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42 results on '"Polte, Jörg"'

5. In situ determination of colloidal gold concentrations with UV-Vis spectroscopy: Limitations and perspectives

Author

Hendel, Thomas, Rademann, Klaus, and Polte, Jörg

Abstract

Resumen del trabajo presentado a la NanoSpain Conference, celebrada en San Sebastián del 7 al 10 de marzo de 2017., The Au(0) concentration in a gold nanoparticle (GNP) solution represents an important information. However, its exact determination often remains a major issue but is crucial for biological applications and their related toxicological discussions. Moreover, for investigations of GNP formation mechanisms, various GNP surface modifications and particle assembling procedures, accurate Au(0) concentration values are required. In this work the UVvis absorbance of colloidal gold nanoparticles at 400 nm is studied and validated as a method to determine Au(0) concentrations in GNP 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 (citrate reduction) was monitored and the Au(0) concentration was determined with a deviation of less than 5%. Hence, a simple, fast, and cheap in situ method for Au(0) determination in GNP solutions is demonstrated that has in the presence of other gold species such as Au(III) an unprecedented accuracy.

Published
2017

8. Fundamental growth principles of colloidal metal nanoparticles

Author

Polte, Jörg

Subjects
540 Chemie, ddc:540
Abstract

In the past few decades, much effort was put into the development of synthetic strategies to produce nanoparticles of different sizes and morphologies and a large number of scientific contributions is dedicated to the characterization and application of metal nanoparticles. In contrast, only few studies deal with particle formation mechanisms. As a consequence, theoretical concepts that describe particle growth processes are very rare and the few existing models are hardly able to explain how synthesis parameters influence the final particle size distribution. This contribution discusses recent experimental results from which a novel growth concept based on colloidal stability is deduced. The growth concept is in contrast to nucleation models and allows a description of colloidal growth processes from a different perspective. It states that for most syntheses the minimal particle size is rather determined by colloidal than thermodynamic stability making a nucleation model irrelevant.

Published
2015
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12. Hydrophobic Nanoreactor Soft-Templating: A Supramolecular Approach to Yolk@Shell Materials

Author

Guiet, Amandine, primary, Göbel, Caren, additional, Klingan, Katharina, additional, Lublow, Michael, additional, Reier, Tobias, additional, Vainio, Ulla, additional, Kraehnert, Ralph, additional, Schlaad, Helmut, additional, Strasser, Peter, additional, Zaharieva, Ivelina, additional, Dau, Holger, additional, Driess, Matthias, additional, Polte, Jörg, additional, and Fischer, Anna, additional

Published
2015
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19. A One-Pot Approach to Mesoporous Metal Oxide Ultrathin Film Electrodes Bearing One Metal Nanoparticle per Pore with Enhanced Electrocatalytic Properties

Author

Guiet, Amandine, primary, Reier, Tobias, additional, Heidary, Nina, additional, Felkel, Diana, additional, Johnson, Benjamin, additional, Vainio, Ulla, additional, Schlaad, Helmut, additional, Aksu, Yilmaz, additional, Driess, Matthias, additional, Strasser, Peter, additional, Thomas, Arne, additional, Polte, Jörg, additional, and Fischer, Anna, additional

Published
2013
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22. Nafion-Free Carbon-Supported Electrocatalysts with Superior Hydrogen Evolution Reaction Performance by Soft Templating.

Author

Bernsmeier, Denis, Bernicke, Michael, Ortel, Erik, Bergmann, Arno, Lippitz, Andreas, Nissen, Jörg, Schmack, Roman, Strasser, Peter, Polte, Jörg, and Kraehnert, Ralph

Subjects
WATER electrolysis, ELECTROCATALYSTS, NAFION, CARBON, HYDROGEN evolution reactions, ELECTRODES
Abstract

Efficient water electrolysis requires electrode coatings with high catalytic activity. Platinum efficiently catalyzes the hydrogen evolution reaction in acidic environments, but is a rare and expensive metal. The activity achieved per metal atom can be increased if small Pt particles are dispersed onto electrically conductive, highly accessible and stable support materials. However, the addition of Nafion, a typical binder material used in the manufacture of electrode coatings, can decrease catalytic activity by the blocking of pores and active surface sites. A new approach is reported for the direct synthesis of highly active Nafion-free Pt/C catalyst films consisting of small Pt nanoparticles supported in size-controlled mesopores of a conductive carbon film. The synthesis relies on the co-deposition of suitable Pt and C precursors in the presence of polymer micelles, which act as pore templates. Subsequent carbonization in an inert atmosphere produces porous catalyst films with controlled film thickness, pore size and particle size. The catalysts clearly outperform all Nafion-based Pt/C catalysts reported in the literature, particularly at high current densities. [ABSTRACT FROM AUTHOR]

Published
2017
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36. Versatile control over size and spacing of small mesopores in metal oxide films and catalytic coatings via templating with hyperbranched core-multishell polymers.

