X-ray Photoelectron Spectroscopy Intensity Ratios Research Articles

Corrosion behavior and surface properties of mild steel in aqueous citrate solutions

The electrochemical and surface properties of mild steel in stagnant aerated solutions containing 5 mM citric acid at pH 5 and pH 9 were investigated by open circuit potential, potentiodynamic polarization and electrochemical impedance measurements combined with X-ray photoelectron spectroscopy (XPS) characterization. The results show that the active dissolution behavior of steel in the citric acid solution at pH 5 is well described using a kinetic model consisting of a modified consecutive mechanism, in parallel with a catalytic path involving the divalent intermediate Fe[(OH)2)] ads . The surface film is composed of FeO, Fe(OH)2, FeOOH and Fe-citrate. Also, the analysis of in-depth distribution of composition indicates a high coverage of the organic layer. In the pH 9 citrate solution, a thin bilayer passive film was formed (with Fe(II) and Fe(III) located mainly in the inner and outer regions, respectively). Its overall composition is Fe3O4/FeOOH. Electrochemical measurements revealed the low protective ability of this film and this was confirmed by a modest XPS intensity ratio of Fe(III)/Fe(II). Moreover, no clear presence of Fe-citrate was detected in the passive film, and so the poor passivation behavior was correlated to insufficient adsorption of the tri-ionized citrate anion. The importance of solution flow to stimulate the adsorption of citrate anion is emphasized.

Chemical in‐depth analysis of (Ca/Sr)F2 core–shell like nanoparticles by X‐ray photoelectron spectroscopy with tunable excitation energy

The fluorolytic sol–gel synthesis is applied with the intention to obtain two different types of core–shell nanoparticles, namely, SrF2–CaF2 and CaF2–SrF2. In two separate fluorination steps for core and shell formation, the corresponding metal lactates are reacted with anhydrous HF in ethylene glycol. Scanning transmission electron microscopy (STEM) and dynamic light scattering (DLS) confirm the formation of particles with mean dimensions between 6.4 and 11.5 nm. The overall chemical composition of the particles during the different reaction steps is monitored by quantitative Al Kα excitation X‐ray photoelectron spectroscopy (XPS). Here, the formation of stoichiometric metal fluorides (MF2) is confirmed, both for the core and the final core–shell particles. Furthermore, an in‐depth analysis by synchrotron radiation XPS (SR‐XPS) with tunable excitation energy is performed to confirm the core–shell character of the nanoparticles. Additionally, Ca2p/Sr3d XPS intensity ratio in‐depth profiles are simulated using the software Simulation of Electron Spectra for Surface Analysis (SESSA). In principle, core–shell like particle morphologies are formed but without a sharp interface between calcium and strontium containing phases. Surprisingly, the in‐depth chemical distribution of the two types of nanoparticles is equal within the error of the experiment. Both comprise a SrF2‐rich core domain and CaF2‐rich shell domain with an intermixing zone between them. Consequently, the internal morphology of the final nanoparticles seems to be independent from the synthesis chronology.

Electron Transfer through a Natural Oxide Layer on Real Metal Surfaces Occurring during Sliding with Polytetrafluoroethylene: Dependence on Heat of Formation of Metal Oxides

