A ZnIn2S4 / SA composite foam and its preparation method and application

Abstract

The present invention discloses a ZnIn2S4 / SA composite foam and its preparation method and application, belonging to the field of material preparation. In the present invention, ZnIn2S4 particles are integrated onto the sodium alginate (SA) carrier to produce the ZnIn2S4 / SA composite foam. The ZnIn2S4 crystals are tightly wrapped on the SA framework, retaining the flower-like structure. Due to the presence of its macropores and highly efficient active centers, it shows great potential in the treatment of Cr(VI). Under visible light irradiation, the maximum photoreduction efficiency of the optimal sample ZS-1 is 93%. When using mixed pollutants (Cr(VI) / dye), the removal efficiency of the ZS-1 sample for Cr(VI) is increased to 98%, and 100% removal effect can be achieved for RhB. In addition, the composite material still maintains good photocatalytic performance and a relatively complete three-dimensional structural scaffold after continuous operation for 6 times, showing excellent reusability and stability.

Description

Technical Field

[0001] The present invention belongs to the field of material preparation, and more specifically, relates to a ZnIn 2 S 4 / SA composite foam and its preparation method and application. Background Art

[0002] ZnIn 2 S 4 is a ternary chalcogenide, which has been widely studied as a photocatalyst due to its suitable band gap, good visible light response and good photochemical stability. However, the inherent disadvantages of bare ZnIn 2 S 4 limit its application performance, such as fast recombination of photogenerated electron-hole pairs, small specific surface area, and insufficient light absorption. Constructing ZnIn 2 S 4 -based composite materials or heterojunctions has been proven to be a feasible method to improve photocatalytic performance and increase active centers. For example, by doping Mo element on ZnIn 2 S 4 to form flower-like hollow microspheres, etc., a hydrogen evolution rate of 4.62 mmol·g -1 ·h -1 was obtained, which is 10 times that of the original sample. A large number of previous studies have shown that ZnIn 2 S 4 is a perfect building block that can maintain a flower-like structure with abundant active centers during the design process of photocatalysts. Integrating ZnIn 2 S 4 on a flexible three-dimensional matrix is very attractive and greatly expands its practical applications.

[0003] People have been committed to preparing three-dimensional and convenient ZnIn 2 S 4 -based photocatalysts, and various materials have been studied as templates or substrates to disperse and anchor ZnIn 2 S 4 . Sodium alginate composed of β-D-mannuronic acid and α-L-guluronic acid is a typical polysaccharide, which has been widely used as a carrier and stabilizer for particulate photocatalysts. CdS nanoparticles were encapsulated into the SA matrix to prepare a flexible aerogel with better performance than the original CdS particles. Cu2O@carbon nanocapsule catalysts were synthesized with SA as a templating agent, which could remove 90% of methyl red within 120 min. One of the challenges in preparing a uniform composite material by combining a semiconductor with SA is to avoid phase separation between the two components and avoid leakage of catalyst particles. Summary of the Invention

[0004] Aiming at the above problems existing in the prior art, the technical problem to be solved by the present invention is to provide a ZnIn 2 S 4 / SA composite foam and its preparation method and application.

[0005] To solve the above technical problems, the technical solutions adopted by the present invention are as follows:

[0006] A preparation method of ZnIn 2 S 4 / SA composite foam: Disperse ZnIn 2 S 4 in deionized water. Under stirring conditions, add sodium alginate powder and CaCl 2 in sequence to form a suspension. Then freeze-dry the suspension, and then wash it with deionized water and freeze-dry it again to obtain the ZnIn 2 S 4 / SA composite foam.

[0007] Further, the preparation method of the ZnIn 2 S 4 / SA composite foam specifically includes the following steps:

[0008] (1) Prepare ZnIn 2 S 4 by solvothermal method;

[0009] (2) Uniformly disperse the prepared ZnIn 2 S 4 in deionized water. Under continuous stirring, add sodium alginate powder to form a suspension, and then add CaCl 2 ;

[0010] (3) After freeze-drying the suspension obtained in step (2), wash it with deionized water and freeze-dry it again to obtain the ZnIn 2 S 4 / SA composite foam.

