Ultra-thin multi-mesoporous SnO₂ nanobowl arrays and preparation method thereof

Abstract

The present invention discloses an ultrathin multi-mesoporous SnO2 nanobowl array and a preparation method thereof. The SnO2 nanobowls are arranged in a regular periodic pattern. Each individual nanobowl is hemispherical, with the bottom of the bowl exposing the silicon substrate, and the exposed surface is circular. Each bowl is connected to six surrounding bowls, forming six irregular triangular gaps. The thickness of the bowl wall is below 20 nm, and the bowl wall is formed by the cross-linking of irregularly shaped nanoparticles with a size below 30 nm, and a large number of mesopores are formed between the particles. Such an ultrathin porous structure has a large specific surface area, which is beneficial to improving the optoelectronic properties of the material. Surface-enhanced Raman scattering (SERS) tests show that it has a large chemical enhancement ability. At the same time, the synthesis method of the present invention has the advantages of simple operation, environmental friendliness, low cost, and easy large-scale synthesis, and is expected to be practically applied.

Description

Technical Field

[0001] The present invention belongs to the field of micro-nano structure preparation, and particularly relates to a method for preparing porous oxide nanomaterials. Background Art

[0002] Tin oxide (SnO2) is an n-type semiconductor material with good electrical conductivity, adsorption capacity, and stability, etc., and is widely used in optoelectronic fields such as surface-enhanced Raman scattering (SERS), gas sensors, photocatalysis, and lithium-ion storage. Compared with the bulk, mesoporous SnO2 combines the characteristics of nanomaterials and a rich porous structure, and has more excellent properties, such as a larger specific surface area.

[0003] However, the development of porous materials has been a long-term challenge. Currently, there are few preparation processes for porous SnO2 nanostructures, mainly including the structure-directing agent method, anodic oxidation method, hydrothermal method, and sol-gel method, etc. The structure-directing agent method mainly uses various structure-directing agent molecules (such as surfactants) to induce the synthesis of mesoporous materials by the interaction with inorganic species. Although this method helps the formation and regulation of the structure and pore size, whether the structure-directing agent is removed by calcination or solvent extraction, the mesoporous structure is difficult to maintain and has poor thermal stability. Porous nanostructures prepared by other methods also have various problems, mainly including uneven pore formation, uncontrollable pore size, and difficulty in applying to large-scale production, etc. These problems have greatly restricted the application of SnO2. Therefore, it is particularly important to develop a green, inexpensive, simple, and operationally flexible preparation method. Summary of the Invention

[0004] The object of the present invention is to overcome the above defects and provide a method for preparing a porous SnO2 micro-nano structure array.

[0005] The technical solution for achieving the object of the present invention is: an ultrathin multi-mesoporous SnO2 nanobowl array, where the SnO2 nanobowls are arranged in a regular periodic pattern to form an array structure. Each individual SnO2 nanobowl is hemispherical, the outer edge of the bowl bottom is in contact with the silicon wafer, and each SnO2 nanobowl is connected to six surrounding SnO2 nanobowls to form six voids. The thickness of the bowl wall is below 20 nm, and the bowl wall is formed by the cross-linking of irregularly shaped SnO2 nanoparticles with a size below 30 nm. A large number of mesopores are formed between the nanoparticles.

[0006] Preferably, each SnO2 nanobowl is connected to six surrounding SnO2 nanobowls through protrusions, and every three protrusions form a void.

[0007] The preparation method of the above ultrathin multi-mesoporous SnO2 nanobowl array mainly includes the following steps:

[0008] (1) Prepare a monolayer array of polystyrene (PS) spheres on a glass slide through self-assembly at the gas-liquid interface, and transfer it to a silicon wafer to obtain a monolayer PS sphere template / silicon wafer;

[0009] (2) Use plasma etching on the monolayer PS sphere template / silicon wafer to regulate the size and gap of the PS spheres;

[0010] (3) Vertically place the etched monolayer PS sphere template / silicon wafer into an aqueous SnCl4 solution, lift it up and dry it;

[0011] (4) Anneal the sample obtained in step (3) in a muffle furnace to obtain an ultrathin multi-mesoporous SnO2 nanobowl array.

[0012] Preferably, in step (2), the etching time is below 18 min, preferably 6 min. As the etching time becomes longer (above 18 min), the diameter of the PS spheres becomes smaller, and the gap formed between the PS spheres becomes larger, resulting in the disconnection of the PS spheres. When the etching time reaches 40 min, the PS spheres disappear, leaving only the silicon wafer.

