A heterojunction ultrathin two-dimensional nanomaterial modified with Co3O4, its preparation method and application

Abstract

A heterojunction ultrathin two-dimensional nanomaterial modified with Co3O4, its preparation method and application belong to the technical field of photocatalytic nanomaterials. By means of a hydrothermal method, ultrathin cobalt hydroxycarbonate nanosheets are prepared and used as a precursor, and then ultrathin Co3O4 nanosheets are prepared by a rapid calcination method; through an adsorption method, Cu 2+ is adsorbed on the surface of the ultrathin Co3O4 nanosheets, and then through secondary calcination, the heterojunction ultrathin two-dimensional nanosheets are obtained. In the present invention, CuO with strong activity and low cost is used as an auxiliary site, and combined with Co3O4 to form a heterojunction, which can adjust the energy band structure with higher efficiency, enhance the migration rate of carriers, and enhance the CO2 photoreduction efficiency.

Description

Technical Field

[0001] The invention belongs to the technical field of photocatalytic nanomaterials, and specifically relates to a Co3O4-modified heterojunction ultra-thin two-dimensional nanomaterial, a preparation method thereof, and an application thereof. Background Art

[0002] In order to alleviate the greenhouse effect and energy dilemma, it is urgent to develop new energy technologies. In this regard, photosynthesis in nature provides us with inspiration: artificial photosynthesis can be used to convert carbon dioxide and water into high-value-added carbon-based fuels and oxygen at normal temperature and pressure, which can not only reduce the concentration of CO2 in the environment and alleviate the problem of global warming, but also produce renewable green energy to achieve effective circulation of carbon elements.

[0003] Common semiconductor photocatalysts include univalent semiconductor photocatalysts, but due to the fixed energy band structure, it is difficult to take into account both a wide light absorption range and strong redox ability at the same time. At the same time, photogenerated electrons and holes are prone to recombination, which makes the quantum efficiency of the photocatalytic reaction low, thereby affecting the yield. The existence of multi-component semiconductor photocatalysts can solve the incompatibility problem well. The improvement of multi-component semiconductor photocatalytic activity can be attributed to the formation of heterojunctions. Different semiconductor materials have different energy band structures. After the heterojunction is formed, not only the band gap width can be adjusted, but also the redox ability can be changed. It can also promote the separation and transfer of photogenerated carriers, while expanding the absorption range of light and improving the stability of the photocatalyst.

[0004] In terms of semiconductor structure, Co3O4 ultra-thin two-dimensional nanosheets can expose a large number of unsaturated active atoms due to their atomic-level thickness, and serve as active centers for photocatalytic reactions, greatly improving the separation and transfer efficiency of photogenerated carriers. However, today, Co3O4 heterojunction materials are still relatively rare, and some Co3O4 composite materials are combined with rare earths and precious metals. Because of its high theoretical capacitance and multi-valence band transitions, it is usually used as an efficient capacitor energy storage material. A few are used as photocatalytic materials and are mainly used in the field of photocatalytic water splitting to produce hydrogen, but are relatively unpopular in the direction of photocatalytic CO2 reduction. Summary of the invention

[0005] The purpose of the present invention is to propose a modified Co3O4 heterojunction ultra-thin two-dimensional nanomaterial and a preparation method and application thereof. The material can be used as a catalyst for photocatalytic CO2 reduction and has good catalytic activity and stability.

[0006] The present invention achieves the above-mentioned purpose through the following technical solutions:

[0007] The present invention provides a method for preparing a Co3O4-modified heterojunction ultra-thin two-dimensional nanomaterial, comprising the following steps:

[0008] (1) First, prepare the cobalt hydroxycarbonate ultrathin nanosheet precursor by the hydrothermal method, and then prepare the Co3O4 ultrathin nanosheet by the rapid calcination method; among them, the rapid calcination method is specifically: place the precursor in an air atmosphere at 300-360 °C for rapid calcination;

[0009] (2) Obtain the metal ion-modified Co3O4 ultrathin two-dimensional nanosheet by the adsorption method, and then obtain the heterojunction ultrathin two-dimensional nanosheet by secondary calcination; among them, the metal ion is Cu 2+ .

[0010] Further improvement lies in that the calcination time of the rapid calcination in step (1) is 4-10 min.

