Markus Kratzer

Adsorption/desorption of H2 and CO on Zn-modified Pd(111)

The adsorption and thermal desorption of H(2) and CO on clean and Zn covered Pd(111) surfaces were studied using temperature programmed desorption (TPD), low energy electron diffraction, and Auger electron spectroscopy. The obtained H(2) and CO-TPD results reveal that thick Zn layers (approximately 10 ML) prepared at low temperature (150 K) block the adsorption of H(2) and CO. However, the ZnPd surface alloy which is formed at temperatures above 300 K shows a different behavior. The amount of hydrogen adsorbed on surface sites is reduced by about 1/2 on the ZnPd surface alloy whereupon the diffusion of hydrogen into the subsurface region is not influenced. The initial sticking coefficient decreases from 0.5 on the clean surface to 0.14 on the ZnPd alloy. The TPD spectra for CO on the ZnPd surface alloy show that the heat of adsorption is shifted to much lower values than on clean Pd, yielding a desorption energy of 71+/-2 kJ mol(-1) at low CO coverages. The saturation coverage equals 0.5 ML which means that each Pd atom of the ZnPd surface alloy is occupied by one CO admolecule. Interestingly, however, the initial sticking coefficient for CO on the ZnPd surface alloy is still unity, as on the clean Pd surface.

Methanol adsorption on Cu(110) and the angular distribution of the reaction products

Integral and angle resolved thermal desorption spectroscopies were used to study methanol adsorption and oxidation on clean and oxygen covered Cu(110) surfaces. Special emphasis was put on the Cu-CuO stripe phase, which forms when the Cu(110) surface is covered with 0.25 ML of oxygen. In the temperature regime between 200 and 300 K associative desorption of methanol and water takes place, showing a normal desorption character with peaks shifting to lower temperature with increasing coverage and with a nearly cosine angular desorption distribution. In the temperature range of about 350 K formaldehyde, hydrogen, and again methanol desorb nearly concomitantly in the form of a very narrow peak (full width at half maximum=10 K), with peaks shifting to higher temperature with increasing methanol coverage. The angular distribution of these peaks is strongly forward focused, indicating activation barriers being involved. In the case of the Cu-CuO stripe phase the angular distribution of the desorption products is clearly different in the [110] and [001] azimuthal directions, demonstrating the influence of the border lines between the copper and the copper oxide stripes on the desorption process.

Manipulating the activation barrier for H2(D2) desorption from potassium-modified palladium surfaces

In this work the permeation and desorption of hydrogen (deuterium) from potassium-modified Pd(111) and polycrystalline palladium surfaces have been studied in the temperature range from 350 to 523 K. Time-of-flight spectroscopy has been used to determine the translational energy distributions of associatively desorbing H(2)(D(2)) molecules as a function of the potassium coverage and additional isotropic O(2) and CO background pressures. It turned out that the energy distribution of the hydrogen desorption flux is thermalized for the clean Pd surfaces but hyperthermal for the potassium-covered surfaces. The activation barrier for adsorption was found to increase with the potassium coverage but to decrease again in the presence of coadsorbates such as O(2) or CO. Especially by choosing different isotropic CO pressures, the effective desorption barrier for hydrogen could be reversibly decreased and increased, which resulted in the equivalent changes of the mean kinetic energies of the desorbing H(2) molecules.

Model reaction studies on vanadium oxide nanostructures on Pd(111)

Deuterium desorption and reaction between deuterium and oxygen to water has been studied on ultrathin vanadium oxide structures prepared on Pd(111). The palladium sample was part of a permeation source, thus enabling the supply of atomic deuterium to the sample surface via the bulk. Different vanadium oxide films have been prepared by e-beam evaporation in UHV under oxygen atmosphere. The structure of these films was determined using low energy electron diffraction and scanning tunneling microscopy. The mean translational energy of the desorption and reaction products has been measured with a time-of-flight spectrometer. The most stable phases for monolayer and submonolayer VOx are particular surface-V2O3 and VO phases at 523 and 700 K, respectively. Thicker films grow in the form of bulk V2O3. The mean translational energy of the desorbing deuterium species corresponds in all cases to the thermalized value. Apparent deviations from this energy distribution could be attributed to different adsorption/desorption and/or accommodation behaviors of molecular deuterium from the gas phase on the individual vanadium oxide films. The water reaction product shows a slightly hyperthermal mean translational energy, suggesting that higher energetic permeating deuterium contributes with higher probability to the water formation.

