Preparation method of Er-doped CdIn2S4 photocatalyst for photocatalytic degradation of methylene blue

Abstract

The invention discloses a preparation method of an Er-doped CdIn2S4 photocatalyst for photocatalytic degradation of methylene blue, which comprises the following steps: adding cadmium chloride, indium chloride tetrahydrate and thioacetamide into deionized water, and carrying out hydrothermal reaction to prepare the CdIn2S4 photocatalyst; in the same reaction system, Er (NO3) 3.5 H2O is added according to different proportions of the molar weight of indium trichloride tetrahydrate, and three samples with different Er doping concentrations are obtained. The photocatalytic performance of CdIn2S4 is effectively regulated and controlled through Er doping, the separation efficiency of photon-generated carriers is remarkably improved, and meanwhile the light absorption capacity of the material is enhanced. More adsorption sites and active sites are exposed on the surface of the modified catalyst, so that the degradation efficiency of methylene blue is greatly improved. The method has the remarkable advantages of being simple in preparation process, low in cost, easy to recycle and the like, and shows a good application prospect in the field of dye wastewater treatment.

Description

TECHNICAL FIELD

[0001] The application relates to the preparation field of an Er-doped CdIn2S4 photocatalyst, in particular to a preparation method of an Er-doped CdIn2S4 photocatalyst for photocatalytic degradation of methylene blue. BACKGROUND

[0002] The printing and dyeing industry, as an important traditional industry in China, plays an important role in the national economy. With the development of chemical synthesis technology, synthetic dyes have gradually replaced natural dyes to become mainstream products. According to statistical data, the global annual output of synthetic dyes exceeds 1 million tons, and although the annual output of dyes in China is decreasing year by year, China is still the largest producer and consumer of dyes in the world.

[0003] However, about 10-20% of dyes are discharged with wastewater during the production and use of dyes, and if the wastewater containing high concentrations of organic pollutants is directly discharged without proper treatment, it will cause serious harm to the water environment.

[0004] Among the synthetic dyes, aromatic azo dyes represented by methylene blue are widely used in the textile, papermaking, leather and other industries due to their excellent dyeing performance, and their use accounts for about 50% of the total amount of dyes. The molecular structure of this type of dye contains stable azo bonds (-N=N-) and aromatic ring structures, making it have strong chemical stability and resistance to biodegradation. More seriously, dyes such as methylene blue have been proven to have a "three-terrible" effect (carcinogenic, teratogenic, and mutagenic), and they can exist in the natural environment for a long time and be enriched through the food chain, ultimately endangering human health.

[0005] Traditional methods for degrading organic dyes include adsorption, coagulation and biodegradation, and these methods have obvious technical limitations in treating this type of dye.

[0006] Although the adsorption method is simple to operate and has low cost, it essentially only transfers the pollutants from the liquid phase to the solid phase, and does not achieve complete degradation of the pollutants, and the adsorbent is difficult to regenerate and can easily cause secondary pollution.

[0007] Although the coagulation method can effectively remove suspended dyes by adding coagulants such as aluminum salts and iron salts, it has poor removal effect on soluble dyes (the removal rate is usually less than 50%), and a large amount of chemical sludge with a water content of up to 95% is produced, which has high subsequent treatment cost and is prone to cause secondary pollution.

[0008] Although the biodegradation method has low operating cost, the degradation efficiency of microorganisms is generally low (the COD removal rate is usually less than 60%) due to the complex and toxic structure of the dye molecules, and strict control of conditions such as pH (6-8) and temperature is required, and the water quality fluctuation adaptability is poor.

[0009] More importantly, the biological treatment system has a long start-up period (usually 15-30 days of sludge acclimation period), a large footprint (3-5 m 2 The reactor area is required to treat 1 ton of wastewater), and the decolorization rate of refractory dyes such as methylene blue is often less than 40%. These inherent defects make it difficult for traditional methods to meet the strict requirements of current dye wastewater treatment.

[0010] Therefore, it is of great significance to find a method for treating methylene blue wastewater that is efficient, green, and simple in process, whether for environmental protection or for human health.