[0013] Preferably, in step (3), the concentration of the aqueous SnCl4 solution is 0.02 M.

[0014] Preferably, in step (4), the annealing temperature is 400 ± 5 °C, and the annealing time is 2 h.

[0015] Compared with the prior art, the innovation of the present invention lies in: the ultrathin multi-mesoporous SnO2 nanobowl array prepared by the present invention is unique in structure. In addition to the ultrathin bowl walls, there are both regularly and periodically arranged nanobowls and nano-triangular gaps, and a large number of mesopores distributed on the bowl walls, giving full play to the advantages of the large specific surface area of the pores. The preparation method is also novel, combining template regulation with wet chemical methods, and has the advantages of simplicity, large synthesis amount, environmental friendliness, low cost, and controllable preparation process. It solves the problems of uneven pore formation, uncontrollable pore size, poor thermal stability, and difficulty in large-scale production.

[0016] The superiority of the present invention will be further elaborated in the following drawings and specific embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 It is a schematic process flow diagram of the present invention.

[0018] Figure 2 It is the morphology of the monolayer PS sphere template / silicon wafer prepared in Example 1 of the present invention at different magnifications, where a: 500 nm, b: 150 nm.

[0019] Figure 3The morphologies of the ultrathin mesoporous SnO2 nanobowl array prepared in Example 1 of the present invention at different magnifications, wherein a and b are images at different magnifications from the front, and c and d are images at different magnifications after being tilted 45°.

[0020] Figure 4 This is the XRD spectrum of the ultrathin mesoporous SnO2 nanobowl array prepared in Example 1 of the present invention.

[0021] Figure 5 This is the SERS spectrum of the ultrathin mesoporous SnO2 nanobowl array of Example 1 of the present invention for 4-mercaptopyridine (4-MPY) molecules.

[0022] Figure 6 This is the morphology of the ultrathin mesoporous SnO2 nanobowl array prepared in Example 2 of the present invention, where a: etching for 0 min, b: etching for 12 min. DETAILED DESCRIPTION

[0023] Combination Figure 1 The present invention provides a method for preparing a porous SnO2 micro-nanostructure array, comprising: first, preparing a PS ball monolayer array on a glass slide by gas-liquid interface self-assembly, transferring it to a silicon wafer to obtain a monolayer PS ball template / silicon wafer, and etching the monolayer PS ball template / silicon wafer with plasma to adjust the size and gap of the PS balls; second, vertically placing the etched monolayer PS ball template / silicon wafer into a precursor solution, pulling and drying; finally, placing the sample in a muffle furnace for annealing. Example 1

[0024] Self-assemble a PS sphere monolayer array on a glass slide and transfer it to a silicon wafer: 100 μL of a 500 nm diameter PS sphere solution was mixed evenly with an equal amount of anhydrous ethanol, and then added dropwise to a glass slide covered with a thin layer of water film. The PS spheres automatically assembled into a monolayer film under the action of the air-liquid interface, and then the liquid on the glass slide was blotted with filter paper and dried naturally (to increase the binding force between the PS spheres). After drying, the glass slide was slowly immersed in water, and the monolayer PS sphere film would float on the water surface. A clean silicon wafer was taken to obtain the monolayer PS sphere template / silicon wafer, and then the monolayer PS sphere template / silicon wafer was placed in a plasma cleaner (PDG-32G-2, Harrick Plasma, American), with a background vacuum of 70 Pa, a power of 18 W, and etching for 6 min.

[0025] Prepare a 0.02 M SnCl4 aqueous solution, place the etched single-layer PS ball template / silicon wafer vertically into the solution and pull it up, then dry it naturally.

[0026] The sample obtained in step (2) was placed in a muffle furnace for annealing at a heating rate of 5 °C / min, an annealing temperature of 400 °C, and a time of 2 h.

[0027] The morphology of the samples was observed using an S-4800 field emission scanning electron microscope (FESEM) from Hitachi, Japan. The XRD patterns of the samples were tested using an X-ray diffractometer (XRD-7000) from Shimadzu, Japan. The optical properties of the samples were analyzed using an In Via laser confocal Raman spectrometer from Renishaw, UK.

