Oak Ridge, TN, United States
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Chen X.,University of California at Berkeley | Choing S.N.,University of California at Berkeley | Aschaffenburg D.J.,University of California at Berkeley | Pemmaraju C.D.,Chemical science Division | And 2 more authors.
Journal of the American Chemical Society | Year: 2017

The initial step of photocatalytic water oxidation reaction at the metal oxide/aqueous interface involves intermediates formed by trapping photogenerated, valence band holes on different reactive sites of the oxide surface. In SrTiO3, these one-electron intermediates are radicals located in Ti-O• (oxyl) and Ti-O•-Ti (bridge) groups arranged perpendicular and parallel to the surface respectively, and form electronic states in the band gap of SrTiO3. Using an ultrafast sub band gap probe of 400 nm and white light, we excited transitions between these radical states and the conduction band. By measuring the time evolution of surface reflectivity following the pump pulse of 266 nm light, we determined an initial radical formation time of 1.3 ± 0.2 ps, which is identical to the time to populate the surface with titanium oxyl (Ti-O•) radicals. The oxyl was separately observed by a subsurface vibration near 800 cm-1 from Ti-O located in the plane right below Ti-O•. Second, a polarized transition optical dipole allows us to assign the 1.3 ps time constant to the production of both Osite radicals. After a 4.5 ps delay, another distinct surface species forms with a time constant of 36 ± 10 ps with a yet undetermined structure. As would be expected, the radicals' decay, specifically probed by the oxyl's subsurface vibration, parallels that of the photocurrent. Our results led us to propose a nonadiabatic kinetic mechanism for generating radicals of the type Ti-O• and Ti-O•-Ti from valence band holes based on their solvation at aqueous interfaces. © 2017 American Chemical Society.

Campbell C.G.,Biosciences and Biotechnology Division | Campbell C.G.,Lawrence Livermore National Laboratory | Kirvel R.D.,Biosciences and Biotechnology Division | Love A.H.,Johnson Wright Inc. | And 5 more authors.
Biosecurity and Bioterrorism | Year: 2012

Decontaminating civilian facilities or large urban areas following an attack with Bacillus anthracis poses daunting challenges because of the lack of resources and proven technologies. Nevertheless, lessons learned from the 2001 cleanups together with advances derived from recent research have improved our understanding of what is required for effective decontamination. This article reviews current decontamination technologies appropriate for use in outdoor environments, on material surfaces, within large enclosed spaces, in water, and on waste contaminated with aerosolized B. anthracis spores. © 2012 Mary Ann Liebert, Inc.

Bugaris D.E.,Northwestern University | Copping R.,Chemical science Division | Tyliszczak T.,Lawrence Berkeley National Laboratory | Shuh D.K.,Chemical science Division | Ibers J.A.,Northwestern University
Inorganic Chemistry | Year: 2010

The compound La2U2Se9 was obtained in high yield from the stoichiometric reaction of the elements in an Sb 2Se3 flux at 1123 K. The compound, which crystallizes in a new structure type in space group Pmma of the orthorhombic system, has a three-dimensional structure with alternating U/Se and La/Se layers attached via three independent, infinite polyselenide chains. The U atom has a monocapped square antiprismatlc coordination of Se atoms, whereas one La atom is bicapped square prismatic and the other La atom is trigonal prismatic. La 2U2Se9 displays an antiferromagnetic transition at TN = 5 K; above 50 K, the paramagnetic behavior can be fit to the Curie-Weiss law, yielding a μeff of 3.10(1) μB/U. The low-temperature specific heat of La2U2Se9 exhibits no anomalous behavior near the Néel temperature that might indicate long-range magnetic ordering or a phase transition. X-ray absorption near-edge structure (XANES) spectra have confirmed the assignment of formal oxidation states of +III for lanthanum and +IV for uranium in La 2U2Se9. ©2010 American Chemical Society.

Parkhill J.A.,Lawrence Berkeley National Laboratory | Head-Gordon M.,Chemical science Division
Journal of Chemical Physics | Year: 2010

Paired, active-space treatments of static correlation are augmented with additional amplitudes to produce a hierarchy of parsimonious and efficient cluster truncations that approximate the total energy. The number of parameters introduced in these models grow with system size in a tractable way: two powers larger than the static correlation model it is built upon: for instance cubic for the models built on perfect pairing, fourth order for a perfect quadruples (PQ) reference, and fifth order for the models built on perfect hextuples. These methods are called singles+doubles (SD) corrections to perfect pairing, PQ, perfect hextuples, and two variants are explored. An implementation of the SD methods is compared to benchmark results for F2 and H2 O dissociation problems, the H4 and H8 model systems, and the insertion of beryllium into hydrogen. In the cases examined even the quartic number of parameters associated with PQSD is able to provide results which meaningfully improve on coupled-cluster singles doubles (CCSD) (which also has quartic amplitudes) and compete with existing multi-reference alternatives. © 2010 American Institute of Physics.

