Scientific Publications 2009
2009. "Spectroscopic Investigation of the Species Involved in the Rhodium-Catalyzed Oxidative Carbonylation of Toluene to Toluic Acid." Physical Chemistry Chemical Physics. PCCP 11(42):9903-9911. doi:10.1039/b906883j Abstract A spectroscopic investigation of complexes used to catalyze the oxidative carbonylation of toluene to para-toluic acid was conducted. Rhodium complexes were analyzed by 103Rh and 13C NMR, UV-visible spectroscopy, and infrared spectroscopy. In the presence of vanadium and oxygen, the resting state of the Rh catalyst was found to exist as a Rh(III) complex with carbonyl and trifluoroacetate ligands, consistent with the structure Rh(CO)2(TFA)3. The complex exhibited a carbonyl peak with an unusual degree of shielding, which resulted in the appearance of the carbonyl peak at an unprecedented upfield position in the 13C NMR spectrum. This shielding was caused by interaction of the carbonyl group with the trifluoroacetate ligand. In the absence of oxygen, the Rh(III) complex reduced to Rh(I), and the reduced form exhibited properties resembling the catalyst precursor. Structures and spectroscopic properties calculated using Density Functional Theory were in good agreement with experimental results. The vanadium co-catalyst was similarly characterized by 51V NMR and UV-visible spectroscopy. The oxidized species corresponded to [(VO2)(TFA)]2, whereas the reduced species corresponded (VO)(TFA)2. The spectroscopic results obtained in this study confirm the identity of the species that have been proposed to be involved in the Rh-catalyzed oxidative carbonylation of toluene to toluic acid.
2009. "Origin of two time-scale regimes in potentiometric titration of metal oxides. A replica kinetic Monte Carlo study." Langmuir 25(12):6841-6848. Abstract Replica Kinetic Monte Carlo simulations were used to study the characteristic time scales of potentiometric titration of the metal oxides and (oxy)hydroxides. The effect of surface heterogeneity and surface transformation on the titration kinetics were also examined. Two characteristic relaxation times are often observed experimentally, with the trailing slower part attributed to surface non-uniformity, porosity, polymerization, amorphization, and other dynamic surface processes induced by unbalanced surface charge. However, our simulations show that these two characteristic relaxation times are intrinsic to the proton binding reaction for energetically homogeneous surfaces, and therefore surface heterogeneity or transformation do not necessarily need to be invoked. However, all such second-order surface processes are found to intensify the separation and distinction of the two kinetic regimes. The effect of surface energetic-topographic non-uniformity, as well dynamic surface transformation, interface roughening/smoothing were described in a statistical fashion. Furthermore, our simulations show that a shift in the point-of-zero charge is expected from increased titration speed and the pH-dependence of the titration measurement error is in excellent agreement with experimental studies.
2009. "Linear Correlation Between Binding Energy and Young’s Modulus in Graphene Nanoribbons." Journal of Applied Physics 106(5):054318-054323. doi:10.1063/1.3211944 Abstract Graphene nanoribbons (GNRs) have been suggested as a promising material for its use as nanoelectromechanical reasonators for highly sensitive force, mass, and charge detection. Therefore the accurate determination of the size-dependent elastic properties of GNRs is desirable for the design of graphene-based nanoelectromechanical devices. In this study we determine the size-dependent Young’s modulus and carbon-carbon binding energy in a homologous series of GNRs, C4n2+6n+2H6n+4 (n=2–12), with the use of all electron first principles computations. An unexpected linearity between the binding energy and Young’s modulus is observed, making possible the prediction of the size-dependent Young’s modulus of GNRs through a single point energy calculation of the GNR ground state. A quantitative-structure-property relationship is derived, which correlates Young’s modulus to the total energy and the number of carbon atoms within the ribbon. In the limit of extended graphene sheets we determine the value of Young’s modulus to be 1.09 TPa, in excellent agreement with experimental estimates derived for graphite and suspended grapheme sheets.
2009. "Achieving Size Independent Hit-Rate in Single Particle Mass Spectrometry." Aerosol Science and Technology 43(4):305-310. Abstract Recent improvements in single particle mass spectrometers make it possible to optically detect, size, and characterize the compositions of individual particles with diameters larger than a micron and smaller than 100 nm. Based on particle detection in two stages of optical detection these instruments generate a precisely timed trigger pulse, which is used to fire the ion generation laser or lasers. Practical experience shows that the wide size range results in small, but significant differences in laser trigger timing between small and large particles. If not treated, the instrument hit-rate becomes size dependent and instrument operator is forced to optimize the instrument for the desired size range, while having to contend with a lower hit-rate for the other. The present paper presents an analysis of the problem, demonstrating that size dependence of laser trigger timing stems from the differences in the particle position within the detection laser beam at the instant of detection. It shows that it is possible to compensate for these differences by generation a laser trigger delay coefficient for individual particles as a function of particle time of flight, i.e. its size. The study also shows that a single function can be used to characterize particles with a wide range of densities.
