S-type heterojunction and preparation method thereof

Abstract

The invention discloses a high-efficiency photocatalytic heterojunction and a preparation method thereof, particularly relates to a defective molybdenum oxide / cadmium sulfide S-type heterojunction and a preparation method thereof, and belongs to the field of photocatalysis. The S-type heterojunction prepared by the invention has the characteristics of self-heating, self-enhancing built-in electric field and full-spectrum absorption. The oxygen vacancy concentration in molybdenum oxide is continuously regulated and controlled by using the oxidation-reduction performance of hydrogen peroxide when the oxygen vacancy concentration is low, the hydrogen peroxide is used as a reducing agent and the hydrogen peroxide is used as an oxidizing agent when the oxygen vacancy concentration is high, so that the built-in electric field intensity, the photothermal effect and the light absorption performance of the heterojunction are optimized. The continuous regulation and control of oxygen vacancies can be realized through a simple process, the environmental friendliness of the reaction is improved, and the method is suitable for batch production. Meanwhile, the obtained S-type heterojunction has high light absorption capacity, excellent charge separation capacity and good self-heating effect, the activity of the photocatalytic material can be obviously improved, and the S-type heterojunction has good application prospects.

Description

Technical Field

[0001] The invention relates to a high-efficiency photocatalytic heterojunction and a preparation method thereof, in particular to a full-spectrum absorption S-type heterojunction with self-heating and self-enhanced internal electric field, belonging to the field of photocatalysis. Background Art

[0002] In recent years, environmental pollution and energy crisis have become the focus of global attention. Photocatalytic technology can use green solar energy to produce clean energy and treat pollutants. It is considered to be the most promising way to solve the energy crisis and environmental pollution. The development of highly active semiconductor photocatalysts is the prerequisite for the practical application of photocatalytic technology.

[0003] Among many photocatalytic materials such as type I heterojunction, type II heterojunction, Schottky junction, and Z-type heterojunction, the S-type heterojunction, which is composed of a reduced semiconductor with a small work function and a high Fermi level and an oxidized semiconductor with a large work function and a low Fermi level, can achieve efficient separation of photogenerated charges and effectively alleviate the serious recombination of photogenerated electrons and holes without damaging the strong redox ability in the photocatalyst. It is a photocatalytic system with competitive advantages. The driving force for the separation of photogenerated charges in the S-type heterojunction is the built-in electric field caused by the difference in the Fermi levels of the reducing semiconductor and the oxidizing semiconductor. The built-in electric field is usually weak, resulting in a weak charge separation ability, which limits the photocatalytic activity of the S-type heterojunction.

[0004] The use of external auxiliary strategies such as external heating, external electromagnetic fields, ultrasound, etc. can significantly improve the separation of photogenerated charges. However, this strategy is introduced from the outside and requires external energy, which will increase energy consumption and equipment complexity. In addition, the light absorption energy of the S-type heterojunction is limited, and most of it can only absorb ultraviolet light and visible light, which greatly limits the use of sunlight. Therefore, it is extremely challenging to develop an S-type heterojunction system with self-heating, self-enhanced internal electricity, and full-spectrum absorption, and there are few reports yet. Summary of the invention

[0005] In view of the shortcomings of the prior art, the purpose of the present invention is to provide a defective molybdenum oxide / cadmium sulfide S-type heterojunction photocatalytic system with self-heating, self-enhanced internal electric field and full-spectrum absorption and a preparation method thereof, which can continuously regulate the concentration of oxygen vacancies and sulfur vacancies through a simple hydrothermal method, thereby achieving photothermal effects and regulation of the built-in electric field, and constructing an efficient S-type heterojunction material.

[0006] A method for preparing a high-efficiency photocatalytic heterojunction comprises the following steps: 1) Preparation of molybdenum oxide with continuously adjustable oxygen vacancies: Mixing metal Mo powder with 1-butanol, then adding hydrogen peroxide and stirring evenly, and then performing a hydrothermal reaction to obtain defective molybdenum oxide; Inevitably, some embodiments also include relevant post-treatment steps, which include centrifuging, washing, and finally drying the precipitate after the hydrothermal reaction is completed.

[0007] 2) Preparation of cadmium sulfide: Cadmium nitrate and thiourea are dissolved in ethylenediamine and subjected to hydrothermal reaction to obtain cadmium sulfide powder with sulfur vacancies; Inevitably, some embodiments also include relevant post-treatment steps, which include centrifuging, washing, and finally drying the precipitate after the hydrothermal reaction is completed.

