Small Polarons and the Janus Nature of TiO_{2} (110)

Polarons are ubiquitous in many semiconductors and have been linked with conductivity and optical response of materials for photovoltaics and heterogeneous catalysis, yet it remains unclear how surface polarons influence adsorption. Here, by modeling the surface of rutile titania using density functional theory, we reveal the effect of small surface polarons on water adsorption, dissociation, and hydrogen bonding. On the one hand, the presence of such polarons significantly suppresses dissociation of water molecules that are bonded directly to polaronic sites. On the other hand, polarons facilitate water dissociation at certain nonpolaronic sites. Furthermore, polarons strengthen hydrogen bonds, which in turn affects water dissociation in hydrogen-bonded overlayer structures. This study reveals that polarons at the rutile surface have complex, multifaceted, Janus-like effects on water adsorption, dissociation, and hydrogen bonding, highlighting the importance of polarons on water structure and dynamics on such surfaces. We expect that many of the physical properties of surface polarons identified here will apply more generally to surfaces and interfaces that can host small polarons, beyond titania

Reduction and oxidation of Au adatoms on the CeO2(111) surface - DFT plus U versus hybrid functionals

Recently we showed that Au atoms may titrate Ce3+ ions in near-surface layers of reduced CeO2(111). This surface contained oxygen vacancies in subsurface position within the topmost O–Ce–O trilayer [Pan et al., Phys. Rev. Lett., 2013, 111, 206101.]. The present work builds upon these findings and discusses additional results obtained using PBE+U and hybrid functionals. These approaches do not predict the same relative stabilities for the various adsorption sites of a single Au adatom at an O-defect concentration of a ¼ ML or 1.984 nm−2. We attribute this discrepancy to a different alignment within the O 2p–Ce 4f gap, i.e. a different order by energy of partially occupied Ce 4f and Au 6s orbitals. The energy offset of these orbitals matters, because the adsorption of Au0(6s1) atop Ce3+(4f1) or atop a subsurface oxygen atom in the first coordination shell of a Ce3+(4f1) involves creation of Au−(6s2) and Ce4+(4f0) ions. The electron transfer to Au is coupled to stabilizing ionic relaxation in the lattice, commonly known as polaronic distortion, reinforcing the Au–Ce bond. The order of 4f and 6s orbitals depends on the density functional approximation and is also strongly influenced by the oxygen defect concentration

Titration of Ce³⁺ Ions in the CeO₂(111) Surface by Au Adatoms

The role of surface and subsurface O vacancies for gold adsorption on crystalline CeO2(111) films has been investigated by scanning tunneling microscopy and density functional theory. Whereas surface vacancies serve as deep traps for the Au atoms, subsurface defects promote the formation of characteristic Au pairs with a mean atom distance of two ceria lattice constants (7.6 Å). Hybrid density functional theory calculations reveal that the pair formation arises from a titration of the two Ce3+ ions generated by a single O vacancy. The Au-Ce3+ bond forms due to a strain effect, as the associated charge transfer from the spacious Ce3+ into the adgold enables a substantial relaxation of the ceria lattice. Also the experimentally determined Au-pair length is reproduced in the calculations, as we find a Ce3+-Ce3+ spacing of two ceria lattice parameters to be energetically preferred. Single Au atoms can thus be taken as position markers for Ce3+ ion pairs in the surface, providing unique information on electron-localization phenomena in reduced ceria

Constitutional isomerism of the linkages in donor acceptor covalent organic frameworks and its impact on photocatalysis

When new covalent organic frameworks COFs are designed, the main efforts are typically focused on selecting specific building blocks with certain geometries and properties to control the structure and function of the final COFs. The nature of the linkage imine, boroxine, vinyl, etc. between these building blocks naturally also defines their properties. However, besides the linkage type, the orientation, i.e., the constitutional isomerism of these linkages, has rarely been considered so far as an essential aspect. In this work, three pairs of constitutionally isomeric imine linked donor acceptor D A COFs are synthesized, which are different in the orientation of the imine bonds D C N A DCNA and D N C A DNCA . The constitutional isomers show substantial differences in their photophysical properties and consequently in their photocatalytic performance. Indeed, all DCNA COFs show enhanced photocatalytic H2 evolution performance than the corresponding DNCA COFs. Besides the imine COFs shown here, it can be concluded that the proposed concept of constitutional isomerism of linkages in COFs is quite universal and should be considered when designing and tuning the properties of COF

Protonated Imine Linked Covalent Organic Frameworks for Photocatalytic Hydrogen Evolution

Covalent organic frameworks COFs have emerged as an important class of organic semiconductors and photocatalysts for the hydrogen evolution reaction HER from water. To optimize their photocatalytic activity, typically the organic moieties constituting the frameworks are considered and the most suitable combinations of them are searched for. However, the effect of the covalent linkage between these moieties on the photocatalytic performance has rarely been studied. Herein, we demonstrate that donor acceptor D A type imine linked COFs can produce hydrogen with a rate as high as 20.7 mmol g 1 h 1 under visible light irradiation, upon protonation of their imine linkages. A significant red shift in light absorbance, largely improved charge separation efficiency, and an increase in hydrophilicity triggered by protonation of the Schiff base moieties in the imine linked COFs, are responsible for the improved photocatalytic performanc