2009. "Beyond single particle mass spectrometry: multidimensional characterisation of individual aerosol particles." International Reviews in Physical Chemistry 28(2):309-358. Abstract The behavior of small aerosol particles depends on a number of their physical and chemical properties, many of which are strongly coupled. The size, internal composition, density, shape, morphology, hygroscopicity, index of refraction, activity as cloud condensation nuclei and ice nuclei, and other attributes of individual particles - all play a role in determining particle properties and their impacts. The traditional particle characterization approaches rely on separate parallel measurements that average over an ensemble of particles of different sizes and/or compositions and later attempt to draw correlations between them. As a result such studies overlook critical differences between particles and bulk and miss the fact that individual particles often exhibit major differences. Here we review the recently developed methods to simultaneously measure in-situ and in real time several of the attributes for individual particles using single particle mass spectrometer, SPLAT or its second generation SPLAT II. We also discuss novel approaches developed for classification, visualization and mining of large datasets produced by the multidimensional single particle characterization.
2009. "Comparison Between Mass Spectra of Individual Organic Particles Generated by UV Laser Ablation and in the IR/UV Two-Step Mode." International Journal of Mass Spectrometry 282(1-2):6-12. Abstract One of the most fundamental aspects of single particle mass spectrometry is that the individual particle mass spectra are first classified and only then averaged. When ions are generated by ablation with a UV laser the mass spectra of particles with identical compositions exhibit large particle to particle fluctuations in the mass spectral intensity pattern. This is particularly true when it comes to particles containing organic molecules. At laser fluence that is sufficient to ionize sulfates many of the organic molecules exhibit high degree of fragmentation, often to the point where they cannot be distinguished from elemental carbon. In contrast, when ion generation is separated into two steps, in which the first step uses infra red to evaporate the semi-volatile components and the second, time delayed UV pulse to ionize the evaporating plume, the quality of the individual particle mass spectra are significantly improved. We present an experimental investigation of the properties and behavior of individual particle mass spectra of organic particles that are generated by ablation and in two steps. We investigate the effect of UV laser fluence and the delay between the two lasers. The study shows that the two step approach yields highly reproducible mass spectra that contain sufficient detail to allow clear molecular assignments. The two step approach also produces 10 times as many ions, and the mass spectral intensity can be related to the amount of organics in the particle. In contrast, ablation generated mass spectra were found to exhibit high degree of fragmentation and particle-to-particle fluctuations.
2009. "SPLAT II: An Aircraft Compatible, Ultra-Sensitive, High Precision Instrument for In-Situ Characterization of the Size and Composition of Fine and Ultrafine Particles." Aerosol Science and Technology 43(5):411-424. Abstract The properties of aerosols depend on the size and internal compositions of the individual particles. The vast majority of atmospheric aerosols are smaller than 200 nm, yet the single particle mass spectrometers, the only instruments that can characterize the size and internal compositions of individual particles, typically detect these small particles with extremely low efficiencies. In this paper we describe a new instrument called SPLAT II that provides unparalleled sensitivity to small particles, detecting 100% of particles that are larger than 125 nm and 40% of 100 nm particles. This instrument also brings an increase by a factor of 10 in temporal resolution, sizing up to 500 particles per second and characterizing the composition of up to 100 of them. SPLAT II uses a two-laser, two-step process to evaporate the particles and generate ions, producing high quality, reproducible mass spectra of the refractive and non-refractive aerosol fractions to yield the complete compositions of individual particles. The instrument control board provides for size dependent delays for lasers’ triggers to eliminate a size dependent hit rate. The mass spectra are recorded with 14-bit vertical resolution and analyzed using custom software packages. The instrument’s high sizing resolution and sensitivity makes it possible to combine it with the differential mobility analyzer(s) and measure particle size, composition, density, dynamic shape factor, hygroscopicity, and fractal dimension.
2009. "Probing the Electronic and Structural Properties of the Niobium Trimer Cluster and its Mono- and Dioxides: Nb30n- and Nb30n (n=0-2)." Journal of Physical Chemistry A 113(16):3866-3875. Abstract We report a photoelectron spectroscopy and density functional theory (DFT) study on the electronic and structural properties of Nb3-, Nb3O-, Nb3O2-, and the corresponding neutrals. Well-resolved photoelectron spectra are obtained for the anion clusters at different photon energies and are compared with DFT calculations to elucidate their structures and chemical bonding. We find that Nb3 - possesses a C2V (3A2) structure, and Nb3 is a scalene Cs (2A′′) triangle. Both Nb3O- and Nb3O are found to have C2V structures, in which the O atom bridges two Nb atoms in a Nb3 triangle. The ground-state of Nb3O2 - is found surprisingly to be a low symmetry C1 (1A) structure, which contains a bridging and a terminal O atom. Molecular orbital analyses are carried out to understand the structures and bonding of the three clusters and provide insights into the sequential oxidation from Nb3- to Nb3O2-. The terminal NbdO unit is common in niobia catalysts, and the Nb3O2- cluster with a NbdO unit may be viewed as a molecular model for the catalytic sites or the initial oxidation of a Nb surface.
