Cobalt tungstate / tin dioxide nanosheet composite material, and preparation method and application thereof

Abstract

This invention belongs to the field of gas-sensitive materials technology, specifically relating to a cobalt tungstate / tin dioxide nanosheet composite gas-sensitive material, its preparation method, and its application. The preparation method includes the following steps: homogeneous precipitation of cobalt salt and tungstate in a precipitant to obtain a CoWO4 precursor; calcination of the CoWO4 precursor in air to obtain CoWO4(P) nanosheets; dispersion of the CoWO4(P) nanosheets and SnO2 powder in an alcohol solvent in a certain proportion; ultrasonication and stirring treatment; and then centrifugation, washing, and drying of the mixture to obtain the final product. This invention employs a homogeneous precipitation-hydrothermal method to synthesize CoWO4 nanosheets with abundant chemically adsorbed oxygen through precipitant regulation, and then composites them with commercial SnO2 through a simple impregnation method to construct a CoWO4 / SnO2 heterojunction gas-sensitive material. The prepared composite material exhibits high sensitivity, excellent selectivity, and stability to triethylamine.

Description

Technical Field

[0001] This invention belongs to the field of gas-sensitive materials technology, specifically relating to a cobalt tungstate / tin dioxide nanosheet composite 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] Triethylamine (TEA) is a volatile organic compound with a strong fishy odor and a key biomarker released during the spoilage of aquatic products. When its concentration reaches or exceeds 5 ppm, it is generally considered to have undergone obvious spoilage and is unfit for consumption in most fish and seafood products, thus serving as a reliable indicator for assessing food freshness.

[0004] Currently, metal-oxide-semiconductor (MOS) gas sensors are widely used for the detection of triethylamine (TEA) due to their advantages such as low cost, mature technology, and rapid response. Among them, nano-tin dioxide (SnO2) is the most commonly used gas-sensitive material system. While it possesses good electron transport characteristics and structural stability, its intrinsic sensing performance for TEA has significant shortcomings: firstly, the SnO2 surface lacks active sites that specifically interact with TEA molecules, making it highly susceptible to interference from common volatile organic compounds such as ethanol, ammonia, and acetone in complex odor environments, severely limiting selectivity; secondly, its surface oxygen adsorption and activation capabilities are weak, resulting in weak response signals to TEA at the ppm to ppb level, with sensitivity far below the requirements of practical applications. For commercial SnO2, due to its large particle size and small specific surface area, the response to TEA is even lower. However, commercial SnO2 offers advantages in both ensuring good electron transport characteristics and large-scale production, as well as significant advantages in long-term operational stability, making it more suitable for practical production and application. Therefore, how to significantly improve the sensitivity and molecular recognition ability of commercial SnO2 to TEA while retaining its advantages of scalable preparation and structural stability remains a key technical challenge that urgently needs to be solved in the field of food safety monitoring. Summary of the Invention

[0005] To address the shortcomings of existing technologies, the present invention aims to provide a cobalt tungstate / tin dioxide nanosheet composite gas-sensitive material, its preparation method, and its applications. This invention employs a homogeneous precipitation-hydrothermal method to synthesize CoWO4 nanosheets rich in chemically adsorbed oxygen through precipitant regulation. These nanosheets are then combined with commercially available SnO2 via a simple impregnation method to construct a CoWO4 / SnO2 heterojunction gas-sensitive material. The prepared composite material exhibits high sensitivity, excellent selectivity, and stability to triethylamine.

[0006] To achieve the above objectives, the present invention is implemented through the following technical solution: In a first aspect, the present invention provides a method for preparing a cobalt tungstate / tin dioxide nanosheet composite gas-sensitive material, comprising the following steps: Cobalt salt and tungstate were homogeneously precipitated in the presence of a precipitant to obtain CoWO4 precursor. The CoWO4 precursor was then calcined in air to obtain CoWO4(P) nanosheets. CoWO4(P) nanosheets and SnO2 powder were dispersed in an alcohol solvent in a certain proportion, and then subjected to ultrasonication and stirring. The mixture was then centrifuged, washed, and dried to obtain the final product.

[0007] Secondly, the present invention provides a cobalt tungstate / tin dioxide nanosheet composite gas-sensitive material, which is prepared by the aforementioned preparation method.

[0008] Thirdly, the present invention provides the application of the cobalt tungstate / tin dioxide nanosheet composite gas-sensitive material as a gas-sensitive material.

[0009] The beneficial effects achieved by one or more embodiments of the present invention described above are as follows: (1) In this invention, CoWO4(P) nanosheets rich in adsorbed oxygen are synthesized by regulating the homogeneous precipitant and pn heterojunction with SnO2, which significantly modulates the band structure and surface electronic state of the material, promotes oxygen molecules to capture electrons from the conduction band of the material to form chemically adsorbed oxygen species, thereby greatly improving the surface oxidation reaction activity of triethylamine. (2) In the prepared CoWO4(P) / SnO2 composite material, CoWO4(P) nanosheets are anchored on the SnO2 surface, forming a large number of heterogeneous interfaces. Although the specific surface area is similar to that of pure SnO2 (~4.1 m²), the composite material has a relatively large surface area. 2 / g), but its performance improvement mainly comes from electronic effects rather than morphological effects, indicating that the present invention achieves precise control of electronic structure; (3) Under CoWO4(P) loading, the proportion of strong Lewis acid sites on the surface of the composite material can reach 75.7%, which can specifically recognize the lone pair electrons of nitrogen atoms in triethylamine molecules, giving the sensor excellent selectivity; (4) The gas sensor prepared by the present invention exhibits a high response value (6.34) to 10 ppm triethylamine at 100℃ and 80%RH, excellent short-term (125 consecutive cycles) and long-term stability (>120 days), low cost, and is suitable for food safety monitoring in actual scenarios such as cold chain logistics and aquatic product markets. Attached Figure Description

[0010] 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.

[0011] Figure 1 The XRD patterns and FTIR spectra of the CoWO4, CoWO4(10P), CoWO4(20P) and CoWO4(30P) samples prepared in Examples 1-4 of this invention are shown in (a) and (b) respectively.

