Preparation Method and Application of a Zn QDs / ZnWO4 Photocatalytic Material

Abstract

The present invention relates to the technical field of photocatalytic materials, and provides a preparation method and application of a Zn QDs / ZnWO4 photocatalytic material. The method includes: (1) mixing zinc acetate and sodium tungstate in the presence of a first solvent, and performing a hydrothermal reaction to obtain a zinc tungstate photocatalytic material; (2) mixing zinc acetate and the zinc tungstate photocatalytic material in the presence of a second solvent, reacting under mercury lamp illumination, and then performing vacuum drying to obtain the Zn QDs / ZnWO4 photocatalytic material. The photocatalytic material of the present invention exhibits excellent photocatalytic reduction activity towards Cr(VI), and the purification efficiency of the wastewater containing Cr(VI) is as high as 99% after 40 minutes of illumination. Moreover, after undergoing ten cyclic experiments, the photocatalytic reduction efficiency of the Zn QDs / ZnWO4 photocatalytic material for the wastewater containing Cr(VI) can still remain above 96%.

Description

Technical Field

[0001] The present invention relates to the technical field of photocatalytic materials, and more specifically, to a preparation method and application of a Zn QDs / ZnWO4 photocatalytic material. Background Art

[0002] Hexavalent chromium Cr(VI) is a harmful water pollutant, which is commonly found in industrial wastewaters such as electroplating, leather, and mining. The harm of Cr(VI) to the ecological environment is extremely serious. First of all, Cr(VI) will damage the soil nutrient structure and endanger the growth of soil vegetation. In addition, Cr(VI) has good solubility and mobility, and it will pollute surface water bodies with the flow of rainwater, thereby endangering the growth of aquatic organisms. Moreover, Cr(VI) will accumulate in the human body through the food chain, thereby endangering our physical health. In particular, Cr(VI) has been listed as a first-class carcinogen by the International Agency for Research on Cancer. If the wastewater containing Cr(VI) is not effectively treated, it will pose a huge threat to the sustainable stability of the entire ecosystem and human health.

[0003] Currently, common methods for treating wastewater containing Cr(VI) include chemical reduction method, precipitation method, adsorption method, membrane separation method, electrolysis method, etc. These traditional treatment methods have disadvantages such as easy generation of secondary pollution, high treatment cost, and high energy consumption. The photocatalytic method can use sunlight to induce the generation of reduction active species such as photogenerated electrons, and reduce harmful Cr(VI) to easily sediment and low-toxic Cr(III), which has the advantages of high efficiency, greenness, energy saving, environmental protection, etc., and is a highly promising method for treating wastewater containing Cr(VI). Chinese Patent with application number 202110247791.8 discloses a preparation of a Sn-α-Fe2O3 / g-C3N4 composite catalyst and its method for photocatalytic reduction of chromium(VI)-containing wastewater. This method uses citric acid as a surfactant to synthesize the Sn-α-Fe2O3 precursor 1, uses melamine and dicyandiamide as raw materials to obtain the g-C3N4 precursor 2 through annealing calcination, and then constructs a heterojunction of the above two precursors by a solvothermal method to prepare the Sn-α-Fe2O3 / g-C3N4 composite catalyst. The Sn-α-Fe2O3 / g-C3N4 composite catalyst has excellent catalytic reduction efficiency for Cr(VI). In addition, Chinese Patent with application number 201710884928.4 discloses a preparation of an Ag@AgHPMo / Ag3VO4 polyoxometalate-based composite visible-light catalyst and its method for treating wastewater containing Cr(VI). This method uses silver nitrate and phosphomolybdic acid as raw materials to prepare the target catalytic material through a hydrothermal-ultraviolet light reduction strategy. The prepared polyoxometalate-based composite material has high photocatalytic reduction efficiency for wastewater containing Cr(VI) under visible light conditions and good stability for recycling.

[0004] The photocatalytic materials disclosed in the above patent documents show good removal activity for treating Cr(VI)-containing wastewater, but there are obvious deficiencies such as complex catalyst preparation processes, low production economic feasibility, and difficulties in large-scale preparation. Summary of the Invention

[0005] In view of this, the purpose of the present invention is to provide a preparation method and application of Zn QDs / ZnWO4 photocatalytic materials. The Zn QDs / ZnWO4 photocatalytic materials provided by the present invention exhibit excellent photocatalytic reduction activity towards Cr(VI), and have excellent photocatalytic activity and cycle stability.

