CN115128136B - Pd-LaFeO for detecting hydrogen sulfide3Gas-sensitive material and application thereof - Google Patents

Abstract

The invention discloses a Pd-LaFeO ₃ gas-sensitive material for detecting hydrogen sulfide and its application. The Pd-LaFeO ₃ gas-sensitive material is a nano material with a mesh structure; and the doping amount of Pd is 3wt%. The preparation method of the Pd-LaFeO ₃ gas-sensitive material is: La ₂ O ₃ , Fe(NO ₃ ) ₃ , PdCl ₂ , PEG and deionized water are mixed, and then HNO ₃ solution is added for mixed dissolution to obtain a mixed solution; the mixed solution is allowed to stand at room temperature, and then heated and stirred to obtain a mixed sol; the mixed sol is pre-sintered, ground and sintered again to obtain Pd-LaFeO _{3 powder. The Pd-LaFeO 3 powder} is then used to prepare a Pd-LaFeO ₃ sensor. The present invention synthesizes Pd-LaFeO ₃ by a sol-gel method, which has a high specific surface area and a high porosity, and is a gas-sensitive material with high response, high selectivity, low detection limit and high long-term stability.

Description

Pd-LaFeO3 gas-sensitive material for detecting hydrogen sulfide and its application

Technical Field

The invention relates to the technical field of perovskite gas-sensitive materials, and in particular to a Pd- _LaFeO3 gas-sensitive material for detecting hydrogen sulfide and applications thereof.

Background Art

H ₂ S is a colorless, highly toxic acidic gas. It has a special smell of rotten eggs, and even low concentrations of H ₂ S can damage the human sense of smell. At high concentrations, it has no smell (because high concentrations paralyze the olfactory nerve). In addition, H ₂ S is flammable and is a typical dangerous gas. H ₂ S gas is released during the decomposition of food and is also the cause of bad breath caused by periodontitis. The H ₂ S content in the exhaled gas of periodontitis patients is about 0.195ppm, and the H ₂ S content in the exhaled gas of healthy people is about 0.105ppm. Using the nose as a means of detecting H ₂ S can be fatal. Therefore, it is very necessary and important to detect extremely low concentrations of H ₂ S gas in a timely manner.

In recent years, the method of using MOSs (metal oxide semiconductor) gas sensors to detect the concentration of target gas has become increasingly popular. For example, smoke sensors in hotels, natural gas alarms in homes, etc. It has been reported that some MOSs as gas-sensitive materials show excellent gas-sensitive responses, such as LaFeO ₃ , SmFeO ₃ , PrFeO ₃ , HoFeO ₃ , NdFeO ₃ , YCoO ₃ , BaSnO ₃ , ZnSnO ₃ , YMnO ₃ , and for H ₂ S, the gas-sensitive materials include Pt-ZnO, Pd-ZnO, CuO/SnO ₂ , Pt-WO ₃ , WO ₃ , Pt-Fe ₂ O ₃ , CuO/CuFe ₂ O ₄ , Ag-SnO ₂ , LaFeO ₃ , YMnO ₃ , Sn-NiO, etc. MOSs, especially ABO ₃ perovskite materials, have the unique advantages of large specific surface area and abundant active sites, which can promote diffusion paths, increase adsorption of target gas molecules, and thus enhance their sensing ability. Although there are many reports on gas sensors detecting H ₂ S, the detection performance of gas-sensitive materials for H ₂ S is generally not good enough, and the detection limit does not reach the PPB level. Therefore, a LaFeO ₃ gas-sensitive material with a detection limit of ppb level is needed, which has high responsiveness, high selectivity and high stability for extremely low concentration H ₂ S gas detection or periodontitis diagnosis.

Summary of the invention

In view of the above prior art, the purpose of the present invention is to provide a Pd-LaFeO ₃ gas-sensitive material for detecting hydrogen sulfide and its application. The present invention synthesizes Pd-LaFeO ₃ by sol-gel method, which has high specific surface area and high porosity and is a gas-sensitive material with high response, high selectivity, low detection limit and high long-term stability.

To achieve the above object, the present invention adopts the following technical solution:

In a first aspect of the present invention, a Pd-LaFeO ₃ gas-sensitive material for detecting H ₂ S is provided. The Pd-LaFeO ₃ gas-sensitive material is a nano-material with a network structure. The doping amount of Pd in the Pd-LaFeO ₃ gas-sensitive material is 3 wt %.

