Papers by Dennis Nordlund
The molecular architecture of asphaltenes is still a matter of debate. Some literature reports pr... more The molecular architecture of asphaltenes is still a matter of debate. Some literature reports provide evidence that the contrast of petroleum asphaltenes versus coal-derived asphaltenes is useful for understanding the governing principles of asphaltene identity. Coal-derived asphaltenes provide an excellent test for understanding the relationship of asphaltene molecular architecture with asphaltene properties. Diffusion measurements have shown that coal-derived asphaltenes are half the size of many crude oil asphaltenes, but there are relatively few studies comparing coal-derived and petroleum asphaltenes using liquid state 13 C NMR. 13 C NMR confirms that the molecular sizes of these coal-derived asphaltenes are smaller than virgin petroleum asphaltenes. DEPT-45 experiments were performed in order to determine the relative amount of nonprotonated and protonated carbon in the aromatic region of the spectrum. In contrast to previous NMR work on asphaltenes that ignored interior bridgehead carbon, we show this is an important component of asphaltenes and that correctly accounting for this carbon enables proper determination of the number of fused rings. XRS data supports interpreting the NMR data with a model that weighs circularly condensed structures more heavily than linearly condensed structures. Significantly more carbon exists in chains at least 9 carbons long in petroleum asphaltenes (g7%) compared to coal-derived asphaltenes (g1%).
Quantum dots (QDs) show promise as the absorber in nanostructured thin film solar cells, but achi... more Quantum dots (QDs) show promise as the absorber in nanostructured thin film solar cells, but achieving high device efficiencies requires surface treatments to minimize interfacial recombination. In this work, lead sulfide (PbS) QDs are grown on a mesoporous TiO 2 film with a crystalline TiO 2 surface, versus one coated with an amorphous TiO 2 layer by atomic layer deposition (ALD). These mesoporous TiO 2 films sensitized with PbS QDs are characterized by X-ray and electron diffraction, as well as X-ray absorption spectroscopy (XAS) in order to link XAS features with structural distortions in the PbS QDs. The XAS features are further analyzed with quantum simulations to probe the geometric and electronic structure of the PbS QD-TiO 2 interface. We show that the anatase TiO 2 surface structure induces PbS bond angle distortions, which increases the energy gap of the PbS QDs at the interface. KEYWORDS: sensitized solar cell, X-ray absorption spectroscopy, X-ray absorption near edge structure, atomic layer deposition, density functional theory, quantum dot
Advanced materials (Deerfield Beach, Fla.), Jan 24, 2015
Electronic coupling and ground-state charge transfer at the C60 /ZnO hybrid interface is shown to... more Electronic coupling and ground-state charge transfer at the C60 /ZnO hybrid interface is shown to localize carriers in the C60 phase. This effect, revealed by resonant X-ray photoemission, arises from interfacial hybridization between C60 and ZnO. Such localization at carrier-selective electrodes and interlayers may lead to severely reduced carrier harvesting efficiencies and increased recombination rates in organic electronic devices.
The microscopic understanding of reactions at surfaces requires an in-depth knowledge of the dyna... more The microscopic understanding of reactions at surfaces requires an in-depth knowledge of the dynamics of elementary processes on an ultrafast timescale. This can be accomplished using an ultrafast excitation to initiate a chemical reaction and then probe the progression of the reaction with an ultrashort x-ray pulse from the FEL. There is a great potential to use atom-specific spectroscopy involving core levels to probe the chemical nature, structure and bonding of species on surfaces. The ultrashort electron pulse obtained in the linear accelerator to feed the X-ray FEL can also be used for generation of coherent synchrotron radiation in the low energy THz regime to be used as a pump. This radiation has an energy close to the thermal excitations of low-energy vibrational modes of molecules on surfaces and phonons in substrates. The coherent THz radiation will be an electric field pulse with a certain direction that can collectively manipulate atoms or molecules on surfaces. In this respect a chemical reaction can be initiated by collective atomic motion along a specific reaction coordinate. If the coherent THz radiation is generated from the same source as the X-ray FEL radiation, full-time synchronization for pump-probe experiments will be possible. The combination of THz and X-ray spectroscopy could be a unique opportunity for FEL facilities to conduct ultrafast chemistry studies at surfaces.
Abstract: H39. 00015: Applications of Light Element X-ray Raman Spectroscopy and Hard X-ray Emiss... more Abstract: H39. 00015: Applications of Light Element X-ray Raman Spectroscopy and Hard X-ray Emission Spectroscopy to the Electronic Structure of Energy Storage Materials: Prospects and Initial Results from the Spectroscopy Program at SSRL
We present a state-of-the-art x-ray Raman spectroscopy end-station recently developed, installed,... more We present a state-of-the-art x-ray Raman spectroscopy end-station recently developed, installed, and operated at the Stanford Synchrotron Radiation Lightsource. The end-station consists of two multicrystal Johann type spectrometers arranged on a Rowland circle of ...
