Gas sensor and method for preparing gas sensor through laser direct writing

Abstract

The invention provides a gas sensor and a method for preparing the gas sensor through laser direct writing. The method comprises the following steps: enabling a metal electrode to be in contact with laser direct writing ink, wherein the laser direct writing ink comprises a nickel source, a porous structure guiding agent and a solvent; laser irradiates a specified contact area of the metal electrode and the laser direct-writing ink, so that the metal electrode absorbs the laser to generate heat so as to form a hydrothermal reaction area in the specified contact area, and the laser direct-writing ink in the hydrothermal reaction area is subjected to a hydrothermal reaction, so that porous nickel hydroxide grows on the metal electrode, and a precursor structure is obtained; and carrying out annealing treatment on the precursor structure so as to convert the porous nickel hydroxide into porous nickel oxide, thereby obtaining the gas sensor. The gas sensor provided by the invention has the porous nickel oxide which has a relatively large specific surface area, so that the effective contact area between the gas sensor and target gas can be increased, and the gas responsivity of the gas sensor is improved.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of laser processing micro-nano devices, and particularly relates to a gas sensor and a method for preparing the gas sensor by laser direct writing. BACKGROUND

[0002] Traditional gas sensors generally have the challenges of large volume, low response, and complex process flow. These inherent limitations hinder their widespread use in portable, distributed, and instant application scenarios. To address the above challenges, the field of gas sensors is rapidly developing towards miniaturization, integration, low power consumption, and high performance. Miniaturization can bring significant advantages, including achieving higher detection accuracy, faster response and recovery time, and lower production cost.

[0003] Nickel oxide is a wide-bandgap p-type semiconductor metal oxide that has attracted increasing attention due to its unique and excellent structure, high chemical stability, high sensitivity, and low cost. The existing nickel oxide thin film or bulk material has limited specific surface area, which limits the gas adsorption-desorption process to the surface. In addition, the current manufacturing technology of nickel oxide gas sensors faces many limitations. The main problems are: first, the manufacturing method of nickel oxide sensors based on traditional semiconductor technology requires expensive equipment such as chemical vapor deposition and complex process flow; second, the manufacturing method of nickel oxide sensors based on chemical hydrothermal synthesis requires long-time hydrothermal synthesis in a reaction kettle, which cannot achieve miniaturized and customized sensor manufacturing. Therefore, it is urgent to develop a new manufacturing technology for nickel oxide sensors to overcome the above limitations and achieve low-cost and customized manufacturing of miniaturized, integrated, and high-performance nickel oxide sensors. SUMMARY

[0004] To solve all or part of the above technical problems, the application provides the following technical solutions: The first aspect of the application provides a method for preparing a gas sensor by laser direct writing, the method comprising: contacting a metal electrode with a laser direct writing ink, the laser direct writing ink comprising a nickel source, a porous structure directing agent, and a solvent; irradiating a specified contact area of the metal electrode with the laser direct writing ink with a laser, causing the metal electrode to absorb the laser and heat up, forming a hydrothermal reaction zone in the specified contact area, and causing the laser direct writing ink in the hydrothermal reaction zone to undergo a hydrothermal reaction, thereby growing porous nickel hydroxide on the metal electrode to obtain a precursor structure; and annealing the precursor structure to convert the porous nickel hydroxide to porous nickel oxide, thereby preparing the gas sensor.

[0005] The gas sensor prepared by the laser direct writing ink can form porous nickel hydroxide, and the porous nickel hydroxide can be converted into porous nickel oxide through annealing treatment, so that the specific surface area of the porous nickel oxide is large, the effective contact area of the gas sensor and the target gas is increased, the gas response of the gas sensor is improved, and the problem that the gas adsorption-desorption process of the nickel oxide film or the bulk nickel oxide material in the prior art is limited to the surface due to the limited specific surface area is solved.

[0006] In some embodiments, the porosity of the porous nickel oxide is 50-200 / µm 2 , and the pore size of the contained pores is 50-300 nm.

[0007] In some embodiments, the concentration of the nickel source in the laser direct writing ink is 2-3 mol / L. The nickel source may, for example, include nickel nitrate, but is not limited thereto.

[0008] In some embodiments, the porous structure directing agent includes ethanolamine, and the volume ratio of the solvent to ethanolamine is 10-20:1.

[0009] In some embodiments, the solvent of the laser direct writing ink includes dimethyl sulfoxide and ethanol in a volume ratio of 5-10:5-10.

[0010] In some embodiments, the method specifically includes: focusing a laser on a specified contact area of the metal electrode and the laser direct writing ink, so as to form the hydrothermal reaction zone.

[0011] In some embodiments, the method specifically includes: growing the porous nickel hydroxide on at least two metal electrodes spaced from each other, respectively, and connecting the porous nickel hydroxide on the at least two metal electrodes to each other, and then performing the annealing treatment, so as to obtain the gas sensor.

[0012] In some embodiments, the method specifically includes: forming a metal electrode on a first substrate, the metal electrode having a one-dimensional, two-dimensional or three-dimensional structure; growing two-dimensional shape porous nickel hydroxide on the metal electrode with a one-dimensional or two-dimensional structure, so as to obtain a gas sensor with two-dimensional shape porous nickel oxide; and / or growing three-dimensional shape porous nickel hydroxide on the metal electrode with a three-dimensional structure, so as to obtain a gas sensor with three-dimensional shape porous nickel oxide.