[0011] Further, step (1) is specifically: Disperse ZnCl 2 , InCl 4 ·H 2 O and thioacetamide in deionized water in sequence to form a mixed solution. Heat the mixed solution at 100 - 120 °C for 1 - 2 h, cool it to room temperature and then centrifuge. Wash the precipitate with deionized water and ethanol, and then vacuum dry it at 50 - 70 °C.

[0012] Further, step (1) is specifically: ZnCl with a molar ratio of 1∶2∶2 2 , InCl 4 ·H 2O and 2 mmol of thioacetamide were successively dispersed in deionized water, and ultrasonic treatment was carried out for 10 - 30 min to obtain a mixed solution; then, the mixed solution was transferred to a reaction kettle, heated at 100 - 120 °C for 1 - 2 h, cooled to room temperature, centrifuged, and the precipitate was washed thoroughly with deionized water and ethanol; finally, the precipitate was dried under vacuum at 50 - 70 °C.

[0013] Further, in the mixed suspension in step (2), the mass ratio of ZnIn 2 S 4 to sodium alginate is 0.25 - 2, and the dosage of the cross-linking agent CaCl 2 is 2 - 10 wt%.

[0014] Further, the freeze-drying temperature in step (3) is -50 to -70 °C.

[0015] Further, in the composite foam, the loading amount of ZnIn 2 S 4 is greater than 20 - 50 wt%.

[0016] The ZnIn 2 S 4 / SA composite foam material prepared by any of the above methods.

[0017] The application of the said ZnIn 2 S 4 / SA composite foam material in the removal of Cr(VI) or organic pollutants.

[0018] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0019] 1) The preparation process of the photocatalyst of the ZnIn 2 S 4 / SA composite foam of the present invention is simple, easy to control, convenient to operate and low in cost.

[0020] 2) For the three-dimensional composite foam of ZnIn 2 S 4 / SA prepared by the present invention, the ZnIn 2 S 4 crystals are closely wrapped on the SA skeleton, retaining the flower-like structure. Due to the presence of its macropores and efficient active centers, it shows great potential in the treatment of Cr(VI). Under visible light irradiation, the maximum Cr(VI) reduction efficiency of the best sample ZS-1 (the mass ratio of ZnIn 2 S 4 :SA is 1:1) is 93%. When using mixed pollutants (Cr(VI) / dye), the removal efficiency of the ZS-1 sample for Cr(VI) is increased to 98%, and 100% removal effect can be achieved for RhB.

[0021] 3) The ZnIn 2 S 4 / SA composite foam has a simple usage method. Just put the catalyst into the sewage and then irradiate it with natural light at normal temperature and pressure; and it has the advantages of being easy to recycle and not causing secondary pollution. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 are digital images and SEM morphology characterization diagrams of different materials, where: (a) is the digital image of ZnIn 2 S 4 powder; (b) is the digital image of SA blank foam; (c) is the digital image of ZS-1 composite foam; (d) is ZnIn 2 S 4 powder SEM image; (e) is the SEM image of SA blank foam; (f)-(i) are the SEM images of ZS-1 composite foam;

[0023] Figure 2 (a) and Figure 2 (b) are the XRD patterns and FT-IR spectra of ZnIn 2 S 4 powder, SA blank foam, and composite foam respectively;

[0024] Figure 3 are the XPS spectra of the samples, where: (a) is the high-resolution spectrum of C 1s; (b) is the high-resolution spectrum of Zn 2p; (c) is the high-resolution spectrum of In 3d; (d) is the high-resolution spectrum of S 2p;

[0025] Figure 4 are the photocatalytic reduction performances of each material under different conditions, where: (a) is the photocatalytic reduction performance of different samples for Cr(VI), experimental conditions: 20mg / L, pH = 6; (b) is the photocatalytic reduction performance of ZS-1 at different Cr(VI) concentrations, experimental conditions: pH = 6; (c) is the photocatalytic reduction performance of ZS-1 at different pH values, experimental conditions: 20mg / L; (d) and (e) are the photocatalytic performances of different samples for the Cr(VI) / RhB mixed system, experimental conditions: 20 / 20mg / L, pH = 6; (f) is the cyclic test diagram of ZS-1 foam;