Electron emission (EE) from real metal surfaces occurring during sliding contact with a polytetrafluoroethylene (PTFE) rider has been investigated using the thermodynamic data of metal oxides and the X-ray photoelectron spectroscopy (XPS) intensity ratio of oxygen/metal on the surfaces. EE was termed triboelectron emission (TriboEE). Rolled metal sheets of 18 types were used. The metal‒oxygen bond energy calculated from the heat of the formation of metal oxide, (D(M–O)), was shown to be a key factor in dividing the EE into two routes, the so-called Schottky effect and the tunnel effect, due to the surface oxide layer. The metals in periodic groups 4 (Ti and Zr), 5 (V, Nb, and Ta), and 6 (Mo and W) maintained higher values of D(M–O), while, moving down the groups, the TriboEE intensity increased, being ascribed to the former route. In groups 10 (Ni, Pd, and Pt) and 11 (Cu, Ag, and Au), the D(M–O) values decreased moving down the groups, but the TriboEE intensity increased significantly, which can be attributed to the latter route. Furthermore, with the increase in the electrical conductivity of metals, the TriboEE intensity became remarkably high, while the D(M–O) value fell rapidly and became almost constant. The XPS results showed that the dependence of the D(M–O) and XPS metal core intensity on the O1s intensity and the XPS intensity ratio of the O1s/metal core was different between groups 10 and 11 and groups 4, 5, and 6. It was concluded that, under the electric field caused on the real metal surface by the friction with PTFE, the electron from metals with small D(M–O) values predominantly tunnels the surface oxide layer as a surface barrier, while with large D(M–O) values, the electron passes over the top of the barrier.

Implantation and erosion of nitrogen in tungsten

Nitrogen puffing is routinely applied in nuclear fusion plasma experiments with tungsten walls to control the amount of power emitted from the plasma by radiation. However, as nitrogen is retained in significant amounts in tungsten it adds some complexity to the plasma-wall interaction. Basic questions concerning the interaction of nitrogen with tungsten, namely the energy and temperature dependent retention of nitrogen implanted into tungsten and the erosion of the formed tungsten nitride by deuterium, are still open. To address these questions, laboratory experiments with a mass-filtered ion source and sample analysis with in situ x-ray photoelectron spectroscopy (XPS) and nuclear reaction analysis were performed. The results of the implantation and erosion measurements were interpreted by means of simulations with a Monte-Carlo code describing the interaction of energetic particles with matter in the binary collision approximation. This required the development of a forward calculation, converting the simulated depth profiles into XPS intensity ratios. With appropriate settings, the experimental implantation and erosion results at ambient temperature are well described by the simulations. However, for increased temperatures it has been observed that there is an unexpected difference between implanting nitrogen into tungsten before heating the sample and implantation into a heated sample. The application of the developed forward calculation is not limited to the problems presented in this work but can be applied especially to all kind of XPS sputter-depth profiling measurements. Finally, simulations with the previously validated Monte-Carlo code are used to extrapolate the presented results on nitrogen retention to energies and particle compositions relevant for fusion experiments. These simulations make quantitative predictions on nitrogen retention in tungsten and on relevant time scales. The simulations also show that recoil implantation of nitrogen by deuterium significantly increases the effective implantation depth of nitrogen.

Effect of method of preparation on hydrodesulphurization activity of Co- or Ni-promoted MoS2/SBA-15 catalysts

Ordered mesoporous material SBA-15 was synthesized and used as a support for the preparation of molybdenum sulphide catalysts through precipitation from homogeneous solution (PFHS) technique with the Mo content varying from 2–12 wt%. The prepared catalysts were evaluated for thiophene hydrodesulphurization catalytic activities at 400°C. Catalysts prepared through PFHS method resulted in highly dispersed MoS2 catalysts, which were inferred from powder X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), low temperature oxygen chemisorptions (LTOC) and BET surface area analysis. The relationship between XPS intensity ratio, oxygen chemisorption and catalytic activities is discussed in terms of highly dispersed nano particles of MoS2 and its consequence in accommodating more promoted atoms at the edge sites.

Determination of the Size of Supported Pd Nanoparticles by X-ray Photoelectron Spectroscopy. Comparison with X-ray Diffraction, Transmission Electron Microscopy, and H2 Chemisorption Methods

Supported palladium nanoparticles with different diameters were synthesized by the water-in-oil microemulsion method using TiO2 as support. The materials were characterized by different physicochemical methods such as X-ray photoelectron spectroscopy (XPS), inductively coupled plasma atomic emission spectroscopy, X-ray diffraction (XRD), transmission electron microscopy (TEM), and H2 chemisorption. The results confirmed that the microemulsion method permits well-dispersed palladium nanoparticles to be obtained. The size of the nanoparticles was estimated by XPS intensity ratios using models proposed by Davis and by Kerkhof and Moulijn and compared with XRD, TEM, and H2 chemisorption analysis. Good accordance of the two models was found for very small Pd particles (smaller than 3 nm). The Kerkhof−Moulijn model seemed to be very sensitive to the small variation in the particle size distribution. The Davis model seemed to be more adequate to determine the size of small and biggest particles as compared with ...

