Ammonia gas sensor based on integration of p-n heterojunction and optical microfibers and preparation method of ammonia gas sensor

Abstract

The invention discloses an ammonia gas sensor based on integration of p-n heterojunction and optical microfibers and a preparation method of the ammonia gas sensor, and belongs to the technical field of gas sensing detection. In order to solve the problems that a traditional sensor is low in integration level and slow in response and an existing optical fiber sensor is insufficient in performance, cooperative enhancement detection is achieved through dynamic regulation and control of a strong evanescent wave field of optical microfibers and carriers of p-n heterojunctions. The surface of the optical microfibers is sequentially modified with an n-type SnO2 nanoparticle layer and a p-type PANI nanoparticle layer to form the p-n heterojunctions; ammonia molecule adsorption causes heterojunction carrier concentration change, and the change is converted into monitorable red shift of a transmission spectrum through strong interaction of evanescent waves and carriers. The invention has the advantages of room temperature work, high sensitivity, ultrafast response / recovery time, low temperature cross sensitivity, high specificity and the like, and can be widely applied to the fields of environmental monitoring, industrial safety and the like.

Description

Technical Field

[0001] This invention belongs to the field of gas sensing and detection technology, specifically relating to an ammonia sensor based on pn heterojunction and optical microfiber integration and its preparation method. By utilizing the synergistic effect of optical microfiber evanescent waves and pn heterojunction, it achieves high sensitivity, room temperature, rapid response, and real-time detection of ammonia, which is applicable to environmental monitoring, industrial safety, agricultural breeding and other fields. Background Technology

[0002] Ammonia is an alkaline gas with a pungent odor, widely present in various fields such as chemical engineering, agriculture, and environmental protection. However, ammonia is also a harmful gas; high concentrations can cause serious damage to the human respiratory system and eyes, and also pollute the environment. Therefore, accurate and rapid detection of ammonia is of great significance.

[0003] Currently, common ammonia gas sensors mainly include electrochemical sensors and semiconductor gas sensors. However, these traditional sensors have some limitations. For example, electrochemical sensors have a short lifespan, operate at high temperatures, and are easily affected by environmental factors; while semiconductor gas sensors have high sensitivity, they have long response times and poor selectivity.

[0004] In recent years, fiber optic sensors have attracted increasing attention in the field of gas sensing due to their advantages such as resistance to electromagnetic interference, small size, and long-distance transmission capability. However, existing fiber optic ammonia sensors still need further improvement in terms of sensitivity, response speed, detection limit, and stability.

[0005] In view of this, the present invention makes full use of the synergistic regulation mechanism of optical field and semiconductor heterojunction carriers to achieve simultaneous optimization of sensor sensitivity, response speed and anti-interference performance, thereby constructing an ammonia sensor based on pn heterojunction and optical microfiber integration. Summary of the Invention

[0006] The purpose of this invention is to propose an ammonia sensor based on the integration of pn heterojunction and optical microfiber and its fabrication method. The sensor achieves synergistic enhancement through the strong evanescent wave field of optical microfiber and the dynamic control of charge carriers in pn heterojunction, thereby solving the problems of low sensitivity, slow response and large temperature interference of traditional sensors.

[0007] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0008] An ammonia sensor based on pn heterojunction and optical microfiber integration is disclosed. The pn heterostructure is formed by sequentially modifying the surface of the optical microfiber with an n-type semiconductor layer and a p-type semiconductor layer. The n-type semiconductor layer is a SnO2 nanoparticle layer, and the p-type semiconductor layer is a polyaniline (PANI) nanoparticle layer. The evanescent wave generated by the optical microfiber during transmission interacts strongly with the charge carriers of the pn heterojunction. The quantitative detection of ammonia content is achieved by controlling the transmission spectrum shift through changes in charge carrier concentration.

[0009] As a preferred embodiment of the present invention, the optical microfiber is prepared by fused taper of a thin-core optical fiber, with a diameter of 12.58 μm, and decomposes into HE during transmission. 11 Fundamental model and HE 12 Higher-order modes form a cosine-type transmission spectrum through two-mode interference. The energy of higher-order modes is concentrated at the edge of the fiber cladding, enhancing the interaction between evanescent waves and surface heterojunctions.

