A three-dimensional ZnSnO3-SnO2 heterojunction composite aerogel for rapid detection of hydrogen at room temperature and a preparation method thereof

Abstract

This invention discloses a three-dimensional ZnSnO3-SnO2 heterojunction composite aerogel for rapid hydrogen detection at room temperature and its preparation method, belonging to the field of gas sensing materials. The composite aerogel is composed of chitosan, ZnSnO3-SnO2 heterojunction nanoparticles, and carboxylated multi-walled carbon nanotubes, wherein the molar ratio of ZnSnO3 to SnO2 is 1:1, ZnSnO3-SnO2 accounts for 30wt%~60wt% of chitosan, and carboxylated multi-walled carbon nanotubes account for 8wt%~16wt% of chitosan. It possesses a hierarchical three-dimensional interconnected porous structure of mesoporous and macroporous components. The carboxylated multi-walled carbon nanotubes form a three-dimensional conductive network and form p-n heterojunctions with ZnSnO3-SnO2, enriching surface oxygen vacancies. In preparation, the multi-walled carbon nanotubes are acidified, co-precipitated, and annealed to obtain ZnSnO3-SnO2, which is then mixed with chitosan and freeze-dried. This aerogel can detect hydrogen rapidly and stably at room temperature without the need for precious metals. It features fast response, good stability, and low power consumption, making it suitable for industrial applications.

Description

Technical Field

[0001] This invention belongs to the field of gas sensing materials, specifically relating to a three-dimensional ZnSnO3-SnO2 heterojunction composite aerogel for rapid detection of hydrogen at room temperature and its preparation method. Background Technology

[0002] Hydrogen energy, with its zero carbon emissions, high calorific value, and renewability, has become a core carrier of global energy transition. With the rapid expansion of the hydrogen economy industrial chain, safety risk control at each stage, from production and preparation to storage, transportation, and distribution to end-use applications, has become particularly critical. While molecular hydrogen (H2) is non-toxic, its low density, high diffusion coefficient, and low ignition energy make it highly susceptible to leakage. Even more concerning is the extremely wide explosive limit range of hydrogen in air (4.0%~75.6%), and its colorless and odorless nature, making real-time, in-situ monitoring of trace amounts of hydrogen in the environment a rigid requirement for ensuring the safety of facilities and personnel.

[0003] Among numerous gas detection technologies, metal-oxide-semiconductor (MOS) sensors are considered one of the most commercially promising hydrogen sensing solutions due to their high sensitivity, simple structure, and low cost. Currently, n-type semiconductors, represented by SnO2, ZnO, WO3, and In2O3, as well as p-type semiconductors such as NiO, have been extensively studied. These materials possess abundant chemically adsorbed oxygen on their surfaces, which can induce changes in carrier concentration through redox reactions between the target gas and the adsorbed oxygen, thereby generating an electrical signal response.

[0004] However, traditional MOS-based hydrogen sensors generally face the "high-temperature operating paradox." Because the surface reaction kinetics of MOS materials are driven by heat, most sensors require temperatures of 200–400°C to achieve sufficient response and recovery speeds. This not only leads to excessively high device power consumption, making it difficult to meet the low-power requirements of IoT nodes, but also poses a potential ignition source risk. Furthermore, long-term high-temperature operation can easily cause material agglomeration and deactivation, reducing long-term stability. To address this issue, methods such as noble metal modification, material morphology control, or the introduction of other materials to construct heterojunctions are commonly employed. However, noble metal modification (such as Pt, Pd, and Au) significantly increases material costs, limiting its large-scale application; while simple morphology control or heterostructure construction, under room temperature conditions without external heating, still generally suffers from slow response speeds and insufficient long-term stability. Therefore, developing a sensing material that does not require noble metals and can rapidly and stably detect hydrogen at room temperature is of significant practical importance. Summary of the Invention

[0005] To address the problems of slow response speed and high dependence on precious metals in existing hydrogen sensors at room temperature, this invention provides a three-dimensional ZnSnO3-SnO2 heterojunction composite aerogel for rapid hydrogen detection at room temperature and its preparation method.