Characterization of Pd–CeO x interaction on α-Al 2O 3 support

The Pd–Ce interaction was studied over CeO 2 (0.3–2.5 wt.%)–Pd (1 wt.%)/α-Al 2O 3 catalysts used in the reforming reaction of CH 4 with CO 2. The samples were characterized by using high resolution transmission electron microscopy (HRTEM) and X-ray photoelectron spectroscopy (XPS). The activity and selectivity behavior was in good agreement with that of other supported metal catalysts (Ni and Pd) modified with different promoters. The preliminary results of HRTEM would indicate that the CeO x forms small crystallites around the Pd particle. The XPS analysis for the regions of Ce 3d and Pd 3d, gives an account of Ce being present mostly as Ce 3+ and a high binding energy for Pd 3d 5/2 (335.3 eV), an evidence of Pd–Ce chemical interaction. The Pd/Al XPS intensity ratios vs. the Pd average particle size, determined by TEM, show an excellent correlation for fresh and used catalyst. These results indicate that the diminution of the Pd/Al ratios was due to Pd sintering. Consequently, the small amounts of CeO x species do not cover the Pd particle, in agreement with the HRTEM results. The overall results stand for the promoter action mechanism of the CeO x for the reforming reaction with CO 2.

The Influence of Water on the Dispersion of Vanadia Supported on Silica SBA-15: A Combined XPS and Raman Study

The influence of water on the dispersion and structure of silica SBA-15-supported vanadia model catalysts has been studied using X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy, which were combined within one experimental setup, as well as UV−vis diffuse reflectance spectroscopy. By performing time-dependent XPS experiments, the influence of UHV/X-ray radiation could be eliminated by extrapolation of the observed temporal changes to t = 0. XPS characterization reveals that the V2p3/2 emission consists of two contributions, which are assigned to vanadia with distinctly different cluster size. Dehydration by treatment in oxygen flow at elevated temperatures leads to a significant increase in total intensity and a substantial redistribution of spectral weight to higher binding energies as a result of an increase in the vanadia dispersion. The V/Si XPS intensity ratio of the dehydrated samples closely follows the corresponding bulk ratio over the whole range of vanadium loadings (0−22 wt % V) st...

Interfacial interaction of in situ Cu growth on tetrasulfide self-assembled monolayer on plasma treated parylene surface

The interfacial reactions of in situ grown Cu and functionalized parylene surfaces were examined using the x-ray photoelectron spectroscopy (XPS). It is observed that bis[3(triethoxysilane)propyl]tetrasulfide (tetrasulfide) forms a self-assembled monolayer (SAM) only on a parylene surface treated with either He plasma or N2 plasma, but not on the as-deposited parylene surface due to its lack of functional groups. The functional groups on the plasma treated parylene surface facilitate the formation of tetrasulfide SAM that subsequently improves the reactivity of parylene to Cu. The XPS spectra show a strong shift (∼1.1eV) of the S 2p peaks from a higher binding energy to a lower binding energy, which suggests the existence of an interaction between sulfide and Cu. In addition, the result of a higher XPS intensity ratio of Cu 2p3∕2∕C 1s for Cu growth on tetrasulfide SAM/plasma treated parylene surface than that of an as-deposited parylene surface indicates that the formation of tetrasulfide moiety (–SSSS–) on functionalized parylene surface leads to the increase of Cu growth rate.

