Fig. 1 shows the powder X-ray diffr

Fig. 1 shows the powder X-ray diffraction (XRD) patterns and the transmission electron microscope (TEM) images of the as-synthesized samples. Comparison of the sample XRD pattern to the pattern simulated from the published ZIF-8 structure data4 indicates that the product is pure-phase ZIF-8 material (Fig. 1(a)). Peak broadening can be clearly observed from the sample XRD pattern, indicating the formation of nanosized crystals. From the Scherrer equation the crystallite size was calculated as B70 nm. The Fourier-transform infrared (FTIR) spectrum (see Fig. S1 in ESIw) also demonstrated that the as-synthesized sample was ZIF-8 material.18 The TEM pictures in Fig. 1(b) and (c) as well as the Field-emission SEM images (see Fig. S2 in ESIw), revealed that the particles were nanocrystals with sharp hexagonal facets. The mean particle size is ca. 85 nm. The polydispersity index is around 0.089 measured from dynamic light scattering. The electron diffraction pattern shown in the inset of Fig. 1(b) confirmed once again that the product was highly crystalline ZIF-8 crystals. The obtained nanoparticles can be well dispersed in methanol to form a stable suspension (Fig. 1(d)) and can be kept for several weeks without settlement. Varying the molar ratio of 2-methylimidazole to zinc nitrate can further reduce the crystal size, but has little effect on the morphology. As the results showed in Fig. S3 in ESIw, the crystal size was reduced from B85 nm to B50 nm when the ratio of 2-methylimidazole tozinc nitrate increased from 70 to 200.