A carbon-based sulfide heterojunction composite photocatalyst and a preparation method and application thereof

Abstract

The application discloses a preparation method of a carbon-based sulfide heterojunction composite light, a catalyst and application thereof; the preparation method comprises the following steps: (1) adding a carbon source and a doping source into a solvent, and reacting under the condition of 160-220 DEG C to obtain nitrogen and sulfur co-doped graphene quantum dots; (2) adding the GQDs into anhydrous ethanol, uniformly mixing under ultrasonic, then adding a tin compound and a sulfur-containing compound, and reacting under the condition of 160-180 DEG C to obtain GQDs / SnS2; (3) adding the GQDs / SnS2 into a graphene oxide aqueous solution, reacting to reduce the GO into rGO, and forming a 2D network structure, meanwhile, the GQDs / SnS2 nanosheet composite material is loaded on a graphene sheet layer; the catalyst has high specific surface area and high conductivity, can effectively inhibit the recombination of photo-generated electrons and holes, widen the light absorption range, improve the photocatalytic activity, and can be used for degrading heavy metals or dyes.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of photocatalysis, in particular to a carbon-based sulfide heterojunction composite photocatalyst and a preparation method and application thereof. BACKGROUND

[0002] In the past decade, with the rapid development of human economy and the rapid growth of energy resources, humanity will face the development problems of energy crisis and environmental pollution. Solving these two problems requires developing new sustainable energy and protecting the environment as the fundamental point. The wastewater discharged in the process of industrial development contains heavy metal pollutants such as cadmium, chromium, lead and other heavy metals. Such wastewater will harm the growth of plants and animals and human health during the discharge process, and has a high carcinogenic possibility, seriously endangering human health.

[0003] At present, the methods for treating heavy metal industrial wastewater include adsorption, chemical oxidation-reduction, photocatalyst degradation, etc. The adsorption method takes a long time to treat heavy metal wastewater and does not meet the discharge standard. The chemical oxidation-reduction method consumes a lot of energy to treat wastewater and may produce toxic by-products, causing secondary pollution to the environment. Photocatalytic degradation is a green and environmentally friendly new energy technology that uses inexhaustible solar energy to photocatalytically degrade heavy metals in wastewater.

[0004] At present, the primary problem of photocatalytic degradation of organic pollutants and heavy metal pollutants is to develop high-efficiency and stable photocatalysts. SnS2 is a layered semiconductor material and a direct band gap semiconductor compound. The band gap of SnS2 is 2.0-2.2 eV, which has a good response to visible light and can efficiently utilize sunlight, making it a very potential visible light-driven photocatalyst for efficient absorption of solar energy. However, pure SnS2 nanosheets have the disadvantages of low separation efficiency of photo-generated electron-hole pairs, fast recombination, and short lifetime, resulting in low photocatalytic efficiency. The preparation of SnS2-based heterojunction composite materials can promote the separation of carriers and improve the photocatalytic activity.

[0005] As a new type of 0D carbon material, graphene quantum dots (GQDs) generally have a size of less than 10 nm, usually exhibit excellent photoelectric properties, and also exhibit quantum confinement effect and boundary effect, thus giving rise to some new physical properties, and are widely used in the field of photocatalysis. Semiconductor photocatalysts modified by GQDs usually exhibit more excellent photocatalytic performance. For example, ZnO / GQDs and Zn-BiVO4 / GQDs composite materials exhibit fast electron separation efficiency, and can improve the efficiency of photocatalytic degradation of organic pollutants and heavy metal pollutants under sunlight. In addition, studies have shown that nitrogen and sulfur heteroatoms doped GQDs can significantly improve the fluorescence properties of quantum dots, and can maximize the photocatalytic activity of semiconductor photocatalysts. Therefore, after GQDs doped with nitrogen and sulfur heteroatoms are combined with SnS2, the photocatalytic activity of SnS2 will be significantly improved.

