X-ray analysis of carbon nanostructures helps material design
Schematic view of carbon structures with pores. © HZB
The intensity of the single peak increases as the chlorination temperature increases and carbons show increased order. © HZB
Nanostructures made of carbon are extremely versatile: they can absorb ions in batteries and supercapacitors, store gases, and desalinate water. How well they cope with the task at hand depends largely on the structural features of the nanopores. A new study from the HZB has now shown that structural changes that occur due to morphology transition with increasing temperature of the synthesis can also be measured directly – using small-angle X-ray scattering. The results have now been published in the journal Carbon.
Optimized nanoporous carbons can serve as electrodes for fast electron and ion transport or improve the performance of energy storage and conversion devices. Thus the tunability of the size, shape, and distribution of pores is highly required. The team at the HZB Institute for Soft Matter and Functional Materials collaborated with a group at the University of Tartu, Estonia, to inquire the nanoarchitecture, inner surface, size, form and distribution of nanopores in dependence of the synthesis conditions.
From 600 to 1000 degrees
Colleagues in Estonia produced a series of nanoporous carbons by reacting a powder of molybdenum carbide (Mo2C) with gaseous chlorine at 600, 700, 800, 900, and 1000 degrees Celsius. Depending on the synthesis conditions chosen, the nanoporous carbon exhibit different properties such as surface area, porosity, electronic and ionic conductivity, hydrophilicity and electrocatalytic activity.
Small Angle X-ray Scattering SAXS
Surface structures were analysed by transmission electron microscopy at the HZB. The interior surface area of nanocarbon materials is usually investigated by adsorption of gas. However, this method is not only comparatively inaccurate, it also contains no information about the shape and size of the pores. For deeper insights, Dr. Eneli Härk and her colleagues at HZB worked with small-angle X-ray scattering, a technique permitting to obtain information on various structural features on the nanometer scale including the mean pore size.
All about nanopores
Small-angle X-ray scattering not only provides information on the precise inner surface area and the average pore size, but also on their angularity, i.e., sharp edges of formed pores, which play a major role for the functionalization of the materials. “The SAXS analysis summarizes over an enormous amount of micropores omitting misleading assumptions thereby directly relating the nanostructural architecture of the material to macroscopic technical parameters under investigation in engineering” Härk explains.
The main aim was to understand structural formation, and electrochemical characteristics of carbon as a function of the synthesis temperature. “For optimal function, not only the high inner surface area is crucial, but the pores should have exactly the right shape, size and distribution”, says Härk.
The study is published in "Carbon" (2019): Carbide Derived Carbons Investigated by Small Angle X-ray Scattering: Inner Surface and Porosity vs. Graphitization; Eneli Härk, Albrecht Petzold, Günter Goerigk, Sebastian Risse, Indrek Tallo, Riinu Härmas, Enn Lust and Matthias Ballauff.
DOI: 10.1016/j.carbon.2019.01.076
- A new way to control the magnetic properties of rare earth elementsThe special properties of rare earth magnetic materials are due to the electrons in the 4f shell. Until now, the magnetic properties of 4f electrons were considered almost impossible to control. Now, a team from HZB, Freie Universität Berlin and other institutions has shown for the first time that laser pulses can influence 4f electrons- and thus change their magnetic properties. The discovery, which was made through experiments at EuXFEL and FLASH, opens up a new way to data storage with rare earth elements.
- BESSY II shows how solid-state batteries degradeSolid-state batteries have several advantages: they can store more energy and are safer than batteries with liquid electrolytes. However, they do not last as long and their capacity decreases with each charge cycle. But it doesn't have to stay that way: Researchers are already on the trail of the causes. In the journal ACS Energy Letters, a team from HZB and Justus-Liebig-Universität, Giessen, presents a new method for precisely monitoring electrochemical reactions during the operation of a solid-state battery using photoelectron spectroscopy at BESSY II. The results help to improve battery materials and design.
- HZB magazine lichtblick - the new issue is out!In his search for the perfect catalyst, HZB researcher Robert Seidel is now getting a tailwind – thanks to a ERC Consolidator Grant. In the cover story, we explain why the X-ray source BESSY II plays an important role for his research.