Resonant Photoemission Measurements Research Articles

Iridium Oxide Coordinatively Unsaturated Active Sites Govern the Electrocatalytic Oxidation of Water

AbstractA special membrane electrode assembly to measure operando X‐ray absorption spectra and resonant photoemission spectra of mesoporous templated iridium oxide films is used. These films are calcined to different temperatures to mediate the catalyst activity. By combining operando resonant photoemission measurements of different films with ab initio simulations these are able to unambiguously distinguish µ2‐O (bridging oxygen) and µ1‐O (terminal oxygen) in the near‐surface regions of the catalysts. The intrinsic activity of iridium oxide scales with the formation of µ1‐O (terminal oxygen) is found. Importantly, it is shown that the peroxo species do not accumulate under reaction conditions. Rather, the formation of µ1‐O species, which are active in O−O bond formation during the OER, is the most oxidized oxygen species observed, which is consistent with an O−O rate‐limiting step. Thus, the oxygen species taking part in the electrochemical oxidation of water on iridium electrodes are more involved and complex than previously stated. This result highlights the importance of employing theory together with true and complementary operando measurements capable of probing different aspects of catalysts surfaces during operation.

On the origin of the improved hydrogen evolution reaction in Mn- and Co-doped MoS2.

In the field of hydrogen production, MoS2 demonstrates good catalytic properties for the hydrogen evolution reaction (HER) which improve when doped with metal cations. However, while the role of sulfur atoms as active sites in the HER is largely reported, the role of metal atoms (i.e. molybdenum or the dopant cations) has yet to be studied in depth. To understand the role of the metal dopant, we study MoS2 thin films doped with Co and Mn ions. We identify the contribution of the electronic bands of the Mn and Co dopants to the integral valence band of the material using in situ resonant photoemission measurements. We demonstrate that Mn and Co dopants act differently: Mn doping favors the shift of the S-Mo hybridized band towards the Fermi level, while in the case of Co doping it is the less hybridized Co band that shifts closer to the Fermi level. Doping with Mn increases the effectiveness of S as the active site, thus improving the HER, while doping with Co introduces the metallic site of Co as the active site, which is less effective in improving HER properties. We therefore clarify the role of the dopant cation in the electronic structure determining the active site for hydrogen adsorption/desorption. Our results pave the way for the design of efficient materials for hydrogen production via the doping route, which can be extended to different catalytic reactions in the field of energy applications.

Temperature driven Mott-Hubbard to charge-transfer insulator transition in hexagonal Sr0.6Ba0.4MnO3

We have probed the electronic structure across the ferroelectric transition temperature (TC = 355 K) of hexagonal Sr0.6Ba0.4MnO3 (SBMO) which shows a magneto-electric effect at room temperature. The valence and conduction bands are characterized by resonant photoemission and X-ray absorption spectroscopy measurements, respectively. The valence band spectrum is found to be hugely modified across TC. Above TC, SBMO behaves like a Mott-Hubbard insulator, whereas at 300 K, it shows a charge-transfer-type insulator behavior. Modulation in the hybridization of Mn-3d and O-2p orbitals due to local structural variations with temperature unravels the rational for the observed electronic structure transition vis a vis magnetic and electrical behaviors of SBMO.

Fermi surface variation of Ce 4f-electrons in hybridization controlled heavy-fermion systems

Ce 3d–4f resonant angle-resolved photoemission measurements on CeCoGe1.2Si0.8 and CeCoSi2 have been performed to understand the Fermi surface topology as a function of hybridization strength between Ce 4f- and conduction electrons in heavy-fermion systems. We directly observe that the hole-like Ce 4f-Fermi surfaces of CeCoSi2 is smaller than that of CeCoGe1.2Si0.8, indicating the evolution of the Ce 4f-Fermi surface with the increase of the hybridization strength. In comparison with LDA calculation, the Fermi surface variation cannot be understood even though the overall electronic structure is roughly explained, indicating the importance of strong correlation effects.

Evidence of coexisting Kondo screening and valence fluctuations in theCePd7/Pd(001) surface alloy

We report on the electronic structure of a clean, well ordered ${\mathrm{CePd}}_{7}$ surface alloy that was grown in a unique $(\sqrt{5}\ifmmode\times\else\texttimes\fi{}\sqrt{5})R26.{6}^{\ensuremath{\circ}}$ structure, as evidenced by low-energy electron diffraction and Auger electron spectroscopy. Resonant photoemission measurements at the $4d\ensuremath{\rightarrow}4f$ absorption edge evidenced strong Ce 4$f$ derived states having two components near the Fermi level. This observation can be explained by considering very different hybridizations between Ce 4$f$ and the Pd 4$d$ electrons in the surface and deeper layers. In the former, the hybridization is weak and leads to a spin-dependent screening with low Kondo temperature, while in the latter the hybridization recovers the value of bulk alloy and thus leads to strong charge fluctuations.

