Near-edge Absorption Fine-structure Spectroscopy Research Articles

Consistent mineral-associated organic carbon chemistry with variable erosion rates in a mountainous landscape

Interactions between organic carbon (OC) and minerals represent a critical mechanism for stabilizing organic matter in soils. Because both mineral weathering and plant productivity are negatively affected by soil erosion, mineral-associated organic carbon (MOC) chemistry is also expected to vary with erosion intensity. Here we show that MOC chemistry, determined by carbon X-ray absorption near-edge fine structure spectroscopy (XANES), exhibits little difference across a large (10-fold) gradient in erosion-derived soil turnover times. Mineral-associated OC chemistry further fails to explain the variation in radiocarbon-based MOC turnover times. Our results suggest that soil OC longevity is largely independent of organic matter chemistry in steep mountainous landscapes where soil development is constrained by erosion.

Structure and electronic structure of van der Waals interfaces at a Au(1 1 1) surface covered with a well-ordered molecular layer of n-alkanes

The structure and electronic structure of a system comprising molecular layers of n-alkane adsorbed on Au(111) were investigated to understand the origin of the interfacial electric dipole layer. These alkane molecules are inert and thus van der Waals (vdW) interaction is the primary adsorptive force. Although these vdW interfaces are regarded as ideal physical adsorption systems unlikely to chemically interact with the metal surface, a large interfacial electric dipole layer over 1 eV is formed. It is important to elucidate the mechanism underlying the generation of such large dipole layers to understand the electrode interface in organic semiconductor devices. Thus, this paper elucidates the details of the vdW interface, in particular between tetratetracontane (TTC) and Au(111). First, the mechanism of growth of the TTC layer up to monolayer thickness was revealed using a low-energy electron-diffraction method. Second, the interface states were newly formed, as determined using angle-resolved photoelectron spectroscopy, X-ray photoemission spectroscopy, and near-edge absorption fine-structure spectroscopy. Our findings indicate, unexpectedly, that a relatively strong inter-orbital interaction governs this vdW interface, similar to a weak chemisorption system rather than a physisorption system.

Quasi-supercontinuum source in the extreme ultraviolet using multiple frequency combs from high-harmonic generation.

A quasi-supercontinuum source in the extreme ultraviolet (XUV) is demonstrated using a table-top femtosecond laser and a tunable optical parametric amplifier (OPA) as a driver for high-harmonic generation (HHG). The harmonic radiation, which is usually a comb of odd multiples of the fundamental frequency, is generated by near-infrared (NIR) laser pulses from the OPA. A quasi-continuous XUV spectrum in the range of 30 to 100 eV is realized by averaging over multiple harmonic comb spectra with slightly different fundamental frequencies and thus different spectral spacing between the individual harmonics. The driving laser wavelength is swept automatically during an averaging time period. With a total photon flux of 4×109 photons/s in the range of 30 eV to 100 eV and 1×107photons/s in the range of 100 eV to 200 eV, the resulting quasi-supercontinuum XUV source is suited for applications such as XUV coherence tomography (XCT) or near-edge absorption fine structure spectroscopy (NEXAFS).

Spectroscopic Evidence of the Improvement of Reactive Iron Mineral Content in Red Soil by Long-Term Application of Swine Manure.

Mineral elements in soil solutions are thought to be the precursor of the formation of reactive minerals, which play an important role in global carbon (C) cycling. However, information regarding the regulation of mineral elements release in soil is scarce. Here, we examined the long-term (i.e., 23 yrs) effects of fertilisation practices on Fe minerals in a red soil in Southern China. The results from chemical analysis and Fourier-transform infrared spectroscopy showed that long-term swine manure (M) treatment released greater amounts of minerals into soil solutions than chemical fertilisers (NPK) treatment, and Fe played a dominant role in the preservation of dissolved organic C. Furthermore, Fe K-edge X-ray absorption near-edge fine structure spectroscopy demonstrated that reactive Fe minerals were mainly composed of less crystalline ferrihydrite in the M-treated soil and more crystalline goethite in the NPK-treated soil. In conclusion, this study reported spectroscopic evidence of the improvement of reactive Femineral content in the M-treated soil colloids when compared to NPK-treated soil colloids.

Electrocatalysis of heat-treated cobalt-porphyrin/carbon for hydrogen peroxide formation

The electrocatalysis of heat-treated Co-TPP/carbon for O2 reduction and H2O2 formation using a fuel cell reactor with an H2SO4 aqueous electrolyte was studied (TPP: 5,10,15,20-tetrakis(phenyl)-21H,23H-porphyrin). Carbon supports, including vapor grown carbon fiber (VGCF), activated carbon (AC), carbon black XC-72 (XC72), and carbon black Black-Pearl-2000/3500 (BP2000/3500), strongly affected the electrocatalysis of the Co-TPP/carbon. The Co-TPP/VGCF heat-treated at 1023K, abbreviated as Co-TPP/VGCF(1073), showed a good electrocatalytic activity for H2O2 formation (1.0moldm−3 with 40% current efficiency (CE)); however, other carbon supports were unfavorable. Electrocatalytic activities were Co-TPP/VGCF(1073)>>Co-TPP/XC72(1073)>Co-TPP/AC(1073)>Co-TPP/BP2000(1073). Their electrocatalysis were studied by rotating ring-disk electrode voltammetry and kinetic studies of the decomposition of H2O2 and the electroreduction of H2O2 to water. These studies indicated that the electrocatalytic activities for the successive reduction of H2O2 to water determined the final H2O2 concentrations and CEs. An active site of heat-treated Co-TPP/VGCF was characterized by X-ray photoelectron spectroscopy, and X-ray absorption near-edge fine structure spectroscopy. Partial pyrolysis of Co-TPP on the carbon surface was essential to generate the active site for the H2O2 formation. A model of the Co active site, CoN2–carbon, was proposed.

