Preparation method of antioxidant titanium electrode, antioxidant titanium electrode and application of antioxidant titanium electrode in electrochemical oxidation reaction

Abstract

The application discloses a preparation method of an oxidation-resistant titanium electrode and the oxidation-resistant titanium electrode prepared by the method. The preparation method comprises the following steps: dispersing ATO nano particles in a dispersion medium to prepare a dispersion liquid, coating the dispersion liquid on the surface of a pretreated titanium substrate, and performing heat treatment to form an oxidation-resistant layer. The particle size of the ATO nano particles is less than 100 nm, and the concentration of the ATO nano particles in the dispersion liquid is 1-100 g / L. The heat treatment is performed at 300-800 DEG C for 1-3 hours in an air atmosphere. The loading of the ATO nano particles is 5-50 mg / cm 2 The application can improve the specific surface area and the cycle life of the titanium electrode under the premise of ensuring the oxidation resistance and corrosion resistance, and can be applied to high-efficiency electrochemical oxidation reaction, synthesis of high-value fine chemicals, meeting the demand of large current density in organic electro-synthesis, and maintaining a high synthesis yield after being used for multiple cycles.

Description

Technical Field

[0001] This invention belongs to the technical field of electrochemistry, specifically relating to a method for preparing an antioxidant titanium electrode, the antioxidant titanium electrode and its application in electrochemical oxidation reactions. Background Technology

[0002] Organic electrochemical synthesis is an advanced chemical synthesis technology that combines electricity and chemical synthesis techniques. It utilizes factors such as current, voltage, and electrode catalysts in electrochemical reactions to chemically synthesize organic molecules. It has the advantages of high efficiency, low energy consumption, and low pollution, and is an important way to promote the green upgrading and transformation of the traditional chemical industry.

[0003] High-efficiency, long-life anode materials are core components of organic electrosynthesis reactors, including carbon-based anodes, lead-based anodes, and titanium-based anodes. Traditional carbon-based anodes are prone to corrosion at high potentials, making it difficult to meet the requirements of high current and long lifespan in practical applications. Lead-based anodes are prone to the dissolution of high-valence lead ions at high potentials, making it difficult to meet environmental protection policy requirements. Titanium anodes have good corrosion resistance, but their surface is easily oxidized at high potentials, forming a passivation layer that hinders the continued electrochemical reaction. Size-stabilized anodes (DSA) use titanium as a substrate with a ruthenium-iridium oxide coating on the surface, offering advantages such as high catalytic activity and good stability. Since its invention in the 1960s, DSA has not only rapidly replaced graphite anodes, significantly increasing the production capacity of the chlor-alkali industry, but has also been gradually applied to electroplating, electrometallurgy, and organic wastewater degradation. However, due to the low oxygen evolution overpotential of the noble metal coatings (RuO2, IrO2, Pt, etc.) used in currently commercially available DSA electrodes, they are not suitable for the highly selective oxidation of organic compounds in aqueous media. Therefore, there is an urgent need to develop antioxidant titanium electrodes suitable for organic electrosynthesis reactions.

[0004] To further improve performance, patents CN105937037A and CN114105260A use conductive polymers such as polyaniline and polypyrrole as antioxidant coatings for titanium electrodes, improving the selectivity and yield of organic electrosynthesis reactions. However, these conductive polymers have low oxidation potentials and still face oxidation corrosion problems similar to those of carbon-based anodes such as graphite at high potentials. Patents CN103014755A, CN107779847A, and CN113789539A use antimony oxide (ATO), a non-precious metal oxide, as an antioxidant coating for titanium electrodes, improving the current efficiency and service life of titanium electrodes in the degradation of organic wastewater. However, the preparation methods of the above-mentioned ATO coatings mainly adopt methods such as thermal decomposition, sol-gel method, and electrodeposition method. The coatings obtained are difficult to meet the requirements of high current density in organic electrosynthesis, limiting their application in organic electrochemical synthesis. Summary of the Invention

[0005] This invention addresses the shortcomings of existing technologies by providing a method for preparing an antioxidant titanium electrode, the antioxidant titanium electrode itself, and its application in electrochemical oxidation reactions, while simultaneously improving the current density and cycle life of the electrochemical oxidation reaction.

