Figure 2. TG (A) and DTG (B) curves under helium flow for pure UiO-66 and UiO-66-nGr composite samples.
TG and DTG curves measured in helium are collected in Figure 2. Based on the thermal stability of the composites upon addition of nGr, they bring indirect information about the change in the chemical composition of our composites. While the linkers in UiO-66 decompose at 460°C, causing the collapsing of the MOF structure [11, 36], this process for all composites takes place at a temperature of about 80°C higher. This shift might be related to the change in a heat capacity upon the addition of the nanographite phase. This change in the heat capacity might also be due to the expected alteration in the samples’ porosity. Interestingly, the decomposition event of the composites overlaps with a small shoulder seen on the UiO-66 DTG curve, which likely suggests the imperfection of our UiO-66 structure. A broad peak between 200 and 400°C is linked to the removal of the residual solvent. This process occurs over a broader temperature range for the composites and might be caused by the interaction of the solvent with the nGr phase.
The results of thermal analysis in air are presented in Figure 1S of the Supplementary Information. From these data, assuming that the missing linkers are the main defects in our materials, and based on the approach of Katz et al. [12], the number of linkers was calculated. The obtained results indicate that 8, 7.8, 7.8 and 7.6 linkers are present per node in UiO-66, UiO-66-nGr, UiO-66-nGr2 and UiO-66-nGr6, respectively. Since the ideal structure is expected to have 12 linkers, all our materials can be considered defective and the level of defects is higher in the composites and increases with an increase in the nGr content.