A method for preparing an MXene / graphene gradient micro-crease structure with strain sensing performance

Abstract

The application discloses a preparation method of a MXene / reduced graphene oxide (rGO) composite gradient micro-crease structure with strain sensing performance. A MXene / GO colloidal solution is prepared, vacuum filtration is carried out, the solution is placed on an inclined platform, a MXene / GO composite film with different gradient changes is obtained through vacuum filtration, the film is placed in hydroiodic acid HI, reduction is carried out, and a MXene / rGO composite film is obtained; an acrylic film with super tensile performance is used as a substrate, the MXene / rGO composite film is transferred to the acrylic film substrate film through a dry method, when the substrate film is retracted together, the film forms a crease structure, and a gradient crease structure is formed from the film with a thickness gradient. The structure can increase the strain range of a sensor, the sensitivity can be improved through crease deformation and micro-cracks generated under different strain conditions, and the sensitivity and the strain range are improved through non-synchronous changes of the gradient structure in the stretching process.

Description

Technical Field

[0001] This invention belongs to the field of flexible electronics technology, specifically relating to a method for preparing an MXene / graphene gradient micro-wrinkled structure with strain sensing properties. This flexible strain sensor can be applied to health monitoring, human interaction, and electronic skin. Background Technology

[0002] Flexible strain sensors, due to their exceptional flexibility, ductility, skin-fitting capability, and superior sensing performance, show great promise in the fields of healthcare, human-machine interfaces, and environmental monitoring. However, among several evaluation metrics for flexible strain sensors, the difficulty in simultaneously improving sensitivity and strain range limits their further development as high-performance strain sensors. When external stress is applied, high sensitivity is typically achieved by disrupting the conductive network, such as through the induction of microcracks, leading to a reduction in the sensing range. Therefore, achieving both high sensitivity and a large strain range in flexible strain sensors remains a significant challenge. Summary of the Invention

[0003] The purpose of this invention is to balance the trade-off between high sensitivity and a wide strain range in the same material, and to provide a flexible strain sensor made of MXene / rGO composite material with a novel gradient micro-fold structure. This invention features a simple structure, stable signal transmission, convenient signal collection and processing, and low cost, while improving both sensitivity and strain range.

[0004] The technical solution adopted in this invention is: a flexible strain sensor of MXene / rGO composite material with a novel gradient micro-fold structure, which is obtained by MXene / rGO composite material through filtration, reduction, pre-stretching and encapsulation.

[0005] Furthermore, the flexible strain sensor of the MXene / rGO composite material with the novel gradient micro-fold structure described above has a mass percentage ratio of MXene:rGO = 1:1.

[0006] A flexible strain sensor made of MXene / rGO composite material with a novel gradient micro-wrinkled structure includes the following steps:

[0007] 1) Mix MXene colloidal solution with graphene oxide (GO) solution at a ratio of 1:1, disperse by ultrasonication to obtain MXene / GO mixture, and then stir thoroughly with a magnetic stirrer to obtain MXene / GO colloidal solution;

[0008] 2) The MXene / GO colloidal solution was vacuum filtered in a vacuum filtration device. The filtration flask was placed on an inclined platform, and the MXene / GO composite film was obtained by vacuum filtration.

[0009] 3) The MXene / GO composite film obtained in step 2) is placed in hydroiodic acid (HI) for reduction to obtain the MXene / rGO composite film;

[0010] 4) The MXene / rGO composite film obtained in step 3) is used to construct a micro-wrinkled structure by a simple pre-stretching method to obtain MXene / rGO composite wrinkled films with different gradient changes.

[0011] 5) Assemble copper electrodes on the surface of the MXene / rGO composite gradient micro-wrinkled thin film obtained in step 4) to obtain an MXene / rGO composite gradient micro-wrinkled strain sensor.

[0012] 6) The MXene / rGO composite gradient micro-wrinkled film with electrodes obtained in step 5) is encapsulated with polydimethylsiloxane (PDMS) to obtain a flexible strain sensor of MXene / rGO composite material with a novel gradient micro-wrinkled structure.

[0013] Further specifying, the specific preparation steps of the MXene / rGO colloidal solution in step 1) are as follows:

[0014] Step 1.1: The GO solution was prepared using a modified Hummers method. 20–25 mL of 98% concentrated sulfuric acid (H₂SO₄) was added to a beaker and cooled to -1–1°C. 0.5–1.5 g of natural graphite was added, and the mixture was stirred for 40–60 min. 4–6 g of potassium permanganate (KMnO₄) was added in portions, and the mixture was stirred for 2–4 h. After stirring, the ice bath was replaced, and the beaker was placed in a constant-temperature water bath for the first heating and stirring, stirring for 40–50 min while bubbling and releasing heat. Then, the beaker was transferred to a high-temperature constant-temperature water bath for the second heating and stirring. 70–90 mL of distilled water was added very slowly in portions, and the mixture was stirred for 10–20 min. Then, 50–70 mL of distilled water was added to dilute the solution. 350–370 g of 5% hydrogen peroxide (H₂O₂) was then added. A golden-yellow color appeared. The solution was immediately centrifuged, and the centrifuged liquid was discarded. The solution was washed until the pH value was around 5–6.