Investigation of electronic states of infinite-layer SrFeO 2 epitaxial thin films by X-ray photoemission and absorption spectroscopies

We investigated the electronic states of a single-crystal SrFeO 2 epitaxial thin film in the valence-band and conduction-band regions using synchrotron-radiation X-ray photoemission and absorption spectroscopies. Fe 2 p–3 d resonant photoemission measurements revealed that the Fe 3 d states have higher densities of states at binding energies of 3–5 eV and 5–8.5 eV in the valence-band region. The O K-edge X-ray absorption spectrum exhibited three peaks in the Fe 3 d-derived conduction band hybridized with O 2 p states; these can be assigned to Fe 3 d xy , 3 d xz + 3 d yz , and 3 d x 2 – y 2. In addition, the indirect bandgap value of the SrFeO 2 film was determined to be 1.3 eV by transmission and absorption spectroscopies.

Depth profile high-energy spectroscopic study of Mn-doped GaN prepared by thermal diffusion

We have performed an in situ depth profile study of Mn-doped GaN prepared by a low temperature thermal diffusion method using photoemission and x-ray absorption spectroscopy. It was revealed from the core-level photoemission measurements that Mn ions are diffused into a deep (∼70 Å) region of the GaN substrates and that the line shapes of Mn 3d partial density of states obtained by resonant photoemission measurements were close to that of Ga1−xMnxN thin films grown by molecular-beam epitaxy. From x-ray absorption spectroscopy and x-ray magnetic circular dichroism measurements at the Mn L edge, it was revealed that the doped Mn ions were in the divalent Mn2+ state and primarily paramagnetic. In magnetization measurements, weak hysteresis was detected in samples prepared using p-type GaN substrates while samples using n-type GaN substrates showed only paramagnetism.

Structure and electronic properties of Fe nanostructures on MgO(0 0 1)

We present and discuss X-ray absorption spectroscopy and resonant photoemission measurements on Fe nanostructures self-assembled on MgO(0 0 1). For Fe coverages below 1 ML equivalent we measured an increase of the Fe L 23 branching ratio and changes in the splitting, widths and relative intensities of the different final states in the L 3M 23M 23 resonant Auger peak. Scanning tunnelling microscopy indicates that the average lateral dimensions of the self-aggregated structures decrease with decreasing Fe amount, from 12 nm at 15 ML nominal Fe amount to 5 nm at 2 ML Fe. This observation allows to interpret the observed changes in the 3d band electronic properties in terms of the evolution of the Fe local atomic coordination from a bulk-like situation to a configuration where low dimensionality effects are significant.

Experimental investigation of the electronic structure ofGd5Ge2Si2 by photoemission and x-ray absorption spectroscopy

The electronic structure of the magnetic refrigerantGd5Ge2Si2 has been experimentally investigated by photoemission and x-ray absorption spectroscopy.The resonant photoemission and x-ray absorption measurements performed across theGd N4,5 and Gd M4,5 edges identify the position of Gd 4f multiplet lines, and assess the 4f occupancy(4f7) and the character of the states close to the Fermi edge. The presence ofGd 5d states in the valence band suggests that an indirect 5d exchangemechanism underlies the magnetic interactions between Gd 4f moments inGd5Ge2Si2. From 175 to 300 K the first 4 eV of the valence band and the Gd partial density of statesdo not display clear variations. A significant change is instead detected in thephotoemission spectra at higher binding energy, around 5.5 eV, likely associated to thevariation of the bonding and antibonding Ge(Si) s bands across the phase transition.

Electronic structure of the charge-density-wave compoundEr5Ir4Si10

Photoemission and x-ray absorption measurements have been performed onEr5Ir4Si10, a compound that exhibits an incommensurate charge-density-wave formation below 155 K.The resonant photoemission and x-ray absorption measurements performed across the ErN54 andEr M5 edge identify the Er 4f multiplet lines, the 4f occupancy and the character of the statesclose to the Fermi edge.

