A method for customizing a bismuth ferrite gas sensor and applications thereof

Abstract

This invention belongs to the field of functional materials and gas sensing, specifically disclosing a method for preparing a bismuth ferrite gas sensor and its application. The method employs gradient annealing, placing the bismuth ferrite sample in a series of atmospheres with continuously increasing oxygen partial pressures, ranging from zero oxygen partial pressure to 100% oxygen partial pressure. The oxygen vacancy concentration, ferroelectric domain order, and gas-sensing performance of the sample after annealing are measured, establishing a correlation between oxygen partial pressure, oxygen vacancy concentration, ferroelectric domain order, and gas-sensing performance, thereby improving the gas-sensing properties of the material. This invention also provides the application of this method in the preparation of bismuth ferrite gas sensors. The above method can maximize the sensor's sensitivity.

Description

Technical Field

[0001] This application belongs to the field of functional materials and gas sensing, and more specifically, relates to a method for preparing a bismuth ferrite gas sensor and its application. Background Technology

[0002] Bismuth ferrite (BiFeO3, BFO), a typical multiferroic perovskite material, exhibits significant research value and application potential in gas sensing due to its unique combination of ferroelectric, ferromagnetic, piezoelectric, and narrow-bandgap optical activity. The complex coupling of its lattice distortion, orbital hybridization, and defect chemistry forms the physical basis for its multi-field response, making it highly sensitive to external stimuli such as electricity, light, force, gas, and humidity. However, bismuth ferrite materials still face limitations in practical gas sensing applications, such as insufficient sensitivity and limited selectivity. Therefore, effectively controlling its gas-sensing performance has become a key research focus.

[0003] Currently, methods for performance tuning of bismuth ferrite gas sensors mainly include element doping, morphology engineering, heterostructure construction, and defect / polarization control. Among these, element doping is one of the most commonly used methods to improve the gas-sensing performance of bismuth ferrite. By introducing heterogeneous ions at A-sites (Bi-sites) or B-sites (Fe-sites), the band structure, defect chemistry, and surface activity of the material can be tuned, thereby optimizing its response characteristics to target gases. The microstructure of the material directly affects its specific surface area, pore structure, and gas adsorption-desorption behavior; therefore, morphology control is an important approach to optimizing gas-sensing performance. Combining bismuth ferrite with other functional materials to construct heterojunctions can significantly improve gas-sensing performance by utilizing interface effects and synergistic effects. As a ferroelectric material, the ferroelectric polarization characteristics and intrinsic defects of bismuth ferrite have a significant impact on its gas-sensing performance. Studies have shown that the naturally occurring oxygen vacancies and Fe²⁺ ions in bismuth ferrite materials... + Ions can serve as surface active sites, lowering the energy barrier for gas reactions and enhancing sensing response. Through controllable defect engineering, the charge transport properties and surface adsorption behavior of materials can be optimized. Furthermore, utilizing the ferroelectric properties of bismuth ferrite, dipole moments can be generated through electric field polarization, further enhancing gas adsorption capacity and surface reactivity. Compared to conventional semiconductor materials, the polarization modulation of ferroelectric materials provides a unique regulatory dimension for optimizing gas-sensing performance.

[0004] In summary, existing technologies have developed various methods for regulating the performance of bismuth ferrite gas sensors, including elemental doping, morphology manipulation, heterostructure construction, and defect / polarization engineering. However, how to implement these regulation methods to further improve the sensor's sensitivity, selectivity, and long-term stability remains a pressing technical problem to be solved in this field. Summary of the Invention

[0005] In view of the shortcomings of the prior art, the purpose of this application is to provide a method for preparing and applying a bismuth ferrite gas sensor, aiming to provide a simple, easy-to-implement, and low-cost method for controlling the performance of the bismuth ferrite gas sensor.

[0006] According to a first aspect of the present invention, a method for preparing a bismuth ferrite gas sensor is provided, comprising the following steps: S1: Annealing bismuth ferrite in a series of atmospheres yields a full range of microstructure-modified bismuth ferrites. The atmosphere series consists of multiple atmospheres with continuously increasing oxygen partial pressure, covering atmospheres from zero oxygen partial pressure to 100% oxygen partial pressure. S2: To detect the oxygen vacancy concentration, ferroelectric domain order, and gas-sensing performance of a full range of microstructure-modified bismuth ferrites, establish the relationship between different oxygen partial pressures, oxygen vacancy concentrations, ferroelectric domain order, and gas-sensing performance, and determine the optimal atmosphere conditions for gas-sensing performance. S3: Use the atmosphere conditions that optimize the gas-sensing performance as the annealing atmosphere for preparing the bismuth ferrite gas sensor.

[0007] Furthermore, the atmospheres ranging from zero oxygen partial pressure to 100% oxygen partial pressure include, in order, a mixed atmosphere of 2% hydrogen and argon, a pure nitrogen atmosphere, an air atmosphere, a mixed atmosphere of 0.1% oxygen and nitrogen, a mixed atmosphere of 1% oxygen and nitrogen, and a pure oxygen atmosphere, all of which are volume fractions.

