Near-edge X-ray Absorption Fine Structure Data Research Articles

X-ray Spectroscopy Study of Defect Contribution to Lithium Adsorption on Porous Carbon.

Lithium adsorption on high-surface-area porous carbon (PC) nanomaterials provides superior electrochemical energy storage performance dominated by capacitive behavior. In this study, we demonstrate the influence of structural defects in the graphene lattice on the bonding character of adsorbed lithium. Thermally evaporated lithium was deposited in vacuum on the surface of as-grown graphene-like PC and PC annealed at 400 °C. Changes in the electronic states of carbon were studied experimentally using surface-sensitive X-ray photoelectron spectroscopy and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. NEXAFS data in combination with density functional theory calculations revealed the dative interactions between lithium sp2 hybridized states and carbon π*-type orbitals. Corrugated defective layers of graphene provide lithium with new bonding configurations, shorter distances, and stronger orbital overlapping, resulting in significant charge transfer between carbon and lithium. PC annealing heals defects, and as a result, the amount of lithium on the surface decreases. This conclusion was supported by electrochemical studies of as-grown and annealed PC in lithium-ion batteries. The former nanomaterial showed higher capacity values at all applied current densities. The results demonstrate that the lithium storage in carbon-based electrodes can be improved by introducing defects into the graphene layers.

The role of Co valence in charge transport in the entropy‐stabilized oxide (Mg0.2Co0.2Ni0.2Cu0.2Zn0.2)O

AbstractMany of the studies on the entropy‐stabilized oxide (Mg0.2Co0.2Ni0.2Cu0.2Zn0.2)O have been heavily application‐based. Previous works have studied effects of cation stoichiometry on the entropy‐driven reaction to form a single phase, but a fundamental exploration of the effects of anion stoichiometry and/or redox chemistry on electrical properties is lacking. Using near‐edge X‐ray absorption fine structure (NEXAFS) and electrical measurements, we show that oxidizing thin film samples of (Mg0.2Co0.2Ni0.2Cu0.2Zn0.2)O affects primarily the valence of Co, leaving the other cations in this high‐entropy system unchanged. This oxidation increases electrical conduction in these thin films, which occurs via small polaron hopping mediated by the Co valence shift from 2+ to a mixed 2+/3+ state. In parallel, we show that bulk samples sintered in an oxygen‐rich atmosphere have a lower activation energy for electrical conduction than those equilibrated in a nitrogen (reducing) atmosphere. Combining feasible defect compensation scenarios with electrical impedance measurements and NEXAFS data, we propose a self‐consistent interpretation of Co redox‐mediated small polaron conduction as the dominant method of charge transfer in this system.

Nanocrystalline cobalt hydroxide oxide: Synthesis and characterization with SQUID, XPS, and NEXAFS

Nanocrystalline cobalt hydroxide oxide (nc-CoOOH) particles were synthesized via a precipitation-oxidation mechanism with different oxidation agents. Since the choice of the oxidative compound is of great practical importance, we compared the properties of the nc-CoOOH particles obtained from the oxidation with O2 and Br2. The particles from both synthesis routes were fully characterized with X-ray powder diffraction (XRPD), infrared spectroscopy (IR), superconducting quantum interface device (SQUID) measurements, X-ray photoelectron spectroscopy (XPS), and near-edge X-ray absorption fine structure (NEXAFS) measurements. It was found that the choice of the oxidation agent greatly affects the properties of the resulting nanoparticles; the most significant differences concern particle size and magnetic moment, due to residual Co2+. Although both, Br2 and O2, lead to the formation of nc-CoOOH, particles obtained from the synthesis involving O2 show a three times larger particle size than their counterparts obtained from the synthesis using Br2 (24 nm vs. 8 nm). Furthermore, using Br2 during the synthesis leads to an 8-fold increase of the amount of residual Co2+ impurities, which, in fact, induce a magnetic moment. A possible enrichment of those Co2+ impurities on the surface of the nanoparticles is discussed based on XPS and NEXAFS data.

