A metastable indium oxide gas sensitive material, a preparation method and application thereof

Abstract

The application discloses a nano-needle self-assembled flower-shaped microsphere metastable indium oxide gas-sensitive material, a gas-sensitive element and a preparation method and application. The nano-needle self-assembled flower-shaped microsphere metastable indium oxide gas-sensitive material has a three-dimensional flower-shaped microsphere formed by nano-needle self-assembly, the diameter of the nano-needle is about 8 nm, the diameter of the flower-shaped microsphere is about 2-3 microns, the main exposed crystal face of the material is (104), and the crystal structure is a metastable rhombohedral corundum phase. The prepared nano-needle self-assembled flower-shaped microsphere metastable indium oxide successfully realizes preparation of pure metastable indium oxide material under mild conditions and realizes application in the field of gas-sensitive detection, especially in the detection of H2S, and has the advantages of low working temperature, high sensitivity, high selectivity and low detection limit. Meanwhile, the synthesis preparation method is simple in synthesis, low in cost and has good industrial production prospects.

Description

Technical Field

[0001] This invention belongs to the field of gas-sensitive materials technology, specifically relating to a metastable indium oxide gas-sensitive material, its preparation method, and its application. Background Technology

[0002] The statements herein provide only background information in relation to this invention and do not necessarily constitute prior art.

[0003] Hydrogen sulfide (H2S) is a colorless, pungent, and asphyxiating gas with a putrid, egg-like odor. Large quantities of H2S are emitted during modern industrial production. H2S is readily soluble in water on mucous membranes, combining with sodium ions to form sodium sulfide, which has a strong irritant effect on mucous membranes, causing conjunctivitis, respiratory inflammation, and even pulmonary edema. After being absorbed by the human body, H2S gas rapidly dissolves into the bloodstream. The portion that is not excreted or oxidized enters tissue cells, interacting with cytochrome oxidases and their disulfide bonds, or binding with ferric iron, inhibiting cellular oxidation processes, causing tissue hypoxia, and leading to systemic poisoning. According to the current H2S emission standard (TJ36-79), the maximum permissible concentration of hazardous substances in industrial production environments is 10 mg / m³. 3 (6.57 ppm). Therefore, designing a gas sensor with high sensitivity and selectivity for H2S gas is of great significance for realizing the detection of industrial H2S emissions and ensuring human health and safety.

[0004] Resistive semiconductor oxide gas sensors are widely used due to their low cost and simple detection methods. Their main detection principle utilizes the redox properties of the analyte gas. After the gas comes into contact with and is adsorbed onto the semiconductor material, an interfacial chemical reaction occurs, affecting the material's electrical conductivity. By recording the changes in the material's electrical signal before and after contact with the analyte gas, the type and concentration of the analyte gas are determined. Indium oxide (InO), as an n-type semiconductor, has relatively wide applications. However, currently used InO mainly exists in a stable cubic crystal phase. Due to the stability of its crystal structure, it also exhibits drawbacks such as poor selectivity and low detection sensitivity. Synthesizing metastable InO with a homogeneous phase under mild conditions is quite difficult. Summary of the Invention

[0005] To address the shortcomings of existing technologies, the purpose of this invention is to provide a metastable indium oxide gas-sensitive material, its preparation method, and its applications.

[0006] To achieve the above objectives, the present invention is implemented through the following technical solution:

[0007] In a first aspect, the present invention provides a method for preparing a metastable indium oxide gas-sensitive material, comprising the following steps:

[0008] Indium salt, alkali, and reducing agent are dissolved and mixed thoroughly. Under an alkaline atmosphere, the indium salt reacts with OH- through stirring. - The reaction preferentially forms In(OH)3 and dissolves in the reaction solvent, followed by a solvothermal reaction. During the solvothermal process, it recrystallizes to form a self-assembled flower-like microsphere structure. The reducing agent promotes the dehydration of In(OH)3 during the solvothermal process to form the InOOH precursor.

