Tubular ZnS / In2O3 solar light activated NO2 gas sensing material and preparation method thereof

Abstract

This invention relates to a tubular ZnS / In₂O₃ sunlight-activated NO₂ gas sensing material and its preparation method, belonging to the technical fields of nanomaterial preparation, atmospheric environment detection, and gas sensors, with promising application prospects. The key feature is the use of indium nitrate pentahydrate as the indium source, with an indium oxide precursor prepared via a simple hydrothermal and calcination method. Then, using zinc acetate dihydrate as the zinc source, an In₂O₃ / ZnS composite material is prepared via a hydrothermal method. This material features spherical ZnS distributed on the inner and outer walls of the tubular In₂O₃, constructing a type II heterojunction, achieving effective separation of photogenerated electrons and holes, and improving gas-sensing performance. Under natural sunlight excitation, it achieves a high sensitivity response to NO₂ at room temperature, with a response value as high as 1088.31 for 10 ppm NO₂ gas, which is 9.2 times its response value under dark and thermally activated (60°C) conditions.

Description

Technical Field

[0001] This invention relates to the technical fields of gas-sensitive material preparation, gas detection, and gas sensors, specifically to a tubular ZnS / In2O3 solar-activated NO2 gas sensing material and its preparation method. Background Technology

[0002] Industrial and agricultural production processes release various harmful gases daily, including H2S, CO, NO2, NH3, H2, CH4, toluene, acetone, ethanol, methanol, and benzene. Among these, H2 and CH4 are explosive, while NO2 is a major contributor to acid rain and photochemical smog. Furthermore, NO2 concentrations exceeding a critical threshold can harm human health. Therefore, developing gas sensors with high precision, high sensitivity, rapid response, good selectivity, low detection limit (LOD), and real-time monitoring capabilities is crucial. To address these diverse detection needs, researchers have developed various types of gas sensors, primarily including resistive, optical, ultrasonic and acoustic, thermoelectric, and electrochemical types. Among these, resistive gas sensors hold a significant position in both academic research and industrial applications due to their simple structure, mature fabrication process, controllable cost, and ease of miniaturization and integration. The sensing materials used in these resistive gas sensors are mainly semiconductor metal oxide nanomaterials, carbon materials, and organic semiconductors. Compared to carbon materials and organic semiconductors, semiconductor metal oxides typically offer higher sensitivity, faster response / recovery speeds, better reversibility and stability, and are cost-effective and structurally simple. However, semiconductor metal oxide-based sensors are often heated to higher temperatures to improve performance, and operating at high temperatures significantly increases the energy consumption, overall size, and cost of gas sensors. Heating to high temperatures can cause changes in the microstructure of the sensing nanomaterials, leading to a decrease in sensing performance. Furthermore, high-temperature sensing has limitations in practical applications. In particular, heating is extremely dangerous for the detection of flammable and explosive gases, posing an explosion risk. Although the advantages of room-temperature gas sensors based on metal oxides have been proven, current room-temperature gas sensors still face several key challenges: insufficient sensitivity under room-temperature operating conditions; slow response and recovery kinetics, with recovery times for some devices reaching several minutes or even longer, limiting real-time monitoring capabilities; and insufficient cyclic stability and reversibility, often resulting in signal attenuation or baseline drift after multiple adsorption-desorption cycles. The electrical conductivity of semiconductor gas-sensitive materials is one of the main factors affecting their room-temperature gas-sensing performance. The conductivity of semiconductor gas-sensitive materials is primarily determined by the concentration of their charge carriers (holes in p-type semiconductors and electrons in n-type semiconductors). Under illumination, when the energy of the incident photon is greater than the band gap (Eg) of the semiconductor metal oxide, the material absorbs the photon energy, thereby generating photogenerated electrons (electrons). - ) and holes (h+ This process leads to a significant increase in carrier concentration, which increases the conductivity of the sensitive layer and reduces the initial resistance, providing a wider range of resistance variation for subsequent gas adsorption and surface reactions, thereby improving the sensing performance of the gas sensor at room temperature.

