High-concentration nanoscale silver colloidal solution and preparing process thereof

Abstract

The present invention relates to a high-concentration nanoscale silver colloidal solution and the preparing process thereof. The colloidal solution of the present invention comprises a high content of silver particles, i.e. approximately 1.5 wt %. The mean size of the nanoscale silver is less than 10 nm. In the preparing process, silver salt, ionic chelating agent, stabilizing agent, reducing agent, solvent and reaction accelerator are homogeneously mixed together. The increase of reaction temperature by external heat source accelerates completed reaction. By using the specified reaction accelerator and chelating agent and under the operating condition of the present invention, high-density silver colloidal solution is obtained while inhibiting particle aggregation. Therefore, the resulting nanoscale silver colloidal solution contains very small-sized particles and the stability thereof is satisfactory.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flowchart of a process for preparing a nanoscale silver colloidal solution according to the present invention;

FIG. 2 is a plot showing the conversion of reaction under three different preparing conditions;

FIG. 3 is a plot showing the particle size distribution of nanoscale silver measured by dynamic light scattering equipment and obtained from a reaction for one hour under the reaction conditions of the present invention;

FIG. 4 is a TEM image of a nanoscale silver paste obtained under urea system according to the present invention and subject to a specified purification procedure followed by dilution; and

FIG. 5 is a TEM image of a nanoscale silver paste obtained under non-urea system according to prior art and subject to a specified purification procedure followed by dilution.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The technical contents of the present invention will now be described in more detail hereinafter with reference to the accompanying drawings that show various embodiments of the invention.

Referring to FIG. 1, a flowchart of a synthetic process according to a preferred embodiment of the present invention is illustrated. The reaction solution used in the present invention principally comprises two components, i.e. Component A and Component B. Component A includes stabilizing agent, reducing agent, reaction accelerator and deionized water. Component B includes metallic salt, ionic chelating agent and deionized water. With stirring and under the low temperature environment below 25° C., Component B is rapidly added to Component A. Then, the mixed solution of Component A and Component B is transferred to the elevated temperature environment of 80˜85° C. as soon as possible to perform a chemical reduction reaction. With continuous stirring, the reaction temperatures inside and outside the reactor are substantially identical. After the reaction is nearly complete, the reaction solution would be transferred to a low temperature environment for storage. It is demonstrated that this method is advantageous for stably maintaining the particle size.

EXAMPLE 1

Preparation of a Nanoscale Silver Colloidal Solution of the Present Invention

The nanoscale silver colloidal solution of the present invention is prepared according to the following steps:

1. 136 g of PVP (MW=40,000) is weighed and dissolved in deionized water (400 ml).

2. 17 g of silver nitrate is weighed and dissolved in deionized water (200 ml).

3. 1.6 g of sodium hydroxide is dissolved in the aqueous solution of PVP (prepared in the step 1).

4. 80 g of urea is dissolved in the aqueous solution of silver nitrate prepared in the step 2.

5. 36 g of glucose is dissolved in the aqueous solution of PVP (prepared in the step 1).

6. With stirring and at room temperature, the aqueous solution of silver nitrate (prepared in the step 2) is rapidly added to the aqueous solution of PVP (prepared in the step 1). The mixed solution is then transferred to a thermostatic bath at 85° C. and reacted for one hour. After the reaction mixture is cooled, the nanoscale silver colloidal solution of the present invention is obtained.

EXAMPLE 2

Effect of Composition Variations on the Results of the Present Invention

I. Objective:

For a purpose of realizing the influences of composition variations on the results of the present invention, the amount of composition used in this example is varied hereinafter. For example, the added amount of sodium hydroxide or urea is adjusted to carry out the reaction. The particle size of resulting nanosalce silver particle and the conversion are examined in order to study the optimized condition of the present preparing method.

II. Means:

The added amount of sodium hydroxide or urea is changed and the preparing steps of the present invention are repeatedly done. After the reaction proceeds for one hour, a trace amount of product is diluted to investigate the particle size distribution using dynamic light scattering (DLS) analysis. Furthermore, the concentration of the residual silver ions of the nanoscale silver paste is measured by using silver ion electrode so as to deduce the conversion of reaction. The reaction conditions used in this example include the conditions 2 and 3 shown in the following table.