[0009] The obtained InOOH precursor was annealed at 300-850℃ for 1-6h. The synthesis of indium oxide with stable metastable crystal structure and micro-nano morphology was controlled by controlling the annealing rate. After annealing, the product was obtained.

[0010] The alkali is selected from one or a combination of urea, ethylenediamine, or hydrazine hydrate.

[0011] The reducing agent is oxalic acid and / or citric acid;

[0012] The molar ratio of indium salt, alkali and reducing agent is 1:1-5:0.5-2.

[0013] Current research on indium oxide-based gas-sensitive materials still faces challenges such as poor sensitivity, poor selectivity, and high operating temperatures. Furthermore, current research on indium oxide mainly focuses on stable cubic phase indium oxide, while the controllable synthesis of metastable rhombic corundum phase indium oxide nanomaterials and their application in the field of gas sensors remain lacking. Therefore, this paper proposes a method for preparing and applying metastable indium oxide materials with self-assembled nanoneedle-like flower-like microspheres.

[0014] By controlling the content and type of reducing agent and the solvothermal conditions in the solvothermal synthesis reaction, a precursor of indium hydroxyl oxide with uniform morphology and crystal structure was first obtained. Then, by controlling the annealing parameters, an indium oxide gas-sensitive material with a unique microstructure and metastable crystal structure was obtained. For gas-sensitive materials, their micro / nanostructure and crystal structure play a decisive role in gas-sensing performance. The metastable indium oxide gas-sensitive material with self-assembled nanoneedle-like microspheres obtained by the present invention through solvothermal reaction and subsequent annealing treatment has a large specific surface area and abundant pore structure, which is beneficial to the adsorption of target gases. In addition, the metastable indium oxide material has higher material activity, and the preferred growth of high-energy crystal faces provides adsorption and reaction sites for gas adsorption and gas-sensing reactions, promoting the improvement of sensitivity.

[0015] Metastable indium oxide precursors were obtained by controlling solvothermal conditions. By controlling annealing conditions, the morphology of the precursors was not damaged during the annealing transformation process, and the unstable metastable phase structure was not stable and prone to phase transformation under excessively high annealing rates, thus obtaining uniform metastable indium oxide materials.

[0016] In some embodiments, the indium salt is InCl3·4H2O or / and In(NO3)3·xH2O.

[0017] In some embodiments, the solvent used in the solvothermal reaction is selected from one or a mixture of two of deionized water or anhydrous ethanol.

[0018] In some embodiments, the molar ratio of indium salt, alkali and reducing agent is 1:1-3:0.5-2.

[0019] In some embodiments, the stirring time for dissolving and mixing the indium salt, alkali, and reducing agent is 10-60 min, and the stirring temperature is 20-50°C.

[0020] In some embodiments, the temperature of the solvothermal reaction is 100-200°C, and the reaction time is 8-24 hours.

[0021] Preferably, the temperature of the solvothermal reaction is 120-180℃, and the reaction time is 12-24h.

[0022] In some embodiments, after the solvothermal reaction is completed, the product is further subjected to centrifugation, washing, and drying.

[0023] Preferably, the drying temperature is 60-100℃ and the drying time is 8-24h.

[0024] In some embodiments, the annealing rate is 0.5-3 °C / min.

[0025] Secondly, the present invention provides a metastable indium oxide gas-sensitive material, which is prepared by the aforementioned preparation method.

[0026] Metastable indium oxide with nanoneedle self-assembly into flower-like microspheres is formed by the self-assembly of nanoneedles with a diameter of about 8 nm to form a flower-like microsphere structure. The microsphere diameter is about 2-3 μm, and the indium oxide crystal structure is a metastable rhombic corundum phase.

[0027] Thirdly, the present invention provides a gas-sensitive element, comprising a ceramic substrate and a gas-sensitive layer attached to the surface of the ceramic substrate, wherein the gas-sensitive layer is prepared from the metastable indium oxide gas-sensitive material.

[0028] The method for preparing a gas-sensitive element involves mixing the above-mentioned gas-sensitive material with water to form a slurry, coating the slurry onto a ceramic substrate, and forming a gas-sensitive sensing layer on the ceramic substrate through a drying process. After thorough drying, a gas-sensitive element is obtained, and then the coated ceramic substrate is welded onto a substrate base to form a sensor.

