De-Tong Jiang

XAFS at the Pacific Northwest Consortium-Collaborative Access Team undulator beamline

The Pacific Northwest Consortium-Collaborative Access Team (PNC-CAT) has begun operating an insertion device beamline at the Advanced Photon Source. The beamline has been extensively used for XAFS studies. This paper summarizes its capabilities, and our initial operational experience. The beamline is based on APS undulator A, and incorporates full undulator scanning. The monochromator is liquid nitrogen cooled and has both Si(111) and Si(311) crystals in a side-by-side configuration. Crystal changes only take a few minutes. The crystals cover the energy range from 3-50 keV with fluxes as high as 2x10(13) ph/sec. Microbeams can be produced using Kirkpatrick-Baez mirrors (spot size 1-3 microm) or tapered capillaries (sub-microm spots). When these optics are combined with a 13-element Ge detector, the beamline provides powerful microbeam imaging and spectroscopy capabilities. Experimental examples from the environmental field and in-situ UHV film growth will be discussed.

Spectroscopic Evidence of Uranium Immobilization in Acidic Wetlands by Natural Organic Matter and Plant Roots

Biogeochemistry of uranium in wetlands plays important roles in U immobilization in storage ponds of U mining and processing facilities but has not been well understood. The objective of this work was to study molecular mechanisms responsible for high U retention by Savannah River Site (SRS) wetland sediments under varying redox and acidic (pH = 2.6-5.8) conditions using U L3-edge X-ray absorption spectroscopy. Uranium in the SRS wetland sediments existed primarily as U(VI) bonded as a bidentate to carboxylic sites (U-C bond distance at ∼2.88 Å), rather than phenolic or other sites of natural organic matter (NOM). In microcosms simulating the SRS wetland processes, U immobilization on roots was 2 orders of magnitude higher than on the adjacent brown or more distant white sands in which U was U(VI). Uranium on the roots were both U(IV) and U(VI), which were bonded as a bidentate to carbon, but the U(VI) may also form a U phosphate mineral. After 140 days of air exposure, all U(IV) was reoxidized to U(VI) but remained as a bidentate bonding to carbon. This study demonstrated NOM and plant roots can highly immobilize U(VI) in the SRS acidic sediments, which has significant implication for the long-term stewardship of U-contaminated wetlands.

Structure of tetracene films on hydrogen-passivated Si(001) studied via STM, AFM, and NEXAFS

Scanning tunneling microscopy STM, atomic force microscopy AFM, and near-edge x-ray absorption fine structure NEXAFS have been used to study the structure of tetracene films on hydrogen-passivated Si001. STM imaging of the films with nominal thickness of three monolayers 3M L exhibits the characteristic “herringbone” molecular packing known from the bulk crystalline tetracene, showing standing molecules on the ab plane. The dimensions and orientation of the herringbone lattice indicate a commensurate structural relationship between the lattice and the crystalline substrate. The corresponding AFM images illustrate that at and above the third layer of the films, the islands are anisotropic, in contrast with the submonolayer fractals, with two preferred growth directions appearing orthogonal to each other. The polarization dependent NEXAFS measurements indicate that the average molecular tilting angle with respect to the surface first increases with the film thickness up to 3 ML, then stabilizes at a value close to the bulk tetracene case afterwards. The combined results indicate a distinct growth morphological change that occurs around a few monolayers of thickness.

Multiple Scattering Debye-Waller Factors for Arsenate

Debye-Waller factors for the As-O-O triangular multiple scattering paths within the arsenate in Na2HAsO4·7H2O are evaluated in terms of magnitude ratio with respect to the Debye-Waller factor of the nearest neighbour As-O shell (σ2As−0−0/σ2As-0). The arsenates are studied under two different levels of distortion from an ideal tetrahedron, i.e. a relatively high distortion in the powder form and a nearly ideal tetrahedron in an aqueous solution at pH 14. The Debye-Waller factor ratio σ2As−0−0/σ2As-0) is found to be 2.0 and 1.9 for the powder and liquid sample, respectively, appearing to be insensitive to the distortion of the arsenate tetrahedron.

XAFS Study of Arsenical Nickel Hydroxide

To Investigate the role played by nickel co-ions in contributing to the stability of arsenic, fluorescence XAFS measurements at both arsenic K-edge and nickel K-edge, respectively, on amorphous arsenical nickel hydroxide, crystalline arsenical nickel hydroxide, and annabergite reference compounds have been carried out. The XAFS results indicate that the arsenic-bearing nickel hydroxides have a well-defined arsenic local structure with multiple coordination shells, suggesting a compound formation mechanism instead of surface adsorption. The degradation of the arsenic local structure in the crystalline arsenical nickel hydroxide is observed. The XAFS of annabergites are compared to that of the arsenical nickel hydroxide and possible structural models are discussed.

In situ XANES & XRD Study of interphasial reaction between uncharged Li2FeSiO4 cathode and LiPF6-based electrolyte

In situ synchrotron radiation XANES and XRD have been carried out on Li2FeSiO4 cathode material in a lithium-ion-battery (LIB) cell. The evolution of the long range lattice structure and the local iron oxidation state has been observed at a charging rate of C/20 for the formation cycle for one Lithium extraction; additional ex situ measurements of the pristine cathode material were taken for comparison. The observed spontaneous interaction between the cathode and the fluorinated electrolyte and the impact of subsequent cycling are discussed.

Structural phase transition in CaH{sub 2} at high pressures

The structural and vibrational properties of CaH2 have been examined up to 30GPa at room temperature. Under ambient conditions, CaH2 has a Pnma (cotunnite-type) structure. A structural phase transformation was observed around 15GPa and completed at 20GPa . The high pressure structure is identified as hexagonal P63/mmc . First-principles calculations reproduced the first-order nature of the transition. Since P63/mmc is a supergroup of Pnma the structural change can be traced back to gradual displacements of the hydrogen atoms from the 4c positions in the cotunnite structure to the special 2a and 2d positions in the hexagonal structure. The observed phase transition pressure is much lower than that predicted for MgH2 .

Structure and Morphology of Tetracene Thin Films on Hydrogen-Terminated Si(001)

Scanning tunneling microscopy (STM), atomic force microscopy (AFM) and near-edge x-ray absorption fine structure (NEXAFS) have been used to study the structure of tetracene films on hydrogen-passivated Si(001). A distinct growth morphology change that occurs around a few monolayers of film thickness was characterized. This coverage-dependent film structural phase transition leads to a molecularly ordered film structure commensurate with the crystalline substrate.