Reductive graphene oxide ink, preparation method thereof and application of reductive graphene oxide ink in ink-jet printing

Abstract

The invention discloses reductive graphene oxide ink and a preparation method and application thereof in ink-jet printing, and belongs to the field of printing electronics, the preparation method of the reductive graphene oxide ink comprises the following steps: mixing deionized water, ethylene glycol and ethanol according to a volume ratio of (2-12): (18-22): (70-76) to obtain a mixed solution; ascorbic acid is added into the mixed solution to serve as a reducing agent, then single-layer graphene oxide powder with the concentration being 0.5-4 mg / ml is added, and a mixed material is obtained; and carrying out ultrasonic treatment on the mixed material, and standing to obtain the reductive graphene oxide ink. The invention also provides application of the ink in ink-jet printing of high-precision patterns. The preparation method of the ink provided by the invention has the characteristics of simplicity, convenience, greenness, no pollution and low cost, meanwhile, high-precision conductive patterns can be printed on a flexible substrate in an ink-jet manner, and possibility is provided for manufacturing high-precision flexible electronic devices and circuits.

Description

Technical Field

[0001] This invention belongs to the field of printed electronics, specifically relating to a reduced graphene oxide ink, its preparation method, and its application in inkjet printing. Background Technology

[0002] Printed electronics manufacturing processes transfer functional materials onto various substrates via printing, finding wide application in printed electronics, energy batteries, and flexible displays. Among these, inkjet printing technology formulates materials into ink, enabling deposition on any substrate. It offers advantages such as digitalization, non-contact operation, high precision, high efficiency, and zero pollution, showing promising prospects for development in flexible electronic devices and circuit manufacturing.

[0003] However, inkjet printing technology places extremely high demands on the fluid properties of the ink used, including viscosity, surface tension, stability, and substrate contact angle. These parameters directly affect the ejection and flight of ink droplets during the printing process, the wetting behavior on the substrate, and the condensation and solidification of ink droplets, ultimately affecting the accuracy of the inkjet printed pattern.

[0004] For conductive inks, research has gradually shifted from inks made of metallic materials such as gold, silver, and copper to graphene systems, which offer a balance between conductivity and cost. However, graphene is difficult to disperse in aqueous solvents, often requiring the use of toxic reagents such as dimethylformamide. While graphene oxide, by introducing hydrophilic oxygen-containing functional groups onto its surface, can be well dispersed in aqueous solvents, its high degree of oxidation introduces numerous structural defects, making it almost an insulator. Therefore, reduction methods are needed to restore the conductivity of inkjet-printed patterns. Inkjet-printed substrates (such as PET) are easily damaged at high temperatures, so high-temperature thermal reduction treatment is generally not considered for inkjet-printed patterns. Chemical reduction methods are also an option, but most chemical reagents, such as hydrazine hydrate, sodium borohydride, and hydroiodic acid, are highly toxic. Summary of the Invention

[0005] The technical problem to be solved by this invention is to address the shortcomings of the above-mentioned technologies by providing a reduced graphene oxide ink with viscosity and surface tension parameters that meet the requirements of inkjet printing, and correspondingly providing a simple, green, pollution-free, and low-cost preparation method. At the same time, it enables high-precision conductive patterns to be printed on flexible substrates by inkjet printing, thus providing the possibility for the manufacture of high-precision flexible electronic devices and circuits.

[0006] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is as follows: This invention provides a method for preparing a reduced graphene oxide ink, comprising the following steps: Deionized water, ethylene glycol, and ethanol are mixed in a volume ratio of (2-12):(18-22):(70-76) to obtain a mixture. Ascorbic acid at a concentration of 5-15 mg / ml is added to the mixture as a reducing agent, followed by monolayer graphene oxide powder at a concentration of 0.5-4 mg / ml, to obtain a final mixture. This mixture is then subjected to ultrasonic treatment and allowed to stand to obtain reduced graphene oxide ink. As a preferred embodiment, the ultrasonic treatment power is 300-800 W, and the ultrasonic treatment time is 4-8 hours; the standing time is 2-4 days at room temperature. After ultrasonic treatment and standing, the ascorbic acid is able to reduce most of the graphene oxide.