[0011] Photocatalysis technology is considered to be the most promising dye wastewater treatment technology due to its unique advantages. This technology uses the electron-hole pairs generated by semiconductor materials under light to degrade organic pollutants into harmless small molecules through a series of redox reactions, with the characteristics of mild reaction conditions, complete degradation, and no secondary pollution. Compared with traditional physical and chemical treatment methods, photocatalysis technology does not require the addition of a large amount of chemical agents, has low energy consumption, and can effectively treat persistent organic pollutants that are difficult to degrade by traditional methods.

[0012] However, existing photocatalysts still face the following problems in practical application:

[0013] (1) The recombination rate of photo-generated electron-hole pairs is too fast. When the recombination is too fast, a large number of electron-hole pairs recombine before participating in the catalytic reaction;

[0014] (2) The photocatalyst has low utilization efficiency of sunlight and a narrow response range to the solar spectrum, and can only effectively utilize specific wavebands in the solar spectrum, resulting in low light energy conversion efficiency and greatly reducing the degradation efficiency of pollutants.

[0015] Therefore, it is a problem that needs to be solved by those skilled in the art to provide a method that can improve the separation efficiency of photo-generated carriers, enhance the light absorption capacity of the material, greatly improve the degradation efficiency of methylene blue, has the advantages of simple preparation process, low cost, easy to reuse, and good application prospect in the field of dye wastewater treatment. SUMMARY

[0016] Therefore, the present application provides a preparation method of an Er-doped CdIn2S4 photocatalyst for photocatalytic degradation of methylene blue.

[0017] To solve the above technical problems, the present application adopts the following technical solutions:

[0018] A preparation method of an Er-doped CdIn2S4 photocatalyst for photocatalytic degradation of methylene blue, comprising the following steps:

[0019] Step 1: Preparation of CdIn2S4

[0020] Step 1.1: Cadmium chloride, indium trichloride tetrahydrate and thioacetamide were added into deionized water, and magnetically stirred until completely dissolved;

[0021] Step 1.2: The mixed solution was transferred into a polytetrafluoroethylene-lined stainless steel autoclave for hydrothermal reaction;

[0022] Step 1.3: The precipitate was collected by centrifugation, washed, and dried;

[0023] Step 1.4: After grinding, CdIn2S4 photocatalyst was obtained;

[0024] Step 2: Preparation of Er-CdIn2S4

[0025] Step 2.1: Cadmium chloride, indium trichloride tetrahydrate and thioacetamide were added into deionized water, and Er(NO3)3·5H2O was added according to 3%, 5% and 7% of the molar amount of indium trichloride tetrahydrate, respectively, and magnetically stirred until completely dissolved;

[0026] Step 2.2: The mixed solution was transferred into a polytetrafluoroethylene-lined stainless steel autoclave for hydrothermal reaction;

[0027] Step 2.3: The precipitate was collected by centrifugation, washed, and dried;

[0028] Step 2.4: After grinding, three samples with different doping concentrations of 3% Er-CdIn2S4, 5% Er-CdIn2S4 and 7% Er-CdIn2S4 were obtained.

[0029] Preferably, in the step 1.1, the amount of substance of cadmium chloride is 1 mmol, the amount of substance of indium trichloride tetrahydrate is 2 mmol, the amount of substance of thioacetamide is 4 mmol, and the amount of deionized water added is 60 mL.

[0030] Preferably, in the step 1.1 and the step 2.1, the magnetic stirring is continued at room temperature at a speed of 600 rpm for 1 hour.

[0031] Preferably, in the step 1.2 and the step 2.2, the volume of the polytetrafluoroethylene-lined stainless steel autoclave is 100 mL, the temperature of the hydrothermal reaction is 180°C, and the time of the hydrothermal reaction is 12 h.

[0032] Preferably, in the step 1.3 and the step 2.3, the centrifugation speed is 8000 rpm, and deionized water and anhydrous ethanol are used for washing respectively for five times.

[0033] Preferably, in the step 1.3 and the step 2.3, the drying temperature is 80°C, and the drying time is 12 h.

[0034] Preferably, in the step 2.1, the amount of substance of cadmium chloride is 1 mmol, the amount of substance of indium trichloride tetrahydrate is 2 mmol, the amount of substance of thioacetamide is 4 mmol, and the amount of deionized water added is 60 mL.

