1s X-ray Absorption Spectroscopy Spectra Research Articles

Soft X-ray absorption and photoemission spectroscopy study of semiconductor oxide nanoparticles for dye-sensitized solar cell: ZnSnO3 and Zn2SnO4

Abstract The electronic structures of the Zn-stannate nanoparticles of ZnSnO3 and Zn2SnO4, which are the potential nano-structured semiconductor oxides for a dye sensitized solar cell (DSSC), have been investigated by employing photoemission spectroscopy (PES) and soft X-ray absorption spectroscopy (XAS), and compared to those of reference materials. The divalent and tetravalent valence states of Zn2+ and Sn4+ ions are confirmed experimentally. The energy levels of both the valence-band and conduction-band edges are determined experimentally. The top of the valence band in PES is slightly higher in Zn2SnO4 than in ZnSnO3. The onset energies of the O 1s XAS spectra of the Zn-stannates are found to be similar to each other, but higher than that of TiO2. The O 1 s XAS spectrum of ZnSnO3 exhibits the higher unoccupied density of states near the bottom of the conduction band than those of Zn2SnO4, SnO2 and ZnO, reflecting the larger number of holes in the Zn 3 d bands of ZnSnO3. Hence, the easier electron transfer is expected from the LUMO (lowest unoccupied molecular orbital) of a dye molecule to the conduction band of ZnSnO3 nanoparticles on the transparent conductive electrode of a DSSC.

Valence states and electronic structures of Co and Mn substituted spin gapless semiconductor PbPdO2

Electronic structures of Pb(Pd0.9T0.1)O2 (T = Mn, Co) spin gapless semiconductors have been investigated by employing soft X-ray absorption spectroscopy (XAS) and photoemission spectroscopy (PES). The valence states of Co and Mn ions are found to be mixed-valent (∼2.7) and tetravalent, respectively. The measured valence-band PES and O 1s XAS spectra show that both PbPdO2 and PbPd0.9Co0.1O2 are small-gap semiconductors. This finding is supported by the calculated band structures, obtained in the density functional theory with the modified Becke-Johnson potential (mBJ) scheme. This work also shows evidence for the existence of the phase separation in Mn-substituted PbPd0.9Mn0.1O2.

High resolution photoemission and x-ray absorption spectroscopy of a lepidocrocite-like TiO2 nanosheet on Pt(110) (1 × 2)

The electronic structure of TiO(2) nanosheets on the Pt(110)-(1 × 2) surface has been investigated by using high resolution photoemission spectroscopy and x-ray absorption spectroscopy (XAS). The Ti 2p XAS spectra of the deposited TiO(2) films have been theoretically evaluated and, from the comparison with the experimental data, the assignment to a lepidocrocite-like structure is confirmed. Coexistence of TiO(2) islands with PtO(2) stripes for incomplete nanosheets is confirmed by high resolution photoemission data. The location of the valence and conduction band edges of the nanosheet has been experimentally determined allowing us to describe in details subtle electronic effects due to the interface with the substrate. The locations of the valence band maximum and the leading peak in the O 1s XAS spectrum indicate a band gap similar to anatase but with the Fermi level closer to mid-gap than found for bulk, n-type TiO(2).

Probing the conduction band edge of transition metal oxides by X-ray absorption spectroscopy

We present a comprehensive picture of how X-ray absorption spectroscopy (XAS) at the O 1s edge, supported by ab initio calculations, can be used to address the electronic properties of the conduction band edges of transition metal oxides. The compounds studied in order to illustrate the method are two of the most versatile transition metal oxides, ZrO 2 and TiO 2. Special attention is paid to the subtler aspects of the approach, discussing in more detail the kind of information provided and also possible shortcomings and complications. It is shown that the interpretation of the relationship between the Fermi level-referenced O 1s PES peak and the O 1s XAS spectrum can change depending on the electronic properties of the material under study. In order to fully understand the PES–XAS relationship, supporting information from calculations is essential.