[0006] The composite aerogel of this invention is composed of chitosan (CS), ZnSnO3-SnO2 heterojunction nanoparticles (ZS), and carboxylated multi-walled carbon nanotubes (c-MWCNTs); wherein the molar ratio of ZnSnO3 to SnO2 is 1:1; the mass fraction of the ZnSnO3-SnO2 heterojunction nanoparticles relative to chitosan is 30wt%~60wt%, and the mass fraction of the carboxylated multi-walled carbon nanotubes relative to chitosan is 8wt%~16wt%; the composite aerogel has a three-dimensional interconnected porous structure with mesoporous-macroporous hierarchies, the carboxylated multi-walled carbon nanotubes form a three-dimensional interconnected conductive network in the chitosan matrix, the ZnSnO3-SnO2 heterojunction nanoparticles are loaded on this conductive network, and pn heterojunctions are formed between the ZnSnO3-SnO2 heterojunction nanoparticles and the carboxylated multi-walled carbon nanotubes.

[0007] The method for preparing the composite aerogel of the present invention includes the following steps: S1. Multi-walled carbon nanotubes (MWCNTs) were dispersed in a mixed acid solution prepared by a volume ratio of concentrated nitric acid (65wt%) and concentrated sulfuric acid (98wt%) of 1:3. The resulting dispersion was refluxed in an oil bath at 70-80℃ for 8 hours to introduce carboxyl functional groups onto the surface of MWCNTs, forming a uniform black suspension. The suspension was filtered and repeatedly washed with deionized water until neutral pH was reached. The solid was collected and dried at 100℃ for 12 hours to obtain carboxylated multi-walled carbon nanotube (c-MWCNT) powder. S2. Zn(NO3)2·6H2O and SnCl4·5H2O were dissolved in a mixed solvent of anhydrous ethanol and deionized water in a volume ratio of 4:6 at a molar ratio of 1:2 to form a transparent precursor solution. NaOH was added stepwise under continuous stirring, with a total amount of 100 mmol of NaOH, first 30 mmol and then 70 mmol. The mixture was stirred at room temperature for 30 minutes. The reaction system was then heated to 80°C and refluxed for 3 hours. After the reaction was completed, the mixture was allowed to cool naturally to room temperature. The resulting precipitate was washed three times each with anhydrous ethanol and deionized water, and dried overnight at 60°C. Finally, the dried powder was heated to 400°C in air at a heating rate of 2°C / min and annealed for 2 hours to obtain ZnSnO3-SnO2 heterojunction nanoparticles with a ZnSnO3 to SnO2 molar ratio of 1:1, denoted as ZS. S3. The ZS is ultrasonically dispersed in deionized water, and c-MWCNT is ultrasonically dispersed in a 1% (v / v) acetic acid solution. Chitosan (degree of deacetylation ≥ 85%) is dissolved in a 1% (v / v) acetic acid solution and stirred at room temperature until completely dissolved. Then, the c-MWCNT dispersion is added and stirred at room temperature for 1 hour to obtain a uniform polymer dispersion. Under continuous stirring, the pre-dispersed ZS suspension is added dropwise to ensure uniform distribution of inorganic components. The resulting mixture is cast into a mold, rapidly frozen in liquid nitrogen, and then freeze-dried to obtain the composite aerogel. The mass fraction of ZS relative to chitosan is adjustable in the range of 30wt% to 60wt%, and the mass fraction of c-MWCNT relative to chitosan is adjustable in the range of 8wt% to 16wt%.

[0008] Preferably, the freeze-drying in step S3 is: first pre-freezing at -10°C for 10 hours, and then freeze-drying at -50°C for 48 hours.

[0009] Preferably, the ZS has a mass fraction of 50 wt% relative to chitosan, and the c-MWCNT has a mass fraction of 14 wt% relative to chitosan, and the resulting composite aerogel is designated as CS / ZS / c-MWCNT-4.

[0010] Compared with the prior art, the present invention has the following advantages: (1) The present invention uses a three-dimensional porous aerogel framework constructed by chitosan, which has a mesoporous-macroporous hierarchical structure (specific surface area 17.1 m² / g, average pore size 19.2 nm), which significantly improves gas diffusion efficiency and is beneficial to the rapid adsorption and desorption of hydrogen.