[0035] Preferably, in the step 2.1, 0.06 mmol, 0.10 mmol and 0.14 mmol of Er(NO3)3·5H2O are added according to 3%, 5% and 7% of the molar amount of indium trichloride tetrahydrate, respectively.

[0036] The present application has the following technical effects relative to the prior art:

[0037] (1) The present application uniformly dopes rare earth element Er into CdIn2S4 by a hydrothermal method, finally forming an Er-CdIn2S4 composite photocatalyst, which utilizes the electron capture effect of Er 3+ ions to significantly inhibit the recombination of photo-generated electron-hole pairs and prolong the carrier lifetime.

[0038] (2) The present application introduces an intermediate energy level in the band gap of CdIn2S4 by Er doping, which widens the visible light absorption range and especially enhances the light absorption capacity in the long wavelength region.

[0039] (3) The present application exposes more adsorption sites and active sites on the surface of the modified catalyst, greatly improving the adsorption and degradation efficiency of methylene blue.

[0040] (4) The present application has a simple preparation process and excellent degradation effect. Under the optimized conditions (pH = 11, catalyst mass 30 mg), the degradation rate of 10 mg / L -1 of methylene blue solution is as high as 99.60% within 180 minutes, and the performance only decreases by 4% after 5 cycles, showing excellent stability and application potential. BRIEF DESCRIPTION OF DRAWINGS

[0041] Figure 1 (a) is the X-ray diffraction (XRD) pattern of the composite sample of the present application;

[0042] Figure 1 (b) is the X-ray photoelectron spectroscopy (XPS) pattern of 5% Er-CIS of the present application;

[0043] Figure 2 (a) is the SEM graph of CIS of the present application;

[0044] Figure 2 (b) is the SEM graph of 5% Er-CIS of the present application;

[0045] Figure 2(c-e) are TEM images of 5% Er-CIS of the present application;

[0046] Figure 2 (f-j) are energy dispersive spectroscopy (EDS) images of the present application;

[0047] Figure 3 (a) is a plot of the degradation of methylene blue for 10 mg L -1 Methylene blue degradation curve;

[0048] Figure 3 (b) is a plot of the first order kinetics curve of the present application;

[0049] Figure 3 (c) is a plot of the kinetic constant distribution of the present application;

[0050] Figure 3 (d) is a plot of the cyclic degradation of methylene blue experiment of the present application;

[0051] Figure 4 (a) is a plot of the degradation of methylene blue for different mass of the present application;

[0052] Figure 4 (b) is a plot of the first order kinetics curve of the present application;

[0053] Figure 4 (c) is a plot of the kinetic constant distribution of the present application;

[0054] Figure 5 (a) is a plot of the degradation of methylene blue for different solution pH of the present application;

[0055] Figure 5 (b) is a plot of the first order kinetics curve of the present application;

[0056] Figure 5 (c) is a plot of the kinetic constant distribution of the present application. DETAILED DESCRIPTION

[0057] The technical solutions in the embodiments of the present application will be apparently and completely described below with the accompanying drawings of the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all the other embodiments obtained by those skilled in the art without creative work belong to the protection scope of the present application.

[0058] The present application discloses a preparation method of an Er-doped CdIn2S4 photocatalyst for photocatalytic degradation of methylene blue, comprising the following steps:

[0059] Step 1: Preparation of CdIn2S4

[0060] Step 1.1: 1 mmol of cadmium chloride (CdCl2), 2 mmol of indium trichloride tetrahydrate (InCl3·4H2O) and 4 mmol of thioacetamide (TAA) were added to 60 mL of deionized water, and continuously stirred at room temperature at a speed of 600 rpm for 1 hour until completely dissolved;

[0061] Step 1.2: Transferred to a 100 mL polytetrafluoroethylene-lined stainless steel reaction kettle, hydrothermal reaction at 180°C for 12 hours;

[0062] Step 1.3: The precipitate was collected by centrifugation at 8000 rpm, washed with deionized water and anhydrous ethanol for five times respectively, and dried at 80°C for 12 hours;

[0063] Step 1.4: After grinding, the CdIn2S4 photocatalyst was obtained;