2009. "Structural Evolution, Sequential Oxidation, and Chemical Bonding in Tritantalum Oxide Clusters: Ta3On- and Ta3On(n=1-8)." Journal of Physical Chemistry A 113(36):9804-9813. Abstract We report a combined photoelectron spectroscopy (PES) and density functional theory (DFT) study on a series of tri-tantalum oxide clusters, Ta3On-. Well-resolved PES spectra are obtained for Ta3On- (n = 1-8) at several detachment photon energies, yielding electronic structure information which is used to compare with the DFT calculations. A trend of sequential oxidation is observed as a function of O content until Ta3O8-, which is a stoichiometric cluster. Extensive DFT calculations are performed in search of the lowest energy structures for both the anions and neutrals. The first three O atoms are shown to successively occupy the bridging sites in the Ta3 triangle. The next three O atoms each occupy a terminal site, with the seventh and eighth O atoms forming a double-bridge and a double-terminal, respectively. The Ta3O7- anion is found to possess a localized electron pair on a single Ta center, making it an interesting molecular model for Ta3+ surface sites. Molecular orbital analyses are performed to elucidate the chemical bonding in the Ta3On- clusters.
2009. "Biomineralization Associated with Microbial Reduction of Fe3+ and Oxidation of Fe2+ in Solid Minerals ." American Mineralogist 94(7):1049-1058. Abstract Iron- reducing and oxidizing microorganisms gain energy through reduction or oxidation of iron, and by doing so they play an important role in geochemical cycling of iron in a wide range of environments. This study was undertaken to investigate iron redox cycling in the deep subsurface by taking an advantage of the Chinese Continental Scientific Deep Drilling project. A fluid sample from 2450 m was collected and Fe(III)-reducing microorganisms were enriched using specific media (pH 6.2). Nontronite, an Fe(III)-rich clay mineral, was used in initial enrichments with lactate and acetate as electron donors under strictly anaerobic condition at the in-situ temperature of the fluid sample (65oC). Instead of a monotonic increase in Fe(II) concentration with time as would have been expected if Fe(III) bioreduction was the sole process, Fe(II) concentration initially increased, reached a peak, but then decreased to a minimum level. Continued incubation revealed an iron cycling with a cycling period of five to ten days. These initial results suggested that there might be Fe(III) reducers and Fe(II) oxidizers in the enrichment culture. Subsequently, multiple transfers were made with an attempt to isolate individual Fe(III) reducers and Fe(II) oxidizers. However, iron cycling persisted after multiple transfers. Additional experiments were conducted to ensure that iron reduction and oxidation was indeed biological. Biological Fe(II) oxidation was further confirmed in a series of roll tubes (with a pH gradient) where FeS and siderite were used as the sole electron donor. The oxidation of FeS occurred only at pH 10, and goethite, lepidocrocite, and ferrihydrite formed as oxidation products. Although molecular evidence (16S rRNA gene analysis) collectively suggested that only a single organism (a strain of Thermoanaerobacter ethanolicus) might be responsible for both Fe(III) reduction and Fe(II) oxidation, we could not rule out the possibility that Fe(III) reduction and Fe(II) oxidation may be accomplished by a consortia of organisms. Nonetheless, our data were definitive in showing that iron redox cycling exists in the deep subsurface.
2009. "An Invisible Bend Sensor Based on Porous Crosslinked Polyelectrolyte Film." Sensors and Actuators. A, Physical 151(2):154-158. doi:10.1016/j.sna.2009.02.034 Abstract This paper describes the fabrication of a porous cross-linked polyelectrolyte membrane and the characterization of its humidity sensitivity performance. Electrostatic self-assembly, combined with acid treatment, and post-deposition annealing produced the membrane. The fabrication process offers the ability to control the thickness of the membrane, as well as enabling the engineering of the humidity sensitivity properties. A transparent humidity sensor was fabricated by integrating the membrane into a capacitive structure. In order to improve the moisture absorption and diffusion, both the polyelectrolyte layer and the electrode were made porous. The membrane was cross-linked to enhance the durability in high humid environments. Such a polyelectrolyte membrane showed high sensitivity to relative humidity variation over a range of 25-99%. The see-through property of the structure adds extra features and benefits to the sensor.
2009. "Application of High-Resolution H-1 MAS NMR Spectroscopy to the Analysis of Intact Bones from Mice Exposed to Gamma Radiation." Radiation Research 172(5):607-616. doi:10.1667/RR1715.1 Abstract A high resolution Magic Angle Spinning (hr-MAS) 1H NMR method was developed to directly measure the metabolite spectrum of bone marrow inside unprocessed mouse femur. The method is rapid and involves minimal sample preparation. The utility of this method was demonstrated by applying it to assess the gamma radiation- induced damage to bone marrow in the femurs of mice. Metabolite profiles were different in the middle and end sections of the femur. In particular, the end section contained a higher content of lipids compared to the middle section. The results obtained from both the middle and end sections of femur clearly show that the total choline content, i.e., the sum of choline, phosphocholine and glycerophosphocholine decreased with increasing dose of gamma radiation when dosed groups were compared with control groups 4 days post irradiation. Further, the content of fatty acids also increased with the increasing radiation dose. Eleven days post 3.0 Gy exposure, many of the metabolites showed a trend toward returning to normal control level. The lipid biomarkers responsive to radiation dosage may provide novel targets for medical countermeasures against radiation damage.
