Thin and uniform silver nanowires, method of synthesis and transparent conductive films formed from the nanowires
Highly uniform and thin silver nanowires are described having average diameters below 20 nm and a small standard deviation of the diameters. The silver nanowires have a high aspect ratio. The silver nanowires can be characterized by a small number of nanowires having a diameter greater than 18 nm as well as with a blue shifted narrow absorption spectrum in a dilute solution. Methods are described to allow for the synthesis of the narrow uniform silver nanowires. Transparent conductive films formed from the thin, uniform silver nanowires can have very low levels of haze and low values of ΔL*, the diffusive luminosity, such that the transparent conductive films can provide little alteration of the appearance of a black background.
1. A method for synthesizing silver nanowires, the method comprising:
forming a reaction solution substantially free of paramagnetic ions comprising a blend of polyol solvent, a polyvinyl pyrrolidone capping agent, chloride anion, bromide anion, and a five-membered aromatic heterocyclic salt comprising an imidazolium, pyrazolium or thiazolium cation;
heating the reaction solution to a peak temperature; and
adding a soluble silver salt.
2. The method of claim 1 , the imidazolium salt represented by the formula:
wherein:
R 1 and R 2 are independently hydrogen, a linear, branched, or cyclic alkyl group, an unsaturated group, an arylalkyl group, with the proviso that R 1 and R 2 are not both hydrogen, and
R 3 is a halogen or any substituent described for R 1 and R 2 .
3. The method of claim 1 , the pyrazolium salt represented by the formula:
wherein:
R 1 and R 2 are independently hydrogen, a linear, branched, or cyclic alkyl group, an unsaturated group, an arylalkyl group, with the proviso that R 1 and R 2 are not both hydrogen, and
R 3 is a halogen or any substituent described for R 1 and R 2 .
4. The method of claim 1 , the thiazolium salt represented by the formula:
wherein:
R 1 and R 2 are independently hydrogen, a linear, branched, or cyclic alkyl group, an unsaturated group, an arylalkyl group, with the proviso that R 1 and R 2 are not both hydrogen, and
R 3 is a halogen or any substituent described for R 1 and R 2 .
5. The method of claim 1 wherein moles of the bromide ion are from about 0.6 to about 15 times moles of the chloride ion.
6. The method of claim 1 wherein the reaction solution is formed in a reactor, and a concentration based on added amounts into the reactor for the heterocyclic cation is from about 0.00001M to about 0.01M.
7. The method of claim 1 wherein the reaction solution is formed in a reactor, and concentrations based on added amounts into the reactor are polyvinyl pyrrolidone capping agent 0.1 wt % to 10 wt %, bromide anion 0.00002M to 0.005M, chloride anion 0.00002M to 0.005M, five-membered aromatic heterocyclic salt 0.00002M to 0.005M, and soluble silver salt 0.0025M to 0.25M.
8. The method of claim 1 wherein the polyvinyl pyrrolidone capping agent comprises a blend of PVP K30 and PVP K90.
9. The method of claim 1 wherein the peak temperature is at least about 145° C.
10. The method of claim 1 wherein the soluble silver salt is added at a temperature within 5° C. of the peak temperature.
11. The method of claim 1 , further comprising purifying silver nanowires from a completed reaction mixture, wherein the silver nanowires have an average diameter of no more than about 18 nm and a standard deviation of the diameter of no more than about 2.5 nm.