Sonochemical coating of nanosized nickel on alumina submicrospheres and the interaction between the nickel and nickel oxide with the substrate

The preparation and coating of nickel on amorphous and crystallized alumina were carried out by employing a sonochemical method. The interaction between the nickel as well as its oxide and the alumina core was then studied by TEM, XRD, DSC, DRS, FTIR, and magnetization measurements. The following were found: (1) Amorphous alumina can provide a great number of active sites for reaction with nickel and can yield a good coating effect in which most of nickel is adhered to the alumina surface tightly, while in the case of the crystallized alumina as substrate, most of nickel particles is distributed in the free space among the alumina submicrospheres. (2) As compared to the unadhered nickel, the adhered nickel has a strong interaction with the alumina core, which can retard the crystallization of elemental nickel and, conversely, promote the formation of the spinel phase NiAl₂O₄. (3) The first stage of the interaction between the nickel and the alumina may be through the isolated hydroxyl groups to form a kind of interface Ni-O-Al bond. The connection positions further become nucleation centers for elemental nickel. After the sample is heated to high temperatures, DRS and IR results showed the diffusion of nickel ions into the vacant tetrahedral sites in alumina. At higher temperature, an inversion process -the substitution of Ni²⁺ ions for part of Al³⁺ at octahedral sites has taken place, and it results in the final formation of the disordered spinel phase. (4) Magnetization measurements show that the as-prepared sonication products are superparamagnetic due to the ultrafine nature of nickel particles.