Electronic structure of Fe- vs. Ru-based dye molecules

In order to explore whether Ru can be replaced by inexpensive Fe in dye molecules for solar cells, the differences in the electronic structure of Fe- and Ru-based dyes are investigated by X-ray absorption spectroscopy and first-principles calculations. Molecules with the metal in a sixfold, octahedral N cage, such as tris(bipyridines) and tris(phenanthrolines), exhibit a systematic downward shift of the N 1s-to-π* transition when Ru is replaced by Fe. This shift is explained by an extra transfer of negative charge from the metal to the N ligands in the case of Fe, which reduces the binding energy of the N 1s core level. The C 1s-to-π* transitions show the opposite trend, with an increase in the transition energy when replacing Ru by Fe. Molecules with the metal in a fourfold, planar N cage (porphyrins) exhibit a more complex behavior due to a subtle competition between the crystal field, axial ligands, and the 2+ vs. 3+ oxidation states.This work was supported by the National Science Foundation (NSF) under Award Nos. CHE-1026245, DMR-1121288 (MRSEC), DMR-0537588 (SRC), and by the (U.S.) Department of Energy (DOE) under Contract Nos. DE-FG02-01ER45917 (end station) and DE-AC02-05CH11231 (ALS). P. L. Cook acknowledges support from the University of Wisconsin System 2012-2013 Applied Research Grant. J. M. García-Lastra and A. Rubio acknowledge financial support from the European Research Council (ERC-2010-AdG-Proposal No. 267374), Spanish Grants (FIS2011-65702-C02-01 and PIB2010US-00652), Grupos Consolidados (IT-319-07), and European Commission project CRONOS (280879-2).Peer Reviewe

Orbital reflectometry

The occupation of d-orbitals controls the magnitude and anisotropy of the inter-atomic electron transfer in transition metal oxides and hence exerts a key influence on their chemical bonding and physical properties. Atomic-scale modulations of the orbital occupation at surfaces and interfaces are believed to be responsible for massive variations of the magnetic and transport properties, but could thus far not be probed in a quantitative manner. Here we show that it is possible to derive quantitative, spatially resolved orbital polarization profiles from soft x-ray reflectivity data, without resorting to model calculations. We demonstrate that the method is sensitive enough to resolve differences of 3 % in the occupation of Ni e_g orbitals in adjacent atomic layers of a LaNiO3-LaAlO3 superlattice, in good agreement with ab-initio electronic-structure calculations. The possibility to quantitatively correlate theory and experiment on the atomic scale opens up many new perspectives for orbital physics in d-electron materials

Resonante und hochenergetische Röntgenbeugungsuntersuchungen von stark korrelierten Elektronensystemen in Übergangsmetalloxiden

