Cölfen, Helmut
0000-0002-1148-0308 Berufsbeschreibung
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Suchergebnisse Publikationen
The Structure, Preparation, Characterization, and Intercalation Mechanism of Layered Hydroxides Intercalated with Guest Anions
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.
Revealing the Formation Mechanism and Optimizing the Synthesis Conditions of Layered Double Hydroxides for the Oxygen Evolution Reaction
Layered double hydroxides (LDHs), whose formation is strongly related to OH- concentration, have attracted significant interest in various fields. However, the effect of the real-time change of OH- concentration on LDHs’ formation has not been fully explored due to the unsuitability of the existing synthesis methods for in situ characterization. Here, the deliberately designed combination of NH3 gas diffusion and in situ pH measurement provides a solution to the above problem. The obtained results revealed the formation mechanism and also guided us to synthesize a library of LDHs with the desired attributes in water at room temperature without using any additives. After evaluating their oxygen evolution reaction performance, we found that FeNi-LDH with a Fe/Ni ratio of 25/75 exhibits one of the best performances so far reported.
Formation Of Amorphous Iron‐Calcium Phosphate With High Stability
Amorphous iron-calcium phosphate (Fe-ACP) plays a vital role in the mechanical properties of teeth of some rodents, which are very hard, but its formation process and synthetic route remain unknown. Here, we report the synthesis and characterization of an iron-bearing amorphous calcium phosphate in the presence of ammonium iron citrate (AIC). The iron is distributed homogeneously on the nanometer scale in the resulting particles. The prepared Fe-ACP particles can be highly stable in aqueous media, including water, simulated body fluid and acetate buffer solution (pH 4). In vitro study demonstrates that these particles have good biocompatibility and osteogenic properties. Subsequently, Spark Plasma Sintering (SPS) is utilized to consolidate the initial Fe-ACP powders. The results show that the hardness of the ceramics increases with the increase of iron content, but an excess of iron leads to a rapid decline in hardness. Calcium iron phosphate ceramics with a hardness of 4 GPa can be achieved, which is higher than that of human enamel. Furthermore, the ceramics composed of iron-calcium phosphates showed enhanced acid resistance. This study provides a novel route to prepare Fe-ACP, and presents the potential role of Fe-ACP in biomineralization and as starting material to fabricate acid-resistant high-performance bioceramics.
Synthesis of ultrathin metal oxide and hydroxide nanosheets using formamide in water at room temperature
The rational preparation of ultrathin two-dimensional metal oxide and hydroxide nanosheets is the first and crucial step towards their utilization in both fundamental research and practical applications. From the perspective of the production cost, the further development of such materials remains a challenging task. Therefore, it is highly desirable to synthesize these materials via a simple and general strategy at room temperature. Besides, less attention has been paid to investigate their growth process, leading to ambiguous formation mechanisms and the lack of guiding principles for designing the targeted ultrathin 2D metal oxides and hydroxides. Here, 6 different ultrathin (<5 nm) 2D metal oxides and hydroxides have been successfully synthesized via a simple precipitation route in formamide aqueous solution at room temperature. Detailed investigations demonstrate that the formation of the ultrathin morphology relies on the inhibition of the z-direction growth by the –NH2 groups of the formamide molecules. These findings broaden the fundamental understanding of 2D material formation mechanisms and inspire interest in extending this strategy to further systems. Our study opens a new avenue for an easy, general and room temperature synthesis of ultrathin 2D metal oxides and hydroxides.
Growth strategy for solution-phase growth of two-dimensional nanomaterials via a unified model
Two-dimensional (2D) materials prepared by a solution-phase growth route exhibit many unique properties and are promising for use in various fields. However, simple, rational and green fabrication of target materials remains challenging due to the lack of guiding principles. Here we propose a universal qualitative model for 2D materials grown for layered and non-layered crystal structures by a solution-phase growth route; both theoretical simulation and experimental results confirm the model’s validity. This model demonstrates that 2D growth can be controlled by only tuning the reaction concentration and temperature, and has been applied to fabricate more than 30 different 2D nanomaterials in water at room temperature and in the absence of additives. Furthermore, the model shows promise for optimizing the experimental design of numerous other 2D nanomaterials.
Toward Understanding the Formation Mechanism and OER Catalytic Mechanism of Hydroxides by In Situ and Operando Techniques
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.
