A nickel molybdate nanocomposite modified with gold nanoparticles and a preparation method thereof

Abstract

This application relates to the technical field of semiconductor gas-sensitive devices, and specifically provides a nickel molybdate nanocomposite modified with gold nanoparticles and a preparation method thereof. The composite material includes spherical nickel molybdate and gold nanoparticles, and the gold nanoparticles are fixedly distributed on the surface of the spherical nickel molybdate. The method includes the following steps: S1, preparing nickel molybdate nanospheres; S2, preparing a nickel molybdate nanocomposite modified with gold nanoparticles. Step S1 includes weighing Ni(NO3)2·6H2O and Na2MoO4·2H2O and dissolving them in deionized water. After complete dissolution, urea and NH4F are added simultaneously, and continuous stirring is carried out until a uniform mixture is formed. Then, it is transferred to a hydrothermal reaction kettle for hydrothermal reaction. The hydrothermal reaction product is centrifuged, washed, and vacuum dried, and finally calcined in air to obtain NiMoO4 nanospheres; Step S2 uses trisodium citrate to reduce HAuCl4 to directly generate gold nanoparticles on the surface of the nickel molybdate nanospheres, and PVP is used as a buffer.

Description

Technical Field

[0001] The present application relates to the technical field of semiconductor gas sensors, and more particularly, to a nickel molybdate nanocomposite modified with gold nanoparticles and a preparation method thereof. Background Art

[0002] Resistive gas sensors convert the type and concentration of a gas to be detected into a change in the resistance of the gas sensor by means of a chemical reaction between the gas to be detected and the gas-sensitive material, thereby achieving the purpose of sensitively detecting the gas to be detected. Gas sensors can be used to detect both toxic and harmful gases at the ppm level and flammable and explosive gases at high percentage concentrations.

[0003] During detection, in an air environment, oxygen will adsorb on the surface of the semiconductor gas-sensitive material, capture electrons inside the semiconductor gas-sensitive material, form oxygen anions, and increase the resistance of the gas sensor. When encountering a reducing gas to be detected, such as flammable gases like ethanol, carbon monoxide, methane, and hydrogen, an oxidation-reduction reaction will occur between the reducing gas and the oxygen anions on the surface of the semiconductor gas-sensitive material, releasing a large number of electrons into the conduction band of the semiconductor gas-sensitive material, reducing the resistance of the gas sensor, and achieving the purpose of detection. The sensitivity of a gas sensor is a measure of the gas-sensing performance of the gas sensor. The higher the sensitivity of the gas sensor, the better the gas-sensing characteristics, which are closely related to the gas-sensitive material of the gas sensor.

[0004] To improve the sensitivity of such traditional semiconductor resistive gas sensors, it is generally achieved by increasing the activity of air and the gas to be detected. That is, the operating temperature of such traditional semiconductor resistive gas sensors is relatively high, mostly around 350°C. This greatly shortens the lifespan of the gas sensor and severely limits the scope of application of the gas sensor, making it difficult to safely and effectively detect flammable and explosive gases. In addition, traditional semiconductor resistive gas sensors have poor selectivity for specific gases.

[0005] In summary, gas sensors prepared based on existing gas-sensitive materials have low sensitivity at room temperature, are difficult to use for detecting flammable and explosive gases, and have poor selectivity. Summary of the Invention

[0006] The purpose of the present invention is to provide, in view of the above-mentioned deficiencies in the prior art, a nickel molybdate nanocomposite modified with gold nanoparticles and a preparation method thereof. By disclosing a new gas-sensitive material and its preparation method, the problems that gas sensors prepared based on existing gas-sensitive materials have low sensitivity at room temperature, are difficult to use for detecting flammable and explosive gases, and have poor selectivity are solved.

[0007] To achieve the above object, the technical concept of the present invention is as follows: The core of improving the sensitivity of the gas sensor device is to increase the degree of the redox reaction during the detection process. The stronger the degree of the redox reaction, the more electrons are constrained and released in the gas sensor device, and the faster the speed of the constraint and release process. As a result, the resistance change of the gas sensor device is more obvious, that is, the higher the sensitivity of the gas sensor device. The traditional semiconductor gas sensor device increases the degree of the redox reaction by enhancing the activity of air and the gas to be measured, that is, a relatively high working temperature is required, so that the gas sensor device cannot work properly at room temperature. This application provides a new gas sensing material - nickel molybdate nanocomposite modified with gold nanoparticles. The surface of the nano-spherical nickel molybdate spheres is modified with gold nanoparticles. The particle sizes of the nickel molybdate spheres and the gold nanoparticles are both in the nanometer scale, and the surfaces of the nickel molybdate spheres and the gold nanoparticles are connected by chemical bonds. The gold nanoparticles are uniformly distributed on the surface of the nickel molybdate spheres. Specifically, under light illumination, local surface plasmon resonance occurs on the metal nanoparticles on the surface of the double-metal oxide semiconductor nickel molybdate, and a strong electric field is generated on the surface of the gold nanoparticles (AuNPs). The strong electric field generates a large number of hot electrons on the surface of the double-metal oxide semiconductor nickel molybdate, so that more oxygen molecules in the air are adsorbed on the surface of the double-metal oxide semiconductor nickel molybdate, thereby constraining more electrons and generating more oxygen anions, that is, the degree of the oxidation process reaction is stronger; when the gas sensor device based on the gas sensing material is in the gas to be measured, on the one hand, the strong electric field makes more gas molecules to be measured gather on the surface of the double-metal oxide semiconductor nickel molybdate, and on the other hand, the strong electric field makes the reduction reaction process of the gas molecules to be measured and the oxygen anions generated in the oxidation process faster, releasing more electrons into the conduction band of the double-metal oxide semiconductor nickel molybdate, that is, more and faster electrons are released in the reduction process, that is, the degree of the oxidation process reaction is stronger; the resistance change of the gas sensor device caused by the oxidation reaction and the reduction process is greater. Therefore, when the gas sensing material of this application is used in the gas sensor device, the sensitivity of the gas sensor device is higher. Since the sensitivity of the gas sensor device is not improved by increasing the activity of air and the gas to be measured, that is, the sensitivity of the gas sensor device based on the gas sensing material of the present invention during operation does not depend on high-temperature conditions, the working temperature of the gas sensor device can be reduced, that is, the gas sensor device based on the gas sensing material prepared by the method of the present invention can work at room temperature and has a high sensitivity.