[0011] (2) Carboxylated multi-walled carbon nanotubes form a three-dimensional interconnected conductive network in the chitosan matrix, which not only effectively inhibits the aggregation of ZnSnO3-SnO2 nanoparticles, but also provides charge transport channels and maximizes the exposure of active sites, thereby accelerating the response and recovery kinetics.

[0012] (3) The present invention utilizes the pn heterojunction formed between ZnSnO3-SnO2 and carboxylated multi-walled carbon nanotubes, as well as the nn heterojunction between ZnSnO3 and SnO2, to synergistically regulate the interfacial charge transfer, significantly enriching oxygen vacancies (the EPR spectrum shows a strong characteristic signal at g=2.0012), further enhancing the sensitivity and stability of the material.

[0013] (4) Based on the above synergistic effect, this invention can achieve rapid detection of hydrogen at room temperature without the need for precious metal modification and external heating. The results of the examples show that its response value to 1000 ppm H2 is 1.99, the response time is only 15 s, the recovery time is 21 s, the detection limit is as low as 14 ppm, and it exhibits excellent long-term stability (retaining 90.4% of the initial response after 31 days) and selectivity. Within the strain range of 0% to 65%, its mechanical properties are superior to the control sample without carbon nanotubes, which is beneficial for maintaining structural integrity. Attached Figure Description

[0014] Figure 1 This is a schematic diagram of the preparation process of the composite aerogel of the present invention, wherein (a) is a preparation diagram of c-MWCNT, (b) is a preparation diagram of ZS, and (c) is an overall preparation process diagram of the composite aerogel. Figure 2 The XRD characterization results are shown in (a), where (b) is the XRD pattern of CS / c-MWCNT aerogel, (c) is the XRD comparison of ZnSn(OH)6 precursor and annealed ZS, and (d) is the XRD pattern of CS / ZS / c-MWCNT composite aerogel. Figure 3 Scanning electron microscope (SEM) images of the CS / ZS-3 and CS / ZS / c-MWCNT-4 composite aerogels at different magnifications; Figure 4 X-ray photoelectron spectroscopy (XPS) spectra of CS / ZS / c-MWCNT aerogel, where (a) is the total spectrum, (b) is the C 1s spectrum, (c) is the Sn 3d spectrum, and (d) is the Zn 2p spectrum. Figure 5 Electron paramagnetic resonance (EPR) spectra of CS / ZS-3 and CS / ZS / c-MWCNT-4 aerogels; Figure 6 N2 adsorption-desorption isotherms and pore size distribution curves of CS / ZS / c-MWCNTs-4 aerogel; Figure 7 Compressive stress-strain curves of CS / ZS-3 and CS / ZS / c-MWCNT-4 aerogels; Figure 8 The following are the sensing performance graphs of CS / ZS sensors with different ZS doping amounts for 1000ppm hydrogen gas, where (a) is the response curve, (b) is a summary graph of response value, response time and recovery time, and (c) is the resistance response curve of CS / ZS-3. Figure 9The following are the sensing performance graphs of CS / ZS / c-MWCNT sensors with different c-MWCNT doping amounts, where (a) and (b) are performance summary graphs, and (c) is a resistance response curve graph. Figure 10 The following are the comprehensive performance graphs of the CS / ZS / c-MWCNT-4 sensor, where (a) is the response graph for different hydrogen concentrations, (b) is the response-concentration linear relationship graph, (c) is the repeatability graph, (d) is the long-term stability graph, (e) is the humidity effect graph, and (f) is the selectivity graph. Detailed Implementation

[0015] To facilitate understanding of the present invention, the technical solutions described below are further explained in conjunction with specific embodiments and accompanying drawings, but the present invention is not limited thereto. All raw materials used in the following embodiments are commercially available. The following embodiments provide specific implementation methods and effect verification of the composite aerogel of the present invention; comparative examples are used to compare with the embodiments to demonstrate the beneficial effects of the present invention.

