A method for in-situ growth of perovskite quantum dots on carbon nitride nanosheets, and products and applications thereof

Abstract

The application discloses a method for in-situ growth of perovskite quantum dots on carbon nitride nanosheets, and products and applications thereof. Perovskite quantum dot precursors are mixed and ground with graphite-phase carbon nitride nanosheets to realize mechanical chemical pre-reaction. Then, low-temperature freezing treatment is performed, and the mixture after the freezing treatment is calcined in an inert atmosphere to realize in-situ synthesis of perovskite quantum dots on the carbon nitride nanosheets. The calcined sample is sequentially subjected to water washing, alcohol washing, and then drying to obtain a perovskite quantum dot / graphite-phase carbon nitride nanosheet composite material. The synthesis of the quantum dots is directly synthesized under high-temperature solid-phase conditions without adding ligands, the quantum dots have few interface defects with the carbon nitride, which is beneficial to carrier transmission and eliminates the influence of ligands on photo-generated carriers.

Description

TECHNICAL FIELD

[0001] The present application relates to a method for compounding perovskite quantum dots and carbon nitride nanosheets, in particular to a method for in-situ growth of perovskite quantum dots on carbon nitride nanosheets without ligand participation, especially to a method for in-situ growth of perovskite quantum dots on carbon nitride nanosheets by high-temperature solid-phase method, and further relates to the obtained perovskite quantum dots / graphitic carbon nitride (g-C3N4) nanosheet composite material and the application of the composite material in the field of photocatalysis. BACKGROUND

[0002] Graphitic carbon nitride is a typical visible light photocatalyst, which has been widely used in various photocatalytic fields in the past 20 years. It is of great concern due to its non-metallic element, non-toxic, environmentally friendly, low cost and simple preparation process. In order to improve the performance of carbon nitride-based photocatalysts, it is crucial to expand the light response and improve the separation and transport efficiency of photo-generated carriers. Since layered carbon nitride materials can be prepared into ultrathin nanosheets, forming a heterojunction with other semiconductor materials is the key to improving photocatalytic efficiency.

[0003] Perovskite quantum dots are a typical semiconductor luminescent material, which can cover the entire visible light region in light absorption. The process of luminescence is just the opposite of photocatalysis. Luminescence is the emission of photons through the recombination of photo-generated electrons and holes, while the photocatalytic process is the separation of photo-generated electrons and holes. These photo-generated electrons and holes participate in the photocatalytic redox reaction, so the luminescence phenomenon will disappear. Due to the excellent absorption properties of perovskite quantum dots, its application in the field of photocatalysis is obvious. The key is to prevent the recombination of photo-generated electrons and holes. On the other hand, in order to obtain high-quality perovskite quantum dots, traditional solution chemical methods usually need to add organic ligands to passivate the surface, and these ligands will affect the migration of photo-generated electrons, so the perovskite quantum dots should be compounded without ligands to obtain high-quality hydrophilic nanoparticles and well-developed interfaces. SUMMARY

[0004] In view of the fact that the existing synthesis technology of perovskite quantum dots is difficult to be directly applied in the field of photocatalysis, the present application provides a method for in-situ growth of perovskite quantum dots on carbon nitride nanosheets. This method makes full use of the secondary high-temperature thermal polymerization process of carbon nitride, and directly embeds high-quality perovskite quantum dot nanocrystals into carbon nitride nanosheets in-situ by high-temperature solid-phase in-situ growth method, to obtain a heterojunction with excellent interface properties. This method is completed under high-temperature solid-phase conditions without ligands, so as to ensure the carrier transport efficiency, and the secondary thermal polymerization of carbon nitride can also prevent the oxidation of perovskite quantum dots to ensure the adjustable light absorption range of perovskite quantum dots. This method is simple in process, good in repeatability, and beneficial to industrial production. The prepared perovskite quantum dots are uniformly embedded on the carbon nitride nanosheets, have good stability, and have good application prospect.

[0005] The specific technical solutions of the present application are as follows:

[0006] A method for in-situ growth of perovskite quantum dots on carbon nitride nanosheets, the method comprising the following steps:

[0007] (1) Mix and grind perovskite quantum dot precursors with graphite phase carbon nitride nanosheets to achieve mechanical chemical pre-reaction;

[0008] (2) Perform low-temperature freezing treatment on the mixture after grinding in step (1);

[0009] (3) Perform calcination on the frozen sample under an inert atmosphere to achieve in-situ synthesis of perovskite quantum dots on carbon nitride nanosheets;

[0010] (4) Wash and dry the calcined sample to obtain a perovskite quantum dot / graphite phase carbon nitride nanosheet composite material.