Synthesis of two-dimensional layered double hydroxides : a systematic overview
Two-dimensional (2D) layered double hydroxides (LDH) are classic materials in fundamental research and practical application. 2D LDH have unique structural features, such as high aspect ratio, high specific surface area, quantum confinement in one direction, layered structure, tunable intercalated anions/interlayer spacing/metal-cation compositions, etc., which endow 2D LDH with excellent chemical and physical properties. Since rational production is the first and crucial step towards their utilization, numerous efforts have been devoted to explore innovative and effective methods for their preparation. This review is a comprehensive overview of synthesis methods and aimed to capture the features of different synthesis methods to enable the selection of a suitable method to achieve the desired LDH characteristics. Therefore, we systematically summarized various preparation methods of 2D LDH, including a top-down approach, bottom-up approach, and decomposition-regrowth strategy. Besides, we listed and compared the important parameters relating to both the synthesis process and the obtained products of various synthesis methods, such as the synthesis temperature/pressure, the used device, the utilized reactants, the presence or absence of pre/post-treatment, the yield, the crystallinity and the lateral size and thickness. Finally, some current challenges and further tasks regarding the synthesis of LDH are presented.
Aufdeckung des Entstehungsmechanismus und Optimierung der Synthesebedingungen von geschichteten Doppelhydroxiden für die Sauerstoffentwicklungsreaktion
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.
A Bioinspired Gelatin–Amorphous Calcium Phosphate Coating on Titanium Implant for Bone Regeneration
Biocompatible and bio-active coatings can enhance and accelerate osseointegration via chemical binding onto substrates. Amorphous calcium phosphate (ACP) has been shown as a precursor to achieve mineralization in vertebrates and invertebrates under the control of biological macromolecules. This work presents a simple bioinspired Gelatin-CaPO4 (Gel-CaP) composite coating on titanium surfaces to improve osseointegration. The covalently bound Gel-CaP composite is characterized as an ACP-Gel compound via SEM, FT-IR, XRD, and HR-TEM. The amorphous compound coating exhibited a nanometer range thickness and improved elastic modulus, good wettability, and nanometric roughness. The amount of grafted carboxyl groups and theoretical thickness of the coatings are also investigated. More importantly, MC3T3 cells, an osteoblast cell line, show excellent cell proliferation and adhesion on the Gel-CaP coating. The level of osteogenic genes is considerably upregulated on Ti with Gel-CaP coatings compared to uncoated Ti, demonstrating that Gel-CaP coatings possess a unique osteogenic ability. To conclude, this work offers a new perspective on functional, bioactive titanium coatings, and Gel-CaP composites can be a low-cost and promising candidate in bone regeneration.
Synthesis of hierarchical transition metal oxyhydroxides in aqueous solution at ambient temperature and their application as OER electrocatalysts
First-row (3d) transition metal oxyhydroxides have attracted increasing attention due to their various advantages. Although investigating the oxidation mechanism and processing such materials into hierarchical architectures are greatly desired for their further development, it remains unclear how the oxidation state change occurs, and efforts to produce hierarchical oxyhydroxides in compliance with high ecological and economic standards have progressed slowly. Here, we describe a facile one-step coprecipitation route for the preparation of hierarchical CoOOH, NiOOH and MnOOH, which involves the diffusion of NH3 originating from ammonium hydroxide solution into an aqueous solution containing metal ion salts and K2S2O8. Comprehensive characterizations by scanning electron microscope, transmission electron microscopy, X-ray diffraction analysis, X-ray photoelectron spectroscopy, ultraviolet-visible spectroscopy and in situ pH measurement demonstrated that K2S2O8 induces the oxidation state change of metal ion species after the start of hydrolysis. Meanwhile, it was found that, benefiting from the OH– concentration gradient created by the NH3 diffusion method and the suitable growth environment provided by the presence of K2S2O8 (high nucleation rate and secondary nucleation), the formation of hierarchical oxyhydroxide structures can be realized in aqueous solution at ambient temperature without the use of heat energy and additional structure-directing agents. The hierarchical CoOOH structures are performed as the electrocatalysts for the oxygen evolution reaction in alkaline media, which exhibit good activity with an overpotential of 320 mV at 10 mA cm−2 and a low Tafel slope of 59.6 mV dec–1, outperforming many congeneric electrocatalysts. Overall, our study not only provides important insights to understand the formation mechanism of hierarchical oxyhydroxides, but also opens up new opportunities for the preparation of hierarchical oxyhydroxides via a facile, green and low-cost method.