[0064] Step 2: Preparation of Er-CdIn2S4

[0065] Step 2.1: 1 mmol of cadmium chloride (CdCl2), 2 mmol of indium trichloride tetrahydrate (InCl3·4H2O) and 4 mmol of thioacetamide (TAA) were added to 60 mL of deionized water, and 3%, 5% and 7% of Er(NO3)3·5H2O (corresponding to 0.06 mmol, 0.10 mmol and 0.14 mmol) of InCl3·4H2O were added respectively, continuously stirred at room temperature at a speed of 600 rpm for 1 hour until completely dissolved;

[0066] Step 2.1: The mixed solution was transferred to a 100 mL polytetrafluoroethylene-lined stainless steel reaction kettle, and hydrothermal reaction was carried out at 180°C for 12 hours;

[0067] Step 2.2: The precipitate was collected by centrifugation at 8000 rpm, washed with deionized water and anhydrous ethanol for five times respectively, and finally dried at 80°C for 12 hours;

[0068] Step 2.3: After grinding, three samples with different doping concentrations of 3% Er-CdIn2S4 (3% Er-CIS), 5% Er-CdIn2S4 (5% Er-CIS) and 7% Er-CdIn2S4 (7% Er-CIS) were obtained.

[0069] Example 1: Structure and morphology characterization of 5% Er-CIS composite

[0070] Firstly, the crystal structure of the 5% Er-CIS composite was analyzed by X-ray diffraction (XRD).

[0071] As Figure 1(a) shows that the XRD pattern of Er-CIS sample is consistent with that of CIS sample, and both are consistent with the standard card (PDF #27-0060) of cubic phase CdIn2S4.

[0072] The diffraction peaks of pure CdIn2S4 at 2θ = 14.1°, 23.2°, 27.2°, 28.5°, 33.0°, 43.3°, 47.4°, 49.7° and 55.5° correspond to crystal faces (111), (220), (311), (222), (400), (511), (440), (531) and (533), respectively.

[0073] With the increase of Er doping concentration, the diffraction peaks in the diffraction pattern show a gradual shift to high angle direction, which is due to the substitution of Er 3+ Substitution of Cd 2+ or In 3+ site, resulting in a decrease in lattice constant. The shift of diffraction peaks confirms the successful doping of Er element into CdIn2S4.

[0074] X-ray photoelectron spectroscopy (XPS) analysis results clearly confirm the presence of Er element in the sample.

[0075] As Figure 1 (b) shows that the characteristic peak of Er is detected in the binding energy range of 0-1200eV, which directly proves that Er has been successfully doped into the CdIn2S4 crystal structure. The results of XRD and XPS show that the composite material is successfully synthesized.

[0076] Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to systematically characterize the morphology and structure of pure CdIn2S4 and 5% Er-CIS composite material.

[0077] As Figure 2 (a) shows that pure CdIn2S4 is a microspherical structure assembled by irregular nanosheets and small blocks, with uniform particle size distribution and an average diameter of about 5μm.

[0078] As Figure 2 (b) shows that after 5% Er doping, the composite material still maintains similar microspherical morphology, and Er doping does not change the basic morphology characteristics of CdIn2S4.

[0079] In Figure 2 (c), it can be seen that the composite is tightly stacked by nanosheets and small blocks, and the stacking of nanoscale structure is more compact.

[0080] As Figure 2(d) As shown, the lattice fringes of 5% Er-CIS are clearly visible, and the lattice spacing is measured to be 0.1755 nm, which is smaller than that of the undoped sample, indicating that Er is successfully doped into the lattice of CdIn2S4, causing lattice shrinkage.

[0081] Energy dispersive spectroscopy (EDS) as shown in (e-j) shows that there are S, In, Cd and Er elements in the composite material 5% Er-CIS, and the Er element is uniformly distributed on the material, further confirming that the Er element is successfully doped into the lattice of CdIn2S4, and the 5% Er-CIS composite photocatalyst is successfully synthesized. Figure 2

[0082] Example 2: Performance of 5% Er-CIS composite in degrading methylene blue

[0083] The performance of Er-CdIn2S4 photocatalyst was evaluated by degrading methylene blue using a 300W xenon lamp with a cutoff filter (λ≥420nm) as a visible light source.