[0003] To date, wide bandgap semiconductors such as ZnO, WO3, and SnO2 have been widely used as metal oxide gas-sensing materials. These materials typically have bandgap widths of 3.0–3.8 eV, which determines that their light absorption is mainly in the ultraviolet band. Therefore, without the assistance of an ultraviolet light source, it is difficult to achieve effective photoexcitation under indoor visible light or natural sunlight. In contrast, In2O3, as a typical transparent conductive oxide (TCO), has a bandgap of approximately 3.16 eV. Although it also falls within the wide bandgap range, it has been actively studied due to its superior conductivity, excellent transparency, and electrical properties. However, pure In2O3 suffers from limited visible light absorption and rapid recombination of photogenerated carriers, which significantly hinders its sensing performance at room temperature. In gas detection, surface reactions are crucial, but most photogenerated electron-hole pairs recombine before participating in surface redox reactions, thus limiting the carrier content that can be used to detect NO2. To address these issues, a promising solution is to construct hierarchical heterojunctions with band structures precisely matched to the visible spectrum (400-700 nm), coupled with one-dimensional nanotube structures, providing abundant light absorption sites and further enhancing optical performance. This approach strengthens visible light absorption, suppresses carrier recombination, and promotes efficient charge separation and transfer.

[0004] Transition metal sulfides are widely used in humidity sensing due to their ordered crystal structure, abundant surface active sites, and excellent charge transport capabilities. Among them, ZnS is a wide-bandgap n-type semiconductor with stable chemical properties and high electron mobility. Although its excellent semiconductor properties have attracted widespread research interest in optoelectronic devices, its inherently high resistivity greatly limits its applicability in traditional thermally activated resistive gas sensors. These defects can be overcome and the photogenerated carrier separation efficiency improved by methods such as morphology modification, heterojunction construction, and noble metal doping. A type II (Type-II) heterojunction structure with band-matching characteristics can be constructed by combining In₂O₃ and ZnS. In₂O₃, a wide-bandgap n-type transparent conductive oxide (Eg = 3.16 eV), has a conduction band bottom (CBM) of approximately -0.52 eV (relative to the vacuum level) and a valence band top (VBM) of approximately 2.88 eV. ZnS, also a wide-bandgap n-type semiconductor (Eg = 3.57 eV), has a CBM of approximately -3.9 eV and a VBM of approximately -7.5 eV. The conduction band position of ZnS is more negative than that of In₂O₃, and its valence band position is more positive. This results in a spontaneous charge redistribution at the interface driven by a built-in electric field, where photogenerated electrons tend to migrate from the ZnS conduction band to the In₂O₃ conduction band and accumulate there. Furthermore, the In₂O₃ / ZnS heterointerface provides abundant chemisorption sites.

[0005] In recent years, research on In2O3-based gas-sensitive materials has been increasing, with synthesized materials exhibiting diverse morphologies and properties. However, research on solar photosensitized gas sensors based on In2O3 / ZnS heterojunctions has yet to be reported. Although there is considerable literature supporting the synthesis of In2O3 and ZnS nanomaterials, research on their photosynthetic gas sensing remains lacking. In2O3, as a typical transparent conductive oxide, has been applied to photoresponsive gas sensors due to its high carrier mobility and excellent visible light transmittance. However, limited by its wide bandgap and the significantly lower power density of sunlight compared to visible and monochromatic light sources provided by xenon lamps, In2O3 has limited photon absorption capacity in the solar spectrum, resulting in low energy utilization in this wavelength range. By constructing In2O3 / ZnS Type-II heterojunctions, not only can the light absorption capacity of both materials be improved in the ultraviolet-visible spectral range, but the built-in electric field formed by the bandgap arrangement can also effectively promote the spatial separation of photogenerated electrons and holes, suppressing recombination losses. This significantly enhances the adsorption of oxygen species (such as O2) on the surface under light. - O - This improves the generation and activation efficiency of NO2, thereby enhancing the material's catalytic oxidation capacity for NO2 and its gas-sensitive response sensitivity.

[0006] The structural design and preparation strategies of In₂O₃ / ZnS composite materials reported in existing literature differ significantly from those in this work. Firstly, regarding the morphology, the In₂O₃ nanotubes form the substrate framework, with ZnS nanospheres distributed on both the inner and outer surfaces of the tube walls, resulting in a higher specific surface area and more active sites. Secondly, the simple hydrothermal method used in this preparation method is more cost-effective. Furthermore, parameters such as material dosage, hydrothermal reaction temperature, and hydrothermal time were optimized through multiple experiments, ensuring high reproducibility of the material's morphology and properties. The key factor in obtaining the nanotube structure in this invention is the hydrothermal synthesis of the In₂O₃ precursor, which is not easily known by researchers in this field through experience but requires extensive experimental work. This two-dimensional structure is one of the key factors contributing to the excellent gas sensing performance of the material prepared in this invention.

