Method for preparing three-dimensional multi-channel gas sensor for h2s gas detection

Abstract

The application relates to a preparation method of a three-dimensional multi-channel gas sensor for H2S gas detection. The Ag-doped WO3 material is prepared by a hydrothermal method. The Ag@WO3 prepared presents a three-dimensional nanoflower shape self-assembled by nanosheets, a three-dimensional hierarchical structure, the electronic and chemical sensitization of the noble metal Ag and the sulfuration reaction of Ag and H2S can promote the high sensitivity of the sensitive layer to the H2S gas. The three-dimensional multi-channel gas sensor can realize the rapid response, high sensitivity and high selectivity detection of the H2S gas under the room temperature condition, and meets the practical application in atmospheric pollution detection.

Description

Technical Field

[0001] The present invention belongs to the technical field of gas sensors, and in particular relates to a method for preparing a three-dimensional multi-channel gas sensor for H2S gas detection. Background Art

[0002] Hydrogen sulfide is a flammable and explosive acidic gas with a rotten egg smell. A large amount of H2S is produced in daily human activities, such as the production of domestic waste, automobile exhaust emissions, coal coking and power plants. [1] When the H2S gas in the air exceeds a certain value, it will cause certain harm to the human body, including respiratory infections, pulmonary edema, headaches, low blood pressure, breathing difficulties, and even death. [2,3] For safety reasons, the American Conference of Governmental Industrial Hygienists has set the threshold for H2S gas at 10 ppm, and the exposure time for humans should not exceed 8 hours. [4] The acceptable environmental limit for H2S (recommended by the U.S. Scientific Advisory Committee on Toxic Air Pollutants) is between 20-100 ppb. [5] In addition, H2S can also be used as a biomarker for the diagnosis of medical diseases such as bad breath, asthma and chronic obstructive pulmonary disease. [6] Therefore, the development of high-performance H2S gas sensors is very necessary to ensure human life safety, protect the living environment and promote the development of smart medicine.

[0003] A gas sensor is a device that converts chemical signals into electrical signals. The key part is the selection of sensitive layer materials. [7] Metal oxide semiconductor (MOS) gas sensors have become the mainstream research material due to their advantages such as easy manufacturing, low cost and high sensitivity. The materials currently used for H2S gas detection mainly include WO3 [8] 、SnO2 [9,10] 、TiO

[11] 、ZnO

[12] 、Co3O4

[13] 、In2O3

[14] wait.

[0004] Among the many MOSs, WO3, as a typical n-type semiconductor, has a band gap of 2.4-2.8eV. Its structure is similar to that of perovskite, with abundant oxygen vacancies and active adsorption sites, which indicates that it has great application prospects in H2S gas detection.

[15] For example, Li et al.

[16] A simple topological transformation strategy was used to synthesize porous two-dimensional WO3 nanosheets with a response value of 35.55 to 2000 ppb H2S at 160 °C. Zhu et al.

[17] The prepared WO3 film has excellent gas sensitivity, repeatability and stability to H2S at 250℃, with a response value of 40.1 to 10ppm H2S.

[0005] Although pure WO3 has good gas sensing performance for H2S, it still has disadvantages such as high operating temperature, poor response value and selectivity. Therefore, it needs to be modified to improve the gas sensing performance of the sensor to meet the needs of practical applications.

[18] .

[0006] It is reported that various precious metal modified WO3 materials have been applied to H2S gas sensors, mainly focusing on Pd [19,20] , Pt [21-27] 、Au[ 28-30] , a few papers reported that Ag modified WO3 [31-33] Ahmad et al.

[31] The synthesized silver-doped tungsten oxide nanoparticle (Ag / WO3) material can detect H2S gas as low as 0.25 ppm at room temperature. Gui et al.

[32] Ag / WO3 / rGO bulk sensor was prepared by microwave-assisted gas-liquid interface method, and the response value to 100ppm H2S at 150℃ was 204.5, which is about 7 times that of WO3 / rGO. Jang et al.

[33] Hollow bimetallic Pt-Ag nanoparticles (NPs) with a size below 10 nm were prepared and sensitized on multilayer SnO-WO3 heterojunction nanofibers (HNFs). The bimetallic catalysts enabled excellent H2S sensitivity for halitosis breath detection.

[0007] It can be seen that the strategy of noble metal modified WO3 gas sensor can fully improve the gas-sensing performance of H2S detection. However, the existing Ag modified WO3 sensing materials usually have high working temperature for detecting H2S gas, or have low sensitivity, narrow detection range and poor selectivity at room temperature.

[0008] To this end, it is necessary to establish a simple, efficient, accurate and reliable sensing preparation method, especially to achieve the detection of H2S gas as low as ppb level at room temperature, and the sensor has high response value, wide detection range, low detection limit, high selectivity and good stability.

