Scientific Publications 2010
2010. "“Smart” Diblock Copolymers as Templates for Magnetic-Core Gold-Shell Nanoparticle Synthesis." Nano Letters 10(1):85-91. doi:10.1021/nl902865v Abstract We report a new strategy for synthesizing temperature-responsive γ-Fe2O3-core/Au-shell nanoparticles (Au-mNPs) from diblock copolymer micelles. The amphiphilic diblock copolymer chains were synthesized using reversible addition-fragmentation chain-transfer (RAFT) with a thermally responsive “smart” poly(N-isopropylacrylamide) (pNIPAAm) block and an amine-containing poly(N,N-dimethylaminoethylacrylamide) (DMAEAm) block that acted as a reducing agent during gold shell formation. The Au-mNPs reversibly aggregated upon heating the solution above the transition temperature of pNIPAAm, resulting in a red-shifted localized surface plasmon resonance.
2010. "Near-field Localization in Plasmonic Superfocusing: a Nanoemitter on a Tip ." Nano Letters 10(2):592–596. doi:10.1021/nl903574a Abstract The effective focusing of light to sub-wavelength dimensions has been a long standing desire in optics, yet proven challenging even resorting to a multitude of new strategies based on near-field effects, surface plasmon polaritons, and metamaterials1-7. The propagation of surface plasmon polaritons (SPP) on a conical tip to obtain nanoconfined light as proposed theoretically has recently emerged as a particularly promising candidate5-10. On the basis of our previous work, having established coupling of SPPs onto gold metal tips and their radiationless transport11, here, using a refined geometry we provide direct evidence of plasmonic nanofocusing into a localized excitation at the apex. As demonstrated by employing scattering-type scanning near-field optical microscopy (s-SNOM) efficient convergence of the optical fields into the 20 nm size apex volume is shown, exceeding the diffraction limit by more than one order of magnitude. The resulting nanoemitter is a point-like axial dipole with corresponding spectral and angular radiative characteristics. The associated linear and nonlinear field concentration as enabled by the unique 3D topology of the conical tip opens up exciting new applications in nano-optics, facilitating ultra-high spatio-temporal optical imaging, attosecond XUV generation, or antenna-based sensing and energy harvesting.
2010. "Decomposition Pathway of Ammonia Borane on the Surface of nano-BN." Journal of Physical Chemistry C 114(32):13935-13941. Abstract Ammonia borane (AB) is under significant investigation as a possible hydrogen storage material. While many chemical additives have been demonstrated to have a significant positive effect on hydrogen release from ammonia borane, many provide additional complications in the regeneration cycle. Mechanically alloyed hexagonal BN (nano-BN) has been shown to facilitate the release of hydrogen from AB at lower temperature, with minimal induction time, less exothermically, and inert nano-BN may be easily removed during any regeneration of the spent AB. The samples were prepared by mechanically alloying AB with nano-BN. Raman spectroscopy indicates that the AB:nano-BN samples are physical mixtures of AB and h-BN. The release of hydrogen from AB:nano-BN mixtures as well as the decomposition products were characterized by 11B magic angle spinning (MAS) solid state NMR, TGA/DSC/MS with 15N labeled AB, and solution 11B NMR spectroscopy. The 11B MAS solid state NMR spectrum shows that diammonate of diborane (DADB) is present in the mechanically alloyed mixture, which drastically shortens the induction period for hydrogen release from AB. Analysis of the TGA/DSC/MS spectra using 15N labeled AB shows that all the borazine (BZ) produced in the reaction comes from AB and that increasing nano-BN surface area results in increased amounts of BZ. However, under high temperature, 150˚C, isothermal conditions, the amount of BZ released was the same as for neat AB. High resolution transmission electron microscopy (HRTEM), selected area diffraction (SAD), and electron energy loss spectroscopy (EELS) of the initial and final nano-BN additive provide evidence for crystallinity loss but not significant chemical changes. The higher concentration of BZ observed for low temperature dehydrogenation of AB:nano-BN mixtures versus neat AB is attributed to a surface interaction that favors the formation of precursors which ultimately result in BZ. This pathway can be avoided through isothermal heating at temperatures >150˚C.
2010. "Relationships Between Complex Core Level Spectra and Materials Properties ." International Journal of Quantum Chemistry 110(15):2752–2764. doi:10.1002/qua.22807 Abstract The XPS of many oxides are quite complex and there may be several peaks of significant intensity for each subshell. These peaks arise from many-electron effects, which normally are treated with configuration interaction (CI) wavefunctions where static correlation effects are taken into account. It is common to use semiempirical methods to determine the matrix elements of the CI Hamiltonian and there are few rigorous CI calculations where parameters are not adjusted to fit experiment. In contrast, we present, in the present work, theoretical XPS spectra obtained with rigorous CI wavefunctions for CeO2 where the XPS are especially complex; several different core levels are studied. This study uses an embedded CeO8 cluster model to represent bulk CeO2 and the relativistic CI wavefunctions are determined using four-component spinors from Dirac-Fock calculations. In particular, we examine the importance of interatomic many-body effects where there is a transfer of electrons from occupied oxygen 2p orbitals into empty cation orbitals as it is common to ascribe the complex XPS to this effect. We also contrast the importance of many-body charge-transfer effects for the isoelectronic cations of Ce4+ and La3+. The long-range goal of this work is to relate the XPS features to the nature of the chemical bonding in CeO2 and we describe our progress toward this goal.