[0012] Figure 2 The following are the quantitative analysis results of (a) full XPS spectrum, (b) high-resolution XPS spectrum of Co 2p, (c) high-resolution XPS spectrum of W 4f, (d) high-resolution XPS spectrum of O 1s, and (e) peak fitting of the CoWO4, CoWO4(10P), CoWO4(20P) and CoWO4(30P) samples prepared in Examples 1-4 of this invention.

[0013] Figure 3 These are transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images of the cobalt tungstate nanosheets and cobalt tungstate / tin dioxide composite gas-sensitive materials prepared in Examples 3 and 6 of this invention. (a) is a TEM image of the CoWO4(P) sample obtained in Example 3, and (bd) are HRTEM images of the CoWO4(P)-SnO2 composite material obtained in Example 6.

[0014] Figure 4 Energy dispersive spectroscopy (EDS) elemental distribution maps of the samples prepared in Examples 3 and 6 of this invention. Among them, (a) is the EDS distribution map of the CoWO4(P) sample obtained in Example 3, and (b) is the EDS distribution map of the CoWO4(P)-SnO2 composite material obtained in Example 6.

[0015] Figure 5 The images of the Au-SnO2 sample prepared in Example 9 of this invention are shown in (ab) transmission electron microscope (TEM) and high-resolution transmission electron microscope (HRTEM) images, and (c) energy dispersive spectroscopy (EDS) elemental distribution map.

[0016] Figure 6 The present invention includes (a) high-resolution XPS spectra of Sn 3d for commercial SnO2, 6 at% Au-SnO2 prepared in Example 9, 2 at% CoWO4(P)-SnO2 prepared in Example 5, 6 at% CoWO4(P)-SnO2 prepared in Example 6, and 10 at% CoWO4(P)-SnO2 prepared in Example 7; (b) high-resolution XPS spectra of O 1s for the above samples; and (c) NH3-TPD curves for commercial SnO2, 6 at% Au-SnO2 prepared in Example 9, and 6 at% CoWO4(P)-SnO2 prepared in Example 6.

[0017] Figure 7(a) UV-Vis DRS; (b) Tauc plot of commercial SnO2, 6 at% Au-SnO2 prepared in Example 9, and 6 at% CoWO4(P)-SnO2 prepared in Example 6.

[0018] Figure 8 (a) Nitrogen adsorption-desorption isotherm (BET) for commercial SnO2, 6 at% Au-SnO2 prepared in Example 9, and 6 at% CoWO4(P)-SnO2 prepared in Example 6; (b) Pore size distribution curve (BJH method).

[0019] Figure 9 The present invention provides the following gas sensors: (a) response values ​​of 10 ppm triethylamine at different operating temperatures under 80% relative humidity for commercial SnO2, 6 at% CoWO4-SnO2 prepared in Example 8, 6 at% Au-SnO2 prepared in Example 9, 2 at% CoWO4(P)-SnO2 prepared in Example 5, 6 at% CoWO4(P)-SnO2 prepared in Example 6, and 10 at% CoWO4(P)-SnO2 prepared in Example 7; (b) response-recovery curves of 10 ppm triethylamine at 100°C and 80% relative humidity.

[0020] Figure 10 The present invention provides the following gas sensors: (a) response-recovery curves of 0.5-20 ppm triethylamine at 100°C for commercial SnO2, 6 at% CoWO4-SnO2 prepared in Example 8, 6 at% Au-SnO2 prepared in Example 9, 2 at% CoWO4(P)-SnO2 prepared in Example 5, 6 at% CoWO4(P)-SnO2 prepared in Example 6, and 10 at% CoWO4(P)-SnO2 prepared in Example 7; (b) response-recovery curves of 6 at% Au-SnO2 and 6 at% CoWO4-SnO2 sensors at 100°C for 1-20 ppm triethylamine; (cd) fitted curves of the response values ​​corresponding to all the above sensors; and (e) selective test results for 100 ppm of various interfering gases (including triethylamine, ethanol, acetone, ammonia, methanol, benzene, and formaldehyde).

[0021] Figure 11The present invention provides the following gas sensors for commercial SnO2, 2 at% CoWO4(P)-SnO2 prepared in Example 5, 6 at% CoWO4(P)-SnO2 prepared in Example 6, and 10 at% CoWO4(P)-SnO2 prepared in Example 7, as well as 6 at% Au-SnO2 prepared in Example 9 and 6 at% CoWO4-SnO2 prepared in Example 8: (a) response-recovery stability curves for 125 consecutive cycles of 10 ppm triethylamine at 80% relative humidity; (b) response-recovery stability curves for 6 consecutive cycles of 10 ppm triethylamine at 80% relative humidity; and (c) long-term stability test results of the 6 at% CoWO4(P)-SnO2 sensor.

[0022] Figure 12 Photos of frozen and refrigerated salmon taken at different times.

[0023] Figure 13 (a) Response-recovery curves of the 6 at% CoWO4(P)-SnO2 gas sensor prepared in Example 6 of this invention to triethylamine (TEA) released from frozen and refrigerated salmon at 0, 2, 4, 6, 8, 16 and 24 hours; (b) Comparison of the changes in triethylamine concentration and spoilage rate of frozen and refrigerated salmon over time during storage.

[0024] Figure 14 This is a schematic diagram of the gas-sensitive element prepared in Embodiment 6 of the present invention. Detailed Implementation

[0025] 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.

[0026] To address the technical problems mentioned in the background art, the present invention provides a method for preparing a cobalt tungstate / tin dioxide nanosheet composite gas-sensitive material, comprising the following steps: Cobalt salt and tungstate were homogeneously precipitated in the presence of a precipitant to obtain CoWO4 precursor. The CoWO4 precursor was then calcined in air to obtain CoWO4(P) nanosheets. CoWO4(P) nanosheets and SnO2 powder were dispersed in an alcohol solvent in a certain proportion, and then subjected to ultrasonication and stirring. The mixture was then centrifuged, washed, and dried to obtain the final product.