[0084] A 10mg / L methylene blue simulated dye wastewater with pH 7.0 was prepared, 100mL of the simulated wastewater was added to a 200mL reactor, and 30mg of photocatalyst was used to degrade methylene blue at room temperature, the solution was stirred in the dark for 60min to balance adsorption, and then the photocatalytic reaction was carried out under a 300W xenon lamp and a 420nm cutoff filter for 3 hours, and the absorbance was measured every 30min.

[0085] The remaining pollutants in the filtered methylene blue degradation solution were analyzed by ultraviolet-visible spectrophotometer (λ=663nm). (Before measuring the absorbance, the sample was filtered with a 0.45μm syringe filter).

[0086] As a preferred embodiment of the present application, the degradation rate of methylene blue is calculated according to the following formula:

[0087] D(%) = [(C0-C t ) / C0]x100%

[0088] Wherein, C0is the initial concentration of methylene blue, C t is the concentration of methylene blue after degradation.

[0089] First, in order to evaluate the photocatalytic activity of the composite material Er-CdIn2S4, we added 30mg of catalyst to 100mL of 10mg / L -1 methylene blue solution.

[0090] As Figure 3 ​(a) shows that after 60 min of adsorption equilibrium, the methylene blue adsorption removal rates of CIS, 3% Er-CIS, 5% Er-CIS and 7% Er-CIS are 5.92%, 4.08%, 5.74% and 5.51%, respectively.

[0091] When visible light irradiation is performed for 180 min, the methylene blue degradation rates of CIS, 3% Er-CIS, 5% Er-CIS and 7% Er-CIS are 68.61%, 81.09%, 92.36% and 81.58%, respectively, and the degradation of methylene blue mainly depends on the photocatalytic process rather than physical adsorption.

[0092] When the doping amount increases from 3% to 5%, the degradation rate is significantly improved, because the appropriate amount of Er doping promotes the effective separation of photo-generated carriers, widens the absorption range of visible light, and thus enhances the photocatalytic activity.

[0093] However, when the doping amount increases from 5% to 7%, the degradation rate decreases, which is due to the fact that too much Er element doping blocks the active sites on CdIn2S4, hindering the adsorption of reactants and the catalytic process. The photocatalytic degradation effect of 5% Er-CIS is the best, and the degradation rate (92.36%) is 1.35 times that of pure CdIn2S4.

[0094] Next, the degradation kinetics is studied, and the degradation first-order kinetics straight line is obtained, as shown in Figure 3 (b), the slope of the first-order kinetics straight line of 5% Er-CIS is the largest, indicating that 5% Er-CIS has the highest degradation efficiency.

[0095] In order to more intuitively observe the slope change, the kinetic constant column chart is made, as shown in Figure 3 (c), it can be seen that the kinetic constants of the catalysts after compounding are all larger than those of the monomers.

[0096] The kinetic constants of CIS, 3% Er-CIS, 5% Er-CIS and 7% Er-CIS are 0.00594 min -1 , 0.00881 min -1 , 0.01348 min -1 and 0.00896 min -1 , respectively, and the kinetic constant of 5% Er-CIS is 2.27 times that of the monomer CdIn2S4 (0.00594 min -1 ).

[0097] In summary, the appropriate amount of Er element doped in CdIn2S4 promotes the effective separation of photo-generated carriers, widens the absorption range of visible light, and thus greatly improves the photocatalytic efficiency.

[0098] In addition, the stability of the 5% Er-CIS composite was also tested, as shown in Figure 3 (d), it can be seen that the degradation rate of the 5% Er-CIS composite decreased by only 4% after five cycles, indicating that the 5% Er-CIS has good stability.

[0099] Next, the effect of the mass of the 5% Er-CIS composite catalyst on the degradation performance of the methylene blue solution was studied, as shown in -1 Fig. 4.

[0100] As shown in Figure 4 (a), when the catalyst mass was 10 mg, 20 mg, 30 mg, 40 mg, and 50 mg, the photocatalytic degradation efficiency of the 5% Er-CIS on methylene blue was 58.58%, 89%, 92.36%, 91.05%, and 90.01%, respectively, and the degradation rate of the 10 mg L -1 methylene blue solution reached the highest when the catalyst mass was 30 mg.

