Synthesis of metal-organic framework films by pore diffusion method.

Metal-organic frameworks (MOFs) presents high controllability in designing the nano-scale pore, and this enable molecular storages, catalysts, gas sensors, gas separation membranes, and electronic devices for next-generation. Therefore, a simple method for film synthesis of MOFs compared with conventional methods $^{\mathrm{[1]}}$ is strongly required. In this paper, we provide pore diffusion method, in which a substrate containing constituent metals of MOF is inserted in solution that includes only linker molecules of MOF. As a result, 2D growth of MOF was effectively enhanced, and the formation of flat and dense MOF films was attained. The growth time, $t$, dependence of film thickness, $d$, can be expressed by the relation of $d \quad =$ Aln($t+$1) $+$ B, where A and B are constants. It means that ionized coppers diffuse through the pores of MOFs and the synthesis reaction proceeds at the MOF/solvent interface. We demonstrated the fabrication of a HKUST-1/Cu-TPA hetero structure by synthesizing a Cu-TPA film continuously after the growth of a HKUST-1 film on the CuO$_{\mathrm{x}}$ substrate. [1] Denise Zacher \textit{et al., Angew. Chem. Int. Ed.} \underline {50}, 176 (2011).