2p X-ray Photoemission Spectroscopy Research Articles

Accelerating the Activation of NO x − on Ru Nanoparticles for Ammonia Production by Tuning Their Electron Deficiency

Accelerating the Activation of NO _x ⁻ on Ru Nanoparticles for Ammonia Production by Tuning Their Electron Deficiency

V 2p core-level spectroscopy of V2+/V3+ mixed valence AV10O15 (A = Ba, Sr) and Ba0.9Sr0.1V13O18

We have studied the surface electronic structure of AV10O15 (A = Ba, Sr) and Ba0.9Sr0.1V13O18 by using x-ray photoemission spectroscopy (XPS). The V 2p XPS shows multiple peaks associated with the charge fluctuation in the system. The relative intensity of each valence component can be extracted by Gaussian and Voigt fittings. The result strongly depends on the number of Gaussians/Voigts. In BaV10O15 and Ba0.9Sr0.1V13O18, the surface state has more V3+ than the bulk. In addition to the presence of surface V3+, the V2.5+ in AV10O15 (A = Ba, Sr) and V2+ in Ba0.9Sr0.1V13O18 systems are observed even in the surface sensitive XPS. The number of Gaussians/Voigts should be as large as four for the V 2p3/2, in order to obtain the results consistent with the bulk sensitive HAXPES study.

Surface chemical reactions induced on pyrite by ion bombardment

Through X-ray photoemission spectroscopy (XPS), we studied the chemical changes induced in a natural crystal of pyrite (FeS2) upon exposure to 4.5 keV He+ beam. We found an important reducing effect induced by ion bombardment leading to the production of iron embedded in the pyrite matrix. Through a combination of the usual Doniach–Sunjic treatment and Factor Analysis of XPS yields, we were able of analyzing the full Fe 2p XPS signal. We could in this way distinguish Fe compounds with the same binding energy for the Fe 2p3/2 yield.Our results show that He+ bombardment disrupts the ionic environment producing S2−2 and S0, Fe2+ and Fe3+ ions, and the reduction to metallic iron. The remaining pyrite matrix does not passivate the embedded iron structures, which are readily oxidized under air exposure. The oxide formed resembled that of magnetite from the XPS point of view. Further He+ bombardment proved to be efficient to reduce the iron oxide back to iron again.

Metal–Semiconductor Transition Concomitant with a Structural Transformation in Tetrahedrite Cu12Sb4S13

The tetrahedrite Cu12Sb4S13 undergoes a metal–semiconductor transition (MST) at TMST = 85 K, whose mechanism remains elusive. Our Cu 2p X-ray photoemission spectroscopy study revealed the monovalent state of Cu ions occupying the two sites in this compound. This fact excludes the possibilities of previously proposed antiferromagnetic order and Jahn–Teller instability inherent in a divalent Cu system. A synchrotron X-ray diffraction study has revealed that the body-centered cubic cell of Cu12Sb4S13 transforms into a body-centered 2a × 2a × 2c tetragonal supercell below TMST, where the cell volume per formula unit expands by 0.25%. We have further studied pressure effects on the MST as well as the effects of the substitution of As for Sb. The application of pressure above 1 GPa completely inhibits the MST and leads to a metallic state, suggesting that the low-temperature structure with a larger volume becomes unstable under pressure. The As substitution also reduces the volume and suppresses the MST but the...

염료감응 태양전지 전극용 반도체 나노 물질의 광전자분광 연구

The electronic structures of the potential candidate semiconductor nanoparticles for dye-sensitized solar cell (DSSC), such as and , have been investigated by employing X-ray photoemission spectroscopy (XPS). The measured X-ray diffraction patterns show that and samples have the single-phase ilmenite-type structure and the inverse spinel structure, respectively. The measured Zn 2p and Sn 3d core-level XPS spectra reveal that the valence states of Zn and Sn ions are divalent (Zn 2+) and tetravalent (Sn 4+), respectively, in both and . On the other hand, the shallow core-level measurements show that the binding energies of Sn 4d and Zn 3d core levels in are lower than those in . This work provides the information on the valence states of Zn and Sn ions and their chemical bonding in and .

