CN114791445A - Noble metal modified composite gas sensor - Google Patents

Abstract

The application relates to the field of semiconductor gas sensors, and specifically provides a noble metal-modified composite gas sensor, comprising a base layer, an electrode layer, a metal oxide semiconductor thin film layer, and a response layer. The electrode layer includes one positive electrode and two negative electrodes. The metal oxide The semiconductor thin film layer includes a first metal oxide semiconductor thin film and two second metal oxide semiconductor thin films, and the response layer includes a graphene thin film and precious metal nanoparticles. The positive electrode is located in the middle of the base layer, the negative electrode is arranged on both sides of the base layer, the positive electrode is covered with a first metal oxide semiconductor film, and the two negative electrodes are covered with a second metal oxide semiconductor film. A graphene film is fixed above the first metal oxide semiconductor film, and precious metal nanoparticles are fixed above the two second metal oxide semiconductor films.

Description

A noble metal modified composite gas sensor

technical field

The present application relates to the field of semiconductor gas sensors, and in particular, to a noble metal-modified composite gas sensor.

Background technique

A gas sensor is a sensor device that can convert relevant information such as the type, concentration, and composition of a gas into a visual electrical signal output. The specific conversion process uses various chemical reactions and various physical mechanisms.

The working principle of the gas sensor is to convert the changes in the microstructure of the gas sensor into a visual electrical signal when it reacts with the gas to be measured. Because there are many types of gases, and different gases generally have their unique properties, even the same series of gases will have subtle differences, so there are many different types of gas sensors with gas-to-electricity conversion function, according to the structure. Materials are divided into semiconductor gas sensors and non-semiconductor gas sensors. A document titled "Room-temperature ammoniagas sensor based on reduced graphene oxide nanocomposites decorated by Ag,Auand Pt nanoparticles" published in the journal "Journal of Alloys and Compounds", published in the journal "Sensors and Actuators A:Physical" , the document titled "Room temperature conductive type metal oxide semiconductor gas sensors for NO ₂ detection", published in the journal "ACS Applied Materials &Interfaces", titled "Low voltage driven sensors based on ZnO nanowires for room temperature detection of NO ₂ and The traditional gas sensors disclosed in the "CO gases" document all need to use high-temperature thermal excitation to ensure gas-sensing characteristics, and the general operating temperature is 250-450 ° C, which not only consumes a lot of power, but also may cause the crystal phase of the material due to the high operating temperature. The change of the gas sensor and the agglomeration growth of the crystal grains reduce the stability and life of the gas sensor, and also limit the application of the gas sensor in the field of flammable and explosive gas detection. Therefore, the sensitivity of the existing gas sensor is low, especially in a low temperature environment.

SUMMARY OF THE INVENTION

The purpose of the present invention is to provide a noble metal modified composite gas sensor to solve the problem of the noble metal modified composite gas sensor in the prior art in view of the above deficiencies in the prior art.

For achieving the above object, the technical scheme adopted in the present invention is as follows:

The present application provides a noble metal modified composite gas sensor, the gas sensor includes a base layer, an electrode layer, a metal oxide semiconductor thin film layer, and a response layer, specifically, the electrode layer includes one positive electrode and two negative electrodes, and the metal oxide semiconductor thin film The layer includes a first metal oxide semiconductor thin film and two second metal oxide semiconductor thin films, and the responsive layer includes a graphene thin film and precious metal nanoparticles. The material of the base layer is aluminum oxide, which plays the role of supporting and protecting the sensor. Three electrodes are fixedly arranged on the base layer, of which the positive electrode is located in the middle position, the negative electrode is arranged on both sides, and the first metal oxide semiconductor is covered above the positive electrode. The two negative electrodes are covered with a second metal oxide semiconductor film, and the two second metal oxide semiconductor films are connected through the first metal oxide semiconductor film. The material of the first metal oxide semiconductor film is a first metal oxide semiconductor, the material of the second metal oxide semiconductor film is a second metal oxide semiconductor, and the materials of the first metal oxide semiconductor and the second metal oxide semiconductor are different , specifically, the Fermi level of the first metal oxide semiconductor is lower than that of the second metal oxide semiconductor. A graphene film is fixed above the first metal oxide semiconductor film, and the graphene film has a porous structure with strong adsorption capacity, which can adsorb air and the gas to be measured on the surface of the first metal oxide semiconductor film to participate in redox reaction. Precious metal nanoparticles are fixedly arranged above the two second metal oxide semiconductor thin films for generating plasmon effect under the action of light.