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[33] JSJang, SJChoi, WTKoo, SJKim, JYCheong, IDKim, ElaborateManipulation for Sub-10nm Hollow Catalyst Sensitized Heterogeneous OxideNanofibers for Room Temperature Chemical Sensors, ACS Applied Materials & Interfaces, 9 (2017) 24821-24829. Summary of the Invention

[0043] The purpose of the present invention is to overcome the shortcomings of the existing technology and provide a method for preparing a three-dimensional multi-channel gas sensor for H2S gas detection. A simple hydrothermal method is used to synthesize a three-dimensional nanoflower-shaped Ag@WO3 material with multiple gas dispersion channels. The three-dimensional hierarchical structure, the electronic and chemical sensitization effects of the precious metal Ag, and the sulfurization reaction between Ag and H2S will significantly improve the gas-sensitive response performance of the Ag@WO3 sensitive layer material to H2S gas.

[0044] The present invention solves the technical problem by the following technical solutions:

[0045] A method for preparing a three-dimensional multi-channel gas sensor for H2S gas detection, the method comprising the following steps:

[0046] S1. Pour a certain amount of ethanol solution into a 100 mL beaker, add a certain amount of WCl6 into the beaker, and stir evenly with a magnetic stirrer to obtain a mixed solution A;

[0047] S2. Add silver nitrate AgNO3 to the mixed solution A and stir evenly with a magnetic stirrer to obtain a mixed solution B;

[0048] S3, add diammonium hydrogen citrate C6H 14 O7N2, and continuously stirred with a magnetic stirrer to fully mix to obtain a precursor solution;

[0049] S4, transferring the precursor solution into a 50 ml polytetrafluoroethylene reaction container and placing it in an oven for hydrothermal reaction;

[0050] S5. After the reaction is completed and cooled naturally, the precipitate is washed by centrifugation with deionized water and anhydrous ethanol to obtain a precipitate C;

[0051] S6. Dry the precipitate C in a constant temperature vacuum drying oven for 6 to 10 hours, and then place it in a muffle furnace for calcination to obtain light yellow Ag-WO3 nanoflower powder.

[0052] Moreover, the magnetic stirring time of S1, S2 and S3 is 25 to 35 minutes.

[0053] Moreover, the oven temperature of S4 is 90-110° C., and the hydrothermal reaction time is 23-25 ​​h.

[0054] Moreover, the centrifugal speed of the S5 is 2000-4000 r / min, the deionized water is centrifuged and washed 2-4 times, and the anhydrous ethanol is centrifuged and washed 2-4 times.

[0055] Moreover, the temperature of the constant temperature vacuum drying oven of S6 is 50-70°C, and the vacuum degree is maintained at 700-800Pa.

[0056] Moreover, the muffle furnace calcination temperature rise rate of S6 is 2°C / min, and the calcination time at 440-460°C is 110-130 min.

[0057] The positive effects that the present invention can produce are:

[0058] 1. The present invention utilizes a solvent thermal method to synthesize a three-dimensional nano-flower-shaped Ag-doped WO3 sensitive layer with multiple gas dispersion channels without adding any template agent, in line with the "green chemistry development concept", reducing the preparation cost and enabling large-scale production.

[0059] 2. This invention combines Ag-doped WO3 nanomaterial preparation technology with modern sensing technology to overcome the shortcomings of existing technologies. In terms of detecting H2S gas, Ag doping significantly improves the reliability of the sensor, including high sensitivity and high selectivity.

[0060] 3. The Ag@WO3 sensor of the present invention has an ultra-high response value (Ra / Rg=11420) to 10ppm H2S gas at room temperature of 25°C, which is about 941 times that of pure WO3. It can continuously detect 1-50-1ppm H2S gas with a detection limit as low as 10ppb and a response time of only 7s. It can cope with complex test environments and has strong engineering practicality. BRIEF DESCRIPTION OF THE DRAWINGS

[0061] Figure 1 a, b are XRD patterns and Fourier transform infrared spectra (FT-IR) of WO3, Ag@WO3-5, Ag@WO3-7, Ag@WO3-9 and Ag@WO3-11 sensors; Figure 1 c, d are Raman spectra of WO3, Ag@WO3-5, Ag@WO3-7, Ag@WO3-9 and Ag@WO3-11 sensors; Figure 1e)~g) are UPS spectra and high-resolution UPS spectra of WO3 and Ag@WO3-7 sensors;

[0062] Figure 2 a is the ultraviolet diffuse reflectance spectrum (UV-Vis) of WO3 and Ag@WO3-7 sensors; Figure 2 b is the Tauc plot of WO3 and Ag@WO3-7 sensors; Figure 2 c is the full XPS spectrum of WO3 and Ag@WO3 sensors, Figure 2 d~f are the fine spectra of W 4f, Ag 3d, and O1s, respectively;

[0063] Figure 3 a~e are SEM images of WO3, Ag@WO3-5, Ag@WO3-7, Ag@WO3-9, and Ag@WO3-11, respectively; Figure 3 f~h are EDS element mappings of W, O, and Ag elements of Ag@WO3-7;

[0064] Figure 4 a, b are TEM and HRTEM images of WO3; Figure 4 c, d are TEM and HRTEM images of Ag@WO3-7;