[0010] As a preferred embodiment of the present invention, the thickness of the SnO2 nanoparticle layer is 100-300 nm, and the thickness of the polyaniline (PANI) nanoparticle layer is 80-200 nm; the depletion layer width of the pn heterojunction can be dynamically adjusted with the change of ammonia concentration, and the evanescent wave propagation characteristics are modulated by the dielectric constant to achieve a linear correlation between the spectral redshift and the ammonia concentration.

[0011] This invention constructs a synergistic mechanism of "optical microfiber evanescent wave field - pn heterojunction carrier modulation - spectral response": the sensor is composed of optical microfibers, an n-type SnO2 layer, and a p-type polyaniline (PANI) layer. In the sensor, the optical microfibers serve as the core light transmission carrier, and their micrometer-scale diameter (12.58 μm) enables the transmission of higher-order modes (HE) in the light field. 12 The energy is concentrated at the cladding edge, forming a strong evanescent wave field, providing an efficient coupling channel for the interaction between photons and semiconductor charge carriers. An n-type SnO2 layer (260 nm) and a p-type PANI layer (130 nm) form a pn heterojunction on the microfiber surface. The depletion layer width at the heterojunction interface dynamically changes with the ammonia concentration, modulating the dielectric constant of the semiconductor layer through charge carrier migration, thereby modulating the propagation characteristics of the evanescent wave.

[0012] This invention achieves highly sensitive ammonia detection through the synergistic effect of optical microfibers and pn heterojunctions. The core control mechanism is mainly manifested in:

[0013] 1. Optical field manipulation: The optical field transmitted in the optical microfiber is split into HE... 11 Fundamental model and HE 12 Higher-order model, HE 12The mode energy is concentrated at the edge of the cladding, generating a strong evanescent wave field (penetrating to a depth of 500 nm) that covers the entire thickness of the pn heterostructure, providing an efficient channel for photon-carrier interactions.

[0014] 2. Carrier dynamic response: Under normal conditions, n-type SnO2 and p-type PANI form a depletion layer with low carrier concentration and stable evanescent wave propagation; upon contact with ammonia, NH3 molecules are protonated to form NH4. + This triggered N in PANI + As atoms migrate toward the depletion layer, the SnO2 carrier concentration decreases, the depletion layer width decreases, and the heterojunction conductivity increases.

[0015] 3. Photo-to-electric signal conversion: Changes in carrier concentration lead to a change in the dielectric constant (ε) of the pn heterojunction. Through the strong interaction between the evanescent wave and the carriers, the photoelectric signal is converted into an electrical signal. 11 with HE 12 The change in the effective refractive index difference of the mode ultimately manifests as a redshift in the transmission spectrum (the amount of redshift is linearly related to the ammonia concentration). Therefore, the redshift can be monitored using a spectrometer to achieve quantitative detection of ammonia concentration.

[0016] This invention also proposes a method for preparing the ammonia sensor, comprising the following steps:

[0017] Step 1, Fabrication of optical microfibers: A fused tapering system was used to fused and taper the thin-core optical fiber. The transmission spectrum was monitored in real time using an optical spectrum analyzer to obtain optical microfibers with a diameter of 12.58 μm. Its higher-order mode HE... 12 Energy is concentrated at the cladding edge to enhance the evanescent wave field;

[0018] Step 2, Surface pretreatment of optical microfibers: The optical microfibers are washed in a piranha solution to expose the surface hydroxyl groups to enhance the electrostatic adsorption capacity of the semiconductor layer. After pretreatment, the surface roughness of the optical fiber is ≤5 nm.

[0019] Step 3, pn heterostructure construction: First, the pretreated optical microfibers are immersed in a SnO2 nanosheet solution. SnO2 nanosheets are deposited through electrostatic forces. The deposition is stopped when the transmission spectrum wavelength shift reaches 6.28 nm, forming a SnO2 layer with a thickness of 260 nm. At this point, HE... 12 The coupling efficiency between the mode and the SnO2 conduction band electrons reached 65%. Then, the optical microfibers modified with the n-type SnO2 layer were immersed in a polyaniline (PANI) nanosheet solution. PANI nanosheets were deposited via electrostatic interaction, and the deposition was stopped when the total redshift of the transmission spectrum reached 10.16 nm, forming a 130 nm thick PANI layer. Finally, a sensor integrating a pn heterojunction and optical microfibers was obtained, with a carrier mobility ≥ 5 cm⁻¹.2 / (V·s).