Adsorption of 5-Fluorouracil on Au(111) and Cu(111) surfaces

The adsorption of 5-Fluorouracil (5FU) on Au(111) and Cu(111) surfaces as a function of molecular coverage and temperature has been studied, using x-ray photoelectron spectroscopy (XPS) and near-edge x-ray absorption fine structure (NEXAFS) spectroscopy. The nature of 5-Fluorouracil bonding with the two substrates is remarkably different. The Cu substrate forms a chemisorbed complex with 5-FU while the Au substrate shows only physisorption. NEXAFS data at the C, N and O K-edge show a strong angular dependence, indicating that 5-FU lies nearly parallel on the inert Au(111) surface, and at a steep angle on the Cu(111) surface. 5-FU is a biomolecule used for cancer treatment and the results are relevant for those using metal surfaces to prepare 5-FU for applications such as drug delivery.

Growth of Epitaxial 3,4,9,10-Perylene Tetracarboxylic Dianhydride on Bi-Terminated Silicon

The epitaxial quality of thin films crucially depends on their interaction with the substrate. Up to now, Ag-terminated Si(111) has been employed as the model substrate for the growth of 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) on semiconductors. In this study, we will show that Bi termination results in PTCDA films of superior epitaxial quality. We have studied the growth of PTCDA on bismuth-passivated Si(111) in detail by means of spot profile analysis of low-energy electron diffraction (SPA-LEED), X-ray photoemission spectroscopy (XPS), near-edge X-ray absorption fine structure (NEXAFS), and scanning tunneling microscopy (STM). The XPS results reveal the presence of intact PTCDA molecules on the surface upon adsorption. NEXAFS data indicate the PTCDA molecules being oriented with their molecular plane parallel to the surface. STM shows a very smooth growth front of the PTCDA film, preserving the step structure of the substrate. High-resolution SPA-LEED data demonstrate the presence of a multidomain surface with a rich variety of PTCDA surface structures, which were identified to be most prominently herring-bone polytypes. However, in the monolayer range, quadratic brick-wall structures and a nearly square-like structure as well as a perylene-like structure have also been found. Despite the simultaneous presence of multiple domains, the individual domains show excellent lateral ordering, with larger domain sizes as compared to the case of Ag-terminated Si(111).

Isotropic thin PTCDA films on GaN(0 0 0 1)

The growth of 3, 4, 9, 10-perylene tetracarboxylic dianhydride (PTCDA) on the Ga-polar GaN(0 0 0 1) surface has been studied by x-ray photoelectron spectroscopy (XPS), spot profile analysis low-energy electron diffraction (SPA-LEED), near edge x-ray absorption fine structure (NEXAFS), and scanning tunneling microscopy (STM). The stoichiometric ratios derived from XPS indicate that the molecules remain intact upon adsorption on the surface. Furthermore, no chemical shifts can be observed in the C 1s and O 1s core levels with progressing deposition of PTCDA, suggesting none or only weak interactions between the molecules and the substrate. NEXAFS data indicate the PTCDA molecules being oriented with their molecular plane parallel to the surface. High-resolution STM shows PTCDA islands of irregular shape on the sub-micron scale, and together with corresponding SPA-LEED data reveals a lateral ordering of the molecules that is compatible with the presence of (1 0 2) oriented PTCDA nano-crystals. SPA-LEED moreover clearly shows the presence of homogeneously distributed rotational domains of two-dimensionally isotropic PTCDA.

Oxidation of rubrene thin films: an electronic structure study.

