A MOF-derived Au-supported In₂O₃ nanotube and its preparation method

Abstract

This invention proposes a MOF-derived Au-supported In₂O₃ nanotube and its preparation method. Using indium nitrate pentahydrate as a raw material, terephthalic acid as an organic ligand, and N-N-dimethylformamide as a solvent, the metal-organic framework material In-MIL-68 is prepared by a solvothermal method. Subsequently, Au is provided by a NaAuCl₄ solution. 3+ Ions were uniformly permeated into the In-MIL-68 cavity, and Au was reduced with sodium borohydride. 3+ Au@In-MIL-68 was obtained, and Au-In2O3 was synthesized by heat treatment. Gas-sensing experiments showed that the response value of Au-loaded In2O3 nanotubes to 50 ppm CO was 18.2, approximately 2.8 times that of pure In2O3. The response and recovery times were ultrashort (37 s / 86 s) to 50 ppm, with a detection limit of 0.75 ppm. The sensing material exhibited good periodicity and stability at an operating temperature of 240 °C. The electronic and chemical sensitization effects generated by Au loading jointly enhanced the gas-sensing effect of In2O3. Density functional theory calculations indicated that Au loading enhanced the gas adsorption energy, and Au-loaded In2O3 significantly increased its adsorption energy for CO, strengthening its interaction with CO.

Description

Technical Field

[0001] This invention belongs to the field of gas sensor technology, specifically relating to a MOF-derived Au-supported In2O3 nanotube for monitoring low concentrations of CO and its preparation method. Background Technology

[0002] Carbon monoxide (CO) is a toxic and harmful air pollutant, typically produced by the incomplete combustion of fuels (including exhaust gases, chemicals, etc.). Studies have shown that carbon monoxide readily binds to hemoglobin, leading to tissue hypoxia. Excessive inhalation can cause numerous poisoning symptoms, with severe cases resulting in coma or even death. According to the World Health Organization's recommendations for carbon monoxide exposure standards, a concentration of 9 ppm in the air can cause poisoning in humans within 8 hours. Furthermore, even low concentrations of CO can have mild effects on the human body, including symptoms such as dizziness and fatigue. Therefore, reducing the detection limit of CO to ppb levels is crucial in daily life and industrial environments. Currently, there is an urgent need for a CO gas sensor with high sensitivity and a low detection limit. Gas sensors based on semiconductor metal oxides are considered one of the most promising detection devices due to their low cost, real-time monitoring, and high response. Among them, In₂O₃, a typical wide-bandgap n-type semiconductor material with low resistivity and high catalytic activity, is one of the most promising CO detection sensing materials. However, CO sensors based on single-component In₂O₃ face challenges related to high detection limits and high operating temperatures.

[0003] Currently, noble metals can be used as active catalysts to modify metal oxides, resulting in chemical and electronic sensitization, which has proven to be an effective method for improving sensing performance. However, noble metal nanoparticles are prone to aggregation during synthesis, thereby reducing the utilization of active sites. Therefore, further research is needed on simple methods that can control the morphology of noble metals while achieving effective doping. Metal-organic frameworks (MOFs) have advantages such as high specific surface area and ultra-high porosity, and their preparation as precursors or templates for nano-metal oxides has become a mature method. Compared with traditional methods, these metal oxides have a higher specific surface area and an ordered porous structure, which is beneficial for generating a high density of exposed active sites. In addition, they are easy to dope with highly uniformly distributed noble metal nanoparticles, which can effectively regulate the electronic structure. After annealing, noble metal MOFs can be easily transformed into highly dispersed noble metal nanoparticle functionalized semiconductor gas-sensitive oxide materials. Summary of the Invention

[0004] (a) Technical problems to be solved

[0005] This invention proposes a MOF-derived Au-supported In2O3 nanotube and its preparation method to solve the technical problem of how to achieve high sensitivity and high selectivity detection of low concentration CO.

[0006] (II) Technical Solution

[0007] To address the aforementioned technical problems, this invention proposes a method for preparing MOF-derived Au-supported In₂O₃ nanotubes. The method for preparing Au-supported In₂O₃ nanotubes includes the following steps:

[0008] S1. Using indium nitrate pentahydrate as raw material, terephthalic acid as organic ligand, and N,N dimethylformamide as solvent, indium nitrate pentahydrate and terephthalic acid were ultrasonically dissolved in N,N dimethylformamide. The resulting mixture was placed in a reaction vessel for solvothermal reaction. The product was centrifuged and washed to obtain MOF material In-MIL-68.

