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- x Al x nanoparticles (0.09 ≤ x ≤ 0.50) by cohydrogenolysis of [Ni(cod) 2 ] ( 1 ) with [(AlCp*) 4 ] ( 2 ) or with [(Me 3 N)AlH 3 ] ( 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 H 2 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 H 2 at 150 °C for up to 8 h. The β-NiAl colloids were treated postsynthesis with 17 O-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 17 O NMR and IR spectroscopy. A series of α-Ni 1- x Al 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- x Al x nanoparticles leads to core−shell particles of the type (Al 2 O 3 ) δ /2 @Ni 1- x Al 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 %.