g-C3N4.

[0047] (2) 25 mg of hexadecyltrimethylammonium bromide (CTAB) and 40 mL of deionized water were mixed into a uniform solution. After the solution became transparent and clear, 0.1649 g of Na2WO4·2H2O was added and stirred until dissolved. Then 0.25 g of g-C3N4 obtained in step (1) was added and ultrasonically dispersed for 30 min. Then 0.4851 g of Bi(NO3)3·5H2O was added and stirred for 2 h. The mixture was transferred to a polytetrafluoroethylene reactor and placed in an electric heated blast drying oven for hydrothermal treatment at 120°C for 24 h. After cooling to room temperature, the product was washed and dried to obtain a Bi2WO6 / g-C3N4 catalyst.

[0048] (3) Take 0.2 g of Bi2WO6 / g-C3N4 catalyst and add it to 35 mL of ethanol, then add 0.1845 g of Cd(CH3COO)2·2H2O and ultrasonically disperse it for 10 min, then stir it in a 70°C water bath for 10 min, and then add a uniform solution of 0.0515 g of thioacetamide (TAA) and 15 mL of ethanol under vigorous stirring, continue to react in a 80°C water bath for 1 h, cool to room temperature, centrifuge, wash and dry at 60°C to obtain CdS QDs / Bi2WO6 / g-C3N4 double heterojunction photocatalyst.

[0049] This embodiment also provides a CdS QDs / Bi2WO6 / g-C3N4 double heterojunction photocatalyst, which is prepared by the above method.

[0050] Example 2

[0051] A method for preparing a CdS QDs / Bi2WO6 / g-C3N4 double heterojunction photocatalyst comprises the following steps:

[0052] (1) Mix urea and NH4Cl in a mass ratio of 3:1, grind thoroughly, place in a 50 mL crucible and place in a muffle furnace, heat from room temperature to 520°C and keep warm for 3 h (heating rate is 3°C / min). After cooling to room temperature and grinding, place the crucible containing the powder in a muffle furnace, heat to 520°C again (heating rate is 3°C / min) and keep warm for 2 h. After the powder is cooled to room temperature, wash it alternately with distilled water and anhydrous ethanol several times, place it in an oven and dry it at 60°C to obtain g-C3N4.

[0053] (2) 30 mg of hexadecyltrimethylammonium bromide (CTAB) and 50 mL of deionized water were prepared into a uniform solution. After the solution was transparent and clear, 0.1810 g of K2WO4·2H2O was added and stirred until dissolved. Then 0.25 g of g-C3N4 obtained in step (1) was added and ultrasonically dispersed for 30 min. Then 50 mL of BiCl3 ethanol solution (wherein the mass of BiCl3 was 0.3153 g) was added and vigorously stirred for 2 h. The mixture was transferred to a polytetrafluoroethylene reactor and placed in an electric heated blast drying oven for hydrothermal treatment at 120°C for 24 h. After cooling to room temperature, the product was washed and dried to obtain a Bi2WO6 / g-C3N4 catalyst.

[0054] (3) Take 0.2 g of Bi2WO6 / g-C3N4 catalyst and add it to 35 mL of ethanol, then add 0.1845 g of Cd(CH3COO)2·2H2O and ultrasonically disperse it for 15 min, then stir it in a 60°C water bath for 15 min, and then add a uniform solution of 0.0515 g of thioacetamide (TAA) and 15 mL of ethanol under vigorous stirring, continue to react in a 60°C water bath for 2 h, cool to room temperature, centrifuge, wash and dry at 60°C to obtain CdS QDs / Bi2WO6 / g-C3N4 double heterojunction photocatalyst.

[0055] This embodiment also provides a CdS QDs / Bi2WO6 / g-C3N4 double heterojunction photocatalyst, which is prepared by the above method.

[0056] Example 3

[0057] A method for preparing a CdS QDs / Bi2WO6 / g-C3N4 double heterojunction photocatalyst comprises the following steps:

[0058] (1) Melamine and NH4Cl were mixed in a mass ratio of 1:1, ground thoroughly, placed in a 50 mL crucible and placed in a muffle furnace, heated from room temperature to 550°C and kept warm for 4 h (heating rate of 5°C / min). After cooling to room temperature and grinding, the crucible containing the powder was placed in a muffle furnace, heated to 550°C again (heating rate of 5°C / min) and kept warm for 2 h. After the powder was cooled to room temperature, it was washed alternately with distilled water and anhydrous ethanol several times, placed in an oven and dried at 60°C to obtain g-C3N4.

