Papers by Rolf Hempelmann
Inelastic light scattering and phonon-confinement in nanocrystalline Y2O3
Berichte der Bunsengesellschaft für physikalische Chemie, Nov 1, 1997
Nanocrystalline powders of Y 2 O 3 with grain sizes in the range between 7 and 40 nm were prepare... more Nanocrystalline powders of Y 2 O 3 with grain sizes in the range between 7 and 40 nm were prepared by the microemulsion technique and by condensation from an inert gas atmosphere. The cubic respectively monoclinic powders were examined by means of Brillouin scattering (consolidated samples) and micro-Raman scattering (powders and consolidated samples). Measurements of the elastic properties of quasi-isotropic pellets yield a longitudinal modulus reduced down to 25% compared with that of the monocrystal and point to optical inhomogeneities in the range of the wavelength of the light. With decreasing dimensions of the crystallites a shift of the characteristic Raman lines to lower frequencies and a distinct broadening of the lines are observed. A trial is made to interpret these features by the phonon-confinement model of Richter et al. [1]. A possibility to characterize the nanocrystalline powders by means of Raman spectroscopy is discussed.
Size dependence of optical phonons in yttria nanoparticles by means of Raman light scattering
Journal de physique, May 1, 2000
ABSTRACT
Synthesis of drug loaded magnetic nanoparticles and their uptake into immune cells
Journal of physics, 2010
Ferrite nanoparticles (Mn0,8Zn0,2Fe2O4) are synthesized by the co-precipitation method and charac... more Ferrite nanoparticles (Mn0,8Zn0,2Fe2O4) are synthesized by the co-precipitation method and characterized by X-ray diffraction, transmission electron microscopy and dynamic light scattering. The particles are functionalized with dextran which is activated via amino or carboxymethyl groups. The chemotherapeutic drug doxorubicin (DOX) is attached to these dextran derivates in different ways. One method is based on the attachment of DOX to amino
Gradients in structure and dynamics of Y 2 O 3 nanoparticles as revealed by X-ray and Raman scattering
Scripta Materialia, May 1, 2001
Photo-Curing of Off-set Printing Inks by Functionalized ZnO Nanoparticles
nano Online, 2016
ABSTRACT ZnO nanoparticles surface functionalized with alpha-oxo-carboxylic acids have been used ... more ABSTRACT ZnO nanoparticles surface functionalized with alpha-oxo-carboxylic acids have been used as photo-initiators for the curing of UV off-set printing inks. Other than commercial molecular photo-initiators ZnO nanoparticles do not migrate in the cured resins, they do not leach out and therefore, even if in contact with food, there is no danger of contamination. ESR and in-situ Raman-spectroscopy have been applied to analyze the kinetics of this free radical bulk photo-polymerization process. Upon UV irradiation the Photo-Kolbe-reaction (decarboxylation of carbonic acids by the photo generated electron hole) takes place as could be confirmed by ESR and GC-MS measurements.
arXiv (Cornell University), Jun 19, 2017
The occurrence of metallic film deposition without external power supply on the copper current co... more The occurrence of metallic film deposition without external power supply on the copper current collector of a zinc air battery half-cell containing zinc slurry is investigated. Therefore, test specimens of miscellaneous materials representing the current collector are immersed in a commercial available zinc slurry as well as an in self-prepared zinc slurry. In case of copper and metals which are more noble (silver and gold), a coating on the respective specimen is obtained. An element mapping of the coated copper specimen is performed by means of scanning electron microscopy (SEM) and energy dispersive x-ray spectroscopy (EDX) identifying that the coating layer consists of zinc and oxygen. In order to clarify the crystal structure and the exact composition, focused ion beam (FIB) and x-ray photoelectron spectroscopy (XPS) measurements are applied, proving that the layer consists of elemental zinc, which, is merely oxidized on the surface. Finally, a reaction mechanism for the reversible zinc film deposition is proposed.