[0027] Precipitants (such as hexamethylenetetramine and acetamide) release OH- through slow release. - or CO3 2-This process allows cobalt salt and tungstate to react uniformly in the solution, avoiding agglomeration caused by excessively high local concentrations, and generating uniformly sized CoWO4 precursor particles. Under hydrothermal conditions of 150-200℃, the precursor forms a sheet-like structure through a dissolution-recrystallization process, laying the foundation for the morphology of the nanosheets after subsequent calcination.

[0028] During calcination, the precursor dehydrates and undergoes a solid-phase reaction to form CoWO4 nanosheets. During calcination, the reducing atmosphere generated by the decomposition of the precipitant can induce the formation of oxygen vacancies and chemically adsorbed oxygen (OC) on the surface of CoWO4, which significantly enhances the gas-sensitive reaction activity.

[0029] Dispersing and stirring CoWO4 (p) nanosheets and SnO2 powder in an alcohol solvent using ultrasound can prevent nanosheet agglomeration and ensure uniform loading on the SnO2 surface, forming a tight heterojunction interface. After CoWO4 (p-type) and SnO2 (n-type) come into contact, a built-in electric field is formed at the interface, which promotes electron transfer and enhances oxygen adsorption capacity and surface oxidation reaction rate.

[0030] Alcohol solvents have low surface tension, which can effectively reduce the interfacial tension between CoWO4(P) nanosheets and reduce particle agglomeration. Through ultrasonic dispersion, ethanol can penetrate into the gaps between the stacked nanosheets, peeling them off layer by layer to form a uniformly dispersed suspension, ensuring high dispersibility when subsequently compounded with SnO2 powder. Ethanol's polarity is between that of water and organic solvents, allowing it to simultaneously wet both CoWO4(P) nanosheets (polar surface) and SnO2 powder (weakly polar surface), reducing the contact resistance at the interface between the two phases. During stirring, ethanol acts as a medium to promote close contact between CoWO4(P) nanosheets and SnO2 particles, forming a stable heterojunction interface and enhancing electron transfer efficiency.

[0031] Alcohol solvents have a low dielectric constant, which can mitigate excessive electrostatic attraction between CoWO4(P) and SnO2, preventing non-uniform loading. Simultaneously, ethanol, as an inert solvent, does not participate in chemical reactions, preventing secondary reactions between residual ions after hydrothermal treatment or calcination and the material surface, ensuring the purity of the composite material. Ethanol has a low boiling point and high volatility, allowing for rapid removal during subsequent centrifugation and drying processes, avoiding the impact of solvent residue on gas-sensing properties. Compared to water solvents, ethanol does not leave hydroxyl groups or hydrogen bonds on the material surface after drying, reducing competition for oxygen adsorption sites. Alcohol solvents can stabilize the surface structure of CoWO4(P) nanosheets through hydrogen bonding, inhibiting structural collapse or phase transitions during dispersion.

[0032] In some embodiments, the molar ratio of cobalt salt to tungstate is 1:0.9-1.1, preferably 1:1.

[0033] The stoichiometric composition of CoWO4 is 1:1. When the molar ratio of cobalt salt to tungstate is close to 1:1, it can ensure that the two ions react fully to form pure phase CoWO4, avoiding the formation of impurity phase due to excess of one component, which would affect the gas-sensing performance of the material.

[0034] In some embodiments, the precipitant is selected from urea, hexamethylenetetramine, acetamide, or oxalic acid; Alternatively, the cobalt salt is selected from at least one of Co(NO3)2·6H2O, CoCl2·6H2O, and Co(CH3COO)2·4H2O; Alternatively, the tungstate is Na2WO4·2H2O or Na2WO4·xH2O.

[0035] Preferably, the precipitant is hexamethylenetetramine or acetamide.

[0036] Hexamethylenetetramine undergoes a hydrolysis reaction under hydrothermal conditions, slowly releasing OH groups. - , so that the OH in the solution - The concentration is increased uniformly to avoid aggregation caused by local supersaturation, resulting in uniformly sized CoWO4 precursor particles; Acetamide: CO3 is gradually released through amide bond hydrolysis. 2- And NH3, and Co 2+ WO4 2- It forms stable coordination intermediates, regulates the crystal growth rate, and promotes the anisotropic growth of lamellar structures.

[0037] During calcination, these two precipitants decompose to produce a reducing atmosphere (e.g., HMT decomposes to produce CO and NH3; acetamide decomposes to produce CH3CN and CO2), which can induce the formation of oxygen vacancies and chemical adsorption of oxygen on the surface of CoWO4, significantly higher than other precipitants.

[0038] The molecular structures of HMT and acetamide can be adsorbed onto the crystal surface via hydrogen bonds or van der Waals forces, inhibiting the growth of specific crystal faces and promoting the formation of rectangular nanosheets. In contrast, strong precipitants such as oxalic acid tend to cause particle agglomeration, forming irregular blocky structures. When CoWO4(P) nanosheets prepared by these two precipitants are combined with SnO2, the pn heterojunction at the interface is more compact, and the electron transfer efficiency is higher.

[0039] In some embodiments, the molar ratio of the precipitant to the cobalt salt is 0.1-10:1, preferably 2-10:1. For example, it can be 0.1:1, 0.5:1, 1:1, 2:1, 3:1, 4:1, 5:1, 6:1, 7:1, 8:1, 9:1, or 10:1.

[0040] In some embodiments, the mass ratio of CoWO4(P) nanosheets to SnO2 powder is 1:2-50. For example, it can be 1:2, 1:3, 1:4, 1:5, 1:6, 1:7, 1:8, 1:9, 1:10, 1:12, 1:14, 1:16, 1:18, 1:20, 1:25, 1:30, 1:35, 1:40, 1:45, or 1:50.

[0041] In some embodiments, the alcohol solvent is selected from at least one of methanol, ethanol, or isopropanol. Anhydrous ethanol is preferred.

