2p X-ray Photoelectron Spectroscopy Research Articles

Activation of room-temperature ferromagnetism in nonstoichiometric TiO2-δ powders by oxygen vacancies

Anatase and rutile TiO2-δ powders are synthesized by the sol-gel method. The hysteresis loops and the zero field-cooled and the field-cooled magnetization curves indicate that reduced TiO2-δ powders exhibit the room-temperature ferromagnetism that becomes stronger with prolonging annealing time in H2/Ar mixture. Analysis of Ti 2p x-ray photoelectron spectroscopy spectra indicates that Ti ions are all in the Ti4+ state and that Ti3+ or Ti2+ ions do not exist in all samples. In addition, analysis of O 1s x-ray photoelectron spectroscopy spectra indicates that the concentration of oxygen vacancies increases with prolonging annealing time. Analysis of ultraviolet-visible absorption spectra also further confirms that the concentration of oxygen vacancies increases with prolonging annealing time. These results indicate that ferromagnetism in pure TiO2-δ powders stems from oxygen vacancies. The possible mechanism on ferromagnetism is discussed.

Methanol partial oxidation on Cu–Zn thin films grown on Ni(1 0 0) surface

Methanol partial oxidation on Cu–Zn thin films epitaxially grown on Ni(1 0 0) surface has been investigated in an ultra high vacuum (UHV) system combined with a high-pressure reaction cell. According to temperature programmed reaction (TPR) and temperature programmed desorption (TPD) in UHV, surface reaction between methanol and pre-adsorbed oxygen on epitaxially grown Cu thin films mainly produced H 2 and CO together with HCHO, although addition of Zn greatly suppressed the formation of HCHO. Annealing of the thin films at 420 K prior to admission of methanol and oxygen reactants dramatically accelerated reaction rate of CO 2 product and the maximum formation of H 2 and CO 2 was observed on the surface of 1 ML Cu with 0.25 ML Zn. On the contrary, the rate of methanol partial oxidation in the high-pressure reaction cell monotonously increased with the thickness of Cu films up to 3 ML. The addition of 0.25 ML Zn increased the reaction rate independent of Cu film thicknesses of 1–6 ML. Post-reaction surface analysis showed Cu agglomeration on the Ni substrate. Cu 2p X-ray photoelectron spectroscopy (XPS) and LMM Auger data reveal the presence of Cu + species in addition to metallic Cu atoms abundant in the films corresponding to the higher reactivity. It is proposed that the oxidation number of Cu is controlled by oxided Zn in Cu–Zn alloy.

Pyrite oxidation in acidic medium: overall reaction pathway

AbstractThis study investigated the oxidation processes of pyrite by dissolved oxygen. Synthetic pyrite suspensions have reacted with dissolved oxygen in a closed‐thermostated system for 20 h and at pH values of 2.0 and 3.5. Special emphasis was made on the characterization of the solid phase formed during the reaction process, using X‐ray photoelectron spectroscopy (XPS) and confocal Raman microspectrometry. The results revealed a strong heterogeneity of the samples with the identification of two different micrometric areas. Eg and Ag Raman pyrite modes were characterized by wavenumber drifts, from 0 to 6, correlated with the shine changes of the micrometric probed area. XPS analyses provided evidence of the distribution of iron and sulfur species at the reacted pyrite surface. The Fe(2p3/2) signals showed an increase in the binding energy range typical of Fe(III)S and Fe(III)O compounds. The S(2p) XPS signals included contribution from disulfide, monosulfide, elemental sulfur, and sulfate. Elemental sulfur should be protected from sublimation under ultrahigh vacuum, possibly by an overlaying of other species such as Fe(III) (oxyhydr)oxide species. An overall reaction pathway of the pyrite in oxidizing (Cr(VI), dissolved oxygen, Fe(III)), and acidic media (pH ⩽3.5) has been established from the present results and previous papers. Copyright © 2008 John Wiley & Sons, Ltd.

