Charging and Chemical Reactivity of Gold Nanoparticles and Adatoms on the (111) Surface of Single-Crystal Magnetite: A Scanning Tunneling Microscopy/Spectroscopy Study

Abstract

We present a scanning tunneling microscopy (STM)/scanning tunneling spectroscopy (STS) study of a model catalyst system consisting of supported gold nanoparticles on a reduced Fe3O4(111) surface in ultrahigh vacuum. Gold forms two electrically distinct nanoparticles on an iron oxide surface upon annealing multilayer Au/Fe3O4(111) at 500 °C for 15 min. I (V) curves taken via STS measurements show that large gold nanoparticles (∼8 nm) exhibit a metallic electronic structure and, thus, are likely neutral. Single gold adatoms appear to be strongly bonded to the oxygen sites of the Fe3O4(111) surface, and tunneling electrons are observed to flow predominantly from the STM tip to the Au adatoms and into the oxygen sites of the surface. The site-specific adsorption of the gold adatoms on oxygen surface atoms and the size-sensitive nature of the electronic structure suggest that Au adatoms are likely positively charged. When this Au/Fe3O4(111) system is dosed with CO at 260 K, adsorption of CO molecules normal to the surface atop the gold adatom sites takes place. CO adsorption on the large Au nanoparticles (∼8 nm) could not be confirmed by STM. These observations indicate that nonmetallic, positively charged Au species may play a key role in reactions involving CO, such as the CO oxidation and the water−gas-shift reaction on Au/metal oxide surfaces.