[0006] To achieve the above purpose, the present invention adopts the following technical solutions:

[0007] A preparation method of Zn QDs / ZnWO4 photocatalytic materials, comprising:

[0008] (1) In the presence of a first solvent, zinc acetate and sodium tungstate are mixed and subjected to a hydrothermal reaction to obtain zinc tungstate photocatalytic materials;

[0009] (2) In the presence of a second solvent, zinc acetate and zinc tungstate photocatalytic materials are mixed and reacted under the condition of mercury light irradiation to obtain Zn QDs / ZnWO4 photocatalytic materials.

[0010] Preferably, the mass ratio of zinc acetate to sodium tungstate is 1:(1 - 3), preferably 1:1.

[0011] Preferably, in step (1), the temperature of the hydrothermal reaction is 140 - 180 °C, and the hydrothermal reaction time is 12 - 18 h.

[0012] Preferably, the mixing in step (1) specifically includes:

[0013] Zinc acetate is mixed with the first solvent to obtain component A; sodium tungstate is mixed with the first solvent to obtain component B; component A and component B are mixed.

[0014] Preferably, the mixing of component A and component B includes:

[0015] Component B is added to component A under stirring conditions.

[0016] Preferably, the first solvent includes ethylene glycol and / or water; the second solvent includes ethylene glycol.

[0017] Preferably, the first solvent is obtained by mixing ethylene glycol and water with a volume ratio of 10:3.

[0018] Preferably, the zinc acetate includes zinc acetate dihydrate; the sodium tungstate includes sodium tungstate dihydrate.

[0019] Preferably, the ethylene glycol, sodium tungstate dihydrate and zinc acetate dihydrate are all of analytical grade.

[0020] Preferably, the step (1) includes:

[0021] Mix ethylene glycol with water to prepare an ethylene glycol solution with a volume ratio of ethylene glycol to deionized water of 10:3; disperse zinc acetate dihydrate in the ethylene glycol solution to obtain component A; disperse sodium tungstate dihydrate in the ethylene glycol solution to obtain component B; add component B to component A under stirring conditions.

[0022] Preferably, the conditions for vacuum drying are: vacuum drying at 70 - 80 °C for 12 - 15 h.

[0023] Preferably, the mass ratio of the zinc acetate and zinc tungstate photocatalytic material is 1:(1 - 10), preferably 1:5.

[0024] Preferably, the time for mercury light irradiation is 5 - 30 min, preferably 10 min.

[0025] Preferably, the mercury light comes from a mercury lamp; the power of the mercury lamp is 400 - 500 W.

[0026] Preferably, the mixing in step (2) is carried out under magnetic stirring conditions; the time for magnetic stirring is 60 - 80 min.

[0027] The present invention also provides a Zn QDs / ZnWO4 photocatalytic material prepared by the preparation method of the Zn QDs / ZnWO4 photocatalytic material described above.

[0028] The present invention also provides an application of the preparation method of the Zn QDs / ZnWO4 photocatalytic material or the Zn QDs / ZnWO4 photocatalytic material in photocatalytic chromium removal.

[0029] For the Zn QDs / ZnWO4 photocatalytic material obtained by the preparation method of the present invention, wherein the zinc quantum dots grow in-situ on the surface of the zinc tungstate cubic blocks, which not only enriches the interfacial composition of the composite material and significantly accelerates the migration efficiency of photo-generated carriers; at the same time, the presence of zinc quantum dots can provide more active sites for the interfacial reduction reaction of Cr(VI). Therefore, the Zn QDs / ZnWO4 photocatalytic material exhibits excellent photocatalytic removal activity for Cr(VI)-containing wastewater.

[0030] The present invention has the following positive effects:

[0031] (1) The Zn QDs / ZnWO4 photocatalytic material provided by the present invention uniformly grows zinc quantum dots on the surface of zinc tungstate cubes through a simple hydrothermal-photochemical reduction method, and develops a composite material suitable for metal quantum dots / polyoxometalates.

[0032] (2) The Zn QDs / ZnWO4 photocatalytic material provided by the present invention introduces zinc quantum dots, providing more active sites for the interfacial reduction reaction of Cr(VI); the multi-site composite interface with zinc quantum dots and zinc tungstate cubes as units can significantly accelerate the separation and migration efficiency of photo-generated carriers, thereby improving its photocatalytic removal performance for Cr(VI)-containing wastewater.