[0013] The method for preparing the metal electrode on the substrate can be any known laser direct writing technology or other micro-nano processing method in the art, as long as the metal electrode of the present application can be formed.

[0014] In some embodiments, the metal electrode is fabricated by laser direct writing, so that the gas sensor can be prepared on the same laser platform.

[0015] In some embodiments, the method for fabricating the metal electrode comprises: The first surface of the first substrate and the second surface of the second substrate are arranged opposite to each other with a spacing, and the spacing between the first surface of the first substrate and the second surface of the second substrate has a micron-level thickness, and the first substrate and the second substrate are light-transmitting substrates; The electrode ink for fabricating the metal electrode is injected into the spacing to form a three-dimensional ink space layer that can be laser printed; the laser is incident on the three-dimensional ink space layer, and irradiates the selected contact position between the electrode ink and the first surface of the first substrate, so as to form the metal electrode with a two-dimensional structure or a three-dimensional structure on the first surface of the first substrate.

[0016] In some embodiments, the method for fabricating the metal electrode specifically comprises: The electrode ink is injected into the spacing to form the three-dimensional ink space layer; The laser is incident on the three-dimensional ink space layer from the third surface of the first substrate, and irradiates the selected contact position between the electrode ink and the first surface of the first substrate, so as to form a first part of the metal electrode on the first surface of the first substrate; The laser is incident on the three-dimensional ink space layer from the fourth surface of the second substrate, and irradiates the selected contact position between the electrode ink and the first part of the metal electrode, so as to integrally form a second part of the metal electrode on the first part of the metal electrode; The third surface is opposite to the first surface, and the fourth surface is opposite to the second surface.

[0017] In some typical embodiments, for example, the above method can be used to form a first part of the metal electrode with a one-dimensional structure on the first surface of the first substrate, and then grow a second part on the first part with the one-dimensional structure, so as to form a metal electrode with a three-dimensional structure. For example, the first part of the metal electrode with the one-dimensional structure is linear, and the second part is grown on the linear first part to form a metal electrode with a convex shape.

[0018] In some embodiments, when the metal electrode is fabricated by laser direct writing, the process parameters for forming the metal electrode with the one-dimensional structure can include a laser power of 1.6-3.67 mW and a scanning speed of 5-50 µm / s; and the process parameters for integrally forming the metal electrode with the three-dimensional structure on the metal electrode with the one-dimensional structure can include a laser power of 0.9-4.1 mW and a scanning speed of 0.1-2 µm / s.

[0019] In some embodiments, after the metal electrode is fabricated, the laser direct writing ink is injected into the interlayer space, the laser is incident on the three-dimensional ink space layer, and the laser direct writing ink and the specified contact area of the metal electrode are irradiated, so as to obtain the precursor structure.

[0020] In some embodiments, the focal point of the laser is relatively moved with the metal electrode in at least one of the x, y, and z directions in the three-dimensional coordinate space, so that the laser sequentially irradiates multiple specified contact areas of the metal electrode and the laser direct writing ink and / or changes the distance between the focal point of the laser and the specified contact area of the metal electrode and the laser direct writing ink, so as to regulate the structure of the precursor structure.

[0021] In some embodiments, at least one or more of the wavelength, power, and scanning speed of the laser is adjusted, so as to regulate the structure of the precursor structure.

[0022] In some typical embodiments, the metal electrode of the one-dimensional structure includes a metal wire. For example, the laser direct writing ink is brought into contact with at least part of the metal wire, and the laser is focused on the metal wire to grow two-dimensional shaped porous nickel hydroxide on the metal wire.

[0023] In some embodiments, the laser parameters for growing two-dimensional shaped porous nickel hydroxide on the metal electrode of the one-dimensional structure or the two-dimensional structure include that the power of the laser is 3-6 mW, and the scanning speed of the laser is 0.5-2 µm / s.

[0024] It should be understood that the grown nickel hydroxide has a certain thickness, so the two-dimensional shaped nickel hydroxide here does not represent an absolute plane. The thickness of the two-dimensional shaped porous nickel hydroxide described in the present application is below 3 µm, for example, the thickness can be 2-3 µm.

[0025] In some embodiments, the metal electrode includes a plurality of parallel metal wires, porous nickel hydroxide is grown on the plurality of parallel metal wires, and / or a plurality of porous nickel hydroxides are grown on one metal wire, and adjacent porous nickel hydroxides are connected to each other.

[0026] In some embodiments, the metal electrode of the three-dimensional structure includes a metal protrusion. For example, the laser direct writing ink is brought into contact with the metal protrusion, and the laser is focused on the metal protrusion to grow three-dimensional shaped porous nickel hydroxide on the metal protrusion.

[0027] In some embodiments, the height of the metal protrusion is 4-6 µm.

[0028] In some embodiments, the diameter of the metal protrusion is 1-3 µm.

[0029] In some embodiments, the metal electrode comprises a plurality of metal protrusions spaced apart from each other with a distance of 2-5 μm.

[0030] In some embodiments, the metal electrode comprises a plurality of metal protrusions arranged in a patterned arrangement. The patterned arrangement can be, for example, a linear arrangement or an array arrangement, but is not limited thereto.

