Preparation And Migration Of Silver Particles On Ordered Surface

Silver is a well-known catalyst for partial epoxidation of ethylene and selective oxidation of methanol to formaldehyde. It is too complicate in real catalyst system to understand and manipulate. The size, structure of the silver clusters and the oxygen species are closely related to the reactivity and selectivity. Recent results illustrate that not only the clusters but also the interaction between clusters and supports played an important role in the activation of a catalytic process, as the Ag/SiO2 catalyst was the highest reactivity in CO oxidation, In our experiments, we chose Ag/HOPG and Ag/Si as model catalytic systems to investigate the properties of the silver cluster. For metal deposited on a weakly interacting substrate like graphite, the impinging atoms are mobile on the surface until they desorbed from surface or are trapped at nucleation sites. Most silver atoms pinned at the edge of steps. We use artificial oxidized pits as holding centers for silver clusters. The STS data verifies that the 3D Ag clusters were non-metal.The growth and migration behaviors of Ag clusters on Si (111)-7 7 surface were investigated using Scanning Tunneling Microscopy (STM) and Auger Energy Spectroscopy (AES) at room temperature. Three types of Ag clusters were observed by topography in the early stages of their growth. The distribution of different clusters in faulted half unit cell and unfaulted half-unit cells was distinctive. The STS in UHV system showed that the band gap of the Ag cluster decreased with the cluster size at room temperature, In further deposition, Ag clusters touch each other through the Ag atoms crossing the dimmer row to form wet layer and Ag islands formed on top of wet layer. AS-t curve illustrated silver grew on Si (111)-7x7 surface according to SK growth mode.At ultra-low coverage, the size and shape of the Ag cluster changed dramatically after exposure to oxygen: Some of the Ag atoms were removed and left ad-ins oxygen, larger Ag clusters formed, and the silver cluster distinctly changed the adsorbed oxygen species during long term observation in situ. At higher coverage, the distribution of Ag clusters' size exhibited Ostwald ripening by interaction with O2- The clusters appear to move easily on the surface until being trapped by adsorbed atomic oxygen and pinned around Si-O sites during the scanning after exposure to oxygen. It was also found that the strong interaction between silver cluster and adsorbed oxygen species on the Si surface causes a distinct change of the property of adsorbed oxygen, i.e. a change of surface ins-ins oxygen into ad-ins oxygen species.