Prof. Dr. Klaus Rademann
Profil
Forschungsthemen10
Advanced Chemistry Teaching, ISAP HU University, intern. Studien- und Ausbildungspartnerschaft
Quelle ↗Förderer: DAAD Zeitraum: 09/2017 - 12/2019 Projektleitung: Prof. Dr. Klaus Rademann
Entwicklung magnetisch leitfähiger Elastomere mit 3D-Druck für induktive Übertrager mit Anwendungsentwicklung
Quelle ↗Förderer: BMWE: ZIM Zeitraum: 07/2018 - 06/2020 Projektleitung: Prof. Dr. Klaus Rademann, Prof. Dr. Nicola Pinna
EXIST CreativeQuantum
Quelle ↗Förderer: Bundesministerium für Forschung, Technologie und Raumfahrt Zeitraum: 03/2010 - 02/2011 Projektleitung: Prof. Dr. Klaus Rademann
Gold und Silber als Cluster in WW mit Lanthanoiden
Quelle ↗Förderer: DFG Sachbeihilfe Zeitraum: 11/2006 - 12/2007 Projektleitung: Prof. Dr. Klaus Rademann
Kristalline Nichtgleichgewichtsphasen: Steuerung und kontrollierte Bildung polymorpher Molekülkristallphasen
Quelle ↗Förderer: DFG Sachbeihilfe Zeitraum: 01/2010 - 12/2012 Projektleitung: Prof. Dr. Klaus Rademann
Metallcluster in Natron-Kalk-Silikatglas: Bildungsmechanismen, Nukleation und Wachstum
Quelle ↗Förderer: DFG Sachbeihilfe Zeitraum: 12/2009 - 01/2011 Projektleitung: Prof. Dr. Klaus Rademann
Nanoparticles of bismuth and bismuth related composites
Quelle ↗Förderer: Alexander von Humboldt-Stiftung Zeitraum: 11/2011 - 12/2014 Projektleitung: Prof. Dr. Klaus Rademann
PPP Indien DST 2017, Projektbezogener Personenaustausch
Quelle ↗Förderer: DAAD Zeitraum: 01/2017 - 12/2018 Projektleitung: Prof. Dr. Klaus Rademann
Reactive Chemical Channels: A joint Experimental-Theoretical Program
Quelle ↗Förderer: German-Israeli Foundation Zeitraum: 01/2007 - 12/2010 Projektleitung: Prof. Dr. Klaus Rademann
SPP 1415/2: Kristalline Nichtgleichgewichtsphasen, Bildungsmechanismen
Quelle ↗Förderer: DFG Schwerpunktprogramm Zeitraum: 03/2013 - 11/2016 Projektleitung: Prof. Dr. Klaus Rademann
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Publikationen25
Top 25 nach Zitationen — Quelle: OpenAlex (BAAI/bge-m3 embedded für Matching).
Chemistry of Materials · 587 Zitationen · DOI
The strategy of utilizing mechanochemical synthesis to obtain metal−organic frameworks (MOFs) with high surface areas is demonstrated for two model systems. The compounds HKUST-1 (Cu3(BTC)2, BTC = 1,3,5-benzenetricarboxylate) and MOF-14 (Cu3(BTB)2, BTB = 4,4′,4′′-benzenetribenzoate) were synthesized by ball milling and characterized by powder X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM) and thermal analysis (DTA/DTG/MS). The specific surface area (SSA) of both compounds was characterized by nitrogen adsorption. To verify these results and to understand how the synthetic conditions influence the pore structure and the surface area, additional small-angle X-ray scattering (SAXS) experiments were carried out. Our investigations confirm that this synthesis approach is a promising alternative method for distinct MOFs. This facile method leads to materials with surface areas of 1713 m2/g, which is comparable to the highest given values in the literature for the respective compounds.
