Self-confined dimension of thermodynamic stability in Co-nanoparticles in fcc and bcc allotropes with a thin amorphous Al2O3 surface layer Original Research Article

Co nanoparticles with a thin amorphous Al2O3 surface layer form in self-confined dimension in 50–90 nm diameter (in shape of a thin platelet or sphere) in fcc or bcc allotrope on co-reducing Co2+ cations dispersed in a mesoporous Al2O3 in H2 gas at 850–1150 K. A thin surface interface, which is formed of Al2O3 with a metal surface, insulates Co particles showing 5% tunnel magnetoresistance at 295 K. It conducts a controlled growth of Co particles in these allotropes. In this case, as per smaller critical size r∗=3.17 and 0.41 nm than 3.85 nm in hcp Co, they succeed to nucleate and grow as stable particles prior to the hcp Co nucleation. Thickness in interface and Al2O3 surface layer grows limited to t≤2rc (rc∼1.9 nm the critical size of nucleation and growth of Al2O3 into a stable crystallite) with the particle growth and in turn maintains its moderate growth rate by inhibiting a fast migration of reaction species through it during the reaction. As a result, fcc or bcc structure grows over an enhanced scale, up to ≤90 nm (∼22 nm otherwise), without a change to hcp structure. Crystal structure, size, morphology, and topology of Al2O3 encapsulated Co particles are analyzed with X-ray diffraction, electron micrograph, and XPS studies. Thermodynamic stability of particles in fcc and bcc allotropes is modeled with their total surface energy, interface energy, and Gibbs free energy. A thin monatomic interface improves r* by 146, 19 and 16% in bcc, fcc and hcp allotropes, respectively. fcc (bcc) structure in spherical shape retains up to a critical dimension of its radius R=37.1 (0.71) nm. An anisotropic shape results in a further larger value of it. At limited thickness δ≤2R, a thin platelet particle thus can grow over an infinite dimension.