Showing papers on "Chemical state published in 1995"

Surface chemical states of barium titanate: Influence of sample processing

Abstract: The composition and chemistry of the near-surface region of BaTiO3 have been studied using x-ray photoelectron spectroscopy (XPS). It is found that the Ba3d photoclectron peak shows two chemical states, one of which is attributed to the bulk perovskite and the other to a special surface state unrelated to contamination. The bulk component is reduced and the surface component increases when the material is annealed at high temperatures (either in reducing or oxidizing atmosphere). Both the components are unaltered if the sample is exposed to air, solvents, or water: processes that lead to adsorption of impurities. The surface peak, therefore, attributed to a relaxation related and not contamination-related state, has been compared with those in other Ba-containing oxides. The oxygen photoelectron peak consists of a normal perovskite peak typical of most titanates and a higher energy component clearly related to surface contamination. Annealing in reducing atmosphere results in drastically different optical and electrical properties, and in chemical reduction of some Ti4+ ions to Ti3+. The overall stoichiometry, however, does not change with annealing atmosphere. These results have been discussed in light of our current understanding of this and other related oxides.

NO Reduction by Activated Carbons. 5. Catalytic Effect of Iron

Abstract: The effect of iron as catalyst of the N0-carbon reaction has been investigated. A coal-derived carbon was loaded with iron using different methods and different precursors. A brief exploratory study was also conducted with pitch-derived carbon fibers. The iron-loaded and/or parent carbons were characterized by physical adsorption of CO 2 (at 0 °C) and N 2 (at -196 °C), X-ray absorption fine structure spectroscopy (XAFS), and chemisorption of CO at 25 °C. The N0-carbon reaction was studied in a fixed-bed flow reactor at atmospheric pressure using two types of experiments: (i) temperature-programmed reaction (TPR) in a NO/He mixture, and (ii) isothermal reaction at 300-600 °C. The reaction products were monitored in both cases, thus allowing detailed oxygen and nitrogen balances to be determined. Iron was found to catalyze NO reduction by carbon through an oxidation/reduction (redox) mechanism similar to that reported previously for potassium- and calcium-catalyzed reaction. Nevertheless, the iron species present on the carbon surface before NO reduction (Fe x O y or Fe0) are less effective than the potassium species (elemental potassium or potassium suboxide) in chemisorbing NO, as a result of which they transfer less oxygen to the carbon active sites. The results show also that the nature of the catalyst precursor, the catalyst preparation conditions and the reducibility of the catalyst by the carbon determine the chemical state of the catalyst, its dispersion and catalyst/substrate contact, and hence control the catalytic activity of iron in NO reduction by carbon

Reaction of 1,1,1,5,5,5‐Hexafluoro‐2,4‐pentanedione (H+hfac) with CuO , Cu2 O , and Cu Films

Abstract: The interactions of copper and copper oxide layers with 1,1,1,5,5,5-hexafluoro-2,4-pentanedione (H + hfac) have been investigated. The results provide supporting evidence for proposed reactions of copper with H + hfac which are thought to be responsible for the vapor-phase etching of copper. Specific reaction products depend on the chemical state of the copper in the film. Reaction of H+hfac with Cu 2+ yields volatile reaction products of Cu II (hfac) 2 and H 2 O, while Cu 1+ yields the same products with a change in the chemical state of the surface to Cu (0) . No reaction is observable between H + hfac and Cu (0) at temperatures studied in these experiments

Chemical states of the molybdenum disilicide (MoSi2) surface

Abstract: The analysis of the oxidation of MoSi2 at room temperature was carried out using X-ray photoelectron spectroscopy. A clean surface of MoSi2 was obtained by sputter-etching the bulk material inside the spectrometer, and the chemical states of the surface were examined with both high and low take-off angles for the conditions of as-etched and exposed to the air for different times. The commercially pure MoSi2 powder was also investigated in the as-received condition. The analysis indicated that the exposure of a clean molybdenum disilicide to the air for just a few minutes led to the formation of SiO2 and MoO2, and this oxidation would persist at least for more than 24 h with the final surface of MoSi2 being covered by a duplex oxide layer of SiO2 + MoO3.

Chemical States of Crystalline Silicon Carbide Surfaces

Abstract: Chemical states of 6H polytype crystalline silicon carbide (6H-SiC) surfaces were investigated using X-ray photoelectron spectroscopy (XPS). Surface contaminants such as C-C, C-H and C-O species were evaluated from C1s photoelectron spectra after treatment under various conditions. Clean SiC surfaces were found to appear after the chemical etching of a thermal oxide ( SiO2) film using a buffered HF solution. Furthermore, to clarify the chemical etching characteristics of SiO2 formed on 6H-SiC substrates, the depth profiling of the SiO2 was also performed using XPS. The C1s peak at a binding energy of about 286.2 eV was detected on the surfaces of the 6H-SiC substrates at the moment when only the SiO2 was completely removed, and the C1s peak was characterized as due to the C-O bonds formed near the SiO2/SiC interface.

