Humidity sensor based on two-dimensional layered film as well as preparation method and application of humidity sensor

Abstract

The invention discloses a humidity sensor based on a two-dimensional layered film as well as a preparation method and application of the humidity sensor, and belongs to the technical field of application of two-dimensional layered films. The two-dimensional layered membrane comprises an ionic liquid modified two-dimensional vermiculite membrane, and the ionic liquid comprises a 1-ethyl-3-methylimidazolium tetrafluoroborate ionic liquid; in an XRD diffraction pattern of the ionic liquid modified two-dimensional vermiculite film, a diffraction peak exists at the position of 6.2 + / -0.2 degrees, the two-dimensional layered film shows impedance which changes along with the change of relative humidity, and the humidity sensor based on the two-dimensional layered film shows good linear resistance response in a specific relative humidity range. In addition, the response and recovery time is remarkably shortened in the humidity and humidity removal processes, and the humidity sensor is expected to become an ideal selection of a humidity sensor which is low in cost, wide in linear range, high in stability and high in reaction speed.

Description

Technical Field

[0001] The present invention belongs to the technical field of applications of two-dimensional layered films, and more specifically, relates to a humidity sensor based on a two-dimensional layered film, a preparation method thereof, and an application thereof. Background Art

[0002] In recent years, humidity sensors have shown their importance in fields such as food processing, chemistry, meteorology, and medicine. Due to the demand for high-precision and fast-response humidity sensors in these fields, the research and development of related technologies have become crucial. A humidity sensor is a device that converts humidity into an electrical signal, and its functional parameters such as impedance and dielectric constant change with the water molecules adsorbed by the active layer.

[0003] Currently, in humidity sensors, the materials used for the active layer include ceramics, organic polymers, and nanomaterials of different dimensions. Compared with other materials, humidity sensors based on two-dimensional nanomaterials have become the preferred choice in device engineering. Two-dimensional nanomaterials have a high surface-to-volume ratio, excellent mechanical properties, and abundant active sites, thus enabling ultrasensitive response. The most representative one is graphene oxide (GO), which has attracted much attention in the field of humidity sensing due to its oxygen-containing functional groups that interact with water molecules. However, GO has no continuity due to the random dispersion of oxidized and unoxidized regions, and the connection or boundary between nanosheets hinders proton transport. In addition, the oxygen-containing functional groups disrupt the conjugated electron state of GO, reducing its response to humidity. Therefore, humidity sensors based on GO have disadvantages such as low sensitivity, narrow humidity detection range, and long response time. For example, GO sensors show a sensing response at 11 - 97% RH, but their response time is long (13 s), which limits their practical applications. Therefore, it is necessary to develop other two-dimensional materials and prepare two-dimensional layered films to achieve humidity sensing with a faster response. Summary of the Invention

[0004] 1. Problems to be Solved

[0005] Aiming at the problems of the existing GO humidity sensors, such as long response time:

[0006] The first object of the present invention is to provide a two-dimensional layered film with humidity response characteristics and a shortened response time;

[0007] The second object of the present invention is to provide a humidity sensor based on a two-dimensional layered film with a shortened response time;

[0008] Another object of the present invention is to provide a humidity sensor applied to the process of oral breathing, including human oral breathing.

[0009] 2. Technical Solutions

[0010] Vermiculite (V), a natural clay mineral, has attracted attention due to its excellent chemical and thermal stability, abundant terrestrial reserves, and low cost. It consists of hydrated aluminum silicate, where aluminum octahedral layers are sandwiched between two silicon tetrahedral layers. Due to the partial replacement of Si 4+ by Al 3+ , the V nanosheets carry a negative charge, which is balanced to electrical neutrality by interlayer cations (such as Mg 2+ , Ca 2+ , K + ). The high negative charge density and chemical functional groups (mainly hydroxyl groups) of vermiculite make it promising as a two-dimensional material with humidity sensing function. However, it is found in this invention that when using unmodified V film as a humidity sensor, the response and recovery times are both long during the humidity increase and decrease processes. It is necessary to further improve this problem to obtain a humidity sensor with smaller response and recovery times during the humidity increase and decrease processes.

[0011] In this invention, the potential of V-[EMIM][BF 4 film prepared by intercalation method using 1-ethyl-3-methylimidazolium tetrafluoroborate IL ([EMIM][BF 4 ) as IL in humidity sensing applications was explored.

[0012] To solve the above problems, the technical solutions adopted in this invention are as follows:

[0013] In the first aspect of this invention, a two-dimensional layered film is provided, including an ionic liquid-modified two-dimensional vermiculite film, and the ionic liquid includes 1-ethyl-3-methylimidazolium tetrafluoroborate ionic liquid; there is a diffraction peak at 2θ = 6.2 ± 0.2° in the XRD diffraction pattern of the ionic liquid-modified two-dimensional vermiculite film. Preferably, there is a diffraction peak at 2θ = 6.2 ± 0.1° in the XRD diffraction pattern of the ionic liquid-modified two-dimensional vermiculite film.