[0029] The mass ratio of gas-sensitive material to solvent is 1:2-5, preferably 1:5.

[0030] The drying conditions are 80-100℃, and the drying time is 4-24h.

[0031] Fourthly, the present invention provides the application of the gas-sensitive element in H2S detection.

[0032] Gas-sensitive testing showed that the metastable indium oxide nanoneedle self-assembled flower-like microsphere metastable material provided by this invention exhibits higher selectivity for H2S detection, with an H2S detection limit of 50 ppb and an operating temperature range of 80-300℃ for the gas-sensitive element.

[0033] The beneficial effects achieved by one or more embodiments of the present invention described above are as follows:

[0034] (1) The metastable indium oxide prepared by the present invention, which is composed of self-assembled nanoneedles and flower-like microspheres, significantly improves the material's ability to detect H2S due to its unique micro-nano morphology. The synthesized indium oxide has a morphology of approximately 2-3 μm diameter flower-like microspheres formed by the self-assembly of nanoneedles with a diameter of 8 nm. The average particle size of the material is approximately 20.8 nm, and it has a large specific surface area (54.52 m²). 2 The material's structure ( / g) and mesoporous structure provide ample adsorption sites and diffusion pathways for gas-sensitive reactions, thus providing favorable prerequisites for improving the material's gas-sensitive performance.

[0035] (2) Metastable materials, due to their high-energy metastable crystal structure, have high chemical activity but are also prone to phase transformation to form a stable crystal structure. Traditional indium oxide synthesis processes are difficult to obtain metastable indium oxide materials with a single crystal structure. This invention controls the synthesis and growth of indium hydroxyl oxide with a flower-like micro / nano structure by regulating the reducing atmosphere and subsequent annealing conditions in solvothermal synthesis. By reducing the annealing rate, the destruction of the material's microstructure during high-temperature annealing and the indium oxide phase transformation from metastable to stable state caused by high temperature are suppressed. As a result, a metastable indium oxide material with a uniform crystal structure and a self-assembled flower-like microsphere structure of nanoneedles is obtained. Compared with the stable structure, the material has higher chemical activity and is more prone to interfacial adsorption reactions. In addition, the synthesized metastable indium oxide material exposes a large number of high-energy, high-defect crystal planes, which have a good promoting effect on the adsorption of the target gas and the occurrence of gas-sensitive reactions.

[0036] (3) The gas-sensitive element composed of the self-assembled flower-like microsphere metastable indium oxide material prepared in this invention was tested by gas sensing. The results showed that the gas-sensitive element exhibited excellent selectivity, sensitivity and low operating temperature (100℃) for H2S, and showed a low detection limit (50 ppb) in the detection of H2S, thus having excellent gas-sensitive performance. Attached Figure Description

[0037] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an improper limitation of the invention.

[0038] Figure 1 The XRD patterns of indium hydroxy oxide, metastable indium oxide, and indium oxide-based materials prepared in Examples 4, 6-9 are shown.

[0039] Figure 2 (a) thermogravimetric analysis and (b) differential scanning calorimetry (DSC) of indium hydroxyoxide prepared in Example 4.

[0040] Figure 3 Scanning electron microscope (SEM) images of indium hydroxyoxide (a) and metastable indium oxide (b) prepared in Examples 4 and 8.

[0041] Figure 4 The images shown are TEM and HRTEM images of the metastable indium oxide material prepared in Example 8, where (a) is a TEM image; (b) is a magnified view of a portion of image (a); and (c) is an HRTEM image.

[0042] Figure 5 BET diagram (a) and BJH diagram (b) of the metastable indium oxide material prepared in Example 8.

[0043] Figure 6 The XPS O 1s fine spectrum of the metastable indium oxide material prepared in Example 8.

[0044] Figure 7 The graph shows the response values ​​of the metastable indium oxide material and indium oxide-based material prepared in Examples 6-9 to 10 ppm H2S at different temperatures.

