Flexible pressure sensor based on silver nanowire / MXene and preparation method

Abstract

The invention discloses a silver nanowire / MXene-based flexible pressure sensor, which comprises a flexible base material layer, a sensing layer and a super-hydrophobic layer, and is characterized in that the sensing layer is a silver nanowire / MXene microsphere layer, and the super-hydrophobic layer is a fluorosilane layer. The preparation method sequentially comprises the following steps: preparing MXene microspheres by using a template method; growing silver nanowires on the surfaces of the MXene microspheres in situ; the flexible base material is soaked in the silver nanowire / MXene microsphere dispersion liquid; fluoroalkyl silane is used for super-hydrophobic finishing; the flexible pressure sensor based on the silver nanowire / MXene is finally prepared by coating conductive silver paste and fixing a copper sheet as an electrode by using conductive epoxy resin, and the flexible pressure sensor can be used in the fields of real-time health monitoring, medical diagnosis, scientific exercise training and the like.

Description

Technical Field

[0001] The invention discloses a flexible pressure sensor based on silver nanowire / MXene, belonging to the field of flexible sensors and functional materials. Background Art

[0002] The first recorded sensor in human history was a thermometer invented by the Byzantines in the 3rd century BC to detect temperature changes. In China, as early as 132 AD, Zhang Heng invented the seismometer to detect the occurrence of earthquakes and estimate their direction.

[0003] Early sensors (Sensor 1.0 era) converted physical quantities / events into easily observable mechanical outputs. Later, with the discovery of electricity and the invention of generators, sensors were designed to convert physical parameters into electrical signals to achieve control functions. For example, the electronic thermometer invented by Warren Johnson in 1883 not only monitors temperature, but also has the function of automatically adjusting the temperature control system. This marked the arrival of the Sensor 2.0 era. Entering the Sensor 3.0 era, the electronics industry has promoted the miniaturization of sensors and their integration with other electronic components, giving rise to smart devices such as smartphones and smart watches, where dozens of sensors together provide an impressive user experience.

[0004] Advances in the fields of the Internet of Things (IoT), Industry 4.0, big data, artificial intelligence (AI), robotics, and digital health have prompted sensors to become more interconnected and intelligent, ushering in the era of Sensor 4.0 (ACS Nano, 2023, 17, 6, 5211–5295).

[0005] Sensors are widely used in human physiological indicator detection, health monitoring, clinical diagnosis, virtual electronics, flexible touch screens, flexible electronic skin, and even industrial robots due to their advantages such as good flexibility, stretchability, low detection limit and stability.

[0006] At present, the main technical challenge of sensors lies in how to design the sensor structure and select suitable flexible sensor morphology, flexible substrate materials, sensitive layer materials and preparation processes, so as to integrate many advantages such as high sensitivity, durability, stability, human-friendliness and cost advantages.

[0007] The sensor forms mainly include 1D sensors based on fibers and yarns, 2D sensors based on fabrics, films and gels, and 3D sensors based on three-dimensional porous materials such as foams and sponges (Advanced Materials, 2024, 36(24): 2312761). However, pressure sensors with high sensitivity, durability and stability are still a huge technical challenge. Summary of the invention

[0008] In view of the above-mentioned shortcomings, the present invention provides a flexible pressure sensor based on silver nanowires / MXene and a preparation method thereof.

[0009] The present invention is achieved through the following technical solutions:

[0010] A flexible pressure sensor based on silver nanowires / MXene comprises a flexible base material layer, a sensing layer and a super-hydrophobic layer, wherein the sensing layer is a silver nanowire / MXene microsphere layer, and the super-hydrophobic layer is a fluorosilane layer.

[0011] A method for preparing a flexible pressure sensor based on silver nanowires / MXene includes:

[0012] (1) Preparation of MXene microspheres using the template method: Use ammonia water to adjust the pH value of two polystyrene microsphere dispersions with different particle sizes to 9-11; then ultrasonically mix the above polystyrene microsphere dispersion and MXene dispersion; place them in a vacuum muffle furnace under argon protection, calcine them at 200℃~300℃ for 1-2h, and the heating rate is 1℃ / min, then heat it to 500℃~600℃ and calcine it for 2-4h, and the heating rate is 1℃ / min, remove the polystyrene microspheres, and obtain MXene microspheres.

