Ultraviolet photoelectron spectroscopy (UPS) of anodic oxide films on Au, Pt, Ru and Ru0.5Ir0.5 alloy

Anodically formed oxide films on polycrystalline Au, Pt and Ru electrodes were investigated using ultraviolet photoelectron spectroscopy (UPS) after removal from the electrolyte (0.1M HClO4) under potential control and transfer to the UHV chamber. The UPS valence-band spectra after oxide formation clearly show a significant reduction of the density of states at the Fermi level to virtually zero for Au and Pt. For Ru the metallic properties of the oxide are reflected by a finite density of electronic states at EF. Rinsing of the emersed electrodes with water removes counter ions on the surface, which give rise to additional emission features overlapping with the O 2p band of the oxide. The results indicate that the oxide on Au is a semiconductor (n-type) with the valence-band edge located 1.3 eV below EF. The oxide on Pt is either semimetallic or semiconducting with a very small band gap (0.2 eV). As a case study, the oxide formation on Ru0.5Ir0.5 has been monitored using UPS. As indicated by the position of the t2g band, the UPS results show a transition from ruthenium oxide to iridium oxide formation upon increasing anodic potentials. Our investigation clearly demonstrates that UPS studies on emersed electrodes provide valuable information about the electronic properties of anodically grown oxide layers.