[0014] As a preference of any technical solution in the first aspect of this invention, the thickness of the ionic liquid-modified two-dimensional vermiculite film is not less than 7 μm. When the film thickness increases, the path for ions to cross the film becomes longer, which will increase the resistance of ion transport, thereby increasing its electrochemical impedance. On the other hand, increasing the thickness may also reduce the current density per unit area, further affecting the kinetics and impedance characteristics of the electrode process. Considering from the perspective of the mechanical strength of the film, increasing the thickness can improve the mechanical stability of the film, but being too thick may lead to a decrease in transport efficiency, and an overly thick film will result in a loss of flexibility and an increase in cost. Therefore, generally, the thickness of the ionic liquid-modified two-dimensional vermiculite film is not greater than 20 μm.

[0015] As a preferred embodiment of any technical solution of the first aspect of the present invention, the contact angle of the two-dimensional vermiculite film modified by the ionic liquid is not greater than 20°.

[0016] The second aspect of the present invention provides a method for preparing the two-dimensional layered film of the first aspect of the present invention, comprising:

[0017] Intercalation, the thermal expansion of vermiculite blocks is intercalated to obtain Li + intercalated vermiculite layers;

[0018] Stripping, Li + The intercalated vermiculite layer is peeled off into a single-layer nanosheet; the single layer mentioned here refers to an extremely thin layer with a thickness of a single atom or molecule in the art. In the present invention, the single-layer nanosheet of vermiculite specifically refers to a nanosheet with a thickness of no more than 2 nm, preferably a nanosheet with a thickness of no more than 1 nm;

[0019] Assembly, assembling the nanosheets into a self-supporting two-dimensional vermiculite film;

[0020] Modification: soaking the self-supporting two-dimensional vermiculite membrane in the 1-ethyl-3-methylimidazolium tetrafluoroborate ionic liquid to obtain the two-dimensional vermiculite membrane modified by the ionic liquid.

[0021] As a preferred embodiment of any technical solution of the second aspect of the present invention, in the modification step, the immersion time is not less than 10 hours, preferably not less than 12 hours; thereafter, the residual ionic liquid is removed by centrifugation at a speed of not less than 2000 rpm.

[0022] The third aspect of the present invention provides a humidity sensor based on a two-dimensional layered film, comprising the two-dimensional layered film described in any technical solution of the first aspect of the present invention, and further comprising:

[0023] A first electrode, wherein the first electrode is a platinum wire electrode, and the platinum wire electrode is in contact with a surface of one side of the two-dimensional vermiculite membrane modified by the ionic liquid along a length direction;

[0024] A second electrode, wherein the second electrode is a platinum wire electrode, and the platinum wire electrode is in contact with a surface of one side of the two-dimensional vermiculite membrane modified by the ionic liquid along a length direction;

[0025] The first electrode and the second electrode are arranged in parallel on the same side of the two-dimensional vermiculite membrane modified by the ionic liquid without contacting each other.

[0026] Specifically, the first electrode is used as an input or detection point of the current; the second electrode is used as an output or corresponding detection point of the current, or vice versa, the second electrode is used as an input or detection point of the current; the first electrode is used as an output or corresponding detection point of the current. The platinum wire electrode is generally composed of pure platinum metal wire, for example, the wire diameter can be 1mm.

[0027] The first electrode and the second electrode are arranged in parallel, and the distance between them is determined by the length of the membrane. The key to choosing the membrane length is to balance the distance between the electrodes and the transmission efficiency. Although a longer membrane length may lead to a longer transmission path and higher impedance for electrons and ions, it also helps to disperse the current and reduce local thermal effects.

[0028] The fourth aspect of the present invention provides an application of the two-dimensional layered membrane described in any technical solution of the first aspect of the present invention or the humidity sensor based on the two-dimensional layered membrane described in any technical solution of the third aspect of the present invention, including detecting the relative humidity of the air by testing the impedance of the two-dimensional vermiculite membrane modified by ionic liquid in the relative air humidity range of 0% to 100% RH at a test frequency of 1 to 10 kHz.

[0029] The present invention verifies the response of a two-dimensional layered film or humidity sensor to relative air humidity at different frequencies.

[0030] The performance of a sensor may vary as the frequency changes. Given that different application scenarios may require the sensor to operate within a specific frequency range, it is critical to test the sensor at multiple frequencies to ensure its proper function within its intended range of use. Testing at different frequencies over a long period of time can reveal information about the stability and reliability of the sensor, especially in extreme or challenging applications. In summary, testing at different frequencies helps to fully understand the performance of the sensor, ensure its suitability for its intended application, and improve its reliability and accuracy under various conditions.