[0008] 3) Preparation of defective molybdenum oxide / cadmium sulfide S-type heterojunction photocatalyst: Defective molybdenum oxide and cadmium sulfide are ultrasonically mixed in a dispersant at a certain mass ratio. After ultrasonic stirring for 0.5-3 hours, the sample is taken out, filtered, dried and collected to obtain an S-type heterojunction material.

[0009] Inevitably, some embodiments also include relevant post-processing steps, and the post-processing steps include filtration and drying.

[0010] In step 1), the concentration of the metal Mo powder in the solution is 0.03-0.12 mol / L, the concentration of the hydrogen peroxide is 15%-30%, and the hydrothermal reaction is carried out at 120-180° C. for 12-24 hours.

[0011] The oxygen vacancy concentration in the defective molybdenum oxide is 10%-30%. In step 2), the molar ratio of cadmium nitrate to thiourea is 1:2-1:4, the concentration of cadmium nitrate is 0.05-0.3 mol / L, and the hydrothermal reaction is carried out at 120-180° C. for 12-24 hours.

[0012] In the cadmium sulfide powder with sulfur vacancies, the concentration of sulfur vacancies is 5%-20%. In step 3), the mass ratio of molybdenum oxide to cadmium sulfide is 5%-25%. In step 3), the dispersant is ethanol or other alcohols.

[0013] Another technical solution of the present invention is to provide a method for preparing an S-type heterojunction, which is prepared by the method described.

[0014] Another technical solution of the present invention is to use the S-type heterojunction prepared by the preparation method in photocatalytic hydrogen production.

[0015] Another technical solution of the present invention is to use the S-type heterojunction prepared by the preparation method in the photocatalytic reduction of hexavalent cadmium.

[0016] Another technical solution of the present invention is to use the S-type heterojunction prepared by the preparation method in photocatalytic CO2 reduction.

[0017] The beneficial effects of the present invention are: (1) A method for preparing an efficient photocatalytic heterojunction, in particular, a full-spectrum absorption S-type heterojunction with self-heating and self-enhanced internal electric field, wherein the S-type heterojunction is composed of defective molybdenum oxide with continuously adjustable oxygen vacancy concentration as an oxidized semiconductor and cadmium sulfide as a reduced semiconductor. Changes in the oxygen vacancy concentration in the molybdenum oxide can cause changes in the Fermi level and photothermal effect of the molybdenum oxide, and changes in the sulfur vacancy concentration in the cadmium sulfide can cause changes in the Fermi level of the cadmium sulfide. The Fermi level difference between the two components can optimize the built-in electric field strength, thereby improving the separation efficiency of photogenerated charges. At the same time, the regulation of the photothermal effect of the molybdenum oxide can increase the reaction temperature of the photocatalytic system, reduce the activation energy of hydrogen production, and improve the photocatalytic activity. In addition, the full-spectrum absorption capacity of the defective molybdenum oxide greatly broadens the light absorption range of the composite material and improves the utilization rate of sunlight.

[0018] (2) The present invention can achieve the purpose of regulating the oxygen vacancy concentration by regulating the amount of hydrogen peroxide, and the amount of sulfur vacancies by regulating the thiourea content. Semiconductors with different vacancy concentrations have different Fermi levels and photothermal effects, which can achieve continuous adjustment of the built-in electric field and photothermal effect, thereby increasing the driving force of charge separation and the temperature of the photocatalytic reaction, thereby improving the photocatalytic activity. At the same time, due to the presence of oxygen vacancy defects, the S-type heterojunction can achieve full-spectrum absorption and enhance the utilization rate of sunlight.

[0019] (3) The oxygen vacancy concentration control method of the present invention is simple, the catalyst preparation is simple, easy to operate, and low cost. The catalyst yield obtained is considerable, which is conducive to its application in the photocatalytic reaction process. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 : is the XRD pattern of the defective molybdenum oxide prepared in Example 1.

[0021] Figure 2 : Transmission electron microscope and high-resolution transmission electron microscope images of the defective molybdenum oxide prepared in Example 1.

[0022] Figure 3 : This is the ESR diagram of the defective molybdenum oxide prepared in Example 1.