[0006] Two-dimensional graphene is widely used in the field of photoelectrocatalysis due to its unique porous structure, high specific surface area, excellent mechanical properties and multi-dimensional electron transmission path. When semiconductor nanomaterials are loaded onto graphene sheets, photo-generated electrons can enter graphene when the material is irradiated with light, which can effectively prevent the recombination of photo-generated electrons and holes, thereby improving the photocatalytic efficiency. Compared with undoped graphene, heteroatom-doped graphene has more excellent electron conduction capacity and light absorption performance, which is conducive to further improving the photocatalytic efficiency.

[0007] Although the application of two-dimensional graphene loaded with semiconductors in the field of photocatalysis has made certain progress, few people have reported the controllable preparation of two-dimensional ternary composite heterojunction materials as multifunctional catalysts for photodegradation of heavy metal pollutants, especially the formation of a heterojunction composite material by doping GQDs and SnS2, and then loading the ternary composite heterojunction material onto doped 2D graphene for photodegradation of heavy metal pollutants. SUMMARY

[0008] The purpose of the present application is to provide a carbon-based sulfide heterojunction composite photocatalyst, a preparation method and application thereof, to solve the problems raised in the above background.

[0009] To achieve the above purpose, the present application provides the following technical solution: a preparation method of a carbon-based sulfide heterojunction composite photocatalyst, comprising the following steps:

[0010] (1) adding a carbon source and a doping source into a solvent, and reacting at 160-220 ℃ to obtain nitrogen and sulfur co-doped graphene quantum dots;

[0011] (2) GQDs is added into anhydrous ethanol, and after ultrasonic mixing, tin compound and sulfur-containing compound are added, and the reaction is carried out at 160-180°C to obtain GQDs / SnS2;

[0012] (3) GQDs / SnS2 is added into a graphene oxide aqueous solution, and the reaction makes GO reduced into rGO and forms a 2D network structure, and meanwhile, GQDs / SnS2 nanosheet composite material is loaded on the graphene sheet layer.

[0013] As preferred, the solvent of step (1) is one or several of water and alcohol solvents;

[0014] The carbon source in step (1) is citric acid, and the molar ratio of the carbon source to the doping source is calculated according to a molar ratio of 1.1 of citric acid to thiourea;

[0015] The doping source in step (1) is one or several of thiourea, thiosemicarbazide and melamine.

[0016] As preferred, the reaction temperature in step (1) is 220°C, the reaction time in step (1) is 8-12h, and the reaction needs to be stirred and the stirring time is 30 min.

[0017] As preferred, the sulfur-containing compound in step (2) is one or several of thiourea, thiosemicarbazide, dimethylthiourea, thioacetamide, thioformamide, thioacetic acid and thiomorpholine;

[0018] The amount of GQDs in step (2) is calculated according to 300-600 ml of anhydrous ethanol per gram (g) of GQDs;

[0019] The molar ratio of tin element in the tin compound to sulfur element in the sulfur-containing compound in step (2) is 1:2;

[0020] The mass ratio of GQDs to tin compound in step (2) is 0.1-0.2:1.745.

[0021] As preferred, the ultrasonic time in step (2) is 60 min, the reaction time in step (2) is 6h, and the reaction needs to be stirred and the stirring time is 60 min.

[0022] As preferred, the reaction process in step (3) needs a crosslinking agent and a reducing agent, and the crosslinking agent and the reducing agent are L-cysteine;

[0023] The ultrasonic time in step (3) is 15 min; the reaction temperature in step (3) is 95°C; and the reaction time in step (3) is 3h.

[0024] As preferred, the centrifugal washing is carried out by using deionized water and anhydrous ethanol in sequence.

[0025] A carbon-based sulfide heterojunction composite photocatalyst is prepared according to the method.

[0026] The application of a carbon-based sulfide heterojunction composite photocatalyst, and the application of the carbon-based sulfide heterojunction composite photocatalyst in degrading heavy metals or dyes.

[0027] As preferred, the heavy metal is chromium, and the photocatalytic material has good photocatalytic degradation performance on potassium dichromate solution under visible light with a wavelength greater than 420 nm.

