Showing papers on "XANES published in 2008"

Assignment of pre‐edge peaks in K‐edge x‐ray absorption spectra of 3d transition metal compounds: electric dipole or quadrupole?

Abstract: The characteristics of pre-edge peaks in K-edge x-ray absorption near edge structure (XANES) spectra of 3d transition metals were reviewed from viewpoints of the selection rule, coordination number, number of d-electrons, and symmetry of the coordination sphere. The contribution of the electric dipole and quadrupole transition to the peaks was discussed on the basis of the group theory, polarized spectra, and theoretical calculations. The pre-edge peak intensity for Td symmetry is larger than those for Oh symmetry for all 3d elements. The intense pre-edge peak for tetrahedral species of 3d transition metals is not due to 1s–3d transition, but transition to the p component in d–p hybridized orbital. The mixing of metal 4p orbitals with the 3d orbitals depends strongly on the coordination symmetry, and the possibility is predictable by group theory. The transition of 1s electron to d orbitals is electric quadrupole component in any of the symmetries. The d–p hybridization does not occur with regular octahedral symmetry, and the weak pre-edge peak consists of 1s–3d electric quadrupole transition. The pre-edge peak intensity for a compound with a tetrahedral center changes as a function of the number of 3d electrons regardless of the kind of element; it is maximized at d0 and gradually decreases to zero at d10. The features of pre-edge peaks in K-edge XANES spectra for 4d elements and the L1-edge for 5d elements are analogous with those for 3d elements, but the pre-edge peak is broadened due to the wide natural width of the core level. Copyright © 2008 John Wiley & Sons, Ltd.

Near-edge x-ray absorption fine-structure investigation of graphene.

Abstract: We report the near-edge x-ray absorption fine-structure (NEXAFS) spectrum of a single layer of graphite (graphene) obtained by micromechanical cleavage of highly ordered pyrolytic graphite on a SiO2 substrate. We utilized a photoemission electron microscope to separately study single-, double-, and few-layers graphene samples. In single- layer graphene we observe a splitting of the pi* resonance and a clear signature of the predicted interlayer state. The NEXAFS data illustrate the rapid evolution of the electronic structure with the increased number of layers.

Hydrothermal Microwave: A New Route to Obtain Photoluminescent Crystalline BaTiO3 Nanoparticles

Abstract: Hydrothermal microwave method was used as a new route to synthesize pure BaTiO3 (BT) nanoparticles at 140°C for 10 min under rapid reacting with stoichiometric Ba/Ti ratio. The crystalline products were characterized by X-ray powder diffraction (XRD) and the structure was refined by the Rietveld method from the tetragonal structure, which was supported by the Ti K-edge X-ray absorption near-edge structure (XANES). The pre-edge of Ti in the XANES spectra indicated that titanium ions are localized in a nonregular octahedron. Typical FT-Raman spectra for tetragonal BaTiO3 nanoparticles presented well-defined peaks, indicating a substantial short-range order in the system. However, a scattering peak at 810 cm−1 was attributed to the presence of lattice OH− groups, commonly found in materials obtained by hydrothermal process. Besides, the peak at 716 cm−1 can be related to eventual Ba2+ defects in the BaTiO3 lattice. BaTiO3 (BT) nanoparticles presented spherical morphology with a non-uniform distribution of pa...

