Use Of XPS Research Articles

A Study of Ethanol Reactions over Pt/CeO2 by Temperature-Programmed Desorption and in Situ FT-IR Spectroscopy: Evidence of Benzene Formation

The reaction of ethanol over unreduced and H2-reduced 1 wt% Pt/CeO2 has been investigated primarily by temperature-programmed desorption (TPD) and in situ Fourier transform infrared spectroscopy (FT-IR). Steady state reactions have been conducted to provide information regarding the kinetics of the above reaction. Characterisation of the catalyst has been achieved through the use of XPS and titration of surface metal sites with CO. XPS studies have shown that the addition of Pt partially reduces the CeO2 surface, as indicated by the decrease in the O/Ce ratio from the stoichiometric value to 1.59. FT-IR studies have shown that ethanol adsorbs dissociatively to form two types of ethoxides (monodentate and bidentate). The presence of Pt resulted in a Δν(C–O)=ca. −20 cm−1, for both species, with respect to observed ethoxide band positions on CeO2 alone. This shift may indicate that these species are perturbed due to the presence of Pt2+ clusters and the surface oxygen vacancies associated with them. The oxidative dehydrogenation of these ethoxide species produces acetaldehyde on both the unreduced and H2-reduced Pt/CeO2 surfaces. Adsorbed crotonaldehyde species were observed and characterised by bands at 1657 and 1632 cm−1 corresponding to ν(C=O) and ν(C=C), respectively. TPD experiments have shown that between unreacted ethanol and acetaldehyde, benzene is produced in appreciable amounts from both the unreduced and H2-reduced surfaces of Pt/CeO2. The formation of benzene has been postulated to proceed via reaction of surface-bound crotonaldehyde and acetaldehyde species. Additional studies involving the adsorption of CO over H2-reduced CeO2 and Pt/CeO2 have been conducted. FT-IR results show that CO does not adsorb appreciably on CeO2. In addition to linear and bridged CO species, a tilted CO species was identified, by a broad band at ca. 1704 cm−1, on Pt/CeO2 following adsorption of CO. Carbonite species (1325, 1297, 1190, and 1082 cm−1) were also detected on Pt/CeO2 following adsorption of CO and subsequent heating. TPD following CO adsorption on both CeO2 and Pt/CeO2 have shown that the predominant desorption product is CO2. CO2 desorbs from both CeO2 and Pt/CeO2 in three temperature domains. The lower desorption temperatures, 400 and 525 K, correspond to oxidation of CO to CO2 and decomposition of formate species, respectively. The high-temperature (700 K, CeO2 and 578 K, Pt/CeO2) CO2 desorptions arise from the decomposition of hydrogenocarbonates and carbonate species.

Characterization of Co-Al-O magnetic thin films by combined use of XPS, XRD and EPMA

Several Co–Al–O alloy thin films with different physical properties were prepared by reactive r.f. sputtering of Co85Al15 and Co70Al30 alloy targets in Ar + O2 atmospheres and were characterized by XPS, x-ray diffraction (XRD) and electron probe microanalysis (EPMA). The alloy films contained metallic Co, mainly of hexagonal phase, and also CoO. The Al oxide in the Co–Al–O thin films did not give any detectable diffraction peaks. The EPMA results showed that [Al]/([Co] + [Al]) ratios, where [M] denotes molar fraction of element M in the alloy, were almost the same as the compositions of the targets used for sputtering. However, quantitative XPS analysis resulted in much higher values of [Al]/([Co] + [Al]) ratios in comparison with the bulk values, suggesting enrichment of Al in the surface region. Aluminium detected by XPS was all in the oxidized state, and was in the form of hydroxide in the top surface region. Cobalt in the topmost region was in the form of oxyhydroxide, which was easily reduced to the metallic state by Ar+ etching. By taking the XPS, XRD and EPMA results into consideration, the difference of Co–Al–O alloys in electric resistance was explained qualitatively by the difference in microstructure of the alloy films, i.e. the difference in thickness of the oxide films on nanoscale particles of metallic Co embedded in an oxidized Al matrix together with the clearance between the nanoscale Co particles. Copyright © 1999 John Wiley & Sons, Ltd.

