Titania Thin Films on Gold and Palladium Surfaces: Structure and Properties

Abstract

Motivated by the catalytic properties of titanium dioxide (TiO2), palladium (Pd) and gold (Au) combined with TiO2, growth and properties of TiOx thin films on Au(111), Pd(100) and Pd(111) have been investigated. The TiOx thin films were grown by chemical vapor deposition (CVD) using titanium (IV)isopropoxide (TTIP) as precursor. For the TiOx structures found on Au(111) the interaction with water have been studied and for the TiOx structures on the two Pd surfaces the oxidation and reduction behavior were studied. High resolution photoelectron spectroscopy (HRPES), scanning tunneling microscopy (STM) and low energy electron diffraction (LEED) was used to characterize the structure of the TiOx thin films. Additionally, density functional theory (DFT) calculations were used to investigate a Ti-Pd alloy formation in the Pd(100) surface. On Au(111) four different TiOx structures were characterized, two partially oxidized wetting layers, Honeycomb (HC) and Pinwheel (PW), and two fully oxidized TiO2 phases, Star and TiO2(B). The HC phase and the star phase are both grown directly by TTIP deposition, while the PW and TiO2(B) phases require a post annealing step. The water interaction with the TiOx phases on Au(111) was found to be different for all four phases, water dissociation was observed on three of the phases, Star, PW and TiO2(B). The Star phase showed a water dissociation comparable to that of rutile TiO2(110), whereas the TiO2(B) phase only showed water dissociation in connection to domain boundaries. The PW phase got significantly oxidized during water dissociation. The growth of TiOx on Pd(100) and Pd(111) follow the same pattern on both surfaces. Initially a surface alloy is observed before a partially oxidized wetting layer is formed. Finally a fully oxidized TiO2 phase is formed on top. Post-oxidation of the reduced wetting layers reveal additional, fully oxidized phases which may be reduced back to the respective partially oxidized wetting layers by annealing. Annealing of the reduced wetting layers results in removal of the oxygen and formation of surface alloys. Although the growth and oxidation/reduction behaviors are similar for the two Pd surfaces, the atomic structures of the different TiOx phases are not the same for the two surface orientations