[0023] Figure 4 : XRD diagram of the high-efficiency photocatalytic heterojunction prepared in Example 2 Figure 5 : UV-Vis image of the high-efficiency photocatalytic heterojunction prepared in Example 2.

[0024] Figure 6 : Photothermal imaging of the high-efficiency photocatalytic heterojunction prepared in Example 3. DETAILED DESCRIPTION

[0025] The present invention is further explained below in conjunction with specific embodiments. It should be understood that these embodiments are only used to illustrate the present invention and are not used to limit the scope of the present invention. After reading the present invention, various equivalent forms of modifications to the present invention by those skilled in the art all fall within the scope defined by the claims attached to this application.

[0026] A method for preparing a high-efficiency photocatalytic heterojunction comprises the following steps: 1) Preparation of molybdenum oxide with continuously adjustable oxygen vacancies: A certain amount of metal Mo powder is mixed with 1-butanol, and then different amounts of hydrogen peroxide are added and stirred for 0.5-3 hours. The obtained yellow solution is transferred to a hydrothermal reactor and reacted at 120-180°C for 8-24 hours. After the reaction is completed, the precipitate is centrifuged, washed, and finally dried to prepare molybdenum oxide with continuously adjustable oxygen vacancy concentration.

[0027] 2) Preparation of cadmium sulfide with continuously adjustable sulfur vacancies: A certain amount of cadmium nitrate and thiourea are dissolved in ethylenediamine to obtain a clear light green solution. The solution is then placed in a hydrothermal reactor and reacted at 120-180°C for 12-24 hours. Finally, bright yellow cadmium sulfide powder with sulfur vacancies is collected, centrifuged, rinsed, and dried.

[0028] 3) Preparation of defective molybdenum oxide / cadmium sulfide S-type heterojunction photocatalyst: The defective molybdenum oxide / cadmium sulfide S-type heterojunction photocatalyst was prepared by an in-situ self-assembly strategy, and the defective molybdenum oxide and cadmium sulfide were ultrasonically mixed in a dispersant at a certain mass ratio. After ultrasonic stirring for 0.5-3 hours, the sample was taken out, filtered, dried and collected to obtain the S-type heterojunction material.

[0029] Preferably, in step 1, the concentration of the metal Mo powder in the solution is 0.03-0.12 mol / L.

[0030] Preferably, in step 1, the concentration of the hydrogen peroxide is 15%-30%.

[0031] Preferably, in step 1, the amount of hydrogen peroxide added is 10%-40% by volume.

[0032] Preferably, in step 1, the drying temperature is 30-80°C.

[0033] Preferably, in step 2, the molar ratio of cadmium nitrate to thiourea is 1:2-1:4.

[0034] Preferably, in step 2, the concentration of cadmium nitrate is 0.05-0.3 mol / L.

[0035] Preferably, in step 3, the mass ratio of defective molybdenum oxide to cadmium sulfide is 5%-25%.

[0036] Preferably, in step 3, the dispersant is ethanol or other alcohols.

[0037] Preferably, in step 3, the concentration of the photocatalyst in the dispersion is 200 g / L-1000 g / L.

[0038] Example 1 1) Preparation of defective molybdenum oxide with continuously adjustable oxygen vacancies: 2 mmol of metal Mo powder was mixed with 24 mL of 1-butanol by ultrasonic mixing for 30 minutes, and then 11 mL of 30 wt% hydrogen peroxide was added and stirred for 0.5 hours. The obtained yellow solution was transferred to a hydrothermal reactor and reacted at 140 ° C for 12 hours. After the reaction, the precipitate was washed with deionized water for 3 times and finally dried in a vacuum drying oven at 80 ° C to prepare defective molybdenum oxide with an oxygen vacancy mass percentage of 24%.

[0039] 2) Preparation of cadmium sulfide: 12.8 mmol of cadmium nitrate and 36 mmol of thiourea were dissolved in 80 mL of ethylenediamine solution to obtain a clear light green solution. The solution was then placed in a Teflon-lined stainless steel autoclave and heated at 150°C for 24 h. Finally, the bright yellow powder was collected, washed three times with deionized water, and dried to obtain the CdS material.

