Synthesis of Au@Ni Core−shell Nanoparticles Using NaBH4 or L-ascorbic Acid as a Reducing agent

Au@Ni core–shell nanoparticles were synthesized using Au nanorods (NRs) as seeds. The resultant crystal structures were characterized using transmission electron microscopic (TEM) and TEM-energy dispersed X-ray spectroscopic (EDS) measurements, and ultraviolet (UV)-visible (Vis)-near infrared (NIR) extinction spectra. Ni shells were grown over AuNRs seeds by reducing Ni(NO3)26H2O using NaBH4 or L-ascorbic acid. When NaBH4 was used, AuNR@Ni nanoparticles having egg-like Ni shells and small spherical Ni aggregate shells were produced. On the other hand, AuNR@Ni nanoparticles having irregular shaped Ni shells were produced by using L-ascorbic acid. The nonoccurrence of layered epitaxial growth for AuNR@Ni particles was attributed to the presence of unstable high-index {250} or {5 12 0} facets in AuNRs and a large lattice mismatch between Au and Ni (13.6%). When Ni shells were formed over AuNRs, characteristic surface plasmon resonance bands of AuNR cores became either weak or disappeared, and a long tail band originating from Ni-shell component was observed in the 350–1150 nm region.