[0033] (3) The Zn QDs / ZnWO4 photocatalytic material provided by the present invention exhibits excellent photocatalytic reduction activity for Cr(VI). The purification efficiency of Cr(VI)-containing wastewater is as high as 99% after 40 minutes of illumination. Moreover, after ten cycles of experiments, the photocatalytic reduction efficiency of the Zn QDs / ZnWO4 photocatalytic material for Cr(VI)-containing wastewater can still remain above 96%, indicating that the Zn QDs / ZnWO4 photocatalytic material provided by the present invention has excellent photocatalytic activity and cycle stability. Description of the Drawings

[0034] Figure 1 XRD spectra of ZnWO4 and Zn QDs / ZnWO4-0.2 samples prepared in Comparative Example 1 and Example 1;

[0035] Figure 2 TEM photograph of the Zn QDs / ZnWO4-0.2 sample prepared in Example 1;

[0036] Figure 3 Adsorption-desorption curves of ZnWO4 (a) and Zn QDs / ZnWO4-0.2 (b) samples prepared in Comparative Example 1 and Example 1;

[0037] Figure 4 Photocatalytic performance curves of ZnWO4 and Zn QDs / ZnWO4 samples prepared in Example 1, Comparative Example 1, Comparative Example 2, and Comparative Example 3 for the reduction of Cr(VI);

[0038] Figure 5 Photocatalytic cycle stability histogram of the Zn QDs / ZnWO4-0.2 sample obtained in Example 1 for the reduction of Cr(VI). Detailed Description of the Invention

[0039] The technical solution of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0040] In order to further illustrate the present invention, the following examples are used to explain it in detail. The raw materials used in the following examples and comparative examples of the present invention are all commercially available products.

[0041] Example 1

[0042] The preparation method of the Zn QDs / ZnWO4 photocatalytic material of this embodiment comprises the following steps:

[0043] (1) 50 mL of ethylene glycol and 15 mL of deionized water were fully mixed to prepare an ethylene glycol solution with a volume ratio of ethylene glycol to deionized water of 10:3;

[0044] (2) Dispersing 5 mmol of zinc acetate dihydrate in 60 mL of ethylene glycol solution to obtain component A; dispersing 5 mmol of sodium tungstate dihydrate in 15 mL of ethylene glycol solution to obtain component B; adding component B to component A under stirring to obtain a white suspension;

[0045] (3) transferring the white suspension to a sealed autoclave, conducting a hydrothermal reaction at 180° C. for 14 h, filtering and washing the resulting precipitate, and vacuum drying at 70° C. for 12 h to obtain a zinc tungstate cubic photocatalytic material;

[0046] (4) Using 10 mL of ethylene glycol as solvent, 0.5 g of zinc tungstate cubic photocatalytic material and 0.1 g of zinc acetate dihydrate were dispersed in a 50 mL quartz tube. Magnetic stirring was performed for 60 min to allow zinc tungstate to be fully dispersed in the alcohol solution of zinc acetate dihydrate. The obtained suspension was placed in a photochemical reactor and reduced with 500 W mercury light for 10 min. As the illumination time increased, it was found that the color of the suspension gradually changed to dark gray, indicating that the zinc ions in the alcohol solution were gradually reduced; the precipitate obtained was filtered and washed, and vacuum dried at 70 ° C for 12 h to obtain the Zn QDs / ZnWO4-0.2 sample.

[0047] Comparative Example 1

[0048] The preparation method of the photocatalytic material of this comparative example comprises the following steps:

[0049] (1) 50 mL of ethylene glycol and 15 mL of deionized water were fully mixed to prepare an ethylene glycol solution with a volume ratio of ethylene glycol to deionized water of 10:3;

[0050] (2) Disperse 5 mmol of zinc acetate dihydrate in 60 mL of ethylene glycol solution to obtain component A; disperse 5 mmol of sodium tungstate dihydrate in 15 mL of ethylene glycol solution to obtain component B; add component B to component A under stirring conditions to obtain a white suspension;

[0051] (3) Transfer the above white suspension to a sealed high-pressure reaction kettle, carry out hydrothermal reaction at 180 °C for 14 h, filter and wash the obtained precipitate, and dry it under vacuum at 70 °C for 12 h to obtain zinc tungstate cubic photocatalytic material, denoted as ZnWO4.

