Spectator and participator processes in the resonant photon-in and photon-out spectra at the Ce L3 edge of CeO2

We study both theoretically and experimentally the photon-in and photon-out spectra of CeO₂, which are caused by the Ce 2p to Ce 5d excitation followed by the three different de-excitation channels: (i) Ce 3d to Ce 2p (denoted by 3d-RXES), (ii) O 2p to Ce 2p (v-RXES), and (iii) Ce 5d to Ce 2p (RIXS). In 3d- and v-RXES, the 5d electron plays a role of a spectator, but in RIXS it is a participator. By extending our single impurity Anderson model (SIAM), which was used recently for our calculations of v- and 3d-RXES spectra of CeO₂, we study the polarization dependence in the spectator and participator spectra, and we perform more detailed calculations for 3d- and v-RXES spectral features, as well as new calculations for the RIXS spectrum with charge transfer excitations. The polarization dependence is different for the spectator and participator spectra; we have no polarization correlation between the incident and emitted photons for the spectator spectra but a strong polarization correlation for the participator spectrum. The theoretical calculations predict that the charge transfer excitations in RIXS occur in the transfer-energy range overlapped with v-RXES, but the RIXS and v-RXES spectra can be discriminated by taking advantage of the different polarization dependence. The overlapped RIXS and v-RXES spectra are observed successfully by our experiments and well reproduced by our SIAM calculations.     

High‐energy‐resolution diced spherical quartz analyzers for resonant inelastic X‐ray scattering

A novel diced spherical quartz analyzer for use in resonant inelastic X‐ray scattering (RIXS) is introduced, achieving an unprecedented energy resolution of 10.53 meV at the Ir L₃ absorption edge (11.215 keV). In this work the fabrication process and the characterization of the analyzer are presented, and an example of a RIXS spectrum of magnetic excitations in a Sr₃Ir₂O₇ sample is shown.     

Electronic excitations in hole-doped La1−xSrxMnO3 studied by resonant inelastic X-ray scattering

We report a resonant inelastic X-ray scattering (RIXS) study on perovskite manganese oxides La_1−xSr_xMnO₃ (x=0, 0.2, and 0.4) at Mn K-absorption edge. Hole-doping effect on the electronic excitations in the strongly correlated electron systems is elucidated by comparing with undoped LaMnO₃. The scattering spectra of metallic La_0.6Sr_0.4MnO₃ show that a salient peak appears in low energies indicating the persistence of the Mott gap. At the same time, the energy gap is partly filled by doping holes and the spectral weight shifts toward lower energies. Though the peak position of the excitations shows weak dispersion in momentum dependence, RIXS intensity changes as a function of the scattering angle (2θ), which is related to the anisotropy. Furthermore, anisotropic temperature dependence is observed in La_0.8Sr_0.2MnO₃ which shows a metal-insulator transition associated with a ferromagnetic transition. We consider that the anisotropy in the RIXS spectra is possibly attributed to the correlation of the orbital degrees of freedom. The anisotropy is large in LaMnO₃ with long-range orbital order, while it is small but finite in hole-doped La_1−xSr_xMnO₃ which indicates persistence of short-range orbital correlation.     

Temperature dependence of the electronic structure of Sr14Cu24O41 studied by resonant inelastic X-ray scattering

We report a resonant inelastic X-ray scattering (RIXS) study of charge excitations in the two-leg ladder Sr₁₄Cu₂₄O₄₁. RIXS spectra at 1–5 eV are found to be dependent on temperature. An intraband excitation of the ladder, which appears as a continuum intensity below the Mott gap, decreases in intensity with temperature. Because the intraband excitation is related to the dynamics of doped holes in the ladder, its decrease of the intraband excitation is attributed to the reduction of the mobile holes in the ladder at low temperature.     

Momentum‐resolved resonant inelastic X‐ray scattering on a single crystal under high pressure

A single‐crystal momentum‐resolved resonant inelastic X‐ray scattering (RIXS) experiment under high pressure using an originally designed diamond anvil cell (DAC) is reported. The diamond‐in/diamond‐out geometry was adopted with both the incident and scattered beams passing through a 1 mm‐thick diamond. This enabled us to cover wide momentum space keeping the scattering angle condition near 90°. Elastic and inelastic scattering from the diamond was drastically reduced using a pinhole placed after the DAC. Measurement of the momentum‐resolved RIXS spectra of Sr_2.5Ca_11.5Cu₂₄O₄₁ at the Cu K‐edge was thus successful. Though the inelastic intensity becomes weaker by two orders than the ambient pressure, RIXS spectra both at the center and the edge of the Brillouin zone were obtained at 3 GPa and low‐energy electronic excitations of the cuprate were found to change with pressure.     

