A composite ammonia gas sensor and a preparation method and application thereof

Abstract

This invention discloses a composite ammonia sensor, its preparation method, and its application, belonging to the field of industrial and agricultural environmental monitoring technology. The preparation method involves producing cicada-like Fe2O3 via a hydrothermal reaction, followed by growing CdS quantum dots on the surface of the cicada-like Fe2O3 using a co-precipitation method to obtain Fe-Cd. Ncs Composite ammonia sensing material, preparation of Fe-Cd Ncs A slurry of composite ammonia sensing material and ethanol was prepared, and then coated onto a tubular electrode ceramic tube, which was subsequently welded onto the hexagonal base of the MQ sensor to obtain Fe-Cd. Ncs Composite ammonia sensor. This invention also discloses Fe-Cd prepared by the above-described method. Ncs A composite ammonia sensor and its application in ammonia residue detection. The beneficial effects of this invention are: constructing a CdS quantum well structure on the surface of a cicada-like Fe2O3, storing electrons inside CdS, endowing it with a high response value, and simultaneously exhibiting extremely high selectivity for ammonia, thus enabling Fe-Cd... Ncs The composite ammonia sensor has strong anti-interference capabilities, good stability, and high sensitivity, enabling it to detect ammonia quickly and promptly.

Description

Technical Field

[0001] This invention relates to the field of industrial and agricultural environmental monitoring technology, and in particular to a composite ammonia sensor, its preparation method, and its application. Background Technology

[0002] Ammonia (NH3), as a basic chemical raw material and a characteristic air pollutant, requires precise and efficient monitoring to ensure industrial production safety and achieve refined environmental pollution control. In industrial settings, ammonia leaks can easily lead to combustion, explosion, and poisoning accidents, directly threatening the safety of personnel and facilities. In the environmental field, it is not only a significant precursor to PM2.5 but also a major nitrogen source for eutrophication in water bodies, posing continuous pressure on the atmospheric environment and ecosystems. Although ambient air quality has been continuously improving, ammonia concentrations in some industrial parks and agricultural and pastoral areas still pose a risk of exceeding standards. Furthermore, in industries such as refrigeration, chemicals, and aquaculture, a pollution migration chain that is difficult to break has formed due to "equipment leakage - air diffusion - environmental deposition."

[0003] Current ammonia detection technology faces severe challenges in achieving a balance between room temperature, high sensitivity, and high selectivity, as existing major technological approaches all have inherent drawbacks that are difficult to overcome. While laboratory offline analysis methods, such as gas chromatography and mass spectrometry, offer extremely high accuracy, they are expensive, complex to operate, and have long analysis cycles, making them completely unsuitable for real-time on-site monitoring. Optical technologies, such as tunable diode laser absorption spectroscopy, possess good selectivity, but their systems are complex and costly, making stable deployment in complex industrial environments difficult. Furthermore, the core drawback of mainstream commercial metal-oxide-semiconductor (MOS) sensors lies in their reliance on high temperatures to drive surface redox reactions. This not only leads to high power consumption and safety hazards but also accelerates material aging and causes non-selective responses to various reducing gases at high temperatures, resulting in cross-interference.

[0004] While significant progress has been made in improving high-performance gas sensing at room temperature, existing research generally relies on specific material systems and optimization strategies. The performance enhancement mechanisms and universality of these methods still face inherent limitations. The controllable preparation and reproducibility of materials remain bottlenecks for practical applications. Performance breakthroughs are often based on heterostructures and noble metal modifications. Their cost, stability, and anti-interference capabilities in real complex atmospheres still require the development of more universal material design strategies.

[0005] The severe inadequacy of gas detection capabilities and accuracy under conditions of interference from other gases has become a key bottleneck restricting real-time gas monitoring and farm ecological assessment. Therefore, developing a new generation of high-performance composite ammonia sensors has urgent practical significance and broad application prospects. Summary of the Invention

[0006] The purpose of this invention is to provide a composite ammonia sensor, its preparation method, and its application. It innovates from the source of the sensing mechanism by regulating the interfacial electronic structure of the sensitive material, achieving high response, high selectivity, and high stability at room temperature. This solves the problem that existing ammonia detection methods have high requirements for equipment and detection technology, and are difficult to quickly detect ammonia residues in industrial and agricultural environments.