[0007] The mechanism of photosensitization of gas-sensitive materials is still very incomplete. For most composite materials, even if their energy level structure matching is very good, they cannot exhibit photosensitization. The reasons for this are currently being explained by researchers based on their own prepared materials, without a universally accepted mechanism. Therefore, although there are reports of pure In2O3 exhibiting certain photosensitization gas-sensitive properties [Wang Xiao-Xue, et al. J. Mater. Chem., 8(29) (2020), 14482-14490], most guest components do not exhibit photosensitization properties when combined with In2O3. Entering the keyword "In2O3 gas sensor" into the Web of Science search engine yields 2172 results, but entering the keyword "In2O3 gas sensor photo" only yields 29 results. Of these, 22 are related to photoexcited gas sensors, including 5 that use visible light excitation [Han Chaohan, et al. Sensor. Actuator. B-Chem. 371(2022) 132448; Wang Tianshuang, et al. Nanoscale 10(10)(2018) 4841-4851; Han Chaohan et al. Sensor. Actuator. B-Chem. 450(2026)139250; Wang Xiao-Xue, et al. J. Mater. Chem. 8(29)(2020) 14482-14490; Meng Fanjian, et al. Chem. Eng. J. 510(2025)]. [161674; ], there are no reports on excitation by sunlight. Therefore, the In2O3 / ZnS composite material prepared in this invention has not been reported as a gas-sensitive material, let alone for achieving photosensitization. More importantly, the In2O3 / ZnS composite material prepared in this invention also exhibits natural solar photosensitization gas-sensitive properties. These are not things that researchers in the field could have predicted based on their extensive research experience.

[0008] The material composition, energy level structure, and material structure obtained by this invention have been carefully designed to successfully construct a naturally sunlight-sensitized gas sensing material. The obtained material exhibits excellent and repeatable gas-sensitizing performance. The material preparation process parameters were determined one by one through long-term, meticulous exploration. Therefore, the product of this invention is novel in morphology, advanced and unique in performance, and original in preparation method. This material has clear application potential in the fields of room-temperature sunlight-sensitized gas sensors, low-power environmental monitoring devices, and flexible semiconductor sensors. Summary of the Invention

[0009] This invention prepares a room-temperature, sunlight-sensitized In₂O₃ / ZnS gas sensing material. The key feature is the use of indium nitrate pentahydrate (In(NO₃)₃·5H₂O) as the indium source for In₂O₃, prepared via a simple hydrothermal method. Indium oxide (In₂O₃) is used as a precursor, and zinc acetate dihydrate (ZnAc₂·2H₂O) is used as the zinc source, which is then combined with In₂O₃ via a hydrothermal method to form a heterojunction. The prepared In₂O₃ / ZnS gas sensing material exhibits a tubular structure, with spherical ZnS distributed on the inner and outer walls of the In₂O₃ nanotubes. This artificially designed type II heterojunction concentrates photogenerated holes on the valence band of ZnS and photogenerated electrons on the conduction band of In₂O₃, thereby reducing the recombination rate of electrons and holes and further improving gas-sensing performance. Under sunlight irradiation, this material exhibits a sensitive response to NO₂ at room temperature (25 ± 3℃). For 10 ppm of NO2 gas, the response value reaches 1088.31 (R g / R a The response value was 9.2 times higher than that obtained under thermal activation conditions (60°C, darkness).