[0031] Further, in the relative air humidity range of 0% to 100% RH, the impedance of the two-dimensional vermiculite membrane modified by the ionic liquid changes linearly with the relative air humidity. It should be noted that the linear change is a segmented linear change. For example, in the embodiment, in the relative air humidity range of 0% to 60% RH, 60% to 70% RH, and 70% to 100% RH, the impedance of the two-dimensional vermiculite membrane modified by the ionic liquid and the relative air humidity respectively show linear changes with different slopes.

[0032] Further, under the test conditions, when the impedance of the two-dimensional vermiculite membrane modified by the ionic liquid decreases, the relative air humidity increases; when the impedance of the two-dimensional vermiculite membrane modified by the ionic liquid increases, the relative air humidity decreases.

[0033] As a preferred embodiment of any technical solution of the fourth aspect of the present invention, the application includes detecting mouth breathing. During mouth breathing, the local relative air humidity changes with the process of exhalation or inhalation, and the two-dimensional layered film or humidity sensor in the present invention can effectively respond to the changes in this process, and thus can be used for detecting the mouth breathing process.

[0034] As a preference of any technical solution of the fourth aspect of the present invention, the mouth breathing includes human mouth breathing.

[0035] 3. Beneficial effects

[0036] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0037] (1) The present invention provides a brand-new two-dimensional layered film, namely V-[EMIM][BF 4 film. There is a diffraction peak at 2θ = 6.2 ± 0.2° in its XRD diffraction pattern, further indicating that the film has an appropriate layer spacing. At the same time, the V-[EMIM][BF 4 film shows impedance that changes with relative humidity and has the application prospect of humidity sensing.

[0038] (2) The humidity sensor based on the V-[EMIM][BF 4 film of the present invention shows good linear resistance responses with different slopes respectively in the ranges of relative air humidity from 0% to 60% RH, from 60% to 70% RH, and from 70% to 100% RH, and maintains stability for up to 300 min at a test frequency of 10 kHz. It is expected to be an ideal choice for humidity sensors with low cost, wide linear range, strong stability, and fast reaction speed.

[0039] (3) Compared with the humidity sensor based on the V film, the humidity sensor based on the V-[EMIM][BF 4 film significantly shortens the response and recovery times during the humidity increase and decrease processes, which are 5 s and 34 s respectively; compared with the GO film, the response time is significantly shortened.

[0040] (4) The humidity sensor based on the V-[EMIM][BF 4 film of the present invention shows applicability in human mouth breathing. Description of the drawings

[0041] Figure 1 It is a schematic diagram of the test device for the electrical performance and breathing response of the humidity sensor.

[0042] Figure 2 It is a photo: (a) thermally expanded V particles; (b) V nanosheet colloidal solution.

[0043] Figure 3 It is the (a) SEM characterization, (b) AFM characterization (tapping mode), and (c) height profile of the V nanosheets.

[0044] Figure 4 It is a photo of the V film in the (a) bent state and (b) natural stretched state.

[0045] Figure 5 For the cross-sectional SEM images of the V film (a) and the V-[EMIM][BF 4 film (b) and the EDS analysis of the corresponding red-marked regions.

[0046] Figure 6 For the water contact angle photos of the V film and the V-[EMIM][BF 4 film.

[0047] Figure 7 For the XRD patterns (a), FT-IR spectra (b), and Raman spectra (c) of the V film and the V-[EMIM][BF 4 film.

[0048] Figure 8 For the impedance responses of the humidity sensors based on V and V-[EMIM][BF 4 film at test frequencies of 1 kHz (a) and 10 kHz (b); Inset: Magnified view of the fitting curve of the sensor based on V-[EMIM][BF 4 film in the range of 70%-100% RH; For the resistance sensitivities of the humidity sensors based on V and V-[EMIM][BF 4 film at test frequencies of 1 kHz (c) and 10 kHz (d); (e) For the resistance stability of the humidity sensor based on V-[EMIM][BF 4 film within 300 min at different RHs; (f) For the dependence of the resistance on the frequency of the sensor based on V-[EMIM][BF 4 film in different RH environments.

[0049] Figure 9 For the impedance responses and recovery times of the humidity sensors based on V (a) and V-[EMIM][BF 4 film (b) at a test frequency of 10 kHz during humidification (84%) and dehumidification (33%); For the schematic diagrams of the multi-cycle impedance responses of the sensors based on V (c) and V-[EMIM][BF 4 film (d) to human breathing at a test frequency of 10 kHz. At a test frequency of 10 kHz, for the impedance response times of the humidity sensors based on V (e) and V-[EMIM][BF 4 film (f) during exhalation and inhalation during mouth breathing.

