Electrodeposition of Zn-Terephthalate MOF Thin Films

Metal organic flamework (MOF) is a class of inorganic / organic compound whose crystalline structure is made by bridging inorganic ions with organic ligands. Due to its open porous structure, special functionalities are anticipated in applications such as sensors, catalysis and batteries. Variation of MOFs is almost unlimited, not only by choice of building blocks but also by change of composition and crystalline structure. We have recently reported microwave assisted hydrothermal synthesis of MOFs with various layered structures, in which Zn 2+ ions are bridged by terephthalic acid (TPA). These Zn-TPA MOFs exhibited proton-selective reversible redox reactions, suggesting their usefulness in membrane-free redox batteries. On the other hand, we have established a method to electrochemically self-assemble ZnO / organic dye hybrid thin films. When there is a favorable chemistry between inorganic and organic constituents, their hybrid nanostructures are spontaneously formed during electrolysis. Self-assembly of Zn-TPA MOFs can therefore be expected by minor addition of TPA into the bath for cathodic electrodeposition of ZnO. Potentiostatic electrolysis at -1.0 V (vs. Ag/AgCl) in O 2 -saturated aqueous electrolyte containing 5 mM ZnCl 2 and TPA up to 500 μM resulted in gradual decrease of cathodic current of oxygen reduction current (Fig. 1). Strong passivation occurs when TPA > 75 μM to reduce the steady-state current down to ca. 0.1 mA cm -2 , as compared to the O 2 transport limit (ca. 1 mA cm -2 for Zn 2+ + O 2 + H 2 O + 2e - → ZnO + H 2 O 2 ). Minor addition of TPA at 5 μM resulted in appearance of thin flake-like deposits together with tiny grains of ZnO, which then changes into entire coverage of the FTO-glass substrate with the flakes at 200 μM while ZnO grains disappear (Fig. 2). X-ray diffraction (XRD) patterns of the deposits revealed that the flakes are those of Zn-TPA layered MOF in a Zn 3 (OH) 4 (TPA)•6H 2 O composition. The passivation is obviously caused by high electrochemical resistance of the electrodeposited MOF towards O 2 reduction, making it difficult to obtain thick MOF films for battery applications. Exploring the conditions to promote continuous electrochemical growth of MOF is under way. Figure 1