Direct observation of the growth of vanadium on TiO2(110)-(1 x 2)

The growth of vanadium and vanadia on the (1 x 2) structure of the TiO2 surface was studied by scanning tunneling microscopy. STM of the clean surface yielded atomically resolved (1 x 1), (1 x 2) and (2 x 2) structures and, in some cases, revealed crystallographic shear planes. A (1 x 2) surface structure was stabilized by heating in vacuum to 1150 K. At low coverages metallic vanadium, evaporated onto this surface held at 300 K, binds preferentially atop the added-rows of the (1 x 2) structure. As the vanadium coverage is increased toward one monolayer, the density of vanadium clusters decorating the (1 x 2) rows increases, forming chains of more or less isolated clusters. At three monolayers coverage vanadium metal forms a granular film which covers the oxide surface uniformly. This film is sufficiently smooth that the underlying step structure of the oxide is still visible. When heated to 600 K this film reorganizes to form needle-like features aligned along the [001] direction, which are separated by multiples of the underlying (1 x 2) periodicity along the [<1(1)over bar>0] direction. With continued heating of the film to 635 K migration of the vanadium into the bulk occurs, reducing the surface concentration of vanadium. As the result of the higher annealing temperatures isolated vanadium clusters, undoubtedly oxidized, decorate the top of the emergent, underlying (1 x 2) TiO2 oxide rows. (C) 1999 Elsevier Science B.V. All rights reserved.