Preferably, the Pd-LaFeO ₃ gas-sensitive material has a specific surface area of 17.53 m ² /g and an average pore diameter of 13.6 nm.

The second aspect of the present invention provides a method for preparing a Pd-LaFeO ₃ gas-sensitive material, comprising the following steps:

(1) La ₂ O ₃ , Fe(NO ₃ ) ₃ , PdCl ₂ , PEG and deionized water are mixed, and HNO ₃ solution is added to mix and dissolve to obtain a mixed solution; the mixed solution is allowed to stand at room temperature, and then heated and stirred to obtain a mixed sol;

(2) The mixed sol obtained in step (1) is pre-sintered, ground and then sintered again to obtain Pd-LaFeO ₃ powder, namely Pd-LaFeO ₃ gas-sensitive material.

Preferably, in step (1), the mass ratio of La ₂ O ₃ , Fe(NO ₃ ) ₃ , PdCl ₂ , PEG, HNO ₃ solution and deionized water is 162.9:242:5.322:100:100:100.

Preferably, the weight average molecular weight of the PEG is 20,000; and the volume concentration of the HNO ₃ solution is 97%.

Preferably, in step (1), the standing time is 3 hours; the heating is water bath heating, the heating temperature is 80° C., and the heating time is 24 hours.

Preferably, in step (2), the pre-sintering temperature is 100° C. and the time is 2 h; and the re-sintering temperature is 800° C. and the time is 6 h.

The third aspect of the present invention provides the use of Pd-LaFeO ₃ gas sensitive material in preparing Pd-LaFeO ₃ gas sensitive sensor.

In a fourth aspect of the present invention, a Pd-LaFeO ₃ gas sensor is provided, which is prepared by the following method: Pd-LaFeO ₃ powder is mixed with deionized water to form a paste slurry, and then the paste slurry is coated on a gas sensitive film, and aged at 200°C for 24 hours to obtain a Pd-LaFeO ₃ gas sensor.

A fourth aspect of the present invention provides the use of a Pd-LaFeO3 gas sensor in detecting H ₂ S gas or in non-invasively diagnosing periodontitis.

Beneficial effects of the present invention:

The present invention adopts a sol-gel method to synthesize Pd-LaFeO ₃ , which is a gas-sensitive material with high response, high selectivity, low detection limit and high long-term stability. It has a high specific surface area and a high porosity. Compared with LaFeO ₃ , Pd-LaFeO ₃ shows a higher response. Compared with other gases, 3wt% doped Pd-LaFeO ₃ has a higher selectivity for H ₂ S. In addition, Pd element doping as a catalyst greatly enhances the surface activity of the gas-sensitive material and greatly shortens the response recovery time. Finally, the present invention uses a gas-sensitive sensor to detect the H ₂ S concentration in the air around shrimps, and compares the results obtained by gas chromatography-mass spectrometry, and the error is within 10%. Experimental results show that Pd element doping greatly improves the response of LaFeO ₃ to H ₂ S gas, and it is feasible and effective to use it as a gas sensor to detect H ₂ S gas.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1: (a-d) Pd-LaFeO ₃ preparation process flow chart; (e) Gas sensor structure diameter - 82 g and gas sensor test system;

Figure 2: (a) XRD spectrum of 3wt% Pd-LaFeO ₃ ; (b) EDS spectrum of 3wt% Pd-LaFeO ₃ ; (cd) SEM images of pure LaFeO ₃ ; (ef) SEM images of 3wt% Pd-LaFeO ₃ ; (g) N2 adsorption-desorption isotherm and pore size distribution of 3wt% Pd-LaFeO ₃ nanocomposite materials (inset); (h) Specific surface area of LaFeO ₃ at different Pd doping levels;

Figure 3: (a) Response of Pd-LaFeO ₃ to 1 ppm H ₂ S at operating temperature; (b) Response of Pd-LaFeO ₃ to different concentrations of H ₂ S at operating temperature; (f) Linear relationship between response and H ₂ S concentration;