[0007] The objective of this invention is achieved through the following technical solution: A method for preparing a composite ammonia sensor includes the following steps: S1. Cicada pupa-like Fe2O3 was prepared by hydrothermal synthesis. S2. Fe-Cd was prepared by mixing cicada pupa-like Fe2O3 with a precursor solution containing Cd, S, and trimercaptopropionic acid (MPA), adding butylamine, and precipitating under a dark environment. Ncs Quantum dot composite ammonia gas sensing material; S3. Put Fe-Cd Ncs Quantum dot composite ammonia sensing material was subjected to ultrasonic and grinding treatment in an ethanol environment to obtain a mixed slurry. The mixed slurry was coated on a tubular electrode ceramic tube and dried to prepare a tubular electrode ceramic tube covered with a sensitive film. S4. Weld the tubular electrode ceramic tube covered with the sensitive film to the hexagonal base of the MQ sensor to obtain Fe-Cd. Ncs Composite ammonia gas sensor.

[0008] Furthermore, step S1 includes: X1. FeCl3•6H2O and water were added to a container in sequence, and Fe precursor solution was prepared in a water bath at 75℃. X2. Add NaOH and water to the container in sequence, stir to dissolve, and then slowly add the Fe precursor solution from step X1. Maintain a water bath environment of 75°C and continue stirring to obtain Fe(OH)3 colloidal solution. X3. Add Na2SO4 and water to the container in sequence, stir to dissolve, and then slowly add the Fe(OH)3 colloidal solution from step X2. Keep the container at 100℃, filter and dry to obtain cicada pupa-like Fe2O3.

[0009] Furthermore, step S2 includes: Y1. Add CdCl2•5H2O, MPA and water to a container in sequence, and then add KOH solution to adjust the pH to 12 to obtain a precursor solution containing MPA and Cd; Y2. Prepare a precursor solution containing S and mix it with a precursor solution containing MPA and Cd to obtain a precursor solution containing Cd, S and MPA; Y3. Fe-Cd was prepared by sequentially adding cicada-like Fe2O3 and butylamine to a precursor solution containing Cd, S, and MPA, followed by precipitation treatment in a dark environment, filtration, and drying. Ncs Quantum dot composite ammonia gas sensing material.

[0010] Furthermore, in step S4, a nickel-chromium alloy heating wire is added to the tubular electrode ceramic tube covered with a sensitive membrane, and the two ends of the nickel-chromium alloy heating wire are welded to the hexagonal base of the MQ sensor.

[0011] The present invention also provides a composite ammonia sensor, wherein the composite ammonia sensor is prepared by the above-described method for preparing Fe-Cd. Ncs Composite ammonia gas sensor.

[0012] This invention also provides an application of a composite ammonia sensor, specifically Fe-Cd. Ncs Application of composite ammonia sensor in ammonia residue detection.

[0013] Furthermore, the steps for detecting residual ammonia using the Fe-CdNcs composite ammonia sensor are as follows: Z1. Put Fe-Cd Ncs A composite ammonia sensor, along with a resistance card and a power supply, forms a series voltage divider system. Ammonia solution is injected into the heating stage of the gas-sensitive testing system to create ammonia atmospheres of varying concentrations. Based on Ohm's law, the corresponding ammonia concentrations in Fe-Cd gas are recorded during a current-voltage scan. Ncs The changing resistance value on the composite ammonia sensor; Z2. Based on the ammonia concentration and the corresponding peak resistance response from step Z1, a standard curve is constructed, and a linear equation is established. Z3. Put Fe-Cd Ncs A composite ammonia sensor is connected to a gas-sensitive testing system. The gas to be tested is injected into the test chamber of the gas-sensitive testing system, and the resistance response peak corresponding to the gas concentration is obtained. This peak value is then used in the linear equation constructed in step Z2 to calculate the ammonia content of the gas to be tested.

[0014] Furthermore, in step Z1, the injection volume of the ammonia solution is calculated using a gas mixing formula, which is as follows:

[0015] Among them, V x V is the injection volume, C is the gas concentration, M is the liquid molecular weight, d is the liquid specific gravity, p is the liquid purity, and T is the liquid concentration. r T represents room temperature. b Test chamber temperature.

[0016] The present invention has the following advantages: 1. The prepared composite ammonia sensor has strong anti-interference ability, good stability and high sensitivity, and can detect ammonia in a timely and rapid manner. It is suitable for rapid and accurate detection of ammonia content in industrial and agricultural production processes.

[0017] 2. Fe2O3 with high specific surface area, resembling a cicada pupa, is prepared by hydrothermal reaction. CdS quantum dots are then grown on the surface of the Fe2O3 via coprecipitation. The quantum well structure formed on the surface of the CdS stores electrons inside. When it comes into contact with a reducing gas, more electrons are released back into the conduction band, giving it a high response value.

