Changes In Chemical State Research Articles

Observation of chemical separation of In3Sb1Te2 thin film during phase transition

We investigated the chemical states of In3Sb1Te2 (IST) thin film using high-resolution X-ray photoelectron spectroscopy (HRXPS) with the synchrotron radiation during in-situ annealing in ultra-high vacuum. To obtain the oxygen-free amorphous IST (a-IST), we performed the mild Ne+ ion sputtering. And also we confirmed the relative a-IST stoichiometry to be 54%:17%:29% based on HRXPS data. At the first and second phase transition temperatures of 350 and 400°C, we observed the dramatic changes of chemical states from a-IST to InSb and the mixture of crystalline-IST and InTe, respectively. There was a depletion of Sb atoms on the surface after annealing at 750°C. We assume that Sb atom is a key for the phase transition in IST. However, chemical state of the Sb in IST is unstable during the phase transition and it will be caused with the non-reversible process by this structural instability.

Influence of operating parameters on surface properties of RF glow discharge oxygen plasma treated TiO2/PET film for biomedical application

In this paper, a thin transparent titania (TiO2) film was coated on the surface of flexible poly(ethylene terephthalate) (PET) film using the sol–gel method. The surface properties of the obtained TiO2/PET film were further improved by RF glow discharge oxygen plasma as a function of exposure time and discharge power. The changes in hydrophilicity of TiO2/PET films were analyzed by contact angle measurements and surface energy. The influence of plasma on the surface of the TiO2/PET films was analyzed by atomic force microscopy (AFM) as well as the change in chemical state and composition that were investigated by X-ray photo electron spectroscopy (XPS). The cytotoxicity of the TiO2/PET films was analyzed using human osteoblast cells and the bacterial eradication behaviors of TiO2/PET films were also evaluated against Staphylococcus bacteria. It was found that the surface roughness and incorporation of oxygen containing polar functional groups of the plasma treated TiO2/PET films increased substantially as compared to the untreated one. Moreover the increased concentration of Ti3+ on the surface of plasma treated TiO2/PET films was due to the transformation of chemical states (Ti4+→Ti3+). These morphological and chemical changes are responsible for enhanced hydrophilicity of the TiO2/PET films. Furthermore, the plasma treated TiO2/PET film exhibited no citotoxicity against osteoblast cells and antibacterial activity against Staphylococcus bacteria which can find application in manufacturing of biomedical devices.

Solar water splitting with p-SiC film on p-Si: Photoelectrochemical behavior and XPS characterization

The electrochemical properties of single-crystalline p-type 3C-SiC films on p-Si substrate were investigated as an electrode in H2SO4 aqueous solutions in dark and under white light illumination. The photoelectrochemical (PEC) measurements indicates the p-type 3C-SiC film on p-Si substrate can generate a cathodic photocurrent as a photocathode, which corresponds to hydrogen production, and generate an anodic photocurrent as a photoanode, which corresponds to oxygen evolution. The surface chemical states of the films were investigated by XPS. In order to observe the surface chemical state changes after PEC test, the range of applied potential to the electrode was divided into three zones: −3.6 to 0 V, 0–1.5 V and 1.5–4 V vs. Ag/AgCl. After separated PEC tests in these three areas, XPS shows the surface of the SiC film in the range of −3.6 to 0 V and 0–1.5 V was stable without oxidation except the band bending occurred. But in the range of 1.5–4 V the film surface was oxidized due to anodic oxidation.

Soft X-ray photoelectron spectroscopic investigation of Au-deposited amorphous In–Ga–Zn–O thin-film surface

Thin Au layers (0.04 nm, 0.06 nm, 0.08 nm, 0.21 nm) were deposited on an amorphous In–Ga–Zn–O (a-IGZO) thin-film surface by thermal evaporation in an ultrahigh vacuum. The chemical-state changes of the Au layer and the a-IGZO surface were investigated by measuring the Au 4f, O 1s, In 3d, Ga 3d and Zn 3d peaks as well as the valence bands by soft X-ray photoelectron spectroscopy. Upon initial deposition, an oxidized Au component formed, whereas a metallic Au component was dominant. At the later deposition stages, the metallic Au component dominated the spectral features. When the Au thickness was thicker than 0.08 nm, a split Au 5d feature was apparent. And as the Au thickness continued to increase, In 3d showed a strong lower-binding-energy (lower-BE) component; the Ga 3d and Zn 3d also showed lower-BE components, but of much reduced intensities relative to In 3d. These results seem to imply that compared with other elements, Au atoms prefer to occupy oxygen vacancies and strongly interact with In atoms.

