PNNL

Abstract

The selective adsorption of small molecules over specific facets plays an important role in the morphology controlled synthesis of metal nanocrystals. In the present work, two kinds of branched nickels, i.e., secondary branching nanoparticles and multipods, have been successfully synthesized with aid of hydrogen. High-resolution transmission electron microscopy (HRTEM) reveals that the synthesized branched nickels with diverse morphologies are mainly due to different growth rates along , and directions: (i) the competitive over-growth along and directions results in secondary branching nanoparticles; (ii) the over-growth only along direction produces multipods. The synthesis mechanisms are further elucidated using ab initio thermodynamics based on density functional theory (DFT) calculations. It is found that Gibbs surface free energies of three low index facets, i.e., (100), (111) and (110), are significantly affected in the presence of hydrogen. At high hydrogen pressure of 14 bar, the multipods are grown under kinetic controlled regime. This is confirmed with DFT calculated diffusion activation barrier of a Ni atom on Ni(111) is lower than that on Ni(110) at high hydrogen coverage condition, implying faster grow rate of the Ni(111) over the Ni(110) facet. The DFT based Wulff construction predicted Ni nanocrystals match well with experimental HRTEM results. Compared with nickel nanoparticles synthesized in the absence of hydrogen, the hydrogen assisted branched Ni nanomaterials, especially for Ni multipods, show superior catalytic activities for hydrogenation reactions of acetophenone and nitrobenzene.