[0011] Further improvement lies in that the specific preparation process of the cobalt hydroxycarbonate ultrathin nanosheet precursor in step (1) is as follows:

[0012] Dissolve cobalt acetate tetrahydrate in a mixed solution of water and ethylene glycol, dropwise add sodium hydroxide solution to it, stir and transfer to a hydrothermal kettle, heat and react at 90-130 °C for 3-6 h. After the reaction is completed, take it out and centrifuge, wash repeatedly with ethanol and deionized water, and freeze-dry at -20 °C to -30 °C. The drying time is about 40-60 h to obtain the cobalt hydroxycarbonate ultrathin nanosheet.

[0013] Further improvement lies in that in the mixed solution of water and ethylene glycol, the mixing volume ratio of water and ethylene glycol is 1:0.5 to 0.5:1, and the dropping rate of the dropped sodium hydroxide is 100-300 μL / s.

[0014] Further improvement lies in that step (2) is specifically:

[0015] Perform ultrasonic treatment on the Co3O4 ultrathin nanosheet to uniformly disperse it in an aqueous solution to obtain suspension A; the concentration of the Co3O4 ultrathin nanosheet in suspension A is 2-10 mg / mL;

[0016] Dropwise add the prepared metal salt solution to suspension A, stir at high speed, separate, wash and freeze-dry, collect the solid to obtain the Co3O4 ultrathin nanosheet with surface-doped metal ions; among them, the metal salt is CuCl2, and the concentration is 0.05-0.4 mg / mL;

[0017] Calcinate the Co3O4 ultrathin nanosheet with surface-doped metal ions in a tube furnace in an oxygen atmosphere at a heating rate of 3-5 °C / min to 300-400 °C, and keep it warm for 2-3 hours to finally obtain the heterojunction ultrathin two-dimensional nanomaterial.

[0018] The present invention provides a modified Co3O4 heterojunction ultrathin two-dimensional nanomaterial prepared by the above preparation method. The heterojunction ultrathin two-dimensional nanomaterial is specifically Co3O4 / CuO, and the content of Cu element is 1-3 wt%.

[0019] The present invention provides an application of the above-mentioned modified Co3O4 heterojunction ultrathin two-dimensional nanomaterial as a photocatalyst in the field of photocatalytic carbon dioxide.

[0020] A further improvement lies in that the heterojunction ultrathin two-dimensional nanomaterial uses Co3O4 / CuO to photocatalytically reduce carbon dioxide to produce methane.

[0021] A further improvement lies in that the reaction system for photocatalytic reduction of carbon dioxide to produce methane is a pure water system.

[0022] The present invention provides a method for photocatalytic production of methane, which includes the following steps:

[0023] (1) Dispersing the above-mentioned heterojunction ultrathin two-dimensional nanomaterial in pure water to obtain a dispersion;

[0024] (2) Spin-coating the dispersion on the surface of a high-borosilicate carrier, drying it and then placing it in a photocatalytic system. Extract the excess gas in the system to a vacuum state, and then introduce high-purity CO2 gas until the system pressure is 80 kPa. Turn on the high-energy xenon lamp for photocatalytic reduction to produce methane.

[0025] The beneficial effects of the present invention are as follows:

[0026] 1. The heterojunction ultrathin two-dimensional nanosheets prepared in the present invention can be used for efficient photocatalytic reduction of carbon dioxide to produce methane, and the catalytic activity is very high. The same type of materials (materials containing CuO or Co3O4) cannot produce methane under the same reaction conditions, and the catalytic reaction is completed in a pure water system without the need for any additives, which is green and environmentally friendly.

[0027] 2. In the preparation method of the present invention, the rapid calcination method used in the first step to prepare ultrathin two-dimensional Co3O4 nanosheets is first proposed in the preparation of Co3O4 nanomaterials. Compared with other reported methods, it has low energy consumption, good controllability, and the rapid calcination method adopted can solve the problem of easy agglomeration during calcination due to strong interplanar forces without destroying the two-dimensional material structure, and can also effectively remove the surfactants adsorbed on the surface.

[0028] 3. By changing the content of Cu 2+ added in the raw materials, the amount of CuO generated in the final product can be regulated, and the size of the heterojunction interface between CuO and Co3O4 will change, thereby the yield of the product can be adjusted.

[0029] 4. The composite material is inexpensive, abundant in source, and its synthesis process is simple, mild and environmentally friendly. Brief Description of the Drawings

[0030] Figure 1 It is an X-ray diffraction (XRD) picture of Co3O4 ultrathin two-dimensional nanosheets;

[0031] Figure 2 It is a TEM picture of Co3O4 ultrathin two-dimensional nanosheets;

[0032] Figure 3 It is a TEM picture of Co3O4 / CuO heterojunction ultrathin two-dimensional nanosheets with a copper content of 1%;

[0033] Figure 4 It is a TEM picture of Co3O4 / CuO heterojunction ultrathin two-dimensional nanosheets with a copper content of 3%;

[0034] Figure 5 It is a Raman spectrum comparing Co3O4 / CuO nanosheets with a copper content of 1% and Co3O4 nanosheets.