Quantitative X-ray photoelectron spectroscopy-based depth profiling of bioleached arsenopyrite surface by Acidithiobacillus ferrooxidans

In supergene environments, microbial activities significantly enhance sulfide oxidation and result in the release of heavy metals, causing serious contamination of soils and waters. As the most commonly encountered arsenic mineral in nature, arsenopyrite (FeAsS) accounts for arsenic contaminants in various environments. In order to investigate the geochemical behavior of arsenic during microbial oxidation of arsenopyrite, (230) surfaces of arsenopyrite slices were characterized after acidic (pH 2.00) and oxidative decomposition with or without an acidophilic microorganism Acidithiobacillus ferrooxidans. The morphology as well as chemical and elemental depth profiles of the oxidized arsenopyrite surface were investigated by scanning electron microscopy and X-ray photoelectron spectroscopy. With the mediation of bacteria, cell-shaped and acicular pits were observed on the reacted arsenopyrite surface, and the concentration of released arsenic species in solution was 50 times as high as that of the abiotic reaction after 10days reaction. Fine-scale XPS depth profiles of the reacted arsenopyrite surfaces after both microbial and abiotic oxidation provided insights into the changes in chemical states of the elements in arsenopyrite surface layers. Within the 450nm surface layer of abiotically oxidized arsenopyrite, Fe(III)-oxides appeared and gradually increased towards the surface, and detectable sulfite and monovalent arsenic appeared above 50nm. In comparison, higher contents of ferric sulfate, sulfite, and arsenite were found in the surface layer of approximately 3μm of the microbially oxidized arsenopyrite. Intermediates, such as Fe(III)-AsS and S0, were detectable in the presence of bacteria. Changes of oxidative species derived from XPS depth profiles show the oxidation sequence is Fe>As=S in abiotic oxidation, and Fe>S>As in microbial oxidation. Based on these results, a possible reaction path of microbial oxidation was proposed in a concept model.

In situ study of the role of substrate temperature during atomic layer deposition of HfO2 on InP

The dependence of the “self cleaning” effect of the substrate oxides on substrate temperature during atomic layer deposition (ALD) of HfO2 on various chemically treated and native oxide InP (100) substrates is investigated using in situ X-ray photoelectron spectroscopy. The removal of In-oxide is found to be more efficient at higher ALD temperatures. The P oxidation states on native oxide and acid etched samples are seen to change, with the total P-oxide concentration remaining constant, after 10 cycles of ALD HfO2 at different temperatures. An (NH4)2 S treatment is seen to effectively remove native oxides and passivate the InP surfaces independent of substrate temperature studied (200 °C, 250 °C and 300 °C) before and after the ALD process. Density functional theory modeling provides insight into the mechanism of the changes in the P-oxide chemical states.

Extended X-ray absorption fine structure spectroscopy of selenium-hyperdoped silicon

Silicon doped with an atomic percent of chalcogens exhibits strong, uniform sub-bandgap optical absorptance and is of interest for photovoltaic and infrared detector applications. This sub-bandgap absorptance is reduced with subsequent thermal annealing indicative of a diffusion mediated chemical change. However, the precise atomistic origin of absorptance and its deactivation is unclear. Herein, we apply Se K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy to probe the chemical states of selenium dopants in selenium-hyperdoped silicon annealed to varying degrees. We observe a smooth and continuous selenium chemical state change with increased annealing temperature, highly correlated to the decrease in sub-bandgap optical absorptance. In samples exhibiting strong sub-bandgap absorptance, EXAFS analysis reveals that the atoms nearest to the Se atom are Si at distances consistent with length scales in energetically favorable Se substitutional-type point defect complexes as calculated by density functional theory. As the sub-bandgap absorptance increases, EXAFS data indicate an increase in the Se-Si bond distance. In specimens annealed at 1225 K exhibiting minimal sub-bandgap absorptance, fitting of the EXAFS spectra indicates that Se is predominantly in a silicon diselenide (SiSe2) precipitate state. The EXAFS study supports a model of highly optically absorbing point defects that precipitate during annealing into structures with no sub-bandgap absorptance.