Interfacial effects in ultra-thin nickel deposits on yttria-stabilized zirconia

Abstract Ultra-thin films with 0.2 ML of Ni vapor-deposited in UHV near RT on yttria-stabilized-zirconia YSZ(1 0 0), were studied by X-ray photoelectron spectroscopy (XPS). Upon extensive annealing of the deposits in UHV between 480 and 850 K, it was found that there exists a critical temperature, below which the deposits tend to be fully oxidized via the substrate and above which they are stable in the metallic state. Nickel oxide formed on YSZ by either substrate or RT gas-phase oxidation is stable in UHV at least up to the critical temperature. This critical temperature appears to be sensitive to the history of the clean substrate treatments, as expressed by the O1s/Zr3d XPS intensity ratio, being higher than 850 K for the fully oxidized state of the clean YSZ surface and falling even below 635 K after repeated Ar+ sputtering cycles and extensive heating of the clean substrate in UHV at 870 K leading to an oxygen-deficient surface.

Characterization of a natural and an electro-oxidized arsenopyrite: a study on electrochemical and X-ray photoelectron spectroscopy

This paper discusses the results of a detailed study on the electrochemistry of an arsenopyrite mineral and a concentrate as well as other mineral species contained in it in a chloride medium (NaCl 1.9 M+HCl 0.1 M) using cyclic voltammetry. The surface modification promoted by the anodic oxidation of arsenopyrite mineral samples (+0.8 V for 1 h) is also analysed. X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy have been used to provide information on the chemical state of natural and electro-oxidized surfaces. The results showed that layers of elemental sulphur are produced on the oxidized surface of arsenopyrite by the formation of an intermediate metal-deficient sulphide Fe 1− x As 1− y S. Other surface oxidation products such as iron oxides, arsenic oxides and oxy-sulphur species have also been detected, confirming the interpretation of the voltammetric studies. The identified surface products show that arsenopyrite oxidation occurs by the diffusion of the metal atoms from the bulk to the interface region and by their interaction with air forming metal oxide layers (mainly iron and arsenic oxides) and leaving a predominant elemental sulphur layer. XPS intensity ratios yielded mineral surface stoichiometry before and after electrochemical treatment. Based on the fitted XPS spectra new data for the binding energies of core electrons in arsenopyrite are proposed: 707.3 eV for the Fe 2p 3/2 level, 162.3 eV for the S 2p 3/2 level and 41.5 eV for the As 3d 5/2 level. The effect that electrochemical pre-treatment of refractory arsenopyrite concentrate has towards the breakdown of the sulphide matrix required for the release of the occluded gold is also discussed, taking into account the results obtained in this study.

Low-Energy Electron Diffraction and X-Ray Photoelectron Spectroscopy Studies of Sb-induced Reconstruction on Si(113)3×2 Surface

We investigated Sb-induced reconstruction on a Si(113)3×2 surface using low-energy electron diffraction (LEED) and X-ray photoelectron spectroscopy (XPS). Depending on the annealing temperature and Sb coverage, 1×1, 1×2+2×, 2×2, and 2×5 structures were observed. The 2×2 and 2×5 structures showed reversible phase transitions to 1×2 and 1×5 structures respectively at high temperature. The relative Sb coverages of the 1×1 and 2×2 structures formed by annealing at about 400 and 700°C were measured from Sb 3d/Si 2p core-level XPS intensity ratios.

Cesium-induced Reconstruction on Si(113)3 × 2 Surface Studied by Low Energy Electron Diffraction and X-ray Photoelectron Spectroscopy

We have investigated Cs-induced reconstruction on the Si(113)3×2 surface using low energy electron diffraction (LEED) and X-ray photoelectron spectroscopy (XPS). For Cs deposition at room temperature, the (3×1) LEED pattern was observed for a wide Cs coverage range. At high substrate temperatures, the (3×1), (1×5+2×) and (2×2) phases were observed with increasing Cs deposition time. The relative Cs saturation coverages of (3×1)-Cs at RT and (2×2)-Cs at 300°C were measured from Cs 3d/Si 2p core level XPS intensity ratios. The results are summarized in a phase diagram as a function of the Cs deposition time.