Condition 1	Condition 2	Condition 3
Formulation	The same as that used in	The same as that used in
of example 1	example 1 except for half	example 1 except for half
	amount of urea	amount of sodium hydroxide

III. Result:

In a case that the added amount of urea is double or half, the resulting particle size is substantially identical to that obtained in the example 1. In example 1, the concentration of urea in the solution is 13 times the quantity of silver nitrate. In principle, four urea molecules may chelate one silver ion to form a stable complex. Therefore, even half amount of urea is satisfied. As the ionic chelating agent, there is a lower limit of use amount thereof. It is recommended that the quantity of urea used in this example is at least four times the concentration of silver nitrate. The conversion, even though the use amount is reduced by 50%, still reaches above 95%. It is noted that more urea would result in higher conversion in the short time period.

Moreover, there are two sources of alkaline ions in the solution, i.e. sodium hydroxide and urea. In a case that the use quantity of urea is constant, half amount of sodium hydroxide may result in reduced conversion in principle. Accordingly, in the preparing processes using reduced amount of sodium hydroxide, the resulting particle size is substantially kept unchanged but the conversion is reduced.

FIG. 2 is a plot showing the conversion of reaction on three different preparing conditions. The upper blocks and the lower blocks shown in FIG. 2 indicate the conversions obtained when the reaction proceeds for one hour and 30 minutes, respectively.

IV. Conclusion:

In a case that a long reaction time is desired, the amount variation of sodium hydroxide has a larger influence on the convention of reaction than that of urea. In the present invention, the major role of urea is to provide chelating protection in the initial stage and facilitate occurrence of nucleation in the middle and later stages. Both capabilities allow for easy control of the particle size.

It is concluded that the provision of sodium hydroxide is responsible for the greatest part of the conversion. Moreover, the key factor for controlling particle size is the relative amount between sodium hydroxide and urea. For achieving the optimized conditions, the concentration ratio of NaOH/AgNO₃ is less than or equal to 0.4 and the concentration ratio of CO(NH₂)₂/AgNO₃ is greater than or equal to 4.

For preparing a 1.5 wt % of nanoscale silver colloidal solution, the most preferred operating condition uses a reactant composition including 1˜5 wt % of silver nitrate, 5˜20 wt % of stabilizing agent, 10˜20 wt % of chelating agent, 1˜5 wt % of reducing agent, 50˜80 wt % of deionized water and 0.1˜1 wt % of reaction accelerator.

In an exemplary operating condition, the ratio of PVP(MW=40,000) to AgNO₃ is 8/1 (gig), the reaction temperature is 85° C., the concentration of AgNO₃ solution is 0.167 M, the concentration of glucose solution is 0.334 M, the concentration of NaOH solution is 0.067 M, and the concentration of urea solution is 2.22 M. It is noted that this exemplary operating condition is one of the candidate operating conditions used in the present invention. The initial concentrations of the respective agents are described by reference. As the content of silver to be prepared is varied, numerous modifications and alterations of the operating conditions may be made while retaining the teachings of the invention.

The mean particle size of less than 10 nm is obtained without difficulties if the reaction of the present composition is carried out under the above described operation conditions. From the particle size distribution measured by dynamic light scattering (DLS) equipment, as shown in FIG. 3, arithmetical mean and standard deviation of the particles would be calculated. The result shows that the particle size of the nanoscale silver paste is 8.5±2.5 nm.

The nanoscale silver paste of the present invention comprises about 14 wt % of stabilizing agent and thus can be further diluted or concentrated depending on the succeeding applications. FIGS. 4 and 5 are transmission electron microscope (TEM) images of the nanoscale silver subject to a specified purification procedure followed by dilution. Although the nanoscale silver is possibly suffered from slight aggregation during dilution, the mean particle size is still controlled below 15 nm, as can be seen in FIG. 4. In contrast, the nanoscale silver obtained from the conventional non-urea system has larger particle size and broader particle size distribution (as shown in FIG. 5).

