A two-dimensional polyoxometalate nanomaterial and its preparation method and application

Abstract

The present invention discloses a two-dimensional polyoxometalate nanomaterial and a preparation method and application thereof. The two-dimensional polyoxometalate nanomaterial has a lamellar structure, and the lamellar structure is composed of polyoxometalate clusters arranged in a hexagonal shape. The material has good electron transmission capability and excellent thermal / chemical stability, and at the same time has a porous lamellar structure and a large specific surface area. When applied to a photodetector, it has excellent photoelectric detection performance in different acid-base environments, exhibits excellent photoelectric response speed, and has good long-term stability in harsh application environments.

Description

Technical Field

[0001] The present invention relates to a two-dimensional polyoxometalate nanomaterial and a preparation method thereof, in particular to a two-dimensional polyoxometalate nanomaterial composed of polyoxometalate clusters and a preparation method thereof, and also to a two-dimensional polyoxometalate nanomaterial used in a photoelectric detector, belonging to the technical field of two-dimensional nanomaterial preparation. Background Art

[0002] In the past two decades, two-dimensional (2D) layered nanomaterials, such as graphene, black phosphorus, transition metal dichalcogenides (TMDs), and transition metal carbides or nitrides (MXene), have attracted great attention due to their wonderful and potential physical and chemical properties. Through the research of these traditional two-dimensional nanomaterials, many exciting results have been achieved in various technical fields. For example, the many excellent and unique properties of graphene make it have broad application prospects in various fields, and the preparation of high-quality, large-area graphene that meets different application conditions is a prerequisite for graphene to exert its unique properties. However, the inability to exist stably in an oxygen environment is still a huge disadvantage of graphene, and there are also a small amount of impurities and defects in the prepared materials. At the same time, the preparation cost is also very high. Similar to graphene, black phosphorus is an emerging 2D material with high carrier mobility, adjustable direct band gap, high electronic switching ratio, etc., which makes it show excellent talents in optoelectronic devices, sensors, lithium-ion batteries, solar cells and biomedicine. However, due to the poor stability of black phosphorus under air exposure conditions, its applicability is greatly limited. Therefore, the main research direction at present is to find new materials to replace two-dimensional layered van der Waals materials such as graphene and black phosphorus and break through the shackles of photodetectors in terms of materials.

[0003] In recent years, the research on optoelectronic materials has mainly focused on the molecular level. Compared with traditional materials, molecular materials have more special structures, such as polyoxometalates (POMs), which make them a perfect model for studying material synthesis, electron conduction and modification. Similar to graphene, polyoxometalates are sub-nanoscale clusters. Due to the rich metal ions, they also have good electron storage capacity and a variety of structures. They are widely used in photochemistry, catalysis, electronics and electrochemistry. At present, polyoxometalate nanomaterials as photodetectors still have problems such as serious recombination of photogenerated electron / hole pairs and easy detachment from the electrode. Summary of the invention

[0004] In view of the shortcomings of the two-dimensional polyoxometalate nanomaterials and photodetector materials in the prior art, the first object of the present invention is to provide a two-dimensional polyoxometalate nanomaterial with regular morphology, uniform size, and easily regulated structure. The material is rich in metal ions, has good electron storage capacity, and has a stable structure and is not easily oxidized. When used as a photodetector material, it exhibits excellent photoelectric response speed and long-term stability.

[0005] The second object of the present invention is to provide a method for preparing a two-dimensional polyoxometalate nanomaterial, which has simple operation, mild reaction conditions and the potential for large-scale industrial production.

[0006] The third object of the present invention is to provide an application of a two-dimensional polyoxometalate nanomaterial, which exhibits excellent photoelectric response speed and long-term stability when applied to photodetector materials, without problems such as serious recombination of photogenerated electron / hole pairs and easy detachment from the electrode.

[0007] In order to achieve the above technical objectives, the present invention provides a two-dimensional polyoxometalate nanomaterial having a layered structure, wherein the layered structure is composed of polyoxometalate clusters arranged in a hexagonal shape.