[0016] Example 1: (1) Preparation of carboxylated multi-walled carbon nanotubes (c-MWCNT): 1g of multi-walled carbon nanotube powder was dispersed in 100mL of mixed acid solution, which was prepared by mixing concentrated nitric acid (65wt%) and concentrated sulfuric acid (98wt%) in a volume ratio of 1:3; the mixture was placed in an oil bath at 70-80℃ and refluxed for 8 hours to introduce carboxyl functional groups on its surface to obtain a uniform black suspension; the suspension was filtered and repeatedly washed with deionized water until the filtrate was neutral; the solid was collected and dried at 100℃ for 12 hours to obtain c-MWCNT powder.

[0017] (2) Preparation of ZnSnO3-SnO2 heterojunction nanoparticles (ZS): Zn(NO3)2·6H2O (3mmol) and SnCl4·5H2O (6mmol) were dissolved in a mixed solvent consisting of anhydrous ethanol (40mL) and deionized water (60mL); NaOH (30mmol) and NaOH (70mmol) were added sequentially under stirring, and the mixture was stirred for 30 minutes. Then, the mixture was refluxed at 80℃ for 3 hours and allowed to cool naturally to room temperature. The product was washed three times each with anhydrous ethanol and deionized water, and dried at 60℃ overnight. Finally, the mixture was heated to 400℃ in air at 2℃ / min and annealed for 2 hours to obtain a powder with a ZnSnO3 to SnO2 molar ratio of 1:1, which was denoted as ZS.

[0018] (3) Preparation of composite aerogel: ZS was ultrasonically dispersed in 10 mL of deionized water, and c-MWCNT was ultrasonically dispersed in 10 mL of 1% acetic acid solution. 0.4 g of chitosan (degree of deacetylation ≥ 85%) was added to the solution containing c-MWCNT and stirred at room temperature for 1 hour to form a uniform polymer dispersion. The pre-dispersed ZS suspension was added dropwise under continuous stirring to ensure uniform distribution of inorganic components. The mixture was cast into a 10 mL mold, rapidly frozen in liquid nitrogen, and then freeze-dried (pre-frozen at -10℃ for 10 hours, then freeze-dried at -50℃ for 48 hours) to obtain composite aerogel. The mass fraction of ZS relative to chitosan was 50 wt%, and the mass fraction of c-MWCNT relative to chitosan was 14 wt%. The resulting sample was denoted as CS / ZS / c-MWCNT-4.

[0019] (4) Hydrogen sensing test: A conductive copper sheet with a radius of 1 cm was used as an electrode. A cylindrical aerogel (2 cm in diameter and 2 cm in height) was sandwiched between two electrode sheets and placed in a 10 L sealed gas chamber. The wires were connected to an electrochemical workstation. The target concentration of hydrogen was prepared using the static gas mixing method. The response value was defined as... ,in Resistance in air Resistance in hydrogen gas; response time With recovery time These are defined as the time required for the resistance change to reach 90% of the total change.

[0020] (5) Structural characterization: XRD ( Figure 2 c) shows that the composite aerogel contains the (002) peak of c-MWCNT, the broadened peak cluster of ZS and the amorphous halo of chitosan, with no impurity peaks. Each component retains its inherent crystal structure and forms a tight interfacial contact; among them, the ZnSn(OH)6 precursor is transformed into a coexistence of SnO2 and ZnSnO3 phases after annealing. Figure 2 b). SEM ( Figure 3 The results show that it exhibits a uniform, well-connected three-dimensional porous network, with ZS nanoparticles uniformly loaded onto the c-MWCNT network. XPS ( Figure 4 The overall spectrum shows five elements: Zn, Sn, O, C, and N; the C 1s spectrum can be fitted to 284.8 eV (CC), 285.6 eV (C=O), 286.7 eV (CO / CN), and 288.5 eV (OC=O); Sn 3d 5 / 2 Sn 3d 3 / 2 The binding energies are 486.9 eV and 495.5 eV respectively (the splitting energy is 8.6 eV), confirming that Sn 4+ Zn 2p 3 / 2 Zn 2p 1 / 2The binding energies are 1022.3 eV and 1045.3 eV respectively (split energy 23.0 eV), confirming that Zn 2+ The nitrogen adsorption-desorption isotherm is type IV with an H3 hysteresis loop, a specific surface area of ​​17.1 m² / g, and an average pore size of 19.2 nm, confirming a mesoporous-macroporous hierarchical structure. EPR ( Figure 5 At g=2.0012, the c-MWCNT exhibits a characteristic symmetrical signal of oxygen vacancies, and its intensity is significantly higher than that of Comparative Example 2 (CS / ZS-3), indicating that the introduction of c-MWCNT effectively enriches oxygen vacancies in the ZS component.