[0028] The application of the carbon-based sulfide heterojunction composite photocatalyst in degrading heavy metals is that the carbon-based sulfide heterojunction composite photocatalyst is added into a solution containing hexavalent chromium ions (Cr6+) and having a pH value of 3-11, and a reaction is carried out under light conditions, so that high-valence chromium (hexavalent chromium) is reduced to low-valence chromium (trivalent chromium), so as to realize the degradation of heavy metal chromium.

[0029] The solution containing hexavalent chromium ions (Cr6+) is preferably a potassium dichromate solution.

[0030] The amount of the carbon-based sulfide heterojunction composite photocatalyst is preferably calculated according to a final concentration of 1-2 mg / ml in the reaction system.

[0031] The concentration of the potassium dichromate solution is preferably 3 mg / L-140 mg / L, further preferably 3-100 mg / L, and more preferably 3-10 mg / L.

[0032] The pH value is 7.

[0033] The light condition is a visible light condition, such as a xenon lamp irradiation condition.

[0034] The reaction time is preferably 1-2 h.

[0035] Compared with the prior art, the present application has the following beneficial effects:

[0036] (1) The present application aims at improving the shortcomings of the existing photocatalytic technology, especially solving the problems of the insufficient photocatalytic performance of SnS2, and designs a preparation method and application of rGO@GQDs / SnS2 heterojunction composite photocatalyst material through the existing semiconductor photocatalyst structure and regulation, which can be used for visible light degradation of heavy metal chromium in wastewater to reduce it to low-toxic trivalent chromium, and solves the problems of SnS2 photoelectron hole rapid recombination, and improves the efficiency of reducing high-valence chromium to low-toxic chromium.

[0037] (2) The present application controls the ratio of S and Sn, selects a suitable sulfur source, and obtains GQDs / SnS2 composite in a polyvinyl fluoride reaction kettle under suitable solvents for several hours, then adds graphene oxide, and selects a suitable strong reducing agent to obtain rGO@GQDs / SnS2 heterojunction composite material with good photocatalytic performance under visible light. (3) The present application uses a hydrothermal method and a low-temperature water bath method to prepare a high-efficiency carbon-based flower-shaped structure heterojunction composite sulfide photocatalyst, which has better stability compared with other transition metal catalysts, greatly improves the response to visible light, can more efficiently utilize solar energy, overcomes the utilization of only 5% of the energy in sunlight by other catalysts, accelerates the rapid separation of photoelectron holes, inhibits the recombination of photoelectron holes, prolongs their life, and improves the photocatalytic performance and stability.

[0038] (4) The present application finds that rGO@GQDs / SnS2 has good degradation effect on heavy chromium solution under neutral pH condition, and finds that a small amount of photocatalyst can also have good oxidation and reduction effect on potassium dichromate wastewater when degrading potassium dichromate solution with different doses of photocatalyst.

[0039] (5) The rGO@GQDs / SnS2 photocatalyst prepared in this invention can be used to degrade potassium dichromate solution with high concentration. The photocatalytic degradation method of potassium dichromate is to use semiconductor composite material to generate photogenerated electron-hole pairs under visible light irradiation (xenon lamp irradiation). By accelerating the separation and transfer of electrons and holes, the photogenerated electrons are used to reduce hexavalent chromium in potassium dichromate solution to trivalent chromium with low or even non-toxicity. In other words, light energy is converted into chemical energy to drive the composite semiconductor photocatalyst to reduce heavy metal chromium in the redox process. This solves the energy consumption problem in the process of reducing heavy metals. The raw materials used are readily available, the requirements for use are low, and it is environmentally friendly and has the advantage of sustainable utilization. It can be applied to the fields of heavy metal reduction (heavy metal ions) and dye (including organic dyes) degradation.

[0040] (6) The experimental method of the present invention is a hydrothermal method. The raw material is dissolved in a solvent in a hydrothermal reactor and reacted at 160-220 °C to obtain uniform nanoparticles. This experiment has the advantages of being simple, convenient and safe, using inexpensive and readily available raw materials, and having low energy consumption, and is therefore suitable for mass production. Attached Figure Description

[0041] Figure 1 The image shows a SEM image of the carbon-based flower-like sulfide heterojunction photocatalyst (rGO@GQDs / SnS2) prepared in Example 1.