[0026] Figure 5It is a comparison diagram of fresh and recycled ZS-1, where: (a) is the XRD diagram of fresh and recycled ZS-1; (b) is the FT-IR spectrum diagram of fresh and recycled ZS-1; (c) is the SEM image of fresh ZS-1; (d) is the SEM image of recycled ZS-1; (e) is the XPS spectrum diagram of fresh and recycled ZS-1; (f) is the high-resolution spectrum diagram of Cr2p of recycled ZS-1;

[0027] Figure 6 It is a possible reaction mechanism diagram of ZS-1 photocatalytic reduction of Cr(VI), where: (a) is the photocurrent spectrum diagram; (b) is the EIS-Nyquist diagram; (c) is the valence band XPS spectrum diagram of ZS-1; (d) is the mechanism diagram of composite foam removing Cr(VI); Detailed implementation manners

[0028] To make the objectives, technical solutions and advantages of the present invention clearer, the present invention will be further described below in conjunction with specific embodiments.

[0029] Example 1: Preparation of ZnIn 2 S 4 / SA-0.25 composite foam

[0030] 1. Preparation of ZnIn 2 S 4 powder photocatalyst: 1 mmol of ZnCl 2 , 2 mmol of InCl 4 ·H 2 O and 2 mmol of thioacetamide (TAA) were successively dispersed in 50 - 70 ml of deionized water and ultrasonicated for 10 - 30 min. Then, the mixed solution was transferred to the inner lining of a Teflon stainless steel autoclave and heated at 100 - 120 °C for 1 - 2 h. After cooling to room temperature, it was centrifuged and washed thoroughly with deionized water and ethanol. Finally, the collected sample was dried overnight under vacuum at 50 - 70 °C to obtain the ZnIn 2 S 4 powder photocatalyst.

[0031] 2. Preparation of SA blank foam photocatalyst: Under continuous magnetic stirring, 0.4 - 0.6 g of SA powder was slowly added to 20 - 50 mL of deionized water. After stirring for 2 - 4 h, with the assistance of an oscillator, the obtained homogeneous solution was poured into a silica gel mold. Then, an aqueous solution of cross-linking agent CaCl 2 with a mass fraction of 2 - 10 wt% was added, and the mixed suspension was freeze-dried overnight at -50 ~ -70 °C. Finally, the obtained foam was washed sequentially with deionized water to remove the excess Ca 2+ , and freeze-dried again to obtain the SA blank foam photocatalyst.

[0032] 3. ZnIn 2 S 4 / SA Composite Foam Photocatalyst Preparation: Disperse a certain amount of the ZnIn 2 S 4 powder synthesized in Step 1 in 20 - 50 mL of deionized water and sonicate for 10 - 30 min. Subsequently, under continuous magnetic stirring, slowly add 0.4 - 0.6 g of SA powder to the above suspension. The mass ratio of ZnIn 2 S 4 to SA in the mixed suspension is 0.25. After stirring for 2 - 4 h, with the assistance of an oscillator, pour the obtained homogeneous solution into a silica gel mold. Then add an aqueous solution of crosslinking agent CaCl 2 with a mass fraction of 2 - 10 wt%, and freeze-dry the mixed suspension at -50 to -70 °C overnight. Finally, wash the obtained foam sequentially with deionized water to remove the excess Ca 2+ , and freeze-dry again to obtain the ZnIn 2 S 4 / SA composite foam, denoted as ZS - 0.25.

[0033] Example 2: Preparation of ZnIn 2 S 4 / SA - 0.5 Composite Foam

[0034] The preparation methods of the ZnIn 2 S 4 powder photocatalyst and the SA blank foam photocatalyst are the same as those in Example 1.

[0035] ZnIn 2 S 4 / SA Composite Foam Photocatalyst Preparation: Disperse a certain amount of the ZnIn 2 S 4 powder synthesized in Step 1 in 20 - 50 mL of deionized water and sonicate for 10 - 30 min. Subsequently, under continuous magnetic stirring, slowly add 0.4 - 0.6 g of SA powder to the above suspension. The mass ratio of ZnIn 2 S 4 to SA in the mixed suspension is 0.5. After stirring for 2 - 4 h, with the assistance of an oscillator, pour the obtained homogeneous solution into a silica gel mold. Then add a crosslinking agent CaCl 2 with a mass fraction of 2 - 10 wt%, and freeze-dry the mixed suspension at -50 to -70 °C overnight. Finally, wash the obtained foam sequentially with deionized water to remove the excess Ca 2+ , and freeze-dry again to obtain the ZnIn 2 S 4 / SA composite foam, denoted as ZS - 0.5.