2010. "Engineering Molecular Transformations for Sustainable Energy Conversion." Industrial and Engineering Chemistry Research 49(21):10183-10199. doi:10.1021/ie101300c Abstract Future strategies for sustainable energy production will undoubtedly require processes and materials that can efficiently convert renewable resources into fuels. Nature’s enzymes can exquisitely integrate highly active catalytic centers within flexible environments that can adaptively guide reactants to products with very high activities and selectivities. They are limited, however, by their stability and ability to integrate into large scale production processes. The design of more robust heterogeneous catalytic materials that mimic the performance of enzymes, however, has been hindered by our limited understanding of how such transformations proceed. The tremendous advances in ab initio quantum mechanical methods, atomistic simulations, and high performance computing that have occurred over the past two decades, however, provide unprecedented ability to track molecular transformations and how they proceed at specific sites and within particular environments. This information together with the advances in in situ spectroscopic methods that follow such transformations can begin to enable the design of atomic surface ensembles and nanoscale reaction environments. This paper provides the author’s perspective on how theory and simulation can be used to move from current onedimensional design efforts based on catalytic descriptors to the design of two-dimensional surfaces, threedimensional reaction environments, and proton-coupled electron transfer systems that mimic enzymes in the transformation of molecules.
2010. "Advanced Spectroscopic Synchrotron Techniques to Unravel the Intrinsic Properties of Dilute Magnetic Oxides: The Case of Co:ZnO." New Journal of Physics 12:Art. No. 013020. doi:10.1088/1367-2630/12/1/013020 Abstract The use of synchrotron-based spectroscopy has revolutionized the way we look at matter. X-ray absorption spectroscopy (XAS) using linear and circular polarized light offers a powerful toolbox of element-specific structural, electronic, and magnetic probes that is especially well suited for complex materials containing several elements. We use the specific example of Zn1−xCoxO (Co:ZnO) to demonstrate the usefulness of combining these XAS techniques to unravel its intrinsic properties. We are able to demonstrate, that as long as phase separation or excessive defect formation is absent Co:ZnO is paramagnetic and we can establish independent quality indicators based on XAS. Samples which show long-range magnetic order fail to meet the quality indicators and complementary experimental techniques such as x-ray diffraction and transmission electron microscopy indeed prove phase separation. By deconvoluting the XAS spectra, the characteristic spectral features of the phase separated materials are derived.
2010. "Anisotropic Paramagnetism of Co-doped ZnO Epitaxial Films." Physical Review. B, Condensed Matter 81(5):Art. No. 054420. doi:10.1103/PhysRevB.81.054420 Abstract We have measured the full temperature dependence of M(H) curves for 5%, 10%, and 15% Co-doped ZnO epitaxial films with high crystalline perfection. Bulk magnetometry reveals pure paramagnetism with anisotropic M(H) curves at low temperatures, whereas the x-ray magnetic circular dichroism measured at the Co K-edge is isotropic. Electron paramagnetic resonance shows that the g-factors are not significantly altered compared to Co2+ impurities in ZnO. The M(H) measurements are compared to simulations using either an effective spin model with zero field splitting D or Brillouin functions with an effective temperature ansatz. Whereas both models reproduce well the H ⊥ c data, for H k c the effective spin model indicates that D is reduced by 75% compared to Co2+ impurities in ZnO. The dependence of the M(H) curves and D on the Co concentration are discussed in terms of magnetic interactions between the Co dopant atoms.
2010. "Tuning the magnetic properties of Zn1-xCoxO films." Journal of Magnetism and Magnetic Materials 322(9-12):1232-1234. doi:10.1016/j.jmmm.2009.04.024 Abstract We compare the magnetic properties of two types of Co-doped ZnO films grown on sapphire with distinct structural quality. SQUID magnetometry as well as element-specific synchrotron studies reveal pure paramagnetic behavior for the samples with the highest structural quality, whereas samples with reduced structural quality exhibit superparamagnetic blocking behavior. In this sample signatures of phase separation are detected by X-ray diffraction and X-ray linear dichroism which accounts for the superparamagnetic blocking.