The strongly correlated electron systems in Ca_{2-x}Sr_xRuO_4, RuSr_2GdCu_2O_8 and YBa_2Cu_3O_{6+x} transition metal oxides were investigated with resonant and high-energy x-ray scattering techniques. The main results and conclusions of the work are briefly summarized in the following. Single crystals of Ca_2RuO_4 and Ca_{1.9}Sr_{0.1}RuO_4 were investigated with resonant x-ray diffraction at the Ru LII and LIII absorption edges, i.e. at photon energies E=2.968 keV and E=2.838 keV, respectively. At the magnetically allowed (1 0 0) and (0 1 1) reciprocal space positions a new ordered phase was discovered above the magnetic phase transition at T_N=110 K. The scattering from this phase has a strongly resonant character, with non-zero intensity only at energies very close to the absorption edges. It is observed exclusively in the sigma-pi polarization channel, hence is characterized by a polarization which is parallel to the scattering plane. The scattering intensity in the new phase decreases smoothly with increasing temperature, without showing any anomalies, and vanishes at a phase transition at approximately 260 K. Based on the polarization and temperature dependences, as well as on complementary muon spin rotation measurements, which indicate no ordered magnetic moment above the Néel temperature, we can draw the conclusion that the resonant scattering in the new phase originates from the ordering of the Ru 4d orbitals. The orbital ordering phase transition is also observed in the strontium-doped Ca_{1.9}Sr_{0.1}RuO_4 system, but at a lower temperature of approximately 130 K. The propagation vector of the orbital order remains unchanged in the doped compound, despite the change of the magnetic structure. Furthermore, an additional resonant signal was observed in both investigated Ca_{2-x}Sr_xRuO_4 systems at the (1 1 0) position, which is forbidden both magnetically and by the space-group. The intensity of this signal decreases smoothly with increasing temperature with no anomalies up to the metal-insulator transition. Based on the temperature and polarization dependences of the scattering intensity at (1 1 0), this can be attributed to the tilt order of the RuO_6 octahedra. Micrometer-sized single crystals of RuSr_2GdCu_2O_8 were investigated with resonant x-ray diffraction at the Ru LII absorption edge. Based on the azimuthal dependence of the scattering intensity at the (1/2 1/2 1/2) magnetic position, the exact direction of the magnetic moment in the material could be determined. The magnetic moment was found to be aligned not along the crystallographic c-direction, as initially suggested by neutron scattering investigations, but instead along a direction which forms an angle of approximately 53 degrees with the c-axis. In addition, the magnetic order parameter of the Ru spin-system was determined. The experimental data indicate a possible influence of the onset of superconductivity on the magnetic order parameter, but this is still to be confirmed. Single crystals of underdoped, optimally doped and overdoped YBa_2Cu_3O_{6+x}, as well as stoichiometric YBa_2Cu_4O_8, were investigated with high-energy non-resonant x-ray diffraction at photon energies of 100 keV and 115 keV. A new superstructure with periodicity equal to four unit cells was discovered in optimally doped YBa_2Cu_3O_{6.92} and overdoped Y_{0.8}Ca_{0.2}Ba_2Cu_3O_{6.95}. The superstructures are practically identical in the two compounds, which have almost the same oxygen content, but very different charge carrier concentrations. This indicates that the superstructures do not originate from electronic stripes in the CuO_2 planes, but rather from oxygen vacancy ordering in the Cu-O chains. This conclusion is supported by two more independent observations: the persistence of the superstructures up to temperatures well above room temperature; and the absence of diffuse scattering features in the stoichiometric YBa_2Cu_4O_8 compound, which contains no oxygen vacancies and should therefore make the observation of stripes easier. If such stripes do exist in YBa_2Cu_3O_{6+x}, then the intensity of the associated diffuse features must be at least one order of magnitude smaller than that of the aboved described signatures of oxygen order. The comparison of the diffuse scattering patterns of YBa_2Cu_3O_{6.92} samples containing different oxygen isotopes (O-16 vs. O-18) revealed that these are identical. This means that variations in the oxygen order in the Cu-O chains cannot be responsible for isotope effects reported in previous work on this material.Die stark korrelierten Elektronensysteme in den Übergangsmetalloxiden Ca_{2-x}Sr_xRuO_4, RuSr_2GdCu_2O_8 und YBa_2Cu_3O_{6+x} wurden mit