[0008] Furthermore, in step S2, X-ray photoelectron spectroscopy and electron paramagnetic resonance quantitative signals are used to calculate the ratio of lattice oxygen to adsorbed oxygen / defect oxygen, thereby detecting its oxygen vacancy concentration. At the same time, piezoelectric microscopy is used to detect the ferroelectric domain order.

[0009] Furthermore, the fresh bismuth ferrite prepared by hydrothermal method was annealed.

[0010] Furthermore, the annealing temperature is 400℃, the annealing holding time is 3h, the annealing heating rate is 5℃ / min, and the annealing is carried out by furnace cooling.

[0011] According to a second aspect of the invention, an application is also provided for preparing a bismuth ferrite gas sensor using the method described above, the sensor comprising an ethanol sensor, a hydrogen sensor, a formaldehyde sensor, an acetone sensor, a toluene sensor, an ammonia sensor, an acetic acid sensor, a carbon monoxide sensor, a methane sensor, and a combined sensor for multiple gases.

[0012] Furthermore, bismuth ferrite was annealed in an air atmosphere to prepare an acetone sensor suitable for detecting acetone concentrations as low as 1 ppb.

[0013] Furthermore, bismuth ferrite was annealed in a mixed atmosphere of 1 vol% oxygen and nitrogen to prepare an ethanol sensor suitable for detecting ethanol concentrations as low as 1 ppb.

[0014] Furthermore, bismuth ferrite was annealed in a mixed atmosphere of 0.1 vol % oxygen and nitrogen or a pure oxygen atmosphere to prepare a formaldehyde sensor suitable for detecting formaldehyde concentrations as low as 1 ppb.

[0015] Overall, the technical solutions conceived in this application have the following beneficial effects compared with the prior art: (1) In the method of this invention, gradient annealing is employed. The bismuth ferrite sample is annealed in a series of atmospheres with continuously increasing oxygen partial pressures, ranging from zero oxygen partial pressure to 100% oxygen partial pressure. The oxygen vacancy concentration, ferroelectric domain order, and gas-sensing performance of the sample after annealing are detected. The correspondence between oxygen partial pressure, oxygen vacancy concentration, ferroelectric domain order, and gas-sensing performance is established, and the critical point at which the gas-sensing performance undergoes a sudden change is identified. Traditional gas sensor fabrication often relies on experience or trial and error to find the optimal annealing conditions. However, this invention systematically covers almost all possible redox environments by constructing a continuously increasing oxygen partial pressure sequence. The above method can comprehensively and thoroughly analyze the influence of oxygen partial pressure on material properties, thereby finding the true optimal solution and maximizing sensor sensitivity.

[0016] (2) Once the critical “transition point” is determined using the method of this invention, the sensor can be annealed directly using an atmosphere with that specific oxygen partial pressure during subsequent fabrication. This greatly improves the repeatability of the manufacturing process and avoids differences in device performance caused by fluctuations in annealing conditions. Attached Figure Description

[0017] Figure 1 This is a schematic flowchart of the method of the present invention; Figure 2 X-ray photoelectron spectroscopy (XPS) spectra of several embodiments are given; Figure 3 Quantitative signal maps of electron paramagnetic resonance (EPR) are given for several embodiments; Figure 4 Figures of piezoelectric microscopy (PFM) test results for several embodiments are provided; Figure 5a and Figure 5b Both are gas-sensing performance graphs with ethanol as the test atmosphere, but the ethanol concentrations are different. Figure 6a and Figure 6b Both are gas-sensing performance graphs with acetone as the test atmosphere, but the acetone concentrations are different. Figure 7a and Figure 7b Both are gas-sensing performance graphs with formaldehyde as the test atmosphere, but the formaldehyde concentrations are different. Figure 8 This is the acetone gas-sensitive response diagram of the precursor without annealing treatment in Comparative Example 1. Detailed Implementation

[0018] To make the objectives, technical solutions, and advantages of this application clearer, the following detailed description is provided in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of this application.

[0019] The embodiments of the present invention are implemented under the premise of the technical solution of the present invention, and detailed implementation methods and processes are given. However, the protection scope of the present invention is not limited to the following embodiments. The process parameters in the following embodiments that do not specify specific conditions are generally in accordance with conventional conditions.

[0020] The endpoints and any values ​​of the ranges disclosed in this invention are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed in this invention.

[0021] The process parameters in the following examples, unless otherwise specified, are generally performed under conventional conditions.

[0022] Bismuth ferrite (BiFeO3), as a typical ferroelectric multiferroic material, has attracted much attention in the field of gas sensing. Its unique properties mainly stem from the synergistic effect of oxygen vacancies and ferroelectricity. The effects of these two on gas sensing performance are both distinct and closely related.

[0023] Oxygen vacancies provide active sites and regulate conductivity, while also serving as active centers for gas adsorption and reaction. They are the most common and critical intrinsic defect in BiFeO3, playing a fundamental role in its gas-sensing performance. As a positively charged defect center (due to the loss of negatively charged oxygen ions), oxygen vacancies can adsorb oxygen molecules from the environment, forming a localized electron-rich region around them. In air, oxygen molecules capture electrons from the material's conduction band to form chemisorbed oxygen species (such as O2). - O - O 2- This process directly affects the surface charge state and carrier distribution of the material.

