Nicholas Winograd

X-ray photoelectron spectroscopic studies of nickel-oxygen surfaces using oxygen and argon ion-bombardment

Abstract We have studied the surface chemistry of the nickel-oxygen system using both temperature changes and ion bombardment as techniques for elucidating the surface structure. The spectra of metallic Ni, NiO and Ni 2 O 3 were characterized from samples prepared directly in the spectrometer. The Ni 2 O 3 species could be distinguished from an authentic Ni(OH) 2 sample from both the X-ray photoelectron lines and the Auger transitions. The oxides of NiO and Ni 2 O 3 could be prepared by bombardment with low energy (400eV) O 2 + ions as well as by exposure of Ni to oxygen at reduced pressure (∼ 100 torr). The Ni 2 O 3 was found to be present on most nickel-oxygen surfaces except those prepared by exposing Ni to air for many hours at high temperature (> 600° C ), indicating that the stability of Ni 2 O 3 decreased as the temperature increased. Exposure of both NiO and Ni 2 O 3 to 400 eV Ar + ion bombardment caused reduction to metallic Ni. This observation has also been noted for several other oxides and a prediction of whether or not reduction should be observed is presented by examining the free energy of formation of the molecule.

Initial and final state effects in the ESCA spectra of cadmium and silver oxides

The factors which influence chemical shifts are examined in order to elucidate the cause of the anomalous chemical shifts for Cd and Ag oxides. The effects of extra‐atomic relaxation are accounted for using a procedure employing experimental Auger and binding energies. Atomic partial ionic charges for some simple Cd, Ag, and Zn compounds are calculated from experimental binding energies using a model which includes the effects of lattice potentials and extra‐atomic relaxation. Inclusion of extra‐atomic relaxation effects did not have a drastic effect on the relative ionicities computed for these selected compounds. However, for CdO, a large extra‐atomic relaxation energy contribution reduces the binding energy by 0.5 eV more than is predicted from nearest neighbor electronegativity arguments.

ESCA studies of metal-oxygen surfaces using argon and oxygen ion-bombardment

Abstract ESCA has been used to monitor alterations of catalytically and electrochemically important metal-oxygen surfaces following exposure to Ar + and O 2 + ion bombardment. This treatment resulted not only in sputtering, but also, in many cases, in reduction to the corresponding metal or lower oxide. A model based on bulk thermodynamic free energy considerations Is proposed to explain this phenomenon. We have also exploited this approach to obtain an in-depth concentration profile of various oxidation states of an element, to selectively prepare desired surface oxide compositio and to aid in interpreting complex O ls spectra. Results obtained from metal-oxygen surfaces for Ni, Ru and Mo are presented. Ni 2 O 3 and RuO 3 , which are gross defect structures of the bulk species, are present on NiO and RuO 2 respectively, with the former being confined to the surface layers. The MoO 2 , on the other hand, is covered with a surface layer of MoO 3 present as a regular crystal structure.

X-Ray photoelectron spectroscopic studies of ruthenium-oxygen surfaces

Abstract The technique of X-ray photoelectron spectroscopy (XPS) has been used to study the complex surface chemistry of the ruthenium-oxygen system as a function of temperature and under the influence of Ar + and O 2 + ion-bombardment. Interaction of molecular oxygen and O 2 + ions with metallic ruthenium produced two forms of oxygen which are not attributable to RuO 2 , RuO 3 and RuO 4 . A variety of species has been identified on RuO 2 . On anhydrous RuO 2 we found a surface layer of RuO 3 present as a gross defect structure of RuO 2 . On commercially available hydrated samples, both the XPS signal for the oxide and for the water could be observed. In addition, a carbon contamination indicated from a mass spectral analysis and microanalysis was characterized as RuOCO 3 . In general the XPS approach was found to be valuable in monitoring the surface concentrations of these species and thus in characterizing the chemical composition of this catalyst surface.

