The evolution of Ni nanoislands on the rutile TiO2 (110) surface with coverage, heating and oxygen treatment

Scanning tunnelling microscopy (STM) and reflection high-energy electron di ffraction (RHEED) have been used to examine in situ the morphology and grow th of nickel nanoislands on the rutile TiO2(110)-p(1 x 1) surface. Such ana lysis revealed growth at temperatures between 295 and 495 K via the Volmer- Weber {V-W) mode of three-dimensional islands, in agreement with thermodyna mic expectations. The Ni islands grew first as small crystalline domes with no preferential alignment with the TiO2(110) surface, but with increasing Ni dose their orientation evolved until almost perfect monocrystalline (110 )(Ni)\ \ (110)(TiO2), [110](Ni)\ \ [001]TiO2 orientation-relations were est ablished. This arrangement has one of the lowest lattice mismatches if an a verage of the two orthogonal surface directions [001](TiO2) and [110](TiO2) is considered. The epitaxial relation persists upon annealing in ultra-hig h vacuum (UHV) to 1065 K and the heat treatment causes the particles to coa rsen and grow in size. When the crystalline islands of nickel are oxidised, the lattice parameters are modified, consistent with the formation of NiO. A new epitaxial relation is established between the overlayer and the subs trate: (001)(layer)\ \ (110)(TiO2), [110](layer)\ \ [001](TiO2). This produ ces a large strain parallel to the [110](TiO2) direction and a very small s train along [001](TiO2) (+0.13% mismatch). UHV annealing of the oxidised ov erlayer reduces the crystallites to islands with lattice parameters corresp onding to nickel metal, but the original crystallography is not recovered. Instead, the nanoislands of reduced nickel maintain the same overlayer orie ntation as for the oxide. A model is presented in which the change in epita xial relation is attributed to the formation of an interfacial NiO layer be tween the reduced Ni islands and TiO2 substrate. The results demonstrate th at the oxidation/reduction treatment of the substrate and overlayer can be used to modify the microstructure of the metal/oxide system, and have impli cations for the use of nickel as a supported metal catalyst. (C) 2001 Elsev ier Science B.V. All rights reserved.