1s X-ray Absorption Spectroscopy Research Articles

The origin of Ti 1s XANES main edge shifts and EXAFS oscillations in the energy storage materials Ti2CT x and Ti3C2T x MXenes

A potential application of two-dimensional (2D) MXenes, such as Ti2CT x and Ti3C2T x , is energy storage devices, such as supercapacitors, batteries, and hydride electrochemical cells, where intercalation of ions between the 2D layers is considered as a charge carrier. Electrochemical cycling investigations in combination with Ti 1s x-ray absorption spectroscopy have therefore been performed with the objective to study oxidation state changes during potential variations. In some of these studies Ti3C2T x has shown main edge shifts in the Ti 1s x-ray absorption near-edge structure. Here we show that these main edge shifts originate from the Ti 4p orbital involvement in the bonding between the surface Ti and the termination species at the fcc-sites. The study further shows that the t 2g –eg crystal field splitting (10Dq) observed in the pre-edge absorption region indicate weaker Ti–C bonds in Ti2CT x and Ti3C2T x compared to TiC and the corresponding MAX phases. The results from this study provide information necessary for improved electronic modeling and subsequently a better description of the materials properties of the MXenes. In general, potential applications, where surface interactions with intercalation elements are important processes, will benefit from the new knowledge presented.

Cu 2p-1s x-ray emission spectroscopy of mineral tetrahedrite Cu12Sb4S13

The electronic structure of mineral tetrahedrite Cu12Sb4S13 showing a metal-semiconductor transition (MST) at TMST = 85 K has been investigated by means of Cu 1s x-ray absorption spectroscopy with the partial fluorescence yield mode (PFY-XAS) and Cu 2p-1s x-ray emission spectroscopy (XES). Remarkable changes are detected in both the PFY-XAS and XES spectra on cooling across TMST. The changes in the spectra indicate that the unoccupied Cu 3d and 4p densities of states (DOS) shift away from the Fermi energy (EF). We conclude that the MST of Cu12Sb4S13 is associated with the decrease of the Cu 3d DOS at EF.

Probing covalency with oxidant K edge x-ray absorption spectroscopy of UF4 and UO2

There are some interesting discrepancies between what has been observed experimentally and the predictions of cluster theory. In particular, the F 1s x-ray absorption spectroscopy (XAS) of uranium tetrafluoride and the U 4d XAS branching ratio for oxidized uranium are two cases in point. These discrepancies may be driven by the overestimation of covalency in cluster calculations.

1s3p Resonant Inelastic X-ray Scattering of Cobalt Oxides and Sulfides

We are developing 1s3p resonant inelastic X-ray scattering (RIXS) as a versatile spectroscopic tool to probe active systems under working conditions. In order to establish detailed reference materials for cobalt-based systems, we have applied 1s X-ray absorption spectroscopy (XAS) and 1s3p RIXS to cobalt oxides (CoO, Co3O4) and sulfides (CoS and CoS2). We show that the Co3+ ions in Co3O4 contain a large nonlocal peak intensity in the pre-edge and that the resonant 3p final state of Co3O4 is dominated by the two exchange-split peaks of the tetrahedral Co2+ site. In contrast, the 3p final state of CoS and CoS2 shows a single asymmetric peak due to the large screening of the 3p-3d exchange interaction. While CoS has a single pre-edge peak, CoS2 contains an additional large nonlocal peak. This indicates that although CoS2 is often indicated as Co2+(S2)2–, it has a very strong sulfur-mediated cobalt–cobalt bonding, similar to the low-spin Co3+ sites in oxides. These electronic structure details reveal the significance of 1s3p RIXS spectroscopy for determining the symmetry, spin and covalence of transition metal compounds, information that is largely inaccessible in conventional K edge XAS. The experimental results are simulated with multiplet calculations that reveal the partial screening of the 3p3d exchange interaction and the presence of nonlocal transitions. We conclude that 1s3p RIXS combined with spectral simulations presents a valuable tool to unravel the electronic properties of functional materials under working conditions.

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.

Investigation of Unoccupied Electronic States near the Fermi Level of Polysilane using Resonant Auger Spectroscopy

Unoccupied electronic states near the Fermi level of poly(dimethylsilane) were probed using Si 1s X-ray absorption spectroscopy (XAS) and Si KL2,3L2,3 resonant Auger spectroscopy (RAS). The measured resonance peaks of XAS spectra near Si K-edge have been assigned in comparison with the discrete variational (DV)-Xα molecular orbital calculations. The rapid delocalization of Si 1s core-excited electron through the empty conduction band was observed along the polymer chain with the energy-dependent RAS measurement, and the electron delocalization time was estimated based on the core-hole clock method.

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.

Oxygen Adsorption Characteristics of Nitrogen-Doped Graphite Revealed By N 1s X-Ray Absorption Spectroscopy

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Probing carbon edge exposure of iron phthalocyanine-based oxygen reduction catalysts by soft X-ray absorption spectroscopy

Recently carbon-based catalysts have attracted much attention for its high ability to catalyze oxygen reduction reaction (ORR) in a cathode for polymer electrolyte fuel cells. Among active site models of the catalyst, edge region of sp2 carbon network is expected to demonstrate high ORR activity comparable to platinum catalysts. However, it is difficult to directly identify the chemical structure of the edge sites by applying conventional structural or electronic probes to actual catalysts. Here, we used C 1s X-ray absorption spectroscopy to observe electronic structure of carbon in iron phthalocyanine-based catalysts, and found a signature of edge exposure below the π* edge, whose intensity is well correlated with the ORR activity. Combined with information about the chemical structure of nitrogen, ORR activity of the catalyst is characterized in terms of the edge exposure.