[0008] This application also provides a preparation method of the nickel molybdate nanocomposite modified with gold nanoparticles, and the method includes the following steps:

[0009] S1, preparing nickel molybdate nanospheres;

[0010] First, weigh 0.0363 - 0.363 g of Ni(NO 3 ) 2 ·6H 2 O and 0.0302 - 0.302 g of Na2 MoO 4 ·2H 2 O is dissolved in 50 - 500 ml of deionized water. After complete dissolution, 0.075 - 0.75 g of urea and 0.0277 - 0.278 g of NH 4 F are added simultaneously, and stirring is continued until a homogeneous mixture is formed. Then, it is transferred to a hydrothermal reaction kettle for hydrothermal reaction. Specifically, the hydrothermal reaction temperature is 120 - 200 °C, and the hydrothermal reaction time is 2 - 12 h, which can ensure the full progress of the hydrothermal reaction. The precipitate obtained from the hydrothermal reaction is centrifuged and washed. Among them, NH 4 F is a surfactant, which can effectively increase the active sites on the surface of nickel molybdate. Urea participates in the reaction, and finally nickel molybdate hydrate is formed. The obtained nickel molybdate hydrate is dried in vacuo at 60 °C for 6 h, and finally calcined. The calcination process lasts for 0.5 - 2 h, and the calcination temperature is 350 - 450 °C, and finally powdery nickel molybdate nanospheres are obtained.

[0011] S2 to prepare AuNPs - modified nickel molybdate nanocomposites.

[0012] Using sodium citrate (Na 3 C 6 H 5 O 7 ·2H 2 O) to reduce HAuCl 4 to directly generate gold nanoparticles on the surface of nickel molybdate nanospheres, using PVP (polyvinylpyrrolidone K30) as a buffer. Specifically, a 0.01 mol / L HAuCl 4 solution is prepared; 0.0218 - 0.218 g of the nickel molybdate nanosphere powder prepared in step S1 is weighed and dissolved in 50 - 500 ml of deionized water; then 0.1 - 1.0 g of PVP and 0.5 - 5.0 ml of the above - prepared 0.01 mol / L HAuCl 4 aqueous solution are added to the above nickel molybdate mixture, and stirring is continued on a magnetic stirrer to make the solution evenly mixed. While stirring, the mixed solution is heated to boiling; then, while stirring, 1 - 10 ml of a 1% Na 3 C 6 H 5 O 7 ·2H 2 O aqueous solution is added, and boiling is continued. After the reaction is completed, it is naturally cooled to room temperature, and then stirred to make it fully mixed. After centrifugation, washing, and drying, it is stored for standby, and thus powdery AuNPs - modified nickel molybdate nanocomposites are obtained.

[0013] In the present invention, the preparation of HAuCl 4The solvent of the solution is one of double-distilled water, triple-distilled water, and high-quality deionized water, and the prepared HAuCl 4 The glass container for the solution must be absolutely clean. Before use, it is washed with acid and rinsed thoroughly with deionized water, and then dried for standby. At the same time, when preparing the HAuCl 4 solution, a metal spatula cannot be used for weighing.

[0014] At the same time, the process of preparing the AuNPs-modified nickel molybdate nanocomposite material by the method of the present invention is simple, pollution-free, efficient, and stable, and the prepared composite material has high crystallinity. The gas sensor device based on the gas-sensitive material prepared by the method of the present invention can work at room temperature, and has high sensitivity, with broad application prospects.

[0015] In order to test the gas-sensing characteristics of the gas-sensitive material prepared by this method, a gas-sensing test device was prepared, and at the same time, the gas-sensing performance of the prepared gas-sensitive material was tested using the HCRK-SD101 four-channel gas-sensing performance test software. The results show that the gas sensor device based on the gas-sensitive material prepared by the method of the present invention has high sensitivity. Since the high sensitivity of detection does not depend on the activity of the background gas or the gas to be detected, it does not need to be carried out at high temperature and can be detected at room temperature, which is particularly significant for the detection of flammable and explosive gases.

[0016] Compared with the prior art, the beneficial effects of the present invention are as follows: Using the prepared AuNPs-modified nickel molybdate nanocomposite material as the gas-sensitive material, under the illumination condition, by utilizing the local surface plasmon resonance effect generated by AuNPs, high-sensitivity detection of the gas to be detected at room temperature is realized, with short response time and good selectivity. Moreover, the preparation method of the present invention is simple, pollution-free, efficient, and stable, the synthesized material has high crystallinity, and the gas sensor device based on the gas-sensitive material prepared by the method of the present invention can work at room temperature, with broad application prospects. Therefore, the gas sensor device based on the gas-sensitive material prepared by the method of the present invention has high sensitivity, is suitable for gas detection at room temperature, can be used for the detection of flammable and explosive gases, and has good selectivity. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 It is a schematic diagram of a preparation method of a gold nanoparticle-modified nickel molybdate nanocomposite material provided by the present invention;

[0018] Figure 2 It is a physical diagram of the nickel molybdate nanosphere material obtained in step S1 in the preparation method of a gold nanoparticle-modified nickel molybdate nanocomposite material provided by the present invention;

[0019] Figure 3The physical diagram of the AuNPs-modified nickel molybdate nanocomposite obtained in step S2 of the preparation method of the gold nanoparticle-modified nickel molybdate nanocomposite provided by the present invention;

[0020] Figure 4 The physical diagram of the gas-sensing test device prepared from the AuNPs-modified nickel molybdate nanocomposite prepared by the preparation method of the gold nanoparticle-modified nickel molybdate nanocomposite provided by the present invention.