[0050] Figure 10 For the schematic diagram of the mechanism of the humidity sensor based on V-[EMIM][BF 4 film. Detailed implementation manners

[0051] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the technical field to which this invention belongs; the term "and / or" as used herein includes any and all combinations of one or more of the related listed items.

[0052] For those not specified in the examples, they are carried out under conventional conditions or conditions recommended by the manufacturer. For reagents or instruments whose manufacturers are not specified, they are all conventional products that can be obtained through commercial purchase.

[0053] Concentrations, amounts, and other numerical data may be presented herein in a range format. It should be understood that such range formats are used merely for convenience and brevity and should be interpreted flexibly as including not only the explicitly recited values as the limits of the range, but also all individual values or sub-ranges subsumed within the stated range as if each value and sub-range were explicitly recited. For example, a numerical range of about 1 to about 4.5 should be interpreted as including not only the explicitly recited limits of 1 to about 4.5, but also the individual numbers (such as 2, 3, 4) and sub-ranges (such as 1 to 3, 2 to 4, etc.). The same principle applies to ranges that recite only one numerical value, such as "less than about 4.5", which should be interpreted as including all of the above values and ranges. In addition, this interpretation should apply regardless of the breadth of the range or feature described.

[0054] The present invention will be further described below in conjunction with specific embodiments.

[0055] Experimental Section

[0056] I. Materials

[0057] Thermally expanded vermiculite was purchased from Shijiazhuang Chenxing Industrial Co., Ltd.; the ionic liquid, 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIM][BF 4 )), sodium chloride (NaCl), and lithium chloride (LiCl) were purchased from Tianjin Siyanshi. In all experiments and characterization processes, high-purity water produced by the Millipore Milli-Q system was used.

[0058] II. Preparation of Liquid Ion-Modified Two-Dimensional Vermiculite Membrane

[0059] 1. Preparation of Li + Intercalated Vermiculite Lamellar

[0060] First, 0.20 g of thermally expanded V bulk particles were mixed with 200 ml of saturated NaCl solution, and then magnetically stirred and refluxed at 120 °C for 24 h to achieve Na + Intercalation. After the reaction, filtration was carried out using a polyethersulfone (PES) membrane, and then washed with a large amount of deionized water to remove excess NaCl. Subsequently, Na+ The intercalated V lamellar is mixed with 200 ml of 2 M LiCl solution, and a magnetic stirring reflux reaction is carried out at 120 °C for 24 h. To remove the excess salts during the reaction process, the product is filtered using the same washing process as above to obtain Li + intercalated V lamellar.

[0061] 2. Preparation of vermiculite single-layer nanosheets (V nanosheets)

[0062] After the ion intercalation is completed, the Li + intercalated V lamellar is dispersed in ultrapure water, and the product is treated by ultrasonic-assisted exfoliation for 30 min to exfoliate it into single-layer nanosheets. Then, it is centrifuged at a speed of 3000 rpm / min for multiple times to remove the unexfoliated V precipitate at the bottom layer of the centrifugation. Then, it is centrifuged at a speed of 10,000 rpm to remove the smaller-sized nanosheets. The obtained colloid is dispersed in ultrapure water to obtain a colloidal solution containing single-layer V nanosheets.

[0063] 3. Assembly of self-supporting two-dimensional vermiculite membranes

[0064] The colloidal solution containing single-layer V nanosheets is assembled into a self-supporting two-dimensional vermiculite membrane (V membrane) by vacuum filtration.

[0065] 4. Preparation of ionic liquid-modified two-dimensional vermiculite membranes (modification)

[0066] The self-supporting V membrane is immersed in 1-ethyl-3-methylimidazolium tetrafluoroborate ionic liquid (IL) at room temperature for 12 hours, and then centrifuged at a speed of 3000 rpm for 100 s to remove the residual ionic liquid, obtaining an ionic liquid-modified two-dimensional vermiculite membrane (V-[EMIM][BF 4 membrane).

[0067] III. Structure and testing method of humidity sensors

[0068] The self-supporting membrane sample is loaded into an in-plane test fixture to form a sensor device, and the two-electrode method is used to test its resistance through an impedance spectrometer, as Figure 1 shown (the humidity in the figure is only for illustration). The specific settings of the electrodes in the two-electrode method are as follows:

[0069] Both the first electrode and the second electrode are platinum wire electrodes, as Figure 1 shown, contacting one side surface of the prepared ionic liquid-modified two-dimensional vermiculite membrane along the length direction; the first electrode and the second electrode are arranged in parallel on the same side of the ionic liquid-modified two-dimensional vermiculite membrane and do not contact. The first electrode is used as the input point of the current; the second electrode is used as the corresponding detection point of the current. The diameter of the platinum wire electrode used is 1 mm.