Use of XPS to investigate surface problems in ULC deep drawing steels

During continuous annealing prior to hot dip galvanising, the surface of ULC ( U ltra L ow C arbon) deep drawing steels undergoes a range of chemical modifications that can affect interface reactions, thus influencing the quality of the metallic protective coating and the product's final user's properties. These phenomena were investigated by XPS, in a high-resolution, fully automatic spectrometer coupled with a tight preparation chamber, where the treatment can be simulated very faithfully, in terms of both heat cycle and protective atmosphere. Under vacuum, the equilibrium (Gibbs) surface segregation of alloying — or tramp — elements such as P, Sn, Sb, As, S and C is clearly the predominant phenomenon. Selective oxidation grows much more significant when annealing is performed under a water vapour containing N 2–5%H 2 protective atmosphere. It involves highly oxidisable elements such as Al, Mn and Si. Though much less intense than under vacuum, Gibbs segregation however remains present, in direct competition with selective oxidation.

The use of XPS and ToF-SIMS to investigate adhesion failure of a cationic radiation cured coating on galvanized steel

Although steel substrates are often galvanized to protect against corrosion, further protection can be achieved by painting the exposed zinc surface. The paint must be durable so as to achieve the maximum possible protection. One of the most important properties of a durable coating system is adhesion, which must not deteriorate over the lifetime of the coating. The use of cationic radiation curing chemistry has shown much promise for the development of high-quality paint systems. The cationic radiation cured coating under investigation showed both good durability and mechanical properties, however, the coating delaminated from the substrate rapidly in a hot, humid environment. XPS and ToF-SIMS analyses of the delaminated interface has shown that the phosphorus hexafluoride anion of the photoinitiator segregates to the interface. On the basis of these results the coating was reformulated with a reduced concentration of the photoinitiator, and the durability was much improved.

Use of XPS for reservoir sandstone wettability evaluation. Application to kaolinite and illite

X-Ray Photoelectron Spectroscopy appears to be an attractive tool for wettability evaluation. Due, however, to carbon contamination, it is not possible to determine a direct relationship between surface carbon content and oil-wettability. Reference clays were aged with crude oil or with crude oil+brine; clay-size fractions from actual core samples were extracted, cleaned and submitted to the same procedure. XPS was used to determine the surface composition of the samples after each treatment. Some carbon was fixed on all the clay fractions after contact with crude oil. After contact with oil+brine, two behaviors were found: either more carbon was fixed than with crude oil, or the quantity of carbon fixed was close to the value before contact. Expected values for elemental surface composition versus bulk composition were calculated for kaolinite and illite, assuming variable thicknesses of carbon coating on the clay surface. The ratios Si/Al, K/Al and O/Al were not modified by a homogeneous carbon deposit: so XPS does not provide an answer about the presence of a homogeneous film or a distribution of patches on the surface, but possible changes in these ratios could be interpreted as a particular location of the carbon. Comparison of these simulation results with experimental results allowed an evaluation of the carbon coating thickness, or of the coverage ratio. Cleaning of the samples, which were more charged in carbon, showed that the major part was weakly retained. The final carbon content was still somewhat higher than before contact, which was consistent with a part of carbon being more strongly adsorbed. The major difference in the behavior of kaolinite and illite was found after aging in the presence of water, and might be related to the surface cations. Comparison of the amounts of surface carbon versus bulk carbon shows two mechanisms: adsorption and retention. Wettability changes are obviously the consequence of both phenomena.

Characterisation of the passive film on iron in phosphate medium by voltammetry and XPS measurements

Passivation of iron in phosphate medium was studied by voltammetry and X-ray photoelectron spectroscopy. The voltammetry at low sweep rate has permitted us to determined the different stages of the passivation. The process of electrochemical passivation obeys a dissolution-precipitation mechanism. The film growth is controlled by the resistance of the solution in the pores. The electrochemical impedance measurements show that the passive film on iron is ann-type semiconductor, highly doped (ND = 4.69 × 1020cm−3) and its flat-band potential was equal to -570 mV/(SSE). The composition depth-profile of the passive films was measured by sequential use of XPS and sputter etching. It was found that the film consisted mainly of both metallic oxides and phosphate.

