Detail:
Abstract: Conformal and oriented metal-organic frameworks (MOF) thin film enclosing arbitrary nanostructures (substrates, nanowires, nanocrystals) are fabricated with precisely controlled thickness and pristine interface. This is achieved by atomic layer deposition of aluminum oxide on the substrates and addition of a tetra-topic porphyrin based linker, 4,4',4'',4'''-(porphyrin-5,10,15,20-tetrayl)- tetrabenzoic acid (H4TCPP), to react with alumina and make MOF [Al2(OH)2TCPP] enclosures. Alumina thickness is precisely controlled from 0.1 to 3 nm, thus allowing control of the MOF thickness from 10 to 50 nm. Electron microscopy and grazing angle X-ray diffraction confirm the order and orientation of the MOF by virtue of the porphyrin units being perpendicular to the substrate surface. These features of the MOF enclosure enable us to use surface-enhanced Raman spectroscopy to directly track the porphyrin metalation process of the MOF enclosing octahedral silver nanocrystals. When metalated with cobalt, the Al2(OH)2TCPP-Co thin film functions as catalyst for the selective and efficient reduction of carbon dioxide to carbon monoxide (CO) in aqueous electrolytes with a selectivity for CO beyond 76% and stability over 7 h with a per-site turnover number (TON) of 1400. This electrochemical catalytic activity is unique to the thin film form of the MOF (below 100 nm) and depends strong on the film thickness. This research presents a generalizable method to produce structurally well-defined nano-MOF composite material that has extraordinary functions.