[0031] In some embodiments, the method specifically comprises: when growing the nickel hydroxide on the metal protrusions, gradually increasing the power of the laser focused on the metal protrusions in steps, the initial power of the laser being 0.9-1.3 mW, the maximum power being 2.5-4.1 mW, the increase from the initial power to the maximum power being achieved through 2-5 power increases, the increment of each power increase being 0.4-0.7 mW, and the scanning time at each power level being 2-5 s.

[0032] The present application has been found through systematic research that, when the metal electrode has a plurality of adjacent metal protrusions, gradually increasing the power of the laser in steps according to the above method can produce nickel hydroxide with protruding structures, and adjacent protruding structures form “overhead holes”, i.e., the nickel hydroxide grown on the metal protrusions tends to form protruding structures having a first portion away from the substrate and a second portion close to the substrate, the first portions of two adjacent protruding structures contact each other, and the second portions do not contact each other to form overhead holes at the bottom close to the substrate. The formation of overhead holes can further increase the effective contact area of the gas sensor with the target gas, improve the gas response, and also improve the conductivity of the device.

[0033] In some preferred embodiments, the laser direct writing ink is brought into contact with a plurality of metal protrusions arranged in a patterned arrangement, the metal electrode and the specified contact area of the laser direct writing ink are irradiated with the laser, and the power of the laser is gradually increased in steps according to the above method to form a precursor structure with overhead holes; the precursor structure is subjected to an annealing treatment to obtain a gas sensor with overhead holes. For example, nickel hydroxide can be grown on a plurality of metal protrusions arranged in an array by gradually increasing the power of the laser in steps, and nickel hydroxide protruding structures arranged in an array can be obtained, adjacent protruding structures are connected to each other and form overhead holes, and in an array, a plurality of overhead holes connected through each other form a three-dimensional interconnected overhead network, further improving the conductivity and gas response of the device.

[0034] In some embodiments, the metal protrusions have a height of 4-6 μm, a diameter of 1-3 μm, and a spacing between adjacent metal protrusions of 2-5 μm. In some embodiments, the nickel hydroxide protrusions have a height of 4-8 μm, a diameter of 3-6 μm, and a spacing between adjacent nickel hydroxide protrusions of 2-5 μm. In some embodiments, the nickel oxide protrusions have a height of 4-8 μm, a diameter of 3-6 μm, and a spacing between adjacent nickel oxide protrusions of 2-5 μm. In some embodiments, the height of the metal protrusions is uniform, and the nickel oxide protrusions form a network of interconnected nickel oxide protrusions. In some embodiments, the height of the metal protrusions is not uniform, and the nickel oxide protrusions do not form a network of interconnected nickel oxide protrusions.

[0035] In some embodiments, the annealing process is performed at a temperature of 350-400 °C.

[0036] In some embodiments, the annealing process is performed for a time period of 1-3 h.

[0037] After the annealing process, the nickel hydroxide is converted to nickel oxide, but the annealing process does not change the porous structure or shape of the nickel hydroxide.

[0038] In some embodiments, the metal electrode is made of platinum. In some embodiments, the electrode ink used to fabricate the metal electrode by the laser direct writing technique includes ferric ammonium oxalate trihydrate, ammonium tetrachloroplatinate, and a solvent. In some embodiments, the ferric ammonium oxalate trihydrate has a concentration of 400-600 mM, the ammonium tetrachloroplatinate has a concentration of 50-100 mM, and the solvent includes water.

[0039] In some embodiments, the gas sensor is prepared by the method of any one of the technical solutions described above.

[0040] In some embodiments, the gas sensor includes a substrate, a metal electrode disposed on the substrate, and a gas sensitive structure disposed on the substrate. The gas sensitive structure includes a plurality of porous nickel oxide protrusions that are in contact with each other and the metal electrode.

[0041] In some embodiments, the porous nickel oxide has a porosity of 50-200 pores per μm 2 and a pore size of 50-300 nm.

[0042] In some embodiments, the porous nickel oxide has a protrusion structure that includes a first portion that is away from the substrate and a second portion that is close to the substrate. The first portions of at least two adjacent protrusion structures are in contact with each other, and the second portions are not in contact with each other, thereby forming an overhanging hole at the bottom close to the substrate.

[0043] In some embodiments, the diameter of the protruding structure is 3-6 μm, and / or the height of the protruding structure is 4-8 μm.

[0044] In some embodiments, the height of the overhanging hole is 2-5 μm.

[0045] In some embodiments, the gas-sensitive structure comprises adjacent overhanging holes formed by a plurality of protruding structures, and the distance between the centers of two adjacent overhanging holes is 2-5 μm.

[0046] In some embodiments, the gas-sensitive structure comprises a plurality of protruding structures arranged in a pattern. The patterned arrangement may, for example, be a linear arrangement or an arrayed arrangement, but is not limited thereto.

[0047] In some embodiments, a plurality of protruding structures form a plurality of overhanging holes that are interconnected to form a three-dimensionally interconnected overhanging network. For example, when the protruding structures are arranged in an array, a plurality of overhanging holes are arranged in sequence in the horizontal and vertical directions of the array, thereby forming overhanging channels in the horizontal and vertical directions, and these overhanging channels are interconnected, i.e., the “three-dimensionally interconnected overhanging network”.

[0048] In some embodiments, the material of the metal electrode comprises platinum.