High resolution electron energy loss spectroscopy of MnO(100) and oxidized MnO(100)

Single crystal MnO(100) substrates can be selectively oxidized to produce Mn2O3- and Mn3O4-like surfaces under mild oxidation/reduction conditions readily accessed under ultrahigh vacuum (UHV). MnO(100) yields a characteristic Mn 2p x-ray photoelectron spectroscopy (XPS) satellite structure and appropriate O/Mn concentrations from O 1s/Mn 2p XPS intensity ratios. Its high resolution electron energy loss (HREEL) spectrum shows a series of Fuchs–Kliewer multiple phonon excitations with a single loss energy of 70.9 meV, characteristic of the cubic manganese monoxide structure. However, the HREEL spectral (HREELS) background is high and the phonons are not as well resolved as those typically observed on comparable metal monoxides. Annealing the MnO(100) substrate at 625 K and 5×10−7 Torr O2 slowly forms Mn2O3, as indicated by O 1s and Mn 2p XPS, and does so without significantly altering the symmetry of the MnO(100) low energy electron diffraction pattern. The MnO(100)-Mn2O3 surface can be selectively reduced to Mn3O4-like composition by heating under UHV to 775 K and to MnO(100) at 1000 K. HREEL spectra for the UHV annealed surfaces are well-resolved, and for the MnO(100)-Mn3O4 substrate a second fundamental phonon loss is observed at 55.6 meV as a result of the lower symmetry of the Mn3O4 spinel structure. The UHV-annealed MnO(100) surface appears to be more highly ordered since its HREELS phonon loss peaks are better resolved. It is also somewhat reduced, however, resulting in a less intense phonon spectrum with a fundamental loss energy of only 65.1 meV.

Particle size and metal-support interaction effects in pumice supported palladium catalysts

Pumice supported palladium catalysts with different metal loading and different metal particle sizes have been analyzed with X-ray photoelectron spectroscopy (XPS). The observed negative shift of the Pd 3d doublet binding energies of all the catalysts with respect to the corresponding binding energies of palladium metal powder suggested an increased electron density at the metal centers. Through the combination of the photoelectron and Auger shifts, yielding the Auger parameter shift, relative to the palladium metal, the charge distribution on the palladium atoms was estimated. The variation of that parameter with particle size is attributed to the interaction between the metal and the support rather than to an intrinsic size effect. The XPS intensity ratios of Pd 3d and Si 2p peaks were well reproduced by the Kerkhof and Moulijn model, especially at lower metal loading. Depletion of sodium, naturally present in pumice, has been observed in the catalysts with lower palladium content. An explanation of the catalytic behavior of these samples, in terms of electronic effects and surface elemental composition, is given.

The adsorption and photochemistry of CD3I on TiO2(110)

The adsorption and photochemistry of CD3I adsorbed on TiO2(110) at ∼110 K has been studied by means of temperature programmed desorption (TPD) and x-ray photoelectron spectroscopy (XPS). Complex desorption behavior is observed in TPD suggesting the presence of several distinct coverage regimes. At submonolayer coverages there are two coexisting phases: one dominated by adsorbate–adsorbate interaction, the other dominated by adsorbate–substrate interactions. The first completed monolayer corresponds to (3.8±0.3)×1014 molecules cm−2 and shows only one desorption peak, although this is broad and extends asymmetrically to high temperature indicative of a changing desorption activation energy. With increasing coverage, a discrete, less tightly bound second layer is formed which slowly rearranges to produce three-dimensional clusters of methyl iodide, as indicated by a sharp reduction in the I (3d5/2)/Ti(2p) XPS intensity ratio. There is evidence that this rearrangement does not involve the first monolayer. Irradiation of 1 ML CD3I by 254 or 334 nm photons causes cleavage of the C–I bond and expulsion of I and C containing species into the vacuum. There is retention of ∼20% of the initial I atoms after irradiation at 254 nm. The photodissociation cross section, σ, of adsorbed CD3 I on TiO2(110) at 254 and 334 nm is calculated to be ∼1.1×10−18 cm2 and ∼1.3 ×10−20 cm2, respectively. At 254 nm, the adsorbate and gas phase σ are similar suggesting photodissociation is dominated by adsorbate excitation, but at 334 nm the adsorbate photodissociation cross section is almost an order of magnitude larger than its gas phase counterpart. This suggests that a second photoexcitation mechanism may be contributing to adsorbate photodissociation, possibly involving photogenerated substrate carriers.

