Figure 6. SEM images of pure UiO-66 (A) and UiO-66-nGr2 composite (B) samples. The assembled wall of nanoparticles on the nGr surface is marked in the bottom middle micrograph.
Figure 7 shows the CO2 adsorption isotherms measured on the four materials at 0, 20 and 40°C. The amounts of CO2adsorbed at atmospheric pressure are compared in Table 2 and are in the range of those reported in the literature [14-18]. Upon the composite formation, a ~20 % increase in the amount of CO2 adsorbed was found. The best performing sample is UiO-66-nGr1, and we link it to its highest volume of pores smaller than 0.5 nm in which the adsorption forces for CO2 are the strongest and where, at the low relative pressure, the density of CO2 is the highest [38]. The amount of CO2 adsorbed on UiO-66-nGr2 and UiO-66-nGr 6 is similar to that on UiO-66-nGr1, which is in agreement with the similarities in the volume of ultramicropores in the composite materials. The CO2 uptake decreased with the increasing temperature, from 3 mmol/g at 0°C to 1.2 mmol/g at 40°C for UiO-66-nGr1, in good agreement with the exothermic nature of CO2 adsorption on these materials.