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 diffraction (RHEED) have been used to examine in situ the morphology and growth of nickel nanoislands on the rutile TiO2(110)-p(1×1) surface. Such analysis revealed growth at temperatures between 295 and 495 K via the Volmer–Weber (V–W) mode of three-dimensional islands, in agreement with thermodynamic 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, [1̄10]Ni∥[001]TiO2 orientation-relations were established. This arrangement has one of the lowest lattice mismatches if an average of the two orthogonal surface directions [001]TiO2 and [110]TiO2 is considered. The epitaxial relation persists upon annealing in ultra-high vacuum (UHV) to 1065 K and the heat treatment causes the particles to coarsen 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 substrate: (001)layer∥(110)TiO2, [11̄0]layer∥[001]TiO2. This produces a large strain parallel to the [11̄0]TiO2 direction and a very small strain along [001]TiO2 (+0.13% mismatch). UHV annealing of the oxidised overlayer reduces the crystallites to islands with lattice parameters corresponding to nickel metal, but the original crystallography is not recovered. Instead, the nanoislands of reduced nickel maintain the same overlayer orientation as for the oxide. A model is presented in which the change in epitaxial relation is attributed to the formation of an interfacial NiO layer between the reduced Ni islands and TiO2 substrate. The results demonstrate that the oxidation/reduction treatment of the substrate and overlayer can be used to modify the microstructure of the metal/oxide system, and have implications for the use of nickel as a supported metal catalyst.