Hilfe von resonanter beziehungsweise hochenergetischer Röntgenbeugung untersucht. Die wichtigsten Ergebnisse der Arbeit werden im Folgenden zusammengefasst. Einkristalle der Verbindungen Ca_2RuO_4 und Ca_{1.9}Sr_{0.1}RuO_4 wurden mittels resonanter Röntgenbeugung an den Ru LII und LIII Absorptionskanten (E=2.968 keV beziehungsweise 2.838 keV) untersucht. An den magnetisch erlaubten Positionen (1 0 0) und (0 1 1) im reziproken Raum wurde oberhalb des magnetischen Phasenübergangs bei T_N=110 K eine neue Phase entdeckt. Die gestreute Intensität wird durch eine gedrehte Strahlungspolarisation gekennzeichnet, die parallel zur Streuebene ist, senkrecht zum Polarisationsvektor der einfallenden Strahlung. Die Temperaturabhängigkeit der Streuintensität in der neuen Phase weist keine Anomalien auf, sondern zeigt bei steigender Temperatur einen langsamen Abfall bis zu einem Phasenübergang bei ungefähr 260 K. Anhand der Polarisations- und Temperaturabhängigkeiten, sowie ergänzender Muonspinrotationsmessungen, die kein geordnetes magnetisches Moment oberhalb der Néel-Temperatur angedeutet haben, kann der Schluss gezogen werden, dass die Orbitalordnung der Ursprung der Streuintensität in der neuen Phase ist. Der Orbitalordnungsphasenübergang ist auch im strontiumdotierten Ca_{1.9}Sr_{0.1}RuO_4 System zu beobachten, allerdings bei einer niedrigeren Temperatur von ungefähr 130 K. Trotz der Änderung der magnetischen Struktur bleibt der Ausbreitungsvektor der Ordnung in der dotierten Verbindung unverändert. Des Weiteren wurde in beiden untersuchten Ca_{2-x}Sr_xRuO_4 Systemen ein zusätzliches Resonanzsignal an der (1 1 0) Position beobachtet, die Anhand der Temperatur- und Azimutwinkelabhängigkeiten auf die Tiltordnung der RuO_6 Oktaeder zurückzuführen ist. Das Hybridsystem RuSr_2GdCu_2O_8 wird durch die seltene Koexistenz langreichweitiger magnetischer Ordnung und Supraleitung in seiner Einheitszelle charakterisiert. Kleine Einkristalle des Materials mit Größe um die 50 Mikrometer wurden mittels resonanter Röntgenbeugung an der Ru LII Absorptionskante untersucht. Anhand der Azimutwinkelabhängigkeit der Resonanzintensität an der magnetischen (1/2 1/2 1/2) Position konnte die Richtung des magnetischen Moments im Material genau bestimmt werden. Es wurde insbesondere gefunden, dass das magnetische Moment nicht entlang der kristallographischen c-Richtung orientiert ist, wie ursprünglich anhand von Neutronenbeugungsuntersuchungen vorgeschlagen, sondern entlang einer Richtung, die ungefähr 53 Grad von der c-Achse entfernt ist. Des Weiteren wurde der magnetische Ordnungsparameter des Ru Spinsystems temperaturabhängig verfolgt. Dabei hat sich gezeigt, dass der Einbruch der Supraleitung möglicherweise einen Einfluss auf die magnetische Ordnung hat, was allerdings noch zu bestätigen ist. Der Hochtemperatursupraleiter YBa_2Cu_3O_{6+x} wurde oft in Hinblick auf die langanhaltende Diskussion über die mögliche Entstehung von sogenannten Streifen-Phasen (stripes) in Kupraten und ihre Rolle bei der Etablierung von Supraleitung untersucht. Unterschiedlich dotierte YBa_2Cu_3O_{6+x}, sowie stoichiometrische YBa_2Cu_4O_8 Einkristalle, wurden mittels hochenergetischer nicht-resonanter Röntgenbeugung bei Photonenenergien von 100 keV beziehungsweise 115 keV untersucht. Eine neue Überstrukturphase mit einer Periodizität von vier Elementarzellen wurde im optimal dotierten YBa_2Cu_3O_{6.92} und im überdotierten Y_{0.8}Ca_{0.2}Ba_2Cu_3O_{6.95} entdeckt. Die Überstrukturreflexe sehen in beiden Verbindungen, die praktisch den gleichen Sauerstoffgehalt aber sehr unterschiedliche Ladungstragerkonzentrationen haben, fast identisch aus. Dies weist darauf hin, dass die Überstrukturen nicht auf Streifen, sondern auf die Sauerstoffordnung zurückzuführen sind. Dieser Schluss wird von zwei zusätzlichen Beobachtungen unterstützt: das Weiterbestehen der Überstrukturen bis zu Temperaturen deutlich höher als Raumtemperatur; und die Abwesenheit der Überstrukturen im stoichiometrischen YBa_2Cu_4O_8 Material, das keine Sauerstofflücken enthält und deshalb die Beobachtung von Streifen erleichtern sollte. Sollten solche Stripes elektronischer Natur in YBa_2Cu_3O_{6+x} existieren, müsste die Intensität der entsprechenden Reflexe um mindestens eine Größenordnung kleiner sein als die der oben beschriebenen Überstrukturreflexe der Sauerstoffordnung. Durch den Vergleich der Streuintensitäten der Überstrukturreflexe von YBa_2Cu_3O_{6.92} Proben, die unterschiedliche Sauerstoffisotope enthalten (O-16 vs. O-18), wurde des Weiteren festgestellt, dass diese identisch sind. Dies bedeutet, dass Änderungen in der Sauerstoffordnung in den Cu-O Ketten nicht für Isotopeffekte in diesem Material verantwortlich sein können