Initial stages of AZ91 Mg alloy micro-arc anodizing: Growth mechanisms and effect on the corrosion resistance

In the framework of the new ecological regulations, micro-arc oxidation (MAO) appears as an alternative to usual processes in the field of corrosion protection of Mg alloys. In this work, the initial stages of anodic layer growth in KOH-based electrolytes are studied up to and beyond the initiation of the micro-arc regime.The properties of the first anodized film preceding the occurrence of the dielectric breakdown (corresponding to the start of the micro-arc regime) are mainly determined by the incorporation of additives (fluorides or silicates) in the film, as shown by in situ electrochemical measurements. Scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and micro-Raman spectroscopy reveal both the change of morphology and chemical state of silicate and fluoride in the anodized layer before and after the micro-arc regime. In terms of electrochemical behaviour, investigated by stationary methods and electrochemical impedance spectroscopy (EIS) in reference corrosive water, the anodic film grown in the silicate medium provides the best corrosion resistance thanks to a thick layer containing Mg2SiO4, whose degradation products seal the porosities of the coating.

Crystal Structures and Electrochemical Properties of Ni-Substituted Li2CuO2 as a Cathode Material

A single orthorhombic phases of Li2CuO2-Li2NiO2 solid solutions were prepared in the range of x = 0 to 0.4 in Li2Cu1-x Ni x O2. An improvement of specific capacity by the Ni-substitution was obtained as reported previously. Our present study clarified the phase relationship and changes in chemical oxidation states of Cu and Ni ions caused by Li-extracting. The XRD patterns using synchrotron radiations showed that CuO, NiO or LiNiO2 phases were not observed for fully charged Li2Cu1-x Ni x O2. It is likely that the substitution of Ni has an effect to depress the formation of these oxides during charging. X-ray absorption spectra for both Ni and Cu K-edges shifted to higher energy from Li2Cu0.6Ni0.4O2 to LiCu0.6Ni0.4O2 and then to lower energy by charging LiCu0.6Ni0.4O2. An irreversible capacity was improved by the substitution of 10 or 20 mole % of divalent Ni for divalent Cu.

A study on the Al doping behavior with sol aging time and its effect on structural and optical properties of sol–gel prepared ZnO thin films

A series of aluminum doped zinc oxide (AZO) thin films were prepared by sol–gel method on glass substrates to investigate the behavior of Al doping with sol aging time (as-prepared to 2weeks). X-ray diffraction analysis revealed a considerable shift in (002) diffraction peak towards higher diffraction angles with increasing sol aging time from as-prepared to 24h which was found to be reverse with further aging. X-ray photoelectron spectroscopy and UV–vis spectroscopy were performed to investigate the change in chemical states of Al/Zn/O atoms and band gap fluctuations respectively. It was found that more Al atoms were doped into ZnO lattice with the increment of sol aging time and saturated level was achieved for ~20–24h. Field emission scanning electron microscope and atomic force microscope characterization showed dense and homogeneous film morphology with decreasing relative surface roughness with sol aging time. Photoluminescence analysis and UV–vis spectroscopy revealed defect free films by exhibiting high optical transmission in the visible regime and found to improve with sol aging time. It could be concluded that solution aging time clearly influence the doping behavior and a suitable aging provides more stability which results into more Al atoms substitution for Zn in ZnO crystal lattice that affect the structural, morphological and optical properties of AZO thin films.