[0065] Figure 5 a is the response diagram of WO3 to 10ppm H2S gas at different working temperatures; Figure 5 b is the response diagram of Ag@WO3-7-11 to 10ppm H2S gas at different working temperatures; Figure 5 c is the resistance curve of the sensor in air at different working temperatures; Figure 5 d is the response recovery curve; Figure 5 e is the continuous response curve graph of 5 trials; Figure 5 f is the resistance curve of WO3 sensor in air at different working temperatures;

[0066] Figure 6 a~e are the response / recovery curves of sensors based on WO3, Ag@WO3-5, Ag@WO3-7, Ag@WO3-9, and Ag@WO3-11 to 10 ppm H2S gas at working temperatures (40 and 25 °C), respectively; Figure 6 f is the response time and response value histogram of all sensors;

[0067] Figure 7a~e are the continuous dynamic sensing characteristics of sensors Ag@WO3-5, Ag@WO3-7, Ag@WO3-, Ag@WO3-11, and Ag@WO3 to 1-50-1ppm H2S gas at 25°C, the response curves of the lower detection limit to H2S gas concentration, and the linear fitting data curve of the Ag@WO3-7 sensor; Figure 7 f is the statistical response diagram of Ag@WO3-7 at 25°C when the H2S concentration is 0.01-1ppm; (3 consecutive test curves when the H2S concentration is 10ppb);

[0068] Figure 8 a, b are the continuous cycle detection curve and response value curve of WO3 sensor to 1-30-1ppm H2S at 40℃, respectively;

[0069] Figure 9 a is the selective response diagram of WO3 (40℃) and Ag@WO3-7 (25℃) sensors to different target gases; Figure 9 b is the response diagram of Ag@WO3-7 sensor to 10 ppm H2S at different relative humidity; Figure 9 c is a bar graph of humidity response; Figure 9 d is the graph of Ag@WO3-7 sensor tested with 10ppm H2S for 10 consecutive cycles at 25℃; Figure 9 d is the long-term stability curve of the Ag@WO3-7 sensor to 10 ppm H2S at 25°C;

[0070] Figure 10 a~c are the responses of Ag@WO3-5, Ag@WO3-9, and Ag@WO3-11 sensors to 10 ppm H2S at room temperature and different relative humidity, respectively; Figure 10 d is the response value curve at different humidity levels;

[0071] Figure 11 Schematic diagram of H2S gas sensing mechanism on Ag@WO3 sensor at room temperature;

[0072] Figure 12 a is the fine spectrum of O1s of Ag@WO3-7 sensor in air; Figure 12 b is the fine spectrum of O1s of Ag@WO3-7 sensor after sulfurization. DETAILED DESCRIPTION

[0073] The present invention will be further described in detail below through specific examples. The following examples are only illustrative and not restrictive, and the scope of protection of the present invention cannot be limited thereto.

[0074] Example 1

[0075] The present invention relates to a method for preparing a three-dimensional multi-channel gas sensor for H2S gas detection. The innovation of the method lies in the following steps:

[0076] (1) Pour 20-40 mL of ethanol into a 100 mL beaker. Then add 1.30-1.34 g of WCl6 and stir with a magnetic stirrer.

[0077] (2) Then add 0-0.011 g of silver nitrate (AgNO3) and stir with a magnetic stirrer;

[0078] (3) Then, add 0.5-0.6g of diammonium hydrogen citrate (C6H 14 O7N2), and continuously stirred with a magnetic stirrer to fully mix to obtain a precursor solution;

[0079] (4) The precursor solution was transferred to a 50 mL polytetrafluoroethylene reaction vessel and placed in an oven for hydrothermal reaction;

[0080] (5) After cooling, the resulting precipitate A was centrifuged and washed with deionized water and anhydrous ethanol;

[0081] (6) Dry in a constant temperature vacuum drying oven for 6-10 hours,

[0082] (7) The product was then calcined in a muffle furnace to obtain pale yellow Ag-WO3 nanoflower powder.

[0083] Example 2

[0084] The present invention relates to a method for preparing a three-dimensional multi-channel gas sensor for H2S gas detection. The innovation of the method lies in the following steps:

[0085] (1) Pour 30 mL of ethanol into a 100 mL beaker. Then add 1.32 g of WCl6 and stir with a magnetic stirrer for 30 minutes to allow it to fully dissolve in the solution.

[0086] (2) Then, 0.007 g of silver nitrate (AgNO3) was added and stirred with a magnetic stirrer for 30 min.

[0087] (3) Then, 0.5655 g of diammonium hydrogen citrate (C6H 14 O7N2), and stirred continuously for 30 min with a magnetic stirrer to fully mix the mixture to obtain a precursor solution.

[0088] (4) The precursor solution was transferred to a 50 mL polytetrafluoroethylene reaction vessel and subjected to hydrothermal reaction at 100 °C for 24 h.

