Galvanic Exchange in Colloidal Metal/Metal-Oxide Core/Shell Nanocrystals

While galvanic exchange is commonly applied to metallic nanoparticles, recently its applicability was expanded to metal-oxides. Here the galvanic exchange is studied in metal/metal-oxide core/shell nanocrystals. In particular Sn/SnO₂ is treated by Ag⁺, Pt²⁺, Pt⁴⁺, and Pd²⁺. The conversion dynamics is monitored by in situ synchrotron X-ray diffraction. The Ag⁺ treatment converts the Sn cores to the intermetallic Ag Sn (x ∼ 4) phase, by changing the core's crystal structure. For the analogous treatment by Pt²⁺, Pt⁴⁺, and Pd²⁺, such a galvanic exchange is not observed. This different behavior is caused by the semipermeability of the naturally formed SnO₂ shell, which allows diffusion of Ag⁺ but protects the nanocrystal cores from oxidation by Pt and Pd ions.