[0021] Figure 5 The XRD diffraction pattern of the nickel molybdate nanosphere material prepared in step S1 of the preparation method of the gold nanoparticle-modified nickel molybdate nanocomposite provided by the present invention;

[0022] Figure 6 The SEM characterization image of the nickel molybdate nanosphere material prepared in step S1 of the preparation method of the gold nanoparticle-modified nickel molybdate nanocomposite provided by the present invention;

[0023] Figure 7 The SEM characterization image of the AuNPs-modified nickel molybdate nanocomposite prepared in step S2 of the preparation method of the gold nanoparticle-modified nickel molybdate nanocomposite provided by the present invention;

[0024] Figure 8 The response and recovery curve of the gas-sensing device prepared from the material prepared by the preparation method of the gold nanoparticle-modified nickel molybdate nanocomposite provided by the present invention. Detailed implementation mode

[0025] In order to make the implementation process of the present invention clearer, the following will be described in detail with reference to the accompanying drawings.

[0026] Example 1:

[0027] The present application provides a new gas-sensing material - a gold nanoparticle-modified nickel molybdate nanocomposite. The surface of the nano-spherical nickel molybdate small balls is modified with gold nanoparticles (AuNPs). The particle sizes of the nickel molybdate small balls and the AuNPs are both in the nanometer range, and the surfaces of the nickel molybdate small balls and the AuNPs are connected by chemical bonds. The AuNPs are evenly distributed on the surface of the nickel molybdate small balls.

[0028] The present invention also provides a preparation method of a gold nanoparticle-modified nickel molybdate nanocomposite, and the method includes the following steps:

[0029] S1, preparing nickel molybdate nanospheres;

[0030] Weigh Ni(NO 3 ) 2 ·6H 2 O and Na 2MoO 4 ·2H 2 O is dissolved in deionized water. After complete dissolution, urea and NH 4 F are added simultaneously, and stirring is continued until a homogeneous mixture is formed. Then, it is transferred to a hydrothermal reaction kettle for hydrothermal reaction. The hydrothermal reaction product is centrifuged, washed, and vacuum dried, and finally calcined in air to obtain NiMoO 4 nanospheres.

[0031] Specifically, first, 0.0302 g of Na 2 MoO 4 ·2H 2 O is weighed and dissolved in 50 ml of deionized water. Stirring is continued until complete dissolution to obtain a Na 2 MoO 4 ·2H 2 O solution with a concentration of 2.5 mmol / L. The concentration of the Na 2 MoO 4 ·2H 2 O solution is crucial for preparing nickel molybdate nanospheres with a loose and porous surface structure and good crystallinity. Changes in concentration will make the morphology and crystallinity of the nickel molybdate nanospheres worse, thus affecting the final gas-sensing properties. Then, 0.0363 g of Ni(NO 3 ) 2 ·6H 2 O of the same amount of substance is added to the above mixed solution, so that divalent nickel ions and molybdate ions can react in a ratio of 1:1. Stirring is continued until the added Ni(NO 3 ) 2 ·6H 2 O is completely dissolved and the mixed solution becomes clear. Then, 0.0277 g of NH 4 F (solution concentration is 15 mmol / L) and 0.075 g of urea (solution concentration is 25 mmol / L) are added together to the mixed solution, and stirring is continued for 1 h to fully dissolve the added NH 4 F and urea. Among them, NH 4 F is a surfactant, which increases the active sites on the surface of nickel molybdate and prepares for the modification of AuNPs. The uniformity and density of the distribution of the active sites determine the distribution of AuNPs, and the uniformity and density of the distribution of AuNPs determine the gas-sensing properties of the synthesized gas-sensing material. The better the uniformity of AuNPs, the stronger the enhancement of the gas-sensing properties of the gas-sensing material. Specifically, when the concentration remains unchanged, optimally, NH 4The ratio of the amounts of F and urea is 0.37. In this way, the action intensity of the surfactant remains constant. If the proportion of the surfactant is too small, the action of the surfactant is not obvious. When the proportion of the surfactant is relatively large, on the one hand, it causes waste of the surfactant, and on the other hand, too much surfactant will produce side effects. Specifically, since NH 4 F is a salt of strong acid and weak base, the pH value of the reaction solution is closely related to the amount of NH 4 F. If the amount of NH 4 F is too large, the pH value of the solution is too low, which has a great impact on the surface morphology and the main exposed crystal plane of nickel molybdate, causing the main exposed crystal plane to change from 220 to 110, and the surface morphology is no longer spherical, so that nickel molybdate nanospheres cannot be obtained, and the action of the surfactant is not obvious either.

