Pt-Catalyzed Growth of Ni Nanoparticles in Aqueous CTAB Solution

NiO-passivated nickel nanoparticles were grown on small platinum seeds, which served as catalysts for the reduction of Ni 2+ with hydrazine, in aqueous solutions of the cationic surfactant cetyltrimethylammonium bromide. Control of particle size was achieved through manipulation of the reduction conditions, among which the molar ratio between nickel salt and platinum seeds was found to be dominant. HRTEM, XPS, and XRD show the polycrystaline structure of the particles and the presence of metallic Ni, as well as NiO x and Ni(OH) 2 . The magnetic properties were studied as a function of size and reveal superparamagnetic response at room temperature for 14 nm diameter nanoparticles and ferromagnetic behavior for the nanoparticles with diameters larger than 24 nm. The ferromagnetic nature of bigger Ni particles was found to induce chain formation, even in the absence of external magnetic fields, which is demonstrated through in situ coating with silica, which results in fossilized nanoparticle chains. Additionally, we propose a mechanism for the growth of nickel on the platinum nanoparticle seeds surface.