ACS Nano · 432 Zitationen · DOI
This contribution provides a comprehensive mechanistic picture of the gold nanoparticle synthesis by citrate reduction of HAuCl4, known as Turkevich method, by addressing five key questions. The synthesis leads to monodisperse final particles as a result of a seed-mediated growth mechanism. In the initial phase of the synthesis, seed particles are formed onto which the residual gold is distributed during the course of reaction. It is shown that this mechanism is a fortunate coincidence created by a favorable interplay of several chemical and physicochemical processes which initiate but also terminate the formation of seed particles and prevent the formation of further particles at later stages of reaction. Since no further particles are formed after seed particle formation, the number of seeds defines the final total particle number and therefore the final size. The gained understanding allows illustrating the influence of reaction conditions on the growth process and thus the final size distribution.
ACS Nano · 393 Zitationen · DOI
Gold nanoparticles (AuNP) were prepared by the homogeneous mixing of continuous flows of an aqueous tetrachloroauric acid solution and a sodium borohydride solution applying a microstructured static mixer. The online characterization and screening of this fast process ( approximately 2 s) was enabled by coupling a micromixer operating in continuous-flow mode with a conventional in-house small angle X-ray scattering (SAXS) setup. This online characterization technique enables the time-resolved investigation of the growth process of the nanoparticles from an average radius of ca. 0.8 nm to about 2 nm. To the best of our knowledge, this is the first demonstration of a continuous-flow SAXS setup for time-resolved studies of nanoparticle formation mechanisms that does not require the use of synchrotron facilities. In combination with X-ray absorption near edge structure microscopy, scanning electron microscopy, and UV-vis spectroscopy the obtained data allow the deduction of a two-step mechanism of gold nanoparticle formation. The first step is a rapid conversion of the ionic gold precursor into metallic gold nuclei, followed by particle growth via coalescence of smaller entities. Consequently it could be shown that the studied synthesis serves as a model system for growth driven only by coalescence processes.
The Journal of Physical Chemistry C · 383 Zitationen · DOI
We present a study on the catalytic reduction of 4-nitrophenol (Nip) to 4-aminophenol (Amp) by sodium borohydride (BH4–) in the presence of metal nanoparticles in aqueous solution. This reaction which proceeds via the intermediate 4-hydroxylaminophenol has been used abundantly as a model reaction to check the catalytic activity of metallic nanoparticles. Here we present a full kinetic scheme that includes the intermediate 4-hydroxylaminophenol. All steps of the reaction are assumed to proceed solely on the surface of metal nanoparticles (Langmuir–Hinshelwood model). The discussion of the resulting kinetic equations shows that there is a stationary state in which the concentration of the intermediate 4-hydroxylaminophenol stays approximately constant. The resulting kinetic expression had been used previously to evaluate the kinetic constants for this reaction. In this stationary state there are isosbestic points in the UV/vis-spectra which are in full agreement with most published data. We compare the full kinetic equations to experimental data given by the temporal decay of the concentration of Nip. Good agreement is found underlining the general validity of the scheme. The kinetic constants derived from this analysis demonstrate that the second step, namely the reduction of the 4-hydroxylaminophenol is the rate-determining step.
Advanced Functional Materials · 307 Zitationen · DOI
Abstract With the technique of synchrotron X‐ray activation, molecule‐like, non‐plasmonic gold and silver particles in soda‐lime silicate glasses can be generated. The luminescence energy transfer between these species and lanthanide(III) ions is studied. As a result, a significant lanthanide luminescence enhancement by a factor of up to 250 under non‐resonant UV excitation is observed. The absence of a distinct gold and silver plasmon resonance absorption, respectively, the missing nanoparticle signals in previous SAXS and TEM experiments, the unaltered luminescence lifetime of the lanthanide ions compared to the non‐enhanced case, and an excitation maximum at 300–350 nm (equivalent to the absorption range of small noble metal particles) indicate unambiguously that the observed enhancement is due to a classical energy transfer between small noble metal particles and lanthanide ions, and not to a plasmonic field enhancement effect. It is proposed that very small, molecule‐like noble metal particles (such as dimers, trimers, and tetramers) first absorb the excitation light, undergo a singlet‐triplet intersystem crossing, and finally transfer the energy to an excited multiplet state of adjacent lanthanide(III) ions. X‐ray lithographic microstructuring and excitation with a commercial UV LED show the potential of the activated glass samples as bright light‐emitting devices with tunable emission colors.