[0042] In some embodiments, the homogeneous precipitation is carried out by a hydrothermal reaction, with the hydrothermal reaction temperature being 150-200℃ and the hydrothermal reaction time being 4-18 h.

[0043] The hydrothermal reaction temperature can be 150℃, 160℃, 170℃, 180℃, 190℃, or 200℃. The hydrothermal reaction time can be 4 h, 5 h, 6 h, 7 h, 8 h, 9 h, 10 h, 11 h, 12 h, 13 h, 14 h, 15 h, 16 h, 17 h, or 18 h.

[0044] In some embodiments, the calcination temperature is 400-600℃, the calcination time is 1-4 h, and the heating rate is 1-10℃ min. -1 .

[0045] The calcination temperature can be 400℃, 450℃, 500℃, 550℃ or 600℃; the calcination time can be 1 h, 2 h, 3 h or 4 h. The heating rate can be 1℃ min -1 2℃ min -1 3℃ min -1 4℃ min -1 5℃ min -1 6℃ min -1 7℃ min -1 8℃ min -1 9℃ min -1 10℃ min -1 .

[0046] In some embodiments, the specific method of ultrasonic and stirring treatment is as follows: first, CoWO4(P) nanosheets and SnO2 powder are ultrasonically dispersed separately in an alcohol solvent for 10-30 min, then the two dispersions are mixed and ultrasonically dispersed for 20-40 min, and finally stirred for 12-36 h.

[0047] By adopting a method of first dispersing and then mixing, the aggregation of CoWO4(P) nanosheets can be effectively avoided, ensuring their high dispersibility on the SnO2 surface, thereby constructing a tight heterojunction interface.

[0048] Preferably, the CoWO4(P) nanosheets have a rectangular sheet structure with an average length of 40-80 nm and a width of 10-30 nm; the SnO2 powder has a particle size of 20-500 nm.

[0049] In some embodiments, the centrifugation speed is 10,000-12,000 rpm and the centrifugation time is 5-20 min.

[0050] In some embodiments, the drying temperature is 60-70°C and the drying time is 10-12 h.

[0051] Secondly, the present invention provides a cobalt tungstate / tin dioxide nanosheet composite gas-sensitive material, which is prepared by the aforementioned preparation method.

[0052] Thirdly, the present invention provides a gas sensor, comprising a substrate and a sensing layer loaded on the surface of the substrate, wherein the sensing layer is made of the cobalt tungstate / tin dioxide nanosheet composite gas-sensitive material.

[0053] In some embodiments, the substrate is made of ceramic.

[0054] Fourthly, the present invention provides the application of the cobalt tungstate / tin dioxide nanosheet composite gas-sensitive material as a gas-sensitive material in the detection of triethylamine.

[0055] In some embodiments, the cobalt tungstate / tin dioxide nanosheet composite gas-sensitive material is used for real-time monitoring of trace amounts of triethylamine released during the spoilage of aquatic products, so as to achieve food safety assessment and freshness early warning.

[0056] The present invention will be further described below with reference to specific embodiments.

[0057] Example 1 A method for preparing cobalt tungstate nanosheet gas-sensitive material includes the following steps: 3 mmol of cobalt nitrate hexahydrate (Co(NO3)2·6H2O) was dissolved in 40 mL of deionized water and stirred continuously to form a homogeneous solution. Separately, 3 mmol of sodium tungstate dihydrate (Na2WO4·2H2O) was dissolved in another 40 mL of deionized water and stirred thoroughly until completely clear. Under vigorous stirring, the sodium tungstate solution was slowly added dropwise to the cobalt nitrate solution, and stirring continued for 30 minutes to obtain a precursor mixture. This mixture was transferred to a PTFE-lined stainless steel reactor and hydrothermally reacted at 160 °C for 6 hours. After the reaction, the mixture was allowed to cool naturally to room temperature. The precipitate was collected by centrifugation, washed five times repeatedly with anhydrous ethanol to remove residual ions, and then dried at 70 °C for 12 hours. The resulting powder was calcined in air at 500 °C for 2 hours at a heating rate of 2 °C / min. -1 Cobalt tungstate nanosheets were obtained and denoted as CoWO4.

[0058] Example 2 A method for preparing cobalt tungstate nanosheet gas-sensitive material includes the following steps: 3 mmol of cobalt nitrate hexahydrate (Co(NO3)2·6H2O) and 10 mmol of hexamethylenetetramine were dissolved together in 40 mL of deionized water and stirred continuously to form a homogeneous solution. Separately, 3 mmol of sodium tungstate dihydrate (Na2WO4·2H2O) was dissolved in another 40 mL of deionized water and stirred thoroughly until completely clear. The sodium tungstate solution was slowly added dropwise to the cobalt-precipitant mixture under vigorous stirring for 30 minutes. The mixture was then subjected to hydrothermal reaction at 150 °C for 6 hours, followed by centrifugation, washing, drying at 60 °C, and calcination at 500 °C for 2 hours at a heating rate of 2 °C / min. -1 The resulting product is denoted as CoWO4(10P).

[0059] Example 3 A method for preparing cobalt tungstate nanosheet gas-sensitive material includes the following steps: 3 mmol of cobalt nitrate hexahydrate (Co(NO3)2·6H2O) and 20 mmol of acetamide were dissolved together in 40 mL of deionized water and stirred continuously to form a homogeneous solution. Separately, 3 mmol of sodium tungstate dihydrate (Na2WO4·2H2O) was dissolved in another 40 mL of deionized water and stirred thoroughly until completely clear. The sodium tungstate solution was slowly added dropwise to the cobalt-precipitant mixture under vigorous stirring for 30 minutes. The mixture was then subjected to hydrothermal reaction at 170 °C for 7 hours, followed by centrifugation, washing, drying (60 °C, 12 h), and calcination (450 °C, 2 h, 1 °C min). -1The resulting product is denoted as CoWO4(20P), which is the preferred CoWO4(P) material of this invention.