[0042] Figure 2 This is a TEM image of the carbon-based flower-like sulfide heterojunction photocatalyst (rGO@GQDs / SnS2) prepared in Example 1.

[0043] Figure 3 The light absorption spectra of the SnS2, GQDs / SnS2 and rGO@GQDs / SnS2 photocatalysts in the examples are shown.

[0044] Figure 4 The band gap diagrams are for the SnS2, GQDs / SnS2 and rGO@GQDs / SnS2 photocatalysts in the examples.

[0045] Figure 5 The diagram shows the activity of the SnS2, GQDs / SnS2 and rGO@GQDs / SnS2 photocatalysts in the examples for the photocatalytic reduction of potassium dichromate under neutral conditions.

[0046] Figure 6 The diagram shows the kinetics of the photocatalytic reduction of potassium dichromate by the SnS2, GQDs / SnS2 and rGO@GQDs / SnS2 photocatalysts in the examples under neutral conditions. Detailed Implementation

[0047] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0048] Please see Figures 1-6 This invention provides a technical solution: a method for preparing a carbon-based sulfide heterojunction composite photocatalyst, comprising the following steps:

[0049] (1) Preparation of graphene quantum dots (GQDs)

[0050] ① Dissolve 2.52g of citric acid and 2.76g of thiourea in 100mL of deionized water, stir until dissolved to form a homogeneous solution, then transfer to a 150mL hydrothermal reactor and react at 180℃ for 6 hours. After the reaction is complete, cool to room temperature, remove the solvent using a rotary evaporator, add anhydrous ethanol to the residue, and then centrifuge at 10000r / min for 5min to obtain nitrogen and sulfur co-doped graphene quantum dots;

[0051] (2) Preparation of GQDs / SnS2

[0052] The GQDs obtained in step (1) ① were added to anhydrous ethanol and ultrasonically mixed. Then, tin compounds and sulfur-containing compounds were added and stirred until homogeneous. The mixture was then reacted at 160–200 °C. After the reaction was completed, the mixture was centrifuged, washed, and dried to obtain GQDs / SnS2.

[0053] (3) Preparation of photocatalyst rGO@GQDs / SnS2

[0054] The GQDs / SnS2 obtained in step (2) was ultrasonically dispersed in an aqueous solution of graphene oxide (GO), and then a crosslinking agent and a reducing agent were added. GO was reduced to rGO by low-temperature heating reaction. At the same time, the GQDs / SnS2 nanosheet composite material was loaded on the graphene sheets. After the reaction was completed, the mixture was centrifuged, washed, and dried to obtain rGO@GQDs / SnS2 heterojunction composite photocatalyst with a 2D network structure.

[0055] The above-mentioned method for preparing medium-sized graphene oxide is as follows: Add 0.5g of sodium nitrate and 23mL of concentrated sulfuric acid to a clean 250mL three-necked flask, and mechanically stir under an ice-water bath; after stirring until the sodium nitrate is completely dissolved, add 1g of graphite powder, then slowly add 3g of potassium permanganate. After the addition is complete, maintain the temperature in a 35℃ water bath; after 1 hour, rapidly add 46mL of deionized water while stirring slowly, raise the temperature to 98℃, maintain for 15min, and then slowly add 170mL of deionized water and 5mL of... 30wt% hydrogen peroxide; after stirring for 1 hour, remove the water bath and allow it to cool naturally to room temperature, then collect the reaction solution; centrifuge the reaction solution, pour off the supernatant, wash with deionized water, centrifuge again, and repeat this process until the pH of the supernatant is neutral, then pour off the supernatant, wash once more with ethanol, centrifuge, collect the lower precipitate, place it in a vacuum oven, and dry it at 45℃. After drying, the product graphene oxide is obtained; the concentration of the graphene oxide aqueous solution is 2 mg / mL.

[0056] A carbon-based sulfide heterojunction composite photocatalyst is prepared by any of the methods described above.