Transient Technique for Identification of True Reaction Intermediates: Hydroperoxide Species in Propylene Epoxidation on Gold/Titanosilicate Catalysts by X-ray Absorption Fine Structure Spectroscopy

Abstract: In situ ultraviolet−visible (UV−vis) diffuse reflectance spectroscopy was used in combination with in situ Ti K-edge X-ray absorption near-edge structure (XANES) to study the formation of Ti-hydroperoxo species during the gas-phase epoxidation of propylene with H2 and O2 at reaction conditions over a Au−Ba/Ti−SiO2 (Ti−TUD) catalyst. The in situ UV−vis measurements showed growth of a signal due to Ti-hydroperoxo species when the catalyst was put in contact with H2/O2/Ar (1/1/8) and C3H6/H2/O2/Ar (1/1/1/7) gas mixtures at 423 K and 0.1 MPa. Changes in the area of the pre-edge peak centered at 4968.9 eV present in the Ti K-edge XANES spectra of the catalyst were used to estimate the Ti-hydroperoxo species coverages (θ) under operating conditions. Transient Ti K-edge XANES experiments with H2/O2/Ar (1/1/8) and C3H6/H2/O2/Ar (1/1/1/7) gas mixtures allowed the estimation of the net epoxidation rate by a novel method involving the determination of dθ/dt. It is shown that the Ti-hydroperoxo species are true inter...

XAFS Study of Tungsten L1- and L3-Edges: Structural Analysis of WO3 Species Loaded on TiO2 as a Catalyst for Photo-oxidation of NH3

Abstract: We analyzed the X-ray absorption fine structure (XAFS) spectra of W L1- and L3-edges in order to determine the structure of various W species on TiO2. Two 2p → 5d transitions were observed in the second derivative of the W L3-edge X-ray absorption near-edge structure (XANES) spectra of all samples containing W (WO3, Na2WO4, Cr2WO6, Ba2NiWO6, (NH4)10W12O41·5H2O, Sc2W3O12, H3PW12O40·13H2O, and TiO2-loaded WO3 samples). These two transitions can be assigned to 5d orbitals split by the ligand field. The split in the 5d orbital has a linear relationship with the area of pre-edge peak of the W L1-edge XANES spectrum. This linear relationship is supported by density functional theory (DFT) calculations of the octahedral and tetrahedral W models. On the basis of this linear relationship, we estimated the structure of various W species on TiO2. The obtained structure is in accordance with the curve fitting analysis of W L3-edge extended X-ray absorption fine structure (EXAFS). Additionally, we found that the octah...

Quantitative organic and light-element analysis of comet 81P/Wild 2 particles using C-, N-, and O-μ-XANES

Abstract: Synchrotron-based soft X-ray micro-analysis was performed on particles extracted from the Stardust aerogel collector in order to obtain detailed organic functional group information on any organic solids captured as part of the Principal Examination suite of analyses for samples from comet 81P/Wild 2. It is observed that cometary organic carbon captured in aerogel is present in a number of different manifestations and often intimately associated with silicates. Carbon X-ray absorption near edge structure (XANES) spectra reveal considerable chemical complexity in all of the organic particles studied so far. Universally, the comet 81P/Wild 2 organic particles contain low concentrations of aromatic and/or olefinic carbon relative to aliphatic and heteroatom-containing functional groups, e.g., amide, carboxyl, and alcohol/ethers. N-XANES confirms the presence and assignments of these functional groups. In general, the XANES data record considerable chemical complexity across the range of organic samples currently analyzed. The atomic ratios, N/C and O/C, derived from XANES data reveal a wide range in heteroatom content; in all cases these elemental ratios are higher than that of primitive meteoritic organic matter. The wide range in chemistry, both in elemental abundances and specific organic functional groups, suggests that the comet 81P/Wild 2 organic solids may have multiple origins.

Structural conditions that leads to photoluminescence emission in SrTiO3: An experimental and theoretical approach

Abstract: Complex cluster [TiO5⋅VOz] and [SrO11⋅VOz] (where VOz=VOX, VO•, VO••) vacancies were identified in disordered SrTiO3 powders prepared by the polymeric precursor method, based on experimental measurements by x-ray absorption near edge structure spectroscopy. The paramagnetic complex states of [TiO5⋅VO•] and [SrO11⋅VO•] with unpaired electrons were confirmed by electron paramagnetic resonance spectroscopy. The disordered powders showed strong photoluminescence at room temperature. Structural defects of disordered powders, in terms of band diagram, density of states, and electronic charges, were interpreted using high-level quantum mechanical calculations in the density functional framework. The four periodic models used here were consistent with the experimental data and explained the presence of photoluminescence.

