Cölfen, Helmut

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Cölfen
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Helmut
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Gerade angezeigt 1 - 10 von 242
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Nanocrystalline and Amorphous Calcium Carbonate from Waste Seashells by Ball Milling Mechanochemistry Processes

2024, Marchini, Chiara, Triunfo, Carla, Greggio, Nicolas, Fermani, Simona, Montroni, Devis, Migliori, Andrea, Gradone, Alessandro, Goffredo, Stefano, Cölfen, Helmut, Falini, Giuseppe

Nanocrystalline calcium carbonate (CaCO3) and amorphous CaCO3 (ACC) are materials of increasing technological interest. Nowadays, they are mainly synthetically produced by wet reactions using CaCO3 reagents in the presence of stabilizers. However, it has recently been discovered that ACC can be produced by ball milling calcite. Calcite and/or aragonite are the mineral phases of mollusk shells, which are formed from ACC precursors. Here, we investigated the possibility to convert, on a potentially industrial scale, the biogenic CaCO3 (bCC) from waste mollusk seashells into nanocrystalline CaCO3 and ACC. Waste seashells from the aquaculture species, namely oysters (Crassostrea gigas, low-Mg calcite), scallops (Pecten jacobaeus, medium-Mg calcite), and clams (Chamelea gallina, aragonite) were used. The ball milling process was carried out by using different dispersing solvents and potential ACC stabilizers. Structural, morphological, and spectroscopic characterization techniques were used. The results showed that the mechanochemical process produced a reduction of the crystalline domain sizes and formation of ACC domains, which coexisted in microsized aggregates. Interestingly, bCC behaved differently from the geogenic CaCO3 (gCC), and upon long milling times (24 h), the ACC reconverted into crystalline phases. The aging in diverse environments of mechanochemically treated bCC produced a mixture of calcite and aragonite in a species-specific mass ratio, while the ACC from gCC converted only into calcite. In conclusion, this research showed that bCC can produce nanocrystalline CaCO3 and ACC composites or mixtures having species-specific features. These materials can enlarge the already wide fields of applications of CaCO3, which span from medical to material science.

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Analytical ultracentrifugation in colloid and polymer science : new possibilities and perspectives after 100 years

2023-07, Cölfen, Helmut

Analytical ultracentrifugation (AUC) is a classical polymer and colloid analysis technique invented by Theodor Svedberg 100 years ago. Modern hard- and software and powerful computers make it now possible to develop the methodology beyond what was possible with this technique before. This perspective aims to describe new possibilities, which extend the possibilities of AUC beyond the classical repertoire of the determination of distributions of sedimentation coefficient, particle size, and molar mass as well as stoichiometries and interaction constants of interacting systems. High-resolution simultaneous characterization of particle size and optical property distributions, investigation of nucleation by reaction in the AUC cell, characterization of particle interactions at a very high concentration, and characterization of complex fluids or osmotic pressures over large concentration ranges even crossing phase boundaries are among the discussed topics. They show that even after 100 years of successful application, AUC still has much yet unexplored potential in colloid and polymer science. Graphical Abstract This perspective paper spans from the days of invention of analytical ultracentrifugation to now including nonmainstream methodology and instrumentation, which has a huge potential for the future. This includes multiwavelength detectors, high-resolution particle size distributions, chemical reactions in the ultracentrifuge, high-concentration work, osmotic pressure distributions, and characterization of complex fluids.

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Silica‐Functionalized Nanolimes for the Conservation of Stone Heritage

2023-04-18, Burgos‐Ruiz, Miguel, Elert, Kerstin, Ruiz‐Agudo, Encarnacion, Cölfen, Helmut, Rodriguez‐Navarro, Carlos

The relatively recent development of nanolimes (i.e., alcoholic dispersions of Ca(OH)2 nanoparticles) has paved the way for new approaches to the conservation of important art works. Despite their many benefits, nanolimes have shown limited reactivity, back-migration, poor penetration, and lack of proper bonding to silicate substrates. In this work a novel solvothermal synthesis process is presented by which extremely reactive nanostructured Ca(OH)2 particles are obtained using calcium ethoxide as the main precursor species. Moreover, it is demonstrated that this material can be easily functionalized with silica-gel derivatives under mild synthesis conditions, thereby preventing particle growth, increasing total specific surface area, enhancing reactivity, modifying colloidal behavior, and functioning as self-integrated coupling agents. Additionally, the formation of calcium silicate hydrate (CSH) nanocement is promoted by the presence of water, resulting in optimal bonding when applied to silicate substrates, as evidenced by the higher reinforcement effect produced on treated Prague sandstone specimens as compared to those consolidated with nonfunctionalized commercial nanolime. The functionalization of nanolimes is not only a promising strategy for the design of optimized consolidation treatments for the cultural heritage, but may also have important implications for the development of advanced nanomaterials for building, environmental, or biomedical applications.