[0036] Example 3: Preparation of ZnIn 2 S 4 / SA-1 Composite Foam

[0037] ZnIn 2 S 4 The preparation methods of the ZnIn

[0038] ZnIn 2 S 4 / SA composite foam photocatalyst and the SA blank foam photocatalyst are the same as those in Example 1. 2 S 4 Preparation of ZnIn 2 S 4 / SA composite foam photocatalyst: Disperse a certain amount of the ZnIn 2 S 2 + powder synthesized in Step 1 in 20 - 50 mL of deionized water and sonicate for 10 - 30 min. Subsequently, under continuous magnetic stirring, slowly add 0.4 - 0.6 g of SA powder to the above suspension. The mass ratio of ZnIn 2 S 4 to SA in the mixed suspension is 1. After stirring for 2 - 4 h, with the assistance of an oscillator, pour the obtained homogeneous solution into a silica gel mold. Then add 2 - 10 wt% cross-linking agent CaCl

[0039] Example 4: Preparation of ZnIn 2 S 4 / SA-2 Composite Foam

[0040] ZnIn 2 S 4 The preparation methods of the ZnIn

[0041] ZnIn 2 S 4 / SA composite foam photocatalyst and the SA blank foam photocatalyst are the same as those in Example 1. 2 S 4 Preparation of ZnIn 2 S 4The mass ratio with SA is 2. After stirring for 2 - 4 h, with the assistance of an oscillator, the obtained homogeneous solution is poured into a silicone mold. Then, 2 - 10 wt% cross - linker CaCl 2 is added, and the mixed suspension is freeze - dried at - 50 to - 70 °C overnight. Finally, the obtained foam is washed sequentially with deionized water to remove the excess Ca 2 + , and freeze - dried again to obtain the ZnIn 2 S 4 / SA composite foam, denoted as ZS - 2.

[0042] Example 5: Characterization and testing of ZnIn 2 S 4 / SA composite foam samples

[0043] The ZnIn 2 S 4 / SA composite foam samples obtained in Examples 1 - 4 were characterized and tested as follows:

[0044] 1. SEM analysis

[0045] SEM analysis was used to observe the microscopic morphology, particle size, and surface distribution of each component of the samples. The samples were generally prepared on silicon wafers for testing. A field - emission scanning electron microscope (JSM - 7600F) was used, and the operating voltage was 30 kV.

[0046] Figure 1 For the digital images and SEM morphology characterization diagrams of different materials, as Figure 1 shown in b and 1e, the SA blank foam has a smooth surface and cross - section, and is rich in macropores. In contrast, Figure 1 as shown in c and 1f, the ZnIn 2 S 4 / SA composite foam is bright yellow, has a rough surface and cross - section, which may be due to the entry of ZnIn 2 S 4 particles into the interior of the SA matrix and wrapping around the SA chains. In addition, as Figure 1 shown in g, the ZnIn 2 S 4 / SA composite foam has many obvious irregular micropores, which are expected to be potential channels for wastewater transfer. As can be seen from Figure 1 h, ZnIn 2 S 4 is evenly distributed on the SA substrate. At Figure 1 a higher magnification in i, it can be observed that the composite foam well retains the ZnIn 2 S 4The integrity of the flower-like structure, to a certain extent, the combination of a highly porous matrix and petal-like photocatalysts can expose more active centers and improve the removal efficiency.

[0047] 2. XRD Analysis

[0048] XRD was used to analyze the crystal phase structure and components of each sample. An X-ray diffractometer (Ultima IV) was used, and the test conditions were Cu target (CuKα) radiation (k = 0.1542 nm) and a working voltage of 40 kV.