Author

Bernsmeier, Denis, Ortel, Erik, Polte, Jörg, Eckhardt, Björn, Nowag, Sabrina, Haag, Rainer, and Kraehnert, Ralph

Abstract

Controlling the pore structure of metal oxide films and supported catalysts is an essential requirement for tuning their functionality and long-term stability. Typical synthesis concepts such as "Evaporation Induced Self Assembly" rely on micelle formation and self assembly. These processes are dynamic in nature and therefore strongly influenced by even slight variations in the synthesis conditions. Moreover, the synthesis of very small mesopores (2-5 nm) and independent control over the thickness of pore walls are very difficult to realize with micelle-based approaches. In this contribution, we present a novel approach for the synthesis of mesoporous metal oxide films and catalytic coatings with ordered porosity that decouples template formation and film deposition by use of hyperbranched core-multishell polymers as templates. The approach enables independent control of pore size, wall thickness and the content of catalytically active metal particles. Moreover, dual templating with a combination of hyperbranched core-multishell polymers and micelles provides facile access to hierarchical bimodal porosity. The developed approach is illustrated by synthesizing one of the most common metal oxides (TiO2) and a typical supported catalyst (PdNP/TiO2). Superior catalyst performance is shown for the gas-phase hydrogenation of butadiene. The concept provides a versatile and general platform for the rational optimization of catalysts based e.g. on computational prediction of optimal pore structures and catalyst compositions. [ABSTRACT FROM AUTHOR]

Published
2014
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40. Antireflective Coatings with Adjustable Refractive Index and Porosity Synthesized by Micelle-Templated Deposition of MgF2Sol Particles

Author

Bernsmeier, Denis, Polte, Jörg, Ortel, Erik, Krahl, Thoralf, Kemnitz, Erhard, and Kraehnert, Ralph

Abstract

Minimizing efficiency losses caused by unwanted light reflection at the interface between lenses, optical instruments and solar cells with the surrounding medium requires antireflective coatings with adequate refractive index and coating thickness. We describe a new type of antireflective coating material with easily and independently tailorable refractive index and coating thickness based on the deposition of colloidal MgF2nanoparticles. The material synthesis employs micelles of amphiphilic block copolymers as structure directing agent to introduce controlled mesoporosity into MgF2film. The coatings thickness can be easily adjusted by the applied coating conditions. The coatings refractive index is determined by the materials porosity, which is controlled by the amount of employed pore template. The refractive index can be precisely tuned between 1.23 and 1.11, i.e., in a range that is not accessible to nonporous inorganic materials. Hence, zero reflectance conditions can be established for a wide range of substrate materials.

Published
2014
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41. Nucleation and Growth of Gold Nanoparticles Studied viain situSmall Angle X-ray Scattering at Millisecond Time Resolution

Author

Polte, Jörg, Erler, Robert, Thünemann, Andreas F., Sokolov, Sergey, Ahner, T. Torsten, Rademann, Klaus, Emmerling, Franziska, and Kraehnert, Ralph

Abstract

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 (∼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 viacoalescence of smaller entities. Consequently it could be shown that the studied synthesis serves as a model system for growth driven only by coalescence processes.

Published
2010
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42. SAXS in combination with a free liquid jet for improved time-resolved in situstudies of the nucleation and growth of nanoparticlesElectronic supplementary information (ESI) available: SAXS and SEM data. See DOI: 10.1039/c0cc03238g.

Author

Polte, Jörg, Erler, Robert, Thünemann, Andreas F., Emmerling, Franziska, and Kraehnert, Ralph

Subjects
*NUCLEATION, *CRYSTAL growth, *NANOPARTICLES, *SMALL-angle X-ray scattering, *CHEMICAL reactions, *ORGANIC synthesis, *COLLOIDAL gold
Abstract

A new setup for fast in situSAXS studies of early stages in the nucleation and growth of colloidal nanoparticles is presented. Evading the disturbing influence of container walls and minimizing the possibility of beam-induced reactions, the benefits of the setup are demonstrated exemplarily for the well-known synthesis of gold nanoparticles viathe Turkevich method. Analysis with the new experimental setup reveals the initial rate of particle formation, and enables analysis of particle growth rates. [ABSTRACT FROM AUTHOR]

Published
2010
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