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Aufdeckung des Entstehungsmechanismus und Optimierung der Synthesebedingungen von geschichteten Doppelhydroxiden für die Sauerstoffentwicklungsreaktion

2023-02, Chen, Zongkun, Wang, Xingkun, Han, Zhongkang, Zhang, Siyuan, Pollastri, Simone, Fan, Qiqi, Qu, Zhengyao, Sarker, Debalaya, Huang, Minghua, Cölfen, Helmut

Geschichtete Doppelhydroxide (LDHs), deren Bildung stark von der OH-Konzentration abhängt, haben in verschiedenen Bereichen großes Interesse geweckt. Die Auswirkung der Echtzeit-Änderung der OH− Konzentration auf die Bildung von LDHs wurde jedoch noch nicht vollständig erforscht, da die bestehenden Synthesemethoden für die in situ Charakterisierung nicht geeignet sind. Hier bietet die gezielt entwickelte Kombination aus NH3-Gasdiffusion und in situ pH-Messung eine Lösung für das oben genannte Problem. Die so erhaltenen Ergebnisse deckten den Bildungsmechanismus auf und führten dazu, dass wir eine Bibliothek von LDHs mit den gewünschten Eigenschaften in Wasser bei Raumtemperatur ohne jegliche Additive synthetisieren konnten. Nach der Bewertung ihrer Sauerstoffentwicklungsreaktionsleistung stellten wir fest, dass FeNi-LDH mit einem Fe/Ni-Verhältnis von 25/75 eine der besten, der bisher berichteten, Leistungen aufweist.

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Localized Crystallization of Calcium Phosphates by Light‐Induced Processes

2023-10-19, Besirske, Patricia, Menichetti, Arianna, Montalti, Marco, García‐Ruiz, Juan Manuel, Winterhalder, Martin, Boneberg, Johannes, Cölfen, Helmut

Medical treatment options for bones and teeth can be significantly enhanced by taking control over the crystallization of biomaterials like hydroxyapatite in the healing process. Light‐induced techniques are particularly interesting for this approach as they offer tremendous accuracy in spatial resolution. However, in the field of calcium phosphates, light‐induced crystallization has not been investigated so far. Here, proof of principle is established to successfully induce carbonate‐hydroxyapatite precipitation by light irradiation. Phosphoric acid is released by a photolabile molecule exclusively after irradiation, combining with calcium ions to form a calcium phosphate in the crystallization medium. 4‐Nitrophenylphosphate (4NPP) is established as the photolabile molecule and the system is optimized and fully characterized. A calcium phosphate is crystallized exclusively by irradiation in aqueous solution and identified as carbonate apatite. Control over the localization and stabilization of the carbonate apatite is achieved by a pulsed laser, triggering precipitation in calcium and 4NPP‐containing gel matrices. The results of this communication open up a wide range of new opportunities, both in the field of chemistry for more sophisticated reaction control in localized crystallization processes and in the field of medicine for enhanced treatment of calcium phosphate containing biomaterials.

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The Structure, Preparation, Characterization, and Intercalation Mechanism of Layered Hydroxides Intercalated with Guest Anions

2023-06-04, Chen, Zongkun, Fan, Qiqi, Huang, Minghua, Cölfen, Helmut

Since the intercalation of anions into layered hydroxides (LHs) has a great impact not only on their nucleation and growth but also on their structure, composition, and size, the intercalation chemistry of LHs has aroused the strong interest of researchers. However, the progress in the fundamental understanding of LHs intercalated with guest anions have not been paralleled by a concomitant development of the preparation and performance improvement of such materials. Considering the guidance of a timely in-depth review for scientists in this area, a systematic introduction about the development that is made on the above-mentioned issues is highly needed but yet missing so far. Herein, recent advances in understanding the chemical composition and structure of LHs intercalated with guest anions are systematically summarized. Meanwhile, typical and emerging bottom-up synthesis methods of LHs intercalated with anions are reviewed, and the potential impact of external reaction parameters on the intercalation of anions into LHs are discussed . Besides, different analytical characterization techniques employed in the examination of guest anion-intercalated LHs are deliberated upon. Finally, although progress is slow in exploring the intercalation mechanism, as many examples as possible are included in this review and inferred the possible intercalation mechanism.