Valence electronic states related toMn2+atLa0.7Sr0.3MnO3surfaces characterized by resonant photoemission

Nonferromagnetic ${\mathrm{Mn}}^{2+}$ ions can be readily formed at the surface of half metallic ${\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}{\mathrm{MnO}}_{3}$ manganite as demonstrated by deoxygenating surface treatments. The $3{d}^{5}$ contribution of these ${\mathrm{Mn}}^{2+}$ ions to the valence-band electronic structure has been characterized using $\mathrm{Mn}(2p)$ to $3d$ resonant photoemission measurements. The ${\mathrm{Mn}}^{2+}$ related $3d$ electrons were found to be stabilized by about $2\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ with respect to the mixed-valence $3d$ states, indicating their strong localization. Active participation of ${\mathrm{Mn}}^{2+}$ states in both spin and charge conductivity processes is therefore excluded. A two-channel picture, including independent ${\mathrm{Mn}}^{3+}∕{\mathrm{Mn}}^{4+}$ and ${\mathrm{Mn}}^{2+}$ channels, emerges from detailed data analysis. Reversible ${\mathrm{Mn}}^{2+}$ formation and straightforward oxygen annealing effects point to a direct bonding between ${\mathrm{Mn}}^{2+}$ and oxygen vacancies that are most probably created at preexisting structural defects. The ${t}_{2g}$ and ${e}_{g}$ states of the mixed valence ${\mathrm{Mn}}^{3+}∕{\mathrm{Mn}}^{4+}$ ions remain unaffected as the ${\mathrm{Mn}}^{2+}$ content increases, indicating a robust ${\mathrm{Mn}}^{3+}∕{\mathrm{Mn}}^{4+}$ channel independent of structural defects.

High-resolution resonance photoemission study ofCeMX(M=Pt,Pd;X=P,As,Sb)

We have performed high-resolution Ce $3d\ensuremath{-}4f$ resonance photoemission (RPES) measurements for low Kondo temperature ${(T}_{K})$ $\mathrm{Ce}\mathrm{MX}$ $(M=\mathrm{P}\mathrm{d},\mathrm{P}\mathrm{t},X=\mathrm{P},\mathrm{A}\mathrm{s},\mathrm{S}\mathrm{b})$ and compared the results with $4d\ensuremath{-}4f$ RPES. The experimental results reveal that the bulk Ce $4f$ electronic structures are remarkably different from those in the surface layer even in low-${T}_{K}$ materials. The non-$4f$ valence-band spectra are well described by the results of the band-structure calculation of isostructural $\mathrm{La}\mathrm{MX}$ by considering the photoionization cross sections. We have analyzed the Ce $4f$ spectra by using a noncrossing approximation calculation based on the single-impurity Anderson model. The calculated results successfully reproduce the experimental $4f$ spectra. We find that the bare $4f$ level shift is the most important factor in explaining the difference between the surface and bulk Ce $4f$ spectra. Moreover, the Ce $4f$ states of $\mathrm{Ce}\mathrm{MX}$ are found to hybridize preferentially with a particular part of the p-d mixed antibonding states.

Temperature-Induced Valence Transition of EuNi2(Si0.25Ge0.75)2Studied by Eu 4d–4fResonant Photoemission and Optical Conductivity

Eu 4 d –4 f resonant photoemission and optical reflectivity measurements of a temperature-induced valence transition material, EuNi 2 (Si 0.25 Ge 0.75 ) 2 , have been performed at several temperatures. The valence transition was observed and the mean valence was evaluated to be 2.37 ±0.02 and 2.75 ±0.25 below and above the valence transition temperature, respectively. The mixing between the Ni 3 d conduction band and the Eu 3+ 4 f hole becomes weak as the temperature increases, showing the valence transition from Eu 3+ to Eu 2+ .

Valence transition of Eu ions in GaN near the surface

The valence states of Eu ions doped in GaN were investigated by the core-level X-ray and resonant photoemission spectroscopy. Although the photoluminescence spectra show the emissions from only trivalent Eu ions, the existence of both divalent and trivalent Eu ions is suggested by the Eu 3d core-level spectra. Taking into account the results of the 4d→4f resonant photoemission measurements, it can be concluded that valence transition from Eu 3+ to Eu 2+ occurs near the surface of GaN.

Direct experimental verification of applicability of single-site model for angle integrated photoemission of small TK concentrated Ce compounds

Bulk-sensitive high-resolution Ce 4f spectra have been obtained from 3d → 4f resonance photoemission measurements on La1−xCexAl2 and La1−xCexRu2 for x=0.0, 0.04, 1.0. The 4f spectra of low-Kondo-temperature (TK) (La,Ce)Al2 are essentially identical except for a slight increase of the Kondo peak with x, which is consistent with a known increase of TK with x. In contrast, the 4f spectra of high-TK (La,Ce)Ru2 show a Kondo-like peak and also a 0.5eV structure which increases strongly with x. The resonance photon-energy dependences of the two contributions are different and the origin of the 0.5eV structure is still uncertain.