Electronic and magnetic nature of shandite-type A 2Co 3S 2[formula omitted

We investigated electronic and magnetic nature of A 2Co 3S 2 ( A = Sn , In ) to reveal two different characteristics of metallic Co–Sn and strongly correlated electronic Co–S bondings. Co 2p X-ray photoelectron spectroscopy (XPS) shows a complicated and mixed-valence like feature with strong satellite. This origin can be explained by two different characteristics of Co–Sn/Co–S bonding nature. This feature is consistent with characteristic electron-density distributions obtained by the maximum-entropy method (MEM)-based pattern fitting analysis and the LSDA + U band calculation.

Preparation and characterization of the new oxides Ca 2−xNa xCo 2O 5

New compounds with layered structure Ca 2− x Na x Co 2O 5 ( x=0.00–0.45) were prepared by citrate sol–gel method. X-ray powder diffraction patterns of the specimens could be indexed by two monoclinic lattices with C2 space group. The Co 2p X-ray photoelectron spectroscopy (XPS) was used to study Co chemical valence. The electrical conductivity and the thermoelectric power for the specimens were measured between 350 and 950 K. Both properties increased with rising temperature in the temperature range measured. The substitution of univalent Na + for divalent Ca 2+ had a remarkable effect on the enhancement of both properties. For Ca 1.55Na 0.45Co 2O 5 specimen, the electrical conductivity and the thermoelectric power reached 68.9 S cm −1 and 165 μV K −1 at 950 K, respectively. Their intriguing thermoelectric properties make them candidates for thermoelectric materials.

Characterization of copper manganite oxide-polypyrrole composite electrodes cathodically polarized in acidic medium

We have studied the electrochemical behaviour induced by polarization in sandwich-type composite electrodes with the structure GC/PPy/PPy(Ox)/PPy where GC stands for glassy carbon, PPy for polypyrrole and Ox for Cu 1.4Mn 1.6O 4 nanoparticles. The electrodes were polarized at −0.45 V/SCE in 0.15 M KCl aqueous solution at pH 2.2 either saturated in Ar or O 2 at 25 °C. The changes occurring on these electrodes were studied using X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (EXAFS and XANES) techniques. In previous work we have shown that when the oxide particles are incorporated into the PPy matrix the Cu + present in the initial oxide suffers dismutation to give Cu 2+ and metallic Cu. In this work we show that the polarized electrodes also reveal the presence of metallic Cu and Cu 2+. The data also show that the oxide particles embedded in the polarized electrodes contain Mn 3+ and Mn 4+, although the Mn 3+/Mn 4+ ratio is different from that found in the fresh electrodes. The Cl 2p XPS data show that in the electrode polarized in O 2 there is an enhancement of the Cl covalent contribution that appears at 200.8 eV (which is already present in the fresh electrode although with a very small intensity). This result suggests that the oxygen reduction reaction leads to an increase of the OH − concentration inside the composite electrode that explains the charge transport in PPy at negative potentials.

Oxidation of Alumina-Supported Co and Co−Pd Model Catalysts for the Fischer−Tropsch Reaction

Cobalt and bimetallic Co-Pd systems are well-known Fischer-Tropsch catalysts. Compared to Co, the bimetallic systems exhibit an increased activity toward CO hydrogenation and methane conversion, attributed to resistance against oxidation. To study the oxidation behavior, model catalysts have been generated by depositing either Co or first Co and subsequently Pd onto a thin epitaxial alumina film grown on NiAl(110). Pure Co particles and bimetallic particles with a Co core and a Pd shell have been studied before and after exposure to oxygen and after thermal treatments, using X-ray photoelectron spectroscopy (XPS), temperatureprogrammed CO desorption (TPD), ferromagnetic resonance (FMR), and infrared reflection absorption spectroscopy (IRAS) in ultrahigh vacuum. Large doses of O2 (1000 langmuirs; 1 langmuir ) 10 -6 Torr‚s) at 300 K lead to complete oxidation of Co particles. Upon annealing to temperatures above 530 K, XPS indicates that the cobalt oxide is mostly reduced by transfer of oxygen to the alumina support, resulting in its thickening. TPD, however, indicates the existence of persistent surface oxygen species, reducing the CO adsorption energy on the particles. Exposures to small doses of O2 (30-50 langmuirs) were also studied by a careful comparison of XPS, TPD, and FMR data. In this case, XPS indicates Co in a metallic state, whereas TPD and FMR indicate oxidic behavior. We conclude that small amounts of non-stoichiometric subsurface oxygen or subsurface and surface oxygen are present which are not detectable in the Co 2p XPS signal but have a pronounced effect on the surface chemistry and the magnetism, i.e., on certain bulk properties. In the case of bimetallic Co/Pd particles, an incomplete Pd shell on the Co particles even promotes oxygen uptake, while only a complete Pd shell inhibits oxidation.

