One-Step Synthesis of Tunable-Size Gold Nanoplates on Graphene Multilayers

Au nanoplates (quasi-two-dimensional single crystals) are most commonly synthesized using a mixture of Au precursors via approaches involving multiple processing steps and the use of seed crystals. Here, we report the synthesis of truncated-hexagonal {111}-oriented micrometer-scale Au nanoplates on graphene multilayers using only potassium tetrabromoaurate (KAuBr₄) as the precursor. We demonstrate that the nanoplate sizes can be controllably varied from tens of nanometers up to a few micrometers by introducing desired concentrations of chloroauric acid (HAuCl₄) to KAuBr₄ and their thicknesses from ∼13 to ∼46 nm with the synthesis time. Through a series of experiments carried out as a function of synthesis time and precursor composition [mixtures of HAuCl₄ and KAuBr₄, KBr, or ionic liquid 1-butyl-3-methylimidazolium bromide ([Bmim]Br)], we identify the optimal HAuCl₄ and KAuBr₄ concentrations and synthesis times that yield the largest and the thinnest size nanoplates. We show that the nanoplates are kinetically limited morphologies resulting from preferential growth of {111} facets facilitated by bromide ions in KAuBr₄ solutions; we suggest that the presence of chloride ions enhances the rate of Au deposition and the relative concentration of chloride and bromide ions determines the shape anisotropy of resulting crystals. Our results provide new insights into the kinetics of nanoplate formation and show that a single precursor containing both Au and Br is sufficient to crystallize nanoplates on graphitic layers, which serve as reducing agent while enabling the nucleation and growth of Au nanoplates. We suggest that a similar approach may be used for the synthesis of nanoplates of other metals on weakly interacting van der Waals layers for, potentially, a variety of new applications.