2009. "Transparent Humidity Sensor Using Cross-Linked Polyelectrolyte Membrane ." IEEE Sensors Journal 9(7):854-857. doi:10.1109/JSEN.2009.2024055 Abstract This paper describes the fabrication of a porous cross-linked polyelectrolyte membrane and the characterization of its humidity sensitivity performance. Electrostatic self-assembly, combined with acid treatment, and post-deposition annealing produced the membrane. The fabrication process offers the ability to control the thickness of the membrane, as well as enabling the engineering of the humidity sensitivity properties. A transparent humidity sensor was fabricated by integrating the membrane between two parallel electrodes. In order to improve the moisture absorption and diffusion, both the polyelectrolyte layer and the electrode were made porous. The membrane was cross-linked to enhance the durability in high humid environments. Such a polyelectrolyte membrane showed high sensitivity to relative humidity variation over a range of 25%–99%. The see-through property of the structure adds extra features and benefits to the sensor.
2009. "Stabilization of Platinum Nanoparticle Electrocatalysts for Oxygen Reduction Using Poly(diallyldimethylammonium chloride)." Journal of Materials Chemistry 19(42):7995 - 8001. doi:10.1039/b912104h Abstract A long-chain polyelectrolyte, poly(diallyldimethylammonium chloride) (PDDA), has been employed to stabilize platinum nanoparticles for oxygen reduction in polymer electrolyte membrane (PEM) fuel cells. Pt nanoparticles were synthesized by reducing H2PtCl6 with NaBH4 in the presence of PDDA and then deposited on carbon support (PDDA-Pt/C). Transmission electron microscope images showed that Pt nanoparticles of PDDA-Pt/C are uniformly dispersed on carbon support with a mean size of about 2.2 nm (2.1 nm for commercial Etek-Pt/C). PDDA-Pt/C exhibited a higher activity towards oxygen reduction reaction (ORR) than Etek-Pt/C. The durability of PDDA-Pt/C was improved by a factor of 2 as compared with Etek-Pt/C. X-ray photoelectron spectroscopy characterization of PDDA-Pt/C revealed the interaction between Pt nanoparticles and PDDA, which increased Pt oxidation potential. PDDA-Nafion ionic crosslinking "entraps" Pt nanoparticles and prevents Pt nanoparticles from migrating/agglomerating on or detaching from carbon support. This provides a promising strategy to improve both the durability and activity of electrocatalysts for fuel cells.
2009. "Damage Profile and Ion Distribution of Slow Heavy Ions in Compounds." Journal of Applied Physics 105(10):104901:1-12. doi:10.1063/1.3118582 Abstract Slow heavy ions inevitably produce a significant concentration of defects and lattice disorder in solids during their slowing-down process via ion-solid interactions. For irradiation effects research and many industrial applications, atomic defect production, ion range and doping concentration are commonly estimated by the Stopping and Range of Ions in Matter (SRIM) code. In this study, ion-induced damage and projectile ranges of low energy Au ions in SiC are determined using complementary ion beam and microscopy techniques. Considerable errors in both disorder profile and ion range predicted by the SRIM code indicate an overestimation of the electronic stopping power, by a factor of 2 in most cases, in the energy region up to 25 keV/nucleon. Such large discrepancies are also observed for slow heavy ions, including Pt, Au and Pb ions, in other functional materials, such as GaN, AlN and SrTiO3. Due to the importance of these materials for advanced device and nuclear applications, better electronic stopping cross section predictions, based on a reciprocity principle developed by Sigmund, is suggested with fitting parameters for possible improvement.
2009. "Ion Technique for Identifying Gamma Detector Candidates." IEEE Transactions on Nuclear Science 56(3):920-925. doi:10.1109/TNS.2008.2011640 Abstract Recent demands for radiation detector materials with better energy resolution at room temperature have prompted research efforts on both accelerated material discovery and efficient analysis techniques. Ions can easily deposit their energy in thin films or small crystals and the radiation response can be used to identify material properties relevant to detector performance. In an effort to identify gamma detector candidates using small crystals or film samples, an ion technique is developed to measure relative light yield and energy resolution of candidate materials and to evaluate radiation detection performance. Employing a unique time-of-flight (TOF) telescope, light yield and energy resolution resulting from ion excitation are investigated over a continuous energy region. The efficiency of this ion technique is demonstrated using both organic (plastic scintillator) and inorganic (CaF2:Eu, YAP:Ce, CsI:Tl and BGO) scintillators.