[0057] The application of the carbon-based sulfide heterojunction composite photocatalyst in the degradation of heavy metals or dyes.

[0058] The heavy metal in question is chromium, and the photocatalytic material exhibits excellent photocatalytic degradation performance of potassium dichromate solution under visible light with a wavelength greater than 420 nm.

[0059] The application of the carbon-based sulfide heterojunction composite photocatalyst in the field of heavy metal degradation involves adding the carbon-based sulfide heterojunction composite photocatalyst to a solution containing hexavalent chromium ions (Cr6+) with a pH value of 3-11, and reacting under light conditions to reduce high-valent chromium (hexavalent chromium) to low-valent chromium (trivalent chromium) in order to achieve the degradation of heavy metal chromium.

[0060] The solution containing hexavalent chromium ions (Cr6+) is preferably a potassium dichromate solution.

[0061] The amount of the carbon-based sulfide composite photocatalyst is calculated based on a final concentration of 1-2 mg / ml in the reaction system; preferably, it is calculated based on a final concentration of 1.33 mg / ml in the reaction system; the concentration of the potassium dichromate solution is 3 mg / L-20 g / L; preferably 3 mg / L-140 mg / L; more preferably 3-100 mg / L; even more preferably 3-10 mg / L; the pH value is preferably 3-7; more preferably 7; the illumination conditions are visible light conditions, such as xenon lamp irradiation; the reaction time is preferably 1-2 hours.

[0062] Example 1

[0063] (1) Preparation of graphene quantum dots (GQDs)

[0064] 2.52 g of citric acid and 2.76 g of thiourea were dissolved in 100 mL of deionized water and stirred until dissolved to form a homogeneous solution. The solution was then transferred to a 150 mL hydrothermal reactor and reacted at 180 °C for 6 hours. After the reaction was completed, the mixture was cooled to room temperature, and the solvent was removed using a rotary evaporator. Anhydrous ethanol was added to the residue, and the mixture was then centrifuged at 10,000 r / min for 5 min to obtain nitrogen- and sulfur-doped graphene quantum dots.

[0065] (2) Preparation of GQDs / SnS2 heterojunction composite photocatalyst:

[0066] 0.002 g of GQDs was added to 20 ml of anhydrous ethanol and sonicated for 1 h. Then, 1.745 g of tin tetrachloride pentahydrate and 0.7513 g of thioacetamide were weighed and added to the solution. 40 ml of anhydrous ethanol was added and the mixture was magnetically stirred for 1 h. The solution was then placed in a reaction vessel and reacted at 160 °C for 6 h. The mixture was then washed with deionized water and alcohol by centrifugation and dried at 60 °C overnight to collect the GQDs / SnS2.

[0067] (3) Preparation of control material SnS2

[0068] SnS2 was prepared under the experimental conditions of step (2) above, without the addition of GQDs / SnS2;

[0069] (4) Preparation of graphene oxide (GO) solution

[0070] The preparation method of graphite oxide was based on the Hummurs method (W.S. Hummers Jr., R.E. Offeman, J.Am. Chem. Soc. 80(6)(1958). 1339-1339.), with appropriate modifications. First, graphite oxide was prepared from natural graphite powder, and then the graphite oxide was exfoliated into uniformly dispersed GO sheets in deionized water by ultrasonic dispersion. The specific experimental steps for preparing graphite oxide are as follows: Add 46 mL of concentrated sulfuric acid (98 wt% H2SO4) to a three-necked reaction flask and place the reaction flask in an ice-water bath with vigorous stirring. Then weigh 1 g of NaNO3 and add it to the reaction flask. Continue stirring in the ice-water bath for 10 min until the NaNO3 is completely dissolved. Then weigh 2 g of natural graphite powder and add it. Keep the ice-water bath constant. Weigh 6 g of KMnO4 and add it slowly in batches, keeping the reaction temperature below 5°C. After the addition is complete, continue stirring in the ice-water bath for 30 min until the temperature no longer rises. Then remove the ice-water bath and place the reaction flask in a water bath at 35±3℃ for 1 hour. The reaction solution will be viscous and dark green. Slowly add 92 mL of deionized water, raise the temperature of the reaction system, and control the temperature not to exceed 98℃. React in a 98℃ oil bath for 15 minutes. While still hot, add 300 mL of deionized water, remove the oil bath, and wait for the reaction solution to cool to room temperature. Add 10 mL of 30% H2O2 (slowly), continue stirring for 1 hour, and then centrifuge at 5000 r / min. Discard the supernatant, and wash the lower precipitate with 10 wt% HCl solution and centrifuge. Repeat the washing and centrifugation 10 times. Finally, wash and centrifuge with deionized water until the pH of the supernatant is neutral. Dry the obtained precipitate in an oven at 80℃ overnight to obtain solid graphite oxide.