During application, under the action of light, surface plasmon resonance occurs on the surface of noble metal nanoparticles, and the electric field energy is concentrated on the surface of noble metal nanoparticles, and hot electrons are generated in the strong electric field excitation. Due to the Fermi level of the first metal oxide semiconductor Lower than the second metal oxide semiconductor, hot electrons flow from the second metal oxide semiconductor films on both sides to the first metal oxide semiconductor film in the middle, so that the electron concentration in the first metal oxide semiconductor film is higher, so that the air The oxygen molecules in the metal oxide semiconductor film can capture the electrons inside the first metal oxide semiconductor film, and the oxygen molecules react with the electrons to produce an oxidation reaction to generate oxygen negative ions, so that the electron concentration inside the first metal oxide semiconductor film is reduced, and the resistance increases sharply; then , so that the gas to be tested is in contact with the sensor. Under the adsorption of the graphene film, the molecules of the gas to be tested are adsorbed on the surface of the first metal oxide semiconductor film and undergo a reduction reaction with the negative oxygen ions. The negative oxygen ions are reduced to oxygen and release electrons at the same time. The released electrons return to the inside of the first metal oxide semiconductor thin film, so that the electron concentration in the first metal oxide semiconductor thin film increases sharply, and the resistance value decreases. The change of resistance is monitored by the electrode to realize the detection of the gas to be measured.

Compared with the prior art, the present invention has the beneficial effects: the present application provides a noble metal modified composite gas sensor. In the present application, the graphene film is used to adsorb air and the gas to be tested, so that the oxygen molecules in the air and the gas molecules to be tested are fully contacted with the surface of the first metal oxide semiconductor film, so that the redox reaction is fully carried out. The metal oxide thin film of the present application is formed by stacking nanomaterials, the nanomaterials have a larger specific surface area, and the oxygen molecules and the gas molecules to be tested are more fully contacted with the surface of the first metal oxide semiconductor thin film, so that the redox reaction is more sufficient. The noble metal nanoparticles generate surface plasmon resonance under illumination, and the hot electrons excited by the strong electric field on the surface of the noble metal nanoparticles flow through the second metal oxide semiconductor film to the final first metal oxide semiconductor film, so that the first metal oxide semiconductor The electron concentration in the film is higher, so that the oxidation reaction intensity with oxygen molecules in the air is stronger, and more negative oxygen ions are generated, and finally the reduction reaction between the negative oxygen ions and the gas molecules to be measured is stronger. The intensity of the redox reaction is greater, so that more electrons are released during the reduction reaction between the negative oxygen ions and the gas molecules to be measured, so that the resistance of the sensor decreases more, so the sensitivity of the gas sensor of the present application is higher.

Description of drawings

FIG. 1 is a schematic diagram of a noble metal-modified composite gas sensor provided by the present invention.

FIG. 2 is a schematic diagram of a top view of an electrode layer of a noble metal-modified composite gas sensor provided by the present invention.

Icons: 1-substrate layer; 2-electrode layer; 3-second metal oxide semiconductor film; 4-first metal oxide semiconductor film; 5-graphene film; 6-precious metal nanoparticles.

Detailed ways

In order to make the implementation process of the present invention clearer, a detailed description will be given below with reference to the accompanying drawings.

The present invention provides a noble metal modified composite gas sensor. The gas sensor includes a base layer 1, an electrode layer 2, a metal oxide semiconductor thin film layer, and a response layer in sequence from bottom to top, wherein the metal oxide semiconductor thin film layer includes a first The metal oxide semiconductor film 4 and the second metal oxide semiconductor film 3, and the response layer includes a graphene film 5 and precious metal nanoparticles 6, as shown in FIG. 1 . The thickness of the base layer 1 varies according to the specific application environment. Specifically, the thickness of the base layer 1 is 0.5 mm-1.0 mm, and the thickness of the base layer 1 in this embodiment is 0.625 mm. The top view shape of the base layer 1 in the vertical direction can be any shape, and the size can support other layer structures. Preferably, the top view shape of the base layer 1 in the vertical direction is a rectangle, which is convenient for preparation; the gas sensor of the present application The size depends on the application needs, specifically, the size of the gas sensor is on the order of centimeters or millimeters. The base layer 1 is made of aluminum oxide. Because aluminum oxide is hard and high temperature resistant, it can support and protect other components. At the same time, the high temperature resistance makes the gas sensor age at high temperature to enhance the stability of the gas sensor. In addition, it is convenient to prepare the electrode layer 2 by etching on the base layer 1. Specifically, electrode material is plated on its surface, and then the corresponding electrode structure is etched. During preparation, the electrode structure can also be directly embedded in the base layer. On the bottom 2.