The performances of organic semiconductor devices are crucially linked with their stability at the ambient atmosphere. The evolution of electronic structures of 20 nm thick rubrene films exposed to ambient environment with time has been studied by UV and X-ray photoemission spectroscopy (UPS and XPS), near edge X-ray absorption fine structure (NEXAFS) spectroscopy, and density functional theory (DFT). XPS, NEXAFS data, and DFT calculated values suggest the formation of rubrene-epoxide and rubrene-endoperoxide through reaction of tetracene backbone with oxygen of ambient environment. Angle dependent XPS measurement indicates that the entire probed depth of the films reacts with oxygen by spending only about 120 min in ambient environment. The HOMO peak of pristine rubrene films almost disappears by exposure of 120 min to ambient environment. The evolution of the valence band (occupied states) and NEXAFS (unoccupied states) spectra indicates that the films become more insulating with exposure as the HOMO-LUMO gap increases on oxidation. Oxygen induced chemical reaction completely destroys the delocalized nature of the electron distribution in the tetracene backbone of rubrene. The results are relevant to the performance and reliability of rubrene based devices in the environment.

Spectroscopic characterization of Cobalt–Phthalocyanine electrocatalysts for fuel cell applications

For polymer–electrolyte–membrane fuel cells (PEM-FC), platinum catalysts on carbon based substrates are state of the art, due to high electrochemical activity and chemical stability in acid electrolytes. High costs of platinum force investigations of alternative catalysts. With respect to the oxygen reduction reaction, possible candidates are transition metal (TM) complexes like TM-porphyrines or TM-phthalocyanines. Pyrolysis on carbon based substrates improves the catalytic activity. Highest activities were found for Fe or Co as transition metal centers.We present results of the pyrolysis of Co-phthalocyanine (CoPc) on carbon black. Besides an investigation of morphology by scanning electron microscopy (SEM) and chemical composition by energy dispersive X-ray spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS) during pyrolysis, we present a near edge X-ray absorption fine structure (NEXAFS) study of the chemical composition for the related annealing steps. This investigation is accompanied by an electrochemical characterization of the catalytic activity and selectivity.The XPS and NEXAFS data indicate a decomposition of the CoN4 (chelate-) complex during pyrolysis. The N1s data for 800°C show graphite-like, formally pyrrolic and pyridinic like features, without any significant contribution of Co. The Co2p data for both, XPS and NEXAFS are dominated by CoO/Co2O3. It can be concluded that during the pyrolysis cobalt particles are formed by the disintegration of CoN4-centers, after pyrolysis these particles become oxidized upon contact with air.

A theoretical study of the XP and NEXAFS spectra of alanine: gas phase molecule, crystal, and adsorbate at the ZnO(101̄0) surface

The adsorption of alanine on the mixed-terminated ZnO(10 ̅10) surface is studied by means of quantum-chemical ab initio calculations. Using a finite cluster model and the adsorption geometry as obtained both by periodic CPMD and embedded cluster calculations, the C1s, N1s and O1s X-ray photoelectron spectra (XPS) and near-edge X-ray absorption fine structure (NEXAFS) spectra are calculated for single alanine molecules on ZnO(10 ̅10). These spectra are compared with the spectra calculated for alanine in the gas phase and in its crystalline form and with experimental XPS and NEXAFS data for the isolated alanine molecule and for alanine adsorbed on ZnO(10 ̅10) at multilayer and monolayer coverage. The excellent agreement between the experimental and calculated XP and NEXAFS spectra confirms the calculated adsorption geometry: A single alanine molecule is bound to ZnO(10 ̅10) in a dissociated bidentate form with the two O atoms of the acid group bound to two Zn atoms of the surface and the proton transferred to one O atom of the surface. Other possible structures, such as adsorption of alanine in one of its neutral or zwitterionic forms in which the proton of the -COOH group remains at this group or is transferred to the amino group, can be excluded since they would give rise to quite different XP spectra. In the multilayer coverage regime, on the other hand, alanine is in its crystalline form as is also shown by the analysis of the XP spectra.