[0035] Figure 6 It is a graph of the yield performance of photocatalytic carbon dioxide reduction to methane for Co3O4 / CuO heterojunction ultrathin two-dimensional nanosheets with copper contents of 1% and 3%. Detailed Description of the Invention

[0036] To further elaborate on the technical means and effects adopted by the present invention to achieve the predetermined invention purpose, the following, in combination with the drawings and embodiments, details the specific implementation manners, structures, features and effects of a modified Co3O4 heterojunction ultrathin two-dimensional nanomaterial and its preparation method proposed according to the present invention as follows.

[0037] 1. Materials

[0038] The methods used in the present invention are all conventional methods known to those skilled in the art unless otherwise specified, and the reagents used, etc., are all commercially available products unless otherwise specified.

[0039] 2. Methods

[0040] 2.1 Preparation of Heterojunction Ultrathin Two-Dimensional Nanomaterials

[0041] (1) Preparation of Co3O4 Ultrathin Nanosheets

[0042] First, prepare the basic cobalt carbonate ultrathin nanosheet precursor by hydrothermal method. The specific preparation process is as follows: Dissolve cobalt acetate tetrahydrate in a mixed solution of water and ethylene glycol, with the volume ratio of water to ethylene glycol being 1:0.5 to 0.5:1. Dropwise add sodium hydroxide solution to it, with the dropping rate of sodium hydroxide being 100 - 300 μL / s. After stirring for about 10 - 60 minutes, transfer it to a polytetrafluoroethylene hydrothermal kettle and heat it at 90 - 130 °C for about 3 - 6 h. After the reaction is completed, take it out for centrifugation, wash it repeatedly with ethanol and deionized water, and freeze-dry it between -20 °C and -30 °C for about 40 - 60 h to obtain the basic cobalt carbonate ultrathin nanosheet;

[0043] Place the obtained basic cobalt carbonate nanosheet precursor in a quartz boat and put it in a tube furnace. Rapidly calcine it in an air atmosphere at 300 - 360 °C that has been pre-heated for 4 - 10 min to prepare Co3O4 ultrathin nanosheets.

[0044] (2) Preparation of metal ion-doped Co3O4 ultrathin nanosheets

[0045] Obtain metal ion-modified Co3O4 ultrathin two-dimensional nanosheets by adsorption method, and then obtain heterojunction ultrathin two-dimensional nanosheets through secondary calcination; among them, the metal ion is Cu 2+ , specifically:

[0046] Perform ultrasonic treatment on Co3O4 ultrathin nanosheets to uniformly disperse them in an aqueous solution to obtain suspension A; gradually add the prepared metal salt solution dropwise to suspension A, stir at high speed, separate, wash, and freeze-dry, and collect the solid to obtain Co3O4 ultrathin nanosheets with metal ions doped on the surface; among them, the metal salt is CuCl2; Calcinate the Co3O4 ultrathin nanosheets with metal ions doped on the surface in a tube furnace in an oxygen atmosphere at a heating rate of 3 - 5 °C / min to 300 - 400 °C, and keep the temperature for 2 - 3 hours to finally obtain the heterojunction ultrathin two-dimensional nanomaterial. Among them, the concentration of Co3O4 ultrathin nanosheets in suspension A is 2 - 10 mg / mL, the metal salt is CuCl2, and the concentration of the metal salt solution is 0.05 - 0.4 mg / mL.

[0047] 2.2 Preparation of Co3O4 / CuO heterojunction ultrathin two-dimensional nanomaterials

[0048] 2.2.1 Co3O4 / CuO heterojunction ultrathin two-dimensional nanomaterials with a copper content of 1%

[0049] Prepare a mixed solution of 20 mL of deionized water and ethylene glycol with a volume ratio of 1:1 and stir vigorously. After 5 minutes, add 1 mmol of cobalt acetate tetrahydrate that has been weighed out and stir for 30 min at the same rotation speed. It is observed that the solution changes from colorless to rose red. Then dissolve 40 mg of flaky sodium hydroxide that has been weighed out in 0.5 mL of deionized water. After it is fully dissolved, add it dropwise to the mixed solution at a dropping rate of 200 μL / s. Observe that the solution changes from rose red to sapphire blue, and then stir at high speed for 20 minutes. After the stirring is completed, transfer the hydrothermal reactor containing the mixed solution to a forced-air drying oven and heat it at a temperature of 100 °C for 5 h. After heating, take it out, centrifuge to obtain a solid product, wash it repeatedly with ethanol and deionized water, and then freeze-dry it at -25 °C for 48 h to obtain cobalt hydroxycarbonate precursor nanosheets.