[0045] Figure 8 The cycling curve of the metastable indium oxide material with self-assembled flower-like microspheres prepared in Example 8 at 100 °C under 10 ppm H2S conditions.

[0046] Figure 9 The image shows the gas-sensitive test results of the self-assembled flower-like microsphere metastable indium oxide nanoneedle material prepared in Example 8 at 100 °C for different concentrations of H2S.

[0047] Figure 10 The images show the gas-sensing test results of the metastable indium oxide material and indium oxide-based material prepared in Examples 6-9 at 100°C for 10 ppm of different gases.

[0048] Figure 11The graph shows the response of the metastable indium oxide nanoneedle self-assembled flower-like microsphere material prepared in Example 8 to 10 ppm H2S at 100 °C over 30 days.

[0049] Figure 12 This is a schematic diagram of the components of the gas-sensitive element prepared in Example 10, where (a) is a schematic diagram of the sensing side; and (b) is a schematic diagram of the heating side.

[0050] In the diagram, 1-platinum wire conductor; 2-gas-sensitive material; 3-sensing layer; 4-heating layer. Detailed Implementation

[0051] It should be noted that the following detailed description is illustrative and intended to provide further explanation of the invention. Unless otherwise specified, all technical and scientific terms used in this invention have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0052] The present invention will be further described below with reference to the embodiments.

[0053] Example 1

[0054] The preparation method of indium oxide-based materials includes the following steps:

[0055] (1) Dissolve 1.0 mmol InCl3·4H2O, 5.0 mmol C2H8N2 and 2.0 mmol C6H8O7 in 30 mL of deionized water and stir at 20 °C for 10 min to form a homogeneous solution.

[0056] (2) The homogeneous solution from step (1) was transferred to a steel reactor lined with polytetrafluoroethylene and heated to 100 °C for 8 h. After the solvothermal reaction was completed, the product was centrifuged and washed, and dried at 60 °C for 10 h to obtain the indium hydroxyl oxide precursor material.

[0057] (3) The indium hydroxy oxide precursor obtained in step (2) was annealed at a temperature of 300 °C for 1 h and an annealing rate of 0.5 °C / min. The product was obtained.

[0058] Example 2

[0059] The preparation method of indium oxide-based materials includes the following steps:

[0060] (1) Dissolve 1.0 mmol In(NO3)3·xH2O, 1 mmol C2H8N2 and 1.0 mmol C2H2O4 in 10 mL of anhydrous ethanol and stir at 50 °C for 60 min to form a homogeneous solution.

[0061] (2) The homogeneous solution from step (1) was transferred to a steel reactor lined with polytetrafluoroethylene and heated to 200 °C for 16 h. After the solvothermal reaction was completed, the product was centrifuged and washed, and dried at 100 °C for 24 h to obtain the indium hydroxyl oxide precursor material.

[0062] (3) The indium hydroxy oxide precursor obtained in step (2) was annealed at a temperature of 550 °C for 3 h and an annealing rate of 3 °C / min. The product was obtained.

[0063] Example 3

[0064] The preparation method of indium oxide-based materials includes the following steps:

[0065] (1) Dissolve 1.0 mmol In(NO3)3·xH2O, 3 mmol C2H8N2 and 0.5 mmol C6H8O7 in 20 mL of deionized water and stir at 20 °C for 30 min to form a homogeneous solution.

[0066] (2) The homogeneous solution from step (1) was transferred to a steel reactor lined with polytetrafluoroethylene and heated to 180 °C for 12 h. After the solvothermal reaction was completed, the product was centrifuged and washed, and dried at 100 °C for 24 h to obtain the indium hydroxyl oxide precursor material.

[0067] (3) The indium hydroxy oxide precursor obtained in step (2) was annealed at a temperature of 850 °C for 6 h and an annealing rate of 2 °C / min. The product was obtained.

