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Showing papers by "Fritz Haber Institute of the Max Planck Society published in 1978"


Journal ArticleDOI
TL;DR: In this article, combined IR reflection-absorption and LEED studies of the room temperature adsorption of carbon monoxide on the (100), (111) and (210) surfaces of palladium are reported.

634 citations


Journal ArticleDOI
TL;DR: In this paper, a transmission electron microscope with a single field condenser objective is used to align the microscope onto the coma-free and dispersion-free pencil axis and does not rely on current commutation.

243 citations


Journal ArticleDOI
TL;DR: In this article, low coverages of vacuum deposited palladium on amorphous silica films have been examined with electron microscopy and photoelectron spectroscopy, and changes in the Pd valence band as a function of increasing particle size could be clearly followed in UPS.

198 citations


Journal ArticleDOI
01 May 1978
TL;DR: In this article, it was shown that the Na+ cations at such not well-defined sites at the walls of the supercage are the centers of dissociative H2S adsorption.
Abstract: IR investigations of H2S adsorption on faujasite-type zeolites with systematically varied Si/Al ratios (1.05 to 3.24) have been carried out in order to elucidate the mechanism of adsorption and the role of the Si/Al ratio. The results suggest that on aluminium-rich faujasite-type zeolites up to Si/Al ratios of about 2.5 the first H2S molecules are adsorbed dissociatively. This chemisorption produces OH groups similar to those of hydrogen faujasite giving rise to bands at 3650 and 3580 cm−1. OH groups related to the 3650 cm−1 band are favored. At least a part of the newly formed OH groups exhibit Bronsted acidity. SH groups are indicated by bands around 2560 cm−1. A good correlation has been found between the concentration of dissociatively formed OH and SH species and the population of certain cation sites. From this the conclusion is drawn that the Na+ cations at such not well-defined sites at the walls of the supercage are the centers of dissociative H2S adsorption. The zeolites of NaY type with Si/A1 > 2.5 adsorb the H2S molecules practically without dissociation. This physical adsorption occurs in the whole range of Si/Al ratios if higher H2S pressures are applied; up to Si/Al ≈ 2.5 both mechanisms then overlap. Preliminary studies of H2S oxidation show that this process is also influenced by the Si/Al ratio.

120 citations


Journal ArticleDOI
TL;DR: In this paper, the formation of the (√2 × √2)R45° structure is accompanied by an increase in the work function, which then decreases again during the formation.

110 citations


Journal ArticleDOI
TL;DR: In this paper, an angular resolved UPS measurement was performed on the (√2 × √2)R45° and compressed structures of the adsorption system CO/Ni(100) and the dependence of normal exit photoemission on angle of photon incidence was found to be a sensitive probe of initial state symmetry.

102 citations


Journal ArticleDOI
01 Dec 1978
TL;DR: In this paper, the covalent, layered semiconductor WSe2 has been investigated in a search for corrosion resistent electrode materials for electrochemical solar cells, and it is shown that the produced photovoltage is not systematically dependent on the redox potential of the redoxide agents but influenced by their chemical reactivity due to a specific interaction with the electrode surface.
Abstract: In a search for corrosion resistent electrode materials for electrochemical solar cells the covalent, layered semiconductor WSe2, has been investigated. Its photoactivity in the visible spectral region (band gap 1.57 eV) involves d-d electron transitions which do not lead to an immediate photocorrosien. An energetically favourable energy band position makes it possible to operate p-type material as photocathodes for energy conversion. Photocell outputs in the order of 15 mA/cm2 and 0.5 V were obtained in 100 mW/cm2 illumination (Xenon-lamp). It is shown that the produced photovoltage is not systematically dependent on the redox potential of the redox agents but influenced by their chemical reactivity due to a specific interaction with the electrode surface. Auf der Suche nach korrosionsfesten Halbleiterelektroden fur elektrochemische Solarzellen ist das Schichtgittermaterial WSe2 untersucht worden. Photonenabsorption im sichtbaren Spektralbereich (Bandabstand 1,57 eV) bewirkt in diesem Halbleiter d-d-Elektronenubergange, die nicht zu einer sofortigen Photokorrosion der Elektrode fuhren. Die energetisch gunstige Position der Bandkanten erlaubt es, p-dotiertes Material als Photokathode fur elektrochemische Energieumwandlung zu benutzen. Bei Belichtung mit 100 mW/cm2 (Xenon-lampe) gab die Photozelle bis zu 15 mA/cm2 Photostrom und bis zu 0,5 V Photospannung ab. Es stellte sich heraus, das die Photospannung nicht in systematischer Weise vom Redoxpotential des Redoxelektrolyten abhangt, sondern von dessen chemischer Reaktivitat, die hier durch eine spezifische Wechselwirkung mit der Elektrodenoberflache beeinflust wird.