[0060] Example 4 A method for preparing cobalt tungstate nanosheet gas-sensitive material includes the following steps: 3 mmol of cobalt nitrate hexahydrate (Co(NO3)2·6H2O) and 30 mmol of oxalic acid were dissolved together in 40 mL of deionized water and stirred continuously to form a homogeneous solution. Separately, 3 mmol of sodium tungstate dihydrate (Na2WO4·2H2O) was dissolved in another 40 mL of deionized water and stirred thoroughly until completely clear. The sodium tungstate solution was slowly added dropwise to the cobalt-precipitant mixture under vigorous stirring for 30 minutes. The mixture was then subjected to a hydrothermal reaction at 170 °C for 8 hours, followed by centrifugation, washing, drying, and calcination (450 °C, 2 h, 1 °C min). -1 The resulting product is denoted as CoWO4(30P).

[0061] Example 5 A method for preparing a cobalt tungstate / tin dioxide composite gas-sensitive material includes the following steps: 151 mg of analytical grade tin dioxide (SnO2) powder was added to 10 mL of anhydrous ethanol and ultrasonically dispersed for 10 minutes to obtain a uniform SnO2 dispersion. Separately, the CoWO4(P) nanosheet powder prepared in Example 3 was added to another 10 mL of anhydrous ethanol and ultrasonically dispersed for 10 minutes based on the theoretical atomic loading of 2 at%, to obtain a CoWO4(P) dispersion. The two dispersions were mixed and ultrasonically treated for another 20 minutes, followed by magnetic stirring at room temperature for 24 hours to promote uniform loading and interfacial bonding of CoWO4(P) nanosheets on the SnO2 surface. After the reaction, the mixture was centrifuged at 10,000 rpm for 10 minutes, the precipitate was collected, and washed three times with anhydrous ethanol to remove residual solvent. The resulting solid was dried at 60 °C for 10 hours to obtain a 2 at% CoWO4(P)-SnO2 composite gas-sensitive material.

[0062] Example 6 A method for preparing a cobalt tungstate / tin dioxide composite gas-sensitive material includes the following steps: 151 mg of analytical grade tin dioxide (SnO2) powder was added to 10 mL of anhydrous ethanol and ultrasonically dispersed for 10 minutes. Separately, the CoWO4(P) nanosheet powder prepared in Example 3 was added to another 10 mL of anhydrous ethanol, and the required mass was calculated based on a theoretical atomic loading of 6 at%, and ultrasonically dispersed for 10 minutes. The two dispersions were mixed, ultrasonicated for 20 minutes, and then magnetically stirred for 24 hours. Subsequently, the mixture was centrifuged (10000 rpm, 10 min), washed three times with ethanol, and dried at 60°C for 10 hours to obtain a 6 at% CoWO4(P)-SnO2 composite gas-sensitive material. This sample exhibited the best triethylamine response characteristics in subsequent performance tests and is a preferred embodiment of the present invention.

[0063] Example 7 A method for preparing a cobalt tungstate / tin dioxide composite gas-sensitive material includes the following steps: 151 mg of analytical grade tin dioxide (SnO2) powder was dispersed in 10 mL of anhydrous ethanol and sonicated for 10 minutes. Separately, the required mass of CoWO4(P) nanosheet powder prepared in Example 3 was calculated based on a theoretical atomic loading of 10 at%, and added to 10 mL of anhydrous ethanol and sonicated for 10 minutes. The two dispersions were mixed, sonicated for 20 minutes, and magnetically stirred for 24 hours. The mixture was then centrifuged (10,000 rpm, 10 min), washed three times with ethanol, and dried at 60°C for 10 hours to obtain a 10 at% CoWO4(P)-SnO2 composite gas-sensitive material.

[0064] Example 8 A method for preparing a cobalt tungstate / tin dioxide composite gas-sensitive material (CoWO4 without precipitant control) includes the following steps: 151 mg of analytical grade tin dioxide (SnO2) powder was added to 10 mL of anhydrous ethanol and ultrasonically dispersed for 10 minutes to obtain a uniform dispersion. Separately, CoWO4 powder (without precipitant) prepared in Example 1 was added to another 10 mL of anhydrous ethanol and ultrasonically dispersed for 10 minutes, based on the theoretical atomic loading of 6 at%. The two dispersions were mixed and ultrasonically treated for 20 minutes, followed by magnetic stirring for 24 hours. After the reaction, the mixture was centrifuged at 10,000 rpm for 10 minutes, the precipitate was collected, washed three times with anhydrous ethanol, and dried at 60°C for 10 hours to obtain a 6 at% CoWO4-SnO2 composite gas-sensitive material. This sample was used for performance comparison with Example 6 (6 at% CoWO4(P)-SnO2) to verify the effect of precipitant regulation on improving the oxidation and gas-sensing properties of the CoWO4 surface.

[0065] Example 9 A method for preparing a gold-modified tin dioxide gas-sensitive material includes the following steps: 151 mg of analytical grade tin dioxide (SnO2) powder was added to 10 mL of anhydrous ethanol and ultrasonically dispersed for 10 minutes. Separately, an appropriate amount of chloroauric acid (HAuCl4·xH2O) was dissolved in 10 mL of anhydrous ethanol, and the required mass was calculated based on a theoretical gold loading of 6 at%, and ultrasonically dispersed for 10 minutes. The two solutions were mixed and ultrasonicated for 20 minutes, then magnetically stirred for 24 hours. The mixture was then centrifuged (10000 rpm, 10 min), washed three times with ethanol, and dried at 60 °C for 10 hours to obtain the Au precursor / SnO2 complex. The dried product was placed in a tube furnace and heated at 1 °C for 1 min under a mixed atmosphere of H2 / N2 (10 vol% H2). -1 The temperature was increased to 300°C and held for 2 hours to reduce Au ions to metallic Au nanoparticles. After natural cooling, a 6 at% Au-SnO2 gas-sensitive material was obtained. This sample served as a control for the noble metal-modified system, used to compare its cost and gas-sensitive performance with the non-noble metal CoWO4(P)-SnO2 system of this invention.