A theoretical and experimental study of the near edge X-ray absorption fine structure (NEXAFS) and X-ray photoelectron spectra (XPS) of nucleobases: Thymine and adenine

Abstract: The core level electron excitation and ionization spectra of thymine and adenine have been investigated by photoabsorption and photoemission spectroscopy, and the results interpreted by means of ab initio calculations using the second-order algebraic–diagrammatic construction (ADC(2)) method for the polarization propagator and the fourth-order ADC method (ADC(4)) for the one-particle Green’s function. The photoabsorption spectra are dominated by transitions from core levels to unoccupied π states, but also show clear structures due to Rydberg transitions. The calculated spectra are in good agreement with the experimental results, and many of the observed structures are assigned.

Ab initiostudy of the physical properties ofγ-Al2O3: Lattice dynamics, bulk properties, electronic structure, bonding, optical properties, and ELNES/XANES spectra

Abstract: Based on the most recently determined noncubic structure for -Al2O3 by Menendez-Proupin and Gutierrez, a comprehensive list of physical properties is investigated theoretically. These include lattice dynamics and phonon spectra, elastic constants and bulk structural parameters, electronic structure and interatomic bonding, optical properties, and x-ray absorption near-edge structure XANES spectra. Compared to similar calculations of -Al2O3, we find a smaller lowest zone-center vibrational mode at 97.6 cm −1, a lower heat capacity, a smaller bulk modulus, and a much larger thermal-expansion coefficient. The threefold bonded O ions introduce highly localized vibrational modes near 751 cm−1. The calculated thermal Gruneisen parameter indicates a strong anharmonicity in -Al2O3. The elastic tensor and the elastic wave velocities are also evaluated showing the longitudinal wave to be nearly isotropic. For the electronic structure, we find that -Al2O3 has a smaller band gap but a refractive index similar to -Al2O3. Highly localized states at the top of the valence band originating from threefold bonded O in the more covalently bonded AlO4 tetrahedra are identified. The calculated Mulliken effective charges and bond order values indicate that the structural model for -Al2O3 has a high degree of disorder. The octahedral unit AlO6 is a stronger polyhedron than the tetrahedral unit AlO4 although the latter has stronger Al–O bonds. The calculated Al-K, Al-L3, and O-K edges for Al and O in -Al2O3 show strong dependence on their local coordination and environments. These results are in good agreement with available experimental data but the effect of the -Al2O3 samples’ porosity should be properly assessed. It is argued that the traditional view that stoichiometric -Al2O3 is a defective spinel with cation vacancies or its variations should be modified. -Al2O3 is better described as an amorphous networklike structure such that the ratio of tetrahedrally coordinated Al to octahedrally coordinated Al is close to 0.6; and the O ions are bonded to Al in either a threefold or fourfold configurations in about equal proportion.

Local Structure of TiO2-Derived Nanotubes Prepared by the Hydrothermal Process

Abstract: The structure of TiO2-derived nanotubes with about 10 nm outer diameter and 5 nm inner diameter and a few hundred nanometers in length prepared by the hydrothermal process in an aqueous NaOH solution was investigated. Especially, the local structure of TiO2-derived nanotubes was analyzed in detail on short-range order by extended X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structure (XANES) measurements. It was found that TiO2-derived nanotubes were mainly composed of layered titanate and the anatase-like structure was partly present in the titanate-based nanotubes. The formation of the anatase-like structure was thought to be caused from the consolidation of nanotubes.

Core level study of alanine and threonine.