Physical Chemistry Chemical Physics · 283 Zitationen · DOI
CTAB-stabilized gold nanoparticles were synthesized by applying the seeding-growth approach in order to gain information about the size dependence of the catalytic reduction of p-nitrophenol to p-aminophenol with sodium borohydride. Five different colloidal solutions of stabilized gold nanoparticles have been characterized by TEM, AFM, UV-Vis, SAXS, and DLS for their particle size distributions. Gold nanoparticles (mean sizes: 3.5, 10, 13, 28, 56 nm diameter) were tested for their catalytic efficiency. Kinetic data were acquired by UV-Vis spectroscopy at different temperatures between 25 and 45 °C. By studying the p-nitrophenol to p-aminophenol reaction kinetics we determined the nanoparticle size which is needed to gain the fastest conversion under ambient conditions in the liquid phase. Unexpectedly, CTAB-stabilized gold nanoparticles with a diameter of 13 nm are most efficient.
Advanced Functional Materials · 263 Zitationen · DOI
Abstract As the most abundant natural polymer, cellulose presents a unique advantage for large‐scale applications. To fully unlock its potential, the introduction of desired functional groups onto the cellulose backbone is required, which can be realized by either chemical bonding or physical surface interactions. This review gives an overview of the chemistry behind the state‐of‐the‐art functionalization methods (e.g., oxidation, esterification, grafting) for cellulose in its various forms, from nanocrystals to bacterial cellulose. The existing and foreseeable applications of the obtained products are presented in detail, spanning from water purification and antibacterial action, to sensing, energy harvesting, and catalysis. A special emphasis is put on the interactions of functionalized cellulose with heavy metals, focusing on copper as a prime example. For the latter, its toxicity can either have a harmful influence on aquatic life, or it can be conveniently employed for microbial disinfection. The reader is further introduced to recent sensing technologies based on functionalized cellulose, which are becoming crucial for the near future especially with the emergence of the internet of things. By revealing the potential of water filters and conductive clothing for mass implementation, the near future of cellulose‐based technologies is also discussed.
Physical Review Letters · 251 Zitationen · DOI
Single-photon ionization thresholds are reported for mercury clusters with up to 70 atoms prepared in a supersonic beam. The size dependence of the ionization potentials provides the first direct experimental evidence that a heretofore undetected size-dependent gradual transition from van der Waals--type to metallic properties occurs for mercury clusters in the size range between 20 to approximately 70 atoms.
Journal of the American Chemical Society · 249 Zitationen · DOI
In any given matrix control over the final particle size distribution requires a constitutive understanding of the mechanisms and kinetics of the particle evolution. In this contribution we report on the formation mechanism of silver nanoparticles embedded in a soda-lime silicate glass matrix. For the silver ion-exchanged glass it is shown that at temperatures below 410 °C only molecular clusters (diameter <1 nm) are forming which are most likely silver dimers. These clusters grow to nanoparticles (diameter >1 nm) by annealing above this threshold temperature of 410 °C. It is evidenced that the growth and thus the final silver nanoparticle size are determined by matrix-assisted reduction mechanisms. As a consequence, particle growth proceeds after the initial formation of stable clusters by addition of silver monomers which diffuse from the glass matrix. This is in contrast to the widely accepted concept of particle growth in metal-glass systems, in which it is assumed that the nanoparticle formation is predominantly governed by Ostwald ripening processes.
ACS Nano · 240 Zitationen · DOI
The formation mechanisms of silver nanoparticles using aqueous silver perchlorate solutions as precursors and sodium borohydride as reducing agent were investigated based on time-resolved in situ experiments. This contribution addresses two important issues in colloidal science: (i) differences and analogies between growth processes of different metals such as gold and silver and (ii) the influence of a steric stabilizing agent on the growth process. The results reveal that a growth due to coalescence is a fundamental growth principle if the monomer-supplying chemical reaction is faster than the actual particle formation.