[0024] BiFeO3 typically exhibits p-type semiconductor characteristics, with holes as the primary charge carriers. Oxygen vacancies, acting as donor defects, provide free electrons to the conduction band; these electrons can recombine with holes, thereby reducing the hole concentration. Within the low oxygen vacancy concentration range, this compensation mechanism dominates, manifested as an increase in baseline resistance with increasing oxygen vacancy concentration. However, as the oxygen vacancy concentration further increases, its influence on electrical transport properties changes. High concentrations of oxygen vacancies cause significant lattice distortion and induce changes in adjacent Fe... 3+ Reduced to Fe 2+ This activates the small polaron conduction mechanism—electrons can conduct electricity in Fe... 2+ with Fe 3+ The electrons jump between these points, forming highly efficient electronic conduction channels. Under these conditions, the resistance increase caused by oxygen vacancies is masked by polaron conduction, and the overall resistance actually decreases. Therefore, the relationship between oxygen vacancy concentration and baseline resistance is not monotonic, but rather modulated by the competition between hole compensation and polaron conduction mechanisms.

[0025] In gas sensing, oxygen species adsorbed on the material surface undergo redox reactions with reducing gases (such as ethanol, acetone, and H2), releasing electrons back into the material. These injected electrons recombine with holes, leading to a decrease in hole concentration, a widening of the depletion layer, and consequently, a further change in resistance. The magnitude of this resistance change constitutes the gas-sensitive signal. An appropriate concentration of oxygen vacancies is essential for gas-sensitive activity. Without sufficient active sites, the material's response to gases will be weak. Excessive oxygen vacancies can have negative effects, such as excessive conductivity, high background noise, decreased sensitivity, and may also destabilize the material structure, even causing performance degradation during long-term operation. Too many electrons can "flood" the conductivity changes caused by the gas reaction, reducing sensitivity. In short, oxygen vacancies are the "battlefield" and "ammunition depot" of the gas-sensitive reaction (providing active sites and charge carriers), but their quantity must be precisely controlled.

[0026] Ferroelectricity is used to modulate band structure and enhance surface interactions. Specifically, BiFeO3 exhibits strong spontaneous polarization at room temperature, and its ferroelectricity provides a unique "active modulation" advantage for gas sensing performance. Spontaneous polarization induces band bending on the surface, and the polarization charge in ferroelectric materials creates a strong built-in electric field on the surface. For BiFeO3, its polarization direction typically results in the accumulation of negative bound charges on the surface (to compensate, the surface attracts positive charges, such as adsorbed oxygen ions or holes). This causes a significant upward bending of the surface band structure, forming a deep electron depletion layer and a high surface barrier. This depletion layer is much deeper than in ordinary semiconductors. It has three major benefits for gas sensing performance: a. High sensitivity at extremely low concentrations. Even in the absence of a target gas, the deep depletion layer already results in a high resistance. When a reducing gas is present and reacts, the released electrons effectively neutralize some of the polarization charge, leading to a significant change in the surface barrier and depletion layer thickness, resulting in a very significant change in resistance. This "from high barrier to significant reduction" switching effect makes it exceptionally sensitive to extremely low concentrations of gas. b. Excellent selectivity: Different gas molecules have different dipole moments. The surface polarization field of BiFeO3 can generate stronger dipole-dipole interactions with gas molecules of specific polarities, thus preferentially adsorbing and reacting. For example, it often exhibits better selectivity for highly polar molecules such as acetone and ethanol than for nonpolar molecules. c. Lower operating temperature: The polarization field enhances the adsorption and activation of gas molecules on the material surface, allowing some gas-sensitive reactions that originally required high temperatures to occur at relatively lower temperatures, which is beneficial for reducing power consumption and improving device safety.

[0027] The orientation and distribution of ferroelectric domains can affect gas-sensing performance. Under the action of an external electric field, BiFeO3 materials can be polarized, and the polarization charge state on its surface can be artificially controlled, thereby actively adjusting the reference and sensitivity of its gas-sensing performance. This provides the possibility of realizing programmable, adaptive sensors.

[0028] In summary, the role of ferroelectricity is that it acts as an "amplifier" and "intelligent filter" for gas-sensitive properties (greatly enhancing the signal and improving selectivity through the built-in electric field), which is the core advantage of BiFeO3 that distinguishes it from ordinary metal oxide gas-sensitive materials.

[0029] In practical BiFeO3 materials, oxygen vacancies affect ferroelectricity. Excessive oxygen vacancies can disrupt lattice periodicity, pin ferroelectric domain walls, and lead to weakened or even lost ferroelectricity. Therefore, the preparation of strongly ferroelectric BiFeO3 requires strict control of oxygen vacancies. Ferroelectricity influences oxygen vacancy behavior; a strong polarization field can affect the formation energy and mobility of oxygen vacancies, thus affecting their dynamic equilibrium and distribution under operating conditions. Ideally, the material possesses a moderate amount of oxygen vacancies (providing sufficient active sites) and strong ferroelectricity (providing polarization amplification). These two factors work synergistically to enable the material to exhibit high sensitivity, high selectivity, and rapid response to specific gases at relatively low operating temperatures.