Operando Phonon Studies Of The Protonation Mechanism In Highly Active Hydrogen Evolution Reaction Pentlandite Catalysts

Synthetic pentlandite (Fe4.5Ni4.5S8) is a promising electrocatalyst for hydrogen evolution, demonstrating high current densities, low overpotential, and remarkable stability in bulk form. The depletion of sulfur from the surface of this catalyst during the electrochemical reaction has been proposed to be beneficial for its catalytic performance, but the role of sulfur vacancies and the mechanism determining the reaction kinetics are still unknown. We have performed electrochemical operando studies of the vibrational dynamics of pentlandite under hydrogen evolution reaction conditions using 57Fe nuclear resonant inelastic X-ray scattering. Comparing the measured Fe partial vibrational density of states with density functional theory calculations, we have demonstrated that hydrogen atoms preferentially occupy substitutional positions replacing pre-existing sulfur vacancies. Once all vacancies are filled, the protonation proceeds interstitially, which slows down the reaction. Our results highlight the beneficial role of sulfur vacancies in the electrocatalytic performance of pentlandite and give insights into the hydrogen adsorption mechanism during the reaction

Highly Selective Plasma-Activated Copper Catalysts For Carbon Dioxide Reduction To Ethylene

There is an urgent need to develop technologies that use renewable energy to convert waste products such as carbon dioxide into hydrocarbon fuels. Carbon dioxide can be electrochemically reduced to hydrocarbons over copper catalysts, although higher efficiency is required. We have developed oxidized copper catalysts displaying lower overpotentials for carbon dioxide electroreduction and record selectivity towards ethylene (60%) through facile and tunable plasma treatments. Herein we provide insight into the improved performance of these catalysts by combining electrochemical measurements with microscopic and spectroscopic characterization techniques. Operando X-ray absorption spectroscopy and cross-sectional scanning transmission electron microscopy show that copper oxides are surprisingly resistant to reduction and copper+ species remain on the surface during the reaction. Our results demonstrate that the roughness of oxide-derived copper catalysts plays only a partial role in determining the catalytic performance, while the presence of copper+ is key for lowering the onset potential and enhancing ethylene selectivity

Segregation Phenomena In Size-Selected Bimetallic Cuni Nanoparticle Catalysts

Surface segregation, restructuring, and sintering phenomena in size-selected copper-nickel nanoparticles (NPs) supported on silicon dioxide substrates were systematically investigated as a function of temperature, chemical state, and reactive gas environment. Using near-ambient pressure (NAP-XPS) and ultrahigh vacuum X-ray photoelectron spectroscopy (XPS), we showed that nickel tends to segregate to the surface of the NPs at elevated temperatures in oxygen- or hydrogen-containing atmospheres. It was found that the NP pretreatment, gaseous environment, and oxide formation free energy are the main driving forces of the restructuring and segregation trends observed, overshadowing the role of the surface free energy. The depth profile of the elemental composition of the particles was determined under operando CO2 hydrogenation conditions by varying the energy of the X-ray beam. The temperature dependence of the chemical state of the two metals was systematically studied, revealing the high stability of nickel oxides on the NPs and the important role of high valence oxidation states in the segregation behavior. Atomic force microscopy (AFM) studies revealed a remarkable stability of the NPs against sintering at temperatures as high as 700 °C. The results provide new insights into the complex interplay of the various factors which affect alloy formation and segregation phenomena in bimetallic NP systems, often in ways different from those previously known for their bulk counterparts. This leads to new routes for tuning the surface composition of nanocatalysts, for example, through plasma and annealing pretreatments

Plasma-Activated Copper Nanocube Catalysts for Efficient Carbon Dioxide Electroreduction to Hydrocarbons and Alcohols