The electrode layer 2 includes three electrodes with a double negative electrode structure, which are respectively fixed on both sides and the middle position of the base layer 1, are arranged in parallel with each other, and do not contact each other, and one end connected to the external circuit protrudes from the base layer 1, and the protruding distance is 0.1 -0.5cm, which is convenient for connecting with external circuits, and the electrodes of the rest are all on the base layer 1. As shown in Figure 1, the electrode in the middle is the positive electrode, and the two electrodes on both sides are negative electrodes, which are fixed on both sides of the electrode layer 2. The distance between the two negative electrodes and the positive electrode in the middle position needs to be determined according to the size of the gas sensor. Ground, the distance between the two negative electrodes and the positive electrode in the middle is 0.5cm-1.0cm, and the distance between the two negative electrodes and the middle positive electrode can be equal or unequal. Preferably, the distance between the two negative electrodes and the middle positive electrode is equal, more preferably , the distance between the two negative electrodes and the middle positive electrode is 0.5cm, so that the symmetry of the sensor is better, the electrochemical reactions at the two negative electrodes tend to be consistent, and the resistance changes are consistent, so that the detected resistance changes are all from the under test. Therefore, it is beneficial to improve the sensitivity, accuracy and stability of the sensor. The materials of the above three electrodes are all noble metal platinum (Pt), and all are interdigitated electrodes, and the interdigitated electrodes can quickly and sensitively capture the weak resistance change of the metal oxide semiconductor thin film. As shown in FIG. 2 , the interdigital electrodes of the present application are composed of periodically arranged finger electrodes. Specifically, the thickness of the interdigital electrodes is 5 μm-10 μm. The width is 30nm-150nm, so that the contact between the interdigital electrode and the metal oxide semiconductor thin film layer is more sufficient, so that the contact resistance between the two is reduced, so that the resistance changes detected by the external circuit connected by the interdigital electrode are all from. The response of the gas is improved, thereby improving the accuracy of gas sensing.

The metal oxide semiconductor thin film layer includes a first metal oxide semiconductor thin film 4 and a second metal oxide semiconductor thin film 3 . The first metal oxide semiconductor thin film 4 is disposed above the positive electrode of the electrode layer 2, and at the same time completely covers the positive electrode in the middle, and is in close contact with the positive electrode, thus reducing the contact resistance and making the detection sensitivity higher. Two second metal oxide semiconductor thin films 3 are provided, which are respectively arranged above the two negative electrodes of the electrode layer 2, and at the same time completely cover the negative electrodes directly below them, and are in close contact with the negative electrodes, which can also reduce the contact resistance, so that the The detection sensitivity is higher; the thickness and size of the two second metal oxide semiconductor thin films 3 can be the same or different, preferably, the thickness and size of the two second metal oxide semiconductor thin films 3 disposed directly above the two negative electrodes It is exactly the same, combined with the electrode structure of the double negative electrode, so that the degree of the electrochemical reaction occurring in the two second metal oxide semiconductor thin films 3 is the same, so that the resistance changes of the two second metal oxide semiconductor thin films 3 are the same, and the resistance changes are the same The sensing accuracy and accuracy of the sensor are high; in addition, the order of the positive and negative electrodes of the two second metal oxide semiconductor thin films 3 and the first metal oxide semiconductor thin film 4 is consistent. Preferably, the positive electrode is arranged directly below the geometric center of the first metal oxide semiconductor thin film 4, and the two negative electrodes are respectively arranged directly below the geometric center of the second metal oxide semiconductor thin film 3, so that due to the better symmetry of the sensor, this The application sensor has stronger stability and longer life.