[0015] Further specifying, in step 1.1, KMnO4 is added in 5 to 6 portions over 25 to 30 minutes.

[0016] Further specifying, the stirring rate for the first heating and stirring in step 1.1 is 800-1000 r / min, and the temperature is controlled at 35-45℃.

[0017] Further specified, the stirring rate of the second heating and stirring in step 1.1 is 400-500 r / min, and the temperature is controlled at 70-90℃.

[0018] Further specify that, in step 1.1, during the second heating and stirring, distilled water is added in three portions of 5-10 mL each, followed by the remaining 55-60 mL of distilled water in three separate portions.

[0019] Step 1.2: The MXene solution was prepared by etching. 3–4 g of lithium fluoride (LiF) and 30–50 mL of 9 mol / L hydrochloric acid (HCl) were added to a reaction vessel, and the vessel was heated and stirred in a constant temperature water bath for the first time. After stirring, 1.5–2.5 g of titanium aluminum carbide (Ti3AlC2) was added in portions. After all the solution was added, the vessel was capped, and the vessel was heated and stirred in a constant temperature water bath for the second time. After stirring, the vessel was centrifuged, and the centrifuged liquid was discarded. Unreacted LiF was first washed away with 2 mol / L HCl, and then washed with distilled water until neutral. The product was poured into a gas washing bottle and sonicated in an ultrasonic bath for 0.5–1.5 h, while simultaneously bubbling the mixture. After the treatment, the product was centrifuged, and the supernatant was the MXene solution.

[0020] Further specified, in step 1.2, the stirring rate for the first heating and stirring is 500-800 r / min, the temperature is controlled at 35-45℃, and the stirring time is 10-20 min.

[0021] Further specifying, in step 1.2, Ti3AlC2 is added in 8 to 10 portions over 25 to 35 minutes.

[0022] Further specifying, in step 1.2, the stirring rate for the second heating and stirring is 500-800 r / min, the temperature is controlled at 35-45℃, and the stirring time is 45-50 h.

[0023] Further, the ventilation airflow rate in step 1.2 is 8–12 m / s, and the ventilation time is 0.5–1.5 h.

[0024] Step 1.3: The GO solution obtained by the modified Hummers method is diluted to 0.1 mg / mL and mixed with MXene colloidal solution with a concentration of 0.1 mg / mL. After stirring for 10-15 min, it is sonicated for 15-25 min and stirred with a magnetic stirrer for 4-6 h to obtain a fully homogeneous MXene / GO colloidal solution.

[0025] Further specifying, during the vacuum filtration process described in step 2), the concentration of the MXene / GO colloidal solution is 0.1 mg / mL, and the volume is 5–7 mL.

[0026] Further specifying, in step 2), the tilt angle of the inclined platform on which the filtration bottle is placed is 10° to 30° during the vacuum filtration process.

[0027] Further specifying, in step 3), the HI reduction process is performed at a temperature of 90℃ to 120℃ for 1.5 to 3 hours.

[0028] Further defining step 4), the fabrication process of the micro-wrinkled structure constructed by the pre-stretching method includes the following steps: using an acrylic film VHB4910 with super-stretching properties as a substrate, the MXene / rGO composite film is transferred to the pre-stretched substrate film by a dry method. When the film shrinks together with the substrate film, the film will form a wrinkled structure, and the film with a thickness gradient will thus form a gradient wrinkled structure.

[0029] Further specifying, step 5) involves assembling copper electrodes onto both sides of the MXene / rGO gradient micro-wrinkled film using conductive silver paste.

[0030] Further specifying, the encapsulation process in step 6) specifically involves: dropping polydimethylsiloxane (PDMS) onto an MXene / rGO gradient micro-wrinkled film with electrodes assembled using a dropper to obtain a flexible strain sensor of the MXene / rGO composite material with a novel gradient micro-wrinkled structure.

[0031] The flexible strain sensor of MXene / rGO composite material with a novel gradient micro-wrinkled structure provided by this invention has applications in health monitoring, human interaction, and electronic skin.

[0032] Compared with the prior art, the present invention has the following beneficial technical effects:

[0033] 1. The flexible strain sensor prepared in this invention achieves high structural sensitivity by causing relative slippage and cracking between stacked MXene nanosheets and rGO nanosheets during the stretching process. Simultaneously, the wrinkled structure contributes to a large strain range.