[0049] Compared with the prior art, the present application has at least the following beneficial effects: The preparation method of the gas sensor provided by the present application is based on laser direct writing ink to prepare porous nickel hydroxide, and the porous nickel hydroxide is converted into porous nickel oxide through annealing treatment. The porous nickel oxide has a large specific surface area, which can increase the effective contact area with the target gas and improve the gas response of the gas sensor.

[0050] The method provided by the present application can realize rapid customized production of gas sensors of different shapes and sizes by controlling laser power, scanning time and scanning path, and provides an excellent solution for small-batch customization of gas sensors. The method for preparing a gas sensor by laser direct writing provided by the present application can realize micron-level structural precision and three-dimensional complexity that are difficult to achieve by existing technologies such as hydrothermal method and simple deposition method, improve the integration and miniaturization level of the gas sensor, and make it adapt to the demand of modern electronic equipment for miniaturized and arrayed sensors. BRIEF DESCRIPTION OF DRAWINGS

[0051] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the accompanying drawings needed to be used in the description of the embodiments or the prior art will be briefly introduced. Obviously, the accompanying drawings in the following description only show some embodiments of the present application, and for those skilled in the art, other drawings can also be obtained from these drawings without any creative effort.

[0052] Figure 1 is a flowchart of preparing a gas sensor according to Embodiment 1 of the present application; Figure 2 is a photo of platinum electrode ink used for printing platinum electrodes in Embodiment 1; Figure 3 is a photo of laser direct writing ink used for growing porous nickel hydroxide in Embodiment 1; Figure 4 is a schematic structural diagram of a gas sensor prepared in Embodiment 1; Figure 5 is a scanning electron microscope (SEM) image of a gas sensor prepared in Embodiment 1; Figure 6 is a flowchart of preparing a gas sensor according to Embodiment 2 of the present application; Figure 7 is a SEM image of platinum wires and platinum needles printed in Embodiment 2; Figure 8 is a schematic structural diagram of a gas sensor prepared in Embodiment 2; Figure 9 is a SEM image of a gas sensor prepared in Embodiment 2; Figure 10 is a repeated response curve of a gas sensor prepared in Embodiment 1 to 100 ppm ethanol gas; Figure 11 is a repeated response curve of a gas sensor prepared in Embodiment 2 to 100 ppm ethanol gas; Figure 12 is a comparison diagram of a single response of a gas sensor prepared in Embodiment 1 and a gas sensor prepared in Embodiment 2 to 100 ppm ethanol gas. DETAILED DESCRIPTION

[0053] The technical solutions of the present application will be described in detail below with specific embodiments, so that those skilled in the art can better understand and implement the technical solutions of the present application. The specific functional details disclosed herein should not be interpreted as limiting, but only as a basis for the claims and for teaching those skilled in the art to employ the representative basis of the present application in different ways in any appropriate detailed embodiment.

[0054] In addition, unless otherwise specified, the various raw materials used in the following examples can be obtained commercially, and the various production and testing equipment used are known in the art, and the methods used are also known in the art.

[0055] Example 1 This example provides a gas sensor and a method for preparing the same, the gas sensor comprising a two-dimensional nickel oxide, and the preparation process thereof is shown in Figure 1 and specifically comprises the following steps: 1. A polyimide tape with a thickness of about 60 pm and a width of 5 mm is attached to both sides of a first glass substrate, a second glass substrate is placed parallel to the tape, and then the second glass substrate is fixed with a polyimide tape, so that a layer of about 60 pm thick interlayer space is formed between the two opposite glass substrates.

[0056] 2. Printing platinum electrode: 1) Prepare the platinum electrode ink needed for printing the platinum electrode. Dissolve ferric ammonium oxalate trihydrate and platinum tetrachloride in deionized water to obtain the platinum electrode ink, wherein the concentration of ferric ammonium oxalate trihydrate is about 500 mM, and the concentration of platinum tetrachloride is about 70 mM, Figure 2 is a physical photo of the platinum electrode ink; 2) Inject the above platinum electrode ink into the interlayer space between the two glass substrates, and focus the laser on the interface between the glass substrate and the platinum electrode ink with the laser incident from the back surface of the first glass substrate to the interlayer space, print platinum lines with a laser power of 2.57 mW and a scanning speed of 30 pm / s, and the spacing between adjacent platinum lines is 4 pm, to form platinum lines on the first glass substrate, and clean the platinum lines after printing.

[0057] In order to facilitate the subsequent test instrument to test the gas sensor, a photoetching mask is used to form a large-area platinum electrode on the first glass substrate. The "large area" facilitates the probe of the test instrument to be aligned and connected, and the platinum line is connected with the large-area platinum electrode. The photoetching mask forms a large-area platinum electrode, which is a prior art, and therefore will not be described here.

[0058] 3. Growth of porous nickel hydroxide: 1) Preparation of laser direct writing ink: 0.785 g of nickel nitrate hexahydrate is dissolved in a mixture of 0.5 mL of dimethyl sulfoxide and 0.5 mL of ethanol, and then 90 uL of ethanolamine is added. Stir until the solution is uniform at room temperature to obtain the laser direct writing ink, Figure 3 is a physical photo of the laser direct writing ink; 2) Inject the above prepared laser direct writing ink into the interlayer space, so that it contacts the platinum wire, and use a laser focused on the selected contact position of the laser direct writing ink and the platinum wire through the glass substrate, and form a micro hydrothermal growth reaction zone on the platinum wire by the heat effect generated by the absorption of the laser by the metal platinum. The laser is printed to form a two-dimensional shaped porous nickel hydroxide at a constant power of 4 mW and a scanning speed of 1 µm / s.