[0059] (2) 35 mg of hexadecyltrimethylammonium bromide (CTAB) and 40 mL of deionized water were prepared into a uniform solution. After the solution became transparent and clear, 0.1649 g of Na2WO4·2H2O was added and stirred until dissolved. Then 0.5 g of g-C3N4 obtained in step (1) was added and ultrasonically dispersed for 30 min. Then 0.4851 g of Bi(NO3)3·5H2O was added and stirred for 2 h. The mixture was transferred to a polytetrafluoroethylene reactor and placed in an electric heated blast drying oven for hydrothermal treatment at 120°C for 24 h. After cooling to room temperature, the product was washed and dried to obtain Bi2WO6 / g-C3N4.

[0060] (3) Take 0.2 g Bi2WO6 / g-C3N4 and add it to 35 mL ethanol, then add 0.2768 g Cd(CH3COO)2·2H2O and ultrasonically disperse it for 15 min, then stir it in a 90 °C water bath for 8 min, and then add a uniform solution prepared by 0.0773 g thioacetamide (TAA) and 15 mL ethanol under vigorous stirring, continue to react in a 90 °C water bath for 1 h, cool to room temperature, centrifuge, wash and dry at 60 °C to obtain CdS QDs / Bi2WO6 / g-C3N4 double heterojunction photocatalyst.

[0061] This embodiment also provides a CdS QDs / Bi2WO6 / g-C3N4 double heterojunction photocatalyst, which is prepared by the above method.

[0062] Example 4

[0063] A method for preparing a CdS QDs / Bi2WO6 / g-C3N4 double heterojunction photocatalyst comprises the following steps:

[0064] (1) Mix urea and NH4Cl in a mass ratio of 3:1, grind thoroughly, place in a 50 mL crucible and place in a muffle furnace, heat from room temperature to 520°C and keep warm for 3 h (heating rate is 3°C / min). After cooling to room temperature and grinding, place the crucible containing the powder in a muffle furnace, heat to 520°C again (heating rate is 3°C / min) and keep warm for 2 h. After the powder is cooled to room temperature, wash it alternately with distilled water and anhydrous ethanol several times, place it in an oven and dry it at 60°C to obtain g-C3N4.

[0065] (2) 30 mg of hexadecyltrimethylammonium bromide (CTAB) and 50 mL of deionized water were prepared into a uniform solution. After the solution was transparent and clear, 0.1810 g of K2WO4·2H2O was added and stirred until dissolved. Then 0.5 g of g-C3N4 obtained in step (1) was added and ultrasonically dispersed for 30 min. Then 50 mL of BiCl3 ethanol solution (wherein the mass of BiCl3 was 0.3153 g) was added and vigorously stirred for 2 h. The mixture was transferred to a polytetrafluoroethylene reactor and placed in an electric heated blast drying oven for hydrothermal treatment at 120°C for 24 h. After cooling to room temperature, the product was washed and dried to obtain Bi2WO6 / g-C3N4.

[0066] (3) Take 0.2 g Bi2WO6 / g-C3N4 and add it to 35 mL ethanol, then add 0.738 g Cd(CH3COO)2·2H2O and ultrasonically disperse it for 10 min, then stir it in a water bath at 80 °C for 10 min, and then add a uniform solution prepared by 0.206 g thioacetamide (TAA) and 20 mL ethanol under vigorous stirring, continue to react in a water bath at 70 °C for 2 h, cool to room temperature, centrifuge, wash and dry at 60 °C to obtain CdS QDs / Bi2WO6 / g-C3N4 double heterojunction photocatalyst.

[0067] This embodiment also provides a CdS QDs / Bi2WO6 / g-C3N4 double heterojunction photocatalyst, which is prepared by the above method.

[0068] Comparative Example 1

[0069] The difference between Comparative Example 1 and Example 1 is that step (1) is omitted and g-C3N4 is not added in step (2), that is, pure Bi2WO6 is prepared by the same method as step (2) of Example 1; then the Bi2WO6 / g-C3N4 in step (3) of Example 1 is replaced by pure Bi2WO6 to obtain a CdS QDs / Bi2WO6 catalyst.

