Nanocomposites and nanomaterials 

Optical spectroscopy of Co-based layered double hydroxides

Lukienko I. M.1, Babkin R. Yu.2, Pashkevich Yu. G.2, Kharchenko Yu. M.1, Wulferding D.3, Lemmens P.3,4, Vieira D.E.L.5, Salak A. N.5
1 B.Verkin Institute for Low Temperature Physics and Engineering of the National Academy of Sciences of Ukraine, 47 Nauky Ave., Kharkiv-61103, Ukraine.
2 O. Galkin Donetsk Institute for Physics and Engineering of the National Academy of Sciences of Ukraine, 46 Nauky Ave., Kyiv 03680, Ukraine.
E-mail:yu.pashkevich@gmail.com
3 Laboratory for Emerging Nanometrology (LENA),
TU Braunschweig, D-38106 Braunschweig, Germany
4  Institute for Condensed Matter Physics, TU Braunschweig, D-38106 Braunschweig, Germany
5  CICECO-Aveiro Institute of Materials, Department of Materials and Ceramics Engineering, University of Aveiro, 3810-193 Aveiro, Portugal.

We report on the optical absorption and Raman scattering studies of nano-flakes of double layered hydroxides (LDH) that consist of trigonally distorted (D3d) octahedral layers of hydroxide OH- pairs, contained Co2+ and Al3+ cations, and anions located between these layers. Due to ability to intercalate different functional organic and inorganic anionic species, LDH are attractive for many applications, such as bioactive nanocomposites in medicine, nanocontainers for self-healing corrosion protection, acid absorbents for air and water treatment, and others. Our studies are mainly focused on dynamics of the hydrogen bonding in a range between room temperature and liquid helium temperature, which is reflected in variation of the d-d-optical transitions and the frequencies of the OH vibrations. 
We consider Co(n)Al-LDHs with different cations ratio n = Co/Al (n = 2, 3 and 4) intercalated with CO32- and NO3-. We conclude that at room temperature the hydrogen bonds subsystem remains in a highly fluctuated dynamic state. The level of fluctuations depends on the cations ratio n. The hydrogen bonds become static at temperatures of liquid nitrogen and liquid helium. The energy of the optical transitions levels has been modeled within original modified crystal field approach. 
The research conducted in the University of Aveiro was supported by project TUMOCS. This project has received funding from the European Union’s Horizon 2020 research and innovation program under the Marie Skіodowska-Curie grant agreement No 645660.