X-ray photoelectron spectroscopic investigation of electrochemically oxidised and reduced platinum surfaces

The growing of oxide layer with the time of anodic oxidation in 1M, 3.5M and 8.8M perchloric acid solutions has been investigated by XPS on smooth and platinised Pt surfaces. Pure metallic Pt surfaces were prepared by acidic digestion, ignition and cathodic reduction, and their electron spectra have been compared. The presence of a single species, probably PtO, can be evaluated from the XPS measurements. Calculated values of PtO film thickness for different time intervals of electrolytic oxidation are given at various concentrations of HClO₄.     

An Auger and X-ray photoelectron spectroscopic study of sodium metal and sodium oxide

Photoelectron and Auger electron measurements have been made on polycrystalline films of sodium metal evaporated in ultra high vacuum, and on Na₂O produced by in-situ oxidation by dry oxygen. Most of the spectra were recorded using Mg Kα (1254 eV) radiation but excitation by 5 keV electrons or monochromatized Al Kα (1487 eV) X-rays was used for specific purposes. Core and valence electron binding energies, photoionization cross-sections relative to Na 1s, KLL and KLV Auger energies and transition probabilities are reported. Energy losses in the metal and oxide are discussed and the relative intensities of surface and bulk plasmon losses have been used to calculate mean electron escape depths in the metal. When corrections were made for experimental geometry, escape depths of 10 Å at 180 eV and 31 Å at 1200 eV were obtained. An escape depth of 23 Å at 980 eV was obtained by Na 1s-Na K-Auger intensity correlation and this is consistent with the plasmon data. Data on Auger satellite lines are presented and, in particular, evidence has been obtained which indicates that a high energy satellite should not be attributed to a plasmon gain mechanism. Valence band influences on the KLV Auger spectra are discussed with reference to the XPS spectrum and other sources of valence band information. Unexpected structure was found in the KLV spectra of the metal which, pending thorough interpretation, offsets the sensitivity and resolution advantages which these spectra otherwise offer for valence band studies.     

Auger and X-ray photoelectron spectroscopic and electrochemical characterization of titanium thin film electrodes

Auger (AES) and X-ray photoelectron spectroscopic (XPS) characterizations of electrochemically oxidized titanium are described. Surface oxides on thin (200–250 Å) vacuum deposited titanium films were formed under conditions of linear potential scan in 1 N KClO₄, 1 N HClO₄ and 1 N H₂SO₄. Current/voltage, capacitance/voltage and surface conductance/voltage relationships confirmed the irreversible formation of the surface oxide at thickness of 20–30 Å/V, for low applied potentials. Post moretem analysis of the thin films by AES and XPS indicated a mixture of metal and metal oxides (TiO₂, Ti₂O₃, TiO) on each surface, with the higher oxide states predominating on the electrochemically oxidized films. Observation of the L_IIIM_2,3M_4,5, N(E) signal shape in the Auger spectra of the potentially oxidized oxidized films showed a suboxide TiO-like surface rather than an TiO₂ surface state. Deconvolution of the $\text{Ti}\text{(2}\text{p}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext><mtext>,\; </mtext><mtext>3</mtext><mtext>2</mtext>}}\text{)}\text{XPS}$ spectra confirmed the coexistence of multiple oxidation states of Ti during electrochemical or atmospheric oxidation of the films. Ion sputtering of each surface was used to characterize the subsurface metal/metal oxide composition and to correlate the oxygen to metal atomic ratio with electrochemical pretreatment.     

Surface analysis of Ni/Al alloys by X-ray photoelectron spectroscopy

The surface composition of various pretreated NiAl alloys was investigated by the XPS method. Surface enrichment of Al is observed in freshly polished samples. Storage in air (or H₂O), especially at elevated temperatures, greatly increases the thickness of the aluminium oxide (or aluminium hydroxide) layer and reduces the nickel intensities correspondingly. Treatment with NaOH dissolves aluminium out of the surface, which is thus left with a high nickel content. The changes in this surface caused by exposure in air and H₂O are observed.     

Near surface composition of some alloys by X-ray photoelectron spectroscopy

Chemical compositions of the alloys of CuNi (Cu_0.10Ni_0.90, Cu_0.30Ni_0.70, Cu_0.70Ni_0.30) and BiSb (Bi_0.80Sb_0.20, Bi_0.64Sb_0.34, Bi_0.55Sb_0.45) are determined by X-ray photoelectron spectroscopy. The stoichiometries are determined and are compared with the bulk compositions. Possible sources of systematic errors contributing to the results are discussed. Errors arising out of preferential etching in these alloys have been investigated. It has been inferred from such studies that the preferential etching does not enrich the surface composition with a particular component for the two systems reported here. Quantitative results of CuNi system indicate that the surface regions of the Cu_0.70Ni_0.30 alloy is Cu-rich, although no such evidence is observed in case of BiSb system.     