[0059] 4) The printed sample is washed in deionized water for 10 minutes and naturally air dried, and then placed in a tube furnace and annealed at a temperature of 400°C for 2h, so that the porous nickel hydroxide is converted into porous nickel oxide, and a gas sensor with two-dimensional nickel oxide is obtained.

[0060] Figure 4 is a structural schematic diagram of the gas sensor prepared in this embodiment, Figure 5 is a scanning electron microscope image of the gas sensor prepared in this embodiment. It can be seen from Figure 4 , Figure 5 that the gas sensor prepared in this embodiment has two large-area platinum electrodes located on the left and right sides, two-dimensional nickel oxide is distributed between the two large-area platinum electrodes, and the large-area platinum electrodes are connected by a platinum wire. The two-dimensional nickel oxide has a plurality of and is arranged in sequence along a first direction (in this embodiment, the first direction is perpendicular to the two-dimensional platinum wire), and adjacent two-dimensional nickel oxides are in contact with each other, wherein the customized arrangement of the two-dimensional nickel oxide can be realized by precisely moving the sample in X, Y and Z directions relative to the laser beam by a three-dimensional moving platform. The length of a single two-dimensional nickel oxide prepared in this embodiment is about 20 µm, the width is about 4 µm, and the height is about 1 µm.

[0061] The laser direct writing ink provided by the present application can obtain porous nickel hydroxide by laser-induced hydrothermal growth, and porous nickel oxide is formed after annealing treatment. The porosity of the finally prepared nickel oxide is about 120 / µm 2 , the pore size of the contained pores is about 50-160 nm, which can effectively increase the contact area of the nickel oxide with the gas and improve the gas response.

[0062] Embodiment 2 The present embodiment provides a gas sensor and a preparation method thereof, which includes three-dimensional nickel oxide, and the preparation process is as shown in Figure 6 , and specifically includes the following steps: 1) Preparation of the substrate structure: a polyimide tape with a thickness of about 60 µm and a width of 5 mm is attached to both sides of the first glass substrate, and the second glass substrate is placed parallel on the tape, and then the second glass substrate is fixed with the polyimide tape, so that an air layer with a thickness of about 60 µm is formed in the middle of the two glass substrates.

[0063] 2) Printing of platinum electrode: 1) Formulate the platinum electrode ink required for printing the platinum electrode, dissolve the ferric ammonium oxalate trihydrate and ammonium tetrachloroplatinate in deionized water to obtain the platinum electrode ink, wherein the concentration of ferric ammonium oxalate trihydrate is about 500 mM, and the concentration of tetrachloroplatinate is about 70 mM; 2) Inject the above platinum electrode ink into the interlayer space between the two glass substrates, and focus the laser on the interface between the glass substrate and the platinum electrode ink at a laser power of 2.57 mW and a scanning speed of 30 µm / s to print the platinum line, and the spacing between adjacent platinum lines is 4 µm; after the platinum line is printed, the sample is turned over, the laser is incident from the surface of the second glass substrate to the interlayer space, and focused on the selected contact area between the platinum line and the laser direct writing ink, and the laser power is 1.6 mW and the scanning speed is 0.4 µm / s to print on the platinum line to form a three-dimensional structure of platinum needle (i.e. the metal protrusion described in the present application) on the platinum line.

[0064] In order to facilitate the subsequent test instrument to test the gas sensor, a large area platinum electrode is formed on the first glass substrate by using a photoetching mask, and the "large area" facilitates the probe of the test instrument to be aligned and connected, and the platinum line is connected with the large area platinum electrode, wherein the photoetching mask forms the large area platinum electrode, which is a prior art, and therefore will not be described here.

[0065] Figure 7 It is the electron microscope image of the platinum line and the platinum needle printed in this embodiment. It can be seen that the three-dimensional structure of the platinum needle extends upward from the platinum line, and based on the interval arrangement of the multiple platinum lines, the formed platinum needles are arranged in an array, and the height of a single platinum needle printed in this embodiment is about 5 µm, the diameter is about 1.5 µm, and the spacing between adjacent platinum needles is about 3.5 µm.

[0066] 3, Growth of porous nickel hydroxide: 1) Formulate the laser direct writing ink: take 0.785 g of nickel nitrate hexahydrate and dissolve it in 0.5 mL of dimethyl sulfoxide and 0.5 mL of ethanol mixed solution, then add 90 uL of ethanolamine, and stir until the solution is uniform at room temperature to obtain the laser direct writing ink.

[0067] 2) Inject the above prepared laser direct writing ink into the interlayer space to contact the platinum needle, and use the laser to focus on the selected contact position between the laser direct writing ink and the platinum needle through the glass substrate, and the power of the laser is first set to 1.15 mW for 4 s, then increased to 1.8 mW for 4 s, and then increased to 2.5 mW for 4 s. Through the heat effect generated by the absorption of the metal platinum to the laser, a micro hydrothermal growth reaction zone is formed on the platinum needle to grow three-dimensional porous nickel hydroxide on the platinum needle.