Photoemission Study of Thymidine Adsorbed on Au(111) and Cu(110)

The adsorption geometry of the DNA nucleoside thymidine on Au(111) and Cu(110) surfaces has been determined from experimental results of soft X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. The XPS of C, N, and O 1s as well as the absorption spectra at the N and O K-edges were measured for monolayer thymidine films, and the nature of the bonding with the two metal surfaces has been determined. The NEXAFS data at the N and O K-edge show a strong angular dependence of the π*/σ* intensity ratios. We conclude that the thymine moiety is lying nearly parallel to the Au(111) surface, while on the Cu(110) surface, it adsorbs at a steep angle.

Synchrotron radiation photoelectron spectroscopy and near-edge X-ray absorption fine structure study on oxidative etching of diamond-like carbon films by hyperthermal atomic oxygen

Surface structural changes of a hydrogenated diamond-like carbon (DLC) film exposed to a hyperthermal atomic oxygen beam were investigated by Rutherford backscattering spectroscopy (RBS), synchrotron radiation photoelectron spectroscopy (SR-PES), and near-edge X-ray absorption fine structure (NEXAFS). It was confirmed that the DLC surface was oxidized and etched by high-energy collisions of atomic oxygen. RBS and real-time mass-loss data showed a linear relationship between etching and atomic oxygen fluence. SR-PES data suggested that the oxide layer was restricted to the topmost surface of the DLC film. NEXAFS data were interpreted to mean that the sp 2 structure at the DLC surface was selectively etched by collisions with hyperthermal atomic oxygen, and an sp 3-rich region remained at the topmost DLC surface. The formation of an sp 3-rich layer at the DLC surface led to surface roughening and a reduced erosion yield relative to the pristine DLC surface.

Assembly and structure of alpha-helical peptide films on hydrophobic fluorocarbon surfaces.

The structure, orientation, and formation of amphiphilic alpha-helix model peptide films on fluorocarbon surfaces has been monitored with sum frequency generation (SFG) vibrational spectroscopy, near-edge x-ray absorption fine structure (NEXAFS) spectroscopy, and x-ray photoelectron spectroscopy (XPS). The alpha-helix peptide is a 14-mer of hydrophilic lysine and hydrophobic leucine residues with a hydrophobic periodicity of 3.5. This periodicity yields a rigid amphiphilic peptide with leucine and lysine side chains located on opposite sides. XPS composition analysis confirms the formation of a peptide film that covers about 75% of the surface. NEXAFS data are consistent with chemically intact adsorption of the peptides. A weak linear dichroism of the amide pi( *) is likely due to the broad distribution of amide bond orientations inherent to the alpha-helical secondary structure. SFG spectra exhibit strong peaks near 2865 and 2935 cm(-1) related to aligned leucine side chains interacting with the hydrophobic surface. Water modes near 3200 and 3400 cm(-1) indicate ordering of water molecules in the adsorbed-peptide fluorocarbon surface interfacial region. Amide I peaks observed near 1655 cm(-1) confirm that the secondary structure is preserved in the adsorbed peptide. A kinetic study of the film formation process using XPS and SFG showed rapid adsorption of the peptides followed by a longer assembly process. Peptide SFG spectra taken at the air-buffer interface showed features related to well-ordered peptide films. Moving samples through the buffer surface led to the transfer of ordered peptide films onto the substrates.

Electron spectroscopy studies of PTCDA onAg∕Si(111)−3×3

The growth of 3,4,9,10-perylene tetracarboxylic dianhydride thin films on the $\mathrm{Ag}∕\mathrm{Si}(111)\text{\ensuremath{-}}\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$ surface has been studied by means of high-resolution photoelectron spectroscopy (PES), near edge x-ray absorption fine structure (NEXAFS) and low-energy electron diffraction (LEED). LEED and NEXAFS data indicate that for the first molecular layer there is a well ordered growth of flat lying molecules in several phases with a preferred ordering relative to the substrate. There is a clear interaction between the perylene core of the molecule and substrate seen from changes of the highest occupied molecular orbital and lowest unoccupied molecular orbital levels in the PES and NEXAFS data. Shifts in the C $1s$ and O $1s$ XPS core levels are interpreted as interactions with the carboxylic groups. There are also new components and changes in the low coverage XPS C $1s$ and O $1s$ core level spectra compared to spectra from thicker, pure molecular films. The new components can be related to two different parts of the molecule. One is located in the carboxylic groups and the other in the perylene core of the molecule. For thicker films the NEXAFS and core level results suggest a mainly layered growth of flat lying molecules.