[0011] Further, the structural formula of the perovskite quantum dots is CsPbX3, where X is a halogen such as chlorine, bromine, iodine, etc.

[0012] Further, the precursors of the perovskite quantum dots are a mixture of cesium halide and lead halide, and the cesium halide can be cesium chloride, cesium bromide, cesium iodide, etc., and the lead halide can be lead chloride, lead bromide, lead iodide, etc.

[0013] Further, in step (1), the amounts of cesium halide and lead halide precursors of the perovskite quantum dots are added in stoichiometric proportions according to the molecular formula of the perovskite quantum dots.

[0014] Further, in step (1), the total mass of the perovskite quantum dot precursors is 4-18 wt% of the graphite phase carbon nitride (g-C3N4) nanosheets, such as 4%, 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, or 18%.

[0015] Further, in step (1), the perovskite quantum dot precursors are mixed with graphite phase carbon nitride nanosheets and then thoroughly ground, and the two are allowed to undergo mechanical chemical pre-reaction through grinding. The mechanical chemical pre-reaction refers to pre-reaction of the precursor raw materials through grinding, with the raw materials linked by hydrogen bonds.

[0016] Further, in step (1), the mixture is ground until the volume expands to 1.2-1.5 times the original volume, such as 1.2 times, 1.3 times, 1.4 times, or 1.5 times, and the grinding generally takes 4-10 hours, such as 4h, 5h, 6h, 7h, 8h, 9h, or 10h.

[0017] Furthermore, in step (2), the low-temperature freezing process is as follows: the mixture is frozen at -15 to -20°C for 4-6 hours, then thawed at room temperature, and then frozen again at -15 to -20°C for 4-6 hours, and then thawed at room temperature.

[0018] Furthermore, in step (3), the mixture after low-temperature freezing is calcined for a period of time at a certain temperature and in an inert atmosphere to complete the thermal polymerization of carbon nitride and realize the in-situ high-temperature solid-phase synthesis of quantum dots. The calcination temperature is 600-750℃, for example 600℃, 610℃, 620℃, 630℃, 640℃, 650℃, 660℃, 670℃, 680℃, 690℃, 700℃, 710℃, 720℃, 730℃, 740℃, 750℃. The holding time is generally 10-40 minutes, for example 10 minutes, 15 minutes, 20 minutes, 25 minutes, 30 minutes, 35 minutes, 40 minutes.

[0019] Furthermore, in step (3), the inert atmosphere can be provided by inert gases such as nitrogen and argon.

[0020] Furthermore, in step (3), the heating rate is generally 5-10℃ / min.

[0021] Furthermore, in step (4), the calcined sample is first washed with water to remove unreacted chemical reagents, and the number of water washings is 1-5 times, for example, 1 time, 2 times, 3 times, 4 times, 5 times, preferably 3 times. After water washing, it is washed with ethanol 1-5 times, for example, 1 time, 2 times, 3 times, 4 times, 5 times, preferably 3 times.

[0022] This invention also provides a perovskite quantum dot / graphitic carbon nitride nanosheet composite material obtained according to the above method. This invention achieves solid-phase in-situ high-temperature synthesis and growth of perovskite quantum dots on carbon nitride nanosheets through two processes: mechanochemical pre-reaction and high-temperature thermal polymerization. The quantum dots exhibit good crystallinity and form heterojunctions with graphitic carbon nitride in the absence of ligands, resulting in superior photocatalytic performance. The composite material obtained by this invention is in the form of ultrathin nanosheets, with a thickness of less than 1 nanometer, and perovskite quantum dot nanoparticles are uniformly embedded in the carbon nitride nanosheets.

[0023] This invention also provides the application of the above-mentioned perovskite quantum dot / graphite phase carbon nitride nanosheet composite material in photocatalysis. This perovskite quantum dot / graphite phase carbon nitride nanosheet composite material can be used as a photocatalyst for photocatalytic water splitting to produce hydrogen, photocatalytic reduction of CO2, photocatalytic hydrogen peroxide production, and chemical fuel conversion.

[0024] Compared with the prior art, the present invention has the following beneficial effects:

[0025] 1. The perovskite quantum dots are synthesized on the g-C3N4 nanosheet by the mechanical chemical prevention and high-temperature thermal polymerization processes. By controlling the reaction process and the proportion of the two phases, the light absorption of the obtained composite material is expanded to the entire visible light range, and a good-performing photocatalyst is obtained.