[0066] Performance testing Figure 1 shows the (a) XRD patterns and (b) FTIR spectra of Examples 1-4 of the present invention. From the XRD patterns ( Figure 1 a) It can be seen that all diffraction peaks of the prepared sample match the standard spectrum of monoclinic CoWO4 (JCPDS No. 72-0479), and no other impurity peaks are observed, indicating high sample purity. The corresponding FTIR spectrum (Figure 1b) further confirms the structural integrity of CoWO4. At 512 cm⁻¹ -1 610 cm -1 and 834 cm -1 A distinct absorption peak appears at [value], corresponding to the vibrational absorption of Co-O, WO, and OWO bonds, respectively. Additionally, [values] appear at 2900–3000 cm⁻¹. -1 The absorption peaks at each position are attributed to hydroxyl groups (-OH). Notably, no characteristic absorption peaks related to carbon were detected in the spectrum, indicating that the precipitant was completely decomposed and oxidized during calcination, and the resulting CoWO4 sample has excellent chemical purity.

[0067] Figure 2 shows the peak fitting quantitative analysis results of (a) the full XPS spectrum, (b) the Co 2p high-resolution XPS spectrum, (c) the W 4f high-resolution XPS spectrum, (d) the O 1s high-resolution XPS spectrum, and (e) the O 1s spectrum for Examples 1-4 of this invention. The XPS full spectrum (Figure 2a) shows obvious characteristic peaks of Co 2p, W 4f, and O 1s, confirming the presence of Co, W, and O elements in the sample; no characteristic peaks of other elements were detected, indicating that the prepared CoWO4 has high chemical purity. In the Co 2p high-resolution spectrum (Figure 2b), the four characteristic peaks at binding energies of 781.3 eV, 783.1 eV, 797.5 eV, and 799.4 eV are attributed to Co, respectively. 3+ 2p 3 / 2 Co 2+ 2p 3 / 2 Co 3+ 2p 1 / 2 and Co 2+ 2p 1 / 2 The high-resolution spectrum of W 4f (Figure 2c) shows characteristic peaks at 35.4 eV, 36.4 eV, 37.4 eV, and 38.5 eV, which, after peak fitting, correspond to W... 5+ 4f 7 / 2 W 6+ 4f 7 / 2 W 5+ 4f 5 / 2 and W 6+ 4f 5 / 2 Orbit. Notably, compared to pure CoWO4, the W 4f characteristic peak of the precipitant-assisted prepared sample shows a significant positive shift towards the high binding energy direction, indicating a decrease in the electron cloud density around tungsten atoms. This positive shift mainly stems from the synergistic effect of precipitant-assisted surface reduction and oxygen vacancy formation. The O 1s high-resolution spectrum (Figure 2d) can be fitted with three characteristic peaks, with binding energy centers located at 530.9 eV, 532.2 eV, and 533.3 eV, respectively, corresponding to lattice oxygen (O). L ), oxygen vacancy (O V ) and chemically adsorbed oxygen (O C ).

[0068] Table 1 shows the quantitative analysis data of each component of O 1s.

[0069] Quantitative analysis of the components of O 1s (Table 1) showed that the CoWO4(20P) sample had the highest proportion of chemically adsorbed oxygen, reaching 30.7%. Since chemiluminescence resistive gas sensors primarily rely on the redox reaction between target gas molecules and surface chemically adsorbed oxygen species to achieve gas-sensing response, the content of chemically adsorbed oxygen is significantly positively correlated with the improvement of gas-sensing performance. Therefore, this invention identified CoWO4(20P) as the optimal sample and denoted it as CoWO4(P) for subsequent performance studies.

[0070] Figure 3 These are transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images of the cobalt tungstate nanosheets and cobalt tungstate / tin dioxide composite gas-sensitive materials prepared in Examples 3 and 6 of this invention. It can be seen from the images that the CoWO4(P) sample obtained in Example 3 exhibits a slender rectangular morphology, with an average length of approximately 60 nm and an average width of approximately 21 nm. Figure 3 a). TEM images of the 6 at% CoWO4(P)-SnO2 composite material show that rectangular cobalt tungstate (CoWO4) nanosheets are uniformly loaded on the tin dioxide (SnO2) surface. Figure 3 b). The corresponding HRTEM images show lattice fringes with interplanar spacings of 0.293 nm and 0.335 nm, corresponding to the (-111) plane of CoWO4(P) and the (110) plane of SnO2, respectively. Figure 3 c). HRTEM images further confirmed the interfacial coupling between CoWO4(P) and SnO2. Figure 3 d).

[0071] Figure 4 The above are energy dispersive spectroscopy (EDS) elemental distribution maps of the samples prepared in Examples 3 and 6 of this invention. The EDS elemental distribution map of the 6 at%CoWO4(P)-SnO2 composite material shows that Co and W are uniformly spatially distributed in the SnO2 matrix, proving that CoWO4(P) has been successfully loaded into the SnO2 matrix.

[0072] Figure 5 The images show (ab) transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images, and (c) energy dispersive spectroscopy (EDS) elemental distribution map of the 6 at% Au-SnO2 sample prepared in Example 9 of this invention. For the 6 at% Au-SnO2 sample, the HRTEM and EDS images show that gold (Au) nanoparticles are uniformly dispersed and loaded on the SnO2 surface, with a particle size range of 25–36 nm; the measured interplanar spacing is 0.236 nm, corresponding to the (111) crystal plane of metallic Au.