Bonding Structure of Silicon Oxynitride Grown by Plasma-Enhanced Chemical Vapor Deposition

Silicon oxynitride films were prepared by plasma enhanced chemical vapor deposition (PECVD) with N2O, NH3, and SiH4 precursors. X-ray photoelectron spectroscopy (XPS) and refractive index characterization showed that different film compositions, from silicon dioxide to stoichiometric silicon nitride, are achievable by tuning the flow rate of nitrous oxide in a mixture of precursor gases. The refractive index of silicon oxynitride (SiON) films is linearly proportional to the nitrogen content of these films, making these films a desirable candidate for optical waveguide applications. In addition, a detailed compositional analysis of the oxynitride films was conducted by XPS. By the Gaussian deconvolution of Si 2p XPS spectra, we confirmed that the structure of PECVD silicon oxynitride films should be in the form of the random bonding and the films should have homogenous optical and electrical characteristics.

Comparative XPS Study of Silver and Copper Surfaces Exposed to Flowing Air Containing Low Concentration of Sulfur Dioxide

Sulfur dioxide (SO2) is a typical gas affecting the atmospheric corrosion of metals, particularly outdoors. Although many studies have investigated the effect of SO2 on the atmospheric corrosion of silver and copper, little is known about the early stage of their corrosion behavior. In this study we analyzed the chemical state of sulfur on silver and copper plates exposed to flowing air containing low concentrations (18 and 148 ppb) of SO2 using X-ray photoelectron spectroscopy (XPS). The shapes of the spectra were the same for both concentrations, indicating that the chemical state of the analyzed elements did not depend on the SO2 concentration. There was a difference in the chemical state of the sulfur between the silver and copper. Three peaks were observed in the S 2p XPS spectra for the silver originating from bisulfite, S2O52-, S2O32-, and S2-. The bisulfite resulted from SO2 dissolution into the surface electrolyte, and the others resulted from the reduction of sulfurous acid. In contrast, only bisulfite was observed in the S 2p XPS spectra for the copper. It also originated from the dissolution of SO2 into the surface electrolyte. The difference in the chemical state of the sulfur between the silver and copper is attributed to the presence of native oxides on the surface, which may have impeded the reduction of bisulfite.

Titania Supported Bimetallic Transition Metal Oxides for Low-Temperature SCR of NO with NH3

High surface area titania supported Mn-Cu and Mn-Cr bimetallic oxides were prepared by wet impregnation method using their nitrate salts. The catalytic activity of these materials was evaluated for selective catalytic reduction (SCR) of NO with NH 3 in the presence of excess oxygen at low temperatures (373-523 K). The surface and bulk properties of the catalysts were examined using X-ray diffraction, BET-surface area, FT-IR, temperature-programmed reduction (TPR), and X-ray photoelectron spectroscopy (XPS) techniques. X-ray diffraction (XRD) studies revealed the formation of definite crystalline compounds MnCrO 3 and CrMn 1.5 O 4 for Mn-Cr/TiO 2 samples and Cu 1.4 Mn 1.6 O 4 and Cu 1.5 Mn 1.5 O 4 for Mn-Cu/TiO 2 samples. The specific surface areas of the prepared catalysts were significantly lower than the pure TiO 2 support. The FT-IR studies indicate that the acidic strength of these catalysts was higher than the corresponding single oxide catalysts. TPR studies suggest that the shift in the reduction peak position toward higher temperatures in bimetallic oxides is probably due to the formation of binary or ternary metal oxide phases, as observed from XRD study. The XPS peak intensities and the corresponding binding energies indicate that the surface concentration of Mn is lower than either Cu or Cr at equal metal loadings. A slight shift of the Mn 2p XPS line toward higher binding energies (642.1 eV) was observed in the case of Mn-Cu/TiO 2 and Mn-Cr/TiO 2 catalysts. This is due to the formation of stoichiometric mixed oxide phases between promoted metal oxides, which was observed in the XRD study. The core level binding energy values indicate that the manganese is present as Mn(IV) in both the catalysts. The activity results of these catalysts were compared with their corresponding single oxide catalysts.