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The nucleation of C–S–H via prenucleation clusters

2023-03-21, Sowoidnich, Thomas, Damidot, Denis, Ludwig, Horst-Michael, Germroth, J., Rosenberg, Rose, Cölfen, Helmut

The nucleation and growth of calcium–silicate–hydrate (C–S–H) is of fundamental importance for the strength development and durability of the concrete. However, the nucleation process of C–S–H is still not fully understood. The present work investigates how C–S–H nucleates by analyzing the aqueous phase of hydrating tricalcium silicate (C3S) by applying inductively coupled plasma-optical emission spectroscopy as well as analytical ultracentrifugation. The results show that the C–S–H formation follows non-classical nucleation pathways associated with the formation of prenucleation clusters (PNCs) of two types. Those PNCs are detected with high accuracy and reproducibility and are two species of the 10 in total, from which the ions (with associated water molecules) are the majority of the species. The evaluation of the density and molar mass of the species shows that the PNCs are much larger than ions, but the nucleation of C–S–H starts with the formation of liquid precursor C–S–H (droplets) with low density and high water content. The growth of these C–S–H droplets is associated with a release of water molecules and a reduction in size. The study gives experimental data on the size, density, molecular mass, and shape and outlines possible aggregation processes of the detected species.

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Toward Understanding the Formation Mechanism and OER Catalytic Mechanism of Hydroxides by In Situ and Operando Techniques

2023-09-25, Chen, Zongkun, Fan, Qiqi, Zhou, Jian, Wang, Xingkun, Huang, Minghua, Jiang, Heqing, Cölfen, Helmut

Developing efficient and affordable electrocatalysts for the sluggish oxygen evolution reaction (OER) remains a significant barrier that needs to be overcome for the practical applications of hydrogen production via water electrolysis, transforming CO2 to value‐added chemicals, and metal‐air batteries. Recently, hydroxides have shown promise as electrocatalysts for OER. In situ or operando techniques are particularly indispensable for monitoring the key intermediates together with understanding the reaction process, which is extremely important for revealing the formation/OER catalytic mechanism of hydroxides and preparing cost‐effective electrocatalysts for OER. However, there is a lack of comprehensive discussion on the current status and challenges of studying these mechanisms using in situ or operando techniques, which hinders our ability to identify and address the obstacles present in this field. This review offers an overview of in situ or operando techniques, outlining their capabilities, advantages, and disadvantages. Recent findings related to the formation mechanism and OER catalytic mechanism of hydroxides revealed by in situ or operando techniques are also discussed in detail. Additionally, some current challenges in this field are concluded and appropriate solution strategies are provided.

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The impact of metakaolin on the hydration of tricalcium silicate : effect of C-A-S-H precipitation

2023-04-28, Sowoidnich, Thomas, Cölfen, Helmut, Rößler, Christiane, Damidot, Denis, Ludwig, Horst-Michael

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A Facile and Rational Method to Tailor the Symmetry of Au@Ag Nanoparticles

2023-03-04, Ni, Bing, Zhou, Jian, Stolz, Levin, Cölfen, Helmut

Precisely controlling the morphologies of plasmonic metal nanoparticles (NPs) is of great importance for many applications. Here, a facile seed-mediated growth method is demonstrated that tailors the morphologies of Au@Ag NPs from cubes/cuboids to chiral truncated cuboids/octahedra, well-defined octahedra, and tetrahedra, via simply increasing the concentrations of AgNO3 and cysteine in the halide surfactant systems. Accordingly, the particle symmetries are also tuned. The method is quite robust where seeds with distinct shapes including irregular ones can all lead to uniform Au@Ag NPs. The evolution of these shapes can be illustrated by a recently proposed symmetry-based kinematic theory (SBKT). Furthermore, SBKT shows a strategy to optimize the preparation of chiral/dissymmetric NPs, and the experimental results confirm such a dissymmetric synthesis strategy. Cuboids and octahedra with corners differently truncated are identified as two different chiral forms. The chirality of the NPs is additionally probed by electrochemistry, where the chiral NPs show enantioselectivity in the oxidation of d- and l-glucose. Altogether, the results gain fundamental insights into tailoring the plasmonic NP morphologies, and also suggest strategies to obtain chiral NPs.