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The O/Ni ratios derived from the X-ray photoelectron (XP) spectra of a number of well-characterised Ni oxides were calculated and found to correspond well to the expected O/Ni ratios for these oxides. The measured ratios, combined with the characteristic XP spectra, provide a higher level of confidence to the assignment of O 1s chemical species, particularly for surfaces with multiple phases present. As examples of this, we discuss the additional information imparted by O/Ni ratios measured for oxides that were electrochemically grown on Ni and NiCr-20% alloy surfaces.

This paper reports several interrelated XPS investigations of the Cr–O system that provide additional interpretation of the Cr 2p3/2 spectra, along with more detail on the reaction mechanisms between O2 gas and H2O vapour with metallic Cr. The Cr 2p3/2 spectra of polycrystalline Cr2O3 contain multiplet structures that bear a strong resemblance to those calculated by Gupta and Sen for the free Cr3+ ion. The O/Cr ratio derived from the corrected Cr3+ and substitutional O2− intensities for all polycrystalline Cr2O3 samples was 1.5. Annealing of freshly cleaved polycrystalline Cr2O3 surfaces above 550 °C resulted in a sharpening of the multiplet structures that may be caused by the increased prevalence of the (0 0 0 1) Cr2O3 orientation on the surface. The slight difference in binding energies between the low energy (0 0 0 1) plane and the other major crystallographic orientations could contribute to the spectral broadening that is observed for the Cr 2p3/2 spectral line shapes. The Cr 2p3/2 spectrum for Cr(OH)3 was isolated from that produced by Cr2O3 and the multiplet structure was qualitatively re-assembled for that of the hydroxide. The Cr 2p3/2 and O 1s spectra of reaction products on polycrystalline metal surfaces were analysed using the above information. The oxidic portion of the Cr 2p3/2 spectra could be cleanly separated from the metallic substrate using spectral subtraction. Reactions of either O2 gas or H2O vapour both produced a thin Cr2O3 film that was deficient in Cr3+ and, depending on the reactant, showed varying concentrations of hypo-stoichiometric Cr moieties. The surface corrected O/Cr ratios calculated for all the oxidised surfaces were 1.7. There was little evidence to support the formation of Cr(OH)3 on these surfaces. For comparison, an aqueous reaction of a Ni–Cr (20%) alloy was studied where both Cr(OH)3 and Cr2O3 components could be analysed. For this example no sub-stoichiometric Cr oxide species were produced.

TM abstract The oxidation of polycrystalline nickel (Ni) metal surfaces with water (H2O) vapour was studied at 25 ◦C and 300 ◦ C using dose rates considerably higher than in previous studies. The reaction rate was followed to a point where the oxide thickness was several nanometers. This was done using X-ray photoelectron spectroscopy (XPS) and modeling of the spectral profiles using QUASES

X-ray photoelectron spectroscopy (XPS) was used to examine the corrosion films formed on electroless nickel (EN) deposits prepared in an EN solution containing two different types and concentrations of bath stabilizers in Ar-purged and oxygenated neutral 5% NaCl solution. Two distinctive corrosion films were observed after long periods of immersion. An enrichment of elemental phosphorus compared to nickel was observed on the surface of the EN deposit produced with no stabilizer and with maleic acid (MA). By comparison, a high degree of surface oxidation was observed on the EN deposit prepared with thiourea. Trace impurities of sulfur in the EN deposit prepared with thiourea appear to result in enhanced corrosion and, in turn, lead to the build-up of corrosion products on the EN surface. By contrast, a P-enriched chemical passivation layer was formed on the EN deposits prepared either without any stabilizer or with MA present.

(Fe(OD)2) layer over an inner oxide (FeO) layer. The extraordinary depth resolution of the ToF-SIMS profiles assisted in identifying the two phases; this resolution was achieved by compensation for surface roughness. A substantial concentration of deuterium was found in the subsurface oxide layer. This observation confirmed previous assessments that the formation of FeO was from the reaction of Fe(OD)2 with outward-diffusing Fe, leaving deuterium as a reaction product. Copyright  2005 John Wiley & Sons, Ltd.

The low temperature (

The surface chemistry associated with pitting corrosion on a nickel-based alloy, Inconel 600, has been studied using imaging x-ray photoelectron spectroscopy. The irregularity of the sample surfaces necessitated a more elaborate background correction than is usually applied in such imaging. Areas of active pitting were found to contain corrosion products that are high in chromium oxide and depleted in nickel and iron. Sites of anodic activity on the surface were able to be defined more clearly using principal component analysis. Elements that are dissolved preferentially are deposited cathodically within well-defined regions whose distance from the anodic pit appears related to the open-circuit potential. Copyright  2005 John Wiley & Sons, Ltd.

The open-circuit corrosion and anodic oxidation behavior of the C-series of Ni-Cr-Mo alloys (C-4, C-276, C-2000, and C-22) and alloy 625 have been studied at 25°C and 75°C in 1.0 mol·L−1 NaCl+1.0 mol·L−1 H2SO4. A combination of open-circuit potential, potentiostatic polarization, and electrochemical impedance spectroscopy were employed in the study. The composition of the films formed was determined by x-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectrometry. Passive oxide film resistances increase and defect oxide film concentrations decrease as films thicken and chromium and molybdenum segregate to the alloy/oxide and oxide/solution interfaces, respectively. The high-chromium alloys exhibit higher film resistances and lower film defect concentrations consistent with the more positive potentials observed on these alloys. The results show that the observed ennoblement in corrosion potentials with time is coupled to the Cr/Mo segregation process and the suppression of defect injection at the alloy/oxide interface. By all measures, C-22 exhibited the best passive properties.