2009. "Response of Materials to Single Ion Events." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 267(8-9):1705-1712. doi:10.1016/j.nimb.2009.01.104 Abstract Response of materials to single radiation events is fundamental to research and many technological applications that involve energetic particles. Ion-solid interactions lead to energy loss of ions, production of electron-hole pairs, and light emission from excitation-induced luminescence. Employing a unique time-of-flight system, material response to single ion irradiation has been utilized to measure electronic energy loss, and to evaluate materials performance for radiation detection. Measurements of electronic energy loss of single ions in a thin ZrO2 foil over a continuous energy range exhibit good agreement with SRIM predictions for He and Be ions. For O and F ions, slight over- and under-estimation of SRIM prediction is evident at energies around 250 (near the stopping maximum) and above 800 keV/nucleon, respectively. For a Si semiconductor detector, its response to single ion irradiation shows that pulse height defect is clear for elements heavier than Si, and nonlinear energy response is significant for all elements at energies below ~ 150 keV/nucleon. For a single crystal CsI:Tl scintillator, the response to H ion events is used to determine relative light yield and absolute energy resolution over a wide energy region, where energy resolution of ~ 5.3% is achieved at 2 MeV.
2009. "Response of Strontium Titanate to Ion and Electron Irradiation." Journal of Nuclear Materials 389(2):303-310. doi:10.1016/j.jnucmat.2009.02.014 Abstract Response of strontium titanate (SrTiO3) to ion and electron irradiation is studied at room temperature. For an accurate energy to depth conversion and a better determination of ion-induced disorder profile from Rutherford backscattering spectrometry measurement, a detailed iterative procedure is described and applied to ion channeling spectra to determine the dechanneling yield and the disorder profiles for the Sr and Ti sublattices. The result shows a large underestimation in disorder depth, ~ 40% at the damage peak, which indicates a large overestimation of the electronic stopping power for 1.0 MeV Au ions in SrTiO3 predicted by the SRIM (Stopping and Range of Ions in Matter) code. Overestimation of heavy ion stopping power may lead to an overestimation of the critical dose for amorphization. The current study also demonstrates possible ionization effects in SrTiO3 under ion and electron irradiation. Pre-amorphized SrTiO3 exhibits strong ionization-induced epitaxial recovery at the amorphous/crystalline interface under electron irradiation.
2009. "Imaging Hindered Rotations of Alkoxy Species on TiO2(110)." Journal of the American Chemical Society 131(49):17926-17932. Abstract We present the first study of the rotational dynamics of organic species on any oxide surface. Specifically, variable-temperature scanning tunneling microscopy (STM) and dispersion-corrected density functional theory are used to study the alkyl chain conformational disorder and dynamics of 1-, 2-, 3- and 4-octoxy on rutile TiO2(110). Initially, the geminate pairs of the octoxy and bridging hydroxyl species are created via octanol dissociation on bridging-oxygen (Ob) vacancy defects. The STM images provide time averaged snapshots of octoxy species rotating among multiple energetically nearly-degenerate configurations accessible at a given temperature. In the calculations we find that the underlying corrugated potential energy surface is a result of the interplay between attractive Van der Waals dispersion forces leading to weak attractive C...Ti and repulsive C...Ob interactions which lead to large barriers of 50-70kJmol-1 for the rotation of the octoxy alkyl chains across the Ob rows. In the presence of the germinal hydroxyl groups we find that the relative populations of the various conformations as well as the rotational barriers are perturbed by the presence of geminate hydroxyl due to additional C...hydroxyl repulsions.
2009. "Water as a Catalyst: Imaging Reactions of O-2 with Partially and Fully Hydroxylated TiO2(110) Surfaces." Journal of Physical Chemistry C 113(5):1908-1916. Abstract The reactions of molecular oxygen with bridging hydroxyl groups, OHb, formed by H2O dissociation on bridging oxygen vacancies of TiO2 (110) are studied at low and high OHb coverages as a function of the O2 exposure, using scanning tunneling microscopy (STM), temperature programmed desorption (TPD), and electron simulated desorption (ESD) techniques. On partially hydroxylated surfaces, the sudden simultaneous disappearance of oxygen vacancies and oxygen adatoms formed by O2 dissociation is observed at high O2 exposures. On fully hydroxylated TiO2 surfaces, which enable us to compare results of STM, TPD and ESD studies, most of OHb’s are removed via reacting with O2. Hence, fully hydroxylated TiO2 surfaces can be converted to nearly stoichiometric surfaces, albeit with some amount of adsorbed molecular water. Formation of mobile H2O molecules and water-assisted diffusion of the reactants plays an important role in the kinetics of the processes on both partially and fully hydroxylated surfaces.
