RESUMO
The valence band and core-level X-ray photoelectron spectroscopy (XPS) of iron and its oxides are reported, and the valence band spectra interpreted by various calculation models. The paper focuses upon the valence band region, which shows significant differences between the metal and the following oxidized iron species: FeO, Fe(3)O(4), alpha-Fe(2)O(3), gamma-Fe(2)O(3), alpha-FeOOH and gamma-FeOOH. The core region is of little analytical value as a means of distinguishing between these species, but the valence band region shows significant differences. These differences are consistent with spectra predicted by cluster and band structure calculations. Cluster calculations are valuable as a means for interpreting the spectra of iron oxides with multiple iron sites and defect characteristics.
RESUMO
The oxidation of clean polycrystalline aluminium foil was performed in a previously described anaerobic cell and studied by core level and valence band X-ray photoelectron spectroscopy. Oxide free sample surfaces were exposed under inert atmosphere at room temperature to increasingly oxidative environments of water vapor and liquid water beginning with an oxide free metal foil each time. Minor oxidative environments were found to produce a film of boehmite with a thinner outer film of gibbsite. Harsher oxidation in liquid water resulted in an oxide film much thicker than that found on the as received sample which contained an inner composition of gamma alumina with an outer film of gibbsite. As expected, the oxide film observed for ultra high purity polycrystalline aluminiurm foil in the as received state was gibbsite. The valence band of the cleaned metal is discussed in terms of residual surface species. The evaluation and determination of these residual surface species is evaluated following an analysis of two different band structure calculations which use different d orbital exponents. It is found that while the density of states is almost unaltered by changes in the d exponent, the predicted spectrum is substantially changed. The work makes extensive use of valence band X-ray photoelectron spectroscopy.
RESUMO
A Ni-Ti alloy with a 50:50 atomic composition has shown exceptional properties as a fixed potential LCEC detector for carbohydrates and related substances. It exhibited excellent sensitivity and superior long-term stability compared to pure Ni. A study was therefore undertaken by means of cyclic voltammetry (CV), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS) to understand the role of Ti and the respective surface oxides of Ni and Ti in the catalytic stability of the detector. CV results showed that Ti is initially oxidized, most likely to TiO(2) in 0.1 M NaOH solution. The oxidation of Ni to nickel(II) oxide also occurs at potentials close to that of Ti. At higher potentials in the range of +0.4 to +0.5 V vs Ag/AgCl reference, nickel(II) oxide undergoes further oxidation to the Ni(III) oxidation state. This state is responsible for the catalysis of carbohydrates, amino acids and other biosubstances. When Ni-Ti and Ni are repetitively CV cycled in the potential range of 0.0 to +0.6 V, a second wave appears at more negative potentials during the reverse cathodic scan for Ni but not for Ni-Ti. SEM images of these two electrodes in the oxidized form show the Ni-Ti surface remains smoother in appearance. This smoothness is consistent with the fact that the thickness of the surface "oxide" layer increases less rapidly, as Ni-Ti is repetitively CV cycled, compared to pure Ni. XPS results for the nature of the surface oxides are consistent with oxidized Ti as TiO(2), Ni(II) predominantly as Ni(OH)(2), and Ni(III) possibly as NiOOH. Possible reasons for Ti stabilizing the Ni-Ti alloy as a LCEC detector are discussed.