[0040] 3) Preparation of defective molybdenum oxide / cadmium sulfide S-type heterojunction photocatalyst: The defective molybdenum oxide / cadmium sulfide S-type heterojunction photocatalyst was prepared by in situ self-assembly strategy. 100 mg of cadmium sulfide and 15 mg of defective molybdenum oxide were ultrasonically mixed in 200 mL of ethanol. After 30 minutes, the sample was taken out, filtered, dried and collected to obtain the S-type heterojunction photocatalyst. The XRD diffraction pattern of the obtained defective molybdenum oxide material is as follows: Figure 1 As shown in Figure 1, the product is pure molybdenum oxide with defects. Figure 2 As shown, it can be seen that the defective molybdenum oxide is a nanosheet structure with abundant oxygen vacancies. ESR graph ( Figure 3 ) indicates that there are abundant oxygen vacancies in MoO.

[0041] Example 2 1) Preparation of defective molybdenum oxide with continuously adjustable oxygen vacancies: 1.5 mmol of metal Mo powder was mixed with 24 mL of 1-butanol by ultrasonic mixing for 30 minutes, and then 15 mL of 30wt% hydrogen peroxide was added and stirred for 1 hour. The obtained yellow solution was transferred to a hydrothermal reactor and reacted at 160°C for 12 hours. After the reaction, the precipitate was washed with deionized water three times and finally dried in a vacuum drying oven at 80°C to prepare defective molybdenum oxide with an oxygen vacancy mass percentage of 23%.

[0042] 2) Preparation of cadmium sulfide: 16 mmol of cadmium nitrate and 36 mmol of thiourea were dissolved in 80 mL of ethylenediamine solution to obtain a clear light green solution. The solution was then placed in a Teflon-lined stainless steel autoclave and heated at 150°C for 24 h. Finally, the bright yellow powder was collected, washed three times with deionized water, and dried to obtain the CdS material.

[0043] 3) Preparation of defective molybdenum oxide / cadmium sulfide S-type heterojunction photocatalyst: The defective molybdenum oxide / cadmium sulfide S-type heterojunction photocatalyst was prepared by in situ self-assembly strategy. 120 mg of cadmium sulfide and 10 mg of defective molybdenum oxide were ultrasonically mixed in 200 mL of ethanol. After 30 minutes, the sample was taken out, filtered, dried and collected to obtain the S-type heterojunction photocatalyst. The XRD pattern of defective molybdenum oxide / cadmium sulfide S-type heterojunction photocatalyst is shown in Figure 4 As shown, it can be seen that there are diffraction peaks of molybdenum oxide and cadmium sulfide in the composite material, and there are no other impurities, indicating that the S-type heterojunction composite material is successfully obtained.

[0044] 20 mg of S-type heterojunction composite material was placed in a reaction container, 8 mL of lactic acid and 80 ml of aqueous solution were added to the reaction container, and the photocatalytic hydrogen production test was carried out under a xenon lamp light source equipped with a 420 nm filter. The photocatalytic hydrogen production activity of the S-type heterojunction composite material is 12.6 mmol / h* g, which is more than 5 times that of pure CdS. The UV-Vis image of the composite material is shown in Figure 2. Figure 5 As shown, it can be seen that the composite material has an obvious full-spectrum absorption effect.

[0045] Example 3 1) Preparation of defective molybdenum oxide with continuously adjustable oxygen vacancies: 2 mmol of metal Mo powder was mixed with 24 mL of 1-butanol by ultrasonic mixing for 30 minutes, and then 10 ml and 15 mL of 30 wt% hydrogen peroxide were added and stirred for 0.5 hours. The obtained yellow solution was transferred to a hydrothermal reactor and reacted at 140 ° C for 12 hours. After the reaction, the precipitate was washed with deionized water for 3 times and finally dried in a vacuum oven at 80 ° C to prepare defective molybdenum oxide with more oxygen vacancies (R-MoO 3-x ) and defective molybdenum oxide with fewer oxygen vacancies (P-MoO 3-x ).

[0046] 2) Preparation of cadmium sulfide: 12.8 mmol of cadmium nitrate and 36 mmol of thiourea were dissolved in 80 mL of ethylenediamine solution to obtain a clear light green solution. The solution was then placed in a Teflon-lined stainless steel autoclave and heated at 150°C for 24 h. Finally, the bright yellow powder was collected, washed three times with deionized water, and dried to obtain the CdS material.