[0006] To achieve the above objectives, the technical solution of the present invention is as follows:

[0007] A method for preparing an antioxidant titanium electrode involves dispersing ATO nanoparticles in a dispersion medium to form a dispersion solution, coating the dispersion solution onto the surface of a pretreated titanium substrate, and then heat-treating it to form an antioxidant layer, thereby obtaining the antioxidant titanium electrode. The ATO nanoparticles have a particle size of less than 100 nm and a concentration of 1–100 g / L in the dispersion solution. The heat treatment is performed at 300–800 °C for 1–3 hours in an air atmosphere. The loading of the ATO nanoparticles is 5–50 mg / cm³. 2 .

[0008] Optionally, the preparation method of the ATO nanoparticles includes: adding SnCl4 and SbCl3 in a molar ratio of (1-50):1 sequentially to an organic solvent and stirring at room temperature for 1-3 hours, heating under reflux at 70-90°C for 1-3 hours, cooling to room temperature and standing for 1-2 days, washing and drying the obtained white sol, and then heating it at 300-800°C for 1-3 hours in an air atmosphere; wherein the organic solvent includes one or more of methanol, ethanol, isopropanol, and n-butanol.

[0009] Optionally, the dispersion medium is a solution of water and organic solvent in a ratio of (0-100):1, wherein the organic solvent includes one or more of methanol, ethanol, n-propanol, isopropanol, and n-butanol.

[0010] Optionally, the coating method includes one or more of spraying, brushing, scraping, and spin coating.

[0011] Optionally, the titanium substrate includes one or more of titanium plates, titanium mesh, titanium fiber felt, powder sintered titanium, and foamed titanium.

[0012] Optionally, the pretreatment of the titanium substrate includes: immersing the titanium substrate in an etching solution for 1 to 60 minutes, and then rinsing to remove the etching solution; the etching solution includes one or more of hydrofluoric acid, nitric acid, sulfuric acid, hydrochloric acid, and oxalic acid solutions.

[0013] Antioxidant titanium electrode prepared by the above-described method.

[0014] The above-mentioned antioxidant titanium electrode is used in electrochemical oxidation reactions.

[0015] Optionally, the electrochemical oxidation reaction is an electrochemical oxidation preparation of organic matter, including the electrochemical oxidation of cyclic sulfites to prepare cyclic sulfates, the electrochemical oxidation of sulfides to prepare sulfoxides and sulfones, and the electrochemical oxidation of sulfoxides to prepare sulfones.

[0016] The beneficial effects of this invention are as follows:

[0017] The process of this invention forms an antioxidant layer on the surface of a titanium substrate, thereby improving the specific surface area and cycle life of the titanium electrode while ensuring its antioxidant and corrosion resistance. This layer can be applied to efficient electrochemical oxidation reactions to synthesize high-value-added fine chemicals, meet the high current density requirements in organic electrosynthesis, and maintain a high synthesis yield even after multiple cycles. Attached Figure Description

[0018] Figure 1 Here is a scanning electron microscope image of the antioxidant titanium electrode prepared in Example 1;

[0019] Figure 2 The image shows a scanning electron microscope image of the antioxidant titanium electrode prepared in Comparative Example 1.

[0020] Figure 3 The graphs show the performance of the antioxidant titanium electrodes prepared in Example 1 and Comparative Example 1 in oxidizing vinyl sulfite.

[0021] Figure 4 The graph shows the double-layer capacitance current of the antioxidant titanium electrodes prepared in Example 1 and Comparative Example 1 as a function of the scan rate. Detailed Implementation

[0022] The present invention will be further explained below with reference to the accompanying drawings and specific embodiments.

[0023] Example 1

[0024] In Example 1, the titanium substrate used was a titanium plate with dimensions of 20×20×0.1mm. It was immersed in a 10% oxalic acid etching solution at 80°C for 60 minutes to remove the surface oxide layer, thereby improving the adhesion of the subsequent antioxidant layer prepared on it. After immersion, it was rinsed with deionized water until clean.

[0025] 0.01 mol SnCl4 and 0.001 mol SbCl3 were sequentially added to 100 mL of isopropanol and stirred at room temperature for 1 hour. The mixture was then heated to reflux at 80°C for 1 hour, cooled to room temperature, and allowed to stand for 1 day. The resulting white sol was washed, dried, and then heated at 500°C for 2 hours in air to obtain ATO nanoparticles. The size of the ATO nanoparticles was measured to be 20-80 nm using scanning electron microscopy.