ECS Electrochemistry Letters
Physical chemistry chemical physics : PCCP, Jan 14, 2015
The present study aims to provide insights into the behavior of LiNi0.4Mn0.4Co0.2O2 (NMC442) and ... more The present study aims to provide insights into the behavior of LiNi0.4Mn0.4Co0.2O2 (NMC442) and LiNi0.4Mn0.4Co0.18Ti0.02O2 (NMC442-Ti02) cathode materials under galvanostatic cycling to high potentials, in the context of previous work which predicted that Ti-substituted variants should deliver higher capacities and exhibit better cycling stability than the unsubstituted compounds. It is found that NMC cathodes containing Ti show equivalent capacity fading but greater specific capacity than those without Ti in the same potential range. When repeatedly charged to the same degree of delithiation, NMC cathodes containing Ti showed better capacity retention. Soft X-ray absorption spectroscopy (XAS) spectra for Mn and Co indicated increased reduction in these elements for NMC cathodes without Ti, indicating that the substitution of Ti for Co acts to suppress the formation of a high impedance rock salt phase at the surface of NMC cathode particles. The results of this study validate the a...
Nano letters, Jan 25, 2015
The development of sodium ion batteries (NIBs) can provide an alternative to lithium ion batterie... more The development of sodium ion batteries (NIBs) can provide an alternative to lithium ion batteries (LIBs) for sustainable, low-cost energy storage. However, due to the larger size and higher m/e ratio of the sodium ion compared to lithium, sodiation reactions of candidate electrodes are expected to differ in significant ways from the corresponding lithium ones. In this work, we investigated the sodiation mechanism of a typical transition metal-oxide, NiO, through a set of correlated techniques, including electrochemical and synchrotron studies, real-time electron microscopy observation, and ab initio molecular dynamics (MD) simulations. We found that a crystalline Na2O reaction layer that was formed at the beginning of sodiation plays an important role in blocking the further transport of sodium ions. In addition, sodiation in NiO exhibits a "shrinking-core" mode that results from a layer-by-layer reaction, as identified by ab initio MD simulations. For lithiation, however...
The oxygen evolution reaction (OER) is a key energy conversion reaction in a number of clean ener... more The oxygen evolution reaction (OER) is a key energy conversion reaction in a number of clean energy technologies, including rechargeable metal-air batteries, electrolysis cells, and solar fuel synthesis. Widespread commercialization of these technologies is desirable but limited by the scarcity and high cost of the best known catalysts for OER, ruthenium and iridium oxides.1 In search of replacements for these active precious metal oxides, recent research has focused on earth-abundant catalysts such as manganese oxides (MnOx)2-4 and cobalt oxides.5-7 Interestingly, OER on both manganese and cobalt oxides has been shown to vary with the nature of underlying support, but the possible effect of the support on the oxidation state of the oxide and the corresponding catalytic activity has not yet been determined.4,6 In our work, we prepare MnOx nanoparticles on a variety of supports and characterize the oxidation state of Mn using ex-situ L-edge X-ray absorption spectroscopy both before a...
Nano Energy, 2015
ABSTRACT Understanding and controlling the surface activities of electrode materials is critical ... more ABSTRACT Understanding and controlling the surface activities of electrode materials is critical for optimizing the battery performance, especially for nanoparticles with high surface area. Na0.44MnO2 is a promising positive electrode material for large-scale sodium-ion batteries. However, its application in grid-scale energy storage requires improvements in cycling stability at high rate. Here, we performed comprehensive surface-sensitive soft x-ray spectroscopic studies of the Na0.44MnO2 electrode. We are able to quantitatively determine the Mn evolution upon the potentials and cycle numbers. We reveal the Mn2+ formation on the top 10 nm of Na0.44MnO2 particles when the electrochemical potential is below 2.6 V, which does not occur in the bulk. A portion of the surface Mn2+ compounds become electrochemically inactive after extended cycles, contributing to the capacity fading. Based on the spectroscopic discoveries, we demonstrate that cycling Na0.44MnO2 above 3 V could efficiently suppress the Mn2+ formation.
Chemistry of Materials, 2015
The Journal of Physical Chemistry Letters, 2015
We present an analysis of ice nucleation kinetics from near-ambient pressure water as temperature... more We present an analysis of ice nucleation kinetics from near-ambient pressure water as temperature decreases below the homogeneous limit T H by cooling micrometer-sized droplets (microdroplets) evaporatively at 10(3)-10(4) K/s and probing the structure ultrafast using femtosecond pulses from the Linac Coherent Light Source (LCLS) free-electron X-ray laser. Below 232 K, we observed a slower nucleation rate increase with decreasing temperature than anticipated from previous measurements, which we suggest is due to the rapid decrease in water's diffusivity. This is consistent with earlier findings that microdroplets do not crystallize at <227 K, but vitrify at cooling rates of 10(6)-10(7) K/s. We also hypothesize that the slower increase in the nucleation rate is connected with the proposed "fragile-to-strong" transition anomaly in water.
