X-ray diffraction and X-ray photoelectron spectroscopy studies of stabilised cobalt nanoparticles with a thin Al2O3 surface layer

Abstract

Cobalt nanoparticles supported in a self confined crystallite dimension of diameter D =34–41 nm, with a thin Al2O3 surface layer were synthesised in a hydrogen gas environment at 975–1125 K temperature by co-reduction of Co2+ ions dispersed in an amorphous AlO(OH).αH2O gel. During heating, the gel decomposes at low temperatures, between 425 and 675 K, resulting in a dispersed high energy structure of Co2+ cations amorphous Al2O3 matrix. An effective reconstructive Co2++H2→Co+2H+ co-reduction reaction occurs at a temperature as low as 975 K, producing Co nanoparticles encapsulated in thin amorphous Al2O3 layers. The formation and existence of an Al2O3 layer (with a limited thickness up to terc, where 2rc≈4.28 nm is the critical dimension in a stable crystallite) over a growing Co particle controls the dynamics of its growth in a self confined dimension in a high energy state of fcc or bcc allotrope. The results pertaining to the Co nanoparticles stabilised by Al2O3 surface film are analysed and discussed in terms of thermal analysis, X-ray diffraction, microstructure and X-ray photoelectron spectroscopy.