[0068] Example 4

[0069] The preparation method of indium hydroxyl oxide precursor material includes the following steps:

[0070] (1) Dissolve 1.0 mmol In(NO3)3·xH2O, 1.3 mmol N2H4·H2O and 1.5 mmol C6H8O7 in a mixed solution of 15 mL anhydrous ethanol and 15 mL deionized water, and stir at 30 °C for 30 min to form a homogeneous solution.

[0071] (2) The homogeneous solution from step (1) was transferred to a steel reactor lined with polytetrafluoroethylene and heated to 180 °C for 12 h. After the solvothermal reaction was completed, the product was centrifuged and washed, and dried at 80 °C for 24 h to obtain the indium hydroxyl oxide precursor material.

[0072] (3) The indium hydroxy oxide precursor obtained in step (2) is not annealed.

[0073] Example 5

[0074] The preparation method of indium oxide-based materials includes the following steps:

[0075] (1) Dissolve 1.0 mmol In(NO3)3·xH2O, 3 mmol CO(NH2)2 and 1.5 mmol C2H2O4 in 30 mL of anhydrous ethanol and stir at 50 °C for 10 min to form a homogeneous solution.

[0076] (2) The homogeneous solution from step (1) was transferred to a steel reactor lined with polytetrafluoroethylene and heated to 160 °C for 18 h. After the solvothermal reaction was completed, the product was centrifuged and washed, and dried at 80 °C for 12 h to obtain the indium hydroxyl oxide precursor material.

[0077] (3) The indium hydroxy oxide precursor obtained in step (2) was annealed at a temperature of 600 °C for 4 h and an annealing rate of 1 °C / min. The product was obtained.

[0078] Example 6

[0079] The preparation method of indium oxide-based materials includes the following steps:

[0080] (1) Dissolve 1.0 mmol In(NO3)3·xH2O, 1.5 mmol CO(NH2)2 and 1.5 mmol C6H8O7 in 20 mL of deionized water and stir at 40 °C for 30 min to form a homogeneous solution.

[0081] (2) The homogeneous solution from step (1) was transferred to a steel reactor lined with polytetrafluoroethylene and heated to 100 °C for 12 h. After the solvothermal reaction was completed, the product was centrifuged and washed, and dried at 80 °C for 12 h to obtain the indium hydroxyl oxide precursor material.

[0082] (3) The indium hydroxy oxide precursor obtained in step (2) was annealed at a temperature of 500 °C for 2 h and an annealing rate of 3 °C / min. The product was obtained.

[0083] Example 7

[0084] The preparation method of indium oxide-based materials includes the following steps:

[0085] (1) Dissolve 1.0 mmol InCl3·4H2O, 1.25 mmol CO(NH2)2 and 0.5 mmol C6H8O7 in 20 mL of deionized water and stir at 30 °C for 60 min to form a homogeneous solution.

[0086] (2) The homogeneous solution from step (1) was transferred to a steel reactor lined with polytetrafluoroethylene and heated to 180 °C for 20 h. After the solvothermal reaction was completed, the product was centrifuged and washed, and dried at 80 °C for 6 h to obtain the indium hydroxyl oxide precursor material.

[0087] (3) The indium hydroxy oxide precursor obtained in step (2) was annealed at a temperature of 650 °C for 2 h and an annealing rate of 0.5 °C / min. The product was obtained.

[0088] Example 8

[0089] The preparation method of indium oxide-based materials includes the following steps:

[0090] (1) Dissolve 1.0 mmol In(NO3)3·xH2O, 1.25 mmol C2H8N2 and 1.25 mmol C6H8O7 in 20 mL of deionized water and stir at 20 °C for 60 min to form a homogeneous solution.

[0091] (2) The homogeneous solution from step (1) was transferred to a steel reactor lined with polytetrafluoroethylene and heated to 100 °C for 12 h. After the solvothermal reaction was completed, the product was centrifuged and washed, and dried at 80 °C for 12 h to obtain the indium hydroxyl oxide precursor material.

[0092] (3) The indium hydroxy oxide precursor obtained in step (2) was annealed at a temperature of 350 °C for 2 h and an annealing rate of 1 °C / min. The product was obtained.

