Ludger Schöttner

Karlsruhe Institute of Technology

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Christof Wöll

Karlsruhe Institute of Technology

Alexei Nefedov

Karlsruhe Institute of Technology

Yuemin Wang

Karlsruhe Institute of Technology

Lars Heinke

Karlsruhe Institute of Technology

Frank Röminger

Heidelberg University

Kai Müller

Federal Institute For Materials Research and Testing

Michael Mastalerz

Heidelberg University

Victor Brosius

University of Colorado Boulder

Dennis Reinhard

Heidelberg University

Oliver Hofmann

Graz University of Technology

Cooperative Institutions

Karlsruhe Institute of Technology

Karlsruhe University of Education

FIZ Karlsruhe – Leibniz Institute for Information Infrastructure

Humboldt-Universität zu Berlin

FZI Research Center for Information Technology

Fraunhofer Institute for Chemical Technology

Heidelberg University

TU Dresden

Robert Bosch (Germany)

Leipzig University

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Photoswitchable nanoporous films by loading azobenzene in metal–organic frameworks of type HKUST-1

Chemical Communications (2017)

Kai Müller Jasmine Wadhwa J. S. Malhi Ludger Schöttner Alexander Welle

Photoswitchable metal–organic frameworks (MOFs), realized by loading with azobenzene molecules, enable the dynamic remote control of their key properties.

Azobenzene

Nanoporous

10.1039/c7cc00961e

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Citations (77)

Doping-Induced Electron Transfer at Organic/Oxide Interfaces: Direct Evidence from Infrared Spectroscopy

The Journal of Physical Chemistry (2020)

Ludger Schöttner Simon Erker Raphael Schlesinger Norbert Koch Alexei Nefedov

Charge transfer at organic/inorganic interfaces critically influences the properties of molecular adlayers. Although for metals such charge transfers are well documented by experimental and theoretical results, in the case of semiconductors, clear and direct evidence for a transfer of electrons or holes from oxides with their typically high ionization energy is missing. Here, we present data from infrared reflection–absorption spectroscopy demonstrating that despite a high ionization energy, electrons are transferred from ZnO into a prototype strong molecular electron acceptor, hexafluoro-tetracyano-naphthoquinodimethane (F₆-TCNNQ). Because there are no previous studies of this type, the interpretation of the pronounced vibrational red shifts observed in the experiment was aided by a thorough theoretical analysis using density functional theory. The calculations reveal that two mechanisms govern the pronounced vibrational band shifts of the adsorbed molecules: electron transfer into unoccupied molecular levels of the organic acceptor and also the bonding between the surface Zn atoms and the peripheral cyano groups. These combined experimental data and the theoretical analysis provide the so-far missing evidence of interfacial electron transfer from high ionization energy inorganic semiconductors to molecular acceptors and indicates that n-doping of ZnO plays a crucial role.

Acceptor

Electron acceptor

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Interaction of Water Molecules with the α-Fe₂O₃(0001) Surface: A Combined Experimental and Computational Study

The Journal of Physical Chemistry C (2019)

Ludger Schöttner Roman Ovcharenko Alexei Nefedov Elena Voloshina Yuemin Wang

The interaction of water with the basal plane (0001) of α-Fe2O3 (hematite) is a fundamental and challenging topic in the fields of surface science and earth science. Despite intensive investigations, many issues remain unclear especially due to the lack of direct spectroscopic evidence. Here, water adsorption on the pristine Fe-terminated α-Fe2O3(0001) surface was investigated by polarization-dependent infrared reflection absorption spectroscopy and X-ray photoelectron spectroscopy in conjunction with calculations from density functional theory. The combined results provide solid evidence that the interaction of water with α-Fe2O3(0001) is dominated by the heterolytic dissociation yielding an OwD species coordinated in atop-configuration to surface Fe3+ and an OsD species resulting from the deuterium/hydrogen transfer to an adjacent substrate O2–. Both isolated hydroxyl groups do not feature any hydrogen bonding, while the intact water molecules were identified as minor species that are bound to surface Fe3+ ions and interact via a relatively strong H-bonding with substrate oxygen. Water adsorption on α-Fe2O3(0001) at 230 and 200 K leads to the formation of water thin films including bilayers and multilayers, which are characterized by different types of intermolecular H-bonds.

