Figure 8: FTIR analysis on ZIF-62 and AgZIF-62 particles
FTIR analysis was also performed on the pure ZIF-62-a and ZIF-62-c, as well as the silver-loaded AgZIF-62-a and AgZIF-62-c and their melted components (denoted as the mAgZIF-62) (Figure 8). The FTIR analysis shows peaks for the C-C in-plane and out of plane bending (600-700 cm-1), C-H out of plane bending (700-1000 cm-1), C-H in-plane bending (1100-1300 cm-1), C-N stretching (1300-1370 cm-1), and C=C stretching (1390-1650 cm-1), which are all present in the ligands used for the formation of ZIF-62. However, no clear FTIR peak changes between the silver-loaded ZIF-62 (AgZIF-62-a and ZIF-62-c), its melted components (mAgZIF-62-a and mAgZIF-62-c), and pure ZIF-62 variants (ZIF-62-a and ZIF-62-c) can be seen. Thus, it is most likely that these interactions were not impacted by the presence of silver ions/molecules in the structure. An extra peak for mAgZIF-62-a that is present at 775cm-1 could be attributed to the out-of-plane deformation of the imidazolate linker contributed by the presence of ZIF-zni. This extra peak was also present in FTIR data reported in the literature for the ZIF-zni phase.[32]
An XPS analysis was also performed to understand the bonding occurring on the surface of the particles. The wide scan XPS analysis, as can be seen in Figure S 2a, shows no obvious new peaks between all the crystalline and melted variations, except for the variations containing Ag. This is consistent with the sample and demonstrates that the Ag has been successfully embedded within the sample. The intensity of Ag peaks relative to other peaks was also diminished in the melted sample when compared to the crystalline sample, which indicated that the Ag species may be embedded within the ZIF-62 melted structures rather than being on the surface. This is because XPS is usually a surface-sensitive analysis, and with the same penetration power, the XPS will not penetrate as deeply into the denser melted/zni forms when compared to the crystalline forms, thus reading less Ag that is situated deeper within the structure. The summary of the XPS peaks found for different species can be found in Figure S 3. No clear changes can be seen for all other species except for Ag. A deeper look into the Ag peak shifts can also be seen in Figure S 2b, in which the non-melted AgZIF-62 variants indicate that the Ag may exist in an ionic AgNO3 form, while the melted mAgZIF-62 variants show a possible oxidised Ag. This may be due to the exposure of Ag on the surface to air, thus resulting in the formation of AgO on its surface. As explained above, the lower penetration depth and surface sensitivity of XPS into the denser heat-treated ZIF-62 may be a reason for the lack of Ag nanoparticle peaks, as the Ag nanoparticle may be deeper embedded within the structure. Thus, the actual state of silver within the melted/zni particles should closely resemble the results seen in previously collected XRD data which shows the conversion of silver ions into silver nanoparticles only at higher temperatures (above 150oC).