[0008] The two-dimensional polyoxometalate nanomaterial of the present invention contains a large number of sub-nanometer polyoxometalate clusters, which are rich in metal ions, have good electron storage capacity, and have a large specific surface area and high chemical affinity. In the polyoxometalate of the present invention, due to the presence of neodymium atoms and tungsten atoms, the neodymium atoms replace the tungsten atoms of the Keggin-type phosphotungstic acid and form four coordination bonds with the surface oxygen atoms, and are connected with two adjacent polyoxometalate clusters to form a zigzag linear arrangement, with a hexagonal layered structure. This highly ordered structure allows the electron sharing of the two-dimensional polyoxometalate nanomaterial to occur between multiple clusters, forming a "super molecule" with interlayer electron delocalization. And the interlayer electron delocalization can effectively reduce the recombination of photogenerated electrons / holes, so that the two-dimensional polyoxometalate nanomaterial can show excellent photoelectric response speed and long-term stability.

[0009] As a preferred solution, the thickness of the lamellar structure is 1.5-5 nm.

[0010] The present invention also provides a method for preparing a two-dimensional polyoxometalate nanomaterial, which comprises the following steps: dripping a solvent containing ammonium cations into a solution of tungsten phosphate and neodymium salt to mix, and then sequentially performing ultrasound and centrifugation to obtain the two-dimensional polyoxometalate nanomaterial.

[0011] The present invention provides neodymium atoms by using neodymium salts, wherein the neodymium atoms replace the tungsten atoms of the keggin-type phosphotungstic acid and form four coordination bonds with the surface oxygen atoms, thereby first forming a polyoxometalate cluster. Then, under the action of electrostatic attraction, ammonium cations spontaneously attach to the clusters, so that the oxygen atoms on the clusters are shielded, while the neodymium atoms are exposed due to electrostatic repulsion, and are connected to two adjacent clusters through the coordination of the neodymium atoms, forming a zigzag linear arrangement, thereby obtaining a hexagonal layered structure.

[0012] As a preferred solution, driven by electrostatic attraction, ammonium cations can spontaneously attach to clusters, the oxygen atoms on the surface are shielded by them, and the neodymium atoms are exposed due to electrostatic repulsion, providing coordination for the connection between clusters.

[0013] As a preferred solution, the ammonium cation is provided by hexadecyltrimethylammonium bromide and / or tetrabutylammonium bromide. If the ammonium cation with a shorter alkyl chain is insufficient to support the two-dimensional porous structure, it will cause the pores to partially collapse. The ammonium cation with a longer alkyl chain preferred by the present invention can support the pores and form a lamellar structure.

[0014] As a preferred solution, the concentration of the solution containing ammonium cations is 0.2-0.5 μmol / L, and the solvent used is chloroform. The use of chloroform in the present invention can fully dissolve the ammonium cations and stabilize the material morphology.

[0015] As a preferred solution, the neodymium salt is neodymium nitrate and neodymium nitrate hydrate; the molar ratio of phosphotungstic acid and neodymium salt is 1: 1 to 5. When the ratio of neodymium salt is too low, the prepared two-dimensional polyoxometalate nanomaterial will be over-stacked, while when the ratio of neodymium salt is too high, the holes in the prepared two-dimensional polyoxometalate nanomaterial will be broken to form a nanobelt structure.

[0016] As a preferred solution, the molar ratio of the ammonium cation to the phosphotungstic acid is 2-3:1.

[0017] As a preferred solution, in the preparation of the two-dimensional polyoxometalate nanomaterials of the present invention, the solvent of the ammonium cation is added dropwise at a rate of 0.5 to 1 mL / s.

[0018] As a preferred solution, the ultrasonic treatment time is 5 to 20 minutes; ultrasound can promote solution mixing and reaction.

[0019] As a preferred solution, the centrifugal conditions are: a rotation speed of 5000-8000 rpm and a time of 5-10 min. Two-dimensional polyoxometalate nanomaterials can be separated by centrifugation.