Abstract: Core level X-ray photoemission spectra (XPS) and near edge X-ray absorption fine structure (NEXAFS) spectra of alanine and threonine in the gas phase have been measured at the carbon, nitrogen, and oxygen K edges and interpreted in the light of theoretical calculations. For the computations, a set of approximations is made which allows sufficiently accurate calculations of several conformers to be performed in reasonable computing time. The accuracy has been checked by comparing results obtained for proline to our previous, higher level calculations. The photoemission spectra at the carbon and oxygen edges are assigned and compared. The nitrogen 1s photoemission peaks show anomalous broadening which we relate to the populations and types of conformers. The carbon K-edge NEXAFS spectra of alanine and threonine are compared with our previous data on glycine and resonances assigned accordingly. The nitrogen K-edge NEXAFS spectra of alanine and threonine do not show measurable effects due to the population of...

Electronic Structure and Chemistry of Iron-Based Metal Oxide Nanostructured Materials : A NEXAFS Investigation of BiFeO3, Bi2Fe4O9, α-Fe2O3, γ-Fe2O3, and Fe/Fe3O4

Abstract: We present a systematic and detailed near edge X-ray absorption fine structure (NEXAFS) experimental investigation of the electronic structure and chemistry of iron-based metal oxide nanostructured (FeMONS) materials including BiFeO3, Bi2Fe4O9, α-Fe2O3, γ-Fe2O3, and Fe/Fe3O4. Correlations of the electronic structure and structural chemistry of these intriguing nanomaterials are presented, ranging from the nano to the bulk scale. In this work, variations in the shape, position, and intensity of the O K-edge and Fe L-edge NEXAFS spectra have been analyzed in terms of electronic structure and surface chemistry of the FeMONS materials as compared with that of the bulk. We hypothesize that surface imperfection and surface strain anisotropies in nanoparticles induce distortion and site inequivalency of the oxygen Oh sites around the Fe ion located close to the surface, resulting in an increase in the degree of multiplicity as well as in nonstoichiometric effects in FeMONS materials.

Formation and cation distribution in supported manganese ferrite nanoparticles: an X-ray absorption study

Abstract: Extended X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structure (XANES) techniques at both Fe and Mn K-edges were used to investigate the formation of MnFe2O4nanoparticles embedded in a silica aerogel matrix as a function of calcination temperature (at 450, 750 and 900 °C). Up to 450 °C, two separated highly-disordered phases of iron and manganese are present. With increasing the temperature (to 750 and 900 °C), the structure of aerogel nanoparticles becomes progressively similar to that of the spinel structure MnFe2O4 (jacobsite). Quantitative determination of cations distribution in the spinel structure shows that aerogels calcined at 750 and 900 °C have a degree of inversion i = 0.20. A pure jacobsite sample synthesised by co-precipitation and used as a reference compound shows a much higher degree of inversion (i = 0.70). The different distribution of iron and manganese cations in the octahedral and tetrahedral sites in pure jacobsite and in the aerogels can be ascribed to partial oxidation of Mn2+ to Mn3+ in pure jacobsite, confirmed by XANES analysis, probably due to the synthesis conditions.

Investigations on the Structural, Morphological, Electrical, and Magnetic Properties of CuFe2O4−NiO Nanocomposites

Abstract: CuFe2O4−xNiO (x = 1, 5, 10, and 20 wt %) nanocomposites have been successfully prepared by a simple combustion method using urea−nitrate precursors. The samples were sintered at different temperatures, namely 600, 800, 1000, and 1100 °C, for 5 h to enhance the compound formation and phase purity, studied by means of XRD patterns. Then the 1100 °C sintered sample was further characterized for its structural (EXAFS, XANES, FT-IR, UV–vis), morphological (SEM, TEM, HRTEM, SAED), electrical (ac conductivity, dielectric constant, dielectric loss tangent), and magnetic (Mossbauer) properties. The EXAFS and XANES studies reveal the formation of NiFe2O4 and CuO, in addition to the existence of CuFe2O4 and NiO phases. A partial substitution of metal cations by nickel ions could also be evidenced. The stretching and bending vibration of the tetrahedral and octahedral complexes have been established from FT-IR spectra. The UV–vis spectra elucidate that the prepared materials are semiconductors and also show the quant...