Partial and complete self-assembled monolayers (SAMs) of octadecylphosphonic acid (OPA) have been deposited onto air-exposed surfaces of the metals copper, silver, gold, iron, silicon and aluminium, as well as onto freshly cleaved, air-exposed surfaces of the minerals muscovite and biotite. The line width of the C(1s) signal in the XPS spectra of the surface narrowed, as the extent of coverage increased to 100%, to a half-width of 0.9 eV. Moreover, the line widths associated with the insulating muscovite substrate also became substantially narrower as OPA coverage increased. Binding energy differences on this chargeshifted surface were found to be more consistent when OPA was used as a charge reference, compared to using adventitious carbon as a reference. OPA coverage of the air-exposed metals copper, silver, gold and iron also produced narrow C(1s) spectra whose binding energies were consistently close to 284.9 eV. The C(1s) binding energy positions on Al and Si samples were charge-shifted by the insulating nature of the thin oxide formed on air exposure, or by the insulating nature of the substrate in the case of the minerals. Correction of the observed C(1s) energy position to 284.9 eV gave sets of elemental binding energies for the substrate materials that were reproducible. Thus, OPA coverage could be a possible alternative candidate for use in charge correction of binding energies of insulating materials. The OPA coverage cases were modelled using the software QUASES  Analyse. For the substrates copper, silver, gold, iron and aluminium, analyses of the metal core line spectra gave OPA overlayer thicknesses close to those measured by AFM (1.6 nm). However, QUASES  analyses of the C(1s) extrinsic backgrounds for the same surfaces required the use of an attenuation length of only 0.4 nm to derive a comparable thickness – much lower than literature values for carbon. This discrepancy is ascribed to the structured nature of the SAM. Copyright  2005 John Wiley & Sons, Ltd.

The oxidation of Fe by water (H2O) vapour was studied using X-ray photoelectron spectroscopy (XPS) and QUASES TM . The results indicated that the oxide formed at 25 � C and 150 � C was thinner than that formed when oxygen (O2) gas was present. The kinetics of the reaction were studied and found to follow a direct logarithmic relationship at both 25 � C and 150 � C. At 150 � C, a change in mechanism occurred after an oxide layer of approximately 0.7 nm had formed. It was determined the mechanism changed from a temperature-independent place-exchange to a temperature-dependent field-driven mechanism. The decreased rate of oxide growth in water vapour compared to that in O2 is proposed to result from the presence of H within the oxide as well as adsorbed to the surface. Its location within the oxide acts to restrict the amount of diffusion that can occur, while its presence on the surface restricts the number of surface states available for further adsorption of H2O. The slow conversion of hydroxide to oxide, as well as differences in transport properties between hydroxides and oxides, were also felt to decrease the thickness of the layer formed compared to equivalent reactions with oxygen. � 2004 Elsevier B.V. All rights reserved.

Oxidation of iron (Fe) by oxygen (O2) at low pressures was investigated using XP spectra and QUASES TM Analyze and Generate software programs. The results indicated that the oxide films produced after short oxygen exposure times contained a mixture of magnetite (Fe3O4) and maghemite (c-Fe2O3) without distinct interfaces between the two being seen. Further investigation suggested that a reaction mechanism involving cationic diffusion was mostly responsible for the formation of Fe3O4, while a mechanism involving anionic diffusion was responsible for c-Fe2O3 growth. The kinetics of the overall reaction were also investigated and found to follow a direct logarithmic relationship with increasing time. The rates of oxide growth were calculated for different pressures studied at the same temperature and found to be the same. This observation was used to conclude that the rate-limiting step was the formation of a thin oxide film by place-exchange and not anionic or cationic diffusion. � 2004 Elsevier B.V. All rights reserved.

QUASES Analyze and Generate were used to model the extrinsic loss structures for XPS spectra of oxide films grown on iron in such a way that their thickness and structure could be determined. The Generate program used in conjunction with spectra of model iron oxides allowed for both magnetite (Fe 3 O 4 ) and maghaemite (γ-Fe 2 O 3 ) structures to be identified in all films studied. These structures were identified as overlying layers in the oxide films and were usually intermixed at their interface. The absence of other iron oxide structures within the film could be tested based on their goodness of fit to the experimental spectrum. Comparison of the thickness values obtained using Generate with those found using nuclear reaction analysis suggested that the Generate results were higher by 20%. This difference likely resulted from the use of a calculated inelastic mean free path value for Fe 2p electrons in the Generate calculation rather than using the real attenuation length. For oxide films whose thickness approached 10 nm, the QUASES results for photoelectron spectra obtained with a Zr achromatic x-ray source were compared with those from the standard Al monochromatic source. In this particular case, the oxide thicknesses obtained using Generate and Analyze were found to be more consistent when the Zr source was used.