[0047] 3) Preparation of defective molybdenum oxide / cadmium sulfide S-type heterojunction photocatalyst: The defective molybdenum oxide / cadmium sulfide S-type heterojunction photocatalyst was prepared by in situ self-assembly strategy. 100 mg of cadmium sulfide and 15 mg of defective molybdenum oxide were ultrasonically mixed in 200 mL of ethanol. After 30 minutes, the sample was taken out, filtered, dried and collected to obtain two different S-type heterojunction photocatalysts. The infrared thermal imaging of the obtained S-type heterojunction photocatalyst is shown in Figure 6 As shown, it is obvious that the defects in molybdenum oxide cause the material to have a significant photothermal effect, which can significantly increase the photothermal temperature of cadmium sulfide and thus improve the photocatalytic activity.

[0048] Take 20 mg of the S-type heterojunction composite material and disperse it in 2 mL of deionized water, then evenly coat the suspension on a frosted quartz sheet and dry it in a vacuum. Place the quartz sheet with the catalyst on the surface in a reaction vessel, then inject CO2 and H2O vapor, and measure the concentration of the product by gas chromatography. The rate of the S-type heterojunction for photocatalytic CO2 reduction to CO is 15.2 μmol / g / h, which is more than 5 times that of pure CdS.

[0049] 20 mg of S-type heterojunction composite material was weighed and dispersed into 50 ml of K2Cr2O7 solution with a Cr(VI) concentration of 20 mg L −1. Before visible light irradiation, the mixture was stirred in the dark for 80 min to reach adsorption-desorption equilibrium. After illumination, 2 ml of the solution was taken out every 5 minutes and centrifuged for UV-visible absorbance test. The residual amount of Cr(VI) in the solution was analyzed by monitoring the change of the 540 nm absorption peak. The reduction rate of Cr(VI) by the S-type heterojunction reached 89% within 30 minutes, which is much higher than that of pure cadmium sulfide (42%).

Claims

1. A method for preparing an S-type heterojunction, characterized in that: The steps include: 1) Preparation of molybdenum oxide with continuously adjustable oxygen vacancies: Mixing metal Mo powder with 1-butanol, then adding hydrogen peroxide and stirring evenly, and then performing a hydrothermal reaction to obtain defective molybdenum oxide; 2) Preparation of cadmium sulfide: Cadmium nitrate and thiourea are dissolved in ethylenediamine and subjected to hydrothermal reaction to obtain cadmium sulfide powder with sulfur vacancies; 3) Preparation of defective molybdenum oxide / cadmium sulfide S-type heterojunction photocatalyst: Defective molybdenum oxide and cadmium sulfide are ultrasonically mixed in a dispersant at a certain mass ratio. After ultrasonic stirring for 0.5-3 hours, the sample is taken out, filtered, dried and collected to obtain an S-type heterojunction material.

2. The method for preparing an S-type heterojunction according to claim 1, characterized in that: In step 1), the concentration of the metal Mo powder in the solution is 0.03-0.12 mol / L, the concentration of the hydrogen peroxide is 15%-30%, and the hydrothermal reaction is carried out at 120-180° C. for 12-24 hours.

3. The method for preparing an S-type heterojunction according to claim 1, characterized in that: The oxygen vacancy concentration in the defective molybdenum oxide is 10%-30%.

4. The method for preparing an S-type heterojunction according to claim 1, characterized in that: In step 2), the molar ratio of cadmium nitrate to thiourea is 1:2-1:4, the concentration of cadmium nitrate is 0.05-0.3 mol / L, and the hydrothermal reaction is carried out at 120-180° C. for 12-24 hours.

5. The method for preparing an S-type heterojunction according to claim 1, characterized in that: In the cadmium sulfide powder with sulfur vacancies, the concentration of sulfur vacancies is 5%-20%.

6. The method for preparing an S-type heterojunction according to claim 1, characterized in that: In step 3), the mass ratio of molybdenum oxide to cadmium sulfide is 5%-25%.

7. A method for preparing an S-type heterojunction, characterized in that: The method is prepared by any one of claims 1 to 6.

8. Use of an S-type heterojunction prepared by the preparation method according to any one of claims 1 to 6 in photocatalytic hydrogen production.

9. Use of an S-type heterojunction prepared by the preparation method according to any one of claims 1 to 6 in photocatalytic reduction of hexavalent cadmium.

10. Use of an S-type heterojunction prepared by the preparation method according to any one of claims 1 to 6 in photocatalytic CO2 reduction.

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