Publications by authors named "Andreas Reyer"

6 Publications

  • Page 1 of 1

Synthesis of amorphous and graphitized porous nitrogen-doped carbon spheres as oxygen reduction reaction catalysts.

Beilstein J Nanotechnol 2020 2;11:1-15. Epub 2020 Jan 2.

Chemistry and Physics of Materials, Salzburg University, A-5020 Salzburg, Austria.

Amorphous and graphitized nitrogen-doped (N-doped) carbon spheres are investigated as structurally well-defined model systems to gain a deeper understanding of the relationship between synthesis, structure, and their activity in the oxygen reduction reaction (ORR). N-doped carbon spheres were synthesized by hydrothermal treatment of a glucose solution yielding carbon spheres with sizes of 330 ± 50 nm, followed by nitrogen doping via heat treatment in ammonia atmosphere. The influence of a) varying the nitrogen doping temperature (550-1000 °C) and b) of a catalytic graphitization prior to nitrogen doping on the carbon sphere morphology, structure, elemental composition, N bonding configuration as well as porosity is investigated in detail. For the N-doped carbon spheres, the maximum nitrogen content was found at a doping temperature of 700 °C, with a decrease of the N content for higher temperatures. The overall nitrogen content of the graphitized N-doped carbon spheres is lower than that of the amorphous carbon spheres, however, also the microporosity decreases strongly with graphitization. Comparison with the electrocatalytic behavior in the ORR shows that in addition to the N-doping, the microporosity of the materials is critical for an efficient ORR.
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http://dx.doi.org/10.3762/bjnano.11.1DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6964652PMC
January 2020

Understanding the Polymerization of Polyfurfuryl Alcohol: Ring Opening and Diels-Alder Reactions.

Polymers (Basel) 2019 Dec 17;11(12). Epub 2019 Dec 17.

Elettra-Sincrotrone Trieste S.C.p.A., Strada Statale 14-km 163,5 in AREA Science Park, 34149 Basovizza, Trieste, Italy.

Polyfurfuryl alcohol (PFA) is one of the most intriguing polymers because, despite its easy polymerization in acid environment, its molecular structure is definitely not obvious. Many studies have been performed in recent decades, and every time, surprising aspects came out. With the present study, we aim to take advantage of all of the findings of previous investigations and exploit them for the interpretation of the completely cured PFA spectra registered with three of the most powerful techniques for the characterization of solid, insoluble polymers: Solid-State C-NMR, Attenuated Total Reflectance (ATR), Fourier Transform Infrared (FTIR) spectroscopy, and UV-resonant Raman spectroscopy at different excitation wavelengths, using both an UV laser source and UV synchrotron radiation. In addition, the foreseen structures were modeled and the corresponding C-NMR and FTIR spectra were simulated with first-principles and semi-empiric methods to evaluate their matching with experimental ones. Thanks to this multi-technique approach, based on complementary analytical tools and computational support, it was possible to conclude that, in addition to the major linear unconjugated polymerization, the PFA structure consists of Diels-Alder rearrangements occurring after the opening of some furanic units, while the terminal moieties of the chain involves γ-lactone arrangements. The occurrence of head-head methylene ether bridges and free hydroxyl groups (from unreacted furfuryl alcohol, FA, or terminal chains) could be excluded, while the conjugated systems could be considered rather limited.
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http://dx.doi.org/10.3390/polym11122126DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6969920PMC
December 2019

Three-Dimensional Electrochemical Axial Lithography on Si Micro- and Nanowire Arrays.

Nano Lett 2018 11 25;18(11):7343-7349. Epub 2018 Oct 25.

Department of Chemistry and Physics of Materials , University of Salzburg , Jakob Haringer Strasse 2A , A-5020 Salzburg , Austria.

A templated electrochemical technique for patterning macroscopic arrays of single-crystalline Si micro- and nanowires with feature dimensions down to 5 nm is reported. This technique, termed three-dimensional electrochemical axial lithography (3DEAL), allows the design and parallel fabrication of hybrid silicon nanowire arrays decorated with complex metal nano-ring architectures in a flexible and modular approach. While conventional templated approaches are based on the direct replication of a template, our method can be used to perform high-resolution lithography on pre-existing nanostructures. This is made possible by the synthesis of a porous template with tunable dimensions that guides the deposition of well-defined metallic shells around the Si wires. The synthesis of a variety of ring architectures composed of different metals (Au, Ag, Fe, and Ni) with controlled sequence, height, and position along the wire is demonstrated for both straight and kinked wires. We observe a strong enhancement of the Raman signal for arrays of Si nanowires decorated with multiple gold rings due to the plasmonic hot spots created in these tailored architectures. The uniformity of the fabrication method is evidenced by a homogeneous increase in the Raman signal throughout the macroscopic sample. This demonstrates the reliability of the method for engineering plasmonic fields in three dimensions within Si wire arrays.
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http://dx.doi.org/10.1021/acs.nanolett.8b03608DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6238956PMC
November 2018

Interface Instability of Fe-Stabilized LiLaZrO versus Li Metal.

