Pit-templated synthesis and oxygen adsorption properties of gold nanostructures on highly oriented pyrolytic graphite

Etch pits on highly oriented pyrolytic graphite (HOPG) were used as templates for the formation of gold nanostructures, which were characterized using STM (scanning tunneling microscopy), AFM (atomic force microscopy), XPS (X-ray photoelectron spectroscopy) and TOF-SIMS (time-of-flight secondary ion mass spectrometry). Controlled production of defects on HOPG by ion bombardment leads to the formation of nanometer-size etch pits by thermal oxidation in a controlled fashion. Etch pits act as nucleation and growth sites for gold nanostructures and also play a role in fixing gold nanostructures in place for study by scanning probe techniques. Hexagonal-shaped, flat-topped, and other gold nanostructures were formed in multilayer etch pits. XPS showed that the atomic percentage of gold on a pitted HOPG surface was higher than that on an unpitted HOPG surface for the same amount of deposited gold after annealing at an elevated temperature, indicating that more HOPG surface area was covered by gold on pitted HOPG than on unpitted HOPG. XPS O 1s spectra showed that there are two chemical states of oxygen on gold-covered HOPG samples, corresponding to oxygen adsorbed on gold nanostructures and oxygen adsorbed on HOPG. Oxygen adsorbs molecularly onto surfaces of gold nanostructures, where the coverage of molecular oxygen on gold nanostructures is 0.25 ML for adsorption at room temperature and atmospheric pressure. In contrast, no oxygen adsorption was observed on the surface of a Au(111) single crystal as reported in the literature. This relatively high molecular oxygen coverage is explained by the small size (20-50 nm) of the gold nanostructures. Additionally, two-dimensional gold nanostructure arrays were produced by depositing gold onto pit-patterned HOPG surfaces: These patterned gold nanostructures on HOPG have potential applications in fields such as catalysis, sensors, and nanoelectronics.