2009. "The Superatom States of Fullerenes and Their Hybridization into the Nearly FreeElectron Bands of Fullerites." ACS Nano 3(4):853-864. doi:10.1021/nn800834k Abstract Motivated by the discovery of the superatom states of C₆₀ molecules, we investigate the factors that influence their energy and wave function hybridization into nearly free electron bands in molecular solids. As the n = 3 solutions of the radial Schro¨dinger equation of the central attractive potential consisting of the shortrange C atom core and the long-range collective screening potentials, respectively, located on the icosahedral C60 molecule shell and within its hollow core, superatom states are distinguished by their atom-like orbitals corresponding to different orbital angular momentum states (l = 0, 1, 2,...). Because they are less tightly bound than the π orbitals, that is, the n = 2 states, which are often exploited in the intermolecular electron transport in aromatic organic molecule semiconductors, superatom orbitals hybridize more extensively among aggregated molecules to form bands with nearly free electron dispersion. The prospect of exploiting the strong intermolecular coupling to achieve metal-like conduction in applications such as molecular electronics may be attained by lowering the energy of superatom states from 3.5 eV for single chemisorbed C₆₀ molecules to below the Fermi level; therefore, we study how the superatom state energies depend on factors such as their aggregation into 1D - 3D solids, cage size, and exo- and endohedral doping by metal atoms. We find, indeed, that if the ionization potential of endohedral atom, such as copper, is sufficiently large, superatom states can form the conduction band in the middle of the gap between the HOMO and LUMO of the parent C₆₀ molecule. Through a plane-wave density functional theory study, we provide insights for a new paradigm for intermolecular electronic interaction beyond the conventional one among the spⁿ hybridized orbitals of the organic molecular solids that could lead to design of novel molecular materials and quantum structures with extraordinary optical and electronic properties.
2009. "Theoretical study of the molecular and electronic structure of methanol on a TiO2(110) surface." Physical Review. B, Condensed Matter and Materials Physics 80(23):235416. doi:10.1103/PhysRevB.80.235416 Abstract We present density-functional-theory calculations of the molecular and electronic structure of methanol adsorption on stoichiometric TiO2(110) surface. We have investigated 11 different molecular and dissociated adsorption structures of CH3OH at 1 monolayer coverage. The relative stabilities of different structures depend on the chemisorption-induced charge transfer, the relative strengths of different types of hydrogen bonds, the steric hindrance between methyl groups and the surface stress. We found the intermolecular hydrogen bonding to play an important role in stabilizing the overlayer. We also investigated the occupied and unoccupied surface electronic structure, and the adsorbate-induced surface dipole moment and work-function changes. The electronic structures show that the highest-occupied molecular orbital of CH3OH is near the valance-band maximum, which reflects the character of CH3OH as a hole scavenger on TiO2 surfaces. The unoccupied partially solvated or “wet” electron states for CH3OH on TiO2 are primarily distributed on H atoms of methyl groups. Despite many different structural motifs, the wet-electron-state energy primarily correlates with the surface dipole moment.
2009. "Automated metal-free multiple-column nanoLC for improved phosphopeptide analysis sensitivity and throughput." Journal of Chromatography B 877(8-9):663-670. Abstract We report on the development and characterization of an automated metal-free nanoscale multiple-capillary system for reversed-phase liquid chromatography-mass spectrometry analysis of phosphopeptides. The system incorporates a capillary column (50 um i.d. × 30 cm, packed with 5 um C18 particles) coupled on-line to a solid phase extraction column (150 um i.d. × 4 cm, packed with 5 um C18 particles). Electrospray ionization tips are constructed on the packed capillary column to couple the reversed-phase liquid chromatographic separation to a linear ion trap tandem mass spectrometer.
2009. "Benchmark Energetic Data in a Model System for Grubbs II Metathesis Catalysis and Their Use for the Development, Assessment, and Validation of Electronic Structure Methods." Journal of Chemical Theory and Computation 5(2):324-333. doi:10.1021/ct800386d Abstract We present benchmark relative energetics in the catalytic cycle of a model system for Grubbs second-generation olefin metathesis catalysts. The benchmark data were determined by a composite approach based on CCSD(T) calculations, and they were used as a training set to develop a new spin-component-scaled MP2 method optimized for catalysis, which is called SCSC-MP2. The SCSC-MP2 method has improved performance for modeling Grubbs II olefin metathesis catalysts as compared to canonical MP2 or SCS-MP2. We also employed the benchmark data to test 17 WFT methods and 39 density functionals. Among the tested density functionals, M06 is the best performing functional. M06/TZQS gives an MUE of only 1.06 kcal/mol, and it is a much more affordable method than the SCSC-MP2 method or any other correlated WFT methods. The best performing meta-GGA is M06-L, and M06-L/DZQ gives an MUE of 1.77 kcal/mol. PBEh is the best performing hybrid GGA, with an MUE of 3.01 kcal/mol; however, it does not perform well for the larger, real Grubbs II catalyst. B3LYP and many other functionals containing the LYP correlation functional perform poorly, and B3LYP underestimates the stability of stationary points for the cis-pathway of the model system by a large margin. From the assessments, we recommend the M06, M06-L, and MPW1B95 functionals for modeling Grubbs II olefin metathesis catalysts. The local M06-L method is especially efficient for calculations on large systems.
