A catalytic combustion type hydrogen sensor with high response sensitivity and a preparation method thereof

Abstract

The present invention provides a catalytic combustion type hydrogen sensor with high response sensitivity and a preparation method thereof. The catalytic combustion type hydrogen sensor includes a catalytic combustion element and a compensation element. Both the catalytic combustion element and the compensation element are planar structures with a mica sheet as an insulating substrate, and a platinum resistor and an alumina thin film carrier are sequentially attached to the front surface of the mica sheet substrate. Among them, a layer of catalyst material is deposited on both the front and back surfaces of the catalytic combustion element. The catalyst material deposited on the front is attached to the outer surface of the alumina thin film carrier, and the catalyst material deposited on the back is attached to the back surface of the mica sheet substrate as a sensitivity enhancing element. For the catalytic combustion type hydrogen sensor proposed by the present invention, the catalytic combustion type main element adopts a planar structure, which has the characteristics of simple preparation process and stable structural performance. At the same time, catalysts are deposited on both the front and back surfaces of the catalytic combustion element, significantly enhancing the catalytic effect and greatly improving the sensitivity of the device.

Description

Technical Field

[0001] The present invention relates to the technical field of gas sensing, and particularly to a catalytic combustion type hydrogen sensor with high response sensitivity and a preparation method thereof. Background Art

[0002] Hydrogen is a highly efficient and pollution-free clean energy source with unparalleled renewability. Developing a hydrogen economy has always been an important means for governments of various countries to achieve energy strategic transformation. However, hydrogen is extremely prone to leakage during production, transmission, and use. When the hydrogen content in the air is between 4% and 75%, it can explode violently when encountering an open flame. At the same time, hydrogen is a colorless and odorless gas, which is not easily detected by the sensory systems of the human body, and the five senses of the human body cannot form an effective early warning for hydrogen. Therefore, one of the key technologies for the development and utilization of hydrogen energy is to develop a highly sensitive, fast-responsive, safe and reliable hydrogen sensor for the detection of hydrogen leakage, which is also a prerequisite for ensuring the safety of hydrogen use.

[0003] Hydrogen fuel cells have become a core technology widely concerned in the field of hydrogen energy. Ensuring the safety of hydrogen used in fuel cells is the basis for the widespread popularization of hydrogen fuel cell vehicles. In the field of hydrogen fuel cell vehicles, there are special requirements for the stability of in-vehicle hydrogen sensors. Existing catalytic combustion type hydrogen sensors have an automatic compensation function, which can maximize the elimination of the problems of baseline drift of the sensor device caused by environmental temperature and humidity changes and the sensing element itself, and can maximize the actual use requirements of in-vehicle hydrogen sensors. However, in in-vehicle applications, the sensing element in traditional catalytic combustion type hydrogen sensors uses a platinum wire resistor with a suspended structure, and frequent mechanical vibrations make the platinum resistance wire prone to breakage, resulting in the inability of the sensor to work. Based on semiconductor micro-nano processing technology, an MEMS catalytic combustion type hydrogen sensor prepared by effectively supporting the catalytic combustion element on the surface of a silicon oxide layer can solve this problem. However, its equipment is expensive and the preparation process is complex, which limits the widespread use of MEMS catalytic combustion type hydrogen sensors in in-vehicle hydrogen fuel cells. Based on this, developing a new type of catalytic combustion type hydrogen sensor with low cost and high performance has important application value. Summary of the Invention

[0004] The purpose of the present invention is to overcome the stability problems of traditional catalytic combustion type hydrogen sensors and the high-cost preparation problems of MEMS catalytic combustion type hydrogen sensors, and provide a catalytic combustion type hydrogen sensor with high response sensitivity and a preparation method thereof. The new type of catalytic combustion type hydrogen sensor of the present invention has the advantages of low cost and high performance.