[0034] 2. The strain sensor prepared by this invention utilizes a pleated structure with gradually varying buckling degrees to increase the strain range of the sensor. The sensitivity is improved by the pleated deformation and microcracks generated under different strain conditions. The asynchronous changes of the gradient structure during the stretching process improve the sensitivity and increase the strain range.

[0035] 3. This invention further constructs a novel gradient micro-folded structure through a simple pre-stretching method. The entire preparation process does not require complex experimental instruments or cumbersome experimental steps.

[0036] 4. The preparation process of this invention is simple, the signal transmission is stable, the price is low and the source is wide, and it is green, environmentally friendly and safe.

[0037] 5. This invention achieves MXene-modified rGO through nucleophilic substitution and dehydration reactions, solving the problems of low sensitivity before crack initiation and a sharp increase in sensitivity upon crack initiation. A thin film with gradually varying thickness is prepared by vacuum filtration, combined with a pre-stretching method to obtain a gradient micro-wrinkled structure. This microscopically exhibits a gradually varying degree of buckling, which increases the strain range of the sensor. Sensitivity can be improved by the wrinkle deformation and microcracks generated under different strain conditions. The asynchronous changes in the gradient structure during stretching further enhance sensitivity and increase the strain range. Flexible strain sensors have been widely used in embedded electronic devices, ergonomics research, robotics, medical devices, and wearable devices. Wearable devices are used to monitor and record users' health and activity levels, and in smart homes for controlling lighting, temperature, and security. Attached Figure Description

[0038] Figure 1 The images show the XRD patterns of the MXene, rGO, and MXene / rGO composite materials prepared in Example 1.

[0039] Figure 2 This is a schematic diagram comparing the sensitivity and strain range of the flexible strain sensors with micro-folded structures ① pure MXene, ② pure rGO prepared in Example 1, and ③ a flexible strain sensor with micro-folded structure MXene / rGO composite material.

[0040] Figure 3 This is a schematic diagram comparing the sensitivity and strain range of the flexible strain sensor made of MXene / rGO composite material with a gradient-free micro-fold structure prepared in Example 2 (①) and the flexible strain sensor made of MXene / rGO composite material with a novel gradient micro-fold structure (②).

[0041] Figure 4 SEM images of the MXene / rGO gradient micro-wrinkled film prepared in Example 2: ① Upper part of the film ② Middle part of the film ③ Lower part of the film. Detailed Implementation

[0042] Example 1

[0043] (I) A flexible strain sensor made of MXene / rGO composite material with micro-wrinkled structure, the preparation method of which is as follows:

[0044] 1. The GO solution was prepared using a modified Hummers method. 20–25 mL of 98% concentrated sulfuric acid (H₂SO₄) was added to a beaker and cooled to -1–1°C. 0.5–1.5 g of natural graphite was added, and the mixture was stirred for 40–60 min. 4–6 g of potassium permanganate (KMnO₄) was added in portions, and the mixture was stirred for 2–4 h. After stirring, the ice bath was replaced, and the mixture was placed in a constant-temperature water bath (35–45°C) for the first heating and stirring at a rate of 800–1000 r / min, with bubbling and exothermic stirring for 40–50 min. Then, the mixture was transferred to a high-temperature constant-temperature water bath (70–90°C) for the second heating and stirring at a rate of 400–500 r / min. 70–90 mL of distilled water was added very slowly in portions, and the mixture was stirred for 10–20 min. Finally, 50–70 mL of distilled water was added to dilute the mixture. Add 350-370g of 5% hydrogen peroxide (H2O2). When a golden yellow color appears, immediately centrifuge and discard the centrifuged liquid. Wash until the pH value is around 5-6.

[0045] MXene solution was prepared by etching. 3–4 g of lithium fluoride (LiF) and 30–50 mL of 9 mol / L hydrochloric acid (HCl) were added to a reaction vessel and heated and stirred for the first time in a constant temperature water bath (35–45°C) at a stirring rate of 500–800 r / min. After stirring, 1.5–2.5 g of titanium aluminum carbide (Ti3AlC2) was added in portions. After all the solution was added, the vessel was capped and heated and stirred for the second time in a constant temperature water bath (35–45°C) at a stirring rate of 500–800 r / min. After stirring, the vessel was centrifuged and the centrifuged liquid was discarded. Unreacted LiF was first washed away with 2 mol / L HCl, and then washed with distilled water until neutral. The product was poured into a gas washing bottle and sonicated in an ultrasonic bath for 0.5–1.5 h, while simultaneously undergoing bubbling treatment. The gas flow rate for bubbling treatment was 8–12 m / s, and the bubbling time was 0.5–1.5 h. After completion, centrifuge the solution, and the supernatant is the MXene solution.