[0010] The photocatalyst designed in this invention is achieved as follows: In₂O₃ synthesized via a hydrothermal method is used as a precursor. The tubular morphology synthesized by this method significantly increases the specific surface area, thereby providing more active sites. Using zinc acetate dihydrate as the zinc source, an In₂O₃ / ZnS composite material is obtained again via a hydrothermal method, distributing it on the inner and outer walls of the nanotubes to improve gas-sensing performance. The synthesis process of this invention is simple and efficient, and its specific steps are as follows: (1) Accurately weigh 0.3-6 g of In(NO3)3·5H2O, dissolve 0.15-3 g of terephthalic acid in a mixed solution of 20-40 mL of N,N-dimethylformamide (DMF) and 5-10 mL of ethanol, and stir on a magnetic stirrer at room temperature until dissolved. (2) Continue stirring the mixture at room temperature for 1 h, add the resulting mixed solution to the polytetrafluoroethylene reactor, fill it to 2 / 3, heat it in an oven at 100-160℃ for 4-10 h, and then cool it to room temperature; (4) Filter the solution and dry the filtered product in a drying oven at 60°C for 24 h to obtain the In2O3 precursor; (5) Place 0.01-0.5 g of In2O3 into a mortar, grind for 5 min, then place it in a muffle furnace and calcine at a heating rate of 2-10℃ / min for 1-10 h to obtain In2O3; (6) Disperse the obtained 0.01-0.3 g In2O3 in 20-50 mL of deionized water, add 0.03-0.2 g Zn(Ac)2·2H2O and 0.06-0.5 g thiourea (CH4N2S), and stir on a magnetic stirrer at room temperature to dissolve Zn(Ac)2·2H2O and CH4N2S; (7) Continue stirring the mixture at room temperature for 1 h, add the resulting mixed solution to the polytetrafluoroethylene reactor, fill it to 2 / 3, heat it in an oven at 100-180℃ for 6-12 h, and then cool it to room temperature; (8) The obtained solution was filtered, the filtered product was washed and placed in a drying oven, and dried at 60 °C for 24 h to obtain the In2O3 / ZnS composite material with type II heterostructure. Attached Figure Description

[0011] Figure 1 This is a SEM image of a tubular In2O3 / ZnS binary composite material; Figure 2 This is a TEM image of a tubular In2O3 / ZnS binary composite material; Figure 3 This is the XRD pattern of the tubular In2O3 / ZnS binary composite material; Figure 4 The figure shows the response recovery curve of the tubular In2O3 / ZnS binary composite material to 10 ppm NO2 at room temperature (25 ± 3℃). This figure also serves as an appendix to the abstract.

[0012] Figure 5 It is about the gas selectivity and stability of tubular In2O3 / ZnS binary composite materials. Detailed Implementation

[0013] The In(NO3)3·5H2O, terephthalic acid (C8H6O4), Zn(Ac)2·2H2O, and CH4N2S used in this invention are all commercially available analytical grade products, and the deionized water is prepared in the laboratory. The glassware and equipment used are commonly used instruments and equipment in the laboratory. Example

[0014] Weigh 0.3 g of In(NO3)3·5H2O and 0.1661 g of C8H6O4 and dissolve them in 20 mL of DMF and 5 mL of ethanol solution. Stir at room temperature for 60 min to dissolve them in the mixed solution. Transfer the resulting mixed solution to a stainless steel reactor lined with polytetrafluoroethylene and heat it in an oven at 100 °C for 4 h. After the hydrothermal reactor cools to room temperature, filter the solution. Wash the filtered product with deionized water and anhydrous ethanol and then dry it in a drying oven at 60 °C for 24 h to obtain the In2O3 precursor material.

[0015] The obtained In2O3 precursor was ground thoroughly in a mortar. The ground sample was then transferred to a crucible and placed in a muffle furnace. The heating and cooling rates were set to 3℃ / min, and the sample was calcined at 450℃ for 2 h. The muffle furnace was allowed to cool naturally to room temperature to obtain tubular In2O3 material.

[0016] Weigh 0.0553 g of In2O3 material and disperse it in 30 mL of deionized water. Add 0.1316 g of Zn(Ac)2·2H2O and 0.2284 g of CH4N2S. Stir at room temperature for 60 min to dissolve the mixture in the solution. Transfer the resulting solution to a stainless steel reactor lined with polytetrafluoroethylene. Place the reactor in an oven and heat at 180 °C for 12 h. After the hydrothermal reactor cools to room temperature, filter the solution. Wash the filtered product with deionized water and anhydrous ethanol and place it in a drying oven. Dry at 60 °C for 24 h to obtain the tubular binary composite material In2O3 / ZnS.

