Dye adsorption in ZIF-8: The importance of external surface area

Two kinds of ZIF-8 crystals have been prepared from the ultrasound-assisted hydrothermal reaction of a stoichiometric metal-ligand precursor solution. The addition of surfactant promoted the formation of nanocrystals having a much larger external surface area compared to the non-promoted one. The two samples were then employed to examine the effects of particle size on dye adsorption. The adsorption capacity of Rhodamine B (RB) and methyl orange (MO) was found directly proportional to the external surface area of the ZIF-8 particles, while that of methylene blue (MB) did not vanish with decreasing external area. The residue adsorption is the most clear-cut evidence confirming the penetration of the MB molecule into the ZIF-8 framework, but not the similarly sized MO molecule. The larger RB molecule was also excluded from the internal pores and cover only the external surface of the crystals. Our calculations suggested that each RB and MO molecule occupied 4.7 and 7.9 nm² of the external surface, respectively. In contrast, MB adsorption occurs both inside the ZIF-8 structure, at a loading of 0.11 molecule per unit cell, and on the external surface with 2.3 nm²/molecule.