Figure 4: Dynamic resistance of LaFeO ₃ with different Pd doping amounts to different H 2 S concentrations; (a) Dynamic resistance of LaFeO ₃ to different H ₂ S concentrations, (b) Dynamic resistance of 1wt% Pd-LaFeO ₃ to different H 2 S concentrations, (c) Dynamic resistance of 3wt% Pd-LaFeO ₃ to different H ₂ S concentrations, (d) Dynamic resistance of 5wt% Pd-LaFeO ₃ to different H ₂ S concentrations;

Figure 5: Response recovery time of Pd-LaFeO ₃ to 1ppm H ₂ S at different operating temperatures; (a) response recovery time of LaFeO ₃ to 1ppm H ₂ S, (b) response recovery time of 1wt% Pd-LaFeO ₃ to 1ppm H ₂ S, (c) response recovery time of 3wt% Pd-LaFeO ₃ to 1ppm H ₂ S, (d) response recovery time of 5wt% Pd-LaFeO ₃ to 1ppm H ₂ S;

Figure 6: Comparison of the selectivity of Pd-LaFeO ₃ to 1 ppm H ₂ S exhaled by humans and several other common gases; (a) Comparison of the selectivity of LaFeO ₃ to 1 ppm H ₂ S and several other common gases, (b) Comparison of the selectivity of 1wt% Pd-LaFeO ₃ to 1 ppm H ₂ S and several other common gases, (c) Comparison of the selectivity of 3wt% Pd-LaFeO ₃ to 1 ppm H ₂ S and several other common gases, (d) Comparison of the selectivity of 5wt% Pd-LaFeO ₃ to 1 ppm H2S and several other common gases;

Figure 7: (a) Resistance curve of Pd-LaFeO ₃ under RH; (b) Response curve of Pd-LaFeO ₃ with RH; (c) Response curve of Pd-LaFeO ₃ with time extension;

Figure 8: (a) Charge state of the material surface at room temperature; (b) Charge state of the material surface at high temperature; (c) Resistance curve of 3wt% Pd-LaFeO ₃ at working temperature; (d) Change of charge state of the material surface after gas injection at high temperature; (e) Change of charge state of the material surface with gas concentration at high temperature;

Figure 9: Dynamic curve of H2S concentration around shrimp body over time.

DETAILED DESCRIPTION

It should be noted that the following detailed descriptions are illustrative and are intended to provide further explanation of the present application. Unless otherwise specified, all technical and scientific terms used herein have the same meanings as those commonly understood by those skilled in the art to which the present application belongs.

As introduced in the background technology section, gas-sensitive materials for detecting H ₂ S gas, such as Pt-ZnO, Pd-ZnO, CuO/SnO ₂ , Pt-WO ₃ , WO ₃ , Pt-Fe ₂ O ₃ , CuO/CuFe ₂ O ₄ , Ag-SnO ₂ , LaFeO ₃ , YMnO ₃ , and Sn-NiO, cannot meet the detection requirements of extremely low concentrations, and the detection limit does not reach the PPB level.

Based on this, the purpose of the present invention is to provide a Pd-LaFeO ₃ gas-sensitive material for detecting hydrogen sulfide and its application. As shown in FIG1 , the present invention first uses La ₂ O ₃ , Fe(NO ₃ ) ₃ , PdCl ₂ , PEG, HNO ₃ solution and deionized water to prepare a sol-gel; then the sol-gel is pre-sintered, ground and re-sintered to prepare a Pd-LaFeO ₃ gas-sensitive material. Finally, the Pd-LaFeO ₃ gas-sensitive material is added to deionized water to prepare a paste slurry, the slurry is coated on a gas-sensitive film, and a Pd-LaFeO ₃ gas-sensitive sensor is obtained after drying. The Pd-LaFeO ₃ nanopowder prepared by the present invention has a mesh structure, so that it has a large specific surface area and high porosity. In addition, the Pd element is doped as a catalyst, which greatly enhances the surface activity of the gas-sensitive material and greatly shortens the response recovery time. Compared with LaFeO ₃ , Pd-LaFeO ₃ shows a higher response. In order to enable those skilled in the art to more clearly understand the technical solution of the present application, the technical solution of the present application will be described in detail below in conjunction with specific embodiments.

The test materials used in the examples of the present invention are all conventional test materials in the art and can be purchased through commercial channels.