A photoemission and resonant photoemission study of Ba deposition at the TiO 2 (110) surface

Results are reported on photoemission and resonant photoemission (ResPES) measurements performed on a Ba-deposited TiO 2 (110) using synchrotron radiation. For coverages less than 1 monolayer (ML) of Ba, only oxidized and no metallic Ba states were detected, and in the valence band spectra a new gap state, attributed to Ti 3+, was seen at 0.8 eV below the Fermi edge and shown by ResPES to give a characteristic 3p→3d resonance of Ti. When the Ba thickness exceeds 1 ML, a resonance of the near-Fermi-edge bandgap state appears, with a maximum found for the Ba 4d→4f resonant transition. The metallic nature of the layers thicker than 1 ML is confirmed by an asymmetry of the Ba 4d peak. Work function measurements showed an abrupt change with Ba thickness when approaching 1 ML, in consistence with the above results.

Photoemission studies of single crystal CuO(100)

Photoemission studies of a cleaved CuO single crystal have been carried out, using station 6.2 at the CLRC Daresbury Laboratory. The (100) face orientation produced by anvil cleaving was determined using Laue back-reflection and LEED. Resonant photoemission measurements at the Cu 3pCu 3d threshold were undertaken and these are compared with existing data from oxidized copper films, and with theoretical configuration interaction calculations for the resonance process. Angle-resolved measurements of the valence band structure have also been performed at different photon energies along three crystal axes, and these are mapped onto the 3D structure of the monoclinic material. This allows general comparison with existing momentum-resolved calculations. The validity of comparisons with both localized and delocalized descriptions of the electronic structure of this highly correlated oxide is discussed.

Fermi-energy 5f spectral weight growth in Y 1− xU xPd 3

High-resolution small-spot resonant photoemission measurements of the Y 1− x U x Pd 3 alloy system have revealed the variation in the U 5f lineshape down to 1% uranium dilution. Qualitatively, the spectrum consists of a peak below E F and a second peak impinging on E F. As x decreases from 1 to 0, the first peak moves toward E F due to a process known as Fermi-level tuning. In addition, the relative intensity of the E F peak is observed to grow in accordance with the onset of Kondo-like transport properties.

Electronic structures of Nd 1−xSr xMnO 3, La 1−xSr xMnO 3 and La 1−x Ca xMnO 3 studied by photoemission and inverse photoemission spectroscopy

We have investigated the electronic structures of perovskite manganese oxides, Nd 0.5Sr 0.5MnO 3, (Nd 0.062Sm 0.938) 0.5Sr 0.5MnO 3, La 0.84Sr 0.16MnO 3 and La 0.7Ca 0.3MnO 3, by X-ray photoemission (XPS) and X-ray bremsstrahlung isochromat spectroscopy in addition to vacuum ultraviolet inverse photoemission. It is found that the VUV-IPES spectra seem to be well explained by the band-structure calculation. Mn 3p-3d resonance photoemission measurements have also been performed, where the contributions of the occupied Mn 3d states are extracted. We observed hybridization between the Mn 3d and O 2p states from the Mn 2p and 3p spectra of all samples, and the Mn spin state from the Mn 3s XPS spectra of Nd 0.5Sr 0.5MnO 3 and (Nd 0.062Sm 0.938) 0.5Sr 0.5MnO 3.

Valence-band and resonance-photoemission study ofLa2CuO4+xsingle crystals

Angle-resolved resonance-photoemission measurements at 15{endash}30 and 100{endash}130 eV photon energy ranges were performed from different principal planes of La{sub 2}CuO{sub 4+x} single crystals. Resonance photoemission near the La-4d threshold shows the presence of states in the valence band associated with La within the energy region 2{endash}5.5 eV. At low photon energies near the La-5p threshold (16.5 eV) with the normal emission aligned in the (001) direction (c{perpendicular} surface), valence-band spectra display nonmonotonic photon energy dependence of a 6-eV binding-energy feature, i.e., beyond the main La density of states in the valence band. Along with the absence of such behavior for normal emission from the other principal planes (100) and (010) (c{parallel} surface geometry), this fact implies that surface termination effects need to be taken into account to explain these results. The binding energy of the resonant feature and the resonance photon energy are slightly different for photon polarization vector parallel and perpendicular to the Cu-O planes of the crystal. Measurements were also done on another La-based compound, La{sub 0.7}Sr{sub 0.3}MnO{sub 3}, having similar La-O plane but much smaller c-axis parameter. This manganese oxide demonstrates the same nonmonotonic dependence of the 6-eV binding-energy feature, but in a wider photon energy region.more » On the basis of this comparison, it is tentatively suggested that the intensity modulation is governed not only by the core-level threshold, but may carry information of bulk Brillouin zones as well. {copyright} {ital 1997} {ital The American Physical Society}« less