[0070] Electrical response test: Use a glove box to provide 0% RH, a saturated LiCl salt solution to provide 12% RH, and a test chamber to provide 30% to 100% RH.

[0071] Humidification and dehumidification transient response test: using saturated MgCl 2 The 33% RH provided by the salt solution is used as the dehumidification environment, and the 84% RH provided by the saturated KCl salt solution is used as the humidification environment. The humidity environment is quickly switched after the resistance value of the sensor is stabilized.

[0072] Human mouth breathing transient response test: Keep the distance between the mouth and the sensor at 2cm, and perform the next mouth breathing transient response test after the resistance value stabilizes.

[0073] 4. Membrane Characterization

[0074] 1. Characterization of V nanosheet solution

[0075] (1) Properties of colloidal solutions of V particles and V nanosheets

[0076] Thermal expansion V minerals present a densely stacked golden layer structure and appear in a granular form, such as Figure 2 (a) shows the thermal expansion of V particles. This layered structure provides good conditions for subsequent ion intercalation, enabling V to efficiently capture and embed ions, thereby achieving the exfoliation of nanosheets. The V nanosheet solution is a light yellow transparent colloid. When irradiated by laser, it shows obvious Tyndall scattering effect, such as Figure 2 (b) as shown.

[0077] (2) SEM and AFM analysis of V nanosheets

[0078] Figure 3 (a) and (b) are the SEM and AFM images of the exfoliated V nanosheets, respectively. It can be seen from the figure that the lateral size of the nanosheets is generally no more than 4 μm and has an irregular geometric shape. The irregular shape may be caused by the ion intercalation and ultrasound-assisted exfoliation process. In addition, the AFM height profile shows that the thickness of the V nanosheet is about 2 nm (e.g. Figure 3 (c)), indicating that the V bulk particles have been successfully exfoliated into V nanosheets. The above SEM and AFM analysis results provide important information about the morphological characteristics and thickness of V nanosheets.

[0079] 2. Characterization of V-film and V-[EMIM][BF4] film

[0080] (1) Characterization of V-film properties

[0081] like Figure 4As shown in (a), the V film is light yellow and has excellent flexibility. The film exhibits excellent mechanical properties and does not leave obvious creases when it returns to its natural stretched state after bending ( Figure 4 (b)). First, the high flexibility of the two-dimensional nanosheets enables them to maintain structural integrity when stretched or bent. Second, the abundant hydroxyl groups between the V film layers promote the formation of a hydrogen bond network, which produces a slip effect between the nanosheets, giving the membrane material the ability to resist folding or breaking under external forces.

[0082] (2) V-film and V-[EMIM][BF 4 SEM analysis of the membrane

[0083] Figure 5 (a) and (b) are V-film and V-[EMIM][BF 4 ] film cross-section SEM image and EDS element analysis results. The SEM cross-section image shows that the V film has a well-stacked layered structure, and the layers are stacked tightly and orderly; while the V-[EMIM][BF 4 The film still retains a layered structure, but the boundaries are blurred and adhesives appear between the layers, which may be caused by the filling of ionic liquids. EDS elemental analysis shows that the V film contains O, Mg, Al, and Si elements, which come from silicon-oxygen tetrahedrons and aluminum-oxygen octahedrons, as well as interlayer cations Mg. 2+ ; and V-[EMIM][BF 4 In addition to the four elements of the V membrane, the membrane also contains two additional elements, N and F, which are derived from the cations of IL [EMIM] + and anions [BF 4 ] - The above analysis results show that IL has been successfully doped into the V film layer.

[0084] (3) Water contact angle analysis based on V film

[0085] Figure 6 The results show that the water contact angles on the surface of V membrane and IL-modified V membrane are 15° and 18°, respectively, both less than 60°, indicating that the two membranes are extremely hydrophilic and water droplets can spread freely on the V membrane. The literature shows that the contact angle can reflect the size of the surface energy and polar components from the side, the latter usually comes from the contribution of Coulomb interactions between dipoles (such as hydrogen bonds), so the superhydrophilic property may be closely related to the hydration structure of interlayer cations. V-[EMIM][BF 4 The hydrophilicity of the membrane may come from the hygroscopicity of IL. The cohesive force between water molecules is lower than the interaction between water molecules and the ionic liquid on the surface of V membrane, which leads to the good hydrophilicity of the composite membrane.