A monochromatic Al K α source and magnetic confinement charge compensation yield an As 3 d line width of 0.52 eV for the bulk insulator As 2 S 3 (orpiment) which is as narrow as the As 3 d line width obtained for the semiconductor FeAsS (arsenopyrite). These results, and the XPS results for silicates, demonstrate that charge broadening of XPS spectra has been overcome for most insulators provided care is taken with sample preparation. XPS core level spectral investigations of these minerals now may be conducted at the same level of detail as have been conducted on semi-conductors and metals. The Si 2 p spectrum of a non-conducting orthosilicate (Mg-rich olivine) displays a total line width of 1.36 eV compared to very high resolution synchrotron XPS line widths of 1.1 eV for an Si 2 p spectrum of gaseous Si(OCH 3 ) 4 , and 1.42 eV for the Si 2 p peak of non-charging SiO 2 thin-films grown on Si metal. These results demonstrate that Si 2 p peak broadening of these silicates does not result from differential charging. Comparison of the Si 2 p spectrum of Si in olivine which contains non-polymerized, independent SiO 4 tetrahedra, with that of a gaseous analogue, Si(OCH 3 ) 4 , also composed of non-polymerized Si atoms tetrahedrally bonded to O, provides insight into the cause of Si 2 p peak broadening for olivine (and probably for all other silicates). The vibrational contributions to the Si 2 p spectrum of Si(OCH 3 ) 4 were used to fit the Si 2 p peak of olivine, with each vibrational contribution broadened to reflect the minimum line width of the XPS instrument. Although the Si 2 p envelope resulting from this procedure yields a reasonable fit to the olivine Si 2 p spectrum, additional minor vibrational contributions may be required to reproduce accurately the Si 2 p spectrum of olivine. Continued development of the technique should allow determination of transition metal oxidation states and other chemical state properties of non-conductor Al-silicates.

Ferrous (Fe2+) and ferric (Fe3+) compounds were investigated by XPS to determine the usefulness of calculated multiplet peaks to fit high-resolution iron 2p3/2 spectra from high-spin compounds. The multiplets were found to fit most spectra well, particularly when contributions attributed to surface peaks and shake-up satellites were included. This information was useful for fitting of the complex Fe 2p3/2 spectra for Fe3O4 where both Fe2+ and Fe3+ species are present. It was found that as the ionic bond character of the iron —ligand bond increased, the binding energy associated with either the ferrous or ferric 2p3/2 photoelectron peak also increased. This was determined to be due to the decrease in shielding of the iron cation by the more increasingly electronegative ligands. It was also observed that the difference in energy between a high-spin iron 2p3/2 peak and its corresponding shake-up satellite peak increased as the electronegativity of the ligand increased. The extrinsic loss spectra for ion oxides are also reported; these are as characteristic of each species as are the photoelectron peaks. Copyright © 2004 John Wiley & Sons, Ltd.

Air oxidation of Fe was compared with and without a pre-exposure to water vapour. Angle-resolved x-ray photoelectron spectroscopy (ARXPS) and QUASES were used to determine the thickness of the oxide layer formed and its composition. The extent of oxidation was found to be much less if the surface was pre-exposed to water rather than air alone. Studies performed using ARXPS were able to show that the hydroxyl-containing layer located at the surface after Fe was exposed to water vapour was located below the surface after exposure to air. This observation suggested that the oxidation of Fe in air is mediated by cation diffusion.

Photoelectron spectra of a number of chromium oxides and other compounds were studied under high spectral resolution conditions chosen to reduce the possibility of differential charging. Some of the suite of Cr(III) compounds chosen for study produced Cr 2p spectra containing fine structure that could be identified with multiplet splitting. The splitting patterns produced were similar for all trivalent binary and ternary oxides and sulphides whose patterns closely reproduced the splitting predicted for the Cr(III) free ion by Gupta and Sen. The fine structure observed for compounds such as chromium (III) chloride had a distinctly different pattern. A number of other chromium (III) compounds were studied that did not exhibit the fine structure described above; nonetheless, well-defined line shapes and reproducible peak centroids were obtained by fitting protocols. The use of such information to determine surface chemistry on chromated steels is described, based on the spectral knowledge of chromium (III) oxides and hydroxides and the chromium (VI) oxide systems.

X-ray photoelectron spectroscopy (XPS) has seen widespread use in applied and basic surface science studies since 1970. Its capabilities appear to be undergoing a series of improvements, due both to technological changes and to the culmination of efforts by the XPS community to solve a series of scientific questions that underlie the technique. This paper describes some of the advances made in the past 5 yr and uses two particular studies to illustrate the improvements: the use of mathematical deconvolution to study the initial and extended oxidation of nickel metal, and the use of XPS imaging to identify electrochemical processes during the pitting corrosion of a nickel-based alloy. Finally, the use of modern synchrotrons in further improving the spectroscopic capabilities of XPS will be described.

The Auger parameter analysis in a conjunction with Au M3-edge XANES has been used to obtain the electronic configurations for Al2Au and Au2Al alloys. The net electron transfer from Al to Au (0.13 e− for Al2Au) has been estimated using the modified final state Auger parameter shifts in combination with the Thomas and Weightman model. The magnitude and direction of charge transfer appear to be in agreement with the differences in electronegativity values of the elements. Further, the opposite sign of the Auger parameter shift for Au and Al indicates that the Au core holes in the alloys are in a better screening environment than the core holes in pure Au while Al in the alloy environment is less screened. Au M3-edge XANES has been used to probe the unoccupied densities of Au 5d states. The appearance of the white-line features in Al2Au and Au2Al indicates the loss of 5d electrons at the Au site due to the charge redistribution caused by intra- [Au(5d)→Au(6s, 6p)] and extra-atomic [Au(5d)→Al(3s, 3p)] orbital hybridization. Al2Au exhibits an increased transfer of charge away from the gold compared to Au2Al due to the greater availability of empty states in the Al valence band. In general, the net charge transfer from Al to Au was substantially compensated by the 5d electron depletion, which is responsible for the shifts of core-level and valence band to higher binding energy. Changes in observed plasmon loss structures could be related to the strength of Au–Al bonding resulting from the localization of valence electrons. This further gives a clear picture of the charge rearrangement between Au and Al during alloying.