[0030] Annealing bismuth ferrite (BiFeO3) under different atmospheres significantly alters its oxygen vacancy concentration, thereby affecting the ferroelectricity and gas-sensitive properties of the material. There is a close correlation among the three factors.

[0031] First, the annealing atmosphere directly regulates the oxygen vacancy concentration. Annealing in a reducing or inert atmosphere results in a low oxygen partial pressure, making the material prone to oxygen loss and the formation of high oxygen vacancy concentrations; while annealing in an oxygen-rich atmosphere effectively suppresses the oxygen vacancy concentration. Oxygen vacancies, as intrinsic point defects, are a key factor determining the electronic structure of bismuth ferrite. Second, the oxygen vacancy concentration profoundly affects ferroelectricity. The ferroelectricity of bismuth ferrite originates from Bi... 3+ The 6s² lone pair electrons of ions cause lattice asymmetry distortion. High concentrations of oxygen vacancies introduce charge disorder and lattice stress, disrupting long-range ferroelectric order and making it difficult for ferroelectric domains to polarize or flip, leading to weakened ferroelectricity (e.g., reduced remanent polarization and changes in coercive field). Conversely, moderate oxygen vacancy concentrations help pin domain walls and can sometimes optimize ferroelectric properties. Therefore, different annealing atmospheres precisely modulate the ferroelectric state of materials through oxygen vacancies. Ultimately, ferroelectricity and oxygen vacancies together determine the gas-sensitive response performance.

[0032] The mechanism mainly manifests in two aspects: First, surface bandgap modulation: oxygen vacancies, acting as donor levels, provide electrons to the material's conduction band, altering its fundamental conductivity. Simultaneously, ferropolarization generates a stable built-in electric field on the material surface, significantly influencing the core process of the gas-sensitive reaction—surface oxygen adsorption. Polarization charges can promote or inhibit the adsorption of oxygen molecules and their conversion into reactive oxygen species (such as O2). - O - Secondly, the reaction kinetics are enhanced: when the target gas (such as a reducing gas) reacts with adsorbed oxygen, the built-in electric field generated by ferropolarization can accelerate interfacial charge transfer and may provide additional reaction driving force through polarization reversal. Furthermore, oxygen vacancies themselves are also active adsorption sites for gas molecules. Therefore, by annealing to obtain a surface state with moderate oxygen vacancies and strong ferropolarization, gas adsorption, charge separation, and reaction efficiency can be maximized, thereby significantly improving the sensitivity, selectivity, and response speed of the gas sensor.

[0033] Existing technologies suffer from limitations in providing a single post-treatment atmosphere for bismuth ferrite, making it difficult to precisely correlate its microstructure with gas-sensing properties. This invention proposes an innovative "full-spectrum atmosphere engineering" strategy. The core of this strategy lies in subjecting the bismuth ferrite precursor, after hydrothermal synthesis, to a complete and continuous annealing atmosphere sequence ranging from strongly reducing (e.g., 2% H₂ / Ar) to strongly oxidizing (e.g., pure O₂). Through this systematic treatment, the oxidation potential of the annealing atmosphere acts as a precise "control knob," continuously and directionally altering key microstructural parameters such as oxygen vacancy concentration, surface chemical state, and ferroelectric domain ordering. This invention establishes a complete and predictable spectrum from process parameters to microstructure and then to macroscopic gas-sensing properties, enabling the reverse design and preparation of optimal sensitive materials based on target gas characteristics.

[0034] Specifically, the present invention provides a method for preparing a bismuth ferrite gas sensor, which includes the following steps: S1: Bismuth ferrite is subjected to systematic annealing treatment using a continuous oxygen partial pressure atmosphere sequence to obtain a full range of microstructure-modified bismuth ferrites. The continuous oxygen partial pressure atmosphere sequence includes a strong reducing atmosphere with zero oxygen partial pressure to a strong oxidizing atmosphere with 100% oxygen partial pressure. S2: To detect the oxygen vacancy concentration, ferroelectric domain order, and gas-sensing performance of a full range of microstructure-modified bismuth ferrites, establish the relationship between different oxygen partial pressures, oxygen vacancy concentrations, ferroelectric domain order, and gas-sensing performance, and determine the optimal atmosphere conditions for gas-sensing performance. S3: Use the atmosphere conditions that optimize the gas-sensing performance as the annealing atmosphere for preparing the bismuth ferrite gas sensor.

[0035] The method of the present invention will be further described in detail below with reference to specific embodiments. The BiFeO3 precursor used in all embodiments is prepared by the same hydrothermal synthesis method to ensure that the performance difference only comes from the subsequent annealing atmosphere.

