Metal oxide and preparation method thereof, and gas sensor

Abstract

The present invention relates to the research field of metal oxide semiconductor materials, specifically to a metal oxide and a preparation method thereof, and a gas sensor. A preparation method of a metal oxide comprises the following steps: reacting a pretreated metal oxide with a surfactant, and placing the metal oxide in a dielectric barrier discharge device close to a negative electrode and not in a discharge region after the reaction is completed. And introducing methane gas into the dielectric barrier discharge device. The metal oxide material is bombarded with methane plasma prepared by the dielectric barrier discharge device, so that the metal oxide powder has a loose and porous structure on the basis of the original structure. The increase in the pore structure can increase the contact area between the metal oxide and the gas, and at the same time, the carbon atoms in the methane plasma are doped on the surface of the metal oxide, which can reduce the reaction activation energy of the metal oxide, enhance the metal oxide's adsorption capacity for gas and electron mobility, and improve the prepared metal oxide's gas sensitivity to gas.

Description

Technical Field

[0001] The present invention relates to the research field of metal oxide semiconductor (MOS) materials, and in particular to a metal oxide and a preparation method thereof, and a gas sensor. Background Art

[0002] With the rapid development of industry, the emission of various gases is increasing. In order to protect the environment and improve the health and living standards of human beings, the detection and control of various gases are becoming more and more important. Metal oxides are compounds composed of metal ions and oxygen ions. Studies have shown that some metal oxides have semiconductor properties. After doping, these metal oxides can realize the functions of n-type and p-type semiconductors. Among them, the doping method is usually to change the electronic structure and band structure of the material by replacing some atoms. Common ones include n-type ZnO, SnO2, WO3, etc.; p-type ones include CuO, etc. The difference between n-type metal oxide semiconductors and p-type metal oxide semiconductors lies in the difference in carriers, n-type is electrons, and p-type is holes. Taking n-type metal oxide semiconductors as an example, its sensing mechanism is: oxygen in the air is adsorbed on the surface of the material, capturing electrons inside the material to form oxygen ions. Due to the reduction of carriers, the resistance of the material increases. When the target gas undergoes an oxidation-reduction reaction with the oxygen ions adsorbed on the surface of the material, the electrons captured by oxygen return to the material, the carriers increase, and the resistance decreases. The p-type metal oxide semiconductor is the opposite: when oxygen captures electrons from the material, the number of holes and carriers increases, and the material resistance decreases. Whether it is an n-type metal oxide semiconductor or a p-type oxide semiconductor, both types of metal oxide semiconductors have good gas sensitivity and different resistances in air and target gas atmospheres, so they are widely used in the detection of various gases.

[0003] However, existing metal oxide semiconductor materials have problems such as high operating temperature, low selectivity, and low sensitivity, which are difficult to meet actual needs. Therefore, researchers have adopted various strategies to improve the gas sensing properties of metal oxides, such as doping, controlling micromorphology, surface modification and other methods. They want to improve the oxygen adsorption sites on the surface of metal oxide materials and provide more contact area for the target gas, thereby improving the sensing performance of metal oxides. However, the sensing performance of the metal oxide materials prepared by these preparation methods depends on the result of the combined action of the doped material and the substrate material, which is unique, making the preparation method only suitable for the synthesis of a certain metal oxide material and not universal. Summary of the invention

[0004] In order to solve the problem that the gas-sensing performance of metal oxides prepared by the prior art is poor and the preparation method of the prior art can only be applied to the synthesis of a certain metal oxide material and is not universal, the present invention provides a preparation method of metal oxide.

[0005] To achieve the above purpose, the technical solution adopted by the present invention is: a method for preparing a metal oxide, which comprises the following steps:

[0006] Pretreating the metal oxide powder, adding the pretreated metal oxide to a surfactant in proportion and stirring, separating the metal oxide powder treated with the surfactant from the surfactant after stirring, and fully washing the metal oxide powder treated with the surfactant with anhydrous ethanol and drying to obtain activated metal oxide powder;

[0007] The activated metal oxide powder is placed on an insulating medium connected to a negative electrode in a dielectric barrier discharge device and the activated metal oxide powder is not in a discharge region of the dielectric barrier discharge device; a certain amount of methane gas is introduced into a reaction chamber in the dielectric barrier discharge device, and an alternating current power supply of the dielectric barrier discharge device is turned on; under the action of the alternating current of the dielectric barrier discharge device, the methane gas is converted into methane plasma, and the activated metal oxide powder is bombarded by the methane plasma to form a metal oxide with a loose porous structure.

[0008] As a further improvement of the above scheme, the dielectric barrier discharge device includes an AC power supply and a reaction chamber, the AC power supply includes a positive electrode and a negative electrode, the positive electrode and the negative electrode are both arranged in the reaction chamber, the positive electrode and the negative electrode are arranged in parallel and have a gap, so that the discharge area is formed between the positive electrode and the negative electrode, and an insulating medium is provided on the positive electrode and the negative electrode, and the length of the insulating medium arranged on the negative electrode is greater than the length of the negative electrode.

[0009] As a further improvement of the above scheme, the activated metal oxide powder is placed on the upper end surface of the insulating medium in contact with the negative electrode and the activated metal oxide powder is not in the discharge area, and the activated metal oxide powder is isolated from the insulating medium by an insulating material.

[0010] As a further improvement of the above solution, the positive electrode and the negative electrode are 8 mm apart, the insulating medium is alumina ceramic, and the thickness of the insulating medium is 2 mm.

[0011] As a further improvement of the above scheme, the pretreatment includes the following steps: first, the metal oxide powder is placed in dilute nitric acid for impurity removal, and then the metal oxide powder treated with dilute nitric acid is placed in sodium carbonate for impurity removal, and the metal oxide powder treated with sodium carbonate is washed multiple times, and the washed metal oxide powder is separated, which is the pretreated metal oxide powder.

