Analysis to reveal dynamical and correlated atomic displacements on gold surfaces depending on various environments
Authors: Aso Ryotaro , Ogawa Yohei , Yoshida Hideto , Takeda Seiji 
Published: December 2016

Gold has a wide range of important applications, such as gold nanoparticles (AuNPs) for catalyst and various gold nanostructures for sensing technology. For the applications, it is necessary to understand the chemical reaction on gold surface in actual environments, at atomic resolution, and at high time resolution. Though gold is chemically stable, it is known that the supported AuNPs of the size smaller than about 5 nm exhibit higher catalytic activity. This partially originates from the small curvature of nanoparticle surfaces, so the gold surface structures such as facets, edges and corners could change in certain environments. Here, we analyze
in‐situ
images of the surface of bulk gold with different curvatures that are acquired using spherical aberration (Cs)‐corrected environmental transmission electron microscopy (ETEM) to derive dynamical and correlated atomic displacements in various environments.

TEM characterization is carried out by Cs‐corrected Titan ETEM G2 apparatus [1], where the accelerating voltage is 300 kV. Figure 1 shows the (E)TEM images of the gold surface with relatively small curvature in various environments (vacuum, oxygen, hydrogen, and nitrogen). In vacuum, the facets of {100} and {111} and the step edge are seen clearly. In contrast, the gold surface is rough in oxygen (oxygen partial pressure:
P
O2
= 100 Pa), where the surface gold atoms move continuously. In hydrogen and nitrogen (
P
H2
,
P
N2
= 100 Pa), the surface is facetted as well as that in vacuum and the gold atoms hardly move on the surface.

To shed light on the change of the gold surface in oxygen, we further investigated the dynamics of surface gold atoms in oxygen by high resolution
in‐situ
ETEM observation with an advanced image acquiring system (Figure 2). By tracking the individual gold atoms in time‐lapse images, we found that gold atoms at the step edge readily migrate on the surface compared to those of the terrace surface. We further found that as the oxygen partial pressure decreases, the gold surface becomes more stable structure. We also investigated the electron irradiation effect behind the dynamical changes of surface structures in gas environments, where the current density of the electron beam is varied from 25 A/cm
2
to 0.1 A/cm
2
. As the current density of the electron beam decreases, the migration of the gold atoms in the surface moderates. The gold surface remains rough even in the very small current density of 0.1 A/cm
2
. Though the electron beam affects the structural changes of the gold surface in oxygen, the analysis result suggests that the surface of bulk gold could interact with oxygen gas molecules to some extent regardless of the electron beam.

The dynamic surface structures of metals in actual environments most likely originate from the interaction between gas molecules and metal atoms on the surface. Full understanding of the dynamical behavior of metal surface in various environments crucially important for application to the nanomaterials and nanodevices. To this end, it is useful to detect the behavior of individual metal atoms at higher temporal resolution with high detection efficiency of electrons in atomic resolution ETEM. We have already successful in capturing the extraordinary atomic migration on gold surfaces by advanced
in‐situ
Cs‐ETEM. We will show some movies in our presentation that show the dynamics of individual surface gold atoms in various environments by time resolution better than 50 ms.