[0052] Example 2

[0053] According to the process of Example 1, the difference is that the mass of zinc acetate dihydrate weighed in step (4) is 0.05 g, and other conditions remain unchanged. The obtained sample is denoted as Zn QDs / ZnWO4-0.1.

[0054] Example 3

[0055] According to the process of Example 1, the difference is that the mass of zinc acetate dihydrate weighed in step (4) is 0.15 g, and other conditions remain unchanged. The obtained sample is denoted as Zn QDs / ZnWO4-0.3.

[0056] Test Example

[0057] Carry out relevant tests on the materials obtained in the examples and comparative examples, including: XRD, TEM, N2 isothermal adsorption and desorption test, photocatalytic performance test for the reduction of Cr(VI), and cyclic stability test for the reduction of Cr(VI). The results are as Figures 1-5 shown.

[0058] Test 1 (XRD): The crystal phase of the Zn QDs / ZnWO4 photocatalytic material was analyzed using a UITIMA IV type X-ray powder diffractometer from Rigaku Corporation of Japan.

[0059] Test 2 (TEM): The morphology and microstructure of the Zn QDs / ZnWO4 photocatalytic material were tested using a Philips CM-120 type transmission electron microscope (TEM, acceleration voltage 200 kV) from the Netherlands.

[0060] Test 3 (N2 isothermal adsorption and desorption test): The specific surface area of the Zn QDs / ZnWO4 photocatalytic material was measured using a Tristar II 3020 type physical adsorption instrument from Micromeritics Instrument Corporation (Shanghai) in the United States. Before the test, it was degassed at 150 °C for 3 hours, and then adsorption-desorption was carried out at the temperature of liquid N2. The specific surface area of the sample was calculated by combining with the BET equation.

[0061] Test 4: In the test for evaluating the photocatalytic reduction performance of Zn QDs / ZnWO4 photocatalytic material for wastewater containing Cr(VI), a 400W metal halide lamp was used as the light source. 50 mg of the photocatalytic material and 50 mL of potassium dichromate solution with a concentration of 20 mg / L were placed in a quartz reaction tube. Before illumination, it was stirred and dispersed in the dark environment for 1 hour, and the circulating cooling water was kept unblocked throughout the experimental process. Samples of the potassium dichromate solution were taken at regular intervals, and the change in its absorbance was measured with a spectrophotometer. The change in the concentration of the potassium dichromate solution was analyzed by the change in the absorption value at the optimal absorption wavelength of the potassium dichromate solution.

[0062] Figure 1 XRD patterns of the ZnWO4 and Zn QDs / ZnWO4-0.2 photocatalytic materials prepared in Comparative Example 1 and Example 1. For ZnWO4, characteristic diffraction peaks were detected at diffraction angles 2θ = 15.5°, 18.9°, 23.8°, 24.5°, 30.4°, 30.7°, 36.4°, 38.3°, 41.3°, 48.7°, 50.2°, 51.6°, 53.6° and 64.7°, corresponding to the (010), (100), (011), (110), (-111), (111), (002), (200), (121), (022), (220), (130), (-202) and (-132) crystal planes of monoclinic ZnWO4 (JCPDS No. 01-089-7624). The characteristic diffraction peaks of the Zn QDs / ZnWO4-0.2 photocatalytic material XRD highly match those of the pure ZnWO4 sample, indicating that the Zn QDs / ZnWO4-0.2 sample generally maintained the crystal structure of zinc tungstate. Since the introduced zinc quantum dots were highly dispersed on the surface of the zinc tungstate cubic blocks, no characteristic diffraction peaks of zinc metal were detected.

[0063] Figure 2 TEM photograph of the Zn QDs / ZnWO4-0.2 photocatalytic material of Example 1. As can be seen from Figure 2 (a), the Zn QDs / ZnWO4-0.2 photocatalytic material generally presented a cubic block morphology. Figure 2 (b) is the HRTEM photograph of the Zn QDs / ZnWO4-0.2 photocatalytic material. The lattice fringes of the cubic blocks can be clearly seen, indicating that the main zinc tungstate cubic blocks have good crystallinity, which is consistent with the previous XRD analysis results. In addition, evenly distributed small black dots were observed on the surface of the zinc tungstate cubic blocks, which are zinc quantum dots attached to the surface of the zinc tungstate cubic blocks after zinc ions were reduced under ultraviolet light illumination.