Resonant inelastic x‐ray scattering from highly correlated dysprosium compounds

Dy₂O₃ and Dy metal's resonant inelastic x‐ray scattering (RIXS) spectra were measured in the Beijing Synchrotron Radiation Facility. As a bulk sensitive probe and two‐photon process, RIXS provides more information on the electronic structure of matter. In this full RIXS experiment, the 2p⁶4fⁿ→ 2p⁵4fⁿ5d¹ (2p⁵4f^{n + 1}5d⁰) → 2p⁶3d⁹4fⁿ5d¹ (2p⁶3d⁹4f^{n + 1}5d⁰) channel of two samples (Dy₂O₃ and Dy metal) was studied. Further comparison shows that there are many differences in the RIXS spectra. Dy metal has only a single resonance and its 5d band is broader than that of Dy₂O₃. In the resonant regime, it has a lower final state energy, whereas in the non‐resonant regime it exceeds Dy₂O₃. This causes a broader bandwidth of the main final state B and a narrower bandwidth in the resonant and non‐resonant regime. The pre‐edge structure in Dy L₃ absorption spectra was also resolved using RIXS, which cannot be seen in conventional XAS owing to 2p core hole lifetime broadening. Copyright © 2005 John Wiley & Sons, Ltd.     

Synthesis and characterization of hydroxyapatite/titania nanocomposites using in situ precipitation technique

Hydroxyapatite/titania nanocomposites were successfully synthesized by in situ precipitation of precursor matters from hydroxyapatite and titania at 70 °C with different hydroxyapatite/titania ratios. X-ray diffraction, Fourier transform infrared spectroscopy, Brunauer–Emmett–Teller surface, scanning and transmission electron microscopes were employed to characterize the prepared nanocomposite powders. X-ray diffraction results indicated that hydroxyapatite and anatase (TiO₂) were the major crystalline phases. By increasing the amount of titania nano-particles, Fourier transform infrared spectroscopy revealed that (PO₄)³⁻ bands at 567, 1033 cm⁻¹ decreased. Brunauer–Emmett–Teller surface results also showed a reduction in surface areas of nanocomposites. Transmission electron microscope observations revealed that the aspect ratio of hydroxyapatite/TiO₂ nanocrystals increased by increasing TiO₂ proportion in nanocomposites. The observed nanorod crystals tended to thin, elongated and plate-like in shape.     

Rotation of the plane of polarization and linear birefringence of sound in hematite below the morin point

The linear antiferromagnetic birefringence of sound in hematite (α-Fe₂O₃) residing in the collinear easy-axis phase (L ‖ C ₃) below the Morin point is experimentally studied. The plane of polarization of a linearly polarized transverse acoustic wave propagating along the trigonal axis C ₃ of a hematite crystal placed in a magnetic field H applied in the basal plane (H ⊥ C ₃, 3.5 ≤ H ≤ 15 kOe) is found to rotate after a temperature-driven orientational phase transition to the easy-axis state. The angle of rotation exhibits a 180° angular dependence on the direction of the magnetic field in the basal plane and varies from zero to ～π/2. Numerical estimates suggest that the conditions necessary for rotation of the plane of polarization through appreciable angles (～π/2) can be satisfied in the easy-axis phase at orientational phase transition temperatures close to the Morin point, which actually takes place in the fields employed. The results obtained are described sufficiently well by the theory of linear antiferromagnetic birefringence of sound (E.A. Turov) and confirm its main conclusions.     

Synthesis and characterization of hollow ��-Fe2O3 sub-micron spheres prepared by sol�Cgel

In this work we report the preparation of magnetic hematite hollow sub-micron spheres (??-Fe₂O₃) by colloidal suspensions of ferric nitrate nine-hydrate (Fe(NO₃)₃·9H₂O) particles in citric acid solution by following the sol?Cgel method. After the gel formation, the samples were annealed at different temperatures in an oxidizing atmosphere. Annealing at 180°C resulted in an amorphous phase, without iron oxide formation. Annealing at 250°C resulted in coexisting phases of hematite, maghemite and magnetite, whereas at 400°C, only hematite and maghemite were found. Pure hematite hollow sub-micron spheres with porous shells were formed after annealing at 600°C. The characterization was performed by X-ray diffraction (XRD), Mössbauer spectroscopy (MS) and scanning electron microscopy (SEM).     

