Martin E. McBriarty

Structural consequences of hydrogen intercalation of epitaxial graphene on SiC(0001)

The intercalation of various atomic species, such as hydrogen, to the interface between epitaxial graphene (EG) and its SiC substrate is known to significantly influence the electronic properties of the graphene overlayers. Here, we use high-resolution X-ray reflectivity to investigate the structural consequences of the hydrogen intercalation process used in the formation of quasi-free-standing (QFS) EG/SiC(0001). We confirm that the interfacial layer is converted to a layer structurally indistinguishable from that of the overlying graphene layers. This newly formed graphene layer becomes decoupled from the SiC substrate and, along with the other graphene layers within the film, is vertically displaced by ∼2.1 A. The number of total carbon layers is conserved during the process, and we observe no other structural changes such as interlayer intercalation or expansion of the graphene d-spacing. These results clarify the under-determined structure of hydrogen intercalated QFS-EG/SiC(0001) and provide a precise model to inform further fundamental and practical understanding of the system.

Redox-driven atomic-scale changes in mixed catalysts: VOX/WOX/α-TiO2 (110)

X-ray studies of vanadium–tungsten mixed-monolayer-oxide catalysts grown on the rutile α-TiO2 (110) single crystal surface show redox behavior not observed for lone supported vanadium or tungsten oxides. Two cases are presented: sub-monolayer (sub-ML) vanadium oxide (vanadia) grown on ML tungsten oxide and ML vanadia grown on sub-ML tungsten oxide. The X-ray standing wave (XSW) and X-ray photoelectron spectroscopy (XPS) observations for both cases show coverage-dependent reversible redox-induced atomic-scale structural and chemical state changes. Atomic force microscopy shows that the mixed VOX/WOX overlayers have a conformal film-like structure in the as-deposited state. XSW analysis in light of XPS reveals that the V and W cations that are uncorrelated with the substrate lattice play an important role in catalytic redox reactions. Distinct differences in the redox-induced changes for these two mixed catalysts result from tuning the ratio of V to W, and relationships are drawn between the catalyst composition, structure, and chemistry. Comparison of these V–W mixed cases and the corresponding unmixed cases reveals a synergistic effect in which the reduction of W can be significantly enhanced by the addition of V.

Trace Uranium Partitioning in a Multiphase Nano-FeOOH System

The characterization of trace elements in minerals using extended X-ray absorption fine structure (EXAFS) spectroscopy constitutes a first step toward understanding how impurities and contaminants interact with the host phase and the environment. However, limitations to EXAFS interpretation complicate the analysis of trace concentrations of impurities that are distributed across multiple phases in a heterogeneous system. Ab initio molecular dynamics (AIMD)-informed EXAFS analysis was employed to investigate the immobilization of trace uranium associated with nanophase iron (oxyhydr)oxides, a model system for the geochemical sequestration of radiotoxic actinides. The reductive transformation of ferrihydrite [Fe(OH)3] to nanoparticulate iron oxyhydroxide minerals in the presence of uranyl (UO2)2+(aq) resulted in the preferential incorporation of U into goethite (α-FeOOH) over lepidocrocite (γ-FeOOH), even though reaction conditions favored the formation of excess lepidocrocite. This unexpected result is supported by atomically resolved transmission electron microscopy. We demonstrate how AIMD-informed EXAFS analysis lifts the strict statistical limitations and uncertainty of traditional shell-by-shell EXAFS fitting, enabling the detailed characterization of the local bonding environment, charge compensation mechanisms, and oxidation states of polyvalent impurities in complex multiphase mineral systems.