Large-scale multi step synthesis method for ultralong silver nanowire with controllable diameter

Abstract

A large-scale multi-step synthesis method for ultralong silver nanowire with controllable diameter, comprises: an ethylene glycol solution containing polyvinylpyrrolidone and sodium chloride is fully heated to obtain a solution with strong reducibility, and then silver nitrate in glycol solution is added for a generation of a large number of crystal seeds; hydrogen peroxide is used to achieve the selection of the crystal seeds for a small amount of crystal seeds with particular sizes; the temperature is immediately raised to increase the reaction rate until the threshold of the etching crystal seeds of nitric acid is broke through; the temperature is lowered for long-timed reaction to slow down the reaction rate, reduce the probability of the isotropic seeds by self-nucleation and promote the absorption of small nucleus in the radial direction of large nucleus or nanowire, thus obtaining the ultralong silver nanowire.

Claims

1. A method of large-scale multi-step synthesis for a silver nanowire with a controllable diameter comprising the following steps: 1) a step of screening of crystal seeds comprising: heating ethylene glycol solution containing a surfactant at a temperature of 140 to 160 C. to obtain a solution I with a strong reducibility; dissolving alkali metal halide in ethylene glycol which is added into the solution I to yield a solution II; dissolving silver nitrate in ethylene glycol for preparing a precursor solution, the concentration of silver nitrate is between 0.001 to 5 mol/L; adding the precursor solution into the solution II that reduces silver nitrate rapidly into a multiple twinned crystal seeds with different sizes and part of isotropic seeds; then adding etching agent into the solution II that results in crystal seeds to etch the isotropic seeds which are not resistant to etching and the multiple various sizes of the twinned crystals, thus screening particular sizes of the multiple twinned crystal seeds; 2) a step reaching a threshold of etching multiple twinned crystal seeds comprising: adding hydrogen peroxide into the reaction system of Step 1, increasing the reaction temperature up to 160-200 C. for a continuous reaction in the temperature until turbidities appearing; wherein increasing the temperature boosts the reaction rate until the threshold of etching multiple twinned crystal seeds is reached for by-products; the by-products including nitric acid; 3) a step of longitudinal growth comprising: decreasing the reaction temperature 60-140 C. for a completed reaction when the turbidities appear; wherein decreasing the reaction temperature reduces the reaction rate and depletes the isotropic seeds generated by self-nucleation, allows more free silver ions to be reduced longitudinally along nanowires for obtaining the silver nanowire; wherein length of the silver nanowire is at least 300 m.

2. The method according to claim 1, wherein the surfactant is polyvinylpyrrolidone (PVP); the concentration of PVP in the reaction system is 0.001-10 mol/L.

3. The method according to claim 1, wherein the alkali metal halide is selected from a group consisting of sodium chloride, potassium chloride, sodium bromide, potassium bromide, sodium iodide and potassium iodide; the concentration of the alkali metal halide is 0.001-10 mmol/L.

4. The method according to claim 1, wherein the etching agent is hydrogen peroxide, which is added in a manner of single-time addition.

5. The method according to claim 1, wherein the concentration of the hydrogen peroxide is 0.001-0.5 mol/L, and the diameter of the silver nanowire is controlled by adjusting the amount of the hydrogen peroxide.

6. The method according to claim 1, wherein the molar ratio of the silver nitrate to PVP is 1:0.1-100.

7. The method according to claim 1, wherein the method can be utilized for a large-scale synthesis and the amount of the silver nitrate in the reaction system can be expanded to over 100 g.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 (a) is a scanning electron microscope (SEM) view of the silver nanowires prepared in case of hydrogen peroxide being 0.1 mmol in embodiments;

(2) FIG. 1 (b) is a scanning electron microscope (SEM) view of the silver nanowires prepared in case of hydrogen peroxide being 0.05 mmol in embodiments;

(3) FIG. 1 (c) is a scanning electron microscope (SEM) view of the silver nanowires prepared in case of hydrogen peroxide being 0.01 mmol in embodiments;

(4) FIG. 1 (d) is a scanning electron microscope (SEM) view of the silver nanowires prepared in case of hydrogen peroxide being 0.005 mmol in embodiments;

(5) FIG. 2 is an optical view (enlarged by 20 times) of the silver nanowires prepared in case of hydrogen peroxide being 0.1 mmol in embodiments.