Large increase in the spin entropy of thermoelectric Ca3Co4O9+δ induced by Ni and Ce co-doping

The effects of Ni and Ce co-doping on the spin entropy in Ca3Co4O9+δ have been carefully studied. It is found that Ni and Ce co-doping is more effective than single rare earth element doping in improving the spin entropy of Ca3Co4O9+δ. The largest magnetothermopower ratio value of 70 % is achieved for Ca2.8Ce0.2Co3.9Ni0.1O9+δ, which is at least two times larger than that of the reported Ca2.5Ce0.5Co4O9+δ, Ca2.8Lu0.2Co4O9+δ, and Ca2.4Gd0.6Co4O9+δ materials. This enhancement proves the remarkable increase in the spin entropy of Ni and Ce co-doped Ca3Co4O9+δ. Co 2p X-ray photoemission spectroscopy shows the decrease in Co4+ concentration induced by Ni and Ce co-doping. The reduction in Co4+ concentration is suggested to result in the enhanced spin entropy from Co site. And coexistence of Ni2+ and Ni3+ provides a new hopping model for transporting spin entropy, contributing to the increase of the total spin entropy. The joint contributions from Co4+ concentration decrease and the new hopping model, both induced by Ni and Ce co-doping, make the spin entropy increase remarkably.

Changes in electronic states of platinum–cobalt alloy catalyst for polymer electrolyte fuel cells by potential cycling

Changes in the electronic states of platinum–cobalt (Pt–Co) alloy catalysts through potential cycling between 0.6 and 1.0V were investigated by X-ray photoemission spectroscopy (XPS) using synchrotron radiation. The electrochemical surface area loss and the particle size growth of the Pt catalyst were larger than those of the Pt–Co alloy catalyst. Pt 4f XPS spectra of the Pt–Co alloy catalyst do not show any change through the potential cycling, indicating that most part of Pt is stable during the potential cycling. Larger amount of Pt(OH)2 existed in the initial MEA of the Pt catalyst than the Pt–Co alloy catalyst, indicating that the Pt catalyst has a tendency to be oxidized. The Pt(OH)2 decreased and metallic platinum increased in the cycle-tested MEA, suggesting that the Pt(OH)2 dissolved and re-deposited as metallic states. The oxidation tendency explains the less durability of the Pt catalyst than the Pt–Co alloy catalyst. Co 2p XPS spectra imply that cobalt is absent on the surface of the catalyst particles and the Pt skin layer is thicker than 1.4nm (4 mono-layers). The absence of the cobalt oxide in the cycle-tested MEA demonstrates that the Pt–Co core under the Pt skin layer is stable during the potential cycling.

Nucleic acid interactions with pyrite surfaces

The study of the interaction of nucleic acid molecules with mineral surfaces is a field of growing interest in organic chemistry, origin of life, material science and biotechnology. We have characterized the adsorption of single-stranded peptide nucleic acid (ssPNA) on a natural pyrite surface, as well as the further adsorption of ssDNA on a PNA-modified pyrite surface. The characterization has been performed by means of reflection absorption infrared spectroscopy (RAIRS), atomic force microscopy (AFM) and X-ray photoemission spectroscopy (XPS) techniques. The N(1s) and S(2p) XPS core level peaks of PNA and PNA + DNA have been decomposed in curve-components that we have assigned to different chemical species. RAIRS spectra recorded for different concentrations show the presence of positive and negative adsorption bands, related to the semiconducting nature of the surface. The combination of the information gathered by these techniques confirms that PNA adsorbs on pyrite surface, interacting through nitrogen-containing groups of the nucleobases and the iron atoms of the surface, instead of the thiol group of the molecule. The strong PNA/pyrite interaction inhibits further hybridization of PNA with complementary ssDNA, contrary to the behavior reported on gold surfaces.

Structural and magnetic characterization of soft-magnetic FeCo alloy nanoparticles

Soft-magnetic FeCo alloy nanoparticles with diameters less than 100 nm are prepared by ball milling. X-ray photoemission spectroscopy (XPS) and X-ray magnetic circular dichroism (XMCD) are used to characterize these particles. While the XPS spectrum from the as-prepared sample clearly shows Co photoemission peaks, no sign of Fe is observed in the same spectrum. However, Fe photoemission peaks appear after 1 h of Ar ion sputtering. A quantitative analysis of the XPS spectra shows an increase of Fe concentration versus sputtering time until the Fe:Co ratio of the bulk alloy is reached. In addition, the narrow scan Fe and Co 2p XPS spectra show that Co is more oxidized than Fe. All these measurements indicate that the nanoparticles have a Co shell and an Fe-rich core. They further demonstrate the usefulness of XPS combined with depth-profiling via sputtering to obtain element- and chemically-sensitive structural information on nanoparticles. XMCD as an element-specific magnetic analysis tool further reveals that Fe and Co are ferromagnetically coupled in these particles. The information obtained is useful for establishing a structure–property relation for the studied material that is expected to have applications as a soft magnetic material at high temperatures.