[0005] To achieve the above object, the present invention proposes a device structure of a novel catalytic combustion type hydrogen sensor, which includes a catalytic combustion element and a compensation element. Both the catalytic combustion element and the compensation element use mica sheets as substrates. At the same time, both the catalytic combustion element and the compensation element include platinum resistors and alumina carriers. The difference is that catalysts (i.e., palladium nanoparticles are deposited) are coated on both surfaces of the catalytic combustion element. When the catalytic combustion element works, hydrogen undergoes flameless combustion under the catalysis of palladium nanoparticles, releasing a large amount of heat, which in turn causes the temperature of the platinum resistor to rise, resulting in an increase in the resistance value of the platinum resistor. Since the temperature is positively correlated with the hydrogen concentration, the change in the platinum resistor value is directly related to the hydrogen concentration, thereby realizing the detection of hydrogen. The compensation element mainly forms a bridge measurement circuit system with the catalytic combustion element to achieve the temperature compensation function of the measurement system. Compared with the traditional catalytic combustion gas sensors prepared by MEMS technology and the filament type catalytic combustion gas sensors, the sensor involved in the present invention has three advantages. First, the planar process on the mica sheet avoids the high cost of the MEMS manufacturing process. Second, the planar structure avoids the breakability of the filament structure, provides more stable device performance, and can ensure stable use under working conditions with frequent mechanical vibrations. Third, there are catalysts on both sides of the catalytic combustion element, which can greatly improve the sensitivity of the device and significantly enhance its detection ability.

[0006] The innovation of the present invention lies in: designing a novel catalytic combustion type hydrogen sensor with low cost and high performance. Mica sheets are used as the substrates of the catalytic elements, and the platinum resistors of the main catalytic combustion elements adopt a planar structure and are supported by the substrates. This catalytic combustion type hydrogen sensor has the characteristics of a planar structure, effectively solving the problem that the suspended platinum resistance wire in the traditional catalytic combustion type hydrogen sensor breaks due to frequent mechanical vibrations in vehicle-mounted applications, resulting in the sensor being unable to work. In addition, this invention patent creatively proposes a novel sensor device structure for improving the sensing performance of catalytic combustion type hydrogen sensors. Catalysts are deposited on both sides of the area containing the catalytic element, and catalytic combustion reactions with hydrogen are further carried out to increase the resistance change value of the platinum resistor of the main catalytic combustion element, thereby improving the sensing performance of the catalytic combustion type hydrogen sensor to hydrogen. The substrate of the catalytic combustion type hydrogen sensor is selected as a heat-resistant mica sheet, and the platinum resistor, alumina carrier, and palladium nanoparticles or thin films are prepared by physical coating methods, avoiding the problems of complex processes and expensive equipment existing in the preparation of gas sensor devices by existing MEMS technologies.

[0007] The described catalytic combustion type hydrogen sensor with high response sensitivity includes a catalytic combustion element and a compensation element. Both the catalytic combustion element and the compensation element have a planar structure with a mica sheet as the insulating substrate, and a platinum resistor and an alumina thin film carrier are sequentially attached to the front surface of the mica sheet substrate. Among them, a layer of catalyst material is deposited on both the front and back sides of the catalytic combustion element. The catalyst material deposited on the front is attached to the outer surface of the alumina thin film carrier, and the catalyst material deposited on the back is attached to the back surface of the mica sheet substrate as a sensitivity enhancing element.

[0008] Further, the catalytic combustion element and the compensation element use the same mica sheet substrate, and catalyst materials are attached to both side surfaces of the catalytic combustion element area, that is, the outer surface of the alumina thin film carrier in the catalytic combustion element area and the back surface of the mica sheet substrate in the catalytic combustion element area are respectively attached with catalyst materials.

[0009] Preferably, the substrate of the catalytic combustion type hydrogen sensor is selected as a mica sheet that can withstand high temperatures. For the determination of the thickness of the mica sheet substrate: First, the mica sheet can effectively support the catalytic combustion main element. Second, it ensures that the heat increased by the sensitivity enhancing element when encountering hydrogen is effectively conducted to the platinum resistor of the catalytic main element. The thickness of the mica sheet can be selected in the range of 1 - 100 μm.

[0010] Preferably, the platinum resistor in the catalytic combustion main element is prepared on the mica sheet substrate by a magnetron sputtering coating device. The platinum resistor prepared by this technology has a planar structure and can be effectively supported by the substrate, effectively solving the problem that the suspended platinum resistor in traditional catalytic combustion sensors is easily broken.

[0011] Preferably, the shape and structure of the platinum resistor are realized through different masks. The thickness of the platinum resistor is controlled within 1 - 10 μm, and the electrode gap of the platinum resistor is controlled within 0.5 - 10 μm. This platinum resistor preparation process avoids semiconductor lithography process, has a lower preparation cost, and can better meet the wide use of in-vehicle hydrogen sensors.