The use of XPS to examine the interaction of poly(acrylic acid) with oxidised metal substrates

Abstract As part of a wider investigation into the use of high resolution (monochromated Al Kα) XPS to probe the interfacial reactions responsible for adhesion, thin films of poly(acrylic acid) have been studied on a variety of different substrates. It is shown that sample degradation, induced by photoelectrons from the metallic substrate, occurs in all cases. This is recognised by the appearance of a component in the C 1s spectrum 1.7 eV removed from the main aliphatic C 1s line. The intensity of this component is shown to scale with the total photoelectron cross-section of the metallic substrate. Through careful peak fitting of the high resolution C 1s spectra three different types of specific interaction between the PAA molecule and the oxidised substrates have been identified: (i) hydrogen bonding of the PAA molecule with strongly acidic sites; (ii) the formation of a carboxylate anion on interaction with weakly basic sites; (iii) nucleophilic attack of the acyl carbon by strongly basic sites. This, in principle, provides a direct estimation of the density of the different classes of acido-basic sites on the oxidised substrates.

Combined use of XPS, IR and EDAX techniques for the characterization of ZrO 2-SiO 2 powders prepared by a sol-gel process

Powder precursor gels with composition xZrO 2-(100 × x)SiO 2, with selected values of x = 8, 14 and 75 mol%, were prepared by a sol-gel chemical processing route, using commercial silica gel and zirconyl chloride. A combination of X-ray photoelectron (XPS) and infrared (IR) spectroscopies has been applied for studying the chemical bondings in silicon, zirconium and oxygen as a function of the nominal zirconia compositions. Furthermore, ZrO 2/SiO 2 ratios based on semiquantitative XPS and energy-dispersive X-ray analyses (EDAX) showed changes of the surface chemical composition in the as-prepared samples as compared with the thermal-treated powders. These changes have been ascribed to some peculiarities in the processing chemical route when it is applied to this kind of composites.

XPS and STS of layered semiconductor MoS x

Joint use of XPS and STS methods for MoS x van der Waals face cleaned by removal of a few layers in air have allowed to determine the connection between change of electronic properties and variation in Mo content for MoS x surface layers. STS results (I–V curves with electron transport across MoS x layers) show the p-type conductivity for MoS 2; p- and n- regions for MoS 1.6. The surface state density is higher for p- MoS 1.6 than for p-MoS 2. From I–V curve with electron transport along layers for MoS 1.6 surface the bandgap above 1.1 eV and |E f - E c|≌ 0.4 eV are determined. For results explanation we assume that: surface states creation occurs due to top layer defects formation under top layer breaks ; n- type regions creation is caused by growth of concentration of donor centers associated with excess Mo atoms.

An XPS study of the mixing effects induced by ion bombardment in composite oxides

XPS has been used to study MxOy-TiO 2 (MxOy: CeO 2 and Al 2O 3) composite systems in powder form submitted to bombardment with ions (Ar + and O + 2) of 3.5 keV kinetic energy. In CeO 2-TiO 2 submitted to Ar + bombardment it is found that cerium is reduced to Ce 3+. This species remains stable against re-oxidation by exposure to O 2, which contrasts with the behaviour of Ce 3+ in partially reduced CeO 2 where under similar conditions cerium is oxidized to Ce 4+. Reduction to Ce 3+ is also observed after O + 2 bombardment which points to the fact that the cerium ions are mixed within the titanium oxide lattice and this is the reason for the stabilization of the Ce 3+ state. In TiO 2-Al 2O 3 the existence of mixing of the two components upon ion bombardment is shown by an accurate assessment of the photoelectron and Auger peaks of the spectra and the evaluation of the Auger parameters of the different elements. The use of XPS as a tool to show the existence of mixing in composite oxides after ion bombardment is discussed.

Use of XPS and Ar+ depth profiling to determine the dispersion degree of Ni in Ni/TiO2 and Ni/SiO2 catalysts

AbstractTwo series of Ni/TiO2 and Ni/SiO2 catalysts prepared by different methods have been characterized by XPS and Ar+ sputtering. Depth profiles have been obtained from the intensity of the Ni 2p photoelectron peaks as a function of the sputtering time. The analysis has been carried out for samples in their precursor, calcined and reduced states, in order to obtain information on the changes in dispersion of the active phase through the whole activation process. In the case of reduced Ni/SiO2 catalysts, comparison with results by other techniques (i.e. H2 chemisorption, x‐ray diffraction and transmission electron microscopy) has shown that the sputtering method may give a semi‐quantitative assessment on the dispersion degree of nickel. However, for Ni/TiO2 catalysts in the SMSI state after reduction in H2 at 773 K only the Ar+ sputtering method was able to provide information on the distribution of nickel. In this case the depth profiles were consistent with partial solution of the nickel in the TiO2 matrix.