[0046] The GO solution obtained by the modified Hummers method was diluted to 0.1 mg / mL and mixed with an MXene colloidal solution of 0.1 mg / mL. After stirring for 10–15 min, the mixture was sonicated for 15–25 min and then stirred with a magnetic stirrer for 4–6 h to obtain a fully homogeneous MXene / GO colloidal solution.

[0047] 2. The MXene / GO colloidal solution obtained in step 1 is subjected to vacuum filtration under a vacuum filtration device. Take 5-7 mL of the MXene / GO colloidal solution and perform vacuum filtration to obtain the MXene / GO composite film.

[0048] 3. Place the MXene / GO composite film obtained in step 2 in hydroiodic acid (HI) and reduce it at 90℃~120℃ for 1.5~3h to obtain the MXene / rGO composite film.

[0049] 4. Using the acrylic film VHB4910 with super-stretch properties as a substrate, the MXene / rGO composite film is transferred to the acrylic film VHB4910 substrate film by dry transfer. When the film shrinks together with the substrate film, the film will form a wrinkled structure.

[0050] 5. Assemble copper electrodes on the surface of the MXene / rGO composite micro-wrinkled film obtained in step 4, and assemble the copper electrodes onto both sides of the MXene / rGO micro-wrinkled film with conductive silver paste to obtain the MXene / rGO composite micro-wrinkled film strain sensor.

[0051] 6. The MXene / rGO composite micro-wrinkled film with electrodes obtained in step 5 is encapsulated by dropping polydimethylsiloxane (PDMS) onto the MXene / rGO composite micro-wrinkled film with electrodes using a dropper to obtain a flexible strain sensor of MXene / rGO composite material with micro-wrinkled structure.

[0052] (II) Comparative Example 1 – Pure MXene flexible strain sensor with micro-folded structure, fabricated by the following method:

[0053] 1. Obtain the MXene solution via etching. Add 3–4 g of lithium fluoride (LiF) and 30–50 mL of 9 mol / L hydrochloric acid (HCl) to a reaction vessel. Place the vessel in a constant temperature water bath (35–45°C) for the first heating and stirring at a rate of 500–800 r / min. After stirring, add 1.5–2.5 g of titanium aluminum carbide (Ti3AlC2) in portions. After all the solution is added, cover the vessel and place it in a constant temperature water bath (35–45°C) for the second heating and stirring at a rate of 500–800 r / min. After stirring, centrifuge and discard the centrifuged liquid. First, wash away any unreacted LiF with 2 mol / L HCl, then wash with distilled water until neutral. Pour the product into a gas washing bottle and sonicate it in an ultrasonic bath for 0.5–1.5 h, while simultaneously performing bubbling treatment. The gas flow rate for bubbling treatment is 8–12 m / s, and the bubbling time is 0.5–1.5 h. After completion, centrifuge the solution, and the supernatant is the MXene solution.

[0054] Dilute it to 0.1 mg / mL and stir for 10–15 min to obtain an MXene colloidal solution.

[0055] 2. The MXene colloidal solution obtained in step 1 is subjected to vacuum filtration under a vacuum filtration device. Take 5-7 mL of the MXene colloidal solution and obtain an MXene film by vacuum filtration.

[0056] 3. Using the acrylic film VHB4910 with super-stretch properties as a substrate, the MXene film is transferred to the acrylic film VHB4910 substrate film by dry transfer. When the film shrinks together with the substrate film, the film will form a wrinkled structure.

[0057] 4. Assemble copper electrodes on the surface of the MXene micro-wrinkled film obtained in step 3, and assemble the copper electrodes onto both sides of the MXene micro-wrinkled film with conductive silver paste to obtain the MXene micro-wrinkled film strain sensor.

[0058] 5. The MXene micro-wrinkled film with electrodes obtained in step 4 is encapsulated by dropping polydimethylsiloxane (PDMS) onto the MXene micro-wrinkled film with electrodes using a dropper to obtain an MXene flexible strain sensor with a micro-wrinkled structure.

[0059] (III) Comparative Example 2 – A pure rGO flexible strain sensor with a micro-folded structure, prepared by the following method:

[0060] 1. Using the modified Hummers method, 20–25 mL of 98% concentrated sulfuric acid (H₂SO₄) is added to a beaker and cooled to -1–1°C. 0.5–1.5 g of natural graphite is added, and the mixture is stirred for 40–60 min. Then, 4–6 g of potassium permanganate (KMnO₄) is added in portions, and the mixture is stirred for 2–4 h. After stirring, the ice bath is replaced, and the mixture is placed in a constant-temperature water bath (35–45°C) for the first heating and stirring at a rate of 800–1000 r / min, with bubbling and exothermic stirring for 40–50 min. Then, the mixture is transferred to a high-temperature constant-temperature water bath (70–90°C) for the second heating and stirring at a rate of 400–500 r / min. 70–90 mL of distilled water is added very slowly in portions, and the mixture is stirred for 10–20 min. Finally, 50–70 mL of distilled water is added to dilute the mixture. Add 350-370g of 5% hydrogen peroxide (H2O2). When a golden yellow color appears, immediately centrifuge and discard the centrifuged liquid. Wash until the pH value is around 5-6.

