First, four types of ZIF-8 with different morphologies were prepared using the following synthetic methods (Fig. 1). Cubic ZIF-8 (denoted as C-ZIF-8) with exposed {100} planes was prepared by the reaction of Zn(NO3)2 and 2-methylimidazole (HMeIm) in water in the presence of hexadecyltrimethylammonium bromide as a surfactant35. Rhombic dodecahedral ZIF-8 (denoted as RD-ZIF-8) with exposed {110} planes were synthesized by the solvothermal reaction of Zn(NO3)2 and HMeIm in methanol35. Scanning electron microscopy (SEM) images of the resulting products clearly revealed the formation of uniform cubes and rhombic dodecahedrons of ZIF-8 (Fig. 1a and b). In addition, powder X-ray diffraction (PXRD) patterns of the products confirmed the formation of well-crystalline ZIF-8 materials, as shown in a representative PXRD pattern for ZIF-8 (Fig. 1e). Energy-dispersive X-ray (EDX) spectra of the products also displayed characteristic elements, including zinc, carbon, and nitrogen, for ZIF-8 (Fig. S1). Furthermore, leaf-shaped ZIF-8 (denoted as L-ZIF-8) was obtained from a two-step synthetic process: 1) the initial construction of two-dimensional (2D) ZIF-L from the reaction of Zn(NO3)2 and HMeIm in water and 2) the subsequent transformation of 2D ZIF-L to three-dimensional L-ZIF-8 through the simple thermal treatment of 2D ZIF-L at 70°C for 30 h36. Finally, plate-shaped ZIF-8 (denoted as P-ZIF-8) was prepared via a reported solvothermal reaction of Zn(NO3)2 and HMeIm in water in the presence of stearic acid (SA) micelles37. The SEM images of the resulting products revealed the formation of thin leaf-shaped ZIF-8 particles (L-ZIF-8, Fig. 1c) and square plates of ZIF-8 (P-ZIF-8, Fig. 1d). The PXRD patterns of these products are representative of ZIF-8 (Fig. 1e). However, additional peaks in the PXRD pattern of P-ZIF-8 were detected because of the presence of SA micelles, which are necessary for the formation of P-ZIF-8. The EDX spectra of L-ZIF-8 and P-ZIF-8 displayed the presence of zinc, carbon, and nitrogen elements (Fig. S1).
The porous properties of the four ZIF-8 samples were analyzed via their N2 sorption isotherms at 77 K (Fig. 2a). C-ZIF-8, RD-ZIF-8, and L-ZIF-8 showed the Type I N2 sorption isotherm, which is typical for ZIF-834; however, P-ZIF-8 displayed non-porous characteristics because of the SA micelles incorporated within P-ZIF-837, as shown in Fig. 2a. No significant differences were observed in the Brunauer–Emmett–Teller (BET) surface areas and total pore volumes of C-ZIF-8, RD-ZIF-8, and L-ZIF-8 (Table S1). For example, the BET surface area and total pore volume of C-ZIF-8 were found to be 1301.1 m2 g-1 and 0.68 cm3 g-1, respectively. In addition, the pore size distributions of the ZIF-8 samples determined by the non-local density functional theory (NLDFT) method revealed the characteristic pore dimension of ZIF-84,34 at ∼11.6 Å for C-ZIF-8, RD-ZIF-8, and L-ZIF-8; however, no critical pore was detected in the case of P-ZIF-8 due to the incorporated SA micelles (Fig. 2b). The surface charges of the four ZIF-8 samples were determined from zeta-potential measurements (Fig. 2c). Generally, ZIF-8 is known to have a positive surface charge because of the exposed metal components (Zn2+) on the external surface38,39. C-ZIF-8, RD-ZIF-8, and L-ZIF-8 displayed characteristic positive charges; however, they had slightly varied potential values of 29.7, 21.0, and 17.7 mV, respectively (Fig. 2c). Among the four samples, C-ZIF-8 had the most positive surface charge (29.7 mV, possibly due to the presence of many metal components exposed on the surface). However, the zeta-potential measurement of P-ZIF-8 revealed that it has a negative surface charge (-37.4 mV, Fig. 2c) because of the co-existing SA micelles. The differences in the surface charges among the four ZIF-8 samples affected their adsorption capacities for the CWA simulants.