[0007] A reduced graphene oxide ink is prepared using the method described above. This reduced graphene oxide ink has a viscosity of 4-8 mPa·s and a surface tension of 25-50 mN / m at room temperature (25°C). Inkjet printing places extremely stringent requirements on the fluid properties of inks; only when the ink's viscosity and surface tension meet these requirements can it be suitable for inkjet printing. Even slight changes in the ink's fluid parameters can affect the droplet ejection behavior, ultimately impacting the quality of inkjet printing. For example, excessively high viscosity can cause discontinuous or even impossible ink ejection from the printhead; excessively low surface tension can lead to excessive wetting of the printhead, easily resulting in long trails and satellite dots. In this invention, by adjusting the relative amounts of ethanol and ethylene glycol, the reduced graphene oxide can be well dispersed in deionized water without the need for additional polymeric stabilizers or dispersants; moreover, the ink wets the substrate well and adheres firmly to the surface after drying, eliminating the need for binders. Compared with existing technologies that require the use of additives such as stabilizers and binders, this invention does not use these additives. On the one hand, it can reduce costs, and on the other hand, it avoids the adverse effects of binders and other additives on the conductivity of the final printed pattern.

[0008] At room temperature, deionized water has too low a viscosity (1 mPa·s) and too high a surface tension (72 mN / m), making inks formulated using only deionized water as a solvent unsuitable. Therefore, it is necessary to adjust the viscosity and surface tension of the ink to change its fluid parameters. Furthermore, in the process of adjusting the ink fluid parameters, the interaction between the additives and the reduced graphene oxide must be considered to obtain a uniformly dispersed reduced graphene oxide ink. The ethylene glycol used in this invention has a high viscosity (20 mPa·s) to increase the system viscosity; ethanol has a low surface tension (22 mN / m) to reduce the system surface tension. Furthermore, both ethylene glycol and ethanol are less polar than water. When graphene oxide is reduced to reduced graphene oxide, the removal of some oxygen-containing groups reduces its hydrophilicity, thus decreasing the dispersibility of reduced graphene oxide in water. The weakly polar environment of the ethylene glycol and ethanol used in this invention better matches the hydrophobic properties of graphene, thereby improving the dispersion ability of reduced graphene oxide in the ink. In addition, the hydroxyl groups in ethylene glycol and ethanol molecules can form hydrogen bonds with the small amount of residual oxygen-containing groups in reduced graphene oxide, further improving its dispersion stability. Because ethanol has a low boiling point and is easily volatile, leaving almost no residue in the printed pattern, the ethanol content in the ink system should be as high as possible within a suitable range. Taking into account the chemical reagents commonly used in experiments, while minimizing the use of toxic reagents and their impact on ink performance, this invention uses a specific ratio of ethylene glycol and ethanol to modify the ink.

[0009] In the reduced graphene oxide ink of the present invention, the content of graphene oxide should be moderate. If the concentration is too low, there will be too little conductive medium in the ink, resulting in poor conductivity of the inkjet printed pattern; if the concentration is too high, it will cause uneven dispersion in the solution, resulting in agglomeration, causing nozzle clogging, and affecting inkjet printing.

[0010] Regarding the selection of reducing agents, ascorbic acid is not only environmentally friendly and has strong reducing power, but it can also be used for reduction at room temperature, and the reduction process is simple. Extensive experimental studies have shown that water-soluble ascorbic acid has minimal impact on the viscosity and surface tension of the ink, and the amount of ascorbic acid used should be moderate. Too low a concentration of ascorbic acid leads to incomplete reduction and a long reduction time; too high a concentration results in a large proportion of residual ink later.

