Oxidation of the polycrystalline gold foil surface and XPS study of oxygen states in oxide layers

Gold oxide films obtained on the surface of polycrystalline gold foil upon oxidation by oxygen activated by a high-frequency discharge have been studied by X-ray photoelectron spectroscopy. High-frequency O₂ activation affords oxide films more than 3–5 nm thick. As follows from Au4f spectra, the surface gold atoms are oxidized to the oxidation state +3. The O1s spectra have a composite shape and are decomposed into four components that characterize nonequivalent states of oxygen in the resulting oxide films. It is assumed that the two major oxygen states (E _b(O1s) = 529.0 and 530.0 eV) correspond to the oxygen atoms in two-and three-dimensional gold oxide Au₂O₃, respectively. The oxygen states characterized by the higher binding energies (E _b(O1s) = 531.8 and 535.2 eV) likely correspond to molecular oxygen in peroxide and superoxide groups, respectively.     

X-ray photoelectron spectroscopic characterization of the acetylene cyclotrimerization catalyst NbCl2(CnHn) (n = 10–12)

The elemental and ionic compositions of the surface of NbCl₂(C _n H _n ) (n = 10–12), an active catalyst for acetylene cyclotrimerization into benzene, have been determined by X-ray photoelectron spectroscopy. Binding energy data for the sample sputtered with argon ions E _b (Nb3d _5/2) = 203.8–204.2 eV) suggest that the oxidation state of niobium in the active catalyst is +2 or +3. The narrow C1s line indicates the equivalence of all carbon atoms, and the corresponding binding energy, E b(C1s) = 284.0 eV, is close to the BE value for cyclic unsaturated hydrocarbons with conjugate double bonds. Interacting with atmospheric oxygen and moisture during sample preparation, niobium ions on the catalyst surface oxidize to their highest oxidation state, +5, characterized by E _b (Nb3d _5/2) = 207.3–207.7 eV. These data suggest that niobium oxychlorides or oxides form ion the sample surface. The catalyst is stable in a high vacuum and undergoes slight charging under the action of an X-ray beam, showing poor dielectric properties.     

Temperature effect on the chemical composition of a polytetrafluoroethylene surface under the irradiation of synchrotron radiation by means of the X-ray photoelectron spectroscopy

The chemical composition and components of a polytetrafluoroethylene (PTFE) surface was investigated as a function of the temperature under the irradiation of synchrotron radiation (SR) by the X-ray photoelectron spectroscopy (XPS). When the temperature of PTFE under the SR irradiation was less than 100 °C, the C-rich surface appeared. With increasing the temperature more than 150 °C, the relative intensity of the F 1s peak to the C 1s peak increased markedly. At the temperatures of 150–180 °C, the C–C component became small and the CF₂ component was dominant. With further increasing the temperature more than 200 °C, CF₃, CF and C–CF components grew in addition to CF₂ component. Based on these XPS results, the temperature effect on the chemical composition and components is discussed.     

In Situ Study of the Selective Oxidation of Methanol to Formaldehyde on Copper

Combined use of X-ray photoelectron spectroscopy (XPS) and in situ mass spectrometry made it possible to simultaneously obtain the O1s spectra and the mass spectrometric signal of formaldehyde (m/z = 30) in the course of heating (420–670 K) of polycrystalline foil in a flow of the reaction mixture of methanol and oxygen (with a total pressure of 0.1 mbar and a ratio of 3/1). It is shown that the O1s spectra contain two lines with E _b = 530.1 and 531.2 eV, whose relative intensities depend on the sample temperature. At a low temperature (420 K) the line with a lower binding energy dominates, whereas sample heating leads to a drastic decrease in its intensity and its replacement by a line with a higher value of E _b. A decrease in the intensity of the latter line occurs at T > 550 K, in the same temperature range as a drastic increase in the intensity of the formaldehyde signal. These lines were assigned on the basis of literature data and data obtained by the authors for the known forms of oxygen on copper and for the intermediate species of the reaction, such as methoxy and formate. The O1s line with E _b = 530.1 eV was assigned to methoxy groups, and the line with E _b = 531.2 eV was assigned to suboxide oxygen. The correlation of the intensity of the XPS signal of suboxide oxygen with the yield of formaldehyde was supported by stationary experiments using in situ XPS that prove its participation in the key step of the selective oxidation of methanol to formaldehyde.     

