Effects of ammonium salts and anionic amphiphiles on the photochemical formation of gold nanorods

The effects of ammonium salts and anionic amphiphiles on the formation and morphology of gold nanorods (NRs) in hexadecyltrimethylammonium bromide (CTAB) solutions were investigated. The addition of a small amount of tetrahexylammonium bromide (THAB) increased the aspect ratio of NRs, while an excess of THAB or tetradodecylammonium bromide (TDAB) decreased the aspect ratio, and concomitantly increased the formation of spherical gold nanoparticles. In contrast, the addition of anionic amphiphiles forming hydrophobic ion-complexes with CTAB did not decrease the aspect ratio, but suppressed the formation of NRs. These results could be attributed to the change in the adsorbed layer of CTAB on the gold NRs.