WO2021246858A1

WO2021246858A1 - Method of preparing silver nanoparticles for use as ink

Info

Publication number: WO2021246858A1
Application number: PCT/MY2020/050181
Authority: WO
Inventors: Aiman Sajidah ABD AZIZ; Nora'zah Abdul Rashid; Syed Muhammad Hafiz SYED MOHD JAAFAR; Suraya Sulaiman; Hing Wah Lee
Original assignee: Mimos Berhad
Priority date: 2020-06-03
Filing date: 2020-11-30
Publication date: 2021-12-09
Also published as: MY196802A

Abstract

The present invention discloses a method of preparing silver nanoparticles colloidal solution to be used as conductive ink. The solution is prepared by mixing silver, water, surface coating agent and silver reducing agent into a silver colloidal mixture. The silver mixture is sonicated with ultrasonic waves using low temperature heating. Polar solvent is added to the mixture. The silver mixture is centrifuged which forms precipitate. The precipitate is grinded to obtain coated silver nanoparticles. The silver nanoparticles is processed into ink and inserted in an inkjet cartridge. The inkjet cartridge can be operated with a printer below 50°C. The overall process involves minimal heat. The surface coating agent is selected from the group consisting of carboxyl, carbonyl, carboxylate and acrylic-based polymer. A single layer of printed conductive pattern can be obtained by using the prepared ink.

Description

METHOD OF PREPARING SILVER NANOPARTICLES FOR USE AS INK

The present invention relates to a method of preparing conductive ink, more particularly to a method of preparing silver nanoparticles for use as printable inkjet.

Conductive ink is a type of ink that conducts electricity. The ink is infused with conductive material, like graphene or metal nanoparticle. It could even be used as printable inkjet to print patterns on a substrate.

Conductive inks can be used in a variety of applications. Silver-based inks are used for printing RFID tags and repair printed circuit boards. The ink is also used as a radio antenna on car windshield.

Conductive inks can be infused with silver nanoparticles. A conventional method for preparing such inks involves sodium borohydride, a hazardous reducing agent. Existing ink involves high temperature above 200^oC to allow silver to diffuse into substrate. High sintering temperature involves cost and it is not suitable for plastic substrates.

Particle stability is a desirable feature for conductive inkjet ink. Particle size below 200 nm is appropriate for inkjet printing. It is challenging to produce a physically monodisperse particle size and chemically stable ink. High polydisperse silver nanoparticle has tendency to cause particle agglomeration over time. Particle agglomeration causes clogging of printer nozzle.

Inkjet inks can be solvent-based or water-based. Solvent-based ink can easily evaporate. Being stable below water boiling point, water-based ink is favoured as the ink does not face evaporation in room temperature.

US7445731B2 discloses a metallic ink incorporating metallic colloidal solution. The metallic colloidal solution is prepared by metal salt reducing agent in water. The dispersion medium is made of solvent of water and water-soluble organic solvent. The metallic particles obtained have a diameter of below 200 nm.

US8227022B3 describes a method of forming aqueous-based metal nanoparticles. The method involves providing an aqueous suspension of metal salt, pre-reducing the metal salt suspension with a water soluble polymer capable of metal reduction to obtain metal nuclei and adding a chemical reducer to form metal nanoparticles in dispersion. The preparation is performed at 100^oC. The nanoparticles obtained have a diameter of below 20 nm.

US20120225126A1 discloses a solid state synthesis method of silver nanoparticles. The method includes mixing a silver salt and a water soluble polymer acting as both a reducing agent and a protecting agent to produce a solid mixture. The solid mixture is vibration milled to form silver nanoparticles within the water soluble polymer. The silver salt may be selected from the group consisting of silver nitrate, silver nitrite, silver acetate, silver lactate, and silver citrate hydrate. The water soluble polymer may include oxygen or nitrogen having lone pair electrons. The prepared nanoparticles have a diameter from 2 to 50 nm.

Conventional silver ink requires high post sintering temperature, up to 200⁰C after printing to allow silver to diffuse onto substrate and to evaporate ink residue on the printed pattern. Multiple printed layers are used to achieve high conductivity. However, multiple layers cause increase of thickness and material cost.

It is important for nanoparticles to be coated to reduce oxidation and maintain its conductivity. It is an object of the invention to prepare and operate homogeneous silver nanoparticles with minimal material and heat.

A method to prepare water-based silver nanoparticles solution to be used as conductive ink is presented. A silver, water, surface coating agent and silver reducing agent are mixed into a mixture to form silver colloidal solution. The mixture is sonicated with ultrasonic waves at low temperature heating of 60 to 70^oC. Polar solvent is added in the mixture. The mixture is centrifuged. Silver particles form precipitate which is then collected. The precipitate is grinded to obtain coated silver nanoparticles.