[0026] 20 mg of the above-mentioned ATO nanoparticles were added to 3 mL of isopropanol and 1 mL of water, and sonicated for 1 hour to prepare a dispersion. The dispersion was then sprayed onto the surface of a titanium plate using a spray gun. After drying, the plate was heated at 500°C for 2 hours in air to obtain a titanium electrode with an ATO antioxidant layer. The ATO nanoparticle loading was 5 mg / cm³. 2 Scanning electron microscope image of the obtained titanium electrode ( Figure 1 The results show that the ATO nanoparticles on the electrode surface form a three-dimensional porous structure with a pore size of 80–120 nm.

[0027] Example 2

[0028] The selection and pretreatment of the titanium substrate are the same as in Example 1.

[0029] 40 mg of the ATO nanoparticles prepared in Example 1 were added to 6 mL of isopropanol and 2 mL of water, and sonicated for 1 hour to prepare a dispersion. The dispersion was then sprayed onto the surface of a titanium plate using a spray gun. After drying, the plate was heated at 500°C for 2 hours in air to obtain a titanium electrode with an ATO antioxidant layer. The ATO nanoparticle loading was 10 mg / cm³. 2 .

[0030] Example 3

[0031] The selection and pretreatment of the titanium substrate are the same as in Example 1.

[0032] 80 mg of the ATO nanoparticles prepared in Example 1 were added to 12 mL of isopropanol and 4 mL of water, and sonicated for 1 hour to prepare a dispersion. The dispersion was then sprayed onto the surface of a titanium plate using a spray gun. After drying, the plate was heated at 500°C for 2 hours in air to obtain a titanium electrode with an ATO antioxidant layer. The ATO nanoparticle loading was 20 mg / cm³. 2 .

[0033] Example 4

[0034] The selection and pretreatment of the titanium substrate are the same as in Example 1.

[0035] 200 mg of the ATO nanoparticles prepared in Example 1 were added to 30 mL of isopropanol and 10 mL of water, and sonicated for 1 hour to prepare a dispersion. The dispersion was then sprayed onto the surface of a titanium plate using a spray gun. After drying, the plate was heated at 500°C in air for 2 hours to obtain a titanium electrode with an ATO antioxidant layer. The ATO nanoparticle loading was 50 mg / cm³. 2 .

[0036] Comparative Example 1

[0037] The selection and pretreatment of the titanium substrate are the same as in Example 1.

[0038] The antioxidant layer was prepared using the sol-gel method: 10 mL of ethylene glycol solution was heated to 60 °C, 30 mmol of citric acid was added and stirred until completely dissolved, then the mixture was heated to 90 °C, 9 mmol of SnCl4 and 1 mmol of SbCl3 were added, and stirring was continued for 30 minutes to obtain a sol-gel solution. The solution was evenly brushed onto a titanium plate, dried in a 120 °C oven for 30 minutes, and then heat-treated in a muffle furnace at 500 °C for 10 minutes. The brushing, drying, and heat-treatment operations were repeated 10 times. Finally, the titanium plate was heat-treated in a muffle furnace at 500 °C for 1 hour to obtain a titanium electrode with an ATO antioxidant layer and a loading of 5 mg / cm³. 2 Scanning electron microscope image of the obtained titanium electrode ( Figure 2 The results show that ATO nanoparticles are densely covering the electrode surface, with a size of 40–60 nm.

[0039] Comparative Example 2

[0040] The selection and pretreatment of the titanium substrate are the same as in Example 1.

[0041] 8 mg of ATO nanoparticles prepared in Example 1 were added to 1.2 mL of isopropanol and 0.4 mL of water, and sonicated for 1 hour to prepare a dispersion. The dispersion was then sprayed onto the surface of a titanium plate using a spray gun. After drying, the plate was heated at 500°C for 2 hours in air to obtain a titanium electrode with an ATO antioxidant layer. The ATO nanoparticle loading was 2 mg / cm³. 2 .

[0042] Comparative Example 3

[0043] The selection and pretreatment of the titanium substrate are the same as in Example 1.

[0044] Preparation of the polyaniline antioxidant layer: An electrolyte containing 0.01 mol / L aniline and 0.05 mol / L sodium sulfate was prepared. A titanium plate was used as the working electrode, a platinum sheet as the counter electrode, and silver / silver chloride as the reference electrode. The current was controlled at 0.5 mA / cm. 2 The process takes 10 minutes to obtain a titanium electrode containing a polyaniline antioxidant layer.