DETAILED DESCRIPTION

(6) step 1, phase of screening of crystal seeds:

(7) heating ethylene glycol solution containing a surfactant at a temperature of 140 to 160 C. to obtain a solution I with a strong reducibility; adding alkali metal halide dissolved in ethylene glycol into the solution I to yield a solution II; dissolving silver nitrate in ethylene glycol for preparing a precursor solution, the concentration of silver nitrate is between 0.001 to 5 mol/L;

(8) adding the precursor solution into the solution II that reduces silver nitrate rapidly into a large number of multiple twinned crystal seeds with different sizes and part of isotropic seeds; adding etching agent into the above solution with a large number of crystal seeds results in preferentially etching the isotropic seeds which are not resistant to etching and the multiple various sizes of the twinned crystals, thus screening limited amount of particular sizes of the multiple twinned crystal seeds;

(9) step 2, phase of breaking through the threshold of etching multiple twinned crystal seeds:

(10) after the addition of etching agent in Step 1, increasing the reaction temperature immediately to 160-200 C. for a continuous reaction in the temperature until turbidities appearing; increasing the temperature significantly boosts the reaction rate until the threshold of etching multiple twinned crystal seeds by few by-products, namely nitric acid, is broken through;

(11) step 3, phase of longitudinal growth:

(12) decreasing the reaction temperature to 60-140 C. for a completed reaction under such condition once the appearance of turbidities; the decrease of temperature, which effectively reduces the reaction rate and depletes the isotropic seeds generated by self-nucleation, allows more free silver ions to be reduced longitudinally along nanowires for obtaining ultralong silver nanowires.

(13) Wherein,

(14) The surfactant is polyvinylpyrrolidone (PVP) or the surfactant is capable of being absorbed on <100> crystal surface, mainly including sodium polystyrene sulphonate (PSS), polyacrylic acid (PAA); the concentration of PVP in the reaction system is 0.001-10 mol/L.

(15) The alkali metal halide used is sodium chloride, potassium chloride, sodium bromide, potassium bromide, sodium iodide or potassium iodide, the concentration of the alkali metal halide is 0.001-10 mmol/L.

(16) The etching agent is hydrogen peroxide, which is added in a manner of single-time addition.

(17) The concentration of the etching agent is 0.001-0.5 mol/L, and the diameter of the silver nanowire is controlled by adjusting the amount of the etching agent.

(18) The molar ratio of the silver nitrate to PVP is 1:0.1-100.

(19) The method can be utilized for a large-scale synthesis and the amount of the silver nitrate in the reaction system can be expanded to over 100 g.

(20) In the present invention, the synthesis of silver nanowires experiences three important phases.

(21) The first phase is to select crystal seeds. The ethylene glycol is fully counterflow heated to produce acetaldehyde with strong reducibility, which reduces the silver ions to generate a large number of multiple twinned crystal seeds and parts of isotropic seeds. The nanowires with nonuniform thickness may be generated without selection as there are either larger seeds or smaller seeds in the solution. Adding a certain amount of hydrogen peroxide can significantly select crystal seeds formed in early reaction. The addition of a large amount of hydrogen peroxide etch isotropic seeds and smaller crystal seeds, which will generate the silver nanowires with thinner diameter, and mainly preserve the larger crystal seeds, resulting in thick silver nanowires; a small amount of hydrogen peroxide only etch the isotropic seeds and smaller crystal seeds are preserved to produce the silver nanowires with thin diameter; therefore, the purpose of controlling the diameter of silver nanowires can be reached by adjusting the amount of hydrogen peroxide in selection of crystal seeds. In addition, when the amount of reaction is enlarged, the isotropic seeds existed in solution are easier to absorb the silver atoms to form nanoparticles, which severely retard the generation of nanowires and impose a fatal effect on the reaction. And the addition of hydrogen peroxide etches most isotropic seeds which are not resistant to etching to increase the yield of nanowires. This plays a critical role in enlarging the amount of reaction and achieving industrial production.

