As a new class of porous materials, metal-organic frameworks (MOFs), constructed from metal ions and organic ligands, have attracted considerable attention. The first step to apply unique features of MOFs such as diverse structures and flexible frameworks is to prepare MOFs as a highly uniform film. [Cu2(pzdc)2(pyz)]n (CPL-1; pzdc = pyrazine-2,3-dicarboxylate; pyz = pyrazine) possessing one-dimensional channels (4 Å× 6 Å) [1] is considered here because of the unique characteristics of CPL-1. Very recently, thin films of several MOFs were prepared on surface-modified substrates by in situ crystallization and layer-by-layer assembly; however, CPL-1 films were not obtained by such methods. Thus, we have developed a new method “repeated in situ crystallization”, combing the in situ crystallization and the secondary growth, to prepare continuous CPL-1 films. [2] In this concept, the crystals formed on the substrates act as seeds for the further growth. The procedure is as follow; the substrates were floated for 2h on the CPL-1 synthesis solution freshly prepared at RT, then the substrates were thoroughly rinsed with distilled water and dried. The procedure (float-wash-dry) was repeated for five cycles. The formation of CPL-1 films was confirmed by X-ray diffraction (XRD) and field emission scanning electron microscopy (FE-SEM). The XRD patterns mainly show the peaks attributed to (0k0) in all of the samples (one to five cycles), and the (0k0) peaks become more intense after repeating the procedure, clearly representing preferential b orientation. FE-SEM images show that the number of crystals increased after repeated growth. After the fifth cycle, no bare surfaces of the substrate were observed. In addition, the influence of the substrates was remarkable at the early stages of the formation of CPL-1 film, however, no difference of the substrates was observed after the fifth cycle. These results suggest that repeated in situ crystallization could increase the density of nuclei on the surface regardless of the chemical or physical natures of the substrates. Compared to previous procedure for preparing MOF films, the present procedure is simple and the obtained CPL-1 films are continuous and uniform. It is expected that the repeated in situ crystallization would be suitable for other MOFs.

Reference

1. S. Kitagawa et al., Angew. Chem. Int. Ed., 2004, 43, 2334.

2. M. Kubo et al., Chem. Mater., 2008, 20, 2887-2889.