X-ray spectroscopy characterization of self-assembled monolayers of nitrile-substituted oligo(phenylene ethynylene)s with variable chain length.

Self-assembled monolayers (SAMs) of nitrile-substituted oligo(phenylene ethynylene) thiols (NC-OPEn) with a variable chain length n (n ranging from one to three structural units) on Au(111) were studied by synchrotron-based high-resolution X-ray photoelectron spectroscopy and near-edge absorption fine-structure spectroscopy. The experimental data suggest that the NC-OPEn molecules form well-defined SAMs on Au(111), with all the molecules bound to the substrate through the gold–thiolate anchor and the nitrile tail groups located at the SAM–ambient interface. The packing density in these SAMs was found to be close to that of alkanethiolate monolayers on Au(111), independent of the chain length. Similar behavior was found for the molecular inclination, with an average tilt angle of ~33–36° for all the target systems. In contrast, the average twist of the OPEn backbone (planar conformation) was found to depend on the molecular length, being close to 45° for the films comprising the short OPE chains and ~53.5° for the long chains. Analysis of the data suggests that the attachment of the nitrile moiety, which served as a spectroscopic marker group, to the OPEn backbone did not significantly affect the molecular orientation in the SAMs.

X-ray absorption and photoemission spectroscopy of zinc protoporphyrin adsorbed on rutile TiO2(110) prepared by in situ electrospray deposition

Zinc-protoporphyrin, adsorbed on the rutile TiO(2)(110) surface, has been studied using photoemission spectroscopy and near-edge absorption fine structure spectroscopy to deduce the nature of the molecule-surface bonding and the chemical environment of the central metal atom. To overcome the difficulties associated with sublimation of the porphyrin molecules, samples were prepared in situ using ultrahigh vacuum electrospray deposition, a technique which facilitates the deposition of nonvolatile and fragile molecules. Monolayers of Zn protoporphyrin are found to bond to the surface via the oxygen atoms of the deprotonated carboxyl groups. The molecules initially lie largely parallel to the surface, reorienting to an upright geometry as the coverage is increased up to a monolayer. For those molecules directly chemisorbed to the surface, the interaction is sufficiently strong to pull the central metal atom out of the molecule.

Orientation and Ordering in Monomolecular Films of Sulfur-Modified Homo-oligonucleotides on Gold

The chemical integrity, packing density, orientation, and ordering in monomolecular films of sulfur-modified single-stranded DNA (ssDNA) on Au{111} were probed by a combination of X-ray photoelectron spectroscopy, angle-resolved near-edge absorption fine structure spectroscopy (at all relevant absorption edges), and infrared reflection absorption spectroscopy. As model systems, short chain (five nucleotides) thymine- and adenine-based homo-oligonucleotides were used. All ssDNA moieties were found to be bound to the substrate by the thiolate anchor, with no physisorbed ssDNA species but some contamination present. The density of the ssDNA moieties in the thymine- and adenine-based films was estimated at ∼4.8 × 1013 and ∼5.9 × 1013 mol/cm−2, respectively. At the same time, the former films exhibited a higher degree of the orientational order as compared to the latter ones. The lack of correlation between the packing density and orientational order is noteworthy and is assumed to be a specific property of sulfur-modified ssDNA films. Such behavior can be related to a special character of inter- and intramolecular interaction in these systems along with the influence of direct interaction between the nucleobases and substrate.

Solution and Surface Composition Gradients via Microfluidic Confinement: Fabrication of a Statistical‐Copolymer‐Brush Composition Gradient

A stable solution composition gradient is generated inside a microchannel by varying the relative flow rates of two solutions that differ in composition. This solution gradient is used to synthesize a surface-grafted statistical-copolymer-brush composition gradient via surface-initiated atom transfer radical polymerization (ATRP; see schematic). Near-edge absorption fine structure spectroscopy confirms the continuous transition of surface composition from one end of the substrate to the other.

Self-Assembled Monolayers from Biphenyldithiol Derivatives: Optimization of the Deprotection Procedure and Effect of the Molecular Conformation

A series of biphenyl-derived dithiol (BDDT) compounds with terminal acetyl-protected sulfur groups and different structural arrangements of both phenyl rings have been synthesized and fully characterized. The different arrangements were achieved by introducing hydrocarbon substituents in the 2 and 2' positions of the biphenyl backbone. The presented model compounds enable the investigation of the correlation between the intramolecular conformation and other physical properties of interest, like, e.g., molecular assembly or electronic transport properties. Here, the ability of these model compounds to form self-assembled monolayers (SAMs) on Au(111) and Ag(111) is investigated in details. The deprotection of the target molecules was performed in situ using either NH4OH or triethylamine (TEA) deprotection agent. The fabricated films were characterized by synchrotron-based high-resolution photoelectron spectroscopy and near-edge absorption fine structure spectroscopy. Whereas the deprotection by NH4OH was found to result in the formation of multilayer films, the deprotection by TEA allowed the preparation of densely packed BDDT SAMs with a noticeably higher orientational order and smaller molecular inclination on Ag than on Au. Introduction of the alkyl bridge between the individual rings of the biphenyl backbone did not lead to a noticeable change in the structure and packing density of the BDDT SAMs as long as the molecule had a planar conformation in the respective SAM. The deviation from this conformation resulted in the deterioration of the film quality and a decrease of the orientational order.