[0012] As a further improvement of the above scheme, the concentration of the dilute nitric acid is 3%, 1g~3g of metal oxide powder can be added to every 5ml of 3% dilute nitric acid, and the reaction time of the added metal oxide powder in the dilute nitric acid solution is 10s-20s.

[0013] As a further improvement of the above scheme, the concentration of the sodium carbonate is 3%, and 1g to 3g of the metal oxide powder separated from the dilute nitric acid can be added to every 5ml of the 3% sodium carbonate solution, and the reaction time of the added metal oxide powder separated from the dilute nitric acid in the sodium carbonate solution is 10s to 20s.

[0014] As a further improvement of the above scheme, the surfactant is a mixture of polyvinyl pyrrolidone, polyoxyethylene (9) stearate, sodium sulfate and ethanol, and the mixing ratio of polyvinyl pyrrolidone, polyoxyethylene (9) stearate, sodium sulfate and ethanol is: 0.1:0.1:20:10; the concentration of sodium sulfate is 0.5%.

[0015] As a further improvement of the above scheme, 1 g of pretreated metal oxide powder can be added to every 10 mL of the surfactant, and the reaction time of the pretreated metal oxide powder and the surfactant is 20 min-30 min.

[0016] As a further improvement of the above solution, the metal oxide includes but is not limited to one of ZnO, SnO2, and WO3.

[0017] As a further improvement of the above scheme, in the dielectric barrier discharge device, the gas pressure of the reaction chamber is 50 Pa, the flow rate of the introduced methane gas is 10 sccm, the voltage of the introduced alternating current is 10 kV, the discharge frequency is 5 kHz, and the reaction time of the methane gas and the activated metal oxide powder is 20 min-30 min.

[0018] As a further improvement of the above solution, the nanostructure of the metal oxide under an electron microscope includes but is not limited to one of nanofibers, nanotubes, nanosheets or nanoflowers.

[0019] A metal oxide is prepared by the above preparation method. The prepared metal oxide has a loose porous structure while maintaining the nanostructure of the metal oxide itself.

[0020] A gas sensor comprises the above-mentioned metal oxide.

[0021] Compared with the prior art, the present invention has the following beneficial effects:

[0022] (1) The present invention prepares methane plasma to modify the activated metal oxide, so that the microstructure of the modified metal oxide changes to form a loose and porous feature. This loose and porous structure can increase the specific surface area and porosity of the metal oxide, so that the metal oxide has more reaction sites with the target gas, thereby greatly improving the gas sensing performance of the metal oxide.

[0023] (2) The methane plasma obtained by treating methane gas with a dielectric barrier discharge device contains carbon atoms and hydrogen. The hydrogen has reducing properties and can undergo redox reactions with the surface of metal oxides, increasing the defects on the surface of the metal oxides, thereby increasing the specific surface area of ​​the prepared metal oxides. The methane plasma contains carbon atom free radicals, which can collide with the metal oxides, making the prepared metal oxides more prone to pore structures. The increase in pore structures can increase the contact area between the metal oxides and the gas. At the same time, the carbon atoms in the methane plasma are doped on the surface of the metal oxides, which can reduce the reaction activation energy of the metal oxides, thereby enhancing the metal oxides' adsorption capacity for gases and electron mobility. The improvement of the above two properties helps to improve the gas sensitivity of the prepared metal oxides to gases.

[0024] (3) The present invention activates the metal oxide by a surfactant, thereby improving the surface charge of the activated metal oxide, activating the surface molecules of the metal oxide, and preventing the agglomeration of the metal oxide particles, thereby making them evenly dispersed. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 This is a schematic structural diagram of the dielectric barrier discharge device provided by the present invention.

[0026] Figure 2 The images of zinc oxide prepared in Example 2 and Comparative Example 1 under a scanning electron microscope are shown in FIG. Figure 2 (a) is a picture of zinc oxide prepared in Comparative Example 1 of the present invention under a scanning electron microscope, Figure 2 (b) is a picture of zinc oxide prepared in Example 2 of the present invention under a scanning electron microscope.

[0027] Figure 3 The figure is a comparison chart of the gas sensing performance of zinc oxide prepared in Example 2 of the present invention and Comparative Example 1.

[0028] Figure 4 The images of the tin oxide prepared in Example 3 and Comparative Example 2 under a scanning electron microscope are shown in FIG. Figure 4 (a) is a picture of the tin oxide prepared in Comparative Example 2 of the present invention under a scanning electron microscope, Figure 4(b) is a picture of the tin oxide prepared in Example 3 of the present invention under a scanning electron microscope.

[0029] Figure 5 The figure is a comparison chart of the gas sensing performance of tin oxide prepared in Example 3 of the present invention and Comparative Example 2.

[0030] Figure 6 The images of tungsten oxide prepared in Example 4 and Comparative Example 3 under a scanning electron microscope are shown in FIG. Figure 6 (a) is a picture of tungsten oxide prepared in comparative example 3 of the present invention under a scanning electron microscope, Figure 6 (b) is a picture of the tungsten oxide prepared in Example 4 of the present invention under a scanning electron microscope.

[0031] Figure 7 It is a comparison chart of the gas sensing properties of tungsten oxide prepared in Example 4 of the present invention and Comparative Example 3.

[0032] In the figure: 11, AC power supply; 111, positive electrode; 112, negative electrode; 113, discharge area; 114, insulating medium; 12, reaction chamber. DETAILED DESCRIPTION

[0033] The present invention is further described below in conjunction with specific implementation methods. It should be noted that, under the premise of no conflict, the various embodiments or technical features described below can be arbitrarily combined to form a new embodiment.