[0011] This invention also provides the application of the aforementioned reduced graphene oxide ink in inkjet printing, comprising the following steps: The reduced graphene oxide ink is ultrasonically treated at 300-800 W for 10-30 min. 1-1.5 ml of the ink is then drawn into a cleaned ink cartridge using a syringe and allowed to stand for 5-20 min. The industrial inkjet printer parameters are then adjusted as follows: substrate and cartridge temperature set to 20-40℃, printhead height set to 0.300-0.500 mm, substrate thickness set to 100-500 μm, pulse waveform suitable for the printhead and ink properties selected, and jet voltage adjusted to 20.00-30.00 V. Finally, the printing parameters are adjusted: droplet spacing set to 10-180 μm, number of layers set to 1-30, and inkjet printing is performed on the flexible substrate. The inkjet-printed flexible substrate is placed in a vacuum drying oven and subjected to thermal reduction at 60-100℃ for 6-8 hours to remove some residual solvent from the printed pattern and simultaneously reduce some of the graphene oxide, thus obtaining a high-precision conductive pattern on the flexible substrate. The conductivity of the high-precision conductive pattern is 0.1-0.15 S / cm; the minimum line spacing in the high-precision conductive pattern is 10 μm, and the minimum line width is 1 μm; preferably, the line spacing in the high-precision conductive pattern is 10-180 μm. The line spacing of the conductive pattern obtained by printing with the ink prepared by this invention can be strictly controlled and adjusted from the micrometer level to the macroscopic level. Compared with the millimeter-level adjustment range of existing technologies, the conductive pattern obtained by this invention has the characteristic of high precision. High-precision grid patterns of different periods can be stacked to form a grid superstructure with a certain thickness through multiple inkjet printing, while ensuring strict periodicity and high precision.

[0012] The present invention has the following beneficial effects: The reduced graphene oxide ink of the present invention is formulated from deionized water, ethylene glycol, ethanol, ascorbic acid and graphene oxide, and has the advantages of being green and environmentally friendly, simple to prepare and low in cost. By precisely controlling the proportion of ink solvent components, the viscosity and surface tension of the ink meet the requirements of inkjet printing, and the reduced graphene oxide can be stably dispersed in the system.

[0013] This invention achieves precise adjustment of inkjet printing instrument parameters and printing parameters, enabling the normal ejection of ink droplets with strictly controllable size, thus ensuring high precision of inkjet printed patterns. Simultaneously, the combination of non-contact printing height and flexible substrate, along with the flexibility of the two-dimensional graphene nanostructure, provides a flexible conductive film resistant to curling and folding. Furthermore, by using an optically transparent flexible substrate, high-transmittance, precise patterns can be achieved, making it applicable to high-end flexible electronic and optoelectronic devices.

[0014] This invention utilizes the dual reduction of graphene oxide through ascorbic acid chemical reduction before printing and thermal reduction after printing, which significantly improves the conductivity of inkjet-printed patterns and has great application prospects in flexible electronic devices and circuit manufacturing. Attached Figure Description

[0015] To accurately illustrate the technical solutions of the embodiments of the present invention, the relevant drawings in the embodiments will be described below. The drawings described below are some embodiments of the present invention. Other researchers in the art can obtain other drawings based on the following drawings without creative effort.

[0016] Figure 1 This is an optical image of the reduced graphene oxide ink prepared in Example 1 of this invention.

[0017] Figure 2 This is a contact angle diagram between the reduced graphene oxide ink and the PET substrate in Embodiment 1 of the present invention.

[0018] Figure 3 This is a droplet ejection diagram of the reduced graphene oxide ink in Embodiment 1 of the present invention.

[0019] Figure 4 In Embodiment 1 of this invention, high-precision line patterns with different spacings are printed on a PET substrate using reduced graphene oxide ink.

[0020] Figure 5 This is a droplet ejection diagram of the reduced graphene oxide ink in Embodiment 2 of the present invention.

[0021] Figure 6 This is a droplet ejection diagram of the reduced graphene oxide ink in Embodiment 3 of the present invention.

[0022] Figure 7 In Embodiment 4 of this invention, a high-precision grid pattern with different spacing is printed on a PET substrate using reduced graphene oxide ink.

[0023] Figure 8 This is a graph showing the changes in sheet resistance and conductivity after reducing a high-precision grid pattern with different spacing printed on a PET substrate using reduced graphene oxide ink, as described in Embodiment 4 of the present invention.

[0024] Figure 9 This is a droplet ejection diagram of the reduced graphene oxide ink in Comparative Example 1 of this invention.

