Abstract
Morphology of Ag nanocrystals (NCs) is essential to the NC application in catalysis, optics, and as antibacterial agents. Therefore, it is important to develop synthetic methods and understand the evaluation of NC morphology in different chemical environments. In this study, we report interesting findings of the morphological change of fivefold-twinned Ag–Au–Ag nanorods (NRs) under the effect of H2O2 both as an oxidant (etchant) and a reductant. At low H2O2 concentration, the reconstruction of Ag–Au–Ag NRs was dominated by the growth along the longitudinal direction of NRs. With the increase of H2O2 concentration, the reconstruction also occurs in the transverse direction, and a clear change in particle morphology was observed. We further systematically studied the mechanism of the reaction. The results showed that the transition of the morphology was a two-step process: (1) the etching of Ag on the seeds and (2) the reduction of Ag2O. In the second step, the reaction kinetics was highly affected by H2O2 concentration. At low H2O2 concentration, the growth mainly occurs along ⟨110⟩. However, at high H2O2 concentration, the reduction of Ag was not facet-selective. Using the developed method, we can prepare various bimetallic NCs (high aspect ratio NRs with abundant pinholes, nanoplates, and other NCs). The effect of the reconstruction process on the surface-enhanced Raman scattering (SERS) performance of NCs was investigated.
Highlights
Noble nanocrystals (NCs) attracted much attention in the past few decades due to their potential applications in cancer therapy, catalysis, optics, and electronics.[1−5] Among noble NCs, Ag−Au bimetallic NCs have excellent optical performances because both metals have fine-tuned localized surface plasmon resonance absorption in the visible and near-infrared region.[6−10] The structure and composition of Ag−Au bimetallic NCs strongly affect their physical and chemical properties.[11−17] the development of new methods and technology for controllably preparing Ag−Au bimetallicNCs is of great interest in both fundamental research of crystal growth and meeting the needs of practical applications
Several reports showed that H2O2 could trigger the shape transformation of Ag NCs from a sphere to a plate.[33−35] In these studies, the mechanism that H2O2 may serve as an oxidant to oxidize Ag nanospheres to Ag(I) and as a reductant to transform Ag(I) to Ag atoms for growing nanoplates was proposed
In the results shown above, we observed the morphological transition of NRs from a low aspect ratio to a high aspect ratio by the treatment of H2O2, which motivates us to analyze the mechanism of the morphological transition
Summary
Noble nanocrystals (NCs) attracted much attention in the past few decades due to their potential applications in cancer therapy, catalysis, optics, and electronics.[1−5] Among noble NCs, Ag−Au bimetallic NCs have excellent optical performances because both metals have fine-tuned localized surface plasmon resonance absorption in the visible and near-infrared region.[6−10] The structure and composition of Ag−Au bimetallic NCs strongly affect their physical and chemical properties.[11−17] the development of new methods and technology for controllably preparing Ag−Au bimetallic. The result showed that only etching was observed, and the regrowth of Ag did not occur because the formation of AgCl provided an energy barrier to reduce Ag(I) to Ag atoms (Figures S10 and S11) Another critical question is how Au decahedral NCs and the amount of H2O2 affect the product shape. Results show that the products evolved from the plate with a large transverse length to high aspect ratio NRs with the introduction of Ag NCs. Previously, Parnklang et al reported that only starting Ag nanospheres smaller than 10 nm could induce the regeneration of Ag atoms because the small size effect causes high reactivity and collision frequency.[33] Otherwise, only etching was observed and Ag(I) could not be reduced back to Ag atoms. The SERS enhancement should be due to the pinhole effect rather than the excitation of the laser
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