Pairing Nitroxyl Radical and Phenazine with Electron-Withdrawing/-Donating Substituents in “Water-in-Ionic Liquid” for High-Voltage Aqueous Redox Flow Batteries
SSRN Electronic Journal
プロトン伝導体SrCe 0.95 Yb 0.05 H 0.02 O 2.985 における準弾性熱中性子散乱実験
Solid State Ionics, 1996
Lecture Notes in Physics, 2008
This assessment presents EPA's most current evaluation of the potential health risks from exposur... more This assessment presents EPA's most current evaluation of the potential health risks from exposure to TCE (trichloroethylene). TCE exposure is associated with several adverse health effects, including neurotoxicity, immunotoxicity, developmental toxicity, liver toxicity, kidney toxicity, endocrine effects, and several forms of cancer. Mechanistic research indicates that TCE-induced carcinogenesis is complex, involving multiple carcinogenic metabolites acting through multiple modes of action. Under EPA's proposed (1996) cancer guidelines, TCE can be characterized as "highly likely to produce cancer in humans." For effects other than cancer, an oral RfD of 3×10 -4 mg/kg-d was based on critical effects in the liver, kidney, and developing fetus. An inhalation RfC of 4×10 -2 mg/m 3 was based on critical effects in the central nervous system, liver, and endocrine system. Several cancer slope factors were developed, with most between 2×10 -2 and 4×10 -1 per mg/kg-d. Several sources of uncertainty have been identified and quantified. The mechanistic information suggests some risk factors that may make some populations more sensitive. There are suggestions that TCE could affect children and adults differently. In addition, several chemicals have the potential to alter TCE's metabolism and clearance and subsequent toxicity; conversely, TCE exposure can augment the toxicity of other chemicals. Widespread environmental exposure to some of TCE's metabolites makes it important to consider the cumulative effect of TCE along with other environmental contaminants. 8/1/01 ii DRAFT-DO NOT CITE OR QUOTE PREFACE This assessment presents EPA's most current evaluation of the potential health risks from exposure to trichloroethylene (TCE). TCE is a chemical solvent used for vapor degreasing of metals and as an intermediate in the manufacture of other chemicals. One of the chemicals most released into the environment, TCE is highly volatile, with most TCE released into the environment finding its way into the air. An important exception occurs when TCE is released into groundwater, where TCE can persist for years because of the limited contact between groundwater and air. When people are exposed to TCE, it is readily absorbed by all exposure routes and is widely distributed throughout the body. The potential health risks of TCE in the environment have caused it to be listed as a chemical of concern across several environmental programs. TCE is listed as a hazardous air pollutant under the Clean Air Act, a toxic pollutant under the Clean Water Act, a contaminant under the Safe Drinking Water Act, a hazardous waste under the Resource Conservation and Recovery Act, and a hazardous substance under the Comprehensive Environmental Response, Compensation, and Liability Act (Superfund). It is a toxic chemical with reporting requirements under the Emergency Planning and Community Right-to-Know Act, and under the Toxic Substances Control Act certain releases must be reported to the Toxics Release Inventory. This assessment draws on 16 state-of-the-science papers published as a supplemental
Revista de Chimie, 2020
A series of Zn ferrite nanoparticles was prepared by varying the concentration of precipitating a... more A series of Zn ferrite nanoparticles was prepared by varying the concentration of precipitating agent (NaOH) in the range of 1 � 5 M. Carboxymethylcellulose (CMC) was used as capping agent for stabilizing the particles and to prevent agglomeration. The synthesis done at low temperature was followed by a thermal treatment at 500ºC for 6 h in air. The crystallite size determined using Scherer formula ranged between 8 - 10 nm while the nanoparticles average size observed by Transmission Electron Microscopy varied in between 7-10 nm showing that the increase of coprecipitation agent concentration influences the particles growth. Vibrating sample magnetometry confirmed the strong influence that nanoparticles morphology and size play on superparamagnetic properties of Zn ferrite.
Front‐End‐of‐Line Integration of Graphene Oxide for Graphene‐Based Electrical Platforms
Advanced Materials Technologies, 2018
Scalable and routine integration of chemically exfoliated, graphene‐based materials such as graph... more Scalable and routine integration of chemically exfoliated, graphene‐based materials such as graphene oxide (GO) and reduced graphene oxide (rGO) into standard microelectronic fabrication is a tremendous technological challenge, blocking their advancement toward real applications. A unique approach for wafer‐scale fabrication of rGO devices by a synergistic combination of chemically exfoliated GO with photolithography processing is realized. Using graphite powder as source material, a GO solution is produced in a newly optimized, low‐temperature exfoliation and desalination protocol, resulting in high‐quality GO and confirmed by various characterization techniques. As substrates, 4 in. Si/SiO2 or glass wafers were first silanized in a well‐controlled, gas‐phase procedure. Large‐area GO thin films are then realized by standard spin‐coating resulting in highly homogeneous, covalently bound layers of controllable thicknesses of 3–7 nm depending on the amount of spin‐coatings. The robust...
3D Nanostructured Multielectrode Arrays: Fabrication, Electrochemical Characterization, and Evaluation of Cell–Electrode Adhesion
Advanced Materials Technologies, 2018
3D nanostructures on top of planar multielectrode array (MEA) electrodes increase the surface are... more 3D nanostructures on top of planar multielectrode array (MEA) electrodes increase the surface area and can offer a tight and stable cell–electrode interface, thus leading to a crucial increase of the signal‐to‐noise ratio during measurement. However, each individual cell type might need specific dimensions and an arrangement of nanostructures that fits ideally to a specific cell type. Therefore, a fabrication method of 3D nanostructured MEA chips based on nanoimprint lithography, gold electroplating, and microstructuring techniques is presented which allows the fabrication of a whole set of MEA chips with different nanostructure layouts in one single approach. The chips are characterized using electrochemical methods, atomic force, and scanning electron microscopy. Furthermore, an impedance measurement method is presented to quantify cell–electrode adhesion of nanostructured and unstructured electrodes using the human embryonic kidney cell line (HEK 293). Double‐layer capacitance, t...