[0045] Application Example 1

[0046] The oxidation performance of vinyl sulfite using the titanium electrodes prepared in Example 1 and Comparative Example 1 was tested using linear sweep voltammetry. The specific steps included: preparing an electrolyte containing 5 mL of 0.5 mol / L sulfuric acid, 5 mL of acetonitrile, 5 mmol / L RuCl3, and 0.5 mol / L vinyl sulfite; using the titanium electrodes prepared in Example 1 and Comparative Example 1 as the working electrodes, a platinum sheet as the counter electrode, and silver / silver chloride as the reference electrode; and testing the current-voltage curves for vinyl sulfite oxidation. Figure 3As can be seen, compared with Comparative Example 1, the titanium electrode prepared in Example 1 exhibits a higher current density for the oxidation of vinyl sulfite at the same potential. Compared with the planar dense structure, the three-dimensional porous structure has a larger electrochemical active area and is conducive to the mass transfer of reactants, thus significantly improving the current density for the oxidation of vinyl sulfite.

[0047] Application Example 2

[0048] The electrochemical active area of ​​the titanium electrodes prepared in Example 1 and Comparative Example 1 was tested using the electric double-layer capacitance method. The specific steps included: using the titanium electrodes prepared in Example 1 and Comparative Example 1 as working electrodes, a platinum sheet as the counter electrode, and silver / silver chloride as the reference electrode, cyclic voltammetry curves were measured at different scan rates in 0.5 mol / L sulfuric acid. The average values ​​of the anodic and cathodic currents were used as the electric double-layer capacitance current, and a curve of the electric double-layer capacitance current versus the scan rate was plotted. Figure 4 The slope of the straight line indicates that the double-layer capacitance of the titanium electrode prepared in Example 1 is 28 mF / cm. 2 The double-layer capacitance of the titanium electrode prepared in Comparative Example 1 is 8 mF / cm. 2 The double-layer capacitance results show that the electrochemically active area of ​​the titanium electrode prepared in Example 1 is 3.5 times that of the titanium electrode prepared in Example 1. This is because the three-dimensional porous structure has a larger electrochemically active area than the planar dense structure.

[0049] Application Example 3

[0050] The titanium electrodes prepared in Examples 1-4 and Comparative Examples 1-3 were used as anodes, platinum-plated titanium felt as cathodes, and Nafion 117 as a proton exchange membrane to assemble an electrochemical reaction apparatus. The apparatus was cooled to 5°C, and an anolyte containing 5 mL of 0.5 mol / L sulfuric acid, 5 mL of acetonitrile, 5 mmol / L RuCl3, and 0.5 mol / L vinyl sulfite was introduced into the cathode at a flow rate of 20 mL / min. The cathode electrolyte containing 10 mL of 0.5 mol / L sulfuric acid was introduced into the cathode at a flow rate of 20 mL / min. A voltage of 2.1 V was applied between the anode and cathode electrodes, and the voltage was stopped after 20 minutes. The anolyte after the reaction was subjected to vacuum distillation, filtration, washing, and drying to obtain vinyl sulfate. The product yield was calculated, and the results are shown in Table 1.

[0051] Vinyl sulfate yield = actual yield / theoretical yield × 100%

[0052] Table 1. Product yields of titanium electrodes prepared in Examples 1-4 and Comparative Examples 1-3 in the electrochemical oxidation reaction to prepare vinyl sulfate.

[0053] Anode number Ethylene sulfate yield (%) Example 1 85 Example 2 86 Example 3 85 Example 4 87 Comparative Example 1 45 Comparative Example 2 83 Comparative Example 3 82

[0054] Table 1 shows that the titanium electrodes with three-dimensional porous structures prepared by the spraying method in Examples 1-4 and Comparative Example 2 all exhibited high product yields in the electrochemical oxidation of vinyl sulfate, indicating that the oxidation performance of vinyl sulfite is closely related to the electrode surface structure and independent of the ATO loading. The titanium electrode prepared in Comparative Example 1 had a lower product yield because the surface structure of the titanium electrode prepared by the sol-gel method was dense, resulting in a smaller specific surface area and a lower current density for oxidizing vinyl sulfite at the same potential, thus leading to a lower product yield. The titanium electrode with the polyaniline antioxidant layer prepared in Comparative Example 3 also showed a high vinyl sulfate yield.