[0061] Dilute it to 0.1 mg / mL and stir for 10–15 min to obtain a GO solution.

[0062] 2. The GO solution obtained in step 1 is subjected to vacuum filtration under a vacuum filtration device. Take 5-7 mL of GO solution and obtain a GO membrane by vacuum filtration.

[0063] 3. Place the GO film obtained in step 2 in hydroiodic acid (HI) and reduce it at 90℃~120℃ for 1.5~3h to obtain rGO film.

[0064] 4. Using the acrylic film VHB4910 with super-stretch properties as a substrate, the rGO composite film is transferred to the acrylic film VHB4910 substrate film by dry method. When the film shrinks together with the substrate film, the film will form a wrinkled structure.

[0065] 5. Assemble copper electrodes on the surface of the rGO micro-wrinkled film obtained in step 4, and assemble the copper electrodes onto both sides of the rGO micro-wrinkled film using conductive silver paste to obtain the rGO micro-wrinkled film strain sensor.

[0066] 6. The rGO micro-wrinkled film with electrodes obtained in step 5 is encapsulated by dripping polydimethylsiloxane (PDMS) onto the rGO micro-wrinkled film with electrodes to obtain a flexible rGO strain sensor with a micro-wrinkled structure.

[0067] (iv) Characterization

[0068] Figure 1 XRD patterns of the prepared MXene, rGO, and MXene / rGO composite materials. Figure 1 The results show that the present invention successfully synthesized the MXene / rGO composite material.

[0069] Example 2

[0070] (I) A novel flexible strain sensor made of MXene / rGO composite material with a gradient micro-wrinkled structure, the fabrication method of which is as follows:

[0071] 1. Using the modified Hummers method, 20–25 mL of 98% concentrated sulfuric acid (H₂SO₄) is added to a beaker and cooled to -1–1°C. 0.5–1.5 g of natural graphite is added, and the mixture is stirred for 40–60 min. Then, 4–6 g of potassium permanganate (KMnO₄) is added in portions, and the mixture is stirred for 2–4 h. After stirring, the ice bath is replaced, and the mixture is placed in a constant-temperature water bath (35–45°C) for the first heating and stirring at a rate of 800–1000 r / min, with bubbling and exothermic stirring for 40–50 min. Then, the mixture is transferred to a high-temperature constant-temperature water bath (70–90°C) for the second heating and stirring at a rate of 400–500 r / min. 70–90 mL of distilled water is added very slowly in portions, and the mixture is stirred for 10–20 min. Finally, 50–70 mL of distilled water is added to dilute the mixture. Add 350-370g of 5% hydrogen peroxide (H2O2). When a golden yellow color appears, immediately centrifuge and discard the centrifuged liquid. Wash until the pH value is around 5-6.

[0072] MXene solution was prepared by etching. 3–4 g of lithium fluoride (LiF) and 30–50 mL of 9 mol / L hydrochloric acid (HCl) were added to a reaction vessel and heated and stirred for the first time in a constant temperature water bath (35–45°C) at a stirring rate of 500–800 r / min. After stirring, 1.5–2.5 g of titanium aluminum carbide (Ti3AlC2) was added in portions. After all the solution was added, the vessel was capped and heated and stirred for the second time in a constant temperature water bath (35–45°C) at a stirring rate of 500–800 r / min. After stirring, the vessel was centrifuged and the centrifuged liquid was discarded. Unreacted LiF was first washed away with 2 mol / L HCl, and then washed with distilled water until neutral. The product was poured into a gas washing bottle and sonicated in an ultrasonic bath for 0.5–1.5 h, while simultaneously undergoing bubbling treatment. The gas flow rate for bubbling treatment was 8–12 m / s, and the bubbling time was 0.5–1.5 h. After completion, centrifuge the solution, and the supernatant is the MXene solution.

[0073] The GO solution was diluted to 0.1 mg / mL and mixed with MXene colloidal solution at a concentration of 0.1 mg / mL. After stirring for 10–15 min, the mixture was sonicated for 15–25 min and then stirred with a magnetic stirrer for 4–6 h to obtain a fully homogeneous MXene / GO colloidal solution.