[0089] (5) After the reaction is completed, the mixture is naturally cooled to room temperature, the supernatant is removed, and the mixture is first centrifuged and washed three times with deionized water and then with anhydrous ethanol to obtain precipitate A.

[0090] (6) The sample was then placed in a constant temperature vacuum drying oven at 70°C and dried for 24 h.

[0091] (7) The WO3 nanoparticles were then calcined in a muffle furnace at 450 °C for 120 min at a heating rate of 2 °C / min to obtain light yellow WO3 nanoparticle powder.

[0092] like Figure 1 The physical structure of the samples was characterized by X-ray diffraction analysis (XRD). Figure 1 As shown in (a). It can be seen from the figure that all the diffraction peaks of WO3 and Ag@WO3 materials can correspond to the monoclinic WO3 phase (JCPDS No.72-0677) without other stray peaks, indicating that the prepared samples are of high purity. In addition, compared with the diffraction peaks of pure phase WO3, Ag@WO3 materials have almost similar diffraction peaks, indicating that the addition of Ag does not change the crystal structure of WO3. In addition, this diffraction peak with similar characteristics is mainly due to the fact that Ag in the composite material is trace. However, a closer look reveals that there is an obvious bulge in the diffraction peak near 35.8°. This is because the diffraction peak at 35.891° corresponds to the (004) crystal plane of the hexagonal structure Ag (JCPDS No.41-1402), proving that Ag exists in the composite material. The functional groups of pure WO3 and Ag@WO3 composite materials were analyzed using Fourier transform infrared spectroscopy (FT-IR), as shown in Figure 2. Figure 1 (b) shows that the vibration peak near 734 cm-1 can be attributed to the WO bond, confirming the presence of WO3. The vibration peaks near 1652.53 and 3742.53 cm-1 belong to the HOH bending mode and OH stretching mode, respectively, which may be due to the re-adsorption of water from the surrounding atmosphere. No Ag vibration peaks are observed in the FTIR spectrum, which may be due to the low amount of Ag added. Figure 1(c) The Raman spectra of WO3 and WO3 and Ag@WO3 are shown. The spectral peaks at 200-500 cm-1 and 700-850 cm-1 are attributed to the bending vibration of δ(O-W-O) and the stretching vibration of υ(W-O-W) of WO3, respectively, indicating the presence of WO3 in the samples, which is consistent with the previous FTIR results. All the Raman spectra of the samples show vibration bands near 263, 317, 712 and 805 cm-1, proving that the sensitive material is monoclinic phase, which is consistent with the XRD results. In addition, the 712 cm-1 mode in the monoclinic WO3 crystal is highly sensitive to the changes in crystal symmetry and lattice distortion. Compared with pure WO3, the peak intensity and width of this band increase, indicating that the oxygen vacancy defects caused by lattice distortion have changed. In the Raman spectra of WO3 and Ag@WO3, the most intense Raman band observed near 805 cm-1 can be attributed to the symmetric stretching vibration of the W-O bond. In addition, by Figure 1 (d) In the Raman spectra of Ag@WO3 materials, an obvious bulge appears near 910 cm-1, which is caused by the doped Ag, again confirming that Ag is successfully doped into WO3. Figure 1 (e-g) The results of ultraviolet photoelectron spectroscopy (UPS) tests of pure WO3 and Ag@WO3-7 nanoflowers are shown. According to the calculation formula of formula 1, and the intercepts of the two tangent lines E cut-off-2 and E cut-off-1 on the x-axis, the work functions (W f ) of the two samples can be obtained. By measuring, it can be known that E cut-off-2 and E cut-off-1 of WO3 and Ag@WO3-7 are 17.295 eV, 17.685 eV and 1.004 eV, 1.112 eV, respectively. Therefore, the work functions of WO3 and Ag@WO3-7 are calculated to be 4.929 eV and 4.647 eV, respectively. It is found that the work function of Ag-doped WO3 is smaller than that of WO3, and the smaller the work function, the easier it is to lose electrons. In addition, due to the strong oxidizing property and electron affinity of oxygen molecules, oxygen molecules will steal electrons from the semiconductor material, so that Ag@WO3-7 loses more electrons, forming a thicker electron depletion layer, increasing the initial resistance value Ra (consistent with the results in Figure 5 c) of the sensor), significantly improving the gas sensing performance of the sensing material.

[0093] W f = hυ - (E cut-off-2 - E f ) = 21.22 - (E cut-off-2 - E cut-off-1 ) (1)

[0094] Figure 2 In this study, the band gap of semiconductor materials was analyzed using 310-950 nm UV diffuse reflectance spectroscopy. Figure 2 (a) is the ultraviolet diffuse reflectance spectrum (UV-Vis) of WO3 and Ag@WO3-7. Figure 2 (b) shows the Tauc plots of WO3 and Ag@WO3-7 nanoflowers. The indirect band gap of the materials is calculated by the extrapolation of formula (2). Figure 2 (b) Middle (αhv) 1 / 2 Calculated with the linear part of hv:

[0095] (αhv) 1 / 2 =A(hυ-Eg) (2)

[0096] Where α is the absorption coefficient, h is Planck's constant, υ is the frequency, A is a constant, and Eg is the semiconductor band gap. The calculated band gap energies for WO3 and Ag@WO3-7 samples are 2.55 eV and 2.34 eV, respectively. Ag doping reduces the WO3 band gap, making it more susceptible to visible light activation, promoting electron migration and generating more oxygen vacancy defects, resulting in superior gas-sensing performance.