Depth-Selective Chemical State Analysis of Fine Particles Using X-ray Absorption

Abstract: We have developed a new chemical state analysis method for the particle powders-depth-selective X-ray absorption the structure spectrometry. Recording both the sample current and the X-ray fluoresence intensity concurrently with the change of incident X-ray energy made it possible to obtain surface (10-100 A) and bulk chemical information on powder samples. Examples are presented for prefractionated coal fly ashes in itch we found chemical state differences owing to the probe depth and the coal origin. The coal fly ash contains S 2- in the bulk and S 6+ in the 10-100 A surface. The ratio of S 2- /S 6+ varies with changes in coal origin, depth from the particle surface, and particle size

Interaction of PbZrxTi1-xO3 (PZT) with Ni: role of surface defects

Abstract: The surface properties of lead zirconate titanate (PZT) and its interaction with a metallic overlayer have been investigated. X-ray photoelectron spectroscopy (XPS) was used to study the composition and chemical states of the substrate and adsorbate ions. Results in PZT were compared to those in iso-structural SrTiO3. It was found that, on polished PZT and SiTiO3 surfaces at room temperature, a fraction of the first monolayer of evaporated Ni is oxidized to form Ni-O type bonds. On the other hand, if the surface is ion bombarded prior to deposition, the extent of this chemical interaction is strongly reduced in both samples even though there is no evidence of oxygen depletion at the surface. The relationship between this behaviour and sputtering-related defects has been studied by quantifying bombardment related changes to obtain defect profiles. In PZT, there is depletion of Pb2+ accompanied by chemical reduction to metallic Pb over a depth of 30 or more monolayers. This species is found to be insensitive to surface adsorbates. In both PZT and SrTiO3, Ti4+ ions in the outer two to three monolayers are reduced to Ti3+. This defect reacts easily with adatoms (metals, atmospheric contaminants, etc) to change back to its original Ti4+ state. Since this defective species acts as a source of electrons, and is common to both solids which show similar adsorption behaviour, it must be responsible for reduced oxidation of Ni on the sputtered surface. The scientific significance of these observations and their implication to practical properties have been discussed.

XPS Analysis of Metal Grain Boundary Surfaces

Abstract: X-ray photoelectron spectroscopy (XPS) can provide a quantitative analysis of the surface and determine the chemical state of elements present. However, until recently the spatial resolution has not been adequate to analyse small particles. As a result, Auger electron spectroscopy has been used to obtain elemental information where good spatial resolution is required. Analysis of grain boundary surfaces has therefore been restricted to this technique, with the result that no chemical-state information has been obtained relating to the elements present on the grain boundary. This paper describes results in which grain boundary surfaces exposed in an imaging XPS instrument have been analysed using XPS. In particular, tin and phosphorus are shown to segregate to the grain boundary surface in the elemental state.

Electronic structures of N2+ and O2+ ion‐implanted Si(100)

Abstract: Electronic structures of N 2 + and O 2 + ion-implanted Si(100) (SiN x , SiO x ) have been investigated by means of x-ray photoelectron spectroscopy (XPS), x-ray-induced Auger electron spectroscopy (XAES) and x-ray absorption near-edge structures (XANES). For the O 2 + ion irradiation, the binding energies of the Si 2p and Si KLL Auger lines for SiO x at x > 0.4 show that SiO 2 is predominantly produced. The XANES spectra of SiO x (x > 0.2) at the Si 2p edge are similar to those of bulk SiO 2 . However, other oxides such as SiO are scarcely observed in these spectra. These observations indicate that the SiO x layer consists of a mixture of Si and SiO 2 islands. On the other hand, chemical shift of the Si 2p and Si XLL Auger lines for SiN x becomes larger gradually with the implantation. Thus, it is considered that such changes are due to the stepwise replacement of the Si-Si bond with the Si-N bond. It implies that SiN x has several chemical states of nitride.

Chemical States, Roles and Interrelations of Additives in BaTiO3 Ceramics

Abstract: Chemical states of Mn in BaTiO3 and interactions between Mn and other dopants were studied mainly by chemical analysis and ESR spectroscopy. Chemical analysis showed that the average valency of Mn doped into BaTiO3 was about 3.4, indicating the occurrence of a significant concentration of oxygen vacancy. The average valency of Mn was scarcely affected by codoping of an equimolar amount of La with Mn, but was markedly lowered by addition of Si. It was assumed that stabilization of La segregated to the grain-boundary region due to La-Mn interaction resulted in suppression of the cation vacancy formation and eventually in protection of adsorbed oxygen ions at the grain boundaries. ESR analysis showed that La3+-Mn3+ associate was a strong trap center compatible with Mn4+. The mean valency of Mn in the grain-boundary region of Mn- and La-codoped BaTiO3 was significantly lower than that in the grain interior.