Analytical Chemistry · 222 Zitationen · DOI
This paper studies the UV-vis absorbance of colloidal gold nanoparticles at 400 nm and validates it as a method to determine Au(0) concentrations in colloidal gold solutions. The method is shown to be valid with restrictions depending on the investigated system. The uncertainty of the determined Au(0) concentration can be up to 30%. This deviation is the result of the combined influence of parameters such as particle size, surface modification, or oxidation state. However, quantifying the influence of these parameters enables a much more precise Au(0) determination for specific systems. As an example, the reduction process of the well-known Turkevich method was monitored and the Au(0) concentration was determined with a deviation of less than 5%. Hence, a simple, fast, easy, and cheap in situ method for Au(0) determination is demonstrated that has in the presence of other gold species such as Au(III) an unprecedented accuracy.
Physical Review Letters · 201 Zitationen · DOI
We advance an experimental method for the characterization of the geometric site distribution of a fractal structure, which rests on the interrogation of direct, long-range, singlet-singlet, intermolecular, electronic energy transfer. Time-resolved picosecond spectroscopy was utilized to study energy transfer from rhodamine B to malachite green doped into a porous glass, resulting in a fractal dimension of $\overline{d}=1.74\ifmmode\pm\else\textpm\fi{}0.12$ for this irregular structure.
Physical Chemistry Chemical Physics · 149 Zitationen · DOI
The gas-phase infrared spectroscopy of V4O10+ produced by laser vaporization has been studied in the spectral region from 7 to 16 µm. Mass-selected V4O10+ cations were stored in a helium filled radio frequency hexadecapole ion trap and excited using tunable infrared radiation from a free electron laser. The photodissociation spectrum was recorded by monitoring the V4O8+ yield (O2 loss) as a function of the excitation wavelength. Two absorption bands at 842 and 1032 cm−1 are observed, which are assigned to resonant excitation of the antisymmetric V–O–V stretching and VO stretching vibrations, respectively. Comparison to recent theoretical and experimental studies indicate that the absorbing species consists of a V4O8+ ionic core weakly bound to an oxygen molecule.
Nanotechnology · 133 Zitationen · DOI
We report the chemistry and photophysics of atomic gold and silver particles in inorganic glasses. By synchrotron irradiation of gold-doped soda-lime silicate glasses we could create and identify unambiguously the gold dimer as a stable and bright luminescing particle embedded in the glassy matrix. The gold dimer spectra coincide perfectly with rare gas matrix spectra of Au(2). The glass matrix is, however, stable for years, and is hence perfectly suited for various applications. If the irradiated gold-doped sample is annealed at 550 degrees C a bright green luminescence can be recognized. Intense 337 nm excitation induces a decrease of the green luminescence and the reappearance of the 753 nm Au(2) emission, indicating a strong interrelationship between both luminescence centers. Time-dependent density functional theory (TD-DFT) calculations indicate that the green luminescence can be assigned to noble metal dimers bound to silanolate centers. These complexes are recognized as the first stages in the further cluster growth process, which has been investigated with small-angle x-ray scattering (SAXS). In silver-doped glasses, Ag(0) atoms can be identified with electron paramagnetic resonance (EPR) spectroscopy after synchrotron activation. Annealing at 300 degrees C decreases the concentration of Ag(1), but induces an intense white light emission with 337 nm excitation. The white luminescence can be decomposed into bands that are attributed to small silver clusters such as Ag(2), Ag(3) and Ag(4), and an additional band matching the green emission of gold-doped glasses.
Angewandte Chemie International Edition · 131 Zitationen · DOI
We present an in situ triple coupling of synchrotron X-ray diffraction with Raman spectroscopy, and thermography to study milling reactions in real time. This combination of methods allows a correlation of the structural evolution with temperature information. The temperature information is crucial for understanding both the thermodynamics and reaction kinetics. The reaction mechanisms of three prototypical mechanochemical syntheses, a cocrystal formation, a C-C bond formation (Knoevenagel condensation), and the formation of a manganese-phosphonate, were elucidated. Trends in the temperature development during milling are identified. The heat of reaction and latent heat of crystallization of the product contribute to the overall temperature increase. A decrease in temperature occurs via release of, for example, water as a by-product. Solid and liquid intermediates are detected. The influence of the mechanical impact could be separated from temperature effects caused by the reaction.