The oxidation behavior of Al2Au and Au2Al alloys during a 3-month laboratory air exposure has been investigated using SIMS, XPS and XANES. The oxide growth on both alloys and Al metal were found to be remarkably similar in the whole exposure range. XPS studies showed a rapid initial growth of oxide over the first 10 days. The oxidation of the aluminum component caused phase transitions at the near-surface to alloys which were more Au-rich. SIMS depth profiling showed that the oxide thickness was considerably greater than that estimated by XPS using a simple overlayer model. Both alloy structures were shown to be epitaxically grown homologues to the base aluminum grain structures; by contrast, oxide structures showed no preferred growth on particular alloy grains. Oxidation of the Au2Al results in a major depletion in the aluminum in the immediately below the oxide. Al L2,3-edge XANES was used to probe the nature of the oxide formed on the alloys as compared to that on pure Al. The chemical characteristics of the oxide was found to be affected by the presence of the gold. This difference in oxide structure is detected at the interface and not through the oxide generally.

The accumulation of adventitious carbon on Al and Au–Al alloy surfaces has been studied using XPS across a wide range of water vapour and air exposures. Carbon growth undergoes several discrete changes with the increase of exposure dose. X-ray irradiation effects on carbon growth were also investigated. Under x-ray irradiation Au–Al surfaces show a faster carbon growth than pure Al and Au metal surfaces. A fivefold increase of carbon contamination rate on an Au-rich Au–Al alloy (Au/Al = 5.0) surface has been observed. Copyright © 2002 John Wiley & Sons, Ltd.

Imaging XPS has been used to determine the distributional patterns of corrosion products of different chemistry on a nickel–copper alloy surface that has been subjected to pitting corrosion. Separate regions within corrosion pits can be identified with chemical processes that are suggested to be anodic and cathodic in nature. The anodic processes are indicated by a thin layer of cuprous oxide covering the base alloy, suggesting the preferential dissolution of nickel. Areas undergoing a cathodic process are indicated by the deposition of nickel as an oxide or hydroxide. Adjacent regions also were studied by XPS and characterized further by low-voltage SEM/energy-dispersive x-ray imaging and time-of-flight SIMS techniques to obtain confirmatory and complementary information. Typically, the anodic and cathodic areas were tens of microns in size. Copyright © 2002 John Wiley & Sons, Ltd.

The interactions of carbon dioxide and carbon monoxide at moderately high pressures with clean iron surfaces have been studied using x-ray photoelectron spectroscopy (XPS) and static time-of-flight secondary ion mass spectrometry (ToF-SIMS). Clean iron surfaces were analysed after exposure to CO2 or CO gas having pressures of 10–104 Pa at 25 °C. Exposure to either gas results first in a thin surface film of FeO. In addition, this oxide is completely or partially covered with a carbonaceous layer. The thickness of the carbon layer is influenced by the type of gas used and its pressure. The ToF-SIMS studies using isotopically labelled 13CO2 under the same reaction conditions show that the labelled carbon is not enriched on the surface and therefore no direct reaction occurs between the labelled carbon and the surface. It is proposed that the influence of either gas on the carbonaceous layer formed may be indirect: these gases may alter the equilibrium partial pressures of other adsorbates on the walls of the vacuum chamber. Copyright © 2002 John Wiley & Sons, Ltd.

The Maximum Entropy Method (MEM) has been used to deconvolute the valence-band XPS spectra of thin film Au–Al alloys. The enhanced resolution allows fine changes in the electronic structures of the thin film alloys to be distinguished with the aid of the Au4f core-level spectra. This particular alloy series allows one to examine the 5d electronic interaction between gold atoms as aluminum is gradually added to the matrix. Aluminum is shown to have a much stronger quenching effect on such interactions than has been found previously for copper.

The initial stages of oxide formation following the interaction of water vapour with polycrystalline aluminium surfaces have been studied using x-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES). Attenuation of the electron signal from the metallic substrate has been used to follow the oxidation kinetics of aluminium as a function of energy and dose of Ar+ ion bombardment and temperature. The effects of energy and doses of Ar+ ion bombardment on oxidation kinetics have been examined in the energy range 1–5 keV and ion dose ranging from 1.3 × 1016 to 3.8 × 1017 ions cm−2. Although the increase in energy of Ar+ ion bombardment slightly modified the oxidation kinetics, the effects of ion dose on oxidation kinetics are shown to be more complex. There is a threshold dose of Ar+ ions above which the surface activity becomes significantly reduced; this is ascribed to cluster formation blocking the surface diffusion pathway in the near-surface region. The oxidation kinetics of aluminium also have been studied in a range of temperature from room temperature up to 573 K. As the temperature increases, the oxidation rate of aluminium decreases due to a decrease in the sticking probability of water molecules on the aluminium surfaces. The observed temperature dependence of the oxidation kinetics is likely to be due to a precursor mechanism of water desorption. Copyright © 2001 John Wiley & Sons, Ltd.

The oxidation of thin-film Au- Al alloy, as well as pure Al, has been studied using XPS across a wide range of water vapour and air exposures (0.1 L to 1 × 1014 L). The alloys and the pure Al all undergo a similar set of oxide growth kinetics. In addition to the three stages of early growth kinetics identified for Al in previous studies, a fourth stage is identified at air exposure doses of >109 L. During this stage, the average composition of the film changes from Al2O3 to Al2O3·H2O. The Al 2p lineshapes for the oxides grown on pure Al and Au-Al alloy are different; the origins of this appear to result, in part, from a change in the Fermi level as the oxide thickens. The Al L2,3 X-ray absorption near-edge structure (XANES) furthermore provides a complementary probe of the different oxide structures on alloy and Al metal. Thus, their chemical characteristics are affected also by the presence of the gold. This difference in oxide structure is detected at the interface and not through the oxide generally. Copyright © 2001 John Wiley & Sons, Ltd.