[0070] Comparative Example 2

[0071] The difference between Comparative Example 2 and Example 1 is that step (2) is omitted, and Bi2WO6 / g-C3N4 in step (3) of Example 1 is replaced by g-C3N4 to obtain CdS QDs / g-C3N4 catalyst.

[0072] Comparative Example 3

[0073] The difference between Comparative Example 3 and Example 1 is that step (3) is omitted, and a Bi2WO6 / g-C3N4 catalyst is obtained.

[0074] Comparative Example 4

[0075] The difference between Comparative Example 4 and Example 1 is that steps (1) and (2) are omitted, and Bi2WO6 / g-C3N4 is not added to ethanol in step (3), that is, pure CdS QDs are prepared by the method of step (3) of Example 1.

[0076] Experimental Example 1 Structural Characterization

[0077] 1. The specific surface area, pore size and pore volume parameters of g-C3N4 prepared in step (1) of Example 1, pure Bi2WO6 prepared in Comparative Example 1, pure CdS QDs prepared in Comparative Example 4, Bi2WO6 / g-C3N4 prepared in Comparative Example 3 and CdS QDs / Bi2WO6 / g-C3N4 prepared in Example 1 are shown in Table 1.

[0078] Table 1 Specific surface area, pore size and pore volume parameters of various catalysts

[0079] Catalysts <![CDATA[Specific surface area (m 2 g -1 )]]> Pore ​​size (nm) <![CDATA[Pore volume (cm 3 g -1 )]]> <![CDATA[g-C3N4]]> 82.57 34.65 0.753 <![CDATA[Bi2WO6]]> 41.67 27.48 0.220 CdS QDs 91.23 32.16 0.652 <![CDATA[Bi2WO6 / g-C3N4]]> 60.40 27.23 0.358 <![CDATA[CdS QDs / Bi2WO6 / g-C3N4]]> 75.14 29.46 0.371

[0080] 2. TEM image of CdS QDs / Bi2WO6 / g-C3N4 double heterojunction photocatalyst obtained in Example 1 Figure 1 shown.

[0081] Test Example 2

[0082] In order to explore the free radical capture activity of the CdS QDs / Bi2WO6 / g-C3N4 double heterojunction photocatalyst for tetracycline degradation obtained in Example 1 of the present invention, ascorbic acid was used to capture O2 - , ammonium oxalate was used to capture h + , using isopropanol to capture ·OH. The specific test method is: add the CdS QDs / Bi2WO6 / g-C3N4 double heterojunction photocatalyst obtained in Example 1 to 100mL of water containing 50mg / L tetracycline, and add different free radical scavengers, and stir for 30min in the dark state to achieve adsorption and desorption equilibrium. The photocatalytic degradation experiment was carried out under the irradiation of a 300W xenon lamp equipped with a 400nm cut-off filter, and the light intensity was 345mW / cm 2 Take 3 mL of the degraded solution at a specific time interval, centrifuge it thoroughly, take the supernatant, filter it with a 0.22 μm filter head, and use a UV-visible spectrophotometer to measure the absorbance of the solution at the characteristic wavelength of the pollutant. Record the change in the absorbance of the pollutant solution with the illumination time. The test results are shown in Figure 2 .

[0083] from Figure 2 It is not difficult to see from the free radical capture experiment results that the degradation rate of tetracycline decreased slightly after adding isopropanol and ammonium oxalate, while the degradation rate of tetracycline dropped sharply to 1.55% after adding ascorbic acid. - Play the most critical role, h + It also participates in the degradation of tetracycline to a certain extent. The order of contribution of the three to the photodegradation reaction can be expressed as follows: O2 - >h + >·OH.

[0084] Test Example 3

[0085] In order to further explore the photocatalytic mechanism of the CdS QDs / Bi2WO6 / g-C3N4 double heterojunction photocatalyst prepared in the present invention, electrochemical tests were carried out on its constituent components - Bi2WO6, CdS QDs and g-C3N4. The positions of the conduction band and valence band of the three semiconductors were determined according to the Mott-schottky curve and valence band spectrum test results. The results are shown in Figure 3 and Figure 4 .

