Surface structure and topology in surface stabilized Co-nanoparticles with a thin Al2O3 amorphous layer

Abstract

Heating an amorphous Co2+:AlO(OH)·αH2O gel under a reducing atmosphere of pure H2 gas at 700–850°C temperature results in surface stabilized Co-metal nanoparticles with a thin Al2O3 ceramic surface layer. At early temperatures during heating, the gel decomposes and disperses in a refined structure in divided Co2+ groups through a matrix of Al2O3 (amorphous). A reconstructive Co2++H2→Co+2H+ reaction operates in the divided groups to result in isolated Co-particles. Formation and existence of Al2O3 layer (in a limited thickness t≤R0, with R0∼4.28nm the critical Al2O3 dimension to grow as a stable crystallite) over growing Co-particles control the process in a high-energy metastable fcc or bcc Co allotrope structure. Average crystallite size thus hardly grows to be as big as 41nm. A large value of surface energy σ=0.790J/m2 in Al2O3, in comparison to 0.234J/m2 in fcc or 0.279J/m2 in hcp Co, inhibits a moderate diffusion of surface atoms and in turn controls the nucleation and growth in small Co-particles. Otherwise, a particle of pure Co metal grows rapidly in the hcp bulk structure. The results of the Al2O3 surface modified fcc and bcc Co are analyzed with X-ray photoelectron spectroscopy in correlation of X-ray diffraction and microstructure.