Limitations of surface analytical techniques for determining the surface composition of bimetallic particles

Calculations of expected X-ray photoelectron spectroscopy intensity ratios for both homogeneous and segregated bimetallic particles demonstrate that a minimum particle size exists, below which reliable differentiation between homogeneous alloy particles and particles in which one component has segregated to the surface cannot be made. This minimum particle size varies between 3 and 8 nm depending on the specific metal combination in question. In cases of partial segregation of one component, the minimum particle size is even larger. The implications of these results for the study of surface segregation in supported bimetallic catalyst particles are discussed.

XAS and XPS study of electronic structure of the trivalent cuprate La2Li0.5Cu0.5O4− δ

Well-characterized stoichiometric La 2 Li 0.5 Cu 0.5 O 4− δ ( δ ≈0.04) has been studied using high resolution CuL 3- and OK-edges absorption (XAS) and Cu 2p 3 2 X-ray photoelectron spectroscopy (XPS) techniques. The system is found not to have any Cu 3 d 8 configuration in its ground state but instead reveals a high density of the holes created by doping of monovalent lithium for the divalent copper in the oxygen 2 p levels resulting in a 3 d 9 L copper configuration by charge transfer. The formal valence of copper in it is found to be as high as ≈2.84 in agreement with chemical analysis. The empirical correlation between the shift of the main XPS line (3 d 10 L) and intensity ratio of the satellite (3 d 9) to the main line is examined and, also, a new correlation between the energy spread of the XPS satellite and the energy separation of the CuL 3 absorption lines due to transitions from (3 d 9) and (3d d 9 L) states is suggested.

Quantitative analysis of X-ray photoelectron spectroscopy intensities for dealuminated zeolites and supported catalysts where the promoter-forming elements also occur in the support

The model developed by Kerkhof and Moulijn for the interpretation of X-ray photoelectron spectroscopy (XPS) intensity ratios of uniformly dispersed catalysts was extended because the promoter consists only of chemical elements which also occur in the support. The model is made to investigate photoemission data of different species of the same chemical element which are indiscriminate with respect to their chemical shifts. The model is applied to a series of dealuminated Y zeolites. Thus, the respective Al:Si XPS intensity ratios of high dealuminated Y zeolites are explained by dispersion of the non-framework aluminium on the mesopores surface.

Structure and reactivity of titania-supported molybdenum and tungsten oxides

TiO 2 supported MoO 3 and WO 3 have been prepared by aqueous impregnation of low-area anatase (∼10 3m 2g −1) and Degussa P-25 (55 m 2g −1) with aqueous solutions of molybdate and tungstate salts, followed by calcination: products have been characterised by temperature-programmed reduction (TPR), Laser Raman spectroscopy (LRS) and X-ray photoelectron spectroscopy (XPS). Reaction of Ti-OH groups on these supports with MoOCl 4, and the form of the variation of Mo/Ti and W/Ti XPS intensity ratios, lead to estimates of Mo and W contents corresponding to monolayers of MoO x and WO x adsorbed species, the surface Mo:Ti and W:Ti ratios being a little more than 0.5. Possible structures are suggested. The techniques employed provide evidence that with MoO 3 contents between one and four monolayer equivalents a “disordered MoO x” phase is formed. A corresponding phase in the case of WO x has not been detected, but in both cases “paracrystalline” oxide phases are seen at higher loadings: these cover 2.5–20% of the monolayer surface and differ in their reducibility from the corresponding bulk oxides. A simple model is presented to account quantitatibely for the variation in XPS intensity ratios with oxide content.