Electrochemical self-assembly (ESA) of metal-organic framework (MOF) thin films of Zn~(2+) ions and terephthalic acids (TPA) in a layered structure and with a composition of Zn_3(OH)_4(TPA)·6H_2O was made possible by addition of TPA into the bath for electrodeposition of ZnO, namely, O_2-saturated aqueous solution of ZnCl_2. The deposition of Zn-TPA MOF, however, resulted in a strong passivation of the electrode towards O_2 reduction to limit the electrochemical growth. Interestingly, chemical precipitation of Zn-TPA MOF was found to take over the film growth to achieve a thickness of about 2 μm in the extended electrolysis, in spite of the negligibly small current. Due to decrease of TPA concentration in the solution bulk, the electrode finally recovered the activity for the O_2 reduction, which, however, resulted in the electrodeposition of ZnO to be overlaid on the Zn-TPA MOF. The electrodeposited Zn-TPA MOF was well adherent to the substrate.
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