ZnS (at) CdS heterojunction and preparation method and application thereof

Abstract

The invention discloses a ZnS-CdS heterojunction as well as a preparation method and application thereof, and belongs to photocatalytic hydrogen production. According to the ZnS (at) CdS heterojunction provided by the invention, CdS is loaded on the surface of ZnS, and the ZnS (at) CdS heterojunction is prepared by a two-step hydrothermal method and has higher photocatalytic hydrogen production capacity, and the hydrogen production capacity of the composite material is 15.57 mmol. H <-1 >. G <-1 >, is 97 times of the hydrogen production capacity (0.16 mmol. H <-1 >. G <-1 >) of pure ZnS, and is 5.2 times of the hydrogen production capacity (2.97 mmol. H <-1 >. G <-1 >) of pure CdS.

Description

Technical Field

[0001] The present invention belongs to the field of photocatalytic hydrogen production, and specifically relates to a ZnS@CdS heterojunction and a preparation method and application thereof. Background Art

[0002] The information disclosed in this background technology section is only intended to enhance the understanding of the overall background of the invention, and should not necessarily be regarded as an admission or any form of suggestion that the information constitutes the prior art already known to a person skilled in the art.

[0003] With the development of economy and the advancement of urbanization, energy shortage and environmental pollution have become two major problems that plague the development of all countries. Hydrogen, as a green and pollution-free new energy source, can be used as a substitute for fossil energy. Therefore, the development of hydrogen energy has important strategic significance. Among them, the use of photocatalytic technology to split water to produce hydrogen does not rely on non-renewable resources, so it has good application prospects. Sulfide-based semiconductor materials are a class of effective semiconductor materials. Among them, the conduction band and valence band positions of ZnS give it a suitable redox potential. The conduction band potential is relatively negative, and the photogenerated electrons have sufficient reduction ability to provide power for the reduction of protons into hydrogen. Therefore, it is widely used in photocatalytic water decomposition to produce hydrogen. However, due to its wide bandgap, poor absorption in the visible light region, and serious photogenerated carrier recombination problems, its application in the field of photocatalysis has been limited. Summary of the invention

[0004] In order to overcome the deficiencies in the prior art, the purpose of the present invention is to provide a ZnS@CdS heterojunction and a preparation method and application thereof. The present invention forms a heterojunction structure by compounding ZnS and CdS, so that when the composite material is light-excited, the photogenerated carriers generated by ZnS and CdS will form a built-in electric field due to the energy band difference between the two, prompting the photogenerated electrons and holes to migrate in different directions, respectively, thereby reducing the recombination of carriers; more photogenerated carriers can reach the catalyst surface to participate in the hydrogen production reaction, thereby improving the efficiency of photocatalytic hydrogen production.

[0005] In order to achieve the above object, the technical solution of the present invention is:

[0006] In a first aspect, the present invention provides a method for preparing a ZnS@CdS heterojunction, comprising the following steps:

[0007] ZnS was prepared by hydrothermal method;

[0008] The cadmium source and the sulfur source were dissolved in water, and then ZnS was added to prepare the ZnS@CdS heterojunction by a hydrothermal method.

[0009] In one or more embodiments, the specific steps of preparing ZnS by hydrothermal method are: dissolving a zinc source and a sulfur source in water and ethanolamine to obtain a mixture, and subjecting the mixture to hydrothermal treatment, separation, washing, and drying.

[0010] Furthermore, the zinc source is zinc nitrate hexahydrate, and the sulfur source is thiourea.

[0011] Further, the zinc source and the sulfur source are dissolved in water and ethanolamine and stirred for 20-60 minutes.

[0012] Furthermore, a stabilizer is added to the mixture, and the mixture is stirred for 20-60 minutes after the stabilizer is added; further preferably, the stabilizer is polyvinyl pyrrolidone.

[0013] Furthermore, the feed ratio of zinc source, sulfur source, water, ethanolamine and polyvinyl pyrrolidone is 2-4g: 0.5-1.5g: 20-60ml: 5-15ml: 0.01-1g, preferably 2-3g: 0.5-1g: 25-35ml: 8-12ml: 0.04-0.06g.

[0014] As a preferred embodiment, the specific steps of preparing ZnS by hydrothermal method are: dissolving zinc source and sulfur source in water and ethanolamine, mixing them evenly, then adding polyvinyl pyrrolidone, hydrothermal treatment, separation, washing, and drying. The mixing evenly is stirring for 20-60 minutes. Adding polyvinyl pyrrolidone is also stirred for 20-60 minutes.