[0020] As a preferred technical solution of the present invention, in step 1 of the preparation method, the two ends of the thin-core optical fiber are first connected to a broadband light source and a spectrometer, respectively. The transmission spectrum is monitored in real time by an optical spectrum analyzer. During the melt tapering process, when HE... 11 with HE 12 When the interference fringe contrast of the mode is ≥15 dB and the number of interference peaks reaches more than 10, tapering is stopped, resulting in an optical microfiber with a diameter of 12.58 μm. Its higher-order mode HE 12 Energy is concentrated at the cladding edge to enhance the evanescent wave field. An automated tapering system is used to perform oxyhydrogen flame melting tapering on thin-core optical fibers, with the flame temperature controlled at 2000 ℃ and the tapering speed at 3.5 mm / min.

[0021] As a preferred embodiment of the present invention, in step 2 of the preparation method, the optical microfibers are washed in a mixture of 98% concentrated sulfuric acid and 30% hydrogen peroxide (piranha solution) with a volume ratio of 7:3 for 30 min, and the surface hydroxyl density is increased to ≥1×10⁻⁶ through a hydroxylation reaction. 14 / cm 2 This enhances the electrostatic adsorption force with semiconductor nanoparticles; after pretreatment, the particles are rinsed with deionized water until pH=7 and then dried with nitrogen.

[0022] As a preferred technical solution of the present invention, in step 3 of the preparation method, the pretreated optical microfiber is first immersed in a 12% SnO2 hydrocolloid nanoparticle dispersion solution to deposit a SnO2 layer, and then immersed in a 3 mg / mL polyaniline (PANI) / N-methylpyrrolidone (NMP) solution to deposit a polyaniline (PANI) layer. The deposition temperature for both steps is 25°C, and the deposition time is dynamically controlled by spectral monitoring. The axial tension of the optical fiber is kept stable during the deposition process to avoid deformation of the microfiber.

[0023] Furthermore, this invention also proposes the application of this ammonia sensor in the quantitative detection of ammonia content. The ammonia sensor is placed in a test gas chamber, and its two ends are connected to a broadband light source and a spectrometer, respectively, to construct a detection system. Ammonia gas of 0-500 ppm is introduced into the test gas chamber, and a broadband light source with a wavelength range of 1250-1650 nm is used for incident illumination. The spectrometer monitors the redshift of the transmission spectrum and constructs a linear relationship curve between the redshift and the ammonia concentration. In actual detection, the ammonia content data is read by measuring the redshift data of the transmission spectrum and based on the constructed linear relationship curve.

[0024] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0025] 1. High Sensitivity: The micro / nano optical fiber has a large specific surface area, which increases the contact area with ammonia gas. Simultaneously, the introduction of the pn junction structure makes the interaction between ammonia molecules and the material more significant, improving the sensor's response sensitivity to ammonia. Detection results show that the detection sensitivity for ammonia is 3.98 pm / ppm, and the limit of detection (LOD) is as low as 15.2 ppm.

[0026] 2. Fast response: The fast charge transfer characteristics of the pn heterojunction combined with the real-time monitoring capability of evanescent waves shorten the response time to 11s and the recovery time to 7s, which is 48% better than the TFBG sensor (21s), solving the problem of response lag in traditional fiber optic sensors.

[0027] 3. Low temperature cross-interference: The synergistic regulation of pn heterojunction and optical microfiber reduces the influence of temperature on the light field. The temperature sensitivity is -6.23 pm / ℃ in the range of 20-100 ℃, and the error of temperature change on the detection result is ≤2%, without the need for an additional temperature compensation module.

[0028] 4. Strong anti-interference capability: Fiber optic sensing technology has the advantage of resisting electromagnetic interference, which enables the sensor to work stably in complex electromagnetic environments.

