The synthesis and SERS study of gold-palladium asymmetric nanostructures

Gold-palladium two-component nanostructures have attracted extensive attentions due to their synergetic coupling between optical and catalytic properties, and have shown important application potential in solar energy conversion and storage, photoelectric catalysis, photoelectric devices, biological imaging, medical diagnosis and treatment and other fields. However, the synthesis of gold-palladium nanomaterials with synergistic properties still face great challenges. The reason is that the palladium can epitaxially overgrow on the Au seed surface due to the lattice match. It means that the growth of palladium on Au seeds often leads to symmetric core-shell gold-palladium structures (Au@Pd). The Pd shell can greatly prevent the plasmonics of Au core by blocking light and deteriorating the surface plasmon resonance performance through the interface damping. One strategy is to grow ultrathin Pd shell (with thickness smaller than 5 nm) to realize the synergistic coupling between plasmonics and catalysis. The other more challenging way is to break the structural symmetry to prepare Janus-type gold-palladium asymmetric nanostructures. The asymmetric spatial arrangement of the two components within one nanoparticle will significantly improve the synergistic coupling effect. In this study, two methods for preparation of gold-palladium nanostructures in solution and on solid surface respectively were developed. The concave cuboid and dumbbell-shaped nanostructures with asymmetric palladium distribution on Au nanorods were prepared by adjusting the concentration of copper ions in the Au-nanorod seed solution for the first time. It was demonstrated that copper ions can compete with CTAB to adsorb on the surface of Au nanorods, which can regulate the interfacial energy of palladium deposition on the surface of Au nanorods. Therefore, the changing of gold-palladium interfacial energy caused the growth mode to be gradually transformed from the epitaxial growth to discontinuous patch on the seed surface, leading to the generation of an array of gold-palladium asymmetric nanostructures, including gold-palladium concave cuboidal nanoparticles and dumbbell-shaped nanoparticles, where palladium was mainly grown on the two ends of Au nanorods. Besides the solution-mediated synthesis, we also developed substrate-based techniques combined colloidal chemistry to prepare gold-palladium nanostructures. This method combined the merits of substrate-based self-assembly techniques and powerful colloidal chemistry to prepare a rich class of nanostructure architectures with unique properties. The monolayer of Au-octahedron was prepared at an air-solid interface and then transfer to a silicon surface. The substrate-immobilized self-assembly monolayer was used as seeds to induce the growth of palladium by dipping into the growth solution. A series of gold-palladium nanostructures with different palladium loadings were prepared on the solid surface with a monolayer of Au-octahedron as seeds by controlling the palladium content in the growing solution. Because of the steric hindrance effect of solid surface, palladium is preferred to nucleate on the surface of Au octahedron away from solid base. Therefore, the fabricated gold-palladium nanostructures have asymmetric spatial distribution of the gold and palladium components as was demonstrated by SEM and XRD analysis. Through the study of the SERS performance of the gold-palladium nanostructures under different palladium loadings, the surface-supported gold-palladium nanostructures with optimal SERS performance was obtained. The catalytic hydrogenation of p-nitrothiophenol into p-aminothiophenol through the optimal gold-palladium asymmetric nanostructure was monitored by in-situ SERS. The experimental results showed that the asymmetric palladium nanostructures prepared in this study realized the synergistic coupling between catalytic and SERS properties. It is expected that the method to synthesize asymmetric nanostructure developed here will play an important role in the field of heterogeneous catalysis.