[0050] Grind the completely dried precursor nanosheet sample to a particle size of ≤5 μm, place it in a pre-heated tubular furnace at 350 °C, and quickly calcine it in an air atmosphere for 6 min to obtain black two-dimensional ultrathin Co3O4 nanosheets.

[0051] Weigh 50 mg of the prepared Co3O4 ultrathin nanosheets and put them into a round-bottom flask, and ultrasonically dissolve them with 10 mL of deionized water. Then pour 0.5 mg of CuCl2 into a centrifuge tube containing 5 mL of deionized water, ultrasonically disperse it completely, and slowly add the above-prepared Cu 2+ solution to the Co3O4 sample dispersion, stir well for 2 h to make Cu 2+ completely adsorbed on the surface of the Co3O4 ultrathin nanosheets. Centrifuge and collect the black product, freeze-dry it, and calcine it in a tubular furnace at 300 °C for 2 h in an oxygen atmosphere with a heating rate of 5 °C / min. The calcined product is washed repeatedly with deionized water and ethanol and dried in vacuo at 50 °C to finally obtain black Co3O4 / CuO heterojunction ultrathin two-dimensional nanosheets with a copper content of 1%.

[0052] Use high-resolution transmission electron microscopy, X-ray diffraction, and Raman spectroscopy to characterize the morphology and phase of the Co3O4 ultrathin two-dimensional nanosheets formed by rapid calcination. As attached Figure 1 is the X-ray diffraction pattern of the Co3O4 nanosheets, which corresponds one by one to the standard peak shape of Co3O4 corresponding to card PDF#43-1003, confirming that the product is Co3O4. As attached Figure 2 is the high-resolution transmission electron microscopy image of the Co3O4 nanosheets. It can be seen that the product has the morphology of ultrathin nanosheets, and the lateral size is approximately distributed in the range of 500 nm. As attached Figure 3 is the transmission electron microscopy image of the Co3O4 / CuO heterojunction ultrathin two-dimensional nanosheets with a copper content of 1%. Its morphology is also ultrathin sheet-like, and there is obvious CuO distributed on the surface of the Co3O4 nanosheets, forming a heterojunction with Co3O4. As attachedFigure 5 Raman spectra comparison of Co3O4 / CuO heterojunction ultrathin two-dimensional nanosheets with 1% copper content and the precursor Co3O4 nanosheets. It can be seen that the loading of a small amount of CuO does not change the phase structure of the Co3O4 matrix.

[0053] 2.2.2 Co3O4 / CuO heterojunction ultrathin two-dimensional nanosheets with 3% copper content

[0054] The implementation steps are the same as in 2.2.1, with the difference being that when weighing the CuCl2 sample, the weighed mass is changed to 1.5 mg, and other conditions remain unchanged.

[0055] As shown in the Figure 4 transmission electron microscopy image of Co3O4 / CuO heterojunction ultrathin two-dimensional nanosheets with 3% copper content, indicating that there is CuO distributed on the surface of Co3O4 nanosheets, showing a small amount of granular shape, forming a heterojunction with Co3O4, but the overall morphology of Co3O4 does not change, and the morphology of the Co3O4 / CuO heterojunction with 3% copper content is also ultrathin sheet-like.

[0056] 2.2.3 Application

[0057] Disperse the Co3O4 / CuO heterojunction nanosheets in pure water, then spin-coat the above dispersion on the surface of a high-borosilicate carrier, dry it and place it in a photocatalytic system. Extract the excess gas in the system to a vacuum state, and then introduce high-purity CO2 gas until the system pressure is 80 kPa. Turn on the high-energy xenon lamp (equipped with a 420 nm filter) for photocatalytic reduction. The reaction equation is CO2 + 2H2O → CH4 + 2O2.

[0058] As shown in the Figure 6 photocatalytic carbon dioxide reduction to methane yield performance graph of Co3O4 / CuO heterojunction ultrathin two-dimensional nanosheets with 1% and 3% copper content. It can be seen that by changing the content of Cu 2+ added in the raw materials, the amount of CuO generated in the final product can be regulated, and the size of the heterojunction interface between CuO and Co3O4 will change, thereby regulating the yield of the product.