Nanocrystalline materials: Nanocrystalline metals and oxides I: Pulsed electrodeposition
Berichte der Bunsengesellschaft für physikalische Chemie, 1997
Three preparation routes are presented which, in a simple and cheap way, allow to produce rather ... more Three preparation routes are presented which, in a simple and cheap way, allow to produce rather large amounts of nanocrystalline solids with controlled particle sizes and narrow particle size distributions: the pulsed electrodeposition for nanocrystalline metals and alloys (this contribution), and the precipitation in microemulsions of nanocrystalline metal fluorides as well as the sol‐gel type hydrolysis in microemulsions for nanocrystalline metal oxides (next contribution).By means of pulsed electrodeposition we have prepared n‐Pd, n‐Cu, n‐Ni, n‐Co and n‐Fe as well as n‐Ni‐Cu and n‐Ni‐Fe alloys. The particle sizes are analysed preferentially using scattering methods; further characterisations concern chemical analyses and density or porosity measurements, respectively. The thermal stability is remarkable; grain growth is not detectable below 150°C.As an example of a particular physical property we present the very fast hydrogen diffusion in nanocrystalline Pd as investigated by m...
Pulse electrodeposited cathode catalyst layers for PEM fuel cells
International Journal of Hydrogen Energy, 2017
Abstract Nanostructured Pt and Pt3Co cathodes for proton exchange membrane fuel cells (PEMFCs) ha... more Abstract Nanostructured Pt and Pt3Co cathodes for proton exchange membrane fuel cells (PEMFCs) have been prepared by pulse electrodeposition. For high utilization the catalyst nanoparticles are directly deposited on the microporous layer (MPL) of a commercial available gas diffusion layer (GDL). In order to increase the hydrophilic nature of the substrate surface and thus improve drastically the electrodeposition process and the fuel cell performance, prior to electrodeposition, the carbon substrate is submitted to O2/Ar plasma activation. Cathodes with different amounts and distributions of Aquivion ionomer within the cathode catalyst layer (CCL) thickness (“homogeneous”, “gradient” and “anti-gradient”), different catalysts (Pt and Pt3Co) at varied plasma duration and catalyst loading have been prepared. The cathodes are analysed via attenuated total reflection (ATR-IR), goniometer, SEM, 0.5 M H2SO4 half-cell and 25 cm2 H2/Air single PEMFC. The highest single fuel cell performance is obtained for 2 min plasma activated Pt3Co cathode.
New catalyst supports prepared by surface modification of graphene- and carbon nanotube structures with nitrogen containing carbon coatings
Journal of Power Sources, 2017
Abstract We present a new and facile method for preparation of nitrogen containing carbon coating... more Abstract We present a new and facile method for preparation of nitrogen containing carbon coatings (NCC) on the surface of graphene- and carbon nanotubes (CNT), which has an increased electronic conductivity. The modified carbon system can be used as catalyst support for electrocatalytic applications, especially for polymer electrolyte membrane fuel cells (PEMFC). The surface modification is performed by impregnating carbon structures with a nitrogen containing ionic liquid (IL) with a defined C:N ratio, followed by a thermal treatment under ambient conditions. We investigate the influence of the main experimental parameters (IL amount, temperature, substrate morphology) on the formation of the NCC. Additionally, the structure and the chemical composition of the resulting products are analyzed by electron microscopic techniques (SEM, TEM), energy disperse X-ray analysis (EDX), X-ray photoelectron spectroscopy (XPS) and hot extraction analysis. The modified surface has a nitrogen content of 29 wt% which decreases strongly at temperatures above 600 °C. The new catalyst supports are used for the preparation of PEMFC anodes which are characterized by polarization measurements and electrochemical impedance spectroscopy (EIS). Compared to unmodified graphene and CNT samples the electronic conductivity of the modified systems is increased by a factor of 2 and shows improved mass transport properties.