[0026] 2. The quantum dots are synthesized directly under high-temperature solid-phase conditions without adding ligands. The quantum dots have good crystallinity, and the interface defects between the quantum dots and the g-C3N4 are few, which is beneficial to the carrier transport. Meanwhile, the ligands required for the solution chemical synthesis are not needed, the influence of the ligands on the photo-generated carriers is eliminated, the light absorption of the g-C3N4 material is expanded, the light response range is expanded, the utilization rate of sunlight is improved, the separation and conversion efficiency of the photo-generated carriers are improved, and the photocatalytic performance of the g-C3N4 is improved.

[0027] 3. The preparation method is simple and easy to implement, no organic solvent is used, the manufacturing cost is low, and the obtained perovskite quantum dot / graphitic-phase g-C3N4 nanosheet composite material has important application prospects in the fields of water splitting hydrogen production, CO2 photocatalytic reduction, photocatalytic hydrogen peroxide production, and chemical fuel conversion. BRIEF DESCRIPTION OF DRAWINGS

[0028] Figure 1 It is an electron microscope photo of the sample obtained in Example 1.

[0029] Figure 2 It is a stability cycle experiment result graph of the water splitting hydrogen production of the sample obtained in Example 1. DETAILED DESCRIPTION

[0030] The application will be further described through examples below, and the following description is only for explaining the application and does not limit the content thereof.

[0031] In the following examples, the g-C3N4 nanosheet is purchased from Suzhou Beikeman Nanotechnology Co., Ltd.

[0032] Example 1

[0033] 1.1 1.74 grams of g-C3N4 commercial nanosheet powder, 0.11 grams of lead bromide, 0.064 grams of cesium bromide, and 2 milliliters of ethanol are added into a marcasite mortar, and grinding is performed in the marcasite mortar along one direction until the solvent is completely volatilized; after the solvent is volatilized, the solvent is continuously added and grinding is continuously performed in the same way. The grinding is performed for 8 hours according to the above-mentioned way until the volume is expanded to 1.3 times of the original volume.

[0034] 1.2 The above-mentioned sample is frozen at minus 18 degrees Celsius for 5 hours, and then is naturally thawed at normal temperature. After thawing, the sample is again frozen at minus 18 degrees Celsius for 5 hours, and then is naturally thawed at normal temperature.

[0035] 1.3 Take the sample obtained in step 1.2, and heat it to 700℃ at a temperature rising rate of 7℃ / min under a nitrogen atmosphere, and keep it for 30 minutes. Then grind the sample into fine powder for standby.

[0036] 1.4 Take the sample of step 1.3, wash it with water and ethanol for 3 times respectively, and dry it to obtain the CsPbBr3 / g-C3N4 composite photocatalyst. Figure 1 The transmission electron microscope (TEM) photo of the obtained sample is shown in the figure. It can be seen from the figure that the sample is in the form of sheet, and the cesium lead bromide nanoparticles are uniformly embedded in the carbon nitride nanosheet, and are uniformly distributed. Figure 1 The inserted figure is an enlarged electron microscope photo of a cesium lead bromide nanoparticle. From the inserted figure, a clear crystal lattice phase can be observed, indicating that the cesium lead bromide quantum dot has good crystallinity.

[0037] Example 2

[0038] 2.1 Put 4.35 grams of g-C3N4 commercial nanosheet powder, 0.11 grams of lead bromide, 0.064 grams of cesium bromide, and 2 milliliters of ethanol into a marver, and grind in one direction in the marver until the solvent is completely volatilized; after the solvent is volatilized, continue to grind by adding solvent in the same way. Grind for 4 hours in the above-mentioned manner until the volume is expanded to 1.2 times the original volume.

[0039] 2.2 Freeze the above-mentioned sample at minus 15 degrees Celsius for 6 hours, and then naturally thaw at room temperature. After thawing, freeze it again at minus 15 degrees Celsius for 6 hours, and then naturally thaw at room temperature.

[0040] 2.3 Take the sample obtained in step 2.2, and heat it to 600℃ at a temperature rising rate of 5℃ / min under a nitrogen atmosphere, and keep it for 40 minutes. Then grind the sample into fine powder for standby.

[0041] 2.4 Take the sample of step 2.3, wash it with water and ethanol for 3 times respectively, and dry it to obtain the CsPbBr3 / g-C3N4 composite photocatalyst. The morphology of the obtained sample is similar to that of Example 1.

[0042] Example 3

[0043] 3.1 Put 0.97 grams of g-C3N4 commercial nanosheet powder, 0.11 grams of lead bromide, 0.064 grams of cesium bromide, and 2 milliliters of ethanol into a marver, and grind in one direction in the marver until the solvent is completely volatilized; after the solvent is volatilized, continue to grind by adding solvent in the same way. Grind for 10 hours in the above-mentioned manner until the volume is expanded to 1.5 times the original volume.