Effects of hole-transport layer homogeneity in organic solar cells – A multi-length scale study

Abstract Irreproducibility is a serious issue in thin film organic photovoltaic (OPV) devices, as smallest local inhomogeneities can change the entire behaviour of identically built devices without showing obvious failure. Inhomogeneities can occur at various steps of device preparation and appear in all layers with different length scales and impact. The hole-transport interlayer (HTL) in OPV devices blocks unwanted electron diffusion to the anode and corrects energetic mismatch between oxide electrode and organic semiconductor. Most commonly used is commercial ink based on poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) colloidal particles. However, exactly these are suspected to cause microscopic inhomogeneities, causing known irreproducibility of device characteristics. Considering PEDOT:PSS’ acidity-caused electrode corrosion, it is questionable how much impact colloids have on device homogeneity. In this report, we give proof that a colloidal HTL does not necessarily cause device inhomogeneity and decreased efficiency, by comparing OPV devices with different HTLs, namely from commercial PEDOT:PSS ink and from MoO 3 , obtained from two liquid precursors, leading to quasi-continuous or colloidal layers. With a combination of X-ray diffraction, atomic force and Kelvin probe microscopy, photoelectron and ambient air photoemission spectroscopy, we discuss the layers’ properties from nano- to macroscale and demonstrate their impact upon implementation into OPV devices, via spatially-resolved characterization.

Principal Factors of Contact Charging of Minerals for a Successful Triboelectrostatic Separation Process – a Review

Triboelectrostatic separation is a promising method used to separate non-conductive minerals. However, the knowledge about the underlying triboelectrification mechanisms is still very limited. Thus, predicting the separation results and finding proper separation parameters are challenging tasks.This article presents a comprehensive summary of phenomena and factors which play a decisive role in the charging behavior of non-conductors and, by implication, the efficiency of the separation process, such as water and adsorbents layers on the surface, surface roughness, humidity, type of contact, etc. The authors hope that this article opens a way for a systematic approach through basic experiments dedicated to a better understanding of triboelectrification processes.ZusammenfassungDie Elektroscheidung nach Triboaufladung stellt eine vielversprechende Methode zur Trennung nicht leitfähiger Mineralphasen dar. Das begrenzte Wissen über die zugrundeliegenden Mechanismen der Triboaufladung macht die Vorhersage von über einzustellende Prozessparameter zu erzielenden Trennergebnissen zu einer herausfordernden Aufgabe.Diese Veröffentlichung gibt einen umfassenden Überblick über Phänomene und Faktoren, die eine entscheidende Rolle beim Aufladeverhalten von Nichtleitern spielen oder spielen können und die damit Einfluss auf die Effizienz der Trennung nehmen. Solche Faktoren können sein: Wasser- und Adsorbatschichten, Oberflächenrauigkeit, Luftfeuchtigkeit, Kontaktart, usw. Die Autoren hoffen, dass dieser Artikel Wege für einen systematischen Ansatz durch grundlegende Experimente aufzeigt und damit zu einem besseren Verständnis der bei der Triboaufladung wirkenden Faktoren beiträgt.

Corrigendum: Epitaxy of highly ordered organic semiconductor crystallite networks supported by hexagonal boron nitride

Corrigendum: Epitaxy of highly ordered organic semiconductor crystallite networks supported by hexagonal boron nitride