[0055] Application Example 4

[0056] The cyclic testing was performed according to the method in Application Example 1, and the yield of vinyl sulfate after each cycle was recorded. The results are shown in Table 2.

[0057] Table 2. Product yields of titanium electrodes prepared in Examples 1-4 and Comparative Examples 1-3 after 10 cycles in the electrochemical oxidation reaction to prepare vinyl sulfate.

[0058]

[0059] As shown in Table 2, the titanium electrodes prepared in Examples 1-4 and Comparative Example 1 all exhibited good antioxidant properties, with the product yield remaining unchanged after 10 cycles. Although the titanium electrode prepared in Comparative Example 2 had high initial performance, the yield of vinyl sulfate decreased to 0 after 7 cycles. This was due to the low ATO loading, which could not effectively inhibit the oxidation of the titanium electrode. The titanium electrode prepared in Comparative Example 3 had high initial performance, but the yield of vinyl sulfate decreased to 0 after 5 cycles. This was because the antioxidant properties of polyaniline were poor, and it could not effectively inhibit the oxidation of the titanium electrode.

[0060] Application Example 5

[0061] An electrochemical reaction apparatus was assembled using the titanium electrode prepared in Example 1 as the anode, a platinum-plated titanium felt as the cathode, and Nafion 117 as the proton exchange membrane. An anolyte containing 5 mL of 0.5 mol / L sulfuric acid, 5 mL of acetonitrile, 5 mmol / L RuCl3, and 0.5 mol / L propylene sulfite was introduced into the cathode at a flow rate of 20 mL / min. The cathode electrolyte containing 10 mL of 0.5 mol / L sulfuric acid was introduced into the cathode at a flow rate of 20 mL / min. A voltage of 2.1 V was applied between the anode and cathode electrodes, and the flow was stopped after 20 minutes. The resulting anolyte was subjected to vacuum distillation, filtration, washing, and drying to obtain propylene sulfate in 92% yield.

[0062] Application Example 6

[0063] An electrochemical reaction apparatus was assembled using the titanium electrode prepared in Example 1 as the anode, a platinum-plated titanium felt as the cathode, and Nafion 117 as the proton exchange membrane. An anolyte containing 5 mL of 0.5 mol / L sulfuric acid, 5 mL of acetonitrile, 5 mmol / L RuCl3, and 0.5 mol / L 4-propylethylene sulfite was introduced into the cathode at a flow rate of 20 mL / min. The cathode electrolyte containing 10 mL of 0.5 mol / L sulfuric acid was introduced into the cathode at a flow rate of 20 mL / min. A voltage of 2.1 V was applied between the anode and cathode electrodes, and the flow was stopped after 20 minutes. The resulting anolyte was subjected to vacuum distillation, filtration, washing, and drying to obtain 4-propylethylene sulfite in 90% yield.

[0064] Application Example 7

[0065] An electrochemical reaction apparatus was assembled using the titanium electrode prepared in Example 1 as the anode, a platinum-plated titanium felt as the cathode, and Nafion 117 as the proton exchange membrane. An anolyte containing 5 mL of 0.5 mol / L sulfuric acid, 5 mL of acetonitrile, 5 mmol / L RuCl3, and 0.5 mol / L methylphenyl sulfoxide was introduced into the cathode at a flow rate of 20 mL / min. The cathode electrolyte containing 10 mL of 0.5 mol / L sulfuric acid was introduced into the cathode at a flow rate of 20 mL / min. A voltage of 2.1 V was applied between the anode and cathode electrodes, and the flow was stopped after 20 minutes. The resulting anolyte was subjected to vacuum distillation, filtration, washing, and drying to obtain methylphenyl sulfoxide in 93% yield.

[0066] Application Example 8

[0067] An electrochemical reaction apparatus was assembled using the titanium electrode prepared in Example 1 as the anode, a platinum-plated titanium felt as the cathode, and Nafion 117 as the proton exchange membrane. An anolyte containing 5 mL of 0.5 mol / L sulfuric acid, 5 mL of acetonitrile, 5 mmol / L RuCl3, and 0.2 mol / L anisole was introduced into the cathode at a flow rate of 20 mL / min. The cathode electrolyte containing 10 mL of 0.5 mol / L sulfuric acid was introduced into the cathode at a flow rate of 20 mL / min. A voltage of 1.8 V was applied between the anode and cathode electrodes, and the flow was stopped after 20 minutes. The resulting anolyte was distilled under reduced pressure, filtered, washed, and dried to obtain methylphenyl sulfoxide in 95% yield.