Carbon 1s binding energy and substituent effects in C-centered radical cations

The carbon 1s binding energies (E _b) measured by X-ray photoelectron spectroscopy or obtained from quantum chemical calculations for 18 series of compounds in which the reaction center is the C atom are analyzed. It was established for the first time that E _b values in carboncentered radical cations depend on the inductive, resonance, and polarizability effects of substituents.     

Roughness and anisotropy effects on wettability of polytetrafluoroethylene and sodium-treated polytetrafluoroethylene

The effect of roughness on wettability of skived polytetrafluoroethylene (PTFE or Teflon) and Na-treated PTFE film were studied by advancing contact angle measurements. The effect of an anisotropic force field of elongated Na-treated PTFE on the shear bond strength were also studied as a function of elongation. The results are analyzed in terms of London dispersion γs^d and Keesom polar γs^p contributions to surface energy γs. It was found that the roughness effect on wettability of PTFE is significant for untreated PTFE and negligible for Na-treated PTFE. Our shear bond strength σ_b analysis of elongated Na-treated PTFE showed that σ_b is influenced by an anisotropic force field and the σ_b increases with the fractional polarity p = γs^p/γs but decreases with the dispersion fraction d = γs^d/γs of solid-vapor surface tension γs = γs^d + γs^p     

Development of sample preparation technique to characterize chemical structure of humin by synchrotron-radiation–based X-ray photoelectron spectroscopy

Carbon-binding state of humin (HM, a non-conductive insoluble organo-mineral humic substance) was successfully characterized for the first time by synchrotron-radiation–based X-ray photoelectron spectroscopy (XPS). Four sample preparation techniques—HM on double-sided carbon tape, indium sheet, copper mesh, and in pellet formed from the mixture of HM and copper powder (Cu) at different mixing ratios (1:1, 1:2, and 1:6 v/v)—were compared. The results show that HM samples prepared using the first three methods had significant charge buildup, which made the interpretation of the XPS spectra impossible because of the shifts in the binding energy of C 1s XPS spectra. Pellets of HM:Cu mixture enhanced the electrical conductivity and reduced charge buildup on the sample surface. Pellets prepared with HM:Cu ratio of 1:1 (v/v) provided the minimum charge buildup and high sensitivity with difference in C 1s spectra regardless of the observing position. The C 1s spectra, estimated by the subtraction of the carbon contamination in Cu, showed the resolution of CC (284.0 eV), C C/C H (285.1 eV), C O (286.3 eV), CO (287.3 eV), and OC O (288.3 eV) and three additional peaks of CF (289.3 eV), CF₂ (290.2 eV), and CF₃ (291.4 eV). Soft X-ray absorption spectroscopic (XAS) analysis further proved the existence of fluoride (F 1s) in HM structure. The detection of fluorinated carbon in HM showed a great advancement of XPS compared with other conventional analyses. X-ray with the incident angle of 0° provided the smallest (nearly negligible) energy shift in the C 1s spectra of HM and did not damage the surface of the sample.     

X-ray photoelectron study of fluorinated graphite intercalation compounds

Intercalated compounds of fluorinated graphite C₂F·yR, where R is benzene, hexafluorobenzene, acetone, or germanium tetrachloride, are studied by X-ray photoelectron spectroscopy. The binding energies of the C1s and F1s inner levels indicate that the C-F chemical bond in fluorinated graphite differs dramatically from the covalent bond in graphite monofluoride. The binding energies of the inner levels in atoms of the graphite fluoride matrix and GeCl₄ are analyzed and it is concluded that there is no chemical binding between the host matrix and the guest molecule. Translated fromZhurnal Struktumoi Khimii, Vol. 39, No. 6, pp. 1127–1133, November–December, 1998.     