Hydrogen bonding of water in 3-methylpyridine studied by O 1s X-ray emission and absorption spectroscopy

O 1s X-ray emission and X-ray absorption spectroscopy is applied to probe hydrogen bonding of water (D(2)O) in 3-methylpyridine. Owing to element selectivity of X-ray spectroscopies the electronic structure of water in the binary mixture was observed selectively. Based on the observed spectral changes associated with hydrogen bonding in O 1s X-ray emission and X-ray absorption spectra, we have investigated the hydrogen bond of the mixture sample over a wide range of D(2)O concentrations (X(D(2)O) = 0.02-1.0) at room temperature under atmospheric pressure.

Size effect on the conduction band orbital character of anatase TiO2 nanocrystals

O 1s x-ray absorption spectroscopy and first principle calculation have been used to probe the size effect on the orbital character and hybridization of the conduction band of anatase TiO2 nanocrystals over two orders of magnitude in diameter (2-200 nm). The appearance and predominance of unoccupied delocalized states derived from the hybridization of antibonding O 2p and Ti 4s rather than 3d is observed when the nanoparticle size approaches the exciton radius. These results provide an experimental evidence of quantum size effect on unoccupied states in anatase TiO2 nanocrystals.

Structure and Electron States of Co-phthalocyanine Interacting With the Cu(111) Surface

We studied the growth and the electronic properties of Co-phthalocyanine (CoPc) molecular layers on Cu(111). Scanning tunnelling microscopy shows that the CoPc molecules adsorb first at the Cu(111) step edges, lying “flat”, i.e., parallel to the substrate. At monolayer coverage, the molecules distribute uniformly on the Cu(111) surface with short-range ordering. Angle-dependent N 1s X-ray absorption spectroscopy (XAS) confirms the overall flat orientation. C 1s, N 1s, core-level photoemission, and Co L3 edge XAS indicate selective participation of these atoms to the interface bonds. Angle-resolved photoemission, for submonolayer coverage, shows that the Cu(111) surface states undergo a shift to lower binding energies. This binding energy shift and the increase in the effective mass indicate charge redistribution at the CoPc/Cu(111) interface. At monolayer saturation, the Cu(111) surface state is quenched, and new molecular states appear at binding energies of 0.9 eV.

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).

Direct Interaction of Water Ice with Hydrophobic Methyl-Terminated Si(111)

The effect of ice adsorption on the electronic structure of hydrophobic methyl-terminated Si(111) surface was studied by C 1s X-ray absorption spectroscopy. The transition to the LUMO orbital of the methyl group is symmetry-forbidden according to the dipole selection rule, which governs the X-ray absorption process. Upon ice adsorption, the LUMO peak intensity is enhanced dramatically, indicating that the ice layers directly perturb the excited state orbitals of the methyl groups, increasing the asymmetric character. This is unexpected since the hydrophobic nature of the surface is confirmed by the three-dimensional growth of ice layers. We propose that the interaction between the hydrophobic methyl surface and adsorbed ice results in the formation of weak hydrogen bonds.

Spectroscopic Identification of a Hydrogen Peroxide-Like Intermediate Formed after Molecular Oxygen Adsorption on Hydrogen Rich Pt(111)

Using polarization dependent O 1s x-ray absorption spectroscopy we identify a hydrogen peroxide-like intermediate on the surface of hydrogenated Pt(111) after exposure of a saturated amount of molecular oxygen. The x-ray absorption spectroscopy investigation further reveals that the O-O molecular axis of the hydrogen peroxide-like intermediate is close to parallel and the O-H group is perpendicular to the surface. The O-O bond length is determined to be 1.43 Aå.

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.

Investigation of Jahn–Teller splitting with O 1s x-ray absorption spectroscopy in strained Nd1−xCaxMnO3 thin films

Electronic structures of strained Nd1−xCaxMnO3 (NCMO) thin films with x=0 to 0.8 are investigated via x-ray absorption spectroscopy (XAS). The obtained O 1s spectra within the photon energy 529–535 eV can be decomposed into eg↑1, eg↑2, t2g↓, and eg↓ bands. Based on the assigned energy levels of these band states, the energies of magnetic exchange, crystal field and Jahn–Teller (JT) splitting are determined. Particularly, the JT splitting is around 0.8 eV, which is observed with O 1s XAS for the first time in NCMO thin films.

Peroxide-like intermediate observed at hydrogen rich condition on Pt(111) after interaction with oxygen

Hydrogen peroxide has been found to form as a byproduct during fuel cell operation, which is linked to the degradation of the ion exchange membrane resulting in a shorter lifetime of the fuel cell. Using O 1s X-ray photoelectron spectroscopy and X-ray absorption spectroscopy we investigated the molecular oxygen adsorption on hydrogenated Pt(111) and found that adsorbed hydrogen promotes the formation of a hydrogen peroxide-like intermediate. Polarization dependent X-ray absorption spectroscopy reveals that the O-O molecular axis is close to parallel and the OH group is perpendicular to the surface.

Spectroscopic investigation on the electronic structure of a 5d band insulator [formula omitted] in proximity to ferroelectric instability: Comparison with [formula omitted] and [formula omitted

We investigated the high-energy electronic structure of a 5d perovskite SrHfO 3 by using optical spectroscopy and O 1s x-ray absorption spectroscopy. From the combined spectra the values of electronic structure parameters are estimated properly. In particular, the crystal field splitting energy, which is closely associated with the p–d hybridization strength, is as high as ∼5 eV, and the Sr 4d bands appear to be strongly mixed with the Hf 5d bands. These findings are discussed in relation to a possible ferroelectric instability in SrHfO 3, and are compared with electronic properties of similar compounds, 3d SrTiO 3 and 4d SrZrO 3.