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 sur face 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, i n some cases, revealed crystallographic shear planes. A (1 x 2) surface str ucture was stabilized by heating in vacuum to 1150 K. At low coverages meta llic vanadium, evaporated onto this surface held at 300 K, binds preferenti ally atop the added-rows of the (1 x 2) structure. As the vanadium coverage is increased toward one monolayer, the density of vanadium clusters decora ting the (1 x 2) rows increases, forming chains of more or less isolated cl usters. At three monolayers coverage vanadium metal forms a granular film w hich covers the oxide surface uniformly. This film is sufficiently smooth t hat the underlying step structure of the oxide is still visible. When heate d 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 hea ting of the film to 635 K migration of the vanadium into the bulk occurs, r educing the surface concentration of vanadium. As the result of the higher annealing temperatures isolated vanadium clusters, undoubtedly oxidized, de corate the top of the emergent, underlying (1 x 2) TiO2 oxide rows. (C) 199 9 Elsevier Science B.V. All rights reserved.