Surface concentration profile and surface energy in binary alloys

An improved formulation for the orientation-dependence of multi-layer surface segregation is developed in terms of bond enthalpy and strain energy minimization. Inclusion of this result in the Gibbs adsorption isotherm yields the surface energy. Sample calculations of the surface energy isotherm for solid AuCu and liquid CuNi alloys are in good agreement with published experimental data. The conditions for the existence of an extremum in the surface energy isotherm, originally derived by Defay et al. from a monolayer model, are re-examined on the basis of a multi-layer model. Their results are found applicable to this more general situation in the absence of strain energy. An orientation-dependence of the extremum surface energy, but not of the composition at which the extremum surface energy occurs, is demonstrated.     

X-ray photoelectron and X-ray Auger electron spectroscopy studies of heavy ion irradiated C60 films

The influence of 200 MeV Au ion irradiation on the surface properties of polycrystalline fullerene films has been investigated. The X-ray photoelectron and X-ray Auger electron spectroscopies are employed to study the ion-induced modification of the fullerene, near the surface region. The shift of C 1s core level and decrease in intensity of shake-up satellite were used to investigate the structural changes (like sp² to sp³ conversion) and reduction of π electrons, respectively, under heavy ion irradiation. Further, X-ray Auger electron spectroscopy was employed to investigate hybridization conversion qualitatively as a function of ion fluence.     

Auger electron/X-ray photoelectron and cathodoluminescent spectroscopic studies of pulsed laser ablated SrAl2O4:Eu,Dy thin films

Auger electron/X-ray photoelectron and cathodoluminescent (CL) spectroscopic studies were conducted on pulsed laser deposited SrAl₂O₄:Eu²⁺,Dy³⁺ thin films and the correlation between the surface chemical reactions and the decrease in the CL intensity was determined. The Auger electron and the CL data were collected simultaneously in a vacuum chamber either maintained at base pressure or backfilled with oxygen gas. The data were collected when the films were irradiated for 14 h with 2 keV electrons. The CL emission peak attributed to the 4f⁶5d¹ → 4f⁷ transitions was observed at ∼521 nm and the CL intensity of the peaks degraded at different rates in different vacuum conditions. X-ray photoelectron spectroscopy (XPS) data collected from degraded films suggest that strontium oxide (SrO) and aliminium oxide (Al₂O₃) were formed on the surface of the film as a result of electron stimulated surface chemical reaction (ESSCR).     

Auger spectroscopic study of the surface composition of Pt/Sn alloys in ultrahigh vacuum and in the presence of oxygen and hydrogen

The surface composition of two Pt/Sn alloys, viz. PtSn and Pt₃Sn, has been followed by means of AES, as a function of annealing in ultrahigh vacuum, oxygen chemisorption and reduction with hydrogen.The results, which were quantitatively interpreted with the aid of a novel calibration technique, reveal the following features: - The surface of PtSn and Pt₃Sn becomes enriched with tin by annealing in vacuum. Ultimate values of 68±5 at% Sn for PtSn and 41±5 at% Sn for Pt₃Sn were attained after annealing at 500°C. - The adsorption of oxygen on the annealed surface of PtSn and Pt₃Sn causes a further enrichment with tin, while severe oxidation of PtSn at 500°C leads to complete disappearance of Pt from the surface. - Oxygen is more strongly and differently bound on a surface containing about 40 at% Sn than on a surface containing about 70 at% Sn. Activated adsorption of oxygen takes place only on the latter. The results suggest the formation of SnO₂ surface complexes on the exposed surface of Pt₃Sn. - Reduction of the alloys at 500°C carries the excess of tin into the bulk and reduces its surface concentration to 35±5 at% for Pt₃Sn and 64±5 at% for PtSn, which is an enrichment of the surface with platinum relative to the annealed state.     

A photoelectron spectroscopic study of the adsorption of CO and CO2 on platinum

The adsorption of CO and CO₂ on platinum has been studied by UV photoelectron spectroscopy using both He I and He II radiation. The modulation of the intensity of the spectral features observed for adsorbed CO as the photon energy is changed is used to assign the observed levels. The results are in reasonable agreement with recent theoretical and experimental work. The levels are observed to shift by different amounts compared to gas phase CO because of chemical binding effects. The adsorption of CO₂ produces spectral features that are shifted by the same amount compared to gas phase CO₂. This, together with the absence of any localized attenuation of the platinum valence band and the low heat of adsorption, indicates that CO₂ is physisorbed on platinum.     