The oxidation kinetics of Mg-, Si- and Fe- implanted aluminum has been studied at room temperature and a water vapour pressure of 2.0◊10'6 Pa using X-ray photoelectron spectroscopy. The elements Mg, Si and Fe were implanted into pure aluminum at low ion doses from 6.0◊1012 to 3.6◊1013 ions cm'2. Secondary ion mass spectrometry (SIMS) depth profiles have shown that the element distributions of Mg and Fe in aluminum are consistent with the theoretical depth profiles calculated using a TRIM program. The implanted elements are distributed in the near surface region with their maximum concentration at a depth of ~50 nm. In the case of implanted Si, however, the SIMS depth profile shows no maximum concentration peak, and the near-surface concentration is much higher than that calculated. This is ascribed to diVusion of Si toward the surface during implantation. The oxidation kinetics of implanted aluminum have shown that surface concentrations of implanted Si, as low as 40 ppm, cause an increase in the Al oxidation rate compared to the pure metal. By contrast, equivalent concentrations of implanted Mg in aluminum result in a lowered oxidation rate until the oxide reaches a thickness of 3 monolayers, after which a rapid increase in oxidation occurs. Implanted Fe in the same concentration range does not cause any change in oxidation rate of aluminum. These oxidation kinetics can be explained on the basis of metal vacancies in the defect structure of aluminum thin oxide films. The eVects of surface damage caused by ion beam during the implantation experiment were also determined by measurements of oxidation kinetics. © 1999 Published by Elsevier Science B.V. All rights reserved.

Surface analysis techniques, including x-ray photoelectron spectroscopy with small-spot analysis (down to 100 μm 2 ) capabilities, field emission scanning Auger microscopy, electron energy-loss spectroscopy in association with these two techniques and micro-Raman spectroscopy, have been used to collect surface information on specific facets of commercially available submillimetre-sized cubic boron nitride (cBN) crystals. With the freedom of choosing a specific facet with a known orientation relative to the probing beam and detector, it is possible to obtain accurate information on the surface compositional structure of a cBN crystal before and after a surface reaction.

The three-way parallel factor analysis (PARAFAC) has been used to decompose a set of XPS (X-ray photoelectron spectroscopy) spectra which result during the oxidation of aluminium surfaces by water vapour. Al(2p) and O(1s) core-level photoelectron lines have been used to follow oxide film growth on clean aluminium surfaces as a function of exposure time and pressure of water vapour. The PARAFAC solution provides new information on elemental processes in the very initial stages of oxidation kinetics, showing new components in the XPS spectrum, as well as their evolution through the range of time and pressure variables. Reaction of H2O vapour with aluminium results in attenuation of the metallic peak, binding energy (BE) at 72.87±0.05 eV, and increase of the oxidic peak, BE at 75.80±0.05 eV. An additional factor is identified, which suggests the formation of an interface metal hydride, with BE at 72.4(4) eV, as well as a concomitant oxide peak at 75.4(3) eV. At pressures above 1.3×10−5 Pa this factor is diminished; this is presumably due to the increase in recombination of atomic hydrogen.

Chemical reactions of the surface of a polypropylene (PP) film in the presence of various combinations of ultraviolet light and ozone gas (UVO) conditions were studied. Exposure of the polymer surface was carried out in a laboratory-scale UVO reactor in which the following parameters could be varied: ozone concentration, wavelength of ultraviolet (UV) radiation, pulsed operation of the UV lamps, the treatment distance between the PP film and the lamps, and water vapor concentration. Advancing and receding contact angle measurements were used to monitor surface energy changes imparted by the treatment. Two spectroscopic techniques, X-ray photoelectron spectroscopy (XPS) and attenuated total reflectance–Fourier transform infrared spectroscopy (ATR–FTIR), were used to monitor changes in the polymer surface chemistry. Oxidation of the PP surface is proposed to occur through two alternate mechanisms: (1) insertion of an O (1D) atom to form ether linkages, or (2) hydrogen abstraction by O (3P), followed either by crosslinking or by reaction with oxygen species to form carbonyl and/or carboxyl functional groups. It was found that reaction 1 dominates initially, but that its rate is reduced by the formation of products from reaction 2. It appears that the ether functional groups produced by reaction 1 are responsible primarily for increased surface energy. Carbonyl, carboxyl, and hydroxyl groups formed in reaction 2 appear to have little additional effect on surface energy; it is proposed that these groups are involved strongly in intramolecular hydrogen bonding, thereby decreasing their availability to contribute to increased surface energy. High-energy UV radiation was found to play only a minor role in the surface modification of PP. Of the narrow range of ozone concentrations studied, no clear relationship was found to exist between ozone concentration and rate of modification of the surface; thus, the concentration of ozone must not affect the relative concentrations of products from the competing reactions. Increased surface oxidation and decreased contact angles were observed when the lamp-to-sample distance was minimized. The presence of water vapor during UVO treatment was found to lead to greater oxygen uptake after short-term treatments but did not result in increased surface energy. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 2489–2501, 1999

Three-way parallel factor analysis (PARAFAC) has been used to decompose a set of x-ray photoelectron spectroscopy (XPS) spectra that result during the oxidation of aluminium surfaces. The Al 2p core-level photoelectron lines have been used to follow oxide film growth on clean aluminium surfaces as a function of exposure time and pressure of water vapour. In this paper, a fine peak structure of the XPS Al 2p spectrum has been extracted using PARAFAC. The PARAFAC solution provides new information on elemental processes in the very initial stages of oxidation kinetics, showing new components in the XPS spectrum as well as their evolution through a range of time and pressure variables. As expected, the reaction of water vapour with aluminium results in attenuation of the metallic peak at binding energy (BE) 72.87±0.05 eV and an increase of the oxidic peak at BE 75.80±0.05 eV. However, an additional factor is also identified, which suggests the formation of an interfacial metal hydride at BE 72.4(4) eV, as well as a concomitant oxide peak at 75.4(3) eV. Both are ascribed to products of the hydrolysis of adsorbed water molecules at the aluminium interface. At pressures above and below 1.3×10−5 Pa this factor is diminished; in the case of higher pressure, this is ascribed to an increase of the recombination of atomic hydrogen. Copyright © 1999 John Wiley & Sons, Ltd.