[0020] The present invention also provides an application of a two-dimensional polyoxometalate nanomaterial, which is used in a photodetector. The material has good electron transport capability and excellent thermal / chemical stability, and exhibits excellent photoelectric response speed and long-term stability when applied to photodetector materials, without the problems of serious recombination of photogenerated electron / hole pairs and easy detachment from electrodes.

[0021] The present invention provides a two-dimensional polyoxometalate nanomaterial for use in preparing a photoelectric detector, comprising the following steps:

[0022] 1): Before use, the glass coated with indium tin oxide (ITO) was ultrasonically treated with deionized water, ethanol and acetone for 10 minutes each.

[0023] 2): 1-10 mg of polyoxometalate nanomaterials are added to 1 mL of polyvinylidene fluoride / chloroform (PVDF / CHCl3) solution and ultrasonically treated for 10-30 minutes to obtain a well-dispersed mixture.

[0024] 3): Then 400 μL of the mixture was dropped onto the ITO glass and dried in a vacuum oven at 40° C. overnight to form the working electrode of the photodetector.

[0025] Compared with the prior art, the technical solution of the present invention has the following beneficial technical effects:

[0026] 1) The two-dimensional polyoxometalate nanomaterial provided by the present invention has an easily controllable structure, good electron transport capability and excellent thermal / chemical stability, and has a porous lamellar structure, thus having a large specific surface area.

[0027] 2) The present invention provides a method for preparing a two-dimensional polyoxometalate nanomaterial, which has the advantages of simple operation, mild reaction conditions, low cost, and potential for large-scale industrial production.

[0028] 3) When a two-dimensional polyoxometalate nanomaterial of the present invention is applied to a photodetector, it has excellent photoelectric detection performance in different acid-base environments, and exhibits excellent photoelectric response speed and good long-term stability in harsh application environments, fast response speed and good long-term stability (more than 90 days). BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 The synthesis process diagram of two-dimensional polyoxometalate nanomaterials is shown in Figure 2. Figure 1 As shown, the two-dimensional polyoxometalate nanomaterial prepared by the present invention is a hexagonal pore structure composed of multiple two-dimensional polyoxometalate clusters.

[0030] Figure 2The transmission electron microscope images at different magnifications of the two-dimensional polyoxometalate nanomaterial prepared in Example 1 are shown. It can be seen from the figure that the two-dimensional polyoxometalate nanomaterial of the present invention is composed of multiple two-dimensional polyoxometalate clusters, and the two-dimensional polyoxometalate clusters are arranged in a hexagonal pore structure.

[0031] Figure 3 This is an atomic force microscope image of the two-dimensional polyoxometalate nanomaterial prepared in Example 1. It can be seen from the image that the thickness of the two-dimensional polyoxometalate nanomaterial is substantially 3 nm.

[0032] Figure 4 This is the thickness distribution diagram of the two-dimensional polyoxometalate nanomaterial prepared in Example 1.

[0033] Figure 5 The photoelectric response performance results of the two-dimensional polyoxometalate nanomaterial prepared in Example 1 as a photodetector, wherein Figure (a) shows the photoresponse behavior of the photodetector based on two-dimensional polyoxometalate in 0.5M KOH electrolyte under different wavelengths and λ irradiation; Figures (b) to (d) are curves of photocurrent, photoresponsivity and specific detection rate corresponding to Figure (a).

[0034] Figure 6 The photoresponse performance of the two-dimensional polyoxometalate nanomaterials and the two-dimensional polyoxometalate clusters prepared in Example 1 as photodetectors is compared, wherein Figure (a) is the photoresponse curve of the two-dimensional polyoxometalate nanomaterials and the polyoxometalate clusters photodetectors at -0.6V in 0.5M KOH electrolyte; Figure (b) is the details of the ON / OFF signal captured from Figure (a); Figure (c) is the comparison curve of the photocurrent at different test wavelengths; and Figure (d) is the photoluminescence spectrum.