Soft x-ray synchrotron radiation spectroscopy study of SrMn1−xRuxO3 perovskites (≤x≤1)

The electronic structures of SrMn1−xRuxO3 (0≤x≤1) have been investigated by employing soft x-ray absorption spectroscopy (XAS). The measured Mn 2p XAS spectra for SrMn1−xRuxO3 show that the valence states of Mn ions change monotonically from Mn4+ for x=0 to ∼Mn3+ for x=0.8, suggesting that the valence states of the substituted Ru ions are higher than 4+. The Ru 3p XAS spectra of SrMn1−xRuxO3 show the existence of pentavalent Ru5+ ions for the low x value. The measured O 1s XAS spectra also support these findings.

X-ray absorption analysis of nitrogen contribution to oxygen reduction reaction in carbon alloy cathode catalysts for polymer electrolyte fuel cells

The electronic structure of nitrogen introduced into various carbon-based cathode catalysts for the polymer electrolyte fuel cell (PEFC) is investigated using X-ray absorption spectroscopy (XAS). The profile of π* peaks at the pre-edge of N 1s XAS spectra is used to determine the chemical state of nitrogen, which can be an indicator of oxygen reduction reaction (ORR) activity. It is found that catalysts with a relatively larger amount of graphite-like nitrogen exhibit a higher ORR activity than those with a relatively larger amount of pyridine-like nitrogen. We propose that effective doping with graphite-like nitrogen is a practical guideline for the synthesis of active carbon alloy catalysts.

Electronic structure of La1−xSrxScO3 probed by soft-x-ray absorption spectroscopy

The electronic structure of La1−xSrxScO3 single crystal has been studied by x-ray absorption spectroscopy (XAS). The conduction band is mainly composed of Sc 3d state hybridized with O 2p state. The O 1s XAS spectrum exhibits the existence of the Sc 3d-defect-induced state under the Sc 3d conduction band. The intensity of Sc 3d-defect-induced state increases with Sr substitution. The energy separation between the top of the valence band and the bottom of the Sc 3d-induced unoccupied state is in accordance with the energy gap that is expected from optical absorption.

Interface electronic states and molecular structure of a triarylamine based hole conductor on rutile TiO2(110)

The molecular and electronic surface structure of a triarylamine based hole-conductor (HC) molecule evaporated onto rutile TiO2(110) single crystal is investigated by means of synchrotron light based photoelectron spectroscopy and x-ray absorption spectroscopy in combination with calculations based on density functional theory. Different amounts of the HC molecule was evaporated spanning the monolayer to multilayer region. The molecular surface structure is investigated and the results indicate that no specific covalent chemical bonding is formed and that the plane formed by the different nitrogens in the HC molecules has a rather small angle versus the TiO2 substrate surface plane. Some molecular ordering also persists in the multilayer region. The experimental core level spectra, valence level spectra, and the N 1s x-ray absorption spectroscopy spectra are well modeled by calculations on an individual molecule. Interestingly, the formation of the TiO2HC interface results in significant binding energy shifts in core levels and valence levels shifting all peaks of a the HC material to the same extent. Smaller shifts were also observed in the substrate core level peaks. The shift is discussed in terms of nanoscale energy level bending and final state hole screening. With respect to electronic applications, specifically in a solid state dye-sensitized solar cell, it is argued that the observed energy level alignment at the TiO2HC interface can act as a hole trap.

Band alignment at the ZrO2∕Si(100) interface studied by photoelectron and x-ray absorption spectroscopy

We present measurements of the Zr and Si core level photoelectron binding energies relative to the Fermi level and the vacuum level under a ZrO2 growth series on Si(100). It is shown that the Zr core level binding energy is most properly referenced to the local vacuum level already from the monolayer regime. This confirms the insulating properties of ZrO2. The Si core levels are referenced to the Fermi level and undergo shifts consistent with the disappearance of the mid-band-gap states originating from the (2×1) reconstruction on the clean Si(100) surface. The use of O 1s x-ray absorption spectroscopy (XAS) to determine the location of the conduction band edge of ZrO2 is discussed with the aid of ab initio calculations. It is demonstrated that the conduction band edge is located at the XAS peak position and that the position relative to the valence band can be determined by aligning the O 1s XAS spectrum to the O 1s photoelectron spectrum. The study thus establishes that photoelectron spectroscopy in conjunction with x-ray absorption spectroscopy forms a most powerful tool for studies of the band alignment at metal oxide–silicon interfaces.