[0021] (6) Sensing performance: The CS / ZS / c-MWCNT-4 sensor has a response value of 1.99 to 1000ppmH2 at room temperature, a response time of 15s, and a recovery time of 21s. Figure 9 Within a wide hydrogen concentration range of 500-10000 ppm, the response monotonically increases with concentration, and the response value shows a strong linear correlation with concentration (R0). 2 =0.9813), according to ,in Baseline noise standard deviation, To calibrate the curve slope, the theoretical detection limit was calculated to be 14 ppm. Figure 10 a, Figure 10 b); The resistive response was highly consistent across multiple consecutive exposure-recovery cycles, with a response value of 1.80 to 1000 ppm H2 on day 31, retaining approximately 90.4% of the initial value. Figure 10 c. Figure 10 d); the response gradually weakened as the relative humidity increased from 20%RH to 80%RH, and the response to H2 was significantly higher than the response to carbon monoxide, methanol, ethanol, ammonia, and formaldehyde. Figure 10 e Figure 10 f). In addition, compression test ( Figure 7 This indicates that its stiffness and load-bearing capacity are superior to those of Comparative Example 2 within the strain range of 0% to 65%, which is beneficial for maintaining structural integrity and stability during long-term operation.

[0022] Examples 2-5: Except for replacing the mass fraction of c-MWCNT relative to chitosan in step (3) with 8%, 10%, 12%, and 16 wt%, respectively, the rest was the same as in Example 1, yielding samples CS / ZS / c-MWCNT-1, CS / ZS / c-MWCNT-2, CS / ZS / c-MWCNT-3, and CS / ZS / c-MWCNT-5, respectively. Figure 9 As shown, with the increase of c-MWCNT doping, the sensor response exhibits a volcano-shaped change that first increases and then decreases. All samples show a decrease in resistance under the action of H2, which conforms to the n-type sensing behavior. The overall performance is best when c-MWCNT is 14wt% (Example 1), and excessive c-MWCNT will lead to a decrease in performance.

[0023] Comparative Examples 1-3: ZS was ultrasonically dispersed in 10 mL of deionized water. 0.4 g of chitosan (degree of deacetylation ≥ 85%) was dissolved in 10 mL of 1% (v / v) acetic acid solution and stirred until completely dissolved. The ZS suspension was slowly added dropwise to the chitosan solution while stirring, cast into a 10 mL mold, rapidly frozen with liquid nitrogen, and then freeze-dried to obtain CS / ZS binary aerogels without c-MWCNTs. The mass fractions of ZS relative to chitosan were 30%, 40%, and 60 wt%, respectively, corresponding to Comparative Example 1 (CS / ZS-1), Comparative Example 2 (CS / ZS-2), and Comparative Example 3 (CS / ZS-4).

[0024] In addition, the CS / ZS binary aerogel with a ZS mass fraction of 50 wt% was designated as CS / ZS-3 and served as the main control (hereinafter referred to as Comparative Example 2). Figure 8 As shown, with the increase of ZS doping, the response of the CS / ZS sensor exhibits a volcano-like change, with CS / ZS-3 having the highest response value (1.036 for 1000ppmH2), but its response time of 111s and recovery time of 148s are significantly slower than those of Example 1 (15s / 21s).

Claims

1. A composite aerogel for detecting hydrogen at room temperature, characterized in that, The composite aerogel is composed of chitosan, ZnSnO3-SnO2 heterojunction nanoparticles and carboxylated multi-walled carbon nanotubes. In the ZnSnO3-SnO2 heterojunction nanoparticles, the molar ratio of ZnSnO3 to SnO2 is 1:1, and its mass fraction relative to chitosan is 30wt%~60wt%; the carboxylated multi-walled carbon nanotubes have a mass fraction relative to chitosan of 8wt%~16wt%. The composite aerogel has a three-dimensional interconnected porous structure with mesoporous-macroporous hierarchies. The carboxylated multi-walled carbon nanotubes form a three-dimensional interconnected conductive network in the chitosan matrix. The ZnSnO3-SnO2 heterojunction nanoparticles are in contact with the carboxylated multi-walled carbon nanotubes and form a pn heterojunction structure at the interface.