[0029] 5. High industrialization potential: The sensor fabrication process does not require high-precision micro-nano equipment, and the material cost is low (SnO2 and PANI are both low-cost semiconductors). It can be mass-produced through automated tapering and deposition, which has significant economic and social benefits. Attached Figure Description

[0030] Figure 1 This is a diagram showing the fabrication process of optical microfibers.

[0031] Figure 2 A process diagram for fabricating sensors by combining optical microfibers with pn heterojunctions.

[0032] Figure 3 This is an image showing the EDS surface scan results of the sensor.

[0033] Figure 4 This is a diagram of a sensor gas testing device.

[0034] Figure 5 The image shows the ammonia gas test spectrum drift (a) and the fitted curve (b) of the sensor.

[0035] Figure 6 This is a diagram of a sensor temperature testing device.

[0036] Figure 7 The temperature test spectral drift plot (a) and the fitting curve (b) of the sensor are shown.

[0037] Figure 8 The image shows the sensor specificity test results.

[0038] Figure 9 The sensor response recovery time (a) and stability graph (b) are shown. Detailed Implementation

[0039] The present invention will be further described in detail below with reference to the embodiments and accompanying drawings.

[0040] Example 1: Sensor Fabrication

[0041] 1. Optical microfiber fabrication: Fine-core optical fiber (core diameter 1.8 μm, cladding diameter 125 μm) was selected, and an automated tapering system (such as...) was used. Figure 1 (As shown) The oxyhydrogen flame temperature was set to 2000 ℃, the tapering speed to 3.5 mm / min, and the tapering length to 30 mm. The spectrum was monitored in real time to HE. 11 / HE 12 When the interference fringes are clear (contrast ≥ 15 dB) and the number of interference peaks reaches more than 10, the tapering is stopped, and microfibers with a diameter of 12.58 μm are obtained.

[0042] 2. Surface pretreatment: Immerse in freshly prepared piranha solution (H2SO4:H2O2=7:3) for 30 min, then rinse with deionized water until pH=7, and blow dry with nitrogen (0.5 m / s).

[0043] 3. SnO2 layer deposition: A 12% SnO2 hydrocolloid nanosheet dispersion solution was prepared (SnO2 nanoparticles were ultrasonically dispersed in deionized water for 30 min, and the pH of the solution was adjusted to 10 by adding KOH solution). The microfibers were immersed in the solution and allowed to stand at 25 °C. The spectrum was monitored in real time. When the wavelength shifted by 6.28 nm, the microfibers were removed and dried at 60 °C for 1 h. The prepared SnO2 layer was 260 nm thick and uniformly covered the surface of the microfibers. Figure 2 (As shown).

[0044] 4. PANI layer deposition: A 3 mg / mL PANI / NMP solution (PANI dissolved in NMP) was prepared. The SnO2-modified optical fiber was immersed in the solution and allowed to stand at 25 °C. The spectrum was monitored in real time. When the total redshift reached 10.16 nm, the fiber was removed and dried at 80 °C for 12 h. The prepared PANI layer was 130 nm thick and tightly bonded to the SnO2 layer. Figure 2 As shown), EDS mapping confirms that Sn, C, O, and N elements are uniformly distributed ( Figure 3 (As shown).

[0045] Example 2: Sensor Performance Testing

[0046] 1. Sensitivity Test: Place the sensor in the test gas chamber (25℃), and connect the spectrometer and broadband light source to its two ends respectively. Figure 4 As shown in the figure, 0-500 ppm ammonia gas was introduced into the test chamber, and a broadband light source (1250-1650 nm) was used for incident illumination. The spectrum was recorded by an optical spectrum analyzer (AQ6370D). The results showed that the redshift increased linearly with concentration (Y=0.00398X-0.0088, R...). 2 =99.5%, sensitivity 3.98 pm / ppm ( Figure 5 As shown), it outperforms the LPFG sensor (1 pm / ppm) and the graphene / PANI fiber optic sensor (-27.8 pm / ppm, lower in absolute value).

[0047] 2. Temperature stability test: Place the sensor in a temperature control chamber, and connect the two ends to the spectrometer and the broadband light source respectively. Figure 6 As shown), tested in the range of 20-100℃, the sensor temperature sensitivity is -6.23 pm / ℃, exhibiting low temperature crosstalk. Figure 7 (As shown).