[0073] Figure 6 The following are high-resolution XPS spectra of Sn 3d for (a) commercial SnO2, Examples 5-7, and Example 9; (b) high-resolution XPS spectra of O 1s for the above samples; and (c) high-resolution XPS spectra of Sn 3d for the NH3 temperature-programmed desorption (NH3-TPD) curves of commercial SnO2, Example 9, and Example 6. Two characteristic peaks appear in the Sn 3d high-resolution XPS spectra, with binding energies of 486.6 eV and 495.0 eV, respectively, corresponding to Sn 3d... 5 / 2 and Sn 3d 3 / 2 Orbital. With increasing CoWO4 loading, both characteristic peaks gradually shifted towards lower binding energies; among them, the negative shift in binding energy of the 6 at% CoWO4(P)-SnO2 sample was significantly greater than that of the Au-modified corresponding sample (6 at% Au-SnO2), indicating that it has a stronger interfacial electronic modulation effect. High-resolution XPS spectral analysis showed that the relative adsorbed oxygen contents of commercial SnO2, 6 at% Au-SnO2, 2 at% CoWO4(P)-SnO2, 6 at% CoWO4(P)-SnO2, and 10 at% CoWO4(P)-SnO2 samples were 5.4%, 10.3%, 15.7%, 18.5%, and 13.3%, respectively. The adsorbed oxygen content of the 6 at% CoWO4(P)-SnO2 sample was significantly higher than that of the 6 at% Au-SnO2 sample, indicating that CoWO4(P) can provide a more efficient catalytic environment for oxygen activation. Three desorption peaks appeared in the NH3 temperature-programmed desorption (NH3-TPD) curve, with temperature ranges of 50–300 °C, 300–500 °C, and 500–600 °C, corresponding to weakly adsorbed NH3, strong Lewis acid sites, and strong Brønsted acid sites, respectively. Importantly, compared with commercial SnO2 and 6 at% Au-SnO2 samples, the 6 at% CoWO4(P)-SnO2 sample had significantly higher densities of strong Lewis acid sites (75.7%) and strong Brønsted acid sites (12.7%).

[0074] Figure 7 (a) UV-Vis DRS; (b) Tauc plot of commercial SnO2, 6 at% Au-SnO2 prepared in Example 9, and 6 at% CoWO4(P)-SnO2 prepared in Example 6. Figure 8(a) Nitrogen adsorption-desorption isotherms (BET) and (b) pore size distribution curves (BJH method) for commercial SnO2, 6 at% Au-SnO2 prepared in Example 9, and 6 at% CoWO4(P)-SnO2 prepared in Example 6. The band gap and specific surface area of ​​the commercial SnO2, 6 at% Au-SnO2, and 6 at% CoWO4(P)-SnO2 samples were measured to be 3.48 eV / 4.3 m, respectively. 2 ·g -1 3.32 eV / 5.1 m 2 ·g -1 and 3.20 eV / 4.1 m 2 ·g -1 It is noteworthy that the band gap of the 6 at% CoWO4(P)-SnO2 sample is narrower than that of the commercial SnO2 and 6 at% Au-SnO2 samples. In contrast, the specific surface area of ​​this sample is basically the same as that of the commercial SnO2, indicating that its performance improvement is mainly due to electronic modulation rather than morphological effects.

[0075] Figure 9 The following are the response values ​​of the commercial SnO2 gas sensors of this invention and Examples 5-9 at 80% relative humidity to 10 ppm triethylamine at different operating temperatures (a); and (b) the response-recovery curves of 10 ppm triethylamine at 100 °C. To better simulate actual meat storage conditions, the gas sensitivity performance tests were conducted at 80% relative humidity (RH). The 6 at% CoWO4(P)-SnO2 gas sensor exhibited the highest response value (6.34) at the lowest operating temperature (100 °C), which is 1.86 times that of the 6 at% Au-SnO2 sensor (response value 3.41 at 160 °C) and 2.23 times that of the commercial SnO2 sensor (response value 2.84 at 260 °C). The dynamic response-recovery curves further demonstrate that, compared with commercial SnO2 sensors, the 6 at% CoWO4(P)-SnO2 sensor exhibits significantly faster response and recovery kinetics, with response and recovery times shortened to 98 s and 52 s, respectively.

[0076] Figure 10 The following are the response-recovery curves of the commercial SnO2 gas sensors of this invention (a) to 0.5-20 ppm triethylamine at the optimal operating temperature; (b) the response-recovery curves of 6 at% Au-SnO2 and 6 at% CoWO4-SnO2 sensors to 1-20 ppm triethylamine at the optimal operating temperature; (cd) the corresponding response value fitting curves for all the above sensors; and (e) the selectivity test results for 100 ppm of various interfering gases (including triethylamine, ethanol, acetone, ammonia, methanol, benzene, and formaldehyde). Figure 10 As shown in Figure a, the 6 at% CoWO4(P)-SnO2 sensor exhibits highly reversible and repeatable response-recovery cycles across the entire test concentration range, enabling reliable detection of both low and high concentrations of triethylamine, demonstrating excellent gas-sensitive repeatability and operational stability. Furthermore, the theoretical detection limits of commercial SnO2, 2 at% CoWO4(P)-SnO2, 6 at% CoWO4(P)-SnO2, 10 at% CoWO4(P)-SnO2, 6 at% Au-SnO2, and 6 at% CoWO4-SnO2 sensors are 595 ppb, 102 ppb, 86 ppb, 168 ppb, 225 ppb, and 273 ppb (10c-d), respectively. Therefore, the theoretical detection limit of the 6 at% CoWO4(P)-SnO2 sensor is significantly lower than that of the SnO2 sensor and its Au-loaded counterpart. Figure 10 As shown in Figure e, the selective response results of the six sensor systems to 10 ppm ethanol, methanol, acetone, xylene, ammonia, trimethylamine and triethylamine show that each sensor has obvious gas recognition capabilities, and the 6 at% CoWO4(P)-SnO2 sensor shows significant and extremely strong selectivity for triethylamine.

[0077] Figure 11 The following figures illustrate the response-recovery stability of the commercial SnO2 gas sensor of this invention (Examples 5-9): (a) response-recovery stability curves for 125 cycles of 10 ppm triethylamine at 80% relative humidity; (b) response-recovery stability curves for 6 cycles of 10 ppm triethylamine at 80% relative humidity; and (c) long-term stability test results of the 6at% CoWO4(P)-SnO2 sensor. As shown, the 6at% CoWO4(P)-SnO2 sensor maintained essentially stable gas-sensing performance during 125 consecutive cycles, far exceeding most reported advanced metal-oxide-semiconductor gas sensors. The long-term stability test curves further confirm the excellent stability of the 6at% CoWO4(P)-SnO2 sensor; after 120 days of continuous operation, its gas-sensing response value remained approximately 91.3% of its initial value.

