Philip M. Tucker

X-Ray photoelectron spectroscopy of iron–oxygen systems

X-Ray photoelectron spectroscopy has been used to study a series of iron oxides. It has been shown that iron metal has a different ionisation energy from a number of iron(III) oxides. Small perturbations to the energy of the iron (III) 2p electrons can be attributed to changes in crystal structure. Multiplet splitting and shake-up in the iron oxides contributes to iron 2p peak widths. The chemisorption of water has a marked effect upon the observed peak profiles; the oxygen 1s peaks due to oxide, hydroxyl, and adsorbed water have been characterised.

X-ray photoelectron spectroscopy of adsorbed oxygen and carbonaceous species on platinum electrodes

Summary XPS measurements on electrochemically oxidised Pt suggest the existence of a single species, possibly Pt(OH) 2 , up to potentials where coulometry indicates a limiting coverage. Under more extreme conditions a species which may be identified with some certainty as PtO 2 is formed in larger amounts. A carbonaceous species can be detected on platinised electrodes dipped in methanol or formic acid. The corresponding C 1s peak is 4 eV more positive than the peak for carbon in hydrocarbons.

X-Ray photoelectron spectroscopy of chromium–oxygen systems

X-Ray photoelectron spectroscopy has been used to study a series of chromium–oxygen compounds. It has been shown that the ionisation energy of the chromium 2p electrons is dependent primarily on the oxidation state of the chromium metal ion, but that small perturbations may be attributed to changes in crystal structure and hence the Madelung potential. Multiplet splitting in chromium(III) compounds contributes to peak widths, and the chemisorption of water and oxygen has a marked effect on the observed peak profiles. In addition such chemisorption apparently contributes to the build up of surface charge, thereby complicating the precise determination of binding energies.

Characterization of iron/oxygen surface reactions by X-ray photoelectron spectroscopy

Abstract The initial stages of oxidation of clean, high-purity, iron foil have been studied using a Kratos ES 300 electron spectrometer with monochromatic X-radiation. Through careful observation of the Fe 2p peak profile, peak energy maximum, and peak satellite structure, it was possible to identify oxide species containing Fe (II) and Fe (III). The results indicated that iron was highly mobile in the Fe (II)/Fe (III) lattice at intermediate oxidation levels and that the iron ions in the octahedral sites of Fe3O4 exhibited a rapid symmetric intervalence electron transfer of the type Fe (II) + Fe (III) → Fe (III) + Fe (II).

Early stages in the oxidation of magnesium, aluminium and magnesium/aluminium alloys: I. Exoelectron emission and long wavelength photoemission

The initial stages in the oxidation of magnesium, aluminium and magnesium/aluminium alloys have been studied using a photoelectron spectrometer (XPS and exoelectron energy spectra) and the diode technique (surface potentials and volumetric adsorption of oxygen). This paper describes and characterises the exoelectron emission and the visible wavelength photoemission which occurs during oxidation. At low oxygen exposures (< 10 L), exoelectrons with a 4 eV energy spread are emitted in the dark from magnesium and magnesium/aluminium alloy. Aluminium shows no such emission. Another, dark pressure-dependent emission of exoelectrons occurs only on magnesium/aluminium alloy and aluminium at high oxygen exposures. The onset of this emission lies 3 eV higher and it again has a 4 eV spread. When the surface is illuminated, it is possible to distinguish photo/exoelectron emission, which occurs during oxygen uptake, and true photoemission which occurs in vacuo. Both these types of emission, which were only recorded on magnesium, show a resolved triplet. Photoelectric work functions as low as 0.3 eV were observed. A patchy surface evidently develops during oxidation. This process, which may correspond to the nucleation of oxide islands, is slower than the chemisorption of of oxygen. The nature of the low work function patches, which we associate with the electron emissions, is therefore very dependent on the oxygen ambient pressure. A tentative energy scheme is given for such “exopatches” which are neither metal nor oxide and contain a curious energy level that must be situated above the vacuum level of the patch, as well as above the metal Fermi level. The energy with which exoelectrons are emitted derives from the heat of adsorption.

