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.