XPS studies on the gold oxide surface layer formation

The initial stage of gold oxide layer formation on the gold electrode surface was investigated in 0.5 M H₂SO₄. X-ray photoelectron spectroscopy (XPS) spectra of pure gold and the anodically polarized gold electrode surface were compared quantitatively. It was found that gold anodic polarization in the E range from ∼1.3 to 2.1 V causes increase in intensity of the XPS spectra at an electron binding energy ε_b=85.9 eV for gold and at ε_b=530 eV for oxygen. These ε_b values correspond to Au³⁺ and O²⁻ oxidation states in hydrous or anhydrous gold oxide. The larger the amount of the anodically formed surface substance the higher is the intensity of the spectrum at the ε_b values mentioned above. It was concluded that gold anodic oxidation, yielding most likely an Au(III) hydroxide surface layer, takes place in the E range of the anodic current wave beginning at E≈1.3 V. At E_B=1.7 V (the potential of the Burshtein minimum) the stationary surface layer consists of 2.5 to 3 molecular layers of Au(OH)₃. The theoretical amount of charge required for the reduction of one molecular layer of Au(OH)₃ is ∼0.15 mC cm⁻², since the Au(OH)₃ molecule is planar and occupies about four atomic sites on the electrode surface.     

The Rydberg Nature And Assignments Of Excited States Of The Water Molecule

We report ab initio theoretical calculation on 32 excited states of H₂ O found to lie below 11.7 eV. Of the eight states observed experimentally, the average discrepancy between theoretical and experimental excitation energies is 0.1 eV. We find that the excited states can each be characterized as arising from an excitation to a Rydberg orbital. Our results indicate that the ? and F? states are both 3d-like excited states rather than one 3d state and one 4s state as previously assumed and similarly for the two Rydberg series joining onto ? and F?. The nsa₁ Rydberg series is found to have a quantum defect of 1.38. joining onto the Ã(¹B₁ state. We have assigned the 9.81 eV transition observed by electron impact as the 1b₁ – 3pb₁ excitation to a ³A₁ state.     

Closed and open-shell atomic oxygen on silver: two distinct patterns of the O<Subscript>1s</Subscript> binding energy and X-ray absorption O <Emphasis Type="Italic">K</Emphasis>-edge spectra as revealed by density functional theory

The electronic structure of atomic oxygen adsorbed species is studied by means of the density functional theory in the context of the ethylene epoxidation on the silver surface. The adsorbed oxygen species are modeled by the Ag₂O molecule either in its closed (¹A₁) or open-shell states (³B₁ and ¹B₁). In both open-shell states the 1s level appears to be lower than that in ¹A₁ by about 2 eV. This is apparently a sequence of the separation of electron pair, occupying the *-type highest occupied molecular orbital (HOMO), decreasing the electron density at the oxygen center. Such variation of the O_1s level for closed and open-shell Ag₂O states seems to explain the X-ray photoelectron spectroscopy (XPS) data concerning two distinct atomic oxygen species on silver surface having the O_1s binding energy of about 528 and 530 eV, called nucleophilic and electrophilic oxygen, respectively. The X-ray absorption O K-edge spectra (XANES) calculated for two types of the Ag₂O states by means of multiple-scattered-X-based approach appears to be in a qualitative agreement with those experimentally recorded for nucleophilic and electrophilic oxygen.     

