Kinetic Understanding of Hydrogen-Mediated Ni Growth: From Metal Precursor Reduction to Branched Nanostructure Formation

Small
gas molecule such as H2 has been widely reported
to be used as the structure-directing agent that assists synthetic
processes of diverse structured metal nanocrystals. Herein, we systematically
investigated the H2-mediated growth mechanisms of branched
Ni (Ni-BN) using a combined experimental and theoretical approach
(molecular dynamics). Two types of kinetic models describing Ni precursor
reduction and Ni nuclei overgrowth forming branched Ni-BN have been
established, which quantitatively illustrate kinetic roles of H2 played during Ni-BN synthesis. On one hand, the H2 significantly promoted Ni precursor reduction rate with the value
being about two times higher than that without H2; on the
other hand, the dissociatively adsorbed atomic H on Ni nuclei greatly
affected the deposition kinetics (deposition rate and deposition equilibrium)
of the reduced Ni atoms. Synergistically, the faster Ni precursor
reduction rate associated with enhanced Ni atom deposition kinetics
resulted in the overgrowth of Ni nuclei along the ⟨111⟩
direction and eventually generating the branched nanostructures. Finally,
a MATLAB program was further developed, which can well simulate nanostructure
Ni-BN. Present work quantitatively illustrated H2-mediated
Ni-BN growth mechanism at the atomic level, which contributes to shedding
more light on small gas molecule directed metal nanocrystals (MNCs)
growth.