[0012] Preferably, to ensure that the planar-structured platinum resistor can better adhere to the mica sheet substrate, a titanium or chromium thin film with a thickness of 1 - 5 nm can be plated on the substrate surface using the same mask as an adhesion layer before the process of depositing the platinum resistor.

[0013] Preferably, the alumina carrier of the catalytic main element is realized by a radio frequency coating device. Similarly, mask is used for area-selective coating, so that the plated alumina thin film covers the entire surface of the platinum resistor, and the thickness of the alumina thin film is controlled within the range of 5 - 50 μm.

[0014] Preferably, for the catalysts required for the catalytic combustion element and the sensitizing element, cluster beam deposition technology (see ACS Applied Materials & Interfaces, 10 (2018) 44603–44613) is used to prepare palladium nanoparticles on both sides of the catalytic combustion element.

[0015] Preferably, to enable the catalytic combustion element to undergo a catalytic combustion reaction with hydrogen, the palladium nanoparticles are prepared by adjusting the cluster beam sputtering parameters to obtain palladium nanoparticles with a size of 5–20 nm, and the deposition range of the palladium nanoparticles covers the entire surface area of the alumina thin film in the catalytic combustion element.

[0016] Preferably, palladium nanoparticles are deposited on the back surface of the substrate of the catalytic combustion element as a sensitizing element for the catalytic combustion type hydrogen sensor. The palladium nanoparticles are prepared using a cluster beam deposition device, and the particle size range can be selected from 1–20 nm. The palladium nanoparticles deposited on the back surface of the substrate ensure that they can cover the area of the catalytic combustion element.

[0017] The preparation method of a catalytic combustion type hydrogen sensor with high response sensitivity includes the following steps:

[0018] 1) Affix the mask plate to the mica surface, and first deposit a metal titanium or chromium adhesion layer on the mica sheet surface by magnetron sputtering coating method, and then further deposit a platinum layer by the same magnetron sputtering coating method to obtain a platinum resistance structure;

[0019] 2) Deposit an alumina thin film carrier layer at the position of the platinum resistance. The alumina thin film carrier layer is formed by radio frequency magnetron sputtering coating method. Similarly, the mask plate is used to achieve selective area coating, and the coating area covers the entire surface where the platinum resistance is distributed;

[0020] 3) Prepare the compensation element and the intermediate of the catalytic combustion element according to the methods in steps 1)–2) above;

[0021] 4) Use a cluster beam deposition device to deposit a layer of palladium nanoparticles on both the front and back sides of the intermediate of the catalytic combustion element. The palladium nanoparticles deposited on the front side adhere to and cover the entire surface area where the alumina thin film is distributed, and the palladium nanoparticles deposited on the back side adhere to and cover the entire back surface of the mica sheet substrate.

[0022] Furthermore, in step 1), the thickness of the titanium or chromium adhesion layer is 1–5 nm; in step 1), a magnetron sputtering coating device is used to deposit the platinum layer, the sputtering power supply is a DC power supply, the sputtering power is 20–40 W, and the argon gas pressure in the sputtering chamber is maintained at 0.5–2 Pa.

[0023] Further, in step 2), a magnetron sputtering coating equipment is used to deposit the alumina thin film carrier layer. The sputtering power supply is a radio frequency power supply, the target material is an alumina target, the sputtering power is 150 - 200 W, and the argon gas pressure in the sputtering chamber is maintained at 0.5 - 2 Pa.

[0024] Further, in step 3), during the process of depositing palladium nanoparticles using a cluster beam deposition equipment, both the sputtering gas and the buffer gas are argon. The flow rates of the sputtering gas and the buffer gas are respectively in the range of 50 - 70 sccm and 75 - 85 sccm, and the sputtering power is 15 - 30 W.

[0025] Compared with the prior art, the beneficial effects achieved by the present invention are as follows:

[0026] 1) The invention proposes a novel catalytic combustion type hydrogen sensor. The sensor selects a mica sheet as the substrate, and uses magnetron sputtering coating to deposit a platinum resistor and an alumina carrier on the surface of the substrate respectively. The prepared catalytic combustion type hydrogen sensor has a planar structure and can operate stably in the frequent mechanical vibration environment of vehicles. At the same time, the catalytic combustion type hydrogen sensor has a low preparation cost and can be widely applied in vehicle-mounted hydrogen fuel cells.