[0048] 3. Specificity test: using Figure 4 The detection system shown (25℃) was purged with 500 ppm of ammonia, ethanol, acetone, toluene, methanol, and isopropanol, respectively. The results showed that the sensor's redshift (2.08 nm) for ammonia was more than 10 times greater than that for the other gases, indicating excellent specificity. Figure 8 (As shown).

[0049] 4. Response / Recovery Time Test: Using... Figure 4 The detection system shown (25℃), when 500 ppm ammonia gas was introduced, recorded a spectrum that reached a stable redshift in 11 s; after purging with dry air, recorded a recovery time to the initial state in 7 s. Figure 9 (as shown in a), much faster than the TFBG sensor (21 s) and the MXene fiber optic sensor (60 s).

[0050] 5. Long-term stability test: The sensor was placed in air for 15 days, and the response to 500 ppm ammonia gas was tested daily. The change in the sensor's redshift was measured, indicating high stability. Figure 9 (as shown in b).

[0051] In summary, this invention solves the problems of low integration and slow response of traditional sensors, as well as the insufficient performance of existing fiber optic sensors. It has advantages such as room temperature operation, high sensitivity, ultrafast response / recovery time, low temperature cross-sensitivity, and high specificity. In particular, it breaks through the bottleneck of sensitivity and response speed through the photon-carrier synergistic mechanism, and has the following industrial application value:

[0052] 1. Environmental monitoring: It can be integrated into the air quality monitoring terminal to realize real-time monitoring of indoor and outdoor ammonia (0-500 ppm) with an accuracy of 15.2 ppm, meeting the requirements of environmental protection standards.

[0053] 2. Industrial safety: Suitable for early warning of ammonia leaks in chemical workshops. The 11-second rapid response can shorten the warning time, and the low temperature sensitivity ensures the detection accuracy under complex working conditions.

[0054] 3. Agricultural breeding: It can be embedded in the environmental monitoring system of the breeding farm to monitor ammonia concentration in real time (related to animal health), and its room temperature working characteristics are adapted to the complex field environment.

Claims

1. An ammonia gas sensor based on pn heterojunction and optical microfiber integration, characterized in that, A pn heterostructure is formed by sequentially modifying the surface of an optical microfiber with an n-type semiconductor layer and a p-type semiconductor layer. The n-type semiconductor layer is a SnO2 nanoparticle layer, and the p-type semiconductor layer is a polyaniline (PANI) nanoparticle layer. The evanescent wave generated by the optical microfiber during transmission interacts strongly with the charge carriers of the pn heterostructure. The quantitative detection of ammonia content is achieved by controlling the transmission spectrum shift through changes in charge carrier concentration.

2. The ammonia sensor based on pn heterojunction and optical microfiber integration as described in claim 1, characterized in that, The optical microfiber is fabricated from a thin-core optical fiber through fused taper drawing, and has a diameter of 12.58 μm. During transmission, it decomposes into HE. 11 Fundamental model and HE 12 Higher-order modes form a cosine-type transmission spectrum through two-mode interference. The energy of higher-order modes is concentrated at the edge of the fiber cladding, enhancing the interaction between evanescent waves and surface heterojunctions.

3. The ammonia sensor based on pn heterojunction and optical microfiber integration as described in claim 1, characterized in that, The thickness of the SnO2 nanoparticle layer is 100-300 nm, and the thickness of the polyaniline (PANI) nanoparticle layer is 80-200 nm. The depletion layer width of the pn heterojunction can be dynamically adjusted with the change of ammonia concentration. By modulating the evanescent wave propagation characteristics through the dielectric constant, a linear correlation between the spectral redshift and the ammonia concentration is achieved.