[0013] (2) In situ growth of silver nanowires on the surface of MXene microspheres: 100 mL of ethylene glycol, 0.4 g of silver nitrate, 0.5 g of PVP, 2.5-12.5 μM NaCl and 10-80 μM KBr were added to a flask and stirred evenly at room temperature. Then a certain amount of the above-mentioned MXene microspheres were added. Under argon protection, the oil bath was heated to 80 °C for 30-60 min, and then continued to heat to 120 °C with magnetic stirring for 2-4 h. The mixture was naturally cooled to room temperature, acetone was slowly added, and centrifuged. Then, deionized water was used to repeatedly wash the mixture to remove ethylene glycol and PVP to obtain silver nanowires / MXene microspheres.

[0014] (3) Immersing the flexible substrate material in the silver nanowire / MXene microsphere dispersion for 30 to 60 minutes and vacuum drying to obtain a flexible substrate material based on silver nanowire / MXene microspheres.

[0015] (4) Superhydrophobic finishing using fluorosilane: The flexible substrate material based on silver nanowires / MXene microspheres was immersed in a fluorosilane methanol solution for a period of time, then taken out and vacuum dried at 80°C for 24 hours to obtain a superhydrophobic flexible substrate material based on silver nanowires / MXene microspheres.

[0016] (5) Conductive silver paste is coated on both sides of the above-mentioned super-hydrophobic flexible substrate material based on silver nanowires / MXene microspheres, and copper sheets are fixed with conductive epoxy resin as electrodes, and finally a flexible pressure sensor based on silver nanowires / MXene is prepared.

[0017] As a preferred embodiment, the molar concentration of the ammonia water in step (1) is 0.1 mol / L.

[0018] As a preferred embodiment, the average particle sizes of the two polystyrene microspheres with different particle sizes in step (1) are 0.5-5 μm and 10-20 μm respectively.

[0019] As a preferred embodiment, in step (1), the volume ratio of the polystyrene microsphere dispersions of two different particle sizes to the MXene dispersion is 1:1:2.

[0020] As a preferred embodiment, the molecular weight of PVP in step (2) is 1,000,000 to 1,500,000.

[0021] As a preferred embodiment, in the step (2), the mass ratio of the MXene microspheres to the silver nitrate is 0.5 to 5.

[0022] As a preferred embodiment, the mass fraction of the silver nanowire / MXene microsphere dispersion in step (3) is 1% to 10%.

[0023] As a preferred solution, the mass percentage of the fluorosilane methanol solution in step (4) is 1 to 5%, and the immersion time is 30 to 90 minutes.

[0024] The flexible matrix material is one or more of knitted fabric, woven fabric, non-woven fabric, textile, film, hydrogel, aerogel and sponge.

[0025] Two-dimensional transition metal carbon / nitride (MXene) is a new type of two-dimensional nanomaterial obtained by etching away the A element from the layered ceramic material MAX phase. Its general formula is M n+1 X n T x , where M represents an early transition metal element (such as Ti, Zr, Nb, Mo, Ta, etc.), n = 1 to 4, X represents a C element, a N element or a C / N element, T x The general formula for surface groups such as O, OH, F, and Cl is M n+1 X n T x .

[0026] MXene for Ti2CT x 、T i3C2T x 、T i3CNT x、T i4N3T x 、T i2NT x 、T i2(CN)T x 、(T i,V)2CT x 、(Ti,Nb)2CT x 、(T i,Nb)4C3T x 、(Ti,V,Zr,Nb,Ta)2CT x 、T iVNbMoC3T x 、T iVCrMoC3T x 、Nb2CT x 、(Nb,Zr)4C3T x 、Hf3C2T x 、Mo2CT x 、Mo2NT x 、(Mo,V)4C3T x 、V2CT x 、V2NT x 、V4C3T x Cr2CT x 、(Cr2V)C2T x 、Ta4C3T x 、(Ta,T i)3C2T x 、Zr3C2T x One or more of .