[0018] 3. The special structure of the constructed CdS quantum dots prevents gas molecules with more complex molecular structures from contacting Fe2O3, so that other gases are only adsorbed on the surface of the material, thus exhibiting extremely high selectivity for ammonia gas with simple structure and small molecular weight. Attached Figure Description

[0019] Figure 1 Fe2O3 and Fe-Cd in the form of cicada pupae Ncs SEM image of quantum dot composite ammonia gas sensing material; Figure 2 for Fe-Cd Ncs Elemental spectrum of quantum dot composite ammonia gas sensing material; Figure 3 for Fe-Cd Ncs TEM image of quantum dot composite ammonia gas sensing material; Figure 4 for Fe-Cd Ncs XPS image of quantum dot composite ammonia gas sensing material; Figure 5 Fe2O3 sensor in the form of cicada pupa and Fe-Cd Ncs Performance test diagram of composite ammonia sensor; Figure 6 Fe2O3 sensor in the form of cicada pupa and Fe-Cd Ncs Standard curve of ammonia concentration versus resistance response peak value of composite ammonia sensor; Figure 7 Fe2O3 sensor in the form of cicada pupa and Fe-Cd Ncs Stability comparison chart of composite ammonia gas sensors.

[0020] Figure 8 Fe2O3 sensor in the form of cicada pupa and Fe-Cd Ncs Selectivity radar chart of composite ammonia sensor. Detailed Implementation

[0021] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. The components of the embodiments of the present invention described and shown in the accompanying drawings can generally be arranged and designed in various different configurations.

[0022] Therefore, the following detailed description of the embodiments of the invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but merely to illustrate selected embodiments of the invention. All other embodiments obtained by those skilled in the art based on the embodiments of the invention without inventive effort are within the scope of protection of the invention.

[0023] It should be noted that, unless otherwise specified, the embodiments and features described in this invention can be combined with each other.

[0024] It should be noted that similar labels and letters in the following figures indicate similar items. Therefore, once an item is defined in one figure, it does not need to be further defined and explained in subsequent figures.

[0025] In the description of this invention, it should be noted that the terms "center," "upper," "lower," "left," "right," "vertical," "horizontal," "inner," and "outer," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings, or the orientation or positional relationship commonly used when the product of this invention is in use, or the orientation or positional relationship commonly understood by those skilled in the art. They are only used for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of this invention. In addition, the terms "first," "second," etc., are only used to distinguish descriptions and should not be construed as indicating or implying relative importance.

[0026] In the description of this invention, it should also be noted that, unless otherwise explicitly specified and limited, the terms "set," "install," "connect," and "link" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection of two components. Those skilled in the art can understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0027] Example 1

[0028] A method for preparing a composite ammonia sensor includes the following steps: S1. Preparation of cicada-pupa-like Fe2O3 by hydrothermal synthesis: X1. Weigh 27.5g of FeCl3•6H2O and place it in a 200mL transparent glass reagent bottle. Add 50mL of water to the 200mL transparent glass reagent bottle, transfer it to a water bath, and heat it in a 75℃ water bath for 5min to obtain the Fe precursor solution. X2. Weigh 10.8g of NaOH and place it in a 100mL transparent glass reagent bottle. Add 50mL of water to the 100mL transparent glass reagent bottle and transfer it to a 75℃ water bath. Stir to dissolve and then slowly add the Fe precursor solution from step X1. Maintain the 75℃ water bath environment and continue stirring for 15min to obtain Fe(OH)3 colloidal solution. X3. Weigh 0.213g of Na2SO4 and place it in a transparent glass reagent bottle. Add 2.5mL of water to the transparent glass reagent bottle, stir to dissolve, and then slowly add the Fe(OH)3 colloidal solution from step X2. Incubate at 100℃ for 4 days, filter, and dry to obtain cicada pupa-like Fe2O3. Figure 1 (a) Figure 1 The SEM image of the pupal-shaped Fe2O3 shown in (b) reveals a relatively stable Fe2O3 morphology with a high specific surface area. This unique morphology endows the material with an extremely high specific surface area, significantly increasing the density of gas contact active sites on its surface. Under the same gas concentration conditions, the pupal-shaped Fe2O3, compared to Fe2O3 materials with traditional morphologies, can capture more gas molecules through physical adsorption. This enhanced adsorption capacity not only improves gas enrichment efficiency and allows for more thorough contact with the gas, but also creates favorable conditions for subsequent electron transfer between gas molecules and the material surface, releasing more effective electrons to participate in redox reactions, thereby significantly improving the gas-sensitive response performance of the material.

