Shape and size transformation of gold nanorods (GNRs) via oxidation process: A reverse growth mechanism

摘要

The anisotropic shape transformation of gold nanorods (GNRs) with H_2O_2 was observed in the presence of "cethyl trimethylammonium bromide" (CTAB). The adequate oxidative dissolution of GNR is provided by the following autocatalytic scheme with H_2O_2: Au~0→Au~+, Au~0 + Au~(n+)→2Au~(3+), n = 1 and 3. The shape transformation of the GNRs was investigated by UV-vis spectroscopy and transmission electron microscopy (TEM). As-synthesised GNRs exhibit transverse plasmon band (TPB) at 523 nm and longitudinal plasmon band (LPB) at 731 nm. Upon H_2O_2 oxidation, the LPB showed a systematic hypsochromic (blue) shift, while TPB stays at ca. 523 nm. In addition, a new emerging peak observed at ca. 390 nm due to Au(III)-CTAB complex formation during the oxidation. TEM analysis of as-synthesised GNRs with H_2O_2 confirmed the shape transformation to spherical particles with 10nm size in 2h, whereas centrifuged nanorod solution showed no changes in the aspect ratio under the same condition. Au~(3+) ions produced from oxidation, complex with excess free CTAB and approach the nanorods preferentially at the end, leading to spatially directed oxidation. This work provides some information to the crystal stability and the growth mechanism of GNRs, as both growth and shortening reactions occur preferentially at the edge of single-crystalline GNRs, all directed by Br~ ions.