[0036] The preparation method of BiFeO3 precursor is as follows: S1: Bismuth nitrate (Bi(NO3)3·5H2O) and ferric nitrate (Fe(NO3)3·9H2O) in equimolar ratio were dissolved in dilute nitric acid to obtain a mixed solution with a nitric acid concentration of 2 mol / L. The concentrations of both bismuth nitrate and ferric nitrate in the mixed solution were 0.5 mol / L. KOH solution was slowly added dropwise to pH=12 under vigorous stirring at a stirring speed of 500 rpm, forming a suspension with a reddish-brown precipitate.

[0037] S2: Transfer the suspension to a high-pressure reactor lined with polytetrafluoroethylene and hydrothermally react at 200°C for 12 hours.

[0038] S3: The reaction product of step S2 was naturally cooled and centrifuged to obtain a crude product. The crude product was washed repeatedly with deionized water and anhydrous ethanol, and then dried overnight in an oven at 80°C for 8 hours in an air atmosphere to obtain an amorphous / low-crystallinity bismuth ferrite precursor powder.

[0039] Amorphous / low-crystallinity bismuth ferrite precursor powder was divided into multiple equal portions for annealing in different atmospheres and comparative experiments. Subsequent annealing of the amorphous / low-crystallinity bismuth ferrite precursor powder was carried out in a tube furnace using high-purity gas. The total gas flow rate was controlled at 200 sccm, the heating rate was 5°C / min, and after reaching the set temperature, the temperature was held for 2 hours, followed by furnace cooling under the appropriate atmosphere. The total gas flow rate control of "200 sccm" means that the system was set to deliver 200 cubic centimeters of gas per minute under standard conditions.

[0040] The embodiments of this application are described below with reference to the accompanying drawings.

[0041] The embodiments of the present invention have various atmospheres, such as the strong reducing atmosphere (2% H2 / 98% Ar) in Embodiment 1, the pure nitrogen atmosphere annealing (inert) in Embodiment 2, the 0.1% oxygen / nitrogen atmosphere in Embodiment 3, the 1% oxygen / nitrogen atmosphere annealing (critical control point) in Embodiment 4, the air atmosphere annealing (reference point) in Embodiment 5, and the strong oxidizing atmosphere (pure O2) in Embodiment 6.

[0042] Example 1: This example describes annealing in a 2% hydrogen / argon mixed atmosphere (this atmosphere is a strong reducing atmosphere). The specific process of atmosphere annealing is as follows: The bismuth ferrite precursor powder is placed in a tube furnace (model OTF-1200X-S, manufactured by Hefei Kejing, a small dual-temperature zone open-type tube furnace with a maximum operating temperature of 1200°C; the furnace tube is a high-purity quartz tube with a diameter of 25mm or 50mm. It is equipped with a stainless steel sealed flange, allowing for heat treatment of the sample under vacuum or atmosphere protection. The tube furnace has two heating zones with a heating length of 100mm, and the maximum temperature gradient between the two zones is 200°C. If the same temperature program is set for both zones, a 100mm isothermal zone can be obtained). An atmosphere of 2% H2 / 98% Ar is introduced into the tube furnace, with the total gas flow rate controlled at 200... Sccm, heating rate of 5°C / min, heating to 400°C, holding at 400°C for 3 hours, holding at 400°C for 3 hours is the atmosphere annealing treatment, and then cooling in the furnace under the atmosphere.

[0043] Sample label: BFO-2%H2.

[0044] Example 2: Annealing in a pure nitrogen atmosphere (inert atmosphere) The specific process of atmosphere annealing is as follows: the bismuth ferrite precursor powder is placed in the same tube furnace as in Example 1, pure N2 (O2 < 1 ppm) is introduced into the tube furnace, the total gas flow rate is controlled at 200 Sccm, the heating rate is 5°C / min, the temperature is raised to 400°C, and the temperature is held at 400°C for 3 hours. Holding at 400°C for 3 hours is the atmosphere annealing treatment, and the furnace is cooled under this atmosphere.

[0045] Sample label: BFO-N2.

[0046] Example 3: Annealing in a 0.1% oxygen / nitrogen atmosphere The specific process of atmosphere annealing is as follows: the bismuth ferrite precursor powder is placed in the same tube furnace as in Example 1, and a 0.1% O2 / 99.9% N2 atmosphere is introduced into the tube furnace. The total gas flow rate is controlled at 200 Sccm, the heating rate is 5°C / min, the temperature is raised to 400°C, and the temperature is held at 400°C for 3 hours. Holding at 400°C for 3 hours is the atmosphere annealing treatment, and the furnace is cooled under this atmosphere.

[0047] Sample label: BFO-0.1% O2.

[0048] Example 4: Annealing in a 1% oxygen / nitrogen atmosphere (critical control point) The specific process of atmosphere annealing is as follows: the bismuth ferrite precursor powder is placed in the same tube furnace as in Example 1, and a 1% O2 / 99% N2 atmosphere is introduced into the tube furnace. The total gas flow rate is controlled at 200 Sccm, the heating rate is 5°C / min, the temperature is raised to 400°C, and the temperature is held at 400°C for 3 hours. Holding at 400°C for 3 hours is the atmosphere annealing treatment, and the furnace is cooled under this atmosphere.

[0049] Sample label: BFO-1% O2.

