Anisotropic shape evolution of large-size Cu nanocrystals prepared in nonionic microemulsion media

We report the anisotropic shape evolution of colloidal copper nanocrystals (Cu NCs) in nonionic microemulsion media by adopting tetraethylammonium hydroxide as a reactant and capping agent in water phase. Relatively large-size Cu NCs were synthesized to confine the growth of Cu NCs below 100 nm. TEM images showed the triangular and hexagonal morphologies of Cu NCs ranging over 55-70 nm after 28 h, while they showed the spherical NCs dominantly under higher concentration of a reducing agent. The surface plasmon resonance (SPR) absorption band of anisotropic Cu NCs was centered at 571 nm with signal enhancement over time. The formation kinetics revealed that the gradual conversion of the Cu seeds into larger NCs during 28 h at room temperature. The NCs were characterized to be Cu rather than CuO or Cu2O by electron diffraction patterns, exhibiting relevant d-spacing figures consistent with the bulk Cu crystal.