17 research outputs found
Relative Reactivity of Biogenic and Chemogenic Uraninite and Biogenic Non-Crystalline U(IV)
Aqueous chemical extractions and X-ray absorption spectroscopy (XAS) analyses were conducted to investigate the reactivity of chemogenic uraninite, nanoparticulate biogenic uraninite, and biogenic monomeric U(IV) species. The analyses were conducted in systems containing a total U concentration that ranged from 1.48 to 2.10 mM. Less than 0.02% of the total U was released to solution in extractions that targeted water-soluble and ion exchangeable fractions. Less than 5% of the total U was solubilized via complexation with a 0.1 M solution of NaF. Greater than 90% of the total U was extracted from biogenic uraninite and monomeric U(IV) after 6 h of reaction in an oxidizing solution of 50 mM K2S2O8. Additional oxidation experiments with lower concentrations (2 mM and 10 mM) of K2S2O8 and 8.2 L-1 dissolved oxygen suggested that monomeric U(IV) species are more labile than biogenic uraninite; chemogenic uraninite was much less susceptible to oxidation than either form of biogenic U(IV). These results suggest that noncrystalline forms of U(IV) may be more labile than uraninite in subsurface environments. This work helps fill critical gaps in our understanding of the behavior of solid-associated U(IV) species in bioremediated sites and natural uranium ore deposits
Relative Reactivity of Biogenic and Chemogenic Uraninite and Biogenic Noncrystalline U(IV)
Inverse Temperature Dependence of Charge Carrier Hopping in Quantum Dot Solids
In
semiconductors, increasing mobility with decreasing temperature
is a signature of charge carrier transport through delocalized bands.
Here, we show that this behavior can also occur in nanocrystal solids
due to temperature-dependent structural transformations. Using a combination
of broadband infrared transient absorption spectroscopy and numerical
modeling, we investigate the temperature-dependent charge transport
properties of well-ordered PbS quantum dot (QD) solids. Contrary to
expectations, we observe that the QD-to-QD charge tunneling rate increases
with decreasing temperature, while simultaneously exhibiting thermally
activated nearest-neighbor hopping behavior. Using synchrotron grazing-incidence
small-angle X-ray scattering, we show that this trend is driven by
a temperature-dependent reduction in nearest-neighbor separation that
is quantitatively consistent with the measured tunneling rate