[0068] Application Example 9

[0069] An electrochemical reaction apparatus was assembled using the titanium electrode prepared in Example 1 as the anode, a platinum-plated titanium felt as the cathode, and Nafion 117 as the proton exchange membrane. An anolyte containing 5 mL of 0.5 mol / L sulfuric acid, 5 mL of acetonitrile, 5 mmol / L RuCl3, and 0.2 mol / L anisole was introduced into the cathode at a flow rate of 20 mL / min. The cathode electrolyte containing 10 mL of 0.5 mol / L sulfuric acid was introduced into the cathode at a flow rate of 20 mL / min. A voltage of 2.1 V was applied between the anode and cathode electrodes, and the flow was stopped after 20 minutes. The resulting anolyte was subjected to vacuum distillation, filtration, washing, and drying to obtain methylphenyl sulfone in 87% yield.

[0070] The above embodiments are only used to further illustrate the preparation method of an antioxidant titanium electrode, the antioxidant titanium electrode and its application in electrochemical oxidation reaction of the present invention. However, the present invention is not limited to the embodiments. Any simple modifications, equivalent changes and modifications made to the above embodiments based on the technical essence of the present invention shall fall within the protection scope of the technical solution of the present invention.

Claims

1. The application of an antioxidant titanium electrode in electrochemical oxidation reactions, characterized in that: The electrochemical oxidation reaction is an electrochemical oxidation preparation of organic compounds, including the electrochemical oxidation of cyclic sulfites to prepare cyclic sulfates, the electrochemical oxidation of sulfides to prepare sulfoxides and sulfones, and the electrochemical oxidation of sulfoxides to prepare sulfones; the preparation method of the antioxidant titanium electrode is as follows: ATO nanoparticles were dispersed in a dispersion medium to prepare a dispersion solution, which was then coated onto the surface of a pretreated titanium substrate. After heat treatment, an antioxidant layer was formed, resulting in an antioxidant titanium electrode. The ATO nanoparticles had a particle size of less than 100 nm and a concentration of 1–100 g / L in the dispersion solution. The heat treatment was performed at 300–800 °C for 1–3 hours in an air atmosphere. The loading of the ATO nanoparticles was 5–50 mg / cm³. 2 .

2. The application of the antioxidant titanium electrode according to claim 1 in electrochemical oxidation reactions, characterized in that, The preparation method of the ATO nanoparticles includes: adding SnCl4 and SbCl3 in a molar ratio of (1~50):1 sequentially to an organic solvent and stirring at room temperature for 1~3 hours, heating under reflux at 70~90℃ for 1~3 hours, cooling to room temperature and standing for 1~2 days, washing and drying the obtained white sol, and then heating at 300~800℃ for 1~3 hours in an air atmosphere; wherein the organic solvent includes one or more of methanol, ethanol, isopropanol, and n-butanol.

3. The application of the antioxidant titanium electrode according to claim 1 in electrochemical oxidation reactions, characterized in that, The dispersion medium is a solution of water and organic solvent in a volume ratio of (0~100):1, wherein the organic solvent includes one or more of methanol, ethanol, n-propanol, isopropanol, and n-butanol.

4. The application of the antioxidant titanium electrode according to claim 1 in electrochemical oxidation reactions, characterized in that, The coating method includes one or more of spraying, brushing, scraping, and spin coating.

5. The application of the antioxidant titanium electrode according to claim 1 in electrochemical oxidation reactions, characterized in that, The titanium substrate includes one or more of the following: titanium plate, titanium mesh, titanium fiber felt, powder sintered titanium, and foamed titanium.

6. The application of the antioxidant titanium electrode according to claim 1 in electrochemical oxidation reactions, characterized in that, The pretreatment of the titanium substrate includes: immersing the titanium substrate in an etching solution for 1 to 60 minutes, and then rinsing to remove the etching solution; the etching solution includes one or more of hydrofluoric acid, nitric acid, sulfuric acid, hydrochloric acid, and oxalic acid solutions.

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