X-ray diffraction and X-ray photoelectron spectroscopy studies of stabilised cobalt nanoparticles with a thin Al_O_3 surface layer

摘要

Cobalt nanoparticles supported in a self confined crystallite dimension of diameter D =34-41 nm, with a thin AI_2O_3 surface layer were synthesised in a hydrogen gas environment at 975-1125 K temperature by co-reduction of Co~(2+) ions dispersed in an amorphous AIO(OH).alpha H_2O gel. During heating, the gel decomposes at low temperatures, between 425 and 675 K, resulting in a dispersed high energy structure of Co~(2+) cations amorphous AI_2O_3 matrix. An effective reconstructive Co~(2+) + H_2->Co + 2H~+ co-reduction reaction occurs at a temperature as low as 975 K, producing Co nanoparticles encapsulated in thin amorphous AI_2O_3 layers. The formation and existence of an AI_2O_3 layer (with a limited thickness up to t<=2r_c, where 2r_capprox =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 AI_2O_3 surface film are analysed and discussed in terms of thermal analysis, X-ray diffraction, microstructure and X-ray photoelectron spectroscopy.