We present an x-ray absorption spectroscopy results for fully broken to a complete H-bond network... more We present an x-ray absorption spectroscopy results for fully broken to a complete H-bond network of water molecules on Ru(0001) by varying the morphology from isolated water molecules via two-dimensional clusters to a fully covered monolayer as probed by scanning tunneling microscopy. The sensitivity of x-ray absorption to the symmetry of H-bonding is further elucidated for the amino (-NH) group in glycine adsorbed on Cu(110) where the E-vector is parallel either to the NH donating an H-bond or to the non-H-bonded NH. The results give further evidence for the interpretation of the various spectral features of liquid water and for the general applicability of x-ray absorption spectroscopy to analyze H-bonded systems.
Physical Review B
We demonstrate the sensitivity of x-ray absorption spectroscopy to hydrogen bonding using as expe... more We demonstrate the sensitivity of x-ray absorption spectroscopy to hydrogen bonding using as experimental model system water on Ru(0001). We stepwise go from fully broken to complete H-bond network by varying the morphology from isolated monomers via two-dimensional clusters to a saturated monolayer as probed by scanning tunneling microscopy. The sensitivity of x-ray absorption to the symmetry of H bonding is further elucidated for the amino (-NH2) group in glycine adsorbed on Cu(110) where the E vector is parallel either to the NH donating an H bond or to the non-H-bonded NH. We show that the pre-edge in the x-ray absorption spectrum is associated with an asymmetric hydrogen-bonding situation while the postedge is directly associated with hydrogen bond formation. The results give further evidence for the much debated interpretation of the various spectral features of liquid water and demonstrate the general applicability of x-ray absorption spectroscopy to analyze H-bonded systems.
The Journal of Physical Chemistry C
Sulfur dimer (S22–) terminated pyrite FeS2{100} surfaces with a low energy electron diffraction (... more Sulfur dimer (S22–) terminated pyrite FeS2{100} surfaces with a low energy electron diffraction (LEED) pattern of 2 × 1 symmetry are reported. The 2 × 1 symmetry correlates with the orientation of the anisotropic surface structure and external symmetry of macroscopic striations on the pyrite cube face. The basic condition to form these surfaces is a mild 200 V Ne+ sputter-cleaning procedure followed by a 570 K anneal of the sample in a 10–7 Torr S2(g) atmosphere. Controlled amounts of surface sulfur monomers (S2–) can be introduced by mild sputtering of the sulfur dimer terminated surfaces. At low monomer concentrations the surface displays the same characteristic 1 × 1 LEED pattern as that for fracture-generated surfaces. With increasing sulfur depletion, a (1/√2 × 1/√2)R45° LEED pattern emerges, and soft X-ray photoelectron spectroscopy (XPS) results show a sulfur dimer deficient near-surface region and a new high binding energy sulfur spectral component suggesting the presence of...
Advanced Materials
The energy of the charge-transfer state formed between electron-donating and electronaccepting ma... more The energy of the charge-transfer state formed between electron-donating and electronaccepting materials, a state that directly absorbs, largely determines the limit of the open-circuit voltage in organic photovoltaic devices. This is described in work by Aram Amassian, Michael D. McGehee and co-workers on page 6076.
Advanced Functional Materials, 2015
Chemical doping of graphene represents a powerful means of tailoring its electronic properties. S... more Chemical doping of graphene represents a powerful means of tailoring its electronic properties. Synchrotron-based X-ray spectroscopy offers an effective route to investigate the surface electronic and chemical states of functionalizing dopants. In this work, a suite of X-ray techniques is used, including near edge X-ray absorption fi ne structure spectroscopy, X-ray photoemission spectroscopy, and photoemission threshold measurements, to systematically study plasma-based chlorinated graphene on different substrates, with special focus on its dopant concentration, surface binding energy, bonding confi guration, and work function shift. Detailed spectroscopic evidence of C-Cl bond formation at the surface of single layer graphene and correlation of the magnitude of p-type doping with the surface coverage of adsorbed chlorine is demonstrated for the fi rst time. It is shown that the chlorination process is a highly nonintrusive doping technology, which can effectively produce strongly p-doped graphene with the 2D nature and long-range periodicity of the electronic structure of graphene intact. The measurements also reveal that the interaction between graphene and chlorine atoms shows strong substrate effects in terms of both surface coverage and work function shift.
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Papers by Dennis Nordlund