[0035] Figure 7 Figures 1 and 2 show the stability test results of the two-dimensional polyoxometalate nanomaterials prepared in Example 1, wherein Figures (a) to (c) show the ON / OFF signals of different selected areas of a brand new detector; Figures (d) to (f) show the ON / OFF signals of different selected areas of the detector after a long-term stability test in 0.5 M KOH electrolyte under simulated sunlight three months later.

[0036] Figure 8 They are transmission electron micrographs of the two-dimensional polyoxometalate nanobelts prepared in Comparative Example 1 and the polyoxometalate clusters in Example 1. As can be seen from the figures, nanobelts and cluster-structured polyoxometalates can be obtained under different metal ratios and without adding ammonium cations.

[0037] Fig. 9The photocurrent density comparison curves of the two-dimensional polyoxometalate nanomaterials prepared in Example 2 and Example 1 at -0.6V in 0.5M KOH electrolyte are shown in the figure. As shown in the figure, both have excellent photoelectric response, and the photoelectric response of the two-dimensional polyoxometalate nanomaterial prepared in Example 1 is better than that of Example 2.

[0038] Fig.10 The photocurrent density comparison curves of the two-dimensional polyoxometalate nanomaterials prepared in Example 3 and Example 1 at -0.6V in 0.5M KOH electrolyte are shown in the figure. As shown in the figure, both have excellent photoelectric response, and the photoelectric response of the two-dimensional polyoxometalate nanomaterial prepared in Example 1 is better than that of Example 3. DETAILED DESCRIPTION

[0039] Many specific details are set forth in the following description to facilitate a full understanding of the present invention. However, the present invention can be implemented in many other ways than those described herein, and those skilled in the art can make similar modifications without violating the connotation of the present invention, so the present invention is not limited to the specific implementation disclosed below.

[0040] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those commonly understood by those skilled in the art of the present invention. The terms used in the specification of the present invention herein are only for the purpose of describing specific embodiments and are not intended to limit the present invention.

[0041] In the embodiments of the present invention, unless otherwise specified, the chemical reagents used can be purchased or prepared by existing preparation methods, and the instruments and equipment used are conventional equipment in the prior art.

[0042] Example 1

[0043] Preparation of two-dimensional polyoxometalate nanomaterials:

[0044] 1) 29 mg of phosphotungstic acid (H3PW 12 O 40 ) and 14 mg of neodymium nitrate hexahydrate (NdN3O9·6H2O) were dissolved in 10 mL of deionized water in a molar ratio of 1:3 and stirred for 30 minutes.

[0045] 2) Dissolve 4.5 mg of hexadecyltrimethylammonium bromide (CTAB) and 4 mg of tetrabutylammonium bromide (TBAB) in 50 mL of chloroform.

[0046] 3) The organic solution obtained in step 2 was slowly added dropwise (dropping speed was 1 mL / s) to the solution obtained in step 1, and the mixed solution was stirred for 10 hours.

[0047] 4) The stirred mixed solution was subjected to ultrasonic treatment for 5 minutes.

[0048] 5) The ultrasonically treated solution was centrifuged at 8000 rpm for 5 minutes. The supernatant was carefully removed, and finally, the precipitate was dried in a vacuum oven at 40° C. overnight to obtain a two-dimensional graphene-like structured polyoxometalate nanomaterial.

[0049] Preparation of two-dimensional polyoxometalate clusters:

[0050] 1) 29 mg of phosphotungstic acid (H3PW 12 O 40 ) and 14 mg of neodymium nitrate hexahydrate (NdN3O9·6H2O) were dissolved in 10 mL of deionized water at a molar ratio of 1:3 and stirred for 30 minutes.

[0051] 2) The stirred mixed solution was ultrasonically treated for 5 minutes.

[0052] 3) The ultrasonically treated solution was centrifuged at 8000 rpm for 5 minutes. The supernatant was carefully removed, and finally, the precipitate was dried in a vacuum oven at 40° C. overnight to obtain a two-dimensional graphene-like structured polyoxometalate nanomaterial.