[0068] 4. The printed sample is cleaned in deionized water for 10 minutes and air dried, then put into a tube furnace, annealed at 400℃ for 2h, so that the porous nickel hydroxide is converted into porous nickel oxide, and a gas sensor with three-dimensional nickel oxide is obtained.

[0069] Figure 8 is a structural schematic diagram of the gas sensor prepared in this embodiment, Figure 9 is a scanning electron microscope image of the gas sensor prepared in this embodiment. It can be seen from Figure 8 , Figure 9 that the gas sensor prepared in this embodiment has two large-area platinum electrodes located on the left and right sides, and the three-dimensional nickel oxide is located between the two large-area platinum electrodes and connected to the large-area platinum electrodes through platinum wires. The three-dimensional nickel oxide has a convex structure, which has a first part away from the substrate and a second part close to the substrate, the first parts of two adjacent convex structures are in contact with each other, and the second parts are not in contact, thereby forming an overhead hole at the bottom close to the substrate. The formation of the overhead hole can further increase the effective contact area of the nickel oxide with the gas. As shown in Figure 9 , since the three-dimensional nickel oxide grows on the basis of the platinum needle, the plurality of three-dimensional nickel oxides also form an array, and overhead holes are formed between adjacent three-dimensional nickel oxides. There are multiple overhead channels in the horizontal and vertical directions of the array, and these overhead channels are interconnected, so that the gas sensor has a three-dimensional interconnected overhead network, further improving the conductivity and gas sensing performance of the device.

[0070] The laser power and duration focused on the three-dimensional platinum needle have an effect on the formation of the overhead hole. The present application gradually increases the laser power in steps and maintains a suitable time at each power level, which helps the formation of the overhead hole. If the laser power is too low or the duration is too short, the single nickel hydroxide grown will not be able to effectively connect with adjacent structures due to insufficient size, thereby hindering the formation of a continuous overhead three-dimensional network. If the laser power is too high or the duration is too long, the nickel hydroxide will grow excessively, covering each other, and the overhead hole formed will be small or not formed, at which time the specific surface area of the nickel oxide is reduced. It should be noted that based on the porous structure of nickel oxide, the gas sensor without overhead holes also has good gas sensing performance, but the gas sensor with overhead holes is a preferred solution, and the gas sensor with three-dimensional interconnected overhead holes is a more preferred solution, at which time the conductivity and gas sensing performance of the device are further improved.

[0071] The present application tests the related performance of the gas sensor prepared in the above embodiment 1 and embodiment 2, and the test method is as follows: The gas sensor was heated by a hot plate to 300°C, and 1.3 μL of ethanol was injected into a micro-heating container in a closed chamber environment, to form a 100 ppm ethanol gas environment in the chamber, to test the response of the gas sensor. The response of the gas sensor was calculated by measuring its resistance in air (Ra) and in ethanol gas (Rg), and the response (S) was defined as S = Rg / Ra.

[0072] Figure 10 , Figure 11 The repeated response curves of the gas sensors prepared in Example 1 and Example 2 to 100 ppm ethanol gas are shown in Figures 2 and 3, respectively. Figure 10 It can be seen from Figure 2 that the average rising response time of the gas sensor prepared in Example 1 is about 60 s, and the average falling recovery time is about 40.8 s. Figure 11 It can be seen from Figure 3 that the gas sensor prepared in Example 2 exhibits faster response and recovery speed, with an average rising response time of about 43.6 s and an average falling recovery time of about 30.8 s. Figure 12 The single response curves of the gas sensors prepared in Example 1 and Example 2 to 100 ppm ethanol gas are shown in Figure 4. Figure 12 It can be seen from Figure 4 that the response of the gas sensor prepared in Example 1 is about 1.75 times, and the response of the gas sensor prepared in Example 2 is significantly improved to about 2.69 times. In summary, whether it is the response or the response / recovery speed, the performance of the gas sensor prepared in Example 2 is significantly better than that of Example 1.

[0073] Example 3 Example 3 is basically the same as Example 2, except that the preparation method of the laser direct writing ink in Example 3 is as follows: nickel nitrate hexahydrate is dissolved in a mixture of 1 mL of dimethyl sulfoxide and 1 mL of ethanol, and then 100 uL of ethanolamine is added, wherein the concentration of nickel nitrate is 2 mol / L, and the solution is stirred at room temperature until it is uniform, to obtain the laser direct writing ink. The rest is the same as Example 2, which will not be repeated here.

[0074] The structure and performance of the gas sensor prepared in Example 3 are comparable to those of Example 2.

[0075] Example 4 Example 4 is basically the same as Example 2, except that the preparation method of the laser direct writing ink in Example 4 is as follows: nickel nitrate hexahydrate is dissolved in a mixture of 0.5 mL of dimethyl sulfoxide and 1 mL of ethanol, and then 100 uL of ethanolamine is added, wherein the concentration of nickel nitrate is 3 mol / L, and the solution is stirred at room temperature until it is uniform, to obtain the laser direct writing ink. The rest is the same as Example 2, which will not be repeated here.

[0076] The gas sensor prepared in Example 4 has a structure and performance similar to those of Example 2.