86 citations


Journal ArticleDOI
01 Feb 1978
TL;DR: In this paper, the MoSe2 layer crystals were successfully used as photoelectrodes in electrochemical solar cells and their mechanism is based on d d phototransitions in the electrode in combination with anodic electron transfer from suitable redox couples in the adjoining electrolyte (Fe2+/3+, I−/I2).
Abstract: n-type MoSe2 layer crystals were successfully used as photoelectrodes in electrochemical solar cells. Their mechanism is based on d d phototransitions in the electrode in combination with anodic electron transfer from suitable redox couples in the adjoining electrolyte (Fe2+/3+, I−/I2). Redox agents with potentials more negative than that of water can be oxidized by holes from the low lying Mo 4d energy band clearly before the anodic onset of the photoreaction with water molecules which would lead to the formation of selenic acid and to a gradual photodissolution of the electrode. In this way it is possible to operate regenerative electrochemical solar cells by-passing corrosive anodic reactions. With MoSe2: I−/I2 solar cells photocurrent densities of 22 mA/cm2, photovoltages of 0.55 V and energy conversion efficiencies between 4 and 5% were obtained in the red and near infrared spectral region. Their stability behaviour was tested at photocurrent densities between 11 and 12 mA/cm2 over a period of more than 1200 hours (50 days). No deterioration of the photoelement was observed. Capacity measurements indicate that reducing agents such as iodide ions are chemisorbed to surface states formed by trapped holes on d-orbitals. The resulting surface charge redistribution causes a relative shift of energy levels which suppresses the reaction with water and induces a space charge polarization that facilitates photoactivated electron transport from iodide. n-leitende MoSe-Schichtkristalle wurden erfolgreich als Photoelektroden in elektrochemischen Solarzellen verwendet. Ihr Mechanismus beruht auf d d-Photoubergangen in der Elektrode in Kombination mit anodischer Elektronenubertragung von geeigneten Redox-Systemen (Fe2+ 3+, I−/I2) im angrenzenden Elektrolyten. Redoxverbindungen mit Potentialen, die negativer als das des Wassers liegen, konnen durch Locher aus dem niedrigliegenden Mo 4 d-Energieband oxidiert werden, bevor die anodische Photoreaktion mit Wasser einsetzt, welche zur Bildung von seleniger Saure und zu einer allmahlichen Auflosung der Elektrode fuhren wurde. Es ist dadurch moglich, regenerative elektrochemische Solarzellen so zu betreiben, das man korrosive anodische Reaktionen vermeidet. Mit MoSe2: I−/I2-Zellen wurden Photostromdichten von 22 mA/cm2, Photopotentiale von 0,55 Volt und im roten und nahen infraroten Spektralbereich Energieumwandlungsausbeuten zwischen 4 und 5% erreicht. Ihr Stabilitatsverhalten wurde bei Photostromdichten zwischen 11 und 12 mA/cm2 uber eine Periode von mehr als 1200 Stunden (50 Tage) getestet. Es war kein Qualitatsabfall des Photoelements zu beobachten. Kapazitatsmessungen weisen darauf hin, das reduzierende Substanzen wie Jodid-Ionen an Oberflachenzustanden chemisorbieren, die durch eingefangene Locher auf d-Orbitalen hervorgerufen werden. Die resultierende Umverteilung der Oberflachenladung verursacht eine relative Verschiebung von Energieniveaus, welche die Reaktion mit Wasser unterdruckt und induziert eine Raumladungspolarisation, die einen photoaktivierten Elektrontransfer von Iodid begunstigt.