The thicknesses of the first metal oxide semiconductor thin film 4 and the second metal oxide semiconductor thin film 3 are the same, ranging from 10 μm to 15 μm, and the two second metal oxide semiconductor thin films 3 are connected through the first metal oxide semiconductor thin film 4 , so that the A space charge region is formed in the contact region of the first metal oxide semiconductor thin film 4 and the second metal oxide semiconductor thin film 3 to increase the resistance change of the gas sensor, thereby improving the sensitivity of the gas sensor. The thickness of the first metal oxide semiconductor film 4 and the second metal oxide semiconductor film 3 are the same, and the plan view shape is rectangular and the length in the direction perpendicular to the paper surface is the same, so that the first metal oxide semiconductor film 4 and the The second metal oxide semiconductor thin film 3 is fully in contact, and the contact boundary between the two is long and easy to prepare, so that the space charge region formed at the contact boundary is large, which is beneficial to improve the sensitivity of the sensor of the present application.

The materials of the first metal oxide semiconductor thin film 4 and the second metal oxide semiconductor thin film 3 need to be selected according to the gas to be measured. Specifically, they can be simple metal oxides such as zinc oxide, tin oxide, iron oxide, copper oxide, etc. It can be a multi-element metal oxide such as iron molybdate and sodium bismuth molybdate. In this application, the materials of the first metal oxide semiconductor thin film 4 and the second metal oxide semiconductor thin film 3 are different metal oxide semiconductors, that is, the material of the first metal oxide semiconductor thin film 4 is the first metal oxide semiconductor, and the material of the first metal oxide semiconductor thin film 4 is the first metal oxide semiconductor. The material of the second metal oxide semiconductor thin film 3 is a second metal oxide semiconductor, wherein the Fermi level of the second metal oxide semiconductor is higher than the Fermi level of the first metal oxide semiconductor, so that the second metal oxide semiconductor can be The electrons in the semiconductor flow to the first metal oxide semiconductor; a space charge region is formed at the contact boundary between the first metal oxide semiconductor film 4 and the second metal oxide semiconductor film 3. Due to the effect of the electric field in the space charge region, the space charge The carrier concentration in the region is very low and the conductivity is very small, which makes the sensor exhibit high resistance characteristics; at the same time, the electron concentration in the first metal oxide semiconductor is large, which can make more oxygen molecules in the air participate in the reduction reaction. , thereby increasing the resistance change of the gas sensor, and finally further improving the detection sensitivity. Specifically, the first metal oxide semiconductor in the present application may be tungsten disulfide, and the second metal oxide semiconductor may be titanium dioxide, which are suitable for detecting reducing gases, such as ethanol, acetone, and the like. The first metal oxide semiconductor thin film 4 and the second metal oxide semiconductor thin film 3 can be prepared by methods such as screen printing, spin coating, dipping, pulling, CVD, sputtering, transfer printing, and the like.

The response layer includes a graphene film 5 and precious metal nanoparticles 6, and the gas sensor of the present invention interacts with the outside gas, air, and light field to be measured through the response layer. The graphene film 5 is fixedly arranged on the side of the first metal oxide semiconductor film 4 away from the electrode layer 2, and the fluffy porous structure of the graphene film 5 is conducive to adsorbing more air and gas molecules to participate in the reaction, thereby increasing the resistance change of the gas sensor. , thereby improving the detection sensitivity. The graphene film 5 has the same area and size as the first metal oxide semiconductor film 4. If the size of the graphene film 5 is too small, a part of the first metal oxide semiconductor film 4 will be exposed to the air and the gas to be measured because there is no graphite. The adsorption of the graphene film 5 to the air and the gas to be measured makes the exposed first metal oxide semiconductor film 4 unable to effectively participate in the sensing process; when the size of the graphene film 5 is too large, due to the strong absorption , will make some of the noble metal nanoparticles 6 on the second metal oxide semiconductor thin film 3 unable to be illuminated by light, so that the plasmon effect cannot be generated, and the detection sensitivity is ultimately reduced.