[0049] Figure 2 a shows the XRD patterns of different samples. As Figure 2 shown in b, pure ZnIn 2 S 4 has diffraction peaks at 21.4°, 27.6°, 30.3°, 39.8°, 47.2°, 52.3° and 55.5°, corresponding to the (006), (102), (104), (108), (110), (116) and (022) crystal planes of hexagonal ZnIn 2 S 4 (JCPDS: 65 - 2023). In the composite foam, the characteristic diffraction peaks of ZnIn 2 S 4 can be detected, and with the decrease in the content of ZnIn 2 S 4 , the peak intensity gradually decreases. The characteristic peaks of SA are not obvious due to its low crystallinity and amorphous nature. At the same time, there is a certain degree of overlap between the (006) crystal plane of ZnIn 2 S 4 and SA. According to the Scherrer equation: D = Kλ / βcosθ, the average crystallite size of the catalysts in the samples was 4.8 nm, 4.2 nm, 3.6 nm and 4.8 nm, corresponding to ZS - 0.25, ZS - 0.5, ZS - 1 and ZS - 2 respectively, all of which are smaller than the 5.0 nm of pure ZnIn 2 S 4 particles. This result clearly shows the positive role of the SA matrix in dispersing and stabilizing ZnIn 2 S 4 particles.

[0050] 3. FT-IR Analysis

[0051] FT-IR was used to test the chemical structure, functional groups and bond state characteristics of the samples, and the chemical composition of the samples could be determined at the molecular level. A Fourier transform infrared spectrometer (Nicolet - 360) was used, with a pure KBr spectrum as the background, and the test was carried out after grinding a certain amount of KBr and the sample evenly.

[0052] Chemical functional groups were analyzed by FT-IR spectroscopy, such as Figure 2 As shown in b, the pure SA foam has a broad absorption band near 3000 - 3700 cm -1 , which comes from the -OH of the alginate backbone. The characteristic peaks at 2917 cm -1 and 1027 cm -1 are attributed to the C-H stretching vibration of the G unit in the SA chain and the C-O-C bond on the glycosidic bond, respectively. In addition, the characteristic peaks at 1592 cm -1 and 1410 cm -1 are caused by the asymmetric and symmetric stretching vibrations of -COO-, which is related to the abundant -COOH in SA. In the FT-IR spectrum of the composite material, the -OH stretching peak has a slight shift and broadening, which can be explained by the strong interaction between the -OH in the polysaccharide matrix and the ZnIn 2 S 4 particles. In addition, the obvious change in the corresponding band of -COOH in the composite material indicates the existence of multiple interactions between the catalyst and the matrix. It can be predicted that the existence of the interaction will lead to the effective separation of photo-generated carriers in the composite material.

[0053] 4. XPS Analysis

[0054] XPS is used to analyze information such as the valence state, proportion, and even the valence band potential of single components of the elements contained in the sample. An X-ray photoelectron spectrometer (AXIS Ultra DLD) was used, and the binding energy of all elements was referenced to the C1s peak of the external contamination carbon source at 284.8 eV.

[0055] XPS tests characterized the chemical composition and chemical state of the surface elements of the ZnIn 2 S 4 / SA composite foam. As Figure 3 shown, the elements C, Zn, In, and S were detected in the ZnIn 2 S 4 / SA composite foam. In Figure 3 a, the characteristic peaks at 284.8 eV, 286.6 eV, and 288.6 eV in the high-resolution C1s spectrum correspond to C-C, C-O, and C=O in the SA molecular structure. As Figure 3 shown in b, the two diffraction peaks at 1020.3 eV and 1043.7 eV belong to Zn2p 2 S 4 of Zn 2+ in ZnIn 3 / 2 and Zn2p 1 / 2 . In Figure 3 c, the peak signals at 444.5 eV and 452.1 eV appear due to In 3+of In3d 5 / 2 and In3d 3 / 2 。In addition, the two characteristic peaks at 161.3 eV and 162.6 eV in the S2p spectrum indicate ZnIn 2 S 4 in S 2- of S2p 3 / 2 and S2p 1 / 2 。In the high-resolution XPS spectra of ZS-1, the characteristic peaks of Zn2p, In3d, and S2p shift significantly towards higher binding energies, indicating that the interaction between SA and ZnIn 2 S 4 particles leads to a decrease in electron density. XPS analysis further confirms the high interfacial binding strength between SA and ZnIn 2 S 4 , enabling the semiconductor particles to be tightly anchored on the surface of the matrix.