[0028] The experimental results show that:

[0029] Figure 2 a and b in [reference] are SEM images of the monolayer PS sphere template / silicon wafer prepared in Example 1 of the present invention at different magnifications. From Figure 2 it can be observed that the diameter of the PS spheres changes from 500 nm to about 400 nm. As the diameter of the PS spheres becomes smaller, the gaps between the PS spheres increase, so that the main bodies of the PS spheres are no longer closely connected. Each PS sphere is connected to the six surrounding PS spheres by short rods about 50 nm long, and six irregular triangular cross-section voids are formed between them (i.e., a void with an inner side length of about 150 nm is formed by three short rods).

[0030] Figure 3 is the morphology of the ultrathin multi-mesoporous SnO2 nanobowl array prepared in Example 1 of the present invention. Among them, Figure 3 a and b in [reference] are images at different magnifications of the front side, Figure 3 c and d in [reference] are images at different magnifications after tilting 45°. From Figure 3 it can be seen that the sample is composed of an SnO2 nanobowl array and irregular triangular voids formed between every three adjacent SnO2 nanobowls. A single SnO2 nanobowl is hemispherical, and the outer edge of the bowl bottom is in contact with the silicon wafer, that is, the silicon wafer under the bowl bottom is exposed, and the exposed surface is circular. The edge of the SnO2 nanobowl is not smooth, but forms 3 pairs of protrusions, and the 3 pairs of protrusions form an irregular triangular void (the side length of the triangular gap is about 180 nm). This is because short rods (about 50 nm long) are formed between the PS spheres after the PS sphere template is etched. These short rods are also wetted in the solution and form a semi-surrounding structure like the PS spheres. After annealing, protrusions about 180 nm long appear. During the drying and solidification process of the SnCl4 solution, at the triangular gap, the capillary action is stronger, the liquid level gradually becomes higher than the surface of the surrounding PS spheres, and the solution is also more likely to be enriched, finally generating "protrusions". The height of the protrusions is higher than the surrounding bowl walls, and the length is longer than the inner side of the short rods. However, as the etching time of the PS template becomes longer, the short rods become shorter, and the protrusions become less obvious (as shown in Figure 6). The diameter of the SnO₂ nanobowls is about 500 nm, which is basically the same as the diameter of the PS spheres in the monolayer PS sphere template / silicon wafer. This indicates that the height of the SnO₂ nanobowls is comparable to the radius of the PS spheres, and they grow on the surface of the PS spheres. The circular vacancy at the bottom is the junction of the PS spheres and the silicon wafer substrate, which is circular with a diameter of about 200 nm. The wall of the SnO₂ nanobowls is composed of irregularly shaped SnO₂ nanoparticles with sizes below 30 nm cross-linked with each other. There are a large number of mesopores between the particles, and the wall thickness is about 10 nm. Figure 4 This is the XRD pattern of the ultrathin multi-mesoporous SnO₂ nanobowl array prepared in Example 1 of the present invention. The pattern is consistent with the standard card (JCPDS 41-1445), proving that the synthesized SnO₂ is in the rutile structure.

[0031] Figure 5 This is the SERS spectrum of the ultrathin multi-mesoporous SnO₂ nanobowl array prepared in Example 1 of the present invention for 4-MPY molecules. Curve 1 and Curve 2 correspond to the sample of Example 1 and the two-dimensional SnO₂ thin film on the silicon wafer respectively (other process steps and process parameters are the same as those in Example 1, lacking step (1), that is, directly dropping the SnCl₄ aqueous solution on the silicon wafer without the PS sphere template and then annealing). It can be seen that the SERS enhancement ability of the ultrathin multi-mesoporous SnO₂ nanobowl array structure is significantly higher than that of the SnO₂ thin film. This is attributed to its unique structure: the porous structure has a large specific surface area, which is conducive to adsorbing a large number of molecules; the nanoparticles have more defects, and the chemical enhancement ability is higher than that of the continuous thin film. Example 2

[0032] Other steps and process conditions are the same as those in Example 1, except that the etching time of the PS sphere template is different. Figure 6 a and b in it correspond to etching times of 0 and 12 min respectively. Combining Figure 6 , and comparing the results of different etching times, it can be seen that the etching time has a great influence on the final size of the nanobowls, but does not change the wall thickness of the bowls. When the etching time is short, the nanobowls are arranged closely, and the walls between the bowls are in contact with each other, and the walls forming the triangular gap in the middle are concave inward. As the etching time increases, the triangular size increases and tends to become an equilateral triangle. Therefore, the morphology of the SnO₂ nanobowl array can be regulated by changing the template etching time.

