Nanocluster defects and their properties on TiO2(110) and (001) surfaces.
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TiO2(110) and (001) have been investigated by low energy electron diffraction (LEED) and scanning tunneling microscopy (STM). A nearly bulk-like (1 × 1) TiO2(110) was produced after cycles of Ar+ sputtering and surface annealing at moderate conditions. However, with an increasing number of preparation cycles, the partially reduced surface was then obtained. The surface was heterogeneous with the formation of dispersed nanometer-sized bright strands terminated with bright nanoclusters on the (1 × 1) terraces. They were identified as substoichiometric (TiOx, x < 2) and stoichiometric defects, respectively. Upon the adsorption of gold, the stoichiometric nanoclusters were observed to be the most active sites for the initial nucleation of Au and the subsequent formation of nanoparticles. The first principles calculations indicated that both geometric and electronic effects of the under-coordinated O atoms of the nanocluster with surface O atoms were responsible for exceptionally strong binding sites for Au nanoparticles. This atomistic model suggests a potentially active site for low temperature CO oxidation by Au nanoparticles. TiO2(001) under similar preparation conditions revealed the so-called latticework reconstruction: row-like linear structures running along  and [1-10] directions. Each row further consisted of bright spots separated by 6.5 Å. In some areas, the rows were separated by 13 Å consistent with the lattice domains of (2r2xr2)R45° observed by LEED. In other areas, the rows were distributed in a more random fashion. Thus various nearest neighbor distances and relative heights of the rows formed different microfacets. From the LEED and STM data, a stoichiometric nanocluster is proposed as the basic building blocks for the latticework reconstruction. It is modeled using six TiO2 units located at bulk-extended positions on TiO2(001) similar to those on TiO2(110). The single-step height clusters can further grow into a linear structure either along  or [1-10], exhibiting many structural traits experimentally observed.