Effect of free carboxylic acid groups in UiO-66 analogues on the adsorption of dyes from water: Plausible mechanisms for adsorption and gate-opening behavior

Herein, we report that the surface charge status, adsorption sites, and adsorption spaces in the pore channels of UiO-66-COOH nanocubes can be modulated by changing the mole ratio of H2BDC:H2BDC-COOH in the reaction system. The adsorption capacity of UiO-66-COOH-2 toward the cationic organic dye methylene blue was found to be much higher than that of other UiO-66-COOH adsorbents, whereas UiO-66-COOH-1 nanocrystals exhibited the highest adsorption capacity toward the anionic organic dye Congo red because of the different adsorption mechanisms involved. The adsorption capacity of an adsorbent is determined by the synergetic interplay between the hydrophobic, electrostatic, and π–π interactions and the adsorption sites and adsorption spaces in the pore channels. Furthermore, for the first time, we found that UiO-66-COOH nanostructures exhibited gate-opening behavior during the dye adsorption process owing to the free –COOH groups in the pore channels. The obtained results provided new insights into the mechanism of organic dye adsorption by metal–organic frameworks (MOFs). Therefore, this assembly strategy offers new research opportunities for preparing functionalized MOFs with new functions and potential applications.