The silver mixture is prepared using 10-18% wt. silver, 55-65% wt. deionised water, 20-30% wt. silver reducing agent, and 1-3% wt. surface coating polymer. The surface coating agent is selected from the group consisting of carboxyl, carbonyl, carboxylate and acrylic-based polymer.

The silver nanoparticles are further processed into ink mixture by adding deionized water, polar solvent mixture and viscosity modifier. The silver nanoparticles ink is filtered using 0.20-0.45 µm membrane disk. The ink mixture has low polydispersity, no agglomeration, and water-based evaporation stability. It can be stored at room temperature.

The silver nanoparticles ink can be inserted in an inkjet cartridge. The inkjet cartridge can be operated by a printer below 50^oC. The overall process involves minimal heat. An ohmic single layer of printed conductive pattern can be obtained by using the prepared ink.

A preferred embodiment of the present invention will be described herein below. In the following description, well known functions are not described in detail since they would obscure the description with unnecessary detail.

The present invention provides a method of preparing silver nanoparticles to be use as conductive ink. Minimal heat is involved to prepare the silver nanoparticles and printing the ink to a substrate.

The main raw materials for the ink are silver, deionised water, surface coating agent and silver reducing agent. Silver solution refers to the group consisting of silver nitrate, silver nitrile, silver acetate, silver lactate, and silver citrate hydrate. Silver reducing agent refers to an agent capable of reducing silver into silver metal through nucleation and subsequent particle growth stages. Reducing agents refers to a group consisting of amine derivatives reducing agent such as monoethanolamine, diethanolamine, triethanolamine, aminoethanolamine but not limited to glycerol, monoethylene glycol, diethylene glycol, polyethylene glycol and ascorbic acid. Surface coating agent is selected from the group consisting of carboxyl, carbonyl, carboxylate and acrylic-based polymer.

Example 1

Surface coating agent, silver salt reducing agent and deionized water is stirred at 400-500 rpm for 30 min at room temperature to form a mixture. Carboxylic coating agent is chosen as a surface coating agent as it can absorb silver particles by forming carbonyl COO- group via covalent bond. The coating reduces oxidation of silver nanoparticles. Amine derivatives based reducing agent preferably diethanolamine is used to reduce silver ion (Ag⁺) into silver metal (Ag⁰).

A concentrated silver solution from 1M to 8M, preferably 5M silver nitrate solution is dropped into the mixture. The mixture is continuously stirred at 400-500 rpm at room temperature. Then, the mixture is stirred at 100 rpm for 21-23 hours at room temperature.

The mixture is prepared using 10-18% wt. silver, 55-65% wt. deionised water, 20-30% wt. reducing agent and 1-3% wt. surface coating polymer. A preferred embodiment of the mixture is prepared using 13.8% wt. silver, 25.6% wt. reducing agent, 1.5% wt. surface coating agent and 59.1% wt. deionized water. Stirring is continuous to create a homogenous mixture.

Silver ion is reduced to silver nanoparticle at room temperature in a liquid state. Silver nanoparticles are grown from dissolved silver metal salt and covalently bonded in surface coating.

Example 2

The mixture is sonicated with ultrasonic waves of 35-80 kHz for 45-60 min using at 60-70^oC.The minimal heat is applied to control the desired size of silver nanoparticle. Silver nanoparticles are homogenized and ripened in this step.

Surface coating polymer transition glass (T_g) happens at 126.5^oC for acrylic-based polymer. Temperature of 65^oC is chosen at half of transition glass to achieve 20 nm size within 1 hour of liquid state ultrasonic treatment. T_g higher than 70% of surface coating is not recommended due to rapid particle aggregation which makes particle too reactive and larger particle is not suitable for inkjet application. Smaller than 20 nm silver nanoparticle is not favored at this point as it will leak through nozzle head of the printer.

Example 3

Polar solvent such as ethanol is added to the mixture and stirred at 100 rpm. Silver particles are purified by centrifuging mixture at 900-1000 rpm for 10-30 mins at room temperature. Contaminant such as nitrate and unreacted chemicals are removed.

Example 4

The centrifuging produces silver particle in the form of black precipitate. The precipitate is collected and weighted. The precipitate is grinded. Deionised water is added until silver mass loading achieve 10% v/v. Silver nanoparticles can be used in water mass loading of 10 to 50% v/v. The silver nanoparticle is kept is an enclosed and dark bottle in room temperature.

The silver nanoparticles produced are homogenous and uniform. Silver particles are also fully coated with surface coating agent. The coating has shell size of 0.1 to 1.0 nm for average nanoparticle size of 20 nm. The silver nanoparticles are stable in water and pH above 7.