[0044] 3.2 Freeze the above-mentioned sample at minus 20 degrees Celsius for 4 hours, and then naturally thaw at room temperature. After thawing, freeze it again at minus 20 degrees Celsius for 4 hours, and then naturally thaw at room temperature.

[0045] 3.3 Take the sample obtained in step 3.2, and heat it to 750°C at a temperature increasing rate of 10°C / min under a nitrogen atmosphere, and keep it for 10 min. Then, grind the sample into fine powder for standby.

[0046] 3.4 Wash the sample obtained in step 3.3 with water and ethanol for 3 times respectively, and dry it to obtain the CsPbBr3 / g-C3N4 composite photocatalyst. The morphology of the obtained sample is similar to that of Example 1.

[0047] Example 4

[0048] 4.1 Put 1.36 g of g-C3N4 commercial nanosheet powder, 0.086 g of lead chloride, 0.05 g of cesium chloride and 2 mL of ethanol into a mortar, and grind them in one direction until the solvent is completely volatilized. After the solvent is volatilized, add the solvent again and continue to grind in the same way. Grind in the above-mentioned way until the volume is expanded to 1.3 times of the original volume.

[0049] 4.2 Freeze the above-mentioned sample at minus 18 degrees Celsius for 5 hours, and then naturally thaw it at room temperature. After thawing, freeze it again at minus 18 degrees Celsius for 5 hours, and then naturally thaw it at room temperature.

[0050] 4.3 Take the sample obtained in step 4.2, and heat it to 700°C at a temperature increasing rate of 7°C / min under a nitrogen atmosphere, and keep it for 30 min. Then, grind the sample into fine powder for standby.

[0051] 4.4 Wash the sample obtained in step 4.3 with water and ethanol for 3 times respectively, and dry it to obtain the CsPbCl3 / g-C3N4 composite photocatalyst. The morphology of the obtained sample is similar to that of Example 1, and the photocatalytic hydrogen production performance is about 85% of that of Example 1, because the absorption wavelength of the cesium lead quantum dot is shortened and the absorption of visible light is narrowed.

[0052] Example 5

[0053] 5.1 Put 2.18 g of g-C3N4 commercial nanosheet powder, 0.14 g of lead iodide, 0.078 g of cesium iodide and 2 mL of ethanol into a mortar, and grind them in one direction until the solvent is completely volatilized. After the solvent is volatilized, add the solvent again and continue to grind in the same way. Grind in the above-mentioned way until the volume is expanded to 1.3 times of the original volume.

[0054] 5.2 Freeze the above-mentioned sample at minus 18 degrees Celsius for 5 hours, and then naturally thaw it at room temperature. After thawing, freeze it again at minus 18 degrees Celsius for 5 hours, and then naturally thaw it at room temperature.

[0055] 5.3 The sample obtained in step 5.2 was heated to 700℃ at a heating rate of 7℃ / min under nitrogen atmosphere and kept for 30min, then the sample was ground into fine powder and reserved.

[0056] 5.4 The sample obtained in step 5.3 was washed with water and ethanol for 3 times respectively, and dried to obtain the CsPbI3 / g-C3N4 composite photocatalyst. The morphology of the obtained sample was similar to that of example 1, and the photocatalytic hydrogen production performance was about 104% of that of example 1. The reason was that the absorption wavelength of the cesium lead iodide quantum dot was lengthened, the absorption region of the visible light was widened, and the efficiency was improved.

[0057] Example 6

[0058] The CsPbBr3 / g-C3N4 composite photocatalyst was prepared according to the method of example 1, except that in step 1.1, the amount of lead bromide was 0.044g, and the amount of cesium bromide was 0.0256g. The morphology of the obtained sample was similar to that of example 1, and the photocatalytic hydrogen production performance was about 70% of that of example 1.

[0059] Example 7

[0060] The CsPbBr3 / g-C3N4 composite photocatalyst was prepared according to the method of example 1, except that in step 1.1, the amount of lead bromide was 0.198g, and the amount of cesium bromide was 0.115g. The morphology of the obtained sample was similar to that of example 1, and the photocatalytic hydrogen production performance was about 60% of that of example 1.