Example

First, La ₂ O ₃ , Fe(NO ₃ ) ₃ , PdCl ₂ , PEG (M _W = 20000) and deionized water were added to a beaker in a mass ratio of 162.9:242:5.322:100:100, and then HNO ₃ (volume concentration of 97%) of the same mass as that of deionized water was added to mix and dissolve (Figures 1a-1b). The mixed solution was placed for 3 hours, then placed in a water bath and stirred at 80°C for 24 hours to obtain a mixed sol, and then the mixed sol was placed in a muffle furnace for pre-sintering at 100°C for 2 hours. After cooling, it was taken out and placed in a planetary grinder for ball milling for 30 minutes, and then placed in a muffle furnace at 800°C and sintered again for 6 hours (1c) to obtain a 3wt% doping amount of Pd-LaFeO ₃ powder.

Pd-LaFeO ₃ powder was mixed with deionized water to form a paste, and then the paste was placed on the gas-sensitive membrane (Figure 1d). A Ni-Cr wire was connected to heat the sensing material to achieve a higher operating temperature. The entire prepared sensor was then placed on an aging bench and aged at 200°C for 24 hours, as shown in Figure 1d, to obtain a 3wt% doped Pd-LaFeO ₃ gas sensor. Afterwards, the prepared sensor was tested for the target gas in a gas sensor test system (Figure 1e).

Comparative Example 1

The difference from the embodiment is that PdCl ₂ is not added, and a pure LaFeO ₃ sensor is prepared.

Comparative Example 2

The difference from the embodiment is that the amount of _PdCl2 added is adjusted to prepare a Pd- _LaFeO3 gas sensor with a doping amount of 1wt%.

Comparative Example 3

The difference from the embodiment is that the amount of _PdCl2 added is adjusted to prepare a Pd- _LaFeO3 gas sensor with a doping amount of 5wt%.

Materials properties and characterization;

Figure 2a is the X-ray diffraction analysis of 3wt% Pd-LaFeO ₃ at 40kV and 40mA (XRD, BrukerD8ADVANCE, CuKα content is ). Compared with the standard card (PDF card: 37-1493), it shows a single phase. The average particle size can be calculated using the Scherrer method. The Scherrer equation is as follows:

Where λ is the wavelength of the x-ray, β is the integral width of the diffraction peak, and θ is the Bragg diffraction angle. The average particle size of Pd-LaFeO ₃ is about 68.7nm. Due to the low doping amount of the Pd element, the characteristic peak cannot be reflected in the XRD spectrum, so the EDS spectrum of 3wt% Pd-LaFeO ₃ is performed to confirm the presence of the Pd element. As can be seen from Figure 2b, the Pd element is doped in the material. Figure 2c-f is a scanning electron microscope (SEM, Hitachi SU8010, 8.0kV) of LaFeO ₃ and 3wt% Pd-LaFeO3 at different magnifications. Undoped LaFeO ₃ presents a common perovskite structure, while 3%wt% Pd-LaFeO ₃ presents a network structure.

The specific surface area and porosity of 3wt% Pd-LaFeO ₃ powder were further analyzed by nitrogen adsorption-desorption measurement. Figure 2g shows that the specific surface area of 3wt% Pd-LaFeO ₃ is 17.53m ² /g and the average pore size is 13.6nm. The specific surface area of LaFeO ₃ with different Pd element doping amounts is shown in Figure 2h. It can be seen that when the Pd element doping amount is 3wt%, the composite powder obtains the maximum specific surface area. Since the doped Pd element can inhibit the growth of MOSs grains, the smaller the grain size, the larger the specific surface area. However, when the Pd element doping amount is too high, the particles will agglomerate in a small range and the specific surface area of the material will decrease. Specific surface area is an important factor for the sensing performance of the material. A larger specific surface area can provide more adsorption sites, which can enhance the reaction between the sensing material and the gas molecules, resulting in a high response to the test gas. Therefore, the 3wt% Pd-LaFeO ₃ material prepared in the embodiment has the largest specific surface area compared with comparative examples 1 to 3.

Test example:

1. Gas Sensing Performance Test

The test circuit of the gas sensor test system is shown in FIG1e. The gas-sensitive materials prepared in the embodiment and comparative examples 1 to 3 are coated on the sensing film. Vc is the power supply voltage, which is constant at 5V. The calculation formula of Rg is as follows:

Gas-sensitive response S is defined as Rg/Ra. Ra is the resistance of the sensor in air, and Rg is the resistance of the gas being measured. The response time is defined as the time required to reach 90% of the maximum value in the rising phase, and the recovery time is defined as the time required to recover 10% of the baseline value in the falling phase. Experimental environment: RH: 20%; ambient temperature: 20℃.