[0029] S2. Fe-Cd was prepared by mixing cicada-pupa-like Fe2O3 with a precursor solution containing Cd, S, and MPA, adding butylamine, and precipitating under a dark environment. Ncs Quantum dot composite ammonia sensing material: Y1. Weigh 1.827g of CdCl2•5H2O into a 200mL transparent glass reagent bottle, then add 2.79mL of MPA and 50mL of water, and add KOH solution to adjust the pH to 12 to obtain a precursor solution containing MPA and Cd. Y2. Weigh 0.304g of thiourea (TU) and place it in a 100mL transparent glass reagent bottle. Add 50mL of water to obtain a precursor solution containing S. Mix the precursor solution with the precursor solution containing MPA and Cd to obtain a precursor solution containing Cd, S and MPA. Y3. Fe-Cd precursor solution containing Cd, S, and MPA was sequentially supplemented with cicada-like Fe2O3 and 15 mL of butylamine. The mixture was allowed to precipitate in a dark environment for 3 days, then filtered and dried to obtain Fe-Cd. Ncs Quantum dot composite ammonia gas sensing material. Fe-Cd prepared by the above co-precipitation method. Ncs SEM image of quantum dot composite ammonia gas sensing material as shown below Figure 1 (c) Figure 1 As shown in (d), a comparison reveals that the previously prepared cicada pupa-like Fe2O3 substrate... Figure 1 (a) and Figure 1 In sample (b), the surface morphology is relatively smooth, the overall structure is clear and complete, and no obvious sediment or foreign particles are observed. However, in Figure 1 (c) and Figure 1 In (d), the Fe2O3 surface modified with CdS quantum dots underwent significant changes: on the one hand, the substrate material exhibited a certain degree of agglomeration, which stemmed from the surface energy modulation during quantum dot growth; on the other hand, a large number of white granular substances, distinct from the substrate material, were uniformly distributed on the originally smooth Fe2O3 surface. These white particles contrasted sharply with the intrinsic gray contrast of Fe2O3, and their high density and uniform size exhibited typical quantum dot loading characteristics. These morphological differences directly prove that CdS quantum dots have been successfully synthesized and firmly attached to the Fe2O3 substrate surface, verifying the Fe-Cd... Ncs Effective construction of composite materials.

[0030] This design first utilizes the unique structure of CdS quantum dot nanoclusters, enabling them to guide electron transfer at the interface and effectively store them within the CdS. When the composite material is exposed to the target gas ammonia (NH3), these bound electrons are released back into the conduction band, resulting in a significantly enhanced response value and achieving a highly sensitive gas-sensitive response. Simultaneously, the CdS quantum dots are uniformly distributed on the Fe2O3 surface in the form of nanoclusters, and their unique structure further endows the composite material with excellent gas-sensitive selectivity. On one hand, the sulfur bonds abundant in CdS quantum dots have a strong chemical affinity for H2S gas, preferentially adsorbing H2S molecules, thus hindering direct contact between H2S and the underlying Fe2O3 and subsequent electron release, reducing H2S interference. On the other hand, the high-density CdS quantum dot clusters on the Fe2O3 surface form a physical barrier, effectively blocking larger gas molecules such as trimethylamine, allowing the underlying Fe2O3 to only fully contact and transfer electrons with smaller molecules like NH3, thereby achieving a dual improvement in both sensitivity and selectivity for the composite ammonia sensor.

[0031] S3. Put Fe-Cd NcsQuantum dot composite ammonia gas sensing material was mixed with a small amount of ethanol in a mortar and then ultrasonically and simultaneously ground for 5 minutes in an ultrasonic chamber to obtain a uniformly dispersed slurry. The slurry was then applied to a ceramic electrode tube with a platinum wire guide using a micro-brush, ensuring uniform coverage of the sensing material without clogging the internal channels of the ceramic tube. After drying, a tubular electrode ceramic tube covered with a sensing film was prepared. Using ethanol as a highly volatile solvent and dispersion medium, the constructed Fe-Cd... Ncs The composite material is uniformly dispersed and coated onto the surface of the ceramic tube of the tubular electrode. During the drying process, the rapid evaporation of ethanol promotes the self-assembly of the material under capillary force, forming a uniform sensitive layer. At the same time, the low surface tension of ethanol can effectively reduce material agglomeration, and its good chemical inertness will not destroy the material activity, ultimately forming a gas-sensitive layer with a stable structure and good electrical contact on the ceramic tube.

[0032] S4. Weld the tubular electrode ceramic tube covered with the sensitive film to the hexagonal base of the MQ sensor to obtain Fe-Cd. Ncs Composite ammonia sensor. In this step, a nickel-chromium alloy heating wire is added to the tubular electrode ceramic tube covered with a sensitive membrane. The nickel-chromium alloy heating wire is passed through the tubular electrode ceramic tube, serving as a Fe-Cd... Ncs The heating unit of the composite ammonia sensor serves two purposes: firstly, it enhances the bonding strength between the sensitive layer and the tubular electrode ceramic tube; secondly, it increases the bonding strength between the sensitive layer and the tubular electrode ceramic tube. The two ends of the nickel-chromium alloy heating wire are welded to pins 3 and 4 of the hexagonal base of the MQ sensor, respectively, while the four original platinum wires of the tubular electrode ceramic tube are welded to pins 1, 2, 5, and 6 of the hexagonal base, thus completing the Fe-Cd... Ncs The overall assembly of the composite ammonia sensor.