[0093] Example 9

[0094] The preparation method of indium oxide-based materials includes the following steps:

[0095] (1) Dissolve 1.0 mmol In(NO3)3·xH2O, 2 mmol C2H8N2 and 1.05 mmol C6H8O7 in a mixed solution of 15 mL deionized water and 5 mL anhydrous ethanol, and stir at 20 °C for 30 min to form a homogeneous solution.

[0096] (2) The homogeneous solution from step (1) was transferred to a steel reactor lined with polytetrafluoroethylene and heated to 180 °C for 14 h. After the solvothermal reaction was completed, the product was centrifuged and washed, and dried at 80 °C for 8 h to obtain the indium hydroxyl oxide precursor material.

[0097] (3) The indium hydroxy oxide precursor obtained in step (2) was annealed at a temperature of 800 °C for 2 h at a rate of 1 °C / min. The product was obtained.

[0098] Example 10

[0099] A method for preparing a gas-sensitive element includes the following steps:

[0100] (1) Add the materials prepared in Examples 1-9 into a mortar, mix them with deionized water at a mass ratio of 1:5 and grind them until they are uniformly mixed to form a slurry. The slurry is uniformly dripped onto the surface of the ceramic substrate to form a gas-sensitive sensing layer. The ceramic substrate is then placed in an 80°C oven to dry for 3-5 min. This process is repeated 3 times. The coated ceramic substrate is then placed in an 80°C oven to dry for 24 h until fully dried. The resulting ceramic substrate is then soldered to the substrate base through four platinum wires to form a gas-sensitive element. Figure 12 The components of the gas-sensitive element are shown. The ceramic substrate used is an alumina substrate. The sensing layer surface of the ceramic substrate is coated with the gas-sensitive material prepared in the above embodiment. The working electrode and heating electrode are located on both sides of the ceramic substrate. Two platinum wires are led out from each side. The ceramic substrate is welded to the base through four platinum wires to form the final gas-sensitive element.

[0101] Figure 1 The XRD patterns of indium hydroxyl oxide, metastable indium oxide, and indium oxide-based materials prepared in Examples 4 and 6-9 are shown. In Example 4, the precursor material is pure InOOH without other impurities. Examples 6-9 synthesized indium oxide materials with different crystal structures, indicating that InOOH can form pure metastable indium oxide under high-temperature annealing. However, if the annealing temperature is too high, the metastable structure will undergo a phase transition and further form a stable indium oxide phase. In addition, the number of (104) exposed crystal planes of the prepared metastable indium oxide increased significantly. The (104) plane is a high-energy, high-defect plane of metastable indium oxide. The large exposure of this crystal plane will promote the occurrence of gas-sensitive reactions at the surface.

[0102] Figure 2The thermogravimetric analysis (a) and differential scanning calorimetry (DSC) curves of indium hydroxyl oxide prepared in Example 4 show that the indium hydroxyl oxide material undergoes three distinct thermal weight loss stages during heating. Approximately 3.38% weight loss occurs between 0-200 °C due to water decomposition; 12.86% weight loss occurs between 200-276 °C due to the decomposition of residual reducing agent and alkali source in the precursor; and 6.76% weight loss occurs between 276-800 °C due to the decomposition of water molecules during the dehydration of indium hydroxyl oxide to form indium oxide. The DSC results show that as the temperature increases, the indium hydroxyl oxide material exhibits three distinct exothermic peaks at 200 °C, 346 °C, and 638 °C, corresponding to the decomposition of indium hydroxyl oxide's water of crystallization, the exothermic transformation of indium hydroxyl oxide to metastable indium oxide, and the latent heat of phase transition from metastable indium oxide to stable indium oxide, respectively. This indicates that pure metastable indium oxide material can be obtained under certain annealing temperature conditions.

[0103] Figure 3 The images show scanning electron microscope (SEM) images of the indium hydroxyl oxide and metastable indium oxide materials prepared in Examples 4 and 8. As can be seen from Figure (a), the examples yielded indium hydroxyl oxide precursor materials with a three-dimensional flower-like microsphere structure, with microsphere diameters of approximately 2-3 μm. After annealing, metastable indium oxide materials with self-assembled nanoneedle-like microspheres were obtained, with diameters of approximately 2-3 μm and nanoneedle diameters of approximately 8 nm.

