Elucidation of Cu–Zn Surface Alloying on Cu(997) by Machine-Learning Molecular Dynamics

The Cu–Zn
surface alloy has been extensively involved in
the investigation of the true active site of Cu/ZnO/Al2O3, the industrial catalyst for methanol synthesis which
remains under controversy. The challenge lies in capturing the interplay
between the surface and reaction under operating conditions, which
can be overcome given that the explicit dynamics of the system is
known. To provide a better understanding of the dynamic of Cu–Zn
surface at the atomic level, the structure and the formation process
of the Cu–Zn surface alloy on Cu(997) were investigated by
machine-learning molecular dynamics (MD). Gaussian process regression
aided with on-the-fly learning was employed to build the force field
used in the MD. The simulation reveals atomistic details of the alloying
process, that is, the incorporation of deposited Zn adatoms to the
Cu substrate. The surface alloying is found to start at upper and
lower terraces near the step edge, which emphasize the role of steps
and kinks in the alloying. The incorporation of Zn at the middle terrace
was found at the later stage of the simulation. The rationalization
of alloying behavior was performed based on statistics and barriers
of various elementary events that occur during the simulation. It
was observed that the alloying scheme at the upper terrace is dominated
by the confinement of Zn step adatoms by other adatoms, highlighting
the importance of step fluctuations in the alloying process. On the
other hand, the alloying scheme at the lower terrace is dominated
by direct exchange between the Zn step adatom and the Cu atom underneath.
The alloying at the middle terrace is dominated by the wave deposition
mechanism and deep confinement of Zn adatoms. The short propagation
of alloyed Zn in the middle terrace was observed to proceed by means
of indirect exchange instead of local exchange as proposed in the
previous scanning tunneling microscopy (STM) observation. The comparison
of migration rate and activation energies to the result of STM observation
is also made. We have found that at a certain distance from the surface,
the STM tip significantly affects the elementary events such as vacancy
formation and direct exchange.