[0025] Figure 10 This is a droplet ejection diagram of the reduced graphene oxide ink in Comparative Example 2 of this invention. Detailed Implementation

[0026] The present invention will be further described below with reference to embodiments, so that those skilled in the art can better understand and implement the present invention, but the embodiments are not intended to limit the present invention.

[0027] Furthermore, unless otherwise specified, the preparation processes in the following embodiments are all conventional methods in the art, and therefore will not be described in detail. The raw materials and reagents used in the following embodiments are all commercially available products.

[0028] Example 1 This embodiment provides a reduced graphene oxide ink, its preparation method, and its application in inkjet printing high-precision line patterns with different pitches (20 μm, 30 μm, 60 μm, and 90 μm). A Dimatix DMP-2831 industrial inkjet printer and a matching Samba (2.4 pL) ink cartridge are used. The specific steps include: (1) Preparation of reduced graphene oxide ink: Deionized water, ethylene glycol and ethanol were mixed in a volume ratio of 6%: 20%: 74%, ascorbic acid with a concentration of 10 mg / ml was added as a reducing agent, and monolayer graphene oxide powder with a concentration of 1 mg / ml was added. Then, the mixture was ultrasonically treated at 500 W for 6 h, and allowed to stand for 2 days after the ultrasonic treatment was completed.

[0029] (2) Setting the pattern for inkjet printing on flexible substrate: The reduced graphene oxide ink in step (1) was ultrasonically treated at 500 W power for 10 min. 1 ml of ink was drawn up with a syringe and injected into the cleaned ink cartridge. The ink was left to stand for 15 min to reach equilibrium. Then, the instrument parameters were adjusted. The temperature of the substrate and ink cartridge was set to 30℃, the printing height was set to 0.350 mm, the substrate thickness was set to 220 μm, the Samba Cartridge Waveform pulse waveform was selected, and the jet voltage was adjusted to 25.00 V. Finally, the printing parameters were adjusted. The droplet spacing was set to 20 μm, 30 μm, 60 μm and 90 μm respectively, and the number of printing layers was set to 1 layer. Then, inkjet printing was performed on a PET flexible substrate with a thickness of 120 μm.

[0030] (3) Reduction treatment of printed pattern: The PET flexible substrate that has been inkjet printed is placed in a vacuum drying oven and thermally reduced at 80°C for 6 h to remove some of the solvent remaining in the printed pattern and at the same time reduce some of the graphene oxide.

[0031] Example 2 This embodiment provides a reduced graphene oxide ink and its preparation method. The difference between this embodiment and Embodiment 1 is that in step (1), deionized water, ethylene glycol, and ethanol are mixed in a volume ratio of 12% : 18% : 70%. Everything else is the same as in Embodiment 1.

[0032] Example 3 This embodiment provides a reduced graphene oxide ink and its preparation method. The difference between this embodiment and Embodiment 1 is that in step (1), deionized water, ethylene glycol, and ethanol are mixed in a volume ratio of 2% : 22% : 76%. Everything else is the same as in Embodiment 1.

[0033] Example 4 This embodiment provides a reduced graphene oxide ink, its preparation method, and its application in inkjet printing of high-precision grid patterns with different spacings (20 μm, 30 μm, 60 μm, and 90 μm). A Dimatix DMP-2831 industrial inkjet printer and a matching Samba (2.4 pL) ink cartridge were used. This embodiment differs from Embodiment 1 in that after completing step (2) of inkjet printing, the PET substrate is rotated 90°, and then the inkjet printer's built-in reference camera positioning system is used to ensure that the previously printed lines are accurately rotated 90° before continuing step (2) of inkjet printing, thereby using mutually perpendicular lines to form a grid pattern. Everything else is the same as in Embodiment 1.

[0034] Example 5 This embodiment provides a reduced graphene oxide ink and its preparation method. The difference between this embodiment and Embodiment 1 is that the ascorbic acid concentration in step (1) is 5 mg / ml, and the graphene oxide concentration is 0.5 mg / ml. Everything else is the same as in Embodiment 1.