[0033] Example 2

[0034] This embodiment provides Fe-Cd prepared by the preparation method described in Example 1. Ncs Composite ammonia gas sensor.

[0035] like Figure 2 As shown in (a), Fe-Cd Ncs The composite ammonia sensing material exhibits a composite structure with Fe2O3 as the substrate and CdS quantum dots uniformly grown on the surface. This morphological feature demonstrates the effective synthesis of the composite material. Specifically, it is composed of... Figure 2 (b) Figure 2 As can be seen in (c), the broad-spectrum and high-density coverage of Fe and O elements clearly indicates the presence of the Fe2O3 substrate. Its complete and continuous distribution provides ample attachment sites and structural support for the subsequent loading of CdS quantum dots. Meanwhile, Figure 2 (d) Figure 2Image (e) shows that Cd and S elements also exhibit a uniform and dispersed distribution, closely adhering to the Fe2O3 substrate surface. This fully confirms that CdS quantum dots have been successfully synthesized and well dispersed on the substrate surface; further combining... Figure 2 As shown in (f), the peak intensity analysis of the elemental energy spectrum reveals that although Fe and O elements dominate in terms of abundance due to their role as the substrate, Cd and S elements still exhibit significant characteristic peak intensities, indicating that CdS quantum dots have been effectively loaded onto the Fe2O3 surface. These multi-dimensional characterization results corroborate each other, not only confirming the successful composite of the Fe2O3 substrate and CdS quantum dots morphologically, but also quantitatively verifying the Fe-Cd composite in terms of elemental distribution and abundance. Ncs Successful construction of composite ammonia gas sensing material.

[0036] like Figure 3 The TEM characterization results shown further reveal Fe-Cd at the microscale. Ncs The fine structure and interfacial composition relationship of composite ammonia gas sensing materials. Figure 3 The low-resolution TEM image provided in (a) clearly shows that the Fe2O3 substrate, exhibiting solid contrast, is covered with a layer of other materials that are evenly distributed and have varying contrast. This obvious composite morphology initially indicates that the CdS component has been successfully loaded onto the Fe2O3 substrate. Further magnification reveals... Figure 3 In the high-resolution TEM image provided in (b), two different lattice fringes can be distinguished more precisely: one set of interplanar spacing is 0.26 nm, corresponding to the lattice characteristics of the Fe2O3 substrate; the other set of interplanar spacing is 0.23 nm, which is highly consistent with the standard value of interplanar spacing of CdS.

[0037] More importantly, the two lattice fringes exhibit a tightly interwoven or adjacent distribution interface, meaning that CdS quantum dots are tightly embedded between the Fe2O3 substrate, forming a good heterogeneous contact interface. This direct observation at a high resolution scale not only confirms the successful synthesis of CdS quantum dots on the Fe2O3 surface but also strongly demonstrates the formation of a tight interfacial contact between the two, thus providing direct structural evidence for the effective construction of quantum well structures. This heterogeneous interface formed at the atomic scale is the key structural basis for realizing the storage of electrons inside CdS and their directional release under the action of a target gas. In summary, the TEM characterization results corroborate the aforementioned SEM / EDS analysis, fully demonstrating the Fe-Cd... Ncs Successful composite material combining Fe2O3 substrate with CdS quantum dots and effective construction of heterogeneous interfaces.

