Tery L. Barr

Nature of the use of adventitious carbon as a binding energy standard

It has become common practice to employ, as a binding energy reference for x‐ray photoelectron spectroscopy studies on nonconductive materials, the C(1s) spectra of the ubiquitous (adventitious) carbon that seems to exhibit an instantaneous presence on all air exposed materials. Despite this commonality, surface scientists, including many practitioners, have expressed substantial concerns about the validity of this approach. A detailed discussion of the method is presented including consideration of the types of materials and the electronic energy states involved, e.g., Fermi edges, vacuum levels, etc., and the couplings that must exist for the referencing method to be correctly applied. A number of other surface environments for which the carbon referencing method may be fallacious are also presented. This leads to a consideration of the electron spectroscopy for chemical analysis results for different types of adventitious species and how the presence of some of these may confuse the use of the method. ...

Recent advances in x‐ray photoelectron spectroscopy studies of oxides

X‐ray photoelectron spectroscopy (XPS) has often played the major role in the chemical characterization of select surface species, but the extension of that role to larger classes of compounds has generally been limited by such problems as cleanliness, charging, and relaxation shifts. In this article we suggest that adequate command of these difficulties has now permitted the collective chemical description of quite diverse surfaces. In this regard the progressive bonding differences of representative, simple group A oxides (e.g., BaO and SiO2 ) are analyzed employing various features in the XPS core level and valence band results. Regular progressions in covalency/ionicity are demonstrated. Extensions of these studies to complex (AzMsOt) oxides (e.g., Na2Al2O4 ) are also described in which progressive alterations of the XPS spectra suggest that during this complex formation the A–O bond is generally made more ionic, whereas the M–O bond becomes more covalent. These results also indicate that for many of ...

The nature of hydrogen in x‐ray photoelectron spectroscopy: General patterns from hydroxides to hydrogen bonding

Important progressive alterations in chemical bonding are often realized through correlations with shifts in the x‐ray photoelectron spectroscopy (XPS) binding energies of key elements. For example, there are useful general XPS shifting schemes for such systems as oxides, nitrides, halides, and even various functional groups in organics. Very general patterns, based upon location in the periodic table, exist for many of these materials even when the structure is not strongly considered. Unfortunately, apparently because of the lack of direct XPS detection of hydrogen, there seems to be no general statements in the literature for describing hydrogen‐containing compounds, despite the fact that synergistic shifts obviously exist in the XPS spectra of elements attached to hydrogen (e.g., for M–O–H vs M–O–M units, where M is a typical metal). While not attempting a complete review paper, in the present work we use XPS shifting patterns to evolve a series of interrelated covalency/ionicity arguments to help exp...

The nature of the relative bonding chemistry in zeolites: An XPS study

Abstract A detailed analysis is provided of the chemical-bonding implications of a series of XPS studies of various zeolite systems. Both XPS core level and valence band results are analyzed. The study demonstrates that tetrahedral sodium aluminate is a relatively ionic oxide, whereas the SiO bonds in silica are much more covalent. The varying combinations of these two species found in different zeolites produce progressive shifting patterns in the XPS results that are indicative of related progressions in the relative covalency/ionicity in the zeolite bonding. The study is employed to track the Bronsted acidity of various HZ zeolite systems including dealuminated (ultra stabilized) Y zeolites. Interconnections are also demonstrated with changing patterns of features related to the Sanderson intermediate electronegativity. A key aspect of this study is that we suggest that it is necessary to consider that the oxygens in these zeolite (and related) systems are subject to extensive polarization.

Studies in differential charging

A detailed examination is presented of the causes, removal, and use of differential charging in x‐ray photoelectron spectroscopy [electron spectroscopy for chemical analysis (ESCA)] and related spectroscopies. Particular attention is devoted to the impact of this phenomenon during the dispersion (or conversely crystallite growth) of metals and semiconductors in insulating matrices. A progressive series of models is described to determine their general and specific roles in the charging process. The effects of depth and interaction are considered for the fields involved (e.g., photoionization centers, detection depth, compensation currents, etc.). Thus, in certain cases, the ESCA may be simultaneously employed to analyze macromorphology, as well as surface and near surface chemistry. Practical examples are described from metals catalysis, corrosion and doped thin films.

A new interpretation of the binding energies in X-ray photoelectron studies of oxides

Abstract Films and coatings applied to different substrates are often various types of oxides. The latter are employed for a number of reasons with particular emphasis on their lack of chemical and electrical porosity. It is generally important therefore to have some measure of the physical and chemical integrity of the resulting oxide films and their interfaces with the substrate. The latter is often realized by using X-ray photoelectron spectroscopy or electron spectroscopy for chemical analysis. In order to accomplish this, it is common practice to utilize the (now generally well accepted) binding energy data from the literature to verify and expand upon ones results. Unfortunately, close scrutiny reveals a number of major discrepancies in these literature results for certain key oxides. Thus, for example, a significant portion of the literature reports the Al 2p level of Al 2 O 3 at ∼75.7 ± 0.3 eV, whereas many others specify ∼74.0 ± 0.2 eV. Perhaps the greatest difficulty with this observation is that it can be shown that, based upon their respective methods of analysis, both results are equally valid. In the present investigation, we examine the extent and nature of this problem. We have discovered that there are particular types of oxides that exhibit this dichotomy in binding energies, whereas others do not. The role played by bonding and morphology in these differences is explored, as is the effect of the different methods of binding energy determination. Finally, we shall describe how the mechanism of the growth of oxides is the principal “culprit” and a lack of understanding of the latter and its electronic implications may lead many to an incorrect interpretation of “true” binding energies.