[0015] Furthermore, during the hydrothermal treatment, the hydrothermal temperature is 150-180° C., preferably 150-170° C.; the hydrothermal time is 2-6 h, preferably 2-4 h.

[0016] Furthermore, the separation, washing and drying are specifically: centrifugal separation, washing with water for multiple times, and drying in an oven at 60-80° C. for 5-12 hours.

[0017] In one or more embodiments, the cadmium source is cadmium nitrate tetrahydrate, and the sulfur source is thiourea.

[0018] In one or more embodiments, the feed ratio of cadmium source, sulfur source, water and ZnS is 0.3-1.5g: 0.05-0.5g: 40-60ml: 0.5-1g, preferably 0.5-0.8g: 0.1-0.2g: 45-55ml: 0.6-0.8g.

[0019] In one or more embodiments, the cadmium source and the sulfur source are dissolved in water and shaken for 20-60 minutes to mix them evenly.

[0020] In one or more embodiments, the hydrothermal method for preparing the ZnS@CdS heterojunction is specifically as follows: the hydrothermal temperature is 150-180° C., preferably 150-170° C.; the hydrothermal time is 2-6 h, preferably 2-4 h.

[0021] As a preferred embodiment, cadmium nitrate tetrahydrate and thiourea are added to water, shaken for a period of time, then ZnS is added and shaken for another 30 minutes, put into a reactor, react at 160° C. for 10 hours, and dry at 60° C. for 12 hours.

[0022] In a second aspect, the present invention provides a ZnS@CdS heterojunction obtained by the above preparation method.

[0023] The ZnS@CdS heterojunction is composited by nanometer-scale ZnS and nanometer-scale CdS, wherein CdS is loaded on the surface of ZnS.

[0024] In one or more embodiments, the molar ratio of Zn:Cd is 6-9:4-1, 7-9:3-1, and more preferably 8-9:2-1.

[0025] In a third aspect, the present invention provides an application of the above ZnS@CdS heterojunction in photocatalytic hydrogen production. Preferably, the photocatalytic hydrogen production is photocatalytic decomposition of water to produce hydrogen.

[0026] In a fourth aspect, the present invention provides a method for photocatalytic hydrogen production, using the above-mentioned ZnS@CdS heterojunction as a photocatalyst, comprising the following steps:

[0027] The photocatalyst was dissolved in a mixed aqueous solution of Na2S and Na2SO3 and irradiated with visible light in a closed environment with an inert atmosphere.

[0028] Preferably, the inert atmosphere is argon.

[0029] Preferably, the concentration of Na2S is 0.2-0.5M, preferably 0.3-0.4M.

[0030] Preferably, the concentration of Na2SO3 is 0.2-0.4M, preferably 0.2-0.3M.

[0031] Preferably, the ratio of the photocatalyst to the mixed aqueous solution of Na2S and Na2SO3 is 40-60 mg: 40-60 ml.

[0032] One or some of the above technical solutions have the following advantages or beneficial effects:

[0033] (1) The present invention provides a ZnS@CdS heterojunction as a photocatalyst for photocatalytic hydrogen production. The composite material obtained by loading CdS on the surface of ZnS by a two-step hydrothermal method has a higher photocatalytic hydrogen production capacity. The hydrogen production capacity of the composite material is 15.57 mmol·h -1 ·g -1 , is the hydrogen production of pure ZnS (0.16mmol·h -1 ·g -1 ) is 97 times that of pure CdS (2.97mmol·h -1 ·g -1 )5.2 times.

[0034] (2) Compared with the existing method using water as solvent and without adding a stabilizer, the hydrothermal preparation process of ZnS provided by the present invention uses water and ethanolamine as solvents and polyvinylpyrrolidone (PVP) as a stabilizer, so that the ZnS@CdS heterojunction obtained by combining the prepared ZnS and CdS has a better photocatalytic hydrogen production ability.

[0035] (3) ZnS and CdS have different energy band structures and bandgap widths. The composite material formed by the combination of the two has a wider light response range. The bandgap width of ZnS is relatively wide, and it mainly absorbs ultraviolet light, but has a weaker absorption capacity for visible light. The bandgap width of CdS is relatively small, and it can absorb part of the visible light. After the composite, it can make full use of ultraviolet light and visible light, increase the efficiency of light utilization, and provide more energy for photocatalytic hydrogen production. Under light excitation, the photogenerated carriers generated by ZnS and CdS will form a built-in electric field due to the energy band difference between the two, causing the photogenerated electrons and holes to migrate in different directions, respectively, reducing the recombination of carriers. More photogenerated carriers can reach the catalyst surface to participate in the hydrogen production reaction, thereby improving the efficiency of photocatalytic hydrogen production. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] The accompanying drawings in the specification, which constitute a part of the present invention, are used to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute improper limitations on the present invention.