[0064] Figure 3N2 isothermal adsorption - desorption test results of the ZnWO4 and Zn QDs / ZnWO4 - 0.2 photocatalytic materials prepared in Example 1 and Comparative Example 1. It can be seen from the figure that the ZnWO4 and Zn QDs / ZnWO4 - 0.2 photocatalytic materials have similar N2 adsorption - desorption isotherms. Hysteresis loops appear between 0.6 and 1.0, and the adsorption type is type IV isotherm. The test results show that the specific surface areas of the ZnWO4 and Zn QDs / ZnWO4 - 0.2 photocatalytic materials are 50.2 m 2 / g and 57 m 2 / g respectively. It can be seen that the introduction of zinc quantum dots effectively increases the specific surface area of zinc tungstate cubes, which is beneficial to exposing more catalytic active sites; the introduction of zinc quantum dots enriches the surface structure of zinc tungstate cubes, which is beneficial to improving the separation efficiency of photogenerated carriers, and thus improves the photocatalytic reduction activity of Zn QDs / ZnWO4 - 0.2 towards Cr(VI).

[0065] Figure 4 Photocatalytic performance curves of the ZnWO4 and Zn QDs / ZnWO4 photocatalytic materials prepared in Example 1, Comparative Example 1, Comparative Example 2 and Comparative Example 3 for the reduction of Cr(VI). The test results show that the ZnWO4 photocatalytic material shows good photocatalytic reduction activity towards Cr(VI). After 40 min of illumination, its catalytic reduction rate towards Cr(VI) is 49%. After introducing zinc quantum dots, the removal efficiency of the obtained Zn QDs / ZnWO4 photocatalytic material towards Cr(VI) is significantly improved. Among them, the ZnQDs / ZnWO4 - 0.2 sample obtained in Example 1 shows the best photocatalytic reduction activity towards Cr(VI). After 40 min of illumination, its catalytic reduction rate towards Cr(VI) is as high as 99%.

[0066] Figure 5 Cyclic stability test results of the best sample Zn QDs / ZnWO4 - 0.2 for photocatalytic reduction of Cr(VI). It can be found that after 10 cyclic tests, its photocatalytic reduction efficiency towards Cr(VI) still remains at about 96%, indicating that ZnQDs / ZnWO4 - 0.2 has very excellent photocatalytic chromium removal stability.

[0067] The above description of the disclosed embodiments enables those skilled in the art to implement or use the present invention. Various modifications to these embodiments will be obvious to those skilled in the art, and the general principles defined herein can be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention will not be limited to these embodiments shown herein, but rather to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A preparation method of ZnQDs / ZnWO4 photocatalytic material, characterized in that, Comprising: (1) In the presence of a first solvent, zinc acetate and sodium tungstate are mixed and subjected to a hydrothermal reaction to obtain a zinc tungstate photocatalytic material; (2) In the presence of a second solvent, zinc acetate and the zinc tungstate photocatalytic material are mixed and reacted under mercury lamp irradiation. Subsequently, vacuum drying is carried out to obtain a ZnQDs / ZnWO4 photocatalytic material; The mass ratio of the zinc acetate to the sodium tungstate is 1:(1 - 3); In the step (1), the temperature of the hydrothermal reaction is 140 - 180 °C, and the hydrothermal reaction time is 12 - 18 h; The first solvent includes ethylene glycol and / or water; the second solvent includes ethylene glycol; The mass ratio of the zinc acetate to the zinc tungstate photocatalytic material is 1:(1 - 10).

2. The preparation method according to claim 1, wherein, The mixing in the step (1) specifically includes: mixing zinc acetate and the first solvent to obtain component A; mixing sodium tungstate and the first solvent to obtain component B; and mixing component A and component B.

3. The preparation method according to claim 1, characterized in that, The conditions for the vacuum drying are: vacuum drying at 70 - 80 °C for 12 - 15 h.

4. The preparation method according to claim 1, characterized in that, The time of the mercury lamp irradiation is 5 - 30 min.

5. The ZnQDs / ZnWO4 photocatalytic material prepared by the method for preparing a ZnQDs / ZnWO4 photocatalytic material according to any one of claims 1 - 4.

6. The application of the ZnQDs / ZnWO4 photocatalytic material prepared by the method for preparing a ZnQDs / ZnWO4 photocatalytic material according to any one of claims 1 - 4 or the ZnQDs / ZnWO4 photocatalytic material according to claim 5 in photocatalytic chromium removal.

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