[0038] like Figure 4The XPS analysis results shown further reveal the Fe-Cd relationship from the perspective of elemental chemical state and bonding environment. Ncs The successful construction of a composite ammonia gas sensing material. Among other things, Figure 4 In the high-resolution Fe 2p spectrum of (a), characteristic peaks at 711.9 eV and 725.5 eV are clearly observed, corresponding to the Fe 2p peaks of Fe2O3, respectively. 3 / 2 With Fe 2p 1 / 2 Furthermore, a distinct satellite peak (718.9 eV) accompanies the two peaks, which is due to Fe. 3+ The characteristic features indicate that the substrate material retains the complete chemical state of Fe2O3; while Figure 4 In the XPS analysis results shown in (b), a noteworthy phenomenon is the significant increase in the intensity of the characteristic peak corresponding to defective oxygen. This change in microstructure is not accidental, but rather lays a crucial foundation for subsequent optimization of gas-sensing performance. Meanwhile, from... Figure 4 In the Cd 3d spectrum of (c), a peak value at 404.9 eV (Cd 3d) can be observed. 5 / 2 ) and 411.6 eV (Cd 3d 3 / 2 The characteristic peaks of ) and Cd in CdS 2+ The standard binding energy corresponds precisely to the synthesis of CdS quantum dots, confirming their successful synthesis; furthermore, Figure 4 The S 2p spectrum of (d) at 161.8 eV (S 2p) 3 / 2 ) and 162.9 eV (S 2p 1 / 2 The presence of typical sulfide characteristic peaks at the () position further confirms that CdS exists in the form of pure sulfides. Based on the above analysis, it can be concluded that Fe-Cd... Ncs Both the Fe2O3 substrate and CdS quantum dots in the composite material retain their respective chemical state characteristics. The two form a closely contacted heterostructure through physical composite without phase transition or impurity phase generation. This result confirms the successful synthesis of the composite ammonia sensing material and the effective construction of its heterostructure interface from the perspective of surface chemical composition.

[0039] Example 3

[0040] This embodiment provides Fe-Cd from Example 2. Ncs Application of composite ammonia sensor in ammonia residue detection.

[0041] The steps for detecting residual ammonia using the Fe-CdNcs composite ammonia sensor are as follows: Z1. Put Fe-Cd NcsA composite ammonia sensor, along with a resistance card and a power supply, forms a series voltage divider system. Ammonia solution is injected into the heating stage of the gas-sensitive testing system to create ammonia atmospheres of varying concentrations. Based on Ohm's law, the corresponding ammonia concentrations in Fe-Cd gas are recorded during a current-voltage scan. Ncs The changing resistance value on the composite ammonia sensor: Gas-sensitive tests use solution evaporation to obtain the gas to be measured. Therefore, it is necessary to calculate the required injection volume of the liquid analyte based on the target gas concentration. Specifically, the injection volume of the ammonia solution is calculated using a gas mixing formula, as follows:

[0042] Among them, V x V is the injection volume, C is the gas concentration, M is the liquid molecular weight, d is the liquid specific gravity, p is the liquid purity, and T is the liquid concentration. r T represents room temperature. b Test chamber temperature. Based on the calculated results, a corresponding volume of ammonia solution was drawn and rapidly injected into the heating stage of the gas-sensitive testing system via a micro-syringe. The liquid vaporized and mixed with air to form the desired concentration of ammonia gas for testing. During the test, the ammonia concentration was adjusted multiple times, and based on Ohm's law, the values ​​of different ammonia concentrations in Fe-Cd were recorded during the current-voltage scan. Ncs The changing resistance value on the composite ammonia sensor.

[0043] like Figure 5 Gas-sensitivity test results for Fe-Cd Ncs The composite ammonia gas sensor exhibits significantly better overall gas sensing performance than unmodified cicada pupa-like Fe2O3 in terms of sensitivity, response recovery characteristics, stability, and linear detection range. Figure 5 Figure (a) shows the cyclic test response curves of the sensor under the condition of five consecutive injections of ammonia water of the same concentration. It can be seen from this that the Fe-Cd modified with CdS quantum dots... Ncs Compared to the unmodified pupal-shaped Fe2O3 sensor, the composite ammonia sensor exhibits a rapid and stable response to each ammonia exposure. Not only does its response value significantly exceed that of the unmodified pupal-shaped Fe2O3 sensor, but its response and recovery times remain short, demonstrating excellent repeatability and short-term stability. This result indicates that the composite material maintains stable electron transport properties and surface adsorption-desorption behavior even after multiple cycles, possessing good potential for practical applications. Figure 5 (b) Figure 5 (c) illustrates the unmodified cicada pupa-like Fe2O3 sensor and Fe-Cd sensor, respectively. Ncs Dynamic response curves of the composite ammonia sensor under concentration gradients from 5 ppm to 200 ppm and corresponding response-concentration fitting relationships. (Comparison) Figure 5 (b) Figure 5 In (c), it can be clearly seen that Fe-Cd Ncs The composite sensor exhibits a significantly superior response value compared to the unmodified pupal-shaped Fe2O3 sensor, and its response value increases systematically with increasing ammonia concentration, demonstrating good concentration dependence across the entire test concentration range. This characteristic offers a clear advantage over existing ammonia sensors. During detection, the resistance response value can be rapidly measured, with a low detection limit and high sensitivity, enabling rapid on-site detection of ammonia.

