Seed-mediated synthesis of branched gold nanocrystals derived from the side growth of pentagonal bipyramids and the formation of gold nanostars

We report the first synthesis of elongated penta-branched gold nanocrystals with a shape resembling that of a star fruit but with sharp ends by a seeding growth approach. Cetyltrimethylammonium bromide (CTAB) capping surfactant was used, and AgNO₃ was added to the last growth solution to promote the formation of the five side branches. Through an investigation of the products collected by adding AgNO₃ into first, second, third, or fourth growth solution, the penta-branched nanocrystals were found to be derived from pentagonal bipyramid-shaped nanocrystals. Side growth over the twin boundaries results in the formation of five elongated branches with the highest point of each branch bisecting the branch into two halves. Smaller penta-branched nanocrystals with sizes of 70-110 nm and more fully developed larger nanocrystals with sizes of 200-350 nm can be readily prepared. The branches possess single-crystalline {111} faces. The intermediate products obtained at various time points were examined and their UV-vis absorption spectra recorded. By replacing CTAB with cetyltrimethylammonium chloride surfactant, and controlling the concentration of bromide ions in the solution, gold nanostars with five symmetrical branches and monodispersed sizes of 120-130 nm were synthesized for the first time.