4. The method for preparing the ammonia sensor according to any one of claims 1-3, characterized in that, Includes the following steps: Step 1, Fabrication of optical microfibers: A fused tapering system was used to fused and taper the thin-core optical fiber. The transmission spectrum was monitored in real time using an optical spectrum analyzer to obtain optical microfibers with a diameter of 12.58 μm. Its higher-order mode HE... 12 Energy is concentrated at the cladding edge to enhance the evanescent wave field; Step 2, Surface pretreatment of optical microfibers: The optical microfibers are washed in a piranha solution to expose the surface hydroxyl groups to enhance the electrostatic adsorption capacity of the semiconductor layer. After pretreatment, the surface roughness of the optical fiber is ≤5 nm. Step 3, pn heterostructure construction: First, the pretreated optical microfibers are immersed in a SnO2 nanosheet solution. SnO2 nanosheets are deposited through electrostatic forces. The deposition is stopped when the transmission spectrum wavelength shift reaches 6.28 nm, forming a SnO2 layer with a thickness of 260 nm. At this point, HE... 12 The coupling efficiency between the mode and the SnO2 conduction band electrons reached 65%. Then, the optical microfibers modified with the n-type SnO2 layer were immersed in a polyaniline (PANI) nanosheet solution. PANI nanosheets were deposited via electrostatic interaction, and the deposition was stopped when the total redshift of the transmission spectrum reached 10.16 nm, forming a 130 nm thick PANI layer. Finally, a sensor integrating a pn heterojunction and optical microfibers was obtained, with a carrier mobility ≥ 5 cm⁻¹. 2 / (V·s).

5. The preparation method according to claim 4, characterized in that, In step 1, the two ends of the thin-core optical fiber are first connected to a broadband light source and a spectrum analyzer, respectively. The transmission spectrum is monitored in real time using an optical spectrum analyzer. During the fusion tapering process, when HE... 11 with HE 12 When the interference fringe contrast of the mode is ≥15 dB and the number of interference peaks reaches more than 10, tapering is stopped, resulting in an optical microfiber with a diameter of 12.58 μm. Its higher-order mode HE 12 Energy is concentrated at the cladding edge to enhance the evanescent wave field.

6. The preparation method according to claim 4 or 5, characterized in that, In step 1, an automatic tapering system is used to perform hydrogen-oxygen flame melting tapering on the thin-core optical fiber, with the flame temperature controlled at 2000 ℃ and the tapering speed at 3.5 mm / min.

7. The preparation method according to claim 4, characterized in that, In step 2, the optical microfibers are washed in a mixture of 98% concentrated sulfuric acid and 30% hydrogen peroxide (piranha solution) at a volume ratio of 7:3 for 30 min, and the surface hydroxyl density is increased to ≥1×10⁻⁶ through a hydroxylation reaction. 14 / cm 2 This enhances the electrostatic adsorption force with semiconductor nanoparticles; after pretreatment, the particles are rinsed with deionized water until pH=7 and then dried with nitrogen.

8. The preparation method according to claim 4, characterized in that, In step 3, the pretreated optical microfibers are first immersed in a 12% SnO2 hydrocolloid nanoparticle dispersion solution to deposit a SnO2 layer, and then immersed in a 3 mg / mL polyaniline (PANI) / N-methylpyrrolidone (NMP) solution to deposit a polyaniline (PANI) layer. The deposition temperature for both steps is 25°C.

9. The application of the ammonia sensor as described in claim 1 in the quantitative detection of ammonia content, characterized in that, An ammonia sensor is placed in a test chamber, with its two ends connected to a broadband light source and a spectrometer, respectively, to construct a detection system. Ammonia gas at concentrations of 0-500 ppm is introduced into the test chamber, incident through a broadband light source with a wavelength range of 1250-1650 nm. The spectrometer monitors the redshift of the transmission spectrum and constructs a linear relationship curve between the redshift and the ammonia concentration. During actual detection, the ammonia content is read based on the measured redshift data and the constructed linear relationship curve.

10. The application as described in claim 9, characterized in that, The ammonia sensor achieves high-sensitivity detection through the strong interaction between evanescent waves and charge carriers in a pn heterojunction. It exhibits a detection sensitivity of 3.98 pm / ppm for ammonia, a detection limit (LOD) as low as 15.2 ppm, a response time of 11 s, and a recovery time of 7 s. The temperature cross-sensitivity of the ammonia sensor is reduced through the synergistic modulation of the pn heterojunction and optical microfibers, achieving a temperature sensitivity of -6.23 pm / ℃ within the 20-100 ℃ range.

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