[0027] 2) By setting catalysts on both surfaces of the catalytic combustion element, the present invention significantly enhances the catalytic effect, resulting in a substantial increase in the sensitivity of the device. In addition, for the catalytic combustion type hydrogen sensor proposed by the present invention, the planar structure adopted by the catalytic combustion type main element has the characteristics of simple process and stable structural performance. Therefore, the novel catalytic combustion type hydrogen sensor proposed by the present invention has the characteristics of low cost, high stability, and high sensing performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 is a schematic structural diagram of the catalytic combustion type hydrogen sensor of the present invention.

[0029] Figure 2a is a comparison chart of real-time current curves of different catalytic combustion type hydrogen sensors at different hydrogen concentrations;

[0030] Figure 2b is a comparison result of the relationship diagram between hydrogen concentration and response degree of different catalytic combustion type hydrogen sensors during analysis;

[0031] Figure 1 In the figure: 1 - mica sheet, 2 - platinum resistor, 3 - alumina thin film, 4 - palladium nanoparticles. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0032] The present invention will be further described below in conjunction with specific embodiments, but the protection scope of the present invention is not limited thereto.

[0033] Embodiment:

[0034] As Figure 1 shown, a catalytic combustion type hydrogen sensor of the present invention includes two elements with basically the same structure: a catalytic combustion element and a compensation element. Both of these elements use a mica sheet 1 as an insulating substrate and are supported by the mica sheet insulating substrate that can withstand high temperatures. The planar platinum resistor 2 serves as a measuring unit, and an alumina thin film 3 covers the platinum resistor 2 as a carrier layer. In comparison Figure 1 therein, a layer of palladium nanoparticles 4 is deposited on both the front and back sides of the catalytic combustion element as a catalyst. The palladium nanoparticles 4 deposited on the front side are attached to the outer surface of the alumina thin film 3, and the palladium nanoparticles 4 deposited on the back side are attached to the back surface of the mica sheet 1 substrate as a sensitivity enhancing element. The compensation element has basically the same structure as the catalytic element, except that there is no catalyst palladium nanoparticles 4.

[0035] Example 1:

[0036] The preparation of a catalytic combustion type hydrogen sensor, in comparison Figure 1 therein, the catalytic combustion element and the compensation element use the same mica sheet substrate, and the method includes the following steps:

[0037] (1) Selection of the substrate: Select a mica sheet with a thickness of 25 μm as the insulating substrate.

[0038] (2) Preparation of the platinum resistor: Before preparing the platinum resistor on the substrate surface, in order to increase the bonding force between the platinum resistor and the substrate surface, a chromium layer with a thickness of about 5 nm is first plated on the mica sheet substrate surface as an adhesion layer. The platinum resistor is prepared by a magnetron sputtering coating device. The sputtering power supply uses a DC power supply, the sputtering power is about 30 W, and the argon gas pressure in the sputtering chamber is maintained at about 1 Pa. The patterns of the adhesion layer and the platinum resistor are realized by selecting a mask plate, and the thickness of the platinum resistor is realized by controlling the coating time, and the thickness is controlled within 5 - 6 μm.

[0039] (3) Deposition of the alumina carrier: The alumina carrier thin film is prepared by a magnetron sputtering coating device. The sputtering power supply uses an RF power supply, the target material is an alumina target, the sputtering power is selected as 180 W, the argon gas pressure in the sputtering chamber is maintained at about 1 Pa, and the film thickness is controlled within about 20 μm. When plating the alumina thin film, the alumina layer is plated at the position of the platinum resistor by masking with a mask plate.

[0040] (4) Deposition of palladium nanoparticles: Palladium nanoparticles are prepared by a cluster beam deposition device. The sputtering gas and the buffer gas are both argon, and the flow rates of the sputtering gas and the buffer gas are 60 sccm and 80 sccm respectively, and the sputtering power is 20 - 25 W. During the deposition process, when preparing a catalytic combustion element with a catalyst on only one side (denoted as a single-sided catalytic element), a suitable mask is used to deposit the palladium nanoparticles on the surface of the alumina film of the catalytic element, avoiding the deposition of palladium nanoparticle catalysts on the alumina carrier of the compensation element. Similarly, when preparing a catalytic combustion element with catalysts on both sides (denoted as a double-sided catalytic element), a suitable mask is used to deposit the palladium nanoparticles on the surface of the alumina film of the catalytic element, avoiding the deposition of palladium nanoparticle catalysts on the alumina carrier of the compensation element, and on the back of the substrate, a suitable mask is used to deposit the palladium nanoparticles in the back region of the catalytic combustion element. The palladium nanoparticles on the back are used as a sensitizing element for the catalytic combustion type hydrogen sensor, avoiding the deposition of palladium nanoparticle catalysts on the back of the substrate of the compensation element. The size of the palladium nanoparticles is controlled to be 5 - 15 nm.