2009. "Thermochemical Kinetics for Multireference Systems: Addition Reactions of Ozone." Journal of Physical Chemistry A 13(19):5786–5799. doi:10.1021/jp811054n Abstract The 1,3-dipolar cycloadditions of ozone to ethyne and ethene provide extreme examples of multireference singlet-state chemistry, and they are examined here to test the applicability of several approaches to thermochemical kinetics of systems with large static correlation. Four different multireference diagnostics are applied to measure the multireference characters of the reactants, products, and transition states; all diagnostics indicate significant multireference character in the reactant portion of the potential energy surfaces. We make a more complete estimation of the effect of quadruple excitations than was previously available, and we use this with CCSDT/CBS estimation of Wheeler et al. (Wheeler, S. E.; Ess, D. H.; Houk, K. N. J. Phys. Chem. A 2008, 112, 1798.) to make new best estimates of the van der Waals association energy, the barrier height, and the reaction energy to form the cycloadduct for both reactions. Comparing with these best estimates, we present comprehensive mean unsigned errors for a variety of coupled cluster, multilevel, and density functional methods. Several computational aspects of multireference reactions are considered: (i) the applicability of multilevel theory, (ii) the convergence of coupled cluster theory for reaction barrier heights, (iii) the applicability of completely renormalized coupled cluster methods to multireference systems, (iv) the treatment by density functional theory, (v) the multireference perturbation theory for multireference reactions, and (vi) the relative accuracy of scaling-type multilevel methods as compared with additive ones. It is found that scaling-type multilevel methods do not perform better than the additive-type multilevel methods. Among the 48 tested density functionals, only M05 reproduces the best estimates within their uncertainty. Multireference perturbation theory based on the complete-active-space reference wave functions constructed using a small number of reaction-specific active orbitals gives accurate forward barrier heights; however, it significantly underestimates reaction energies.
2009. "Mouse-Specific Tandem IgY7-SuperMix Immunoaffinity Separations for Improved LC-MS/MS Coverage of the Plasma Proteome ." Journal of Proteome Research 8(11):5387-5395. Abstract We report on a customized mouse specific SuperMix immunoaffinity column and strategy for separating low abundance proteins from high and moderate abundance proteins in mouse plasma. When applied in tandem with a mouse IgY7 column that removes the seven most abundant proteins in blood, the SuperMix column captures >100 additional moderate abundance proteins, thus allowing significant enrichment of low abundance proteins in the flow-through fraction. A side-by-side comparison of results obtained from 2D-LC-MS/MS analyses of flow-through samples from IgY7 and SuperMix columns revealed a nearly two-fold improvement in the overall proteome coverage. Detection of low abundance proteins was also enhanced, as evidenced by a more than two-fold increase in the coverage of cytokines, growth factors, and other low abundance proteins. Moreover, the tandem separations are automated, reproducible, and allow effective identification of protein abundance differences from LC-MS/MS analyses. Considering the overall reproducibility and increased sensitivity using the IgY7-SuperMix separation system, we anticipate broad applications of this strategy for biomarker discovery using mouse models.
2009. "Improving Electrocatalysts for O2 Reduction by Fine-Tuning the Pt-Support Interaction: Pt Monolayer on the Surfaces of a Pd3Fe(111) Single-Crystal Alloy." Journal of the American Chemical Society 131(35):12755-12762. doi:10.1021/ja9039746 Abstract We improved the effectiveness of Pt monolayer electrocatalysts for the oxygen-reduction reaction (ORR) using a novel approach to fine-tuning the Pt monolayer interaction with its support, exemplified by an annealed Pd3Fe(111) single-crystal alloy support having a segregated Pd layer. Low-energy ion scattering and low-energy electron diffraction studies revealed that a segregated Pd layer, with the same structure as Pd (111), is formed on the surface of high-temperature-annealed Pd3Fe(111). This Pd layer is considerably more active than Pd(111); its ORR kinetics is comparable to that of a Pt(111) surface. The enhanced catalytic activity of the segregated Pd layer compared to that of bulk Pd apparently reflects the modification of Pd surface’s electronic properties by underlying Fe. The Pd3Fe(111) suffers a large loss in ORR activity when the subsurface Fe is depleted by potential cycling (i.e., repeated excursions to high potentials in acid solutions). The Pd3Fe(111) surface is an excellent substrate for a Pt monolayer ORR catalyst, as verified by its enhanced ORR kinetics on PTML/Pd/Pd3Fe(111). Our density functional theory studies suggest that the observed enhancement of ORR activity originates mainly from the destabilization of OH binding and the decreased Pt-OH coverage on the Pt/Pd/Pd3Fe(111) surface. The activity of PtML/Pd(111) and Pt(111) is limited by OH removal, whereas the activity of PtML/Pd/Pd3Fe(111) is limited by the O-O bond scission, which places these two surfaces on the two sides of the volcano plot.
2009. "Electrochemical Performance and Stability of the Cathode for Solid Oxide Fuel Cells. I. Cross Validation of Polarization Measurements by Impedance Spectroscopy and Current-Potential Sweep." Journal of the Electrochemical Society 157(2):B220-B227. Abstract The aim of this paper is to address three issues in solid oxide fuel cells: (1) cross-validation of the polarization of a single cell measured using both dc and ac approaches, (2) the precise determination of the total areal specific resistance (ASR), and (3) understanding cathode polarization with LSCF cathodes. The ASR of a solid oxide fuel cell is a dynamic property, meaning that it changes with current density. The ASR measured using ac impedance spectroscopy (low frequency interception with real Z´ axis of ac impedance spectrum) matches with that measured from a dc IV sweep (the tangent of dc i-V curve). Due to the dynamic nature of ASR, we found that an ac impedance spectrum measured under open circuit voltage or on a half cell may not represent cathode performance under real operating conditions, particularly at high current density. In this work, the electrode polarization was governed by the cathode activation polarization; the anode contribution was negligible.