(22) The second phase is to break through the threshold of etching multiple twinned crystal seeds. During the process of reducing the silver ions by acetaldehyde, hydrogen ions are produced, which combine with the nitrate ions to form nitric acid etching multiple twinned crystal seeds; however, after the selection by hydrogen peroxide the amount of crystal seeds considerably decrease in the reaction system, at the same time nitric acid gradually accumulates. In such condition, most multiple twinned crystal seeds in solution will be etched so as to obtain no products. Thus, after the addition of hydrogen peroxide, temperature should be immediately raised to increase reaction rate so as to break through the threshold of etching silver nanowires; if the temperature is not raised, the solution will gradually become clear under the etching effect of nitric acid, that is to say, most of silver nanowires are etched.

(23) The third phase is to lower temperature for long-time reaction. The decrease of temperature after breaking through reverse-etching slows down the reaction rate, thus promoting the bigger ones to eat the smaller ones. Due to the decrease of the probability of self-nucleation forming isotropic seeds in the low temperature environment, most of the residual silver ions are reduced radially along a small amount of survival crystal seeds. The small amounts of the crystal seeds increase the amount of silver ions assigned to each crystal seed, causing the generated silver nanowire is longer than that generated by traditional method. Moreover, small crystal nucleus gradually absorbs on the radial direction of large nucleus or the silver nanowires such that the high-yield ultralong thick nanowire is produced. Otherwise, the reaction rate in high temperature facilitates self-nucleation to generate isotropic seeds that may preferentially absorb silver ions for growth, thereby retarding the longitudinal growth of nanowires. Simultaneously, the amount of crystal nucleus in the reaction system increases and the silver sources used for the growth of nanowires decreases, thus the produced wire being shorter.

(24) The multi-step method proposed by the present invention is different from the traditional one-step method, constructing the most advantageous external conditions for all different reaction phases and fully meeting the demand for different phases of the growth of nanowires. The process of the increase and decrease of temperature just satisfies different demands for reaction rate in different phases. In traditional methods, there are also such methods in which high temperature or low temperature remains the whole reaction phases. However, these two kinds of heating manner can only meet the reaction requirements of nanowires at a certain phase instead of fitting the law of nanowire growth, so both the diameter size and yield of the prepared nanowires cannot achieve the effects similar to the nanowires prepared by the present invention.

(25) The present invention is further described by means of specific embodiments and comparison examples below:

EMBODIMENTS

(26) 1. 0.5 g polyvinylpyrrolidone is dissolved in 150 ml ethylene glycol and then heated under 150 C.

(27) 2. Sodium chloride solution in ethylene glycol with concentration of 0.01 mol/L is configured to be used later;

(28) 0.15 g silver nitrate is dissolved in ethylene glycol and to be used later.

(29) 3. 0.3 ml sodium chloride-ethylene glycol is added into the solution;

(30) 4. The solution of silver nitrate is added, followed by dripping of 0.1 mmol hydrogen peroxide.

(31) 5. The temperature is rapidly raised to 175 C. until turbidities appear in the solution.

(32) 6. The temperature is lowered to 130 C. for full reaction, then the silver nanowires can be obtained by natural cooling down.

(33) The SEM picture of silver nanowires prepared by these embodiments is illustrated in FIG. 1 (a) and the optical picture in FIG. 2. When the amount of hydrogen peroxide is respectively adjusted to 0.05 mmol, 0.01 mmol, 0.005 mmol with other conditions being unchanged, the SEM pictures of silver nanowires prepared are shown in FIG. 1 (b), 1 (c), 1 (d), respectively.

(34) As shown in FIGS. 1 (a) and 2, the silver nanowires prepared by the embodiments has an average diameter of 600 nm and length of more than 300 m, wherein the diameter is 3-11 times and the length is longer than that of the silver nanowires made by the traditional synthetic method.

(35) As shown in FIGS. 1 (b), 1 (c) and 1 (d), the obtained silver nanowires have an average diameter of 300 nm, 150 nm, 50 nm, respectively. The silver nanowires show the characteristics of the length up to hundreds of microns, higher aspect ratio, uniform diameters, good dispersion and precise control of diameter within a narrow range. In addition, modifications can be made by those skilled in the art within the spirit of the present invention, and those modifications made according to the spirit of the present invention should be contained within the range claimed for protection by the present invention.