Study of the growth of NiO on highly oriented pyrolytic graphite by X-ray absorption spectroscopy

In this work, we present a X-ray absorption spectroscopy (XAS) study of the growth of NiO on highly oriented pyrolytic graphite (HOPG). NiO has been grown by reactive evaporation of metallic Ni in an oxygen atmosphere (2 × 10 −5 Torr) at room temperature. We paid special attention to the study of the early stages of growth. Both, Ni 2p and O 1s core-level XAS spectra were measured. For large NiO coverages, the spectra resemble that of a NiO single crystal, thus indicating the formation of a stoichiometric NiO thin film on the HOPG substrate. The Ni 2p XAS spectra remain similar during the whole growth process, indicating that Ni atoms are present in the high spin Ni 2+ form, as supported by multiplet calculations. In contrast, for low coverages the line-shape of the O 1s XAS spectra differ strongly from that of bulk NiO. Cluster calculations of the spectra in octahedral and pyramidal symmetries support the formation of nanometric planar NiO islands at the graphite steps as previously observed by atomic force microscopy (AFM) in this system.

Combined X-ray Absorption Spectroscopy and Density Functional Theory Examination of Ferrocene-Labeled Peptides

A combination of soft X-ray absorption spectroscopy (XAS) measurements and StoBe density functional theory (DFT) calculations has been used to study the electronic structures of the ferrocene-labeled peptides Fc-Pro(n)-OBz (n = 1-4). Excellent agreement between the measured and the simulated data is observed in all cases, and the origin of all major spectral features was assigned. The breaking of the degeneracy of the ferrocene 3e(2u)-like unoccupied molecular orbital under the influence of a substituent attached to a Cp ring was observed experimentally. The influence of the bonding environment on the O 1s and N 1s XAS spectra was examined. A corrected assignment of one of the major features in the Fe 2p XAS spectra of ferrocene is proposed and supported by the DFT simulations, as well as the measured spectra.

Soft X-ray spectroscopy of nucleobases, B-DNA and ferrocene–proline conjugates

Soft X-ray absorption spectroscopy (XAS) and soft X-ray emission spectroscopy (XES) were used to study B-DNA and its nucleobases adenine (A), guanine (G), cytosine (C) and thymine (T), as well as ferrocene carboxylic acid, ferrocenoyldiproline benzyl ester (Fc-Pro 2-OBz), and ferrocenoyltriproline benzyl ester (Fc-Pro 3-OBz). Weighted summations of the C 1s and N 1s spectra of the bases are compared to those of B-DNA, and the influence of buffer solutions on the B-DNA spectra is examined. The C 1s XAS spectra of the ferrocene compounds are compared, and sample degradation is shown through comparison of C 1s XAS spectra measured at different times.

XPS and XAS investigation of condensed and adsorbed n-octane on a Cu(110) surface

The electronic structure of n-octane adsorbed on Cu(110) is studied by using X-ray absorption spectroscopy (XAS) and X-ray photoelectron spectroscopy (XPS) in combination with cluster model calculations in the framework of density functional theory (DFT). The molecule is found to be well oriented on the surface, which is seen from the high degree of XAS dichroism. Saturated hydrocarbons are commonly considered to physisorb on metals such as Cu(110), but still the C 1s XAS spectra reveal large changes in the electronic structure of the adsorbed octane relative to the free molecule. We find that the XAS resonances corresponding to the molecular Rydberg-valence states are strongly quenched upon adsorption and that there is a significant hybridization of the molecular valence orbitals with the metal bands. In addition to a precise interpretation of the XAS spectra, we present details on the molecular orbital structure of the adsorbed octane molecule. We also discuss shifts in the relative binding energies of the chemically inequivalent carbon atoms in octane upon adsorption, which lead to a narrower XPS spectrum for the adsorbate than the condensed phase spectrum due to the existence of a new relaxation channel.