Example 5

The low polydisperse silver nanoparticles are processed into an inkjet mixture. Silver nanoparticles are mixed with deionised water and polar solvent mixture containing aqueous vehicle and viscosity modifier at room temperature. About 50 to 65% v/v of water over 20 to 25% v/v polar solvent is used. Ethylene glycol, glycol or propylene glycol of 10 to 25% v/v can be used. The ink formulation preferably contains silver nanoparticles dispersed in deionised water, methanol as polar solvent mixture and 17% v/v ethylene glycol as viscosity modifier. Low agglomeration happens because the silver nanoparticles are coated with polymeric matrix having carboxyl group. The ink mixture is homogenized for 30 to 60 seconds at room temperature. The silver nanoparticles are filtered with 0.20-0.45 µm membrane disk. The ink viscosity is measured and stored in a dark bottle at room temperature.

Example 6

The ink mixture is homogenized for 60 seconds at room temperature. The ink is inserted in inkjet cartridge and assembled to an inkjet nozzle head. The printer is ready to print the ink on a surface such as a flexible platform.

Example 7

A flexible substrate is assembled on the inkjet printer platform. The printer is set to print using printer platform and head at temperature below 50^oC. The act of printing and annealing pattern is performed simultaneously below 50^oC. Electrical measurement is performed on the printed pattern. Low sintering temperature is done simultaneously during printing to improve ink drying. A method of forming a layer of printable conductive pattern using silver nanoparticles onto a flexible substrate is described.

A comparison between the proposed ink and commercially available ink is shown in Table 1.

Characteristic	Proposed ink	Commercial ink
Average particle size (nm)	20	16
Ink viscosity (cP)	4-8	2-3
Average thickness (nm)	500 (single layer)	1700 (multiple layers)
Resistivity (µΩcm)	22.4	42.3
Conductivity (S/cm)	4.51*10⁴	2.06*10⁴
Sheet resistance (Ω/□)	0.4	0.5
Polydispersity index (%)	27.8	25.3
pH	8-9	6-8

The proposed formulated ink, printed as a single layer with at least 500 nm thickness has showed better conductivity compared to commercial ink that have multiple layers at 1700 nm. Hence, less silver nanoparticles is used to achieve higher conductivity compared to conventional ink.

Accordingly, a method of preparing silver nanoparticles for use as conductive ink is presented. The method uses minimal heat to prepare nanoparticles and operate the ink. Cost and ease of handling can be improved. Oxidation of silver at elevated temperature is prevented.

It is to be understood that although the invention has been described with reference to particular embodiments, the embodiments are merely illustrative of the principles and applications of the present invention, as changes and modifications may become apparent to those skilled in the art, without departing from the scope of the present invention as defined by the appended claims.

Claims

A method of preparing silver nanoparticles for use as ink, comprising:
mixing water, surface coating agent and silver reducing agent into a mixture;
mixing silver in the mixture;
sonicating the silver mixture with ultrasonic waves at 60 to 70^oC;
mixing polar solvent in the silver mixture;
centrifuging the silver mixture;
collecting precipitate from the silver mixture, wherein precipitate formed in the mixture are coated silver nanoparticles; and
grinding the precipitate to obtain coated silver nanoparticles.
The method as claimed in claim 1, wherein the silver mixture is prepared using 10-18% wt. silver, 20-30% wt. reducing agent, and 1-3% wt. surface coating polymer.
The method as claimed in claim 1, wherein the silver mixture is prepared using 13.8% wt. silver, 25.6% wt. reducing agent, 1.5% wt. surface coating polymer and 59.1% wt. water.
The method as claimed in claim 1, wherein the mixing of surface coating agent is performed with the surface coating agent selected from the group consisting of carboxyl, carbonyl, carboxylate and acrylic-based polymer.
The method as claimed in claim 1, further comprising adding water until silver concentration is 10 to 50% v/v.
The method as claimed in claim 5, further comprising:
mixing the silver nanoparticles, water, polar solvent mixture and viscosity modifier into an ink mixture; and
filtering silver nanoparticles ink with 0.20-0.45µm membrane disk;
wherein the ink mixture contains 50 to 65% v/v silver nanoparticles, 20 to 25% v/v polar solvent and 10 to 25% v/v viscosity modifier.
The method as claimed in claim 6, further comprising inserting the filtered ink in an inkjet cartridge.
The method as claimed in claim 7, further comprising operating the inkjet cartridge in a printer below 50^oC.
The method as claim in claim 6, wherein mixing of viscosity modifier is performed with 17% v/v ethylene glycol.
A layer of printed conductive pattern obtainable by process of any preceding claims.