[0041] According to the above preparation method, two different catalytic combustion type hydrogen sensors can be prepared. One of the catalytic combustion elements is a single-sided catalytic element, and the other catalytic combustion element is a double-sided catalytic element.

[0042] Hydrogen response performance test:

[0043] For the two different catalytic combustion type hydrogen sensors prepared by the above method (one contains a single-sided catalytic element and the other contains a double-sided catalytic element), the hydrogen sensing performance test is carried out. A voltage of about 3 - 5 V is applied to the platinum resistors of the two different catalytic combustion type hydrogen sensors respectively to ensure that the initial temperature of the catalytic combustion element is about 160 °C, reaching the starting temperature for the catalytic combustion reaction between palladium nanoparticles and hydrogen. In order to explore the sensitization effect of the sensitizing element, the two different catalytic combustion type hydrogen sensors (one contains a single-sided catalytic element and the other contains a double-sided catalytic element) are respectively placed in the test chamber, and hydrogen concentrations of 4000, 10000, 20000, and 40000 ppm are respectively introduced into the chamber. During the whole test process, the current value of the platinum resistor is monitored in real time. Under this test method, the comparison diagram of the real-time current curves of different catalytic combustion type hydrogen sensors (one contains a single-sided catalytic element and the other contains a double-sided catalytic element) at different hydrogen concentrations is as Figure 2a shown. Further, the difference in the response performance of different catalytic combustion type hydrogen sensors is compared through the sensor response formula. The formula for calculating the sensor response is as follows:

[0044] Response where: I 0is the initial current value (i.e., the sensor current value in an air environment at an initial temperature of about 160 °C of the catalytic combustion element), and I is the sensor current value exposed to a hydrogen environment. Therefore, according to Figure 2a the real-time current curves at different hydrogen concentrations in Figure 2b it is possible to calculate the corresponding relationship between the sensor responsiveness and the hydrogen concentration. The comparison results of the relationship diagrams between the hydrogen concentration and the responsiveness of different catalytic combustion type hydrogen sensors (one containing a single-sided catalytic element and the other containing a double-sided catalytic element) during analysis are as shown in Figure 2b It can be seen that: compared with the single-sided catalytic element, double-sided coating with a catalyst can significantly improve the response performance of the catalytic combustion element to hydrogen. For example, at a hydrogen concentration of 40,000 ppm, the responsiveness of the catalytic combustion main element (i.e., containing a single-sided catalytic element) and the catalytic combustion main element + sensitizing element (i.e., containing a double-sided catalytic element) are 10% and 27.7% respectively, that is, the sensitizing element improves the response performance of the catalytic combustion main element to hydrogen by about 270%.

[0045] The present invention proposes a new type of catalytic combustion type hydrogen sensor, in which the device with double-sided coating of the catalyst can greatly improve the response performance of the catalytic element to hydrogen. At the same time, this catalytic combustion type hydrogen sensor has a planar structure and can work stably for a long time in the frequent mechanical vibration environment of vehicles, and also avoids the complex preparation process of the MEMS type catalytic combustion type hydrogen sensor. In summary, the new type of catalytic combustion type hydrogen sensor proposed by the present invention has the characteristics of low cost, high stability and high hydrogen sensitivity, and has important practical application value in the safety monitoring of vehicle-mounted hydrogen fuel cells.

[0046] The content described in this specification is only a list of the implementation forms of the inventive concept, and the protection scope of the present invention should not be regarded as limited to the specific forms stated in the embodiments.