Chemistry of Materials · 116 Zitationen · DOI
Metal nanoparticles have attracted much attention due to their unique properties. Size control provides an effective key to an accurate adjustment of colloidal properties. The common approach to size control is testing different sets of parameters via trial and error. The actual particle growth mechanisms, and in particular the influences of synthesis parameters on the growth process, remain a black box. As a result, precise size control is rarely achieved for most metal nanoparticles. This contribution presents an approach to size control that is based on mechanistic knowledge. It is exemplified for a common silver nanoparticle synthesis, namely, the reduction of AgClO4 with NaBH4. Conducting this approach allowed a well-directed modification of this synthesis that enables, for the first time, the size-controlled production of silver nanoparticles 4–8 nm in radius without addition of any stabilization agent.
Crystal Growth & Design · 108 Zitationen · DOI
The liquid-assisted grinding cocrystallization of theophylline with benzamide leading to polymorphic compounds was investigated. A solvent screening with 17 different solvents was performed. The dipole moment of the solvent used in the synthesis determines the structure of the polymorphic product. A detailed investigation leads to the determination of the kinetically and thermodynamically favored product. In situ observations of the formation pathway during the grinding process of both polymorphs show that the thermodynamically favored cocrystal is formed in a two-step mechanism with the kinetic cocrystal as an intermediate.
Review of Scientific Instruments · 107 Zitationen · DOI
We present dynamic plowing nanolithography on polymethylmethacrylate films, performed with a scan-linearized atomic force microscope able to scan up to 250 μm with high resolution. Modifications of the surface are obtained by plastically indenting the film surface with a vibrating tip. By changing the oscillation amplitude of the cantilever, i.e., the indentation depth, surfaces can be either imaged or modified. A program devoted to the control of the scanning process is also presented. The software basically converts the gray scale of pixel images into voltages used to control the dither piezo driving cantilever oscillations. The advantages of our experimental setup and the dependence of lithography efficiency on scanning parameters are discussed. Some insights into the process of surface modifications are presented.
Physical Review B · 104 Zitationen · DOI
Antimony nanoparticles grown on highly oriented pyrolytic graphite and molybdenum disulfide were used as a model system to investigate the contact-area dependence of frictional forces. This system allows one to accurately determine both the interface structure and the effective contact area. Controlled translation of the antimony nanoparticles (areas between 10 000 and $110\phantom{\rule{0.2em}{0ex}}000\phantom{\rule{0.3em}{0ex}}{\mathrm{nm}}^{2}$) was induced by the action of the oscillating tip in a dynamic force microscope. During manipulation, the power dissipated due to tip-sample interactions was recorded. We found that the threshold value of the power dissipation needed for translation depends linearly on the contact area between the antimony particles and the substrate. Assuming a linear relationship between dissipated power and frictional forces implies a direct proportionality between friction and contact area. Particles about $10\phantom{\rule{0.2em}{0ex}}000\phantom{\rule{0.3em}{0ex}}{\mathrm{nm}}^{2}$ in size, however, were found to show dissipation close to zero. To explain the observed behavior, we suggest that structural lubricity might be the reason for the low dissipation in the small particles, while elastic multistabilities might dominate energy dissipation in the larger particles.