[0074] 2. The MXene / GO colloidal solution obtained in step 1 is subjected to vacuum filtration in a vacuum filtration device. The filtration flask is placed on an inclined platform with an inclination angle of 10° to 30°. 5 to 7 mL of MXene / GO colloidal solution is taken and vacuum filtered to obtain the MXene / GO composite film.

[0075] 3. Place the MXene / GO composite film obtained in step 2 in hydroiodic acid (HI) and reduce it at 90℃~120℃ for 1.5~3h to obtain the MXene / rGO composite film.

[0076] 4. Using the acrylic film VHB4910 with super-stretchable properties as a substrate, the MXene / rGO composite film is transferred to the acrylic film VHB4910 substrate film by dry transfer. When the film shrinks together with the substrate film, the film will form a wrinkled structure, and the film with thickness gradient will thus form a gradient wrinkled structure.

[0077] 5. Assemble copper electrodes on the surface of the MXene / rGO composite gradient micro-wrinkled film obtained in step 4. Assemble the copper electrodes on both sides of the MXene / rGO micro-wrinkled film with conductive silver paste to obtain the MXene / rGO composite gradient micro-wrinkled strain sensor.

[0078] 6. The MXene / rGO composite gradient micro-wrinkled film with electrodes obtained in step 4 is encapsulated. Polydimethylsiloxane (PDMS) is dropped onto the MXene / rGO composite micro-wrinkled film with electrodes using a dropper to obtain a flexible strain sensor of MXene / rGO composite material with micro-wrinkled structure.

[0079] (II) A flexible strain sensor made of MXene / rGO composite material with a gradient-free micro-wrinkle structure, the fabrication method of which is as follows:

[0080] 1. Using the modified Hummers method, 20–25 mL of 98% concentrated sulfuric acid (H₂SO₄) is added to a beaker and cooled to -1–1°C. 0.5–1.5 g of natural graphite is added, and the mixture is stirred for 40–60 min. Then, 4–6 g of potassium permanganate (KMnO₄) is added in portions, and the mixture is stirred for 2–4 h. After stirring, the ice bath is replaced, and the mixture is placed in a constant-temperature water bath (35–45°C) for the first heating and stirring at a rate of 800–1000 r / min, with bubbling and exothermic stirring for 40–50 min. Then, the mixture is transferred to a high-temperature constant-temperature water bath (70–90°C) for the second heating and stirring at a rate of 400–500 r / min. 70–90 mL of distilled water is added very slowly in portions, and the mixture is stirred for 10–20 min. Finally, 50–70 mL of distilled water is added to dilute the mixture. Add 350-370g of 5% hydrogen peroxide (H2O2). When a golden yellow color appears, immediately centrifuge and discard the centrifuged liquid. Wash until the pH value is around 5-6.

[0081] MXene solution was prepared by etching. 3–4 g of lithium fluoride (LiF) and 30–50 mL of 9 mol / L hydrochloric acid (HCl) were added to a reaction vessel and heated and stirred for the first time in a constant temperature water bath (35–45°C) at a stirring rate of 500–800 r / min. After stirring, 1.5–2.5 g of titanium aluminum carbide (Ti3AlC2) was added in portions. After all the solution was added, the vessel was capped and heated and stirred for the second time in a constant temperature water bath (35–45°C) at a stirring rate of 500–800 r / min. After stirring, the vessel was centrifuged and the centrifuged liquid was discarded. Unreacted LiF was first washed away with 2 mol / L HCl, and then washed with distilled water until neutral. The product was poured into a gas washing bottle and sonicated in an ultrasonic bath for 0.5–1.5 h, while simultaneously undergoing bubbling treatment. The gas flow rate for bubbling treatment was 8–12 m / s, and the bubbling time was 0.5–1.5 h. After completion, centrifuge the solution, and the supernatant is the MXene solution.

[0082] Dilute it to 0.1 mg / mL and mix it with MXene colloidal solution with a concentration of 0.1 mg / mL. Stir for 10-15 min, then sonicate for 15-25 min, and stir with a magnetic stirrer for 4-6 h to obtain MXene / GO colloidal solution.

[0083] 2. The MXene / GO colloidal solution obtained in step 1 is subjected to vacuum filtration under a vacuum filtration device. Take 5-7 mL of the MXene / GO colloidal solution and obtain the MXene / GO composite film by vacuum filtration.

[0084] 3. Place the MXene / GO composite film obtained in step 2 in hydroiodic acid (HI) and reduce it at 90℃~120℃ for 1.5~3h to obtain the MXene / rGO composite film.

[0085] 4. Using the acrylic film VHB4910 with super-stretch properties as a substrate, the MXene / rGO composite film is transferred to the acrylic film VHB4910 substrate film by dry transfer. When the film shrinks together with the substrate film, the film will form a wrinkled structure.