[0059] 2.3 Conclusion

[0060] The present invention provides a good scheme for the simple synthesis of two-dimensional ultrathin heterojunction nanosheets. Through inexpensive Cu metal and a simple secondary calcination method, two-dimensional nanosheets with an optimized energy band structure and significant significance for photocatalytic carbon dioxide reduction to methane are synthesized.

[0061] By combining Co3O4 and CuO, the present invention applies Co3O4 in the field of photocatalytic CO2 reduction where it has no inherent advantages. After modification, the performance is significantly improved. Moreover, a series of raw materials used are environmentally friendly and low-cost. The product synthesized by photocatalysis is methane, which is an important industrial raw material and has the highest yield among catalysts of the same type.

[0062] The above-described embodiments merely represent several implementation manners of the present invention. The description is relatively specific and detailed, but it should not be construed as a limitation on the scope of the patent of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the concept of the present invention, several modifications and improvements can still be made, and these all belong to the protection scope of the present invention.

Claims

1. Application of a modified Co3O4 heterojunction ultrathin two-dimensional nanomaterial as a photocatalyst in photocatalytic reduction of carbon dioxide to produce methane, characterized in that, The preparation method of the heterojunction ultrathin two-dimensional nanomaterial includes the following steps: (1) First, prepare a cobalt hydroxycarbonate ultrathin nanosheet precursor by a hydrothermal method, and then use a rapid calcination method to prepare Co3O4 ultrathin nanosheets; wherein, the rapid calcination method is specifically: place the precursor in an air atmosphere at 300-360 °C for rapid calcination; (2) The metal ion-modified Co3O4 ultrathin two-dimensional nanosheets are obtained by an adsorption method, and then the heterojunction ultrathin two-dimensional nanosheets are obtained through secondary calcination; wherein the metal ion is Cu 2+ .

2. The application according to claim 1, wherein In the step (1), the calcination time of the rapid calcination is 4-10 min.

3. The application according to claim 1, characterized in that, The specific preparation process of the cobalt hydroxycarbonate ultrathin nanosheet precursor in the step (1) is as follows: Dissolve cobalt acetate tetrahydrate in a mixed solution of water and ethylene glycol, dropwise add sodium hydroxide solution thereto, stir and transfer it to a hydrothermal autoclave, heat and react at 90-130 °C for 3-6 h, after the reaction is completed, take it out and centrifuge, wash it repeatedly with ethanol and deionized water, and freeze-dry at -20 °C to -30 °C to obtain cobalt hydroxycarbonate ultrathin nanosheets.

4. The application according to claim 3, characterized in that, In the mixed solution of water and ethylene glycol, the mixing volume ratio of water and ethylene glycol is 1:0.5-0.5:1, and the dropping rate of the dropped sodium hydroxide is 100-300 μL / s.

5. The application according to claim 1, wherein The step (2) is specifically: Perform ultrasonic treatment on the Co3O4 ultrathin nanosheets to uniformly disperse them in an aqueous solution to obtain suspension A, and the concentration of Co3O4 ultrathin nanosheets in suspension A is 2-10 mg / mL; Dropwise add the prepared metal salt solution to suspension A, stir at high speed, separate, wash and freeze-dry, collect the solid to obtain Co3O4 ultrathin nanosheets with metal ions doped on the surface; wherein, the metal salt is CuCl2, and the concentration of the CuCl2 solution is 0.05-0.4 mg / mL; Calcine the Co3O4 ultrathin nanosheets with metal ions doped on the surface in a tube furnace in an oxygen atmosphere at a heating rate of 3-5 °C / min to 300-400 °C, and keep the temperature for 2-3 hours to finally obtain the heterojunction ultrathin two-dimensional nanomaterial.

6. The application according to claim 1, characterized in that, The heterojunction ultrathin two-dimensional nanomaterial is specifically Co3O4 / CuO, and the content of Cu is 1-3 wt%.

7. The application according to claim 1, wherein The reaction system for photocatalytic reduction of carbon dioxide to produce methane is a pure water system.

8. The application according to claim 1, characterized in that, The heterojunction ultrathin two-dimensional nanomaterial is used as a photocatalyst for photocatalytic reduction of carbon dioxide to produce methane, which specifically includes the following steps: (1) Disperse the heterojunction ultrathin two-dimensional nanomaterial in pure water to obtain a dispersion; (2) Spin-coat the dispersion on the surface of a high-borosilicate carrier, dry it and place it in a photocatalytic system, extract the excess gas in the system to a vacuum state, then introduce high-purity CO2 gas until the system pressure is 80 kPa, and turn on the high-energy xenon lamp for photocatalytic reduction to produce methane.

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