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

Towards Understanding Excited-State Properties of Organic Molecules Using Time-Resolved Soft X-ray Absorption Spectroscopy.

The extension of the pump-probe approach known from UV/VIS spectroscopy to very short wavelengths together with advanced simulation techniques allows a detailed analysis of excited-state dynamics in organic molecules or biomolecular structures on a nanosecond to femtosecond time level. Optical pump soft X-ray probe spectroscopy is a relatively new approach to detect and characterize optically dark states in organic molecules, exciton dynamics or transient ligand-to-metal charge transfer states. In this paper, we describe two experimental setups for transient soft X-ray absorption spectroscopy based on an LPP emitting picosecond and sub-nanosecond soft X-ray pulses in the photon energy range between 50 and 1500 eV. We apply these setups for near-edge X-ray absorption fine structure (NEXAFS) investigations of thin films of a metal-free porphyrin, an aggregate forming carbocyanine and a nickel oxide molecule. NEXAFS investigations have been carried out at the carbon, nitrogen and oxygen K-edge as well as on the Ni L-edge. From time-resolved NEXAFS carbon, K-edge measurements of the metal-free porphyrin first insights into a long-lived trap state are gained. Our findings are discussed and compared with density functional theory calculations.

Beyond Nitrogen in the Oxygen Reduction Reaction on Nitrogen-Doped Carbons: A NEXAFS Investigation.

Polymer electrolyte membrane fuel cells require cheap and active electrocatalysts to drive the oxygen reduction reaction. Nitrogen-doped carbons have been extensively studied regarding their oxygen reduction reaction. The work at hand looks beyond the nitrogen chemistry and brings to light the role of oxygen. Nitrogen-doped nanocarbons were obtained by a radio-frequency plasma route at 0, 100, 250, and 350 W. The lateral size of the graphitic domain, determined from Raman spectroscopy, showed that the nitrogen plasma treatment decreased the crystallite size. Synchrotron radiation photoelectron spectroscopy showed a similar nitrogen chemistry, albeit the nitrogen concentration increased with the plasma power. Lateral crystallite size and several nitrogen moieties were plotted against the onset potential determined from oxygen reduction reaction curves. There was no correlation between the electrochemical activity and the sample structure, as determine from Raman and synchrotron radiation photoelectron spectroscopy. Near-edge X-ray absorption fine structure (NEXAFS) was performed to unravel the carbon and nitrogen local structure. A difference analysis of the NEXAFS spectra showed that the oxygen surrounding the pyridinic nitrogen was critical in achieving high onset potentials. The work shows that there were more factors at play, other than carbon organization and nitrogen chemistry.

Alteration of Biochar Carbon Chemistry during Soil Incubations: SR‐FTIR and NEXAFS Investigation

Near‐edge X‐ray absorption fine structure (NEXAFS) spectroscopy and synchrotron radiation based Fourier‐transform infrared (SR‐FTIR) microspectroscopy were utilized to systematically study the aging of three biochars under similar controlled conditions by tracking changes in the C chemistry of biochar in the presence and absence of a Typic Xerorthent soil. By utilizing both NEXAFS and SR‐FTIR, differences in the initial biochar C functional group composition due to feedstock (aromatic C was greater in walnut shell biochar than softwood feedstock) and pyrolysis temperature (no long‐range‐ordered C in wood feedstock made at 410°C compared with 510°C) were confirmed. The data provided spectroscopic evidence corroborating both the conceptual biphasic model for biochar degradation and the power model of organic matter continuum mass loss as biochar ages due to a more labile aliphatic biochar portion and an aromatic portion that is oxidized more slowly. Incubations in the presence and absence of soil revealed a decrease in the ratio of the 287.6 eV peak (aliphatic C) relative to the 285.5 eV peak (aromatic C) during the incubations. Binding through functional groups present on the aged biochar surfaces (e.g., quinones, phenols, carbonyls) as well as the physical protection of the biochar by the soil appears to retard biochar surface decomposition. This study provides high‐resolution spectroscopic data on discrete points on the biochar in addition to interactions between the soil and the biochar under conditions of minimal sample disturbance and destruction that corroborates current biochar stability and turnover models.

Near-Edge X-Ray Absorption Fine-Structure Investigation of Graphene

We report the near-edge x-ray absorption fine-structure (NEXAFS) spectrum of a single layer of graphite (graphene) obtained by micromechanical cleavage of highly ordered pyrolytic graphite on a SiO2 substrate. We utilized a photoemission electron microscope to separately study single-, double-, and few-layers graphene samples. In single-layer graphene we observe a splitting of the pi resonance and a clear signature of the predicted interlayer state. The NEXAFS data illustrate the rapid evolution of the electronic structure with the increased number of layers.