Magnetic and electronic nature of layered manganese oxysulfide Sr2CuMnO3S

We have synthesized and investigated the magnetic and electronic nature of layered manganese oxysulfide Sr2CuMnO3S which crystallizes in an unusual intergrowth structure with Cu2S2 and pyramidal MnO5 layers separated by Sr ions. A Cu 2p x-ray photoemission spectroscopy (XPS) spectrum of Sr2CuMnO3S indicates that the Cu ion is in the monovalent state, or Cu+. This feature of the Mn 2p XPS spectrum of Sr2CuMnO3S is similar to that of the parent compound La3+Mn3+O3 of colossal magnetoresistance materials. The MnO5 cluster model analysis suggests that the electronic nature of the pyramidal MnO5 layers belongs to a strongly correlated electron system, and the system is the so-called Mott insulator resulting from the strongly correlated electron nature. Magnetic susceptibility (χ) exhibits low-dimensional antiferromagnetic ordering of Mn3+ in pyramidal MnO5 layers. Fitting the χ data to Curie–Weiss law provides evidence of strong second neighbor Mn–Mn interaction.

Electronic structure investigation of the room temperature coadsorption of oxygen and potassium on Ni(100): from oxygen submonolayer coverage to saturated NiO/Ni(100) via an Ni(100)-(3×3)-(K+O) structure.

We have investigated the room temperature potassium-promoted oxidation of Ni(100) by means of O 1s and K 2p X-ray photoemission spectroscopy (XPS), relative work function and O 1s X-ray absorption spectroscopy (XAS) measurements. Within the potassium coverage range we examine in this study ( θ K=0.10–0.25 ML), coadsorption of about 0.5 ML of oxygen results in the formation of a (3×3) pattern which is peculiar to the coadsorption of alkalis and oxygen on Ni(100). This structure is stable against further oxygen uptake or against adsorption of other species such as CO. However, by means of O 1s XPS and XAS observations we determine an overall potassium-enhanced oxidation rate of about two orders of magnitude. The O 1s XPS data show that oxygen coadsorbed with K on Ni(100) is still bonded primarily to Ni and thus rule out the formation of potassium oxide or other K x O y compounds such as peroxide or superoxide. The line-shape analysis of the K 2p XPS data show that upon oxygen coadsorption the alkali metal layer undergoes a metal–insulator type of transition. The O 1s XPS data rule out the possibility that this transition is due to a large charge transfer between coadsorbed species so other mechanisms are discussed. The work function changes induced by O on the K-precovered surface are consistent with the formation of a bilayered structure in which oxygen is adsorbed underneath the alkali metal layer at K coverages above the work function minimum. However, we find that the alkali-induced minimum of the work function is not related to the occurrence of structural properties, in particular the formation of the (3×3) structure, or electronic properties such as the metal–insulator transition of the K layer. Therefore a coplanar alkali–oxygen coadsorption model cannot be ruled out. The O 1s XAS data show that the presence of potassium on Ni(100) lowers the O coverage for the onset of the formation of cubic NiO.

Electron correlation and charge transfer at the Ni/Co interface

The evolving magnetism and electronic structure at the Ni/Co interface have been studied using x-ray absorption spectroscopy (XAS) and x-ray photoemission spectroscopy (XPS) with circularly polarized x rays. Deposition of ultrathin Ni films on thin films of Co grown on Cu(001) results in an intensity enhancement across the Co L2.3 absorption edge. By comparison, the intensity of the Ni L2.3 edge decreases as a function of Ni film thickness. The relative changes in the Ni and Co XAS intensities are interpreted as an electronic charge transfer from the Co to the Ni. Distinct changes in the Co 2p XAS and XPS line shapes after addition of the Ni overlayer imply a modification of the Co 3d electron correlation due to the charge transfer. The change in the electronic structure is related to the interface magnetism using magnetic circular dichroism sum rule analysis.

Copper phthalocyanine on Si(111)-7 × 7 and Si(001)-2 × 1 surfaces: an X-ray photoemission spectroscopy and synchrotron X-ray absorption spectroscopy study

