Scanning Tunneling Microscopy Study of Titanium Oxide Nanocrystals Prepared on Au(111) by Reactive-Layer-Assisted Deposition

Abstract

We report on an scanning tunneling microscopy study of the nanocrystallite phases of TiO(2) 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 H(2)O. 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.

Growth of TiO2(B)(001) on Au(111) by chemical vapor deposition

“…TiO 2 (B) is observed in the low coverage regime on both Pt(111) (at 2 MLE) and on Au(111) (at about 1.4 MLE), where the influence of the substrate is bigger. Furthermore, a surface-reconstructed rutile island has been observed on Au(111) after preparation with several techniques [24,34,39]. This is considered to be the thermodynamically favored TiO 2 structure on Au.…”

“…The component labeled "1-2" is related to components "1" and "2" in the Ti 2p 3/2 spectra and component "3" in the O 1s appears in concomitance with "3" in the Ti 2p 3/2 . Component "4" is needed to obtain a satisfactory fit of the O 1s spectra and its origin is uncertain [34][35][36].…”

Ultrathin Oxide Films on Au(111) Substrates

“…The oxidation process may vary for different methods, e.g. in reactive-layer-assisted deposition (RLAD), Ti is evaporated onto a Au(111) substrate pre-deposited with a layer of H 2 O or N 2 O and post-annealing then gives rise to TiO x structures [23,24]. RLAD has also been reported for the CeO x /Au(111) system where a pre-deposited H 2 O or NO 2 layer was used [25].…”

“…A variety of oxide thin films have been fabricated on the Au(111) surface including TiO x [18,23,24,27], FeO x [22,34], VO x [35][36][37][38], CoO [39][40][41], MoO 3 [28,42,43], MgO [19,[44][45][46], CeO x [25], ZnO [47] and BaO x [48]. The growth of ultrathin oxide films on the Au(111) surface usually lifts the herringbone reconstruction due to the strong interaction between the film and the substrate.…”

Alumina coated nickel nanoparticles as a highly active catalyst for dry reforming of methane

“…However, the formed shells often decreased the catalyst activity probably due to mass transfer limitation or active site coverage. Therefore, shell formation around active catalysts (also called "inverse catalysts") [25][26][27] in a more controlled manner is necessary, to inhibit catalyst sintering but also maintain catalytic activity. Moreover, the preparation of inverse catalysts can give the opportunity to enhance selectivity of catalysts by partial coverage of catalytic active but unselective sites.…”