Carbon Speciation of Diesel Exhaust and Urban Particulate Matter NIST Standard Reference Materials with C(1s) NEXAFS Spectroscopy

The U.S. National Institute of Standards and Technology (NIST) provides a number of particulate matter (PM) standard reference materials (SRM) for use in environmental and toxicological methodology and research. We present here the first analysis with respect to the molecular structure of the carbon in three such NIST SRM samples, i.e., diesel engine exhaust soot from heavy duty equipment engines (SRM 1650), diesel soot from a forklift engine (SRM 2975), and urban PM collected in St. Louis, MO (SRM 1648), with near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. The NEXAFS spectra of the two diesel soot samples appear quite similar, while they differ significantly from the urban PM spectrum, in agreement with X-ray diffraction data published recently. Such comparison is made in terms of aromatic and aliphatic carbon species, as well as by a general comparison with graphitic materials. Both diesel soot SRM samples contain basic graphitic structures, but the presence of exciton resonance and extended X-ray absorption fine structure oscillations in SRM 1650 and the lack therof in SRM 2975 suggest that SRM 1650 is the more graphitic material. The presence of polycyclic aromatic hydrocarbons, which have a characteristic NEXAFS resonance at the same position as graphite, can obscure the graphitic character of soot, unless an extraction of the organic matter is made. Our NEXAFS data do not suggest that the urban PM sample SRM 1648 contains a substantial amount of graphite-like material.

X-PEEM/NEXAFS and AFM of polypyrrole and copper micro-patterns on insulating fluoropolymer substrates

Micro-patterns (80 μm and 10 μm) of copper and semi-conducting polypyrrole on insulating fluorinated ethylene propylene substrates were characterized using synchrotron-based X-ray Photoemission Electron Microscopy (X-PEEM), Near Edge X-ray Absorption Fine Structure (NEXAFS), and Atomic Force Microscopy (AFM). Electronic states in the polypyrrole are verified using the NEXAFS data, and sample degradation upon irradiation is addressed. X-PEEM images show homogeneous distributions of the corresponding elements in the patterns. They do not exhibit dichroic effects and give information about the growth of copper and polypyrrole (i.e. nucleation of Cu, overgrowth of PPy, formation of PPy granules). AFM results are used to verify the topography of the patterns and support the findings on pattern growth.

Growth and electronic structure of vanadium on α-Al 2O 3(0001)

The interaction of metallic vanadium with the α-Al 2O 3(0001) surface was studied in the submonolayer-to-multilayer coverage regime using X-ray photoemission spectroscopy (XPS) and near-edge X-ray absorption fine structure (NEXAFS). At low coverage the V 2p photoemission spectra reveal a strong interaction of vanadium with alumina, thus indicating the formation of a vanadium oxide. The NEXAFS data, both the line-shape and the position of the vanadium L-edge, are consistent with the formation of V 2O 3. A comparison of the results of both spectroscopies reveals that vanadium seems to be highly dispersed at low coverage, and final state effects on the core level ionization energies, e.g. cluster size effects, have to be considered. At higher amounts of vanadium deposition a metallic vanadium overlayer forms on top of a vanadia/Al 2O 3 interface. In an ambient of 5×10 −5 Torr oxygen metallic vanadium oxidizes rapidly, even at 300 K, and the formation of V 2O 3 is observed.