Atomic structures of supersaturated ZnO-Al2O3 solid solutions

Abstract: Supersaturated ZnO–Al2O3 (>20at.% Al) thin films are grown by pulsed laser deposition technique on silica glass substrates at 600°C. They are characterized by combining x-ray diffraction, Al-K edge x-ray absorption near edge structures (XANESs), high resolution transmission electron microscope (TEM) imaging, TEM analysis, and a series of first principles calculations. The films are composed of textured wurtzite grains with c planes parallel to the substrate. The distance between c planes expands significantly when the Al concentration is greater than 10at.%. The expansion disappears after annealing the films at above 800°C. High density of dislocationlike defects is found in the as deposited film. Any segregation of Al cannot be detected either at the grain boundaries or inside the grains. The lattice expansion toward c axis and the experimental XANES can be satisfactorily explained by taking a hypothetical homologous model with the composition of (ZnO)3(Al2O3) as the local environment of Al in the supers...

Molecular orientation dependent interfacial dipole at the F16CuPc∕CuPc organic heterojunction interface

Abstract: In situ synchrotron-based near-edge x-ray absorption fine structure measurements and photoemission spectroscopy have been used to investigate the effect of molecular orientation on the interfacial dipole and the energy level alignment at the interfaces of organic heterojunctions comprising copper-hexadecafluoro-phthalocyanine (F16CuPc) on both standing-up and lying-down copper(II) phthalocyanine (CuPc) thin films. It is found that F16CuPc thin films adopt the same molecular orientation of the underlying CuPc thin films. An interfacial dipole of 0.45eV forms at the interface of lying-down F16CuPc∕CuPc on highly ordered pyrolytic graphite. In contrast, a much larger interfacial dipole of 1.35eV appears at the interface of standing-up F16CuPc∕CuPc on octane-1-thiol terminated Au(111).

Thin film structure of tetraceno[2,3-b]thiophene characterized by grazing incidence X-ray scattering and near-edge X-ray absorption fine structure analysis.

Abstract: Understanding the structure−property relationship for organic semiconductors is crucial in rational molecular design and organic thin film process control. Charge carrier transport in organic field-effect transistors predominantly occurs in a few semiconductor layers close to the interface in contact with the dielectric layer, and the transport properties depend sensitively on the precise molecular packing. Therefore, a better understanding of the impact of molecular packing and thin film morphology in the first few monolayers above the dielectric layer on charge transport is needed to improve the transistor performance. In this Article, we show that the detailed molecular packing in thin organic semiconductor films can be solved through a combination of grazing incidence X-ray diffraction (GIXD), near-edge X-ray absorption spectra fine structure (NEXAFS) spectroscopy, energy minimization packing calculations, and structure refinement of the diffraction data. We solve the thin film structure for 2 and 20 ...

Cobalt-catalyzed hydrogen desorption from the LiNH2–LiBH4 system

Abstract: A doping of 5 wt% CoCl2 considerably decreases the dehydrogenation temperature of a mixture of LiNH2 and LiBH4. More that 8 wt% of hydrogen can be released at ca. 155 °C. X-Ray absorption near edge structure (XANES) spectroscopy indicated the formation of metallic Co after ball milling CoCl2 with LiNH2 and LiBH4. Extended X-ray absorption fine structure (EXAFS) spectroscopy measurements revealed that Co particles have poor crystallinity and are finely dispersed in the sample, which could lead to a high catalytic efficiency.