[0086] (4) V-film and V-[EMIM][BF4 Microstructural Analysis of the Membrane

[0087] Figure 7 (a) shows the change in the layer spacing of the V membrane before and after IL modification. After IL intercalation, the interlayer distance of the composite membrane increases from 1.28 nm (diffraction peak at 2θ value of 6.90°) to 1.4 nm (diffraction peak at 2θ value of 6.17°). Combining the above characterization results, we confirm the presence of IL between the layers of the V membrane. The increase in the layer spacing may be due to the interaction between the volume of the [BF 4 — anion and the V nanosheets. Subsequently, the interaction between IL and V nanosheets was investigated in depth using FT-IR ( Figure 7 (b)). The V membrane shows a high-intensity broad peak in the range of 3600 - 3200 cm -1 , which is due to the hydrogen bonding of hydroxyl groups in the material. The literature shows that the peak at 3570 cm -1 is attributed to the stretching vibration of the hydroxyl group coordinated with the central atom in the octahedron. The peaks at 3400 cm -1 and 3210 cm -1 correspond to the stretching vibration of water, and the peak at 3400 cm -1 is related to the water absorbed in V. At 1643 and 1126 cm -1 , the characteristic peaks of the deformation vibration of the O - H bond of water molecules and the stretching vibration of the Si - O bond in the SiO 4 tetrahedron are observed respectively. The above peaks are also observed in the FT-IR spectrum of the V-[EMIM][BF 4 membrane, but some peaks shift to higher wavenumbers (blue shift), which may be caused by the formation of hydrogen bonds between the cations in IL and the hydroxyl groups on the V nanosheets. In the infrared spectrum of the V-[EMIM][BF 4 membrane, three characteristic peaks related to IL are also observed, especially in the fingerprint region related to [EMIM] + . At 2990 cm -1 , 1570 cm -1 and 1166 cm -1 , the characteristic peaks of the stretching vibration of the C - H in the alkyl chain, the stretching of the imidazole ring skeleton, and the in-plane stretching vibration of the C - H in the imidazole ring are observed respectively.

[0088] Figure 7 (c) shows the Raman spectra of the V membrane and the V-[EMIM][BF 4 membrane in the region below 1200 cm -1 . The spectral bands of both membranes show highly similar spectral characteristics in the tested wavenumber range. Peaks are located at approximately 675 cm -1 , 357 cm -1 , 190 cm​-1 and 100 cm -1 The characteristic peaks near indicate the presence of silicate minerals, and these mineral characteristics usually appear in vermiculite. Specifically, the high-intensity peak at 675 cm -1 is related to the deformation vibration of the Si-O tetrahedron, while the peak at 357 cm -1 is related to the symmetric vibration of the O-Mg-O structural unit. The peak at 190 cm -1 is related to the deformation symmetric mode of the non-degenerate vibration of Mg-O in the octahedron. The weak peak at about 100 cm -1 is attributed to the vibration mode of the interlayer cations. The characteristic peak at 1091 cm -1 corresponds to the stretching vibration of the Si-O bond, while the peak near 555 cm -1 is related to the cooperative vibration of the SiO 4 group in the tetrahedron and the transitional or free vibration mode of the OH group. In the Raman spectrum of the V-[EMIM][BF 4 film, two IL-related characteristic peaks are also observed, located at 800 cm -1 and 442 cm -1 , corresponding to the stretching vibration of the B-F bond and the bending vibration of the CH 2 -N bond, respectively. These Raman results confirm the presence of [EMIM][BF 4 in the V film.

[0089] V. Humidity Sensor Performance Analysis Based on V Film and V-[EMIM][BF 4 Film

[0090] To study the electrical characteristics of humidity sensors based on V and V-[EMIM][BF 4 films, their resistance responses at different relative humidities were measured at test frequencies of 1 kHz and 10 kHz, as shown in Figure 8 (a) and (b). In Figure 8 (a), at a test frequency of 1 kHz, both sensors show good linear responses in the range of 0% to 70% RH. The resistance of the humidity sensor based on the V film linearly decreases from 1.39×10 7 Ω at 0% RH to 1.04×10 6 Ω at 70% RH. In comparison, the resistance of the sensor based on the V-[EMIM][BF 4 film decreases more significantly in the same humidity range, from 2.26×10 7 Ω to 1.24×10 6Ω. As the relative humidity increases, the resistance of both sensors shows a downward trend, which is due to the decrease in the resistance of the assembled nanoflakes. Based on the V-[EMIM][BF 4 film sensor shows a higher resistance compared to the V film sensor at low relative humidity because the introduction of the IL layer increases the spacing, resulting in an overall increase in film thickness and thus an extension of the proton transport path. At high relative humidity conditions, the resistance of both sensors tends to be the same because the interlayer is fully occupied by a large number of water molecules, forming a physically adsorbed water layer that can be ionized into protons, thus offsetting the adverse effect of the increased film thickness on the proton transport path. Similarly, Figure 8 (b) shows that within the range of 0% to 60% RH, both sensors exhibit good linearity at a test frequency of 10 kHz. It is worth noting that the sensor based on the V-[EMIM][BF 4 film also shows good linearity within the range of 70% to 100% RH.