Qiao Z.-A.,Chemical science Division | Zhang P.,Chemical science Division | Chai S.-H.,Chemical science Division | Chi M.,Center for Nanophase Material science | And 5 more authors.
Journal of the American Chemical Society | Year: 2014

Here we describe a lab-in-a-shell strategy for the preparation of multifunctional core-shell nanospheres consisting of a core of metal clusters and an outer microporous silica shell. Various metal clusters (e.g., Pd and Pt) were encapsulated and confined in the void space mediated by the entrapped polymer dots inside hollow silica nanospheres acting first as complexing agent for metal ions and additionally as encapsulator for clusters, limiting growth and suppressing the sintering. The Pd clusters encapsulated in hybrid core-shell structures exhibit exceptional size-selective catalysis in allylic oxidations of substrates with the same reactive site but different molecular size (cyclohexene ∼0.5 nm, cholesteryl acetate ∼1.91 nm). The solvent-free aerobic oxidation of diverse hydrocarbons and alcohols was further carried out to illustrate the benefits of such an architecture in catalysis. High activity, outstanding thermal stability and good recyclability were observed over the core-shell nanocatalyst. © 2014 American Chemical Society.

Wu C.,Harbin Institute of Technology | Wu C.,Vanderbilt University | Skelton A.A.,Vanderbilt University | Skelton A.A.,University of Dayton | And 4 more authors.
Langmuir | Year: 2012

The binding of a negatively charged residue, aspartic acid (Asp) in tripeptide arginine-glycine-aspartic acid, onto a negatively charged hydroxylated rutile (110) surface in aqueous solution, containing divalent (Mg 2+, Ca 2+, or Sr 2+) or monovalent (Na +, K +, or Rb +) cations, was studied by molecular dynamics (MD) simulations. The results indicate that ionic radii and charges will significantly affect the hydration, adsorption geometry, and distance of cations from the rutile surface, thereby regulating the Asp/rutile binding mode. The adsorption strength of monovalent cations on the rutile surface in the order Na + > K + > Rb + shows a reverselyotropic trend, while the divalent cations on the same surface exhibit a regularlyotropic behavior with decreasing crystallographic radii (the adsorption strength of divalent cations: Sr 2+ > Ca 2+ > Mg 2+). The Asp side chain in NaCl, KCl, and RbCl solutions remains stably H-bonded to the surface hydroxyls and the inner-sphere adsorbed compensating monovalent cations act as a bridge between the COO - group and the rutile, helping to trapthe negatively charged Asp side chain on the negatively charged surface. In contrast, the mediating divalent cations actively participate in linking the COO - group to the rutile surface; thus the Asp side chain can remain stably on the rutile (110) surface, even if it is not involved in any hydrogen bonds with the surface hydroxyls. Inner- and outer-sphere geometries are all possible mediation modes for divalent cations in bridging the peptide to the rutile surface. © 2012 American Chemical Society.

Predota M.,University of South Bohemia | Machesky M.L.,Illinois State Water Survey | Wesolowski D.J.,Chemical science Division | Cummings P.T.,Oak Ridge National Laboratory | Cummings P.T.,Vanderbilt University
Journal of Physical Chemistry C | Year: 2013

Adsorption of Rb+, Na+, Sr2+, and Cl - on hydroxylated (110) rutile surfaces was studied by molecular dynamics (MD) simulations. Our previous work was extended to the range of surface charge densities from-0.2 to +0.1 C/m2 (from-0.4 to +0.1 C/m2 for Sr2+) and to temperatures of 25, 150, and 250 C. These conditions can be linked to experimental surface charge and pH values from macroscopic titrations of rutile powders with surfaces dominated by 110 crystal planes. Simulations revealed that Na+ and Sr2+ adsorb closer to the surface, shifting from predominately bidentate to tetradentate inner-sphere binding with increasing temperature, whereas Rb+ binding is predominately tetradentate at all temperatures. These differences are related to hydration energies, which must be partially overcome for inner-sphere binding and which decrease with increasing temperature and are lowest for Rb+. The interaction of Cl- with the rutile surface is generally less than that for cations because of repulsion by surface oxygen atoms. These MD results provide molecular-level context for the trends observed in our corresponding macroscopic surface charge titrations. Titration curves steepen in the order Rb+ < Na+ < Sr2+, reflecting the adsorption interactions related to ion charge, radius, and hydration energy. © 2013 American Chemical Society.

