Crystalline noble metal nanostructures and methods for their preparation

1. A method for making a crystalline anisotropic palladium, platinum, or gold nanostructure, comprising:

(a) adsorbing bromide to one or more surfaces of a seed to provide a seed having adsorbed bromide, wherein the seed is selected from the group consisting of palladium, platinum, and gold seeds;

(b) reducing a precursor at a rate sufficient to add palladium, platinum, or gold metal atoms to the seed having adsorbed bromide; and

(c) oxidatively etching one surface of the seed having adsorbed bromide to break symmetry to promote growth of the seed along an axis of the seed to provide an anisotropic palladium, platinum, or gold nanostructure.

2. The method of claim 1 , wherein the seed is a palladium seed and has face-centered cubic (fcc) symmetry.

3. The method of claim 1 , wherein the seed is a palladium seed and is a single crystal.

4. The method of claim 1 , wherein the seed is a multiple twinned crystal.

5. The method of claim 1 , wherein the precursor is selected from the group consisting of Na 2 PdCl 4 and Na 2 PdBr 4 .

6. The method of claim 1 , wherein reducing the precursor comprises treating the precursor with a reducing agent selected from the group consisting of ethylene glycol and di(ethylene glycol).

7. The method of claim 1 , wherein adsorbing bromide to one or more surfaces of the seed comprises altering the order of free energies of the surfaces to promote the formation of {100} and {110} facets.

8. The method of claim 1 , wherein adsorbing bromide to one or more surfaces of the seed comprises treating the seed with potassium bromide.

9. The method of claim 1 , wherein oxidatively etching one surface of the seed comprises breaking symmetry to promote growth of the seed along an axis of the seed to provide an anisotropic nanostructure having elongated {100} and {110} facets.

10. The method of claim 1 , wherein oxidatively etching one surface of the seed comprises removing bromide from a surface of the seed.

11. The method of claim 10 , wherein oxidatively etching one surface of the seed comprises removing bromide from a surface of the seed with oxygen, chloride, or water.

12. The method of claim 1 further comprising adding a capping agent to the seed.

13. The method of claim 12 , wherein the capping agent is a poly(vinylpyrrolidone).

14. The method of claim 1 , wherein an aspect ratio of the anisotropic nanostructure is from about 1.1 to about 20.

15. The method of claim 1 , wherein the anisotropic nanostructure is a palladium or platinum nanobar.

16. The method of claim 1 , wherein the anisotropic nanostructure is a palladium or gold nanorod.

17. The method of claim 1 , wherein the anisotropic nanostructure is a palladium or gold five-fold twinned nanorod.