[0032] Then, the obtained mixed solution is put into a hydrothermal reaction kettle for hydrothermal reaction. The precipitate obtained from the hydrothermal reaction is centrifuged and washed several times with a mixed solution of ethanol and deionized water in a ratio of 1:3 to remove impurity ions such as sodium ions and nitrate ions until the pH value of the solution becomes 6.8 - 7.0, and the washing process ends. Specifically, nickel nitrate is a salt of strong acid and weak base, and its pH value is 4.0 - 5.0, that is, the mixed solution put into the hydrothermal reaction kettle is weakly acidic. Under acidic conditions, nickel molybdate hydrate is formed. Ethanol and deionized water are used to remove impurity ions such as sodium ions and nitrate ions through multiple washings, making the pH value of the mixed solution become 6.8 - 7.0, indicating that the impurity ions have been removed completely. More specifically, the hydrothermal reaction is carried out at 120 - 200 °C for 2 - 12 h. In this embodiment, the hydrothermal reaction is carried out at 160 °C for 12 h. At such a temperature, nickel molybdate can nucleate with a relatively high nucleation rate and good crystallinity, and the hydrothermal reaction process is relatively complete. More specifically, the surfactant NH 4 F effectively increases the active sites on the surface of nickel molybdate, preparing for step S2. Urea participates in the reaction. Urea dissolves in deionized water and decomposes into NH 3 and CO 2 . NH 3 further decomposes into NH 4 + and OH - . CO 2 can be hydrolyzed into CO 3 2- and H + . And Ni 2+ is easily reacted with OH- to form Ni(OH) χ precipitate, and reacts with MoO 4 2- under acidic conditions to form nickel molybdate hydrate. More specifically, the equation of the reaction process is: N 2 H 4 CO + H 2 O → NH3 +CO 2 , CO 2 +H 2 O → CO 3 2- +2H + , NH 3 +H 2 O → NH 4 + +OH⁻, Ni(NO 3 ) 2 +Na 2 MoO 4 → Ni 2+ +MoO 4 2- +NaNO 3 +H 2 O, Ni 2+ +χOH - → Ni(OH) χ , Ni(OH) χ +MoO 4 2- +2H + → NiMoO 4 ·χH 2 O。

[0033] Finally, the generated nickel molybdate hydrate is vacuum dried at 60 °C for 6 h in a vacuum drying oven to avoid contamination of the generated nickel molybdate hydrate by other impurities in the air, and then placed in a muffle furnace. Specifically, it is placed in a crucible in the muffle furnace and calcined in the air and reacts with oxygen. Finally, nickel molybdate nanospheres are obtained. The calcination process can form more oxygen vacancies on the surface of the nickel molybdate nanospheres, achieving the effect of sensitizing the material, making the redox reaction process on the gas-sensitive material more intense during the detection process, thereby improving the sensitivity of the gas-sensitive device of the gas-sensitive material prepared by the method of the present invention. Specifically, it is calcined at 350 - 450 °C for 0.5 - 2 h in an air atmosphere. Specifically, the calcination temperature is 400 °C, which can completely decompose the nickel molybdate hydrate into nickel molybdate crystals and water and dry the moisture. The calcination time is 2 h, which can make the above reaction fully completed, obtaining the nickel molybdate nanospheres as shown in Figure 2 Figure

[0034] The prepared nickel molybdate is spherical, with a diameter of several hundred nanometers. Its surface has a loose and porous structure and good crystallinity. The small particle size and loose porous structure effectively increase the specific surface area and porosity of the gas-sensitive material. On the one hand, it provides a prerequisite for preparing AuNPs in step S2, enabling more AuNPs to stably adhere to the surface of nickel molybdate nanospheres; on the other hand, the porous and loose structure allows oxygen molecules and gas molecules to be detected in the air to quickly diffuse into the interior of nickel molybdate nanospheres, and the large specific surface area can adsorb more oxygen molecules and gas molecules to be detected, thereby accelerating the rate of redox reactions and reducing the response time and recovery time of the gas-sensitive reaction. That is, the gas-sensitive device made of the gas-sensitive material prepared by the method of the present invention has a high sensitivity.

[0035] Na 2 MoO 4 ·2H 2 The concentration of the O solution is 2.5 mmol / L; the amount of substance of Ni(NO 3 ) 2 ·6H 2 O is the same as that of Na 2 MoO 4 ·2H 2 O; the concentration of NH 4 F in the solution is 15 mmol / L; the concentration of urea is 25 mmol / L; the ratio of the amounts of NH 4 F and urea is 0.37; the hydrothermal reaction temperature is 160 °C and the reaction time is 12 h; the combined effect of these parameters makes the prepared nickel molybdate nanospheres have a diameter of several hundred nanometers, a loose and porous structure on the surface, and good crystallinity. Different hydrothermal reaction temperatures, hydrothermal reaction times, different drug concentrations, and different ratios will generate different crystals with the main exposed crystal planes, and thus the morphology, surface activity, nucleation situation, etc. of the nickel molybdate nanospheres are all different. The parameters of the present application are essential for preparing nickel molybdate nanospheres with a size of hundreds of nanometers, a loose and porous structure on the surface, and good crystallinity. Figure 5 This is the XRD diffraction pattern of the prepared nickel molybdate nanospheres. Among them, the upper part is the test result of the nickel molybdate nanosphere material prepared in this example, and the lower part is the peak position corresponding to the standard card. The XRD diffraction peaks of the nickel molybdate nanosphere material are completely consistent with the standard card, indicating that the material prepared in step S1 is nickel molybdate. Figure 6 This is the SEM characterization image of the nickel molybdate nanosphere material prepared in step S1, indicating that the prepared nickel molybdate is indeed in the nanosphere morphology. That is, the material prepared in step S1 of the present invention is nickel molybdate nanospheres.

[0036] S2. Prepare a nickel molybdate nanocomposite modified with AuNPs.