X-ray photoelectron spectroscopic study of the interaction of supported metal catalysts with NO<Subscript>x</Subscript>

The reactions of the platinum and rhodium model catalysts applied to aluminum oxide with NO_x (10 Torr NO + 10 Torr O₂) were studied by X-ray photoelectron spectroscopy. The reaction conducted at room temperature formed on the surface of the oxide support the NO _3,s ^? nitrate ions characterized by the N1s line at 407.4 eV and O1s line at 533.1 eV and the NO _2,s ^? nitrite ions characterized by the N1s line with a binding energy of 404.7 eV. At the same time, the Pt4f and Rh3d lines of the supported platinum particles are shifted toward higher binding energies by 0.5–1.0 eV and 0.7–1.2 eV, respectively. It is assumed that the binding energies increase due to changes in the chemical state of the platinum metal in which oxygen is dissolved. The reaction of NO_x with Pt/Al₂O₃ at 200°C forms platinum oxide defined by the Pt4f _7/2 line with a binding energy of 72.3 eV.     

A study of the electronic structure of polyvinylsiloxane (CH<Superscript>2</Superscript>CHSiO<Subscript>1.5</Subscript>)<Subscript><Emphasis Type="Italic">n</Emphasis></Subscript> by X-ray photoelectron spectroscopy and quantum chemical modeling in the DFT approximation

The electronic structure of polyvinylsiloxane polymeric chains (Si₂O₃(CHCH₂)₂)_n is studied by X-ray photoelectron spectroscopy and quantum chemistry in the DFT approximation. The binding energy of C and O ls electrons occupying inequivalent positions in the polymer coincides within the experimental accuracy. The binding energies for C and O (284.9 eV and 532.4 eV) and for Si2p-electrons (102.7 eV) well agree with the values for related compounds. The experimental data for the binding energy are reproduced in HF and DFT calculations only with the extended 6–311**(d) basis set. The highest occupied levels of the polymer are fy orbitals of vinyl groups     

State of palladium in palladium-aluminosilicate catalysts as studied by XPS and the catalytic activity of the catalysts in the deep oxidation of methane

Palladium catalysts based on Siralox and AS aluminosilicate supports for the deep oxidation of methane were studied. With the use of XRD analysis, it was found that they were heterophase systems consisting of an amorphous aluminosilicate and γ-Al₂O₃ stabilized against agglomeration. It was found that the catalytic activity of palladium-aluminosilicate catalysts in the deep oxidation of methane at 500°C depended on the support precalcination temperature. X-ray photoelectron spectroscopy (XPS) was used to study the states of the AS-30 aluminosilicate support calcined at 600, 800, or 1000°C and palladium supported on it. It was found that the action of an acid impregnation solution of palladium nitrate on the aluminosilicate calcined at 800°C resulted in a structural rearrangement of the aluminosilicate surface. This rearrangement resulted in the stabilization of both palladium oxide and palladium metal particles at surface defects and the incorporation of these particles into the aluminosilicate after catalyst calcination. As a result, an anomalous decrease in catalytic activity was observed in aluminosilicate samples calcined at 800°C. According to XPS data, palladium in the catalyst was stabilized in the following three phases: metal (E _b(Pd 3d _5/2) = 334.8 eV), oxide (E _b(Pd 3d _5/2) = 336.8 eV), and “interaction” (E _b(Pd 3d _5/2) = 335.8 eV) phases. The ratio between these phases depended on support and catalyst calcination temperatures. The interaction phase, which consisted of PdO_x clusters stabilized in the aluminosilicate structure, was responsible for the retention of activity after calcination at high temperatures (800°C). Based on an analysis of XPS data, it was hypothesized that palladium in the interaction phase occurred in a charged state with the formal charge on the Pd atom close to 1 + (δ+ phase).     

