XANES reflects coordination change and underlying surface disorder of zinc adsorbed to silica

Zinc K-edge X-ray absorption near-edge structure (XANES) spectroscopy of Zn adsorbed to silica and Zn-bearing minerals, salts and solutions was conducted to explore how XANES spectra reflect coordination environment and disorder in the surface to which a metal ion is sorbed. Specifically, XANES spectra for five distinct Zn adsorption complexes (Znads) on quartz and amorphous silica [SiO2(am)] are presented from the Zn–water–silica surface system: outer-sphere octahedral Znads on quartz, inner-sphere octahedral Znads on quartz, inner-sphere tetrahedral Znads on quartz, inner-sphere octahedral Znads on SiO2(am) and inner-sphere tetrahedral Znads on SiO2(am). XANES spectral analysis of these complexes on quartz versus SiO2(am) reveals that normalized peak absorbance and K-edge energy position generally decrease with increasing surface disorder and decreasing Zn–O coordination. On quartz, the absorption-edge energy of Znads ranges from 9663.0 to 9664.1 eV for samples dominated by tetrahedrally versus octahedrally coordinated species, respectively. On SiO2(am), the absorption-edge energy of Znads ranges from 9662.3 to 9663.4 eV for samples dominated by tetrahedrally versus octahedrally coordinated species, respectively. On both silica substrates, octahedral Znads presents a single K-edge peak feature, whereas tetrahedral Znads presents two absorbance features. The energy space between the two absorbance peak features of the XANES K-edge of tetrahedral Znads is 2.4 eV for Zn on quartz and 3.2 eV for Zn on SiO2(am). Linear combination fitting of samples with a mixture of Znads complex types demonstrates that the XANES spectra of octahedral and tetrahedral Znads on silica are distinct enough for quantitative identification. These results suggest caution when deciphering Zn speciation in natural samples via linear combination approaches using a single Znads standard to represent sorption on a particular mineral surface. Correlation between XANES spectral features and prior extended X-ray absorption fine structure (EXAFS) derived coordination environments for these Znads on silica samples provides insight into Zn speciation in natural systems with XANES compatible Zn concentrations too low for EXAFS analysis.

XAFS analysis of Arsenic bound in holocellulose extracted from organic-rich contaminated sediments

The arsenic bound in holocellulose, a precursor of humic substances extracted from organic contaminated sediments, was investigated using XANES (x-ray adsorption near-edge structure) and EXAFS (extended x-ray absorption fine structure) with fluorescence mode. The most abundant arsenic bound in holocellulose was As-O in the first coordination sphere. Sulphur and carbon were also found in a neighbouring coordination shell around arsenic. The arsenic oxidation state was judged to be As (III) by As K edge XANES spectra as a shift to higher absorption edge energy with the increasing formal oxidation state. This arsenic speciation and bounding were well matched with biochemical mechanisms of arsenic absorption into plants.

An unconventional method for measuring the TcL3-edge of technetium compounds

TcL3-edge XANES spectra have been collected on powder samples of SrTcO3(octahedral Tc4+) and NH4TcO4(tetrahedral Tc7+) immobilized in an epoxy resin. Features in the TcL3-edge XANES spectra are compared with the pre-edge feature of the TcK-edge as well as other 4dtransition metalL3-edges. Evidence of crystal field splitting is obvious in the TcL3-edge, which is sensitive to the coordination number and oxidation state of the Tc cation. The TcL3absorption edge energy difference between SrTcO3(Tc4+) and NH4TcO4(Tc7+) shows that the energy shift at the TcL3-edge is an effective tool for studying changes in the oxidation states of technetium compounds. The TcL3-edge spectra are compared with those obtained from Mo and Ru oxide standards with various oxidation states and coordination environments. Most importantly, fitting the TcL3-edge to component peaks can provide direct evidence of crystal field splitting that cannot be obtained from the TcK-edge.

X-ray absorption near-edge structure anomalous behaviour in structures with buried layers containing silicon nanocrystals

Substructure and phase composition of silicon suboxide films containing silicon nanocrystals and implanted with carbon have been investigated by means of the X-ray absorption near-edge structure technique with the use of synchrotron radiation. It is shown that formation of silicon nanocrystals in the films' depth (more than 60 nm) and their following transformation into silicon carbide nanocrystals leads to abnormal behaviour of the X-ray absorption spectra in the elementary silicon absorption-edge energy region (100–104 eV) or in the silicon oxide absorption-edge energy region (104–110 eV). This abnormal behaviour is connected to X-ray elastic backscattering on silicon or silicon carbide nanocrystals located in the silicon oxide films depth.

Optical Properties of Amorphous and Nanostructure Si/SiO2 Quantum Wells

Amorphous Si/SiO2quantum wells have been obtained at room temperature with atomic precision using magnetron sputtering. The Si/SiO2layer structure induces the higher optical transmittance at the visible wavelength region with increasing layer numbers. The tentative absorption coefficients are evaluated for integrated Si thicknesses. The absorption edge energy dependency on Si layer thickness E0= 1.61 + 0.75d-2is in accordance with effective mass theory for thicknesses 0.5 < d < 6nm. Quantum confinement effects of the Si/SiO2nanostructure layer are confirmed from optical transmittance and reflectance spectra.

MICROWAVE-ASSISTED HYDROTHERMAL SYNTHESIS, OPTICAL AND ELECTROCHEMICAL PROPERTIES OF AgBi(MoO4)2 NANOSPHERES

In this paper, we report the successful synthesis of novel AgBi(MoO4)2 nanospheres via a simple, rapid and reliable microwave-assisted hydrothermal approach, employing AgNO3 , Bi(NO3)3 ⋅ 5H2O and Na2MoO4 ⋅ 2H2O as starting materials. The phases and morphologies of the products were characterized by powder X-ray diffraction (XRD), energy dispersive spectrometry (EDS), X-ray photoelectron spectrum (XPS), selected area electron diffraction (SAED), transmission electron microscopy (TEM), and scanning electron microscopy (SEM). Optical property of the product gives the value of optical absorption edge energy to be 2.88 eV. Moreover, experimental result showed that obvious increase of the reduction peak was observed employing AgBi(MoO4)2 /GCE as working electrode in the presence of H2O2 molecule, which indicated that the as-prepared AgBi(MoO4)2 nanospheres exhibited excellent electrocatalytic activity to H2O2 .