Joint use of XPS and Auger techniques for the identification of chemical state of copper in spent catalysts

AbstractX‐ray photoelectron and x‐ray‐induced Auger spectroscopic analyses of various copper‐containing catalysts have been used to reveal the chemical state of copper during on‐stream in the dehydrogenation of methanol to methyl formate. Three catalyst systems have been selected for this purpose: (1) supported preparations (Cu/S, where S = ZrO2, SiO2, ZnO, MgO or graphite); (2) mixed oxides of the type LaMn1‐xCuxO3 (x = 0.4–0.6); (3) copper‐containing bentonite and sepiolite. The binding energies of the Cu 2p levels in all catalysts after reaction correspond to reduced Cu species, i.e. Cuo and/or Cu+. However, the precise differentiation between these reduced species is only feasible by measuring the Cu L3VV x‐ray‐induced Auger parameter. Acocordingly, the energies of the modified Auger parameter for type (1) and (2) catalysts were close to that of Cuo, while they correspond to Cu+ species for type (3) preparations.

An investigation of transition metal clusters deposited on graphite and metal oxide substrates by a combined use of XPS, UPS and Auger spectroscopy

Cu and Ni clusters deposited on graphite and metal oxide (TiO 2, Al 2O 3) supports have been investigated by X-ray and UV photoelectron spectroscopy as well as Auger electron spectroscopy. The 2p core-level binding energy of the cluster increases with the decrease in the cluster size and the maximum shift in the case of the smallest cluster is 0.8–1.4 eV (relative to the bulk), the actual value depending on the substrate. The metal LMM Auger line shifts to lower kinetic energies with the decrease in the cluster size and this shift is considerably larger (1.4–3.3 eV) than that of the 2p binding energy. The shifts in the position and intensities of the valence bands with the cluster size have also been studied; the 3d band intensity approaches zero for the smallest cluster size studied by us, the number of atoms in the smallest cluster being in the range 25–75. One of the conclusions from the study is that the magnitude of the shifts in the core-level binding energy and the Auger kinetic energy due to the decrease in the cluster size depends on the substrate, being largest on an insulating substrate such as Al 2O 3 and least on graphite.

An investigation of transition metal clusters deposited on graphite and metal oxide substrates by a combined use of XPS, UPS and Auger spectroscopy

Cu and Ni clusters deposited on graphite and metal oxide (TiO 2, Al 2O 3) supports have been investigated by X-ray and UV photoelectron spectroscopy as well as Auger electron spectroscopy. The 2p core-level binding energy of the cluster increases with the decrease in the cluster size and the maximum shift in the case of the smallest cluster is 0.8–1.4 eV (relative to the bulk), the actual value depending on the substrate. The metal LMM Auger line shifts to lower kinetic energies with the decrease in the cluster size and this shift is considerably larger (1.4–3.3 eV) than that of the 2p binding energy. The shifts in the position and intensities of the valence bands with the cluster size have also been studied; the 3d band intensity approaches zero for the smallest cluster size studied by us, the number of atoms in the smallest cluster being in the range 25–75. One of the conclusions from the study is that the magnitude of the shifts in the core-level binding energy and the Auger kinetic energy due to the decrease in the cluster size depends on the substrate, being largest on an insulating substrate such as Al 2O 3 and least on graphite.

The Use of X-Ray Photoelectron Spectroscopy in Materials Science

AbstractThis review will attempt to show how XPS now makes an important contribution to Materials Science and to highlight the developments which have brought it to this position. XPS is now a mature technique for surface analysis but it has in addition a major role as a specialised tool, being essential to studies in which derivitization methods are used to tag surface groups.The requirements of users in this field have led to the development of X-ray sources which were not envisaged in the early development of the spectroscopy. The usual sources of aluminium Kα and magnesium Kα have limitations for those elements beyond magnesium in the periodic table which would have the Is lino as the principal peak - aluminium, silicon, oulphur and phosphorus for example. Higher energy sources such as silicon Kα or zirconium and silver Lα have made it possible to utilise the Is lines up to chlorine and have the additional advantage that a strong and well resolved series of Auger lines also becomes available. The higher energy radiations are thus particularly suited to the determination of relaxation energies in materials by use of relative shifts between the photo- and Auger lines of the spectrum. Such has been the utility of such relaxation energies that use is often made of Auger lines derived from the Bremmstrahlung component of the normal x-ray sources to make a similar measurement. This measurement is used in the study of insulating ceramics in which electrostatic charging makes measurement of binding energies uncertain.Modern materials technology is particularly concerned with the manufacture of composites; particulate, fibre and laminate composites are all well known and the key to their success often lies within the interface between the phases. Transfer of load across the interface places particular requirements on adhesion at the phase boundary and an understanding of the locus of failure during destructive testing is crucial to the development of satisfactory bonding processes. In coated and laminated products there is no problem in the use of XPS, with its excellent chemical sensitivity but there is a problem of increasing magnitude in fibre and particulate composites as the substructures become finer. This stems, of course, from the difficulty of providing a focused source of X-rays of sufficient magnitude. Imaging XPS is slowly becoming a reality with several systems having a capability of 10μm now available, and one of the markets for such instruments is that of composite materials.There are important areas of Materials Science in which XPS has been displaced by other techniques such as SIMS. One such area is that of polymer surface analysis. The selectivity of XPS for substituent groups in the surface region is not good. Derivitization methods have made an impact, enabling acidic or basic groups to be determined, but SIMS, which has the ability to detach molecular clusters, has obvious advantages which will become increasingly exploited aa the problems of charging become solved. Until then however XPS will continue to find a role in polymer research and development.