[0101] When the catalyst mass increased from 10 mg to 30 mg, the degradation rate increased, because more 5% Er-CIS was dispersed in the solution, increasing the number of exposed active sites and enhancing the photocatalytic efficiency.

[0102] However, when the mass exceeded 30 mg (such as 40-50 mg), the degradation rate decreased, which was attributed to the decrease in active sites caused by excessive catalyst aggregation, and the enhanced scattering of light by the excess particles, which prevented the light from penetrating deep into the solution, reducing the utilization of light energy.

[0103] Therefore, 30 mg was the optimal catalyst mass, achieving a balance between the exposure of active sites and the efficiency of light utilization.

[0104] Then, the kinetics was studied, as shown in Figure 4 (b-c), it can be seen that the degradation process of the composite conforms to the first-order kinetics curve, and the catalyst mass of 30 mg showed the highest kinetic constant (k = 0.01348 min -1 ), which was 2.9 times, 1.1 times, 1.05 times, and 1.06 times that of the catalyst mass of 10 mg (0.00465 min -1 ), 20 mg (0.01228 min -1 ), 40 mg (0.01277 min -1 ), and 50 mg (0.01266 min -1 ), respectively.

[0105] The pH of the solution affects the surface charge state of the 5% Er-CIS, thereby affecting its ability to adsorb and catalytically degrade methylene blue (MB).

[0106] As Figure 5 (a) shown, 30 mg of 5% Er-CIS catalyst was added to 100 mL of 10 mg L -1 Methylene blue solution was tested, and it can be seen that when the initial pH value of the solution is 3, 5, 7, 8, 9 and 11, the corresponding photocatalytic degradation efficiency is 77.62%, 87.66%, 92.36%, 98.96% and 99.60% respectively.

[0107] The experimental results show that under strong alkaline conditions of pH = 11, the degradation efficiency of the catalyst on the methylene blue solution is the highest.

[0108] The change of the degradation efficiency of 5% Er-CIS under different pH values is mainly due to the synergistic effect of the charge state of the catalyst surface and the morphology of the MB molecule.

[0109] Since the MB molecule exists in the form of a cation in the solution, when in acidic conditions, the catalyst surface will adsorb H + + in water, which will carry a positive charge, and at this time, electrostatic repulsion will be generated with the MB cation, and due to the electrostatic repulsion effect, the adsorption of the catalyst on the pollutants is not conducive, thereby reducing the degradation efficiency;

[0110] When the solution is alkaline, the catalyst surface carries a negative charge, and due to the electrostatic adsorption effect, the adsorption of the catalyst is enhanced, thereby improving the degradation efficiency.

[0111] Then, the degradation process under different pH conditions was analyzed kinetically (b-c). Figure 5

[0112] The experimental data shows that when pH = 11, not only the degradation efficiency is the highest (99.60%), but also the reaction kinetic constant reaches the maximum value (k = 0.02483 min -1 ), which is 4.1 times, 3.03 times, 1.84 times and 1.1 times of that under the conditions of pH = 3 (0.00606 min -1 ), pH = 5 (0.00819 min -1 ), pH = 7 (0.01348 min -1 ) and pH = 9 (0.02271 min -1 ) respectively.

[0113] This kinetic result is highly consistent with the change trend of the degradation efficiency, and the degradation rate in the alkaline environment is more than 98.5%, which further proves that the alkaline environment has a significant promoting effect on the photocatalytic reaction.

[0114] The present application first prepares Er 3+ ​The introduction of ions forms new electron capture centers in CdIn2S4, effectively capturing and stabilizing photo-generated electrons, effectively inhibiting the recombination of photo-generated electron-hole pairs, and significantly prolonging the carrier lifetime; secondly, by doping Er in CdIn2S4, an intermediate energy level is introduced, which significantly widens the absorption range of the catalyst in the visible light region, especially enhances the light absorption capacity in the long wavelength region, and greatly improves the solar utilization rate; finally, the surface of the modified catalyst exposes more adsorption sites and active sites, providing sufficient adsorption and reaction sites for methylene blue molecules. The Er-CdIn2S4 photocatalyst of the application is prepared by a simple one-step hydrothermal method, has the advantages of simple process, low cost and easy reuse, and has good application prospect in the field of dye wastewater treatment.