[0037] Figure 1 This is the XRD pattern of the ZnS@CdS heterojunction prepared in Example 1 of the present invention;

[0038] Figure 2 This is a SEM image of the ZnS@CdS heterojunction prepared in Example 1 of the present invention;

[0039] Figure 3 is the SEM image of ZnS;

[0040] Figure 4 is the SEM image of CdS;

[0041] Figure 5 The amount of hydrogen produced by photocatalytic hydrogen production of the materials prepared in the examples of the present invention and the comparative examples;

[0042] Figure 6 This is the cyclic stability of the ZnS@CdS heterojunction prepared in Example 1 of the present invention when used as a photocatalyst. DETAILED DESCRIPTION

[0043] In order to enable those skilled in the art to more clearly understand the technical solution of the present invention, the technical solution of the present invention will be described in detail below in conjunction with specific embodiments.

[0044] Example 1

[0045] The pink ZnS sample was prepared by hydrothermal method. First, 2.38g of zinc nitrate hexahydrate and 0.609g of thiourea were dissolved in 30ml of deionized water and 10ml of ethanolamine, stirred for 40 minutes, and then 0.05g of polyvinyl pyrrolidone was added and stirred for 40 minutes. The mixture was transferred to a polytetrafluoroethylene liner and heated at 160℃ for 4h. The solid powder was centrifuged, washed four times with deionized water, and dried in an oven at 75℃ for 10h.

[0046] The preparation method of ZnS@CdS composite material (the molar ratio of Zn:Cd is 8:2) is as follows: 0.616g of cadmium nitrate tetrahydrate and 0.152g of thiourea are added to 50ml of water, shaken for 30min, then 0.779g of ZnS is added and shaken for another 30min, put into a reactor, react at 160°C for 10h, and dry at 60°C for 12h.

[0047] Example 2

[0048] The difference from Example 1 is that Zn:Cd (molar ratio) = 9:1. The specific steps are:

[0049] The pink ZnS sample was prepared by hydrothermal method. First, 2.38g of zinc nitrate hexahydrate and 0.609g of thiourea were dissolved in 30ml of deionized water and 10ml of ethanolamine, stirred for 40 minutes, and then 0.05g of polyvinyl pyrrolidone was added and stirred for 40 minutes. The mixture was transferred to a polytetrafluoroethylene liner and heated at 160℃ for 4h. The solid powder was centrifuged, washed four times with deionized water, and dried in an oven at 75℃ for 10h.

[0050] The preparation method of the ZnS@CdS composite material is as follows: 0.308 g of cadmium nitrate tetrahydrate and 0.076 g of thiourea are added to 50 ml of water, shaken for 30 minutes, then 0.876 g of ZnS is added and shaken for another 30 minutes, put into a reactor, react at 160° C. for 10 hours, and dry at 60° C. for 12 hours.

[0051] Example 3

[0052] The difference from Example 1 is that Zn:Cd (molar ratio) = 7:3. The specific steps are:

[0053] The pink ZnS sample was prepared by a hydrothermal method. First, 2.38 g of zinc nitrate hexahydrate and 0.609 g of thiourea were dissolved in 30 ml of deionized water and 10 ml of ethanolamine, stirred for 40 minutes, and then 0.05 g of polyvinyl pyrrolidone was added and stirred for 40 minutes. The mixture was transferred to a polytetrafluoroethylene liner and heated at 160 °C for 4 h. The obtained solid powder was centrifuged, washed four times with deionized water, and dried in an oven at 75 °C for 10 h.

[0054] The preparation method of the ZnS@CdS composite material is as follows: 0.925 g of cadmium nitrate tetrahydrate and 0.228 g of thiourea are added into 50 ml of water, shaken for 30 minutes, then 0.681 g of ZnS is added and shaken for another 30 minutes, and then put into a reactor, reacted at 160° C. for 10 hours, and dried at 60° C. for 12 hours.

[0055] Example 4

[0056] The difference from Example 1 is that the molar ratio of Zn:Cd is 6:4. The specific steps are:

[0057] The pink ZnS sample was prepared by hydrothermal method. First, 2.38g of zinc nitrate hexahydrate and 0.609g of thiourea were dissolved in 30ml of deionized water and 10ml of ethanolamine, stirred for 40 minutes, and then 0.05g of polyvinyl pyrrolidone was added and stirred for 40 minutes. The mixture was transferred to a polytetrafluoroethylene liner and heated at 160℃ for 4h. The solid powder was centrifuged, washed four times with deionized water, and dried in an oven at 75℃ for 10h.