[0091] To quantitatively evaluate the humidity responsiveness of the sensors based on V and V-[EMIM][BF 4 films, we evaluated the resistance sensitivity (R = ΔR / R 0 , where ΔR represents the change in resistance and R 0 represents the resistance at 0% RH) and the sensitivity (S = ΔR / ΔH, where ΔR represents the change in resistance sensitivity and ΔH represents the change in humidity). Figure 8 (c) and (d) respectively show the change curves of the resistance sensitivity in the range of 0% RH to 100% RH at test frequencies of 1 kHz and 10 kHz. The responsiveness of the sensor is determined by the slope obtained from curve fitting. At a test frequency of 1 kHz, the sensitivity values of the humidity sensors based on V and V-[EMIM][BF 4 films are 0.013 and 0.014 respectively within the linear range of 0 - 70% RH ( Figure 8 (c)). It is worth noting that at a test frequency of 10 kHz, the humidity sensor based on the V-[EMIM][BF 4 film exhibits a linear resistance response in two regions, covering 0% to 60% RH and 70% to 100% RH respectively, with corresponding sensitivities of 0.015 and 0.002 ( Figure 8 (d)). In addition, the stability of the impedance response of the sensor based on the V-[EMIM][BF 4 film was evaluated at a test frequency of 10 kHz and in different relative humidity environments (30%, 60%, 80% RH) within 300 min, as shown in Figure 8 (e). The results show that the output of the sensor is very stable and the fluctuating error can be ignored. As shown in Figure 8(f) shows the frequency dependence of the resistance of the composite film-based sensor in different relative humidity environments (30%, 60%, and 80% RH). As the frequency and humidity increase, the resistance decreases significantly, which may be due to the formation of a continuous water layer in the two-dimensional nanochannels at higher humidity.

[0092] We further investigated the impedance response time and recovery time of the humidity sensors based on V and V-[EMIM][BF 4 films during the humidification and dehumidification processes. The response time of the sensor is defined as the time required for the change in resistance or capacitance to reach 90% during the water molecule adsorption process, while the recovery time is the duration during the water molecule desorption process. The impedance response time and recovery time of the humidity sensor based on the V film are 10 and 104 s, respectively ( Figure 9 (a)). In contrast, the response time and recovery time of the sensor based on the V-[EMIM][BF 4 film are shorter, 5 and 34 s, respectively ( Figure 9 (b)). Doping with IL significantly shortens the sensor response and recovery times, which benefits from the increased layer spacing, promoting the absorption and desorption of more water molecules. In addition, within the two-dimensional nanochannels, the ultrafast transport of water molecules as proton transfer carriers reduces the sensor response and recovery times, enabling a rapid sensing response.

[0093] VI. Application of the sensor for human oral breathing

[0094] Based on the transient humidity response performance of the sensors based on V and V-[EMIM][BF 4 films described above, we then demonstrated their applicability in detecting human oral breathing ( Figure 9 (c) and (d)). Under the action of exhaled air, the resistance of the sensor decreases; conversely, the resistance increases during inhalation. As can be seen from Figure 9 (c) and (d), the sensors based on V and V-[EMIM][BF 4 films show good repeatability in the impedance response to human breathing during five test cycles. The impedance response times of the two sensors to exhalation are approximately the same, about 0.8 s ( Figure 9 (e) and (f)). However, there is a significant difference in the impedance recovery time during inhalation. The recovery time of the sensor based on the V-[EMIM][BF 4 film is almost half that of the sensor based on the V film. The reason for the significant reduction in the recovery time is similar to the difference in the transient response of the sensor during the humidification and dehumidification processes described above. The ultrafast transport of water molecules in the IL-modified film promotes the rapid desorption of water molecules, and the proton transfer is blocked, causing the resistance of the sensor to quickly recover to the high resistance value at low humidity.

[0095] VII. Analysis of Proton Transport Mechanism of Sensors Based on V-[EMIM][BF 4 Membranes

[0096] Figure 10 For the humidity sensor based on V-[EMIM][BF 4 membrane, the schematic diagram of the sensing mechanism is shown. The V nanosheets carry negative charges and attract some cations in the IL through electrostatic interaction, forming a high-density IL layer near the nanochannel wall, and the IL shows a layered distribution in the membrane. In addition to the electrostatic interaction, the + polar groups of [EMIM]

[0097] H + +H 2 O → H 2 O + H +

[0098] can form hydrogen bonds with the hydroxyl groups on the vermiculite nanosheets, providing an additional transport path for proton transport in the composite membrane. Under low relative humidity conditions, water molecules enter the two-dimensional nanochannels and bind to the polar groups of the IL modified on the V channel surface through double hydrogen bonds, forming the first physically adsorbed water layer with local mobility. Although the first hydration can increase the layer spacing, the increase is limited. According to the Grotthuss mechanism, protons may undergo the following hopping transfer in this physically adsorbed water layer:

[0099] H 2 O + H 2 O → H 3 O + + OH - .