Surface sensitivity of Auger-electron spectroscopy and X-ray photoelectron spectroscopy

A convenient measure of surface sensitivity in Auger-electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS) is the mean escape depth (MED). If the effects of elastic-electron scattering are neglected, the MED is equal to the electron inelastic mean free path (IMFP) multiplied by the cosine of the emission angle with respect to the surface normal, and depends on the material and electron energy of interest. An overview is given here of recent calculations of IMFPs for 50–2000 eV electrons in a range of materials. This work has led to the development of a predictive formula based on the Bethe equation for inelastic electron scattering in matter from which IMFPs can be determined. Estimates show, however, that elastic-electron scattering can significantly modify the MED. Thus, for AES, the MED will be reduced by up to about 35%. For XPS, however, the MED can be changed by up to ±30% for common measurement conditions although it can be much larger (by up to a factor of 2) for near-grazing emission angles. Ratios of MED values, calculated with elastic scattering considered and neglected for XPS from the 3s, 3p, and 3d subshells of silver with Mg Kα X-rays are approximately constant (to about 10%) over a range of emission angles that varies from 40° to 60° depending on the subshell and the angle of X-ray incidence. Recommendations are given on how to determine the optimum range of emission angles for satisfactory analysis of angle-resolved XPS (ARXPS) data. Definitions are included of three terms often used for describing surface sensitivity (IMFP, MED, and effective attenuation length (EAL)), and examples are given of the varying magnitudes of these quantities for different analytical conditions.     

Surface sensitivity and angular dependence of X-ray photoelectron spectra

By using an ultra-high vacuum X-ray photoelectron spectrometer designed specifically for surface studies, the surface sensitivity (ratio of surface to volume signal) of the technique was found to be enhanced by approximately one order of magnitude at low angles of electron emission measured from the surface plane. While the $\text{Mo(3d}{}_{\mathrm{<mtext>5</mtext><mtext>2</mtext>}}$) peak intensity from a clean polycrystalline target of molybdenum was independent of angle, the $\text{Cs(3d}{}_{\mathrm{<mtext>5</mtext><mtext>2</mtext>}}$) intensity from an adsorbed cesium layer on molybdenum increased and the $\text{Mo(3d}{}_{\mathrm{<mtext>5</mtext><mtext>2</mtext>}}$) signal from the molybdenum substrate decreased as the angle of emission was decreased. A model is presented to account for the angular dependence and it is proposed that the angular dependence of spectral intensities can be used to differentiate surface from bulk species.     

Surface composition of native gold and Ag/Au alloys

The surface composition of Ag/Au alloys has been studied by X-ray photoelectron spectroscopy (XPS). Surface segregation of Ag is observed, particularly for alloys having low Ag content. The content of Ag in the first surface monolayer is in accordance with data from ion scattering and with a theoretical model for segregation in ordered solutions. The surface of native gold was found to be enriched with silver in the case of mine gold, and the degree of segregation was significantly higher than for an alloy having similar bulk composition. The surface of mine gold taken from an oxidized zone contained less silver than did that of usual mine gold. The surface of placer gold was depleted of silver as compared to the bulk. These data show the surface composition of native gold to differ markedly from that of an alloy having the same bulk Ag content, and to depend on the genesis of the native gold sample.     

Surface depth analysis for fluorinated block copolymer films by X-ray photoelectron spectroscopy using C60 cluster ion beam

X-ray photoelectron spectroscopy (XPS) using fullerene (C₆₀) cluster ion bombardment was applied to films of a fluorinated block copolymer. Spectra so obtained were essentially different from those using Ar ion beam. Structure in the surface region with the depth down to 60 nm drawn on the basis of XPS with C₆₀ beam was essentially the same as the one drawn by the result using dynamic secondary ion mass spectrometry, which is a well-established method for the depth analysis of polymers. This implies that XPS using C₆₀ beam enables one to gain access to the depth analysis of structure in polymer films with the depth range over the analytical depth of conventional XPS, that is, three times inelastic mean-free path of photoelectrons.     

X-ray photoelectron spectroscopic studies on initial oxidation of iron and manganese mono-silicides

Initial oxidation of iron and manganese mono-silicides (FeSi and MnSi) surfaces was studied by X-ray photoelectron spectroscopy (XPS). Clean surfaces of these silicides were prepared by fracturing in an ultra high vacuum, and then the fractured surfaces were oxidized by exposing to high-purity oxygen at pressures up to 1.3 Pa. For the clean FeSi surface, positive chemical shifts of the Fe 2p_3/2 and Si 2p peaks from elemental Fe and Si were 0.5 eV and 0.1 eV, respectively. For the clean MnSi surface, a negative chemical shift of the Si 2p peak from elemental Si was 0.1 eV. Iron on the FeSi surface was oxidized at an oxygen pressure of 1.3 Pa, whereas the silicon was oxidized under the pressure of 1.3 × 10⁻⁶ Pa, indicating that oxidation of silicon occurred prior to that of iron. Manganese and silicon on the MnSi were simultaneously oxidized in the range from 1.3 × 10⁻⁶ Pa to 1.3 × 10⁻³ Pa; however, over the pressure of 1.3 Pa, the oxidation of manganese occurs prior to that of silicon. These oxidation behaviors at low oxygen pressures were similar to those of the FeSi and MnSi fractured in air.     