Magnetic-field-induced coupling between one-magnon and two-magnon states in TMMC

Magnetic excitations of a typical one-dimensional Heisenberg antiferromagnet (CD₃)₄NMnCl₃ (TMMC) are studied theoretically for a finite field applied to the easy plane. The importance of the magnetic-field-induced coupling between one-magnon and 2-magnon states is pointed out in connection with the recent neutron experiments by Heilmann et al..     

The sub-bandgap energy loss satellites in the RIXS spectra of beryllium compounds

Resonant X-ray inelastic scattering spectra have been measured in BeO, phenakite (Be₂SiO₄) and chrysoberyl (BeAl₂O₄) with the excitation energy near the beryllium K edge.The RIXS spectra excited in the vicinity of the Be 1s core resonance show two principal features: the scattering on a valence excitation (which at higher excitation energies verges into the characteristic K_α emission), and a remarkably strong energy loss sideband to the elastic scattering peak. The energy loss shoulder appears to result from lattice relaxation in the absorption site. The comparison of the RIXS spectra of phenakite, chrysoberyl and BeO shows that the strength of the low energy sideband differs greatly; it is strongest in BeO and weakest in phenakite. The Si 2p RIXS spectra of phenakite also display a similar strong sub-bandgap energy loss tail.To gain further insight to this process, transitions in a system with a single vibrational mode have been modelled. The phonon relaxation has been simulated empirically by “smearing” the photoabsortion-populated vibrational levels with lower levels. This simple model is able to qualitatively explain this wide energy loss shoulder.     

Crystal field and low energy excitations measured by high resolution RIXS at the L3 edge of Cu, Ni and Mn

Resonant inelastic x-ray scattering in the soft x-ray regime has been profiting much from technical advances that have lowered considerably the instru- mental linewidth. At the ADRESS beam line of the Swiss Light Source the SAXES spectrometer can be used to measure RIXS spectra at the L edges of the 3d transition metals with unprecedented energy resolution, of the order of 100 meV for Mn, Ni and Cu. We present here some preliminary spectra on CuO, malachite, NiO, NiCl₂, MnO and LaMnO₃. The dd excitations are very well resolved allowing accurate experimental evaluation of 3d state energy splitting. The low energy scale becomes accessible opening the way to the study of collective excitations in strongly correlated electron systems, like magnons and orbitons.     

Selective XANES spectroscopy from RIXS contour maps

Kβ resonant inelastic X-ray scattering (RIXS) spectra of Mn and Ga compounds are studied to examine different types of selective XANES. RIXS spectra of MnO in Mn Kβ_1,3 region and of GaCl₂ in Ga Kβ₂ region are plotted as contour maps over wide energy ranges in both excitation and emission. Through analyses of RIXS contour maps by the use of the Kramers-Heisenberg equation, spin-selective XANES spectra are deduced for MnO. In the case of a mixed valence compound GaCl₂ selective XANES of Ga³⁺ ion can be obtained by making use of a large difference in transition probabilities.     

Preparation of superhydrophobic films on titanium as effective corrosion barriers

Stable superhydrophobic films were prepared on the electrochemical oxidized titania/titanium substrate by a simple immersion technique into a methanol solution of hydrolyzed 1H,1H,2H,2H-perfluorooctyltriethoxysilane [CF₃(CF₂)₅(CH₂)₂Si(OCH₂CH₃)₃, PTES] for 1 h at room temperature followed by a short annealing at 140 °C in air for 1 h. The surface morphologies and chemical composition of the film were characterized by means of water contact angle (CA), field emission scanning electron microscopy (FESEM), atomic force microscope (AFM) and X-ray photoelectron spectroscopy (XPS). The water contact angle on the surface of this film was measured to be as high as 160°. SEM images showed that the resulting surfaces exhibited special hierarchical structure. The special hierarchical structure along with the low surface energy leads to the high surface superhydrophobicity. The corrosion resistance ability and durance property of the superhydrophobic film in 3.5 wt.% NaCl solution was evaluated by the electrochemical impedance spectroscopy (EIS). The anticorrosion properties of the superhydrophobic film are compared to those of unmodified pure titanium and titania/titanium substrates. The results showed that the superhydrophobic film provides an effective corrosion resistant coating for the titanium metal even with immersion periods up to 90 d in the 3.5 wt.% NaCl solution, pointing to promising future applications.     

Morin transition suppression in Polycrystalline 57Hematite (α-Fe2O3) exposed to 56Fe(II)

We used the isotope selectivity of ⁵⁷Fe Mössbauer spectroscopy to investigate changes in the magnetic properties of polycrystalline hematite exposed to ferrous iron (Fe(II)). We found that sorption of ⁵⁶Fe(II), followed by interfacial electron exchange, alters the bulk magnetic properties of ⁵⁷hematite. After reaction with ⁵⁶Fe(II), we observed partial suppression of the Morin transition of ⁵⁷hematite to below 13 K. This is significantly lower than the Morin temperature (T _M) of ～230 K measured for isotopically enriched polycrystalline ⁵⁷hematite, as well as the T _M of 264?±?2 K reported for normal polycrystalline hematite.     