[0034] In the description of the present invention, it should be noted that for directional words, such as the terms "center", "lateral", "longitudinal", "length", "width", "thickness", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", "clockwise", "counterclockwise" and the like indicating directions and positional relationships are based on the directions or positional relationships shown in the drawings, which are only for the convenience of describing the present invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific direction, be constructed and operated in a specific direction, and cannot be understood as limiting the specific protection scope of the present invention. The terms "first", "second", etc. in the specification and claims of the present invention are used to distinguish similar objects, and are not necessarily used to describe a specific order or sequence. The terms "including" and "having" in the specification and claims of the present invention and any variations thereof are intended to cover non-exclusive inclusions, for example, a process, method, system, product or device including a series of steps or units is not necessarily limited to those steps or units clearly listed, but may include other steps or units that are not clearly listed or inherent to these processes, methods, products or devices.

[0035] Example 1

[0036] A method for preparing a metal oxide comprises the following steps:

[0037] (1) Pre-treating the metal oxide powder, the pre-treatment comprising the following steps: firstly placing the metal oxide powder in dilute nitric acid for impurity removal, then placing the metal oxide powder treated with dilute nitric acid in sodium carbonate for impurity removal, and washing the metal oxide powder treated with sodium carbonate for multiple times, and separating the washed metal oxide powder, i.e. the pre-treated metal oxide powder.

[0038] (2) Prepare surfactant: Mix 0.1 g of polyvinyl pyrrolidone and 0.1 g of polyoxyethylene (9) stearate in 20 mL of 0.5% sodium sulfate solution, then add 10 mL of ethanol, stir well and set aside.

[0039] (3) Add 1 g of the metal oxide powder pretreated in step (1) to 10 mL of the surfactant prepared in step (2), stir with a magnetic stirrer for 20 min, and then separate the metal oxide powder and the surfactant by centrifuge. The metal oxide powder treated with the surfactant is thoroughly washed with anhydrous ethanol, and then the cleaned metal oxide powder is placed in a vacuum drying oven and dried at 80° C. for 1 h to obtain an activated metal oxide powder.

[0040] (4) The metal oxide powder activated in step (3) is placed on an insulating medium connected to a negative electrode in a dielectric barrier discharge device, and the activated metal oxide powder is not in a discharge region of the dielectric barrier discharge device. Then, methane gas with a flow rate of 10 sccm is introduced into the reaction chamber of the dielectric barrier discharge device, and an AC power supply of the dielectric barrier discharge device is connected. Under the action of the AC power of the dielectric barrier discharge device, the methane gas is converted into methane plasma, and the activated metal oxide powder is bombarded by the methane plasma to form metal oxide.

[0041] Among them, in the actual operation process, the pretreatment step of the metal oxide can be divided into the following two cases: (1-1) When the purity of the metal oxide is higher than 98%, at this time, it is only necessary to directly wash the metal oxide powder with anhydrous ethanol and deionized water in sequence for multiple times, and then separate the washed metal oxide powder from the deionized water by a centrifuge to obtain the pretreated metal oxide powder. (1-2) When the purity of the metal oxide is lower than 98%, it is necessary to first add the metal oxide powder to 3% dilute nitric acid, and then separate the metal oxide powder treated with dilute nitric acid by a centrifuge after ultrasonic oscillation, and then add the metal oxide powder treated with dilute nitric acid to a sodium carbonate solution, and after a period of reaction, separate the sodium carbonate solution by a centrifuge to obtain the metal oxide powder treated with the sodium carbonate solution; then wash the metal oxide powder treated with the sodium carbonate solution with anhydrous ethanol, and then wash it with deionized water for multiple times until the pH value of the washing solution after washing is neutral, and then separate the washed metal oxide powder from the deionized water to obtain the pretreated metal oxide powder.

[0042] Among them, in the pretreatment method (1-2), the concentration of dilute nitric acid is 3%. 1g-3g of metal oxide powder can be added to every 5ml of 3% dilute nitric acid, and the reaction time of the added metal oxide powder in the dilute nitric acid solution is 10s-20s.

[0043] The concentration of sodium carbonate is 3%. 1g-3g of metal oxide powder separated from dilute nitric acid can be added to every 5ml of 3% sodium carbonate solution, and the reaction time of the added metal oxide powder separated from dilute nitric acid in the sodium carbonate solution is 10s-20s.

[0044] In the pretreatment process, dilute nitric acid is used to remove impurities that may exist on the surface of the metal oxide powder. After the dilute nitric acid treatment, the metal oxide powder is treated with sodium carbonate. Sodium carbonate can not only further remove impurities in the metal oxide powder after the dilute nitric acid treatment, but also react with the dilute nitric acid remaining on the metal oxide powder. The carbon dioxide generated by the reaction can make the metal oxide powder in the sodium carbonate solution more dispersed, thereby facilitating the further removal of impurities in the metal oxide powder.

[0045] In the pretreatment process, the metal oxide powder is washed multiple times with anhydrous ethanol and deionized water in order to remove substances attached to the metal oxide powder.

[0046] It should be noted that in the pretreatment method (1-2), when the metal oxide powder is washed multiple times with anhydrous ethanol and deionized water, the standard for washing clean is whether the pH value of the deionized water after the last wash is neutral. If the pH value of the deionized water after the last wash is neutral, the surface metal oxide powder is washed clean. The reason for this judgment is that the pretreatment method (1-2) is to remove impurities from the metal oxide powder with dilute nitric acid and sodium carbonate in turn, and dilute nitric acid is acidic and sodium carbonate is alkaline. Only when these two substances are washed clean, the pH value of the deionized water after washing is neutral. In addition, in the process of washing with anhydrous ethanol and deionized water, the washing liquid after washing may also contain the neutral substance sodium nitrate, which is a strong acid and strong base salt and is soluble in water. Most of it can also be washed away, and a small amount of residual strong acid and strong base salt can play the same role as sodium sulfate in the surfactant.