Oxidation of UO2 fuel pellets in air at 503 and 543 K studied using X-ray photoelectron spectroscopy and X-ray diffraction

Abstract An understanding of the low temperature oxidation behaviour of UO2 pellets in air is important in the unlikely event of gas ingress to a fuel can during handling or storage. The main parameter of concern is the production time of U3O8 particulate as a function of temperature. Factors which affect the UO2 → U3O8 transformation have been investigated by sequentially oxidising UO2 fuel pellets in air at 503 and 543 K and monitoring the growth of U3O and U3O7 using X-ray photoelectron spectroscopy, X-ray diffraction and scanning electron microscopy. Initially oxidation proceeded at a linear rate by the inward diffusion of oxygen to form a complete layer of substoichiometric U3O7. This phase was tetragonal with a c a ratio of 1.015, significantly less than the value of 1.03 measured on UO2 powder when oxidised under identical conditions. This difference and the preferred orientation exhibited by surface grains were caused by growth stresses induced in the pellet surface. Both intergranular and transgranular cracking occurred and became nucleation sites for the growth of U3O8. The linear oxidation period associated with U3O7 growth was much shorter at 543 than at 503 K and U3O8 nucleated earlier. Spallation and the production of particulate were only observed during the formation of U3O8 when a 30% increase in volume arose from the U3O7 → U3O8 phase change.

X-Ray photoelectron spectroscopy of some uranium oxide phases

X-Ray photoelectron spectra of a variety of uranium oxides have been studied and compared with the spectrum obtained from the pure metal. The oxide phases studied were UO2·0, UO2·1, UO2·2, U4O9, U3O7, U3O8, and γ-UO3 where the formal oxidation number of uranium alters from IV to VI. Significant chemical shifts observed for U 4f and O 1s peaks are explained by consideration of chemical structure and changes in uranium and oxygen oxidation states. The surface oxidation of uranium dioxide powder in air has been studied by comparing the spectra with those of UO2 pellets and higher uranium oxides.

High resolution lmm auger electron spectra of some first row transition elements

Abstract Auger spectra have been recorded from elements of the first transition series using a hemispherical analyser. Highly resolved LMM spectra were obtained showing for the first time the composite nature of these peaks for many of the elements studied. The recorded spectra show a general similarity for the elements Sc → Zn but interesting differences emerge. At the beginning of the transition period the L3 based transitions have the relative intensities L3 M2,3, M2,3 > L3 M2,3, M4,5 L3 M4,5, M4,5 whereas towards the end of the series the order L3 M4,5, M4,5 > L3 M2,3, M4,5 L3 M2,3, M2,3 is observed. Pronounced chemical shifts have been observed upon oxidation. The spectra are interpreted in terms of an L -S coupling scheme and the fine structure discussed in terms of effects produced by multiplet splitting.

Surface oxidation of nickel metal as studied by X-Ray photoelectron spectroscopy

X-ray photoelectron spectroscopy has been used to study the oxidation of polycrystalline nickel metal. The results indicate that the oxidation process takes place in three stages; associative adsorption of molecular oxygen, followed by the combination of oxygen atoms with surface nickel atoms and, ultimately, the formation of bulk oxide. At room temperature only the first two stages can be detected. For exposures below 1 L the O 1s photoelectron spectrum is considered to be characteristic of an associatively adsorbed oxygen species, but for exposures above this value evidence for the formation of a monolayer of “NiO” is suggested by the development of an O 1s peak at 529.9 eV. Incorporation of oxygen into the nickel lattice is observed at temperatures >500°K. The activation energy for this place-exchange process was estimated at 1.80±0.06 eV.

The behaviour of uranium oxides in low partial pressures of O2 studied using X-ray photoelectron spectroscopy

Abstract Non-stoichiometry, induced by exposure to oxygen and vacuum has been investigated for the uranium oxides UO 2 , U 4 O 9 and U 3 O 8 using X-ray photoelectron spectroscopy. Each stoichiometric uranium oxide is shown to display a unique X-ray photoelectron spectrum which can be altered quite dramatically by changes in stoichiometry and explained by a defect cluster model.