[0097] X-ray photoelectron spectroscopy (XPS) was used to characterize the elemental composition and chemical valence state of the samples, such as Figure 2 (cf). Figure 2 (c) The full XPS spectrum shows that the WO3 sample has three main peaks: W 4f, C 1s, and O 1s, with corresponding binding energies at 36.05, 284.02, and 530.09 eV, respectively. The Ag@WO3 sample, on the other hand, exhibits four main peaks: W 4f, C 1s, O 1s, and Ag 3d, confirming the successful doping of Ag into the WO3 material. C was used as a reference in the XPS experiment. Figure 2 (d) is the fine spectrum of W 4f. Two strong peaks are found at the binding energies of 7.10eV and 34.98eV in the pure WO3 sample, corresponding to W 4f 5 / 2 and W4f 7 / 2 W, indicating W +6 Compared with pure WO3, the W 4f 5 / 2 and W4f 7 / 2 The binding energy peaks of Ag@WO3 material have shifted significantly to the positive direction, indicating that Ag doping has changed the chemical environment of WO3. Figure 2 (e) Ag 3d 3 / 2 and 3D 5 / 2They are mainly concentrated near the binding energies of 374.08eV and 368.08eV, with a fixed difference of 6eV between them, proving the existence of Ag in WO3 nanoflowers. Figure 2 (f) is the fine spectrum of O1s. Three peaks are fitted at the binding energies of 529.75eV, 531.09eV and 532.97eV for WO3, representing lattice oxygen (O L ), oxygen vacancies (O V ) and chemically adsorbed oxygen (O C Compared with pure WO3, the O1s peak of Ag@WO3 shifted negatively, and the oxygen vacancy (O V ) increases. In addition, by calculating their peak areas, it can be seen that the lattice oxygen (O L ) and oxygen vacancies (O V ) accounts for as much as 95.57%, which is beneficial to the improvement of gas sensing performance. This is because, on the one hand, the increase of oxygen vacancies is conducive to increasing active sites. On the other hand, the lattice distortion caused by the introduction of Ag helps to optimize the electronic structure. The surface lattice oxygen (O L ) is more easily released and actively participates in the gas-sensing reaction, thus contributing to the high response value of the material. Therefore, compared with other samples, the Ag@WO3-7 gas sensor has the best gas-sensing performance, which is consistent with the gas-sensing test results later.

[0098] Figure 3 In order to explore the microstructure of the samples, we performed scanning electron microscopy (SEM) characterization on WO3 and Ag@WO3. Figure 3 shown. Figure 3 (a) is the SEM image of WO3 nanoflowers. Figure 3 (b) shows SEM images of WO3 nanoflowers with varying Ag doping levels. It can be clearly observed that all samples exhibit a nanoflower structure formed by self-assembly of nanosheets. Measurements show that the diameter of the WO3 and Ag@WO3 spheres is approximately 2.3 μm. With increasing Ag doping levels, the size of the composite nanosheets decreases, and the entire nanoflower surface becomes increasingly fragmented, likely due to Ag doping within the WO3 lattice. Figure 3 (fi) is the element composition image, in which W, O, and Ag elements are evenly and densely distributed in the Ag@WO3 composite material.

[0099] Figure 4 The internal state and morphology of the samples were studied using transmission electron microscopy (TEM). The morphology of nanoflowers and nanoflower clusters can be seen from Figure 4 As observed in (a, c), the interior of the nanoflower is solid as can be seen from the comparison of bright field and dark field images. Figure 4(b) is the HRTEM image of pure phase WO3, with lattice spacing of 0.377nm and 0.385nm corresponding to the (020) and (002) crystal planes of monoclinic WO3, respectively. Figure 4 (d) is the HRTEM image of pure Ag@WO3. Compared with pure WO3, the lattice spacing is slightly increased. This is because Ag is added to the WO3 crystal and replaces a small part of the W site, resulting in WO3 lattice distortion, which is conducive to the increase of oxygen vacancies and active adsorption sites.