[0104] Figure 4 The images show TEM and HRTEM images of the metastable indium oxide material prepared in Example 8. As can be seen from Figure (a), the morphology of the prepared metastable indium oxide material is a three-dimensional flower-like microsphere structure with self-assembled nanoneedles, and the size of the microspheres is about 2-3 μm. The magnified view in Figure (b) shows a large number of nanoneedle-like structures on the surface of the flower-like microspheres, and the diameter of the nanoneedles is about 8 nm. In the HRTEM image in Figure (c), the (104), (110), and (012) planes of the metastable rhombic corundum phase indium oxide can be observed by measuring the interplanar spacing, which shows the successful preparation of the metastable indium oxide material. In addition, the (104) plane of the metastable indium oxide is a high-energy, high-defect plane, and the exposure of the (104) crystal plane is conducive to the occurrence of gas-sensitive reactions at the surface.

[0105] Figure 5 The BET and BJH diagrams for the metastable indium oxide material prepared in Example 8 are shown in Figure (a). From Figure (a), the specific surface area of ​​the material can be calculated to be 54.52 m². 2 As shown in Figure (b), the most prominent pores in the obtained material are mesopores with a pore size of approximately 7.7 nm. The large specific surface area and abundant pore structure can effectively enhance the adsorption of the target gas and provide additional gas diffusion channels, thus contributing to the improvement of gas sensing performance.

[0106] Figure 6 The XPS O 1s fine spectrum of the metastable indium oxide material prepared in Example 8 shows that the O 1s fine spectrum includes three spin-coupled peaks: lattice oxygen, oxygen vacancies, and chemisorbed oxygen. The proportions of oxygen vacancies and chemisorbed oxygen are 44.56% and 16.48%, respectively. Abundant oxygen vacancy defects provide sufficient oxygen adsorption sites to participate in the gas-sensing reaction; while chemisorbed oxygen can directly participate in the gas-sensing reaction as oxygen ions. High contents of oxygen vacancies and chemisorbed oxygen are beneficial to improving gas-sensing performance.

[0107] Figure 7 The graphs show the response values ​​of the nanoneedle-assembled flower-like microsphere metastable indium oxide material and indium oxide-based material prepared in Examples 6-9 to 10 ppm H2S at different temperatures. It can be seen that the prepared nanoneedle-assembled flower-like microsphere metastable indium oxide material exhibits the best response to H2S at an operating temperature of 100 °C, with a response value of approximately 794 for 10 ppm H2S. This is higher than that of the prepared pure stable indium oxide material and the stable / metastable dual-phase coexistence material, demonstrating that the prepared metastable indium oxide material exhibits superior H2S detection capability.

[0108] Figure 8 The cyclic curve of the metastable indium oxide nanoneedle self-assembled flower-like microsphere material prepared in Example 8 at 100 °C for 10 ppm H2S shows that the response value of the prepared material to H2S is relatively stable in five cycles, and the response recovery process does not produce obvious fluctuations, indicating that the prepared material has excellent stability in the detection of H2S.

[0109] Figure 9 The image shows the gas-sensitive test results of the metastable indium oxide nanoneedle self-assembled flower-like microsphere material prepared in Example 8 at 100 °C for different concentrations of H2S. It can be seen that the material's response value to H2S increases with the increase of H2S concentration. The material can still produce a relatively obvious response to 50 ppb H2S, indicating that the prepared gas-sensitive material has the ability to detect low concentrations of H2S and the ability to dynamically detect H2S.

[0110] Figure 10 The images show the gas-sensing test results of the nanoneedle self-assembled flower-like microsphere metastable indium oxide material and indium oxide-based material prepared in Examples 6-9 at 100 °C for 10 ppm of different gases. It can be seen that the prepared nanoneedle self-assembled flower-like microsphere metastable indium oxide material and indium oxide-based material both exhibit higher response values ​​and selectivity to H2S. In addition, the prepared nanoneedle self-assembled flower-like microsphere metastable indium oxide material has higher selectivity to H2S, indicating that the prepared gas-sensing material has a certain anti-interference ability when detecting H2S.