[0009] S2. Disperse In-MIL-68 in deionized water, and add NaAuCl4 aqueous solution to the In-MIL-68 dispersion to provide Au. 3+ Ions, Au 3+ Ions were uniformly permeated into the cavity of In-MIL-68. After stirring, an appropriate amount of NaBH4 aqueous solution was added dropwise to the mixture. After stirring, Au was removed through NaBH4. 3+ The product was reduced to Au nanoparticles, collected by centrifugation, and vacuum dried to obtain Au@In-MIL-68.

[0010] S3. Heat-treat Au@In-MIL-68 to obtain Au-supported In2O3 nanotubes.

[0011] Further, in step S1, the mass of indium nitrate pentahydrate is 200 mg, terephthalic acid is 50 mg, and N,N dimethylformamide is 30 mL.

[0012] Furthermore, in step S1, the solvothermal reaction time is 4 hours and the solvothermal temperature is 100°C.

[0013] Further, in step S2, the mass of In-MIL-68 is 80 mg, the concentration of NaAuCl4 aqueous solution is 55 mM and the volume is 27 μl, and the concentration of NaBH4 aqueous solution is 26 mM and the volume is 42 μl.

[0014] Furthermore, in step S3, the heat treatment temperature is 500℃ and the heat treatment time is 4h.

[0015] Furthermore, this invention also proposes a MOF-derived Au-supported In2O3 nanotube, which is prepared using the above method.

[0016] Furthermore, Au is in a trivalent state, and the doping amount of Au is 0.1–0.3 wt%.

[0017] In addition, the present invention also proposes a method for fabricating an Au-loaded In2O3 gas sensor for CO detection. The method includes mixing the above-mentioned Au-loaded In2O3 nanotubes with deionized water, grinding them into a paste in agate slurry, applying the paste to interdigitated electrodes, and annealing at 240°C for 12 hours to obtain the Au-loaded In2O3 gas sensor.

[0018] Furthermore, this invention also proposes an Au-loaded In2O3 gas sensor for CO detection, which is prepared using the above method.

[0019] Furthermore, this invention also proposes a method for detecting low concentrations of CO in an industrial environment, using the aforementioned Au-loaded In2O3 gas sensor.

[0020] (III) Beneficial Effects

[0021] This invention proposes a MOF-derived Au-supported In₂O₃ nanotube and its preparation method. Using indium nitrate pentahydrate as a raw material, terephthalic acid as an organic ligand, and N,N-dimethylformamide as a solvent, the metal-organic framework material In-MIL-68 is prepared by a solvothermal method. Subsequently, NaAuCl₄ solution is used to provide Au... 3+ Ions were uniformly permeated into the In-MIL-68 cavity, and Au was reduced with sodium borohydride. 3+ Au@In-MIL-68 was obtained, and Au-In2O3 was obtained by heat treatment. Gas-sensing experiments showed that the Au-supported In2O3 nanotubes had a response value of 18.2 to 50 ppm CO, approximately 2.8 times that of pure In2O3. The response and recovery times were ultra-short (37 s / 86 s) to 50 ppm, with a detection limit of 0.75 ppm. The sensing material exhibited good periodicity and stability at an operating temperature of 240 °C. The electronic and chemical sensitization effects generated by Au loading jointly enhanced the gas-sensing effect of In2O3. Density functional theory calculations showed that Au loading enhanced the gas adsorption energy, and Au-supported In2O3 significantly increased its adsorption energy for CO, enhancing its interaction with CO. The Au-supported In2O3 nanotubes of this invention have potential applications in the detection of low-concentration CO. Attached Figure Description

[0022] Figure 1 Thermogravimetric analysis curves of the prepared metal-organic framework material in In-MIL-68;

[0023] Figure 2XRD diffraction patterns of Au-doped In2O3 gas-sensitive materials prepared with different doping amounts;

[0024] Figure 3 Scanning electron microscope image of the Au-doped In₂O₃ gas-sensitive material prepared in Example 2;

[0025] Figure 4 Transmission electron microscopy image of the Au-doped In₂O₃ gas-sensitive material prepared in Example 2;

[0026] Figure 5 Electrochemical impedance spectroscopy of the Au-doped In₂O₃ gas-sensitive materials prepared in Examples 1-3;

[0027] Figure 6 Line graphs showing the response of Au-doped In₂O₃ gas-sensitive materials prepared in Examples 1-3 to 50 ppm of carbon monoxide in the temperature range of 180-260 °C.

[0028] Figure 7 The response curves of the Au-doped In₂O₃ gas-sensitive material prepared in Example 2 to 50–0.75 ppm of carbon monoxide at 240 °C;

[0029] Figure 8 The response curve of the Au-doped In₂O₃ gas-sensitive material prepared in Example 2 to 50 ppm of carbon monoxide at 240 °C;

[0030] Figure 9 The response curves of the Au-doped In2O3 gas-sensitive material prepared in Example 2 to 50 ppm carbon monoxide and four other interfering gases at 240 °C.