[0078] Figure 12 Photos of frozen and refrigerated salmon taken at different times. Neither sample showed significant changes in appearance within the first 16 hours, highlighting the necessity of real-time freshness testing to ensure food safety.

[0079] Figure 13(a) Response-recovery curves of the 6 at% CoWO4(P)-SnO2 gas sensor prepared in Example 6 of this invention to triethylamine (TEA) released from frozen and refrigerated salmon at 0, 2, 4, 6, 8, 16, and 24 hours; (c) Comparison of the changes in triethylamine concentration and spoilage rate of frozen and refrigerated salmon over time during storage. The dynamic response-recovery curves of the 6 at% CoWO4(P)-SnO2 sensor to triethylamine released from two salmon samples. This sensor exhibits a significant and reversible response to gradually increasing triethylamine concentrations, confirming its ability to continuously track the freshness of salmon. Figure 13 The results (a) show the real-time response values ​​and corresponding quantitative results for triethylamine concentration: refrigerated salmon released detectable triethylamine within the second hour of storage, while the release of triethylamine from frozen salmon within the first four hours of storage was negligible. A linear fit was performed on the triethylamine concentration (y) and storage time (x), yielding the following correlation: y = 0.48x - 1.30 for frozen salmon and y = 0.70x - 0.64 for refrigerated salmon. Since 5 ppm of triethylamine is considered the standard for food to be unusable, based on the above correlation, the theoretical edible threshold for frozen salmon was estimated to be 13.1 hours, and for refrigerated salmon, it was estimated to be 8.1 hours.

[0080] Figure 14 This is a schematic diagram of the gas-sensitive element prepared in Embodiment 6 of the present invention. The gas-sensitive element includes: a gold electrode, a platinum (Pt) wire, a gas-sensitive material, and a ceramic substrate.

[0081] 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 cobalt tungstate / tin dioxide nanosheet composite gas-sensitive material, characterized in that: Includes the following steps: Cobalt salt and tungstate were homogeneously precipitated in the presence of a precipitant to obtain CoWO4 precursor. The CoWO4 precursor was then calcined in air to obtain CoWO4(P) nanosheets. CoWO4(P) nanosheets and SnO2 powder were dispersed in an alcohol solvent in a certain proportion, and then subjected to ultrasonication and stirring. The mixture was then centrifuged, washed, and dried to obtain the final product.

2. The method for preparing the cobalt tungstate / tin dioxide nanosheet composite gas-sensitive material according to claim 1, characterized in that: The molar ratio of cobalt salt to tungstate is 1:0.9-1.1, preferably 1:1; Alternatively, the precipitant may be selected from urea, hexamethylenetetramine, acetamide, or oxalic acid; Alternatively, the cobalt salt is selected from at least one of Co(NO3)2·6H2O, CoCl2·6H2O, and Co(CH3COO)2·4H2O; Alternatively, the tungstate is Na2WO4·2H2O or Na2WO4·xH2O; Preferably, the precipitant is hexamethylenetetramine or acetamide.

3. The method for preparing the cobalt tungstate / tin dioxide nanosheet composite gas-sensitive material according to claim 1, characterized in that: The molar ratio of the precipitant to the cobalt salt is 0.1-10:1, preferably 2-10:1; Alternatively, the mass ratio of the CoWO4(P) nanosheets to SnO2 powder is 1:2-50.

4. The preparation method of the cobalt tungstate / tin dioxide nanosheet composite gas-sensitive material according to claim 1, characterized in that: The alcohol solvent is selected from at least one of methanol, ethanol, or isopropanol; Preferably, the alcohol solvent is anhydrous ethanol.

5. The method for preparing the cobalt tungstate / tin dioxide nanosheet composite gas-sensitive material according to claim 1, characterized in that: The homogeneous precipitation is carried out by hydrothermal reaction, with a temperature of 150-200℃ and a reaction time of 4-18 h. Alternatively, the calcination temperature is 400-600℃, the calcination time is 1-4 h, and the heating rate is 1-10℃ / min. -1 .

6. The method for preparing the cobalt tungstate / tin dioxide nanosheet composite gas-sensitive material according to claim 1, characterized in that: The specific method of ultrasonic and stirring treatment is as follows: first, CoWO4(P) nanosheets and SnO2 powder are ultrasonically dispersed separately in an alcohol solvent for 10-30 min, then the two dispersions are mixed and ultrasonically dispersed for 20-40 min, and finally stirred for 12-36 h. Preferably, the CoWO4(P) nanosheets have a rectangular sheet structure with an average length of 40-80 nm and a width of 10-30 nm; the SnO2 powder has a particle size of 20-500 nm. Alternatively, the centrifugation speed is 10000-12000 rpm, and the centrifugation time is 5-20 min; Alternatively, the drying temperature is 60-70℃, and the drying time is 10-12 h.

7. A cobalt tungstate / tin dioxide nanosheet composite gas-sensitive material, characterized in that: It is prepared by any one of the preparation methods described in claims 1-6.

8. A gas-sensitive sensor, characterized in that: It includes a ceramic substrate and a sensing layer loaded on the surface of the substrate, wherein the material of the sensing layer is the cobalt tungstate / tin dioxide nanosheet composite gas-sensitive material as described in claim 7.

9. The cobalt tungstate / tin dioxide nanosheet composite gas-sensitive material of claim 7, or the gas-sensitive sensor of claim 8, as a gas-sensitive material for detecting triethylamine.

10. The cobalt tungstate / tin dioxide nanosheet composite gas-sensitive material of claim 7, or the gas-sensitive sensor of claim 8, is used for real-time monitoring of trace amounts of triethylamine released during the spoilage of aquatic products, so as to achieve food safety assessment and freshness early warning.

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