Significance of sensitization process in electroless deposition of Ni on nanosized Al2O3 powders

The electroless deposition of Ni on nanosized α-Al2O3 powders of about 150 nm diameter has been studied by employing SnCl2 sensitization and PdCl2 activation approach. The bath for electroless deposition was prepared by using NiCl2·6H2O, C6H5NaO7·H2O, NH4Cl and NaH2PO2·H2O, and the deposition was carried out at a bath temperature of 70 °C with a pH value of 8.5. The morphology, microstructure and phase structure of Ni-coated α-Al2O3 nanopowders were characterized with scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffractometry (XRD), respectively. It is found that, the sensitization process followed by repetitive resining greatly affects the morphology of Ni deposited in the form of nanoparticles onto the ultrafine alumina particles, different from the conventional continuous electroless film/coating deposited on coarser particles. The resining of the SnCl2 sensitized ultrafine α-Al2O3 particles prior to PdCl2 activation process leads to varied amount of deposited Ni particles with diameters of 50–80 nm on the Al2O3. It is revealed that, subsequent reduction process (activation) from Pd2+ to Pd is linked to the original sites of Sn2+ by simultaneous oxidation process from Sn2+ to Sn4+ on the previously sensitized α-Al2O3 nanopowders. Consequently, the form of deposited Ni correlated closely to the surface distribution of reduced Pd which may be continuous or discrete determined predominantly by the density of adsorbed Sn2+ on the powders during sensitization process. It demonstrates a possibility of depositing different distribution structures of metals on nanosized α-Al2O3 powders through controlling SnCl2 sensitization process.