Dehydrogenation of isopropyl alcohol on modified cobalt catalyst

The effects of plasmochemical processing and of Ce, K, and Hf additives on the rate of dehydrogenation for isopropyl alcohol on a 5 wt % Co/SiO₂ catalyst is studied under static and flow conditions. Glow discharge plasma in O₂ and Ar and high-frequency electrodeless plasma in H₂ (HF-H₂) are used. Except for one sample containing Hf, an increase in catalytic activity is observed due to the formation of new active centers. The change in the composition of the initial catalyst’s surface after treatment with Ce and with oxygen, argon, and HF-H₂ plasmas is determined by means of X-ray photoelectron spectroscopy. The change in the size and shape of Co particles after treating the catalyst with HF-H₂ plasma and Ce is determined via X-ray phase analysis. It is suggested that the new catalytic centers formed after treatment in O₂ and Ar plasma contain carbon atoms with C1s bond energies of 282.1 eV; after treatment with HF-H₂ plasma, active centers contain hydrogen and carbon atoms with C1s bond energies of 282.5 eV; with cerium, the C1s bond energy is 297.7 eV.     

Copolymers Containing 1-Methyl-2-phenyl-imidazole Moieties as Permanent Dipole Generating Units: Synthesis,Spectroscopic, Electrochemical,and Photovoltaic Properties

New donor–acceptor conjugated alternating or random copolymers containing 1-methyl-2-phenylbenzimidazole and benzothiadiazole (P1), diketopyrrolopyrrole (P4), or both acceptors (P2) are reported. The specific feature of these copolymers is the presence of a permanent dipole-bearing moiety (1-methyl-2-phenyl imidazole (MPI)) fused with the 1,4-phenylene ring of the polymer main chain. For comparative reasons, polymers of the same main chain but deprived of the MPI group were prepared, namely, P5 with diketopyrrolopyrrole and P3 with both acceptors. The presence of the permanent dipole results in an increase of the optical band gap from 1.51 eV in P3 to 1.57 eV in P2 and from 1.49 eV in P5 to 1.55 eV in P4. It also has a measurable effect on the ionization potential (IP) and electrochemical band gap (E_g^CV), leading to their decrease from 5.00 and 1.83 eV in P3 to 4.92 and 1.79 eV in P2 as well as from 5.09 and 1.87 eV in P5 to 4.94 and 1.81 eV in P4. Moreover, the presence of permanent dipole lowers the exciton binding energy (E_b) from 0.32 eV in P3 to 0.22 eV in P2 and from 0.38 eV in P5 to 0.26 eV in P4. These dipole-induced changes in the polymer properties should be beneficial for photovoltaic applications. Bulk heterojunction solar cells fabricated from these polymers (with PC₇₁BM acceptor) show low series resistance (r_s), indicating good electrical transport properties. The measured power conversion efficiency (PCE) of 0.54% is limited by the unfavorable morphology of the active layer.     

On the Structure of Graphite Fluoride

The structure of graphite fluoride, (C₂F)_n has been investigated by X-ray analyses, solid state ¹⁹F-n.m.r., and electron microscopy for well characterized and crystallized samples obtained from natural graphite or HOPG (highly oriented pyrolytic graphite). On the basis of the present results and structural properties derived from previous works, (C₂F)_n has a layered structure of stage-2 which belongs hexagonal to the system with C_3h symmetry. Detailed discussions on the symmetry both for (CF)_n and (C₂F)_n have led to possible stacking sequences each unit cell of graphite fluoride should require. The ideal structure of (C₂F)_n is a hexagonal crystal lattice with a = b = 2.5 Å; c = 16.2 Å, and a plausible stacking sequence of AB/B′A′/ with I_c (identity period) = 8.09 Å. The layered structure of (CF)_n is of stage-1 with A/A′/ stacking sequence.     