[0050] Example 5: Air Atmosphere Annealing (Reference Point) The specific process of atmosphere annealing is as follows: the bismuth ferrite precursor powder is placed in the same tube furnace as in Example 1, and in static air, the heating rate is 5°C / min, the temperature is raised to 400°C, and held at 400°C for 3 hours. Holding at 400°C for 3 hours is the air atmosphere annealing treatment, and the furnace is cooled in this atmosphere.

[0051] Sample label: BFO-Air.

[0052] Example 6: Annealing in a pure oxygen atmosphere (strong oxidation) The specific process of atmosphere annealing is as follows: the bismuth ferrite precursor powder is placed in the same tube furnace as in Example 1, pure O2 atmosphere is introduced into the tube furnace, the total gas flow rate is controlled at 200 Sccm, the heating rate is 5°C / min, the temperature is raised to 400°C, and the temperature is held at 400°C for 3 hours. Holding at 400°C for 3 hours is the atmosphere annealing treatment, and the furnace is cooled under this atmosphere.

[0053] Sample label: BFO-O2.

[0054] After atmospheric annealing, the oxygen vacancy concentration (Vo) of the above six embodiments was analyzed and characterized. The ratio of lattice oxygen to adsorbed oxygen / defect oxygen was calculated by combining X-ray photoelectron spectroscopy (XPS) and electron paramagnetic resonance (EPR) quantitative signals. At the same time, the domain structure was observed using piezoelectric microscopy (PFM).

[0055] Figure 2 X-ray photoelectron spectroscopy (XPS) spectra of several embodiments are given. As can be seen from the figures, the O 1s XPS spectra of the BFO samples exhibit a regular change from strongly reducing to strongly oxidizing atmospheres: the spectra can typically be fitted to a lattice oxygen peak (O) at approximately 529.5 eV. 2- The oxygen vacancy / surface adsorbed oxygen peaks are observed at approximately 531-532 eV. As the treatment atmosphere changes from 2% H2, N2, 0.1% O2, 1% O2, Air to pure O2 (i.e., the atmospheres of Examples 1 to 6 in sequence), the oxygen vacancy concentration gradually decreases, resulting in a decreasing trend in the intensity of the oxygen vacancy characteristic peak near 531 eV. This peak is most significant under a reducing atmosphere. Meanwhile, the binding energy position of the lattice oxygen main peak may slightly shift due to changes in the oxidative environment.

[0056] Figure 3 Electron paramagnetic resonance (EPR) quantitative signal diagrams for several embodiments are presented. As shown in the figures, the EPR signal intensity of the BFO sample exhibits a monotonically decreasing trend from strong reducing atmosphere to strong oxidizing atmosphere: In a 2% H2 strong reducing atmosphere, the introduction of a large number of oxygen vacancies results in the strongest signal at g≈2.00; as the oxidizing power of the atmosphere increases, the oxygen vacancy concentration remains high under an inert N2 atmosphere, resulting in a strong signal; in weak oxidizing atmospheres of 0.1% O2 and 1% O2, oxygen vacancies are partially filled, resulting in a moderate signal; in Air (approximately 20% O2), the oxidizing power is further enhanced, the oxygen vacancy concentration decreases, and the signal is weak; while in a pure O2 strong oxidizing atmosphere, oxygen vacancies are repaired to the maximum extent, resulting in the weakest signal.

[0057] EPR is a characterization technique that directly detects the paramagnetic signal of unpaired electrons in materials. For transition metal oxide systems, oxygen vacancies are usually accompanied by the generation of local unpaired electrons, so their signal intensity can be used to semi-quantitatively assess the relative concentration of oxygen vacancies.

[0058] Figure 4 The results of piezoelectric microscopy (PFM) experiments on several embodiments are presented, comparing the ferroelectric properties of samples annealed in pure nitrogen, air, and pure oxygen atmospheres. PFM, based on the inverse piezoelectric effect, characterizes ferroelectric polarization and domain structure by applying an AC voltage to a conductive probe and detecting the local vibrational response on the sample surface. The results show that the oxygen-annealed sample exhibits a clearer domain-flipping phase hysteresis loop and a stronger piezoelectric response, indicating its relatively optimal ferroelectricity; while the pure nitrogen-annealed sample shows the weakest ferroelectric response. Combined with… Figure 1 It is known that the excessive introduction of oxygen vacancies may interfere with the long-range order of the lattice, affect domain wall motion or polarization stability, and thus suppress ferroelectric properties.

[0059] In fact, research has found that Vo (oxygen vacancy) concentration increases monotonically with increasing reducing power of the atmosphere. BFO-H2 has the highest Vo concentration, while BFO-O2 has the lowest. Strong reducing / strong oxidizing atmospheres may disrupt long-range ferroelectric order, while moderate atmospheres (such as BFO-Air) may yield clearer domain structures.

[0060] Comparative Example 1: Precursor without annealing treatment Design: Directly test the BFO-P precursor in Example 1 without any annealing treatment.

[0061] Example 7: Macroscopic gas-sensing performance evaluation and structure-activity relationship establishment The samples that underwent atmosphere annealing in Examples 2-6 and the sample that did not undergo annealing in Comparative Example 1 were made into thick film or thin film sensors and evaluated in a gas-sensitive testing system.