[0071] The aqueous dispersion of GO was obtained by dispersing graphite oxide in water and then sonicating it. The specific experimental steps are as follows: 200 mg of graphite oxide was weighed and dispersed in 100 mL of deionized water, and sonicated for 1 hour to obtain a brownish-red GO dispersion with a GO concentration of 2 mg / mL.

[0072] (5) Preparation of rGO@GQDs / SnS2 heterojunction composite photocatalyst

[0073] 10 mL of GO solution (2 mg / mL) was placed in a 20 mL glass bottle, 200 mg of L-cysteine ​​was added, and the solution was stirred until dissolved. Then, 10 mg of the GQDs / NiFe2O4 heterojunction composite photocatalyst prepared in step 3 was added. The bottle was then sonicated for 15 min to ensure that GO, L-cysteine, and GQDs / SnS2 were fully adsorbed and evenly dispersed in the system. The bottle was then placed in a 95 °C oil bath for 3 hours. After the reaction, the solution was washed with deionized water and alcohol by centrifugation, dried at 60 °C overnight, and the rGO@GQDs / SnS2 heterojunction composite photocatalyst was collected.

[0074] Example 2

[0075] The difference between this embodiment and Embodiment 1 is that when synthesizing the GQDs / SnS2 heterojunction composite material using the in-situ synthesis method, the amount of GQDs added is 0.0005g, while the other steps are exactly the same.

[0076] Example 3

[0077] The difference between this embodiment and Embodiment 1 is that when synthesizing the GQDs / SnS2 heterojunction composite material using the in-situ synthesis method, the amount of GQDs added is 0.001g, while the other steps are exactly the same.

[0078] Example 4

[0079] The difference between this embodiment and Embodiment 1 is that when synthesizing the GQDs / SnS2 heterojunction composite material using the in-situ synthesis method, the amount of GQDs added is 0.0015g, while the other steps are exactly the same.

[0080] Example 5

[0081] The difference between this embodiment and Embodiment 1 is that when synthesizing the GQDs / SnS2 heterojunction composite material using the in-situ synthesis method, the amount of GQDs added is 0.0025g, while the other steps are exactly the same.

[0082] Effect Example

[0083] (1) The absorption spectra, band gaps and concentrations of the precursor (SnS2) and heterojunction composite photocatalyst (GQDs / SnS2) in Example 1, as well as the final heterojunction composite photocatalyst rGO@GQDs / SnS2, were tested using scanning electron microscopy (SEM) and ultraviolet-visible absorbance spectrophotometer (UV-Vis-NIR DRS).

[0084] The results are as follows Figures 1-4 As shown: From Figure 1 It can be seen that the synthesized heterojunction composite photocatalyst rGO@GQDs / SnS2 has a nanosheet flower-like structure with an aesthetically pleasing morphology; fromFigure 2 It can be seen that GQDs are uniformly loaded on the surface of SnS2 nanosheets; Figure 3 The light absorption spectrum in the image shows that rGO@GQDs / SnS2 has increased its absorption of visible light. Figure 4 The band gap diagram shows that the band gap of the composite rGO@GQDs / SnS2 is smaller, indicating that the absorption capacity of visible light is stronger and that it can effectively prevent the recombination of photogenerated carriers and improve the photocatalytic degradation performance.