[0047] The metal oxide prepared through the above series of steps has a loose porous structure while maintaining its original nanostructure. This loose porous structure can increase the specific surface area and porosity of the metal oxide, so that the metal oxide has more reaction sites with the target gas, thereby greatly improving the gas sensing performance of the metal oxide.

[0048] The metal oxide prepared through the above series of steps is used to make a gas sensor. Due to its loose porous structure, it can have more reaction sites with the target gas, which can greatly improve the gas sensing performance of the prepared gas sensor.

[0049] In the preparation method, methane plasma is prepared to modify the metal oxide powder after surfactant activation, so that the microstructure of the modified metal oxide is changed to form a loose and porous feature, thereby improving the gas sensitivity of the prepared metal oxide.

[0050] It is understandable that the methane plasma obtained by treating methane gas with a dielectric barrier discharge device contains carbon atoms and hydrogen. The hydrogen has reducing properties and can undergo redox reactions with the surface of the metal oxide, increasing the defects on the surface of the metal oxide, thereby increasing the specific surface area of ​​the prepared metal oxide. The methane plasma contains carbon atom free radicals, which can collide with the metal oxide, making the prepared metal oxide more prone to pore structure. The increase in pore structure can increase the contact area between the metal oxide and the gas. At the same time, the carbon atoms in the methane plasma are doped on the surface of the metal oxide, which can reduce the reaction activation energy of the metal oxide, thereby enhancing the metal oxide's adsorption capacity for gas and electron mobility. The improvement of the above two properties is conducive to improving the gas sensitivity of the prepared metal oxide to gas.

[0051] It is understandable that the reason why carbon atoms are doped on the surface of metal oxides is that the presence of carbon atoms is due to the carbon-containing groups in the methane plasma, and the methane gas is consumed after the metal oxide is treated, and no carbon agglomeration and accumulation is observed after the treatment (because carbon is a black solid at room temperature), which proves that carbon atoms are doped on the surface of the metal oxide.

[0052] Among them, the pore structure of the prepared metal oxide is due to the impact of particles in the methane plasma and the redox reaction between hydrogen and the metal oxide in the methane plasma, so that the pore structure of the metal oxide prepared by the preparation method of this embodiment increases, its porosity is improved, and the specific surface area of ​​the metal oxide increases. The increase in specific surface area can increase the contact area between the metal oxide and the gas, so that the reaction sites of the metal oxide and the gas increase, thereby improving the gas sensitivity of the prepared metal oxide to the gas.

[0053] In this embodiment, the surfactant prepared in step (2) will be arranged in a directional manner after being adsorbed on the surface of the metal oxide powder, specifically: the hydrophobic group faces outward and the hydrophilic group faces the inside of the metal oxide powder. The hydrophilic group is adsorbed on the surface of the metal oxide powder by electrostatic action, so it will improve the charge on the surface of the metal oxide powder. At the same time, its hydrophilic group adsorbed on the surface of the metal oxide powder will have an electrostatic shielding effect, thereby reducing the interaction of the surface charges of the metal oxide powder. According to the above principle, in this embodiment, after being activated by the surfactant, the surface charge of the activated metal oxide can be improved, the surface molecules of the metal oxide can be activated, and the agglomeration of the metal oxide particles can be prevented, so that it is evenly dispersed.

[0054] In this embodiment, the surfactant includes polyvinyl pyrrolidone, polyoxyethylene (9) stearate, a sodium sulfate solution with a concentration of 0.5%, and ethanol. Polyvinyl pyrrolidone can have a synergistic effect with polyoxyethylene (9) stearate and the sodium sulfate solution, which is specifically manifested in the following aspects: 1. In terms of reducing the surface tension of metal oxides, polyvinyl pyrrolidone and polyoxyethylene (9) stearate both have the ability to reduce surface tension. Polyvinyl pyrrolidone can adsorb on the surface of metal oxides through its special molecular structure, thereby changing the charge distribution on the surface of the metal oxide, thereby changing the arrangement of molecules on the surface of the metal oxide. This arrangement also increases the adsorption of the lipophilic group of polyoxyethylene (9) stearate on the surface of the metal oxide, further reducing the surface tension of the metal oxide, and also affecting the distribution of the charge on the surface of the metal oxide to a certain extent. Sodium sulfate is a strong acid and strong base salt, which can be completely ionized in ethanol solution. Therefore, sodium sulfate can affect the arrangement of surfactant molecules on the surface of metal oxides through the electrostatic effect of charged positive and negative ions, so that the active molecules of the configured surfactant can be distributed more evenly on the surface of metal oxides, thereby achieving the synergistic effect of the three to reduce the surface tension of metal oxides. The reduction of the surface tension of metal oxides makes it easier to disperse the prepared metal oxide powder particles and less likely to agglomerate, making the subsequent methane plasma treatment of metal oxides more uniform. At the same time, the reduction of surface tension makes the surface molecules of the prepared metal oxides more active and the particles more active, making them easier to react with substances. Second, in terms of stability: polyvinyl pyrrolidone can increase the stability of emulsions or dispersion systems and prevent particle agglomeration. Stearic acid polyoxyethylene (9) ester helps to form a stable interface film; the ions in the sodium sulfate solution reduce the electrostatic repulsion between particles through electrostatic shielding, enhance the stability of the system, and prevent breakdown by AC voltage.

