Controlled formation of ultrafine nickel particles on well-defined hematite particles

Ultrafine metallic Ni particles were precipitated on well-defined hematite particles of different shapes (spindles, pseudocubes, spheres, and platelets) by reducing nickel acetylacetonate with NABH₄ in the presence of these carriers (molar ratio of Ni/hematite, 1/20) in 2-propanol at 355 K for 10 min. The deposited amorphous Ni particles on spindle-type, pseudocubic and spherical hematites were about 4 nm in mean diameter and of narrow size distribution. They were free from nickel boride, as determined by X-ray diffractometry and electron microscopy in combination with heat treatment of the powder at 673 K for 2 h in N₂ atmosphere. On hexagonal hematite platelets, the deposited Ni particles were larger (i.e. about 20 nm) and of a much broader size distribution than those on the other types of hematite. It appears that the rough surfaces of the spindle-type, pseudocubic, and spherical carriers favor the stabilization of Ni particles in contrast to the smooth surface of the platelet-type hematite. The formation mechanism of the ultrafine Ni on colloidal hematite is discussed.