[0037] Using trisodium citrate (Na 3 C6 H 5 O 7 ·2H 2 O) Reduction of HAuCl 4 The method was used to directly generate gold nanoparticles on the surface of nickel molybdate nanospheres, using PVP (polyvinyl pyrrolidone K30) as a buffer. 4 The concentration of the solution, Na 3 C 6 H 5 O 7 ·2H 2 The concentration of O solution, the concentration of nickel molybdate mixed solution, and the concentration of PVP, that is, the corresponding ratio, is crucial to the generated AuNPs and the modification effect of AuNPs. Slight changes will lead to significant differences in the gas-sensing properties of the prepared AuNPs-modified nickel molybdate nanocomposites.

[0038] First, prepare HAuCl with a concentration of 0.01 mol / L 4 Solution, HAuCl 4 The concentration of the solution is related to Na 3 C 6 H 5 O 7 ·2H 2 O solution, the ratio of the two can determine the size of the generated AuNPs; weigh 0.0218g of the nickel molybdate nanosphere powder prepared in step S1, dissolve it in 50ml of deionized water, and ultrasonically disperse it for 15min to make NiMoO 4 The powder is fully dispersed in deionized water to obtain a nickel molybdate mixed solution. Specifically, the concentration of the nickel molybdate mixed solution is 2.0 mol / L, the concentration of the acid nickel nanosphere solution is greater than 2.0 mol / L, and the modification effect is not obvious. The concentration of the acid nickel nanosphere solution is less than 2.0 mol / L, and the corresponding AuNPs are more, resulting in unnecessary cost waste. At the same time, too many AuNPs will also affect the contact between the gas to be measured and the background gas and the gas-sensitive material, making the redox reaction weaker, which is not conducive to improving the performance of the gas-sensitive material.

[0039] Next, take 0.1g PVP as a buffer and 0.5ml of the above prepared HAuCl with a concentration of 0.01mol / L 4An aqueous solution. The two are added to the above-mentioned nickel molybdate mixed solution simultaneously and continuously stirred on a magnetic stirrer for 30 min to make the solution uniformly mixed. While stirring, the mixed solution is heated to boiling. The added PVP is used as a buffer, which can effectively prevent the aggregation of AuNPs, thereby promoting its composite with nickel molybdate nanospheres, making AuNPs uniformly dispersed on the surface of nickel molybdate nanospheres, and thus generating local surface plasmon resonance under light irradiation. A strong electric field is generated on and near the surface of AuNPs. The uniform dispersion of AuNPs enables a strong interaction between the strong electric field and the electrons in nickel molybdate nanospheres, the oxygen anions on the surface of nickel molybdate nanospheres, oxygen molecules, and the gas molecules to be detected. Therefore, the intensity of the redox reaction is stronger, and the resistance change in the gas-sensitive material is larger. Thus, the gas-sensitive device based on the gas-sensitive material prepared by the method of the present invention has a higher sensitivity. Specifically, in the case of the HAuCl 4 aqueous solution remaining unchanged, the mass ratio of PVP to the volume of the HAuCl 4 aqueous solution is 1:5. Such a ratio can effectively prevent the aggregation of AuNPs. At the same time, if the content of PVP is too high, excessive PVP molecules will wrap and attach to the surface of HAuCl 4 , hindering its reaction with Na 3 C 6 H 5 O 7 ·2H 2 O, making AuNPs uniformly dispersed on the surface of nickel molybdate nanospheres, and the modification effect is better. The aggregation of AuNPs will make AuNPs not dispersed enough. On the one hand, the surface of some nickel molybdate nanospheres is not modified by AuNPs. On the other hand, the specific surface area of AuNPs is small, and the local surface plasmon effect generated under light irradiation is not strong enough, resulting in a weak strong electric field, which cannot effectively enhance the process of the redox reaction, and ultimately makes the gas-sensitive characteristics of the gas-sensitive material poor. Excessive PVP hinders the redox reaction for the preparation of AuNPs, the size of the generated AuNPs becomes larger, and the amount of AuNPs becomes less, so the modification effect on the nickel molybdate material is not ideal, ultimately affecting the gas-sensitive performance.

[0040] Next, 1 ml of an aqueous solution of Na 3 C 6 H 5 O 7 ·2H 2 O with a concentration of 1% is added to the boiling mixed solution during stirring. The concentration of the aqueous solution of Na 3 C 6 H 5 O 7 ·2H 2 O is 1%, which is the optimal concentration obtained through multiple experiments. At the same time, it is also related to HAuCl 4is closely related to the concentration of the solution, Na 3 C 6 H 5 O 7 ·2H 2 O and the ratio of HAuCl 4 needs to be strictly controlled. Otherwise, it will lead to a large difference in the size of the prepared AuNPs, making the performance of the prepared gas-sensitive material unstable; continue to heat and boil for 15 min, and it can be observed that the light green HAuCl 4 and nickel molybdate mixture quickly turns grayish green after the addition of Na 3 C 6 H 5 O 7 ·2H 2 O, then turns black, and finally gradually stabilizes into dark green. This process takes about 2 - 3 min. Since the color of the AuNPs solution is closely related to its size, the color is lighter when the size is smaller and darker when the size is larger. When the reaction occurs, at the beginning, the amount of Na 3 C 6 H 5 O 7 ·2H 2 O participating in the reaction is less, and the size of the generated AuNPs is larger, and the color is black. As the reaction proceeds, the amount of Na 3 C 6 H 5 O 7 ·2H 2 O participating in the reaction is more, and the size of the generated AuNPs is smaller, and the color gradually becomes lighter until it finally becomes stable orange - red. And the NiMoO 4 solution is light green, so the mixture is dark green. Therefore, the generation of dark green indicates the successful preparation of the mixed solution of small - sized AuNPs and NiMoO 4 ; Na 3 C 6 H 5 O 7 ·2H 2 O can reduce HAuCl 4 to AuNPs. In the present invention, the size of the obtained AuNPs is 10 nm - 150 nm. Preferably, the size of the AuNPs is 10 nm - 20 nm because when the size of the AuNPs is too large, the composite effect with NiMoO 4 is poor, and NiMoO 4There are fewer AuNPs on the surface, and the plasmonic effect is weaker, resulting in a poor enhancement effect; when the size of AuNPs is too small, the surface plasmonic effect of individual AuNPs is weak, and the aggregated electric field intensity is weak, making the enhancement of the sensing process on the redox process not obvious, and thus the enhancement effect on the sensitivity of the gas sensor is not obvious; specifically, the size of the prepared AuNPs is closely related to the proportion of the dosage of Na 3 C 6 H 5 O 7 ·2H 2 O and HAuCl 4 dosage. More specifically, the higher the proportion of Na 3 C 6 H 5 O 7 ·2H 2 O, the smaller the size of the prepared AuNPs. More specifically, when the size of the prepared AuNPs is 10 - 20 nm, the volume ratio of the dosages of HAuCl 4 and Na 3 C 6 H 5 O 7 ·2H 2 O is 1:2.