[0100] Figure 5 In this study, a series of Ag-doped WO3 materials were assembled into gas sensors for the sensing characteristics of H2S gas at RT (25°C). First of all, the operating temperature will seriously affect the gas sensitivity of the gas sensor, mainly because the working principle of the metal oxide semiconductor gas sensor is surface controlled. In order to test the operating temperature of WO3 and Ag@WO3 sensors, they were placed at 20-60 and 25-65°C respectively and exposed to 10ppm H2S gas atmosphere (such as Figure 5 (a and b)). It can be clearly seen that the response values ​​of all sensors show a "volcano-shaped" trend with increasing temperature. This is because the increase in operating temperature allows H2S molecules to obtain more energy to overcome the activation energy barrier and react with oxygen. However, if the temperature is too high, the gas molecules will move faster, and the accelerated reaction will also be accompanied by the desorption of gas molecules, so the response value will decrease. Among them, the best operating temperature of WO3 is 40°C, and the response value is Ra / Rg=18.50. The Ag@WO3-5-11 sensor has the highest induction response value of H2S molecules at 35°C, and the response values ​​are 3468.10, 31863.23, 9624.17, and 23943.20 respectively. Among them, the response value of Ag@WO3-7 is the highest, which is about 1723 times that of pure WO3. Figure 5 As can be seen from the image (de), the Ag@WO3-7 sensor's response time is only 5 seconds at 35°C. However, after five cycles of testing with 10ppm H2S gas, the sensor's response is not particularly stable. Furthermore, compared to 35°C, operating at room temperature (25°C) results in lower power consumption, thus avoiding resource waste. It is particularly noteworthy that even at room temperature (25°C), the Ag@WO3-7 sensor's response to 10ppm H2S gas remains as high as 11420, approximately 941 times that of a WO3 sensor (Ra / Rg = 12.14, RT) at room temperature. Therefore, all subsequent tests were conducted at room temperature. Figure 5c is the curve of the sensor's resistance changing with the operating temperature. It can be clearly seen that the initial resistance value (Ra) of all sensors decreases with increasing temperature, showing the typical characteristics of semiconductor materials. This can be attributed to the fact that higher temperature increases the energy of semiconductor material electrons, making it easier for electrons to cross from the conduction band to the valence band, and the carrier concentration increases sharply, thus reducing the resistance. More importantly, compared with pure WO3 ( Figure 5 (f)), the initial resistance value of the Ag@WO3 sensor is significantly higher, which is beneficial to increase the response amplitude space of the sensor and show better gas sensing performance.

[0101] In addition, the response / recovery capabilities of WO3 and Ag@WO3 sensors to detect H2S gas were also investigated. Figure 6 As shown. The response time of the WO3 sensor to 10ppm H2S gas at 40℃ is 30s. Compared with WO3, the response time of Ag@WO3-5, Ag@WO3-7, Ag@WO3-9 and Ag@WO3-11 sensors to 10ppm H2S at room temperature is significantly shortened, which are 8, 7, 7 and 10s respectively. More noteworthy is that with the increase of Ag doping amount, the gas sensitive response value increases, but when the doping amount of AgNO3 reaches 0.009g, the gas sensitive response decreases, indicating that the optimal AgNO3 doping amount of WO3 is 0.007g. The enhancement of the gas sensitive response of Ag@WO3 can be attributed to the electronic sensitization and chemical sensitization of Ag precious metal, which promotes the adsorption and dissociation process of O2 and H2S on the surface of the material and shortens the response time of the sensor. After the reaction is complete, the target gas is transferred to air. Because Ag@WO3's H2S desorption capacity is weak at low operating temperatures and its natural recovery time is long, a short pulse current is applied externally to the sensor to accelerate the separation of H2S gas molecules from the Ag@WO3 surface due to the Joule heating effect. To begin the desorption process, the WO3 and Ag@WO3 sensors are heated at 80°C and 200°C, respectively, for 60 seconds before the operating temperature is lowered to its initial state. In subsequent tests, we used this method to achieve full H2S recovery at low temperatures.

[0102] Continuous response dynamic cycle test is an important indicator to examine the gas sensing performance of the sensor. Figure 7 Figures ad are the resistance change test curves of Ag@WO3-5, Ag@WO3-7, Ag@WO3-9, and Ag@WO3-11 in response to continuous changes in H2S gas concentration from 1 to 50 to 1 ppm. It can be clearly seen that all sensors respond quickly and significantly once exposed to H2S gas, demonstrating typical n-type semiconductor behavior. Furthermore, the sensor's response amplitude increases with increasing H2S gas concentration. Compared to pure WO3 ( Figure 8a), Ag@WO3 sensor has higher response value, faster response capability, and wider detection range. More notably, all sensors have almost the same response value corresponding to different concentrations of H2S gas in continuous response dynamic cycle test, indicating excellent short-term reproducibility. In addition, Figure 7 e further depicts the response values ​​of the Ag@WO3 sensors to 1-50ppm H2S gas at room temperature. It can be seen that Ag@WO3-5, Ag@WO3-7, Ag@WO3-9 and Ag@WO3-11 sensors have obvious saturation sensitivity to high concentrations of H2S gas. More importantly, the response value of the Ag@WO3 sensor is significantly higher than that of WO3 ( Figure 8 b), among which, the response value of the Ag@WO3-7 sensor is the largest and shows a certain linear relationship with the H2S gas concentration. Specifically, in the range of 1-10ppm H2S concentration, the linear fitting curve is y=897.17+1029.06x(R 2 =0.9931), in the range of 10-50ppm H2S concentration, the linear fitting curve is y=9581.58+152.36x(R 2 =0.9869). Figure 7 f is the detection limit test diagram of the Ag@WO3-7 sensor. The obvious response signal to low concentration H2S (10 ppb, Ra / Rg = 1.5) indicates that the sensor can detect trace H2S concentrations at the ppb level and has good stability (as shown in the inset: the test curve of the original data of 10 ppb H2S detected three times in a row).