ESCA studies of the coordination state of aluminium in oxide environments

X-Ray photoelectron spectroscopy (known as XPS or ESCA) can differentiate between tetrahedral and octahedral Al atoms in inorganic solids. Although primarily used for the study of aluminosilicates, this semi-quantitative technique is applicable to other materials, irrespective of whether they contain aluminium in one or more coordinations with respect to oxygen. If less sensitive to the coordination of Al, the range of ESCA, a surface-oriented technique, is thus not very different from that of the bulk-oriented solid-state NMR. The procedures described here are successful with any high-resolution ESCA system, but are greatly facilitated by flood gun technology which removes insulator-induced charging shifts. Areas of potential difficulty are also discussed.

XPS study of the cBN–TiC system

Sintered cubic boron nitride is widely used in various industrial applications because of its extreme wear and corrosion resistance, thermal and electrical properties. In order to obtain composite materials with these optimal properties it is important to elucidate whether chemical reactions occur at boron nitride/bonding phase interfaces. Some of these systems were then subjected to physical and thermal alteration This paper summarizes theoretical and experimental studies on the cBN–TiC 1:1 molar ratio. From theoretical calculations it follows that TiC reacts with boron nitride forming two new phases, TiB2 and TiN. Experimentally CBN–TiC composites were prepared by hot pressing, and the samples were subsequently heat treated. The samples were characterized after heat treatment using transmission electron microscopy and X-ray diffraction.

cBN–TiN, cBN–TiC composites: chemical equilibria, microstructure and hardness mechanical investigations

Abstract This paper summarizes theoretical and experimental studies of cBN–TiN and cBN–TiC of cBN:TiN/TiC molar ratio 1:1 and 2:1. Theoretical calculations show that, at temperatures between 1000 and 1400°C, TiN reacts with BN forming one new phase, TiB2, and that TiC reacts with cBN forming two new phases, TiB2 and TiC0.8N0.2.. Experimental cBN–TiC/TiN composites were prepared by high pressure hot pressing and the samples were subsequently heat treated. After heat treatment, sinters of cBN–TiN/TiC were characterized using transmission electron microscopy and X-ray diffraction. The samples exhibited a dense polycrystalline structure, and a thin layer of fine TiB2 was visible at the BN–binder interface. It was found that hardness decreased significantly after heat treatment.

X-ray diffraction studies of oxidized high-palladium alloys.

OBJECTIVE The purpose of this study was to use x-ray diffraction (XRD) to obtain new information about the oxide layers on four representative oxidized high-palladium alloys. METHODS Cast specimens of two Pd-Cu-Ga alloys and two Pd-Ga alloys, with both polished and etched surfaces and air-abraded surfaces, were subjected to oxidation procedures recommended by the manufacturers. The specimens were analyzed by x-ray diffraction using CuK alpha radiation, and the peaks were compared to appropriate Joint Committee on Powder Diffraction Standards (JCPDS). RESULTS The surface preparation procedure had a profound effect on the phases present in the oxide layers. For the specimens that had been polished and etched, CuGa2O4 and beta-Ga2O3 were detected on the 79Pd-10Cu-9Ga-2Au alloy, whereas SnO2 and CuGa2O4 were detected on the 76Pd-10Cu-5.5Ga-6Sn-2Au alloy. The oxide layers on both Pd-Cu-Ga alloys contained Cu2O1 and the oxide layer on the 76Pd-10Cu-5.5Ga-6Sn-2Au alloy may contain beta-Ga2O3. The principal phase in the oxide layers on both Pd-Ga alloys that had been polished and etched was ln2O3, which exhibited extreme preferred orientation. No other phase was detected in the oxide layer on the 85Pd-10Ga-2Au-1Ag-1 ln alloy, whereas beta-Ga2O3 was found in the oxide layer on the 75Pd-6Ga-6Au-6Ag-6.5ln alloy. For the air-abraded specimens, beta-Ga2O2 was not present in the oxide layers on the Pd-Cu-Ga alloys, and beta-Ga2O3 was the major phase in the oxide layers on the Pd-Ga alloys. Palladium oxide(s) in varying amounts were detected for both surface preparations of the Pd-Cu-Ga alloys and for the air-abraded Pd-Ga alloys. Except for the 76Pd-10Cu-5.5Ga-6Sn-2Au alloy, the oxide layer caused minimal change in the lattice parameter of the palladium solid solution compared to that for the as-cast alloy. SIGNIFICANCE Knowledge of the phases found in the oxide layers on these high-palladium alloys is of fundamental importance for interpreting differences in the adherence of dental porcelain to the metal substrates under static and dynamic conditions, and may provide guidance in the development of new high-palladium alloys with improved metal-ceramic bonding.