Vanadium Oxide Oligomers and Ordered Monolayers Supported on CeO2(111): Structure and Stability Studied by Density Functional Theory

Structures and stabilities of vanadium oxide oligomers as well as two candidate structures for a monolayer on the CeO2(111) surface have been studied by density functional theory, employing a genetic algorithm to determine the global energy minimum structures. These ceria-supported structures have predominantly 4-fold coordinated V5+ ions with V═O groups in common. The agglomeration of VO2 clusters deposited on the surface is a strongly exothermic process, particularly when ring structures with three or six VO2 units are formed that are commensurate with the close-packed surface-terminating oxygen layer. The VO2 and V2O5 monolayers feature larger coordination numbers (5, 6) of V and contain V atoms without V═O groups. Relative to oligomers, VO2 and V2O5 monolayer structures with and without oxygen defects are thermodynamically more stable. This, together with the fact that flat “monolayer” clusters are preferred to taller “bilayer” clusters, indicates the preference for a complete two-dimensional wetting of the ceria support

Programmable photocatalytic activity of multivariate covalent organic frameworks used as metallaphotocatalysts

The multivariate approach allows to incorporate several functionalities into a single covalent organic framework (COF) and consequently allows the construction of bifunctional materials for cooperative catalysis. The well-defined structure of such multivariate COFs is furthermore ideally suited for structure-activity relationship studies. We report a series of multivariate COFs that contain acridine- and 2,2’-bipyridine linkers connected through 1,3,5-benzenetrialdehyde derivatives. The acridine motif is responsible for broad light absorption, while the bipyridine unit enables complexation of nickel catalysts. These features enable usage of the framework materials as catalysts for light-mediated carbon–heteroatom cross-couplings. Variation of the node units shows that the catalytic activity correlates to the keto-enamine tautomer isomerism. This allows switching between high charge-carrier mobility and persistent, localized charge-separated species depending on the nodes, a tool to tailor the materials for specific reactions. Moreover, nickel-loaded COFs are recyclable and catalyze cross-couplings even using red light irradiation

Einfluss von Verzahnungsabweichungen auf das Passungs- und Laufverhalten von Zahnwellen-Verbindungen und Zahnkupplungen. Funktionsgerechtes Toleranzsystem und betriebsnahe Mess- und Lehrenverfahren Wissenschaftliche Zuarbeit zur Normung von Toleranzen und Messverfahren auf europaeischer Basis. Abschlussbericht

SIGLEAvailable from TIB Hannover: F98B1814 / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekStiftung Volkswagenwerk, Hannover (Germany)DEGerman

Instrumente und Handlungsmoeglichkeiten der Bundeslaender zum Klimaschutz. Bd. 1: Analyseband. Bd. 2: Materialband. Bd. 3: Rechtsgutachten

Im Analyseband (Band 1) werden die rechtlichen und politischen Rahmenbedingungen der Landesklimaschutzpolitik dargestellt, gefolgt von einer Analyse dieser Politik hinsichtlich ihrer Entwicklung, Ziele und Schwerpunkte, bestehenden Hindernissen, zukuenftiger Ausrichtung, Handlungsoptionen und Beispiele fuer die Erschliessung neuer Taetigkeitsgebiete der Laender. Der Materialband (Band 2) enthaelt neben umfassenden Daten zur Schilderung der Situation auch von den Laendern ausgewaehlte, erfolgreiche Beispiele fuer die Durchfuehrung von Klimaschutzmassnahmen. In einem Rechtsgutachten (Band 3) werden die verfassungsrechtlichen Grundlagen der Kompetenzverteilung zwischen Bund und Laendern auf dem Gebiet des Klimaschutzes detailliert dargelegt und Moeglichkeiten der Umsetzung von Klimaschutzpolitik durch Landesgesetzgebung sowie gesetzesvollziehende und nicht-gesetzesvollziehende Verwaltungstaetigkeit untersucht. (orig./CB)In the analysis volume, (vol. 1), the legal and political frameworks governing the greenhouse gas reduction policy adopted by the Federal states of Germany are explained. The evolution of the policies, goals pursued and points of major emphasis, the orientation for the future and existing impediments for implementation are analysed. Volume 2 contains a comprehensive baseline data library characterising the present situation, as well as examples selected by the Federal states, illustrating policy schemes which proved to be successful. The legal opinion presented in volume 3 explains the distribution of competences for climate policy of the Federal Government and the state governments in accordance with constitutional law, as well as channels of implementing policy schemes via legislation issued by the Federal states, or administrative action in execution of the law, or other administrative action. (orig./CB)Published in 3 volumesAvailable from TIB Hannover: RN 8908(2000,331,1): RN 8908(2000,331,2): RN 8908(2000,331,3) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEBundesministerium fuer Umwelt, Naturschutz und Reaktorsicherheit, Bonn (Germany)DEGerman