Lipid Imaging with Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS)

Fundamental advances in secondary ion mass spectrometry (SIMS) now allow for the examination and characterization of lipids directly from biological materials. The successful application of SIMS-based imaging in the investigation of lipids directly from tissue and cells are demonstrated. Common complications and technical pitfalls are discussed. In this review, we examine the use of cluster ion sources and cryogenically compatible sample handling for improved ion yields and to expand the application potential of SIMS. Methodological improvements, including pre-treating the sample to improve ion yields and protocol development for 3-dimensional analyses (i.e. molecular depth profiling), are also included in this discussion. New high performance SIMS instruments showcasing the most advanced instrumental developments, including tandem MS capabilities and continuous ion beam compatibility, are described and the future direction for SIMS in lipid imaging is evaluated.

Low energy ion impact phenomena on single crystal surfaces

The dynamics of a solid bombarded by a 600 eV Ar+ ion have been studied classically by computer simulation. The model uses a crystallite of about 250 atoms described by pair potentials derived from elastic constants and which reproduce the surface binding energy of the solid. The relative calculated yield of secondary atom emission from the three low index faces of Cu follow the previously determined experimental order (111) > (100) > (110). We find major differences in the sputtering mechanisms for these faces. On (110), the impacted atom is ejected most frequently, while on (111) and (100) it almost never leaves the solid. We report the energy distribution of the sputtered particles for each face. The simulation successfully predicts the shape of the curve including the low energy maximum which is observed experimentally near 2 eV. In addition our model shows that many low energy atoms attempt to leave the crystal but are subsequently trapped to the solid at large distances from their original sites. This mechanism of radiation enhanced diffusion inevitably occurs in conjunction with sputtering or any other heavy secondary particle emission or scattering process.

XPS spectroscopic study of potentiostatic and galvanostatic oxidation of Pt electrodes in H2SO4 and HClO4

Abstract The surface oxides produced from potentiostatic and galvanostatic oxidation of Pt electrodes in HClO4 and H2SO4 are examined using X-ray photoelectron spectroscopy. The oxide I species produced as the initial oxidation product by successively more anodic potentiostatic oxidation in 0.2 M HClO4 is found to have a Pt2+ oxidation state, a binding energy characteristic of neither PtO, Pt(OH)2 or PtO2, and a limiting thickness of 8 A. Galvanostatic oxidation in HClO4 and H2SO4 is found to produce PtO2·H2O as an unlimiting growth oxide or a limiting growth oxide layer depending on the concentration of the acid electrolyte. The incorporation of the acid electrolyte anion in the surface layer is shown to have an effect on which type of oxide layer is produced. X-ray decomposition and chemical modification by Ar+ stripping are shown to produce chemical artifacts complicating any interpretation of a Pt oxide surface layer.

Multiphoton resonance ionization of sputtered neutrals: a novel approach to materials characterization

Abstract Ionization of neutral atoms sputtered from ion bombarded solids by multiphoton resonance ionization has been demonstrated for the first time. The ionization efficiency is several orders of magnitude greater than other post-ionization methods. This approach should find applications in the characterization of the chemistry and structure of solid surfaces and in the trace analysis of a wide variety of materials.

Performance characteristics of a chemical imaging time-of-flight mass spectrometer.

A chemical imaging time-of-flight secondary ion mass spectrometer is described. It consists of a liquid metal ion gun, medium energy resolution reflectron mass analyzer, liquid nitrogen cooled sample stage, preparation chamber and dual stage entry port. Unique features include compatibility with laser postionization experiments, large field of view, cryogenic sample handling capability and high incident ion beam current. Instrument performance is illustrated by the characterization of scanning electron microscopy grids, silver and functionalized polystyrene beads and the postionization of an organic overlayer on a gold substrate.

X-ray photoelectron spectroscopic binding energy shifts due to matrix in alloys and small supported metal particles

Abstract Core level binding energies of metals in the following alloys and metal-dispersed or -implanted materials have been studied: AuAg, PtAg, PtC and AuSiO 2 . Their shifts from the binding energies of pure species (referenced to the Fermi level) are interpreted to consist of a matrix shift resulting from differences in crystal field potential, relaxation energy and work function as well as a chemical shift due to differences in valence electron density. By estimating the matrix shift using implanted Ar in the related pure materials, the chemical shift could be isolated from the observed binding energy shift. In all cases studied, the matrix shift is greater than the chemical shift and its contribution to the binding energy shifts is in the opposite direction.