[0027] Flexible pressure sensors include piezoresistive, capacitive, piezoelectric and triboelectric types. The flexible pressure sensor prepared by the present invention is a typical piezoresistive sensor. When an external force is applied, the silver nanowire @MXene microspheres are compressed, the number of point contacts between the silver nanowire @MXene microspheres increases, and the conductive path increases. When the external force is released, the distance between the silver nanowire @MXene microspheres returns to its original state, and the flexible pressure sensor returns to its original state. When external pressure acts on the flexible pressure sensor, the resistance will change accordingly. Resistance is positively correlated with pressure, and the magnitude of pressure can be sensed by measuring the change in resistance. The introduction of the silver nanowire @MXene microsphere structure makes the flexible pressure sensor highly sensitive and multifunctional.

[0028] The flexible pressure sensor prepared by the present invention has a wide range of applications and has extremely sensitive feedback for actions such as finger bending, swallowing, wrist bending, mouse clicking, and pulse. It can be used for sports behavior monitoring, human-computer interaction, breathing, heart rate, pulse and other health monitoring. In the future, it can be used in real-time health monitoring, medical diagnosis, scientific sports training and other fields. For example, real-time monitoring of the pulse, breathing, heartbeat and other physiological parameters of newborns or critically ill elderly people. Based on Internet technology, it can better manage physical health through functions such as remote care and health data management.

[0029] Compared with the prior art, the advantages of the present invention are:

[0030] (1) After the pH value is adjusted by ammonia water, the surface of polystyrene microspheres is positively charged, and the negatively charged MXene can be adsorbed by electrostatic action. X , and two MXene microspheres of different sizes are formed by segmented calcination. (2) Silver nanowires are directly grown in situ on the surface of MXene microspheres. The two MXene microspheres of different sizes and the in situ grown silver nanowires are synergistic, and can exert the effect of 1+1 being far greater than 2, constructing a multi-level, interconnected three-dimensional porous microstructure, which is more sensitive to pressure perception. The sensitivity and durability of the assembled flexible pressure sensor are greatly improved. (3) The porous skeleton structure of the flexible matrix material and the high conductivity and synergistic effect of silver nanowires and MXene microspheres provide a three-dimensional conductive network path during the compression process. When subjected to external stimuli, the microcracks, deformation and contact of the flexible matrix material skeleton in the conductive silver nanowire / MXene microsphere layer will cause the resistance of the three-dimensional conductive network structure to change, making the flexible pressure sensor have higher sensitivity, shorter response time and long-term durability. (4) The silver nanowire / MXene microsphere layer plays a skeleton support role, which improves the mechanical strength of the flexible matrix material. (5) Fluorosilane is encapsulated on the surface of the silver nanowire / MXene microsphere layer, which greatly improves the environmental stability and tolerance of the silver nanowire and MXene, giving the flexible pressure sensor excellent superhydrophobic properties. In addition to the normal air environment, the flexible pressure sensor is not afraid of changes in sweat and humidity. It can still work normally in a humid environment or even underwater environment, expanding the working environment of the sensor. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 It is a schematic diagram of the preparation process of the present invention.

[0032] Figure 2 The MXene (Ti3C2T X ) scanning transmission electron microscopy image.

[0033] Figure 3 The MXene (Ti3C2T X )Surface Zeta potential.

[0034] Figure 4 The silver nanowire / MXene-based flexible pressure sensor prepared in Example 1 of the present invention is used to monitor the human pulse in real time. DETAILED DESCRIPTION

[0035] The present invention is further described below in conjunction with specific implementation methods.

[0036] Embodiment 1:

[0037] The MAX phase Ti3AlC2 was etched with LiF and HCl to remove the Al layer and obtain the accordion-structured Ti3C2T X Then, single-layer Ti3C2T X Nanosheets. The specific steps are as follows: slowly pour 40 mL of 9 mol / L HCl into a polytetrafluoroethylene beaker, add 2 g of Li F, and magnetically stir at 400 rpm at room temperature for 30 minutes to completely dissolve Li F. Then slowly add 2 g of Ti3AlC2 powder to the etching solution and magnetically stir at room temperature for 24 hours. After the reaction is completed, wash with deionized water several times and centrifuge at 3500 rpm until the pH value is ≥6. Then redisperse the black precipitate in a 1:1 water / ethanol mixed solvent and centrifuge at 10000 rpm for 10 minutes. Wash the resulting precipitate with deionized water, continue to centrifuge at 3500 rpm for several times, collect the supernatant, and you can get a single layer of Ti3C2T X Nanosheet colloidal solution. X The nanosheet colloidal solution was protected by argon and stored at 4°C for future use. X TEM images of nanosheets are shown in Figure 2 .