75 citations


Journal ArticleDOI
TL;DR: In this paper, several Raman bands below about 220 cm −1 are found, and the two strongest, at 194 ± 0.5 cm − 1 and 120.5 ± 0.5 cm−1, could clearly be assigned to Ag 2 and Ag 3 respectively.

72 citations


Journal ArticleDOI
01 Oct 1978
TL;DR: Using a refined technique, thin homogeneous RuO2 films were prepared reproducibly on highly polished Ti surfaces by thermal decomposition of ruthenium salts The produced electrodes were investigated with electrochemical techniques Chronocoulometry experiments were found to be particularly useful because they permitted a clear separation of surface and bulk processes.
Abstract: Using a refined technique, thin, homogeneous RuO2 films were prepared reproducibly on highly polished Ti surfaces by thermal decomposition of ruthenium salts The produced electrodes were investigated with electrochemical techniques Chronocoulometry experiments were found to be particularly useful because they permitted a clear separation of surface and bulk processes – The oxidation state of the surface Ru atoms was found to change in the potential range between hydrogen and oxygen evolution in 1 M H2SO4 from +2 to +6 Hydrogen adsorption is proposed at potentials more cathodic than +04 V (RHE) The associated bulk diffusion processes are discussed

66 citations


Journal ArticleDOI
TL;DR: In this paper, the width of the Pd valence band was found to be a sensitive function of particle size and the number of particles in a Pd array was investigated with UV photoelectron spectroscopy.

Journal ArticleDOI
TL;DR: In this paper, a quantitative analysis of the reflectance of CdS crystals in the spectral range of the A-exciton is presented, and measured spectra for the isotropic and the mixed-mode orientation of the crystal are fitted by calculations based on the model of excitonic polaritons.
Abstract: A careful quantitative analysis of the reflectance of CdS crystals in the spectral range of the A-exciton is presented. Measured spectra for the isotropic and the mixed-mode orientation of the crystal are fitted by calculations based on the model of excitonic polaritons. Spatial dispersion and Pekar's abc, damping, exciton-free surface layers, and oblique incidence of light are considered simultaneously. The good agreement between theory and experiment confirms the model, the still existing discrepancies are due to the simplifying assumption of a homogeneous surface layer. The formulae needed are summarized, and the influence of the different polariton parameters on the calculated reflectivity spectra are discussed. Es wird eine sorgfaltige Analyse des Reflexionsvermogens von CdS-Kristallen im Spektralbereich des A-Exzitons durchgefuhrt. Die gemessenen Spektren fur die isotrope Anordnung und die mixed-mode-Anordnung des Kristalls werden durch Rechnungen auf der Grundlage des Modells der Exzitonenpolaritonen angepast. Dabei werden raumliche Dispersion und Pekars abc, Dampfung, exzitonenfreie Oberflachenschicht und schrager Lichteinfall berucksichtigt. Die gute Ubereinstimmung zwischen Theorie und Experiment bestatigt das Modell, die noch existierenden Abweichungen beruhen auf der vereinfachenden Annahme einer homogenen Oberflachenschicht. Die benotigten Formeln werden zusammengefast und der Einflus der verschiedenen Polaritonen-parameter auf die berechneten Reflexionsspektren diskutiert.

Journal ArticleDOI
TL;DR: In this article, it was shown that alladium layers in the thickness range 0.1-15 monolayers were deposited by evaporation onto the polar ZnO(0001)Zn and (0001)O surfaces.