[0055] In this embodiment, the activated metal oxide powder is placed on the insulating medium connected to the negative electrode in the dielectric barrier discharge device (i.e., close to the negative electrode), and the activated metal oxide powder is not in the discharge area of ​​the dielectric barrier discharge device. By placing the activated metal oxide powder in a non-discharge area, the activated metal oxide powder can be protected from being broken down by high AC voltage while being bombarded by methane plasma. At the same time, the purpose of placing it near the negative electrode is because in the dielectric barrier discharge device, the negative electrode position is mainly bombarded by ions, while the positive electrode position is mainly bombarded by electrons, and in this scheme, the main change is to change the microstructure of the metal oxide, so in this embodiment, the activated metal oxide powder is placed near the negative electrode position so that it can be bombarded by larger ions in the methane plasma, and the metal oxide can be prepared under the bombardment of ions in the methane plasma.

[0056] Please refer to Figure 1 The dielectric barrier discharge device includes an AC power supply and a reaction chamber, and the AC power supply is used to provide AC power. The AC power supply includes a positive electrode and a negative electrode, and the positive electrode and the negative electrode are both arranged in the reaction chamber. The positive electrode and the negative electrode are arranged in parallel and have a gap, so that a discharge area is formed between the positive electrode and the negative electrode. An insulating medium is provided on the positive electrode and the negative electrode, and the length of the insulating medium provided on the negative electrode is greater than the length of the negative electrode. The length of the insulating medium is greater than the length of the negative electrode, so that the metal oxide can be placed on the insulating medium connected to the negative electrode and is not located in the discharge area.

[0057] It is understandable that in this embodiment, the particles in the plasma collide with the surface of the metal oxide and undergo a redox reaction. In the bombardment process, if the particle energy is too high, the metal oxide will be broken down, destroying the structural advantages of the metal oxide body; if the particle energy is too low, the purpose of bombarding a large number of pores cannot be achieved. Therefore, in the actual operation process, the particle energy in the plasma needs to be regulated according to the actual metal oxide being processed.

[0058] It is understood that the activated metal oxide powder is placed on the upper end surface of the insulating medium in contact with the negative electrode and the activated metal oxide powder is not in the discharge region (such as Figure 1 The red part in the middle may be the placement position of the metal oxide powder), and the activated metal oxide powder is isolated from the insulating medium by the insulating material.

[0059] The distance between the positive electrode and the negative electrode is 8 mm. The insulating medium is alumina ceramic, and the thickness of the insulating medium is 2 mm.

[0060] The surfactant is a mixture of polyvinyl pyrrolidone, polyoxyethylene (9) stearate, sodium sulfate and ethanol. The mixing ratio of polyvinyl pyrrolidone, polyoxyethylene (9) stearate, sodium sulfate and ethanol is 0.1:0.1:20:10. Among them, polyvinyl pyrrolidone and polyoxyethylene (9) stearate are solid, and sodium sulfate and ethanol are liquid. Therefore, in this embodiment, the mixing ratio of polyvinyl pyrrolidone, polyoxyethylene (9) stearate, sodium sulfate and ethanol is 0.1g:0.1g:20mL:10mL. The concentration of sodium sulfate is 0.5%.

[0061] In this embodiment, 1 g of pretreated metal oxide powder can be added to every 10 mL of surfactant, and the reaction time of the pretreated metal oxide powder and the surfactant is 20 min-30 min.

[0062] The metal oxide includes but is not limited to one of ZnO, SnO2, and WO3.

[0063] The metal oxide treated by the preparation method of this embodiment mainly has carbon-containing groups and redox reactions with hydrogen on the surface. The collision and doping of the carbon-containing groups are formed by the bombardment of methane plasma, which is a physical reaction and has stable properties, because in the actual operation process, other metal oxides can also use the dielectric barrier discharge principle to generate methane plasma, and then bombard the activated metal oxide with methane plasma to achieve it. In the reaction process with hydrogen, its reducibility is used, and all metal oxides have oxidizing properties, so this characteristic is also generally applicable. In summary, in the treatment process of metal oxides in this embodiment, the properties participating in the reaction are all common characteristics of metal oxides, so the preparation method of this embodiment can be generally applicable to all metal oxides.

[0064] The nanostructure of the metal oxide under an electron microscope includes, but is not limited to, one of nanofibers, nanotubes, nanosheets or nanoflowers.

[0065] In the dielectric barrier discharge device, the gas pressure of the reaction chamber is 50Pa, the flow rate of the introduced methane gas is 10sccm, the voltage of the connected alternating current is 10kV, the discharge frequency is 5kHz, and the reaction time of the methane gas and the activated metal oxide powder is 20min-30min.

[0066] In the actual process, there are many types of materials for preparing gas sensors, but the common metal oxides with good gas sensitivity are mainly ZnO, SnO2, and WO3. Therefore, the following examples use these three representative metal oxides for preparation experiments to verify the conclusions obtained in the present invention.

[0067] Example 2

[0068] A method for preparing zinc oxide comprises the following steps:

[0069] (1) Add 2 g of ZnO (nanofiber) powder with a purity of less than 98% to 10 mL of 3% dilute nitric acid, and then use a centrifuge to separate the ZnO (nanofiber) powder treated with dilute nitric acid after ultrasonic vibration for 10 seconds. Then, add the ZnO (nanofiber) powder treated with dilute nitric acid to 10 mL of 3% sodium carbonate solution, react for 10 seconds, and then use a centrifuge to separate the ZnO (nanofiber) powder treated with sodium carbonate solution.

[0070] (2) The ZnO (nanofiber) powder treated with sodium carbonate solution in step (1) is first washed with anhydrous ethanol, and then washed with deionized water for multiple times until the pH value is neutral, and finally separated to obtain the pretreated ZnO (nanofiber) powder.

[0071] (3) Prepare a surfactant: mix 0.1 g of polyvinyl pyrrolidone and 0.1 g of polyoxyethylene (9) stearate in 20 mL of 0.5% sodium sulfate solution, then add 10 mL of ethanol and stir well for later use.