[0086] 5. Assemble copper electrodes on the surface of the MXene / rGO composite micro-wrinkled film obtained in step 4, and assemble the copper electrodes onto both sides of the MXene / rGO micro-wrinkled film with conductive silver paste to obtain the MXene / rGO composite micro-wrinkled film strain sensor.

[0087] 6. The MXene / rGO composite micro-wrinkled film with electrodes obtained in step 5 is encapsulated by dropping polydimethylsiloxane (PDMS) onto the MXene / rGO composite micro-wrinkled film with electrodes using a dropper to obtain a flexible strain sensor of MXene / rGO composite material with micro-wrinkled structure.

[0088] Figure 1 The XRD patterns of MXene, rGO, and MXene / rGO composites prepared in Example 1 are shown. The crystal structures of MXene, rGO, and MXene / rGO composites were analyzed using X-ray diffraction. The XRD pattern of monolayer MXene shows the highest diffraction peak at 7°, while the highest diffraction peak of rGO is at 24.45°. After MXene combines with GO and is then reduced, characteristic peaks of both MXene and rGO are detected in the mixture, indicating successful composite formation. A broad, weak peak at 25.35° is observed in the XRD pattern of the composite material, indicating that the oxygen functional groups on the surface have been removed, and the graphene oxide nanosheets have been reduced to form rGO.

[0089] Figure 2This diagram illustrates the comparison of sensitivity and strain range of the flexible strain sensors prepared in Example 1: ① pure MXene, ② pure rGO, and ③ MXene / rGO composite material with micro-wrinkled structures. The resistance of the thin films and their real-time changes under different strains were recorded using a digital multimeter. ① The strain range of the pure MXene flexible strain sensor with micro-wrinkled structures is 200%. Within the strain range of 0–130%, GF = 2.35, and within the strain range of 130%–200%, GF = 11.73. ② The strain range of the pure rGO flexible strain sensor with micro-wrinkled structures is 200%. Within the strain range of 0–160%, GF = 1.39, and within the strain range of 160%–200%, GF = 78.50. ③ The strain range of the MXene / rGO composite material flexible strain sensor with micro-wrinkled structures is 200%. Within the strain range of 0–120%, GF = 19.68, and within the strain range of 120%–200%, GF = 172.61. The results showed that the combination of the two improved the sensitivity of the sensor, therefore the MXene / rGO composite material was selected.

[0090] Figure 3 This is a schematic diagram comparing the sensitivity and strain range of the flexible strain sensor made of MXene / rGO composite material with a gradient-free micro-wrinkled structure (①) and the flexible strain sensor made of MXene / rGO composite material with a novel gradient-wrinkled structure (②) prepared in Example 2. The resistance of the thin film and its real-time changes under different strains were recorded using a digital multimeter. The strain range of the flexible strain sensor made of MXene / rGO composite material with a gradient-free micro-wrinkled structure is 200%. Within the strain range of 0–120%, GF = 19.68, and within the strain range of 120%–200%, GF = 172.61. The strain range of the flexible strain sensor made of MXene / rGO composite material with a novel gradient-wrinkled structure is 200%. Within the strain range of 0–120%, GF = 30.22, within the strain range of 120%–160%, GF = 1016.63, within the strain range of 160%–180%, GF = 12914.30, and within the strain range of 180%–200%, GF = 47420.48. This demonstrates that gradient structures significantly improve the sensitivity of the sensor.

[0091] Figure 4 SEM images of the MXene / rGO gradient micro-wrinkled film prepared in Example 2: ① Upper part of the film, ② Middle part of the film, ③ Lower part of the film. The surface microstructure of the film was characterized using scanning electron microscopy. A gradient structure is clearly visible, with the number of wrinkles decreasing from top to bottom.

Claims

1. A method for fabricating an MXene / rGO gradient micro-folded structure with strain sensing properties, characterized in that, The preparation method is carried out according to the following steps: Step 1: Combine the MXene colloidal solution with the graphene oxide (GO) solution, disperse by ultrasonication to obtain an MXene / GO mixture, and stir thoroughly to obtain an MXene / GO colloidal solution; Step 2: Vacuum filter the MXene / GO colloidal solution in a vacuum filtration device. Place the filtration flask on an inclined platform and obtain MXene / GO composite films with different gradient changes through vacuum filtration. Step 3: The MXene / GO composite film is placed in hydroiodic acid (HI) for reduction to obtain the MXene / rGO composite film. Step 4: Using the super-stretchable acrylic film VHB4910 as a substrate, the MXene / rGO composite film is transferred onto the acrylic film VHB4910 substrate film by dry transfer. When the film shrinks together with the substrate film, the film will form a wrinkled structure, and the film with thickness gradient will thus form a gradient wrinkled structure.