[0100] In the present invention, the V-[EMIM][BF 4 membrane was successfully prepared by the ionic liquid intercalation method, and its application potential in humidity sensing was explored. V-[EMIM][BF 4The mold has a hydrophilic surface and an enlarged interlayer distance, which is beneficial to enhancing the adsorption and desorption of water molecules. Based on the V-[EMIM][BF 4 film humidity sensors exhibit good linear resistance responses in the ranges of 0%-60%RH, 60%-70%RH, and 70%-100%RH respectively, and maintain stability for up to 300 min at a test frequency of 10 kHz. In addition, compared with the sensors based on the V film, the sensors based on the V-[EMIM][BF 4 film have significantly shorter response and recovery times during the humidification and dehumidification processes, which are 5 s and 34 s respectively. Therefore, the sensor demonstrates applicability in human mouth breathing. The above research results indicate that the two-dimensional vermiculite film modified with ionic liquid is an ideal choice for manufacturing humidity sensors with low manufacturing cost, wide linear range, strong stability, and fast reaction speed.

[0101] The above content is a schematic description of the present invention and its implementation manners. This description is not restrictive. What is shown in the embodiments is only one of the implementation manners of the present invention, and the actual implementation manners are not limited thereto. Therefore, if those of ordinary skill in the art are inspired by it and design implementation manners and embodiments similar to the technical solution without creative efforts without departing from the purpose of the present invention, they shall fall within the protection scope of the present invention.

Claims

1. A two-dimensional layered film, characterized in that, it includes an ionic liquid-modified two-dimensional vermiculite film, and the ionic liquid includes 1-ethyl-3-methylimidazolium tetrafluoroborate ionic liquid; there is a diffraction peak at 2θ = 6.2 ± 0.2° in the XRD diffraction pattern of the ionic liquid-modified two-dimensional vermiculite film.

2. The two-dimensional layered film according to claim 1, characterized in that, the thickness of the ionic liquid-modified two-dimensional vermiculite film is not less than 7 μm.

3. A preparation method of the two-dimensional layered film according to claim 1 or 2, characterized in that, it includes: Intercalation: ionically intercalate the thermally expanded vermiculite blocks to obtain Li + Intercalated vermiculite lamellar materials; Exfoliate to separate Li + Exfoliate the intercalated vermiculite layers into single-layer nanosheets; Assembly, assembling nanosheets into a self-supporting two-dimensional vermiculite film; Modification, immersing the self-supporting two-dimensional vermiculite film in the 1-ethyl-3-methylimidazolium tetrafluoroborate ionic liquid to obtain an ionic liquid-modified two-dimensional vermiculite film.

4. The preparation method of the two-dimensional layered film according to claim 3, characterized in that, in the modification step, the immersion time is not less than 10 h, and then the residual ionic liquid is removed by centrifugation at a speed of not less than 2000 rpm.

5. A humidity sensor based on the two-dimensional layered film, characterized in that, it contains the two-dimensional layered film according to claim 1 or 2, and further includes: A first electrode, the first electrode is a platinum wire electrode, and the platinum wire electrode contacts one side surface of the ionic liquid-modified two-dimensional vermiculite film along the length direction; A second electrode, the second electrode is a platinum wire electrode, and the platinum wire electrode contacts one side surface of the ionic liquid-modified two-dimensional vermiculite film along the length direction; The first electrode and the second electrode are arranged in parallel on the same side of the ionic liquid-modified two-dimensional vermiculite film and do not contact.

6. An application of the two-dimensional layered film according to any one of claims 1 to 2 or the humidity sensor based on the two-dimensional layered film according to claim 5, characterized in that, it includes detecting the relative air humidity by measuring the impedance of the ionic liquid-modified two-dimensional vermiculite film in the range of relative air humidity of 0% - 100% RH at a test frequency of 1 - 10 kHz.

7. The application according to claim 6, characterized in that, in the range of relative air humidity of 0% - 100% RH, the impedance of the ionic liquid-modified two-dimensional vermiculite film changes linearly with the relative air humidity.

8. The application according to claim 6 or 7, characterized in that, under the test conditions, when the impedance of the ionic liquid-modified two-dimensional vermiculite film decreases, the relative air humidity increases; when the impedance of the ionic liquid-modified two-dimensional vermiculite film increases, the relative air humidity decreases.

9. The application according to any one of claims 6 to 8, characterized in that, the application includes detecting mouth breathing.

10. The application according to claim 9, characterized in that, the mouth breathing includes human mouth breathing.

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