[0111] Figure 11 The graph shows the response value of the metastable indium oxide nanoneedle self-assembled flower-like microspheres prepared in Example 8 to 10 ppm H2S at 100 ℃ over 30 days. It can be seen that the response value of the gas-sensitive material fluctuates slightly but does not decrease significantly during the 30-day test period, indicating that the prepared material has a certain stability and long-term detection capability in the long-term detection of H2S.

[0112] Figure 12 This is a schematic diagram of the components of the gas-sensitive element prepared in Example 10. The gas-sensitive element consists of a ceramic substrate and a gas-sensitive material. The ceramic substrate is an alumina substrate, with a sensing layer and a heating layer on each side. The sensing layer carries the gas-sensitive material, and its surface is coated with the gas-sensitive material prepared in this embodiment of the invention. The heating layer heats the ceramic substrate, providing the operating temperature required for gas detection. The ceramic substrate is welded to a base via four platinum wires to form a sensor.

[0113] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing a metastable indium oxide gas-sensitive material, characterized in that: Includes the following steps: Dissolve and mix the indium salt, alkali, and reducing agent thoroughly. Under an alkaline atmosphere, stir to allow the indium salt to react with OH-. - The reaction forms In(OH)3, which dissolves in the reaction solvent. Then, a solvothermal reaction occurs, during which the substance recrystallizes to form a self-assembled flower-like microsphere structure. The reducing agent promotes the dehydration of In(OH)3 during the solvothermal process to form a pure InOOH precursor. The obtained InOOH precursor was annealed at 350℃ for 1-6 hours to obtain metastable indium oxide material with self-assembled flower-like microspheres of nanoneedles. The base is selected from ethylenediamine or hydrazine hydrate or a combination thereof; The reducing agent is citric acid; The molar ratio of indium salt, alkali, and reducing agent is 1:1-3:0.5-2; The annealing rate is 0.5-3℃ / min.

2. The method for preparing metastable indium oxide gas-sensitive material according to claim 1, characterized in that: The indium salt is InCl3·4H2O or / and In(NO3)3·xH2O.

3. The method for preparing metastable indium oxide gas-sensitive material according to claim 1, characterized in that: The solvent used in the solvothermal reaction is selected from one or a mixture of two of deionized water or anhydrous ethanol.

4. The method for preparing metastable indium oxide gas-sensitive material according to claim 1, characterized in that: The stirring time for dissolving and mixing indium salt, alkali and reducing agent is 10-60 minutes, and the stirring temperature is 20-50℃.

5. The method for preparing metastable indium oxide gas-sensitive material according to claim 1, characterized in that: The temperature of the solvothermal reaction is 100-200℃, and the reaction time is 8-24h.

6. The method for preparing metastable indium oxide gas-sensitive material according to claim 1, characterized in that: The temperature of the solvothermal reaction is 120-180℃, and the reaction time is 12-24h.

7. The method for preparing metastable indium oxide gas-sensitive material according to claim 1, characterized in that: After the solvothermal reaction is completed, the product is further subjected to centrifugation, washing and drying.

8. The method for preparing metastable indium oxide gas-sensitive material according to claim 7, characterized in that: The drying temperature is 60-100℃, and the drying time is 8-24h.

9. A metastable indium oxide gas-sensitive material, characterized in that: It is prepared by any one of the preparation methods described in claims 1-8.

10. A gas-sensitive element, characterized in that: It includes a ceramic substrate and a gas-sensitive layer attached to the surface of the ceramic substrate. The gas-sensitive layer is made of the metastable indium oxide gas-sensitive material as described in claim 9. The gas-sensitive element exhibits high selectivity for H2S and a low operating temperature of 100 °C. The detection limit for H2S is 50 ppb.

11. The application of the gas-sensitive element according to claim 10 in H2S detection.

Images