[0044] Z2. Using the ammonia concentration from step Z1 as the x-axis and the corresponding peak resistance response as the y-axis, a standard curve is created by fitting the linear equation using Origin software, and the linear equation is constructed as follows: R p =n+mX ammonia Where m is the slope, n is the intercept, and X ammonia R represents the concentration of ammonia gas. p This is the response value.

[0045] like Figure 6 The comparison of the standard curves shown can be seen that Fe-Cd Ncs The significant advantages of the composite ammonia sensor in quantitative detection performance. From Figure 6 As can be seen in (a), the unmodified cicada pupa-shaped Fe2O3 sensor detects ammonia, and its response value shows a general linear relationship with the concentration. The regression equation for the change in resistance response after fitting is R = 14.55 + 0.13443X. ammonia The correlation coefficient was 0.863; from Figure 6 As can be seen in (b), Fe-Cd Ncs The composite ammonia sensor has significant advantages; the regression equation for the change in resistance response after fitting is R = 23.694 + 0.43323X. ammonia The correlation coefficient was 0.998. Comparing the two sets of regression equations, it can be found that Fe-Cd Ncs The composite ammonia sensor exhibits excellent detection performance for ammonia, significantly outperforming the unmodified pupal-shaped Fe2O3 sensor in terms of detection linearity, sensitivity, and quantitative accuracy. This superior linear response characteristic lays a solid data foundation for the subsequent development of portable ammonia detection instruments based on this sensor, enabling precise quantification of ammonia concentration in practical applications and meeting detection needs in various scenarios, from environmental monitoring to industrial safety.

[0046] Z3. Put Fe-Cd NcsA composite ammonia sensor is connected to a gas-sensitive testing system. The gas to be tested is injected into the test chamber of the gas-sensitive testing system, and the resistance response peak corresponding to the gas concentration is obtained. This peak value is then used in the linear equation constructed in step Z2 to calculate the ammonia content of the gas to be tested.

[0047] This embodiment further focuses on Fe-Cd Ncs The stability and selectivity of the composite ammonia sensor were tested.

[0048] Stability test: Fe-Cd Ncs A composite ammonia sensor and an unmodified cicada pupa-shaped Fe2O3 sensor were connected to a gas-sensitive testing system. For a continuous 15-day period, the same concentration of ammonia gas was injected into the test chamber three times a day (morning, afternoon, and evening), and the sensor response values ​​were repeatedly tested. Response data was recorded for each test, and the arithmetic mean of the three test results for that day was used as the valid response value for that day, to minimize the random effects of instantaneous environmental fluctuations and operational errors. Figure 7 As shown, the results indicate that during continuous testing for up to 15 days, Fe-Cd Ncs The response value of the composite ammonia sensor did not show significant decay or drift, and the overall trend was close to a horizontal straight line. In contrast, the unmodified pupal-shaped Fe2O3 sensor began to decay in the later stages, which fully demonstrates the effectiveness of Fe-Cd. Ncs The sensor maintains a stable and repeatable output signal even under prolonged continuous operation. This excellent long-term stability is mainly attributed to Fe-Cd. Ncs Structural characteristics of the composite material: On the one hand, the heterogeneous interface constructed between CdS quantum dots and Fe2O3 substrate has high structural stability and is not prone to peeling or agglomeration under long-term gas adsorption-desorption cycle; on the other hand, the abundant active sites on the material surface are evenly distributed and stable, which can maintain consistent adsorption behavior of ammonia in repeated tests, avoiding performance degradation caused by surface passivation or deactivation of active sites.

[0049] Selective testing: Fe-Cd Ncs A composite ammonia sensor and an unmodified cicada-like Fe2O3 sensor were connected to a gas-sensitive testing system. First, ammonia water was injected onto the heating stage to create an ammonia atmosphere with a concentration of 100 ppm. The sensor's baseline response value to this target gas was recorded as a reference for subsequent comparative analysis. Based on this, four common volatile organic compounds—ethanol, acetone, trimethylamine, and hydrogen sulfide—were introduced sequentially as interfering gases to systematically examine the sensor's response behavior under a multi-component background. For each introduced interfering substance, three independent tests were performed under the same conditions, and the average value was taken to eliminate random errors. The results are as follows: Figure 8 As shown in the radar chart, the significant optimization of sensor selectivity brought about by CdS quantum dot modification can be clearly observed through intuitive graphical comparison. Figure 8 Image (a) shows the response distribution of the unmodified cicada-like Fe2O3 sensor to ammonia and various interfering gases. Its radar image covers a large area, indicating that the sensor exhibits varying degrees of cross-response to multiple gases such as ethanol, acetone, trimethylamine, and hydrogen sulfide, suggesting that its selectivity needs improvement. In contrast, from... Figure 8 As can be seen in (b), Fe-Cd formed after CdS quantum dot modification... Ncs The composite sensor significantly reduces the area occupied by its radar image, resulting in a more compact graphic profile. It maintains a prominent response peak only on the coordinate axis corresponding to ammonia, while effectively suppressing the response to other interfering gases to a low level.