[0031] Figure 10 denoted as CO adsorption energy on In2O3 and Au-doped In2O3 surfaces. Detailed Implementation

[0032] To make the objectives, contents, and advantages of the present invention clearer, the specific embodiments of the present invention will be described in further detail below with reference to the accompanying drawings and examples.

[0033] Example 1

[0034] Preparation of Au-doped In₂O₃ gas-sensitive material with an Au doping content of 0.1 wt%.

[0035] S1. Weigh 50 mg of terephthalic acid (H2BDC) and ultrasonically disperse it in 30 ml of N-N dimethylformamide (DMF) until it is completely dissolved.

[0036] S2. Add 200 mg of indium nitrate pentahydrate (In(NO3)3·5H2O) to the solution in step S1, and continue sonicating for 2 hours until it is completely dissolved. At this time, the solution turns milky white.

[0037] S3. Transfer the milky white solution from step S2 to a reaction vessel and react at 100°C for 4 hours.

[0038] S4. After the reactor has cooled naturally, wash it three times with deionized water and anhydrous ethanol, collect the product, and dry it overnight at 60°C to obtain the metal-organic framework material In-MIL-68.

[0039] S5. Weigh 80 mg of In-MIL-68 and disperse it in 5 mL of deionized water. Then, add 13.5 μl of NaAuCl4 aqueous solution (55 mM) to the In-MIL-68 dispersion and stir for 0.5 h.

[0040] S6. Add an appropriate amount of sodium borohydride aqueous solution (26mM) dropwise to the mixture, stir for 1 hour, and allow Au to settle. 3+ It is reduced to gold nanoparticles (NPs).

[0041] S7. Collect the product by centrifugation with deionized water and anhydrous ethanol, and dry it under vacuum at 60°C for 24 h to obtain Au@In-MIL-68.

[0042] S8. Annealing Au@In-MIL-68 at 500℃ for 4h yielded a 0.1wt% Au-doped In2O3 sample.

[0043] Example 2

[0044] Preparation of Au-doped In₂O₃ gas-sensitive material with an Au doping content of 0.2 wt%.

[0045] Similar to Example 1, only the amount of NaAuCl4 aqueous solution added was changed, that is, 27.0 μl of NaAuCl4 aqueous solution was added to the In-MIL-68 dispersion.

[0046] Example 3

[0047] Preparation of Au-doped In₂O₃ gas-sensitive material with an Au doping content of 0.3 wt%.

[0048] Similar to Example 1, only the amount of NaAuCl4 aqueous solution added was changed, that is, 41.5 μl of NaAuCl4 aqueous solution was added to the In-MIL-68 dispersion.

[0049] The fabrication process of the Au-doped In₂O₃ gas sensor is as follows:

[0050] (1) Place the obtained Au-doped In2O3 gas-sensitive material in an agate mortar, add an appropriate amount of deionized water, and grind it into a paste with appropriate strength. (2) Use a pipette to drop the paste from (1) onto a cleaned Ag-Pd electrode (14mm×7mm) and let it air dry naturally at room temperature. (3) Before testing, the gas sensor should be continuously aged at 240℃ for 12 hours to make the resistance of the material more stable and to obtain better gas-sensitive test results.

[0051] Figure 1 The TGA analysis curves obtained in atmospheric air represent the decomposition process of the In-MIL-68 precursor obtained in Example 1. The results show significant mass loss in the temperature range of 400℃ to 500℃, which is attributed to the thermal decomposition of the organic framework in the MOF precursor. The mass change of the sample tends to stabilize after 500℃; therefore, 500℃ was chosen as the heat treatment temperature.

[0052] Figure 2 The XRD diffraction patterns of the Au-doped In2O3 gas-sensitive materials obtained in Examples 1-3 are shown in the figure. The figure shows that the diffraction peaks at 21.5°, 30.6°, 35.5°, 41.6°, 45.7°, 51.1°, and 60.7° are consistent with the standard In2O3 spectrum (PDF#89-4595), corresponding to the (211), (222), (400), (332), (431), (440), and (622) crystal planes of cubic In2O3, respectively. More notably, the (111) crystal intensity of Au (JCPDS 89-3697) continuously increases with the increase of Au doping quality, which preliminarily indicates that AuNPs have been successfully loaded into In2O3 nanotubes.