[0072] (4) Add 1 g of the ZnO (nanofiber) powder pretreated in step (2) to 10 mL of the surfactant prepared in step (3), and stir for 20 min. Then separate the stirred ZnO (nanofiber) powder, and wash the separated ZnO (nanofiber) powder thoroughly with anhydrous ethanol, and then place it in a vacuum drying oven at 80 ° C for 1 h to obtain activated ZnO (nanofiber) powder.

[0073] (5) Place the ZnO (nanofiber) powder activated in step (4) close to the negative electrode and not in the discharge area. Using the dielectric barrier discharge principle, stainless steel is used as the positive electrode and copper is used as the negative electrode, with a distance of 8 mm between the two. Double-layer alumina ceramic is used as the insulating medium, with a thickness of 2 mm and a spacing of 4 mm. Turn on the vacuum pump, maintain the pressure in the reaction chamber at 50 Pa, and introduce methane gas at a rate of 10 sccm. Connect 10 kV AC, discharge frequency 5 kHz, react at room temperature for 20 minutes, and take out after the reaction is completed to obtain zinc oxide with a loose porous structure.

[0074] Example 3

[0075] A method for preparing tin oxide comprises the following steps:

[0076] (1) 2 g of SnO2 (nanosphere) powder with a purity higher than 98% was first washed with anhydrous ethanol, and then washed with deionized water for multiple times, and finally the SnO2 (nanosphere) powder was separated, which was the pretreated SnO2 (nanosphere) powder.

[0077] (2) Prepare a surfactant: mix 0.1 g of polyvinyl pyrrolidone and 0.1 g of polyoxyethylene (9) stearate in 20 mL of 0.5% sodium sulfate solution, then add 10 mL of ethanol and stir well.

[0078] (3) Add 1 g of the SnO2 (nanosphere) powder pretreated in step (1) to 10 mL of the surfactant prepared in step (3), stir for 20 min, and then separate. The separated SnO2 (nanosphere) powder is thoroughly washed with anhydrous ethanol, and then placed in a vacuum drying oven at 80°C for 1 h to obtain activated SnO2 (nanosphere) powder.

[0079] (4) Place the SnO2 (nanosphere) powder activated in step (3) close to the negative electrode and not in the discharge area. Using the dielectric barrier discharge principle, stainless steel is used as the positive electrode and copper is used as the negative electrode, with a distance of 8 mm between the two. Double-layer alumina ceramic is used as the insulating medium, with a thickness of 2 mm and a spacing of 4 mm. Turn on the vacuum pump, maintain the pressure in the reaction chamber at 50 Pa, and introduce methane gas at a rate of 10 sccm. Connect 10 kV AC, discharge frequency 5 kHz, react at room temperature for 20 minutes, and take out after the reaction is completed to obtain tin oxide with a loose porous structure.

[0080] Example 4

[0081] A method for preparing tungsten oxide comprises the following steps:

[0082] (1) 2 g of WO3 (nanoflower) powder with a purity higher than 98% was first washed with anhydrous ethanol, and then washed with deionized water for multiple times, and finally the WO3 (nanoflower) powder was separated, which was the pretreated WO3 (nanoflower) powder.

[0083] (2) Prepare surfactant: Mix 0.1 g polyvinyl pyrrolidone and 0.1 g polyoxyethylene (9) stearate in 20 mL of 0.5% sodium sulfate solution, then add 10 mL of ethanol and stir well.

[0084] (3) Add 1 g of the WO3 (nanoflower) powder pretreated in step (1) to 10 mL of the surfactant prepared in step (2), stir for 20 min, and then separate. The separated WO3 (nanoflower) powder is thoroughly washed with anhydrous ethanol, and then placed in a vacuum drying oven at 80 ° C for 1 h to obtain activated WO3 (nanoflower) powder.

[0085] (4) Place the activated WO3 (nanoflower) powder in step (3) close to the negative electrode but not in the discharge area. Using the dielectric barrier discharge principle, stainless steel is used as the positive electrode and copper is used as the negative electrode, with a distance of 8 mm between the two. Double-layer alumina ceramic is used as the insulating medium, with a thickness of 2 mm and a spacing of 4 mm. Turn on the vacuum pump, maintain the pressure in the reaction chamber at 50 Pa, and introduce methane gas at a rate of 10 sccm. Pass 10 kV AC, discharge frequency 5 kHz, react at room temperature for 20 minutes, and take out after the reaction is completed to obtain tungsten oxide with a loose porous structure.

[0086] Comparative Example 1

[0087] Comparative Example 1 also prepares zinc oxide, and its preparation steps are the same as those of Example 2, except that step (5) is not performed when preparing zinc oxide in Comparative Example 1.

[0088] Comparative Example 2

[0089] Comparative Example 2 also prepares tin oxide, and its preparation steps are the same as those of Example 3, except that step (4) is not performed when preparing tin oxide in Comparative Example 3.

[0090] Comparative Example 3

[0091] Comparative Example 3 also prepares tungsten oxide, and its preparation steps are the same as those of Example 4, except that step (4) is not performed when preparing tungsten oxide in Comparative Example 4.

[0092] Comparative Example 4

[0093] Comparative Example 4 also prepares zinc oxide, but the difference is that in Comparative Example 4, oxygen plasma is prepared by a radio frequency discharge system in the prior art, and then the prepared oxygen plasma is used to modify zinc oxide. The power of the radio frequency discharge system is 350 W, the fluctuation frequency is 13.56 MHz, the rate of oxygen introduction is 10 sccm, and the reaction time is 30 min.