The two second metal oxide semiconductor thin films 3 are provided with precious metal nanoparticles 6 on one side away from the electrode layer 2. The materials of the precious metal nanoparticles 6 are precious metal materials such as gold Au, silver Ag or platinum Pt, and the particle size is 10nm-50nm, In this way, under the condition of illumination, surface plasmon resonance is generated on the surface of the noble metal nanoparticles 6, a strong electric field is generated on the surface of the noble metal nanoparticles 6, and the strong local electric field excites the generated hot electrons, and the generated hot electrons are generated by the noble metal nanoparticles 6. The surface of the metal oxide semiconductor is transferred to the second metal oxide semiconductor film 3, which increases the thickness of the space charge region between the first metal oxide semiconductor and the second metal oxide semiconductor, making the gas sensor resistance change more drastic, so this application gas The sensitivity of the sensor is high. In addition, the strong electric field on the surface of the noble metal nanoparticles 6 will make the gas molecules more easily adsorbed by the graphene film 5 on the surface of the first metal oxide semiconductor film 4, thereby making the redox reaction stronger, thereby improving the sensitivity of the gas sensor of the present application . The graphene film 5 can be prepared by a chemical vapor deposition method, and the precious metal nanoparticles can be prepared by an in-situ reduction method.

In application, the surface of the gas sensor of the present application is irradiated with light, and the two negative electrodes and one positive electrode of the electrode layer 2 are respectively connected to the negative electrode and the positive electrode of the external power supply. Since the Fermi level of the second metal oxide semiconductor is higher than that of the first metal oxide semiconductor, the first metal oxide semiconductor film 4 is equivalent to a p-type semiconductor, and the second metal oxide semiconductor film 3 is equivalent to an n-type semiconductor. The current flows in from the positive electrode of the electrode layer 2 and flows out from the two negative electrodes, that is, the test current flows in from the first metal oxide semiconductor film 4 and flows out from the second metal oxide semiconductor film 3, and the space charge region increases with the increase of the applied bias voltage. Widening, the carrier concentration in the space charge region is very low, and the electrical conductivity is very small, which makes the sensor exhibit high resistance characteristics. Under the action of the applied bias voltage, more electrons flow from the second metal oxide semiconductor film 3 into the first metal oxide semiconductor film 4, so that the electron concentration in the first metal oxide semiconductor film 4 is larger, which can make the air More oxygen is involved in the redox reaction, and at the same time, the degree of electron exchange between the gas to be measured and the first metal oxide semiconductor thin film 4 is stronger, so the gas sensor of the present application has a higher sensitivity.

Light is irradiated on the surface of the gas sensor of the present application, surface plasmon resonance is generated on the surface of the noble metal nanoparticles 6, a strong electric field is generated on the surface of the noble metal nanoparticles 6, and the hot electrons excited by the strong electric field flow from the noble metal nanoparticles 6 to the hot electrons. The second metal oxide semiconductor thin film 3 . Since the Fermi level of the second metal oxide semiconductor is higher than that of the first metal oxide semiconductor, the hot electrons flow from the second metal oxide semiconductor film 3 to the first metal through the space charge region at the contact. The oxide semiconductor thin film 4, so that the electron concentration in the first metal oxide semiconductor thin film 4 is high, and the electron holes are separated in the space charge region, so that the resistance monitored by the electrodes changes drastically. A higher concentration of electrons can produce a sufficient oxidation reaction with oxygen in the air, so that more negative oxygen ions can be produced.

First, the gas sensor of the present application is brought into contact with the air. The graphene film 5 is a porous structure and has a strong adsorption effect. A large number of oxygen molecules are adsorbed on the surface of the first metal oxide semiconductor film 4, and the oxygen molecules capture the first metal oxide semiconductor. The electrons in the film 4 produce an oxidation reaction to generate oxygen negative ions; the high concentration of electrons in the first metal oxide semiconductor film 4 makes the oxygen molecules capture more electrons, so that more oxygen in the air participates in the redox reaction, and at the same time The intensity of the oxidation reaction is enhanced, thereby generating more negative oxygen ions to prepare for the reduction reaction. The more negative oxygen ions generated, the higher the detection sensitivity of the gas sensor of the present application.

Then, the gas sensor of the present application is contacted with the gas to be measured, and through the adsorption of the graphene film 5 of the porous structure, the gas molecules to be measured undergo a reduction reaction with the negative oxygen ions on the surface of the first metal oxide semiconductor thin film 4, and the negative oxygen ions are reduced to Oxygen releases electrons, and the released electrons return to the first metal oxide semiconductor thin film 4, so that the electron concentration inside the sensor increases sharply, so that the resistance monitored by the electrode drops sharply, thereby judging the type and concentration of the gas to be measured. Different types of gas to be tested have different degrees of reduction reaction with negative oxygen ions, the number and speed of electrons released during the reduction process are different, and the change of resistance is different; The speed and quantity of electrons released are different, and the resistance changes are also different. Therefore, the type and concentration of the gas to be measured can be judged by the resistance change.