[0038] Zeta potential measurement of Ti3C2T X Zeta potential, see Figure 3 Under different pH values ​​(2-11), Ti3C2T X The surface Zeta potential is negative.

[0039] The pH value of two polystyrene microsphere dispersions with average particle sizes of 0.5 μm and 10 μm was adjusted to 9 using ammonia water with a molar concentration of 0.1 mol / L. The surface Zeta potential was positive.

[0040] The polystyrene microsphere dispersion and Ti3C2T X The dispersion was ultrasonically mixed and placed in a vacuum muffle furnace under argon protection, calcined at 200°C for 1 h at a heating rate of 1°C / min, and then heated to 500°C for 2 h at a heating rate of 1°C / min to remove the polystyrene microspheres and obtain MXene microspheres.

[0041] 100mL ethylene glycol, 0.4g silver nitrate, 0.5g PVP, 2.5μM NaCl and 10μM KBr were added to the flask and stirred evenly at room temperature, wherein the molecular weight of PVP was 1,000,000. Then 0.2g of the above MXene microspheres were added. Under argon protection, the oil bath was heated to 80°C for 30min, then continued to heat to 120°C with magnetic stirring for 8h, cooled naturally to room temperature, acetone was slowly added, centrifuged, and then repeatedly washed with deionized water to remove ethylene glycol and PVP, and silver nanowires / MXene microspheres with an average diameter of 50nm and an average length of 120μm were obtained.

[0042] Pure cotton woven fabric was immersed in a silver nanowire / MXene microsphere dispersion with a mass fraction of 1% for 30 minutes and vacuum dried to obtain cotton fabric based on silver nanowire / MXene microspheres.

[0043] The cotton fabric based on silver nanowires / MXene microspheres was immersed in a 1% by mass 1H,1H,2H,2H-perfluorooctyltriethoxysilane methanol solution for 30 minutes, then taken out and vacuum dried at 80°C for 24 hours to obtain a superhydrophobic cotton fabric based on silver nanowires / MXene microspheres.

[0044] Conductive silver paste was coated on both sides of the superhydrophobic cotton fabric based on silver nanowires / MXene microspheres, and copper sheets were fixed with conductive epoxy resin as electrodes, finally preparing a flexible pressure sensor based on silver nanowires / MXene.

[0045] The synergistic effect of silver nanowires and MXene microspheres improves the sensing performance of flexible pressure sensors, making the sensitivity of flexible pressure sensors based on silver nanowires / MXene microspheres (192.8 kPa -1 ) than the flexible pressure sensor based on a single silver nanowire (76.4 kPa -1 ) and a flexible pressure sensor based on a single MXene microsphere (103.2 kPa -1 ) are excellent. Detection limit 0.014Pa, response time 7ms, cycle number 20,000.

[0046] Figure 4 The silver nanowire / MXene microsphere flexible pressure sensor was demonstrated to be used for real-time monitoring of human pulse. It can be seen that the silver nanowire / MXene microsphere flexible pressure sensor has high sensitivity and can obtain tiny human physiological activity signals, and is expected to play a huge role in the field of human health monitoring in the future.

[0047] Embodiment 2:

[0048] The pH values ​​of two polystyrene microsphere dispersions with average particle sizes of 3 μm and 15 μm were adjusted to 10 using ammonia water with a molar concentration of 0.1 mol / L. Then, the polystyrene microsphere dispersion and Ti3C2T X The dispersion was ultrasonically mixed and placed in a vacuum muffle furnace under argon protection, calcined at 250°C for 3 h at a heating rate of 1°C / min, and then heated to 550°C for 10 h at a heating rate of 1°C / min to remove the polystyrene microspheres and obtain MXene microspheres.

[0049] 100 mL of ethylene glycol, 0.4 g of silver nitrate, 0.5 g of PVP, 8 μM NaCl and 50 μM KBr were added to the flask and stirred evenly at room temperature, wherein the molecular weight of PVP was 1,200,000. Then 0.4 g of the above MXene microspheres were added. Under argon protection, the oil bath was heated to 80 ° C for 45 min, then continued to heat to 120 ° C with magnetic stirring for 10 h, cooled naturally to room temperature, acetone was slowly added, centrifuged, and then repeatedly washed with deionized water to remove ethylene glycol and PVP to obtain silver nanowires / MXene microspheres.

