Tanel Käämbre

Photocatalytic activity of non-stoichiometric ZnFe2O4 under visible light irradiation

Nanostructured zinc ferrites with different excess iron contents (ZnFe2+zO4, where z = 0.00, 0.05, 0.10 and 0.15) have been synthesized using the sol–gel auto-combustion method. The effect of excess iron on the structural, optical and visible light photocatalytic activity of zinc ferrite samples has been investigated. X-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS), x-ray magnetic circular dichroism (XMCD), Brunauer–Emmett–Teller theory, scanning electron microscopy (SEM), diffuse reflectance spectroscopy (UV–Vis) and photoluminescence spectroscopy were used to characterize the synthesized non-stoichiometric ZnFe2O4 powders. The XRD patterns demonstrated that the samples consist of single phase spinel structure with crystallite sizes of ~25 nm. SEM analysis indicated that the nanosized particles grow together in porous clusters with a size of several microns. The XPS and XMCD analyses revealed that the excess iron ion substitutes Zn2+ in tetrahedral sites and in octahedral sites Fe2+ can be found in addition to Fe3+, which could be created to restore the overall charge balance in the crystal lattice. Stoichiometric zinc ferrite (ZnFe2+zO4, z = 0.00) exhibited higher photocatalytic activity (40%) than the excess iron sample (ZnFe2+zO4, z = 0.15, i.e. 4%) under visible light irradiation for 3 h. This was explained by the formation of Fe2+ in the octahedral sites of excess iron ZnFe2O4; these species act as recombination centres.

Magnetic circular dichroism in X-ray fluorescence of Heusler alloys at threshold excitation

The results of fluorescence measurements of magnetic circular dichroism (MCD) in Mn L2,3 X-ray emission and absorption for Heusler alloys NiMnSb and Co2MnSb are presented. Very intense resonance Mn ...

A straightforward and “green” solvothermal synthesis of Al doped zinc oxide plasmonic nanocrystals and piezoresistive elastomer nanocomposite

Plasmonic oxide nanocrystals hold great promise in a wide range of applications, for which the availability of scalable and “green” synthesis methods is prerequisite, whereas until recently an excellent response has been demonstrated only for samples prepared through intricate synthesis paths. We report here a simple ethanol solvothermal synthesis route of Al doped ZnO plasmonic nanocrystals (Zn1−xAlxO) at doping levels of x up to 0.15. The obtained Al doped ZnO samples consisted of nanoparticles and short nanorods with a diameter of around 10 nm at x = 0.15 doping level while reaching aspect ratio levels of 50 for lower doping levels. Detailed structural studies using powder X-ray diffraction Rietveld refinement, X-ray absorption and photoelectron spectroscopies show that all samples maintain the structure of the phase-pure zincite with the space group P63mc. The resulting powders exhibit strong infrared absorption while remaining largely transparent for visible light, enabling the preparation of transparent colloidal dispersions. Furthermore, as a test of applicability in a practical device, the nanocrystals were used to prepare transparent piezoresistive Zn0.925Al0.075O–polydimethylsiloxane composites. The prepared sensor material exhibits excellent repeatable and reproducible piezoresistive behaviour.

Non-destructive chemical analysis of sandwich structures by means of soft X-ray emission

Abstract Soft X-ray emission spectroscopy provides information about elemental composition, including the light elements, as well as the chemical bonding. The probe depth reaches hundreds of nanometers but under certain conditions considerable surface sensitivity can be attained. For electron impact excitation, the probe depth can be varied by variation of the electron energy, and by variation of the detection angle. The highest surface sensitivity is obtained when the emission angle is near the critical angle for total reflection. In this case the effective probe depth is of the order of a few wavelengths, typically around 3 run for 3d transition element L emission. In the present study we show the feasability of using the angular dependence as a means to study the thickness and composition of layered samples by comparing the experimental data to model calculations based on the X-ray optical constants of the constituent materials.