Journal ArticleDOI
TL;DR: In this paper, the excitation spectrum of the green emission band of ZnO:Cu crystals, zero phonon lines have been detected for the first time, and the experimental data are compatible with a model of the investigated centre in which the ground state is Cu2+ and the excited state is a Cu+ with a loosely bound hole.
Abstract: In the excitation spectrum of the green emission band of ZnO:Cu crystals, zero phonon lines have been detected for the first time. In the liquid-helium temperature range, a line with Δ ν = 2 cm−1 at λ = 433.5 nm coincides with the well-known zero-phonon line of the emission. Additional lines are located at 432.2 and 431.4 nm. In a distance of 525 cm−1 from these zero-phonon lines, phonon satellites superpose on the low-energy part of the broad excitation band. On its high-energy slope pronounced structures appear in the excitonic region. With selected highly-doped crystals, the new zero-phonon lines could also be detected in absorption. Zeeman experiments produce anisotropic splittings of the common ground state of all three lines with g∥ = 0.7 ± 0.1 and g⊥ = 1.5 ± 0.1 which are approximately the values known from emission. Different anisotropic g-factors around g = 2 are derived for the terminal states of the absorption lines. The experimental data are compatible with a model of the investigated centre in which the ground state is Cu2+ and the excited state is a Cu+ with a loosely bound hole: (Cu+)+ [3]. As to the position of the impurity levels within the forbidden gap, still two possibilities exist: the excited hole could either be localized near the oxygen neighbours so that a depth of some tenth of an eV results, or effective-mass-like states could be involved in a distance of only a few meV above the top of the valence band.

Journal ArticleDOI
TL;DR: In this paper, it was shown that paracrystalline α-Fe can be formed by groups of X2FeO4 with trivalent cations X such as Al, Sc, Cr, etc.

Journal ArticleDOI
01 Dec 1978-Nature
TL;DR: In this article, a mechanism is suggested for the liberation of electrical charge from underground: electrochemical glow discharge, which was suggested in the case of the Friuli earthquake in northeastern Italy.
Abstract: REPORTS of strange animal behaviour in my home village, which was struck by the Friuli earthquake in northeastern Italy (6 May 1976) convinced me that aerosol particles were electrostatically charged before the quake and that the physiological effects were essentially caused by an increase of the ion concentration in the air resulting in reactions such as the serotonin irritation syndrome. A mechanism is suggested here for the liberation of electrical charge from underground: electrochemical glow discharge.

Journal ArticleDOI
01 May 1978
TL;DR: In this paper, a transformation of the surface of CdSe and CdS single crystal photoanodes under illumination in sulfide/polysulfide redox electrolytes to polycrystalline or amorphous cdS has been found.
Abstract: A transformation of the surface of CdSe and CdS single crystal photoanodes under illumination in sulfide/polysulfide redox electrolytes to polycrystalline or amorphous CdS has been found. A reaction mechanism is proposed and its thermodynamics are discussed. The discovered transformation could cause problems for the application of these systems for electrochemical solar cells. Es wurde eine Umwandlung der Oberflache von einkristallinen CdS- und CdSe-Elektroden in polykristallines oder amorphes CdS unter Belichtung in Anwesenheit von Sulfid/Polysulfid-Redoxelektrolyt gefunden. Ein Reaktionsmechanismus wird vorgeschlagen und die Thermodynamik der Auflosungsprozesse diskutiert. Die beschriebene Umwandlung der Oberflachen wirft Probleme beim Einsatz dieser Materialien in elektrochemischen Solarzellen auf