Claims

1. A catalytic combustion type hydrogen sensor with high response sensitivity, characterized in that it includes a catalytic combustion element and a compensation element. Both the catalytic combustion element and the compensation element are planar structures with a mica sheet as the insulating substrate, and a platinum resistor and an alumina thin film carrier are sequentially attached to the front surface of the mica sheet substrate; among them, a layer of catalyst material is deposited on both the front and back surfaces of the catalytic combustion element. The catalyst material deposited on the front surface is attached to the outer surface of the alumina thin film carrier, and the catalyst material deposited on the back surface is attached to the back surface of the mica sheet substrate as a sensitivity enhancing element; the catalyst materials deposited on both the front and back surfaces of the catalytic combustion element are both palladium catalysts; the palladium catalyst is palladium nanoparticles. The deposition range of the palladium nanoparticles deposited on the front surface of the catalytic combustion element covers the entire surface area of the alumina thin film carrier, and the particle size of the palladium nanoparticles deposited on the front surface is 5 - 20 nm; the deposition range of the palladium nanoparticles deposited on the back surface of the catalytic combustion element covers the entire back surface of the mica sheet substrate in the area of the catalytic combustion element, and the particle size of the palladium nanoparticles deposited on the back surface is 1 - 20 nm; The preparation method of the catalytic combustion type hydrogen sensor with high response sensitivity as described in claim 1 includes the following steps: 1) Attach a mask plate to the mica surface. First, deposit metal titanium or chromium on the mica sheet surface as an adhesion layer by magnetron sputtering coating method, and then further deposit a platinum layer by the same magnetron sputtering coating method to obtain a platinum resistor structure; 2) Deposit an alumina thin film carrier layer at the position of the platinum resistor. The alumina thin film carrier layer is formed by radio frequency magnetron sputtering coating method, and the same mask plate is used to achieve selective area coating, and the coating area covers the entire surface where the platinum resistor is distributed; 3) According to the methods in steps 1) - 2) above, prepare the compensation element and the intermediate of the catalytic combustion element; 4) Use a cluster beam deposition device to deposit a layer of palladium nanoparticles on both the front and back surfaces of the intermediate of the catalytic combustion element. The palladium nanoparticles deposited on the front surface adhere to and cover the entire surface area where the alumina thin film is distributed, and the palladium nanoparticles deposited on the back surface adhere to and cover the entire back surface of the mica sheet substrate.

2. The catalytic combustion type hydrogen sensor with high response sensitivity as described in claim 1, characterized in that the catalytic combustion element and the compensation element use the same mica sheet substrate, and catalyst materials are attached to both side surfaces in the area of the catalytic combustion element, that is, the outer surface of the alumina thin film carrier in the area of the catalytic combustion element and the back surface of the mica sheet substrate in the area of the catalytic combustion element are respectively attached with catalyst materials.

3. The catalytic combustion type hydrogen sensor with high response sensitivity as described in claim 1, characterized in that the thickness of the mica sheet substrate is 1 - 100 μm, the thickness of the platinum resistor is 1 - 10 μm, and the electrode gap of the platinum resistor is controlled within 0.5 - 10 μm; the thickness of the alumina thin film carrier is in the range of 5 - 50 μm, and the alumina thin film carrier covers the entire surface of the platinum resistor.

4. The catalytic combustion type hydrogen sensor with high response sensitivity as described in claim 1, characterized in that In step 1), the thickness of the titanium or chromium adhesion layer is 1 to 5 nm; in step 1), a magnetron sputtering coating device is used to deposit the platinum layer, the sputtering power supply is a DC power supply, the sputtering power is 20 to 40 W, and the argon gas pressure in the sputtering chamber is maintained at 0.5 to 2 Pa.

5. A catalytic combustion type hydrogen sensor with high response sensitivity as described in claim 1, characterized in that in step 2), a magnetron sputtering coating device is used to deposit the alumina thin film carrier layer, the sputtering power supply is a radio frequency power supply, the target material is an alumina target, the sputtering power is 150 to 200 W, and the argon gas pressure in the sputtering chamber is maintained at 0.5 to 2 Pa.

6. A catalytic combustion type hydrogen sensor with high response sensitivity as described in claim 1, characterized in that in step 3), during the deposition of palladium nanoparticles using a cluster beam deposition device, both the sputtering gas and the buffer gas are argon, the flow rates of the sputtering gas and the buffer gas are respectively in the range of 50 to 70 sccm and 75 to 85 sccm, and the sputtering power is 15 to 30 W.

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