Nanoparticles and nanowires: synchrotron spectroscopy studies

Abstract: This paper reviews the research in nanomaterials conducted in our laboratory in the last decade using conventional and synchrotron radiation techniques. While preparative and conventional characterisation techniques are described, emphasis is placed on the analysis of nanomaterials using synchrotron radiation. Materials of primary interests are metal nanoparticles and semiconductor nanowires and naonribbons. Synchrotron techniques based on absorption spectroscopy such as X-ray absorption fine structures (XAFS), which includes X-ray absorption near edge structures (XANES) and extended X-ray absorption fine structures (EXFAS), and de-excitation spectroscopy, including X-ray excited optical luminescence (XEOL), time-resolved X-ray excited optical luminescence (TRXEOL) and X-ray emission spectroscopy (XES) are described. We show that the tunability, brightness, polarisation and time structure of synchrotron radiation are providing unprecedented capabilities for nanomaterials analysis. Synchrotron studies of prototype systems such as gold nanoparticles, 1-D nanowires of group IV materials, C, Si and Ge as well as nanodiamond, and compound semiconductors, ZnS, CdS, ZnO and related materials are used to illustrate the power and unique capabilities of synchrotron spectroscopy in the characterisation of local structure, electronic structure and optical properties of nanomaterials.

The C 1s and N 1s near edge x-ray absorption fine structure spectra of five azabenzenes in the gas phase.

Abstract: Near edge x-ray absorption fine structure spectra have been measured and interpreted by means of density functional theory for five different azabenzenes (pyridine, pyridazine, pyrimidine, pyrazine ...

X-ray absorption spectroscopy of hemes and hemeproteins in solution: multiple scattering analysis.

Abstract: A full quantitative analysis of Fe K-edge X-ray absorption spectra has been performed for hemes in two porphynato complexes, that is, iron(III) tetraphenylporphyrin chloride (Fe(III)TPPCl) and iron(III) tetraphenylporphyrin bis(imidazole) (Fe(III)TPP(Imid)2), in two protein complexes whose X-ray structure is known at atomic resolution (1.0 A), that is, ferrous deoxy-myoglobin (Fe(II)Mb) and ferric aquo-myoglobin (Fe(III)MbH2O), and in ferric cyano-myoglobin (Fe(III)MbCN), whose X-ray structure is known at lower resolution (1.4 A). The analysis has been performed via the multiple scattering approach, starting from a muffin tin approximation of the molecular potential. The Fe-heme structure has been obtained by analyzing independently the Extended X-ray Absorption Fine Structure (EXAFS) region and the X-ray Absorption Near Edge Structure (XANES) region. The EXAFS structural results are in full agreement with the crystallographic values of the models, with an accuracy of +/- 0.02 A for Fe-ligand distances, and +/-6 degrees for angular parameters. All the XANES features above the theoretical zero energy (in the lower rising edge) are well accounted for by single-channel calculations, for both Fe(II) and Fe(III) hemes, and the Fe-N p distance is determined with the same accuracy as EXAFS. XANES evaluations of Fe-5th and Fe-6th ligand distances are determined with 0.04-0.07 A accuracy; a small discrepancy with EXAFS (0.01 to 0.05 A beyond the statistical error), is found for protein compounds. Concerns from statistical correlation among parameters and multiple minima in the parameter space are discussed. As expected, the XANES accuracy is slightly lower than what was found for polarized XANES on Fe(III)MbCN single crystal (0.03-0.04 A), and states the actual state-of-the-art of XANES analysis when used to extract heme-normal parameters in a solution spectrum dominated by heme-plane scattering.