[0056] Example 6: Photocatalytic reduction of Cr(VI) or organic pollutants by ZnIn 2 S 4 / SA composite foam

[0057] 1. Photocatalytic reduction performance of different samples for Cr(VI)

[0058] The light source for the reaction is a 300 W xenon lamp with a UV filter (CEL-HXF300, λ > 420 nm, light intensity of 100 mW / cm 2 ). K 2 Cr 2 O 7 is used as a hexavalent chromium simulated pollutant to prepare a Cr(VI) solution, and the reaction conditions are room temperature and a pH of approximately 6. Specific steps: Weigh 40 mg of the composite foam and add it to 40 mL of a 20 mg / L Cr(VI) aqueous solution, and continuously stir. Before the light is turned on, stir for 1 h in the dark to reach the adsorption equilibrium. After turning on the light, sample by syringe at certain time intervals (each sampling volume is approximately 3 mL), and filter through a 0.22 μm filter head to remove the catalyst solid particles to obtain the test solution. Finally, measure the absorbance at 540 nm by the diphenylcarbazide spectrophotometric (DPC) coloring method and convert the Cr(VI) concentration through the calibration curve to obtain the reduction performance of each material.

[0059] Figure 4a shows the photocatalytic reduction performance of different samples. The visible-light photocatalytic reduction of Cr(VI) to Cr(III) can be used to evaluate the performance of composite catalysts. In the absence of a photocatalyst and light, the concentration of Cr(VI) hardly changes. As a control experiment, SA blank foam material was used for the photocatalytic reduction of Cr(VI). The SA blank foam material has poor reduction ability for Cr(VI), only 5%, indicating that pure SA has extremely low negative surface activity and weak light response ability. For ZnIn 2 S 4 powder, the reduction ability of Cr(VI) is about 85% after 180 min of light irradiation. However, due to the formation of a uniform photocatalyst coating on the surface of the porous SA matrix structure, the composite foam exhibits various outstanding behaviors in removing Cr(VI). Generally speaking, the photocatalytic performance of the composite foam shows an upward trend with the increase of ZnIn 2 S 4 content. However, the reduction ability of ZS-1 (93%) is higher than that of ZS-2 (80%), indicating that the excessive photocatalyst in the composite material is a recombination center rather than an electron channel. Therefore, the optimal composite foam is ZnIn 2 S 4 : ZS-1 with a mass ratio of ZnIn

[0060] 2. Photocatalytic reduction performance of ZS-1 at different Cr(VI) concentrations

[0061] The light source for the reaction is a 300W xenon lamp with a UV filter (CEL-HXF300, λ > 420 nm, light intensity is 100 mW / cm 2 ). K a Cr 2 O 7 is used as a hexavalent chromium simulated pollutant to prepare the Cr(VI) solution. The reaction conditions are room temperature and pH of about 6. Specific steps: Weigh 40 mg of the composite foam and add it to 40 mL of Cr(VI) aqueous solutions with concentrations of 5 mg / L, 10 mg / L, 20 mg / L, and 40 mg / L in turn and stir continuously. Before the light irradiation starts, stir for 1 h in the dark to reach the adsorption equilibrium. After turning on the light, sample by syringe at certain time intervals (the sampling volume each time is about 3 mL), and filter out the catalyst solid particles through a 0.22 μm filter head to obtain the test solution. Finally, measure the absorbance at 540 nm by diphenylcarbazide spectrophotometry (DPC) coloring method and convert the Cr(VI) concentration through the calibration curve to obtain the reduction performance of ZS-1 at each Cr(VI) concentration.

[0062] Figure 4b shows the photocatalytic reduction performance of ZS-1 at different Cr(VI) concentrations. The results indicate that as the initial Cr(VI) concentration increases, the removal efficiency decreases, which may be due to the reduction of active centers.

[0063] 3. Photocatalytic reduction performance of ZS-1 at different pH values

[0064] The light source for the reaction is a 300 W xenon lamp with a UV filter (CEL-HXF300, λ > 420 nm, light intensity of 100 mW / cm 2 ). Using K 2 Cr 2 O 7 as the hexavalent chromium simulated pollutant to prepare the Cr(VI) solution, the reaction conditions are room temperature and the concentration of Cr(VI) aqueous solution is 20 mg / L. Specific steps: Weigh 40 mg of the composite foam and add it to 40 mL of Cr(VI) aqueous solutions with pH values of 2, 4, 6, 8, and 10 in sequence and continuously stir. Before the light irradiation starts, stir for 1 h in the dark to reach the adsorption equilibrium. After turning on the light, sample by syringe at certain time intervals (the sampling volume each time is about 3 mL), and filter through a 0.22 μm filter head to remove the catalyst solid particles to obtain the test solution. Finally, measure the absorbance at 540 nm by the diphenylcarbazide spectrophotometry (DPC) coloring method and convert the Cr(VI) concentration through the standard curve, and then obtain the reduction performance at each pH value.

