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A. Müller, T. Krahl, J. Radnik, A. Wagner, C. Kreyenschulte, W.S.M. Werner, B. Ritter, E. Kemnitz, W. Unger:
"Chemical in-depth analysis of (Ca/Sr)F₂ core-shell like nanoparticles by X-ray photoelectron spectroscopy with tunable excitation energy";
Surface and Interface Analysis, 53 (2021), 494 - 508.

The fluorolytic sol-gel synthesis is applied with the intention to obtain two different
types of core-shell nanoparticles, namely, SrF2-CaF2 and CaF2-SrF2. In two separate
fluorination steps for core and shell formation, the corresponding metal lactates are
reacted with anhydrous HF in ethylene glycol. Scanning transmission electron
microscopy (STEM) and dynamic light scattering (DLS) confirm the formation of
particles with mean dimensions between 6.4 and 11.5 nm. The overall chemical composition
of the particles during the different reaction steps is monitored by quantitative
Al Kα excitation X-ray photoelectron spectroscopy (XPS). Here, the formation of
stoichiometric metal fluorides (MF2) is confirmed, both for the core and the final
core-shell particles. Furthermore, an in-depth analysis by synchrotron radiation XPS
(SR-XPS) with tunable excitation energy is performed to confirm the core-shell
character of the nanoparticles. Additionally, Ca2p/Sr3d XPS intensity ratio in-depth
profiles are simulated using the software Simulation of Electron Spectra for Surface
Analysis (SESSA). In principle, core-shell like particle morphologies are formed but
without a sharp interface between calcium and strontium containing phases.
Surprisingly, the in-depth chemical distribution of the two types of nanoparticles is
equal within the error of the experiment. Both comprise a SrF2-rich core domain and
CaF2-rich shell domain with an intermixing zone between them. Consequently, the
internal morphology of the final nanoparticles seems to be independent from the
synthesis chronology.

metal fluorides, nanoparticles, sol-gel synthesis, synchrotron radiation, X-ray photoelectron spectroscopy

http://dx.doi.org/10.1002/sia.6937

https://publik.tuwien.ac.at/files/publik_295144.pdf