[0086] Figure 3 Mott-schottky curves of Bi2WO6, CdS QDs and g-C3N4, Figure 4 The photocatalytic mechanism diagram of CdS QDs / Bi2WO6 / g-C3N4 double heterojunction photocatalyst. Since the conduction band potential of g-C3N4 and CdS QDs is higher than that of O2 / ·O2 - The potential of the catalyst is more negative, and the electrons in their respective conduction bands can reduce the O2 adsorbed on the catalyst surface to ·O2 with strong oxidizing ability. - , and then decompose the antibiotics into non-toxic small molecules. Similarly, since the valence band potential of Bi2WO6 is only +2.36eV, which is lower than the potential position of H2O / ·OH (+2.4eV), the holes on its valence band cannot produce ·OH and can only migrate to the surface of Bi2WO6 to oxidize organic pollutants into small molecules. After the catalyst is excited by light, the photogenerated holes of g-C3N4 and CdS QDs respectively recombine with the photogenerated electrons of Bi2WO6 at the contact interface under the action of its own built-in electric field, thereby extending the lifetime of the photogenerated electrons of g-C3N4 and CdS QDs and producing more ·O2 - , thus being able to more efficiently degrade various antibiotics, showing Figure 4 Double heterojunction catalytic mechanism shown.

[0087] Test Example 4

[0088] In order to explore the effect of the catalysts obtained in Examples 1-4 and Comparative Examples 1-3 of the present invention on removing antibiotic pollutants in antibiotic wastewater, the catalysts obtained in Examples 1-4 and Comparative Examples 1-3 were added to antibiotic wastewater with a tetracycline concentration of 50 mg / L, and the addition ratio of each group of catalysts was 0.5 g / L, and the mixture was stirred at 1000 rpm for uniform mixing. The specific experimental method was the same as that of Experimental Example 2, except that no free radical scavenger was added. The degradation activity results of each group of catalysts on tetracycline are shown in FIG. Figure 5 shown.

[0089] like Figure 5As shown, after 12 minutes of photocatalytic degradation, compared with the degradation rates of tetracycline by CdS QDs / Bi2WO6 catalyst, CdS QDs / g-C3N4 catalyst and Bi2WO6 / g-C3N4 catalyst obtained in Comparative Examples 1-3 (70.6%, 72.4% and 40.2%, respectively), the degradation rate of tetracycline by CdS QDs / Bi2WO6 / g-C3N4 double heterojunction photocatalyst prepared by the present invention is significantly higher, reaching 80.9%. This is because CdS QDs, Bi2WO6 and / g-C3N4 form a Figure 1 The unique double heterostructure shown effectively promotes the separation and migration of photogenerated electron-hole pairs, reduces the recombination probability, and thus significantly improves the photocatalytic activity. This charge separation mechanism not only prolongs the lifetime of photogenerated carriers, but also enhances the redox ability of the catalyst, allowing more tetracycline molecules to be photocatalytically degraded.

[0090] The above results show that the CdS QDs / Bi2WO6 / g-C3N4 double heterojunction photocatalyst prepared by the present invention exhibits excellent performance in tetracycline degradation, with a fast degradation rate and a high degradation rate, and has potential application value in environmental pollution control.

[0091] Test Example 5

[0092] In order to demonstrate the degradation universality of the CdS QDs / Bi2WO6 / g-C3N4 double heterojunction photocatalyst of the present invention and the change of the absorbance of each antibiotic over time when the CdS QDs / Bi2WO6 / g-C3N4 double heterojunction photocatalyst degrades various antibiotics, the CdS QDs / Bi2WO6 / g-C3N4 double heterojunction photocatalyst obtained in Example 1 was added to wastewater containing tetracycline, levofloxacin, norfloxacin and oxytetracycline hydrochloride, respectively, wherein the concentrations of tetracycline, levofloxacin, norfloxacin and oxytetracycline hydrochloride in the wastewater were all 0.5 g / L. The photodegradation activity of various antibiotics was tested by the method of Experimental Example 4, and the results are shown in FIG. Figure 6 The absorbance variation of each antibiotic over time when CdS QDs / Bi2WO6 / g-C3N4 double heterojunction photocatalyst degrades various antibiotics can be found in Figure 7 .

[0093] from Figure 6 It can be seen that the CdS QDs / Bi2WO6 / g-C3N4 double heterojunction photocatalyst prepared in the present invention has excellent degradation activity for tetracycline, levofloxacin, norfloxacin and oxytetracycline hydrochloride within 12 minutes, and the degradation rate is not less than 80%, indicating that the CdS QDs / Bi2WO6 / g-C3N4 double heterojunction catalyst of the present invention has good universality for the degradation of antibiotics.

