Figure 7: In situ THz IR analysis
on ZIF-62 and AgZIF-62 at a temperature range between 30oC to 300 oC for sample a) ZIF-62-a,
b) ZIF-62-c, c) AgZIF-62-a, d)AgZIF-62-c. For each plot, from bottom to
top, the sample is heated from 30oC to
300oC, before cooling back down to
30oC.
In order to further
understand the silver interactions within the structure of the ZIF-62
and its role in the ZIF-62 structural transformation, the samples were
probed using THz IR (Figure 7). There were not many changes or
differences between different benzimidazole ratios of ZIF-62 particles
with no silver loading (ZIF-62-a and ZIF-62-c). Only three main clear
peaks can be observed in both cases, a peak at around 280-330
cm-1 which indicates the Zn-N stretching movements, a
peak around 650-700 cm-1, indicating in-plane and
out-of-plane aromatic ring deformations, and a peak at around 750-800
cm-1 indicating C-H bending. In the presence of
silver, however, some peak intensifies with new peaks forming at
different benzimidazole ratio. For the lower benzimidazole ratio
(AgZIF-62-a), the peaks responsible for all three groups present in the
normal ZIF-62-a start to intensify greatly above 200oC. A clear peak broadening for the peaks responsible
for Zn-N stretching movements (280-330 cm-1) and C-H
bending can be seen, which may indicate additional interaction at these
sites. Additional stronger peaks also start to emerge between 100-200
cm-1, which represents the 4-membered and 6-membered
ring collective motions, especially with the peak around 150-200
cm-1. This may indicate the formation of
ZIF-zni-related interactions as this formation was not observed for
AgZIF-62-c. AgZIF-62-c also undergoes similar intensification for the
three main peaks observed for the normal ZIF-62-c. Some intensifications
of the peaks between 400-600 cm-1 may also indicate a
stronger interaction with the benzimidazole ligand within the structure,
as data from the literature shows no significant peak between this
wavenumber region for Zn and Imidazole
precursors.[26] This may also indicate that the
silver is strongly interacting with the benzimidazole ligands at higher
temperatures, which further explains the change of thermal dynamics
between lower and higher benzimidazole ratio AgZIF-62 (AgZIF-62-a
forming ZIF-zni while AgZIF-62-c was able to fully melt). The stronger
interaction of Ag with the benzimidazole ligand may have promoted the
stacking of imidazolate ligand at a lower temperature, thus promoting
the formation of ZIF-zni in the environment with a lower benzimidazole
amount. An additional peak can also be seen for AgZIF-62-c at a
wavenumber range between 650-700 cm-1 at 300oC, which may indicate an additional interaction
within the aromatic ring, leading to an additional aromatic ring
deformation peak. This additional peak is highly likely to be attributed
to the interaction of Ag within the AgZIF-62-c structure as this
additional peak was not seen in any of the normal ZIF-62 structures
(ZIF-62-a and ZIF-62-c), and also from THz data reported in the
literature on ZIF-62 variants.[26] This proves
that Ag is strongly interacting with the ligand structure within the
AgZIF-62 crystals, but at different sites depending on the final
formation results (ZIF-zni for AgZIF-62-a and fully melted ZIF for
AgZIF-62-c).