Electronic structure of Ti metal and TiO 2 powder studied by hard and soft (Cu Kα 1 and Al Kα1) X-ray photoelectron and Auger spectroscopy

High-energy X-ray photoelectron spectroscopy (XPS) is of particular importance for minimizing the effects of surface contamination by increasing photoelectron escape depths. In this study high-resolution high-energy Cu Kα 1 and soft Al Kα 1 XPS and Auger electron spectroscopy were used to compare the electronic structure of Ti in TiO 2 powder and Ti metal. The Ti 1s in TiO 2 XPS line is narrower and more symmetric than in Ti metal. A comparison of the relative intensities of the L 23M 23M 45 and L 23M 23M 23 Auger transitions in Ti metal and TiO 2 is consistent with the expected transfer of Ti 3d electrons away from the Ti site in the oxide. The satellites accompanying the Ti 1s XPS line excited by Cu Kα 1 X-rays occur at the same energies as the satellites accompanying the Ti 2s and 2p XPS lines excited by Al Kα 1 X-rays indicating that they do not depend on the core-level, the experimental resolution or inelastic scattering processes.

Wettability of polypropylene films coated with SiO x plasma deposited layers

Surface wettability of polypropylene (PP) films has been significantly improved by the deposition of thin SiO x layers, elaborated from a mixture of hexamethyldisiloxane (HMDSO) and oxygen in a microwave DECR (distributed electron cyclotron resonance) plasma reactor (2.45 GHz, 400 W). Contact angle measurements, Atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS) were used to study, respectively, the change in surface free energy, surface topography and chemical composition due to the deposition of thin SiO x layers. Contact angle measurements revealed that the water contact angle value is reduced from about 99° for untreated PP surface to nearly 30° for coated PP film. The polar component of the surface energy is considerably increased from 0.5 to 35 mJ/m 2. For films deposited in the presence of VUV photons, AFM images showed the growth of irregular structure due to substrate etching effect and the deconvolution results of the Si 2p XPS peaks indicated that the proportion of the partly oxidized silicon environment was the major component. However, for films deposited without the presence of VUV radiation, typical SiO 2 agglomerates can clearly be seen on PP surface at a thickness of about 29 nm and the proportion of the silicon dioxide was the major component.

Characterization of COOH-Terminated Self-Assembled Monolayers and Adsorption Efficiency of DNA Molecules Studied by X-ray Photoelectron Spectroscopy and Near-Edge X-ray Absorption Fine Structure Spectroscopy

Self-assembled monolayers (SAMs) of three thiol derivatives with a COOH group at the terminus of the molecule were prepared on Au(111) substrates as templates for immobilization of DNA molecules. The adsorption states and the structures were examined by X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. S 2p XPS results showed that these molecules are mostly chemisorbed on the Au(111) surface through S–Au bonds. The molecular orientation in these SAMs was elucidated using the polarization dependence of the C K-edge NEXAFS spectra. These results suggested that the film structure and the molecular packing differ among these SAMs. We compared the amounts of the amino-terminated DNA molecules adsorbed on these SAMs estimated from the peak intensity of the N 1s XP spectra. These results show that the adsorption efficiency of the DNA molecules depends on the structures of the COOH-terminated SAMs.