Heterolysis

10.1021/acs.jpcc.8b08819

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Citations (33)

Continuous microfluidic synthesis of colloidal ultrasmall gold nanoparticles:in situstudy of the early reaction stages and application for catalysis

Reaction Chemistry & Engineering (2017)

Ghazal Tofighi Henning Lichtenberg Jan Pesek Thomas L. Sheppard Wu Wang

The formation of gold nanoparticles in the first 2–20 ms of the reaction was studiedin situwith XAS using microfluidics.

10.1039/c7re00114b

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Defects as Color Centers: The Apparent Color of Metal–Organic Frameworks Containing Cu²⁺-Based Paddle-Wheel Units

ACS Applied Materials & Interfaces (2017)

Kai Müller Karin Fink Ludger Schöttner Meike Koenig Lars Heinke

As in the case of other semiconducting materials, optical and electronic properties of metal-organic frameworks (MOFs) depend critically on defect densities and defect types. We demonstrate here that, in addition to the influence of imperfections on MOF chemical properties like guest binding energies and catalytic activity, the optical properties of these crystalline molecular solids also crucially depend on deviations from the perfect crystalline structure. By recording UV-vis absorption spectra for MOF thin films of particularly high quality, we demonstrate that low-defect samples of an important MOF, HKUST-1, are virtually colorless. Electronic structure calculations of the excited states by employing complete active space self-consistent field (CASSCF) calculations show that the d-d excitations in defects result in the typical green color of the MOF material synthesized by conventional methods.

10.1021/acsami.7b12045

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Citations (64)

Doping-Induced Electron Transfer at Organic/Oxide Interfaces: Direct Evidence from Infrared Spectroscopy

The Journal of Physical Chemistry C (2020)

Ludger Schöttner Simon Erker Raphael Schlesinger Norbert Koch Alexei Nefedov

Charge transfer at organic/inorganic interfaces critically influences the properties of molecular adlayers. Although for metals such charge transfers are well documented by experimental and theoretical results, in the case of semiconductors, clear and direct evidence for a transfer of electrons or holes from oxides with their typically high ionization energy is missing. Here, we present data from infrared reflection-absorption spectroscopy demonstrating that despite a high ionization energy, electrons are transferred from ZnO into a prototype strong molecular electron acceptor, hexafluoro-tetracyano-naphthoquinodimethane (F6-TCNNQ). Because there are no previous studies of this type, the interpretation of the pronounced vibrational red shifts observed in the experiment was aided by a thorough theoretical analysis using density functional theory. The calculations reveal that two mechanisms govern the pronounced vibrational band shifts of the adsorbed molecules: electron transfer into unoccupied molecular levels of the organic acceptor and also the bonding between the surface Zn atoms and the peripheral cyano groups. These combined experimental data and the theoretical analysis provide the so-far missing evidence of interfacial electron transfer from high ionization energy inorganic semiconductors to molecular acceptors and indicates that n-doping of ZnO plays a crucial role.

Acceptor

Electron acceptor

10.1021/acs.jpcc.9b08768

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Synthesis ofpara-Aryl-Substituted Salicyldialdehydes

European Journal of Organic Chemistry (2015)

Dennis Reinhard Ludger Schöttner Victor Brosius Frank Röminger Michael Mastalerz

Abstract para ‐Substituted salicyldialdehydes are versatile molecular building blocks for porous organic cages, fluorescent dyes, spirobenzopyrans, homooxacalixarenes, dinuclear metal complexes, macrocycles, and cryptands. To date, no procedures for the synthesis of para ‐aryl‐substituted salicyldialdehydes have been reported, which would broaden the scope of the above‐mentioned topics, especially in the field of sensors and dyes, because the aromatic π systems can be enlarged and absorption or fluorescent events can be fine‐tuned by various substituents. Herein, two synthetic routes to para ‐aryl‐substituted salicyldialdehydes with both electron‐withdrawing and ‐donating aromatic units are reported.