Figure 3a shows the response curves of LaFeO ₃ with different Pd doping amounts to 1ppm H2S with operating temperature. For all samples, the highest response was obtained at 120°C. The highest responses to 1ppm H2S were 8.26 (0wt% Pd), 17.85 (1wt% Pd), 36.29 (3wt% Pd) and 23.26 (5wt% Pd), respectively. It can be seen that the response has increased by more than 4 times compared with before doping with Pd element. Figures 3b-e show the response of all samples to 0.1-1ppm _H2S at operating temperature. It can also be seen that for any concentration of _H2S , the optimal operating temperature is 120°C. The response of LaFeO ₃ under different Pd element doping amounts is shown in Table 1.

Table 1 Response values of LaFeO ₃ with different Pd doping amounts to H ₂ S gas.

Note: The values in the table represent the response values to H ₂ S gas.

The relationship between the sensitivity of the material and the gas concentration is very important, and a highly fitted relationship can be used to predict the response value under a given gas concentration. The response relationship between Pd-LaFeO ₃ and different H ₂ S concentrations is shown in Figure 3f. It can be seen that for both undoped and doped Pd-LaFeO ₃ , the response has a good linear relationship with the gas concentration, and the R2 value is greater than 98%.

Repeatability is another important characteristic that determines whether a gas-sensitive material is good or not. For Pd-LaFeO ₃ , the repeatability of the response to different concentrations of H ₂ S gas is shown in Figures 4a-d. The repetitive process is as follows: When the resistance value of the gas-sensitive material is stable, H ₂ S gas is injected into the reaction chamber, and the resistance of the material increases immediately. After a period of time, the resistance stabilizes, and then the H ₂ S gas is removed, and the resistance of the material immediately decreases and can be restored to the initial state. For different concentrations of H ₂ S gas, after removing the H ₂ S gas, the resistance of the gas-sensitive material can be restored to the initial value every time, indicating that the material prepared in the embodiment has excellent repeatability.

In the present invention, at different operating temperatures, the response recovery time of the sensors prepared in the embodiment and comparative examples 1 to 3 are all different, indicating that the operating temperature affects the chemical reaction on the surface of the material. The response recovery time of the sensors prepared in the embodiment and comparative examples 1 to 3 is shown in Figures 5a-d. It can be seen that before reaching 120°C, the response recovery time increases with the increase of the operating temperature, and after 120°C, the response recovery time decreases with the further increase of the operating temperature. This may be due to the fact that before the optimal operating temperature, the adsorption rate of the gas molecules is higher than the desorption rate, and the number of oxygen ions and _H2S gas molecules adsorbed on the surface of the material increases, resulting in an increase in the reaction time. As the operating temperature increases, the adsorption rate and the desorption rate remain balanced at the optimal operating temperature, and the number of _H2S gas molecules and oxygen ions adsorbed on the surface of the material reaches a maximum value. At this operating temperature, the reaction time also reaches a maximum value. As the operating temperature further increases, the desorption rate of the gas molecules is higher than the adsorption rate, the reactants become less, and the reaction time becomes shorter. In addition, the reaction recovery time of 3wt% Pd- _LaFeO3 prepared in Example 1 is shortened by 2 times compared with _LaFeO3 prepared in Comparative Example 1.

In practical applications, it is common to detect a certain gas in a mixture, especially H ₂ S gas in the breath of real people. Therefore, the selectivity of the gas-sensitive material to a certain gas determines its practical application value. The selectivity comparison of 3wt% Pd-LaFeO ₃ to 1ppm H ₂ S and several other common gases exhaled by humans is shown in Figure 6a-d. It can be seen that compared with other gases, 3wt% Pd-LaFeO ₃ shows higher selectivity for H ₂ S gas. In particular, for common gases exhaled by humans such as N ₂ , O ₂ , NO, CO ₂ , CO, etc., the response can be ignored, so that H ₂ S in human exhaled gas can be detected more accurately.