[0053] The morphology and structure of the 0.2% Au-In2O3 obtained in Example 2 were studied using scanning electron microscopy and transmission electron microscopy analysis results. Figure 3 and 4 As shown, the results indicate that 0.2% Au-In2O3 has a hollow porous morphology.

[0054] Figure 5 Electrochemical impedance spectroscopy (EIS) measurements were performed on Au-In₂O₃ and In₂O₃ obtained in Examples 1-3. The results show that 0.2% Au-In₂O₃ has the smallest impedance radius, demonstrating that 0.2% Au-In₂O₃ has higher conductivity, which is beneficial for charge transport.

[0055] Figure 6The gas-sensing response of the Au-In₂O₃ gas-sensitive materials obtained in Examples 1-3 to 50 ppm CO is shown in the temperature range of 180–260 °C. It can be clearly seen that the gas-sensing response of the three groups of samples exhibits a volcanic trend with temperature, determining the optimal operating temperature as 240 °C. Furthermore, due to different Au doping amounts, the sensing performance of In₂O₃ is improved to varying degrees, with the 0.2% Au-In₂O₃ exhibiting the largest gas-sensing response.

[0056] Figure 7 The following table shows the gas-sensing response of the 0.2% Au-In₂O₃ gas-sensitive material obtained in Example 2 to CO concentrations ranging from 50 to 0.75 ppm at an operating temperature of 240°C. It can be seen that the gas-sensing response values ​​change with decreasing concentration, being 18.2, 12.3, 9.8, 7.1, 4.9, 3.4, and 1.7, respectively. Meanwhile, the detection limit of the 0.2% Au-In₂O₃ sensor is 0.75 ppm, indicating that this sensor has good potential for practical detection of low-concentration CO.

[0057] Figure 8 The response curve of the 0.2% Au-In2O3 gas-sensitive material obtained in Example 2 to 50ppm CO at an operating temperature of 240℃ clearly shows that the response time to reach 18.2 is 37s, and the recovery time is 81s.

[0058] Figure 9 The image shows the response curves of the 0.2% Au-In2O3 gas-sensitive material prepared in Example 2 at 240°C to 500 ppm carbon monoxide and four other interfering gases. The results show that the 0.2% Au-In2O3 sensor has a significantly higher response to CO than to other interfering gases, indicating that the 0.2% Au-In2O3 sensor has good selectivity for CO.

[0059] Figure 10 The adsorption energies of CO on the surfaces of the Au-In2O3 and In2O3 gas-sensitive sensing materials prepared in Example 2 are -0.31 eV and -0.63 eV, respectively. The more negative the adsorption energy, the stronger the adsorption capacity of the material for gas and the better its gas sensitivity.

[0060] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the technical principles of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for detecting low concentrations of CO in an industrial environment, characterized in that, The detection method uses an Au-loaded In2O3 gas sensor; wherein, the fabrication method of the Au-loaded In2O3 gas sensor includes mixing Au-loaded In2O3 nanotubes with deionized water, grinding them into a paste in agate slurry, applying the paste to interdigitated electrodes, and annealing at 240°C for 12 hours to obtain the Au-loaded In2O3 gas sensor. The preparation method of the Au-supported In2O3 nanotubes includes the following steps: S1. Using indium nitrate pentahydrate as raw material, terephthalic acid as organic ligand, and N,N dimethylformamide as solvent, indium nitrate pentahydrate and terephthalic acid were ultrasonically dissolved in N,N dimethylformamide. The resulting mixture was placed in a reaction vessel for solvothermal reaction. The product was centrifuged and washed to obtain MOF material In-MIL-68. The mass of indium nitrate pentahydrate was 200 mg, terephthalic acid was 50 mg, and N,N dimethylformamide was 30 mL. The solvothermal reaction time was 4 h, and the solvothermal temperature was 100 °C. S2. Disperse In-MIL-68 in deionized water, and add NaAuCl4 aqueous solution to the In-MIL-68 dispersion to provide Au. 3+ Ions, Au 3+ Ions were uniformly permeated into the cavity of In-MIL-68. After stirring, an appropriate amount of NaBH4 aqueous solution was added dropwise to the mixture. After stirring, Au was removed through NaBH4. 3+ The product was reduced to Au nanoparticles, collected by centrifugation and vacuum dried to obtain Au@In-MIL-68; wherein, the mass of In-MIL-68 was 80 mg, the concentration of NaAuCl4 aqueous solution was 55 mM and the volume was 27 µL, and the concentration of NaBH4 aqueous solution was 26 mM and the volume was 42 µL. S3. Au@In-MIL-68 was heat-treated to obtain Au-supported In2O3 nanotubes; wherein the heat treatment temperature was 500℃ and the heat treatment time was 4h.

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