Since the present application makes the gas sensor contact the air first, the oxygen molecules in the air react with the electrons of the first metal oxide semiconductor film 4 to undergo oxidation reaction, and a large amount of negative oxygen ions are generated, and at the same time, the process consumes the first metal oxide semiconductor film. 4, the resistance value of the sensor becomes higher; then the gas sensor of the present application is contacted with the gas to be measured, so that a reduction reaction occurs between the molecules of the gas to be measured and a large amount of negative oxygen ions generated, because the number of negative oxygen ions is large, The larger the concentration is, the more sufficient the reduction reaction is. During the process, electrons are released, and the electron concentration in the first metal oxide semiconductor film 4 increases, so that the resistance value of the sensor drops sharply. sufficient, more electrons are released in the reduction reaction process, therefore, the resistance value of the sensor drops sharply, and the sensitivity of the gas sensor of the present application is high. Since the process does not require the gas molecules to be tested to have high activity, the sensitivity of the sensor of the present application is higher at low temperature, and of course, the sensitivity at high temperature is higher. Specifically, the low temperature of the present application refers to 25°C-100°C °C.

The first metal oxide semiconductor thin film 4 and the second metal oxide semiconductor thin film 3 in the present application are both formed by stacking nanomaterials, wherein the nanomaterials include nanosheets, nanospheres, nanorods, etc. The surface of the oxide semiconductor thin film 4 refers to the surface of the aforementioned nanomaterials. The specific surface area of the nanomaterial is higher, so that the contact between the air and the gas to be measured and the first metal oxide semiconductor thin film 4 is more sufficient, so that the oxidation reaction and reduction reaction are more sufficient, and at the same time, the working temperature of the gas sensor can be lowered, which is more suitable for Gas sensing at low temperatures.

The above are only preferred embodiments of the present invention, and are not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and changes. 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 (10)

1. A noble metal-modified composite gas sensor, characterized in that the sensor comprises a base layer, an electrode layer, a metal oxide semiconductor thin film layer, and a response layer in sequence from bottom to top, and the electrode layer comprises three electrodes, so The three electrodes are fixedly arranged on the base layer and are not in contact with each other, the metal oxide semiconductor thin film layer is fixedly arranged on the side of the electrode layer away from the base layer, and the metal oxide semiconductor thin film The layer includes a first metal oxide semiconductor film and two second metal oxide semiconductor films, which completely cover the three electrodes respectively, and graphite is fixedly arranged on the side of the first metal oxide semiconductor film away from the electrode layer. The two second metal oxide semiconductor thin films are fixedly provided with precious metal nanoparticles on one side away from the electrode layer. 2 . The noble metal-modified composite gas sensor according to claim 1 , wherein the three electrodes are arranged in parallel on the middle and both sides of the base layer, wherein the positive electrode is in the middle position, and the electrodes on the two sides are viewed from the middle position. 3 . for the two negative poles. 3 . The noble metal modified composite gas sensor according to claim 2 , wherein the first metal oxide semiconductor thin film completely covers the positive electrode, and the two second metal oxide semiconductor thin films completely cover the Describe the two negative poles. 4 . The noble metal modified composite gas sensor according to claim 3 , wherein the materials of the two second metal oxide semiconductor thin films are two different metal oxide semiconductors. 5 . 5 . The noble metal modified composite gas sensor according to claim 4 , wherein the thicknesses of the two second metal oxide semiconductor thin films are the same. 6 . 6 . The noble metal modified composite gas sensor according to claim 5 , wherein the three electrodes are all composed of periodically arranged finger electrodes. 7 . 7 . The noble metal modified composite gas sensor according to claim 6 , wherein the thicknesses of the three electrodes are all 5 μm-10 μm. 8 . 8 . The noble metal modified composite gas sensor according to claim 7 , wherein the area of the graphene film is the same as that of the first metal oxide semiconductor film. 9 . 9 . The noble metal modified composite gas sensor according to claim 8 , wherein the material of the base layer is aluminum oxide. 10 . 10 . The noble metal-modified composite gas sensor according to claim 9 , wherein the noble metal nanoparticles are made of gold, silver or platinum. 11 .

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