X-ray photoelectron spectroscopic studies on phase identification and quantification of nickel aluminides

Core-level XPS spectra for clean surfaces of Ni₃Al, NiAl, and NiAl₃ alloys were studied. The clean surfaces were obtained by fracturing in the ultra-high vacuum chamber. The positive chemical shifts of Ni 2p_3/2 peak for NiAl and NiAl₃ from Ni metal were 0.2 and 1.0 eV, respectively. The negative shift for Al 2p peak and the positive shift for Ni 3p peaks increased with the decreasing concentration of the corresponding elements. The peak position of the bulk plasmon loss peak for Al 2s peak shifted toward higher energy side, and further, the intensity ratio decreased with the decrease in aluminum concentration. Both the peak intensity ratios of Al 2p to Ni 3p determined by factor analysis and convenient separation are proportional to the atomic ratio of aluminum to nickel. The results indicate that the intensity ratio of Al 2p to Ni 3p determined by these two methods can be applied to the quantification for the surface of the nickel-aluminum alloys.     

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

We have studied the surface chemistry of the nickel-oxygen system using both temperature changes and ion bombardment as techniques for elucidating the surface structure. The spectra of metallic Ni, NiO and Ni₂O₃ were characterized from samples prepared directly in the spectrometer. The Ni₂O₃ species could be distinguished from an authentic Ni(OH)₂ sample from both the X-ray photoelectron lines and the Auger transitions. The oxides of NiO and Ni₂O₃ could be prepared by bombardment with low energy (400eV) O₂⁺ ions as well as by exposure of Ni to oxygen at reduced pressure (～ 100 torr). The Ni₂O₃ was found to be present on most nickel-oxygen surfaces except those prepared by exposing Ni to air for many hours at high temperature (> 600°C), indicating that the stability of Ni₂O₃ decreased as the temperature increased. Exposure of both NiO and Ni₂O₃ to 400 eV Ar⁺ ion bombardment caused reduction to metallic Ni. This observation has also been noted for several other oxides and a prediction of whether or not reduction should be observed is presented by examining the free energy of formation of the molecule.     

X-ray photoelectron spectroscopic study of the adsorption of N2 and NO on tungsten

X-ray photoelectron spectroscopy (ESCA) has been used in a study of N₂ and NO adsorbed on a polycrystalline tungsten ribbon. The sample was flash cleaned under ultrahigh vacuum conditions, and cooled to either 300 K or 100 K for the adsorption studies. Large chemical shifts, as great as 8 eV, were observed between the N (1s) spectra associated with the weakly chemisorbed γ-nitrogen states and the strongly chemisorbed β-nitrogen states. Chemical shifts in both the N(1s) and O(1s) spectra suggest that NO is largely non-dissociatively chemisorbed at 100 K. In general, the binding energies of N(1s) and O(1s) electrons in the adsorbed layers are smaller than the binding energies for the same atoms in small gaseous molecules. In addition, the binding energies associated with the weakly-bound states of NO and N₂ are invariably greater than the binding energies associated with strongly chemisorbed species.     

Study of the interface Si-nc/SiO2 by infrared spectroscopic ellipsometry and X-ray photoelectron spectroscopy

SiO_x films (1<x<2), 0.5 μm thick, have been elaborated by electron-gun evaporation. A thermal annealing of these films induced a phase separation leading to the formation of Si nanocrystals embedded in a SiO₂ matrix. These films have been studied by infrared spectroscopic ellipsometry and by X-ray photoelectron spectroscopy (XPS). The effective dielectric function of the thin films has been extracted in the 600–5000 cm⁻¹ range which allowed us to deduce the dielectric function of the matrix surrounding the Si-nc. A study of the Transverse Optical (TO) vibration mode has revealed the presence of SiO_x into the matrix. Before XPS measurements, the films have been etched in fluorhydric acid to remove the superficial SiO₂ layer formed during air exposure. The Si 2p core-level emission has been recorded. The decomposition of the Si 2p peak into contributions of the usual five tetrahedrons Si-(Si_4−nO_n) (n=0–4) has also revealed the presence of a SiO_x phase. Consistency between infra-red and XPS results is discussed.