[0050] The PET film was immersed in a 5% by mass silver nanowire / MXene microsphere dispersion for 45 minutes and vacuum dried to obtain a PET film based on silver nanowires / MXene microspheres. The above-mentioned flexible matrix material based on silver nanowires / MXene microspheres was immersed in a 3% by mass 1H,1H,2H,2H-perfluorooctyltriethoxysilane methanol solution for 60 minutes, then taken out and vacuum dried at 80°C for 24 hours to obtain a super-hydrophobic PET film based on silver nanowires / MXene microspheres. Conductive silver paste was coated on both sides of the above-mentioned super-hydrophobic PET film based on silver nanowires / MXene microspheres, and a copper sheet was fixed with a conductive epoxy resin as an electrode to finally prepare a flexible pressure sensor based on silver nanowires / MXene. Its sensitivity is 208kPa -1 , detection limit 0.04Pa, response time 10ms, cycle number 18,000, able to withstand ultrasonic washing for more than 30 minutes. It can detect the pulse signal of the wrist artery in real time and has excellent linearity and sensitivity.

[0051] Example 3

[0052] The pH values ​​of two polystyrene microsphere dispersions with average particle sizes of 5 μm and 20 μm were adjusted to 11 using ammonia water with a molar concentration of 0.1 mol / L. Then, the polystyrene microsphere dispersion and Ti3C2T XThe dispersion was ultrasonically mixed and placed in a vacuum muffle furnace under argon protection. It was calcined at 300°C for 2 h with a heating rate of 1°C / min, and then heated to 600°C for 4 h with a heating rate of 1°C / min to remove the polystyrene microspheres and obtain MXene microspheres.

[0053] 100 mL of ethylene glycol, 0.4 g of silver nitrate, 0.5 g of PVP, 12.5 μM NaCl and 80 μM KBr were added to the flask and stirred evenly at room temperature, wherein the molecular weight of PVP was 1,500,000. Then 2 g of the above MXene microspheres were added. Under argon protection, the oil bath was heated to 80 ° C for 60 min, then continued to heat to 120 ° C with magnetic stirring for 12 h, cooled naturally to room temperature, acetone was slowly added, centrifuged, and then repeatedly washed with deionized water to remove ethylene glycol and PVP to obtain silver nanowires / MXene microspheres.

[0054] The porous polyurethane sponge was immersed in a 10% by mass fraction of silver nanowire / MXene microsphere dispersion for 60 minutes and vacuum dried to obtain a polyurethane sponge based on silver nanowire / MXene microspheres. The above-mentioned polyurethane sponge based on silver nanowire / MXene microspheres was immersed in a 5% by mass percentage of 1H,1H,2H,2H-perfluorooctyltriethoxysilane methanol solution for 90 minutes, then taken out and vacuum dried at 80°C for 24 hours to obtain a super-hydrophobic polyurethane sponge based on silver nanowire / MXene microspheres. Conductive silver paste was coated on both sides of the above-mentioned super-hydrophobic polyurethane sponge based on silver nanowire / MXene microspheres, and a copper sheet was fixed with a conductive epoxy resin as an electrode to finally prepare a flexible pressure sensor based on silver nanowire / MXene. Its sensitivity is 225kPa -1 , detection limit 0.011Pa, response time as low as 5ms, and cycle number 16,000. The high porosity of polyurethane sponge helps to improve the sensitivity of flexible pressure sensors.

[0055] 1H,1H,2H,2H-perfluorooctyltriethoxysilane reduces the surface energy of the material, and the micro-nano rough structure formed by the silver nanowires / MXene microspheres increases the roughness of the flexible substrate material, thereby making it have excellent superhydrophobic properties. As can be seen from Table 1, the contact angles of Examples 1, 2, and 3 to water, hexadecane solution, sodium hydroxide solution, and nitric acid solution are all greater than 150°, thereby improving the adaptability of the flexible pressure sensor to the environment.

[0056] Table 1 Contact angles of Examples 1, 2, and 3 to water, hexadecane solution, sodium hydroxide solution, and nitric acid solution

[0057]

[0058]

[0059] Obviously, the above embodiments of the present invention are merely examples for clearly explaining the present invention, and are not intended to limit the implementation methods of the present invention. For those skilled in the art, other different forms of changes or modifications can be made based on the above description. It is impossible to list all implementation methods here. Any obvious changes or modifications derived from the technical solution of the present invention are still within the scope of protection of the present invention.