2010. "Formation, isomerization, and dissociation of alpha-carbon-centered and pi-centered glycylglycyltryptophan radical cations." Journal of Physical Chemistry B 114(6):2270-2280. doi:10.1021/jp908599a Abstract Gas phase fragmentations of two isomeric radical cationic tripeptides of glycylglycyltryptophan-G•GW+ and [GGW]•+—with well-defined initial radical sites at the α-carbon atom and the 3-methylindole ring, respectively, have been studied using collision-induced dissociation (CID), density functional theory (DFT), and Rice-Ramsperger-Kassel-Marcus (RRKM) theory. Substantially different low-energy CID spectra were obtained for these two isomeric GGW structures, suggesting that they did not interconvert on the time scale of these experiments. DFT and RRKM calculations were used to investigate the influence of the kinetics, stabilities, and locations of the radicals on the competition between the isomerization and dissociation channels. The calculated isomerization barrier between the GGW radical cations (>35.4 kcal/mol) was slightly higher than the barrier for competitive dissociation of these species (<30.5 kcal/mol); the corresponding microcanonical rate constants for isomerization obtained from RRKM calculations were all considerably lower than the dissociation rates at all internal energies. Thus, interconversion between the GGW isomers examined in this study cannot compete with their fragmentations.
2010. "High-Resolution Mass Spectroscopic Analysis of Secondary Organic Aerosol Generated by Ozonolysis of Isoprene." Atmospheric Environment 44(8):1032-1042. doi:10.1016/j.atmosenv.2009.12.019 Abstract The chemical composition of secondary organic aerosol (SOA) generated from the ozonolysis of isoprene (C5H8) in the presence of an OH scavenger was examined using high-resolution electrospray ionization mass spectrometry (ESI-MS). The chemical composition of SOA is complex, with more than 1000 assigned peaks observed in the positive and negative ion mode spectra. Only a small fraction of peaks corresponds to known products of isoprene oxidation, such as pyruvic acid, glycolic acid, methylglyoxal, etc. The absolute majority of the detected peaks correspond to highly oxidized oligomeric constituents of SOA, with an average O:C molar ratio of ~0.6. The corresponding organic mass (OM) to organic oxygen (OO) ratio is OM/OO∼2.4. Approximately 8% of oxygen atoms in SOA are in the form of peroxides as quantified with an iodide test. Double bond equivalency (DBE) factors, representing the sum of all double bonds and rings, increase by 1 for every 2-3 additional carbon atoms in the molecule. The prevalent oligomer building blocks are therefore carbonyls or carboxylic acids with a C2-C3 skeleton. Kendrick analysis suggests that simple aldehydes, specifically formaldehyde, acetaldehyde, and methylglyoxal can serve as monomeric building blocks in the observed oligomers. The large number of reactive functional groups, especially organic peroxides and carbonyls, suggests that isoprene/O3 SOA should be prone to chemical and photochemical aging.
2010. "Preferential CO Oxidation in Hydrogen: Reactivity of Core-Shell Nanoparticles." Journal of the American Chemical Society 132(21):7418-7428. doi:10.1021/ja101108w Abstract We report on the first-principles-guided design, synthesis, and characterization of core-shell nanoparticle (NP) catalysts made of a transition metal core (M ) Ru, Rh, Ir, Pd, or Au) covered with a ∼1-2 monolayer thick shell of Pt atoms (i.e., a M@Pt core-shell NP). An array of experimental techniques, including X-ray diffraction, Fourier transform infrared spectroscopy, high resolution transmission electron microscopy, and temperature-programmed reaction, are employed to establish the composition of the synthesized NPs. Subsequent studies of these NPs’ catalytic properties for preferential CO oxidation in hydrogen-rich environments (PROX), combined with Density Functional Theory (DFT)-based mechanistic studies, elucidate important trends and provide fundamental understanding of the reactivity of Pt shells as a function of the core metal. Both the PROX activity and selectivity of several of these M@Pt core-shell NPs are significantly improved compared to monometallic and bulk nonsegregated bimetallic nanoalloys. Among the systems studied, Ru@Pt core-shell NPs exhibit the highest PROX activity, where the CO oxidation is complete by 30 °C (1000 ppm CO in H2). Therefore, despite their reduced Pt content, M@Pt core-shell NPs afford the design of more active PROX catalysts. DFT studies suggest that the relative differences in the catalytic activities for the various core-shell NPs originate from a combination of (i) the relative availability of CO-free Pt surface sites on the M@Pt NPs, which are necessary for O2 activation, and (ii) a hydrogen-mediated low-temperature CO oxidation process that is clearly distinct from the traditional bifunctional CO oxidation mechanism.
2010. "Synthesis and Properties of Nano Zeolitic Imidazolate Frameworks." Chemical Communications 46(27):4878-4880. Abstract Nano sized zeolitic imidazolate frameworks [nZIF-8] with excellent chemical and thermal stability has been synthesized at room temperature by simple mixing of 2-methylimidazole and zinc nitrate hexahydrate in methanol/ 1% high molecular weight poly(diallyldimethylammonium chloride) solution for 24 hrs