[0050] Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing a composite ammonia sensor, characterized in that, Includes the following steps: S1. Cicada pupa-like Fe2O3 was prepared by hydrothermal synthesis. S2. Fe-Cd was prepared by mixing cicada-pupa-like Fe2O3 with a precursor solution containing Cd, S, and MPA, adding butylamine, and precipitating under a dark environment. Ncs Quantum dot composite ammonia gas sensing material; S3. Put Fe-Cd Ncs Quantum dot composite ammonia sensing material was subjected to ultrasonic and grinding treatment in an ethanol environment to obtain a mixed slurry. The mixed slurry was coated on a tubular electrode ceramic tube and dried to prepare a tubular electrode ceramic tube covered with a sensitive film. S4. Weld the tubular electrode ceramic tube covered with the sensitive film to the hexagonal base of the MQ sensor to obtain Fe-Cd. Ncs Composite ammonia gas sensor.

2. The method for preparing a composite ammonia sensor according to claim 1, characterized in that, Step S1 includes: X1. FeCl3•6H2O and water were added to a container in sequence, and Fe precursor solution was prepared in a water bath at 75℃. X2. Add NaOH and water to the container in sequence, stir to dissolve, and then slowly add the Fe precursor solution from step X1. Maintain a water bath environment of 75°C and continue stirring to obtain Fe(OH)3 colloidal solution. X3. Add Na2SO4 and water to the container in sequence, stir to dissolve, and then slowly add the Fe(OH)3 colloidal solution from step X2. Keep the container at 100℃, filter and dry to obtain cicada pupa-like Fe2O3.

3. The method for preparing a composite ammonia sensor according to claim 1, characterized in that, Step S2 includes: Y1. Add CdCl2•5H2O, MPA and water to a container in sequence, and then add KOH solution to adjust the pH to 12 to obtain a precursor solution containing MPA and Cd; Y2. Prepare a precursor solution containing S and mix it with a precursor solution containing MPA and Cd to obtain a precursor solution containing Cd, S and MPA; Y3. Fe-Cd was prepared by sequentially adding cicada-like Fe2O3 and butylamine to a precursor solution containing Cd, S, and MPA, followed by precipitation treatment in a dark environment, filtration, and drying. Ncs Quantum dot composite ammonia gas sensing material.

4. The method for preparing a composite ammonia sensor according to claim 1, characterized in that, In step S4, a nickel-chromium alloy heating wire is added to the tubular electrode ceramic tube covered with a sensitive membrane, and the two ends of the nickel-chromium alloy heating wire are welded to the hexagonal base of the MQ sensor.

5. A composite ammonia gas sensor, characterized in that, Fe-Cd is prepared by the method for preparing a composite ammonia sensor according to any one of claims 1-4. Ncs Composite ammonia gas sensor.

6. An application of a composite ammonia sensor, characterized in that, Fe-Cd as described in claim 5 Ncs Application of composite ammonia sensor in ammonia residue detection.

7. The application of the composite ammonia sensor according to claim 6, characterized in that, The Fe-Cd Ncs The steps for detecting residual ammonia using a composite ammonia sensor are as follows: Z1. Put Fe-Cd Ncs A composite ammonia sensor, along with a resistance card and a power supply, forms a series voltage divider system. Ammonia solution is injected into the heating stage of the gas-sensitive testing system to create ammonia atmospheres of varying concentrations. Based on Ohm's law, the corresponding ammonia concentrations in Fe-Cd gas are recorded during a current-voltage scan. Ncs The changing resistance value on the composite ammonia sensor; Z2. Based on the ammonia concentration and the corresponding peak resistance response from step Z1, a standard curve is constructed, and a linear equation is established. Z3. Put Fe-Cd Ncs A composite ammonia sensor is connected to a gas-sensitive testing system. The gas to be tested is injected into the test chamber of the gas-sensitive testing system, and the resistance response peak corresponding to the gas concentration is obtained. This peak value is then used in the linear equation constructed in step Z2 to calculate the ammonia content of the gas to be tested.

8. The application of the composite ammonia sensor according to claim 7, characterized in that, In step Z1, the injection volume of the ammonia solution is calculated using a gas mixing formula, which is as follows: Among them, V x V is the injection volume, C is the gas concentration, M is the liquid molecular weight, d is the liquid specific gravity, p is the liquid purity, and T is the liquid concentration. r T represents room temperature. b Test chamber temperature.

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