[0094] Figure 7 The graph shows the change of absorbance of each antibiotic over time when CdS QDs / Bi2WO6 / g-C3N4 double heterojunction photocatalyst degrades various antibiotics, where (a), (b), (c), and (d) correspond to oxytetracycline hydrochloride, norfloxacin, levofloxacin, and tetracycline, respectively. Figure 7 It can be seen that in the dark adsorption-desorption equilibrium process before illumination, the absorption peaks of various antibiotics such as levofloxacin did not shift significantly under the action of the catalyst, indicating that the adsorption process did not change the structure of the antibiotics. However, after 2-3 minutes of light irradiation, the absorbance of various antibiotic solutions showed a sharp decline at wavelengths of about 275nm and 357nm, and as the illumination time increased, the position of the absorption peak shifted significantly, indicating that the CdS QDs / Bi2WO6 / g-C3N4 double heterojunction photocatalyst of the present invention effectively degraded various antibiotics.

[0095] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit them. The basic principles and main features of the present invention have been described above with specific implementation schemes. On the basis of the present invention, some modifications or replacements may be made thereto, but these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of protection claimed by the present invention.

Claims

1. A method for preparing a CdS QDs / Bi2WO6 / g-C3N4 double heterojunction photocatalyst, characterized in that: The following steps are involved: The Bi2WO6 / g-C3N4 catalyst and cadmium salt are added to a solvent for ultrasonic dispersion and stirring, and then an alcohol solution containing a sulfur source is added to carry out a water bath reaction; the product is collected by centrifugation, washed and dried to obtain the product.

2. The method for preparing the CdS QDs / Bi2WO6 / g-C3N4 double heterojunction photocatalyst according to claim 1, characterized in that: The dosage ratio of the Bi2WO6 / g-C3N4 catalyst to the cadmium salt is 1g:(3-14)mmol.

3. The method for preparing the CdS QDs / Bi2WO6 / g-C3N4 double heterojunction photocatalyst according to claim 1, characterized in that: The added amounts of the cadmium salt and the sulfur source are such that the molar ratio of Cd atoms to S atoms is 1:(1.1-1.2).

4. The method for preparing the CdS QDs / Bi2WO6 / g-C3N4 double heterojunction photocatalyst according to claim 1, characterized in that: The mass ratio of CdS QDs to Bi2WO6 / g-C3N4 in the CdS QDs / Bi2WO6 / g-C3N4 double heterojunction photocatalyst is (0.5-2):

1.

5. The method for preparing the CdS QDs / Bi2WO6 / g-C3N4 double heterojunction photocatalyst according to claim 1, characterized in that: The cadmium salt is any one of (CH3COO)2Cd·2H2O, CdCl2, and Cd(NO3)2; the sulfur source is any one of thioacetamide, thiourea, and sodium sulfide.

6. The method for preparing the CdS QDs / Bi2WO6 / g-C3N4 double heterojunction photocatalyst according to claim 1, characterized in that: The ultrasonic dispersion time is 10-15 min; the stirring temperature is 60-90° C. and the stirring time is 8-15 min.

7. The method for preparing the CdS QDs / Bi2WO6 / g-C3N4 double heterojunction photocatalyst according to claim 1, characterized in that: The temperature of the water bath reaction is 60-90° C. and the reaction time is 1-2 h.

8. The method for preparing the CdS QDs / Bi2WO6 / g-C3N4 double heterojunction photocatalyst according to claim 1, characterized in that: The mass ratio of g-C3N4 to Bi2WO6 in the Bi2WO6 / g-C3N4 catalyst is (0.05-2):1; the preparation process of the Bi2WO6 / g-C3N4 catalyst includes: adding tungstate and g-C3N4 to a hexadecyltrimethylammonium bromide solution, adding bismuth salt for hydrothermal reaction after ultrasonic dispersion, and obtaining the catalyst after centrifugation, washing and drying.

9. A CdS QDs / Bi2WO6 / g-C3N4 double heterojunction photocatalyst, characterized in that: The method is prepared by any one of claims 1 to 8.

10. The use of the CdS QDs / Bi2WO6 / g-C3N4 double heterojunction photocatalyst according to claim 9, characterized in that: Used for photocatalytic degradation of antibiotic pollutants in water.

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