Microporous and Mesoporous Materials · 100 Zitationen · DOI
Berichte der Bunsengesellschaft für physikalische Chemie · 99 Zitationen · DOI
Abstract A new and versatile photoelectron‐photoion coincidence spectrometer has been developed and advanced to facilitate for the first time the investigation of the valence electronic structure of isolated metal atom clusters up to a mass range of approximately 40000 amu. The neutral cluster distribution, which is synthesized in a doubly skimmed seeded supersonic molecular beam, is probed 130 mm down‐stream by the pulsed and monochromatized output of a newly developed sub‐nanosecond vacuum‐UV/UV‐flashlamp in the photon energy range between 3 and 11.8 eV. Single photoelectrons are energy analyzed and time‐correlated detected either in a field‐free or in a magnetic type time‐of‐flight spectrometer. The corresponding cluster cations are mass analyzed and counted in coincidence in a modified Wiley‐McLaren time‐of‐flight mass spectrometer. The data acquisition electronics is capable of recording and storing simultaneously photoion‐photoelectron coincidence spectra of all those neutral clusters that have sufficient intensity in the molecular beam. First photoelectron spectroscopic results for mass selected mercury clusters up to Hg 78 are used to discuss the performance of this new technique. — A photoionization mass spectrometry study of sodium cluster beams has revealed shell closings at the sizes being predicted by the cluster shell model. For the case of large mercury clusters a gradual transition from van der Waals type bonding to metallic cohesion occurs in the size range between 20 and 70 atoms. The occurrence of this transition, which has been observed by energy resolved mass spectrometry, is strongly confirmed by photoelectron‐photoion coincidence spectroscopy. Moreover, the photoelectron spectra clearly indicate that the evolution of the valence band structure toward the metallic state is almost fully accomplished for clusters larger than Hg 70 . The gradually size dependent transition is found to be induced by the rapid increase of the averaged number of nearest neighbours. Hence, the transition can be explained semi‐quantitatively in the framework of the tight‐binding approximation.
Langmuir · 98 Zitationen · DOI
In this work, we report the formation and growth mechanisms of gold nanoparticles (AuNPs) in eco-friendly deep eutectic solvents (DES; choline chloride and urea). AuNPs are synthesized on the DES surface via a low-energy sputter deposition method. Detailed small angle X-ray scattering (SAXS), UV-Vis, and cryogenic transmission electron microscopy (cryo-TEM) investigations show the formation of AuNPs of 5 nm diameter. Data analysis reveals that for a prolonged gold-sputtering time there is no change in the size of the particles. Only the concentration of AuNPs increases linearly in time. More surprisingly, the self-assembly of AuNPs into a first and second shell ordered system is observed directly by in situ SAXS for prolonged gold-sputtering times. The self-assembly mechanism is explained by the templating nature of DES combined with the equilibrium between specific physical interaction forces between the AuNPs. A disulfide-based stabilizer, bis((2-mercaptoethyl)trimethylammonium) disulfide dichloride, was applied to suppress the self-assembly. Moreover, the stabilizer even reverses the self-assembled or agglomerated AuNPs back to stable 5 nm individual particles as directly evidenced by UV-Vis. The template behavior of DES is compared to that of nontemplating solvent castor oil. Our results will also pave the way to understand and control the self-assembly of metallic and bimetallic nanoparticles.
Physical Chemistry Chemical Physics · 87 Zitationen · DOI
Mechanochemical reactions often result in 100% yields of single products, making purifying procedures obsolete. Mechanochemistry is also a sustainable and eco-friendly method. The ever increasing interest in this method is contrasted by a lack in mechanistic understanding of the mechanochemical reactivity and selectivity. Recent in situ investigations provided direct insight into formation pathways. However, the currently available theories do not predict temperature T as an influential factor. Here, we report the first determination of an apparent activation energy for a mechanochemical reaction. In a temperature-dependent in situ study the cocrystallisation of ibuprofen and nicotinamide was investigated as a model system. These experiments provide a pivotal step towards a comprehensive understanding of milling reaction mechanisms.
Missing Piece of the Mechanism of the Turkevich Method: The Critical Role of Citrate Protonation
2016Chemistry of Materials · 75 Zitationen · DOI
This contribution investigates the growth mechanism of the Turkevich method. The experimental results provide the missing piece of the mechanistic puzzle which enables the actual control of particle growth in the commonly used Turkevich method. Applying the gained knowledge, the boundary conditions for a successful Turkevich synthesis are deduced. Moreover, the conditions under which the Turkevich method is highly reproducible are derived. Following these conditions, the Turkevich synthesis is modified to reveal small monodisperse particles with an unprecedented reproducibility of ±0.1 nm.
Electronic spectroscopy of fluorobenzene Van der Waals molecules by resonant two-photon ionization
1983Chemical Physics · 73 Zitationen · DOI
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- Name
- Prof. Dr. Klaus Rademann
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- Prof. Dr.
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- Mathematisch-Naturwissenschaftliche Fakultät
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- Institut für Chemie
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