[0053] Example 2

[0054] The difference between this embodiment and embodiment 1 is that the amount of neodymium nitrate hexahydrate (NdN3O9·6H2O) is replaced with 22 mg, and the other conditions are the same.

[0055] Example 3

[0056] The difference between this embodiment and embodiment 1 is that the amount of neodymium nitrate hexahydrate (NdN3O9·6H2O) is replaced by 4.7 mg, and the other conditions are the same.

[0057] Comparative Example 1

[0058] Preparation of two-dimensional polyoxometalate nanoribbons:

[0059] 1) 29 mg of phosphotungstic acid (H3PW 12 O 40 ) and 45 mg of neodymium nitrate hexahydrate (NdN3O9·6H2O) were dissolved in 10 mL of deionized water at a molar ratio of 1:10 and stirred for 30 minutes.

[0060] 2) Dissolve 4.5 mg of hexadecyltrimethylammonium bromide (CTAB) and 4 mg of tetrabutylammonium bromide (TBAB) in 50 mL of chloroform.

[0061] 3) The organic solution obtained in step 2 was slowly added dropwise (addition speed was 0.5-1 mL / s) to the solution obtained in step 1, and the mixed solution was stirred for 14 hours.

[0062] 4) The stirred mixed solution was subjected to ultrasonic treatment for 5 minutes.

[0063] 5) The ultrasonically treated solution was centrifuged at 8000 rpm for 5 minutes. The supernatant was carefully removed, and finally, the precipitate was dried in a vacuum oven at 40°C overnight to obtain a polyoxometalate nanomaterial with a nanobelt structure. Figure 8 (a) It can be seen that when the proportion of neodymium salt is too high, the polyoxometalate clusters will cause holes to break and cannot be arranged in an orderly hexagonal pore structure to form a lamellar structure, but a band structure.

[0064] Application Example 1

[0065] The glass coated with indium tin oxide (ITO) was ultrasonically treated with deionized water, ethanol and acetone for 10 minutes. 10 mg of the two-dimensional polyoxometalate clusters or two-dimensional polyoxometalate nanomaterials prepared in Example 1 were added to 1 mL of polyvinylidene fluoride / chloroform (PVDF / CHCl3) solution and ultrasonically treated for 30 minutes to obtain a fully dispersed mixture. Then 400 μL of the mixture was dropped onto the ITO glass and dried in a vacuum oven at 40°C overnight to form the working electrode of the photodetector.

[0066] The performance of the prepared photodetector was tested, such as Figure 5 to Figure 7 shown. Figure 5 (a) shows that the two-dimensional polyoxometalate nanomaterial as a photodetector has obvious switching signals in the 400nm incident light range. Figure 5 (b) shows that the photocurrent density increases with the gradual increase of light intensity at each wavelength. For example, under 350nm light, when the light intensity is 11.20mWcm -2 Increased to 60.10 mW cm -2 When the photocurrent density increases from 0.21uA cm -2 Increase to 0.40uA cm -2 Higher photoresponsivity can be obtained by higher photocurrent density or lower light intensity. Due to the low growth rate of photocurrent density, the photoresponse value decreases with increasing light intensity, such as Figure 5 (c). Detectivity is another important factor in evaluating the performance of photodetectors, such as Figure 5 As shown in (d), the variation trend of the specific detectivity is the same as that of the photoresponsivity. When the light is irradiated at level I at 350 nm, the maximum value of the specific detectivity is 4.58×10 10Jones, showed that two-dimensional polyoxometalate photodetectors have greater detection potential for lower-intensity ultraviolet light.