[0065] Figure 4 c shows the photocatalytic reduction performance of ZS-1 at different pH values. The pH value of the aqueous solution is one of the key factors affecting the reduction effect of Cr(VI), and this study has carried out a detailed exploration on this. Under acidic conditions, ZS-1 has a relatively high removal efficiency for Cr(VI), but at pH = 8, the degradation rate of Cr(VI) is still 71%, further proving that the ZS-1 composite foam has a relatively high removal efficiency for Cr(VI).

[0066] 4. Performance of photocatalytic Cr(VI) / RhB mixed systems of different samples

[0067] The light source for the reaction is a 300 W xenon lamp with a UV filter (CEL-HXF300, λ > 420 nm, light intensity of 100 mW / cm 2 )。Using K 2 Cr 2 O 7Prepare a Cr(VI) solution using hexavalent chromium as a simulated pollutant and a RhB solution using rhodamine B (RhB) as a dye simulated pollutant. The normal reaction conditions are room temperature and a pH of approximately 6. Specific steps: Weigh 40 mg of the composite foam and add it to 40 mL of an aqueous solution of Cr(VI) / RhB at 20 / 20 mg / L and stir continuously. Before the light is turned on, stir for 1 h in the dark to achieve adsorption equilibrium. After turning on the light, sample through a syringe at certain time intervals (each sample volume is approximately 3 mL), and filter through a 0.22 μm filter head to remove catalyst solid particles to obtain the test solution. Finally, measure the absorbance at 540 nm by diphenylcarbazide spectrophotometry (DPC) coloring method and convert the Cr(VI) concentration through the calibration curve. Measure the absorbance at 554 nm and convert the RhB concentration through the calibration curve to obtain the reduction performance of each material for the Cr(VI) / RhB mixed system.

[0068] Figure 4 Figures d and e show the photocatalytic performance of different samples for the Cr(VI) / RhB mixed system. As is well known, industrial wastewater usually includes heavy metal pollution and refractory organic pollutants (such as dyes). Therefore, a mixed solution composed of Cr(VI) and RhB is used to evaluate the photocatalytic performance of the composite foam for the synergistic removal of pollutants. As Figure 4 shown in Figure d, it can be observed that the removal rate of Cr(VI) is increased after adding RhB compared with the single system. When the concentration of Cr(VI) / RhB is 20 / 20 mg / L, the best removal efficiencies of ZS-1 are 98% and 100% respectively. There are two reasons for the improved photocatalytic activity of the hybrid foam in the mixed system. On the one hand, the composite foam preferentially adsorbs the RhB dye with cationic properties. The surface of the composite foam becomes positive, generating electrostatic attraction to adsorb Cr 2 O 7 2 - anions. On the other hand, in the mixed system, RhB consumes photo-generated holes, and Cr(VI), as a photo-generated electron acceptor, effectively prevents electron-hole recombination. It can be inferred that the polysaccharide matrix of sodium alginate can release active centers in time for further reactions, thus realizing the synergistic removal of Cr(VI) and RhB. In addition, the best composite foam is still ZnIn 2 S 4 :ZS-1 with a mass ratio of ZnIn:S to SA of 1:1.

[0069] 5. Recycling test

[0070] To test the stability and reusability of the composite foam, the reacted ZS-1 foam can be easily recovered from the solution with tweezers, and the used foam is washed with deionized water and ethanol and freeze-dried for the next cycle. Inductively coupled plasma optical emission spectrometer ICP-OES (AA900T) can be used for qualitative and quantitative analysis of all metal elements and some non-metal elements in samples in the fields of geology, environmental protection, chemical engineering, biology, medicine, food, metallurgy, agriculture, etc.