Selective catalytic oxidation of ammonia to nitrogen over CuO-CeO2 mixed oxides prepared by surfactant-templated method

The selective catalytic oxidation of ammonia to nitrogen (NH3-SCO) has been studied over CuO-CeO2 mixed oxides. The active Cu component was doped into the CeO2 by surfactant-templated method. The finely dispersed CuO, Cu-O-Ce solid solution and bulk CuO species were detected in CuO-CeO2 mixed oxides. When the Cu loading was 10wt% and the calcination temperature was 500°C, CuO-CeO2 catalyst exhibited the highest molar ratio of the finely dispersed CuO species and the smallest CeO2 particles in size, and simultaneously possessed the highest level of activity. The finely dispersed CuO species was the main adsorbed sites of NH3 molecules, and the NH3(ad) could be further activated and transformed into NHx species by ceria under the roles of quick change of chemical state in near-surface region and the strong electron state interaction in CuO-CeO2 catalysts. The synergetic interaction between the two components played an important role in NH3 activation and oxidation. In addition, the activated intermediates (NHx) could also react with lattice oxygen provided by Cu-O-Ce solid solution to form N2, N2O and H2O, which was confirmed by XPS, EPR and NH3-TPR analysis. Moreover, gas oxygen could refill the oxygen vacancies to replenish the lattice oxygen consumed by NHx species. The Cu-O-Ce solid solution promoted the activation of gas oxygen as well as the formation and migration of lattice oxygen in NH3-SCO reaction, and the formed rapid reduction–oxidation cycle was essential for the higher activity of NH3 oxidation.

Catalysts Transform While Molecules React: An Atomic-Scale View.

We explore how the atomic-scale structural and chemical properties of an oxide-supported monolayer (ML) catalyst are related to catalytic behavior. This case study is for vanadium oxide deposited on a rutile α-TiO2(110) single-crystal surface by atomic layer deposition (ALD) undergoing a redox reaction cycle in the oxidative dehydrogenation (ODH) of cyclohexane. For measurements that require a greater effective surface area, we include a comparative set of ALD-processed rutile powder samples. In situ single-crystal X-ray standing wave (XSW) analysis shows a reversible vanadium oxide structural change through the redox cycle. Ex situ X-ray photoelectron spectroscopy (XPS) shows that V cations are 5+ in the oxidized state and primarily 4+ in the reduced state for both the (110) single-crystal surface and the multifaceted surfaces of the powder sample. In situ diffuse reflectance infrared Fourier transform spectroscopy, which could only achieve a measurable signal level from the powder sample, indicates that these structural and chemical state changes are associated with the change of the V═O vanadyl group. Catalytic tests on the powder-supported VOx revealed benzene as the major product. This study not only provides atomic-scale models for cyclohexane molecules interacting with V sites on the rutile surface but also demonstrates a general strategy for linking the processing, structure, properties, and performance of oxide-supported catalysts.

Biological Change of Chemical States of Actinides and Lanthanides-effects of Organic Acids-

In this study, we have examined the chemical states change of lanthanides (Lns) of La, Ce, Pr, Eu, and actinides (Ans) of Th(IV), Pu(IV) in the complexes with organic acids of citric acids or desferrioxamine B (DFO) by the interaction with Pseudomonas fluorescens cells under the resting condition at pH 4 – 9. In the adsorption experiments the distribution coefficients (Kd) were determined. The oxidation states of Ce in the solution were determined by X-ray near edge fine structure (XAFS). In the absence of organic materials, Kd of Eu(III) increased gradually with pH up to 7; above 7, the Kd decreased with an increase in pH. In the presence of citric acid, we found that Kd was lower than that in the absence of organic materials, reflecting the formation of Eu(III)-citrate complexes competes with the Eu(III)-cell-surface complexes. For Ln-DFO complex, the higher Kds of La, Ce, and Pr were obtained at lower pHs on P. fluorescens cells. The Kd of Ce was lower than that of its neighboring REEs, La(III) and Pr(III). XAFS analysis showed that Ce exists as the Ce(IV)-DFO complex in higher pH than 6. Thus, the pH dependence of lower Kd of Ce is predominantly dependent on the stability of Ce(IV)-DFO complex. For the adsorption of Ans, the Kds of Th and Pu were larger than Eu, being inverse relationship to the stability constants of DFO complex. These results indicate that presence of organic acids affect the chemical states of Ln and An in the environments.