The chemical bonding and molecular orientation of copper phthalocyanine (CuN 8C 32H 16), deposited on clean single crystal silicon substrates of (111) and (001) orientation, were studied using X-ray photoemission spectroscopy (XPS) of the Cu 2p, C 1s, and N 1s core-levels and synchrotron radiation X-ray absorption spectroscopy (XAS) at the Cu L 2,3 edges. In the monolayer range, the strong interaction of the molecule with the Si (111) surface is mostly evidenced by changes in the Cu 2p XPS spectra that indicate a partial reduction (up to 30%) of the Cu(II) atoms to Cu 3d 10. The linear dichroism of the XAS spectra shows that those molecules, whose copper is still Cu(II), lie down on the surface. On the other hand, the bonding of the molecule on Si(001) is weaker, as Cu 2p XPS spectra do not exhibit Cu(II) reduction, while XAS spectroscopy indicates a random orientation of the molecules with respect to the surface. N 1s XPS spectra exhibit, on both surfaces, very intense satellites which present, on Si(111), a sharp dependence on the electron take-off angle. With the growth of thicker films, the XPS line shapes of the molecular solid are recovered, and the average tilt angle of the molecules (around 70°) does not depend on the chosen Si surface. We conclude that the structure of the first deposited layer strongly depends on the specific substrate, but does not affect the successive growth of thick layers.

Electron energy levels in Nd2-xCexCuO4: A study by valence- and core-level photoemission spectroscopy.

Valence- and core-level photoemission spectra of vacuum-annealed polycrystalline ${\mathrm{Nd}}_{2\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ce}}_{\mathit{x}}$${\mathrm{CuO}}_{4}$ ceramics have been studied over the composition range 0x0.20. The cleaned surfaces give simple O1s core line shapes in x-ray photoemission spectroscopy (XPS) and metallic samples with x values in excess of 0.1 display x-ray and Ne(I) excited valence-region spectra showing a distinct Fermi-edge cutoff. Valence-band and core-level binding energies are inconsistent with filling of a rigid conduction band. Moreover, there is little change in the Cu 2p XPS line shape across the series. This observation is discussed in relation to the Larsson-Sawatzky model. The Cu 2p core-level data for ${\mathrm{Nd}}_{2\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ce}}_{\mathit{x}}$${\mathrm{CuO}}_{4}$ are compared with those for ${\mathrm{La}}_{2\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{CuO}}_{4}$. The experimental work reveals a stabilization of Cu 3d levels in ${\mathrm{Nd}}_{2}$${\mathrm{CuO}}_{4}$ as compared with ${\mathrm{La}}_{2}$${\mathrm{CuO}}_{4}$ and suggests a negative charge-transfer parameter \ensuremath{\Delta} in the former compound.

Nonlocal screening effects in 2p x-ray photoemission spectroscopy core-level line shapes of transition metal compounds.

We show that the 2p core-level line shapes of transition metal (TM) compounds are strongly influenced by the presence of more TM ions. Using calculations of clusters, involving more than one TM ion we explain the double peaked main line of the Ni 2p spectrum of NiO as well as the difference in line shape compared with Ni impurities in MgO and show the sensitivity of the line shape in NiO to surrounding defects. This intersite charge transfer screening also explains the wide asymmetric shape of the main line of the Cu 2p spectrum of CuO and the high-${\mathit{T}}_{\mathit{c}}$ compounds.

X-ray photoemission and Auger-electron spectroscopic study of the electronic structure of intercalation compounds MxTiS2 (M=Mn, Fe, Co, and Ni).

The electronic structure of intercalation compounds ${M}_{x}$${\mathrm{TiS}}_{2}$ (M=Mn, Fe, Co, and Ni) has been studied by x-ray photoemission spectroscopy (XPS) and Auger-electron spectroscopy. The intercalated M 3d--derived spectra in the valence-band region can be interpreted in terms of multiplet and satellite structures, suggesting that the intra-atomic Coulomb and exchange energies for the M 3d electrons and the M 3d--S 3p hybridization dominate the M 3d-band width. The core-level XPS spectra of the guest 3d atoms also exhibit multiplet and satellite structures, from which these atoms are found to be in high-spin divalent states, except for Co, which is in the low-spin divalent state. The host Ti 3d-- and S 3p--derived electronic states, on the other hand, are basically well described by the band theory. Auger-electron spectra also support the strong electron correlation of the M 3d states and its insignificance for the host Ti 3d and S 3p states. Itinerant behaviors of the M 3d electrons as observed in the magnetic, thermal, and transport properties combined with the present results suggest that correlated M 3d bands are formed in these compounds as a result of strong hybridization with the host electronic states. The Ti 2p core-level XPS spectra of ${M}_{x}$${\mathrm{TiS}}_{2}$ are shown to consist of poorly screened and well-screened peaks. As for ${\mathrm{TiS}}_{2}$, the Ti 2p XPS spectrum does not show well-screened peaks because of the absence of conduction electrons that can fill the screening orbital, whereas this orbital appears to be filled in the initial state of Auger-electron emission.