Organometallic synthesis of colloidal α-/β-NiAl nanoparticles and selective aluminum oxidation in α-Ni_1-xAl_x nanoalloys

A novel soft chemical synthesis of Ni_1-xAl_x nanoparticles (0.09 ≤ x ≤ 0.50) by cohydrogenolysis of [Ni(cod) ₂] (1) with [(AlCp*)₄] (2) or with [(Me ₃N)AlH₃] (3) in nonaqueous solution is presented (cod = 1,5-cyclooctadiene, Cp* = 1,2,3,4,5-cyclopentadienyl). The treatment of equimolar amounts of 1 and 2 in mesitylene solution under 3 bar of H₂ at 150°C gave a brown-black colloidal solution of intermetallic β-NiAl particles, characterized by transmission electron microscopy/energy dispersive X-ray analysis (TEM/EDX) and powder X-ray diffraction (XRD). The solution was stable under 3 bar of H₂ at 150°C for up to 8 h. The β-NiAl colloids were treated postsynthesis with ¹⁷O-enriched 1-adamantanecarboxylic acid (ACA) as a surface capping group, giving nearly monodisperse α-NiAl colloids that were stable under argon at room temperature for weeks. The coordination of ACA at the α-NiAl surface was studied by ¹⁷O NMR and IR spectroscopy. A series of α-Ni _1-xAl_x samples with a variety of compositions (x = 0.50, 0.33, 0.25, 0.17, 0.09) were prepared analogously, and the samples were characterized by means of elemental analysis (ICP), XRD, and TEM/EDX. Air oxidation of α-Ni_1-xAl_x nanoparticles leads to core-shell particles of the type (Al₂O₃) _δ/2@Ni_1-xAl_x-δ (0.09 ≤ x ≤ 0.50; x > δ). XRD, X-ray photoelectron spectroscopy (XPS), and X-ray absorption spectroscopy (XAS) analyses showed that the Ni core is protected against oxidation down to an Al content of about 10 atom %.