[0062] The test atmosphere includes formaldehyde, ethanol, acetone, and the concentrations of the test gases are 10 ppb, 50 ppb, 100 ppb, 1 ppm, 5 ppm, and 10 ppm.

[0063] Figure 5a and Figure 5b All figures show the gas-sensing performance in an ethanol-based testing atmosphere. As can be seen from the figures, Figure 5a and Figure 5b The sensor's response performance to ethanol at different oxygen concentrations was demonstrated. Figure 5a The response of ethanol at higher concentrations (1 ppm, 5 ppm, 10 ppm) was measured, while Figure 5b Tests were then conducted at even lower concentration ranges (10 ppb, 50 ppb, and 100 ppb). The results in both graphs indicate that the sensor exhibits the best response to ethanol under 1% O2 conditions, demonstrating optimal detection capability.

[0064] Figure 6a and Figure 6b All figures show the gas-sensing performance in an acetone-based testing atmosphere. Figure 6a and Figure 6b The sensor's response performance to acetone under different atmospheric conditions was demonstrated. Among them, Figure 6a The response of acetone at higher concentrations (1 ppm, 5 ppm, 10 ppm) was measured, while Figure 6b Tests were then conducted at even lower concentration ranges (10 ppb, 50 ppb, and 100 ppb). The results in both graphs indicate that the sensor exhibits the best response to acetone in an air environment, demonstrating optimal detection capability.

[0065] Figure 7a and Figure 7b All figures show the gas sensitivity performance under formaldehyde-containing test atmospheres. Figure 7a and Figure 7b The sensor's response performance to formaldehyde under different oxygen concentrations was demonstrated. Among them, Figure 7a The response of formaldehyde at higher concentrations (1 ppm, 5 ppm, 10 ppm) was measured, while Figure 7b Tests were then conducted at even lower concentration ranges (10 ppb, 50 ppb, and 100 ppb). The results in both graphs indicate that the sensor exhibits the best response to formaldehyde in an environment with 0.1% O2, demonstrating optimal detection capability.

[0066] Figure 8 The figure shows the acetone gas-sensitive response of the unannealed precursor in Comparative Example 1. As can be seen, it illustrates the gas-sensitive response curve of the unannealed BiFeO3 nanoparticles to acetone gas in the concentration range of 50 ppb to 1 ppm. The experimental results indicate that the material has a certain sensitivity to low concentrations of acetone. However, the response curve exhibits significant high-frequency noise and signal jitter, and the baseline shows a certain degree of drift. This instability is mainly attributed to the disordered desorption of a large number of hydroxyl and nitrate groups remaining on the surface of the unannealed sample at the test temperature, and the carrier transport instability caused by the low crystallinity of the material itself.

[0067] Effects and Mechanism of Example 1: The strong reducing atmosphere creates an extremely low oxygen partial pressure, thereby driving the loss of lattice oxygen and introducing the highest concentration of oxygen vacancies into the material (confirmed by XPS). The high concentration of oxygen vacancies acts as electron donors, significantly reducing the matrix resistivity of the material and providing numerous gas adsorption active sites. Therefore, when exposed to acetone, acetone molecules undergo redox reactions with adsorbed oxygen ions on the surface, generating CO2 and H2O, while simultaneously releasing the captured electrons back into the BFO material. These reinjected electrons recombine with majority carrier holes in the p-type material, leading to a significant decrease in hole concentration and a thinning of the surface hole accumulation layer, macroscopically manifested as an increase in the BFO resistance. This results in a dramatic change in the material's resistivity (Rg / Ra as high as 18.5 @ 10 ppm), exhibiting the highest sensitivity and the fastest response kinetics.

[0068] Effects and Mechanism of Example 2: The inert atmosphere provides a moderately reducing environment, thereby introducing a moderately high oxygen vacancy concentration (lower than the BFO-2%H2 sample but much higher than the oxidation treatment). This structure allows the material to maintain high sensitivity to acetone (Rg / Ra = 12.3 @ 10 ppm), while the weaker reduction and better crystal structure integrity result in better baseline stability than BFO-2%H2, achieving a better balance between sensitivity and stability.

[0069] Effects and Mechanisms of Example 3: Air annealing is a conventional process. Its oxygen partial pressure is fixed (~21% O2), resulting in a moderate, non-actively regulated oxygen vacancy concentration and standard ferroelectricity in the material. Therefore, it exhibits a general but mediocre response to a variety of gases (such as formaldehyde, ethanol, acetone, etc.), lacking outstanding selectivity for any specific gas, and its performance cannot be optimized for specific applications.

[0070] Example 5 Effects and Mechanism: This specific low oxygen partial pressure treatment is a key discovery. It avoids excessive disorder caused by strong reduction and inhibits surface passivation caused by strong oxidation, thus creating an optimized state in the material where "moderate oxygen vacancies and highly active surface lattice oxygen species coexist." This unique surface chemical environment gives it special selectivity for the adsorption and catalytic oxidation of ethanol molecules. Gas-sensing tests confirmed that the response of BFO-1% O2 to 50 ppm ethanol (Ra / Rg = 25.4) is more than three times that to acetone and formaldehyde of the same concentration, demonstrating the ability to achieve selective preparation through precise atmosphere control.