Since the nanoscale silver paste of the present invention comprises about 14 wt % of stabilizing agent, the nanoscale silver paste is very stable at low temperature. Even though most of the stabilizing agent is removed after purification and concentration, the content of silver in the solution reaches up to 20% by weight. In addition, the trace amount of stabilizing agent adsorbed on the particle surface may facilitate stabilizing the particles for several months.

According to the investigation on the further applications, low-concentration nanoscale silver solution has germicidal efficacy. After dilution, the nanoscale silver can be directly incorporated into some materials such as the clothing fibers, plastic material and the like to serve as the antibacterial agent. Furthermore, the nanoscale silver can be incorporated into organic paint via phase inversion. In addition to the germicidal efficacy, highly purified nanoscale silver paste exhibits excellent conductivity to be industrially used for IC wire printing, conductive ink-jet printing or electrical conduction or used as thermal adhesive.

While the invention has been described by way of example and in terms of a preferred embodiment, it is to be understood that the invention is not limited thereto. To the contrary, it is intended to cover various modifications and similar arrangements and procedures, and the scope of the appended claims therefore should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements and procedures.

Claims

1. A nanoscale silver composition comprising nanoscale silver particles having a mean particle size of less than 10 nm, wherein said mean particle size of said nanoscale silver particle maintains 20 nm or less for at least 180 days at room temperature.

2. The nanoscale silver composition of claim 1, comprising at least 5% w/w of stabilizing agent.

3. The nanoscale silver composition of claim 2, wherein said stabilizing agent is selected from a group consisting of sodium dodecyl sulphate (SDS), polyvinyl pyrrolidone (PVP) and polyvinyl alcohol (PVA).

4. The nanoscale silver composition of claim 1, comprising 1˜5% w/w of metallic silver.

5. A preparing process of a nanoscale silver composition, comprising steps of: a) adding a stabilizing agent, a reducing agent and a reaction accelerator into deionized water to form an aqueous solution;b) dissolving a chelating agent in an aqueous solution of silver salt and deionized water to form a stable complex;c) rapidly adding the aqueous solution obtained in step b) to the aqueous solution obtained in step a) with stirring and at a low temperature environment below 25° C.; andd) rapidly transferring the mixed solution to an elevated temperature environment of 80˜85° C. and effecting a reaction, thereby producing nanoscale silver particles having a mean particle size of less than 10 nm, wherein said particle size of said nanoscale silver particle maintains 20 nm or less for at least 180 days at room temperature.

6. The preparing process of claim 5, wherein said stabilizing agent is selected from a group consisting of sodium dodecyl sulphate (SDS), polyvinyl pyrrolidone (PVP) and polyvinyl alcohol (PVA).

7. The preparing process of claim 5, wherein said stabilizing agent is used in an amount of 5˜20% w/w.

8. The preparing process of claim 5, wherein said reducing agent is selected from a group consisting of sodium borohydride (NaBH4), hydrazine (N2H4), formaldehyde (HCHO), glucose (C6H12O6.H2O) and trisodium citrate (2Na3C6H5O7.11H2O).

9. The preparing process of claim 5, wherein said reducing agent is used in an amount of 1˜5% w/w.

10. The preparing process of claim 5, wherein said reaction accelerator is a alkaline agent capable of generating hydroxide ions (OH) in water.

11. The preparing process of claim 5, wherein said reaction accelerator is used in an amount of 0.1˜1% w/w.

12. The preparing process of claim 5, wherein said chelating agent is urea.

13. The preparing process of claim 5, wherein said chelating agent is used in an amount of 10˜20% w/w.

14. The preparing process of claim 5, wherein said silver salt is selected from a group consisting of silver nitrate, barium sulfate and silver perchlorate.

15. The preparing process of claim 5, wherein said reaction is effected at said elevated temperature environment of 80˜85° C. for 0.1˜1 hour.

16. The preparing process of claim 5, wherein the concentration ratio of said reducing agent to said silver salt is ranged from 1:2.5 to 1:10.

17. The preparing process of claim 5, wherein the concentration ratio of said chelating agent to said silver salt is ranged from 4:1 to 10:1.