The adsorption of CEES on the four ZIF-8 samples was first analyzed at room temperature by using a jar-in-jar setup4 (Scheme 1). Small jars containing ZIF-8 samples and CEES were placed together in a large jar, and ZIF-8 samples exposed to CEES vapors for various time periods were analyzed by 1H NMR spectroscopy to quantify the uptake amounts of CEES on the ZIF-8 samples. The ZIF-8 samples exposed to CEES vapors for various periods were digested in a mixed deuterated solvent of CDCl3 and acetic acid-d4. Peak integrations of CEES and HMeIm molecules were used to determine the amount of CEES per gram of ZIF-8 (Figs. S2–S4). The adsorption graphs showing the relationship between the exposure time and uptake amounts of CEES for the four ZIF-8 samples are shown in Fig. 3a. No significant CEES adsorption was observed on P-ZIF-8, expectedly because of its non-porous nature. The adsorption of CEES on C-ZIF-8, RD-ZIF-8, and L-ZIF-8 almost saturated within 4 h. Moreover, the adsorption capacities of the abovementioned three ZIF-8 samples were slightly different; the adsorption capacity of C-ZIF-8 was found to be the highest at 460 mg of CEES per gram of ZIF-8 (460 mg/g). This adsorption capacity of C-ZIF-8 was much higher than those of other porous materials, such as carbon (74 mg/g)13 and zeolite (108.5 mg/g)14. The adsorption capacities of RD-ZIF-8 and L-ZIF-8 were 440 and 421 mg/g, respectively (Fig. 3b). The difference in the adsorption capacities of the three ZIF-8 samples can be attributed to their different surface charges; the positive charge of the ZIF-8 samples seems to improve their effective interaction with CEES. The positive charge of ZIF-8 enhances the effective polar interaction with the electron-rich sulfur atoms in CEES7,15,40. The adsorption of CEES on the ZIF-8 samples was also verified through IR spectroscopy; the spectra show the representative bands7,41 for CEES at 1262.2 and 1213.0 cm-1 (Fig. S5). The EDX spectra of the ZIF-8 samples except P-ZIF-8 confirmed the incorporation of CEES into ZIF-8, as shown by the detection of sulfur and chlorine elements (Fig. 4). In addition, the SEM images and PXRD patterns of the ZIF-8 samples after the exposure to and adsorption of CEES revealed no critical morphological and structural changes (Figs. S6 and S7).
Furthermore, the DMMP adsorption properties of the four ZIF-8 samples were analyzed by measuring the uptake amounts of DMMP at several time points. The ZIF-8 samples exposed to DMMP vapors for several time points were digested in a mixed deuterated solvent; next, the peak integrations of DMMP and HMeIm molecules were used to determine the amounts of DMMP in the ZIF-8 samples (Figs. S8–S10). The adsorption graphs showing the uptake amounts of DMMP for the four ZIF-8 samples are shown in Fig. 3c. No adsorption of DMMP was observed in the case of P-ZIF-8, similar to that of CEES, owing to its non-porous nature. The adsorption of DMMP on the other three ZIF-8 samples was saturated after 5 days. The time required for saturation of DMMP adsorption was much longer than that of CEES adsorption (4 h) because of the lower vapor pressure of DMMP. The vapor pressures of DMMP and CEES at 25°C are 0.96 and 3.4 mmHg42,43, respectively. The adsorption capacities of the three ZIF-8 samples were slightly different, and the DMMP adsorption capacity of C-ZIF-8 was the highest at 530 mg of DMMP per gram of ZIF-8 (530 mg/g). This value is considerably higher than those of other porous materials, including porous carbon and other MOFs3,7,10. The DMMP adsorption capacities of RD-ZIF-8 and L-ZIF-8 were found to be 412 and 383 mg/g (Fig. 3d). The IR spectra and EDX spectra of the ZIF-8 samples except P-ZIF-8 confirmed the effective adsorption of DMMP on the ZIF-8 samples (Figs. S11 and 4). There were no significant morphological or structural changes after DMMP adsorption, as shown in the SEM images and PXRD patterns (Figs. S12 and S13).
In addition, the recyclability of C-ZIF-8 for CEES adsorption was tested by conducting three successive adsorption experiments (Fig. 5a). The CEES adsorption capacity of C-ZIF-8 was well preserved during the three cycles. In addition, the SEM image and PXRD pattern (Fig. 5b and c) of C-ZIF-8 after three cycles revealed no critical morphological or structural changes during the adsorption process.