2009. "Characterization of Dispersed Heteropoly Acid on Mesoporous Zeolite Using Solid-State P-31 NMR Spin-Lattice Relaxation." Journal of the American Chemical Society 131(28):9715-9721. doi:10.1021/ja901317r Abstract Dispersion and quantitative characterization of supported catalysts is a grand challenge in catalytic science. In this paper, heteropoly acid H3PW12O40 (HPA) is dispersed on mesoporous zeolite silicalite-1 derived from hydrothermal synthesis using carbon black nanoparticle templates, and the catalytic activity is studied for 1-butene isomerization. The HPAs supported on conventional zeolite and on mesoporous zeolite exhibit very different activities and thus provide good model systems to investigate the structure dependence of the catalytic properties. The HPA on mesoporous silicalite-1 shows enhanced catalytic activity for 1-butene isomerization, while HPA on conventional silicalite-1 exhibits low activity. To elucidate the structural difference, supported HPA catalysts are characterized using a variety of techniques, including 31P magic angle spinning nuclear magnetic resonance, and are shown to contain a range of species on both mesoporous and conventional zeolites. However, contrary to studies reported in the literature, conventional NMR techniques and chemical shifts alone do not provide sufficient information to distinguish the dispersed and aggregated surface species. The dispersed phase and the nondispersed phase can only be unambiguously and quantitatively characterized using spin-lattice relaxation NMR techniques. The HPA supported on mesoporous zeolite contains a fast relaxation component related to the dispersed catalyst, giving a much higher activity, while the HPA supported on conventional zeolite has essentially only the slow relaxation component with very low activity. The results obtained from this work demonstrate that the combination of spinning sideband fitting and spin-lattice relaxation techniques can provide detailed structural information on not only the Keggin structure for HPA but also the degree of dispersion on the support.
2009. "Molecular Depth Profiling of Sucrose Films: A Comparative Study of C₆₀n⁺ Ions and Traditional Cs⁺ and O₂⁺ Ions." Analytical Chemistry 81(20):8272-8279. doi:10.1021/ac900553z Abstract Time-of-flight secondary ion mass spectrometry (ToF-SIMS) depth profiling of sucrose thin films were investigated using 10 keV C60+, 20 keV C602+, 30 keV C603+, 250 eV, 500 eV and 1000 eV Cs+ and O2+ as sputtering ions. With C60n+ ions, the molecular ion signal initially decreases, and reaches a steady-state that is about 38-51% of its original intensity, depending on the energy of the C60n+ ions. On the contrary, with Cs+ and O2+ sputtering, molecular ion signals decrease quickly to the noise level, even using low energy (250 eV) sputtering ions. In addition, the sucrose/Si interface by C60+ sputtering is much narrower than that of Cs+ and O2+ sputtering. To understand the mechanisms of sputtering-induced damage by these ions, X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) were used to characterize the bottoms of these sputter craters. XPS data show very little chemical change in the C60+ sputter crater, while considerable amorphous carbon was found in the O2+ and Cs+ sputter craters, indicating extensive decomposition of the sucrose molecules. AFM images show a very flat bottom in the C60+ sputter crater, while the Cs+ and O2+ sputter crater bottoms are significantly rougher than that of the C60+ sputter crater. Based on above data, we developed a simple model to explain different damage mechanisms during sputtering process.
2009. "Chemisorption of a Molecular Oxygen On the UN(0 0 1) Surface: Ab Initio Calculations." Journal of Nuclear Materials 393(3):504-507. doi:10.1016/j.jnucmat.2009.07.010 Abstract The results of DFT GGA calculations on oxygen molecules adsorbed upon the (0 0 1) surface of uranium mononitride (UN) are presented and discussed. We demonstrate that O2 molecules oriented parallel to the substrate can dissociate either (i) spontaneously when the molecular center lies above the surface hollow site or atop N ion, (ii) with the activation barrier when a molecule sits atop the surface U ion. This explains fast UN oxidation in air.
2009. "First Principles Calculations of Oxygen Adsorption on the UN(001) Surface." Surface Science 603(1):50-53. doi:10.1016/j.susc.2008.10.019 Abstract Fabrication, handling and disposal of nuclear fuel materials require comprehensive knowledge of their surface morphology and reactivity. Due to unavoidable contact with air components (even at low partial pressures), UN samples contain considerable amount of oxygen impurities affecting fuel properties. In this study we focus on reactivity of the energetically most stable (001) substrate of uranium nitride towards the atomic oxygen as one of initial stages for further UN oxidation. The basic properties of O atoms adsorbed on the UN(001) surface are simulated here combining the two first principles calculation methods based on the plane wave basis set and that of the localized orbitals.