Ag sensitized TiO2 and NiFe2O4 three-component nanoheterostructures: synthesis, electronic structure and strongly enhanced visible light photocatalytic activity

This study reports on the synthesis and characterisation of two- and three-component visible light active photocatalytic nanoparticle heterostructures, based on TiO2 and NiFe2O4 and sensitized with Ag. We observe that a Ag content as small as 1 at% in the TiO2/NiFe2O4 heterostructure increases by more than an order of magnitude the rate constant for the visible light photocatalytic process. We rationalise this in terms of the measured structure and electronic structure data of the binary and ternary combinations of the component materials and focus on details, which show that an optimised deposition sequence is vital for attaining high values of photocatalytic efficiency, because the charge transfer across the interfaces appears to be sensitive to where the Ag is loaded in the heterostructure. The overall higher visible light photocatalytic activity of the TiO2/Ag/NiFe2O4 heterostructure was observed and is attributed to enhanced charge carrier separation efficiency and migration via vectorial electron transfer.

Density functional simulation of resonant inelastic X-ray scattering experiments in liquids: acetonitrile

In this paper we report an experimental and computational study of liquid acetonitrile (H3C-C[triple bond, length as m-dash]N) by resonant inelastic X-ray scattering (RIXS) at the N K-edge. The experimental spectra exhibit clear signatures of the electronic structure of the valence states at the N site and incident-beam-polarization dependence is observed as well. Moreover, we find fine structure in the quasielastic line that is assigned to finite scattering duration and nuclear relaxation. We present a simple and light-to-evaluate model for the RIXS maps and analyze the experimental data using this model combined with ab initio molecular dynamics simulations. In addition to polarization-dependence and scattering-duration effects, we pinpoint the effects of different types of chemical bonding to the RIXS spectrum and conclude that the H2C-C[double bond, length as m-dash]NH isomer, suggested in the literature, does not exist in detectable quantities. We study solution effects on the scattering spectra with simulations in liquid and in vacuum. The presented model for RIXS proved to be light enough to allow phase-space-sampling and still accurate enough for identification of transition lines in physical chemistry research by RIXS.

Resonant Mn L emission spectra of layered manganite La1.2Sr1.8Mn2O7

Resonant Mn L α,β soft X-ray emission spectra of a colossal magnetoresistive manganese oxide La 1.2 Sr 1.8 Mn 2 O 7 have been measured at the Mn L 2,3 -edges excited by monochromatized undulator radiation. The excitation energy dependence of the spectra has been analyzed in terms of d – d and charge-transfer excitations. The experimental spectra are reproduced by the multiplet calculation for the Mn 3+ configuration, which is consistent with the dominant occupancy of the 3 d x 2 - y 2 orbital and probably a phase separation.

Resonant photoemission of CoCl2

Abstract We present the results of a study of the resonant photoemission from polycrystalline in situ evaporated thin film of CoCl2 excited at 2p photoabsorption edges of Co and Cl. At the Co 2p→3d resonance strong enhancement was observed for the Co 3p, valence band photoemission and for spectral regions of L23M23M23, L23M23M45 and L23M45M45 Auger transitions. The results of valence band resonant photoemission is discussed in terms of the available calculations. The valence band resonant spectra exhibit no enhancement in connection to the Cl 2p edge which shows that the Cl 2p absorption pre-edge structure is formed by d- and s-like final delocalized states.

Substrate-induced effects in the creation and decay of potassium 2p core excitations in ultrathin films of KCl on copper

Substrate-induced effects on the transport properties in thin KCl films on the Cu(100) surface have been studied using K + 2p photoelectron, photoabsorption and resonant Auger spectra. The measurements were performed at different KCl coverages ranging from a partial monolayer to a thick bulk-like film. The morphology and layer thickness were estimated from an analysis of the electron energy loss structure of the K + 2p and Cl − 2p photoelectron peaks, and from the variations in the photoelectron peak fine structure. The resonant Auger spectator decay spectra measured at the photoabsorption resonances show that the significant differences between the spectra of the solid and of thin layers are related to the charge delocalization from the K + 3d excited state into the metal substrate. The core-hole-clock approach yields an estimate of 1.5 fs for the corresponding charge transfer time at a single monolayer KCl coverage. (Some figures in this article are in colour only in the electronic version)

Insulating surface layer on single crystal K3C60

Abstract.Using angle-dependent photoemission spectra of core and valence levels we show that metallic, single crystal K3C60 is terminated by an insulating or weakly-conducting surface layer. We attribute this to the effects of strong intermolecular correlations combined with the average surface charge state. Several controversies on the electronic structure are thereby resolved.