[0103] Figure 9 In the field of gas sensors, the gas selectivity is another key aspect in evaluating their practical applications. Figure 9 As shown in a, the responses of WO3 and Ag@WO3-7 sensors to different gases, methanol, ethanol, ethylene glycol, NO2, CO, SO2 and H2S, were tested, where the concentration of organic gas was 100ppm and the concentration of inorganic gas was 10ppm. The selection of these control gas groups took into account the main harmful gases present in industrial environments or the atmosphere, which may interfere with the detection of H2S. It can be clearly seen from the figure that the responses of WO3 and Ag@WO3-7 sensors to H2S gas are significantly high, and the response value of Ag@WO3-7 sensor is the highest, confirming its good selectivity. In addition, in order to discuss the effect of ambient humidity on the performance of gas sensors, we compared the response recovery curves of Ag@WO3-7 sensor to 10ppm H2S gas at ambient relative humidity of 20-68% ( Figure 9bc). As the relative humidity of the environment increases, the sensor's response amplitude and initial resistance value both decrease. This is because water molecules in the air compete with H2S gas molecules for adsorption on the active sites on the material surface, thereby reducing the sensor's response value. In addition, water molecules in the air also react with oxygen ions adsorbed on the material surface, releasing electrons back into the material's bulk, thereby reducing the sensor's initial resistance value. At the same time, we also tested the impact of changes in relative humidity on the sensor response value of Ag@WO3-5, Ag@WO3-9, and Ag@WO3-11 sensors. Figure 10 As shown in Figures ac, all sensors show the same Ra and sensitivity decrease. Under different ambient humidity, the response value of Ag@WO3-7 sensor is still the best ( Figure 10 d). Figure 9 Figure d shows the repeatability of the Ag@WO3-7 sensor under 10 ppm H2S gas at 25°C, tested ten times in a row. The figure demonstrates good repeatability and minimal fluctuation. During the 30-day long-term stability test of the Ag@WO3-7 sensor, the baseline response remained stable with a slight downward trend, demonstrating its excellent long-term stability and making it a viable candidate for long-term H2S detection. This slight decrease in response may be due to the incomplete desorption of H2S gas from the material after exposure, resulting in the partial formation of WS2, Ag2S, and other substances.

[0104] Figure 11 In most MOS sensors, the gas sensing mechanism is usually explained by the adsorption-desorption model. Therefore, as a typical n-type MOS, the surface morphology of WO3 is closely related to its gas sensing performance. From the previous SEM and TEM results, we can see that the Ag@WO3 composite material is a nanoflower with good gas permeability. This special structure can provide more active sites and gas diffusion channels for gas adsorption. In order to better explain the sensing process, Figure 11 The electron depletion layer, electron transport and band structure changes of the sensor in air and H2S environments are shown.

[0105] When the material is placed in the air, due to the strong oxidizing and electron affinity of oxygen, oxygen molecules will capture electrons in the material to form chemically adsorbed oxygen ions (O2 - , O - and O 2- )(Reaction 4-6). In addition, Li et al. also proposed that the surface chemically adsorbed oxygen in the gas sensing process may come from the lattice oxygen of the oxide itself (O L), the previous XPS O1s analysis results and gas-sensing performance tests are consistent with this view. When Ag and WO3 are in contact with each other, because they have different Fermi levels and work functions, electrons will transfer from the conduction band of Ag to the contact interface until the Fermi levels are equal. Due to the downward band bending of WO3 and the interface barrier, the depletion region becomes thicker. Therefore, this electron sensitization effect will increase the Ra value. In addition, according to the previous UPS and UV-Vis results, it can be seen that Ag doping into the WO3 crystal brings about lattice distortion, which reduces the work function and band gap of the material and increases the oxygen vacancy content, which is conducive to the formation of a thicker electron depletion layer, increases the initial resistance Ra of the material, and provides a wider space for the resistance change of the gas sensor. The test conditions of this study were carried out at room temperature, so the reaction of reaction formula 4 mainly occurred.

[0106]

[0107] O - (ads)+e - →O 2- (ads)(T>300℃) (6)

[0108] Since H2S has strong reducing properties, when the sensor is exposed to H2S gas, H2S molecules will react with the surface adsorbed oxygen ions (O2 - ) reacts to generate SO2 and H2O, releasing electrons to return to the material body, thinning the depletion layer, thereby reducing the resistance of the material and obtaining a response signal (Reaction Formula 5). Because Ag has a chemical sensitization mechanism, it can increase the response signal and speed up the response. Specifically, on the one hand, Ag as a catalyst can activate H2S gas molecules and reduce the activation energy of H2S gas participating in the gas-sensitive reaction. On the other hand, Ag can also promote the adsorption of more oxygen molecules to form chemically adsorbed oxygen ions through the "overflow effect", and then overflow to form a high concentration of oxygen ions on the surface of the WO3 material. A large number of oxygen ions can react with more H2S gas, thereby significantly improving the gas-sensitive sensing performance.