Electronic band structure of solid CO2 as determined from the hv-dependence of photoelectron emission

Photoelectron energy distribution curves from solid CO₂ have been determined for excitation energies from hv = 14 up to 40 eV using synchrotron radiation. A 1:1 correspondence to the gas-phase photoelectron spectrum is observed for the occupied molecular orbitals. The vertical binding energies E_B^v (E_VAC = 0) and widths (fwhm) of the valence bands of solid CO₂ are determined to be 13.0 and 0.95 eV (1π_g); 16.7 and 1.1 eV (1π_u); 17.6 and 0.85 eV (3σ_u) and 18.8 and 0.8 eV (4σ_g) for the individual bands respectively. The partial photoemission cross sections differ importantly from those of the gas phase in exhibiting pronounced maxima at 5.2 eV (1π_g), 4.4–5.3 eV (1π_u + 3σ_u) and 4.2 eV (4σ_g) above the vacuum level, which is attributed to effects of high density of final (conduction-band) states. Further weaker maxima are observed at higher photon energies. Contrary to the case for the gas phase, the resonances are unperturbed in the solid by degenerate autoionizing molecular Rydberg states. The molecular origin of the resonances in the continuum is discussed and related to X-ray absorption spectra, electron-scattering data and to theoretical cross-section calculations. It is shown that the same set of resonances is observed in the different experiments. The resonances occur however at different energies due to different Coulomb interactions. The photoemission results presented provide also a key to the hitherto unexplained optical spectrum of solid CO₂ in the VUV range, making possible an assignment of the structures observed to Frenkel-type excitons (hv ≤ 15 eV) and interband transitions (hv ? 15 eV).     

Chemisorption of CO on Pd(100): An lcgto-lsd cluster study

LCGTO-LSD model potential calculations have been performed for CO interacting with two-, four-, and eight-atom clusters of Pd, chosen to model the bridge site of the (100) surface. The geometry and vibrational frequencies are not very sensitive to the cluster size. For Pd₈ + CO we obtain d_C—O = 1.18 Å (1.13 ± 0.1 exp.), d_Pd—C = 1.87 Å (1.93 ± 0.07 exp.), and (uncoupled) estimates for ω_C—O = 1828 cm^?1 (1895 exp.) and ω_Pd—CO = 454 cm^?1 (339 exp.) Binding energies of 4.8, 3.8, and 2.6 eV are calculated, respectively, for Pd₂ + CO, Pd₄ + CO, and Pd₈ + CO which may be compared with the experimental initial heat of adsorption of 1.6 eV. Ionization potentials for CO-derived levels are in excellent agreement with experiment (relative to ?_F: 4σ (-11.0 eV, -11.2 exp.); 5σ (-8.0, ?8.2 exp.); 1π [?7.5 (b₁), ?7.3 (b₂), ?7.5 exp.]). The main negative ion states of 2π* character are calculated at 2.8 eV (b₁) and 2.7 eV (b₂) above E_F. Other states with appreciable 2π* character are found near 5 eV. These may be compared with inverse photoemission results which show a broad peak centered at 4.8 eV. Interactions of the 4σ, 5σ, 1π, and 2π* orbitals of CO with the metal are discussed. The 4σ and 5σ levels are highly mixed, each receiving appreciable contributions from the 4σ and 5σ orbitals of isolated CO. This is discussed in relation to the dispersion of the 4σ and 5σ levels observed in UPS and to the photon-energy dependent intensities of the 4σ and 5σ resonances. The 2π* component of the backbonding comes through several levels in the upper part of the d band which contain small 2π* contributions in bonding combination with Pd d orbitals. The main 2π* orbitals (contaminated by small antibonding contributions from the metal) are empty (see above).     

XPS C 1s binding energies for fluorocarbon–hydrocarbon microblock copolymers

Fluorocarbon–hydrocarbon microblock copolymers –(CF₂)_n–(CH₂)_m– (n = 4, 6, 8; m = 6, 8, 10) were synthesized. Binding energies of the C 1s and F 1s peaks of these copolymers were measured using x‐ray photoelectron spectroscopy. The binding energy of the C 1s peaks of the carbon atoms of the hydrocarbon segments was set at 285.0 eV as the binding energy reference. Unexpectedly, the binding energy of the C 1s peak corresponding to the CF₂ group of the microblock copolymers was determined to be ～291.4 eV, which is ～0.8 eV lower than that of the CF₂ group of tetrafluoroethylene. Copyright © 2003 John Wiley & Sons, Ltd.