Preparation of 1,3,5-trithia-2,4,6-triazapentalenyl films on gold surfaces

Thin films of 1,3,5-trithia-2,4,6-triazapentalenyl (TTTA) were prepared on gold surfaces by “dry” and “wet” processes and were investigated by X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), and surface plasmon resonance (SPR) spectroscopy. Vacuum deposition and self-assembly method were used as dry and wet processes, respectively. In the dry process, the S 2p XPS spectra show the formation of S Au bond between TTTA molecules and gold surfaces. In addition, UPS measurements revealed that TTTA molecules are closely packed without decomposition. In the wet process, the adsorption kinetics monitored by SPR indicates that TTTA molecules are strongly bound to gold surfaces. The thickness of TTTA films from ethanol and hexane solutions was 1.3 nm and 1.8 nm, respectively, which suggests multilayer formation.

Reactive adsorption of thiophene on Au(1 1 1) from solution

The adsorption state of thiophene on Au(1 1 1) formed with the immersion method was investigated by X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure (NEXAFS). The S 2p XPS spectra indicate that the thiolate, not thiophene, is adsorbed on the Au surface with the Au–S bond. The results of C K-edge NEXAFS reveal that the adsorbed species on Au(1 1 1) are mainly saturated hydrocarbons. This is in clear contrast with the vacuum-deposited thiophene on Au(1 1 1), in which thiophene is molecularly adsorbed on the surface.

Chain length dependence of adsorption structure of COOH-terminated alkanethiol SAMs on Au(1 1 1)

We studied adsorption structures and adsorption states of COOH-terminated alkanethiols with different alkyl chain lengths in self-assembled monolayers (SAMs) by X-ray photoelectron spectroscopy (XPS), scanning tunneling microscopy (STM), and near-edge X-ray absorption finestructure (NEXAFS) spectroscopy. In the SAM of 11-mercapto-undecanoic acid on an Au(1 1 1) surface, formation of a well-ordered monolayer was confirmed from the results of the S 2p XPS and STM observation, while this was not the case in the SAM of 6-mercapto-haxanoic acid. C K-edge NEXAFS results suggested that the orientation or conformation of the alkyl chain differs between two SAMs. These results indicate that the intermolecular interaction of the alkyl chains plays an important role for the formation of a well-ordered monolayer of COOH-terminated alkanethiols on Au(1 1 1).

Characterization of Cr/SiO 2 catalysts and ethylene polymerization by XPS

Cr/SiO 2 catalysts with 1 or 3 wt.% Cr loadings and different chromium precursors were characterized by X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). A method to determine chromium species in the sample was developed through the decomposition of the Cr 2p XPS spectrum in Cr 6+ and Cr 3+ standard spectra. The results of the binding energy from the Cr 2p region and of the distribution of chromium species allowed to evaluate the dynamic photo-reduction of the surface chromium species during XPS analysis. Photo-reduction of surface Cr 6+ to Cr 3+ species was verified for all samples supported in silica, depending on the precursor and chromium content. Bulk CrO 3 and Cr 2O 3 standards did not reveal variation in the binding energy of Cr 2p 3/2, but a physical mixture of CrO 3 with SiO 2 presented photo-reduction. The behavior of this mixture resembled to the catalysts and suggests the participation of the surface hydroxyls of silica in the photo-reduction process. XPS intensity measurements for assessing dispersion of chromium oxide were used to compare the calcined and reduced catalysts to different chromium precursors. Polyethylene chains were detected by in situ XPS, while oligomerization products were not observed.

Spectroscopic study on sorption of hydrogen sulfide by means of red soil

This paper reports the results of the characterization of red soils in relation to the sorption of H 2S from coal gas at 500 °C by spectroscopic techniques in order to provide more information on red soils’ structural change both before and after reaction. In addition, by-products analysis has also been studied using Fourier transform infrared (FTIR) spectroscopy. Before and after the experiments the red soils were characterized with X-ray powder diffraction (XRPD), energy dispersion spectrum (EDS), X-ray photoelectron spectroscopy (XPS) and FTIR spectroscopy. XRPD results indicate that iron oxide species disappear from the original to reacted red soil. EDS analysis shows that a significant amount of sulfur is present in the reacted red soil, which is in agreement with the results of the elemental analysis and the calculated value based on breakthrough curve. XPS regression fitting results further indicate that sulfur retention may be associated with the iron oxides. S 2p XPS fittings point out that the major sulfur species present in the reacted red soil are composed of S −2, elemental sulfur, polysulfide, sulfite and sulfate. Additionally, the binding energy of iron shifts to a lower position for the reacted red soil, which indicates that iron oxides in the original red soil have been converted into iron sulfide. Appreciable amounts of the by-products CO 2, SO 2 and COS are detected by on-line FTIR spectroscopy during the initial and later stages of the sorption process. The formation of CO 2 is related to the water-shift reaction, and SO 2 is probably attributable to the reaction of organic matters and H 2S. The concentration of COS is quantified by GC/FPD and found it to be about 350 ppm, which is close to the equilibrium concentration of the reaction of inlet CO and H 2S at a temperature of 500 °C.