[0041] Finally, heat continuously, and keep the mixed solution boiling for 30 min to make the reduction reaction proceed sufficiently. After the reaction is completed, stop heating, cool the mixed solution to room temperature, and continue stirring for 1 h to make the generated AuNPs fully adhere to the nickel molybdate nanospheres. After that, centrifuge, wash, and dry to obtain the final product, as Figure 3 shown in the physical picture of the nickel molybdate nanocomposite modified with AuNPs. Specifically, during centrifugation, wash several times with a 1:3 mixture of ethanol and deionized water. Each time, pour out the centrifuged solution and re - add the 1:3 ethanol and deionized water until the pH of the solution becomes 6.8 - 7, and the washing process ends. Then, dry in vacuum at 60 °C for 6 h to avoid the influence of other impurities in the air on the generated material. Finally, obtain the nickel molybdate nanocomposite modified with AuNPs, and the physical picture is as Figure 3 shown. Figure 7 This is the SEM characterization image of the prepared nickel molybdate nanocomposite modified with AuNPs. Among them, the spherical nickel molybdate nanoparticles become more loose and porous on the surface after being modified with AuNPs, which indicates that the nickel molybdate nanocomposite modified with AuNPs is indeed prepared in step S2.

[0042] In addition, prepare HAuCl 4The solvent of the solution is double-distilled water or triple-distilled water, or high-quality deionized water. Impurity ions in the water have a great influence on the synthesis process of AuNPs. Using double-distilled water or triple-distilled water or high-quality deionized water can avoid the influence of impurity ions such as hydrogen ions and hydroxide ions in the solvent on AuNPs, and effectively reduce the influence of the solvent on the synthesis result. Due to the high requirement for cleanliness, dust particles and oil stains and other particles have a great influence on the HAuCl 4 solution. The glass container for preparing the HAuCl 4 solution must be absolutely clean. Before use, it is washed with acid and rinsed thoroughly with deionized water, and then dried for later use. HAuCl 4 has a strong corrosive effect on metals. Therefore, a metal spatula cannot be used to weigh when preparing the HAuCl 4 solution.

[0043] In order to test the gas-sensing characteristics of the nickel molybdate nanocomposite modified with AuNPs prepared by the method of the present invention, the nickel molybdate nanocomposite modified with AuNPs obtained in step S2 is thoroughly ground in a quartz mortar, and then a certain amount of terpineol is added and grinding is continued until a uniform paste mixture is formed, which has a certain fluidity. Then, the above paste mixture is evenly coated on the gas-sensing test device by screen printing. In this way, at high temperature, the terpineol in the mixture volatilizes, and the remaining gas-sensing material can form a dense, flat and uniform-thickness gas-sensing material film. The resistance measurement of such a gas-sensing material film is more accurate. Therefore, the accuracy of the gas-sensing device is higher. Then, it is placed on a hot plate and annealed after solidification. The annealing temperature is 300-400 °C, preferably 400 °C, and the annealing time is 1-4 h, preferably 4 h. Annealing in air can generate oxygen vacancies on the surface of the gas-sensing material, so that the adsorption process and desorption process on the surface of the gas-sensing material are more sufficient, that is, the oxidation process and reduction process are faster, thereby improving the response recovery speed. At 400 °C, it can evaporate hydrates while introducing oxygen vacancies, and the annealing time of 4 h can form more oxygen vacancies. After annealing, it is aged for 24 h, and then the gas-sensing test can be carried out. The physical diagram of the prepared gas-sensing test element is as shown in Figure 4 . More specifically, the substrate of the gas-sensing test device is made of alumina (Al 2 O 3 ), and a platinum (Pt) electrode is covered on the substrate for detecting the resistance change of the gas-sensing test device.