Potassium spin polarization lifetime for a 30-carbon chain siloxane film

The siloxane film derived from the 30-carbon chain triacontyltrichlorosilane (TCTS) is studied as an anti-relaxation coating for atomic vapor cells. The longitudinal spin relaxation lifetime of optically pumped potassium atoms in the presence of TCTS is measured and the average number of non-relaxing atom-wall collisions, or bounces, enabled by the coated surface is determined. X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) of TCTS were performed to investigate changes in chemical states and surface morphology of TCTS arising from K atom deposition on the film surface. TCTS was found to give approximately 530 bounces. Following lifetime measurements, K2p signals were clearly observed in XPS spectra. AFM images display non-preferential K deposition on the TCTS surface, however additional AFM studies with a TCTS surface exposed to Rb atoms show deposition occurs along surface defects. In agreement, Rb is found to preferentially deposit along the step edges of an 18-carbon chain monolayer film derived from 1-Octadecene. Finally, AFM indicates a much smoother surface for a tetracontane coating relative to TCTS. The importance of siloxane surface morphology versus film thickness with respect to coating performance is discussed.

Olivine as tar removal catalyst in biomass gasification: Catalyst dynamics under model conditions

Olivine ((Mg,Fe)2SiO4) has been extensively explored as an active bed material for catalytic cracking of tars during gasification of biomass in dual fluidized bed reactors. It is known that both the elemental composition, addition of Fe and high temperature calcinations influence the catalytic properties of this mineral. However, it is not clear how olivine responds to the fairly hostile environments present during gasification or what chemical state Fe takes during operation. We have investigated the stability of Austrian olivine under model conditions, resembling those in a gasifier. Powder samples were heated to 750°C in a quartz-tube flow-reactor and sequentially exposed to oxidizing (O2, H2O, CO2) or reducing gases (CO, H2) or mixtures thereof, for various durations of time. Significant changes in phase composition of the material, depending on the gas composition and the duration of the treatments, were found using X-ray photo-electron spectroscopy (XPS), X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS) and scanning electron microscopy (SEM). A large fraction of the Fe in the investigated material is present as free Fe-phases, which are sensitive to changes in the gas environment. After exposure to oxidizing gases, the free Fe phases are: Fe2O3 and Fe3O4 or MgFe2O4. Upon exposure to reducing gases, the iron oxides are converted into Fe0 and Fe3C and formation of graphitic carbon is observed. In addition, the elemental composition of the surface changes dramatically depending on the gas composition. After exposure to oxidizing environments, the amount of Fe at the surface is twice as high as after reduction. Both the change in chemical state of the Fe-phases, the amount of surface Fe and the build-up of surface carbon are fast processes under the applied conditions and significant changes are observed on the time scale of one minute. These observations have important implications for olivine as a tar cracking catalyst, especially when used in dual fluidized bed gasifiers. The fast reduction of the iron oxides upon switching from oxidizing to reducing conditions shows that olivine transports oxygen from the combustor into the gasifier. Furthermore, the catalytic properties of Fe depend strongly on its chemical state. Therefore, the catalytic function of olivine depends strongly on the gas environment and on the catalysts residence time in the gasifier. Finally, both the decreasing amount of surface-Fe and the carbon deposition observed after exposing olivine to reducing conditions can result in significant catalyst deactivation.

New instrument for the study of ‘the kg, mise en pratique’: first results on the correlation between the change in mass and surface chemical state

In order to study the practical means of a dissemination of the redefined kilogram, the mass laboratory of METAS has built a new and unique instrument, which combines high precision mass measurement and element-specific surface chemical analysis by x-ray photoelectron spectroscopy (XPS) under identical conditions. It is especially designed for the analysis of large samples such as real 1 kg standards and artefacts of up to 100 mm in diameter. In this paper, we first provide a detailed description of the apparatus. In a second step we demonstrate its performance by applying various processes, such as the transfer from ambient to vacuum, cleaning by low-pressure hydrogen plasma, transfer from vacuum to ambient and recontamination in air with time. Real 1 kg PtIr standards and surface artefacts have been monitored at individual steps gravimetrically, and in vacuum additionally by XPS. Values for the gain and loss of mass due to the application of different processes are provided. A model for the short-term recontamination after cleaning is presented, showing the initial growth of contaminants is self-limited.

