Noble metal coated silver nanowires

Abstract

Metal nanowires with uniform noble metal coatings are described. Two methods, galvanic exchange and direct deposition, are disclosed for the successful formation of the uniform noble metal coatings. Both the galvanic exchange reaction and the direct deposition method benefit from the inclusion of appropriately strong binding ligands to control or mediate the coating process to provide for the formation of a uniform coating. The noble metal coated nanowires are effective for the production of stable transparent conductive films, which may comprise a fused metal nanostructured network.

What is claimed is: 
     
         1 . A method for forming noble metal coated silver nanowires by galvanic exchange, the method comprising:
 gradually adding a coating solution comprising a mixture of noble metal ions and complexing ligands to a heated reaction solution comprising a dispersed blend of silver nanowires and a polymer capping agent to gradually replace silver with noble metal to form an intact silver core with a noble metal coating.   
     
     
         2 . The method of  claim 1  wherein the mixture undergoes no observable reaction in 24 hours at room temperature. 
     
     
         3 . The method of  claim 1  wherein the complexing ligand is selected from the group consisting of nitrite, diethyl amine, ethylene diamine, nitrilotriacetic acid, iminobis(methylene phosphonic acid), aminotris(methylene phosphonic acid), ethylene diamine tetraacetic acid (EDTA), 1,3-propylenediaminetetraacetic acid (1,3-PDTA), triethylene tetramine, tri(2-aminoethyl) amine, diethylenetriaminepentaacetic acid, 1,2-cyclohexanediaminotetraacetic acid, iminodiacetic acid, methyliminodiacetic acid, N-(2-acetamide) iminoacetic acid, N-(2-carboxyethyl) iminodiacetic acid, N-(2-carboxymethyl)imino dipropionic acid, picolinic acid, dipicolinic acid, histidine, combinations thereof. 
     
     
         4 . The method of  claim 1  wherein the coating solution is added gradually to the reaction solution over a period of at least an hour and the reaction solution is heated to a temperature of at least about 60° C. 
     
     
         5 . The method of  claim 4  wherein the noble metal source solution comprises at least about 2 ligand molecules per noble metal ion. 
     
     
         6 . The method of  claim 1  wherein the reaction solution further comprises a capping polymer, wherein the coating solution comprises noble metal ions at from about 0.0001 M to about 5 M and wherein the reaction solution comprises from about 0.01 wt% to about 3.5 wt% silver nanowires and from about 0.01 wt% to about 1.5 wt% capping polymer. 
     
     
         7 . The method of  claim 1  further comprising harvesting noble metal coated silver nanowires having from about 0.05 wt% to about 15 wt% noble metal. 
     
     
         8 . A collection of noble metal coated silver nanowires having a diameter along the length of the nanowire with a standard deviation of no more than about 10 percent of the average diameter and having at least about 0.02 atomic percent noble metal. 
     
     
         9 . The collection of noble metal coated silver nanowires of  claim 8  comprising at least about 0.1 atomic percent noble metal. 
     
     
         10 . The collection of noble metal coated silver nanowires of  claim 8  wherein the noble metal is gold, platinum or a combination thereof.