[0035] Example 6 This embodiment provides a reduced graphene oxide ink and its preparation method. The difference between this embodiment and Embodiment 1 is that the ascorbic acid concentration in step (1) is 15 mg / ml, and the graphene oxide concentration is 4 mg / ml. Everything else is the same as in Embodiment 1.

[0036] Comparative Example 1 This comparative example provides a reduced graphene oxide ink and its preparation method. The difference between this comparative example and Example 2 is that in step (1), deionized water, ethylene glycol, and ethanol are mixed in a volume ratio of 2% : 28% : 70%. Everything else is the same as in Example 1.

[0037] Comparative Example 2 This comparative example provides a reduced graphene oxide ink and its preparation method. The difference between this comparative example and Example 2 is that in step (1), deionized water, ethylene glycol, and ethanol are mixed in a volume ratio of 2% : 18% : 80%. Everything else is the same as in Example 1.

[0038] The relevant parameters of the reduced graphene oxide ink prepared in Example 1 were characterized, and the results are shown in Table 1 below. The ink has a viscosity of 5.52 mPa·s, a surface tension of 25.716 mN / m, and an average particle size of 1193.87 nm (less than 2 μm), which meets the requirements of the Samba printhead and will not cause nozzle clogging. The ink has a Zeta potential of -17.17 mV, a flow coefficient of -0.34 (μ / s) / (V / cm), and a dispersion coefficient of 0.293. Figure 1 The optical image of the reduced graphene oxide ink shows that the ink system has high stability, good dispersibility, and good flowability. The contact angle between the ink and the PET substrate is shown in the image. Figure 2 As shown, the average contact angle is 24.9°. The relatively small contact angle indicates that the ink spreads easily on the PET substrate, which is beneficial for improving the resolution of inkjet printed patterns and eliminating the coffee ring effect. In summary, combined with the above discussion... Figure 3 It can be seen that the ejected ink droplets always remain single and fall vertically, and the ejection is continuous and stable, as expected. Scanning electron microscopy was used to characterize the high-precision line patterns with different spacings printed on the PET substrate in Example 1 above, such as... Figure 4 As shown, the periodic lines printed by inkjet printing are clearly regular. The line width is approximately 1 μm.

[0039] It should be noted that, in other embodiments, by adjusting the printing parameters and ink composition, the minimum line spacing can reach 10 μm, and the line spacing can be precisely and arbitrarily adjusted within the range of 10-180 μm.

[0040] Table 1. Relevant parameters of reduced graphene oxide ink

[0041] Similarly, within the scope of this invention, the solvent ratio and solute concentration of the reduced graphene oxide ink can be varied. The fluid parameters of the reduced graphene oxide inks with different formulations are shown in Table 2 below. It can be seen that the viscosity and surface tension of the reduced graphene oxide inks prepared in the examples in the table meet the requirements. Figure 5 and Figure 6 This indicates that the ink was ejected normally.

[0042] Table 2. Ink fluid parameters for different formulations in each embodiment.

[0043] Similarly, scanning electron microscopy was used to characterize the high-precision grid patterns with different spacings printed on the PET substrate in Example 4 above, such as... Figure 7 As shown, the periodic grid of inkjet printing is uniform in size and the lines are regular and clear.

[0044] The minimum grid spacing of this invention can reach 10 μm, and the grid spacing can be precisely and arbitrarily adjusted within the range of 10-180 μm. Based on the preceding discussion and analysis, the high precision of the aforementioned inkjet-printed periodic patterns stems not only from the accurate adjustment of printing parameters but also from the superior performance of the formulated ink. For example, suitable viscosity and surface tension ensure normal droplet ejection, and a suitable substrate contact angle suppresses the generation of the coffee ring effect.

[0045] Meanwhile, although inkjet-printed patterns are two-dimensional structures, they can be printed multiple times to achieve multi-layer stacking of printed patterns, forming a strictly periodic superstructure with a certain thickness.