A Mechanochemical Model of Actin Filaments

In eukaryotic cells, actin filaments are involved in important processes such as motility, division, cell shape regulation, contractility, and mechanosensation. Actin filaments are polymerized chains of monomers, which themselves undergo a range of chemical events such as ATP hydrolysis, polymerization, and depolymerization. When forces are applied to F-actin, in addition to filament mechanical deformations, the applied force must also influence chemical events in the filament. We develop an intermediate-scale model of actin filaments that combines actin chemistry with filament-level deformations. The model is able to compute mechanical responses of F-actin during bending and stretching. The model also describes the interplay between ATP hydrolysis and filament deformations, including possible force-induced chemical state changes of actin monomers in the filament. The model can also be used to model the action of several actin-associated proteins, and for large-scale simulation of F-actin networks. All together, our model shows that mechanics and chemistry must be considered together to understand cytoskeletal dynamics in living cells.

Study on the retention behavior of hydrogen isotopes and the change of chemical states of boron film exposed to hydrogen plasma in LHD

The behavior of hydrogen retention and the change of chemical states of boron film exposed to hydrogen plasma in LHD were investigated. The sample was prepared in LHD, and atomic concentrations for the boron film after hydrogen plasma exposure were changed from 75% for boron, 15% for carbon and 8% for oxygen to 53%, 18% and 22%, respectively. BC bond was a major chemical state of the boron film after hydrogen plasma exposure, although abundance of BB bond was the highest before the plasma exposure. Total hydrogen retention measured by TDS was evaluated to be 1.7×1020Hm−2, and the retentions of hydrogen as BHB, BH and BCH bonds were, respectively, 4.8×1019, 7.2×1019 and 5.2×1019Hm−2. It was concluded that the hydrogen retention could be estimated by taking account not only of chemical states of impurities, but also of hydrogen depth profile.

XPS characterization and photoelectrochemical behaviour of p-type 3C-SiC films on p-Si substrates for solar water splitting

The electrochemical (EC) properties of single-crystalline p-type 3C-SiC films on p-Si substrates were investigated as electrodes in H2SO4 aqueous solutions in dark and under white light illumination. Before EC tests, the SiC films were etched by HF solution and aqua-regia–HF solution, respectively, and then investigated by x-ray photoelectron spectroscopy (XPS) including one untreated SiC sample. After EC tests, XPS was also applied to investigate the surface chemical state changes. The EC measurements indicate that the p-type 3C-SiC films on p-Si substrates can generate a cathodic photocurrent as the photocathode, which corresponds to hydrogen production, and generate an anodic photocurrent as the photoanode, which corresponds to oxygen evolution. XPS shows the surface of all the SiC films was oxidized due to anodic oxidation applied by a positive bias during the EC test, which indicates the formation of silicon oxides, CO2 or CO and carbonates.

Platinum Nanoparticles on Gallium Nitride Surfaces: Effect of Semiconductor Doping on Nanoparticle Reactivity

Platinum nanoparticles supported on n- and p-type gallium nitride (GaN) are investigated as novel hybrid systems for the electronic control of catalytic activity via electronic interactions with the semiconductor support. In situ oxidation and reduction were studied with high pressure photoemission spectroscopy. The experiments revealed that the underlying wide-band-gap semiconductor has a large influence on the chemical composition and oxygen affinity of supported nanoparticles under X-ray irradiation. For as-deposited Pt cuboctahedra supported on n-type GaN, a higher fraction of oxidized surface atoms was observed compared to cuboctahedral particles supported on p-type GaN. Under an oxygen atmosphere, immediate oxidation was recorded for nanoparticles on n-type GaN, whereas little oxidation was observed for nanoparticles on p-type GaN. Together, these results indicate that changes in the Pt chemical state under X-ray irradiation depend on the type of GaN doping. The strong interaction between the nanoparticles and the support is consistent with charge transfer of X-ray photogenerated free carriers at the semiconductor-nanoparticle interface and suggests that GaN is a promising wide-band-gap support material for photocatalysis and electronic control of catalysis.