Figure 6: DSC heat flow analysis
of pure and Silver doped ZIF-62 a)Initial upscan, and b) the second
upscan. Small arrow indicating the melting peak (Tm) for
the initial upscan and glass transition peak (Tg) for
the second upscan.
Highlighted region in a) indicates a possible region for Ag nanoparticle
formation.
A DSC analysis (Figure 6) was used to further probe the behaviour of
these particles. For the lower bIm ratio samples, there were additional
peaks at temperatures ca. 179 oC and 238oC, which may indicate the interaction of Ag within
the structure, leading to the formation of ZIF-zni. The dip at around
200 oC to 330 oC in Figure 6a may
indicate the formation of Ag nanoparticles, which coincides with the
previous XRD structure. These peaks are also not present in the pure
ZIF-62, which further indicates the role of silver in the observation of
this behaviour. It is also interesting to note that the peaks seen at
earlier temperatures for the lower bIm ratio of silver-doped AgZIF-62
were not observed at the higher bIm ratio, which may indicate the
absence of ZIF-zni formation. The melting peak can be seen for all cases
in the first DSC upscan, with each respective ZIF-62 and AgZIF-62 of the
same benzimidazole ratio having an almost similar melting point. The
ZIF-62-a and AgZIF-62-a can be seen to have a melting peak, with
Tm at around 374 oC and 373oC, respectively, while ZIF-62-c and AgZIF-62-c have a
melting temperature, Tm at 413 oC and
419 oC respectively. A clearer melting peak can also
be observed for the higher benzimidazole ratio ZIF-62, which may
indicate clearer and better melting at a higher benzimidazole ratio. An
exothermic peak was also seen at around 373 oC for
AgZIF-62-a, after the melting peak ends, which may indicate the
recrystallisation back into the dense crystalline ZIF-zni phase right
after the melting process occurs. This peak forms immediately after the
observed melting point, which may also indicate an accelerated process
of the conversion of the remaining ZIF-62 to ZIF-zni during melting,
most probably due to the better flowability of silver interacting with
the whole ZIF-62-a structure.
This analysis shows that the Ag to bIm ratio is very important to
control the melting of ZIF-62, either to form a complete melt or to form
a recrystallized and more thermodynamically stable ZIF-zni. It is also
important to note that the Ag compounds in all AgZIF-62 variants start
to form Ag nanoparticles at temperatures between 150oC to 200 oC.