[0085] (2) The photocatalyst rGO@GQDs / SnS2 prepared in Example 1 was used to photocatalytically reduce potassium dichromate under neutral conditions. The photocatalytic degradation performance was tested using visible light with a wavelength greater than 420 nm. That is, the photocatalytic material was driven by xenon lamp irradiation to generate electron-hole pair transitions and separations to degrade potassium dichromate. The specific experimental steps are as follows:

[0086] 1) Preparation of potassium dichromate solution (purity 99%) to be treated: The concentration during the experiment was 100 mg / L;

[0087] 2) Prepare 0.1M NaOH and HCl solutions respectively to adjust the pH value of the solutions; the pH value of the experiment is 7;

[0088] 3) Take the potassium dichromate solution to be treated (60 ml) and the rGO@GQDs / SnS2 photocatalytic material prepared in Example 2 (80 mg) into a photocatalytic double-walled beaker;

[0089] 4) Connect the condenser tube to the photocatalytic double-walled beaker, with the condensate entering from the bottom and exiting from the top, to carry out the dark reaction for 60 minutes. After 60 minutes of dark reaction, conduct the photo-reaction experiment using a 300W xenon lamp.

[0090] 5) Samples were taken every 10 minutes during the dark reaction (60 min) and light reaction (90 min) to measure the remaining concentration of the degraded solution using a UV-Vis spectrophotometer and plot the relevant images. The absorbance of the solution was recorded at a wavelength of 540 nm for data plotting. The remaining concentration of the degraded solution is proportional to the product of absorbance A and the concentration c of the absorbing substance and the optical path length b of the absorption cell, according to Beer's Law. When the unit of c is g / L and the unit of b is cm, then A = abc, and the proportionality constant a is called the absorption coefficient, with the unit L / g·cm. When the unit of c is mol / L and the unit of b is cm, then A = εbc, and the proportionality constant ε is called the molar absorption coefficient, with the unit L / mol·cm. Numerically, ε is equal to the product of a and the molar mass of the absorbing substance. After processing, the concentration of the remaining concentration can be obtained. Figure 5 .

[0091] 6) The experimental data obtained in 5) were processed and calculated using the first kinetic equation lnC0 / Ct=kt, where C0 is the solution concentration at dark adsorption equilibrium and Ct is the solution concentration at any time t during the illumination period. Figure 6 .

[0092] Figure 5 The figure shows the lnCt / C0 ratio of three photocatalysts, SnS2, GQDs / SnS2, and rGO@GQDs / SnS2, to reduce a 100 mg / L hexavalent chromium solution under the same experimental conditions. As can be seen from the figure, rGO@GQDs / SnS2 can completely reduce the hexavalent chromium solution of this concentration after 30 min of light irradiation, compared with the other two photocatalysts, exhibiting very good photocatalytic performance.

[0093] Figure 6 The first equation kinetic graphs of the reduction of 100 mg / L hexavalent chromium solution by three photocatalysts, SnS2, GQDs / SnS2, and rGO@GQDs / SnS2, are shown in the figure. From the R2 and k values ​​in the figure, it can be seen that the kinetic graphs of the three photocatalysts conform to the first equation of kinetics. At the same time, the reaction rate constant of rGO@GQDs / SnS2 is 205.1 times and 6.8 times that of SnS2 and GQDs / SnS2, respectively, which shows a very significant improvement effect.

[0094] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for preparing a carbon-based sulfide heterojunction composite photocatalyst, characterized by: The method comprises the following steps: (1) adding a carbon source and a doping source into a solvent to obtain nitrogen and sulfur co-doped graphene quantum dots (GQDs) by reaction at 160-220℃; the doping source is thiourea or thiosemicarbazide; (2) adding the GQDs into anhydrous ethanol, mixing uniformly by ultrasonic, adding a tin compound and a sulfur-containing compound, and performing reaction at 160-180℃ to obtain GQDs / SnS2; (3) dispersing the GQDs / SnS2 obtained in step (2) into a graphene oxide (GO) aqueous solution by ultrasonic, adding a crosslinking agent and a reducing agent, reducing the GO to rGO by low-temperature heating reaction, and loading the GQDs / SnS2 nanosheet composite material on the graphene sheet at the same time, centrifuging, washing and drying to obtain a rGO@GQDs / SnS2 heterojunction composite photocatalyst with a 2D network structure.