Altman A.B.,University of California at Berkeley | Pemmaraju C.D.,Chemical science Division | Camp C.,University of California at Berkeley | Arnold J.,University of California at Berkeley | And 4 more authors.
Journal of the American Chemical Society | Year: 2015

Polarized aluminum K-edge X-ray absorption near edge structure (XANES) spectroscopy and first-principles calculations were used to probe electronic structure in a series of (BDI)Al, (BDI)AlX2, and (BDI)AlR2 coordination compounds (X = F, Cl, I; R = H, Me; BDI = 2,6-diisopropylphenyl-β-diketiminate). Spectral interpretations were guided by examination of the calculated transition energies and polarization-dependent oscillator strengths, which agreed well with the XANES spectroscopy measurements. Pre-edge features were assigned to transitions associated with the Al 3p orbitals involved in metal-ligand bonding. Qualitative trends in Al 1s core energy and valence orbital occupation were established through a systematic comparison of excited states derived from Al 3p orbitals with similar symmetries in a molecular orbital framework. These trends suggested that the higher transition energies observed for (BDI)AlX2 systems with more electronegative X1- ligands could be ascribed to a decrease in electron density around the aluminum atom, which causes an increase in the attractive potential of the Al nucleus and concomitant increase in the binding energy of the Al 1s core orbitals. For (BDI)Al and (BDI)AlH2 the experimental Al K-edge XANES spectra and spectra calculated using the eXcited electron and Core-Hole (XCH) approach had nearly identical energies for transitions to final state orbitals of similar composition and symmetry. These results implied that the charge distributions about the aluminum atoms in (BDI)Al and (BDI)AlH2 are similar relative to the (BDI)AlX2 and (BDI)AlMe2 compounds, despite having different formal oxidation states of +1 and +3, respectively. However, (BDI)Al was unique in that it exhibited a low-energy feature that was attributed to transitions into a low-lying p-orbital of b1 symmetry that is localized on Al and orthogonal to the (BDI)Al plane. The presence of this low-energy unoccupied molecular orbital on electron-rich (BDI)Al distinguishes its valence electronic structure from that of the formally trivalent compounds (BDI)AlX2 and (BDI)AlR2. The work shows that Al K-edge XANES spectroscopy can be used to provide valuable insight into electronic structure and reactivity relationships for main-group coordination compounds. © 2015 American Chemical Society.

Feng G.,Vanderbilt University | Jiang D.-E.,Chemical science Division | Cummings P.T.,Vanderbilt University | Cummings P.T.,Oak Ridge National Laboratory
Journal of Chemical Theory and Computation | Year: 2012

Recent experiments have revealed that onion-like carbons (OLCs) offer high energy density and charging/discharging rates when used as the electrodes in supercapacitors. To understand the physical origin of this phenomenon, molecular dynamics simulations were performed for a room-temperature ionic liquid near idealized spherical OLCs with radii ranging from 0.356 to 1.223 nm. We find that the surface charge density increases almost linearly with the potential applied on electric double layers (EDLs) near OLCs. This leads to a nearly flat shape of the differential capacitance versus the potential, unlike the bell or camel shape observed on planar electrodes. Moreover, our simulations reveal that the capacitance of EDLs on OLCs increases with the curvature or as the OLC size decreases, in agreement with experimental observations. The curvature effect is explained by dominance of charge overscreening over a wide potential range and increased ion density per unit area of electrode surface as the OLC becomes smaller. © 2012 American Chemical Society.

PubMed | University of Lausanne, Lawrence Berkeley National Laboratory, Chemical science Division. and Molecular Foundry and.
Type: Journal Article | Journal: Proceedings of the National Academy of Sciences of the United States of America | Year: 2015

The photoreductive dissolution of Mn(IV) oxide minerals in sunlit aquatic environments couples the Mn cycle to the oxidation of organic matter and fate of trace elements associated with Mn oxides, but the intrinsic rate and mechanism of mineral dissolution in the absence of organic electron donors is unknown. We investigated the photoreduction of -MnO2 nanosheets at pH 6.5 with Na or Ca as the interlayer cation under 400-nm light irradiation and quantified the yield and timescales of Mn(III) production. Our study of transient intermediate states using time-resolved optical and X-ray absorption spectroscopy showed key roles for chemically distinct Mn(III) species. The reaction pathway involves (i) formation of Jahn-Teller distorted Mn(III) sites in the octahedral sheet within 0.6 ps of photoexcitation; (ii) Mn(III) migration into the interlayer within 600 ps; and (iii) increased nanosheet stacking. We propose that irreversible Mn reduction is coupled to hole-scavenging by surface water molecules or hydroxyl groups, with associated radical formation. This work demonstrates the importance of direct MnO2 photoreduction in environmental processes and provides a framework to test new hypotheses regarding the role of organic molecules and metal species in photochemical reactions with Mn oxide phases. The timescales for the production and evolution of Mn(III) species and a catalytic role for interlayer Ca(2+) identified here from spectroscopic measurements can also guide the design of efficient Mn-based catalysts for water oxidation.

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