[0115] The above is only a preferred embodiment of the application, and does not limit the technical scope of the application in any way, so any slight modification, equivalent change and modification of the above embodiment according to the technical essence of the application still belongs to the scope of the technical solution of the application.

Claims

1. A method for preparing an Er-doped CdIn2S4 photocatalyst for photocatalytic degradation of methylene blue, characterized in that: The steps include: Step 1: Preparation of CdIn2S4 Step 1.1: Add cadmium chloride, indium trichloride tetrahydrate, and thioacetamide to deionized water and stir magnetically until completely dissolved. Step 1.2: Transfer to a polytetrafluoroethylene-lined stainless steel reactor for hydrothermal reaction; Step 1.3: Collect the precipitate by centrifugation, wash, and dry; Step 1.4: After grinding, the CdIn2S4 photocatalyst is obtained; Step 2: Preparation of Er-CdIn2S4 Step 2.1: Add cadmium chloride, indium trichloride tetrahydrate, and thioacetamide to deionized water, and add Er(NO3)3·5H2O at 3%, 5%, and 7% of the molar amount of indium trichloride tetrahydrate, respectively, and stir magnetically until completely dissolved; Step 2.2: Transfer the mixed solution to a polytetrafluoroethylene-lined stainless steel reactor for hydrothermal reaction; Step 2.3: Collect the precipitate by centrifugation, wash, and dry; Step 2.4: After grinding, three samples with different doping concentrations of 3% Er-CdIn2S4, 5% Er-CdIn2S4 and 7% Er-CdIn2S4 were obtained.

2. The method for preparing an Er-doped CdIn2S4 photocatalyst for photocatalytic degradation of methylene blue according to claim 1, characterized in that: In the step 1.1, the amount of cadmium chloride is 1 mmol, the amount of indium trichloride tetrahydrate is 2 mmol, the amount of thioacetamide is 4 mmol, and the amount of deionized water added is 60 mL.

3. The method for preparing an Er-doped CdIn2S4 photocatalyst for photocatalytic degradation of methylene blue according to claim 1, characterized in that: In the steps 1.1 and 2.1, magnetic stirring was continued at room temperature at a rotation speed of 600 rpm for 1 hour.

4. The method for preparing an Er-doped CdIn2S4 photocatalyst for photocatalytic degradation of methylene blue according to claim 1, characterized in that: In the steps 1.2 and 2.2, the volume of the polytetrafluoroethylene-lined stainless steel reactor is 100 mL, the temperature of the hydrothermal reaction is 180° C., and the time of the hydrothermal reaction is 12 h.

5. The method for preparing an Er-doped CdIn2S4 photocatalyst for photocatalytic degradation of methylene blue according to claim 1, characterized in that: In the steps 1.3 and 2.3, the centrifugal speed is 8000 rpm, and the samples are washed five times with deionized water and five times with anhydrous ethanol, respectively.

6. The method for preparing an Er-doped CdIn2S4 photocatalyst for photocatalytic degradation of methylene blue according to claim 1, characterized in that: In the step 1.3 and the step 2.3, the drying temperature is 80° C. and the drying time is 12 h.

7. The method for preparing an Er-doped CdIn2S4 photocatalyst for photocatalytic degradation of methylene blue according to claim 1, characterized in that: In the step 2.1, the amount of cadmium chloride is 1 mmol, the amount of indium trichloride tetrahydrate is 2 mmol, the amount of thioacetamide is 4 mmol, and the amount of deionized water added is 60 mL.

8. The method for preparing an Er-doped CdIn2S4 photocatalyst for photocatalytic degradation of methylene blue according to claim 1, characterized in that: In the step 2.1, 0.06 mmol, 0.10 mmol and 0.14 mmol of Er(NO 3 ) 3 ·5H 2 O were added respectively according to 3%, 5% and 7% of the molar amount of indium trichloride tetrahydrate.

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