2. The composite aerogel according to claim 1, characterized in that, The ZnSnO3-SnO2 heterojunction nanoparticles have a mass fraction of 50 wt% relative to chitosan, and the carboxylated multi-walled carbon nanotubes have a mass fraction of 14 wt% relative to chitosan.

3. The composite aerogel according to claim 1, characterized in that, The aerogel is enriched with oxygen vacancies, and its electron paramagnetic resonance spectrum exhibits a characteristic symmetric signal of oxygen vacancies at g=2.0012.

4. The composite aerogel according to claim 1, characterized in that, The composite aerogel exhibits a type IV nitrogen adsorption-desorption isotherm with an H3 type hysteresis loop, a specific surface area of ​​17.1 m² / g, and an average pore size of 19.2 nm.

5. The composite aerogel according to claim 1, characterized in that, The composite aerogel is cylindrical with a diameter of 2 cm and a height of 2 cm; conductive electrodes are connected to both ends of the composite aerogel to form a room temperature hydrogen sensing element.

6. A method for preparing the composite aerogel according to claim 1, characterized in that, Includes the following steps: S1. Disperse multi-walled carbon nanotubes in a mixed acid solution prepared by concentrated nitric acid and concentrated sulfuric acid in a volume ratio of 1:3, and reflux in an oil bath at 70-80℃ for 8 hours to introduce carboxyl groups on the surface. After filtration, washing with deionized water until neutral, and drying, carboxylated multi-walled carbon nanotubes are obtained. S2. Zn(NO3)2·6H2O and SnCl4·5H2O were dissolved in a mixed solvent of anhydrous ethanol and deionized water at a molar ratio of 1:

2. NaOH was added stepwise and the mixture was refluxed at 80°C. After washing and drying, the mixture was annealed in air to obtain ZnSnO3-SnO2 heterojunction nanoparticles with a molar ratio of ZnSnO3 to SnO2 of 1:

1. S3. The ZnSnO3-SnO2 heterojunction nanoparticles and carboxylated multi-walled carbon nanotubes are ultrasonically dispersed separately, then mixed evenly with an acetic acid solution of chitosan, cast into a mold, pre-frozen in liquid nitrogen at -10°C for 10 hours, and then freeze-dried at -50°C for 48 hours to obtain the composite aerogel, wherein the mass fraction of the ZnSnO3-SnO2 heterojunction nanoparticles relative to chitosan is 50 wt%, and the mass fraction of the carboxylated multi-walled carbon nanotubes relative to chitosan is 14 wt%. The ZnSnO3-SnO2 heterojunction nanoparticles have a mass fraction of 30wt% to 60wt% relative to chitosan, and the carboxylated multi-walled carbon nanotubes have a mass fraction of 8wt% to 16wt% relative to chitosan.

7. The preparation method according to claim 6, characterized in that, In step S1, the concentrated nitric acid has a mass fraction of 65 wt%, and the concentrated sulfuric acid has a mass fraction of 98 wt%; the drying temperature is 100°C, and the drying time is 12 hours.

8. The preparation method according to claim 6, characterized in that, In step S2, the volume ratio of anhydrous ethanol to deionized water is 4:6; the total amount of NaOH is 100 mmol, with 30 mmol added first and then 70 mmol added; the reflux reaction time is 3 hours.

9. The preparation method according to claim 6, characterized in that, The annealing in step S2 is as follows: heating to 400°C at a heating rate of 2°C / min and holding at that temperature for 2 hours.

10. The preparation method according to claim 6, characterized in that, In step S3, the degree of deacetylation of the chitosan is ≥85%, and the chitosan is dissolved in a 1% (v / v) acetic acid solution; the carboxylated multi-walled carbon nanotubes are ultrasonically dispersed in a 1% (v / v) acetic acid solution.

Images