Team
The focus of our research group is on the so-called molecular switches. Compounds which can change their properties by an external stimulus and can thus be used as sensors or, in the future, data storage devices. Mößbauer spectroscopy has been established as a further research focus in the group. This method allows a precise investigation of the above mentioned switchable compounds. Miniaturized Mößbauer spectrometers have already been successfully used on various space missions.
Molecular switches
The spin crossover phenomenon (SCO) describes the reversible transition between different electronic configurations of a coordination compound through external stimuli (such as temperature, pressure, light, chemical environment, etc.) and was first observed in the early 1930s by Cambi and Malatesta through a temperature-dependent change in the magnetic susceptibility of Fe(III) coordination compounds. A previously unknown stimulus, hard X-ray radiation, was discovered by Prof. Dr. Renz during his doctoral studies and is known as the strong-field LIESST effect. He was able to demonstrate that spin-crossover can be induced and stabilized by light of different wavelengths.
For many years, spin-crossover compounds have attracted increasing interest in coordination chemistry due to their potential for miniaturizing electronic components. The SCO is accompanied by significant property changes (volume, color, magnetism, etc.), which can be used in various nanoscale applications such as data storage or sensors. However, to date, only a few technical applications based on the spin-crossover effect have been developed.
The Renz research group works on the synthesis and characterization of new spin-crossover compounds to better understand the relationship between ligand, metal center, and switching behavior. The goal is to synthesize molecular switches with tailored switching behavior and morphology for testing in novel specialized applications.
The synthesized and characterized spin-crossover compounds can be used in composites as smart materials in a variety of ways, such as in coatings, fibers, or 3D-printed components. For this purpose, the Renz group maintains a collaboration with the group of Prof. Dr. Sindelar at Hannover University of Applied Sciences and Arts. One method for producing one-dimensional polymer fibers in the nanoscale range is called electrospinning, which the group of Prof. Dr.-Ing. Sindelar is specialized in. The electrospinning process is based on the effect of electrostatic forces on free charge carriers in a polymeric liquid. By applying a high-voltage source between a filled syringe and a collector, the polymeric liquid is highly accelerated, elongated, and after the solvent evaporates, collected as a nanofiber. The high surface area of the fibers is attractive for applications such as sensors or filters.
Mößbauer spectroscopy
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NASA/JPL/Cornell University
Mössbauer spectroscopy is a highly precise method for analysing various elements. It is based on the Mössbauer effect, the recoil-free nuclear resonance absorption of gamma rays, and is named after its discoverer, Rudolf Mößbauer (1929–2011). Using this spectroscopic method, bonding and magnetic properties of solids can be examined. In research, Mössbauer spectroscopy primarily investigates iron and tin, as well as their compounds, with 57Fe and 119Sn being the Mössbauer-active isotopes.
Over the years, Mössbauer spectroscopy has been further developed and refined. Advances in electronics and mechanical engineering have enabled the miniaturization of Mössbauer spectrometers. Göstar Klingelhöfer (1956–2019) and his team from the Johannes Gutenberg University in Mainz and the Technical University in Darmstadt developed the miniaturized Mössbauer spectrometer MIMOS II, which was used on the Mars rovers Spirit and Opportunity during the Mars missions. These provided insights into the large water reserves found on Mars in the past. The Renz group is working on improving the devices established by Mr. Klingelhöfer. For this purpose, a cooperation with Hannover University of Applied Sciences and Arts has been established. Currently, in addition to spin-crossover compounds, a variety of minerals such as meteorites, lunar return samples, and analogue samples of terrestrial origin are being analysed. As part of a project on the safe disposal of radioactive waste, the suitability of various filler materials is also being evaluated.
Hydrogen storage materials
One of the new research topics of the Renz group is the storage of hydrogen in solid materials based on carbon nitride. When using hydrogen as an energy carrier, there are still inadequately solved problems for large-scale use, particularly an energetically efficient storage in the industrial and transport sectors. Therefore, the research of new hydrogen storage systems is of enormous relevance for an efficient design of the clean energy transition in all sectors and their integration. A promising material for hydrogen storage is graphitic carbon nitride (g-C₃N₄, see Figure 1). It is environmentally friendly, easy to synthesize, and possesses excellent chemical and physical properties. The Renz group’s focus is the research on the surface modification of g-C₃N₄ to achieve the highest possible storage density, as the specific surface area of g-C₃N₄ is very low. Additionally, there is an effort to incorporate coordination compounds into the matrix to increase the storage capacity of the material and to enable targeted switching of the hydrogen’s storage and release.
Over the years, Mößbauer spectroscopy has been further developed and refined. New technologies offered the possibility of miniaturization. Göstar Klingelhöfer and his research group from the Johannes Gutenberg University in Mainz and the Technical University in Darmstadt thus developed the miniaturized Mößbauer spectrometer MIMOS II, which was used on the rovers Spirit and Opportunity during the Mars missions. They provided information about the water that once existed on Mars. The Renz group is working on improving the devices established by Mr Klingelhöfer.
Cooperations
Head of the group
Prof. Dr. Dr. h. c. Franz Renz
Address
Callinstraße 1
30167 Hannover
30167 Hannover
Building
Room