[0061] Comparative Example 1

[0062] 1.1 g-C3N4 powder 1.74g, lead bromide 0.11g, cesium bromide 0.064g, ethanol 2mL were added into a maroon mortar, and grinding was carried out in one direction in the maroon mortar. After the solvent was completely volatilized, the solvent was added for continuous grinding. The grinding was stopped after 2h, and the mixture did not swell in volume;

[0063] 1.2 The same as example 1.2;

[0064] 1.3 The same as example 1.3;

[0065] 1.4 The same as example 1.4;

[0066] The electron microscope photograph showed that the cesium lead bromide quantum dots in the obtained sample were unevenly distributed on the carbon nitride, and the quantum dots and the carbon nitride were not well combined. The photocatalytic performance was poor, similar to that of pure carbon nitride, and the surface of the two materials was not well combined.

[0067] Comparative Example 2

[0068] The product was prepared in the same manner as in Example 2, except that in step 2.3, the temperature was raised to 800℃ at a rate of 5℃ / min under a nitrogen atmosphere for 20 minutes. The sample was black in color. There was almost no photocatalytic hydrogen production.

[0069] Comparative Example 3

[0070] The product was prepared in the same manner as in Example 3, except that step 3.2 was omitted and the milled sample was directly calcined.

[0071] The sample was loose and partially agglomerated. The sample could not be uniformly dispersed during the water splitting process, and the photocatalytic properties were poor.

[0072] Verification of photocatalytic performance

[0073] The samples of the examples and comparative examples were used as photocatalysts for photocatalytic hydrogen production experiments. The experiments were performed in a conventional water system hydrogen production device. The light source was a xenon lamp with full spectrum, and the irradiation light source was controlled in the visible light region by a filter. The catalyst was added to the reactor, and 1% chloroplatinic acid solution was added as a cocatalyst. The catalyst was the photocatalyst prepared in the above examples and comparative examples, commercially available CsPbBr3 quantum dots, or g-C3N4 nanosheet powder in step 1.1 of Example 1.

[0074] The hydrogen production was detected by gas chromatography, and then the hydrogen production rate at 2.5h was calculated. The hydrogen production rate formula was: the molar amount of hydrogen / (the mass of the catalyst x 2.5h). The results are shown in Table 1.

[0075]

[0076] The stability experiment was performed using the photocatalyst of Example 1 as an example. The experimental steps are shown above. The hydrogen production time was 2.5h each time, and the experiment was repeated 4 times. The catalyst used in the first experiment was reused in the subsequent experiments. The experimental results are shown in Figure 2 As can be seen from the figure, the hydrogen production efficiency of the catalyst did not decrease after 4 experiments, indicating that the photocatalyst of the present application has good recyclable stability.

Claims

1. A method for in-situ growth of perovskite quantum dots on carbon nitride nanosheets, characterized in that: Includes the following steps: (1) The perovskite quantum dot precursor was mixed and ground with graphitic carbon nitride nanosheets to achieve a mechanochemical pre-reaction; (2) The mixture after grinding in step (1) is subjected to low-temperature freezing treatment; (3) The frozen sample was calcined in an inert atmosphere to achieve in-situ synthesis of perovskite quantum dots on carbon nitride nanosheets; (4) The calcined sample was washed and dried to obtain a perovskite quantum dot / graphite phase carbon nitride nanosheet composite material; In step (1), the mixture is ground until its volume expands to 1.2-1.5 times its original size; In step (2), the low-temperature freezing process is as follows: the mixture is frozen at -15 to -20°C for 4-6 hours, then thawed at room temperature, and then frozen again at -15 to -20°C for 4-6 hours, and then thawed at room temperature. In step (3), the calcination temperature is 600-750℃.

2. The method according to claim 1, characterized in that: The perovskite quantum dot has the structural formula CsPbX3, where X is a halogen.

3. The method according to claim 1, characterized in that: The precursors of perovskite quantum dots are a mixture of cesium halide and lead halide.

4. The method according to claim 1 or 3, characterized in that: The amount of perovskite quantum dot precursor used is 4-18 wt% of graphitic carbon nitride nanosheets.

5. The method according to claim 1, characterized in that: In step (1), the grinding time is 4-10 hours.

6. The method according to claim 1, characterized in that: In step (3), the heat preservation time is 10-40 minutes.

7. The method according to claim 1 or 6, characterized in that: In step (3), the heating rate is 5-10℃ / min.

8. The method according to claim 1, characterized in that: In step (4), during washing, water washing is performed first, followed by alcohol washing.

9. A perovskite quantum dot / graphite-phase carbon nitride nanosheet composite material and its application as a photocatalyst, characterized in that: The perovskite quantum dot / graphite phase carbon nitride nanosheet composite material is obtained by the method described in any one of claims 1-8 for in-situ growth of perovskite quantum dots on carbon nitride nanosheets.

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