[0109]

[0110] Due to the reverse outflow of oxygen ions, a small amount of Ag may be oxidized to Ag2O (Reaction Equation 8). Due to the low Gibbs free energy of Reaction Equation 9, Ag2O reacts with H2S to form metallic Ag2S, significantly reducing the material's resistance. This is the fundamental reason why the Ag@WO3 composite material has high selectivity for H2S gas.

[0111]

[0112] Ag2O+H2S→Ag2S+H2O (9)

[0113] In addition, in order to verify the gas sensing mechanism, the changes in XPS O1s peak of Ag@WO3-7 sensor before and after exposure to H2S gas were studied, such as Figure 12 shown. Figure 12 a) is the O 1s fine spectrum of the Ag@WO3-7 sensor in air, lattice oxygen (O L ), oxygen vacancies (O V ) and chemically adsorbed oxygen (O C ) are located at 530.14, 530.50 and 532.63 eV, respectively, with peak areas of 56.05%, 39.53% and 4.43%, respectively. When exposed to H2S gas, the lattice oxygen (O L ), oxygen vacancies (O V ) and chemically adsorbed oxygen (OC) both shifted negatively ( Figure 12 b)). More notably, the 1s peak area of ​​Ag@WO3-7 O also changed after sulfurization, which were 48.34%, 38.06% and 13.60% respectively, and the lattice oxygen (O L ) and oxygen vacancies (O V ) content decreased, which once again confirmed that the lattice oxygen (O L ) are also involved in the gas sensing process.

[0114] When the sensor is exposed to air after the reaction with H2S, the desorption rate of H2S from the material surface is very slow at room temperature due to the strong bonding between Ag and S. Therefore, a DC heating power supply is required to provide an additional heating voltage. According to the Joule heating effect, heating will accelerate the oxidation of Ag2S and promote the desorption of H2S gas from the material surface (Reaction 10).

[0115] Ag2S+O2→Ag2O+H2S (10).

[0116] Although the embodiments and drawings of the present invention are disclosed for illustrative purposes, those skilled in the art will understand that various substitutions, changes and modifications are possible without departing from the spirit and scope of the present invention and the appended claims. Therefore, the scope of the present invention is not limited to the contents disclosed in the embodiments and drawings.

Claims

1. A method for preparing a three-dimensional multi-channel gas sensor for H2S gas detection, characterized by: The steps of the method are: S1. Pour a certain amount of ethanol solution into a 100 mL beaker, add 1.30-1.34 g of WCl6 into the beaker, and stir evenly with a magnetic stirrer to obtain a mixed solution A. S2. Add 0-0.011 g of silver nitrate AgNO3 to the mixed solution A and stir evenly with a magnetic stirrer to obtain a mixed solution B; S3, add diammonium hydrogen citrate C6H 14 O7N2, and continuously stirred with a magnetic stirrer to fully mix to obtain a precursor solution; S4, transferring the precursor solution into a 50 ml polytetrafluoroethylene reaction container and placing it in an oven for hydrothermal reaction; S5. After the reaction is completed and cooled naturally, the precipitate is washed by centrifugation with deionized water and anhydrous ethanol to obtain a precipitate C; S6. Dry the precipitate C in a constant temperature vacuum drying oven for 6 to 10 hours, and then place it in a muffle furnace for calcination to obtain light yellow Ag-WO3 nanoflower powder.

2. The method for preparing a three-dimensional multi-channel gas sensor for H2S gas detection according to claim 1, characterized in that: The magnetic stirring time of S1, S2 and S3 is 25 to 35 minutes.

3. The method for preparing a three-dimensional multi-channel gas sensor for H2S gas detection according to claim 1, characterized in that: The oven temperature of S4 is 90-110° C., and the hydrothermal reaction time is 23-25 ​​h.

4. The method for preparing a three-dimensional multi-channel gas sensor for H2S gas detection according to claim 1, characterized in that: The centrifugal speed of the S5 is 2000-4000 r / min, and the deionized water is centrifuged and washed 2-4 times, and the anhydrous ethanol is centrifuged and washed 2-4 times.

5. The method for preparing a three-dimensional multi-channel gas sensor for H2S gas detection according to claim 1, characterized in that: The temperature of the constant temperature vacuum drying oven of S6 is 50-70°C, and the vacuum degree is maintained at 700-800Pa.

6. The method for preparing a three-dimensional multi-channel gas sensor for H2S gas detection according to claim 1, characterized in that: The muffle furnace calcination temperature rise rate of S6 is 2°C / min, and the calcination time at 440-460°C is 110-130 minutes.

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