Effects of arrival rate and gas pressure on the chemical bonding and composition in titanium nitride films prepared on Si(100) substrates by ion beam and vapor deposition

Thin titanium nitride films were prepared at room temperature by titanium metal vapor deposition on silicon substrates with simultaneous irradiation by a 2 keV nitrogen ion beam. Arrival rate ratios, ARR(N/Ti), defined as the ratio of the flux of incident atomic nitrogen particles in the ion beam relative to the flux of titanium atoms transported to the substrate, ranged from 0.17 to 2.5. The gas pressure in the vacuum chamber was maintained at 1.3×10−3 or 6.7×10−3Pa during the deposition and irradiation process. Analyses of Ti 2p x-ray photoelectron spectroscopy spectra indicated the presence of metal Ti0, nitride TiN, oxide TiO2, oxynitride TiNxOy, and carbide TiC phases. The Ti0 phase was observed exclusively and predominantly in the films prepared at 1.3×10−3Pa and ARR(N∕Ti)=0.17, 0.21, and 0.28, and the TiN phase is major in the others, as confirmed by the x-ray diffractometry analyses. The chemical composition ratio N/Ti in the films prepared at 1.3×10−3Pa increased linearly with increasing ARR(N/Ti) up to ARR(N∕Ti)=0.42 and tended to be constant with further increase in ARR(N/Ti), while this ratio in the films prepared at 6.7×10−3Pa was almost constant independently of ARR(N/Ti), similar to the constant value observed at 1.3×10−3Pa and higher ARR(N/Ti). This dependence may be understood by comparison with the flux of evaporated titanium atoms, the flux of nitrogen in the beam, and the impingement rate of nitrogen gas in the vacuum chamber, evaluated through the kinetic theory of gases. On the other hand, titanium is known to be one of the chemically active materials which form stable compounds with gases by chemisorption, this fact leading to considerable incorporation of contaminant oxygen and carbon in the depositing titanium film.

The interactions of thiophene with polycrystalline UO 2

We have studied the adsorption and desorption of thiophene on polycrystalline UO 2 as function of coverage, over the temperature range 100–640 K, using X-ray photoelectron spectroscopy (XPS), temperature programmed desorption (TPD) and electron stimulated desorption (ESD). Thiophene is found to adsorb molecularly on stoichiometric UO 2. C 1s and S 2p XPS spectra are measured at different thiophene exposures and at different temperatures; they show no evidence for the presence of dissociation fragments, confirming that thiophene adsorbs and desorbs molecularly on a polycrystalline stoichiometric UO 2 surface. The variation of the S 2p and C 1s intensity as function of exposure, together with ESD measurements of O + as function of exposure, can be connected to the growth mode of a thiophene film on UO 2; the thiophene film converts from a flat-lying configuration to an inclined structure as coverage increases. The effects of X-rays, UV, and electron irradiation on thiophene films have been studied in two different coverage regimes, monolayer and multilayer. Irradiation leads to a modification of thiophene films, and appreciable concentrations of species stable to 640 K are present on the surface for both regimes. The XPS results suggest that irradiation induces polymerization and oligomerization, as well as formation of thiolates and dissociation fragments of thiophene. The adsorption and reactivity of thiophene on defective UO 2 surfaces have also been studied. The O vacancies and defects in the oxide surface cause cleavage of C–H and C–S bonds leading to the dissociation of thiophene at temperatures as low as 100 K. These results illustrate the important role played by O vacancies in the chemistry of thiophene over an oxide surface.