J Phys Chem C Nanomater Interfaces 2018 Feb 27;122(7):3780-3785. Epub 2018 Jan 27.

Department of Chemistry and Physics of Materials, University of Salzburg, Salzburg, Austria.

The interface stability versus Li represents a major challenge in the development of next-generation all-solid-state batteries (ASSB), which take advantage of the inherently safe ceramic electrolytes. Cubic LiLaZrO garnets represent the most promising electrolytes for this technology. The high interfacial impedance versus Li is, however, still a bottleneck toward future devices. Herein, we studied the electrochemical performance of Fe-stabilized LiLaZrO (LLZO:Fe) versus Li metal and found a very high total conductivity of 1.1 mS cm at room temperature but a very high area specific resistance of ∼1 kΩ cm. After removing the Li metal electrode we observe a black surface coloration at the interface, which clearly indicates interfacial degradation. Raman- and nanosecond laser-induced breakdown spectroscopy reveals, thereafter, the formation of a 130 μm thick tetragonal LLZO interlayer and a significant Li deficiency of about 1-2 formula units toward the interface. This shows that cubic LLZO:Fe is not stable versus Li metal by forming a thick tetragonal LLZO interlayer causing high interfacial impedance.
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http://dx.doi.org/10.1021/acs.jpcc.7b12387DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5847116PMC
February 2018

Investigation of Mass-Produced Substrates for Reproducible Surface-Enhanced Raman Scattering Measurements over Large Areas.

ACS Appl Mater Interfaces 2017 Aug 24;9(30):25445-25454. Epub 2017 Jul 24.

Department of Chemistry and Physics of Materials, University of Salzburg , Jakob-Haringer-Strasse 2a, 5020 Salzburg, Austria.

Surface-enhanced Raman scattering (SERS) is a versatile spectroscopic technique that suffers from reproducibility issues and usually requires complex substrate fabrication processes. In this article, we report the use of a simple mass production technology based on Blu-ray disc manufacturing technology to prepare large area SERS substrates (∼40 mm) with a high degree of homogeneity (±7% variation in Raman signal) and enhancement factor of ∼6 × 10. An industrial high throughput injection molding process was used to generate periodic microstructured polymer substrates coated with a thin Ag film. A short chemical etching step produces a highly dense layer of Ag nanoparticles at the polymer surface, which leads to a large and reproducible Raman signal. Finite difference time domain simulations and cathodoluminescence mapping experiments suggest that the sample microstructure is responsible for the generation of SERS active nanostructures around the microwells. Comparison with commercial SERS substrates demonstrates the validity of our method to prepare cost-efficient, reliable, and sensitive SERS substrates.
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http://dx.doi.org/10.1021/acsami.7b06002DOI Listing
August 2017

Structural and Raman spectroscopic characterization of pyroxene-type compounds in the CaCuZnGeO solid-solution series.

Acta Crystallogr B Struct Sci Cryst Eng Mater 2017 Jun 1;73(Pt 3):419-431. Epub 2017 Jun 1.

Department of Chemistry and Physics of Materials, Division of Materials Science and Mineralogy, University of Salzburg, Hellbrunnerstr. 34, Salzburg 5020, Austria.

Pyroxene-type germanate compounds with the composition CaCuGeO-CaZnGeO have been synthesized via a solid-state ceramic sintering route. Phase-pure polycrystalline and small single-crystal material was obtained all over the series, representing a complete solid-solution series. Differential thermal analysis, single-crystal X-ray diffraction and Raman spectroscopy were used to characterize phase stability, phase changes and structural alterations induced by the substitution of Cu with Zn. Whereas pure CaCuGeO exhibits P2/c symmetry with a strong distortion of the M1 octahedra and two different Ge sites, one of them with an unusual fivefold coordination, the replacement of Cu by Zn induces a chemically driven phase change to the C2/c symmetry. The phase change takes place around Zn contents of 0.12 formula units and is associated with large changes in the unit-cell parameters. Here, the increase of c by as much as 3.2% is remarkable and it is mainly controlled by an expansion of the tetrahedral chains. Further differences between the P2/c and C2/c structures are a more regular chain of edge-sharing M1 octahedra as a consequence of more and more reduced Jahn-Teller distortion and a less kinked, symmetry-equivalent tetrahedral chain. The coordination of the Ca site increases from sevenfold to eightfold with large changes in the Ca-O bond lengths during the phase change. Raman spectroscopy was mainly used to monitor the P2/c to C2/c phase change as a function of composition, but also as a function of temperature and to follow changes in specific Raman modes throughout the solid-solution series.
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http://dx.doi.org/10.1107/S205252061700381XDOI Listing
June 2017