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).