[0044] This application uses the HCRK-SD101 four-channel gas-sensing performance testing software to conduct gas-sensing tests on the gas-sensing performance of the prepared gas-sensing materials. The testing process is carried out at room temperature (25 °C). At the same time, a light source is set directly above the gas-sensing material, and the light emitted by the light source irradiates on the gas-sensing material. Specifically, the light source can be a visible light source or an ultraviolet light source. During the test, the background gas is clean air, and the gas to be measured is ethanol gas. By controlling the ratio of the flow rates of the background gas and the gas to be measured, the concentration of the gas to be measured is controlled. The results of the measured response-recovery curve are as Figure 8 shown by the curve represented by the solid square in. The sensitivity of the gas-sensing test device to 100 ppm ethanol at room temperature is 10, the response time is 30 s, and the recovery time is 100 s. The sensitivity is higher than the result of 45.6%-100% of the sensitivity of ethanol gas at room temperature disclosed by Kuchi, PS et al. in the article titled "A novel room temperature ethanol sensor based on PbS:SnS 2 nanocomposite with enhanced ethanol sensing properties". Specifically, 45.6% means the sensitivity is 1.456, and 100% means the sensitivity is 2, both of which are less than 10 of this embodiment. Specifically, the AuNPs on the surface of the gas-sensing material undergo local surface plasmon resonance under the action of light irradiation, generating a strong electric field on the surface of the AuNPs. The strong electric field generates a large number of hot electrons on the surface of nickel molybdate, thereby adsorbing more oxygen molecules in the air on the surface of nickel molybdate, making the intensity of the oxidation reaction stronger, capable of capturing more electrons in nickel molybdate, increasing the detected resistance, and generating more oxygen anions; in ethanol, under the action of the strong electric field, more gas molecules of ethanol are adsorbed on the surface of nickel molybdate nanospheres, enabling the gas molecules of ethanol to react with the oxygen anions generated during the oxidation process. At the same time, the presence of the strong electric field makes the intensity of the reduction reaction stronger, and the reduction process rapidly releases more electrons, causing the detected resistance to decrease rapidly; that is, at the same concentration of the gas to be measured, the change in the resistance of the gas-sensing device based on the gas-sensing material prepared by the method of the present invention is greater and faster, meaning higher sensitivity. This makes it possible to achieve highly sensitive monitoring of the gas-sensing device at room temperature, greatly reducing the dependence of the gas-sensing device on a high-temperature environment.

[0045] It should be noted that ethanol is used as an example in the testing process, which does not mean that the gas-sensitive material prepared by the method of the present invention can only be used for the detection of ethanol. The gas-sensitive material prepared by the method of the present invention is applicable to the detection of reducing gases such as ethanol, methanol, formaldehyde, and n-butanol. The gas-sensitive material prepared by the method of the present invention has almost no response to carbon monoxide reducing gas. Therefore, the gas-sensitive material prepared by the method of the present invention has good selectivity. Specifically, compared with traditional single metal oxides, nickel molybdate prepared by the present invention is a multi-component transition metal oxide, and at the same time forms a heterojunction with AuNPs modified thereon. Since NiMoO4 itself is a catalyst with excellent performance, due to the synergistic catalytic effect of Mo and Ni existing therein, it is considered to be a good catalyst for promoting the partial oxidation of hydrocarbons and the oxidative dehydrogenation of alkanes. Therefore, it can selectively adsorb the gas to be detected. More specifically, refer to the article named "NiMoO 4 Selective Oxidation Catalysts Containing Excess MoO 3 for the Conversionof C 4 Hydrocarbons to Maleic Anhydride”. Since the selective adsorption of the gas to be detected by the gas-sensitive material is improved, the gas selectivity of the gas-sensitive device based on the gas-sensitive material of the present invention is also improved.

[0046] Example 2:

[0047] The difference between this example and Example 1 is as follows:

[0048] In step S1, 0.151 g of Na 2 MoO 4 ·2H 2 O is dissolved in 250 ml of deionized water. Correspondingly, the mass of Ni(NO 3 ) 2 ·6H 2 O is 0.182 g. After stirring for 20 min, 0.139 g of NH 4 F and 0.375 g of urea are added to the mixed solution to make it fully dissolved. The temperature of the hydrothermal reaction is 120 °C and the time is 10 h; the calcination temperature is 450 °C and the time is 1 h. The rest is the same as in Example 1.

[0049] In step S2, 0.109 g of the nickel molybdate nanosphere powder prepared in step S1 is dissolved in 250 ml of deionized water. After ultrasonic dispersion, 0.5 g of PVP and 2.5 ml of the prepared HAuCl solution with a concentration of 0.01 mol / L are taken 4An aqueous solution. The two are added to the mixed solution simultaneously, and continuously stirred on a magnetic stirrer to make the solution uniformly mixed. While stirring, the mixed solution is heated to boiling. During stirring, 5 ml of an aqueous solution of Na with a concentration of 1% is added to the mixed solution 3 C 6 H 5 O 7 ·2H 2 O aqueous solution. The rest is the same as in Example 1

[0050] The annealing temperature during the preparation of the gas-sensing test device is 350 °C, and the annealing time is 1 h. Using the HCRK-SD101 four-channel gas-sensing performance test software, the gas-sensing test of the prepared gas-sensing test device is carried out. The results of the test response recovery curve are as Figure 8 shown by the curve represented by the solid circles in. The sensitivity of the gas-sensing test device to 100 ppm ethanol at room temperature is 10.2, the response time is 25 s, and the recovery time is 90 s

[0051] Example 3:

[0052] The difference between this example and Example 1 is that

[0053] In step S1, 0.302 g of Na 2 MoO 4 ·2H 2 O is dissolved in 500 ml of deionized water. Correspondingly, the mass of Ni(NO 3 ) 2 ·6H 2 O is 0.363 g. Stir until it is uniformly mixed. 0.278 g of NH 4 F and 0.75 g of urea are added to the mixed solution together to make it fully dissolved. The hydrothermal reaction temperature is 200 °C and the time is 2 h; the calcination temperature is 350 °C and the time is 0.5 h. The rest is the same as in Example 1