Journal ArticleDOI
01 Apr 1978
TL;DR: In this paper, the method of glow-discharge polymerization was applied to deposit highly cross-linked polymer films of 100-1000 A thickness on steel, platinum, and glassy carbon electrodes.
Abstract: The method of glow-discharge polymerisation was applied to deposit highly cross-linked polymer films of 100-1000 A thickness on steel, platinum, and glassy carbon electrodes. These films were characterized by electrical, electrochemical, and optical techniques and found to be stable during electrolysis in liquid solutions of both inert and redox electrolytes. Moreover, interesting correlations between the electrochemical properties of the film-covered electrodes, studied by ac and dc techniques, and the conduction properties of metal-film-metal junctions using the same films, could be established. – Under suitable and well-defined preparation conditions the films possess sufficiently high conductivity that their electrocatalytic properties can be exploited. Insertion of metal clusters or chelate polymers is likely to improve the electrocatalytic activity, and the film electrode systems are thus promising, e. g. in fuel cells and other practical applications. Mit der Methode der Gasentladungs-Polymerisation wurden auf Metall- und Glaskohlenstoffelektroden quervernetzte Polymerfilme von etwa 100-1000 A Dicke aufgebraucht. Sie waren bei der Elektrolyse in wasrigen Elektrolyten stabil. – Das elektrochemische Verhalten der polymerbedeckten Elektroden wurde mit Wechsel- und Gleichstrommethoden in inerten und Redox-Elektrolyten untersucht. Auserdem wurden die Festkorpereigenschaften der Polymerschichten untersucht und mit den Ergebnissen der elektrochemischen Messungen korreliert. – Die Polymerschichten haben fur die Sauerstoff-Reduktion eine gewisse elektrokatalytische Wirksamkeit. Die katalytische Wirksamkeit last sich wahrscheinlich durch Einbau von Metallen in die Schicht, etwa in Form von Clustern oder Chelat-Polymeren, noch steigern. Damit konnten diese Elektroden auch fur eine praktische Verwendung in Betracht kommen, etwa fur Brennstoffzellen.

Journal ArticleDOI
TL;DR: A review on recent developments in field ionization mass spectrometry of inorganic compounds can be found in this article, where different mechanisms of ion formation at surfaces and under the influence of extremely high electric fields, field dependent chemical reactivity at surfaces, and ion desorption from surfaces are discussed.

Journal ArticleDOI
TL;DR: The influence on the DQE of the pulse height distribution of the single electron video signals and of a multiple counting are eliminated by means of special electronic components.

Journal ArticleDOI
01 Sep 1978
TL;DR: In this paper, the influence of anionic micelles (SDS) on the kinetics of diffusion controlled outer sphere electron transfer reactions between transition metal complexes has been investigated, and it was shown that SDS interaction with the reactants can cause ion association (e.g. Os(dipy)) as well as through hydrophobic forces.
Abstract: We report a systematic investigation of the influence of anionic micelles (SDS) on the kinetics of diffusion controlled outer sphere electron transfer reactions between transition metal complexes. — The micelles interact with the reactants through electrostatic repulsion (e. g. IrCl62) or attraction which can cause ion association (e. g. Os(dipy)) as well as through hydrophobic forces (e. g. Fe(phen)2(CN)20). The rate constants kET of these reactions range in the order of 1010 M−1 s−1 in aqueous solution and are decreased by more than four orders of magnitude if these forces superimpose. — By variation of the SDS concentrations and the ionic strengths electrostatic influences were largely eliminated. It was thus possible to establish a hydrophobic barrier in electron transfer reactions between complexes which are solubilized in the hydrophobic core of the micelles and substitution inert complexes in the bulk solution. Wir berichten uber eine systematische Untersuchung des Einflusses von anionischen Mizellen (NaDS) auf die Kinetik schneller, in wasriger Losung meist diffusionskontrollierter outer-sphere Elektronenubertragungsreaktionen zwischen ubergangsmetallkomplexen. — Die Mizellen wechselwirken mit den Reaktanden durch elektrostatische Abstosung (z. B. IrCl62−) oder Anziehung, die zur Bildung von Ionenassoziaten (z. B. Os(dipy)) fuhren kann, sowie durch hydrophobe Krafte (z. B. Fe(phen)2(CN)20). Die Geschwindigkeitskonstanten. kET, dieser Reaktionen von etwa 1010 M−1 s−1 in H2O werden beim Zusammenwirken dieser Krafte bis zu vier Zehnerpotenzen erniedrigt. — Durch Variation von NaDS-Konzentration und Ionenstarke konnten elektrostatische Einflusse weitgehend ausgeschlossen werden. und eine hydrophobe Barriere bei der Elektronenubertragung zwischen den in Mizellen gelosten Komplexen und den Reaktionspartnern in der Losung nachgewiesen werden.