[0094] Comparative Example 5

[0095] Comparative Example 5 also prepares zinc oxide, but the difference is that in Comparative Example 5, hydrogen plasma is prepared by a radio frequency discharge system in the prior art, and then the prepared hydrogen plasma is used to modify zinc oxide. The power of the radio frequency discharge system is 350 W, the fluctuation frequency is 13.56 MHz, the rate of hydrogen introduction is 10 sccm, and the reaction time is 30 min.

[0096] The zinc oxide prepared in Example 2 and the zinc oxide prepared in Comparative Example 1 were placed under a scanning electron microscope and observed as follows: Figure 2 Results shown.

[0097] in Figure 2 (a) is the result of observing the zinc oxide prepared in Comparative Example 1 under a scanning electron microscope. Figure 2 (b) is the result of observation of zinc oxide prepared in Example 2 under a scanning electron microscope. Figure 2 (b) and Figure 2 (a) By comparison, it can be seen that the surface particles of zinc oxide nanofibers that have not been bombarded with methane plasma are arranged more closely (i.e., comparative example 1), while the zinc oxide nanofibers bombarded with methane plasma have many evenly distributed pores on the surface while maintaining the original structure (i.e., example 2). At the same time, since methane contains carbon atoms, after it attaches to the surface of zinc oxide, the surface defects increase, thereby reducing the activation energy of the reaction between the prepared zinc oxide and the gas and increasing the reaction rate.

[0098] The zinc oxide prepared in Example 2 and the zinc oxide prepared in Comparative Example 1 were tested for gas sensing performance, and the test data obtained are as follows: Figure 3 shown.

[0099] By comparing these two gas-sensitive performance test curves, it can be seen that the response value of the zinc oxide prepared in Example 2 to 20ppm formaldehyde at 300°C is increased by 32, and the response time is increased by 5s. This is mainly because the zinc oxide prepared in Example 2 has more pores, which increases the specific surface area of ​​the prepared zinc oxide, thereby increasing the area in contact with the gas, greatly improving its adsorption capacity, and achieving more sensitive detection of other targets. At the same time, since methane contains carbon atoms, after being bombarded and attached to the surface of zinc oxide, the surface defects of zinc oxide increase, thereby reducing the activation energy of the reaction between the prepared zinc oxide and the gas and increasing the reaction rate. Under the combined effect of increasing the pores and increasing the reaction rate, the gas-sensitive performance of the prepared zinc oxide is improved.

[0100] The tin oxide prepared in Example 3 and the tin oxide prepared in Comparative Example 2 were placed under a scanning electron microscope and observed as follows: Figure 4 Results shown.

[0101] in Figure 4 (a) is the result of observing the tin oxide prepared in Comparative Example 2 under a scanning electron microscope. Figure 4 (b) is the result of observation of the tin oxide prepared in Example 3 under a scanning electron microscope. Figure 4 (b) and Figure 4 (a) By comparison, it can be seen that the surface of the tin oxide nanosphere structure that has not been bombarded with methane plasma is relatively smooth (Comparative Example 2), while the tin oxide nanosphere structure bombarded with methane plasma has many evenly distributed pores on the surface while maintaining the original structure (ie, Example 3). At the same time, since methane contains carbon atoms, they attach to the surface of tin oxide after bombardment, increasing the surface defects, thereby reducing the activation energy of the reaction between the prepared tin oxide and the gas and increasing the reaction rate.

[0102] The tin oxide prepared in Example 3 and the tin oxide prepared in Comparative Example 2 were tested for gas sensing performance, and the test data obtained are as follows: Figure 5 shown.

[0103] By comparing these two gas-sensitive performance test curves, it can be seen that the response value of the tin oxide prepared in Example 3 to 10ppm formaldehyde at 220°C is increased by 21, and the response time is increased by 9s. This is mainly because the tin oxide prepared in Example 3 has more pores, which increases the specific surface area of ​​the prepared tin oxide, thereby increasing the area in contact with the gas, greatly improving its adsorption capacity, and achieving more sensitive detection of other targets. At the same time, since methane contains carbon atoms, after being bombarded and attached to the surface of tin oxide, the surface defects of tin oxide increase, thereby reducing the activation energy of the reaction between the prepared tin oxide and the gas and increasing the reaction rate. Under the combined effect of increasing the pores and increasing the reaction rate, the gas-sensitive performance of the prepared tin oxide is improved.

[0104] The tungsten oxide prepared in Example 4 and the tungsten oxide prepared in Comparative Example 3 were placed under a scanning electron microscope and observed as follows: Figure 6 Results shown.

[0105] in Figure 6 (a) is the result of observing the tungsten oxide prepared in Comparative Example 3 under a scanning electron microscope. Figure 6 (b) is the result of observation of tungsten oxide prepared in Example 4 under a scanning electron microscope. Figure 6 (b) and Figure 6 (a) By comparison, it can be seen that the surface of the tungsten oxide nanoflower structure that has not been bombarded by methane plasma is smoother (i.e., comparative example 3), while the tungsten oxide nanosphere structure that has been bombarded by methane plasma has many evenly distributed pores on the surface while maintaining the original structure (i.e., example 4). At the same time, since methane contains carbon atoms, they attach to the surface of tungsten oxide after bombardment, increasing the surface defects, thereby reducing the activation energy of the reaction between the prepared tungsten oxide and the gas and increasing the reaction rate.

[0106] The gas sensing performance of the tungsten oxide prepared in Example 4 and the tungsten oxide prepared in Comparative Example 3 were tested respectively. The test data obtained are as follows: Figure 7 shown.