[0067] To better compare the performance of photodetectors, we compared the photoresponses of 2D polyoxometalate nanomaterials and polyoxometalate cluster photodetectors, e.g. Figure 6 . Figure 6 (a) The photocurrent density of both detectors increases gradually with the increase of light intensity. Figure 6 As shown in (b-c), the photodetection performance of the two-dimensional polyoxometalate nanomaterial photodetector is significantly enhanced compared to that of the polyoxometalate cluster, and the photocurrent density is increased by about 2.35 times at level IV. As mentioned above, the charge is evenly distributed in the two-dimensional polyoxometalate, making its electronic excitation energy lower than that of a single cluster, resulting in large-scale electron delocalization and better electron transport. Figure 6 The photoluminescence spectrum of (d) clearly shows that under the two-dimensional morphology, a significant fluorescence quenching intensity can be found, revealing the process of suppressing charge recombination and enhancing charge transfer in two-dimensional layered polyoxometalates, which can significantly accelerate the transport of photoinduced electron / hole pairs, thereby further improving the performance of two-dimensional polyoxometalate photodetectors.

[0068] Figure 7 The current-time curves shown show a detailed comparison of the photocurrent density at different time periods. Obviously, both the newly prepared photodetector and the one after three months of storage can find highly reproducible and stable on / off signals throughout the process. After careful evaluation, the photocurrent density of the newly prepared photodetector only dropped by about 0.08% after 10,000s of cycling. At the same time, compared with the photodetector after 3 months of storage, the attenuation can reach 18.2%, indicating that the two-dimensional polyoxometalate photodetector has good cycle stability. And it has an ultra-stable fast response and recovery speed in the full cycle stability test, revealing the great potential of two-dimensional polyoxometalate photodetectors in practical applications.

[0069] The above is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily thought of by any technician familiar with the art within the technical scope disclosed in the present invention should be included in the protection scope of the present invention. Therefore, the protection scope of the present invention shall be based on the protection scope of the claims.

Claims

1. A two-dimensional polyoxometalate nanomaterial, characterized in that: It has a lamellar structure; the lamellar structure is composed of polyoxometalate clusters arranged in a hexagonal shape; The preparation process of the two-dimensional polyoxometalate nanomaterial is as follows: a solution containing ammonium cations is added dropwise to a mixed solution containing tungsten phosphate and neodymium salt, and then ultrasonication and centrifugation are performed in sequence to obtain the two-dimensional polyoxometalate nanomaterial; the ammonium cations are provided by hexadecyltrimethylammonium bromide and / or tetrabutylammonium bromide; the molar ratio of phosphotungstic acid and neodymium salt is 1:1-5.

2. A two-dimensional polyoxometalate nanomaterial according to claim 1, characterized in that: The thickness of the lamella structure is 1.5-5 nm.

3. The method for preparing a two-dimensional polyoxometalate nanomaterial according to claim 1 or 2, characterized in that: The solution containing ammonium cations is added dropwise to a mixed solution containing tungsten phosphate and neodymium salt, and then ultrasonicated and centrifuged in sequence to obtain the product; the ammonium cations are provided by hexadecyltrimethylammonium bromide and / or tetrabutylammonium bromide; and the molar ratio of phosphotungstic acid and neodymium salt is 1:1-5.

4. The method for preparing a two-dimensional polyoxometalate nanomaterial according to claim 3, characterized in that: The concentration of the solution containing ammonium cations is 0.2-0.5 μmol / L, and the solvent used is chloroform.

5. The method for preparing a two-dimensional polyoxometalate nanomaterial according to claim 3, characterized in that: The neodymium salt is neodymium nitrate and / or a hydrate of neodymium nitrate.

6. The method for preparing a two-dimensional polyoxometalate nanomaterial according to claim 4 or 5, characterized in that: The molar ratio of the ammonium cation to the phosphotungstic acid is 2-3:

1.

7. The method for preparing a two-dimensional polyoxometalate nanomaterial according to claim 3, characterized in that: The ultrasonic time is 5 to 20 minutes.

8. The method for preparing a two-dimensional polyoxometalate nanomaterial according to claim 3, characterized in that: The centrifugal conditions are: a rotation speed of 5000-8000 rpm and a time of 5-10 min.

9. The use of a two-dimensional polyoxometalate nanomaterial according to claim 1 or 2, characterized in that: For photodetectors.

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