The relative humidity (RH) in the environment is also a factor that cannot be ignored in the application of gas sensors. Figure 7a shows the change of the resistance of Pd-LaFeO ₃ with RH. For the sensors prepared in the embodiment and comparative examples 1 to 3, the resistance decreases with RH, but the reduction ratio is different. In the range of 20 to 90% RH, the reduction ratios are 44.13% (Comparative Example 1), 34.1% (Comparative Example 2), 19.46% (Example) and 26.46% (Comparative Example 3), respectively. It shows that the resistance of 3wt% Pd-LaFeO ₃ prepared in the embodiment shows the highest RH adaptability. RH will also affect the response of the gas-sensitive material to the target gas. Figure 7b shows the response of Pd-LaFeO ₃ to 1ppm H ₂ S under RH conditions. It can be seen that the response decreases with the increase of RH. Before 50% RH, the response is little affected by it. But after 50% RH, the response drops sharply. It shows that the gas sensor prepared by the present invention can be used in a low RH environment without considering the influence of RH. This will greatly expand its practical application field.

Long-term stability is another important characteristic of gas-sensitive materials. The higher the long-term stability, the longer the replacement cycle of the gas-sensitive material, the more economic and energy advantages it has. Figure 7c shows the long-term stability of Pd- _LaFeO3 within 30 days. The experimental data were obtained every 2 days. It can be seen that all responses decreased slightly over time, but the proportion of decrease was different. The proportions of decrease were 11.45% (Comparative Example 1), 3.6% (Comparative Example 2), 0.65% (Example) and 1.63% (Comparative Example 3), respectively. It can be seen that the long-term stability of 3wt% Pd- _LaFeO3 prepared in Example 1 is more than 17 times that of _LaFeO3 prepared in Comparative Example 1. The 3wt% Pd- _LaFeO3 prepared by the present invention shows a higher advantage in long-term stability.

2. Sensing mechanism analysis

Figure 8 shows the reaction mechanism of the whole experiment in this work. At room temperature (20℃), for p-type semiconductors, the main carrier of Pd-LaFeO ₃ is holes (h ^· ) (Figure 8). According to the Kroger-Vink defect symbol, holes are mainly composed of The ionization reaction may be as follows:

Oxygen molecules adsorbed on the surface of Pd-LaFeO ₃ continuously capture electrons from the material, resulting in an increase in hole counts (Figure 8b). At room temperature, the rate at which oxygen molecules capture electrons is very small and has little effect on the resistance value. However, as the operating temperature gradually increases, the capture rate on the surface of Pd-LaFeO ₃ also increases. Therefore, the resistance decreases with increasing operating temperature (Figure 8c). In addition, the work function of Pd is greater than that of LaFeO ₃ , and the free electrons on the surface of LaFeO ₃ are more easily transferred to Pd nanoparticles and form a depletion layer, which increases the resistance of the material in air. The reaction of oxygen molecules on the surface of Pd-LaFeO ₃ with free electrons may be as follows:

ads represents the state of oxygen adsorption on the surface of LaFeO _3. After the introduction of H ₂ S gas molecules, they will be adsorbed to the surface of LaFeO ₃ and react with oxygen ions (Figure 8d). The adsorption and desorption of H ₂ S gas molecules on the Pd-LaFeO ₃ surface exist simultaneously. The adsorption and desorption rates increase with the increase of operating temperature, and before the operating temperature reaches the optimal temperature, the adsorption rate is greater than the desorption rate. Therefore, the number of H ₂ S molecules adsorbed on the surface of the material increases, and the reaction between H ₂ S molecules and oxygen ions becomes more intense, resulting in an increase in response. When the operating temperature exceeds the optimal temperature, the adsorption rate of Pd-LaFeO ₃ for H ₂ S molecules is lower than the desorption rate, and the reaction intensity of H ₂ S molecules and oxygen ions decreases, resulting in a decrease in response. In addition, at the optimal temperature, as the concentration of H ₂ S gas molecules increases, the number of H ₂ S molecules adsorbed on the Pd-LaFeO ₃ surface increases, resulting in an increase in response (Figure 8e). However, the number of free electrons on the surface of Pd-LaFeO ₃ is not infinite, and the energy required for electron transitions inside Pd-LaFeO ₃ is also increasing. Therefore, the response (R _g /R _a ) increases with the concentration of H ₂ S gas molecules, but the rate of increase decreases. In addition, when free electrons are released from adsorbed oxygen ions to Pd-LaFeO ₃ , the width of the depletion layer caused by Pd becomes narrower, resulting in a large change in resistance. The reaction between H ₂ S molecules and oxygen ions may be like this:

e ^- +h ⁺ →null

Application Examples

H ₂ S is considered to be the most important gas released by the decomposition of marine food. The gas sensor prepared in the embodiment and gas chromatography-mass spectrometry (GC-MS) were used to detect the concentration of H ₂ S around the shrimp body over time, as shown in Figure 9. There were 9 shrimps in the experiment, and the length of each shrimp was about 10-16 cm. It can be seen that the concentration of H ₂ S increased with the increase of death time. The H 2 S concentration measured by the gas sensor was greater than the H ₂ S concentration measured by GC-MS at any time, which shows that there are other gases in the air around the shrimp, which can affect the gas sensor, but the effect is very small. By comparing the H ₂ S concentrations measured by the two methods, the error is within 10%.

The present invention adopts the sol-gel method to synthesize Pd-LaFeO ₃ , which has a large specific surface area and high porosity, and improves the response to a certain extent. The response of 3wt% Pd-LaFeO ₃ to H ₂ S is more than 4 times higher than that before doping with Pd element, and the long-term stability is more than 17 times that of pure LaFeO _3. Moreover, the response recovery time of 3wt% Pd-LaFeO ₃ is shortened by 2 times compared with pure LaFeO _3. In addition, the doping of Pd element as a catalyst greatly improves the RH adaptability and selectivity of the material. Finally, Pd-LaFeO ₃ is very accurate in detecting the H ₂ S gas concentration in the air around shrimp, and the error is less than 10% compared with the result obtained by GC-MS. The experimental results fully demonstrate the advantages of noble metal doping in improving the performance of gas-sensitive materials and the great potential of Pd-LaFeO ₃ in H ₂ S detection.

The above description is only the preferred embodiment of the present application and is not intended to limit the present application. For those skilled in the art, the present application may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims (6)

1. The Pd-LaFeO ₃ gas-sensitive material for detecting H ₂ S is characterized in that the Pd-LaFeO ₃ gas-sensitive material is a nano material with a net structure; the doping amount of Pd in the Pd-LaFeO ₃ gas-sensitive material is 3wt%;

The specific surface area of the Pd-LaFeO ₃ gas-sensitive material is 17.53 m ²/g, and the average pore diameter is 13.6 nm;

the Pd-LaFeO ₃ gas-sensitive material is prepared by the following method:

(1) Mixing La ₂O₃、Fe(NO₃)₃、PdCl₂, PEG and deionized water, and then adding HNO ₃ solution for mixing and dissolving to obtain a mixed solution; standing the mixed solution at room temperature, and then heating and stirring to obtain mixed sol;

(2) And (3) presintering the mixed sol obtained in the step (1), grinding and then sintering again to obtain Pd-LaFeO ₃ powder, namely the Pd-LaFeO ₃ gas-sensitive material.

2. The Pd-LaFeO ₃ gas sensitive material according to claim 1, wherein in step (1), the mass ratio of the La ₂O₃、Fe(NO₃)₃、PdCl₂、PEG、HNO₃ solution to deionized water is 162.9:242:5.322:100:100:100.

3. The Pd-LaFeO ₃ gas sensitive material according to claim 2, wherein the weight average molecular weight of the PEG is 20000; the volume concentration of the HNO ₃ solution is 97%.

4. The Pd-LaFeO ₃ gas sensitive material according to claim 1, wherein in step (1), the time of rest is 3h; the heating is water bath heating, the heating temperature is 80 ℃, and the heating time is 24 hours.

5. The Pd-LaFeO ₃ gas sensitive material according to claim 1, wherein in step (2), the pre-sintering temperature is 100 ℃ for 2 hours; the temperature of the re-sintering is 800 ℃ and the time is 6 hours.

6. The application of the Pd-LaFeO ₃ gas-sensitive material in preparing a sensor for detecting H ₂ S gas or noninvasively diagnosing periodontitis, wherein the sensor is prepared by the following method:

the Pd-LaFeO ₃ gas-sensitive material according to any one of claims 1-5 is mixed with deionized water to prepare paste slurry, then the paste slurry is coated on a gas-sensitive film, and the gas-sensitive film is aged for 24 hours at 200 ℃ to obtain the sensor for noninvasively diagnosing periodontitis.