Thermodynamic and kinetic roles of H2 in structure evolution of urchin-like Co: A density functional theory study

Abstract Small gas molecules acting as good capping agents play important roles in controlling the morphologies and surface structures of metal nanocrystals. In the present work, the thermodynamic and kinetic roles of H2 molecules in the morphology of Co nanocrystals were systematically studied by density functional theory (DFT). The Gibbs surface free energies of Co(100), Co(110), and Co(111) at different hydrogen surface coverages were determined by ab initio thermodynamics. The phase diagram of stable H coverage on each plane was obtained and morphology evolution of the Co nanocrystals with various surface hydrogen coverages was predicted by the Wulff construction. Addition of H2 changes the facet stability, generating diverse morphological Co nuclei. The kinetic role of H2 in adatom Co surface diffusion at different H coverages was investigated by DFT. The results suggest that surface H hinders Co surface diffusions, except for Co(100) at 0.56 monolayer coverage. The projected density of states gives deeper insight into the electronic structures of Co adatoms with addition of the surface H atoms, which affect its surface diffusion ability.