Carbon dioxide electroreduction to chemicals and fuels powered by renewable energy sources is considered a promising path to address climate change and energy storage needs. We have developed highly active and selective copper (Cu) nanocube catalysts with tunable Cu(100) facet and oxygen/chlorine ion content by low-pressure plasma pretreatments. These catalysts display lower overpotentials and higher ethylene, ethanol, and <i>n</i>-propanol selectivity, resulting in a maximum Faradaic efficiency (FE) of ∼73% for C₂ and C₃ products. Scanning electron microscopy and energy-dispersive X-ray spectroscopy in combination with quasi-<i>in situ</i> X-ray photoelectron spectroscopy revealed that the catalyst shape, ion content, and ion stability under electrochemical reaction conditions can be systematically tuned through plasma treatments. Our results demonstrate that the presence of oxygen species in surface and subsurface regions of the nanocube catalysts is key for achieving high activity and hydrocarbon/alcohol selectivity, even more important than the presence of Cu(100) facets

Highly active single-layer MoS2 catalysts synthesized by swift heavy ion irradiation

Two-dimensional molybdenum-disulfide (MoS2) catalysts can achieve high catalytic activity for the hydrogen evolution reaction upon appropriate modification of their surface. The intrinsic inertness of the compound's basal planes can be overcome by either increasing the number of catalytically active edge sites or by enhancing the activity of the basal planes via a controlled creation of sulfur vacancies. Here, we report a novel method of activating the MoS2 surface using swift heavy ion irradiation. The creation of nanometer-scale structures by an ion beam, in combination with the partial sulfur depletion of the basal planes, leads to a large increase of the number of low-coordinated Mo atoms, which can form bonds with adsorbing species. This results in a decreased onset potential for hydrogen evolution, as well as in a significant enhancement of the electrochemical current density by over 160% as compared to an identical but non-irradiated MoS2 surface.Peer reviewe

Operando Evolution of the Structure and Oxidation State of Size-Controlled Zn Nanoparticles during CO₂ Electroreduction

We explored the size-dependent activity and selectivity of Zn nanoparticles (NPs) for the electrochemical CO₂ reduction reaction (CO₂RR). Zn NPs ranging from 3 to 5 nm showed high activity and selectivity (∼70%) for CO production, whereas those above 5 nm exhibited bulk-like catalytic properties. In addition, a drastic increase in hydrogen production was observed for the Zn NPs below 3 nm, which is associated with the enhanced content of low-coordinated sites on small NPs. The presence of residual cationic Zn species in the catalysts was also revealed during CO₂RR via <i>operando</i> X-ray absorption fine-structure spectroscopy measurements. Such species are expected to play a role in the selectivity trends obtained. Our findings can serve as guidance for the development of highly active and CO-selective Zn-based catalysts for CO₂RR

Segregation Phenomena in Size-Selected Bimetallic CuNi Nanoparticle Catalysts

Surface segregation, restructuring, and sintering phenomena in size-selected copper–nickel nanoparticles (NPs) supported on silicon dioxide substrates were systematically investigated as a function of temperature, chemical state, and reactive gas environment. Using near-ambient pressure (NAP-XPS) and ultrahigh vacuum X-ray photoelectron spectroscopy (XPS), we showed that nickel tends to segregate to the surface of the NPs at elevated temperatures in oxygen- or hydrogen-containing atmospheres. It was found that the NP pretreatment, gaseous environment, and oxide formation free energy are the main driving forces of the restructuring and segregation trends observed, overshadowing the role of the surface free energy. The depth profile of the elemental composition of the particles was determined under <i>operando</i> CO₂ hydrogenation conditions by varying the energy of the X-ray beam. The temperature dependence of the chemical state of the two metals was systematically studied, revealing the high stability of nickel oxides on the NPs and the important role of high valence oxidation states in the segregation behavior. Atomic force microscopy (AFM) studies revealed a remarkable stability of the NPs against sintering at temperatures as high as 700 °C. The results provide new insights into the complex interplay of the various factors which affect alloy formation and segregation phenomena in bimetallic NP systems, often in ways different from those previously known for their bulk counterparts. This leads to new routes for tuning the surface composition of nanocatalysts, for example, through plasma and annealing pretreatments