[0017] Scanning electron microscope images of tubular In2O3 / ZnS composite materials are shown below. Figure 1 As shown, smaller zinc sulfide nanoparticles are uniformly dispersed and firmly attached to the interior and surface of indium oxide nanotubes. This unique heterostructure not only retains the inherent morphological characteristics of each component but also promotes the formation of a tight interfacial contact between indium oxide and zinc sulfide nanoparticles, which greatly increases the specific surface area of ​​the prepared material. Figure 2 The TEM image of the tubular binary In₂O₃ / ZnS composite material confirms the successful bonding of In₂O₃ and ZnS nanoparticles, forming a nanocomposite structure. Furthermore, this nanocomposite material is densely packed with ZnS nanoparticles, which significantly increases the number of available active sites. The X-ray diffraction analysis results of the prepared tubular binary In₂O₃ / ZnS composite material are shown below. Figure 3 As shown, by comparing with the standard card, it can be seen that the prepared material is mainly composed of In2O3 and ZnS; Figure 4The response-recovery curve of a gas sensor based on a tubular binary In2O3 / ZnS composite material for 10 ppm NO2 gas under sunlight irradiation at room temperature (25 ± 3℃) shows a response value of 1088.31 (R²). g / R a The response value was 9.2 times higher than that obtained under thermal activation conditions (60°C, darkness); Figure 5 The graph shows the selectivity and stability of the gas sensor based on the tubular In₂O₃ / ZnS binary composite material. The sensor's response to NO₂ gas is significantly higher than that of other test gases, confirming the excellent specificity of the prepared In₂O₃ / ZnS gas sensor for NO₂ gas at room temperature. Furthermore, the sensor underwent eight performance tests over 29 days with 10 ppm NO₂ gas under sunlight irradiation. The initial resistance and response values ​​of the sensor showed only minimal fluctuations, with relative standard deviations (RSDs) of 0.09 and 0.05, respectively, indicating excellent long-term stability.

[0018] Of course, there are many other embodiments of the present invention. Without departing from the spirit and essence of the present invention, those skilled in the art can make various corresponding changes and modifications according to the present invention, but these corresponding changes and modifications should all fall within the protection scope of the appended claims.

Claims

1. A tubular ZnS / In2O3 solar-activated NO2 gas sensing material, characterized in that, Indium oxide (In₂O₃) was prepared using indium nitrate pentahydrate (In(NO₃)₃·5H₂O) as the indium source via a simple hydrothermal and calcination method. Using the prepared In₂O₃ as a precursor and zinc acetate dihydrate (ZnAc₂·2H₂O) as the zinc source, ZnS was composited onto In₂O₃ via a secondary hydrothermal method. The resulting In₂O₃ / ZnS gas sensing material exhibits a tubular structure, with spherical ZnS particles distributed on the inner and outer walls of the tubular In₂O₃, forming a type II heterojunction. This promotes the aggregation of photogenerated holes on the valence band of ZnS and photogenerated electrons on the conduction band of In₂O₃, thus achieving effective separation of photogenerated electrons and holes. This further enhances the gas-sensing performance, enabling the In₂O₃ / ZnS gas sensing material to achieve a high-sensitivity response to NO₂ at room temperature under natural sunlight excitation. Under sunlight irradiation, the material exhibits a high sensitivity to 10⁻⁶ NO₂ at room temperature (25 ± 3℃). The response value of NO2 gas at ppm is as high as 1088.31 (R g / R a The response value obtained under dark and thermal activation (60°C) conditions is 9.2 times that obtained under these conditions.

2. A method for preparing the tubular ZnS / In2O3 sunlight-excited NO2 gas sensing material as described in claim 1, comprising the following specific steps: (1) Preparation of In2O3 precursor Accurately weigh 0.3-6 g of In(NO3)3·5H2O, dissolve 0.15-3 g of terephthalic acid in a mixed solution of 20-40 mL of N,N-dimethylformamide (DMF) and 5-10 mL of ethanol, stir at room temperature for 1 h, add the resulting mixed solution to a polytetrafluoroethylene reactor, and hydrothermally react in an oven at 100-160℃ for 4-10 h. After cooling to room temperature, filter, wash and dry, grind in a mortar for 5 min, and then calcine in a muffle furnace at 450 ℃ for 1-10 h to remove terephthalic acid, obtaining the In2O3 precursor; (2) Preparation of tubular ZnS / In2O3 gas sensing material Disperse 0.01-0.5 g of In2O3 precursor in 20-50 mL of deionized water, add 0.03-0.2 g of Zn(Ac)2·2H2O and 0.06-0.5 g of thiourea (CH4N2S), and stir at room temperature for 1 h after Zn(Ac)2·2H2O and CH4N2S are completely dissolved. Add the mixture to a polytetrafluoroethylene reactor and react in an oven at 100-180℃ for 6-12 h. After cooling to room temperature, filter the precipitate, wash and dry to obtain the In2O3 / ZnS composite material.

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