Local structure and dynamics of two novel metal-organic framework materials investigated with Raman spectroscopy and neutron scattering techniques
Typ
Examensarbete för masterexamen
Master's Thesis
Master's Thesis
Program
Nanotechnology (MPNAT), MSc
Publicerad
2024
Författare
Reinsfelt, David
Modellbyggare
Tidskriftstitel
ISSN
Volymtitel
Utgivare
Sammanfattning
Metal-Organic Frameworks (MOFs) show a wide variety of new applications in areas
such as energy storage, catalysis, sensor technology and drug delivery, due to
their inherent porous structures and facile tunability. Some MOFs also show structural
flexibility upon temperature variation, which opens up even more possibilities
for applications. However, the local structure and dynamics, underpinning this
flexibility, are often unclear. This thesis deals with a study of the local structure
and dynamics of two newly discovered MOFs, namely La2(CPB) and Ce2(CPB),
where CPB refers to the hexagon shaped hexatopic linker 1,2,3,4,5,6-hexakis(4- carboxyphenyl)
benzene. La2(CPB), also called CTH-17, has previously proven to show
gate opening effects under CO2 gas sorption measurements by drastically increase
the adsorption after certain pressure values. Changes in the crystal structure in the
form of twisted phenyl rings and stretching in the rod direction of the rod-shaped
CTH-17 have also been observed when measured with SCXRD in 90 K, 300 K and
500 K. This study seeks to confirm these observed structure changes by studying
local atomic vibrations, using Raman spectroscopy and inelastic neutron scattering
(INS). The goal is not only to observe the static material structures before and after
undergone the mentioned changes observed with SCXRD measurements, but also
to observe the structural dynamics upon temperature variation.
The samples were synthesized with a solvothermal synthesis method which yielded
La2(CPB) and Ce2(CPB) as the majority phase with variable amounts of La or Ceformate
as side phases. Both Raman and INS spectra of La2(CPB) and Ce2(CPB)
show negligible differences, indicating that their local structure and dynamics are
very similar. Room temperature Raman spectroscopy measurements of activated
La2(CPB) and Ce2(CPB) samples show photoluminescence. The variable temperature
Raman spectroscopy results show indications of DMF leaving the porous structures
around 200-300 ◦C. Linearly shifting frequency trends with increased temperature
is observed for several bands indicating thermal expansion, and a big irreversible
shift at 300 ◦C is observed indicating a new structural phase. Indications
of luminescence in combination with DMF leaving the structure is also observed.
The similarities between La2(CPB) and Ce2(CPB) suggests that changing the
metal ion nodes in this type of hexagonal shaped MOF from La to a similar metal
such as Ce, does not alter the local structure and dynamics. The supposed observed
thermal expansion and structural phase shift for both La2(CPB) and Ce2(CPB)
could explain the gate opening effect during CO2 gas sorption measurements by
showing that the porous structure changes upon increased temperature which can
create more space for molecules to enter the pores.