[0058] The preparation method of the ZnS@CdS composite material is as follows: 1.233 g of cadmium nitrate tetrahydrate and 0.304 g of thiourea are added to 50 ml of water, shaken for 30 minutes, then 0.584 g of ZnS is added and shaken for another 30 minutes, put into a reactor, react at 160° C. for 10 hours, and dry at 60° C. for 12 hours.

[0059] Comparative Example 1

[0060] The preparation process of CdS material is:

[0061] 6.16 g of cadmium nitrate tetrahydrate and 1.52 g of thiourea were added to 50 ml of water, shaken for 30 min, transferred to a polytetrafluoroethylene liner, heated at 160 °C for 10 h, and the obtained solid powder was centrifuged, washed four times with deionized water, and dried in an oven at 60 °C for 12 h.

[0062] Comparative Example 2

[0063] Different from Example 1, no stabilizer PVP (polyvinyl pyrrolidone) was added, and the solvents were water and ethanolamine, specifically:

[0064] The pink ZnS sample was prepared by hydrothermal method. First, 2.38g of zinc nitrate hexahydrate and 0.609g of thiourea were dissolved in 30ml of deionized water and 10ml of ethanolamine, stirred for 40 minutes, transferred to a polytetrafluoroethylene liner, heated at 160℃ for 4h, and the obtained solid powder was centrifuged, washed four times with deionized water, and dried in an oven at 75℃ for 10h.

[0065] The preparation method of ZnS@CdS composite material is as follows: 0.616g of cadmium nitrate tetrahydrate and 0.152 thiourea are added into 50ml of water, shaken for 30 minutes, then 0.779g of ZnS is added and shaken for another 30 minutes, put into a reactor, react at 160°C for 10 hours, and dry at 60°C for 12 hours.

[0066] Comparative Example 3

[0067] The difference from Example 1 is that a stabilizer PVP (polyvinyl pyrrolidone) is added, but the solvent is only water, and ethanolamine is not added. Specifically:

[0068] The white ZnS sample was prepared by a hydrothermal method. First, 2.38 g of zinc nitrate hexahydrate and 0.609 g of thiourea were dissolved in 40 ml of deionized water (the pH of the water was adjusted in advance to pH = 12.50), stirred for 40 minutes, and then 0.05 g of polyvinyl pyrrolidone was added and stirred for 40 minutes. The mixture was transferred to a polytetrafluoroethylene liner and heated at 160°C for 4 hours. The obtained solid powder was centrifuged, washed four times with deionized water, and dried in an oven at 75°C for 10 hours.

[0069] The preparation method of the ZnS@CdS composite material is as follows: 0.616 g of cadmium nitrate tetrahydrate and 0.152 g of thiourea are added to 50 ml of water, shaken for 30 minutes, then 0.779 g of ZnS is added and shaken for another 30 minutes, put into a reactor, react at 160° C. for 10 hours, and dry at 60° C. for 12 hours.

[0070] Figure 1 The XRD diagram of the ZnS@CdS heterojunction prepared in Example 1 of the present invention. The crystal structure and phase structure of the synthesized CdS and ZnS nanomaterials were analyzed by X-ray diffraction (XRD). It can be seen from the figure that the diffraction peaks of pure ZnS and CdS are good, and there are no impurity peaks, indicating that the purity of the synthesized material is high. As the Zn content of other CdS@ZnS composites with different ratios continues to increase, their diffraction peaks gradually tend to the peak shape of ZnS, which can also prove that the required catalyst has been successfully synthesized.

[0071] Figure 2 This is a SEM image of the ZnS@CdS heterojunction prepared in Example 1 of the present invention. It can be seen from the figure that ZnS presents a relatively small and aggregated granular structure, and the particle morphology of CdS is relatively regular, similar to a dendrite structure. Compared with ZnS, the synthesized composite material has a larger particle size and better dispersion, which may be due to the production of the composite material, which changes its surface morphology. The ZnS@CdS heterojunction is not a core-shell structure, but CdS is loaded on ZnS.

[0072] Figure 5 The photocatalytic hydrogen production of the materials prepared in the present invention and the comparative example is shown in the figure. It can be seen from the figure that when the prepared composite material Zn:Cd=8:2, the best hydrogen production effect reaches 15.57mmol g -1 h -1 It is 97 times the hydrogen production of pure ZnS and 5.7 times the hydrogen production of pure CdS, and it is also the best among all the ratios. It can also be seen from the figure that the same Zn:Cd=8:2, but the solvent is changed (7.51mmol g -1 h -1 ) and without stabilizer (pvp) (13.69mmol g -1 h -1 ) also changes its hydrogen production, which can further prove the superiority of the preparation method described in the present invention.