[0107] By comparing these two gas-sensitive performance test curves, it can be seen that the response value of the tungsten oxide prepared in Example 4 to 20ppm formaldehyde at 325°C is increased by 11, and the response time is increased by 6s. This is mainly because the tungsten oxide prepared in Example 4 has more pores, which increases the specific surface area of ​​the prepared tungsten oxide, thereby increasing the area in contact with the gas, greatly improving its adsorption capacity, and achieving more sensitive detection of other targets. At the same time, since methane contains carbon atoms, after being bombarded and attached to the surface of tungsten oxide, the surface defects of tungsten oxide increase, thereby reducing the activation energy of the reaction between the prepared tungsten oxide and the gas and increasing the reaction rate. Under the combined effect of increasing the pores and increasing the reaction rate, the gas-sensitive performance of the prepared tungsten oxide is improved.

[0108] By observing the structure of zinc oxide prepared in Example 2, Comparative Example 4 and Comparative Example 5, and testing its gas sensitivity, the following conclusions were obtained: the microstructure of zinc oxide treated with hydrogen plasma changed, but the gas sensitivity was not significantly improved; the structure of zinc oxide treated with oxygen plasma changed, and the gas sensitivity was significantly improved, but the response time was extended by nearly 20 seconds. The reason for this may be that the radius of oxygen plasma is small, and the bombarded holes are small, which takes time for gas diffusion, so the response time is longer.

[0109] The above describes the basic principles, main features and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The above embodiments and descriptions only describe the principles of the present invention. The present invention may be subject to various changes and improvements without departing from the spirit and scope of the present invention. These changes and improvements fall within the scope of the present invention to be protected. The scope of protection claimed by the present invention is defined by the attached claims and their equivalents.

Claims

1. A method for preparing a metal oxide, characterized in that: It includes the following steps: Pre-treating the metal oxide powder, adding the pre-treated metal oxide to the surfactant in proportion and stirring, separating the metal oxide powder treated with the surfactant from the surfactant after stirring, and fully washing the metal oxide powder treated with the surfactant with anhydrous ethanol and drying to obtain activated metal oxide powder; Placing the activated metal oxide powder on an insulating medium connected to a negative electrode in a dielectric barrier discharge device and not being in a discharge region of the dielectric barrier discharge device; then introducing a certain amount of methane gas into a reaction chamber in the dielectric barrier discharge device, and connecting an alternating current power supply to the dielectric barrier discharge device; under the action of the alternating current of the dielectric barrier discharge device, the methane gas is converted into methane plasma, and the activated metal oxide powder is bombarded by the methane plasma to form a metal oxide with a loose porous structure; The surfactant is a mixture of polyvinyl pyrrolidone, polyoxyethylene (9) stearate, sodium sulfate and ethanol, wherein the mixing ratio of polyvinyl pyrrolidone, polyoxyethylene (9) stearate, sodium sulfate and ethanol is 0.1:0.1:20:10; the concentration of sodium sulfate is 0.5%; 1 g of the pretreated metal oxide powder is added to every 10 mL of the surfactant, and the reaction time of the pretreated metal oxide powder and the surfactant is 20 min-30 min.

2. The method for preparing a metal oxide according to claim 1, wherein: The dielectric barrier discharge device comprises an AC power supply and a reaction chamber, wherein the AC power supply comprises a positive electrode and a negative electrode, wherein the positive electrode and the negative electrode are both arranged in the reaction chamber, wherein the positive electrode and the negative electrode are arranged in parallel and have a gap therebetween, so that the discharge region is formed between the positive electrode and the negative electrode, wherein an insulating medium is arranged on the positive electrode and the negative electrode, and wherein the length of the insulating medium arranged on the negative electrode is greater than the length of the negative electrode.

3. The method for preparing a metal oxide according to claim 2, characterized in that: The activated metal oxide powder is placed on the upper end surface of the insulating medium in contact with the negative electrode and is not in the discharge region. The activated metal oxide powder is isolated from the insulating medium by an insulating material.

4. The method for preparing a metal oxide according to claim 2, wherein: The positive electrode and the negative electrode are 8 mm apart, the insulating medium is alumina ceramic, and the thickness of the insulating medium is 2 mm.

5. The method for preparing a metal oxide according to claim 1, wherein: The pretreatment comprises the following steps: firstly, placing the metal oxide powder in dilute nitric acid to remove impurities, then placing the metal oxide powder treated with dilute nitric acid in sodium carbonate to remove impurities, washing the metal oxide powder treated with sodium carbonate for multiple times, and separating the washed metal oxide powder, i.e. the pretreated metal oxide powder; The concentration of the dilute nitric acid is 3%, 1 g to 3 g of metal oxide powder is added to every 5 ml of the dilute nitric acid with a concentration of 3%, and the reaction time of the added metal oxide powder in the dilute nitric acid solution is 10 s to 20 s; The concentration of the sodium carbonate is 3%, and 1g to 3g of the metal oxide powder separated from the dilute nitric acid is added to every 5ml of the 3% sodium carbonate solution, and the reaction time of the added metal oxide powder separated from the dilute nitric acid in the sodium carbonate solution is 10s to 20s.

6. The method for preparing a metal oxide according to claim 1, characterized in that: The metal oxide includes one of ZnO, SnO2, and WO3; And / or, the nanostructure of the metal oxide under an electron microscope includes one of nanofibers, nanotubes, nanosheets or nanoflowers.

7. The method for preparing a metal oxide according to claim 1, characterized in that: In the dielectric barrier discharge device, the gas pressure of the reaction chamber is 50 Pa, the flow rate of the introduced methane gas is 10 sccm, the voltage of the introduced alternating current is 10 kV, the discharge frequency is 5 kHz, and the reaction time of the methane gas and the activated metal oxide powder is 20 min-30 min.

8. A metal oxide, characterized in that The metal oxide is prepared by the preparation method of the metal oxide as described in any one of claims 1 to 7. The prepared metal oxide has a loose porous structure while maintaining the nanostructure of the metal oxide itself.

9. A gas sensor, characterized in that: It comprises the metal oxide according to claim 8, wherein the metal oxide has a loose porous structure.

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