Effect of cobalt doping and annealing on properties of titania thin films prepared by sol-gel process

Undoped and cobalt doped titania (TiO₂) thin films have been prepared on Si(1 0 0) monocrystal and quartz substrate using the sol-gel deposition method and annealed in air at 450, 550, 650, 750, 850, 950 and 1050 °C. Several experimental techniques (AFM, XRD, Raman spectroscopy, XRR, EDX, XPS, XAS, UV-VIS spectroscopy) have been used to characterize these films. Further more the degree of light induced hydrophilicity was estimated by measuring the contact angle of a water droplet on the film. Increase of the annealing temperature and in smaller degree also cobalt doping predispose titania crystallite growth. The rutile phase was detected at lower temperatures in the cobalt doped films than in the undoped titania films. Cobalt in the cobalt doped TiO₂ was seen to be in Co²⁺ oxidation state, mainly in CoTiO₃ phase when films were annealed at temperatures higher than 650 °C. Cobalt compounds segregated into the sub-surface region and to the surface of the titania, where they formed islands. Cobalt doping red-shifted the fundamental absorption edge further into the visible range, however it did not enhance the light induced hydrophilicity of the thin film surface as compared to the undoped titania thin films.     

Application of Mössbauer spectroscopy in phase analysis of corrosion products from nuclear power plants

Magnetic phases of corrosion products from nuclear power plant WWER-440 (Jaslovské Bohunice) steam generators and secondary circuit pipes were investigated using transmission Mössbauer spectroscopy technique. The main components of secondary circuit’s corrosion products are magnetite (Fe₃O₄) and hematite (α-Fe₂O₃) in different ratios. The results can be used for optimal control and maintenance of WWER-440 nuclear reactors as well as for optimisation of water chemistry regime.     

Boron and nitrogen co-doped titania with enhanced visible-light photocatalytic activity for hydrogen evolution

A visible-light boron and nitrogen co-doped titania (B-N-TiO₂) photocatalyst was prepared by sol-gel method with titanium tetra-n-butyl oxide, urea and boric acid as precursors. The photocatalyst was characterized by Fourier Transform Infrared (FT-IR), UV-vis diffusive reflectance spectroscopy (DRS), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), BET and electrochemistry method. Photocatalytic activity for hydrogen production over platinized B-N-TiO₂ under visible-light (λ ≥ 420 nm) irradiation was investigated. In nitrogen doped titania (N-TiO₂) NTiO bond is formed, which extends the absorption edge to the visible-light region. A part of doping boron enters into titania lattice and most of the boron exists at the surface of the catalyst. The crystallite size of B-N-TiO₂ decreases compared to N-TiO₂, while its photocurrent and the surface hydroxyl group increase. Furthermore, doping boron could act as shallow traps for photoinduced electrons to prolong the life of the electrons and holes. Therefore, the visible-light activity of B-N-iO₂ increases greatly compared with that of N-TiO₂.     

Synthesis and Characterization of a Novel Ternary Hematite Nanocomposites Structure with Fullerene and 2D‐Electrochemical Reduced Graphene Oxide for Superior Photoelectrochemical Performance

In this study, a novel ternary hematite nanocomposites photoanode structure with superior photoelectrochemical (PEC) performance consisting of fullerene (C₆₀) and 2D‐electrochemical reduced graphene oxide (eRGO) used as the effective surface passivators is developed. The introduction of both the electron scavenging C₆₀ and highly conducting eRGO has mitigated the high interfacial recombination rate of hematite and led to the superior enhancement in PEC performance. UV–vis analysis reveals that the incorporation of C₆₀ and eRGO can provide a stronger light absorption at the visible light (400 nm < λ < 700 nm) and near infrared (IR) region (λ > 700 nm). Through the electrochemical impedance spectroscopy measurements, it can be concluded that the introduction of C₆₀ and eRGO onto hematite photoanode improves electron transfer and collection, reduces charge‐carrier recombination efficiency, and enhances PEC activity. The resultant ternary hematite photoanode structure exhibits 16.8‐fold enhancement in photocurrent density and 0.8‐fold reduction in charge transfer resistance when compared to the bare hematite structure only. This study has shown that the application of C₆₀, 2D‐eRGO, or in combination as a ternary structure provides the plasmonic effect that can enhance the PEC performance in hematite photoanode structure.