Synthesis and characterization of silica-, meso-silica- and their functionalized silica-coated copper oxide nanomaterials
Journal of Sol-Gel Science and Technology, 2016
Silica- or meso-silica- or silica-meso-silica-coated copper oxide microspheres were prepared base... more Silica- or meso-silica- or silica-meso-silica-coated copper oxide microspheres were prepared based on base hydrolysis of tetraethyl orthosilicate in the presence of CuO and CTAB. Functionalization with amine or thiol organofunctional groups was conducted onto the surface of silica-meso-silica-coated copper oxide microspheres (Scheme 1). The silica-coated CuO composites and their amine- or thiol-functionalized materials have been characterized by TEM, XRD, TGA, FTIR and UV/Vis. TEM analysis showed that the CuO nanoparticles were encapsulated and dispersed into the silica or meso-silica microspheres. XRD analysis indicated that the size of CuO nanoparticles has decreased after coating with silica precursors. TGA and FTIR results indicated that the meso-silica-coated copper oxide materials have been successfully grafted by amine and thiol organofunctional groups.Scheme 1Description of formation of silica-coated or encapsulation of metal oxide nanomaterialsGraphical Abstract
Enhanced stability of multilayer graphene-supported catalysts for polymer electrolyte membrane fuel cell cathodes
Journal of Power Sources, 2015
ABSTRACT MLG-composites were applied as highly durable catalyst support for PEMFC cathodes. Remar... more ABSTRACT MLG-composites were applied as highly durable catalyst support for PEMFC cathodes. Remarkable positive effect of MLG-composites in term of the platinum utilization was shown. They lead to more than 9 times higher power density in comparison to the commercial standard. High electrochemical stability of MLG-containing cathodes was demonstrated. They showed no degradation but even a 13% higher power output after the AAP tests. a b s t r a c t One of the biggest challenges in the field of polymer electrolyte membrane fuel cells (PEMFC) is to enhance the lifetime and the long-term stability of PEMFC electrodes, especially of cathodes, furthermore , to reduce their platinum loading, which could lead to a cost reduction for efficient PEMFCs. These demands could be achieved with a new catalyst support architecture consisting of a composite of carbon structures with significant different morphologies. A highly porous cathode catalyst support layer is prepared by addition of various carbon types (carbon black particles, multi-walled carbon nanotubes (MWCNT)) to multilayer graphene (MLG). The reported optimized cathodes shows extremely high durability and similar performance to commercial standard cathodes but with 89% lower Pt loading. The accelerated aging protocol (AAP) on the membrane electrode assemblies (MEA) shows that the presence of MLG increases drastically the durability and the Pt-extended electrochemical surface area (ECSA). In fact, after the AAP slightly enhanced performance can be observed for the MLG-containing cathodes instead of a performance loss, which is typical for the commercial carbon-based cathodes. Furthermore, the presence of MLG drastically decreases the ECSA loss rate. The MLG-containing cathodes show up to 6.8 times higher mass-normalized Pt-extended ECSA compared to the commercial standard systems.
Luminescence properties of Mn and Ni doped ZnS nanoparticles synthesized by capping agent
Journal of Materials Science: Materials in Electronics, 2014
ABSTRACT ZnS and transition metal (Mn and Ni) doped ZnS were synthesized by a simple chemical met... more ABSTRACT ZnS and transition metal (Mn and Ni) doped ZnS were synthesized by a simple chemical method using alkyl hydroxyl ethyl dimethyl ammonium chloride (HY) as capping agent. The structural and optical properties were studied using various techniques. FTIR and X-ray diffraction can be used to identify the chemical bonding and crystal structure. The X-ray diffraction (XRD) analysis show that the particles are in cubic structure. The mean size of the nanoparticles calculated through Scherrer equation is in the range of 5–2.5 nm. Elemental dispersive (EDX) analysis of doped samples reveals the presence of doping ions. The transmission electron microscopic (TEM) studies show that the synthesized particles are in spherical shape. Optical characterization of both undoped and doped samples was carried out by ultraviolet–visible (UV–Vis) and photoluminescence (PL) spectroscopy. The absorption spectra of all the samples are blue shifted from the bulk ZnS.
An approach to cyclic plasticity and deformation-induced structure changes of electrodeposited nickel
Materials Science and Engineering: A, 2005
Bulk nickel samples, produced by electrodeposition, resulting in different initial structure prop... more Bulk nickel samples, produced by electrodeposition, resulting in different initial structure properties, were experimentally studied by X-ray diffraction, as well as by scanning and transmission electron microscopy. Attempts are made to correlate the mechanical behaviour during cyclic plastic deformation with the response of the microstructure. A special effort is made to examine the influence of grain size and internal stresses
Synthesis and microstructure of nanostructured Al/Al2O3 (H)-composite
Journal of Materials Science, 1996
By means of a metal organic chemical vapour deposition, starting from a single precursor compound... more By means of a metal organic chemical vapour deposition, starting from a single precursor compound [tBuOAlH2]2, an aluminium/aluminium hydrido-oxide composite has been synthesized in thin layers. Electron micrographs indicate self-similarity. A detailed analysis by small angle neutron scattering directly yields a surface fractal microstructure with a fractal dimension of 2.26; in the framework of a surface fractal model we determine the relevant particle dimensions and their size distribution.
Uploads
Papers by Rolf Hempelmann