X-ray diffraction and X-ray photoelectron spectroscopy studies of stabilised cobalt nanoparticles with a thin Al 2 O 3 surface layer

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 2r_c≈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.