Claims

1. A flexible pressure sensor based on silver nanowires / MXene, characterized in that: It mainly includes a flexible base material layer, a sensing layer and a super-hydrophobic layer. The sensing layer is a silver nanowire / MXene microsphere layer, and the super-hydrophobic layer is a fluorosilane layer.

2. The method for preparing a flexible pressure sensor based on silver nanowires / MXene according to claim 2, characterized in that: The preparation steps include: (1) Using ammonia water, the pH value of two polystyrene microsphere dispersions with different particle sizes is adjusted to 9-11; then the polystyrene microsphere dispersion and the MXene dispersion are ultrasonically mixed to be uniform; placed in a vacuum muffle furnace under argon protection, calcined at a temperature of 200°C to 300°C for 1-2 hours, with a heating rate of 1°C / min, and then heated to 500°C to 600°C for calcination for 2-4 hours, with a heating rate of 1°C / min, to remove the polystyrene microspheres and obtain MXene microspheres; (2) 100 mL of ethylene glycol, 0.4 g of silver nitrate, 0.5 g of PVP, 2.5-12.5 μM NaCl and 10-80 μM KBr were added to a flask and stirred evenly at room temperature. Then a certain amount of the above-mentioned MXene microspheres were added. Under argon protection, the oil bath was heated to 80 ° C for 30-60 min, and then continued to heat to 120 ° C with magnetic stirring for 2-4 h. The mixture was naturally cooled to room temperature, acetone was slowly added, and centrifuged. Then, deionized water was used to repeatedly wash to remove ethylene glycol and PVP to obtain silver nanowires / MXene microspheres; (3) immersing the flexible substrate material in the silver nanowire / MXene microsphere dispersion for 30 to 60 minutes, and vacuum drying to obtain a flexible substrate material based on silver nanowire / MXene microspheres; (4) immersing the above-mentioned flexible matrix material based on silver nanowires / MXene microspheres in a fluorosilane methanol solution for a period of time, taking it out, and vacuum drying it at 80° C. for 24 hours to obtain a super-hydrophobic flexible matrix material based on silver nanowires / MXene microspheres; (5) Conductive silver paste is coated on both sides of the above-mentioned super-hydrophobic flexible substrate material based on silver nanowires / MXene microspheres, and copper sheets are fixed with conductive epoxy resin as electrodes, and finally a flexible pressure sensor based on silver nanowires / MXene is prepared.

3. The method for preparing a flexible pressure sensor based on silver nanowires / MXene according to claim 2, characterized in that: The molar concentration of ammonia water in step (1) is 0.1 mol / L.

4. The method for preparing a flexible pressure sensor based on silver nanowires / MXene according to claim 2, characterized in that: The average particle sizes of the two polystyrene microspheres with different particle sizes in step (1) are 0.5-5 μm and 10-20 μm respectively.

5. The method for preparing a flexible pressure sensor based on silver nanowires / MXene according to claim 2, characterized in that: In the step (1), the volume ratio of the polystyrene microsphere dispersions of two different particle sizes to the MXene dispersion is 1:1:

2.

6. The method for preparing a flexible pressure sensor based on silver nanowires / MXene according to claim 2, characterized in that: The molecular weight of PVP in step (2) is 1,000,000 to 1,500,000.

7. The method for preparing a flexible pressure sensor based on silver nanowires / MXene according to claim 2, characterized in that: In the step (2), the mass ratio of the MXene microspheres to the silver nitrate is 0.5 to 5.

8. The method for preparing a flexible pressure sensor based on silver nanowires / MXene according to claim 2, characterized in that: The mass fraction of the silver nanowire / MXene microsphere dispersion in step (3) is 1% to 10%.

9. The method for preparing a flexible pressure sensor based on silver nanowires / MXene according to claim 2, characterized in that: In the step (4), the mass percentage of the fluorosilane methanol solution is 1-5%, and the immersion time is 30-90 minutes.

10. A flexible pressure sensor based on silver nanowires / MXene prepared by the preparation method according to any one of claims 2 to 9.

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