The oxidation kinetics of Mg-, Si-, and Fe-implanted aluminum has been studied at room temperature and a water vapor pressure of 2.0 {times} 10{sup {minus}6} Pa using X-ray photoelectron spectroscopy. The elements Mg, Si, and Fe were implanted into high-purity aluminum at low ion doses from 6.0 {times} 10{sup 12} to 3.6 {times} 10{sup 13} ions {center_dot} cm{sup {minus}2}. Secondary ion mass spectrometry (SIMS) depth profiles have shown that the element distributions of Mg and Fe in aluminum are consistent with the theoretical depth profiles calculated using a TRIM (transport of ions in matter) program. The implanted elements are distributed in the near-surface region with their maximum concentration at the depth of {approximately} 50 nm. In the case of implanted Si, however, SIMS depth profile shows no maximum concentration peak and the near-surface concentration is much higher than that calculated. This is ascribed to diffusion of Si toward the surface during implantation. Oxidation kinetics of implanted aluminum have shown that surface concentrations of implanted Si, as low as 40 ppm, cause an increase in the Al oxidation rate compared to the pure metal. By contrast, implanted Mg, surface concentration from 20 to 120 ppm, results in a lowered oxidation ratemore » until the oxide reaches a thickness of {approximately}0.55 nm, and after that a rapid increase in oxidation occurs. Implanted Fe does not cause any change in the oxidation rate of aluminum. These oxidation kinetics can be explained on the basis of metal vacancies in the defect structure of aluminum thin oxide films. The effects of surface damage caused by the ion beam during the implantation experiment were also determined by measurements of oxidation kinetics.« less

The surface oxidation behavior at room temperature of two thin films of the alloys Al2Au and Au2Al was investigated using X-ray photoelectron spectroscopy (XPS). Relatively large chemical shifts of both Au4f and Al2p photoelectron lines with the alloy composition assist the identification of additional alloy phases. The spectral resolution has been improved by the use of the Maximum Entropy Method which removes the external contributions to the photoelectron linewidth. In the case of Au2Al, as much as 80% of the surface phase is transformed into two new alloy phases. This is equivalent to a transformed layer thickness of 10 nm. For the Al2Au, oxidation also results in a partial phase transition to a gold-enriched alloy, but the transition region extends only to an estimated depth of 2 nm.

The long-term oxide growth kinetics under atmospheric ambient conditions has been studied for pure magnesium and several magnesium alloys. Oxide thicknesses in the range of 1–15 nm are measured using X-ray photoelectron spectroscopy, making use of the inelastic mean free paths for magnesium and MgO. On pure magnesium, the oxide growth rate is slow, following logarithmic kinetics during exposure periods as long as 10 months. On samples containing iron or manganese as inclusions, an increased oxidation rate was noted following exposure periods as short as several hours. Evidence of localised oxidation around such inclusions could be observed from secondary ion mass spectrometry (SIMS) images of the near-surface region. High purity aluminum–magnesium alloys exhibit lower oxide growth rates than pure magnesium; the initially thicker oxide film contains aluminum ions which are believed to increase the activation energy for ion movement.

The maximum entropy method (MEM) is applied to the deconvolution of x-ray photoelectron spectra. This method provides the least-biased estimate of the unbroadened spectrum by using the Shannon information content as the regularizing functional. The large-scale, non-linear optimization problem is solved using a robust variable metric sequential-quadratic programming (SQP) algorithm implemented on a personal computer (PC). The program is tested on simulated spectra and then shown to provide reliable resolution enhancement of measured spectra by unfolding a measured instrumental resolution function. Typical resolution enhancements of 50% are achievable in < 15 min of computer time.

The maximum entropy method (MEM) is a new procedure for deconvoluting exciting line width and instrument resolution contributions from XPS spectra. MEM has been used to obtain additional detail from XPS 2p spectra collected from pyrite, marcasite and arsenopyrite surfaces which have been exposed by fracturing under UHV conditions. The resultant spectra have line widths that are nearly 50% narrower than the original spectra. Deconvoluted S 2p and Fe 2 p 3 2 spectra are shown to contain contributions from species located in disrupted lattice sites near the fracture surface as well as species on the outermost fracture surface. Iron 2p spectra are also found to exhibit fine structure which is attributed to multiplet splitting.

X-ray photoelectron spectroscopy (XPS) has been used to follow oxide film growth on pure magnesium and pure aluminum surfaces across 12 orders of magnitude exposure to water vapour and humid air. Both Mg and Al exhibited logarithmic-type oxide growth kinetics. Oxide film growth was observed to continue well into the ambinent exposure range, suggesting the possibility for modifying the oxide film chemistry while still under atmospheric conditions. The chemistry of the films was monitored as a function of exposure by examining the changes in the core-level O 1s spectra and the associated oxidised metal/oxygen stoichiometry. It was observed that both Mg and Al surfaces initially formed partially hydrated oxide surfaces which became progressively more hydrated at longer exposures.