Al K-Edge Near-Edge X-ray Absorption Fine Structure (NEXAFS) Study on the Coordination Structure of Aluminum in Minerals and Y Zeolites

Curved-wave−multiple-scattering cluster calculations with the FEFF6 code were used to interpret experimental AlK-edge near-edge X-ray absorption fine structure (NEXAFS) spectra of various minerals and Y zeolites for energies ∼15 eV above threshold. Octahedral, tetrahedral, and square planar geometries of Al can be easily distinguished from each other utilizing characteristic features in the NEXAFS data. NEXAFS line shapes are used for determining the geometrical conformations of Al atoms in Y zeolites with one or more conformational geometries. In the H−Y zeolite, separate contributions to the NEXAFS from tetrahedrally and octahedrally coordinated Al atoms are identified. The differences in the octahedrally coordinated Al spectra in the H−Y zeolite compared with spectra for standard octahedrally coordinated Al compounds can be attributed to the presence of very small nonregular clusters of octahedrally coordinated Al dispersed over the zeolite. However, the presence of some pentacoordinated Al cannot be e...

Interaction of magnesium with oriented diphenyl-carbonate films

Abstract The interaction of evaporated magnesium with thin diphenyl-carbonate (DPC) films deposited on a Ag(111) single crystal substrate has been investigated by X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure (NEXAFS). Vacuum-sublimated DPC molecules form well-ordered and oriented layers on Ag(111) substrates with the carbonate group tilted by about 20° and the phenyl rings oriented parallel to the substrate as deduced from the NEXAFS data. DPC forms a weak physisorptive bond with the Ag substrate. In contrast, at the interface to the metallic Mg overlayer reactive bonding occurs via the carbonate groups. Upon annealing the Mg/DPC heterointerface from 150 K to about room temperature, Mg diffuses into the organic film to form a strongly interacting network of Mg–DPC charge transfer complexes.

Investigation of the Molecular Motion of Self-Assembled Fatty Acid Films

We have investigated self-assembled stearic acid films adsorbed on an Al2O3 film by means of electron paramagnetic resonance (EPR) as well as near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. The stearic acid molecules are orientated in the film as shown by the NEXAFS data. Doping of the film with spin-labels in order to make the films accessible to EPR spectroscopy distorts the structure only slightly as long as the concentration of the spin-labels is low. The temperature-dependent EPR spectra reveal a distinct dependence of the onset of the rotational motion of the spin-label with variation of the location of the spin-label along the chain. The analysis of the EPR line shape allows to gain insight into the internal motion of the molecular chain.

NEXAFS Characterization and Reactivity Studies of Bimetallic Vanadium Molybdenum Oxynitride Hydrotreating Catalysts

The surface and bulk compositions of vanadium molybdenum oxynitride (V{sub 2}MoO{sub 1.7}N{sub 2.4}), prepared by temperature-programmed reaction (TPR) of vanadium molybdenum oxide (V{sub 2}MoO{sub 8}) with ammonia, have been characterized using near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. The NEXAFS data were recorded at the K-edges of nitrogen and oxygen, the L-edge of vanadium, and the M-edge of molybdenum. The nitrogen K-edge region of V-Mo oxynitride shows the characteristic NEXAFS features of early-transition-metal nitrides, although these features are different from those of either VN or Mo{sub 2}N. Furthermore, comparison of the electron yield and fluorescence yield measurements also reveals that the oxidation state is different for vanadium near the surface region and for vanadium in the bulk, which is estimated to be 2.8 {+-} 0.3 and 3.8 {+-} 0.3, respectively. The oxidation state of bulk molybdenum is also estimated to be 4.4 {+-} 0.3. The X-ray diffraction pattern shows that the bulk phase of the bimetallic oxide is different from the pure monometallic oxide phases but the oxynitride has a cubic structure that resembles the pure vanadium and molybdenum nitride phases. The V-Mo oxide as prepared shows a preferential orientation of [001] crystallographic planes which is lost during the nitridationmore » process. This shows that the solid state transformation V{sub 2}MoO{sub 8} {yields} V{sub 2}MoO{sub 1.7}N{sub 2.4} is not topotactic. 27 refs., 8 figs., 1 tab.« less