AVS 50th International Symposium
    Surface Science Thursday Sessions
       Session SS3-ThM

Paper SS3-ThM10
Surface Structure and Properties of Vanadia Supported on Anatase TiO@sub 2@(001)

Thursday, November 6, 2003, 11:20 am, Room 328

Session: Oxide Structure, Growth, and Defects
Presenter: W. Gao, Yale University
Authors: W. Gao, Yale University
E.I. Altman, Yale University
Correspondent: Click to Email
We have successfully grown epitaxial anatase TiO@sub 2@(001) thin films on LaAlO@sub 3@(001) substrates using oxygen plasma assisted molecular beam epitaxy (OPA-MBE). A two-domain (1x4)/(4x1) reconstruction was observed by reflection high energy electron diffraction (RHEED) and low energy electron diffraction (LEED). With OPA-MBE, monolayer to multi-layer vanadia was deposited on the reconstructed anatase (001) surface. Photoelectron spectroscopy (UPS and XPS) was used to characterize the chemical state of the vanadia. XPS results showed that the vanadia was predominantly in the 5+ oxidation state after deposition of a monolayer at 525 K, while UPS showed a small peak in the TiO@sub 2@ bandgap consistent with a small amount of V@super 4+@. After 1 ML vanadia was deposited on anatase (001), the anatase (1x4)/(4x1) LEED and RHEED patterns were replaced by (1x1) patterns indicating that the vanadia lifts the reconstruction and suggesting that the monolayer is pseudomorphic. The reducibility of the monolayer was studied by monitoring XPS peak shifts as a function of annealing temperature. At temperatures between 625 K to 875 K, the vanadia was reduced to the 3+ oxidation state; these results were very similar to vanadia on SiO@sub 2@. XPS results also showed a decrease in the intensity of the V peaks with increasing temperature. At ~875 K, nearly no vanadium was detected by XPS and the (1x4)/(4x1) pattern of the bare surface was restored suggesting that V migrated into the bulk. Vanadia growth beyond the first monolayer caused the RHEED pattern to fade and produced no discernible LEED patterns suggesting that pseudomorphic growth cannot be continued beyond 1 ML. The structural evolution of the surface as vanadia was deposited was also characterized by scanning tunneling microscopy (STM).