Copper- and chromium-impregnated and non-impregnated charcoals have been characterized by X-ray photoelectron spectroscopy and scanning electron microscopy, showing the composition and distribution of elements in the charcoal substrate. Water (as D2O) was reacted with dried charcoals and its gradual thermal desorption was studied by mass spectrometry. The impregnated charcoal retains substantially more water over a wider temperature span. It is suggested that Cr3+, in particular, increases the number of high-energy adsorption sites. Thermal desorption also results in decomposition of metal hydroxides and reduction of Cu2+ to Cu+ and Cr6+ to Cr3+.

A number of binary oxides, sulphide compounds and complex minerals have been analysed by x-ray photoelectron spectroscopy to give the relative intensities of major photoelectron lines. Where possible, compound surfaces were prepared by fracturing in vacuo or in inert gas to avoid contamination. Alternatively, some of these compounds have undergone a mild thermal treatment in vacuo to remove much of the surface contamination which normally affects such intensity measurements. The experimental line intensity ratios were corrected only by their particular photoelectron cross-section and inelastic mean free paths. For most oxides and complex minerals studied, good agreement is obtained with the expected stoichiometry, particularly when using photoelectron lines of higher kinetic energy. This tends to support the use of a simple background approximation when measuring specifically with those oxides involving non-transition elements. The sulphides studied mostly involved transition metals ; agreement between calculated and actual stoichiometry was only modest. Some differences in measured surface stoichiometry occurred as a result of the ambient conditions within which the fracture was made ; fracture in vacuo produced a different S/M ratio than did fracture in an inert gas environment.

The gas phase downstream products of an air glow discharge have been measured, using absorption and emission spectroscopies, as a function of plasma power, air flow rate, and distance from the plasma. In addition, the reaction of these products with a linear low density polyethylene (LLDPE) polymer surface has been followed using x-ray photoelectron spectroscopy (XPS). At higher air flow rates (>300 sccm), the primary reactive species is confirmed to be O( 3 P) atomic oxygen. Some O( 3 P) is generated in the plasma itself, but more appears to be formed in the downstream region, because of dissociation of molecules in their excited states. At low flow rates, the concentration of O( 3 P) is strongly depleted at the sample position, but other atomic oxygen states become more prominent. O( 5 S) and O( 3 S) are two states which are identified. XPS studies of the polyethylene surface reacted at high flow rates shows oxygen functionalities that are likely the result of an initiation by hydrogen abstraction. At low flow rates, the products suggest initiation by oxygen insertion. Thus, changes in flow rate can result in major changes to the polymer surface chemistry.

X-ray photoelectron spectroscopy (XPS) was used to examine the corrosion films formed on nanocrystalline (nc) and amorphous Ni-P alloys in 0.1 M H 2 SO 4 . Neither the nc nor the amorphous alloys were found to exhibit passivity. An enrichment of elemental P compared to Ni was observed at the surface of both materials, suggesting that Ni is preferentially dissolved during anodic polarization. The high volume fraction of grain boundaries and triple junctions on the nc specimens resulted in enhanced dissociative adsorption of oxygen and hydroxyl species from solution. The films formed were, however, non-protective because the defective nc surfaces also facilitated atom dissolution and oxidation of surface P atoms from hypophosphite to soluble phosphate anions. At higher applied potentials, the thick, porous films formed on the nc specimens provided a small kinetic barrier to further dissolution, resulting in slightly lowered anodic current densities compared with amorphous Ni-P.

A mass-separated low-energy ion beam system was used to deliver pure OH+ and NH+ to 15nm thick polystyrene films on silicon in ultrahigh vacuum. This was done in an effort to produce specific surface chemical functional groups. X-ray photoelectron spectroscopy showed that when the bombardment energy of OH+ exceeded 10 eV, or the dose was higher than 1 × 1016 ions cm−2, a mixture of COH, CCO and CCOOH groups was produced, along with severe damage to the aromatic rings. However, for bombardment at 10 eV with a dose of 1 × 1016 ions cm−2, only COH (or COR) groups were found. Similarly, bombardment with NH+ at 10 eV and a dose 1 × 1016 ions cm−2 induced incorporation of a single nitrogen-containing functionality. The C 1s data indicated that the major chemical functionality on such surfaces in a CNH2 (or CNHR) group with a minor component of C(NH2)2. Hence, surface functionality can indeed be controlled by altering the molecular nature, energy and dose of the bombarding species.

The room temperature interaction of water vapour with polycrystalline magnesium specimens deliberately contaminated with very low levels of iron (120 to 700 ppm by weight) has been examined using Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectrometry (SIMS). The initial adsorption and subsequent reaction of the water molecules with the iron-containing specimens was shown to be retarded during the very early stages. The influence of iron on the uptake of oxygen on magnesium surfaces was not found to be a clear function of iron concentration. This was likely due to the fact that the solubility limit of iron in magnesium ( ~ 55 wt ppm) was exceeded in each specimen, suggesting that iron in magnesium solid solution strongly influences the oxidation process. The oxide growth rate at longer exposures, during the bulk thickening regime of the oxidation process, was found to be unaffected by the iron content. Segregation of iron into the oxide film was, however, detected by static secondary ion mass spectrometry (SSIMS). Finally, the saturated oxide films formed on Fe-containing magnesium specimens were shown by XPS to be more “defective” than those formed on pure magnesium.

