Visualization of gaseous iodine adsorption on single zeolitic imidazolate framework-90 particles.

Zeolitic imidazolate frameworks (ZIFs) are very useful as high-capacity iodine (I2) adsorbents. The adsorption performance is usually probed by measuring a statistical average property over an entire sample consisting of a large number of ZIF particles, leaving the interparticle heterogeneity information among individuals. Here we report a dark-field microscopy (DFM) method to visualize gaseous I2 adsorption on single ZIF-90 particles in situ and in real time. The adsorption of I2 is found to alter the scattering spectrum of ZIF-90 particles, inducing a distinct color change from bluewhite to yellow. According to correlating the adsorption amount of gaseous I2 with the change of B value from DFM images, we quantitatively image the adsorption process and estimate the related kinetic parameters at the single particle level. Single particle measurements clarify the large particle-to-particle heterogeneity in adsorption reactivity and significant adsorption activity improvement of ZIF-90 after introduction of linker defects, which provides a microscopic understanding of the structure-activity relationship. We further demonstrate the capacity of this strategy for studying gaseous I2 adsorption on single ZIF-91 particle as a derivative of ZIF-90 to illustrate the generality.