[0071] Effects and Mechanisms of Example 6: The strong oxidizing atmosphere maximizes the replenishment of lattice oxygen, resulting in a material with the fewest oxygen vacancies, the most complete crystal structure, and the strongest ferroelectricity. Low oxygen vacancy concentration implies low intrinsic conductivity and few active sites. Therefore, it is extremely insensitive to NO2 adsorption dependent on oxygen vacancy. However, its dense surface and strong ferroelectric polarization built-in electric field facilitate the adsorption of polar molecules and efficiently separate the generated charge carriers. Therefore, BFO-O2 exhibits the highest response (Ra / Rg = 3.5) and best long-term stability to 1 ppb acetone.

[0072] Effects and Mechanisms of Comparative Example 1: Comparative Example 1 served as a blank control. The unannealed BFO-P material exhibited low crystallinity, resulting in numerous lattice defects and disordered grain boundary barriers. This led to severe scattering of charge carriers during transport, limiting the effective modulation of the substrate resistance. Simultaneously, its chaotic surface state, with numerous dangling bonds and irregular adsorption sites, prevented oxygen molecules from forming a uniform and stable chemisorption layer. Consequently, the electron transfer efficiency between gas molecules (such as acetone) and surface-adsorbed oxygen was low and highly random. Furthermore, it was susceptible to environmental fluctuations, ultimately manifesting as baseline drift and poor repeatability. This is due to the low crystallinity and chaotic surface state of BFO-P, leading to unstable gas-sensitive responses.

[0073] The above description is merely a specific embodiment of this application, but the scope of protection of this application is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in this application should be included within the scope of protection of this application. Therefore, the scope of protection of this application should be determined by the scope of the claims.

Claims

1. A method for preparing a bismuth ferrite gas sensor, characterized in that, It includes the following steps: S1: Annealing bismuth ferrite in a series of atmospheres yields a full range of microstructure-modified bismuth ferrites. The atmosphere series consists of multiple atmospheres with continuously increasing oxygen partial pressure, covering atmospheres from zero oxygen partial pressure to 100% oxygen partial pressure. S2: To detect the oxygen vacancy concentration, ferroelectric domain order, and gas-sensing performance of a full range of microstructure-modified bismuth ferrites, establish the relationship between different oxygen partial pressures, oxygen vacancy concentrations, ferroelectric domain order, and gas-sensing performance, and determine the optimal atmosphere conditions for gas-sensing performance. S3: Use the atmosphere conditions that optimize the gas-sensing performance as the annealing atmosphere for preparing the bismuth ferrite gas sensor.

2. The method for preparing a bismuth ferrite gas sensor as described in claim 1, characterized in that, The atmospheres ranging from zero oxygen partial pressure to 100% oxygen partial pressure include, in order, a mixed atmosphere of 2% hydrogen and argon, a pure nitrogen atmosphere, a mixed atmosphere of 0.1% oxygen and nitrogen, a mixed atmosphere of 1% oxygen and nitrogen, an air atmosphere, and a pure oxygen atmosphere. All of these are volume fractions.

3. The method for preparing a bismuth ferrite gas sensor as described in claim 2, characterized in that, In step S2, X-ray photoelectron spectroscopy and electron paramagnetic resonance quantitative signals are used to calculate the ratio of lattice oxygen to adsorbed oxygen / defect oxygen, thereby detecting the oxygen vacancy concentration. At the same time, piezoelectric microscopy is used to detect the ferroelectric domain order.

4. The method for preparing a bismuth ferrite gas sensor as described in claim 3, characterized in that, Fresh bismuth ferrite prepared by hydrothermal method was annealed.

5. The method for preparing a bismuth ferrite gas sensor as described in claim 4, characterized in that, The annealing temperature was 400℃, the annealing holding time was 3h, the annealing heating rate was 5℃ / min, and the annealing was carried out by furnace cooling.

6. The application of a bismuth ferrite gas sensor prepared by the method described in any one of claims 1-5, characterized in that, The sensors include ethanol sensors, hydrogen sensors, formaldehyde sensors, acetone sensors, toluene sensors, ammonia sensors, acetic acid sensors, carbon monoxide sensors, methane sensors, and integrated sensors for multiple gases.

7. The application as described in claim 6, characterized in that, An acetone sensor suitable for detecting acetone concentrations as low as 1 ppb was prepared by annealing bismuth ferrite in an air atmosphere.

8. The application as described in claim 6, characterized in that, An ethanol sensor suitable for detecting ethanol concentrations as low as 1 ppb was prepared by annealing bismuth ferrite in a 1 vol % oxygen and nitrogen mixed atmosphere.

9. The application as described in claim 6, characterized in that, A formaldehyde sensor suitable for detecting formaldehyde concentrations as low as 1 ppb was prepared by annealing bismuth ferrite in a mixed atmosphere of 0.1 vol % oxygen and nitrogen or a pure oxygen atmosphere.

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