Method of encapsulating pigment flakes with a metal oxide coating

1. A method of encapsulating pigment flakes with a metal oxide coating, the method comprising:

mixing pigment flakes with a solvent;

adding a metal salt to the solvent; and

adding a reducing agent to the solvent, so as to encapsulate the pigment flakes with the metal oxide coating, wherein the metal salt is a precursor to the metal oxide coating.

2. The method of claim 1 , wherein the metal oxide coating consists essentially of a metal oxide.

3. The method of claim 1 , wherein the metal salt is a zinc salt, a zirconium salt, a tin salt, a cerium salt, or a mixture thereof, and the metal coating is a zinc oxide coating, a zirconium oxide coating, a tin oxide coating, a cerium oxide coating, or a mixture thereof.

4. The method of claim 1 , wherein the metal salt is a zinc salt, and the metal oxide coating is a zinc oxide coating.

5. The method of claim 4 , wherein the zinc oxide coating consists essentially of zinc oxide.

6. The method of claim 1 , wherein the metal salt is a metal nitrate, a metal sulfate, a metal phosphate, a metal acetate, a metal chloride, or a mixture thereof.

7. The method of claim 1 , wherein the solvent is water, an alcohol, an ester, or a mixture thereof.

8. The method of claim 1 , wherein the reducing agent is a hydride reducing agent or a borane complex reducing agent.

9. The method of claim 1 , wherein the reducing agent is sodium borohydride.

10. The method of claim 1 , further comprising:

adding a surfactant to the solvent to facilitate dispersion of the pigment flakes in the solvent.

11. The method of claim 1 , wherein the metal salt dissolves in the solvent to provide metal cations, and the reducing agent reduces the metal cations.

12. The method of claim 1 , wherein the metal oxide coating fully encapsulates each individual flake of the pigment flakes.

13. The method of claim 1 , wherein the pigment flakes include at least one metal layer with at least one exposed surface.

14. The method of claim 1 , wherein the pigment flakes include:

a metal layer having a top surface, a bottom surface, and at least one side surface; and

dielectric layers covering the top surface and the bottom surface of the metal layer, but not the at least one side surface of the metal layer.

15. The method of claim 14 , wherein the metal layer is formed of aluminum, and the dielectric layers are each formed of magnesium fluoride.

16. The method of claim 1 , wherein the pigment flakes comprise one of three-layer pigment flakes, five-layer pigment flakes, or seven-layer pigment flakes.

17. The method of claim 1 , wherein the metal salt comprises one or more of a transition metal salt, a main-group metal salt, or a rare-earth metal salt.

18. The method of claim 1 , wherein adding the reducing agent comprises:

dripping the reducing agent into the solvent.

19. The method of claim 1 , further comprising:

washing the pigment flakes with ethanol after adding the reducing agent to the solvent.