The room temperature interaction of water vapour with two polycrystalline MgAl alloy surfaces, Mg-3.0wt%Al and Mg-8.5wt%Al, has been examined in detail using Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS). By using calibrated doses of deuterated water vapour, three stages of early oxide growth were recognised: dissociative chemisorption during low exposures (up to ≈ 0.5 langmuirs (L)); oxide nucleation and island growth during intermediate exposures (≈ 0.523.0 L); and slow, diffusion-controlled bulk oxide thickening after coalescence of the oxide islands. AES lineshape analysis indicated that water vapour dissociates on MgAl surfaces, leading to the simultaneous adsorption of oxygen atoms on both Mg and Al atoms present on the surface. Modelling results suggested that the rate of oxide nucleation and growth was enhanced on MgAl surfaces compared to on pure Mg surfaces, particularly at higher Al contents. In addition, it was found that magnesium oxide islands were preferentially nucleated at Mg-rich regions and subsequently grew to cover the mixed MgA-O chemisorbed precursor layer. At longer exposures, XPS indicated that Al 3+ ions were incorporated into the growing oxide film, resulting in an increased activation energy for ion movement and a pronounced decrease in the rate of bulk thickening. The surface concentration of aluminum was observed to decrease slowly during the bulk thickening regime. The results have been interpreted based on the Cabrera-Mott theory of low temperature oxidation.

This work explores the use of time-of-flight secondary ion mass spectrometry to characterize films on Monel 400 steam generator tubing that has been subjected to simulated CANDU reactor start-up conditions. The results are compared with those obtained using more traditional techniques, including scanning Auger microscopy and x-ray photoelectron spectroscopy. Copyright © 2002 John Wiley & Sons, Ltd.

Surface characterization of supported molybdenum oxide thin films is undertaken following calcination and reduction. In situ and ex situ laser Raman spectroscopy (LRS) and X-ray photoelectron spectroscopy (XPS) are the analytical tools used to probe the catalyst surfaces. Spectral peak broadening of Mo(VI) oxide films has been observed on electrically conducting thin films deposited on active and nonactive surfaces. This peak broadening decreases as the film thickness increases. Reduction of very thin (2 nm) amorphous films of Mo(VI) oxide on planar alumina does not result in lower oxidation states. In somewhat thicker films (4 nm), Mo 7 O 24 6- and a Mo 6 O 19 2- specis are identified following H 2 treatments

Surface characterization of supported molybdenum oxide thin films is undertaken following sulfidation treatments with H[sub 2]S. Surface characterization is carried out by in situ and ex situ laser Raman spectroscopy (LRS) and X-ray photoelectron spectroscopy (XPS). Sulfidation of thin-film molybdenum oxides supported on planar alumina and graphite produces two sulfides species: MoS[sub 2] and a reduced form with a binding energy lower than MoS[sub 2]. The S(2p) spectra also show polysulfide species at higher binding energies to MoS[sub 2]. Sulfidation of thicker, octahedral MoO[sub 3] thin films results only in MoS[sub 2]. Extensive prereduction of octahedral MoO[sub 2] prior to sulfidation is shown not to be important in the conversion of oxide to sulfide phases. 52 refs., 6 figs., 1 tab.

Surfaces of fine polystyrene (PS) and polymethyl methacrylate (PMMA) powders were modified by exposure to the downstream products of a nitrogen or oxygen microwave plasma. The effects of nitrogen and oxygen incorporation in the powder surface were studied with emphasis on variations in the triboelectric properties of the powder. X‐ray photoelectron spectroscopy was utilized to determine the changes in surface elemental composition. After nitrogen plasma treatment, the C 1s peak shapes suggested the formation of amines in the case of PS, and the formation of imines and amides in the case of PMMA. Oxygen plasma treatment appears to result in the formation of hydroxyl and carbonyl groups on the surfaces of both PS and PMMA. After treatment with a nitrogen or oxygen plasma, the charge‐to‐mass ratio (Q/M) of PS and PMMA powders in contact with carrier particles was measured using the cage blowoff method. The surface charge density (Q/A) was calculated from Q/M. The Q/A of nitrogen plasma‐treated PS powder was ...

The initial oxide formation on polycrystalline magnesium surfaces at room temperature has been quantitatively followed from the earliest stage using Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS). Clean surfaces were prepared by sputtering with a minimum dose of argon ions to avoid the creation of heavily disordered crystal planes. Crystallographic orientations of various grain faces were determined using the electron back scattering diffraction technique in order to allow AES measurements to be made on grains of known orientation. By using calibrated doses of deuterated water vapour, three stages of early oxide growth were recognised: (1) a chemisorption stage during doses up to approximately 0.7 langmuir (L) ( 1 L = 1.3 × 10 −4 Pa · s); (2) a rapid oxide nucleation and island growth stage which is complete by about 5 L at an average island height of four monolayers and (3) a slow, diffusion-controlled growth stage after coalescence of the islands. Modelling results suggest that the rate of initial oxygen uptake is faster on grain faces that have more open-packed, high index orientations, particularly in the second (island nucleation and growth) stage. In addition, the slow growth stage has been adequately described by a logarithmic type growth law for exposures up to 1 × 10 6 L, suggesting a limiting oxide thickness in water vapour of approximately 11 monolayers of MgO. The slow growth process was found to be controlled by the movement of metal cations through the oxide film from the metal/oxide interface to the oxide/gas interface. Finally, the role of hydrogen in the oxide film was studied using XPS and nuclear reaction analysis (NRA). The results indicated that hydrogen was present in the film only in small relative amounts, likely as hydroxyl groups trapped at the metal/oxide interface. Detailed XPS spectra showed two distinct types of oxygen: one of these is assigned to be oxygen in a normal MgO lattice position; the other is ascribed to oxygen in a “defective” chemical environment.