Interpretation of surface textural changes in SrTiO3 ex‐oxalate samples using XPS and IR spectroscopies

AbstractXPS, IR and x‐ray diffraction (XRD) have been used to study high‐surface‐area ‘ex‐oxalate’ polycrystalline SrTiO3 materials. Changes of surface area and pore size distribution are induced during lixiviation with a 0.1 M HCl solution, which removes carbonate impurities. The study shows the presence in the original sample of a microcrystalline SrCO3 phase at its surface and isolated CO species, these latter remaining even after outgassing at 773 K for 15 h. However, a complete elimination of the SrCO3 and other carbonate species at the surface is achieved by heating the acid‐lixiviated sample at 773 K, although carbonate species still remain in the bulk of the SrTiO3 material after this treatment. The usefulness of the combined use of XPS and IR for the characterization of surface and bulk species in this type of powdered material is pointed out during this study.

Nitrosoarene complexes of rhodium(III), iridium(III), copper(I) and mercury(II). Use of XPS in determining the mode of bonding to transition metals

Some previously synthesized nitrosoarene complexes of known coordination mode have been studied by XPS spectroscopy. New nitrosoarene complexes of Rh III, Ir III, Cu I and Hg II have been synthesized and studied by both X-ray photoelectron and IR spectroscopy. A correlation was found between the value Δ E B(O-N) and the bonding mode of the nitroso ligand.

New approaches to failure analysis using XPS sputter‐depth profiles

AbstractThe methodology of failure analysis and the use of XPS sputter‐depth profiles in failure analysis is illustrated in three examples: a titanium adhesive bond, a multilayer optical filter, and an ohmic contact. XPS sputter‐depth profiles supplement those obtained using AES and are essential to the failure analysis of a non‐conducting sample or one that is easily damaged by an electron beam. XPS sputter‐depth profiles also allow chemical state determination and improved quantification, especially for samples in which the Auger signals of different elements overlap.

The physical and chemical nature of sensitization centers left from hot spots caused in triaminotrinitrobenzene by shock or impact

Samples of triaminotrinitrobenzene (TATB), a well-known explosive, were taken to the brink of ignition by either underwater shock or impact and were investigated for the generation of hot spots. SEM was used for detecting, locating, and measuring the size of the hot spot remnants. These were found to be tiny ragged holes in the explosive with a fine deposit of debris near them. By use of XPS, a specially surface-sensitive technique, it was found that the debris consisted of furoxan and furazan derivatives of TATB produced from its decomposition. The furoxans are far more sensitive than TATB and constitute sensitization centers where reaction an easily restart during handling of the explosive. The hot spot sites were of micron size for the impacted samples and an order of magnitude smaller for the underwater-shocked samples.

Surface treatment of phlogopite particles for composite applications: XPS and DRIFT studies

AbstractThe use of XPS and diffuse reflectance fourier transform infrared spectroscopy (DRIFT) to characterize mica surfaces modified with a sulfonylazide alkoxysilane coupling agent in the range of 0.2 to 1.0% (w/w) is discussed. XPS and DRIFT provide a description of chemical composition of the surfaces as well as quantitative analysis of the amount of silane present. Both techniques correlate linearly with the amount of surface treatment. However, DRIFT is more sensitive and faster and does not require an ultra high vaccum system.