Although gold nanorods capped with hexadecyltrimethylammonium bromide (CTAB) have been prepared through the seed-mediated method for their use in diagnostics and therapeutics, the toxicity of AuNRs@CTAB limits their practical applications in the biomedical field. In this work, the synthesis and tuning of gold nanorods at very low concentrations of CTAB (as low as 0.008 M) was successfully achieved by using the seed-mediated method. Furthermore, we managed to optimize the growth conditions by changing the amount of seeds, AgNO 3 , and/or HCl. At low CTAB concentrations, gold nanorods with tunable size and aspect ratio, high monodispersity, and high purity were obtained and studied by UV–vis spectroscopy, transmission electron microscopy, and Mie–Gans theoretical calculations. This work revealed a method of nanoparticle growth that may be extended to synthesize other nanomaterials such as Ag, Cu, Pd, and Pt at such low CTAB concentrations.
Bimetallic nanostructures are interesting materials for many applications ranging from plasmonics to catalysis. However, synthesizing bimetallic nanorods with good control over composition, morphology and maintaining an anisotropic shape is challenging. Here, we have shown a reproducible approach to the synthesize the bimetallic core-shell nanorods within a stabilizing mesoporous silica shell and with an Au core and a Pd metal shell with excellent control over their properties. Specifically, we demonstrate that the thickness and morphology of the Pd metal shell (rough/smooth) can be precisely tuned and studied the plasmonic properties of the resulting bimetallic nanostructures in detail. Our approach can be naturally extended to different or more than two metals leading to rod-shaped metal particles with highly tunable materials properties that are of interest in their use in optical and catalytic applications.
Demonstrating asymmetric (AuNR–Pt)–Ag tri-metallic nanostructures by a two-step seed-mediated method. The shell thickness was controlled by the amount of AgNO3.
Ir-CoFeB-based synthetic antiferromagnets (SAFs) are potential candidates as the free layer of the next-generation magnetic tunnel junctions (MTJs) for high speed and density memories due to their perpendicular magnetic anisotropy and strong interlayer exchange coupling. However, the field-free spin–orbit torque (SOT) switching of Ir-CoFeB-based SAFs has rarely been reported, especially in the Co/Ir/CoFeB system with high anti-interference capability and being readily integrated with MTJs. In this paper, SOT-induced magnetization switching and SOT efficiency in Co/Ir/CoFeB SAFs with perpendicular anisotropy and tunable exchange coupling are systemically investigated. A full field-free switching of perpendicular Co/Ir/CoFeB SAFs is realized by depositing them onto crystal miscut Al2O3 substrates, which induce a tilted magnetic anisotropy. Furthermore, by introducing crystalline MgO or amorphous HfO2/SiO2 as the seed layers, the source of the tilted magnetic anisotropy was proved to be from the transverse asymmetry caused by the crystal miscut. Moreover, the crystal miscut enhances the SOT efficiency. The findings provide an approach to reliable field-free switching and high SOT efficiency of Ir-CoFeB-based SAFs for memories as well as logics with low power, fast speed, and high density.
Gold nanorods (AuNRs) have shown excellent performance in various fields such as biocatalysis, optical imaging, chemistry and medicine. Although bimetallic nanostructures based on gold nanorods have been widely used, how to effectively control the growth of the second metal is still a big challenge. To solve this problem, we develop this method to control the symmetric overgrowth of Ag shell based on gold nanorods. Here, we use K2PtCl4 to be a precursor to form the AuNRs-Pt. And then AuNRs-Pt were used as seeds to form symmetric AuNRs-Pt-Ag by the addition of the AgNO3 precursor. The resulting products possess core-shell nanostructures and stronger localized surface plasmon resonances (LSPRs). Our approach can be widely extended to two or three metals in different shapes, which can be used in optical and catalytic applications.
The localized surface plasmon resonances (LSPRs) in plasmonic nanoparticles have been used in accelerating photocatalytic reactions under light illumination. To improve the catalytic performance, bimetallic nanoparticles composed of a plasmonic core and a catalytic shell, where LSPR-excited hot electrons and the intrinsic catalytic active sites work synergistically, have attracted much attention. Despite progress in designing bimetallic catalysts, balancing the strong LSPR and catalytic sites remains challenging. Here, a trimetallic nanostructure containing a gold nanorod (AuNR) core and a silver−platinum (AgPt) hollow alloy shell was designed and used as a plasmon-mediated photocatalyst for methylene blue reduction reaction. Specifically, the AuNRs covered by a thin layer of Ag (Au@Ag) were used as the template to deposit Pt, forming a Au@AgPt trimetallic nanostructure. By changing the Ag and/or Pt precursor concentration, the external layer could be varied from AgPt heterostructure and AgPt hollow alloy shell to AuAgPt alloy dendrites. Using methylene blue as a model system, the photocatalytic reduction reaction was studied by adding the obtained nanoparticles as catalysts under visible and near-infrared light irradiation. The optimal photocatalytic performance of the trimetallic nanoparticles was seen with the AgPt hollow alloy shell, and the reaction rate is ∼7 times higher than that of the reaction without catalysts and ∼3 times higher than that of monometallic AuNRs, bimetallic Au@Ag, and Au@Pt nanorods. Plasmon energy transfer from the AuNRs to the AgPt layer, which enhances the charge-carrier generation, is responsible for outstanding photocatalytic performance. The approach used here to synthesize Au@AgPt trimetallic nanostructures is suitable for the design of other multimetallic photocatalysts.
The roles of CTAB and Ag+ have been discovered and given us a deeper understanding of the seed-mediated method in the gold nanorods synthesis. Former work used binary surfactants CTAB + NaOL (sodium oleate) to greatly improve the dimensional tunability and monodispersity of gold nanorods. However, they only used a few of the concentration combinations of the binary surfactants, and the influence of NaOL under this method has not been systematically studied. In this work, we carried out systematic experiments under the variation of NaOL and used transmission electron microscopy and UV–vis-near-infrared spectroscopy to monitor the growth process of the gold nanorods. The results showed that the NaOL contributed to the symmetry breaking process. We discovered the ideal ranges of NaOL concentration under different concentrations of CTAB (10–40 mM). Lower concentrations of NaOL produced many impurities, such as Au spheres, while higher concentrations of NaOL led to the decrease of monodispersity of the obtained gold nanorods. A growth model based on the balance of diffusion/reduction of the growth solution has been proposed in order to explain the formation of the gold nanorods.
The method by using hexadecyltrimethylammonium bromide (CTAB) and sodium oleate (NaOL) as binary surfactant mixture to synthesize high quality gold nanorods (AuNRs) has been one of the most widely used methods in this field. But the specific influence on the obtained AuNRs of these two surfactants has not been systematically studied in previous works. Here, we carry out series of experiments and characterized the results. By using the UV-VIR spectrophotometer and transmission electron microscope, we found the higher CTAB concentrations will lead to the larger aspect ratios of the obtained AuNRs. In addition, the final products under this method are more sensitive to the NaOL concentration.
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