[0033] According to the above research results, it can be known that the strategy of combining template regulation with the wet chemical method proposed by the present invention is feasible, with simple, green, low-cost, macro-scale preparation, and controllable preparation process. The prepared ultra-thin multi-mesoporous SnO2 nanobowl array structure has both regularly periodically arranged nanobowls and nanotriangular gaps, and a large number of mesopores distributed on the bowl walls, and the bowl walls are ultra-thin, which can fully utilize the advantages such as large specific surface area. It avoids the disadvantages of the existing SnO2 nanomaterial synthesis technology, such as uneven pore formation, uncontrollable pore size, poor thermal stability, and difficulty in large-scale production, and is expected to be further applied in practice.

Claims

1. An ultra-thin multi-mesoporous SnO₂ nanobowl array, characterized in that, The SnO₂ nanobowls are arranged in a regular periodic pattern to form an array structure. Each individual SnO₂ nanobowl is hemispherical in shape. The outer edge of the bottom of the bowl makes contact with the silicon wafer. Each SnO₂ nanobowl is connected to six surrounding SnO₂ nanobowls, forming six voids. The bowl wall is composed of irregularly shaped SnO₂ nanoparticles with a size below 30 nm that are cross-linked with each other, and a large number of mesopores are formed between the nanoparticles; It is prepared by the following steps: (1) Prepare a monolayer array of PS spheres on a glass slide through gas-liquid interface self-assembly and transfer it to a silicon wafer to obtain a monolayer PS sphere template / silicon wafer: Mix the PS sphere solution evenly with an equal amount of absolute ethanol, and dropwise add it onto a glass slide covered with a thin water film. The PS spheres automatically assemble into a monolayer film under the action of the gas-liquid interface. Then, use filter paper to absorb the liquid on the glass slide and let it dry naturally. After drying, slowly immerse the glass slide in water, and the monolayer PS sphere film will float on the water surface. A clean silicon wafer can be used to pick it up to obtain the monolayer PS sphere template / silicon wafer; (2) Use plasma etching on the monolayer PS sphere template / silicon wafer to regulate the size and gap of the PS spheres; (3) Vertically place the etched monolayer PS sphere template / silicon wafer into an aqueous solution of SnCl₄ with a concentration of 0.02 M, lift it up and let it dry; (4) Anneal the sample obtained in step (3) in a muffle furnace to obtain an SnO₂ ultra-thin multi-mesoporous SnO₂ nanobowl array.

2. The ultrathin multi-mesoporous SnO2 nanobowl array according to claim 1, wherein The thickness of the bowl wall is below 20 nm.

3. The ultra-thin multi-mesoporous SnO2 nanobowl array according to claim 1, wherein Each SnO₂ nanobowl is connected to six surrounding SnO₂ nanobowls through protrusions, and every three protrusions form a void.

4. The preparation method of the ultra-thin multi-mesoporous SnO₂ nanobowl array according to any one of claims 1-3, characterized in that, It includes: (1) Prepare a monolayer array of PS spheres on a glass slide through gas-liquid interface self-assembly and transfer it to a silicon wafer to obtain a monolayer PS sphere template / silicon wafer: Mix the PS sphere solution evenly with an equal amount of absolute ethanol, and dropwise add it onto a glass slide covered with a thin water film. The PS spheres automatically assemble into a monolayer film under the action of the gas-liquid interface. Then, use filter paper to absorb the liquid on the glass slide and let it dry naturally. After drying, slowly immerse the glass slide in water, and the monolayer PS sphere film will float on the water surface. A clean silicon wafer can be used to pick it up to obtain the monolayer PS sphere template / silicon wafer; (2) Use plasma etching on the monolayer PS sphere template / silicon wafer to regulate the size and gap of the PS spheres; (3) Vertically place the etched monolayer PS sphere template / silicon wafer into an aqueous solution of SnCl₄ with a concentration of 0.02 M, lift it up and let it dry; (4) Anneal the sample obtained in step (3) in a muffle furnace to obtain an SnO₂ ultra-thin multi-mesoporous SnO₂ nanobowl array.

5. The method according to claim 4, characterized in that In step (2), the etching time is below 18 min, preferably 6 min.

6. The method according to claim 4, wherein In step (4), the annealing temperature is 400 ± 5 °C, and the annealing time is 2 h.

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