[0071] Figure 4 Figure f is the cyclic test diagram of ZS-1 foam. 40 mg of ZS-1 foam is added to 40 mL of Cr(VI) solution with a concentration of 20 mg / L, and the visible light λ > 420 nm. After 6 cyclic operations, the photocatalytic performance of ZS-1 remains good with a slight decrease, which confirms the good stability and recyclability of the composite foam. According to the detection results of ICP-OES, it is found that zinc and indium elements are almost absent, indicating that there will be no photocorrosion or shedding of semiconductor particles during use. As Figure 5 shown in Figures 6a and 5b, the XRD and FT-IR spectra of the foam after multiple cycles show no obvious changes, indicating that neither the crystal structure nor the phase of the ZS-1 foam is damaged. In addition, a weak peak of Cr2p was also observed in the XPS spectrum of the used ZS-1 ( Figure 5 Figure 8e). In Figure 5 Figure f, the high-resolution XPS spectrum verifies that the heavy metal Cr(VI) is successfully reduced to Cr(III) after the reaction.

[0072] 6. Photocatalytic reduction mechanism

[0073] Electrochemical tests in a typical three-electrode mode were carried out on a CHI760E electrochemical workstation, including transient photocurrent response and electrochemical impedance spectroscopy (EIS), which can be used to study the possible reaction mechanism of ZS-1 photocatalytic reduction of Cr(VI).

[0074] Figure 6 Figure 20 is the possible reaction mechanism diagram of ZS-1 photocatalytic reduction of Cr(VI). As is well known, the three-dimensional structure foam as a carrier can largely enhance the interfacial contact of the supported photocatalyst, significantly overcome the dissolution and aggregation of particles, and improve the photocatalytic performance. In the photocurrent spectrum Figure 6 Figure 21a and the alternating current impedance spectrum Figure 6 Figure 22b, ZS-1 shows a higher photocurrent response and a smaller arc radius than ZnIn 2 S 4 , proving effective charge transfer. In Figure 6 Figure 23c, ZnIn 2 S 4The valence band (VB) potential is 1.49 eV. Combining with the Eg value, the conduction band potential is calculated to be -0.84 eV. Therefore, a small fraction of e- can convert O 2 to ·O 2 - (E(O 2 / ·O 2 - ) = -0.33 eV). In the hybrid system, photo-generated H + and ·O 2 - can oxidize organic dye pollutants into small molecules. As Figure 6 shown in d, reduction and adsorption are the two main mechanisms for the complete removal of Cr(VI) by the ZS-1 three-dimensional composite foam, especially reduction plays a crucial role in the water purification process. Based on mechanisms such as electrostatic interaction and surface complex formation, a small fraction of Cr(VI) can be adsorbed onto the composite foam in the dark. Due to the hydrophilicity of the polysaccharide matrix, the excited electrons generated by the uniformly distributed photocatalyst can quickly transfer to Cr(VI), enabling the occurrence of Cr(VI) reduction in the solution (E(Cr(VI) / Cr(III)) = 0.57 eV).

Claims

1. ZnIn 2 S 4 / SA composite foam in the application of synergistic removal of Cr(VI) and RhB, the ZnIn 2 S 4 / SA composite foam preparation method, Including the following steps: (1) Disperse ZnCl with a molar ratio of 1:2:2 2 , InCl 4 ·H 2 O and thioacetamide in deionized water in sequence, and ultrasonically treat for 10 - 30 min to obtain a mixed solution; then, transfer the mixed solution to a reaction kettle, heat at 100 - 120 °C for 1 - 2 h, after cooling to room temperature, centrifuge and thoroughly wash the precipitate with deionized water and ethanol; finally, vacuum dry the precipitate under the condition of 50 - 70 °C; (2) Disperse the prepared ZnIn 2 S 4 evenly in deionized water. Under continuous stirring, add sodium alginate powder to form a suspension. The mass ratio of ZnIn 2 S 4 to sodium alginate in the mixed suspension is 1:

1. Then add CaCl 2 . The dosage of the crosslinking agent CaCl 2 is 2 - 10 wt%. (3) The suspension obtained in step (2) is freeze-dried and then washed with deionized water, and freeze-dried at -70 to -50 °C to obtain ZnIn 2 S 4 / SA composite foam. In the composite foam, the loading amount of ZnIn 2 S 4 is 20 - 50 wt%.

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