XPS sputter depth profiling of the chemical states for SrTiO3/Si interface by O2+ ion beams

Abstract: Because the chemical states of the elements in SrTiO3 thin film on Si are reduced by argon ion beam bombardment, it was impossible to sputter depth profile the chemical states of SrTiO3 thin film by argon ion beams In this paper, it is reported that the undistorted chemical states of Ti and Si at the SrTiO3/Si interface can be determined with oxygen ion beams at the appropriate 70° angle of incidence, with which either metallic Ti is not oxidized or Ti in SrTiO3 is not reduced Under the sputter depth profiling conditions, the chemical state of Ti at the SrTiO3/Si interface could be successfully characterized and the effects of post-annealing at high temperature on the chemical state of Ti were studied A significant number of Ti atoms in the metallic state were observed at the SrTiO3/Si interface without any post-annealing but all of them were oxidized to the Ti4+ chemical state after 2 h post-annealing at the temperatures above 600°C under oxygen flow The dielectric properties of SrTiO3 thin films on Si were well correlated to the oxidation state of Ti and the broadening of the interface SiO2 layer induced by post-annealing at high temperature

Potentiostatic study of the passivation of aluminium alloys in aqueous electrolyte media

Abstract: The anodic oxidation of Al 5083 and Al 6061 has been studied by steady state and transient polarization measurements in deoxygenated chloride solution containing sulphate ions at 22°C. Complementary to the electrochemical studies, techniques such as scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) were applied to investigate the morphological structure and chemical state of the electrode surface. The data suggests that a porous film of MgO and possibly Fe2O3 (for Al 6061) is formed on the electrode surface in addition to the aluminium oxide film (Al2O3). Some mechanisms for the growth of surface films on both alloys are discussed.

Depth selective chemical state analysis of fly ash with simultaneous XANES measurement of total electron and X-ray fluorescence yields

Abstract: We have developed a method to obtain both surface ( A ) and bulk chemical state of powder samples simultaneously. This method is applied to analyze depth profile of fly ash samples and find that the chemical state of surface sulfur is different from that of bulk sulfur.

Chemical state mapping on material surfaces with the X1A second-generation scanning photoemission microscope (X1A SPEM-II)

Abstract: The second generation scanning photoelectron microscope at beamline X1A of the National Synchrotron Light Source (X1A SPEM-II) is designed for spatially resolved elemental and chemical analysis by x ray photoelectron spectroscopy (XPS) on material surfaces. This microscope can focus photon energies between 300 to 800 eV with submicron spatial resolution. Multiple photoelectron images can be acquired simultaneously with the use of a hemispherical sector analyzer (HSA) with multi-channel detection (MCD), which enables a technique called 'parallel imaging for chemical state mapping' (PICSM). The PICSM technique was demonstrated using a Si/SiO2 pattern.© (1995) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Effect of surface layer treatment of lead-silicate glasses on their composition and structure

Abstract: The method of x-ray electron spectroscopy is used for investigating the effect of preliminary treatment (ion bombardment, polishing, sintering with soluble glass, etching in an acid solution) on the composition and chemical state of elements in the surface layer of lead-silicate glasses and on the results of their heat treatment in hydrogen.

The Analysis of Chemical State of Obliquely Evaporated Co-Ni-O Film by Auger Electron Spectroscopy

Abstract: Obliquely evaporated Co-O and Co-Ni-O films show different coercivity dependences on saturation magnetization. To clarify the origin of this difference, we introduced numerical analysis in Auger electron spectroscopy and analyzed the chemical state (metal or oxide) of the Co-Ni-O magnetic layer. In the Co-Ni-O magnetic layer, cobalt was oxidized preferentially compared to nickel. Therefore the compositional ratio of metallic Co to Ni in the magnetic layer was assumed to be different from that of source material. This preferential oxidation of cobalt was considered to be the cause of the difference in coercivi ty dependences on saturation magnetization between Co-O and Co-Ni-O.

Local density of states analysis by Auger electron spectroscopy

Abstract: In order to measure the local density of states (LDOS) on each depth profile of the GaAs/Si interface by Auger electron spectroscopy, the Auger line shapes are carefully processed and the Auger line shape analysis is performed with the help of factor analysis. A series of corrections to remove the distortions of the peak shape are carried out. There are two chemical states coexisting in the interface. Some of the Si atoms will bond with As and transfer 0.3 p electrons per Si atom to As, and the others will keep the chemical state of pure Si with Si–Si bond. The top of the valence band of Si has a slight shift due to the bonding between Si and As, and the amplitude of the LDOS has a significant decrease.