Speciation of deeply buriedTiOxnanolayers with grazing-incidence x-ray fluorescence combined with a near-edge x-ray absorption fine-structure investigation

Nondestructive methods based on electron emission may encounter serious difficulties when probing the chemical state of deeply buried nanolayers due to restricted information depth. The purpose of the present work is to evaluate to which extent photon emission can overcome these restrictions. Grazing-incidence x-ray fluorescence combined with a near-edge x-ray absorption fine-structure investigation (GIXRF-NEXAFS) offers access to depth-resolving analysis of buried nanolayers with respect to both the chemical speciation and the layer composition. By varying the angle of incidence, the penetration depth can be tuned from a few to several hundreds of nanometers. The information depth of the emitted fluorescence radiation is in the same general range as the soft x-ray regime. Initial measurements were performed on nominally 30 nm thick titanium nanolayers oxidized to different extents and buried below 5 nm carbon layers. These layered structures were produced by means of ion beam sputtering deposition. The plane grating monochromator beamline for undulator radiation in the laboratory of the Physikalisch-Technische Bundesanstalt at the electron storage ring BESSY II provides tunable radiation of both well-known flux and high spectral purity for GIXRF-NEXAFS studies. The current results confirm that GIXRF-NEXAFS has the potential to substantially contribute to the speciation of deeply buried nanolayers. The analysis of measurements at a constant incident angle demonstrated that it is not possible to find an angle of incidence for the NEXAFS region to ensure a stable penetration depth. However, appropriate angular corrections can ensure a constant mean penetration depth, in particular, in the vicinity of absorption edges.

Self-assembling peptides: A combined XPS and NEXAFS investigation on the structure of two dipeptides Ala–Glu, Ala–Lys

The two dipeptides AE ( Lalanine – Lglutamic acid) and AK ( Lalanine – Llysine), that constitute the “building blocks” of the 16-unit self-complementary amphiphilic oligopeptide EAK16, have been investigated by XPS (X-ray photoelectron spectroscopy) and NEXAFS (near-edge X-ray absorption fine structure) spectroscopy. Thin films of both dipeptides on TiO 2, a distinguished biocompatible surface, were prepared by incubation from aqueous solutions. Thick films of dipeptides on inert Au substrates were also studied for comparison. The chemical structure and composition were investigated by XPS spectroscopy; furthermore, molecular orientation of dipeptides on TiO 2 was checked by angular dependent NEXAFS measurements at both C–K and N–K edges. In order to yield some insight on adsorption geometry and molecular orientation MD (molecular dynamic) simulations were also carried out. The performed molecular and electronic characterization of AE and AK provides an excellent model for the interpretation of more complex peptide spectra.

Electronic structure of platinum complex/Zn-porphyrinato assembled macrosystems, related precursors and model molecules, as probed by X-ray absorption spectroscopy (NEXAFS): theory and experiment

Macromolecular arrays containing Zn-diethynylporphyrin coordinated to platinum tethers were studied using near-edge X-ray absorption fine structure (NEXAFS) spectroscopy at the N K-edge. Porphin (Pf), 2,8,12,18-tetraethyl-5,15-diethynyl-3,7,13,17-tetramethylporphyrin (PfNH) and 2,8,12,18-tetraethyl-5,15-diethynyl-3,7,13,17-tetramethylporphyrinatozinc(II) (Zn-Pf2), have been taken as experimental models. The resonances assignment is supported by static-exchange (STEX) calculations for the porphin (Pf) and Zn-porphin (Zn-Pf1) molecules. By calculation, four π* resonances occur for porphin (Pf) and two for Zn-Pf1. Experimentally we detect four resonances for porphin and PfNH, and two for the Zn-complexed ones. They can be assigned to core excitations to a couple of energy split π* orbitals for each not equivalent N atom. Our calculations justify the observed low π* intensity as due to an extended delocalization of the π* orbitals. The NEXAFS investigations on platinum complex/Zn-porphyrinato assembled macrosystems (i.e. Pt-ZnPf1, Pt-ZnPf2 and Pt-ZnPf3) show the influence of the Pt tethers bound to the porphyrin through the ethynyl moiety on the electronic structure of the tetrapyrrolic cycle.

NEXAFS investigations on ordered adsorbate layers of large aromatic molecules

It is a scientific challenge to understand and control the adsorption of large organic molecules on single crystal substrates on the same level as that of small molecules or atoms. This is discussed on the basis of one of the most suitable techniques for this purpose, near edge X-ray absorption fine structure (NEXAFS). As examples we present data for highly symmetric and flat aromates without (perylene) or with reactive groups (PTCDA, PTCDI, NDCA) on various single crystal surfaces such as Ag(111), Ni(111) and Si(111). Preparation parameters such as coverage, temperature and preadsorbate have been varied to study the bonding and molecular orientation on the surface together with the layer growth behaviour. In most cases the experimental comparison between different molecules and simple calculations help for the interpretation of the complicated data but ad hoc Xα-SW calculations which have been carried out for the largest molecule presently possible (NDCA) are required for a more detailed understanding of the rich spectral features.

Direct differentiation of surface and bulk compositions of powder catalysts: application of electron-yield and fluorescence-yield NEXAFS to LixNi1−xO

We have applied near-edge X-ray absorption fine structure (NEXAFS) to characterize the surface and bulk properties of LixNi1−xO catalysts. In our experimental set-up, NEXAFS spectra of powder materials could be obtained by measuring the intensity of either electron-yield or fluorescence-yield. While the electron-yield method is sensitive only to the top few atomic layers, the fluorescence-yield method can detect species up to a few thousands angstroms deep into the bulk structure. The ability to distinguish surface and bulk compositions is demonstrated in studies of a number of Li0.5Ni0.5O samples, of which the surface compositions vary as a function of preparation procedures. In addition, NEXAFS investigations following the reaction of LixNi1−xO with CH4 have also been carried out and the results indicate that the initial surface reaction intermediates are Li2CO3.