[0054] In step S2, 0.218 g of the nickel molybdate nanosphere powder prepared in step S1 is dissolved in 500 ml of deionized water. After ultrasonic dispersion, 1.0 g of PVP and 5 ml of a prepared aqueous solution of HAuCl with a concentration of 0.01 mol / L are taken, and the two are added to the mixed solution simultaneously. Continuously stir on a magnetic stirrer to make the solution uniformly mixed. While stirring, the mixed solution is heated to boiling. During stirring, 10 ml of an aqueous solution of Na with a concentration of 1% is added to the mixed solution 4 An aqueous solution. The rest is the same as in Example 1 3 C 6 H 5 O 7 ·2H 2 O aqueous solution. The rest is the same as in Example 1

[0055] The annealing temperature is 300 °C and the annealing time is 2 h when preparing the gas-sensitive test device. Using the HCRK-SD101 four-channel gas-sensitive performance test software, the gas-sensitive test of the prepared gas-sensitive test device is carried out. The results of the test response-recovery curve are as Figure 8 shown by the curve represented by the solid triangles in. The sensitivity of the gas-sensitive test device to 100 ppm ethanol at room temperature is 9.5, the response time is 35 s, and the recovery time is 120 s.

[0056] In the three embodiments of the present invention, the concentrations of each solution are the same. Specifically, the volumes of each solution are different. The test results show that the gas-sensitive characteristics of the gas-sensitive materials prepared in the three embodiments are all good, and the sensitivities of the corresponding gas-sensitive test devices are all high, all higher than the sensitivity of ethanol gas at room temperature of 1.456-2 disclosed in the article titled "A novel room temperature ethanol sensor based on PbS:SnS 2 nanocomposite with enhanced ethanol sensing properties". At the same time, the response time is not longer than 35 s and the recovery time is not longer than 120 s. It also shows that the method for preparing the gas-sensitive material of the present invention is stable, efficient, and has good repeatability.

[0057] Generally, as the temperature rises, the activities of the gas to be measured and the background gas are higher, so the intensity of the redox reaction is stronger, that is, the higher the temperature, the higher the sensitivity of the gas-sensitive device. Due to different ideas for improving sensitivity, the gas-sensitive device based on the gas-sensitive material prepared by the method of the present invention has a higher sensitivity at room temperature and an even higher sensitivity at high temperature. The gas-sensitive material prepared by the method of the present invention can work normally at 25-160 °C and has a high sensitivity. In particular, it can be used for the detection of flammable and explosive gases at room temperature.

[0058] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, the present invention can have various changes and modifications. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A nickel molybdate nanocomposite modified with gold nanoparticles, Characterized in that, The composite material comprises a nickel molybdate material and gold nanoparticles. The nickel molybdate material is in the shape of a loose and porous sphere, the particle size of the nickel molybdate material is in the nanometer range, the gold nanoparticles are in the shape of spheres, the particle size of the gold nanoparticles is in the nanometer range, the gold nanoparticles are distributed on the surface of the nickel molybdate material, and the gold nanoparticles and the nickel molybdate material are fixedly connected by chemical bonds; The composite material is prepared by the following method: S1, preparing nickel molybdate nanospheres; S2, preparing a nickel molybdate nanocomposite modified with gold nanoparticles; Step S1 includes weighing Na 2 MoO 4 ·2H 2 O and dissolving it in deionized water, followed by stirring. After complete dissolution, Ni(NO 3 ) 2 ·6H 2 O is added. After complete dissolution again, urea and NH 4 F are added simultaneously, and stirring is carried out again. After sufficient dissolution, the resulting mixed solution is transferred to a hydrothermal reaction kettle for hydrothermal reaction. The product of the hydrothermal reaction is centrifuged, washed, vacuum dried, and calcined to obtain powdery nickel molybdate nanospheres; The step S2 includes preparing HAuCl 4 solution, taking the nickel molybdate nanospheres prepared in the step S1, dissolving them in deionized water to obtain a nickel molybdate mixture, then taking polyvinylpyrrolidone K30 and the HAuCl 4 solution and adding them to the nickel molybdate mixture to obtain a mixture, continuously stirring, heating the mixture to boiling while stirring, and adding Na 3 C 6 H 5 O 7 ·2H 2 O aqueous solution, continuously boiling, and after the reduction reaction is completed, naturally cooling to room temperature, and obtaining a gold nanoparticle-modified nickel molybdate nanocomposite after centrifugation, washing, and vacuum drying; The Na 2 MoO 4 ·2H 2 O and the Ni(NO 3 ) 2 ·6H 2 O have the same molar mass; the temperature of the hydrothermal reaction is 120 - 200 °C, and the time of the hydrothermal reaction is 2 - 12 h; the temperature of the calcination is 350 - 450 °C, and the time of the vacuum drying is 0.5 - 2 h; the solution concentration of NH 4 F is 15 mmol / L, the solution concentration of urea is 25 mmol / L, and the ratio of the dosage of NH 4 F to the dosage of urea is 0.37; The concentration of the HAuCl 4 solution is 0.01 mol / L, the concentration of the nickel molybdate mixture is 2.0 mol / L, the mass ratio of polyvinylpyrrolidone K30 to the volume of the HAuCl 4 aqueous solution is 1:5, the concentration of the Na 3 C 6 H 5 O 7 ·2H 2 O aqueous solution is 1%, and the volume ratio of the dosage of the HAuCl 4 and the Na 3 C 6 H 5 O 7 ·2H 2 O is 1:

2.

2. Use of the nickel molybdate nanocomposite modified with gold nanoparticles according to claim 1, Characterized in that, The composite material can be applied to the detection of reducing gases and can be used in gas sensors.

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