Scanning tunneling microscopy study of titanium oxide nanocrystals prepared on Au(111) by reactive-layer-assisted deposition

We report on an scanning tunneling microscopy study of the nanocrystallite phases of TiO2 formed via reactive-layer-assisted deposition in ultrahigh vacuum. The synthesis used reaction of a thin layer of water, on a Au(111) substrate at 130 K, with low-coverage vapor-deposited Ti. The effects of annealing temperature and reactant coverage were investigated. Large-scale (> 20 nm) patterns in the surface distribution of nanoparticles were observed with the characteristic length-scale of the pattern correlating with the thickness of the initial layer of H2O. The phenomenon is explained as being due to the formation of droplets of liquid water at temperatures between 130 and 300 K. After the surface was annealed to 400 K, the individual titania nanoparticles formed by this process had diameters of 0.5-1 nm. When the surface was annealed to higher temperatures, nanoparticles coalesced and for annealing temperatures of 900 K compact nanocrystals formed with typical dimensions of 5-20 nm. Three distinct classes of nanocrystallites were observed and their atomic structure and composition investigated and discussed.