2. The method for preparing a carbon-based sulfide heterojunction composite photocatalyst according to claim 1, characterized in that: the solvent in step (1) is one or more of water, ethanol, ethylene glycol, glycerol, 1,2-propanediol, 1,3-propanediol, n-butanol and isobutyl alcohol; the carbon source in step (1) is one or more of citric acid, glucose and maltose, and the molar ratio of the carbon source to the doping source is 1:1-5.

3. The method for preparing a carbon-based sulfide heterojunction composite photocatalyst according to claim 1, characterized in that: the reaction temperature in step (1) is 180-220℃, and the reaction time in step (1) is 8-12h.

4. The method of claim 1, wherein the carbon-based sulfide heterojunction composite photocatalyst is prepared by the following steps: (1) preparing a carbon-based sulfide photocatalyst; (2) preparing a metal oxide photocatalyst; and (3) mixing the carbon-based sulfide photocatalyst and the metal oxide photocatalyst. the sulfur-containing compound in step (2) is one or more of thiourea, thiosemicarbazide, dimethylthiourea, thioacetamide, thioformamide, thioacetic acid and thiomorpholine; the amount of GQDs in step (2) is 300-600ml of anhydrous ethanol per gram of GQDs; the molar ratio of tin in the tin compound to sulfur in the sulfur-containing compound in step (2) is 1:2; the mass ratio of GQDs to the tin compound in step (2) is 0.1-0.2:1.

745.

5. The method of claim 1, wherein the carbon-based sulfide heterojunction composite photocatalyst is prepared by the following steps: (1) preparing a carbon-based sulfide photocatalyst; (2) preparing a metal oxide photocatalyst; and (3) mixing the carbon-based sulfide photocatalyst and the metal oxide photocatalyst. the ultrasonic time in step (2) is 30-60min, and the reaction time in step (2) is 4-6h.

6. The method of claim 1, wherein the carbon-based sulfide heterojunction composite photocatalyst is prepared by the following steps: (1) preparing a carbon-based sulfide photocatalyst; (2) preparing a metal oxide photocatalyst; and (3) mixing the carbon-based sulfide photocatalyst and the metal oxide photocatalyst. the crosslinking agent and the reducing agent are required in the reaction process in step (3), and the crosslinking agent and the reducing agent are L-cysteine; the ultrasonic time in step (3) is 10-30min, the reaction temperature in step (3) is 90-95℃, and the reaction time in step (3) is 1-4h.

7. The method for preparing a carbon-based sulfide heterojunction composite photocatalyst according to claim 1, characterized in that: the centrifugal washing in step (3) is carried out with deionized water and anhydrous ethanol in sequence.

8. A carbon-based sulfide heterojunction composite photocatalyst, characterized by: The method is prepared according to any one of claims 1-7.

9. Use of a carbon-based sulfide heterojunction composite photocatalyst according to claim 8, characterized in that: The application of the carbon-based sulfide heterojunction composite photocatalyst in degrading heavy metals or dyes.

10. The use of a carbon-based sulfide heterojunction composite photocatalyst according to claim 9, characterized in that: The heavy metal is chromium, and the application is adding the carbon-based sulfide composite photocatalyst into a solution containing hexavalent chromium ions at a pH value of 3-11, and performing reaction under light to reduce hexavalent chromium to trivalent chromium, so as to realize degradation of the heavy metal chromium. The solution containing hexavalent chromium ions is potassium dichromate solution. The amount of the carbon-based sulfide heterojunction composite photocatalyst is calculated according to a final concentration of 1-2mg / ml in the reaction system. The concentration of the potassium dichromate solution is 3mg / L-20g / L. The reaction time is 1-2 hours. The reaction time is 1-2 hours.

Images