2. The method for fabricating an MXene / rGO gradient micro-fold structure with strain sensing properties according to claim 1, characterized in that, The MXene / GO colloidal solution described in step one is achieved through the following steps: Step 1.1: The GO solution was prepared using a modified Hummers method. 98% concentrated sulfuric acid (H2SO4) was cooled to -1 to 1°C, natural graphite was added, and the mixture was stirred. Potassium permanganate (KMnO4) was added in portions while stirring. After stirring, the ice bath was replaced, and the solution was placed in a constant temperature water bath for the first heating and stirring. Bubbling and exothermic stirring was performed for 40 to 50 minutes. Then, the solution was transferred to a high-temperature constant temperature water bath for the second heating and stirring. Distilled water was added very slowly in portions while stirring. After dilution with distilled water, 5% hydrogen peroxide (H2O2) was added. When a golden yellow color appeared, the solution was immediately centrifuged and the centrifuged liquid was discarded. The solution was washed until the pH value was 5 to 6. Step 1.2: The MXene colloidal solution was prepared by etching. Lithium fluoride (LiF) and hydrochloric acid (HCl) were added to the reaction vessel and heated and stirred for the first time in a constant temperature water bath. After stirring, titanium aluminum carbide (Ti3AlC2) was added in portions. The mixture was sealed and heated and stirred for the second time in a constant temperature water bath. After stirring, the mixture was centrifuged and the centrifuged liquid was discarded. Unreacted LiF was first washed with HCl until neutral. The product was poured into a gas washing bottle and placed in an ultrasonic instrument for sonication. At the same time, the mixture was purged with air and bubbled. After the process, the mixture was centrifuged, and the supernatant was the MXene colloidal solution. Step 1.3: The GO solution obtained by the modified Hummers method was diluted to 0.1 mg / mL and mixed with MXene colloidal solution with a concentration of 0.1 mg / mL. The mixture was stirred, sonicated, and stirred again until homogeneous to obtain the MXene / GO colloidal solution.

3. The method for fabricating an MXene / rGO gradient micro-fold structure with strain sensing properties according to claim 2, characterized in that, In step 1.1, KMnO4 is added in 5 to 6 portions over 25 to 30 minutes.

4. The method for fabricating an MXene / rGO gradient micro-folded structure with strain sensing properties according to claim 2, characterized in that, In step 1.1, the stirring rate for the first heating and stirring is 800~1000 r / min and the temperature is 35~45℃. The stirring rate for the second heating and stirring is 400~500 r / min and the temperature is 70~90℃. During the second heating and stirring, 70~90 mL of distilled water is added, first in three portions of 5~10 mL, and then in three portions of the remaining distilled water.

5. The method for fabricating an MXene / rGO gradient micro-fold structure with strain sensing properties according to claim 2, characterized in that, In step 1.2, the stirring rate for the first heating and stirring is 500~800 r / min, the temperature is 35~45℃, and the stirring time is 10~20 min. The stirring rate for the second heating and stirring is 500~800 r / min, the temperature is 35~45℃, and the stirring time is 45~50 h.

6. The method for fabricating an MXene / rGO gradient micro-fold structure with strain sensing properties according to claim 2, characterized in that, In step 1.2, Ti3AlC2 is added in 8 to 10 portions over 25 to 35 minutes.

7. The method for fabricating an MXene / rGO gradient micro-fold structure with strain sensing properties according to claim 2, characterized in that, In step 1.2, the ventilation airflow rate for the ventilation and bubbling treatment is 8~12 m / s, and the ventilation time is 0.5~1.5h.

8. The method for fabricating an MXene / rGO gradient micro-fold structure with strain sensing properties according to claim 2, characterized in that, In step 1.3, the stirring rate for the first stirring is 600~800 r / min and the stirring time is 10~15 min. The stirring rate for the second stirring is 600~800 r / min and the stirring time is 4~6 h.

9. The method for fabricating an MXene / rGO gradient micro-fold structure with strain sensing properties according to claim 1, characterized in that, The MXene / GO composite films with different gradients described in step two are achieved through the following steps: vacuum filtration is performed in a vacuum filtration device, the filtration flask is placed on an inclined platform, 5-7 mL of MXene / GO colloidal solution is taken and vacuum filtered to obtain MXene / GO composite films, and the gradient distribution of film thickness is adjusted by adjusting the angle of the inclined platform from 10° to 30°.

10. The method for fabricating an MXene / rGO gradient micro-fold structure with strain sensing properties according to claim 1, characterized in that, In the HI reduction process described in step three, the reduction temperature is between 90℃ and 120℃, and the time is between 1.5 and 3 hours.

Images