[0077] Example 5 Example 5 is basically the same as Example 2, except that in Example 5, the step of growing porous nickel hydroxide is as follows: the laser direct writing ink prepared above is injected into the interlayer space to contact the platinum needle, and a laser is focused through the glass substrate at the selected contact position of the laser direct writing ink and the platinum needle, with the power of the laser first set to 1.3 mW for 5 s, then increased to 1.7 mW for 5 s, then to 2.1 mW for 5 s, and then to 2.5 mW for 5 s. The rest is the same as in Example 2, and is not repeated here.

[0078] The gas sensor prepared in Example 5 also has a three-dimensional nickel oxide and aerial hole structure similar to that of Example 2, and has a performance similar to that of Example 2.

[0079] Example 6 Example 6 is basically the same as Example 2, except that in Example 6, the step of growing porous nickel hydroxide is as follows: the laser direct writing ink prepared above is injected into the interlayer space to contact the platinum needle, and a laser is focused through the glass substrate at the selected contact position of the laser direct writing ink and the platinum needle, with the power of the laser first set to 0.9 mW for 5 s, then increased to 1.6 mW for 5 s, then to 2.3 mW for 5 s, then to 3.0 mW for 2 s, then to 3.7 mW for 2 s, and then to 4.1 mW for 2 s. The rest is the same as in Example 2, and is not repeated here.

[0080] The gas sensor prepared in Example 6 also has a three-dimensional nickel oxide and aerial hole structure similar to that of Example 2, and has a performance similar to that of Example 2.

[0081] Example 7 Example 7 differs from Example 2 in that in Example 7, the step of growing porous nickel hydroxide is as follows: the laser direct writing ink prepared above is injected into the interlayer space to contact the platinum needle, and a laser is focused through the glass substrate at the selected contact position of the laser direct writing ink and the platinum needle, with the power of the laser kept constant at 2.5 mW for 12 s. The rest is the same as in Example 2, and is not repeated here.

[0082] Comparative Example 2 and Example 7 show that when growing nickel hydroxide on a three-dimensional platinum needle, using a constant power can cause the platinum needle to collapse due to overheating or structural stress, and if the laser power is increased in steps too quickly or by too large an amount at a time, a similar effect to using a constant laser power can be achieved, i.e., the three-dimensional platinum electrode can easily overheat or collapse due to structural stress, and the effect is worse than in Example 2.

[0083] Example 8 Example 8 is different from Example 2 in that the holding time of each power level is 20 s in the step of growing porous nickel hydroxide of Example 8. The rest is the same as Example 2, which is not described here.

[0084] It is found from Comparative Example 2 and Example 7 that when the laser duration is too long, the nickel hydroxide structure will grow excessively, resulting in covering each other and filling the adjacent empty space, which is not conducive to the formation of the empty hole. Compared with Example 2, the specific surface area of the material decreases, and the gas sensor prepared also has a certain decline compared with Example 2.

[0085] Comparative Example 1 Comparative Example 1 is different from Example 2 only in that no ethanolamine is added in the nickel-based laser direct ink of Comparative Example 1, and the rest is the same as Example 2, which is not described here.

[0086] It is found that the nickel hydroxide printed by the ink prepared in Comparative Example 1 has a dense surface without pore distribution, and the surface area is much smaller than that of the porous nickel hydroxide, resulting in poor responsivity of the gas sensor.

[0087] In summary, the preparation method of the gas sensor provided by the present application is based on laser direct writing ink to prepare porous nickel hydroxide, and the porous nickel hydroxide is converted into porous nickel oxide through annealing treatment. The porous nickel oxide has a large specific surface area, which can increase the effective contact area with the target gas and improve the gas responsivity of the gas sensor. Further, the three-dimensional porous nickel hydroxide is obtained by laser-induced hydrothermal growth, and the three-dimensional nickel oxide is formed after annealing. The three-dimensional nickel oxide not only has a high porosity itself, but also the formation of the empty hole can increase the effective area of the nickel oxide in contact with the target gas, thereby improving the gas responsivity. Compared with the method for preparing a gas sensor by using multi-step etching, deposition or hydrothermal method in the prior art, the method based on laser direct writing provided by the present application has a simple process step. From the preparation of the metal electrode to the growth of the nickel hydroxide, it can be realized on the same laser platform through program control, which greatly simplifies the manufacturing process, reduces the dependence on the clean room environment and the use of chemical reagents.

[0088] In addition, the present inventors have also carried out tests with other raw materials, process operations and process conditions described in the present specification with reference to the foregoing examples, and all ideal results have been obtained.

[0089] The aspects, embodiments, features and examples of the present application should be considered illustrative, for the purpose of explanation, illustration and description of the present application, but not for the purpose of limiting the present application, and the scope of the present application is only defined by the claims.

[0090] While the application has been described with reference to the illustrative embodiments, those skilled in the art will appreciate that various modifications, omissions, and / or additions can be made without departing from the spirit or scope of the application. Additionally, many modifications can be made to adapt a particular situation or material to the teachings of the application without departing from its scope. Therefore, the application is not intended to be limited to the particular embodiments disclosed, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein. Further, to the extent that the terms first, second, etc. are used herein to describe various elements, these are generally used only as labels, and are not intended to impose numerical requirements on their objects.