Electronic structure and temperature-induced paramagnetism inLaCoO3

We have studied the electronic structure of ${\mathrm{LaCoO}}_{3}$ by photoemission spectroscopy and x-ray absorption spectroscopy (XAS). The Co 2p core-level and valence-band photoemission spectra display satellite structures indicating a strong electron-correlation effect. The Co 2p core-level photoemission, the valence-band photoemission, and the O 1s XAS spectra have been analyzed using a configuration-interaction cluster model for the initial-state configurations of the low-spin (LS: $^{1}\mathrm{A}_{1}$), intermediate-spin (IS: $^{3}\mathrm{T}_{1}$) and high-spin (HS: $^{5}\mathrm{T}_{2}$) states and their mixtures. The ground state of ${\mathrm{LaCoO}}_{3}$ in the LS state is found to have heavily mixed ${\mathrm{d}}^{6}$ and ${\mathrm{d}}^{7}$L- character, reflecting the strong covalency. The magnetic susceptibility has been analyzed for various level orderings of the LS, IS, and HS states. From the analyses of the photoemission spectra and the magnetic susceptibility data, the temperature-induced paramagnetism in ${\mathrm{LaCoO}}_{3}$ above \ensuremath{\sim}90 K is most likely due to a gradual LS-to-IS transition.

A photoelectron spectroscopy and x-ray absorption study of single crystal with adsorbed Cs: on the origin of the states affected by electron doping and evidence for spatially resolved electron doping

The influence of electron doping, via deposition of small amounts of Cs, on the electronic structure of has been studied by high-resolution photoelectron spectroscopy (PES) and x-ray absorption spectroscopy (XAS), utilizing synchrotron radiation. The changes in the electronic structure were monitored by PES of the valence band and of the O 1s, Bi 4f, Bi 5d, Ca 2p and Sr 3d core levels, and by XAS at the O 1s, Cu 2p and Ca 2p edges. The experimental data suggest that the loss of the Fermi edge and the loss of spectral intensity down to about 2 eV below the Fermi level, and the substantial loss of spectral intensity of the pre-edge structure in the O 1s XAS spectrum are mainly due to annihilation of states with O 2p character in the Cu - O layer. It is evident from bulk- and surface-sensitive XAS spectra that the electron doping by Cs affects the electronic structure more strongly close to the surface. This implies that the doping occurs locally and that the charge transfer between the different layers in the unit cell is not uniform. Thus it seems possible to alter the electronic properties of locally through spatially resolved electron doping. When larger amounts of Cs are deposited, a chemical reaction occurs which causes a disruption of the Bi - O layer. This deposition regime is characterized by the presence of caesium oxide and reduced (metallic) Bi.

The interaction of nitrogen with titanium studied by soft X-ray absorption spectroscopy: adsorption versus implantation

The interaction of nitrogen with titanium has been studied by means of N 1s X-ray absorption spectroscopy (XAS). It is shown that N 1s XAS spectra provide useful chemical information on this system. The spectra of nitrogen adsorbed on Ti indicate strong Ti3dN2p and Ti4SPN2p interactions. The Ti3d states are split by crystal field effects and are strongly hybridized to N2p orbitais. In comparison with a N 1s XAS spectrum from a reference TiN sample the spectra indicate a clear chemical similarity between the nitrogen chemisorbed layer on Ti and TiN bulk. In the case of the spectra of N + implanted in Ti, similar features to the above, at low primary beam energies (250 eV), are observed whereas at higher energies (above 1 keV) the spectra show a sharp quasi-atomic absorption peak which is attributed to weakly bonded N atoms in the bulk. The spectra also reflect disorder in the implanted layer and the two-dimensional character of the chemisorbed layer.