X-ray absorption spectroscopic characterization of a cytochrome P450 compound II derivative

Abstract: The cytochrome P450 enzyme CYP119, its compound II derivative, and its nitrosyl complex were studied by iron K-edge x-ray absorption spectroscopy. The compound II derivative was prepared by reaction of the resting enzyme with peroxynitrite and had a lifetime of ≈10 s at 23°C. The CYP119 nitrosyl complex was prepared by reaction of the enzyme with nitrogen monoxide gas or with a nitrosyl donor and was stable at 23°C for hours. Samples of CYP119 and its derivatives were studied by x-ray absorption spectroscopy at temperatures below 140 (K) at the Advanced Photon Source of Argonne National Laboratory. The x-ray absorption near-edge structure spectra displayed shifts in edge and pre-edge energies consistent with increasing effective positive charge on iron in the series native CYP119 < CYP119 nitrosyl complex < CYP119 compound II derivative. Extended x-ray absorption fine structure spectra were simulated with good fits for k = 12 Å−1 for native CYP119 and k = 13 Å−1 for both the nitrosyl complex and the compound II derivative. The important structural features for the compound II derivative were an iron-oxygen bond length of 1.82 Å and an iron-sulfur bond length of 2.24 Å, both of which indicate an iron-oxygen single bond in a ferryl-hydroxide, FeIVOH, moiety.

Systematic Oxidation of Polystyrene by Ultraviolet-Ozone, Characterized by Near-Edge X-ray Absorption Fine Structure and Contact Angle

Abstract: The process of implanting oxygen in polystyrene (PS) via exposure to ultraviolet-ozone (UV-O) was systematically investigated using the characterization technique of near-edge X-ray absorption fine structure (NEXAFS). Samples of PS exposed to UV-O for 10-300 s and washed with isopropanol were analyzed using the carbon and oxygen K-edge NEXAFS partial electron yields, using various retarding bias voltages to depth-profile the oxygen penetration into the surface. Evaluation of reference polymers provided a scale to quantify the oxygen concentration implanted by UV-O treatment. We find that ozone initially reacts with the double bonds on the phenyl rings, forming carbonyl groups, but within 1 min of exposure, the ratio of double to single oxygen bonds stabilizes at a lower value. Oxygen penetrates the film with relative ease, creating a fairly uniform distribution of oxygen within at least the first 4 nm (the effective depth probed by NEXAFS here). Before oxygen accumulates in large concentrations, however, it preferentially degrades the uppermost layer of the film by removing oxygenated low-molecular-weight oligomers. The failure to accumulate high concentrations of oxygen is seen in the nearly constant carbon edge jump, the low concentration of oxygen even at 5 min exposure (58% of that in poly(4-acetoxystyrene), the polymer with the most similarities to UV-O-treated PS), and the relatively high contact angles. At 5 min exposure the oxygen concentration contains ca. 7 atomic % oxygen. The oxygen species that are implanted consist predominantly of single O-C bonds and double O=C bonds but also include a small fraction of O-H. UV-O treatment leads a plateau after 2 min exposure in the water contact angle hysteresis, at a value of 67 +/- 2 degrees , due primarily to chemical heterogeneity. Annealing above T(g) allows oxygenated species to move short distances away from the surface but not diffuse further than 1-2 nm.

Photosynthetic water oxidation at elevated dioxygen partial pressure monitored by time-resolved X-ray absorption measurements

Abstract: The atmospheric dioxygen (O2) is produced at a tetramanganese complex bound to the proteins of photosystem II (PSII). To investigate product inhibition at elevated oxygen partial pressure (pO2 ranging from 0.2 to 16 bar), we monitored specifically the redox reactions of the Mn complex in its catalytic S-state cycle by rapid-scan and time-resolved X-ray absorption near-edge spectroscopy (XANES) at the Mn K-edge. By using a pressure cell for X-ray measurements after laser-flash excitation of PSII particles, we found a clear pO2 influence on the redox reactions of the Mn complex, with a similar half-effect pressure as determined (2–3 bar). However, XANES spectra and the time courses of the X-ray fluorescence collected with microsecond resolution suggested that the O2 evolution transition itself (S3⇒S0+O2) was not affected. Additional (nonstandard) oxidation of the Mn complex at high pO2 explains our experimental findings more readily. Our results suggest that photosynthesis at ambient conditions is not limited by product inhibition of the O2 formation step.