[0046] The sheet resistance of the high-precision grid pattern with different spacing printed on the PET substrate in Example 4 was tested using a four-probe method after reconstruction. Simultaneously, the sheet resistance was determined according to the formula σ = 1 / (R sh · t) Calculate its conductivity, the result is as follows Figure 8 As shown, with the increase of grid spacing, the sheet resistance of the pattern increases, while the conductivity decreases. The conductivity of the printed pattern is approximately 0.1-0.15 S / cm. This demonstrates that the present invention utilizes both chemical and thermal reduction of ascorbic acid to perform a dual reduction of graphene oxide, making the originally insulating graphene conductive. This allows for multiple inkjet printing processes or the use of other conductive inks to further improve the conductivity of the printed pattern. In summary, the present invention provides an effective and feasible method for the subsequent fabrication of high-precision electronic devices and circuits on flexible substrates.

[0047] The solvent ratios for reduced graphene oxide inks, which are not within the scope of this invention, were also investigated. Different solvent ratios significantly affect parameters such as ink viscosity and surface tension, thereby directly affecting droplet ejection and ultimately the quality of inkjet printed patterns. Figure 9 As shown, the ink droplets are difficult to eject (extremely low ejection height) because the proportion of ethylene glycol in the reduced graphene oxide ink prepared in Comparative Example 1 is too high, resulting in excessively high viscosity of the ink system. This leads to poor ink flow and uneven ink ejection, making it unsuitable for inkjet printing. Similarly, as... Figure 10 As shown, the proportion of ethanol in the reduced graphene oxide ink prepared in Comparative Example 2 was too high, and the surface tension of the ink system was too low, resulting in long trailing ink droplets. Moreover, the excess ink droplets formed satellite dots, making it unsuitable for inkjet printing.

[0048] Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

Claims

1. A method for preparing a reduced graphene oxide ink, characterized in that, Includes the following steps: Deionized water, ethylene glycol, and ethanol were mixed in a volume ratio of (2-12):(18-22):(70-76) to obtain a mixture; ascorbic acid was added to the mixture as a reducing agent, and then monolayer graphene oxide powder with a concentration of 0.5-4 mg / ml was added to obtain a mixture material; the mixture material was subjected to ultrasonic treatment and allowed to stand to obtain reduced graphene oxide ink.

2. The method for preparing the reduced graphene oxide ink according to claim 1, characterized in that, The ultrasonic treatment power is 300-800 W, and the ultrasonic treatment time is 4-8 h.

3. The method for preparing the reduced graphene oxide ink according to claim 1, characterized in that, The settling time is 2-4 days, and the temperature is room temperature.

4. The method for preparing the reduced graphene oxide ink according to claim 1, characterized in that, The concentration of ascorbic acid in the mixture is 5-15 mg / ml.

5. A reducing graphene oxide ink, characterized in that, The reduced graphene oxide ink is prepared by the preparation method as described in any one of claims 1 to 4.

6. The reduced graphene oxide ink according to claim 5, characterized in that, The reduced graphene oxide ink has a viscosity of 4-8 mPa·s and a surface tension of 25-50 mN / m at room temperature.

7. The application of the reduced graphene oxide ink as described in claim 5 in inkjet printing, characterized in that, Includes the following steps: After ultrasonically mixing the reduced graphene oxide ink, it is transferred to an ink cartridge and inkjet printed on a flexible substrate using an industrial inkjet printer. The flexible substrate after inkjet printing is then subjected to thermal reduction treatment, which yields a high-precision conductive pattern on the flexible substrate. The conductivity of the high-precision conductive pattern is 0.1-0.15 S / cm. The minimum line spacing in the high-precision conductive pattern is 10 μm, and the minimum line width is 1 μm.

8. The application according to claim 7, characterized in that, The parameters of the industrial inkjet printer are set as follows: substrate and ink cartridge temperature is set to 20-40℃, printhead printing height is set to 0.300-0.500 mm, substrate thickness is set to 100-500 μm, a pulse waveform suitable for the printhead and ink properties is selected, and the jetting voltage is adjusted to 20.00-30.00 V; finally, the printing parameters are adjusted as follows: droplet spacing is set to 10-180 μm, and the number of printing layers is set to 1-30 layers.

9. The application according to claim 7, characterized in that, The heat reduction treatment is carried out at a temperature of 60-100℃ for 6-8 hours in a vacuum drying oven.

10. The application according to claim 9, characterized in that, The line spacing in the high-precision conductive pattern is 10-180 μm.

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