[0073] Application Example 1

[0074] The process of photocatalytic hydrogen production is as follows: First, weigh 50 mg of the sample of Example 1 and add it to 50 ml of a mixed aqueous solution of 0.35 M Na2S and 0.25 M Na2SO3, and stir evenly. Expel the air by blowing argon gas, and exhaust for 20 minutes. Seal, then place the sample in a circular tank and fix it, turn on the magnetic stirrer, turn on the condensation device, irradiate with a xenon lamp under visible light (420-800 nm), take samples every 2 hours, and use a gas chromatograph with Ar as the carrier gas to detect the amount of hydrogen produced.

[0075] Figure 6 The cyclic stability of the ZnS@CdS heterojunction prepared in Example 1 of the present invention when used as a photocatalyst. In order to test the stability of the photocatalyst, four cyclic hydrogen production experiments were conducted, and the difference in hydrogen production between different cycles was not large, indicating that the catalytic activity of the synthesized photocatalyst did not decrease significantly during repeated use, reflecting good stability.

[0076] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for preparing a ZnS@CdS heterojunction, characterized in that: The following steps are involved: ZnS was prepared by hydrothermal method; The cadmium source and the sulfur source were dissolved in water, and then ZnS was added to prepare the ZnS@CdS heterojunction by a hydrothermal method.

2. The preparation method according to claim 1, characterized in that: The specific steps of preparing ZnS by hydrothermal method are: dissolving zinc source and sulfur source in water and ethanolamine to obtain a mixture, and then hydrothermally treating, separating, washing and drying the mixture.

3. The preparation method according to claim 2, characterized in that: The zinc source is zinc nitrate hexahydrate, and the sulfur source is thiourea; Preferably, the zinc source and the sulfur source are dissolved in water and ethanolamine and stirred for 20-60 minutes; Preferably, a stabilizer is also added to the mixture, and the mixture is stirred for 20-60 minutes after the stabilizer is added; further preferably, the stabilizer is polyvinyl pyrrolidone; Preferably, the feed ratio of zinc source, sulfur source, water, ethanolamine and polyvinyl pyrrolidone is 2-4g:0.5-1.5g:20-60ml:5-15ml:0.01-1g, preferably 2-3g:0.5-1g:25-35ml:8-12ml:0.04-0.06g.

4. The preparation method according to claim 2, characterized in that: During the hydrothermal treatment, the hydrothermal temperature is 150-180° C., preferably 150-170° C.; the hydrothermal time is 2-6 h, preferably 2-4 h.

5. The preparation method according to claim 1, characterized in that: The cadmium source is cadmium nitrate tetrahydrate, and the sulfur source is thiourea; Preferably, the feed ratio of cadmium source, sulfur source, water and ZnS is 0.3-1.5g: 0.05-0.5g: 40-60ml: 0.5-1g, preferably 0.5-0.8g: 0.1-0.2g: 45-55ml: 0.6-0.8g; Preferably, the cadmium source and the sulfur source are dissolved in water and shaken for 20-60 minutes.

6. The preparation method according to claim 1, characterized in that: The hydrothermal method for preparing the ZnS@CdS heterojunction is specifically as follows: the hydrothermal temperature is 150-180° C., preferably 150-170° C.; the hydrothermal time is 2-6 hours, preferably 2-4 hours.

7. A ZnS@CdS heterojunction obtained by the preparation method according to any one of claims 1 to 6.

8. The ZnS@CdS heterojunction according to claim 7, characterized in that: The ZnS@CdS heterojunction is composited with ZnS and CdS, wherein CdS is loaded on the surface of ZnS; Preferably, the molar ratio of Zn:Cd is 6-9:4-1, preferably 7-9:3-1, and more preferably 8-9:2-1.

9. Use of the ZnS@CdS heterojunction obtained by the preparation method according to any one of claims 1 to 6 or the ZnS@CdS heterojunction according to claim 7 or 8 in photocatalytic hydrogen production; Preferably, the photocatalytic hydrogen production is photocatalytic decomposition of water to produce hydrogen.

10. A method for preparing hydrogen produced by photocatalytic decomposition of water, characterized in that: The ZnS@CdS heterojunction prepared by any one of claims 1 to 6 or the ZnS@CdS heterojunction according to claim 7 or 8 is used as a photocatalyst, comprising the following steps: The photocatalyst was dissolved in a mixed aqueous solution of Na2S and Na2SO3 and irradiated with visible light in a closed environment with an inert atmosphere.

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