Growth and properties of vanadia on anatase titanium dioxide(001) and (101) surfaces

Due to its unique catalytic properties and elusive structure, the growth and structure of vanadia on epitaxial anatase (001) and (101) films have been studied by oxygen plasma assisted molecular beam epitaxy, reflection high-energy electron diffraction (RHEED), low energy electron diffraction (LEED), scanning tunneling microscopy (STM), photoelectron spectroscopy, x-ray diffraction (XRD) and scanning transmission electron microscopy (STEM).; For anatase (001) a (1x4)/(4x1) reconstruction was observed by RHEED and LEED. X-ray photoelectron spectroscopy (XPS) showed predominantly V5+ after depositing 1 monolayer (ML) at 525 K. At this coverage, the (1x4)/(4x1) diffraction patterns were replaced by (1x1) patterns indicating that vanadia lifts the reconstruction and suggesting that the monolayer is pseudomorphic. At 525 K, continued vanadia growth caused the RHEED pattern to fade, suggesting that V2O5 epitaxy cannot be continued beyond ML. At 750 K, the (1x1) pattern remained clear for 20 nm of vanadia, indicating that the monolayer is epitaxial. Both XRD and STEM confirmed the formation of epitaxial vanadia layers. The stoichiometry of the film was VO2. At 800 K, a c(2x2) termination was observed. This suggests that the V5+ at 1 ML is due to 0.5 ML of terminal oxygen which allows both the anatase structure and the 5+ oxidation state.; The anatase (101) surface is the lowest energy surface and was grown on both stepped and flat LaAlO3 (101) substrates where the lattice matching along the [010] direction is excellent but poor along [101¯]; steps accommodate the lattice strain. Both XRD and STEM indicated that the films grew with anatase (102) planes parallel to the interface. Both RHEED and LEED, however, showed patterns expected for anatase (101) surfaces. Further, STM images revealed the oblique unit cell expected for anatase (101) along with many parallel steps. It is suggested that the film surface facets towards (101) to reduce the surface energy.; The diffraction patterns of the anatase (101) film remain the same after the first monolayer vanadia deposition, suggesting an epitaxial layer. Further deposition of vanadium results in 3-D growth. The results for vanadia growth on this surface are also compared with growth on the (001) surface.