Cryptand

Borylation

10.1002/ejoc.201500228

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Structural Evolution of α-Fe2O3(0001) Surfaces Under Reduction Conditions Monitored by Infrared Spectroscopy

Frontiers in Chemistry (2019)

Ludger Schöttner Alexei Nefedov Chengwu Yang Stefan Heißler Yuemin Wang

The precise determination of the surface structure of iron oxides (hematite and magnetite) is a vital prerequisite to understand their unique chemical and physical properties under different conditions. Here, the atomic structure evolution of the hematite (0001) surface under reducing conditions was tracked by polarization-resolved infrared reflection absorption spectroscopy (IRRAS) using carbon monoxide (CO) as a probe molecule. The frequency and intensity of the CO stretch vibration is extremely sensitive to the valence state and electronic environments of surface iron cations. Our comprehensive IRRAS results provide direct evidence that the monocrystalline, stoichiometric α-Fe2O3(0001) surface is single Fe-terminated. The initial reduction induced by annealing at elevated temperatures produces surface oxygen vacancies, where the excess electrons are localized at adjacent subsurface iron ions (5-fold coordinated). A massive surface restructuring occurs upon further reduction by exposing to atomic hydrogen followed by Ar+-sputtering and annealing under oxygen poor conditions. The restructured surface is identified as a Fe3O4(111)/Fe1-xO(111)-biphase exposing both, Fe3+ and Fe2+ surface species. Here the well-defined surface domains of Fe3O4(111) exhibit a Feoct2-termination, while the reduced Fe1-xO(111) is Fe2+(oct)-terminated. These findings are supported by reference IRRAS data acquired for CO adsorption on magnetite (111) and (001) monocrystalline surfaces.

10.3389/fchem.2019.00451

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Citations (27)

ZnO@ZIF-8: Gas sensitive core-shell hetero-structures show reduced cross-sensitivity to humidity

Sensors and Actuators B Chemical (2019)

Sreeja Sreedharan Nair Navas Illyaskutty Benjamin Tam A. Özgür Yazaydın Katja Emmerich

Nanorod

Kirkendall Effect

Gaseous diffusion

10.1016/j.snb.2019.127184

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Citations (41)

Dissolving uptake-hindering surface defects in metal–organic frameworks

Chemical Science (2018)

Kai Müller Nina Vankova Ludger Schöttner Thomas Heine Lars Heinke

Metal-organic frameworks (MOFs) have unique properties which make them perfectly suited for various adsorption and separation applications; however, their uses and efficiencies are often hindered by their limited stability. When most MOFs are exposed to water or humid air, the MOF structure, in particular at the surface, is destroyed, creating surface defects. These surface defects are surface barriers which tremendously hinder the uptake and release of guest molecules and, thus, massively decrease the performance in any application of MOFs. Here, the destruction by exposure to water vapor is investigated by using well-defined MOF films of type HKUST-1 as a model system for uptake experiments with different-sized probe molecules as well as for spectroscopic investigations, complemented by density functional theory calculations of the defect structure. In addition to the characterization of the surface defects, it is found that the pristine MOF structure can be regenerated. We show that the surface defects can be dissolved by exposure to the synthesis solvent, here ethanol, enabling fast uptake and release of guest molecules. These findings show that the storage of MOF materials in a synthesis solvent results in healing of surface defects and enables ideal performance of MOF materials.

10.1039/c8sc03735c

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Citations (60)