Au nanobipyramids (NBPs) are important nanostructures which attract much attention due to their unique structure, optical, and catalytic properties. The controlled synthesis of Au NBPs and corresponding mechanistic study are highly desirable for both fundamental research and practical applications. Herein, we demonstrate a strategy that large sized Au decahedra with well-defined shape act as seeds for growing NBPs. Furthermore, through using different sized decahedra seeds with edge from 25 to 49 nm, various sized NBPs can be easily prepared (longitudinal length from 110 to 210 nm; transverse length from 36 to 70 nm). Our study provides hard evidence for the growth of NBPs that they surely stem from the overgrowth on penta-twinned decahedra. Because these used large size seeds have well-defined shape and structure, the growth of the NBPs can be easily determined. Results show that the formation of NBPs is primarily determined by the molar ratio of Au(3+) and Au seeds (MRAA). MRAA less than 4 only causes size enhancement and no significant shape change. In cases of MRAA higher than 4 and lower than 8, quasi-nanorods are produced. When MRAA range from 8 to 10, NBPs form and the yield is higher than 90%. The effect of reaction time and temperature also are vital to the growth of NBPs. These prepared NBPs are found to exhibit excellent surface enhanced Raman scattering (SERS) performance because of many present hotspots, edges, steps, and tips on their surfaces.

Au nanostructures attract much attention due to their potential applications in many fields. The controlled synthesis is critical to their properties modulation and applications. AgNO3-assisted synthesis is a widely used method for controllably preparing Au nanostructures in aqueous system. Herein, the effect of AgNO3 on the growth of Au nanostructures in polyol is studied. We observe an unusual effect that AgNO3 can induce the formation of pentatwinned Au nanostructures (nanorods and decahedra) and block the growth of Au nanorods. More interestingly, this blocking effect can be tuned through controlling the amount of AgNO3. A moderate amount of AgNO3 facilitates the formation of Au nanorods. A large amount of AgNO3 completely blocks the growth of nanorods and favors the formation of high quality decahedra (decahedra can be considered as nanorods with 0 nm longitudinal length). Besides, this blocking effect also allows preparation of different high-index-faceted Au nanobipyramids. These prepared Au nanostructures further serve as starting templates to fabricate other heterostructured Au/Ag nanomaterials, such as Ag-Au-Ag segmental nanorods, Au@Ag core-shelled nanostructures. The prepared nanostructures exhibit size- and structure-dependent catalytic performance in the reduction of p-nitrophenol to p-aminophenol by sodium borohydride.

We present a general route for synthesizing M@ZnO nanoparticles (NPs) by using ascorbic acid (AA) to induce deposition of ZnO on various shaped and structured cationic-surfactant-capped NP surfaces (noble, magnetic, semiconductor, rod-like, spherical, cubic, dendrite, alloy, core@shell). The results show that the complexing (AA and Zn(2+)) and cooperative effects (AA and CTAB) play important roles in the formation of polycrystalline ZnO shells. Besides, the growth kinetics of M@ZnO was systematically studied. It was found that the slow growth rate favors the successful formation of uniform core@ZnO NPs with relatively loose shells. An appropriate growth rate allows achieving high quality M@ZnO NPs with dense shells. However, very fast growth causes significant additional nucleation and the formation of pure ZnO NPs. This general method is suitable for preparing M@ZnO using seed NPs prepared in both water and organic phases. It might be an alternative route for functionalizing NPs for bioapplications (ZnO is biocompatible), modulating material properties as designed, or synthesizing template materials for building other nanostructures.

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