Claims

1. A method of fabricating a gas sensor by laser direct writing, characterized in that, The method comprises: contacting a metal electrode with a laser direct writing ink, the laser direct writing ink comprising a nickel source, a porous structure directing agent and a solvent; irradiating a specified contact area of the metal electrode with the laser direct writing ink with a laser, causing the metal electrode to absorb the laser and heat up, to form a hydrothermal reaction zone at the specified contact area, and causing the laser direct writing ink in the hydrothermal reaction zone to undergo a hydrothermal reaction, thereby growing porous nickel hydroxide on the metal electrode, to obtain a precursor structure; and annealing the precursor structure, to convert the porous nickel hydroxide into porous nickel oxide, thereby preparing the gas sensor.

2. The method of claim 1, wherein, The method specifically comprises: focusing the laser on a specified contact area of the metal electrode with the laser direct writing ink, thereby forming the hydrothermal reaction zone; and / or, the method specifically comprises: growing the porous nickel hydroxide on at least two metal electrodes spaced apart from each other, respectively, and connecting the porous nickel hydroxide on the at least two metal electrodes to each other, before the annealing, thereby preparing the gas sensor; and / or, the method specifically comprises: forming a metal electrode on a first substrate, the metal electrode having a one-dimensional, two-dimensional or three-dimensional structure; growing two-dimensional shaped porous nickel hydroxide on the metal electrode having a one-dimensional or two-dimensional structure, thereby obtaining a gas sensor having two-dimensional shaped porous nickel oxide; and / or, growing three-dimensional shaped porous nickel hydroxide on the metal electrode having a three-dimensional structure, thereby obtaining a gas sensor having three-dimensional shaped porous nickel oxide.

3. The method of claim 2, wherein: arranging a first surface of a first substrate and a second surface of a second substrate opposite to and spaced apart from each other, and causing the interlayer space between the first surface of the first substrate and the second surface of the second substrate to have a micron level thickness, the first substrate and the second substrate being light-transmitting substrates; injecting electrode ink used for preparing the metal electrode into the interlayer space, to form a laser printable three-dimensional ink space layer, causing the laser to be incident on the three-dimensional ink space layer, and irradiating the selected contact position of the electrode ink and the first surface of the first substrate, thereby forming the metal electrode having a two-dimensional structure or a three-dimensional structure on the first surface of the first substrate.

4. The method of claim 3, wherein: injecting the electrode ink into the interlayer space to form the three-dimensional ink space layer; causing the laser to be incident on the three-dimensional ink space layer from a third surface of the first substrate, and irradiating the selected contact position of the electrode ink and the first surface of the first substrate, thereby forming a first part of the metal electrode on the first surface of the first substrate; causing the laser to be incident on the three-dimensional ink space layer from a fourth surface of the second substrate, and irradiating the selected contact position of the electrode ink and the first part of the metal electrode, thereby integrally forming a second part of the metal electrode on the first part of the metal electrode; wherein the third surface is opposite to the first surface, and the fourth surface is opposite to the second surface.

5. The method according to claim 3 or 4, characterized in that: after the preparation of the metal electrode is completed, injecting the laser direct writing ink into the interlayer space, and irradiating the specified contact area of the laser direct writing ink and the metal electrode, thereby obtaining the precursor structure.

6. The method of claim 2, wherein, The laser parameters for growing the two-dimensional porous nickel hydroxide on the one-dimensional or two-dimensional metal electrode include: the power of the laser is 3-6 mW, and the scanning speed of the laser is 0.5-2 µm / s.

7. The method of claim 2, wherein, The three-dimensional metal electrode includes metal protrusions.

8. The method of claim 7, wherein: The height of the metal protrusion is 4-6 µm, and the diameter is 1-3 µm; and / or, the metal electrode includes a plurality of metal protrusions spaced apart from each other with a spacing of 2-5 µm; and / or, the metal electrode includes a plurality of metal protrusions arranged in a pattern.

9. The method according to claim 7 or 8, characterized in that, The method comprises: When growing the nickel hydroxide on the metal protrusion, the power of the laser is increased stepwise, the initial power of the laser is 0.9-1.3 mW, the maximum power is 2.5-4.1 mW, the power is increased from the initial power to the maximum power by 2-5 times, the increment of each power increase is 0.4-0.7 mW, and the scanning time at each power level is 2-5 s.

10. The method of claim 1, wherein: The temperature of the annealing treatment is 350-400 °C, and / or the time of the annealing treatment is 1-3 h; And / or, the concentration of the nickel source in the laser direct writing ink is 2-3 mol / L; And / or, the porous structure directing agent includes ethanolamine, and the volume ratio of the solvent to ethanolamine is 10-20:1; And / or, the solvent includes dimethyl sulfoxide and ethanol in a volume ratio of 5-10:5-10; and / or the porosity of the porous nickel oxide is 50-200 holes per µm 2 having a pore size of 50-300 nm.

11. A gas sensor, characterized by It is prepared by the method of any one of claims 1-10.

12. A gas sensor, characterized by The gas sensing structure includes a plurality of porous nickel oxide continuously distributed and in contact with each other, and the porous nickel oxide is electrically connected to the metal electrode.

13. The gas sensor of claim 12, wherein: The porosity of the porous nickel oxide is 50-200 / µm 2 having a pore size of 50-300 nm; And / or, the porous nickel oxide is a protrusion structure, the protrusion structure includes a first part away from the substrate and a second part close to the substrate, the first parts of at least two adjacent protrusion structures are in contact with each other, and the second parts are not in contact with each other to form an overhead hole at the bottom close to the substrate.

Images