Journal ArticleDOI
TL;DR: In this paper, the c(2×2) Se overlayer on Ni(001) was studied by angular resolved UPS using unpolarised and polarised HeI (21.2 eV) radiation.

Journal ArticleDOI
TL;DR: In spite of the inevitable chemical damage at high doses, virtually all molecules in orientations that exhibited the characteristic protamine structure could still be recognized in dark field scanning transmission or fixed beam bright field at 100-200 e/A2.

Journal ArticleDOI
01 Oct 1978
TL;DR: In this paper, the sensitization photocurrent at single crystalline ZnO electrodes in contact with an aqueous solution of crystal violet has been investigated during excitation by a 1 msec laser pulse.
Abstract: The sensitization photocurrent at single crystalline ZnO electrodes in contact with an aqueous solution of crystal violet has been investigated during excitation by a 1 msec laser pulse. The photocurrent shows relaxation phenomena which can be explained by an involvement of “surface states. They capture electrons from the excited dye molecules followed by electron injection into the conduction band or recombination” with the oxidized dye molecules. – The quantum yield for the electron injection is in the order of 2%. This relatively low value a high efficiency of the recombination via surface states. – In the presence of hydroquinone, a slight increase of the quantum observed which is attributed to a photoreduction of the excited dye molecules. The reduced crystal violet molecule can inject an electron into the conduction band from its ground state. Es wurden sensibilisierte Photostrome an einkristallinen ZnO-Elektroden im Kontakt mit wasrigen, Kristallviolett enthaltenden Elektro. Die Photostrome wahrend eines Laser-Lichtpulses von 1 msec Dauer zeigen Relaxationserscheinungen. Diese werden Einfang von Elektronen aus angeregten Farbstoffmolekulen in Oberflachenzustanden mit anschliesender Injektion dieser Elektronen in das Letiungsband oder Rekombination mit den oxidierten Molekulen erklart. – Die Quantenausbeute fur Elektroneninjektionen der Grosenordnung von 2%. Das deutet auf eine sehr grose Effektivitat der Rekombination uber Oberflachenzustande hin. – von Hydrochinon wird ein geringer Anstieg der Quantenausbeute beobachtet. Dies wird einer Photoreduktion der an Farbstoffmolekule zugeschrieben, wobei angenommen wird, das das reduzierte Kristallviolett-Molekul direkt ein Elektron in das Leitungsband injizieren kann.


Journal ArticleDOI
TL;DR: Radiation damage on a holey carbon foil was investigated in an electron microscope with a superconducting lens system, where the temperature of the specimen and its environment initially was 4 K, and the diameter of a hole increased 5 nm due to an electron dose of 2 X 10(4) As/cm2.

Journal ArticleDOI
TL;DR: In this article, the authors measured the electroreflectance in an electrochemical cell on a variety of copper single crystal faces at normal incidence of the light and observed striking crystallographic and polarisation anisotropies which cannot be explained within a free electron model.


Journal ArticleDOI
TL;DR: In this article, the authors showed that the appearance potentials of rare gas ions displayed no detectable field dependence and showed that energy deficits are not influenced by polarization energies, and that the local potential at the point of formation, rather than the energetics of the process, determined the critical energy loss of the field ions.

Journal ArticleDOI
TL;DR: The quantum efficiency of photooxidation of a number of xanthene dyes at ZnO single crystal electrodes has been found to depend on the solution concentration of a triplet quenching agent, FeCN 4−6, providing evidence for triplet state participation in the oxidation reaction as discussed by the authors.